PTPN11/Corkscrew activates local presynaptic MAPK signaling to regulate Synapsin, synaptic vesicle pools, and neurotransmission strength, with a dual requirement in neurons and glia

Cytoplasmic protein tyrosine phosphatase (PTP) non-receptor type 11 (PTPN11) and Drosophila homolog Corkscrew (Csw) regulate the mitogen-activated protein kinase (MAPK) pathway via a conserved autoinhibitory mechanism. Disease causing loss-of-function (LoF) and gain-of-function (GoF) mutations both disrupt this autoinhibition to potentiate MAPK signaling. At the Drosophila neuromuscular junction (NMJ) glutamatergic synapse, LoF/GoF mutations elevate transmission strength and reduce activity-dependent synaptic depression. In both sexes of LoF/GoF mutations, the synaptic vesicles (SV) colocalized Synapsin phosphoprotein tether is highly elevated at rest, but quickly reduced with stimulation, suggesting a larger SV reserve pool with greatly heightened activity-dependent recruitment. Transmission electron microscopy of mutants reveals an elevated number of SVs clustered at the presynaptic active zones, suggesting that the increased vesicle availability is causative for the elevated neurotransmission. Direct neuron-targeted extracellular signal-regulated kinase (ERK) GoF phenocopies both increased local presynaptic MAPK/ERK signaling and synaptic transmission strength in mutants, confirming the presynaptic regulatory mechanism. Synapsin loss blocks this elevation in both presynaptic PTPN11 and ERK mutants. However, csw null mutants cannot be rescued by wildtype Csw in neurons: neurotransmission is only rescued by expressing Csw in both neurons and glia simultaneously. Nevertheless, targeted LoF/GoF mutations in either neurons or glia alone recapitulates the elevated neurotransmission. Thus, PTPN11/Csw mutations in either cell type is sufficient to upregulate presynaptic function, but a dual requirement in neurons and glia is necessary for neurotransmission. Taken together, we conclude PTPN11/Csw acts in both neurons and glia, with LoF and GoF similarly upregulating MAPK/ERK signaling to enhance presynaptic Synapsin-mediated SV trafficking. Significance statement Noonan syndrome (NS) is, by far, the most common RASopathy; a group of clinically-classified genetic syndromes caused by MAPK pathway alterations: it affects 1 in every 1000-2000 people. Patients present with cognitive deficits caused by PTPN11 mutations; with gain-of-function the most common basis for NS, and loss-of-function resulting in NS with multiple lentigines (NSML). We find NS/NSML patient-derived LoF/GoF PTPN11 mutations, as well as Drosophila homolog corkscrew LoF/GoF mutations, all increase presynaptic MAPK signaling, Synapsin turnover, and synaptic vesicle availability at presynaptic release sites. Surprisingly, we find PTPN11/corkscrew to be required in both glia and neurons to control neurotransmission strength. These findings suggest disease interventions manipulating presynaptic vesicle trafficking mechanisms, as well as therapeutic strategies targeting both glia and neurons.