Elucidating molecular mechanisms of lysosomal dysfunction underlying progranulin deficiency

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PROJECT SUMMARY Progranulin (PGRN) encoded by the gene GRN is a secreted pleiotropic protein implicated in several processes including inflammation, extracellular signaling, and neuronal survival. Heterozygous, loss of function GRN mutations cause the neurodegenerative disorder frontotemporal dementia (FTD). Genetic variants in GRN that decrease circulating levels of PGRN also increase the risk of developing Alzheimer’s disease or Parkinson’s disease. These genetic discoveries demonstrate that PGRN is important for neuronal health, but it is unclear why the loss of PGRN leads to neurodegeneration. Interestingly, recently identified homozygous GRN mutations cause neuronal ceroid lipofuscinosis (CLN11). This group of lysosomal storage disorders (LSD) presents with neurodegeneration, cognitive deterioration, and lipofuscinosis in multiple tissues. The discovery that that homozygous GRN mutations cause an LSD combined with the observation that FTD-GRN patients share CLN11 pathological features, strongly suggests lysosomal dysfunction may underlie both disorders. This proposal will help elucidate the molecular role of Progranulin in the lysosome underlying mechanisms of neurodegeneration. PGRN traffics to the lysosome where it is processed into subunit granulins. It has been suggested that granulins are neurotoxic and pro-inflammatory, but recent work from the Kukar lab and others indicates that granulins play a homeostatic role in the lysosome. Data shows that granulins are stable in the lysosome compared to full length PGRN, and that granulins are decreased in human FTD-GRN cells and brain tissue. Furthermore, the complete loss of PGRN and granulins leads to accumulation of ceramides in the brain. These data suggest that granulins may be a bioactive component of lysosomal function involved in lipid degradation, yet the ability of individual granulins to ameliorate phenotypes of PGRN deficiency in vivo in unknown. Aim 1 will assess whether the expression of individual granulins is sufficient to rescue dysregulated lysosome function, and inflammation in Grn-/- mice. Progranulin has been implicated in the inflammatory response and is highly expressed in microglia, the brain’s resident immune cells. PGRN is involved in microglia activation, phagocytosis, migration, and synapse pruning. Moreover, the loss of PGRN has been shown to cause early and selective impairments in the maturation of lysosomal cathepsins, and accumulation of lipid droplets in murine microglia suggesting that microglial lysosomes may be particularly vulnerable to the loss of PGRN. Homeostatic microglial function is critical for brain health, however the impact of PGRN deficiency on human microglia is unknown. Aim 2 will assess the functional effects of PGRN deficiency on human microglia leveraging a novel patient derived iPSC line and isogenic controls developed by the Kukar lab. These experiments will be the first to directly assess the role of granulins on the molecular, and pathological, phenotypes of PGRN-deficient systems. Evaluating the bioactivity of PGRN and granulins will elucidate disease-relevant molecular pathways, and lead to the development of effective and precise therapeutics to treat diseases caused by PGRN/granulin deficiency.