Retinal neuroepithelium-derived BMP4 is dispensable for anterior and posterior ocular development

Abstract

Purpose : Heterozygous Bmp4 mutations in humans and mice cause anterior and posterior ocular dysgenesis, such as iridocorneal adhesions, variable IOP, abnormal retinal vasculature, and optic nerve head malformation. BMP4 is expressed in several ocular tissues including the developing retina, but the precise spatiotemporal source of BMP4 required for eye development is undefined. Here, we test whether retinal neuroepithelium-derived BMP4 is required for ocular development.

Methods : Either αCre (anterior segment; AS) or Six3Cre (posterior segment) was used to conditionally delete Bmp4 from murine embryonic ocular neuroepithelium. An inducible transgenic line (CMVCreER) was also used to remove Bmp4 from the adult eye. All conditional mutants were compared to Bmp4 heterozygous mice and wild-type littermates. Morphological, molecular, and functional assays were performed on adult mice, including plastic histology (n≥3), immunohistochemistry (n≥3), slit lamp imaging (n≥24), fluorescein angiography (n≥6), and IOP measurement (n≥14). Significance was defined as p≤0.05.

Results : As previously reported, Bmp4^+/- mice had abnormalities in both the anterior and posterior eye. In contrast, Bmp4^fl/fl; αCre⁺ mutants had normal AS development. Slit lamp images and histological analysis (3μm plastic sections) showed standard morphology of all AS structures. α-SMA staining indicated normal trabecular meshwork development. There was no statistical difference in IOP between mutant (15.89±0.53 mmHg, n=82) and control mice (15.62±0.45 mmHg, n=84), implying normal ciliary body, trabecular meshwork, and Schlemm’s canal function. Similarly, Bmp4^fl/fl; Six3Cre⁺ mutants appeared to have normal posterior development. Histological analysis showed typical gross morphology of the retina and optic nerve head. Retinal vasculature was also unaffected. Despite sustained expression of BMP4, its removal from adult eyes using Bmp4^fl/fl;CMVCreER⁺ mice had no impact on structural or functional maintenance. Slit lamp images, morphology, α-SMA staining, retinal vasculature, and IOP (2-way-ANOVA, n≥14 per group per time point) were all indistinguishable from controls.

Conclusions : Though BMP4 is clearly crucial for eye development, these results suggest that embryonic retinal neuroepithelium-derived BMP4 is not a critical source. Moreover, the adult deletion studies indicate that BMP4 is not required for ocular maintenance.

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