Although myosin II is known to play an important role in cell migration, little is known about its specific functions. We have addressed the function of one of the isoforms of myosin II, myosin IIB, by analyzing the movement and mechanical characteristics of fibroblasts where this protein has been ablated by gene disruption. Myosin IIB null cells displayed multiple unstable and disorganized protrusions, although they were still able to generate a large fraction of traction forces when cultured on flexible polyacrylamide substrates. However, the traction forces were highly disorganized relative to the direction of cell migration. Analysis of cell migration patterns indicated an increase in speed and decrease in persistence, which were likely responsible for the defects in directional movements as demonstrated with Boyden chambers. In addition, unlike control cells, mutant cells failed to respond to mechanical signals such as compressing forces and changes in substrate rigidity. Immunofluorescence staining indicated that myosin IIB was localized preferentially along stress fibers in the interior region of the cell. Our results suggest that myosin IIB is involved not in propelling but in directing the cell movement, by coordinating protrusive activities and stabilizing the cell polarity. INTRODUCTIONThe functional roles of myosin II in nonmuscle cells have been an important topic of investigation. Although its involvement in cytokinesis has been investigated in detail (Robinson and Spudich, 2000), there is also strong evidence that myosin II plays a role in cell migration. For example, although myosin II null mutants of Dictyostelium are capable of migration, they display a lower migration speed and a loss of forward bias in protrusion compared with wild-type cells (Wessels et al., 1988), particularly on surfaces of increased adhesiveness (Jay et al., 1995).Fibroblast migration involves a number of controlled and coordinated processes, including protrusion, adhesion, translocation, and detachment (Lauffenburger and Horwitz, 1996;Mitchison and Cramer, 1996;Sheetz et al., 1998). It is commonly accepted that translocation of the cell body and detachment from the substrate require contractile forces (Jay et al., 1995;Wolenski, 1995;Anderson et al., 1996;Svitkina et al., 1997). Consistent with the idea, treatment of cells with myosin II inhibitors causes relaxation of traction forces and impairment of cell migration (Pelham and Wang, 1999). Equally important is a guidance mechanism in response to environmental cues. In addition to chemotaxis, fibroblasts show profound responses in morphology, traction forces, and motility rates, to physical signals (Pelham and Wang, 1997;Lo et al., 2000). They are also able to steer their migration toward substrates of high rigidity (Lo et al., 2000). Because the detection of such physical characteristics as rigidity cannot be achieved through purely chemical means, the cell must invoke a contractile mechanism that probes the environment. Myosin II may be involved in such a sensing stru...

Upon encountering an antigen, motile T cells stop crawling, change morphology and ultimately form an 'immunological synapse'. Although myosin motors are thought to mediate various aspects of this process, the molecules involved and their exact roles are not defined. Here we show that nonmuscle myosin heavy chain IIA, or MyH9, is the only class II myosin expressed in T cells and is associated with the uropod during crawling. MyH9 function is required for maintenance of the uropod and for T cell motility but is dispensable for synapse formation. Phosphorylation of MyH9 in its multimerization domain by T cell receptor-generated signals indicates that inactivation of this motor may be a key step in the 'stop' response during antigen recognition.

Regional myocardial blood flow can be measured accurately and noninvasively from serially acquired and reoriented short-axis 13N-ammonia images, thus overcoming limitations inherent to the use of transaxially acquired images and permitting a more complete evaluation of regional blood flows throughout the left ventricular myocardium.