Increased caspase activation and decreased TDP-43 solubility in progranulin knockout cortical cultures

Null mutations in progranulin (GRN) are associated with frontotemporal lobar degeneration characterized by intraneuronal accumulation of TAR DNA-binding protein-43 (TDP-43). However, the mechanism by which GRN deficiency leads to neurodegeneration remains largely unknown. In primary cortical neurons derived from Grn knockout (Grn(-/-) ) mice, we found that Grn-deficiency causes significantly reduced neuronal survival and increased caspase-mediated apoptosis, which was not observed in primary mouse embryonic fibroblasts derived from Grn(-/-) mice. Also, neurons derived from Grn(-/-) mice showed an increased amount of pTDP-43 accumulations. Furthermore, proteasomal inhibition with MG132 caused increased caspase-mediated TDP-43 fragmentation and accumulation of detergent-insoluble 35- and 25-kDa C-terminal fragments in Grn(-/-) neurons and mouse embryonic fibroblasts. Interestingly, full-length TDP-43 also accumulated in the detergent-insoluble fraction, and caspase-inhibition prevented MG132-induced generation of TDP-43 C-terminal fragments but did not block the pathological conversion of full-length TDP-43 from soluble to insoluble species. These data suggest that GRN functions as a survival factor for cortical neurons and GRN-deficiency causes increased susceptibility to cellular stress. This leads to increased aggregation and accumulation of full-length TDP-43 along with its C-terminal derivatives by both caspase-dependent and independent mechanisms.