Histone deacetylase 9 is a negative regulator of adipogenic differentiation

Differentiation of preadipocytes into mature adipocytes capable of efficiently storing lipids is an important regulatory mechanism in obesity. Here, we examined the involvement of histone deacetylases (HDACs) and histone acetyltransferases (HATs) in the regulation of adipogenesis. We find that among the various members of the HDAC and HAT families, only HDAC9 exhibited dramatic down-regulation preceding adipogenic differentiation. Preadipocytes from HDAC9 gene knock-out mice exhibited accelerated adipogenic differentiation, whereas HDAC9 overexpression in 3T3-L1 preadipocytes suppressed adipogenic differentiation, demonstrating its direct role as a negative regulator of adipogenesis. HDAC9 expression was higher in visceral as compared with subcutaneous preadipocytes, negatively correlating with their potential to undergo adipogenic differentiation in vitro. HDAC9 localized in the nucleus, and its negative regulation of adipogenesis segregates with the N-terminal nuclear targeting domain, whereas the C-terminal deacetylase domain is dispensable for this function. HDAC9 co-precipitates with USF1 and is recruited with USF1 at the E-box region of the C/EBPα gene promoter in preadipocytes. Upon induction of adipogenic differentiation, HDAC9 is down-regulated, leading to its dissociation from the USF1 complex, whereas p300 HAT is up-regulated to allow its association with USF1 and accumulation at the E-box site of the C/EBPα promoter in differentiated adipocytes. This reciprocal regulation of HDAC9 and p300 HAT in the USF1 complex is associated with increased C/EBPα expression, a master regulator of adipogenic differentiation. These findings provide new insights into mechanisms of adipogenic differentiation and document a critical regulatory role for HDAC9 in adipogenic differentiation through a deacetylase-independent mechanism.

Diminished cardiac fibrosis in heart failure is associated with altered ventricular arrhythmia phenotype

OBJECTIVES

We sought to define the role of interstitial fibrosis in the proarrhythmic phenotype of failing ventricular myocardium.

BACKGROUND

Multiple cellular events that occur during pathological remodeling of the failing ventricle are implicated in the genesis of ventricular tachycardia (VT), including interstitial fibrosis. Recent studies suggest that ventricular fibrosis is reversible, and current anti-remodeling therapies attenuate ventricular fibrosis. However, the role of interstitial fibrosis in the proarrhythmic phenotype of failing ventricular myocardium is currently not well defined.

METHODS

Class II histone deacetylases (HDACs) have been implicated in promoting collagen biosynthesis. As these enzymes are inhibited by protein kinase D1 (PKD1), we studied mice with cardiomyocyte-specific transgenic over-expression of a constitutively active mutant of PKD1 (caPKD). caPKD mice were compared with animals in which cardiomyopathy was induced by severe thoracic aortic banding (sTAB). Hearts were analyzed by echocardiographic and electrocardiographic means. Interstitial fibrosis was assessed by histology and quantified biochemically. Ventricular arrhythmias were induced by closed-chest, intracardiac pacing.

RESULTS

Similar degrees of hypertrophic growth, systolic dysfunction and mortality were observed in the two models. In sTAB mice, robust ventricular fibrosis was readily detected, but myocardial collagen content was significantly reduced in caPKD mice. As expected, VT was readily inducible by programmed stimulation in sTAB mice and VT was less inducible in caPKD mice. Surprisingly, episodes of VT manifested longer cycle lengths and longer duration in caPKD mice.

CONCLUSION

Attenuated ventricular fibrosis is associated with reduced VT inducibility, increased VT duration, and significantly longer arrhythmia cycle length.

Modulation of K-Ras-dependent lung tumorigenesis by MicroRNA-21

Lung cancer is the leading cause of cancer-related deaths in the world, and non-small-cell lung cancer (NSCLC) accounts for 80% of cases. MicroRNA-21 (miR-21) expression is increased and predicts poor survival in NSCLC. Although miR-21 function has been studied in vitro with cancer cell lines, the role of miR-21 in tumor development in vivo is unknown. We utilize transgenic mice with loss-of-function and gain-of-function miR-21 alleles combined with a model of NSCLC to determine the role of miR-21 in lung cancer. We show that overexpression of miR-21 enhances tumorigenesis and that genetic deletion of miR-21 partially protects against tumor formation. MiR-21 drives tumorigenesis through inhibition of negative regulators of the Ras/MEK/ERK pathway and inhibition of apoptosis.

MicroRNA-206 delays ALS progression and promotes regeneration of neuromuscular synapses in mice

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by loss of motor neurons, denervation of target muscles, muscle atrophy, and paralysis. Understanding ALS pathogenesis may require a fuller understanding of the bidirectional signaling between motor neurons and skeletal muscle fibers at neuromuscular synapses. Here, we show that a key regulator of this signaling is miR-206, a skeletal muscle-specific microRNA that is dramatically induced in a mouse model of ALS. Mice that are genetically deficient in miR-206 form normal neuromuscular synapses during development, but deficiency of miR-206 in the ALS mouse model accelerates disease progression. miR-206 is required for efficient regeneration of neuromuscular synapses after acute nerve injury, which probably accounts for its salutary effects in ALS. miR-206 mediates these effects at least in part through histone deacetylase 4 and fibroblast growth factor signaling pathways. Thus, miR-206 slows ALS progression by sensing motor neuron injury and promoting the compensatory regeneration of neuromuscular synapses.

MicroRNAs miR-143 and miR-145 modulate cytoskeletal dynamics and responsiveness of smooth muscle cells to injury

Vascular injury triggers dedifferentiation and cytoskeletal remodeling of smooth muscle cells (SMCs), culminating in vessel occlusion. Serum response factor (SRF) and its coactivator, myocardin, play a central role in the control of smooth muscle phenotypes by regulating the expression of cytoskeletal genes. We show that SRF and myocardin regulate a cardiovascular-specific microRNA (miRNA) cluster encoding miR-143 and miR-145. To assess the functions of these miRNAs in vivo, we systematically deleted them singly and in combination in mice. Mice lacking both miR-143 and miR-145 are viable and do not display overt abnormalities in smooth muscle differentiation, although they show a significant reduction in blood pressure due to reduced vascular tone. Remarkably, however, neointima formation in response to vascular injury is profoundly impeded in mice lacking these miRNAs, due to disarray of actin stress fibers and diminished migratory activity of SMCs. These abnormalities reflect the regulation of a cadre of modulators of SRF activity and actin dynamics by miR-143 and miR-145. Thus, miR-143 and miR-145 act as integral components of the regulatory network whereby SRF controls cytoskeletal remodeling and phenotypic switching of SMCs during vascular disease.

Calcium/calmodulin-dependent protein kinase II links ER stress with Fas and mitochondrial apoptosis pathways

ER stress-induced apoptosis is implicated in various pathological conditions, but the mechanisms linking ER stress-mediated signaling to downstream apoptotic pathways remain unclear. Using human and mouse cell culture and in vivo mouse models of ER stress-induced apoptosis, we have shown that cytosolic calcium resulting from ER stress induces expression of the Fas death receptor through a pathway involving calcium/calmodulin-dependent protein kinase IIgamma (CaMKIIgamma) and JNK. Remarkably, CaMKIIgamma was also responsible for processes involved in mitochondrial-dependent apoptosis, including release of mitochondrial cytochrome c and loss of mitochondrial membrane potential. CaMKII-dependent apoptosis was also observed in a number of cultured human and mouse cells relevant to ER stress-induced pathology, including cultured macrophages, endothelial cells, and neuronal cells subjected to proapoptotic ER stress. Moreover, WT mice subjected to systemic ER stress showed evidence of macrophage mitochondrial dysfunction and apoptosis, renal epithelial cell apoptosis, and renal dysfunction, and these effects were markedly reduced in CaMKIIgamma-deficient mice. These data support an integrated model in which CaMKII serves as a unifying link between ER stress and the Fas and mitochondrial apoptotic pathways. Our study also revealed what we believe to be a novel proapoptotic function for CaMKII, namely, promotion of mitochondrial calcium uptake. These findings raise the possibility that CaMKII inhibitors could be useful in preventing apoptosis in pathological settings involving ER stress-induced apoptosis.

Regulation of GLUT4 expression in denervated skeletal muscle

Denervation by sciatic nerve resection causes decreased muscle glucose transporter 4 (GLUT4) expression, but little is known about the signaling events that cause this decrease. Experiments were designed to test the hypothesis that decreased GLUT4 expression in denervated muscle occurs because of decreased calcium/CaMK activity, which would then lead to decreased activation of the transcription factors myocyte enhancer factor 2 (MEF2) and GLUT4 enhancer factor (GEF), which are required for normal GLUT4 expression. GLUT4 mRNA was elevated in mice expressing constitutively active CaMK isoform IV (CaMKIV) and decreased by denervation. Denervation decreased GEF binding to the promoter and the content of GEF in the nucleus, but there was no change in either MEF2 binding or MEF2 protein content. Expression of a MEF2-dependent reporter gene did not change in denervated skeletal muscle. To determine the domains of the GLUT4 promoter that respond to denervation, transgenic mice expressing the chloramphenicol acetyl transferase (CAT) reporter gene driven by different lengths of the human GLUT4 promoter were denervated. Using several different promoter/reporter gene constructs, we found that all areas of the GLUT4 promoter were truncated or missing, except for the MEF2 binding domain and the basal promoter. All of the GLUT4 promoter/CAT reporter constructs evaluated responded normally to denervation. Our data lead us to conclude that decreased CaMK activity is not the reason for decreased GLUT4 content in denervated muscle and that negative control of GLUT4 expression is not mediated through the MEF2 or GEF-binding domains. These findings indicate that withdrawal of a GEF- or MEF2-dependent signal is not likely a major determinant of the denervation effect on GLUT4 expression. Thus, the response to denervation may be mediated by other elements present in the basal promoter of the GLUT4 gene.

microRNA-133a regulates cardiomyocyte proliferation and suppresses smooth muscle gene expression in the heart

MicroRNAs (miRNAs) modulate gene expression by inhibiting mRNA translation and promoting mRNA degradation, but little is known of their potential roles in organ formation or function. miR-133a-1 and miR-133a-2 are identical, muscle-specific miRNAs that are regulated during muscle development by the SRF transcription factor. We show that mice lacking either miR-133a-1 or miR-133a-2 are normal, whereas deletion of both miRNAs causes lethal ventricular-septal defects in approximately half of double-mutant embryos or neonates; miR-133a double-mutant mice that survive to adulthood succumb to dilated cardiomyopathy and heart failure. The absence of miR-133a expression results in ectopic expression of smooth muscle genes in the heart and aberrant cardiomyocyte proliferation. These abnormalities can be attributed, at least in part, to elevated expression of SRF and cyclin D2, which are targets for repression by miR-133a. These findings reveal essential and redundant roles for miR-133a-1 and miR-133a-2 in orchestrating cardiac development, gene expression, and function and point to these miRNAs as critical components of an SRF-dependent myogenic transcriptional circuit.

Calsarcin-2 deficiency increases exercise capacity in mice through calcineurin/NFAT activation

The composition of skeletal muscle, in terms of the relative number of slow- and fast-twitch fibers, is tightly regulated to enable an organism to respond and adapt to changing physical demands. The phosphatase calcineurin and its downstream targets, transcription factors of the nuclear factor of activated T cells (NFAT) family, play a critical role in this process by promoting the formation of slow-twitch, oxidative fibers. Calcineurin binds to calsarcins, a family of striated muscle-specific proteins of the sarcomeric Z-disc. We show here that mice deficient in calsarcin-2, which is expressed exclusively by fast-twitch muscle and encoded by the myozenin 1 (Myoz1) gene, have substantially reduced body weight and fast-twitch muscle mass in the absence of an overt myopathic phenotype. Additionally, Myoz1 KO mice displayed markedly improved performance and enhanced running distances in exercise studies. Analysis of fiber type composition of calsarcin-2-deficient skeletal muscles showed a switch toward slow-twitch, oxidative fibers. Reporter assays in cultured myoblasts indicated an inhibitory role for calsarcin-2 on calcineurin, and Myoz1 KO mice exhibited both an excess of NFAT activity and an increase in expression of regulator of calcineurin 1-4 (RCAN1-4), indicating enhanced calcineurin signaling in vivo. Taken together, these results suggest that calsarcin-2 modulates exercise performance in vivo through regulation of calcineurin/NFAT activity and subsequent alteration of the fiber type composition of skeletal muscle.

Maintenance of cardiac energy metabolism by histone deacetylase 3 in mice

Histone deacetylase (HDAC) inhibitors show remarkable therapeutic potential for a variety of disorders, including cancer, neurological disease, and cardiac hypertrophy. However, the specific HDAC isoforms that mediate their actions are unclear, as are the physiological and pathological functions of individual HDACs in vivo. To explore the role of Hdac3 in the heart, we generated mice with a conditional Hdac3 null allele. Although global deletion of Hdac3 resulted in lethality by E9.5, mice with a cardiac-specific deletion of Hdac3 survived until 3-4 months of age. At this time, they showed massive cardiac hypertrophy and upregulation of genes associated with fatty acid uptake, fatty acid oxidation, and electron transport/oxidative phosphorylation accompanied by fatty acid-induced myocardial lipid accumulation and elevated triglyceride levels. These abnormalities in cardiac metabolism can be attributed to excessive activity of the nuclear receptor PPARalpha. The phenotype associated with cardiac-specific Hdac3 gene deletion differs from that of all other Hdac gene mutations. These findings reveal a unique role for Hdac3 in maintenance of cardiac function and regulation of myocardial energy metabolism.

TrkB regulates hippocampal neurogenesis and governs sensitivity to antidepressive treatment

Adult hippocampal neurogenesis is stimulated by chronic administration of antidepressants (ADs) and by voluntary exercise. Neural progenitor cells (NPCs) in the dentate gyrus (DG) that are capable of continuous proliferation and neuronal differentiation are the source of such structural plasticity. Here we report that mice lacking the receptor tyrosine kinase TrkB in hippocampal NPCs have impaired proliferation and neurogenesis. When exposed to chronic ADs or wheel-running, no increase in proliferation or neurogenesis is observed. Ablation of TrkB also renders these mice behaviorally insensitive to antidepressive treatment in depression- and anxiety-like paradigms. In contrast, mice lacking TrkB only in differentiated DG neurons display typical neurogenesis and respond normally to chronic ADs. Thus, our data establish an essential cell-autonomous role for TrkB in regulating hippocampal neurogenesis and behavioral sensitivity to antidepressive treatments, and support the notion that impairment of the neurogenic niche is an etiological factor for refractory responses to an antidepressive regimen.

The endothelial-specific microRNA miR-126 governs vascular integrity and angiogenesis

Endothelial cells play essential roles in maintenance of vascular integrity, angiogenesis, and wound repair. We show that an endothelial cell-restricted microRNA (miR-126) mediates developmental angiogenesis in vivo. Targeted deletion of miR-126 in mice causes leaky vessels, hemorrhaging, and partial embryonic lethality, due to a loss of vascular integrity and defects in endothelial cell proliferation, migration, and angiogenesis. The subset of mutant animals that survives displays defective cardiac neovascularization following myocardial infarction. The vascular abnormalities of miR-126 mutant mice resemble the consequences of diminished signaling by angiogenic growth factors, such as VEGF and FGF. Accordingly, miR-126 enhances the proangiogenic actions of VEGF and FGF and promotes blood vessel formation by repressing the expression of Spred-1, an intracellular inhibitor of angiogenic signaling. These findings have important therapeutic implications for a variety of disorders involving abnormal angiogenesis and vascular leakage.

HDAC4 inhibits cell-cycle progression and protects neurons from cell death

HDAC4 is a Class II histone deacetylase (HDAC) that is highly expressed in the brain, but whose functional significance in the brain is not known. We show that forced expression of HDAC4 in cerebellar granule neurons protects them against low potassium-induced apoptosis. HDAC4 also protects HT22 neuroblastoma cells from death induced by oxidative stress. HDAC4-mediated neuroprotection does not require its HDAC catalytic domain and cannot be inhibited by chemical inhibitors of HDACs. Neuroprotection by HDAC4 also does not require the Raf-MEK-ERK or the PI-3 kinase-Akt signaling pathways and occurs despite the activation of c-jun, an event that is generally believed to condemn neurons to die. The protective action of HDAC4 occurs in the nucleus and is mediated by a region that contains the nuclear localization signal. HDAC4 inhibits the activity of cyclin-dependent kinase-1 (CDK1) and the progression of proliferating HEK293T and HT22 cells through the cell cycle. Mice-lacking HDAC4 have elevated CDK1 activity and display cerebellar abnormalities including a progressive loss of Purkinje neurons postnatally in posterior lobes. Surviving Purkinje neurons in these lobes have duplicated soma. Furthermore, large numbers of cells within these affected lobes incorporate BrdU, indicating cell-cycle progression. These abnormalities along with the ability of HDAC4 to inhibit CDK1 and cell-cycle progression in cultured cells suggest that neuroprotection by HDAC4 is mediated by preventing abortive cell-cycle progression.

The MEF2D transcription factor mediates stress-dependent cardiac remodeling in mice

The adult heart responds to excessive neurohumoral signaling and workload by a pathological growth response characterized by hypertrophy of cardiomyocytes and activation of a fetal program of cardiac gene expression. These responses culminate in diminished pump function, ventricular dilatation, wall thinning, and fibrosis, and can result in sudden death. Myocyte enhancer factor-2 (MEF2) transcription factors serve as targets of the signaling pathways that drive pathological cardiac remodeling, but the requirement for MEF2 factors in the progression of heart disease in vivo has not been determined. MEF2A and MEF2D are the primary MEF2 factors expressed in the adult heart. To specifically determine the role of MEF2D in pathological cardiac remodeling, we generated mice with a conditional MEF2D allele. MEF2D-null mice were viable, but were resistant to cardiac hypertrophy, fetal gene activation, and fibrosis in response to pressure overload and beta-chronic adrenergic stimulation. Furthermore, we show in a transgenic mouse model that forced overexpression of MEF2D was sufficient to drive the fetal gene program and pathological remodeling of the heart. These results reveal a unique and important function for MEF2D in stress-dependent cardiac growth and reprogramming of gene expression in the adult heart.

Stimulation of calcineurin Aalpha activity attenuates muscle pathophysiology in mdx dystrophic mice

Calcineurin activation ameliorates the dystrophic pathology of hindlimb muscles in mdx mice and decreases their susceptibility to contraction damage. In mdx mice, the diaphragm is more severely affected than hindlimb muscles and more representative of Duchenne muscular dystrophy. The constitutively active calcineurin Aalpha transgene (CnAalpha) was overexpressed in skeletal muscles of mdx (mdx CnAalpha*) mice to test whether muscle morphology and function would be improved. Contractile function of diaphragm strips and extensor digitorum longus and soleus muscles from adult mdx CnAalpha* and mdx mice was examined in vitro. Hindlimb muscles from mdx CnAalpha* mice had a prolonged twitch time course and were more resistant to fatigue. Because of a slower phenotype and a decrease in fiber cross-sectional area, normalized force was lower in fast- and slow-twitch muscles of mdx CnAalpha* than mdx mice. In the diaphragm, despite a slower phenotype and a approximately 35% reduction in fiber size, normalized force was preserved. This was likely mediated by the reduction in the area of the diaphragm undergoing degeneration (i.e., mononuclear cell and connective and adipose tissue infiltration). The proportion of centrally nucleated fibers was reduced in mdx CnAalpha* compared with mdx mice, indicative of improved myofiber viability. In hindlimb muscles of mdx mice, calcineurin activation increased expression of markers of regeneration, particularly developmental myosin heavy chain isoform and myocyte enhancer factor 2A. Thus activation of the calcineurin signal transduction pathway has potential to ameliorate the mdx pathophysiology, especially in the diaphragm, through its effects on muscle degeneration and regeneration and endurance capacity.

Overexpression of pyruvate dehydrogenase kinase 4 in heart perturbs metabolism and exacerbates calcineurin-induced cardiomyopathy

The heart adapts to changes in nutritional status and energy demands by adjusting its relative metabolism of carbohydrates and fatty acids. Loss of this metabolic flexibility such as occurs in diabetes mellitus is associated with cardiovascular disease and heart failure. To study the long-term consequences of impaired metabolic flexibility, we have generated mice that overexpress pyruvate dehydrogenase kinase (PDK)4 selectively in the heart. Hearts from PDK4 transgenic mice have a marked decrease in glucose oxidation and a corresponding increase in fatty acid catabolism. Although no overt cardiomyopathy was observed in the PDK4 transgenic mice, introduction of the PDK4 transgene into mice expressing a constitutively active form of the phosphatase calcineurin, which causes cardiac hypertrophy, caused cardiomyocyte fibrosis and a striking increase in mortality. These results demonstrate that cardiac-specific overexpression of PDK4 is sufficient to cause a loss of metabolic flexibility that exacerbates cardiomyopathy caused by the calcineurin stress-activated pathway.

Regulation of atrial natriuretic peptide secretion by a novel Ras-like protein

Atrial cardiomyocytes, neurons, and endocrine tissues secrete neurotransmitters and peptide hormones via large dense-core vesicles (LDCVs). We describe a new member of the Ras family of G-proteins, named RRP17, which is expressed specifically in cardiomyocytes, neurons, and the pancreas. RRP17 interacts with Ca²⁺-activated protein for secretion-1 (CAPS1), one of only a few proteins known to be associated exclusively with LDCV exocytosis. Ectopic expression of RRP17 in cardiomyocytes enhances secretion of atrial natriuretic peptide (ANP), a regulator of blood pressure and natriuresis. Conversely, genetic deletion of RRP17 in mice results in dysmorphic LDCVs, impaired ANP secretion, and hypertension. These findings identify RRP17 as a component of the cellular machinery involved in regulated secretion within the heart and potential mediator of the endocrine influence of the heart on other tissues.

Myosin accumulation and striated muscle myopathy result from the loss of muscle RING finger 1 and 3

Maintenance of skeletal and cardiac muscle structure and function requires precise control of the synthesis, assembly, and turnover of contractile proteins of the sarcomere. Abnormalities in accumulation of sarcomere proteins are responsible for a variety of myopathies. However, the mechanisms that mediate turnover of these long-lived proteins remain poorly defined. We show that muscle RING finger 1 (MuRF1) and MuRF3 act as E3 ubiquitin ligases that cooperate with the E2 ubiquitin-conjugating enzymes UbcH5a, -b, and -c to mediate the degradation of beta/slow myosin heavy chain (beta/slow MHC) and MHCIIa via the ubiquitin proteasome system (UPS) in vivo and in vitro. Accordingly, mice deficient for MuRF1 and MuRF3 develop a skeletal muscle myopathy and hypertrophic cardiomyopathy characterized by subsarcolemmal MHC accumulation, myofiber fragmentation, and diminished muscle performance. These findings identify MuRF1 and MuRF3 as key E3 ubiquitin ligases for the UPS-dependent turnover of sarcomeric proteins and reveal a potential basis for myosin storage myopathies.

Histone deacetylase degradation and MEF2 activation promote the formation of slow-twitch myofibers

Skeletal muscle is composed of heterogeneous myofibers with distinctive rates of contraction, metabolic properties, and susceptibility to fatigue. We show that class II histone deacetylase (HDAC) proteins, which function as transcriptional repressors of the myocyte enhancer factor 2 (MEF2) transcription factor, fail to accumulate in the soleus, a slow muscle, compared with fast muscles (e.g., white vastus lateralis). Accordingly, pharmacological blockade of proteasome function specifically increases expression of class II HDAC proteins in the soleus in vivo. Using gain- and loss-of-function approaches in mice, we discovered that class II HDAC proteins suppress the formation of slow twitch, oxidative myofibers through the repression of MEF2 activity. Conversely, expression of a hyperactive form of MEF2 in skeletal muscle of transgenic mice promotes the formation of slow fibers and enhances running endurance, enabling mice to run almost twice the distance of WT littermates. Thus, the selective degradation of class II HDACs in slow skeletal muscle provides a mechanism for enhancing physical performance and resistance to fatigue by augmenting the transcriptional activity of MEF2. These findings provide what we believe are new insights into the molecular basis of skeletal muscle function and have important implications for possible therapeutic interventions into muscular diseases.

Regulation of skeletal muscle sarcomere integrity and postnatal muscle function by Mef2c

Myocyte enhancer factor 2 (MEF2) transcription factors cooperate with the MyoD family of basic helix-loop-helix (bHLH) transcription factors to drive skeletal muscle development during embryogenesis, but little is known about the potential functions of MEF2 factors in postnatal skeletal muscle. Here we show that skeletal muscle-specific deletion of Mef2c in mice results in disorganized myofibers and perinatal lethality. In contrast, neither Mef2a nor Mef2d is required for normal skeletal muscle development in vivo. Skeletal muscle deficient in Mef2c differentiates and forms normal myofibers during embryogenesis, but myofibers rapidly deteriorate after birth due to disorganized sarcomeres and a loss of integrity of the M line. Microarray analysis of Mef2c null muscles identified several muscle structural genes that depend on MEF2C, including those encoding the M-line-specific proteins myomesin and M protein. We show that MEF2C directly regulates myomesin gene transcription and that loss of Mef2c in skeletal muscle results in improper sarcomere organization. These results reveal a key role for Mef2c in maintenance of sarcomere integrity and postnatal maturation of skeletal muscle.

Calcineurin-A alpha activation enhances the structure and function of regenerating muscles after myotoxic injury

Calcineurin signaling is essential for successful muscle regeneration. Although calcineurin inhibition compromises muscle repair, it is not known whether calcineurin activation can enhance muscle repair after injury. Tibialis anterior (TA) muscles from adult wild-type (WT) and transgenic mice overexpressing the constitutively active calcineurin-A alpha transgene under the control of the mitochondrial creatine kinase promoter (MCK-CnA alpha*) were injected with the myotoxic snake venom Notexin to destroy all muscle fibers. The TA muscle of the contralateral limb served as the uninjured control. Muscle structure was assessed at 5 and 9 days postinjury, and muscle function was tested in situ at 9 days postinjury. Calcineurin stimulation enhanced muscle regeneration and altered levels of myoregulatory factors (MRFs). Recovery of myofiber size and force-producing capacity was hastened in injured muscles of MCK-CnA alpha* mice compared with control. Myogenin levels were greater 5 days postinjury and myocyte enhancer factor 2a (MEF2a) expression was greater 9 days postinjury in muscles of MCK-CnA alpha* mice compared with WT mice. Higher MEF2a expression in regenerating muscles of MCK-CnA alpha* mice 9 days postinjury may be related to an increase of slow fiber genes. Calcineurin activation in uninjured and injured TA muscles slowed muscle contractile properties, reduced fatigability, and enhanced force recovery after 4 min of intermittent maximal stimulation. Therefore, calcineurin activation can confer structural and functional benefits to regenerating skeletal muscles, which may be mediated in part by differential expression of MRFs.

MEF2C transcription factor controls chondrocyte hypertrophy and bone development

Chondrocyte hypertrophy is essential for endochondral bone development. Unexpectedly, we discovered that MEF2C, a transcription factor that regulates muscle and cardiovascular development, controls bone development by activating the gene program for chondrocyte hypertrophy. Genetic deletion of Mef2c or expression of a dominant-negative MEF2C mutant in endochondral cartilage impairs hypertrophy, cartilage angiogenesis, ossification, and longitudinal bone growth in mice. Conversely, a superactivating form of MEF2C causes precocious chondrocyte hypertrophy, ossification of growth plates, and dwarfism. Endochondral bone formation is exquisitely sensitive to the balance between MEF2C and the corepressor histone deacetylase 4 (HDAC4), such that bone deficiency of Mef2c mutant mice can be rescued by an Hdac4 mutation, and ectopic ossification in Hdac4 null mice can be diminished by a heterozygous Mef2c mutation. These findings reveal unexpected commonalities in the mechanisms governing muscle, cardiovascular, and bone development with respect to their regulation by MEF2 and class II HDACs.