Progranulin deficiency promotes neuroinflammation through cathepsin B-mediated activation of the NLRP3 inflammasome
Haploinsufficiency of progranulin (resulting from mutations in the GRN gene) is a major genetic cause of frontotemporal dementia (FTD), which is characterized by profound changes in personality and behavior, progressive impairments in executive function, speech, language and/or movement, TDP-43 pathology, lysosomal dysfunction, and neuroinflammation. Studies in progranulin-deficient mouse models have demonstrated the critical role of neuroinflammation in disease pathogenesis, as deletion of NF-kB or complement factors prevents FTD-associated behavioral deficits. While progranulin is known to be anti-inflammatory, the precise molecular mechanism by which its deficiency triggers inflammatory cascades remain unclear. Here, we investigate the involvement of the NLRP3 inflammasome in progranulin-deficient models, including mice, human microglial cells, and patient fibroblasts. In Grn mice, we found increased levels of several inflammasome-related proteins, including procaspase-1 and cleaved IL-1{beta}. Similarly, GRN knockout (KO) microglia cells showed significant increases in levels of procaspase-1, NLRP3, and cleaved IL-1{beta}. Moreover, KO cells had increased NLRP3 oligomerization, suggesting that progranulin negatively regulates inflammasome assembly. We also investigated the potential link between lysosomal dysfunction and NLRP3 inflammasome activation in progranulin deficiency. We found that pharmacological inhibition of cathepsin B reduced NLRP3 inflammasome oligomerization as well as inflammatory cytokine expression in GRN KO cells. Additionally, siRNA knockdown of cathepsin B reduced levels of procaspase-1 and cleaved IL-1{beta} in GRN KO cells. Finally, we found that heterozygous GRN fibroblasts exhibited similar increases in procaspase-1 and cleaved IL-1{beta} levels, and these increases were corrected by treatment with an antisense oligonucleotide (ASO) that increase progranulin protein levels. Overall, our results demonstrate that progranulin deficiency promotes NLRP3 inflammasome assembly and activity through cathepsin B-mediated signaling, thereby establishing a mechanistic link between lysosomal dysfunction and increased inflammation. The ability of the GRN-targeting ASO to correct NLRP3-mediated inflammation highlights a promising therapeutic strategy for FTD-GRN and inflammation in other neurodegenerative disorders linked to progranulin deficiency.