No abstract

Cells are frequently required to move in a local environment that physically restricts locomotion, such as during extravasation or metastatic invasion. In order to model these events, we have developed an assay in which vegetative Dictyostelium amoebae undergo chemotaxis under a layer of agarose toward a source of folic acid [Laevsky, G. and Knecht, D. A. (2001). Biotechniques 31, 1140-1149]. As the concentration of agarose is increased from 0.5% to 3% the cells are increasingly inhibited in their ability to move under the agarose. The contribution of myosin II and actin cross-linking proteins to the movement of cells in this restrictive environment has now been examined. Cells lacking myosin II heavy chain (mhcA -) are unable to migrate under agarose overlays of greater than 0.5%, and even at this concentration they move only a short distance from the trough. While attempting to move, the cells become stretched and fragmented due to their inability to retract their uropods. At higher agarose concentrations, the mhcA -cells protrude pseudopods under the agarose, but are unable to pull the cell body underneath. Consistent with a role for myosin II in general cortical stability, GFP-myosin dynamically localizes to the lateral and posterior cortex of cells moving under agarose. Cells lacking the essential light chain of myosin II (mlcE -), have no measurable myosin II motor activity, yet were able to move normally under all agarose concentrations. Mutants lacking either ABP-120 or α-actinin were also able to move under agarose at rates similar to wild-type cells. We hypothesize that myosin stabilizes the actin cortex through its cross-linking activity rather than its motor function and this activity is necessary and sufficient for the maintenance of cortical integrity of cells undergoing movement in a restrictive environment. The actin cross-linkers α-actinin and ABP-120 do not appear to play as major a role as myosin II in providing this cortical integrity. Research Article 3762 measurable actin activated ATPase activity (Chen et al., 1995;Xu et al., 1996). While not generally thought of as an actin cross-linking protein, myosin II minifilaments presumably also have this capability (Wachsstock et al., 1994;Humphrey et al., 2002). Mutants lacking myosin II (mhcA -) are able to accomplish both random and chemotactic motility; however, they move slowly and have defects in pseudopod extension (Peters et al., 1988;Wessels et al., 1988). Although mhcA -cells are able to aggregate, they are unable to complete the developmental program .The developmental defect of mhcA -cells appears to be due to their inability to move in a restrictive environment . During early development, cells acquire surface adhesion proteins and so movement occurs while cells are continually making and breaking adhesive contacts with their neighbors as well as the substratum. Unlike movement on a planar substratum, this form of motility is analogous to the movement of metastatic cancer cells away from a primary tumor, or the extravasation of immune...

Under-agarose chemotaxis has been used previously to assess the ability of neutrophils to respond to gradients of chemoattractant. We have adapted this assay to the chemotactic movement of Dictyostelium amoebae in response to folic acid. Troughs are used instead of wells to increase the area along which the cells can be visualized and to create a uniform front of moving cells. Imaging the transition zone where the cells first encounter the agarose, we find that the cells move perpendicular to the gradient and periodically manage to squeeze under the agarose and move up the gradient. As cells exit the troughs, their cross-sectional area increases as the cells become flattened. Three-dimensional reconstruction of confocal optical sections through GFP-labeled cells demonstrates that the increase in cross-sectional area is due to the flattening of the cells. Since the cells locally deform the agarose and become deformed by it, the concentration of the agarose, and therefore its stiffness, should affect the ability of the cells to migrate. Consistent with this hypothesis, cells in 0.5% agarose move faster and are less flat than cells under 2% agarose. Cells do not exit the troughs and move under 3% agarose at all. Therefore, this assay can be used to compare and quantify the ability of different cell types or mutant cell lines to move in a restrictive environment.

No abstract

The complexity of intracellular signaling requires both a diversity of molecular players and the sequestration of activity to unique compartments within the cell. Recent findings on the role of liquid-liquid phase separation provide a distinct mechanism for spatial segregation of proteins to regulate signaling pathway crosstalk. Here we discover that DACT1 is induced by TGF-β and forms protein condensates in the cytoplasm to repress Wnt signaling. These condensates do not localize to any known organelles but rather exist as phase-separated proteinaceous cytoplasmic bodies. Deletion of intrinsically disordered domains within the DACT1 protein eliminates its ability to both form protein condensates and suppress Wnt signaling. Isolation and mass spectrometry analysis of these particles revealed a complex of protein machinery that sequesters Casein Kinase 2, a Wnt pathway activator. We further demonstrate that DACT1 condensates are maintained in vivo and that DACT1 is critical to breast and prostate cancer bone metastasis.

The kinetics of binding, uptake and degradation of bacteria by vegetative Dictyostelium amoeba using Escherichia coli expressing the recombinant fluorescent protein DsRed have been characterized. There are significant advantages to using DsRed-expressing bacteria for phagocytosis assays. Stable expression of the fluorescent protein, DsRed, provides living bacteria with a bright internal fluorescent signal that is degradable in the phagolysosomal pathway. Unlike assays with chemically labelled bacteria or latex beads, the bacteria are alive and possess a natural, unaltered external surface for receptor interaction. Dictyostelium cells rapidly bind and phagocytose DsRed bacteria. Pulse-chase experiments show that the signal derived from DsRed is degraded with a half-life of approximately 45 min. To distinguish internalized bacteria from those bound to the surface, an assay was developed in which sodium azide was used to release surface-bound particles. Surprisingly, surface particle release appears to be independent of myosin II function. Using this assay it was shown that the uptake of bacteria into cells is extremely rapid. After 1 min incubation, 20 % of the signal is derived from internalized bacteria. The proportion of the signal from internalized bacteria increases gradually and reaches 50 % at steady state. This assay will be useful in investigations of the molecular machinery of phagocytosis and post-internalization vesicle trafficking.

We have developed a novel method, (ECIS/taxis), for monitoring cell movement in response to chemotactic and chemokinetic factors. In this system, cells migrate in an under-agarose environment, and their positions are monitored using the electric cell-substrate impedance sensor technology to measure the impedance change at a target electrode, that is lithographed onto the substrate, as the cells arrive at the target. In the studies reported here, Dictyostelium discoideum was used as a prototypical, motile eukaryotic cell. Using the ECIS/taxis system, the arrival of cells at the target electrode was proportional to the dose offolate used to stimulate the cells and could be assessed by changes in resistance at the electrode. ECIS/taxis was readily able to distinguish between wild-type cells and a mutant that is deficient in its chemotactic response. Finally, we have shown that an agent that interferes with chemotactic motility leads to the delayed arrival of cells at the target electrode. The multi-well assay configuration allows for simultaneous automated screening of many samples for chemotactic or anti-chemotactic activity. This assay system is compatible with measurements of mammalian cell movement and should be valuable in the assessment of both agonists and antagonists of cell movement.

We present a multimodal optical microscope that incorporates six imaging modalities on one common platform. The imaging modalities include three staring modes, optical quadrature microscopy (OQM), differential interference contrast (DIC) microscopy, and epi-fluorescence microscopy, and three scanning modes, confocal reflectance microscopy (CRM), confocal fluorescence microscopy (CFM), and two-photon microscopy (2PM). OQM reconstructs the amplitude and phase of an optically transparent specimen within a modified Mach-Zehnder configuration. DIC microscopy images the phase gradient along a specified direction of an optically transparent specimen. CRM detects index of refraction changes that modulate backscatter. Epi-fluorescence microscopy, CFM, and 2PM detect endogenous and exogenous fluorophores within a specimen. The scanning modes are inherently capable of producing three-dimensional (3-D) images due to optical sectioning and localized probing. Illumination and imaging are performed coaxially with minimal changes of optical components between modes. Multimodal images of embryos are shown to demonstrate the microscope's imaging capabilities.