An ultrastructural study of the effects of acidic phospholipid substitutions on calcium phosphate precipitation in anionic liposomes

Precipitation of Calcium Phosphates and Calcium Carbonates in the Presence of Differently Charged Liposomes

Liposomes (lipid vesicles) are often considered to be a versatile tool for the synthesis of advanced materials, as they allow various control mechanisms to tune the materials’ properties. Among diverse materials, the synthesis of calcium phosphates (CaPs) and calcium carbonates (CaCO3) using liposomes has attracted particular attention in the development of novel (bio)materials and biomineralization research. However, the preparation of materials using liposomes has not yet been fully exploited. Most of the liposomes used have been anionic and/or zwitterionic, while data on the influence of cationic liposomes are limited. Therefore, the aim of this study was to investigate and compare the influence of differently charged liposomes on CaPs and CaCO3 formation. Zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), negatively charged 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS), and positively charged 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine (EPC) lipids were used to prepare the respective liposomes. The presence of liposomes during the spontaneous precipitation of CaPs and CaCO3 affected both the precipitation and transformation kinetics, as well as the morphology of the precipitates formed. The most prominent effect was noted for both materials in the presence of DMPS liposomes, as (nano) shell structures were formed in both cases. The obtained results indicate possible strategies to fine-tune the precipitation process of CaPs and CaCO3, which may be of interest for the production of novel materials.

Precipitation of Calcium Carbonate Inside Giant Unilamellar Vesicles Composed of Fluid-Phase Lipids

Biomineralization of CaCO₃ commonly involves the formation of amorphous CaCO₃ precursor particles that are produced in a confined space surrounded by a lipid bilayer. While the influence of confinement itself has been investigated with different model systems, the impact of an enclosing continuous lipid bilayer on CaCO₃ formation in a confined space is still poorly understood as appropriate model systems are rare. Here, we present a new versatile method based on droplet-based microfluidics to produce fluid-phase giant unilamellar vesicles (GUVs) in the presence of high CaCl₂ concentrations. These GUVs can be readily investigated by means of confocal laser scanning microscopy in combination with bright-field microscopy, demonstrating that the formed CaCO₃ particles are in conformal contact with the fluid-phase lipid bilayer and thus suggesting a strong interaction between the particle and the membrane. Atomic force microscopy adhesion studies with membrane-coated spheres on different CaCO₃ crystals corroborated this notion of a strong interaction between the lipids and CaCO₃.

Crystallization in Confinement

Many crystallization processes of great importance, including frost heave, biomineralization, the synthesis of nanomaterials, and scale formation, occur in small volumes rather than bulk solution. Here, the influence of confinement on crystallization processes is described, drawing together information from fields as diverse as bioinspired mineralization, templating, pharmaceuticals, colloidal crystallization, and geochemistry. Experiments are principally conducted within confining systems that offer well-defined environments, varying from droplets in microfluidic devices, to cylindrical pores in filtration membranes, to nanoporous glasses and carbon nanotubes. Dramatic effects are observed, including a stabilization of metastable polymorphs, a depression of freezing points, and the formation of crystals with preferred orientations, modified morphologies, and even structures not seen in bulk. Confinement is also shown to influence crystallization processes over length scales ranging from the atomic to hundreds of micrometers, and to originate from a wide range of mechanisms. The development of an enhanced understanding of the influence of confinement on crystal nucleation and growth will not only provide superior insight into crystallization processes in many real-world environments, but will also enable this phenomenon to be used to control crystallization in applications including nanomaterial synthesis, heavy metal remediation, and the prevention of weathering.

Effect of size at the nanoscale and bilayer rigidity on skin diffusion of liposomes

This study reports the effect of liposome particle size at the nanoscale and bilayer deformability on the permeation through MatTek human skin equivalents and provides a comparative quantitative measure through calculation of diffusion coefficients. Exploring DOPC and DPPC fluorescent liposomes, our results demonstrate the faster diffusion of 50 nm liposomes compared with 100 and 200 nm liposomes when the lipid bilayer remains the same. Diffusion kinetics of the 50 nm particles appear not to depend on the rigidity of the lipid layer, whereas diffusion of particles larger than 100 nm is significantly affected by the rigidity of the bilayer, and DOPC liposomes diffuse faster than their DDPC equivalents. Our results suggest that liposomes composed of a rigid bilayer can be expected to remain intact after passing through the stratum corneum.

In vivo assessment of the osteointegrative potential of phosphatidylserine-based coatings

The successful implantation of titanium-based implants for orthopaedic and dental applications is often hindered because of their mobility, which arises because of a lack of direct binding of the metal surface to the mineral phase of the surrounding bone. Ceramic coatings, although ensuring the integration of the implant within the tissue, are unstable and carry risks of delamination and of failure. Recently, a novel biomimetic approach has been developed where porous titanium implants are coated with calcium-binding phospholipids able to catalyse the nucleation of discrete apatite crystals after only 30 min incubation in simulated body fluids. The present work assesses the osteointegrative potential of this new class of coatings in an in vivo rabbit model and compares its performance with those of bare porous titanium and hydroxyapatite-coated titanium. The data obtained show that phosphatidylserine-based coatings, whilst resorbing, drive the growing bone into apposition with the metal surface. This is in contrast to the case of bare titanium.

Calcium-binding phospholipids as a coating material for implant osteointegration

Among the many biomolecules involved in the bone mineralization processes, anionic phospholipids play an important role because of their ability to bind calcium. In particular, phosphatidylserine is a natural component of the plasmalemma and of the matrix vesicles generated from the osteoblast membrane to create nucleation centres for calcium phosphate crystal precipitation. In the present work, we demonstrate that calcium-binding phospholipids can be used as biomimetic coating materials for improving the osteointegration of metal implants. Relatively thick phosphatidylserine-based coatings were deposited on titanium coupons by dip-coating. Upon dehydration in a simulated body fluid phospholipids were quickly crosslinked by calcium and re-arranged into a three-dimensional matrix able to induce rapid formation of a calcium phosphate mineral phase. The rate of mineralization was shown to be dependent on the adopted coating formulation. In the attempt to closely mimic the cell membrane composition, heterogeneous formulations based on the mixing of anionic phospholipids (either phosphatidylserine or phosphatidylinositol) with phosphatidylcholine and cholesterol were synthesized. However, surface plasmon resonance studies as well as scanning electron microscopy and elemental analysis demonstrated that the homogeneous phosphatidylserine coating was a more effective calcification environment than the heterogeneous formulations.

Anomalous solubility behavior of the antibiotic ciprofloxacin encapsulated in liposomes: a 1H-NMR study

Many drugs are weak bases and can be accumulated into liposomes in response to a pH gradient to achieve high internal drug concentrations. This study is aimed at gaining an understanding of the relationship between the retention of the fluoroquinolone antibiotic ciprofloxacin in liposomes and the intraliposomal form and location of this drug. 1H-NMR spectroscopy was used to probe the interactions experienced by ciprofloxacin following uptake into large unilamellar liposomes (LUV). It is shown that ciprofloxacin is located in the aqueous interior of the liposomes and is self-associated in the form of small stacks. It does not precipitate out of solution even though the intraliposomal ciprofloxacin concentration can exceed its solubility in aqueous solutions by almost two orders of magnitude. The results also indicate that little entrapped ciprofloxacin partitions into the inner monolayer of the LUV. As a result of the lack of precipitation and rapid exchange properties, ciprofloxacin can respond quickly to changes in electrochemical equilibria such as depletion of the pH gradient. This provides a rationale for the rapid leakage of this drug in response to serum destabilization or depletion of the pH gradient.

Persistence of complexed acidic phospholipids in rapidly mineralizing tissues is due to affinity for mineral and resistance to hydrolytic attack: in vitro data

Acidic phospholipids, complexed with calcium and inorganic phosphate, are components of extracellular matrix vesicles. Both the complexed acidic phospholipids and matrix vesicles have previously been shown to serve as hydroxyapatite (HA) nucleators in solution and when implanted in a muscle pouch. The present study supplies evidence that complexed acidic phospholipids can persist in mineralizing tissues both because of their affinity for HA and because of their resistance to hydrolysis by phospholipase A2. Calcium-phosphatidylserine-phosphate complex (CPLX-PS) synthesized with 14C-labeled phosphatidylserine (PS) was used to measure CPLX-PS affinity for HA using a Langmuir adsorption isotherm model. The affinity was shown to be higher and more specific than that of PS itself (K = 8.66 ml/mumol; N, the number of binding sites = 20.4 mumol/m2 as compared with previously reported values for PS of K = 3.33 ml/mumol, and N = 4.87 mumol/m2). Incorporated into synthetic liposomes and incubated in a calcium phosphate solution in which mineralization is induced by an ionophore, CPLX-PS showed behavior distinct from free PS. As previously reported, PS in these liposomes totally blocked HA formation. On the other hand, CPLX-PS in similar concentrations had a varied response, having no effect, slightly inhibiting, or actually promoting HA formation. CPLX-PS was also shown to be a poorer substrate for phospholipase A2 than PS, with Km = 4.63 mM for CPLX-PS and Km = 0.27 mM for PS; and Vmax = 0.029 ml/minute for CPLX-PS and Vmax = 0.066 ml/minute for PS. These data explain how complexed acidic phospholipids may persist in the growth plate and facilitate initial mineral deposition.

Effect of 1-hydroxyethylidene-1,1-bisphosphonate on membrane-mediated calcium phosphate formation in model liposomal suspensions

The bisphosphonate, 1-hydroxyethylidene-1,1-bisphosphonate (HEBP), was examined for its effect on calcium phosphate precipitation in pH 7.4, 22 degrees C suspensions of 7:2:1 PC:phosphatidylcholine (PC):dicetylphosphate (DCP):cholesterol (Chol) and 7:1:1 PC:phosphatidylserine (PS):Chol liposomes. HEBP (0.5-50 mumol/l) in the suspending medium had little, if any, effect on precipitation that formed inside phosphate-rich (50 mmol/l) aqueous interiors of liposomes as a result of ionophore (X-537A) driven 2.25 mmol/l Ca2+ influxes from the medium. On the other hand, HEBP had a significant negative impact on the subsequent spread of the precipitate into the surrounding medium when the latter was made metastable with 1.5 mmol/l total inorganic phosphate (PO4). The inhibitory effect of HEBP was more strongly felt in the 7PC:1PS:1Chol liposomal suspensions, with only 1 mumol/l HEBP needed to effectively block extraliposomal precipitation compared to 7.5 mumol/l for 7PC:2DCP:1Chol suspensions. Direct encapsulation of HEBP (1-1000 mumol/l) together with PO4 in the aqueous cores of 7PC:2DCP:1Chol liposomes reduced somewhat (approximately 30%) intraliposomal yields and delayed but did not block extraliposomal precipitate development. These results provide a possible physicochemical explanation for the suppression of matrix vesicle initiated mineralization in ectopically-induced osteoid tissue of HEBP treated mice [1]. In particular, the liposome results suggest that membrane phosphatidylserine interactions with mineral may enhance HEBP's effectiveness in vivo.

Biomineralization: new directions in crystal science

Effective protocols for controlling crystal structure, size, and morphology attract considerable interest given the requirement for particles of modal size and shape in many areas of materials fabrication and the importance of crystallochemical selectivity in determining the exploitable properties of inorganic solids. For this reason biomineralization merits particular attention since many biominerals are deposited in a highly controlled manner to produce crystals which are uniformly sized and crystallographically unique. Studies of biominerals have revealed that while a complex array of strategies have evolved for regulating their formation, one feature is common to the biological paradigm; interactions between organized biopolymeric assemblies and the nascent inorganic solids play a pivotal role in controlling the crystallization process. In order to gain a better understanding of the molecular interactions which take place at organic-inorganic interface and address the fundamental chemical problems of biomineralization, a crystal chemical approach has been adopted. Organized organic assemblies (phospholipid vesicles, Langmuir monolayers, polypetide templates) of precise molecular design (head group identity, packing conformation, peptide sequence, etc.) were assayed for their effectiveness in controlling the nucleation and growth of inorganic solids. This work has established that through systematic changes in the nature of the organic matrix the size, crystallographic orientation, and growth of the mineral phase can be controlled. Critical to this process was the translation of specific molecular information at the organic-inorganic interface: epitaxial alignment, stereochemical complementarity, and electrostatic interactions were an essential feature of this recognition event.

Mixed phospholipid liposome calcification

Synthetic lipid vesicle (liposome) suspensions have been used to experimentally model many of the calcium phosphate precipitation steps observed in matrix vesicle (MV) calcification. In particular, precipitate development in liposomes can be made to preferentially follow the progression seen in MV, i.e. to occur initially in intraliposomal spaces and then to expand into the surrounding suspending medium. This paper reviews results from studies by us which show that certain phospholipid (PL) constituents of the liposomal membrane can modulate this progression. Of greatest relevance to MV calcification is the observation that phosphatidylserine and sphingomyelin, two lipids selectively enriched in MV, slow the expansion of the precipitation from inside to outside the liposome.