Revealing Compartmentalized Diffusion in Living Cells with Interferometric Scattering Microscopy

Electrophoretic Deposition Interferometric Scattering Mass Photometry

Interferometric scattering microscopy (iSCAT) has rapidly developed as a quantitative tool for the label-free detection of single macromolecules and nanoparticles. In practice, this measurement records the interferometric scattering signal of individual nanoparticles in solution as they land and stick on a coverslip, exhibiting an intensity that varies linearly with particle volume and an adsorption rate that reflects the solution-phase transport kinetics of the system. Together, such measurements provide a multidimensional gauge of the particle size and concentration in solution over time. However, the landing kinetics of particles in solution also manifest a measurement frequency limitation imposed by the slow long-range mobility of particle diffusion to the measurement interface. Here we introduce an effective means to overcome the inherent diffusion-controlled sampling limitation of spontaneous mass photometry. We term this methodology electrophoretic deposition interferometric scattering microscopy (EPD-iSCAT). This approach uses a coverslip supporting a conductive thin film of indium tin oxide (ITO). Charging this ITO film to a potential of around +1 V electrophoretically draws charged nanoparticles from solution and binds them in the focal plane of the microscope. Regulating this potential offers a direct means of controlling particle deposition. Thus, we find for a 0.1 nM solution of 50 nm polystyrene nanoparticles that the application of +1 V to an EPD-iSCAT coverslip assembly drives an electrophoretic deposition rate constant of 1.7 s-1 μm-2 nM-1. Removal of the potential causes deposition to cease. This user control of EPD-iSCAT affords a means to apply single-molecule mass photometry to monitor long-term changes in solution, owing to slow kinetic processes. In contrast with conventional coverslips chemically derivatized with charged thin films, EPD-iSCAT maintains a deposition rate that varies linearly with the bulk concentration.

Precise characterization of nanometer-scale systems using interferometric scattering microscopy and Bayesian analysis

Interferometric scattering microscopy can image the dynamics of nanometer-scale systems. The typical approach to analyzing interferometric images involves intensive processing, which discards data and limits the precision of measurements. We demonstrate an alternative approach: modeling the interferometric point spread function and fitting this model to data within a Bayesian framework. This approach yields best-fit parameters, including the particle's three-dimensional position and polarizability, as well as uncertainties and correlations between these parameters. Building on recent work, we develop a model that is parameterized for rapid fitting. The model is designed to work with Hamiltonian Monte Carlo techniques that leverage automatic differentiation. We validate this approach by fitting the model to interferometric images of colloidal nanoparticles. We apply the method to track a diffusing particle in three dimensions, to directly infer the diffusion coefficient of a nanoparticle without calculating a mean-square displacement, and to quantify the ejection of DNA from an individual lambda phage virus, demonstrating that the approach can be used to infer both static and dynamic properties of nanoscale systems.

Implications of different membrane compartmentalization models in particle-based in silico studies

Studying membrane dynamics is important to understand the cellular response to environmental stimuli. A decisive spatial characteristic of the plasma membrane is its compartmental structure created by the actin-based membrane-skeleton (fences) and anchored transmembrane proteins (pickets). Particle-based reaction-diffusion simulation of the membrane offers a suitable temporal and spatial resolution to analyse its spatially heterogeneous and stochastic dynamics. Fences have been modelled via hop probabilities, potentials or explicit picket fences. Our study analyses the different approaches' constraints and their impact on simulation results and performance. Each of the methods comes with its own constraints; the picket fences require small timesteps, potential fences might induce a bias in diffusion in crowded systems, and probabilistic fences, in addition to carefully scaling the probability with the timesteps, induce higher computational costs for each propagation step.

Axial profiling of interferometric scattering enables an accurate determination of nanoparticle size

Interferometric scattering (iSCAT) microscopy has undergone significant development in recent years. It is a promising technique for imaging and tracking nanoscopic label-free objects with nanometer localization precision. The current iSCAT-based photometry technique allows quantitative estimation for the size of a nanoparticle by measuring iSCAT contrast and has been successfully applied to nano-objects smaller than the Rayleigh scattering limit. Here we provide an alternative method that overcomes such size limitations. We take into account the axial variation of iSCAT contrast and utilize a vectorial point spread function model to uncover the position of a scattering dipole and, consequently, the size of the scatterer, which is not limited to the Rayleigh limit. We found that our technique accurately measures the size of spherical dielectric nanoparticles in a purely optical and non-contact way. We also tested fluorescent nanodiamonds (fND) and obtained a reasonable estimate for the size of fND particles. Together with fluorescence measurement from fND, we observed a correlation between the fluorescent signal and the size of fND. Our results showed that the axial pattern of iSCAT contrast provides sufficient information for the size of spherical particles. Our method enables us to measure the size of nanoparticles from tens of nanometers and beyond the Rayleigh limit with nanometer precision, making a versatile all-optical nanometric technique.

DNA-Protein Interactions Studied Directly Using iSCAT Imaging of GNP-Tagged Proteins

Many protein interactions with DNA are reliant on the presence of specific DNA sequences, adducts, or structures. In bulk-phase experiments such DNA features are facile to include in a study. For single-molecule imaging this can be more difficult, because the constraints of the assay limit the variety of adducts that can be used. Surface-immobilized DNA provides an ideal compromise, and the use of interferometric scattering microscopy allows for high-speed imaging of these interactions. Furthermore, this technique offers the ability to identify binder stoichiometry and the composition of protein complexes. Its implementation is relatively simple; however data analysis and deconvolution are more challenging. In this chapter we examine how this technique is implemented and reveal software that can be used to deconvolute the images. Altogether, we hope to make this technique more accessible for studying specific DNA-protein interactions on tailored substrates.

Scattering-based Light Microscopy: From Metal Nanoparticles to Single Proteins

Our ability to detect, image, and quantify nanoscopic objects and molecules with visible light has undergone dramatic improvements over the past few decades. While fluorescence has historically been the go-to contrast mechanism for ultrasensitive light microscopy due to its superior background suppression and specificity, recent developments based on light scattering have reached single-molecule sensitivity. They also have the advantages of universal applicability and the ability to obtain information about the species of interest beyond its presence and location. Many of the recent advances are driven by novel approaches to illumination, detection, and background suppression, all aimed at isolating and maximizing the signal of interest. Here, we review these developments grouped according to the basic principles used, namely darkfield imaging, interferometric detection, and surface plasmon resonance microscopy.

Fast Leaps between Millisecond Confinements Govern Ase1 Diffusion along Microtubules

Diffusion is the most fundamental mode of protein translocation within cells. Confined diffusion of proteins along the electrostatic potential constituted by the surface of microtubules, although modeled meticulously in molecular dynamics simulations, has not been experimentally observed in real-time. Here, interferometric scattering microscopy is used to directly visualize the movement of the microtubule-associated protein Ase1 along the microtubule surface at nanometer and microsecond resolution. Millisecond confinements of Ase1 and fast leaps between these positions of dwelling preferentially occurring along the microtubule protofilaments are resolved, revealing Ase1's mode of diffusive translocation along the microtubule's periodic surface. The derived interaction potential closely matches the tubulin-dimer periodicity and the distribution of the electrostatic potential on the microtubule lattice. It is anticipated that mapping the interaction landscapes for different proteins on microtubules, finding plausible energetic barriers of different positioning and heights, can provide valuable insights into regulating the dynamics of essential cytoskeletal processes, such as intracellular cargo trafficking, cell division, and morphogenesis, all of which rely on diffusive translocation of proteins along microtubules.

Breakdown of the Stokes-Einstein relation in supercooled water: the jump-diffusion perspective

Although water is the most ubiquitous liquid it shows many thermodynamic and dynamic anomalies. Some of the anomalies further intensify in the supercooled regime. While many experimental and theoretical studies have focused on the thermodynamic anomalies of supercooled water, fewer studies explored the dynamical anomalies very extensively. This is due to the intricacy of the experimental measurement of the dynamical properties of supercooled water. Violation of the Stokes-Einstein relation (SER), an important relation connecting the diffusion of particles with the viscosity of the medium, is one of the major dynamical anomalies. In absence of experimentally measured viscosity, researchers used to check the validity of SER indirectly using average translational relaxation time or α-relaxation time. Very recently, the viscosity of supercooled water was accurately measured at a wide range of temperatures and pressures. This allowed direct verification of the SER at different temperature-pressure thermodynamic state points. An increasing breakdown of the SER was observed with decreasing temperature. Increasing pressure reduces the extent of breakdown. Although some well-known theories explained the above breakdown, a detailed molecular mechanism was still elusive. Recently, a translational jump-diffusion (TJD) approach has been able to quantitatively explain the breakdown of the SER in pure supercooled water and an aqueous solution of methanol. The objective of this article is to present a detailed and state-of-the-art analysis of the past and present works on the breakdown of SER in supercooled water with a specific focus on the new TJD approach for explaining the breakdown of the SER.

Information-Efficient, Off-Center Sampling Results in Improved Precision in 3D Single-Particle Tracking Microscopy

In this work, we present a 3D single-particle tracking system that can apply tailored sampling patterns to selectively extract photons that yield the most information for particle localization. We demonstrate that off-center sampling at locations predicted by Fisher information utilizes photons most efficiently. When performing localization in a single dimension, optimized off-center sampling patterns gave doubled precision compared to uniform sampling. A ~20% increase in precision compared to uniform sampling can be achieved when a similar off-center pattern is used in 3D localization. Here, we systematically investigated the photon efficiency of different emission patterns in a diffraction-limited system and achieved higher precision than uniform sampling. The ability to maximize information from the limited number of photons demonstrated here is critical for particle tracking applications in biological samples, where photons may be limited.

Adaptive dynamic range shift (ADRIFT) quantitative phase imaging

Quantitative phase imaging (QPI) with its high-contrast images of optical phase delay (OPD) maps is often used for label-free single-cell analysis. Contrary to other imaging methods, sensitivity improvement has not been intensively explored because conventional QPI is sensitive enough to observe the surface roughness of a substrate that restricts the minimum measurable OPD. However, emerging QPI techniques that utilize, for example, differential image analysis of consecutive temporal frames, such as mid-infrared photothermal QPI, mitigate the minimum OPD limit by decoupling the static OPD contribution and allow measurement of much smaller OPDs. Here, we propose and demonstrate supersensitive QPI with an expanded dynamic range. It is enabled by adaptive dynamic range shift through a combination of wavefront shaping and dark-field QPI techniques. As a proof-of-concept demonstration, we show dynamic range expansion (sensitivity improvement) of QPI by a factor of 6.6 and its utility in improving the sensitivity of mid-infrared photothermal QPI. This technique can also be applied for wide-field scattering imaging of dynamically changing nanoscale objects inside and outside a biological cell without losing global cellular morphological image information.

Fluorescence-Combined Interferometric Scattering Imaging Reveals Nanoscale Dynamic Events of Single Nascent Adhesions in Living Cells

Focal adhesions (FAs) are dynamic protein nanostructures that form mechanical links between cytoskeletal actin fibers and the extracellular matrix. Here, we demonstrate that interferometric scattering (iSCAT) microscopy, a high-speed and time-unlimited imaging technique, can uncover the real-time dynamics of nanoscopic nascent adhesions (NAs). The high sensitivity and stability of the iSCAT signal enabled us to trace the whole life span of each NA spontaneously nucleated under a lamellipodium. Such high-throughput and long-term image data provide a unique opportunity for statistical analysis of adhesion dynamics. Moreover, we directly revealed that FAs play critical roles in both the extrusion of filopodia as nucleation sites on the leading edge and the one-dimensional transport of cargos along cytoskeletal fibers as fiber docking sites. These experimental results show that iSCAT is a sensitive tool for tracking real-time dynamics of nanoscopic objects involved in endogenous and exogenous biological processes in living cells.

High-Precision Protein-Tracking With Interferometric Scattering Microscopy

The mobility of proteins and lipids within the cell, sculpted oftentimes by the organization of the membrane, reveals a great wealth of information on the function and interaction of these molecules as well as the membrane itself. Single particle tracking has proven to be a vital tool to study the mobility of individual molecules and unravel details of their behavior. Interferometric scattering (iSCAT) microscopy is an emerging technique well-suited for visualizing the diffusion of gold nanoparticle-labeled membrane proteins to a spatial and temporal resolution beyond the means of traditional fluorescent labels. We discuss the applicability of interferometric single particle tracking (iSPT) microscopy to investigate the minutia in the motion of a protein through measurements visualizing the mobility of the epidermal growth factor receptor in various biological scenarios on the live cell.

Differential Diffusional Properties in Loose and Tight Docking Prior to Membrane Fusion

Fusion of biological membranes, although mediated by divergent proteins, is believed to follow a common pathway. It proceeds through distinct steps, including docking, merger of proximal leaflets (stalk formation), and formation of a fusion pore. However, the structure of these intermediates is difficult to study because of their short lifetime. Previously, we observed a loosely and tightly docked state preceding leaflet merger using arresting point mutations in SNARE proteins, but the nature of these states remained elusive. Here, we used interferometric scattering (iSCAT) microscopy to monitor diffusion of single vesicles across the surface of giant unilamellar vesicles (GUVs). We observed that the diffusion coefficients of arrested vesicles decreased during progression through the intermediate states. Modeling allowed for predicting the number of tethering SNARE complexes upon loose docking and the size of the interacting membrane patches upon tight docking. These results shed new light on the nature of membrane-membrane interactions immediately before fusion.

Point spread function in interferometric scattering microscopy (iSCAT). Part I: aberrations in defocusing and axial localization

Interferometric scattering (iSCAT) microscopy is an emerging label-free technique optimized for the sensitive detection of nano-matter. Previous iSCAT studies have approximated the point spread function in iSCAT by a Gaussian intensity distribution. However, recent efforts to track the mobility of nanoparticles in challenging speckle environments and over extended axial ranges has necessitated a quantitative description of the interferometric point spread function (iPSF). We present a robust vectorial diffraction model for the iPSF in tandem with experimental measurements and rigorous FDTD simulations. We examine the iPSF under various imaging scenarios to understand how aberrations due to the experimental configuration encode information about the nanoparticle. We show that the lateral shape of the iPSF can be used to achieve nanometric three-dimensional localization over an extended axial range on the order of 10 µm either by means of a fit to an analytical model or calibration-free unsupervised machine learning. Our results have immediate implications for three-dimensional single particle tracking in complex scattering media.

Three-dimensional interferometric scattering microscopy via remote focusing technique

Interferometric scattering (iSCAT) microscopy enables us to track nm-sized objects with high spatial and temporal resolutions and permits label-free imaging of biomolecules. Its superb sensitivity, however, comes at a cost by several downsides, such as slow three-dimensional imaging and limited vertical tracking. Here, we propose a new method, Remote Focusing-iSCAT (RF-iSCAT) microscopy, to visualize a volume specimen by imaging sections at different depths without translation of either the objective lens or sample stage. We demonstrate the principle of RF-iSCAT by determining the z-position of submicrometer beads by translating the reference mirror instead. RF-iSCAT features an unprecedentedly long range of vertical tracking and permits fast but vibration-free vertical scanning. We anticipate that RF-iSCAT would enhance the utility of iSCAT for dynamics study.

Out-of-Plane Nanoscale Reorganization of Lipid Molecules and Nanoparticles Revealed by Plasmonic Spectroscopy

Lipid bilayers assembled on solid substrates have been extensively studied with single-molecule resolution as the constituent molecules diffuse in 2D; however, the out-of-plane motion is typically ignored. Here we present the subnanometer out-of-plane diffusion of nanoparticles attached to hybrid lipid bilayers (HBLs) assembled on metal surfaces. The nanoscale cavity formed between the Au nanoparticle and Au film provides strongly enhanced optical fields capable of locally probing HBLs assembled in the gaps. This allows us to spectroscopically resolve the nanoparticles assembled on bilayers, near edges, and in membrane defects, showing the strong influence of charged lipid rafts. Nanoparticles sitting on the edges of the HBL are observed to flip onto and off of the bilayer, with flip energies of ∼10 meV showing how thermal energies dynamically modify lipid arrangements around a nanoparticle. We further resolve the movement of individual lipid molecules by doping the HBL with low concentrations of Texas Red (TxR) dye-labeled lipids.

Monovalent and Oriented Labeling of Gold Nanoprobes for the High-Resolution Tracking of a Single-Membrane Molecule

Single-molecule tracking is a powerful method to study molecular dynamics in living systems including biological membranes. High-resolution single-molecule tracking requires a bright and stable signal, which has typically been facilitated by nanoparticles due to their superb optical properties. However, there are concerns about using a nanoparticle to label a single molecule because of its relatively large size and the possibility of cross-linking multiple target molecules, both of which could affect the original molecular dynamics. In this work, using various labeling schemes, we investigate the effects using nanoparticles to measure the diffusion of single-membrane molecules. By conjugating a low density of streptavidin (sAv) to gold nanoparticles (AuNPs) of different sizes (10, 15, 20, 30, and 40 nm), we isolate and quantify the effect of the particle size on the diffusion of biotinylated lipids in supported lipid bilayers (SLBs). We find that single sAv tends to cross-link two biotinylated lipids, leading to a much slower diffusion in SLBs. We further demonstrate a simple and robust strategy for the monovalent and oriented labeling of a single lipid molecule with a AuNP by using naturally dimeric rhizavidin (rAv) as a bridge, thus connecting the biotinylated nanoparticle surface and biotinylated target molecule. The rAv-AuNP conjugate demonstrates fast and free diffusion in SLBs (2-3 μm²/s for rAv-AuNP sizes of 10-40 nm), which is comparable to the diffusion of dye-labeled lipids, indicating that the adverse size and cross-linking effects are successfully avoided. We also note that the diffusion of dye-labeled lipids critically depends on the choice of dye, which could report different diffusion coefficients by about 20% (2.2 μm²/s of ATTO647N and 2.6 μm²/s of ATTO532). By comparing the diffusion of the uniformly and randomly oriented labeling of a single lipid molecule with a AuNP, we conclude that oriented labeling is favorable for measuring the diffusion of single-membrane molecules. Our work shows that the measured diffusion of the membrane molecule is highly sensitive to the molecular design of the cross-linker for labeling. The demonstrated approach of monovalent and oriented AuNP labeling provides the opportunity to study single-molecule membrane dynamics at much higher spatiotemporal resolutions and, most importantly, without labeling artifacts.

Interferometric Scattering Microscopy: Seeing Single Nanoparticles and Molecules via Rayleigh Scattering

Fluorescence microscopy has been the workhorse for investigating optical phenomena at the nanometer scale but this approach confronts several fundamental limits. As a result, there have been a growing number of activities toward the development of fluorescent-free imaging methods. In this Mini Review, we demonstrate that elastic scattering, the most ubiquitous and oldest optical contrast mechanism, offers excellent opportunities for sensitive detection and imaging of nanoparticles and molecules at very high spatiotemporal resolution. We present interferometric scattering (iSCAT) microscopy as the method of choice, explain its theoretical foundation, discuss its experimental nuances, elaborate on its deep connection to bright-field imaging and other established microscopies, and discuss its promise as well as challenges. A showcase of numerous applications and avenues made possible by iSCAT demonstrates its rapidly growing impact on various disciplines concerned with nanoscopic phenomena.

Cytoplasmic Protein Imaging with Mid-Infrared Photothermal Microscopy: Cellular Dynamics of Live Neurons and Oligodendrocytes

Mid-infrared photothermal microscopy has been suggested as an alternative to conventional infrared microscopy because in addition to the inherent chemical contrast available upon vibrational excitation, it can feasibly achieve spatial resolution at the submicrometer level. Furthermore, it has substantial potential for real-time bioimaging for the observation of cellular dynamics without photodamage or photobleaching of fluorescent labels. We performed real-time imaging of oligodendrocytes to investigate cellular dynamics throughout the life cycle of a cell, revealing details of cell division and apoptosis, as well as cellular migration. In the case of live neurons, we observed a photothermal contrast associated with traveling protein complexes on an axon, which correspond to the transport of vesicles from the cell body to the dendritic branches of the neuron through the cytoskeleton. We anticipate that mid-infrared photothermal imaging will be of great use for gaining insights into the field of biophysical science, especially with regard to cellular dynamics and functions.

Interferometric Scattering Microscopy

Interferometric scattering microscopy (iSCAT) is an extremely sensitive imaging method based on the efficient detection of light scattered by nanoscopic objects. The ability to, at least in principle, maintain high imaging contrast independent of the exposure time or the scattering cross section of the object allows for unique applications in single-particle tracking, label-free imaging of nanoscopic (dis)assembly, and quantitative single-molecule characterization. We illustrate these capabilities in areas as diverse as mechanistic studies of motor protein function, viral capsid assembly, and single-molecule mass measurement in solution. We anticipate that iSCAT will become a widely used approach to unravel previously hidden details of biomolecular dynamics and interactions.