Notch and BMP Signaling Ligands in Ocular Development: AnAnalysis of Altered Spatiotemporal Expression and GeneDosage

Ocular development requires precise interactions between the neuroepithelium, surface ectoderm, mesoderm, and neural crest-derived cells. By way of physical and chemical contact with one another, these cell types work in concert throughout embryonic and early postnatal development to form the multiple, specialized tissues of the eye. Abnormalities in ocular tissue morphogenesis can lead to congenital diseases of both the anterior and posterior eye. Two genes encoding ligands within the Notch and Bone Morphogenic Protein (BMP) signaling pathways, Jag1 and Bmp4, respectively, have been implicated in ocular dysgenesis. However, their precise spatiotemporal requirements in eye development are undefined. The trabecular meshwork (TM) is a tiny tissue responsible for the majority of aqueous humor drainage in the front of the eye, and thus regulates intraocular pressure. Previous literature has shown that single heterozygous null mutations of Jag1 and Bmp4 result in TM hypoplasia, yet the timing and underlying mechanisms causing this developmental dysgenesis are unknown. Surprisingly, the cellular mechanisms driving normal TM formation are also poorly understood. To thoroughly examine both wild-type as well as Jag1+/– and Bmp4+/– mice, I assessed tissue morphology, cellular proliferation, apoptotic cell death, and markers for TM differentiation over the full time course of TM development. I found that postnatal day 1 marks an important stage in TM cellular differentiation, yet the phenotypic abnormalities detected in adult Jag1 +/– and Bmp4+/– mice do not arise until later stages. In order to determine the tissue-specific requirements of JAG1 and BMP4 during early ocular development, I next examined multiple genetically modified mouse strains in which these genes were conditionally deleted at specific early time points in the optic cup of the mouse eye. Previous studies had established that both ligands are expressed in the margins of the optic cup during embryogenesis, a position consistent with reported anterior eye defects in the heterozygotes. Surprisingly, morphological, molecular, and functional analysis revealed that both Jag1 and Bmp4 are dispensable for ocular development when individually removed from the optic cup neuroepithelium. These results indicate that this important signaling occurs outside of the optic cup spatially and/or temporally during early eye development. Together, these data demonstrate the complexity of spatiotemporal expression patterns and gene dosage effects of Jag1 and Bmp4 throughout ocular development. This work has provided novel insight into the molecular mechanisms underlying early eye morphogenesis and malformation.