Insights into the mechanisms of myosin and kinesin molecular motors from the single-molecule unbinding force measurements

Simulated actin reorganization mediated by motor proteins

Cortical actin networks are highly dynamic and play critical roles in shaping the mechanical properties of cells. The actin cytoskeleton undergoes significant reorganization in many different contexts, including during directed cell migration and over the course of the cell cycle, when cortical actin can transition between different configurations such as open patched meshworks, homogeneous distributions, and aligned bundles. Several types of myosin motor proteins, characterized by different kinetic parameters, have been involved in this reorganization of actin filaments. Given the limitations in studying the interactions of actin with myosin in vivo, we propose stochastic agent-based models and develop a set of data analysis measures to assess how myosin motor proteins mediate various actin organizations. In particular, we identify individual motor parameters, such as motor binding rate and step size, that generate actin networks with different levels of contractility and different patterns of myosin motor localization, which have previously been observed experimentally. In simulations where two motor populations with distinct kinetic parameters interact with the same actin network, we find that motors may act in a complementary way, by tuning the actin network organization, or in an antagonistic way, where one motor emerges as dominant. This modeling and data analysis framework also uncovers parameter regimes where spatial segregation between motor populations is achieved. By allowing for changes in kinetic rates during the actin-myosin dynamic simulations, our work suggests that certain actin-myosin organizations may require additional regulation beyond mediation by motor proteins in order to reconfigure the cytoskeleton network on experimentally-observed timescales.

Cell shape is dictated by a scaffolding network called the cytoskeleton. Actin filaments, a main component of the cytoskeleton, are found predominantly at the periphery of the cell, where they organize into different patterns in response to various stimuli, such as progression through the cell cycle. The actin filament reorganizations are mediated by motor proteins from the myosin superfamily. Using a realistic stochastic model that simulates actin filament and motor protein dynamics and interactions, we systematically vary motor protein kinetics and investigate their effect on actin filament organization. Using novel measures of spatial organization, we quantify conditions under which motor proteins, either alone or in combination, can produce the different actin filament organizations observed in vitro and in vivo. These results yield new insights into the role of motor proteins, as well as into how multiple types of motors can work collectively to produce specific actomyosin network patterns.

Neuromechanobiology: An Expanding Field Driven by the Force of Greater Focus

The brain processes information by transmitting signals through highly connected and dynamic networks of neurons. Neurons use specific cellular structures, including axons, dendrites and synapses, and specific molecules, including cell adhesion molecules, ion channels and chemical receptors to form, maintain and communicate among cells in the networks. These cellular and molecular processes take place in environments rich of mechanical cues, thus offering ample opportunities for mechanical regulation of neural development and function. Recent studies have suggested the importance of mechanical cues and their potential regulatory roles in the development and maintenance of these neuronal structures. Also suggested are the importance of mechanical cues and their potential regulatory roles in the interaction and function of molecules mediating the interneuronal communications. In this review, the current understanding is integrated and promising future directions of neuromechanobiology are suggested at the cellular and molecular levels. Several neuronal processes where mechanics likely plays a role are examined and how forces affect ligand binding, conformational change, and signal induction of molecules key to these neuronal processes are indicated, especially at the synapse. The disease relevance of neuromechanobiology as well as therapies and engineering solutions to neurological disorders stemmed from this emergent field of study are also discussed.

Modelling cytoskeletal transport by clusters of non-processive molecular motors with limited binding sites

Molecular motors are responsible for intracellular transport of a variety of biological cargo. We consider the collective behaviour of a finite number of motors attached on a cargo. We extend previous analytical work on processive motors to the case of non-processive motors, which stochastically bind on and off cytoskeletal filaments with a limited number of binding sites available. Physically, motors attached to a cargo cannot bind anywhere along the filaments, so the number of accessible binding sites on the filament should be limited. Thus, we analytically study the distribution and the velocity of a cluster of non-processive motors with limited number of binding sites. To validate our analytical results and to go beyond the level of detail possible analytically, we perform Monte Carlo latticed based stochastic simulations. In particular, in our simulations, we include sequence preservation of motors performing stepping and binding obeying a simple exclusion process. We find that limiting the number of binding sites reduces the probability of non-processive motors binding but has a relatively small effect on force-velocity relations. Our analytical and stochastic simulation results compare well to published data from in vitro and in vivo experiments.

Mechanical role of actin dynamics in the rheology of the Golgi complex and in Golgi-associated trafficking events

BACKGROUND

In vitro studies have shown that physical parameters, such as membrane curvature, tension, and composition, influence the budding and fission of transport intermediates. Endocytosis in living cells also appears to be regulated by the mechanical load experienced by the plasma membrane. In contrast, how these parameters affect intracellular membrane trafficking in living cells is not known. To address this question, we investigate here the impact of a mechanical stress on the organization of the Golgi complex and on the formation of transport intermediates from the Golgi complex.

RESULTS

Using confocal microscopy, we visualize the deformation of Rab6-positive Golgi membranes applied by an internalized microsphere trapped in optical tweezers and simultaneously measure the corresponding forces. Our results show that the force necessary to deform Golgi membranes drops when actin dynamics is altered and correlates with myosin II activity. We also show that the applied stress has a long-range effect on Golgi membranes, perturbs the dynamics of Golgi-associated actin, and induces a sharp decrease in the formation of Rab6-positive vesicles from the Golgi complex as well as tubulation of Golgi membranes.

CONCLUSIONS

We suggest that acto-myosin contractility strongly contributes to the local rigidity of the Golgi complex and regulates the mechanics of the Golgi complex to control intracellular membrane trafficking.

Mechanoregulation of h2-calponin gene expression and the role of Notch signaling

The essential role of mechanical signals in regulating the function of living cells is universally observed. However, how mechanical signals are transduced in cells to regulate gene expression is largely unknown. We previously demonstrated that the gene encoding h2-calponin (Cnn2) is sensitively regulated by mechanical tension. In the present study, mouse genomic DNA containing the Cnn2 promoter was cloned, and a nested set of 5' truncations was studied. Transcriptional activity of the Cnn2 promoter-reporter constructs was examined in transfected NIH/3T3, HEK293, and C2C12 cells for their responses to the stiffness of culture substrate. The results showed significant transcriptional activities of the -1.00- and -1.24-kb promoter constructs, whereas the -0.61-kb construct was inactive. The -1.38-, -1.57-, and -2.12-kb constructs showed higher transcriptional activity, whereas only the -1.57- and -2.12-kb constructs exhibited repression of expression when the host cells were cultured on low stiffness substrate. Internal deletion of the segment between -1.57 and -1.38 kb in the -2.12-kb promoter construct abolished the low substrate stiffness-induced repression. Site-specific deletion or mutation of an HES-1 transcription factor binding site in this region also abolished this repression effect. The level of HES-1 increased in cells cultured under a low tension condition, corresponding to the down-regulation of h2-calponin. h2-Calponin gene expression is further affected by the treatment of cells with Notch inhibitor and activator, suggesting an upstream signaling mechanism.

Nanogel surface coatings for improved single-molecule imaging substrates

Surfaces that resist protein adsorption are important for many bioanalytical applications. Bovine serum albumin (BSA) coatings and multi-arm poly(ethylene glycol) (PEG) coatings display low levels of non-specific protein adsorption and have enabled highly quantitative single-molecule (SM) protein studies. Recently, a method was developed for coating a glass with PEG-BSA nanogels, a promising hybrid of these two low-background coatings. We characterized the nanogel coating to determine its suitability for SM protein experiments. SM adsorption counting revealed that nanogel-coated surfaces exhibit lower protein adsorption than covalently coupled BSA surfaces and monolayers of multi-arm PEG, so this surface displays one of the lowest degrees of protein adsorption yet observed. Additionally, the nanogel coating was resistant to DNA adsorption, underscoring the utility of the coating across a variety of SM experiments. The nanogel coating was found to be compatible with surfactants, whereas the BSA coating was not. Finally, applying the coating to a real-world study, we found that single ligand molecules could be tethered to this surface and detected with high sensitivity and specificity by a digital immunoassay. These results suggest that PEG-BSA nanogel coatings will be highly useful for the SM analysis of proteins.