ERK1/2 Signaling is Required for Lens Cell Survival and Fiber Cell Differentiation during Development

Abstract

Purpose: : Accumulated evidence has shown that FGFRs play a key role in vertebrate lens development. FGFR can activate multiple downstream signal transduction pathways. The contribution of individual signaling event to lens development is not entirely clear. We know that ERK1 is dispensable for lens development and ERK2 is required for cell proliferation at the lens germinative zone. The present work was designed to study the role of ERK1/2 in lens development.

Methods: : Erk2 conditional knockout (Erk2^CKO) mice were first generated by using Erk2-flox alleles and the Le-cre driver. Erk1/2 double knockout (Erk1/2^DKO) mice were generated by breeding the Erk2^CKO mice into the Erk1^-/- background. Lens development in the Erk1/2^DKO mice was compared to WT mice by histology and immunostaining to investigate the changes in lens cell proliferation, differentiation, and survival.

Results: : Erk1/2 deletion did not affect lens induction and Pax-6 expression, but caused severe lens hypoplasia. Apoptosis was dramatically increased in the lens of Erk1/2^DKO mice, a major defect also seen in the LeCre-Fgfr1/2 and MLR10Cre-Fgfr1/2/3 conditional knockoutmice. In contrast to the WT lens at E12.5, the prospective primary fiber cells in Erk1/2^DKO lens lack cell elongation, remained in the columnar shape and continued to express cell proliferation markers. However, α, β and γ crystallin expression was detected in a few primary fiber cells at the center of the Erk1/2^DKO lens at E14.5. Evidently, secondary fiber cell differentiation was completely abolished in the Erk1/2^DKO lens. The fiber differentiation defects in the Erk1/2^DKO mice closely resemble those in the MLR10Cre-Fgfr1/2/3 triple conditional knockout mice, suggesting that ERK1/2-signaling activated by FGFRs is essential for lens fiber differentiation. Furthermore, loss of ERK1/2 in the lens severely decreased but did not eliminate cell proliferation, suggesting that other signaling activity may also be involved in regulating cell proliferation.

Conclusions: : ERK1/2 activation is not required for lens induction but is essential for cell survival and fiber differentiation during lens development. Our results suggest that, from the lens pit stage onward, the function of FGFR in lens cell survival and differentiation is mainly mediated through the ERK1/2 signaling.

Keywords: development • transgenics/knock-outs • genetics