A Two-Immunoglobulin-Domain Transmembrane Protein Mediates an Epidermal-Neuronal Interaction to Maintain Synapse Density.

Synaptic maintenance is essential for neural circuit function. In the C. elegans locomotor circuit, motor neurons are in direct contact with the epidermis. Here, we reveal a novel epidermal-neuronal interaction mediated by a two-immunoglobulin domain transmembrane protein, ZIG-10, that is necessary for maintaining cholinergic synapse density. ZIG-10 is localized at the cell surface of epidermis and cholinergic motor neurons, with high levels at areas adjacent to synapses. Loss of zig-10 increases the number of cholinergic excitatory synapses and exacerbates convulsion behavior in a seizure model. Mis-expression of zig-10 in GABAergic inhibitory neurons reduces GABAergic synapse number, dependent on the presence of ZIG-10 in the epidermis. Furthermore, ZIG-10 interacts with the tyrosine kinase SRC-2 to regulate the phagocytic activity of the epidermis to restrict cholinergic synapse number. Our studies demonstrate the highly specific roles of non-neuronal cells in modulating neural circuit function, through neuron-type-specific maintenance of synapse density.