Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

The COVID-19 pandemic: a target for surfactant therapy?

INTRODUCTION

The dramatic impact of COVID-19 on humans worldwide has initiated an extraordinary search for effective treatment approaches. One of these is the administration of exogenous surfactant, which is being tested in ongoing clinical trials.

AREAS COVERED

Exogenous surfactant is a life-saving treatment for premature infants with neonatal respiratory distress syndrome. This treatment has also been tested for acute respiratory distress syndrome (ARDS) with limited success possibly due to the complexity of that syndrome. The 60-year history of successes and failures associated with surfactant therapy distinguishes it from many other treatments currently being tested for COVID-19 and provides the opportunity to discuss the factors that may influence the success of this therapy.

EXPERT OPINION

Clinical data provide a strong rationale for using exogenous surfactant in COVID-19 patients. Success of this therapy may be influenced by the mechanical ventilation strategy, the timing of treatment, the doses delivered, the method of delivery and the preparations utilized. In addition, future development of enhanced preparations may improve this treatment approach. Overall, results from ongoing trials may not only provide data to indicate if this therapy is effective for COVID-19 patients, but also lead to further scientific understanding and improved treatment strategies.

Distribution and Co-localization of endosome markers in cells

Clathrin mediated endocytosis is one pathway for internalization of extracellular nano materials into cells [1, 2]. In this pathway, proteins attached to receptors and the internalized materials are encapsulated in clathrin coated membrane vesicles that subsequently fuse with or transform into intracellular compartments (early and late endosomes) as their contents are being directed to the lysosomes for degradation. The following proteins are commonly used to mark the pathway at various stages: Rab5 (early endosome), Rab7 (late endosome), and LAMP-1 (lysosome). In this work, we studied the distribution and co-localization of these marker proteins in two cell lines (C2C12 and A549) to determine whether these markers are unique for specific endosome types or whether they can co-exist with other markers. We estimate the densities and sizes of the endosomes containing the three markers, as well as the number of marker antibodies attached to each endosome. We determine that the markers are not unique to one endosome type but that the extent of co-localization is different for the two cell types. In fact, we find endosomes that contain all three markers simultaneously. Our results suggest that the use of these proteins as specific markers for specific endosome types should be reevaluated. This was the first successful use of triple image cross correlation spectroscopy to qualitatively and quantitatively study the extent of interaction among three different species in cells and also the first experimental study of three-way interactions of clathrin mediated endocytic markers.

Characterizing the inhibition of α-synuclein oligomerization by a pharmacological chaperone that prevents prion formation by the protein PrP

Aggregation of the disordered protein α-synuclein into amyloid fibrils is a central feature of synucleinopathies, neurodegenerative disorders that include Parkinson's disease. Small, pre-fibrillar oligomers of misfolded α-synuclein are thought to be the key toxic entities, and α-synuclein misfolding can propagate in a prion-like way. We explored whether a compound with anti-prion activity that can bind to unfolded parts of the protein PrP, the cyclic tetrapyrrole Fe-TMPyP, was also active against α-synuclein aggregation. Observing the initial stages of aggregation via fluorescence cross-correlation spectroscopy, we found that Fe-TMPyP inhibited small oligomer formation in a dose-dependent manner. Fe-TMPyP also inhibited the formation of mature amyloid fibrils in vitro, as detected by thioflavin T fluorescence. Isothermal titration calorimetry indicated Fe-TMPyP bound to monomeric α-synuclein with a stoichiometry of 2, and two-dimensional heteronuclear single quantum coherence NMR spectra revealed significant interactions between Fe-TMPyP and the C-terminus of the protein. These results suggest commonalities among aggregation mechanisms for α-synuclein and the prion protein may exist that can be exploited as therapeutic targets.

Early stages of aggregation of engineered α-synuclein monomers and oligomers in solution

α-Synuclein is a protein that aggregates as amyloid fibrils in the brains of patients with Parkinson's disease and dementia with Lewy bodies. Small oligomers of α-synuclein are neurotoxic and are thought to be closely associated with disease. Whereas α-synuclein fibrillization and fibril morphologies have been studied extensively with various methods, the earliest stages of aggregation and the properties of oligomeric intermediates are less well understood because few methods are able to detect and characterize early-stage aggregates. We used fluorescence spectroscopy to investigate the early stages of aggregation by studying pairwise interactions between α-synuclein monomers, as well as between engineered tandem oligomers of various sizes (dimers, tetramers, and octamers). The hydrodynamic radii of these engineered α-synuclein species were first determined by fluorescence correlation spectroscopy and dynamic light scattering. The rate of pairwise aggregation between different species was then monitored using dual-color fluorescence cross-correlation spectroscopy, measuring the extent of association between species labelled with different dyes at various time points during the early aggregation process. The aggregation rate and extent increased with tandem oligomer size. Self-association of the tandem oligomers was found to be the preferred pathway to form larger aggregates: interactions between oligomers occurred faster and to a greater extent than interactions between oligomers and monomers, indicating that the oligomers were not as efficient in seeding further aggregation by addition of monomers. These results suggest that oligomer-oligomer interactions may play an important role in driving aggregation during its early stages.

Structural characteristics and membrane interactions of tandem α-synuclein oligomers

Pre-fibrillar oligomers of α-synuclein are thought to be pathogenic molecules leading to neurotoxicity associated with Parkinson’s disease and other neurodegenerative disorders. However, small oligomers are difficult to isolate for study. To gain better insight into the properties of small α-synuclein oligomers, we investigated engineered oligomers of specific size (dimers, tetramers, and octamers) linked head-to-tail in tandem, comparing the behavior of the oligomers to monomeric α-synuclein. All oligomeric constructs remained largely disordered in solution, as determined from dynamic light scattering and size-exclusion chromatography. Electron microscopy revealed that each construct could aggregate to form fibrils similar to those formed by monomeric α-synuclein. The interactions with large unilamellar vesicles (LUVs) composed of negatively-charged lipids differed depending on size, with smaller oligomers forming more extensive helical structure as determined by CD spectroscopy. Monitoring the influx of a fluorescence bleaching agent into vesicles showed that larger oligomers were somewhat more effective at degrading vesicular integrity and inducing membrane permeabilization.

Amyloids of alpha-synuclein affect the structure and dynamics of supported lipid bilayers

Interactions of monomeric alpha-synuclein (αS) with lipid membranes have been suggested to play an important role in initiating aggregation of αS. We have systematically analyzed the distribution and self-assembly of monomeric αS on supported lipid bilayers. We observe that at protein/lipid ratios higher than 1:10, αS forms micrometer-sized clusters, leading to observable membrane defects and decrease in lateral diffusion of both lipids and proteins. An αS deletion mutant lacking amino-acid residues 71-82 binds to membranes, but does not observably affect membrane integrity. Although this deletion mutant cannot form amyloid, significant amyloid formation is observed in the wild-type αS clusters. These results suggest that the process of amyloid formation, rather than binding of αS on membranes, is crucial in compromising membrane integrity.

Characterization of phospholipid-encapsulated gold nanoparticles: a versatile platform to study drug delivery and cellular uptake mechanisms

The data presented in this work aim to provide a comprehensive characterization of lipid-coated gold nanoparticles. We show that it is possible to envelop gold nanoparticles with a coating of lipids during the formation of the gold nanoparticles, that the gold in these lipid-coated gold nanoparticles is crystalline and the size and shape can be controlled by the reaction conditions (within limits), that the lipid coating corresponds to a thickness consistent with the formation of a bilayer, that the bilayer can include fluorescent probes that, while quenched, can be used to trace the fate of the gold nanoparticles in cellular systems, that their surface charge, and hence their overall stability in solution, is influenced by the lipid coating, and that while some lipid dyes may exchange among the particles, they are sufficiently stable to exchange to permit their use as tracers in cell studies. We believe this is the most comprehensive characterization of these systems to date.

Lipid-coated gold nanoparticles promote lamellar body formation in A549 cells

Gold nanoparticles (GNPs) have been applied as diagnostic and therapeutic agents because they can be targeted, localized, and be heated to cause cell death. However, their use has been limited by their relatively low biocompatibility. In this work, we coated the GNPs' surface by a biocompatible phospholipid bilayer composed of 1-stearoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (SOPG). We tested their interaction with A549 cells to investigate their uptake and intracellular fate as well as the response of the cells to the presence of the GNPs. We used flow cytometry and confocal microscopy to show that the SOPG coated GNPs were readily taken up by the A549 cells. Transmission electron microscopy (TEM) images and fluorescence images further showed that the number of granular structures in the cells was increased following exposure to the lipid coated GNPs. Co-localization experiments demonstrated that SOPG coated GNPs localize in acidic compartments in a time dependent manner and that the number of these increase as the cells are exposed to the GNPs suggesting that they induce formation of lamellar bodies (LBs) which in A549 cells in turn can serve as a means of exporting the GNPs.

A modified squeeze-out mechanism for generating high surface pressures with pulmonary surfactant

The exact mechanism by which pulmonary surfactant films reach the very low surface tensions required to stabilize the alveoli at end expiration remains uncertain. We utilized the nanoscale sensitivity of atomic force microscopy (AFM) to examine phospholipid (PL) phase transition and multilayer formation for two Langmuir-Blodgett (LB) systems: a simple 3 PL surfactant-like mixture and the more complex bovine lipid extract surfactant (BLES). AFM height images demonstrated that both systems develop two types of liquid condensed (LC) domains (micro- and nano-sized) within a liquid expanded phase (LE). The 3 PL mixture failed to form significant multilayers at high surface pressure (π while BLES forms an extensive network of multilayer structures containing up to three bilayers. A close examination of the progression of multilayer formation reveals that multilayers start to form at the edge of the solid-like LC domains and also in the fluid-like LE phase. We used the elemental analysis capability of time-of-flight secondary ion mass spectrometry (ToF-SIMS) to show that multilayer structures are enriched in unsaturated PLs while the saturated PLs are concentrated in the remaining interfacial monolayer. This supports a modified squeeze-out model where film compression results in the hydrophobic surfactant protein-dependent formation of unsaturated PL-rich multilayers which remain functionally associated with a monolayer enriched in disaturated PL species. This allows the surface film to attain low surface tensions during compression and maintain values near equilibrium during expansion.

Photon counting histogram analysis for two-dimensional systems

Photon counting statistics in 3D photon counting histogram analysis for one-photon excitation is a function of the number of molecules of particular brightness in the excitation-detection volume of a confocal microscope. In mathematical form that volume is approximated by a three-dimensional Gaussian function which is embedded in the PCH theoretical equations. PCH theory assumes that a molecule can be found anywhere inside the excitation-detection volume with equal probability. However, one can easily imagine systems in which this assumption is violated because molecules are constrained by the geometry of the sample. For example, molecules on a surface or in a membrane would be constrained to two dimensions. To enable the analysis of such systems by PCH, the theoretical framework requires modification. Herein, we present an extension of the PCH analysis to systems where molecules exist in thin structures that are effectively two-dimensional. The method, aptly called two-dimensional photon counting histogram (2D PCH), recovers the number of fluorescent particles per unit area and their molecular brightness. Both theoretical background and experimental results are presented. The theory was tested using computer-simulated and experimental 2D PCHs obtained from confocal experiments. We demonstrate that this modification of the theoretical framework provides a tool to extract data that reveal states of aggregation, surface photophysics, and reactivity.

Trapping of BMP receptors in distinct membrane domains inhibits their function in pulmonary arterial hypertension

Bone morphogenetic proteins (BMPs) are pleiotrophic growth factors that influence diverse processes such as skeletal development, hematopoiesis, and neurogenesis. They play crucial roles in diseases such as pulmonary arterial hypertension (PAH). In PAH, mutants of the BMP type II receptors (BMPR2) were detected, and their functions were impaired during BMP signaling. It is thought that expression levels of these receptors determine the fate of BMP signaling, with low levels of expression leading to decreased Smad activation in PAH. However, our studies demonstrate, for the first time, that the localization of receptors on the plasma membrane, in this case BMPR2, was misdirected. Three BMPR2 mutants, D485G, N519K, and R899X, which are known to be involved in PAH, were chosen as our model system. Our results show that all three BMPR2 mutants decreased BMP-dependent Smad phosphorylation and Smad signaling. Although the three mutants reached the cell membrane and their expression was lower than that of BMPR2, they formed smaller clusters and associated differently with membrane domains, such as caveolae and clathrin-coated pits. The disruption of these domains restored the Smad signaling of D485G and N519K to the level of wild-type BMPR2, showing that these mutants were trapped in the domains, rather than just expressed at a lower level on the surface. Therefore, new treatment options for PAH should also target receptor localization, rather than just expression level.

A ToF-SIMS study of the lateral organization of lipids and proteins in pulmonary surfactant systems

Pulmonary surfactant is a complex lipid-protein mixture whose main function is to reduce the surface tension at the air-liquid interface of alveoli to minimize the work of breathing. The exact mechanism by which surfactant monolayers and multilayers are formed and how they lower surface tension to very low values during lateral compression remains uncertain. We used time-of-flight secondary ion mass spectrometry to study the lateral organization of lipids and peptide in surfactant preparations ranging in complexity. We show that we can successfully determine the location of phospholipids, cholesterol and a peptide in surfactant Langmuir-Blodgett films and we can determine the effect of cholesterol and peptide addition. A thorough understanding of the lateral organization of PS interfacial films will aid in our understanding of the role of each component as well as different lipid-lipid and lipid-protein interactions. This may further our understanding of pulmonary surfactant function.

Casein kinase 2 beta-subunit is a regulator of bone morphogenetic protein 2 signaling

Bone morphogenetic proteins (BMPs) play a crucial role during embryonic development and regulate processes as diverse as neurogenesis, skeletal formation, and hematopoesis. They signal through a hetero-oligomer complex of BMP receptors. Binding of the ligand to the receptors activates several pathways, including Smad and p38. BMP signaling is controlled in the extracellular space, the plasma membrane, and the intracellular space; however, the mechanism of receptor signaling at the plasma membrane and proteins that regulate this process still need to be identified. The experiments presented here identify the protein kinase casein kinase II (CK2) as a BMP receptor type Ia (BRIa) interacting protein. Fluorescence resonance energy transfer revealed that this interaction occurs at the plasma membrane. BMP2 stimulation of C2C12 cells leads to the release of CK2 from BRIa. Blocking this interaction with specific peptides that inhibit the binding sites for CK2 on BRIa demonstrated a redistribution of BRIa on the plasma membrane. Signaling was initiated once CK2 was released from BRIa, leading to the mineralization of C2C12 cells. These data suggest that CK2 is a negative regulator of BMP signaling and osteoblast differentiation.

Recent advances in alveolar biology: some new looks at the alveolar interface

This article examines the manner in which some new methodologies and novel concepts have contributed to our understanding of how pulmonary surfactant reduces alveolar surface tension. Investigations utilizing small angle X-ray diffraction, inverted interface fluorescence microscopy, time of flight-secondary ion mass spectroscopy, atomic force microscopy, two-photon fluorescence microscopy and electrospray mass spectroscopy are highlighted and a new model of ventilation-induced acute lung injury described. This contribution attempts to emphasize how these new approaches have resulted in a fuller appreciation of events presumably occurring at the alveolar interface.

Effects of second order photobleaching on recovered diffusion parameters from fluorescence photobleaching recovery

In the original theoretical development of fluorescence photobleaching recovery with circular or Gaussian laser intensity profiles (Axelrod et al., 1976, Biophys. J.) the bleaching process is assumed to obey first order kinetics in the fluorescent probe. While this is reasonable in most cases where oxygen participates in the photolysis reaction, some processes may obey second order kinetics in the fluorophore concentration due to dimerization. Accordingly, we present here an analysis of the fluorescence recovery when the photobleaching process is taken to be second order in the probe. Analytical solutions for small bleaching levels indicate that the fluorescence recovery curve is very similar to that measured following a bleaching process first order in the probe. Numerical solutions for moderate bleaching levels show that the recovery is qualitatively similar, but quantitatively different. Because the shape of the recovery curve provides no evidence as to the order of photobleaching, we recommend continued use of the previous theoretical analysis. However, it must be borne in mind that the diffusion coefficient is increasingly underestimated as the extent of photobleaching is increased. The true diffusion coefficient is obtained in the limit of small levels of photobleaching. Estimates of the fractional recovery are not affected by this approach.

Solution phase interactions controlled ordered arrangement of gold nanoparticles in dried state

Gold (Au) nanoparticles (NPs) were synthesized in the presence of water soluble biomolecules such as DNA, chitosan, phospholipids, and BSA by using seed-mediated approach at room temperature. All reactions produced mostly spherical geometries with comparable size (< or = 20 nm). The NPs were arranged in a typical pearl-necklace type arrangement except in the presence of BSA. Different measurements such as UV-visible, TEM, XRD, and XPS were used to characterize the Au NPs. Fluorescence spectroscopy was used to identify the interactions between biomolecules and blank (uncapped) Au NPs in aqueous colloidal solutions. It was concluded that the favorable interactions between Au NPs and biomolecules in aqueous phase, in fact, drive them into pearl-necklace type arrangement in the dried state.

FRET reveals novel protein-receptor interaction of bone morphogenetic proteins receptors and adaptor protein 2 at the cell surface

Bone morphogenetic proteins (BMPs) are involved with a wide range of processes including apoptosis, differentiation, and proliferation. Several different pathways such as Smad, p38, and PI3/Akt are activated by BMPs. Signaling is transduced by BMP receptors (BMPRs) of type I and type II that are serine/threonine kinase receptors. BMPRs shuttle between membrane domains such as caveolae enriched with caveolin-1 beta-isoform and caveolae of the caveolin-1 alpha/beta-isoforms. It is hypothesized that there are other membrane domains to which the receptors localize. We used immunoprecipitation, Western blots, image cross-correlation spectroscopy, and fluorescence resonance energy transfer to investigate the interaction of BMPRs with proteins in clathrin-coated pits (CCPs). Our data indicate that these domains are associated with at least two of the BMPRs: BRIa and BRII. For the first time, to our knowledge, we showed what we believe are specific interactions between BRIa and BRII with a key component of CCPs, adaptor protein 2. Further, disruption of CCPs resulted in increased BRIa aggregation at the cell surface and activation of the BMP pathway even in the absence of BMP2. Therefore, CCPs seem to function as a negative regulatory membrane domain for BMP pathway activation.

Chemical mapping of ceramide distribution in sphingomyelin-rich domains in monolayers

The incorporation of ceramide in phase-separated monolayers of ternary lipid mixtures has been studied by a combination of atomic force microscopy (AFM), fluorescence, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Replacement of a fraction of the sphingomyelin by ceramide in DOPC/SM/cholesterol monolayers leads to changes in the SM-cholesterol-rich liquid-ordered domains. AFM shows the formation of heterogeneous domains with small raised islands that are assigned to a ceramide-rich gel phase. ToF-SIMS provides conclusive evidence for the localization of SM and ceramide in ordered domains and shows that ceramide is heterogeneously distributed in small islands throughout the domains. The results indicate the utility of combining AFM and ToF-SIMS for understanding compositions of phase-separated membranes.

Current perspectives in pulmonary surfactant--inhibition, enhancement and evaluation

Pulmonary surfactant (PS) is a complicated mixture of approximately 90% lipids and 10% proteins. It plays an important role in maintaining normal respiratory mechanics by reducing alveolar surface tension to near-zero values. Supplementing exogenous surfactant to newborns suffering from respiratory distress syndrome (RDS), a leading cause of perinatal mortality, has completely altered neonatal care in industrialized countries. Surfactant therapy has also been applied to the acute respiratory distress syndrome (ARDS) but with only limited success. Biophysical studies suggest that surfactant inhibition is partially responsible for this unsatisfactory performance. This paper reviews the biophysical properties of functional and dysfunctional PS. The biophysical properties of PS are further limited to surface activity, i.e., properties related to highly dynamic and very low surface tensions. Three main perspectives are reviewed. (1) How does PS permit both rapid adsorption and the ability to reach very low surface tensions? (2) How is PS inactivated by different inhibitory substances and how can this inhibition be counteracted? A recent research focus of using water-soluble polymers as additives to enhance the surface activity of clinical PS and to overcome inhibition is extensively discussed. (3) Which in vivo, in situ, and in vitro methods are available for evaluating the surface activity of PS and what are their relative merits? A better understanding of the biophysical properties of functional and dysfunctional PS is important for the further development of surfactant therapy, especially for its potential application in ARDS.

Image correlation spectroscopy

Membrane domains, such as caveolae and clathrin-coated pits, regulate cell signaling and protein internalization in the plasma membrane. Fluorescence imaging and microscopy provide an opportunity to determine the receptor protein dynamics of membrane microdomains. The family of image correlation spectroscopy (ICS) techniques provides powerful tools with which to measure the aggregation, clustering, and dynamics of proteins in the plasma membrane. ICS is used to calculate the cluster density and the degree of aggregation of plasma membrane proteins, whereas image cross-correlation spectroscopy (ICCS) measures the fraction of colocalization of two proteins. Dynamic image correlation spectroscopy (DICS) can be used to analyze protein dynamics on the cell surface during live-cell imaging.

Studies of distribution, location and dynamic properties of EGFR on the cell surface measured by image correlation spectroscopy

In this work, we have studied the distribution and dynamic properties of Epidermal Growth Factor (EGF) receptors in the plasma membrane of fixed and live cells as well as the extent of co-localization of this transmembrane protein with proteins specific for three-membrane microdomains: membrane rafts, caveolae and clathrin-coated pits. This was achieved using a family of image-processing tools called image correlation spectroscopy (ICS), image cross-correlation spectroscopy (ICCS) and dynamic image correlation spectroscopy (DICS). Our results indicate that EGFR is diffusely distributed on the cell surface at 37 degrees C and aggregates as the temperature is lowered to 4 degrees C. This aggregation takes place within 15 min and is reversible. Changes in temperature also affect the diffusion of EGFR by two orders of magnitude. The dynamic properties of EGFR are similar to the dynamic properties of a GPI-anchored protein known to be present in membrane rafts, which motivated us to explore the extent of co-localization of EGFR with this membrane raft protein using ICCS. Our results indicate that more than half of the EGFR population is present in membrane rafts and smaller percentages are present in caveolae and clathrin-coated pits.

Metal nanoparticle pollutants interfere with pulmonary surfactant function in vitro

Reported associations between air pollution and pulmonary and cardiovascular diseases prompted studies on the effects of gold nanoparticles (Au NP) on pulmonary surfactant function. Low levels (3.7 mol % Au/lipid, 0.98% wt/wt) markedly inhibited adsorption of a semisynthetic pulmonary surfactant (dipalmitoyl-phosphatidylcholine (DPPC)/palmitoyl-oleoyl-phosphatidylglycerol/surfactant protein B (SP-B); 70:30:1 wt %). Au NP also impeded the surfactant's ability to reduce surface tension (gamma) to low levels during film compression and to respread during film expansion. Transmission electron microscopy showed that Au NP generated by a seed-growth method were spherical with diameters of approximately 15 nm. Including palmitoyl-oleoyl-phosphatidylglycerol appeared to coat the NP with at least one lipid bilayer but did not affect NP shape or size. Similar overall observations occurred with dimyristoyl phosphatidylglycerol. Dipalmitoyl-phosphatidylglycerol was less effective in NP capping, although similar sized NP were formed. Including SP-B (1% wt/wt) appears to induce the formation of elongated strands of interacting threads with the fluid phosphatidylglycerols (PG). Including DPPC resulted in formation of aggregated, less spherical NP with a larger size distribution. With DPPC, strand formation due to SP-B was not observed. Agarose gel electrophoresis studies demonstrated that the aggregation induced by SP-B blocked migration of PG-coated NP. Migration was also influenced by the fluidity of the PGs. It is concluded that Au NP can interact with and sequester pulmonary surfactant phospholipids and, if inhaled from the atmosphere, could impede pulmonary surfactant function in the lung.

Thermodynamic studies of bovine lung surfactant extract mixing with cholesterol and its palmitate derivative

Langmuir film behavior of bovine lipid extract surfactant (BLES), mixed with cholesterol (CHOL) and cholesterol palmitate (CHOLP), has been studied by surface pressure (pi)-area (A) measurements. Associative interactions, observed for both systems, were less favored at lower BLES content. The presence of unsaturated phospholipids and surfactant proteins in BLES favored the association. Miscibility of BLES was better with CHOLP than with CHOL at all compositions, indicating more compact packing of the BLES-CHOLP than of the BLES-CHOL system. The most stable mixtures were found at 30-40 mol% CHOL and at low pi and at 20-25 mol% CHOLP but at higher pi. These results suggest that BLES-CHOL miscibility is better at low pi and low CHOL concentrations, while BLES-CHOLP miscibility is better at high pi and high CHOLP concentrations.

Effect of cholesterol on the biophysical and physiological properties of a clinical pulmonary surfactant

Pulmonary surfactant is a complex mixture of lipids and proteins that forms a surface-active film at the air-water interface of alveoli capable of reducing surface tension to near 0 mN/m. The role of cholesterol, the major neutral lipid component of pulmonary surfactant, remains uncertain. We studied the physiological effect of cholesterol by monitoring blood oxygenation levels of surfactant-deficient rats treated or not treated with bovine lipid extract surfactant (BLES) containing zero or physiological amounts of cholesterol. Our results indicate no significant difference between BLES and BLES containing cholesterol immediately after treatment; however, during ventilation, BLES-treated animals maintained higher PaO2 values compared to BLES+cholesterol-treated animals. We used a captive bubble tensiometer to show that physiological amounts of cholesterol do not have a detrimental effect on the surface activity of BLES at 37 degrees C. The effect of cholesterol on topography and lateral organization of BLES Langmuir-Blodgett films was also investigated using atomic force microscopy. Our data indicate that cholesterol induces the formation of domains within liquid-ordered domains (Lo). We used time-of-flight-secondary ion mass spectrometry and principal component analysis to show that cholesterol is concentrated in the Lo phase, where it induces structural changes.

Comparison study of live cells by atomic force microscopy, confocal microscopy, and scanning electrochemical microscopy

In this report, three kinds of scanning probe microscopy techniques, atomic force microscopy (AFM), confocal microscopy (CM), and scanning electrochemical microscopy (SECM), were used to study live cells in the physiological environment. Two model cell lines, CV-1 and COS-7, were studied. Time-lapse images were obtained with both contact and tapping mode AFM techniques. Cells were more easily scratched or moved by contact mode AFM than by tapping mode AFM. Detailed surface structures such as filamentous structures on the cell membrane can be obtained and easily discerned with tapping mode AFM. The toxicity of ferrocenemethanol (Fc) on live cells was studied by CM in reflection mode by recording the time-lapse images of controlled live cells and live cells with different Fc concentrations. No significant change in the morphology of cells was caused by Fc. Cells were imaged by SECM with Fc as the mediator at a biased potential of 0.35 V (vs. Ag/AgCl with a saturated KCl solution). Cells did not change visibly within 1 h, which indicated that SECM was a noninvasive technique and thus has a unique advantage for the study of soft cells, since the electrode scanned above the cells instead of in contact with them. Reactive oxygen species (ROS) generated by the cells were detected and images based on these chemical species were obtained. It is demonstrated that SECM can provide not only the topographical images but also the images related to the chemical or biochemical species released by the live cells.Key words: live cells, atomic force microscopy, confocal microscopy, scanning electrochemical microscopy.

Lamellar phase supported synthesis of colloidal gold nanoparticles, nanoclusters, and nanowires

Gold nanoparticles (Au NP) have been synthesized in aqueous phase under ambient conditions in the presence of a series of various cationic double chain as well as dimeric (gemini) surfactants. The spacer chain and twin tail length of these surfactants has been systematically varied to see the effect of hydrophobicity on their capping ability. It has been observed that the increase in the length of spacer chain (from 12-2-12 to 12-6-12) and twin tails (from 10-2-10 to 14-2-14) significantly increases the lamellar phase formation and which in return acts as a wonderful template to accommodate the NP in the form of nanoclusters and nanowires. The lamellar phase practically facilitates the nucleation of Au degrees and produces large NP (15 +/- 2 nm). All reactions have also been carried out in the presence of beta-cyclodextrin (CYC) which has strong ability to complex with surfactant tail. The presence of CYC induces a tendency to form nanowire and it is more prominent in the case of surfactants with longer spacer group.

Spatially controlled cell adhesion via micropatterned surface modification of poly(dimethylsiloxane)

Spatial control of cell growth on surfaces can be achieved by the selective deposition of molecules that influence cell adhesion. The fabrication of such substrates often relies upon photolithography and requires complex surface chemistry to anchor adhesive and inhibitory molecules. The production of simple, cost-effective substrates for cell patterning would benefit numerous areas of bioanalytical research including tissue engineering and biosensor development. Poly(dimethylsiloxane) (PDMS) is routinely used as a biomedical implant material and as a substrate for microfluidic device fabrication; however, the low surface energy and hydrophobic nature of PDMS inhibits its bioactivity. We present a method for the surface modification of PDMS to promote localized cell adhesion and proliferation. Thin metal films are deposited onto PDMS through a physical mask in the presence of a gaseous plasma. This treatment generates topographical and chemical modifications of the polymer surface. Removal of the deposited metal exposes roughened PDMS regions enriched with hydrophilic oxygen-containing species. The morphology and chemical composition of the patterned substrates were assessed by optical and atomic force microscopies as well as X-ray photoelectron spectroscopy. We observed a direct correlation between the surface modification of PDMS and the micropatterned adhesion of fibroblast cells. This simple protocol generates inexpensive, single-component substrates capable of directing cell attachment and growth.

Fibronectin organization under and near cells

Polymerization of soluble fibronectin molecules results in fibres that are visible as networks using fluorescently labelled fibronectin protomers or by antibody labelling. Displacement of fibres composed of modified protomers in living cells provides information regarding matrix structure, organization, and movement. A static analysis of fibronectin structures and patterns of organization provide insight into their reorganization during adhesion and motility. Confocal microscopy and atomic force microscopy (AFM) reveal fibronectin-containing networks aligned in arrays perpendicular to the retracting cell edge and in apparently disordered networks of fibres under the cell. The change in patterns suggests a reorganization of fibronectin from disordered arrays used for adhesion into ordered arrays during movement of the cell. Comparison of confocal images with corresponding AFM images confirms that the fibres left on the surface as the cell moves away do contain fibronectin. The orientation of these fibres relative to the tail (uropod) and the receding edges of the cell leads us to propose that cells generate a force on the fibres that exceeds the adhesion force of the fibres to the surface causing them to pull fibronectin fibres into straight arrays. However, when the fibres are parallel to the direction of pull, the fibres remain attached to the surface. The data supports the hypothesis that disorganized, linear fibres are the product of Fn polymerization induced by the cell beneath it and serve to adhere the cell to the substrate as the cell spreads, whereas arrays of fibres found outside the cell are formed as existing fibrils and reorganize during cell motility.

Photolithographically patterned surface modification of poly(dimethylsiloxane) via UV-initiated graft polymerization of acrylates

Patterned surface modification of poly(dimethylsiloxane) (PDMS) is achieved by combining ultraviolet-initiated graft polymerization (UV-GP) and photolithography. Poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) patterns were grafted onto PDMS with micrometer-scale feature edge resolution. The morphology and chemical composition of the grafted layers were assessed by optical and atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and XPS imaging. AFM section analyses demonstrated the deposition of 33 +/- 1 and 62 +/- 8 nm thick patterned films of PAA and PMAA, respectively. Spatially resolved C 1s XPS provided images of carboxylic acid functionalities, verifying the patterned deposition of acrylate films on PDMS. These observations demonstrate the general usefulness of UV-GP and photolithography for micropatterning.

Dynamics of GPI-anchored proteins on the surface of living cells

Rather than being distributed homogeneously on the cell surface, proteins are probably aggregated in clusters or in specific domains. Some of these domains (lipid rafts) have lipid compositions, which differ from their surrounding membrane. They have been implicated in cell signaling, cell adhesion, and cholesterol homeostasis. Estimates of their size vary from 40 to 350 nm in diameter depending on the study and cell type used. Rafts are enriched in glycosphingolipids and cholesterol and appear to be in a more ordered lipid phase. Although there is some knowledge of their function in cell signaling, less is known about their assembly and dynamics in cells at various temperatures. We use image correlation spectroscopy and dynamic image correlation spectroscopy to study the clustering and diffusion of glycosylphosphatidylinositol (GPI)-anchored proteins within the plasma membrane of living cells at various temperatures. We find that GPI-anchored proteins occur both as monomers and in clusters at the cell surface. The propensities to cluster as well as the diffusion coefficient of these clusters are strongly temperature dependent. At 37 degrees C the GPI-anchored proteins are highly dynamic with a lower state of clustering than at lower temperatures.

Dynamics and interaction of caveolin-1 isoforms with BMP-receptors

Caveolae are small invaginations of the cell membrane that are thought to play a role in important physiological functions such as cell surface signaling, endocytosis and intracellular cholesterol transport. Caveolin-1 is a key protein in these domains and contributes to the organization of cholesterol and saturated lipids within these vesicular invaginations of the plasma membrane. Caveolae are thought to be involved in the signaling of tyrosine kinase receptors and serine threonine receptors. In this article we focus on the involvement of caveolae in the signal transduction of bone morphogenetic proteins (BMPs). BMPs play important roles during embryonic development and especially in chondrogenesis, osteogenesis, neurogenesis and hematopoiesis. The initiation of the signal tranduction starts by the binding of a BMP to a corresponding set of BMP receptors. Using image cross-correlation spectroscopy, we show that the BMP receptors BRIa and BRII colocalize with caveolin-1 isoforms alpha and beta on the cell surface. BRIa colocalizes predominantly with the caveolin-1 alpha isoform. Coexpression of BRII leads to a redistribution of BRIa into domains enriched in caveolin-1 beta. After stimulation with BMP-2, BRIa moves back into the region with caveolin-1 alpha. BRII is expressed in regions enriched in caveolin-1 alpha and beta. Stimulation of cells with BMP-2 leads to a redistribution of BRII into domains enriched in caveolin-1 alpha. Immunoprecipitation studies using transfected COS-7 cells indicate that BRII binds to caveolin-1 alpha and beta. The binding of BRII to caveolin-1 was verified using A431 cells. Stimulation of starved A431 cells with BMP-2 lead to a release of caveolin-1 from the BMP receptors. We show further that the caveolin-1 beta isoform inhibits BMP signaling whereas the alpha isoform does not.

Disparate effects of two phosphatidylcholine binding proteins, C-reactive protein and surfactant protein A, on pulmonary surfactant structure and function

C-reactive protein (CRP) and surfactant protein A (SP-A) are phosphatidylcholine (PC) binding proteins that function in the innate host defense system. We examined the effects of CRP and SP-A on the surface activity of bovine lipid extract surfactant (BLES), a clinically applied modified natural surfactant. CRP inhibited BLES adsorption to form a surface-active film and the film's ability to lower surface tension (γ) to low values near 0 mN/m during surface area reduction. The inhibitory effects of CRP were reversed by phosphorylcholine, a water-soluble CRP ligand. SP-A enhanced BLES adsorption and its ability to lower γ to low values. Small amounts of SP-A blocked the inhibitory effects of CRP. Electron microscopy showed CRP has little effect on the lipid structure of BLES. SP-A altered BLES multilamellar vesicular structure by generating large, loose bilayer structures that were separated by a fuzzy amorphous material, likely SP-A. These studies indicate that although SP-A and CRP both bind PC, there is a difference in the manner in which they interact with surface films.

Signal transduction of bone morphogenetic protein receptors

Bone morphogenetic proteins (BMPs) play a crucial role during all stages of embryonic development. Although only two major signaling pathways have been characterized (the p38 and Smad pathways), the BMP signaling is complex and includes several negative feedback mechanisms. This article reviews the current state of BMP receptor signaling and provides a summary of the crosstalk of the BMP receptor pathway with other major signaling pathways.

Effect of Acute Lung Injury on Structure and Function of Pulmonary Surfactant Films

The structural and functional alterations in pulmonary surfactant that occur during acute lung injury were studied using rat lung surfactant large aggregates (LA) isolated from normal nonventilated lungs (N), and from standard ventilated (V) and injuriously ventilated (IV) excised lungs. N lungs inflated significantly better than IV lungs, with V lungs intermediate. Although IV LA phosphatidylcholine levels were unchanged, cholesterol and protein were elevated. V LA exhibited PC/cholesterol and PC/protein ratios intermediate between N and IV. In contrast to total cholesterol and protein levels, these ratios were not significantly different from IV LA. N and V LA, but not IV LA, adsorbed rapidly and were able to generate surface pressures (pi) near 70 mN/m during surface area reduction at 37 degrees C on a captive bubble tensiometer. Langmuir-Wilhelmy surface balance studies at 23 degrees C showed N LA films consistently attained pi approaching 70 mN/m during ten compression-expansion cycles. IV films were less effective and failed to achieve high pi consistently after the sixth cycle. V films were intermediate. Epifluorescence studies revealed compression of adsorbed N LA films formed well-defined liquid-condensed (LC) domains, but fewer, smaller domains were observed with IV films and, to a lesser extent, V films. Atomic force microscopy on Langmuir-Blodgett N films transferred at pi = 30 mN/m showed high, well-defined LC domains. IV films showed thinner, intermediate height, possibly fluid domains, which contain large numbers of small higher domains with heights corresponding to LC domains. V films were intermediate. We conclude that acute lung injury induced by hyperventilation, and to a lesser extent standard ventilation, of excised lungs alters surfactant surface activity and the ability of natural surfactant to form surface structures at the air-water interface.

Analyzing protein-protein interactions in cell membranes

Interactions among membrane proteins regulate numerous cellular processes, including cell growth, cell differentiation and apoptosis. We need to understand which proteins interact, where they interact and to which extent they interact. This article describes a set of novel approaches to measure, on the surface of living cells, the number of clusters of proteins, the number of proteins per cluster, the number of clusters or membrane domains that contain pairs of interacting proteins and the fraction of one protein species that interacts with another protein within these domains. These data can then be interpreted in terms of the function of the protein-protein interactions.

Analyzing for Co-Localization of Proteins at a Cell Membrane

Cell-to-cell communication is mediated by molecular interactions at the surface of the cell by soluble ligands released from distant cells or by cell surface molecules on adjacent cells. These interactions lead to activation of intracellular signaling pathways that subsequently can lead to activation of specific genes. This signal transduction process controls cellular activities as diverse as proliferation, differentiation and apoptosis, so we must understand the underlying molecular events in detail in order to understand broader questions related to development, uncontrolled growth in tumors, tissue regeneration and use of stem cells to name a few. Binding of a ligand in the extracellular space to a transmembrane receptor constitutes the first crucial step for activation of a signaling pathway within the cell. This binding can either lead to oligomerization of individual receptors, to reorganization of existing clusters of receptors or to changes in the protein conformations, which in turn results in recruitment of signaling molecules in the cytoplasm. While different membrane receptors activate different downstream signaling pathways, some receptors can activate more than one pathway and a particular pathway can be activated by different receptors. It appears that these processes are regulated either by agonists and antagonists in the extracellular medium, by receptor-receptor interactions in the membrane or by a number of signaling mediators in the cytoplasm of the cell. Our work has focused on understanding how the intermolecular interactions in the membrane can control the signal transduction process: Are there specialized structures on the surface that facilitate receptor-receptor interactions? Do the receptors exist as monomers or pre-existing complexes that enhance the probability of activation? Do different receptors associate in the same domains or are there distinct organizational principles for each receptor type. In order to address these questions, we seek to develop tools that allow us to examine intermolecular interactions and reactions directly on the cell surface, particularly on live cells in culture or in tissue. This review discusses some of the approaches that are currently available and highlights some of the key advantages and disadvantages they represent with particular focus on image cross correlation spectroscopy as a relatively new quantitative tool developed by us to address some of these issues.

Caveolin-1 isoform reorganization studied by image correlation spectroscopy

Caveolae are small, flask shaped invaginations in the cell membrane. They are thought to play a crucial role in cell signaling, endocytosis and intracellular cholesterol transport. Caveolin-1, 2 and 3 are key proteins, which are important for the formation of the invaginations on the cell surface. Caveolin-1 exists in two isoforms: caveolin-1 alpha (a) and caveolin-1 beta (beta). Little is known about the difference between these two isoforms, and less in known about their role in cell signaling. Bone morphogenetic proteins IBMPs) are a subfamily of the TGF beta superfamily and their response is mediated by serine/threonine kinase receptors. Epidermal growth factor (EGF) is known to signal through tyrosine kinase receptors of the ErbB family. Here we report on the aggregation and association of caveolin-1 isoforms with these receptors and the effect of BMP and EGF activation on caveolin-1 distribution in A431 cells. Our data, obtained by application of a family of image correlation spectroscopy tools, indicate that BMP and EGF stimulation lead to a rearrangement of the caveolin-1 isoforms on the cell surface. BMP as well as EGF stimulation leads to a rearrangement of the caveolin-1 P isoform into domains enriched in the caveolin-1 alpha isoform. We further show that about 20-30% of the caveolin-1 present at the surface of the cells co-localize with the EGF and BMP receptors. Using a reporter gene assay sensitive to the activation of the BMP pathway, we show that overexpression of caveolin-1beta inhibits signaling. Our data suggest that the two isoforms of caveolin-1 play different roles on the cell surface and that caveolae are dynamic structures.

β1integrins are distributed in adhesion structures with fibronectin and caveolin and in coated pits

Integrins are found in adhesion structures, which link the extracelullar matrix to cytoskeletal proteins. Here, we attempt to further define the distribution of β1integrins in the context of their association with matrix proteins and other cell surface molecules relevant to the endocytic process. We find that β1integrins colocalize with fibronectin in fibrillar adhesion structures. A fraction of caveolin is also organized along these adhesion structures. The extracellular matrix protein laminin is not concentrated in these structures. The α4β1integrin exhibits a distinct distribution from other β1integrins after cells have adhered for 1 h to extracellular matrix proteins but is localized in adhesion structures after 24 h of adhesion. There are differences between the fibronectin receptors: α5β1integrins colocalize with adaptor protein-2 in coated pits, while α4β1integrins do not. This parallels our earlier observation that of the two laminin receptors, α1β1and α6β1, only αaβ1integrins colocalize with adaptor protein-2 in coated pits. Calcium chelation or inhibition of mitogen-activated protein kinase kinase, protein kinase C, or src did not affect localization of α1β1and α5β1integrins in coated pits. Likewise, the integrity of coated-pit structures or adhesion structures is not required for integrin and adaptor protein-2 colocalization. This suggests a robust and possibly constitutive interaction between these integrins and coated pits.Key words: adhesion, endocytosis, extracellular matrix, microscopy, confocal, signalling.

Effect of the distribution and clustering of the type I A BMP receptor (ALK3) with the type II BMP receptor on the activation of signalling pathways

Bone morphogenetic proteins (BMPs) play an important role during embryonic development, especially in chondrogenesis, osteogenesis, neurogenesis and hematopoiesis. There are over 19 BMPs known in mammalians, but only three BMP-type-I receptors and three BMP-type-II receptors are known so far to mediate these responses. Previous reports provide evidence to support that oligomerisation of BMP receptors influences the activation of the downstream BMP signalling pathways, the Smad or the p38 MAPK pathway. To further explore the importance of BMP receptor clustering in signalling, image correlation spectroscopy has been used to investigate the clustering and distribution of BMP receptors at the surface of the cell membrane. Here we demonstrate that the co-expression of the BMP-type-II receptor (BRII) influences the aggregation and the distribution of the BMP-type-Ia receptor (BRIa) in COS7 cells and in A431 cells. We also demonstrate that BMP-2 stimulation of the cells leads to a rearrangement of receptor complexes at the cell surface. Using A431 cells and limb bud-derived mesenchymal cells, we show that co-expression of the BRII and a constitutive active BRIa-ca is necessary for the activation of the Smad pathway. Importantly using a kinase-inactive BRII the rearrangement of BRIa is blocked. Together, these findings suggest that rearrangement of the receptors at the cell surface prior to forming preformed ligand independent complexes plays a critical role in activation of the Smad pathway. It also suggests further that the kinase activity of BRII is needed for signalling beyond the activation of BRIa at the GS domain.

Isolation of bright aggregate fluctuations in a multipopulation image correlation spectroscopy system using intensity subtraction

Image correlation spectroscopy allows sensitive measurement of the spatial distribution and aggregation state of fluorescent membrane macro molecules. When studying a single population system (i.e., aggregates of similar brightness), an accurate measure can be made of the aggregate number per observation area, but this measurement becomes much more complex in a distributed population system (i.e., bright and faint aggregates). This article describes an alternate solution that involves extraction of the bright aggregate population information. This novel development for image correlation spectroscopy, termed intensity subtraction analysis, uses sequential uniform intensity subtraction from raw confocal images. Sequential intensity subtraction results in loss of faint aggregate fluctuations that are smaller in magnitude than fluctuations due to the brightest aggregates. The resulting image has correlatable fluctuations originating from only the brightest population, permitting quantification of this population's distribution and further cross-correlation measurements. The feasibility of this technique is demonstrated using fluorescent microsphere images and biological samples. The technique is further used to examine the spatial distribution of a plasma-membrane-labeled fluorescent synthetic ganglioside, and to cross-correlate this probe with various membrane markers. The evidence provided demonstrates that bright aggregates of the fluorescent ganglioside are associated with clathrin-coated pits, membrane microvilli, and detergent-resistant membranes.

Structural alterations of phospholipid film domain morphology induced by cholesterol

Structures of the monolayer films of dipalmitoylphosphatidylcholine (DPPC) mixed with different amounts of cholesterol were studied at air-water interface using surface pressure-area measurements, epifluorescence microscopy and atomic force microscopy (AFM). Pure DPPC, cholesterol or DPPC-cholesterol mixtures were dissolved in organic solvents with a small amount of fluorescently labeled phospholipid probe (NBD-PC) and spread onto the air-water interface. Surface pressure-area isotherms and epifluorescence microscopy of such films at the air-water interface suggested that DPPC undergoes a gas to fluid to condensed phase transition, while cholesterol undergoes a gas to solid-like transition. A shift of the surface pressure-area curve to lower area per molecule was observed when cholesterol was mixed with DPPC. Epifluorescence microscopy showed the formation of spiral shaped domains for mixed monolayers. Increase in cholesterol content abolished domain characteristics possibly due to fluidizing property of cholesterol. AFM measurements of monolayers, transferred onto freshly cleaved mica by Langmuir-Blodgett technique, revealed the alterations caused by cholesterol on the gel and fluid domains of such films. AFM measurements re-established similar trend in domain characteristics as evidenced in epifluorescence microscopy.