Optical tweezers map spatiotemporal force generation in active actin-microtubule composites

The cytoskeleton is a dynamic network of proteins, including actin, microtubules, and their associated motor proteins that enables essential cellular processes such as motility, division, and growth. Previous study has shown co-entangled actin-microtubule networks driven by myosin exhibit organized and controlled contraction dynamics. In contrast, similar actomyosin networks that lack microtubules undergo fast, multidirectional motion and network rupturing. However, the underlying motor-generated forces that give rise to these different behaviors is poorly understand. Here, we combine optical tweezers microrheology, fluorescence microscopy and photo-controlled myosin activity, to measure local contractile forces in active co-entangled actin-microtubule composites. By measuring the forces on beads that are held fixed at different positions throughout the network, with a particular focus on force generation at the network boundary, we determine the stress that the composites can self-generate, and how these stresses propagate through the network from the contractile edge. By simutaneously imaging the actively restructuring filaments in the composite we directly correlate structure and deformation fields of the network to force generation.