Disruption of anterior segment development by TGF‐β1 overexpression in the eyes of transgenic mice

Abstract

Previous experiments showed that transgenic mice expressing a secreted selfactivating transforming growth factor (TGF) -␤1 did not show a phenotype in the lens and cornea until postnatal day 21, when anterior subcapsular cataracts, sporadic thickening of the corneal stroma, and thinning of the corneal epithelium were noted (Srinivasan et al., 1998). To examine the effects of higher concentrations of TGF-␤1 on the lens and cornea, we constructed transgenic mice harboring the strong, lens-specific chicken ␤B1-crystallin promoter driving an activated porcine TGF-␤1 gene. In contrast to the earlier study, the transgenic mice had microphthalmic eyes with closed eyelids. Already at embryonic day (E) 13.5, the future cornea of the transgenic mice was threefold thicker than that of wild-type littermates due to increased proliferation of corneal stromal mesenchyme cells. Staining of fibronectin and thrombospondin-1 was increased in periocular mesenchyme. At E17.5, the thickened transgenic corneal stroma was vascularized and densely populated by abundant star-shaped, neural cell adhesion molecule-positive cells of mesenchymal appearance surrounded by irregular swirls of collagen and extracellular matrix. The corneal endothelium, anterior chamber, and stroma of iris/ciliary body did not develop, and the transgenic cornea was opaque. Fibronectin, perlecan, and thrombospondin-1 were elevated, whereas type VI collagen decreased in the transgenic corneal stroma. Stromal mesenchyme cells expressed ␣-smooth muscle actin as did lens epithelial cells and cells of the retinal pigmented epithelium. By E17.5, lens fiber cells underwent apoptotic cell death that was followed by apoptosis of the entire anterior lens epithelium between E18.5 and birth. Posteriorly, the vitreous humor was essentially absent; however, the retina appeared relatively normal. Thus, excess TGF-␤1, a mitogen for embryonic corneal mesenchyme, severely disrupts corneal and lens differentiation. Our findings profoundly contrast with the mild eye phenotype observed with presumably lower levels of ectopic TGF-␤ and illustrate the complexity of TGF-␤ utilization and the importance of dose when assessing the effects of this growth factor.

Corneal development associated with eyelid opening

“…Their data suggest that Pax6 is necessary for epithelial stratification; however, it is unclear if an environmental cue is required to stimulate the action of Pax6 in association with eyelid opening. Other factors that have been demonstrated to be involved in corneal development are TGF-β1 (Flugel-Koch et al, 2002), notch1 (Nicolas et al, 2003), TGF-α (Reneker et al, 2000), the forkhead/winged-helix gene (Kidson et al, 1999), and IκB kinase α (Yoshida et al, 2000). Intriguingly, both overexpression of TGF-α and deletion of forkhead lead to the lack of an endothelial cell layer.…”

Update on Anterior Segment Development with Emphasis on Genetics and Correlation with Pathogenesis of Developmental & Primary Open Angle Glaucoma

“…The authors in that study have also speculated that in the TGFα and EGF transgenic mice, over stimulation of the TGF receptor in the corneal mesenchymal cells may have inhibited the TGFB/SMAD signaling pathway, which is essential for corneal endothelial specification and differentiation [15]. Several studies have shown that TGFB signaling appears to be essential for the formation of normal cornea during early eye development [36,37,38]. The TGFB super-family of secreted signaling molecules has an influence on several biological processes including: cell proliferation, cell differentiation, cell death, bone morphogenesis and wound repair [16].…”

“…Their corneas were thickened. The transgenic corneal stroma was vascularized and densely populated by star-shaped cells indicating that the stromal cells retained a mesenchymal phenotype [38].…”

“…Then it has been more recently suggested to be involved in the regulation of TGFB1 signaling during the development of the anterior segment or vice versa [38]. On the other hand, more studies have been conducted for better understanding of the potential role of CYP1B1 in the pathogenesis of several PCG phenotypes in various geographical locations [82].…”

Sma- and Mad-related Protein 7 (Smad7) Is Required for Embryonic Eye Development in the Mouse

“…It has been proposed that TGF-␤ signaling can regulate the specification of the cranial neural crest and the establishment of the normal ocular anterior segments. Reduction of TGF-␤ signaling is linked to the human disorder Axenfeld-Rieger anomaly by regulating Pitx2 at the late embryonic stage (20,(51)(52)(53). However, in Smad7 mutant embryos, the broadened optic disc appears only for a short time.…”