Abstract
Purpose:
Lactate is an important metabolite that is a critical energy source for retinal neurons. Extracellular lactate increases retinal blood flow via angiogenesis and vasodilation. Lactate shuttle is also critical for retinal function: glucose is metabolized into lactate in Muller cells and the Muller-cell-derived lactate serves as the energy source for retinal neurons. Recently, a cell-surface receptor for lactate has been described in non-retinal tissues. The receptor, GPR81, is linked to G-proteins Gi/Go, eliciting intracellular signaling with a decrease in cAMP and/or an increase in calcium. Given the critical role for lactate in retina, we examined the expression and regulation of Gpr81 in mouse retina to determine the relevance of the receptor to retinal lactate biology.
Methods:
Gpr81 expression was examined by qPCR and immunofluorescence (IF). Neuronal localization of the receptor was determined by assessing co-localization with the neuronal marker NeuN. Polarized distribution of Gpr81 in RPE was assessed using the monocarboxylate transporter MCT1 as a marker for RPE apical membrane. Relative expression of Gpr81 in primary cultures of mouse RPE, Muller cells, and retinal ganglion cells was also studied. Retinal Gpr81 expression was investigated in mouse models of hemochromatosis and streptozotocin (STZ)-induced diabetes.
Results:
Gpr81 is expressed in mouse retina in all cell types. The expression is comparable in RPE and ganglion cells but lower in Muller cells. The expression co-localizes with NeuN in ganglion cells. In RPE, the expression is restricted to the apical membrane. Gpr81 expression is increased in retina in two different mouse models of hemochromatosis: Hfe-/- mouse and Hjv-/- mouse, and the increase is higher in Hjv-/- mouse retina than in Hfe-/- mouse retina. STZ-induced diabetes is also associated with a marked upregulation of Gpr81 in the retina.
Conclusions:
Gpr81 is expressed widely in murine retina, suggesting extracellular actions of lactate in target cells through a cell-surface receptor. The apical membrane localization in RPE suggests that it is the retinal lactate, not the circulating lactate, that is relevant to Gpr81 signaling in this cell. The upregulation in hemochromatosis and diabetes indicates that lactate/Gpr81 signaling is likely a critical determinant in the retinal pathology of iron overload and diabetes.
Keywords: 498 diabetes •
688 retina •
674 receptors