Phosphorylation of presynaptic PLPPR3 controls synaptic vesicle release

Phospholipid-phosphatase-related protein 3 (PLPPR3) belongs to a family of transmembrane proteins highly expressed in the nervous system where it regulates critical axonal growth processes during guidance, filopodia formation, and branching. However, little is known regarding its role in synapses and the signaling events regulating PLPPR3 function. Here, we identify 26 high-confidence phosphorylation sites in the intracellular domain of PLPPR3 using mass spectrometry. Biochemical characterization established one of these-S351-as a bona fide phosphorylation site of protein kinase A (PKA). PLPPR3 is enriched at presynaptic terminals, and deletion of PLPPR3 results in increased depolarization-induced synaptic vesicle release in hippocampal neurons. This tonic inhibitory signal toward depolarization-induced presynaptic activity is corrected by expression of PLPPR3 intracellular domain, but not a S351A phospho-dead mutant, in Plppr3 ^-/- hippocampal neurons. We propose that PLPPR3 phosphorylation under the control of PKA activity is a signaling integrator of presynaptic activity in hippocampal neurons.