Abstract
BackgroundLigand-driven modulation of the mitochondrial translocator protein 18 kDa (TSPO) was recently described to dampen the neuroinflammatory response of microglia in a retinal light damage model resulting in protective effects on photoreceptors. We characterized the effects of the TSPO ligand XBD173 in the postischemic retina focusing on changes in the response pattern of the major glial cell types of the retina—microglia and Müller cells.MethodsRetinal ischemia was induced by increasing the intraocular pressure for 60 min followed by reperfusion of the tissue in mice. On retinal cell types enriched via immunomagnetic separation expression analysis of TSPO, its ligand diazepam-binding inhibitor (DBI) and markers of glial activation were performed at transcript and protein level using RNA sequencing, qRT-PCR, lipid chromatography-mass spectrometry, and immunofluorescent labeling. Data on cell morphology and numbers were assessed in retinal slice and flatmount preparations. The retinal functional integrity was determined by electroretinogram recordings.ResultsWe demonstrate that TSPO is expressed by Müller cells, microglia, vascular cells, retinal pigment epithelium (RPE) of the healthy and postischemic retina, but only at low levels in retinal neurons. While an alleviated neurodegeneration upon XBD173 treatment was found in postischemic retinae as compared to vehicle controls, this neuroprotective effect of XBD173 is mediated putatively by its action on retinal glia. After transient ischemia, TSPO as a marker of activation was upregulated to similar levels in microglia as compared to their counterparts in healthy retinae irrespective of the treatment regimen. However, less microglia were found in XBD173-treated postischemic retinae at 3 days post-surgery (dps) which displayed a more ramified morphology than in retinae of vehicle-treated mice indicating a dampened microglia activation. Müller cells, the major retinal macroglia, show upregulation of the typical gliosis marker GFAP. Importantly, glutamine synthetase was more stably expressed in Müller glia of XBD173-treated postischemic retinae and homeostatic functions such as cellular volume regulation typically diminished in gliotic Müller cells remained functional.ConclusionsIn sum, our data imply that beneficial effects of XBD173 treatment on the postischemic survival of inner retinal neurons were primarily mediated by stabilizing neurosupportive functions of glial cells.
Highlights
Ligand-driven modulation of the mitochondrial translocator protein 18 kDa (TSPO) was recently described to dampen the neuroinflammatory response of microglia in a retinal light damage model resulting in protective effects on photoreceptors
We showed that a modified gliotic activation leading to a stable Kir4.1 channel function in reactive Müller cells is beneficial for the survival of inner retinal neurons in the postischemic retina [6]
translocator protein kDa (TSPO) upregulation in distinct retinal cell types of the ischemic retina Performing cell type-specific expression analysis at transcript and protein level from microglia, vascular cells, Müller glia, and retinal neurons (Fig. 1a), we found that TSPO is expressed at the highest levels in Müller glia and vascular cells in the healthy neuroretina (Fig. 1b)
Summary
Ligand-driven modulation of the mitochondrial translocator protein 18 kDa (TSPO) was recently described to dampen the neuroinflammatory response of microglia in a retinal light damage model resulting in protective effects on photoreceptors. Müller cells maintain the retinal homeostasis fulfilling a number of functions including ion and cell volume regulation, neurotransmitter recycling, and expression of neuroprotective factors [1,2,3]. They are highly responsive to pathological triggers, a reaction termed Müller cell gliosis. We demonstrated that this Müller cell gliosis includes changes in their electrophysiological properties (e.g., downregulation of Kir4.1 potassium channels) and a concomitant loss of their ability of volume regulation resulting in a constrained ion and volume homeostasis of the Müller cells with likely consequences on other retinal structures [2, 6]. We showed that a modified gliotic activation leading to a stable Kir4.1 channel function in reactive Müller cells is beneficial for the survival of inner retinal neurons in the postischemic retina [6]
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