Ecto-5'-nucleotidase (CD73) regulates bone formation and remodeling during intramembranous bone repair in aging mice

Synergistic effect of ROS-generating polydopamine on drug-induced bone tissue regeneration

A PHD (prolyl hydroxylase) inhibitor, 1,4-dihydrophenonthrolin-4-one-3-carboxylic acid (1,4-DPCA), is a drug that can artificially promote tissue regeneration by enhancing metabolic activity through the upregulation of hypoxia inducible factor 1 subunit alpha (Hif-1α) under normoxic conditions. This study presents a novel design methodology for a drug delivery system to maximize the regenerative effect of 1,4-DPCA. Specifically, by encapsulating 1,4-DPCA in polydopamine (PDA) that generates reactive oxygen species (ROS), the combined effects of Hif-1α upregulation and the induction of cellular antioxidant defense mechanisms by localized ROS can significantly enhance tissue regeneration. The study confirmed that each material (PDA and 1,4-DPCA) triggers a positive synergistic effect on the regenerative mechanisms. As a result, the use of a PDA drug delivery system loaded with 1,4-DPCA showed approximately six times greater bone regeneration compared to the control (no treatment) in a mouse calvarial defect model.

Translational Experimental Basis of Indirect Adenosine Receptor Agonist Stimulation for Bone Regeneration: A Review

Bone regeneration remains a significant clinical challenge, often necessitating surgical approaches when healing bone defects and fracture nonunions. Within this context, the modulation of adenosine signaling pathways has emerged as a promising therapeutic option, encouraging osteoblast activation and tempering osteoclast differentiation. A literature review of the PubMed database with relevant keywords was conducted. The search criteria involved in vitro or in vivo models, with clear methodological descriptions. Only studies that included the use of indirect adenosine agonists, looking at the effects of bone regeneration, were considered relevant according to the eligibility criteria. A total of 29 articles were identified which met the inclusion and exclusion criteria, and they were reviewed to highlight the preclinical translation of adenosine agonists. While preclinical studies demonstrate the therapeutic potential of adenosine signaling in bone regeneration, its clinical application remains unrealized, underscoring the need for further clinical trials. To date, only large, preclinical animal models using indirect adenosine agonists have been successful in stimulating bone regeneration. The adenosine receptors (A1, A2A, A2B, and A3) stimulate various pathways, inducing different cellular responses. Specifically, indirect adenosine agonists act to increase the extracellular concentration of adenosine, subsequently agonizing the respective adenosine receptors. The agonism of each receptor is dependent on its expression on the cell surface, the extracellular concentration of adenosine, and its affinity for adenosine. This comprehensive review analyzed the multitude of indirect agonists currently being studied preclinically for bone regeneration, discussing the mechanisms of each agonist, their cellular responses in vitro, and their effects on bone formation in vivo.

The Role of the Receptor Activator of Nuclear Factor Kappa-B Ligand/Osteoprotegerin Ratio in Vascular Diseases: A Therapeutic Approach

Cardiovascular and bone diseases contribute independently to mortality and global health. The exact mechanisms involved in the pathophysiology shared between bone and vascular diseases are not well defined. Endothelial cells and osteoblasts communicate during osteogenesis, thus establishing a connection between angiogenesis and osteogenesis. One shared mechanism may involve osteoprotegerin (OPG) and its ligand Receptor Activator of NF-κB Ligand (RANKL). The RANKL/OPG ratio is an important modulator for the skeletal, immunological, and vascular systems. OPG levels are elevated due to either osteogenic causes or inflammatory responses in the vasculature. The data obtained from clinical and in vitro studies support the role of the RANKL/OPG ratio as a potential marker for the progression of endothelial damage. Therefore, determining the therapeutic approaches for the targeting RANKL/OPG ratio and evaluating its usage as a biomarker in cardiovascular and bone pathophysiology are needed. By integrating the protective and disease-causing role of OPG with its ligand, this review outlines the role of the RANKL/OPG ratio at the molecular level. We also consider targeted therapeutic approaches.

The protective role of CD73 in periodontitis: preventing hyper-inflammatory fibroblasts and driving osteoclast energy metabolism

Introduction

Periodontitis is an immune-mediated inflammatory disease affecting almost half of the adult population and is the leading cause of tooth loss in the United States. The role of extracellular nucleotide signaling including nucleotide metabolizing enzyme CD73 adds an important layer of interaction of purine mediators capable of orchestrating inflammatory outcomes. CD73 is able to catabolize 5'-adenosine monophosphate into adenosine at the extracellular level, playing a critical role in regulating many processes under physiological and pathological conditions. Here, we explored the role of CD73 in ligature-induced periodontitis in vivo comparing wild-type C57Bl/6J and CD73-deficient mice.

Methods

We assessed gingival levels of inflammatory cytokines in vivo and in murine gingival fibroblasts in vitro, as well as bone loss, and RANKL-induced osteoclastogenesis. We have also analyzed CD73 mRNA in samples derived from patients diagnosed with severe periodontitis.

Results

Our results in mice show that lack of CD73 resulted in increased inflammatory cytokines and chemokines such as IL-1β, IL-17, Cxcl1 and Cxcl2 in diseased gingiva relative to the healthy-controls and in comparison with the wild type. CD73-deficient gingival fibroblasts also manifested a defective healing response with higher MMP-13 levels. CD73-deficient animals also showed increased osteoclastogenesis in vitro with increased mitochondrial metabolism typified by excessive activation of oxidative phosphorylation, increased mitochondrial membrane potential and accumulation of hydrogen peroxide. Micro-CT analysis revealed that lack of CD73 resulted in decreased bone mineral density, decreased trabecular bone volume and thickness as well as decreased bone volume in long bones. CD73 deficiency also resulted in increased alveolar bone loss in experimental periodontitis. Correlative studies of gingival samples from severe (Grade C) periodontitis showed decreased levels of CD73 compared to healthy controls, further supporting the relevance of our murine results.

Conclusion

In conclusion, CD73 appears to play a protective role in the gingival periodontal tissue and bone homeostasis, regulating hyper-inflammatory state of stromal fibroblasts and osteoclast energy metabolism and being an important candidate for future target therapies to prevent or control immune-mediated inflammatory and osteolytic diseases.

Multi-Functional Small Molecule Alleviates Fracture Pain and Promotes Bone Healing

Bone injuries such as fractures are one major cause of morbidities worldwide. A considerable number of fractures suffer from delayed healing, and the unresolved acute pain may transition to chronic and maladaptive pain. Current management of pain involves treatment with NSAIDs and opioids with substantial adverse effects. Herein, we tested the hypothesis that the purine molecule, adenosine, can simultaneously alleviate pain and promote healing in a mouse model of tibial fracture by targeting distinctive adenosine receptor subtypes in different cell populations. To achieve this, a biomaterial-assisted delivery of adenosine is utilized to localize and prolong its therapeutic effect at the injury site. The results demonstrate that local delivery of adenosine inhibited the nociceptive activity of peripheral neurons through activation of adenosine A1 receptor (ADORA1) and mitigated pain as demonstrated by weight bearing and open field movement tests. Concurrently, local delivery of adenosine at the fracture site promoted osteogenic differentiation of mesenchymal stromal cells through adenosine A2B receptor (ADORA2B) resulting in improved bone healing as shown by histological analyses and microCT imaging. This study demonstrates the dual role of adenosine and its material-assisted local delivery as a feasible therapeutic approach to treat bone trauma and associated pain.

Could hypoxia rehabilitate the osteochondral diseased interface? Lessons from the interplay of hypoxia and purinergic signals elsewhere

The osteochondral unit comprises the articular cartilage (90%), subchondral bone (5%) and calcified cartilage (5%). All cells present at the osteochondral unit that is ultimately responsible for matrix production and osteochondral homeostasis, such as chondrocytes, osteoblasts, osteoclasts and osteocytes, can release adenine and/or uracil nucleotides to the local microenvironment. Nucleotides are released by these cells either constitutively or upon plasma membrane damage, mechanical stress or hypoxia conditions. Once in the extracellular space, endogenously released nucleotides can activate membrane-bound purinoceptors. Activation of these receptors is fine-tuning regulated by nucleotides' breakdown by enzymes of the ecto-nucleotidase cascade. Depending on the pathophysiological conditions, both the avascular cartilage and the subchondral bone subsist to significant changes in oxygen tension, which has a tremendous impact on tissue homeostasis. Cell stress due to hypoxic conditions directly influences the expression and activity of several purinergic signalling players, namely nucleotide release channels (e.g. Cx43), NTPDase enzymes and purinoceptors. This review gathers experimental evidence concerning the interplay between hypoxia and the purinergic signalling cascade contributing to osteochondral unit homeostasis. Reporting deviations to this relationship resulting from pathological alterations of articular joints may ultimately unravel novel therapeutic targets for osteochondral rehabilitation. At this point, one can only hypothesize how hypoxia mimetic conditions can be beneficial to the ex vivo expansion and differentiation of osteo- and chondro-progenitors for auto-transplantation and tissue regenerative purposes.

Extracellular adenosine signaling in bone health and disease

Purinergic signaling is a key molecular pathway in the maintenance of bone health and regeneration. P1 receptor signaling, which is activated by extracellular adenosine, has emerged as a key metabolic pathway that regulates bone tissue formation, function, and homeostasis. Extracellular adenosine is mainly produced by ectonucleotidases, and alterations in the function of these enzymes or compromised adenosine generation can result in bone disorders, such as osteoporosis and impaired fracture healing. This mini review discusses the key role played by adenosine in bone health and how its alterations contribute to bone diseases, as well as potential therapeutic applications of exogenous adenosine to combat bone diseases like osteoporosis and injury.

Novel alendronate-CGS21680 conjugate reduces bone resorption and induces new bone formation in post-menopausal osteoporosis and inflammatory osteolysis mouse models

Loss of bone is a common medical problem and, while it can be treated with available therapies, some of these therapies have critical side effects. We have previously demonstrated that CGS21680, a selective A2A adenosine receptor agonist, prevents bone loss, but its on-target toxicities (hypotension, tachycardia) and frequent dosing requirements make it unusable in the clinic. We therefore generated a novel alendronate-CGS21680 conjugate (MRS7216), to target the agonist to bone where it remains for long periods thereby diminishing the frequency of administration and curtailing side effects. MRS7216 was synthesized from CGS21680 by sequential activation of the carboxylic acid moiety and reacting with an appropriate amino acid (PEG, alendronic acid) under basic conditions. MRS7216 was tested on C57BL/6J (WT) mice with established osteoporosis (OP) and WT or A2A KO mice with wear particle-induced inflammatory osteolysis (OL). Mice were treated weekly with MRS7216 (10mg/kg). Bone formation was studied after in vivo labeling with calcein/Alizarin Red, and μCT and histology analyses were performed. In addition, human primary osteoblasts and osteoclasts were cultured using bone marrow discarded after hip replacement. Receptor binding studies demonstrate that MRS7216 efficiently binds the A2A adenosine receptor. MRS7216-treated OP and OL mice had significant new bone formation and reduced bone loss compared to vehicle or alendronate-treated mice. Histological analysis showed that MRS7216 treatment significantly reduced osteoclast number and increased osteoblast number in murine models. Interestingly, cultured human osteoclast differentiation was inhibited, and osteoblast differentiation was stimulated by the compound indicating that MRS7216 conjugates represent a novel therapeutic approach to treat osteoporosis and osteolysis.

CD73 Attenuates Alcohol-Induced Liver Injury and Inflammation via Blocking TLR4/MyD88/NF-κB Signaling Pathway

Background

Alcoholic liver disease (ALD) is liver damage caused by long-term drinking. Inflammation plays a central role in the progression of ALD. CD73 is a ubiquitously expressed glycosylphosphatidylinositol-anchored glycoprotein that is a key enzyme that converts ATP into adenosine. Evidence has shown that CD73 plays an important role in many diseases, but the role and mechanism of CD73 in alcohol-induced liver injury and inflammation is still unclear.

Methods

The alcohol-induced liver injury and inflammation mouse model was established. The rAAV9-CD73 was used to overexpress CD73. Isolation of primary macrophages (MΦ) from the liver was conducted. The effects of CD73 on alcohol-induced liver injury and inflammation were evaluated by quantitative real‑time PCR, Western blotting, ELISA, and immunohistochemical assay. Flow cytometry was used to detect the cell cycle and apoptosis.

Results

Our results showed that overexpression of CD73 can reduce alcohol-induced liver damage, lipid accumulation, and the secretion of inflammatory cytokines. pEX3-CD73 can promote RAW264.7 cells proliferation and inhibit apoptosis via suppressing the activation of TLR4/MyD88/NF-κB signaling pathway. Inhibition of TLR4 further enhanced the anti-inflammatory effect of overexpression of CD73.

Conclusion

Overexpression of CD73 can reduce alcohol-induced liver injury and inflammation. CD73 may serve as a potential therapeutic target for ALD.

Bone marrow CD73+ mesenchymal stem cells display increased stemness in vitro and promote fracture healing in vivo

Mesenchymal stem cells (MSCs) are multipotent and considered to be of great potential for regenerative medicine. We could show recently (Breitbach, Kimura et al. 2018) that a subpopulation of MSCs as well as sinusoidal endothelial cells (sECs) in the bone marrow (BM) of CD73-EGFP reporter mice could be labeled in vivo.

We took advantage of this model to explore the plasticity and osteogenic potential of CD73-EGFP⁺ MSCs in vitro and their role in the regenerative response upon bone lesion in vivo. Herein we show that isolated CD73-EGFP⁺ MSCs displayed more pronounced stemness and stronger in vitro differentiation capacity into the osteogenic lineage compared to CD73-EGFP⁻ MSCs. In a bone fracture model, endogenous BM-resident CD73-EGFP⁺ MSCs were found to migrate to the fracture site and differentiate into cartilage and bone cells. Our analysis also showed that CD73-EGFP⁺ sECs contributed to the neovascularization of the fracture site. In addition, grafting of CD73-EGFP⁺ MSCs into acute bone lesions revealed their capacity to differentiate into chondrocytes and osteocytes in vivo and their contribution to callus formation in the regeneration process of fracture healing. Thus, CD73⁺ MSCs display enhanced stemness and osteogenic differentiation potential in vitro and in vivo illustrating a prominent role of the CD73⁺ MSC subpopulation to promote fracture repair.

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CD73+ mesenchymal stem cells (MSCs) display pronounced proliferation potential.

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CD73+ MSCs show high in vitro osteogenic differentiation capacity.

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Endogenous bone marrow-resident CD73+ MSCs contribute to fracture healing.

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Grafted CD73+ MSCs promote bone formation upon fracture repair.

Resolution of inflammation in bone regeneration: From understandings to therapeutic applications

Impaired bone healing occurs in 5-10% of cases following injury, leading to a significant economic and clinical impact. While an inflammatory response upon injury is necessary to facilitate healing, its resolution is critical for bone tissue repair as elevated acute or chronic inflammation is associated with impaired healing in patients and animal models. This process is governed by important crosstalk between immune cells through mediators that contribute to resolution of inflammation in the local healing environment. Approaches modulating the initial inflammatory phase followed by its resolution leads to a pro-regenerative environment for bone regeneration. In this review, we discuss the role of inflammation in bone repair, the negative impact of dysregulated inflammation on bone tissue regeneration, and how timely resolution of inflammation is necessary to achieve normal healing. We will discuss applications of biomaterials to treat large bone defects with a specific focus on resolution of inflammation to modulate the immune environment following bone injury, and their observed functional benefits. We conclude the review by discussing future strategies that could lead to the realization of anti-inflammatory therapeutics for bone tissue repair.

Simulated Microgravity Remodels Extracellular Matrix of Osteocommitted Mesenchymal Stromal Cells

The extracellular matrix (ECM) is the principal structure of bone tissue. Long-term spaceflights lead to osteopenia, which may be a result of the changes in composition as well as remodeling of the ECM by osteogenic cells. To elucidate the cellular effects of microgravity, human mesenchymal stromal cells (MSCs) and their osteocommitted progeny were exposed to simulated microgravity (SMG) for 10 days using random positioning machine (RPM). After RPM exposure, an imbalance of MSC collagen/non-collagen ratio at the expense of a decreased level of collagenous proteins was detected. At the same time, the secretion of proteases (cathepsin A, cathepsin D, MMP3) was increased. No significant effects of SMG on the expression of stromal markers and cell adhesion molecules on the MSC surface were noted. Upregulation of COL11A1, CTNND1, TIMP3, and TNC and downregulation of HAS1, ITGA3, ITGB1, LAMA3, MMP1, and MMP11 were detected in RPM exposed MSCs. ECM-associated transcriptomic changes were more pronounced in osteocommitted progeny. Thus, 10 days of SMG provokes a decrease in the collagenous components of ECM, probably due to the decrease in collagen synthesis and activation of proteases. The presented data demonstrate that ECM-associated molecules of both native and osteocommitted MSCs may be involved in bone matrix reorganization during spaceflight.

In Search of a Role for Extracellular Purine Enzymes in Bone Function

Bone is one of the major tissues that undergoes continuous remodeling throughout life, thus ensuring both organic body growth during development and protection of internal organs as well as repair of trauma during adulthood. Many endogenous substances contribute to bone homeostasis, including purines. Their role has increasingly emerged in recent decades as compounds which, by interacting with specific receptors, can help determine adequate responses of bone cells to physiological or pathological stimuli. Equally, it is recognized that the activity of purines is closely dependent on their interconversion or metabolic degradation ensured by a series of enzymes present at extracellular level as predominantly bound to the cell membrane or, also, as soluble isoforms. While the effects of purines mediated by their receptor interactions have sufficiently, even though not entirely, been characterized in many tissues including bone, those promoted by the extracellular enzymes providing for purine metabolism have not been. In this review, we will try to circumstantiate the presence and the role of these enzymes in bone to define their close relationship with purine activities in maintaining bone homeostasis in normal or pathological conditions.

Extracellular purines and bone homeostasis

Maintenance of a healthy skeleton is highly dependent on an intricate regulation of bone metabolism, as changes in the balance between bone formation and bone resorption leads to bone loss, bone fragility and ultimately bone fractures. During the last three decades it has become increasingly evident that physiological release of purines in the extracellular space is imperative for bone homeostasis and is orchestrated via the network of purinoceptors. Adenosine derivatives are released locally in the skeleton either by the bone forming osteoblasts or the bone degrading osteoclasts actioned directly by processes like mechanical loading and indirectly by systemic hormones. Adenosine derivatives directly affect the bone cells by their action on the membranal receptors or have co-stimulatory actions with bone active hormones such as parathyroid hormone or the gut hormones. Any deviations leading to increased levels of extracellular adenosine derivatives in the bone tissue such as in pathologic situations, trigger complex pathways with opposing effects on tissue health as presented by studies involving a range of model organisms. Pathological conditions where skeletal purinergic signaling is affected are following tissue injury like microdamage and macroscopic fractures; and during inflammatory processes where nucleotides and nucleosides play an important part in the pathophysiological skeletal response. Moreover, adenosine derivatives also play an important role in the interaction between malignant cells and bone cells in several types of cancers involving the skeleton, such as but not limited to multiple myeloma and bone osteolysis. Much knowledge has been gained over the last decades. The net- resulting phenotype of adenosine derivatives in bone (including the ratio of ATP to Adenosine) is highly dependent on CD39 and CD73 enzymes together with the expression and activity of the specific receptors. Thus, each component is important in the physiological and pathophysiological processes in bone. Promising perspectives await in the future in treating skeletal disorders with medications targeting the individual components of the purinergic signaling pathway.

Identification of ALP+/CD73+ defining markers for enhanced osteogenic potential in human adipose-derived mesenchymal stromal cells by mass cytometry

BACKGROUND

The impressive progress in the field of stem cell research in the past decades has provided the ground for the development of cell-based therapy. Mesenchymal stromal cells obtained from adipose tissue (AD-MSCs) represent a viable source for the development of cell-based therapies. However, the heterogeneity and variable differentiation ability of AD-MSCs depend on the cellular composition and represent a strong limitation for their use in therapeutic applications. In order to fully understand the cellular composition of MSC preparations, it would be essential to analyze AD-MSCs at single-cell level.

METHOD

Recent advances in single-cell technologies have opened the way for high-dimensional, high-throughput, and high-resolution measurements of biological systems. We made use of the cytometry by time-of-flight (CyTOF) technology to explore the cellular composition of 17 human AD-MSCs, interrogating 31 markers at single-cell level. Subcellular composition of the AD-MSCs was investigated in their naïve state as well as during osteogenic commitment, via unsupervised dimensionality reduction as well as supervised representation learning approaches.

RESULT

This study showed a high heterogeneity and variability in the subcellular composition of AD-MSCs upon isolation and prolonged culture. Algorithm-guided identification of emerging subpopulations during osteogenic differentiation of AD-MSCs allowed the identification of an ALP+/CD73+ subpopulation of cells with enhanced osteogenic differentiation potential. We could demonstrate in vitro that the sorted ALP+/CD73+ subpopulation exhibited enhanced osteogenic potential and is moreover fundamental for osteogenic lineage commitment. We finally showed that this subpopulation was present in freshly isolated human adipose-derived stromal vascular fractions (SVFs) and that could ultimately be used for cell therapies.

CONCLUSION

The data obtained reveal, at single-cell level, the heterogeneity of AD-MSCs from several donors and highlight how cellular composition impacts the osteogenic differentiation capacity. The marker combination (ALP/CD73) can not only be used to assess the differentiation potential of undifferentiated AD-MSC preparations, but also could be employed to prospectively enrich AD-MSCs from the stromal vascular fraction of human adipose tissue for therapeutic applications.

Bone Marrow and Adipose Tissue Adenosine Receptors Effect on Osteogenesis and Adipogenesis

Adenosine is an extracellular signaling molecule that is particularly relevant in times of cellular stress, inflammation and metabolic disturbances when the levels of the purine increase. Adenosine acts on two G-protein-coupled stimulatory and on two G-protein-coupled inhibitory receptors, which have varying expression profiles in different tissues and conditions, and have different affinities for the endogenous ligand. Studies point to significant roles of adenosine and its receptors in metabolic disease and bone health, implicating the receptors as potential therapeutic targets. This review will highlight our current understanding of the dichotomous effects of adenosine and its receptors on adipogenesis versus osteogenesis within the bone marrow to maintain bone health, as well as its relationship to obesity. Therapeutic implications will also be reviewed.

Signaling of the Purinergic System in the Joint

The joint is a complex anatomical structure consisting of different tissues, each with a particular feature, playing together to give mobility and stability at the body. All the joints have a similar composition including cartilage for reducing the friction of the movement and protecting the underlying bone, a synovial membrane that produces synovial fluid to lubricate the joint, ligaments to limit joint movement, and tendons for the interaction with muscles. Direct or indirect damage of one or more of the tissues forming the joint is the foundation of different pathological conditions. Many molecular mechanisms are involved in maintaining the joint homeostasis as well as in triggering disease development. The molecular pathway activated by the purinergic system is one of them.The purinergic signaling defines a group of receptors and intermembrane channels activated by adenosine, adenosine diphosphate, adenosine 5’-triphosphate, uridine triphosphate, and uridine diphosphate. It has been largely described as a modulator of many physiological and pathological conditions including rheumatic diseases. Here we will give an overview of the purinergic system in the joint describing its expression and function in the synovium, cartilage, ligament, tendon, and bone with a therapeutic perspective.

Human Dental Pulp Stem Cells in Rat Mandibular Bone Defects

This study aimed to evaluate the use of human dental pulp stem cells (hDPSCs) in non-critical-sized mandibular bone defects in rats. hDPSCs from permanent teeth were isolated and engrafted in mandibular bone defects in rats for 7, 14, and 28 days; bone defects without cells formed the control group. Samples were evaluated by scanning electron microscopy (SEM), light microscopy (hematoxylin and eosin staining), and the regeneration area was measured by the Image J program. Before surgery procedures, the human dental pulp cells were characterized as dental pulp stem cells: fusiform morphology, plastic-adherent; expression of CD105, CD73, and CD90; lack of expression of CD45 and CD34, and differentiated into osteoblasts, adipocytes, and chondroblasts. The results indicated that within 7 days the control group presented a pronounced bone formation when compared with the treated group (p < 0.05). After 14 days, the treated group showed an increase in bone formation, but with no statistical difference among the groups (p > 0.05). In the final evaluated period there was no difference between the control group and the treated group (p > 0.05). There was a significant difference between 7 and 14 days (p < 0.05) and between 7 and 28 days (p < 0.05) in the treated group. In conclusion, there is no evidence that the use of hDPSCs in the conditions of this study could improve bone formation in non-critical-sized mandibular bone defects.

Dysregulation of ectonucleotidase-mediated extracellular adenosine during postmenopausal bone loss

Adenosine and its receptors play a key role in bone homeostasis and regeneration. Extracellular adenosine is generated from CD39 and CD73 activity in the cell membrane, through conversion of adenosine triphosphate to adenosine monophosphate (AMP) and AMP to adenosine, respectively. Despite the relevance of CD39/CD73 to bone health, the roles of these enzymes in bona fide skeletal disorders remain unknown. We demonstrate that CD39/CD73 expression and extracellular adenosine levels in the bone marrow are substantially decreased in animals with osteoporotic bone loss. Knockdown of estrogen receptors ESR1 and ESR2 in primary osteoprogenitors and osteoclasts undergoing differentiation showed decreased coexpression of membrane-bound CD39 and CD73 and lower extracellular adenosine. Targeting the adenosine A2B receptor using an agonist attenuated bone loss in ovariectomized mice. Together, these findings suggest a pathological association of purine metabolism with estrogen deficiency and highlight the potential of A2B receptor as a target to treat osteoporosis.

Effect of cell culture density on dental pulp-derived mesenchymal stem cells with reference to osteogenic differentiation

Dental pulp stem cells (DPSCs) are a good source for tissue regeneration, however, the number of DPSCs in the pulp tissue is limited. Cell propagation is essential for tissue engineering using DPSCs and the cell culture conditions may affect the properties of DPSCs. The purpose of this study was to analyze the effect of cell culture condition, especially dense culture condition, on the property and differentiation pathway of DPSCs. We cultured DPSCs under sparse (sDPSCs; 5 × 10³ cells/cm²) or dense (dDPSCs; 1 × 10⁵ cells/cm²) conditions for 4 days and compared their properties. The populations of CD73⁺ and CD105⁺ cells were significantly decreased in dDPSCs. Both groups showed multi-differentiation potential, but mineralized nodule formation was enhanced in dDPSCs. The phosphorylation of focal adhesion kinase (FAK) and phosphoinositide 3-kinase (PI3K) proteins was promoted in dDPSCs, and alkaline phosphatase (ALP) mRNA expression in dDPSCs was abolished in the presence of pan-PI3K and FAK inhibitors. dDPSCs implanted into mouse bone cavities induced more mineralized tissue formation than sDPSCs and control. These findings indicate that dense culture conditions modified the properties of DPSCs and gave rise to osteogenic-lineage commitment via integrin signaling and suggest that dense culture conditions favor the propagation of DPSCs to be used for mineralized tissue regeneration.

[A novel calcium phosphate cement pre-loaded with chitosan and small molecule adenosine for repairing large cranial defects in rats]

OBJECTIVE

To evaluate the effect of a novel biomaterial in repairing large cranial defects in rats.

METHODS

Eighteen SD rats were used to establish rat modes of large cranial defect (8 mm in diameter). The rat models were randomized into 3 groups and the cranial defects were repaired using different scaffold materials, namely CPC paste prepared with distilled water (CPC control group), CPC paste mixed with 10% chitosan (CPC/CN group), or CPC paste with 10% chitosan and 300 mg adenosine (CPC/CN/AD group). The defects were examined 12 weeks after the surgery with X-ray, CT, HE staining and quantitative assessments.

RESULTS

X-ray showed that the defect was repaired in all the groups. The fracture line became obscure and the defects were almost fully repaired by regenerated bone tissues in CPC/CN/AD group, which was consistent with CT findings. In all the 3 groups, HE staining revealed the presence of new bones in the defects and new vessels in and around the new bones without inflammatory cells. The new bone area was significantly greater in CPC/CN/AD group than in CPC/CN group and CPC control group (P<0.05). The new vessel density was the highest in CPC/CN/AD group (P>0.05) but similar between CPC/CN group and CPC control group (P>0.05).

CONCLUSION

This novel calcium phosphate cement pre-loaded with chitosan and small molecule adenosine can better promote bone regeneration than calcium phosphate cement for repairing large bone defects to serve as a good replacement material for bone regeneration.