Molecular pathway underlying bouton stabilization by Semaphorin4D during inhibitory synapse formation

Abstract

Changes in inhibitory connections are essential for experience-dependent circuit adaptations. Defects in inhibitory synapses are linked to neurodevelopmental disorders, but the molecular processes underlying inhibitory synapse formation are not well understood. Here we use high resolution two-photon microscopy in organotypic hippocampal slices to examine the signaling pathways induced by the postsynaptic signaling molecule Semaphorin4D (Sema4D) during inhibitory synapse formation. By monitoring changes in individual GFP-labeled presynaptic boutons we found that the primary action of Sema4D is to induce stabilization of presynaptic boutons within tens of minutes. Stabilizing boutons rapidly recruited synaptic vesicles, which was followed by accumulation of postsynaptic gephyrin. Newly formed inhibitory synapses were complete and functional after 24 hours, as determined by electrophysiology and immunohistochemistry. We further showed that Sema4D signaling is regulated by network activity and can induce a local increase in bouton density, suggesting a possible role in circuit adaptation. We further examined the intracellular signaling cascade triggered by Sema4D and found that bouton stabilization occurred through rapid remodeling of actin, and this could be mimicked by the actin-depolymerizing drug Latrunculin B or by reducing ROCK activity. The intracellular signaling cascade required activation of the receptor tyrosine kinase MET, which is a well-known autism risk factor. Our immunohistochemistry data suggests that MET may be localized to presynaptic inhibitory axons. Together, our data yield important insights in the molecular pathway underlying activity-dependent Sema4D-induced synapse formation and reveal a novel role for MET in inhibitory synapses.\n\nSignificance StatementGABAergic synapses provide the main inhibitory control of neuronal activity in the brain. We make important steps in unraveling the molecular processes that take place when formation of inhibitory synapses is triggered by a specific signaling molecule, Sema4D. We find that this process depends on network activity and involves specific remodeling of the intracellular actin cytoskeleton. We also reveal a previously unknown role for MET in inhibitory synapses. As defects in GABAergic synapses have been implied in many brain disorders, and mutations in MET are strong risk factors for autism, our findings urge for a further investigation of the role of MET at inhibitory synapses.