Trans-synaptic interaction with mGluR6 contributes to ELFN1 presynaptic enrichment in rod photoreceptors.

At the glutamatergic synapses between rod photoreceptors and ON-type bipolar cells, neurotransmitter is detected by the postsynaptic metabotropic glutamate receptor mGluR6. This receptor forms trans-synaptic interactions with ELFN1, a presynaptic cell adhesion molecule expressed in rods, and ELFN1 is important for mGluR6 localization at bipolar cell dendritic tips. Here, we show that in mice of either sex lacking mGluR6, the presynaptic localization of ELFN1 is disrupted. In rods of mGluR6 null mice, ELFN1 is still restricted to the axon terminal spherules, but is only partially co-localized with synapses. The ELFN1 localization defect is rescued by expressing mGluR6-EGFP in ON-bipolar cells. In vitro binding experiments demonstrated that the leucine-rich repeat (LRR) and LRR C-terminal cap (LRRCT) regions of the ELFN1 extracellular domain are necessary and sufficient for binding to all of the group III mGluRs, including mGluR6. ELFN1-flag expressed in rods of wild-type mice is correctly localized at presynapses, co-localizing with the postsynaptic marker TRPM1 in the outer plexiform layer. Deletion of the LRRCT domain abolished trafficking of ELFN1-flag to rod spherules, whereas deletion of other parts of the ELFN1 extracellular domain did not prevent axonal trafficking or correct presynaptic localization. Our results demonstrate bidirectional mutual regulation of presynaptic enrichment of ELFN1 and postsynaptic enrichment of mGluR6 at photoreceptor synapses.Significance statement Metabotropic glutamate receptors (mGluRs) play important roles at synapses throughout the central nervous system. The group III mGluRs participate in trans-synaptic interactions with ELFN synaptic adhesion molecules, which regulate synapse formation, mGluR recruitment, and mGluR function. In the retina, mGluR6 detects neurotransmitter at synapses between photoreceptors and depolarizing bipolar cells. Unlike conventional synapses, where postsynaptic ELFN1 interacts with presynaptic mGluRs, ELFN1 is located at presynapses in photoreceptors and interacts with postsynaptic mGluR6. ELFN1 knockout leads to mGluR6 mislocalization. Here, we show that loss of mGluR6 also disrupts ELFN1 localization. These results demonstrate a bidirectional role for the ELFN1-mGluR6 complex in mediating synaptic enrichment of both parties, which may have broad implications for formation and function of excitatory synapses.