Long-term storage of surface-adsorbed protein machines

The potential of myosin and actin in nanobiotechnology

Since the late 1990s, efforts have been made to utilize cytoskeletal filaments, propelled by molecular motors, for nanobiotechnological applications, for example, in biosensing and parallel computation. This work has led to in-depth insights into the advantages and challenges of such motor-based systems, and has yielded small-scale, proof-of-principle applications but, to date, no commercially viable devices. Additionally, these studies have also elucidated fundamental motor and filament properties, as well as providing other insights obtained from biophysical assays in which molecular motors and other proteins are immobilized on artificial surfaces. In this Perspective, I discuss the progress towards practically viable applications achieved so far using the myosin II-actin motor-filament system. I also highlight several fundamental pieces of insights derived from the studies. Finally, I consider what may be required to achieve real devices in the future or at least to allow future studies with a satisfactory cost-benefit ratio.

An automated in vitro motility assay for high-throughput studies of molecular motors

Molecular motors, essential to force-generation and cargo transport within cells, are invaluable tools for powering nanobiotechnological lab-on-a-chip devices. These devices are based on in vitro motility assays that reconstitute molecular transport with purified motor proteins, requiring a deep understanding of the biophysical properties of motor proteins and thorough optimization to enable motility under varying environmental conditions. Until now, these assays have been prepared manually, severely limiting throughput. To overcome this limitation, we developed an in vitro motility assay where sample preparation, imaging and data evaluation are fully automated, enabling the processing of a 384-well plate within less than three hours. We demonstrate the automated assay for the analysis of peptide inhibitors for kinesin-1 at a wide range of concentrations, revealing that the IAK domain responsible for kinesin-1 auto-inhibition is both necessary and sufficient to decrease the affinity of the motor protein for microtubules, an aspect that was hidden in previous experiments due to scarcity of data.

Preparation of liposomes containing small gold nanoparticles using electrostatic interactions

The development of liposome-nanoparticle colloid systems offers a versatile approach towards the manufacture of multifunctional therapeutic platforms. A strategy to encapsulate small metallic nanoparticles (<4nm) within multilamellar vesicles, effected by exploiting electrostatic interactions was investigated. Two liposome-gold nanoparticle (lipo-GNP) systems were prepared by the reverse-phase evaporation method employing cationic or anionic surface functionalised particles in combination with oppositely charged lipid compositions with subsequent post-formulation PEGylation. Structural characterisation using electron microscopy and elemental analysis revealed a regular distribution of GNPs between adjacent lipid bilayers of intact liposomes. Nanoparticle encapsulation efficacy of the two lipo-GNP systems was revealed to be significantly different (p=0.03), evaluated by comparing the ratio of measured lipid to gold concentration (loading content) determined by a colorimetric assay and atomic emission spectroscopy, respectively. It was concluded that the developed synthetic strategy is an effective approach for the preparation of liposome-nanoparticle colloids with potential to control the relative concentration of encapsulated particles to lipids by providing favourable electrostatic interactions.

Magnetic capture from blood rescues molecular motor function in diagnostic nanodevices

BACKGROUND

Introduction of effective point-of-care devices for use in medical diagnostics is part of strategies to combat accelerating health-care costs. Molecular motor driven nanodevices have unique potentials in this regard due to unprecedented level of miniaturization and independence of external pumps. However motor function has been found to be inhibited by body fluids.

RESULTS

We report here that a unique procedure, combining separation steps that rely on antibody-antigen interactions, magnetic forces applied to magnetic nanoparticles (MPs) and the specificity of the actomyosin bond, can circumvent the deleterious effects of body fluids (e.g. blood serum). The procedure encompasses the following steps: (i) capture of analyte molecules from serum by MP-antibody conjugates, (ii) pelleting of MP-antibody-analyte complexes, using a magnetic field, followed by exchange of serum for optimized biological buffer, (iii) mixing of MP-antibody-analyte complexes with actin filaments conjugated with same polyclonal antibodies as the magnetic nanoparticles. This causes complex formation: MP-antibody-analyte-antibody-actin, and magnetic separation is used to enrich the complexes. Finally (iv) the complexes are introduced into a nanodevice for specific binding via actin filaments to surface adsorbed molecular motors (heavy meromyosin). The number of actin filaments bound to the motors in the latter step was significantly increased above the control value if protein analyte (50-60 nM) was present in serum (in step i) suggesting appreciable formation and enrichment of the MP-antibody-analyte-antibody-actin complexes. Furthermore, addition of ATP demonstrated maintained heavy meromyosin driven propulsion of actin filaments showing that the serum induced inhibition was alleviated. Detailed analysis of the procedure i-iv, using fluorescence microscopy and spectroscopy identified main targets for future optimization.

CONCLUSION

The results demonstrate a promising approach for capturing analytes from serum for subsequent motor driven separation/detection. Indeed, the observed increase in actin filament number, in itself, signals the presence of analyte at clinically relevant nM concentration without the need for further motor driven concentration. Our analysis suggests that exchange of polyclonal for monoclonal antibodies would be a critical improvement, opening for a first clinically useful molecular motor driven lab-on-a-chip device.

Ultrafast molecular motor driven nanoseparation and biosensing

Portable biosensor systems would benefit from reduced dependency on external power supplies as well as from further miniaturization and increased detection rate. Systems built around self-propelled biological molecular motors and cytoskeletal filaments hold significant promise in these regards as they are built from nanoscale components that enable nanoseparation independent of fluidic pumping. Previously reported microtubule-kinesin based devices are slow, however, compared to several existing biosensor systems. Here we demonstrate that this speed limitation can be overcome by using the faster actomyosin motor system. Moreover, due to lower flexural rigidity of the actin filaments, smaller features can be achieved compared to microtubule-based systems, enabling further miniaturization. Using a device designed through optimization by Monte Carlo simulations, we demonstrate extensive myosin driven enrichment of actin filaments on a detector area of less than 10 μm², with a concentration half-time of approximately 40 s. We also show accumulation of model analyte (streptavidin at nanomolar concentration in nanoliter effective volume) detecting increased fluorescence intensity within seconds after initiation of motor-driven transportation from capture regions. We discuss further optimizations of the system and incorporation into a complete biosensing workflow.

Sample solution constraints on motor-driven diagnostic nanodevices

The last decade has seen appreciable advancements in efforts towards increased portability of lab-on-a-chip devices by substituting microfluidics with molecular motor-based transportation. As of now, first proof-of-principle devices have analyzed protein mixtures of low complexity, such as target protein molecules in buffer solutions optimized for molecular motor performance. However, in a diagnostic work-up, lab-on-a-chip devices need to be compatible with complex biological samples. While it has been shown that such samples do not interfere with crucial steps in molecular diagnostics (for example antibody-antigen recognition), their effect on molecular motors is unknown. This critical and long overlooked issue is addressed here. In particular, we studied the effects of blood, cell lysates and solutions containing genomic DNA extracts on actomyosin and kinesin-microtubule-based transport, the two biomolecular motor systems that are most promising for lab-on-a-chip applications. We found that motor function is well preserved at defined dilutions of most of the investigated biological samples and demonstrated a molecular motor-driven label-free blood type test. Our results support the feasibility of molecular-motor driven nanodevices for diagnostic point-of-care applications and also demonstrate important constraints imposed by sample composition and device design that apply both to kinesin-microtubule and actomyosin driven applications.

Transportation of nanoscale cargoes by myosin propelled actin filaments

Myosin II propelled actin filaments move ten times faster than kinesin driven microtubules and are thus attractive candidates as cargo-transporting shuttles in motor driven lab-on-a-chip devices. In addition, actomyosin-based transportation of nanoparticles is useful in various fundamental studies. However, it is poorly understood how actomyosin function is affected by different number of nanoscale cargoes, by cargo size, and by the mode of cargo-attachment to the actin filament. This is studied here using biotin/fluorophores, streptavidin, streptavidin-coated quantum dots, and liposomes as model cargoes attached to monomers along the actin filaments (“side-attached”) or to the trailing filament end via the plus end capping protein CapZ. Long-distance transportation (>100 µm) could be seen for all cargoes independently of attachment mode but the fraction of motile filaments decreased with increasing number of side-attached cargoes, a reduction that occurred within a range of 10–50 streptavidin molecules, 1–10 quantum dots or with just 1 liposome. However, as observed by monitoring these motile filaments with the attached cargo, the velocity was little affected. This also applied for end-attached cargoes where the attachment was mediated by CapZ. The results with side-attached cargoes argue against certain models for chemomechanical energy transduction in actomyosin and give important insights of relevance for effective exploitation of actomyosin-based cargo-transportation in molecular diagnostics and other nanotechnological applications. The attachment of quantum dots via CapZ, without appreciable modulation of actomyosin function, is useful in fundamental studies as exemplified here by tracking with nanometer accuracy.

Antibodies covalently immobilized on actin filaments for fast myosin driven analyte transport

Biosensors would benefit from further miniaturization, increased detection rate and independence from external pumps and other bulky equipment. Whereas transportation systems built around molecular motors and cytoskeletal filaments hold significant promise in the latter regard, recent proof-of-principle devices based on the microtubule-kinesin motor system have not matched the speed of existing methods. An attractive solution to overcome this limitation would be the use of myosin driven propulsion of actin filaments which offers motility one order of magnitude faster than the kinesin-microtubule system. Here, we realized a necessary requirement for the use of the actomyosin system in biosensing devices, namely covalent attachment of antibodies to actin filaments using heterobifunctional cross-linkers. We also demonstrated consistent and rapid myosin II driven transport where velocity and the fraction of motile actin filaments was negligibly affected by the presence of antibody-antigen complexes at rather high density (>20 µm(-1)). The results, however, also demonstrated that it was challenging to consistently achieve high density of functional antibodies along the actin filament, and optimization of the covalent coupling procedure to increase labeling density should be a major focus for future work. Despite the remaining challenges, the reported advances are important steps towards considerably faster nanoseparation than shown for previous molecular motor based devices, and enhanced miniaturization because of high bending flexibility of actin filaments.

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.