Incorporation of Functionalized Calcium Phosphate Nanoparticles in Living Cells

Intracellular calcium (Ca2+) is a key signaling element that is involved in a great variety of fundamental biological processes. Thus, Ca2+ deregulation would be involved in the cancer cell progression and damage of mitochondrial membrane and DNA, which lead to apoptosis and necrosis. In this study, we have prepared amorphous calcium phosphate nanoparticles (ACP NPs) for studied their incorporation by endocytosis or electroporation to epithelial, endothelial and fibroblast cells (MCF-7, HUVEC and COS-1 cells, respectively). Our results showed that internalized ACP NPs have cytotoxic effects as a consequence of the increase of the intracellular calcium content. The endocytosis pathways showed a greater cytotoxic effect since calcium ions could easily be released from the nanoparticles and be accumulated in the lysosomes and mitochondria. In addition, the cytotoxic effect could be reversed when calcium ion was chelated with ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA). Modification of ACP NPs by coating with different compounds based on phosphates was also evaluated. The results indicated a reduction of the cytotoxic effect, in the order polyphosphate < phosphonic acid < orthophosphate. A differential cytotoxic effect of ACP-NPs was observed in function of the cell type; the cytotoxic effect can be ordered as i.e., HUVEC > COS-1 > MCF-7. The greater cytotoxic effect caused by the increase of intracellular calcium that is observed in normal cells and the greater resistance of cancer cells suggests new perspectives for cancer research.

Biominerals Formed by DNA and Calcium Oxalate or Hydroxyapatite: A Comparative Study

Biominerals formed by DNA and calcium oxalate (CaOx) or hydroxyapatite (HAp), the most important and stable phase of calcium phosphate) have been examined and compared using a synergistic combination of computer simulation and experimental studies. The interest of this comparison stems from the medical observation that HAp- and CaOx-based microcalcifications are frequently observed in breast cancer tissues, and some of their features are used as part of the diagnosis. Molecular dynamics simulations show that (1) the DNA double helix remains stable when it is adsorbed onto the most stable facet of HAp, whereas it undergoes significant structural distortions when it is adsorbed onto CaOx; (2) DNA acts as a template for the nucleation and growth of HAp but not for the mineralization of CaOx; and (3) the DNA double helix remains stable when it is encapsulated inside HAp nanopores, but it becomes destabilized when the encapsulation occurs into CaOx nanopores. Furthermore, CaOx and HAp minerals containing DNA molecules inside and/or adsorbed on the surface have been prepared in the lab by mixing solutions containing the corresponding ions with fish sperm DNA. Characterization of the formed minerals, which has been focused on the identification of DNA using UV-vis spectroscopy, indicates that the tendency to adsorb and, especially, encapsulate DNA is much smaller for CaOx than for HAp, which is in perfect agreement with results from molecular dynamics simulations. Finally, quantum mechanical calculations have been performed to rationalize these results in terms of molecular interactions, evidencing the high affinity of Ca²⁺ toward oxalate anions in an aqueous environment.

Probing the surface structure of hydroxyapatite through its interaction with hydroxyl: a first-principles study

Understanding the interaction of the hydroxyapatite (HAp) surface with hydroxyl originating from either the alkalescent physiological environment or HAp itself is crucial for the development of HAp-based biomaterials. Periodical density functional theory calculations were carried out in this study to explore the interaction of the HAp (100), (010) and (001) facets with hydroxyl. Based on a comparison study of Ca-rich, PO4-rich and Ca-PO4-OH mixed surfaces, the interaction pattern, interaction energy and effect of an additional water molecule on the Ca-OH interaction were comprehensively studied. The formation of CaOH on the Ca-rich surface was energetically favored on (100) and (001), while Ca(OH)2 was energetically favored on (010). The Ca-water interaction was competitive, but had lower interaction energy than Ca-OH. Furthermore, Ca-O bonding and its influence on the OH stretching vibration were analyzed. Our calculations suggest that the hydroxyl-coated surface structure is more appropriate than the commonly used Ca-terminated surface model for studying HAp surface activity in its service environments.

Complex interplay of interatomic bonding in a multi-component pyrophosphate crystal: K2Mg (H2P2O7)2·2H2O

The electronic structure and interatomic bonding of pyrophosphate crystal K₂Mg (H₂P₂O₇)₂·2H₂O are investigated for the first time showing complex interplay of different types of bindings. The existing structure from single-crystal X-ray diffraction is not sufficiently refined, resulting in unrealistic short O─H bonds which is rectified by high-precision density functional theory (DFT) calculation. K₂Mg (H₂P₂O₇)₂·2H₂O has a direct gap of 5.22 eV and a small electron effective mass of 0.14 m_e. Detailed bond analysis between every pair of atoms reveals the complexity of various covalent, ionic, hydrogen bonding and bridging bonding and their sensitive dependence on structural differences. The K--O bonds are much weaker than Mg--O bonds and contributions from the hydrogen bonds are non-negligible. Quantitative analysis of internal cohesion in terms of total bond order density and partial bond order density divulges the relative importance of different types of bonding. The calculated optical absorptions show multiple peaks and a sharp Plasmon peak at 23 eV and a refractive index of 1.44. The elastic and mechanical properties show features unique to this low-symmetry crystal. Phonon calculation gives vibrational frequencies in agreement with reported Raman spectrum. These results provide new insights indicating that acidic pyrophosphates could have a variety of unrealized applications in advanced technology.

Amorphous polyphosphate-hydroxyapatite: A morphogenetically active substrate for bone-related SaOS-2 cells in vitro

There is increasing evidence that inorganic calcium-polyphosphates (polyP) are involved in human bone hydroxyapatite (HA) formation. Here we investigated the morphology of the particles, containing calcium phosphate (CaP) with different concentrations of various Na-polyP concentrations, as well as their effects in cell culture. We used both SaOS-2 cells and human mesenchymal stem cells. The polymeric phosphate readily binds calcium ions under formation of insoluble precipitates. We found that addition of low concentrations of polyP (<10wt.%, referred to the CaP deposits) results in an increased size of the HA crystals. Surprisingly, at higher polyP concentrations (>10wt.%) the formation of crystalline HA is prevented and amorphous polyP/HA hybrid particles with a size of ≈50nm are formed, most likely consisting of polyP molecules linked via Ca(2+) bridges to the surface of the CaP deposits. Further studies revealed that the polyP-CaP particles cause a strong upregulation of the expression of the genes encoding for two marker proteins of bone formation, collagen type I and alkaline phosphatase. Based on their morphogenetic activity the amorphous polyP-CaP particles offer a promising material for the development of bone implants, formed from physiological inorganic precursors/polymers.

STATEMENT OF SIGNIFICANCE

Hydroxyapatite (HA) is a naturally occurring mineral of vertebrate bone. Natural HA, a bio-ceramic material which is crystalline to different scale, has been used as a biomaterial to fabricate scaffolds for in situ bone regeneration and other tissue engineering purposes. In contrast to natural HA, synthetic apatite is much less effective. In general, while HA is bioactive, its interaction and biocompatibility with existing bone tissue is low. These properties have been attributed to a minimal degradability in the physiological environment. In the present study we introduce a new Ca-phosphate (CaP) fabrication technology, starting from calcium chloride and dibasic ammonium phosphate with the HA characteristic Ca/P molar ratio of 10:6 and report that after addition >10% (by weight) of polyphosphate (polyP) amorphous CaP/HA samples were obtained. Those samples elicits strong morphogenetic activity let us to conclude that polyP/HA-based material might be beneficial for application as bone substitute implant.