Rotation of endosomes demonstrates coordination of molecular motors during axonal transport

Fluorescent Nanodiamonds for Quantum Sensing in Biology

Fluorescent nanodiamonds exhibiting outstanding optical and biocompatible properties are the subject of increased studies and attention in physics and biology. The nitrogen-vacancy center in diamonds with unique quantum properties at room temperature is sensitive to physical properties such as magnetic field, electric field, temperature, and pressure. By taking advantage of the NV center and high sensitivity that arises from the intrinsic quantum properties of spins in nanodiamonds, which are extensively employed in quantum sensing, bio-imaging, and bio-sensing. In this review, the selected topic mainly focuses on the surface functionalization of nanodiamonds and the recent progress in applying nanodiamonds as quantum sensors for intracellular orientation tracking, temperature sensing, and notably nuclear magnetic resonance and electron spin resonance applications.

Nanometer-resolution tracking of single cargo reveals dynein motor mechanisms

Cytoplasmic dynein is essential for intracellular transport. Despite extensive in vitro characterizations, how the dynein motors transport vesicles by processive steps in live cells remains unclear. To dissect the molecular mechanisms of dynein, we develop optical probes that enable long-term single-particle tracking in live cells with high spatiotemporal resolution. We find that the number of active dynein motors transporting cargo switches stochastically between one and five dynein motors during long-range transport in neuronal axons. Our very bright optical probes allow the observation of individual molecular steps. Strikingly, these measurements reveal that the dwell times between steps are controlled by two temperature-dependent rate constants in which two ATP molecules are hydrolyzed sequentially during each dynein step. Thus, our observations uncover a previously unknown chemomechanical cycle of dynein-mediated cargo transport in living cells.

Digitally Assisted Single-Particle Tracking for Accurate Analysis of Complicated Cargo Transport Dynamics in Microtubule Networks

Intracellular transport is a fundamental process crucial for cellular function, driven by the coordinated action of motor proteins that move cargo along microtubule tracks. Traditional tracking methods primarily focus on cargo trajectories, often overlooking rotational dynamics and their impact on cargo interactions with the complex microtubule network. To address this limitation, we introduced a digitally assisted single-particle tracking (dSPT) method that significantly advances the angular resolution of intracellular cargo dynamics. By integrating intensity measurements with advanced digital classification algorithms to process defocused half-plane image patterns captured through bifocal parallax microscopy, this approach extends the angular resolution range from the conventional method to a full 0-360° range, even in heterogeneous cellular environments, while maintaining high spatial and temporal resolutions. In intracellular transport events, we directly observed the accurate determination of the chiral rotational directions and precise calculation of the step angles. When combined with super-resolution radial fluctuation (SRRF) imaging to achieve higher-resolution microtubule imaging, our dSPT technique enables in vivo investigations of cargo dynamics during intracellular transport. To validate this, we studied the rotational dynamics of the cargo in microtubule confinement. Furthermore, we identified characteristic patch-searching patterns in the microtubule network, where cargo exhibited a combined motion pattern of confined and hopping diffusion to navigate through the constraints imposed by the microtubules.

3D real-time single particle tracking using two-photon fluorescence from bright dye-based organic nanoparticles

This paper addresses the use of ultrabright dye-based fluorescent organic nanoparticles in a 3D single-particle tracking two-photon microscopy setup. The nanoparticles consist of an assembly of quadrupolar dyes, presenting a large two-photon absorption cross-section. They exhibit low photobleaching, crucial for long-term tracking, and their high brightness allows nanometer localization precision. Their small size compared to previously used nonlinear inorganic nanocrystals, stable structure at physiological temperature, and adjustable optical properties make them promising tools for further biological research. This study highlights their potential to track dynamic processes with precision and stability, paving the way for exploring cellular processes at the nanoscale.

Probing Twist-Induced Endocytotic Membrane Fission using Anisotropic Gold Homodimers

Membrane fission involves a crucial step of lipid remodeling, in which the dynamin collar constricts and severs the tubulated lipid membrane at the neck of budding vesicles. Nevertheless, the difficulty in accurately determining the rotational dynamics of live endocytotic vesicles poses a limit on the elucidation of dynamin-induced membrane remodeling for endocytotic vesicle scission. Herein, we designed a DNA-modified gold homodimer (AuHD)-based anisotropic plasmonic probe with uniform surface chemistry, minimizing orientational fluctuation within vesicle encapsulation. Using AuHDs as cargos to image the dynamics of cargo-containing vesicles during endocytosis, we showed that, prior to detachment from plasma membrane, the cargo-containing vesicles underwent multiple intermittent twists of ~4° angular orientation relative to plasma membrane with a ~0.2 s dwell time. These findings suggest that the membrane torques resulting from dynamin actions in vivo constitute the pathway to membrane fission, potentially shedding light on how dynamin-mediated lipid remodeling orchestrates membrane fission.

Temporal Patterns of Angular Displacement of Endosomes: Insights into Motor Protein Exchange Dynamics

The material transport system, facilitated by motor proteins, plays a vital role in maintaining a non-equilibrium cellular state. However, understanding the temporal coordination of motor protein activity requires an advanced imaging technique capable of measuring 3D angular displacement in real-time. In this study, a Fourier transform-based plasmonic dark-field microscope has been developed using anisotropic nanoparticles, enabling the prolonged and simultaneous observation of endosomal lateral and rotational motion. A sequence of discontinuous 3D angular displacements has been observed during the pause and run phases of transport. Notably, a serially correlated temporal pattern in the intermittent rotational events has been demonstrated during the tug-of-war mechanism, indicating Markovian switching between the exploitational and explorational modes of motor protein exchange prior to resuming movement. Alterations in transition frequency and the exploitation-to-exploration ratio upon dynein inhibitor treatment highlight the relationship between disrupted motor coordination and reduced endosomal transport efficiency. Collectively, these results suggest the importance of orchestrated temporal motor protein patterns for efficient cellular transport.

Can repetitive mechanical motion cause structural damage to axons?

Biological structures have evolved to very efficiently generate, transmit, and withstand mechanical forces. These biological examples have inspired mechanical engineers for centuries and led to the development of critical insights and concepts. However, progress in mechanical engineering also raises new questions about biological structures. The past decades have seen the increasing study of failure of engineered structures due to repetitive loading, and its origin in processes such as materials fatigue. Repetitive loading is also experienced by some neurons, for example in the peripheral nervous system. This perspective, after briefly introducing the engineering concept of mechanical fatigue, aims to discuss the potential effects based on our knowledge of cellular responses to mechanical stresses. A particular focus of our discussion are the effects of mechanical stress on axons and their cytoskeletal structures. Furthermore, we highlight the difficulty of imaging these structures and the promise of new microscopy techniques. The identification of repair mechanisms and paradigms underlying long-term stability is an exciting and emerging topic in biology as well as a potential source of inspiration for engineers.

Deep Learning-Assisted Automated Multidimensional Single Particle Tracking in Living Cells

The translational and rotational dynamics of anisotropic optical nanoprobes revealed in single particle tracking (SPT) experiments offer molecular-level information about cellular activities. Here, we report an automated high-speed multidimensional SPT system integrated with a deep learning algorithm for tracking the 3D orientation of anisotropic gold nanoparticle probes in living cells with high localization precision (<10 nm) and temporal resolution (0.9 ms), overcoming the limitations of rotational tracking under low signal-to-noise ratio (S/N) conditions. This method can resolve the azimuth (0°-360°) and polar angles (0°-90°) with errors of less than 2° on the experimental and simulated data under S/N of ∼4. Even when the S/N approaches the limit of 1, this method still maintains better robustness and noise resistance than the conventional pattern matching methods. The usefulness of this multidimensional SPT system has been demonstrated with a study of the motions of cargos transported along the microtubules within living cells.

Repetitive CREB-DNA interactions at gene loci predetermined by CBP induce activity-dependent gene expression in human cortical neurons

Neuronal activity-dependent transcription plays a key role in plasticity and pathology in the brain. An intriguing question is how neuronal activity controls gene expression via interactions of transcription factors with DNA and chromatin modifiers in the nucleus. By utilizing single-molecule imaging in human embryonic stem cell (ESC)-derived cortical neurons, we demonstrate that neuronal activity increases repetitive emergence of cAMP response element-binding protein (CREB) at histone acetylation sites in the nucleus, where RNA polymerase II (RNAPII) accumulation and FOS expression occur rapidly. Neuronal activity also enhances co-localization of CREB and CREB-binding protein (CBP). Increased binding of a constitutively active CREB to CBP efficiently induces CREB repetitive emergence. On the other hand, the formation of histone acetylation sites is dependent on CBP histone modification via acetyltransferase (HAT) activity but is not affected by neuronal activity. Taken together, our results suggest that neuronal activity promotes repetitive CREB-CRE and CREB-CBP interactions at predetermined histone acetylation sites, leading to rapid gene expression.

Coordination, cooperation, competition, crowding and congestion of molecular motors: Theoretical models and computer simulations

Cytoskeletal motor proteins are biological nanomachines that convert chemical energy into mechanical work to carry out various functions such as cell division, cell motility, cargo transport, muscle contraction, beating of cilia and flagella, and ciliogenesis. Most of these processes are driven by the collective operation of several motors in the crowded viscous intracellular environment. Imaging and manipulation of the motors with powerful experimental probes have been complemented by mathematical analysis and computer simulations of the corresponding theoretical models. In this article, we illustrate some of the key theoretical approaches used to understand how coordination, cooperation and competition of multiple motors in the crowded intra-cellular environment drive the processes that are essential for biological function of a cell. In spite of the focus on theory, experimentalists will also find this article as an useful summary of the progress made so far in understanding multiple motor systems.

In Vivo Fast Nonlinear Microscopy Reveals Impairment of Fast Axonal Transport Induced by Molecular Motor Imbalances in the Brain of Zebrafish Larvae

Cargo transport by molecular motors along microtubules is essential for the function of eukaryotic cells, in particular neurons in which axonal transport defects constitute the early pathological features of neurodegenerative diseases. Mainly studied in motor and sensory neurons, axonal transport is still difficult to characterize in neurons of the brain in absence of appropriate in vivo tools. Here, we measured fast axonal transport by tracing the second harmonic generation (SHG) signal of potassium titanyl phosphate (KTP) nanocrystals (nanoKTP) endocytosed by brain neurons of zebrafish (Zf) larvae. Thanks to the optical translucency of Zf larvae and to the perfect photostability of nanoKTP SHG, we achieved a high scanning speed of 20 frames (of ≈90 μm × 60 μm size) per second in Zf brain. We focused our study on endolysosomal vesicle transport in axons of known polarization, separately analyzing kinesin and dynein motor-driven displacements. To validate our assay, we used either loss-of-function mutations of dynein or kinesin 1 or the dynein inhibitor dynapyrazole and quantified several transport parameters. We successfully demonstrated that dynapyrazole reduces the nanoKTP mobile fraction and retrograde run length consistently, while the retrograde run length increased in kinesin 1 mutants. Taking advantage of nanoKTP SHG directional emission, we also quantified fluctuations of vesicle orientation. Thus, by combining endocytosis of nanocrystals having a nonlinear response, fast two-photon microscopy, and high-throughput analysis, we are able to finely monitor fast axonal transport in vivo in the brain of a vertebrate and reveal subtle axonal transport alterations. The high spatiotemporal resolution achieved in our model may be relevant to precisely investigate axonal transport impairment associated with disease models.

Motor generated torque drives coupled yawing and orbital rotations of kinesin coated gold nanorods

Kinesin motor domains generate impulses of force and movement that have both translational and rotational (torque) components. Here, we ask how the torque component influences function in cargo-attached teams of weakly processive kinesins. Using an assay in which kinesin-coated gold nanorods (kinesin-GNRs) translocate on suspended microtubules, we show that for both single-headed KIF1A and dimeric ZEN-4, the intensities of polarized light scattered by the kinesin-GNRs in two orthogonal directions periodically oscillate as the GNRs crawl towards microtubule plus ends, indicating that translocating kinesin-GNRs unidirectionally rotate about their short (yaw) axes whilst following an overall left-handed helical orbit around the microtubule axis. For orientations of the GNR that generate a signal, the period of this short axis rotation corresponds to two periods of the overall helical trajectory. Torque force thus drives both rolling and yawing of near-spherical cargoes carrying rigidly-attached weakly processive kinesins, with possible relevance to intracellular transport.

Two GPSes in a Ball: Deciphering the Endosomal Tug-of-War Using Plasmonic Dark-Field STORM

Live video recording of intracellular material transport is a promising means of deciphering the fascinating underlying mechanisms driving life at the molecular level. Such technology holds the key to realizing real-time observation at appropriate resolutions in three-dimensional (3D) space within living cells. Here, we report an optical microscopic method for probing endosomal dynamics with proper spatiotemporal resolution within 3D space in live cells: plasmonic dark-field STORM (pdf-STORM). We first confirmed that pdf-STORM has a spatial resolution comparable to that of scanning electron microscopy. Additionally, by observing two optical probes within a single organelle, we were able to track rotational movements and demonstrate the feasibility of using pdf-STORM to observe the angular displacements of an endosome during a "tug-of-war" over an extended period. Finally, we show various biophysical parameters of the hitherto unelucidated dynamics of endosomes-angular displacement is discontinuous and y-axis movement predominates and follows a long-tail distribution.

Spatiotemporal three-dimensional transport dynamics of endocytic cargos and their physical regulations in cells

Intracellular transport, regulated by complex cytoarchitectures and active driving forces, is crucial for biomolecule translocations and relates to many cellular functions. Despite extensive knowledge obtained from two-dimensional (2D) experiments, the real three-dimensional (3D) spatiotemporal characteristics of intracellular transport is still unclear. With 3D single-particle tracking, we comprehensively studied the transport dynamics of endocytic cargos. With varying timescale, the intracellular transport changes from thermal-dominated 3D-constrained motion to active-dominated quasi-2D motion. Spatially, the lateral motion is heterogeneous with peripheral regions being faster than perinuclear regions, while the axial motion is homogeneous across the cells. We further confirmed that such anisotropy and heterogeneity of vesicle transport result from actively directed motion on microtubules. Strikingly, inside the vesicles, we observed endocytic nanoparticles make diffusive motions on their inner membranes when microtubules are absent, suggesting endocytic cargos are normally localized at the inner vesicle membranes through a physical connection to the microtubules outside during transport.

Imaging Dynamic Processes in Multiple Dimensions and Length Scales

Optical microscopy has become an invaluable tool for investigating complex samples. Over the years, many advances to optical microscopes have been made that have allowed us to uncover new insights into the samples studied. Dynamic changes in biological and chemical systems are of utmost importance to study. To probe these samples, multidimensional approaches have been developed to acquire a fuller understanding of the system of interest. These dimensions include the spatial information, such as the three-dimensional coordinates and orientation of the optical probes, and additional chemical and physical properties through combining microscopy with various spectroscopic techniques. In this review, we survey the field of multidimensional microscopy and provide an outlook on the field and challenges that may arise. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Freestanding nanomaterials for subcellular neuronal interfaces

Current technological advances in neural probing and modulation have enabled an extraordinary glimpse into the intricacies of the nervous system. Particularly, nanomaterials are proving to be an incredibly versatile platform for neurological applications owing to their biocompatibility, tunability, highly specific targeting and sensing, and long-term chemical stability. Among the most desirable nanomaterials for neuroengineering, freestanding nanomaterials are minimally invasive and remotely controlled. This review outlines the most recent developments of freestanding nanomaterials that operate on the neuronal interface. First, the different nanomaterials and their mechanisms for modulating neurons are explored to provide a basis for how freestanding nanomaterials operate. Then, the three main applications of subcellular neuronal engineering-modulating neuronal behavior, exploring fundamental neuronal mechanism, and recording neuronal signal-are highlighted with specific examples of current advancements. Finally, we conclude with our perspective on future nanomaterial designs and applications.

Imaging Intracellular Trafficking in Neurons of C. elegans

Axonal transport is an essential component of neuronal function. Several neurodegenerative disorders have been associated with defects in cargo transport. Thus, studying axonal transport is important to understand such disorders. Live imaging of fluorescently labeled cargo is a prevailing technique to study properties of axonal transport. C. elegans is both transparent and genetically amenable, making it an excellent model system to study axonal transport. In this chapter, we describe protocols to live image several neuronal cargo in vivo in C. elegans neurons.

Dynamin-dependent vesicle twist at the final stage of clathrin-mediated endocytosis

Dynamin plays an important role in clathrin-mediated endocytosis (CME) by cutting the neck of nascent vesicles from the cell membrane. Here through using gold nanorods as cargos to image dynamin action during live CME, we show that near the peak of dynamin accumulation, the cargo-containing vesicles always exhibit abrupt, right-handed rotations that finish in a short time (~0.28 s). The large and quick twist, herein named the super twist, is the result of the coordinated dynamin helix action upon GTP hydrolysis. After the super twist, the rotational freedom of the vesicle drastically increases, accompanied with simultaneous or delayed translational movement, indicating that it detaches from the cell membrane. These observations suggest that dynamin-mediated scission involves a large torque generated by coordinated actions of multiple dynamins in the helix, which is the main driving force for vesicle scission.

Neuronal Trafficking of the Amyloid Precursor Protein-What Do We Really Know?

Alzheimer's disease (AD) is the most common type of dementia, contributing to 60-80% of cases. It is a neurodegenerative disease that usually starts symptomless in the first two to three decades and then propagates into a long-term, irreversible disease, resulting in the progressive loss of memory, reasoning, abstraction and language capabilities. It is a complex disease, involving a large number of entangled players, and there is no effective treatment to cure it or alter its progressive course. Therefore, a thorough understanding of the disease pathology and an early diagnosis are both necessary. AD has two significant pathological hallmarks: extracellular senile plaques composed of amyloid β-peptide (Aβ) and intracellular neurofibrillary tangles composed of hyperphosphorylated tau protein, and the aggregation of Aβ, which starts in earlier stages, is usually claimed to be the primary cause of AD. Secretases that cleave Aβ precursor protein (APP) and produce neurotoxic Aβ reside in distinct organelles of the cell, and current concepts suggest that APP moves between distinct intracellular compartments. Obviously, APP transport and processing are intimately related processes that cannot be dissociated from each other, and, thus, how and where APP is transported determines its processing fate. In this review, we summarize critical mechanisms underlying neuronal APP transport, which we divide into separate parts: (1) secretory pathways and (2) endocytic and autophagic pathways. We also include two lipoprotein receptors that play essential roles in APP transport: sorting-related receptor with A-type repeats and sortilin. Moreover, we consider here some major disruptions in the neuronal transport of APP that contribute to AD physiology and pathology. Lastly, we discuss current methods and technical difficulties in the studies of APP transport.

Association of Nanodiamond Rotation Dynamics with Cell Activities by Translation-Rotation Tracking

Correlated translation-orientation tracking of single particles can provide important information for understanding the dynamics of live systems and their interaction with the probes. However, full six-dimensional (6D) motion tracking has yet to be achieved. Here, we developed synchronized 3D translation and 3D rotation tracking of single diamond particles based on nitrogen-vacancy center sensing. We first performed 6D tracking of diamond particles attached to a giant plasma membrane vesicle to demonstrate the method. Quantitative analysis of diamond particles' motion allowed elimination of the geometric effect and revealed the net rotation on the vesicle. 6D tracking was then applied to measure live cell dynamics. Motion characteristics of nanodiamonds on cell membranes under various controlled physiological conditions suggest that the nanodiamonds' rotation is associated with cell metabolic activities. Our technique extends the toolbox of single particle tracking and provides a unique solution to problems where correlated analysis of translation and rotation is critical.

Individual Plasmonic Nanoprobes for Biosensing and Bioimaging: Recent Advances and Perspectives

With the advent of nanofabrication techniques, plasmonic nanoparticles (PNPs) have been widely applied in various research fields ranging from photocatalysis to chemical and bio-sensing. PNPs efficiently convert chemical or physical stimuli in their local environment into optical signals. PNPs also have excellent properties, including good biocompatibility, large surfaces for the attachment of biomolecules, tunable optical properties, strong and stable scattering light, and good conductivity. Thus, single optical biosensors with plasmonic properties enable a broad range of uses of optical imaging techniques in biological sensing and imaging with high spatial and temporal resolution. This work provides a comprehensive overview on the optical properties of single PNPs, the description of five types of commonly used optical imaging techniques, including surface plasmon resonance (SPR) microscopy, surface-enhanced Raman scattering (SERS) technique, differential interference contrast (DIC) microscopy, total internal reflection scattering (TIRS) microscopy, and dark-field microscopy (DFM) technique, with an emphasis on their single plasmonic nanoprobes and mechanisms for applications in biological imaging and sensing, as well as the challenges and future trends of these fields.

Micro- and Nano-Devices for Studying Subcellular Biology

Cells are complex machines whose behaviors arise from their internal collection of dynamically interacting organelles, supramolecular complexes, and cytoplasmic chemicals. The current understanding of the nature by which subcellular biology produces cell-level behaviors is limited by the technological hurdle of measuring the large number (>103 ) of small-sized (<1 μm) heterogeneous organelles and subcellular structures found within each cell. In this review, the emergence of a suite of micro- and nano-technologies for studying intracellular biology on the scale of organelles is described. Devices that use microfluidic and microelectronic components for 1) extracting and isolating subcellular structures from cells and lysate; 2) analyzing the physiology of individual organelles; and 3) recreating subcellular assembly and functions in vitro, are described. The authors envision that the continued development of single organelle technologies and analyses will serve as a foundation for organelle systems biology and will allow new insight into fundamental and clinically relevant biological questions.

Resolving cargo-motor-track interactions with bifocal parallax single-particle tracking

Resolving coordinated biomolecular interactions in living cellular environments is vital for understanding the mechanisms of molecular nanomachines. The conventional approach relies on localizing and tracking target biomolecules and/or subcellular organelles labeled with imaging probes. However, it is challenging to gain information on rotational dynamics, which can be more indicative of the work done by molecular motors and their dynamic binding status. Herein, a bifocal parallax single-particle tracking method using half-plane point spread functions has been developed to resolve the full-range azimuth angle (0-360°), polar angle, and three-dimensional (3D) displacement in real time under complex living cell conditions. Using this method, quantitative rotational and translational motion of the cargo in a 3D cell cytoskeleton was obtained. Not only were well-known active intracellular transport and free diffusion observed, but new interactions (tight attachment and tethered rotation) were also discovered for better interpretation of the dynamics of cargo-motor-track interactions at various types of microtubule intersections.

Single-Nanoparticle Orientation Sensing by Deep Learning

This paper describes a computational imaging platform to determine the orientation of anisotropic optical probes under differential interference contrast (DIC) microscopy. We established a deep-learning model based on data sets of DIC images collected from metal nanoparticle optical probes at different orientations. This model predicted the in-plane angle of gold nanorods with an error below 20°, the inherent limit of the DIC method. Using low-symmetry gold nanostars as optical probes, we demonstrated the detection of in-plane particle orientation in the full 0-360° range. We also showed that orientation predictions of the same particle were consistent even with variations in the imaging background. Finally, the deep-learning model was extended to enable simultaneous prediction of in-plane and out-of-plane rotation angles for a multibranched nanostar by concurrent analysis of DIC images measured at multiple wavelengths.

Nano-enabled cellular engineering for bioelectric studies

Engineered cells have opened up a new avenue for scientists and engineers to achieve specialized biological functions. Nanomaterials, such as silicon nanowires and quantum dots, can establish tight interfaces with cells either extra- or intracellularly, and they have already been widely used to control cellular functions. The future exploration of nanomaterials in cellular engineering may reveal numerous opportunities in both fundamental bioelectric studies and clinic applications. In this review, we highlight several nanomaterials-enabled non-genetic approaches to fabricating engineered cells. First, we briefly review the latest progress in engineered or synthetic cells, such as protocells that create cell-like behaviors from nonliving building blocks, and cells made by genetic or chemical modifications. Next, we illustrate the need for non-genetic cellular engineering with semiconductors and present some examples where chemical synthesis yields complex morphology or functions needed for biointerfaces. We then provide discussions in detail about the semiconductor nanostructure-enabled neural, cardiac, and microbial modulations. We also suggest the need to integrate tissue engineering with semiconductor devices to carry out more complex functions. We end this review by providing our perspectives for future development in non-genetic cellular engineering.

Kinesin and dynein use distinct mechanisms to bypass obstacles

Kinesin-1 and cytoplasmic dynein are microtubule (MT) motors that transport intracellular cargoes. It remains unclear how these motors move along MTs densely coated with obstacles of various sizes in the cytoplasm. Here, we tested the ability of single and multiple motors to bypass synthetic obstacles on MTs in vitro. Contrary to previous reports, we found that single mammalian dynein is highly capable of bypassing obstacles. Single human kinesin-1 motors fail to avoid obstacles, consistent with their inability to take sideways steps on to neighboring MT protofilaments. Kinesins overcome this limitation when working in teams, bypassing obstacles as effectively as multiple dyneins. Cargos driven by multiple kinesins or dyneins are also capable of rotating around the MT to bypass large obstacles. These results suggest that multiplicity of motors is required not only for transporting cargos over long distances and generating higher forces, but also for maneuvering cargos on obstacle-coated MT surfaces.

Cargos Rotate at Microtubule Intersections during Intracellular Trafficking

Intracellular cargos are transported by molecular motors along actin and microtubules, but how their dynamics depends on the complex structure of the cytoskeletal network remains unclear. In this study, we investigated this longstanding question by measuring simultaneously the rotational and translational dynamics of cargos at microtubule intersections in living cells. We engineered two-faced particles that are fluorescent on one hemisphere and opaque on the other and used their optical anisotropy to report the rotation of cargos. We show that cargos undergo brief episodes of unidirectional and rapid rotation while pausing at microtubule intersections. Probability and amplitude of the cargo rotation depend on the geometry of the intersecting filaments. The cargo rotation is not random motion due to detachment from microtubules, as revealed by statistical analyses of the translational and rotational dynamics. Instead, it is an active rotation driven by motor proteins. Although cargos are known to pause at microtubule intersections, this study reveals a different dimension of dynamics at this seemingly static state and, more importantly, provides direct evidence showing the correlation between cargo rotation and the geometry of underlying microtubule intersections.

An atlas of nano-enabled neural interfaces

Advances in microscopy and molecular strategies have allowed researchers to gain insight into the intricate organization of the mammalian brain and the roles that neurons play in processing information. Despite vast progress, therapeutic strategies for neurological disorders remain limited, owing to a lack of biomaterials for sensing and modulating neuronal signalling in vivo. Therefore, there is a pressing need for developing material-based tools that can form seamless biointerfaces and interrogate the brain with unprecedented resolution. In this Review, we discuss important considerations in material design and implementation, highlight recent breakthroughs in neural sensing and modulation, and propose future directions in neurotechnology research. Our goal is to create an atlas for nano-enabled neural interfaces and to demonstrate how emerging nanotechnologies can interrogate neural systems spanning multiple biological length scales.

Tracking the rotation of single CdS nanorods during photocatalysis with surface plasmon resonance microscopy

Rotational dynamics of anisotropic nanomaterials reveals and regulates their behaviors and functions in diverse fields ranging from nanomotors, biomechanics, and enzymatic catalysis to microrheology. An optical imaging technique that is suitable for all kinds of anisotropic nanoobjects, regardless of its inherent optical property, is thus highly desirable and it is yet to be demonstrated. In the present work, by taking a nonfluorescent and nonplasmonic CdS nanorod as an example, we demonstrate the capability of a recently developed surface plasmon resonance microscopy for determining the orientation of single anisotropic nanomaterials with arbitrary chemical composition and morphology.

While rotational dynamics of anisotropic nanoobjects has often been limited in plasmonic and fluorescent nanomaterials, here we demonstrate the capability of a surface plasmon resonance microscopy (SPRM) to determine the orientation of all kinds of anisotropic nanomaterials. By taking CdS nanorods as an example, it was found that two-dimensional Fourier transform of the asymmetrical wave-like SPRM image resulted in a peak in its angular spectrum in k space. Consistency between the peak angle and the geometrical orientation of the nanorod was validated by both in situ scanning electron microscope characterizations and theoretical calculations. Real-time monitoring of the rotational dynamics of single CdS nanorods further revealed the accelerated rotation under appropriate reaction conditions for photocatalyzed hydrogen generation. The driving force was attributed to the asymmetric production of hydrogen molecules as a result of inhomogeneous distribution of reactive sites within the nanorod. The present work not only builds the experimental and theoretical connections between the orientation of anisotropic nanomaterials and its SPRM images; the general suitability of SPRM also sheds light on broad types of nonfluorescent and nonplasmonic anisotropic nanoobjects from semiconductors to bacteria and viruses.