In many organ systems, cells projecting hundreds of beating cilia, called multiciliate cells, produce a vigorous fluid flow that transports biological materials along luminal surfaces. Multiciliate cells populate the respiratory and reproductive tracts, and the ventricles of the brain, and the flow they produce has significant implications for human health. To be effective in organ function, ciliary flow has to direct along a specific axis: in the lung, for example, flow propels mucus out of rather than deeper into the airways. To produce directed flow, developing epithelia need to acquire a planar axis, thus orienting cilia beating within a cell, as well as between cells. To determine how multiciliate cells acquire planar cell polarity (PCP), we have pioneered a model system, namely the X. laevis larval skin. Multiciliate cells begin to differentiate in the developing skin soon after gastrulation, producing a vigorous ciliary flow that invariably is directed from anterior to posterior. A global patterning event that occurs during gastrulation is known to fix the direction of ciliary flow, but nothing is known about the mechanisms that mediate this patterning event as is the case in all other known examples of PCP. In this exploratory proposal, we will test a new model where the direction of planar polarity is dictated in part by the orientation of tensile stress that occurs in the tissue during embryogenesis. In the case of the skin, this tensile stress occurs during gastrulation via the forces generated by mesoderm during involution and axial elongation. Specifically, we will employ a device, called the tractor pull, to apply oriented stress to isolated developing skin, at stages when the planar axis is normally established. These experiments will extend on preliminary findings, determine the parameters by which oriented stress can specify a planar axis, and determine whether oriented stress works upstream or in parallel with the PCP signaling pathway. In sum, the experiments proposed here will provide an important proof of principle, thus establishing a new model for studying how forces in the embryo sculpt and pattern tissues.
Multiciliate cells play important roles in human health by generating directed fluid flow in the brain, lung and reproductive tract, but the mechanisms that orient flow direction in relation to an organ axis are poorly understood. To study these mechanisms, the proposed research will employ a model system to test a novel hypothesis where tensile stress incurred by a developing tissue acts to orient the direction of ciliary flow. Results from the proposed experiments will potentially establish a new paradigm for how the direction of ciliary flow is established during development, and will have implications for diagnosis and treatment of human disease that affect ciliated epithelia, such as the ciliary defects that occurs during primary ciliary dyskinesia and Kartegener's syndrome.