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 funct...