Restricted expression of the neuronal intermediate filament protein plasticin during zebrafish development.

In the adult goldfish visual pathway, expression of the neuronal intermediate filament (nIF) protein plasticin is restricted to differentiating retinal ganglion cells (RGCs) at the margin of the retina. Following optic nerve injury, plasticin expression is elevated transiently in all RGCs coincident with the early stages of axon regeneration. These results suggest that plasticin may be expressed throughout the nervous system during the early stages of axonogenesis. To test this hypothesis, we analyzed plasticin expression during zebrafish (Danio rerio) neuronal development. By using immunocytochemistry and in situ hybridization, we found that plasticin is expressed in restricted subsets of early zebrafish neurons. Expression coincides with axon outgrowth in projection neurons that pioneer distinct axon tracts in the embryo. Plasticin is expressed first in trigeminal, Rohon-Beard, and posterior lateral line ganglia neurons, which are among the earliest neurons to initiate axonogenesis in zebrafish. Plasticin is expressed also in reticulospinal neurons and in caudal primary motoneurons. Together, these neurons establish the first behavioral responses in the embryo. Plasticin expression also coincides with initial RGC axonogenesis and progressively decreases after RGC axons reach the tectum. At later developmental stages, plasticin is expressed in a subset of the cranial nerves. The majority of plasticin-positive neurons are within or project axons to the peripheral nervous system. Our results suggest that plasticin subserves the changing requirements for plasticity and stability during axonal outgrowth in neurons that project long axons.