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Pachiappan Arjunan, Jaya P. Gnanaprakasam, Sudha Ananth, Michelle A. Romej, Veeranan-Karmegam Rajalakshmi, Puttur D. Prasad, Pamela M. Martin, Mariappan Gurusamy, Muthusamy Thangaraju, Yangzom D. Bhutia, Vadivel Ganapathy; Increased Retinal Expression of the Pro-Angiogenic Receptor GPR91 via BMP6 in a Mouse Model of Juvenile Hemochromatosis. Invest. Ophthalmol. Vis. Sci. 2016;57(4):1612-1619. doi: https://doi.org/10.1167/iovs.15-17437.
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© ARVO (1962-2015); The Authors (2016-present)
Hemochromatosis, an iron-overload disease, occurs as adult and juvenile types. Mutations in hemojuvelin (HJV), an iron-regulatory protein and a bone morphogenetic protein (BMP) coreceptor, underlie most of the juvenile type. Hjv−/− mice accumulate excess iron in retina and exhibit aberrant vascularization and angiomas. A succinate receptor, GPR91, is pro-angiogenic in retina. We hypothesized that Hjv−/− retinas have increased BMP signaling and increased GPR91 expression as the basis of angiomas.
Expression of GPR91 was examined by qPCR, immunofluorescence, and Western blot in wild-type and Hjv−/− mouse retinas and pRPE cells. Influence of excess iron and BMP6 on GPR91 expression was investigated in ARPE-19 cells, and wild-type and Hjv−/− pRPE cells. Succinate was used to activate GPR91 and determine the effects of GPR91 signaling on VEGF expression. Signaling of BMP6 was studied by the expression of Smad1/5/8 and pSmad4, and the BMP-target gene Id1. The interaction of pSmad4 with GPR91 promoter was studied by ChIP.
Expression of GPR91 was higher in Hjv−/− retinas and RPE than in wild-type counterparts. Unexpectedly, BMP signaling was increased, not decreased, in Hjv−/− retinas and RPE. Bone morphogenetic protein 6 induced GPR91 in RPE, suggesting that increased BMP signaling in Hjv−/− retinas was likely responsible for GPR91 upregulation. Exposure of RPE to excess iron and succinate as well as BMP6 and succinate increased VEGF expression. Bone morphogenetic protein 6 promoted the interaction of pSmad4 with GPR91 promoter in RPE.
G-protein-coupled receptor 91 is a BMP6 target and Hjv deletion enhances BMP signaling in retina, thus underscoring a role for excess iron and hemochromatosis in abnormal retinal vascularization.
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