Publications

Hydroxyapatite nanoparticles are popular tools in bone regeneration, but they have also been used for gene delivery and as anticancer drugs. Understanding their mechanism of action, particularly for the latter application, is crucial to predict their toxicity. To this end, we aimed to elucidate the importance of nanoparticle membrane interactions in the cytotoxicity of MG-63 cells using two different types of nanoparticles. In addition, conventional techniques for studying nanoparticle internalisation were evaluated and compared with newer and less exploited approaches. Hydroxyapatite and magnesium-doped hydroxyapatite nanoparticles were used as suspensions or compacted as specular discs. Comparison between cells seeded on the discs and those supplemented with the nanoparticles allowed direct interaction of the cell membrane with the material to be ruled out as the main mechanism of toxicity. In addition, standard techniques such as flow cytometry were inconclusive when used to assess nanoparticles toxicity. Interestingly, the use of intracellular calcium fluorescent probes revealed the presence of a high number of calcium-rich vesicles after nanoparticle supplementation in cell culture. These structures could not be detected by transmission electron microscopy due to their liquid content. However, by using cryo-soft X-ray imaging, which was used to visualise the cellular ultrastructure without further treatment other than vitrification and to quantify the linear absorption coefficient of each organelle, it was possible to identify them as multivesicular bodies, potentially acting as calcium stores. In the study, an advanced state of degradation of the hydroxyapatite and magnesium-doped hydroxyapatite nanoparticles within MG-63 cells was observed. Overall, we demonstrate that the combination of fluorescent calcium probes together with cryo-SXT is an excellent approach to investigate intracellular calcium, especially when found in its soluble form.Copyright © 2022 The Author(s). Published by Elsevier B.V. All rights reserved.

JTD Keywords: adsorption, cryo-soft x-ray tomography, cytotoxicity, expression, flow cytometry, internalisation, intracellular calcium, magnesium, nano, nanomaterials, nanoparticles, proliferation, protein corona, ultrastructure, Calcium-phosphate nanoparticles, Cryo-soft x-ray tomography, Flow cytometry, Hydroxyapatite, Internalisation, Intracellular calcium, Nanoparticles

Biomaterials can interact with cells directly, that is, by direct contact of the cells with the material surface, or indirectly, through soluble species that can be released to or uptaken from the surrounding fluids. However, it is difficult to characterise the relevance of this fluid-mediated interaction separately from the topography and composition of the substrate, because they are coupled variables. These fluid-mediated interactions are amplified in the case of highly reactive calcium phosphates (CaPs) such as biomimetic calcium deficient hydroxyapatite (CDHA), particularly in static in vitro cultures. The present work proposes a strategy to decouple the effect of ion exchange from topographical features by adjusting the volume ratio between the cell culture medium and biomaterial (VCM/VB). Increasing this ratio allowed mitigating the drastic ionic exchanges associated to the compositional changes experienced by the material exposed to the cell culture medium. This strategy was validated using rat mesenchymal stem cells (rMSCs) cultured on CDHA and beta-tricalcium phosphate (β-TCP) discs using different VCM/VB ratios. Whereas in the case of β-TCP the cell response was not affected by this ratio, a significant effect on cell adhesion and proliferation was found for the more reactive CDHA. The ionic exchange, produced by CDHA at low VCM/VB, altered cell adhesion due to the reduced number of focal adhesions, caused cell shrinkage and further rMCSs apoptosis. This was mitigated when using a high VCM/VB, which attenuated the changes of calcium and phosphate concentrations in the cell culture medium, resulting in rMSCs spreading and a viability over time. Moreover, rMSCs showed an earlier expression of osteogenic genes on CDHA compared to sintered β-TCP when extracellular calcium fluctuations were reduced. Statement of Significance: Fluid mediated interactions play a significant role in the bioactivity of calcium phosphates. Ionic exchange is amplified in the case of biomimetic hydroxyapatite, which makes the in vitro characterisation of cell-material interactions especially challenging. The present work proposes a novel and simple strategy to explore the mechanisms of interaction of biomimetic and sintered calcium phosphates with mesenchymal stem cells. The effects of topography and ion exchange are analysed separately by modifying the volume ratio between cell culture medium and biomaterial. High ionic fluctuations interfered in the maturation of focal adhesions, hampering cell adhesion and leading to increased apoptosis and reduced proliferation rate.

JTD Keywords: Calcium phosphates, Mesenchymal stem cells, Intracellular calcium, Cell adhesion

JTD Keywords: Optogenetics, Genetically encoded biosensors, Chloride imaging, pH monitoring, Intracellular Ca, Optopharmacology, Photochromic ligands, Photoswitchable tethered ligands