Molecular mechanisms of polarized cell behaviour during axis elongation

Jessica Yu 1, 2 ; Rodrigo Fernandez-Gonzalez 1, 2, 3, 4

 1. Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario, Canada; 2. Translational Biology and Engineering Program, Ted Rogers Centre for Heart Research, University of Toronto, Toronto, Ontario, Canada; 3. Cell and Systems Biology, University of Toronto, Toronto, Ontario, Canada; 4. Developmental and Stem Cell Biology Program, Hospital for Sick Kids, Toronto, Ontario, Canada

Axis elongation is a conserved process in which animals establish the head-to-tail body axis. Defective axis elongation in humans leads to anencephaly, a lethal congenital disorder in which the brain does not form. In Drosophila, axis elongation is driven by directional cell rearrangements and oriented cell divisions. We recently reported that mesectoderm cells, a population of glial precursors in Drosophila, divide parallel to the head-to-tail axis of the embryo to reduce strain generated by cell rearrangements and facilitate axis elongation. However, the molecular mechanisms that orient mesectoderm cell behaviours are unclear. Using spinning disk confocal microscopy and quantitative image analysis, we found that the polarity protein Par-3 localized specifically to new junctions between daughter mesectoderm cells after division. Consequently, Par-3 became asymmetrically distributed, and accumulated in junctions separating anterior and posterior mesectoderm neighbours. In addition, the molecular motor non-muscle Myosin II and its upstream regulator Rho-kinase were also asymmetrically distributed in mesectoderm cells following division, forming supracellular cables parallel to the head-to-tail axis on either side of the tissue. The polarization of both myosin and Par-3 was lost when embryos were treated with Y-27632, an inhibitor of Rho-kinase, and cells no longer divided with a bias along the head-to-tail axis, with some cells dividing perpendicular to the monolayer plane. Injection of RNAi targeting Par-3 yielded similar results, including loss of myosin polarity, indicating that Par-3 directs myosin localization in mesectoderm cells. Our results suggest that Rho-kinase and Par-3 organizes mesectoderm cell polarity, and is critical to establish oriented cell behaviours and tissue boundaries. We are currently investigating the interdependency of Myosin and Par-3 planar polarity in the mesectoderm, and its effects on junctional stability, tissue mechanics, and architecture when either polarity is impaired.