Adhesion between epithelial cells enables the remarkable mechanical behavior of epithelial tissues during morphogenesis. However, it remains unclear how cell-cell adhesion influences mechanics in both static and dynamically flowing confluent epithelial tissues. Here, we systematically modulate E-cadherin-mediated adhesion in the Drosophila embryo and study the effects on the mechanical behavior of the germband epithelium before and during dramatic tissue remodeling and flow associated with body axis elongation. Before axis elongation, we find that increasing E-cadherin levels produces tissue comprising more elongated cells and predicted to be more fluid-like, providing reduced resistance to tissue flow. During axis elongation, we find that the dominant effect of E-cadherin is tuning the speed at which cells proceed through rearrangement events. Before and during axis elongation, E-cadherin levels influence patterns of actomyosin-dependent forces, supporting the notion that E-cadherin tunes tissue mechanics in part through effects on actomyosin. Notably, the effects of ∼4-fold changes in E-cadherin levels on overall tissue structure and flow are relatively weak, suggesting that the system is tolerant to changes in absolute E-cadherin levels over this range where an intact tissue is formed. Taken together, these findings reveal dual-and sometimes opposing-roles for E-cadherin-mediated adhesion in controlling tissue structure and dynamics in vivo, which result in unexpected relationships between adhesion and flow in confluent tissues.

How pulsed contractile dynamics drive the remodeling of cell and tissue topologies in epithelial sheets has been a key question in development and disease. Due to constraints in imaging and analysis technologies, studies that have described the in vivo mechanisms underlying changes in cell and neighbor relationships have largely been confined to analyses of planar apical regions. Thus, how the volumetric nature of epithelial cells affects force propagation and remodeling of the cell surface in three dimensions, including especially the apical-basal axis, is unclear. Here, we perform lattice light sheet microscopy (LLSM)-based analysis to determine how far and fast forces propagate across different apical-basal layers, as well as where topological changes initiate from in a columnar epithelium. These datasets are highly time- and depth-resolved and reveal that topology-changing forces are spatially entangled, with contractile force generation occurring across the observed apical-basal axis in a pulsed fashion, while the conservation of cell volumes constrains instantaneous cell deformations. Leading layer behaviors occur opportunistically in response to favorable phasic conditions, with lagging layers \"zippering\" to catch up as new contractile pulses propel further changes in cell topologies. These results argue against specific zones of topological initiation and demonstrate the importance of systematic 4D-based analysis in understanding how forces and deformations in cell dimensions propagate in a three-dimensional environment.

Geometric criteria can be used to assess whether cell intercalation is active or passive during the convergent extension of tissue.

Planar polarity is a commonly observed phenomenon in which proteins display a consistent asymmetry in their subcellular localization or activity across the plane of a tissue. During animal development, planar polarity is a fundamental mechanism for coordinating the behaviors of groups of cells to achieve anisotropic tissue remodeling, growth, and organization. Therefore, a primary focus of developmental biology research has been to understand the molecular mechanisms underlying planar polarity in a variety of systems to identify conserved principles of tissue organization. In the early Drosophila embryo, the germband neuroectoderm epithelium rapidly doubles in length along the anterior-posterior axis through a process known as convergent extension (CE); it also becomes subdivided into tandem tissue compartments through the formation of compartment boundaries (CBs). Both processes are dependent on the planar polarity of proteins involved in cellular tension and adhesion. The enrichment of actomyosin-based tension and adherens junction-based adhesion at specific cell-cell contacts is required for coordinated cell intercalation, which drives CE, and the creation of highly stable cell-cell contacts at CBs. Recent studies have revealed a system for rapid cellular polarization triggered by the expression of leucine-rich-repeat (LRR) cell-surface proteins in striped patterns. In particular, the non-uniform expression of Toll-2, Toll-6, Toll-8, and Tartan generates local cellular asymmetries that allow cells to distinguish between cell-cell contacts oriented parallel or perpendicular to the anterior-posterior axis. In this review, we discuss (1) the biomechanical underpinnings of CE and CB formation, (2) how the initial symmetry-breaking events of anterior-posterior patterning culminate in planar polarity, and (3) recent advances in understanding the molecular mechanisms downstream of LRR receptors that lead to planar polarized tension and junctional adhesion.

Anteroposterior (AP) elongation of the vertebrate body plan is driven by convergence and extension (C&E) gastrulation movements in both the mesoderm and neuroectoderm, but how or whether molecular regulation of C&E differs between tissues remains an open question. Using a zebrafish explant model of AP axis extension, we show that C&E of the neuroectoderm and mesoderm can be uncoupled ex vivo, and that morphogenesis of individual tissues results from distinct morphogen signaling dynamics. Using precise temporal manipulation of BMP and Nodal signaling, we identify a critical developmental window during which high or low BMP/Nodal ratios induce neuroectoderm- or mesoderm-driven C&E, respectively. Increased BMP activity similarly enhances C&E specifically in the ectoderm of intact zebrafish gastrulae, highlighting the in vivo relevance of our findings. Together, these results demonstrate that temporal dynamics of BMP and Nodal morphogen signaling activate distinct morphogenetic programs governing C&E gastrulation movements within individual tissues.

Mediolateral cell intercalation is a morphogenetic strategy used throughout animal development to reshape tissues. Dorsal intercalation in the Caenorhabditis elegans embryo involves the mediolateral intercalation of two rows of dorsal epidermal cells to create a single row that straddles the dorsal midline, and thus is a simple model to study cell intercalation. Polarized protrusive activity during dorsal intercalation requires the C. elegans Rac and RhoG orthologs CED-10 and MIG-2, but how these GTPases are regulated during intercalation has not been thoroughly investigated. In this study, we characterized the role of the Rac-specific guanine nucleotide exchange factor (GEF) TIAM-1 in regulating actin-based protrusive dynamics during dorsal intercalation. We found that TIAM-1 can promote formation of the main medial lamellipodial protrusion extended by intercalating cells through its canonical GEF function, whereas its N-terminal domains function to negatively regulate the generation of ectopic filiform protrusions around the periphery of intercalating cells. We also show that the guidance receptor UNC-5 inhibits these ectopic filiform protrusions in dorsal epidermal cells and that this effect is in part mediated via TIAM-1. These results expand the network of proteins that regulate basolateral protrusive activity during directed rearrangement of epithelial cells in animal embryos.

Convergent extension (CE) requires the coordinated action of the planar cell polarity (PCP) proteins1,2 and the actin cytoskeleton,3,4,5,6 but this relationship remains incompletely understood. For example, PCP signaling orients actomyosin contractions, yet actomyosin is also required for the polarized localization of PCP proteins.7,8 Moreover, the actin-regulating Septins play key roles in actin organization9 and are implicated in PCP and CE in frogs, mice, and fish5,6,10,11,12 but execute only a subset of PCP-dependent cell behaviors. Septin loss recapitulates the severe tissue-level CE defects seen after core PCP disruption yet leaves overt cell polarity intact.5 Together, these results highlight the general fact that cell movement requires coordinated action by distinct but integrated actin populations, such as lamella and lamellipodia in migrating cells13 or medial and junctional actin populations in cells engaged in apical constriction.14,15 In the context of Xenopus mesoderm CE, three such actin populations are important, a superficial meshwork known as the \"node-and-cable\" system,4,16,17,18 a contractile network at deep cell-cell junctions,6,19 and mediolaterally oriented actin-rich protrusions, which are present both superficially and deeply.4,19,20,21 Here, we exploited the amenability of the uniquely \"two-dimensional\" node and cable system to probe the relationship between PCP proteins, Septins, and the polarization of this actin network. We find that the PCP proteins Vangl2 and Prickle2 and Septins co-localize at nodes, and that the node and cable system displays a cryptic, PCP- and Septin-dependent anteroposterior (AP) polarity in its organization and dynamics.

BACKGROUND: Noncanonical Wnts are morphogens that can elevate intracellular Ca2+, activate the Ca2+/calmodulin-dependent protein kinase, CaMKII, and promote cell movements during vertebrate gastrulation.
RESULTS: Zebrafish express seven CaMKII genes during embryogenesis; two of these, camk2b1 and camk2g1, are necessary for convergent extension (CE) cell movements. CaMKII morphant phenotypes were observed as early as epiboly. At the 1-3 somite stage, neuroectoderm and paraxial cells remained unconverged in both morphants. Later, somites lacked their stereotypical shape and were wider, more closely spaced, and body gap angles increased. At 24hpf, somite compression and notochord undulation coincided with a shorter and broader body axis. A camk2b1 crispant was generated which phenocopied the camk2b1 morphant. The levels of cell proliferation, apoptosis and paraxial and neuroectodermal markers were unchanged in morphants. Hyperactivation of CaMKII during gastrulation by transient pharmacological intervention (thapsigargin) also caused CE defects. Mosaically expressed dominant-negative CaMKII recapitulated these phenotypes and showed significant midline bifurcation. Finally, the introduction of CaMKII partially rescued Wnt11 morphant phenotypes.
CONCLUSIONS: Overall, these data support a model whereby cyclically activated CaMKII encoded from two genes enables cell migration during the process of CE.

Shape changes of epithelia during animal development, such as convergent extension, are achieved through concerted mechanical activity of individual cells. While much is known about the corresponding large scale tissue flow and its genetic drivers, fundamental questions regarding local control of contractile activity on cellular scale and its embryo-scale coordination remain open. To address these questions, we develop a quantitative, model-based analysis framework to relate cell geometry to local tension in recently obtained timelapse imaging data of gastrulating Drosophila embryos. This analysis provides a systematic decomposition of cell shape changes and T1-rearrangements into internally driven, active, and externally driven, passive, contributions. Our analysis provides evidence that germ band extension is driven by active T1 processes that self-organize through positive feedback acting on tensions. More generally, our findings suggest that epithelial convergent extension results from controlled transformation of internal force balance geometry which combines the effects of bottom-up local self-organization with the top-down, embryo-scale regulation by gene expression.

Caudal developmental defects, including caudal regression, caudal dysgenesis and sirenomelia, are devastating conditions affecting the skeletal, nervous, digestive, reproductive and excretory systems. Defects in mesodermal migration and blood supply to the caudal region have been identified as possible causes of caudal developmental defects, but neither satisfactorily explains the structural malformations in all three germ layers. Here, we describe caudal developmental defects in transmembrane protein 132a (Tmem132a) mutant mice, including skeletal, posterior neural tube closure, genitourinary tract and hindgut defects. We show that, in Tmem132a mutant embryos, visceral endoderm fails to be excluded from the medial region of early hindgut, leading directly to the loss or malformation of cloaca-derived genitourinary and gastrointestinal structures, and indirectly to the neural tube and kidney/ureter defects. We find that TMEM132A mediates intercellular interaction, and physically interacts with planar cell polarity (PCP) regulators CELSR1 and FZD6. Genetically, Tmem132a regulates neural tube closure synergistically with another PCP regulator Vangl2. In summary, we have identified Tmem132a as a new regulator of PCP, and hindgut malformation as the underlying cause of developmental defects in multiple caudal structures.