Staphylococcal lipoteichoic acid promotes osteogenic differentiation of mouse mesenchymal stem cells by increasing autophagic activity

D-alanine suppressed osteoclastogenesis derived from bone marrow macrophages and downregulated ERK/p38 signalling pathways

OBJECTIVES

D-alanine is a residue of the backbone structure of Type Ⅰ Lipoteichoic acid (LTA), which is a virulence factor in inflammation caused by gram-positive bacteria. However, the role of D-alanine in infectious bone destruction has not been investigated. We aimed to explore the role of D-alanine in the proliferation, apoptosis, and differentiation of osteoclasts.

DESIGN

Mouse bone marrow-derived macrophages (BMMs) were isolated as osteoclast precursors and stimulated with D-alanine. Cell proliferation and apoptosis were detected using CCK-8 and flow cytometry, respectively. The formation of osteoclasts morphologically observed by tartrate-resistant acid phosphatase staining (TRAP) and immunofluorescence staining. The expressions of osteoclastogenic genes were measured by real-time RT-PCR. The protein expressions of osteoclastogenic markers, p38, and ERK1/2 MAPK signalling were measured by western blot. The expression level of soluble Sema4D was detected via enzyme-linked immunosorbent assay (ELISA).

RESULTS

The cell proliferation of BMMs was significantly inhibited by D-alanine in a dose-dependent manner. Apoptosis of BMMs was markedly activated with the stimulation of D-alanine. The differentiation of BMMs into osteoclasts was significantly inhibited by D-alanine, and the gene and protein expressions of NFATc1, c-Fos, and Blimp decreased. Western blot showed that D-alanine inhibited the phosphorylated p38 and ERK1/2 signalling pathways of BMMs. Moreover, the expression level of soluble Sema4D significantly decreased in the supernatant of BMMs due to the D-alanine intervention.

CONCLUSION

D-alanine plays a pivotal role in the inhibition of RANKL-induced osteoclastogenesis and might become a potential therapeutic drug for bone-resorptive diseases.

Tough physically crosslinked poly(vinyl alcohol)-based hydrogels loaded with collagen type I to promote bone regeneration in vitro and in vivo

Poly(vinyl alcohol) (PVA) hydrogels exhibit great potential as ideal biomaterials for tissue engineering, owing to their non-toxicity, high water content, and strong biocompatibility. However, limited mechanical strength and low bioactivity have constrained their application in bone tissue engineering. In this study, we have developed a tough PVA-based hydrogel using a facile physical crosslinking method, comprising of PVA, tannic acid (TA), and hydroxyapatite (HA). Systematic experiments were conducted to examine the physicochemical properties of PVA/HA/TA hydrogels, including their compositions, microstructures, and mechanical and rheological properties. The results demonstrated that the PVA/HA/TA hydrogels possessed the porous microstructures and excellent mechanical properties. Furthermore, collagen type I (ColI) was used to further improve the biocompatibility and bioactivity of PVA/HA/TA hydrogels. In vitro experiments revealed that PVA/HA/TA/COL hydrogel could offer a suitable microenvironment for the growth of MC3T3-E1 cells and promote their osteogenic differentiation. Meanwhile, the PVA/HA/TA/COL hydrogel demonstrated the ability to promote bone regeneration and osteointegration in a rat femoral defect model. This study provides a potential strategy for the use of PVA-based hydrogels in bone tissue engineering.

Insight into the effect of biomaterials on osteogenic differentiation of mesenchymal stem cells: A review from a mitochondrial perspective

Bone damage may be triggered by a variety of factors, and the damaged area often requires a bone graft. Bone tissue engineering can serve as an alternative strategy for repairing large bone defects. Mesenchymal stem cells (MSCs), the progenitor cells of connective tissue, have become an important tool for tissue engineering due to their ability to differentiate into a variety of cell types. The precise regulation of the growth and differentiation of the stem cells used for bone regeneration significantly affects the efficiency of this type of tissue engineering. During the process of osteogenic induction, the dynamics and function of localized mitochondria are altered. These changes may also alter the microenvironment of the therapeutic stem cells and result in mitochondria transfer. Mitochondrial regulation not only affects the induction/rate of differentiation, but also influences its direction, determining the final identity of the differentiated cell. To date, bone tissue engineering research has mainly focused on the influence of biomaterials on phenotype and nuclear genotype, with few studies investigating the role of mitochondria. In this review, we provide a comprehensive summary of researches into the role of mitochondria in MSCs differentiation and critical analysis regarding smart biomaterials that are able to "programme" mitochondria modulation was proposed. STATEMENT OF SIGNIFICANCE: : • This review proposed the precise regulation of the growth and differentiation of the stem cells used to seed bone regeneration. • This review addressed the dynamics and function of localized mitochondria during the process of osteogenic induction and the effect of mitochondria on the microenvironment of stem cells. • This review summarized biomaterials which affect the induction/rate of differentiation, but also influences its direction, determining the final identity of the differentiated cell through the regulation of mitochondria.

Alpinetin alleviates osteoporosis by promoting osteogenic differentiation in BMSCs by triggering autophagy via PKA/mTOR/ULK1 signaling

Osteoporosis, a systemic bone disease that is characterized by a reduction in bone mass and destruction of bone microstructure, is becoming a serious problem worldwide. Bone marrow mesenchymal stem cells (BMSCs) can differentiate into bone-forming osteoblasts, and play an important role in maintaining homeostasis of bone metabolism, thus being a potential therapeutic target for osteoporosis. Although the phytochemical alpinetin (APT) has been reported to possess a variety of pharmacological activities, it is still unclear whether APT can influence the osteogenic differentiation of on BMSCs and if it can improve osteoporosis. In this study, we found that APT treatment was able to enhance osteogenic differentiation levels of human BMSCs in vitro and mouse ones in vivo as revealed by multiple osteogenic markers including increased alkaline phosphatase activity and osteocalcin expression. Mechanistically, the protein kinase A (PKA)/mTOR/ULK1 signaling was involved in the action of APT to enhance the osteogenic differentiation of BMSCs. In addition, oral administration of APT significantly mitigated the bone loss in a dexamethasone-induced mouse model of osteoporosis through strengthening PKA signaling and autophagy. Altogether, these data demonstrate that APT promotes osteogenic differentiation in BMSCs by augmenting the PKA/mTOR/ULK1 autophagy signaling, highlighting its potential therapeutic application for treating osteoporotic diseases.

Autophagy Plays Multiple Roles in the Soft-Tissue Healing and Osseointegration in Dental Implant Surgery-A Narrative Review

Dental endo-osseous implants have become a widely used treatment for replacing missing teeth. Dental implants are placed into a surgically created osteotomy in alveolar bone, the healing of the soft tissue lesion and the osseointegration of the implant being key elements to long-term success. Autophagy is considered the major intracellular degradation system, playing important roles in various cellular processes involved in dental implant integration. The aim of this review is an exploration of autophagy roles in the main cell types involved in the healing and remodeling of soft tissue lesions and implant osseointegration, post-implant surgery. We have focused on the autophagy pathway in macrophages, endothelial cells; osteoclasts, osteoblasts; fibroblasts, myofibroblasts and keratinocytes. In macrophages, autophagy modulates innate and adaptive immune responses playing a key role in osteo-immunity. Autophagy induction in endothelial cells promotes apoptosis resistance, cell survival, and protection against oxidative stress damage. The autophagic machinery is also involved in transporting stromal vesicles containing mineralization-related factors to the extracellular matrix and regulating osteoblasts' functions. Alveolar bone remodeling is achieved by immune cells differentiation into osteoclasts; autophagy plays an important and active role in this process. Autophagy downregulation in fibroblasts induces apoptosis, leading to better wound healing by improving excessive deposition of extracellular matrix and inhibiting fibrosis progression. Autophagy seems to be a dual actor on the scene of dental implant surgery, imposing further research in order to completely reveal its positive features which may be essential for clinical efficacy.

Changes of Migration, Immunoregulation and Osteogenic Differentiation of Mesenchymal Stem Cells in Different Stages of Inflammation

Bone infection has always been the focus of orthopedic research. Mesenchymal stem cells (MSCs) are the natural progenitors of osteoblasts, and the process of osteogenesis is triggered in response to different signals from the extracellular matrix. MSCs exert important functions including secretion and immune regulation and also play a key role in bone regeneration. The biological behavior of MSCs in acute and chronic inflammation, especially the transformation between acute inflammation and chronic inflammation, has aroused great interest among researchers. This paper reviews the recent literature and summarizes the behavior and biological characteristics of MSCs in acute and chronic inflammation to stimulate further research on MSCs and treatment of bone diseases.

Autophagy is involved in neurofibromatosis type I gene-modulated osteogenic differentiation in human bone mesenchymal stem cells

Neurofibromatosis type I (NF1) is an autosomal dominant genetic disease that is caused by mutations in the NF1 gene. Various studies have previously demonstrated that the mTOR complex 1 signaling pathway is essential for the NF1-modulated osteogenic differentiation of bone mesenchymal stem cells (BMSCs). Additionally, the mTOR signaling pathway plays a notable role in autophagy. The present study hypothesized that NF1 could modulate the osteogenic differentiation of BMSCs by regulating the autophagic activities of BMSCs. In the present study, human BMSCs were cultured in an osteogenic induction medium. The expression of the NF1 gene was either knocked down or overexpressed by transfection with a specific small interfering RNA (siRNA) targeting NF1 or the pcDNA3.0 NF1-overexpression plasmid, respectively. Autophagic activities of BMSCs (Beclin-1, P62, LC3B I, and LC3B II) were determined using western blotting, electron microscopy, acridine orange (AO) staining and autophagic flux/lysosomal detection by fluorescence microscopy. In addition, the autophagy activator rapamycin (RAPA) and inhibitor 3-methyladenine (3-MA) were used to investigate the effects of autophagy on NF1-modulated osteogenic differentiation in BMSCs. Inhibiting NF1 with siRNA significantly decreased the expression levels of autophagy markers Beclin-1 and LC3B-II, in addition to osteogenic differentiation markers osterix, runt-related transcription factor 2 and alkaline phosphatase. By contrast, overexpressing NF1 with pcDNA3.0 significantly increased their levels. Transmission electron microscopy, AO staining and autophagic flux/lysosomal detection assays revealed that the extent of autophagosome formation was significantly decreased in the NF1-siRNA group but significantly increased in the NF1-pcDNA3.0 group when compared with the NC-siRNA and pcDNA3.0 groups, respectively. In addition, the activity of the PI3K/AKT/mTOR pathway [phosphorylated (p)-PI3K, p-AKT, p-mTOR and p-p70S6 kinase] was significantly upregulated in the NF1-siRNA group compared with the NC-siRNA group, and significantly inhibited in the NF1-pcDNA3.0 group, compared with the pcDNA3.0 group. The knockdown effects of NF1-siRNA on the autophagy and osteogenic differentiation of BMSCs were reversed by the autophagy activator RAPA, while the overexpression effects of NF1-pcDNA3.0 on the autophagy and osteogenic differentiation of BMSCs were reversed by the autophagy inhibitor 3-MA. In conclusion, results from the present study suggest at the involvement of autophagy in the NF1-modulated osteogenic differentiation of BMSCs. Furthermore, NF1 may partially regulate the autophagic activity of BMSCs through the PI3K/AKT/mTOR signaling pathway.

The role of autophagy in the process of osseointegration around titanium implants with micro-nano topography promoted by osteoimmunity

Osteoimmunity plays an important role in the process of implant osseointegration. Autophagy is a conservative metabolic pathway of eukaryotic cells, but whether the interaction between autophagy and osteoimmunity plays a key role in osseointegration remains unclear. In this study, we prepared smooth titanium disks and micro-nano topography titanium disks, to study the immune microenvironment of RAW264.7 cells, and prepared the conditioned medium to study the effect of immune microenvironment on the osteogenesis and autophagy of MC3T3-E1 cells. Autophagy inhibitor 3-MA was used to inhibit autophagy to observe the change of expression of osteogenic markers. The results showed that the micro-nano topography titanium disks could stimulate RAW264.7 cells to differentiate into M2 type, forming an anti-inflammatory immune microenvironment; compared with the control group, the anti-inflammatory immune microenvironment promoted the proliferation and differentiation of osteoblasts better. The anti-inflammatory immune environment activated the autophagy level of osteoblasts, while the expression of osteogenic markers was down-regulated after inhibition of autophagy. These results indicate that anti-inflammatory immune microenvironment can promote cell proliferation and osteogenic differentiation, autophagy plays an important role in this process. This study further explains the mechanism of implant osseointegration in osteoimmune microenvironment, and provides reference for improving implant osseointegration.

Regulation of Bone Cell Differentiation and Activation by Microbe-Associated Molecular Patterns

Gut microbiota has emerged as an important regulator of bone homeostasis. In particular, the modulation of innate immunity and bone homeostasis is mediated through the interaction between microbe-associated molecular patterns (MAMPs) and the host pattern recognition receptors including Toll-like receptors and nucleotide-binding oligomerization domains. Pathogenic bacteria such as Porphyromonas gingivalis and Staphylococcus aureus tend to induce bone destruction and cause various inflammatory bone diseases including periodontal diseases, osteomyelitis, and septic arthritis. On the other hand, probiotic bacteria such as Lactobacillus and Bifidobacterium species can prevent bone loss. In addition, bacterial metabolites and various secretory molecules such as short chain fatty acids and cyclic nucleotides can also affect bone homeostasis. This review focuses on the regulation of osteoclast and osteoblast by MAMPs including cell wall components and secretory microbial molecules under in vitro and in vivo conditions. MAMPs could be used as potential molecular targets for treating bone-related diseases such as osteoporosis and periodontal diseases.

Electrospun polyamide-6/chitosan nanofibers reinforced nano-hydroxyapatite/polyamide-6 composite bilayered membranes for guided bone regeneration

Periodontal defect poses a significant challenge in orthopedics. Guided Bone Regeneration (GBR) membrane is considered as one of the most successful methods applied to reconstruct alveolar bone and then to achieve periodontal defect repair/regeneration. In this paper, a novel polyamide-6/chitosan@nano-hydroxyapatite/polyamide-6 (PA6/CS@n-HA/PA6) bilayered tissue guided membranes by combining a solvent casting and an electrospinning technique was designed. The developed PA6/CS@n-HA/PA6 composites were characterized by a series of tests. The results show that n-HA/PA6 and electrospun PA6/CS layers are tightly bound by molecular interaction and chemical bonding, which enhances the bonding strength between two distinct layers. The porosity and adsorption average pore diameter of the PA6/CS@n-HA/PA6 membranes are 36.90 % and 22.61 nm, respectively. The tensile strength and elastic modulus of PA6/CS@n-HA/PA6 composites are 1.41 ± 0.18 MPa and 7.15 ± 1.09 MPa, respectively. In vitro cell culture studies demonstrate that PA6/CS@n-HA/PA6 bilayered scaffolds have biological safety, good bioactivity, biocompatibility and osteoconductivity.

Lipoteichoic Acid Accelerates Bone Healing by Enhancing Osteoblast Differentiation and Inhibiting Osteoclast Activation in a Mouse Model of Femoral Defects

Lipoteichoic acid (LTA) is a cell wall component of Gram-positive bacteria. Limited data suggest that LTA is beneficial for bone regeneration in vitro. Thus, we used a mouse model of femoral defects to explore the effects of LTA on bone healing in vivo. Micro-computed tomography analysis and double-fluorochrome labeling were utilized to examine whether LTA can accelerate dynamic bone formation in vivo. The effects of LTA on osteoblastogenesis and osteoclastogenesis were also studied in vitro. LTA treatment induced prompt bone bridge formation, rapid endochondral ossification, and accelerated healing of fractures in mice with femoral bone defects. In vitro, LTA directly enhanced indicators of osteogenic factor-induced MC3T3-E1 cell differentiation, including alkaline phosphatase activity, calcium deposition and osteopontin expression. LTA also inhibited osteoclast activation induced by receptor activator of nuclear factor-kappa B ligand. We identified six molecules that may be associated with LTA-accelerated bone healing: monocyte chemoattractant protein 1, chemokine (C-X-C motif) ligand 1, cystatin C, growth/differentiation factor 15, endostatin and neutrophil gelatinase-associated lipocalin. Finally, double-fluorochrome, dynamic-labeling data indicated that LTA significantly enhanced bone-formation rates in vivo. In conclusion, our findings suggest that LTA has promising bone-regeneration properties.

Orthosilicic Acid Accelerates Bone Formation in Human Osteoblast-Like Cells Through the PI3K-Akt-mTOR Pathway

Silicon is one of the essential trace elements in the human body; the deficiency of which may lead to bone diseases. Numerous animal experiments have shown that an appropriate increase in the intake of silicon is beneficial to enhancing bone density and toughness to prevent osteoporosis. However, the molecular mechanisms of the silicon-mediated osteogenesis process have not been sufficiently clarified. In this study, we determined the possible osteogenesis-related mechanisms of orthosilicic acid at a molecular level. We detected the relevant pathway and osteogenic indicators by immunofluorescence (IF), Western blot, alkaline phosphatase (ALP) staining (using 5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium [BCIP/NBT]), ALP enzyme labeling method, osteocalcin (OCN), and N-terminal propeptide of type 1 procollagen (P1NP) enzyme-linked immunosorbent assay (ELISA). We found that orthosilicic acid is capable of enhancing the expression of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K), phospho-protein kinase B (P-Akt), phospho-mammalian target of rapamycin (P-mTOR), and related osteogenic markers (runt-related transcription factor 2 [RUNX2], type I collagen [COL1], ALP, OCN, and P1NP). However, with the addition of PI3K-Akt-mTOR pathway-specific inhibitor LY294002, the expression of PI3K, P-Akt, P-mTOR, RUNX2, COL1, ALP, OCN, and P1NP decreased. The results indicated that the PI3K-Akt-mTOR pathway played a positive regulatory role in the process of orthosilicic acid-mediated osteogenesis in vitro.

Interleukin-37 inhibits osteoclastogenesis and alleviates inflammatory bone destruction

Excessive osteoclast formation is one of the important pathological features of inflammatory bone destruction. Interleukin‐37 (IL‐37) is an anti‐inflammatory agent that is present throughout the body, but it displays low physiological retention. In our study, high levels of the IL‐37 protein were detected in clinical specimens from patients with bone infections. However, the impact of IL‐37 on osteoclast formation remains unclear. Next, IL‐37 alleviated the inflammatory bone destruction in the mouse in vivo. We used receptor activator of nuclear factor‐κB ligand and lipopolysaccharide to trigger osteoclastogenesis under physiological and pathological conditions to observe the role of IL‐37 in this process and explore the potential mechanism of this phenomenon. In both induction models, IL‐37 exerted inhibitory effects on osteoclast differentiation and bone resorption. Furthermore, IL‐37 decreased the phosphorylation of inhibitor of κBα and p65 and the expression of nuclear factor of activated T cells c1, while the dimerization inhibitor of myeloid differentiation factor 88 reversed the effects. These data provide evidence that IL‐37 modulates osteoclastogenesis and a theoretical basis for the clinical application of IL‐37 as a treatment for bone loss–related diseases.

Enterococcus faecalis lipoteichoic acid regulates macrophages autophagy via PI3K/Akt/mTOR pathway

Enterococcus faecalis (E. faecalis) infection is considered an important etiological factor for the development of persistent apical periodontitis (PAP), but the exact mechanisms of autophagy between E. faecalis and immune cells remain unknown. In this study, we elucidated how E. faecalis lipoteichoic acid (LTA) is associated with macrophages autophagy. We found that E. faecalis LTA apparently activated macrophage autophagy with significant increase of autophagosomes and autophagy relative protein. Meanwhile, we noticed significantly decreasing expression of p-Akt and p-mTOR. However, these effect were absent in macrophages knockdown of Beclin1. In summary, these findings suggested E. faecalis LTA may increased macrophages autophagy via inhibiting PI3K/Akt/mTOR pathway and this process was Beclin1 dependent.