Ex vivo expansion of skeletal muscle satellite cells

摘要 因为分化的、有功能的肌纤维中的所有细胞核都已经永久地退出了细胞周期，卫星 当需要组织生长、修复或再生时，细胞群充当新核的来源。卫星 细胞在动态平衡期间处于静止的非增殖状态，夹在细胞膜之间。 和分化的肌纤维的基膜。在这种状态下，它们表达卫星干细胞转录 Pax7因子，但MyoD家族的转录因子很少或没有。为应对伤害或疾病，卫星 细胞被激活以重新进入细胞周期，上调MyoD和其他肌源性转录因子，以及 作为成肌细胞扩张以产生新的肌源性细胞，然后致力于终末分化并融合成新的 或现有的肌纤维。通过一小部分卫星细胞后代尚不清楚的机制 将重新进入静止状态并重新填充干细胞库。这个过程是快速和高效的， 在大多数情况下，足够在一生中保持肌肉质量和功能。然而，在这种情况下 肌肉退行性疾病，如杜氏肌营养不良症或大量创伤，导致 肌肉体积丧失，内源性卫星细胞不足或无法修复肌肉导致 长期病理学。该领域的一个长期目标是分离患者或供者来源的卫星细胞。 以及在体外扩增它们(可能与修复营养不良蛋白基因等操作相一致) 然后将它们移植到治疗上，然而这种方法还没有取得良好的成功。一大专业 障碍是在体外维持卫星细胞及其后代处于增殖的祖细胞状态的困难： 即使在高有丝分裂原条件下，它们也倾向于终末分化，甚至那些 保持增殖状态的干细胞在很大程度上失去了干细胞特性(例如，在体内作为卫星细胞自我更新的能力)。我们 有令人兴奋的新数据表明，在没有细胞表面信号分子ephin-A5的情况下， 成肌细胞不仅不会发生终末分化，而且在高密度生长时会发生分化。 退出细胞周期，表达高水平的Pax7，因此至少在两个关键字上类似于静止的卫星细胞 敬重。当以低密度重新传代时，这些细胞将重新进入细胞周期，并在没有 致力于终端差异化。为了将这一结果用于潜在的翻译应用程序，我们 建议尝试暂时和可逆地抑制(而不是删除)ePhin-A5，以允许成肌细胞 在不损失分化的情况下在培养中扩增，理想情况下是在植入时增强干细胞特性。 我们选择开发RNA适配子作为ePhin-A5抑制剂，以便利用适配子的优势。 结合的高度特异性，缺乏免疫原性，潜在的额外的化学功能，以及 产生的速度和成本效益。如果成功，这项技术有可能显著 推进基于细胞的治疗DMD和体积性肌肉损失，并通过允许 研究人员希望在延长的培养时间内产生大量的祖细胞。

SUMMARY Because all nuclei in differentiated, functional myofibers have permanently exited the cell cycle, the satellite cell population acts as a source of new nuclei when tissue growth, repair, or regeneration is required. Satellite cells during homeostasis rest in a quiescent, nonproliferative state sandwiched between the cell membrane and the basal lamina of a differentiated myofiber. In this state they express the satellite stem cell transcription factor Pax7 but little to no transcription factors of the MyoD family. In response to injury or disease, satellite cells are activated to re-enter the cell cycle, upregulate MyoD and other myogenic transcription factors, and expand as myoblasts to generate new myogenic cells, then commit to terminal differentiation and fuse into new or existing myofibers. Through mechanisms that are not yet well understood a fraction of satellite cell progeny will re-enter the quiescent state and repopulate the stem cell pool. This process is rapid and highly effective, and in most cases is sufficient to maintain muscle mass and function throughout life. However, in the case of muscle degenerative diseases such as Duchenne's muscular dystrophy or massive trauma leading to volumetric muscle loss, endogenous satellite cells are insufficient or unable to repair the muscle leading to long-term pathology. A longstanding goal in the field has been isolating patient- or donor-derived satellite cells and expanding them ex vivo (potentially in concert with manipulations such as repair of the dystrophin gene) then engrafting them therapeutically, however this approach has not yet met with good success. One major hurdle is the difficulty of maintaining satellite cells and their progeny in a proliferative progenitor state in vitro: they tend to commit to terminal differentiation even under high-mitogen conditions, and even those cells that remain proliferative largely lose their stem cell character (e.g., ability to self-renew as satellite cells in vivo). We have exciting new data showing that in the absence of the cell surface signaling molecule ephrin-A5, myoblasts will not only not commit to terminal differentiation but they will, when grown at high densities, instead exit the cell cycle and express high levels of Pax7, thus resembling quiescent satellite cells in at least two key respects. When repassaged at low density, these cells will re-enter the cell cycle and expand again without committing to terminal differentiation. To leverage this result into a potential translational application, we propose to attempt to transiently and reversibly inhibit (rather than delete) ephrin-A5, to allow myoblast expansion in culture without loss to differentiation and ideally to enhance stem cell character on engraftment. We have chosen to develop RNA aptamers as ephrin-A5 inhibitors in order to take advantage of aptamers' high specificity of binding, absence of immunogenicity, potential for additional chemical functionalization, and speed and cost-effectiveness to generate. If successful, this technique has the potential to significantly advance cell-based therapies for DMD and volumetric muscle loss and to accelerate basic science by allowing researchers to generate of large numbers of progenitor cells over extended culture periods.