Karine Guevorkian

Aspiration of biological viscoelastic drops.

Spherical cellular aggregates are in vitro systems to study the physical and biophysical properties of tissues. We present a novel approach to characterize the mechanical properties of cellular aggregates using a micropipette aspiration technique. We observe an aspiration in two distinct regimes: a fast elastic deformation followed by a viscous flow. We develop a model based on this viscoelastic behavior to deduce the surface tension, viscosity, and elastic modulus. A major result is the increase of the surface tension with the applied force, interpreted as an effect of cellular mechanosensing.

Spreading dynamics and wetting transition of cellular aggregates.

We study the spreading of spheroidal aggregates of cells, expressing a tunable level of E-cadherin molecules, on glass substrates decorated with mixed fibronectin and polyethylene glycol. We observe the contact area by optical interferometry and the profile by side-view microscopy. We find a universal law of aggregate spreading at short times, which we interpret through an analogy with the spreading of viscoelastic droplets. At long times, we observe either partial wetting or complete wetting, with a precursor film of cells spreading around the aggregate with two possible states. In strongly cohesive aggregates this film is a cellular monolayer in the liquid state, whereas in weakly cohesive aggregates, cells escape from the aggregate, forming a 2D gas. The escape of isolated cells is a physical mechanism that appears also to be present in the progression of a noninvasive tumor into a metastatic malignant carcinoma, known as the epithelial-mesenchymal transition.

Swimming Paramecium in magnetically simulated enhanced, reduced, and inverted gravity environments

Earths gravity exerts relatively weak forces in the range of 10–100 pN directly on cells in biological systems. Nevertheless, it biases the orientation of swimming unicellular organisms, alters bone cell differentiation, and modifies gene expression in renal cells. A number of methods of simulating different strength gravity environments, such as centrifugation, have been applied for researching the underlying mechanisms. Here, we demonstrate a magnetic force-based technique that is unique in its capability to enhance, reduce, and even invert the effective buoyancy of cells and thus simulate hypergravity, hypogravity, and inverted gravity environments. We apply it to Paramecium caudatum, a single-cell protozoan that varies its swimming propulsion depending on its orientation with respect to gravity, g. In these simulated gravities, denoted by fgm, Paramecium exhibits a linear response up to fgm = 5 g, modifying its swimming as it would in the hypergravity of a centrifuge. Moreover, experiments from fgm = 0 to −5 g show that the response is symmetric, implying that the regulation of the swimming speed is primarily related to the buoyancy of the cell. The response becomes nonlinear for fgm >5 g. At fgm = 10 g, many paramecia “stall” (i.e., swim in place against the force), exerting a maximum propulsion force estimated to be 0.7 nN. These findings establish a general technique for applying continuously variable forces to cells or cell populations suitable for exploring their force transduction mechanisms.

Mechanosensitive shivering of model tissues under controlled aspiration

During embryonic development and wound healing, the mechanical signals transmitted from cells to their neighbors induce tissue rearrangement and directional movements. It has been observed that forces exerted between cells in a developing tissue under stress are not always monotonically varying, but they can be pulsatile. Here we investigate the response of model tissues to controlled external stresses. Spherical cellular aggregates are subjected to one-dimensional stretching forces using micropipette aspiration. At large enough pressures, the aggregate flows smoothly inside the pipette. However, in a narrow range of moderate aspiration pressures, the aggregate responds by pulsed contractions or “shivering.” We explain the emergence of this shivering behavior by means of a simple analytical model where the uniaxially stretched cells are represented by a string of Kelvin–Voigt elements. Beyond a deformation threshold, cells contract and pull on neighboring cells after a time delay for cell response. Such an active behavior has previously been found to cause tissue pulsation during dorsal closure of Drosophila embryo.

Varying the effective buoyancy of cells using magnetic force

We introduce a magnetic force buoyancy variation (MFBV) technique that employs intense inhomogeneous magnetic fields to vary the effective buoyancy of cells and other diamagnetic systems in solution. Nonswimming Paramecia have been suspended, forced to sediment and driven to rise in solution using MFBV. Details of their response to MFBV have been used to determine the magnetic susceptibility of a single Paramecium. The use of MFBV as a means by which to suspend cell cultures indefinitely is also described.

In situ imaging of micro-organisms in intense magnetic fields

This article describes a setup suitable for the in situ visualization and imaging of swimming unicellular organisms in intense magnetic fields at a constant temperature. It fits within a 31 T maximum field, 50 mm bore resistive magnet at the National High Magnetic Field Laboratory. The main optical component is a 6 mm side view borescope, which provides an optical axis perpendicular to the magnetic field allowing us to monitor the motion of the micro-organisms parallel to the field. The system has a maximum resolution of 20μm. We will present images of swimming paramecia in magnetic field obtained with this setup and show that the resolution is adequate for tracking purposes.