Bone Tissue Homeostasis Research Articles

Loss of Runx2 in Gli1 + Osteogenic Progenitors Prevents Bone Loss Following Ovariectomy

Abstract Osteoporosis is a metabolic bone disorder characterized by low bone mass and bone mineral density. It is the most prevalent bone disease and a common cause of fracture in aging adults. Low bone mass, as seen in osteoporosis, results from an imbalance between osteoblast and osteoclast activity. Gli1+ cells are indispensable to the maintenance of bone tissue homeostasis. These cells give rise to osteoprogenitors and are present at the osteogenic fronts of long bones in adult mice. Runx2 is a key regulator of osteogenesis and plays a crucial role in osteoblastic differentiation and maturation during development. However, its function in maintaining adult bone tissue homeostasis remains unclear. In this study, we investigated the role of Runx2 in maintaining adult bone homeostasis in the context of ovariectomy-induced estrogen deficiency, a model for postmenopausal osteoporosis. Our results show that deletion of Runx2 in the Gli1+ osteogenic progenitor population prevents loss of both cortical and trabecular bone mass and mineralization after ovariectomy. At the cellular level, loss of Runx2 leads to a decrease in osteoclast activity. Our study indicates that Runx2 is essential for maintaining adult bone tissue homeostasis by regulating osteoclast differentiation.

Wnt3a-induced LRP6 phosphorylation enhances osteoblast differentiation to alleviate osteoporosis through activation of mTORC1/β-catenin signaling

ObjectiveOsteoporosis (OP) is a common cause of morbidity and mortality in older individuals. The importance of Wnt3a in osteogenic activity and bone tissue homeostasis is well known. Here, we explored the possible molecular mechanism by which Wnt3a mediates the LRP6/mTORC1/β-catenin axis to regulate osteoblast differentiation in OP. MethodsOP-related key genes were identified through a bioinformatics analysis. A ROS17/2.8 cell differentiation system for rat osteogenic progenitors and a rat model of senile OP were constructed for in vitro and in vivo mechanism verification. ResultsBioinformatics analysis revealed that LRP6 was poorly expressed in OP and may play a key role in the occurrence of OP by affecting osteoblast differentiation. LRP6 knockdown inhibited osteoblast differentiation in an in vitro model. In addition, Wnt3a promoted osteoblast differentiation by inducing LRP6 phosphorylation. Moreover, LRP6 promoted mTORC1 expression, which indirectly promoted β-catenin expression, thus promoting osteoblast differentiation. Finally, an in vivo assay revealed that LRP6 inhibition improved OP. ConclusionOur study provides evidence that Wnt3a induces phosphorylation of LRP6 to activate the mTORC1/β-catenin axis, thus promoting osteoblast differentiation and ultimately improving OP in aged rats.

Macrophage-derived amphiregulin promoted the osteogenic differentiation of chondrocytes through EGFR/Yap axis and TGF-β activation

Endochondral ossification represents a crucial biological process in skeletal development and bone defect repair. Macrophages, recognized as key players in the immune system, are now acknowledged for their substantial role in promoting endochondral ossification within cartilage. Concurrently, the epidermal growth factor receptor (EGFR) ligand amphiregulin (Areg) has been documented for its contributory role in restoring bone tissue homeostasis post-injury. However, the mechanism by which macrophage-secreted Areg facilitates bone repair remains elusive. In this study, the induction of macrophage depletion through in vivo administration of clodronate liposomes was employed in a standard open tibial fracture mouse model to assess bone healing using micro-computed tomography (micro-CT) analysis, histomorphology, and ELISA serum evaluations. The investigation revealed sustained expression of Areg during the fracture healing period in wild-type mice. Macrophage depletion significantly reduced the number of macrophages on the local bone surface and vital organs. This reduction led to diminished Areg secretion, decreased collagen production, and delayed fracture healing. However, histological and micro-CT assessments at 7 and 21 days post-local Areg treatment exhibited a marked improvement of bone healing compared to the vehicle control. In vitro studies demonstrated an increase of Areg secretion by the Raw264.7 cells upon ATP stimulation. Indirect co-culture of Raw264.7 and ATDC5 cells indicated that Areg overexpression enhanced the osteogenic potential of chondrocytes, and vice versa. This osteogenic promotion was attributed to Areg's activation of the membrane receptor EGFR in the ATDC5 cell line, the enhanced phosphorylation of transcription factor Yap, and the facilitation of the expression of bioactive TGF-β by chondrocytes. Collectively, this research elucidates the direct mechanistic effects of macrophage-secreted Areg in promoting bone homeostasis following bone injury.

HIV Modulates Osteoblast Differentiation via Upregulation of RANKL and Vitronectin.

Bone loss is a prevalent characteristic among people with HIV (PWH). We focused on mesenchymal stem cells (MSCs) and osteoblasts, examining their susceptibility to different HIV strains (R5- and X4-tropic) and the subsequent effects on bone tissue homeostasis. Our findings suggest that MSCs and osteoblasts are susceptible to R5- and X4-tropic HIV but do not support productive HIV replication. HIV exposure during the osteoblast differentiation process revealed that the virus could not alter mineral and organic matrix deposition. However, the reduction in runt-related transcription factor 2 (RUNX2) transcription, the increase in the transcription of nuclear receptor activator ligand kappa B (RANKL), and the augmentation of vitronectin deposition strongly suggested that X4- and R5-HIV could affect bone homeostasis. This study highlights the HIV ability to alter MSCs' differentiation into osteoblasts, critical for maintaining bone and adipose tissue homeostasis and function.

Adenosine diphosphate released from stressed cells triggers mitochondrial transfer to achieve tissue homeostasis.

Cell-to-cell mitochondrial transfer has recently been shown to play a role in maintaining physiological functions of cell. We previously illustrated that mitochondrial transfer within osteocyte dendritic network regulates bone tissue homeostasis. However, the mechanism of triggering this process has not been explored. Here, we showed that stressed osteocytes in mice release adenosine diphosphate (ADP), resulting in triggering mitochondrial transfer from healthy osteocytes to restore the oxygen consumption rate (OCR) and to alleviate reactive oxygen species accumulation. Furthermore, we identified that P2Y2 and P2Y6 transduced the ADP signal to regulate osteocyte mitochondrial transfer. We showed that mitochondrial metabolism is impaired in aged osteocytes, and there were more extracellular nucleotides release into the matrix in aged cortical bone due to compromised membrane integrity. Conditioned medium from aged osteocytes triggered mitochondrial transfer between osteocytes to enhance the energy metabolism. Together, using osteocyte as an example, this study showed new insights into how extracellular ADP triggers healthy cells to rescue energy metabolism crisis in stressed cells via mitochondrial transfer in tissue homeostasis.

Effect of genetically engineered drugs in aseptic necrosis of the femoral head in rats

Bone tissue is a dynamic structure with a metabolic function. The maintenance of bone homeostasis is carried out due to the continuous process of its renewal, remodeling. At the same time, a number of pathological processes, such as ischemic catastrophe, can lead to a violation of the balance of maintaining the constancy of the bone structure. One of these diseases is aseptic necrosis of the femoral head. The presented study analyzes the dynamics of expression of genes involved in maintaining bone tissue homeostasis, changes in the histological picture during the development of aseptic necrosis of the femoral head in laboratory rats that did not receive genetically engineered drugs and against the background of the use of inhibitors of biological action IL-6, TNF-α. After induction of aseptic necrosis in the proximal epiphysis of the femur, the histological picture in animals of different groups was not the same. More preserved bone architectonics and a larger volume of bone plates were recorded in rats receiving genetically engineered drugs compared to animals without the introduction of biological agents. The latter also had the most vivid picture of osteodestruction with increased expression of proinflammatory cytokine genes. In animals, against the background of the use of drugs of inhibitors of the biological action of IL-6, TNF-α, from the second week after induction of aseptic necrosis of the mRNA, the profile of the spongiose bone of the proximal epiphysis of the femur tended to increase the expression of osteoreparation genes. At the same time, the greatest inhibition of osteoclastogenesis gene expression was obtained in rats after injection of a monoclonal antibody to the IL-6 receptor.

Osteomesopyknosis associated with a novel ALOX5 variant that impacts the RANKL pathway.

Bone tissue homeostasis relies on the coordinated activity of the bone-forming osteoblasts and bone-resorbing osteoclasts. Osteomesopyknosis is considered a distinctive rare sclerosing skeletal disorder of unelucidated pathophysiology and presumably autosomal dominant transmission. However, the causal genes are unknown. We present a case report encompassing clinical assessments, imaging studies, and whole-exome sequencing analysis, complemented by functional invitro experiments. This new case of osteomesopyknosis was associated with a missense ALOX5 variant predicted to induce protein misfolding and proteasomal degradation. Transfection experiments demonstrated that the variant was associated with reduced protein levels restored by proteasomal inhibition with bortezomib. Likewise, gene expression analysis showed that the mutated gene was associated with a decreased RANKL/OPG ratio, which is a critical driver of osteoclast precursor differentiation. Our data indicate impaired bone resorption as the underlying mechanism of this rare osteosclerosis, implicating ALOX5 pathogenic variants as potential etiological factors.

Assessment of the dalargin effect on the level of vitamin D and melatonin in blood serum in patients with malignant bone tumors and polymorbidity

Background. Involved in many cascades of biological regulatory reactions, calcium, vitamin D and melatonin create a single metabolic pattern, maintaining the dynamic balance of bone tissue homeostasis. Nevertheless, these regulatory molecules, as structural and functional elements of bone tissue, play a crucial role in various cellular processes associated with oncogenesis and tumor progression. Synthetic peptide dalargin has wound healing, reparative, anti-inflammatory, antioxidant, immunomodulatory, anticarcinogenic, antitumor, and antimetastatic effects. The purpose of the study was to investigate the synthetic neuropeptide dalargin influence on the dynamics of vitamin D and melatonin blood serum le­vels as part of the treatment algorithm of patients with bone malignant tumors of the lower extremities and pelvis with polymorbidity and endoprosthetics. Materials and methods. The study inclu­ded a control (first) group and a main (second) group of clinical observation of patients with malignant bone lesions. The serum melatonin content was measured by enzyme immunoassay using reagent kits (No. RE54021) from IBL-Hamburg GmbH (Germany). Serum levels of 25-hydroxycalciferol (25(OH)D) were determined by the immunochemiluminescent method on an ARCHITECT 25-OH Vitamin D Controls automatic analyzer. Results. In patients who received dalargin, more pronounced positive dynamics were established in the normalization of 25(ОН)D and melatonin levels, and there was no exacerbation of concomitant diseases. In contrast to the data of patients who did not receive dalargin, in the group where it was used, there were no relapses or metastases of the tumor du­ring 14 months. A comparison of the 25(ОН)D and melatonin le­vels in patients 6–7 months and 12–14 months after treatment shows a tendency to recovery of indicators only in patients who were additionally treated with dalargin (p < 0.01). Conclusions. The study showed the possibility of using the neuropeptide dalargin to increase the effectiveness of the treatment of patients with primary malignant tumors with the burden of polymorbidity by influencing the levels of vitamin D and melatonin in the blood serum.

Treatment with Raloxifene Induces the Expression of Kisspeptin, Insulin, and Androgen Receptors in Bones of Castrated Adult Female Rats.

Objective To evaluate the effects of estrogen, raloxifene and genistein on the expression of KISS1 (kisspeptin), KISS1R (kisspeptin receptor), AR (androgen receptor) and INSR (insulin receptor) in the bones of ovariectomized rats. Methods Forty-eight adult rats were randomly divided into 6 groups, containing 8 animals each: G1-nonovariectomized control; G2-ovariectomized and treated with conjugated equine estrogens (50 µg/Kg/day); G3-ovariectomized and treated with raloxifene (0.75 mg/kg/day); G4-ovariectomized animal that received soy extract with genistein (300 mg/kg/day); G5-ovariectomized animal that received estrogen and genistein; and G6-ovariectomized animal that received estrogen and raloxifene. Three months after surgery, the castrated animals received the drugs orally daily for 120 days. All animals were sacrificed after this period, by deepening the anesthesia. The left tibia was removed for total RNA extraction and analysis of gene expression of KISS1 , KISS1R , AR and INSR , by quantitative real-time polymerase chain reaction (qRT-PCR). Results KISS1 was not detected in any of the treated groups. KISS1R , INSR and AR showed higher expression in the G3 group ( p < 0.001), while lower levels of transcripts for these genes were observed in G4 and G5. G2 animals showed hypoexpression of the evaluated genes. Conclusion The results indicate that raloxifene, alone or combined with estrogen, was able to induce the expression of genes associated with the recovery of bone tissue homeostasis in ovariectomized rats.

Single nucleotide polymorphism of osteoprotegerin as a possible biomarker of rheumatoid arthritis in Bashkir population of Chelyabinsk region

Rheumatoid arthritis (RA) is a multifactorial autoimmune rheumatic disease of unknown etiology characterized by chronic erosive arthritis. The protein osteoprotegerin (OPG) is a member of bone tissue homeostasis (RANK/RANKL/OPG) which is responsible for the regulation of osteoclast differentiation and osteolysis. The altered binding of RANKL and OPG is one of the causes of many diseases with increased production of pro-inflammatory cytokines, including rheumatoid arthritis. Polymorphic variants of the genes that control protective reactions could affect the level of production for encoded proteins and, thus, changing the course of immune response in RA. G1181C SNP in osteoprotegerin gene leads to disruption of its transcriptional activity and conformation of the protein itself, which can lead to an imbalance of pro- and anti-inflammatory cytokines. We analyzed the relationship between the TNFRSF11B gene polymorphism at the 1181 G C position and the risk of developing RA in the Bashkir population. The analysis was based on a molecular genetic study of single nucleotide polymorphism in groups of patients with rheumatoid arthritis and conditionally healthy individuals of the Bashkir population of the Chelyabinsk Region. Statistical evaluation was carried out using standard criteria generally accepted in immunogenetics. The 1181*GC polymorphism of the osteoprotegerin gene is characterized by interpopulation differences, which confirms the importance of using an ethnically identical comparison group to assess the association with a predisposition to multifactorial pathology. We showed that the carriage of genotype 1181 G/C was increased in the group of Bashkirs with rheumatoid arthritis. This genotype could be considered a biomarker of susceptibility to RA. No differences in the SNP allelic frequencies and genotypes were found among women with RA. Our research is a part of comprehensive assessment of the interaction of cytokines and their receptors from the TNFαsuperfamily as an immunogenetic component of RA genesis.

Epigenetic Reprogramming via Synergistic Hypomethylation and Hypoxia Enhances the Therapeutic Efficacy of Mesenchymal Stem Cell Extracellular Vesicles for Bone Repair.

Mesenchymal stem cells (MSCs) are a promising cell population for regenerative medicine applications, where paracrine signalling through the extracellular vesicles (EVs) regulates bone tissue homeostasis and development. MSCs are known to reside in low oxygen tension, which promotes osteogenic differentiation via hypoxia-inducible factor-1α activation. Epigenetic reprogramming has emerged as a promising bioengineering strategy to enhance MSC differentiation. Particularly, the process of hypomethylation may enhance osteogenesis through gene activation. Therefore, this study aimed to investigate the synergistic effects of inducing hypomethylation and hypoxia on improving the therapeutic efficacy of EVs derived from human bone marrow MSCs (hBMSCs). The effects of the hypoxia mimetic agent deferoxamine (DFO) and the DNA methyltransferase inhibitor 5-azacytidine (AZT) on hBMSC viability was assessed by quantifying the DNA content. The epigenetic functionality was evaluated by assessing histone acetylation and histone methylation. hBMSC mineralisation was determined by quantifying alkaline phosphate activity, collagen production and calcium deposition. EVs were procured from AZT, DFO or AZT/DFO-treated hBMSCs over a two-week period, with EV size and concentration defined using transmission electron microscopy, nanoflow cytometry and dynamic light scattering. The effects of AZT-EVs, DFO-EVs or AZT/DFO-EVs on the epigenetic functionality and mineralisation of hBMSCs were evaluated. Moreover, the effects of hBMSC-EVs on human umbilical cord vein endothelial cells (HUVECs) angiogenesis was assessed by quantifying pro-angiogenic cytokine release. DFO and AZT caused a time-dose dependent reduction in hBMSC viability. Pre-treatment with AZT, DFO or AZT/DFO augmented the epigenetic functionality of the MSCs through increases in histone acetylation and hypomethylation. AZT, DFO and AZT/DFO pre-treatment significantly enhanced extracellular matrix collagen production and mineralisation in hBMSCs. EVs derived from AZT/DFO-preconditioned hBMSCs (AZT/DFO-EVs) enhanced the hBMSC proliferation, histone acetylation and hypomethylation when compared to EVs derived from AZT-treated, DFO-treated and untreated hBMSCs. Importantly, AZT/DFO-EVs significantly increased osteogenic differentiation and mineralisation of a secondary hBMSC population. Furthermore, AZT/DFO-EVs enhanced the pro-angiogenic cytokine release of HUVECs. Taken together, our findings demonstrate the considerable utility of synergistically inducing hypomethylation and hypoxia to improve the therapeutic efficacy of the MSC-EVs as a cell-free approach for bone regeneration.

Bone Marrow Adipose Tissue: Regulation of Osteoblastic Niche, Hematopoiesis and Hematological Malignancies.

Bone marrow adipose tissue (BMAT) creates a specific microniche within multifunctional bone marrow (BM) ecosystem which imposes changes in surrounding cells and at systemic level. Moreover, BMAT contributes to spatial and temporal separation and metabolic compartmentalization of BM, thus regulating BM homeostasis and diseases. Recent findings have identified novel progenitor subsets of bone marrow adipocytes (BMAd)s recruited during the BM adipogenesis within different skeletal and hematopoietic stem cell niches. Potential of certain mesenchymal BM cells to differentiate into both osteogenic and adipogenic lineages, contributes to the complex interplay of BMAT with endosteal (osteoblastic) niche compartments as an important cellular player in bone tissue homeostasis. Targeting and ablation of BMAT cells at certain states might be an optional and promising strategy for improvement of bone health. Additionally, recent findings demonstrated spatial distribution of BMAds related to hematopoietic cells and pointed out important functional roles in the vital processes such as long-term hematopoiesis. BM adipogenesis appears to be an emergency phenomenon that follows the production of hematopoietic stem and progenitor cell niche factors, thus regulating physiological, stressed, and malignant hematopoiesis. Lipolytic and secretory activity of BMAds can influence survival and proliferation of hematopoietic cells at different maturation stages. Due to their different lipid status, constitutive and regulated BMAds are important determinants of normal and malignant hematopoietic cells. Further elucidation of cellular and molecular players involved in BMAT expansion and crosstalk with malignant cells is of paramount importance for conceiving the new therapies for improvement of BM health.

Hyperlipidemia impacts osteogenesis via lipophagy.

The mechanism of the impact of hyperlipidemia on bone tissue homeostasis is unclear, and the role of lipophagy is yet to be investigated. This study investigated changes in lipophagy and osteogenesis levels under hyperlipemic conditions and explored the effects of lipophagy on bone regeneration. In vivo, femurs of mice with diet-induced moderate hyperlipidemia were ground out with a ball drill to create defects. In vitro, mouse osteoblast cell lines were grown in two different concentrations of the high-fat medium. We found that at hyperphysiological of lipid conditions, activation of lipophagy restored osteoblast function in a way, and similar results were observed in mice with diet-induced hyperlipidemia. Still, at suprahyperphysiological concentrations of lipid culture, the activation of lipophagy further inhibited osteogenesis, and inhibition of autophagy instead promoted osteogenesis to a small extent. These results demonstrate that lipophagy functions differently in diverse high-fat environments, suggesting that cellular and organismal changes in response to high-fat stimuli are dynamic. This may provide new ideas for improving bone dysfunction caused by lipid metabolism disorders.

Anti-sclerostin antibodies: a new frontier in fragility fractures treatment.

Bone fragility is the determinant of the increased risk of minimal trauma fracture and must be treated with a multimodal approach that includes pharmacological therapy, physical exercise, and adequate nutrition. Pharmacological therapy, to date based on the administration of antiresorptive drugs, such as bisphosphonates and denosumab, or osteoanabolic drugs, such as teriparatide and abaloparatide, has shown to be effective in reducing the risk of fracture in osteoporotic patients. In the context of the cellular and molecular mechanisms that regulate bone metabolism, the discovery of the Wnt signaling pathway and its role in bone tissue homeostasis has allowed the identification of sclerostin as an inhibitor of osteoblastic activity and simultaneously as a stimulator of osteoclastic activity. Therefore, the use of a monoclonal antibody, romosozumab, against this protein has been tested as a potential drug with a dual action, stimulating bone neo-apposition and inhibiting bone resorption. The efficacy of romosozumab has been demonstrated in numerous clinical trials against both placebo and other drugs commonly used in the treatment of patients affected by osteoporosis. The advantages of this drug lie above all in its rapid action which makes it particularly suitable in clinical situations where it is necessary to improve bone strength very quickly due to the imminent risk of fragility fracture. Clinical studies and guidelines suggest romosozumab as an initial drug in an ideal sequential approach from osteoanabolic to antiresorptive drugs. Some aspects of cardiovascular safety remain to be fully investigated, therefore its use in osteoporotic patients at high cardiovascular risk should be avoided until further data become available.

Influence and Role of Vitamin D on Dental Implants

Dental implants are one of the methods for replacing missing teeth and their use has become an important part of dentistry. Dental implants have many advantages over traditional fixed partial dentures, including a high success rate of over 97% for 10 years among certain others. Vitamin D is involved in the calcium and phosphate metabolism of bone tissue in the body and in the maxillofacial region. This information can help dentists reduce the risk of complications after different types of surgery, like dental implant surgery and bone grafting. Due to its involvement in bone metabolic processes and regulation of the immune system, vitamin D is currently of particular interest to dentists performing implant procedures. It is believed that the correct concentration of this prohormone is positively correlated with the process of osseointegration. Numerous studies show that vitamin D is potentially important for the process of postoperative tissue repair, as well as the integration of the implant with bone tissue and bone homeostasis around the implant after it is loaded with a prosthetic crown. Furthermore, adequate serum levels of vitamin D may improve peri-implant bone healing. To date, only a few studies have investigated the possible association between serum vitamin D levels and early dental implant failure hence further clinical research is need of time to provide evidence -based results for the role of vitamin D in dental implants. The purpose of this research is to review the available information about influence and role of vitamin D on dental implants.

Molecular and cellular features of femoral head avascular necrosis: in vivo study

Aim. To study the molecular and cellular features of femoral head avascular necrosis in the rat model.Materials and Methods. Femoral head avascular necrosis was surgically induced in 8 rats with the 4-week follow-up. Then, the animals have been euthanised, and we performed gross, radiological, and histological examination of avascular and intact contralateral femoral heads. Systemic inflammation was assessed using enzyme-linked immunosorbent assay, and pro-inflammatory cytokines (interleukin-1β, interleukin-6, and tumor necrosis factor α). The proteomic profile of healthy and necrotic femoral heads was interrogated using sodium dodecyl sulfate polyacrylamide gel electrophoresis and ultra-high performance liquid chromatography-tandem mass spectrometry with ion mobility (TimsToF Pro).Results. Aseptic necrosis of the femoral head was successfully induced in all rats. Serum levels of pro-inflammatory cytokines (interleukin-1β and interleukin-6) were higher in rats with femoral head avascular necrosis as compared with healthy rats. Among the major proteins revealed at proteomic profiling were those involved in maintaining bone tissue homeostasis, calcium phosphate metabolism, angiogenesis, hematopoiesis, cell-cell interactions, chaperones, cartilage matrix proteins, collagen synthesis, and lipid metabolism. In bones with avascular necrosis, we have also found proteins regulating the inflammatory response and oxidative stress. Sodium dodecyl sulfate polyacrylamide gel electrophoresis indicate that the development of avascular osteonecrosis was accompanied by an overexpression of oxidative stress proteins, anaerobic glycolysis, and non-specific inflammatory response along with the downregulation of molecules responsible for angiogenesis, chondrogenesis, calcium phosphate metabolism, collagen synthesis, and cartilage matrix.Conclusion. Femoral head avascular necrosis is accompanied by non-specific inflammation, oxidative stress, and lipid peroxidation all presumably developed because of hypoxia and together contributing to bone destruction.

EMILIN1 deficiency causes arterial tortuosity with osteopenia and connects impaired elastogenesis with defective collagen fibrillogenesis

EMILIN1 (elastin-microfibril-interface-located-protein-1) is a structural component of the elastic fiber network and localizes to the interface between the fibrillin microfibril scaffold and the elastin core. How EMILIN1 contributes to connective tissue integrity is not fully understood. Here, we report bi-allelic EMILIN1 loss-of-function variants causative for an entity combining cutis laxa, arterial tortuosity, aneurysm formation, and bone fragility, resembling autosomal-recessive cutis laxa type 1B, due to EFEMP2 (FBLN4) deficiency. In both humans and mice, absence of EMILIN1 impairs EFEMP2 extracellular matrix deposition and LOX activity resulting in impaired elastogenesis, reduced collagen crosslinking, and aberrant growth factor signaling. Collagen fiber ultrastructure and histopathology in EMILIN1- or EFEMP2-deficient skin and aorta corroborate these findings and murine Emilin1-/- femora show abnormal trabecular bone formation and strength. Altogether, EMILIN1 connects elastic fiber network with collagen fibril formation, relevant for both bone and vascular tissue homeostasis.

Osteoimmunology: an interdisciplinary approach to studying the relationships between immune and bone cells

In this review, we discuss molecular and cellular mechanisms underlying cross-talk between immune cells and bone cells, both in healthy conditions and in some diseases. We provide short description of the main cell populations of bone tissue, i.e., osteoblasts, osteoclasts, osteocytes, bone marrow macrophages, OsteoMacs, and their effects on immune cells during bone modeling and remodeling. The data are presented on regulatory molecular pathways of bone marrow cell activity, T and B cells, macrophages, and formation of “endosteal niche” by the bone cells. We describe the key system of bone tissue homeostasis: RANK/RANKL/ OPG, which regulates differentiation of osteoclasts and bone destruction. In addition, RANK/RANKL/ OPG system modulates maturation and activity of various T and B cell subsets. We present the data on pleiotropic effects of T cells, B cells, dendritic cells, macrophage subpopulations, Tregs, NK cells, neutrophils upon differentiation and function of osteoblasts and osteoclasts. These effects promote accumulation and maintenance of the bone mass. We describe mechanisms of these effects based on direct cell-to-cell contacts and various soluble mediators and intracellular signaling pathways. A brief characteristic of some diseases is provided with concomitant dysfunction of immune cells and bone cells which play a decisive pathogenetic role (fractures, rheumatoid arthritis, periodontitis, postmenopausal osteoporosis, multiple myeloma). It was shown that the destructive bone inflammation, both in RA and periodontitis, leads to loss of bone mass, being featured by similar pathophysiological mechanisms involving immune and bone cell populations. Therapy of these diseases requires newer treatment strategies aimed not only at pro-inflammatory cytokines, but for increased bone resorption. We describe involvement of activated T cells, their cytokines into the pathogenesis of postmenopausal osteoporosis, thus providing a rationale for the novel term of “immunoporosis”, coined in 2018. The relationships between multiple myeloma cells and bone marrow microenvironment are provided. This cross-talk is based on contact cell-cell interactions, as well as due to effects of soluble mediators upon osteoclasts, stromal cells, and osteoblasts. These effects result in osteolysis, loss of bone mass, and myeloma progression. In conclusion, the relationships between the immune and bone cell populations suggest that they function as an entire regulatory system. This consideration provides a framework for the development of new therapeutic targets for the treatment of bone and immune system disorders.

The role of the endocannabinoid system in tooth eruption: An ex vivo study.

The aim was to characterise the endocannabinoid system (ECS) in the dental pulp of teeth at different stages of eruption. Pulp of: erupted premolars (EPM), third molars in pre-eruptive (PThM), intraosseous (IThM) and eruptive stages (EThM) (n= 12 each group) were used. Messenger RNA expression of components of the ECS as cannabinoid receptors (CBr1 and CBr2), and anandamide synthetizing (NAPE-PLD) and degradation (FAAH) enzymes were measured by RT-PCR. Data were analysed using Student's t-test for comparisons between two groups and one-way analysis of variance and Tukey's post-test for multiple comparisons (statistical significance: p< 0.05). mRNA expression of CBr2, NAPE-PLD and FAAH was similar in the studied stages, was lower in IThM than in PThM and EThM, and the lowest in EThM (p< 0.01); of note, CBr2 mRNA expression was not detected in EThM. CBr1 mRNA did not differ significantly between IThM and PThM but was lower in EThM (p< 0.01). The absence of CBr2 and presence of CBr1 in EThM suggest the involvement of the ECS via CBr1 as a mediator of tooth and bone tissue homeostasis during tooth eruption.

Gut Microbiota Ecosystem Governance of Host Inflammation, Mitochondrial Respiration and Skeletal Homeostasis.

Osteoporosis and osteoarthritis account for the leading causes of musculoskeletal dysfunction in older adults. Senescent chondrocyte overburden, inflammation, oxidative stress, subcellular organelle dysfunction, and genomic instability are prominent features of these age-mediated skeletal diseases. Age-related intestinal disorders and gut dysbiosis contribute to host tissue inflammation and oxidative stress by affecting host immune responses and cell metabolism. Dysregulation of gut microflora correlates with development of osteoarthritis and osteoporosis in humans and rodents. Intestinal microorganisms produce metabolites, including short-chain fatty acids, bile acids, trimethylamine N-oxide, and liposaccharides, affecting mitochondrial function, metabolism, biogenesis, autophagy, and redox reactions in chondrocytes and bone cells to regulate joint and bone tissue homeostasis. Modulating the abundance of Lactobacillus and Bifidobacterium, or the ratio of Firmicutes and Bacteroidetes, in the gut microenvironment by probiotics or fecal microbiota transplantation is advantageous to suppress age-induced chronic inflammation and oxidative damage in musculoskeletal tissue. Supplementation with gut microbiota-derived metabolites potentially slows down development of osteoarthritis and osteoporosis. This review provides latest molecular and cellular insights into the biological significance of gut microorganisms and primary and secondary metabolites important to cartilage and bone integrity. It further highlights treatment options with probiotics or metabolites for modulating the progression of these two common skeletal disorders.