Incorporation of NF-L into keratin filaments in transfected epithelial cells

NEUROFILAMENTS are the characteristic intermediate filaments of mature neurons; during development and in some neuronal cell lines and neuroendocrine tumors, neurofilament proteins are expressed together with vimentin or cytokeratins. In some cell types, the filamentous arrays formed by neurofilaments and vimentin or cytokeratins do not coincide. However, individual neurofilament proteins co-assemble with vimentin in transfected non-neuronal cells. In order to determine whether individual neurofilament proteins could also co-assemble with cytokeratins in a cellular environment, the light chain of neurofilaments, NF-L, was transfected into MCF-7 cells, in which the only cytoplasmic intermediate filament proteins expressed are cytokeratins. In transfected MCF-7 cells, human NF-L was localized to a prominent filamentous network. This pattern most probably reflected the incorporation of NF-L into the endogenous keratin cytoskeleton as it is unlikely to be due to human NF-L self-assembly since, like its rodent counterpart, human NF-L accumulated into punctate aggregates when transiently transfected in intermediate filament-deficient SW13vim(-) cells. These results suggest the existence of a specific mechanism of segregation of neurofilaments and keratin filaments in some cell types.