Abstract 309: Two SoxD Family Transcription Factors Influence Heart Function at Baseline and After Cardiac Overload

Background: Sox6, a member of the SoxD family of transcription factors, has recently been established as a mediator of the actions of MyomiRs miR-208b and 499 in skeletal muscle. Sox6 suppresses the expression of slow skeletal muscle genes and its deletion promotes skeletal muscle fiber switching in mice. However, the role of Sox6 in the heart remains elusive. Given the importance of MyomiRs in regulating cardiac stress responses and the regulation of Sox6 by MyomiRs in the heart, we postulate that Sox6 is a central regulator of cardiac hypertrophy and heart failure.

Results: Because of functional redundancy of Sox6 and Sox5 in multiple tissues, Sox6 fl/fl mice were crossbred with Sox5 fl/fl mice and mice expressing CRE recombinase under the control of the alphaMHC gene promoter to obtain Sox5 fl/fl / Sox6 fl/fl /CRE+ ( Sox5/6 cKO) animals or CRE- controls. Sox5/6 cKO animals were viable and born at expected Mendelian ratios. At baseline, Sox5/6 cKO mice did not show any signs of hypertrophy or cardiac failure as assessed by echocardiography at the age of 9 weeks. While cardiac size and heart function measured by echocardiography was unaltered in mice aged 4-5 months, cardiac catheterization revealed significant decreases in contractility in Sox5/6 cKO animals. In addition, qPCR analysis of hearts showed increased expression of slow myosin Myh7 and Nppa , a marker of cardiac stress, in Sox5/6 cKO animals compared to controls. Following cardiac pressure overload for 3 weeks by transverse aortic constriction (TAC), significant decreases in cardiac function, as measured by echocardiography, were apparent in Sox5/6 cKO compared to control mice. Sox5/6 cKO mice subjected to TAC also showed signs of cardiac failure such as increased wet lung weight.

Conclusion: Cardiac-specific deletion of Sox6 and Sox5 lead to reduction of cardiac function at baseline and exacerbated susceptibility to the development of cardiac failure after cardiac pressure overload. The contribution of each individual transcription factor to the observed phenotypes is currently being investigated.

Abstract 328: Med13 regulates cardiac metabolism

Background: The heart is a metabolic organ that primarily utilizes fatty acids as energy substrate. While it is well established that the heart is metabolically flexible, the transcriptional network regulating cardiac metabolism is only partially understood. We have previously demonstrated that cardiac overexpression of Med13, a component of the Mediator Complex that regulates transcription, results in a lean phenotype with enhanced basal metabolic rates. We now investigate the mechanisms contributing to metabolic changes in mice with cardiac over-expression of Med13(Med13cTg).

Methods and Results: Cardiac fludeoxyglucose (18F-FDG)-PET imaging analysis revealed that Med13cTg hearts take up more glucose than wild type littermates. To determine pathways responsible for enhanced glucose uptake, ventricles from Med13cTg mice were subjected to RNA-seq and metabolomic analysis. The expression of fatty acid oxidation genes was decreased in Med13cTg hearts, accompanied by an increase in acyl CoA and a decrease in acetyl CoA. These data suggest that beta oxidation is decreased in Med13cTg hearts. Mitochondria function was therefore determined in Med13cTg hearts by performing electron-flow analyses and assessing oxygen consumption rates. Indeed, oxygen consumption rates were decreased in mitochondria isolated from Med13cTg hearts. Expression of Krebs Cycle genes and corresponding intermediary metabolites were also decreased in Med13cTg hearts, suggesting decreased flux through this pathway as well.

Conclusions: Overexpression of Med13 in the heart increases glucose uptake and decreases fatty acid oxidation in the heart. We speculate that Med13 transcriptionally regulates key mediators of cardiac metabolism. The mechanisms by which this occurs are currently under investigation.

Abstract 011: Transcriptional Basis Of Epicardial Progenitor Cell Activation During Heart Development, Regeneration And Repair

The epicardium encapsulates the heart and functions as a source of multipotent progenitor cells and paracrine factors essential for cardiac development and repair. Injury of the adult heart results in reactivation of epicardial progenitor cells, which reengages a developmental gene program, but the underlying transcriptional basis has not been delineated. We established a mouse embryonic heart organ culture and gene expression system that facilitated the identification of epicardial enhancers activated during heart development and injury. Epicardial activation of these enhancers depends on a combinatorial transcriptional code centered on C/EBP, HOX, MEIS, and GRAINYHEAD families of transcription factors. Furthermore, disruption of C/EBP signaling in the adult epicardium reduced injury-induced neutrophil infiltration and improved cardiac function. These findings reveal a transcriptional basis for epicardial progenitor cell activation during heart development and injury, providing a platform for enhancing cardioprotection and regeneration.

Myomaker is a membrane activator of myoblast fusion and muscle formation

Fusion of myoblasts is essential for the formation of multi-nucleated muscle fibres. However, the identity of muscle-specific proteins that directly govern this fusion process in mammals has remained elusive. Here we identify a muscle-specific membrane protein, named myomaker, that controls myoblast fusion. Myomaker is expressed on the cell surface of myoblasts during fusion and is downregulated thereafter. Overexpression of myomaker in myoblasts markedly enhances fusion, and genetic disruption of myomaker in mice causes perinatal death due to an absence of multi-nucleated muscle fibres. Remarkably, forced expression of myomaker in fibroblasts promotes fusion with myoblasts, demonstrating the direct participation of this protein in the fusion process. Pharmacological perturbation of the actin cytoskeleton abolishes the activity of myomaker, consistent with previous studies implicating actin dynamics in myoblast fusion. These findings reveal a long-sought myogenic fusion protein that controls mammalian myoblast fusion and provide new insights into the molecular underpinnings of muscle formation.

Postsynaptic FMRP bidirectionally regulates excitatory synapses as a function of developmental age and MEF2 activity

Rates of synapse formation and elimination change over the course of postnatal development, but little is known of molecular mechanisms that mediate this developmental switch. Here, we report that the dendritic RNA-binding protein fragile X mental retardation protein (FMRP) bidirectionally and cell autonomously regulates excitatory synaptic function, which depends on developmental age as well as function of the activity-dependent transcription factor myocyte enhancer factor 2 (MEF2). The acute postsynaptic expression of FMRP in CA1 neurons of hippocampal slice cultures (during the first postnatal week, P6-P7) promotes synapse function and maturation. In contrast, the acute expression of FMRP or endogenous FMRP in more mature neurons (during the second postnatal week; P13-P16) suppresses synapse number. The ability of neuronal depolarization to stimulate MEF2 transcriptional activity increases over this same developmental period. Knockout of endogenous MEF2 isoforms causes acute postsynaptic FMRP expression to promote, instead of eliminate, synapses onto 2-week-old neurons. Conversely, the expression of active MEF2 in neonatal neurons results in a precocious FMRP-dependent synapse elimination. Our findings suggest that FMRP and MEF2 function together to fine tune synapse formation and elimination rates in response to neuronal activity levels over the course of postnatal development.

A pathway involving HDAC5, cFLIP and caspases regulates expression of the splicing regulator polypyrimidine tract binding protein in the heart

Polypyrimidine tract binding protein (PTB) regulates pre-mRNA splicing, having special relevance for determining gene expression in the differentiating muscle. We have previously shown that PTB protein abundance is progressively reduced during heart development without reduction of its own transcript. Simultaneous reduction of histone deacetylase (HDAC) expression prompted us to investigate the potential link between these events. HDAC5-deficient mice have reduced cardiac PTB protein abundance, and HDAC inhibition in myocytes causes a reduction in endogenous expression of cellular FLICE-like inhibitory protein (cFLIP) and caspase-dependent cleavage of PTB. In agreement with this, cardiac PTB expression is abnormally high in mice with cardiac-specific executioner caspase deficiency, and cFLIP overexpression prevents PTB cleavage in vitro. Caspase-dependent cleavage triggers further fragmentation of PTB, and these fragments accumulate in the presence of proteasome inhibitors. Experimental modification of the above processes in vivo and in vitro results in coherent changes in the alternative splicing of genes encoding tropomyosin-1 (TPM1), tropomyosin-2 (TPM2) and myocyte enhancer factor-2 (MEF2). Thus, we report a pathway connecting HDAC, cFLIP and caspases regulating the progressive disappearance of PTB, which enables the expression of the adult variants of proteins involved in the regulation of contraction and transcription during cardiac muscle development.

C/EBP transcription factors mediate epicardial activation during heart development and injury

The epicardium encapsulates the heart and functions as a source of multipotent progenitor cells and paracrine factors essential for cardiac development and repair. Injury of the adult heart results in reactivation of a developmental gene program in the epicardium, but the transcriptional basis of epicardial gene expression has not been delineated. We established a mouse embryonic heart organ culture and gene expression system that facilitated the identification of epicardial enhancers activated during heart development and injury. Epicardial activation of these enhancers depends on a combinatorial transcriptional code centered on CCAAT/enhancer binding protein (C/EBP) transcription factors. Disruption of C/EBP signaling in the adult epicardium reduced injury-induced neutrophil infiltration and improved cardiac function. These findings reveal a transcriptional basis for epicardial activation and heart injury, providing a platform for enhancing cardiac regeneration.

An essential role for histone deacetylase 4 in synaptic plasticity and memory formation

Histone deacetylases (HDACs), a family of enzymes involved in epigenetic regulation, have been implicated in the control of synaptic plasticity, as well as learning and memory. Previous work has demonstrated administration of pharmacological HDAC inhibitors, primarily those targeted to class I HDACs, enhance learning and memory as well as long-term potentiation. However, a detailed understanding of the role of class II HDACs in these processes remains elusive. Here, we show that selective loss of Hdac4 in brain results in impairments in hippocampal-dependent learning and memory and long-term synaptic plasticity. In contrast, loss of Hdac5 does not impact learning and memory demonstrating unique roles in brain for individual class II HDACs. These findings suggest that HDAC4 is a crucial positive regulator of learning and memory, both behaviorally and at the cellular level, and that inhibition of Hdac4 activity may have unexpected detrimental effects to these processes.

A cardiac microRNA governs systemic energy homeostasis by regulation of MED13

Obesity, type 2 diabetes, and heart failure are associated with aberrant cardiac metabolism. We show that the heart regulates systemic energy homeostasis via MED13, a subunit of the Mediator complex, which controls transcription by thyroid hormone and other nuclear hormone receptors. MED13, in turn, is negatively regulated by a heart-specific microRNA, miR-208a. Cardiac-specific overexpression of MED13 or pharmacologic inhibition of miR-208a in mice confers resistance to high-fat diet-induced obesity and improves systemic insulin sensitivity and glucose tolerance. Conversely, genetic deletion of MED13 specifically in cardiomyocytes enhances obesity in response to high-fat diet and exacerbates metabolic syndrome. The metabolic actions of MED13 result from increased energy expenditure and regulation of numerous genes involved in energy balance in the heart. These findings reveal a role of the heart in systemic metabolic control and point to MED13 and miR-208a as potential therapeutic targets for metabolic disorders.

microRNA-206 promotes skeletal muscle regeneration and delays progression of Duchenne muscular dystrophy in mice

Skeletal muscle injury activates adult myogenic stem cells, known as satellite cells, to initiate proliferation and differentiation to regenerate new muscle fibers. The skeletal muscle-specific microRNA miR-206 is upregulated in satellite cells following muscle injury, but its role in muscle regeneration has not been defined. Here, we show that miR-206 promotes skeletal muscle regeneration in response to injury. Genetic deletion of miR-206 in mice substantially delayed regeneration induced by cardiotoxin injury. Furthermore, loss of miR-206 accelerated and exacerbated the dystrophic phenotype in a mouse model of Duchenne muscular dystrophy. We found that miR-206 acts to promote satellite cell differentiation and fusion into muscle fibers through suppressing a collection of negative regulators of myogenesis. Our findings reveal an essential role for miR-206 in satellite cell differentiation during skeletal muscle regeneration and indicate that miR-206 slows progression of Duchenne muscular dystrophy.

In vivo analysis of MEF2 transcription factors in synapse regulation and neuronal survival

MEF2 (A-D) transcription factors govern development, differentiation and maintenance of various cell types including neurons. The role of MEF2 isoforms in the brain has been studied using in vitro manipulations with only MEF2C examined in vivo. In order to understand specific as well as redundant roles of the MEF2 isoforms, we generated brain-specific deletion of MEF2A and found that Mef2aKO mice show normal behavior in a range of paradigms including learning and memory. We next generated Mef2a and Mef2d brain-specific double KO (Mef2a/dDKO) mice and observed deficits in motor coordination and enhanced hippocampal short-term synaptic plasticity, however there were no alterations in learning and memory, Schaffer collateral pathway long-term potentiation, or the number of dendritic spines. Since previous work has established a critical role for MEF2C in hippocampal plasticity, we generated a Mef2a, Mef2c and Mef2d brain-specific triple KO (Mef2a/c/dTKO). Mef2a/c/d TKO mice have early postnatal lethality with increased neuronal apoptosis, indicative of a redundant role for the MEF2 factors in neuronal survival. We examined synaptic plasticity in the intact neurons in the Mef2a/c/d TKO mice and found significant impairments in short-term synaptic plasticity suggesting that MEF2C is the major isoform involved in hippocampal synaptic function. Collectively, these data highlight the key in vivo role of MEF2C isoform in the brain and suggest that MEF2A and MEF2D have only subtle roles in regulating hippocampal synaptic function.

MicroRNA-214 protects the mouse heart from ischemic injury by controlling Ca²⁺ overload and cell death

Early reperfusion of ischemic cardiac tissue remains the most effective intervention for improving clinical outcome following myocardial infarction. However, abnormal increases in intracellular Ca²⁺ during myocardial reperfusion can cause cardiomyocyte death and consequent loss of cardiac function, referred to as ischemia/reperfusion (IR) injury. Therapeutic modulation of Ca²⁺ handling provides some cardioprotection against the paradoxical effects of restoring blood flow to the heart, highlighting the significance of Ca²⁺ overload to IR injury. Cardiac IR is also accompanied by dynamic changes in the expression of microRNAs (miRNAs); for example, miR-214 is upregulated during ischemic injury and heart failure, but its potential role in these processes is unknown. Here, we show that genetic deletion of miR-214 in mice causes loss of cardiac contractility, increased apoptosis, and excessive fibrosis in response to IR injury. The cardioprotective roles of miR-214 during IR injury were attributed to repression of the mRNA encoding sodium/calcium exchanger 1 (Ncx1), a key regulator of Ca²⁺ influx; and to repression of several downstream effectors of Ca²⁺ signaling that mediate cell death. These findings reveal a pivotal role for miR-214 as a regulator of cardiomyocyte Ca²⁺ homeostasis and survival during cardiac injury.

Exercise-induced BCL2-regulated autophagy is required for muscle glucose homeostasis

Exercise has beneficial effects on human health, including protection against metabolic disorders such as diabetes. However, the cellular mechanisms underlying these effects are incompletely understood. The lysosomal degradation pathway, autophagy, is an intracellular recycling system that functions during basal conditions in organelle and protein quality control. During stress, increased levels of autophagy permit cells to adapt to changing nutritional and energy demands through protein catabolism. Moreover, in animal models, autophagy protects against diseases such as cancer, neurodegenerative disorders, infections, inflammatory diseases, ageing and insulin resistance. Here we show that acute exercise induces autophagy in skeletal and cardiac muscle of fed mice. To investigate the role of exercise-mediated autophagy in vivo, we generated mutant mice that show normal levels of basal autophagy but are deficient in stimulus (exercise- or starvation)-induced autophagy. These mice (termed BCL2 AAA mice) contain knock-in mutations in BCL2 phosphorylation sites (Thr69Ala, Ser70Ala and Ser84Ala) that prevent stimulus-induced disruption of the BCL2-beclin-1 complex and autophagy activation. BCL2 AAA mice show decreased endurance and altered glucose metabolism during acute exercise, as well as impaired chronic exercise-mediated protection against high-fat-diet-induced glucose intolerance. Thus, exercise induces autophagy, BCL2 is a crucial regulator of exercise- (and starvation)-induced autophagy in vivo, and autophagy induction may contribute to the beneficial metabolic effects of exercise.

Abstract P244: Transcriptional Regulation of Cardiac Gene Expression by Med13 Alters Global Energy Balance

Aberrant cardiac metabolism is associated with obesity, type 2 diabetes and heart failure. The heart requires highly efficient metabolism to maintain the levels of ATP needed for contractility and pump function, however little is known about the role of the heart as a metabolic organ. Nuclear hormone receptors, such as thyroid hormone receptor play an important role in cardiovascular disease by significantly altering expression of genes involved in maintaining metabolic activity. The Mediator, a large multiprotein complex functions as a hub to control gene expression through association with transcriptional activators and repressors. We tested the hypothesis that Med13, a component of the Mediator complex, regulates cardiac function in a gain-of-function mouse model. Trangsenic mice overexpressing Med13 in the heart are lean, have increased energy expenditure, are resistant to high fat diet-induced obesity and have enhanced cardiac contractility. Microarray analysis and biochemical assays show that in vivo and in vitro Med13 selectively inhibits nuclear hormone receptor target genes of energy metabolism. These results implicate the Mediator complex regulates energy balance and cardiac contractility and suggests that the heart may function as a key component of mammalian energy homeostasis.

Specific control of pancreatic endocrine β- and δ-cell mass by class IIa histone deacetylases HDAC4, HDAC5, and HDAC9

OBJECTIVE

Class IIa histone deacetylases (HDACs) belong to a large family of enzymes involved in protein deacetylation and play a role in regulating gene expression and cell differentiation. Previously, we showed that HDAC inhibitors modify the timing and determination of pancreatic cell fate. The aim of this study was to determine the role of class IIa HDACs in pancreas development.

RESEARCH DESIGN AND METHODS

We took a genetic approach and analyzed the pancreatic phenotype of mice lacking HDAC4, -5, and -9. We also developed a novel method of lentiviral infection of pancreatic explants and performed gain-of-function experiments.

RESULTS

We show that class IIa HDAC4, -5, and -9 have an unexpected restricted expression in the endocrine β- and δ-cells of the pancreas. Analyses of the pancreas of class IIa HDAC mutant mice revealed an increased pool of insulin-producing β-cells in Hdac5(-/-) and Hdac9(-/-) mice and an increased pool of somatostatin-producing δ-cells in Hdac4(-/-) and Hdac5(-/-) mice. Conversely, HDAC4 and HDAC5 overexpression showed a decreased pool of insulin-producing β-cells and somatostatin-producing δ-cells. Finally, treatment of pancreatic explants with the selective class IIa HDAC inhibitor MC1568 enhances expression of Pax4, a key factor required for proper β-and δ-cell differentiation and amplifies endocrine β- and δ-cells.

CONCLUSIONS

We conclude that HDAC4, -5, and -9 are key regulators to control the pancreatic β/δ-cell lineage. These results highlight the epigenetic mechanisms underlying the regulation of endocrine cell development and suggest new strategies for β-cell differentiation-based therapies.

Regulation of insulin-like growth factor signaling by Yap governs cardiomyocyte proliferation and embryonic heart size

The Hippo signaling pathway regulates growth of the heart and other tissues. Hippo pathway kinases influence the activity of various targets, including the transcriptional coactivator Yap, but the specific role of Yap in heart growth has not been investigated. We show that Yap is necessary and sufficient for embryonic cardiac growth in mice. Deletion of Yap in the embryonic mouse heart impeded cardiomyocyte proliferation, causing myocardial hypoplasia and lethality at embryonic stage 10.5. Conversely, forced expression of a constitutively active form of Yap in the embryonic heart increased cardiomyocyte number and heart size. Yap activated the insulin-like growth factor (IGF) signaling pathway in cardiomyocytes, resulting in inactivation of glycogen synthase kinase 3β, which led to increased abundance of β-catenin, a positive regulator of cardiac growth. Our results point to Yap as a critical downstream effector of the Hippo pathway in the control of cardiomyocyte proliferation and a nexus for coupling the IGF, Wnt, and Hippo signaling pathways with the developmental program for heart growth.

Mice lacking microRNA 133a develop dynamin 2–dependent centronuclear myopathy

MicroRNAs modulate cellular phenotypes by inhibiting expression of mRNA targets. In this study, we have shown that the muscle-specific microRNAs miR-133a-1 and miR-133a-2 are essential for multiple facets of skeletal muscle function and homeostasis in mice. Mice with genetic deletions of miR-133a-1 and miR-133a-2 developed adult-onset centronuclear myopathy in type II (fast-twitch) myofibers, accompanied by impaired mitochondrial function, fast-to-slow myofiber conversion, and disarray of muscle triads (sites of excitation- contraction coupling). These abnormalities mimicked human centronuclear myopathies and could be ascribed, at least in part, to dysregulation of the miR-133a target mRNA that encodes dynamin 2, a GTPase implicated in human centronuclear myopathy. Our findings reveal an essential role for miR-133a in the maintenance of adult skeletal muscle structure, function, bioenergetics, and myofiber identity; they also identify a potential modulator of centronuclear myopathies.