Cyclophilin D, regulator of the mitochondrial permeability transition, impacts bone development and fracture repair.

Mitochondrial Permeability Transition Pore (MPTP) and its key positive regulator, Cyclophilin D (CypD), control activity of cell oxidative metabolism important for differentiation of stem cells of various lineages including osteogenic lineage. Our previous work (Sautchuk et al., 2022) showed that CypD gene, Ppif, is transcriptionally repressed during osteogenic differentiation by regulatory Smad transcription factors in BMP canonical pathway, a major driver of osteoblast (OB) differentiation. Such a repression favors closure of the MPTP, priming OBs to higher usage of mitochondrial oxidative metabolism. The physiological role of CypD/MPTP regulation was demonstrated by its inverse correlation with BMP signaling in aging and bone fracture healing in addition to the negative effect of CypD gain-of-function (GOF) on bone maintenance. Here we show evidence that CypD GOF also negatively affects bone development and growth as well as fracture healing in adult mice. Developing craniofacial and long bones presented with delayed ossification and decreased growth rate, respectively, whereas in fracture, bony callus volume was diminished. Given that Genome Wide Association Studies showed that PPIF locus is associated with both body height and bone mineral density, our new data provide functional evidence for the role of PPIF gene product, CypD, and thus MPTP in bone growth and repair.

Natural Compounds for Bone Remodeling: Targeting osteoblasts and relevant signaling pathways.

In the process of bone metabolism and bone remodeling, bone marrow mesenchymal stem cells (BM-MSCs) differentiate into osteoblasts (OBs) under certain conditions to enable the formation of new bone, and normal bone reconstruction and pathological bone alteration are closely related to the differentiation and proliferation functions of OBs. Osteogenic differentiation of BM-MSCs involves multiple signaling pathways, which function individually but interconnect intricately to form a complex signaling regulatory network. Natural compounds have fewer adverse effects than chemically synthesized drugs, optimize bone health, and are more suitable for long-term use. In this paper, we focus on OBs, summarize the current research progress of signaling pathways related to OBs differentiation, and review the molecular mechanisms by which chemically synthesized drugs with potential anti-osteoporosis properties regulate OBs-mediated bone formation.

sEV-mediated Lipid Droplets Transferred from Bone Marrow Adipocytes promote ferroptosis and impair Osteoblast Function.

Osteoporosis is linked to increased bone marrow adipocyte (BMAd) proliferation, which displaces bone-forming cells and alters the local environment. The impact of BMAd lipid droplets on bone health and osteoblast function remains unclear. This study investigates the interplay between BMAd-derived lipid droplets and osteoblast functionality, focusing on ferroptosis pathways. Osteoblast cultures were treated with conditioned media from adipocytes to simulate in vivo conditions. High-throughput mRNA sequencing and Western blot analysis were used to profile changes in gene expression and protein levels related to ferroptosis, oxidative phosphorylation, and osteogenic markers. Cellular assays assessed the direct impact of lipid droplets on osteoblast activity. Results showed that osteoblasts exposed to adipocyte-conditioned media had increased intracellular lipid droplet accumulation, upregulation of ferroptosis-related genes and proteins, and downregulation of oxidative phosphorylation and osteoblast differentiation markers. Treatment with ferroptosis inhibitors reversed the detrimental effects on osteoblasts, indicating the functional relevance of this pathway in osteoporosis. BMAd-derived lipid droplets contribute to osteoblast dysfunction through ferroptosis induction. Inhibiting ferroptosis could preserve osteoblast function and combat osteoporosis-related bone issues, suggesting that modulating lipid metabolism and redox balance in bone cells may be promising for future treatments.

Examining the level of inflammatory cytokines TNF-α and IL-8 produced by osteoblasts differentiated from dental pulp stem cells.

BACKGROUND: The use of dental pulp stem cells (DPSCs) in clinical applications instead of bone marrow stem cells is a very promising method capable of significantly changing the future of medical treatment. If further studies prove that DPSCs and the cells differentiated from them do not stimulate the immune system, these cells can be used more reliably in treatment of autoimmune diseases. METHODS: In this research, we examined the isolated DPSCs and differentiated osteoblasts from them in medium without inflammatory stimulants in terms of TLR3 and TLR4 gene expression and inflammatory cytokines, including TNF-α and IL-8 using qRT-PCR, and measured the concentration of inflammatory cytokines IL-8 and TNF-α produced by these two types of cells through ELISA. RESULTS: The obtained results showed that the expression level of inflammatory cytokines IL-8 and TNF-α in differentiated osteoblasts is significantly different as compared with DPSCs. However, no significant difference was observed in TLR-4 expression between two groups. An increase in TNF-α expression level was found to directly correlate with an increase in the expression of IL-8. The concentration of cytokine TNF-α in osteoblasts was significantly higher than that of IL-8 in DPSCs. CONCLUSION: In comparison to DPSCs, osteoblast cells first lead to inflammatory responses. These responses reduce overtime. However, DPSCs retain their immunomodulatory properties and do not show inflammatory responses.

Levels of B-ALP and TRAP-5b in a patient with short root anomaly: a case report.

The purpose of this study was to observe the changes in bone-specific alkaline phosphatase (B-ALP) and tartrate-resistant acid phosphatase-5b (TRAP-5b) in a patient diagnosed with short root anomaly (SRA). The detailed clinical data and history of related clinical symptoms of the SRA patient were retrieved. Oral examination showed that the shape and color of the tooth crown were normal. Tooth 11 and 12 were missing, and the mobility degree of other teeth was II-III. Panoramic radiograph examination showed that the root length only reached the neck of the tooth. Laboratory results showed that blood spectrum, chromosome and trace elements were normal. Endocrinological evaluation indicated that hormone levels were within normal limits; however, both B-ALP and TRAP-5b were higher than the normal range. The present case shows that SRA may be related to an imbalance in osteoblast/osteoclast metabolism, which provides a new direction for the etiological research of this disease.

ED-71 Ameliorates Bone Loss in Type 2 Diabetes Mellitus by Enhancing Osteogenesis Through Upregulation of the Circadian Rhythm Coregulator BMAL1.

Purpose: Bone loss is a common complication of type 2 diabetes mellitus (T2DM). Circadian rhythms play a significant role in T2DM and bone remodeling. Eldecalcitol (ED-71), a novel active vitamin D analog, has shown promise in ameliorating T2DM. We aimed to investigate whether the circadian rhythm coregulator BMAL1 mediates the anti-osteoporotic effect of ED-71 in T2DM and its associated mechanisms. Methods: A T2DM mouse model was established using high-fat diet (HDF) and streptozotocin (STZ) injection, and blood glucose levels were monitored weekly. HE staining, Masson staining, and Micro-CT were performed to assess the changes in bone mass. IHC staining and IF staining were used to detect osteoblast status and BMAL1 expression and RT-qPCR was applied to detect the change of oxidative stress factors. In vitro, high glucose (HG) stimulation was used to simulate the cell environment in T2DM. RT-qPCR, Western blot, IF, ALP staining and AR staining were used to detect osteogenic differentiation and SIRT1/GSK3ß signaling pathway. DCFH-DA staining was used to detect reactive oxygen species (ROS) levels. Results: ED-71 increased bone mass and promoted osteogenesis in T2DM mice. Moreover, ED-71 inhibited oxidative stress and promoted BMAL1 expression in osteoblasts The addition of STL1267, an agonist of the BMAL1 transcriptional repressor protein REV-ERB, reversed the inhibitory effect of ED-71 on oxidative stress and the promotional effect on osteogenic differentiation. In addition, ED-71 facilitated SIRT1 expression and reduced GSK3ß activity. The inhibition of SIRT1 with EX527 partially attenuated ED-71's effects, whereas the GSK3ß inhibitor LiCl further enhanced ED-71's positive effects on BMAL1 expression. Conclusion: ED-71 ameliorates bone loss in T2DM by upregulating the circadian rhythm coregulator BMAL1 and promoting osteogenesis through inhibition of oxidative stress. The SIRT1/GSK3ß signaling pathway is involved in the regulation of BMAL1.

Effect of Huoxue Jiegu compound capsule on osteoblast differentiation and fracture healing by regulating the PI3K/Akt/mTOR signaling pathway in rabbits.

Objective: To examine and talk about the mechanism of the Huoxue Jiegu compound capsule's effects on osteoblasts and the PI3K/Akt/mTOR signal pathway in rabbits suffering from tibial fractures. Method: In vitro, CCK8 was used to assess the survival rates. Alizarinred staining was used to evaluate mineralized nodules. ALP staining was used to observe the osteoblasts. qRT-PCR was used to determine the mRNA expression of the bone formation-related factors BMP-2, bFGF, and TGF-ß. In vivo, three groups of nine male rabbits each were randomly assigned to three groups: the Model group, the Huoxue Jiegu compound capsule group (HXJGC group), and the inhibitor group (HXJGC+3-MA), four weeks following the intervention. HE staining was employed to examine the rabbits' bone histology. immunohistochemistry was employed to examine the relative expression of the proteins VEGF and LC3-II. Western Blot was utilized to examine the relative expression of the proteins Beclin-1, LC3-II/Ⅰ, p62, p-PI3K, p-AKT, and p-mTOR. Results: Compared to the control group, the medium- and high-dose groups exhibited considerably higher survival rates (P < 0.05), as well as enhanced cell proliferation and differentiation (P < 0.05) and more pronounced mineralized nodules. (P < 0.05), but the low-dose groups showed no appreciable variation. In the low, medium, and high-dose groups, there was a substantial reduction in the expression of bFGF mRNA, whereas the levels of BMP-2 and TGF-ß mRNA were considerably higher than in the control group (P < 0.05). In vivo, after four weeks of treatment, the model control group and inhibito group had a large amount of fibrous hyperplasia accompanied by bleeding and a small amount of inflammatory cell infiltration. But in the HXJGC group, new cartilage appeared, and the surface of the cartilage was smooth and flat. Beclin-1 and LC3-II/I expression in the HXJGC+3 MA group was significantly lower than in the HXJGC and Model groups (P < 0.05). The HXJGC group showed lower p62 expression than the HXJGC+3 MA and model groups (P < 0.05). The HXJGC group exhibited significantly reduced levels of p-PI3K, p-AKT, and p-mTOR expression in comparison to HXJGC+3 MA groups (P < 0.05). Conclusion: Rabbits with tibial fractures can be treated with HXJGC, which can control the expression of the PI3K/Akt/mTOR signal pathway. It can promote the differentiation and maturation of osteoblasts at the fracture end of rabbits, accelerate the recovery of fractures, and achieve the purpose of treating the disease.

Gallein but not fluorescein enhances the PGD2-stimulated synthesis of osteoprotegerin and interleukin-6 in osteoblasts: Amplification of osteoprotegerin/interleukin-6 by gallein.

Gallein, a small molecule related to fluorescein, is established as an inhibitor of Gßγ subunits to inhibit G protein (Gs) signaling. This agent is providing a potential therapeutic strategy to ameliorate organ dysfunctions especially involved in inflammation, however; the effects on bone metabolism have not yet been clarified. Prostaglandins (PGs) play important roles as autacoids including osteoblasts, and d-type prostanoid (DP) receptor, a member of G protein-coupled receptor specific to PGD2, is expressed on osteoblasts. We previously reported that prostaglandin D2 (PGD2) induces the syntheses of osteoprotegerin (OPG) and interleukin-6 (IL-6), essential factors in bone remodelling process, and p38 mitogen-activated protein kinase (MAPK), c-Jun N-terminal kinase (JNK), and p44/p42 MAPK are involved in the signal transduction of PGD2 in osteoblast-like MC3T3-E1 cells. Thus, we investigated in this study that the effect and the underlying mechanism of gallein, an inhibitor Gßɤ subunits, on the syntheses of OPG and IL-6 induced by PGD2 in these cells. The cultured cells were treated with gallein or fluorescein, a structurally related compound inactive to Gßɤ subunits, and subsequently stimulated with PGD2. Not fluorescein but gallein amplified the PGD2-stimulated releases of OPG and IL-6. Gallein enhanced the PGD2-upregulated mRNA expression levels of OPG and IL-6. Regarding the signaling mechanism, gallein did not affect the PGD2-induced phosphorylation of p38 MAPK, JNK, or p42 MAPK. In conclusion, gallein upregulates the PGD2-stimulated syntheses of OPG and IL-6 by the specific effect to inhibit Gßγ subunits in osteoblasts, but the effect is not exerted at the upstream of p38 MAPK, JNK, or p44/p42 MAPK activation.

KIAA0753 enhances osteoblast differentiation suppressed by diabetes.

Diabetes-related bone loss represents a significant complication that persistently jeopardizes the bone health of individuals with diabetes. Primary cilia proteins have been reported to play a vital role in regulating osteoblast differentiation in diabetes-related bone loss. However, the specific contribution of KIAA0753, a primary cilia protein, in bone loss induced by diabetes remains unclear. In this investigation, we elucidated the pivotal role of KIAA0753 as a promoter of osteoblast differentiation in diabetes. RNA sequencing demonstrated a marked downregulation of KIAA0753 expression in pro-bone MC3T3 cells exposed to a high glucose environment. Diabetes mouse models further validated the downregulation of KIAA0753 protein in the femur. Diabetes was observed to inhibit osteoblast differentiation in vitro, evidenced by downregulating the protein expression of OCN, OPN and ALP, decreasing primary cilia biosynthesis, and suppressing the Hedgehog signalling pathway. Knocking down KIAA0753 using shRNA methods was found to shorten primary cilia. Conversely, overexpression KIAA0753 rescued these changes. Additional insights indicated that KIAA0753 effectively restored osteoblast differentiation by directly interacting with SHH, OCN and Gli2, thereby activating the Hedgehog signalling pathway and mitigating the ubiquitination of Gli2 in diabetes. In summary, we report a negative regulatory relationship between KIAA0753 and diabetes-related bone loss. The clarification of KIAA0753's role offers valuable insights into the intricate mechanisms underlying diabetic bone complications.

Wnt family members regulating osteogenesis and their origins.

Wnt signaling plays an important role in the regulation of bone metabolism. Wnt activates the ß-catenin-mediated canonical pathway and ß-catenin-independent non-canonical pathway. When Wnt ligands bind to the co-receptors low density lipoprotein receptor-related protein (Lrp)5 or Lrp6, and a seven-transmembrane receptor frizzled, the canonical pathway is activated. On the other hand, when Wnt ligands bind to the receptor complex consisting of the co-receptor receptor tyrosine kinase-like orphan receptor (Ror)1 and Ror2 or Ryk and frizzled, the non-canonical pathway is activated. An analysis of loss-of-function and gain-of-function mutations in molecules involved in Wnt signaling (ligands, receptors, and inhibitors) has revealed the mechanisms by which Wnt signaling regulates bone metabolism. In this review, based on transcriptome analyses of Wnt expression in bone tissues including single cell RNA sequence analysis and previous literatures, we herein introduce and discussed the latest findings on the mechanisms by which Wnt ligand mutations impair bone metabolism, especially bone formation.

Regulation of Osteogenic and Angiogenic Markers in Alkali Treated Titanium for Hard Tissue Engineering Applications.

Titanium (Ti) and Ti alloys are of great interest in bone and dental tissue engineering applications due to their biocompatibility, corrosion resistance, and close mechanical properties to natural bone. However, the formation of fibrous tissue prevents osteointegration and results in implant loosening. Thus, physical and chemical methods are used to improve the surface properties of Ti. This study aimed to understand the role of alkali treatment conditions, including alkali medium concentration, temperature, rotation speed, and post-heat treatment. Our results showed that alkali treatment using 5 and 10 molar sodium hydroxide solution allows the formation of web-like microstructure. However, a higher concentration of 15 molar resulted in cracks along the surface. Interaction between the human fetal osteoblast cells (hfOBs) and Ti samples showed that heat treatment is necessary for increased cellular proliferation, which was not significantly different at later time points compared to the polished Ti. Alkali heat treatment did not induce inflammatory reactions at later time points. It showed an increase in vascular endothelial growth factor, osteoprotegerin/nuclear factor kappa-В ligand ratio, and osteocalcin expression, which is evidence for accelerated osteoblast cell maturation and bone remodeling in surface-modified samples. Together, these data show that alkali treatment using 5 or 10 molar of NaOH followed by heat treatment may have therapeutic effect and assist with bone tissue integration with Ti implant.

Investigating the Promising P28 Peptide-Loaded Chitosan/Ceramic Bone Scaffolds for Bone Regeneration.

Bone has the ability to heal itself; however, bone defects fail to heal once the damage exceeds a critical size. Bone regeneration remains a significant clinical challenge, with autograft considered the ideal bone graft material due to its sufficient porosity, osteogenic cells, and biological growth factors. However, limitations to bone grafting, such as limited bone stock and high resorption rates, have led to a great deal of research into developing bone graft substitutes. The P28 peptide is a small molecule bioactive biomimetic alternative to mimic the bone morphogenetic protein 2 (BMP-2). In this study, we investigated the potential of P28-loaded hybrid scaffolds to mimic the natural bone structure for enhancing the bone regeneration process. We hypothesized that the peptide-loaded scaffolds and nude scaffolds both have the potential to promote bone healing, and the bone healing process is accelerated by the release of the peptide. To verify our hypothesis, C2C12 cells were evaluated for the presence of calcium deposits by histological stain at 7 and 14 days in cultures with hybrid scaffolds. Total RNA was isolated from C2C12 cells cultured with hybrid scaffolds for 7 and 14 days to assess osteoblast differentiation. The project findings demonstrated that the hybrid scaffold could enhance osteoblast differentiation and significantly improve the therapeutic effects of the scaffold in bone regeneration.

Multifunctional gallium doped bioactive glasses: a targeted delivery for antineoplastic agents and tissue repair against osteosarcoma.

Osteosarcoma (OS) is the mostly commonly occurring primary bone cancer. Despite comprehensive treatment programs including neoadjuvant chemotherapy and tumour resection, survival rates have not improved significantly since the 1970s. Survival rates are dramatically reduced for patients who suffer a local recurrence. Furthermore, primary bone cancer patients are at increased risk of bone fractures. Consequently, there is an urgent need for alternative treatment options. In this paper we report the development of novel gallium doped bioactive glass that selectively kill bone cancer cells whilst simultaneously stimulating new bone growth. Here we show, using a combination of 3-(4.5-dimethylthiazol-2-yl)-2.5-diphenyltetrazolium bromide, LIVE/DEAD assays and image analysis, that bioactive glasses containing gallium oxide are highly toxic and reduce both the proliferation and migration of bone cancer cells (Saos-2) in a dose dependant manner. Glasses containing 5 mol% gallium oxide reduced the viability of OS cells by 99% without being cytotoxic to the non-cancerous normal human osteoblasts (NHOst) control cells. Furthermore, Fourier transform infrared and energy-dispersive x-ray spectroscopy results confirmed the formation of an amorphous calcium phosphate/hydroxyapatite like layer on the surface of the bioactive glass particulates, after 7 d incubating in simulated body fluid, indicating the early stages of bone formation. These materials show significant potential for use in bone cancer applications as part of a multimodal treatment.

Role of oxytocin in bone.

Oxytocin (OT) is a posterior pituitary hormone that, in addition to its role in regulating childbirth and lactation, also exerts direct regulatory effects on the skeleton through peripheral OT and oxytocin receptor (OTR). Bone marrow mesenchymal stem cells (BMSCs), osteoblasts (OB), osteoclasts (OC), chondrocytes, and adipocytes all express OT and OTR. OT upregulates RUNX2, BMP2, ALP, and OCN, thereby enhancing the activity of BMSCs and promoting their differentiation towards OB rather than adipocytes. OT also directly regulates OPG/RANKL to inhibit adipocyte generation, increase the expression of SOX9 and COMP, and enhance chondrocyte differentiation. OB can secrete OT, exerting influence on the surrounding environment through autocrine and paracrine mechanisms. OT directly increases OC formation through the NκB/MAP kinase signaling pathway, inhibits osteoclast proliferation by triggering cytoplasmic Ca2+ release and nitric oxide synthesis, and has a dual regulatory effect on OCs. Under the stimulation of estrogen, OB synthesizes OT, amplifying the biological effects of estrogen and OT. Mediated by estrogen, the OT/OTR forms a feedforward loop with OB. Apart from estrogen, OT also interacts with arginine vasopressin (AVP), prostaglandins (PGE2), leptin, and adiponectin to regulate bone metabolism. This review summarizes recent research on the regulation of bone metabolism by OT and OTR, aiming to provide insights into their clinical applications and further research.

Low-molecular-weight estrogenic phytoprotein suppresses osteoporosis development through positive modulation of skeletal estrogen receptors.

Age-related osteoporosis is a metabolic skeletal disorder caused by estrogen deficiency in postmenopausal women. Prolonged use of anti-osteoporotic drugs such as bisphosphonates and FDA-approved anti-resorptive selective estrogen receptor modulators (SERMs) has been associated with various clinical drawbacks. We recently discovered a low-molecular-weight biocompatible and osteoanabolic phytoprotein, called HKUOT-S2 protein (32 kDa), from Dioscorea opposita Thunb that can accelerate bone defect healing. Here, we demonstrated that the HKUOT-S2 protein treatment can enhance osteoblasts-induced ossification and suppress osteoporosis development by upregulating skeletal estrogen receptors (ERs) ERα, ERß, and GPR30 expressions in vivo. Also, HKUOT-S2 protein estrogenic activities promoted hMSCs-osteoblasts differentiation and functions by increasing osteogenic markers, ALP, and RUNX2 expressions, ALP activity, and osteoblast biomineralization in vitro. Fulvestrant treatment impaired the HKUOT-S2 protein-induced ERs expressions, osteoblasts differentiation, and functions. Finally, we demonstrated that the HKUOT-S2 protein could bind to ERs to exert osteogenic and osteoanabolic properties. Our results showed that the biocompatible HKUOT-S2 protein can exert estrogenic and osteoanabolic properties by positively modulating skeletal estrogen receptor signaling to promote ossification and suppress osteoporosis. Currently, there is no or limited data if any, on osteoanabolic SERMs. The HKUOT-S2 protein can be applied as a new osteoanabolic SERM for osteoporosis treatment.

Hdac3 deficiency limits periosteal reaction associated with Western diet feeding in female mice.

Diet-induced obesity is associated with enhanced systemic inflammation that limits bone regeneration. HDAC inhibitors are currently being explored as anti-inflammatory agents. Prior reports show that myeloid progenitor-directed Hdac3 ablation enhances intramembranous bone healing in female mice. In this study, we determined if Hdac3 ablation increased intramembranous bone regeneration in mice fed a high-fat/high-sugar (HFD) diet. Micro-CT analyses demonstrated that HFD-feeding enhanced the formation of periosteal reaction tissue of control littermates, reflective of suboptimal bone healing. We confirmed enhanced bone volume within the defect of Hdac3-ablated females and showed that Hdac3 ablation reduced the amount of periosteal reaction tissue following HFD feeding. Osteoblasts cultured in a conditioned medium derived from Hdac3-ablated cells exhibited a four-fold increase in mineralization and enhanced osteogenic gene expression. We found that Hdac3 ablation elevated the secretion of several chemokines, including CCL2. We then confirmed that Hdac3 deficiency increased the expression of Ccl2. Lastly, we show that the proportion of CCL2-positve cells within bone defects was significantly higher in Hdac3-deficient mice and was further enhanced by HFD. Overall, our studies demonstrate that Hdac3 deletion enhances intramembranous bone healing in a setting of diet-induced obesity, possibly through increased production of CCL2 by macrophages within the defect.

Erythropoietin regulates osteoclast formation via up-regulating PPARγ expression.

Erythropoietin (EPO), expressed in red blood progenitor cells, primarily regulates erythropoiesis by binding to its receptor. Besides anemia, recent studies have identified new therapeutic indications for EPO that are not connected to red blood cell formation. Elevated EPO levels harm bone homeostasis in adult organisms and are associated with increased osteoclast; however, the underlying molecular mechanisms remain unclear. This study demonstrated that EPO enhanced osteoclast differentiation and bone resorption in vitro. We showed that EPO promoted osteoclast formation by up-regulating PPARγ expression through activating the Jak2/ERK signaling pathway. Consistently, PPARγ antagonists rescued the hyperactivation of osteoclasts due to EPO, while PPARγ agonists reversed the EMP9-mediated decrease in osteoclast differentiation. Further, exposing female mice to EPO for two months led to a decrease in bone mass and increased osteoclast numbers. The present results suggested that EPO promotes osteoclastogenesis by regulating the Jak2/ERK/ PPARγ signaling pathway. From a clinical perspective, the risk of compromised bone health should be considered when using EPO to treat anemia in post-operative patients with intertrochanteric fractures of the femur, as it could significantly impact the patient's recovery and quality of life.

Melatonin Regulates Osteoblast Differentiation through the m6A Reader hnRNPA2B1 under Simulated Microgravity.

Recent studies have confirmed that melatonin and N6-methyladenosine (m6A) modification can influence bone cell differentiation and bone formation. Melatonin can also regulate a variety of biological processes through m6A modification. Heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNPA2B1) serves as a reader of m6A modification. In this study, we used the hindlimb unloading model as an animal model of bone loss induced by simulated microgravity and used 2D clinorotation to simulate a microgravity environment for cells on the ground. We found that hnRNPA2B1 was downregulated both in vitro and in vivo during simulated microgravity. Further investigations showed that hnRNPA2B1 could promote osteoblast differentiation and that overexpression of hnRNPA2B1 attenuated the suppression of osteoblast differentiation induced by simulated microgravity. We also discovered that melatonin could promote the expression of hnRNPA2B1 under simulated microgravity. Moreover, we found that promotion of osteoblast differentiation by melatonin was partially dependent on hnRNPA2B1. Therefore, this research revealed, for the first time, the role of the melatonin/hnRNPA2B1 axis in osteoblast differentiation under simulated microgravity. Targeting this axis may be a potential protective strategy against microgravity-induced bone loss and osteoporosis.

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.

LMK-235 suppresses osteoclastogenesis and promotes osteoblastogenesis by inhibiting HDAC4.

Osteoblasts and osteoclasts play an important role in maintaining the structural integrity of bone tissue, in which osteoclasts degrade bone structure and osteoblasts restore bone tissue. The imbalance of osteoblast and osteoclast function can lead to many bone-related diseases, such as osteoporosis and inflammatory osteolysis. The drug that can both promote bone formation and inhibit bone loss will be able to treat those diseases. In this study, it was found that LMK-235, an selective HDAC4/5 inhibitor, inhibited the differentiation and maturation of osteoclasts by regulating NF-κB and p-Smad2/3 signaling pathways via inhibition of HDAC4. At the same time, we found that LMK-235 promoted osteoblast mineralization by upregulating Runx2 expression via inhibition of HDAC4. In vivo, LMK-235 was able to alleviate lipopolysaccharide (LPS)-induced calvarial osteolysis and promote the repair of bone defects. Taken together, LMK-235 suppresses osteoclast differentiation and promotes osteoblast formation by inhibiting HDAC4. This may provide a valuable treatment for bone diseases caused by abnormal osteoclast bone resorption and osteoblast bone regeneration.