The Martin lab is interested in how tissues get into shape. Understanding tissue shape requires understanding how cells generate force and how cells work together to collectively sculpt a tissue. We have elucidated how cells generate force and how this force is propagated to the tissue-level to fold a tissue. In addition, we are interested in how tissue integrity is regulated and also investigate mechanisms that regulate the epithelial-to-mesenchymal transition or EMT. Finally, work in the lab also addresses the interplay between tissue forces and cell division.
Much like folding paper can generate more complex forms, two-dimensional sheets of polarized cells called epithelia can be folded to sculpt tissues. Apical constriction is a cell shape change that is associated with tissue folding, tube formation (e.g. the neural tube), and cell extrusion from epithelial layers.
Described below is work from the Martin lab that has shown how tissues fold.
a) Cellular mechanism of force generation
The presence of myosin is often correlated with constriction during tissue morphogenesis, but it was unclear how myosin generates force. In muscle, actin and myosin are highly organized in a structure called a sarcomere. We discovered that apical constriction also depends on a polarized organization of the actin and myosin cortex (Nature Cell Biology, Jul. 2013). In contrast to muscle, epithelial cells have actin and myosin radially polarized relative to the apical surface of the cell forming a structure we termed the ‘radial sarcomere’ (Developmental Cell, Nov. 2016).
Like muscle, we have also shown that apical apical constriction velocity and tissue folding rate were proportional to the in vitro ATPase activity. This result was from a collaboration with Jim Sellers’ lab, NIH that biochemically characterized the wild-type Drosophila myosin 2 and a series of myosin 2 mutants with motor activity compromised to varying degrees (eLife, Dec. 2016)
b) Transmitting contractile forces across a tissue
For contractility to change tissue shape, forces have to be transmitted between cells in the tissue. Adherens junctions, which contain the transmembrane adhesion molecule E-cadherin, link neighboring epithelial cells to each other. In addition, force transmission requires that the actin cortex is linked to the adherens junctions.
Using live imaging, we showed that the apical actin cortex spontaneously releases adherens junctions and then rapidly reattaches. We found that genes involved in actin turnover are critical to reattach the apical actin cortex to adherens junctions (Developmental Cell, Dec. 2015). Disrupting actin turnover prevents rapid reattachment to adherens junctions, which causes a loss of force balance.
Our result provides insight into a mammalian birth defect, spina bifida. Spina bifida is a birth defect that results from incomplete folding and closure of the developing brain and spinal cord. Spina bifida is associated with mutations in a gene that mediates actin turnover, Cofilin (Gurniak et al., Dev. Biol., 2005).
c) The interplay between cell signaling and form
The RhoA GTPase is a key signaling molecule that activates contractility. RhoA is activated by Guanine nucleotide exchange factors (GEFs) and inactivated by GTPase activating proteins (GAPs). Precise regulation of RhoA is required at both the cell and tissue level for fold a tissue.
Using a reverse genetic screen, we identified a RhoGAP that is expressed in embryonic furrow, which we named Cumberland GAP (C-GAP). C-GAP is required for the proper spatial and temporal control of RhoA and myosin. This uncovers a surprising role for a RhoA inhibitor promoting contractility (JCB, Aug. 2016).
Contractility must also be temporally and spatially regulated across the tissue. We found that the onset of tissue folding is associated with cells changing from a competitive behavior to a more cooperative behavior (Nature Communications, May 2015). Once this temporal transition occurs, myosin accumulated as a gradient across the tissue. This contractile gradient is important for folding and is the result of a gradient in transcription (Development, May 2017).
d) Mechanosensing and collective cell behavior
Forces that are transmitted across a tissue can feed-back on cells to regulate their behavior. We have used laser cutting experiments to show that epithelial tension is anisotropic in the ventral furrow and that this tension causes cells to direct force along a specific axis of the embryo. This demonstrates that cells “feel” and “respond” to tissue-level forces and that macroscopic properties such as embryo shape can influence cell behavior (i.e., force generation)(Nature Communications, May 2017).
Apical constriction can constrict cells and fold tissues, but it can also cause cells to extrude from an epithelium. What determines which of these two outcomes occurs? In a genetic screen, we discovered that depleting the Abelson non receptor tyrosine kinase causes cells that normally undergo apical constriction to abnormally extrude from the epithelium during tissue folding. Interestingly, this extrusion is associated with an EMT-like loss of apical-basal polarity. The EMT-like loss of apical-basal polarity and extrusion in Abelson mutants depends on the gene Enabled. Enabled is implicated in cancer cell metastasis (Philippar et al., Dev. Cell, 2008). Our work (MBoC, Jul. 2016) suggests a possible mechanism by which Enabled mediates metastasis.
Cell Division and Morphogenesis
Cell division is critical to grow a tissue. Epithelial sheet growth requires that cell division occurs in the plane of the sheet. We are studying the forces that allow cell division to occur in the correct orientation to grow an epithelium. To address this problem, we are utilizing live imaging of the early Drosophila embryo to directly visualize cell divisions and observe cell shape changes during mitosis.
During embryonic development, cell division must be coordinated with tissue shape changes to ensure proper form. The Martin lab is interested in the interplay between various processes that happen during cell division and those that drive tissue shape change. In some cases, it appears that cell division can interfere with morphogenetic processes that change cell shape and we are investigating how these processes are coordinated and what happens when they are not.