Runt-related transcription factor 1 is required for murine osteoblast differentiation and bone formation

Regulation of Skeletal Development and Maintenance by Runx2 and Sp7

Runx2 (runt related transcription factor 2) and Sp7 (Sp7 transcription factor 7) are crucial transcription factors for bone development. The cotranscription factor Cbfb (core binding factor beta), which enhances the DNA-binding capacity of Runx2 and stabilizes the Runx2 protein, is necessary for bone development. Runx2 is essential for chondrocyte maturation, and Sp7 is partly involved. Runx2 induces the commitment of multipotent mesenchymal cells to osteoblast lineage cells and enhances the proliferation of osteoprogenitors. Reciprocal regulation between Runx2 and the Hedgehog, fibroblast growth factor (Fgf), Wnt, and parathyroid hormone-like hormone (Pthlh) signaling pathways and Dlx5 (distal-less homeobox 5) plays an important role in these processes. The induction of Fgfr2 (Fgf receptor 2) and Fgfr3 expression by Runx2 is important for the proliferation of osteoblast lineage cells. Runx2 induces Sp7 expression, and Runx2+ osteoprogenitors become Runx2+Sp7+ preosteoblasts. Sp7 induces the differentiation of preosteoblasts into osteoblasts without enhancing their proliferation. In osteoblasts, Runx2 is required for bone formation by inducing the expression of major bone matrix protein genes, including Col1a1 (collagen type I alpha 1), Col1a2, Spp1 (secreted phosphoprotein 1), Ibsp (integrin binding sialoprotein), and Bglap (bone gamma carboxyglutamate protein)/Bglap2. Bglap/Bglap2 (osteocalcin) regulates the alignment of apatite crystals parallel to collagen fibrils but does not function as a hormone that regulates glucose metabolism, testosterone synthesis, and muscle mass. Sp7 is also involved in Co1a1 expression and regulates osteoblast/osteocyte process formation, which is necessary for the survival of osteocytes and the prevention of cortical porosity. SP7 mutations cause osteogenesis imperfecta in rare cases. Runx2 is an important pathogenic factor, while Runx1, Runx3, and Cbfb are protective factors in osteoarthritis development.

The potential therapeutic role of curcumin in osteoporosis treatment: based on multiple signaling pathways

Osteoporosis is a common chronic metabolic bone disease caused by disturbances in normal bone metabolism and an imbalance between osteoblasts and osteoclasts. Osteoporosis is characterized by a decrease in bone mass and bone density, leading to increased bone fragility. Osteoporosis is usually treated with medications and surgical methods, but these methods often produce certain side effects. Therefore, the use of traditional herbal ingredients for the treatment of osteoporosis has become a focus of attention and a hot topic in recent years. Curcumin, widely distributed among herbs such as turmeric, tulip, and curcuma longa, contains phenolic, terpenoid, and flavonoid components. Modern pharmacological studies have confirmed that curcumin has a variety of functions including antioxidant and anti-inflammatory properties. In addition, curcumin positively regulates the differentiation and promotes the proliferation of osteoblasts, which play a crucial role in bone formation. Multiple studies have shown that curcumin is effective in the treatment of osteoporosis as it interacts with a variety of signaling pathway targets, thereby interfering with the formation of osteoblasts and osteoclasts and regulating the development of osteoporosis. This review summarized the key signaling pathways and their mechanisms of action of curcumin in the prevention and treatment of osteoporosis and analyzed their characteristics and their relationship with osteoporosis and curcumin. This not only proves the medicinal value of curcumin as a traditional herbal ingredient but also further elucidates the molecular mechanism of curcumin's anti-osteoporosis effect, providing new perspectives for the prevention and treatment of osteoporosis through multiple pathways.

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.

A newly identified gene Ahed plays essential roles in murine haematopoiesis

The development of haematopoiesis involves the coordinated action of numerous genes, some of which are implicated in haematological malignancies. However, the biological function of many genes remains elusive and unknown functional genes are likely to remain to be uncovered. Here, we report a previously uncharacterised gene in haematopoiesis, identified by screening mutant embryonic stem cells. The gene, 'attenuated haematopoietic development (Ahed)', encodes a nuclear protein. Conditional knockout (cKO) of Ahed results in anaemia from embryonic day 14.5 onward, leading to prenatal demise. Transplantation experiments demonstrate the incapacity of Ahed-deficient haematopoietic cells to reconstitute haematopoiesis in vivo. Employing a tamoxifen-inducible cKO model, we further reveal that Ahed deletion impairs the intrinsic capacity of haematopoietic cells in adult mice. Ahed deletion affects various pathways, and published databases present cancer patients with somatic mutations in Ahed. Collectively, our findings underscore the fundamental roles of Ahed in lifelong haematopoiesis, implicating its association with malignancies.

Runx1 alleviates osteoarthritis progression in aging mice

With rates growing quickly with age, osteoarthritis (OA) is the most common cause of chronic disability in aging persons. The discomfort and reduced motion associated with osteoarthritis have a significant impact on quality of life, and there is no known solution. Runt-related transcription factor 1(Runx1) has been shown to play a protective role in the development of osteoarthritis by promoting chondrogenesis. We had created models of ageing mice with osteoarthritis by anterior cruciate ligament transection (ACLT) and analyzed the effects of intra-articular injection of adeno-associated virus/Runx1 (AAV/Runx1) on the models. The results showed that the AAV/Runx1-group maintained better articular cartilage integrity and retained more proteoglycan than the OA group after injection of AAV-Runx1. The markers related to pathological changes in cartilage were downregulated, while the markers related to physiological changes in cartilage were upregulated. This suggests that Runx1 may impede OA progression on the knee joint of ageing mice, potentially playing a protective role in OA and becoming a probable treatment target for osteoarthritis among ageing patients in the future.

Cbfβ regulates Wnt/β-catenin, Hippo/Yap, and Tgfβ signaling pathways in articular cartilage homeostasis and protects from ACLT surgery-induced osteoarthritis

As the most common degenerative joint disease, osteoarthritis (OA) contributes significantly to pain and disability during aging. Several genes of interest involved in articular cartilage damage in OA have been identified. However, the direct causes of OA are poorly understood. Evaluating the public human RNA-seq dataset showed that CBFB (subunit of a heterodimeric Cbfβ/Runx1, Runx2, or Runx3 complex) expression is decreased in the cartilage of patients with OA. Here, we found that the chondrocyte-specific deletion of Cbfb in tamoxifen-induced Cbfbf/f;Col2a1-CreERT mice caused a spontaneous OA phenotype, worn articular cartilage, increased inflammation, and osteophytes. RNA-sequencing analysis showed that Cbfβ deficiency in articular cartilage resulted in reduced cartilage regeneration, increased canonical Wnt signaling and inflammatory response, and decreased Hippo/Yap signaling and Tgfβ signaling. Immunostaining and western blot validated these RNA-seq analysis results. ACLT surgery-induced OA decreased Cbfβ and Yap expression and increased active β-catenin expression in articular cartilage, while local AAV-mediated Cbfb overexpression promoted Yap expression and diminished active β-catenin expression in OA lesions. Remarkably, AAV-mediated Cbfb overexpression in knee joints of mice with OA showed the significant protective effect of Cbfβ on articular cartilage in the ACLT OA mouse model. Overall, this study, using loss-of-function and gain-of-function approaches, uncovered that low expression of Cbfβ may be the cause of OA. Moreover, Local admission of Cbfb may rescue and protect OA through decreasing Wnt/β-catenin signaling, and increasing Hippo/Yap signaling and Tgfβ/Smad2/3 signaling in OA articular cartilage, indicating that local Cbfb overexpression could be an effective strategy for treatment of OA.

Human umbilical cord mesenchymal stem cells overexpressing RUNX1 promote tendon-bone healing by inhibiting osteolysis, enhancing osteogenesis and promoting angiogenesis

BACKGROUND

Rotator cuff injury (RCI) is a common shoulder injury, which is difficult to be completely repaired by surgery. Hence, new strategies are needed to promote the healing of tendon-bone.

OBJECTIVE

We aimed to investigate the effect of human umbilical cord mesenchymal stem cells (hUC-MSCs) overexpressing RUNX1 on the tendon-bone healing after RCI, and to further explore its mechanism.

METHODS

Lentiviral vector was used to mediate the overexpression of RUNX1. RUNX1-overexpressed UCB-MSCs (referred to as MSC-RUNX1) were co-cultured with osteoclasts, and TRAP staining was performed to observe the formation of osteoclasts. Then MSC-RUNX1 was cultured in osteogenic differentiation medium, Alizarin red staining was conducted to detect osteogenic differentiation. The expression of markers of osteogenesis and osteoclast was detected by RT-qPCR. EA. hy926 cells were co-cultured with MSC-RUNX1. Transwell assay was used to detect the migration, and the expression of angiogenesis related-genes VEGF and TGF-β was detected by RT-qPCR. The rat rotator cuff reconstruction model was established and MSCs were injected at the tendon-bone junction. Biomechanical test and micro-CT scanning were performed, and HE, Masson and Alcian Blue staining were used for histological evaluation of tendon-bone healing. TUNEL and PCNA immunofluorescence (IF) staining were performed to evaluate apoptosis and proliferation at the tendon-bone healing site. The levels of TNF-α, IL-6 and IL-8 in serum were detected by ELISA. The expression of CD31 and Endomucin that related to angiogenesis was detected by IF. Safranin O-fast and TRAP/CD40L immunohistochemical staining were used to assess the levels of osteoclasts and osteoblasts at the tendon-bone healing site.

RESULTS

hUC-MSCs overexpressing RUNX1 inhibited osteoclast formation and promoted osteogenic differentiation. MSC-RUNX1 could promote the migration and tube formation of EA. hy926 cells, and up-regulate the levels of VEGF and TGF-β. Model mice treated with MSC-RUNX1 partially restored the biomechanical indexes. Treatment of MSC-RUNX1 obviously increased the bone density, accompanied by the formation of new bone. In vivo experiments showed that MSC-RUNX1 treatment could promote tendon-bone healing and inhibit inflammatory response in rats. MSC-RUNX1 treatment also promoted angiogenesis at the tendon-bone healing site, while inhibiting osteoclast formation and promoting osteogenic differentiation.

CONCLUSION

hUC-MSCs overexpressing RUNX1 can inhibit the formation of osteoclasts and differentiation of osteoblasts, promote angiogenesis and inhibit inflammation, thereby promoting tendon-bone healing after RCI.

[Latest Findings on the Role of RUNX1 in Bone Development and Disorders]

Runt-related transcription factor (RUNX1) is a transcription factor closely involved in hematopoiesis. RUNX1 gene mutation plays an essential pathogenic role in the initiation and development of hematological tumors, especially in acute myeloid leukemia. Recent studies have shown that RUNX1 is also involved in the regulation of bone development and the pathological progression of bone-related diseases. RUNX1 promotes the differentiation of mesenchymal stem cells into chondrocytes and osteoblasts and modulates the maturation and extracellular matrix formation of chondrocytes. The expression of RUNX1 in mesenchymal stem cells, chondrocytes, and osteoblasts is of great significance for maintaining normal bone development and the mass and quality of bones. RUNX1 also inhibits the differentiation and bone resorptive activities of osteoclasts, which may be influenced by sexual dimorphism. In addition, RUNX1 deficiency contributes to the pathogenesis of osteoarthritis, delayed fracture healing, and osteoporosis, which was revealed by the RUNX1 conditional knockout modeling in mice. However, the roles of RUNX1 in regulating the hypertrophic differentiation of chondrocytes, the sexual dimorphism of activities of osteoclasts, as well as bone loss in diabetes mellitus, senescence, infection, chronic inflammation, etc, are still not fully understood. This review provides a systematic summary of the research progress concerning RUNX1 in the field of bone biology, offering new ideas for using RUNX1 as a potential target for bone related diseases, especially osteoarthritis, delayed fracture healing, and osteoporosis.

IFT20 and WWTR1 govern bone homeostasis via synchronously regulating the expression and stability of TβRII in osteoblast lineage cells

Balance of bone and marrow fat formation is critical for bone homeostasis. The imbalance of bone homeostasis will cause various bone diseases, such as osteoporosis. However, the precise mechanisms governing osteoporotic bone loss and marrow adipose tissue (MAT) accumulation remain poorly understood. By analysis of publicly available databases from bone samples of osteoporosis patients, we found that the expression of intraflagellar transport 20 (IFT20) and WW domain containing transcription regulator 1 (WWTR1) were significantly downregulated in osteoblast lineage cells. Additionally, we found that double deletions of IFT20 and WWTR1 in osteoblasts resulted in a significant accumulation of MAT and bone loss. Moreover, IFT20 and WWTR1 deficiency in osteoblasts exacerbated bone-fat imbalance in ovariectomy (OVX)- and high-fat-diet (HFD)-induced osteoporosis mouse models. Mechanistically, we found that deletions of IFT20 and WWTR1 in osteoblasts synergistically inhibited osteogenesis and promoted adipogenesis and osteoclastogenesis. We also found that IFT20 interacted with TGF-β receptor type II (TβRII) to enhance TβRII stability by blocking c-Cbl-mediated ubiquitination and degradation of TβRII. WWTR1 transcriptionally upregulated TβRII expression by directly binding its promoter. These findings indicate that targeting IFT20/WWTR1 may be a potential therapeutic strategy for the treatment of osteoporosis.

Fgf10 mutant newts regenerate normal hindlimbs despite severe developmental defects

In amniote limbs, Fibroblast Growth Factor 10 (FGF10) is essential for limb development, but whether this function is broadly conserved in tetrapods and/or involved in adult limb regeneration remains unknown. To tackle this question, we established Fgf10 mutant lines in the newt Pleurodeles waltl which has amazing regenerative ability. While Fgf10 mutant forelimbs develop normally, the hindlimbs fail to develop and downregulate FGF target genes. Despite these developmental defects, Fgf10 mutants were able to regenerate normal hindlimbs rather than recapitulating the embryonic phenotype. Together, our results demonstrate an important role for FGF10 in hindlimb formation, but little or no function in regeneration, suggesting that different mechanisms operate during limb regeneration versus development.

Deficiency of Cbfβ in articular cartilage leads to osteoarthritis-like phenotype through Hippo/Yap, TGFβ, and Wnt/β-catenin signaling pathways

Osteoarthritis (OA) is the most prevalent degenerative joint disorder, causing physical impairments among the elderly. Core binding factor subunit β (Cbfβ) has a critical role in bone homeostasis and cartilage development. However, the function and mechanism of Cbfβ in articular cartilage and OA remains unclear. We found that Cbfβf/fAggrecan-CreERT mice with Cbfβ-deficiency in articular cartilage developed a spontaneous osteoarthritis-like phenotype with articular cartilage degradation. Immunofluorescence staining showed that Cbfβf/fAggrecan-CreERT mice exhibited a significant increase in the expression of articular cartilage degradation markers and inflammatory markers in the knee joints. RNA-sequencing analysis demonstrated that Cbfβ orchestrated Hippo/Yap, TGFβ/Smad, and Wnt/β-catenin signaling pathways in articular cartilage, and Cbfβ deficiency resulted in the abnormal expression of downstream genes involved in maintaining articular cartilage homeostasis. Immunofluorescence staining results showed Cbfβ deficiency significantly increased active β-catenin and TCF4 expression while reducing Yap, TGFβ1, and p-Smad 2/3 expression. Western blot and qPCR validated gene expression changes in hip articular cartilage of Cbfβ-deficient mice. Our results demonstrate that deficiency of Cbfβ in articular cartilage leads to an OA-like phenotype via affecting Hippo/Yap, TGFβ, and Wnt/β-catenin signaling pathways, disrupting articular cartilage homeostasis and leading to the pathological process of OA in mice. Our results indicate that targeting Cbfβ may be a potential therapeutic target for the design of novel and effective treatments for OA.

The roles and regulatory mechanisms of TGF-β and BMP signaling in bone and cartilage development, homeostasis and disease

Transforming growth factor-βs (TGF-βs) and bone morphometric proteins (BMPs) belong to the TGF-β superfamily and perform essential functions during osteoblast and chondrocyte lineage commitment and differentiation, skeletal development, and homeostasis. TGF-βs and BMPs transduce signals through SMAD-dependent and -independent pathways; specifically, they recruit different receptor heterotetramers and R-Smad complexes, resulting in unique biological readouts. BMPs promote osteogenesis, osteoclastogenesis, and chondrogenesis at all differentiation stages, while TGF-βs play different roles in a stage-dependent manner. BMPs and TGF-β have opposite functions in articular cartilage homeostasis. Moreover, TGF-β has a specific role in maintaining the osteocyte network. The precise activation of BMP and TGF-β signaling requires regulatory machinery at multiple levels, including latency control in the matrix, extracellular antagonists, ubiquitination and phosphorylation in the cytoplasm, nucleus-cytoplasm transportation, and transcriptional co-regulation in the nuclei. This review weaves the background information with the latest advances in the signaling facilitated by TGF-βs and BMPs, and the advanced understanding of their diverse physiological functions and regulations. This review also summarizes the human diseases and mouse models associated with disordered TGF-β and BMP signaling. A more precise understanding of the BMP and TGF-β signaling could facilitate the development of bona fide clinical applications in treating bone and cartilage disorders.

Bone Fracture-healing Properties and UPLC-MS Analysis of an Enriched Flavonoid Fraction from Oxystelma esculentum

Oxystelma esculentum has been used as a folk medicine to treat jaundice, throat infections, and skin problems. In the current study, the bone fracture-healing properties of a flavonoid-enriched fraction (Oxy50-60F) of O. esculentum were investigated in Swiss mice using a drill-hole injury model. Oxy50-60F (1 mg/kg/day, 5 mg/kg/day, and 10 mg/kg/day) was administered orally (from the next day) after a 0.6 mm drill-hole injury in mice femur mid-diaphysis for 7 days and 14 days. Parathyroid hormone (40 µg/kg; 5 times/week) was given subcutaneously as the positive control. Confocal imaging for bone regeneration, micro-architecture of femur bones, ex vivo mineralization, hematoxyline and eosin staining, measurement of reactive oxygen species, and gene expression of osteogenic and anti-inflammatory genes were studied. Quercetin, kaempferol, and isorhamnetin glycosides were identified in the active fraction using mass spectrometry techniques. Our results confirm that Oxy50-60F treatment promotes fracture healing and callus formation at drill-hole sites and stimulates osteogenic and anti-inflammatory genes. Oxy50-60F administration to fractured mice exhibited significantly better micro-CT parameters in a dose-dependent manner and promoted nodule mineralization at days 7 and 14 post-injury. Oxy50-60F also prevents ROS generation by increasing expression of the SOD2 enzyme. Overall, this study reveals that Oxy50-60F has bone regeneration potential in a cortical bone defect model, which supports its use in delayed-union and non-union fracture cases.

Cbfβ regulates Wnt/β-catenin, Hippo/Yap, and TGFβ signaling pathways in articular cartilage homeostasis and protects from ACLT surgery-induced osteoarthritis

As the most common degenerative joint disease, osteoarthritis (OA) contributes significantly to pain and disability during aging. Several genes of interest involved in articular cartilage damage in OA have been identified. However, the direct causes of OA are poorly understood. Evaluating the public human RNA-seq dataset showed that Cbfβ, (subunit of a heterodimeric Cbfβ/Runx1,Runx2, or Runx3 complex) expression is decreased in the cartilage of patients with OA. Here, we found that the chondrocyte-specific deletion of Cbfβ in tamoxifen-induced Cbfβf/fCol2α1-CreERT mice caused a spontaneous OA phenotype, worn articular cartilage, increased inflammation, and osteophytes. RNA-sequencing analysis showed that Cbfβ deficiency in articular cartilage resulted in reduced cartilage regeneration, increased canonical Wnt signaling and inflammatory response, and decreased Hippo/YAP signaling and TGF-β signaling. Immunostaining and western blot validated these RNA-seq analysis results. ACLT surgery-induced OA decreased Cbfβ and Yap expression and increased active β-catenin expression in articular cartilage, while local AAV-mediated Cbfβ overexpression promoted Yap expression and diminished active β-catenin expression in OA lesions. Remarkably, AAV-mediated Cbfβ overexpression in knee joints of mice with OA showed the significant protective effect of Cbfβ on articular cartilage in the ACLT OA mouse model. Overall, this study, using loss-of-function and gain-of-function approaches, uncovered that low expression of Cbfβ may be the cause of OA. Moreover, Local admission of Cbfβ may rescue and protect OA through decreasing Wnt/β-catenin signaling, and increasing Hippo/Yap signaling and TGFβ/Smad2/3 signaling in OA articular cartilage, indicating that local Cbfβ overexpression could be an effective strategy for treatment of OA.

Advances in the roles of ATF4 in osteoporosis

Osteoporosis (OP) is characterized by reduced bone mass, decreased strength, and enhanced bone fragility fracture risk. Activating transcription factor 4 (ATF4) plays a role in cell differentiation, proliferation, apoptosis, redox balance, amino acid uptake, and glycolipid metabolism. ATF4 induces the differentiation of bone marrow mesenchymal stem cells (BM-MSCs) into osteoblasts, increases osteoblast activity, and inhibits osteoclast formation, promoting bone formation and remodeling. In addition, ATF4 mediates the energy metabolism in osteoblasts and promotes angiogenesis. ATF4 is also involved in the mediation of adipogenesis. ATF4 can selectively accumulate in osteoblasts. ATF4 can directly interact with RUNT-related transcription factor 2 (RUNX2) and up-regulate the expression of osteocalcin (OCN) and osterix (Osx). Several upstream factors, such as Wnt/β-catenin and BMP2/Smad signaling pathways, have been involved in ATF4-mediated osteoblast differentiation. ATF4 promotes osteoclastogenesis by mediating the receptor activator of nuclear factor κ-B (NF-κB) ligand (RANKL) signaling. Several agents, such as parathyroid (PTH), melatonin, and natural compounds, have been reported to regulate ATF4 expression and mediate bone metabolism. In this review, we comprehensively discuss the biological activities of ATF4 in maintaining bone homeostasis and inhibiting OP development. ATF4 has become a therapeutic target for OP treatment.

Runt-related Transcription Factors and Gene Regulatory Mechanisms in Skeletal Development and Diseases

PURPOSE OF REVIEW

Runt-related transcription factors (RUNX) play critical roles in skeletal development, metabolism, and diseases. In mammals, three RUNX members, namely RUNX1, RUNX2, and RUNX3, play distinct and redundant roles, although RUNX2 is a dominant factor in skeletal development and several skeletal diseases. This review is to provide an overview of the current understanding of RUNX-mediated transcriptional regulation in different skeletal cell types.

RECENT FINDINGS

Advances in chromatin immunoprecipitation and next-generation sequencing (ChIP-seq) have revealed genome-wide RUNX-mediated gene regulatory mechanisms, including their association with cis-regulatory elements and putative target genes. Further studies with genome-wide analysis and biochemical assays have shed light on RUNX-mediated pioneering action and involvements of RUNX2 in lipid-lipid phase separation. Emerging multi-layered mechanisms of RUNX-mediated gene regulations help us better understanding of skeletal development and diseases, which also provides clues to think how genome-wide studies can help develop therapeutic strategies for skeletal diseases.

RUN(X) out of blood: emerging RUNX1 functions beyond hematopoiesis and links to Down syndrome

BACKGROUND

RUNX1 is a transcription factor and a master regulator for the specification of the hematopoietic lineage during embryogenesis and postnatal megakaryopoiesis. Mutations and rearrangements on RUNX1 are key drivers of hematological malignancies. In humans, this gene is localized to the 'Down syndrome critical region' of chromosome 21, triplication of which is necessary and sufficient for most phenotypes that characterize Trisomy 21.

MAIN BODY

Individuals with Down syndrome show a higher predisposition to leukemias. Hence, RUNX1 overexpression was initially proposed as a critical player on Down syndrome-associated leukemogenesis. Less is known about the functions of RUNX1 in other tissues and organs, although growing reports show important implications in development or homeostasis of neural tissues, muscle, heart, bone, ovary, or the endothelium, among others. Even less is understood about the consequences on these tissues of RUNX1 gene dosage alterations in the context of Down syndrome. In this review, we summarize the current knowledge on RUNX1 activities outside blood/leukemia, while suggesting for the first time their potential relation to specific Trisomy 21 co-occurring conditions.

CONCLUSION

Our concise review on the emerging RUNX1 roles in different tissues outside the hematopoietic context provides a number of well-funded hypotheses that will open new research avenues toward a better understanding of RUNX1-mediated transcription in health and disease, contributing to novel potential diagnostic and therapeutic strategies for Down syndrome-associated conditions.

A Novel RUNX1 Genetic Variant Identified in a Young Male with Severe Osteoporosis

This case describes a young man with an unusual cause of severe osteoporosis and markedly deranged bone microarchitecture resulting in multiple fractures. A potentially pathogenic germline variant in the runt-related transcription factor 1 (RUNX1) gene was discovered by a focused 51-gene myeloid malignancy panel during investigation for his unexplained normochromic normocytic anemia. Further bone-specific genetic testing and a pedigree analysis were declined by the patient. Recent experimental evidence demonstrates that RUNX1 plays a key role in the regulation of osteogenesis and bone homeostasis during skeletal development, mediated by the bone morphogenic protein and Wnt signaling pathways. Therefore, rarer causes of osteoporosis, including those affecting bone formation, should be considered in young patients with multiple unexpected minimal trauma fractures. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

MicroRNA-181a-5p Curbs Osteogenic Differentiation and Bone Formation Partially Through Impairing Runx1-Dependent Inhibition of AIF-1 Transcription

BACKGRUOUND

Evidence has revealed the involvement of microRNAs (miRNAs) in modulating osteogenic differentiation, implying the promise of miRNA-based therapies for treating osteoporosis. This study investigated whether miR-181a-5p influences osteogenic differentiation and bone formation and aimed to establish the mechanisms in depth.

METHODS

Clinical serum samples were obtained from osteoporosis patients, and MC3T3-E1 cells were treated with osteogenic induction medium (OIM) to induce osteogenic differentiation. miR-181a-5p-, Runt-related transcription factor 1 (Runx1)-, and/or allograft inflammatory factor-1 (AIF-1)-associated oligonucleotides or vectors were transfected into MC3T3-E1 cells to explore their function in relation to the number of calcified nodules, alkaline phosphatase (ALP) staining and activity, expression levels of osteogenesis-related proteins, and apoptosis. Luciferase activity, RNA immunoprecipitation, and chromatin immunoprecipitation assays were employed to validate the binding relationship between miR-181a-5p and Runx1, and the transcriptional regulatory relationship between Runx1 and AIF-1. Ovariectomy (OVX)-induced mice were injected with a miR-181a-5p antagonist for in vivo verification.

RESULTS

miR-181a-5p was highly expressed in the serum of osteoporosis patients. OIM treatment decreased miR-181a-5p and AIF-1 expression, but promoted Runx1 expression in MC3T-E1 cells. Meanwhile, upregulated miR-181a-5p suppressed OIM-induced increases in calcified nodules, ALP content, and osteogenesis-related protein expression. Mechanically, miR-181a-5p targeted Runx1, which acted as a transcription factor to negatively modulate AIF-1 expression. Downregulated Runx1 suppressed the miR-181a-5p inhibitor-mediated promotion of osteogenic differentiation, and downregulated AIF-1 reversed the miR-181a-5p mimic-induced inhibition of osteogenic differentiation. Tail vein injection of a miR-181a-5p antagonist induced bone formation in OVX-induced osteoporotic mice.

CONCLUSION

In conclusion, miR-181a-5p affects osteogenic differentiation and bone formation partially via the modulation of the Runx1/AIF-1 axis.

Hematopoietic Cell Autonomous Disruption of Hematopoiesis in a Germline Loss-of-function Mouse Model of RUNX1-FPD

RUNX1 familial platelet disorder (RUNX1-FPD) is a hematopoietic disorder caused by germline loss-of-function mutations in the RUNX1 gene and characterized by thrombocytopathy, thrombocytopenia, and an increased risk of developing hematologic malignancies, mostly of myeloid origin. Disease pathophysiology has remained incompletely understood, in part because of a shortage of in vivo models recapitulating the germline RUNX1 loss of function found in humans, precluding the study of potential contributions of non-hematopoietic cells to disease pathogenesis. Here, we studied mice harboring a germline hypomorphic mutation of one Runx1 allele with a loss-of-function mutation in the other Runx1 allele (Runx1 L148A/- mice), which display many hematologic characteristics found in human RUNX1-FPD patients. Runx1 L148A/- mice displayed robust and pronounced thrombocytopenia and myeloid-biased hematopoiesis, associated with an HSC intrinsic reconstitution defect in lymphopoiesis and expansion of myeloid progenitor cell pools. We demonstrate that specific deletion of Runx1 from bone marrow stromal cells in Prrx1-cre;Runx1 fl/fl mice did not recapitulate these abnormalities, indicating that the hematopoietic abnormalities are intrinsic to the hematopoietic lineage, and arguing against a driving role of the bone marrow microenvironment. In conclusion, we report a RUNX1-FPD mouse model faithfully recapitulating key characteristics of human disease. Findings do not support a driving role of ancillary, non-hematopoietic cells in the disruption of hematopoiesis under homeostatic conditions.

The roles of Runx1 in skeletal development and osteoarthritis: A concise review

Runt-related transcription factor-1 (Runx1) is well known for its functions in hematopoiesis and leukemia but recent research has focused on its role in skeletal development and osteoarthritis (OA). Deficiency of the Runx1 gene is fatal in early embryonic development, and specific knockout of Runx1 in cell lineages of cartilage and bone leads to delayed cartilage formation and impaired bone calcification. Runx1 can regulate genes including collagen type II (Col2a1) and X (Col10a1), SRY-box transcription factor 9 (Sox9), aggrecan (Acan) and matrix metalloproteinase 13 (MMP-13), and the up-regulation of Runx1 improves the homeostasis of the whole joint, even in the pathological state. Moreover, Runx1 is activated as a response to mechanical compression, but impaired in the joint with the pathological progress associated with osteoarthritis. Therefore, interpretation about the role of Runx1 could enlarge our understanding of key marker genes in the skeletal development and an increased understanding of Runx1 could be helpful to identify treatments for osteoarthritis. This review provides the most up-to-date advances in the roles and bio-mechanisms of Runx1 in healthy joints and osteoarthritis from all currently published articles and gives novel insights in therapeutic approaches to OA based on Runx1.

Circular RNA CircSLC8A1 contributes to osteogenic differentiation in hBMSCs via CircSLC8A1/miR-144-3p/RUNX1 in periprosthetic osteolysis

Circular RNAs (circRNAs) are often found in eukaryocyte and have a role in the pathogenesis of a variety of human disorders. Our related research has shown the differential expression of circRNAs in periprosthetic osteolysis (PPOL). However, the involvement of circRNAs in the exact process is yet unknown. CircSLC8A1 expression was evaluated in clinical samples and human bone marrow mesenchymal stem cells (hBMSCs) in this investigation using quantitative real-time PCR. In vitro and in vivo studies were conducted to explicate its functional role and pathway. We demonstrated CircSLC8A1 is involved in PPOL using gain- and loss-of-function methods. The association of CircSLC8A1 and miR-144-3p, along with miR-144-3p and RUNX1, was predicted using bioinformatics. RNA pull-down and luciferase assays confirmed it. The impact of CircSLC8A1 in the PPOL-mouse model was also investigated using adeno-associated virus. CircSLC8A1 was found to be downregulated in PPOL patients' periprosthetic tissues. Overexpression of CircSLC8A1 promoted osteogenic differentiation (OD) and inhibited apoptosis of hBMSCs in vitro. The osteogenic markers of RUNX1, osteopontin (OPN) and osteocalcin (OCN) were significantly upregulated in hBMSCs after miR-144-3p inhibitor was transferred. Mechanistic analysis demonstrated that CircSLC8A1 directly bound to miR-144-3p and participated in PPOL through the miR-144-3p/RUNX1 pathway in hBMSCs. Micro-CT and quantitative analysis showed that CircSLC8A1 markedly inhibited PPOL, and osteogenic markers (RUNX1, OPN and OCN) were significantly increased (P<0.05) in the mice model. Our findings prove that CircSLC8A1 exerted a regulatory role in promoting osteogenic differentiation in hBMSCs, and CircSLC8A1/miR-144-3p/RUNX1 pathway may provide a potential target for prevention of PPOL.

Different Requirements of CBFB and RUNX2 in Skeletal Development among Calvaria, Limbs, Vertebrae and Ribs

RUNX proteins, such as RUNX2, regulate the proliferation and differentiation of chondrocytes and osteoblasts. Haploinsufficiency of RUNX2 causes cleidocranial dysplasia, but a detailed analysis of Runx2+/- mice has not been reported. Furthermore, CBFB is required for the stability and DNA binding of RUNX family proteins. CBFB has two isoforms, and CBFB2 plays a major role in skeletal development. The calvaria, femurs, vertebrae and ribs in Cbfb2-/- mice were analyzed after birth, and compared with those in Runx2+/- mice. Calvarial development was impaired in Runx2+/- mice but mildly delayed in Cbfb2-/- mice. In femurs, the cortical bone but not trabecular bone was reduced in Cbfb2-/- mice, whereas both the trabecular and cortical bone were reduced in Runx2+/- mice. The trabecular bone in vertebrae increased in Cbfb2-/- mice but not in Runx2+/- mice. Rib development was impaired in Cbfb2-/- mice but not in Runx2+/- mice. These differences were likely caused by differences in the indispensability of CBFB and RUNX2, the balance of bone formation and resorption, or the number and maturation stage of osteoblasts. Thus, different amounts of CBFB and RUNX2 were required among the bone tissues for proper bone development and maintenance.

Runx1 is a key regulator of articular cartilage homeostasis by orchestrating YAP, TGFβ, and Wnt signaling in articular cartilage formation and osteoarthritis

Runt-related transcription factor 1 (Runx1) plays a key role in cartilage formation, but its function in articular cartilage formation is unclear. We generated non-inducible and inducible Runx1-deficient mice (Runx1f/fCol2α1-Cre and Runx1f/fCol2α1-CreER mice) and found that chondrocyte-specific Runx1-deficient mice developed a spontaneous osteoarthritis (OA)-like phenotype and showed exacerbated articular cartilage destruction under OA, characterized by articular cartilage degradation and cartilage ossification, with decreased Col2α1 expression and increased Mmp13 and Adamts5 expression. RNA-sequencing analysis of hip articular cartilage from the Runx1f/fCol2α1-Cre mice compared to that from wild-type mice and subsequent validation analyses demonstrated that Runx1 is a central regulator in multiple signaling pathways, converging signals of the Hippo/Yap, TGFβ/Smad, and Wnt/β-catenin pathways into a complex network to regulate the expression of downstream genes, thereby controlling a series of osteoarthritic pathological processes. RNA-sequencing analysis of mutant knee joints showed that Runx1's role in signaling pathways in articular cartilage is different from that in whole knee joints, indicating that Runx1 regulation is tissue-specific. Histopathologic analysis confirmed that Runx1 deficiency decreased the levels of YAP and p-Smad2/3 and increased the levels of active β-catenin. Overexpression of Runx1 dramatically increased YAP expression in chondrocytes. Adeno-associated virus-mediated Runx1 overexpression in the knee joints of osteoarthritic mice showed the protective effect of Runx1 on articular cartilage damaged in OA. Our results notably showed that Runx1 is a central regulator of articular cartilage homeostasis by orchestrating the YAP, TGFβ, and Wnt signaling pathways in the formation of articular cartilage and OA, and targeting Runx1 and its downstream genes may facilitate the design of novel therapeutic approaches for OA.

Bisphenol A inhibits osteogenic activity and causes bone resorption via the activation of retinoic acid-related orphan receptor α

Bisphenol A (BPA) has deleterious effects on bone metabolism; however, its underlying mechanism has not yet been comprehensively understood. Here, we investigated whether RORα plays an important role in BPA-induced bone resorption both in vitro and in vivo. We found that BPA (0.1-1 μM) inhibited osteogenic activity (including ALP activity and mineralization), decreased the expression levels of osteoblast markers (such as RUNX2, OSX, and ALP) in human MG-63 osteoblast-like osteosarcoma cells, and inhibited spontaneous vertebral formation in zebrafish larvae. Additionally, BPA diminished β-glycerophosphate-induced osteoblast differentiation and vertebral formation, while simultaneously downregulating the expression levels of RUNX2a, OSX, and ALP. Furthermore, molecular docking data showed that a hydroxyl group of BPA dominantly binds to the H3 (ALA70) and/or H5 (ARG107) of RORα-ligand binding domain with hydrogen bonding (ALA330 and/or ARG367 in the full length of RORα, respectively), which another hydroxyl group of BPA fits into H3, H6, and H7 elements with non-covalent interactions, resulting in the activation of RORα. However, an RORα inverse agonist potently inhibited BPA-induced anti-osteogenic activity and vertebral formation in zebrafish larvae, concomitant with inhibition of osteogenic gene expression. Overall, our findings reveal that BPA inhibits osteoblast differentiation and bone formation by activating RORα. These results suggest that BPA exposure (0.1-1 μM) can cause various bone-resorptive diseases, such as osteoporosis.

Sexually Dimorphic Increases in Bone Mass Following Tissue-specific Overexpression of Runx1 in Osteoclast Precursors

Many metabolic bone diseases arise as a result excessive osteoclastic bone resorption, which has motivated efforts to identify new molecular targets that can inhibit the formation or activity of these bone-resorbing cells. Mounting evidence indicates that the transcription factor Runx1 acts as a transcriptional repressor of osteoclast formation. Prior studies using a conditional knockout approach suggested that Runx1 in osteoclast precursors acts as an inhibitor of osteoclastogenesis; however, the effects of upregulation of Runx1 on osteoclast formation remain unknown. In this study, we investigated the skeletal effects of conditional overexpression of Runx1 in preosteoclasts by crossing novel Runx1 gain-of-function mice (Rosa26-LSL-Runx1) with LysM-Cre transgenic mice. We observed a sex-dependent effect whereby overexpression of Runx1 in female mice increased trabecular bone microarchitectural indices and improved torsion biomechanical properties. These effects were likely mediated by delayed osteoclastogenesis and decreased bone resorption. Transcriptomics analyses during osteoclastogenesis revealed a distinct transcriptomic profile in the Runx1-overexpressing cells, with enrichment of genes related to redox signaling, apoptosis, osteoclast differentiation, and bone remodeling. These data further confirm the antiosteoclastogenic activities of Runx1 and provide new insight into the molecular targets that may mediate these effects.

SUMOylation in Skeletal Development, Homeostasis, and Disease

The modification of proteins by small ubiquitin-related modifier (SUMO) molecules, SUMOylation, is a key post-translational modification involved in a variety of biological processes, such as chromosome organization, DNA replication and repair, transcription, nuclear transport, and cell signaling transduction. In recent years, emerging evidence has shown that SUMOylation regulates the development and homeostasis of the skeletal system, with its dysregulation causing skeletal diseases, suggesting that SUMOylation pathways may serve as a promising therapeutic target. In this review, we summarize the current understanding of the molecular mechanisms by which SUMOylation pathways regulate skeletal cells in physiological and disease contexts.

Knockout and Double Knockout of Cathepsin K and Mmp9 reveals a novel function of Cathepsin K as a regulator of osteoclast gene expression and bone homeostasis

Cathepsins play a role in regulation of cell function through their presence in the cell nucleus. However, the role of Cathepsin K (Ctsk) as an epigenetic regulator in osteoclasts remains unknown. Our data demonstrated that Ctsk-/-Mmp9-/- mice have a striking phenotype with a 5-fold increase in bone volume compared with WT. RNA-seq analysis of Ctsk-/- , Mmp9-/- and Ctsk-/-/Mmp9-/- osteoclasts revealed their distinct functions in gene expression regulation, including reduced Cebpa expression, increased Nfatc1 expression, and in signaling pathways activity regulation. Western blots and qPCR data validated these changes. ATAC-seq profiling of Ctsk-/- , Mmp9-/-, and Ctsk-/-/Mmp9-/- osteoclasts indicated the changes resulted from reduced chromatin openness in the promoter region of Cebpa and increased chromatin openness in Nfatc1 promoter in Ctsk-/-/Mmp9-/- osteoclasts compared to that in osteoclasts of WT, Ctsk/- and Mmp9-/- . We found co-localization of Ctsk with c-Fos and cleavage of H3K27me3 in wild-type osteoclasts. Remarkably, cleavage of H3K27me3 was blocked in osteoclasts of Ctsk-/- and Ctsk-/-/Mmp9-/- mice, suggesting that Ctsk may epigenetically regulate distinctive groups of genes' expression by regulating proteolysis of H3K27me3. Ctsk-/-/Mmp9-/- double knockout dramatically protects against ovariectomy induced bone loss. We found that Ctsk may function as an essential epigenetic regulator in modulating levels of H3K27me3 in osteoclast activation and maintaining bone homeostasis. Our study revealed complementary and unique functions of Ctsk as epigenetic regulators for maintaining osteoclast activation and bone homeostasis by orchestrating multiple signaling pathways and targeting both Ctsk and Mmp9 is a novel therapeutic approach for osteolytic diseases such as osteoporosis.

A review of osteoarthritis signaling intervention using small-molecule inhibitors

Numerous small-molecule inhibitors (SMIs) have been approved as adjuvant or first-line therapies for malignancies. Based on cancer treatment using SMIs, next-generation SMIs that can be used to optimize the therapeutic index, overcome drug resistance, and establish combination therapies are in development. Osteoarthritis (OA) is the most common chronic joint disease with senescence, and there are various approaches to OA treatment; however, the gold standard treatment is controversial. Therefore, in this manuscript, we demonstrated the potential of using SMIs in OA treatment and described the general strategies for using SMIs in OA treatment.

Age-Progressive and Gender-Dependent Bone Phenotype in Mice Lacking Both Ebf1 and Ebf2 in Prrx1-Expressing Mesenchymal Cells

Ebfs are a family of transcription factors regulating the differentiation of multiple cell types of mesenchymal origin, including osteoblasts. Global deletion of Ebf1 results in increased bone formation and bone mass, while global loss of Ebf2 leads to enhanced bone resorption and decreased bone mass. Targeted deletion of Ebf1 in early committed osteoblasts leads to increased bone formation, whereas deletion in mature osteoblasts has no effect. To study the effects of Ebf2 specifically on long bone development, we created a limb bud mesenchyme targeted Ebf2 knockout mouse model by using paired related homeobox gene 1 (Prrx1) Cre. To investigate the possible interplay between Ebf1 and Ebf2, we deleted both Ebf1 and Ebf2 in the cells expressing Prrx1. Mice with Prrx1-targeted deletion of Ebf2 had a very mild bone phenotype. However, deletion of both Ebf1 and Ebf2 in mesenchymal lineage cells lead to significant, age progressive increase in bone volume. The phenotype was to some extent gender dependent, leading to an increase in both trabecular and cortical bone in females, while in males a mild cortical bone phenotype and a growth plate defect was observed. The phenotype was observed at both 6 and 12 weeks of age, but it was more pronounced in older female mice. Our data suggest that Ebfs modulate bone homeostasis and they are likely able to compensate for the lack of each other. The roles of Ebfs in bone formation appear to be complex and affected by multiple factors, such as age and gender.

Therapeutic Treatments for Osteoporosis-Which Combination of Pills Is the Best among the Bad?

Osteoporosis is a chronical, systemic skeletal disorder characterized by an increase in bone resorption, which leads to reduced bone density. The reduction in bone mineral density and therefore low bone mass results in an increased risk of fractures. Osteoporosis is caused by an imbalance in the normally strictly regulated bone homeostasis. This imbalance is caused by overactive bone-resorbing osteoclasts, while bone-synthesizing osteoblasts do not compensate for this. In this review, the mechanism is presented, underlined by in vitro and animal models to investigate this imbalance as well as the current status of clinical trials. Furthermore, new therapeutic strategies for osteoporosis are presented, such as anabolic treatments and catabolic treatments and treatments using biomaterials and biomolecules. Another focus is on new combination therapies with multiple drugs which are currently considered more beneficial for the treatment of osteoporosis than monotherapies. Taken together, this review starts with an overview and ends with the newest approaches for osteoporosis therapies and a future perspective not presented so far.

Excessive osteoclast activation by osteoblast paracrine factor RANKL is a major cause of the abnormal long bone phenotype in Apert syndrome model mice

The fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling pathway plays important roles in the development and growth of the skeleton. Apert syndrome caused by gain‐of‐function mutations of FGFR2 results in aberrant phenotypes of the skull, midface, and limbs. Although short limbs are representative features in patients with Apert syndrome, the causative mechanism for this limb defect has not been elucidated. Here we quantitatively confirmed decreases in the bone length, bone mineral density, and bone thickness in the Apert syndrome model of gene knock‐in Fgfr2^S252W/+ (EIIA‐Fgfr2^S252W/+) mice. Interestingly, despite these bone defects, histological analysis showed that the endochondral ossification process in the mutant mice was similar to that in wild‐type mice. Tartrate‐resistant acid phosphatase staining revealed that trabecular bone loss in mutant mice was associated with excessive osteoclast activity despite accelerated osteogenic differentiation. We investigated the osteoblast–osteoclast interaction and found that the increase in osteoclast activity was due to an increase in the Rankl level of osteoblasts in mutant mice and not enhanced osteoclastogenesis driven by the activation of FGFR2 signaling in bone marrow‐derived macrophages. Consistently, Col1a1‐Fgfr2^S252W/+ mice, which had osteoblast‐specific expression of Fgfr2 S252W, showed significant bone loss with a reduction of the bone length and excessive activity of osteoclasts was observed in the mutant mice. Taken together, the present study demonstrates that the imbalance in osteoblast and osteoclast coupling by abnormally increased Rankl expression in Fgfr2^S252W/+ mutant osteoblasts is a major causative mechanism for bone loss and short long bones in Fgfr2^S252W/+ mice.

Kindlin-2 deletion in osteoprogenitors causes severe chondrodysplasia and low-turnover osteopenia in mice

BACKGROUND

Our recent studies demonstrate that the focal adhesion protein Kindlin-2 exerts crucial functions in the mesenchymal stem cells, mature osteoblasts and osteocytes in control of early skeletal development and bone homeostasis in mice. However, whether Kindlin-2 plays a role in osteoprogenitors remains unclear.

MATERIALS AND METHODS

Mice lacking Kindlin-2 expression in osterix (Osx)-expressing cells (i.e., osteoprogenitors) were generated. Micro-computerized tomography (μCT) analyses, histology, bone histomorphometry and immunohistochemistry were performed to determine the effects of Kindlin-2 deletion on skeletal development and bone mass accrual and homeostasis. Bone marrow stromal cells (BMSCs) from mutant mice (Kindlin-2 fl/fl ; Osx Cre ) and control littermates were isolated and determined for their osteoblastic differentiation capacity.

RESULTS

Kindlin-2 was highly expressed in osteoprogenitors during endochondral ossification. Deleting Kindlin-2 expression in osteoprogenitors impaired both intramembranous and endochondral ossifications. Mutant mice displayed multiple severe skeletal abnormalities, including unmineralized fontanel, limb shortening and growth retardation. Deletion of Kindlin-2 in osteoprogenitors impaired the growth plate development and largely delayed formation of the secondary ossification center in the long bones. Furthermore, adult mutant mice displayed a severe low-turnover osteopenia with a dramatic decrease in bone formation which exceeded that in bone resorption. Primary BMSCs isolated from mutant mice exhibited decreased osteoblastic differentiation capacity.

CONCLUSIONS

Our study demonstrates an essential role of Kinlind-2 expression in osteoprogenitors in regulating skeletogenesis and bone mass accrual and homeostasis in mice.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

This study reveals that Kindlin-2 through its expression in osteoprogenitor cells controls chondrogenesis and bone mass. We may define a novel therapeutic target for treatment of skeletal diseases, such as chondrodysplasia and osteoporosis.

Runx1 protects against the pathological progression of osteoarthritis

Runt-related transcription factor-1 (Runx1) is required for chondrocyte-to-osteoblast lineage commitment by enhancing both chondrogenesis and osteogenesis during vertebrate development. However, the potential role of Runx1 in joint diseases is not well known. In the current study, we aimed to explore the role of Runx1 in osteoarthritis induced by anterior cruciate ligament transaction (ACLT) surgery. We showed that chondrocyte-specific Runx1 knockout (Runx1f/fCol2a1-Cre) aggravated cartilage destruction by accelerating the loss of proteoglycan and collagen II in early osteoarthritis. Moreover, we observed thinning and ossification of the growth plate, a decrease in chondrocyte proliferative capacity and the loss of bone matrix around the growth plate in late osteoarthritis. We overexpressed Runx1 by adeno-associated virus (AAV) in articular cartilage and identified its protective effect by slowing the destruction of osteoarthritis in cartilage in early osteoarthritis and alleviating the pathological progression of growth plate cartilage in late osteoarthritis. ChIP-seq analysis identified new targets that interacted with Runx1 in cartilage pathology, and we confirmed the direct interactions of these factors with Runx1 by ChIP-qPCR. This study helps us to understand the function of Runx1 in osteoarthritis and provides new clues for targeted osteoarthritis therapy.

FoxO3a cooperates with RUNX1 to promote chondrogenesis and terminal hypertrophic of the chondrogenic progenitor cells

FoxO transcription factors (FoxOs) have recently been shown to protect against chondrocyte dysfunction and modulate cartilage homeostasis in osteoarthritis. The mechanism underlying of FoxOs regulate chondrocyte differentiation remains unknown. Runt related transcription factor 1 (RUNX1) mediated both chondrocyte and osteoblast differentiation. Our data showed that FoxO3a and RUNX1 are co-expressed in ATDC5 cells and undifferentiated mesenchyme cells and have similar high levels in chondrocytes undergoing transition from proliferation to hypertrophy. Overexpression of FoxO3a in ATDC5 cells or mouse mesenchymal cells resulted in a potent induction of the chondrocyte differentiation markers. Knockdown FoxO3a or RUNX1 potently inhibits the expressions of chondrocyte differentiation markers, including Sox9, Aggrecan, Col2, and hypertrophic chondrocyte markers including RUNX2, ColX, MMP13 and ADAMTs-5 in ATDC5 cells. Co-immunoprecipitation showed that FoxO3a binds the transcriptional regulator RUNX1. Immunohistochemistry showed that FoxO3a and RUNX1 are highly co-expressed in the proliferative chondrocytes of the growth plates in the hind limbs of newborn mice. Collectively, we revealed that FoxO3a cooperated with RUNX1 promoted chondrocyte differentiation through enhancing both early chondrogenesis and terminal hypertrophic of the chondrogenic progenitor cells, indicating FoxO3a interacting with RUNX1 may be a therapeutic target for the treatment of osteoarthritis and other bone diseases.

Runx1/miR-26a/Jagged1 signaling axis controls osteoclastogenesis and alleviates orthodontically induced inflammatory root resorption

BACKGROUND

MicroRNAs (miRNAs) are involved in the regulation of osteoclast biology and several pathogenic progression. This study aimed to identify the role of miR-26a in osteoclastogenesis and orthodontically induced inflammatory root resorption(OIIRR).

METHODS

Rat orthodontic tooth movement (OTM) model was established by ligating a closed coil spring between maxillary first molar and incisor, and 50 g orthodontic force was applied to move upper first molar to middle for 7 days. Human periodontal ligament (hPDL) cells were isolated from periodontium of healthy donors, and then subjected to compression force (CF) for 24 h to mimic an in vitro OTM model. The levels of associated factors in vivo and in vitro were measured subsequently.

RESULT

The distance of tooth movement was increased and root resorption pits were occurred in rat OTM model. The expression of miR-26a was decreased in vivo and vitro experiments. CF treatment enhanced the secretion of inflammatory factors receptor activator of nuclear factor-kappa B ligand (RANKL) and IL-6, osteoclast marker levels, and the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts, while miR-26a overexpression reversed these results. Furthermore, miR-26a overexpression inhibited the osteoclastogenesis and rescued the root resorption in OTM rats through inhibition of Jagged1. Additionally, Runx1 could bind to miR-26a promoter and promote its expression, thereby suppressing the osteoclastogenesis.

CONCLUSION

We concluded that Runx1/miR-26a/Jagged1 signaling axis restrained osteoclastogenesis and alleviated OIIRR.

TLE4 Is a Critical Mediator of Osteoblast and Runx2-Dependent Bone Development

Healthy bone homeostasis hinges upon a delicate balance and regulation of multiple processes that contribute to bone development and metabolism. While examining hematopoietic regulation by Tle4, we have uncovered a previously unappreciated role of Tle4 on bone calcification using a novel Tle4 null mouse model. Given the significance of osteoblasts in both hematopoiesis and bone development, this study investigated how loss of Tle4 affects osteoblast function. We used dynamic bone formation parameters and microCT to characterize the adverse effects of Tle4 loss on bone development. We further demonstrated loss of Tle4 impacts expression of several key osteoblastogenic genes, including Runx2, Oc, and Ap, pointing toward a potential novel mechanism for Tle4-dependent regulation of mammalian bone development in collaboration with the RUNX family members.

Probiotics Stimulate Bone Formation in Obese Mice via Histone Methylations

Rationale: Manipulation of the gut microbiome can prevent pathologic bone loss. However, the effects of probiotics on mitochondrial epigenetic remodeling and skeletal homeostasis in the high-fat diet (HFD)-linked obesity remains to be explored. Here, we examined the impact of probiotics supplementation on mitochondrial biogenesis and bone homeostasis through the histone methylation mechanism in HFD fed obese mice.

Methods: 16S rRNA gene sequencing was performed to study the microbiota composition in the gut and microbial dysbiosis in obese mouse model. High resolution (microPET/CT) imaging was performed to demonstrate the obese associated colonic inflammation. Obese-associated upregulation of target miRNA in osteoblast was investigated using a microRNA qPCR array. Osteoblastic mitochondrial mass was evaluated using confocal imaging. Overexpression of mitochondrial transcription factor (Tfam) was used to investigate the glycolysis and mitochondrial bioenergetic metabolism using Tfam-transgenic (Tg) mice fed on HFD. The bone formation and mechanical strength was evaluated by microCT analysis and three-point bending analysis.

Results: High-resolution imaging (µ-CT) and mechanical testing revealed that probiotics induced a significant increase of trabecular bone volume and bone mechanical strength respectively in obese mice. Probiotics or Indole-3-propionic acid (IPA) treatment directly to obese mice, prevents gut inflammation, and improved osteoblast mineralization. Mechanistically, probiotics treatment increases mitochondrial transcription factor A (Tfam) expression in osteoblasts by promoting Kdm6b/Jmjd3 histone demethylase, which inhibits H3K27me3 epigenetic methylation at the Tfam promoter. Furthermore, Tfam-transgenic (Tg) mice, fed with HFD, did not experience obesity-linked reduction of glucose uptake, mitochondrial biogenesis and mineralization in osteoblasts.

Conclusions: These results suggest that the probiotics mediated changes in the gut microbiome and its derived metabolite, IPA are potentially be a novel agent for regulating bone anabolism via the gut-bone axis.

Pitavastatin prevents ovariectomy-induced osteoporosis by regulating osteoclastic resorption and osteoblastic formation

Excessive osteoclast activity, along with relatively weak osteoblast function, is strongly associated with bone disease. Therefore, studies to identify novel anti-osteoporosis candidates with dual actions of inhibiting osteoclastogenesis and increasing osteoblastogenesis may provide an ideal approach for treating osteoporosis. Pitavastatin, an inhibitor of 3-hydroxy-3 methyl-glutaryl coenzyme A reductase, has demonstrated various pharmacological activities, including anti-inflammation, bone anabolic effects, vasodilation, and inhibition of revascularization; however, the precise effects and mechanisms of pitavastatin on the regulation of osteoblast and osteoclast activity need to be comprehensively elucidated. Herein, we demonstrated that pitavastatin is a potential candidate for treating osteoporosis by enhancing osteoblast differentiation and bone growth and inhibiting osteoclast differentiation and bone resorption. Pitavastatin exerted dose-dependent inhibitory effects on receptor activator of nuclear factor kappa-B ligand-induced osteoclast formation, bone resorption, and osteoclast-specific marker gene expression. These inhibitory effects were achieved by inhibiting the Akt, NF-κB, and mitogen-activated protein kinase (p38, ERK, and JNK) signaling pathways, resulting in the downregulation of major transcription factors c-Fos and NFATc1. Furthermore, pitavastatin potentially stimulated osteoblast differentiation by activating alkaline phosphatase (ALP), enhancing mineralization by Alizarin Red S, and increasing the expression of osteoblastogenic marker genes such as runt-related transcription factor 2, ALP, osteocalcin, and collagen type 1 alpha. Furthermore, we evaluated the therapeutic potential of pitavastatin in ovariectomy-induced systematic bone loss based on micro-computed tomography and histological analysis of femurs. Our findings demonstrated a new function and mechanism for pitavastatin in bone remodeling, indicating its potential as a therapeutic candidate in treating osteoporosis by inhibiting osteoclastic resorption and promoting osteoblastic formation.

Insulin growth factor-1 promotes the proliferation and osteogenic differentiation of bone marrow mesenchymal stem cells through the Wnt/β-catenin pathway

Bone marrow mesenchymal stem cells (BMSCs) are stem cells that exist in bone marrow tissue and have osteogenic differentiation potential. Insulin growth factor-1 (IGF-1) plays a key role in the proliferation and osteogenic differentiation of BMSCs. However, the specific mechanism of IGF-1 in cell proliferation and osteogenic differentiation remains unclear. In the present study, BMSCs were transfected with lentivirus carrying the siRNA-Wnt3a gene, and the Wnt3a level in BMSCs was revealed to be reduced by western blotting, real-time quantitative polymerase chain reaction and immunofluorescence detection. Then, BMSCs were treated with 80 ng/ml IGF-1 in complete medium for 5 days. CCK-8 and cell cycle assays revealed that cell proliferation was significantly decreased in the siRNA-Wnt3a group than in the control group. The protein and mRNA levels of β-catenin and cyclin D1 were significantly downregulated in the siRNA-Wnt3a group compared with the control group. In addition, BMSCs were treated with IGF-1 in osteogenic differentiation medium for 7 and 21 days, and alkaline phosphatase staining and Alizarin Red staining demonstrated significantly reduced osteogenic differentiation ability in the siRNA-Wnt3a group compared with the control group. Furthermore, the protein and mRNA levels of β-catenin, RUNX2, and OPN were downregulated compared with the control group. Our findings revealed that IGF-1 promoted the proliferation and differentiation of BMSCs at least partially through the Wnt/β-catenin pathway. These findings provided new insight into the clinical treatment of bone disease.

Long Non-coding RNA 332443 Inhibits Preadipocyte Differentiation by Targeting Runx1 and p38-MAPK and ERK1/2-MAPK Signaling Pathways

Long non-coding RNAs (lncRNAs) have emerged as integral regulators of pathophysiological processes, but their specific roles and mechanisms in adipose tissue development remain largely unknown. Here, through microarray analysis, co-expression, and tissue specific analysis of adipocyte tissues after fasting for 72 h, we found that Lnc-FR332443 expression was dramatically decreased, as well as the expression of Runx1. The UCSC database and Ensembl database indicated that Lnc-FR332443 is the antisense lncRNA of Runx1. Lnc-FR332443 and Runx1 are highly enriched in adipose tissue and downregulated during adipogenic differentiation. Adipose tissue-specific knockdown of Lnc-FR332443 increased fat mass in vivo, and specific knockdown of Lnc-FR332443 in 3T3-L1 preadipocytes promoted adipogenic differentiation. In this process, Runx1 expression was decreased when Lnc-FR332443 was downregulated in adipocytes or 3T3-L1 preadipocytes, and vice versa, when Lnc-FR332443 was upregulated, the expression of Runx1 was increased. However, overexpression of Runx1 decreased the expression of the adipocyte cell marker genes PPARγ, C/EBPα and FABP4 significantly, while not affected the expression of Lnc-FR332443. Mechanistically, Lnc-FR332443 positively regulates Runx1 expression in mouse adipocytes and suppresses adipocyte differentiation by attenuating the phosphorylation of MAPK-p38 and MAPK-ERK1/2 expression. Thus, this study indicated that Lnc-FR332443 inhibits adipogenesis and which might be a drug target for the prevention and treatment of obesity.

ASCL1 represses a SOX9+ neural crest stem-like state in small cell lung cancer

ASCL1 is a neuroendocrine lineage-specific oncogenic driver of small cell lung cancer (SCLC), highly expressed in a significant fraction of tumors. However, ∼25% of human SCLC are ASCL1-low and associated with low neuroendocrine fate and high MYC expression. Using genetically engineered mouse models (GEMMs), we show that alterations in Rb1/Trp53/Myc in the mouse lung induce an ASCL1+ state of SCLC in multiple cells of origin. Genetic depletion of ASCL1 in MYC-driven SCLC dramatically inhibits tumor initiation and progression to the NEUROD1+ subtype of SCLC. Surprisingly, ASCL1 loss promotes a SOX9+ mesenchymal/neural crest stem-like state and the emergence of osteosarcoma and chondroid tumors, whose propensity is impacted by cell of origin. ASCL1 is critical for expression of key lineage-related transcription factors NKX2-1, FOXA2, and INSM1 and represses genes involved in the Hippo/Wnt/Notch developmental pathways in vivo. Importantly, ASCL1 represses a SOX9/RUNX1/RUNX2 program in vivo and SOX9 expression in human SCLC cells, suggesting a conserved function for ASCL1. Together, in a MYC-driven SCLC model, ASCL1 promotes neuroendocrine fate and represses the emergence of a SOX9+ nonendodermal stem-like fate that resembles neural crest.

[The Roles of RUNX1 in the Proliferation and Osteogenic and Adipogenic Differentiation of Dental Pulp Stem Cells]

OBJECTIVE

To investigate the influence of Runt-related transcription factor 1 (RUNX1) on the proliferation, osteogenic differentiation and adipogenic differentiation of dental pulp stem cells (DPSC) in vitro.

METHODS

DPSCs were transfected through lentiviral vector carrying the target gene RUNX1 and green fluorescent protein (GFP). After 48 h, transfection efficiency was determined with the fluorescent marking of GFP and Western blot. The effect of the overexpression of RUNX1 on DPSC proliferation and colony formation was determined with CCK-8 and colony formation assay; cell cycle of DPSC was detected by flow cytometry. RUNX1 siRNA was transfected into the DPSCs. After mineralized induction, the effect of RUNX1 overexpression/silencing on the osteogenetic differentiation of DPSC was tested by alkaline phosphatase (ALP) staining and alizarin red staining. After adipogenic induction, oil red O staining was done in order to observe the effect of overexpression/silencing of RUNX1 on the adipogenic differentiation of DPSC.

RESULTS

RUNX1 protein was overexpressed in DPSC after lentiviral transfection. Fluorescent test showed successful transfection of lentiviral transfection and over 70% of the cells showed stable expression of GFP protein. The proliferation and colony-formation efficiency of DPSC was enhanced significantly and the proportion of DPSCs in the S phase was significantly increased in the RUNX1-overexpessed group ( P<0.05). ALP activity and mineralized nodule formation ability increased, while lipid droplets decreased in the RUNX1-overexpessed group ( P<0.05). ALP activity and mineralized nodule formation ability decreased, while lipid droplets increased in the RUNX1 knockdown group ( P<0.05) .

CONCLUSION

RUNX1 promotes DPSC proliferation and osteogenic differentiation while it inhibits DPSC adipogenic differentiation.

Regulation of Osteoblast Differentiation by Cytokine Networks

Osteoblasts, which are bone-forming cells, play pivotal roles in bone modeling and remodeling. Osteoblast differentiation, also known as osteoblastogenesis, is orchestrated by transcription factors, such as runt-related transcription factor 1/2, osterix, activating transcription factor 4, special AT-rich sequence-binding protein 2 and activator protein-1. Osteoblastogenesis is regulated by a network of cytokines under physiological and pathophysiological conditions. Osteoblastogenic cytokines, such as interleukin-10 (IL-10), IL-11, IL-18, interferon-γ (IFN-γ), cardiotrophin-1 and oncostatin M, promote osteoblastogenesis, whereas anti-osteoblastogenic cytokines, such as tumor necrosis factor-α (TNF-α), TNF-β, IL-1α, IL-4, IL-7, IL-12, IL-13, IL-23, IFN-α, IFN-β, leukemia inhibitory factor, cardiotrophin-like cytokine, and ciliary neurotrophic factor, downregulate osteoblastogenesis. Although there are gaps in the body of knowledge regarding the interplay of cytokine networks in osteoblastogenesis, cytokines appear to be potential therapeutic targets in bone-related diseases. Thus, in this study, we review and discuss our osteoblast, osteoblast differentiation, osteoblastogenesis, cytokines, signaling pathway of cytokine networks in osteoblastogenesis.

Runx1 is a central regulator of osteogenesis for bone homeostasis by orchestrating BMP and WNT signaling pathways

Runx1 is highly expressed in osteoblasts, however, its function in osteogenesis is unclear. We generated mesenchymal progenitor-specific (Runx1f/fTwist2-Cre) and osteoblast-specific (Runx1f/fCol1α1-Cre) conditional knockout (Runx1 CKO) mice. The mutant CKO mice with normal skeletal development displayed a severe osteoporosis phenotype at postnatal and adult stages. Runx1 CKO resulted in decreased osteogenesis and increased adipogenesis. RNA-sequencing analysis, Western blot, and qPCR validation of Runx1 CKO samples showed that Runx1 regulates BMP signaling pathway and Wnt/β-catenin signaling pathway. ChIP assay revealed direct binding of Runx1 to the promoter regions of Bmp7, Alk3, and Atf4, and promoter mapping demonstrated that Runx1 upregulates their promoter activity through the binding regions. Bmp7 overexpression rescued Alk3, Runx2, and Atf4 expression in Runx1-deficient BMSCs. Runx2 expression was decreased while Runx1 was not changed in Alk3 deficient osteoblasts. Atf4 overexpression in Runx1-deficient BMSCs did not rescue expression of Runx1, Bmp7, and Alk3. Smad1/5/8 activity was vitally reduced in Runx1 CKO cells, indicating Runx1 positively regulates the Bmp7/Alk3/Smad1/5/8/Runx2/ATF4 signaling pathway. Notably, Runx1 overexpression in Runx2-/- osteoblasts rescued expression of Atf4, OCN, and ALP to compensate Runx2 function. Runx1 CKO mice at various osteoblast differentiation stages reduced Wnt signaling and caused high expression of C/ebpα and Pparγ and largely increased adipogenesis. Co-culture of Runx1-deficient and wild-type cells demonstrated that Runx1 regulates osteoblast-adipocyte lineage commitment both cell-autonomously and non-autonomously. Notably, Runx1 overexpression rescued bone loss in OVX-induced osteoporosis. This study focused on the role of Runx1 in different cell populations with regards to BMP and Wnt signaling pathways and in the interacting network underlying bone homeostasis as well as adipogenesis, and has provided new insight and advancement of knowledge in skeletal development. Collectively, Runx1 maintains adult bone homeostasis from bone loss though up-regulating Bmp7/Alk3/Smad1/5/8/Runx2/ATF4 and WNT/β-Catenin signaling pathways, and targeting Runx1 potentially leads to novel therapeutics for osteoporosis.