Retinal bipolar cells provide the major conduit for the transmission of visual information from photoreceptors to the output neurons of the retina. They can release a brief burst of neurotransmitter to signal a change in contrast, and they release neurotransmitter continuously to provide an analog read-out of luminance. To perform these functions, they must maintain a stockpile of fusion-competent or primed synaptic vesicles. In the extreme case that this stockpile is depleted, vision would cease, while modest decrements would alter the encoding of visual information. These sequelae are offset by the Ca2+-accelerated recruitment of vesicles to the primed state. However, the mechanism(s) by which Ca2+ facilitates recruitment to the primed state at bipolar cell ribbon-style synapses is poorly-understood, and the unusual molecular composition of ribbon-style synapses prevents the simple extrapolation of information gleaned from other systems. At the same time, this unusual molecular composition presents the opportunity to identify novel mechanisms that can be exploited for therapeutic gain. In this research program, we consider the unique properties of two unusual proteins proposed to regulate bipolar cell neurotransmitter release, syntaxin3B and CaBP5, and use these as a guide to uncover novel Ca2+-dependent mechanisms that regulate vesicle priming and enable bipolar cells to maintain their critical signaling abilities. In additio, we will determine whether the newly identified pathways comprise the mysterious, alternate priming pathway of retinal ribbon-style synapses. To provide this information, we use a three-pronged approach. Experiments using state-of-the-art biophysical approaches in reduced preparations will allow us to perform in-depth, mechanistic studies. Experiments performed at the tissue level will allow us to study the regulation of a biochemical correlate of priming by these pathways in response to physiological stimuli. Studies performed at the circuit level will allow us to define the consequences of these pathways on synaptic transmission from bipolar cells to postsynaptic neurons. These approaches will be supported by the use of genetically-modified animals, designer peptides, phospho-specific antibodies and pharmacological manipulations. Results will yield new information about the Ca2+-dependent pathways in bipolar cells that regulate the transfer of visual information across the vertebrate retina and open the door to the selective modulation of the visual pathway.
In this research program, we identify molecular mechanisms that regulate synaptic output at retinal ribbon synapses. These synapses, essential for vision, relay the fundamental signals we use to perceive the visual world. Knowledge of their function is important for developing new strategies to restore useful vision in people with blinding disorders.
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