Delivering Progranulin to Astrocytic Lysosomes Promotes Growth of Co-Cultured Neurons

Progranulin (GRN) mutations, most of which cause progranulin haploinsufficiency, are a major genetic cause of frontotemporal dementia (FTD). Restoring progranulin to people with GRN mutations is a promising therapeutic strategy and understanding progranulin's mechanism of action may enable the design of optimal progranulin-based therapies. Progranulin is constitutively secreted and interacts with several receptors, but is also taken up and trafficked to lysosomes where it is necessary for maintaining normal lysosomal function. Progranulin promotes neuronal growth and survival, but it is not clear if these actions are mediated by extracellular signaling or by regulation of lysosomal function. In previous work we showed that progranulin acts in neuronal lysosomes to promote neuronal survival. In this study we investigated the mechanism by which progranulin promotes neuronal growth using lentiviral vectors expressing either progranulin (PGRN) or a non-secreted, lysosome-targeted progranulin (L-PGRN) in rat primary hippocampal neurons and astrocytes. Using lentiviral vectors driven by non-selective (PGK), neuron-selective (hSyn), or astrocyte-selective (GFAP) promoters, we found that delivering L-PGRN to astrocytes, but not neurons, promoted dendritic outgrowth in primary hippocampal cultures. L-PGRN-transduced astrocytes grown on transwell inserts also promoted the growth of co-cultured neurons. RNA sequencing of astrocytes indicated that L-PGRN downregulated transcriptomic pathways associated with cellular reactivity. Analysis of astrocyte conditioned medium showed that transduction with L-PGRN reduced the secretion of PAI-1, a protease inhibitor that inhibits neuronal outgrowth in hippocampal cultures. Collectively, these data indicate that delivering progranulin to astrocytic lysosomes may inhibit the secretion of factors that restrain neuronal outgrowth. Consistent with this hypothesis, depleting astrocytes from hippocampal cultures increased dendritic outgrowth and occluded the pro-growth effects of L-PGRN. These data show that under these culture conditions, progranulin secretion is not required to promote dendritic outgrowth. Instead, progranulin increased dendritic outgrowth by a non-cell autonomous mechanism involving actions in astrocytic lysosomes. These data add to a growing body of evidence that progranulin may act on astrocytes to promote neuronal health.