Autophagy activation facilitates mechanical stimulation-promoted osteoblast differentiation and ameliorates hindlimb unloading-induced bone loss

FoxO3 Regulates Mouse Bone Mesenchymal Stem Cell Fate and Bone-Fat Balance During Skeletal Aging

Age-related osteoporosis is characterized by an imbalance between osteogenic and adipogenic differentiation in bone mesenchymal stem cells (BMSCs). Forkhead box O 3 (FoxO3) transcription factor is involved in lifespan and cell differentiation. In this study, we explore whether FoxO3 regulates age-related bone loss and marrow fat accumulation. The expression levels of FoxO3 in BMSCs during aging were detected in vivo and in vitro. To explore the role of FoxO3 in osteogenic and adipogenic differentiation, primary BMSCs were isolated from young and aged mice. FoxO3 expression was modulated by adenoviral vector transfection. The role of FoxO3 in bone-fat balance was evaluated by alizarin red S staining, oil red O staining, quantitative reverse transcription-polymerase chain reaction, Western blot, and histological analysis. Age-related bone loss and fat deposit are associated with downregulation of FoxO3. Overexpression of FoxO3 alleviated age-related bone loss and marrow fat accumulation in aged mice. Mechanistically, FoxO3 reduced adipogenesis and enhanced osteogenesis of BMSCs via downregulation of PPAR-γ and Notch signaling, respectively. In conclusion, FoxO3 is an essential factor controlling the fate of BMSCs and is a potential target for the prevention of age-related osteoporosis.

Quadriceps muscle atrophy after non-invasive anterior cruciate ligament injury: evidence linking to autophagy and mitophagy

Introduction: Anterior cruciate ligament (ACL) injury is frequently accompanied by quadriceps muscle atrophy, a process closely linked to mitochondrial health and mitochondria-specific autophagy. However, the temporal progression of key quadricep atrophy-mediating events following ACL injury remains poorly understood. To advance our understanding, we conducted a longitudinal study to elucidate key parameters in quadriceps autophagy and mitophagy. Methods: Long-Evans rats were euthanized at 7, 14, 28, and 56 days after non-invasive ACL injury that was induced via tibial compression overload; controls were not injured. Vastus lateralis muscle was extracted, and subsequent immunoblotting analysis was conducted using primary antibodies targeting key proteins involved in autophagy and mitophagy cellular processes. Results: Our findings demonstrated dynamic changes in autophagy and mitophagy markers in the quadriceps muscle during the recovery period after ACL injury. The early response to the injury was characterized by the induction of autophagy at 14 days (Beclin1), indicating an initial cellular response to the injury. Subsequently, at 14 days we observed increase in the elongation of autophagosomes (Atg4B), suggesting a potential remodeling process. The autophagosome flux was also augmented between 14- and 28 days (LC3-II/LC3-I ratio and p62). Notably, at 56 days, markers associated with the elimination of damaged mitochondria were elevated (PINK1, Parkin, and VDAC1), indicating a possible ongoing cellular repair and restoration process. Conclusion: These data highlight the complexity of muscle recovery after ACL injury and underscore the overlooked but crucial role of autophagy and mitophagy in promoting the recovery process.

The Role of the Kynurenine Pathway in the Pathophysiology of Frailty, Sarcopenia, and Osteoporosis

Tryptophan is an essential nutrient required to generate vitamin B3 (niacin), which is mainly involved in energy metabolism and DNA production. Alterations in tryptophan metabolism could have significant effects on aging and musculoskeletal health. The kynurenine pathway, essential in tryptophan catabolism, is modulated by inflammatory factors that are increased in older persons, a process known as inflammaging. Osteoporosis, sarcopenia, osteosarcopenia, and frailty have also been linked with chronically increased levels of inflammatory factors. Due to the disruption of the kynurenine pathway by chronic inflammation and/or changes in the gut microbiota, serum levels of toxic metabolites are increased and are associated with the pathophysiology of those conditions. In contrast, anabolic products of this pathway, such as picolinic acid, have demonstrated a positive effect on skeletal muscle and bone. In addition, physical activity can modulate this pathway by promoting the secretion of anabolic kynurenines. According to the evidence collected, kynurenines could have a promising role as biomarkers for osteoporosis sarcopenia, osteosarcopenia, and frailty in older persons. In addition, some of these metabolites could become important targets for developing new pharmacological treatments for these conditions.

Exercise improved bone health in aging mice: a role of SIRT1 in regulating autophagy and osteogenic differentiation of BMSCs

Introduction

This study was designed to investigate the effect of running exercise on improving bone health in aging mice and explore the role of the SIRT1 in regulating autophagy and osteogenic differentiation of Bone marrow Mesenchymal Stem Cells (BMSCs).

Methods

Twelve-month-old male C57BL/6J mice were used in this study as the aging model and were assigned to treadmill running exercise for eight weeks. Non-exercise male C57BL/6J mice of the same old were used as aging control and five-month-old mice were used as young controls. BMSCs were isolated from mice and subjected to mechanical stretching stimulation in vitro.

Results

The results showed that aging mice had lower bone mass, bone mineral density (BMD), and autophagy than young mice, while running exercise improved BMD and bone mass as well as upregulated autophagy in bone cells. Mechanical loading increased osteogenic differentiation and autophagy in BMSCs, and knockdown of SIRT1 in BMSCs demonstrated that SIRT1-regulated autophagy involved the mechanical loading activation of osteogenic differentiation.

Conclusion

Taken together, this study revealed that exercise improved bone health during aging by activating bone formation, which can be attributed to osteogenic differentiation of BMSCs through the activation of SIRT1-mediated autophagy. The mechanisms underlying this effect may involve mechanical loading.

Osteocytes autophagy mediated by mTORC2 activation controls osteoblasts differentiation and osteoclasts activities under mechanical loading

Autophagy is an important mechanosensitive response for cellular homeostasis and survival in osteocytes. However, the mechanism and its effect on bone metabolism have not yet clarified. The objective of this study was to evaluate how compressive cyclic force (CCF) induced autophagic response in osteocytes and to determine the effect of mechanically induced-autophagy on bone cells including osteocytes, osteoblasts, and osteoclasts. Autophagic puncta observed in MLO-Y4 cells increased after exposure to CCF. The upregulated levels of the LC3-II isoform and the degradation of p62 further confirmed the increased autophagic flux. Additionally, ATP synthesis and release, osteocalcin (OCN) expression, and cell survival increased in osteocytes as well. The Murine osteoblasts MC3T3-E1 cells and RAW 264.7 macrophage cells were cultured in conditioned medium collected from MLO-Y4 cells subjected to CCF. The concentration of FGF23 increased and the concentrations of SOST and M-CSF and RANKL/OPG ratio decreased significantly in the conditioned medium. Moreover, the promotion of osteogenic differentiation in MC3T3-E1 cells and inhibition of osteoclastogenesis and function in RAW 264.7 cells were significantly attenuated when osteocytes autophagy was inhibited by siAtg7. Our findings suggested that CCF induced protective autophagy in osteocytes and subsequently enhanced osteocytes survival and osteoblasts differentiation and downregulated osteoclasts activities. Further study revealed that CCF induced autophagic response in osteocytes through mechanistic target of rapamycin complex 2 (mTORC2) activation. In conclusion, CCF-induced osteocytes autophagy upon mTORC2 activation promoted osteocytes survival and osteogenic response and decreased osteoclastic function. Thus, osteocytes autophagy will provide a promising target for better understanding of bone physiology and treatment of bone diseases.

Fluid Shear Stress Regulates Osteogenic Differentiation via AnnexinA6-Mediated Autophagy in MC3T3-E1 Cells

Fluid shear stress (FSS) facilitates bone remodeling by regulating osteogenic differentiation, and extracellular matrix maturation and mineralization. However, the underlying molecular mechanisms of how mechanical stimuli from FSS are converted into osteogenesis remain largely unexplored. Here, we exposed MC3T3-E1 cells to FSS with different intensities (1 h FSS with 0, 5, 10, and 20 dyn/cm² intensities) and treatment durations (10 dyn/cm² FSS with 0, 0.5, 1, 2 and 4 h treatment). The results demonstrate that the 1 h of 10 dyn/cm² FSS treatment greatly upregulated the expression of osteogenic markers (Runx2, ALP, Col I), accompanied by AnxA6 activation. The genetic ablation of AnxA6 suppressed the autophagic process, demonstrating lowered autophagy markers (Beclin1, ATG5, ATG7, LC3) and decreased autophagosome formation, and strongly reduced osteogenic differentiation induced by FSS. Furthermore, the addition of autophagic activator rapamycin to AnxA6 knockdown cells stimulated autophagy process, and coincided with more expressions of osteogenic proteins ALP and Col I under both static and FSS conditions. In conclusion, the findings in this study reveal a hitherto unidentified relationship between FSS-induced osteogenic differentiation and autophagy, and point to AnxA6 as a key mediator of autophagy in response to FSS, which may provide a new target for the treatment of osteoporosis and other diseases.

Autophagy Regulates Osteogenic Differentiation of Human Periodontal Ligament Stem Cells Induced by Orthodontic Tension

Tooth movement is the core of orthodontics. Osteogenesis of the tension side under orthodontic force has great significance on tooth movement and stability, which involves complex mechanical and biological signal transduction. However, the mechanism remains unclear. Through in vitro cell studies, we observed the increased expression levels of osteogenesis-related factors and autophagy-related factors during the osteogenic differentiation of mesenchymal stem cells induced by orthodontic force. The change trend of autophagy-related factors and osteogenesis-related factors is similar, which indicates the involvement of autophagy in osteogenesis. In the study of autophagy-related gene ATG7 silenced cells, the expression level of autophagy was significantly inhibited, and the expression level of osteogenesis-related factors also decreased accordingly. Through drug regulation, we observed that the increase of autophagy level could effectively promote osteogenic differentiation, while the decrease of the autophagy level inhibited this process to some extent. Therefore, autophagy plays an important role in the osteogenic differentiation of mesenchymal stem cells induced by orthodontic force, which provides a novel idea useful for orthodontic treatment in promoting periodontal tissue remodeling and accelerating tooth movement.

Autophagy Is Possibly Involved in Osteoblast Responses to Mechanical Loadings

Both mechanical loading and autophagy play important roles in regulating bone growth and remodeling, but the relationship between the two remains unclear. In this study, we examined bone structure with micro-CT imaging and measured bone mechanical properties with three-point bending experiments using bones from wild-type (WT) mice and conditional knockout (cKO) mice with Atg7 deletion in their osteoblasts. We found that the knockout mice had significantly less bone volume, bone thickness, bone ultimate breaking force, and bone stiffness compared to wild-type mice. Additionally, bone marrow cells from knockout mice had reduced differentiation and mineralization capacities in terms of alkaline phosphatase and calcium secretion, as well as Runx2 and osteopontin expression. Knockout mice also had significantly less relative bone formation rate due to mechanical loading. Furthermore, we found that the osteoblasts from wild-type mice had stronger responses to mechanical stimulation compared to autophagy-deficient osteoblasts from knockout mice. When inhibiting autophagy with 3 MA in wild-type osteoblasts, we found similar results as we did in autophagy-deficient osteoblasts. We also found that mechanical loading-induced ATP release is able to regulate ERK1/2, Runx2, alkaline phosphatase, and osteopontin activities. These results suggest that the ATP pathway may play an important role in the possible involvement of autophagy in osteoblast mechanobiology.

Concentrated Growth Factors Promote hBMSCs Osteogenic Differentiation in a Co-Culture System With HUVECs

Osteogenesis is a complex physiologic process that occurs during bone regeneration. This process requires several growth factors that act on bone marrow-derived mesenchymal stem cells (BMSCs). Concentrated growth factor (CGF) is a new-generation platelet-rich derivative that is an appealing autologous material for application in tissue repair and bone regenerative medicine because it contains a variety of fibrin and growth factors. In this study, the effects of CGF on the proliferation and osteogenic differentiation of hBMSCs and human umbilical vein endothelial cells (HUVECs) were explored with in vitro cell co-culture experiments. HBMSCs and HUVECs were directly co-cultured at the ratio of 1:2 under different concentrations (0, 2, 5, 10, 20%) of CGF for 7 days. Alkaline phosphatase (ALP) staining and quantitative reverse transcription polymerase chain reaction were used to detect the effects of CGF on the expression of osteogenic genes (ALP, osteocalcin [OCN], type I collagen [COL-1], Runt-related transcription factor 2 [RUNX2]) and connexin 43 (CX43). RNA sequencing was used to explore potential regulated genes and signaling pathways that affect the osteogenesis of co-cultured hBMSCs exposed to CGF. The results showed higher expressions of ALP, COL-1, RUNX2, OCN, and CX43 in the direct co-culture group containing 10% CGF compared to the direct co-culture group without CGF and the indirect co-culture group. In summary, 10% CGF can significantly promote osteogenesis in hBMSCs directly co-cultured with HUVECs. Intercellular communication between the direct co-culture of hBMSCs and HUVECs through CX43 may be one of the main regulatory mechanisms.

Current Aspects on the Pathophysiology of Bone Metabolic Defects during Progression of Scoliosis in Neurofibromatosis Type 1

Neurofibromatosis type 1 (NF1), which is the most common phacomatoses, is an autosomal dominant disorder characterized by clinical presentations in various tissues and organs, such as the skin, eyes and nervous and skeletal systems. The musculoskeletal implications of NF1 include a variety of deformities, including scoliosis, kyphoscoliosis, spondylolistheses, congenital bony bowing, pseudarthrosis and bone dysplasia. Scoliosis is the most common skeletal problem, affecting 10-30% of NF1 patients. Although the pathophysiology of spinal deformities has not been elucidated yet, defects in bone metabolism have been implicated in the progression of scoliotic curves. Measurements of Bone Mineral Density (BMD) in the lumbar spine by using dual energy absorptiometry (DXA) and quantitative computer tomography (QCT) have demonstrated a marked reduction in Z-score and osteoporosis. Additionally, serum bone metabolic markers, such as vitamin D, calcium, phosphorus, osteocalcin and alkaline phosphatase, have been found to be abnormal. Intraoperative and histological vertebral analysis confirmed that alterations of the trabecular microarchitecture are associated with inadequate bone turnover, indicating generalized bone metabolic defects. At the molecular level, loss of function of neurofibromin dysregulates Ras and Transforming Growth factor-β1 (TGF-β1) signaling and leads to altered osteoclastic proliferation, osteoblastic activity and collagen production. Correlation between clinical characteristics and molecular pathways may provide targets for novel therapeutic approaches in NF1.

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.

TCP/PLGA composite scaffold loaded rapamycin in situ enhances lumbar fusion by regulating osteoblast and osteoclast activity

The purpose of this study was to develop a novel β-tricalcium phosphate (TCP)/poly (D,L-lactic-co-glycolic acid) (PLGA) composite scaffold loaded with rapamycin that can regulate the activity of osteoblasts and osteoclasts for lumbar fusion. The TCP/PLGA composite scaffold was fabricated by cryogenic three-dimensional printing techniques and then loaded with rapamycin in situ. The structural surface morphology of the composite scaffold was tested with scanning electron microscope. To evaluate the biocompatibility of the composite scaffold in vitro, bone marrow mesenchymal stem cells (BMSCs) were cultured on the TCP/PLGA composite scaffold slide and tested with Live/Dead Viability Kit. The effect of rapamycin on osteoclast and osteoblast was studied with staining and Western blotting. The in vitro results showed that the rapamycin-loaded TCP/PLGA composite scaffold showed good biocompatibility with BMSC and released rapamycin obviously promoted the osteoblast differentiation and mineralization. In vivo study, the TCP/PLGA composite scaffold loaded with rapamycin were implanted in lumbar fusion model and study with micro-computed tomography scanning, hematoxylin-eosin, Masson, and immune-histological staining, to evaluate the effect of rapamycin on bone fusion. The in vivo results demonstrated that rapamycin-loaded TCP/PLGA composite scaffold could enhance bone formation by regulating osteoblast and osteoclast activity, respectively. In this study, the TCP/PLGA composite scaffold loaded with rapamycin was confirmed to provide great compatibility and improved performance in lumbar fusion by regulating osteoblastic and osteoclastic activity and would be a promising composite biomaterial for bone tissue engineering.

Oxidative stress induces downregulation of TP53INP2 and suppresses osteogenic differentiation of BMSCs during osteoporosis through the autophagy degradation pathway

Oxidative stress plays an important role in the pathogenesis of osteoporosis and impaired bone formation. However, the mechanisms behind which oxidative stress represses bone formation remains unclear. TP53INP2, a target of the tumor suppressor p53, is ubiquitously expressed in various cell types including BMSCs and contributes to autophagosome formation by recruiting ubiquitinated substrates to autophagosomes for degradation. However, little is known about its function in BMSCs and its relation to osteoporosis. In this study, first, we verified that the expression of TP53INP2 was persistently decreased in BMSCs derived from osteoporosis patients and OVX mice, and that the antioxidant N-acetylcysteine could ameliorate this decreased TP53INP2 level in vitro. Second, we identified that the mRNA and protein levels of TP53INP2 decreased in BMSCs under H₂O₂ induced oxidative stress in a dose-dependent manner, with resultant co-location of LC3 and TP53INP2. Additionally, the autophagy-lysosome system was involved in the degradation process of TP53INP2 and applying autophagy inhibitors (Baf-A1) could significantly increase the TP53INP2 levels in murine and human BMSCs under oxidative stress. Third, gain- and loss-of-function assays revealed that knockdown of TP53INP2 inhibited osteogenic differentiation of BMSCs, while overexpression of TP53INP2 promoted osteogenic differentiation of BMSCs in vitro. Further analysis demonstrated that TP53INP2 promoted osteogenic differentiation of BMSCs by activating Wnt/β-cantenin signaling. DKK1, an inhibitor of Wnt signaling, resulted in osteogenic defects of BMSCs that had over-expressed TP53INP2. Lithium, a Wnt/β-catenin activator, improved the mineralization ability in TP53INP2-knockdown BMSCs. Moreover, restoring TP53INP2 levels recovered OVX-derived BMSCs osteogenic differentiation and attenuated bone loss in OVX mice. Taken together, our study indicated that oxidative stress-induced downregulation of TP53INP2 suppressed osteogenic differentiation of BMSCs during osteoporosis and was mediated by the autophagy degradation pathway. These findings may introduce a novel therapeutic target for osteoporosis.

l-cysteine-modified chiral gold nanoparticles promote periodontal tissue regeneration

Gold nanoparticles (AuNPs) with surface-anchored molecules present tremendous potential in tissue regeneration. However, little is known about chiral-modified AuNPs. In this study, we successfully prepared L/D-cysteine-anchored AuNPs (L/D-Cys-AuNPs) and studied the effects of chiral-modified AuNPs on osteogenic differentiation and autophagy of human periodontal ligament cells (hPDLCs) and periodontal tissue regeneration. In vitro, more L-Cys-AuNPs than D-Cys-AuNPs tend to internalize in hPDLCs. L-Cys-AuNPs also significantly increased the expression of alkaline phosphatase, collagen type 1, osteocalcin, runt-related transcription factor 2, and microtubule-associated protein light chain 3 II and decreased the expression of sequestosome 1 in hPDLCs compared to the expression levels in the hPDLCs treated by D-Cys-AuNPs. In vivo tests in a rat periodontal-defect model showed that L-Cys-AuNPs had the greatest effect on periodontal-tissue regeneration. The activation of autophagy in L-Cys-AuNP-treated hPDLCs may be responsible for the cell differentiation and tissue regeneration. Therefore, compared to D-Cys-AuNPs, L-Cys-AuNPs show a better performance in cellular internalization, regulation of autophagy, cell osteogenic differentiation, and periodontal tissue regeneration. This demonstrates the immense potential of L-Cys-AuNPs for periodontal regeneration and provides a new insight into chirally modified bioactive nanomaterials.

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L/D-Cys-AuNPs exert a chirality-dependent effect on hPDLCs.

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L-Cys-AuNPs efficiently induced osteogenic differentiation in hPDLCs.

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L-Cys-AuNPs significantly improved periodontal tissue regeneration.

The miRNA-15b/USP7/KDM6B axis engages in the initiation of osteoporosis by modulating osteoblast differentiation and autophagy

Osteoporosis is a metabolic disease that results from oxidative stress or inflammation in renal disorders. microRNAs (miRNAs) are recently implicated to participate in osteoporosis, but the mechanism remains largely unexplored. Herein, we aimed to explore the potential role of miR-15b in osteoblast differentiation and autophagy in osteoporosis. We established osteoporosis models through ovariectomy and determined that miR-15b was highly expressed whereas USP7 and KDM6B were poorly expressed in tissue of osteoporosis mice. Treatment of silenced miR-15b resulted in the elevation of decreased bone mineral density (BMD), the maximum elastic stress and the maximum load of osteoporosis mice. In osteoblasts, miR-15 overexpression decreased proliferation but suppressed the cell differentiation and autophagy, accompanied with decreased expression of USP7. Mechanistically, miR-15 bound and inhibited USP7 expression, while overexpression of USP7 promoted autophagy of osteoblasts. USP7, importantly, strengthened the stability of KDM6B and promoted KDM6B expression. MG132 protease inhibitor increased KDM6B and USP7 expression in osteoblasts. Silencing of KDM6B reversed the promoting effect on autophagy and proliferation induced by overexpression of USP7. Taken altogether, miR-15b inhibits osteoblast differentiation and autophagy to aggravate osteoporosis by targeting USP7 to regulate KDM6B expression.

The mTOR/ULK1 signaling pathway mediates the autophagy-promoting and osteogenic effects of dicalcium silicate nanoparticles

A novel bioactive inorganic material containing silicon, calcium and oxygen, calcium silicate (Ca₂SiO₄, C₂S) with a CaO-SiO₂ ingredient, has been identified as a potential candidate for artificial bone. Autophagy has an essential function in adult tissue homoeostasis and tumorigenesis. However, little is known about whether silicate nanoparticles (C₂S NPs) promote osteoblastic differentiation by inducing autophagy. Here we investigated the effects of C₂S NPs on bone marrow mesenchymal stem cell differentiation (BMSCs) in osteoblasts. Furthermore, we identified the osteogenic gene and protein expression in BMSCs treated with C₂S NPs. We found that autophagy is important for the ability of C₂S NPs to induce osteoblastic differentiation of BMSCs. Our results showed that treatment with C₂S NPs upregulated the expression of BMP2, UNX2, and OSX in BMSCs, and significantly promoted the expression of LC3 and Beclin, while P62 (an autophagy substrate) was downregulated. C₂S NP treatment could also enhance Alizarin red S dye (ARS), although alkaline phosphatase (ALP) activity was not significantly changed. However, all these effects could be partially reversed by 3-MA. We then detected potential signaling pathways involved in this biological effect and found that C₂S NPs could activate autophagy by suppressing mTOR and facilitating ULK1 expression. Autophagy further activated β-catenin expression and promoted osteogenic differentiation. In conclusion, C₂S NPs promote bone formation and osteogenic differentiation in BMSCs by activating autophagy. They achieve this effect by activating mTOR/ULK1, inducing autophagy, and subsequently triggering the WNT/β-catenin pathway to boost the differentiation and biomineralization of osteoblasts.

The neurofibromatosis type I gene promotes autophagy via mTORC1 signalling pathway to enhance new bone formation after fracture

Bone fracture is one of the most common injuries. Despite the high regenerative capacity of bones, failure of healing still occurs to near 10% of the patients. Herein, we aim to investigate the modulatory role of neurofibromatosis type I gene (NF1) to osteogenic differentiation of bone marrow–derived mesenchymal stem cells (BMSCs) and new bone formation after fracture in a rat model. We studied the NF1 gene expression in normal and non‐union bone fracture models. Then, we evaluated how NF1 overexpression modulated osteogenic differentiation of BMSCs, autophagy activity, mTORC1 signalling and osteoclastic bone resorption by qRT‐PCR, Western blot and immunostaining assays. Finally, we injected lentivirus‐NF1 (Lv‐NF1) to rat non‐union bone fracture model and analysed the bone formation process. The NF1 gene expression was significantly down‐regulated in non‐union bone fracture group, indicating NF1 is critical in bone healing process. In the NF1 overexpressing BMSCs, autophagy activity and osteogenic differentiation were significantly enhanced. Meanwhile, the NF1 overexpression inhibited mTORC1 signalling and osteoclastic bone resorption. In rat non‐union bone fracture model, the NF1 overexpression significantly promoted bone formation during fracture healing. In summary, we proved the NF1 gene is critical in non‐union bone healing, and NF1 overexpression promoted new bone formation after fracture by enhancing autophagy and inhibiting mTORC1 signalling. Our results may provide a novel therapeutic clue of promoting bone fracture healing.

Biological Factors, Metals, and Biomaterials Regulating Osteogenesis through Autophagy

Bone loss raises great concern in numerous situations, such as ageing and many diseases and in both orthopedic and dentistry fields of application, with an extensive impact on health care. Therefore, it is crucial to understand the mechanisms and the determinants that can regulate osteogenesis and ensure bone balance. Autophagy is a well conserved lysosomal degradation pathway, which is known to be highly active during differentiation and development. This review provides a revision of the literature on all the exogen factors that can modulate osteogenesis through autophagy regulation. Metal ion exposition, mechanical stimuli, and biological factors, including hormones, nutrients, and metabolic conditions, were taken into consideration for their ability to tune osteogenic differentiation through autophagy. In addition, an exhaustive overview of biomaterials, both for orthopedic and dentistry applications, enhancing osteogenesis by modulation of the autophagic process is provided as well. Already investigated conditions regulating bone regeneration via autophagy need to be better understood for finely tailoring innovative therapeutic treatments and designing novel biomaterials.

Recombinant Irisin Prevents the Reduction of Osteoblast Differentiation Induced by Stimulated Microgravity through Increasing β-Catenin Expression

Background: Irisin, a novel exercise-induced myokine, was shown to mediate beneficial effects of exercise in osteoporosis. Microgravity is a major threat to bone homeostasis of astronauts during long-term spaceflight, which results in decreased bone formation. Methods: The hind-limb unloading mice model and a random position machine are respectively used to simulate microgravity in vivo and in vitro. Results: We demonstrate that not only are bone formation and osteoblast differentiation decreased, but the expression of fibronectin type III domain-containing 5 (Fdnc5; irisin precursor) is also downregulated under simulated microgravity. Moreover, a lower dose of recombinant irisin (r-irisin) (1 nM) promotes osteogenic marker gene (alkaline phosphatase (Alp), collagen type 1 alpha-1(ColIα1)) expressions, ALP activity, and calcium deposition in primary osteoblasts, with no significant effect on osteoblast proliferation. Furthermore, r-irisin could recover the decrease in osteoblast differentiation induced by simulated microgravity. We also find that r-irisin increases β-catenin expression and partly neutralizes the decrease in β-catenin expression induced by simulated microgravity. In addition, β-catenin overexpression could also in part attenuate osteoblast differentiation reduction induced by simulated microgravity. Conclusions: The present study is the first to show that r-irisin positively regulates osteoblast differentiation under simulated microgravity through increasing β-catenin expression, which may reveal a novel mechanism, and it provides a prevention strategy for bone loss and muscle atrophy induced by microgravity.

Kynurenine inhibits autophagy and promotes senescence in aged bone marrow mesenchymal stem cells through the aryl hydrocarbon receptor pathway

Osteoporosis is an age-related deterioration in bone health that is, at least in part, a stem cell disease. The different mechanisms and signaling pathways that change with age and contribute to the development of osteoporosis are being identified. One key upstream mechanism that appears to target a number of osteogenic pathways with age is kynurenine, a tryptophan metabolite and an endogenous Aryl hydrocarbon receptor (AhR) agonist. The AhR signaling pathway has been reported to promote aging phenotypes across species and in different tissues. We previously found that kynurenine accumulates with age in the plasma and various tissues including bone and induces bone loss and osteoporosis in mice. Bone marrow mesenchymal stem cells (BMSCs) are responsible for osteogenesis, adipogenesis, and overall bone regeneration. In the present study, we investigated the effect of kynurenine on BMSCs, with a focus on autophagy and senescence as two cellular processes that control BMSCs proliferation and differentiation capacity. We found that physiological levels of kynurenine (10 and 100 μM) disrupted autophagic flux as evidenced by the reduction of LC3B-II, and autophagolysosomal production, as well as a significant increase of p62 protein level. Additionally, kynurenine also induced a senescent phenotype in BMSCs as shown by the increased expression of several senescence markers including senescence associated β-galactosidase in BMSCs. Additionally, western blotting reveals that levels of p21, another marker of senescence, also increased in kynurenine-treated BMSCs, while senescent-associated aggregation of nuclear H3K9me3 also showed a significant increase in response to kynurenine treatment. To validate that these effects are in fact due to AhR signaling pathway, we utilized two known AhR antagonists: CH-223191, and 3',4'-dimethoxyflavone to try to block AhR signaling and rescue kynurenine /AhR mediated effects. Indeed, AhR inhibition restored kynurenine-suppressed autophagy levels as shown by levels of LC3B-II, p62 and autophagolysosomal formation demonstrating a rescuing of autophagic flux. Furthermore, inhibition of AhR signaling prevented the kynurenine-induced increase in senescence associated β-galactosidase and p21 levels, as well as blocking aggregation of nuclear H3K9me3. Taken together, our results suggest that kynurenine inhibits autophagy and induces senescence in BMSCs via AhR signaling, and that this may be a novel target to prevent or reduce age-associated bone loss and osteoporosis.

Increased Gene Expression of RUNX2 and SOX9 in Mesenchymal Circulating Progenitors Is Associated with Autophagy during Physical Activity

Lack of physical exercise is considered an important risk factor for chronic diseases. On the contrary, physical exercise reduces the morbidity rates of obesity, diabetes, bone disease, and hypertension. In order to gain novel molecular and cellular clues, we analyzed the effects of physical exercise on differentiation of mesenchymal circulating progenitor cells (M-CPCs) obtained from runners. We also investigated autophagy and telomerase-related gene expression to evaluate the involvement of specific cellular functions in the differentiation process. We performed cellular and molecular analyses in M-CPCs, obtained by a depletion method, of 22 subjects before (PRE RUN) and after (POST RUN) a half marathon performance. In order to prove our findings, we performed also in vitro analyses by testing the effects of runners' sera on a human bone marrow-derived mesenchymal stem (hBM-MSC) cell line. PCR array analyses of PRE RUN versus POST RUN M-CPC total RNAs put in evidence several genes which appeared to be modulated by physical activity. Our results showed that physical exercise promotes differentiation. Osteogenesis-related genes as RUNX2, MSX1, and SPP1 appeared to be upregulated after the run; data showed also increased levels of BMP2 and BMP6 expressions. SOX9, COL2A1, and COMP gene enhanced expression suggested the induction of chondrocytic differentiation as well. The expression of telomerase-associated genes and of two autophagy-related genes, ATG3 and ULK1, was also affected and correlated positively with MSC differentiation. These data highlight an attractive cellular scenario, outlining the role of autophagic response to physical exercise and suggesting new insights into the benefits of physical exercise in counteracting chronic degenerative conditions.

Mitochondrial Phosphoenolpyruvate Carboxykinase Regulates Osteogenic Differentiation by Modulating AMPK/ULK1-Dependent Autophagy

Mitochondrial phosphoenolpyruvate carboxykinase (PCK2) is a rate‐limiting enzyme that plays critical roles in multiple physiological processes. The decompensation of PCK2 leads to various energy metabolic disorders. However, little is known regarding the effects of PCK2 on osteogenesis by human mesenchymal stem cells (hMSCs). Here, we report a novel function of PCK2 as a positive regulator of MSCs osteogenic differentiation. In addition to its well‐known role in anabolism, we demonstrate that PCK2 regulates autophagy. PCK2 deficiency significantly suppressed autophagy, leading to the impairment of osteogenic capacity of MSCs. On the other hand, autophagy was promoted by PCK2 overexpression; this was accompanied by increased osteogenic differentiation of MSCs. Moreover, PCK2 regulated osteogenic differentiation of MSCs via AMP‐activated protein kinase (AMPK)/unc‐51 like autophagy activating kinase 1(ULK1)‐dependent autophagy. Collectively, our present study unveiled a novel role for PCK2 in integrating autophagy and bone formation, providing a potential target for stem cell‐based bone tissue engineering that may lead to improved therapies for metabolic bone diseases. stem cells2019;37:1542–1555

Autophagy in bone homeostasis and the onset of osteoporosis

Autophagy is an evolutionarily conserved intracellular process, in which domestic cellular components are selectively digested for the recycling of nutrients and energy. This process is indispensable for cell homeostasis maintenance and stress responses. Both genetic and functional studies have demonstrated that multiple proteins involved in autophagic activities are critical to the survival, differentiation, and functioning of bone cells, including osteoblasts, osteocytes, and osteoclasts. Dysregulation at the level of autophagic activity consequently disturbs the balance between bone formation and bone resorption and mediates the onset and progression of multiple bone diseases, including osteoporosis. This review aims to introduce the topic of autophagy, summarize the understanding of its relevance in bone physiology, and discuss its role in the onset of osteoporosis and therapeutic potential.

HIF-1α Regulates Glucocorticoid-Induced Osteoporosis Through PDK1/AKT/mTOR Signaling Pathway

Long-term and high dose glucocorticoid treatment can cause decreased viability and function of osteoblasts, which leads to osteoporosis and osteonecrosis. In this study, we investigated the role and mechanism of action of HIF-1α in glucocorticoid-induced osteogenic inhibition in MC3T3-E1 cells. Our results showed that HIF-1α protein expression was reduced when MC3T3-E1 cells were exposed to dexamethasone (Dex) at varying concentrations ranging from 10^-9 to 10^-6 M. PDK1 expression was also decreased in MC3T3-E1 cells after dexamethasone treatment. MC3T3-E1 cells when treated with the glucocorticoid receptor antagonist RU486 along with dexamethasone showed enhanced HIF-1α expression. In addition, upregulated expression of HIF-1α was capable of promoting the osteogenic ability of MC3T3-E1 cells and PDK1 expression. However, the HIF-1α antagonist 2-methoxyestradiol (2-ME) had a reverse effect in MC3T3-E1 cells exposed to dexamethasone. Furthermore, the PDK1 antagonist dichloroacetate could repress the osteogenic ability of MC3T3-E1 cells, although HIF-1α was upregulated when transduced with adenovirus-HIF-1α construct. The PDK1 agonist PS48 was able to promote the osteogenic ability of MC3T3-E1 cells treated with dexamethasone. Importantly, the protein levels of p-AKT and p-mTOR were increased in MC3T3-E1 cells treated with dexamethasone after PS48 treatment. in vivo, the PDK1 agonist PS48 could maintain the bone mass of mice treated with dexamethasone. This study provides a new understanding of the mechanism of glucocorticoid-induced osteoporosis.

Effect of rapamycin on bone mass and strength in the α2(I)-G610C mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is commonly caused by heterozygous type I collagen structural mutations that disturb triple helix folding and integrity. This mutant‐containing misfolded collagen accumulates in the endoplasmic reticulum (ER) and induces a form of ER stress associated with negative effects on osteoblast differentiation and maturation. Therapeutic induction of autophagy to degrade the mutant collagens could therefore be useful in ameliorating the ER stress and deleterious downstream consequences. To test this, we treated a mouse model of mild to moderate OI (α2(I) G610C) with dietary rapamycin from 3 to 8 weeks of age and effects on bone mass and mechanical properties were determined. OI bone mass and mechanics were, as previously reported, compromised compared to WT. While rapamycin treatment improved the trabecular parameters of WT and OI bones, the biomechanical deficits of OI bones were not rescued. Importantly, we show that rapamycin treatment suppressed the longitudinal and transverse growth of OI, but not WT, long bones. Our work demonstrates that dietary rapamycin offers no clinical benefit in this OI model and furthermore, the impact of rapamycin on OI bone growth could exacerbate the clinical consequences during periods of active bone growth in patients with OI caused by collagen misfolding mutations.

Enhanced gene delivery in tumor cells using chemical carriers and mechanical loadings

Intracellular delivery of DNA is considered a challenge in biological research and treatment of diseases. The previously reported transfection rate by commercially available transfection reagents in cancer cell lines, such as the mouse lung tumor cell line (TC-1), is very low. The purpose of this study is to introduce and optimize an efficient gene transfection method by mechanical approaches. The combinatory transfection effect of mechanical treatments and conventional chemical carriers is also investigated on a formerly reported hard-to-transfect cell line (TC-1). To study the effect of mechanical loadings on transfection rate, TC-1 tumor cells are subjected to uniaxial cyclic stretch, equiaxial cyclic stretch, and shear stress. The TurboFect transfection reagent is exerted for chemical transfection purposes. The pEGFP-N1 vector encoding the green fluorescent protein (GFP) expression is utilized to determine gene delivery into the cells. The results show a significant DNA delivery rate (by ~30%) in mechanically transfected cells compared to the samples that were transfected with chemical carriers. Moreover, the simultaneous treatment of TC-1 tumor cells with chemical carriers and mechanical loadings significantly increases the gene transfection rate up to ~ 63% after 24 h post-transfection. Our results suggest that the simultaneous use of mechanical loading and chemical reagent can be a promising approach in delivering cargoes into cells with low transfection potentials and lead to efficient cancer treatments.