ER Network Shaping Mechanisms in the Hereditary Spastic Paraplegias

神经遗传学分支细胞生物学部分的研究重点是一些神经退行性疾病的分子机制，包括线粒体疾病、肌张力障碍和遗传性痉挛性截瘫（HSPs）。这些疾病折磨着数以百万计的美国人，随着时间的推移，病情逐渐恶化，其中许多人的治疗选择有限。我们的实验室正在研究这些疾病的遗传形式，使用分子和细胞生物学方法研究疾病基因的突变如何最终导致细胞功能障碍。

Research in the Cell Biology Section, Neurogenetics Branch focuses on the molecular mechanisms underlying a number of neurodegenerative disorders, including mitochondrial disorders, dystonia, and the hereditary spastic paraplegias (HSPs). These disorders, which together afflict millions of Americans, worsen insidiously over a number of years, and treatment options are limited for many of them. Our laboratory is investigating inherited forms of these disorders, using molecular and cell biology approaches to study how mutations in disease genes ultimately result in cellular dysfunction. In this project, we are focusing on the HSPs. One major research theme involves the characterization and functional analysis of the hereditary spastic paraplegia type 3A (SPG3A) protein, atlastin-1. In 2009, we reported in the journal Cell that atlastin-1 is a member of a ubiquitous family of GTPases that interact with two families of ER shaping proteins to generate the tubular endoplasmic reticulum (ER) network. Interestingly, atlastin-1 interacts with the SPG31 protein REEP1, which is an ER shaping protein, as well as the SPG4 protein spastin, a microtubule-severing ATPase. In 2010, we published a study in the Journal of Clinical Investigation demonstrating that these three proteins interact with one another to organize the tubular ER network in conjunction with the microtubule cytoskeleton. Since SPG3A, SPG4, and SGP31 account for well over 50% of all HSP cases, we suggest ER network defects as the predominant neuropathologic mechanism for the HSPs. This is supported by the recent identification of numerous other HSP proteins that regulate ER morphology. Over the past year, we have continued to develop animal models for SPG31 (knock out) and SPG3A (knockout and knock in) to evaluate the extent of ER morphology changes using both in vivo and ex vivo studies. We are employing both high-throughput electron microscopy (in collaboration with Dr. Mark Terasaki) and super-resolution confocal microscopy to examine the changes in tubular ER within neuronal axons in response to these genetic manipulations. In addition, we have identified interactions of these proteins with several other proteins mutated in the HSPs, expanding the number of HSP cases related to defects in ER network formation. Last, we are actively generating in situ models for many of the HSPs through the production of patient-derived, induced pluripotent stem cells that are then differentiated into telecephalic neurons, in collaboration with Dr. Xue-Jun Li. A key aspect of ER function possibly related to disease pathogenesis is the formation of lipid droplets, and this is an area of emphasis for our cellular and organismal studies. We are in the process of completing several studies tying changes in ER morphology to alterations in lipid droplet biogenesis. As part of these studies, we have used CRISPR and TALEN technologies to knock out all three atlastin isoforms from cell lines such as HeLa and NIH-3T3. We are also utilizing advanced imaging techniques such as CT scans and MR spectroscopy to study changes in fat tissue in HSP mouse models non-invasively. Taken together, we expect that our studies will advance our understanding of the molecular pathogenesis of the HSPs. Such an understanding at the molecular and cellular levels will hopefully lead to novel treatments to prevent the progression of these disorders.