CharcotMarieTooth病是最常见的遗传性神经系统疾病之一,影响来自周围神经系统的运动和/或感觉神经元或雪旺细胞的轴突,由100多个基因引起。我们之前鉴定了FGD4中的突变,作为CMT4H的原因,CMT的常染色体隐性脱髓鞘形式。FGD4编码FRABIN,GDP/GTP核苷酸交换因子,特别是对于小的GTPasecdc42。值得注意的是,来自CMT4H患者的神经显示出由局灶性髓鞘过多引起的过度冗余髓鞘形成,称为外翻,提示FRABIN可能在控制PNS髓鞘形成中发挥作用。为了深入了解FGD4/FRABIN在雪旺细胞髓鞘形成中的作用,我们生成了一个敲除小鼠模型(Fgd4SC-/-),在施万细胞中条件消融Fgd4。我们表明,雪旺氏细胞中FRABIN的特异性缺失导致体外异常的髓鞘形成,在背根神经节神经元/雪旺氏细胞共培养以及体内,Fgd4SC-/-小鼠的坐骨神经远端。我们观察到,这些髓鞘形成缺陷与NRG1III型/ERBB2/3信号通路的一些相互作用者的上调有关,众所周知,这可以确保周围神经系统中适当的髓鞘形成水平。基于酵母双杂交筛选,我们确定SNX3是FRABIN的新伙伴,参与调节内吞运输。有趣的是,我们发现FRABIN的缺失会损害内吞运输,这可能导致NRG1III型/ERBB2/3信号传导和髓鞘形成缺陷.使用RNA-Seq体外,我们已经确定了失调途径的新的潜在效应物,比如ERBIN,RAB11FIP2和MAF,从而提供线索以了解FRABIN如何通过胆固醇合成促进适当的ERBB2运输或甚至髓鞘膜添加。最后,我们发现,使用烟酸治疗的NRG1III型/ERBB2/3通路的适当水平的重建减少了CMT4H小鼠神经中的髓磷脂折叠。总的来说,我们的工作揭示了FRABIN在调节NRG1III型/ERBB2/3NRG1信号传导和髓鞘形成中的新作用,并开辟了基于调节NRG1III型/ERBB2/3通路的未来治疗策略,以减少CMT4H病理和更普遍的其他脱髓鞘CharcotMarieTooth病.
Charcot-Marie-Tooth (CMT) disease is one of the most common inherited neurological disorders, affecting either axons from the motor and/or sensory neurons or Schwann cells of the peripheral nervous system (PNS) and caused by more than 100 genes. We previously identified mutations in FGD4 as responsible for CMT4H, an autosomal recessive demyelinating form of CMT disease. FGD4 encodes FRABIN, a GDP/GTP nucleotide exchange factor, particularly for the small GTPase Cdc42. Remarkably, nerves from patients with CMT4H display excessive redundant myelin figures called outfoldings that arise from focal hypermyelination, suggesting that FRABIN could play a role in the control of PNS myelination. To gain insights into the role of FGD4/FRABIN in Schwann cell myelination, we generated a knockout mouse model (Fgd4SC-/-), with conditional ablation of Fgd4 in Schwann cells. We show that the specific deletion of FRABIN in Schwann cells leads to aberrant myelination in vitro, in dorsal root ganglia neuron/Schwann cell co-cultures, as well as in vivo, in distal sciatic nerves from Fgd4SC-/- mice. We observed that those myelination defects are related to an upregulation of some interactors of the NRG1 type III/ERBB2/3 signalling pathway, which is known to ensure a proper level of myelination in the PNS. Based on a yeast two-hybrid screen, we identified SNX3 as a new partner of FRABIN, which is involved in the regulation of endocytic trafficking. Interestingly, we showed that the loss of FRABIN impairs endocytic trafficking, which may contribute to the defective NRG1 type III/ERBB2/3 signalling and myelination. Using RNA-Seq, in vitro, we identified new potential effectors of the deregulated pathways, such as ERBIN, RAB11FIP2 and MAF, thereby providing cues to understand how FRABIN contributes to proper ERBB2 trafficking or even myelin membrane addition through cholesterol synthesis. Finally, we showed that the re-establishment of proper levels of the NRG1 type III/ERBB2/3 pathway using niacin treatment reduces myelin outfoldings in nerves of CMT4H mice. Overall, our work reveals a new role of FRABIN in the regulation of NRG1 type III/ERBB2/3 NRG1signalling and myelination and opens future therapeutic strategies based on the modulation of the NRG1 type III/ERBB2/3 pathway to reduce CMT4H pathology and more generally other demyelinating types of CMT disease.