Ultrastructural identification of storage compartments and localization ofactivity-dependent secretion of neurotrophin 6 in hippocampal neurons

A modulatory role of neurotrophins (NTs) in activity-dependent neuronal pla sticity by pre- and postsynaptic mechanisms is now well established. In thi s context, it is important to identify the storage compartments and to loca lize the precise site(s) and mechanism of NT secretion in order to deduce t he spatial and temporal availability of NTs. We approached these questions at the ultrastructural level, exploiting the unique property of NT6 to bind tightly to heparan sulfate proteoglycans at the neuronal surface (R. Gotz at al., 1994, Nature 372, 266-269), permitting the localization of secretio n sites excluding diffusion artifacts. The myc tagging of NT6 permitted glu taraldehyde fixation and hence good preservation of the membrane structure, permitting immunogold labeling of NT6myc at the neuronal surface. NT6myc i s preferentially secreted from neurites compared to neuronal cell bodies. I n agreement with light-microscopic observations, the ultrastructural locali zation of NT6myc by postembedding procedures showed a predominant localizat ion in ER-like membrane-confined compartments, partially associated with mi crotubules.