GARLH regulates neuroligin preference for excitatory versus inhibitory synapses

Synaptic specificity is governed by precise combinations of cell adhesion proteins that stabilize pre- and postsynaptic sites and appropriate neurotransmitter receptors. The postsynaptic neuroligins NL1/3 and NL2/3/4 localize to excitatory and inhibitory synapses, respectively, and regulate the corresponding neurotransmitter receptors. However, the exact molecular mechanisms that determine synaptic specificity via defined combinations of neuroligins and neurotransmitter receptors remain unclear. We found that all neuroligin isoforms form a tripartite complex with GABA_A receptors and GARLH4 protein, with isoform-specific preferences, and that NL1, previously thought to be restricted to excitatory synapses, is also present at inhibitory synapses. In the absence of inhibitory synapse-specific NL2/4, NL1/3 increasingly assembles with GARLH4/GABA_A receptors and relocates to inhibitory synapses. Moreover, forced interaction between NL1 and GARLH4 redirects their localization to inhibitory synapses. These findings demonstrate that GARLHs regulate the synaptic specificity of neuroligins, providing the key link between neuroligins and inhibitory GABA_A receptors.