Mechanisms of Glutamate Metabolic Signaling in Retinal Glial (Müller) Cells

Abstract

Retinal Müller (glial) cells metabolize glucose to lactate, which is preferentially taken up by photoreceptor neurons as fuel for their oxidative metabolism. We explored whether lactate supply to neurons is a glial function controlled by neuronal signals. For this, we used subcellular fluorescence imaging and either amperometric or optical biosensors to monitor metabolic responses simultaneously from mitochondrial and cytosolic compartments of individual Müller cells from salamander retina. Our results demonstrate that lactate production and release is controlled by the combined action of glutamate and NH(4)(+), both at micromolar concentrations. Transport of glutamate by a high-affinity carrier can produce in Müller cells a rapid rise of glutamate concentration. In our isolated Müller cells, glutamine synthetase (GS) converted transported glutamate to glutamine that was released. This reaction, predominant when enough NH(4)(+) is available, was limited at micromolar concentrations of NH(4)(+), and more glutamate entered then as substrate into the mitochondrial tricarboxylic acid cycle (TCA). Increased production of glutamine by GS leads to increased utilization of ATP, some of which is generated glycolytically. Methionine sulfoximine, a specific inhibitor of GS, suppressed the stimulatory effect of glutamate and NH(4)(+) on glycolysis and induced massive entry of glutamate into the TCA cycle. The rate of glutamine production 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 Müller glial cells is to nourish retinal neurons and to metabolize the neurotoxic ammonia and glutamate.

NH4+ triggers the release of astrocytic lactate via mitochondrial pyruvate shunting

“…It seems likely that NH + 4 may also have long-term roles in the control of astrocytic glycolysis and similar mechanisms could be present in other glial cells. For example, retinal Müller cells have been shown to increase their production of lactate in response to NH + 4 , but only in the presence of glutamate (49). Cooperativity might also help to explain the present observation that astrocytes in brain slices, likely exposed to some glutamate, were found to be more sensitive to NH + 4 than astrocytes in culture.…”

Mitochondria Contribute to NADPH Generation in Mouse Rod Photoreceptors

“…The results in the present paper indicate that, at least for the generation of NADPH, lactate may be an unlikely extracellular metabolic substrate for photoreceptors. A far more likely metabolic substrate appears to be glutamine, which can also be generated by Muller cells (51). Consistent with such a role, glutamine transport plays a significant role in vivo, because intravitreal injections of 4-CIN result in a decrease in glutamine metabolites in the photoreceptors (52).…”

Metabolic adaptation and neuroprotection differ in the retina and choroid in a piglet model of acute postnatal hypoxia

“…Interestingly, the increased lactate production in the retina may arise from an increased metabolic rate from Müller glial cells. Lactate shuttling from Müller glial cells to photoreceptors has been described to fuel mitochondrial oxidative metabolism and glutamate resynthesis in these cells (31,32). Thus, as it occurs from astrocytes to neurons, it is likely that transport of lactate to the neighboring cell is part of a symbiotic mechanism that favors metabolic competence in adverse (or energy demanding) conditions, in this specific case under an hypoxic event.…”