Active alignment of microtubules with electric fields

No observable non-thermal effect of microwave radiation on the growth of microtubules

Despite widespread public interest in the health impact of exposure to microwave radiation, studies of the influence of microwave radiation on biological samples are often inconclusive or contradictory. Here we examine the influence of microwave radiation of frequencies 3.5 GHz, 20 GHz and 29 GHz on the growth of microtubules, which are biological nanotubes that perform diverse functions in eukaryotic cells. Since microtubules are highly polar and can extend several micrometres in length, they are predicted to be sensitive to non-ionizing radiation. Moreover, it has been speculated that tubulin dimers within microtubules might rapidly toggle between different conformations, potentially participating in computational or other cooperative processes. Our data show that exposure to microwave radiation yields a microtubule growth curve that is distorted relative to control studies utilizing a homogeneous temperature jump. However, this apparent effect of non-ionizing radiation is reproduced by control experiments using an infrared laser or hot air to heat the sample and thereby mimic the thermal history of samples exposed to microwaves. As such, no non-thermal effects of microwave radiation on microtubule growth can be assigned. Our results highlight the need for appropriate control experiments in biophysical studies that may impact on the sphere of public interest.

Building on-chip cytoskeletal circuits via branched microtubule networks

Controllable platforms to engineer robust cytoskeletal scaffolds have the potential to create novel on-chip nanotechnologies. Inspired by axons, we combined the branching microtubule (MT) nucleation pathway with microfabrication to develop "cytoskeletal circuits." This active matter platform allows control over the adaptive self-organization of uniformly polarized MT arrays via geometric features of microstructures designed within a microfluidic confinement. We build and characterize basic elements, including turns and divisions, as well as complex regulatory elements, such as biased division and MT diodes, to construct various MT architectures on a chip. Our platform could be used in diverse applications, ranging from efficient on-chip molecular transport to mechanical nano-actuators. Further, cytoskeletal circuits can serve as a tool to study how the physical environment contributes to MT architecture in living cells.

Functional material-mediated wireless physical stimulation for neuro-modulation and regeneration

Nerve injuries and neurological diseases remain intractable clinical challenges. Despite the advantages of stem cell therapy in treating neurological disorders, uncontrollable cell fates and loss of cell function in vivo are still challenging. Recently, increasing attention has been given to the roles of external physical signals, such as electricity and ultrasound, in regulating stem cell fate as well as activating or inhibiting neuronal activity, which provides new insights for the treatment of neurological disorders. However, direct physical stimulations in vivo are short in accuracy and safety. Functional materials that can absorb energy from a specific physical field exerted in a wireless way and then release another localized physical signal hold great advantages in mediating noninvasive or minimally invasive accurate indirect physical stimulations to promote the therapeutic effect on neurological disorders. In this review, the mechanism by which various physical signals regulate stem cell fate and neuronal activity is summarized. Based on these concepts, the approaches of using functional materials to mediate indirect wireless physical stimulation for neuro-modulation and regeneration are systematically reviewed. We expect that this review will contribute to developing wireless platforms for neural stimulation as an assistance for the treatment of neurological diseases and injuries.

Flow patterns and defect dynamics of active nematic liquid crystals under an electric field

The effects of an electric field on the flow patterns and defect dynamics of two-dimensional active nematic liquid crystals are numerically investigated. We found that field-induced director reorientation causes anisotropic active turbulence characterized by enhanced flow perpendicular to the electric field. The average flow speed and its anisotropy are maximized at an intermediate field strength. Topological defects in the anisotropic active turbulence are localized and show characteristic dynamics with simultaneous creation of two pairs of defects. A laning state characterized by stripe domains with alternating flow directions is found at a larger field strength near the transition to the uniformly aligned state. We obtained periodic oscillations between the laning state and active turbulence, which resembles an experimental observation of active nematics subject to anisotropic friction.

Cytoelectric Coupling: Electric fields sculpt neural activity and "tune" the brain's infrastructure

We propose and present converging evidence for the Cytoelectric Coupling Hypothesis: Electric fields generated by neurons are causal down to the level of the cytoskeleton. This could be achieved via electrodiffusion and mechanotransduction and exchanges between electrical, potential and chemical energy. Ephaptic coupling organizes neural activity, forming neural ensembles at the macroscale level. This information propagates to the neuron level, affecting spiking, and down to molecular level to stabilize the cytoskeleton, "tuning" it to process information more efficiently.

Construction of Molecular Robots from Microtubules for Programmable Swarming

Swarm robotics has been attracting much attention in recent years in the field of robotics. This chapter describes a methodology for the construction of molecular swarm robots through precise control of active self-assembly of microtubules (MTs). Detailed protocols are presented for the construction of molecular robots through conjugation of DNA to MTs and demonstration of swarming of the MTs. The swarming is mediated by DNA-based interaction and photoirradiation which act as processors and sensors respectively for the robots. Furthermore, the required protocols to utilize the swarming of MTs for molecular computation is also described.

Design of Mechanical and Electrical Properties for Multidirectional Control of Microtubules

Microtubule (MT)-motor systems show promise as nanoscale actuator platforms for performing molecular manipulations in nanobiotechnology and micro total analysis systems. These systems have been demonstrated to exert a variety of functions, including the concentration, transportation, and detection of molecular cargos. Although gliding direction control of MTs is necessary for these applications, most direction control methods are currently conducted using micro/nanofabricated guiding structures and/or flow, magnetic, and electric field forces. These control methods force all MTs to exhibit identical gliding behaviors and destinations. In this chapter, we describe an active multidirectional control method for MT without guiding tracks. The bottom-up molecular design allowed MTs to be guided in designated directions under an electric field in a microfluidic device. By designing the stiffness and surface charge density of MTs, three types of MT (Stiff-MT, Soft-MT, and Charged soft-MT) with different mechanical and electrical properties are prepared. The gliding directions within an electric field are predicted according to the measured stiffness and electrophoretic mobility. Finally, the Stiff-MTs are separated from Soft-MTs and Charged soft-MTs with a microfluidic sorter.

Revealing and Attenuating the Electrostatic Properties of Tubulin and Its Polymers

Tubulin is an electrostatically negative protein that forms cylindrical polymers termed microtubules, which are crucial for a variety of intracellular roles. Exploiting the electrostatic behavior of tubulin and microtubules within functional microfluidic and optoelectronic devices is limited due to the lack of understanding of tubulin behavior as a function of solvent composition. This work displays the tunability of tubulin surface charge using dimethyl sulfoxide (DMSO) for the first time. Increasing the DMSO volume fractions leads to the lowering of tubulin's negative surface charge, eventually causing it to become positive in solutions >80% DMSO. As determined by electrophoretic mobility measurements, this change in surface charge is directionally reversible, i.e., permitting control between -1.5 and + 0.2 cm²  (V s)^-1 . When usually negative microtubules are exposed to these conditions, the positively charged tubulin forms tubulin sheets and aggregates, as revealed by an electrophoretic transport assay. Fluorescence-based experiments also indicate that tubulin sheets and aggregates colocalize with negatively charged g-C₃ N₄ sheets while microtubules do not, further verifying the presence of a positive surface charge. This study illustrates that tubulin and its polymers, in addition to being mechanically robust, are also electrically tunable.

All Wired Up: An Exploration of the Electrical Properties of Microtubules and Tubulin

Microtubules are hollow, cylindrical polymers of the protein α, β tubulin, that interact mechanochemically with a variety of macromolecules. Due to their mechanically robust nature, microtubules have gained attention as tracks for precisely directed transport of nanomaterials within lab-on-a-chip devices. Primarily due to the unusually negative tail-like C-termini of tubulin, recent work demonstrates that these biopolymers are also involved in a broad spectrum of intracellular electrical signaling. Microtubules and their electrostatic properties are discussed in this Review, followed by an evaluation of how these biopolymers respond mechanically to electrical stimuli, through microtubule migration, electrorotation and C-termini conformation changes. Literature focusing on how microtubules act as nanowires capable of intracellular ionic transport, charge storage, and ionic signal amplification is reviewed, illustrating how these biopolymers attenuate ionic movement in response to electrical stimuli. The Review ends with a discussion on the important questions, challenges, and future opportunities for intracellular microtubule-based electrical signaling.

Voltage-gated ion channels mediate the electrotaxis of glioblastoma cells in a hybrid PMMA/PDMS microdevice

Transformed astrocytes in the most aggressive form cause glioblastoma, the most common cancer in the central nervous system with high mortality. The physiological electric field by neuronal local field potentials and tissue polarity may guide the infiltration of glioblastoma cells through the electrotaxis process. However, microenvironments with multiplex gradients are difficult to create. In this work, we have developed a hybrid microfluidic platform to study glioblastoma electrotaxis in controlled microenvironments with high throughput quantitative analysis by machine learning-powered single cell tracking software. By equalizing the hydrostatic pressure difference between inlets and outlets of the microchannel, uniform single cells can be seeded reliably inside the microdevice. The electrotaxis of two glioblastoma models, T98G and U-251MG, requires an optimal laminin-containing extracellular matrix and exhibits opposite directional and electro-alignment tendencies. Calcium signaling is a key contributor in glioblastoma pathophysiology but its role in glioblastoma electrotaxis is still an open question. Anodal T98G electrotaxis and cathodal U-251MG electrotaxis require the presence of extracellular calcium cations. U-251MG electrotaxis is dependent on the P/Q-type voltage-gated calcium channel (VGCC) and T98G is dependent on the R-type VGCC. U-251MG electrotaxis and T98G electrotaxis are also mediated by A-type (rapidly inactivating) voltage-gated potassium channels and acid-sensing sodium channels. The involvement of multiple ion channels suggests that the glioblastoma electrotaxis is complex and patient-specific ion channel expression can be critical to develop personalized therapeutics to fight against cancer metastasis. The hybrid microfluidic design and machine learning-powered single cell analysis provide a simple and flexible platform for quantitative investigation of complicated biological systems.

A Confirmation for the Positive Electric Charge of Bio-Molecular Motors through Utilizing a Novel Nano-Technology Approach In Vitro

Molecular motors are microtubule-based proteins which contribute to many cell functions, such as intracellular transportation and cell division. The details of the nature of the mutual interactions between motors and microtubules still needs to be extensively explored. However, electrostatic interaction is known as one of the key factors making motor-microtubule association possible. The association rate of molecular motors to microtubules is a way to observe and evaluate the charge of the bio-motors in vivo. Growing evidence indicates that microtubules with distinct structural compositions in terms of beta tubulin isotypes carry different charges. Therefore, the electrostatic-driven association rate of motors–microtubules, which is a base for identifying the charge of motors, can be more likely influenced. Here, we present a novel method to experimentally confirm the charge of molecular motors in vitro. The offered nanotechnology-based approach can validate the charge of motors in the absence of any cellular components through the observation and analysis of the changes that biomolecular motors can cause on the dynamic of charged microspheres inside a uniform electric field produced by a microscope slide-based nanocapacitor. This new in vitro experimental method is significant as it minimizes the intracellular factors that may interfere the electric charge that molecular motors carry.

Cytoskeletal Disruption after Electroporation and Its Significance to Pulsed Electric Field Therapies

Pulsed electric fields (PEFs) have become clinically important through the success of Irreversible Electroporation (IRE), Electrochemotherapy (ECT), and nanosecond PEFs (nsPEFs) for the treatment of tumors. PEFs increase the permeability of cell membranes, a phenomenon known as electroporation. In addition to well-known membrane effects, PEFs can cause profound cytoskeletal disruption. In this review, we summarize the current understanding of cytoskeletal disruption after PEFs. Compiling available studies, we describe PEF-induced cytoskeletal disruption and possible mechanisms of disruption. Additionally, we consider how cytoskeletal alterations contribute to cell-cell and cell-substrate disruption. We conclude with a discussion of cytoskeletal disruption-induced anti-vascular effects of PEFs and consider how a better understanding of cytoskeletal disruption after PEFs may lead to more effective therapies.

Pause of the target gliding microtuble on the virtual cathode

We report the transient response of gliding microtubules on a virtual cathode. In vivo activities, microtubule-kinesin systems are known to act as motor proteins with respect to cell motility cytokinesis and cellular transport by hydrolyzing ATP molecules. With development of in vitro assays, motor proteins have been attracting much attention as a key component for highly efficient nano-transportation systems. The molecular functions based on structural states are affected by changing the ionic condition of the molecular functions and by changing the electrical field in solution because of electrical charges of the molecules. The virtual cathode, which was generated on the SiN display surface by a low energy electron beam, locally induced electrochemical reactions and electric field around the targeted molecules on the display surface, and then the gliding motions of the targeted microtubules were regulated. In this study, we demonstrated that the virtual cathode display temporally stops a selected gliding microtubule by only applying the virtual cathode to the microtubule. The pause mode of the microtubule was easily canceled by simply turning the virtual cathode off, and then the gliding motion was restarted.

Local direction change of surface gliding microtubules

In vitro gliding assay, microtubule translocation by kinesin motor proteins on a surface, has been used as an engineering tool in analyte detection, molecular cargo transport, and other applications. Although controlling the moving direction is often necessary to realize these applications, current direction control methods focus largely on lithographic microfabrication of tracks or external fields on the microtubules. These methods are effective, but are relatively complicated. In addition, they cannot target particular microtubules without affecting others. In this study, we propose a facile approach that can make local direction changes for selected microtubules using a polystyrene particle as a circular motion center and a DNA double helix with streptavidin as a capture arm. The DNA arm captures a microtubule in the close proximity of the immobilized particle via biotin-streptavidin interaction and changes the moving direction ~10° on average. In contrast, no significant direction changes are observed other than random variations with streptavidin-less DNA arms (normal distribution centered at 0°), similar to regular motility assay. The particle-assisted local direction change scheme is compared with a flow field-based ensemble method. The combination of flow and kinesin interactions with each microtubule exerts a force to change the direction, ultimately aligning it to the flow field, regardless of its initial direction. A simple model based on the force balance predicts the time needed for such an alignment. Overall, the particle-based local scheme is distinct and different from ensemble methods such as crossflow that changes directions of all microtubules in the field, thus offering unique utility in engineering applications.

Tubulin's response to external electric fields by molecular dynamics simulations

Tubulin heterodimers are the building blocks of microtubules and disruption of their dynamics is exploited in the treatment of cancer. Electric fields at certain frequencies and magnitudes are believed to do the same. Here, the tubulin dimer’s response to external electric fields was determined by atomistic simulation. External fields from 50 to 750 kV/cm, applied for 10 ns, caused significant conformational rearrangements that were dependent upon the field’s directionality. Charged and flexible regions, including the α:H1-B2 loop, β:M-loop, and C-termini, were susceptible. Closer inspection of the α:H1-B2 loop in lower strength fields revealed that these effects were consistent and proportional to field strength, and the findings indicate that external electric fields modulate the stability of microtubules through conformational changes to key loops involved in lateral contacts. We also find evidence that tubulin’s curvature and elongation are affected, and external electric fields may bias tubulin towards depolymerization.

Targeted Activation of Molecular Transportation by Visible Light

Regulated transportation of nanoscale objects with a high degree of spatiotemporal precision is a prerequisite for the development of targeted molecular delivery. In vitro integration of the kinesin-microtubule motor system with synthetic molecules offers opportunities to develop controllable molecular shuttles for lab-on-a-chip applications. We attempted a combination of the kinesin-microtubule motor system with push-pull type azobenzene tethered inhibitory peptides (azo-peptides) through which reversible, spatiotemporal control over the kinesin motor activity was achieved locally by a single, visible wavelength. The fast thermal relaxation of the cis-isomers of azo-peptides offered us quick and complete resetting of the trans-state in the dark, circumventing the requirement of two distinct wavelengths for two-way switching of kinesin-driven microtubule motility. Herein, we report the manipulation of selected, single microtubule movement while keeping other microtubules at complete rest. The photoresponsive inhibitors discussed herein would help in realizing complex bionanodevices.

Unraveling the mechanistic effects of electric field stimulation towards directing stem cell fate and function: A tissue engineering perspective

Electric field (EF) stimulation can play a vital role in eliciting appropriate stem cell response. Such an approach is recently being established to guide stem cell differentiation through osteogenesis/neurogenesis/cardiomyogenesis. Despite significant recent efforts, the biophysical mechanisms by which stem cells sense, interpret and transform electrical cues into biochemical and biological signals still remain unclear. The present review critically analyses the variety of EF stimulation approaches that can be employed to evoke appropriate stem cell response and also makes an attempt to summarize the underlying concepts of this notion, placing special emphasis on stem cell based tissue engineering and regenerative medicine. This review also discusses the major signaling pathways and cellular responses that are elicited by electric stimulation, including the participation of reactive oxygen species and heat shock proteins, modulation of intracellular calcium ion concentration, ATP production and numerous other events involving the clustering or reassembling of cell surface receptors, cytoskeletal remodeling and so on. The specific advantages of using external electric stimulation in different modalities to regulate stem cell fate processes are highlighted with explicit examples, in vitro and in vivo.

Control of molecular shuttles by designing electrical and mechanical properties of microtubules

Kinesin-driven microtubules have been focused on to serve as molecular transporters, called "molecular shuttles," to replace micro/nanoscale molecular manipulations necessitated in micro total analysis systems. Although transport, concentration, and detection of target molecules have been demonstrated, controllability of the transport directions is still a major challenge. Toward broad applications of molecular shuttles by defining multiple moving directions for selective molecular transport, we integrated a bottom-up molecular design of microtubules and a top-down design of a microfluidic device. The surface charge density and stiffness of microtubules were controlled, allowing us to create three different types of microtubules, each with different gliding directions corresponding to their electrical and mechanical properties. The measured curvature of the gliding microtubules enabled us to optimize the size and design of the device for molecular sorting in a top-down approach. The integrated bottom-up and top-down design achieved separation of stiff microtubules from negatively charged, soft microtubules under an electric field. Our method guides multiple microtubules by integrating molecular control and microfluidic device design; it is not only limited to molecular sorters but is also applicable to various molecular shuttles with the high controllability in their movement directions.

Calcium-independent disruption of microtubule dynamics by nanosecond pulsed electric fields in U87 human glioblastoma cells

High powered, nanosecond duration, pulsed electric fields (nsPEF) cause cell death by a mechanism that is not fully understood and have been proposed as a targeted cancer therapy. Numerous chemotherapeutics work by disrupting microtubules. As microtubules are affected by electrical fields, this study looks at the possibility of disrupting them electrically with nsPEF. Human glioblastoma cells (U87-MG) treated with 100, 10 ns, 44 kV/cm pulses at a frequency of 10 Hz showed a breakdown of their interphase microtubule network that was accompanied by a reduction in the number of growing microtubules. This effect is temporally linked to loss of mitochondrial membrane potential and independent of cellular swelling and calcium influx, two factors that disrupt microtubule growth dynamics. Super-resolution microscopy revealed microtubule buckling and breaking as a result of nsPEF application, suggesting that nsPEF may act directly on microtubules.

Multi scale modeling of 2450MHz electric field effects on microtubule mechanical properties

Microtubule (MT) rigidity and response to 2450MHz electric fields were investigated, via multi scale modeling approach. For this purpose, six systems were designed and simulated to consider all types of feasible interactions between α and β monomers in MT, by using all atom molecular dynamics method. Subsequently, coarse grain modeling was used to design different lengths of MT. Investigation of effects of external 2450MHz electric field on MT showed MT less rigidity in the presence of such field, which may perturb its functions. Moreover, an additional computational setup was designed to study effects of 2450MHz field on MT response to AFM tip. It was found, more tip velocity led to MT faster transformation and less time was required to change MT elastic response to plastic one, applying constant radius. Moreover it was observed smaller tip caused to increase required time to change MT elastic response to plastic one, considering constant velocity. Furthermore, exposing MT to 2450MHz field led to no significant changes in MT response to AFM tip, but quick change in MT elastic response to plastic one.

Somato-dendritic decoupling as a novel mechanism for protracted cortical maturation

BACKGROUND

Both human and animal data indicate that disruption of the endogenously slow maturation of temporal association cortical (TeA) networks is associated with abnormal higher order cognitive development. However, the neuronal mechanisms underlying the endogenous maturation delay of the TeA are poorly understood.

RESULTS

Here we report a novel form of developmental plasticity that is present in the TeA. It was found that deep layer TeA neurons, but not hippocampal or primary visual neurons, exist in a protracted 'embryonic-like' state through a mechanism involving reduced somato-dendritic communication and a non-excitable somatic membrane. This mechanism of neural inactivity is present in intact tissue and shows a remarkable transition into an active somato-dendritically coupled state. The quantity of decoupled cells diminishes in a protracted and age-dependent manner, continuing into adolescence.

CONCLUSIONS

Based on our data, we propose a model of neural plasticity through which protracted compartmentalization and decoupling in somato-dendritic signalling plays a key role in controlling how excitable neurons are incorporated into recurrent cortical networks independent of neurogenesis.

Dynamic formation of a microchannel array enabling kinesin-driven microtubule transport between separate compartments on a chip

Microtubules driven by kinesin motors have been utilised as "molecular shuttles" in microfluidic environments with potential applications in autonomous nanoscale manipulations such as capturing, separating, and/or concentrating biomolecules. However, the conventional flow cell-based assay has difficulty in separating bound target molecules from free ones even with buffer flushing because molecular manipulations by molecular shuttles take place on a glass surface and molecular binding occurs stochastically; this makes it difficult to determine whether molecules are carried by molecular shuttles or by diffusion. To address this issue, we developed a microtubule-based transport system between two compartments connected by a single-micrometre-scale channel array that forms dynamically via pneumatic actuation of a polydimethylsiloxane membrane. The device comprises three layers-a control channel layer (top), a microfluidic channel layer (middle), and a channel array layer (bottom)-that enable selective injection of assay solutions into a target compartment and dynamic formation of the microchannel array. The pneumatic channel also serves as a nitrogen supply path to the assay area, which reduces photobleaching of fluorescently labelled microtubules and deactivation of kinesin by oxygen radicals. The channel array suppresses cross-contamination of molecules caused by diffusion or pressure-driven flow between compartments, facilitating unidirectional transport of molecular shuttles from one compartment to another. The method demonstrates, for the first time, efficient and unidirectional microtubule transport by eliminating diffusion of target molecules on a chip and thus may constitute one of the key aspects of motor-driven nanosystems.

Control of microtubule trajectory within an electric field by altering surface charge density

One of challenges for using microtubules (MTs) driven by kinesin motors in microfluidic environments is to control their direction of movement. Although applying physical biases to rectify MTs is prevalent, it has not been established as a design methodology in conjunction with microfluidic devices. In the future, the methodology is expected to achieve functional motor-driven nanosystems. Here, we propose a method to guide kinesin-propelled MTs in multiple directions under an electric field by designing a charged surface of MT minus ends labeled with dsDNA via a streptavidin-biotin interaction. MTs labeled with 20-bp or 50-bp dsDNA molecules showed significantly different trajectories according to the DNA length, which were in good agreement with values predicted from electrophoretic mobilities measured for their minus ends. Since the effective charge of labeled DNA molecules was equal to that of freely dispersed DNA molecules in a buffer solution, MT trajectory could be estimated by selecting labeling molecules with known charges. Moreover, the estimated trajectory enables to define geometrical sizes of a microfluidic device. This rational molecular design and prediction methodology allows MTs to be guided in multiple directions, demonstrating the feasibility of using molecular sorters driven by motor proteins.

Electrical control of kinesin-microtubule motility using a transparent functionalized-graphene substrate

We report a new strategy to selectively localize and control microtubule translocation via electrical control of microtubules using a microfabricated channel on a functionalized-graphene electrode with high transparency and conductivity. A patterned SU-8 film acts as an insulation layer which shields the electrical field generated by the graphene underneath while the localized electric field on the exposed graphene surface guides the negatively charged microtubules. This is the first report showing that functionalized graphene can support and control microtubule motility.

Paclitaxel-conjugated PAMAM dendrimers adversely affect microtubule structure through two independent modes of action

Paclitaxel (Taxol) is an anticancer drug that induces mitotic arrest via microtubule hyperstabilization but causes side effects due to its hydrophobicity and cellular promiscuity. The targeted cytotoxicity of hydrophilic paclitaxel-conjugated polyamidoamine (PAMAM) dendrimers has been demonstrated in cultured cancer cells. Mechanisms of action responsible for this cytotoxicity are unknown, that is, whether the cytotoxicity is due to paclitaxel stabilization of microtubules, as is whether paclitaxel is released intracellularly from the dendrimer. To determine whether the conjugated paclitaxel can bind microtubules, we used a combination of ensemble and single microtubule imaging techniques in vitro. We demonstrate that these conjugates adversely affect microtubules by (1) promoting the polymerization and stabilization of microtubules in a paclitaxel-dependent manner, and (2) bundling preformed microtubules in a paclitaxel-independent manner, potentially due to protonation of tertiary amines in the dendrimer interior. Our results provide mechanistic insights into the cytotoxicity of paclitaxel-conjugated PAMAM dendrimers and uncover unexpected risks of using such conjugates therapeutically.

Optimization of isopolar microtubule arrays

Isopolar arrays of aligned cytoskeletal filaments are components in a number of designs of hybrid nanodevices incorporating biomolecular motors. For example, a combination of filament arrays and motor arrays can form an actuator or a molecular engine resembling an artificial muscle. Here, isopolar arrays of microtubules are fabricated by flow alignment, and their quality is characterized by their degree of alignment. We find, in agreement with our analytical models, that the degree of alignment is ultimately limited by thermal forces, while the kinetics of the alignment process are influenced by the flow strength, the microtubule stiffness, the gliding velocity, and the tip length. Strong flows remove microtubules from the surface and reduce the filament density, suggesting that there is an optimal flow strength for the fabrication of ordered arrays.

Orienting actin filaments for directional motility of processive myosin motors

To utilize molecular motors in manmade systems, it is necessary to control the motors' motion. We describe a technique to orient actin filaments so that their barbed ends point in the same direction, enabling same-type motors to travel unidirectionally. Myosin-V and myosin-VI were observed to travel, respectively, toward and away from the filaments' barbed ends. When both motors were present, they occasionally passed each other while "walking" in opposite directions along single actin filaments.

Differential interference contrast polarization anisotropy for tracking rotational dynamics of gold nanorods

We describe differential interference contrast (DIC) polarization anisotropy for tracking rotational dynamics of gold nanorod (AuNR) probes. DIC polarization anisotropy enabled us to reveal the unidirectional clockwise circular translocation of an AuNR attached to a kinesin-driven microtubule and to precisely determine the real-time orientation of the AuNR during the dynamic process.

Magnetic manipulation of actin orientation, polymerization, and gliding on myosin using superparamagnetic iron oxide particles

The actin cytoskeleton controls cell shape, motility, as well as intracellular molecular trafficking. The ability to remotely manipulate actin is therefore highly desirable as a tool to probe and manipulate biological processes at the molecular level. We demonstrate actin manipulation by labeling actin filaments with superparamagnetic iron oxide particles (IOPs) and applying a uniform magnetic field to affect actin orientation, polymerization and gliding on myosin. We show for the first time magnetic manipulation of magnetizable actin filaments at the molecular level while gliding on a bed of myosin molecules and during polymerization. A model for the magnetic alignment and guiding mechanism is proposed based on the torque from the induced molecular anisotropy due to interactions between neighboring IOPs distributed along magnetically labeled actin molecules.

Measuring the number and spacing of molecular motors propelling a gliding microtubule

The molecular motor gliding assay, in which a microtubule or other filament moves across a surface coated with motors, has provided much insight into how molecular motors work. The kinesin-microtubule system is also a strong candidate for the job of nanoparticle transporter in nanotechnology devices. In most cases, several motors transport each filament. Each motor serves both to bind the microtubule to a stationary surface and to propel the microtubule along the surface. By applying a uniform transverse force of 4-19 pN to a superparamagnetic bead attached to the trailing end of the microtubule, we have measured the distance d between binding points (motors). The average value of d was determined as a function of motor surface density σ. The measurements agree well with the scaling model of Duke, Holy, and Liebler, which predicts that (d)~σ(-2/5) if 0.05≤σ≤20 μm(-2) [Phys. Rev. Lett. 74, 330 (1995)]. The distribution of d fits an extension of the model. The radius of curvature of a microtubule bent at a binding point by the force of the magnetic bead was ≈1 μm, 5000-fold smaller than the radius of curvature of microtubules subjected only to thermal forces. This is evidence that at these points of high bending stress, generated by the force on the magnetic bead, the microtubule is in the more flexible state of a two-state model of microtubule bending proposed by Heussinger, Schüller, and Frey [Phys. Rev. E 81, 021904 (2010)].

In silico design and testing of guiding tracks for molecular shuttles powered by kinesin motors

We present an extended computer simulation method which allows in silico design and testing of guiding tracks for molecular shuttles powered by kinesin motors. The simulation reproduced molecular shuttle movements under external forces and dissociation of shuttles from guiding track surfaces. The simulation was validated by comparing the simulation results with the corresponding experimental ones. Using the simulation, track designers can change guiding track designs, choose guiding methods, tune the strength of external forces, and test the module performance. This simulation would significantly reduce the effort expended in designing guiding tracks for molecular shuttles powered by kinesin motors.

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.

A nano-needle/microtubule composite gliding on a kinesin-coated surface for target molecule transport

An alternative method of micro/nano-transport has been achieved by using motor proteins. Microtubules on a kinesin-coated surface have potential to act as a nano-transport system. When microtubules are used as carriers, either cargo or cargo linkers are attached on the microtubule surface. Such cargo attachments can significantly affect kinesin motion. To deal with the difficulty caused by molecular attachment to the microtubule surface, the cargo loading and transport mechanism should be separated. In this work, we propose to use micromachined needles as cargo carriers which then can be transported on microtubules. Because of the separation of needle functionalization and transport mechanism, functionalization of the needles can proceed without any effect on the microtubule structure, significantly increasing the possible types of cargo. We have fabricated silicon needles in mass numbers using a simple and effective method and have shown that the microtubule-needle composites are transported without affecting the kinesin activity.

Imaging capability of patterned liquid crystals

We demonstrate the ability to make high resolution arbitrary patterned optical retarders using liquid crystal polymer (LCP). Contact lithography is used to define unique LCP alignment domains. Patterned LCP retarders are imaged between crossed polarizers to determine pattern visibility as a function of feature size. It was determined that patterned retarders for wavelengths between 250 nm and 2500 nm can be constructed with feature sizes as small as 4 microm. We also showed that multiple patterns can be created on the same substrate using a combination of patterned LCP and opaque features. Our process has applications in displays, double-patterning lithography, and imaging polarimetry.

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.

Surface-bound casein modulates the adsorption and activity of kinesin on SiO2 surfaces

Conventional kinesin is routinely adsorbed to hydrophilic surfaces such as SiO(2). Pretreatment of surfaces with casein has become the standard protocol for achieving optimal kinesin activity, but the mechanism by which casein enhances kinesin surface adsorption and function is poorly understood. We used quartz crystal microbalance measurements and microtubule gliding assays to uncover the role that casein plays in enhancing the activity of surface-adsorbed kinesin. On SiO(2) surfaces, casein adsorbs as both a tightly bound monolayer and a reversibly bound second layer that has a dissociation constant of 500 nM and can be desorbed by washing with casein-free buffer. Experiments using truncated kinesins demonstrate that in the presence of soluble casein, kinesin tails bind well to the surface, whereas kinesin head binding is blocked. Removing soluble casein reverses these binding profiles. Surprisingly, reversibly bound casein plays only a moderate role during kinesin adsorption, but it significantly enhances kinesin activity when surface-adsorbed motors are interacting with microtubules. These results point to a model in which a dynamic casein bilayer prevents reversible association of the heads with the surface and enhances association of the kinesin tail with the surface. Understanding protein-surface interactions in this model system should provide a framework for engineering surfaces for functional adsorption of other motor proteins and surface-active enzymes.

Biomolecular motor-driven molecular sorter

We have developed a novel, microfabricated, stand-alone microfluidic device that can efficiently sort and concentrate (bio-)analyte molecules by using kinesin motors and microtubules as a chemo-mechanical transduction machine. The device removes hundreds of targeted molecules per second from an analyte stream by translocating functionalized microtubules with kinesin across the stream and concentrating them at a horseshoe-shaped collector. Target biomolecule concentrations increase up to three orders of magnitude within one hour of operation.

Models of protein linear molecular motors for dynamic nanodevices

Protein molecular motors are natural nano-machines that convert the chemical energy from the hydrolysis of adenosine triphosphate into mechanical work. These efficient machines are central to many biological processes, including cellular motion, muscle contraction and cell division. The remarkable energetic efficiency of the protein molecular motors coupled with their nano-scale has prompted an increasing number of studies focusing on their integration in hybrid micro- and nanodevices, in particular using linear molecular motors. The translation of these tentative devices into technologically and economically feasible ones requires an engineering, design-orientated approach based on a structured formalism, preferably mathematical. This contribution reviews the present state of the art in the modelling of protein linear molecular motors, as relevant to the future design-orientated development of hybrid dynamic nanodevices.

Single-molecule observation of anomalous electrohydrodynamic orientation of microtubules

We use fluorescence microscopy to measure the orientation and shape of microtubules-which serve as a model system for semiflexible rods-that are electrophoretically driven. Surprisingly, a bimodal orientation distribution is observed, with microtubules in either parallel or perpendicular orientations to the electric field. The occupancy of these states varies nonmonotonically with the microtubule length L and the electric field E. We also observe a surprising bending deformation of microtubules. Interestingly, all data collapse onto a universal scaling curve when the average alignment is plotted as a function of B proportional, variantEL3, which reflects the ratio between the driving force and a restoring elastic force. Our results have important implications for the interpretation of electrical birefringence experiments and, more generally, for a better understanding of the electrokinetics of rods.

Microtubule alignment and manipulation using AC electrokinetics

The kinesin-microtubule system plays an important role in intracellular transport and is a model system for integrating biomotor-driven transport into microengineered devices. AC electrokinetics provides a novel tool for manipulating and organizing microtubules in solution, enabling new experimental geometries for investigating and controlling the interactions of microtubules and microtubule motors in vitro. By fabricating microelectrodes on glass substrates and generating AC electric fields across solutions of microtubules in low-ionic-strength buffers, bundles of microtubules are collected and aligned and the electrical properties of microtubules in solution are measured. The AC electric fields result in electro-osmotic flow, electrothermal flow, and dielectrophoresis of microtubules, which can be controlled by varying the solution conductivity, AC frequency, and electrode geometry. By mapping the solution conductivity and AC frequency over which positive dielectrophoresis occurs, the apparent conductivity of taxol-stabilized bovine-brain microtubules in PIPES buffer is measured to be 250 mS m(-1). By maximizing dielectrophoretic forces and minimizing electro-osmotic and electrothermal flow, microtubules are assembled into opposed asters. These experiments demonstrate that AC electrokinetics provides a powerful new tool for kinesin-driven transport applications and for investigating the role of microtubule motors in development and maintenance of the mitotic spindle.

Nanomechanical model of microtubule translocation in the presence of electric fields

Research efforts in recent years have been directed toward actively controlling the direction of translocation of microtubules on a kinesin-coated glass surface with E-fields (electric fields), opening up the possibility of engineering controllable nanodevices that integrate microtubules and motor proteins into their function. Here, we present a detailed, biophysical model that quantitatively describes our observations on the steering of microtubules by electric fields. A sudden application of an electric field parallel to the surface and normal to the translocation direction of a microtubule bends the leading end toward the anode, because Coulombic (electrophoretic) forces are dominant on negatively charged microtubules. Modeling this bending as a cantilever deflection with uniform loading requires accurate mechanical and electrical properties of microtubules, including their charge density, viscous drag, and flexural rigidity. We determined the charge density of microtubules from measurements of the electrophoretic mobility in a "zero flow" capillary electrophoresis column and estimate it to be 256 e(-) per micron of length. Viscous drag forces on deflecting microtubules in electroosmotic flows were studied theoretically and experimentally by directly characterizing flows using a caged dye imaging method. The flexural rigidity of microtubules was measured by applying E-fields to microtubules with biotinylated segments that were bound to streptavidin-coated surfaces. From the calculated loading, and the Bernoulli-Euler curvature and moment equation, we find that the flexural rigidity of microtubules depends on their length, suggesting microtubules are anisotropic. Finally, our model accurately predicts the biophysical properties and behavior of microtubules directed by E-fields, which opens new avenues for the design of biomolecular nanotransport systems.

Self-contained, biomolecular motor-driven protein sorting and concentrating in an ultrasensitive microfluidic chip

We developed a molecular sorter that operates without external power or control by integrating the microtubule-based, biological motor kinesin into a microfluidic channel network to sort, transport, and concentrate molecules. In our devices, functionalized microtubules that capture analyte molecules are steered along kinesin-coated microchannel tracks toward a collector structure, concentrated, and trapped. Using fluorescent analyte molecules and nanoliter sample volumes, we demonstrated 14 fM sensitivity, even in the presence of high concentrations of other proteins.

Polarity orientation of microtubules utilizing a dynein-based gliding assay

The motor protein dynein was introduced into a nanotransport system. We oriented microtubules by their polarity, and immobilized them based on a dynein-microtubule gliding assay system. This system achieved unidirectional transport of kinesin-coated microbeads. In contrast to conventional kinesin-based orientation systems, the dynein-based system allowed the reverse motion of microtubules, resulting in an inversion of the orientation of microtubule polarity and thus reverse transport of kinesin-coated microbeads. This combined kinesin- and dynein-based system constitutes a new means to facilitate the bidirectional orientation of microtubules and transport of cargos in a nanofluidic system.

Microtubule curvatures under perpendicular electric forces reveal a low persistence length

The mechanics of microtubules, cylindrical protein filaments that constitute the cytoskeleton, have been well characterized on long length scales. Here, we investigate the persistence length of short (approximately 0.1 microm) ends of microtubules by measuring the trajectories of kinesin-propelled microtubules under perpendicular electric forces. We relate the measured trajectory curvatures to the biased thermal fluctuations of the leading microtubule end, and upon including all electrohydrodynamic forces, we find that the persistence length of the microtubule ends is only 0.08 +/- 0.02 mm. This is significantly shorter than the well established value of approximately 4-8 mm that is measured for long microtubules. Our data are in good agreement with recent theoretical predictions that microtubules mechanically behave as a loose assembly of independent protofilaments on these short length scales.