doi: 10.1161/CIRCULATIONAHA.105.578153
(Circulation. 2005;112:2891-2893.)
© 2005 American Heart Association, Inc.
Editorial |
Heart Failure Research Continues to Reveal the Flaws in Nature’s Unintelligent Design
From Whitaker Cardiovascular Institute and Molecular Stress Response Unit, Department of Medicine, Boston University School of Medicine, Boston, Mass.
Correspondence to Douglas B. Sawyer, MD, PhD, Whitaker Cardiovascular Institute and Molecular Stress Response Unit, Department of Medicine, Boston University School of Medicine, Boston, MA 02118. E-mail douglas.sawyer{at}bmc.org
Key Words: Editorials • genes • heart failure
Current therapy of patients with systolic heart failure focuses on protecting the patient from the chronic activation of hemodynamic reflexes that are activated by decreased tissue perfusion. The same renin-angiotensin, aldosterone, and adrenergic agonists that work in concert to dynamically regulate salt and water homeostasis and tissue perfusion throughout life become the cause of tissue edema, cardiac remodeling, and disease progression in the setting of impaired cardiac function. Although these and other regulators of cardiovascular homeostasis are complex and "highly evolved," their pathophysiologic role in heart failure points out a design flaw of nature. From the point of view of an evolutionary biologist, this arguably "unintelligent design" flaw can be recognized as a consequence of natural selection. Survival of individuals with the ability to dynamically regulate cardiac output, regional blood flow, and salt and water homeostasis in a stressful environment favors a robust neurohormonal system. Although less obvious, it seems reasonable to argue that the now clearly recognized deleterious effects of the neurohormonal response to decreased cardiac function would exert significant negative selection pressure. This latter argument is based on the fact that cardiac injury is rare before reproduction. Fortunately, years of inspired research have unraveled these design flaws, and pharmacological agents have been developed for heart failure patients to protect them from this example of "nature gone bad." Thank God!?
Article p 2930
The story of serum response factor (SRF), highlighted in a study by Parlakian et al in this issue of Circulation,1 illustrates how another of nature’s marvelously complex but flawed designs can contribute to the pathogenesis of heart failure. SRF is a transcription factor in the MADS family, which derives its name from the conserved nature of the basic structure of this DNA binding protein across diverse species: MCM-1 from yeast, Agamous and Deficiens from plants, and SRF from animals (for recent review, see Messenguy and Dubois2). SRF was first identified in 1988, as a transcriptional regulator that binds to the serum response element (SRE) in response to addition of serum to the culture media of cell lines.3 The response of most cell lines to the complex mix of growth factors in serum is to switch from a differentiated to a proliferative phenotype. The induction of c-fos, an immediate early gene, is critical for the transition from quiescent to proliferative states. Focusing on the SRE described in the promoter for c-fos, Norman et al3 were able to identify SRF as a specific DNA binding protein that was required for induction of c-fos expression and cell proliferation. They found that SRF was not only ubiquitously expressed but also highly conserved, because they were able to detect expression of SRF in fruit flies and frogs using the human sequence as a probe.
The SRE required for SRF binding is a 10-base-long sequence with the simple pattern CC(AT)6GG, described as an inverted repeat flanking an A/T rich core and shown to be identical to what had been described as a muscle-specific CArG element.4 SRF binds to the CArG element as a dimer. A look at the crystal structure of the MADS domain of SRF dimer binding to CArG5 is helpful in understanding the highly conserved nature of SRF and its homologs (Figure, panel A). Critical for SRF dimerization and function are C-terminal basic residues that bind to the CC/GG bases of the CArG, residues that make up the 
-helix that binds to the (AT)6 segment, and the subsequent ß-sheet structure that stabilizes the dimer. A comparison of SRF homologs among eukaryotes demonstrates that the most conserved residues of SRF reside in these critical portions of the molecule (Figure, panel B).
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Looking at the elegant structure and conservation of SRF/SRE interactions across species, it is difficult not to digress for a moment and ponder the current discussion in America over how to educate our children (including the future generation of physicians and scientists). Proponents of both "intelligent design" theory and evolution will likely admit that the structure of the SRF/SRE complex is quite pleasing to the eye. Evolutionary biologists would argue that the high degree of conservation across species of the SRF MADS box is driven by natural selection toward an efficient and specific SRF/SRE interaction. At the level of the single-cell organism (such as yeast), this selection is easiest to understand. Strains of yeast that can most rapidly switch between growth and quiescent states in response to changes in the environment would obviously fair better than those less dynamic. Moving to more complex organisms, the process of organogenesis as well as wound repair requires similarly rapid and specific changes between proliferative and differentiated states of cells. Highly conserved DNA binding proteins like SRF appear to play central roles in these changes.
Of course, with the proper coloring and perspective, such as used in panel A of the Figure, the SRF dimer might appear to some as a couple doing the tango along the DNA backbone, the designer’s blueprint. Furthermore, I suppose one could argue that the molecular similarities of SRF from yeast to man are simply the application of the same design principle to a broad set of organisms, and leave it at that. But does this imply that the success of all species is equally important to the designer? Most bread, beer, and wine enthusiasts would agree that the creation of Saccharomyces cerevisiae was one of Nature’s finest moments, by whatever means; however, I have yet to meet anyone who would say the same for Aspergillus sp.
Missing from the rendition in panel A of the Figure is a large portion of SRF that includes the so-called activation domain. Whereas the MADS box brings SRF monomers together specifically at the CArG element, the activation domain forms a complex with transcriptional coactivators or repressors that determine the transcriptional activity of a given SRF-responsive gene. Via recruitment of these other factors, SRF coordinates the activation of some genes with the simultaneous repression of others in response to environmental cues for growth. As understanding grows of how this is coordinated, so too does one’s impression of SRF as a beautiful example of biological complexity. In smooth muscle cells in the differentiated state, the SRF dimer interacts with the transcriptional coactivator myocardin, which allows transcription of smooth muscle contractile proteins.6 Growth stimuli induce phosphorylation of a family of transcription factors known as ternary complex factors (TCF), which allows these to interact with SRF and induce a family of growth factor–inducible genes involved in cell proliferation. Wang and colleagues7 have shown that phosphorylation of the TCF Elk-1 revealed SRF binding activity that directly competed with SRF/myocardin interactions, thereby suppressing transcription of muscle-specific genes. The competition between Elk-1 and myocardin for SRF binding creates a central "toggle switch" in vascular smooth muscle cells, controlling gene expression that determines whether a cell will proliferate versus maintain a differentiated state.
In contrast to most cell types, the adult differentiated cardiac myocyte has limited proliferative capacity and is "terminally differentiated." Thus, it remains unclear whether SRF serves a similar "toggle" function in the adult myocyte. During embryonic development of the heart, SRF is required for myocardial maturation, as demonstrated by both conditional knockout8,9 and dominant-negative strategies.10 Parlakian et al1 now extend this prior work to demonstrate that persistent SRF activity is required for maintenance of normal sarcomeric structure and contractile function in the adult heart (discussed further below). Thus, it is unclear whether SRF in the adult myocyte under normal physiological conditions ever switches positions to activate an undifferentiated state.
Under the pathological condition of heart failure, however, it appears that SRF can toggle "off" via a unique mechanism. Chang et al11 examined the expression pattern of SRF in the human failing heart. They found that whereas normal SRF protein migrates as a 65-kDa protein on gel electrophoresis, the majority of SRF from the failing heart existed as 55 or 32 kDa, consistent with SRF proteolysis in the failing heart. Using a molecular approach, they went on to show that SRF can be cleaved to a 32-kDa product by the enzyme caspase 3, which they and others have shown is activated in the failing heart. Not only was the cleaved SRF (SRF-N) inactive in transcriptional assays, but simultaneous expression of cleaved and full-length SRF demonstrated a dominant-negative effect of caspase-proteolyzed SRF. Because the 32-kDa cleaved SRF includes the MADS box, without the activation domain, the authors proposed that cleaved SRF in failing cardiac myocytes becomes a dominant-negative form, suppressing SRF responsive genes by binding to CArG elements without recruitment of transcriptional coregulators.
Parlakian et al1 add to the literature supporting a role for SRF breakdown in the pathophysiology of heart failure progression with an elegant experiment. Using a transgenic approach with a heart-specific, drug-inducible Cre-Lox system, they were able to delete SRF specifically from cardiac myocytes in the adult mouse. In the absence of SRF, the mouse went on to develop a dilated cardiomyopathy and death within 10 weeks of SRF gene disruption. Through a series of functional, histological, and molecular studies, they demonstrate that in the absence of SRF, myocytes lose sarcomeric proteins, without hypertrophic compensation. Perhaps this result is not particularly surprising in light of the well-established role of SRF in regulation of cardiac gene expression. Moreover, the effect of SRF deletion on the adult heart can only be considered indirect evidence that SRF cleavage to a dominant-negative fragment is pathophysiologic in the progression of heart failure. Nevertheless, this study is an important step confirming the critical role of SRF in maintaining the adult heart.
The number of genes regulated by SRF that are important for cardiac function continues to grow as newer methods are applied to scan the genome.12 Perhaps not surprisingly, the activity of SRF must be tightly regulated, with increases in SRF having similarly deleterious effects as too little. Zhang and colleagues10 have reported that overexpression of SRF leads to the development of a dilated cardiomyopathy, with early mortality and impaired ventricular function.
However complex and fascinating, one should wonder about the "intelligence" behind the design of a system in which the activity of a single protein provides such broad transcriptional regulation. Moreover, without redundant factors that can compensate for SRF function, the ability of cardiac myocytes to reverse the function of SRF by proteolytic cleavage would appear to be a major design flaw in the setting of systolic dysfunction. Whether there is some unknown "adaptive" function for caspase-dependent SRF cleavage remains to be determined. Further studies along these lines will help interpret the proteolytic processing of SRF that occurs in heart failure. Perhaps with further research, as for the chronic activation of the neurohormonal cascade in heart failure, this design flaw will offer a new target for therapy and new hope for patients with chronic heart failure.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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 Top References |
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1. Parlakian A, Charvet C, Escoubet B, Mericskay M, Molkentin JD, Gary-Bobo G, De Windt LJ, Ludosky M-A, Paulin D, Daegelen D, Tuil D, Li Z. Temporally controlled onset of dilated cardiomyopathy through disruption of the SRF gene in adult heart. Circulation. 2005; 112: 2930–2939.
2. Messenguy F, Dubois E. Role of MADS box proteins and their cofactors in combinatorial control of gene expression and cell development. Gene. 2003; 316: 1–21.[CrossRef][Medline] [Order article via Infotrieve]
3. Norman C, Runswick M, Pollock R, Treisman R. Isolation and properties of cDNA clones encoding SRF, a transcription factor that binds to the c-fos serum response element. Cell. 1988; 55: 989–1003.[CrossRef][Medline] [Order article via Infotrieve]
4. Boxer LM, Prywes R, Roeder RG, Kedes L. The sarcomeric actin CArG-binding factor is indistinguishable from the c-fos serum response factor. Mol Cell Biol. 1989; 9: 515–522.
5. Pellegrini L, Tan S, Richmond TJ. Structure of serum response factor core bound to DNA. Nature. 1995; 376: 490–498.[CrossRef][Medline] [Order article via Infotrieve]
6. Wang Z, Wang DZ, Pipes GC, Olson EN. Myocardin is a master regulator of smooth muscle gene expression. Proc Natl Acad Sci U S A. 2003; 100: 7129–7134.
7. Wang Z, Wang DZ, Hockemeyer D, McAnally J, Nordheim A, Olson EN. Myocardin and ternary complex factors compete for SRF to control smooth muscle gene expression. Nature. 2004; 428: 185–189.[CrossRef][Medline] [Order article via Infotrieve]
8. Parlakian A, Tuil D, Hamard G, Tavernier G, Hentzen D, Concordet JP, Paulin D, Li Z, Daegelen D. Targeted inactivation of serum response factor in the developing heart results in myocardial defects and embryonic lethality. Mol Cell Biol. 2004; 24: 5281–5289.
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Circulation 2005 112: 2930-2939.
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