Sorting of internalized neurotrophins into an endocytic transcytosis pathway via the Golgi system: Ultrastructural analysis in retinal ganglion cells

Subcellular pathways and accumulation of internalized radiolabeled neurotro phins NGF, BDNF, and NT-3 were examined in retinal ganglion cells (RGCs) of chick embryos by using quantitative electron microscopic autoradiography. All three neurotrophins accumulated in endosomes and multivesicular bodies. BDNF and NGF also concentrated at the plasma membrane, whereas NT-3 accumu lated transiently in the Golgi system. The enhanced targeting of NT-3 to th e Golgi system correlated with the anterograde axonal transport of this neu rotrophin. Anterograde transport of NT-3, but not its internalization, was significantly attenuated by the tyrosine kinase (trk) inhibitor K252a. Abol ishment of trk activity with K252a shifted NT-3 (and BDNF) away from the Go lgi system and into a lysosomal pathway, indicating that trk activity regul ated sorting of the ligand-receptor complex. Cross-linking of neurotrophins and immunoprecipitation with antibodies to the neurotrophin receptors p75, trkA, trkB, and trkC showed that the large majority of exogenous, receptor -bound NT-3 was bound to trkC in RGC somata, but during anterograde transpo rt in the optic nerve most receptor-bound NT-3 was associated with p75, and after arrival and release in the optic tectum transferred to presumably po stsynaptic trkC. These results reveal remarkable and unexpected differences in the intracellular pathways and fates of different neurotrophins within the same cell type. They provide first evidence for an endocytic pathway of internalized neurotrophic factors via the Golgi system before anterograde transport and transcytosis. The results challenge the belief that after int ernalization all neurotrophins are rapidly degraded in lysosomes.