FORCES PRODUCED BY MYOSIN AND ACTIN-CONTAINING SUBCELLULAR STRUCTURES

Kevin Burton and D. Lansing Taylor
Center for Light Microscope Imaging & Biotechnology, Carnegie Mellon University, Pittsburgh, PA

Forces generated at the subcellular level in the range of nanonewtons to micronewtons have been correlated with the dynamics of myosin and actin- containing cytoskeletal structures and locomotory protrusions. Several types of light microscopy have been used to monitor forces applied to transparent elastic substrata (Harris, Wild and Stopak, Science 208:177, 1980; Burton and Taylor, Nature 385:450, 1997) and simultaneously to visualize cellular force-generating structures. In stationary fibroblasts, myosin II is incorporated along growing stress fibers (diameter ~1 micron) as force is generated at their termini where focal adhesions form. During cell division in fibroblasts, filopodia (diameter ~0.8 microns) frequently protrude, and those that adhere to the silicone substrata have been observed to apply pulling forces, whereas those that do not adhere are retracted into the cell body. Although filopodia are known to contain myosin I, and this motor may play a role in protrusion, the force for retraction may actually be generated at the base of the filopodium within the cortex where myosin II is located. Tubular cytoplasmic protrusions (diameter ~3 microns) also execute the same sequence of events as filopodia: extend, adhere, and pull. In addition, even blebs (2-2.5 micron surface herniations) have frequently been observed to exert force. In rapidly locomoting keratocytes, punctate distortions (0.6-0.7 microns) can appear anywhere in the elastic substratum under the lamellipodium, even at the leading edge, demonstrating the application of forces at those locations. The current method for simultaneously measuring forces at many locations uses a continuous substratum, and improvement will come from development of micro-arrays of discrete transducers for accurately measuring nanonewton forces at sub-micron resolution in two and three dimensions.