Role of survivin in the regulation of cell proliferation and differentiation in vertebrate lens development

Introduction: The lens is an unusual and transparent tissue. This transparency of the lens depends on a precise and fine regulation of cell proliferation and differentiation when this control is disrupted cataract arises. Previous studies have identified regulators of the cell cycle in lens epithelial and fibre cells or in the identification of the main components of fibre cell differentiation. However, few studies have been carried out in order to understand how proliferation and differentiation are regulated and coordinated in the vertebrate lens. Survivin, also called Birc5, is the smallest member of the inhibitor of apoptosis protein (IAP) family. Survivin functions at a pivotal junction of the cell cycle/apoptosis balance and is vital in maintaining normal tissue homeostasis. Purpose: The purpose of the present study was the analysis of the expression of Survivin in the lens during development in order to provide evidence that Survivin may have an important role as a regulator of cell proliferation and differentiation during lens development. Methods: In order to clarify the role of Survivin in vertebrate lens development three models were used: embryonic chick lens, chick epithelial dissociated primary cell culture and postnatal mouse lens. A thorough spatio-temporal analysis of Survivin expression in the developing lens was carried out. RT-PCR, RT-QPCR, Western Blot and immunocytochemistry were used to study the expression of Survivin during lens development. Proliferation (PCNA) and denucleation pattern (TUNEL) were correlated with dynamic changes in the pattern of expression of Survivin during lens development. Results: Survivin expression was detected in the three models studied. In embryonic chick lens model, Survivin expression was developmentally regulated with high peak at early embryonic lens stages of development (ED6). Finally, Survivin protein expression was absent of the lens before the denucleation in the fibre cells were observed at ED 16. In the chick lens epithelial dissociated cell culture, Survivin expression was associated with the increase of proliferation in cell culture. Survivin was detected in presence of denucleation and down regulation of Survivin was in coincidence of increase of denucleation at day 8 of cell culture. In postnatal mouse lens development, gene expression analysis confirmed the results observed in the other two models. The main difference was observed at protein level. The WB showed an additional band to Survivin wild type at NB and Pn7. Also, an increase of Survivin wild type was observed at Pn7, in coincidence with decrease in proliferation. Conclusions: Overall, the results presented suggest that Survivin is an essential survival factor in vivo and in vitro during the development of the vertebrate lens and suggest that Survivin expression is critical to maintaining cell survival in cells that are continuously proliferating and in those younger lens fibre cells that withdraw from the cell cycle, but not in cells in quiescence i.e. the central lens epithelial cells, after ED 12 in chicken and at Pnl4 in mouse lenses