Nucleus pulposus cells express HIF-1α under normoxic culture conditions: A metabolic adaptation to the intervertebral disc microenvironment

ARRB1 inhibits extracellular matrix degradation and apoptosis of nucleus pulposus cells by promoting autophagy and attenuates intervertebral disc degeneration

OBJECTIVE

Intervertebral disc degeneration (IVDD) is the leading cause of lower back pain (LBP). β-arrestin 1 (ARRB1) is a multifunctional protein that regulates numerous pathological processes. The aim of this study was to investigate the role of ARRB1 in IVDD.

METHODS

The expression of ARRB1 in nucleus pulposus (NP) of rats with IVDD was assayed. Next, rat nucleus pulposus cells (NPCs) were infected with lentiviruses containing shArrb1 (LV-shArrb1) and overexpressing Arrb1 (LV-oeArrb1). The roles of Arrb1 in serum-deprived NPCs were investigated by measuring apoptosis, extracellular matrix degradation, and autophagic flux. For experiments in vivo, LV-oeArrb1 lentivirus was injected into the NP tissues of IVDD rats to evaluate the effects of Arrb1 overexpression on NP.

RESULTS

In the NP tissues of IVDD rats, ARRB1 and cleaved caspase-3 expression increased, and the ratio of LC3II/LC3I protein expression was upregulated. Arrb1 knockdown aggravated extracellular matrix degradation, cellular apoptosis, and impairment of autophagic flux in rat NPCs under serum-deprived conditions, whereas Arrb1 overexpression significantly reversed these effects. ARRB1 interacted with Beclin 1, and Arrb1 knockdown suppressed the formation of the Beclin1-PIK3C3 core complex. The autophagy inhibitor 3-methyladenine (3-MA) offset the protective effects of Arrb1 overexpression in serum-deprived NPCs. Furthermore, Arrb1 overexpression inhibited apoptosis and extracellular matrix degradation, promoted autophagy in NP, and delayed the development of IVDD in rats.

CONCLUSION

ARRB1 prevents extracellular matrix degradation and apoptosis of NPCs by upregulating autophagy and ameliorating IVDD progression, presenting an innovative strategy for the treatment of IVDD.

Overactivation of NF-kB pathway can induce apoptosis by down-regulating glycolysis in human degenerative nucleus pulposus cells

Intervertebral disc herniation, a prevalent condition in spinal surgery that frequently results in low back pain and lower limb dysfunction, significantly impacting patients' quality of life. Several factors, including spine biomechanics, biology, nutrition, injury, and abnormal inflammatory responses, have been associated with the development of intervertebral disc herniation. Among these factors, abnormal inflammatory responses have received considerable attention as a crucial mediator of both clinical symptoms and disease progression during the intervertebral disc herniation process. However, the underlying mechanisms of inflammation-induced intervertebral disc herniation remain inadequately explored. The NF-κB (Nuclear Factor-κB) pathway plays a central role in regulating the expression of proinflammatory cytokines. Research on intervertebral disc herniation has suggested that NF-κB can activate the NLRP3 inflammasome, thereby exacerbating intervertebral disc degeneration. Targeting the NF-κB pathway has shown promise in alleviating disc degeneration and associated pain. Previous research indicated that the upregulation of the NF-κB pathway, achieved through the inhibition of A20 (zinc finger protein A20), accelerated intervertebral disc herniation. In the present study, we observed that increased activation of NF-κB pathway activation suppressed the glycolysis process in nucleus pulposus cells (NPCs), leading to NPC apoptosis. Conversely, inhibition of the NF-κB pathway overactivated promoted the restoration of glycolysis and reversed NPC apoptosis, especially when treated with Lipopolysaccharide (LPS).

Mesenchymal Stem Cell-Derived Extracellular Vesicles Protect Rat Nucleus Pulposus Cells from Oxidative Stress

BACKGROUND

Oxidative stress (OS) is mainly associated with the pathogenesis of intervertebral disc (IVD) degeneration; it causes nucleus pulposus cells (NPCs) to undergo senescence and triggers autophagy and apoptosis. This study aims to evaluate the regeneration potential of extracellular vesicles (EVs) derived from human umbilical cord-mesenchymal stem cells (hUC-MSCs) in an in vitro rat NPC-induced OS model.

DESIGN

NPCs were isolated from rat coccygeal discs, propagated, and characterized. OS was induced by hydrogen peroxide (H2O2), which is confirmed by 2,7-dichlorofluorescein diacetate (H2DCFDA) assay. EVs were isolated from hUC-MSCs and characterized by analyzing the vesicles using fluorescence microscope, scanning electron microscope (SEM), atomic force microscope (AFM), dynamic light scattering (DLS), and Western blot (WB). The in vitro effects of EVs on migration, uptake, and survival of NPCs were determined.

RESULTS

SEM and AFM topographic images revealed the size distribution of EVs. The phenotypes of isolated EVs showed that the size of EVs was 403.3 ± 85.94 nm, and the zeta potential was -0.270 ± 4.02 mV. Protein expression analysis showed that EVs were positive for CD81 and annexin V. Treatment of NPCs with EVs reduced H2O2-induced OS as evidenced by a decrease in reactive oxygen species (ROS) levels. Co-culture of NPCs with DiI-labeled EVs showed the cellular internalization of EVs. In the scratch assay, EVs significantly increased NPC proliferation and migration toward the scratched area. Quantitative polymerase chain reaction analysis showed that EVs significantly reduced the expression of OS genes.

CONCLUSION

EVs protected NPCs from H2O2-induced OS by reducing intracellular ROS generation and improved NPC proliferation and migration.

HIF-1α protects nucleus pulposus cells from oxidative stress-induced mitochondrial impairment through PDK-1

The pathogenesis of intervertebral disc degeneration (IVDD) involves complex signaling networks and various effector molecules, and our understanding of the pathogenesis of IVDD is limited. Hypoxia inducible factor-1α (HIF-1α) is closely related to IVDD, and there is excessive oxidative stress concurrent with IVDD. In this study, we found that HIF-1α could protect nucleus pulposus cells from excessive oxidative stress by reversing the imbalance between oxidants and antioxidants and thus mitigating the oxidative stress-induced mitochondrial impairment. With further exploration, we found that pyruvate dehydrogenase kinase 1 (PDK-1) was involved in the protective effect of HIF-1α on nucleus pulposus cells under oxidative stress. We suggested that HIF-1α could preserve the mitochondrial integrity and activate glycolysis in nucleus pulposus cells via PDK-1, and the addition of DCA, a PDK-1 inhibitor, could blunt the protective effect of HIF-1α. In addition, the HIF-1α/PDK-1 regulatory axis was also confirmed in vivo through HIF-1α knockout mice model. Therefore, we propose that HIF-1α protects nucleus pulposus cells from excessive oxidative stress by maintaining the mitochondrial integrity and glycolysis via PDK-1, thus enriching the insight into the protective mechanism of HIF-1α against IVDD, and providing a novel therapeutic target for the treatment of IVDD.

Glutamine suppresses senescence and promotes autophagy through glycolysis inhibition-mediated AMPKα lactylation in intervertebral disc degeneration

Regulating metabolic disorders has become a promising focus in treating intervertebral disc degeneration (IDD). A few drugs regulating metabolism, such as atorvastatin, metformin, and melatonin, show positive effects in treating IDD. Glutamine participates in multiple metabolic processes, including glutaminolysis and glycolysis; however, its impact on IDD is unclear. The current study reveals that glutamine levels are decreased in severely degenerated human nucleus pulposus (NP) tissues and aging Sprague-Dawley (SD) rat nucleus pulposus tissues, while lactate accumulation and lactylation are increased. Supplementary glutamine suppresses glycolysis and reduces lactate production, which downregulates adenosine-5'-monophosphate-activated protein kinase α (AMPKα) lactylation and upregulates AMPKα phosphorylation. Moreover, glutamine treatment reduces NP cell senescence and enhances autophagy and matrix synthesis via inhibition of glycolysis and AMPK lactylation, and glycolysis inhibition suppresses lactylation. Our results indicate that glutamine could prevent IDD by glycolysis inhibition-decreased AMPKα lactylation, which promotes autophagy and suppresses NP cell senescence.

Increased HIF-2α activity in the nucleus pulposus causes intervertebral disc degeneration in the aging mouse spine

Hypoxia-inducible factors (HIFs) are essential to the homeostasis of hypoxic tissues. Although HIF-2α, is expressed in nucleus pulposus (NP) cells, consequences of elevated HIF-2 activity on disc health remains unknown. We expressed HIF-2α with proline to alanine substitutions (P405A; P531A) in the Oxygen-dependent degradation domain (HIF-2αdPA) in the NP tissue using an inducible, nucleus pulposus-specific K19CreERT allele to study HIF-2α function in the adult intervertebral disc. Expression of HIF-2α in NP impacted disc morphology, as evident from small but significantly higher scores of degeneration in NP of 24-month-old K19CreERT; HIF-2αdPA (K19-dPA) mice. Noteworthy, comparisons of grades within each genotype between 14 months and 24 months indicated that HIF-2α overexpression contributed to more pronounced changes than aging alone. The annulus fibrosus (AF) compartment in the 14-month-old K19-dPA mice exhibited lower collagen turnover and Fourier transform-infrared (FTIR) spectroscopic imaging analyses showed changes in the biochemical composition of the 14- and 24-month-old K19-dPA mice. Moreover, there were changes in aggrecan, chondroitin sulfate, and COMP abundance without alterations in NP phenotypic marker CA3, suggesting the overexpression of HIF-2α had some impact on matrix composition but not the cell phenotype. Mechanistically, the global transcriptomic analysis showed enrichment of differentially expressed genes in themes closely related to NP cell function such as cilia, SLIT/ROBO pathway, and HIF/Hypoxia signaling at both 14- and 24-month. Together, these findings underscore the role of HIF-2α in the pathogenesis of disc degeneration in the aged spine.

Involvement of DJ-1 in the pathogenesis of intervertebral disc degeneration via hexokinase 2-mediated mitophagy

Intervertebral disc degeneration (IDD) is an important pathological basis for degenerative spinal diseases and is involved in mitophagy dysfunction. However, the molecular mechanisms underlying mitophagy regulation in IDD remain unclear. This study aimed to clarify the role of DJ-1 in regulating mitophagy during IDD pathogenesis. Here, we showed that the mitochondrial localization of DJ-1 in nucleus pulposus cells (NPCs) first increased and then decreased in response to oxidative stress. Subsequently, loss- and gain-of-function experiments revealed that overexpression of DJ-1 in NPCs inhibited oxidative stress-induced mitochondrial dysfunction and mitochondria-dependent apoptosis, whereas knockdown of DJ-1 had the opposite effect. Mechanistically, mitochondrial translocation of DJ-1 promoted the recruitment of hexokinase 2 (HK2) to damaged mitochondria by activating Akt and subsequently Parkin-dependent mitophagy to inhibit oxidative stress-induced apoptosis in NPCs. However, silencing Parkin, reducing mitochondrial recruitment of HK2, or inhibiting Akt activation suppressed DJ-1-mediated mitophagy. Furthermore, overexpression of DJ-1 ameliorated IDD in rats through HK2-mediated mitophagy. Taken together, these findings indicate that DJ-1 promotes HK2-mediated mitophagy under oxidative stress conditions to inhibit mitochondria-dependent apoptosis in NPCs and could be a therapeutic target for IDD.

OPA1 protects intervertebral disc and knee joint health in aged mice by maintaining the structure and metabolic functions of mitochondria

Due to their glycolytic nature and limited vascularity, nucleus pulposus (NP) cells of the intervertebral disc and articular chondrocytes were long thought to have minimal reliance on mitochondrial function. Recent studies have challenged this long-held view and highlighted the increasingly important role of mitochondria in the physiology of these tissues. We investigated the role of mitochondrial fusion protein OPA1 in maintaining the spine and knee joint health in aging mice. OPA1 knockdown in NP cells altered mitochondrial size and cristae shape and increased the oxygen consumption rate without affecting ATP synthesis. OPA1 governed the morphology of multiple organelles, and its loss resulted in the dysregulation of NP cell autophagy. Metabolic profiling and 13 C-flux analyses revealed TCA cycle anaplerosis and altered metabolism in OPA1-deficient NP cells. Noteworthy, Opa1 AcanCreERT2 mice showed age- dependent disc, and cartilage degeneration and vertebral osteopenia. Our findings suggest that OPA1 regulation of mitochondrial dynamics and multi-organelle interactions is critical in preserving metabolic homeostasis of disc and cartilage.

Teaser

OPA1 is necessary for the maintenance of intervertebral disc and knee joint health in aging mice.

Increased HIF-2α Activity in the Nucleus Pulposus Causes Intervertebral Disc Degeneration in the Aging Mouse Spine

Hypoxia-inducible factors (HIFs) are essential to the homeostasis of hypoxic tissues. Although HIF-2α, is expressed in nucleus pulposus (NP) cells, consequences of elevated HIF-2 activity on disc health remains unknown. We expressed HIF-2α with proline to alanine substitutions (P405A;P531A) in the Oxygen-dependent degradation domain (HIF-2αdPA) in the NP tissue using an inducible, nucleus pulposus-specific K19 CreERT allele to study HIF-2α function in the adult intervertebral disc. Expression of HIF-2α in NP impacted disc morphology, as evident from small but significantly higher scores of degeneration in NP of 24-month-old K19 CreERT ; HIF-2α dPA (K19-dPA) mice. Noteworthy, comparisons of grades within each genotype between 14 months and 24 months indicated that HIF-2α overexpression contributed to more pronounced changes than aging alone. The annulus fibrosus (AF) compartment in the 14-month-old K19-dPA mice exhibited lower collagen turnover and Fourier transform-infrared (FTIR) spectroscopic imaging analyses showed changes in the biochemical composition of the 14-and 24-month-old K19-dPA mice. Moreover, there were changes in aggrecan, chondroitin sulfate, and COMP abundance without alterations in NP phenotypic marker CA3, suggesting the overexpression of HIF-2α had some impact on matrix composition but not the cell phenotype. Mechanistically, the global transcriptomic analysis showed enrichment of differentially expressed genes in themes closely related to NP cell function such as cilia, SLIT/ROBO pathway, and HIF/Hypoxia signaling at both 14- and 24-months. Together, these findings underscore the role of HIF-2α in the pathogenesis of disc degeneration in the aged spine.

The mitophagy receptor BNIP3 is critical for the regulation of metabolic homeostasis and mitochondrial function in the nucleus pulposus cells of the intervertebral disc

The contribution of mitochondria to the metabolic function of hypoxic NP cells has been overlooked. We have shown that NP cells contain networked mitochondria and that mitochondrial translocation of BNIP3 mediates hypoxia-induced mitophagy. However, whether BNIP3 also plays a role in governing mitochondrial function and metabolism in hypoxic NP cells is not known. BNIP3 knockdown altered mitochondrial morphology, and number, and increased mitophagy. Interestingly, BNIP3 deficiency in NP cells reduced glycolytic capacity reflected by lower production of lactate/H+ and lower ATP production rate. Widely targeted metabolic profiling and flux analysis using 1-2-13C-glucose showed that the BNIP3 loss resulted in redirection of glycolytic flux into pentose phosphate and hexosamine biosynthesis as well as pyruvate resulting in increased TCA flux. An overall reduction in one-carbon metabolism was noted suggesting reduced biosynthesis. U13C-glutamine flux analysis showed preservation of glutamine utilization to maintain TCA intermediates. The transcriptomic analysis of the BNIP3-deficient cells showed dysregulation of cellular functions including membrane and cytoskeletal integrity, ECM-growth factor signaling, and protein quality control with an overall increase in themes related to angiogenesis and innate immune response. Importantly, we observed strong thematic similarities with the transcriptome of a subset of human degenerative samples. Last, we noted increased autophagic flux, decreased disc height index and aberrant COL10A1/collagen X expression, signs of early disc degeneration in young adult bnip3 knockout mice. These results suggested that in addition to mitophagy regulation, BNIP3 plays a role in maintaining mitochondrial function and metabolism, and dysregulation of mitochondrial homeostasis could promote disc degeneration.Abbreviations: ECAR extracellular acidification rate; HIF hypoxia inducible factor; MFA metabolic flux analysis; NP nucleus pulposus; OCR oxygen consumption rate; ShBnip3 short-hairpin Bnip3.

GLUT1 is redundant in hypoxic and glycolytic nucleus pulposus cells of the intervertebral disc

Glycolysis is central to homeostasis of nucleus pulposus (NP) cells in the avascular intervertebral disc. Since the glucose transporter, GLUT1, is a highly enriched phenotypic marker of NP cells, we hypothesized that it is vital for the development and postnatal maintenance of the disc. Surprisingly, primary NP cells treated with 2 well-characterized GLUT1 inhibitors maintained normal rates of glycolysis and ATP production, indicating intrinsic compensatory mechanisms. We showed in vitro that NP cells mitigated GLUT1 loss by rewiring glucose import through GLUT3. Of note, we demonstrated that substrates, such as glutamine and palmitate, did not compensate for glucose restriction resulting from dual inhibition of GLUT1/3, and inhibition compromised long-term cell viability. To investigate the redundancy of GLUT1 function in NP, we generated 2 NP-specific knockout mice: Krt19CreERT Glut1fl/fl and Foxa2Cre Glut1fl/fl. There were no apparent defects in postnatal disc health or development and maturation in mutant mice. Microarray analysis verified that GLUT1 loss did not cause transcriptomic alterations in the NP, supporting that cells are refractory to GLUT1 loss. These observations provide the first evidence to our knowledge of functional redundancy in GLUT transporters in the physiologically hypoxic intervertebral disc and underscore the importance of glucose as the indispensable substrate for NP cells.

Baicalein alleviates TNF-α-induced apoptosis of human nucleus pulposus cells through PI3K/AKT signaling pathway

BACKGROUND

Nucleus pulposus (NP) cell apoptosis contributed to disc degeneration. Baicalein, a natural steroid saponin, has been demonstrated to have anti-inflammatory, antiapoptotic, and antioxidative effects in various diseases. However, little is known about the roles of baicalein in intervertebral disc degeneration.

METHODS

To evaluate the roles of baicalein in disc degeneration and its specific mechanism, human NP cells were incubated with TNF-α and various concentrations of baicalein. Cell viability, extracellular matrix protein expression, catabolic factors, degree of apoptosis, inflammatory factors, and related signaling pathways were evaluated by western blotting, fluorescence immunostaining, TUNEL staining, and reverse transcription PCR.

RESULTS

Baicalein inhibited TNF-α-activated apoptotic signaling and catabolic activity in NP cells. Baicalein promoted PI3K/Akt signaling and attenuated the level of apoptosis-related markers in TNF-α-stimulated human NP cells.

CONCLUSION

Our work provides that baicalein attenuates TNF-α-activated apoptosis in human NP cells through promoting the PI3K/Akt pathway, indicating that baicalein is a new potential candidate for clinical therapy to attenuate disc degeneration.

Biomaterials delivery strategies to repair degenerated intervertebral discs by regulating the inflammatory microenvironment

Intervertebral disc degeneration (IVDD) is one of the leading causes of lower back pain. Although IVDD cannot directly cause death, it can cause pain, psychological burdens, and economic burdens to patients. Current conservative treatments for IVDD can relieve pain but cannot reverse the disease. Patients who cannot tolerate pain usually resort to a strategy of surgical resection of the degenerated disc. However, the surgical removal of IVDD can affect the stability of adjacent discs. Furthermore, the probability of the reherniation of the intervertebral disc (IVD) after surgery is as high as 21.2%. Strategies based on tissue engineering to deliver stem cells for the regeneration of nucleus purposes (NP) and annulus fibrosus (AF) have been extensively studied. The developed biomaterials not only locally withstand the pressure of the IVD but also lay the foundation for the survival of stem cells. However, the structure of IVDs does not provide sufficient nutrients for delivered stem cells. The role of immune mechanisms in IVDD has recently become clear. In IVDD, the IVD that was originally in immune privilege prevents the attack of immune cells (mainly effector T cells and macrophages) and aggravates the disease. Immune regulatory and inflammatory factors released by effector T cells, macrophages, and the IVD further aggravate IVDD. Reversing IVDD by regulating the inflammatory microenvironment is a potential approach for the treatment of the disease. However, the biological factors modulating the inflammatory microenvironment easily degrade in vivo. It makes it possible for different biomaterials to modulate the inflammatory microenvironment to repair IVDD. In this review, we have discussed the structures of IVDs and the immune mechanisms underlying IVDD. We have described the immune mechanisms elicited by different biological factors, including tumor necrosis factors, interleukins, transforming growth factors, hypoxia-inducible factors, and reactive oxygen species in IVDs. Finally, we have discussed the biomaterials used to modulate the inflammatory microenvironment to repair IVDD and their development.

Nutrient metabolism of the nucleus pulposus: A literature review

Cells take in, consume, and synthesize nutrients for numerous physiological functions. This includes not only energy production but also macromolecule biosynthesis, which will further influence cellular signaling, redox homeostasis, and cell fate commitment. Therefore, alteration in cellular nutrient metabolism is associated with pathological conditions. Intervertebral discs, particularly the nucleus pulposus (NP), are avascular and exhibit unique metabolic preferences. Clinical and preclinical studies have indicated a correlation between intervertebral degeneration (IDD) and systemic metabolic diseases such as diabetes, obesity, and dyslipidemia. However, a lack of understanding of the nutrient metabolism of NP cells is masking the underlying mechanism. Indeed, although previous studies indicated that glucose metabolism is essential for NP cells, the downstream metabolic pathways remain unknown, and the potential role of other nutrients, like amino acids and lipids, is understudied. In this literature review, we summarize the current understanding of nutrient metabolism in NP cells and discuss other potential metabolic pathways by referring to a human NP transcriptomic dataset deposited to the Gene Expression Omnibus, which can provide us hints for future studies of nutrient metabolism in NP cells and novel therapies for IDD.

Slow twitch paraspinal muscle dysregulation in adolescent idiopathic scoliosis exhibiting HIF-2α misexpression

Background

Adolescent idiopathic scoliosis (AIS) refers to a three-dimensional spinal deformity which has a typical onset during adolescence. In most cases, the cause of the deformity cannot be clearly identified. Unbalanced paraspinal muscle activity in AIS patients was reported and hypoxia was implicated to regulate myogenesis. This study aims to investigate the association between myogenesis/muscle toning and HIF-αs activity in the pathogenesis of AIS.

Methods

HIF-αs expression was examined by enzyme-linked immunosorbent assay and western blot in paraspinal myoblasts isolated from 18 subjects who underwent deformity correction surgery. QPCR was conducted to measure the gene expression levels of perinatal muscle fiber markers MYH3, MYH8; slow twitch muscle fiber markers MHY7; fast twitch muscle fiber markers MYH4; and myogenic regulatory factors MYF5 and MYOG. Slow and fast twitch muscle fiber composition in concave/convex paraspinal musculature of AIS subjects was evaluated by immunostaining of myosin heavy chain type I (MyHC I) and myosin heavy chain type II (MyHC II).

Results

Reduced HIF-2α induction under hypoxia was found in paraspinal myoblast culture of 33% AIS subjects. We detected a suppression of perinatal and slow twitch muscle fiber associated genes, but not fast twitch muscle fiber-associated genes and myogenic regulatory factors in HIF-2α misexpressed AIS myoblasts. Distinct reduction of slow twitch muscle fiber was evidenced in convex paraspinal musculature, suggesting an asymmetric expression of slow twitch muscle fiber in HIF-2α misexpressed AIS patients.

Conclusions

This study indicates an association of abnormal HIF-2α expression in paraspinal myoblasts and a disproportionate slow twitch muscle fiber content in the convexity of the curvature in a subset of AIS subjects, suggesting HIF-2α dysregulation as a possible risk factor for AIS. The role of HIF-2α in paraspinal muscle function during spinal growth and its relevance in AIS prognosis warrants further investigation.

Orientin downregulating oxidative stress-mediated endoplasmic reticulum stress and mitochondrial dysfunction through AMPK/SIRT1 pathway in rat nucleus pulposus cells in vitro and attenuated intervertebral disc degeneration in vivo

Although considered as a major contributor to low back pain (LBP), intervertebral disc degeneration (IVDD) has poor medical and surgical treatments. Various studies have revealed that endoplasmic reticulum (ER) stress and extracellular matrix (ECM) degeneration play a vital role in initiating and developing the progression of IVDD. Moreover, restoration of SIRT1/AMPK was confirmed to prevent IVDD and damage via maintaining ER and extracellular homeostasis. In addition, orientin (Ori) has been shown to upregulate SIRT1. However, the effect of Ori in nucleus pulposus cells (NPCs) is not determined. Hence, in this study we aim to explore the function of Ori in IVDD pathological model. The results demonstrate that Ori treatment in vitro increased SIRT1/AMPK in NPCs, maintained ECM and ER balance and decreased oxidative stress (OS) response. Ori rescued the disordered homeostasis stimulated by tert-butyl hydroperoxide (TBHP), and its function can be inhibited by thapsigargin (TG). Compound C and EX-527, inhibitors of AMPK and SIRT1 counteracted the Ori-mediated ER stress elimination. These results confirm that Ori exerts its effects by upregulating AMPK and SIRT1. Puncture-stimulated IVDD rats were used to show that Ori attenuates the pathological development in vivo. In all, we partly unveil the underlying mechanisms of Ori in IVDD.

Interleukin-18 Inhibition Protects Against Intervertebral Disc Degeneration via the Inactivation of Caspase-3/9 Dependent Apoptotic Pathways

BACKGROUND

The present study was designed to identify and understand the potential effectiveness of therapeutic target in intervertebral disc degeneration (IVDD) and its regulation mechanism.

METHODS

The role and mechanism of interleukin-18 (IL-18) in the disease were investigated. The IVDD degenerative nucleus pulposus (NP) tissues from the human and mouse models were used.A total of three groups of Male BALB/c mice were randomly made i.e control, IVDD, and IVDD+Ad-shIL-18 groups. After Ad-shIL-18 transfection, the expression of ECM synthesis related protein Aggrecan (ACAN) and Collagen II, apoptotic effector Caspases (Caspase-3, 8, 9, 12 and Cleaved-Caspase 3, 8, 9, 12), pro-apoptotic gene Bax and anti-apoptotic factors Bcl-2 in NP cells of the human were evaluated.

RESULTS

The results of our study revealed that the mRNA and protein expression levels of IL-18 were notably increased in the NP tissues of IVDD patients and mice models. In the IVDD mice model, Ad-sh-IL-18 treatment reversed the IVDD progression. The levels of Aggrecan and Collagen II, contributing to ECM degradation in NP cells, were also significantly increased. Additionally, Ad-sh-IL-18 could inhibit the NP cell's apoptosis via regulating the caspase-3/9 pathway.

CONCLUSION

The IL-18 knockdown via the caspase-3/9 pathway, might reduce the NP cell's death as well as the imbalance between catabolism and anabolism of ECM in IVDD.

SERPINA1 is a hub gene associated with intervertebral disc degeneration grade and affects the nucleus pulposus cell phenotype through the ADIRF-AS1/miR-214-3p axis

Long noncoding RNAs (lncRNAs) and miRNAs have been reported to participate in intervertebral disc degeneration (IDD) progression. However, the key lncRNA-miRNA axis and its corresponding affected hub genes in IDD remain unknown. In this study, weighted gene coexpression network analysis (WGCNA) was first used to determine the key gene cluster and hub genes implicated in IDD progression. The expression levels of ADIRF-AS1, miR-214-3p, and SERPINA1 in nucleus pulposus (NP) tissues were detected. The ADIRF-AS1/miR-214-3p/SERPINA1 axis was identified, and its effects on the proliferation, senescence, and apoptosis of NP cells were investigated in vitro and in vivo. SERPINA1 overexpression in NP cells promoted cell viability and inhibited cell apoptosis and senescence. Moreover, SERPINA1 regulated the IDD grade in rat models. The lncRNA ADIRF-AS1 was downregulated in high-grade degeneration NP tissues and positively correlated with SERPINA1. ADIRF-AS1 overexpression attenuated cellular degenerative changes in NP cells. miR-214-3p directly bound to SERPINA1 and ADIRF-AS1 and negatively regulated ADIRF-AS1 expression. miR-214-3p inhibition exerted similar effects on cellular degenerative changes in NP cells to SERPINA1 or ADIRF-AS1 overexpression. Furthermore, miR-214-3p overexpression partially reversed the effects of ADIRF-AS1 overexpression. Collectively, these data suggest that ADIRF-AS1 overexpression could mitigate IDD by binding to miR-214-3p to upregulate SERPINA1. Additional studies (especially those using an axial loading-induced IDD animal model) will be needed to further validate the role of the ADIRF-AS1/miR-214-3p/SERPINA1 signaling axis in IDD progression.

Single-Cell RNA-Seq Analysis of Cells from Degenerating and Non-Degenerating Intervertebral Discs from the Same Individual Reveals New Biomarkers for Intervertebral Disc Degeneration

In this study, we used single-cell transcriptomic analysis to identify new specific biomarkers for nucleus pulposus (NP) and inner annulus fibrosis (iAF) cells, and to define cell populations within non-degenerating (nD) and degenerating (D) human intervertebral discs (IVD) of the same individual. Cluster analysis based on differential gene expression delineated 14 cell clusters. Gene expression profiles at single-cell resolution revealed the potential functional differences linked to degeneration, and among NP and iAF subpopulations. GO and KEGG analyses discovered molecular functions, biological processes, and transcription factors linked to cell type and degeneration state. We propose two lists of biomarkers, one as specific cell type, including C2orf40, MGP, MSMP, CD44, EIF1, LGALS1, RGCC, EPYC, HILPDA, ACAN, MT1F, CHI3L1, ID1, ID3 and TMED2. The second list proposes predictive IVD degeneration genes, including MT1G, SPP1, HMGA1, FN1, FBXO2, SPARC, VIM, CTGF, MGST1, TAF1D, CAPS, SPTSSB, S100A1, CHI3L2, PLA2G2A, TNRSF11B, FGFBP2, MGP, SLPI, DCN, MT-ND2, MTCYB, ADIRF, FRZB, CLEC3A, UPP1, S100A2, PRG4, COL2A1, SOD2 and MT2A. Protein and mRNA expression of MGST1, vimentin, SOD2 and SYF2 (p29) genes validated our scRNA-seq findings. Our data provide new insights into disc cells phenotypes and biomarkers of IVD degeneration that could improve diagnostic and therapeutic options.

CB2R Attenuates Intervertebral Disc Degeneration by Delaying Nucleus Pulposus Cell Senescence through AMPK/GSK3β Pathway

Nucleus pulposus (NP) cell (NPC) senescence is one of the main causes of intervertebral disc degeneration (IVDD). However, the underlying mechanism of NPC senescence is still unclear. The cannabinoid type 2 receptor (CB2R) is a member of the cannabinoid system and plays an important role in antioxidative stress, anti-inflammatory and antisenescence activities. In this study, we used a hydrogen peroxide (H₂O₂)-induced NPC senescence model and a rat acupuncture IVDD model to explore the role of CB2R in IVDD in vitro and in vivo. First, we confirmed that the expression of p16INK4a in the NP tissues of IVDD patients and rat acupuncture IVDD models obviously increased accompanied by a decrease in CB2R expression. Subsequently, we found that activation of CB2R significantly reduced the number of SA-β-gal positive cells and suppressed the expression of p16INK4a and senescence-related secretory phenotypes [SASP, including matrix metalloproteinase 9 and 13 (MMP9, MMP13) and high mobility group protein b1 (HMGB1)]. In addition, activation of CB2R promoted the expression of collagen type II (Col-2) and SRY-Box transcription factor 9 (SOX9), inhibit the expression of collagen type X (Col-X), and restore the balance of extracellular matrix (ECM) metabolism. In addition, the AMPK/GSK3β pathway was shown to play an important role in CB2R regulation of NPC senescence. Inhibition of AMPK expression reversed the effect of JWH015 (a CB2R agonist). Finally, we further demonstrated that in the rat IVDD model, the AMPK/GSK3β pathway was involved in the regulation of CB2R on NPC senescence. In conclusion, our experimental results prove that CB2R plays an important role in NPC senescence. Activation of CB2R can delay NPC senescence, restore the balance of ECM metabolism, and attenuate IVDD.

Spheroid-Based Tissue Engineering Strategies for Regeneration of the Intervertebral Disc

Degenerative disc disease, a painful pathology of the intervertebral disc (IVD), often causes disability and reduces quality of life. Although regenerative cell-based strategies have shown promise in clinical trials, none have been widely adopted clinically. Recent developments demonstrated that spheroid-based approaches might help overcome challenges associated with cell-based IVD therapies. Spheroids are three-dimensional multicellular aggregates with architecture that enables the cells to differentiate and synthesize endogenous ECM, promotes cell-ECM interactions, enhances adhesion, and protects cells from harsh conditions. Spheroids could be applied in the IVD both in scaffold-free and scaffold-based configurations, possibly providing advantages over cell suspensions. This review highlights areas of future research in spheroid-based regeneration of nucleus pulposus (NP) and annulus fibrosus (AF). We also discuss cell sources and methods for spheroid fabrication and characterization, mechanisms related to spheroid fusion, as well as enhancement of spheroid performance in the context of the IVD microenvironment.

Unfolded protein response alleviates acid-induced premature senescence by promoting autophagy in nucleus pulposus cells

Acid-induced cellular senescence is a critical underlying mechanism of intervertebral disc (IVD) degeneration (IDD). Acid stimulation activates a variety of biological changes including autophagy, endoplasmic reticulum stress, and related unfolded protein response (UPR), which are important regulators of cellular senescence. However, the precise mechanism of acid-mediated UPR and autophagy in nucleus pulposus cell (NPC) senescence has not been fully elucidated. In this study, we used acid to mimic the acidic microenvironment of IVD, and rat NPCs were cultured with or without autophagy or UPR signaling small-interfering RNAs. The related proteins and genes were assessed by immunofluorescence staining assay, Western blot analyses, and quantitative real-time polymerase chain reaction to monitor the activation of these signals and classify the molecular mechanisms underlying the correlation between autophagy and UPR pathway. Cell cycle analyses, senescence-associated β-galactosidase staining, gene expression, and immunoblotting analyses were performed to observe NPC senescence. Results showed that acid stimulation not only induced NPC senescence, but also initiated UPR and autophagy. Silencing the binding immunoglobulin protein signaling of UPR or autophagy signaling promoted rat NPC senescence. Knock-down of the UPR also blocked NPC autophagy. Taken together, UPR inhibits NPC senescence under acidic condition by activating autophagy. Hence, UPR-dependent autophagy could be an effective biologic target for the treatment of IDD in the future.

Inhibition of aberrant Hif1α activation delays intervertebral disc degeneration in adult mice

The intervertebral disc (IVD) is the largest avascular tissue. Hypoxia-inducible factors (HIFs) play essential roles in regulating cellular adaptation in the IVD under physiological conditions. Disc degeneration disease (DDD) is one of the leading causes of disability, and current therapies are ineffective. This study sought to explore the role of HIFs in DDD pathogenesis in mice. The findings of this study showed that among HIF family members, Hif1α was significantly upregulated in cartilaginous endplate (EP) and annulus fibrosus (AF) tissues from human DDD patients and two mouse models of DDD compared with controls. Conditional deletion of the E3 ubiquitin ligase Vhl in EP and AF tissues of adult mice resulted in upregulated Hif1α expression and age-dependent IVD degeneration. Aberrant Hif1α activation enhanced glycolytic metabolism and suppressed mitochondrial function. On the other hand, genetic ablation of the Hif1α gene delayed DDD pathogenesis in Vhl-deficient mice. Administration of 2-methoxyestradiol (2ME2), a selective Hif1α inhibitor, attenuated experimental IVD degeneration in mice. The findings of this study show that aberrant Hif1α activation in EP and AF tissues induces pathological changes in DDD, implying that inhibition of aberrant Hif1α activity is a potential therapeutic strategy for DDD.

Visfatin promotes intervertebral disc degeneration by inducing IL-6 expression through the ERK/JNK/p38 signalling pathways

Visfatin reportedly induces the expression of proinflammatory cytokines. Severe grades of intervertebral disc disease (IVDD) exhibit higher expression of visfatin than mild ones. However, the direct relationship between visfatin and IVDD remains to be elucidated. This study aimed to clarify whether stimulation of visfatin in IVDD is mediated by IL-6. To investigate the role of visfatin in IVDD, a rat model of anterior disc puncture was established by injecting visfatin or PBS using a 27-gauge needle. Results revealed an obvious aggravation of the histological morphology of IVDD in the visfatin group. On treating human NP cellswith visfatin, the levels of collagenII and aggrecan decreased and those of matrix metallopeptidase 3 and IL-6 gradually increased. A rapid increase in ERK, JNK, and p38 phosphorylation was also noted after visfatin treatment. Compared to those treated with visfatin alone, NP cells pretreated with ERK1/2, JNK, and p38 inhibitors or siRNA targeting p38, ERK, and JNK exhibited a significant suppression of IL-6. Our data represent the first evidence that visfatin promotes IL-6 expression in NP cells via the JNK/ERK/p38-MAPK signalling pathways. Further, our findings suggest epidural fat and visfatin as potential therapeutic targets for controlling IVDD-associated inflammation.

Cell sources proposed for nucleus pulposus regeneration

Lower back pain (LBP) occurs in 80% of adults in their lifetime; resulting in LBP being one of the biggest causes of disability worldwide. Chronic LBP has been linked to the degeneration of the intervertebral disc (IVD). The current treatments for chronic back pain only provide alleviation of symptoms through pain relief, tissue removal, or spinal fusion; none of which target regenerating the degenerate IVD. As nucleus pulposus (NP) degeneration is thought to represent a key initiation site of IVD degeneration, cell therapy that specifically targets the restoration of the NP has been reviewed here. A literature search to quantitatively assess all cell types used in NP regeneration was undertaken. With key cell sources: NP cells; annulus fibrosus cells; notochordal cells; chondrocytes; bone marrow mesenchymal stromal cells; adipose-derived stromal cells; and induced pluripotent stem cells extensively analyzed for their regenerative potential of the NP. This review highlights: accessibility; expansion capability in vitro; cell survival in an IVD environment; regenerative potential; and safety for these key potential cell sources. In conclusion, while several potential cell sources have been proposed, iPSC may provide the most promising regenerative potential.

Ganoderic Acid A Attenuates IL-1β-Induced Inflammation in Human Nucleus Pulposus Cells Through Inhibiting the NF-κB Pathway

Intervertebral disc (IVD) degeneration is a major cause of low back pain associated with several pathological changes in the IVD, including dysfunction of nucleus pulposus (NP) cells. Ganoderic Acid A (GAA), one of triterpenoid extracts of Ganoderma lucidum (G. lucidum), has been reported to possess anti-inflammatory effect. In the current study, we aimed to evaluate the effect of Ganoderic Acid A (GAA) on the interleukin-1β (IL-1β)-induced inflammation in human NP cells. Our results showed that the IL-1β-stimulated production of inflammatory mediators including nitric oxide (NO), prostaglandin E2 (PGE2), inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX)-2 were suppressed by GAA. In addition, treatment of NP cells with GAA significantly inhibited the production of inflammatory cytokines tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in IL-1β-stimulated human NP cells. GAA improved the reduced expression levels of extracellular matrix (ECM) proteins, collagen II and aggrecan in IL-1β-stimulated human NP cells. GAA also alleviated IL-1β-induced the levels of matrix metalloproteinase (MMP)-3 and MMP-13. Furthermore, GAA inhibited the IL-1β-induced upregulation of the phosphorylation of p65 and downregulation of IκBα. Taken together, these findings indicated that GAA alleviated IL-1β-induced inflammation and ECM degradation in NP cells through regulating NF-κB pathway.

Increased Expression of Integrin Alpha 6 in Nucleus Pulposus Cells in Response to High Oxygen Tension Protects against Intervertebral Disc Degeneration

The destruction of the low oxygen microenvironment in nucleus pulposus (NP) cells played a critical role in the pathogenesis of intervertebral disc degeneration (IVDD). The purpose of this study was to determine the potential role of integrin alpha 6 (ITG α6) in NP cells in response to high oxygen tension (HOT) in IVDD. Immunofluorescence staining and western blot analysis showed that the levels of ITG α6 expression were increased in the NP tissue from IVDD patients and the IVDD rat model with mild degeneration, which were reduced as the degree of degeneration increases in severity. In NP cells, the treatment of HOT resulted in upregulation of ITG α6 expression, which could be alleviated by blocking the PI3K/AKT signaling pathway. Further studies found that ITG α6 could protect NP cells against HOT-induced apoptosis and oxidative stress and protect NP cells from HOT-inhibited ECM protein synthesis. Upregulation of ITG α6 expression by HOT contributed to maintaining NP tissue homeostasis through the interaction with hypoxia-inducible factor-1α (HIF-1α). Furthermore, silencing of ITG α6 in vivo could obviously accelerate puncture-induced IVDD. Taken together, these results revealed that the increase of ITG α6 expression by HOT in NP cells might be a protective factor in IVD degeneration as well as restore NP cell function.

Hypoxic Preconditioning Enhances Bone Marrow-Derived Mesenchymal Stem Cell Survival in a Low Oxygen and Nutrient-Limited 3D Microenvironment

OBJECTIVE

Skeletal tissues such as intervertebral disc and articular cartilage possess limited innate potential to regenerate, in part due to their avascularity and low cell density. Despite recent advances in mesenchymal stem cell (MSC)-based disc and cartilage regeneration, key challenges remain, including the sensitivity of these cells to in vivo microenvironmental stress such as low oxygen and limited nutrition. The objective of this study was to investigate whether preconditioning with hypoxia and/or transforming growth factor-β 3 (TGF-β3) can enhance MSC survival and extracellular matrix production in a low oxygen and nutrient-limited microenvironment.

DESIGN

MSCs from multiple bovine donors were preconditioned in monolayer in normoxia or hypoxia, with or without TGF-β3, and the global effects on gene expression were examined using microarrays. Subsequently, the effects of preconditioning on MSC survival and extracellular matrix production were examined using low oxygen and nutrient-limited pellet culture experiments.

RESULTS

Hypoxic preconditioning resulted in upregulation of genes associated with growth, cell-cell signaling, metabolism, and cell stress response pathways, and significantly enhanced MSC survival for all donors in low oxygen and nutrient-limited pellet culture. In contrast, TGF-β3 preconditioning diminished survival. The nature and magnitude of the effects of preconditioning with either hypoxia or TGF-β3 on glycosaminoglycan production were donor dependent.

CONCLUSIONS

These results strongly support the use of hypoxic preconditioning to improve postimplantation MSC survival in avascular tissues such as disc and cartilage.

Comparison of the differentiation abilities of bone marrow-derived mesenchymal stem cells and adipose-derived mesenchymal stem cells toward nucleus pulposus-like cells in three-dimensional culture

Nucleus pulposus cell (NPC) transplantation can be a potential therapeutic approach for intervertebral disc degeneration (IDD). However, low cell viability has restricted the therapeutic capacity of NPCs, and sources of natural NPCs are limited. Bone marrow-derived mesenchymal stem cells (BMSCs) and adipose-derived mesenchymal stem cells (ADSCs) can be differentiated toward NPC-like cells. However, it is unknown whether there are differences in the abilities of these two cell types to differentiate into NPC-like cells, or which cell type exhibits the best differentiation ability. The present study compared the abilities of BMSCs and ADSCs to differentiate toward NPC-like cells with or without a 3D culture system to lay a foundation for stem cell transplantation therapy for IDD. BMSCs were isolated from the rat whole bone marrow cell using the repeated adherent culture method. ADSCs were isolated from rat adipose tissues in the subcutaneous inguinal region using the enzyme digestion method. Cells were identified using flow cytometry. Cell viability was assessed via Cell Counting Kit-8 assays, and reverse transcription-quantitative PCR and western blotting were carried out to evaluate the expression of NPC markers and chondrocyte-specific genes. Glycosaminoglycans (GAGs) and proteoglycans were examined via Alcian blue and safranin O staining, respectively. ADSCs in 3D culture displayed the highest cell proliferative ability, compared with the 2D culture system and BMSC culture. In addition, ADSCs in 3D culture exhibited increased GAG and proteoglycan synthesis than BMSCs. Compared with BMSCs in 3D culture, ADSCs in 3D culture exhibited higher mRNA and protein expression of NPC marker genes (hypoxia-inducible factor 1-α, glucose transporter 1) and chondrocyte-specific genes (Sox-9, aggrecan and type II collagen). The present findings indicated that ADSCs exhibited a better ability to differentiate into NPC-like cells in 3D culture compared with BMSCs, which may be of value for the regeneration of intervertebral discs using cell transplantation therapy.

The role of HIF proteins in maintaining the metabolic health of the intervertebral disc

The physiologically hypoxic intervertebral disc and cartilage rely on the hypoxia-inducible factor (HIF) family of transcription factors to mediate cellular responses to changes in oxygen tension. During homeostatic development, oxygen-dependent prolyl hydroxylases, circadian clock proteins and metabolic intermediates control the activities of HIF1 and HIF2 in these tissues. Mechanistically, HIF1 is the master regulator of glycolytic metabolism and cytosolic lactate levels. In addition, HIF1 regulates mitochondrial metabolism by promoting flux through the tricarboxylic acid cycle, inhibiting downsteam oxidative phosphorylation and controlling mitochondrial health through modulation of the mitophagic pathway. Accumulation of metabolic intermediates from HIF-dependent processes contribute to intracellular pH regulation in the disc and cartilage. Namely, to prevent changes in intracellular pH that could lead to cell death, HIF1 orchestrates a bicarbonate buffering system in the disc, controlled by carbonic anhydrase 9 (CA9) and CA12, sodium bicarbonate cotransporters and an intracellular H⁺/lactate efflux mechanism. In contrast to HIF1, the role of HIF2 remains elusive; in disorders of the disc and cartilage, its function has been linked to both anabolic and catabolic pathways. The current knowledge of hypoxic cell metabolism and regulation of HIF1 activity provides a strong basis for the development of future therapies designed to repair the degenerative disc.

NF-κB signalling pathways in nucleus pulposus cell function and intervertebral disc degeneration

Intervertebral disc degeneration (IDD) is a common clinical degenerative disease of the spine. A series of factors, such as inflammation, oxidative stress and mechanical stress, promote degradation of the extracellular matrix (ECM) of the intervertebral discs (IVD), leading to dysfunction and structural destruction of the IVD. Nuclear factor-κB (NF-κB) transcription factor has long been regarded as a pathogenic factor of IDD. Therefore, NF-κB may be an ideal therapeutic target for IDD. As NF-κB is a multifunctional functional transcription factor with roles in a variety of biological processes, a comprehensive understanding of the function and regulatory mechanism of NF-κB in IDD pathology will be useful for the development of targeted therapeutic strategies for IDD, which can prevent the progression of IDD and reduce potential risks. This review discusses the role of the NF-κB signalling pathway in the nucleus pulposus (NP) in the process of IDD to understand pathological NP degeneration further and provide potential therapeutic targets that may interfere with NF-κB signalling for IDD therapy.

Hepatocyte growth factor regulates HIF-1α-induced nucleus pulposus cell proliferation through MAPK-, PI3K/Akt-, and STAT3-mediated signaling

Intervertebral discs are important for maintaining mobility and offer support to the body trunk. If these discs lose their biomechanical features, lower back pain can occur. We previously reported that hepatocyte growth factor (HGF) promotes cell proliferation and suppresses apoptosis, inflammation, and matrix degradation in nucleus pulposus (NP) cells. In the present study, we investigated the molecular mechanisms of how HGF promotes the proliferation of NP cells in hypoxic conditions. Hypoxic stimulation promoted modest cell proliferation, which was further upregulated by HGF. Expression of hypoxia-inducible factor (HIF-1α) protein, which contributes to the maintenance of homeostasis in NP cells, was also upregulated in hypoxia-treated cell groups; HGF further increased HIF-1α expression in NP cells. Additionally, knockdown of HIF-1α expression significantly reduced the proliferation of NP cells. An MAPK inhibitor inhibited the expression of HIF-1α and pERK, as well as cell proliferation in a dose-dependent manner. Similarly, inhibiting the PI3K/Akt and STAT3 pathways also decreased the expression of HIF-1α and cell proliferation. These results show that under hypoxic conditions, HGF promotes NP cell proliferation via HIF-1α-, MAPK-, PI3K/Akt-, and STAT3-mediated signaling which is involved in this pathway. The control of these signaling pathways may be a target for potential therapeutic strategies for the treatment of disc degeneration in hypoxic conditions.

Regeneration in Spinal Disease: Therapeutic Role of Hypoxia-Inducible Factor-1 Alpha in Regeneration of Degenerative Intervertebral Disc

The intervertebral disc (IVD) is a complex joint structure comprising three primary components—namely, nucleus pulposus (NP), annulus fibrosus (AF), and cartilaginous endplate (CEP). The IVD retrieves oxygen from the surrounding vertebral body through CEP by diffusion and likely generates ATP via anaerobic glycolysis. IVD degeneration is characterized by a cascade of cellular, compositional, structural changes. With advanced age, pronounced changes occur in the composition of the disc extracellular matrix (ECM). NP and AF cells in the IVD possess poor regenerative capacity compared with that of other tissues. Hypoxia-inducible factor (HIF) is a master transcription factor that initiates a coordinated cellular cascade in response to a low oxygen tension environment, including the regulation of numerous enzymes in response to hypoxia. HIF-1α is essential for NP development and homeostasis and is involved in various processes of IVD degeneration process, promotes ECM in NP, maintains the metabolic activities of NP, and regulates dystrophic mineralization of NP, as well as angiogenesis, autophagy, and apoptosis during IVD degeneration. HIF-1α may, therefore, represent a diagnostic tool for early IVD degeneration and a therapeutic target for inhibiting IVD degeneration

Cartilage Endplate Stem Cells Transdifferentiate Into Nucleus Pulposus Cells via Autocrine Exosomes

Stem cells derived from cartilage endplate (CEP) cells (CESCs) repair intervertebral disc (IVD) injury; however, the mechanism remains unclear. Here, we evaluated whether CESCs could transdifferentiate into nucleus pulposus cells (NPCs) via autocrine exosomes and subsequently inhibit IVD degeneration. Exosomes derived from CESCs (CESC-Exos) were extracted and identified by ultra-high-speed centrifugation and transmission electron microscopy. The effects of exosomes on the invasion, migration, and differentiation of CESCs were assessed. The exosome-activating hypoxia-inducible factor (HIF)-1α/Wnt pathway was investigated using lenti-HIF-1α and Wnt agonists/inhibitors in cells and gene ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analysis in normal and degenerated human CEP tissue. The effects of GATA binding protein 4 (GATA4) on transforming growth factor (TGF)-β expression and on the invasion, migration, and transdifferentiation of CESCs were investigated using lenti-GATA4, TGF-β agonists, and inhibitors. Additionally, IVD repair was investigated by injecting CESCs overexpressing GATA4 into rats. The results indicated that CESC-Exos promoted the invasion, migration, and differentiation of CESCs by autocrine exosomes via the HIF-1α/Wnt pathway. Additionally, increased HIF-1α enhanced the activation of Wnt signaling and activated GATA4 expression. GATA4 effectively promoted TGF-β secretion and enhanced the invasion, migration, and transdifferentiation of CESCs into NPCs, resulting in promotion of rat IVD repair. CESCs were also converted into NPCs as endplate degeneration progressed in human samples. Overall, we found that CESC-Exos activated HIF-1α/Wnt signaling via autocrine mechanisms to increase the expression of GATA4 and TGF-β1, thereby promoting the migration of CESCs into the IVD and the transformation of CESCs into NPCs and inhibiting IVDD.

RACK1 mediates the advanced glycation end product-induced degradation of HIF-1α in nucleus pulposus cells via competing with HSP90 for HIF-1α binding

Hyperglycemia can drive advanced glycation end product (AGE) accumulation and associated nucleus pulposus cell (NPC) dysfunction, but the basis for this activity has not been elucidated. Hypoxia-inducible factor-1α (HIF-1α) is subject to cell-type-specific AGE-mediated regulation. In the current study, we assessed the mechanistic relationship between AGE accumulation and HIF-1α degradation in NPCs. Immunohistochemical staining of degenerated nucleus pulposus (NP) samples was used to assess AGE levels. AGE impact on NPC survival and glycolysis-related gene expression was assessed via 3-(4,5)-dimethylthiazol(-z-y1)-3,5-di-phenyltetrazolium bromide assay and quantitative reverse-transcription polymerase chain reaction (qRT-PCR), while HIF-1α expression in NPCs following AGE treatment was monitored via Western blot analysis and qRT-PCR. Additionally, a luciferase reporter assay was used to monitor HIF-1α transcriptional activity. The importance of the receptor for activated C-kinase 1 (RACK1) as a mediator of HIF-1α degradation was evaluated through gain- and loss-of-function experiments. Competitive binding of RACK1 and HSP90 to HIF-1α was evaluated via immunoprecipitation. Increased AGE accumulation was evident in NP samples from diabetic patients, and AGE treatment resulted in reduced HIF-1α protein levels in NPCs that coincided with reduced HIF-1α transcriptional activity. AGE treatment impaired the stability of HIF-1α, leading to its RACK1-mediated proteasomal degradation in a manner independent of the canonical PHD-mediated degradation pathway. Additionally, RACK1 competed with HSP90 for HIF-1α binding following AGE treatment. AGE treatment of NPCs leads to HIF-1α protein degradation. RACK1 competes with HSP90 for HIF-1α binding following AGE treatment, resulting in posttranslational HIF-1α degradation. These results suggest that AGE is an intervertebral disc degeneration risk factor, and highlight potential avenues for the treatment or prevention of this disease.

Inhibition of the P53/P21 Pathway Attenuates the Effects of Senescent Nucleus Pulposus Cell-Derived Exosomes on the Senescence of Nucleus Pulposus Cells

Objective

The purpose of this paper is to investigate the effects of senescent nucleus pulposus cell (NPC)‐derived exosomes (SNPC‐Exo) and the roles of the P53/P21 pathway on the senescence of NPC.

Methods

The senescent phenotypes of NPC were induced by interleukin‐1β treatment. SNPC‐Exo was extracted from the culture medium of senescent NPC and purified by differential centrifugation. The structure of SNPC‐Exo was identified by transmission electron microscopy and western blot analysis was used to determine the exosomal marker proteins CD63 and Tsg101. Western blot analysis was performed to determine the relative expression levels of P16, P21, and P53 in NPC. Senescence‐associated β‐galactosidase (SA‐β‐gal) staining was used to stain the senescent NPC and a phase contrast microscope was used to observe and count the SA‐β‐gal staining of NPC. The proliferation of SNPC‐Exo‐treated NPC was assessed using growth curve analysis and the colony formation assay. The cell cycle of SNPC‐Exo‐treated NPC was determined by flow cytometry. NPC were transfected with siRNA to knock down P53 and P21 expression.

Results

Interleukin‐1β‐treated NPC had a higher percentage of SA‐β‐gal positive cells (45%) than the control group (20%) and showed an increase in the relative expression of P16, P21, and P53 (P < 0.05). SNPC‐Exo were positive for exosomal marker protein CD63 and Tsg 101 and negative for calnexin, and successfully internalized as previously described. SNPC‐Exo‐treated NPC showed an increase in the relative expression of P21 and P53 (P < 0.05). Compared with the control group, the SNPC‐Exo‐treated NPC showed a lower growth rate (3 times lower on the 5th day and 2 times lower on the 7th day), fewer colony‐forming units (12.0%), and a higher percentage of SA‐β‐gal‐positive NPC (50.0%). The SNPC‐Exo‐treated NPC contained more G1 phase cells (68.0%) and fewer S phase (15.5%) cells than the control group (53.0% in G1 phase, 33.5% in S phase). The expression of P21 and P53 significantly decreased in SNPC‐exo‐treated NPC after siRNA transfection (P < 0.05), followed by a higher growth rate (2 times higher on the 5th day and 1.5 times higher on the 7th day) and lower percentage of SA‐β‐gal‐positive NPC (22.5%). Moreover, the inhibition of the P53/P21 pathway promoted the SNPC‐Exo‐treated NPC to enter the S phase (from 15.5% to 25.3%).

Conclusion

The inhibition of the P53/P21 pathway attenuated the senescence of NPC induced by SNPC‐Exo.

Acid-sensing ion channels regulate nucleus pulposus cell inflammation and pyroptosis via the NLRP3 inflammasome in intervertebral disc degeneration

Objective

Lactate accumulation is an important factor in the intervertebral disc degeneration (IVDD). Currently, the effect and underlying mechanism of action of lactate on nucleus pulposus (NP) cell inflammation during IVDD are unclear. Previous studies have found that the NLRP3 inflammasome plays an important role in the regulation of NP inflammation. This study focused on the regulation of acid‐sensitive ion channels (ASICs) in relation to inflammation and the effect of NLRP3 on pyroptosis levels in NP cells under acidic conditions.

Design

For the in vitro experiments, human NP cells were exposed to 6 mM lactate solution; different groups were either treated with NLRP3 inhibitor or transfected with siRNA against NLRP3, siRNA against ASC or a mix of these, and mRNA and protein expression levels were then assessed. For the in vivo experiment, varying concentrations of lactate were injected into rat intervertebral discs and examined via magnetic resonance imaging (MRI) and histological staining.

Results

Extracellular lactate promoted NLRP3 inflammasome activation and degeneration of the NP extracellular matrix; furthermore, it increased the levels of inflammation and pyroptosis in the NP. Lactate‐induced NLRP3 inflammasome activation was blocked by ASIC inhibitors and NLRP3 siRNA.

Conclusions

Extracellular lactate regulates levels of intercellular reactive oxygen species (ROS) through ASIC1 and ASIC3. ROS activate the NF‐κB signalling pathway, thus promoting NLRP3 inflammasome activation and IL‐1β release, both of which promote NP degeneration.

Dioscin Attenuates Interleukin 1β (IL-1β)-Induced Catabolism and Apoptosis via Modulating the Toll-Like Receptor 4 (TLR4)/Nuclear Factor kappa B (NF-κB) Signaling in Human Nucleus Pulposus Cells

Background

Nucleus pulposus (NP) cell dysfunction and apoptosis contribute to disc degeneration. Dioscin, a natural steroid saponin, has been demonstrated to have anti-inflammatory, antiapoptotic, and antioxidative effects in various diseases. However, little is known about the roles of dioscin in intervertebral disc degeneration.

Material/Methods

To evaluate the roles of dioscin in disc degeneration and its specific mechanism, human NP cells were incubated with IL-1β and various concentrations of dioscin. Cell viability, extracellular matrix protein expression, catabolic factors, degree of apoptosis, inflammatory factors, and related signaling pathways were evaluated by western blotting, fluorescence immunostaining, TUNEL staining, and reverse transcription PCR.

Results

Dioscin inhibited IL-1β-activated apoptotic signaling and catabolic activity in NP cells. Dioscin suppressed TLR4/NF-0κB signaling, and attenuated the level of inflammatory mediators (IL-6, TNF-α) in IL-1β-stimulated human NP cells.

Conclusions

Our work provides the first evidence that dioscin attenuates IL-1β-activated inflammation and catabolic activity in human NP cells through inhibiting the TLR4/NF-κB pathway, indicating that dioscin is a new potential candidate for clinical therapy to attenuate disc degeneration.

A possible injectable tissue engineered nucleus pulposus constructed with platelet-rich plasma and ADSCs in vitro

BACKGROUND

Injectable tissue engineered nucleus pulposus is a new idea for minimally invasive repair of degenerative intervertebral disc. The platelet-rich plasma (PRP) and adipose-derived stromal cells (ADSCs) could be harvested from autologous tissue easily. PRP contains numerous autologous growth factors and has reticulate fibrous structure which may have the potential to make ADSCs differentiate into nucleus pulposus-like cells. The goal of this study was to explore the feasibility of constructing a possible injectable tissue engineered nucleus pulposus with PRP gel scaffold and ADSCs.

METHODS

After identification with flow cytometry, the rabbit ADSCs were seeded into PRP gel and cultured in vitro. At the 2nd, 4th, and 8th week, the PRP gel/ADSCs complex was observed by macroscopy, histological staining, BrdU immunofluorescence, and scanning electron microscopy. The glycosaminoglycans (GAG) in the PRP gel/ADSCs complex were measured by safranin O staining with spectrophotometry. In PRP gel/ADSCs complex, gene expression of HIF-1α, aggrecan, type II collagen were tested by RT-PCR. The injectability of this complex was evaluated.

RESULTS

Macroscopically, the complex was solidified into gel with smooth surface and good elasticity. The safranin O dye was almost no positive staining at 2nd week; however, the positive staining of extracellular matrix was enhanced obviously at 4th and 8th week. The HE staining and SEM demonstrated that the cells were well-distributed in the reticulate scaffold. BrdU immunofluorescence showed that ADSCs can survive and proliferate in PRP gel at each time points. The level of GAG at 4th week was higher than those at 2nd week (P < 0.05), and significant difference was also noted between 4th and 8th week (P < 0.05). HIF-1α, aggrecan, type II collagen gene expression at 4th week were much more than those at 2nd week (P < 0.05), and significant differences were also noted between 4th and 8th week (P < 0.05). The flow rate of complex was 0.287 mL/min when passed through the 19-gauge needle with the 100 mmHg injection pressure.

CONCLUSIONS

Our preliminary findings suggest that the PRP gel make it possible for rabbit ADSCs differentiated into nucleus pulposus-like cells after coculture in vitro. According to the results, it is a better feasible method for construction of autologous injectable tissue engineered nucleus pulposus.

Hypoxic Regulation of Mitochondrial Metabolism and Mitophagy in Nucleus Pulposus Cells Is Dependent on HIF-1α-BNIP3 Axis

Nucleus pulposus (NP) cells reside in an avascular and hypoxic microenvironment of the intervertebral disc and are predominantly glycolytic due to robust HIF-1 activity. It is generally thought that NP cells contain few functional mitochondria compared with cells that rely on oxidative metabolism. Consequently, the contribution of mitochondria to NP cell metabolism and the role of hypoxia and HIF-1 in mitochondrial homeostasis is poorly understood. Using mitoQC reporter mice, we show for the first time to our knowledge that NP cell mitochondria undergo age-dependent mitophagy in vivo. Mechanistically, in vitro studies suggest that, under hypoxic conditions, mitochondria in primary NP cells undergo HIF-1α-dependent fragmentation, controlled by modulating the levels of key proteins DRP1 and OPA1 that are involved in mitochondrial fission and fusion, respectively. Seahorse assays and steady state metabolic profiling coupled with [1-2-¹³ C]-glucose flux analysis revealed that in hypoxia, HIF-1α regulated metabolic flux through coordinating glycolysis and the mitochondrial TCA cycle interactions, thereby controlling the overall biosynthetic capacity of NP cells. We further show that hypoxia and HIF-1α trigger mitophagy in NP cells through the mitochondrial translocation of BNIP3, an inducer of receptor-mediated mitophagy. Surprisingly, however, loss of HIF-1α in vitro and analysis of NP-specific HIF-1α null mice do not show a decrease in mitophagic flux in NP cells but a compensatory increase in NIX and PINK1-Parkin pathways with higher mitochondrial number. Taken together, our studies provide novel mechanistic insights into the complex interplay between hypoxia and HIF-1α signaling on the mitochondrial metabolism and quality control in NP cells. © 2020 American Society for Bone and Mineral Research.

Correlation between the HIF-1α/Notch signaling pathway and Modic changes in nucleus pulposus cells isolated from patients with low back pain

BACKGROUND

The HIF-1α/Notch signaling pathway regulates cell proliferation, apoptosis, and metabolism in the intervertebral discs (IVDs) and is implicated in disc degeneration. The nucleus pulposus (NP) is an important structure adjacent to the IVDs. However, the role of the HIF-1α/Notch signaling pathway in NP cells obtained from patients with different Modic changes (MCs) remains unclear. The purpose of the present study was to investigate the role of HIF-1α and components of the Notch pathway in the NP obtained from patients with various MCs.

METHODS

A total of 85 NP tissue samples were collected from patients undergoing diskectomy for the treatment of low back pain. The NP tissues were divided into four groups based on the adjacent endplate degeneration, namely, MC I, II, III, and negative MC groups. The expression of HIF-1α and Notch-related components was measured and compared.

RESULTS

The expression of HIF-1α, Notch1, and Notch2 was gradually increased in the MC I and MC II groups compared with that in the negative MC group. HIF-1α and Notch-related components were rarely detected in the MC III group.

CONCLUSIONS

The expression of HIF-1α/Notch increased in the NP cells of patients with MC I and MC II. HIF-1α and Notch-related components are potential biomarkers and the HIF-1α/Notch signaling pathway may serve as a promising therapeutic target for disc degeneration in patients with MCs.

Nanofibrous spongy microspheres to deliver rabbit mesenchymal stem cells and anti-miR-199a to regenerate nucleus pulposus and prevent calcification

Lower back pain is mainly caused by intervertebral disc degeneration, in which calcification is frequently involved. Here novel nanofibrous spongy microspheres (NF-SMS) are used to carry rabbit bone marrow mesenchymal stromal cells (MSCs) to regenerate nucleus pulposus tissues. NF-SMS are shown to significantly enhance the MSC seeding, proliferation and differentiation over control microcarriers. Furthermore, a hyperbranched polymer (HP) with negligible cytotoxicity and high microRNA (miRNAs) binding affinity is synthesized. The HP can complex with anti-miR-199a and self-assemble into "double shell" polyplexes which are able to achieve high transfection efficiency into MSCs. A double-emulsion technique is used to encapsulate these polyplexes in biodegradable nanospheres (NS) to enable sustained anti-miR-199 delivery. Our results demonstrate that MSC/HP-anti-miR-199a/NS/NF-SMS constructs can promote the nucleus pulposus (NP) phenotype and resist calcification in vitro and in a subcutaneous environment. Furthermore, injection of MSC/HP-anti-miR-199a/NS/NF-SMS can stay in place, produce functional extracellular matrix, maintain disc height and prevent intervertebral disc (IVD) calcification in a rabbit lumbar degeneration model.

Differential Effect of Long-Term Systemic Exposure of TNFα on Health of the Annulus Fibrosus and Nucleus Pulposus of the Intervertebral Disc

The inflammatory cytokine tumor necrosis factor alpha (TNFα) is considered to play a key role in the pathogenesis of intervertebral disc disease. To evaluate the importance of this cytokine we examined the inflammatory environment and spinal phenotype of 9-month-old human TNFα overexpressing transgenic (hTNFα-TG) mice. The mice evidenced increased circulating levels of interleukin-1β (IL-1β), IL-2, keratinocyte chemoattractant/human growth-regulated oncogene (KC/GRO), and monocyte chemoattractant protein-1 (MCP-1) along with thinning of the cortical and trabecular vertebral bone. Surprisingly, although the nucleus pulposus (NP) of these mice was intact and healthy, the caudal annulus fibrosus (AF) evidenced robust cell death and immune cell infiltration. Despite these differences, there were no obvious alterations in the collagen or aggrecan content in the NP and AF. However, there was a reduction in cartilage oligomeric matrix protein (COMP), suggesting destabilization of the AF matrix. Microarray analysis of the NP from hTNFα-TG mice cells revealed minimal changes in global gene expression. These findings lend support to the notion that NP tissue is isolated from systemic inflammation. In contrast, the severe AF phenotype suggests that systemic inflammation interferes with AF health, predisposing discs to herniation as opposed to directly causing NP degeneration. © 2020 American Society for Bone and Mineral Research.

Lactate Efflux From Intervertebral Disc Cells Is Required for Maintenance of Spine Health

Maintenance of glycolytic metabolism is postulated to be required for health of the spinal column. In the hypoxic tissues of the intervertebral disc and glycolytic cells of vertebral bone, glucose is metabolized into pyruvate for ATP generation and reduced to lactate to sustain redox balance. The rise in intracellular H⁺ /lactate concentrations are balanced by plasma-membrane monocarboxylate transporters (MCTs). Using MCT4 null mice and human tissue samples, complemented with genetic and metabolic approaches, we determine that H⁺ /lactate efflux is critical for maintenance of disc and vertebral bone health. Mechanistically, MCT4 maintains glycolytic and tricarboxylic acid (TCA) cycle flux and intracellular pH homeostasis in the nucleus pulposus compartment of the disc, where hypoxia-inducible factor 1α (HIF-1α) directly activates an intronic enhancer in SLC16A3. Ultimately, our results provide support for research into lactate as a diagnostic biomarker for chronic, painful, disc degeneration. © 2019 American Society for Bone and Mineral Research.

Bio-conjugation of platelet-rich plasma and alginate through carbodiimide chemistry for injectable hydrogel therapies

Alginate is a highly tailorable, biocompatible polymer whose properties can be tuned to mimic the properties of native nucleus pulposus (NP) tissue. Platelet-rich plasma (PRP) is a highly accessible, inexpensive, and readily available mix of pro-regenerative factors. By functionalizing alginate with PRP, a mechanically optimized, bioactive alginate NP analogue may stimulate NP cells to proliferate and accumulate matrix over a longer period of time than if the PRP were solely encapsulated within the hydrogel. In this study, PRP was chemically bound to alginate using carbodiimide chemistry and mechanically, physically, and cytologically compared to plain alginate as well as alginate containing free-floating lyophilized PRP. The alginates were mechanically and physically characterized; PRP-conjugated alginate had similar mechanical properties to controls and had the benefit of retained PRP proteins within the hydrogel. Human nucleus pulposus cells (hNPCs) were seeded within the modified alginates and cultured for 14 days. Quantification data of glycosaminoglycans suggests that PRP-incorporated alginate has the potential to increase ECM production within the characterized alginate constructs, and that PRP-functionalized alginate can retain protein within the hydrogel over time. This is the first study to functionalize the milieu of PRP proteins onto alginate and characterize the mechanical and physical properties of the modified alginates. This study also incorporates hNPCs into the characterized PRP-modified alginates to observe phenotypic maintenance when encapsulated within the in situ gelling constructs.

Mechanosensitive transcriptional coactivators MRTF-A and YAP/TAZ regulate nucleus pulposus cell phenotype through cell shape

Cells of the adult nucleus pulposus (NP) are critically important in maintaining overall disc health and function. NP cells reside in a soft, gelatinous matrix that dehydrates and becomes increasingly fibrotic with age. Such changes result in physical cues of matrix stiffness that may be potent regulators of NP cell phenotype and may contribute to a transition toward a senescent and fibroblastic NP cell with a limited capacity for repair. Here, we investigate the mechanosignaling cues generated from changes in matrix stiffness in directing NP cell phenotype and identify mechanisms that can potentially preserve a biosynthetically active, juvenile NP cell phenotype. Using a laminin-functionalized polyethylene glycol hydrogel, we show that when NP cells form rounded, multicell clusters, they are able to maintain cytosolic localization of myocardin-related transcription factor (MRTF)-A, a coactivator of serum-response factor (SRF), known to promote fibroblast-like behaviors in many cells. Upon preservation of a rounded shape, human NP cells similarly showed cytosolic retention of transcriptional coactivator Yes-associated protein (YAP) and its paralogue PDZ-binding motif (TAZ) with associated decline in activation of its transcription factor TEA domain family member-binding domain (TEAD). When changes in cell shape occur, leading to a more spread, fibrotic morphology associated with stronger F-actin alignment, SRF and TEAD are up-regulated. However, targeted deletion of either cofactor was not sufficient to overcome shape-mediated changes observed in transcriptional activation of SRF or TEAD. Findings show that substrate stiffness-induced promotion of F-actin alignment occurs concomitantly with a flattened, spread morphology, decreased NP marker expression, and reduced biosynthetic activity. This work indicates cell shape is a stronger indicator of SRF and TEAD mechanosignaling pathways than coactivators MRTF-A and YAP/TAZ, respectively, and may play a role in the degeneration-associated loss of NP cellularity and phenotype.-Fearing, B. V., Jing, L., Barcellona, M. N., Witte, S. E., Buchowski, J. M., Zebala, L. P., Kelly, M. P., Luhmann, S., Gupta, M. C., Pathak, A., Setton, L. A. Mechanosensitive transcriptional coactivators MRTF-A and YAP/TAZ regulate nucleus pulposus cell phenotype through cell shape.

Mitochondrial NDUFA4L2 attenuates the apoptosis of nucleus pulposus cells induced by oxidative stress via the inhibition of mitophagy

The main pathological mechanism of intervertebral disc degeneration (IVDD) is the programmed apoptosis of nucleus pulposus (NP) cells. Oxidative stress is a significant cause of IVDD. Whether mitophagy is induced by strong oxidative stress in IVDD remains to be determined. This study aimed to investigate the relationship between oxidative stress and mitophagy and to better understand the mechanism of IVDD in vivo and in vitro. To this end, we obtained primary NP cells from the human NP and subsequently exposed them to TBHP. We observed that oxidative stress induced mitophagy to cause apoptosis in NP cells, and we suppressed mitophagy and found that NP cells were protected against apoptosis. Interestingly, TBHP resulted in mitophagy through the inhibition of the HIF-1α/NDUFA4L2 pathway. Therefore, the upregulation of mitochondrial NDUFA4L2 restricted mitophagy induced by oxidative stress. Furthermore, the expression levels of HIF-1α and NDUFA4L2 were decreased in human IVDD. In conclusion, these results demonstrated that the upregulation of NDUFA4L2 ameliorated the apoptosis of NP cells by repressing excessive mitophagy, which ultimately alleviated IVDD. These findings show for the first time that NDUFA4L2 and mitophagy may be potential therapeutic targets for IVDD.

A study in rats highlights the role of mitochondria in intervertebral disc degeneration (IVDD), one of the most important and prevalent predisposing factors for lower back pain. Previous studies have shown that in IVDD, oxidative stress results in the gradual loss of cells in the inner part of vertebral discs which cushion the space between vertebrae. Sheng-Dan Jiang and Lei-Sheng Jiang at Shanghai Jiaotong University School of Medicine found that oxidative stress in these cells causes the selective degradation of mitochondria by preventing the expression of a protein that is essential for mitochondrial function. Overexpressing this protein in the intervertebral discs of rats with IVDD alleviated degeneration, suggesting that restoring mitochondrial function could be an effective therapeutic strategy for easing the pain associated with the condition.

Nucleus pulposus primary cilia alter their length in response to changes in extracellular osmolarity but do not control TonEBP-mediated osmoregulation

The nucleus pulposus (NP) cells adapt to their physiologically hyperosmotic microenvironment through Tonicity-responsive enhancer binding protein (TonEBP/nuclear factor of activated T-cell5 [NFAT5])-mediated osmoregulation. Primary cilia in different organs serve diverse roles including osmosensing, but its contribution to NP cell osmoadaptive response is unknown. A high percentage of cultured primary NP cells possessed primary cilia that changed length in response to osmotic stimuli. Stable silencing of Intraflagellar Transport 88 (Ift88) or Kinesin Family Member 3 A (Kif3a) to inhibit the formation of primary cilia did not affect hyperosmotic upregulation of TonEBP. While ShKif3a blocked hyperosmotic increase of TonEBP-Transactivation Domain (TAD) activity, overall the knockdown of either gene did not alter the hyperosmotic status of proximal promoter activities and transcription of key TonEBP targets. On the other hand, a small decrease in TonEBP level under hypoosmotic condition was attenuated by Ift88 or Kif3a knockdown. Noteworthy, none of the TonEBP target genes were responsive to hypoosmotic stimulus in control and Ift88 or Kif3a knockdown cells, suggesting the primary role of TonEBP in the hyperosmotic adaptation of NP cells. Similarly, in Kif3a null mouse embryonic fibroblasts (MEFs), the overall TonEBP-dependent hyperosmotic responses were preserved. Unlike NP cells, TonEBP targets were responsive to hypoosmolarity in wild-type MEFs, and these responses remained intact in Kif3a null MEFs. Together, these results suggest that primary cilia are dispensable for TonEBP-dependent osmoadaptive response.

CircERCC2 ameliorated intervertebral disc degeneration by regulating mitophagy and apoptosis through miR-182-5p/SIRT1 axis

The molecular mechanism of intervertebral disc degeneration (IVDD) remains unclear. This study aimed to investigate the role of circular RNAs (circRNAs) in the pathogenesis of IVDD. We sued nucleus pulposus (NP) tissues of patients, tert-butyl hydroperoxide (TBHP) stimulated NP cells (NPCs), and IVDD rat model to explore the interaction between circERCC2 and miR-182-5p/SIRT1 axis. The results showed that downregulation of circERCC2 increased the level of miR-182-5p and decreased the level of SIRT1 in degenerative NP tissues in vivo as well as in TBHP-stimulated NPCs in vitro. Treatment of SIRT1-si activated apoptosis and inhibited mitophagy. Moreover, miR-182-5p-si could regulate the mitophagy and the apoptosis of NPCs by targeting SIRT1. The effects of circERCC2 on NPCs and IVDD rat model were mediated by miR-182-5p/SIRT1 axis. In conclusion, this study provides the first evidence that circERCC2 could ameliorate IVDD through miR-182-5p/SIRT1 axis by activating mitophagy and inhibiting apoptosis, and suggests that circERCC2 is a potentially effective therapeutic target for IVDD.

Lessons learned from intervertebral disc pathophysiology to guide rational design of sequential delivery systems for therapeutic biological factors

Intervertebral disc (IVD) degeneration has been associated with low back pain, which is a major musculoskeletal disorder and socio-economic problem that affects as many as 600 million patients worldwide. Here, we first review the current knowledge of IVD physiology and physiopathological processes in terms of homeostasis regulation and consecutive events that lead to tissue degeneration. Recent progress with IVD restoration by anti-catabolic or pro-anabolic approaches are then analyzed, as are the design of macro-, micro-, and nano-platforms to control the delivery of such therapeutic agents. Finally, we hypothesize that a sequential delivery strategy that i) firstly targets the inflammatory, pro-catabolic microenvironment with release of anti-inflammatory or anti-catabolic cytokines; ii) secondly increases cell density in the less hostile microenvironment by endogenous cell recruitment or exogenous cell injection, and finally iii) enhances cellular synthesis of extracellular matrix with release of pro-anabolic factors, would constitute an innovative yet challenging approach to IVD regeneration.