Neighborly relations: cadherins and mechanotransduction

Cell–cell adhesions are sites where cells experience and resist tugging forces. It has long been postulated, but not directly tested, that cadherin adhesion molecules may serve in mechanotransduction at cell–cell contacts. In this issue, Le Duc et al. (2010. J. Cell Biol. doi: 10.1083/jcb.201001149) provide direct evidence that E-cadherin participates in a mechanosensing pathway that regulates the actomyosin cytoskeleton to modulate cell stiffness in response to pulling force

Light-activated Frizzled7 reveals a permissive role of non-canonical wnt signaling in mesendoderm cell migration

10.7554/eLife.42093.001Non-canonical Wnt signaling plays a central role for coordinated cell polarization and directed migration in metazoan development. While spatiotemporally restricted activation of non-canonical Wnt-signaling drives cell polarization in epithelial tissues, it remains unclear whether such instructive activity is also critical for directed mesenchymal cell migration. Here, we developed a light-activated version of the non-canonical Wnt receptor Frizzled 7 (Fz7) to analyze how restricted activation of non-canonical Wnt signaling affects directed anterior axial mesendoderm (prechordal plate, ppl) cell migration within the zebrafish gastrula. We found that Fz7 signaling is required for ppl cell protrusion formation and migration and that spatiotemporally restricted ectopic activation is capable of redirecting their migration. Finally, we show that uniform activation of Fz7 signaling in ppl cells fully rescues defective directed cell migration in fz7 mutant embryos. Together, our findings reveal that in contrast to the situation in epithelial cells, non-canonical Wnt signaling functions permissively rather than instructively in directed mesenchymal cell migration during gastrulation

Multicomponent Analysis of Junctional Movements Regulated by Myosin II Isoforms at the Epithelial Zonula Adherens

The zonula adherens (ZA) of epithelial cells is a site of cell-cell adhesion where cellular forces are exerted and resisted. Increasing evidence indicates that E-cadherin adhesion molecules at the ZA serve to sense force applied on the junctions and coordinate cytoskeletal responses to those forces. Efforts to understand the role that cadherins play in mechanotransduction have been limited by the lack of assays to measure the impact of forces on the ZA. In this study we used 4D imaging of GFP-tagged E-cadherin to analyse the movement of the ZA. Junctions in confluent epithelial monolayers displayed prominent movements oriented orthogonal (perpendicular) to the ZA itself. Two components were identified in these movements: a relatively slow unidirectional (translational) component that could be readily fitted by least-squares regression analysis, upon which were superimposed more rapid oscillatory movements. Myosin IIB was a dominant factor responsible for driving the unilateral translational movements. In contrast, frequency spectrum analysis revealed that depletion of Myosin IIA increased the power of the oscillatory movements. This implies that Myosin IIA may serve to dampen oscillatory movements of the ZA. This extends our recent analysis of Myosin II at the ZA to demonstrate that Myosin IIA and Myosin IIB make distinct contributions to junctional movement at the ZA

Drosophila TNF modulates tissue tension in the embryo to facilitate macrophage invasive migration

Migrating cells penetrate tissue barriers during development, inflammatory responses, and tumor metastasis. We study if migration in vivo in such three-dimensionally confined environments requires changes in the mechanical properties of the surrounding cells using embryonic Drosophila melanogaster hemocytes, also called macrophages, as a model. We find that macrophage invasion into the germband through transient separation of the apposing ectoderm and mesoderm requires cell deformations and reductions in apical tension in the ectoderm. Interestingly, the genetic pathway governing these mechanical shifts acts downstream of the only known tumor necrosis factor superfamily member in Drosophila, Eiger, and its receptor, Grindelwald. Eiger-Grindelwald signaling reduces levels of active Myosin in the germband ectodermal cortex through the localization of a Crumbs complex component, Patj (Pals-1-associated tight junction protein). We therefore elucidate a distinct molecular pathway that controls tissue tension and demonstrate the importance of such regulation for invasive migration in vivo

Satb2 acts as a gatekeeper for major developmental transitions during early vertebrate embryogenesis

Zygotic genome activation (ZGA) initiates regionalized transcription underlying distinct cellular identities. ZGA is dependent upon dynamic chromatin architecture sculpted by conserved DNA-binding proteins. However, the direct mechanistic link between the onset of ZGA and the tissue-specific transcription remains unclear. Here, we have addressed the involvement of chromatin organizer Satb2 in orchestrating both processes during zebrafish embryogenesis. Integrative analysis of transcriptome, genome-wide occupancy and chromatin accessibility reveals contrasting molecular activities of maternally deposited and zygotically synthesized Satb2. Maternal Satb2 prevents premature transcription of zygotic genes by influencing the interplay between the pluripotency factors. By contrast, zygotic Satb2 activates transcription of the same group of genes during neural crest development and organogenesis. Thus, our comparative analysis of maternal versus zygotic function of Satb2 underscores how these antithetical activities are temporally coordinated and functionally implemented highlighting the evolutionary implications of the biphasic and bimodal regulation of landmark developmental transitions by a single determinant

Friction forces position the neural anlage

During embryonic development, mechanical forces are essential for cellular rearrangements driving tissue morphogenesis. Here, we show that in the early zebrafish embryo, friction forces are generated at the interface between anterior axial mesoderm (prechordal plate, ppl) progenitors migrating towards the animal pole and neurectoderm progenitors moving in the opposite direction towards the vegetal pole of the embryo. These friction forces lead to global rearrangement of cells within the neurectoderm and determine the position of the neural anlage. Using a combination of experiments and simulations, we show that this process depends on hydrodynamic coupling between neurectoderm and ppl as a result of E-cadherin-mediated adhesion between those tissues. Our data thus establish the emergence of friction forces at the interface between moving tissues as a critical force-generating process shaping the embryo

Emerging role of mechanical forces in cell fate acquisition

Mechanical forces are now recognized as key cellular effectors that together with genetic and cellular signals physically shape and pattern tissues and organs during development. Increasing efforts are aimed toward understanding the less explored role of mechanical forces in controlling cell fate decisions in embryonic development. Here we discuss recent examples of how differential forces feedback into cell fate specification and tissue patterning. In particular, we focus on the role of actomyosin-contractile force generation and transduction in affecting tissue morphogenesis and cell fate regulation in the embryo

Transactivation of E2F-Regulated Genes by Polyomavirus Large T Antigen: Evidence for a Two-Step Mechanism

Polyomavirus large T antigen transactivates a variety of genes whose products are involved in S phase induction. These genes are regulated by the E2F family of transcription factors, which are under the control of the pocket protein retinoblastoma protein and its relatives p130 and p107. The viral protein causes a dissociation of E2F-pocket protein complexes that results in transactivation of the genes. This reaction requires the N-terminal binding site for pocket proteins and the J domain that binds chaperones. We found earlier that a mutation of the zinc finger located within the C-terminal domain, a region assumed to function mainly in the replication of viral DNA, also interferes with transactivation. Here we show that binding of the histone acetyltransferase coactivator complex CBP/p300-PCAF to the C terminus correlates with the ability of large T antigen to transactivate genes. This interaction results in promoter-specific acetylation of histones. Inactive mutant proteins with changes within the C-terminal domain were nevertheless able to dissociate the E2F pocket protein complexes, indicating that this dissociation is a necessary but insufficient step in the T antigen-induced transactivation of genes. It has to be accompanied by a second step involving the T antigen-mediated recruitment of a histone acetyltransferase complex