Mechanical coordination and mechano-biochemical feedback in epithelial folding - during embryogenesis, the formation of body plan is largely driven by remodeling of epithelial tissues

While epithelial morphogenesis is regulated biochemically by genes and molecules, IT is also an intrinsically mechanical process controlled by cellular forces and tissue mechanical properties. Despite significant progress in deciphering the genetic and biochemical determinants of force generation, little is known about how tissue mechanical properties are regulated and how this regulation impact morphogenesis. The proposed study addresses these questions in the context of epithelial folding, a fundamental tissue construction mechanism in development. Using drosophila gastrulation as a model, our recent studies have elucidated how local tissue mechanical properties and global mechanical coordination are exploited by the embryo to facilitate mesoderm invagination, and how these apparent "passive" tissue properties are regulated by active cellular processes and feedback mechanisms. During gastrulation, mesoderm precursor cells constrict their cell apex and subsequently invaginate from the surface of the embryo by forming a ventral furrow. We found that as the tissue constricts apically, the tissue interior behaves as a viscous continuum and forms a laminar flow. This viscous response involves prompt cell surface expansion that relies on the function of the PI 4-kinase four wheel drive (FWD). In addition, we found that the transition from apical constriction to mesoderm invagination relies on compressive forces generated in the surrounding ectodermal tissues, which facilitate tissue folding by promoting mechanical bistability in the mesoderm. This long-range mechanical coordination is contingent on effective mechanical coupling between the mesoderm and the ectoderm. Finally, we found that the embryo employs a feedback mechanism involving RAB11-mediated vesicle trafficking to reinforce the physical integrity of the supracellular actomyosin network, allowing the tissue to promptly adapt to a rapid increase in tissue tension. In the proposed study, we will use a multipronged approach combining genetics, optogenetics, quantitative live-imaging, cell biology and biophysics to (1) investigate the mechanism underlying RAB11-medaited feedback regulation of actomyosin during apical constriction, (2) determine how PI-4 kinases and their product, PI4P, regulate force-induced cell surface expansion and how cell surface "expandability" determines local and global tissue mechanical properties, and (3) determine how tissue compression is generated in the ectoderm to promote long-range mechanical coordination during mesoderm invagination. Successful completion of our research goals will advance scientific knowledge by elucidating the regulatory networks and physical principles that dictate how tissues generate, sense and respond to mechanical forces to establish proper form and function in development.