A Presynaptic Phosphosignaling Hub for Lasting Homeostatic Plasticity

Stable function of networks in the brain requires that synapses adapt their strength to levels of neuronal activity in a durable manner and failures to do so result in cognitive disorders. How such persistent and homeostatic regulation of synaptic connections may be biologically implemented in mammalian synapses remains poorly understood. Here, we have employed optical recordings of vesicle release from single synapses, super-resolution microscopy, and phosphoproteomics and reveal that the level of homeostatic regulation of transmitter release is stored in the phosphorylation state of a single serine (1045) of the central active zone organizer protein RIM1. While we also discovered other phosphosites in RIM1 with differential effects on neurotransmitter output, we only found S1045 to be necessary and sufficient for expression of silencing-induced HP. Furthermore, we show that S1045 is kept phosphorylated by regulated expression of the serine arginine protein kinase 2 (SRPK2) which binds to RIM1 and effectively targets this (and other) AZ proteins. SRPK2-induced upscaling of synaptic release involved formation of additional RIM1 nanoclusters and docked vesicles at the AZ, was not observed in the absence of RIM1 and occluded by RIMS1045E. Our data suggest that SRPK2 and RIM1 present a presynaptic phosphosignalling hub and that the spatial association of a kinase and AZ protein can achieve the persistent regulation of transmitter release required for the homeostatic balance of synaptic coupling of neuronal networks.