Expression and phosphorylation of the mid‐sized neurofilament protein NF‐M during chick spinal cord neurogenesis

The middle molecular weight polypeptide of neurofilaments (NF‐M) is modified posttranslationally by extensive phosphorylation. This modification is slow in mature neurons, requiring approximately 24–48 hr for completion and probably occurs outside of the cell soma (Bennett and DiLullo: J Cell Biol 100:1799, 1985c). Thus, NF‐M synthesis and phosphorylation are separate events both temporally and spatially. Although it is known that NF‐M is among the earliest neuron‐specific gene products to be expressed during nervous system development, it is not known what the temporal relationship is between the initiation of NF‐M translation and its phosphorylation. To address this question, we have produced an antiserum against the dephosphorylated form of NF‐M (NF‐M130) and have used this antiserum, together with a previously characterized antiserum against completely phosphorylated NF‐M (NF‐M160), in an immunohistochemical examination of neurogenesis and the initial period of neuronal differentiation in chick spinal cord. We found that (1) nonphosphorylated and partially phosphorylated NF‐M cannot be detected prior to the completion of the terminal mitosis; (2) most postmitotic neuroblasts begin expressing NF‐M as they commence migration, but do not contain the completely phosphorylated polypeptide until some time after completion of migration; and (3) those precursor cells of a subpopulation of neuroblasts that begin expressing completely phosphorylated NF‐M during their terminal cell cycle (Bennett and DiLullo: Dev Biol 107:94, 1985a) contain no detectable nonphosphorylated or partially phosphorylated NF‐M. These cells probably complete the phosphorylation step more rapidly than do mature neurons.