Role of growth factors in regulating of lens fibre differentiation

The lens is a good developmental biology model owing to its simplicity and distinct polarity. The anterior lens compartment constitutes a monolayer of cuboidal-shaped epithelial cells, which overly a mass of precisely aligned elongated fibre cells. Lens growth throughout life is mediated by proliferation of epithelial cells and their subsequent differentiation into fibres, to form a transparent and tightly-packed lens. During lens growth and development, cell proliferation is mainly restricted to a specialised region at the periphery of the lens epithelium known as the gerrninative zone. The progeny of these cell divisions migrate posteriorly into the transitional zone, where they initiate their differentiation into the lens fibre cells. Based on this, the lens epithelial cells can therefore be regarded as the progenitors of the lens fibre cells. The fate of lens cells is regulated by growth factors found in the ocular environment, the aqueous and vitreous humour, which bathes the lens anteriorly and posteriorly, respectively. Growth factors in vitreous regulate lens differentiation by activating intracellular signalling pathways, which lead to the expression of fibre-specific proteins. However, the mechanism by which growth factors regulate lens fibre differentiation has yet to be elucidated. In order to investigate the mechanisms involved in promoting cell differentiation, the present study characterised the downstream signalling pathways induced by a range of growth factors and compared these to those induced by the vitreous humour, the source of these endogenous growth factors. Using this approach, we showed that two major signalling pathways, the ERKl/2 and Pl3—K/Akt signalling, are required for lens fibre differentiation, with the duration of ERKl/Z phosphorylation determining lens cell fate: transient ERKl/2 phosphorylation being associated with lens cell proliferation and sustained ERKl/2 phosphorylation being required for lens fibre differentiation. In addition, a combination of growth factors in Vitreous are thought to contribute to the distinct ERKl/2 and PI3-K/Akt signalling required for lens fibre differentiation. Furthermore, in lens cells, we identified a negative regulator of the ERK1/2 signalling pathway, Spry2, which may be important for defining the cell fate. Overexpression of Spry2 in vitro and in vivo can block cell elongation during lens fibre differentiation. Taken together, the findings presented in this thesis illustrate that the lens fibre differentiation process is complex and is regulated by multiple signalling pathways activated by various growth factors and is effectively monitored by intracellular signalling regulators