In the heart of the MEF2 transcription network: novel downstream effectors as potential targets for the treatment of cardiovascular disease

This editorial refers to ‘The MEF2 transcriptional target DMPK induces loss of sarcomere structure and cardiomyopathy’ by A. Damanafshan et al., pp. 1474–1486. Although heart failure management has improved during recent years, cardiovascular disease remains the leading cause of death worldwide, highlighting the need for better therapeutic strategies. The transcription factor myocyte enhancer factor 2 (MEF2) is essential for the cardiac gene regulatory network, and increasing evidence suggests its role in pathological cardiac remodelling, although the underlying mechanisms remain poorly understood.1 The MEF2 family is comprised of four members (MEF2A–D), which share a highly conserved N-terminal MADS box and an adjacent MEF2-specific domain.1 The MADS box binds to A/T-rich DNA target sequences in muscle-specific genes and is responsible for homodimerization and heterodimerization, while the MEF2-specific domain is required for cofactor binding, modulating MEF2 function as either activator or repressor of gene expression. In contrast, their C-terminal regions, containing nuclear localization signals and transcriptional activation domains, are highly divergent and targets for post-translational modifications regulating MEF2 function. All MEF2 family members are expressed during cardiac development, while MEF2A and MEF2B are the most abundant isoforms in adult heart.1 MEF2 plays essential roles in the development and function of the heart, but despite of their similar structure, gain- and loss-of function studies have revealed widely different functions of MEF2 isoforms. MEF2A ablation in mice resulted in perinatal death of most animals, while cardiac mitochondrial deficiency and conduction defects were observed in mice surviving to adulthood.1 Knockout of MEF2C caused embryonic lethality due to defective cardiac morphogenesis, while MEF2C appear to be dispensable after early embryonic development.1 In fact, siRNA-mediated MEF2C silencing attenuated hypertrophy and fibrosis in response to transaortic constriction (TAC)-induced mechanical pressure overload.2 In contrast to MEF2A and MEF2C, mice deficient for MEF2B3 and MEF2D4 are viable and display normal cardiac structure and function, and MEF2D ablation protected against cardiac remodelling and dysfunction following TAC and chronic β-adrenergic stimulation.4 On the other hand, overexpression of MEF2 family members in neonatal cardiomyocytes resulted in cardiomyocyte elongation and sarcomere disorganization in vitro and dilated cardiomyopathy in vivo.4–6 Based on these observations, MEF2 has been suggested as a therapeutic target to prevent cardiac dilation and dysfunction during heart failure. However, while expression of a dominant negative MEF2C mutant reduced cardiac dilation and prevented cardiac dysfunction in calcineurin transgenic mice exhibiting severe cardiac hypertrophy,5 it resulted in impaired mitochondrial function, oxidative stress, and cardiac dysfunction following TAC.7 In contrast, blocking of stress-induced MEF2 acetylation through the use of a small molecular probe interfering with MEF2 cofactor binding was found to be sufficient to prevent, and even revert cardiac hypertrophy and restore cardiac function following TAC.8 Based on the conflicting results obtained from MEF2-targeting therapeutic approaches, suggesting a role of MEF2 in both the adaptive and maladaptive response to cardiac stress, much focus has been on the identification of MEF2 downstream targets responsible for pathological cardiac remodelling. So far, more than 80 direct transcriptional targets of MEF2 have been identified,1 some of which are listed in Figure 1. In particular, Rarres2 was recently identified as a novel MEF2 target gene regulated by β-adrenergic signalling and up-regulated in response to pressure overload.9 Chemerin, which is encoded by Rarres2, has previously been associated with apoptosis and its overexpression resulted in cardiomyocyte hypertrophy in vitro, suggesting its potential role in cardiac remodelling. Schematic overview of the signalling pathways that can activate MEF2 as well as selected direct MEF2 target genes identified in the heart1 classified into functional classes. Schematic overview of the signalling pathways that can activate MEF2 as well as selected direct MEF2 target genes identified in the heart1 classified into functional classes. In this issue of Cardiovascular Research, Damanafshan et al. report myotonic dystrophy protein kinase (DMPK) as a novel direct transcriptional target of MEF2.10 Taking advantage of a ventricular muscle cell line allowing for inducible activation of constitutive active MEF2, the authors identified 75 genes that were differentially expressed following MEF2 activation. Among these was DMPK, which was considered of particular interest as its overexpression has been reported to cause disruption of sarcomere structure in vitro.11,12 Otherwise, DMPK, which belongs to the Rho family of kinases, is best known from its implication in myotonic dystrophy 1 (DM1), a multi-systemic neuromuscular disorder caused by expansion of cytosione-thymine-guanine (CTG) repeats in the 3′ untranslated region (3’ UTR) of DMPK, resulting in accumulation of toxic DMPK transcripts in nuclear foci, leading to abnormal transcription and splicing of numerous genes.13 DMPK was confirmed as a direct MEF2 target by ChIP analysis, and specifically the DMPK E isoform was found to be up-regulated upon MEF2 activation as well as in failing human and mouse hearts. DMPK knockdown prevented MEF2-induced sarcomere degeneration, but not cardiomyocyte elongation in vitro. Conversely, overexpression of the DMPK E isoform resulted in disruption of sarcomere structure both in vitro and in vivo, leading to fibrosis, cardiomyopathy, and premature death of DMPK E transgenic mice. This effect, which was associated with decreased sarcomeric gene expression, was shown to be dependent on DMPK kinase activity and, at least partially, attributed to inhibition of serum response factor (SRF), a MADS box transcription factor essential for sarcomeric gene expression.14 DMPK was found to inhibit the transcriptional activity of SRF through phosphorylation of two residues in its DNA binding MADS box domain, causing translocation of SRF from the nucleus to the cytoplasm, thereby decreasing the expression levels of its target genes, causing sarcomere disruption. Thus, in conclusion, the authors demonstrate that MEF2-induced sarcomere degeneration is mediated by DMPK through inhibition of SRF transcriptional activity, leading to decreased sarcomeric gene expression. The novel role of DMPK as a mediator of adverse structural remodelling during heart failure development brings up the question whether DMPK knockdown may represent a potential therapy in end-stage heart failure patients. DMPK knockout in mice was previously reported to be associated with cardiac conduction defects and skeletal myopathy.15 However, in a recent study DMPK knockdown and knockout was demonstrated to have no effect on cardiac function or muscle strength,15 suggesting that DMPK silencing does not have adverse effects. On the other hand, DMPK ablation did not prevent TAC-induced cardiac dysfunction, although it remains to be determined whether DMPK inhibition may have an effect during end-stage heart failure associated with up-regulation of DMPK E and sarcomere disruption. A potential therapy would be facilitated by the availability of antisense oligonucleotides targeting DMPK, which are already being tested in clinical trials for their ability to mitigate RNA toxicity in DM1 patients.16 This work was supported by the Italian Space Agency (ASI; DC-DTE-2011-2013 to M.L.B.) and the Italian Ministry of Education, Universities, and Research (MiUR)—National Research Programme (PNR)—Italian National Research Council (CNR) flagship project on ageing 2012–14 (to M.L.B.). Conflict of interest: none declared.