Neurons of the Olfactory Epithelium in Adult Rats Contain Vimentin

In the developing nervous system, the intermediate filament protein vimentin is found in the proliferating neuroepithelium and neural crest. As development proceeds, postmitotic neurons cease vimentin expression and neurofilament proteins begin to accumulate. We have shown that olfactory receptor neurons deviate from the general pattern of neuronal intermediate filament expression, in that they continue to express vimentin or a highly vimentin-like protein rather than neurofilament proteins in the adult rat. With light-microscopic immunohistochemistry, three independently derived antibodies to vimentin label all portions of the primary olfactory projection, including the sensory neuron cell bodies in the olfactory epithelium, the fascicles of the olfactory nerve, and their axonal arbors in the glomeruli of the olfactory bulb. In contrast, anti-neurofilament antisera stain only rare scattered receptor cells and a small number of axons in the olfactory nerve. Electron-microscopic immunohistochemistry shows dense staining of olfactory axons with anti-vimentin. The vimentin-like immunoreactive material in the olfactory nerve layer was characterized by SDS-PAGE and by immunoblotting. On immunoblots of homogenates of the olfactory nerve, the anti-vimentin monoclonal antibody SBV-21 (Blose et al., 1984) stains only a single protein of Mr = 55 kDa. This band comigrates with vimentin in crude cytoskeletal material from the neonatal rat brain prepared according to the method of Dahl et al. (1981). SBV-21 does not stain neurofilament triplet proteins or glial fibrillary acidic protein, which are also present in these blots. These results demonstrate that the vast majority of olfactory receptor neurons and their axons contain vimentin or a protein of similar immunological character and electrophoretic mobility, while identifiable expression of neurofilament proteins is confined to a very small subpopulation. Hence, the switch in intermediate filament proteins that normally accompanies neuronal maturation is arrested in most olfactory neurons, and a "juvenile" biochemical marker is retained. This population of neurons is also unique among mammalian neurons in several other respects, including that olfactory neurons die during normal adult life or following injury and then are replaced from a proliferating pool of stem cells.