David Knecht, University of Connecticut
Much has been learned about the molecular mechanisms and regulatory networks underlying cell motility. However, much of this work has been done by examining cells moving on a planar non-biological surface with no physical restrictions. Little is known about in vivo motility which is carried out in an ill-defined extracellular matrix, on top of sheets of other cells or within masses of cells. In Dictyostelium, development occurs when starved amoebae gather together in groups of 100-100,000 cells to construct a multicellular organism. We have used this system to investigate the mechanism of movement of individual cells within the multicellular mass.
We have developed techniques in which individual cells are labeled with a fluorescent dye and then mixed with an excess of unlabeled cells and imaged with a confocal microscope during development. The merged transmitted light and fluorescence images provide a time-lapse record of the behavior of individual cells in either 3 or 4 dimensions. When wild-type cells are imaged in aggregation streams, their movement is surprisingly like that of cells moving as individuals on a coverslip. The cells change shape and change neighbors as they move in spite of the 3 different adhesion systems that can hold cells together. Early in development, when the cells are moving as individuals, the movement is pulsatile, coordinated by cAMP waves. Once the cells are in multicellular streams, pulsatile movement is difficult to detect. The cells in the center of the stream tend to move slightly faster than those at the edge. Mixing mutants lacking myosin II with wild type cells has provided new insights into the role of myosin II in multicellular motility. The mutant cells become distorted by forces applied by wild-type cells and are pushed to the edges of the streams and aggregates. If two of the adhesion systems are genetically removed, the myosin mutant phenotype is now suppressed such that the mutant cells now integrate into the stream and contribute to all parts of the developing organism. Our hypothesis is that during multicellular motility, cells need a requisite stiffness generated by acto-myosin interaction in order to offset the adhesive forces between cells. If the light chain of myosin II is removed, the mutant cells are also able to contribute to development and move normally even though the contractile activity of the myosin motor is undetectable. This result implies that the actin cross-linking activity of myosin minifilaments, rather than the motor, is sufficient to provide this cortical stiffness.