Coordinating tissue surface contraction and basement membrane reorganisation to shape an organ in three-dimensions

Most of our organs contain cells that adhere to one another through lateral adhesion contacts to form sheets, called epithelia. In these sheets, cells all point in the same direction. Therefore, an epithelium presents two distinct surfaces - a top and a bottom surface. This top-bottom orientation is im-portant because it underpins the ability of epithelia to control the absorption of nutrients (intestine), oxygen (lung) or the filtration of salts and toxins (kid-ney or liver). In addition to this top-bottom orientation, epithelia also adopt specific shapes. When an epithelial sheet closes onto itself, it forms a tube or cavity. The oesophagus, intestine, lung, kidney and stomach are all tubu-lar epithelial structures. With this new research proposal, we will investigate how cells coordinate changes in their shape to generate these structures, vital for organ function. Typically, epithelia are shaped through a combination of internal force-generating machineries, which power cell deformation, and external constraints, including mechanical. Recent work in our laboratory has identified a novel contractile machinery that can power the reduction of the bottom surface of an epithelium, to shape the whole tissue as a dome. Here, we will characterise this new contractile machinery in organ formation. Moreover, to understand what induces contraction of the basal tissue surface as a whole, we will also need to determine how cells interact to coordinate their contraction. Our recent work indicates that calcium fluxes between cells promote this coordination. We will determine how these calcium fluxes are generated and how they regulate cell basal contraction. Altogether, we expect our work establishing new pathways in epithelial tissue shape control, to be broadly relevant to our understanding of epithelial tissue development, vital to supporting organ function.

The objective of this proposal is to understand how cells coordinate changes in their morphology to shape an epithelium in 3D. Reducing the apical or basal surface area of epithelial cells is a key mechanism of epithelial tissue deformation, which underpins formation of folds, tubes, and cavities that are essential for organ function. While progress continues to be made in our understanding of the pathways that induce changes in cell apical area, we know comparatively much less about the pathways that induce cell basal constriction. Addressing this gap in our understanding of epithelial morphogenesis is required if we want to understand how complex 3D structures arise in organogenesis. In studying this, using the genetically amenable fly eye, we made the surprising discovery that cell basal constriction involves a contractile machinery that differs from the non-muscle MyosinII cytoskeleton that powers apical constriction. Cell basal constriction involves factors typically required to support skeletal-muscle contraction. Thus, alternative contractile cytoskeletons operate in epithelial cell constriction, depending on the tissue surface considered. Capitalizing on this discovery, we will i) characterize this new contractile cytoskeleton in epithelial morphogenesis, ii) determine how cells coordinate their basal constriction to achieve tissue level contraction, and iii) examine the relationship between tissue surface contraction and basement membrane organization. We expect that the principles we uncover will be of general importance, since basal surface contraction is a conserved mechanism of epithelial tissue shape determination. By focusing on a genetically tractable animal model system of epithelial morphogenesis, this proposal aims to elucidate how a force-generating machinery working within cells, and biomechanical regulation from the micro-environment come together to induce the functional organization of a complex tissue, in health and disease.