Patterning morphogenesis through the planar cell polarity pathway
Dr Danelle Devenport, Princeton University, USA
How cells assemble into precise spatial patterns from undifferentiated progenitors is a fundamental but still poorly understood question in developmental biology and tissue engineering. Using the mouse embryonic skin as a model system, which is decorated with regularly spaced, globally polarized hair follicles (HFs) that arise through self-organized epidermal-dermal signaling and planar polarized morphogenesis, the Devenport lab has established methods to perform long-term live imaging of epidermal development to capture the individual and collective cell behaviors that drive polarized morphogenesis of mammalian hair follicles. Using cell tracking methods to monitor the behaviors of every cell within developing hair placodes over the course of polarization, this live imaging approach revealed an unanticipated and novel pattern of collective cell movements that generates both morphological and cell fate asymmetry of developing follicles. The spatial patterning of hair follicle progenitors through Wnt and Shh pathways establish a morphogenetic program of collective cell motion. Moreover, this morphogenetic program displays unanticipated robustness, being able to withstand perturbations to spatial patterning though feedback between cell fate specification and cell motility.
Revealing functional interactions between PAR proteins and the cytoskeleton in C. elegans zygote
Dr Josana Rodrigues, Newcastle University, UK
C. elegans zygote polarity relies on the asymmetric distribution of key polarity effectors, the PAR proteins, which in turn drive the zygotes’ asymmetric cell division leading to the germline and somatic cell precursors. Zygote polarisation is triggered by the sperm-donated centrosome via two semi-redundant pathways. First, the centrosome reorganises the cortical acto-myosin meshwork, inducing a cortical flow away from the newly defined posterior pole. This flow transports a subset of PAR proteins to the anterior half of the zygote. Second, centrosomal-microtubules (MT) induce the membrane loading of another set of PAR proteins at the posterior. Anteriorly and posteriorly localised PARs mutually antagonise each other, further ensuring their asymmetric distribution. The existence of these two pathways confers robustness, but also makes it hard to tease these mechanisms apart and has impeded the identification of regulatory components for the MT pathway. We reasoned that knock-down of MT-pathway regulators in a mutant strain where the acto-myosin flow is perturbed, should lead to strong polarity defects and lethality that are not observed when knocked-down in a wild-type strain. Using this strategy we are identifying MT-pathway candidates and their characterisation is revealing new roles for microtubules in zygote polarity. I will discuss the unexpected role of a chromokinesin in cell polarity establishment.
Left-right asymmetry of the heart : forming the right loop
Dr Sigolène Meilhac, Institut Imagine, Institut Pasteur, France
Left-right partitioning of the heart underlies the double blood circulation. Impairment of left-right patterning leads to heterotaxy, including severe cardiac malformations. Asymmetric heart morphogenesis is initiated in the embryo, by the rightward looping of the cardiac tube, which determines cardiac chamber alignment. Whereas the molecular cascade breaking the symmetry has been well characterised, how Nodal signalling is sensed by precursor cells to generate asymmetric organogenesis remains unknown. Heart looping has been previously analysed simply as a direction. The associated 3D shape changes have now been reconstructed and quantified in the mouse. In combination with cell labelling and computer simulations, a model of heart looping has been proposed, centred on the buckling of the tube growing between fixed poles, which functions as a random asymmetry generator. Sequential and opposite left-right asymmetries have been identified at the poles, which bias the buckling, thus leading to a helical shape. Manipulating Nodal signalling in time and space shows that it is not involved in the buckling, but that it is required transiently in heart precursors, to amplify and coordinate asymmetries at the heart tube poles and thus generate a robust helical shape. Laterality defects are often partially penetrant, and thus it is a challenge to correlate embryonic anomalies with specific congenital defects. A multimodality imaging pipeline has been developed to phenotype laterality defects at multiple stages and at multiple scales within a single individual. These tools and model provide a novel framework to analyse the origin of complex congenital heart defects.
Mechanics and mechanisms of tube formation
Dr Katja Röper, Medical Research Council Laboratory of Molecular Biology, UK
We study the dynamic behaviour of epithelial sheets of cells during organ formation, in particular during the formation of tubular organs, using the formation of the tubes of the salivary glands in the Drosophila embryo as our main model system. These tubes form through a process of budding, and we have recently uncovered that cell behaviours across the tissue primordium, the placode, are highly patterned during the initial formation of the tube from a flat epithelial sheet. Within the apical domain, isotropic constriction near the invagination point combines with polarised cell intercalation away from the invagination point. In 3D, this was due to strong wedging of cells near the pit, as well as tilting towards it, and interleaving of cells across the tissue. I will discuss these findings in the light of the analysis of mutants that fail proper tube formation and that allow us to dissect the contribution of different behaviours as well as their potential mechanical interplay. We know that apical constriction in the placodal cells is driven by an apical medial acto-myosin network that depends on an intact longitudinal microtubule cytoskeleton. This microtubule network becomes acentrosomal concomitant with apical constriction, and we can now show that this change in organisation is driven by a two-pronged mechanism, combining loss of nucleation capacity at centrosomes with release of controsomal microtubules through severing followed by selective stabilisation of new free minus ends within the apical domain.