Distinct Roles of Individual E2Fs in Induction of Cell Cycle Entry in Postmitotic Lens Fiber Cells of Transgenic Mice

Abstract

Abstract: : Purpose: Our previous transgenic studies have shown that lens-specific expression of E2F1 or E2F2 induces post-mitotic fiber cells to re-enter the cell cycle and to subsequently endergo programmed cell death. In this study, we designed experiments to assay for the roles that E2F family members E2F3a, E2F3b, E2F4 and E2F5 play in regulation of cell cycle entry and cell death in the lens. Methods: E2F3a, E2F3b, E2F4 and E2F5 cDNAs were linked to the mouse αA-crystallin promoter either in CPV2 or DREAM, and used for microinjection to generate transgenic mice. E2F5 mice were cross-mated with E2F3a or E2F2 transgenic mice. Single and double transgenic mice were characterized by histology, in situ hybridization, BrdU incorporation and TUNEL assay. Results: E2F3a transgenic mice show small eyes and cataracts, similar to E2F1 and E2F2 mice. About three weeks after birth, cataracts were found in the E2F4 but not the E2F3b or E2F5 transgenic lenses. At embryonic day 15.5 (E15.5), expression of the E2F3a, E2F3b, E2F4 and E2F5 transgenes was observed in lens fiber cells, or in lens epithelial and fiber cells. A few BrdU positive fiber cells were detected in the E2F3a and E2F4 lenses. The fiber cells of the E2F3b or E2F5 mice did not show BrdU incorporation. Upregulation of cyclin E expression was found in the lens fiber cells expressing E2F3a or E2F4. No apoptotic fiber cell nuclei or upregulation of p21 expression was detected in transgenic lenses expressing E2F3a, E2F3b, E2F4 or E2F5. Expression of E2F5 appears to inhibit cell cycle reentry induced by E2F3a but not by E2F2. Conclusions: Overexpression of E2F3a or E2F4 in the lens can induce cell cycle re-entry but not cell death, while overexpression of E2F3b or E2F5 is not sufficient to alter cell cycle control in the ocular lens. E2F5 may repress the function of E2F3a but not E2F2.

Keywords: transcription factors • proliferation • apoptosis/cell death