CYK-1/Formin activation in cortical RhoA signaling centers promotes organismal left-right symmetry breaking

'Three signals - three body axes' as patterning principle in bilaterians

In vertebrates, the three orthogonal body axes, anteroposterior (AP), dorsoventral (DV) and left-right (LR) are determined at gastrula and neurula stages by the Spemann-Mangold organizer and its equivalents. A common feature of AP and DV axis formation is that an evolutionary conserved interplay between growth factors (Wnt, BMP) and their extracellular antagonists (e.g. Dkk1, Chordin) creates signaling gradients for axial patterning. Recent work showed that LR patterning in Xenopus follows the same principle, with R-spondin 2 (Rspo2) as an extracellular FGF antagonist, which creates a signaling gradient that determines the LR vector. That a triad of anti-FGF, anti-BMP, and anti-Wnt governs LR, DV, and AP axis formation reveals a unifying principle in animal development. We discuss how cross-talk between these three signals confers integrated AP-DV-LR body axis patterning underlying developmental robustness, size scaling, and harmonious regulation. We propose that Urbilateria featured three orthogonal body axes that were governed by a Cartesian coordinate system of orthogonal Wnt/AP, BMP/DV, and FGF/LR signaling gradients.

Dynamics of multicellular swirling on micropatterned substrates

Chirality plays a crucial role in biology, as it is highly conserved and fundamentally important in the developmental process. To better understand the relationship between the chirality of individual cells and that of tissues and organisms, we develop a generalized mechanics model of chiral polarized particles to investigate the swirling dynamics of cell populations on substrates. Our analysis reveals that cells with the same chirality can form distinct chiral patterns on ring-shaped or rectangular substrates. Interestingly, our studies indicate that an excessively strong or weak individual cellular chirality hinders the formation of such chiral patterns. Our studies also indicate that there exists the influence distance of substrate boundaries in chiral patterns. Smaller influence distances are observed when cell-cell interactions are weaker. Conversely, when cell-cell interactions are too strong, multiple cells tend to be stacked together, preventing the formation of chiral patterns on substrates in our analysis. Additionally, we demonstrate that the interaction between cells and substrate boundaries effectively controls the chiral distribution of cellular orientations on ring-shaped substrates. This research highlights the significance of coordinating boundary features, individual cellular chirality, and cell-cell interactions in governing the chiral movement of cell populations and provides valuable mechanics insights into comprehending the intricate connection between the chirality of single cells and that of tissues and organisms.

Chirality-induced Lineage Enforcement of Mechanosensitive Mesenchymal Stem Cells Across Germ Layer Boundaries

Mesenchymal to epithelial transition (MET) is instrumental in embryogenesis, tissue repair, and wound healing while the epithelial to mesenchymal transition (EMT) plays role in carcinogenesis. Alteration in microenvironment can modulate cellular signaling and induce EMT and MET. However, modulation of microenvironment to induce MET has been relatively less explored. In this work, effect of matrix stiffness in mediating MET in umbilical cord-derived mesenchymal stem cells (UCMSC) is investigated. Differential segregation of cell fate determinant proteins is one of the key factors in mediating altered stem cell fates through MET even though the genesis of apicobasal polarity remains ambiguous. Herein, it is also attempted to decipher if microenvironment-induced asymmetric cell division has a role to play in driving the cells toward MET. UCMSC cultured on stiffer PDMS matrices resulted in significantly (p < 0.05) higher expression of mechanotransduction proteins. It was also observed that stiffer matrices mediated significant (p < 0.05) upregulation of the polarity proteins and cell fate determinant protein, and epithelial marker proteins over lesser stiff substrates. On the contrary, expression of inflammatory and mesenchymal markers was reduced significantly (p < 0.05) on the stiffer matrices. Cell cycle analysis showed a significant increase in the G1 phase among the cells seeded on stiffer matrices. Transcriptomic studies validated higher expression of epithelial markers genes and lower expression of EMT markers. The transition from mesenchymal to epithelial phenotype depending on the gradation in matrix stiffness is successfully demonstrated. A computational machine learning model was developed to validate stiffness-MET correlation with 94% accuracy. The cross-boundary trans-lineage differentiation capability of MSC on bioengineered substrates can be used as a potential tool in tissue regeneration, organogenesis, and wound healing applications. In our present study, we deciphered the correlation between YAP/TAZ mechanotransduction pathway, EMT signaling pathway, and asymmetric cell division in mediating MET in MSC in a substrate stiffness-dependent manner. It is inferred that the stiffer PDMS matrices facilitate the transition from mesenchymal to epithelial state of MSC. Further, our study also proposed a scoring system to sort MSC from an intermediate hybrid E/M population while undergoing graded MET on matrices of different stiffnesses using a machine learning technique. This proposed scoring system can provide information regarding the E/M state of MSC on different bioengineered constructs based on their biophysical properties which may help in the proper choice of biomaterials in complex tissue-engineering applications.

OSGN-1 is a conserved flavin-containing monooxygenase required to stabilize the intercellular bridge in late cytokinesis

Cytokinesis is the last step of cell division and is regulated by the small GTPase RhoA. RhoA activity is required for all steps of cytokinesis, including prior to abscission when daughter cells are ultimately physically separated. Like germ cells in all animals, the Caenorhabditis elegans embryonic germline founder cell initiates cytokinesis but does not complete abscission, leaving a stable intercellular bridge between the two daughter cells. Here, we identify and characterize C. elegans OSGN-1 as a cytokinetic regulator that promotes RhoA activity during late cytokinesis. Sequence analyses and biochemical reconstitutions reveal that OSGN-1 is a flavin-containing monooxygenase (MO). Genetic analyses indicate that the MO activity of OSGN-1 is required to maintain active RhoA at the end of cytokinesis in the germline founder cell and to stabilize the intercellular bridge. Deletion of OSGIN1 in human cells results in an increase in binucleation as a result of cytokinetic furrow regression, and this phenotype can be rescued by expressing a catalytically active form of C. elegans OSGN-1, indicating that OSGN-1 and OSGIN1 are functional orthologs. We propose that OSGN-1 and OSGIN1 are conserved MO enzymes required to maintain RhoA activity at the intercellular bridge during late cytokinesis and thus favor its stability, enabling proper abscission in human cells and bridge stabilization in C. elegans germ cells.

Axis convergence in C. elegans embryos

Embryos develop in a surrounding that guides key aspects of their development. For example, the anteroposterior (AP) body axis is always aligned with the geometric long axis of the surrounding eggshell in fruit flies and worms. The mechanisms that ensure convergence of the AP axis with the long axis of the eggshell remain unresolved. We investigate axis convergence in early C. elegans development, where the nascent AP axis, when misaligned, actively re-aligns to converge with the long axis of the egg. We identify two physical mechanisms that underlie axis convergence. First, bulk cytoplasmic flows, driven by actomyosin cortical flows, can directly reposition the AP axis. Second, active forces generated within the pseudocleavage furrow, a transient actomyosin structure similar to a contractile ring, can drive a mechanical re-orientation such that it becomes positioned perpendicular to the long axis of the egg. This in turn ensures AP axis convergence. Numerical simulations, together with experiments that either abolish the pseudocleavage furrow or change the shape of the egg, demonstrate that the pseudocleavage-furrow-dependent mechanism is a major driver of axis convergence. We conclude that active force generation within the actomyosin cortical layer drives axis convergence in the early nematode.

Chiral growth of adherent filopodia

Adherent filopodia are elongated finger-like membrane protrusions, extending from the edges of diverse cell types and participating in cell adhesion, spreading, migration, and environmental sensing. The formation and elongation of filopodia are driven by the polymerization of parallel actin filaments, comprising the filopodia cytoskeletal core. Here, we report that adherent filopodia, formed during the spreading of cultured cells on galectin-8-coated substrates, tend to change the direction of their extension in a chiral fashion, acquiring a left-bent shape. Cryoelectron tomography examination indicated that turning of the filopodia tip to the left is accompanied by the displacement of the actin core bundle to the right of the filopodia midline. Reduction of the adhesion to galectin-8 by treatment with thiodigalactoside abolished this filopodia chirality. By modulating the expression of a variety of actin-associated filopodia proteins, we identified myosin-X and formin DAAM1 as major filopodia chirality promoting factors. Formin mDia1, actin filament elongation factor VASP, and actin filament cross-linker fascin were also shown to be involved. Thus, the simple actin cytoskeleton of filopodia, together with a small number of associated proteins are sufficient to drive a complex navigation process, manifested by the development of left-right asymmetry in these cellular protrusions.

Actin polymerisation and crosslinking drive left-right asymmetry in single cell and cell collectives

Deviations from mirror symmetry in the development of bilateral organisms are common but the mechanisms of initial symmetry breaking are insufficiently understood. The actin cytoskeleton of individual cells self-organises in a chiral manner, but the molecular players involved remain essentially unidentified and the relationship between chirality of an individual cell and cell collectives is unclear. Here, we analysed self-organisation of the chiral actin cytoskeleton in individual cells on circular or elliptical patterns, and collective cell alignment in confined microcultures. Screening based on deep-learning analysis of actin patterns identified actin polymerisation regulators, depletion of which suppresses chirality (mDia1) or reverses chirality direction (profilin1 and CapZβ). The reversed chirality  is mDia1-independent but requires the function of actin-crosslinker α-actinin1. A robust correlation between the effects of a variety of actin assembly regulators on chirality of individual cells and cell collectives is revealed. Thus, actin-driven cell chirality may underlie tissue and organ asymmetry.

Three-dimensional chiral morphodynamics of chemomechanical active shells

Morphogenesis of active shells such as cells is a fundamental chemomechanical process that often exhibits three-dimensional (3D) large deformations and chemical pattern dynamics simultaneously. Here, we establish a chemomechanical active shell theory accounting for mechanical feedback and biochemical regulation to investigate the symmetry-breaking and 3D chiral morphodynamics emerging in the cell cortex. The active bending and stretching of the elastic shells are regulated by biochemical signals like actomyosin and RhoA, which, in turn, exert mechanical feedback on the biochemical events via deformation-dependent diffusion and inhibition. We show that active deformations can trigger chemomechanical bifurcations, yielding pulse spiral waves and global oscillations, which, with increasing mechanical feedback, give way to traveling or standing waves subsequently. Mechanical feedback is also found to contribute to stabilizing the polarity of emerging patterns, thus ensuring robust morphogenesis. Our results reproduce and unravel the experimentally observed solitary and multiple spiral patterns, which initiate asymmetric cleavage in Xenopus and starfish embryogenesis. This study underscores the crucial roles of mechanical feedback in cell development and also suggests a chemomechanical framework allowing for 3D large deformation and chemical signaling to explore complex morphogenesis in living shell-like structures.

How torque on formins is relaxed strongly affects cellular swirling

Chirality is a common and essential characteristic at varied scales of living organisms. By adapting the rotational clutch-filament model we previously developed, we investigate the effect of torque relaxation of a formin on cellular chiral swirling. Since it is still unclear how the torque on a formin is exactly relaxed, we probe three types of torque relaxation, as suggested in the literature. Our analysis indicates that, when a formin periodically undergoes positive and negative rotation during processive capping to relax the torque, cells hardly rotate. When the switch between the positive and the negative rotation during the processive capping is randomly regulated by the torque, our analysis indicates that cells can only slightly rotate either counterclockwise or clockwise. When a formin relaxes the torque by transiently loosening its contact either with the membrane at its anchored site or with the actin filament, we find that cells can prominently rotate either counterclockwise or clockwise, in good consistency with the experiment. Thus, our studies indicate that how the torque on a formin is relaxed strongly affects cellular swirling and suggest an efficient type of torque relaxation in switching cellular swirling.

Symmetry-breaking of animal cytokinesis

Cytokinesis is a mechanism that separates dividing cells via constriction of a supramolecular structure, the contractile ring. In animal cells, three modes of symmetry-breaking of cytokinesis result in unilateral cytokinesis, asymmetric cell division, and oriented cell division. Each mode of cytokinesis plays a significant role in tissue patterning and morphogenesis by the mechanisms that control the orientation and position of the contractile ring relative to the body axis. Despite its significance, the mechanisms involved in the symmetry-breaking of cytokinesis remain unclear in many cell types. Classical embryologists have identified that the geometric relationship between the mitotic spindle and cell cortex induces cytokinesis asymmetry; however, emerging evidence suggests that a concerted flow of compressional cell-cortex materials (cortical flow) is a spindle-independent driving force in spatial cytokinesis control. This review provides an overview of both classical and emerging mechanisms of cytokinesis asymmetry and their roles in animal development.

Septins and a formin have distinct functions in anaphase chiral cortical rotation in the Caenorhabditis elegans zygote

Many cells and tissues exhibit chirality that stems from the chirality of proteins and polymers. In the Caenorhabditis elegans zygote, actomyosin contractility drives chiral rotation of the entire cortex circumferentially around the division plane during anaphase. How contractility is translated to cell-scale chirality, and what dictates handedness, are unknown. Septins are candidate contributors to cell-scale chirality because they anchor and cross-link the actomyosin cytoskeleton. We report that septins are required for anaphase cortical rotation. In contrast, the formin CYK-1, which we found to be enriched in the posterior in early anaphase, is not required for cortical rotation but contributes to its chirality. Simultaneous loss of septin and CYK-1 function led to abnormal and often reversed cortical rotation. Our results suggest that anaphase contractility leads to chiral rotation by releasing torsional stress generated during formin-based polymerization, which is polarized along the cell anterior–posterior axis and which accumulates due to actomyosin network connectivity. Our findings shed light on the molecular and physical bases for cellular chirality in the C. elegans zygote. We also identify conditions in which chiral rotation fails but animals are developmentally viable, opening avenues for future work on the relationship between early embryonic cellular chirality and animal body plan.

CYK-1/Formin activation in cortical RhoA signaling centers promotes organismal left-right symmetry breaking

Proper left-right symmetry breaking is essential for animal development, and in many cases, this process is actomyosin-dependent. In Caenorhabditis elegans embryos active torque generation in the actomyosin layer promotes left-right symmetry breaking by driving chiral counterrotating cortical flows. While both Formins and Myosins have been implicated in left-right symmetry breaking and both can rotate actin filaments in vitro, it remains unclear whether active torques in the actomyosin cortex are generated by Formins, Myosins, or both. We combined the strength of C. elegans genetics with quantitative imaging and thin film, chiral active fluid theory to show that, while Non-Muscle Myosin II activity drives cortical actomyosin flows, it is permissive for chiral counterrotation and dispensable for chiral symmetry breaking of cortical flows. Instead, we find that CYK-1/Formin activation in RhoA foci is instructive for chiral counterrotation and promotes in-plane, active torque generation in the actomyosin cortex. Notably, we observe that artificially generated large active RhoA patches undergo rotations with consistent handedness in a CYK-1/Formin-dependent manner. Altogether, we conclude that CYK-1/Formin-dependent active torque generation facilitates chiral symmetry breaking of actomyosin flows and drives organismal left-right symmetry breaking in the nematode worm.