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

ERLIN 1和ERLIN 2是约40 kDa的同源蛋白，其组装成异源寡聚或同源寡聚高分子量复合物。ERLIN属于SPFH蛋白家族，已提出其组织具有不同脂质和蛋白质组成的膜微区。ERLIN定位于内质网（ER），结合胆固醇，并促进特定底物的ER相关降解。在遗传性痉挛性截瘫（HSP）患者中发现了ERLIN 1和ERLIN 2基因的功能缺失突变，HSP是一种遗传异质性神经系统疾病，其特征是中枢运动轴突的进行性变性。缺乏ERLINs如何导致HSP仍然是个谜。我们认为，作为重要的蛋白质和脂质膜支架，ERLINs可能在ER的形态发生和脂滴（LD）形成中发挥作用，这两个过程已经与HSP有关。与此假设一致，我们最近发现，在HeLa细胞中ERLIN复合物耗尽后，外周ER经历了显着的结构重排，其特征在于替代ER小管的扁平片状结构的增殖。此外，我们确定ERLIN 1和ERLIN 2都与BSCL 2基因的蛋白产物seipin相互作用，并促进seipin在ER中的稳定性。Seipin参与LD生物发生和维持ER与LD之间的接触，并且本身在某些形式的HSP中突变。在这里，我们的目标是研究ERLIN复合物调节ER形态发生的分子机制，通过使用候选分子方法和无偏见的蛋白质组学和脂质组学策略的组合。我们将定义ERLINs在ER中的蛋白质和脂质支架功能，以及它们如何影响特定蛋白质（如seipin）在ER膜中的稳定性。最后，为了研究ERLIN复合物在体内神经系统中的作用，并评估体外鉴定的改变的途径是否与长运动轴突的变性相关，我们将开发和表征Erlin 1缺陷型、Erlin 2缺陷型和双Erlin 1/2缺陷型小鼠。

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.