ctor Pax6 can act as a transcriptional regulator of cF-crystallin promoter. Pax6 was detected in 24211709 the more mature lens fiber cells of aA-CLEF 
mice. It is very well possible that Pax6 participates in the repression of c-crystallin gene expression, thus contributing to the lower levels of c-crystallin protein in adult aA-CLEF lenses. Disorganized distribution of fiber cell nuclei in aA-CLEF embryonic lenses was the first indication of impaired differentiation of fiber cells. The persistence of fiber cell differentiation markers in the fiber cell compartment of transgenic aA-CLEF lenses suggested delayed differentiation of fiber cells, which was shifted from the transitional zone to the central part of the lens. This is reminiscent of the effect of b-catenin stabilization within the entire lens that resulted, among other effects, in delayed differentiation of lens DMXB-A biological activity epithelial cells. Since Wnt/b-catenin signaling is known to regulate the cell cycle, we examined 23033494 Wnt/b-Catenin Signaling in Lens Fiber Cells the expression of cell cycle promoting factors cyclin D1 and cyclin D2, and the expression of negative regulators of the cell cycle, cyclin-dependent kinase inhibitors p27Kip1 and p57Kip2. Cyclin D1 and cyclin D2 are closely related G1 cyclins. Importantly, cyclin D1 is also a known target of the Wnt/b-catenin signaling pathway. We thus expected its altered expression pattern due to the Wnt/b-catenin signaling activation in lens fiber cells. Cyclin D1 and cylin D2 are normally expressed in proliferating lens epithelial cells and in equatorial lens fiber cells because they are required for G1 to S phase transitional, whereas their expression is absent from fiber cells that have already completed the differentiation. As cyclin-dependent kinase inhibitor p27Kip1 is required together with p57Kip2 for inhibition of Cdk’s responsible for G1/S transition, p27Kip1 expression coincides with the cell cycle exit during the fiber cell differentiation. Since expression of p27Kip1 and p57Kip2 in the transitional zone appears to require the activity of transcription factor Prox1, aberrant expression of p27Kip1 at E16.5 in the central part of the lens may very well be due to the stronger Prox1 expression in this compartment in aA-CLEF lenses. It was shown previously that the loss of b-catenin in the entire lens results in downregulation of cyclin D1 in lens epithelial cells, and at the same time in apparent upregulation of cyclin D1 in the more matured differentiating fiber cells, a somewhat surprisingly contradictory effect. It is well established that active Wnt/b-catenin signaling induces cyclin D1 and thereby triggers G1-progression, preventing cell cycle exit to G0. It is therefore not surprising that activation of Wnt/bcatenin in fiber cells in aA-CLEF mice induces an increase of cyclin D1 and cyclin D2 in the more matured differentiating fiber cells. Analysis of R26pFucci2 mouse embryos at E13.5 provides convincing visual confirmation that lens epithelial cells are in S/G2/M phase, whereas differentiating fiber cells in the lens equatorial region are in G1 phase. It is likely that the differentiating fiber cells can be kept in G1 by the aberrantly present cyclin D1 and cyclin D2, activated via Wnt/b-catenin signaling at E13.5 in aA-CLEF mice. In summary, we have shown here that ectopic activation of Wnt/b-catenin signaling in lens fiber cells during embryonic development results in delayed fiber cell differentiation and cataract formation. A more dire