Loss of Progranulin Results in Increased Pan-Cathepsin Activity and Reduced LAMP1 Lysosomal Protein

Mutations in the progranulin (PGRN) encoding gene, GRN, cause familial frontotemporal dementia (FTD) and neuronal ceroid lipofuscinosis (NCL) and PGRN is also implicated in Parkinson's disease (PD). These mutations result in decreased PGRN expression. PGRN is highly expressed in peripheral immune cells and microglia and regulates cell growth, survival, repair, and inflammation. When PGRN is lost, the lysosome becomes dysfunctional, but the exact mechanism by which PGRN plays a role in lysosome function and how this contributes to inflammation and degeneration is not entirely understood. To better understand the role of PGRN in regulating lysosome function, this study examined how loss of GRN impacts total LAMP1 protein expression and cathepsin activities. Using mouse embryonic fibroblasts (MEFs), immunocytochemistry and immunoblotting assays were performed to analyze fluorescent signal from LAMP1 (lysosomal marker) and BMV109 (marker for pan-cathepsin activity). GRN^-/- MEFs exhibit increased expression of pan-cathepsin activity relative to GRN^+/+ MEFs, and significantly impacts expression of LAMP1. The significant increase in pan-cathepsin activity in the GRN^-/- MEFs confirms that PGRN loss does alter cathepsin expression, which may be a result of compensatory mechanisms happening within the cell. Using NTAP PGRN added to GRN^-/- MEFs, specific cathepsin activity is rescued. Further investigations should include assessing LAMP1 and BMV109 expression in microglia from GRN^-/- mice, in the hopes of understanding the role of PGRN in lysosomal function in immune cells of the central nervous system and the diseases in which it is implicated.