The role of SIK1 in myogenic differentiation and skeletal muscle repair

描述(由申请人提供)：许多信号转导通路涉及控制骨骼肌发育和出生后骨骼肌生长过程中的肌源性指定和分化。越来越清楚的是，这种发育程序是在成人骨骼肌修复过程中由常驻肌肉干细胞或卫星细胞重现的。因此，对促进肌源性分化的信号转导通路的透彻理解可能会导致新的治疗方法的发展，以促进肌肉在各种病理状态下的修复或生长。第二信使cAMP及其细胞效应器在肌肉发育过程中受到动态调节，但对cAMP在肌肉细胞中发挥作用的特定靶点知之甚少。为了解决这个问题，我们专注于cAMP诱导的转录途径，它影响肌肉分化和骨骼肌修复。我们发现了一个这样的转录靶点，盐诱导蛋白激酶1(SIK1)，它是一种催化II类组蛋白脱乙酰酶的磷酸化并允许肌肉特异基因表达的酶。SIK1基因在发育中的体节中表达，SIK1功能对小鼠肌细胞和骨骼肌的存活起重要作用。然而，对于该酶本身是如何在未分化的成肌细胞中进行调节的，它的功能是否在肌肉发育过程中是必需的，或者肌纤维中SIK1的缺失是否会导致肌病，人们知之甚少。拟议的实验将检验这样一个假设，即在肌源性分化和肌肉修复期间，MEF2活动的适当时机需要SIK1的诱导。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的MEF2活性重要。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，来检验SIK1对II类HDAC的调节是成肌细胞分化过程中的关键步骤的假设。这一假说的一个推论是，SIK1活动是整个肌肉纤维发育或修复所必需的。这一假说将在肌源性前体细胞和分化的肌纤维中缺乏SIK1表达的小鼠身上得到验证。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。这些实验的数据将确定SIK1是否是肌肉分化和肌肉修复所必需的，并将揭示这种酶在骨骼肌母细胞中正常调控的分子机制。作为cAMP信号的靶点，SIK1是生肌程序的信号依赖调节器。SIK1或其调节因子可作为促进人类患者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.