Stability of diether C25,25 liposomes from the hyperthermophilic archaeon Aeropyrum pernix K1

Influence of archaeal lipids isolated from Aeropyrum pernix K1 on physicochemical properties of sphingomyelin-cholesterol liposomes

We investigated the influence of archaeal lipids (C25,25) isolated from thermophilic archaeon Aeropyrum pernix K1 on physicochemical properties of liposomes comprised of egg sphingomyelin (SM) and cholesterol (CH) using fluorescence emission anisotropy, calcein release studies, dynamic light scattering, transmission electron microscopy and phase analysis light scattering. The 2 mol% addition of archaeal lipids enabled formation of small unilamellar vesicles by sonication while also having significant effect on reducing mean size, polydispersity index and zeta potential of C25,25/SM/CH vesicles. Increasing the ratio of C25,25 lipids in mixture of C25,25/SM/CH decreased lipid ordering parameter in dose dependent manner at different temperatures. We also demonstrated that adding 15 mol% C25,25 to SM/CH mixture will cause it to notably interact with fetal bovine serum which could make them a viable alternative adjuvant to synthetic ether-linked lipids in development of advanced liposomal vaccine delivery systems. The prospect of combining the proven strengths of SM/CH mixtures with the unique properties of C25,25 opens exciting possibilities for advancing drug delivery technologies, promising to yield formulations that are both highly effective and adaptable to a range of therapeutic applications.

Doxorubicin loaded thermostable nanoarchaeosomes: a next-generation drug carrier for breast cancer therapeutics

Breast cancer has a poor prognosis due to the toxic side effects associated with high doses of chemotherapy. Liposomal drug encapsulation has resulted in clinical success in enhancing chemotherapy tolerability. However, the formulation faces severe limitations with a lack of colloidal stability, reduced drug efficiency, and difficulties in storage conditions. Nanoarchaeosomes (NA) are a new generation of highly stable nanovesicles composed of the natural ether lipids extracted from archaea. In our study, we synthesized and characterized the NA, evaluated their colloidal stability, drug release potential, and anticancer efficacy. Transmission electron microscopy images have shown that the NA prepared from the hyperthermophilic archaeon Aeropyrum pernix K1 was in the size range of 61 ± 3 nm. The dynamic light scattering result has confirmed that the NA were stable at acidic pH (pH 4) and high temperature (70 °C). The NA exhibited excellent colloidal stability for 50 days with storage conditions at room temperature. The cell viability results have shown that the pure NA did not induce cytotoxicity in NIH 3T3 fibroblast cells and are biocompatible. Then NA were loaded with doxorubicin (NAD), and FTIR and UV-vis spectroscopy results have confirmed high drug loading efficiency of 97 ± 1% with sustained drug release for 48 h. The in vitro cytotoxicity studies in MCF-7 breast cancer cell lines showed that NAD induced cytotoxicity at less than 10 nM concentration. Fluorescence-activated cell sorting (FACS) results confirmed that NAD induced late apoptosis in nearly 92% of MCF-7 cells and necrosis in the remaining cells with cell cycle arrest at the G0/G1 phase. Our results confirmed that the NA could be a potential next-generation carrier with excellent stability, high drug loading efficiency, sustained drug release ability, and increased therapeutic efficacy, thus reducing the side effects of conventional drugs.

Unraveling the multiplicity of geranylgeranyl reductases in Archaea: potential roles in saturation of terpenoids

The enzymology of the key steps in the archaeal phospholipid biosynthetic pathway has been elucidated in recent years. In contrast, the complete biosynthetic pathways for proposed membrane regulators consisting of polyterpenes, such as carotenoids, respiratory quinones, and polyprenols remain unknown. Notably, the multiplicity of geranylgeranyl reductases (GGRs) in archaeal genomes has been correlated with the saturation of polyterpenes. Although GGRs, which are responsible for saturation of the isoprene chains of phospholipids, have been identified and studied in detail, there is little information regarding the structure and function of the paralogs. Here, we discuss the diversity of archaeal membrane-associated polyterpenes which is correlated with the genomic loci, structural and sequence-based analyses of GGR paralogs.

Advances in Physicochemical and Biochemical Characterization of Archaeosomes from Polar Lipids of Aeropyrum pernix K1 and Stability in Biological Systems

Archaeosomes are vesicles made from archaeal lipids. They are characterized by remarkable thermostability, resistance to enzymatic degradation, long-term stability, and immunomodulatory properties. In this review the current status of physicochemical properties of archaeal lipids and their stability in biological systems is presented, focusing on total polar lipids from Aeropyrum pernix K1. The isolated total polar lipids from Aeropyrum pernix K1 consist exclusively of glycerol ether lipids with isoprenoid groups attached to glycerol via ether linkages. More specifically, the two major polar lipids extracted from the membranes are C_25,25-achaetidyl(glucosyl)inositol and C_25,25-achaetidylinositol. An overview of the results of the effects of temperature and pH on the stability, structural organization, fluidity, and permeability of archaeosomes composed of pure C_25,25 was examined by a combination of techniques, including fluorescence emission spectroscopy, electron paramagnetic resonance, differential scanning calorimetry, and confocal microscopy. We also compared the physicochemical properties of pure vesicles composed of either archaeal lipids or conventional lipids (e.g., 1,2-dipalmitoyl-sn-glycero-3-phosphocholine) with mixed vesicles composed of both lipid types. Archaeal lipids are discussed in terms of their potential use as a targeted drug delivery system based on the results of in vivo and cytotoxicity studies.

Vesicular and Planar Membranes of Archaea Lipids: Unusual Physical Properties and Biomedical Applications

Liposomes and planar membranes made of archaea or archaea-like lipids exhibit many unusual physical properties compared to model membranes composed of conventional diester lipids. Here, we review several recent findings in this research area, which include (1) thermosensitive archaeosomes with the capability to drastically change the membrane surface charge, (2) MthK channel's capability to insert into tightly packed tetraether black lipid membranes and exhibit channel activity with surprisingly high calcium sensitivity, and (3) the intercalation of apolar squalane into the midplane space of diether bilayers to impede proton permeation. We also review the usage of tetraether archaeosomes as nanocarriers of therapeutics and vaccine adjuvants, as well as the biomedical applications of planar archaea lipid membranes. The discussion on archaeosomal therapeutics is focused on partially purified tetraether lipid fractions such as the polar lipid fraction E (PLFE) and glyceryl caldityl tetraether (GCTE), which are the main components of PLFE with the sugar and phosphate removed.

Contribution of headgroup and chain length of glycerophospholipids to thermal stability and permeability of liposomes loaded with calcein

Biological membranes are complex systems that are composed of lipids, proteins and carbohydrates. They are difficult to study, so it is established practice to use lipid vesicles that consist of closed 'shells' of phospholipid bilayers as model systems to study various functional and structural aspects of lipid organisation. To define the effects of the structural properties of lipid vesicles on their phase behaviour, we investigated their headgroup and chain length, and the chemical bonds by which their acyl chains are attached to the glycerol moiety of glycerophospholipid species, in terms of phase transition temperature, enthalpy change and calcein permeability. We used differential scanning calorimetry to measure the temperature and enthalpy changes of phase transition, and fluorescence to follow calcein release through the bilayer structure. Our data show that longer acyl chains increase the stability of the lipid bilayers, whereas higher salt concentrations decrease the thermal stability and widen the phase transitions of these lipid bilayers. We discuss the possible reasons for the observed phase transition behaviour.

New Insights Into the Polar Lipid Composition of Extremely Halo(alkali)philic Euryarchaea From Hypersaline Lakes

We analyzed the polar membrane lipids of 13 strains of halo(alkali)philic euryarchaea from hypersaline lakes. Nine belong to the class Halobacteria, representing two functional groups: aerobic polysaccharide utilizers and sulfur-respiring anaerobes. The other four strains represent halo(alkali)philic methanogens from the class Methanomicrobia and a recently discovered class Methanonatronarchaeia. A wide range of polar lipids were detected across the 13 strains including dialkyl glycerol diethers (archaeols), membrane-spanning glycerol tetraethers and diether-based cardiolipins. The archaeols contained a range of core lipid structures, including combinations of C20 and C25 isoprenoidal alkyl chains, unsaturations, and hydroxy moieties. Several diether lipids were novel, including: (a) a phosphatidylglycerolhexose (PG-Gly) headgroup, (b) a N,N,N-trimethyl aminopentanetetrol (APT)-like lipid with a methoxy group in place of a hydroxy group on the pentanetetrol, (c) a series of polar lipids with a headgroup with elemental composition of either C12H25NO13S or C12H25NO16S2, and (d) novel cardiolipins containing a putative phosphatidylglycerolphosphate glycerophosphate (PGPGP) polar moiety. We found that the lipid distribution of the 13 strains could be generally separated into two groups, the methanogens (group) and the Halobacteria (class) based on the presence of specific core lipids. Within the methanogens, adaption to a high or more moderate salt concentration resulted in different ratios of glycerol dialkyl glycerol tetraethers (GDGTs) to archaeol. The methanogen Methanosalsum natronophilum AME2T had the most complex diether lipid composition of any of the 13 strains, including hydroxy archaeol and macrocyclic archaeol which we surmise is an order-specific membrane adaption. The zwitterionic headgroups APT and APT-Me were detected only in the Methanomicrobiales member Methanocalculus alkaliphilus AMF2T which also contained the highest level of unsaturated lipids. Only alkaliphilic members of the Natrialbales order contained PGPGP cardiolipins and the PG-Gly headgroup. The four analyzed neutrophilic members of the Halobacteria were characterized by the presence of sulfur-containing headgroups and glycolipids. The presence of cardiolipins with one or more i-C25 alkyl chains, generally termed extended archaeol (EXT-AR), in one of the Methanonatronarchaeia strains was unexpected as only one other order of methanogenic archaea has been reported to produce EXT-AR. We examined this further by looking into the genomic potential of various archaea to produce EXT-AR.

Biosynthetic machinery for C25,C25-diether archaeal lipids from the hyperthermophilic archaeon Aeropyrum pernix

Archaea that thrive in harsh environments usually produce membrane lipids with specific structures such as bipolar tetraether lipids. Only a few genera of archaea, which are hyperthermophiles or halophiles, are known to utilize diether lipids with extended, C₂₅ isoprenoid hydrocarbon chains. In the present study, we identify two prenyltransferases and a prenyl reductase responsible for the biosynthesis of C₂₅,C₂₅-diether lipids in the hyperthermophilic archaeon Aeropyrum pernix. These enzymes are more specific to C₂₅ isoprenoid chains than to C₂₀ chains, which are used for the biosynthesis of ordinary C₂₀,C₂₀-diether archaeal lipids. The recombinant expression of these enzymes with two known archaeal enzymes allows the production of C₂₅,C₂₅-diether archaeal lipids in the cells of Escherichia coli.

Specific electrical capacitance and voltage breakdown as a function of temperature for different planar lipid bilayers

The breakdown voltage and specific electrical capacitance of planar lipid bilayers formed from lipids isolated from the membrane of archaeon Aeropyrum pernix K1 as a function of temperature were studied and compared with data obtained previously in MD simulation studies. Temperature dependence of breakdown voltage and specific electrical capacitance was measured also for dipalmitoylphosphatidylcholine (DPPC) bilayers and bilayers formed from mixture of diphytanoylphosphocholine (DPhPC) and DPPC in ratio 80:20. The breakdown voltage of archaeal lipids planar lipid bilayers is more or less constant until 50°C, while at higher temperatures a considerable drop is observed, which is in line with the results from MD simulations. The breakdown voltage of DPPC planar lipid bilayer at melting temperature is considerably higher than in the gel phase. Specific electrical capacitance of planar lipid bilayers formed from archaeal lipids is approximately constant for temperatures up to 40°C and then gradually decreases. The difference with MD simulation predictions is discussed. Specific electrical capacitance of DPPC planar lipid bilayers in fluid phase is 1.75 times larger than that of the gel phase and it follows intermediated phases before phase transition. Increase in specific electrical capacitance while approaching melting point of DPPC is visible also for DPhPC:DPPC mixture.

Effect of superparamagnetic iron oxide nanoparticles on fluidity and phase transition of phosphatidylcholine liposomal membranes

Superparamagnetic iron oxide nanoparticles (SPIONs) with multifunctional properties have shown great promise in theranostics. The aim of our work was to compare the effects of SPIONs on the fluidity and phase transition of the liposomal membranes prepared with zwitterionic phosphatidylcholine lipids. In order to study if the surface modification of SPIONs has any influence on these membrane properties, we have used four types of differently functionalized SPIONs, such as: plain SPIONs (primary size was shown to bê11 nm), silica-coated SPIONs, SPIONs coated with silica and functionalized with positively charged amino groups or negatively charged carboxyl groups (the primary size of all the surface-modified SPIONs was ~20 nm). Small unilamellar vesicles prepared with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine lipids and multilamellar vesicles prepared with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine lipids were encapsulated or incubated with the plain and surface-modified SPIONs to determine the fluidity and phase transition temperature of the bilayer lipids, respectively. Fluorescent anisotropy and differential scanning calorimetric measurements of the liposomes that were either encapsulated or incubated with the suspension of SPIONs did not show a significant difference in the lipid ordering and fluidity; though the encapsulated SPIONs showed a slightly increased effect on the fluidity of the model membranes in comparison with the incubated SPIONs. This indicates the low potential of the SPIONs to interact with the nontargeted cell membranes, which is a desirable factor for in vivo applications.

Bending elasticity modulus of giant vesicles composed of aeropyrum pernix k1 archaeal lipid

Thermally induced shape fluctuations were used to study elastic properties of giant vesicles composed of archaeal lipids C_25,25-archetidyl (glucosyl) inositol and C_25,25-archetidylinositol isolated from lyophilised Aeropyrum pernix K1 cells. Giant vesicles were created by electroformation in pure water environment. Stroboscopic illumination using a xenon flash lamp was implemented to remove the blur effect due to the finite integration time of the camera and to obtain an instant picture of the fluctuating vesicle shape. The mean weighted value of the bending elasticity modulus k_c of the archaeal membrane determined from the measurements meeting the entire set of qualification criteria was (1.89 ± 0.18) × 10⁻¹⁹ J, which is similar to the values obtained for a membrane composed of the eukaryotic phospholipids SOPC (1.88 ± 0.17) × 10⁻¹⁹ J and POPC (2.00 ± 0.21) × 10⁻¹⁹ J. We conclude that membranes composed of archaeal lipids isolated from Aeropyrum pernix K1 cells have similar elastic properties as membranes composed of eukaryotic lipids. This fact, together with the importance of the elastic properties for the normal circulation through blood system, provides further evidence in favor of expectations that archaeal lipids could be appropriate for the design of drug delivery systems.

Structural properties of archaeal lipid bilayers: small-angle X-ray scattering and molecular dynamics simulation study

Aeropyrum pernix is an aerobic hyperthermophilic archaeon that grows in harsh environmental conditions and as such possesses unique structural and metabolic features. Its membrane interfaces with the extreme environment and is the first line of defense from external factors. Therefore, lipids composing this membrane have special moieties that increase its stability. The membrane of A. pernix is composed predominantly of two polar lipids 2,3-di-O-sesterterpanyl-sn-glicerol-1-phospho-1'(2'-O-α-D-glucosyl)-myo-inositol (AGI) and 2,3-di-O-sesterterpanyl-sn-glicerol-1-phospho-myo-inositol (AI). Both have methyl branches in their lipid tails and ether linkages and carbohydrates in their headgroup. These moieties significantly affect the structure and dynamics of the bilayer. To provide a molecular level insight into these characteristics, we used here Molecular Dynamics (MD) simulations of lipid bilayers of composition similar to those of the archaeal membranes. First, we show that the electron density profiles along the normal to the bilayers derived from the simulations are in good agreement with the profiles obtained by the small-angle X-ray scattering (SAXS) technique, which provides confidence in the force fields used. Analyses of the simulation data show that the archaeal lipid bilayers are less hydrated than conventional phosphatidylcholine (PC) lipids and that their structure is not affected by the salt present in the surrounding solution. Furthermore, the lateral pressure in their hydrophobic core, due to the presence of the branched tails, is much higher than that at PC-based lipid bilayers. Both the methyl branched tails and the special headgroup moieties contribute to slow drastically the lateral diffusion of the lipids. Furthermore, we found that the lipid head groups associate via hydrogen bonding, which affects their reorientational dynamics. All together, our data provide links between the microscopic properties of these membranes and their overall stability in harsh environments.

Influence of nanoparticle-membrane electrostatic interactions on membrane fluidity and bending elasticity

The aim of this work is to investigate the effect of electrostatic interactions between the nanoparticles and the membrane lipids on altering the physical properties of the liposomal membrane such as fluidity and bending elasticity. For this purpose, we have used nanoparticles and lipids with different surface charges. Positively charged iron oxide (γ-Fe2O3) nanoparticles, neutral and negatively charged cobalt ferrite (CoFe2O4) nanoparticles were encapsulated in neutral lipid 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine and negatively charged 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine lipid mixture. Membrane fluidity was assessed through the anisotropy measurements using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Though the interaction of both the types of nanoparticles reduced the membrane fluidity, the results were more pronounced in the negatively charged liposomes encapsulated with positively charged iron oxide nanoparticles due to strong electrostatic attractions. X-ray photoelectron spectroscopy results also confirmed the presence of significant quantity of positively charged iron oxide nanoparticles in negatively charged liposomes. Through thermally induced shape fluctuation measurements of the giant liposomes, a considerable reduction in the bending elasticity modulus was observed for cobalt ferrite nanoparticles. The experimental results were supported by the simulation studies using modified Langevin-Poisson-Boltzmann model.

Interaction between dipolar lipid headgroups and charged nanoparticles mediated by water dipoles and ions

In this work, a theoretical model describing the interaction between a positively or negatively charged nanoparticle and neutral zwitterionic lipid bilayers is presented. It is shown that in the close vicinity of the positively charged nanoparticle, the zwitterionic lipid headgroups are less extended in the direction perpendicular to the membrane surface, while in the vicinity of the negatively charged nanoparticle, the headgroups are more extended. This result coincides with the calculated increase in the osmotic pressure between the zwitterionic lipid surface and positively charged nanoparticle and the decrease of osmotic pressure between the zwitterionic lipid surface and the negatively charged nanoparticle. Our theoretical predictions agree well with the experimentally determined fluidity of a lipid bilayer membrane in contact with positively or negatively charged nanoparticles. The prospective significance of the present work is mainly to contribute to better understanding of the interactions of charged nanoparticles with a zwitterionic lipid bilayer, which may be important in the efficient design of the lipid/nanoparticle nanostructures (like liposomes with encapsulated nanoparticles), which have diverse biomedical applications, including targeted therapy (drug delivery) and imaging of cancer cells.

Cytotoxicity and uptake of archaeosomes prepared from Aeropyrum pernix lipids

Archaeon Aeropyrum pernix K1 is an obligate aerobic hyperthermophilic organism with C25,25-archeol membrane lipids with head groups containing inositol. Interactions of archaeosomes, liposomes prepared from lipids of A. pernix, with mammalian cells in vitro were studied. In vitro cytotoxicity was tested on five different cell lines: rodent mouse melanoma cells (B16-F1) and Chinese hamster ovary (CHO) cells, and three human cell lines—epithelial colorectal adenocarcinoma cells (CACO-2), liver hepatocellular carcinoma cell line (Hep G2) and endothelial umbilical vein cell line (EA.hy926). Archaeosomes were nontoxic to human Hep G2, CACO-2 and mildly toxic to rodent CHO and B16-F1 cells but showed strong cytotoxic effect on EA.hy926 cells. Confocal microscopy revealed that archaeosomes are taken up by endocytosis. The uptake of archaeosomes and the release of loaded calcein are more prominent in EA.hy926 cells, which is in line with high toxicity toward these cells. The mechanisms of uptake, release and action in these cells as well as in vivo functioning have to be further studied for possible targeted drug delivery.

Morphology, biophysical properties and protein-mediated fusion of archaeosomes

As variance from standard phospholipids of eubacteria and eukaryotes, archaebacterial diether phospholipids contain branched alcohol chains (phytanol) linked to glycerol exclusively with ether bonds. Giant vesicles (GVs) constituted of different species of archaebacterial diether phospholipids and glycolipids (archaeosomes) were prepared by electroformation and observed under a phase contrast and/or fluorescence microscope. Archaebacterial lipids and different mixtures of archaebacterial and standard lipids formed GVs which were analysed for size, yield and ability to adhere to each other due to the mediating effects of certain plasma proteins. GVs constituted of different proportions of archaeal or standard phosphatidylcholine were compared. In nonarchaebacterial GVs (in form of multilamellar lipid vesicles, MLVs) the main transition was detected at T_m = 34. 2°C with an enthalpy of ΔH = 0.68 kcal/mol, whereas in archaebacterial GVs (MLVs) we did not observe the main phase transition in the range between 10 and 70°C. GVs constituted of archaebacterial lipids were subject to attractive interaction mediated by beta 2 glycoprotein I and by heparin. The adhesion constant of beta 2 glycoprotein I – mediated adhesion determined from adhesion angle between adhered GVs was in the range of 10⁻⁸ J/m². In the course of protein mediated adhesion, lateral segregation of the membrane components and presence of thin tubular membranous structures were observed. The ability of archaebacterial diether lipids to combine with standard lipids in bilayers and their compatibility with adhesion-mediating molecules offer further evidence that archaebacterial lipids are appropriate for the design of drug carriers.

Effect of growth medium pH of Aeropyrum pernix on structural properties and fluidity of archaeosomes

The influence of pH (6.0; 7.0; 8.0) of the growth medium of Aeropyrum pernix K1 on the structural organization and fluidity of archaeosomes prepared from a polar-lipid methanol fraction (PLMF) was investigated using fluorescence anisotropy and electron paramagnetic resonance (EPR) spectroscopy. Fluorescence anisotropy of the lipophilic fluorofore 1,6-diphenyl-1,3,5-hexatriene and empirical correlation time of the spin probe methylester of 5-doxylpalmitate revealed gradual changes with increasing temperature for the pH. A similar effect has been observed by using the trimethylammonium-6-diphenyl-1,3,5-hexatriene, although the temperature changes were much smaller. As the fluorescence steady-state anisotropy and the empirical correlation time obtained directly from the EPR spectra alone did not provide detailed structural information, the EPR spectra were analysed by computer simulation. This analysis showed that the archaeosome membranes are heterogeneous and composed of several regions with different modes of spin-probe motion at temperatures below 70°C. At higher temperatures, these membranes become more homogeneous and can be described by only one spectral component. Both methods indicate that the pH of the growth medium of A. pernix does not significantly influence its average membrane fluidity. These results are in accordance with TLC analysis of isolated lipids, which show no significant differences between PLMF isolated from A. pernix grown in medium with different pH.