Interactions between actin filament networks and the focal adhesions to which they are linked results in the generation of forces. These forces may be exerted internally through actin bundle tension and filament network dynamics or externally as the cell pushes on its surroundings. A number of studies, discussed below, have focused on measuring the protrusive forces generated by lamellipodia .
A study by Prass et al described the ‘whole cell stall force’ by placing an atomic force microscopy cantilever in the path of migrating keratocytes. This measurement represents the maximum force applied by the lamellipodia before the leading edge can no longer overcome the opposing force of the cantilever. At the moment this point is reached the process of cell body translocation stalls. In the case of migrating keratocytes, the maximal whole cell stall force was 40nN .
Importantly, observations from this study revealed that the force applied to the cantilever was generated by only a small region (3µM) of the lamellipodia, with the adjacent portions continuing to crawl forward, moving around the object. This not only highlights the dynamic nature of the actin cytoskeletal network within lamellipodia, but also illustrates that the total force is generated by independent filament components. In this case it was calculated that approximately 4pN of force was generated by each filament and with a whole cell stall force of 40nN, approximately 100 filaments were estimated to be generating the protrusive force per 1µM of the leading edge .
Protrusive forces have also been measured in non-migratory cells, with a higher resolution of detection through the use of optical tweezers. In one such study by Cojac et al, the forces exerted by lamellipodia during neuronal differentiation were compared to those produced by filopodia and found to differ significantly. The forces produced by filopodia were consistently measured to be no larger than 2pN and were exerted for varying lengths of time of up to 15 seconds. in contrast the forces produced by lamellipodia ranged greatly from less than 1pN to more than 20pN – a force strong enough to displace trapped beads. Furthermore, these forces were exerted for an equally wide range of time, from less than 1 second to 30 seconds .
In another study conducted using optical tweezers, by Shahapure et al, the protrusive force of lamellipodia in rat dorsal ganglia (DRG) was measured, within a millisecond and picoNewton range . In this case the maximal pressure exerted by DRG lamellipodia was ranged between 20 to 80pN/µm2. The ever-changing and complex nature of the generation of these forces was also noted. This was apparent from the alternating phases of rapid growth and retraction at the leading edge, further complicated by the influence of signals generated at adhesion sites. Adhesion sites in this case were in some cases associated with a transient inversion of lamellipodial velocity as well as transient retractions, all of which impinge on force generation .
Although each of the studies described above provide key quantitative insights into the total protrusive force generated by lamellipodia, it remains difficult to determine the specific contributions of the mechanisms driving these forces within each biological system. This is highlighted by the in vivo work of Prass et al, where numerous factors such as actin polymerization, the influence of motor proteins, local osmotic pressure and mechanical and chemical stimuli all influence lamellipodial protrusion .
The processes contributing to force generation during lamellipodial protrusion are varied in both their mechanisms and in their contribution. In some cases these processes will be redundant, whilst in other cases they will be crucial. Understanding which process to isolate, control or measure in an in vitro setting is a factor that currently limits our ability to properly define force generation. In line with this thinking, Shahapure et al recognized the fact that the dynamic nature of the lamellipodia causes the force it generates to be equally dynamic.