The Effects of Lactate Receptor G Protein-Coupled Receptor 81 (GPR81) on the Integrity of the Choroidal Vasculature

Rather than simply a body waste, lactate exhibits various biological activities, including pro-angiogenesis, anti-inflammation, and anti-lipolysis functions, via its receptor G protein-coupled receptor GPR81 (i.e., hydroxycarboxylic acid receptor 1, HCA1). Despite being well studied in cancers, the potential involvement of GPR81 in the eye remains insufficiently investigated. Given that the retina (and perhaps also the outer retina) is one of the most energy-demanding tissues in the human body and that retinal metabolism dysregulation is linked to several pathologies including age-related macular degeneration (AMD), a leading cause of blindness in elderly people in industrialized countries, we are interested in the lactate receptor GPR81 and its potential functions in the outer retina. Intriguingly, GPR81 was abundantly expressed in the retinal pigment epithelium (RPE) layer of the outer retina, which is a key element in maintaining homeostasis of the outer retina including the choroidal vasculature and photoreceptors.  The pro-angiogenic function of GPR81 activation was confirmed by choroid sprout assay (ex vivo) and choroidal thickness measurement in mice given intravitreal injections of lactate (in vivo). In contrast, by evaluating the choroidal thickness, we found that GPR81-/- (i.e., KO) mice displayed choroidal degeneration followed by aberrant neovascularization. Compared to age-matched WT mice, young GPR81-/- mice (<P30) displayed a dramatically reduced choroidal vasculature, which was mediated by activation of the integrated stress response (ISR) pathway, an elaborate mechanism employed by cells to adapt to various stresses. Growth factor microarray analysis revealed a general decrease in translation in GPR81-/- mice, indicating the presence of ER stress, which was further confirmed by the increased levels of ER stress markers (BiP and PDI). Moreover, these mice also showed a significant elevation of oxidative stress evidenced by significantly increased levels of reactive active species (ROS) and 8-isoprostane as well as lower levels of Nrf2 and SOD activity. Therefore, ER stress and oxidative stress in GPR81-/- mice worked synergistically to activate the ISR pathway, as suggested by increased levels of phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2α), ATF4, and transcription of its downstream target genes. Inhibition of the ISR pathway reversed the choroidal thinning in GPR81-/- mice at P12, mediated by a partial restoration of protein synthesis, particularly of the pro-angiogenic factors such as VEGF A and HGF. The termination of the ISR pathway in GPR81-/- mice was at P30, as evidenced by the increased level of growth arrest and DNA damage-inducible protein 34 (GADD34) and the restoration of general translation. The subsequent increase of global translation in GPR81-/- mice, however, eventually triggered the development of pathological choroidal neovascularization (CNV), the hallmark of “wet” AMD, at P180. Compared to age-matched WT mice, these GPR81-/- RPEs displayed several forms of degeneration manifesting as the disappearance of some RPE junctions as well as the appearance of ghost RPEs. At one year of age, some choroidal vessels were observed growing into the RPE layer in GPR81-/- mice. All these data suggest that aged GPR81-/- mouse could be a mouse model of CNV. Taken together, our study provides the first evidence for the therapeutic potential of targeting the metabolic receptor GPR81 in “wet” AMD through the simultaneous regulation of oxidative stress, ER stress, and resultant ISR pathway activation. The GPR81-/- mouse can be used as a mouse model of CNV.

Keyphrases: G protein-coupled receptor 81 (GPR81), Hydroxycarboxylic acid receptor 1 (HCA1), Lactate, Choroidal neovascularization (CNV), Aged-related macular degeneration (AMD), Choroidal thickening, Choroidal thinning, Oxidative stress, Stress ER, ER stress, Integrated stress response (ISR), Metabolism dysregulation, Outer retina, Retinal pigment epithelium (RPE)