Final Report Summary - WING MORPHOGENESIS (A physical basis for wing morphogenesis and planar cell polarity)
The central problem in developmental biology is to understand the mechanisms that determine tissue size, shape and polarity. We addressed this problem in the Drosophila pupal wing - at this stage, epithelial remodelling generates the final wing shape as cells in the hinge region of the wing contract, and cells in the more distal wing blade flow anisotropically towards the hinge. These tissue flows elongate and narrow the wing blade in its proximal distal (PD) axis (Aigouy et al., 2010). At the end of the process, the planar polarity of the tissue becomes morphologically obvious with the emergence of distally oriented wing hairs. Our goal was to understand quantitatively how wing shape emerges from the interactions of many constituent cells with specific patterns of mechanical properties, and how the shape of the wing is coordinated with its tissue (planar) polarity.
To tackle this problem, we used novel imaging and image analysis methods to identify and track every cell in the wing during the entire process. We quantified patterns of cell shape changes, cell divisions, cell rearrangements and cell deaths that occur as the wing changes its shape and developed novel theoretical methods to quantify the contributions of each of these cellular events to the shape change of the wing. By mechanically perturbing developing wings, we were able to distinguish which of these cellular events were actively controlled, and which were produced in response to epithelial stresses. These studies showed that the shaping of the pupal wing results from the interplay of patterns of autonomous cellular force generation, the resulting emergence of anisotropic tissues stresses, and the cellular responses to these stresses. A key insight that emerged from this work was that tissue stresses allow cells to communicate, coordinate their behaviors, and compensate for perturbations. This mechanical communication is key to the robustness and reproducibility of wing shape and size (Etournay et al., 2015).
A second major achievement has been to understand the emergence of planar cell polarity (PCP) patterns in the developing wing, and the bidirectional interplay between oriented epithelial morphogenesis and PCP. The Core PCP pathway is a key organizer of planar polarity in the wing. Core PCP proteins localize to apical adherens junctions, and form polarized asymmetric cortical domains that couple the polarity of adjacent cells. At the time that wing hairs form, Core PCP domains are globally aligned along the proximal distal axis of the wing with distal polarity. However nothing was known about how this pattern arises. Our work revealed that the distal Core PCP pattern in pupal wings evolves from an earlier pattern that is specified by organizer regions during larval growth – this initial pattern is shifted to face distally by the oriented morphogenesis that occurs at pupal stages. The distal shift in the Core PCP pattern is not only essential for the distal orientation of wing hairs – it also facilitates stress-induced epithelial remodelling along the proximal distal axis. Thus, there is a bidirectional interplay between Core PCP and epithelial remodeling: oriented remodelling shifts global patterns of planar polarity, and reorientation of planar polarity promotes the completion of epithelial remodelling.
Taken together, the work supported by this grant has revealed the physical basis of wing morphogenesis, and uncovered a key role for tissue stresses in coordinating cell behaviour to produce reproducible tissue shape changes. It has explained how planar cell polarity systems mediate cellular responses to tissue stress and how they couple tissue shape and tissue polarity.
To tackle this problem, we used novel imaging and image analysis methods to identify and track every cell in the wing during the entire process. We quantified patterns of cell shape changes, cell divisions, cell rearrangements and cell deaths that occur as the wing changes its shape and developed novel theoretical methods to quantify the contributions of each of these cellular events to the shape change of the wing. By mechanically perturbing developing wings, we were able to distinguish which of these cellular events were actively controlled, and which were produced in response to epithelial stresses. These studies showed that the shaping of the pupal wing results from the interplay of patterns of autonomous cellular force generation, the resulting emergence of anisotropic tissues stresses, and the cellular responses to these stresses. A key insight that emerged from this work was that tissue stresses allow cells to communicate, coordinate their behaviors, and compensate for perturbations. This mechanical communication is key to the robustness and reproducibility of wing shape and size (Etournay et al., 2015).
A second major achievement has been to understand the emergence of planar cell polarity (PCP) patterns in the developing wing, and the bidirectional interplay between oriented epithelial morphogenesis and PCP. The Core PCP pathway is a key organizer of planar polarity in the wing. Core PCP proteins localize to apical adherens junctions, and form polarized asymmetric cortical domains that couple the polarity of adjacent cells. At the time that wing hairs form, Core PCP domains are globally aligned along the proximal distal axis of the wing with distal polarity. However nothing was known about how this pattern arises. Our work revealed that the distal Core PCP pattern in pupal wings evolves from an earlier pattern that is specified by organizer regions during larval growth – this initial pattern is shifted to face distally by the oriented morphogenesis that occurs at pupal stages. The distal shift in the Core PCP pattern is not only essential for the distal orientation of wing hairs – it also facilitates stress-induced epithelial remodelling along the proximal distal axis. Thus, there is a bidirectional interplay between Core PCP and epithelial remodeling: oriented remodelling shifts global patterns of planar polarity, and reorientation of planar polarity promotes the completion of epithelial remodelling.
Taken together, the work supported by this grant has revealed the physical basis of wing morphogenesis, and uncovered a key role for tissue stresses in coordinating cell behaviour to produce reproducible tissue shape changes. It has explained how planar cell polarity systems mediate cellular responses to tissue stress and how they couple tissue shape and tissue polarity.