Velocity-dependent actomyosin ATPase cycle revealed by in vitro motility assay with kinetic analysis

Cell Behavioral Dynamics as a Cue in Optimizing Culture Stabilization in the Bioprocessing of Pluripotent Stem Cells

Pluripotent stem cells (PSCs) are important for future regenerative medicine therapies. However, in the production of PSCs and derivatives, the control of culture-induced fluctuations in the outcome of cell quality remains challenging. A detailed mechanistic understanding of how PSC behaviors are altered in response to biomechanical microenvironments within a culture is necessary for rational bioprocessing optimization. In this review, we discuss recent insights into the role of cell behavioral and mechanical homeostasis in modulating the states and functions of PSCs during culture processes. We delineate promising ways to manipulate the culture variability through regulating cell behaviors using currently developed tools. Furthermore, we anticipate their potential implementation for designing a culture strategy based on the concept of Waddington's epigenetic landscape that may provide a feasible solution for tuning the culture quality and stability in the bioprocessing space.

Collective effects of yeast cytoplasmic dynein based microtubule transport

Teams of cortically anchored dyneins pulling at microtubules (MTs) are known to be essential for aster, spindle and nuclear positioning during cell division and fertilization. While the single-molecule basis of dynein processivity is now better understood, the effect of increasing numbers of motors on transport is not clear. Here, we examine the collective transport properties of a Saccharomyces cerevisiae cytoplasmic dynein fragment, widely used as a minimal model, by a combination of quantitative MT gliding assays and stochastic simulations. We find both MT lengths and motor densities qualitatively affect the degree of randomness of MT transport. However, the directionality and velocity of MTs increase above a threshold number of motors (N) interacting with a filament. To better understand this behavior, we simulate a gliding assay based on a model of uniformly distributed immobilized motors transporting semi-flexible MTs. Each dynein dimer is modeled as an effective stochastic stepper with asymmetric force dependent detachment dynamics, based on single-molecule experiments. Simulations predict increasing numbers of motors (N) result in a threshold dependent transition in directionality and transport velocity and a monotonic decrease in effective diffusivity. Thus both experiment and theory show qualitative agreement in the emergence of coordination in transport above a threshold number of motor heads. We hypothesize that the phase-transition like property of this dynein could play a role in vivo during yeast mitosis, when this dynein localizes to the cortex and pulls astral MTs of increasing length, resulting in correct positioning and orientation of the nucleus at the bud-neck.

Correlated receptor transport processes buffer single-cell heterogeneity

Cells typically vary in their response to extracellular ligands. Receptor transport processes modulate ligand-receptor induced signal transduction and impact the variability in cellular responses. Here, we quantitatively characterized cellular variability in erythropoietin receptor (EpoR) trafficking at the single-cell level based on live-cell imaging and mathematical modeling. Using ensembles of single-cell mathematical models reduced parameter uncertainties and showed that rapid EpoR turnover, transport of internalized EpoR back to the plasma membrane, and degradation of Epo-EpoR complexes were essential for receptor trafficking. EpoR trafficking dynamics in adherent H838 lung cancer cells closely resembled the dynamics previously characterized by mathematical modeling in suspension cells, indicating that dynamic properties of the EpoR system are widely conserved. Receptor transport processes differed by one order of magnitude between individual cells. However, the concentration of activated Epo-EpoR complexes was less variable due to the correlated kinetics of opposing transport processes acting as a buffering system.

Cell surface receptors translate extracellular ligand concentrations to intracellular responses. Receptor transport between the plasma membrane and other cellular compartments regulates the number of accessible receptors at the plasma membrane that determines the strength of downstream pathway activation at a given ligand concentration. In cell populations, pathway activation strength and cellular responses vary between cells. Understanding origins of cell-to-cell variability is highly relevant for cancer research, motivated by the problem of fractional killing by chemotherapies and development of resistance in subpopulations of tumor cells. The erythropoietin receptor (EpoR) is a characteristic example of a receptor system that strongly depends on receptor transport processes. It is involved in several cellular processes, such as differentiation or proliferation, regulates the renewal of erythrocytes, and is expressed in several tumors. To investigate the involvement of receptor transport processes in cell-to-cell variability, we quantitatively characterized trafficking of EpoR in individual cells by combining live-cell imaging with mathematical modeling. Thereby, we found that EpoR dynamics was strongly dependent on rapid receptor transport and turnover. Interestingly, although transport processes largely differed between individual cells, receptor concentrations in cellular compartments were robust to variability in trafficking processes due to the correlated kinetics of opposing transport processes.

A small-molecule modulator of cardiac myosin acts on multiple stages of the myosin chemomechanical cycle

Mavacamten, formerly known as MYK-461 is a recently discovered novel small-molecule modulator of cardiac myosin that targets the underlying sarcomere hypercontractility of hypertrophic cardiomyopathy, one of the most prevalent heritable cardiovascular disorders. Studies on isolated cells and muscle fibers as well as intact animals have shown that mavacamten inhibits sarcomere force production, thereby reducing cardiac contractility. Initial mechanistic studies have suggested that mavacamten primarily reduces the steady-state ATPase activity by inhibiting the rate of phosphate release of β-cardiac myosin-S1, but the molecular mechanism of action of mavacamten has not been described. Here we used steady-state and presteady-state kinetic analyses to investigate the mechanism of action of mavacamten. Transient kinetic analyses revealed that mavacamten modulates multiple steps of the myosin chemomechanical cycle. In addition to decreasing the rate-limiting step of the cycle (phosphate release), mavacamten reduced the number of myosin-S1 heads that can interact with the actin thin filament during transition from the weakly to the strongly bound state without affecting the intrinsic rate. Mavacamten also decreased the rate of myosin binding to actin in the ADP-bound state and the ADP-release rate from myosin-S1 alone. We, therefore, conclude that mavacamten acts on multiple stages of the myosin chemomechanical cycle. Although the primary mechanism of mavacamten-mediated inhibition of cardiac myosin is the decrease of phosphate release from β-cardiac myosin-S1, a secondary mechanism decreases the number of actin-binding heads transitioning from the weakly to the strongly bound state, which occurs before phosphate release and may provide an additional method to modulate myosin function.

Maximum limit to the number of myosin II motors participating in processive sliding of actin

In this work, we analysed processive sliding and breakage of actin filaments at various heavy meromyosin (HMM) densities and ATP concentrations in IVMA. We observed that with addition of ATP solution, the actin filaments fragmented stochastically; we then determined mean length and velocity of surviving actin filaments post breakage. Average filament length decreased with increase in HMM density at constant ATP, and increased with increase in ATP concentration at constant HMM density. Using density of HMM molecules and length of actin, we estimated the number of HMM molecules per actin filament (N) that participate in processive sliding of actin. N is solely a function of ATP concentration: 88 ± 24 and 54 ± 22 HMM molecules (mean ± S.D.) at 2 mM and 0.1 mM ATP respectively. Processive sliding of actin filament was observed only when N lay within a minimum lower limit (Nmin) and a maximum upper limit (Nmax) to the number of HMM molecules. When N < Nmin the actin filament diffused away from the surface and processivity was lost and when N > Nmax the filament underwent breakage eventually and could not sustain processive sliding. We postulate this maximum upper limit arises due to increased number of strongly bound myosin heads.

Single carbon nanotube-based reversible regulation of biological motor activity

Because of their small size and high thermal conductivity, carbon nanotubes (CNTs) are excellent candidates for exploring heat transfer at the level of individual molecules in biological research. With a view toward examining the thermal regulation of single biomolecules, we here developed single CNTs as a new platform for observing the motile activity of myosin motors. On multiwall CNTs (diameter ∼170 nm; length ∼10 μm) coated with skeletal-muscle myosin, the ATP-driven sliding of single actin filaments was clearly observable. The normal sliding speed was ∼6 μm/s. Locally irradiating one end of the CNT with a red laser (642 nm), without directly irradiating the active myosin motors, accelerated the sliding speed to ∼12 μm/s, indicating the reversible activation of protein function on a single CNT in real time. The temperature along the CNT, which was estimated from the temperature-dependence of the sliding speed, decreased with the distance from the irradiated spot. Using these results with the finite element method, we calculated a first estimation of the thermal conductivity of multiwall CNTs in solution, as 1540 ± 260 (Wm(-1) K(-1)), which is consistent with the value estimated from the width dependency of multiwall CNTs and the length dependency of single-wall CNTs in a vacuum or air. The temporal regulation of local temperature through individual CNTs should be broadly applicable to the selective activation of various biomolecules in vitro and in vivo.

Single turnovers of fluorescent ATP bound to bipolar myosin filament during actin filaments sliding

The nucleotide turnover rates of bipolar myosin thick filament along which actin filament slides were measured by the displacement of prebound fluorescent ATP analog 2'(3')-O-[N-[2-[(Cy3)]amindo]ethyl] carbamoyl]-adenosine 5' triphosphate (Cy3-EDA-ATP) upon flash photolysis of caged ATP. The fluorescence of the thick filament where actin filament slides decayed with two exponential processes. The slower rate constant was the same as that without actin filament. Along bipolar myosin thick filament, actin filaments slide at a fast speed towards the central bare zone (forward), but more slowly away from the bare zone (backward). The displacement rate constant of fluorescent ATP from the myosin filament where actin filament moved forward was 5.0 s(-1), whereas the rate constant where the actin filament slid backward was 1.7 s(-1). These findings suggest that the slow ADP release rate is responsible for the slow backward sliding movement.