Near-field scanning optical microscopy, a siren call to biology

Live endothelial cells imaged by Scanning Near-field Optical Microscopy (SNOM): capabilities and challenges

The scanning near-field optical microscopy (SNOM) shows a potential to study details of biological samples, since it provides the optical images of objects with nanometric spatial resolution (50-200 nm) and the topographic information at the same time. The goal of this work is to demonstrate the capabilities of SNOM in transmission configuration to study human endothelial cells and their morphological changes, sometimes very subtle, upon inflammation. Various sample preparations were tested for SNOM measurements and promising results are collected to show: 1) the influence of α tumor necrosis factor (TNF-α) on EA.hy 926 cells (measurements of the fixed cells); 2) high resolution images of various endothelial cell lines, i.e. EA.hy 926 and HLMVEC (investigations of the fixed cells in buffer environment); 3) imaging of live endothelial cells in physiological buffers. The study demonstrate complementarity of the SNOM measurements performed in air and in liquid environments, on fixed as well as on living cells. Furthermore, it is proved that the SNOM is a very useful method for analysis of cellular morphology and topography. Changes in the cell shape and nucleus size, which are the symptoms of inflammatory reaction, were noticed in TNF-α activated EA.hy 926 cells. The cellular structures of submicron size were observed in high resolution optical images of cells from EA.hy 926 and HLMVEC lines.

Extending Near-Field Scanning Optical Microscopy for Biological Studies

Near-field scanning optical microscopy (NSOM) is a scanning probe technique that enables optical measurements to be conducted with nanometric spatial resolution. In addition to high-resolution optical information, NSOM yields a topographic mapping of sample features that enables a direct comparison to be made between surface topography and the optical signal. For the biological sciences, where sophisticated fluorescence labeling protocols have been developed, the simultaneously collected fluorescence and topography information measured with NSOM offers a potentially powerful tool. The progress in implementing NSOM on unfixed, viable samples, however, has been slowed over concerns regarding sample heating and problems associated with damaging forces generated during imaging. Here, we discuss recent measurements that show sample heating to be modest and not limiting for NSOM applications on viable tissues. In addition, we highlight recent work on developing new NSOM probes that have been demonstrated to be amenable with imaging unfixed samples under buffered conditions. These developments now enable the high resolution of NSOM to be applied to areas in the biological sciences that were previously inaccessible. (JALA 2006;11:268–72)

Graphene-Based Platform for Infrared Near-Field Nanospectroscopy of Water and Biological Materials in an Aqueous Environment

Scattering scanning near-field optical microscopy (s-SNOM) has emerged as a powerful nanoscale spectroscopic tool capable of characterizing individual biomacromolecules and molecular materials. However, applications of scattering-based near-field techniques in the infrared (IR) to native biosystems still await a solution of how to implement the required aqueous environment. In this work, we demonstrate an IR-compatible liquid cell architecture that enables near-field imaging and nanospectroscopy by taking advantage of the unique properties of graphene. Large-area graphene acts as an impermeable monolayer barrier that allows for nano-IR inspection of underlying molecular materials in liquid. Here, we use s-SNOM to investigate the tobacco mosaic virus (TMV) in water underneath graphene. We resolve individual virus particles and register the amide I and II bands of TMV at ca. 1520 and 1660 cm(-1), respectively, using nanoscale Fourier transform infrared spectroscopy (nano-FTIR). We verify the presence of water in the graphene liquid cell by identifying a spectral feature associated with water absorption at 1610 cm(-1).

Rapid quantification of live cell receptors using bioluminescence in a flow-based microfluidic device

The number of receptors expressed by cells plays an important role in controlling cell signaling events, thus determining its behaviour, state and fate. Current methods of quantifying receptors on cells are either laborious or do not maintain the cells in their native form. Here, a method integrating highly sensitive bioluminescence, high precision microfluidics and small footprint of lensfree optics is developed to quantify cell surface receptors. This method is safe to use, less laborious, and faster than the conventional radiolabelling and near field scanning methods. It is also more sensitive than fluorescence based assays and is ideal for high throughput screening. In quantifying β(1) adrenergic receptors expressed on the surface of H9c2 cardiomyocytes, this method yields receptor numbers from 3.12 × 10(5) to 9.36 × 10(5) receptors/cell which are comparable with current methods. This can serve as a very good platform for rapid quantification of receptor numbers in ligand/drug binding and receptor characterization studies, which is an important part of pharmaceutical and biological research.

Analysis of light scattering from human breast tissue using a custom dual-optical scanning near-field optical microscope

In this paper we introduce a custom scanning near-field optical microscope (SNOM) that simultaneously collects reflection and transmission near-field images along with topography. This dual-optical SNOM uses a bent probe, which allows for axial reflection imaging, accurate surface scanning, and easy identification of topographic artifacts. Using this novel dual-optical SNOM, we image desiccated and non-desiccated human breast epithelial tissue. By comparing the simultaneous SNOM images, we isolate the effects of tissue morphology and variations in refractive indices on the forward- and back-scattering of light from the tissue. We find that the reduction in back-scattering from tissue, relative to the glass slide, is caused by dense packing of the scattering sites in the cytoplasm (morphology) in the desiccated tissue and a thin-film of water adhering to the glass slide (refractive index) in the non-desiccated tissue sample. Our work demonstrates the potential of our customized dual-optical SNOM system for label-free tissue diagnostics.

Class I MHC molecules as probes of membrane patchiness: from biophysical measurements to modulation of immune responses

Here I summarize decades of work using the biophysics of class I MHC molecules to probe the patchiness and heterogeneity of cell surfaces. This program began as a study of membranes generally. MHC molecules were a convenient probe. However, in recent years, it has become clear that the lateral distribution, clustering, of class I MHC molecules in the membrane affects their recognition by effector CTL. This offers the possibility of enhancing or reducing T-cell recognition of targets by altering the clustering of their membrane proteins.

Near-field optical study of protein transport kinetics at a single nuclear pore

The kinetics of proteins passing through individual nuclear pore complexes (NPCs) of the nuclear envelope (NE) was studied using near-field scanning optical microscopy (NSOM) in combination with fluorescence correlation spectroscopy (FCS). The NSOM probe was placed over a single pore in an unsupported native NE to observe fluorescence-labeled NTF2 moving in the transport channel. A correlation analysis of the arising fluorescence fluctuations enabled us to characterize the translocation as driven by Brownian motion and to determine the related kinetic constants. Though trapped in the pore, NTF2 turned out to be highly mobile within a large axial extension. Our findings support the idea that molecules in transit interact with NPC proteins containing phenylalanine-glycine-repeat domains at the periphery of the channel. NSOM-FCS may help to understand the facilitated translocation in more detail and offers a new way to study single molecule mobility on a nanoscale.

Near-field scanning optical microscopy detects nanoscale glycolipid domains in the plasma membrane

The localization of asialo-GM1 in ordered membrane raft domains in HeLa cells has been examined using a combination of membrane fractionation and fluorescence imaging. The glycolipid is enriched in Triton X-100 insoluble membrane fractions that contain high concentrations of cholesterol and caveolin-1 but is also found in detergent soluble membrane fractions. Near-field fluorescence microscopy shows that a fraction of the asialo-GM1 is localized in small nanoscale clusters that have an upper limit for the average diameter of approximately 90 nm and are partially colocalized with caveolae membrane domains. In addition to clusters, a diffuse, non-clustered population of asialo-GM1 is observed and is hypothesized to correspond to glycolipid isolated in detergent soluble membrane fractions.

Do-it-yourself guide: how to use the modern single-molecule toolkit

Single-molecule microscopy has evolved into the ultimate-sensitivity toolkit to study systems from small molecules to living cells, with the prospect of revolutionizing the modern biosciences. Here we survey the current state of the art in single-molecule tools including fluorescence spectroscopy, tethered particle microscopy, optical and magnetic tweezers, and atomic force microscopy. We also provide guidelines for choosing the right approach from the available single-molecule toolkit for applications as diverse as structural biology, enzymology, nanotechnology and systems biology.

Vertebrate membrane proteins: structure, function, and insights from biophysical approaches

Membrane proteins are key targets for pharmacological intervention because they are vital for cellular function. Here, we analyze recent progress made in the understanding of the structure and function of membrane proteins with a focus on rhodopsin and development of atomic force microscopy techniques to study biological membranes. Membrane proteins are compartmentalized to carry out extra- and intracellular processes. Biological membranes are densely populated with membrane proteins that occupy approximately 50% of their volume. In most cases membranes contain lipid rafts, protein patches, or paracrystalline formations that lack the higher-order symmetry that would allow them to be characterized by diffraction methods. Despite many technical difficulties, several crystal structures of membrane proteins that illustrate their internal structural organization have been determined. Moreover, high-resolution atomic force microscopy, near-field scanning optical microscopy, and other lower resolution techniques have been used to investigate these structures. Single-molecule force spectroscopy tracks interactions that stabilize membrane proteins and those that switch their functional state; this spectroscopy can be applied to locate a ligand-binding site. Recent development of this technique also reveals the energy landscape of a membrane protein, defining its folding, reaction pathways, and kinetics. Future development and application of novel approaches during the coming years should provide even greater insights to the understanding of biological membrane organization and function.

Near-field scanning optical microscopy to identify membrane microdomains

Near-field scanning optical microscopy (NSOM) allows optical imaging with a spatial resolution that is significantly better than the diffraction limited resolution achievable with conventional optical microscopy. NSOM has the potential to study the nanoscale organization of membrane surfaces and ultimately to resolve questions concerning lipid rafts in both model-supported membranes and cellular membranes. Supported phospholipid monolayers and bilayers of phase separated binary and ternary lipid mixtures that model the composition of lipid rafts in natural membranes have been studied by NSOM. The results illustrate the ability of NSOM measurements with 50-100 nm probe apertures to obtain detailed nanoscale information for small, closely spaced domains, and the utility of two-color experiments to probe localization of raft markers in supported membranes.

Characterization of power induced heating and damage in fiber optic probes for near-field scanning optical microscopy

Tip-induced sample heating in near-field scanning optical microscopy (NSOM) is studied for fiber optic probes fabricated using the chemical etching technique. To characterize sample heating from etched NSOM probes, the spectra of a thermochromic polymer sample are measured as a function of probe output power, as was previously reported for pulled NSOM probes. The results reveal that sample heating increases rapidly to approximately 55-60 degrees C as output powers reach approximately 50 nW. At higher output powers, the sample heating remains approximately constant up to the maximum power studied of approximately 450 nW. The sample heating profiles measured for etched NSOM probes are consistent with those previously measured for NSOM probes fabricated using the pulling method. At high powers, both pulled and etched NSOM probes fail as the aluminum coating is damaged. For probes fabricated in our laboratory we find failure occurring at input powers of 3.4+/-1.7 and 20.7+/-6.9 mW for pulled and etched probes, respectively. The larger half-cone angle for etched probes ( approximately 15 degrees for etched and approximately 6 degrees for pulled probes) enables more light delivery and also apparently leads to a different failure mechanism. For pulled NSOM probes, high resolution images of NSOM probes as power is increased reveal the development of stress fractures in the coating at a taper diameter of approximately 6 microm. These stress fractures, arising from the differential heating expansion of the dielectric and the metal coating, eventually lead to coating removal and probe failure. For etched tips, the absence of clear stress fractures and the pooled morphology of the damaged aluminum coating following failure suggest that thermal damage may cause coating failure, although other mechanisms cannot be ruled out.

Detectors for single-molecule fluorescence imaging and spectroscopy

Single-molecule observation, characterization and manipulation techniques have recently come to the forefront of several research domains spanning chemistry, biology and physics. Due to the exquisite sensitivity, specificity, and unmasking of ensemble averaging, single-molecule fluorescence imaging and spectroscopy have become, in a short period of time, important tools in cell biology, biochemistry and biophysics. These methods led to new ways of thinking about biological processes such as viral infection, receptor diffusion and oligomerization, cellular signaling, protein-protein or protein-nucleic acid interactions, and molecular machines. Such achievements require a combination of several factors to be met, among which detector sensitivity and bandwidth are crucial. We examine here the needed performance of photodetectors used in these types of experiments, the current state of the art for different categories of detectors, and actual and future developments of single-photon counting detectors for single-molecule imaging and spectroscopy.

Rafts in oligodendrocytes: Evidence and structure–function relationship

The plasma membrane of eukaryotic cells exhibits lateral inhomogeneities, mainly containing cholesterol and sphingomyelin, which provide liquid-ordered microdomains (lipid "rafts") that segregate membrane components. Rafts are thought to modulate the biological functions of molecules that become associated with them, and as such, they appear to be involved in a variety of processes, including signal transduction, membrane sorting, cell adhesion and pathogen entry. Although still a matter of ongoing debate, evidence in favor of the presence of these microdomains is gradually accumulating but a consensus on issues like their size, lifetime, composition, and biological significance has yet to be reached. Here, we provide an overview of the evidence supporting the presence of rafts in oligodendrocytes, the myelin-producing cells of the central nervous system, and discuss their functional significance. The myelin membrane differs fundamentally from the plasma membrane, both in lipid and protein composition. Moreover, since myelin membranes are unusually enriched in glycosphingolipids, questions concerning the biogenesis and functional relevance of microdomains thus appear of special interest in oligodendrocytes. The current picture of rafts in oligodendrocytes is mainly based on detergent methods. The robustness of such data is discussed and alternative methods that may provide complementary data are indicated.

The significance of lipid composition for membrane activity: New concepts and ways of assessing function

In the last decade or so, it has been realised that membranes do not just have a lipid-bilayer structure in which proteins are embedded or with which they associate. Structures are dynamic and contain areas of heterogeneity which are vital for their formation. In this review, we discuss some of the ways in which these dynamic and heterogeneous structures have implications during stress and in relation to certain human diseases. A particular stress is that of temperature which may instigate adaptation in poikilotherms or appropriate defensive responses during fever in mammals. Recent data emphasise the role of membranes in sensing temperature changes and in controlling a regulatory loop with chaperone proteins. This loop seems to need the existence of specific membrane microdomains and also includes association of chaperone (heat stress) proteins with the membrane. The role of microdomains is then discussed further in relation to various human pathologies such as cardiovascular disease, cancer and neurodegenerative diseases. The concept of modifying membrane lipids (lipid therapy) as a means for treating such pathologies is then introduced. Examples are given when such methods have been shown to have benefit. In order to study membrane microheterogeneity in detail and to elucidate possible molecular mechanisms that account for alteration in membrane function, new methods are needed. In the second part of the review, we discuss ultra-sensitive and ultra-resolution imaging techniques. These include atomic force microscopy, single particle tracking, single particle tracing and various modern fluorescence methods. Finally, we deal with computing simulation of membrane systems. Such methods include coarse-grain techniques and Monte Carlo which offer further advances into molecular dynamics. As computational methods advance they will have more application by revealing the very subtle interactions that take place between the lipid and protein components of membranes - and which are so essential to their function.

Detecting microdomains in intact cell membranes

Current models for cellular plasma membranes focus on spatial heterogeneity and how this heterogeneity relates to cell function. In particular, putative lipid raft membrane domains have been postulated to exist based in large part on the results that a significant fraction of the membrane is detergent insoluble and that molecules facilitating key membrane processes like signal transduction are often found in the detergent-resistant membrane fraction. Yet, the in vivo existence of lipid rafts remains extremely controversial because, despite being sought for more than a decade, evidence for their presence in intact cell membranes is inconclusive. In this review, a variety of experimental techniques that have been or might be used to look for lipid microdomains in intact cell membranes are described. Experimental results are highlighted and the strengths and limitations of different techniques for microdomain identification and characterization are assessed.

Imaging the secretory pathway: The past and future impact of live cell optical techniques

Classically, the secretory pathway has been studied using a combination of electron microscopic, biochemical and genetic approaches. In the last 20 years with the arrival of molecular biology and epitope tagging, fluorescence microscopy has become more important than previously. Moreover, with the common availability of Green Fluorescent Protein (GFP) and confocal microscopes in the last 10 years, live cell imaging has become a major experimental approach. This review highlights the impact of the recent introduction of single-cell quantitative time-lapse imaging and photobleach techniques on the study of the secretory pathway, and the potential impact of those optical techniques which may play a significant future role in the study of the Golgi apparatus and the secretory pathway. Particular attention is paid to techniques (Fluorescence Resonance Energy Transfer, Fluorescence Correlation Spectroscopy) which can monitor protein-protein interactions in living cells.

Membrane domains

Considerable evidence shows that lateral inhomogeneities in lipid composition and physical properties exist in biological membranes. These membrane lipid domains are proposed to play important roles in processes such as signal transduction and membrane traffic. However, there is not at present an adequate description of the nature of these lipid domains in terms of their size, abundance, composition, or dynamics. We discuss the current analyses of the properties and function of membrane domains in cells and compare their properties with chemically simpler model membrane systems that can be understood in greater detail.

Near-field scanning fluorescence microscopy study of ion channel clusters in cardiac myocyte membranes

Near-field scanning optical microscopy (NSOM) has been used to study the nanoscale distribution of voltage-gated L-type Ca2+ ion channels, which play an important role in cardiac function. NSOM fluorescence imaging of immunostained cardiac myocytes (H9C2 cells) demonstrates that the ion channel is localized in small clusters with an average diameter of 100 nm. The clusters are randomly distributed throughout the cell membrane, with some larger fluorescent patches that high-resolution images show to consist of many small closely-spaced clusters. We have imaged unstained cells to assess the contribution of topography-induced artifacts and find that the topography-induced signal is <10% of the NSOM fluorescence intensity. We have also examined the dependence of the NSOM signal intensity on the tip-sample separation to assess the contributions from fluorophores that are significantly below the cell surface. This indicates that chromophores > approximately 200 nm below the probe will have negligible contributions to the observed signal. The ability to quantitatively measure small clusters of ion channels will facilitate future studies that examine changes in protein localization in stimulated cells and during cardiac development. Our work illustrates the potential of NSOM for studying membrane domains and protein localization/colocalization on a length scale which exceeds that available with optical microscopy.

Study of the lignin model compound supramolecular structure by combination of near-field scanning optical microscopy and atomic force microscopy

In this paper, we present a nanoscale study of the supramolecular structure of the dehydrogenate polymer (ZL-DHP) lignin model compound. The combination of near-field scanning optical microscopy (NSOM or SNOM) and atomic force microscopy (AFM) has been utilized to explore physicochemical properties of the lignin model compound on a scale ranging from individual macromolecules to globular supramolecular assemblies. By utilizing NSOM in transmission mode, the optical inhomogeneity in the lignin supramolecular structure has been observed for the first time. In particular, the transmission-mode NSOM images reveal a combination of hollow and layered supramolecular globular structure in the lignin model compound. Through the paired use of TappingMode and pulsed-mode AFM, we have also confirmed the existence of regions with different rheological properties on the single lignin model compound supramolecular assembly.

Heterogeneous molecular distribution in supported multicomponent lipid bilayers

Membrane domains contribute important structural and functional attributes to biological membranes. We describe the heterogeneous nanoscale distribution of lipid molecules within microscale membrane domains in multicomponent lipid bilayers composed of dipalmitoylphosphatidylcholine (DPPC), dilauroylphosphatidylcholine (DLPC), and cholesterol (chol). The lipids were labeled with the fluorescent lipid analogues Bodipy-PC and DiI-C20:0 to identify the distribution of individual membrane components. We used a near-field scanning optical microscope (NSOM) at room temperature to identify the nanoscale structures in the membrane. Simultaneous multicolor NSOM imaging at the emission maxima of the fluorescent analogues revealed a patchy distribution of Bodipy-PC and DiI-C20:0 indicative of phase separations in the bilayer. In a cholesterol-free system (DPPC/DLPC = 1:1), NSOM images proved that the two phosphatidylcholine molecules can coexist in domains at the micrometer level but form nanoscopic patches within the domains; DPPC occurs at the edge of the domains, whereas DLPC is present throughout the domains. In the presence of cholesterol (DPPC/DLPC = 7:3, chol = 18.9%), the two lipid molecules were more miscible but incomplete phase separations also occurred. The average domain sizes were 140-200 nm, well below the resolution capabilities of diffraction-limited light microscopy techniques; the domains were unresolvable by confocal microscopy. Our high-resolution NSOM studies of membrane domain behavior provide a better understanding of complex membrane phase phenomena in multicomponent biological membranes.

Dynamic, yet structured: The cell membrane three decades after the Singer-Nicolson model

The fluid mosaic membrane model proved to be a very useful hypothesis in explaining many, but certainly not all, phenomena taking place in biological membranes. New experimental data show that the compartmentalization of membrane components can be as important for effective signal transduction as is the fluidity of the membrane. In this work, we pay tribute to the Singer-Nicolson model, which is near its 30th anniversary, honoring its basic features, "mosaicism" and "diffusion," which predict the interspersion of proteins and lipids and their ability to undergo dynamic rearrangement via Brownian motion. At the same time, modifications based on quantitative data are proposed, highlighting the often genetically predestined, yet flexible, multilevel structure implementing a vast complexity of cellular functions. This new "dynamically structured mosaic model" bears the following characteristics: emphasis is shifted from fluidity to mosaicism, which, in our interpretation, means nonrandom codistribution patterns of specific kinds of membrane proteins forming small-scale clusters at the molecular level and large-scale clusters (groups of clusters, islands) at the submicrometer level. The cohesive forces, which maintain these assemblies as principal elements of the membranes, originate from within a microdomain structure, where lipid-lipid, protein-protein, and protein-lipid interactions, as well as sub- and supramembrane (cytoskeletal, extracellular matrix, other cell) effectors, many of them genetically predestined, play equally important roles. The concept of fluidity in the original model now is interpreted as permissiveness of the architecture to continuous, dynamic restructuring of the molecular- and higher-level clusters according to the needs of the cell and as evoked by the environment.

Direct imaging of domains in the Lbeta' state of 1,2-dipalmitoylphosphatidylcholine bilayers

A near-field scanning optical microscope was used to study domain formation and evolution in single-component supported lipid bilayers in the gel (L(beta') state. Results on 1,2-dipalmitoylphosphatidylcholine (DPPC) bilayers on glass substrates at room temperature are presented. The domain structure is determined by means of the optical anisotropy of the sample, which arises because DPPC molecules are tilted at theta approximately 32 degrees with respect to the bilayer normal [J. F. Nagle and S. Tristram-Nagle, Biochim. Biophys. Acta 1469, 159 (2000)]. From the measurements we obtain the difference in the index of refraction for the directions parallel and perpendicular to the acyl chains of the lipid molecules, Delta(n)=0.37+/-0.12, in good agreement with calculated and measured values. Direct evidence of the existence of domains in the L(beta') state is provided. These domains, defined as the correlation of the tilt angle theta, are found to be 1-2 microm across. Furthermore, it was found that they are robust under single-lipid-molecule diffusion, remaining unchanged over periods of hundreds of minutes.