Molecular Mechanism of Cone Photoreceptor Death in Diseases of the Retina
Humans rely on cones for vision in daylight and on rods in very dim light. Many forms of blindness are due to mutations in genes that are expressed only in the rod photoreceptors. To look at models for these disorders, we have acquired several strains of mice suffering from loss of vision caused by a mutant gene expressed only in rods. However, these mice exhibit both rod and cone degeneration. In some models, the rods die very quickly, within 1 week, but much more slowly (as long as 6 months) in others. In each case, cones die after most of the rods have died. We have used microarrays to find genes that are up- or down-regulated relative to age-matched controls. We are particularly interested in those genes that change their level of expression during the onset of cone death in all of the models. The genes discovered to change to date are involved in retinal metabolism. This finding led to an examination of whether manipulating signaling through the mTOR pathway might alter the rate of cone death. We found that the rate of cone death can be changed through signaling via the insulin receptor. We are now continuing to study whether metabolic dysregulation might drive the cone death. In addition, as cones are in a hyperoxic environment after rods die, and show signs of oxidation, we have developed AAV gene therapy vectors to deliver genes that fight oxidation as a way to preserve cone activity and survival.
Cell Fate Determination in the Vertebrate Retina
We are interested in the mechanisms that direct development and degeneration of the central nervous system (CNS) of vertebrates. We are focusing our studies on the vertebrate retina, a relatively simple and well-characterized area of the CNS. We have used genomics approaches to characterize genes with dynamic temporal patterns to better determine which genes are candidates for playing a role in cell fate determination. We are now trying to determine how the retina uses this large repertoire of genes to form this complex tissue of >60 neuronal cell types. We are particularly interested in the diversification of the different types of interneurons because these cells form critical elements in retinal circuitry. In addition, we are interested in the mechanisms that direct formation of the photoreceptor cells because they are crucial for vision and are the target of many genetic diseases that lead to blindness. To study these processes, we perform gain- and loss-of-function studies using the genes that we have identified. In addition, we carry out lineage studies wherein we mark individual progenitor cells in vivo and analyze the types of neurons produced. Of interest is whether progenitor cells produce cells that are connected in various types of retinal circuits. We are also using them to learn more about retinal circuitry. Finally, we use an additional approach to study the regulatory elements in the genome that direct specific expression of particular genes to probe the genetic networks that produce the different types of retinal neurons.
Transsynaptic Viral Tracers
The connections among neurons within the central nervous system are so numerous, and complex, that it has been difficult to map and quantify them. We have been developing viral tools to allow one to map connections in vivo. The tools are based on vesicular stomatitis virus, which is a rapidly replicating virus similar to rabies virus. It can replicate and spread transsynaptically in mice and in a broad range of other organisms, including fish and birds. We have several projects aimed at further characterizing the activity of these tracers as well as expanding their capabilities.