Impact of Schwann Cell Pathology on Axon Structure and Function

轴突和雪旺细胞(SCs)之间的相互作用推动着轴突的形成、功能和维持 有髓神经对有效的跳跃传导和神经功能是必不可少的。外在的 来自轴突和基底板的信号协同驱动一系列SC转录的表达 因子(TF)，最终导致Egr2的表达，Egr2是PNS髓鞘形成的主要调节因子。Egr2是必需的 对于干细胞来说，从早幼粒阶段，当它们一次包裹轴突时，进入髓鞘阶段，当它们 上调髓鞘成分，形成多层髓鞘。髓鞘干细胞依次重组 轴突进入不同的区域，特别是兰维尔结节，并增加轴突大小。这些集体变化 通过跳跃传导实现并优化动作电位的传播。这项规定的重要性 SCs的轴突生物学突出表现为与后天和遗传相关的残疾(例如，Charcot Marie 髓鞘干细胞(MSCs)的牙齿(CMT)障碍。由各种SC突变导致的CMT 脱髓鞘/髓鞘功能障碍以神经传导速度慢为特征，常伴有神经传导阻滞 (NCB)。CMTs的病理特征通常包括炎症、肥厚性改变、轴突减少 直径和(远端)轴突丢失。由此导致的神经残疾从轻微到严重不等。演示SC 缺陷导致了这一系列的细胞病理，以及什么是CMTs临床谱系的基础机制 具有重要翻译意义的关键问题。询问SC病理对轴突生物学的影响 并导致临床缺陷，我们正在研究两种关键的SC蛋白，Egr2和G偶联的小鼠 蛋白质受体GPR126已被删除。这两种蛋白的条件性敲除(CKO)可阻止SCs 在早髓化阶段，并阻止他们形成髓鞘的能力。然而，这些小鼠有非常不同的表型： GPR126 cKO受轻微影响，寿命正常，而Egr2 cKO呈进行性瘫痪 在3-4个月大的时候就会死亡。正如预期的那样，NCV在两个突变体中都明显缓慢。然而，只有 Egr2 cKO展示了坦率的NCB，这可能是他们严重残疾的驱动因素。相应地，这些突变体有非常多的 不同的神经病理。Egr2的cKOS神经明显发炎、肥大，其轴突 与GPR126个cKO相比，明显更小。为了进一步阐明这些突变体之间的差异，我们 将研究：i)炎症在其各自表型中的作用以及为什么这些髓鞘功能障碍的干细胞 区别地激活炎症，II)检查这些SC突变调节轴突的机制 DIAMETER和III)使用单核RNAseq来表征影响 轴突生物学及其在神经传导中的作用。这些研究应该提供重要的新见解，了解如何 SC病理影响轴突的生物学和功能，并可能导致新的治疗策略来改善 有髓纤维功能障碍。

Reciprocal interactions between axons and Schwann cells (SCs) drive the formation, function, and maintenance of myelinated nerves, which are essential for effective saltatory conduction and neurologic function. Extrinsic signals from the axon and the basal lamina (BL) cooperatively drive expression of a series of SC transcription factors (TFs), culminating in the expression of Egr2, the master regulator of PNS myelination. Egr2 is required for SCs to advance from the promyelinating stage, when they wrap axons once, to myelination, when they upregulate myelin components and form the multilamellar myelin sheath. Myelinating SCs in turn re-organize axons into distinct domains, in particular the node of Ranvier, and increase axon size. These collective changes enable and optimize action potential propagation by saltatory conduction. The importance of this regulation of axon biology by SCs is underscored by the disability associated with acquired and inherited (e.g., Charcot Marie Tooth (CMT) disorders of myelinating SCs (mSCs). CMTs that result from various SC mutations cause de/dysmyelination characterized by slow nerve conduction velocity (NCV), often with nerve conduction block (NCB). Pathologic features of CMTs typically include inflammation, hypertrophic changes, reduced axon diameters and (distal) axon loss. The resulting neurological disability can range from minor to severe. How SC defects drive this array of cellular pathologies and what mechanisms underlie the clinical spectrum of CMTs are key questions with important translational implications. To interrogate how SC pathology impacts axon biology and leads to clinical defects, we are characterizing mice in which two key SC proteins, Egr2 and the G coupled protein receptor, Gpr126 have been deleted. Conditional knockouts (cKOs) of either of these proteins arrest SCs at the promyelinating stage and blocks their ability to form myelin. Yet these mice have very different phenotypes: Gpr126 cKOs are mildly affected and have a normal life span whereas Egr2 cKOs are progressively paralyzed and moribund by 3-4 months of age. As expected, NCV is markedly slow in both mutants. However, only the Egr2 cKOs exhibit frank NCB, a likely driver of their severe disability. Correspondingly, these mutants have very distinct nerve pathologies. Egr2 cKOs nerves are markedly inflamed, hypertrophic, and their axons are significantly smaller as compared to Gpr126 cKOs. To further elucidate differences between these mutants, we will investigate: i) the role of inflammation in their respective phenotypes and why these dysmyelinating SCs differentially activate inflammation, ii) examine the mechanisms by which these SC mutations regulate axon diameter and iii) use single nuclei RNAseq to characterize changes in the transcriptomes of SCs that impact axon biology and function in nerve conduction. These studies should provide important new insights into how SC pathology impacts axon biology and function and may lead to new therapeutic strategies to ameliorate disability in disorders of myelinated fibers.