Using Single-Protein Tracking to Study Cell Migration

Novel imaging methods and force probes for molecular mechanobiology of cytoskeleton and adhesion

Detection and conversion of mechanical forces into biochemical signals is known as mechanotransduction. From cells to tissues, mechanotransduction regulates migration, proliferation, and differentiation in processes such as immune responses, development, and cancer progression. Mechanosensitive structures such as integrin adhesions, the actin cortex, ion channels, caveolae, and the nucleus sense and transmit forces. In vitro approaches showed that mechanosensing is based on force-dependent protein deformations and reorganizations. However, the mechanisms in cells remained unclear since cell imaging techniques lacked molecular resolution. Thanks to recent developments in super-resolution microscopy (SRM) and molecular force sensors, it is possible to obtain molecular insight of mechanosensing in live cells. We discuss how understanding of molecular mechanotransduction was revolutionized by these innovative approaches, focusing on integrin adhesions, actin structures, and the plasma membrane.

Forces generated by lamellipodial actin filament elongation regulate the WAVE complex during cell migration

Actin filaments generate mechanical forces that drive membrane movements during trafficking, endocytosis and cell migration. Reciprocally, adaptations of actin networks to forces regulate their assembly and architecture. Yet, a demonstration of forces acting on actin regulators at actin assembly sites in cells is missing. Here we show that local forces arising from actin filament elongation mechanically control WAVE regulatory complex (WRC) dynamics and function, that is, Arp2/3 complex activation in the lamellipodium. Single-protein tracking revealed WRC lateral movements along the lamellipodium tip, driven by elongation of actin filaments and correlating with WRC turnover. The use of optical tweezers to mechanically manipulate functional WRC showed that piconewton forces, as generated by single-filament elongation, dissociated WRC from the lamellipodium tip. WRC activation correlated with its trapping, dwell time and the binding strength at the lamellipodium tip. WRC crosslinking, hindering its mechanical dissociation, increased WRC dwell time and Arp2/3-dependent membrane protrusion. Thus, forces generated by individual actin filaments on their regulators can mechanically tune their turnover and hence activity during cell migration.