The role of SIK1 in myogenic differentiation and skeletal muscle repair

描述（申请人提供）：在发育和出生后骨骼肌生长过程中，许多信号转导通路涉及肌原性规范和分化的控制。越来越清楚的是，这种发育程序在成人骨骼肌的修复过程中由常驻肌肉干细胞或卫星细胞重演。因此，对促进肌源性分化的信号转导途径的透彻理解可能会导致新疗法的发展，以促进各种人类病理状态下的肌肉修复或生长。第二信使cAMP及其细胞效应物在肌肉发育过程中受到动态调节，但对cAMP在肌肉细胞中介导其作用的具体靶点知之甚少。为了解决这个问题，我们将重点放在camp诱导的影响肌分化和骨骼肌修复的转录途径上。我们确定了一个这样的转录靶点，盐诱导激酶1 (SIK1)，这是一种催化II类组蛋白去乙酰化酶磷酸化的酶，并允许表达肌肉特异性基因。Sik1 mRNA在发育体中表达，Sik1功能对小鼠肌细胞和骨骼肌的存活至关重要。然而，对于该酶本身在未分化的成肌细胞中是如何调节的，其功能在肌肉发育过程中是否必需，以及肌纤维中Sik1缺失是否会导致肌病，我们知之甚少。拟议的实验将验证在肌源性分化和肌肉修复过程中，MEF2活动的适当时机需要SIK1诱导的假设。我们将研究SIK1稳定性的分子决定因素，并测试这种调节机制是否对限制未分化成肌细胞中MEF2活性很重要。我们还将通过检测缺乏SIK1的原代成肌细胞的分化以及卫星细胞特异性缺失SIK1的小鼠的表型特征，来验证SIK1对II类hdac的调控是成肌细胞分化的关键步骤这一假设。这一假设的一个推论是，SIK1活性是肌纤维发育或修复所必需的。这一假设将在肌原性前细胞和分化肌纤维中缺乏Sik1表达的小鼠中进行验证。我们的遗传策略将允许明确确定SIK1促进肌肉修复的细胞类型。从这些实验中得到的数据将确定SIK1是否为成肌细胞分化和肌肉修复所必需，并将揭示该酶在骨骼肌细胞中正常调节的分子机制。作为cAMP信号的靶标，SIK1是肌生成程序的信号依赖性调节剂。SIK1或其调节因子可作为促进人类骨骼肌再生和修复的治疗靶点。

DESCRIPTION (provided by applicant): Many signal transduction pathways have been implicated in control of myogenic specification and differentiation during development and postnatal skeletal muscle growth. It has also become increasingly clear that such developmental programs are recapitulated during repair of adult skeletal muscle by resident muscle stem cells, or satellite cells. Thus, a thorough understanding of signal transduction pathways that promote myogenic differentiation could lead to the development of new therapeutics to promote muscle repair or growth in a variety of human pathologic states. The second messenger cAMP and its cellular effectors are dynamically regulated during muscle development, but little is known about the specific targets of cAMP that mediates its effects in muscle cells. To address this question, we focus on cAMP-induced transcriptional pathways that affect myogenic differentiation and skeletal muscle repair. We identified one such transcriptional target, Salt- Inducible Kinase 1 (SIK1), which is an enzyme that catalyzes phosphorylation of class II histone deacetylases and allows expression of muscle specific genes. Sik1 mRNA is expressed in developing somites and SIK1 function is important for survival of myocytes and skeletal muscle in mice. However, little is known about how the enzyme itself is regulated in undifferentiated myoblasts, whether its function is required during muscle development, or whether Sik1 deletion in myofibers will cause myopathy. The proposed experiments will test the hypothesis that SIK1 induction is required for appropriate timing of MEF2 activity during myogenic differentiation and muscle repair. We will investigate molecular determinants of SIK1 stability and test whether this regulatory mechanism is important for limiting MEF2 activity in undifferentiated myoblasts. We will also test the hypothesis that SIK1 regulation of class II HDACs is a crucial step during myoblast differentiation by examining differentiation of primary myoblasts lacking Sik1 and by characterizing phenotypes in mice with satellite cell-specific deletion of Sik1. A corollary to this hypothesis is that SIK1 activity is required for full muscle fiber development or repair. This hypothesis will be tested in mice lacking Sik1 expression in myogenic precursor cells and differentiated myofibers. Our genetic strategy will allow unequivocal determination of the cell type in which SIK1 acts to promote muscle repair. The data resulting from these experiments will establish whether SIK1 is necessary for myogenic differentiation and muscle repair and will reveal the molecular mechanisms by which this enzyme is normally regulated in skeletal myoblasts. As a target of cAMP signaling, SIK1 is a signal-dependent modulator of the myogenic program. SIK1 or its regulators could serve as therapeutic targets to promote skeletal muscle regeneration and repair in human patients.