Distinct mechanisms of planar polarization by the core and Fat-Dachsous planar polarity pathways in the Drosophila wing

Epithelial polarization by the planar cell polarity complex is exclusively non-cell autonomous

For cells to polarize collectively along a tissue plane, asymmetrically localized planar cell polarity (PCP) complexes must form intercellular contacts between neighboring cells. Yet, it is unknown whether asymmetric segregation of PCP complexes requires cell-cell contact, or if cell autonomous, antagonistic interactions are sufficient for polarization. To test this, we generated mouse chimeras consisting of dual PCP-reporter cells mixed with unlabeled cells that cannot form PCP bridges. In the absence of intercellular interactions, PCP proteins failed to polarize cell autonomously. Rather, PCP-mediated contacts along single cell-cell interfaces were sufficient to sort PCP components to opposite sides of the junction, independent of junction orientation. Thus, intercellular binding of PCP complexes is the critical step that initiates sorting of opposing PCP complexes to generate asymmetry.

Fat4 intracellular domain controls internalization of Fat4/Dchs1 planar polarity membrane complexes

The Fat/Dachsous (Ft/Ds) pathway is a highly conserved pathway regulating planar cell polarity (PCP) across different animal species. Proteins from the Ft and Ds family are large transmembrane protocadherins that form heterophilic complexes on the boundaries between cells. Fat4 and Dchs1, the main mammalian homologs of this pathway, have been implicated in PCP in various epithelial tissues and were shown to form extremely stable complexes at the boundaries between cells. It is unclear, however, what are the dynamics controlling such stable boundary complexes, and how the formation and internalization of these complexes is regulated. Here, we use quantitative live imaging to elucidate the role of the intracellular domains (ICDs) of Fat4 and Dchs1 in regulating Fat4/Dchs1 complex dynamics. We show that removing the ICD of Fat4 results in a reduction of both trans-endocytosis of Dchs1 into the Fat4 cells and boundary accumulation of Fat4/Dchs1 complexes, but does not affect the diffusion of the complexes at the boundary. We further show that the ICD of Fat4 controls the internalization rate of Fat4/Dchs1 complexes. Finally, we find that while actin polymerization is required for the accumulation at the boundary of Fat4/Dchs1 complexes, we do not identify correlations between Fat4/Dchs1 complexes and local actin accumulation. Overall, we suggest that the Fat4 ICD is important for the internalization and plasticity of the highly stable Fat4/Dchs1 complexes associated with PCP.

Tissue shear as a cue for aligning planar polarity in the developing Drosophila wing

Planar polarity establishment in epithelia requires interpretation of directional tissue-level information at cellular and molecular levels. Mechanical forces exerted during tissue morphogenesis are emerging as crucial tissue-level directional cues, yet the mechanisms by which they regulate planar polarity are poorly understood. Using the Drosophila pupal wing, we confirm that tissue stress promotes proximal-distal (PD) planar polarity alignment. Moreover, high tissue stress anisotropy can reduce the rate of accumulation and lower the stability on cell junctions of the core planar polarity protein Frizzled (Fz). Notably, under high tissue stress anisotropy, we see an increased gradient of cell flow, characterised by differential velocities across adjacent cell rows. This promotes core protein turnover at cell-cell contacts parallel to the flow direction, possibly via dissociation of transmembrane complexes by shear forces. We propose that gradients of cell flow play a critical role in establishing and maintaining PD-oriented polarity alignment in the developing pupal wing.

Fat-Dachsous planar polarity function requires two distinct heterophilic cadherin-cadherin binding interactions

Fat and Dachsous are evolutionarily conserved atypical cadherins that regulate polarized cell behaviors. In the Drosophila wing, they interact heterophilically between neighboring cells, localize asymmetrically to opposite cell ends, and control wing shape by regulating oriented cell rearrangements and divisions. Fat and Dachsous have 34 and 27 cadherin repeats, respectively, and previous work has identified trans interactions between their first four cadherin repeats. Here, we identify a second heterophilic binding site in their C-terminal cadherin repeats and show the conservation of this binding site in human Fat4 and Dachsous1. We provide evidence that both N- and C-terminal binding sites regulate the stability of Fat-Dachsous binding interactions and show that the N-terminal binding sites are partly dispensable for Fat-Dachsous function in vivo. Finally, we provide in vivo confirmation that the N-terminal repeats interact in an anti-parallel manner. We propose that multiple binding sites promote the clustering of Fat and Dachsous into a lattice-like array.

Perspectives on polarity - exploring biological asymmetry across scales

In this Perspective, Journal of Cell Science invited researchers working on cell and tissue polarity to share their thoughts on unique, emerging or open questions relating to their field. The goal of this article is to feature 'voices' from scientists around the world and at various career stages, to bring attention to innovative and thought-provoking topics of interest to the cell biology community. These voices discuss intriguing questions that consider polarity across scales, evolution, development and disease. What can yeast and protists tell us about the evolution of cell and tissue polarity in animals? How are cell fate and development influenced by emerging dynamics in cell polarity? What can we learn from atypical and extreme polarity systems? How can we arrive at a more unified biophysical understanding of polarity? Taken together, these pieces demonstrate the broad relevance of the fascinating phenomenon of cell polarization to diverse fundamental biological questions.

Molecular symmetry breaking in the Frizzled-dependent planar polarity pathway

The core planar polarity pathway consists of six proteins that form asymmetric intercellular complexes that segregate to opposite cell ends in developing tissues and specify polarized cell structures or behaviors. Within these complexes, the atypical cadherin Flamingo localizes on both sides of intercellular junctions, where it interacts homophilically in trans via its cadherin repeats, whereas the transmembrane proteins Frizzled and Strabismus localize to the opposite sides of apposing junctions. However, the molecular mechanisms underlying the formation of such asymmetric complexes are poorly understood. Using a novel tissue culture system, we determine the minimum requirements for asymmetric complex assembly in the absence of confounding feedback mechanisms. We show that complexes are intrinsically asymmetric and that an interaction of Frizzled and Flamingo in one cell with Flamingo in the neighboring cell is the key symmetry-breaking step. In contrast, Strabismus is unable to promote homophilic Flamingo trans binding and is only recruited into complexes once Frizzled has entered on the opposite side. This interaction with Strabismus requires intact intracellular loops of the seven-pass transmembrane domain of Flamingo. Once recruited, Strabismus stabilizes the intercellular complexes together with the three cytoplasmic core proteins. We propose a model whereby Flamingo exists in a closed conformation and binding of Frizzled in one cell results in a conformational change that allows its cadherin repeats to interact with a Flamingo molecule in the neighboring cell. Flamingo in the adjacent cell then undergoes a further change in the seven-pass transmembrane region that promotes the recruitment of Strabismus.

Local coordination of epithelial planar polarity in the maintenance and regeneration of the adult rat airway

The maintenance of planar polarity in airway multiciliated cells (MCCs) has been poorly characterized. We recently reported that the direction of ciliary beating in a surgically inverted tracheal segment remained inverted beyond the time required for the turnover of cells, without adjustment to global distal-to-proximal polarity. We hypothesized that the local maintenance of tissue-level polarity occurs via locally reproduced cells. To provide further insight regarding this hypothetical property, we performed allotransplantation of an inverted tracheal segment between wild-type (donor) and tdTomato-expressing (host) rats, with and without scratching the mucosa of the transplants. The origin of cells in the transplants was assessed using tdTomato-specific immunostaining. Ciliary movement and structures were observed by high-speed video and electron microscopy to analyze MCC orientations. Variabilities in the orientations of closely and distantly located MCCs were analyzed to evaluate the local- and broad-scale coordination of polarity, respectively. The epithelium was maintained by donor-derived cells in the non-scratched inverted transplant over 6 months, beyond one cycle of turnover. The inverted orientation of MCCs was also maintained throughout the non-scratched transplant. MCCs regenerated in the scratched transplant were derived from the host and exhibited diverse orientations across the transplant. However, the orientations of adjacent regenerated MCCs were often coordinated, indicating that airway MCCs can locally coordinate their orientations. A steady-state airway may maintain MCC orientation by locally reproducing MCCs via the local coordination of polarity. This local coordination enables the formation and maintenance of tissue-level polarity in small regions after mucosal injury.

Wnt 2022 EMBO | the Company of Biologists workshop and Yamada conference

Wnt2022 was held on November 15th-19th, 2022, in Awaji Yumebutai International Conference Center, Hyogo Prefecture, Japan, as an in-person meeting for the first time in last 3 years. Wnt signaling is a highly conserved pathway among various species. Since Wnt1 was discovered in 1982, a number of studies using many model animals and human samples have revealed that Wnt signaling plays crucial roles in embryonic development, tissue morphogenesis, and regeneration, as well as many other physiological and pathological processes. Since the year 2022 marks the 40th anniversary of Wnt research, we aimed to look back at our research progress and discuss the future direction of this field. The scientific program consisted of plenary lectures, invited talks, short talks selected from abstracts, and poster sessions. Whereas several different Wnt meetings have been held almost every year in Europe and the United States, this was the first Wnt meeting convened in Asia. Therefore, Wnt2022 was highly anticipated to bring together leaders and young scientists from Europe, the United States, and especially Asia and Oceania. In fact, 148 researchers from 21 countries attended this meeting. Although there were travel and administrative restrictions due to COVID-19, the meeting was highly successful in enabling face-to-face discussions.