NEUROTROPHIN-4 5 (NT-4/5) AND BRAIN-DERIVED NEUROTROPHIC FACTOR (BDNF) ACT AT LATER STAGES OF CEREBELLAR GRANULE CELL-DIFFERENTIATION/

The developing cerebellum expresses genes which encode for both neurot rophins and their receptors, The present study was designed to determi ne at what stages during cerebellar granule cell neurogenesis neurotro phin family molecules may act, We report here that in purified, well-c haracterized granule cell cultures (Gao et al., 1991; 1992), none of t he neurotrophins stimulated proliferation of granule cell precursors o r rescued phenotypic defect of mutant weaver granule cell precursors i n the initiation of neuronal differentiation, However, neurotrophin-4/ 5 (NT-4/5) and BDNF, but not neurotrophin-3 (NT-3) or NGF, promoted ne urite extension and survival of differentiated cerebellar granule cell s, Both of these effects were blocked by the specific inhibitor for Tr k tyrosine kinases, K-252a. NT-4/5 and BDNF also enhanced neurite exte nsion by weaver granule cells which were rescued by wild-type granule cells during differentiation, Moreover, TrkB immunohistochemistry perf ormed on sections of the developing wildtype and weaver cerebella reve aled that only differentiated granule cells, but not the precursor cel ls, make high levels of TrkB receptor. These findings together suggest that NT-4/5 and BDNF promote the maturation and maintenance of differ entiated granule cells, effects which are downstream to the weaver gen e, Since no additive effects were seen with the combination of NT-4/5 and BDNF, it seems likely that the two neurotrophins activate the same receptor trkB for signal transduction.