TGFbeta promotes Wnt expression during cataract development.

TGFbeta induces lens epithelial cells to undergo epithelial mesenchymal transition (EMT) and many changes with characteristics of fibrosis including posterior capsular opacification (PCO). Consequently much effort is directed at trying to block the damaging effects of TGFbeta in the lens. To do this effectively it is important to know the key signaling pathways regulated by TGFbeta that lead to EMT and PCO. Given that Wnt signaling is involved in TGFbeta-induced EMT in other systems, this study set out to determine if Wnt signaling has a role in regulating this process in the lens. Using RT-PCR, in situ hybridization and immunolocalization this study clearly shows that Wnts 5a, 5b, 7b, 8a, 8b and their Frizzled receptors are upregulated in association with TGFbeta-induced EMT and cataract development. Both rat in vitro and mouse in vivo cataract models show similar profiles for the Wnt and Frizzled mRNAs and proteins that were assessed. Currently it is not clear if the canonical beta-catenin/TCF signaling pathway, or a non-canonical pathway, is activated in this context. Overall, the results from the current study indicate that Wnt signaling is involved in TGFbeta-induced EMT and development of fibrotic plaques in the lens.

Lithium stabilizes the polarized lens epithelial phenotype and inhibits proliferation, migration, and epithelial mesenchymal transition.

Posterior capsule opacification (PCO) is a common complication of cataract surgery caused by epithelial mesenchymal transition (EMT) and aberrant lens cell growth. One path to prevention depends on maintaining the quiescent lens epithelial phenotype. Here we report that lithium chloride (LiCl) is a potent stabilizer of the lens epithelial phenotype. In lens epithelial explants (controls), at low cell density, cells readily depolarized, spread out, and proliferated. By contrast, in the presence of LiCl, cells did not spread out or exhibit migratory behaviour. Using concentrations of 1-30 mM LiCl we also showed that cell proliferation is inhibited in a dose-dependent manner. Confocal microscopy and immunohistochemistry for ZO-1 and E-cadherin showed that LiCl treatment maintained tight junctions at the apical margins of cells. Taken together with measurements of cell heights, this showed that the cells in LiCl-treated explants maintained the apical baso-lateral polarity and cobblestone-like packing that is characteristic of lens epithelial cells in vivo. Significantly, the effects of LiCl also extended to blocking the potent EMT/cataract-promoting effects of transforming growth factor beta (TGFbeta) on lens epithelial cells. In TGFbeta-treated explants, cells progressively dissociated from one another, taking on various elongated spindle shapes and strongly expressing alpha-smooth muscle actin (alpha-SMA). These features are characteristic of PCO. In both rat and human capsulorhexis explants, LiCl treatment effectively blocked the accumulation of alpha-SMA and maintained the cells in a polarized, adherent, cobblestone-packed monolayer. These findings highlight the feasibility of applying molecular strategies to stabilize lens epithelial cells and prevent aberrant differentiation and growth that leads to cataract.

A role for Wnt/planar cell polarity signaling during lens fiber cell differentiation?

Wnt signaling through frizzled (Fz) receptors plays key roles in just about every developmental system that has been studied. Several Wnt-Fz signaling pathways have been identified including the Wnt/planar cell polarity (PCP) pathway. PCP signaling is crucial for many developmental processes that require major cytoskeletal rearrangements. Downstream of Fz, PCP signaling is thought to involve the GTPases, Rho, Rac and Cdc42 and regulation of the JNK cascade. Here we report on the localization of these GTPases and JNK in the lens and assess their involvement in the cytoskeletal reorganisation that is a key element of FGF-induced lens fiber cell differentiation.

Spatial and temporal expression of Wnt and Dickkopf genes during murine lens development.

Recent studies indicate a role for Wnt signalling in regulating lens cell differentiation (Stump et al., 2003). To further our understanding of this, we investigated the expression patterns of Wnts and Wnt signalling regulators, the Dickkopfs (Dkks), during murine lens development. In situ hybridisation showed that Wnt5a, Wnt5b, Wnt7a, Wnt7b, Wnt8a and Wnt8b genes are expressed throughout the early lens primordia. At embryonic day 14.5 (E14.5), Wnt5a, Wnt5b, Wnt7a, Wnt8a and Wnt8b are reduced in the primary fibres, whereas Wnt7b remains strongly expressed. This trend persists up to E15.5. At later embryonic stages, Wnt expression is predominantly localised to the epithelium and elongating cells at the lens equator. As fibre differentiation progresses, Wnt expression becomes undetectable in the cells of the lens cortex. The one exception is Wnt7b, which continues to be weakly expressed in cortical fibres. This pattern of expression continues through to early postnatal stages. However, by postnatal day 21 (P21), expression of all Wnts is distinctly weaker in the central lens epithelium compared with the equatorial region. This is most notable for Wnt5a, which is barely detectable in the central lens epithelium at P21. Dkk1, Dkk2 and Dkk3 have similar patterns of expression to each other and to the majority of the Wnts during lens development. This study shows that multiple Wnt and Dkk genes are expressed during lens development. Expression is predominantly in the epithelial compartment but is also associated, particularly in the case of Wnt7b, with early events in fibre differentiation.