Fatty acid alteration and the lateral diffusion of lipids in the plasma membrane of keratinocytes

Turnover and flow of the cell membrane for cell migration

The role of cell membrane dynamics in cell migration is unclear. To examine whether total cell surface area changes are required for cell migration, Dictyostelium cells were flattened by agar-overlay. Scanning electron microscopy demonstrated that flattened migrating cells have no membrane reservoirs such as projections and membrane folds. Similarly, optical sectioning fluorescence microscopy showed that the cell surface area does not change during migration. Interestingly, staining of the cell membrane with a fluorescent lipid analogue demonstrated that the turnover rate of cell membrane is closely related to the cell migration velocity. Next, to clarify the mechanism of cell membrane circulation, local photobleaching was separately performed on the dorsal and ventral cell membranes of rapidly moving cells. The bleached zones on both sides moved rearward relative to the cell. Thus, the cell membrane moves in a fountain-like fashion, accompanied by a high membrane turnover rate and actively contributing to cell migration.

Universal Approach to FRAP Analysis of Arbitrary Bleaching Patterns

The original approach to calculating diffusion coefficients of a fluorescent probe from Fluorescence Recovery After Photobleaching (FRAP) measurements assumes bleaching with a circular laser beam of a Gaussian intensity profile. This method was used without imaging the bleached cell. An empirical equation for calculating diffusion coefficients from a rectangular bleaching geometry, created in a confocal image, was later published, however a single method allowing the calculation of diffusion coefficients for arbitrary geometry does not exist. Our simulation approach allows computation of diffusion coefficients regardless of bleaching geometry used in the FRAP experiment. It accepts a multiple-frame TIFF file, representing the experiment as input, and simulates the (pure) diffusion of the fluorescent probes (2D random walk) starting with the first post-bleach frame of the actual data. It then fits the simulated data to the real data and extracts the diffusion coefficient. We validate our approach using a well characterized diffusing molecule (DiIC18) against well-established analytical procedures. We show that the algorithm is able to calculate the absolute value of diffusion coefficients for arbitrary bleaching geometries, including exaggeratedly large ones. It is provided freely as an ImageJ plugin, and should facilitate quantitative FRAP measurements for users equipped with standard fluorescence microscopy setups.

Independent mobility of proteins and lipids in the plasma membrane of Escherichia coli

Fluidity is essential for many biological membrane functions. The basis for understanding membrane structure remains the classic Singer-Nicolson model, in which proteins are embedded within a fluid lipid bilayer and able to diffuse laterally within a sea of lipid. Here we report lipid and protein diffusion in the plasma membrane of live cells of the bacterium Escherichia coli, using Fluorescence Recovery after Photobleaching (FRAP) and Total Internal Reflection Fluorescence (TIRF) microscopy to measure lateral diffusion coefficients. Lipid and protein mobility within the membrane were probed by visualizing an artificial fluorescent lipid and a simple model membrane protein consisting of a single membrane-spanning alpha-helix with a Green Fluorescent Protein (GFP) tag on the cytoplasmic side. The effective viscosity of the lipid bilayer is strongly temperature-dependent, as indicated by changes in the lipid diffusion coefficient. Surprisingly, the mobility of the model protein was unaffected by changes in the effective viscosity of the bulk lipid, and TIRF microscopy indicates that it clusters in segregated, mobile domains. We suggest that this segregation profoundly influences the physical behaviour of the protein in the membrane, with strong implications for bacterial membrane function and bacterial physiology.

Mobility of photosynthetic proteins

The mobility of photosynthetic proteins represents an important factor that affects light-energy conversion in photosynthesis. The specific feature of photosynthetic proteins mobility can be currently measured in vivo using advanced microscopic methods, such as fluorescence recovery after photobleaching which allows the direct observation of photosynthetic proteins mobility on a single cell level. The heterogeneous organization of thylakoid membrane proteins results in heterogeneity in protein mobility. The thylakoid membrane contains both, protein-crowded compartments with immobile proteins and fluid areas (less crowded by proteins), allowing restricted diffusion of proteins. This heterogeneity represents an optimal balance as protein crowding is necessary for efficient light-energy conversion, and protein mobility plays an important role in the regulation of photosynthesis. The mobility is required for an optimal light-harvesting process (e.g., during state transitions), and also for transport of proteins during their synthesis or repair. Protein crowding is then a key limiting factor of thylakoid membrane protein mobility; the less thylakoid membranes are crowded by proteins, the higher protein mobility is observed. Mobility of photosynthetic proteins outside the thylakoid membrane (lumen and stroma/cytosol) is less understood. Cyanobacterial phycobilisomes attached to the stromal side of the thylakoid can move relatively fast. Therefore, it seems that stroma with their active enzymes of the Calvin-Benson cycle, are a more fluid compartment in comparison to the rather rigid thylakoid lumen. In conclusion, photosynthetic protein diffusion is generally slower in comparison to similarly sized proteins from other eukaryotic membranes or organelles. Mobility of photosynthetic proteins resembles restricted protein diffusion in bacteria, and has been rationalized by high protein crowding similar to that of thylakoids.

Measuring diffusion of lipid-like probes in artificial and natural membranes by raster image correlation spectroscopy (RICS): use of a commercial laser-scanning microscope with analog detection

The heterogeneity in composition and interaction within the cellular membrane translates into a wide range of diffusion coefficients of its constituents. Therefore, several complementary microfluorimetric techniques such as fluorescence correlation spectroscopy (FCS), fluorescence recovery after photobleaching (FRAP) and single-particle tracking (SPT) have to be applied to explore the dynamics of membrane components. The recently introduced raster image correlation spectroscopy (RICS) offers a much wider dynamic range than each of these methods separately and allows for spatial mapping of the dynamic properties. RICS is implemented on a confocal laser-scanning microscope (CLSM), and the wide dynamic range is achieved by exploiting the inherent time information carried by the scanning laser beam in the generation of the confocal images. The original introduction of RICS used two-photon excitation and photon counting detection. However, most CLSM systems are based on one-photon excitation with analog detection. Here we report on the performance of such a commercial CLSM (Zeiss LSM 510 META) in the study of the diffusion of the fluorescent lipid analog 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indodicarbocyanine perchlorate (DiI-C(18)(5)) both in giant unilamellar vesicles and in the plasma membrane of living oligodendrocytes, i.e., the myelin-producing cells of the central nervous system. It is shown that RICS on a commercial CLSM with analog detection allows for reliable results in the study of membrane diffusion by removal of unwanted correlations introduced by the analog detection system. The results obtained compare well with those collected by FRAP and FCS.

Diffusion coefficient of fluorescent phosphatidylinositol 4,5-bisphosphate in the plasma membrane of cells

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) controls a surprisingly large number of processes in cells. Thus, many investigators have suggested that there might be different pools of PIP(2) on the inner leaflet of the plasma membrane. If a significant fraction of PIP(2) is bound electrostatically to unstructured clusters of basic residues on membrane proteins, the PIP(2) diffusion constant, D, should be reduced. We microinjected micelles of Bodipy TMR-PIP(2) into cells, and we measured D on the inner leaflet of fibroblasts and epithelial cells by using fluorescence correlation spectroscopy. The average +/- SD value from all cell types was D = 0.8 +/- 0.2 microm(2)/s (n = 218; 25 degrees C). This is threefold lower than the D in blebs formed on Rat1 cells, D = 2.5 +/- 0.8 microm(2)/s (n = 26). It is also significantly lower than the D in the outer leaflet or in giant unilamellar vesicles and the diffusion coefficient for other lipids on the inner leaflet of these cell membranes. The simplest interpretation is that approximately two thirds of the PIP(2) on inner leaflet of these plasma membranes is bound reversibly.

Using fluorescence recovery after photobleaching to measure lipid diffusion in membranes

The lateral diffusion of lipids is crucial to the biogenesis and function of biological membranes. In this chapter, approaches for observing the lateral diffusion of lipids using fluorescence recovery after photobleaching are described. The procedures described can be carried out with a standard laser-scanning confocal microscope. The membrane of interest is stained with a lipophilic fluorophore or fluorescent lipid analog. The confocal laser spot is then used to photobleach fluorescence in a small region of the sample. Subsequent spread and recovery of the bleach, reports on the diffusion of the fluorophore. The results provide a measure of membrane fluidity, and the extent to which lipid diffusion in the membrane might be constrained. Fluorescence recovery after photobleaching measurements may be carried out in vivo, in systems ranging from bacteria to mammalian cells, or in vitro in isolated membrane fragments. Procedures for preparing the biological sample, performing the measurement, and quantitative data analysis are explained.

Regional cardiac ganglia projections in the guinea pig heart studied by postmortem DiI tracing

Our purpose was to identify and localize intrinsic cardiac ganglia innervating distinct regions of the heart using postmortem tracing of nerve projections with DiI, a method not previously used to study the intrinsic cardiac nervous system. We also investigated the possibility of collateral innervation of myocardium and intrinsic ganglia. In isolated paraformaldehyde-fixed guinea pig hearts, crystals of DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) were inserted into the posterior ventricular myocardium below the atrioventricular groove, the right atrium, or the left ventricular septum. Hearts were placed in the dark at 37 degrees C for 2-14 weeks to allow DiI diffusion within neuronal membranes. Labeled neurons were observed in intracardiac ganglia after at least 4 weeks of dye exposure. Labeling was restricted to the inferior-most ganglia (those near the atrioventricular groove) when DiI was inserted into the posterior ventricular myocardium and to ganglia near the sinus node after right atrial DiI placement. Application of DiI to the left ventricular septum resulted in neuron labeling in ganglia primarily in the interatrial septum near the atrioventricular node. After 8 weeks, DiI-labeled nerve fibers and varicosities were seen surrounding unlabeled neurons in some ganglia, suggesting that axons terminating in or passing through the DiI application site in posterior ventricular tissue had collateral branches innervating these ganglia. These results indicate that intrinsic innervation of major cardiac subdivisions is accomplished by regionally segregated cardiac ganglia. Also, tracing with DiI has provided evidence for collateral nerve projections that could be the substrate for novel intracardiac regulatory circuits.

Low mobility of phosphatidylinositol 4,5-bisphosphate underlies receptor specificity of Gq-mediated ion channel regulation in atrial myocytes

We have shown previously that cardiac G protein-gated inwardly rectifying K+ (GIRK) channels are inhibited by Gq protein-coupled receptors (GqPCRs) via phosphatidylinositol 4,5-bisphosphate (PIP2) depletion in a receptor-specific manner. To investigate the mechanism of receptor specificity, we examined whether the activation of GqPCRs induces localized PIP2 depletion. When we applied endothelin-1 to the bath, GIRK channel activities recorded in cell-attached patches were not changed, implying that PIP2 signal is not diffusible but is a localized signal. To test this possibility, we directly measured lateral diffusion by introducing fluorescence-labeled phosphoinositides to a small area of the membrane with patch pipettes. After pipettes were attached, phosphatidylinositol 4-monophosphate or phosphatidylinositol diffused rapidly to the entire membrane, whereas PIP2 was confined to the membrane patch inside the pipette. The confinement of PIP2 was disrupted after cytochalasin D treatment, suggesting that the cytoskeleton is responsible for the low mobility of PIP2. The diffusion coefficient (D) of PIP2 in the plasma membrane measured with the fluorescence recovery after photobleaching technique was 0.00039 microm2/s (n = 6), which is markedly lower than D of phosphatidylinositol (5.8 microm2/s, n = 5). Simulation of PIP2 concentration profiles by the diffusion model confirms that when D is small, the kinetics of PIP2 depletion at different distances from phospholipase C becomes similar to the characteristic kinetics of GIRK inhibition by different agonists. These results imply that PIP2 depletion is localized adjacent to GqPCRs because of its low mobility, and that spatial proximity of GqPCR and the target protein underlies the receptor specificity of PIP2-mediated signaling.

In vitro skin irritation: facts and future. State of the art review of mechanisms and models

The skin is the main target tissue for exogenous noxes, protecting us from harmful environmental hazards, UV-irradiation and endogenous water loss. It is composed of three layers, whereas the outermost epidermis is a squamous epithelium that mainly consists of keratinocytes. These cells execute a terminal differentiation, which finally results in the assembly of the stratum corneum. This layer, consisting of cornified keratinocytes, is an effective barrier against a vast number of substances. Apart of this, keratinocytes play crucial roles in the immune surveillance and the initiation, modulation and regulation of inflammation in the epidermis. Regarding cutaneous inflammatory reactions, skin irritation is one of the most common adverse effect in humans. For reasons of human safety assessment new chemicals are still evaluated for irritant potentials by application to animals followed by visible changes such as erythema and oedema. Testing for skin irritation in animals potentially cause them pain and discomfort. Furthermore, the results are not always predictive for those found in humans. In order to replace animal testing and to improve the prediction of irritants, the cosmetic and toiletry industry, in Europe represented by Colipa, develops and uses several alternative in vitro test systems. In this respect, the use of in vitro reconstructed organotypic skin equivalents are mostly favored, because of their increasingly close resemblance to human skin. Due to ethical and scientific questions and on account of the 7th amendment of the European Council Directive 76/768/EEC, the authors see the requirement to drive the development of alternative tests for irritants. Therefore, this article centres on cosmetic ingredients and provides the readership an overview of the state of art of cellular mechanisms of skin irritation and summarizes the results of the commonly used skin equivalents to evaluate irritation in vitro.

Creating biological membranes on the micron scale: forming patterned lipid bilayers using a polymer lift-off technique

We present a new method for creating patches of fluid lipid bilayers with conjugated biotin and other compounds down to 1 microm resolution using a photolithographically patterned polymer lift-off technique. The patterns are realized as the polymer is mechanically peeled away in one contiguous piece in solution. The functionality of these surfaces is verified with binding of antibodies and avidin on these uniform micron-scale platforms. The biomaterial patches, measuring 1 micro m-76 microm on edge, provide a synthetic biological substrate for biochemical analysis that is approximately 100x smaller in width than commercial printing technologies. 100 nm unilamellar lipid vesicles spread to form a supported fluid lipid bilayer on oxidized silicon surface as confirmed by fluorescence photobleaching recovery. Fluorescence photobleaching recovery measurements of DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiIC(18)(3))) stained bilayer patches yielded an average diffusion coefficient of 7.54 +/- 1.25 microm(2) s(-1), equal to or slightly faster than typically found in DiI stained cells. This diffusion rate is approximately 3x faster than previous values for bilayers on glass. This method provides a new means to form functionalized fluid lipid bilayers as micron-scale platforms to immobilize biomaterials, capture antibodies and biotinylated reagents from solution, and form antigenic stimuli for cell stimulation.

Lipid diffusion in the thylakoid membranes of the cyanobacterium Synechococcus sp.: effect of fatty acid desaturation

Thylakoid membranes are crucial to photosynthesis in cyanobacteria and plants. In cyanobacteria, genetic modification of membrane lipid composition strongly influences cold tolerance and susceptibility to photoinhibition. We have used fluorescence recovery after photobleaching to measure the diffusion of a lipid-soluble fluorescent marker in cells of the cyanobacterium Synechococcus sp. PCC 7942. We have compared the wild-type strain with a transformant with an increased level of fatty acid unsaturation. The transformant showed a six-fold increase in the diffusion coefficient for the fluorescent marker at growth temperature. This is the first direct measurement of lipid diffusion in a photosynthetic membrane.

HDL-mediated cholesterol uptake and targeting to lipid droplets in adipocytes

Adipocytes express high levels of the HDL scavenger receptor class B type I in a differentiation-dependent manner. We thus have analyzed the routes of HDL cholesterol trafficking at different phases of adipocyte differentiation in the 3T3-L1 cell line. One novel and salient feature of this paper is the observation of a widespread distribution in the cell cytoplasm of Golgi markers, caveolin-2, and a fluorescent cholesterol analog NBD-cholesterol (NBD-chol), observed in the early phases of adipocyte formation, clearly distinct from that observed in mature fat cells (i.e., with fully formed lipid vesicles). Thus, in cells without visible lipid droplets, Golgi markers (Golgi 58K, Golgin 97, trans-Golgi network 38, Rab 6, and BODIPY-ceramide), caveolin-2, and NBD-chol all colocalize in a widespread distribution in the cell. In contrast, when lipid droplets are fully formed at latter stages, these markers clearly are distributed to distinct cell compartments: a compact juxtanuclear structure for the Golgi markers and caveolin-2, while NDB-chol concentrates in lipid droplets. In addition, disorganization of the Golgi using three different agents (Brefeldin, monensin, and N-ethyl-maleimide) drastically reduces NBD-chol uptake at different phases of adipocyte formation, strongly suggesting that the Golgi apparatus plays a critical role in HDL-mediated NBD uptake and routing to lipid droplets.

Agonist-induced PIP(2) hydrolysis inhibits cortical actin dynamics: regulation at a global but not at a micrometer scale

Phosphatidylinositol 4, 5-bisphosphate (PIP(2)) at the inner leaflet of the plasma membrane has been proposed to locally regulate the actin cytoskeleton. Indeed, recent studies that use GFP-tagged pleckstrin homology domains (GFP-PH) as fluorescent PIP(2) sensors suggest that this lipid is enriched in membrane microdomains. Here we report that this concept needs revision. Using three distinct fluorescent GFP-tagged pleckstrin homology domains, we show that highly mobile GFP-PH patches colocalize perfectly with various lipophilic membrane dyes and, hence, represent increased lipid content rather than PIP(2)-enriched microdomains. We show that bright patches are caused by submicroscopical folds and ruffles in the membrane that can be directly visualized at approximately 15 nm axial resolution with a novel numerically enhanced imaging method. F-actin motility is inhibited significantly by agonist-induced PIP(2) breakdown, and it resumes as soon as PIP(2) levels are back to normal. Thus, our data support a role for PIP(2) in the regulation of cortical actin, but they challenge a model in which spatial differences in PIP(2) regulation of the cytoskeleton exist at a micrometer scale.

Lateral diffusion coefficients in membranes measured by resonance energy transfer and a new algorithm for diffusion in two dimensions

We describe measurements of lateral diffusion in membranes using resonance energy transfer. The donor was a rhenium (Re) metal-ligand complex lipid, which displays a donor decay time near 3 micros. The long donor lifetime resulted in an ability to measure lateral diffusion coefficient below 10(-8) cm(2)/s. The donor decay data were analyzed using a new numerical algorithm for calculation of resonance energy transfer for donors and acceptors randomly distributed in two dimensions. An analytical solution to the diffusion equation in two dimensions is not known, so the equation was solved by the relaxation method in Laplace space. This algorithm allows the donor decay in the absence of energy transfer to be multiexponential. The simulations show that mutual lateral diffusion coefficients of the donor and acceptor on the order of 10(-8) cm(2)/s are readily recovered from the frequency-domain data with donor decay times on the microsecond timescale. Importantly, the lateral diffusion coefficients and acceptor concentrations can be recovered independently despite correlation between these parameters. This algorithm was tested and verified using the donor decays of a long lifetime rhenium lipid donor and a Texas red-lipid acceptor. Lateral diffusion coefficients ranged from 4.4 x 10(-9) cm(2)/s in 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG) at 10 degrees C to 1.7 x 10(-7) cm(2)/s in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) at 35 degrees C. These results demonstrated the possibility of direct measurements of lateral diffusion coefficients using microsecond decay time luminophores.

Agonist-dependent immobilization of chimeric bombesin/GRP receptors: dependence on c-Src activity and dissociation from internalization

G-protein-coupled receptors (GPCRs) are membrane proteins that exhibit a decreased mobile fraction compared to a freely mobile plasma membrane protein. Recently, interest has focused on proteins other than heterotrimeric G-proteins that interact with GPCRs as scaffolding structures that affect receptor signal transduction. In order to investigate the physical state of receptors before and after agonist, we used fluorescence recovery after photobleaching of the bombesin/gastrin-releasing peptide (GRP) receptor fused to the intrinsically fluorescent green fluorescent protein (GFP-GRP receptor) expressed in KNRK cells to measure both the fraction of mobile receptors and their diffusion rate before and after agonist stimulation. In live cells at 37 degrees C, addition of GRP (100 nM) caused a rapid decrease in GFP-GRP receptor mobile fraction from 0.8 +/- 0.1 to 0.49 +/- 0.05, which was independent of endocytosis. Concurrently, the remaining mobile GFP-GRPreceptors showed an increase in the diffusion rate with the half-time of fluorescent recovery, tau(1/2) = 46 +/- 7 s for untreated cells, decreasing to tau(1/2) = 30 +/- 6 s for cells treated with GRP. Prior treatment with the Src-specific inhibitor PP-2 (10 microM) blocked GFP-GRP receptor immobilization while treatment with the inactive analog PP-3 (10 microM) did not affect receptor immobilization. These data suggest that agonist-bound GPCR have increased plasma membrane diffusion rates but an increased affinity for immobilization into a multiprotein complex that is mediated by Src activity.

Actin dynamics at the living cell submembrane imaged by total internal reflection fluorescence photobleaching

Although reversible chemistry is crucial to dynamical processes in living cells, relatively little is known about relevant chemical kinetic rates in vivo. Total internal reflection/fluorescence recovery after photobleaching (TIR/FRAP), an established technique previously demonstrated to measure reversible biomolecular kinetic rates at surfaces in vitro, is extended here to measure reversible biomolecular kinetic rates of actin at the cytofacial (subplasma membrane) surface of living cells. For the first time, spatial imaging (with a charge-coupled device camera) is used in conjunction with TIR/FRAP. TIR/FRAP imaging produces both spatial maps of kinetic parameters (off-rates and mobile fractions) and estimates of kinetic correlation distances, cell-wide kinetic gradients, and dependences of kinetic parameters on initial fluorescence intensity. For microinjected rhodamine actin in living cultured smooth muscle (BC3H1) cells, the unbinding rate at or near the cytofacial surface of the plasma membrane (averaged over the entire cell) is measured at 0.032 +/- 0.007 s(-1). The corresponding rate for actin marked by microinjected rhodamine phalloidin is very similar, 0.033 +/- 0.013 s(-1), suggesting that TIR/FRAP is reporting the dynamics of entire filaments or protofilaments. For submembrane fluorescence-marked actin, the intensity, off-rate, and mobile fraction show a positive correlation over a characteristic distance of 1-3 microm and a negative correlation over larger distances greater than approximately 7-14 microm. Furthermore, the kinetic parameters display a statistically significant cell-wide gradient, with the cell having a "fast" and "slow" end with respect to actin kinetics.

Organotypic keratinocyte cocultures in defined medium with regular epidermal morphogenesis and differentiation

Skin equivalents formed by keratinocytes cocultured with fibroblasts embedded in collagen lattices represent promising tools for mechanistic studies of skin physiology, for pharmacotoxicologic testing, and for the use as skin substitutes in wound treatment. Such cultures would be superior in defined media to avoid interference with components of serum or tissue extracts. Here we demonstrate that a defined medium (supplemented keratinocyte defined medium) supports epidermal morphogenesis in organotypic cocultures equally well as serum-containing medium (mixture of Ham's F12 and Dulbecco's modified Eagle's medium), as documented by hallmarks of the epidermal phenotype studied by immunofluorescence and electron microscopy. In both cases regularly structured, orthokeratinized epithelia evolved with similar kinetics. Morphology in mixture of Ham's F12 and Dulbecco's modified Eagle's medium was slightly hyperplastic, and keratins 1 and 10 synthesis less co-ordinated than in supplemented keratinocyte defined medium, but a consistently inverted sequence of expression of keratins 1 and 10 was found in either medium. The late differentiation markers filaggrin, involucrin, keratin 2e, and transglutaminase 1 corresponded in their typical distribution in upper suprabasal layers. Keratin 16 persisted under both conditions indicating the activated epidermal state. Keratinocyte proliferation was comparable in both media, whereas fibroblast multiplication and proliferation was delayed and reduced in supplemented keratinocyte defined medium. In both media, ultrastructural features of epidermal differentiation as well as reconstitution of a basement membrane occurred similarly. Immature lamellar bodies and cytoplasmatic vacuoles, however, indicated an impaired lipid metabolism in supplemented keratinocyte defined medium. Nevertheless, these defined organotypic cocultures provide a suitable basis for in vitro skin models to study molecular mechanisms of tissue homeostasis and for use in pharmacotoxicologic testing.