Rhonda Bassel-Duby

Transcriptional co-activator PGC-1α drives the formation of slow-twitch muscle fibres

The biochemical basis for the regulation of fibre-type determination in skeletal muscle is not well understood. In addition to the expression of particular myofibrillar proteins, type I (slow-twitch) fibres are much higher in mitochondrial content and are more dependent on oxidative metabolism than type II (fast-twitch) fibres. We have previously identified a transcriptional co-activator, peroxisome-proliferator-activated receptor-γ co-activator-1 (PGC-1α), which is expressed in several tissues including brown fat and skeletal muscle, and that activates mitochondrial biogenesis and oxidative metabolism. We show here that PGC-1α is expressed preferentially in muscle enriched in type I fibres. When PGC-1α is expressed at physiological levels in transgenic mice driven by a muscle creatine kinase (MCK) promoter, a fibre type conversion is observed: muscles normally rich in type II fibres are redder and activate genes of mitochondrial oxidative metabolism. Notably, putative type II muscles from PGC-1α transgenic mice also express proteins characteristic of type I fibres, such as troponin I (slow) and myoglobin, and show a much greater resistance to electrically stimulated fatigue. Using fibre-type-specific promoters, we show in cultured muscle cells that PGC-1α activates transcription in cooperation with Mef2 proteins and serves as a target for calcineurin signalling, which has been implicated in slow fibre gene expression. These data indicate that PGC-1α is a principal factor regulating muscle fibre type determination.

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.

Heart repair by reprogramming non-myocytes with cardiac transcription factors

The adult mammalian heart possesses little regenerative potential following injury. Fibrosis due to activation of cardiac fibroblasts impedes cardiac regeneration and contributes to loss of contractile function, pathological remodelling and susceptibility to arrhythmias. Cardiac fibroblasts account for a majority of cells in the heart and represent a potential cellular source for restoration of cardiac function following injury through phenotypic reprogramming to a myocardial cell fate. Here we show that four transcription factors, GATA4, HAND2, MEF2C and TBX5, can cooperatively reprogram adult mouse tail-tip and cardiac fibroblasts into beating cardiac-like myocytes in vitro. Forced expression of these factors in dividing non-cardiomyocytes in mice reprograms these cells into functional cardiac-like myocytes, improves cardiac function and reduces adverse ventricular remodelling following myocardial infarction. Our results suggest a strategy for cardiac repair through reprogramming fibroblasts resident in the heart with cardiogenic transcription factors or other molecules.

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.

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.

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.

Histone Deacetylase 5 Epigenetically Controls Behavioral Adaptations to Chronic Emotional Stimuli

Previous work has identified alterations in histone acetylation in animal models of drug addiction and depression. However, the mechanisms which integrate drugs and stress with changes in chromatin structure remain unclear. Here, we identify the activity-dependent class II histone deacetylase, HDAC5, as a central integrator of these stimuli with changes in chromatin structure and gene expression. Chronic, but not acute, exposure to cocaine or stress decreases HDAC5 function in the nucleus accumbens (NAc), a major brain reward region, which allows for increased histone acetylation and transcription of HDAC5 target genes. This regulation is behaviorally important, as loss of HDAC5 causes hypersensitive responses to chronic, not acute, cocaine or stress. These findings suggest that proper balance of histone acetylation is a crucial factor in the saliency of a given stimulus and that disruption of this balance is involved in the transition from an acute adaptive response to a chronic psychiatric illness.

MicroRNA-206 Delays ALS Progression and Promotes Regeneration of Neuromuscular Synapses in Mice

An Innervative Small RNA Amyotrophic lateral sclerosis (ALS) is a relentless disease characterized by progressive degeneration of motor neurons that control muscle movement, leading to muscle atrophy and paralysis. Williams et al. (p. 1549; see the Perspective by Brown) show that a small noncoding RNA that is selectively expressed in skeletal muscle, miR-206, senses motor neuron injury or loss and helps ameliorate resultant muscle damage by promoting regeneration of neuromuscular synapses. Expression of miR-206 was dramatically induced in a mouse model of ALS, and when this RNA was removed from mice by genetic manipulation, the disease progressed at a faster rate. The salutary effects of miR-206 appear to be mediated through a signaling pathway in muscle cells involving histone deacetylase 4 and a fibro-blast growth factor modulator, activation of which leads to release of factors that promote nerve-muscle interactions. A small noncoding RNA promotes nerve-muscle interactions in response to motor neuron injury and slows disease progression. 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.

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.

MEF2 responds to multiple calcium‐regulated signals in the control of skeletal muscle fiber type

Different patterns of motor nerve activity drive distinctive programs of gene transcription in skeletal muscles, thereby establishing a high degree of metabolic and physiological specialization among myofiber subtypes. Recently, we proposed that the influence of motor nerve activity on skeletal muscle fiber type is transduced to the relevant genes by calcineurin, which controls the functional activity of NFAT (nuclear family of activated T cell) proteins. Here we demonstrate that calcineurin‐dependent gene regulation in skeletal myocytes is mediated also by MEF2 transcription factors, and is integrated with additional calcium‐regulated signaling inputs, specifically calmodulin‐dependent protein kinase activity. In skeletal muscles of transgenic mice, both NFAT and MEF2 binding sites are necessary for properly regulated function of a slow fiber‐specific enhancer, and either forced expression of activated calcineurin or motor nerve stimulation up‐regulates a MEF2‐dependent reporter gene. These results provide new insights into the molecular mechanisms by which specialized characteristics of skeletal myofiber subtypes are established and maintained.