INVESTIGATING THE ROLE OF PLANAR CELL POLARITY DURING MURINE HAIR PLACODE POLARIZATION

Abstract

Breaking symmetry is essential for the development of complex structures at almost every scale, from macromolecular to cellular to organismal. One such symmetry breaking process is establishment of planar cell polarity (PCP), the coordinated orientation and alignment of cells within the plane of a tissue. This is through the asymmetric localization of the three transmembrane proteins, Frizzled-6, Vangl2, and Celsr1. PCP is required for collective cell behaviors such as alignment of single-cell protrusions of the Drosophila wing to multicellular structures such as the mammalian hair follicle. However, while the components of the PCP pathway have largely been identified, how they regulate downstream cytoskeletal dynamics to generate collective cell movements in multicellular structures is not well understood. Our previous work has shown that PCP and downstream myosin is required for counter-rotational cell flows that drive polarized hair follicle morphogenesis. Here, using live- and fixed-imaging combined with genetic and pharmacological, and continuum mechanics modeling we investigate the role of PCP at the apical and basal surface in generating cell flows. We find that PCP does not correlate with apical junction behavior but is required for basal protrusions. This suggests that basal crawling may drive collective cell migration to polarize the follicle. We then test this hypothesis through continuum mechanics modeling and find that basal forces are sufficient to drive apical flows. We then find that through inhibition cytoskeletal regulators of branched actin polymerization that basal crawling is required for hair follicle polarization. We then explore PCP organization at the cell membrane and the role of PCP in tissue-wide alignment of hair placode morphogenesis. We conclude by discussing open questions with respect to PCP-directed placode morphogenesis and new avenues to investigate such questions.