Focal Adhesion Kinase (FAK) Is Required for Postnatal Lens Development

Abstract

Purpose:: Focal adhesion kinase (FAK) is a key regulator of cell-matrix-cytoskeletal interaction and is strongly expressed in the developing and adult lens. While FAK has been described in other systems to guide fundamental aspects of cell behavior, such as migration, differentiation, proliferation and polarity, its role in the developing lens remains unexplored.

Methods:: Deletion of FAK was carried out by crossing mice containing a conditional floxed FAK allele to a Nestin-Cre line with robust expression in the lens at embryonic day 15. Visualization of individual lens epithelial and fiber cells was further achieved by mating these animals to a mosaic GFP reporter mouse line. The resulting lens phenotype was assayed using laser scanning confocal microscopy of vibratome sections from early postnatal mice.

Results:: FAK deletion in the lens results in numerous defects in epithelial and fiber cell development. The morphology of FAK-/- epithelial cells lacked extensive basolateral protrusions and exhibited an elongated morphology. These cells also showed an increase in SMA-1 staining, reflecting a mild epithelial to mesenchymal transition. While the lens capsule was intact at postnatal day 1, aberrant secretion of extracellular matrix was present throughout the epithelial layer. Fiber cell differentiation occurred normally, but they failed to properly degrade their nuclei. The endfeet of these cells remained stuck to the capsule, preventing proper detachment and posterior suture formation. As new membrane was added over time, the fiber cells acquired a convex morphology and expelled the lens core through the posterior capsule. Following rupture, the lens degraded and was absent in adult eyes.

Conclusions:: Proper lens development requires FAK signaling. Our results suggest that FAK regulates lens epithelial cell morphology, prevents epithelial to mesenchymal differentiation and is required for the migration of fiber cell endfeet along the capsule and detachment at the posterior suture. Lastly, FAK may play an important role in organization of the lens capsule extracellular matrix.

Keywords: extracellular matrix • signal transduction • transgenics/knock-outs