Disease-related rod domain E1841K mutation in the Myh9 (non-muscle myosin IIA/NMIIA) gene suppresses anisotropic tension required for hexagonal packing of mouse lens meridional row cells.

The mouse ocular lens is an excellent vertebrate model system for studying cell packing and cell shape changes during tissue morphogenesis. The lens is composed of epithelial and fiber cells. During differentiation, the lens epithelial cells transform from randomly packed cells to hexagon-shaped, hexagonally-packed cells to form meridional row (MR) cells. The MR cells further differentiate and elongate into fiber cells, maintaining their hexagon shapes as new layers of fiber cells are formed. We have previously reported that the human disease-related rod domain NMIIA(Myh9)-E1841K mutation, which impairs the bipolar filament assembly of NMIIA, significantly disrupts MR hexagonal cell shape and hexagonal packing. To determine how NMIIA regulates cell shape and organization during mouse lens cellular differentiation, we studied F-actin and NMIIA localization and force distribution at the membrane of MR cells by confocal microscopy and computational analyses, respectively. We observed that the NMIIA-E1841K mutation disrupts the ratio of the F-actin membrane to cytoplasmic distribution in MR cells, resulting in greater F-actin associated with the membrane. Employing Bayesian Mechanical Inference of the imaging data, we discovered that MR cells in control lenses exhibit anisotropic junctional tension, in which relative tension is more concentrated at the anterior-posteriorly (AP) oriented edges. In contrast, MR cells in mutant lenses show isotropic junctional tension on all sides. In addition, immunofluorescence of MR cells also demonstrates anisotropic enrichment of NMIIA and N-cadherin at AP-oriented sides of control MR cells, but equal distributions on all sides of mutant MR cells. Together, our data suggests that the E1841K mutation results in altered F-actin, NMIIA, and N-cadherin distributions. These alterations disrupt the pattern of mechanical forces within the tissue, leading to irregular cell shapes and disordered packing structures during mouse lens epithelial cell differentiation and morphogenesis.