Expression of neurotrophin-3 (NT-3) and anterograde axonal transport of endogenous NT-3 by retinal ganglion cells in chick embryos

Anterograde axonal transport of neurotrophins has been demonstrated recentl y, but to date such transport has only been shown for brain-derived neurotr ophic factor and no other endogenous neurotrophin. Endogenous neurotrophin- 3 (NT-3) protein is present in the ganglion cell layer of the chicken retin a, as well as the superficial layers of the optic tectum. NT-3 immunolabel in these tectal layers is largely reduced or abolished after treatment of t he eye with colchicine or monensin, demonstrating that endogenous NT-3 is t ransported to the optic tectum by retinal ganglion cells (RGCs). Reverse tr anscription-PCR analysis of RGCs purified to 100% shows that RGCs, but not tectal cells, express NT-3 mRNA. Blockade of the intercellular transfer of NT-3 within the retina does not reduce the anterograde transport of endogen ous NT-3 to the tectum, indicating that a major fraction of the anterograde ly transported NT-3 is produced by RGCs rather than taken up from other ret inal cells. Immunolabel for the neurotrophin receptor p75, but not trkB or trkC, in the superficial tectum coincides with the NT-3 label. The p75 labe l in the neuropil of superficial tectal layers is largely reduced or elimin ated by injection of monensin in the eye, indicating that p75 protein is ex ported along RGC axons to the retinotectal terminals and may act as a neuro trophin carrier. These results show that NT-3 is produced by RGCs and that some of this NT-3 is transported anterogradely along the axons to the super ficial layers of the tectum, possibly to regulate the survival, synapse for mation, or dendritic growth of tectal neurons.