Activation of retinal Müller cells in response to glucose variability

Purpose In the earliest stages of diabetic retinopathy (DR), a dysfunction of Muller cells, characterized by high levels of glial fibrillary acidic protein (GFAP), and aquaporins (AQP), has been observed. Although chronic hyperglycemia causes the activation of Muller cells, the effect of glycemic fluctuations is yet unknown. The aim of the study was to analyze the impact of glucose variability on rat retinal Muller cells (rMC-1) adapted to either normal (5 mM) or high (25 mM) glucose levels. Methods rMC-1 were cultured in a medium containing either 5 mM (N cells) or 25 mM of glucose (H cells) and then incubated for 96 h in a medium containing (a) low glucose (either 1-3 or 5 mM), (b) basal glucose (either 5 or 25 mM), (c) high glucose (either 25 or 45 mM), (d) basal and high glucose alternated every 24 h; (e) low- and high glucose alternated every 24 h; (f) basal glucose with episodes of low glucose for 30 min twice a day. Muller cells activation was evaluated by measuring the levels of GFAP, AQP4, and phospho-active extracellular signal-regulated kinase (pERK). Results Under both basal and high glucose concentrations rMC-1 were viable, but their response to glucose excursions was different. In N cells kept under normal (5 mM) glucose, a significant glial activation was measured not only in response to constant high glucose but also to alternating low/high glucose. In H cells, adapted to 25 mM glucose, a significant response was observed only after exposition to a lower (5 mM) glucose concentration. Conclusion Our results highlight Muller cells activation in response to glucose variability and a different susceptibility depending on the basal glucose conditions.