Notch signaling pathway regulates proliferation and differentiation of immortalized Müller cells under hypoxic conditions in vitro

Previous studies have indicated that Müller glia in chick and fish retinas can re-enter the cell cycle, express progenitor genes, and regenerate neurons via the Notch signaling pathway in response to retinal damage or growth factors. Here, we investigated the role of Notch signaling and the effect of hypoxia, as a means to induce retinal damage, on the proliferation of an immortalized Müller cell line (rMC-1 cells). Our data showed that rMC-1 cells expressed Müller glia and neural and retinal progenitor markers but did not express neuronal or retinal markers. Hypoxia increased rMC-1 cell proliferation by activating the positive cell-cycle regulators, cyclins A and D1, as well as the neural and retinal progenitor markers, Notch1, Hes1, nestin, Sox2, Msi1, Pax6, and NeuroD1. However, hypoxia did not significantly influence the expression of Müller glial markers GS, CRALBP, and cyclin D3 or the death of the rMC-1 cells. The increase in cell proliferation induced by hypoxia was greatly attenuated by blocking Notch signaling with the inhibitor DAPT, resulting in the reduced expression of positive cell-cycle regulators (cyclins A and D1) and neural and retinal progenitor markers (Notch1, Hes1, Sox2, Pax6, and NeuroD1). Blockade of the Notch signaling pathway by DAPT after hypoxia promoted the differentiation of rMC-1 cells to neurons, as demonstrated by the induction of neural marker (Tuj1), retinal amacrine (Syntaxin1), and retinal ganglion cell (Brn3b) markers, although the expression of the latter marker was low. Taken together, our data indicate that Notch signaling is required for proliferation under hypoxic conditions either by activating the positive cell-cycle regulators or by skewing their de-differentiation towards a neural progenitor lineage. These findings indicate that the Notch signaling pathway regulates hypoxia-induced proliferation and differentiation of Müller glia.