Essential role of BMPs in FGF-induced secondary lens fiber differentiation

It is widely accepted that vitreous humor-derived FGFs are required for the differentiation of anterior lens epithelial cells into crystallin-rich fibers. We show that BMP2, 4, and 7 can induce the expression of markers of fiber differentiation in primary lens cell cultures to an extent equivalent to FGF or medium conditioned by intact vitreous bodies (VBCM). Abolishing BMP2/4/7 signaling with noggin inhibited VBCM from upregulating fiber marker expression. Remarkably, noggin and anti-BMP antibodies also prevented purified FGF (but not unrelated stimuli) from upregulating the same fiber-specific proteins. This effect is attributable to inhibition of BMPs produced by the lens cells themselves. Although BMP signaling is required for FGF to enhance fiber differentiation, the converse is not true. Expression of noggin in the lenses of transgenic mice resulted in a postnatal block of epithelial-to-secondary fiber differentiation, with extension of the epithelial monolayer to the posterior pole of the organ. These results reveal the central importance of BMP in secondary fiber formation and show that although FGF may be necessary for this process, it is not sufficient. Differentiation of fiber cells, and thus proper vision, is dependent on cross-talk between the FGF and BMP signaling pathways.