THE SOLUBLE N-ETHYLMALEIMIDE-SENSITIVE FACTOR ATTACHED PROTEIN-RECEPTOR COMPLEX IN GROWTH CONES - MOLECULAR ASPECTS OF THE AXON TERMINAL DEVELOPMENT

Soluble N-ethylmaleimide-sensitive factor attached protein (SNAP) rece ptor (SNARE) mechanisms are thought to be involved in two important pr ocesses in axonal growth cones: (1) membrane expansion for axonal grow th and (2) vesicular membrane fusion for mature synaptic transmission. We investigated the localization and interactions among the proteins involved in SNARE complex formation in isolated growth cone particles (GCP) from forebrain. We demonstrated that the SNARE complex is presen t in GCPs morphologically without synaptic vesicles (SVs) and associat ed with growth cone vesicles. However, the apparently SV-free GCP was lacking in the regulatory mechanisms inhibiting SNARE complex formatio n proposed in SV fusion, i.e., the association of synaptotagmin with t he SNARE complex, and vesicle-associated membrane protein (VAMP)-synap tophysin complex formation. The core components of the SNARE complex ( syntaxin, SNAP-25, and VAMP) accumulated for several days before postn atal day 7, when SVs first appeared, and preceded the accumulation of marker proteins such as synaptophysin, SV2, and V-ATPase. Our present results suggest that the SNARE mechanism for vesicular transmitter rel ease is not fully functional in growth cones before the appearance of SVs, but the SNARE mechanism is working for membrane expansion in grow th cones, which supports our recent report. We concluded that the regu lation of the SNARE complex in growth cones is different from that in mature presynaptic terminals and that this switching may be one of the key steps in development from the growth cone to the presynaptic term inal.