Endothelial cell-derived Indian Hedgehog targets multipotent perivascular cells in the adult choroid and prevents retinal damage

Abstract

Purpose : Recent publications support the notion that microvascular endothelial cells (ECs) constitute tissue-specific instructive niches key for the maintenance and regeneration of adult organs (Rafii et al, Nature 2016). This could be relevant to the etiology of Age Related Macular Degeneration (AMD) as patients with this disease exhibit choroidal thinning and vascular dropout. Hence, we investigated whether choroid ECs secrete specific factors that regulate the homeostasis of the outer retina to test the hypothesis that functional defects of choroid ECs may be important in the etiology and pathogenesis of AMD.

Methods : RNAseq analysis of purified choroid, neural retina, liver, heart and lung ECs from adult mice identified Indian hedgehog (Ihh) as a gene specifically expressed by choroid ECs. To identify Hedgehog (Hh)-responding cells, we made use of a reporter albino Gli1^+/eGFP mouse that expresses GFP under the control of the Gli1 promoter. To test the role of choroidal Ihh in retinal function in adult mice, we generated a tamoxifen inducible EC-specific Ihh KO mouse. We analyzed visual function by electroretinography and OptoMotor Response in the absence or presence of moderate retinal stress induced by low doses of intravenous sodium iodate.

Results : We found that adult choroid ECs express Ihh at 300x higher levels than all other tested ECs. Immunofluorescence assays on eye sections from Gli1^+/eGFP reporter mice revealed Hh-responding cells surrounding the choriocapillaris but not around deeper choroidal blood vessels. Molecular characterization of sorted perivascular choroid GFP-positive cells and in vitro differentiation assays suggested a mesenchymal stem cell (MSC)-like identity. EC-specific Ihh KO mice displayed more severe visual function disruption than control mice after sodium iodate-induced retinal stress.

Conclusions : Past studies by several laboratories suggest an association between defects in the Hh-signaling pathway and AMD, a prevalent and incurable disease affecting the elderly. Here, we demonstrate the existence of choroidal ECs that express Ihh and perivascular MSC-like cells expressing Gli1, suggesting a key role of Ihh in choroid homeostasis. EC-specific Ihh deletion promotes higher susceptibility to retinal damage, suggesting that disruption of this Hh-pathway may participate in the onset of AMD.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.