Abstract
IF (intermediate filament) proteins can be cleaved by caspases to generate proapoptotic fragments as shown for desmin. These fragments can also cause filament aggregation. The hypothesis is that disease-causing mutations in IF proteins and their subsequent characteristic histopathological aggregates could involve caspases. GFAP (glial fibrillary acidic protein), a closely related IF protein expressed mainly in astrocytes, is also a putative caspase substrate. Mutations in GFAP cause AxD (Alexander disease). The overexpression of wild-type or mutant GFAP promotes cytoplasmic aggregate formation, with caspase activation and GFAP proteolysis. In this study, we report that GFAP is cleaved specifically by caspase 6 at VELD²²&sup5; in its L12 linker domain in vitro. Caspase cleavage of GFAP at Asp²²&sup5; produces two major cleavage products. While the C-GFAP (C-terminal GFAP) is unable to assemble into filaments, the N-GFAP (N-terminal GFAP) forms filamentous structures that are variable in width and prone to aggregation. The effect of N-GFAP is dominant, thus affecting normal filament assembly in a way that promotes filament aggregation. Transient transfection of N-GFAP into a human astrocytoma cell line induces the formation of cytoplasmic aggregates, which also disrupt the endogenous GFAP networks. In addition, we generated a neo-epitope antibody that recognizes caspase-cleaved but not the intact GFAP. Using this antibody, we demonstrate the presence of the caspase-generated GFAP fragment in transfected cells expressing a disease-causing mutant GFAP and in two mouse models of AxD. These findings suggest that caspase-mediated GFAP proteolysis may be a common event in the context of both the GFAP mutation and excess.