Jean-Paul Borg, Arnaud Echard, Nathan (Nate) Goehring, André Le Bivic (organisateur), Pierre-François Lenne, Ian G. Macara, Jean-Léon Maître, Sophie Martin, Fernando Martín-Belmonte, Mireille Montcouquiol, Edwin Munro, Caren Norden, Ewa Paluch, Rytis Prekeris, Josana Rodriguez, Bénédicte Sanson, Daniel St Johnston (organisateur), Ulrich (Ulli) Tepass, John Wallingford, Chiara Zurzolo
by André Le Bivic
25 – 30 June, 2018
Twenty researchers (biologists and physicists including 3 young researchers starting their own group) from Europe, the United States and Canada met to discuss the relationship between cell polarity and morphogenesis, as these two biological phenomena are intimately related. The field of cell polarity investigates how cells establish asymmetries to make one side of the cell different from the other, whereas morphogenesis analyses how cells change shape and position to generate complex structures, such as tissues and organs, during the process of development. Cell polarity underpins morphogenesis, because cells must organise polarised actin structures on specific sides in order to change shape and to move, whereas morphogenesis impinges on polarity, because changes in the distribution or activity of polarity factors can alter cell shape, and these changes in turn must lead to changes in the sizes of polarised domains within cells. The meeting was conducted under “Chatham house” rules, in which each speaker was encouraged to present their unpublished results and future plans in the style of a lab meeting, on the understanding that they would remain confidential. This proved a very effective way to encourage discussion of the links between these fields and how information sharing could catalyse advances in research in both areas. All participants meeting reported finding the meeting very stimulating and it led to several new collaborations between the participants.
Keywords : Cell polarity, actin cytoskeleton, polarity complexes, early development, epithelia, neurons
The first day of the meeting focused on the best characterised systems to study cell polarity: anterior-posterior axis formation in C. elegans (Edwin Munro, Nate Goehring, Josana Rodriguez) and Drosophila (Daniel St Johnston), polarised growth in yeast (Sophie Martin) and blastocoel formation in the Mouse (Jean-Léon Maître). The worm talks revealed the importance of crosstalk between polarity factors and the acto-myosin cytoskeleton, as cortical flows are necessary for both the establishment and maintenance of polarity, while the regulation of clustering versus the diffusion of active polarity factors are also essential. These processes can only be understood by mathematical modelling, which also reveals that there is a minimum cell size over which the mutual antagonism between diffusing Par proteins can polarise cells. Drosophila axis formation also depends on the PAR proteins and on acto-myosin activity, but in a different way from C. elegans, highlighting the plasticity of polarity systems in different contexts. This theme, which extended throughout the meeting, was also raised by Sophie Martin’s talk, who described how Cdc42 is activated by different mechanisms in different contexts to polarise fission yeast cells and how these mechanisms could be dissected using optogenetic approaches. The last talk of the day from Jean-Léon Maître discussed how the emergence of apical-basal polarity in mouse blastomeres controls the decision between inner cell mass and trophectoderm in a complex process that depends on cortical contractility and spindle orientation and feedback. Jean-Leon then went on to describe the process of blastocoel formation through cavitation, which breaks symmetry in the embryo.
The first session of second day considered how the polarised arrangement of acto-myosin generates forces within cells, how these forces are regulated and how this drives morphogenetic changes. Ewa Paluch described her studies on the organisation of the cell cortex and how it relates to cortical tension. She reported found that cells round in mitosis up because their cortical tension increases, but paradoxically this correlates with a decrease in the thickness of the cortex. More detailed analysis suggests that the key factors determining cortical tension are myosin activity, steric inhibition of the entry of myosin into the cortex, actin filament length and the degree to which the filaments are cross-linked by actin-binding proteins. Bénédicte Sanson introduced the topic of in Drosophila and discussed the role of apical-basal polarity factors in the formation of para-segmental furrows, before presenting evidence that proteins at tricellular junctions play a particularly important role in controlling cell shape changes during Drosophila anterior-posterior axis elongation. Pierre-Francois Lenne continued the theme of Drosophila axis elongation, with an analysis of how forces change the shape of cells. Elegant experiments with optical tweezers show that cells are elastic on short time scales, i.e they return to their original shape after deformation, but show viscous shape changes in response to imposed deformations on longer time scales. The acto-myosin pulses that drive germband elongation in Drosophila occur on these longer time scales, explaining why they cell shapes are irreversibly altered during this process. John Wallingford carried on the theme of axis elongation by reporting how the planar cell polarity (PCP) pathway controls convergent extension of the anterior-posterior axis in Xenopus. His group found that PCP proteins show dynamic enrichments at shrinking cell-cell junctions that correlate with increased levels of Cadherin and actomyosin. Ulrich Tepass considered how Drosophila neural stem cells (neuroblasts) undergo a programmed loss of epithelial polarity to delaminate from the primary embryonic epithelium. This process is driven by pulsatile acto-myosin contractions, but only the cells that can endocytose polarity and adhesion proteins decrease their apical domains and delaminate.
The next set of talks examined how secretory vesicles are targeted to the correct domains in polarised epithelial cells and how this relates to the cortical polarity factors. Chiara Zurzolo described a new pathway that directs the trafficking of GPI-linked proteins to the apical domain of epithelial cells, as well as providing an illuminating aside on the structure of tunnelling nanotubes. Ian Macara then reported that the Par-3 polarity protein interacts with the conserved exocyst complex, which helps catalyse vesicle fusion with the plasma membrane. Elegant single molecule imaging revealed that the exocyst arrives at the plasma membrane shortly before vesicle fusion and suggested that there are many exocyst complexes per vesicle. Rytis Preteris, Arnaud Echard and Fernando Martín-Belmonte addressed the question of how the apical domain is established when cultured cells form lumen-filled cysts in 3D matrices. Rytis reported that Cingulin associates with the microtubules in the midbody to target secretion of apical factors to the site of cell division. Arnaud found that Rab35 is recruited to the cleavage furrow between dividing cells and functions in a parallel pathway to target secretion to the apical initiation site, in part by regulating actin organisation. Finally, Fernando described the essential roles of several actin regulators in this process and how their knock-down could lead to a complete inversion of polarity.
Caren Norden took the discussion from tissue culture systems into the fish retina and hindbrain, where she has analysed how nuclei move towards the apical domain prior to division in the process of interkinetic nuclear migration, presenting evidence that the mechanism of nuclear movement relates to the shape of the tissue. André Le Bivic presented some beautiful super-resolution images that showed the relationships between the localisations of polarity factors and the organisation of the actin cytoskeleton. He then went on to analyse the evolutionary origin of polarised epithelia and presented very intriguing data on epithelia-like tissues in sponges.
The final session returned to the topic of planar polarity with a talk from Jean-Paul Borg on the role the PCP protein Vangl2 in cancer metastasis and a fascinating presentation from Mireille Montcouquiol on how the cochlear cells of the ear become planar polarised. This revealed an unexpected relationship between the planar polarity pathway and the proteins involved in spindle orientation and apical-basal polarity, which made us all realise that we have a lot still to discover about how the polarity is adapted in different cell-types and contexts.
Everyone had a very enjoyable and stimulating five days at les Treilles, thanks to the wonderful environment, the excellent food and support and the outstanding science that was presented. The meeting provoked intense discussions and many new ideas that will help move this interdisciplinary area of research forward in the next few years.