LRRTM4: A Novel Regulator of Presynaptic Inhibition and Ribbon Synapse Arrangements of Retinal Bipolar Cells

LRRTM4 is a transsynaptic adhesion protein regulating glutamatergic synapse assembly on dendrites of central neurons. In the mouse retina, we find that LRRTM4 is enriched at GABAergic synapses on axon terminals of rod bipolar cells (RBCs). Knockout of LRRTM4 reduces RBC axonal GABA(A) and GABA(c) receptor clustering and disrupts presynaptic inhibition onto RBC terminals. LRRTM4 removal also perturbs the stereotyped output synapse arrangement at RBC terminals. Synaptic ribbons are normally apposed to two distinct postsynaptic "dyad" partners, but in the absence of LRRTM4, "monad" and "triad" arrangements are also formed. RBCs from retinas deficient in GABA release also demonstrate dyad mis-arrangements but maintain LRRTM4 expression, suggesting that defects in dyad organization in the LRRTM4 knockout could originate from reduced GABA receptor function. LRRTM4 is thus a key synapse organizing molecule at RBC terminals, where it regulates function of GABAergic synapses and assembly of RBC synaptic dyads.