Hypoxia regulates sumoylation pathways in intervertebral disc cells: implications for hypoxic adaptations

A novel mechanism of FBXW7 in combating intervertebral disc degeneration: Mitigating ferroptosis in nucleus pulposus cells through the regulation of mitophagy

With the aging of the global population, the prevalence of intervertebral disc degeneration (IVDD) disease is gradually increasing. This disease not only leads to a substantial reduction in the quality of life of patients but also imposes a considerable burden on the health care system. At present, the understanding of its pathogenesis is relatively limited, and in-depth research is urgently needed to identify effective treatment methods. One of the main causes of IVDD is the compression of the spine caused by body weight. The objective of this study was to investigate the potential regulatory mechanism underlying IVDD induced by excessive compression. Moreover, to investigate whether FBXW7 is involved in the regulation of mitophagy and ferroptosis, we used 1 MPa pressure to induce nucleus pulposus cell (NPC) degeneration and then constructed plasmids or small interfering RNAs to overexpress or knock down FBXW7. In addition, in vivo animal experiments were performed to verify the function of FBXW7. We found that FBXW7 expression was decreased in degenerative NP tissues. Compression promoted the initiation of mitophagy, but blocked autophagic flux and ultimately caused ferroptosis in NPCs. However, overexpression of FBXW7 can activate mitophagy, improve autophagic flux, and alleviate ferroptosis. Moreover, FBXW7 can bind to mTOR and promote its ubiquitination and degradation, thus increasing the expression of PINK1 and Parkin. Taken together, the results of both in vitro and in vivo experiments suggested that FBXW7 induced mitophagy, alleviated ferroptosis, and delayed IVDD via the mTOR signaling pathway.

Alginate vs. Hyaluronic Acid as Carriers for Nucleus Pulposus Cells: A Study on Regenerative Outcomes in Disc Degeneration

Intervertebral disc degeneration is a leading cause of chronic low back pain, affecting millions globally. Regenerative medicine, particularly cell-based therapies, presents a promising therapeutic strategy. This study evaluates the comparative efficacy of two biomaterials-hyaluronic acid (HA) and alginate-as carriers for nucleus pulposus (NP) cell transplantation in a beagle model of induced disc degeneration. NP cells were isolated, cultured, and injected with either HA or alginate into degenerated discs, with saline and non-cell-loaded carriers used as controls. Disc height index, T2-weighted MRI, and histological analyses were conducted over a 12-week follow-up period to assess reparative outcomes. Imaging revealed that both carrier and cell-loaded treatments improved outcomes compared to degenerative controls, with cell-loaded carriers consistently outperforming carrier-only treated discs. Histological assessments supported these findings, showing trends toward extracellular matrix restoration in both treatment groups. While both biomaterials demonstrated reparative potential, HA showed greater consistency in supporting NP cells in promoting disc regeneration. These results underscore HA's potential as a superior carrier for NP cell-based therapies in addressing disc degeneration.

Oxygen-regulated post-translation modifications as master signalling pathway in cells

Oxygen is essential for viability in mammalian organisms. However, cells are often exposed to changes in oxygen availability, due to either increased demand or reduced oxygen supply, herein called hypoxia. To be able to survive and/or adapt to hypoxia, cells activate a variety of signalling cascades resulting in changes to chromatin, gene expression, metabolism and viability. Cellular signalling is often mediated via post-translational modifications (PTMs), and this is no different in response to hypoxia. Many enzymes require oxygen for their activity and oxygen can directly influence several PTMS. Here, we review the direct impact of changes in oxygen availability on PTMs such as proline, asparagine, histidine and lysine hydroxylation, lysine and arginine methylation and cysteine dioxygenation, with a focus on mammalian systems. In addition, indirect hypoxia-dependent effects on phosphorylation, ubiquitination and sumoylation will also be discussed. Direct and indirect oxygen-regulated changes to PTMs are coordinated to achieve the cell's ultimate response to hypoxia. However, specific oxygen sensitivity and the functional relevance of some of the identified PTMs still require significant research.

Insights into the underlying pathogenesis and therapeutic potential of endoplasmic reticulum stress in degenerative musculoskeletal diseases

Degenerative musculoskeletal diseases are structural and functional failures of the musculoskeletal system, including osteoarthritis, osteoporosis, intervertebral disc degeneration (IVDD), and sarcopenia. As the global population ages, degenerative musculoskeletal diseases are becoming more prevalent. However, the pathogenesis of degenerative musculoskeletal diseases is not fully understood. Previous studies have revealed that endoplasmic reticulum (ER) stress is a stress response that occurs when impairment of the protein folding capacity of the ER leads to the accumulation of misfolded or unfolded proteins in the ER, contributing to degenerative musculoskeletal diseases. By affecting cartilage degeneration, synovitis, meniscal lesion, subchondral bone remodeling of osteoarthritis, bone remodeling and angiogenesis of osteoporosis, nucleus pulposus degeneration, annulus fibrosus rupture, cartilaginous endplate degeneration of IVDD, and sarcopenia, ER stress is involved in the pathogenesis of degenerative musculoskeletal diseases. Preclinical studies have found that regulation of ER stress can delay the progression of multiple degenerative musculoskeletal diseases. These pilot studies provide foundations for further evaluation of the feasibility, efficacy, and safety of ER stress modulators in the treatment of musculoskeletal degenerative diseases in clinical trials. In this review, we have integrated up-to-date research findings of ER stress into the pathogenesis of degenerative musculoskeletal diseases. In a future perspective, we have also discussed possible directions of ER stress in the investigation of degenerative musculoskeletal disease, potential therapeutic strategies for degenerative musculoskeletal diseases using ER stress modulators, as well as underlying challenges and obstacles in bench-to-beside research.

MicroRNA‑25 protects nucleus pulposus cells against apoptosis via targeting SUMO2 in intervertebral disc degeneration

It has been reported that microRNA (miRNA/miR)‑25 is downregulated in patients with intervertebral disc degeneration (IVDD). However, the potential role of miR‑25 in IVDD remains unclear. Therefore, the present study aimed to investigate the effects of miR‑25 on human intervertebral disc nucleus pulposus cells (NPCs). The expression levels of miR‑25 and those of small ubiquitin‑related modifier 2 (SUMO2) were determined in human nucleus pulposus (NP) tissues by reverse transcription‑quantitative PCR (RT‑qPCR) and western blot analyses. Subsequently, the potential interaction between miR‑25 and SUMO2 was validated via dual‑luciferase reporter assay and RNA pull‑down assay with biotinylated miRNA. The effects of miR‑25 on NPC proliferation and apoptosis were evaluated using Cell Counting Kit‑8 assay, 5‑ethynyl‑2'‑deoxyuridine incorporation assay, and flow cytometry. The results showed that miR‑25 was downregulated in patients with IVDD. In addition, miR‑25 increased the proliferation of NPCs and inhibited their apoptosis. Furthermore, the current study verified that miR‑25 could directly target SUMO2 and regulate its expression via the p53 signaling pathway. Additionally, the effects of miR‑25 on NPCs were abrogated following SUMO2 overexpression. Overall, the results of the present study demonstrated that miR‑25 could promote the proliferation and inhibit the apoptosis of NPCs via targeting SUMO2, suggesting that miR‑25 may be a potential target in the treatment of IVDD.

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.

Nucleus Pulposus Cell Conditioned Medium Promotes Mesenchymal Stem Cell Differentiation into Nucleus Pulposus-Like Cells under Hypoxic Conditions

Low back pain (LBP) is a major physical and socioeconomic challenge worldwide. Nucleus pulposus (NP) is directly associated with LBP due to intervertebral disc (IVD) degeneration. IVD degeneration is mainly caused by structural and matrix-related changes within the IVD occurring during aging and degeneration. Mesenchymal stem cells (MSCs) can differentiate into multiple mesenchymal lineages under specific stimulatory conditions. This study is aimed at evaluating the effectiveness of the nucleus pulposus cell (NPC) conditioned medium for promoting the expression of MSCs and at confirming the expression of healthy NP phenotypic markers recently recommended by the Spine Research Interest Group. Expression was investigated using quantitative polymerase chain reaction (qPCR) and western blotting under normoxic and hypoxic conditions. qPCR and western blotting demonstrated significant upregulation of NP marker expression in MSCs cultured under hypoxic conditions and treated with the 50% or 100% NPC conditioned medium, compared with those cultured under normoxic conditions. Upregulation was highest in the presence of the 100% NPC conditioned medium compared with the control group (aggrecan, p < 0.01; brachyury, p < 0.05; collagen II, p < 0.001; KRT8, p < 0.01; KRT19, p < 0.001; and Shh, p < 0.01). The expression levels of genes in MSCs treated with the 50% NPC conditioned medium also showed upregulation compared with the control group (collagen II, p < 0.05; KRT8, p < 0.05; and KRT19, p < 0.01). These findings suggested that the NPC conditioned medium stimulated MSC differentiation into an NP-like phenotype with distinct characteristics. The results could inform strategies for IVD regeneration.

Sumoylation enhances the activity of the TGF-β/SMAD and HIF-1 signaling pathways in keloids

Excessive fibrosis and extracellular matrix deposition resulting from upregulation of target genes expression mediated by transforming growth factor-beta (TGF-β)/SMAD and hypoxia inducible factor-1 (HIF-1) signaling pathways are the main mechanisms that drive keloid formation. Sumoylation is a protein posttranslational modification that regulates the function of proteins in many biological processes. In the present study, we aimed to investigate the mechanism underlying the effects of sumoylation on the TGF-β/SMAD and HIF-1 signaling pathways in keloids. We used 2-D08 to block sumoylation and silenced the expression of sentrin sumo-specific protease 1 (SENP1) to enhance sumoylation in human foreskin fibroblasts (HFFs) and human keloid fibroblasts (HKFs). We also reduced and increased intracellular SUMO1 levels by silencing SUMO1 and transfecting cells with a SUMO1 overexpression lentivirus, respectively. Sumoylation has the ability to amplify TGF-β/SMAD and HIF-1 signals in keloids, while SUMO1, especially the SUMO1-RanGAP1 complex, is the key molecule affecting the TGF-β/SMAD and HIF-1 signaling pathways. In addition, we also found that hypoxia promotes sumoylation in keloids and that HIF-1α is covalently modified by SUMO1 at Lys 391 and Lys 477 in HKFs. In summary, we elucidated the role and molecular mechanism of sumoylation in the formation of keloids, providing a new perspective for a potential therapeutic target of keloids.

Desumoylation of aggrecan and collagen II facilitates degradation via aggrecanases in IL-1β-mediated osteoarthritis

Background: Aggrecan plays a crucial role in the ability of tissues to withstand compressive loads during the pathological progression of osteoarthritis (OA). Progressive loss of aggrecan from cartilage may result in exposure of the collagen matrix and can lead to its disintegration by metalloproteases. Although aggrecanases are expressed constitutively in human chondrocytes, the degradation of aggrecan is induced by proinflammatory cytokines; however, little is known about the underlying mechanisms. Methods: Human primary chondrocytes from OA patients or healthy donors and human chondrogenic SW1353 cells were cultured and stimulated with IL-1β in vitro, the mRNA expressions and protein levels of MMP-13, ADAMTS-4, ADAMTS-5, SENP1, and SENP2 were determined using real time PCR and Western blot, respectively. The localizations of aggrecan and Col-II, as well as the SUMOylation modification of these proteins were analyzed using immunofluorescence and immunoprecipitation assays, respectively. Results: Our results showed that a proinflammatory cytokine interleukin-1β induced the OA model and desumoylation of aggrecan and collagen type II because the small ubiquitin-like modifier 2/3 (SUMO2/3) was co-localized with aggrecan and collagen type II proteins and interacted physically with them. Mechanistic studies have shown that knockdown of SUMO2/3 expression can significantly enhance the rate of degradation of aggrecan and collagen type II at both the mRNA and protein levels in the OA model. In addition, SUMO-specific protease 2 (SENP2) plays important roles in the desumoylation of aggrecan, while knockdown of SENP2 can protect aggrecan and collagen type II. Clinical assays have shown that OA patients have higher SENP2 levels than healthy controls, and the SENP2 level correlates negatively with both aggrecan and collagen type II levels. Conclusion: SENP2 desumoylates aggrecan and collagen type II proteins in the inflammation induced OA, and SENP2 expression correlates with OA progression.

Hypoxia-induced Slug SUMOylation enhances lung cancer metastasis

Background

The Slug-E-cadherin axis plays a critical role in non-small-cell lung cancers (NSCLCs) where aberrant upregulation of Slug promotes cancer metastasis. Now, the post-translational modifications of Slug and their regulation mechanisms still remain unclear in lung cancer. Hence, exploring the protein linkage map of Slug is of great interest for investigating the scenario of how Slug protein is regulated in lung cancer metastasis.

Methods

The Slug associated proteins, Ubc9 and SUMO-1, were identified using yeast two-hybrid screening; and in vitro SUMOylation assays combined with immunoprecipitation and immunoblotting were performed to explore the detail events and regulations of Slug SUMOylation. The functional effects of SUMOylation on Slug proteins were examined by EMSA, reporter assay, ChIP assay, RT-PCR, migration and invasion assays in vitro, tail vein metastatic analysis in vivo, and also evaluated the association with clinical outcome of NSCLC patients.

Results

Slug protein could interact with Ubc9 and SUMO-1 and be SUMOylated in cells. Amino acids 130–212 and 33–129 of Slug are responsible for its binding to Ubc9 and protein inhibitor of activated STAT (PIAS)y, respectively. SUMOylation could enhance the transcriptional repression activity of Slug via recruiting more HDAC1, resulting in reduced expression of downstream Slug target genes and enhanced lung cancer metastasis. In addition, hypoxia could increase Slug SUMOylation through attenuating the interactions of Slug with SENP1 and SENP2. Finally, high expression Slug and Ubc9 levels were associated with poor overall survival among NSCLC patients.

Conclusions

Ubc9/PIASy-mediated Slug SUMOylation and subsequent HDAC1 recruitment may play a crucial role in hypoxia-induced lung cancer progression, and these processes may serve as therapeutic targets for NSCLC.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0996-8) contains supplementary material, which is available to authorized users.

Nucleus pulposus cell apoptosis is attenuated by CDMP-2 through regulating oxidative damage under the hyperosmotic environment

Disc nucleus pulposus (NP) cell experiences periodic osmolarity alterations during daily activities, which has been proved to affect cell biology in vitro The present study was aimed to investigate the effects of cartilage-derived morphogenetic protein-2 (CDMP-2) on NP cell apoptosis under the hyperosmolarity culture and the potential mechanism. Isolated rat NP cells were cultured in the in situ-osmolarity medium or hyperosmolarity medium for 3 days. CDMP-2 was added into the hyperosmolarity medium to investigate its effects on NP cell apoptosis. Cell apoptosis rate, caspase-3 activity, gene expression of Bcl-2, Bax, and caspase-3, and protein expression of Bcl-2, Bax, and cleaved caspase-3 were analyzed to evaluate NP cell apoptosis. Additionally, the intracellular reactive oxygen species (ROS) and the total superoxide dismutase (SOD) activity were analyzed to investigate the potential role of oxidative damage in this process. In the hyperosmolarity culture, NP cells showed a significantly increased cell apoptosis rate and caspase-3 activity, an up-regulated expression of Bax and caspase-3/cleaved-caspase-3 and a down-regulated expression of Bcl-2. However, CDMP-2 partly inhibited these effects of hyperosmolarity culture on NP cells. Additionally, the hyperosmolarity culture significantly increased ROS content and decreased the total SOD activity compared with the in situ-osmolarity culture, whereas exogenous CDMP-2 partly decreased the ROS content and increased the total SOD activity in the hyperosmolarity culture. In conclusion, CDMP-2 is effective in attenuating hyperosmolarity environment-induced NP cell apoptosis, and this process may be mediated through inhibiting oxidative stress damage. The present study indicates that CDMP-2 may be helpful to retard hyperosmolarity niche-mediated disc degeneration.

Silencing SUMO2 promotes protection against degradation and apoptosis of nucleus pulposus cells through p53 signaling pathway in intervertebral disc degeneration

Objective: Intervertebral disc degeneration (IDD), as a common cause of back pain, is related to the promotion of cellular senescence and reduction in proliferation. Based on recent studies, small ubiquitin-related modifier (SUMO) proteins have been implicated in various biological functions. Therefore, in the present study, we investigated the effects of SUMO2 on proliferation and senescence of nucleus pulposus cells (NPCs) via mediation of p53 signaling pathway in rat models of IDD.

Methods: After the establishment of rat models of IDD for the measurement of positive expression of SUMO2/3 protein, the mRNA and protein levels of SUMO2, molecular phenotype [matrix metalloproteinase-2 (MMP-2) and hypoxia-inducible factor-1α (HIF-1α)] and p53 signaling pathway-related genes [p21, murine double minute-2 (MDM2), growth arrest and DNA-damage-inducible protein 45 α (GADD45α), cyclin-dependent kinase 2/4 (CDK2/4), and CyclinB1] were determined, followed by the detection of cell proliferation, cell cycle, apoptosis, and cell senescence.

Results: The rat models of IDD were successfully constructed. The results obtained showed that there was a higher positive expression of SUMO2/3 protein in IDD rats. Moreover, the silencing of the SUMO2 gene decreases the levels of SUMO2, p53, p21, MDM2, GADD45α, MMP-2, and HIF-1α expressions and p53 phosphorylation level while it increases the levels of CDK2/4 and CyclinB1 expressions. In addition, SUMO2 gene silencing enhances proliferation and suppresses apoptosis and cell senescence of NPCs.

Conclusion: In conclusion, SUMO2 gene silencing promotes proliferation, and inhibits the apoptosis and senescence of NPCs in rats with IDD through the down-regulation of the p53 signaling pathway. Thus, SUMO2 is a potential target in the treatment of IDD.

Knockdown of RWD domain containing 3 inhibits the malignant phenotypes of glioblastoma cells via inhibition of phosphoinositide 3-kinase/protein kinase B signaling

Glioblastoma is the most common and malignant primary brain tumor. RWD domain containing 3 (RWDD3) has been previously reported to serve a promoting role in pituitary tumors. However, the exact role of RWDD3 in glioblastoma remains unclear. Therefore, the present study aimed to investigate the expression levels of RWDD3 in human glioblastoma tissues and cell lines, as well as to examine the regulatory mechanism of RWDD3 underlying glioblastoma growth and metastasis. The results revealed that RWDD3 was significantly upregulated in glioblastoma tissues compared with normal brain tissues, while high expression of RWDD3 was associated with a shorter survival time of glioblastoma patients. The expression levels of RWDD3 were also higher in the glioblastoma cell lines compared with the normal human astrocyte cell line. Subsequent to knockdown of RWDD3, the proliferation of glioblastoma U87 and U251 cells was significantly decreased, possibly due to the cell cycle arrest at G1 phase, as well as the increased cell apoptosis. Furthermore, downregulation of RWDD3 also suppressed U87 and U251 cell invasion by inhibiting the expression levels of matrix metalloproteinase 2 (MMP2) and MMP9. Molecular mechanism investigation demonstrated that knockdown of RWDD3 significantly downregulated the activity of the phosphoinositide 3-kinase/protein kinase B (PI3K/AKT) signaling pathway. Activation of PI3K/AKT signaling prevented the suppressive effects of RWDD3 downregulation on glioblastoma cell proliferation and migration, concurrent with increased protein levels of MMP2 and MMP9. In conclusion, the current study demonstrated for the first time that inhibition of RWDD3 expression inhibited glioblastoma progression, at least partly, via suppressing the PI3K/AKT signaling activity, and thus RWDD3 may be a novel potential therapeutic target for glioblastoma.

Acid-Sensing Ion Channel 1a Regulates Fate of Rat Nucleus Pulposus Cells in Acid Stimulus Through Endoplasmic Reticulum Stress

Acid-sensing ion channel 1a (ASIC1a) participates in human intervertebral disc degeneration (IVDD) and regulates the destiny of nucleus pulposus cells (NPCs) in acid stimulus. However, the mechanism of ASIC1a activation and its downstream pathway remain unclear. Endoplasmic reticulum (ER) stress also participates in the acid-induced apoptosis of NPCs. The main purpose of this study was to investigate whether there is any connection between ASIC1a and ER stress in an acid-induced nucleus pulposus degeneration model. The IVDs of Sprague-Dawley rats were stained by immunohistochemical staining to evaluate the expression of ASIC1a in normal and degenerated rat nucleus pulposus. ASIC1a expression was also quantified by quantitative real-time-polymerase chain reaction and Western blotting analysis. NPCs were exposed to the culture media with acidity at pH 7.2 and 6.5 for 24 h, with or without 4-phenylbutyrate (4-PBA, a blocker of the ER stress pathway). Cell apoptosis was examined by Annexin V/Propidium Iodide (PI) staining and was quantified using flow cytometry analysis. ASIC1a-mediated intracellular calcium was determined by Ca²⁺ imaging using Fura-2-AM. Acidity-induced changes in ER stress markers were studied using Western blotting analysis. In vivo, ASIC1a expression was upregulated in natural degeneration. In vitro, acid stimulus increased intracellular calcium levels, but this effect was blocked by knockdown of ASIC1a, and this reversal was partly inhibited by 4-PBA. In addition, blockade of ASIC1a reduced expression of ER stress markers, especially the proapoptotic markers. ASIC1a partly regulates ER stress and promotes apoptosis of NPCs under acid stimulus and may be a novel therapeutic target in IVDD.

Hypothermia inhibits the proliferation of bone marrow-derived mesenchymal stem cells and increases tolerance to hypoxia by enhancing SUMOylation

Hypothermia therapy has a positive effect on patients with severe brain injury. Recent studies have shown that mild hypothermia increases the survival of bone marrow-derived mesenchymal stem cells (BMSCs) in a hypoxic environment; however, the underlying mechanisms are not yet fully understood. Small ubiquitin-like modifiers (SUMOs) are sensitive to temperature stress reactions and are considered to exert a protective effect. In this study, we examined the protective effects of hypothermia on BMSCs in terms of SUMO protein modification. First, we found that mild hypothermia inhibited the proliferation and differentiation of BMSCs and increased cell tolerance to a hypoxic environment. Second, hypothermia significantly increased the levels of SUMO modification of multiple proteins in BMSCs. The knockdown of SUMO1/2/3 induced the rapid aging of the BMSCs, while the inhibition of the SUMO-conjugating enzyme, Ubc9, reduced cell proliferation and increased the proportion of BMSCs differentiating into nerve cells. Moreover, the tolerance of BMSCs to the hypoxic environment was significantly decreased. Lastly, we investigated 4 reported SUMO target proteins, anti-proliferating cell nuclear antigen, octamer-binding transcription factor 4, p53 and hypoxia-inducible factor-1α, to confirm that SUMO modification was indeed involved in maintaining the proliferation, inhibiting differentiation and enhancing the resistance of BMSCs against adverse conditions. Taken together, our results indicate that the SUMO pathway is involved in the response to hypothermic stress, and that SUMOylation may be an important protective mechanism against hypothermia for the survival of BMSCs under unfavorable conditions.

Formation, function, and exhaustion of notochordal cytoplasmic vacuoles within intervertebral disc: current understanding and speculation

Notochord nucleus pulposus cells are characteristic of containing abundant and giant cytoplasmic vacuoles. This review explores the embryonic formation, biological function, and postnatal exhaustion of notochord vacuoles, aiming to characterize the signal network transforming the vacuolated nucleus pulposus cells into the vacuole-less chondrocytic cells. Embryonically, the cytoplasmic vacuoles within vertebrate notochord originate from an evolutionarily conserved vacuolation process during neurulation, which may continue to provide mechanical and signal support in constructing a mammalian intervertebral disc. For full vacuolation, a vacuolating specification from dorsal organizer cells, synchronized convergent extension, well-structured notochord sheath, and sufficient post-Golgi trafficking in notochord cells are required. Postnatally, age-related and species-specific exhaustion of vacuolated nucleus pulposus cells could be potentiated by Fas- and Fas ligand-induced apoptosis, intolerance to mechanical stress and nutrient deficiency, vacuole-mediated proliferation check, and gradual de-vacuolation within the avascular and compression-loaded intervertebral disc. These results suggest that the notochord vacuoles are active and versatile organelles for both embryonic notochord and postnatal nucleus pulposus, and may provide novel information on intervertebral disc degeneration to guide cell-based regeneration.

Endoplasmic Reticulum Stress Is Involved in Nucleus Pulposus Degeneration and Attenuates Low pH-Induced Apoptosis of Rat Nucleus Pulposus Cells

The microenvironment of degenerative intervertebral disk (IVD) is characteristic of a high concentration of lactic acid and low pH levels, whereas the underlying mechanism has not been clearly defined. Endoplasmic reticulum (ER) is the hub of interactions between environmental signals and cellular biological functions, the malfunction of which is closely involved in the pathogenesis of multiple disorders, including IVD degeneration (IVDD). This research mainly aims at exploring what role ER stress plays in the natural process of IVDD and pH-induced apoptosis of rat nucleus pulposus (NP) cells (NPCs). The IVD of Sprague-Dawley rats at different ages was stained by Hematoxylin-Eosin staining to visualize the histocytological changes during the nature process of IVDD. Immunohistochemical staining was performed to evaluate the expression of ER stress markers within normal and degenerated NP. The ER stress markers were also quantified by quantitative real-time-polymerase chain reaction (PCR) and Western blotting analysis, respectively. NPCs were exposed to the culturing media with acidity of pH 7.4, 7.0, 6.5, or 6.0 for 24-72 h, with or without the supplement of 4-phenylbutyrayte (4-PBA, the blocker of ER stress pathways). Changes in cell viability were evaluated by CCK-8 assay and neutral red assay, whereas apoptosis was stained by Annexin-V/PI staining and quantified by flow cytometry analysis. The acidity-induced changes in the expression of ER stress markers were studied by immunofluorescent staining, qRT-PCR, and Western blotting analysis. In vivo, the expression of GRP78 and XBP1 was downregulated whereas CHOP and Caspase12 were upregulated in natural degeneration. In vitro, low pH induced apoptosis of rat NPCs; prolonged exposure of acid reduced cell viability and caused upregulation of ER stress markers. 4-PBA was used to alleviate ER stress, and it promoted acid-induced apoptosis of NPCs. ER stress is involved in NP natural degeneration and attenuates low-pH-induced apoptosis of NPCs.

The influence of oxygen concentration on the extracellular matrix production of human nucleus pulposus cells during isolation-expansion process

Nucleus pulposus (NP) cells locate in the center of avascular intervertebral discs, and thus have presumably adapted to a hypoxic environment. The purpose of this study was to investigate the influences of hypoxic condition, during isolation-expansion of human NP cells, on the cellular proliferation and extracellular matrix (ECM) synthesis in later three-dimensional cultures. Human NP tissues were obtained from patients who underwent lumbar disc surgeries. Immediately after retrieval, NP tissues from each patient were divided into two aliquots for in vitro cultivation either under classical normoxic (21% O₂ ) or hypoxic (3.5% O₂ ) condition. After isolation-expansion processes, microtissues of NP cells were formed and the analysis was performed after one-week culture. Experiments of pretreatment with TGF-β1 or lovastatin were designed to investigate if the isolation-expansion conditions affect the responsiveness to later exogenous treatments. Hypoxic isolation-expansion stimulated NP cell proliferation during monolayer culture. Hypoxia also upregulated mRNA levels of SOX9 and HIF-1α but downregulated type X collagen as well as improved aggrecan and type II collagen synthesis. Although TGF-β1 had no substantial effect, lovastatin pretreatment showed a greater enhancement on type II collagen expression in hypoxic group. Normoxia negatively affected the biochemical composition of regenerated ECM attributable to downregulation of SOX9 and HIF-1α, while hypoxia enhanced cellular proliferation, improved matrix production, and maintained a functional phenotype of NP cells. Hypoxic isolation-expansion of human NP cells is important to achieve better regenerative cells for later cultivation or cell transplantation. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1575-1582, 2017.