Loss of Translocator Protein 18 kDa (TSPO) in the retinal pigment epithelium (RPE) of mice may lead to impaired visual function and abnormal RPE morphology

Abstract

Purpose : TSPO is upregulated in microglia, astrocytes and to some extent in neurons of damaged areas in the pathologic brain. Modulation of TSPO with specific ligands yields positive outcomes in paradigms of brain diseases and retinal degeneration. This beneficial effect of TSPO ligands is mainly attributed to the dampening of microglia activation. Recently, however, we found that TSPO is strongly and constitutively expressed also in the RPE. Here we address the role of TSPO in the mouse RPE for retinal homeostasis.

Methods : RPE-specific knock-out of Tspo was achieved by breeding BEST1-Cre with Tspo^flox/flox mice. Specificity of CRE activity was tested in reporter mice. Tspo^flox/flox;BEST1-Cre (RPE^ΔTspo), littermates Tspo^flox/flox (RPE^Ctrl) and BEST1-Cre mice were followed up for up to 6-months of age by fundus imaging, optical coherence tomography (OCT) and electroretinography (ERG). Morphology was analyzed by light microscopy on semi-thin plastic sections. Tspo knock-out was tested by immunofluorescence (IF). Gene expression was determined by real-time PCR.

Results : IF showed that TSPO was strongly expressed in the RPE of adult wild type mice. Tspo mRNA was detected in the RPE and neural retina as early as at post-natal day 1. Expression in the RPE increased during post-natal development. BEST1-Cre activity was restricted to the RPE but patchy, resulting in a mixed population of RPE cells that were or were not depleted of TSPO in RPE^ΔTspo mice. Fundus appearance and retinal layering were not affected but ERG recordings revealed a trend toward reduced b-wave amplitudes in RPE^ΔTspo mice. Although RPE^ΔTspo mice had a normal retinal morphology they developed droplet-like structures in the RPE. The number of affected cells and droplets per cells increased from 2- to 6-months of age.

Conclusions : Lack of TSPO in the mouse RPE might have an impact on retinal function without grossly affecting retinal morphology or fundus appearance. The development of droplet-like inclusions in the RPE of RPE^ΔTspo mice suggests an involvement of TSPO in RPE metabolism. Further characterization of the droplets may help to unveil the function of TSPO in the RPE and determine how it might influence retinal function.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.