Abstract
Purpose :
Translocator protein 18 kDa (TSPO), a mitochondrial membrane protein, is predominantly expressed by non-neuronal cells in the retina. TSPO ligands, known for reducing microglial reactivity, enhance neuronal preservation upon retinal injury, though their exact mode of action remains unclear. TSPO seems primarily involved in cholesterol shuttling to the inner mitochondrial membrane, the initial site of steroidogenesis, hinting at neuroprotection via increased steroid hormone production. Previous studies demonstrated a beneficial effect of knockout (KO) of microglial TSPO, leading to the hypothesis that the neuroprotective effects of TSPO ligands are mainly mediated by Müller cells.
Methods :
To investigate Müller cell-specific TSPO functions in transient retinal ischemia/reperfusion, we utilized cell type-specific recombination for conditional TSPO knockout. We assessed retinal damage by immunohistochemistry and evaluated retinal function with electroretinography (ERG). Metabolic changes of TSPO-KO Müller cells were investigated using the JC-1 assay, to assess mitochondrial membrane potential (MMP), volumetric measurements in response to hypoosmolarity and fluorescence lifetime imaging of NAD(P)H, revealing changes in energy metabolism. Complementary single cell RNA sequencing provided insights into transcriptional alterations.
Results :
Post-ischemia, Müller cell TSPO-KO retinas show an increased neuron loss and impaired electrical response to light as compared to wild types. Furthermore, TSPO-KO Müller cells exhibited an altered physiology: (I) reduced MMP stress tolerance in homeostatic conditions, (ii) inability to compensate for hypoosmotic stress post-ischemia, and (iii) a metabolic switch from oxidative phosphorylation to glycolysis in response to retinal damage. Preliminary transcriptomic profiling indicates an increased fatty acid metabolism and stress response.
Conclusions :
Our results underscore the vital role of TSPO in Müller cells in maintaining retinal homeostasis and the cells’ adaptation to tissue damage. The loss of Müller cell TSPO negatively influences glial mitochondrial health, supporting the cell type-specific action of TSPO. Our findings suggest that indeed TSPO acts in a cell type-specific manner and that the neuroprotective effects of TSPO ligands are mediated through Müller cells.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.