“It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life.” — Lewis Wolpert
During embryonic development, masses of cells undergo dramatic rearrangements to organize into separate layers that will give rise to different parts of the body. This incredibly dynamic process is called gastrulation and is driven by cell shape changes that collectively deform the tissue. Our lab is interested in imaging the dynamics of these cell shape changes and determining how mechanical forces are generated that drive massive tissue movements. We study these questions using the embryonic development of the fruit fly, Drosophila melanogaster, where cell shape changes and cytoskeletal dynamics can be readily imaged, quantified, and functionally dissected using a multidisciplinary approach.
Myosin supracellular meshwork in the ventral furrow
Cytoskeletal dynamics during cell shape change
Tissue movements throughout development are driven by collective cell shape changes. For example, a sheet of polarized columnar cells, called an epithelium, can bend and fold when cells constrict at their apical end, a process known as apical constriction. During Drosophila gastrulation, apical constriction is thought to drive the invagination of cells along the ventral midline, which will eventually form the mesoderm (much of which becomes the muscle) of the fly. Apical constriction was widely believed to result from the gradual purse-string-like contraction of a belt of actin filaments and myosin (type II) motors around the apical circumference of the cell. However, simultaneous live imaging of both cell shape and myosin demonstrated that a dynamic contractile meshwork of actin and myosin spans the apical cortex of these epithelial cells. Importantly, these actomyosin contractions are pulsed, with phases of rapid constriction (contraction) interrupted by pauses where the constricted state is maintained (stabilization). Using a combination of traditional cell biology and genetics, RNAi, and quantitative analysis, we showed that the cell apex constricts incrementally like a ratchet. Our lab is further investigating the molecular mechanisms of this ratchet-like constriction.
Cells that undergo apical constriction during gastrulation do not do so in isolation. Greater than 1,000 cells undergo apical constriction in the ventral furrow and these cells are connected to each other through E-cadherin containing adherens junctions. We have found that adherens junctions are required to link actomyosin fibers into a supracellular meshwork and that this meshwork is required to generate tensile force across the tissue. Using a laser to cut actomyosin fibers in the meshwork, we measured an anisotropy in this tissue-level tension and showed that this tension influences the direction in which cells constrict. This illustrates how mechanical forces influence individual cell behavior in a tissue. Our lab is interested in how this supracellular meshwork is formed and how forces are propagated between the cells of a tissue. In addition, we are interested in how forces generated at the apex of an epithelial cell are transmitted basally to influence overall cell shape.
Coordination of cellular behavior
Our work and others’ demonstrating the influence of mechanical forces on cell behavior suggests that cells could influence each other through mechanical communication during tissue morphogenesis. Interestingly, contraction pulses do not occur simultaneously in all cells, but often occur asynchronously in adjacent cells. Our lab is interested in how pulses are coordinated across the tissue and whether cell-cell neighbors influence each other to coordinate morphogenesis.
Chanet S., Miller, C. J., Vaishnav, E., Ermentrout, B., Davidson, L. A., Martin, A. C. Actomyosin meshwork mechanosensing enables tissue shape to orient cell force. Nature Communications. (2017). Accepted.
Vasquez, C.G., S.M. Heissler, N. Billington, J.R. Sellers, and A.C. Martin. 2016. Drosophila non-muscle myosin II motor activity determines the rate of tissue folding. Elife. 5. PMCID: PMC5201417
Coravos, J.S., and A.C. Martin. 2016. Apical sarcomere-like actomyosin contracts nonmuscle Drosophila epithelial cells. Dev Cell. 39:346-358.
Mason, F.M., S. Xie, C.G. Vasquez, M. Tworoger, and A.C. Martin. 2016. RhoA GTPase inhibition organizes contraction during epithelial morphogenesis. J Cell Biol. 214:603-617. PMID: 27551058.
Xie S, Mason FM, Martin AC. Loss of Gα12/13 exacerbates apical area dependence of actomyosin contractility. Mol Biol Cell. 2016 Nov 7;27(22):3526-3536. PubMed PMID: 27489340; PubMed Central PMCID: PMC5221585.
Jodoin JN, Martin AC. Abl suppresses cell extrusion and intercalation during epithelium folding. Mol Biol Cell. 2016 Sep 15;27(18):2822-32. doi: 10.1091/mbc.E16-05-0336. PubMed PMID: 27440923; PubMed Central PMCID: PMC5025269.
Jodoin, J.N., J.S. Coravos, S. Chanet, C.G. Vasquez, M. Tworoger, E.R. Kingston, L.A. Perkins, N. Perrimon, and A.C. Martin. 2015. Stable force balance between epithelial cells arises from F-actin turnover. Dev Cell. 35:685-697. PMCID: PMC4699402
Xie S., Martin A. C. Intracellular signalling and intercellular coupling coordinate heterogeneous contractile events to facilitate tissue folding. Nature Communications, 2015;6:7161.
Vasquez C. G., Tworoger M., Martin A. C. Dynamic myosin phosphorylation regulates contractile pulses and tissue integrity during epithelial morphogenesis. J. Cell Biol, 2014 Aug 4;206(3):435-50. PMC4121972.
Mason F. M., Tworoger M., Martin A. C. Apical domain polarization promotes actin-myosin assembly to drive ratchet-like apical constriction. Nat Cell Biol, 2013;15(8):926-36. PMC3736338.