Caspase cleavage of GFAP produces an assembly-compromised proteolytic fragment that promotes filament aggregation.

[alexander disease]

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 VELD Â²Â²âµ in its L 12 linker domain in vitro . Caspase cleavage of GFAP at Asp Â²Â²âµ 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 .