One of the fundamental tasks of basic retina research is to understand how the retina - a thin sheet of light- sensitive tissue at the back of te eye - is responsible for processing and transmitting signals arising from the absorption of photons. This process begins at the very first synapse in the retina, which transmits signals from rod and cone photoreceptors to second-order neurons. The signaling capabilities of photoreceptors depend the synaptic ribbon, a specialized structure found in a variety of neurosensory cells responsible for providing a supply of primed vesicles to support signaling by tonic vesicle release. The objective of this proposal is to explore the fundamental signaling capabilities of the cone ribbon synapse by examining how synaptic ribbons are replenished with vesicles and how that replenishment mechanism contributes to early stages of visual processing in the retina. Previous studies have described the ways in which retinal ribbon synapses are capable of signaling contrast and luminance in a manner dependent on the kinetics of synaptic vesicle replenishment. Recent work in the Thoreson lab has shown that the kinetics of replenishment at the cone ribbon is also accelerated by calcium (Ca2+), pointing to a Ca2+-dependent mechanism for regulating the encoding of photoreceptor signals. At the Calyx of Held, an auditory relay synapse, acceleration of replenishment by calcium depends on the signaling molecule calmodulin. Calmodulin may similarly regulate the Ca2+- dependence of replenishment at photoreceptor synapses, but this possibility has yet to be tested.
Aim 1 will test the hypothesis that calmodulin is responsible for a fast, Ca2+-dependent mechanism of vesicle replenishment at the cone ribbon. Additionally, replenishment has been suggested as a major determinant of kinetic-encoding responses by the photoreceptor synapse, yet this also remains untested.
Aim 2 will test the hypothesis that a fast calmodulin-dependent mode of replenishment is responsible for encoding and transmitting the timing of visual responses. These goals will be accomplished using a variety of electrophysiological techniques such as electroretinogram and single and paired whole-cell recordings as well as with live imaging techniques such as confocal calcium imaging, TIRF microscopy, and single-particle tracking with quantum dots to assess synaptic signaling by cones. Specific pharmacological compounds, delivered directly to the photoreceptor or applied to the entire retina, will be used to manipulate signaling by calmodulin and Ca2+-dependent processes. Proper encoding and transmission of signals by photoreceptors is crucial to vision. Understanding retinal function in health as well as disease is important when implementing therapeutic approaches that integrate with the existing retinal network such as stem cells or retinal implants. Moreover, an understanding of the mechanisms regulating vesicle resupply and trafficking in photoreceptors, as this proposal seeks to provide, is important in understanding the pathophysiology of several retinal degenerative diseases, as synaptic proteins with roles in the neurotransmission have been implicated in various retinopathies.

Public Health Relevance

The goal of this project is to study signaling by photoreceptor neurons in the vertebrate retina, which perform the essential first step in vision by detecting light, converting it to a neural signal, and transmitting it to other retinal neurons before it is ultimately routed to the brain for conscious visual perception. This project builds on other recent research and explores the specific mechanisms used by photoreceptors to encode and transmit this signal by testing for the involvement of one specific molecule called calmodulin in regulating these processes. Because disruption of the mechanisms regulating signaling by photoreceptors are involved in diseases of the retina that ultimately lead to blindness, and because properly treating retinal disease relies on an understanding of the healthy retina, these studies are relevant to public health and potential therapeutic treatments.

Agency
National Institute of Health (NIH)
Institute
National Eye Institute (NEI)
Type
Postdoctoral Individual National Research Service Award (F32)
Project #
1F32EY023864-01A1
Application #
8706596
Study Section
Special Emphasis Panel (ZRG1)
Program Officer
Agarwal, Neeraj
Project Start
2014-07-01
Project End
2017-06-30
Budget Start
2014-07-01
Budget End
2015-06-30
Support Year
1
Fiscal Year
2014
Total Cost
Indirect Cost
Name
University of Nebraska Medical Center
Department
Ophthalmology
Type
Schools of Medicine
DUNS #
City
Omaha
State
NE
Country
United States
Zip Code
68198
Wen, Xiangyi; Van Hook, Matthew J; Grassmeyer, Justin J et al. (2018) Endocytosis sustains release at photoreceptor ribbon synapses by restoring fusion competence. J Gen Physiol 150:591-611
Van Hook, Matthew J; Babai, Norbert; Zurawski, Zack et al. (2017) A Presynaptic Group III mGluR Recruits G??/SNARE Interactions to Inhibit Synaptic Transmission by Cone Photoreceptors in the Vertebrate Retina. J Neurosci 37:4618-4634
Thoreson, Wallace B; Van Hook, Matthew J; Parmelee, Caitlyn et al. (2016) Modeling and measurement of vesicle pools at the cone ribbon synapse: Changes in release probability are solely responsible for voltage-dependent changes in release. Synapse 70:1-14
Warren, Ted J; Van Hook, Matthew J; Tranchina, Daniel et al. (2016) Kinetics of Inhibitory Feedback from Horizontal Cells to Photoreceptors: Implications for an Ephaptic Mechanism. J Neurosci 36:10075-88
Warren, Ted J; Van Hook, Matthew J; Supuran, Claudiu T et al. (2016) Sources of protons and a role for bicarbonate in inhibitory feedback from horizontal cells to cones in Ambystoma tigrinum retina. J Physiol 594:6661-6677
Cork, Karlene M; Van Hook, Matthew J; Thoreson, Wallace B (2016) Mechanisms, pools, and sites of spontaneous vesicle release at synapses of rod and cone photoreceptors. Eur J Neurosci 44:2015-27
Chen, Minghui; Van Hook, Matthew J; Thoreson, Wallace B (2015) Ca2+ Diffusion through Endoplasmic Reticulum Supports Elevated Intraterminal Ca2+ Levels Needed to Sustain Synaptic Release from Rods in Darkness. J Neurosci 35:11364-73
Van Hook, Matthew J; Thoreson, Wallace B (2015) Weak endogenous Ca2+ buffering supports sustained synaptic transmission by distinct mechanisms in rod and cone photoreceptors in salamander retina. Physiol Rep 3:
Plog, Stephanie; Klymiuk, Nikolai; Binder, Stefanie et al. (2015) Naturally Occurring Deletion Mutants of the Pig-Specific, Intestinal Crypt Epithelial Cell Protein CLCA4b without Apparent Phenotype. PLoS One 10:e0140050
Van Hook, Matthew J; Parmelee, Caitlyn M; Chen, Minghui et al. (2014) Calmodulin enhances ribbon replenishment and shapes filtering of synaptic transmission by cone photoreceptors. J Gen Physiol 144:357-78

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