Functional Phases in Cell Attachment and Spreading

There are many types of cellular motility and inherent in each motile process is a series of steps that must be performed for the cell to accomplish the desired function. Underlying each motility step is a set of proteins that must be activated in the right place for the correct period. In the case of cell attachment and spreading, cells perform several tasks that appear to occur in series rather than in parallel, which results in the cell transitioning between a number of distinct phases. For each phase, there are significant differences in protein activities, which belie differences in function for each of the phases. In the isotropic mode of attachment and spreading, we observed four distinct phases: suspension, early spreading, contractile spreading and fully spread. Suspension cells often exhibit a basal level of motility, in which they extend and retract large finger- like projections presumably to explore the environment. In early isotropic spreading, cells have committed to spreading on the surface and there is stimulated actin assembly with relatively little contraction of the assembled filaments. Over a very short period, cells transition to contractile spreading that is characterized by periodic contractions that test the rigidity of the surface. When the cell is fully spread, extension activity is significantly decreased and focal complexes start to assemble near the cell periphery. Transitions between the phases occur quite rapidly with dramatic changes in the activity of many cellular components. The highly reproducible characteristics of behavior in each of the phases indicates that the cells have only a few modes of spreading behavior and comparisons between cellular activities should be made between cells in the same phase.