Tracking actomyosin at fluorescence check points

Polymer-Carrying Ability of Actin Filaments Interacting with Myosin Motors in a Biological Motility System In Vitro

The reconstituted motility system of actin-myosin is expected to be used in bioinspired transport devices, in which carried materials are attached to either moving actin filaments or walking myosin molecules. However, the dependence of the ability to transport on the size of the attached materials is still inadequately understood. Here, as carried materials, polyethylene glycols (PEGs) of various sizes are covalently bound to actin filaments, and the motility of PEG-attached filaments on a heavy meromyosin immobilized on a glass surface is observed via fluorescence microscopy. Full attachment of 2 kDa PEG, with an approximately 2 nm gyration radius, decreases the velocity and fraction of moving actin filaments by approximately 10% relative to unattached filaments. For the 5 kDa PEG, the fraction of moving filaments is decreased by approximately 70% even when the filaments contain only 20% PEG-attached actin. The attachment of both sizes of PEGs suppresses the actin-activated ATPase activity at the same level. These results suggest that actin filaments can carry PEGs up to 2 kDa having the same size as actin monomers, while the rate of ATP hydrolysis is limited. The size-dependence may provide a criterion for material delivery via actin filaments in nanotransport applications. This article is protected by copyright. All rights reserved.

Something has to give: scaling combinatorial computing by biological agents exploring physical networks encoding NP-complete problems

On-chip network-based computation, using biological agents, is a new hardware-embedded approach which attempts to find solutions to combinatorial problems, in principle, in a shorter time than the fast, but sequential electronic computers. This analytical review starts by describing the underlying mathematical principles, presents several types of combinatorial (including NP-complete) problems and shows current implementations of proof of principle developments. Taking the subset sum problem as example for in-depth analysis, the review presents various options of computing agents, and compares several possible operation 'run modes' of network-based computer systems. Given the brute force approach of network-based systems for solving a problem of input size C, 2C solutions must be visited. As this exponentially increasing workload needs to be distributed in space, time, and per computing agent, this review identifies the scaling-related key technological challenges in terms of chip fabrication, readout reliability and energy efficiency. The estimated computing time of massively parallel or combinatorially operating biological agents is then compared to that of electronic computers. Among future developments which could considerably improve network-based computing, labelling agents 'on the fly' and the readout of their travel history at network exits could offer promising avenues for finding hardware-embedded solutions to combinatorial problems.

Parallel computation with molecular-motor-propelled agents in nanofabricated networks

The combinatorial nature of many important mathematical problems, including nondeterministic-polynomial-time (NP)-complete problems, places a severe limitation on the problem size that can be solved with conventional, sequentially operating electronic computers. There have been significant efforts in conceiving parallel-computation approaches in the past, for example: DNA computation, quantum computation, and microfluidics-based computation. However, these approaches have not proven, so far, to be scalable and practical from a fabrication and operational perspective. Here, we report the foundations of an alternative parallel-computation system in which a given combinatorial problem is encoded into a graphical, modular network that is embedded in a nanofabricated planar device. Exploring the network in a parallel fashion using a large number of independent, molecular-motor-propelled agents then solves the mathematical problem. This approach uses orders of magnitude less energy than conventional computers, thus addressing issues related to power consumption and heat dissipation. We provide a proof-of-concept demonstration of such a device by solving, in a parallel fashion, the small instance {2, 5, 9} of the subset sum problem, which is a benchmark NP-complete problem. Finally, we discuss the technical advances necessary to make our system scalable with presently available technology.

Sensing protein antigen and microvesicle analytes using high-capacity biopolymer nano-carriers

Lab-on-a-chip systems with molecular motor driven transport of analytes attached to cytoskeletal filament shuttles (actin filaments, microtubules) circumvent challenges with nanoscale liquid transport. However, the filaments have limited cargo-carrying capacity and limitations either in transportation speed (microtubules) or control over motility direction (actin). To overcome these constraints we here report incorporation of covalently attached antibodies into self-propelled actin bundles (nanocarriers) formed by cross-linking antibody conjugated actin filaments via fascin, a natural actin-bundling protein. We demonstrate high maximum antigen binding activity and propulsion by surface adsorbed myosin motors. Analyte transport capacity is tested using both protein antigens and microvesicles, a novel class of diagnostic markers. Increased incubation concentration with protein antigen in the 0.1-100 nM range (1 min) reduces the fraction of motile bundles and their velocity but maximum transportation capacity of >1 antigen per nm of bundle length is feasible. At sub-nanomolar protein analyte concentration, motility is very well preserved opening for orders of magnitude improved limit of detection using motor driven concentration on nanoscale sensors. Microvesicle-complexing to monoclonal antibodies on the nanocarriers compromises motility but nanocarrier aggregation via microvesicles shows unique potential in label-free detection with the aggregates themselves as non-toxic reporter elements.

Nanowire-imposed geometrical control in studies of actomyosin motor function

Recently, molecular motor gliding assays with actin and myosin from muscle have been realized on semiconductor nanowires coated with Al2O3. This opens for unique nanotechnological applications and novel fundamental studies of actomyosin motor function. Here, we provide a comparison of myosin-driven actin filament motility on Al2O3 to both nitrocellulose and trimethylchlorosilane derivatized surfaces. We also show that actomyosin motility on the less than 200 nm wide tips of arrays of Al2O3-coated nanowires can be used to control the number, and density, of myosin-actin attachment points. Results obtained using nanowire arrays with different inter-wire spacing are consistent with the idea that the actin filament sliding velocity is determined both by the total number and the average density of attached myosin heads along the actin filament. Further, the results are consistent with buckling of long myosin-free segments of the filaments as a factor underlying reduced velocity. On the other hand, the findings do not support a mechanistic role in decreasing velocity, of increased nearest neighbor distance between available myosin heads. Our results open up for more advanced studies that may use nanowire-based structures for fundamental investigations of molecular motors, including the possibility to create a nanowire-templated bottom-up assembly of 3D, muscle-like structures.

Molecular motor transport through hollow nanowires

Biomolecular motors offer self-propelled, directed transport in designed microscale networks and can potentially replace pump-driven nanofluidics. However, in existing systems, transportation is limited to the two-dimensional plane. Here we demonstrate fully one-dimensional (1D) myosin-driven motion of fluorescent probes (actin filaments) through 80 nm wide, Al2O3 hollow nanowires of micrometer length. The motor-driven transport is orders of magnitude faster than would be possible by passive diffusion. The system represents a necessary element for advanced devices based on gliding assays, for example, in lab-on-a-chip systems with channel crossings and in pumpless nanosyringes. It may also serve as a scaffold for bottom-up assembly of muscle proteins into ordered contractile units, mimicking the muscle sarcomere.

Molecular motor propelled filaments reveal light-guiding in nanowire arrays for enhanced biosensing

Semiconductor nanowire arrays offer significant potential for biosensing applications with optical read-out due to their high surface area and due to the unique optical properties of one-dimensional materials. A challenge for optical read-out of analyte-binding to the nanowires is the need to efficiently collect and detect light from a three-dimensional volume. Here we show that light from fluorophores attached along several μm long vertical Al2O3 coated gallium phosphide nanowires couples into the wires, is guided along them and emitted at the tip. This enables effective collection of light emitted by fluorescent analytes located at different focal planes along the nanowire. We unequivocally demonstrate the light-guiding effect using a novel method whereby the changes in emitted fluorescence intensity are observed when fluorescent cytoskeletal filaments are propelled by molecular motors along the wires. The findings are discussed in relation to nanobiosensor developments, other nanotechnological applications, and fundamental studies of motor function.

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.