Geometric control of Myosin-II orientation during axis elongation

The actomyosin cytoskeleton is a crucial driver of morphogenesis. Yet how the behavior of large scale cytoskeletal patterns in deforming tissues emerges from the interplay of geometry, genetics, and mechanics remains incompletely understood. Convergent extension flow in D. melanogaster embryos provides the opportunity to establish a quantitative understanding of the dynamics of anisotropic non-muscle myosin II. Cell-scale analysis of protein localization in fixed embryos suggests that there are complex rules governing how the control of myosin anisotropy is regulated by gene expression patterns. However, technical limitations have impeded quantitative and dynamic studies of this process at the whole embryo level, leaving the role of geometry open. Here we combine in toto live imaging with quantitative analysis of molecular dynamics to characterize the distribution of myosin anisotropy and corresponding genetic patterning. We found pair rule gene expression continuously deformed, flowing with the tissue frame. In contrast, myosin anisotropy orientation remained nearly static, aligned with the stationary dorsal-ventral axis of the embryo. We propose myosin recruitment by a geometrically defined static source, potentially related to the embryo-scale epithelial tension, and account for transient deflections by the interplay of cytoskeletal turnover with junction reorientation by flow. With only one parameter, this model quantitatively accounts for the time course of myosin anisotropy orientation in wild-type, twist, and even-skipped embryos as well as embryos with perturbed egg geometry. Geometric patterning of the cytoskeleton suggests a simple physical strategy to ensure a robust flow and formation of shape.

A novel role of PRL in regulating epithelial cell density by inducing apoptosis at confluence

Maintaining proper epithelial cell density is essential for the survival of multicellular organisms. While regulation of cell density through apoptosis is well known, its mechanistic details remain elusive. Here, we report the involvement of membrane-anchored phosphatase of regenerating liver (PRL), originally known for its role in cancer malignancy, in this process. In epithelial MDCK cells, upon confluence, doxycycline-induced expression of PRL upregulated apoptosis, reducing the cell density. This could be circumvented by artificially reducing the cell density via stretching the cell-seeded silicon chamber. Moreover, siRNA-mediated knockdown of endogenous PRL blocked apoptosis, leading to greater cell density. Mechanistically, PRL promoted apoptosis by upregulating the translation of E-cadherin and activating TGF-β pathway. Morpholino-mediated inhibition of PRL expression in zebrafish embryos caused developmental defect with reduced apoptosis and increased epithelial cell density during convergent extension. This study revealed a novel role of PRL in regulating density-dependent apoptosis in vertebrate epithelium.

Vangl2-environment interaction causes severe neural tube defects, without abnormal neuroepithelial convergent extension

Planar cell polarity (PCP) signalling is vital for initiation of mouse neurulation, with diminished convergent extension (CE) cell movements leading to craniorachischisis, a severe neural tube defect (NTD). Some humans with NTDs also have PCP gene mutations but these are heterozygous, not homozygous as in mice. Other genetic or environmental factors may interact with partial loss of PCP function in human NTDs. We found that reduced sulfation of glycosaminoglycans interacts with heterozygosity for the Lp allele of Vangl2 (a core PCP gene), to cause craniorachischisis in cultured mouse embryos, with rescue by exogenous sulphate. We hypothesised this glycosaminoglycan-PCP interaction may regulate CE but, surprisingly, DiO labeling of the embryonic node demonstrates no abnormality of midline axial extension in sulfation-depleted Lp/+ embryos. Positive-control Lp/Lp embryos show severe CE defects. Abnormalities were detected in the size and shape of somites that flank the closing neural tube in sulfation-depleted Lp/+ embryos. We conclude that failure of closure initiation can arise by a mechanism other than faulty neuroepithelial CE, with possible involvement of matrix-mediated somite expansion, adjacent to the closing neural tube.

Optogenetic dissection of the roles of actomyosin in the mechanics underlying tissue fluidity

Rapid epithelial tissue flows are essential to building and shaping developing embryos. However, it is not well understood how the mechanical properties of tissues and the forces driving them to flow are jointly regulated to accommodate rapid tissue remodeling. To dissect the roles of actomyosin in the mechanics of epithelial tissue flows, here we use two optogenetic tools, optoGEF and optoGAP, to manipulate Rho/Rho-kinase signaling and actomyosin contractility in the germband epithelium, which flows via convergent extension during Drosophila body axis elongation. The ability to perturb actomyosin across the tissue allows us to analyze the effects of actomyosin on cell rearrangements, tissue tensions, and tissue mechanical properties. We find that either optogenetic activation or deactivation of Rho1 signaling and actomyosin contractility at the apical surface of the germband disrupts cell rearrangements and tissue-level flows. By probing mechanical tensions in the tissue using laser ablation and predicting tissue mechanical properties from cell packings, we find that actomyosin influences both the anisotropic forces driving tissue flow and the mechanical properties of the tissue resisting flow, leading to complex relationships between actomyosin activity and tissue-level flow. Moreover, our results indicate that changes in the distribution of medial and junctional myosin in the different perturbations alter tissue tension and cell packings in distinct ways, revealing how junctional and medial myosin have differential roles in promoting and orienting cell rearrangements to tune tissue flows in developing embryos.

Visceral organ morphogenesis via calcium-patterned muscle contractions

How organs achieve their final shape is a problem at the interface between physics and developmental biology. Organs often involve multiple interacting tissue layers that must be coordinated to orchestrate the complex shape changes of development. Intense study has uncovered genetic and physical ingredients driving the form of monolayer tissue. Yet, tracing dynamics across tissue layers and across scales -- from cell to tissue, to entire organs -- remains an outstanding challenge. Here, we study the midgut of Drosophila embryos as a model visceral organ to reconstruct in toto the dynamics of multi-layer organ formation in vivo. Using light-sheet microscopy, genetics, computer vision, and tissue cartography, we extract individual tissue layers to map the time course of shape across scales. We identify the kinematic mechanism driving the shape change due to tissue layer interactions by linking out-of-plane motion to active contraction patterns, revealing a convergent extension process in which cells deform as they flow into deepening folds. Acute perturbations of contractility in the muscle layer using non-neuronal optogenetics reveals that these contraction patterns are due to muscle activity, which induces cell shape changes in the adjacent endoderm layer. This induction cascade relies on high frequency calcium pulses in the muscle layer, under the control of hox genes. Inhibition of targets of calcium involved in myosin phosphorylation abolishes constrictions. Our study of multi-layer organogenesis reveals how genetic patterning in one layer triggers a dynamic molecular mechanism to control a physical process in the adjacent layer, orchestrating whole-organ shape change.

Stretch-induced endogenous electric fields drive neural crest directed collective cell migration in vivo

Directed collective cell migration (dCCM) is essential for morphogenesis. Cell clusters migrate in inherently complex in vivo environments composed of chemical, electrical, mechanical as well as topological features. While these environmental factors have been shown to allow dCCM in vitro, our understanding of dCCM in vivo is mostly limited to chemical guidance. Thus, despite its wide biological relevance, the mechanisms that guide dCCM in vivo remain unclear. To address this, we study endogenous electric fields in relation to the migratory environment of the Xenopus laevis cephalic neural crest, an embryonic cell population that collectively and directionally migrates in vivo. Combining bioelectrical, biomechanical and molecular tools, we show that endogenous electric fields drive neural crest dCCM via electrotaxis in vivo. Moreover, we identify the voltage-sensitive phosphatase 1 (Vsp1) as a key component of the molecular mechanism used by neural crest cells to transduce electric fields into a directional cue. Furthermore, Vsp1 function is specifically required for electrotaxis, being dispensable for cell motility and chemotaxis. Finally, we reveal that endogenous electric fields are mechanoelectrically established. Mechanistically, convergent extension movements of the neural fold generate membrane tension, which in turn opens stretch-activated channels to mobilise the ions required to fuel electric fields. Overall, our results reveal a mechanism of cell guidance, where electrotaxis emerges from the mechanoelectrical and molecular interplay between neighbouring tissues. More broadly, our data contribute to validate the, otherwise understudied, functions of endogenous bioelectrical stimuli in morphogenetic processes.

A Dominant Heterozygous Mutation in COG4 Causes Saul–Wilson Syndrome, a Primordial Dwarfism, and Disrupts Zebrafish Development via Wnt Signaling

Saul–Wilson syndrome (SWS) is a rare, skeletal dysplasia with progeroid appearance and primordial dwarfism. It is caused by a heterozygous, dominant variant (p.G516R) in COG4, a subunit of the conserved oligomeric Golgi (COG) complex involved in intracellular vesicular transport. Our previous work has shown the intracellular disturbances caused by this mutation; however, the pathological mechanism of SWS needs further investigation. We sought to understand the molecular mechanism of specific aspects of the SWS phenotype by analyzing SWS-derived fibroblasts and zebrafish embryos expressing this dominant variant. SWS fibroblasts accumulate glypicans, a group of heparan sulfate proteoglycans (HSPGs) critical for growth and bone development through multiple signaling pathways. Consistently, we find that glypicans are increased in zebrafish embryos expressing the COG4p.G516R variant. These animals show phenotypes consistent with convergent extension (CE) defects during gastrulation, shortened body length, and malformed jaw cartilage chondrocyte intercalation at larval stages. Since non-canonical Wnt signaling was shown in zebrafish to be related to the regulation of these processes by glypican 4, we assessed wnt levels and found a selective increase of wnt4 transcripts in the presence of COG4p.G516R. Moreover, overexpression of wnt4 mRNA phenocopies these developmental defects. LGK974, an inhibitor of Wnt signaling, corrects the shortened body length at low concentrations but amplifies it at slightly higher concentrations. WNT4 and the non-canonical Wnt signaling component phospho-JNK are also elevated in cultured SWS-derived fibroblasts. Similar results from SWS cell lines and zebrafish point to altered non-canonical Wnt signaling as one possible mechanism underlying SWS pathology.

Gpr125 Marks Distinct Cochlear Cell Types and Is Dispensable for Cochlear Development and Hearing

The G protein-coupled receptor (GPR) family critically regulates development and homeostasis of multiple organs. As a member of the GPR adhesion family, Gpr125 (Adgra3) modulates Wnt/PCP signaling and convergent extension in developing zebrafish, but whether it is essential for cochlear development in mammals is unknown. Here, we examined the Gpr125lacZ/+ knock-in mice and show that Gpr125 is dynamically expressed in the developing and mature cochleae. From embryonic day (E) 15.5 to postnatal day (P) 30, Gpr125-β-Gal is consistently expressed in the lesser epithelial ridge and its presumed progenies, the supporting cell subtypes Claudius cells and Hensen’s cells. In contrast, Gpr125-β-Gal is expressed transiently in outer hair cells, epithelial cells in the lateral cochlear wall, interdental cells, and spiral ganglion neurons in the late embryonic and early postnatal cochlea. In situ hybridization for Gpr125 mRNA confirmed Gpr125 expression and validated loss of expression in Gpr125lacZ/lacZ cochleae. Lastly, Gpr125lacZ/+ and Gpr125lacZ/lacZ cochleae displayed no detectable loss or disorganization of either sensory or non-sensory cells in the embryonic and postnatal ages and exhibited normal auditory physiology. Together, our study reveals that Gpr125 is dynamically expressed in multiple cell types in the developing and mature cochlea and is dispensable for cochlear development and hearing.

Severe neural tube defects due to failure of closure initiation can arise without abnormality of neuroepithelial convergent extension

Planar cell polarity (PCP) signalling is vital for initiation of neural tube closure in mice, with diminished convergent extension (CE) cell movements leading to a severe form of neural tube defect (NTD), termed craniorachischisis (CRN). Some human NTDs are also associated with PCP gene mutations, but affected individuals are generally heterozygous, whereas PCP homozygosity or compound heterozygosity is needed to produce CRN in mice. This suggests human NTDs may involve other genetic or environmental factors, that interact with partial loss of PCP function. We found that reduced sulfation OF glycosaminoglycans (GAGs) interacts with heterozygosity for the Lp allele of Vangl2 (a core PCP gene), to cause CRN in mice. Here, we hypothesised that this GAG-PCP interaction may regulate convergent extension movements, and hence lead to severe NTDs in the context of only partial loss of PCP function. Both Lp and null alleles of Vangl2 gave similar findings. Culture of E8.5 embryos in the presence of chlorate (a GAG sulfation inhibitor), or enzymatic cleavage of GAG chains, led to failure of NT closure initiation in the majority of Lp/+ embryos, whereas few +/+ littermates exhibited CRN. The chlorate effect was rescued by exogenous sulphate. Surprisingly, DiO labeling of the embryonic node demonstrated no abnormality of midline axial extension in chlorate-treated Lp/+ embryos that developed CRN. In contrast, positive control Lp/Lp embryos displayed severe convergent extension defects in this assay. Morphometric analysis of the closure initiation site revealed abnormalities in the size and shape of somites that flank the closing neural tube in chlorate-treated Lp/+ embryos. We conclude that severe NTDs involving failure of closure initiation can arise by a mechanism other than faulty neuroepithelial convergent extension. Matrix-mediated expansion of somites, flanking the closing neural tube, may be required for closure initiation.