Contemporary morphogenesis

Epithelia change shape extensively during embryo development and this requires the remodeling of the contacts between cells, which include adherens and occluding junctions. Whereas there is much knowledge about how bicellular contacts are remodeled, less is known about tricellular contacts, the epithelial cell’s “corners”. Cell behaviours important to shape tissues such as polarised cell intercalation, cell division and cell extrusion require the remodelling of cell-cell contacts and pose a conformational problem at tricellular junctions that has started to be addressed. Also there is evidence that tricellular vertices may be important for tension sensing and force transmission during epithelial remodelling. Until recently, resident proteins at tricellular contacts were known for occluding junctions, both in vertebrates (angulins and tricellulins) and in invertebrates (Gliotactin, Anakonda, M6), but not for adherens junctions. We have now identified the adhesion molecule Sidekick, known before for its role in synaptogenesis in the visual system, as the first known component of tricellular adherens junctions (tAJs) in Drosophila epithelia. The identification of Sidekick gives an opportunity to study the role of tricellular adherens junctions in morphogenesis. Clonal analysis showed that two cells, rather than three cells, contributing Sdk are sufficient for tAJs localization. Super-resolution imaging using structured illumination reveals that Sdk proteins form string-like structures at vertices, suggesting an adhesion plaque there. Postulating that Sdk may have a role in epithelia where adherens junctions are actively remodeled, we analyzed the phenotype of sdk null mutant embryos during Drosophila axis extension, using quantitative methods. We find that apical cell shapes are abnormal in sdk mutants, suggesting a defect in tissue remodeling during convergence and extension. Moreover, adhesion at apical vertices is compromised in rearranging cells, with apical tears in the cortex forming and persisting throughout axis extension, especially at the centers of rosettes. Finally, we show that polarized cell intercalation is decreased in sdk mutants. Mathematical modeling of the cell behaviours supports the notion that the T1 transitions of polarized cell intercalation are delayed in sdk mutants, in particular in rosettes. We propose that this delay, in combination with a change in the mechanical properties of the converging and extending tissue, causes the abnormal apical cell shapes in sdk mutant embryos.

Throughout the lifespan of an organism, tissues are remodeled to sculpt organs and organisms and to maintain tissue integrity and homeostasis. Apical constriction is a ubiquitous cell shape change of epithelial tissues that promotes epithelia folding and cell/tissue invagination in a variety of contexts. Apical constriction promotes tissue bending by changing the shape of constituent cells from a columnar-shape to a wedge-shape. Drosophila gastrulation is one of the classic examples of apical constriction, where cells constrict to fold the primitive epithelial sheet and internalize cells that will give rise to internal organs. Studies of Drosophila gastrulation have illustrated the intricate spatial and temporal organization of the proteins that drive apical constriction. Apical constriction of presumptive mesoderm cells occurs via repeated contractile pulses, specifically via the pulsed accumulation of myosin II motors. Contractile pulses are stabilised to promote incremental apical constriction, similar to a ratchet. Furthermore, the cytoskeleton exhibits clear spatial organization. The upstream signals that regulate myosin II activity exhibit are polarized to the center of the apical domain (medioapical). Actin turnover regulated by the microtubule cytoskeleton is important to maintain medioapical actomyosin connected to peripheral adherens junctions. Finally, apical actomyosin fibers connect between cells to form a supracellular cytoskeletal network. Intercellular connections in this cytoskeletal network are guided by tissue geometry and result in anisotropic tension that promotes a furrow shape. 

Morphogenesis is a process by which the embryo is reshaped into the final form of a developed animal. Tissue morphogenesis is under the control of genes which expression follows precise and instructive patterns that can extend from the anterior to the posterior (AP) or from the dorsal to the ventral (DV) axis of the embryo. While much work has been done in understanding how AP and DV patterning independently control morphogenesis, little is known on how cross-patterning functions.  We use the Drosophila embryo as a model system and focus on the process of tissue folding, process that is vital for the animal since folding defects can impair neurulation in vertebrates and gastrulation in all animals which are organized into the three germ layers. Past work has shown that an acto-myosin meshwork spanning the apical-medial side of prospective mesoderm cells and under the control of the embryo DV patterning plays a key role in mesoderm invagination. Nevertheless, experimental evidence and theoretical simulations have argued that apical constriction per se is not sufficient for invagination. In our lab we have uncovered a cell junctional lateral network under the control of both AP and DV patterning. This contractile network generates tension along the apical-basal axis and within the tissue plane 10-15 µm inside the mesoderm epithelium initiating lateral cell intercalation. Lateral forces in mesoderm cells seem to play a multivalent role both driving mesoderm extension and invagination. Finally, by implementing 4D multi-view light sheet imaging, infra-red femtosecond ablation to perturb the cytoskeleton and optogenetics to synthetically control tissue morphology, this work shines new light on the origin and functions of a novel mechanism responsible for simultaneous tissue elongation and folding.

Epithelial tissues form chemical and mechanical barriers in animal bodies, and must therefore maintain their integrity to function. This is a particular challenge during development, when new cells are being added to the tissue. Work in a number of systems shows that one answer to this challenge is cell reintegration: epithelial cells can be born protruding from the sheet, then reincorporate into it. The Bergstralh lab is working to understand this process. A previous study demonstrated that reintegration in the Drosophila follicular epithelium relies in part on Neuroglian (Nrg) a homophilic adhesion molecule that promotes axonal growth and pathfinding. Current research demonstrates that Nrg coordinates with another neuronal adhesion molecule, Fasciclin 3 (Fas3), and with the juxtamembrane spectrin-based cytoskeleton to drive reintegration. These proteins are likely to provide a traction force for the reintegrating cell, analogous to their function in the nervous system. Accumulating evidence suggests that this assembly is evolutionarily conserved, and also acts to maintain tissue integrity in proliferating vertebrate epithelia.

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.

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 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.

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.