Lipid lateral diffusion and membrane organization

What's past is prologue: FRAP keeps delivering 50 years later

Fluorescence recovery after photobleaching (FRAP) has emerged as one of the most widely utilized techniques to quantify binding and diffusion kinetics of biomolecules in biophysics. Since its inception in the mid-1970s, FRAP has been used to address an enormous array of questions including the characteristic features of lipid rafts, how cells regulate the viscosity of their cytoplasm, and the dynamics of biomolecules inside condensates formed by liquid-liquid phase separation. In this perspective, I briefly summarize the history of the field and discuss why FRAP has proven to be so incredibly versatile and popular. Next, I provide an overview of the extensive body of knowledge that has emerged on best practices for quantitative FRAP data analysis, followed by some recent examples of biological lessons learned using this powerful approach. Finally, I touch on new directions and opportunities for biophysicists to contribute to the continued development of this still-relevant research tool.

Microscopic Picture of Calcium-Assisted Lipid Demixing and Membrane Remodeling Using Multiscale Simulations

The specificity of anionic phospholipids-calcium ion interaction and lipid demixing has been established as a key regulatory mechanism in several cellular signaling processes. The mechanism and implications of this calcium-assisted demixing have not been elucidated from a microscopic point of view. Here, we present an overview of atomic interactions between calcium and phospholipids that can drive nonideal mixing of lipid molecules in a model lipid bilayer composed of zwitterionic (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)) and anionic (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS)) lipids with computer simulations at multiple resolutions. Lipid nanodomain formation and growth were driven by calcium-enabled lipid bridging of the charged phosphatidylserine (PS) headgroups, which were favored against inter-POPS dipole interactions. Consistent with several experimental studies of calcium-associated membrane sculpting, our analyses also suggest modifications in local membrane curvature and cross-leaflet couplings as a response to such induced lateral heterogeneity. In addition, reverse mapping to a complementary atomistic description revealed structural insights in the presence of anionic nanodomains, at timescales not accessed by previous computational studies. This work bridges information across multiple scales to reveal a mechanistic picture of calcium ion's impact on membrane biophysics.

Effect of an Antimicrobial Peptide on Lateral Segregation of Lipids: A Structure and Dynamics Study by Neutron Scattering

Antimicrobial peptides are one of the most promising classes of antibiotic agents for drug-resistant bacteria. Although the mechanisms of their action are not fully understood, many of them are found to interact with the target bacterial membrane, causing different degrees of perturbations. In this work, we directly observed that a short peptide disturbs membranes by inducing lateral segregation of lipids without forming pores or destroying membranes. Aurein 1.2 (aurein) is a 13-amino acid antimicrobial peptide discovered in the frog Litoria genus that exhibits high antibiotic efficacy. Being cationic and amphiphilic, it binds spontaneously to a membrane surface with or without charged lipids. With a small-angle neutron scattering contrast matching technique that is sensitive to lateral heterogeneity in membrane, we found that aurein induces significant lateral segregation in an initially uniform lipid bilayer composed of zwitterionic lipid and anionic lipid. More intriguingly, the lateral segregation was similar to the domain formed below the order-disorder phase-transition temperature. To our knowledge, this is the first direct observation of lateral segregation caused by a peptide. With quasi-elastic neutron scattering, we indeed found that the lipid lateral motion in the fluid phase was reduced even at low aurein concentrations. The reduced lateral mobility makes the membrane prone to additional stresses and defects that change membrane properties and impede membrane-related biological processes. Our results provide insights into how a short peptide kills bacteria at low concentrations without forming pores or destroying membranes. With a better understanding of the interaction, more effective and economically antimicrobial peptides may be designed.

Content and activity of the testis-specific isoform of angiotensin-converting enzyme are reduced in frozen-thawed bull spermatozoa

Sperm cryopreservation and thawing reduces fertility and alters the content and function of various sperm proteins. Previously, we reported that a testes-specific isoform of angiotensin-converting enzyme (tACE) was required for capacitation of bovine spermatozoa. The aim of the present study was to determine effects of sperm cryopreservation and thawing on the content, activity and localisation of tACE in bovine spermatozoa. Relative median fluorescence intensity (flow cytometry) was greater (P<0.01), tACE content (110 kDa protein) in sperm proteins was higher (P<0.01) and there was greater tACE enzyme activity (mean (±s.e.m.) 0.16±0.01 vs 0.06±0.02UmL-1; P<0.01) in fresh versus frozen-thawed spermatozoa (n=6 bulls). In fresh spermatozoa, tACE was immunolocalised in the acrosomal and principal piece regions of the sperm head and tail respectively. However, in frozen-thawed spermatozoa, there were four patterns of localisation: most frozen-thawed spermatozoa (64%) had fluorescence in the acrosomal ridge, whereas in 17% and 9% of spermatozoa the signal was limited to the post-acrosomal region and the equatorial segment respectively; in the remainder (10%), there was no signal. We conclude that cryopreservation and thawing decrease the content and activity of tACE and cause it to be translocated to other parts of the sperm head.

Versatile cellular uptake mediated by catanionic vesicles: simultaneous spontaneous membrane fusion and endocytosis

Lactose-derived catanionic vesicles offer unique opportunities to overcome cellular barriers. These potential nanovectors, very easy to formulate as drug delivery systems, are able to encapsulate drugs of various hydrophilicity. This article highlights versatile interaction mechanisms between these catanionic vesicles, labeled with hydrophilic and amphiphilic fluorescent probes, and a mammalian cell line, Chinese Hamster Ovary. Confocal microscopy and flow cytometry techniques show that these vesicles are internalized by cells through cellular energy dependent processes, as endocytosis, but are simultaneously able to spontaneously fuse with cell plasma membranes and release their hydrophilic content directly inside the cytosol. Such innovative and polyvalent nanovectors, able to deliver their content via different internalization pathways, would positively be a great progress for the coadministration of drugs of complementary efficiency.

Metabolomics and systems pharmacology: why and how to model the human metabolic network for drug discovery

Metabolism represents the 'sharp end' of systems biology, because changes in metabolite concentrations are necessarily amplified relative to changes in the transcriptome, proteome and enzyme activities, which can be modulated by drugs. To understand such behaviour, we therefore need (and increasingly have) reliable consensus (community) models of the human metabolic network that include the important transporters. Small molecule 'drug' transporters are in fact metabolite transporters, because drugs bear structural similarities to metabolites known from the network reconstructions and from measurements of the metabolome. Recon2 represents the present state-of-the-art human metabolic network reconstruction; it can predict inter alia: (i) the effects of inborn errors of metabolism; (ii) which metabolites are exometabolites, and (iii) how metabolism varies between tissues and cellular compartments. However, even these qualitative network models are not yet complete. As our understanding improves so do we recognise more clearly the need for a systems (poly)pharmacology.

Virus assembly and plasma membrane domains: which came first?

Viral assembly is a key step in the virus life cycle. In this review, we focus mainly on the ability of retroviruses, especially HIV-1, to assemble at the plasma membrane of their host cells. The assembly process of RNA enveloped viruses necessitates a fine orchestration between the different viral components and specific interactions between viral proteins and lipids of the host cell membrane. Searching for a comparison with another RNA enveloped virus, we refer to influenza virus to show how it could share (or not) some common features with HIV-1 assembly since both viruses are believed to assemble mainly in raft microdomains. We also discuss the role of RNA and the cellular actin cytoskeleton in enhancing these viral assembly processes. Finally, based on the literature and on new results we have obtained by molecular docking, we propose another mechanism for HIV-1 assembly in membrane domains. This mechanism involves the trapping of acidic lipids by the viral Gag protein by means of ionic protein-lipid interactions, inducing thereby formation of acidic lipid-enriched microdomains (ALEM).

Advanced fluorescence microscopy techniques--FRAP, FLIP, FLAP, FRET and FLIM

Fluorescence microscopy provides an efficient and unique approach to study fixed and living cells because of its versatility, specificity, and high sensitivity. Fluorescence microscopes can both detect the fluorescence emitted from labeled molecules in biological samples as images or photometric data from which intensities and emission spectra can be deduced. By exploiting the characteristics of fluorescence, various techniques have been developed that enable the visualization and analysis of complex dynamic events in cells, organelles, and sub-organelle components within the biological specimen. The techniques described here are fluorescence recovery after photobleaching (FRAP), the related fluorescence loss in photobleaching (FLIP), fluorescence localization after photobleaching (FLAP), Förster or fluorescence resonance energy transfer (FRET) and the different ways how to measure FRET, such as acceptor bleaching, sensitized emission, polarization anisotropy, and fluorescence lifetime imaging microscopy (FLIM). First, a brief introduction into the mechanisms underlying fluorescence as a physical phenomenon and fluorescence, confocal, and multiphoton microscopy is given. Subsequently, these advanced microscopy techniques are introduced in more detail, with a description of how these techniques are performed, what needs to be considered, and what practical advantages they can bring to cell biological research.

Measuring immune receptor mobility by fluorescence recovery after photobleaching

The coordinated effort of cells in the immune system relies heavily on surface receptor interactions. Immune receptor mobility provides vital information on the function and responses of immune cells, and these measurements shed light on their interactions with other membrane, cytosolic, and extracellular matrix proteins. These measurements can be obtained using the fluorescence recovery after photobleaching (FRAP) technique in living cells. We describe here general approaches for FRAP using green fluorescent protein fusion proteins.

The anchoring protein SAP97 retains Kv1.5 channels in the plasma membrane of cardiac myocytes

Membrane- associated guanylate kinase proteins (MAGUKs) are important determinants of localization and organization of ion channels into specific plasma membrane domains. However, their exact role in channel function and cardiac excitability is not known. We examined the effect of synapse-associated protein 97 (SAP97), a MAGUK abundantly expressed in the heart, on the function and localization of Kv1.5 subunits in cardiac myocytes. Recombinant SAP97 or Kv1.5 subunits tagged with green fluorescent protein (GFP) were overexpressed in rat neonatal cardiac myocytes and in Chinese hamster ovary (CHO) cells from adenoviral or plasmidic vectors. Immunocytochemistry, fluorescence recovery after photobleaching, and patch-clamp techniques were used to study the effects of SAP97 on the localization, mobility, and function of Kv1.5 subunits. Adenovirus-mediated SAP97 overexpression in cardiac myocytes resulted in the clustering of endogenous Kv1.5 subunits at myocyte-myocyte contacts and an increase in both the maintained component of the outward K+current, IKur(5.64 ± 0.57 pA/pF in SAP97 myocytes vs. 3.23 ± 0.43 pA/pF in controls) and the number of 4-aminopyridine-sensitive potassium channels in cell-attached membrane patches. In live myocytes, GFP-Kv1.5 subunits were mobile and organized in clusters at the basal plasma membrane, whereas SAP97 overexpression reduced their mobility. In CHO cells, Kv1.5 channels were diffusely distributed throughout the cell body and freely mobile. When coexpressed with SAP97, Kv subunits were organized in plaquelike clusters and poorly mobile. In conclusion, SAP97 regulates the K+current in cardiac myocytes by retaining and immobilizing Kv1.5 subunits in the plasma membrane. This new regulatory mechanism may contribute to the targeting of Kv channels in cardiac myocytes.

Characterization of the liquid-ordered state by proton MAS NMR

We investigated if magic angle spinning (MAS) 1H NMR can be used as a tool for detection of liquid-ordered domains (rafts) in membranes. In experiments with the lipids SOPC, DOPC, DPPC, and cholesterol we demonstrated that 1H MAS NMR spectra of liquid-ordered domains (lo) are distinctly different from liquid-disordered (ld) and solid-ordered (so) membrane regions. At a MAS frequency of 10 kHz the methylene proton resonance of hydrocarbon chains in the ld phase has a linewidth of 50 Hz. The corresponding linewidth is 1 kHz for the lo phase and several kHz for the so phase. According to results of 1H NMR dipolar echo spectroscopy, the broadening of MAS resonances in the lo phase results from an increase in effective strength of intramolecular proton dipolar interactions between adjacent methylene groups, most likely because of a lower probability of gauche/trans isomerization in lo. In spectra recorded as a function of temperature, the onset of lo domain (raft) formation is seen as a sudden onset of line broadening. Formation of small domains yielded homogenously broadened resonance lines, whereas large lo domains (diameter >0.3 microm) in an ld environment resulted in superposition of the narrow resonances of the ld phase and the much broader resonances of lo. 1H MAS NMR may be applied to detection of rafts in cell membranes.

5-Dodecanoylaminofluorescein as a probe for the determination of critical micelle concentration of detergents using fluorescence anisotropy

A method for determining the critical micelle concentration (CMC) of various detergents based on fluorescence polarization (anisotropy) of the lipophilic probe 5-dodecanoylaminofluorescein is presented. Nonionic, cationic, anionic, and steroid-based detergents can all be evaluated by this method and the determined CMC values of selected detergents agree well with those reported in the literature. In addition, we report the CMC of domiphen bromide, whose CMC value has not previously been described. In the case of ionic detergents, the method described is particularly sensitive at discerning changes in the CMC with increasing ionic strength of the medium and can discriminate detergent CMCs in 5 mM versus 25 mM buffering components. The described fluorescence polarization technique allows very low (submicromolar) concentrations of probe to be employed, thus minimizing the perturbation of micelle formation by 5-dodecanoylaminofluorescein insertion.

Dynamic Confinement of NK2 Receptors in the Plasma Membrane

A functional fluorescent neurokinin NK2 receptor, EGFP-NK2, was previously used to follow, by fluorescence resonance energy transfer measurements in living cells, the binding of its fluorescently labeled agonist, bodipy-neurokinin A (NKA). Local agonist application suggested that the activation and desensitization of the NK2 receptors were compartmentalized at the level of the plasma membrane. In this study, fluorescence recovery after photobleaching experiments are carried out at variable observation radius (vrFRAP) to probe EGFP-NK2 receptor mobility and confinement. Experiments are carried out at 20 degrees C to maintain the number of receptors constant at the cell surface during recordings. In the absence of agonist, 35% EGFP-NK2 receptors diffuse within domains of 420 +/- 80 nm in radius with the remaining 65% of receptors able to diffuse with a long range lateral diffusion coefficient between the domains. When cells are incubated with a saturating concentration of NKA, 30% EGFP-NK2 receptors become immobilized in small domains characterized by a radius equal to 170 +/- 50 nm. Biochemical experiments show that the confinement of EGFP-NK2 receptor is not due to its association with rafts at any given time. Colocalization of the receptor with beta-arrestin and transferrin supports that the small domains, containing 30% of activated EGFP-NK2, correspond to clathrin-coated pre-pits. The similar amount of confined EGFP-NK2 receptors found before and after activation (30-35%) is discussed in term of putative transient interactions of the receptors with preexisting scaffolds of signaling molecules.

Membrane order conservation in raft and non-raft regions of hepatocyte plasma membranes from thermally acclimated rainbow trout

Homeoviscous adaptation (HVA), the thermal conservation of membrane fluidity/order at different body temperatures, has been observed to varying degrees in different membranes. However, HVA has not been studied in raft and non-raft regions of the plasma membrane (PM) separately. Rafts are ordered PM microdomains implicated in signal transduction, membrane traffic and cholesterol homeostasis. Using infrared spectroscopy, we measured order in raft-enriched PM (raft) and raft-depleted PM (RDPM) isolated from hepatocytes of rainbow trout (Oncorhynchus mykiss) acclimated to 5 and 20 degrees C. We found approximately 130% and 90% order compensation in raft and RDPM, respectively, suggesting their independent regulation. Raft was more ordered than RDPM in the warm-acclimated trout, a difference fully explained by a 58% enrichment of cholesterol, compared to RPDM. Unexpectedly, raft and RDPM from cold-acclimated trout did not differ in cholesterol content or order. Freezing the membrane samples during preparation had no effect on order. Treatment with cyclodextrin depleted cholesterol by 36%, 56%, and 55%, producing significant decreases in order in raft and RDPM from warm-acclimated trout and RDPM from cold-acclimated trout, respectively. However, a 69% depletion of cholesterol from raft from cold-acclimated trout had no significant effect on order. This result, and the lack of a difference in order between raft and RDPM, suggests that raft and non-raft PM in cold-acclimated trout are not spatially segregated by phase separation due to cholesterol.

Effect of incubating human sperm at room temperature on capacitation-related events

OBJECTIVE

To determine the effect of human sperm incubation at room temperature (20 degrees C) upon capacitation-related events.

DESIGN

Prospective study.

SETTING

Basic research laboratory.

PATIENT(S)

Semen samples were obtained from normozoospermic volunteers. Human follicular fluid (hFF) was collected from women undergoing assisted reproductive treatment.

INTERVENTION(S)

Spermatozoa were incubated for up to 18 hours at 20 degrees C and/or 37 degrees C.

MAIN OUTCOME MEASURE(S)

Protein tyrosine phosphorylation patterns, development of hyperactivated motility, and induction of acrosome reaction (AR) in response to hFF.

RESULT(S)

Spermatozoa incubated for 18 hours at 20 degrees C showed an array of tyrosine phosphorylated proteins similar to noncapacitated cells. After incubation at 20 degrees C, the percentage of spermatozoa displaying hyperactivated motility and undergoing acrosomal loss in response to hFF was significantly lower when compared with cells kept the same time at 37 degrees C. Conversely, spermatozoa incubated overnight at 37 degrees C could respond to hFF, either at 37 degrees C or 20 degrees C. When preincubation at 20 degrees C was followed by sperm exposure to 37 degrees C, capacitation-related events could be activated. In capacitated cells (16 hours at 37 degrees C), 2-hour incubation at 20 degrees C led to a significant decrease in acrosome reaction inducibility, suggesting sperm decapacitation.

CONCLUSION(S)

Human sperm incubation at room temperature does not allow capacitation, although it does not affect hFF-induced acrosome reaction in capacitated cells. The blocking effect is overcome when spermatozoa are exposed to 37 degrees C.

Role of sphingolipids in the biogenesis of membrane domains

In recent years, a huge interest in sphingolipid- and cholesterol-enriched membrane domains has risen, after their involvement in fundamental membrane-associated events such as signal transmission, cell adhesion and lipid/protein sorting was postulated. Theoretical considerations and several experimental data suggest that sphingolipids play an important role in the biogenesis and function of domains. In fact, their physicochemical features, different from those of other membrane lipids, allow their interaction either with other sphingolipids or with other membrane components and external ligands. Owing to these features, sphingolipids may undergo segregation and represent a nucleation point for co-clustering with other lipids and proteins in a complex, functional domain. Moreover, sphingolipids confer dynamic properties on domains, a fundamental feature for the modulation of their postulated functions.

Insertion of fluorescent fatty acid probes into the outer membranes of the pathogenic spirochaetes Treponema pallidum and Borrelia burgdorferi

The authors examined the ability of octadecanoyl (C(18)), hexadecanoyl (C(16)) and dodecanoyl (C(12)) fatty acid (FA) conjugates of 5-aminofluorescein (OAF, HAF and DAF, respectively) to insert into the outer membranes (OMs) of Treponema pallidum, Borrelia burgdorferi and Escherichia coli. Biophysical studies have demonstrated that these compounds stably insert into phospholipid bilayers with the acyl chain within the hydrophobic interior of the apical leaflet and the hydrophilic fluorescein moiety near the phospholipid head groups. Consistent with the known poor intrinsic permeability of the E. coli OM to hydrophobic compounds and surfactants, E. coli was not labelled with any of the FA probes. OAF inserted more readily into OMs of B. burgdorferi than into those of T. pallidum, although both organisms were completely labelled at concentrations at or below 2 microg ml(-1). Intact spirochaetes were labelled with OAF but not with antibodies against known periplasmic antigens, thereby confirming that the probe interacted exclusively with the spirochaetal OMs. Separate experiments in which organisms were cooled to 4 degrees C (i.e. below the OM phase-transition temperatures) indicated that labelling with OAF was due to insertion of the probe into the OMs. B. burgdorferi, but not T. pallidum, was labelled by relatively high concentrations of HAF and DAF. Taken as a whole, these findings support the prediction that the lack of lipopolysaccharide renders T. pallidum and B. burgdorferi OMs markedly more permeable to lipophilic compounds than their Gram-negative bacterial counterparts. The data also raise the intriguing possibility that these two pathogenic spirochaetes obtain long-chain FAs, nutrients they are unable to synthesize, by direct permeation of their OMs.

In situ imaging of detergent-resistant membranes by atomic force microscopy

Purified detergent-resistant membranes (DRMs) are powerful tools for the biochemical study of plasma membrane domains. To what extent these isolated DRMs correspond to native membrane domains remains, however, a matter of debate. The most immediate question to be answered concerns the in situ size range of DRMs, a determination that escapes classical microscopy techniques. In this study we show that in situ three-dimensional images of a material as fragile as Triton X-100-treated cells can be obtained, in buffer, by tapping mode atomic force microscopy. These images establish that, prior to the isolation procedure, the detergent plasma membrane fragments form domains whose size frequently exceeds 15-20 microm(2). This DRMs size range is about 1 order of magnitude higher than that estimated for the larger microdomains of living cells, which strongly suggests that membrane microdomains rearrange into larger DRMs during Triton X-100 treatment. Concomitantly, the images also reveal the presence of the cytoskeleton, which is resistant to detergent extraction, and suggest that, in situ, DRMs are associated with the membrane cytoskeleton.

Partitioning of amphiphiles between coexisting ordered and disordered phases in two-phase lipid bilayer membranes

The partition coefficients (K(P)) of a series of single-chain and double-chain fluorescent amphiphiles, between solid ordered (P(beta') and L(beta)) and liquid disordered (L(alpha) of the type l(d)) lipid phases coexisting in the same lipid bilayer, was studied using steady-state fluorescence emission anisotropy. The single-chain amphiphiles were N-(7-nitrobenzoxa-2, 3-diazol-4-yl)-alkylamines, and the double-chain amphiphiles were N-(7-nitrobenzoxa-2, 3-diazol-4-yl)-phosphatidylethanolamines with chain lengths of 12-18 carbon atoms. Saturated 18-carbon alkyl/acyl chain compounds were also compared with Delta(9)-cis unsaturated chains of the same chain length. The fluorescence anisotropy of the probes was examined in lipid bilayers (multilamellar vesicles) prepared from an equimolar mixture of dilauroylphosphatidylcholine and distearoylphosphatidylcholine and studied as a function of temperature through the entire temperature range of coexistence of ordered gel phases and a disordered fluid phase in this system. The unsaturated chain amphiphiles partitioned exclusively into the fluid phase whenever this phase was present, as did the saturated chain amphiphiles with the shortest chains (C(12:0)), while K(P) ranges between 1 and 2, in favor of the L(beta) solid phase, for the amphiphiles with long saturated (C(18:0)) alkyl/acyl chains, with intermediate behavior for the intermediate chain lengths. All probes appeared to be totally excluded from P(beta') solid (gel) phases. The technique was also used to determine partitioning of some of the probes between coexisting liquid ordered (cholesterol-containing) (l(o)) and liquid disordered (l(d)) L(alpha) phases. In this case the ratio of signal amplitude to noise allowed us to obtain a qualitative, but not quantitative, measure of the phase partitioning of the probes. We conclude that the partitioning behavior of the probes examined between coexisting l(o) and l(d) phases is qualitatively similar to that observed between solid ordered and liquid disordered phases.

Detection of lipid domains in docasahexaenoic acid-rich bilayers by acyl chain-specific FRET probes

A major problem in defining biological membrane structure is deducing the nature and even existence of lipid microdomains. Lipid microdomains have been defined operationally as heterogeneities in the behavior of fluorescent membrane probes, particularly the fluorescence resonance energy transfer (FRET) probes 7-nitrobenz-2-oxa-1,3-diazol-4-yl-diacyl-sn-glycero-3-phosphoethan olamine (N-NBD-PE) and (N-lissamine rhodamine B sulfonyl)-diacyl-snglycero-3-phosphoethanolamine (N-Rh-PE). Here we test a variety of N-NBD-PEs and N-Rh-PEs containing: (a) undefined acyl chains, (b) liquid crystalline- and gel-state acyl chains, and (c) defined acyl chains matching those of phase separated membrane lipids. The phospholipid bilayer systems employed represent a liquid crystalline/gel phase separation and a cholesterol-driven fluid/fluid phase separation; phase separation is confirmed by differential scanning calorimetry. We tested the hypothesis that acyl chain affinities may dictate the phase into which N-NBD-PE and N-Rh-PE FRET probes partition. While these FRET probes were largely successful at tracking liquid crystalline/gel phase separations, they were less useful in following fluid/fluid separations and appeared to preferentially partition into the liquid-disordered phase. Additionally, partition measurements indicate that the rhodamine-containing probes are substantially less hydrophobic than the analogous NBD probes. These experiments indicate that acyl chain affinities may not be sufficient to employ acyl chain-specific N-NBD-PE/N-Rh-PE FRET probes to investigate phase separations into biologically relevant fluid/fluid lipid microdomains.

Transverse and lateral mobility in outer hair cell lateral wall membranes

Cochlear outer hair cell (OHC) electromotility is associated with the cell's lateral wall. The lateral wall contains two distinct membranes: the plasma membrane (PM) and the subsurface cisternae (SSC). We explored biophysical characteristics of these lipid structures using membrane-specific fluorescent dyes. We have previously demonstrated that di-8-ANEPPS stains the PM while NBD-C6-ceramide partitions to the SSC. In this report we show that NBD-cholesterol also partitions to the SSC. Transmigration of the SSC dyes across the PM was visualized under confocal microscopy, after separating the two membranes using the micropipette aspiration technique. The transverse mobility of NBD-cholesterol was faster than that of NBD-C6-ceramide. We then measured the lateral mobility of the dyes within both the PM and the SSC using fluorescence recovery after photobleaching (FRAP). The diffusion coefficients at 12 37 degrees C and the activation energies for diffusion were found to be similar to those of other biological membranes. These data indicate that both the PM and the SSC are membranes in the fluid phase, with no evidence of temperature-dependent phase transitions. Our observations are consistent with a fluid-mosaic model of the lateral wall membranes.

Alteration of the lateral organization of the plasma membrane of Chinese hamster ovary cells by synthetic lipopeptide, Pam3Cys-Ser-Lys4

The cationic lipohexapeptide (S)-[2, 3-bis(palmitoyloxy)-(2RS)-propyl]-N-palmitoyl-(R)-Cys-(S)-Ser-(S)- Lys 4-OH, trihydrochloride (Pam3Cys-Ser-Lys4) is a synthetic analog of the triacylated N-terminal part of bacterial lipoproteins. In this study we addressed the question of whether Pam3Cys-Ser-Lys4 could modify the organization of the plasma membrane of Chinese hamster ovary cells. 1-Acyl-2-[6-(7-nitro-2-1, 3-benzoxadiazol-4-yl)amino]caproyl]-sn-glycero-3-phosphocholine (C6-NBD-PC) diffusion was followed by fluorescence recovery after photobleaching experiments carried out on the plasma membrane of Chinese hamster ovary cells. Incubation of cells in the presence of Pam3Cys-Ser-Lys4 induced an increase in the lateral diffusion coefficient and in the immobile fraction of C6-NBD-PC probes. Various control experiments have shown that the increase in the immobile fraction was not due to probe internalization induced by Pam3Cys-Ser-Lys4. Back-exchange experiments showed that a good correlation exists between the fractions of immobilized probes and nonextractable probes in the plasma membrane of Chinese hamster ovary cells. A useful way to analyze the origin of probe immobilization (micrometer-sized domains or aggregated patches of proteins) is to carry out fluorescence recovery after photobleaching experiments at variable observation radii. This type of experiment, carried out on the plasma membrane of Chinese hamster ovary cells incubated with Pam3Cys-Ser-Lys4, confirmed that the lipopeptide induced the aggregation of proteins of Chinese hamster ovary plasma membrane. Lipids which were trapped inside these aggregates were thus prevented from diffusing at long range in the plasma membrane plane and behave as an immobile fraction.

Hyperactivation of hamster sperm motility by temperature-dependent tyrosine phosphorylation of an 80-kDa protein

Protein phosphorylation and dephosphorylation are believed to play key roles in regulation of sperm motility. Here we examine the effect of temperature on hamster sperm motility and protein tyrosine phosphorylation status. As in previous work, a decrease from 37 degrees C to 22 degrees C caused loss of hyperactivated motility. We now find that cooling also produces a dephosphorylation of several 48-80-kDa flagellar peptides. A return to 37 degrees C restored hyperactivation but resulted in rephosphorylation of only an 80-kDa protein. Conversely, hyperactivation and phosphorylation of the 80-kDa component were insensitive to incubation temperature for sperm incubated with the protein phosphatase inhibitor, calyculin A, or for sperm demembranated by detergent extraction. These results strongly indicate that the temperature-sensitive tyrosine phosphorylation status of an 80-kDa sperm flagellar peptide explains the sensitivity of hyperactivation to temperature.

Glycolipid-enriched caveolae and caveolae-like domains in the nervous system

Recent years have been characterized by a booming interest in research on caveolae and caveolae-like membrane domains. The interest in this subject grew further, when their involvement in fundamental membrane-associated events, such as signal transmission and lipid/protein sorting, was postulated. Substantial progress has been reached in understanding the biological role of membrane domains in eukaryotic cells. The neuron, however, which perhaps represents one of the greatest challenges to research on membrane traffic and function, has only been partially investigated. The purpose of the present review is to survey this issue in the nervous system. We confine ourselves to the presence of membrane domains in the nervous system and discuss this in the context of three facts: first, glycolipids are peculiarly enriched in both caveolae and caveolae-like domains and are particularly abundant in the nervous system; second, the neuron is characterized by a basic dual polarity, similar in this respect to other polarized cells, where the role of glycolipid-enriched domains for lipid/protein sorting has been better ascertained; and third, neurons evolved from, and are related to, simpler eukaryotic cells, allowing us to find analogies with more investigated nonneuronal cells.

Frequency-dependent capacitance of the apical membrane of frog skin: dielectric relaxation processes

Impedance analysis of the isolated epithelium of frog skin (northern Rana pipiens) was carried out in the frequency range between 0.1 Hz and 5.5 kHz while Na+ transport was abolished. Under these conditions, the impedance is determined almost completely by the dielectric properties of the apical membranes of the cells and the parallel shunt resistance. The modeling of the apical membrane impedance function required the inclusion of dielectric relaxation processes as originally described by. J. Chem. Phys. 9:341-351), where each process is characterized by a dielectric increment, relaxation frequency, and power law dependence. We found that the apical plasma membrane exhibited several populations of audio frequency dielectric relaxation processes centered at 30, 103, 2364, and 6604 Hz, with mean capacitive increments of 0.72, 1.00, 0.88, and 0.29 microF/cm2, respectively, that gave rise to dc capacitances of 1.95 +/- 0.06 microF/cm2 in 49 tissues. Capacitance was uncorrelated with large ranges of parallel shunt resistance and was not changed appreciably within minutes by K+ depolarization and hence a decrease in basolateral membrane resistance. A significant linear correlation existed between the dc capacitance and Na+ transport rates measured as short-circuit currents (Cadc = 0.028 Isc + 1.48; Isc between 4 and 35 microA/cm2) before inhibition of transport by amiloride and substitution of all Na+ with NMDG (N-methyl-D-glucamine) in the apical solution. The existence of dominant audio frequency capacitive relaxation processes complicates and precludes unequivocal interpretation of changes of capacitance in terms of membrane area alone when capacitance is measured at audio frequencies.

Effects of sauna on sperm movement characteristics of normal men measured by computer-assisted sperm analysis

The effects of sauna exposure on sperm movement characteristics and other semen parameters were evaluated using computer-assisted sperm analysis (CASA). A significant (p < 0.01) decrease in average path velocity (VAP), curvilinear velocity (VCL) and amplitude of lateral head displacement (ALH) was found after exposure to sauna for 2 weeks. The altered parameters returned to their original values within 1 week after cessation of sauna exposure. Mean values for semen volume, sperm count, percentage motility, sperm morphology and sperm penetration assay (SPA) were not statistically different during and after sauna, when compared to the corresponding control values. The results suggest that increasing scrotal temperature by sauna causes a reversible decrease in sperm movement parameters.

Measurement of the lateral diffusion of dipalmitoylphosphatidylcholine adsorbed on silica beads in the absence and presence of melittin: a 31P two-dimensional exchange solid-state NMR study

31P two-dimensional exchange solid-state NMR spectroscopy was used to measure the lateral diffusion, D(L), in the fluid phase of dipalmitoylphosphatidylcholine (DPPC) in the presence and absence of melittin. The use of a spherical solid support with a radius of 320 +/- 20 nm, on which lipids and peptides are adsorbed together, and a novel way of analyzing the two-dimensional exchange patterns afforded a narrow distribution of D(L) centered at a value of (8.8 +/- 0.5) x 10(-8) cm2/s for the pure lipid system and a large distribution of D(L) spanning 1 x 10(-8) to 10 x 10(-8) cm2/s for the lipids in the presence of melittin. In addition, the determination of D(L) for nonsupported DPPC multilamellar vesicles (MLVs) suggests that the support does not slow down the lipid diffusion and that the radii of the bilayers vary from 300 to 800 nm. Finally, the DPPC-melittin complex is stabilized at the surface of the silica beads in the gel phase, opening the way to further study of the interaction between melittin and DPPC.

Evaluation of solute permeation through the stratum corneum: lateral bilayer diffusion as the primary transport mechanism

Solute permeation across human stratum corneum (SC) was examined in terms of the fundamental bilayer transport properties. A mathematical model was developed to describe the macroscopic SC permeation via the interkeratinocyte lipid domain in terms of (i) the structure and dimensions of the SC, and (ii) the microscale lipid bilayer transport properties, which include the bilayer/water partition coefficient, the lateral diffusion coefficient, the interfacial transbilayer mass transfer coefficient, and the intramembrane transbilayer mass transfer coefficient. The relative importance of the diffusive resistances associated with the bilayer transport properties was evaluated with the model and experimental data. Lateral diffusion coefficients in SC lipid bilayers were calculated from 120 human skin permeability measurements, and compared with previously reported measurements made in SC-extracted lipids. Good qualitative and quantitative agreement was observed, indicating that, in the context of the model, the diffusive resistance associated with lateral diffusion is sufficient to explain the overall resistance of solute permeation through the SC. A similar analysis shows that the diffusive resistance associated with interfacial transbilayer transport is not capable of explaining the experimental permeation values, thus supporting this finding. The lateral diffusion analysis also revealed a bifunctional size dependence of transport within the SC, with a strong size dependence for small solutes (<300 Da) and a weak size dependence for larger solutes.

Lipid domains in the membrane: thermotropic properties of sphingomyelin vesicles containing GM1 ganglioside and cholesterol

The thermotropic behavior of palmitoylsphingomyelin vesicles containing GM1 ganglioside and cholesterol has been investigated by high-sensitivity differential scanning calorimetry. The thermograms exhibited by binary palmitoylsphingomyelin/GM1 mixtures are resolvable into two components. The relative contribution of the minor component, undetectable in the absence of ganglioside, to the total enthalpy and its transition temperature (>40 degrees C) increase with the concentration of the glycolipid embedded in the vesicles. These data suggest the occurrence of lateral phase separation and that more ordered, higher melting GM1 ganglioside-enriched domains are present within the sphingomyelin bilayer. Studies on binary sphingomyelin/cholesterol mixtures confirmed the known tendency of the sterol to decrease the total enthalpy of sphingomyelin, forming cholesterol-enriched domains. The thermograms exhibited by ternary sphingomyelin/ganglioside/cholesterol mixtures in variable proportions (up to 20% molar GM1 or Chol) displayed, on increasing the content of either the sterol or the ganglioside, features addressable to sphingomyelin/cholesterol (peaks centered at temperature </=40 degrees C, decrease of enthalpy) or to sphingomyelin/GM1 mixtures (peaks centered at a temperature >40 degrees C), respectively. This trend was confirmed by deconvolution analysis, showing that the thermograms are resolvable into components addressable to GM1-enriched and to cholesterol-enriched domains. Taken all together, the results show that the architectural features of sphingomyelin bilayers are strongly dependent on the presence of GM1 ganglioside and cholesterol, whose presence is leading to the formation of separate, GM1-enriched and cholesterol-enriched distinct domains. Ganglioside-sphingomyelin and sphingomyelin-cholesterol, together with mutual ganglioside-ganglioside, interactions could contribute to maintain a network of bonds extending to proteins, forming specialized membrane domains, such as caveolae, or others, whose experimental clues are the glycolipid-enriched detergent-insoluble fractions that can be isolated from cell membranes.

Hypothesis: the meeting place model for prion disease

Prions are responsible for spongiform diseases such as scrapie and bovine spongiform encephalopathy. It is now generally accepted that the disease mechanism involves the conversion from the normal form, PrPC, to the pathogenic form, PrPSc, and that this isoform is infectious. In the case of scrapie, 15 different forms of the disease have been described and some of these different phenotypes can be conferred by infectious prions that are themselves encoded by normal genes. We propose here that a prion with an altered structure has a correspondingly altered preference for lipids; this altered preference creates a proteolipid domain containing different lipids and other factors such as chaperonins and enzymes responsible for post-translational modifications. Normal prions associated with this abnormal domain adopt the conformation dictated by its lipidic composition (and by the other factors present) and so acquire the lipidic preference of the original pathogenic prions. These transformed prions could then create new proteolipid domains. This process may be considered as semi-conservative replication in which prion and lipids are analogous to the Watson and Crick strands and the proteolipid domain to the double helix itself.

Lateral diffusion of small compounds in human stratum corneum and model lipid bilayer systems

An image-based technique of fluorescence recovery after photobleaching (video-FRAP) was used to measure the lateral diffusion coefficients of a series of nine fluorescent probes in two model lipid bilayer systems, dimyristoylphosphatidylcholine (DMPC) and DMPC/cholesterol (40 mol%), as well as in human stratum corneum-extracted lipids. The probes were all lipophilic, varied in molecular weight from 223 to 854 Da, and were chosen to characterize the lateral diffusion of small compounds in these bilayer systems. A clear molecular weight dependence of the lateral diffusion coefficients in DMPC bilayers was observed. Values ranged from 6.72 x 10(-8) to 16.2 x 10(-8) cm2/s, with the smaller probes diffusing faster than the larger ones. Measurements in DMPC/cholesterol bilayers, which represent the most thorough characterization of small-solute diffusion in this system, exhibited a similar molecular weight dependence, although the diffusion coefficients were lower, ranging from 1.62 x 10(-8) to 5.60 x 10(-8) cm2/s. Lateral diffusion measurements in stratum corneum-extracted lipids, which represent a novel examination of diffusion in this unique lipid system, also exhibited a molecular weight dependence, with values ranging from 0.306 x 10(-8) to 2.34 x 10(-8) cm2/s. Literature data showed that these strong molecular weight dependencies extend to even smaller compounds than those examined in this study. A two-parameter empirical expression is presented that describes the lateral diffusion coefficient in terms of the solute's molecular weight and captures the size dependence over the range examined. This study illustrates the degree to which small-molecule lateral diffusion in stratum corneum-extracted lipids can be represented by diffusion in DMPC and DMPC/cholesterol bilayer systems, and may lead to a better understanding of small-solute transport across human stratum corneum.

Fluorescence-quenching study of percolation and compartmentalization in two-phase lipid bilayers

Fluorescence quenching of a lipid-labeled fluorophore by a lipid spin-labeled quencher has been studied experimentally in two-component, two-phase phosphatidylcholine bilayers to examine the effect of phase connection and disconnection on quenching. Both fluorophore and quencher prefer the fluid phase. At the percolation threshold, the point at which the fluid phase becomes subdivided into may small disconnected domains, the quenching drops abruptly. This decrease in quenching is a function of the fluid-phase fraction and is due to the heterogeneous distribution of fluorophores and quenchers over the fluid-phase domains. Computer simulations of the system were carried out with a triangular lattice divided into closed compartments of variable size and reactant occupancy. The simulations demonstrate that the degree of quenching is reduced in the disconnected systems and that the reduction is correlated with the size of the disconnected domains. The combination of experimental data with simulations leads to the conclusion that at constant temperature the size of fluid-phase domains, nfluid, in the region of the coexistence of the fluid and gel phases is proportional to the fluid fraction, Xfluid. This is in a qualitative agreement with a previous electron spin resonance study of interlipid spin-spin interactions in the same two-component, two-phase bilayer system.

Synergistic effects of chemical enhancers and therapeutic ultrasound on transdermal drug delivery

The effects of (i) a series of chemical enhancers and (ii) the combination of these enhancers and therapeutic ultrasound (1 MHz, 1.4 W/cm2, continuous) on transdermal drug transport are investigated. A series of chemical enhancer formulations, including (i) polyethylene glycol 200 dilaurate (PEG), (ii) isopropyl myristate (IM), (iii) glycerol trioleate (GT), (iv) ethanol/pH 7.4 phosphate buffered saline in a 1:1 ratio (50% EtOH), (v) 50% EtOH saturated with linoleic acid (LA/EtOH), and (vi) phosphate buffered saline (PBS), as a control, are evaluated using corticosterone as a model drug. LA/EtOH is the most effective of these enhancers, increasing the corticosterone flux by 900-fold compared to that from PBS. Therapeutic ultrasound (1 MHz, 1.4 W/cm2, continuous) increases the corticosterone permeability from all of the enhancers examined by up to 14-fold (LA/EtOH) and increases the corticosterone flux from the saturated solutions by up to 13,000-fold (LA/EtOH), relative to that from PBS. Similar enhancements are obtained with LA/EtOH with and without ultrasound for four other model drugs, dexamethasone, estradiol, lidocaine, and testosterone. The permeability enhancements for all of these drugs resulting from the addition of linoleic acid to 50% EtOH increase with increasing drug molecular weight. Likewise, the permeability enhancement attained by ultrasound and LA/EtOH relative to passive EtOH exhibits a similar size dependence. A mechanistic explanation of this size dependence is provided. It is suggested that bilayer disordering agents, such as linoleic acid and ultrasound, transform the SC lipid bilayers into a fluid lipid bilayer phase or create a separate bulk oil phase. The difference in diffusivity of a given solute in SC bilayers and in either fluid bilayers or bulk oil is larger for larger solutes, thereby producing greater enhancements for larger solutes.

The spatial distribution of phospholipids and glycolipids in the membrane of the bacterium Micrococcus luteus varies during the cell cycle

Recently, we have developed a photocrosslinking approach which uses anthracene as a photoactivatable group and which allows us to determine the lateral distribution of lipids in membranes quantitatively. In synchronous cultures of the gram-positive bacterium Micrococcus luteus, this approach shows that the spatial distribution of phosphatidylglycerol and dimannosyldiacylglycerol, the two major lipids in the bacterial membrane, varies greatly during the cell cycle. Minimum heterogeneity was observed during cell growth while maximum heterogeneity was detected during cell division.

Aggregates of saturated phospholipids at the air-water interface

Surface viscosities of phospholipid/steroid mixtures at the air-water interface were measured by means of an oscillating pendulum. Phospholipids studied included 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC); 1,2-dipalmitoyl-sn-glycero-3-phosphodimethylethanolamine (DPPDME); 1,2-dipalmitoyl-sn-glycero-3-phosphomonomethylethanolamine (DPPMME); 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE); 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DPPG); 1-palmitoyl-2-elaidyl-sn-glycero-3-phosphocholine and 1-palmitoyl-2-linelaidyl-sn-glycero-3-phosphocholine. Each saturated phospholipid was studied in the presence of cholesterol, DPPC was also investigated in the presence of 5-androsten-3beta-ol; cholestanol; 5-cholestene;5alpha-cholesten-3beta-ol methyl ether; coprostanol; 7,(5alpha)-cholesten-3beta-ol; desmosterol; epicholestanol; lanosterol and lophenol. The surface viscosities of the trans-unsaturated phosphatidylcholines (PC) were undetectable by this method and in this respect they resembled cis- unsaturated PC. The surface viscosities of saturated phospholipids were very high but were reduced by low concentrations of steroids. Interpretation of the results for DPPC/cholesterol mixtures indicates that DPPC functions at the air-water interface as a one-dimensional linear aggregate. At 50 mN/m and 22 degrees C the average structure contains approximately 300 DPPC molecules. DPPDME acts similarly but DPPMME, DPPE and DPPG differ from DPPC in their response to cholesterol. All of the steroids examined except 5-androsten-3beta-ol and the lanosterol mixture paralleled cholesterol in their interaction with DPPC an indication that phospholipid/steroid interactions modifying surface viscosity have less stringent requirements for the steroid structure than interactions measured as condensation in surface area.

Hypothesis: chromosome separation in Escherichia coli involves autocatalytic gene expression, transertion and membrane-domain formation

To explain how daughter chromosomes are separated into discrete nucleoids and why chromosomes are partitioned with pole preferences, I propose that differential gene expression occurs during DNA replication in Escherichia coli. This differential gene expression means that the daughter chromosomes have different patterns of gene expression and that cell division is not a simple process of binary fission. Differential gene expression arises from autocatalytic gene expression and creates a separate proteolipid domain around each developing chromosome via the coupled transcription-translation-insertion of proteins into membranes (transertion). As these domains are immiscible, daughter chromosomes are simultaneously replicated and separated into discrete nucleoids. I also propose that the partitioning relationship between chromosome age and cell age arises because the poles of cells have a proteolipid composition that favours transertion from one nucleoid rather than from the other. This hypothesis forms part of an ensemble of related hypotheses which attempt to explain cell division, differentiation and wall growth in bacteria in terms of the physical properties and interactions of the principal constituents of cells.

Lateral inhomogeneous lipid membranes: theoretical aspects

The physical concepts underlying the lateral distribution of the components forming a lamellar assembly of amphiphiles are discussed in this review. The role of amphiphiles' molecular structure and/or aqueous environment (ionic strength, water soluble substances) on formation and stability of lateral patterns is investigated. A considerable effort is devoted to the analysis of the properties of patterned structure which can be different from those of randomly mixed multi-component lamellae. Examples include adhesion and fusion among laterally inhomogeneous bilayers, enhanced interfacial adsorption of ions and polymers, enhanced transport across the bilayer, modified mechanical properties, local stabilization of non-planar geometries (pores, edges) and related phenomena (electroporation, budding transition and so on). Furthermore, an analysis of chemical reactivity within or at the water interface of a laterally inhomogeneous bilayer is briefly discussed. A link between these concepts and experimental findings taken from the biological literature is attempted throughout the review.

Laser cytometric analysis of FIV-induced injury in astroglia

Glia are the predominant brain cells infected by the lentiviruses human immunodeficiency virus (HIV) and feline immunodeficiency virus (FIV). The importance of astrocytes in maintenance of central nervous system homeostasis suggests that astrocytes are likely to play a strategic role in the progression of neurological disease in lentiviral-infected patients. In consideration of this postulate, the ability of FIV to cause injury by infection of cultured feline astroglia was examined via vital fluorescence assays. Intracellular Ca2+ homeostasis, plasma membrane permeability and fluidity, and cytosolic glutathione (GSH) levels were evaluated. Although basal intracellular Ca2+ was not significantly different between groups, FIV-infected astroglia displayed both a significant delay in development of peak Ca2+ levels following ionophore application and a decrease in the amount of Ca2+ released from intracellular stores. Plasma membrane lipid mobility was increased in FIV-infected cells within 24 h of infection. Glutathione levels were affected in a dose dependent fashion. With a standard viral inoculum there was a decrease in GSH which became significant after 8 days postinfection. With a high inoculum dose there was rapid loss of cell viability with an increase in GSH in surviving cells. We have identified several cellular processes altered in FIV-infected astroglia and our findings suggest that FIV-infection of feline astroglia affects cellular membranes, both structurally and functionally.

Cheek cell membrane fluidity measured by fluorescence recovery after photobleaching and steady-state fluorescence anisotropy

Membrane fluidity of human cheek cells was determined using fluorescence recovery after photobleaching (FRAP) and steady-state fluorescence anisotropy. The FRAP data showed that the lateral diffusion coefficient (D) and mobile fraction (%R) of lipid in the plasma membrane of control cells were 2.01 x 10(-9) cm2/sec and 54.25%, respectively. Trypsin treatment increased D and %R to 6.4 x 10(-9) cm2/sec and 72.15%. In contrast, the anisotropy (r) for control cells was 0.270 which remained unchanged by trypsin treatment. The results show that diffusion of lipids in the plane of the membrane is restricted by trypsin-sensitive barriers.

Lipid domains and lipid/protein interactions in biological membranes

In the fluid mosaic model of membranes, lipids are organized in the form of a bilayer supporting peripheral and integral proteins. This model considers the lipid bilayer as a two-dimensional fluid in which lipids and proteins are free to diffuse. As a direct consequence, both types of molecules would be expected to be randomly distributed within the membrane. In fact, evidences are accumulating to indicate the occurrence of both a transverse and lateral regionalization of membranes which can be described in terms of micro- and macrodomains, including the two leaflets of the lipid bilayer. The nature of the interactions responsible for the formation of domains, the way they develop and the time- and space-scale over which they exist represent today as many challenging problems in membranology. In this report, we will first consider some of the basic observations which point to the role of proteins in the transverse and lateral regionalization of membranes. Then, we will discuss some of the possible mechanisms which, in particular in terms of lipid/protein interactions, can explain lateral heterogenities in membranes and which have the merit of providing a thermodynamic support to the existence of lipid domains in membranes.

Lipid domains in model and biological membranes

Lipid domains that occur within biological of model membranes encompass a variety of structures with very different lifetimes. The separation of membrane lipids into compositional domains can be due to lateral phase separation, immiscibility within a single phase, or interaction of lipids with integral or peripheral proteins. Lipid domains can affect the extent and rate of reactions in the membrane and provide sites for the activity of specialized proteins. Domains are likely to be involved in the process of lipid sorting to various cellular membranes, as well as in other processes which involve membrane budding or invagination.

Diffusion and chemical reactions in phase-separated membranes

The biological membrane may be viewed as a two-dimensional solvent system, the lipid bilayer, in which the membrane components are either dissolved (intrinsic) or to the surface of which they are adsorbed (extrinsic). The solvent bilayer is made up of a large number of lipid chemical species derived from a few lipid classes. Experience with model systems has shown that in mixed lipid bilayers immiscibility of components is the rule rather than the exception. This suggests that the bilayer in a biological membrane is not a homogenous two-dimensional fluid but rather a heterogenous system consisting of a mosaic of co-existing phase domains in which the phases differ both chemically and physically from each other. A consequence of this is the physical separation of membrane components, including proteins, based on their phase solubility. The percolation in such a phase-separated system then determines the range over which free lateral diffusion is possible and bimolecular reactions can occur. Phase percolation and long-range translational diffusion have been studied in model systems using the fluorescence recovery after photobleaching (FRAP) technique, and theoretical work shows that bimolecular reaction yields can be seriously reduced in phase-separated membranes. Transitions between percolating and non-percolating states in biomembranes is proposed as a potential trigger mechanism in the control of membrane physiology.

Rapid alterations in lateral mobility of lipids in the plasma membrane of activated human megakaryocytes

In the present study we measured membrane fluidity as the lateral mobility of the lipid probe 1,1'-ditetradecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate by fluorescence recovery after photobleaching in the plasma membrane of a single megakaryocyte, the progenitor cell of platelets. Megakaryocytes after 13 days in culture (maturation stage III) had a lateral diffusion coefficient (D) of (4.56 +/- 0.10) x 10(-9) cm2/s and a mobile fraction of 65 +/- 2% (means +/- SEM, n = 140). Megakaryocytes isolated from rib had a similar D and mobile fraction. Stimulation with alpha-thrombin (1-10 U/ml) induced a dose-dependent decrease in D to (3.40 +/- 0.22) x 10(-9) cm2/s between 1-5 min after stimulation (P < 0.001). The mobile fraction did not change. A similar decrease in D was found following stimulation with ADP (20 microM) and ionomycin (100 nM). Modulation of calpain I activity with calpain I inhibitor or tetracain had no effect. Pretreatment with cytochalasin B or colchicine decreased D to (3.64 +/- 0.29) x 10(-9) cm2/s (P < 0.003) and (3.96 +/- 0.18) x 10(-9) cm2/s (P < 0.013) respectively. After stimulation D decreased further in cytochalasin-treated cells (3.37 +/- 0.16) x 10(-9) cm2/s (P < 0.020) but remained at the same level in colchicine-treated cells. Both treatments increased the mobile fraction to 73-75% in stimulated megakaryocytes (P < 0.03). These data indicate that the diffusion velocity of lipids in megakaryocytes is low and decreases further after stimulation. These changes are independent of calpain I. Treatments that decrease the cytoskeletal mass and thereby increase the mobility of proteins in the plasma membrane increase the number of lipids that participate in this process.