Mechanisms of glutamate metabolic signaling in retinal glial (Muller) cells

Retinal Muller (glial) cells metabolize glucose to lactate, which is prefer entially taken up by photoreceptor neurons as fuel for their oxidative meta bolism. We explored whether lactate supply to neurons is a glial function c ontrolled by neuronal signals. For this, we used subcellular fluorescence i maging and either amperometric or optical biosensors to monitor metabolic r esponses simultaneously from mitochondrial and cytosolic compartments of in dividual Muller cells from salamander retina. Our results demonstrate that lactate production and release is controlled by the combined action of glut amate and NH4+, both at micromolar concentrations. Transport of glutamate b y a high-affinity carrier can produce in Muller cells a rapid rise of gluta mate concentration. In our isolated Muller cells, glutamine synthetase (GS) converted transported glutamate to glutamine that was released. This react ion, predominant when enough NH4+ is available, was limited at micromolar c oncentrations of NH4+, and more glutamate entered then as substrate into th e mitochondrial tricarboxylic acid cycle (TCA). Increased production of glu tamine by GS leads to increased utilization of ATP, some of which is genera ted glycolytically. Methionine sulfoximine, a specific inhibitor of GS, sup pressed the stimulatory effect of glutamate and NH4+ on glycolysis and indu ced massive entry of glutamate into the TCA cycle. The rate of glutamine pr oduction also determined the amount of pyruvate transaminated by glutamate to alanine. Lactate, alanine, and glutamine can be taken up and metabolized by photoreceptor neurons. We conclude that a major function of Muller glia l cells is to nourish retinal neurons and to metabolize the neurotoxic ammo nia and glutamate.