The regulated secretion and vectorial targeting of neurotrophins in neuroendocrine and epithelial cells

The varied roles that neurotrophins play in the development and activity-dependent plasticity of the nervous system presumably require that the sites and quantity of neurotrophin release be precisely regulated. As a step toward understanding how different neurotrophins are sorted and secreted by neurons, we expressed nerve growth factor (NGF), brain-derived neurotrophic factor, and neurotrophin-3 in cell lines used as models for neuronal protein sorting. All three neurotrophins were secreted by a regulated pathway in transfected AtT-20 and PC12 neuroendocrine cells, with a 3-6-fold increase in neurotrophin release in response to 8-bromo-cAMP or depolarization, respectively. To determine if the propeptide directs the intracellular sorting of mature NGF, we examined mutants in which regions spanning the propeptide were deleted. These mutants underwent regulated release in every case in which expression could be detected. Similarly, NGF sorting was not significantly altered by mutations which specifically abolished N-glycosylation or proteolytic processing sites within the NGF precursor. Finally, we found that all three neurotrophins were secreted 65-75% basolaterally by polarized Madin-Darby canine kidney epithelial cells. These findings suggest that the determinants of regulated neurotrophin secretion lie within the mature neurotrophin moiety and that NGF, brain-derived neurotrophic factor, and neurotrophin-3 are likely to be sorted similarly and released in a regulated manner by neurons.