AP-2β is required for formation of the murine trabecular meshwork and Schlemm's canal

Previously, we have shown that Tfap2b, the gene encoding transcription factor AP-2β, is needed for normal mouse eye development. Specifically, targeted loss of Tfap2b in neural crest cells (NCCs)¹ and their derivatives, particularly the periocular mesenchyme (POM), resulted in anterior segment defects affecting the cornea and angle tissue. These defects were further associated with an increase in intraocular pressure (IOP). The present study investigates the underlying changes in embryonic and postnatal POM cell development and differentiation caused by loss of AP-2β in the NCCs, particularly in the structures that control aqueous outflow, using Wnt1Cre^+/-; Tfap2b^-/lox; tdTomato^lox/+ mice (AP-2β neural crest cell knockout or AP-2β NCC KO). Toluidine blue-stained sections and ultrathin sections stained with uranyl acetate and lead citrate were used to assess morphology and ultrastructure, respectively. Immunohistochemistry of KO and control eyes was performed at embryonic day (E) 15.5, E18.5, postnatal day (P) 1, P7 and P14 using phospho-histone H3 (PH3), α-smooth muscle actin (α-SMA), myocilin and endomucin antibodies, as well as a TUNEL assay. Conditional deletion of AP-2β in the NCC-derived POM resulted in defects that appeared during both embryogenesis and postnatal stages. Fate mapping of the knockout cells in the mutants revealed that the POM migrated appropriately into the eye during embryogenesis. However, during postnatal stages a significant reduction in POM proliferation in the angle region was observed in the mutants compared to controls. This was accompanied by a lack of expression of appropriate trabecular meshwork and Schlemm's canal markers. This is the first study to show that AP-2β is required for development and differentiation of the trabecular meshwork and Schlemm's canal. Together, these defects likely contributed to the elevated intraocular pressure (IOP) previously reported in the AP-2β NCC KO mice.