SRPK2 regulates active zone ultrastructure and RIM1 phosphorylation to tune presynaptic function and plasticity

Abstract

Chemical synapses communicate through the release of neurotransmitter-filled synaptic vesicles into the synaptic cleft. Synapses are able to dynamically adapt to the changes in the neuronal network by strengthening or weakening synaptic connections. How these changes that can range from milliseconds to days are mediated is still unknown. Potential mechanisms to achieve this dynamic regulation are the posttranslational modifications of proteins involved in synaptic transmission. Many active zone (AZ) proteins are known phospho-proteins and in the last years it was postulated that protein phosphorylation, especially in the presynapse, is regulated during neuronal activity.<br /> Here, we characterized a novel presynaptic kinase, the Serine-Arginine Protein Kinase 2 (SRPK2), and investigated its interaction with proteins of the AZ and its role in synaptic plasticity. SRPK2 was identified as a new integral component of the synaptic cytomatrix. For the first time, the phosphoproteome of SRPK2 was determined and RIM1 was identified as a direct target of SRPK2. Superresolution microscopy revealed that SRPK2 controls RIM1 nanoclusters and thereby potential release sites. Moreover, presynaptic SRPK2 and RIM1 clusters increase during homeostatic plasticity. We therefore propose that SRPK2 regulates the clustering of RIM1 by controlling its phosphorylation status by direct and indirect phosphorylation. The increase in synaptic strength during homeostatic plasticity is mediated by the translocation of SRPK2 to the presynapse resulting in the recruitment of RIM1 molecules to form new RIM1 nanoclusters.