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Neurofilament (NF) proteins constitute the major intermediate filament type in adult neurons. They are made up by the copolymerization of the neurofilament light (NF-L, 61 kDa), medium (NF-M, 90 kDa), and heavy (NF-H, 115 kDa) proteins. Although neurofilaments play a crucial role in neuronal growth, organization, shape, and plasticity, their expression pattern and cellular distribution in the developing neurons remain unknown. In this study, we have produced a rabbit polyclonal antibody specific to NF-M and investigated expression of NF-M in cultured cortical neurons. Immunostaining of 12 and 24 h cultures revealed strong expression of NF-M in axonal growth cone and in the region of a soma toward the axon. Doublestaining of 4 and 14 DIV corical neurons with NF-M and PSD95 antibodies revealed that both axon and dendrites were stained intensely with NF-M antibody, and that NF-M immunostaining along dendrites is often punctate and colocalize with PSD95 puncta, indicating that the puncta represent postsynaptic spines. Presence of NF-M in the postsynaptic spine was also indicated by immunoblot analysis of the postsynaptic density fraction. Taken together, our results show intensive targeting of NF-M into axons in the early axonal development, and into spines in mature neurons, indicating its important functions in axon and spine development.
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