Mammalian rod and cone photoreceptors are indispensible for vision. They convert light into electrical response, which is then propagated to the downstream neurons, the ON-bipolar cells. Deficits in synaptic communication with the ON-bipolar neurons are known to cause congenital stationary blindness in humans, a condition characterized by poor light sensitivity and frequent co-morbidity with many other ocular conditions. Our long-term goal is to elucidate molecular and cellular mechanisms of signal transmission at photoreceptor to ON-bipolar synapse with the hope to better understand blinding conditions and devising strategies for their treatment. During synaptic transmission, light-induced suppression of the neurotransmitter glutamate release from the photoreceptors is translated into the depolarization of the ON-BC by the prototypic G protein cascade that includes the receptor mGluR6, the G protein G?o and the effector channel TRPM1. In addition, several new synaptic proteins have emerged to be essential in the process but their functions are poorly understood. Increasing evidence suggests that the elements in this signaling cascade are scaffolded together into a ?signalosome? and that the central role in this organization is played by the mGluR6. We have found that mGluR6 signaling in the ON-bipolar neurons is synergistically regulated by two RGS proteins: RGS7 and RGS11 that play essential role in synaptic transmission. We demonstrated that RGS proteins integrate into the mGluR6 cascade via interactions with specialized adaptor proteins at the synapses. We have further discovered that mGluR6 signaling complex in ON-BC neurons is also physically integrated with presynaptic release machinery of photoreceptors via the interaction with a novel cell-adhesion molecule ELFN1. These observations lead to the central hypothesis of the proposal that precise synaptic communication of photoreceptors with the downstream ON-bipolar neurons requires assembly of the signaling complex where interactions between individual elements are tightly orchestrated. We plan on testing this hypothesis by pursuing two complementary Specific Aims that will: 1) determine the mechanisms governing integration of RGS complex into mGluR6 signaling cascade and 2) delineate the role of trans-synaptic coordination of mGluR6 complex with the photoreceptors. The strategy proposed to address these aims will entail a synergistic combination of biochemical, molecular biological, and physiological approaches, each exploiting the existence of a powerful array of reagents and animal models. Better understanding of the mGluR6 pathway regulation will yield important insights into the mechanisms of synaptic transmission and may suggest novel nodes of intervention for therapeutic strategies designed to treat inherited types of night blindness.
Normal vision is hinged on the ability of photoreceptors to transmit their light excitation to downstream neurons in the visual circuit. Failure of this synaptic communication is a leading cause of congenital forms of night blindness. The work proposed herein will yield a clearer understanding of molecular mechanisms by which photoreceptors communicate with other retina neurons. This is expected to increase our understanding of blinding retina diseases and could offer new strategies for their amelioration.
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