Endogenous OptoRhoGEFs reveal biophysical principles of epithelial tissue furrowing.

During development, epithelia function as malleable substrates that undergo extensive remodeling to shape developing embryos. Optogenetic control of Rho signaling provides an avenue to investigate the mechanisms of epithelial morphogenesis, but transgenic optogenetic tools can be limited by variability in tool expression levels and deleterious effects of transgenic overexpression on development. Here, we use CRISPR/Cas9 to tag Drosophila RhoGEF2 and Cysts/Dp114RhoGEF with components of the iLID/SspB optogenetic heterodimer, permitting light-dependent control over endogenous protein activities. Using quantitative optogenetic perturbations, we uncover a dose-dependence of tissue furrow depth and bending behavior on RhoGEF recruitment, revealing mechanisms by which developing embryos can shape tissues into particular morphologies. We show that at the onset of gastrulation, furrows formed by cell lateral contraction are oriented and size-constrained by a stiff basal actomyosin layer. Our findings demonstrate the use of quantitative, 3D-patterned perturbations of cell contractility to precisely shape tissue structures and interrogate developmental mechanics.

Tissue flows are tuned by actomyosin-dependent mechanics in developing embryos.

Rapid epithelial tissue flows are essential to building and shaping developing embryos. However, the mechanical properties of embryonic epithelial tissues and the factors that control these properties are not well understood. Actomyosin generates contractile tensions and contributes to the mechanical properties of cells and cytoskeletal networks in vitro, but it remains unclear how the levels and patterns of actomyosin activity contribute to embryonic epithelial tissue mechanics in vivo. To dissect the roles of cell-generated tensions in the mechanics of flowing epithelial tissues, we use optogenetic tools to manipulate actomyosin contractility with spatiotemporal precision in the Drosophila germband epithelium, which rapidly flows during body axis elongation. We find that manipulating actomyosin-dependent tensions by either optogenetic activation or deactivation of actomyosin alters the solid-fluid mechanical properties of the germband epithelium, leading to changes in cell rearrangements and tissue-level flows. Optogenetically activating actomyosin leads to increases in the overall level but decreases in the anisotropy of tension in the tissue, whereas optogenetically deactivating actomyosin leads to decreases in both the level and anisotropy of tension compared to in wild-type embryos. We find that optogenetically activating actomyosin results in more solid-like (less fluid-like) tissue properties, which is associated with reduced cell rearrangements and tissue flow compared to in wild-type embryos. Optogenetically deactivating actomyosin also results in more solid-like properties than in wild-type embryos but less solid-like properties compared to optogenetically activating actomyosin. Together, these findings indicate that increasing the overall tension level is associated with more solid-like properties in tissues that are relatively isotropic, whereas high tension anisotropy fluidizes the tissue. Our results reveal that epithelial tissue flows in developing embryos involve the coordinated actomyosin-dependent regulation of the mechanical properties of tissues and the tensions driving them to flow in order to achieve rapid tissue remodeling.

Using optogenetics to link myosin patterns to contractile cell behaviors during convergent extension.

Distinct patterns of actomyosin contractility are often associated with particular epithelial tissue shape changes during development. For example, a planar-polarized pattern of myosin II localization regulated by Rho1 signaling during Drosophila body axis elongation is thought to drive cell behaviors that contribute to convergent extension. However, it is not well understood how specific aspects of a myosin pattern influence the multiple cell behaviors, including cell intercalation, cell shape changes, and apical cell area fluctuations, that simultaneously occur during morphogenesis. Here, we developed two optogenetic tools, optoGEF and optoGAP, to activate or deactivate Rho1 signaling, respectively. We used these tools to manipulate myosin patterns at the apical side of the germband epithelium during Drosophila axis elongation and analyzed the effects on contractile cell behaviors. We show that uniform activation or inactivation of Rho1 signaling across the apical surface of the germband is sufficient to disrupt the planar-polarized pattern of myosin at cell junctions on the timescale of 3-5 min, leading to distinct changes in junctional and medial myosin patterns in optoGEF and optoGAP embryos. These two perturbations to Rho1 activity both disrupt axis elongation and cell intercalation but have distinct effects on cell area fluctuations and cell packings that are linked with changes in the medial and junctional myosin pools. These studies demonstrate that acute optogenetic perturbations to Rho1 activity are sufficient to rapidly override the endogenous planar-polarized myosin pattern in the germband during axis elongation. Moreover, our results reveal that the levels of Rho1 activity and the balance between medial and junctional myosin play key roles not only in organizing the cell rearrangements that are known to directly contribute to axis elongation but also in regulating cell area fluctuations and cell packings, which have been proposed to be important factors influencing the mechanics of tissue deformation and flow.

Manipulating the Patterns of Mechanical Forces That Shape Multicellular Tissues.

During embryonic development, spatial and temporal patterns of mechanical forces help to transform unstructured groups of cells into complex, functional tissue architectures. Here, we review emerging approaches to manipulate these patterns of forces to investigate the mechanical mechanisms that shape multicellular tissues, with a focus on recent experimental studies of epithelial tissue sheets in the embryo of the model organism Drosophila melanogaster.

Biophysical control of the cell rearrangements and cell shape changes that build epithelial tissues.

Epithelial cell rearrangements and cell shape changes are fundamental mechanisms by which cells build and shape elaborate and diverse tissue architectures from simple tissue sheets. These cell behaviors are regulated by a complex interplay between physical and biochemical mechanisms, many of which have been uncovered in recent studies in Drosophila. While the regulation of these cell behaviors is still under investigation, emerging technologies are being used to gain experimental control over these behaviors, opening new possibilities for designing and engineering tissue structures. Analysis of the biophysical mechanisms governing cell shape and movement will be crucial for understanding morphogenesis and for harnessing this knowledge to build tissues of precise shapes and structures for basic science and engineering applications.

Presence of stromal cells in a bioengineered tumor microenvironment alters glioblastoma migration and response to STAT3 inhibition.

Despite the increasingly recognized importance of the tumor microenvironment (TME) as a regulator of tumor progression, only few in vitro models have been developed to systematically study the effects of TME on tumor behavior in a controlled manner. Here we developed a three-dimensional (3D) in vitro model that recapitulates the physical and compositional characteristics of Glioblastoma (GBM) extracellular matrix (ECM) and incorporates brain stromal cells such as astrocytes and endothelial cell precursors. The model was used to evaluate the effect of TME components on migration and survival of various patient-derived GBM cell lines (GBM10, GBM43 and GBAM1) in the context of STAT3 inhibition. Migration analysis of GBM within the 3D in vitro model demonstrated that the presence of astrocytes significantly increases the migration of GBM, while presence of endothelial precursors has varied effects on the migration of different GBM cell lines. Given the role of the tumor microenvironment as a regulator of STAT3 activity, we tested the effect of the STAT3 inhibitor SH-4-54 on GBM migration and survival. SH-4-54 inhibited STAT3 activity and reduced 3D migration and survival of GBM43 but had no effect on GBM10. SH-4-54 treatment drastically reduced the viability of the stem-like line GBAM1 in liquid culture, but its effect lessened in presence of a 3D ECM and stromal cells. Our results highlight the interplay between the ECM and stromal cells in the microenvironment with the cancer cells and indicate that the impact of these relationships may differ for GBM cells of varying genetic and clinical histories.