Investigating the regulation of lipid homeostasis and lysosomal function in neurodegenerative disease by prosaposin and progranulin in C. elegans

神经退行性疾病是对现代健康的最大威胁之一，而对于这些复杂和进行性的疾病，几乎没有可用的治疗方法。这项建议调查了原颗粒(PGRN)的基础生物学，它是一种溶酶体蛋白，其单倍性不足会导致阿尔茨海默病相关痴呆(ADRD)额颞叶痴呆(FTD)[1]。虽然PGRN缺乏是FTD的主要遗传原因之一，但其功能尚不完全清楚。为了进一步了解FTD的病因和病理生理学，有必要确定溶酶体中PGRN的内源性功能。PGRN与另一种溶酶体前蛋白PSAP(PSAP)有着千丝万缕的联系，PSAP也与原颗粒蛋白FTD有关[2]。PSAP被裂解成鞘磷脂酶激活蛋白，称为皂苷，它催化溶酶体中的鞘脂代谢。皂苷的丧失扰乱了溶酶体脂肪酶的活性，导致鞘脂堆积和溶酶体功能障碍。有趣的是，神经鞘脂代谢紊乱最近已成为ADRD发病和发展的关键危险因素，包括FTD[3-11]。此外，PGRN被认为与ADRD相关的脂质平衡失调有关[12，13]，然而，其潜在的分子机制尚不清楚。总之，这表明了FTD风险因子PGRN通过PSAP调节脂质稳态的途径，这反过来又有助于ADRD的病理生理。尽管皂苷在鞘脂代谢中起着明显的作用，但皂苷从PSAP中释放出来，从而调节鞘脂酶、鞘脂动态平衡和下游溶酶体功能的机制尚不清楚。我推测PSAP裂解成皂苷受到溶酶体酶、FTD风险因子PGRN和年龄的调节，PSAP裂解的调节在脂质稳态和溶酶体功能中起着关键作用。为了验证这一假设，我提出了几个创新的实验，使用重组人蛋白在体外、人类细胞系，以及在秀丽线虫体内的行为和成像技术中。我将利用成熟的体外蛋白酶功能和线虫表型的方法，以及新开发的技术来测量体内溶酶体的pH和脂肪含量。我将确定PGRN如何调节PSAP裂解为皂苷，如果PGRN通过调节PSAP影响脂平衡，以及脂平衡改变如何影响溶酶体功能。这项研究的成功完成将填补关于溶酶体中FTD危险因子PGRN和PSAP之间的关系和功能的基础生物学知识的空白，并通过PSAP的溶酶体功能阐明PGRN和ADRD中脂类平衡改变之间的关键联系。这项建议的成功完成将扩大我们对FTD的理解，因为它产生了关于内溶酶体系统在蛋白质和脂肪稳态中的内源性功能的基础知识，这可能是产生新的诊断技术、预防策略和治疗FTD、AD和其他ADRD的关键。 关键词：FTD、原颗粒、神经退行性变、衰老、溶酶体、鞘磷脂、丙皂苷、线虫

Neurodegenerative diseases are one of the greatest threats to modern health, and there are few treatments available for these complex and progressive diseases. This proposal investigates the basic biology underlying progranulin (PGRN), a lysosomal protein whose haploinsufficiency causes the Alzheimer’s Disease-Related Dementia (ADRD) frontotemporal dementia (FTD) [1]. While PGRN deficiency is one of the leading genetic causes of FTD, its function is not completely understood. To further our understanding of the causes and pathophysiology of FTD, it’s imperative to determine the endogenous functions of PGRN in the lysosome. PGRN is inextricably linked with another lysosomal pro-protein, prosaposin (PSAP), which is also implicated in progranulin-FTD [2]. PSAP is cleaved into sphingolipidase activator proteins called saposins, which catalyze sphingolipid metabolism in the lysosome. Loss of saposins perturbs lysosomal lipase activity, resulting in accumulation of sphingolipids and lysosomal dysfunction. Interestingly, perturbed sphingolipid metabolism has recently emerged as a critical risk factor for the onset and progression of ADRDs, including FTD [3-11]. Moreover, PGRN has been implicated in ADRD-related dysregulation in lipid homeostasis [12, 13], however, the underlying molecular mechanisms are unknown. Together, this suggests a pathway where the FTD-risk factor PGRN regulates lipid homeostasis via PSAP, which in turn contributes to ADRD pathophysiology. Despite the clear role for saposins in sphingolipid metabolism, the mechanism by which saposins are released from PSAP, and thus available to modulate sphingolipidases, sphingolipid homeostasis, and downstream lysosomal function, is not well understood. I hypothesize that PSAP cleavage into saposins is subject to regulation by lysosomal enzymes, the FTD-risk factor PGRN, and age, and that regulation of PSAP cleavage plays a crucial role in lipid homeostasis and lysosome function. To test this hypothesis, I propose a several innovative experiments with recombinant human proteins in vitro, human cell lines, and in in vivo behavioral and imaging techniques in Caenorhabditis elegans. I will utilize well-established protocols for both in vitro protease function and C. elegans phenotypes alongside newly developed techniques to measure lysosomal pH and lipid content in vivo. I will determine how PGRN regulates the cleavage of PSAP into saposins, if PGRN influences lipid homeostasis via its regulation of PSAP, and how altered lipid homeostasis impacts lysosomal function. Successful completion of this proposal will fill a gap in knowledge about the basic biology underlying the relationship and function of the FTD-risk factors PGRN and PSAP in the lysosome and elucidate a critical link between PGRN and altered lipid homeostasis in ADRD via the lysosomal function of PSAP. Successful completion of this proposal will expand our understanding of FTD by generating fundamental knowledge on the endogenous function of the endolysosomal system in protein and lipid homeostasis, which could be the key to the generation of novel diagnostic techniques, prevention strategies, and treatments for FTD, AD, and other ADRDs. Key Words: FTD, progranulin, neurodegeneration, aging, lysosome, sphingolipid, prosaposin, C. elegans