Active Cytoskeletal Composites Display Emergent Tunable Contractility and Restructuring

The cytoskeleton is a model active matter system that controls diverse cellular processes from division to motility. While both active actomyosin dynamics and actin-microtubule interactions are key to the cytoskeleton's versatility and adaptability, an understanding of their interplay is lacking. Here, we couple microscale experiments with mechanistic modeling to elucidate how connectivity, rigidity, and force-generation affect emergent material properties in in vitro composites of actin, tubulin, and myosin. We use time-resolved differential dynamic microscopy and spatial image autocorrelation to show that ballistic contraction occurs in composites with sufficient flexibility and motor density, but that a critical fraction of microtubules is necessary to sustain controlled dynamics. Our active double-network models reveal that percolated actomyosin networks are essential for contraction, but that networks with comparable actin and microtubule densities can uniquely resist mechanical stresses while simultaneously supporting substantial restructuring. Our findings provide a much-needed blueprint for designing cytoskeleton-inspired materials that couple tunability with resilience and adaptability.

Myosin-driven actin-microtubule networks exhibit self-organized contractile dynamics

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. While actomyosin networks are extensively studied, how interactions between actin and microtubules, ubiquitous in the cytoskeleton, influence actomyosin activity remains an open question. Here, we create a network of co-entangled actin and microtubules driven by myosin II. We combine dynamic differential microscopy, particle image velocimetry, and particle tracking to show that both actin and microtubules undergo ballistic contraction with unexpectedly indistinguishable characteristics. This contractility is distinct from faster disordered motion and rupturing that active actin networks exhibit. Our results suggest that microtubules enable self-organized myosin-driven contraction by providing flexural rigidity and enhanced connectivity to actin networks. Beyond the immediate relevance to cytoskeletal dynamics, our results shed light on the design of active materials that can be precisely tuned by the network composition.

Myosin-driven actin-microtubule networks exhibit self-organized contractile dynamics

The cytoskeleton is a dynamic network of proteins, including actin, microtubules, and myosin, that enables essential cellular processes such as motility, division, mechanosensing, and growth. While actomyosin networks are extensively studied, how interactions between actin and microtubules, ubiquitous in the cytoskeleton, influence actomyosin activity remains an open question. Here, we create a network of co-entangled actin and microtubules driven by myosin II. We combine dynamic differential microscopy, particle image velocimetry and particle-tracking to show that both actin and microtubules in the network undergo ballistic contraction with surprisingly indistinguishable characteristics. This controlled contractility is distinct from the faster turbulent motion and rupturing that active actin networks exhibit. Our results suggest that microtubules can enable self-organized myosin-driven contraction by providing flexural rigidity and enhanced connectivity to actin networks. These results provide important new insight into the diverse interactions cells can use to tune activity, and offer a powerful platform for designing multifunctional materials with well-regulated activity.

The Influence of Familiarization Sessions on the Stability of Ramp and Ballistic Isometric Torque in Older Adults

Ramp isometric contractions determine peak torque (PT) and neuromuscular activation (NA), and ballistic contractions can be used to evaluate rate of torque development (RTD) and electrical mechanical delay (EMD). The purposes of this study were to assess the number of sessions required to stabilize ramp and ballistic PT and to compare PT and NA between contractions in older adults. Thirty-five older men and women (age 63.7 ± 3.7 yr, body mass 64.3 ± 10.7 kg, height 159.2 ± 6.6 cm) performed 4 sessions of unilateral ramp and ballistic isometric knee extension, 48 hr apart. PT significantly increased (main time effectp< .05) from the first to the third session, with no further improvements thereafter. There was a trend toward higher PT in ballistic than in ramp contractions. No difference between contraction types on EMG values was observed. Therefore, the authors suggest that 3 familiarization sessions be performed to correctly assess PT. In addition, PT, NA, RTD, and EMD can be assessed with ballistic contraction in older adults.

Ballistic Movement: Muscle Activation and Neuromuscular Adaptation

Movements that are performed with maximal velocity and acceleration can be considered ballistic actions. Ballistic actions are characterized by high firing rates, brief contraction times, and high rates of force development. A characteristic triphasic agonist/antagonist/agonist electromyographic (EMG) burst pattern occurs during ballistic movement, wherein the amount and intensity of antagonist coactivation is variable. In conditions of low-grade tonic muscular activity, a premovement EMG depression (PMD; or silent period, PMS) can occur in agonist muscles prior to ballistic contraction. The agonist PMD period may serve to potentiate the force and velocity of the following contraction. A selective activation of fast twitch motor units may occur in ballistic contractions under certain movement conditions. Finally, high-velocity ballistic training induces specific neuromuscular adaptations that occur as a function of the underlying neurophysiological mechanisms that subserve ballistic movement. Key words: electromyography, motor control, training adaptation, motor unit