The planar cell polarity (PCP) signaling pathway polarizes epithelial cells along an axis parallel to the epithelial sheet to generate front-back asymmetry at the tissue level. Failures in PCP signaling underlie developmental defects and diseases such as neural tube and heart defects, deafness, and contribute to cancer. Six core PCP proteins assemble into large, asymmetric clusters at cell-cell junctions. In the canonical picture of this pathway, Van Gogh (Vang) and Prickle (Pk) recruited to the proximal side of the cell-cell junction while Frizzled (Fz), Diego (Dgo), and Dishevelled (Dsh) are recruited to the distal side. Flamingo (Fmi) forms intercellular bridges connecting polarity between adjacent cells. Neither the potential requirement for clusters nor their detailed organization are understood. To explore these questions, we used total internal reflection fluorescence (TIRF) microscopy to image live GFP-tagged PCP proteins in the Drosophila pupal wing disc. Quantification of these data allowed us to count the number of a given PCP protein in individual complexes. Using this approach, we find that the molecular size distribution of PCP complexes follows an exponential function, suggestive of a single underlying growth mechanism. As expected, for Fmi, these data reveal equivalent size distributions and numbers of clusters on the proximal and distal sides of individual cells. In contrast, Fz and Pk are highly polarized, being almost absent on the proximal and distal sides, respectively. On the distal size, we find Vang is present in numbers equal to Fz, Fmi, Dgo, and Dsh, a finding not anticipated based on previous confocal imaging. On the proximal side, Vang exists in 3x higher numbers relative to other components. Finally, we show that mutations that block Dsh oligomerization decrease average cluster sizes and result in PCP phenotypic defects, demonstrating a requirement for large clusters in polarization. In conclusion, our findings provide a quantitative insight into how the PCP mechanism generates stable polarization during early embryonic development.

Speaker 2: Mr. Dominic Devlin (University of Auckland)
Title: Evolution of morphogenesis can drive the emergence of stem to non-stem cell differentiation

Abstract: Complex multicellularity is characterised by irreversible differentiation from stem cells to non-stem cells (i.e., stem-cell-systems). This research explores why stem-cell-systems repeatedly evolve among multicellular organisms. Current theoretical research often assumes that a cell becomes irreversibly differentiated when it specialises heavily in a somatic task. We question this assumption by investigating the interplay between morphogenesis and irreversible differentiation through stem-cell-systems. To undertake this research, we developed a multi-scale computational model of multicellular developmental evolution. Our findings reveal that the emergence of stem-cell-systems is facilitated by the evolution of morphogenesis. Moreover, we demonstrate a bidirectional relationship, as organisms with stem-cell-systems have much more reproducible morphologies than organisms without stem-cell-systems. By linking morphological evolution to stem-cell-systems, this research offers valuable insights into the fundamental mechanisms driving the evolution of complex developmental processes, such as body elongation and branching morphogenesis.