Synapsin: A Novel Insight for PKA Phospho-Domains in Inhibiting Release Probability

Abstract

Synapsin (Syn), a family of phosphoproteins are found abundantly in neurons of both vertebrate and invertebrate systems. Syn is located in the nervous terminals and regulates the availability of synaptic vesicles by reversible association with the actin cytoskeleton and other vesicles via phosphorylation and dephosphorylation cycle. Syn deficiencies have been found in several mental disorders including Bipolar and Maniac Spectrum Disorders, Epilepsy, and short-term memory deficiencies. Evidence indicates that the development of short-term memory requires the phosphorylation of Syn domains via Protein Kinase-Activated by cAMP (PKA) during nerve activity. However, the neurophysiological effect of the PKA phosphorylations of Syn in synaptic transmission and plasticity is not clear. We determined the effects of PKA-phosphorylation domains of Syn in synaptic function at Drosophila neuromuscular junction of third-instar larvae. For this, we scrutinize the synaptic transmission of wild-type, Syn KO and its neuronal rescues with normal Syn and with a PKA phospho-incompetent mutant. Transgenic animals containing the UAS-Syn constructs were expressed under the control of the neuronal driver elav-Gal-4. Synaptic transmission activity was measured by electrophysiological recording of the postsynaptic compartment by two-electrode voltage-clamp. Synaptic responses were evoked by paradigms of nerve stimulation controlled by a programmable stimulator. Our work in Drosophila indicates that Syn is required for normal synaptic transmission and short-term synaptic memory by modulating nerve-evoked release probability at rest and during high demand, in agreement with the previous evidence in other model systems. Interestingly, neuronal rescues indicate that Syn may operate at the last step of vesicle fusion process by inhibiting the nerve-evoked fusion via PKA phosphorylation and promoting it in its non-phosphorylated form.