Overexpression of the Inwardly Rectifying Potassium Channel Kir4.1 or Kir4.1 Tyr9 Asp in Müller Cells Exerts Neuroprotective Effects in an Experimental Glaucoma Model

Downregulation of the inwardly rectifying potassium channel Kir4.1 is a key step for inducing retinal Müller cell activation and interaction with other glial cells, which is involved in retinal ganglion cell apoptosis in glaucoma. Modulation of Kir4.1 expression in Müller cells may therefore be a potential strategy for attenuating retinal ganglion cell damage in glaucoma. In this study, we identified seven predicted phosphorylation sites in Kir4.1 and constructed lentiviral expression systems expressing Kir4.1 mutated at each site to prevent phosphorylation. Following this, we treated Müller glial cells in vitro and in vivo with the mGluR I agonist DHPG to induce Kir4.1 or Kir4.1 Tyr9Asp overexpression. We found that both Kir4.1 and Kir4.1 Tyr9Asp overexpression inhibited activation of Müller glial cells. Subsequently, we established a rat model of chronic ocular hypertension by injecting microbeads into the anterior chamber and overexpressed Kir4.1 or Kir4.1 Tyr9Asp in the eye, and observed similar results in Müller cells in vivo as those seen in vitro. Both Kir4.1 and Kir4.1 Tyr9Asp overexpression inhibited Müller cell activation, regulated the balance of Bax/Bcl-2, and reduced the mRNA and protein levels of pro-inflammatory factors, including interleukin-1β and tumor necrosis factor-α. Furthermore, we investigated the regulatory effects of Kir4.1 and Kir4.1 Tyr9Asp overexpression on the release of pro-inflammatory factors in a co-culture system of Müller glial cells and microglia. In this co-culture system, we observed elevated adenosine triphosphate concentrations in activated Müller cells, increased levels of translocator protein (a marker of microglial activation), and elevated interleukin-1β mRNA and protein levels in microglia induced by activated Müller cells. These changes could be reversed by Kir4.1 and Kir4.1 Tyr9Asp overexpression in Müller cells. Kir4.1 overexpression, but not Kir4.1 Tyr9Asp overexpression, reduced the number of proliferative and migratory microglia induced by activated Müller cells. Collectively, these results suggest that the tyrosine residue at position nine in Kir4.1 may serve as a functional modulation site in the retina in an experimental model of glaucoma. Kir4.1 and Kir4.1 Tyr9Asp overexpression attenuated Müller cell activation, reduced ATP/P2X receptor-mediated interactions between glial cells, inhibited microglial activation, and decreased the synthesis and release of pro-inflammatory factors, consequently ameliorating retinal ganglion cell apoptosis in glaucoma.