Anna Korol

Overlapping expression patterns and redundant roles for AP‐2 transcription factors in the developing mammalian retina

Background: We have previously shown that the transcription factor AP‐2α (Tcfap2a) is expressed in postmitotic developing amacrine cells in the mouse retina. Although retina‐specific deletion of Tcfap2a did not affect retinogenesis, two other family members, AP‐2β and AP‐2γ, showed expression patterns similar to AP‐2α. Results: Here we show that, in addition to their highly overlapping expression patterns in amacrine cells, AP‐2α and AP‐2β are also co‐expressed in developing horizontal cells. AP‐2γ expression is restricted to amacrine cells, in a subset that is partially distinct from the AP‐2α/β‐immunopositive population. To address possible redundant roles for AP‐2α and AP‐2β during retinogenesis, Tcfap2a/b‐deficient retinas were examined. These double mutants showed a striking loss of horizontal cells and an altered staining pattern in amacrine cells that were not detected upon deletion of either family member alone. Conclusions: These studies have uncovered critical roles for AP‐2 activity in retinogenesis, delineating the overlapping expression patterns of Tcfap2a, Tcfap2b, and Tcfap2c in the neural retina, and revealing a redundant requirement for Tcfap2a and Tcfap2b in horizontal and amacrine cell development. Developmental Dynamics 241:814–829, 2012.

Matrix Metalloproteinase-9–Null Mice Are Resistant to TGF-β–Induced Anterior Subcapsular Cataract Formation

Epithelial-mesenchymal transition (EMT) is associated with fibrotic diseases in the lens, such as anterior subcapsular cataract (ASC) formation. Often mediated by transforming growth factor (TGF)-β, EMT in the lens involves the transformation of lens epithelial cells into a multilayering of myofibroblasts, which manifest as plaques beneath the lens capsule. TGF-β-induced EMT and ASC have been associated with the up-regulation of two matrix metalloproteinases (MMPs): MMP-2 and MMP-9. The current study used MMP-2 and MMP-9 knockout (KO) mice to further determine their unique roles in TGF-β-induced ASC formation. Adenoviral injection of active TGF-β1 into the anterior chamber of all wild-type and MMP-2 KO mice led to the formation of distinct ASC plaques that were positive for α-smooth muscle actin, a marker of EMT. In contrast, only a small proportion of the MMP-9 KO eyes injected with adenovirus-expressing TGF-β1 exhibited ASC plaques. Isolated lens epithelial explants from wild-type and MMP-2 KO mice that were treated with TGF-β exhibited features indicative of EMT, whereas those from MMP-9 KO mice did not acquire a mesenchymal phenotype. MMP-9 KO mice were further bred onto a TGF-β1 transgenic mouse line that exhibits severe ASC formation, but shows a resistance to ASC formation in the absence of MMP-9. These findings suggest that MMP-9 expression is more critical than MMP-2 in mediating TGF-β-induced ASC formation.

Dual function of TGFβ in lens epithelial cell fate: implications for secondary cataract

TGFβ signaling is linked to posterior capsule opacification (PCO), the most common complication of cataract surgery. TGFβ can induce primary lens epithelial cells to differentiate into two disparate, PCO-causing abnormal cell phenotypes, a variation of the TGFβ paradox. Analysis of the signaling pathways downstream of TGFβ reveals novel therapeutic targets for PCO.

The Lens Capsule: Synthesis, Remodeling, and MMPs

The lens capsule is an amorphous, elastic structure that encapsulates the ocular lens. It is secreted by cells of the lens and is composed primarily of type IV collagen and laminin along with additional extracellular matrix (ECM) components such as entactin/nidogen, heparin sulfate proteoglycans (HSPG), and secreted protein acidic and rich in cysteine (SPARC), which act to stabilize the lens capsule structure. As the lens grows during development and with age, new capsular lamellae are synthesized, deposited, and organized by the lens epithelial and fiber cells. The main receptors of lens cells that adhere them to the ECM of the capsule are the integrins, heterodimeric transmembrane cell adhesion molecules. These adhesion molecules also act as bidirectional signaling molecules, mediating signals between the lens and the surrounding ocular media. The composition and arrangement of both the ECM of the lens capsule and the integrins are altered in fibrotic cataracts such as posterior capsule opacification (PCO) and anterior subcapsular cataract (ASC). This includes the aberrant deposition of ECM components not normally expressed in the lens capsule and a corresponding change in the profile of integrins expressed in the lens. The matrix metalloproteinases (MMPs), a family of matrix-degrading enzymes, have been shown to release growth factors from the lens capsule and activate receptors. They are also aberrantly expressed in PCO and ASC and their inhibition has been shown to suppress events involved in fibrotic cataract formation including lens epithelial cell migration, capsular contraction, and the transformation of cells into myofibroblasts.