Property and activity of mycoloyl esters of methyl glucoside and trehalose. Effect on mitochondrial oxidative phosphorylation related to organization of suspensions and to acyl-chain structures

Development of vizantin, a safe immunostimulant, based on the structure-activity relationship of trehalose-6,6'-dicorynomycolate

Vizantin, 6,6'-bis-O-(3-nonyldodecanoyl)-α,α'-trehalose, was developed as a safe immunostimulator on the basis of a structure-activity relationship (SAR) study with trehalose 6,6'-dicorynomycolate (TDCM). It was possible to synthesize vizantin on a large scale more easily than in the case of TDCM, and the compound exhibited more potent prophylactic effect on experimental lung metastasis of B16-F0 melanoma cells. Because vizantin stimulated human macrophages, it is a promising candidate for clinical application.

Structure of the cell envelope of corynebacteria: importance of the non-covalently bound lipids in the formation of the cell wall permeability barrier and fracture plane

With the recent success of the heterologous expression of mycobacterial antigens in corynebacteria, in addition to the importance of these bacteria in biotechnology and medicine, a better understanding of the structure of their cell envelopes was needed. A combination of molecular compositional analysis, ultrastructural appearance and freeze-etch electron microscopy study was used to arrive at a chemical model, unique to corynebacteria but consistent with their phylogenetic relatedness to mycobacteria and other members of the distinctive suprageneric actinomycete taxon. Transmission electron microscopy and chemical analyses showed that the cell envelopes of the representative strains of corynebacteria examined consisted of (i) an outer layer composed of polysaccharides (primarily a high-molecular-mass glucan and arabinomannans), proteins, which include the mycoloyltransferase PS1, and lipids; (ii) a cell wall glycan core of peptidoglycan-arabinogalactan which may contain other sugar residues and was usually esterified by corynomycolic acids; and (iii) a typical plasma membrane bilayer. Freeze-etch electron microscopy showed that most corynomycolate-containing strains exhibited a main fracture plane in their cell wall and contained low-molecular-mass porins, while the fracture occurred within the plasma membrane of strains devoid of both corynomycolate and pore-forming proteins. Importantly, in most strains, the amount of cell wall-linked corynomycolates was not sufficient to cover the bacterial surface; interestingly, the occurrence of a cell wall fracture plane correlated with the amount of non-covalently bound lipids of the strains. Furthermore, these lipids were shown to spontaneously form liposomes, indicating that they may participate in a bilayer structure. Altogether, the data suggested that the cell wall permeability barrier in corynebacteria involved both covalently linked corynomycolates and non-covalently bound lipids of their cell envelopes.

Molecular packing of cord factor and its interaction with phosphatidylinositol in mixed monolayers

Cord factor (trehalose 6,6'-dimycolate, CF) is a glycolipid located in the outer mycobacterial cell wall that is implicated in the pathogenesis of mycobacteria. Furthermore, CF is a convenient model for studying mycolic acid residues, the major lipid constituents of the mycobacterial cell wall that are believed to form a barrier against drug penetration. The surface properties of CF and its interactions with phosphatidylinositol (PI) have been investigated using the monolayer technique. During compression/expansion/recompression cycles, CF monolayers switch from a loosely packed to a more tightly packed structure. The change in surface properties suggests a molecular rearrangement, perhaps involving interdigitation of long and short chains of the CF molecules. In CF-PI monolayers, maximal lateral packing density occurs between 0.5 and 0.7 mole fraction CF, which is close to the relative composition of mycolic acid residues and shorter-chain lipids in the mycobacterial cell wall. Low concentrations of CF increase the order in PI monolayers, consistent with CF toxicity involving rigidification of cell membranes.

Mycobacteria glycolipids as potential pathogenicity effectors: alteration of model and natural membranes

Four mycobacterial wall glycolipids were tested for their effects on phospholipidic liposome organization and passive permeability and on oxidative phosphorylation of isolated mitochondria. From fluorescence polarization of diphenylhexatriene performed on liposomes it was concluded that the two trehalose derivatives (dimycoloyltrehalose and polyphthienoyltrehalose) rigidified the fluid state of liposomes, the triglycosyl phenolphthiocerol slightly fluidized the gel state, while the peptidoglycolipid ("apolar" mycoside C) just shifted the phase transition temperature upward. Dimycoloyltrehalose was without effect on liposome passive permeability, as estimated from dicarboxyfluorescein leak rates, and polyphthienoyltrehalose and triglycosyl phenolphthiocerol slightly decreased leaks, while mycoside C dramatically increased leaks. Activity of these lipids on mitochondrial oxidative phosphorylation was examined. The two trehalose derivatives have been tested previously: both had the same type of inhibitory activity, dimycoloyltrehalose being the most active. Triglycosyl phenolphthiocerol was inactive. Mycoside C was very active, with effects resembling those of classical uncouplers: this suggested that its activity on mitochondria was related to its effect on permeability. All these membrane alterations were called nonspecific because it is likely that they result from nonspecific lipid-lipid interactions, and not from recognition between specific molecular structures. Such nonspecific interactions could be at the origin of some of the effects of mycobacteria glycolipids on cells of the immune system observed in the last few years.

Polyphthienoyl trehalose, glycolipids specific for virulent strains of the tubercle bacillus

Phthienoic acids constitute a family of dextro-rotary odd-numbered unsaturated fatty acids isolated exclusively from virulent strains of human and bovine tubercle bacilli. In the bacterial cell they are not free and a search for their linked form in complex wall lipids of Mycobacterium tuberculosis (strain Canetti) showed that they esterified trehalose. Structural elucidation of the major phthienoyl trehalose showed the occurrence of five acyl residues located at 2, 2', 3', 4 and 6' positions of trehalose. The acyl substituents were mainly 2,4,6-trimethyl tetracos-2-enoic acid (C27 phthienoic acid) accompanied by its homologs. In addition to these branched fatty acids, straight-chain C16 and C18 acyls composed about 20% of the substituents. The proposed structure is a new one, both for the mycobacterial-specific glycolipid and for the substituted positions on trehalose. Other minor acyl trehaloses were detected in M. tuberculosis (strain Canetti), differing from the major component by the occurrence of an additional hydroxy fatty acid (3-hydroxy-2,4,6-trimethyl tetracosanoic acid) or by the number of acyl substituents. The major glycolipid presented a weak activity in vitro on mitochondrial oxidative phosphorylation. These glycolipids and phthienoic acids could serve as virulence indicators.

Lipid A monosaccharide analogues inhibiting oxidative phosphorylation

Three acyl-glucosamine analogues of "lipid A" and an acyl-glucose analogue of "cord factor" were synthesized and their activity was tested on isolated rat liver mitochondria. The 4 glycolipids slightly inhibited succinate-supported active respiration and strongly inhibited glutamate-supported active respiration. The most potent inhibitors were the two diacylated compounds which are the most hydrophobic. Phosphorylation was also impaired. Comparison of our results with the few published data about the effects of lipid A on mitochondria indicated that the two diacylated glucosamines were as active as their natural model. The minimal requirements to obtain a glycolipid structure with an activity resembling that of lipid A is discussed.

Dicorynomycoloyl trehalose activity: Comparison of the activity of α,α′- and β,β′-trehalose derivatives on mitochondrial oxidative phosphorylation

6,6'-Dicorynomycoloyl esters of alpha,alpha'- or beta;beta'-trehalose were synthesized and tested on isolated rat liver mitochondria. In contrast to the well known site-II-specific uncoupling effect of the alpha,alpha'-trehalose derivative, the beta,beta'-trehalose derivative exhibited only non-specific inhibition of active respiration in the presence of glutamate. It is proposed that this unexpected difference between the two isomers could arise from conformational differences in the carbohydrate moiety of the glycolipids, which would render the beta,beta' isomer unable to recognize targets specific to the alpha,alpha'-trehalose glycolipid.

Mycobacteria arabinolipids as potential endotoxins: their activity on mitochondrial oxidative phosphorylation

Two natural arabinolipids from the cell wall of Mycobacterium tuberculosis (5-mycoloyl-D-arabinose and 5-mycoloyldiarabinoside) and two synthetic arabinolipids (5-mycoloyl-L-arabinose and 5-mycoloyl(2'-hydroxyethyl)L-arabinoside) were tested on rat liver mitochondria. 5-Mycoloyl D- and L-rabinose had the same low activity, while 5-mycoloyldiarabinoside efficiently lowered the ADP/O ratio, inhibited active respiration and increased controlled respiration. The synthetic 5-mycoloyl(2'-hydroxyethyl)arabinoside had an intermediate efficiency. Due to their activity on mitochondria, the arabinolipids of the mycobacterial cell wall can be considered as potential endotoxins.

Polymyxin B-induced phase separation and acyl chain interdigitation in phosphatidylcholine/phosphatidylglycerol mixtures

Monolayers, fluorescence polarization, differential scanning calorimetry and X-ray diffraction experiments have been carried out to examine the effect of the polypeptide antibiotic polymyxin B on the phase behaviour of dipalmitoylphosphatidylglycerol (DPPG) either pure or mixed with dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC). It is shown that in both phosphatidylglycerol alone and phosphatidylglycerol/phosphatidylcholine mixtures, polymyxin B can induce either phase separation between lipid domains of various compositions or interdigitation of the acyl chains in the solid state, without segregation of the two lipids. Phase separation was observed by fluorescence and differential scanning calorimetry after addition of the antibiotic to vesicles composed of mixtures of DMPC and DPPG in conditions where polymyxin B did not saturate phosphatidylglycerol (DPPG to polymyxin B molar ratio, Ri, higher than 15). Phase separation was also observed in mixed monolayers of DPPC and of the 5:1 DPPG/polymyxin B complex, at high surface pressure. Acyl chain interdigitation was observed by X-ray diffraction in both 5:1 DPPG/polymyxin B mixtures and preformed 5:5:1 DMPC/DPPG/polymyxin B mixture, in which the antibiotic saturates phosphatidylglycerol (Ri 5). In both cases, raising the temperature gave rise to a complex double-peaked phase transition by differential scanning calorimetry, from the interdigitating phase to a normal L alpha lamellar phase. As it is known that polymyxin B does not interact with phosphatidylcholine, the data presented show that, when phosphatidylcholine and phosphatidylglycerol are mixed together, a phase perturbation such as acyl chain interdigitation, which normally affects only phosphatidylglycerol, is also felt by phosphatidylcholine.

Effect of pH, mono- and divalent cations on the mixing of phosphatidylglycerol with phosphatidylcholine. A monolayer (pi, delta V) and fluorescence study

The mixing of various molecular species of phosphatidylglycerol and phosphatidylcholine differing in their acyl chain lengths has been studied both in monolayers (pi, delta V), and in water dispersions (fluorescence polarization) with varying pH and ionic strength of the aqueous phase and in the presence of the divalent cations Mg2+ and Ca2+. In dilauroylphosphatidylglycerol/dipalmitoylphosphatidylcholine mixtures, both in monolayers and in water dispersions, no phase separation was detected at pH 2.9 where phosphatidylglycerol was protonated. With dipalmitoylphosphatidylglycerol/dipalmitoylphosphatidylcholine mixtures, in monolayers and at the same pH, no phase separation was detected for surface pressures below pi = 40 mN.m-1. In monolayers, and under ionic conditions such that phosphatidylglycerol was ionized (pH 5.6, 10 mM NaCl) miscibility was observed with dilauroylphosphatidylglycerol and dipalmitoylphosphatidylcholine and also with dipalmitoylphosphatidylglycerol and dilauroylphosphatidylcholine. Varying the ionic strength did not alter the miscibility of these lipids. The divalent cations Mg2+ and Ca2+ did not modify that of dilauroylphosphatidylglycerol with dilauroylphosphatidylcholine or with dipalmitoylphosphatidylcholine. Both in monolayers and in water dispersions, dipalmitoylphosphatidylglycerol and dilauroylphosphatidylcholine appeared to be at least partly miscible, in the presence of magnesium. Only in the presence of calcium and at high surface pressure might the monolayer data account for phase separation between these two lipids. The data presented demonstrate the existence of strong cohesive forces between phosphatidylcholine and phosphatidylglycerol with a marked influence of the former on the physical state of the latter. From an analysis of the delta V data, it is suggested that intrafacial hydrogen bonds may play a significant role in stabilizing phosphatidylcholine/phosphatidylglycerol mixtures.

Phase behaviour in monolayers and in water dispersions of mixtures of dimannosyl diacylglycerol with phosphatidylglycerol

Mixtures of dimannosyl diacylglycerol, extracted from the membrane of Micrococcus luteus, with synthetic dipalmitoyl phosphatidylglycerol or with samples of phosphatidylglycerol and phosphatidylinositol, extracted from the same bacterium, have been studied. Through a monolayer (pi, delta V) study and from fluorescence polarization data relative to diphenylhexatriene embedded in vesicles of the mixed lipids, it is shown that the glycolipid interacts with the phospholipids. These interactions are independent of the structure and physical state of the phospholipid acyl chains, of the lipid molecular packing and of the nature of the cations (monovalent, bivalent) present in the aqueous phase. No phase separation was detected, either in monolayers or in water dispersions. Furthermore, the data presented demonstrated a marked influence of the glycolipid on the phase behaviour of phosphatidylglycerol, both in the presence of monovalent (Na+, K+) and bivalent (Ca2+, Mg2+) cations. This point is of particular interest with regard to the highly rigid phase this phospholipid is known to assume in the presence of bivalent cations. It is then suggested that the glycolipid could act as a regulator of the membrane fluidity by preventing a too high rigidity of the lipid phase when bivalent cations are present at the membrane surface.

Evidence for penetration in liposomes and in mitochondrial membranes of a fluorescent analogue of cord factor

A fluorescent analogue of cord factor, a glycolipid toxin of mycobacteria, has been synthesized and its interactions with liposomes and isolated mitochondria have been studied. This compound, methyl alpha-D-6[12-(9-anthroyl)stearoyl]glucoside, is shown to be active against oxidative phosphorylation. When spread as a monolayer at the air-water interface, it forms a well organized phase and it strongly interacts with phosphatidylcholine. Addition of phosphatidylcholine liposomes or of isolated mitochondria to a water disperson of this fluorescent cord factor analogue results in a large increase of the fluorescence intensity. Moreover, the glycolipid probes for the temperature-dependent phase transition of the added suspensions. It is thus suggested that this cord factor analogue penetrates within mitochondrial membranes, a result which is discussed with respect to our previous conclusions concerning the way natural cord factors can interact with these organelles.