1. No category

Potential New Aging Model Schmidtea Mediterranea Undergoes Gradual
Senescence, Experiences Accelerated Senescence at Elevated Temperature
by
David A Bertin
A Thesis
presented to
The University of Guelph
In partial fulfillment of requirements
for the degree of
Master of Science
in
Molecular and Cellular Biology
Guelph, Ontario, Canada
© David A Bertin, April 2015
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ABSTRACT
POTENTIAL NEW AGING MODEL SCHMIDTEA MEDITERRANEA UNDERGOES GRADUAL
SENESCENCE, EXPERIENCES ACCELERATED SENESCENCE AT ELEVATED TEMPERATURE
David A Bertin
Advisor:
University of Guelph, 2015
Dr. Ray Lu
Current model organisms for aging research, commonly C. elegans and D. melanogaster,
possess significant limitations such as non-aging life stages, loss of 10-15% of genes compared
to non-Ecydosozoans, and low or absent tissue repair by stem cells. An emerging model
organism, Schmidtea mediterranea, does not have these limitations. This flatworm species may
be a much better representation of human aging while still having the desirable traits of a
simple model organism. This study characterized normal aging through demographic study
illustrating that this flatworm species, from a demographic standpoint, is indeed a suitable
model. A number of additional relationships were shown including the reduction of lifespan and
reproductive output resulting from elevated growth temperature, and high reproductive output
correlates with long life.
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ACKNOWLEDGEMENTS
I would firstly like to express gratitude to my family for their love, advice, and support
throughout these past years. Without them I wouldn’t have made it nearly this far. I would also
like to thank past lab colleagues including Mandi Martyn, Cheryl Cragg, Adam McCluggage,
Talya Amor, Karin Olek, Wan Kong Yip, Minghua Zeng, and Alex Caruso for their training, advice,
and shared knowledge. Finally, I wish to thank Dr. Ray Lu and my advisory committee members
Drs. Terry Van Raay, and Doug Fudge for helping me see the way, turning obstacles into
avenues, and providing fresh perspective and indispensable advice.
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Table of Contents
Acknowledgements.........................................................................................................................iii
List of Figures ..................................................................................................................................vi
List of Tables .................................................................................................................................. vii
List of Abbreviations ..................................................................................................................... viii
1. Introduction ................................................................................................................................ 1
1.1 Evolutionary causes of aging................................................................................................. 1
1.1.1 Mutation accumulation ................................................................................................. 2
1.1.2 Antagonistic pleiotropy.................................................................................................. 2
1.1.3 Disposable soma theory................................................................................................. 3
1.1.4 Heat dissipation limit ..................................................................................................... 4
1.2 Aging mechanisms and interventions ................................................................................... 6
1.2.1 Insulin/insulin-like growth factor (IGF-1) signaling........................................................ 6
1.2.2 Multiplex stress resistance ............................................................................................ 9
1.2.3 Non-genetic interventions ........................................................................................... 11
1.2.4 Germline ablation ........................................................................................................ 12
1.2.5 Caloric restriction ......................................................................................................... 13
1.3 Schmidtea mediterranea as a model organism for aging ................................................... 13
1.3.1 Planarians in research .................................................................................................. 13
1.3.2 Replicative neoblast stem cell population ................................................................... 15
1.3.3 Ecdysozoan gene loss ................................................................................................... 16
1.3.4 Diapause and dauer states, larval instars .................................................................... 18
2. Objectives.................................................................................................................................. 20
3. Materials and Methods ............................................................................................................. 21
3.1 General culture conditions ................................................................................................. 21
3.1.1 Growth media and feeding .......................................................................................... 21
3.1.2 Age-synchronized colonies .......................................................................................... 21
3.2 Demographic colony establishment ................................................................................... 22
3.2.1 Collection of anterior lines........................................................................................... 22
3.2.2 Demographic data curation ......................................................................................... 23
3.2.3 Lifespan data analysis and model fitting ..................................................................... 23
3.3 Measurement of global protein carbonylation .................................................................. 23
3.4 Measurement of available ATP ........................................................................................... 24
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3.5 Cloning of Smed-age-1 ........................................................................................................ 26
3.5.1 RNA isolation and first strand synthesis ...................................................................... 26
3.5.2 Cloning into L4440 dsRNA expression vector .............................................................. 26
3.6 Antibody characterization................................................................................................... 28
3.6.1 Antibody generation .................................................................................................... 28
3.6.2 Western blotting .......................................................................................................... 28
3.6.3 Epitope preincubation ................................................................................................. 28
3.7 siRNA knockdown ............................................................................................................... 29
3.7.1 In vitro expression of age-1 dsRNA .............................................................................. 29
3.7.2 Feeding protocol .......................................................................................................... 30
3.7.3 Quantitative reverse transcription PCR (qRT-PCR) ...................................................... 30
4. Results ....................................................................................................................................... 31
4.1 S. mediterranea kept at an elevated growth temperature shows a reduced lifespan ...... 31
4.1.1 Survival data shows a significant difference between elevated and ambient
temperature cohorts............................................................................................................. 31
4.1.2 Model fitting shows that a Gompertz model best describes lifespan at elevated
temperature .......................................................................................................................... 34
4.2 Elevated growth temperature decreases reproductive rate at all lifespans ...................... 37
4.3 Long-lived ambient temperature worms show a resurgence in weekly reproductive rate in
late life ...................................................................................................................................... 39
4.4 Reproductive events become more frequent with each successive event when at elevated
temperature but not at ambient temperature......................................................................... 41
4.5 Global protein carbonylation does not significantly increase with advancing age ............ 43
4.6 ATP availability decreases with advancing age ................................................................... 43
4.7 SMED-AGE-1 presents as a 140 kDa species by western blot ............................................ 46
4.8 siRNA knockdown of age-1 reduces expression levels to 20 % of endogenous levels and
lasts between four and five weeks ........................................................................................... 48
5. Discussion.................................................................................................................................. 49
6. Conclusions and Future Directions ........................................................................................... 54
7. References ................................................................................................................................ 58
Appendix ....................................................................................................................................... 64
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LIST OF FIGURES
Figure 1: Simplified diagram illustrating how IIS can affect multiple downstream signaling
pathways
Figure 2: Overlap of IIS pathway and dauer formation pathway (DAF)
Figure 3: A partial cladogram showing the gene-loss event
Figure 4: Single measurements of standard concentrations and ratios of ATP and ADP
Figure 5: Diagram illustrating the cloning strategy for Smed-age-1 full-length coding sequence
Figure 6: Lifespan distributions for ambient (20 °C) and elevated (26 ± 0.5 °C) cohorts
Figure 7: Survivorship curve for ambient (20 °C) and elevated (26 ± 0.5 °C) cohorts
Figure 8: Total reproduction rate by life span with LOESS spline by treatment
Figure 9: Reproduction and survival rates for short-lived and long-lived groups of worms
Figure 10: Median time to next reproductive event plotted against current reproductive event
Figure 11: Two putative aging biomarkers were tested for use with S. mediterranea
Figure 12: Epitope preincubation of polyclonal rabbit α-AGE1 antibody (GenScript)
Figure 13: RNAi knockdown of Smed-age-1 duration and intensity
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LIST OF TABLES
Table 1: Key data points extracted from demographic study of S. mediterranea including cohort
size, average (avg), high, and low values for reproduction data
Table 2: Gompertz coefficients and mortality rate doubling time
Table 3: Oligonucleotide primers used for PCR amplification and cloning of age-1
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LIST OF ABBREVIATIONS
ADP
AKT/PKB
AMP
AMPK
AP
ATP
BMR
CA
Ct
DAF
DNP
dsRNA
dT
FOXO
Gapdh
IGF-1
IIS
kDa
LAM
LC50
LOESS
MA
MCS
MRDT
Nox
PBS
PCR
PI3K
PIP2
PIP3
PTEN
PVDF
qRT-PCR
RAS
RLU
RNAi
ROS
RT
SDS
siRNA
TOR
TORC
Adenosine diphosphate
Ak strain thymoma/protein kinase B
adenosine monophosphate
AMP-activated protein kinase
antagonistic pleiotropy
adenosine triphosphate
basal metabolic rate
caloric restriction
cycle threshold
dauer formation
2,4-dinitrophenol
double-stranded RNA
deoxythymidine
forkhead box class O
glyceraldehydes 3-phosphate dehydrogenase
insulin-like growth factor 1
insulin/IGF signaling
kilodalton
longevity assurance mechanism
lethal concentration, 50% mortality
local regression
mutation accumulation
multiple cloning site
mortality rate doubling time
NADPH oxidase
phosphate buffered saline
polymerase chain reaction
phosphatidylinositol-3-kinase
phosphatidylinositol 4,5-bisphosphate
phosphatidylinositol (3,4,5)-trisphosphate
phosphatase and tensin homolog
polyvinylidene fluoride
quantitative reverse-transcription PCR
rat sarcoma family
relative light unit
RNA interference
reactive oxygen species
room temperature
sodium dodecyl sulfate
small interfering RNA
target of rapamycin
target of rapamycin complex
viii
1. INTRODUCTION
1.1 Evolutionary causes of aging
To date, many hypotheses have been put forward to describe the aging process in terms
of causes and effects involving a variety of intracellular and intercellular processes. There are a
number of theories regarding the evolution of aging (Gavrilov & Gavrilova 2002; Hughes &
Reynolds 2005), as well as a great wealth of information regarding the mechanisms by which it
is thought to occur (Hughes & Reynolds 2005; Liu & Rando 2011). There exists a large body of
research on individual mechanisms impacting senescence, such as the involvement of reactive
oxygen species (ROS), nutrient sensing and caloric restriction (CR), DNA damage and mutation
accumulation, and telomere length, among many others (Medvedev 1990). From an
evolutionary perspective, senescence is thought to be an inevitable consequence of natural
selection (Hughes & Reynolds 2005). If we consider a population of a non-senescent organism,
its lifespan must be determined by extrinsic (ie. environmental) causes such as predation and
physical hazards. Therefore, there exists an expected lifespan for this species regardless of their
immortality. Natural selection would tend to favour those individuals that invest more
resources into reproduction in this expected lifespan at the expense of somatic maintenance in
order to maximize genetic contribution to the next generation. This would drive the evolution
of an intrinsic (ie. age-related) lifespan similar to or earlier than that specified by extrinsic
causes. The review also provides experimental and survey data to support this hypothesis. For
example, populations of a given species in high predation environments show earlier
senescence patterns than those in low predation environments (Hughes & Reynolds 2005). As a
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consequence of this selection process several phenomena are proposed to occur resulting in
the complex aging process observed in contemporary organisms.
1.1.1 Mutation accumulation
Due to the asymmetric distribution of selection strength throughout lifespan, mutation
accumulation (MA) was proposed as a specific evolutionary cause of senescence (Medawar
1952). This theory proposes that any mutations having deleterious effects late in life will have
little or no selective pressure against them and may accumulate in the genome by genetic drift.
Since reproductive effort is primarily focused during early life, natural selection will act most
strongly on genes with effects before or during this productive period. An example which fits
this model is the mutant allele causing Huntington’s disease. Since the effects of the disease are
post-reproductive there is little selection pressure and the allele persists in the population. This
theory fits with regard to the aforementioned development of intrinsic mortality to optimize
fitness, and also includes the special case of antagonistic pleiotropy.
1.1.2 Antagonistic pleiotropy
An additional level of complexity gives rise to the concept of antagonistic pleiotropy (AP)
(Williams 1957). Like MA, AP refers to the accumulation of late-acting deleterious genes during
natural selection. The difference is that in AP the genes in question have beneficial early effects
and detrimental late effects. An example of such a gene is any member of the Nox family. These
genes produce physiological levels of ROS required for a number of cellular functions but also
contribute to late-life pathologies such as hypertension and atherosclerosis (Lambeth 2007).
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1.1.3 Disposable soma theory
An organism’s lifespan may not be limited only by late-acting detrimental effects. It is
thought that active, energy-intensive mechanisms exist for maintaining the body with age. Such
a mechanism is known as a longevity assurance mechanism, or LAM (Speakman & Krol 2010).
The premise of the disposable soma theory (Kirkwood 1977) is that an individual organism has a
limited amount of available energy. This energy must be shared between reproductive output
and the LAMs, both of which are energy-intensive processes. This observation supports the
theory that small animals tend to reproduce early and live short lives (Medawar 1952). Smaller
animals tend to have a higher extrinsic mortality risk. As such, a small animal with a high rate of
extrinsic mortality ought to invest more energy into reproduction before death occurs
extrinsically. It follows that this high and early investment in reproduction comes at the cost of
energy expenditure on LAMs. In larger organisms the risk of extrinsic mortality is low leaving
ample time for reproduction while allotting more energy into LAMs resulting in a relatively
longer lifespan and lower rate of aging. To clarify this idea we can consider the evolution of a
large animal from a small ancestor (Speakman & Krol 2010). Initially extrinsic mortality is high,
reproduction is early and in high numbers, late-acting detrimental mutations accumulate, and
aging is rapid. As the size of the organism increases over generations, extrinsic mortality
decreases and selection begins to favour lower intrinsic mortality while selecting against AP and
MA effects. All of these changes favour longer lifespan and reproductive investment is reduced.
While a decrease in reproductive output may seem counterintuitive from an evolutionary
perspective, the large animal tends to have fewer offspring with a higher likelihood of survival
compared to the many offspring at a lower likelihood of survival seen in the small animal. For
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example, Asian elephants generally produce single offspring with 80-90 % surviving to
adulthood (Mumby et al. 2013), and giant pacific octopuses produce around 56 000 offspring
with less than 0.004% surviving to adulthood (Cosgrove & McDaniel 2009).
1.1.4 Heat dissipation limit
A novel theory contradicting the disposable soma theory is the heat dissipation limit
proposed recently by Speakman and Krol (2010). These researchers reject the assumptions
made by the disposable soma theory and instead argue that the limiting factor concerning
reproductive output is the ability to dissipate waste heat. The rejection of this theory is
primarily due to the failure to show high-energy requirements of supposed LAMs. Many cellular
processes have been implicated in the modulation of aging, none of which when altered to
prolong life show any increase in energy demand (van Voorhies et al. 2003). Further to this,
larger animals tend to have a lower basal metabolic rate (BMR) per unit mass than smaller
animals. This is contradictory to the idea that larger animals have active energy sink LAMs as is
required by the disposable soma theory (Speakman & Krol 2010). The authors also argue that
during times of reproduction energy reserves are often unlimited resulting in an energy surplus
which can support both somatic maintenance and reproduction. Although some of these points
are conjectural and lack significant support, the authors reference some convincing studies
done on the subject. In a number of food supplementation studies less than a third reported
increases in litter/clutch size, and these increases were minimal. This information does suggest
that food energy is not a limiting resource in terms of reproductive output. The final premise
offered to dismiss the idea of a limited amount of available energy concerns the winter coat
seen in many endotherms. This coat serves to prevent heat loss and conserve energy during a
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time of limited resources. The authors presume that if energy was limited year-round, the
animals would maintain the coat year-round to conserve energy. This argument is weak in that
the authors fail to consider that as weather warms a thick coat becomes an energy burden by
requiring active cooling. Despite these cursory arguments against the disposable soma theory,
the arguments given in favour of the heat dissipation limit are fairly strong. It is proposed that
the low surface area to volume ratio of larger animals results in a lower capacity to dissipate
heat which in turn would limit the amount of energy expended on a task like reproduction. As
such, selection will not favour large animals with high investment in early reproduction. In
studies on mice, there exists a maximum limit of lactation which cannot be exceeded by
increasing food intake. This maximum was not exceeded by increasing litter size or forcing the
mothers to expend energy by exercise, but was only exceeded by decreasing the environmental
temperature. Mice kept at low temperatures produced more milk and larger offspring, and
those at thermoneutral conditions produced less milk and smaller offspring, supporting the
heat dissipation limit as the limiting factor in energy usage. Another key finding notes that
metabolic rate scales to the surface area to volume ratio (Speakman & Krol 2010).
These theories and observations are best understood by again considering the evolution
of a larger size in a small endotherm.. As the animal evolves a larger size the ability to dissipate
heat is lessened. This results in a reduction in reproductive investment in order to reduce the
production of heat since reproduction is energy intensive. Since reproductive investment is
reduced or delayed there is now a selective pressure against the negative effects of AP and MA.
This selective pressure may also encourage LAMs provided that their energy requirement, and
subsequent contributed heat generation, is low. The end result is that as organisms evolve into
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larger forms, reproductive output is reduced and longevity is enhanced due to changing
selection pressure on AP, MA, and LAM effects.
Without further testing it is premature accept the heat dissipation theory. This theory
only pertains to endotherms and therefore provides only a partial explanation in the evolution
of senescence. The lifespan of ectotherms also correlates size with extrinsic mortality risk,
lifespan and reproductive output. Although heat dissipation ability appears to be a factor in
endotherm life history evolution, it has much less relevance to ectotherms where similar
phenomena are observed. This suggests that heat dissipation plays only a small part in lifespan
determination.
1.2 Aging mechanisms and interventions
1.2.1 Insulin/insulin-like growth factor (IGF-1) signaling
Molecular mechanisms have been linked to the aging process and many pathways have
been shown to affect lifespan (Friedman & Johnson 1988; Kenyon et al. 1993; Wei et al. 2008).
A key pathway with numerous regulatory components is the insulin/IGF signaling (IIS) pathway.
Multiple mutants have been identified that lack functional components of this pathway and
subsequently show a change in longevity (Friedman & Johnson 1988; Gerisch et al. 2001; Wei et
al. 2008). By way of the IIS and other converging signaling pathways a variety of cellular
processes are modulated. These include general stress resistance, cell cycling, apoptotic
control, proliferation, and translational control. As such, single changes in signaling can produce
disparate cellular responses (Figure 1). Insulin-like signals trigger an activation of
phosphatidylinositol-3-kinase (PI3K) of which AGE-1 makes up the catalytic subunit. PI3K
6
phosphorylates phosphatidylinositol 4,5-bisphosphate (PIP2) to form phosphatidylinositol
(3,4,5)-trisphosphate (PIP3). This action is opposed by PTEN which maintains a low PIP3 level in
the absence of an insulin-like signal. PIP3 then activates AKT (also known as PKB) which in turn
regulates multiple downstream cellular activities.
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Figure 1: Simplified diagram illustrating how IIS can affect multiple downstream signaling
pathways. Those shown (proliferation, cell cycling, apoptosis, translation) are modulated by
AKT (also known as PKB). “CS” denotes catalytic subunit, “PH” denotes pleckstrin homology
domain. Adapted from “Development_IGF-1 receptor signaling” by GeneGo, Inc.
http://www.genego.com/maps/540_map.png.
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1.2.2 Multiplex stress resistance
The most potent single gene influencing lifespan is known as age-1 which encodes a
class-I PI3K catalytic subunit as seen in Figure 1 (Ayyadevara et al. 2008). The AGE-1 and DAF
(abnormal dauer formation) pathways converge at DAF-16 nuclear transport and subsequent
activation (Figure 2). Consistent with the idea of cross-talk between intracellular signaling
pathways presented above, C. elegans age-1 null mutants show resistance to a variety of
cellular stresses in addition to the 10-fold lifespan extension. These null worms show
significantly higher resistance to thermal stress, oxidative stress, and electrophilic stress relative
to wild-type (Ayyadevara et al. 2008). Furthermore, the degree of resistance is often correlated
with the degree of lifespan extension associated with the various age-1 alleles (Ayyadevara et
al. 2008). Other studies have also shown that mutants of IIS components including age-1 and
members of the daf gene family show resistance to UV exposure (Murakami & Johnson 1996)
and have significantly higher lethal concentration (LC50) values for the heavy metals copper
and cadmium (Barsyte et al. 2001).
This concept of lifespan extension via suppression of IIS is consistent with the disposable
soma theory of aging. Under the assumptions of the theory, one would expect that the
alteration of a nutrient sensing pathway could influence resource allocation, and thus lifespan.
A lack of functional AGE-1 or DAF-2 increases DAF-16 nuclear translocation resulting in
transcriptional activation of target genes (Shmookler Reis et al. 2009). This DAF-16 activation
also occurs during periods of stress. In effect, the age-1 or daf-2 mutant acts as if the cell is
always under stress. Following the disposable soma theory, during times of stress resources will
be shifted away from reproduction and towards somatic maintenance. In this case, insufficient
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Figure 2: Overlap of IIS pathway and dauer formation pathway (DAF). A. IIS pathway
contributing to DAF-16 activation and longevity. With permission from Neumann-Haefelin et al.
(2008) B. Influence of DAF proteins on longevity and dauer formation. Also shown is AGE-1
which similarly to DAF-2 inhibits DAF-16 nuclear localization. Adapted with permission from
Cold Spring Harbor C elegans II Section IV, Subsection B: Genetic Pathways, Figure 3, p. 754
(1997).
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suppression of DAF-16 activity results in a shift towards somatic maintenance, and therefore,
lifespan extension. It has also been observed that extended lifespan by age-1 mutation reduces
fertility and hence reproductive output (Friedman & Johnson 1988).
Oxidative stress, among other cellular stresses, has major influence on some of the
accumulative changes that are thought to contribute to aging (Sohal 2002). This type of stress
leads to the accumulation of oxidized proteins in the form of carbonylated amino acid side
chains (Stadtman 1993). These modified proteins have impaired function and contribute to
physiological decline (Sohal 2002). The quantity of available metabolic energy also provides a
measure for the activity of the IIS pathway. The relationship of energy balance and lifespan has
been shown to be mediated through AAK-2 protein in C. elegans (Apfeld et al. 2004). Both of
these aspects of cell physiology have successfully been used as biomarkers of aging in C.
elegans, providing a good measure of true age.
1.2.3 Non-genetic interventions
A number of chemical compounds have been identified that affect aging in laboratory
animals. Rapamycin is able to extend lifespan possibly via activation of FOXO (DAF-16) through
inhibition of TORC2 complex (Shmookler Reis et al. 2009). Another more commonly known
compound is resveratrol. Naturally found in red wine, resveratrol has been shown to promote
longevity in C. elegans (Bass et al. 2007). Several other compounds have been shown to
promote longevity in laboratory conditions including SRT1720 (Milne et al. 2007; Zarse et al.
2010). Both resveratrol and SRT1720 exert this effect by activation of sirtuins. Sirtuins have
been shown to extend lifespan in yeast, possibly through gene silencing via histone
deacetylation, and increased sirtuin levels are seen in calorie restricted mice (David Adams &
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Klaidman 2007). Although chemical modifiers of aging look very alluring, findings have been
mixed overall. Resveratrol has been shown to extend lifespan in some studies (Zarse et al.
2010), but has also been shown to have no effect, or a negative effect in others (Bass et al.
2007). Another issue regarding the use of chemical interventions is the physiologically relevant
dosage. In many cases the compounds were studied at doses too high for practical therapeutic
use (Zarse et al. 2010). In the case of resveratrol, replication of dosage in humans would require
drinking over 12 000 glasses of red wine (David Adams & Klaidman 2007). Resveratrol has more
recently been thought to target a variety of other signaling molecules apart from sirtuins. One
of these targets is AMP-activated protein kinase (AMPK) which acts to regulate energy balance
via carbon utilization and ATP production (Kaeberlein 2010). Interestingly though, these
compounds have been shown to augment lifespan through IIS or by mimicking the effect of
caloric restriction (Wood et al. 2004; Kaeberlein 2010). The fact that these compounds interact
with those pathways already shown to alter lifespan lends credence to the claim of lifeextending pharmaceuticals, but many obstacles exist which currently limit clinical applicability.
1.2.4 Germline ablation
Due to the integrative nature of the molecular pathways influencing aging, it comes as
little surprise that the germline is intimately related to the aging process. The original age-1
mutant strain showed lifespan extension as well as a marked decrease in fertility (Friedman &
Johnson 1988). In a reverse cause and effect fashion, ablation of the germ-line can lead to
lifespan extension (Berman & Kenyon 2006). The results of these studies are consistent with the
disposable soma theory and the division of resources; extending lifespan requires energy
normally allotted to reproduction. By ablating the germline, the energy is made available for
12
somatic maintenance and long life. Likewise, extending lifespan robs the germline of energy
and results in reduced fertility.
1.2.5 Caloric restriction
Dietary restriction has long been known to extend lifespan (McCay et al. 1935). In more
recent studies particular mechanisms of action have been proposed. When grown in calorie
limiting media, C. elegans larvae were shown to induce a stress response involving DAF-16.
Similar to age-1 null mutants, which exhibit constitutive nuclear localization of DAF-16,
starvation induces an acute stress response which also causes nuclear DAF-16 localization
(Weinkove et al. 2006). As such, caloric restriction results in increased longevity and stress
resistance by invoking the same DAF-16 transcription factor and stress response as seen in IIS.
A more recent study has implicated the Ras, Tor, and Sch9 signaling pathways in the mediation
of the effects of caloric restriction (Wei et al. 2008). These pathways act to integrate nutrient
sensing pathways like the IIS with cellular growth and division. Although currently very complex
and convoluted, the specific contributions of pathways and their individual protein components
are being probed, and their involvement in stress resistance and lifespan extension is being
uncovered.
1.3 Schmidtea mediterranea as a model organism for aging
1.3.1 Planarians in research
A range of planarian species have been studied for their many remarkable properties
throughout the past two centuries. Planarian regenerative capacity was first studied by
zoologist George Shaw in 1790, and was thoroughly evaluated by T. H. Morgan in 1898.
13
Although planarians have classically been studied for their regenerative properties, there has
been a more recent surge in experimental usage due to interest in stem cells. Adult planarians
maintain a population of pluripotent stem cells known as neoblasts throughout the majority of
their body tissues which orchestrate regeneration and other processes (Baguñà et al. 1989).
Currently, major planarian research is being performed focusing on parasitology (Collins III &
Newmark 2013), regeneration (King & Newmark 2012), apoptosis (Bender et al. 2012), and
development (Molina et al. 2011; Auletta et al. 2012).
Schmidtea mediterranea has a fairly small (4.8 x 108 bp), stable, diploid genome with
few transposable elements (Saló 2006). Other species of this phylum are known to be mixoploid
with varying ploidy states for somatic and germ cells (Saló 2006). S. mediterranea is also
available in sexual or asexual strains as determined by a simple Robertsonian translocation (De
Vries et al. 1984). The asexual strain is particularly useful in that the generation of clonal lines
can be simple and rapid. These genetically uniform lines can be produced by simply allowing an
individual worm to asexually bud or by manually cutting and allowing the worm to regenerate.
Planarians possess many other more typical traits seen in model organisms: they are small, kept
in a simple low salt solution, reproduce at high rates, only need to be fed weekly, and flourish
at room temperature. Recently, a multitude of molecular interventions have been established
for this species including an optimized whole mount in situ hybridization protocol, simple RNA
interference using an expression vector system with application by feeding, and specialized
protocols for other mainstream procedures like protein/DNA/RNA isolation from tissue
(Sanchez Alvarado & Newmark 1999; Pearson et al. 2009). The genome sequencing project and
14
corresponding database is also a highly valuable tool for study design or in silico study (Robb et
al. 2008).
The study of aging has heretofore been performed primarily using C. elegans and D.
melanogaster, and also to some extent using mice. Although effective initial choices, each
model organism possesses noteworthy shortcomings which can impede longevity studies.
Although mice have value in being more closely related to humans, colony maintenance is
expensive, lifespan is relatively long, and experimental manipulations can be complex. The flies
and nematodes, both of the superphylum Ecdysozoa, have more desirable traits such as short
lifespan, simple colony maintenance, and ease in molecular interventions, but also have their
own serious shortcomings. The Ecdysozoans have life history states, dauer and diapause in
addition to metamorphosis in Drosophila, which have no parallel in the mammalian life history,
and somatic cells are largely, or entirely post-mitotic. Furthermore, it has been shown that C.
elegans and Drosophila underwent a significant gene loss event wherein at least 7-9% of genes
were lost compared to the chordates (Technau et al. 2005). Taking these characteristics into
consideration, S. mediterranea is proposed as a new model organism for aging study which may
provide information about the role of unique, physiologically relevant processes on senescence.
1.3.2 Replicative neoblast stem cell population
The primary feature making the planarian (S. mediterranea; terms will herein be used
interchangeably) a preferred model organism for aging studies is the presence of pluripotent
neoblasts. In C. elegans, age-1 null alleles affect AKT signaling which is required for proper cell
replication (Shmookler Reis et al. 2009). In the entirely post-mitotic C. elegans this does not
pose a problem, but may be complicated when actively dividing cells are involved. Planarians
15
have an actively replicating cell population. Since organismal aging is thought to occur as a
result of the accumulation of senescent cells in tissues (Campisi 2005), the role of tissue
regeneration by the action of stem cells must also be important.
1.3.3 Ecdysozoan gene loss
As mentioned, planarians have not suffered the gene loss observed in the Ecdysozoans,
C. elegans and Drosophila (Technau et al. 2005) which occurred more recently than the most
common ancestor of the Ecdysozoans and the Lophotrochozoans. Some of these lost genes
may be involved in the aging process or in stress resistance. Future genetic screens would
benefit greatly by having this body of genes common to both humans and planarians, but not
Ecdysozoans. By using a model organism that underwent this gene loss, we exclude any
pertinent genes that may be included in this set. The gene loss is illustrated in Figure 3.
16
Figure 3: A partial cladogram showing the gene-loss event. Relatedness between the Ecdysozoa
(C. elegans and D. melanogaster), the Chordata (H. sapiens), Lophotrochozoa (planarians; S.
mediterranea), and the outgroup Cnidaria (Hydra spp) is shown. The proposed gene loss event
occurred after the most recent common ancestor between Lophotrochozoa and Ecdysozoa
indicated by the asterisk. Cladogram was compiled from Nielsen (2001) and Maddison and
Schulz (2007).
17
1.3.4 Diapause and dauer states, larval instars
Diapause in flies and the dauer state in nematodes are both stages induced during
unfavourable environmental conditions wherein the organism appears not to age (Austad
2009). In C. elegans, the organism can remain in dauer state for several months (Weinkove et
al. 2006) whereas the normal, non-dauer lifespan of the worm is only about fifteen days
(Ayyadevara et al. 2008). After a return to favourable conditions these organisms are able to
exit from these stages and resume their lifespan; That is, their biological clock resumes ticking
upon reintroduction from dauer/diapause.
Similarly, the flies and nematodes both have discrete developmental stages demarcated
by molting during growth. Drosophila undergo three larval stages, pupate, and then eclose as
an adult fly, while nematodes undergo four larval stages which may or may not include dauer
formation depending on available resources and temperature.
Planaria have no periods of growth arrest as the Ecdysozoan model organisms do, nor
do they develop through discrete life stages. These stages have no human equivalent and as
such may present as confounding factors. For example, the original age-1 null allele is
considered a dauer constitutive mutation; that is, unless grown at low temperature all mutants
remain permanently in the dauer stage (Ayyadevara et al. 2008). In fact, experimenters must
raise the mutants at low temperature until all larval stages are complete. Only then may the
worms be brought to normal growth temperatures and exhibit long lifespan. Thus the observed
lifespan extension could be due to dauer-type stress resistance mechanisms being in effect
while the worm is not physiologically or behaviourally in the dauer state. If this is the case,
would this mutation have the same effect in an organism without such growth arrested states?
18
In fact, some life span extension has been seen in higher eukaryotes but with less potency by
knocking out the mammalian homolog of igf-1 receptor daf-2 (Holzenberger et al. 2003). It
appears that some inference to mammalian systems can be made from studies done in C.
elegans, but there must be some additional processes that make up the difference between the
25% lifespan increase seen in mice and the 1000% increase seen in C. elegans. We propose that
these processes exist in S. mediterranea and are awaiting discovery.
19
2. OBJECTIVES
In order to make full use of S. mediterranea as a model organism for aging studies, some
groundwork must be completed. We must first characterize normal aging in this species; for
example, even the most basic information such as the lifespan of the species is unknown. In
fact, many researchers consider the worms immortal based on their endless regeneration
capacity (Torre & Ghigo 2015). This study aimed to characterize normal aging using a
demographic study based on previous work performed by Mouton et al. (2009). Furthermore,
reproductive data was collected and described for this population as reproductive rate has long
been shown to correlate with lifespan (Chen et al. 2007; Mukhopadhyay & Tissenbaum 2007).
The study also aimed to adapt or create aging biomarkers to be used as a tool to measure the
impact of experimentation on lifespan.
20
3. MATERIALS AND METHODS
3.1 General culture conditions
3.1.1 Growth media and feeding
All planarians were initially maintained in a 1 x Montjuïch solution (1.6 mM NaCl, 1.0
mM CaCl2, 1.0 mM MgSO4, 0.1 mM MgCl2, 0.1 mM KCl, 1.2 mM NaHCO3, pH 7.0) as specified by
Cebria and Newmark (2005). The demographic colony responded well to this medium and it
was used for the duration of the study. The mixed age source colony and age-synchronized
colonies experienced periodic bacterial or protozoal infection and in addition to antibiotic
treatment (gentamycin, 50 μg/mL) were switched to a 0.5 g/L Sea Salt Mixture (Instant Ocean,
United Pet Group, Blacksburg, VA). Planarians were kept entirely in the dark when not being
handled.
Planarians were fed a weekly diet of 1-2 hours ad libitum feeding on pureed veal liver.
The liver was dissected and major connective tissue and blood vessels were removed. The
remaining tissue was blended in a Waring blender and frozen in 5 cm Petri plates at -80 °C.
After 1-2 hours of feeding excess liver was discarded and the worms transferred to fresh
growth medium and clean containers.
3.1.2 Age-synchronized colonies
In order to test biomarkers of aging, groups of age-synchronized worms were
established. Initially, before the lifespan of the species was known, age-synchronized colonies
were established every two months. As the demographic study progressed and the lifespan
appeared to be much longer than initially predicted, this collection was changed to once every
21
four months. These colonies were maintained in ~175 mL cups with population sizes beginning
at around 100 individuals. In order to preserve the uniform age of each colony, fission products
(buds) were removed weekly and recorded for future analysis and/or tracking of the colony
health.
3.2 Demographic colony establishment
The demographic study was conducted by following a previous study of the marine
planarian, Macrostomum lignano (Mouton et al. 2009). Individual posterior fragments were
selected from the population at large and were kept, individually, in 3 mL of media in a well of a
12-well tissue culture plate. Each week the worms were fed 6 μL liver puree for 1-2 hours, any
buds were removed and recorded, and any dead worms were assessed for cause of death and
also recorded. Occasionally worms would die due to physical injury during handling,
desiccation, infection, etc. These individuals were censored and their lifespan and reproductive
data were not used in the final analysis.
3.2.1 Collection of anterior lines
Based on the results of classic and modern studies, two hypotheses were made about S.
mediterranea. Firstly, this organism undergoes senescence as observed in other asexual
metazoans (Martinez & Levinton 1992). Secondly, the posterior fragment of transverse fission is
immortal while the anterior fragment senesces (Sonneborn 1930; Newmark & Alvarado 2002).
It is obvious that one of the two fragments must be immortal, otherwise the species would die
out within one lifespan. All age-synchronized colonies used in this study were established
through the collection of anterior fission products which were assigned an age of zero.
22
3.2.2 Demographic data curation
Lifespan and reproduction data were noted weekly during feedings. The data were
entered into a database program written in Pelles C IDE (Version 6.00.4, Copyright Pelle Orinius
1999-2009). The data were stored in a Microsoft Excel file for export and further analysis.
3.2.3 Lifespan data analysis and model fitting
Basic lifespan data from the elevated temperature cohort were analyzed using
mathematical model fitting procedures described by Broadbent (1958) and Pletcher (1999). The
standard temperature cohort has not reached 90th percentile in terms of lifespan and therefore
cannot be processed by this method. Additional statistical analyses were conducted by Dr.
Daniel Gillis from the Department of Mathematics and Statistics, University of Guelph. Analyses
were conducted using the freeware statistical program R (v3.1.2) (R Core Team 2014). Packages
downloaded for analyses and data visualization included ggplot2 (v1.0.0) (Wickham & Chang
2013), survival (v2.37-7) (Therneau 2013), multcomp (v1.3-7) (Hothorn et al. 2008), gmodels
(v2.15.4.1) (Warnes 2012), and contrast (v0.19) (Kuhn et al. 2013). All plots were produced
using ggplot2, with the exception of Figure 7, which also required the ggsurv function (code
freely available at the R-statistics blog [url: http://www.r-statistics.com/2013/07/creatinggood-looking-survival-curves-the-ggsurv-function/]).
3.3 Measurement of global protein carbonylation
Lysates were collected by homogenizing whole worms in 100 μL modified Orii buffer
(62.5 mM Tris-HCl buffer pH 6.8, 2% SDS, 10% glycerol, 2% β-mercaptoethanol) (Orii et al. 2005)
per mg of worm tissue. The lysate was then treated with OxyBlot™ Kit (EMD Millipore, Billerica,
23
MA, USA) components to convert carbonyl groups into 2,4-dinitrophenylhydrazone (DNPhydrazone). The samples were then slot blotted onto nitrocellulose membranes using a 24 slot
vacuum blotter (Whatman, GE Healthcare, Baie d’Urfe, QC). The membranes were blocked and
probed with an α-DNP antibody (in kit) at 1:150 dilution in block or a monoclonal mouse α-βactin antibody (Sigma-Aldrich, St. Louis, MO, USA) at 1:5000 dilution in block. Membranes were
treated with secondary antibody, 1:300 kit secondary or 1:10 000 HRP conjugated α-mouse
(Thermo Fisher Scientific, Waltham, MA) for DNP and β-actin respectively. Membranes were
treated with ECL Plus (GE Healthcare, Baie d’Urfe, QC) and signal was detected with Molecular
Imager® ChemiDoc™ XRS System (Bio-Rad Laboratories, Hercules, CA).
3.4 Measurement of available ATP
Lysates were produced by rapidly homogenizing whole worms in 10 μL ice cold PBS per
mg of tissue with a pellet pestle (Sigma-Aldrich, St. Louis, MO, USA). Large cellular debris was
removed by centrifugation. A small amount of supernatant was snap frozen in liquid nitrogen
where it remained frozen until immediately before assay. Using the EnzyLight™ ADP/ATP Ratio
Assay Kit (BioAssay Systems, Hayward, CA, USA), ATP concentration was measured relative to
ADP concentration using a Turner TD-20e Luminometer (Promega Corporation, Fitchburg, WI).
Initially, a standard curve was performed to confirm that the concentrations were within range
of detection (Figure 4).
24
Figure 4: Single measurements of standard concentrations and ratios of ATP and ADP. A)
ADP:ATP ratio measured for three concentrations. B) RLU measurements for ADP and ATP
independently, normalized to the 1x sample. 1x – 1 μM ATP and 0.2 μM ADP, 10x – 10 μM ATP
and 2 μM ADP, 25x – 25 μM ATP and 5 μM ADP, 100x – 100 μM ATP and 20 μM ADP. The 100x
sample saturated the luminometer and is not shown. The expected measurement based on
concentration or ratio is shown.
25
3.5 Cloning of Smed-age-1
3.5.1 RNA isolation and first strand synthesis
Total RNA was isolated from whole organisms using TRIzol reagent (Thermo Fisher
Scientific, Waltham, MA). The mRNA was reverse-transcribed using a SuperScript® III Reverse
Transcriptase (Thermo Fisher Scientific, Waltham, MA) and oligo-dT primers (Sigma-Aldrich, St.
Louis, MO, USA).
3.5.2 Cloning into L4440 dsRNA expression vector
Figure 5 depicts the cloning process with a list of primers provided in Table 3. The age-1
cDNA was amplified using three sets of overlapping primers, each encompassing around 1000
base pairs referred to as fragments A, B and C. Each fragment was amplified using the
respective primer sets with the addition of a 5’ HindIII recognition sequence on fragment C and
a 3’ NheI recognition site on fragment A. After silica column purification (Bio Basic Canada Inc.,
Markham, ON) fragment B and C were joined by a single polymerase reaction utilizing the
forward primer for fragment B, the reverse primer for fragment C, and the overlapping region
between them. Fragment BC was then subcloned into the multiple cloning site (MCS) of the
L4440 plasmid making use of an internal XbaI site and the added HindIII site. Finally, fragment A
was subcloned into the L4440-BC plasmid using the same internal XbaI site. NheI and XbaI
produce compatible ends (5’ CTAG) which allowed the insertion of fragment A which was then
screened for proper orientation. L4440 was a gift from Andrew Fire (Addgene plasmid # 1654).
26
Figure 5: Diagram illustrating the cloning strategy for Smed-age-1 full-length coding sequence.
27
3.6 Antibody characterization
3.6.1 Antibody generation
Rabbit polyclonal antibodies were raised using peptides NEPEGPTETDQSLSC for SmedWI-1, a neoblast marker, and CNNRKKKFGVNRERV for Smed-AGE-1. Both antibodies were
produced and affinity purified by Genscript (Piscataway, NJ). Both antibodies were affinity
purified.
3.6.2 Western blotting
Whole planarians were homogenized in a modified Laemmli buffer (62.5 mM Tris-HCl
pH 6.8, 10% glycerol, 2% SDS, 10% β-mercaptoethanol) previously used by Orii et al. (2005).
Tissues were homogenized using a pellet pestle (Sigma-Aldrich, St. Louis, MO, USA) boiled for
10 minutes and centrifuged to pellet larger debris. Samples were separated by electrophoresis
on an 8% SDS-PAGE gel and wet transferred to PVDF membrane. Membranes were blocked in
5% skim milk powder in PBS-Tween (140 mM NaCl, 3 mM KCl, 5 mM NaHPO4, 2 mM KH2PO4,
0.1% Tween-20, pH 7.4) for 1 hour at room temperature (RT), incubated in primary antibody
diluted in blocking solution 1 hour to overnight, and secondary antibody in blocking solution for
1 hour. Blots were treated with ECL Plus (GE Healthcare, Baie d’Urfe, QC) and exposed to
Hyperfilm ECL (GE Healthcare, Baie d’Urfe, QC) and developed.
3.6.3 Epitope preincubation
Initial tests of the α-AGE-1 antibody showed a large number of distinct bands. In order
to distinguish the non-specific bands from those identifying the protein of interest, epitope
preincubation was used. The primary antibody solution was treated with an excess of the same
28
peptide used for antibody generation, ie. the epitope. This should block the antibody from
binding the AGE-1 protein and therefore the change in banding pattern will identify the specific
target polypeptide.
3.7 siRNA knockdown
A simple system for knocking down gene expression was developed for use in C. elegans
(Timmons & Fire 1998; Timmons et al. 2001), and has since been adapted for use in planarians
(Sanchez Alvarado & Newmark 1999). The L4440 vector was used in this study with the gene of
interest inserted in the multiple cloning site set between flanking T7 promoters. These
promoters provide locations to initiate transcription both upstream and downstream of the
gene thereby producing two fully complementary RNA molecules which then anneal to form a
duplex. This transcription may occur in vitro as done in this study, or may be produced by
RNase-deficient E. coli strain HT115. This dsRNA is fed to the subject inducing RNA interference
leading to reduction of mRNA and protein levels by siRNA.
3.7.1 In vitro expression of age-1 dsRNA
Large amounts of starting material are produced by PCR amplification of the gene of
interest using primers designed against the T7 promoter. This DNA is purified and then
transcribed in vitro using T7 polymerase. The original DNA template is degraded using DNaseI
treatment. The final dsRNA is purified and analyzed for quality by spectrophotometry
(NanoDrop 8000, Thermo Fisher Scientific, Waltham, MA) and by non-denaturing agarose gel.
29
3.7.2 Feeding protocol
A mixture of liver puree and dsRNA is made fresh at a concentration of 100 ng/mL of
liver plus 1 % food dye. This mixture is fed to a number of worms for 1-2 h and worms which
have eaten become stained and may be identified by colour and retained while those that have
not eaten are discarded.
3.7.3 Quantitative reverse transcription PCR (qRT-PCR)
The amount of mRNA was measured by qRT-PCR using a SYBR based system. Total RNA
was reverse transcribed using qScript supermix (Quanta BioSciences, Inc., Gaithersburg, MD)
and qPCR was used to determine relative abundance of mRNA using PerfeCTa® SYBR® Green
FastMix® (Quanta BioSciences, Inc., Gaithersburg, MD) and an Applied Biosystems 7300 RealTime PCR System (Waltham, MA). Data were analyzed using the ΔΔCt method (Livak &
Schmittgen 2001).
30
4. RESULTS
4.1 S. mediterranea kept at an elevated growth temperature shows a reduced
lifespan
4.1.1 Survival data shows a significant difference between elevated and ambient temperature
cohorts
In addition to basic characterization of flatworm lifespan, the elevated temperature
cohort (26 ± 0.5 °C) was kept in an attempt to shorten lifespan as observed previously for
ectotherms (Ragland & Sohal 1975; Miquel et al. 1976; Klass 1977). Worms were kept
individually and age at death was recorded. These data are presented in Figure 6 as a lifespan
distribution and some key statistics are presented in Table 1. A Wilcoxon Signed-Rank test
verifies a non-zero shift between the curves with a p-value of 7.4 x 10-10. This statistical test is a
non-parametric test used to compare samples which are not assumed to be normally
distributed. A non-zero shift means that the data distributions are significantly different.
Survival curves for each cohort are also presented in Figure 7. A Cox proportional hazards
model yields a parameter level of -0.812 at a p-value of 1.2 x 10-8, which indicates that the
worms at elevated temperature die at approximately 2.3x the rate of the ambient worms. This
statistical model relates the amount of time passing (lifespan) before an event occurs (death) in
relation to one or more covariates (temperature). The parameter level returned describes the
relative difference in the amount of time (lifespan) between the two groups where the fold
difference is a function of the parameter level (= eparameter level).
31
Table 1: Key data points extracted from demographic study of S. mediterranea including cohort
size, average (avg), high, and low values for reproduction data.
32
Figure 6: Lifespan distributions for ambient (20 °C) and elevated (26 ± 0.5 °C) cohorts. Density
specifies the proportion of all worms falling into each age range. The median lifespan group for
the ambient cohort is 90-120 weeks, and for the elevated temperature cohort is 90 weeks.
33
4.1.2 Model fitting shows that a Gompertz model best describes lifespan at elevated
temperature
The aging process is defined by characteristic physiological changes occurring over the
lifespan of an organism eventually ending with death. At every point in the lifespan of a
population there exists a mortality rate which is a reflection of these physiological changes. The
mortality rate is a measure of normal aging for a given population and is represented by a
mortality distribution. For current model organisms used to study aging this information is
available and represents normal aging for the species in question. This information describes
the changes in mortality rate that occur in an aging population. To develop S. mediterranea as a
model for aging research, this demographic information needs to be developed. In order to
collect these mortality data and infer important parameters such as initial mortality rate and
mortality rate doubling time, a longitudinal study was performed based on a previous design
with modification (Mouton et al. 2009). Two large cohorts were established, one at ambient
laboratory temperature (20 °C) and another at an elevated temperature (26 ± 0.5 °C). Deaths
were recorded and mortality models were fitted to the data (Broadbent 1958; Wilson 1994).
The mortality curve generated (Figure 7) shows a typical sigmoid shape consistent with the
commonly observed exponential increase in mortality following reproductive age and the long
tail characteristic of exceptionally long-lived individuals. The ambient/room temperature (RT)
cohort study is in progress as of the writing of this manuscript whereas the elevated
temperature cohort has surpassed the 90th percentile. As such, the mortality data of the
elevated temperature cohort was fit to a Gompertz distribution, a probability distribution
commonly applied to lifespan, with the coefficients of the fit presented in Table 2. A Gompertz
34
coefficient (b) of 0.145 gives a mortality rate doubling time (MRDT) of 4.79 time intervals (r)
which equates to 0.46 years. This means that the likelihood of death at any given moment
doubles every 0.46 years. This value falls within the range indicative of gradual senescence
(Finch 1990).
Table 2: Gompertz coefficients and mortality rate doubling time. Ambient cohort remains to be
determined (TBD) due to exceptionally long lived individuals.
35
Figure 7: Survivorship curve for ambient (20 °C) and elevated (26 ± 0.5 °C) cohorts. The curves
were generated using lifespan data and a Cox Proportional Hazards Model. Median survival is
indicated for each cohort, ambient being 81 weeks and elevated 55 weeks.
36
4.2 Elevated growth temperature decreases reproductive rate at all lifespans
The effect of temperature on lifespan, mortality rate and distribution is displayed in
Figures 6 and 7. Since lifespan and reproduction are closely linked, we sought to examine the
effect of temperature and lifespan on reproductive rate by plotting total reproductive rate
against lifespan for each growth temperature (Figure 8). A LOESS curve, a non-parametric
technique combining multiple regression models, was fit to the data to show trends. Our first
observation is that each cohort has a unique profile but it is clear that total reproduction rate is
higher for all lifespans at ambient temperature compared to the elevated temperature cohort.
Another observation is that in the ambient cohort, peak total reproductive rate occurs near 50
weeks of age but remains relatively stable for all lifespans in the elevated temperature cohort.
37
Figure 8: Total reproduction rate by life span with LOESS spline by treatment. Treatments
include ambient (20 °C) and elevated (26 ± 0.5 °C) temperatures. Each data point represents an
individual planarian and its lifelong reproductive rate. Note that for the elevated temperature
cohort, there were 13 individuals who lived ~52 weeks or less that had no reproduction.
38
4.3 Long-lived ambient temperature worms show a resurgence in weekly
reproductive rate in late life
In addition to examining the relationship between lifespan and total reproductive
output, a more detailed analysis was performed by subdividing the ambient temperature
cohort into short-lived (bottom 25th percentile) and long-lived (top 25th percentile) groups. Both
weekly survival rate and weekly reproduction rate were plotted (Figure 9). The short-lived
group shows a steady and rapid decline in both reproduction rate and survival rate with
increasing age. The long-lived group shows a much slower fall in survival rate and in late life
shows a resurgence in reproductive rate.
39
Figure 9: Reproduction and survival rates for short-lived and long-lived groups of worms. Shortlived (SL: bottom 25th percentile of lifespan) and long-lived (LL: top 25th percentile of lifespan)
ambient temperature worms were analyzed for reproductive rate and survival rate through
rolling intervals of 11 weeks beginning at week 3. The reproductive rate and survival rate for
each interval are plotted. Confidence intervals are shaded for each LOESS curve. A Poisson
regression was applied and tested by Tukey’s post hoc analysis showing a significant difference
between short-lived and long-lived reproductive rate. In addition, long-lived reproduction rate
began increasing after reaching 100 weeks of age.
40
4.4 Reproductive events become more frequent with each successive event when at
elevated temperature but not at ambient temperature
In addition to analyzing the frequency and magnitude of reproduction, we also wanted
to determine the interval between reproductive events for each cohort. We hypothesized that
planaria at higher temperatures would have a longer overall time interval between
reproductive events given the overall lower lifelong reproductive rate, and a shorter time
interval between successive reproductive events in order to explain the increase in total
reproduction at longer lifespans shown in Figure 8. Figure 10 shows the median time between
reproductive events for each cohort. In support of our hypothesis, reproductive events become
gradually more frequent with advancing age at elevated temperatures contrary to the relatively
constant interval observed in the ambient cohort
41
Figure 10: Median time to next reproductive event plotted against current reproductive event.
Numbers below each line indicate the number of individuals used to calculate each median.
Treatments include ambient (20 °C; gray) and elevated (26 ± 0.5 °C; red) temperatures. Note: At
the beginning of the experiment, 100 planaria at the elevated temperature had approximately
9 weeks between reproductive events, while 96 planaria at the ambient temperature cohort
had 3 weeks between reproductive events. Also note that the longest lived planarian at the
elevated temperature reproduces once every 6 weeks.
42
4.5 Global protein carbonylation does not significantly increase with advancing age
We have shown by demographic analysis that this planarian species undergoes
senescence. Several established biomarkers of aging in other species were examined for
applicability in this species. Such biomarkers will be useful as an instantaneously measurable
metric for cellular senescence, and will also provide insight into the underlying mechanisms of
senescence. In a number of species cellular proteins undergo irreversible oxidative damage and
accumulate with advancing age (Levine 2002; Nystrom 2005) typically forming aldehyde or
ketone groups on side chains of lysine, arginine, proline and threonine residues (Stadtman
1993). Although the cause of this accumulation is currently unknown, elevated levels of protein
carbonyl content are observed in a number of studies occurring in a number of tissues and
organisms (Levine 2002). To examine this biomarker, carbonylated proteins were quantified
and normalized (Figure 11A) using a kit (Protein Carbonyl Content Assay Kit, Abcam). No
significant difference was observed between young (4-5 week old) and old (122 week old)
samples.
4.6 ATP availability decreases with advancing age
Another well established biomarker concerns available metabolic energy as measured
by ATP concentration (Collins et al. 2008). The concentration of ATP is a product of catabolic
processes and represents the available energy content of the cell which itself represents the
metabolic status of the cell. An age-dependent decline in ATP concentration was observed in C.
elegans and this decline can be blocked by interfering with daf family genes (Braeckman et al.
2002). ATP content was measured in four age groups using a luminescent assay. Since the
luminometer returns relative light units ADP content was also measured to normalize ATP
43
content against in order to compare different samples. A statistically significant increase in the
ratio, which implies a decrease in ATP concentration, was observed over the course of
approximately 125 weeks but there was no significant difference between any lesser time span
(Figure 11B).
44
Figure 11: Two putative aging biomarkers were tested for use with S. mediterranea. (A) Global
protein carbonylation was quantified using the OxyBlot™ Kit and (B) ADP:ATP ratio was
measured using EnzyLight™ ADP/ATP Ratio Assay Kit. There is a significant difference in
ADP:ATP ratio between the youngest (2-5 weeks old) and oldest (127-131 weeks old) animals.
45
4.7 SMED-AGE-1 presents as a 140 kDa species by western blot
The Smed-age-1 coding sequence is 3081 base pairs corresponding to a 1026 amino acid
polypeptide. This polypeptide is predicted to have a molecular weight of 119 kDa by Compute
pI/Mw tool hosted by ExPASy resource portal (Gasteiger et al. 2003). Western blotting using
our polyclonal α-AGE-1 antibody results in a characteristic banding pattern of three, always
present, major bands and up to six minor bands which may or not be visible depending on total
protein concentration and antibody dilution. The largest of the three major bands is eliminated
through pre-incubation of peptide epitope, and is not eliminated in the non-specific peptide
(WI1) negative control (Figure 12). This band presents as a ~140 kDa species, running ~20 kDa
larger than expected, which may be a result of post-translational modification or differences in
buffer between the ladder and the lysates.
46
Figure 12: Epitope preincubation of polyclonal rabbit α-AGE1 antibody (GenScript). An arrow
indicates which band corresponds to the protein of interest. Primary antibody dilution in
blocking solution is given plus the corresponding peptide epitope added. A size standard is
given on the left with sizes expressed in kDa.
47
4.8 siRNA knockdown of age-1 reduces expression levels to 20 % of endogenous
levels and lasts between four and five weeks
To assess the effects of genetic knockdowns on aging it is useful to know how long a
single treatment will last. Worms were fed dsRNA representing a large portion of the age-1
coding sequence. Individuals were harvested weekly and mRNA levels measured by qRT-PCR.
Knockdown of age-1 was reduced to approximately 20 % of normal levels and persisted for at
least four weeks (Figure 13).
Figure 13: RNAi knockdown of Smed-age-1 duration and intensity. qRT-PCR measurements of
age-1 mRNA quantity were taken weekly and normalized to gapdh after RNAi feeding.
Expression drops by 80% for 4-5 weeks before returning to or exceeding wildtype levels.
48
5. DISCUSSION
When modeling human aging it is advantageous to use a model system with a similar
aging rate; we have shown that this is true for S. mediterranea. The MRDT, the length of time
required for mortality rate to double, of 0.46 years is desirable in that, while considered to
describe gradual senescence (between 0.3 to 10 year MRDT), the organism still has a high
mortality rate compared to humans with an MRDT of around 8 years (Finch 1990; Wilson 1994).
This organism permits the study of aging including the effects of stem cells on tissue
replacement and repair while still senescing at a high rate compared to humans. As such, S.
mediterranea could provide novel insight into mammalian aging more quickly and easily than
using mammalian models. The results of this work represent the first demographic
characterization for this species, and only the second for all flatworm species. The resultant
data provide a framework that future investigators can use to design and execute aging studies.
The mortality distributions presented in Figure 7 illustrate the typical sigmoid shape
observed for other gradually senescing species including rodents, birds, and domestic dogs
(Finch & Pike 1996), showing a decline in mortality rate at advanced age represented as a long
tail. This sigmoid shape is also observed in populations of C. elegans (Stroustrup et al. 2013) and
Drosophila (Linford et al. 2013), which are both categorized as rapidly senescing species. Based
on the Gompertz coefficients produced by the model fitting procedure for the elevated
temperature cohort, S. mediterranea exhibits the characteristics of a good model for aging
research since its mortality rate doubling time falls in the same category as humans (Finch et al.
1990). In addition, the elevated temperature cohort illustrates the effect of temperature on
senescence as was previously observed in other ectotherms both in wild and laboratory
49
conditions (Miquel et al. 1976; Klass 1977). Despite their exceptional regenerative capability
and ability to maintain telomere ends (Tan et al. 2012), this study shows that planarians do
undergo senescence. In contrast, a lack of senescence has been demonstrated in hydra, a genus
also characterized by asexually reproducing individuals with a high degree of stem cell activity
(Martinez 1998). Future research could focus on differences in these organisms that may cause
flatworms to age while hydra appear non-senescent.
The use of S. mediterranea as a model organism for aging research is largely predicated
on the presence of actively dividing totipotent stem cells known as neoblasts. These cells
represent approximately 30 % of total cell population in these planarians (Baguñà et al. 1989).
The importance of stem cell activity and tissue replacement in organismal aging is suggested by
a number of studies in a few species of planaria (Lange 1968; Nimeth et al. 2002; De Mulder et
al. 2010) in which ablation of all stem cells is accomplished by RNAi, by γ-irradiation, or
chemical inhibition of the cell cycle. Upon removal of the stem cell population lifespan is
drastically shortened. Senescent cells exhibit characteristic changes in physiology including
adapting a secretory phenotype and exhibiting changes in cell morphology. There also exist
changes in gene expression patterns (Coppe et al. 2008). Targeted apoptosis of senescent cells
in mice has been shown to extend lifespan (Baker et al. 2011). These findings may also indicate
cellular replacement and tissue repair act as an opposing force to cellular senescence and
therefore organismal senescence. The exceptional lifespan of S. mediterranea, especially when
compared to other simple ectotherms such as C. elegans, is consistent with this proposed
involvement of stem cells in aging. This discrepancy in lifespan between S. mediterranea and
the closely related species P. velata, with a lifespan of one to two months (Austad 2009), is
50
worth further investigation. Any differences in stem cell number or function could further
implicate their involvement in aging.
Kirkwood’s disposable soma theory (Kirkwood 1977) proposes a trade-off between
reproduction and somatic maintenance as a result of limiting resources. The analysis of
reproduction and lifespan presented here is in concordance with this concept. Figure 8
illustrates an overall fall in total reproductive rate with advancing age, although when grouped
by lifespan (Figure 9) we see long-lived individuals have a resurgence in weekly reproductive
rate in late life. The fact that this late-life increase in weekly reproductive rate in the long-lived
group has little effect on total reproductive rate in all groups (Figure 8) could indicate a distinct
subpopulation experiencing this phenomenon. In addition to the overall declining trend in total
reproduction rate, each cohort displays a unique profile in terms of the shape of each LOESS
spline. The ambient cohort shows a prominent peak around the 50 week lifespan point. This
means that individuals with this lifespan tend to have the highest total reproductive rate. This is
consistent with the “live fast, die young” concept proposed by Kirkwood. In contrast, the
elevated temperature cohort appears to peak later and much more modestly at around 100
weeks. This peak is also preceded by a trough around the 50 week lifespan. This trough and
subsequent delay in peak reproductive rate may be due to an adjustment period associated
with the elevated growth condition. When this colony was established, new offspring were
taken from an ambient colony and placed for growth at the elevated temperature. Under new
growth conditions, the worms may have undergone physiological or epigenetic changes as
acclimation. Further analysis could confirm the existence of this adjustment period by
51
comparing newly introduced elevated temperature individuals with those that have been
maintained at elevated temperature for a number of generations.
Time between reproductive events is displayed in Figure 10. Despite the overall lower
lifelong reproductive rate at elevated temperatures, this cohort shows a decreasing trend in
timing between reproductive events. With each reproductive event the time until each
successive event lessens. At first this appears to contradict the general trend of decreasing
reproduction with age, but this decreasing rate actually supports the upward trend observed in
Figure 8 after week 50. This analysis also only includes lifespan indirectly and only looks at the
timing between events. For example, the third reproductive event for one individual may occur
at age 5 weeks while occurring at age 40 weeks for another worm. This trend is not observed in
the ambient cohort, which may in fact show a slight increase in time between events. It is
possible that the downward trend in the elevated cohort reveals the existence of a distinct
subpopulation of high output, high frequency reproductive worms. Such a population would be
obscured by lower output worms until their reproductive period ceases. This phenomenon may
also be present in the ambient cohort but is obscured by the rest of the population. Perhaps
these subpopulations could be identified and isolated by observing individual reproductive
rates and then those groups with low reproductive rates could be compared to those with high
reproductive rates..
In order to make full use of S. mediterranea as an aging model, biomarkers of aging will
need to be confirmed or discovered. A number of biomarkers that correlate with senescence
have already been established (Dimri et al. 1995; Krishnamurthy et al. 2004; Collins et al. 2008;
Park et al. 2009). These biomarkers represent consistent changes with age involving behaviour,
52
tissue organization, metabolism, gene expression and many other facets of physiology (Collins
et al. 2008). This planarian species is a relatively long-lived species, which presents a challenge
since longitudinal studies of lifespan-altering interventions are impractical. Biomarkers present
an alternative metric for assessing the effect of such interventions. We investigated ATP
availability and global protein carbonylation, two established biomarkers in other aging models,
as possible biomarkers for aging in S. mediterranea (Dimri et al. 1995; Krishnamurthy et al.
2004; Collins et al. 2008; Park et al. 2009). The results of these studies show that neither is an
effective biomarker in this species (Figure 11). Global protein carbonylation shows no change
between two extremes of age and ATP availability as assessed by ADP:ATP ratio only shows a
modest change between two extremes in age. An effective biomarker would serve to measure
fine distinctions between ages on a practical timescale on the order of several weeks or months
at the most. This is necessary to measure the effect of various interventions on the aging
process in a reasonable amount of time. One feature of note is that in both assays the older
samples tend to have a smaller standard error. In addition to the aforementioned data
concerning total reproductive rate, this could also suggest the existence of subpopulations of
worms that age at a different rate than the rest of the population. This would interfere with the
function of these aging biomarkers and may explain the lack of observable differences. Since
the planarian strain used is highly inbred to the point of possibly being clonal, epigenetic
changes may well be responsible for the variation seen in this species.
53
6. CONCLUSIONS AND FUTURE DIRECTIONS
This study intended to complete the first steps required to establish S. mediterranea as
a model organism for aging research. Demographic analysis represents an essential part of this
process. We have shown that this species falls into the same category of senescence as humans
and has provided a thorough understanding of the senescent profile, including MRDT,
maximum and median lifespan. For future study this work will serve as an important reference
tool for experimental design. In addition, the change and general decline in reproductive output
is consistent with a senescing species.
The two putative biomarkers, global protein carbonylation and ATP availability, showed
limited utility. The small amount of change seen in ADP:ATP ratio was over a period of two and
a half years which is too long to be practical. It is likely that the cellular changes that occur in
other organisms are obscured by the stem cell activity of these worms. It has been shown that
telomere length is maintained and telomerase activity is high in these worms (Tan et al. 2012),
so perhaps other processes are at work in neoblasts reducing or preventing characteristic
accumulative changes. To address the role of neoblasts in aging, a few approaches could be
taken. One could impair neoblast function by siRNA against cell cycle genes, or by γ-irradiation
followed by measurement of putative biomarkers which have not yet been established for S.
mediterranea such as fluorescent pigment accumulation, morphological changes, accumulation
of DNA damage, etc. (Collins et al. 2008).
Several additional biomarkers were tested in this study. A number of physical and
chemical stressors were implemented and measured by physiological and behavioural changes.
The stressors tested included heat, ultraviolet light, oxidative stress caused by hydrogen
54
peroxide exposure, and electrophilic stress caused by N-ethylmaleimide which represents the
end result of biological lipid peroxidation. These treatments were tested at a variety of
concentrations and exposure times, and the effects were measured by behavioural
observations. General observations were made and it was decided that self-righting behaviour
would serve as an appropriate means of measurement. In an unstressed condition each worm
will instantaneously right itself when inverted. As applied stress is intensified this reaction time
is reduced until reaching unresponsiveness. Although the general trend is present, there was
immense variability within the control groups at a given stress dosage. This variability also
occurred in the time elapsed from treatment until death. As such, no experimental
manipulation was performed.
An important conclusion of this work is that S. mediterranea experiences senescence
including reproductive decline despite a lack of measured biochemical changes with age, most
notably telomere length (Tan et al. 2012). Many have considered telomere shortening, the
agent specifying the Hayflick limit, a key feature limiting lifespan. These conflicting results raise
the question, what causes aging in these worms if not these well-established accumulative or
degenerative changes? This seems to indicate that a major piece of the puzzle is still missing.
This species reproduces by transverse fission producing asymmetrical products. By
necessity, one of these fragments must be immortal in order for the species to survive, and
because we see senescence, the other fragment must be mortal. The differences between
these two fragments could provide an enormous amount of insight into senescence.
Furthermore, if the anterior fragment ages and the posterior does not, we encounter an
interesting problem: After transverse fission the posterior fragment must undergo some sort of
55
rejuvenating process. Such processes may also be unique to this organism and anti-aging in
nature. If we can discover the processes by which this organism can rejuvenate, we can infer
that the systems or pathways acted upon must be progeric and lead to senescence.
Furthermore, we can look for similar processes in mammals or possibly use their mechanism in
combating human age-related disease.
There has been a strong association with stress response and aging (Minois 2000). As
such, it would be beneficial to also gain an understanding of the planarian stress response.
Once again, the stress response has not been studied in planarians and a great deal of work
must be done to understand how these worms respond to various types of stress before any
real experimentation can be done. A good starting point would be a comparison of heat shock
response with or without age-1 knockdown since the C. elegans age-1 knockout showed
multiplex stress resistance (Kenyon et al. 1993).
Further biomarker testing is also warranted. This study only considered two biochemical
attributes, both of which have close ties to metabolism: carbonylation occurs as a result of
metabolic ROS byproducts and ADP:ATP ratio represents available cellular energy. A number of
other biomarkers have been shown to be relevant in C. elegans and it would be well worth
testing and confirming the effectiveness of several more. We have shown that this species does
senesce, so it must be true that some aspects of cell physiology are changing and would serve
as biomarkers of aging. Whether testing biomarkers already demonstrated in other organisms
or searching for novel markers, it is important to continue the search. These markers will have
great value in quantifying the effect of age-related interventions and would be great tools for
56
screening subpopulations to distinguish the short-lived from the long-lived either
independently, or in concert with the aforementioned reproductive rate assessment.
Some fundamental tools for the study of aging using this model system have been
established by this study. Future longitudinal studies may rely on these demographic data to
determine appropriate sample sizes and experimental timelines. Several biomarkers have been
tested and ruled out for high resolution age-group discrimination. This may inform future
experimenters of potential pitfalls and provide guidelines for design. This collection of findings
provides a firm starting point for aging studies in a unique organism with features relevant to
human aging and possibly leading to valuable novel discoveries in this field of study.
57
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APPENDIX
Table 3: Oligonucleotide primers used for PCR amplification and cloning of age-1. Bolded text
represents the addition of a restriction site.
64