R-spondin 3 deletion induces Erk phosphorylation to enhance Wnt signaling and promote bone formation in the appendicular skeleton

Mapk7 enhances osteogenesis and suppresses adipogenesis by activating Lrp6/β-catenin signaling axis in mesenchymal stem cells

The lineage commitment and differentiation of mesenchymal stem cells (MSCs) play a crucial role in bone homeostasis. MAPK7 (Mitogen-activated protein kinase 7), a member of MAPK family, controls cell differentiation, proliferation and survival. However, the specific role of Mapk7 in regulating osteogenic and adipogenic differentiation of MSCs remains to be determined. In this study, depletion of Mapk7 in MSCs by crossing Prx1-Cre mice to Mapk7flox/flox resulted in severe low bone mass and accumulation of fat in bone marrow exhibiting osteoporosis (OP) in mice. Mapk7 promoted osteogenic differentiation and inhibited adipogenic differentiation of MSCs after knocking out and over-expressing Mapk7 in vitro. Mechanistically, Mapk7 activated Wnt/β-catenin signaling by phosphorylating Lrp6 at Ser1490, which ultimately enhanced osteogenesis and suppressed adipogenesis of MSCs. This is of great clinical and scientific significance for understanding biological function of Mapk7 and developing potential therapeutic targets for treatment of MSCs differentiation imbalance related bone diseases, such as, osteoporosis.

Isolation and characterization of bone mesenchymal cell small extracellular vesicles using a novel mouse model

Extracellular vesicles (EVs) are key mediators of cell-cell communication and are involved in transferring specific biomolecular cargo to recipient cells to regulate their physiological functions. A major challenge in the understanding of EV function in vivo is the difficulty ascertaining the origin of the EV particles. The recent development of the "Snorkel-tag," which includes EV-membrane-targeted CD81 fused to a series of extra-vesicular protein tags, can be used to mark EVs originating from a specific source for subsequent isolation and characterization. We developed an in vivo mouse model, termed "CAGS-Snorkel," which expresses the Snorkel-tag under the control of the Cre-lox system, and crossed this mouse with either Prx1-Cre (mesenchymal progenitors) or Ocn-Cre (osteoblasts/osteocytes) and isolated Snorkel-tagged EVs from the mouse bone marrow plasma using a magnetic bead affinity column. miRNA-sequencing was performed on the isolated EVs, and although similar profiles were observed, a few key miRNAs involved in bone metabolism (miR-106b-5p, miRs-19b-3p, and miRs-219a-5p) were enriched in the Ocn-derived relative to the Prx1-derived EV subpopulations. To characterize the effects of these small EVs on a bone cell target, cultured mouse bone marrow stromal cells were treated with Prx1 or Ocn EVs, and mRNA-sequencing was performed. Pathways involved in ossification, bone development, and extracellular matrix interactions were regulated by both EV subpopulations, whereas a few pathways including advanced glycation end-products signaling were uniquely regulated in the Ocn EV subpopulation, underlying important biological effects of specific EV subpopulations within the bone marrow microenvironment. These data demonstrate that EV isolation in vivo using the CAGS-Snorkel mouse model is a useful tool in characterizing the cargo and understanding the biology of tissue-specific EVs. Moreover, while bone mesenchymal cell populations share a common EV secretory profile, we uncover key differences based on the stage of osteoblastic differentiation that may have important biological consequences.

Jawbone periosteum-derived cells with high osteogenic potential controlled by R-spondin 3

The jawbone periosteum, the easily accessible tissue responding to bone repair, has been overlooked in the recent development of cell therapy for jawbone defect reconstruction. Therefore, this study aimed to elucidate the in vitro and in vivo biological characteristics of jawbone periosteum-derived cells (jb-PDCs). For this purpose, we harvested the jb-PDCs from 8-week-old C57BL/6 mice. The in vitro cultured jb-PDCs (passages 1 and 3) contained skeletal stem/progenitor cells and exhibited clonogenicity and tri-lineage differentiation capacity. When implanted in vivo, the jb-PDCs (passage 3) showed evident ectopic bone formation after 4-week subcutaneous implantation, and active contribution to repair the critical-size jawbone defects in mice. Molecular profiling suggested that R-spondin 3 was strongly associated with the superior in vitro and in vivo osteogenic potentials of jb-PDCs. Overall, our study highlights the significance of comprehending the biological characteristics of the jawbone periosteum, which could pave the way for innovative cell-based therapies for the reconstruction of jawbone defects.

Signaling pathways associated with Lgr6 to regulate osteogenesis

Fracture management largely relies on the bone's inherent healing capabilities and, when necessary, surgical intervention. Currently, there are limited osteoinductive therapies to promote healing, making targeting skeletal stem/progenitor cells (SSPCs) a promising avenue for therapeutic development. A limiting factor for this approach is our incomplete understanding of the molecular mechanisms governing SSPCs' behavior. We have recently identified that the Leucine-rich repeat-containing G-protein coupled receptor 6 (Lgr6) is expressed in sub-populations of SSPCs, and is required for maintaining bone volume during adulthood and for proper fracture healing. Lgr family members (Lgr4-6) are markers of stem cell niches and play a role in tissue regeneration primarily by binding R-Spondin (Rspo1-4). This interaction promotes canonical Wnt (cWnt) signaling by stabilizing Frizzled receptors. Interestingly, our findings here indicate that Lgr6 may also influence cWnt-independent pathways. Remarkably, Lgr6 expression was enhanced during Bmp-mediated osteogenesis of both human and murine cells. Using biochemical approaches, RNA sequencing, and bioinformatic analysis of published single-cell data, we found that elements of BMP signaling, including its target gene, pSMAD, and gene ontology pathways, are downregulated in the absence of Lgr6. Our findings uncover a molecular interdependency between the Bmp pathway and Lgr6, offering new insights into osteogenesis and potential targets for enhancing fracture healing.

Genetic predisposition to bone mineral density and their health conditions in East Asians

Osteoporosis, a condition defined by low BMD (typically < -2.5 SD), causes a higher fracture risk and leads to significant economic, social, and clinical impacts. Genome-wide studies mainly in Caucasians have found many genetic links to osteoporosis, fractures, and BMD, with limited research in East Asians (EAS). We investigated the genetic aspects of BMD in 86 716 individuals from the Taiwan Biobank and their causal links to health conditions within EAS. A genome-wide association study (GWAS) was conducted, followed by observational studies, polygenic risk score assessments, and genetic correlation analyses to identify associated health conditions linked to BMD. GWAS and gene-based GWAS studies identified 78 significant SNPs and 75 genes related to BMD, highlighting pathways like Hedgehog, WNT-mediated, and TGF-β. Our cross-trait linkage disequilibrium score regression analyses for BMD and osteoporosis consistently validated their genetic correlations with BMI and type 2 diabetes (T2D) in EAS. Higher BMD was linked to lower osteoporosis risk but increased BMI and T2D, whereas osteoporosis linked to lower BMI, waist circumference, hemoglobinA1c, and reduced T2D risk. Bidirectional Mendelian randomization analyses revealed that a higher BMI causally increases BMD in EAS. However, no direct causal relationships were found between BMD and T2D, or between osteoporosis and either BMI or T2D. This study identified key genetic factors for bone health in Taiwan, and revealed significant health conditions in EAS, particularly highlighting the genetic interplay between bone health and metabolic traits like T2D and BMI.

Cell signaling and transcriptional regulation of osteoblast lineage commitment, differentiation, bone formation, and homeostasis

The initiation of osteogenesis primarily occurs as mesenchymal stem cells undergo differentiation into osteoblasts. This differentiation process plays a crucial role in bone formation and homeostasis and is regulated by two intricate processes: cell signal transduction and transcriptional gene expression. Various essential cell signaling pathways, including Wnt, BMP, TGF-β, Hedgehog, PTH, FGF, Ephrin, Notch, Hippo, and Piezo1/2, play a critical role in facilitating osteoblast differentiation, bone formation, and bone homeostasis. Key transcriptional factors in this differentiation process include Runx2, Cbfβ, Runx1, Osterix, ATF4, SATB2, and TAZ/YAP. Furthermore, a diverse array of epigenetic factors also plays critical roles in osteoblast differentiation, bone formation, and homeostasis at the transcriptional level. This review provides an overview of the latest developments and current comprehension concerning the pathways of cell signaling, regulation of hormones, and transcriptional regulation of genes involved in the commitment and differentiation of osteoblast lineage, as well as in bone formation and maintenance of homeostasis. The paper also reviews epigenetic regulation of osteoblast differentiation via mechanisms, such as histone and DNA modifications. Additionally, we summarize the latest developments in osteoblast biology spurred by recent advancements in various modern technologies and bioinformatics. By synthesizing these insights into a comprehensive understanding of osteoblast differentiation, this review provides further clarification of the mechanisms underlying osteoblast lineage commitment, differentiation, and bone formation, and highlights potential new therapeutic applications for the treatment of bone diseases.

Latest Progress of LIPUS in Fracture Healing: A Mini-Review

The use of low-intensity pulsed ultrasound (LIPUS) for promoting fracture healing has been Food and Drug Administration (FDA)-approved since 1994 due to largely its non-thermal effects of sound flow sound radiation force and so on. Numerous clinical and animal studies have shown that LIPUS can accelerate the healing of fresh fractures, nonunions, and delayed unions in pulse mode regardless of LIPUS devices or circumstantial factors. Rare clinical studies show limitations of LIPUS for treating fractures with intramedullary nail fixation or low patient compliance. The biological effect is achieved by regulating various cellular behaviors involving mesenchymal stem/stromal cells (MSCs), osteoblasts, chondrocytes, and osteoclasts and with dose dependency on LIPUS intensity and time. Specifically, LIPUS promotes the osteogenic differentiation of MSCs through the ROCK-Cot/Tpl2-MEK-ERK signaling. Osteoblasts, in turn, respond to the mechanical signal of LIPUS through integrin, angiotensin type 1 (AT1), and PIEZO1 mechano-receptors, leading to the production of inflammatory factors such as COX-2, MCP-1, and MIP-1β fracture repair. LIPUS also induces CCN2 expression in chondrocytes thereby coordinating bone regeneration. Finally, LIPUS suppresses osteoclast differentiation and gene expression by interfering with the ERK/c-Fos/NFATc1 cascade. This mini-review revisits the known effects and mechanisms of LIPUS on bone fracture healing and strengthens the need for further investigation into the underlying mechanisms.

Aromatase deficiency in transplanted bone marrow cells improves vertebral trabecular bone quantity, connectivity, and mineralization and decreases cortical porosity in murine bone marrow transplant recipients

Estradiol is an important regulator of bone accumulation and maintenance. Circulating estrogens are primarily produced by the gonads. Aromatase, the enzyme responsible for the conversion of androgens to estrogen, is expressed by bone marrow cells (BMCs) of both hematopoietic and nonhematopoietic origin. While the significance of gonad-derived estradiol to bone health has been investigated, there is limited understanding regarding the relative contribution of BMC derived estrogens to bone metabolism. To elucidate the role of BMC derived estrogens in male bone, irradiated wild-type C57BL/6J mice received bone marrow cells transplanted from either WT (WT(WT)) or aromatase-deficient (WT(ArKO)) mice. MicroCT was acquired on lumbar vertebra to assess bone quantity and quality. WT(ArKO) animals had greater trabecular bone volume (BV/TV p = 0.002), with a higher trabecular number (p = 0.008), connectivity density (p = 0.017), and bone mineral content (p = 0.004). In cortical bone, WT(ArKO) animals exhibited smaller cortical pores and lower cortical porosity (p = 0.02). Static histomorphometry revealed fewer osteoclasts per bone surface (Oc.S/BS%), osteoclasts on the erosion surface (ES(Oc+)/BS, p = 0.04) and low number of osteoclasts per bone perimeter (N.Oc/B.Pm, p = 0.01) in WT(ArKO). Osteoblast-associated parameters in WT(ArKO) were lower but not statistically different from WT(WT). Dynamic histomorphometry suggested similar bone formation indices' patterns with lower mean values in mineral apposition rate, label separation, and BFR/BS in WT(ArKO) animals. Ex vivo bone cell differentiation assays demonstrated relative decreased osteoblast differentiation and ability to form mineralized nodules. This study demonstrates a role of local 17β-estradiol production by BMCs for regulating the quantity and quality of bone in male mice. Underlying in vivo cellular and molecular mechanisms require further study.

Upregulation of RSPO3 via targeted promoter DNA demethylation inhibits the progression of cholangiocarcinoma

BACKGROUND

Cholangiocarcinoma (CCA) refers to a collection of malignant tumors that develop from the biliary epithelium. Extensive clinical evidence and epidemiological observations indicate a concerning increase in both the incidence and mortality rates of CCA. Surgical resection is currently the sole available cure for CCA. However, it is unfortunate that only a fraction of patients has access to surgery at the time of diagnosis. Moreover, there is a high incidence of cancer recurrence after resection, and systemic treatments have limited efficacy. Therefore, the identification of novel biomarkers for CCA-targeted molecular therapy remains a crucial task in oncology research.

RESULTS

Our study demonstrated that low expression of RSPO3 was associated with poorer survival rates in patients with CCA. We found that the RSPO3 promoter DNA was hypermethylated in CCA, which was correlated with the low expression of RSPO3. The expression of RSPO3 was influenced by the balance between the DNA methyltransferase DNMT3a and the DNA demethylase TET1 in CCA. In vitro and in vivo experiments showed that targeting RSPO3 promoter DNA methylation using dCas9DNMT3a promoted tumorigenicity of CCA, while targeted RSPO3 promoter DNA demethylation using dCas9TET1CD inhibited CCA tumorigenicity. Additionally, in our primary CCA model, knockdown of Rspo3 promoted CCA progression, whereas overexpression of Rspo3 inhibited CCA progression.

CONCLUSIONS

Our findings suggest that increased methylation and decreased expression of RSPO3 may indicate a poor prognosis in CCA. Restoring RSPO3 expression by targeting promoter DNA demethylation could offer insights for precise treatment of CCA.

Effects of adipocyte-specific Dkk1 deletion on bone homeostasis and obesity-induced bone loss in male mice

The link between obesity and low bone strength has become a significant medical concern. The canonical Wnt signaling pathway is a key regulator of mesenchymal stem cell differentiation into either osteoblasts or adipocytes with active Wnt signaling promoting osteoblastogenesis. Our previous research indicated that Dickkopf-1 (Dkk1), a Wnt inhibitor, is upregulated in bone tissue in obesity and that osteoblast-derived Dkk1 drives obesity-induced bone loss. However, Dkk1 is also produced by adipocytes, but the impact of adipogenic Dkk1 on bone remodeling and its role in obesity-induced bone loss remain unclear. Thus, in this study, we investigated the influence of adipogenic Dkk1 on bone homeostasis and obesity-induced bone loss in mice. To that end, deletion of Dkk1 in adipocytes was induced by tamoxifen administration into 8-week-old male Dkk1fl/fl;AdipoQcreERT2 mice. Bone and fat mass were analyzed at 12 and 20 weeks of age. Obesity was induced in 8-week-old male Dkk1fl/fl;AdipoQcre mice with a high-fat diet (HFD) rich in saturated fats for 12 weeks. We observed that 12-week-old male mice without adipogenic Dkk1 had a significant increase in trabecular bone volume in the vertebrae and femoral bones. While histological and serological bone formation markers were not different, the number of osteoclasts and adipocytes was decreased in the vertebral bones of Dkk1fl/fl;AdipoQcre-positive mice. Despite the increased bone mass in 12-week-old male mice, at 20 weeks of age, there was no difference in the bone volume between the controls and Dkk1fl/fl;AdipoQcre-positive mice. Also, Dkk1fl/fl;AdipoQcre-positive mice were not protected from HFD-induced bone loss. Even though mRNA expression levels of Sost, another important Wnt inhibitor, in bone from Dkk1-deficient mice fed with HFD were decreased compared to Dkk1-sufficient mice on an HFD, this did not prevent the HFD-induced suppression of bone formation. In conclusion, adipogenic Dkk1 may play a transient role in bone mass regulation during adolescence, but it does not contribute to bone homeostasis or obesity-induced bone loss later in life.