Synapsin dynamics and function in living hippocampal synapses

The essence of brain function is the modulation of information transfer between neurons over time scales ranging from milliseconds to years. Central synapses exhibit vastly diverse functional properties and exquisitely fine tuned information transfer. Effective signaling and information transfer require synergistic modulation of both pre- and post-synaptic properties. The focus of this thesis is to elucidate the underlying molecular mechanisms of a family of highly conserved neurospecific phosphoproteins at presynaptic nerve terminals, the synapsins, in the regulation of synaptic transmission.; First, we characterized the synaptic vesicle recycling properties in hippocampal nerve terminals derived from synapsin II-, I/II-, and III-deficient mice. Synapsin II- and I/II-deficient mice exhibited qualitatively similar, but quantitatively different phenotypes, including functional recycling pool sizes, kinetics of vesicle pool turnover, rate of endocytosis, rate of repriming, and frequency dependence of vesicle pool turnover efficiencies. In contrast, synapsin III-deficient mice exhibited completely opposite phenotypes from those of synapsin II- and I/II-deficient mice.; Next, we demonstrated that synapsins dynamically dissociate from synaptic vesicles, and disperse into axons during synaptic activity. We developed a real-time assay to provide quantitative measurements of this activity using green fluorescent protein (GFP)-labeled synapsin Ia. Using various mutated forms of synapsin Ia that prevent phosphorylation at specific sites, we showed that synapsin Ia dynamics is regulated by calcium-dependent phosphorylation, which in turn modulates vesicle pool turnover.; Finally, we investigated the physiological significance of phosphorylation and dephosphorylation events in synapsin Ia during synaptic activity, as well as how synapsin Ia would function in response to neuronal activity at different firing rates. GFP-synapsin Ia exhibited stimulation-frequency-dependent dynamics at presynaptic nerve terminals, which correlated well with frequency-dependent kinetics of synaptic vesicle pool turnover. Mutation of the CaM kinase phosphorylation sites showed the strongest effect on the slowing of vesicle pool turnover at low stimulation frequency (5 Hz), whereas mutation of the MAP kinase sites had maximal effect at high stimulation frequency (20 Hz).; Our observations strongly suggest that synapsin Ia modulates synaptic strength over a wide range of firing frequencies at presynaptic nerve terminals, and it employs different phosphorylation sites that are exquisitely fine tuned for different ranges of stimulation frequencies to accomplish this task.