Synaptic vesicle life cycle and synaptic turnover

Abstract

Cholinergic synaptic vesicles contain a mixture of soluble low molecular mass constituents. Besides acetylcholine these include Ca 2+, ATP, GTP, small amounts of ADP and AMP, and also the diadenosine polyphosphates Ap 4A and Ap 5A. In synaptic vesicles isolated from the electric ray these diadenosine polyphosphates occur in mmol concentrations and might represent a novel cotransmitter. The membrane proteins of cholinergic synaptic vesicles presumably are identical to those in other types of electron-lucent synaptic vesicles. A presumptive exception are the transmitter-specific carriers. The life cycle of the synaptic vesicle in intact neurons and in situ was investigated by analysis of all cytoplasmic membrane compartments that share membrane integral proteins with synaptic vesicles. The results suggest that the synaptic vesicle membrane compartment might originate from the trans-Golgi network and, after cycles of exo- and endocytosis in the nerve terminal, might fuse into an endosomal membrane compartment early on retrograde transport. Tracer experiments using membrane proteins and soluble contents suggest that the synaptic vesicle membrane compartment does not intermix with the presynaptic plasma membrane on repeated cycles of exo- and endocytosis if low frequency stimulation is applied. A cDNA has been isolated from the electric ray electric lobe that codes for o-rab3, a small GTP-binding protein highly homologous to mammalian rab3. While abundant in the nerve terminals of the electric organ and at the neuromuscular junction this protein occurs only in limited subpopulations of nerve terminals in electric ray brain. Immunocytochemical analysis using the colloidal gold technique and a monospecific antibody against o-rab3 suggests that the GTP-binding protein remains attached to recycling synaptic vesicles. No evidence was found for a major contribution of an intraterminal endosomal sorting compartment involved in synaptic vesicle recycling.