Gap junctions and biophysical regulation of bone cell differentiation

Classical cannabinoid receptors as target in cancer-induced bone pain: a systematic review, meta-analysis and bioinformatics validation

To test the hypothesis that genetic and pharmacological modulation of the classical cannabinoid type 1 (CB1) and 2 (CB2) receptors attenuate cancer-induced bone pain, we searched Medline, Web of Science and Scopus for relevant skeletal and non-skeletal cancer studies from inception to July 28, 2022. We identified 29 animal and 35 human studies. In mice, a meta-analysis of pooled studies showed that treatment of osteolysis-bearing males with the endocannabinoids AEA and 2-AG (mean difference [MD] - 24.83, 95% confidence interval [95%CI] - 34.89, - 14.76, p < 0.00001) or the synthetic cannabinoid (CB) agonists ACPA, WIN55,212-2, CP55,940 (CB1/2-non-selective) and AM1241 (CB2-selective) (MD - 28.73, 95%CI - 45.43, - 12.02, p = 0.0008) are associated with significant reduction in paw withdrawal frequency. Consistently, the synthetic agonists AM1241 and JWH015 (CB2-selective) increased paw withdrawal threshold (MD 0.89, 95%CI 0.79, 0.99, p < 0.00001), and ACEA (CB1-selective), AM1241 and JWH015 (CB2-selective) reduced spontaneous flinches (MD - 4.85, 95%CI - 6.74, - 2.96, p < 0. 00001) in osteolysis-bearing male mice. In rats, significant increase in paw withdrawal threshold is associated with the administration of ACEA and WIN55,212-2 (CB1/2-non-selective), JWH015 and AM1241 (CB2-selective) in osteolysis-bearing females (MD 8.18, 95%CI 6.14, 10.21, p < 0.00001), and treatment with AM1241 (CB2-selective) increased paw withdrawal thermal latency in males (mean difference [MD]: 3.94, 95%CI 2.13, 5.75, p < 0.0001), confirming the analgesic capabilities of CB1/2 ligands in rodents. In human, treatment of cancer patients with medical cannabis (standardized MD - 0.19, 95%CI - 0.35, - 0.02, p = 0.03) and the plant-derived delta-9-THC (20 mg) (MD 3.29, CI 2.24, 4.33, p < 0.00001) or its synthetic derivative NIB (4 mg) (MD 2.55, 95%CI 1.58, 3.51, p < 0.00001) are associated with reduction in pain intensity. Bioinformatics validation of KEGG, GO and MPO pathway, function and process enrichment analysis of mouse, rat and human data revealed that CB1 and CB2 receptors are enriched in a cocktail of nociceptive and sensory perception, inflammatory, immune-modulatory, and cancer pathways. Thus, we cautiously conclude that pharmacological modulators of CB1/2 receptors show promise in the treatment of cancer-induced bone pain, however further assessment of their effects on bone pain in genetically engineered animal models and cancer patients is warranted.

Translation of biophysical environment in bone into dynamic cell culture under flow for bone tissue engineering

Bone is a dynamic environment where osteocytes, osteoblasts, and mesenchymal stem/progenitor cells perceive mechanical cues and regulate bone metabolism accordingly. In particular, interstitial fluid flow in bone and bone marrow serves as a primary biophysical stimulus, which regulates the growth and fate of the cellular components of bone. The processes of mechano-sensory and -transduction towards bone formation have been well studied mainly in vivo as well as in two-dimensional (2D) dynamic cell culture platforms, which elucidated mechanically induced osteogenesis starting with anabolic responses, such as production of nitrogen oxide and prostaglandins followed by the activation of canonical Wnt signaling, upon mechanosensation. The knowledge has been now translated into regenerative medicine, particularly into the field of bone tissue engineering, where multipotent stem cells are combined with three-dimensional (3D) scaffolding biomaterials to produce transplantable constructs for bone regeneration. In the presence of 3D scaffolds, the importance of suitable dynamic cell culture platforms increases further not only to improve mass transfer inside the scaffolds but to provide appropriate biophysical cues to guide cell fate. In principle, the concept of dynamic cell culture platforms is rooted to bone mechanobiology. Therefore, this review primarily focuses on biophysical environment in bone and its translation into dynamic cell culture platforms commonly used for 2D and 3D cell expansion, including their advancement, challenges, and future perspectives. Additionally, it provides the literature review of recent empirical studies using 2D and 3D flow-based dynamic cell culture systems for bone tissue engineering.

Silencing NTPDase3 activity rehabilitates the osteogenic commitment of post-menopausal stem cell bone progenitors

BACKGROUND

Endogenously released adenine and uracil nucleotides favour the osteogenic commitment of bone marrow-derived mesenchymal stromal cells (BM-MSCs) through the activation of ATP-sensitive P2X7 and UDP-sensitive P2Y₆ receptors. Yet, these nucleotides have their osteogenic potential compromised in post-menopausal (Pm) women due to overexpression of nucleotide metabolizing enzymes, namely NTPDase3. This prompted us to investigate whether NTPDase3 gene silencing or inhibition of its enzymatic activity could rehabilitate the osteogenic potential of Pm BM-MSCs.

METHODS

MSCs were harvested from the bone marrow of Pm women (69 ± 2 years old) and younger female controls (22 ± 4 years old). The cells were allowed to grow for 35 days in an osteogenic-inducing medium in either the absence or the presence of NTPDase3 inhibitors (PSB 06126 and hN3-B3_s antibody); pre-treatment with a lentiviral short hairpin RNA (Lenti-shRNA) was used to silence the NTPDase3 gene expression. Immunofluorescence confocal microscopy was used to monitor protein cell densities. The osteogenic commitment of BM-MSCs was assessed by increases in the alkaline phosphatase (ALP) activity. The amount of the osteogenic transcription factor Osterix and the alizarin red-stained bone nodule formation. ATP was measured with the luciferin-luciferase bioluminescence assay. The kinetics of the extracellular ATP (100 µM) and UDP (100 µM) catabolism was assessed by HPLC RESULTS: The extracellular catabolism of ATP and UDP was faster in BM-MSCs from Pm women compared to younger females. The immunoreactivity against NTPDase3 increased 5.6-fold in BM-MSCs from Pm women vs. younger females. Selective inhibition or transient NTPDase3 gene silencing increased the extracellular accumulation of adenine and uracil nucleotides in cultured Pm BM-MSCs. Downregulation of NTPDase3 expression or activity rehabilitated the osteogenic commitment of Pm BM-MSCs measured as increases in ALP activity, Osterix protein cellular content and bone nodule formation; blockage of P2X7 and P2Y₆ purinoceptors prevented this effect.

CONCLUSIONS

Data suggest that NTPDase3 overexpression in BM-MSCs may be a clinical surrogate of the osteogenic differentiation impairment in Pm women. Thus, besides P2X7 and P2Y₆ receptors activation, targeting NTPDase3 may represent a novel therapeutic strategy to increase bone mass and reduce the osteoporotic risk of fractures in Pm women.

Current strategies for enhancement of the bioactivity of artificial ligaments: A mini-review

Background and objective

Anterior cruciate ligament (ACL) reconstruction calls for artificial ligaments with better bioactivity, however systematic reviews regarding bioactivity enhancement strategies, technologies, and perspectives of artificial ligaments have been rarely found.

Methods

Research papers, reviews, and clinical reports related to artificial ligaments were searched and summarized the current status and research trends of artificial ligaments through a systematic analysis.

Results

Having experienced ups and downs since the very first record of clinical application, artificial ligaments differing in material, and fabrication methods have been reported with different clinical performances. Various manufacturing technologies have developed and realized scaffold- and cell-based strategies. Despite encouraging in-vivo and in-vitro test results, the clinical results of such new designs need further clinical examinations.

Conclusion

As the demand for ACL reconstruction dramatically increases, novel artificial ligaments with better osteoinductivity and mechanical performance are promising.

The translational potential of this article

To develop novel artificial ligaments simultaneously possessing excellent osteoinductivity and satisfactory mechanical performance, it is important to grab a glance at recent research advances. This systematic analysis provides researchers and clinicians with comprehensive and comparable information on artificial ligaments, thus being of clinical translational significance.

Histochemical assessment on osteocytic osteolysis in lactating mice fed with a calcium-insufficient diet

OBJECTIVES

This study aimed to examine if feeding lactating mice a calcium-insufficient diet while simultaneously administering alendronate (ALN) could potentially induce osteocytic osteolysis.

METHODS

Lactating mice were fed calcium (Ca)-insufficient diets with or without ALN administration, and then their femurs were examined for TRAP and ALP, and observed by Kossa staining and transmission electron microscopy (TEM). Mice that had been fed a Ca-insufficient diet were then fed a ⁴⁴Ca-containinig diet, and their tibial sections were examined by isotope microscopy.

RESULTS

Mice fed a Ca-insufficient diet had a reduced number of TRAP-positive osteoclasts after ALN administration. ALN-treated, lactating mice fed a Ca-insufficient diet had enlarged lacunae in their cortical bones, and TEM imaging demonstrated expanded regions between osteocytes and lacunar walls. In ALN-treated lactating mice fed a Ca-insufficient diet, huge areas of demineralized bone matrix occurred, centered around blood vessels in the cortical bone. Isotope microscopy showed ⁴⁴Ca in the vicinity of the osteocytic lacunae, and in the broad, previously demineralized region around the blood vessels in the cortical bone of lactating mice fed a ⁴⁴Ca-sufficient diet.

CONCLUSIONS

Bone demineralization likely takes place in the periphery of the osteocytic lacunae and in the broad regions around the blood vessels of lactating mice when they are exposed to severely reduced serum Ca through a Ca-insufficient diet coupled with ALN administration.

Matrix Vesicle-Mediated Mineralization and Osteocytic Regulation of Bone Mineralization

Bone mineralization entails two mineralization phases: primary and secondary mineralization. Primary mineralization is achieved when matrix vesicles are secreted by osteoblasts, and thereafter, bone mineral density gradually increases during secondary mineralization. Nearby extracellular phosphate ions (PO43−) flow into the vesicles via membrane transporters and enzymes located on the vesicles’ membranes, while calcium ions (Ca2+), abundant in the tissue fluid, are also transported into the vesicles. The accumulation of Ca2+ and PO43− in the matrix vesicles induces crystal nucleation and growth. The calcium phosphate crystals grow radially within the vesicle, penetrate the vesicle’s membrane, and continue to grow outside the vesicle, ultimately forming mineralized nodules. The mineralized nodules then attach to collagen fibrils, mineralizing them from the contact sites (i.e., collagen mineralization). Afterward, the bone mineral density gradually increases during the secondary mineralization process. The mechanisms of this phenomenon remain unclear, but osteocytes may play a key role; it is assumed that osteocytes enable the transport of Ca2+ and PO43− through the canaliculi of the osteocyte network, as well as regulate the mineralization of the surrounding bone matrix via the Phex/SIBLINGs axis. Thus, bone mineralization is biologically regulated by osteoblasts and osteocytes.

Bone Remodeling Process Based on Hydrostatic and Deviatoric Strain Mechano-Sensing

A macroscopic continuum model intended to provide predictions for the remodeling process occurring in bone tissue is proposed. Specifically, we consider a formulation in which two characteristic stiffnesses, namely the bulk and shear moduli, evolve independently to adapt the hydrostatic and deviatoric response of the bone tissue to environmental changes. The formulation is deliberately simplified, aiming at constituting a preliminary step toward a more comprehensive modeling approach. The evolutive process for describing the functional adaptation of the two stiffnesses is proposed based on an energetic argument. Numerical experiments reveal that it is possible to model the bone remodeling process with a different evolution for more than one material parameter, as usually done. Moreover, the results motivate further investigations into the subject.

Osteocytic Osteolysis in PTH-treated Wild-type and Rankl-/- Mice Examined by Transmission Electron Microscopy, Atomic Force Microscopy, and Isotope Microscopy

To demonstrate the ultrastructure of osteocytic osteolysis and clarify whether osteocytic osteolysis occurs independently of osteoclastic activities, we examined osteocytes and their lacunae in the femora and tibiae of 11-week-old male wild-type and Rankl^-/- mice after injection of human parathyroid hormone (PTH) [1-34] (80 µg/kg/dose). Serum calcium concentration rose temporarily 1 hr after PTH administration in wild-type and Rankl^-/- mice, when renal arteries and veins were ligated. After 6 hr, enlargement of osteocytic lacunae was evident in the cortical bones of wild-type and Rankl^-/- mice, but not so in their metaphyses. Von Kossa staining and transmission electron microscopy showed broadly demineralized bone matrix peripheral to enlarged osteocytic lacunae, which contained fragmented collagen fibrils and islets of mineralized matrices. Nano-indentation by atomic force microscopy revealed the reduced elastic modulus of the PTH-treated osteocytic perilacunar matrix, despite the microscopic verification of mineralized matrix in that region. In addition, ⁴⁴Ca deposition was detected by isotope microscopy and calcein labeling in the eroded osteocytic lacunae of wild-type and Rankl^-/- mice. Taken together, our findings suggest that osteocytes can erode the bone matrix around them and deposit minerals on their lacunar walls independently of osteoclastic activity, at least in the murine cortical bone. (J Histochem Cytochem 68: -XXX, 2020).

Adult mesenchymal stem cells: is there a role for purine receptors in their osteogenic differentiation?

The role played by mesenchymal stem cells (MSCs) in contributing to adult tissue homeostasis and damage repair thanks to their differentiation capabilities has raised a great interest, mainly in bone regenerative medicine. The growth/function of these undifferentiated cells of mesodermal origin, located in specialized structures (niches) of differentiated organs is influenced by substances present in this microenvironment. Among them, ancestral and ubiquitous molecules such as adenine-based purines, i.e., ATP and adenosine, may be included. Notably, extracellular purine concentrations greatly increase during tissue injury; thus, MSCs are exposed to effects mediated by these agents interacting with their own receptors when they act/migrate in vivo or are transplanted into a damaged tissue. Here, we reported that ATP modulates MSC osteogenic differentiation via different P2Y and P2X receptors, but data are often inconclusive/contradictory so that the ATP receptor importance for MSC physiology/differentiation into osteoblasts is yet undetermined. An exception is represented by P2X7 receptors, whose expression was shown at various differentiation stages of bone cells resulting essential for differentiation/survival of both osteoclasts and osteoblasts. As well, adenosine, usually derived from extracellular ATP metabolism, can promote osteogenesis, likely via A2B receptors, even though findings from human MSCs should be implemented and confirmed in preclinical models. Therefore, although many data have revealed possible effects caused by extracellular purines in bone healing/remodeling, further studies, hopefully performed in in vivo models, are necessary to identify defined roles for these compounds in favoring/increasing the pro-osteogenic properties of MSCs and thereby their usefulness in bone regenerative medicine.

Metabonomic-Transcriptome Integration Analysis on Osteoarthritis and Rheumatoid Arthritis

Purpose

This study is aimed at exploring the potential metabolite/gene biomarkers, as well as the differences between the molecular mechanisms, of osteoarthritis (OA) and rheumatoid arthritis (RA).

Methods

Transcriptome dataset GSE100786 was downloaded to explore the differentially expressed genes (DEGs) between OA samples and RA samples. Meanwhile, metabolomic dataset MTBLS564 was downloaded and preprocessed to obtain metabolites. Then, the principal component analysis (PCA) and linear models were used to reveal DEG-metabolite relations. Finally, metabolic pathway enrichment analysis was performed to investigate the differences between the molecular mechanisms of OA and RA.

Results

A total of 976 DEGs and 171 metabolites were explored between OA samples and RA samples. The PCA and linear module analysis investigated 186 DEG-metabolite interactions including Glycogenin 1- (GYG1-) asparagine_54, hedgehog acyltransferase- (HHAT-) glucose_70, and TNF receptor-associated factor 3- (TRAF3-) acetoacetate_35. Finally, the KEGG pathway analysis showed that these metabolites were mainly enriched in pathways like gap junction, phagosome, NF-kappa B, and IL-17 pathway.

Conclusions

Genes such as HHAT, GYG1, and TRAF3, as well as metabolites including glucose, asparagine, and acetoacetate, might be implicated in the pathogenesis of OA and RA. Metabolites like ethanol and tyrosine might participate differentially in OA and RA progression via the gap junction pathway and phagosome pathway, respectively. TRAF3-acetoacetate interaction may be involved in regulating inflammation in OA and RA by the NF-kappa B and IL-17 pathway.

Pro-inflammatory Cytokines and Osteocytes

PURPOSE OF REVIEW

An elevated level of pro-inflammatory cytokines in inflammatory conditions causes bone loss and disrupts vital organ function. Osteocytes comprise > 95% of the cellular component in bone tissue, produce a range of cytokines and signaling molecules, and influence bone and other organ function. In this review, we hypothesized that an elevated level of pro-inflammatory cytokines in inflammatory conditions affects osteocyte survival and function thereby possibly amplifying inflammation, and causing bone loss and non-bone clinical complications.

RECENT FINDINGS

Several studies have reported that the elevated level of pro-inflammatory cytokines in inflammatory conditions alters osteocyte mechanosensitivity, causes osteocyte apoptosis, and modulates osteocyte-derived production of various inflammatory cytokines and signaling molecules. Cytokines and signaling molecules released from osteocytes affect surrounding bone cells and distant organ function in a paracrine and endocrine fashion. Inflammatory diseases including diabetes, chronic kidney disease, rheumatoid arthritis, and periodontitis affect osteocyte survival and function, and upregulate osteocyte-derived expression of sclerostin, RANKL, TNFα, FGF23, DKK1, and other signaling molecules.

Joint diseases: from connexins to gap junctions

Connexons form the basis of hemichannels and gap junctions. They are composed of six tetraspan proteins called connexins. Connexons can function as individual hemichannels, releasing cytosolic factors (such as ATP) into the pericellular environment. Alternatively, two hemichannel connexons from neighbouring cells can come together to form gap junctions, membrane-spanning channels that facilitate cell-cell communication by enabling signalling molecules of approximately 1 kDa to pass from one cell to an adjacent cell. Connexins are expressed in joint tissues including bone, cartilage, skeletal muscle and the synovium. Indicative of their importance as gap junction components, connexins are also known as gap junction proteins, but individual connexin proteins are gaining recognition for their channel-independent roles, which include scaffolding and signalling functions. Considerable evidence indicates that connexons contribute to the function of bone and muscle, but less is known about the function of connexons in other joint tissues. However, the implication that connexins and gap junctional channels might be involved in joint disease, including age-related bone loss, osteoarthritis and rheumatoid arthritis, emphasizes the need for further research into these areas and highlights the therapeutic potential of connexins.

Electronic Cigarette Smoke Impairs Normal Mesenchymal Stem Cell Differentiation

Electronic cigarettes (e-cigarettes) are promoted as low-risk alternatives to combustible cigarettes. However, the effects of chronic inhalation of potential toxicants emitted by ecigarettes remain largely unexamined. It is conceivable that smoking-induced chronic diseases result in cellular injury, in the absence of effective repair by stem cells. This study evaluates the effect of cigarette and e-cigarette aerosol extracts on the survival and differentiation of bone marrow-derived mesenchymal stem cells (MSCs). MSC growth and osteogenic differentiation were examined after exposure to smoke extracts. Data revealed detrimental effects of both cigarette and e-cigarette extracts on MSC morphology and growth. Levels and activity of alkaline phosphatase, an osteogenic marker, decreased and induction of osteoblastic differentiation was impaired. Both smoke extracts prevented osteogenic differentiation from progressing, evident by decreased expression of terminal osteogenic markers and mineralization. Elevated levels of reactive oxygen species (ROS) were detected in cells exposed to smoke extracts. Moreover, decreased differentiation potential was concomitant with severe down-regulation of Connexin 43 expression, leading to the loss of gap junction-mediated communication, which together with elevated ROS levels, could explain decreased proliferation and loss of differentiation potential. Hence, e-cigarettes present similar risk as combustible cigarettes with respect to tissue repair impairment.

Chronological immunolocalization of sclerostin and FGF23 in the mouse metaphyseal trabecular and cortical bone

To assess the chronological participation of sclerostin and FGF23 in bone metabolism, this study tracked the immunolocalization of sclerostin and FGF23 in the metaphyses of murine long bones from embryonic day 18 (E18) through 1 day after birth, 1 week, 2 weeks, 4 weeks, 8 weeks, and 20 weeks of age. We have selected two regions in the metaphyseal trabeculae for assessing sclerostin and FGF23 localization: close to the chondro-osseous junction, i.e., bone modeling site even in the adult animals, and the trabecular region distant from the growth plate, where bone remodeling takes place. As a consequence, sclerostin-immunopositive osteocytes could not be observed in both close and distant trabecular regions early at the embryonic and young adult stages. However, osteocytes gradually started to express sclerostin in the distant region earlier than in the close region of the trabeculae. Immunoreactivity for FGF23 was observed mainly in osteoblasts in the early stages, but detectable in osteocytes in the later stages of growth in trabecular and cortical bones. Fgf23 was weakly expressed in the embryonic and neonatal stages, while the receptors, Fgfr1c and αKlotho were strongly expressed in femora. At the adult stages, Fgf23 expression became more intense while Fgfr1c and aKlotho were weakly expressed. These findings suggest that sclerostin is secreted by osteocytes in mature bone undergoing remodeling while FGF23 is synthesized by osteoblasts and osteocytes depending on the developmental/growth stage. In addition, it appears that FGF23 acts in an autocrine and paracrine fashion in fetal and neonatal bones.

A paradigm shift for bone quality in dentistry: A literature review

PURPOSE

The aim of this study was to present the current concept of bone quality based on the proposal by the National Institutes of Health (NIH) and some of the cellular and molecular factors that affect bone quality.

STUDY SELECTION

This is a literature review which focuses on collagen, biological apatite (BAp), and bone cells such as osteoblasts and osteocytes.

RESULTS

In dentistry, the term "bone quality" has long been considered to be synonymous with bone mineral density (BMD) based on radiographic and sensible evaluations. In 2000, the NIH proposed the concept of bone quality as "the sum of all characteristics of bone that influence the bone's resistance to fracture," which is completely independent of BMD. The NIH defines bone quality as comprising bone architecture, bone turnover, bone mineralization, and micro-damage accumulation. Moreover, our investigations have demonstrated that BAp, collagen, and bone cells such as osteoblasts and osteocytes play essential roles in controlling the current concept of bone quality in bone around hip and dental implants.

CONCLUSION

The current concept of bone quality is crucial for understanding bone mechanical functions. BAp, collagen and osteocytes are the main factors affecting bone quality. Moreover, mechanical loading dynamically adapts bone quality. Understanding the current concept of bone quality is required in dentistry.

Synchronous disruption of anisotropic arrangement of the osteocyte network and collagen/apatite in melanoma bone metastasis

Cancer metastasis to bones increases the risk of fragility fracture by altering bone metabolism and disrupting bone structure. Osteocytes, which organize a dense network that is closely linked with the circumambient matrix, play a key role in regulation of bone microstructure and material properties. The aim of this study was to elucidate the influence of cancer metastasis on the organization of the osteocyte network and collagen/biological apatite (BAp) microstructure in the context of osteocyte/matrix coupling. Using a mouse model intracardially injected with B16F10 melanoma cells or vehicle, the geometric and metabolic changes to osteocytes were analyzed by nano-computed tomography (nano-CT) and histology, and the alignment of collagen fibrils and BAp was analyzed by birefringence measurement and microbeam-X-ray diffraction, respectively. The material properties of bones were further analyzed with nanoindentation method. These experiments revealed that the osteocyte network was markedly disorganized in cancer-bearing bone tissues. The osteocytes showed a variety of residing states in the lacunae; some lacunae were osteolytic while some were replete with immature matrix, suggesting significant disruption in osteocyte/matrix coupling. Collagen/BAp microstructure was also disorganized in cancer-bearing bones as observed by significant decreases in the preferential alignment of both collagen fibrils and BAp; the latter was further shown to be significantly correlated with Young's modulus. The present study revealed that the disruption in the arrangement of the osteocyte network and collagen/BAp microstructure and the deterioration of mechanical function occurred synchronously during cancer bone metastasis.

In vitro osteocytic microdamage and viability quantification using a microloading platform

Bone remodeling is a process in which bone is resorbed by osteoclasts and formed by osteoblasts. This is normally a paired process, although it can be disrupted by changes in mechanical load. One theory is that osteocytes play a key role in the cellular regulation of this process. Mechanotransduction studies, which investigate how cells convert mechanical stimuli into biophysical effects and cellular activity, offer one way to investigate this theory. Mechanotransduction work is commonly done by applying an isolated mechanical load to cells grown in vitro, and quantifying the response. While in vitro work does not fully replicate the natural environment, it does allow the study of isolated factors. In this study, a mechanical loading platform was designed, fabricated, and characterized for bone mechanotransduction studies. This platform was designed to tent cell-seeded substrates from below, loading using out of plane distension. This introduced a nonuniform strain profile, enabling the study of cells cultured under identical conditions and variable strains as a function of substrate location. An alphanumerically gridded polydimethylsiloxane well substrate was designed and fabricated for cellular loading experiments. Following initial characterization, a study was run to quantify the cellular activity of osteocyte-like MLO-Y4 cells as a function of strain field. The results indicated that regions with lower strains led to an increase in cellular activity while higher strains led to a reduction in cellular activity. This demonstrated that cells could be exposed to mechanically-induced microdamage using the microloading platform.

Radiolucent inflammatory implant periapical lesions: a review of the literature

PURPOSE

To discuss the terminology, etiopathogenesis, and treatment of radiolucent inflammatory implant periapical lesions.

MATERIALS AND METHODS

An electronic search for relevant articles published in the English literature in the PubMed database.

RESULTS

Bacterial contamination of the apical portion of the implant either from a preexisting dental periapical infection or from a periapical lesion of endodontic origin of an adjacent tooth is the probable causative factor. Aseptic bone necrosis owing to overheating of the bone during preparation of osteotomies, or compression of the bone at the apex of the implant owing to excessive tightening, may also play a role. The histopathological features are of a mixed inflammatory cell infiltrate on a background of granulation tissue consistent with either a granuloma or an abscess as may be found at the apex of a nonvital tooth. Treatment consists of immediate and aggressive surgical debridement, chemical detoxification of the apical portion of the exposed implant surface, and systemic antibiotics with or without a bone regenerative procedure.

CONCLUSION

A radiolucent inflammatory implant periapical lesion is analogous to either a granuloma or an abscess as may be found at the apex of a nonvital tooth.

Mesenchymal Stem Cells Ageing: Targeting the "Purinome" to Promote Osteogenic Differentiation and Bone Repair

Mesenchymal stem cells (MSCs) are multipotent cells that can differentiate into bone forming cells. Such ability is compromised in elderly individuals resulting in bone disorders such as osteoporosis, also limiting their clinical usage for cell transplantation and bone tissue engineering strategies. In bone marrow niches, adenine and uracil nucleotides are important local regulators of osteogenic differentiation of MSCs. Nucleotides can be released to the extracellular milieu under both physiological and pathological conditions via (1) membrane cell damage, (2) vesicle exocytosis, (3) ATP-binding cassette transporters, and/or (4) facilitated diffusion through maxi-anion channels, hemichannels or ligand-gated receptor pores. Nucleotides and their derivatives act via adenosine P1 (A1 , A2A , A2B , and A3 ) and nucleotide-sensitive P2 purinoceptors comprising ionotropic P2X and G-protein-coupled P2Y receptors. Purinoceptors activation is terminated by membrane-bound ecto-nucleotidases and other ecto-phosphatases, which rapidly hydrolyse extracellular nucleotides to their respective nucleoside 5'-di- and mono-phosphates, nucleosides and free phosphates, or pyrophosphates. Current knowledge suggests that different players of the "purinome" cascade, namely nucleotide release sites, ecto-nucleotidases and purinoceptors, orchestrate to fine-tuning regulate the activity of MSCs in the bone microenvironment. Increasing studies, using osteoprogenitor cell lines, animal models and, more recently, non-modified MSCs from postmenopausal women, raised the possibility to target chief components of the purinergic signaling pathway to regenerate the ability of aged MSCs to differentiate into functional osteoblasts. This review summarizes the main findings of those studies, prompting for novel therapeutic strategies to control ageing disorders where bone destruction exceeds bone formation, like osteoporosis, rheumatoid arthritis, and fracture mal-union. J. Cell. Physiol. 231: 1852-1861, 2016. © 2016 Wiley Periodicals, Inc.

HOW DO BONE CELLS SENSE MECHANICAL LOADING?

Influenced by gravidity, bone tissue experiences stronger or lighter deformation according to the strength of the activities of daily life. Activities resulting in impact are particularly known to stimulate osteogenesis, thus reducing bone mass loss. Knowing how bone cells recognize the mechanical deformation imposed to the bone and trigger a series of biochemical chain reactions is of crucial importance for the development of therapeutic and preventive practices in orthopaedic activity. There is still a long way to run until we can understand the whole process, but current knowledge has shown a strong progression, with researches being conducted focused on therapies. For a mechanical sign to be transformed into a biological one (mechanotransduction), it must be amplified at cell level by the histological structure of bone tissue, producing tensions in cell membrane proteins (integrins) and changing their spatial structure. Such change activates bindings between these and the cytoskeleton, producing focal adhesions, where cytoplasmatic proteins are recruited to enable easier biochemical reactions. Focal adhesion kinase (FAK) is the most important one being self-activated when its structure is changed by integrins. Activated FAK triggers a cascade of reactions, resulting in the activation of ERK-1/2 and Akt, which are proteins that, together with FAK, regulate the production of bone mass. Osteocytes are believed to be the mechanosensor cells of the bone and to transmit the mechanical deformation to osteoblasts and osteoclasts. Ionic channels and gap junctions are considered as intercellular communication means for biochemical transmission of a mechanical stimulus. These events occur continuously on bone tissue and regulate bone remodeling.

Brucella abortus Invasion of Osteocytes Modulates Connexin 43 and Integrin Expression and Induces Osteoclastogenesis via Receptor Activator of NF-κB Ligand and Tumor Necrosis Factor Alpha Secretion

Osteoarticular brucellosis is the most common localization of human active disease. Osteocytes are the most abundant cells of bone. They secrete factors that regulate the differentiation of both osteoblasts and osteoclasts during bone remodeling. The aim of this study is to determine if Brucella abortus infection modifies osteocyte function. Our results indicate that B. abortus infection induced matrix metalloproteinase 2 (MMP-2), receptor activator for NF-κB ligand (RANKL), proinflammatory cytokines, and keratinocyte chemoattractant (KC) secretion by osteocytes. In addition, supernatants from B. abortus-infected osteocytes induced bone marrow-derived monocytes (BMM) to undergo osteoclastogenesis. Using neutralizing antibodies against tumor necrosis factor alpha (TNF-α) or osteoprotegerin (OPG), RANKL's decoy receptor, we determined that TNF-α and RANKL are involved in osteoclastogenesis induced by supernatants from B. abortus-infected osteocytes. Connexin 43 (Cx43) and the integrins E11/gp38, integrin-α, integrin-β, and CD44 are involved in cell-cell interactions necessary for osteocyte survival. B. abortus infection inhibited the expression of Cx43 but did not modify the expression of integrins. Yet the expression of both Cx43 and integrins was inhibited by supernatants from B. abortus-infected macrophages. B. abortus infection was not capable of inducing osteocyte apoptosis. However, supernatants from B. abortus-infected macrophages induced osteocyte apoptosis in a dose-dependent manner. Taken together, our results indicate that B. abortus infection could alter osteocyte function, contributing to bone damage.

A histological study of mineralised tissue formation around implants with 3D culture of HMS0014 cells in Cellmatrix Type I-A collagen gel scaffold in vitro

We cultured HMS0014 Yub621b cells within a 3D collagen gel scaffold (Cellmatrix Type I-A) and aimed to study the fate and contribution of human bone-derived mesenchymal stem cells (MSCs) in the guided bone regeneration(GBR)-engineered tissue which has developed around the titanium (Ti) test dental implant (IP) in vitro. The light microscopy (LM) and transmission electron microscopy (TEM) results of the peri-IP tissue indicated that collagen fibrils of the Cellmatrix Type I-A gel were accumulated and fabricated to provide a 3D meshwork for proliferation and differentiation of the HMS0014 cells in the top (cell) layer; mineralisation of the GBR tissue had commenced since day 1 and became markedly deposited between days 7 and 14 of the experiment. TEM observation revealed sedimentation of cement line at the periphery of the interwoven Cellmatrix fibres and fibrils in the ECM scaffold of the GBR tissue; matrix vesicle-mediated and appositional collagen-mediated mineralisation were identified in the peri-IP ECM scaffold. The fine structure study of the plurimorphic osteoblast(Ob)-like osteogeneic cells demonstrated numerous membranous organelles related with vesicular trafficking, secretion and endocytosis in the cytoplasm; well-developed cytoskeleton networks and intercellular junctional complexes were also observed. The specimens on fluorescence immunohistochemistry (IHC) by confocal laser-scanning microscopy (LSM) showed the expression of LC3 and Cx43 associated with autophagic-lysosomal degeneration pathway and signal conduction mediated with gap junctions (GJS) in maintaining tissue homeostasis of the Ob-like cells which grew and degenerated in the 3D scaffold. Results from this in vitro study suggest that Ob-like HMS0014 cells actively regulate turnover of the peri-IP ECM to recapitulate the development and formation of osteoid tissue-engineered material which might contribute to augment osseointegration around the dental implant.

The effect of mechanical stimulation on mineralization in differentiating osteoblasts in collagen-I scaffolds

Developing a viable and functional bone scaffold in vitro that is capable of surviving and bearing mechanical load in vivo requires an understanding of the cell biology of osteoprogenitor cells, particularly how they are influenced by mechanical stimulation during cell differentiation and maturation. In this study, mechanical load was applied using a modified FlexCell plate to impart confined compression to collagen-I scaffolds seeded with undifferentiated murine embryonic stem cells. The activity, presence, and expression of osteoblast-cadherin (OB-Cad) and connexin-43, as well as various pluripotent and osteogenic markers were examined at 5-30 days of differentiation as cells were stimulated to differentiate to osteoblasts with and without applied mechanical load. Fluorescence recovery after photobleaching, immunofluorescence, viability, von Kossa, and real-time polymerase chain reaction assessments revealed that mechanical prestimulation of this cell-seeded scaffold altered the expression of OB-Cad and connexin-43 and resulted in significant differences in the structure and organization of mineralization present in the collagen matrix. Specifically, cells in gels that were loaded for 40 h after 5 days of differentiation and then left to fully differentiate for 30 days produced a highly structured honeycomb-shaped mineralization in the matrix; an outcome that was previously shown to be indicative of late osteoblast/early osteocyte activity. This study highlights the potential of mechanical load to accelerate differentiation and enhance osteoblast communication and function during the differentiation process, and highlights a time point of cell differentiation within this scaffold to apply load in order to most effectively transduce a mechanical signal.

Biological Events in Periodontal Ligament and Alveolar Bone Associated with Application of Orthodontic Forces

Orthodontic force-induced stresses cause dynamic alterations within the extracellular matrix and within the cytoskeleton of cells in the periodontal ligament and alveolar bone, mediating bone remodelling, ultimately enabling orthodontic tooth movement. In the periodontal ligament and alveolar bone, the mechanically induced tensile strains upregulate the expression of osteogenic genes resulting in bone formation, while mechanically induced compressive strains mediate predominantly catabolic tissue changes and bone resorption. In this review article we summarize some of the currently known biological events occurring in the periodontal ligament and in the alveolar bone in response to application of orthodontic forces and how these facilitate tooth movement.

Impact of Gap Junctional Intercellular Communication on MLO-Y4 Sclerostin and Soluble Factor Expression

Bone remodeling is a continual process in which old bone is resorbed by osteoclasts and new bone is formed by osteoblasts, providing a mechanism for bones' ability to adapt to changes in its mechanical environment. While the role of osteoblasts and osteoclasts in bone remodeling is well understood, the cellular regulation of bone remodeling is unclear. One theory is that osteocytes, found within bone, play an important role in controlling the bone remodeling response. Osteocytes possess gap junctions, narrow channels that extend between nearby cells and allow communication between cells via the transfer of small molecules and ions. This work investigated the potential role of gap junctional intercellular communication in bone remodeling by exposing osteocyte-like MLO-Y4 cells to mechanical strains and quantifying the expression of soluble factors, including sclerostin, a protein closely associated with bone remodeling. The soluble factors and sclerostin expression were further examined after inhibiting gap junctional intercellular communication to study the impact of the communication. At supraphysiologic strains, the inhibition of gap junctional intercellular communication led to increases in sclerostin expression relative to cells in which communication was present, indicating that the communication may play a significant role in regulating bone remodeling.

Histological and molecular-biological analyses of poly(3-hydroxybutyrate) (PHB) patches for enhancement of bone regeneration

Tissue engineered cell-seeded constructs with poly(3)hydroxybutyrate (PHB) induced ectopic bone formation after implantation into the back muscle of rats. The objective of our in vivo study was to evaluate the osteogenic potential of pure PHB patches in surgically created cranial defects. For this, PHB patches were analyzed after implantation in surgically created defects on the cranium of adult male rats. After healing periods of 4, 8 and 12 weeks, the bone tissue specimens containing PHB patches were processed and analyzed histologically as well as molecular-biologically. After 4 weeks, the PHB patches were completely embedded in connective tissue. Eight weeks after PHB insertion, bone regeneration proceeding from bearing bone was found in 50% of all treated animals, whereas all PHB treated cavities showed both bone formation and embedding of the patches in bone 12 weeks after surgery. Furthermore, all slices showed pronounced development of blood vessels. Histomorphometric analysis presented a regenerated bone mean value between 46.4 ± 16.1% and 54.2 ± 19.3% after 4-12 weeks of healing. Caveolin-1 staining in capillary-like structures showed a 1.16-1.38 fold increased expression in PHB treated defects compared to controls. Real-time RT-PCR analyses showed significantly lower expressions of Alpl, Col1a1 and VEGFA in cranium defects after treatment with PHB patches compared to untreated bony defects of the same cranium. Within the limits of the presented animal investigation, it could conclude that the tested PHB patches featured a good biocompatibility and an osteoconductive character.

Sclerostin is differently immunolocalized in metaphyseal trabecules and cortical bones of mouse tibiae

Sclerostin, an osteocyte-derived molecule, has been reported to serve as a negative regulator of osteoblastic activity as well as bone remodeling. However, there is no report that verified the regional difference for sclerostin synthesis, and in this study we have investigated immunolocalization of sclerostin by comparing dentin matrix protein (DMP) 1, an osteocyte-derived factor broadly expressed in tibial metaphyses and cortical bone. In metaphyseal primary trabecules, a site of bone modeling, strong DMP1-reactivity was observed in osteocytic lacunar-canalicular system (OLCS), while faint staining for sclerostin was visible only in a few osteocytes. In secondary trabecules, in which bone remodeling begins, some osteocytes showed intense sclerostin-immunopositivity, though there were many DMP1-positive osteocytes. In cortical bone, there were more osteocytes reactive for sclerostin, when compared with those in the secondary trabecules. Silver impregnation verified that immature, primary trabecules contained randomly-oriented OLCS, while mature, cortical bone showed geometrically well-arrangement of OLCS. Taken together, though DMP1 is broadly synthesized in bone, sclerostin appears to be abundantly synthesized in regular OLCS of cortical bone, but less produced in irregular OLCS as seen in primary trabecules, indicating the regional difference for sclerostin synthesis.

Inhibition of GSK-3β rescues the impairments in bone formation and mechanical properties associated with fracture healing in osteoblast selective connexin 43 deficient mice

Connexin 43 (Cx43) is the most abundant gap junction protein in bone and is required for osteoblastic differentiation and bone homeostasis. During fracture healing, Cx43 is abundantly expressed in osteoblasts and osteocytes, while Cx43 deficiency impairs bone formation and healing. In the present study we selectively deleted Cx43 in the osteoblastic lineage from immature osteoblasts through osteocytes and tested the hypothesis that Cx43 deficiency results in delayed osteoblastic differentiation and impaired restoration of biomechanical properties due to attenuated β-catenin expression relative to wild type littermates. Here we show that Cx43 deficiency results in alterations in the mineralization and remodeling phases of healing. In Cx43 deficient fractures the mineralization phase is marked by delayed expression of osteogenic genes. Additionally, the decrease in the RankL/ Opg ratio, osteoclast number and osteoclast size suggest decreased osteoclast bone resorption and remodeling. These changes in healing result in functional deficits as shown by a decrease in ultimate torque at failure. Consistent with these impairments in healing, β-catenin expression is attenuated in Cx43 deficient fractures at 14 and 21 days, while Sclerostin (Sost) expression, a negative regulator of bone formation is increased in Cx43cKO fractures at 21 days, as is GSK-3β, a key component of the β-catenin proteasomal degradation complex. Furthermore, we show that alterations in healing in Cx43 deficient fractures can be rescued by inhibiting GSK-3β activity using Lithium Chloride (LiCl). Treatment of Cx43 deficient mice with LiCl restores both normal bone formation and mechanical properties relative to LiCl treated WT fractures. This study suggests that Cx43 is a potential therapeutic target to enhance fracture healing and identifies a previously unknown role for Cx43 in regulating β-catenin expression and thus bone formation during fracture repair.

Osteocyte density in the peri-implant bone of implants retrieved after different time periods (4 weeks to 27 years)

Bone tissue is characterized by a constant turnover in response to mechanical stimuli, and osteocytes play an essential role in bone mechanical adaptation. However, little to no information has been published regarding osteocyte density as a function of implantation time in vivo. The aim of this retrospective histological study was to evaluate the osteocyte density of the peri-implant bone in implants retrieved because of different reasons in a time period from 4 weeks to 27 years. A total of 18 samples were included in the present study. Specimens were divided into 3 groups depending on the loading history of the implants: loading between 4 weeks and 7 months (group 1); loading between 1 and 5 years (group 2); loading between 14 and 27 years (group 3). All the samples were histologically evaluated and osteocyte density was obtained using the ratio of the number of osteocytes to the bone-area (mm(2) ). The osteocyte density values significantly increased in the Group 2 (1-5 years) compared with Group 1 (4 weeks-7 months), and significantly decreased in the Group 3 (14-27 years) compared to Group 2. No significant differences were detected between Group 1 and Group 3. The decrease in osteocyte density observed in samples that were in vivo for long periods of time under loading is possibly because of the fact that once the bone structure is well aligned and biomechanically competent, a lower number of osteocytes are necessary to keep the tissue homeostasis under loading.

Potential risk of asymptomatic osteomyelitis around mandibular third molar tooth for aged people: a computed tomography and histopathologic study

The purpose of this study was to explore the relationship between bone mineral density and histopathological features of mandibular alveolar bone evaluated quantitatively by Hounsfield units [HU] and by histopathology in human subjects. Fifty-six mandibular molars were extracted in 50 patients. Computed tomography was obtained preoperatively, and a cortical bone biopsy was obtained on the extracted sites for histopathological evaluation. The mean cortical and cancellous bone radiodensity was 1846 ± 118 HU and 926 ± 436 HU, respectively. There was no correlation between age and cortical bone HU (r = -0.004, P = 0.976); however, the correlation between age and cancellous bone HU was significant (r = 0.574, P<0.0000). Significant differences in the cancellous bone between young (0-30 years), middle (31-60 years) and old patient groups (61< years) were evident (P<0.05), whereas the cortical bone presented no significant differences. The histopathological evaluation showed that the young patient group had relatively few osteomyelitis, whereas the old patient group showed 100% focal sclerotic osteomyelitis regardless of the fact that the patients had no clinical symptoms. The mean osteocyte number/unit bone area was 170.7 ± 82.2. Negative correlation between age and osteocyte number was significant (r = -0.51, P<0.0001). Mean lacunae numbers/unit cortical bone area were 413.1 ± 130 with non-significant negative correlation (r = -0.257, P = 0.056). The mean empty lacunae numbers/cortical bone were 242.5±145, with no correlation (r = 0.081, P = 0.559). The young patients had high osteocyte number, whereas the old patients showed reduction of the osteocytes in the cortical bone (P < 0.05). Bone quality might correlate better to viable cell numbers, which influenced the osseous healing. It is suggested that the outermost layer of cortical bone may have lost its cellular activities over the years due to chronic infection, which may have provoked sclerotic changes in the cancellous bone around tooth.

Gap junction and hemichannel functions in osteocytes

Cell-to-cell and cell-to-matrix communication in bone cells mediated by gap junctions and hemichannels, respectively, maintains bone homeostasis. Gap junctional communication between cells permits the passage of small molecules including calcium and cyclic AMP. This cell-to-cell communication occurs between bone cells including osteoblasts, osteoclasts and osteocytes, and is important in both bone formation and bone resorption. Connexin (Cx) 43 is the predominant gap junction protein in bone cells, and facilitates the communication of cellular signals either through docking of gap junctions between two cells, or through the formation of un-paired hemichannels. Systemic deletion of Cx43 results in perinatal lethality, so conditional deletion models are necessary to study the postnatal role of gap junctions in bone. These models provide the opportunity to determine the role of gap junctions in specific bone cells, notably the osteocyte. In this review, we summarize the key roles that gap junctions and hemichannels in osteocytes play in bone cell response to many stimuli including mechanical loading, intracellular and extracellular stimuli, such as parathyroid hormone, PGE2, plasma calcium levels and pH, as well as in maintaining osteocyte survival.

In vitro and in vivo approaches to study osteocyte biology

Osteocytes, the most abundant cell population of the bone lineage, have been a major focus in the bone research field in recent years. This population of cells that resides within mineralized matrix is now thought to be the mechanosensory cell in bone and plays major roles in the regulation of bone formation and resorption. Studies of osteocytes had been impaired by their location, resulting in numerous attempts to isolate primary osteocytes and to generate cell lines representative of the osteocytic phenotype. Progress has been achieved in recent years by utilizing in vivo genetic technology and generation of osteocyte directed transgenic and gene deficiency mouse models. We will provide an overview of the current in vitro and in vivo models utilized to study osteocyte biology. We discuss generation of osteocyte-like cell lines and isolation of primary osteocytes and summarize studies that have utilized these cellular models to understand the functional role of osteocytes. Approaches that attempt to selectively identify and isolate osteocytes using fluorescent protein reporters driven by regulatory elements of genes that are highly expressed in osteocytes will be discussed. In addition, recent in vivo studies utilizing overexpression or conditional deletion of various genes using dentin matrix protein (Dmp1) directed Cre recombinase are outlined. In conclusion, evaluation of the benefits and deficiencies of currently used cell lines/genetic models in understanding osteocyte biology underlines the current progress in this field. The future efforts will be directed towards developing novel in vitro and in vivo models that would additionally facilitate in understanding the multiple roles of osteocytes.

Gap-junction-mediated Communication in Human Periodontal Ligament Cells

Periodontal tissue homeostasis depends on a complex cellular network that conveys cell-cell communication. Gap junctions (GJs), one of the intercellular communication systems, are found between adjacent human periodontal ligament (hPDL) cells; however, the functional GJ coupling between hPDL cells has not yet been elucidated. In this study, we investigated functional gap-junction-mediated intercellular communication in isolated primary hPDL cells. SEM images indicated that the cells were in contact with each other via dendritic processes, and also showed high anti-connexin43 (Cx43) immunoreactivity on these processes. Gap-junctional intercellular communication (GJIC) among hPDL cells was assessed by fluorescence recovery after a photobleaching (FRAP) analysis, which exhibited dye coupling between hPDL cells, and was remarkably down-regulated when the cells were treated with a GJ blocker. Additionally, we examined GJs under hypoxic stress. The fluorescence recovery and expression levels of Cx43 decreased time-dependently under the hypoxic condition. Exposure to GJ inhibitor or hypoxia increased RANKL expression, and decreased OPG expression. This study shows that GJIC is responsible for hPDL cells and that its activity is reduced under hypoxia. This is consistent with the possible role of hPDL cells in regulating the biochemical reactions in response to changes in the hypoxic environment.

17β estradiol regulation of connexin 43-based gap junction and mechanosensitivity through classical estrogen receptor pathway in osteocyte-like MLO-Y4 cells

Connexin 43 (Cx43) plays an essential role in osteocyte mechanotransduction. Although estrogen involves in the adaptive responses of bone cells to mechanical loadings, its effects on osteocytic Cx43-based gap junction intercellular communication (GJIC) remain obscure. We found that 17β estradiol (E2) up-regulated Cx43, and enhanced GJIC in osteocyte-like MLO-Y4 cells in fluorescence recovery after photobleaching (FRAP) assay. Combination of E2 pre-treatment and oscillating fluid flow (OFF) further enhanced Cx43 expression and mitogen-activated protein kinase (MAPK) phosphorylation, comparing to E2 or OFF treatment alone. Both blocking of classical estrogen receptors (ERα/β) by fulvestrant and ERα knockdown by small interfering RNA inhibited E2-mediated Cx43 increase, while a GPR30-specific agonist G-1 failed to promote Cx43 expression. Our results suggest that the presence of E2 enhanced Cx43-based GJIC mainly via ERα/β pathway, and sensitized osteocytes to mechanical loading.

Ex vivo real-time observation of Ca(2+) signaling in living bone in response to shear stress applied on the bone surface

Bone cells respond to mechanical stimuli by producing a variety of biological signals, and one of the earliest events is intracellular calcium ([Ca(2+)](i)) mobilization. Our recently developed ex vivo live [Ca(2+)](i) imaging system revealed that bone cells in intact bone explants showed autonomous [Ca(2+)](i) oscillations, and osteocytes specifically modulated these oscillations through gap junctions. However, the behavior and connectivity of the [Ca(2+)](i) signaling networks in mechanotransduction have not been investigated in intact bone. We herein introduce a novel fluid-flow platform for probing cellular signaling networks in live intact bone, which allows the application of capillary-driven flow just on the bone explant surface while performing real-time fluorogenic monitoring of the [Ca(2+)](i) changes. In response to the flow, the percentage of responsive cells was increased in both osteoblasts and osteocytes, together with upregulation of c-fos expression in the explants. However, enhancement of the peak relative fluorescence intensity was not evident. Treatment with 18 α-GA, a reversible inhibitor of gap junction, significantly blocked the [Ca(2+)](i) responsiveness in osteocytes without exerting any major effect in osteoblasts. On the contrary, such treatment significantly decreased the flow-activated oscillatory response frequency in both osteoblasts and osteocytes. The stretch-activated membrane channel, when blocked by Gd(3+), is less affected in the flow-induced [Ca(2+)](i) response. These findings indicated that flow-induced mechanical stimuli accompanied the activation of the autonomous [Ca(2+)](i) oscillations in both osteoblasts and osteocytes via gap junction-mediated cell-cell communication and hemichannel. Although how the bone sense the mechanical stimuli in vivo still needs to be elucidated, the present study suggests that cell-cell signaling via augmented gap junction and hemichannel-mediated [Ca(2+)](i) mobilization could be involved as an early signaling event in mechanotransduction.

Osteoblast and osteocyte-specific loss of Connexin43 results in delayed bone formation and healing during murine fracture healing

Connexin43 (Cx43) plays an important role in osteoblastic differentiation in vitro, and bone formation in vivo. Mice with osteoblast/osteocyte-specific loss of Cx43 display decreased gap junctional intercellular communication (GJIC), bone density, and cortical thickness. To determine the role of Cx43 in fracture healing, a closed femur fracture was induced in Osteocalcin-Cre+; Cx43(flox/flox) (Cx43cKO) and Cre-; Cx43(flox/flox) (WT) mice. We tested the hypothesis that loss of Cx43 results in decreased bone formation and impaired healing following fracture. Here, we show that osteoblast and osteocyte-specific deletion of Cx43 results in decreased bone formation, bone remodeling, and mechanical properties during fracture healing. Cx43cKO mice display decreased bone volume, total volume, and fewer TRAP+ osteoclasts. Furthermore, loss of Cx43 in mature osteoblasts and osteocytes results in a significant decrease in torsional rigidity between 21 and 35 days post-fracture, compared to WT mice. These studies identify a novel role for the gap junction protein Cx43 during fracture healing, suggesting that loss of Cx43 can result in both decreased bone formation and bone resorption. Therefore, enhancing Cx43 expression or GJIC may provide a novel means to enhance bone formation during fracture healing.

Calcium phosphate phases integrated in silica/collagen nanocomposite xerogels enhance the bioactivity and ultimately manipulate the osteoblast/osteoclast ratio in a human co-culture model

A human co-culture model of osteoblasts and osteoclasts, derived from bone marrow stromal cells and monocytes respectively, was used to characterize the influence of biomaterial modification on the bioactivity and ultimately the ratio of bone-forming to bone-resorbing cells cultivated directly on the surface. Nanocomposites of silica and collagen have been shown to function as skeletal structures in nature and were reproduced in vitro by using a sol-gel approach. The resulting xerogels exhibit a number of features that make it a valuable system for the development of innovative materials for bone substitution applications. In the present study, the incorporation of different calcium phosphate phases in silica/collagen-based gels was demonstrated to enhance the bioactivity of these samples. This ability of the biomaterial to precipitate calcium phosphate on the surface when incubated in simulated body fluids or cell culture medium is generally considered to an advantageous property for bone substitution materials. By co-cultivating human osteoblasts and osteoclasts up to 42 days on the xerogels, we demonstrate that the long-term ratio of these cell types depends on the level of bioactivity of the substrate samples. Biphasic silica/collagen xerogels exhibited comparably low bioactivity but encouraged proliferation of osteoblasts in comparison to osteoclast formation. A balanced ratio of both cell types was detected for moderately bioactive triphasic xerogels with 5% calcium phosphate. However, enhancing the bioactivity of the xerogel samples by increasing the calcium phosphate phase percentage to 20% resulted in a diminished number of osteoblasts in favor of osteoclast formation. Quantitative evaluation was carried out by biochemical methods (calcium, DNA, ALP, TRAP 5b) as well as RT-PCR (ALP, BSP II, OC, RANKL, TRAP, CALCR, VTNR, CTSK), and was supported by confocal laser scanning microscopy (cell nuclei, actin, CD68, TRAP) as well as scanning electron microscopy.

Macro and microfluidic flows for skeletal regenerative medicine

Fluid flow has a great potential as a cell stimulatory tool for skeletal regenerative medicine, because fluid flow-induced bone cell mechanotransduction in vivo plays a critical role in maintaining healthy bone homeostasis. Applications of fluid flow for skeletal regenerative medicine are reviewed at macro and microscale. Macroflow in two dimensions (2D), in which flow velocity varies along the normal direction to the flow, has explored molecular mechanisms of bone forming cell mechanotransduction responsible for flow-regulated differentiation, mineralized matrix deposition, and stem cell osteogenesis. Though 2D flow set-ups are useful for mechanistic studies due to easiness in in situ and post-flow assays, engineering skeletal tissue constructs should involve three dimensional (3D) flows, e.g., flow through porous scaffolds. Skeletal tissue engineering using 3D flows has produced promising outcomes, but 3D flow conditions (e.g., shear stress vs. chemotransport) and scaffold characteristics should further be tailored. Ideally, data gained from 2D flows may be utilized to engineer improved 3D bone tissue constructs. Recent microfluidics approaches suggest a strong potential to mimic in vivo microscale interstitial flows in bone. Though there have been few microfluidics studies on bone cells, it was demonstrated that microfluidic platform can be used to conduct high throughput screening of bone cell mechanotransduction behavior under biomimicking flow conditions.

Connexin 43 deficiency attenuates loss of trabecular bone and prevents suppression of cortical bone formation during unloading

Connexin 43 (Cx43) is the most abundant gap junction protein in bone and has been demonstrated as an integral component of skeletal homeostasis. In the present study, we sought to further refine the role of Cx43 in the response to mechanical unloading by subjecting skeletally mature mice with a bone-specific deletion of Cx43 (cKO) to 3 weeks of mechanical unloading via hindlimb suspension (HLS). The HLS model was selected to recapitulate the effects of skeletal unloading due to prolonged bed rest, reduced activity associated with aging, and spaceflight microgravity. At baseline, the cortical bone of cKO mice displayed an osteopenic phenotype, with expanded cortices, decreased cortical thickness, decreased bone mineral density, and increased porosity. There was no baseline trabecular phenotype. After 3 weeks of HLS, wild-type (WT) mice experienced a substantial decline in trabecular bone volume fraction, connectivity density, trabecular thickness, and trabecular tissue mineral density. These deleterious effects were attenuated in cKO mice. Conversely, there was a similar and significant amount of cortical bone loss in both WT and cKO. Interestingly, mechanical testing revealed a greater loss of strength and rigidity for cKO during HLS. Analysis of double-label quantitative histomorphometry data demonstrated a substantial decrease in bone formation rate, mineralizing surface, and mineral apposition rate at both the periosteal and endocortical surfaces of the femur after unloading of WT mice. This suppression of bone formation was not observed in cKO mice, in which parameters were maintained at baseline levels. Taken together, the results of the present study indicate that Cx43 deficiency desensitizes bone to the effects of mechanical unloading, and that this may be due to an inability of mechanosensing osteocytes to effectively communicate the unloading state to osteoblasts to suppress bone formation. Cx43 may represent a novel therapeutic target for investigation as a countermeasure for age-related and unloading-induced bone loss.

The effect of osteocyte apoptosis on signalling in the osteocyte and bone lining cell network: a computer simulation

Osteocytes play a critical role in the regulation of bone remodelling by translating strain due to mechanical loading into biochemical signals transmitted through the interconnecting lacuno-canalicular network to bone lining cells (BLCs) on the bone surface. This work aims to examine the effects of disruption of that intercellular communication by simulation of osteocyte apoptosis in the bone matrix. A model of a uniformly distributed osteocyte network has been developed that simulates the signalling through the network to the BLCs based on strain level. Bi-directional and asymmetric communication between neighbouring osteocytes and BLCs is included. The effect of osteocyte apoptosis is examined by preventing signalling at and through the affected cells. The simulation shows that apoptosis of only 3% of the osteocyte cells leads to a significant reduction in the peak signal at the BLCs. Furthermore, experiments with the model confirm how important the location and density of the apoptotic osteocytes are to the signalling received at the bone surface. With 5% and 9% osteocyte apoptosis, the mean peak BLC levels were reduced by 25% and 37% respectively. Such a significant reduction in the signal at the BLCs may explain a possible mechanism that leads to the increased remodelling and eventual bone loss observed with osteoporosis. More generally, it provides a unique framework for a broader exploration of the role of osteocyte and bi-directional and asymmetric cell-cell communication in mechanotransduction, and the effects of disruption to that communication.

Fibroblasts regulate osteoblasts through gap junctional communication

BACKGROUND AIMS

Fibroblasts are present in most tissues of the body, playing an active role in the regulation of homeostasis in such tissues. While fibroblast heterotypic interactions are acknowledged in the regeneration of tissues such as skin and periodontal ligament, their role in bone regeneration is far from being understood. We hypothesized that fibroblasts could influence osteoblasts, and as connexin 43 is the predominant connexin in both cell types, we speculated that those heterotypic interactions could occur through gap junctional communication (GjC).

METHODS

Direct co-cultures of human mesenchymal stromal cell (hMSC)-derived osteoblasts and human dermal fibroblasts (hDFb) were established in the presence and absence of the GjC inhibitor α-glycyrrhetinic acid. Communication between osteoblasts and hDFb via GjC was verified by transference of the gap junction-permeable dye calcein-AM. Cell proliferation was assessed by dsDNA quantification, while osteogenic differentiation was evaluated by measuring alkaline phosphatase (ALP) activity and the expression of osteogenic markers by real-time polymerase chain reaction (PCR).

RESULTS

The amount of calcein-AM transferred between the different cell types decreased when α-glycyrrhetinic acid was used. While the proliferation of the hMSC-derived osteoblasts was not affected by the presence of the hDFb, the level of osteogenic markers such as ALP activity and osteocalcin in transcripts in osteoblasts was severely diminished. This effect was partially reversed by adding α-glycyrrhetinic acid to the co-cultures.

CONCLUSIONS

The results strongly suggest that fibroblasts regulate osteoblast behavior partially through GjC. This information could be critical for predicting the outcome of strategies aimed at promoting bone regeneration as, for example, in bone tissue-engineering approaches.

Connexin43 and pannexin1 channels in osteoblasts: who is the "hemichannel"?

Osteoblasts sense and respond to mechanical stimuli in a process involving influx and release of large ions and signaling molecules. Unapposed gap junction hemichannels formed of connexin43 (Cx43) have been proposed as a major route for such exchange, in particular for release of ATP and prostaglandin E₂ (PGE₂) in osteocytes. However, we have found that Cx43-null osteoblasts have unaltered, mechanically induced PGE₂ release and ATP-induced YoPro dye uptake. In contrast, PGE₂ release in response to fluid shear stress is abolished in P2X₇ receptor (P2X₇R)-null osteoblasts, and ATP-induced dye uptake is attenuated following treatment of wild-type cells with a P2X₇R or Pannexin1 (Panx1) channel blocker. These data indicate that Panx1 channels, in concert with P2X₇R, likely form a molecular complex that performs the hemichannel function in osteoblast mechanosignaling.

Demonstration of osteocytic perilacunar/canalicular remodeling in mice during lactation

Osteoclasts are thought to be solely responsible for the removal of bone matrix. However, we show here that osteocytes can also remove bone matrix by reversibly remodeling their perilacunar/canalicular matrix during the reproductive cycle. In contrast, no osteocytic remodeling was observed with experimental unloading despite similar degrees of bone loss. Gene array analysis of osteocytes from lactating animals revealed an elevation of genes known to be utilized by osteoclasts to remove bone, including tartrate-resistant acid phosphatase (TRAP) and cathepsin K, that returned to virgin levels upon weaning. Infusion of parathyroid hormone-related peptide (PTHrP), known to be elevated during lactation, induced TRAP activity and cathepsin K expression in osteocytes concurrent with osteocytic remodeling. Conversely, animals lacking the parathyroid hormone type 1 receptor (PTHR1) in osteocytes failed to express TRAP or cathepsin K or to remodel their osteocyte perilacunar matrix during lactation. These studies show that osteocytes remove mineralized matrix through molecular mechanisms similar to those utilized by osteoclasts.

The canalicular system and the osteoblast domain in human secondary osteons

The lacunar-canalicular system in human secondary osteons was examined by two complementary techniques: light microscopy analysis of undecalcified thick sections and the SEM cortex-fractured surface technique. Unlike the earlier definitions of 'osteoblastic domain' presented as the matrix volume produced by osteoblasts in the process of osteon infilling, this study measured the domain by the length of osteoblast dendritic processes. The domain extension was defined along radial vectors advancing from the reversal line towards the central canal. According to their lengths, domains were divided into three classes: peripheral, intermediate and internal. The mean length of peripheral domains was significantly shorter than those of the intermediate and internal domains. This suggests that the infilling process is modulated by an initial preparatory phase characterised by osteoblast adhesion to the wall of the cutting cone, and a limited matrix synthesis, followed by a regular matrix volume apposition organised in concentric layers. In addition to the radial canaliculae arranged along converging vectors in planes perpendicular to the central canal, we distinguished a further class of canaliculae, the equatorial canaliculae originating from the major perimeter of the lacuna and spreading out radially in the plane of the same lacuna (therefore, perpendicularly to the radial canaliculae). The whole lacunar-canalicular network was structured as a closed system around the vascular axis of the central canal with very few canaliculae crossing the reversal line and connecting the neighbouring osteons. These anatomical observations contribute to our knowledge of lacunar-canalicular system development.