Tracking movements of lipids and Thy1 molecules in the plasmalemma of living fibroblasts by fluorescence video microscopy with nanometer scale precision

Non-invasive study of the three-dimensional structure of nanoporous triblock terpolymer membranes

Nanoporous media are of great importance for drug delivery or filtration. Typically the pore structure of such media is characterized using high-resolution techniques such as electron microscopy or atomic force microscopy. However, these techniques are restricted to the surface of the material and/or are highly invasive. In a proof-of-concept experiment we have employed three-dimensional single-particle orbit tracking for testing the three-dimensional pore structure of a liquid filled nanoporous polystyrene-block-polyisoprene-block-poly(N-isopropylacrylamide) (PS-b-PI-b-PNiPAAm) triblock terpolymer membrane. Using fluorescent tracers with a diameter of about 10% of the relevant void structures, the tracking experiments yielded results that were comparable to those obtained from reference experiments using environmental scanning electron microscopy (eSEM). This testifies that single-particle orbit tracking can serve as a useful non-invasive alternative for characterising the structure of nanoporous materials.

Fluorescent Polymer Nanoparticles Based on Dyes: Seeking Brighter Tools for Bioimaging

Speed, resolution and sensitivity of today's fluorescence bioimaging can be drastically improved by fluorescent nanoparticles (NPs) that are many-fold brighter than organic dyes and fluorescent proteins. While the field is currently dominated by inorganic NPs, notably quantum dots (QDs), fluorescent polymer NPs encapsulating large quantities of dyes (dye-loaded NPs) have emerged recently as an attractive alternative. These new nanomaterials, inspired from the fields of polymeric drug delivery vehicles and advanced fluorophores, can combine superior brightness with biodegradability and low toxicity. Here, we describe the strategies for synthesis of dye-loaded polymer NPs by emulsion polymerization and assembly of pre-formed polymers. Superior brightness requires strong dye loading without aggregation-caused quenching (ACQ). Only recently several strategies of dye design were proposed to overcome ACQ in polymer NPs: aggregation induced emission (AIE), dye modification with bulky side groups and use of bulky hydrophobic counterions. The resulting NPs now surpass the brightness of QDs by ≈10-fold for a comparable size, and have started reaching the level of the brightest conjugated polymer NPs. Other properties, notably photostability, color, blinking, as well as particle size and surface chemistry are also systematically analyzed. Finally, major and emerging applications of dye-loaded NPs for in vitro and in vivo imaging are reviewed.

Measuring a diffusion coefficient by single-particle tracking: statistical analysis of experimental mean squared displacement curves

We provide experimental results on the accuracy of diffusion coefficients obtained by a mean squared displacement (MSD) analysis of single-particle trajectories. We have recorded very long trajectories comprising more than 1.5 × 10(5) data points and decomposed these long trajectories into shorter segments providing us with ensembles of trajectories of variable lengths. This enabled a statistical analysis of the resulting MSD curves as a function of the lengths of the segments. We find that the relative error of the diffusion coefficient can be minimized by taking an optimum number of points into account for fitting the MSD curves, and that this optimum does not depend on the segment length. Yet, the magnitude of the relative error for the diffusion coefficient does, and achieving an accuracy in the order of 10% requires the recording of trajectories with about 1000 data points. Finally, we compare our results with theoretical predictions and find very good qualitative and quantitative agreement between experiment and theory.

Setup for single-particle orbit tracking: artifacts and corrections

We report on an experimental setup for single-particle orbit tracking, which allows following fluorescent nanoparticles for more than 10 min with a temporal resolution of 4 ms and a dynamic position accuracy of better than 10 nm. On a model sample--20 nm sized fluorescent polymer beads in glycerol--we will illustrate how artifacts caused by unavoidable experimental shortcomings (might) obscure the experimental result and how misinterpretations can be prevented.

Dynamic partitioning of a glycosyl-phosphatidylinositol-anchored protein in glycosphingolipid-rich microdomains imaged by single-quantum dot tracking

Recent experimental developments have led to a revision of the classical fluid mosaic model proposed by Singer and Nicholson more than 35 years ago. In particular, it is now well established that lipids and proteins diffuse heterogeneously in cell plasma membranes. Their complex motion patterns reflect the dynamic structure and composition of the membrane itself, as well as the presence of the underlying cytoskeleton scaffold and that of the extracellular matrix. How the structural organization of plasma membranes influences the diffusion of individual proteins remains a challenging, yet central, question for cell signaling and its regulation. Here we have developed a raft-associated glycosyl-phosphatidyl-inositol-anchored avidin test probe (Av-GPI), whose diffusion patterns indirectly report on the structure and dynamics of putative raft microdomains in the membrane of HeLa cells. Labeling with quantum dots (qdots) allowed high-resolution and long-term tracking of individual Av-GPI and the classification of their various diffusive behaviors. Using dual-color total internal reflection fluorescence (TIRF) microscopy, we studied the correlation between the diffusion of individual Av-GPI and the location of glycosphingolipid GM1-rich microdomains and caveolae. We show that Av-GPI exhibit a fast and a slow diffusion regime in different membrane regions, and that slowing down of their diffusion is correlated with entry in GM1-rich microdomains located in close proximity to, but distinct, from caveolae. We further show that Av-GPI dynamically partition in and out of these microdomains in a cholesterol-dependent manner. Our results provide direct evidence that cholesterol-/sphingolipid-rich microdomains can compartmentalize the diffusion of GPI-anchored proteins in living cells and that the dynamic partitioning raft model appropriately describes the diffusive behavior of some raft-associated proteins across the plasma membrane.

Chromatin dynamics in interphase cells revealed by tracking in a two-photon excitation microscope

Increasing evidence points to a dynamical compartmentalization of the cell nucleus, yet the mechanisms by which interphase chromatin moves and is positioned within nuclei remain unclear. Here, we study the dynamics of chromatin in vivo applying a novel particle-tracking method in a two-photon microscope that provides approximately 10-fold higher spatial and temporal resolutions than previous measurements. We followed the motion of a chromatin sequence containing a lac-operator repeat in cells stably expressing lac repressor fused with enhanced green fluorescent protein, observing long periods of apparent constrained diffusion interrupted by relatively abrupt jumps of approximately 150 nm lasting 0.3-2 s. During these jumps, the particle moved an average of four times faster than in the periods between jumps and in paths more rectilinear than predicted for random diffusion motion. Additionally, the jumps were sensitive to the temperature and absent after ATP depletion. These experimental results point to an energy-dependent mechanism driving fast motion of chromatin in interphase cells.

3-D particle tracking in a two-photon microscope: application to the study of molecular dynamics in cells

We developed a method for tracking particles in three dimensions designed for a two-photon microscope, which holds great promise to study cellular processes because of low photodamage, efficient background rejection, and improved depth discrimination. During a standard cycle of the tracking routine (32 ms), the laser beam traces four circular orbits surrounding the particle in two z planes above and below the particle. The radius of the orbits is half of the x,y-width of the point spread function, and the distance between the z planes is the z-width of the point spread function. The z-position is adjusted by moving the objective with a piezoelectric-nanopositioner. The particle position is calculated on the fly from the intensity profile obtained during the cycle, and these coordinates are used to set the scanning center for the next cycle. Applying this method, we were able to follow the motion of 500-nm diameter fluorescent polystyrene microspheres moved by a nanometric stage in either steps of 20-100 nm or sine waves of 0.1-10 microm amplitude with 20 nm precision. We also measured the diffusion coefficient of fluorospheres in glycerol solutions and recovered the values expected according to the Stokes-Einstein relationship for viscosities higher than 3.7 cP. The feasibility of this method for live cell measurements is demonstrated studying the phagocytosis of protein-coated fluorospheres by fibroblasts.

Quantitative intercomparison of transmission electron microscopy, flow cytometry, and epifluorescence microscopy for nanometric particle analysis

Nanometric biological particles such as viruses have received increased attention in a wide range of scientific fields. Evaluation of viral contributions to environmental processes and the use of viruses in medical applications such as gene therapy require viruses to be routinely and accurately enumerated. There are a variety of existing techniques for counting viruses, namely, plaque assays, transmission electron microscopy (TEM), epifluorescence microscopy (EFM), and flow cytometry (FCM); each has advantages and disadvantages. While there have been attempts to intercompare some of these techniques to determine the most effective means to count viruses, no previous study used a technique-independent standard for quantitative comparison of collection efficiency, accuracy, and precision. In this work, polystyrene nanospheres were used as standards for the intercomparison of performance characteristics for TEM, EFM, FCM, as well as a custom-built flow cytometer (the Single Nanometric Particle Enumerator, SNaPE). EFM and SNaPE exhibited the highest degree of accuracy and precision, with particle concentrations deviating < or =5% from true and relative errors less than half that of TEM, EFM and SNaPE are also significantly more time and cost efficient than TEM.

Rapid enumeration of respiratory viruses

Virus detection and enumeration has become increasingly important in fields ranging from medicine and biotechnology to environmental science. Although there are a wide variety of techniques that can be used to count viruses, there is demand for a rapid and more accurate means for virus enumeration. In this work, the performance of a flow cytometer that was designed and custom-built specifically for rapid detection of single viruses was evaluated. The instrument, designated a single nanometric particle enumerator (SNaPE), was characterized and calibrated using fluorescent polystyrene nanospheres. The reliability of the instrument with respect to virus enumeration was demonstrated for three medically relevant viruses, adenovirus-5, respiratory syncytial virus, and influenza A, treated with a fluorescent nucleotide stain. In each case, the SNaPE yielded a virus particle concentration consistent with, but slightly lower than, transmission electron microscopy (TEM) results, as expected. In addition, on the basis of calibration of signal intensity, the average peak height for a given virus was correlated with genome size, as expected. In contrast to time-consuming analyses such as TEM and plaque titers, SNaPE analysis of pure virus samples (including sample handling, data collection, and data processing) can be completed within 1 h.

Deformation-enhanced fluctuations in the red cell skeleton with theoretical relations to elasticity, connectivity, and spectrin unfolding

To assess local elasticity in the red cell's spectrin-actin network, nano-particles were tethered to actin nodes and their constrained thermal motions were tracked. Cells were both immobilized and controllably deformed by aspiration into a micropipette. Since the network is well-appreciated as soft, thermal fluctuations even in an unstressed portion of network were expected to be many tens of nanometers based on simple equipartition ideas. Real-time particle tracking indeed reveals such root-mean-squared motions for 40-nm fluorescent beads either tethered to actin directly within a cell ghost or connected to actin from outside a cell via glycophorin. Moreover, the elastically constrained displacements are significant on the scale of the network's internodal distance of approximately 60-80 nm. Surprisingly, along the aspirated projection-where the network is axially extended by as much as twofold or more-fluctuations in the axial direction are increased by almost twofold relative to motions in the unstressed network. The molecular basis for such strain softening is discussed broadly in terms of force-driven transitions. Specific considerations are given to 1) protein dissociations that reduce network connectivity, and 2) unfolding kinetics of a localized few of the red cell's approximately 10(7) spectrin repeats.

Lateral mobility and anchoring of recombinant GABAA receptors depend on subunit composition

The clustering of type A gamma-aminobutyric acid receptors (GABA(A)R) at discrete and functionally significant domains on the nerve cell surface is an important determinant in the integration of synaptic inputs. To discern the role that the subunits of the GABA(A)R play in determining the receptor's cell surface topography and mobility, the alpha1, beta1, beta3, and gamma2s subunits were transfected into COS7, HEK293, and PC12 cells and the distribution and cell surface mobility of these recombinant receptors were examined. Our results show that alpha1 subunits are retained in the endoplasmic reticulum while beta1 and beta3 subunits are sorted to the plasma membrane where they form clusters. Co-expression and co-assembly of alpha1 and beta3 subunits result in the rescue of intracellular alpha1 subunits, which are transported as alphabeta subunit complexes to the cell surface where they formed clusters. Fluorescence photobleach recovery and single particle tracking of recombinant receptors show that, despite clustering, beta3 subunit homooligomers are mobile within a cell surface domain. Inclusion of alpha1 in beta3 or beta3gamma2s complexes, however, dramatically reduces the receptor's lateral mobility in COS 7 and PC12 cells and anchors GABA(A)Rs on the cell surface, suggesting the formation of a direct link to a component of the cytoskeleton. The mobility of recombinant receptors that include the alpha1 subunit mirrors the mobility of GABA(A)Rs on cell bodies and dendrites of cortical and spinal cord neurons. The results suggest that incorporation of alpha1 subunits give rise to a population of GABA(A)Rs that are immobilized on the cell surface.

Assays using digital fluorescence: 1985-1998

Luminescence continues to provide comprehensive literature surveys which will be published in most issues. These are a continuation of the literature surveys begun in 1986 in the Journal of Bioluminescence and Chemiluminescence which, up until 1998, encompassed more than 6000 references cited by year or specialized topic. With this newly named journal these searches are expanding to reflect the journal's wider scope. In future we will cover all fundamental and applied aspects of biological and chemical luminescence and include not only bioluminescence and chemiluminescence but also fluorescence, time resolved fluorescence, electrochemiluminescence, phosphorescence, sonoluminescence, lyoluminescence and triboluminescence. The compilers would be pleased to receive any comments from the readership. Contact by e-mail: L.J. Kricka: larry_kricka@path1a.med.upenn.edu or P.E. Stanley: Stanley@LUMIWEB.COM Copyright 1999 John Wiley & Sons, Ltd.

Anomalous diffusion of major histocompatibility complex class I molecules on HeLa cells determined by single particle tracking

Single-particle tracking (SPT) was used to determine the mobility characteristics of MHC (major histocompatibility complex) class I molecules at the surface of HeLa cells at 22 degrees C and on different time scales. MHC class I was labeled using the Fab fragment of a monoclonal antibody (W6/32), covalently bound to either R-phycoerythrin or fluorescent microspheres, and the particles were tracked using high-sensitivity fluorescence imaging. Analysis of the data for a fixed time interval suggests a reasonable fit to a random diffusion model. The best fit values of the diffusion coefficient D decreased markedly, however, with increasing time interval, demonstrating the existence of anomalous diffusion. Further analysis of the data shows that the diffusion is anomalous over the complete time range investigated, 4-300 s. Fitting the results obtained with the R-phycoerythrin probe to D = D0talpha-1, where Do is a constant and t is the time, gave D0 = (6.7 +/- 4.5) x 10(-11) cm2 s-1 and alpha = 0.49 +/- 0.16. Experiments with fluorescent microspheres were less reproducible and gave slower anomalous diffusion. The R-phycoerythrin probe is considered more reliable for fluorescent SPT because it is small (11 x 8 nm) and monovalent. The type of motion exhibited by the class I molecules will greatly affect their ability to migrate in the plane of the membrane. Anomalous diffusion, in particular, greatly reduces the distance a class I molecule can travel on the time scale of minutes. The present data are discussed in relation to the possible role of diffusion and clustering in T-cell activation.

Structural mosaicism on the submicron scale in the plasma membrane

The lateral mobility of the neural cell adhesion molecule (NCAM) was examined using single particle tracking (SPT). Various isoforms of human NCAM, differing in their ectodomain, their membrane anchorage mode, or the size of their cytoplasmic domain, were expressed in National Institutes of Health 3T3 cells and C2C12 muscle cells. On a 6.6-s time scale, SPT measurements on both transmembrane and glycosylphosphatidylinositol (GPI) anchored isoforms of NCAM expressed in 3T3 cells could be classified into mobile (Brownian diffusion), slow diffusion, corralled diffusion, and immobile subpopulations. On a 90-s time scale, SPT studies in C2C12 cells revealed that 40-60% of transfected NCAM was mobile, whereas a smaller fraction (approximately 10-30%) experienced much slower diffusion. In addition, a fraction of approximately 30% of both transfected GPI and transmembrane isoforms and endogenous NCAM isoforms in C2C12 cells experienced transient confinement for approximately 8 s within regions of approximately 300-nm diameter. Diffusion within both these and the slow diffusion regions was anomalous, consistent with movements through a dense field of obstacles, whereas diffusion outside these regions was normal. Thus the membrane appears as a mosaic containing regions that permit free diffusion as well as regions in which NCAM is transiently confined to small or more extended domains. These results, including a large, freely diffusing fraction, similar confinement of transmembrane and GPI isoforms, a significant slowly diffusing fraction, and relatively large interdomain distances, are at some variance with the membrane skeleton fence model (Kusumi and Sako, 1996). Possible revisions to the model that incorporate these data are discussed.

Single-particle tracking: the distribution of diffusion coefficients

In single-particle tracking experiments, the diffusion coefficient D may be measured from the trajectory of an individual particle in the cell membrane. The statistical distribution of single-trajectory diffusion coefficients is examined by Monte Carlo calculations. The width of this distribution may be useful as a measure of the heterogeneity of the membrane and as a test of models of hindered diffusion in the membrane. For some models, the distribution of the short-range diffusion coefficient is much narrower than the observed distribution for proteins diffusing in cell membranes. To aid in the analysis of single-particle tracking measurements, the distribution of D is examined for various definitions of D and for various trajectory lengths.

Single-particle tracking: applications to membrane dynamics

Measurements of trajectories of individual proteins or lipids in the plasma membrane of cells show a variety of types of motion. Brownian motion is observed, but many of the particles undergo non-Brownian motion, including directed motion, confined motion, and anomalous diffusion. The variety of motion leads to significant effects on the kinetics of reactions among membrane-bound species and requires a revision of existing views of membrane structure and dynamics.

Single particle tracking of cell-surface HLA-DR molecules using R-phycoerythrin labeled monoclonal antibodies and fluorescence digital imaging

The mobility of cell surface MHC molecules and their ability to form dynamic associations may be related to the physiological status of the cell and to the potential to bind effector T lymphocytes. To investigate these properties, we have prepared HLA DR specific monoclonal antibodies coupled in a 1:1 mole ratio to the fluorescent phycobiliprotein, R-phycoerythrin (PE). We show that these small particles can be sequentially imaged using a cooled slow-scan charge coupled device camera and hence can be used for single particle tracking experiments. We have applied this technique to investigate the movements of HLA DR molecules on fibroblasts transfected with human DR alpha and DR beta genes. PE-IgG was bound to the transfected fibroblasts and particle tracks were obtained by sequential imaging over a period of typically 30 minutes. Analysis of particle tracks revealed the presence of directed motion and domain-limited diffusion in addition to random diffusion. The contributions of these three types of motion showed cell to cell variability. Velocities of directed motion were of the order of 2 nm second-1 whilst domain diameters were in the range 200–800 nm. Diffusion coefficients for random diffusion were in the range 1 × 10(−13)-5 × 10(−12) cm2 second-1. The higher mobilities were observed for the lower intensity fluorescent spots, which possibly correspond to images of single particles. Much lower mobility was observed with a cell where the spot intensities were approximately double that of the lower intensity spots. These spots could be images of double particles implying the association of at least two HLA DR alpha beta dimers. These data are relevant to the study of MHC class II cell surface redistribution and antigen presentation in specific immunity.

Single-particle tracking: effects of corrals

Structural proteins of the membrane skeleton are thought to form "corrals" at the membrane surface, and these corrals may restrict lateral diffusion of membrane proteins. Recent experimental developments in single-particle tracking and laser trapping make it possible to examine the corral model in detail. Techniques to interpret these experiments are presented. First, escape times for a diffusing particle in a corral are obtained from Monte Carlo calculations and analytical solutions for various corral sizes, shapes, and escape probabilities, and reduced to a common curve. Second, the identification of corrals in tracking experiments is considered. The simplest way to identify corrals is by sight. If the walls are impermeable enough, a trajectory fills the corral before the diffusing particle escapes. The fraction of distinct sites visited before escape is calculated for corrals of various sizes, shapes, and escape probabilities, and reduced to a common curve. This fraction is also a measure of the probability that the diffusing species will react with another species in the corral before escaping. Finally, the effect of the sampling interval on the measurement of the short-range diffusion coefficient is examined.