The histone deacetylase HDAC4 connects neural activity to muscle transcriptional reprogramming

HDAC4 integrates PTH and sympathetic signaling in osteoblasts

Parathyroid hormone (PTH) and the sympathetic tone promote Rankl expression in osteoblasts and osteoclast differentiation by enhancing cyclic adenosine monophosphate production through an unidentified transcription factor for PTH and through ATF4 for the sympathetic tone. How two extracellular cues using the same second messenger in the same cell elicit different transcriptional events is unknown. In this paper, we show that PTH favors Rankl expression by triggering the ubiquitination of HDAC4, a class II histone deacetylase, via Smurf2. HDAC4 degradation releases MEF2c, which transactivates the Rankl promoter. Conversely, sympathetic signaling in osteoblasts favors the accumulation of HDAC4 in the nucleus and its association with ATF4. In this context, HDAC4 increases Rankl expression. Because of its ability to differentially connect two extracellular cues to the genome of osteoblasts, HDAC4 is a critical regulator of osteoclast differentiation.

Advances in the understanding of skeletal muscle weakness in murine models of diseases affecting nerve-evoked muscle activity, motor neurons, synapses and myofibers

Disease processes and trauma affecting nerve-evoked muscle activity, motor neurons, synapses and myofibers cause different levels of muscle weakness, i.e., reduced maximal force production in response to voluntary activation or nerve stimulation. However, the mechanisms of muscle weakness are not well known. Using murine models of amyotrophic lateral sclerosis (SOD1(G93A) transgenic mice), congenital myasthenic syndrome (AChE knockout mice and Musk(V789M/-) mutant mice), Schwartz-Jampel syndrome (Hspg2(C1532YNEO/C1532YNEO) mutant mice) and traumatic nerve injury (Neurotomized wild-type mice), we show that the reduced maximal activation capacity (the ability of the nerve to maximally activate the muscle) explains 52%, 58% and 100% of severe weakness in respectively SOD1(G93A), Neurotomized and Musk mice, whereas muscle atrophy only explains 37%, 27% and 0%. We also demonstrate that the impaired maximal activation capacity observed in SOD1, Neurotomized, and Musk mice is not highly related to Hdac4 gene upregulation. Moreover, in SOD1 and Neurotomized mice our results suggest LC3, Fn14, Bcl3 and Gadd45a as candidate genes involved in the maintenance of the severe atrophic state. In conclusion, our study indicates that muscle weakness can result from the triggering of different signaling pathways. This knowledge may be helpful in designing therapeutic strategies and finding new drug targets for amyotrophic lateral sclerosis, congenital myasthenic syndrome, Schwartz-Jampel syndrome and nerve injury.

HDAC6 contributes to pathological responses of heart and skeletal muscle to chronic angiotensin-II signaling

Little is known about the function of the cytoplasmic histone deacetylase HDAC6 in striated muscle. Here, we addressed the role of HDAC6 in cardiac and skeletal muscle remodeling induced by the peptide hormone angiotensin II (ANG II), which plays a central role in blood pressure control, heart failure, and associated skeletal muscle wasting. Comparable with wild-type (WT) mice, HDAC6 null mice developed cardiac hypertrophy and fibrosis in response to ANG II. However, whereas WT mice developed systolic dysfunction upon treatment with ANG II, cardiac function was maintained in HDAC6 null mice treated with ANG II for up to 8 wk. The cardioprotective effect of HDAC6 deletion was mimicked in WT mice treated with the small molecule HDAC6 inhibitor tubastatin A. HDAC6 null mice also exhibited improved left ventricular function in the setting of pressure overload mediated by transverse aortic constriction. HDAC6 inhibition appeared to preserve systolic function, in part, by enhancing cooperativity of myofibrillar force generation. Finally, we show that HDAC6 null mice are resistant to skeletal muscle wasting mediated by chronic ANG-II signaling. These findings define novel roles for HDAC6 in striated muscle and suggest potential for HDAC6-selective inhibitors for the treatment of cardiac dysfunction and muscle wasting in patients with heart failure.

The interplay between microRNAs and histone deacetylases in neurological diseases

Neurological conditions, such as Alzheimer's disease and stroke, represent a prevalent group of devastating illnesses with few treatments. Each of these diseases or conditions is in part characterized by the dysregulation of many genes, including those that code for microRNAs (miRNAs) and histone deacetylases (HDACs). Recently, a complex relationship has been uncovered linking miRNAs and HDACs and their ability to regulate one another. This provides a new avenue for potential therapeutics as the ability to reinstate a careful balance between miRNA and HDACs has lead to improved outcomes in a number of in vitro and in vivo models of neurological conditions. In this review, we will discuss recent findings on the interplay between miRNAs and HDACs and its implications for pathogenesis and treatment of neurological conditions, including amyotrophic lateral sclerosis, Alzheimer's disease, Huntington's disease and stroke.

Expression changes in human skeletal muscle miRNAs following 10 days of bed rest in young healthy males

AIM

Studies in humans show global changes in mRNA and protein expression occur in human skeletal muscle during bed rest. As microRNAs are important regulators of expression, we analysed the global microRNA expression changes in human muscle following 10 days of sustained bed rest, with the rationale that miRNAs play key roles in atrophy of skeletal muscle.

METHODS

We analysed expression of miRNA and selected target proteins before and after 10 days of bed rest in biopsies obtained from the vastus lateralis muscle of 6 healthy males.

RESULTS

Fifteen of 152 miRNAs detected in human muscle tissue were differentially expressed, and all of them with exception of two were downregulated. The downregulated miRNAs include the following: miR-206, a myomir involved in function and maintenance of skeletal muscle; miR-23a, involved in insulin response and atrophy defence; and several members of the let-7 family involved in cell cycle, cell differentiation and glucose homeostasis. Predicted gene targets of these miRNAs are members of the MAPK, TNF receptor, ALK1, TGF-beta receptor and SMAD signalling pathways. All of these pathways were previously indicated to be involved in skeletal muscle response to physical inactivity. We also measured protein expression of selected miRNA targets and observed a decrease in HDAC4.

CONCLUSION

Our data demonstrate that miRNAs in postural muscles are affected by sustained inactivity and unloading, as induced by prolonged bed rest, and hence are potentially involved in regulation of skeletal muscle adjustments to inactivity. We also propose new miRNAs involved in regulation of biological processes in adult human muscle.

mTORC1 promotes denervation-induced muscle atrophy through a mechanism involving the activation of FoxO and E3 ubiquitin ligases

Skeletal muscle mass and function are regulated by motor innervation, and denervation results in muscle atrophy. The activity of mammalian target of rapamycin complex 1 (mTORC1) is substantially increased in denervated muscle, but its regulatory role in denervation-induced atrophy remains unclear. At early stages after denervation of skeletal muscle, a pathway involving class II histone deacetylases and the transcription factor myogenin mediates denervation-induced muscle atrophy. We found that at later stages after denervation of fast-twitch muscle, activation of mTORC1 contributed to atrophy and that denervation-induced atrophy was mitigated by inhibition of mTORC1 with rapamycin. Activation of mTORC1 through genetic deletion of its inhibitor TSC1 (tuberous sclerosis complex 1) sensitized mice to denervation-induced muscle atrophy and suppressed the kinase activity of Akt, leading to activation of FoxO transcription factors and increasing the expression of genes encoding E3 ubiquitin ligases atrogin [also known as MAFbx (muscle atrophy F-box protein)] and MuRF1 (muscle-specific ring finger 1). Rapamycin treatment of mice restored Akt activity, suggesting that the denervation-induced increase in mTORC1 activity was producing feedback inhibition of Akt. Genetic deletion of the three FoxO isoforms in skeletal muscle induced muscle hypertrophy and abolished the late-stage induction of E3 ubiquitin ligases after denervation, thereby preventing denervation-induced atrophy. These data revealed that mTORC1, which is generally considered to be an important component of anabolism, is central to muscle catabolism and atrophy after denervation. This mTORC1-FoxO axis represents a potential therapeutic target in neurogenic muscle atrophy.

Altered gene expression patterns in muscle ring finger 1 null mice during denervation- and dexamethasone-induced muscle atrophy

Muscle atrophy can result from inactivity or unloading on one hand or the induction of a catabolic state on the other. Muscle-specific ring finger 1 (MuRF1), a member of the tripartite motif family of E3 ubiquitin ligases, is an essential mediator of multiple conditions inducing muscle atrophy. While most studies have focused on the role of MuRF1 in protein degradation, the protein may have other roles in regulating skeletal muscle mass and metabolism. We therefore systematically evaluated the effect of MuRF1 on gene expression during denervation and dexamethasone-induced atrophy. We find that the lack of MuRF1 leads to few differences in control animals, but there were several significant differences in specific sets of genes upon denervation- and dexamethasone-induced atrophy. For example, during denervation, MuRF1 knockout mice showed delayed repression of metabolic and structural genes and blunted induction of genes associated with the neuromuscular junction. In the latter case, this pattern correlates with blunted HDAC4 and myogenin upregulation. Lack of MuRF1 caused fewer changes in the dexamethasone-induced atrophy program, but certain genes involved in fat metabolism and intracellular signaling were affected. Our results demonstrate a new role for MuRF1 in influencing gene expression in two important models of muscle atrophy.

Skeletal muscle denervation causes skeletal muscle atrophy through a pathway that involves both Gadd45a and HDAC4

Skeletal muscle denervation causes muscle atrophy via complex molecular mechanisms that are not well understood. To better understand these mechanisms, we investigated how muscle denervation increases growth arrest and DNA damage-inducible 45α (Gadd45a) mRNA in skeletal muscle. Previous studies established that muscle denervation strongly induces Gadd45a mRNA, which increases Gadd45a, a small myonuclear protein that is required for denervation-induced muscle fiber atrophy. However, the mechanism by which denervation increases Gadd45a mRNA remained unknown. Here, we demonstrate that histone deacetylase 4 (HDAC4) mediates induction of Gadd45a mRNA in denervated muscle. Using mouse models, we show that HDAC4 is required for induction of Gadd45a mRNA during muscle denervation. Conversely, forced expression of HDAC4 is sufficient to increase skeletal muscle Gadd45a mRNA in the absence of muscle denervation. Moreover, Gadd45a mediates several downstream effects of HDAC4, including induction of myogenin mRNA, induction of mRNAs encoding the embryonic nicotinic acetylcholine receptor, and, most importantly, skeletal muscle fiber atrophy. Because Gadd45a induction is also a key event in fasting-induced muscle atrophy, we tested whether HDAC4 might also contribute to Gadd45a induction during fasting. Interestingly, however, HDAC4 is not required for fasting-induced Gadd45a expression or muscle atrophy. Furthermore, activating transcription factor 4 (ATF4), which contributes to fasting-induced Gadd45a expression, is not required for denervation-induced Gadd45a expression or muscle atrophy. Collectively, these results identify HDAC4 as an important regulator of Gadd45a in denervation-induced muscle atrophy and elucidate Gadd45a as a convergence point for distinct upstream regulators during muscle denervation and fasting.

Muscle histone deacetylase 4 upregulation in amyotrophic lateral sclerosis: potential role in reinnervation ability and disease progression

Amyotrophic lateral sclerosis is a typically rapidly progressive neurodegenerative disorder affecting motor neurons leading to progressive muscle paralysis and death, usually from respiratory failure, in 3-5 years. Some patients have slow disease progression and prolonged survival, but the underlying mechanisms remain poorly understood. Riluzole, the only approved treatment, only modestly prolongs survival and has no effect on muscle function. In the early phase of the disease, motor neuron loss is initially compensated for by collateral reinnervation, but over time this compensation fails, leading to progressive muscle wasting. The crucial role of muscle histone deacetylase 4 and its regulator microRNA-206 in compensatory reinnervation and disease progression was recently suggested in a mouse model of amyotrophic lateral sclerosis (transgenic mice carrying human mutations in the superoxide dismutase gene). Here, we sought to investigate whether the microRNA-206-histone deacetylase 4 pathway plays a role in muscle compensatory reinnervation in patients with amyotrophic lateral sclerosis and thus contributes to disease outcome differences. We studied muscle reinnervation using high-resolution confocal imaging of neuromuscular junctions in muscle samples obtained from 11 patients with amyotrophic lateral sclerosis, including five long-term survivors. We showed that the proportion of reinnervated neuromuscular junctions was significantly higher in long-term survivors than in patients with rapidly progressive disease. We analysed the expression of muscle candidate genes involved in the reinnervation process and showed that histone deacetylase 4 upregulation was significantly greater in patients with rapidly progressive disease and was negatively correlated with the extent of muscle reinnervation and functional outcome. Conversely, the proposed regulator of histone deacetylase 4, microRNA-206, was upregulated in both patient groups, but did not correlate with disease progression or reinnervation. We conclude that muscle expression of histone deacetylase 4 may be a key factor for muscle reinnervation and disease progression in patients with amyotrophic lateral sclerosis. Specific histone deacetylase 4 inhibitors may then constitute a therapeutic approach to enhancing motor performance and slowing disease progression in amyotrophic lateral sclerosis.

Nuclear calcium signaling regulates nuclear export of a subset of class IIa histone deacetylases following synaptic activity

In neurons, dynamic changes in the subcellular localization of histone deacetylases (HDACs) are thought to contribute to signal-regulated gene expression. Here we show that in mouse hippocampal neurons, synaptic activity-dependent nucleo-cytoplasmic shuttling is a common feature of all members of class IIa HDACs, which distinguishes them from other classes of HDACs. Nuclear calcium, a key regulator in neuronal gene expression, is required for the nuclear export of a subset of class IIa HDACs. We found that inhibition of nuclear calcium signaling using CaMBP4 or increasing the nuclear calcium buffering capacity by means of expression of a nuclear targeted version of parvalbumin (PV.NLS-mC) led to a build-up of HDAC4 and HDAC5 in the cell nucleus, which in the case of PV.NLS-mC can be reversed by nuclear calcium transients triggered by bursts of action potential firing. A similar nuclear accumulation of HDAC4 and HDAC5 was observed in vivo in the mouse hippocampus following stereotaxic delivery of recombinant adeno-associated viruses expressing either CaMBP4 or PV.NLS-mC. The modulation of HDAC4 activity either by RNA interference-mediated reduction of HDAC4 protein levels or by expression of a constitutively nuclear localized mutant of HDAC4 leads to changes in the mRNA levels of several nuclear calcium-regulated genes with known functions in acquired neuroprotection (atf3, serpinb2), memory consolidation (homer1, arc), and the development of chronic pain (ptgs2, c1qc). These results identify nuclear calcium as a regulator of nuclear export of HDAC4 and HDAC5. The reduction of nuclear localized HDACs represents a novel transcription-promoting pathway stimulated by nuclear calcium.

Epigenetic regulation of muscle phenotype and adaptation: a potential role in COPD muscle dysfunction

Quadriceps muscle dysfunction occurs in one-third of patients with chronic obstructive pulmonary disease (COPD) in very early stages of their condition, even prior to the development of airway obstruction. Among several factors, deconditioning and muscle mass loss are the most relevant contributing factors leading to this dysfunction. Moreover, epigenetics, defined as the process whereby gene expression is regulated by heritable mechanisms that do not affect DNA sequence, could be involved in the susceptibility to muscle dysfunction, pathogenesis, and progression. Herein, we review the role of epigenetic mechanisms in muscle development and adaptation to environmental factors such as immobilization and exercise, and their implications in the pathophysiology and susceptibility to muscle dysfunction in COPD. The epigenetic modifications identified so far include DNA methylation, histone acetylation and methylation, and non-coding RNAs such as microRNAs (miRNAs). In the present review, we describe the specific contribution of epigenetic mechanisms to the regulation of embryonic myogenesis, muscle structure and metabolism, immobilization, and exercise, and in muscles of COPD patients. Events related to muscle development and regeneration and the response to exercise and immobilization are tightly regulated by epigenetic mechanisms. These environmental factors play a key role in the outcome of muscle mass and function as well as in the susceptibility to muscle dysfunction in COPD. Future research remains to be done to shed light on the specific target pathways of miRNA function and other epigenetic mechanisms in the susceptibility, pathogenesis, and progression of COPD muscle dysfunction.

Acetylation and deacetylation--novel factors in muscle wasting

We review recent evidence that acetylation and deacetylation of cellular proteins, including transcription factors and nuclear cofactors, may be involved in the regulation of muscle mass. The level of protein acetylation is balanced by histone acetyltransferases (HATs) and histone deacetylases (HDACs) and studies suggest that this balance is perturbed in muscle wasting. Hyperacetylation of transcription factors and nuclear cofactors regulating gene transcription in muscle wasting may influence muscle mass. In addition, hyperacetylation may render proteins susceptible to degradation by different mechanisms, including intrinsic ubiquitin ligase activity exerted by HATs and by dissociation of proteins from cellular chaperones. In recent studies, inhibition of p300/HAT expression and activity and stimulation of SIRT1-dependent HDAC activity reduced glucocorticoid-induced catabolic response in skeletal muscle, providing further evidence that hyperacetylation plays a role in muscle wasting. It should be noted, however, that although several studies advocate a role of hyperacetylation in muscle wasting, apparently contradictory results have also been reported. For example, muscle atrophy caused by denervation or immobilization may be associated with reduced, rather than increased, protein acetylation. In addition, whereas hyperacetylation results in increased degradation of certain proteins, other proteins may be stabilized by increased acetylation. Thus, the role of acetylation and deacetylation in the regulation of muscle mass may be both condition- and protein-specific. The influence of HATs and HDACs on the regulation of muscle mass, as well as methods to modulate protein acetylation, is an important area for continued research aimed at preventing and treating muscle wasting.

Beside the MEF2 axis: unconventional functions of HDAC4

The class IIa deacetylase HDAC4 is unequivocally known as a negative regulator of MEF2-dependent transcription. In the past years several works have allowed us to understand how different signals, mirroring specific environmental circumstances keep in check the repressive action of HDAC4 against MEF2s. At the same time, pieces of evidence have gradually accumulated about HDAC4 activities emancipated from MEF2s. The aim of this review is to discuss about these "unconventional functions" of HDAC4.

Amino acid starvation induces reactivation of silenced transgenes and latent HIV-1 provirus via down-regulation of histone deacetylase 4 (HDAC4)

The epigenetic silencing of exogenous transcriptional units integrated into the genome represents a critical problem both for long-term gene therapy efficacy and for the eradication of latent viral infections. We report here that limitation of essential amino acids, such as methionine and cysteine, causes selective up-regulation of exogenous transgene expression in mammalian cells. Prolonged amino acid deprivation led to significant and reversible increase in the expression levels of stably integrated transgenes transcribed by means of viral or human promoters in HeLa cells. This phenomenon was mediated by epigenetic chromatin modifications, because histone deacetylase (HDAC) inhibitors reproduced starvation-induced transgene up-regulation, and transcriptome analysis, ChIP, and pharmacological and RNAi approaches revealed that a specific class II HDAC, namely HDAC4, plays a critical role in maintaining the silencing of exogenous transgenes. This mechanism was also operational in cells chronically infected with HIV-1, the etiological agent of AIDS, in a latency state. Indeed, both amino acid starvation and pharmacological inhibition of HDAC4 promoted reactivation of HIV-1 transcription and reverse transcriptase activity production in HDAC4(+) ACH-2 T-lymphocytic cells but not in HDAC4(-) U1 promonocytic cells. Thus, amino acid deprivation leads to transcriptional derepression of silenced transgenes, including integrated plasmids and retroviruses, by a process involving inactivation or down-regulation of HDAC4. These findings suggest that selective targeting of HDAC4 might represent a unique strategy for modulating the expression of therapeutic viral vectors, as well as that of integrated HIV-1 proviruses in latent reservoirs without significant cytotoxicity.

A direct HDAC4-MAP kinase crosstalk activates muscle atrophy program

Prolonged deficits in neural input activate pathological muscle remodeling, leading to atrophy. In denervated muscle, activation of the atrophy program requires HDAC4, a potent repressor of the master muscle transcription factor MEF2. However, the signaling mechanism that connects HDAC4, a protein deacetylase, to the atrophy machinery remains unknown. Here, we identify the AP1 transcription factor as a critical target of HDAC4 in neurogenic muscle atrophy. In denervated muscle, HDAC4 activates AP1-dependent transcription, whereas AP1 inactivation recapitulates HDAC4 deficiency and blunts the muscle atrophy program. We show that HDAC4 activates AP1 independently of its canonical transcriptional repressor activity. Surprisingly, HDAC4 stimulates AP1 activity by activating the MAP kinase cascade. We present evidence that HDAC4 binds and promotes the deacetylation and activation of a key MAP3 kinase, MEKK2. Our findings establish an HDAC4-MAPK-AP1 signaling axis essential for neurogenic muscle atrophy and uncover a direct crosstalk between acetylation- and phosphorylation-dependent signaling cascades.

Multifaceted genomic risk for brain function in schizophrenia

Recently, deriving candidate endophenotypes from brain imaging data has become a valuable approach to study genetic influences on schizophrenia (SZ), whose pathophysiology remains unclear. In this work we utilized a multivariate approach, parallel independent component analysis, to identify genomic risk components associated with brain function abnormalities in SZ. 5157 candidate single nucleotide polymorphisms (SNPs) were derived from genome-wide array based on their possible connections with SZ and further investigated for their associations with brain activations captured with functional magnetic resonance imaging (fMRI) during a sensorimotor task. Using data from 92 SZ patients and 116 healthy controls, we detected a significant correlation (r=0.29; p=2.41 × 10(-5)) between one fMRI component and one SNP component, both of which significantly differentiated patients from controls. The fMRI component mainly consisted of precentral and postcentral gyri, the major activated regions in the motor task. On average, higher activation in these regions was observed in participants with higher loadings of the linked SNP component, predominantly contributed to by 253 SNPs. 138 identified SNPs were from known coding regions of 100 unique genes. 31 identified SNPs did not differ between groups, but moderately correlated with some other group-discriminating SNPs, indicating interactions among alleles contributing toward elevated SZ susceptibility. The genes associated with the identified SNPs participated in four neurotransmitter pathways: GABA receptor signaling, dopamine receptor signaling, neuregulin signaling and glutamate receptor signaling. In summary, our work provides further evidence for the complexity of genomic risk to the functional brain abnormality in SZ and suggests a pathological role of interactions between SNPs, genes and multiple neurotransmitter pathways.

Molecular control of neuromuscular junction development

Skeletal muscle innervation is a multi-step process leading to the neuromuscular junction (NMJ) apparatus formation. The transmission of the signal from nerve to muscle occurs at the NMJ level. The molecular mechanism that orchestrates the organization and functioning of synapses is highly complex, and it has not been completely elucidated so far. Neuromuscular junctions are assembled on the muscle fibers at very precise locations called end plates (EP). Acetylcholine receptor (AChR) clusterization at the end plates is required for an accurate synaptic transmission. This review will focus on some mechanisms responsible for accomplishing the correct distribution of AChRs at the synapses. Recent evidences support the concept that a dual transcriptional control of AChR genes in subsynaptic and extrasynaptic nuclei is crucial for AChR clusterization. Moreover, new players have been discovered in the agrin-MuSK pathway, the master organizer of postsynaptical differentiation. Mutations in this pathway cause neuromuscular congenital disorders. Alterations of the postynaptic apparatus are also present in physiological conditions characterized by skeletal muscle wasting. Indeed, recent evidences demonstrate how NMJ misfunctioning has a crucial role at the onset of age-associated sarcopenia.

Class IIa HDACs: from important roles in differentiation to possible implications in tumourigenesis

Histone deacetylases (HDACs) are important regulators of gene expression. Specific structural features and distinct regulative mechanisms rationalize the separation of the 18 different human HDACs into four classes. The class II comprises a heterogeneous group of nuclear and cytosolic HDACs involved in the regulation of several cellular functions, not just limited to transcriptional repression. In particular, HDAC4, 5, 7 and 9 belong to the subclass IIa and share many transcriptional partners, including members of the MEF2 family. Genetic studies in mice have disclosed the fundamental contribution of class IIa HDACs to specific developmental/differentiation pathways. In this review, we discuss about the recent literature, which hints a role of class IIa HDACs in the development, growth and aggressiveness of cancer cells.

Myogenin regulates denervation-dependent muscle atrophy in mouse soleus muscle

Muscle inactivity due to injury or disease results in muscle atrophy. The molecular mechanisms contributing to muscle atrophy are poorly understood. However, it is clear that expression of atrophy-related genes, like Atrogin-1 and MuRF-1, are intimately tied to loss of muscle mass. When these atrophy-related genes are knocked out, inactive muscles retain mass. Muscle denervation stimulates muscle atrophy and Myogenin (Myog) is a muscle-specific transcription factor that is highly induced following muscle denervation. To investigate if Myog contributes to muscle atrophy, we have taken advantage of conditional Myog null mice. We show that in the denervated soleus muscle Myog expression contributes to reduced muscle force, mass, and cross-sectional area. We found that Myog mediates these effects, at least in part, by regulating expression of the Atrogin-1 and MuRF-1 genes. Indeed Myog over-expression in innervated muscle stimulates Atrogin-1 gene expression and Myog over-expression stimulates Atrogin-1 promoter activity. Thus, Myog and the signaling cascades regulating its induction following muscle denervation may represent novel targets for therapies aimed at reducing denervation-induced muscle atrophy.

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.

Skeletal muscle IP3R1 receptors amplify physiological and pathological synaptic calcium signals

Ca(2+) release from internal stores is critical for mediating both normal and pathological intracellular Ca(2+) signaling. Recent studies suggest that the inositol 1,4,5-triphosphate (IP(3)) receptor mediates Ca(2+) release from internal stores upon cholinergic activation of the neuromuscular junction (NMJ) in both physiological and pathological conditions. Here, we report that the type I IP(3) receptor (IP(3)R(1))-mediated Ca(2+) release plays a crucial role in synaptic gene expression, development, and neuromuscular transmission, as well as mediating degeneration during excessive cholinergic activation. We found that IP(3)R(1)-mediated Ca(2+) release plays a key role in early development of the NMJ, homeostatic regulation of neuromuscular transmission, and synaptic gene expression. Reducing IP(3)R(1)-mediated Ca(2+) release via siRNA knockdown or IP(3)R blockers in C2C12 cells decreased calpain activity and prevented agonist-induced acetylcholine receptor (AChR) cluster dispersal. In fully developed NMJ in adult muscle, IP(3)R(1) knockdown or blockade effectively increased synaptic strength at presynaptic and postsynaptic sites by increasing both quantal release and expression of AChR subunits and other NMJ-specific genes in a pattern resembling muscle denervation. Moreover, in two mouse models of cholinergic overactivity and NMJ Ca(2+) overload, anti-cholinesterase toxicity and the slow-channel myasthenic syndrome (SCS), IP(3)R(1) knockdown eliminated NMJ Ca(2+) overload, pathological activation of calpain and caspase proteases, and markers of DNA damage at subsynaptic nuclei, and improved both neuromuscular transmission and clinical measures of motor function. Thus, blockade or genetic silencing of muscle IP(3)R(1) may be an effective and well tolerated therapeutic strategy in SCS and other conditions of excitotoxicity or Ca(2+) overload.

A systems biology approach identifies molecular networks defining skeletal muscle abnormalities in chronic obstructive pulmonary disease

Chronic Obstructive Pulmonary Disease (COPD) is an inflammatory process of the lung inducing persistent airflow limitation. Extensive systemic effects, such as skeletal muscle dysfunction, often characterize these patients and severely limit life expectancy. Despite considerable research efforts, the molecular basis of muscle degeneration in COPD is still a matter of intense debate. In this study, we have applied a network biology approach to model the relationship between muscle molecular and physiological response to training and systemic inflammatory mediators. Our model shows that failure to co-ordinately activate expression of several tissue remodelling and bioenergetics pathways is a specific landmark of COPD diseased muscles. Our findings also suggest that this phenomenon may be linked to an abnormal expression of a number of histone modifiers, which we discovered correlate with oxygen utilization. These observations raised the interesting possibility that cell hypoxia may be a key factor driving skeletal muscle degeneration in COPD patients.

Chronic Obstructive Pulmonary Disease (COPD) is a major life threatening disease of the lungs, characterized by airflow limitation and chronic inflammation. Progressive reduction of the body muscle mass is a condition linked to COPD that significantly decreases quality of life and survival. Physical exercise has been proposed as a therapeutic option but its utility is still a matter of debate. The mechanisms underlying muscle wasting are also still largely unknown. The results presented in this paper show that diseased muscles are largely unable to coordinate the expression of muscle remodelling and bioenergetics pathways and that the cause of this phenomena may be tissue hypoxia. These findings contrast with current hypotheses based on the role of chronic inflammation and show that a mechanism based on an oxygen driven, epigenetic control of these two important functions may be an important disease mechanism.

Treatment with trichostatin A initiated after disease onset delays disease progression and increases survival in a mouse model of amyotrophic lateral sclerosis

Recent studies suggest that progressive motoneuron death in amyotrophic lateral sclerosis (ALS) is non-cell autonomous and may involve the participation of non-neuronal cells such as glial cells and skeletal muscle. Therefore, a drug that targets motoneurons as well as neighboring non-neuronal cells might be a potential therapeutic strategy to delay disease progression in ALS. Trichostatin A (TSA), a histone deacetylase (HDAC) inhibitor, has shown protective effects in multiple cell types implicated in ALS by resetting gene transcription profiles through increased histone acetylation. To test whether TSA could serve as a potential therapeutic agent, we intraperitoneally injected TSA from postnatal day 90 (P90), after disease symptoms appear, until P120 or the end-stage in SOD1-G93A mice. We found that TSA ameliorated motoneuron death and axonal degeneration in SOD1-G93A mice. Reduced gliosis and upregulation of the glutamate transporter (GLT-1) were also observed in the spinal cord of TSA-treated SOD1-G93A mice. In addition, TSA ameliorated muscle atrophy and neuromuscular junction (NMJ) denervation, which are the pathological characteristics of ALS found in skeletal muscle. Improved morphology in TSA-treated SOD1-G93A mice was accompanied by enhanced motor functions as assessed by rota-rod and grip strength analyses. Furthermore, TSA treatment significantly increased the mean survival duration after the treatment by 18% and prolonged lifespan by 7%. Our findings suggest that TSA may provide a potential therapy to slow disease progression as well as to enhance motor performance to improve the quality of life for ALS patients.

p300 Acetyltransferase activity differentially regulates the localization and activity of the FOXO homologues in skeletal muscle

The Forkhead Box O (FOXO) transcription factors regulate diverse cellular processes, and in skeletal muscle are both necessary and sufficient for muscle atrophy. Although the regulation of FOXO by Akt is well evidenced in skeletal muscle, the current study demonstrates that FOXO is also regulated in muscle via the histone acetyltransferase (HAT) activities of p300/CREB-binding protein (CBP). Transfection of rat soleus muscle with a dominant-negative p300, which lacks HAT activity and inhibits endogenous p300 HAT activity, increased FOXO reporter activity and induced transcription from the promoter of a bona fide FOXO target gene, atrogin-1. Conversely, increased HAT activity via transfection of either wild-type (WT) p300 or WT CBP repressed FOXO activation in vivo in response to muscle disuse, and in C2C12 cells in response to dexamethasone and acute starvation. Importantly, manipulation of HAT activity differentially regulated the expression of various FOXO target genes. Cotransfection of FOXO1, FOXO3a, or FOXO4 with the p300 constructs further identified p300 HAT activity to also differentially regulate the activity of the FOXO homologues. Markedly, decreased HAT activity strongly increased FOXO3a transcriptional activity, while increased HAT activity repressed FOXO3a activity and prevented its nuclear localization in response to nutrient deprivation. In contrast, p300 increased FOXO1 nuclear localization. In summary, this study provides the first evidence to support the acetyltransferase activities of p300/CBP in regulating FOXO signaling in skeletal muscle and suggests that acetylation may be an important mechanism to differentially regulate the FOXO homologues and dictate which FOXO target genes are activated in response to varying atrophic stimuli.

HDACs in skeletal muscle remodeling and neuromuscular disease

Skeletal muscle is made of heterogeneous myofibers with different contractile and metabolic properties. The diverse functionality of myofibers enables skeletal muscle to carry out different tasks from maintaining body posture to performing active movements. In addition to motility, skeletal muscle, which constitutes 40% of body mass, is also a key target of insulin action and performs an essential function in glucose metabolism. Adult skeletal muscle is a highly adaptive organ system and can undergo specific changes in contractile and metabolic properties to meet different functional demands. This plasticity of myofibers reflects a highly coordinated change in gene expression program that is controlled by neural activity. The capacity for on-demand remodeling confers skeletal muscle the remarkable adaptability important for animal survival; its dysregulation, however, could contribute to muscle and metabolic diseases. How neural activity dictates transcriptional programming to modify muscle functionality and diversity is a fundamental issue. Recent studies have identified members of class IIa HDACs as important effectors in both physiological and pathological muscle remodeling. By way of modifying myofiber properties, pharmacological manipulation of IIa HDACs activity could have potential therapeutic utility in the treatment of muscle disorders.

Histone modifications and exercise adaptations

The spatial association between genomic DNA and histone proteins within chromatin plays a key role in the regulation of gene expression and is largely governed by post-translational modifications to histone proteins, particularly H3 and H4. These modifications include phosphorylation, acetylation, and mono-, di-, and tri-methylation, and while some are associated with transcriptional repression, acetylation of lysine residues within H3 generally correlates with transcriptional activation. Histone acetylation is regulated by the balance between the activities of histone acetyl transferase (HAT) and histone deacetylase (HDAC). In skeletal muscle, the class II HDACs 4, 5, 7, and 9 play a key role in muscle development and adaptation and have been implicated in exercise adaptations. As just one example, exercise results in the nuclear export of HDACs 4 and 5, secondary to their phosphorylation by CaMKII and AMPK, two kinases that are activated during exercise in response to changes in sarcoplasmic Ca(2+) levels and energy status, in association with increased GLUT4 expression in human skeletal muscle. Unraveling the complexities of the so-called "histone code" before and after exercise is likely to lead to a greater understanding of the regulation of exercise/activity-induced alterations in skeletal muscle gene expression and reinforce the importance of skeletal muscle plasticity in health and disease.

Targeting the correct HDAC(s) to treat cognitive disorders

Changes in gene expression in the brain may underlie cognitive deficits inherent to normal aging and neurodegenerative disease. However, the mechanisms underlying pathological alterations in the brain transcriptome are incompletely understood. Epigenetic mechanisms such as DNA methylation and histone acetylation have been shown to be important for memory processes in the adult brain. There is accumulating evidence that altered chromatin plasticity and histone acetylation are also involved in cognitive aging, neurodegeneration, and neuropsychiatric diseases. Inhibitors of histone deacetylase (HDAC) exhibit neuroprotective and neuroregenerative properties in animal models of various brain diseases. As such, targeting of HDACs seems to be a promising therapeutic strategy. In this review, we discuss the specific roles of each HDAC protein and the possible function of distinct histone modifications. We hope that this knowledge will aid in the development of diagnostic tools and in designing more potent and specific treatment for neurological disorders targeting selective HDAC proteins.

Myogenin and class II HDACs control neurogenic muscle atrophy by inducing E3 ubiquitin ligases

Maintenance of skeletal muscle structure and function requires innervation by motor neurons, such that denervation causes muscle atrophy. We show that myogenin, an essential regulator of muscle development, controls neurogenic atrophy. Myogenin is upregulated in skeletal muscle following denervation and regulates expression of the E3 ubiquitin ligases MuRF1 and atrogin-1, which promote muscle proteolysis and atrophy. Deletion of myogenin from adult mice diminishes expression of MuRF1 and atrogin-1 in denervated muscle and confers resistance to atrophy. Mice lacking histone deacetylases (HDACs) 4 and 5 in skeletal muscle fail to upregulate myogenin and also preserve muscle mass following denervation. Conversely, forced expression of myogenin in skeletal muscle of HDAC mutant mice restores muscle atrophy following denervation. Thus, myogenin plays a dual role as both a regulator of muscle development and an inducer of neurogenic atrophy. These findings reveal a specific pathway for muscle wasting and potential therapeutic targets for this disorder.

Regional and subcellular distribution of HDAC4 in mouse brain

Histone deacetylases (HDACs) are part of a system that links epigenetic control of gene expression to a variety of environmental stimuli. Some HDACs, including HDAC4, shuttle between the cytoplasm and nucleus in response to physiological cues such as calcium signaling. HDAC4 mRNA is enriched in the brain, but the regional and subcellular protein expression pattern of HDAC4 is not known. Here we show that HDAC4 is more highly expressed in some brain regions than in others. HDAC4 is present in the perikaryial cytoplasm of most neurons but its nuclear localization is variable. In some areas, such as the dentate gyrus, nuclear expression is not detectable, whereas in other areas some neuronal nuclei contain HDAC4 immunoreactivity whereas others do not. In the cytoplasm, HDAC4 immunoreactivity is punctate. Some of these puncta are present in dendritic spines where the strongest immunoreactivity is associated with the postsynaptic density. These data demonstrate that the regional and subcellular distribution of HDAC4 is heterogeneous and raise the possibilities that HDAC4 acts on nonhistone substrates in dendritic spines or that it shuttles between spine and nucleus to coordinate synaptic activity with gene expression.

Chemical phylogenetics of histone deacetylases

The broad study of histone deacetylases in chemistry, biology and medicine relies on tool compounds to derive mechanistic insights. A phylogenetic analysis of class I and II histone deacetylases (HDACs) as targets of a comprehensive, structurally diverse panel of inhibitors revealed unexpected isoform selectivity even among compounds widely perceived as nonselective. The synthesis and study of a focused library of cinnamic hydroxamates allowed the identification of, to our knowledge, the first nonselective HDAC inhibitor. These data will guide a more informed use of HDAC inhibitors as chemical probes and therapeutic agents.

The microtubule-associated histone deacetylase 6 (HDAC6) regulates epidermal growth factor receptor (EGFR) endocytic trafficking and degradation

Histone deacetylase 6 (HDAC6) is a microtubule-associated deacetylase with tubulin deacetylase activity, and it binds dynein motors. Recent studies revealed that microtubule acetylation affects the affinity and processivity of microtubule motors. These unique properties implicate a role for HDAC6 in intracellular organelle transport. Here, we show that HDAC6 associates with the endosomal compartments and controls epidermal growth factor receptor (EGFR) trafficking and degradation. We found that loss of HDAC6 promoted EGFR degradation. Mechanistically, HDAC6 deficiency did not cause aberrant EGFR internalization and recycling. Rather, it resulted in accelerated segregation of EGFR from early endosomes and premature delivery of EGFR to the late endosomal and lysosomal compartments. The deregulated EGFR endocytic trafficking was accompanied by an increase in microtubule-dependent movement of EGFR-bearing vesicles, revealing a novel regulation of EGFR vesicular trafficking and degradation by the microtubule deacetylase HDAC6.

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.

Parathyroid hormone-related peptide represses chondrocyte hypertrophy through a protein phosphatase 2A/histone deacetylase 4/MEF2 pathway

The maturation of immature chondrocytes to hypertrophic chondrocytes is regulated by parathyroid hormone-related peptide (PTHrP). We demonstrate that PTHrP or forskolin administration can block induction of collagen X-luciferase by exogenous Runx2, MEF2, and Smad1 in transfected chondrocytes. We have found that PTHrP/forskolin administration represses the transcriptional activity of MEF2 and that forced expression of MEF2-VP16 can restore expression of the collagen X reporter in chondrocytes treated with these agents. PTHrP/forskolin induces dephosphorylation of histone deacetylase 4 (HDAC4) phospho-S246, which decreases interaction of HDAC4 with cytoplasmic 14-3-3 proteins and promotes nuclear translocation of HDAC4 and repression of MEF2 transcriptional activity. We have found that forskolin increases the activity of an HDAC4 phospho-S246 phosphatase and that forskolin-induced nuclear translocation of HDAC4 was reversed by the protein phosphatase 2A (PP2A) antagonist, okadaic acid. Finally, we demonstrate that knockdown of PP2A inhibits forskolin-induced nuclear translocation of HDAC4 and attenuates the ability of this signaling molecule to repress collagen X expression in chondrocytes, indicating that PP2A is critical for PTHrP-mediated regulation of chondrocyte hypertrophy.

Nitric oxide deficiency determines global chromatin changes in Duchenne muscular dystrophy

The present study provides evidence that abnormal patterns of global histone modification are present in the skeletal muscle nuclei of mdx mice and Duchenne muscular dystrophy (DMD) patients. A combination of specific histone H3 modifications, including Ser-10 phosphorylation, acetylation of Lys 9 and 14, and Lys 79 methylation, were found enriched in muscle biopsies from human patients affected by DMD and in late-term fetuses, early postnatal pups, or adult mdx mice. In this context, chromatin immunoprecipitation experiments showed an enrichment of these modifications at the loci of genes involved in proliferation or inflammation, suggesting a regulatory effect on gene expression. Remarkably, the reexpression of dystrophin induced by gentamicin treatment or the administration of nitric oxide (NO) donors reversed the abnormal pattern of H3 histone modifications. These findings suggest an unanticipated link between the dystrophin-activated NO signaling and the remodeling of chromatin. In this context, the regulation of class IIa histone deacetylases (HDACs) 4 and 5 was found altered as a consequence of the reduced NO-dependent protein phosphatase 2A activity, indicating that both NO and class IIa HDACs are important for satellite cell differentiation and gene expression in mdx mice. In conclusion, this work provides the first evidence of a role for NO as an epigenetic regulator in DMD.

A histone deacetylase 4/myogenin positive feedback loop coordinates denervation-dependent gene induction and suppression

Muscle activity contributes to formation of the neuromuscular junction and affects muscle metabolism and contractile properties through regulated gene expression. However, the mechanisms coordinating these diverse activity-regulated processes remain poorly characterized. Recently, it was reported that histone deacetylase 4 (HDAC4) can mediate denervation-induced myogenin and nicotinic acetylcholine receptor gene expression. Here, we report that HDAC4 is not only necessary for denervation-dependent induction of genes involved in synaptogenesis (nicotinic acetylcholine receptor and muscle-specific receptor tyrosine kinase) but also for denervation-dependent suppression of genes involved in glycolysis (muscle-specific enolase and phosphofructokinase). In addition, HDAC4 differentially regulates genes involved in muscle fiber type specification by inducing myosin heavy chain IIA and suppressing myosin heavy chain IIB. Consistent with these regulated gene profiles, HDAC4 is enriched in fast oxidative fibers of innervated tibialis anterior muscle and HDAC4 knockdown enhances glycolysis in cultured myotubes. HDAC4 mediates gene induction indirectly by suppressing the expression of Dach2 and MITR that function as myogenin gene corepressors. In contrast, HDAC4 is directly recruited to myocyte enhancer factor 2 sites within target promoters to mediate gene suppression. Finally, we discovered an HDAC4/myogenin positive feedback loop that coordinates gene induction and repression underlying muscle phenotypic changes after muscle denervation.

A highly conserved molecular switch binds MSY-3 to regulate myogenin repression in postnatal muscle

Myogenin is the dominant transcriptional regulator of embryonic and fetal muscle differentiation and during maturation is profoundly down-regulated. We show that a highly conserved 17-bp DNA cis-acting sequence element located upstream of the myogenin promoter (myogHCE) is essential for postnatal repression of myogenin in transgenic animals. We present multiple lines of evidence supporting the idea that repression is mediated by the Y-box protein MSY-3. Electroporation in vivo shows that myogHCE and MSY-3 are required for postnatal repression. We further show that, in the C2C12 cell culture system, ectopic MSY-3 can repress differentiation, while reduced MSY-3 promotes premature differentiation. MSY-3 binds myogHCE simultaneously with the homeodomain protein Pbx in postnatal innervated muscle. We therefore propose a model in which the myogHCE motif operates as a switch by specifying opposing functions; one that was shown previously is regulated by MyoD and Pbx and it specifies a chromatin opening, gene-activating function at the time myoblasts begin to differentiate; the other includes MYS-3 and Pbx, and it specifies a repression function that operates during and after postnatal muscle maturation in vivo and in myoblasts before they begin to differentiate.

The deacetylase HDAC4 controls myocyte enhancing factor-2-dependent structural gene expression in response to neural activity

Histone deacetylase 4 (HDAC4) binds and inhibits activation of the critical muscle transcription factor myocyte enhancer factor-2 (MEF2). However, the physiological significance of the HDAC4-MEF2 complex in skeletal muscle has not been established. Here we show that in skeletal muscle, HDAC4 is a critical modulator of MEF2-dependent structural and contractile gene expression in response to neural activity. We present evidence that loss of neural input leads to concomitant nuclear accumulation of HDAC4 and transcriptional reduction of MEF2-regulated gene expression. Cell-based assays show that HDAC4 represses structural gene expression via direct binding to AT-rich MEF2 response elements. Notably, using both surgical denervation and the neuromuscular disease amyotrophic lateral sclerosis (ALS) model, we found that elevated levels of HDAC4 are required for efficient repression of MEF2-dependent structural gene expression, indicating a link between the pathological induction of HDAC4 and subsequent MEF2 target gene suppression. Supporting this supposition, we show that ectopic expression of HDAC4 in muscle fibers is sufficient to induce muscle damage in mice. Our study identifies HDAC4 as an activity-dependent regulator of MEF2 function and suggests that activation of HDAC4 in response to chronically reduced neural activity suppresses MEF2-dependent gene expression and contributes to progressive muscle dysfunction observed in neuromuscular diseases.

Cellular and molecular mechanisms underlying age-related skeletal muscle wasting and weakness

Some of the most serious consequences of ageing are its effects on skeletal muscle. The term 'sarcopenia' describes the slow but progressive loss of muscle mass with advancing age and is characterised by a deterioration of muscle quantity and quality leading to a gradual slowing of movement and a decline in strength. The loss of muscle mass and strength is thought to be attributed to the progressive atrophy and loss of individual muscle fibres associated with the loss of motor units, and a concomitant reduction in muscle 'quality' due to the infiltration of fat and other non-contractile material. These age-related changes in skeletal muscle can be largely attributed to the complex interaction of factors affecting neuromuscular transmission, muscle architecture, fibre composition, excitation-contraction coupling, and metabolism. Given the magnitude of the growing public health problems associated with sarcopenia, there is considerable interest in the development and evaluation of therapeutic strategies to attenuate, prevent, or ultimately reverse age-related muscle wasting and weakness. The aim is to review our current understanding of some of the cellular and molecular mechanisms responsible for age-related changes in skeletal muscle.