Lactate receptor GPR81 modulates the integrated stress response in the developing outer retina

Abstract

Purpose : The retinal pigment epithelium (RPE) plays a critical role in maintaining the homeostasis of the outer retina. Our recent observations have shown that the lactate receptor G-protein coupled receptor 81 (GPR81) is abundantly expressed in the RPE. Interestingly, GPR81KO primary RPE cells undergo apoptosis in prolonged serum starvation while this does not occur in RPE cells expressing GPR81. Mechanisms involved in this process are still unknown. To our knowledge, prolonged starvation could induce the integrated stress response (ISR). Herein, we investigated whether GPR81 in RPE cells is implicated in modulating stress response and the consequences of its deletion in the outer retina by using GPR81KO mice.

Methods : GPR81KO and wild type (WT) mice were generated on a background of C57BL/6J mice. Choroidal vasculature thickness was evaluated in eye cryosections from GPR81 KO and WT mice stained with lectins at different ages. Pro-angiogenic response of lactate was evaluated in ex-vivo (choroid sprouting assay) and in vivo (intravitreal injections) in the outer retina from GPR81KO and WT mice. Gene expression of factors and protein levels were evaluated by qPCR and Western blot in primary RPE cultures under serum starvation, and in choroidal/RPE complex from GPR81KO and WT mice.

Results : Choroidal vascular thickness was significantly (p<0.05) reduced in GPR81KO compared to WT mice. Intravitreal injections of lactate significantly (p<0.01) increased the choroidal vascular thickness in WT mice but not in GPR81KO mice. Pro-angiogenic effects of lactate also occurred in the choroidal sprouting assay. Interestingly, choroidal/RPE complex from GPR81KO showed a reduced histone deacetylase (HDACs) activity and higher levels of histone acetylation associated to an elevated expression of genes implicated in the ISR. Furthermore, the translation initiation factor eIF2α, a master regulator of cell adaption to various forms of stress, was significantly phosphorylated, while the downstream activating translation factor 4 (ATF4) and its target genes including Trib3 and Chop were significantly elevated, suggesting that the ISR pathway eIF2α/ATF4 is activated in the absence of GPR81 receptor.

Conclusions : These findings suggested that GPR81 located in RPE cells is implicated in the regulation of the integrated stress response and also plays an important role in maintaining the integrity of the choroidal vasculature.

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