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

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

As cure rates for childhood cancers have improved, there has been an increasing awareness of the late health consequences of childhood cancer therapies. One outcome associated with significant morbidity and mortality for these survivors is the development of subsequent neoplasms, unique from recurrence of the original childhood malignancy.¹ Survivors with a subsequent neoplasm are more likely to report adverse general and mental health outcomes² and have increased hospitalization rates³ compared with survivors without a subsequent neoplasm. Subsequent malignant neoplasms are the most common nonrelapse cause of late mortality, accounting for approximately one-half of all nonrelapse deaths among 5-year survivors.^1,4

Skeletal muscle development is controlled by the myocyte enhancer factor (MEF2) and myogenic basic helix-loop-helix (bHLH) families of transcription factors, which associate and synergistically activate muscle gene expression. Muscle differentiation is further reinforced by positive-feedback loops in which myogenic bHLH proteins activate their own expression and the expression of MEF2, while MEF2 stimulates expression of myogenic bHLH genes and the Mef2c gene. Here we describe a myogenic negative-feedback loop that consists of MEF2 proteins and the transcriptional repressor histone deacetylase 9 (HDAC9). We show that the HDAC9 gene is a direct transcriptional target of MEF2 in vitro and in vivo. HDAC9 can associate with MEF2 proteins and suppress their transcriptional activity. The transcriptional repressor HDAC9 thus forms a negative-feedback loop in the transcriptional circuitry of muscle differentiation.

Smooth muscle cells (SMCs) display remarkable phenotypic diversity and plasticity and can readily switch between proliferative and differentiated states in response to extracellular cues. In an effort to identify novel transcriptional regulators of smooth muscle phenotypes, we compared the gene expression profiles of arterial and venous SMCs by microarray-based transcriptional profiling. Among numerous genes displaying distinct expression patterns in these two SMC types, we discovered an expressed sequence tag encoding a previously uncharacterized zinc finger protein belonging to the PRDM (PRDI-BF1 and RIZ homology domain) family of chromatin-remodeling proteins and named it PRISM (PR domain in smooth muscle). PRISM is expressed in a variety of smooth muscle-containing tissues and displays especially robust expression in the cardiac outflow tract and descending aorta during embryogenesis. PRISM is localized to the nucleus and contains an amino-terminal PR domain and four Krüppel-like zinc fingers at the carboxy terminus. We show that PRISM acts as a transcriptional repressor by interacting with class I histone deacetylases and the G9a histone methyltransferase, thereby identifying PRISM as a novel SMC-restricted epigenetic regulator. Overexpression of PRISM in cultured primary SMCs induces genes associated with the proliferative smooth muscle phenotype while repressing regulators of differentiation, including myocardin and GATA-6. Conversely, small interfering RNA-mediated knockdown of PRISM slows cell growth and induces myocardin, GATA-6, and markers of SMC differentiation. We conclude that PRISM acts as a novel epigenetic regulator of SMC phenotypic plasticity by suppressing differentiation and maintaining the proliferative potential of vascular SMCs.