Osteoclastogenic differentiation of human precursor cells over micro- and nanostructured hydroxyapatite topography

A variable mineralization time and solution concentration intervene in the microstructure of biomimetic mineralized collagen and potential osteogenic microenvironment

The absence of a conducive bone formation microenvironment between fractured ends poses a significant challenge in repairing large bone defects. A promising solution is to construct a bone formation microenvironment that mimics natural bone tissue. Biomimetic mineralized collagen possesses a chemical composition and microstructure highly similar to the natural bone matrix, making it an ideal biomimetic bone substitute material. The microstructure of biomimetic mineralized collagen is influenced by various factors, and its biomineralization and microstructure, in turn, affect its physicochemical properties and biological activity. We aimed to utilize mineralization time and solution concentration as variables and employed the polymer-induced liquid precursor strategy to fabricate mineralized collagen with diverse microstructures, to shed light on how mineralization parameters impact the material microstructure and physicochemical properties. We also investigated the influence of microstructure and physicochemical properties on cell biocompatibility and the bone-forming microenvironment. Through comprehensive characterization, we examined the physical and chemical properties of I-EMC under various mineralization conditions and assessed the in vitro and in vivo biocompatibility and osteogenic performance. By investigating the relationship between mineralization parameters, material physicochemical properties, and osteogenic performance, we revealed how microstructures influence cellular behaviors like biocompatibility and osteogenic microenvironment. Encouragingly, mineralization solutions with varying concentrations, stabilized by polyacrylic acid, successfully produced intrafibrillar and extrafibrillar mineralized collagen. Compared to non-mineralized collagen, all mineralized samples demonstrated improved bone-forming performance. Notably, samples prepared with a 1× mineralization solution exhibited relatively smooth surfaces with even mineralization. Extending the mineralization time enhanced the degree of mineralization and osteogenic performance. Conversely, samples prepared with a 2× mineralization solution had rough surfaces with large calcium phosphate particles, indicating non-uniform mineralization. Overall, our research advances the potential for commercial production of mineralized collagen protein products, characterized by dual biomimetic properties, and their application in treating various types of bone defects.

Differentiation-inducing effect of osteoclast microgrooves for the purpose of three-dimensional design of regenerated bone

In vivo bone remodeling is promoted by the balance between osteoclast and osteoblast activity. Conventional research on bone regeneration has mainly focused on increasing osteoblast activity, with limited studies on the effects of scaffold topography on cell differentiation. Here, we examined the effect of microgroove-patterned substrate with spacings ranging from 1 to 10 μm on the differentiation of rat bone marrow-derived osteoclast precursors. Tartrate-resistant acid phosphatase (TRAP) staining and relative gene expression quantification showed that osteoclast differentiation was enhanced in substrate with 1 µm microgroove spacing compared with that in the other groups. Additionally, the ratio of podosome maturation stages in substrate with 1 μm microgroove spacing exhibited a distinct pattern, which was characterized by an increase in the ratio of belts and rings and a decrease in that of clusters. However, myosin II abolished the effects of topography on osteoclast differentiation. Overall, these showed that the reduction of myosin II tension in the podosome core by an integrin vertical vector increased podosome stability and promoted osteoclast differentiation in substrates with 1 μm microgroove spacing, including that microgroove design plays an important role in scaffolds for bone regeneration. STATEMENT OF SIGNIFICANCE: Reduction of myosin II tension in the podosome core, facilitated by an integrin vertical vector, resulted in an enhanced osteoclast differentiation, concomitant with an increase in podosome stability within 1-μm-spaced microgrooves. These findings are anticipated to serve as valuable indicators for the regulation of osteoclast differentiation through the manipulation of biomaterial surface topography in tissue engineering. Furthermore, this study contributes to the lucidation of the underlying mechanisms governing cellular differentiation by providing insights into the impact of the microtopographical environment.

Monocyte Differentiation on Atomic Layer-Deposited (ALD) Hydroxyapatite Coating on Titanium Substrate

Hydroxyapatite (HA; Ca₁₀(PO₄)₆(OH)₂) coating of bone implants has many beneficial properties as it improves osseointegration and eventually becomes degraded and replaced with new bone. We prepared HA coating on a titanium substrate with atomic layer deposition (ALD) and compared monocyte differentiation and material resorption between ALD-HA and bone. After stimulation with macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappa-B ligand (RANKL), human peripheral blood monocytes differentiated into resorbing osteoclasts on bovine bone, but non-resorbing foreign body cells were observed on ALD-HA. The analysis of the topography of ALD-HA and bone showed no differences in wettability (water contact angle on ALD-HA 86.2° vs. 86.7° on the bone), but the surface roughness of ALD-HA (Ra 0.713 µm) was significantly lower compared to bone (Ra 2.30 µm). The cellular reaction observed on ALD-HA might be a consequence of the topographical properties of the coating. The absence of resorptive osteoclasts on ALD-HA might indicate inhibition of their differentiation or the need to modify the coating to induce osteoclast differentiation.

Nanotopographical cues for regulation of macrophages and osteoclasts: emerging opportunities for osseointegration

Nanotopographical cues of bone implant surface has direct influences on various cell types during the establishment of osseointegration, a prerequisite of implant bear-loading. Given the important roles of monocyte/macrophage lineage cells in bone regeneration and remodeling, the regulation of nanotopographies on macrophages and osteoclasts has arisen considerable attentions recently. However, compared to osteoblastic cells, how nanotopographies regulate macrophages and osteoclasts has not been properly summarized. In this review, the roles and interactions of macrophages, osteoclasts and osteoblasts at different stages of bone healing is firstly presented. Then, the diversity and preparation methods of nanotopographies are summarized. Special attentions are paid to the regulation characterizations of nanotopographies on macrophages polarization and osteoclast differentiation, as well as the focal adhesion-cytoskeleton mediated mechanism. Finally, an outlook is indicated of coordinating nanotopographies, macrophages and osteoclasts to achieve better osseointegration. These comprehensive discussions may not only help to guide the optimization of bone implant surface nanostructures, but also provide an enlightenment to the osteoimmune response to external implant.

Distinct Morphologies of Bone Apatite Clusters in Endochondral and Intramembranous Ossification

Bone apatite crystals grow in clusters, but the microstructure of these clusters is unknown. This study compares the structural and compositional differences between bone apatite clusters formed in intramembranous (IO) and endochondral ossification (EO). Calvaria (IO) and femurs (EO) are isolated from mice at embryonic days (E) 14.5 to 15.5 and post-natal days (P) 6 to 7, respectively. Results show that the initially formed bone apatite clusters in EO (≅1.2 µm² ) are >10 times larger than those in IO (≅0.1 µm² ), without significant changes in ion composition. In IO (E14.5 calvarium), early minerals are formed inside matrix vesicles (MVs). In contrast, in EO (P6 femur epiphysis), no MVs are observed, and chondrocyte-derived plasma membrane nanofragments (PMNFs) are the nucleation site for mineralization. Apatite cluster size difference is linked with the different nucleation sites. Moreover, an alkaline pH and slow P supply into a Ca-rich microenvironment are suggested to facilitate apatite cluster growth, as demonstrated in a biomimetic mineralization system. Together, the results reveal for the first time the distinct and exquisite microstructures of bone apatite clusters in IO and EO, and provide insightful inspirations for the design of more efficient materials for bone tissue engineering and repair.

Etidronate-based organic salts and ionic liquids: In vitro effects on bone metabolism

Bisphosphonates are a class of drugs widely used for the treatment of several pathologies associated with increased bone resorption. Although displaying low oral bioavailability, these drugs have the ability to accumulate in bone matrix, where the biological effects are exerted. In the present work, four mono- and dianionic Etidronate-based Organic Salts and Ionic Liquids (Eti-OSILs) were developed by combination of this drug with the superbases 1,1,3,3-tetramethylguanidine (TMG) and 1,5-diazabicyclo(4.3.0)non-5-ene (DBN) as cations, aiming to improve not only the physicochemical properties of this seminal bisphosphonate, but also its efficacy in the modulation of cellular behavior, particularly on human osteoclasts and osteoblasts. It was observed that some of the developed compounds, in particular the dianionic ones, presented very high water solubility and diminished or absent polymorphism. Also, several of them appeared to be more cytotoxic against human breast and osteosarcoma cancer cell lines while retaining low toxicity to normal cells. Regarding bone cells, a promotion of an anabolic state was observed for all Eti-OSILs, primarily for the dianionic ones, which leads to an inhibition of osteoclastogenesis and an increase in osteoblastogenesis. The observed effects resulted from differential modulation of intracellular signaling pathways by the Eti-OSILs in comparison with Etidronate. Hence, these results pave the way for the development of more efficient and bioavailable ionic formulations of bisphosphonates aiming to effectively modulate bone metabolism, particularly in the case of increased bone resorption.

Physical Gold Nanoparticle-Decorated Polyethylene Glycol-Hydroxyapatite Composites Guide Osteogenesis and Angiogenesis of Mesenchymal Stem Cells

In this study, polyethylene glycol (PEG) with hydroxyapatite (HA), with the incorporation of physical gold nanoparticles (AuNPs), was created and equipped through a surface coating technique in order to form PEG-HA-AuNP nanocomposites. The surface morphology and chemical composition were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–Vis spectroscopy (UV–Vis), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and contact angle assessment. The effects of PEG-HA-AuNP nanocomposites on the biocompatibility and biological activity of MC3T3-E1 osteoblast cells, endothelial cells (EC), macrophages (RAW 264.7), and human mesenchymal stem cells (MSCs), as well as the guiding of osteogenic differentiation, were estimated through the use of an in vitro assay. Moreover, the anti-inflammatory, biocompatibility, and endothelialization capacities were further assessed through in vivo evaluation. The PEG-HA-AuNP nanocomposites showed superior biological properties and biocompatibility capacity for cell behavior in both MC3T3-E1 cells and MSCs. These biological events surrounding the cells could be associated with the activation of adhesion, proliferation, migration, and differentiation processes on the PEG-HA-AuNP nanocomposites. Indeed, the induction of the osteogenic differentiation of MSCs by PEG-HA-AuNP nanocomposites and enhanced mineralization activity were also evidenced in this study. Moreover, from the in vivo assay, we further found that PEG-HA-AuNP nanocomposites not only facilitate the anti-immune response, as well as reducing CD86 expression, but also facilitate the endothelialization ability, as well as promoting CD31 expression, when implanted into rats subcutaneously for a period of 1 month. The current research illustrates the potential of PEG-HA-AuNP nanocomposites when used in combination with MSCs for the regeneration of bone tissue, with their nanotopography being employed as an applicable surface modification approach for the fabrication of biomaterials.

Surface modification of titanium manufactured through selective laser melting inhibited osteoclast differentiation through mitogen-activated protein kinase signaling pathway

Selective laser melting used in manufacturing custom-made titanium implants becomes more popular. In view of the important role played by osteoclasts in peri-implant bone resorption and osseointegration, we modified selective laser melting-manufactured titanium surfaces using sandblasting/alkali-heating and sandblasting/acid-etching, and investigated their effect on osteoclast differentiation as well as their underlying mechanisms. The properties of the surfaces, including elements, roughness, wettability and topography, were analyzed. We evaluated the proliferation and morphology of primary mouse bone marrow-derived monocytes, as well as induced osteoclasts derived from bone marrow-derived monocytes, on samples. Then, osteoclast differentiation was determined by the tartrate-resistant acid phosphatase activity assay, calcitonin receptors immunofluorescence staining and the expression of osteoclast-related genes. The results showed that sandblasting/alkali-heating established nanonet structure with the lowest water contact angle, and both sandblasting/alkali-heating and sandblasting/acid-etching significantly decreased surface roughness and heterogeneity compared with selective laser melting. Surface modifications of selective laser melting-produced titanium altered bone marrow-derived monocyte morphology and suppressed bone marrow-derived monocyte proliferation and osteoclastogenesis in vitro (sandblasting/alkali-heating>sandblasting/acid-etching>selective laser melting). These surface modifications reduced the activation of extracellular signal-regulated kinase and c-Jun N-terminal kinases compared to native-selective laser melting. Sandblasting/alkali-heating additionally blocked tumor necrosis factor receptor-associated factor 6 recruitment. The results suggested that sandblasting/alkali-heating and sandblasting/acid-etching modifications on selective laser melting titanium could inhibit osteoclast differentiation through suppressing extracellular signal-regulated kinase and c-Jun N-terminal kinase phosphorylation in mitogen-activated protein kinase signaling pathway and provide a promising technique which might reduce peri-implant bone resorption for optimizing native-selective laser melting implants.

Effects of hydroxyapatite surface nano/micro-structure on osteoclast formation and activity

The surface structure of calcium phosphate (CaP) ceramic plays an important role in its osteoinductivity; however, little is known about its effects on osteoclastogenesis. In this study, an intramuscular implantation model suggested a potential relationship between hydroxyapatite (HA)-induced bone formation and osteoclast appearance in the non-osseous site, which might be modulated by scaffold surface structure. Then, three dense HA discs with different grain sizes from biomimetic nanoscale (∼100 nm) to submicron scale (∼500 nm) were fabricated via distinct sintering procedures, and their impacts on osteoclastic differentiation of RAW 264.7 macrophages under RANKL stimulation were further investigated. Our results showed that compared with the ones in the submicron-scale dimension, nano-structured HA discs markedly impaired osteoclastic formation and function, as evidenced by inhibited cell fusion, reduced osteoclast size, less-defined actin ring, increased osteoclast apoptosis, suppressed expression of osteoclast specific genes and proteins, decreased TRAP-positive cells, and hampered resorption activity. This demonstrated that the surface structure of CaP ceramics has a great influence on osteoclastogenesis, which might be further related to its osteoinductive capacity. These findings might not only help us gain insight into biomolecular events during CaP-involved osteoinduction, but also offer a principle for designing orthopaedic implants with an ability of regulating both osteogenesis and osteoclastogenesis to achieve the desired performance.

Assessment of the Inner Surface Microstructure of Decellularized Cortical Bone by a Scanning Electron Microscope

The microstructural changes of bones, which form a hierarchy of skeletal tissue, vary, depending on their condition, and are affected by the behaviors of bone cells. The purpose of this study is to assess the microstructural changes in the inner femoral surface of Sprague Dawley rats according to the conditions using a scanning electron microscope. Microstructural differences on the endocortical surface were observed in the characteristics of osteocytic canaliculi, bone fibers, and surface roughness, showing a rougher surface in old adults and an osteoporosis model by quantitative comparison. These results could be helpful for developing a basic understanding of the microstructural changes that occur on the bone surface under various conditions.

Effect of calcium phosphate heparinization on the in vitro inflammatory response and osteoclastogenesis of human blood precursor cells

The immobilization of natural molecules on synthetic bone grafts stands as a strategy to enhance their biological interactions. During the early stages of healing, immune cells and osteoclasts (OC) modulate the inflammatory response and resorb the biomaterial, respectively. In this study, heparin, a naturally occurring molecule in the bone extracellular matrix, was covalently immobilized on biomimetic calcium-deficient hydroxyapatite (CDHA). The effect of heparin-functionalized CDHA on inflammation and osteoclastogenesis was investigated using primary human cells and compared with pristine CDHA and beta-tricalcium phosphate (β-TCP). Biomimetic substrates led to lower oxidative stresses by neutrophils and monocytes than sintered β-TCP, even though no further reduction was induced by the presence of heparin. In contrast, heparinized CDHA fostered osteoclastogenesis. Optical images of stained TRAP positive cells showed an earlier and higher presence of multinucleated cells, compatible with OC at 14 days, while pristine CDHA and β-TCP present OC at 21-28 days. Although no statistically significant differences were found in the OC activity, microscopy images evidenced early stages of degradation on heparinized CDHA, compatible with osteoclastic resorption. Overall, the results suggest that the functionalization with heparin fostered the formation and activity of OC, thus offering a promising strategy to integrate biomaterials in the bone remodelling cycle by increasing their OC-mediated resorption.

Differential effects of antiepileptic drugs on human bone cells

Antiepileptic drugs (AED) have been associated to in vivo deleterious consequences in bone tissue. The present work aimed to characterize the cellular and molecular effects of five different AED on human osteoclastogenesis and osteblastogenesis. It was observed that the different drugs had the ability to differentially modulate both processes, in a way dependent on the identity and dose of the AED. Shortly, valproic acid stimulated either osteoclastogenesis and osteoblastogenesis, whereas carbamazepine, gabapentin, and lamotrigine revealed an opposite behavior; topiramate elicited a decrease of osteoclast development and an increase in osteoblast differentiation. This is the first report describing the direct effects of different AED on human primary bone cells, which is a very important issue, because these drugs are usually consumed in long-term therapeutics, with acknowledged in vivo effects in bone tissue.

Emerging Strategies for Stem Cell Lineage Commitment in Tissue Engineering and Regenerative Medicine

Stem cells have transformed the fields of tissue engineering and regenerative medicine, and their potential to further advance these fields cannot be overstated. The stem cell niche is a dynamic microenvironment that determines cell fate during development and tissue repair following an injury. Classically, stem cells were studied in isolation of their microenvironment; however, contemporary research has produced a myriad of evidence that shows the importance of multiple aspects of the stem cell niche in regulating their processes. In the context of tissue engineering and regenerative medicine studies, the niche is an artificial environment provided by culture conditions. In vitro culture conditions may involve coculturing with other cell types, developing specific biomaterials, and applying relevant forces to promote the desired lineage commitment. Considerable advance has been made over the past few years toward directed stem cell differentiation; however, the unspecific differentiation of stem cells yielding a mixed population of cells has been a challenge. In this review, we provide a systematic review of the emerging strategies used for lineage commitment within the context of tissue engineering and regenerative medicine. These strategies include scaffold pore-size and pore-shape gradients, stress relaxation, sonic and electromagnetic effects, and magnetic forces. Finally, we provide insights and perspectives into future directions focusing on signaling pathways activated during lineage commitment using external stimuli.