Synapse maintenance refers to the process of preserving the number and pattern of synaptic connections, which is balanced by synapse formation and synapse elimination. Too few or too many synapses have been associated with various neurological disorders, such as autism spectrum disorders or schizophrenia. While the mechanisms of synapse formation during development have been widely investigated, how established synaptic connections are maintained for the lifetime of an organism is still under debate. Using the C. elegans locomotor circuit as a model system, we have previously shown that ZIG-10, an immunoglobulin-domain superfamily member, is required to constrain the density of excitatory synapses, but not inhibitory synapses (1). Here, we report two new signaling molecules differentially required for ZIG-10-mediated maintenance of excitatory synapses. We show that a novel guanylate kinase, MAGU-2, is required for ZIG-10 to restrict synapse density. Null mutants in
magu-2 display an increase in excitatory synapses, similar to
zig-10 null mutants, but double mutants of
magu-2 and
zig-10 display a similar number of synapses as compared to either single mutant. This suggests that MAGU-2 and ZIG-10 function in the same pathway. Our previous data showed that ZIG-10 localizes to the cell surface of excitatory neurons and epidermal cells. In the epidermis ZIG-10 activates the phagocytosis pathway to eliminate excitatory synapses. MAGU-2 displays a similar expression pattern to ZIG-10, but is only required in either neurons or epidermis. KCA-1 is a MAGU-2 interacting protein (2); however, in contrast to MAGU-2, a deletion mutation in
kca-1 reduces the excitatory synapse density in
zig-10 mutants. This suggests that in the absence of ZIG-10, KCA-1 drives excessive synapse formation. Therefore, we hypothesize that the ZIG-10 normally opposes KCA-1 to maintain proper synapse density. Overall, our data uncover how optimal neuronal connectivity is maintained by coordinating the formation and elimination of synapses. Further understanding of the ZIG-10/MAGU-2 pathway will provide new insights into how different cell types within the nervous system interact in order to maintain synaptic connections. 1. S. J. Cherra, 3rd, Y. Jin, A Two-Immunoglobulin-Domain Transmembrane Protein Mediates an Epidermal-Neuronal Interaction to Maintain Synapse Density. Neuron 89, 325-336 (2016). 2. T. Koorman et al., A combined binary interaction and phenotypic map of C. elegans cell polarity proteins. Nat Cell Biol 18, 337-346 (2016).