A Major Role In Cardiomyopathy

Published Date: 02 Nov 2017

the heart muscle characterized by cardiac dysfunction. Several miRNAs including

those involved in heart development are found to be dysregulated in cardiomyopathy.

These miRNAs act either directly or indirectly by controlling the genes involved in

normal development and functioning of the heart. Indirectly it also targets modifier

genes and genes involved in signaling pathways. In this review, the miRNAs involved

in heart development, including dysregulation of miRNA which regulate various genes,

modifiers and notch signaling pathway genes leading to cardiomyopathy are

discussed. A study of these miRNAs would give an insight into the mechanisms

involved in the processes of heart development and disease. Apart from this, the

information gathered from these studies would also generate suitable therapeutic

targets in the form of antagomirs which are chemically engineered oligonucleotides

used for silencing miRNAs.

MICRO RNA REGULATION DURING CARDIAC DEVELOPMENT AND

REMODELING IN CARDIOMYOPATHY

Abstract

miRNAs have been found to play a major role in cardiomyopathy, which is a disease of the heart

muscle characterized by cardiac dysfunction. Several miRNAs including those involved in heart

development are found to be dysregulated in cardiomyopathy. These miRNAs act either directly

or indirectly by controlling the genes involved in normal development and functioning of the

heart. Indirectly it also targets modifier genes and genes involved in signaling pathways. In this

review, the miRNAs involved in heart development, including dysregulation of miRNA which

regulate various genes, modifiers and notch signaling pathway genes leading to cardiomyopathy

are discussed. A study of these miRNAs would give an insight into the mechanisms involved in

the processes of heart development and disease. Apart from this, the information gathered from

these studies would also generate suitable therapeutic targets in the form of antagomirs which are

chemically engineered oligonucleotides used for silencing miRNAs.

Introduction

Cardiomyopathy is a disease of the heart muscle characterized by cardiac dysfunction,

arrhythmia, heart failure and sudden death. It is a major cause of morbidity and mortality

worldwide. Cardiomyopathies can be classified into 2 major groups based on predominant organ

involvement: Primary cardiomyopathies (genetic, nongenetic, acquired) are those confined to

heart muscle and Secondary cardiomyopathies which show pathological myocardial involvement

as part of a large number and variety of generalized systemic (multiorgan) disorders (Elliott P et

al., 2008 ; Barry J. M et al, 2006).

ManuscriptmiRNAs have been found to play a major role in cardiomyopathy, which is a disease of the heart

muscle characterized by cardiac dysfunction. Several miRNAs including those involved in heart

development are found to be dysregulated in cardiomyopathy. These miRNAs act either directly

or indirectly by controlling the genes involved in normal development and functioning of the

heart. Indirectly it also targets modifier genes and genes involved in signaling pathways. In this

review, the miRNAs involved in heart development, including dysregulation of miRNA which

regulate various genes, modifiers and notch signaling pathway genes leading to cardiomyopathy

are discussed. A study of these miRNAs would give an insight into the mechanisms involved in

the processes of heart development and disease. Apart from this, the information gathered from

these studies would also generate suitable therapeutic targets in the form of antagomirs which are

chemically engineered oligonucleotides used for silencing miRNAs.

Introduction

Cardiomyopathy is a disease of the heart muscle characterized by cardiac dysfunction,

arrhythmia, heart failure and sudden death. It is a major cause of morbidity and mortality

worldwide. Cardiomyopathies can be classified into 2 major groups based on predominant organ

involvement: Primary cardiomyopathies (genetic, nongenetic, acquired) are those confined to

heart muscle and Secondary cardiomyopathies which show pathological myocardial involvement

as part of a large number and variety of generalized systemic (multiorgan) disorders (Elliott P et

al., 2008 ; Barry J. M et al, 2006).

It is also considered as a disease of sarcomere and cytoskeleton and recent studies have revealed

new facets about the role of micro RNAs in cardiac development and remodeling. A growing

body of exciting evidence suggests that miRNAs are regulators of cardiovascular cell

differentiation, growth, proliferation, angiogenesis and apoptosis. It has provided glimpses of

undiscovered regulatory mechanisms and potential therapeutic targets for the treatment of

cardiomyopathies.

miRNA, an endogenous ~22nt RNA, plays important regulatory roles in animals and plants.

They cleave the target mRNA or translationally repress them. The first miRNA discovered, was

lin-4 in C. elegans, which was found to produce a pair of short RNA transcripts that regulate the

timing of larval development by translational repression of lin-14, which encodes a nuclear

protein (Lee R.C et al. 1993). Since the discovery of let-7, thousands of miRNAs have been

identified in organisms as diverse as viruses, worms, and primates through random cloning and

sequencing or computational prediction. A hepta nucleotide sequence at the 5’ end of the

miRNA is essential in the base pairing specificity with the 3’end of the mRNA (Gregory P. A., et

al. 2008).

Role of miRNA in heart development and function:

Role of miRNAs in heart development was identified in Zebrafish and mice by targeting the

Dicer protein which is the main component of miRNA machinery. Zebrafish lacking maternal

and endogenous Dicer exhibited heart developmental defects (Giraldez A. J et al. 2005 ;

Wienholds E. et al. 2003). Cardiac-specific deletion of Dicer in mice resulted in pericardial

edema and poorly developed ventricular myocardium resulting in embryonic lethality (Zhao Y.

et al 2007). Adult mice lacking Dicer in the myocardium revealed high incidence of sudden

death, cardiac hypertrophy, reactivation of the fetal cardiac gene program (da Costa Martins P. A

et al 2008). Further studies also showed that additional miRNAs, miR1, miR133, miR1/133

(bicistronic), miR21 and miR138 are involved in the regulation of heart development.

miR-1/miR-133: The most abundant miRNA in cardiac myocytes and also the first miRNA

implicated in heart development was miR-1(Zhao Y. et al 2007). miR-1 and -133 are expressed

in cardiac and skeletal muscles and are transcriptionally regulated by the myogenic

differentiation factors MyoD, Mef2, and serum response factor (SRF) (Chen J. F et al 2008,

Kwon C et al. 2005 ; Lagos-Quintana M et al 2001 ; Rao P. K et al 2006 ; Sokol N. S et al

2005 ; Zhao Y et al 2005).

Targeted deletion of the muscle-specific miRNA, miR-1-2, was found to cause ventricular septal

defects (VSD) in mice embryos resulting in immediate death and in some embryos it caused

pericardial edema which contributes to embryonic mortality (Yu X et al 2008). In mice that

survived postnatally, some died within months due to rapid dilatation of the heart and ventricular

dysfunction, while many others suffered sudden cardiac death because of abnormalities in

cardiac conduction and repolarization. miR-1-2 also directly targets irx5, which is known to

repress the potassium channel, Kcnd2, ensuring coordinated cardiac repolarization (Costantini D.

L et al 2005). In adult miR-1-2 mutants, cardiomyocyte division continues postnatally due to

abnormalities in cell-cycle leading to hyperplasia of the heart.

miR-1 levels are low during cardiac development but seem to increase as development

progresses (Zhao Y et al 2005). In mice, overexpression of miR-1 under the control of the

myosin heavy chain (MHC) promoter negatively regulates cardiac growth, partly by inhibiting

implicated in heart development was miR-1(Zhao Y. et al 2007). miR-1 and -133 are expressed

in cardiac and skeletal muscles and are transcriptionally regulated by the myogenic

differentiation factors MyoD, Mef2, and serum response factor (SRF) (Chen J. F et al 2008,

Kwon C et al. 2005 ; Lagos-Quintana M et al 2001 ; Rao P. K et al 2006 ; Sokol N. S et al

2005 ; Zhao Y et al 2005).

Targeted deletion of the muscle-specific miRNA, miR-1-2, was found to cause ventricular septal

defects (VSD) in mice embryos resulting in immediate death and in some embryos it caused

pericardial edema which contributes to embryonic mortality (Yu X et al 2008). In mice that

survived postnatally, some died within months due to rapid dilatation of the heart and ventricular

dysfunction, while many others suffered sudden cardiac death because of abnormalities in

cardiac conduction and repolarization. miR-1-2 also directly targets irx5, which is known to

repress the potassium channel, Kcnd2, ensuring coordinated cardiac repolarization (Costantini D.

L et al 2005). In adult miR-1-2 mutants, cardiomyocyte division continues postnatally due to

abnormalities in cell-cycle leading to hyperplasia of the heart.

miR-1 levels are low during cardiac development but seem to increase as development

progresses (Zhao Y et al 2005). In mice, overexpression of miR-1 under the control of the

myosin heavy chain (MHC) promoter negatively regulates cardiac growth, partly by inhibiting

translation of heart and neural crest derivative-2 protein, Hand2, which is involved in ventricular

myocyte expansion (Zhao Y et al 2005). In Drosophila, dmiR-1(miR-1 of drosophila) plays an

important role in differentiation of cardiac progenitor cells by targeting transcripts of delta, a

ligand involved in Notch signaling pathway, which regulates the expansion of cardiac and

muscle progenitor cells (Kwon C et al 2005).

vaguely defined. This particular miRNA is stress induced and is a regulator of cardiac growth

(Van Rooij E et al 2006). Hence its interaction with the HSP-70 gene family needs to be

elucidated, as HSP is also a cardiac specific development gene. Sabatel et al(2011) showed that

over-expression of miR-21 reduced endothelial cell proliferation, migration, and tube formation

by targeting RhoB, whereas knockdown of miR-21 led to an opposite effect (Sabatel C et al

2011)

miR-138:

embryo, but within the zebrafish heart, it is specifically expressed in the ventricular chamber

(Morton S. U et al 2008). Disruption of miR-138 function led to the expansion of the

atrioventricular canal into the ventricle and failure of maturation of ventricular cardiomyocytes.

It has thus been suggested that other region-specific miRNAs may reinforce known signaling and

transcriptional networks that establish patterns of gene expression throughout the

developing heart tube (Heitzler P. & Simpson P. 1991). Hence the relation between modifier

genes and miRs during embryogenesis needs to be substantiated.

miRNA in cardiomyopathy and cardiac remodeling:

miRNA mediated gene repression is an important regulatory mechanism to modulate

fundamental cellular processes such as the cell cycle, growth, proliferation, phenotype, death,

which in turn have major influences on pathophysiological outcomes like cardiac fibrosis,

hypertrophy, angiogenesis, and heart failure(Catalucci D et al 2009 ; Thum T et al 2007 ; Thum

T et al 2008). Although miRNAs are highly expressed in heart, their role in heart diseases

especially cardiomyopathies still remains unclear.

Multiple aberrant miRNA expression is a remarkable characteristic of the hypertrophic heart (Y

Cheng et al 2007). The dysregulation and the time course changes of these multiple aberrantly

expressed miRNAs match the complex process of cardiac hypertrophy formation in which

several genes have been reported to be dysregulated (Dorn II G. W., Hahn H. S. 2004).

Determining the effects of these dysregulated miRNAs in cardiac hypertrophy is the prerequisite

for a novel research on cardiomyopathy.

It was reported that a cardiac-specific knockout of the Dicer gene leads to rapidly progressive

dilated cardiomyopathy (DCM), heart failure, and postnatal lethality. Dicer expression decreased

in end-stage human DCM and heart failure. These findings suggest that Dicer function and

miRNAs play critical roles in normal cardiac function and heart diseases especially during heart

failure (Chen J. F et al 2008).

miR-1/miR-133: miR-133 and miR-1 play critical roles in cardiac hypertrophy. In human and

mouse models of cardiac hypertrophy, decreased expression of both miR-133 and miR-1 is

reported. In vitro, overexpression of miR-133 or miR-1 inhibited cardiac hypertrophy. In

contrast, suppression of miR-133 induced hypertrophy. In vivo inhibition of miR-133 by a single

infusion of an antagomir caused marked and sustained cardiac hypertrophy. RhoA, a GDP-GTP

exchange protein (regulator of cardiac hypertrophy); Cdc42, a signal transduction kinase

(implicated in hypertrophy); and Nelf-A/WHSC2, a nuclear factor (involved in cardiogenesis)

are all targets of miR-133 (Carè A et al 2007).

When overexpressed in normal or infarcted rat hearts, miR-1 aggravates arrhythmogenesis and

elimination of miR-1 by an antisense inhibitor relieved arrhythmogenesis. The target of miR-1

was found to be gap junction protein a1 (GJA1) which encodes Cx43, the main cardiac gap

junction channel important for conductance in the ventricles (Luo X et al 2007) and potassium

inwardly-rectifying channel, subfamily J, member 2 (KCNJ2) (Yang B et al 2007).

miR-21: Inhibition of miR-21 in neonatal rat cardiomyocytes by transfecting locked nucleic acid

(LNA)-modified antisense oligonucleotides(miR-LNA) to miR-21 or miR-18b induced myocyte

hypertrophy while transfection of miR-21 and miR-18b duplexes slightly decreased

cardiomyocyte size and decreased hypertrophy thus suggesting their role in regulating

mouse models of cardiac hypertrophy, decreased expression of both miR-133 and miR-1 is

reported. In vitro, overexpression of miR-133 or miR-1 inhibited cardiac hypertrophy. In

contrast, suppression of miR-133 induced hypertrophy. In vivo inhibition of miR-133 by a single

infusion of an antagomir caused marked and sustained cardiac hypertrophy. RhoA, a GDP-GTP

exchange protein (regulator of cardiac hypertrophy); Cdc42, a signal transduction kinase

(implicated in hypertrophy); and Nelf-A/WHSC2, a nuclear factor (involved in cardiogenesis)

are all targets of miR-133 (Carè A et al 2007).

When overexpressed in normal or infarcted rat hearts, miR-1 aggravates arrhythmogenesis and

elimination of miR-1 by an antisense inhibitor relieved arrhythmogenesis. The target of miR-1

was found to be gap junction protein a1 (GJA1) which encodes Cx43, the main cardiac gap

junction channel important for conductance in the ventricles (Luo X et al 2007) and potassium

inwardly-rectifying channel, subfamily J, member 2 (KCNJ2) (Yang B et al 2007).

miR-21: Inhibition of miR-21 in neonatal rat cardiomyocytes by transfecting locked nucleic acid

(LNA)-modified antisense oligonucleotides(miR-LNA) to miR-21 or miR-18b induced myocyte

hypertrophy while transfection of miR-21 and miR-18b duplexes slightly decreased

cardiomyocyte size and decreased hypertrophy thus suggesting their role in regulating

cardiomyocyte hypertrophy. The molecular basis of this regulation still needs to be established.

Hence the role of miR-21 in chromatin modelling of cardiomyocytes cannot be overlooked

(Tatsuguchi M et al 2007).

A study showed an increased expression of miR-21, miR-23, miR-24, miR-125b, miR-195, miR199a,

and miR-214, and decreased expression of miR-29c, miR-93, miR-150, and miR-181b in

mice subjected to chemicals which induced cardiac hypertrophy. Upon transfection into

cardiomyocytes, five of the miRNAs that were upregulated, viz. miR-23a, miR-23b, miR-24,

miR-195, and miR-214, were found to be capable of inducing hypertrophic growth. Whereas

transfection of miR-199a resulted in elongated spindle shapes myocytes reminiscent of the

elongated cardiac myocytes observed in dilated cardiomyopathy.

Cardiac specific overexpression of miR-195 was found to cause death in the first 2 weeks after

birth due to heart failure because of cardiac dilatation and in the rats that survived initially,

induction of cardiac growth with disorganization of cardiomyocytes occurred at 2 weeks of age,

which later progressed to a dilated cardiac hypertrophic phenotype by 6 weeks of age thus

suggesting the critical role played by miR-195 in cardiac remodeling (Van Rooij E et al 2006).

miR-29: The miR-29 family, which is downregulated after myocardial infarction, inhibits the

expression of several collagens and extracellular matrix proteins, thereby contributing to scar

formation and fibrosis, as seen in DCM, during heart failure.

miR-208: miR-208 is required for the development of cardiac hypertrophy and myocardial

fibrosis and it is also a positive regulator of MHC gene expression (van Rooij E et al 2007).

expression of several collagens and extracellular matrix proteins, thereby contributing to scar

formation and fibrosis, as seen in DCM, during heart failure.

miR-208: miR-208 is required for the development of cardiac hypertrophy and myocardial

fibrosis and it is also a positive regulator of MHC gene expression (van Rooij E et al 2007).

Regulation of Modifiers by miRNAs:

Stress is a major etiologic factor that may contribute to heart diseases. Stress overload can cause

tissue injury; cardiomyocyte death is considered an important cellular basis for stress-induced

injury in cardiomyopathies (Feuerstein G. Z., Young P. R. 2000). miR-199 family is rapidly

downregulated in cardiac myocytes under hypoxic conditions, relieving the repression of sirtuin

1 and hypoxia inducible factor 1-a in a model of hypoxia preconditioning. The miRNA that

repeatedly showed dynamic regulation after cellular stress is miR-21, which promotes cardiac

hypertrophy and fibrosis in response to pressure overload (Rane S et al 2009).

Under stress a change in expression of HSP70 in rat myocardium is observed. HSP70 protects

cardiomyocyte from stress induced injury by inhibiting Fas-mediated apoptosis (Basu N et al

2001). The levels of miR-1 was found to increase significantly in response to oxidative stress

which later reduced the levels of HSP70 favoring cardiomyocyte apoptosis, while decreased

levels of miR-1 favored cardiomyocyte survival (Xu C et al 2007).

The members of TGF-ß family have been found to have a cardioprotective role and are highly

induced in affected hearts. Their putative roles during atherogenesis, infarct healing, cardiac

repair and left ventricular remodeling have been proposed (Os I et al 2002). miR24a and

miR34a seem to have a strong and specific regulatory effect on TGF ß while miR-373 and miR34b

have a constitutive role (Schultz N et al 2011).

Stress is a major etiologic factor that may contribute to heart diseases. Stress overload can cause

tissue injury; cardiomyocyte death is considered an important cellular basis for stress-induced

injury in cardiomyopathies (Feuerstein G. Z., Young P. R. 2000). miR-199 family is rapidly

downregulated in cardiac myocytes under hypoxic conditions, relieving the repression of sirtuin

1 and hypoxia inducible factor 1-a in a model of hypoxia preconditioning. The miRNA that

repeatedly showed dynamic regulation after cellular stress is miR-21, which promotes cardiac

hypertrophy and fibrosis in response to pressure overload (Rane S et al 2009).

Under stress a change in expression of HSP70 in rat myocardium is observed. HSP70 protects

cardiomyocyte from stress induced injury by inhibiting Fas-mediated apoptosis (Basu N et al

2001). The levels of miR-1 was found to increase significantly in response to oxidative stress

which later reduced the levels of HSP70 favoring cardiomyocyte apoptosis, while decreased

levels of miR-1 favored cardiomyocyte survival (Xu C et al 2007).

The members of TGF-ß family have been found to have a cardioprotective role and are highly

induced in affected hearts. Their putative roles during atherogenesis, infarct healing, cardiac

repair and left ventricular remodeling have been proposed (Os I et al 2002). miR24a and

miR34a seem to have a strong and specific regulatory effect on TGF ß while miR-373 and miR34b

have a constitutive role (Schultz N et al 2011).

The renin-angiotensin system (RAS) is one of the most important modifiers in cardiomyopathy.

The angiotensin converting enzyme(ACE) is the key enzyme, involved in conversion of

angiotensin I to angiotensin II which is responsible for cardiac hypertrophy and heart failure

(Kawaguchi H. 2003) and miR145 has been found to regulate this enzyme(Small E et al 2010).

Hence it can be hypothesized that miRNAs have both positive and negative roles in

cardiomyopathies, especially in a heart failure phenotype; either as modifiers or by gene-gene

interaction and/or regulators of cardiogenesis. One such regulation could be in

signaling/transduction pathways.

Regulation of Signaling pathways by miRNAs and their role in cardiomyopathies

Understanding complex diseases like cardiomyopathies not only requires identification of genes

and upregulation/downregulation of miRNAs, but also of the proteins that are regulated and

signaling pathways that are affected by these miRNAs. Various intercellular signaling pathways

have been implicated in the control of cardiogenesis viz. Notch signaling, FGF signaling, BMP

signaling, Wnt/b-catenin signaling, Wnt/JNK pathways etc.

Notch signaling and cardiogenesis:

Notch signaling mediates numerous developmental cell fate decisions in organisms ranging from

flies to humans, resulting in the generation of multiple cell types from equipotential precursors.

Notch signaling is also involved in angiogenesis and vasculogenesis. Notch signaling is a highly

conserved and a complex mechanism initiated by the interaction of Notch receptors with their

Understanding complex diseases like cardiomyopathies not only requires identification of genes

and upregulation/downregulation of miRNAs, but also of the proteins that are regulated and

signaling pathways that are affected by these miRNAs. Various intercellular signaling pathways

have been implicated in the control of cardiogenesis viz. Notch signaling, FGF signaling, BMP

signaling, Wnt/b-catenin signaling, Wnt/JNK pathways etc.

Notch signaling and cardiogenesis:

Notch signaling mediates numerous developmental cell fate decisions in organisms ranging from

flies to humans, resulting in the generation of multiple cell types from equipotential precursors.

Notch signaling is also involved in angiogenesis and vasculogenesis. Notch signaling is a highly

conserved and a complex mechanism initiated by the interaction of Notch receptors with their

ligands both of which are transmembrane proteins whose extracellular domains are composed of

epidermal growth factor (EGF) like repeats (Eiraku M et al 2005). The Notch receptors include

the Notch1-4 in mammals and Notch in Drosophila. The Notch ligands include classical ligands

such as Jagged –Jag1 and Jag2 -and Delta-like -DLL1, DLL3 and DLL4 -as well as several

atypical ligands DNER, F3/Contactin1, NB-3/Contactin6 and Delta-like 1 homologue.

The Notch pathway is intricately involved in the development of the cardiovascular system. One

of the major functions of Notch signaling is its ability to influence cell fate decisions during

development (Kwon C et al 2005). Several of the Notch pathway components have been linked

to the vascular system development, including Jagged1, Notch1, Notch2, Notch4 and

presenilin(Eiraku M et al 2005 ; Bray S. J et al 2008). It was reported that the Notch ligand and

receptor expression is restricted to either the endothelial or vascular SMC during different stages

of development. This is demonstrated by Notch1, Notch4 and Dll4 which are initially present in

the embryo in all blood cells and is later restricted to arteries. Similarly Notch2 expression is

restricted to the pulmonary artery (Bruckner K et al 2000). The epithelial-mesenchymal

transitions, a potential source of mesenchymal stem cells in the adult vasculature and cardiac

valves, may occur as a result of Notch activation by Jag1, which represses the activation of Wnt

pathway. Preferential expression of Jagged1 in the endothelial cells of injured blood vessels may

induce high levels of Notch receptors in neighboring smooth muscle cells and reduce contact

inhibition and cell adhesion through a reduction in cadherin levels indicating that Jagged1 may

be involved in the de-differentiation of vascular cells and the cellular proliferation phase

characteristic for atherosclerosis (Ivey K. N et al 2008).

It has been reported that constitutive activation of the Notch pathway significantly reduces

cardiac differentiation. The Notch1 receptor is responsible for the blockade of cardiogenesis.

Notch1 also is involved in the suppression of catrdiomyocyte differentiation. It has also been

proposed that inhibition of cardiogenesis by Notch signalling is carried out by blocking

mesodermal differentiation (Sethupathy P. Et al 2006). Hence Notch signaling pathway which is

known to influence cardiogenesis and heart development, in conjunction with miRNAs, needs to

be elucidated.

Notch signalling and miRNA in cardiomyopathy

miRNA regulation is essential for normal Notch signalling. Default repression by miRNAs does

not necessarily have to target core pathway components; it may be equally effective when it

intercepts their transcriptional targets as shown by the default repression of the E (spl) and

Bearded (Brd) gene clusters whose activation is dependent on signalling by Notch in Drosophila.

This is a highly redundant system, in which families of related miRNAs (miR-2, miR-4, miR-7,

miR-11 and miR-79) promiscuously target a family of related mRNAs, preventing aberrant

deployment of Notch-mediated developmental programmes (Sabatel C et al 2011). Regulation of

the expansion of cardiac and muscle progenitor cells is carried out by the notch ligand Delta, and

this is targeted for repression by dmiR-1 (Rao P. K et al 2006 ; Sethupathy P et al 2006 ; Zhao Y

et al 2005). Several conserved putative miR-1-binding sites were found in the 3’-UTR of the

gene encoding Delta (Artavanis-Tsakonas S et al 1999 ; Corbin V et al 1991 ; Heitzler P. &

Simpson P. 1991). It was also found that miR-1 fine-tunes Notch ligand Delta that is critically

involved in differentiation of cardiac and somatic muscle progenitors and targets a pathway

miRNA regulation is essential for normal Notch signalling. Default repression by miRNAs does

not necessarily have to target core pathway components; it may be equally effective when it

intercepts their transcriptional targets as shown by the default repression of the E (spl) and

Bearded (Brd) gene clusters whose activation is dependent on signalling by Notch in Drosophila.

This is a highly redundant system, in which families of related miRNAs (miR-2, miR-4, miR-7,

miR-11 and miR-79) promiscuously target a family of related mRNAs, preventing aberrant

deployment of Notch-mediated developmental programmes (Sabatel C et al 2011). Regulation of

the expansion of cardiac and muscle progenitor cells is carried out by the notch ligand Delta, and

this is targeted for repression by dmiR-1 (Rao P. K et al 2006 ; Sethupathy P et al 2006 ; Zhao Y

et al 2005). Several conserved putative miR-1-binding sites were found in the 3’-UTR of the

gene encoding Delta (Artavanis-Tsakonas S et al 1999 ; Corbin V et al 1991 ; Heitzler P. &

Simpson P. 1991). It was also found that miR-1 fine-tunes Notch ligand Delta that is critically

involved in differentiation of cardiac and somatic muscle progenitors and targets a pathway

essential for progenitor cell specification and asymmetric cell division. Introduction of miR-133

allows cardiac tissue formation, but the tissue is disorganized and does not lead to chamber

formation. It has thus been shown that miR-1 and miR-133 function antagonistically to each

other whenever miR-1 shifts the development of the stem cells towards a cardiac fate and miR133

inhibits this event. The cardiac fate achieved by miR-1 is by transcriptional repression of

Dll-1, which is the mammalian ortholog of Delta in Drosophila(Atsuhiko I et al 2011). It has also

been reported that the members of the Hairy family, particularly Hrt2/Hey2, involved in heart

disease, are themselves regulated by miR-1-2 and members of the Hairy family are

transcriptional repressors which mediate Notch signalling. The effect produced by miR-1-2 on

Hey2 is also seen on Hand1, involved in Notch pathway, which is a bHLH transcription factor

involved in ventricular development and septation that, in combination with Hand2 (a paralog of

Hand1), is known to regulate expansion of the embryonic cardiac ventricles (Kwon C et al 2005 ;

Jiang Q et al 2009 ; Rusconi, J. C. & Corbin, V. 1998 ; Sabatel C et al 2011 ; Sapir A et al 2005).

miR-1-2 appears to be involved in the regulation of diverse cardiac and skeletal muscle

functions, including cellular proliferation, differentiation, cardiomyocyte hypertrophy, cardiac

conduction and arrhythmias (Han Z. & Bodmer R. 2003). Hence miRs regulating the Notch

signaling pathway which is involved in cardiac development, differentiation and ultimately

cardiomyopathy, needs to be evaluated.

Conclusion

In the preceding discussion, the involvement of miRNAs in regulating developmental processes

in the heart and their involvement in cardiomyopathies via sarcomeric genes, modifiers and

In the preceding discussion, the involvement of miRNAs in regulating developmental processes

in the heart and their involvement in cardiomyopathies via sarcomeric genes, modifiers and

signaling pathways such as the Notch pathway is reviewed. Mutations in sarcomeric genes are

the primary causatives of the disease, whereas the modifiers determine the severity.

The miRNAs regulating these genes thus play an important role in development and disease. The

roles played by several miRNAs have been elucidated, but an in depth analysis of the miRNAs,

and the genes that encode them and also the genes targeted by them is essential to bring forward

the complex interplay that occurs during development and disease causation. Notch pathway is

involved in the development of cardiovascular system, as it promotes cell proliferation and

apoptosis. Many miRNA are known to regulate the Notch pathway and the dysregulation of

these miRNA affects cell proliferation, differentiation, cardiac conduction, leading to cardiac

hypertrophy and arrhythmias. But the information available in this context is still obscure.

Further studies are necessary to identify other miRNAs involved in regulation of notch pathway.

A study of miRNAs would also give us potential therapeutic targets in the form of antagomirs

which are used for silencing miRNAs that are implicated in the manifestation of

cardiomyopathies. Complete revelation of the roles played by miRNA may give crucial insights

into many of the mysteries of the human heart.