Role of ERK2 in Lens Cell Proliferation and Differentiation during Development

Abstract

Purpose: : Growth factor plays an essential role in lens cell proliferation, differentiation and survival. The well characterized Raf-MEK-ERK kinase cascade is one of the major signaling events activated by the growth factor receptor. The lens expresses two ERK isoforms (ERK1 and ERK2). The present work was designed to study the role of ERK2 on lens development.

Methods: : The Cre/loxP system was used to delete ERK2 by crossing the LeCre transgenic mice to the Erk2^flox mice. The LeCre-Erk2^flox/flox mice, referred as ERK2-CKO mice, were compared by histology and immuno-staining to the control Erk2^flox/flox mice to study the changes in lens cell proliferation, differentiation, and survival.

Results: : ERK immuno-intensity was reduced in the ERK2-CKO lens. The mutant lens was smaller, which was evident as early as embryonic day 14 (E14). Cell proliferation markers, such as expression of Ki67 and PCNA, and incorporation of BrdU, were not notably changed. However, the number of cells expressing phospho-histone 3, another cell proliferation marker, was significantly reduced in the lens epithelial layer of the ERK2-CKO mice. This result suggests that ERK2 deletion can directly affect mitosis in the lens epithelium. When cell death was examined by TUNEL assay and expression of active caspase-3, it was obvious that loss of ERK2 induced apoptosis in the lens epithelial cells. Despite the severe defects in the lens epithelium, fiber cell differentiation was induced at the transition zone and the expression patterns of β and γ-crystallins were normal. However, lens fibers started to degenerate postnatally and the lens epithelial cells failed to differentiate into the fiber cells in postnatal day 8 (P8) ERK2-CKO lens.

Conclusions: : Loss of ERK2 in mouse lens caused significant decrease in the phospho-histone 3 level and increase of apoptosis in the epithelial layer, suggesting that the level of ERK activity is crucial for epithelial cell proliferation and survival during lens development. The effect of ERK2 deletion on fiber differentiation was a delayed response, implying that reduction of ERK2 activity could indirectly inhibit fiber cell differentiation.

Keywords: development • transgenics/knock-outs • proliferation