Light is a profoundly important regulator of biology and behavior in a variety of organisms. For decades, rods and cones were thought to be the only photosensitive cells in the retina, conveying light signals to the brain for visual processing via retinal ganglion cells (RGCs), the only projection neurons from the retina. However, recent results indicate that a small population of RGCs are intrinsically photosensitive (ip)RGCs (~2-4% of RGCs) and express the photopigment melanopsin (Opn4). These atypical photoreceptive ganglion cells subserve primarily non-image forming (NIF) functions such as circadian photoentrainment and the pupillary light reflex (PLR). There is now substantial evidence that ipRGCs of diverse morphological, electrophysiological and molecular origins mediate distinct behaviors. However, the developmental processes giving rise to the ipRGCs that mediate these distinct behaviors are poorly understood. A series of organized processes occur during development in which progenitor cells generate all retinal cell types through the expression of various transcription factors at discrete timepoints. Math5 (Atoh7) is a basic helix-loop-helix proneural transcription factor that is necessary for mammalian RGC determination. Recent results indicate that, in contrast to conventional RGCs, ipRGCs continue to be generated in the developing retina long after Math5 is downregulated, indicating that determination of some ipRGCs may be Math5 independent (referred to as non-Math5). Given that ipRGCs are also photoreceptors, it is possible that at least some of the non-Math5 ipRGCs rely on transcription factors responsible for specifying the classical photoreceptors, rods and cones, during development. The central goal of this proposal is to characterize the developmental programs that determine ipRGC fate. I will test the specific hypothesis that ipRGCs develop from distinct molecular lineages to mediate distinct functions. My preliminary data indicate that only 50% of ipRGCs arise from the Math5 lineage and that non- Math5 ipRGCs express the transcription factor Crx, a transcription factor critical for development of the classical photoreceptors. These preliminary data challenge the long-held dogma that Math5 is necessary for the development of all RGCs and reveal the unexpected finding that ipRGCs may develop from transcription factors specifying both RGCs (Math5) and photoreceptors (Crx). In addition, if Crx contributes to ipRGC development, this will challenge the current belief that mammalian RGCs and photoreceptors develop from independent lineages.
The research outlined in this proposal seeks to identify the developmental programs that give rise to a specialized type of ganglion cell in the retina called intrinsically photosensitive retinal ganglion cells. These cells are critical for mediation of functions such as photoentrainment of the circadian rhythm and the pupillary light reflex. Specifically, this research seeks to identify factors involved in development of these cells and whether cells that develop from distinct programs mediate distinct behaviors.
|Schmidt, Tiffany M; Alam, Nazia M; Chen, Shan et al. (2014) A role for melanopsin in alpha retinal ganglion cells and contrast detection. Neuron 82:781-8|