Notch gain of function in mouse periocular mesenchyme downregulates FoxL2 and impairs eyelid levator muscle formation, leading to congenital blepharophimosis

Notch signaling is pivotal for the morphogenesis and homeostasis of many tissues. We found that aberrant Notch activation in mouse neural-crest-derived periocular mesenchymal cells (POMCs), which contribute to the formation of corneal and eyelid stroma, results in blepharophimosis. Compound transgenic mice overexpressing the Notch1 intracellular domain (N1-ICD) in POMCs (POMC(N1-ICD)) showed relatively minor effects on the cornea, but increased cell apoptosis and decreased cell proliferation during eyelid morphogenesis. Eyelid closure at E15.5 and eyelid formation at birth were incomplete. In further analyses, overexpression of N1-ICD impaired eyelid levator smooth muscle formation by downregulating the transcription factor FoxL2. This is similar to the effect of haploinsufficiency of FOXL2 in humans, which results in type II BPES (blepharophimosis, ptosis and epicanthus inversus syndrome). In vitro studies showed that FoxL2 expression is augmented by a low dose of N1-ICD but was downregulated by a high dose, depending on the extent of Hes-1 and Hey-1 activation. Moreover, transfection of CMV-FoxL2 enhanced alpha-SMA promoter activity. These data strongly imply that a physiologically low level of Notch1 is crucial for proper FoxL2 expression in POMCs, which is, in turn, essential for Mueller muscle formation and normal eyelid development.