Naringin alleviates H2O2-induced apoptosis via the PI3K/Akt pathway in rat nucleus pulposus-derived mesenchymal stem cells

Naringin safeguards vertebral endplate chondrocytes from apoptosis and NLRP3 inflammasome activation through SIRT3-mediated mitophagy

AIM

The degradation of the cartilage endplate (CEP) plays a critical role in the initiation and progression of intervertebral disc degeneration (IVDD), a disease closely associated with inflammation and oxidative stress. Naringin (NGN), a flavonoid compound derived from citrus fruits, has been shown to exhibit significant anti-inflammatory and antioxidant properties. This suggests a promising avenue for NGN's application in IVDD therapy. This study aims to elucidate the therapeutic effects and underlying mechanisms of NGN on CEP degeneration, contributing to the formulation of evidence-based treatment strategies for IVDD.

METHODS

In vivo, we developed an intervertebral disc degeneration (IVDD) model in mice by excising the bilateral facet joints and surrounding ligaments, and evaluated the effects of naringin using HE staining and Micro-CT analysis. In vitro, endplate chondrocytes were isolated and subjected to TBHP to replicate the IVDD pathological condition. The protective effects of NGN on these cells were confirmed through immunofluorescence, Western Blot, and flow cytometry.

RESULTS

In vivo, NGN effectively mitigated IVDD progression and CEP calcification in mice. In vitro, NGN enhanced mitophagy and suppressed NLRP3 inflammasome activation through the SIRT3/FOXO3a/Parkin pathway. Furthermore, NGN safeguarded chondrocytes against apoptosis and calcification triggered by oxidative stress, in addition to mitigating the degradation of the extracellular matrix. However, silencing SIRT3 negated NGN's protective influence on chondrocytes.

CONCLUSION

Our study demonstrated that NGN effectively shields chondrocytes from apoptosis and NLRP3 inflammasome activation by facilitating SIRT3-mediated mitophagy. These insights could pave the way for innovative approaches in the prevention and management of IVDD.

Exploring the effects of naringin on oxidative stress-impaired osteogenic differentiation via the Wnt/β-catenin and PI3K/Akt pathways

This study aimed to explore naringin's potential to promote the osteogenic differentiation of MC3T3-E1 under oxidative stress. It delved into Nar's connection with the Wnt/β-catenin and PI3K/Akt signaling pathways. Initially, 2911 OP-related genes were analyzed, revealing close ties with the PI3K/Akt and Wnt pathways alongside oxidative stress. Nar's potential targets-ESR1, HSP90AA1, and ESR2-were identified through various databases and molecular docking studies confirmed Nar's affinity with ESR1 and HSP90AA1. Experiments established optimal concentrations for Nar and H2O2. H2O2 at 0.3 mmol/L damaged MC3T3-E1 cells, alleviated by 0.1 µmol/L Nar. Successful establishment of oxidative stress models was confirmed by DCFH-DA probe and NO detection. Nar exhibited the ability to enhance osteogenic differentiation, counteracting oxidative damage. It notably increased osteoblast-related protein expression in MC3T3-E1 cells under oxidative stress. The study found Nar's positive influence on GSK-3β phosphorylation, β-catenin accumulation, and pathway-related protein expression, all critical in promoting osteogenic differentiation. The research concluded that Nar effectively promotes osteogenic differentiation in MC3T3-E1 cells under oxidative stress. It achieved this by activating the Wnt/β-catenin and PI3K/Akt pathways, facilitating GSK-3β phosphorylation, and enhancing β-catenin accumulation, pivotal in osteogenesis.

Phillyrin reduces ROS production to alleviate the progression of intervertebral disc degeneration by inhibiting NF-κB pathway

BACKGROUND

Intervertebral disc degeneration (IDD) is an increasingly important cause of low back pain (LBP) that results in substantial health and economic burdens. Inflammatory pathway activation and the production of reactive oxygen species (ROS) play vital roles in the progression of IDD. Several studies have suggested that phillyrin has a protective role and inhibits inflammation and the production of ROS. However, the role of phillyrin in IDD has not been confirmed.

PURPOSE

The purpose of this study was to investigate the role of phillyrin in IDD and its mechanisms.

STUDY DESIGN

To establish IDD models in vivo, ex-vivo, and in vitro to verify the function of phillyrin in IDD.

METHOD

The effects of phillyrin on extracellular matrix (ECM) degeneration, inflammation, and oxidation in nucleus pulposus (NP) cells were assessed using immunoblotting and immunofluorescence analysis. Additionally, the impact of phillyrin administration on acupuncture-mediated intervertebral disc degeneration (IDD) in rats was evaluated using various techniques such as MRI, HE staining, S-O staining, and immunohistochemistry (IHC).

RESULT

Pretreatment with phillyrin significantly inhibited the IL-1β-mediated reduction in the degeneration of ECM and apoptosis by alleviating activation of the NF-κB inflammatory pathway and the generation of ROS. In addition, in vivo and ex-vivo experiments verified the protective effect of phillyrin against IDD.

CONCLUSION

Phillyrin can attenuate the progression of IDD by reducing ROS production and activating inflammatory pathways.

Emerging role and therapeutic implications of p53 in intervertebral disc degeneration

Lower back pain (LBP) is a common degenerative musculoskeletal disease that imposes a huge economic burden on both individuals and society. With the aggravation of social aging, the incidence of LBP has increased globally. Intervertebral disc degeneration (IDD) is the primary cause of LBP. Currently, IDD treatment strategies include physiotherapy, medication, and surgery; however, none can address the root cause by ending the degeneration of intervertebral discs (IVDs). However, in recent years, targeted therapy based on specific molecules has brought hope for treating IDD. The tumor suppressor gene p53 produces a transcription factor that regulates cell metabolism and survival. Recently, p53 was shown to play an important role in maintaining IVD microenvironment homeostasis by regulating IVD cell senescence, apoptosis, and metabolism by activating downstream target genes. This study reviews research progress regarding the potential role of p53 in IDD and discusses the challenges of targeting p53 in the treatment of IDD. This review will help to elucidate the pathogenesis of IDD and provide insights for the future development of precision treatments.

Naringin commonly acts via hormesis

The present paper provides the first integrative assessment of the capacity of naringin and its metabolite, naringenin, to induce hormetic dose responses within a broad range of experimental biomedical models. The findings indicate that these agents commonly induced protective effects that are typically mediated via hormetic mechanisms leading to biphasic dose-response relationships. The maximum protective effects are generally modest, 30-60 % greater than control group values. The range of experimental findings with these agents has been reported for models with various neurodegenerative diseases, nucleus pulpous cells (NPCs) located within intravertebral discs, several types of stem cells (i.e., bone marrow, amniotic fluid, periodontal, endothelial) as well as cardiac cells. These agents also were effective within preconditioning protocols protecting against environmental toxins such as ultraviolet radiation (UV), cadmium, and paraquat. The mechanism(s) by which the hormetic responses mediates these biphasic dose responses is complex but commonly involves the activation of nuclear factor erythroid 2-related factor (Nrf2), an increasingly recognized regulator of cellular resistance to oxidants. Nrf2 appears to play a role in controlling the basal and induced expression of an array of antioxidant response element-dependent genes to regulate oxidant exposure's physiological and pathophysiological outcomes. Hence its importance in the assessment of toxicologic and adaptive potential is likely to be significant.

Oxidative stress in intervertebral disc degeneration: Molecular mechanisms, pathogenesis and treatment

Low back pain (LBP) is a leading cause of labour loss and disability worldwide, and it also imposes a severe economic burden on patients and society. Among symptomatic LBP, approximately 40% is caused by intervertebral disc degeneration (IDD). IDD is the pathological basis of many spinal degenerative diseases such as disc herniation and spinal stenosis. Currently, the therapeutic approaches for IDD mainly include conservative treatment and surgical treatment, neither of which can solve the problem from the root by terminating the degenerative process of the intervertebral disc (IVD). Therefore, further exploring the pathogenic mechanisms of IDD and adopting targeted therapeutic strategies is one of the current research hotspots. Among the complex pathophysiological processes and pathogenic mechanisms of IDD, oxidative stress is considered as the main pathogenic factor. The delicate balance between reactive oxygen species (ROS) and antioxidants is essential for maintaining the normal function and survival of IVD cells. Excessive ROS levels can cause damage to macromolecules such as nucleic acids, lipids, and proteins of cells, affect normal cellular activities and functions, and ultimately lead to cell senescence or death. This review discusses the potential role of oxidative stress in IDD to further understand the pathophysiological processes and pathogenic mechanisms of IDD and provides potential therapeutic strategies for the treatment of IDD.

The Development of Naringin for Use against Bone and Cartilage Disorders

Bone and cartilage disorders are the leading causes of musculoskeletal disability. There is no absolute cure for all bone and cartilage disorders. The exploration of natural compounds for the potential therapeutic use against bone and cartilage disorders is proving promising. Among these natural chemicals, naringin, a flavanone glycoside, is a potential candidate due to its multifaceted pharmacological activities in bone and cartilage tissues. Emerging studies indicate that naringin may promote osteogenic differentiation, inhibit osteoclast formation, and exhibit protective effects against osteoporosis in vivo and in vitro. Many signaling pathways, such as BMP-2, Wnt/β-catenin, and VEGF/VEGFR, participate in the biological actions of naringin in mediating the pathological development of osteoporosis. In addition, the anti-inflammatory, anti-oxidative stress, and anti-apoptosis abilities of naringin also demonstrate its beneficial effects against bone and cartilage disorders, including intervertebral disc degeneration, osteoarthritis, rheumatoid arthritis, bone and cartilage tumors, and tibial dyschondroplasia. Naringin exhibits protective effects against bone and cartilage disorders. However, more efforts are still needed due to, at least in part, the uncertainty of drug targets. Further biological and pharmacological evaluations of naringin and its applications in bone tissue engineering, particularly its therapeutic effects against osteoporosis, might result in developing potential drug candidates.

Oxidative Stress and Intervertebral Disc Degeneration: Pathophysiology, Signaling Pathway, and Therapy

Intervertebral disc degeneration (IDD), characterized as decreased proteoglycan content, ossification of endplate, and decreased intervertebral height, is one of the major reasons of low back pain, which seriously affects the quality of life and also brings heavy economic burden. However, the mechanisms leading to IDD and its therapeutic targets have not been fully elucidated. Oxidative stress refers to the imbalance between oxidation and antioxidant systems, between too many products of reactive oxygen species (ROS) and the insufficient scavenging function. Excessive ROS can damage cell lipids, nucleic acids and proteins, which has been proved to be related to the development of a variety of diseases. In recent years, an increasing number of studies have reported that oxidative stress is involved in the pathological process of IDD. Excessive ROS can accelerate the IDD process via inducing the pathological activities, such as inflammation, apoptosis, and senescence. In this review, we focused on pathophysiology and molecular mechanisms of oxidative stress-induced IDD. Moreover, the present review also summarized the possible ideas for the future therapy strategies of oxidative stress-related IDD.

Global research status and hot trends in stem cells therapy for Intervertebral disc degeneration: A bibliometric and clinical study analysis

Background: Stem cells (SCs) therapy for intervertebral disc degeneration (IDD) has been studied for nearly 20 years and it is an important part of regenerative medicine and tissue engineering research, as well as a current research hotspot and challenge. Although the volume of literature has shown an annual growth trend, there is no literature available for bibliometric and clinical analysis of the content of multiple databases in this field.

Methods: The articles were obtained from the WOSCC, Scopus, Pubmed, and ClinicalTrials on 27 December 2021. Three scientometric software (VOSviewer 1.6.17, CiteSpace 5.8.R.1 and Scimago Graphica) were used to perform bibliometric and knowledge-map analysis.

Results: We included 867 articles from WOSCC, 716 articles from Scopus and 6 clinical studies from ClinicalTrials for literature analysis. Our results showed that China was the country with the highest number of publications, with the United States (US) being the leader in terms of international collaborations and the number of citations. Sakai D, Grad S and Hoyland JA had made outstanding contributions for their high productivity and the quality articles. Spine was the most published and most cited journal, in addition to Spine Journal and Biomaterials, which were also more authoritative journals and had received high citations. All of them had received high citations. Keyword co-occurrence studies suggested that the current hotspots were in mechanistic studies, including inflammation, apoptosis, exosome, autophagy, and others. Some studies had also investigated tissue-engineered scaffolds of SCs to better repair degenerated discs. Clinical studies were relatively scarce. Direct injection of Mesenchymal Stem Cells (MSCs) into degenerated discs for the treatment of Degenerative disc disease (DDD) was the current direction of research.

Conclusion: This study demonstrates the global research hotspots, trends and clinical use of SCs in the treatment of IDD. It can help scholars to quickly understand the current status and hotspots of research in this field, and also provide some guidance and reference for those who are currently researching in this area.

Recent Advances in Managing Spinal Intervertebral Discs Degeneration

Low back pain (LBP) represents a frequent and debilitating condition affecting a large part of the global population and posing a worldwide health and economic burden. The major cause of LBP is intervertebral disc degeneration (IDD), a complex disease that can further aggravate and give rise to severe spine problems. As most of the current treatments for IDD either only alleviate the associated symptoms or expose patients to the risk of intraoperative and postoperative complications, there is a pressing need to develop better therapeutic strategies. In this respect, the present paper first describes the pathogenesis and etiology of IDD to set the framework for what has to be combated to restore the normal state of intervertebral discs (IVDs), then further elaborates on the recent advances in managing IDD. Specifically, there are reviewed bioactive compounds and growth factors that have shown promising potential against underlying factors of IDD, cell-based therapies for IVD regeneration, biomimetic artificial IVDs, and several other emerging IDD therapeutic options (e.g., exosomes, RNA approaches, and artificial intelligence).

The Role of Flavonoids in the Osteogenic Differentiation of Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) play an important role in developing bone tissue engineered constructs due to their osteogenic and chondrogenic differentiation potential. MSC-based tissue engineered constructs are generally considered a safe procedure, however, the long-term results obtained up to now are far from satisfactory. The main causes of these therapeutic limitations are inefficient homing, engraftment, and directional differentiation. Flavonoids are a secondary metabolite, widely existed in nature and have many biological activities. For a long time, researchers have confirmed the anti-osteoporosis effect of flavonoids through in vitro cell experiments, animal studies. In recent years the regulatory effects of flavonoids on mesenchymal stem cells (MSCs) differentiation have been received increasingly attention. Recent studies revealed flavonoids possess the ability to modulate self-renewal and differentiation potential of MSCs. In order to facilitate further research on MSCs osteogenic differentiation of flavonoids, we surveyed the literature published on the use of flavonoids in osteogenic differentiation of MSCs, and summarized their pharmacological activities as well as the underlying mechanisms, aimed to explore their promising therapeutic application in bone disorders and bone tissue engineered constructs.

1,25(OH)2D3 Mitigates Oxidative Stress-Induced Damage to Nucleus Pulposus-Derived Mesenchymal Stem Cells through PI3K/Akt Pathway

Intervertebral disc degeneration (IVDD) is one of the main causes of low back pain. The local environment of the degenerated intervertebral disc (IVD) increases oxidative stress and apoptosis of endogenous nucleus pulposus-derived mesenchymal stem cells (NPMSCs) and weakens its ability of endogenous repair ability in degenerated IVDs. A suitable concentration of 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) has been certified to reduce oxidative stress and cell apoptosis. The current study investigated the protective effect and potential mechanism of 1,25(OH)₂D₃ against oxidative stress-induced damage to NPMSCs. The present results showed that 1,25(OH)₂D₃ showed a significant protective effect on NPMSCs at a concentration of 10⁻¹⁰ M for 24 h. Protective effects of 1,25(OH)₂D₃ were also exhibited against H₂O₂-induced NPMSC senescence, mitochondrial dysfunction, and reduced mitochondrial membrane potential. The Annexin V/PI apoptosis detection assay, TUNEL assay, immunofluorescence, western blot, and real-time quantitative polymerase chain reaction assay showed that pretreatment with 1,25(OH)₂D₃ could alleviate H₂O₂-induced NPMSC apoptosis, including the apoptosis rate and the expression of proapoptotic-related (Caspase-3 and Bax) and antiapoptotic-related (Bcl-2) proteins. The intracellular expression of p-Akt increased after pretreatment with 1,25(OH)₂D₃. However, these protective effects of 1,25(OH)₂D₃ were significantly decreased after the PI3K/Akt pathway was inhibited by the LY294002 treatment. In vivo, X-ray, MRI, and histological analyses showed that 1,25(OH)₂D₃ treatment relieved the degree of IVDD in Sprague–Dawley rat disc puncture models. In summary, 1,25(OH)₂D₃ efficiently attenuated oxidative stress-induced NPMSC apoptosis and mitochondrial dysfunction via PI3K/Akt pathway and is a promising candidate treatment for the repair of IVDD.

Improved Wound Healing by Naringin Associated with MMP and the VEGF Pathway

This study aims to investigate the wound-healing effectiveness of the phenolic compound, naringin, both in vitro and in vivo. Male mice were shaved on their dorsal skin under isoflurane, a biopsy punch was made in four symmetrical circular resection windows (6 mm) to induce a wound. These excision wounds were used to study the topical effects of naringin in terms of various biochemical, molecular, and histological parameters. We observed a significant recovery in the wound area. Increased levels of MMP-2, 9, 14, TIMP-2, VEGF-A, and VEGF-R1 were induced by naringin in the HaCaT cells. The time course experiments further revealed that levels of VEGF-A and B increased within 36 h; whereas levels of VEGF-C decreased. In line with this, VEGF-R3 levels, but not VEGF-R1 and 2 levels, increased soon after stimulation; although the increase subsided after 36 h. Additionally, naringin cream upregulated wound healing in vitro. The blockage of VEGF by Bevacizumab abolished the function of naringin cream on cell migration. Histological alterations in the wounded skin were restored by naringin cream, which accelerated wound healing via upregulated expression of growth factors (VEGF-A, B, and C and VEGF-R3), and thus increased MMP-2, 9, 14 expressions.

Naringin Inhibits Apoptosis Induced by Cyclic Stretch in Rat Annular Cells and Partially Attenuates Disc Degeneration by Inhibiting the ROS/NF-κB Pathway

Oxidative stress and apoptosis play important roles in the pathogenesis of various degenerative diseases. Previous studies have shown that naringin can exert therapeutic effects in multiple degenerative diseases by resisting oxidative stress and inhibiting apoptosis. Although naringin is effective in treating degenerative disc disease, the underlying mechanism remains unclear. This study is aimed at investigating the effects of naringin on oxidative stress, apoptosis, and intervertebral disc degeneration (IVDD) induced by cyclic stretch and the underlying mechanisms in vitro and in vivo. Abnormal cyclic stretch was applied to rat annulus fibrosus cells, which were then treated with naringin, to observe the effects of naringin on apoptosis, oxidative stress, mitochondrial function, and the nuclear factor- (NF-) κB signaling pathway. Subsequently, a rat model of IVDD induced by dynamic and static imbalance was established to evaluate the effects of naringin on the degree of degeneration (using imaging and histology), apoptosis, and oxidative stress in the serum and the intervertebral disc. Naringin inhibited the cyclic stretch-induced apoptosis of annulus fibrosus cells, reduced oxidative stress, improved mitochondrial function, enhanced the antioxidant capacity, and suppressed the activation of the NF-κB signaling pathway. Additionally, it reduced the degree of IVDD (evaluated using magnetic resonance imaging) and the level of oxidative stress and inhibited apoptosis and p-P65 expression in the intervertebral discs of rats. Thus, naringin can inhibit cyclic stretch-induced apoptosis and delay IVDD, and the underlying mechanism may be related to the inhibition of oxidative stress and activation of the NF-κB signaling pathway. Naringin may be an effective drug for treating degenerative disc disease.

Urolithin a alleviates oxidative stress-induced senescence in nucleus pulposus-derived mesenchymal stem cells through SIRT1/PGC-1α pathway

BACKGROUND

In degenerative intervertebral disc (IVD), an unfavorable IVD environment leads to increased senescence of nucleus pulposus (NP)-derived mesenchymal stem cells (NPMSCs) and the inability to complete the differentiation from NPMSCs to NP cells, leading to further aggravation of IVD degeneration (IDD). Urolithin A (UA) has been proven to have obvious effects in delaying cell senescence and resisting oxidative stress.

AIM

To explore whether UA can alleviate NPMSCs senescence and to elucidate the underlying mechanism.

METHODS

In vitro, we harvested NPMSCs from rat tails, and divided NPMSCs into four groups: the control group, H₂O₂ group, H₂O₂ + UA group, and H₂O₂ + UA + SR-18292 group. Senescence-associated β-Galactosidase (SA-β-Gal) activity, cell cycle, cell proliferation ability, and the expression of senescence-related and silent information regulator of transcription 1/PPAR gamma coactivator-1α (SIRT1/ PGC-1α) pathway-related proteins and mRNA were used to evaluate the protective effects of UA. In vivo, an animal model of IDD was constructed, and X-rays, magnetic resonance imaging, and histological analysis were used to assess whether UA could alleviate IDD in vivo.

RESULTS

We found that H₂O₂ can cause NPMSCs senescence changes, such as cell cycle arrest, reduced cell proliferation ability, increased SA-β-Gal activity, and increased expression of senescence-related proteins and mRNA. After UA pretreatment, the abovementioned senescence indicators were significantly alleviated. To further demonstrate the mechanism of UA, we evaluated the mitochondrial membrane potential and the SIRT1/PGC-1α pathway that regulates mitochondrial function. UA protected mitochondrial function and delayed NPMSCs senescence by activating the SIRT1/PGC-1α pathway. In vivo, we found that UA treatment alleviated an animal model of IDD by assessing the disc height index, Pfirrmann grade and the histological score.

CONCLUSION

In summary, UA could activate the SIRT1/PGC-1α signaling pathway to protect mitochondrial function and alleviate cell senescence and IDD in vivo and vitro.

Natural Products of Pharmacology and Mechanisms in Nucleus Pulposus Cells and Intervertebral Disc Degeneration

Intervertebral disc degeneration (IDD) is one of the main causes of low back pain (LBP), which severely reduces the quality of life and imposes a heavy financial burden on the families of affected individuals. Current research suggests that IDD is a complex cell-mediated process. Inflammation, oxidative stress, mitochondrial dysfunction, abnormal mechanical load, telomere shortening, DNA damage, and nutrient deprivation contribute to intervertebral disc cell senescence and changes in matrix metabolism, ultimately causing IDD. Natural products are widespread, structurally diverse, afford unique advantages, and exhibit great potential in terms of IDD treatment. In recent years, increasing numbers of natural ingredients have been shown to inhibit the degeneration of nucleus pulposus cells through various modes of action. Here, we review the pharmacological effects of natural products on nucleus pulposus cells and the mechanisms involved. An improved understanding of how natural products target signalling pathways will aid the development of anti-IDD drugs. This review focuses on potential IDD drugs.

[Research progress of endogenous repair strategy in intervertebral disc]

Objective

To review the research progress of endogenous repair strategy (ERS) in intervertebral disc (IVD).

Methods

The domestic and foreign literature related to ERS in IVD in recent years was reviewed, and its characteristics, status, and prospect in the future were summarized.

Results

The key of ERS in IVD is to improve the vitality of stem/progenitor cells in IVD or promote its migration from stem cell Niche to the tissue that need to repair. These stem/progenitor cells in IVD are derived from nucleus pulposus, annulus fibrosus, and cartilaginous endplate, showing similar biological characteristics to mesenchymal stem cells including the expression of the specific stem/progenitor cell surface markers and gene, and also the capacity of multiple differentiations potential. However, the development, senescence, and degeneration of IVD have consumed these stem/progenitor cells, and the harsh internal microenvironment further impair their biological characteristics, which leads to the failure of endogenous repair in IVD. At present, relevant research mainly focuses on improving the biological characteristics of endogenous stem/progenitor cells, directly supplementing endogenous stem/progenitor cells, biomaterials and small molecule compounds to stimulate the endogenous repair in IVD, so as to improve the effect of endogenous repair.

Conclusion

At present, ERS has gotten some achievements in the treatment of IVD degeneration, but its related studies are still in the pre-clinical stage. So further studies regarding ERS should be carried out in the future, especially in vivo experiments and clinical transformation.

Targeting mitochondrial dysfunction with small molecules in intervertebral disc aging and degeneration

The prevalence of rheumatic and musculoskeletal diseases (RMDs) including osteoarthritis (OA) and low back pain (LBP) in aging societies present significant cost burdens to health and social care systems. Intervertebral disc (IVD) degeneration, which is characterized by disc dehydration, anatomical alterations, and extensive changes in extracellular matrix (ECM) composition, is an important contributor to LBP. IVD cell homeostasis can be disrupted by mitochondrial dysfunction. Mitochondria are the main source of energy supply in IVD cells and a major contributor to the production of reactive oxygen species (ROS). Therefore, mitochondria represent a double-edged sword in IVD cells. Mitochondrial dysfunction results in oxidative stress, cell death, and premature cell senescence, which are all implicated in IVD degeneration. Considering the importance of optimal mitochondrial function for the preservation of IVD cell homeostasis, extensive studies have been done in recent years to evaluate the efficacy of small molecules targeting mitochondrial dysfunction. In this article, we review the pathogenesis of mitochondrial dysfunction, aiming to highlight the role of small molecules and a selected number of biological growth factors that regulate mitochondrial function and maintain IVD cell homeostasis. Furthermore, molecules that target mitochondria and their mechanisms of action and potential for IVD regeneration are identified. Finally, we discuss mitophagy as a key mediator of many cellular events and the small molecules regulating its function.

Small molecule-based treatment approaches for intervertebral disc degeneration: Current options and future directions

Low back pain (LBP) is a major reason for disability, and symptomatic intervertebral disc (IVD) degeneration (IDD) contributes to roughly 40% of all LBP cases. Current treatment modalities for IDD include conservative and surgical strategies. Unfortunately, there is a significant number of patients in which conventional therapies fail with the result that these patients remain suffering from chronic pain and disability. Furthermore, none of the current therapies successfully address the underlying biological problem - the symptomatic degenerated disc. Both spinal fusion as well as total disc replacement devices reduce spinal motion and are associated with adjacent segment disease. Thus, there is an unmet need for novel and stage-adjusted therapies to combat IDD. Several new treatment options aiming to regenerate the IVD are currently under investigation. The most common approaches include tissue engineering, growth factor therapy, gene therapy, and cell-based treatments according to the stage of degeneration. Recently, the regenerative activity of small molecules (low molecular weight organic compounds with less than 900 daltons) on IDD was demonstrated. However, small molecule-based therapy in IDD is still in its infancy due to limited knowledge about the mechanisms that control different cell signaling pathways of IVD homeostasis. Small molecules can act as anti-inflammatory, anti-apoptotic, anti-oxidative, and anabolic agents, which can prevent further degeneration of disc cells and enhance their regeneration. This review pursues to give a comprehensive overview of small molecules, focusing on low molecular weight organic compounds, and their potential utilization in patients with IDD based on recent in vitro, in vivo, and pre-clinical studies.

6-gingerol protects nucleus pulposus-derived mesenchymal stem cells from oxidative injury by activating autophagy

BACKGROUND

To date, there has been no effective treatment for intervertebral disc degeneration (IDD). Nucleus pulposus-derived mesenchymal stem cells (NPMSCs) showed encouraging results in IDD treatment, but the overexpression of reactive oxygen species (ROS) impaired the endogenous repair abilities of NPMSCs. 6-gingerol (6-GIN) is an antioxidant and anti-inflammatory reagent that might protect NPMSCs from injury.

AIM

To investigate the effect of 6-GIN on NPMSCs under oxidative conditions and the potential mechanism.

METHODS

The cholecystokinin-8 assay was used to evaluate the cytotoxicity of hydrogen peroxide and the protective effects of 6-GIN. ROS levels were measured by 2´7´-dichlorofluorescin diacetate analysis. Matrix metalloproteinase (MMP) was detected by the tetraethylbenzimidazolylcarbocyanine iodide assay. TUNEL assay and Annexin V/PI double-staining were used to determine the apoptosis rate. Additionally, autophagy-related proteins (Beclin-1, LC-3, and p62), apoptosis-associated proteins (Bcl-2, Bax, and caspase-3), and PI3K/Akt signaling pathway-related proteins (PI3K and Akt) were evaluated by Western blot analysis. Autophagosomes were detected by transmission electron microscopy in NPMSCs. LC-3 was also detected by immunofluorescence. The mRNA expression of collagen II and aggrecan was evaluated by real-time polymerase chain reaction (RT-PCR), and the changes in collagen II and MMP-13 expression were verified through an immunofluorescence assay.

RESULTS

6-GIN exhibited protective effects against hydrogen peroxide-induced injury in NPMSCs, decreased hydrogen peroxide-induced intracellular ROS levels, and inhibited cell apoptosis. 6-GIN could increase Bcl-2 expression and decrease Bax and caspase-3 expression. The MMP, Annexin V-FITC/PI flow cytometry and TUNEL assay results further confirmed that 6-GIN treatment significantly inhibited NPMSC apoptosis induced by hydrogen peroxide. 6-GIN treatment promoted extracellular matrix (ECM) expression by reducing the oxidative stress injury-induced increase in MMP-13 expression. 6-GIN activated autophagy by increasing the expression of autophagy-related markers (Beclin-1 and LC-3) and decreasing the expression of p62. Autophagosomes were visualized by transmission electron microscopy. Pretreatment with 3-MA and BAF further confirmed that 6-GIN-mediated stimulation of autophagy did not reduce autophagosome turnover but increased autophagic flux. The PI3K/Akt pathway was also found to be activated by 6-GIN. 6-GIN inhibited NPMSC apoptosis and ECM degeneration, in which autophagy and the PI3K/Akt pathway were involved.

CONCLUSION

6-GIN efficiently decreases ROS levels, attenuates hydrogen peroxide-induced NPMSCs apoptosis, and protects the ECM from degeneration. 6-GIN is a promising candidate for treating IDD.

Inhibiting Heat Shock Protein 90 Protects Nucleus Pulposus-Derived Stem/Progenitor Cells From Compression-Induced Necroptosis and Apoptosis

Nucleus pulposus-derived stem/progenitor cells (NPSCs) provide novel prospects for the regeneration of degenerated intervertebral disc (IVD). Nevertheless, with aging and degeneration of IVD, the frequency of NPSCs markedly decreases. Excessive cell death could be the main reason for declined frequency of NPSCs, however, the exact mechanisms remain elusive. Thus, the present study was undertaken to explore the mechanisms of compression-induced NPSCs death, and the effects of heat shock protein 90 (HSP90) on NPSCs survival. Here, we found that compression could trigger receptor-interacting protein kinase 1 (RIPK1)/receptor-interacting protein kinase 3 (RIPK3)/mixed lineage kinase domain-like protein (MLKL)-mediated necroptosis of NPSCs. Furthermore, we found that elevated expression of HSP90 was involved in compression-induced NPSCs death, and inhibiting HSP90 could dramatically attenuate compression-induced necroptosis of NPSCs via regulating the expression and activity of RIPK1/RIPK3/MLKL, and alleviating the mitochondrial dysfunction (mitochondrial membrane potential loss and ATP depletion) and oxidative stress [production of mitochondrial reactive oxygen species (ROS), cellular total ROS and malondialdehyde, and downregulation of superoxide dismutase 2]. Besides necroptosis, compression-induced apoptosis of NPSCs was also attenuated by HSP90 inhibition. In addition, we found that enhanced expression of HSP70 contributed to the cytoprotective effects of inhibiting HSP90. More encouragingly, our results demonstrated that inhibiting HSP90 could also mitigate the exhaustion of NPSCs in vivo. In conclusion, RIPK1/RIPK3/MLKL-mediated necroptosis participates in compression-induced NPSCs death. Furthermore, targeting HSP90 to simultaneously inhibit necroptosis and apoptosis of NPSCs might be an efficient strategy for preventing the death of NPSCs, thus rescuing the endogenous repair capacity of NP tissue.

Insights of stem cell-based endogenous repair of intervertebral disc degeneration

Low back pain has become more prevalent in recent years, causing enormous economic burden for society and government. Common therapies used in clinics including conservative treatment and surgery can only relieve pain. Subsequent cell-based treatment such as mesenchymal stem cell transplantation poses problems such as short duration of therapeutic effect and tumorigenesis. Recently, the discovery and identification of stem cell niche and stem/progenitor cells in intervertebral disc bring increased attention to endogenous repair strategy. Therefore, we review the studies involving endogenous repair strategy and present the characteristics and current status of this treatment. Meanwhile, we also discuss the strategy and perspective of endogenous repair strategy in future.

Heme Oxygenase-1-Mediated Autophagy Protects against Oxidative Damage in Rat Nucleus Pulposus-Derived Mesenchymal Stem Cells

Although endogenous nucleus pulposus-derived mesenchymal stem cell- (NPMSC-) based regenerative medicine has provided promising repair strategy for intervertebral disc (IVD) degeneration, the hostile microenvironments in IVD, including oxidative stress, can negatively affect the survival and function of the NPMSCs and severely hinder the endogenous repair process. Therefore, it is of great importance to reveal the mechanisms of the endogenous repair failure caused by the adverse microenvironments in IVD. The aim of this study was to investigate the effect of oxidative stress on the rat NPMSCs and its underlying mechanism. Our results demonstrated that oxidative stress inhibited cell viability, induced apoptosis, and increased the production of reactive oxygen species (ROS) in NPMSCs. In addition, the results showed that the expression level of heme oxygenase-1 (HO-1) increased at an early stage but decreased at a late stage when NPMSCs were exposed to oxidative stress, and the oxidative damages of NPMSCs could be partially reversed by promoting the expression of HO-1. Further mechanistic analysis indicated that the protective effect of HO-1 against oxidative damage in NPMSCs was mediated by the activation of autophagy. Taken together, our study revealed that oxidative stress could inhibit cell viability, induce apoptosis, and increase ROS production in NPMSCs, and HO-1-mediated autophagy might act as a protective response to the oxidative damage. These findings might enhance our understanding on the mechanism of the endogenous repair failure during IVD degeneration and provide novel research direction for the endogenous repair of IVD degeneration.