P59(FYN) IN RAT-BRAIN IS LOCALIZED IN DEVELOPING AXONAL TRACTS AND SUBPOPULATIONS OF ADULT NEURONS AND GLIA

Expression of the c-fyn proto-oncogene was analysed in the developing and adult rat brain by subcellular fractionation and immunocytochemist ry using polyclonal antibodies specific for its protein product (p59fy n). Immunoperoxidase staining revealed widespread localization of p59f yn in developing axonal tracts throughout the fetal (E18) rat brain. f yn immunoreactivity was not observed in most axon-rich regions of the adult brain, but continued to be expressed at elevated levels in the a dult olfactory and vomeronasal systems. At other sites in the adult ra t brain, fyn immunoreactivity was restricted to cell bodies of neurona l subpopulations, especially those in brain stem and hypothalamic nucl ei, and to subpopulations of glial cells along axonal tracts in the me dulla, optic nerve and white matter of the spinal cord. In the periphe ral nervous system, fyn staining was also prominent in Schwann cells. Subcellular fractionation of fetal and adult rat brain confirmed the i mmunocytochemical localization, demonstrating an enrichment of p59fyn in membranes from a fetal brain fraction containing nerve growth cones , and lower levels in adult brain where there was only a small enrichm ent in synaptosomal membranes. The developmental regulation of p59fyn suggests that the fyn tyrosine kinase may serve separate cellular role s in axonal growth and specialized functions of mature neurons and gli a.