Fast molecular tracking maps nanoscale dynamics of plasma membrane lipids

Dynamic organizing principles of the plasma membrane that regulate signal transduction: commemorating the fortieth anniversary of Singer and Nicolson's fluid-mosaic model

The recent rapid accumulation of knowledge on the dynamics and structure of the plasma membrane has prompted major modifications of the textbook fluid-mosaic model. However, because the new data have been obtained in a variety of research contexts using various biological paradigms, the impact of the critical conceptual modifications on biomedical research and development has been limited. In this review, we try to synthesize our current biological, chemical, and physical knowledge about the plasma membrane to provide new fundamental organizing principles of this structure that underlie every molecular mechanism that realizes its functions. Special attention is paid to signal transduction function and the dynamic aspect of the organizing principles. We propose that the cooperative action of the hierarchical three-tiered mesoscale (2-300 nm) domains--actin-membrane-skeleton induced compartments (40-300 nm), raft domains (2-20 nm), and dynamic protein complex domains (3-10 nm)--is critical for membrane function and distinguishes the plasma membrane from a classical Singer-Nicolson-type model.

Three-dimensional real-time tracking of nanoparticles at an oil-water interface

Single-particle tracking with real-time feedback control can be used to study three-dimensional nanoparticle transport dynamics. We apply the method to study the behavior of adsorbed nanoparticles at a silicone oil-water interface in a microemulsion system over a range of particles sizes from 24 nm to 2000 nm. The diffusion coefficient of large particles (>200 nm) scales inversely with particle size, while smaller particles exhibit an unexpected increase in drag force at the interface. The technique can be applied in the future to study three-dimensional dynamics in a range of systems, including complex fluids, gels, biological cells, and geological media.

Measuring the size and charge of single nanoscale objects in solution using an electrostatic fluidic trap

Measuring the size and charge of objects suspended in solution, such as dispersions of colloids or macromolecules, is a significant challenge. Measurements based on light scattering are inherently biased to larger entities, such as aggregates in the sample, because the intensity of light scattered by a small object scales as the sixth power of its size. Techniques that rely on the collective migration of species in response to external fields (electric or hydrodynamic, for example) are beset with difficulties including low accuracy and dispersion-limited resolution. Here, we show that the size and charge of single nanoscale objects can be directly measured with high throughput by analysing their thermal motion in an array of electrostatic traps. The approach, which is analogous to Millikan's oil drop experiment, could in future be used to detect molecular binding events with high sensitivity or carry out dynamic single-charge resolved measurements at the solid/liquid interface.

Raf inhibitors target ras spatiotemporal dynamics

BACKGROUND

The lateral segregation of Ras proteins into transient plasma membrane nanoclusters is essential for high-fidelity signal transmission by the Ras mitogen-activated protein kinase (MAPK) cascade. In this spatially constrained signaling system, the dynamics of Ras nanocluster assembly and disassembly control MAPK signal output.

RESULTS

We show here that BRaf inhibitors paradoxically activate CRaf and MAPK signaling in Ras transformed cells by profoundly dysregulating Ras nanocluster dynamics. Specifically, BRaf inhibitors selectively enhance the plasma membrane nanoclustering of oncogenic K-Ras and N-Ras but have no effect on H-Ras nanoclustering. Raf inhibitors are known to drive the formation of stable BRaf-CRaf and CRaf-CRaf dimers. Our results demonstrate that the presence of two Ras-binding domains in a single Raf dimer is sufficient and required to increase Ras nanoclustering, indicating that Raf dimers promote K- and N-Ras nanocluster formation by crosslinking constituent Ras proteins. Ras crosslinking increases the fraction of K-Ras and N-Ras in their cognate nanoclusters, leading to an increase in MAPK output from the plasma membrane. Intriguingly, increased MAPK signaling in BRaf inhibited cells is accompanied by significantly decreased Akt activation. We show that this signal pathway crosstalk results from a novel mechanism of competition between stabilized Raf dimers and p110α for recruitment to Ras nanoclusters.

CONCLUSIONS

Our findings reveal that BRaf inhibitors disrupt Ras nanocluster dynamics with significant, yet divergent, consequences for MAPK and PI3K signaling.

Archipelago architecture of the focal adhesion: membrane molecules freely enter and exit from the focal adhesion zone

The focal adhesion (FA) is an integrin-based structure built in/on the plasma membrane, mechanically linking the extracellular matrix with the termini of actin stress fibers, providing key scaffolds for the cells to migrate in tissues. The FA was considered as a micron-scale, massive assembly of various proteins, although its formation and decomposition occur quickly in several to several 10 s of minutes. The mechanism of rapid FA regulation has been a major mystery in cell biology. Here, using fast single fluorescent-molecule imaging, we found that transferrin receptor and Thy1, non-FA membrane proteins, readily enter the FA zone, diffuse rapidly there, and exit into the bulk plasma membrane. Integrin β3 also readily enters the FA zone, and repeatedly undergoes temporary immobilization and diffusion in the FA zone, whereas approximately one-third of integrin β3 is immobilized there. These results are consistent with the archipelago architecture of the FA, which consists of many integrin islands: the membrane molecules enter the inter-island channels rather freely, and the integrins in the integrin islands can be rapidly exchanged with those in the bulk membrane. Such an archipelago architecture would allow rapid FA formation and disintegration, and might be applicable to other large protein domains in the plasma membrane.

Mechanisms underlying the confined diffusion of cholera toxin B-subunit in intact cell membranes

Multivalent glycolipid binding toxins such as cholera toxin have the capacity to cluster glycolipids, a process thought to be important for their functional uptake into cells. In contrast to the highly dynamic properties of lipid probes and many lipid-anchored proteins, the B-subunit of cholera toxin (CTxB) diffuses extremely slowly when bound to its glycolipid receptor GM(1) in the plasma membrane of living cells. In the current study, we used confocal FRAP to examine the origins of this slow diffusion of the CTxB/GM(1) complex at the cell surface, relative to the behavior of a representative GPI-anchored protein, transmembrane protein, and fluorescent lipid analog. We show that the diffusion of CTxB is impeded by actin- and ATP-dependent processes, but is unaffected by caveolae. At physiological temperature, the diffusion of several cell surface markers is unchanged in the presence of CTxB, suggesting that binding of CTxB to membranes does not alter the organization of the plasma membrane in a way that influences the diffusion of other molecules. Furthermore, diffusion of the B-subunit of another glycolipid-binding toxin, Shiga toxin, is significantly faster than that of CTxB, indicating that the confined diffusion of CTxB is not a simple function of its ability to cluster glycolipids. By identifying underlying mechanisms that control CTxB dynamics at the cell surface, these findings help to delineate the fundamental properties of toxin-receptor complexes in intact cell membranes.

Obstructed diffusion propagator analysis for single-particle tracking

We describe a method for the analysis of the distribution of displacements, i.e., the propagators, of single-particle tracking measurements for the case of obstructed subdiffusion in two-dimensional membranes. The propagator for the percolation cluster is compared with a two-component mobility model against Monte Carlo simulations. To account for diffusion in the presence of obstacle concentrations below the percolation threshold, a propagator that includes the transient motion in finite percolation clusters and hopping between obstacle-induced compartments is derived. Finally, these models are shown to be effective in the analysis of Kv2.1 channel diffusive measurements in the membrane of living mammalian cells.

Nanoscale localization sampling based on nanoantenna arrays for super-resolution imaging of fluorescent monomers on sliding microtubules

Sub-diffraction-limited imaging of fluorescent monomers on sliding microtubules in vitro by nanoscale localization sampling (NLS) is reported. NLS is based on periodic nanohole antenna arrays that create locally amplified electromagnetic hot spots through surface plasmon localization. The localized near-field hot spot temporally samples microtubular movement for enhanced spatial resolution. A fourfold improvement in spatial resolution compared to conventional wide-field microscopy is demonstrated. The resolution enhancement is achieved by imaging rhodamine-labeled microtubules that are sampled by the hot spots to provide sub-diffraction-limited images at 76 nm resolution in the direction of movement and 135 nm orthogonally. The intensity distribution produced by the NLS is measured to be broader than that of conventional imaging, which is consistent with the improvement of imaging resolution. Correlation studies between neighboring nanoantennas are also performed. This confirms the possibility of measuring microtubular transport dynamics. NLS can be useful for moving objects that have a high labeling density or for performing fluctuation spectroscopy in small volumes, and may allow "super-resolution on demand" by customizing nanoantenna structures for specific resolution needs.

Coupling exo- and endocytosis: an essential role for PIP₂ at the synapse

Chemical synapses are specialist points of contact between two neurons, where information transfer takes place. Communication occurs through the release of neurotransmitter substances from small synaptic vesicles in the presynaptic terminal, which fuse with the presynaptic plasma membrane in response to neuronal stimulation. However, as neurons in the central nervous system typically only possess ~200 vesicles, high levels of release would quickly lead to a depletion in the number of vesicles, as well as leading to an increase in the area of the presynaptic plasma membrane (and possible misalignment with postsynaptic structures). Hence, synaptic vesicle fusion is tightly coupled to a local recycling of synaptic vesicles. For a long time, however, the exact molecular mechanisms coupling fusion and subsequent recycling remained unclear. Recent work now indicates a unique role for the plasma membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP(2)), acting together with the vesicular protein synaptotagmin, in coupling these two processes. In this work, we review the evidence for such a mechanism and discuss both the possible advantages and disadvantages for vesicle recycling (and hence signal transduction) in the nervous system. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

Tuning membrane phase separation using nonlipid amphiphiles

Lipid phase separation may be a mechanism by which lipids participate in sorting membrane proteins and facilitate membrane-mediated biochemical signaling in cells. To provide new tools for membrane lipid phase manipulation that avoid direct effects on protein activity and lipid composition, we studied phase separation in binary and ternary lipid mixtures under the influence of three nonlipid amphiphiles, vitamin E (VE), Triton-X (TX)-100, and benzyl alcohol (BA). Mechanisms of additive-induced phase separation were elucidated using coarse-grained molecular dynamics simulations of these additives in a liquid bilayer made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dilinoleoyl-sn-glycero-3-phosphocholine [corrected]. From simulations, the additive's partitioning preference, changes in membrane thickness, and alterations in lipid order were quantified. Simulations showed that VE favored the DPPC phase but partitioned predominantly to the domain boundaries and lowered the tendency for domain formation, and therefore acted as a linactant. This simulated behavior was consistent with experimental observations in which VE promoted lipid mixing and dispersed domains in both gel/liquid and liquid-ordered/liquid-disordered systems. From simulation, BA partitioned predominantly to the DUPC phase, decreased lipid order there, and thinned the membrane. These actions explain why, experimentally, BA promoted phase separation in both binary and ternary lipid mixtures. In contrast, TX, a popular detergent used to isolate raft membranes in cells, exhibited equal preference for both phases, as demonstrated by simulations, but nonetheless, was a strong domain promoter in all lipid mixtures. Further analysis showed that TX increased membrane thickness of the DPPC phase to a greater extent than the DUPC phase and thus increased hydrophobic mismatch, which may explain experimental observation of phase separation in the presence of TX. In summary, these nonlipid amphiphiles provide new tools to tune domain formation in model vesicle systems and could provide the means to form or disperse membrane lipid domains in cells, in addition to the well-known methods involving cholesterol enrichment and sequestration.

Membrane mechanisms for signal transduction: the coupling of the meso-scale raft domains to membrane-skeleton-induced compartments and dynamic protein complexes

Virtually all biological membranes on earth share the basic structure of a two-dimensional liquid. Such universality and peculiarity are comparable to those of the double helical structure of DNA, strongly suggesting the possibility that the fundamental mechanisms for the various functions of the plasma membrane could essentially be understood by a set of simple organizing principles, developed during the course of evolution. As an initial effort toward the development of such understanding, in this review, we present the concept of the cooperative action of the hierarchical three-tiered meso-scale (2-300 nm) domains in the plasma membrane: (1) actin membrane-skeleton-induced compartments (40-300 nm), (2) raft domains (2-20 nm), and (3) dynamic protein complex domains (3-10nm). Special attention is paid to the concept of meso-scale domains, where both thermal fluctuations and weak cooperativity play critical roles, and the coupling of the raft domains to the membrane-skeleton-induced compartments as well as dynamic protein complexes. The three-tiered meso-domain architecture of the plasma membrane provides an excellent perspective for understanding the membrane mechanisms of signal transduction.

Fluorescent lipids: functional parts of fusogenic liposomes and tools for cell membrane labeling and visualization

In this paper a rapid and highly efficient method for controlled incorporation of fluorescent lipids into living mammalian cells is introduced. Here, the fluorescent molecules have two consecutive functions: First, they trigger rapid membrane fusion between cellular plasma membranes and the lipid bilayers of their carrier particles, so called fusogenic liposomes, and second, after insertion into cellular membranes these molecules enable fluorescence imaging of cell membranes and membrane traffic processes. We tested the fluorescent derivatives of the following essential membrane lipids for membrane fusion: Ceramide, sphingomyelin, phosphocholine, phosphatidylinositol-bisphosphate, ganglioside, cholesterol, and cholesteryl ester. Our results show that all probed lipids could more efficiently be incorporated into the plasma membrane of living cells than by using other methods. Moreover, labeling occurred in a gentle manner under classical cell culture conditions reducing cellular stress responses. Staining procedures were monitored by fluorescence microscopy and it was observed that sphingolipids and cholesterol containing free hydroxyl groups exhibit a decreased distribution velocity as well as a longer persistence in the plasma membrane compared to lipids without hydroxyl groups like phospholipids or other artificial lipid analogs. After membrane staining, the fluorescent molecules were sorted into membranes of cell organelles according to their chemical properties and biological functions without any influence of the delivery system.

STED nanoscopy reveals molecular details of cholesterol- and cytoskeleton-modulated lipid interactions in living cells

Details about molecular membrane dynamics in living cells, such as lipid-protein interactions, are often hidden from the observer because of the limited spatial resolution of conventional far-field optical microscopy. The superior spatial resolution of stimulated emission depletion (STED) nanoscopy can provide new insights into this process. The application of fluorescence correlation spectroscopy (FCS) in focal spots continuously tuned down to 30 nm in diameter distinguishes between free and anomalous molecular diffusion due to, for example, transient binding of lipids to other membrane constituents, such as lipids and proteins. We compared STED-FCS data recorded on various fluorescent lipid analogs in the plasma membrane of living mammalian cells. Our results demonstrate details about the observed transient formation of molecular complexes. The diffusion characteristics of phosphoglycerolipids without hydroxyl-containing headgroups revealed weak interactions. The strongest interactions were observed with sphingolipid analogs, which showed cholesterol-assisted and cytoskeleton-dependent binding. The hydroxyl-containing headgroup of gangliosides, galactosylceramide, and phosphoinositol assisted binding, but in a much less cholesterol- and cytoskeleton-dependent manner. The observed anomalous diffusion indicates lipid-specific transient hydrogen bonding to other membrane molecules, such as proteins, and points to a distinct connectivity of the various lipids to other membrane constituents. This strong interaction is different from that responsible for forming cholesterol-dependent, liquid-ordered domains in model membranes.

Membrane organization and lipid rafts

Cell membranes are composed of a lipid bilayer, containing proteins that span the bilayer and/or interact with the lipids on either side of the two leaflets. Although recent advances in lipid analytics show that membranes in eukaryotic cells contain hundreds of different lipid species, the function of this lipid diversity remains enigmatic. The basic structure of cell membranes is the lipid bilayer, composed of two apposing leaflets, forming a two-dimensional liquid with fascinating properties designed to perform the functions cells require. To coordinate these functions, the bilayer has evolved the propensity to segregate its constituents laterally. This capability is based on dynamic liquid-liquid immiscibility and underlies the raft concept of membrane subcompartmentalization. This principle combines the potential for sphingolipid-cholesterol self-assembly with protein specificity to focus and regulate membrane bioactivity. Here we will review the emerging principles of membrane architecture with special emphasis on lipid organization and domain formation.

Hierarchical mesoscale domain organization of the plasma membrane

Based on recent single-molecule imaging results in the living cell plasma membrane, we propose a hierarchical architecture of three-tiered mesoscale (2-300nm) domains to represent the fundamental functional organization of the plasma membrane: (i) membrane compartments of 40-300nm in diameter due to the partitioning of the entire plasma membrane by the actin-based membrane skeleton 'fence' and transmembrane protein 'pickets' anchored to the fence; (ii) raft domains (2-20nm); and (iii) dimers/oligomers and greater complexes of membrane-associated proteins (3-10nm). The basic molecular interactions required for the signal transduction function of the plasma membrane can be fundamentally understood and conveniently summarized as the cooperative actions of these mesoscale domains, where thermal fluctuations/movements of molecules and weak cooperativity play crucial roles.

Single-molecule investigations of the stringent response machinery in living bacterial cells

The RelA-mediated stringent response is at the heart of bacterial adaptation to starvation and stress, playing a major role in the bacterial cell cycle and virulence. RelA integrates several environmental cues and synthesizes the alarmone ppGpp, which globally reprograms transcription, translation, and replication. We have developed and implemented novel single-molecule tracking methodology to characterize the intracellular catalytic cycle of RelA. Our single-molecule experiments show that RelA is on the ribosome under nonstarved conditions and that the individual enzyme molecule stays off the ribosome for an extended period of time after activation. This suggests that the catalytically active part of the RelA cycle is performed off, rather than on, the ribosome, and that rebinding to the ribosome is not necessary to trigger each ppGpp synthesis event. Furthermore, we find fast activation of RelA in response to heat stress followed by RelA rapidly being reset to its inactive state, which makes the system sensitive to new environmental cues and hints at an underlying excitable response mechanism.

Lipid-mediated endocytosis

Receptor-mediated endocytosis is used by a number of viruses and toxins to gain entry into cells. Some have evolved to use specific lipids in the plasma membrane as their receptors. They include bacterial toxins such as Shiga and Cholera toxin and viruses such as mouse polyoma virus and simian virus 40. Through multivalent binding to glycosphingolipids, they induce lipid clustering and changes in membrane properties. Internalization occurs by unusual endocytic mechanisms involving lipid rafts, induction of membrane curvature, trans-bilayer coupling, and activation of signaling pathways. Once delivered to early endosomes, they follow diverse intracellular routes to the lumen of the ER, from which they penetrate into the cytosol. The role of the lipid receptors is central in these well-studied processes.

An Adaptive Anti-Brownian ELectrokinetic trap with real-time information on single-molecule diffusivity and mobility

We present the design and implementation of an adaptive Anti-Brownian ELectrokinetic (ABEL) trap capable of extracting estimates of the diffusion coefficient and mobility of single trapped fluorescent nanoscale objects such as biomolecules in solution. The system features rapid acousto-optic scanning of a confocal excitation spot on a 2D square lattice to encode position information on the arrival time of each detected photon, and Kalman filter-based signal processing unit for refined position estimation. We demonstrate stable trapping of multisubunit proteins (D ≈ 22 μm(2)/s) with a count rate of 6 kHz for as long as 15 s and small single-stranded DNA molecules (D ≈ 118 μm(2)/s) at a 15 kHz count rate for seconds. Moreover, we demonstrate real-time measurement of diffusion coefficient and electrokinetic mobility of trapped objects, using adaptive tuning of the Kalman filter parameters.

Detecting nanodomains in living cell membrane by fluorescence correlation spectroscopy

Cell membranes actively participate in numerous cellular functions. Inasmuch as bioactivities of cell membranes are known to depend crucially on their lateral organization, much effort has been focused on deciphering this organization on different length scales. Within this context, the concept of lipid rafts has been intensively discussed over recent years. In line with its ability to measure diffusion parameters with great precision, fluorescence correlation spectroscopy (FCS) measurements have been made in association with innovative experimental strategies to monitor modes of molecular lateral diffusion within the plasma membrane of living cells. These investigations have allowed significant progress in the characterization of the cell membrane lateral organization at the suboptical level and have provided compelling evidence for the in vivo existence of raft nanodomains. We review these FCS-based studies and the characteristic structural features of raft nanodomains. We also discuss the findings in regards to the current view of lipid rafts as a general membrane-organizing principle.

Spot variation fluorescence correlation spectroscopy allows for superresolution chronoscopy of confinement times in membranes

Resolving the dynamical interplay of proteins and lipids in the live-cell plasma membrane represents a central goal in current cell biology. Superresolution concepts have introduced a means of capturing spatial heterogeneity at a nanoscopic length scale. Similar concepts for detecting dynamical transitions (superresolution chronoscopy) are still lacking. Here, we show that recently introduced spot-variation fluorescence correlation spectroscopy allows for sensing transient confinement times of membrane constituents at dramatically improved resolution. Using standard diffraction-limited optics, spot-variation fluorescence correlation spectroscopy captures signatures of single retardation events far below the transit time of the tracer through the focal spot. We provide an analytical description of special cases of transient binding of a tracer to pointlike traps, or association of a tracer with nanodomains. The influence of trap mobility and the underlying binding kinetics are quantified. Experimental approaches are suggested that allow for gaining quantitative mechanistic insights into the interaction processes of membrane constituents.

Nanoscale fluorescence correlation spectroscopy on intact living cell membranes with NSOM probes

Characterization of molecular dynamics on living cell membranes at the nanoscale is fundamental to unravel the mechanisms of membrane organization and compartmentalization. Here we demonstrate the feasibility of fluorescence correlation spectroscopy (FCS) based on the nanometric illumination of near-field scanning optical microscopy (NSOM) probes on intact living cells. NSOM-FCS applied to fluorescent lipid analogs allowed us to reveal details of the diffusion hidden by larger illumination areas. Moreover, the technique offers the unique advantages of evanescent axial illumination and straightforward implementation of multiple color excitation. As such, NSOM-FCS represents a powerful tool to study a variety of dynamic processes occurring at the nanometer scale on cell membranes.

Microsecond single-molecule tracking (μsSMT)

Here we report on a method to track individual molecules on nanometer length and microsecond timescales using an optical microscope. Our method is based on double-labeling of a molecule with two spectrally distinct fluorophores and illuminating it with laser pulses of different wavelengths that partially overlap temporally. We demonstrate our method by using it to resolve the motion of short DNA oligomers in solution down to a timescale of 100 μs.

Phase separation in lipid membranes

Cell membranes show complex behavior, in part because of the large number of different components that interact with each other in different ways. One aspect of this complex behavior is lateral organization of components on a range of spatial scales. We found that lipid-only mixtures can model the range of size scales, from approximately 2 nm up to microns. Furthermore, the size of compositional heterogeneities can be controlled entirely by lipid composition for mixtures such as 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/cholesterol or sphingomyelin (SM)/DOPC/POPC/cholesterol. In one region of special interest, because of its connection to cell membrane rafts, nanometer-scale domains of liquid-disordered phase and liquid-ordered phase coexist over a wide range of compositions.

Electrons, photons, and force: quantitative single-molecule measurements from physics to biology

Single-molecule measurement techniques have illuminated unprecedented details of chemical behavior, including observations of the motion of a single molecule on a surface, and even the vibration of a single bond within a molecule. Such measurements are critical to our understanding of entities ranging from single atoms to the most complex protein assemblies. We provide an overview of the strikingly diverse classes of measurements that can be used to quantify single-molecule properties, including those of single macromolecules and single molecular assemblies, and discuss the quantitative insights they provide. Examples are drawn from across the single-molecule literature, ranging from ultrahigh vacuum scanning tunneling microscopy studies of adsorbate diffusion on surfaces to fluorescence studies of protein conformational changes in solution.

Molecular strategies to read and write at the nanoscale with far-field optics

Diffraction prevents the focusing of ultraviolet and visible radiations within nanoscaled volumes and, as a result, the imaging and patterning of nanostructures with conventional far-field illumination. Specifically, the irradiation of a fluorescent or photosensitive material with focused light results in the simultaneous excitation of multiple chromophores distributed over a large area, relative to the dimensions of single molecules. It follows that the spatial control of fluorescence and photochemical reactions with molecular precision is impossible with conventional illumination configurations. However, the photochemical and photophysical properties of organic chromophores can be engineered to overcome diffraction in combination with patterned or reiterative illumination. These ingenious strategies offer the opportunity to confine excited chromophores within nanoscaled volumes and, therefore, restrict fluorescence or photochemical reactions within subdiffraction areas. Indeed, information can be "read" in the form of fluorescence and "written" in the form of photochemical products with resolution down to the nanometre level on the basis of these innovative approaches. In fact, these promising far-field optical methods permit the convenient imaging of biological samples and fabrication of miniaturized objects with unprecedented resolution and can have long-term and profound implications in biomedical research and information technology.

Membrane rafts in plant cells

Over the past five years, the structure, composition and possible functions of membrane raft-like domains on plant plasma membranes (PM) have been described. Proteomic analyses have indicated that a high proportion of proteins associated with detergent-insoluble membranes (DIMs), supposed to contain raft-like domains isolated from the PM, might be involved in signalling pathways. Recently, the dynamic association of specific proteins with the DIM fraction upon environmental stress has been reported. Innovative imaging methods have shown that lateral segregation of lipids and proteins exists at the nanoscale level in the plant PM, correlating detergent insolubility and membrane-domain localization of presumptive raft proteins. These data suggest a role for plant rafts as signal transduction platforms, similar to those documented for mammalian cells.

Three-dimensional tracking of single fluorescent particles with submillisecond temporal resolution

Observing dynamics at the nanoscale requires submillisecond time resolution. Notably, in studying biological systems, three-dimensional (3D) trajectories of fluorescently labeled objects such as viruses or transport vesicles often need to be acquired with high temporal resolution. Here, we present a novel instrument that combines scanning-free multiplane detection at 3.2 kHz frame rate and single photon sensitivity with optimized beam-steering capabilities. This setup enables ultrafast 3D localization with submillisecond time resolution and nanometer localization precision. We demonstrate 3D tracking of single fluorescent particles at speeds of up to 150 nm/ms over several seconds and large volumes. By focused excitation of only the particle of interest, while avoiding confocal pinholes, the system realizes maximum detection efficiency at minimal laser irradiation. These features, combined with the avoidance of stage movement, provide high live-sample compatibility for future biomedical applications.

Time-resolved three-dimensional molecular tracking in live cells

We report a method for tracking individual quantum dot (QD) labeled proteins inside of live cells that uses four overlapping confocal volume elements and active feedback once every 5 ms to follow three-dimensional molecular motion. This method has substantial advantages over three-dimensional molecular tracking methods based upon charge-coupled device cameras, including increased Z-tracking range (10 μm demonstrated here), substantially lower excitation powers (15 μW used here), and the ability to perform time-resolved spectroscopy (such as fluorescence lifetime measurements or fluorescence correlation spectroscopy) on the molecules being tracked. In particular, we show for the first time fluorescence photon antibunching of individual QD labeled proteins in live cells and demonstrate the ability to track individual dye-labeled nucleotides (Cy5-dUTP) at biologically relevant transport rates. To demonstrate the power of these methods for exploring the spatiotemporal dynamics of live cells, we follow individual QD-labeled IgE-FcεRI receptors both on and inside rat mast cells. Trajectories of receptors on the plasma membrane reveal three-dimensional, nanoscale features of the cell surface topology. During later stages of the signal transduction cascade, clusters of QD labeled IgE-FcεRI were captured in the act of ligand-mediated endocytosis and tracked during rapid (~950 nm/s) vesicular transit through the cell.

Cell cholesterol homeostasis: mediation by active cholesterol

Recent evidence suggests that the major pathways mediating cell cholesterol homeostasis respond to a common signal: active membrane cholesterol. Active cholesterol is the fraction that exceeds the complexing capacity of the polar bilayer lipids. Increments in plasma membrane cholesterol exceeding this threshold have an elevated chemical activity (escape tendency) and redistribute via diverse transport proteins to both circulating plasma lipoproteins and intracellular organelles. Active cholesterol thereby prompts several feedback responses. It is the substrate for its own esterification and for the synthesis of regulatory side-chain oxysterols. It also stimulates manifold pathways that down-regulate the biosynthesis, curtail the ingestion and increase the export of cholesterol. Thus, the abundance of cell cholesterol is tightly coupled to that of its polar lipid partners through active cholesterol.