The RhoGAP-myosin Myo9b regulates ocular lens pit morphogenesis

Background During eye development the lens placode invaginates to form the lens pit. Further bending of lens epithelium and separation from ectoderm leads eventually to a spherical lens vesicle with enclosed extracellular fluid. Changes in epithelial morphology involve the actin cytoskeleton and its regulators. The myosin Myo9b is simultaneously an actin-based motor and Rho GTPase-activating protein that regulates actin cytoskeleton organization. Myo9b-deficient adult mice and embryos were analyzed for eye malformations and alterations in lens development. Results Myo9b-deficient mice showed a high incidence of microphthalmia and cataracts with occasional blepharitis. Formation of the lens vesicle during embryonic lens development was disordered in virtually all embryos. Lens placode invagination was less deep and gave rise to a conical structure instead of a spherical pit. At later stages either no lens vesicle was formed or a significantly smaller one that was not enclosed by the optic cup. Expression of the cell fate marker Pax6 was not altered. Staining of adherens junctions and F-actin was most intense at the tip of conical invaginations, suggesting that mechanical forces are not properly coordinated between epithelial cells that form the pit. Conclusions Myo9b is a critical regulator of ocular lens vesicle morphogenesis during eye development.