SIGNALING MECHANISMS OF RETINAL BIPOLAR CELLS

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The signaling pathway in retinal ON-bipolar originates with a unique metabotropic glutamate receptor, mGluR6, which is found exclusively on the dendrites of ON-bipolar cells (ON-BPCs). Via the G-protein GO, mGluR6 is coupled to the activity of an unidentified cation channel such that the light-induced decrease in glutamate causes an increase in channel activity and concomitant membrane depolarization. This sequence of events resembles the well studied signal transduction pathway of photoreceptor outer segments, in which photoexcitation of rhodopsin is coupled via the G-protein, transducin, to the closure of a cGMP-gated cation channel. In the outer segment, the kinetics of the light response is largely determined by the lifetime of activated transducin. In G-protein mediated responses, the rate-limiting step in the termination of the response is usually the hydrolysis of GTP by the G protein ¿ subunit. The hydrolytic process is accelerated by interaction with regulator of G-protein signaling (RGS) proteins. In photoreceptor outer segments, the light response is terminated by rapid deactivation of transducin by the G¿5-RGS9-R9AP complex, and mutations in the genes encoding these proteins severely impair vision by slowing recovery after light flashes. We have identified two similar complexes, G¿5-RGS7 and G¿5-RGS11 in ON-BPC dendrites. We have shown that R9AP is required for stable expression of G¿5-RGS11 in ON-bipolar cell dendrites. Furthermore, using the electroretinogram, we found that the G¿5-RGS11R9AP complex accelerates the initial ON-bipolar cell response to light.