Mpp3 is Required for Maintenance of Adherens Junctions in the Retina during Light Exposure

Abstract

Purpose: : Membrane Palmitoylated Protein 3 (MPP3) is a member of the Membrane Associated Guanylate Kinase (MAGUK) family and is expressed in retina and brain. In the retina, MPP3 is localized at the subapical region adjacent to the outer limiting membrane (OLM) and in the outer plexiform layer. MPP3 and CRB1 interact directly with PALS1/MPP5 and may form a protein complex or mutual exclusive complexes since MPP3 does not bind CRB1. The aim of this study is to further unravel the role of MPP3 in the retina.

Methods: : We have generated Mpp3 conditional knockout (cKO) mice. We have crossed Mpp3 cKO mice with Rx-Cre mice, to specifically abolish Mpp3 in the retina. To investigate the effect of deletion of MPP3 on retinal morphology and localization of other proteins in the retina, we performed immunohistochemical, morphological and EM analyses on these retinas. Additionally, we have performed functional analyses using OCT, SLO and ERG. We have exposed six months old Mpp3 cKO mice to moderate levels of light to investigate whether this induced a more severe retinal degeneration.

Results: : In retinas from Mpp3^F/F RxCre^Tg/+ mice up to six months of age we detected a mild degeneration phenotype: At foci ectopic photoreceptor nuclei were detected in the subretinal space, suggesting that the integrity of the OLM might be compromised at these areas and occasionally rosettes were found. Exposure of six months old Mpp3^F/F RxCre^Tg/+ mice to moderate levels (3000 lux) of white light clearly increased the number of degenerative spots in the retina.Immunohistochemical analysis revealed that levels of PALS1 at the OLM seemed to be decreased in Mpp3^F/F RxCre^Tg/+ mice compared to control. Additionally, at areas where the integrity of the OLM was compromised we observed a loss of Nectin1 and other adherens junction markers.

Conclusions: : These data suggest that MPP3 is dispensable for proper morphological development of the retina, but might be required for stabilizing PALS1. Additionally, we hypothesize that MPP3, like CRB1, is required for the maintenance of adherens junctions during light exposure. These data suggest that Mpp3 might be a novel candidate gene for inherited retinopathies.

Keywords: retinal development • retinal degenerations: cell biology • retinitis