TY - JOUR T1 - Intracellular Proteolysis of Progranulin Generates Stable, Lysosomal Granulins That Are Haploinsufficient in Patients with Frontotemporal Dementia Caused by <em>GRN</em> Mutations JF - eneuro JO - eNeuro DO - 10.1523/ENEURO.0100-17.2017 SP - ENEURO.0100-17.2017 AU - Christopher J. Holler AU - Georgia Taylor AU - Qiudong Deng AU - Thomas Kukar Y1 - 2017/08/09 UR - http://www.eneuro.org/content/early/2017/08/09/ENEURO.0100-17.2017.abstract N2 - Homozygous or heterozygous mutations in the GRN gene, encoding progranulin (PGRN), cause neuronal ceroid lipofuscinosis (NCL) or frontotemporal dementia (FTD), respectively. NCL and FTD are characterized by lysosome dysfunction and neurodegeneration, indicating PGRN is important for lysosome homeostasis in the brain. PGRN is trafficked to the lysosome where its functional role is unknown. PGRN can be cleaved into seven 6 kDa proteins called granulins (GRNs), however little is known about how GRNs are produced or if levels of GRNs are altered in FTD-GRN mutation carriers. Here, we report the identification and characterization of antibodies that reliably detect several human GRNs by immunoblot and immunocytochemistry. Using these tools, we find that endogenous GRNs are present within multiple cell lines and are constitutively produced. Further, extracellular PGRN is endocytosed and rapidly processed into stable GRNs within lysosomes. Processing of PGRN into GRNs is conserved between humans and mice and is modulated by sortilin expression and cysteine protease activity. Induced lysosome dysfunction caused by alkalizing agents or increased expression of TMEM106B inhibit processing of PGRN into GRNs. Finally, we find that multiple GRNs are haploinsufficient in primary fibroblasts and cortical brain tissue from FTD-GRN patients. Taken together, our findings raise the interesting possibility that GRNs carry out critical lysosomal functions and that loss of GRNs should be explored as an initiating factor in lysosomal dysfunction and neurodegeneration caused by GRN mutations.Significance Statement Progranulin (PGRN) plays a critical, yet undefined role in lysosome function. PGRN is cleaved into 6kDa proteins called granulins (GRNs), but this process is poorly understood. We find that PGRN is proteolytically processed into stable, lysosomal GRNs, implying that GRNs may have a functional role in the lysosome, and are not toxic as previously proposed. Moreover, deficiency of GRNs in frontotemporal dementia (FTD) caused by GRN mutations may play a causal role in the development of lysosome dysfunction that underlies FTD-GRN, which paves the way for testing GRN replacement as a therapeutic strategy. Finally, potential drug candidates to treat FTD-GRN should evaluate their effect on the production of both PGRN and GRNs in the brain. ER -