H I S TO R I C A L P E R S P E C T I V E

Eric N. Olson is in the Department of Molecular Biology, University of TexasSouthwestern Medical Center at Dallas, 6000 Harry Hines Blvd., Dallas, Texas75390-9148, USA.e-mail: eric.olson@utsouthwestern.edu

NATURE MEDICINE VOLUME 10 | NUMBER 5 | MAY 2004 467

The heart is the first organ to form in the embryo, and allsubsequent events in the life of the organism depend on itsfunction. Inherited mutations in cardiac regulatory genes giverise to congenital heart disease, the most common form ofhuman birth defects, and abnormalities of the adult heartrepresent the most prevalent cause of morbidity and mortalityin the industrialized world. The past decade has marked atransition from physiological and functional studies of theheart toward a deeper understanding of cardiac function (anddysfunction) at genetic and molecular levels. These discoverieshave provided new therapeutic approaches for prevention andpalliation of cardiac disease and have raised new questions,challenges and opportunities for the future.

Widely regarded as one of the most influential and far-reachingworks ever published in physiology and medicine, William Harvey’sDe Motu Cordis (On the Motion of the Heart) serves as the foundationof contemporary cardiovascular biology. In it, Harvey prescientlydelineated the basic principles of cardiac function and development,which remain the focus of intense investigation even today. Amonghis controversial ideas were the concepts of the heart as a muscularpump in which the right and left ventricular chambers performeddistinctive functions, the circulatory system as a closed circuit, andthe conservation in structure and function of the heart from insectsto humans.

Since Harvey’s time, the workings of the heart have been docu-mented in intricate detail. However, the past decade has marked atransition from physiological and functional studies of the hearttoward a deeper understanding of cardiac function (and dysfunc-tion) at genetic and molecular levels. This transition has providedinsights into the mechanisms of heart development and disease andhas fueled new therapeutic opportunities for the prevention and pal-liation of cardiac pathogenesis. Nevertheless, cardiovascular diseaseremains the number one cause of death in the western world,accounting for an estimated 960,000 deaths per year in the United

States1. Indeed, one in five Americans is afflicted with some form ofcardiac disease, and the numbers are increasing. Thus, much remainsto be learned.

This review looks back at some of the discoveries in cardiac biol-ogy over the past decade and considers their potential impact onfuture therapies for congenital and acquired heart diseases. Becauseof the breadth and depth of the field, it has been possible only totouch on representative areas rather than to provide a comprehen-sive review. In reflecting on the decade, it is remarkable how the mol-ecules and mechanisms uncovered during this period have refinedand reaffirmed the original proposals of William Harvey, consideredheretical in 1628. One wonders how the discoveries of today willaffect the field 400 years from now.

Heart development and congenital heart diseaseThe heart is the first organ to form and function in the embryo, andall subsequent events in the life of the organism depend on the heart’sability to match its output with the organism’s demands for oxygenand nutrients. Abnormalities in heart formation, the most commonform of human birth defects, afflict nearly 1% of newborns, and theirfrequency in spontaneously aborted pregnancies is estimated to betenfold higher2.

In fruit fly as in vertebrate embryos, the initial commitment ofmesodermal progenitor cells to a cardiac fate is dependent on signal-ing between adjacent tissues. Members of the bone morphogeneticprotein (BMP) family have positive roles and wingless proteins haveboth positive and negative roles in establishment of the cardiac line-age3–6. Entry of cells into the cardiac lineage in response to the appro-priate signals is coupled to the expression of a set of transcriptionfactors that initiates the program for cardiac gene expression anddrives the morphogenic events involved in formation of the multi-chambered heart.

Although the embryological events in heart formation have beendocumented for centuries, it was not until the early 1990s that thegenetic underpinnings of cardiac morphogenesis began to revealthemselves. A notable breakthrough in this regard was the discoveryof the homeobox gene tinman (tin), required for the formation of theprimitive heart in the fruit fly Drosophila melanogaster7,8. A mam-malian ortholog of tinman, called Nkx2-5 or Csx, is expressed in car-diac muscle cells from the onset of embryonic heart formation untiladulthood9,10. Although highly conserved and restricted to the car-diac lineage, Nkx2-5 is dispensable for establishment of the cardiaclineage. Instead, mice lacking Nkx2-5 die during mid-embryogenesisfrom abnormalities in growth and development of the left ventricularchamber of the heart11, which suggests that Nkx2-5 serves a differentfunction than tinman or that other genes have redundant functions inthe specification of cardiac cell fate.

A decade of discoveries in cardiac biologyEric N Olson

The heart of creatures is the foundation of life, the Prince of all, thesun of their microcosm, from where all vigor and strength does flow.—William Harvey, De Motu Cordis, 1628

CELEBRATING OUR TENTH YEAR©

2004

Nat

ure

Pub

lishi

ng G

roup

ht

tp://

ww

w.n

atur

e.co

m/n

atur

emed

icin

e

H I S TO R I C A L P E R S P E C T I V E

In contrast to skeletal muscle—in which a single transcription factor,MyoD, is sufficient to activate the full program of muscle differentia-tion—cardiac muscle differentiation is dependent on combinations oftranscription factors. In mammals as well as fruit flies, the MADS-boxfactor myocyte enhancer factor-2 (MEF2), in conjunction with othertranscription factors, directly activates the expression of genes encod-ing myofibrillar proteins12,13. Similarly, serum response factor (SRF), arelated MADS-box factor, associates with an array of transcription fac-tors including Nkx2-5, GATA4 and myocardin to control the expres-sion of muscle structural genes whose products are incorporated intothe contractile apparatus, such as actin, myosin and troponins14,15. Thehomeodomain protein Irx4 also activates ventricular genes while sup-pressing atrial genes in the ventricular chambers16. The analysis of reg-ulatory DNA sequences responsible for cardiac transcription hasrevealed an unexpected complexity of regulation in which individualgenes are often controlled by multiple independent enhancers thatdirect expression in highly restricted patterns in the developing heart17.In addition to activating subordinate genes involved in cardiac func-tion, cardiac transcription factors amplify and maintain their expres-sion within mutually reinforcing transcriptional circuits involvingpositive feedback loops and protein-protein interactions.

Recent studies indicate that two populations of cardiac progeni-tors, referred to as the primary and secondary heart fields, con-tribute to the developing heart in vertebrates18. Cardiac myocytesbecome organized into a linear heart tube that undergoes rightwardlooping in response to an axial signaling system that establishesasymmetry across the left-right axis of the embryo19. Subsequentballoon-like growth of the looped heart tube and septation yield themultichambered heart (Fig. 1). The right (pulmonary) and left (systemic) ventricular and atrial chambers of the heart have distinctfunctions and must be separated by an interventricular and intera-trial septum in order for blood to be delivered to and from the lungsand body, respectively. Human congenital cardiac malformations inwhich one ventricle is underdeveloped suggested that the two ven-tricular chambers might form independently according to distinctdevelopmental programs. Important insight into the genetic cir-cuitry of ventricular development was provided by the discovery oftwo chamber-restricted basic helix-loop-helix transcription factors,HAND1 and HAND2, that are expressed in the left and right ven-tricular chambers, respectively20. Knockout mice lacking HAND2do not form a right ventricle and mice lacking both genes form nei-ther ventricle21. These mutant mice, the first to demonstrate that asingle-gene mutation could result in the ablation of an entire seg-ment of the heart, supported the notion that the heart is assembledin a modular fashion, with each compart-ment governed by unique genetic programs.The segmental pattern of heart develop-ment is also exemplified by the phenotypeof mice lacking the homeobox gene Isl1, inwhich entire regions of the heart areabsent22. Mutations in other cardiac controlgenes in mice and zebrafish further substan-tiate such a model23.

Growth and maturation of cardiacmyocytes within the developing ventricularchambers depends on complex signalingevents between adjacent cell layers. Signalingby neuregulins from the endocardium, a thinlayer of cells lining the cardiac chambers, tothe ErbB receptor in the myocardium isrequired for growth of the ventricular layer24.

Mutations that alter this signaling interaction result in a thin-walledmyocardium and embryonic death. Similarly, interference withneuregulin signaling in the adult heart by antibodies to ErbB, used inbreast cancer therapy, can lead to severe contractile dysfunction25. Theepicardial layer of the heart also serves as a rich source of signals thatstimulate cardiac growth, such as retinoic acid and as yet unidentifiedpeptide growth factors26.

Formation of the cardiac valves, which are essential for unidirec-tional blood flow, requires signaling by TGF-β family members fromthe myocardium to localized swellings of the endocardium, known ascardiac cushions (Fig. 1 and ref. 27). Disruption in this signalingmechanism in knockout mice lacking cardiac expression of BMPs orBMP receptors results in valve abnormalities and ventricular-septaldefects, which account for a majority of congenital cardiacdefects28,29. Neural crest cells from the pharyngeal arches also popu-late the developing heart and are important in the formation of valvesand the interventricular septum and in the patterning of the outflowtract and great arteries30,31.

Each heartbeat and synchronized round of cardiac contraction andrelaxation is controlled by a specialized cluster of cells in the rightatrium, called the sinoatrial node, which functions as the pacemaker(Fig. 1). Electrical impulses are propagated through the heart by thecardiac conduction system and by direct cell-cell coupling of cardiacmyocytes. The embryonic origins of the cell lineages that form the car-diac conduction system have been unclear until recently, when it wasshown that Purkinje cells are derived from a subpopulation of ventric-ular cardiomyocytes in response to signaling by endothelin-1 andneuregulins32,33. Fate mapping of cardiac conduction lineages in micehas yielded insights into the transcriptional pathways responsible forthe formation of this specialized cardiac structure and the molecularbasis for cardiac conduction defects34.

Mutations in cardiac transcription factors, the genes they regulateand the genes that regulate them have been identified in individualswith congenital heart disease35. Humans and mice heterozygous formissense mutations in Nkx2-5 show a spectrum of structural andfunctional cardiac defects that include atrial-septal and ventricular-septal abnormalities and conduction defects36. The molecular basis ofthese defects is unclear, but it seems likely that they arise from subtledeficits in the growth or patterning of cardiac myocytes duringembryogenesis. Holt-Oram syndrome, characterized by heart andlimb defects, has been attributed to mutations in the T-box gene TBX5(ref. 37). In addition, missense mutations in GATA4 that impair itsinteraction with TBX5 have been shown to cause ventricular-septaldefects in humans38.

468 VOLUME 10 | NUMBER 5 | MAY 2004 NATURE MEDICINE

Figure 1 Heart development. The looping heart tube gives rise to the ventricular and atrial chambers.Localized swellings of the endocardium, known as the cardiac cushions, give rise to the valves inresponse to signaling from the myocardium. Components of the cardiac conduction system within themature four-chambered heart are shown. A-V, atrioventricular; LA, left atrium; LV, left ventricle; RA,right atrium; RV, right ventricle; S-A, sinoatrial.

©20

04 N

atur

e P

ublis

hing

Gro

up

http

://w

ww

.nat

ure.

com

/nat

urem

edic

ine

H I S TO R I C A L P E R S P E C T I V E

Despite these detailed descriptions ofabnormal cardiac phenotypes, importantgaps in our understanding of developmentalcontrol mechanisms remain. Only a handfulof target genes regulated by cardiac transcrip-tion factors have been identified, and howthese ultimately relate to cardiac malforma-tions is unclear. It should also be kept in mindthat most conclusions about the functions ofcardiac control genes have been based on phe-notypes resulting from homozygous genedeletion in model organisms, whereas mostdisease-causing mutations in humans are het-erozygous. Congenital heart defects inhumans also show remarkable variability inpenetrance and expressivity, indicative ofmodifier genes and environmental influencesin determining disease phenotypes. Muchremains to be learned about the identities andmechanisms of action of such modifiers.

Insights into cardiac development promiseto have impact on human disease in manyways, most of which have yet to be realized.The identification of cardiac control genespermits genetic screening for mutations inaffected individuals and families. However,the ultimate hope is to exploit our understanding of cardiac develop-mental biology to design therapies for the correction or prevention ofcongenital heart defects. The involvement of developmental controlmechanisms in the pathogenesis of the adult heart also points to thetherapeutic potential of such strategies.

Abnormalities of the adult heartJust as the developing heart is highly prone to malfunctions, the adultheart is susceptible to abnormalities that perturb its growth and con-tractile function. Diverse forms of hemodynamic stress, includingmyocardial infarction, hypertension, aortic stenosis and valvular dys-function, can provoke the heart to undergo pathological hypertrophicgrowth and remodeling. Traditionally thought to represent an adap-tive mechanism to normalize ventricular wall stress in response toinjury or myocyte loss, prolonged hypertrophy can lead to diastolicand, later, systolic heart failure and cardiac sudden death fromarrhythmias. Systolic heart failure, a complex clinical syndrome char-acterized by impaired cardiac output and circulatory congestion, isthe primary cause of human morbidity and mortality, affecting anestimated 5 million Americans with a 5-year mortality rate of about50%1. Over the past decade, there have been major advances in theidentification of genes and signaling pathways underlying these car-diac abnormalities, many of which are triggered by the mishandlingof ions by cardiac muscle cells.

Signaling pathways and cardiac hypertrophyIn contrast to developmental growth of the heart, which occursthrough hyperplasia of cardiac muscle cells, growth of the heart afterbirth occurs primarily through hypertrophy—an increase in the sizeof cardiac muscle cells. Normal growth of the heart postnatally or inconditioned athletes—so called ‘physiological’ hypertrophy—enhances cardiac output to meet increased metabolic demands and ismolecularly distinct from pathological hypertrophy in response tostress signals and injury. Pathological hypertrophy triggers a meta-bolic transition in the heart from an oxidative toward a more gly-

colytic metabolism characteristic of the fetal stage and is accompa-nied by the activation of a fetal cardiac gene program that includesgenes whose products regulate cardiac contractility and calcium han-dling39. Activation of the fetal gene program and concomitant repres-sion of corresponding adult cardiac genes correlates with eventualcardiac demise40. Thus, there has been great interest in decipheringthe mechanisms that couple stress signaling to the fetal gene programand in therapeutic perturbation of these pathways. Indeed, a series ofrecent studies has shown that pharmacological or genetic blockade ofpathological cardiac signaling in the face of unremitting cardiac stresspreserves cardiac function, pointing to the potential efficacy of anti-hypertrophic therapeutic strategies in the setting of pathological cardiac hypertrophy41–43.

While some stress signals result in cardiac hypertrophy, whichcan progress to left ventricular dilatation and heart failure in whichthe heart is unable to pump sufficient blood to meet the metabolicdemands of the body, others can cause cardiac dilatation and fail-ure without an intermediate hypertrophic stage. The balancebetween cell survival and apoptotic pathways appears to be a majordeterminant of the transition from hypertrophy to ventriculardilatation44,45. Cytokine signaling involving the gp130 receptor andits downstream effectors has been shown to be required for survivalof adult cardiomyocytes; its absence results in massive cardiacapoptosis in response to stress46. Strategies for suppressing apopto-sis and promoting the survival of cardiac myocytes representattractive therapeutic opportunities for treatment of heart failure,dilated cardiomyopathies and post–myocardial infarction cardiacremodeling.

Many neurohormonal signals act through the G-protein Gαq tostimulate cardiac hypertrophy47. There has been major progresstoward unraveling calcium-dependent signaling pathways that drivehypertrophic growth of the adult heart (Fig. 2). Among the mostintensely scrutinized has been the calcineurin pathway48. The cal-cineurin phosphatase is activated by sustained elevation of intracellu-lar calcium and dephosphorylates a variety of cellular substrates,

NATURE MEDICINE VOLUME 10 | NUMBER 5 | MAY 2004 469

)

Figure 2 Hypertrophic signaling pathways that influence the growth of the adult heart. Neurohormonalsignals or biomechanical stress can cause pathological cardiac hypertrophy, characterized by fetal geneactivation and remodeling. Multiple interacting signaling pathways have been shown to connect stresssignals with cardiac gene expression through a set of signal-dependent transcription factors.

©20

04 N

atur

e P

ublis

hing

Gro

up

http

://w

ww

.nat

ure.

com

/nat

urem

edic

ine

H I S TO R I C A L P E R S P E C T I V E

including the NFAT transcription factor, which is sufficient to inducehypertrophy when dephosphorylated by calcineurin. The calcineurinpathway is integrated with other pathological signaling systemsincluding those controlled by stress-responsive MAP kinases and cal-cium-dependent kinases, such as protein kinase C, calcium- andcalmodulin-dependent kinases and cyclin-dependent kinases49–52.Recently, these pathways have been shown to converge in the nucleuswith the phosphorylation of a subclass of histone deacetylases (classII HDACs), which act as signal-responsive regulators of the fetal geneprogram and cardiac growth53. Mice lacking the class II HDACHDAC9 are sensitized to hypertrophic signals and develop massivehearts when stressed. By contrast, expression of signal-resistantHDACs in cardiac myocytes renders them insensitive to hypertrophicsignaling. Many of the same transcription factors involved in buildingthe embryonic heart (such as MEF2, GATA4 and SRF) serve as end-points for hypertrophic signaling pathways and mediators of fetalgene reactivation.

Although there is broad, though not universal, agreement thataberrations in calcium handling are responsible for triggering patho-logical cardiac growth, the mechanisms that enable cardiac myocytesto distinguish between the calcium pools involved in contraction andtranscription-dependent remodeling remain to be defined. It isassumed that calcium compartmentalization and distinct patterns ofcalcium concentration waveforms trigger specific signal-transductionpathways that are otherwise insensitive to the moment-to-momentfluctuations in calcium concentration in the cardiac myocyte.

Over the past decade, there have been significant advances in thetreatment of heart failure through slowing or reversal of pathologicalcardiac remodeling. Heart failure and hypertrophy are accompaniedby the upregulation and secretion of atrial and brain natriuretic pep-tides by the heart, which signal through cell-surface receptors coupledto guanylyl cyclase54. The resultant activation of protein kinase G bycGMP has been shown to suppress cardiac hypertrophy and fetal geneactivation through mechanisms that are only beginning to beunveiled. The natriuretic peptides act as powerful antihypertrophicagonists and ameliorate pathophysiological responses to heart failureby promoting natriuresis, diuresis and peripheral vasodilation.Natriuretic peptides have recently shown effectiveness in treatment ofheart failure54. Interference with neurohormonal signaling throughthe use of angiotensin-converting enzyme (ACE) inhibitors,angiotensin blockers, aldosterone antagonists and beta blockers alsoimproves survival in individuals with heart failure, in part by attenu-ating stress signaling in the heart55.

β-Adrenergic signalingSignaling by adrenergic agonists stimulates cardiac performance.Binding of catecholamines to seven-transmembrane-spanning recep-tors, which couple through G-proteins to adenylate cyclase, results inthe elevation of intracellular cAMP and activation of protein kinase A(PKA) (reviewed in ref. 56). The β-adrenergic receptor kinase (β-ARK) is also activated in response to β-adrenergic receptor (β-AR)signaling and phosphorylates the cytoplasmic tail of the receptor,triggering a diminution in receptor signaling (Fig. 3 and ref. 56).

Multiple steps in β-AR signaling are perturbed in the failingheart57,58. Chronic activation of the sympathetic nervous system, acommon response to heart failure, causes an increase in circulatingcatecholamines, with resulting desensitization and downregulation ofcardiac β-AR receptors. β-ARK expression has also been reported toincrease in the failing heart, further dampening β-AR signaling56.Cardiac expression of a dominant-negative β-ARK mutant, which canrestore β-AR signaling, prevents pathological hypertrophy and theprogression to heart failure in several animal models of heart failure43.These findings point to the loss of β-AR signaling in the failing heart asa key mechanism of pathogenesis. Paradoxically, however, β-AR block-ers, which would be predicted to further exacerbate this loss in β-ARsignaling in the heart, have shown remarkable efficacy in improvingsurvival and cardiac function and in preventing pathological remodel-ing in individuals with heart failure59. In spite of a detailed under-standing of β-AR signaling, the mechanisms responsible for thesalutary effects of beta blockers remain unclear.

Calcium handling and cardiac functionCalcium is central to the control of cardiac growth and contractile func-tion. Thus, it is not surprising that abnormalities in calcium handlinghave been implicated in many forms of adult cardiac disease. Duringeach heartbeat, calcium enters the cardiac muscle cell through L-typecalcium channels (Fig. 3). The increase in intracellular calcium triggersfurther calcium release from the sarcoplasmic reticulum through theryanodine receptor (RyR), raising the free intracellular calcium concen-tration approximately tenfold60. Binding of calcium to troponin C in thecontractile apparatus initiates muscle contraction (systole). Reuptake ofcalcium into the sarcoplasmic reticulum by the sarcoplasmic reticulumCa2+-ATPase (SERCA) allows for cardiac relaxation (diastole). The abil-ity of SERCA to pump calcium back into the sarcoplasmic reticulum isgoverned by its interaction with phospholamban, a small modulatoryprotein within the membrane of the sarcoplasmic reticulum61. In theunphosphorylated state, phospholamban inhibits calcium uptake by

470 VOLUME 10 | NUMBER 5 | MAY 2004 NATURE MEDICINE

Figure 3 Calcium signaling during cardiaccontraction. Cardiac contraction is initiated by theinflux of calcium through L-type calcium channels(LTCC) in the cell membrane. The rise inintracellular calcium triggers further calciumrelease from the sarcoplasmic reticulum by theryanodine receptor (RyR). Calcium then associateswith troponin C in the sarcomere and stimulatescontraction (systole). Release of calcium from thesarcomere causes relaxation (diastole) and itsreuptake into the sarcoplasmic reticulum by SERCA.Signaling from the β-adrenergic receptor (β-AR)activates adenylyl cyclase (AC), generating cAMP,which activates protein kinase A (PKA). PKAphosphorylates the RyR, resulting in dissociation ofFKBP12.6, and phospholamban (PLB), which mod-ulates the activity of SERCA. Activation of β-ARK byβ-AR signaling creates a negative feedback loop.

©20

04 N

atur

e P

ublis

hing

Gro

up

http

://w

ww

.nat

ure.

com

/nat

urem

edic

ine

H I S TO R I C A L P E R S P E C T I V E

SERCA. Signaling by PKA leads to phosphorylation of phospholamban,which diminishes its inhibitory activity, thereby allowing enhancedactivity of SERCA and promoting cardiac relaxation.

The RyR is a tetrameric channel that associates with multiple signal-ing molecules, including the FK-506 binding protein FKBP12.6, PKAand protein phosphatases-1 and -2A (ref. 62). Interaction withFKBP12.6 stabilizes the channel in a closed state and reduces its activ-ity62. Phosphorylation of the RyR by PKA results in dissociation ofFKBP12.6 and increased sensitivity to calcium-induced activation(Fig. 3). Mice lacking FKBP12.6 or humans with mutations in the geneencoding RyR that reduce its affinity for FKBP12.6 have ‘leaky’ cal-cium channels with increased probability of being open and have anelevated susceptibility to exercise-induced cardiac arrhythmias63. RyRreceptors have also been reported to be hyperphosphorylated by PKAin failing human hearts, which could potentially contribute to abnor-malities in calcium handling and contractility. The increase in RyRphosphorylation in the failing heart is surprising in light of the down-regulation of β-AR signaling. The mechanism responsible for thiseffect remains to be defined, but may involve a decline in activity ofPP1 rather than an increase in activity of PKA.

In the failing heart, calcium uptake and release by the sarcoplasmicreticulum are diminished owing to a decrease in SERCA expression andactivity, resulting in diastolic and systolic dysfunction. The notion thataberrant calcium handling contributes to the pathogenesis of the fail-ing heart was supported by the finding that the heart failure phenotypeof mice lacking the cardiac LIM domain protein MLP was completelyabrogated by homozygous deletion of the gene encoding phospholam-ban, which apparently allowed for enhanced calcium reuptake by thesarcoplasmic reticulum64. Similarly, overexpression of SERCA can pre-serve cardiac function in certain rodent models of heart failure65. Thesefindings catalyzed interest in the possibility of introducing a dominant-negative mutated form of phospholamban into the failing heartthrough gene therapy, as a means of restoring sarcoplasmic reticulumcalcium uptake and contractility. However, later studies have reportedthat the apparent beneficial effects of phospholamban ablation areobserved in only a subset of mouse models of heart failure66. Moresobering is the recent discovery of phospholamban loss-of-functionmutations in families with hereditary heart failure67,68. These findingsraise significant concerns about such an approach and underscore thepotential pitfalls in directly extrapolating findings from the mouse tothe human heart. The mere fact that heart rates in mice and humansdiffer by an order of magnitude points to fundamental differences incardiac control mechanisms in the two species. Nevertheless, it is obvi-ous that functional abnormalities of SERCA, phospholamban or bothare central to the pathogenesis of heart failure.

Abnormal calcium signaling has also been implicated in myocardialstunning—a form of mechanical dysfunction that follows a transientischemic episode in the heart69. Reperfusion of transiently ischemicmyocardium is accompanied by calcium overload, which activates cal-cium-dependent calpain proteases that degrade components of thesarcomere, resulting in contractile depression. Free radicals also con-tribute to pathogenesis associated with myocardial stunning andischemia–reperfusion injury.

Hypertrophic and dilated cardiomyopathiesIn addition to signaling pathways triggered by extracellular stimuli, acommon cause of cardiomyopathy is mutations affecting contractileand structural proteins. The contractile apparatus is comprised of par-allel actin filaments anchored at a repeating structure known as the Z-disk. Although traditionally viewed as a static structure, recent studieshave revealed a more dynamic role for the Z-disk as a ‘stretch sensor’

and integrator of signaling pathways, particularly those involved intransmission of biomechanical stress70. Numerous signaling mole-cules have been found to associate with the Z-disk, but much remainsto be learned about the mechanisms for signal transduction from thisstructure and the nature of the protein-protein interactions that occurat this site.

Mutations affecting myosin heavy chain, myosin binding proteinand other components of the contractile apparatus have been shownto cause hypertrophic cardiomyopathy in humans and animal mod-els71–75. Dilated cardiomyopathy, characterized by an increase in sizeof the ventricular chamber, thin ventricular walls and frequently car-diac conduction abnormalities, has been attributed to abnormalities inforce transmission during cardiac contraction. Mutations affectingdystrophin and its associated proteins76,77, as well as cardiac actin,were the first to be shown to cause dilated cardiomyopathy. Later,mutations affecting other components of the sarcomere and thecytoskeleton (desmin, lamin A and C) were also identified in individu-als with dilated cardiomyopathy.

In each of the above cases, the mutant genes encode ‘poison peptides’that become incorporated into the sarcomere or cellular scaffolds andthereby perturb cardiac function. Although the catastrophic conse-quences of such mutations on cardiac growth and function are undis-puted, the mechanisms whereby the mutations disrupt cardiac functionremain vague. There is evidence to suggest that mutations in genesencoding sarcomere proteins alter calcium handling by the cardiacmyocyte, which may activate downstream signaling cascades that mod-ify cardiac function and gene expression78. The ability of a phospholam-ban-null mutation to restore cardiac function in mice lacking thesarcomere protein MLP is consistent with the notion that abnormalitiesin calcium handling contribute to pathogenesis in the absence of MLP64.However, the phospholamban knockout does not rescue cardiomyopa-thy resulting from other sarcomere abnormalities. Numerous examplesin which gender influences the severity of cardiomyopathic phenotypeshave also been reported79. The underlying mechanisms of such effectsare unknown and represent fascinating problems for the future.

Electrical conduction and arrhythmiasThe electrical impulse of each heartbeat is propagated through theheart by direct cell-cell coupling of cardiac muscle cells and through thecardiac conduction system comprising the atrioventricular node, His-bundle branches and Purkinje fibers. Abnormalities in cardiac rhythm,referred to as arrhythmias, account for more than 250,000 deaths peryear in the United States1. Cardiac arrhythmias are commonly associ-ated with underlying cardiac pathology, such as cardiomyopathies ormyocardial infarction, but can also occur in an otherwise healthy heartowing to inherited mutations in genes encoding ion channels and ionchannel–interacting proteins that perturb ion fluxes80,81.

The first gene shown to be responsible for lethal cardiac arrhythmiasin humans was the sodium-channel gene SCN5A82. Mutations in thisgene are responsible for long-QT syndrome, in which the actionpotential of the cardiac myocyte is abnormally prolonged. This muta-tion results in an inability of the channel to maintain the correct bal-ance of inward and outward currents. Loss-of-function mutationsaffecting voltage-gated potassium channels have also been identifiedin long-QT syndrome83,84

. Downregulation of K+ channels alsoaccompanies heart failure and is thought to contribute to the suscepti-bility of patients with heart failure to lethal cardiac arrhythmias. Drug-induced arrhythmias are also due to perturbation in K+ channelfunction, possibly owing to otherwise innocuous polymorphisms inthe ion-channel genes that confer enhanced sensitivity to drugs thatblock K+ channels. Long-QT syndrome is generally inherited in an

NATURE MEDICINE VOLUME 10 | NUMBER 5 | MAY 2004 471

©20

04 N

atur

e P

ublis

hing

Gro

up

http

://w

ww

.nat

ure.

com

/nat

urem

edic

ine

H I S TO R I C A L P E R S P E C T I V E

autosomal dominant manner, such that even relatively subtle alter-ations in the protein encoded by one mutant allele can result in susceptibility to lethal arrhythmias.

Mutations affecting the gap-junction proteins connexin40 and con-nexin43 also cause arrhythmias by disturbing cell–cell interactions85.In addition, arrhythmias have been attributed to mutations in mouseNkx2-5 (ref. 86), Tbx5 (ref. 87) and Sp4 (encoding the zinc-finger pro-tein HF-1b, or trans-acting transcription factor-4)88. In these cases, itis thought that abnormalities in the development of conduction sys-tem cell lineages, as well as in expression of target genes encoding pro-teins involved in cardiac conduction, are responsible forarrhythmogenic defects. Important areas for future investigationinclude the discovery of antiarrhymogenic drugs that act on ion chan-nels and the identification of additional genes encoding proteins thatare directly involved in arrhythmia or that modify the expressivity ofthe disease.

Stem cells and possibilities for cardiac regenerationIt has been generally accepted that cardiac myocytes lose the ability todivide after birth. Thus, loss of cardiac myocytes from apoptosis ornecrosis after myocardial infarction results in irreparable damage tothe adult heart, eventual heart failure and early death. However, aseries of recent studies has challenged this notion and suggested possi-bilities for repair of the damaged myocardium using stem cells.

Injection of bone marrow cells and, most recently, of stem cellsderived from cloned mouse embryos directly into the zone borderingthe dead portion of the heart after myocardial infarction have beenreported to promote cardiac regeneration and improve cardiac func-tion in animal models89,90. There has been disagreement regarding theefficiency with which exogenous stem cells can colonize the heart andadopt a cardiomyocyte cell fate91–96. Nevertheless, phase 1 clinical tri-als for patients who have had myocardial infarction have been initi-ated, and claims of improvement in cardiac function after intracardiacinjection of bone marrow cells have been made97. Injection of micewith specific cytokines has also been reported to stimulate the mobi-lization of bone marrow cells and repair of the infarcted heart, withresultant improvement in function and survival89. It remains to bedetermined whether bone marrow cells home specifically to the site ofcardiac injury or, alternatively, become dispersed throughout the body,with those in a region of damage being stimulated by local cues to pro-liferate or transdifferentiate into cardiac myocytes. It is also question-able whether the reported salutary effects of injected bone marrowcells on cardiac function reflect the conversion of stem cells to a car-diac fate or an alternative mechanism.

Potential hurdles in such cell-based therapies are the requirementsthat new myocytes become seamlessly integrated into the damagedmyocardium without becoming a substrate for arrhythmogenesis, andthat they be able to induce neovascularization of the regeneratedmyocardium. In this regard, initial studies with injected skeletal mus-cle cells showed promise, but this approach is unlikely to be suitablefor long-term therapy owing to the failure of the injected cells tobecome electrically coupled to the rest of the heart and to the intrinsicdifferences in contractile properties between cardiac and skeletal mus-cle cells98. Embryonic stem cells also can form teratomas in vivo, whichraises another set of therapeutic hurdles.

The heart has been reported to harbor a resident population of self-renewing stem cells able to give rise to cardiac myocytes, smooth mus-cle cells and endothelial cells99,100. Despite the possible existence ofthis population and the ability of bone marrow cells to contribute tocardiac repair, these intrinsic mechanisms are inadequate on their ownto restore cardiac function to a failing heart. Thus, strategies for coax-

ing cells toward the cardiac lineage, stimulating proliferation of post-mitotic cardiac myocytes, or both are needed. Irrespective of the con-troversies surrounding the conversion of adult stem cells to cardiacmyocytes (or lack thereof), such cell-based therapies will be inade-quate to meet the needs of all individuals worldwide requiring cardiacrepair. Thus, the development of small molecules capable of stimulat-ing cardiac cell fate decisions by activating normal developmental con-trol mechanisms represents an especially appealing approach foreventual cardiac regeneration101.

Scar formation, a common consequence of myocardial infarction,prevents reparative growth of the myocardium. The zebrafish, likecertain amphibians, has the ability to regenerate the ventricularmyocardium without forming a scar. This form of regenerativegrowth involves the formation of a blastema followed by de-differen-tiation of cardiac myocytes and repair102. A strain of mice defective inscarring has also been reported to undergo cardiac regeneration103.The ability to use zebrafish to perform genetic screens for mutationsthat affect cardiac regeneration represents a promising avenue forfuture exploration.

Looking to the futureDespite centuries of fascination by physicians and philosophers and,most recently, physiologists and molecular biologists, the heartremains one of the most mysterious works of nature. Although thepast decade has been marked by unprecedented advances towardrevealing its secrets, each advance has raised new questions, challengesand opportunities for the future.

In spite of dramatic progress on many fronts, numerous holes in ourcurrent knowledge remain to be filled. At present we have only a crudegenetic blueprint of cardiac development, with a multitude of detailsremaining to be added. How diverse cardiac cell fates are specified andhow the heart adopts its complex and integrated structure representmajor frontiers. The mechanisms that link mutations affecting specificproteins to cardiac phenotypes also remain vague in most cases, as dothe details of the cell-autonomous and non-cell-autonomous signal-ing mechanisms that control cardiac growth and function. Given thatsurgical intervention remains the primary therapy for congenital aswell as adult heart disease, there are significant opportunities for thedevelopment of new small-molecule therapeutics for restoration andmaintenance of cardiac function.

Future advances in cardiac biology and molecular medicine will bedriven by the integration of new technologies at the interfaces of mul-tiple disciplines. For example, mapping of cardiac disease genes, once adaunting task, has been facilitated by the completion of the humangenome sequence. The ability to identify modifiers and genetic poly-morphisms that affect cardiac disease phenotypes has provided insightinto mechanisms of disease and has paved the way toward the eventualdevelopment of individualized therapies. Techniques for transcrip-tional profiling and proteomics now permit the comprehensive analy-sis of disease-dependent changes in gene expression, and the ability toinactivate genes efficiently in model organisms such as zebrafish hasopened opportunities to rapidly determine the functions of genes thatcontrol cardiac function and dysfunction. Sophisticated new imagingmethods are permitting the analysis of genetic and chemical influenceson cardiac function at the subcellular, cellular and whole-organ levels.The continued unveiling of signaling pathways responsible for normaland abnormal cardiac growth, apoptosis and survival, and of proteinsinvolved in the regulation of ion fluxes and contractility, has providedfertile ground for drug development. Powerful new approaches incombinatorial chemistry coupled with high-throughput screeninghave accelerated the identification of small molecules with the poten-

472 VOLUME 10 | NUMBER 5 | MAY 2004 NATURE MEDICINE

©20

04 N

atur

e P

ublis

hing

Gro

up

http

://w

ww

.nat

ure.

com

/nat

urem

edic

ine

H I S TO R I C A L P E R S P E C T I V E

tial to modify cardiac development and disease101,104, and furtheroptimization of technologies for gene delivery and stem cell therapypromises to provide alternative means of repairing injured hearts.

In De Motu Cordis, William Harvey anticipated the discoveries andrichness of biology that lay ahead in the field:

The current opportunities in the field make Harvey’s vision andoptimism for the future timelier than ever.

ACKNOWLEDGEMENTSI am grateful to past and present members of my lab for insightful comments andscientific contributions.

COMPETING INTERESTS STATEMENTThe author declares that he has no competing financial interests.

This material is part of Nature Medicine’s 10 year anniversary series. For morecontent related to these special focus issues, please see http://www.nature.com/nm/special_focus/anniversary/index.html

Published online at http://www.nature.com/naturemedicine/

1. American Heart Association. Heart Disease and Stroke Statistics: 2004 Update(American Heart Association, Dallas, Texas, USA, 2003).

2. Hoffman, J.I. Incidence of congenital heart disease: II. Prenatal incidence. Pediatr.Cardiol. 16, 155–165 (1995).

3. Schneider, V.A. & Mercola, M. Wnt antagonism initiates cardiogenesis in Xenopus lae-vis. Genes Dev. 15, 304–315 (2001).

4. Schultheiss, T.M., Burch, J.B. & Lassar, A.B. A role for bone morphogenetic proteins inthe induction of cardiac myogenesis. Genes Dev. 11, 451–462 (1997).

5. Pandur, P., Lasche, M., Eisenberg, L.M. & Kuhl, M. Wnt-11 activation of a non-canon-ical Wnt signalling pathway is required for cardiogenesis. Nature 418, 636–641(2002).

6. Marvin, M.J., Di Rocco, G., Gardiner, A., Bush, S.M. & Lassar, A.B. Inhibition of Wntactivity induces heart formation from posterior mesoderm. Genes Dev. 15, 316–327(2001).

7. Bodmer, R. The gene tinman is required for specification of the heart and visceral mus-cles in Drosophila. Development 118, 719–729 (1993).

8. Azpiazu, N. & Frasch, M. tinman and bagpipe: two homeo box genes that determinecell fates in the dorsal mesoderm of Drosophila. Genes Dev. 7, 1325–1340 (1993).

9. Lints, T.J., Parsons, L.M., Hartley, L., Lyons, I. & Harvey, R.P. Nkx-2.5: a novel murinehomeobox gene expressed in early heart progenitor cells and their myogenic descen-dants. Development 119, 419–431 (1993).

10. Komuro, I. & Izumo, S. Csx: a murine homeobox-containing gene specifically expressedin the developing heart. Proc. Natl. Acad. Sci. USA 90, 8145–8149 (1993).

11. Lyons, I. et al. Myogenic and morphogenetic defects in the heart tubes of murineembryos lacking the homeo box gene Nkx2-5. Genes Dev. 9, 1654–1666 (1995).

12. McKinsey, T.A., Zhang, C.L. & Olson, E.N. MEF2: a calcium-dependent regulator ofcell division, differentiation and death. Trends Biochem. Sci. 27, 40–47 (2002).

13. Morin, S., Charron, F., Robitaille, L. & Nemer, M. GATA-dependent recruitment ofMEF2 proteins to target promoters. EMBO J. 19, 2046–2055 (2000).

14. Belaguli, N.S. et al. Cardiac tissue enriched factors serum response factor and GATA-4are mutual coregulators. Mol. Cell. Biol. 20, 7550–7558 (2000).

15. Wang, D. et al. Activation of cardiac gene expression by myocardin, a transcriptionalcofactor for serum response factor. Cell 105, 851–862 (2001).

16. Bao, Z.Z., Bruneau, B.G., Seidman, J.G., Seidman, C.E. & Cepko, C.L. Regulation ofchamber-specific gene expression in the developing heart by Irx4. Science 283,1161–1164 (1999).

17. Kelly, R.G., Zammit, P.S. & Buckingham, M.E. Cardiosensor mice and transcriptionalsubdomains of the vertebrate heart. Trends Cardiovasc. Med. 9, 3–10 (1999).

18. Kelly, R.G. & Buckingham, M.E. The anterior heart-forming field: voyage to the arterialpole of the heart. Trends Genet. 18, 210–216 (2002).

19. Kramer, K.L. & Yost, H.J. Cardiac left-right development: are the early steps conserved?Cold Spring Harb. Symp. Quant. Biol. 67, 37–43 (2002).

20. Srivastava, D., Cserjesi, P. & Olson, E.N. A subclass of bHLH proteins required for car-diac morphogenesis. Science 270, 1995–1999 (1995).

21. Yamagishi, H. et al. The combinatorial activities of Nkx2.5 and dHAND are essentialfor cardiac ventricle formation. Dev. Biol. 239, 190–203 (2001).

22. Cai, C.L. et al. Isl1 identifies a cardiac progenitor population that proliferates prior todifferentiation and contributes a majority of cells to the heart. Dev. Cell 5, 877–889(2003).

23. Fishman, M.C. & Olson, E.N. Parsing the heart: genetic modules for organ assembly.Cell 91, 153–156 (1997).

24. Garratt, A.N., Ozcelik, C. & Birchmeier, C. ErbB2 pathways in heart and neuraldiseases. Trends Cardiovasc. Med. 13, 80–86 (2003).

25. Chien, K.R. Myocyte survival pathways and cardiomyopathy: implications fortrastuzumab cardiotoxicity. Semin. Oncol. 27, 9–14; discussion 92–100 (2000).

26. Stuckmann, I., Evans, S. & Lassar, A.B. Erythropoietin and retinoic acid, secretedfrom the epicardium, are required for cardiac myocyte proliferation. Dev. Biol.255, 334–349 (2003).

27. Gitler, A.D., Lu, M.M., Jiang, Y.Q., Epstein, J.A. & Gruber, P.J. Molecular markersof cardiac endocardial cushion development. Dev. Dyn. 228, 643–650 (2003).

28. Gaussin, V. et al. Endocardial cushion and myocardial defects after cardiacmyocyte-specific conditional deletion of the bone morphogenetic protein receptorALK3. Proc. Natl. Acad. Sci. USA 99, 2878–2883 (2002).

29. Kim, R.Y., Robertson, E.J. & Solloway, M.J. Bmp6 and Bmp7 are required forcushion formation and septation in the developing mouse heart. Dev. Biol. 235,449–466 (2001).

30. Farrell, M., Waldo, K., Li, Y.X. & Kirby, M.L. A novel role for cardiac neural crest inheart development. Trends Cardiovasc. Med. 9, 214–220 (1999).

31. Epstein, J.A. & Buck, C.A. Transcriptional regulation of cardiac development:implications for congenital heart disease and DiGeorge syndrome. Pediatr. Res.48, 717–724 (2000).

32. Gourdie, R.G., Wei, Y., Kim, D., Klatt, S.C. & Mikawa, T. Endothelin-induced con-version of embryonic heart muscle cells into impulse-conducting Purkinje fibers.Proc. Natl. Acad. Sci. USA 95, 6815–6818 (1998).

33. Rentschler, S. et al. Neuregulin-1 promotes formation of the murine cardiac con-duction system. Proc. Natl. Acad. Sci. USA 99, 10464–10469 (2002).

34. Rentschler, S. et al. Visualization and functional characterization of the develop-ing murine cardiac conduction system. Development 128, 1785–1792 (2001).

35. Srivastava, D. & Olson, E.N. A genetic blueprint for cardiac development. Nature407, 221–226 (2000).

36. Schott, J.J. et al. Congenital heart disease caused by mutations in the transcrip-tion factor NKX2-5. Science 281, 108–111 (1998).

37. Basson, C.T. et al. Mutations in human TBX5 [corrected] cause limb and cardiacmalformation in Holt-Oram syndrome. Nat. Genet. 15, 30–35 (1997).

38. Garg, V. et al. GATA4 mutations cause human congenital heart defects and revealan interaction with TBX5. Nature 424, 443–447 (2003).

39. Olson, E.N. & Schneider, M.D. Sizing up the heart: development redux in disease.Genes Dev. 17, 1937–1956 (2003).

40. Lowes, B.D. et al. Myocardial gene expression in dilated cardiomyopathy treatedwith beta-blocking agents. N. Engl. J. Med. 346, 1357–1365 (2002).

41. Antos, C.L. et al. Activated glycogen synthase-3β suppresses cardiac hypertrophyin vivo. Proc. Natl. Acad. Sci. USA 99, 907–912 (2002).

42. Rothermel, B.A. et al. Myocyte-enriched calcineurin-interacting protein, MCIP1,inhibits cardiac hypertrophy in vivo. Proc. Natl. Acad. Sci. USA 98, 3328–3333(2001).

43. Koch, W.J. et al. Cardiac function in mice overexpressing the β-adrenergic recep-tor kinase or a β-ARK inhibitor. Science 268, 1350–1353 (1995).

44. MacLellan, W.R. & Schneider, M.D. Death by design. Programmed cell death incardiovascular biology and disease. Circ. Res. 81, 137–144 (1997).

45. Chien, K.R. Genomic circuits and the integrative biology of cardiac diseases.Nature 407, 227–232 (2000).

46. Hirota, H. et al. Loss of a gp130 cardiac muscle cell survival pathway is a criticalevent in the onset of heart failure during biomechanical stress. Cell 97, 189–198(1999).

47. Dorn, G.W., II & Brown, J.H. Gq signaling in cardiac adaptation and maladapta-tion. Trends Cardiovasc. Med. 9, 26–34 (1999).

48. Molkentin, J.D. et al. A calcineurin-dependent transcriptional pathway for cardiachypertrophy. Cell 93, 215–228 (1998).

49. Frey, N., McKinsey, T.A. & Olson, E.N. Decoding calcium signals involved in car-diac growth and function. Nat. Med. 6, 1221–1227 (2000).

50. Sugden, P.H. Signalling pathways in cardiac myocyte hypertrophy. Ann. Med. 33,611–622 (2001).

51. Molkentin, J.D. & Dorn, I.G., II. Cytoplasmic signaling pathways that regulate car-diac hypertrophy. Annu. Rev. Physiol. 63, 391–426 (2001).

52. Sano, M. et al. Activation and function of cyclin T–Cdk9 (positive transcriptionelongation factor-b) in cardiac muscle-cell hypertrophy. Nat. Med. 8, 1310–1317(2002).

53. Zhang, C.L. et al. Class II histone deacetylases act as signal-responsive repressorsof cardiac hypertrophy. Cell 110, 479–488 (2002).

54. McFarlane, S.I., Winer, N. & Sowers, J.R. Role of the natriuretic peptide system incardiorenal protection. Arch. Intern. Med. 163, 2696–2704 (2003).

55. Konstam, M.A. Improving clinical outcomes with drug treatment in heart failure:what have trials taught? Am. J. Cardiol. 91, 9D–14D (2003).

56. Rockman, H.A., Koch, W.J. & Lefkowitz, R.J. Seven-transmembrane-spanningreceptors and heart function. Nature 415, 206–212 (2002).

57. Packer, M. The neurohormonal hypothesis: a theory to explain the mechanism ofdisease progression in heart failure. J. Am. Coll. Cardiol. 20, 248–254 (1992).

58. Dorn, G.W., II. Adrenergic pathways and left ventricular remodeling. J. Card. Fail.8, S370–S373 (2002).

59. Bristow, M. Antiadrenergic therapy of chronic heart failure: surprises and new

NATURE MEDICINE VOLUME 10 | NUMBER 5 | MAY 2004 473

Reflecting on every part of medicine, physiology, pathology, semei-otics and therapeutics, when I see how many questions can beanswered, how many doubts resolved, how much obscurity illus-trated by the truth we have declared, the light we have made to shine,I see a field of such vast extent that my whole life, perchance, wouldnot suffice for its completion.

©20

04 N

atur

e P

ublis

hing

Gro

up

http

://w

ww

.nat

ure.

com

/nat

urem

edic

ine

H I S TO R I C A L P E R S P E C T I V E

opportunities. Circulation 107, 1100–1102 (2003).60. Bers, D.M. Cardiac excitation-contraction coupling. Nature 415, 198–205

(2002).61. Schmidt, A.G., Edes, I. & Kranias, E.G. Phospholamban: a promising therapeutic

target in heart failure? Cardiovasc. Drugs Ther. 15, 387–396 (2001).62. Marx, S.O. et al. PKA phosphorylation dissociates FKBP12.6 from the calcium

release channel (ryanodine receptor): defective regulation in failing hearts. Cell101, 365–376 (2000).

63. Wehrens, X.H. et al. FKBP12.6 deficiency and defective calcium release channel(ryanodine receptor) function linked to exercise-induced sudden cardiac death.Cell 113, 829–840 (2003).

64. Minamisawa, S. et al. Chronic phospholamban-sarcoplasmic reticulum calciumATPase interaction is the critical calcium cycling defect in dilated cardiomyopa-thy. Cell 99, 313–322 (1999).

65. Miyamoto, M.I. et al. Adenoviral gene transfer of SERCA2a improves left-ventric-ular function in aortic-banded rats in transition to heart failure. Proc. Natl. Acad.Sci. USA 97, 793–798 (2000).

66. Song, Q. et al. Rescue of cardiomyocyte dysfunction by phospholamban ablationdoes not prevent ventricular failure in genetic hypertrophy. J. Clin. Invest. 111,859–867 (2003).

67. Haghighi, K. et al. Human phospholamban null results in lethal dilated car-diomyopathy revealing a critical difference between mouse and human. J. Clin.Invest. 111, 869–876 (2003).

68. Schmitt, J.P. et al. Dilated cardiomyopathy and heart failure caused by a muta-tion in phospholamban. Science 299, 1410–1413 (2003).

69. Bolli, R. & Marban, E. Molecular and cellular mechanisms of myocardial stun-ning. Physiol. Rev. 79, 609–634 (1999).

70. Hoshijima, M., Pashmforoush, M., Knoll, R. & Chien, K.R. The MLP family ofcytoskeletal Z disc proteins and dilated cardiomyopathy: a stress pathway modelfor heart failure progression. Cold Spring Harb. Symp. Quant. Biol. 67, 399–408(2002).

71. Seidman, J.G. & Seidman, C. The genetic basis for cardiomyopathy: from muta-tion identification to mechanistic paradigms. Cell 104, 557–567 (2001).

72. Maass, A., Konhilas, J.P., Stauffer, B.L. & Leinwand, L.A. From sarcomeric muta-tions to heart disease: understanding familial hypertrophic cardiomyopathy. ColdSpring Harb. Symp. Quant. Biol. 67, 409–415 (2002).

73. Maron, B.J. Hypertrophic cardiomyopathy. Lancet 350, 127–133 (1997).74. Dalloz, F., Osinska, H. & Robbins, J. Manipulating the contractile apparatus:

genetically defined animal models of cardiovascular disease. J. Mol. Cell. Cardio.33, 9–25 (2001).

75. Olson, T.M., Michels, V.V., Thibodeau, S.N., Tai, Y.S. & Keating, M.T. Actin muta-tions in dilated cardiomyopathy, a heritable form of heart failure. Science 280,750–752 (1998).

76. Towbin, J.A. & Bowles, N.E. The failing heart. Nature 415, 227–233 (2002).77. Li, D. et al. Desmin mutation responsible for idiopathic dilated cardiomyopathy.

Circulation 100, 461–464 (1999).78. Fatkin, D. et al. An abnormal Ca2+ response in mutant sarcomere protein-medi-

ated familial hypertrophic cardiomyopathy. J. Clin. Invest. 106, 1351–1359(2000).

79. Leinwand, L.A. Sex is a potent modifier of the cardiovascular system. J. Clin.Invest. 112, 302–307 (2003).

80. Keating, M.T. & Sanguinetti, M.C. Molecular and cellular mechanisms of cardiacarrhythmias. Cell 104, 569–580 (2001).

81. Marban, E. Cardiac channelopathies. Nature 415, 213–218 (2002).82. Wang, Q. et al. SCN5A mutations associated with an inherited cardiac arrhyth-

mia, long QT syndrome. Cell 80, 805–811 (1995).

83. Sanguinetti, M.C., Jiang, C., Curran, M.E. & Keating, M.T. A mechanistic linkbetween an inherited and an acquired cardiac arrhythmia: HERG encodes the IKrpotassium channel. Cell 81, 299–307 (1995).

84. Splawski, I., Tristani-Firouzi, M., Lehmann, M.H., Sanguinetti, M.C. & Keating,M.T. Mutations in the hminK gene cause long QT syndrome and suppress IKsfunction. Nat. Genet. 17, 338–340 (1997).

85. Kirchhoff, S. et al. Abnormal cardiac conduction and morphogenesis in con-nexin40 and connexin43 double-deficient mice. Circ. Res. 87, 399–405(2000).

86. Kasahara, H. et al. Progressive atrioventricular conduction defects and heart fail-ure in mice expressing a mutant Csx/Nkx2.5 homeoprotein. J. Clin. Invest. 108,189–201 (2001).

87. Bruneau, B.G. et al. A murine model of Holt-Oram syndrome defines roles of theT-box transcription factor Tbx5 in cardiogenesis and disease. Cell 106, 709–721(2001).

88. Nguyen-Tran, V.T. et al. A novel genetic pathway for sudden cardiac death viadefects in the transition between ventricular and conduction system cell line-ages. Cell 102, 671–682 (2000).

89. Orlic, D. et al. Mobilized bone marrow cells repair the infarcted heart, improvingfunction and survival. Proc. Natl. Acad. Sci. USA 98, 10344–10349 (2001).

90. Lanza, R. et al. Regeneration of the infected heart with stem cell derived bynuclear transplantation. Circ. Res. 94, 820–827 (2004)

91. Murry, C.E., Whitney, M.L., Laflamme, M.A., Reinecke, H. & Field, L.J. Cellulartherapies for myocardial infarct repair. Cold Spring Harb. Symp. Quant. Biol. 67,519–526 (2002).

92. Pasumarthi, K.B. & Field, L.J. Cardiomyocyte cell cycle regulation. Circ. Res. 90,1044–1054 (2002).

93. Murry, C.E. et al. Haematopoietic stem cells do not transdifferentiate into cardiacmyocytes in myocardial infarcts. Nature 428, 664–668 (2004).

94. Balsam, L.B. et al. Haematopoietic stem cells adopt mature haematopoietic fatesin ischaemic myocardium. Nature 428, 668–675 (2004).

95. Chien, K.R. Stem cells: lost in translation. Nature 428, 607–608 (2004).96. Nygren, J.M. et al. Bone marrow–derived hematopoietic cells generate cardiomy-

ocytes at a low frequency through cell fusion, but not transdifferentiation. Nat.Med. 10, 494–501 (2004).

97. Assmus, B. et al. Transplantation of progenitor cells and regeneration enhance-ment in acute myocardial infarction (TOPCARE-AMI). Circulation 106,3009–3017 (2002).

98. Reinecke, H., Poppa, V. & Murry, C.E. Skeletal muscle stem cells do not transdif-ferentiate into cardiomyocytes after cardiac grafting. J. Mol. Cell. Cardiol. 34,241–249 (2002).

99. Beltrami, A.P. et al. Adult cardiac stem cells are multipotent and support myocar-dial regeneration. Cell 114, 763–776 (2003).

100. Oh, H. et al. Cardiac progenitor cells from adult myocardium: homing, differenti-ation, and fusion after infarction. Proc. Natl. Acad. Sci. USA 100,12313–12318 (2003).

101. Wu, X., Ding, S., Ding, Q., Gray, N.S. & Schultz, P.G. Small molecules thatinduce cardiomyogenesis in embryonic stem cells. J. Am. Chem. Soc. 126,1590–1591 (2004).

102. Poss, K.D., Wilson, L.G. & Keating, M.T. Heart regeneration in zebrafish. Science298, 2188–2190 (2002).

103. Leferovich, J.M. et al. Heart regeneration in adult MRL mice. Proc. Natl. Acad.Sci. USA 98, 9830–9835 (2001).

104. Bush, E. et al. A small molecular activator of cardiac hypertrophy uncovered in achemical screen for modifiers of the calcineurin signaling pathway. Proc. Natl.Acad. Sci. USA 101, 2870–2875 (2004).

474 VOLUME 10 | NUMBER 5 | MAY 2004 NATURE MEDICINE

©20

04 N

atur

e P

ublis

hing

Gro

up

http

://w

ww

.nat

ure.

com

/nat

urem

edic

ine