Insights from a metabolic chimera: Pyruvate Kinase and Aspartate-Glutamate Carrier distributions reveal key metabolic links between Neurons and Glia in the retina.

Abstract

The development of ophthalmology and vision research has established an abundance of knowledge on the process of vision. The study of the retina has contributed information about neuronal and biochemical signaling mechanisms that convert the detection of light to a neuronal signal. However, what fuels the process of photo-transduction remains unknown. Perhaps this is what makes the observation that so many metabolic diseases have the unusual distinction of leading to retinal degeneration so intriguing. The goal of my research has been to understand the unique metabolic regulation of the photoreceptor to establish what implications this has for the basal health of the retina. Aerobic glycolysis is a metabolic phenomenon typically associated with tumor cells which has also been observed within the retina. This metabolic process has been attributed to expression of a unique isoform of pyruvate kinase, PKM2. In order to determine how aerobic glycolysis influences overall retina metabolism, we sought to determine the localization of PKM2 within the retina and thereby which cells within the retina are reliant on aerobic glycolysis. Our work revealed a unique localization of PKM2 to photoreceptor cells and exclusion of pyruvate kinase in Müller glia of the retina. Combined with the complimentary expression of a glutamate aspartate transporter in the photoreceptor and absence from the Müller glia, the distribution of these enzymes are well positioned to shuttle lactate from glycolytic photoreceptors and drive glutamate turnover in the Müller glia. We established through studies of retina metabolic flux that the demand for aerobic glycolysis in the photoreceptor cell is used to fuel the neuronal glutamate/glutamine cycle that is essential to neuronal signaling within the retina. In the course of our studies of retina metabolism, we sought to optimally culture isolated photoreceptor cells and resolve what is required for outer-segment retention and preservation of photoreceptor cell function. Following up on previous observations that Dark Adaptation improves outer segment retention, we tested the effects of light adaptation on isolated outer-segments. Our experiments indicated that outer segment morphology was unaffected by light adaptation, but could be modified by culture conditions. Preliminary experiments with optical tweezers revealed a previously unobserved tether on photoreceptor outer segments that may introduce a tension component necessary for maintenance of outer segment morphology and photoreceptor cell synthesis of outer segment discs. We believe that refinement of these techniques for cultivation of primary intact photoreceptor cells will allow us to delve further into the unique metabolism of the individual cell types of the retina and how the individual demands of cells in the retina may impact the overall effects of metabolic catastrophe on retina degeneration.