Expression of two developmentally regulated brain-specific proteins is correlated with late outgrowth of the pyramidal tract.

The regulation of axon outgrowth is not well understood. In previous studies, however, axon elongation has been well correlated with expression of a small number of growth-associated proteins (GAPs). To identify other proteins whose expression could be correlated with axon outgrowth during development of CNS pathways, monoclonal antibodies were raised against growth cone particles isolated from neonatal hamster brains. Two of these antibodies recognized a brain-specific 33 kDa protein associated with intracellular membranes of axons and growth cones. Immunoblotting demonstrated a sharp developmental decline in levels of the protein in hamster brain during the first postnatal week and a more gradual decline thereafter. Immunocytochemical studies with the antibodies revealed ubiquitous staining of the neuropil during the first several days, which by the end of the first week became restricted to a few later-maturing pathways. Staining was most intense in the pyramidal tract and was well correlated with axon outgrowth, which continues until 14 d in this pathway. These results suggest that the 33 kDa protein may, like previously identified GAPs, play a role in axon elongation. Late outgrowth of the hamster pyramidal tract is also correlated with expression of another developmentally regulated protein, the high-molecular-weight neurofilament subunit (NF-H). Immunostaining with a monoclonal antibody that recognized phosphorylated NF-H demonstrated that this subunit does not begin to appear in the late-maturing pyramidal tract fibers until several weeks after birth, in striking contrast to intense immunoreactivity of other spinal cord pathways from postnatal day 1. This finding suggests that specific pathways may have a highly idiosyncratic time course for expression of neurofilament subunits.