My general interest lies in the biophysics of living cells both at the single cell and tissue level. At the single cell level, my research aims to understand the biological and physical mechanisms that power cell motility within three-dimensional environments such as connective tissue. Other research looks at the biophysics and biology of cell protrusions known as blebs.
At the tissue level, my research is investigating what single cell properties influence tissue properties using simple cellular aggregates such as cysts. Another aspect of this research is the design of in vitro systems to study simple, yet important, morphogenetic events such as cell sheet invagination.
To investigate these questions, the laboratory combines modern molecular and cell biological techniques with biophysical measurement and micromanipulation techniques derived from nanotechnology, microfluidic technology, and computational modelling.
My laboratory is funded by the Royal Society, the Biotechnology and Biological Sciences Research Council, and the Human Frontier Science Program.
There are often openings for PhD and post-doctoral positions in the lab, if you are interested please get in touch with me directly.
Charras G.T., Hu C.-K., Coughlin M., and Mitchison T.J., “Re-assembly of a contractile actin cortex in cell blebs”, Journal of Cell Biology, 175(3), 477-490 (2006). [download PDF file]
The cortex is a 0.1 to 1 µm thick layer of actin and associated proteins that underlies the cell membrane and determines the shape of animal cells. Whereas the biology of some actin structures (lamellipodia, filopodia) is well understood, the events leading to the formation and subsequent regulation of a submembranous actin cortex are not due to the lack of a good model system. In this paper, we use blebs as model systems to study actin cortex assembly. We identify the proteins involved in cortex assembly as well as their dynamics and ultrastructural arrangement.
Charras G.T., Yarrow J.C., Horton M.A., Mahadevan L., and Mitchison T.J., “Non-Equilibration of Pressure in Blebbing Cells”, Nature, 435, 365-9 (2005). [download PDF file]
Current models for protrusive motility in animal cells focus on cytoskeleton-based mechanisms, where localized protrusion is driven by local regulation of actin biochemistry. In plants and fungi, protrusion is driven primarily by hydrostatic pressure. For hydrostatic pressure to drive localized protrusion in animal cells, it would have to be locally regulated, but current models treating cytoplasm as an incompressible viscoelastic continuum or viscous liquid require that hydrostatic pressure equilibrates essentially instantaneously over the whole cell. Here, we use cell blebs as reporters of local pressure in the cytoplasm. When we locally perfuse blebbing cells with cortex-relaxing drugs to dissipate pressure on one side, blebbing continues on the untreated side, implying non-equilibration of pressure on scales of ~10um and ~10sec. We can account for localization of pressure by considering the cytoplasm as a contractile, elastic network infiltrated by cytosol. Motion of the fluid relative to the network generates spatially heterogenous transients in the pressure field, and can be described in the framework of poroelasticity.
Charras G.T. and Horton M.A., “Determination of Cellular Strains by Combined Atomic Force Microscopy and Finite Element Modelling”, Biophysical Journal, 82(6), 2970-81 (2002). [download PDF file]
Many organs adapt to their mechanical environment as a result of physiological change or disease. Cells are both the detectors and effectors of this process. Though many studies have been performed in vitro to investigate the mechanisms of detection and adaptation to mechanical strains, the cellular strains remain unknown and results from different stimulation techniques cannot be compared. By combining experimental determination of cell profiles and elasticities by atomic force microscopy with finite element modeling and computational fluid dynamics, we report the cellular strain distributions exerted by common whole-cell straining techniques and from micromanipulation techniques, hence enabling their comparison.
Neutrophils are the primary cells of the immune system responsible for detecting and preventing bacterial infections, as well as driving inflammation. Neutrophils circulate freely in the bloodstream, and when passing through an inflamed region attach the blood vessel wall, traverse the endothelium (transendothelial migration), and migrate through the connective tissue to the site of infection (chemotaxis). Here a neutrophil (in red) is shown migrating through a 10x3µm microfluidic channel towards a source of chemoattractant (in blue). Scale bar = 5µm. In collaboration with Dr Irimia, Massachusetts General Hospital
Figure 1: Neutrophil ChemotaxisFigure 1: Neutrophil Chemotaxis
Dissociated cells of the animal pole of xenopus embryos display characteristic dynamics of the cell membrane, known as circus movements. In these cells, a local delamination of the cell membrane from the cytoskeleton (a bleb) can propagate around the cell as a traveling wave which circles the cell periphery multiple times. This wave progresses through cycles of delamination. In this image, the blastomere has been injected with RNA encoding the PH domain of phospholipase C _ tagged with GFP, which highlights the cell membrane. Three separate time points showing the progression of the traveling wave have been superimposed in the following colours: red t=0s, green t=5s, blue t=10s. Scale bar=10µm.
Figure 2: Circus movements in xenopus blastomeres
Given appropriate culture conditions, some cells can form structures akin to a football with each patch of the football being a cell. These structures are known as cysts and are a good model of multicellular structure as they are amenable to genetic manipulation, mechanical testing, and comprise sufficiently few cells (~100) that they can be modelled computationally. In this figure, a phase contrast image of a MDCK cell cyst is shown. The thickness of the rim of this cyst is approximately 10 µm and comprises only one cell. In the top left corner, the shadow of a micropipette used to deform the cyst can be distinguished.
Figure 3: MDCK cell cyst
Blebs are spherical cellular protrusions that occur in many physiological situations. Two distinct phases make up the life of a bleb each of which have their own biology and physics: expansion, which lasts ~30s, and retraction, which lasts ~2min. During expansion, the cell membrane delaminates from the actin cortex and fills with cytosol. Growth stalls as an actin cortex reforms under the bleb membrane, and retraction starts, driven by myosin-II. In this figure, the cell membrane has been removed with detergent and the actin cytoskeleton stabilized with phalloidin. The cell has been imaged using scanning electron microscopy revealing the cage-like structure of actin filaments within retracting blebs.
Figure 4: Actin cytoskeleton of a blebbing cell