Actomyosin motility on nanostructured surfaces

Surface Topography Induces and Orients Nematic Swarms of Active Filaments: Considerations for Lab-On-A-Chip Devices

Surface-bound molecular motors can drive the collective motion of cytoskeletal filaments in the form of nematic bands and polar flocks in reconstituted gliding assays. Although these "swarming transitions" are an emergent property of active filament collisions, they can be controlled and guided by tuning the surface chemistry or topography of the substrate. To date, the impact of surface topography on collective motion in active nematics is only partially understood, with most experimental studies focusing on the escape of a single filament from etched channels. Since the late 1990s, significant progress has been made to utilize the nonequilibrium properties of active filaments and create a range of functional nanodevices relevant to biosensing and parallel computation; however, the complexity of these swarming transitions presents a challenge when attempting to increase filament surface concentrations. In this work, we etch shallow, linear trenches into glass substrates to induce the formation of swarming nematic bands and investigate the mechanisms by which surface topography regulates the two-dimensional (2D) collective motion of driven filamentous actin (F-actin). We demonstrate that nematic swarms only appear at intermediate trench spacings and vanish if the trenches are made too narrow, wide, or tortuous. To rationalize these results, we simulate the F-actin as self-propelled, semiflexible chains subject to a soft, spatially modulated potential that encodes the energetic cost of bending a filament along the edge of a trench. In our model, we hypothesize that an individual filament experiences a penalty when its projected end-to-end distance is smaller than the trench spacing ("bending and turning"). However, chains that span the channel width glide above the trenches in a force- and torque-free manner ("crowd-surfing"). Our simulations demonstrate that collections of filaments form nematic bands only at intermediate trench spacings, consistent with our experimental findings.

Soft confinement of self-propelled rods: simulation and theory

We present an analytical framework for evolving the dynamics of active rods under any periodic external potential, including confining channels and arrays of harmonic traps. As a proof of concept, we analyze the structure and dispersion of self-propelled rods under a soft, periodic one-dimensional (1D) confinement potential and under a two-dimensional (2D) periodic radial harmonic trap. While passive rods and polymers nematically order under 1D confinement, their diffusive transport along the director is limited by thermal diffusion. In contrast, self-propelled rods can generate large convective fluxes when combined with nematic ordering, producing a strong dispersion along the director. Combining theory and simulation, we demonstrate that nematic alignment and self-propulsion generates an exponential enhancement in active diffusivity along the director, in contrast to passive rods that experience at most a 2-fold increase.

Regeneration of Assembled, Molecular-Motor-Based Bionanodevices

The guided gliding of cytoskeletal filaments, driven by biomolecular motors on nano/microstructured chips, enables novel applications in biosensing and biocomputation. However, expensive and time-consuming chip production hampers the developments. It is therefore important to establish protocols to regenerate the chips, preferably without the need to dismantle the assembled microfluidic devices which contain the structured chips. We here describe a novel method toward this end. Specifically, we use the small, nonselective proteolytic enzyme, proteinase K to cleave all surface-adsorbed proteins, including myosin and kinesin motors. Subsequently, we apply a detergent (5% SDS or 0.05% Triton X100) to remove the protein remnants. After this procedure, fresh motor proteins and filaments can be added for new experiments. Both, silanized glass surfaces for actin-myosin motility and pure glass surfaces for microtubule-kinesin motility were repeatedly regenerated using this approach. Moreover, we demonstrate the applicability of the method for the regeneration of nano/microstructured silicon-based chips with selectively functionalized areas for supporting or suppressing gliding motility for both motor systems. The results substantiate the versatility and a promising broad use of the method for regenerating a wide range of protein-based nano/microdevices.

Controlled Surface Silanization for Actin-Myosin Based Nanodevices and Biocompatibility of New Polymer Resists

Molecular motor-based nanodevices require organized cytoskeletal filament guiding along motility-promoting tracks, confined by motility-inhibiting walls. One way to enhance motility quality on the tracks, particularly in terms of filament velocity but also the fraction of motile filaments, is to optimize the surface hydrophobicity. We have investigated the potential to achieve this for the actin-myosin II motor system on trimethylchlorosilane (TMCS)-derivatized SiO₂ surfaces to be used as channel floors in nanodevices. We have also investigated the ability to supress motility on two new polymer resists, TU7 (for nanoimprint lithography) and CSAR 62 (for electron beam and deep UV lithography), to be used as channel walls. We developed a chemical-vapor deposition tool for silanizing SiO₂ surfaces in a controlled environment to achieve different surface hydrophobicities (measured by water contact angle). In contrast to previous work, we were able to fabricate a wide range of contact angles by varying the silanization time and chamber pressure using only one type of silane. This resulted in a significant improvement of the silanization procedure, producing a predictable contact angle on the surface and thereby predictable quality of the heavy meromyosin (HMM)-driven actin motility with regard to velocity. We observed a high degree of correlation between the filament sliding velocity and contact angle in the range 10-86°, expanding the previously studied range. We found that the sliding velocity on TU7 surfaces was superior to that on CSAR 62 surfaces despite similar contact angles. In addition, we were able to suppress the motility on both TU7 and CSAR 62 by plasma oxygen treatment before silanization. These results are discussed in relation to previously proposed surface adsorption mechanisms of HMM and their relationship to the water contact angle. Additionally, the results are considered for the development of actin-myosin based nanodevices with superior performance with respect to actin-myosin functionality.

Direct measurement of cortical force generation and polarization in a living parasite

Apicomplexa is a large phylum of intracellular parasites that are notable for the diseases they cause, including toxoplasmosis, malaria, and cryptosporidiosis. A conserved motile system is critical to their life cycles and drives directional gliding motility between cells, as well as invasion of and egress from host cells. However, our understanding of this system is limited by a lack of measurements of the forces driving parasite motion. We used a laser trap to measure the function of the motility apparatus of living Toxoplasma gondii by adhering a microsphere to the surface of an immobilized parasite. Motion of the microsphere reflected underlying forces exerted by the motile apparatus. We found that force generated at the parasite surface begins with no preferential directionality but becomes directed toward the rear of the cell after a period of time. The transition from nondirectional to directional force generation occurs on spatial intervals consistent with the lateral periodicity of structures associated with the membrane pellicle and is influenced by the kinetics of actin filament polymerization and cytoplasmic calcium. A lysine methyltransferase regulates both the magnitude and polarization of the force. Our work provides a novel means to dissect the motile mechanisms of these pathogens.

Polymer surface properties control the function of heavy meromyosin in dynamic nanodevices

The actin-myosin system, responsible for muscle contraction, is also the force-generating element in dynamic nanodevices operating with surface-immobilized motor proteins. These devices require materials that are amenable to micro- and nano-fabrication, but also preserve the bioactivity of molecular motors. The complexity of the protein-surface systems is greatly amplified by those of the polymer-fluid interface; and of the structure and function of molecular motors, making the study of these interactions critical to the success of molecular motor-based nanodevices. We measured the density of the adsorbed motor protein (heavy meromyosin, HMM) using quartz crystal microbalance; and motor bioactivity with ATPase assay, on a set of model surfaces, i.e., nitrocellulose, polystyrene, poly(methyl methacrylate), and poly(butyl methacrylate), poly(tert-butyl methacrylate). A higher hydrophobicity of the adsorbing material translates in a higher total number of HMM molecules per unit area, but also in a lower uptake of water, and a lower ratio of active per total HMM molecules per unit area. We also measured the motility characteristics of actin filaments on the model surfaces, i.e., velocity, smoothness and deflection of movement, determined via in vitro motility assays. The filament velocities were found to be controlled by the relative number of active HMM per total motors, rather than their absolute surface density. The study allowed the formulation of the general engineering principles for the selection of polymeric materials for the manufacturing of dynamic nanodevices using protein molecular motors.

Influence of Surface Topographical Cues on the Differentiation of Mesenchymal Stem Cells in Vitro

Adult stem cell research has been advanced in recent years because of the cells' attractive abilities of self-renewal and differentiation. Topography of materials is one of the key features that can be harnessed to regulate stem cell behaviors. Stem cells can interact with underlying material through nanosized integrin receptors. Therefore, the manipulation of topographical cues at a nanoscale level can be employed to modulate the cell fate. In this review, we focus our discussion on the different surface topographical cues, especially, with an emphasis on the viral nanoparticle-coated materials, and their effects on stem cell differentiation.

Plasma treatment of PDMS for applications of in vitro motility assays

In vitro motility assays are readily used to simplify the complex environments within the cell and in muscle tissue. These assays have afforded considerable insight into the fundamentals of their underlying biophysics, interactions with cargo, intracellular regulation, and motor cooperation/competition. Extension of the standard in vitro motility assay into a more automated and cost-effective fluidic design while providing availability to the scientific community without expertise in lithographic fabrication is critical for the continued advancement of the field. In this work, we utilized a standard plasma cleaner to oxidize the widely prevalent material polydimethylsiloxane (PDMS) to create flow cells that could be used for in vitro motility assays. Our analysis indicated that a 40 min pre-treatment of the PDMS with plasma exposure resulted in optimal bundle motility. This finding was attributed to the condition at which the least amount of oxygen permeates the PDMS slab, enters the motility buffer, and oxidizes the motor proteins. Based on these findings, we developed a method for constructing microfluidic devices from glass and plasma-treated PDMS molds in which motility could be observed.

Directional transport by nonprocessive motor proteins on fascin-cross-linked actin arrays

In this study, the unidirectional transport of heavy meromyosin (HMM)-coated beads is demonstrated on fascin-cross-linked actin arrays. The streptavidin-coated surface was properly blocked to prevent nonspecific binding of F-actin and, as a result, a high population of long gelsolin-actin complexes was suspended in the medium for subsequent processes. A flow field was utilized to lay down F-actin aligned along the direction of the flow and fascin cross-linked laid F-actin to prevent F-actin resuspension. When HMM-coated beads came into contact with the fascin-cross-linked actin arrays, they started to move in the same direction over long distances. Because of the nonprocessive nature of myosin II motor protein, the bead size limited the number of HMM heads on the area in contact with F-actin arrays, which resulted in beads traveling at different velocities according to their sizes. Furthermore, this study demonstrates the patterning of actin arrays, which could serve as a basis for the development of applications.

Tracking actomyosin at fluorescence check points

Emerging concepts for on-chip biotechnologies aim to replace microfluidic flow by active, molecular-motor driven transport of cytoskeletal filaments, including applications in bio-simulation, biocomputation, diagnostics, and drug screening. Many of these applications require reliable detection, with minimal data acquisition, of filaments at many, local checkpoints in a device consisting of a potentially complex network of channels that guide filament motion. Here we develop such a detection system using actomyosin motility. Detection points consist of pairs of gold lines running perpendicular to nanochannels that guide motion of fluorescent actin filaments. Fluorescence interference contrast (FLIC) is used to locally enhance the signal at the gold lines. A cross-correlation method is used to suppress errors, allowing reliable detection of single or multiple filaments. Optimal device design parameters are discussed. The results open for automatic read-out of filament count and velocity in high-throughput motility assays, helping establish the viability of active, motor-driven on-chip applications.

Actin filament motility induced variation of resonance frequency and rigidity of polymer surfaces studied by quartz crystal microbalance

This contribution reports on the quantification of the parameters of the motility assays for actomyosin system using a quartz crystal microbalance (QCM). In particular, we report on the difference in the observed resonance frequency and dissipation of a quartz crystal when actin filaments are stationary as opposed to when they are motile. The changes in QCM measurements were studied for various polymer-coated surfaces functionalized with heavy meromyosin (HMM). The results of the QCM experiments show that the HMM-induced sliding velocity of actin filaments is modulated by a combination of the viscoelastic properties of the polymer layer including the HMM motors.

Translational actomyosin research: fundamental insights and applications hand in hand

This review describes the development towards actomyosin based nanodevices taking a starting point in pioneering studies in the 1990s based on conventional in vitro motility assays. References are given to parallel developments using the kinesin–microtubule motor system. The early developments focused on achieving cargo-transportation using actin filaments as cargo-loaded shuttles propelled by surface-adsorbed heavy meromyosin along micro- and nanofabricated channels. These efforts prompted extensive studies of surface–motor interactions contributing with new insights of general relevance in surface and colloid chemistry. As a result of these early efforts, a range of complex devices have now emerged, spanning applications in medical diagnostics, biocomputation and formation of complex nanostructures by self-organization. In addition to giving a comprehensive account of the developments towards real-world applications an important goal of the present review is to demonstrate important connections between the applied studies and fundamental biophysical studies of actomyosin and muscle function. Thus the manipulation of the motor proteins towards applications has resulted in new insights into methodological aspects of the in vitro motiliy assay. Other developments have advanced the understanding of the dynamic materials properties of actin filaments.

FLUCTUATIONS OF SINGLE CONFINED ACTIN FILAMENTS

Thermal fluctuations of individual actin filaments confined in rectangular microchannels with dimensions similar to the mesh size of the cytoskeleton in eukaryotic cells are studied experimentally using fluorescence microscopy and theoretically by a combination of analytical methods and Monte Carlo simulations. Compared to freely fluctuating filaments, long filaments confined in narrow channels exhibit enhanced tangent correlations and a characteristic shape of their correlation function. The tangent correlation function is calculated analytically by approximating the confining geometry by a parabolic potential. This approximation is validated by Monte Carlo simulations. For the quantitative analysis of experimental data additional corrections for image analysis effects have to be included, for which we provide a modified analytical approximation formula which is corroborated by simulations. This allows us to obtain both the persistence length LPdescribing the bending rigidity of the polymer and the deflection length λ characterizing confinement effects from fits to the experimental data. Our results confirm the scaling relation λ ∝ d2/3between the deflection length and the channel width d.

Towards the application of cytoskeletal motor proteins in molecular detection and diagnostic devices

Over the past ten years, great advancements have been made towards using biomolecular motors for nanotechnological applications. In particular, devices using cytoskeletal motor proteins for molecular transport are maturing. First efforts towards designing such devices used motor proteins attached to micro-structured substrates for the directed transport of microtubules and actin filaments. Soon thereafter, the specific capture, transport and detection of target analytes like viruses were demonstrated. Recently, spatial guiding of the gliding filaments was added to increase the sensitivity of detection and allow parallelization. Whereas molecular motor powered devices have not yet demonstrated performance beyond the level of existing detection techniques, the potential is great: Replacing microfluidics with transport powered by molecular motors allows integration of the energy source (ATP) into the assay solution. This opens up the opportunity to design highly integrated, miniaturized, autonomous detection devices. Such devices, in turn, may allow fast and cheap on-site diagnosis of diseases and detection of environmental pathogens and toxins.

Application of poly(amidoamine) dendrimers for use in bionanomotor systems

The study and utilization of bionanomotors represents a rapid and progressing field of nanobiotechnology. Here, we demonstrate that poly(amidoamine) (PAMAM) dendrimers are capable of supporting heavy meromyosin dependent actin motility of similar quality to that observed using nitrocellulose, and that microcontact printing of PAMAM dendrimers can be exploited to produce tracks of active myosin motors leading to the restricted motion of actin filaments across a patterned surface. These data suggest that the use of dendrimer surfaces will increase the applicability of using protein biomolecular motors for nanotechnological applications.

Myosin motors drive long range alignment of actin filaments

The bulk alignment of actin filament sliding movement, powered by randomly oriented myosin molecules, has been observed and studied using an in vitro motility assay. The well established, actin filament gliding assay is a minimal experimental system for studying actomyosin motility. Here, we show that when the assay is performed at densities of actin filaments approaching those found in living cells, filament gliding takes up a preferred orientation. The oriented patterns of movement that we have observed extend over a length scale of 10-100 microm, similar to the size of a mammalian cell. We studied the process of filament alignment and found that it depends critically upon filament length and density. We developed a simple quantitative measure of filament sliding orientation and this enabled us to follow the time course of alignment and the formation and disappearance of oriented domains. Domains of oriented filaments formed spontaneously and were separated by distinct boundaries. The pattern of the domain structures changed on the time scale of several seconds and the collision of neighboring domains led to emergence of new patterns. Our results indicate that actin filament crowding may play an important role in structuring the leading edge of migrating cells. Filament alignment due to near-neighbor mechanical interactions can propagate over a length scale of several microns; much greater than the size of individual filaments and analogous to a log drive. Self-alignment of actin filaments may make an important contribution to cell polarity and provide a mechanism by which cell migration direction responds to chemical cues.

Utilization of myosin and actin bundles for the transport of molecular cargo

The utilization of motor proteins for the movement and assembly of synthetic components is currently a goal of nanoengineering research. Application of the myosin actin motor system for nanotechnological uses has been hampered due to the low flexural rigidity of individual F-actin filaments. Here it is demonstrated how actin bundling can be used to affect the translational behavior of myosin-propelled filaments, transport molecules across a motor-patterned surface, and that the movement of bundled actin can be regulated photonically. These data suggest that actin bundling may significantly improve the applicability of the myosin motor for future nanotechnological applications.

Molecular Motors as Components of Future Medical Devices and Engineered Materials

A new frontier in the development of prosthetic devices is the design of nanoscale systems which replace, augment, or support individual cells. Similar to cells, such devices will require the ability to generate mechanical movement, either for transport or actuation. Here, the development of nanoscale transport systems, which integrate biomolecular motors, is reviewed. To date, close to 100 publications have explored the design of such “molecular shuttles” based on the integration of synthetic molecules, nano- and microparticles, and micropatterned structures with kinesin and myosin motors and their associated cytoskeletal filaments, microtubules, and actin filaments. Tremendous progress has been made in addressing the key challenges of guiding, loading, and controlling the shuttles, providing a foundation for the exploration of applications in medicine and engineering.

Diffusion dynamics of motor-driven transport: gradient production and self-organization of surfaces

The interaction between cytoskeletal filaments (e.g., actin filaments) and molecular motors (e.g., myosin) is the basis for many aspects of cell motility and organization of the cell interior. In the in vitro motility assay (IVMA), cytoskeletal filaments are observed while being propelled by molecular motors adsorbed to artificial surfaces (e.g., in studies of motor function). Here we integrate ideas that cytoskeletal filaments may be used as nanoscale templates in nanopatterning with a novel approach for the production of surface gradients of biomolecules and nanoscale topographical features. The production of such gradients is challenging but of increasing interest (e.g., in cell biology). First, we show that myosin-induced actin filament sliding in the IVMA can be approximately described as persistent random motion with a diffusion coefficient (D) given by a relationship analogous to the Einstein equation (D = kT/gamma). In this relationship, the thermal energy (kT) and the drag coefficient (gamma) are substituted by a parameter related to the free-energy transduction by actomyosin and the actomyosin dissociation rate constant, respectively. We then demonstrate how the persistent random motion of actin filaments can be exploited in conceptually novel methods for the production of actin filament density gradients of predictable shapes. Because of regularly spaced binding sites (e.g., lysines and cysteines) the actin filaments act as suitable nanoscale scaffolds for other biomolecules (tested for fibronectin) or nanoparticles. This forms the basis for secondary chemical and topographical gradients with implications for cell biological studies and biosensing.

Harnessing biological motors to engineer systems for nanoscale transport and assembly

Living systems use biological nanomotors to build life's essential molecules--such as DNA and proteins--as well as to transport cargo inside cells with both spatial and temporal precision. Each motor is highly specialized and carries out a distinct function within the cell. Some have even evolved sophisticated mechanisms to ensure quality control during nanomanufacturing processes, whether to correct errors in biosynthesis or to detect and permit the repair of damaged transport highways. In general, these nanomotors consume chemical energy in order to undergo a series of shape changes that let them interact sequentially with other molecules. Here we review some of the many tasks that biomotors perform and analyse their underlying design principles from an engineering perspective. We also discuss experiments and strategies to integrate biomotors into synthetic environments for applications such as sensing, transport and assembly.

Comparing guiding track requirements for myosin- and kinesin-powered molecular shuttles

The design of nanoscale transport systems utilizing motor proteins as engines has advanced rapidly. Here, actin/myosin- and microtubule/kinesin-based molecular shuttles are compared with respect to their requirements for track designs. To this end, the trajectory persistence length of actin filaments gliding on myosin-coated surfaces has been experimentally determined to be equal to 8.8 +/- 2 microm. This measurement complements an earlier determination of the trajectory persistence length of microtubules gliding on kinesin-coated surfaces and enables a comparison of the accessible track designs for kinesin and myosin motor-powered systems. Despite the 200-fold smaller stiffness of actin filaments compared to that of microtubules, the dimensions of myosin tracks for actin filaments have to be quite similar to the dimensions of kinesin tracks for microtubules (radii larger than 200 nm and widths smaller than 0.9 microm compared to 600 nm and 19 microm). The difference in gliding speed is shown to require additional consideration in the design of track modules.

Guidance of actin filament elongation on filament-binding tracks

Biomolecular motors, which convert chemical energy into mechanical work in intracellular processes, have high potential in bionanotechnology in vitro as molecular shuttles or nanoscale actuators. In this context, guided elongation of actin filaments in vitro could be used to lay tracks for myosin motor-based shuttles or to direct nanoscale actuators based on actin filament end-tracking motors. To guide the direction of filament polymerization on surfaces, microcontact printing was used to create tracks of chemically modified myosin, which binds to, but cannot exert force on, filaments. These filament-binding tracks captured nascent filaments from solution and guided the direction of their subsequent elongation. The effect of track width and protein surface density on filament alignment and elongation rate was quantified. These results indicate that microcontact printing is a useful method for guiding actin filament polymerization in vitro for biomolecular motor-based applications.

Surface hydrophobicity modulates the operation of actomyosin-based dynamic nanodevices

We studied the impact of surface hydrophobicity on the motility of actin filaments moving on heavy-meromyosin (HMM)-coated surfaces. Apart from nitrocellulose (NC), which is the current standard for motility assays, all materials tested are good candidates for microfabrication: hydrophilic and hydrophobic glass, poly(methyl methacrylate) (PMMA), poly(tert-butyl methacrylate) (PtBuMA), and a copolymer of O-acryloyl acetophenone oxime with a 4-acryloyloxybenzophenone (AAPO). The most hydrophilic (hydrophilic glass, contact angle 35 degrees) and the most hydrophobic (PtBuMA, contact angle 78 degrees) surfaces do not maintain the motility of actin filaments, presumably because of the low density of adsorbed HMM protein or its high levels of denaturation, respectively. The velocity of actin filaments presents higher values in the middle of this "surface hydrophobicity motility window" (NC, PMMA), and a bimodal distribution, which is more apparent at the edges of this motility window (hydrophobic glass and AAPO). A molecular surface analysis of HMM and its S1 units suggests that the two very different, temporally separated conformations of the HMM heads could exacerbate the surface-modulated protein behavior, which is common to all microdevices using surface-immobilized proteins. An explanation for the above behavior proposes that the motility of actin filaments on HMM-functionalized surfaces is the result of the action of three populations of motors, each in a different surface-protein conformation, that is, HMM with both heads working (high velocities), working with one head (low velocities), and fully denatured HMM (no motility). It is also proposed that the molecularly dynamic nature of polymer surfaces amplifies the impact of surface hydrophobicity on protein behavior. The study demonstrates that PMMA is a good candidate for the fabrication of future actomyosin-driven dynamic nanodevices because it induces the smoothest motility of individual nano-objects with velocities comparable with those obtained on NC.

Contact angle measurements by confocal microscopy for non-destructive microscale surface characterization

Contact angle measurements are of great importance in surface characterization but the practical use has often been limited to macroscopic dimensions (millimeters). Therefore, we have developed a confocal microscopy method that allows non-destructive measurements of both low (<30 degrees ) and high (30 degrees -90 degrees ) contact angles. Low contact angles were measured by reconstructing the drop profile from the interference patterns in droplets condensed from atmospheric humidity. At higher contact angles water droplets with a small amount of fluorescein were sprayed onto the surfaces and 3D-image stacks were recorded and used to extract the contact angle. Suitable drop sizes were between a few up to about 50 mum radius, using a 40x magnification objective. Using drops >10 micrometers radius for microcontact angle measurements a good correlation was obtained between measured micro- and macrocontact angles. After microcontact angle measurements the surfaces were rinsed and heavy meromyosin motor fragments were adsorbed to the surface. Importantly, the sensitive actin propelling function of these motor proteins was not affected by the previous contact angle measurements using fluorescent droplets. This suggests that the methodology should be suitable for non-destructive characterization of different parts of micropatterned surfaces being developed for biological assays.

Cell Sensing and Response to Micro- and Nanostructured Surfaces Produced by Chemical and Topographic Patterning

Chemical and topographic substrate surface patterning is recognized as a powerful tool for regulating cell functions. We discuss the relative role of scale and pattern of chemically and topographically patterned surfaces in regulating cell behavior. Chemical patterning achieved using spatial cell-adhesive molecular organization regulates different cell functions depending on its scale (micropattern for cell patterning and derived cell functions, nanopattern for collective cell functions such as adhesion, proliferation, and differentiation). In chemical patterning, a direct and specific cell-sensing mechanism such as integrin-ligand binding governs. Alternatively, topographic modification affects different cell functions depending on its pattern (anisotropic ridges and grooves for contact-guided cell alignment, isotropic textures having randomly or evenly distributed topographic features for collective functions). For all topographic patterns, micro- or nanotopographic scale determines whether specific cell reactions occur. If the topography effect were assessed under the same surface chemistry, cell adaptation processes would play a major role in cell sensing and response to topography, largely independent of mediation via differences in adsorbed proteins. Controlling scale and pattern in chemical and topographic substrate patterning would help significantly to develop purpose-specific cell-regulating cues in various biomedical applications, including tissue engineering, implants, cell-based biosensors, microarrays, and basic cell biology.

Protein Linear Molecular Motor-Powered Nanodevices

Myosin–actin and kinesin–microtubule linear protein motor systems and their application in hybrid nanodevices are reviewed. Research during the past several decades has provided a wealth of understanding about the fundamentals of protein motors that continues to be pursued. It has also laid the foundations for a new branch of investigation that considers the application of these motors as key functional elements in laboratory-on-a-chip and other micro/nanodevices. Current models of myosin and kinesin motors are introduced and the effects of motility assay parameters, including temperature, toxicity, and in particular, surface effects on motor protein operation, are discussed. These parameters set the boundaries for gliding and bead motility assays. The review describes recent developments in assay motility confinement and unidirectional control, using micro- and nano-fabricated structures, surface patterning, microfluidic flow, electromagnetic fields, and self-assembled actin filament/microtubule tracks. Current protein motor assays are primitive devices, and the developments in governing control can lead to promising applications such as sensing, nano-mechanical drivers, and biocomputation.

Actin filament guidance on a chip: toward high-throughput assays and lab-on-a-chip applications

Biological molecular motors that are constrained so that function is effectively limited to predefined nanosized tracks may be used as molecular shuttles in nanotechnological applications. For these applications and in high-throughput functional assays (e.g., drug screening), it is important that the motors propel their cytoskeletal filaments unidirectionally along the tracks with a minimal number of escape events. We here analyze the requirements for achieving this for actin filaments that are propelled by myosin II motor fragments (heavy meromyosin; HMM). First, we tested the guidance of HMM-propelled actin filaments along chemically defined borders. Here, trimethylchlorosilane (TMCS)-derivatized areas with high-quality HMM function were surrounded by SiO(2) domains where HMM did not bind actin. Guidance along the TMCS-SiO(2) border was almost 100% for filament approach angles between 0 and 20 degrees but only about 10% at approach angles near 90 degrees . A model (Clemmens, J.; Hess, H.; Lipscomb, R.; Hanein, Y.; Bohringer, K. F.; Matzke, C. M.; Bachand, G. D.; Bunker, B. C.; Vogel, V. Langmuir 2003, 19, 10967-10974) accounted for essential aspects of the data and also correctly predicted a more efficient guidance of actin filaments than previously shown for kinesin-propelled microtubules. Despite the efficient guidance at low approach angles, nanosized (<700 nm wide) TMCS tracks surrounded by SiO(2) were not effective in guiding actin filaments. Neither was there complete guidance along nanosized tracks that were surrounded by topographical barriers (walls and roof partially covering the track) unless there was also chemically based selectivity between the tracks and surroundings. In the latter case, with dually defined tracks, there was close to 100% guidance. A combined experimental and theoretical analysis, using tracks of the latter type, suggested that a track width of less than about 200-300 nm is sufficient at a high HMM surface density to achieve unidirectional sliding of actin filaments. In accord with these results, we demonstrate the long-term trapping of actin filaments on a closed-loop track (width < 250 nm). The results are discussed in relation to lab-on-a-chip applications and nanotechnology-assisted assays of actomyosin function.

Selective spatial localization of actomyosin motor function by chemical surface patterning

We have previously described the efficient guidance and unidirectional sliding of actin filaments along nanosized tracks with adsorbed heavy meromyosin (HMM; myosin II motor fragment). In those experiments, the tracks were functionalized with trimethylchlorosilane (TMCS) by chemical vapor deposition (CVD) and surrounded by hydrophilic areas. Here we first show, using in vitro motility assays on nonpatterned and micropatterned surfaces, that the quality of HMM function on CVD-TMCS is equivalent to that on standard nitrocellulose substrates. We further examine the influences of physical properties of different surfaces (glass, SiO(2), and TMCS) and chemical properties of the buffer solution on motility. With the presence of methylcellulose in the assay solution, there was HMM-induced actin filament sliding on both glass/SiO(2) and on TMCS, but the velocity was higher on TMCS. This difference in velocity increased with decreasing contact angles of the glass and SiO(2) surfaces in the range of 20-67 degrees (advancing contact angles for water droplets). The corresponding contact angle of CVD-TMCS was 81 degrees. In the absence of methylcellulose, there was high-quality motility on TMCS but no motility on glass/SiO(2). This observation was independent of the contact angle of the glass/SiO(2) surfaces and of HMM incubation concentrations (30-150 microg mL(-)(1)) and ionic strengths of the assay solution (20-50 mM). Complete motility selectivity between TMCS and SiO(2) was observed for both nonpatterned and for micro- and nanopatterned surfaces. Spectrophotometric analysis of HMM depletion during incubation, K/EDTA ATPase measurements, and total internal reflection fluorescence spectroscopy of HMM binding showed only minor differences in HMM surface densities between TMCS and SiO(2)/glass. Thus, the motility contrast between the two surface chemistries seems to be attributable to different modes of HMM binding with the hindrance of actin binding on SiO(2)/glass.

Simulating molecular shuttle movements: towards computer-aided design of nanoscale transport systems

Molecular shuttles based on the motor protein kinesin and microtubule filaments have the potential to extend the lab-on-a-chip paradigm to nanofluidics by enabling the active, directed and selective transport of molecules and nanoparticles. Based on experimentally determined parameters, in particular the trajectory persistence length of a microtubule gliding on surface-adhered kinesin motors, we developed a Monte-Carlo simulation, which models the transport properties of guiding structures, such as channels, rectifiers and concentrators, and reproduces the properties of several experimentally realized systems. Our tool facilitates the rational design of individual guiding structures as well as whole networks, and can be adapted to the simulation of other nanoscale transport systems.

Highly selective directed assembly of functional actomyosin on Au surfaces

The capability of assembling biomotors onto specific locations of solid substrates is a key for development of biomotor-based nanomechanical systems. We developed a method to direct the assembly of the heavy meromyosin fragment from rabbit skeletal muscle myosin onto specific locations of Au substrates utilizing surface molecular patterns. In this strategy, chemically directed patterns of streptavidin were achieved to direct highly specific assembly of biotinylated heavy meromyosin on the substrates--a strategy applicable for patterning a variety of biotinylated molecules--while BSA was utilized to avoid nonspecific adsorption. In vitro motility assays of filament sliding were used to confirm functionality of assembled actomyosin.

Silanized surfaces for in vitro studies of actomyosin function and nanotechnology applications

We have previously shown that selective heavy meromyosin (HMM) adsorption to predefined regions of nanostructured polymer resist surfaces may be used to produce a nanostructured in vitro motility assay. However, actomyosin function was of lower quality than on conventional nitrocellulose films. We have therefore studied actomyosin function on differently derivatized glass surfaces with the aim to find a substitute for the polymer resists. We have found that surfaces derivatized with trimethylchlorosilane (TMCS) were superior to all other surfaces tested, including nitrocellulose. High-quality actin filament motility was observed up to 6 days after incubation with HMM and the fraction of motile actin filaments and the velocity of smooth sliding were generally higher on TMCS than on nitrocellulose. The actomyosin function on TMCS-derivatized glass and nitrocellulose is considered in relation to roughness and hydrophobicity of these surfaces. The results suggest that TMCS is an ideal substitute for polymer resists in the nanostructured in vitro motility assay. Furthermore, TMCS derivatized glass also seems to offer several advantages over nitrocellulose for HMM adsorption in the ordinary in vitro motility assay.

Motor-protein "roundabouts": microtubules moving on kinesin-coated tracks through engineered networks

Nanotechnology promises to enhance the functionality and sensitivity of miniaturized analytical systems. For example, nanoscale transport systems, which are driven by molecular motors, permit the controlled movement of select cargo along predetermined paths. Such shuttle systems may enhance the detection efficiency of an analytical system or facilitate the controlled assembly of sophisticated nanostructures if transport can be coordinated through complex track networks. This study determines the feasibility of complex track networks using kinesin motor proteins to actively transport microtubule shuttles along micropatterned surfaces. In particular, we describe the performance of three basic structural motifs: (1) crossing junctions, (2) directional sorters, and (3) concentrators. We also designed track networks that successfully sort and collect microtubule shuttles, pointing the way towards lab-on-a-chip devices powered by active transport instead of pressure-driven or electroosmotic flow.

In vitro sliding of actin filaments labelled with single quantum dots

We recently refined the in vitro motility assay for studies of actomyosin function to achieve rectified myosin induced sliding of actin filaments. This paves the way, both for detailed functional studies of actomyosin and for nanotechnological applications. In the latter applications it would be desirable to use actin filaments for transportation of cargoes (e.g., enzymes) between different predetermined locations on a chip. We here describe how single quantum dot labelling of isolated actin filaments simultaneously provides handles for cargo attachment and bright and photostable fluorescence labels facilitating cargo detection and filament tracking. Labelling was achieved with preserved actomyosin function using streptavidin-coated CdSe quantum dots (Qdots). These nanocrystals have several unique physical properties and the present work describes their first use for functional studies of isolated proteins outside the cell. The results, in addition to the nanotechnology developments, open for new types of in vitro assays of isolated biomolecules.