Cystatin B function and Progressive Myoclonus Epilepsy (EPM1).

Mutations of the cystatin B gene (cstb) are described in the great majority of EPM1 patients. In mammals CSTB is a ubiquitous protein but the disease affects the CNS only (Lehesjoki AE. 2003). We have recently shown that, in vivo, CSTB has a polymeric structure and interacts with cytoskeletal proteins involved in vesicular transport and that overexpression of CSTB in neuroblastoma tissue culture cells generates toxic cytoplasmic aggregates (Cipollini et al., 2008). Furthermore, in rat cerebellum, CSTB is expressed in the Purkinje cells and radial glia only (Riccio et al., 2005). Although CSTB, in vitro, shows antiprotease activity, our results suggest the existence of additional role(s) possibly due to its interaction with partner proteins involved in cytoskeletal functions. Under suitable conditions, the CSTB monomer polymerizes in vitro generating amyloid fibers that grow by dimer addition. In an in vitro purified system the native CSTB monomer polymerizes following addition of copper and HSP70 only without requirement for ATP. We discuss the effect of the co-transfection of HSP70 and CSTB cDNAs on CSTB aggregate formation in neuroblastoma cells. We are also defining the interaction domains of HSP70/CSTB with each other. The possible relevance of these findings on the EPM1 disease will be discussed.