The ERLINs as endoplasmic reticulum membrane scaffolds and their role in hereditary spastic paraplegia

ERLIN1和ERLIN2是一种约40 kDa的同源蛋白，它们聚集在杂寡体或同源低聚体的高分子复合体中。ERLIN属于SPFH蛋白家族，被认为是组织具有不同脂肪和蛋白质组成的膜微域的蛋白质。ERLIN定位于内质网(ER)，结合胆固醇，并促进特定底物与ER相关的降解。在遗传性痉挛截瘫(HSP)患者中发现了ERLIN1和ERLIN2基因的功能丧失突变。HSP是一种以中枢运动轴突进性退化为特征的遗传异质性神经疾病。缺乏ERLIN是如何导致HSP的仍然是个谜。我们认为，作为重要的蛋白质和脂膜支架，ERLINs可能在内质网的形态发生和脂滴(LD)的形成中发挥作用，这两个过程已经与HSP联系在一起。与这一假说一致，我们最近发现，在HeLa细胞中Erlin复合体耗尽后，外周内质网经历了显著的结构重排，其特征是替代内质网小管的扁平片状结构的增殖。此外，我们证实ERLIN1和ERLIN2都与BSCL2基因的蛋白产物Seipin相互作用，并促进内质网中Seipin的稳定性。Seipin参与LD的生物发生和维持内质网和LDS之间的联系，并且本身在某些形式的HSP中发生突变。在这里，我们旨在研究Erlin复合体调控ER形态发生的分子机制，通过结合候选分子方法和无偏倚的蛋白质组学和脂质组学策略。我们将确定ERLIN在内质网中的蛋白质和脂肪支架功能，以及它们如何影响特定蛋白质(如Seipin)在内质网膜上的稳定性。最后，为了研究体内Erlin复合体在神经系统中的作用，并评估在体外发现的改变的通路是否与长运动轴突变性有关，我们将建立和表征Erlin1缺陷、Erlin2缺陷和Erlin1/2双重缺陷的小鼠。

ERLIN1 and ERLIN2 are homologous proteins of about 40 kDa that assemble in hetero-oligomeric or homo-oligomeric high-molecular weight complexes. ERLINs belong to the SPFH family of proteins, which have been proposed to organize membrane microdomains with distinct lipid and protein composition. The ERLINs localize to the endoplasmic reticulum (ER), bind cholesterol, and promote ER-associated degradation of specific substrates. Loss-of-function mutations in ERLIN1 and ERLIN2 genes have been found in patients affected by hereditary spastic paraplegia (HSP), a genetically heterogeneous neurological disease characterized by progressive degeneration of central motor axons. How lack of ERLINs leads to HSP remains enigmatic. We propose that, as crucial protein and lipid membrane scaffolds, the ERLINs may play a role in both morphogenesis of the ER and lipid droplet (LD) formation, two processes that have already been linked to HSP. In agreement with this hypothesis, we recently found that upon depletion of the ERLIN complexes in HeLa cells, the peripheral ER undergoes a remarkable structural rearrangement, characterized by a proliferation of flat sheet-like structures that substitute ER tubules. Furthermore, we established that both ERLIN1 and ERLIN2 interact with seipin, the protein product of the BSCL2 gene, and promote seipin stability in the ER. Seipin is involved in LD biogenesis and in maintaining the contacts between the ER and the LDs, and is itself mutated in some forms of HSP. Here, we aim to investigate the molecular mechanisms by which the ERLIN complexes regulate ER morphogenesis by using a combination of candidate molecule approaches and of unbiased proteomics and lipidomics strategies. We will define the protein and lipid scaffolding functions of ERLINs in the ER and how they affect stability in the ER membrane of specific proteins, such as seipin. Finally, to investigate the role of the ERLIN complexes in vivo in the nervous system and assess if altered pathways identified in vitro are relevant for the degeneration of long motor axons, we will develop and characterize Erlin1-deficient, Erlin2-deficient, and double Erlin1/2-deficient mice.