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MicroRNAs and Aneurysm Formation
Reinier A. Boon and Stefanie Dimmeler*
Aneurysms occur in large arteries and are characterized by pathological
widening of the vessel and thinning of the vessel wall. In the past decade,
microRNAs (miRs) have emerged as key regulators of biological processes,
and they were recently shown to be involved in aneurysm formation. A few
miRs have been proposed to play a role in aneurysm development, such as
miR-21, miR-26, and miR-143/145. Several recent studies describe the
involvement of miR-29 in aneurysm formation by post-transcriptionally
repressing the expression of extracellular matrix proteins. Therapeutic
inhibition of miR-29 using anti-miRs attenuates experimental aneurysm
formation in mice. This review provides an overview of the upstream
regulation of miR-29 as well as the downstream targets of miR-29. It also
discusses the potential clinical use for miR-29 inhibitors and the role of
other miRs involved in aneurysm formation. (Trends Cardiovasc Med
2011;21:172-177) © 2011 Elsevier Inc. All rights reserved.
• Introduction
In the past decade, microRNAs (miRs)
have been shown to be key regulators of
virtually all biological processes (Bartel
2009). miRs are noncoding RNA molecules that, in contrast to mRNAs, are not
translated into proteins. Being only
20-23 nt long, miRs post-transcriptionally inhibit mRNAs by attenuating protein translation and inducing mRNA
degradation. Target mRNA recognition
by miRs that are incorporated in RNAinduced silencing complexes (RISC) is
facilitated through partially complementary Watson-Crick base pairing. RISCincorporated miRs most frequently bind
to the 3’ UTR of mRNAs, but they can
also bind to the coding region or 5’ UTRs
of their target genes. The main determinant of target specificity is the so-called
seed sequence, usually nucleotides 2-8 of
Reinier A. Boon and Stefanie Dimmeler are at
the Institute of Cardiovascular Regeneration,
Center for Molecular Medicine, Goethe University, 60596 Frankfurt, Germany.
*Address correspondence to: Stefanie Dimmeler, Theodor-Stern-Kai 7, 60596 Frankfurt
am Main, Germany. Tel.: (⫹49) 69 6301 6667;
fax: (⫹49) 69 6301 83462; e-mail: dimmeler@
em.uni-frankfurt.de.
© 2011 Elsevier Inc. All rights reserved.
1050-1738/$-see front matter
172
the miR, that does fully complementary
base pair with the target mRNA sequence. Due to this relatively short target specificity region, one given miR can
have more than 100 target mRNAs.
miRs are endogenously encoded in the
genome and either reside within introns
or are located in between other genes
(intergenic). The intron-embedded miRs
are often co-regulated with their hostgene, but they can also be under control of
their own promoter, as is the case for the
intergenic miRs. To further increase the
genomic complexity, miRs are often transcribed in clusters. These so-called primary miRs are processed into precursor
miRs (pre-miRs) by an enzyme called
Drosha. Pre-miRs are then further cleaved
into the mature miRs by Dicer and incorporated in RISC. Expression of miRs is
also regulated on the level of these processing steps by, for example, Smad proteins (Davis et al. 2008).
• miRs in the Cardiovascular System
The importance of miRs for vascular
homeostasis became clear with the generation of mice with endothelial-specific
depletion of Dicer that displayed postnatal angiogenesis defects (Suárez et al.
2008). miRs also play a crucial role in
vascular smooth muscle cell (VSMC)
physiology, as exemplified by the embryonic lethality of mice with a VSMCspecific depletion of Dicer (Albinsson et
al. 2010). Many miRs have been identified to control crucial biological processes, also in the cardiovascular system
(Bonauer et al. 2010). For example, miR143 and miR-145 were shown to control
smooth muscle cell phenotype (Cordes
et al. 2009), and miR-126 regulates endothelial cell functions (Fish et al. 2008,
Van Solingen et al. 2009, Wang et al.
2008b). Likewise, miRs are involved in
diseases of the cardiovascular system,
such as atherosclerosis (Zernecke et al.
2009) and chronic heart failure (van
Rooij et al. 2006). Consequently, modulation of miR functions is a promising
strategy for therapeutic intervention.
For example, inhibition of miR-92a improved recovery after acute myocardial
infarction in mice by augmenting angiogenesis (Bonauer et al. 2009), and miR143/145, which was recently shown to
act anti-atherosclerotic in a paracrine
manner, may well be used to inhibit
atherosclerosis (Boettger et al. 2009,
Elia et al. 2009, Hergenreider et al.
2012).
• Aneurysm Formation
An aneurysm is a pathological widening
that occurs mainly in large arteries and
most frequently in the abdominal section of the aorta. The main risk factor
for aneurysm formation is aging, with
an approximately 8% incidence in the
general population aged 65 years or older; however, other risk factors, such as
smoking, male gender, and the presence
of atherosclerosis, contribute as well
(Baxter et al. 2008, Singh et al. 2001).
Aneurysms are characterized by ongoing
inflammation, induction of matrix degradation enzymes, and gradual thinning of
the vascular wall, which may lead to acute
rupture, an event with a very high mortality rate (Baxter et al. 2008). Aneurysms
usually develop in predilected sites of the
arterial tree, the most common site being
the abdominal aorta (AAA), but aneurysms are also known to occur in the
ascending part of the thoracic aorta (TAA)
or, for example, in the internal carotid
artery in the brain. Whereas AAAs are
clearly age-dependent, TAAs are less agedependent and often have a genetic basis,
usually in the form of mutations in extracellular matrix (ECM) components (Lindsay and Dietz 2011).
TCM Vol. 21, No. 6, 2011
Most types of aneurysms are characterized by common molecular processes that
underlie inflammation and ECM perturbation. Accelerated by risk factors, influx
of inflammatory cells leads to local degradation of ECM components by secreted
enzymes such as matrix metalloproteinases (MMPs) (Chase and Newby 2003).
Many different MMPs are involved in aneurysm formation and degrade the ECM
proteins elastin and fibronectin and various collagen proteins. Degradation of the
ECM leads to apoptosis of VSMCs, which
stimulates influx of inflammatory cells,
aggravating the aneurysm. Loss of ECM
also perturbs the structural integrity of the
vessel wall, thereby weakening the vessel,
which can lead to rupture.
Relatively common genetic causes of
aneurysm formation include Marfan
syndrome and Loeys-Dietz syndrome
(Lindsay and Dietz 2011). Marfan syn-
drome is caused by mutations in FBN-1,
which encodes fibrillin 1, a component
of the extracellular matrix, causing increased transforming growth factor-␤
(TGF-␤) signaling and perturbed connective tissue function that weakens the
vessel wall. On the other hand, LoeysDietz syndrome is a multigenetic disease caused by mutations in TGFBR1,
TGFBR2, or MADH3, which are all components of the TGF-␤ signaling pathway.
• miR-29 in Aneurysm Formation
Experiments with VSMC-specific Dicer
depletion emphasize the importance of
miRs for VSMC homeostasis (Albinsson
et al. 2010), and it is therefore likely that
miRs also play a prominent role in aneurysm formation, which is characterized by VSMC dysfunction. Indeed, it
was recently shown that miR-29 plays a
pivotal role in the formation of aneu-
Healthy / Normal
miR-29
ECM
Collagens
Elastin
Fibrillins
Others
mRNA
protein
Diseased / Aged
miR-29
ECM
Collagens
Elastin
Fibrillins
Others
mRNA
protein
Therapy / Intervention
LNA-29
miR-29
ECM
Collagens
Elastin
Fibrillins
Others
mRNA
protein
Figure 1. Inhibition of miR-29 induces extracellular matrix synthesis and attenuates aneurysm formation.
In the healthy vessel, extracellular matrix (ECM) is produced by smooth muscle cells and fibroblasts, which
increase the structural integrity of the vessel wall. miR-29 functions as an endogenous brake on the
expression of ECM components such as collagens, elastin, and fibrillins. In diseased situations, the
expression of miR-29 is altered, which may result in perturbation of the synthesis of ECM. Therapeutic
inhibition of miR-29 in the vessel wall augments the production of ECM components and ameliorates
aneurysm formation.
TCM Vol. 21, No. 6, 2011
rysms (Boon et al. 2011, Maegdefessel et
al. 2012b, Merk et al. 2012). These studies report that inhibition of miR-29 reduces aneurysm formation in different
murine models. Specifically, inhibition
of the entire miR-29 family was shown
to prevent angiotensin II (Ang II)–induced dilation of the aorta of aged wildtype mice (Boon et al. 2011). miR-29b
inhibitors reduced aneurysm in the porcine pancreatic elastase (PPE) infusion
model in C57Bl6 mice and, albeit to a
minor extent, in the Ang II infusion model
in ApoE⫺/⫺ mice (Maegdefessel et al.
2012b). Similar results were shown in genetic models using Marfan (Fbn1C1039G/⫹)
mice, in which miR-29b blockade prevented early aneurysm development and
aortic wall apoptosis (Merk et al. 2012). So
far, all studies have used locked nucleic
acid (LNA)-DNA antisense oligonucleotides to inhibit miR-29 family members
that were administered intravenously at
8-20 mg/kg. In addition, overexpression of
miR-29b induced severe aneurysm expansion in two different murine models (Maegdefessel et al. 2012b).
All of these studies point to the same
molecular mechanism: miR-29 posttranscriptionally regulates the expression levels of multiple targets with a
function in the ECM, and therapeutic
inhibition of miR-29 improves the structural integrity of the vessel wall (Figure
1). It was previously shown that in the
heart, miR-29 targets various components of the ECM, such as collagens,
elastin, and fibrillins (van Rooij et al.
2008). These ECM components are also
induced after inhibition of miR-29 in the
vasculature (Boon et al. 2011, Maegdefessel et al. 2012b, Merk et al. 2012).
Interestingly, inhibition of miR-29 can
also be used to augment elastin expression in cells from patients haploinsufficient for elastin and to increase elastin
deposition in bioengineered vessels
(Zhang et al. 2012).
In addition to targeting ECM structural components, miR-29 also targets
the anti-apoptotic protein MCL-1 (Mott
et al. 2007) and, paradoxically, MMP2
(Steele et al. 2010). Indeed, decreased
MCL-1 protein was found in Marfan
mice, and inhibition of miR-29 rescued
apoptosis (Merk et al. 2012), which
could contribute to the therapeutic effects of miR-29 inhibition. However,
173
Table 1.
Comparison of studies describing miR-29 expression in experimental aneurysms
Disease model
Aging
18-Month-old mice
DNA damage
Marfan syndrome
Fibulin-4R/R mice
Fibulin-1C1039G/⫹ mice
Angiotensin II
Ang II in aged wild-type mice
Ang II in young ApoE⫺/⫺
Matrix disruption
Elastase infusion
MMP2 was not changed after inhibition
of miR-29 in wild-type mice (Boon et al.
2011) and was even reduced in the PPEinduced aneurysm model (Maegdefessel
et al. 2012b). This is important because
therapeutic benefit of anti-miRs against
miR-29 relies on upregulation of ECM
synthesis, which could potentially be
counteracted by an upregulation of
MMP2. The observation that MMP2 expression is not induced or even downregulated after miR-29 inhibition could
be the result of less invasion of inflammatory cells, which express high levels
of MMP2 (Oviedo-Orta et al. 2008). An
alternative explanation may be the observation that miR-29 targets the DNA
methyltransferase DNMT3B that epigenetically silences MMP2 and MMP9
(Chen et al. 2011). MMP9 was also consistently reduced by miR-29 inhibition
in two studies (Boon et al. 2011, Maegdefessel et al. 2012b).
Physiologically, miR-29 likely functions as an endogenous brake on the
expression of ECM proteins, thereby
dampening profibrotic effects. Consistently, it has been described that miR-29
inhibition in vivo induces fibrosis in various organs, including heart (van Rooij
et al. 2008), liver (Roderburg et al. 2011),
and kidney (Qin et al. 2011), characterized by increased deposition of ECM.
Table 2.
Regulated miR (fold change)
Reference
miR-29a,b,c (⫹1.6)
miR-29a,b,c (⫹2.5)
Boon et al. (2011)
Ugalde et al. (2011)
miR-29a,b,c (⫹2.5)
miR-29b (⫹5.9)
Boon et al. (2011)
Merk et al. (2012)
miR-29b (⫹1.3)
miR-29b (–2.3)
Boon et al. (2011)
Maegdefessel et al. (2012b)
miR-29b (–2.5)
Maegdefessel et al. (2012b)
Regulation of miR-29 Expression
Although various studies report a therapeutic benefit of miR-29 inhibition in
different aneurysm models (Boon et al.
2011, Maegdefessel et al. 2012b, Merk et
al. 2012, Zhang et al. 2012), the regulation of miR-29 is variable: Some studies
describe the miR-29 family members to
be upregulated in diseased arteries,
whereas some studies show downregulation of miR-29 members (Table 1).
The miR-29 family consists of three
members—miR-29a, -b, and – c—which
are expressed in two clusters. One copy
of miR-29b is co-transcribed from the
genome with miR-29a, whereas the
other miR-29b copy is transcribed with
miR-29c. Boon et al. (2011) showed that
aging induces the primary miR-29b129a cluster, whereas the primary miR29b2-29c cluster is not affected by aging.
However, all mature cluster members—
miR-29a, miR-29b, and miR-29c—were
increased in the aorta of aged mice,
indicating a post-transcriptional control
of miR-29 processing. Consistently, all
mature members of the cluster were
increased in Zmpste24-null mice, a
mouse model of Hutchinson-Gilford
progeria that exhibits accelerated aging
and recapitulates many symptoms of
normal aging (Ugalde et al. 2011). Moreover, all three family members were
shown to be increased in the aorta of
genetic models such as Fib4⫺/⫺ mice
(Boon et al. 2011), and miR-29b was
significantly upregulated in Marfan
(Fbn1C1039G/⫹) mice (Merk et al. 2012).
Together, these data suggest that age
and genetic models of aneurysms augment miR-29 expression. However, in
other models of aneurysm formation, the
results are more heterogeneous: Whereas
Ang II infusion in aged wild-type mice
increased miR-29b (but not miR-29a and
miR-29c) expression (Boon et al. 2011),
both primary miRs and the mature miR29b were significantly decreased after
elastase infusion and miR-29b levels were
reduced after Ang II infusion in ApoE⫺/⫺
mice (Maegdefessel et al. 2012b).
In human aneurysms, miR-29b (but
not miR-29a and miR-29c) was upregulated in thoracic aneurysms in one study
(N ⫽ 109), whereas it was not regulated
in another study (N ⫽ 25) and was
downregulated in abdominal aortic aneurysms (N ⫽ 15) (Table 2). An additional recent report describes the association of altered miR-29 levels with
aneurysm formation in human thoracic
aneurysms (Jones et al. 2011), and using
a bioinformatics approach, miR-29 was
proposed to contribute to aneurysm formation (Liao et al. 2011). It is difficult to
pinpoint from where these discrepancies
arise. All studies use the entire aorta
Comparison of studies describing miR-29 expression in human aneurysms
Regulated miR (fold change)
Type of aneurysm
N
29a
29b
29c
Reference
Thoracic aorta
Thoracic aorta (bicuspid aortic valves)
Thoracic aorta (tricuspid aortic valves)
Abdominal aorta
25
79
30
15
–5.0
NR
NR
NR
NR
⫹1.8
⫹1.7
–2.3
NR
NR
NR
NR
Jones et al. (2011)
Boon et al. (2011)
Boon et al. (2011)
Maegdefessel et al. (2012b)
NR, not regulated.
174
TCM Vol. 21, No. 6, 2011
Transcriptio
Inducers
Aging
P53
Ang II
TGFb
Marfan
n
?
(Boon)
(Ugalde)
(Boon)
(Merk)
(Merk)
Repressors
NFkB
Losartan
Ang II
TGFb
Elastase
(Wang)
(Merk)
(Maegdef.)
(Maegdef.)
(Maegdef.)
?
MCL1
MMP2
pri-miR-29
pre-miR-29
miR-29
ECM
Collagens
Elastin
Fibrillins
Others
gene
mRNA
protein
Figure 2. The expression of miR-29 is controlled by multiple pathways. Transcription of the two primary
miR-29 clusters (pri-miR-29a/b1 and pri-miR-29b2/c) results in expression of the mature miR-29a, miR-29b,
and miR-29c, which post-transcriptionally inhibit gene expression. Regulation of miR-29 expression is
complex and involves transcriptional activators and repressors. Furthermore, miR-29 levels can also be
regulated by processing of the primary transcripts or the precursor transcripts.
specimen for RNA isolation, but these
samples also contain the full heterogeneous mix of cells of which the aorta is
composed: fibroblasts, VSMCs, endothelial cells, and inflammatory cells. Differences in disease state and the presence or
severity of atherosclerotic lesions can alter
the cellular composition, which potentially alters miR-29 levels in the specimen.
Moreover, the differences between
studies may in part be explained by the
various underlying mechanisms that
control miR-29 expression and processing. One of the established cellular
mechanisms of post-transcriptional miR
control comprises the TGF-␤ signaling
cascade (Davis et al. 2008). TGF-␤ signaling is a pivotal factor in aneurysm
formation and was shown to repress
miR-29 in the heart (van Rooij et al.
2008), lungs (Cushing et al. 2011), kidney (Qin et al. 2011), and human aortic
fibroblasts (Maegdefessel et al. 2012b).
However, TGF-␤ did not regulate
miR-29 expression in human smooth
muscle cells in vitro (Maegdefessel et al.
2012b), and treatment of Marfan mice
with TGF-␤ blockers even decreased
miR-29b levels (Merk et al. 2012), suggesting that TGF-␤ signaling augments
miR-29b expression in the vasculature of
genetic models of aneurysm formation in
vivo. Therefore, it is tempting to speculate
that miR-29 transcription and processing
is controlled by TGF-␤ in a context-dependent manner. In addition to TGF-␤ signaling, miR-29 expression is controlled by
p53, which augments miR-29 expression
TCM Vol. 21, No. 6, 2011
(Ugalde et al. 2011) and likely contributes
to the age-dependent upregulation of the
miR-29 cluster members (Boon et al.
2011). Finally, miR-29 is regulated by NF␬B, which transcriptionally represses the
primary miR-29b2-29c cluster by acting
through YY1 and the Polycomb group
(Wang et al. 2008a).
Together, miR-29 family members are
regulated by multiple signaling cascades
at the level of both transcription and processing (Figure 2), and it has been shown
that the miR-29 family members, due to
the slight variances in sequence, have different stabilities and intracellular localization (Hwang et al. 2007, Zhang et al.
2011). One may speculate that those signaling cascades are context-dependent effective in disease models such as aging
(p53 activation and apoptosis), Marfan
syndrome (TGF-␤ activation), and abdominal aneurysm formation (inflammation).
Given these complex signaling pathways,
which in part have opposing effects on
individual miR-29 family members, it
might not be surprising that different expression profiles are observed in different
disease models. Particularly in clinical cohorts, the patient characteristics (eg, age
and gender) as well as treatment (eg, losartan was shown to decrease miR-29 in
Marfan mice [Merk et al. 2012]) may additionally influence the net result.
• Other miRs Involved in Aneurysms
In addition to miR-29, other miRs have
been described to play a role in aneu-
rysm pathology. One of the first reports
alluding to a role for miRs in aneurysm
formation described the mechanism by
which miR-143/145 regulates VSMC
function (Elia et al. 2009). The authors
showed that in human thoracic aneurysms, miR-143 and miR-145 were expressed at lower levels compared to
healthy thoracic aortas, which correlated with VSMC function. Another
study pointing to the role of miR-143/
145 in maintaining VSMC function described the VSMC-specific deletion of
Dicer in mice leading to VSMC contractile dysfunction that could be rescued by
miR-143/145 restoration (Albinsson et
al. 2010). Whether restoration of miR143/145 levels ameliorates aneurysm
formation has not been reported.
Employing in vitro experiments with
VSMCs, Leeper and colleagues (2011)
found that miR-26 promotes a synthetic phenotype through regulation of
SMAD1 and SMAD4. In two mouse
models of aneurysm formation, miR-26
was shown to be decreased, which is
counterintuitive but may represent a
failing endogenous rescue mechanism.
Another pivotal regulator of VSMC phenotype, miR-21, was shown to inhibit aneurysm formation in mice (Maegdefessel et
al. 2012a). miR-21 overexpression protected against aneurysm progression,
whereas inhibition of miR-21 further augmented ongoing aneurysm formation.
Mechanistically, the authors identified the
miR-21 target PTEN, a negative regulator
of Akt, to be responsible for aggravating
aneurysm formation.
• Putative Clinical Intervention
One of the promising aspects of the
rapid progress being made in the
microRNA field in general and in aneurysm research specifically is the high
level of clinical applicability. Whereas
clinical application of miRs or so-called
mimics is still problematic, the use of
miR inhibitors or anti-miRs is highly
efficient and these have been successfully used in many preclinical studies
(van Rooij 2011) and even in a phase II
clinical trial (miR-122; trial number
NCT01200420). Therefore, of the various miRs discussed in this review,
miR-29 inhibition—for example, by using LNA anti-miRs or, alternatively, cholesterol-modified antagomirs (Krützfeldt
et al. 2005)—seems to be the most promising clinical application. Because sys175
temic inhibition may cause side effects
such as fibrosis (van Rooij et al. 2008) or
tumor growth (Iorio and Croce 2012),
local application of miR-29 anti-miRs
would be advantageous. Local application of therapeutics is well-established
in the vasculature, so this should be
relatively easily adaptable for delivery of
miR-29 anti-miRs with, for example,
coated stents or balloons. Whether prolonged local inhibition of miR-29 leads
to too much remodeling or otherwise
alters vessel wall properties remains to
be determined. The elution rate of local
miR-29 inhibitors needs to be carefully
titrated to avoid overremodeling.
• Acknowledgments
The authors are financially supported by
the European Research Council (Advanced grant “Angiomirs”) and the
LOEWE Center for “Cell and Gene Therapy,” which is supported by the state of
Hesse.
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PII S1050-1738(12)00116-8
TCM
Aptamer Binding and Neutralization
of ␤1-Adrenoceptor Autoantibodies:
Basics and a Vision of Its Future in
Cardiomyopathy Treatment夡
Annekathrin Haberland, Gerd Wallukat, and Ingolf Schimke*
Autoantibodies directed against the second extracellular receptor loop
of the ␤1 receptor (␤1-ECII-AABs) that belong to the superfamily of G
protein– coupled receptors have been frequently found in patients with
idiopathic dilated cardiomyopathy, Chagas’ cardiomyopathy, and peripartum cardiomyopathy and have been clearly evidenced to be related
to disease pathogenesis. Consequently, specific proteins or peptides
used as binders in immunoapheresis or as in vivo neutralizers of
␤1-ECII-AABs have been suggested for patient treatment. Aptamers,
which are target specifically selected short single- or double-stranded
RNA or DNA sequences, are a recently introduced new molecule class
applicable to bind and neutralize diverse molecule species, including
antibodies. This article reviews selection technologies and characteristics of aptamers with respect to a single-stranded DNA aptamer recently
identified as having a very high affinity against ␤1-ECII-AABs. The
potential of this aptamer for the elimination of ␤1-ECII-AABs and in
vivo neutralization is critically analyzed in view of its potential for
future use in cardiomyopathy treatment. (Trends Cardiovasc Med
2011;21:177-182) © 2011 Elsevier Inc. All rights reserved.
• Introduction
The number of diseases showing an etiological or accompanying involvement
Annekathrin Haberland and Ingolf Schimke are
at the Pathobiochemie und Medizinische Chemie, Charité - Universitätsmedizin Berlin, 10117
Berlin, Germany; Gerd Wallukat is at the MaxDelbrück-Centrum für Molekulare Medizin
Berlin-Buch, 13125 Berlin, Germany.
夡
This work was supported by the European
Regional Development Fund (10141685; Berlin, Germany) and Stiftung Pathobiochemie,
Deutsche Gesellschaft für Klinische Chemie
TCM Vol. 21, No. 6, 2011
of autoantibodies in their pathogenesis
is constantly growing. In addition to the
classic autoimmune diseases such as
myasthenia gravis, lupus erythematound Laboratoriumsmedizin (66/2007) and 48/
2011, Germany).
*Address correspondence to: Ingolf Schimke,
PhD, Pathobiochemie und Medizinische Chemie, CC11, Charité Universitätsmedizin Berlin,
Charitéplatz 1, 10117 Berlin, Germany; e-mail:
[email protected].
© 2011 Elsevier Inc. All rights reserved.
1050-1738/$-see front matter
sus, and Graves’ disease, increasingly
more diseases can be traced back to an
autoantibody involvement. Among these
diseases, there are also disorders of the
heart and circulatory system. Whereas
autoantibodies usually play a role in the
destruction of their targets and target
tissues, the situation is different for the
autoantibodies of a distinct group of
heart and circulatory diseases. In these
cases, so-called “functional autoantibodies” have received an increasing amount
of attention. These functional autoantibodies are agonistically directed against
G protein– coupled receptors (GPCRs) of
the heart and circulatory system.
GPCRs are the largest gene superfamily in humans (Jacoby et al. 2006) and
cover approximately 800 different receptors. All these receptors possess seven
transmembrane domains. Historically,
the GPCRs are subdivided into three
groups—the rhodopsin-like receptors
(family A), the peptide receptors (family
B), and the receptors of the GABA and
glutamate type (family C). Their common feature is the downstream signal
cascade exploiting the GTP-binding proteins.
The huge variety of GPCRs allows the
regulation of a plethora of physiological
processes. The GPCR family includes
receptors used in sensory perception,
cell growth and movement, as well as
receptors engaged in physiological regulation via neurotransmitters and hormones. It is this last category of GPCRs
that is the target of autoantibodies in the
case of cardiomyopathies. Given the
critical role of the ␤1-adrenergic receptor in the regulation of heart chronotropy and inotropy, it can be readily appreciated that a prolonged activation of
such receptors via autoantibodies would
have dramatic consequences.
Table 1 lists a variety of GPCRs for
which autoantibodies have been identified
in patients with heart and circulatory diseases. The epitopes and the respective diseases are also given. Because of the central
role of GPCRs in the physiological regulation of cells, tissues, and organs and, in
case of misregulation, the wide variety of
pathogenic consequences, it is not surprising that stimulating or inhibiting autoantibodies can be the cause of disease.
In the case of cardiomyopathies, autoantibodies directed against the first or
second extracellular loop of the adrenergic ␤1 receptor (␤1-AABs) have been
177