Coordination of actin-myosin contractions

Pulsatile actin-myosin contractions fold tissues

During Drosophila gastrulation, around 1,200 mesoderm cells collectively contract to form a fold in the tissue. The force necessary for this morphogenesis is (partly) generated by actin-myosin contractions at the apical surfaces of the mesoderm cells.

These actin-myosin contractions are pulsatile, meaning they occur in discrete steps, driven by the phosphorylation dynamics of myosin; each pulse of myosin activation leads to a discrete constriction of the cell apical surface.

How are discrete pulses spatiotemporally coordinated?

I became interested in how a large, mechanically coupled sheet of epithelial cells coordinate these discrete contraction events in a way that eventually propagates tissue tension across a millimeter in length.

I built computational methods to extract when and where these contractions occurred in the embryo, using a combination of image analysis and time-series analysis. (Xie & Martin, 2015)

Then, using machine learning, Monte Carlo methods, and spatial statistical analysis, I found that neighboring pulses are coordinated within the epithelium. Cells that are next to contractions are more likely to mount a contraction of their own. Contractions that are next to other contractions are also more likely to be irreversible.

How does a contracting epithelium tolerate heterogeneity in cell size?

Using my computational method, I analyzed a class of developmental mutants who fail to mount ‘coherent’ contractions. In these mutants, a subset of cells in the mesoderm successfully constrict their apical surface, while the rest of the cells end up with expanded apices, resulting in failed gastrulation.

These mutations encode genes in the Drosophila Gα12/13 pathway, including the gene concertina (cta), which activates RhoA GTPase to control actin-myosin dynamics.

I found that, contrary to previous models that suggested these pathways propagated a contraction signal throughout the tissue, these pathways actually acted cell-autonomously. They ensured that the apical cortex in each contracting cell is robustly organized to withstand heterogeneity in apical sizes. In cta mutants, cells that are initially larger than their neighbors cannot sustain enough tension throughout its apical cortext, and will be pulled apart by its contracting neighbors.

Thus, this pathway buffers the contracting mesoderm against heterogeneity in cell size and ensures robust morphogenesis. (Xie et al., 2016)

References

2016

Mol. Biol. Cell

Loss of Gα12/13 exacerbates apical area dependence of actomyosin contractility

Shicong Xie, Frank M Mason, and Adam C Martin

Molecular Biology of the Cell, Apr 2016

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During development, coordinated cell shape changes alter tissue shape. In the Drosophila ventral furrow and other epithelia, apical constriction of hundreds of epithelial cells folds the tissue. Genes in the Gα12/13 pathway coordinate collective apical constriction, but the mechanism of coordination is poorly understood. Coupling live-cell imaging with a computational approach to identify contractile events, we discovered that differences in constriction behavior are biased by initial cell shape. Disrupting Gα12/13 exacerbates this relationship. Larger apical area is associated with delayed initiation of contractile pulses, lower apical E-cadherin and F-actin levels, and aberrantly mobile Rho-kinase structures. Our results suggest that loss of Gα12/13 disrupts apical actin cortex organization and pulse initiation in a sizedependent manner. We propose that Gα12/13 robustly organizes the apical cortex despite variation in apical area to ensure the timely initiation of contractile pulses in a tissue with heterogeneity in starting cell shape.

2015

Nat. Comm.

Intracellular signalling and intercellular coupling coordinate heterogeneous contractile events to facilitate tissue folding.

Shicong Xie, and Adam C Martin

Nature communications, Apr 2015

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Cellular forces generated in the apical domain of epithelial cells reshape tissues. Recent studies highlighted an important role for dynamic actomyosin contractions, called pulses, that change cell and tissue shape. Net cell shape change depends on whether cell shape is stabilized, or ratcheted, between pulses. Whether there are different classes of contractile pulses in wild-type embryos and how pulses are spatiotemporally coordinated is unknown. Here we develop a computational framework to identify and classify pulses and determine how pulses are coordinated during invagination of the Drosophila ventral furrow. We demonstrate biased transitions in pulse behaviour, where weak or unratcheted pulses transition to ratcheted pulses. The transcription factor Twist directs this transition, with cells in Twist-depleted embryos exhibiting abnormal reversed transitions in pulse behaviour. We demonstrate that ratcheted pulses have higher probability of having neighbouring contractions, and that ratcheting of pulses prevents competition between neighbouring contractions, allowing collective behaviour.