Trophoblast glycoprotein is required for efficient synaptic vesicle exocytosis from retinal rod bipolar cells

IntroductionRod bipolar cells (RBCs) faithfully transmit light-driven signals from rod photoreceptors in the outer retina to third order neurons in the inner retina. Recently, significant work has focused on the role of leucine-rich repeat (LRR) proteins in synaptic development and signal transduction at RBC synapses. We previously identified trophoblast glycoprotein (TPBG) as a novel transmembrane LRR protein localized to the dendrites and axon terminals of RBCs.MethodsWe examined the effects on RBC physiology and retinal processing of TPBG genetic knockout in mice using immunofluorescence and electron microscopy, electroretinogram recording, patch-clamp electrophysiology, and time-resolved membrane capacitance measurements.ResultsThe scotopic electroretinogram showed a modest increase in the b-wave and a marked attenuation in oscillatory potentials in the TPBG knockout. No effect of TPBG knockout was observed on the RBC dendritic morphology, TRPM1 currents, or RBC excitability. Because scotopic oscillatory potentials primarily reflect RBC-driven rhythmic activity of the inner retina, we investigated the contribution of TPBG to downstream transmission from RBCs to third-order neurons. Using electron microscopy, we found shorter synaptic ribbons in TPBG knockout axon terminals in RBCs. Time-resolved capacitance measurements indicated that TPBG knockout reduces synaptic vesicle exocytosis and subsequent GABAergic reciprocal feedback without altering voltage-gated Ca2+ currents.DiscussionTPBG is required for normal synaptic ribbon development and efficient neurotransmitter release from RBCs to downstream cells. Our results highlight a novel synaptic role for TPBG at RBC ribbon synapses and support further examination into the mechanisms by which TPBG regulates RBC physiology and circuit function. Proposed model for the effects of TPBG-KO on synaptic vesicle exocytosis and reciprocal feedback in RBC axon terminals. (Left) In a WT RBC, membrane depolarization opens voltage-gated Ca2+ channels. The influx of Ca2+ triggers vesicle exocytosis and glutamate release into the synaptic cleft. This glutamate is detected by downstream AII-ACs and A17-ACs. The A17-AC then releases GABA into the synaptic cleft which hyperpolarizes the RBC. (Right) In a TPBG-KO RBC, Ca2+ influx is normal. However, in the absence of TPBG, synaptic vesicle exocytosis is reduced, resulting in reduced ERG OPs and reciprocal feedback.