We derive most of our information about the world through our visual system by means of rod and cone photoreceptors. The human retina has one type of rod for dim light vision, and three types of cone cells that allow color discrimination. Despite the similarities in their morphology and mechanism of light detection, rods and cones have distinct functional properties. Rods are extremely sensitive (they can detect a single photon of light!) and thus are perfectly suited for dim light conditions. They saturate in moderate light, however, and are not able to respond to light through most of the day. Cones, on the other hand, are 30-100 times less sensitive than rods and do not respond to light in dim light conditions (this is why we don’t see colors at night). However, cones are capable of adapting to an extremely wide range of light conditions, rendering them perfectly suited as our daytime photoreceptors. Our lab is interested in the mechanisms that determine the functional properties of mammalian rod and cone photoreceptors. We use a battery of tools, from single-cell and isolated retina recordings, to live electroretinogram and behavior experiments with wild type and genetically modified mice. While the emphasis of our studies is on our daytime photoreceptors, the cones, we are also investigating some aspects of rod phototransduction. Some of the ongoing projects in the lab are:
Cone photoreceptors function under daylight conditions and are essential for color perception and vision with high temporal and spatial resolution. One of the fundamental mysteries of the human visual system is the continuous function of cone photoreceptors in bright daylight and t heir rapid dark adaptation. As visual pigment is destroyed, or bleached, by light, cones require its rapid regeneration, which in turn involves rapid recycling of the pigment’s chromophore. We have recently demonstrated the function of a novel pathway in the amphibian, mouse, primate, and human retinas that promotes such rapid recycling of chromophore, pigment regeneration, and dark adaptation specifically in cones, but not in rods. This mechanism is independent of the pigment epithelium and instead relies on the glial (Mller) cells in the retina. We are currently characterizing this pathway in the mammalian retina and the mechanisms that restrict its function to cones. This research is funded by an RO1 grant from the National Eye Institute.
The ability of cones to adapt over a wide range of light intensities is critical for their function as our daytime photoreceptors. Their wide dynamic range implies the existence of powerful adaptation mechanisms of the cone phototransduction cascade. Photoreceptor light adaptation is mediated by the decline in calcium upon photoactivation. However, the molecular mechanisms by which lowering calcium exerts negative feedback on cone phototransduction are not known. Using physiological tools and genetically modified mice, we are investigating how mammalian cone phototransduction is modulated by the calcium-binding proteins recoverin and guanylyl cyclase activating proteins (GCAPs). We are investigating how the deletion of each of these proteins affects the photoresponse gain and kinetics in mouse cones in darkness as well as their ability to adapt to background light. This research is funded by an R21 grant from the National Eye Institute.
Mutations in several of the genes involved in the turnover of chromophore can cause a delayed clearance of all-trans retinal from photoreceptors following exposure to bright light. Both all-trans retinal and its aggregate N-retinylidene-N-retinylethanolamine (A2-E) are believed to cause retinal degenerative disorders. In humans, mutations in the photoreceptor-specific ABC chromophore transporter (ABCR, also known as ABCA4), located in the photoreceptor outer segments, lead to impaired visual function, including slower rod dark adaptation and accumulation of A2-E and lipofuscin. Such mutations have been associated with visual disorders including autosomal recessive Stargardt disease and cone-rod dystrophy. As part of a multi-institutional project, we are currently working on developing pharmacological treatments targeting the turnover of chromophore in an effort to prevent its accumulation and retinal degeneration. This research is funded by an R24 grant from the National Eye Institute.
In addition to the projects detailed above, we are also involved in a number of collaborative projects investigating various aspects of rod and cone phototransduction. Some of these include:
- The retina visual cycle in the all-cone Nrl knockout and hybrid-rod Nr2e3 knockout mouse retinas (in collaboration with Joe Corbo from Department of Pathology & Immunology and Carter Cornwall from Boston University School of Medicine, Boston);
- The functional expression of rod or cone transducin alpha in gene therapy and comparison of the signaling properties of rod and cone transducins (in collaboration with William Hauswirth from the University of Florida, Gainesville);
- The role of rod transducin gamma in phototransduction (in collaboration with Oleg Kisselev from Saint Louis University, Saint Louis);
- The role of apo-opsin phosphorylation by GRK in preventing retinal degeneration (in collaboration with Rosalie Crouch from Medical University of South Carolina, Charleston);
- The role of pigment phosphorylation by G-protein receptor kinase (GRK) in determining the rate-limiting step for photoresponse termination in rods and cones (in collaboration with Shahrokh Khani from the Schepens Eye Institute, Boston);
- The role of Rp1-like protein (Rp1L1) in mouse rod formation, signaling, and degeneration (in collaboration with Jian Zuo from St. Jude Children’s Research Hospital, Memphis).
- Razafsky D., Ward C., Potter C., Zhu W., Xue Y., Kefalov V.J., Fong L.G., Young S.G., Hodzic D. (2016) Lamin B1 and lamin B2 are long-lived proteins with distinct functions in retinal development. Mol Biol Cell 27:1928-37. PDF
- Frederiksen R., Nymark S., Kolesnikov A.V., Berry J.D., Adler L., Koutalos Y., Kefalov V.J., Cornwall M.C. (2016) Rhodopsin kinase and arrestin binding control the decay of photoactivated rhodopsin and dark adaptation of mouse rods. J Gen Physiol. 148:1-11.Sato S, Kefalov VJ. (2016) PDF
- SatoS., Kefalov V.J. cis Retinol oxidation regulates photoreceptor access to the retina visual cycle and cone pigment regeneration. J Physiol. 594:6753-6765. PDF
- Sakurai K., Vinberg F., Wang T., Chen J., Kefalov V.J. (2016) The Na+/Ca2+, K+ exchanger 2 modulates mammalian cone phototransduction. Sci Rep 6:32521. doi: 10.1038/srep32521. PDF
Postdoctoral Position Available
An NIH-funded postdoctoral position is available immediately to study the physiological properties of mammalian photoreceptors. We are seeking an energetic, highly motivated PhD, MD, or MD/PhD with solid publication record and vision research-related experience in neuroscience, electrophysiology, biophysics, or biochemistry. Experience in molecular biology is a plus and good analytical and communication skills are essential. The specific project will be tailored to the interests and qualifications of the applicant, but will focus broadly on using electrophysiological tools to study the phototransduction cascade, adaptation, and the visual cycle of mammalian photoreceptors, and their link to retinal degeneration and visual disorders. For details on recent research click the publications tab. The successful candidate will have access to state-of-the-art lab equipment and departmental core facilities, as well as to the great research community in one of the top medical schools in the USA. To apply, please email cover letter, curriculum vitae, and list of references to: Dr. Vladimir Kefalov (firstname.lastname@example.org), Ophthalmology & Visual Sciences, Washington University School of Medicine, 660 S. Euclid Ave, Box 8096, Saint Louis, MO 63110.
Student Position Available
We are always happy to welcome bright and motivated students. We are a small but rather productive and collaborative lab that offers friendly and supportive environment for mastering the art of visual neuroscience and photoreceptor electrophysiology. Funding is available for selected students dissertation research. Please contact Vladimir if you are interested in discussing a rotation in the lab.