Controls Bone Homeostasis Research Articles

Cortactin controls bone homeostasis through regulating the differentiation of osteoblasts and osteoclasts.

Cortactin (CTTN), a cytoskeletal protein and substrate of Src kinase, is implicated in tumor aggressiveness. However, its role in bone cell differentiation remains unknown. The current study revealed that CTTN was upregulated during osteoblast and adipocyte differentiation. Functional experiments demonstrated that CTTN promoted the in vitro differentiation of mesenchymal stem/progenitor cells into osteogenic and adipogenic lineages. Mechanistically, CTTN was able to stabilize the protein level of mechanistic target of rapamycin kinase (mTOR), leading to the activation of mTOR signaling. In-depth investigation revealed that CTTN could bind with casitas B lineage lymphoma-c (c-CBL) and counteract the function of c-CBL, a known E3 ubiquitin ligase responsible for the proteasomal degradation of mTOR. Silencing c-Cbl alleviated the impaired differentiation of osteoblasts and adipocytes caused by CTTN siRNA, while silencing mTOR mitigated the stimulation of osteoblast and adipocyte differentiation induced by CTTN overexpression. Notably, transplantation of CTTN-silenced bone marrow stromal cells (BMSCs) into the marrow of mice led to a reduction in trabecular bone mass, accompanied by a decrease in osteoblasts and an increase in osteoclasts. Furthermore, CTTN-silenced BMSCs expressed higher levels of receptor activator of nuclear factor κB ligand (RANKL) than control BMSCs did and promoted osteoclast differentiation when cocultured with bone marrow-derived osteoclast precursor cells. This study provides evidence that CTTN favors osteoblast differentiation by counteracting the c-CBL-induced degradation of mTOR and inhibits osteoclast differentiation by downregulating the expression of RANKL. It also suggests that maintaining an appropriate level of CTTN expression may be advantageous for maintaining bone homeostasis.

OR29-01 Deleting Salt Inducible Kinases In Osteoprogenitor Cells Increases Trabecular Bone

Abstract Disclosure: G. Papaioannou: None. A. Morellato Alcantara: None. K. Strauss: None. H.M. Kronenberg: None. M.N. Wein: Other; Self; Radius Health, Inc, research funding and patent royalties, holds equity in and is a scientific advisory board member of Relation Therapeutics, consultant for AstraZeneca, Guidepoint LLC, and Aditum Bio. Introduction: Osteoblasts are continuously generated by the differentiation of skeletal progenitor cells. Osx positive cells are committed osteoprogenitors. Ebf3 positive cells are a group of bone marrow stromal cells marked by CXCL12 expression that are thought to contribute to osteoblasts in adult mice. Salt-inducible kinases (SIKs) are key kinases that control the skeletal actions of parathyroid hormone. We previously reported that deletion of SIK genes in osteoblasts/osteocytes (using Dmp1-Cre) or ubiquitously (using Ub-CreERt2) causes dramatic increases of trabecular bone and stromal cells. However, the specific osteoblast progenitor populations through which SIKs control bone homeostasis downstream of PTH signaling are still to be determined. Methods: SIK genes were deleted from Osx or Ebf3 positive cells in 3-day-old, 6- or 9-week-old mice using the CreERt2 (tamoxifen inducible) system. OsxCreERt2 mice were crossed with animals harboring floxed Sik2 and Sik3 genes. Ebf3CreERt2 mice were crossed with Sik1, Sik2, and Sik3 floxed mice. These mice also bear a tdTomato reporter that labels cells and their descendants upon Cre recombination. After sacrifice, tibiae were fixed in paraformaldehyde or formalin and decalcified in EDTA for frozen or paraffin sections respectively. Results: In OsxCreERt2; Sik2f/f;Sik3f/f;R26R-Tomato mice tamoxifen was injected at 6 weeks old or P3 and tibias were harvested 4 weeks later. In both cases there was a dramatic increase in trabecular bone mass, that filled a large part of the bone marrow, and an increase in stromal cells. Most of the osteoblasts and many of the stromal cells were tomato positive, indicating that they derive from the osteoprogenitors in which SIKs were deleted. In Ebf3CreERt2;Sik1f/f;Sik2f/f;Sik3f/f;R26R-Tomato mice, tamoxifen was injected at 9-weeks of age and tibiae examined 3 and 9 weeks later, or tamoxifen was given at 3 days of age and tibiae examined 4 weeks later. There were no obvious differences in H/E staining and no significant differences in the number or localization of cells labeled with tomato reporter. The number of tomato positive cells was small (compared to the OsxCreER model) at all ages examined. Some of these cells, were lining the bone, presumably becoming osteoblasts, but interestingly did not increase their number despite SIK deletion and in contrast with the OsxCreER model. Conclusion: Conditional deletion of SIK genes in Osx positive cells leads to a dramatic increase of trabecular bone and stromal cells in long bones suggesting a crucial role for SIKs in more committed osteoprogenitors and their descendants. The SIK pathway may not play a role in the subset of mesenchymal progenitor cells labeled with Ebf3CreERt2. Presentation: Sunday, June 18, 2023

The Role and Mechanism of MicroRNA 21 in Osteogenesis: An Update

MicroRNAs are short, single-stranded ribonucleic acids expressed endogenously in the body to regulate gene expression at the post-translational level, with exogenous microRNA offering an attractive approach to therapy. Among the myriad microRNA candidates involved in controlling bone homeostasis and remodeling, microRNA 21 (miR21) is the most abundant. This paper discusses the studies conducted on the role and mechanism of human miR21 (hsa-miR21) in the regulation of bones and the various pathways mediated by miR21, and explores the feasibility of employing exogenous miR21 as a strategy for promoting osteogenesis. From the literature review, it was clear that miR21 plays a dual role in bone metabolism by regulating both bone formation and bone resorption. There is substantial evidence to date from both in vitro and in vivo studies that exogenous miR21 can successfully accelerate new bone synthesis in the context of bone loss due to injury or osteoporosis. This supports the exploration of applications of exogenous miR21 in bone regenerative therapy in the future.

Low-intensity pulsed ultrasound regulates osteoblast-osteoclast crosstalk via EphrinB2/EphB4 signaling for orthodontic alveolar bone remodeling.

Background: The limited regenerative potential of periodontal tissue remains a challenge in orthodontic treatment, especially with respect to alveolar bone remodeling. The dynamic balance between the bone formation of osteoblasts and the bone resorption of osteoclasts controls bone homeostasis. The osteogenic effect of low-intensity pulsed ultrasound (LIPUS) is widely accepted, so LIPUS is expected to be a promising method for alveolar bone regeneration. Osteogenesis is regulated by the acoustic mechanical effect of LIPUS, while the cellular perception, transduction mode and response regulation mechanism of LIPUS stimuli are still unclear. This study aimed to explore the effects of LIPUS on osteogenesis by osteoblast-osteoclast crosstalk and the underlying regulation mechanism. Methods: The effects of LIPUS on orthodontic tooth movement (OTM) and alveolar bone remodeling were investigated via rat model by histomorphological analysis. Mouse bone marrow mesenchymal stem cells (BMSCs) and bone marrow monocytes (BMMs) were purified and used as BMSC-derived osteoblasts and BMM-derived osteoclasts, respectively. The osteoblast-osteoclast co-culture system was used to evaluate the effect of LIPUS on cell differentiation and intercellular crosstalk by Alkaline phosphatase (ALP), Alizarin Red S (ARS), tartrate-resistant acid phosphatase (TRAP) staining, real-time quantitative PCR, western blotting and immunofluorescence. Results: LIPUS was found to improve OTM and alveolar bone remodeling in vivo, promote differentiation and EphB4 expression in BMSC-derived osteoblasts in vitro, particularly when cells were directly co-cultured with BMM-derived osteoclasts. LIPUS enhanced EphrinB2/EphB4 interaction between osteoblasts and osteoclasts in alveolar bone, activated the EphB4 receptor on osteoblasts membrane, transduced LIPUS-related mechanical signals to the intracellular cytoskeleton, and gave rise to the nuclear translocation of YAP in Hippo signaling pathway, thus regulating cell migration and osteogenic differentiation. Conclusions: This study shows that LIPUS modulates bone homeostasis by osteoblast-osteoclast crosstalk via EphrinB2/EphB4 signaling, which benefits the balance between OTM and alveolar bone remodeling.

Strategies of Macrophages to Maintain Bone Homeostasis and Promote Bone Repair: A Narrative Review.

Bone homeostasis (a healthy bone mass) is regulated by maintaining a delicate balance between bone resorption and bone formation. The regulation of physiological bone remodeling by a complex system that involves multiple cells in the skeleton is closely related to bone homeostasis. Loss of bone mass or repair of bone is always accompanied by changes in bone homeostasis. However, due to the complexity of bone homeostasis, we are currently unable to identify all the mechanisms that affect bone homeostasis. To date, bone macrophages have been considered a third cellular component in addition to osteogenic spectrum cells and osteoclasts. As confirmed by co-culture models or in vivo experiments, polarized or unpolarized macrophages interact with multiple components within the bone to ensure bone homeostasis. Different macrophage phenotypes are prone to resorption and formation of bone differently. This review comprehensively summarizes the mechanisms by which macrophages regulate bone homeostasis and concludes that macrophages can control bone homeostasis from osteoclasts, mesenchymal cells, osteoblasts, osteocytes, and the blood/vasculature system. The elaboration of these mechanisms in this narrative review facilitates the development of macrophage-based strategies for the treatment of bone metabolic diseases and bone defects.

Phospholipase D2 controls bone homeostasis by modulating M-CSF-dependent osteoclastic cell migration and microtubule stability

Phospholipase D2 (PLD2), a signaling protein, plays a central role in cellular communication and various biological processes. Here, we show that PLD2 contributes to bone homeostasis by regulating bone resorption through osteoclastic cell migration and microtubule-dependent cytoskeletal organization. Pld2-deficient mice exhibited a low bone mass attributed to increased osteoclast function without altered osteoblast activity. While Pld2 deficiency did not affect osteoclast differentiation, its absence promoted the migration of osteoclast lineage cells through a mechanism involving M-CSF-induced activation of the PI3K–Akt–GSK3β signaling pathway. The absence of Pld2 also boosted osteoclast spreading and actin ring formation, resulting in elevated bone resorption. Furthermore, Pld2 deletion increased microtubule acetylation and stability, which were later restored by treatment with a specific inhibitor of Akt, an essential molecule for microtubule stabilization and osteoclast bone resorption activity. Interestingly, PLD2 interacted with the M-CSF receptor (c-Fms) and PI3K, and the association between PLD2 and c-Fms was reduced in response to M-CSF. Altogether, our findings indicate that PLD2 regulates bone homeostasis by modulating osteoclastic cell migration and microtubule stability via the M-CSF-dependent PI3K–Akt–GSK3β axis.

Long noncoding RNAs in mesenchymal stromal/stem cells osteogenic differentiation: Implications in osteoarthritis pathogenesis.

This letter focuses on a recently published article that provided an exceptional description of the effect of epigenetic modifications on gene expression patterns related to skeletal system remodeling. Specifically, it discusses a novel modality of epigenetic regulation, the long noncoding RNAs (lncRNAs), and provides evidence of their involvement in mesenchymal stromal/stem cells osteo-/adipo-genic differentiation balance. Despite focus on lncRNAs, there is an emerging cross talk between lncRNAs and miRNAs interaction as a novel mechanism in the regulation of the function of the musculoskeletal system, by controlling bone homeostasis and bone regeneration, as well as the osteogenic differentiation of stem cells. Thus, we touched on some examples to demonstrate this interaction. In addition, we believe there is still much to discover from the effects of lncRNAs on progenitor and non-progenitor cell differentiation. We incorporated data from other published articles to review lncRNAs in normal progenitor cell osteogenic differentiation, determined lncRNAs involved in osteoarthritis pathogenesis in progenitor cells, and provided a review of lncRNAs in non-progenitor cells that are differentially regulated in osteoarthritis. In conclusion, we really enjoyed reading this article and with this information we hope to further our under standing of lncRNAs and mesenchymal stromal/stem cells regulation.

Abstract 5723: JDP2 interacts with p53 and enhances p53 transactivation and stability

Abstract USAAP-1 protein mainly consists of proteins belonging to c-Jun, c-Fos, activating transcription factor (ATF), and Jun dimerization protein (JDP) family. In addition to AP-1 site, JDP2 also binds to cAMP responsive element (CRE) site in numerous cis-elements of the target genes. JDP2 has been shown to be involved in cancer development and cell-cycle regulation, suppression of adipocyte differentiation, and controlling bone homeostasis and antibacterial immunity. ATF3 and JDP2 share high similarity in C-terminal domains with identities of 65%. Previous studies have demonstrated that ATF3 can bind to p53 and regulate p53 transcriptional activity and stability. Though couple of studies have shown that JDP2 inhibits expression of p53 and p53 represses the transcriptional activity of JDP2 promoter; however, whether JDP2 directly interacts and regulates p53 transactivation remains largely unknown. Herein, we demonstrate for the first time that JDP2 can directly interact with p53. First, we found that both ATF3 and JDP2 can bind to p53 and the C-terminus of JDP2 is required for p53 binding. Secondly, in p53-null H1299 cells, JDP2 dose-dependently enhances p53 transactivation using p53RE-Luc (14X). Moreover, we demonstrated that JDP2 dose-dependently decreases p53 protein level in consistence with the previous reports. Finally, JDP2 increases p53 stability in CHX chase experiments. Taken together, our results demonstrate that JDP2 directly binds to p53 and enhances p53 transactivation and stability, suggesting that JDP2 is a novel regulator of p53. Citation Format: Wei-Hsiung Yang, Kasey Price, Leticia Cardoso, William Yang, Chiung-Min Wang, Richard Yang. JDP2 interacts with p53 and enhances p53 transactivation and stability [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5723.

IMPLICATION OF BONE MARROW ADIPOSE TISSUE IN BONE HOMEOSTASIS DURING OSTEOARTHRITIS

Purpose: Osteoarthritis (OA) is the most common age-related musculoskeletal disease with no curative treatment so far. Recently, it has been observed that bone remodeling is accompanied by an increase in the number of medullary adipocytes at the expense of osteoblasts in age-associated diseases. Some studies suggest that adipocytes from the bone marrow could play a role in bone homeostasis, yet their role in OA is to be discovered. In the present study, we hypothesize that bone marrow adipocytes (BMAds) are crucial cell partners in the control of bone homeostasis and that their physiological functions are deregulated in OA.

Effect of inflammation on bones in diabetic patients with periodontitis via RANKL/OPG system-A review.

Diabetes mellitus and periodontitis are inflammatory diseases, the severity of inflammation results in the progression and persistence of both the disorders and affects bones. Diabetic complications aggravate in diabetic subjects having periodontitis; similarly, diabetic patients are more prone to developing gingivitis and periodontitis. Periodontal and diabetic inflammation disturbs bone homeostasis, which possibly involves both innate and adaptive immune responses. The pathogenic processes that link the two diseases are the focus of much research and it is likely that upregulated inflammation arising from each condition adversely affects the other. RANKL/OPG pathway plays a prominent role in periodontal and diabetic inflammation and bone resorption. This review article summarises the literature on the link between inflammatory cytokines and the prevalence of disturbed bone homeostasis in diabetic patients with periodontitis. An extensive search was done in PubMed, Scopus, Medline and Google Scholar databases between April 2003 and May 2021. A total of 27 articles, including pilot studies, case-control studies, cross-sectional studies, cohort studies, randomized control trials, longitudinal studies, descriptive studies and experimental studies, were included in our literature review. Since RANKL/OPG are cytokines and have immune responses, regulating these cytokines expression will help control diabetes, periodontitis and bone homeostasis. The growing evidence of bone loss and increased fracture risk in diabetic patients with periodontitis makes it imperative that health professionals carry out planned treatment focusing on monitoring oral health in diabetic patients; bone markers should also be evaluated in patients with chronic periodontitis with an impaired glycemic state.

Epigenetics of Osteoporosis

AbstractFragile bone is the root cause of osteoporosis. For inherited or acquired reasons, the fragile bone does not provide sufficient fracture resistance to withstand the physical strains of a normal lifestyle. Accordingly, clinical characteristics consist of fragility fractures that occur during daily life activities or low energy trauma. Hip fractures and vertebral fractures are so called "major osteoporotic fractures”, that also cause the highest burden of disease. Although the clinical osteoporosis manifestations are relatively uniform, there is a vast spectrum of underlying molecular causes. Impaired bone formation, accelerated bone loss, and impaired lifetime adaptive regeneration according to physical impact characterize the cruder facets of osteoporosis. The signaling cascades that govern bone formation and metabolism may be altered by genetically or epigenetically inherited defects or acquired epigenetic changes due to tissue aging and/or underlying diseases. While molecular genetics and mechanisms and specific osteoporosis treatments have made impressive progress over the last three decades, there is still an urgent need to better understand the role of epigenetics in this disease.Epigenetic mechanisms such as covalent modifications of DNA, histones, or essential core factors like the osteogenic transcription factors (e. g., RUNX2) and inhibitory modulators of osteogenic WNT-signaling (e. g., Dickkopf-1 (DKK-1), sclerostin (SOST)) are all intricately implicated in developmental bone formation and adaptive regeneration and remodeling processes throughout adult life. These mechanisms are accompanied by chromatin architecture and gene expression changes of small (e. g., microRNAs (miRs)) and long, noncoding RNAs (lncRNAs). The timely execution of these mechanisms either facilitates or inhibits bone formation and remodeling. Together, epigenetic mechanisms controlling bone homeostasis widen the spectrum of potential dysregulations that can cause osteoporosis and open new avenues for therapeutic interventions.Apart from the core mechanisms of bone formation and regeneration, recent research revealed that tissue-resident cells of the immune system such as tissue-specific macrophages, myeloid precursors, and lymphocytes have surprisingly fundamental influence on tissue regeneration, including bone. Those tissue resident cells are also subject to epigenetic changes and may substantially contribute to the development of disease. Epigenetic constellations can be inherited, but the dynamic epigenetic mechanisms involved in physiological processes of tissue regeneration may also be affected by pathologies such as cellular aging and senescence. Recently, several studies aimed at identifying DNA methylation signatures in peripheral blood leukocytes from osteoporosis patients that reveal novel disease mechanisms and potential targets for diagnosis and treatment. Overall, these studies rendered, however, yet inconclusive results.By contrast, studies using bone marrow-derived skeletal progenitors identified transcriptome changes in osteoporosis patients, which could have epigenetic reasons in the absence of genetic causes. Respective changes may be related to the local milieu in bone and bone marrow as a kind of segmental attitude of a specific tissue acquired through tissue aging and/or supported by underlying aging-associated diseases such as arteriosclerosis or aging of cells of the immune system.In summary, there is cumulating evidence linking epigenetic factors to the pathogenesis of aging-associated osteoporosis. However, we are currently still limited in our knowledge with respect to the causal traits that are common, inherited, or acquired in a lifetime in the respective tissues and cells involved in bone formation and regeneration. During the following years, the field will most certainly learn more about molecular processes and factors that can be targeted therapeutically and/or used as biomarkers for risk assessment.

Molecular genetics and epigenetics of temporomandibular disorder

Temporomandibular joint (TMJ) has an important role in stomatognathic system. Its role during function is facilitated from rotation and translation movement. Any deviation from TMJ normal anatomy and movement could lead into either clicking, crepitus, or pain in preauricular area. These sign and symptoms, which are widely referred as TMJ Temporomandibular joint disorder (TMD), extremely common in world population. Several genes have been identified contribute in susceptibility towards TMD. Genetic polymorphism are a form of gene sequences variance that is found in more than 1% of world population. Epigenetics is an interaction between internal and external environments that leads to a change in chromatin structures that switches the gene expression on and off. There are several factors that posibly affect the genetic polymorphisms in TMD such as; serotonin, cathecolamine, estrogen, folate, human leukocyte antigen (HLA), extracellular matrix, transcription factors, transforming growth factor beta (TGFβ), epithelial growth factor, β-catenin, and discoidin. Epigenetic mechanisms such as DNA methylation, histone modification, and microRNA are found in chondrocyte of TMD patients. In a temporomandibular joint, miRNA-140 controls bone homeostasis especially on the articular remodeling. Genetic molecular and epigenetic study will benefit in diagnosis and treatment of TMD patient. The aim of this paper is author want to inform about molecular genetics and epigenetics of TMD.

Hedgehog Signaling Controls Bone Homeostasis by Regulating Osteogenic/Adipogenic Fate of Skeletal Stem/Progenitor Cells in Mice

Skeletal stem/progenitor cells (SSPCs) can differentiate into osteogenic or adipogenic lineage. The mechanism governing lineage allocation of SSPCs is still not completely understood. Hedgehog (Hh) signaling plays an essential role in specifying osteogenic fate of mesenchymal progenitors during embryogenesis. However, it is still unclear whether Hh signaling is required for lineage allocation of SSPCs in postnatal skeleton, and whether its dysregulation is related to age-related osteoporosis. Here, we demonstrated that Hh signaling was activated in metaphyseal SSPCs during osteogenic differentiation in the adult skeleton, and its activity decreased with aging. Inactivation of Hh signaling by genetic ablation of Smo, a key molecule in Hh signaling, in Osx-Cre-targeted SSPCs and hypertrophic chondrocytes led to decreased bone formation and increased bone marrow adiposity, two key pathological features of age-related osteoporosis. Moreover, we found that the bone-fat imbalance phenotype caused by Smo deletion mainly resulted from aberrant allocation of SSPCs toward adipogenic lineage at the expense of osteogenic differentiation, but not due to accelerated transdifferentiation of chondrocytes into adipocytes. Mechanistically, we found that Hh signaling regulated osteoblast versus adipocyte fate of SSPCs partly through upregulating Wnt signaling. Thus, our results indicate that Hh signaling regulates bone homeostasis and age-related osteoporosis by acting as a critical switch of cell fate decisions of Osx-Cre-targeted SSPCs in mice and suggest that Hh signaling may serve as a potential therapeutic target for the treatment of osteoporosis and other metabolic bone diseases. © 2021 American Society for Bone and Mineral Research (ASBMR).

MicroRNA-146a controls age-related bone loss.

Bone loss is one of the consequences of aging, leading to diseases such as osteoporosis and increased susceptibility to fragility fractures and therefore considerable morbidity and mortality in humans. Here, we identify microRNA‐146a (miR‐146a) as an essential epigenetic switch controlling bone loss with age. Mice deficient in miR‐146a show regular development of their skeleton. However, while WT mice start to lose bone with age, animals deficient in miR‐146a continue to accrue bone throughout their life span. Increased bone mass is due to increased generation and activity of osteoblasts in miR‐146a‐deficient mice as a result of sustained activation of bone anabolic Wnt signaling during aging. Deregulation of the miR‐146a target genes Wnt1 and Wnt5a parallels bone accrual and osteoblast generation, which is accompanied by reduced development of bone marrow adiposity. Furthermore, miR‐146a‐deficient mice are protected from ovariectomy‐induced bone loss. In humans, the levels of miR‐146a are increased in patients suffering fragility fractures in comparison with those who do not. These data identify miR‐146a as a crucial epigenetic temporal regulator which essentially controls bone homeostasis during aging by regulating bone anabolic Wnt signaling. Therefore, miR‐146a might be a powerful therapeutic target to prevent age‐related bone dysfunctions such as the development of bone marrow adiposity and osteoporosis.

PERK controls bone homeostasis through the regulation of osteoclast differentiation and function

Osteoclasts are multinucleated giant cells with the ability to degrade bone tissue, and are closely related to abnormal bone metabolic diseases. Endoplasmic reticulum (ER) is an organelle responsible for protein modification, quality control, and transportation. The accumulation of unfolded or misfolded proteins in ER cavity induces ER stress. Double-stranded RNA-dependent protein kinase-like ER kinase (PERK) is an ER stress-sensing protein, which is ubiquitous in eukaryotic cells. Systemic PERK knockout mice show severe bone loss, suggesting that PERK is of great significance for maintaining the normal growth and development of bone tissue, but the role of PERK in osteoclastogenesis is still unclear. In this study, we found that PERK was significantly activated during RANKL-induced osteoclast differentiation; knockdown of PERK by siRNA and inhibition of PERK by GSK2606414, respectively, had significant negative regulatory effects on the formation and bone resorption of osteoclasts. PERK inhibitor GSK2606414 down-regulated the mRNA levels and protein expression of osteoclast differentiation marker genes, and inhibited RANKL-induced activation of Mitogen-activated protein kinase (MAPK) and nuclear factor κB (NF-κB) pathways. Treatment with PERK inhibitor GSK2606414 in ovariectomized mouse model significantly suppressed bone loss and osteoclast formation. Thapsigargin activated ER stress to enhance autophagy, while GSK2606414 had a significant inhibitory effect on autophagy flux and autophagosome formation. Antioxidant N-acetylcysteine (NAC) could inhibit the expression of PERK phosphorylation, osteoclast-related proteins and autophagy-related proteins, but the use of PERK activator CCT020312 can reverse inhibition effect of NAC. Our findings demonstrate a key role for PERK in osteoclast differentiation and suggest its therapeutic potential.

CCR5 deficiency in aged mice causes a decrease in bone mass

The CC chemokine receptor 5 (CCR5) is a G protein-coupled receptor that regulates chemotaxis and effector functions of immune cells. It also serves as the major co-receptor for the entry of human immunodeficiency virus (HIV). Recently, CCR5 inhibitors have been developed and used for the treatment or prevention of HIV infections. Additionally, it has been identified that CCR5 controls bone homeostasis by regulating osteoclastogenesis and the communication between osteoblasts and osteoclasts. However, the effects of CCR5 inhibition on bone tissue in elderly patients are unknown. This study aimed to examine the bone phenotype of aged CCR5 knockout (KO) mice. Femoral and tibial bones were isolated from 12-month and 18-month old wild-type (WT) and CCR5 KO mice, and microcomputed tomography and histology analyses were performed. Twelve-month-old CCR5 KO mice exhibited a decreased trabecular bone mass and cortical bone thickness in both femoral and tibial bones compared with agematched WT mice. Eighteen-month-old mice also showed a decreased trabecular bone mass in femurs compared with control WT mice, but not in tibial bones. Unlike in 12-month-old mice, the cortical margin of femurs and tibias in 18-month-old mice were rough, likely because they were aggravated by the deficiency of CCR5. Overall, our data suggest that the deficiency of CCR5 with aging can cause severe bone loss. When CCR5 inhibitors or CCR5 inactivating technologies are used in elderly patients, a preventive strategy for bone loss should be considered.

Focal adhesion proteins Pinch1 and Pinch2 regulate bone homeostasis in mice.

Mammalian focal adhesion proteins Pinch1 and Pinch2 regulate integrin activation and cell-extracellular matrix adhesion and migration. Here, we show that deleting Pinch1 in osteocytes and mature osteoblasts using the 10-kb mouse Dmp1-Cre and Pinch2 globally (double KO; dKO) results in severe osteopenia throughout life, while ablating either gene does not cause bone loss, suggesting a functional redundancy of both factors in bone. Pinch deletion in osteocytes and mature osteoblasts generates signals that inhibit osteoblast and bone formation. Pinch-deficient osteocytes and conditioned media from dKO bone slice cultures contain abundant sclerostin protein and potently suppress osteoblast differentiation in primary BM stromal cells (BMSC) and calvarial cultures. Pinch deletion increases adiposity in the BM cavity. Primary dKO BMSC cultures display decreased osteoblastic but enhanced adipogenic, differentiation capacity. Pinch loss decreases expression of integrin β3, integrin-linked kinase (ILK), and α-parvin and increases that of active caspase-3 and -8 in osteocytes. Pinch loss increases osteocyte apoptosis in vitro and in bone. Pinch loss upregulates expression of both Rankl and Opg in the cortical bone and does not increase osteoclast formation and bone resorption. Finally, Pinch ablation exacerbates hindlimb unloading-induced bone loss and impairs active ulna loading-stimulated bone formation. Thus, we establish a critical role of Pinch in control of bone homeostasis.

Vdr expression in osteoclast precursors is not critical in bone homeostasis

The long-recognized role of the vitamin D endocrine system is to maintain stable serum calcium concentrations, which are ensured by a complex interplay between parathyroid gland, kidney, intestine, and bone. However, although VDR is expressed in osteoclastogenic cells, the contribution of VDR-mediated signaling to osteoclast differentiation and activity remains undefined. We therefore deleted Vdr expression efficiently and specifically in myeloid cells by use of M lysozyme-driven Cre expression, which targets granulocytes, monocytes, macrophages and osteoclasts (Vdrmyel− mice). Bone and calcium homeostasis were investigated under basal conditions and in conditions of increased bone remodeling, by feeding Vdrmyel− and Vdrmyel+ (wildtype) mice either a normal (1%) or a low (0.02%) calcium diet from weaning onwards. Vdrmyel− mice developed normally and were normocalcemic at the age of 8 weeks, both at the normal and the low calcium diet. No differences in trabecular or cortical bone mass were observed between Vdrmyel− mice and their wildtype littermates. Dietary calcium restriction resulted in a comparable reduction of trabecular bone mass (40%) and cortical thickness (48%) in Vdrmyel− and Vdrmyel+ mice, pointing to a massive transfer of calcium from the bone to the serum. In agreement with these results, osteoclastic differentiation of hematopoietic cells of Vdrmyel− mice, either induced by M-CSF and RANKL, or cocultured with osteoblasts, occurred as efficiently as osteoclastogenesis from Vdrmyel+ mice. In conclusion, our data do not support a role for osteoclastic Vdr signaling in the control of bone homeostasis.

PIP5k1β controls bone homeostasis through modulating both osteoclast and osteoblast differentiation.

PIP5k1β is crucial to the generation of phosphotidylinosotol (4, 5)P2. PIP5k1β participates in numerous cellular activities, such as B cell and platelet activation, cell phagocytosis and endocytosis, cell apoptosis, and cytoskeletal organization. In the present work, we aimed to examine the function of PIP5k1β in osteoclastogenesis and osteogenesis to provide promising strategies for osteoporosis prevention and treatment. We discovered that PIP5k1β deletion in mice resulted in obvious bone loss and that PIP5k1β was highly expressed during both osteoclast and osteoblast differentiation. Deletion of the gene was found to enhance the proliferation and migration of bone marrow-derived macrophage-like cells to promote osteoclast differentiation. PIP5k1β−/− osteoclasts exhibited normal cytoskeleton architecture but stronger resorption activity. PIP5k1β deficiency also promoted activation of mitogen-activated kinase and Akt signaling, enhanced TRAF6 and c-Fos expression, facilitated the expression and nuclear translocation of NFATC1, and upregulated Grb2 expression, thereby accelerating osteoclast differentiation and function. Finally, PIP5k1β enhanced osteoblast differentiation by upregulating master gene expression through triggering smad1/5/8 signaling. Therefore, PIP5k1β modulates bone homeostasis and remodeling.

The Emerging Role of Osteocytes in Cancer in Bone.

ABSTRACTAdvances in the last decade have established the osteocyte, the most abundant cell in bone, as a dynamic and multifunctional cell capable of controlling bone homeostasis by regulating the function of both osteoblasts and osteoclasts. In addition, accumulating evidence demonstrates that osteocyte function is altered in several skeletal disorders, and targeting osteocytes and their derived factors improves skeletal health. Despite the remarkable progress in our understanding of osteocyte biology, there has been a paucity of information regarding the role of osteocytes in the progression of cancer in bone. Exciting, recent discoveries suggest that tumor cells communicate with osteocytes to generate a microenvironment that supports the growth and survival of cancer cells and stimulates bone destruction. This review features these novel findings and discussions regarding the impact of chemotherapy on osteocyte function and the potential of targeting osteocytes for the treatment of cancer in bone. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.