FOXO3 protects nucleus pulposus cells against apoptosis under nutrient deficiency via autophagy

Identification of Autophagy-Related Genes Involved in Intervertebral Disc Degeneration by Microarray Data Analysis

BACKGROUND

Nucleus pulposus cells survive in a hypoxic, acidic, nutrient-poor, and hypotonic microenvironment. Consequently, they maintain low proliferation and undergo autophagy to protect themselves from cellular stress. Therefore, we aimed to identify autophagy-related biomarkers involved in intervertebral disc degeneration pathogenesis.

METHODS

Autophagy-related differentially expressed genes were derived from the intersection between the public GSE147383 microarray data set to identify differentially expressed genes and online databases to identify autophagy-related genes. Furthermore, we assessed their biological functions with gene annotation and enrichment analysis in the Metscape portal. Then, the STRING database and Cytoscape software allowed inferring a protein-protein interaction (PPI) network and identifying hub genes. In addition, to predict transcription factors that may regulate the hub genes, we used the GeneMANIA website. Finally, the competing endogenous RNA prediction tools and Cytoscape were also used to construct an mRNA-miRNA-lncRNA network.

RESULTS

A total of 123 autophagy-related differentially expressed genes were identified, they were mainly involved in phosphoinositide 3-kinase-Akt signaling, autophagy animal, and apoptosis pathways. Nine were identified as hub genes (PTEN, MYC, CTNNB1, JUN, BECN1, ERBB2, FOXO3, ATM, and FN1) and 36 transcription factors were associated with them. Finally, an autophagy-associated competing endogenous RNA network was constructed based on the 9 hub genes.

CONCLUSIONS

Nine hub genes were identified and a network of competing endogenous RNA associated with autophagy was established. They can be used as autophagy-related biomarkers of intervertebral disc degeneration and for further exploration.

Microglial forkhead box O3a deficiency attenuates LPS-induced neuro-inflammation and depressive-like behaviour through regulating the expression of peroxisome proliferator-activated receptor-γ

BACKGROUND AND PURPOSE

Depression is closely linked with microglial activation and neuro-inflammation. Peroxisome proliferator-activated receptor-γ (PPAR-γ) plays an important role in M2 activation of microglia. Forkhead box (FOX) O3a has been implicated in the regulation of mood-relevant behaviour. However, little is known about the inflammatory mechanisms of in the microglia of the brain. Here, we have investigated the role of microglial FOXO3a/PPAR-γ in the development of depression.

EXPERIMENTAL APPROACH

The effect of FOXO3a on microglia inflammation was analysed in vitro and in lipopolysaccharide (LPS)-induced depression-like behaviours in vivo. ChIP-seq and Dual-luciferase reporter assays were used to confirm the interaction between FOXO3a and PPAR-γ. Behavioural changes were measured, while inflammatory cytokines, microglial phenotype and morphological properties were determined by ELISA, qRT-PCR, western blotting and immunostaining.

KEY RESULTS

Overexpression of FOXO3a significantly attenuated expression of PPAR-γ and enhanced the microglial polarization towards the M1 phenotype, while knockdown of FOXO3a had the opposite effect. FOXO3a binds to the promoters of PPAR-γ and decreases its transcription activity. Importantly, deacetylation and activation of FOXO3a regulate LPS-induced neuro-inflammation by inhibiting the expression of PPAR-γ in microglia cells, supporting the antidepressant potential of histone deacetylase inhibitors. Microglial FOXO3a deficiency in mice alleviated LPS-induced neuro-inflammation and depression-like behaviours but failed to reduce anxiety behaviour, whereas pharmacological inhibition of PPAR-γ by GW9662 restored LPS-induced microglial activation and depressive-like behaviours in microglial FOXO3a-deficient mice.

CONCLUSION AND IMPLICATIONS

FOXO3a/PPAR-γ axis plays an important role in microglial activation and depression, identifying a new therapeutic avenue for the treatment of major depression.

Glucose depletion decreases cell viability without triggering degenerative changes in a physiological nucleus pulposus explant model

Although the etiology of intervertebral disc degeneration is still unresolved, the nutrient paucity resulting from its avascular nature is suspected of triggering degenerative processes in its core: the nucleus pulposus (NP). While severe hypoxia has no significant effects on NP cells, the impact of glucose depletion, such as found in degenerated discs (0.2-1 mM), is still uncertain. Using a pertinent ex-vivo model representative of the unique disc microenvironment, the present study aimed, therefore, at determining the effects of "degenerated" (0.3 mM) glucose levels on bovine NP explant homeostasis. The effects of glucose depletion were evaluated on NP cell viability, apoptosis, phenotype, metabolism, senescence, extracellular matrix anabolism and catabolism, and inflammatory mediator production using fluorescent staining, RT-qPCR, (immuno)histology, ELISA, biochemical, and enzymatic assays. Compared to the "healthy" (2 mM) glucose condition, exposure to the degenerated glucose condition led to a rapid and extensive decrease in NP cell viability associated with increased apoptosis. Although the aggrecan and collagen-II gene expression was also downregulated, NP cell phenotype, and senescence, matrix catabolism, and inflammatory mediator production were not, or only slightly, affected by glucose depletion. The present study provided evidence for glucose depletion as an essential player in NP cell viability but also suggested that other microenvironment factor(s) may be involved in triggering the typical shift of NP cell phenotype observed during disc degeneration. The present study contributes new information for better understanding disc degeneration at the cellular-molecular levels and thus helps to develop relevant therapeutical strategies to counteract it.

M6A methylation-regulated autophagy may be a new therapeutic target for intervertebral disc degeneration

Degeneration of intervertebral discs is considered one of the most important causes of low back pain and disability. The intervertebral disc (IVD) is characterized by its susceptibility to various stressors that accelerate the senescence and apoptosis of nucleus pulposus cells, resulting in the loss of these cells and dysfunction of the intervertebral disc. Therefore, how to reduce the loss of nucleus pulposus cells under stress environment is the main problem in treating intervertebral disc degeneration. Autophagy is a kind of programmed cell death, which can provide energy by recycling substances in cells. It is considered to be an effective method to reduce the senescence and apoptosis of nucleus pulposus cells under stress. However, further research is needed on the mechanisms by which autophagy of nucleus pulposus cells is regulated under stress environments. M6A methylation, as the most extensive RNA modification in eukaryotic cells, participates in various cellular biological functions and is believed to be related to the regulation of autophagy under stress environments, may play a significant role in nucleus pulposus responding to stress. This article first summarizes the effects of various stressors on the death and autophagy of nucleus pulposus cells. Then, it summarizes the regulatory mechanism of m6A methylation on autophagy-related genes under stress and the role of these autophagy genes in nucleus pulposus cells. Finally, it proposes that the methylation modification of autophagy-related genes regulated by m6A may become a new treatment approach for intervertebral disc degeneration, providing new insights and ideas for the clinical treatment of intervertebral disc degeneration.

FOXO3-Activated HOTTIP Sequesters miR-615-3p away from COL2A1 to Mitigate Intervertebral Disc Degeneration

In this study, knockout of FOXO3 was found to impair intervertebral disc maturation and homeostasis in postnatal mice as well as facilitating extracellular matrix degradation. RNA sequencing can uncover disease-related gene expression and investigate disease pathophysiology. High-throughput transcriptome sequencing and experimental validations were used to identify the essential gene and mechanism involved in intervertebral disc degeneration (IDD). Nucleus pulposus (NP) tissue samples were collected from the mice with conditional knockout of FOXO3 (FOXO3 KO) for high-throughput sequencing, followed by screening of differentially expressed lncRNAs and mRNAs. The mRNAs were subjected to GO and KEGG enrichment analyses. Interactions among FOXO3, HOTTIP, miR-615-3p, and COL2A1 were analyzed. NP cells were subjected to a series of mimics, inhibitors, overexpression plasmids, and shRNAs to validate the mechanisms of FOXO3 in controlling HOTTIP/miR-615-3p/COL2A1 in IDD. Mechanistically, FOXO3 transcriptionally activated HOTTIP, facilitated the competitive HOTTIP binding to miR-615-3p, and increased the expression of the miR-615-3p target gene COL2A1. Thus, NP cell proliferation was induced, cell apoptosis was diminished, resulting in delayed development of IDD. Based on these data, the transcription factor FOXO3 may decrease miR-615-3p binding to COL2A1 and up-regulate COL2A1 expression by activating HOTTIP transcription, which in turn inhibits NP cell apoptosis and promotes its proliferation, to prevent the degradation of intervertebral disc matrix and maintain the normal physiological function of intervertebral disc, thereby preventing the occurrence and development of IDD.

Scientific Bibliometric and Visual Analysis of Studies on Autophagy in Intervertebral Disc Degeneration Based on Web of Science

OBJECTIVE

To analyze the current research trends and potential mechanisms related to the role of autophagy in intervertebral disc degeneration (IVDD) and to provide new ideas for future research in this field.

METHODS

All articles on IVDD and autophagy were retrieved and extracted from the Web of Science (WoS) core collection database. The results were evaluated and visualized using the bibliometric Web site, CiteSpace, and VOSviewer software, including annual articles published, countries, institutions, authors, journals, research areas, funding agencies, citations, and keywords.

RESULTS

From January 1, 2011, to December 31, 2022, 323 reviews and original articles were included, and the overall trend in the number of articles was increasing rapidly. China and the United States were the countries with the most scientific research achievements. The 323 articles received a total number of citations of 6949, with an H index of 43 and an average citation of 21.51. The top publication country, institution, author, journal, research area, and funding agency were China, Huazhong University of Science and Technology, Cao Yang of Tongji Medical College, Oxidative Medicine and Cellular Longevity, cell biology, and National Natural Science Foundation of China, respectively. Most of the keywords were associated with the mechanisms and regulatory networks of autophagy. In addition, with increasing evidence showing the key role of autophagy in IVDD, therapy, signaling pathway, and mitophagy are emerging as new research hot spots that should be paid more attention.

CONCLUSIONS

This study provided a scientific perspective on autophagy in IVDD and elucidated the current research status and hot spots in this field. The mechanism of autophagy and the application of regulating autophagy in the treatment of IVDD deserve further research.

Role of omentin-1 in susceptibility to anxiety and depression like behaviors

Neuro-inflammation and blood-brain barrier (BBB) dysfunction are associated with depression. Evidence shows that adipokines enter the brain from the circulation, which regulates depressive behaviors. Omentin-1 is a newly identified adipocytokine that has anti-inflammatory effects, but little is known about its role in neuro-inflammation and mood-relevant behavior. Our results showed omentin-1 knockout mice (Omentin-1-/-) increased susceptibility to anxiety and depressive-like behaviors, which are associated with abnormalities of cerebral blood flow (CBF) and impaired BBB permeability. Moreover, omentin-1 depletion significantly increased hippocampal pro-inflammatory cytokines (IL-1β, TNFα, IL-6), caused microglial activation, inhibited hippocampus neurogenesis, and resulted in autophagy impairment by dysregulating ATG genes. Omentin-1 deficiency also sensitized mice to the behavioral changes induced by lipopolysaccharide (LPS), suggesting that omentin-1 could rescue neuro-inflammation by acting as an anti-depressant. Our in vitro microglia cell culture data confirmed that recombinant omentin-1 suppresses microglial activation and pro-inflammatory cytokine expression induced by LPS. Our study suggests that omentin-1 can be used as a promising therapeutic agent for the prevention or treatment of depression by providing a barrier-promoting effect and an endogenous anti-inflammatory balance to downregulate the proinflammatory cytokines.

FOXO3-Activated circFGFBP1 Inhibits Extracellular Matrix Degradation and Nucleus Pulposus Cell Death via miR-9-5p/BMP2 Axis in Intervertebral Disc Degeneration In Vivo and In Vitro

(1) Background: intervertebral disc degeneration (IVDD) defined as the degenerative changes in intervertebral disc is characterized by extracellular matrix (ECM) degradation and death in nucleus pulposus (NP) cells. (2) Methods: The model of IVDD was established in male Sprague Dawley rats using a puncture of a 21-gauge needle at the endplates located in the L4/5 intervertebral disc. Primary NP cells were stimulated by 10 ng/mL IL-1β for 24 h to mimic IVDD impairment in vitro. (3) Results: circFGFBP1 was downregulated in the IVDD samples. circFGFBP1 upregulation inhibited apoptosis and extracellular matrix (ECM) degradation and promoted proliferation in IL-1β-stimulated NP cells. Additionally, circFGFBP1 upregulation mitigated the loss of NP tissue and the destruction of the intervertebral disc structure in vivo during IVDD. FOXO3 could bind to the circFGFBP1 promoter to enhance its expression. circFGFBP1 upregulated BMP2 expression in NP via sponging miR-9-5p. FOXO3 enhanced the protection of circFGFBP1 in IL-1β-stimulated NP cells, whereas a miR-9-5p increase partly reversed the protection. miR-9-5p downregulation contributed to the survival of IL-1β-stimulated NP cells, which was partially reversed by BMP2 silence. (4) Conclusions: FOXO3 could activate the transcription of circFGFBP1 via binding to its promoter, which resulted in the enhancement of BMP2 via sponging miR-9-5p and then inhibited apoptosis and ECM degradation in NP cells during IVDD.

Quiescence preconditioned nucleus pulposus stem cells alleviate intervertebral disc degeneration by enhancing cell survival via adaptive metabolism pattern in rats

Quiescence is a cellular state of reversible growth arrest required to maintain homeostasis and self-renewal. Entering quiescence allows the cells to remain in the non-dividing stage for an extended period of time and enact mechanisms to protect themselves from damage. Due to the extreme nutrient-deficient microenvironment in the intervertebral disc (IVD), the therapeutic effect of cell transplantation is limited. In this study, nucleus pulposus stem cells (NPSCs) were preconditioned into quiescence through serum starvation in vitro and transplanted to repair intervertebral disc degeneration (IDD). In vitro, we investigated apoptosis and survival of quiescent NPSCs in a glucose-free medium without fetal bovine serum. Non-preconditioned proliferating NPSCs served as controls. In vivo, the cells were transplanted into a rat model of IDD induced by acupuncture, and the intervertebral disc height, histological changes, and extracellular matrix synthesis were observed. Finally, to elucidate the mechanisms underlying the quiescent state of NPSCs, the metabolic patterns of the cells were investigated through metabolomics. The results revealed that quiescent NPSCs decreased apoptosis and increased cell survival when compared to proliferating NPSCs both in vitro and in vivo, as well as maintained the disc height and histological structure significantly better than that by proliferating NPSCs. Furthermore, quiescent NPSCs have generally downregulated metabolism and reduced energy requirements in response to a switch to a nutrient-deficient environment. These findings support that quiescence preconditioning maintains the proliferation and biological function potential of NPSCs, increases cell survival under the extreme environment of IVD, and further alleviates IDD via adaptive metabolic patterns.

Extracellular vesicles derived from mesenchymal stem cells confer protection against intervertebral disc degeneration through a microRNA-217-dependent mechanism

OBJECTIVE

Extracellular vesicles released by mesenchymal stem cells (MSC-EVs) can be applied to alleviate intervertebral disc degeneration (IVDD) by curbing apoptosis of nucleus pulposus cells (NPCs). The current study aims to evaluate the effect of MSC-EVs on NPC apoptosis and IVDD and the related regulatory mechanisms involving microRNA (miR)-217.

METHOD

Expression of miR-217 was examined in tumor necrosis factor-α (TNF-α)-induced NPCs and MSC-EVs, followed by identification in the relationship between miR-217, enhancer of zeste homolog 2 (EZH2) and forkhead box O-3 (FOXO3). After isolation of EVs from MSCs and subsequent co-culture with NPCs, we assessed effects of miR-217 on NPC viability, autophagy, senescence and apoptosis along with extracellular matrix (ECM) degradation. Further in vivo experiments were conducted in rat models of IVDD to substantiate the effect of miR-217 on IVDD.

RESULTS

Poor miR-217 expression was found in TNF-α-induced NPCs, while high miR-217 expression was identified in MSC-EVs (P < 0.05). MSC-EVs transferred miR-217 to NPCs and increased its expression, thus attenuating NPC apoptosis and ECM degradation (elevated collagen II and aggrecan but reduced MMP13 and ADAMTS5) (P < 0.05). miR-217 targeted EZH2, and EZH2 bound to the FOXO3 promoter and consequently downregulated its expression. FOXO3 restrained NPC apoptosis and ECM degradation by stimulating cell autophagy (P < 0.05). Furthermore, in vivo experimental results confirmed the suppressive role of miR-217 shuttled by MSC-EVs in IVDD.

CONCLUSION

Overall, the delivery of miR-217 may be a novel mechanism underlying the effect of MSC-EVs on NPC apoptosis and ECM degradation following IVDD.

Mitophagy—A New Target of Bone Disease

Bone diseases are usually caused by abnormal metabolism and death of cells in bones, including osteoblasts, osteoclasts, osteocytes, chondrocytes, and bone marrow mesenchymal stem cells. Mitochondrial dysfunction, as an important cause of abnormal cell metabolism, is widely involved in the occurrence and progression of multiple bone diseases, including osteoarthritis, intervertebral disc degeneration, osteoporosis, and osteosarcoma. As selective mitochondrial autophagy for damaged or dysfunctional mitochondria, mitophagy is closely related to mitochondrial quality control and homeostasis. Accumulating evidence suggests that mitophagy plays an important regulatory role in bone disease, indicating that regulating the level of mitophagy may be a new strategy for bone-related diseases. Therefore, by reviewing the relevant literature in recent years, this paper reviews the potential mechanism of mitophagy in bone-related diseases, including osteoarthritis, intervertebral disc degeneration, osteoporosis, and osteosarcoma, to provide a theoretical basis for the related research of mitophagy in bone diseases.

p300 arrests intervertebral disc degeneration by regulating the FOXO3/Sirt1/Wnt/β-catenin axis

The transcription factor p300 is reportedly involved in age-associated human diseases, including intervertebral disc degeneration (IDD). In this study, we investigate the potential role and pathophysiological mechanism of p300 in IDD. Clinical tissue samples were collected from patients with lumbar disc herniation (LDH), in which the expression of p300, forkhead box O3 (FOXO3), and sirtuin 1 (Sirt1) was determined. Nucleus pulposus cells (NPCs) isolated from clinical degenerative intervertebral disc (IVD) tissues were introduced with oe-p300, oe-FOXO3, Wnt/β-catenin agonist 1, C646 (p300/CBP inhibitor), or si-p300 to explore the functional role of p300 in IDD and to characterize the relationship between p300 and the FOXO3/Sirt1/Wnt/β-catenin pathway. Also, we established a rat IDD model by inducing needle puncture injuries in the caudal IVDs for further verification of p300 functional role. We found that p300 was downregulated in the clinical tissues and NPCs of IDD. Overexpression of p300 promoted the proliferation and autophagy of NPCs while inhibiting cell apoptosis, which was associated with FOXO3 upregulation. p300 could increase the expression of FOXO3 by binding to the Sirt1 promoter, and thus, contributed to inactivation of the Wnt/β-catenin pathway. In vivo results further displayed that p300 slowed down the progression of IDD by disrupting the Wnt/β-catenin pathway through the FOXO3/Sirt1 axis. Taken together, we suggest that p300 can act to suppress IDD via a FOXO3-dependent mechanism, highlighting a potential novel target for treatment of IDD.

CircARHGAP12 Triggers Mesenchymal Stromal Cell Autophagy to Facilitate its Effect on Repairing Diabetic Wounds by Sponging miR-301b-3p/ATG16L1 and miR-301b-3p/ULK2

Circular RNAs have been confirmed to play vital roles in the development of human diseases. Nevertheless, their effects on modulating mesenchymal stromal cells (MSCs) to heal diabetic wounds are still elusive. In this study, our data revealed that MSCs treated with high glucose displayed an evident reduction in circARHGAP12 expression, whereas autophagy mediated by circARHGAP12 suppressed high glucose-triggered apoptosis of MSCs. Mechanistically, circARHGAP12 was capable of directly interacting with miR-301b-3p and subsequently sponged microRNA to modulate the expression of the miR-301b-3p target genes ATG16L1 and ULK2 and the downstream signaling pathway. Moreover, circARHGAP12 promoted the survival of MSCs in diabetic wounds in vivo and accelerated wound healing. Collectively, these results suggest that circARHGAP12/miR-301b-3p/ATG16L1 and circARHGAP12/miR-301b-3p/ULK2 regulatory networks might be an underlying therapeutic target for MSCs in diabetic wound healing.

RETREG1-mediated ER-phagy activation induced by glucose deprivation alleviates nucleus pulposus cell damage via ER stress pathway

Accumulating evidence indicates that ER-phagy serves as a key adaptive regulatory mechanism in response to various stress conditions. However, the exact mechanisms underlying ER-phagy in the pathogenesis of intervertebral disc degeneration remain largely unclear. In the present study, we demonstrated that RETREG1-mediated ER-phagy is induced by glucose deprivation (GD) treatment, along with ER stress activation and cell function decline. Importantly, ER-phagy was shown to be crucial for cell survival under GD conditions. Furthermore, ER stress was suggested as an upstream event of ER-phagy upon GD treatment and upregulation of ER-phagy could counteract the ER stress response. Therefore, our findings indicate that RETREG1-mediated ER-phagy activation protects against GD treatment-induced cell injury via modulating ER stress in human nucleus pulposus cells.

Role of autophagy in intervertebral disc degeneration

Intervertebral disc degeneration (IDD) is a leading contributor to low back pain. The intervertebral disc (IVD) is composed of three tissue types: the central gelatinous nucleus pulposus (NP) tissue, the surrounding annulus fibrosus (AF) tissue, and the inferior and superior cartilage endplates. The IVD microenvironment is hypoxic, acidic, hyperosmotic, and low in nutrients because it is mostly avascular. The cellular processes that underlie IDD initiation and progression are still poorly understood. Specifically, a lack of understanding regarding NP cell metabolism and physiology hinders the development of effective therapeutics to treat IDD patients. Autophagy is a vital intracellular degradation process that removes damaged organelles, misfolded proteins, and intracellular pathogens and recycles the degraded components for cellular energy and function. NP cells have adapted to survive within their harsh tissue microenvironment using processes that are largely unknown, and we postulate autophagy is one of these undiscovered mechanisms. In this review, we describe unique features of the IVD tissue, review how physiological stressors impact autophagy in NP cells in vitro, survey the current understanding of autophagy regulation in the IVD, and assess the relationship between autophagy and IDD. Published studies confirm autophagy markers are present in IVD tissue, and IVD cells can regulate autophagy in response to cellular stressors in vitro. However, data are still lacking to determine the exact mechanisms regulating autophagy in IVD cells. More in-depth research is needed to establish whether autophagy is necessary to maintain IVD cell health and validate autophagy as a relevant therapeutic target for treating IDD.

LncRNA nuclear receptor subfamily 2 group F member 1 antisense RNA 1 (NR2F1-AS1) aggravates nucleus pulposus cell apoptosis and extracellular matrix degradation

Emerging reports uncover that long noncoding RNAs (lncRNAs) help regulate intervertebral disc degeneration (IVDD). Here, we probe the function of lncRNA nuclear receptor subfamily 2 group F member 1 antisense RNA 1 (NR2F1-AS1) in IVDD. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) was applied to verify the expression of NR2F1-AS1 and miR-145-5p in nucleus pulposus (NP) tissues from IVDD patients or NP cells dealt with IL-1β or TNF-α. Flow cytometry or the TdT-mediated dUTP nick end labeling (TUNEL) assay was performed to validate the apoptosis of NP cells with selective regulation of NR2F1-AS1 and miR-145-5p. ECM-related genes, FOXO1, Bax, and Bcl2 were evaluated by qRT-PCR or Western blot (WB). The targeted relationships between NR2F1-AS1 and miR-145-5p, miR-145-5p and FOXO1 were testified by the dual-luciferase reporter assay and the RNA immunoprecipitation (RIP) assay. Our outcomes substantiated that NR2F1-AS1 was up-regulated, while miR-145-5p was down-regulated in intervertebral disc tissues of IVDD patients or NP cells treated with IL-1β or TNF-α. Besides, overexpressing NR2F1-AS1 intensified ECM degradation and NP cell apoptosis induced by IL-1β, while knocking down NR2F1-AS1 or up-regulating miR-145-5p reversed IL-1β-mediated effects in NP cells. Meanwhile, NR2F1-AS1 choked miR-145-5p and abated its effects in NP cells. This study confirms that NR2F1-AS1 modulates IVDD progression by up-regulating the FOXO1 pathway through the sponge of miR-145-5p as a competitive endogenous RNA (ceRNA).

Role of AMPK mediated pathways in autophagy and aging

AMPK is an important kinase regulating energy homeostasis and also a key protein involved in a variety of signal transduction pathways. It plays a vitally regulatory role in cellular senescence. Activation of AMPK can delay or block the aging process, which is of great significance in the treatment of cardiovascular diseases and other aging related diseases, and provides a potential target for new indications such as Alzheimer's disease. Therefore, AMPK signaling pathway plays an important role in aging research. The in-depth study of AMPK activators will provide more new directions for the treatment of age-related maladies and the development of innovative drugs. Autophagy is a process that engulfs and degrades own cytoplasm or organelles. Thereby, meeting the metabolic demands and updating certain organelles of the cell has become a hotspot in the field of anti-aging in recent years. AMPK plays an important role between autophagy and senescence. In our review, the relationship among AMPK signaling, autophagy and aging will be clarified through the interaction between AMPK and mTOR, ULK1, FOXO, p53, SIRT1, and NF -κB.

Mitophagy in degenerative joint diseases

Mitochondrial dysfunction is involved in aging and multiple degenerative diseases, including intervertebral disc degeneration (IVDD) and osteoarthritis (OA). Thus, the maintenance of mitochondria homeostasis and function is important. Mitophagy, a process that selectively clears damaged or dysfunctional mitochondria through autophagic machinery, functions to maintain mitochondrial quality control and homeostasis. IVDD and OA are similar joint diseases involving the degradation of cartilaginous tissues that are mainly caused by oxidative stress, cell apoptosis and extracellular matrix (ECM) degradation. Over the past decade, accumulating evidence indicates the essential role of mitophagy in the pathogenesis of IVDD and OA. Importantly, strategies by the regulation of mitophagy exert beneficial effects in the pre-clinical experiments. Given the importance and novelty of mitophagy, we provide an overview of mitophagy pathways and discuss the roles of mitophagy in IVDD and OA. We also highlight the potential of targeting mitophagy for the treatment of degenerative joint diseases.

Abbreviations: AD: Alzheimer disease; AF: annulus fibrosus; ADORA2A/A2AR: adenosine A2a receptor; AMBRA1: autophagy and beclin 1 regulator 1; BMSCs: bone marrow mesenchymal stem cells; BNIP3: BCL2 interacting protein 3; BNIP3L/NIX: BCL2/adenovirus E1B interacting protein 3-like; CDH6: cadherin 6; CEP: cartilaginous endplates; circRNA: circular RNA; DNM1L/DRP1: dynamin 1-like; ECM: extracellular matrix; HIF1A: hypoxia inducible factor 1: alpha subunit; IL1B: interleukin 1 beta; IMM: inner mitochondrial membranes; IVDD: intervertebral disc degeneration; MAPK8/JNK: mitogen-activated protein kinase 8; MFN1: mitofusin 1; MFN2: mitofusin 2; MIA: monosodium iodoacetate; RHOT/MIRO: ras homolog family member T; MMP: mitochondrial transmembrane potential; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; NFE2L2: nuclear factor: erythroid 2 like 2; NP: nucleus pulposus; OA: osteoarthritis; OPA1: OPA1: mitochondrial dynamin like GTPase; OPTN: optineurin; PRKN: parkin RBR E3 ubiquitin protein ligase; PD: Parkinson disease; PGAM5: PGAM family member 5; PPARGC1A/PGC-1A: peroxisome proliferator activated receptor: gamma: coactivator 1 alpha; PHF23: PHD finger protein 23; PINK1: PTEN induced putative kinase 1; ROS: reactive oxygen species; SfMSCs: synovial fluid MSCs; SIRT1: sirtuin 1; SIRT2: sirtuin 2; SIRT3: sirtuin 3; SQSTM1/p62: sequestosome 1; TNF: tumor necrosis factor; Ub: ubiquitin; UBL: ubiquitin-like; VDAC: voltage-dependent anion channel.