Fig. 7.
MAPK1 activation is required for cell proliferation in the germinative zone during lens development. MAPK1 plays a unique role in promoting high mitotic activity in the lens germinative zone. Growth factors (such as FGF and PDGF) released from the anterior margin of the retina (blue) stimulate the RTK-Ras-MAPK1 pathway in the adjacent lens cells (red) through a paracrine mechanism. Cell cycle regulatory proteins downstream of MAPK1 identified in this study include cyclin D1, phospho-H3 and survivin. Upregulation of MAPK3 activity in Mapk1CKO lens cannot compensate for the loss of MAPK1. MAPK1 activity in lens epithelium (yellow) is also essential for cell survival during development.