Regulatory role of hypoxia inducible factor in the biological behavior of nucleus pulposus cells

Regulation of endoplasmic reticulum stress on autophagy and apoptosis of nucleus pulposus cells in intervertebral disc degeneration and its related mechanisms

Intervertebral disc degeneration (IVDD) is a common and frequent disease in orthopedics, which seriously affects the quality of life of patients. Endoplasmic reticulum stress (ERS)-regulated autophagy and apoptosis play an important role in nucleus pulposus (NP) cells in IVDD. Hypoxia and serum deprivation were used to induce NP cells. Cell counting kit-8 (CCK-8) assay was used to detect cell activity and immunofluorescence (IF) was applied for the appraisement of glucose regulated protein 78 (GRP78) and green fluorescent protein (GFP)-light chain 3 (LC3). Cell apoptosis was detected by flow cytometry and the expression of LC3II/I was detected by western blot. NP cells under hypoxia and serum deprivation were induced by lipopolysaccharide (LPS), and intervened by ERS inhibitor (4-phenylbutyric acid, 4-PBA) and activator (Thapsigargin, TP). Then, above functional experiments were conducted again and western blot was employed for the evaluation of autophagy-, apoptosis and ERS-related proteins. Finally, NP cells under hypoxia and serum deprivation were stimulated by LPS and intervened using apoptosis inhibitor z-Val-Ala-DL-Asp-fluoromethyl ketone (Z-VAD-FMK) and autophagy inhibitor 3-methyladenine (3-MA). CCK-8 assay, IF, flow cytometry and western blot were performed again. Besides, the levels of inflammatory cytokines were measured with enzyme-linked immunosorbent assay (ELISA) and the protein expressions of programmed death markers were estimated with western blot. It showed that serum deprivation induces autophagy and apoptosis. ERS was significantly activated by LPS in hypoxic and serum deprivation environment, and autophagy and apoptosis were significantly promoted. Overall, ERS affects the occurrence and development of IVDD by regulating autophagy, apoptosis and other programmed death.

Hypoxia regulates adipose mesenchymal stem cells proliferation, migration, and nucleus pulposus-like differentiation by regulating endoplasmic reticulum stress via the HIF-1α pathway

OBJECTIVE

Hypoxia can promote stem cell proliferation and migration through HIF-1α. Hypoxia can regulate cellular endoplasmic reticulum (ER) stress. Some studies have reported the relationship among hypoxia, HIF-α, and ER stress, however, while little is known about HIF-α and ER stress in ADSCs under hypoxic conditions. The purpose of the study was to investigate the role and relationship of hypoxic conditions, HIF-1α and ER stress in regulating adipose mesenchymal stem cells (ADSCs) proliferation, migration, and NPC-like differentiation.

METHOD

ADSCs were pretreated with hypoxia, HIF-1α gene transfection, and HIF-1α gene silence. The ADSCs proliferation, migration, and NPC-like differentiation were assessed. The expression of HIF-1α in ADSCs was regulated; then, the changes of ER stress level in ADSCs were observed to investigate the relationship between ER stress and HIF-1α in ADSCs under hypoxic conditions.

RESULT

The cell proliferation and migration assay results show that hypoxia and HIF-1α overexpression can significantly increase the ADSCs proliferation and migration, while HIF-1α inhibition can significantly decrease the ADSCs proliferation and migration. The HIF-1α and co-cultured with NPCs played an important role in the directional differentiation of ADSCs into NPCs. The hypoxia-regulated ER stress in ADSCs through the HIF-1α pathway, thereby regulating the cellular state of ADSCs, was also observed.

CONCLUSION

Hypoxia and HIF-1α play important roles in proliferation, migration, and NPC-like differentiation of ADSCs. This study provides preliminary evidence that HIF-1α-regulated ER stress thus affects ADSCs proliferation, migration, and differentiation. Therefore, HIF-1α and ER may serve as key points to improve the efficacy of ADSCs in treating disc degeneration.

Exosomes and exosomal miRNAs: A new therapy for intervertebral disc degeneration

Low back pain has been found as a major cause of global disease burden and disability. Intervertebral disc degeneration is recognized as the vital factor causing low back pain. Intervertebral disc degeneration has a complex mechanism and cannot be avoided. Traditional strategies for the treatment of intervertebral disc degeneration cannot meet the needs of intervertebral disc regeneration, so novel treatment methods are urgently required. Exosomes refer to extracellular vesicles that can be released by most cells, and play major roles in intercellular material transport and information transmission. MicroRNAs have been identified as essential components in exosomes, which can be selectively ingested by exosomes and delivered to receptor cells for the regulation of the physiological activities and functions of receptor cells. Existing studies have progressively focused on the role of exosomes and exosomal microRNAs in the treatment of intervertebral disc degeneration. The focus on this paper is placed on the changes of microenvironment during intervertebral disc degeneration and the biogenesis and mechanism of action of exosomes and exosomal microRNAs. The research results and deficiencies of exosomes and exosomal microRNAs in the regulation of apoptosis, extracellular matrix homeostasis, inflammatory response, oxidative stress, and angiogenesis in intervertebral disc degeneration are primarily investigated. The aim of this paper is to identify the latest research results, potential applications and challenges of this emerging treatment strategy.

Intervertebral Disk Degeneration: The Microenvironment and Tissue Engineering Strategies

Intervertebral disk degeneration (IVDD) is a leading cause of disability. The degeneration is inevitable, and the mechanisms are complex. Current therapeutic strategies mainly focus on the relief of symptoms, not the intrinsic regeneration of the intervertebral disk (IVD). Tissue engineering is a promising strategy for IVDD due to its ability to restore a healthy microenvironment and promote IVD regeneration. This review briefly summarizes the IVD anatomy and composition and then sets out elements of the microenvironment and the interactions. We rationalized different scaffolds based on tissue engineering strategies used recently. To fulfill the complete restoration of a healthy IVD microenvironment, we propose that various tissue engineering strategies should be combined and customized to create personalized therapeutic strategies for each individual.

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

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.

Multiscale Regulation of the Intervertebral Disc: Achievements in Experimental, In Silico, and Regenerative Research

Intervertebral disc (IVD) degeneration is a major risk factor of low back pain. It is defined by a progressive loss of the IVD structure and functionality, leading to severe impairments with restricted treatment options due to the highly demanding mechanical exposure of the IVD. Degenerative changes in the IVD usually increase with age but at an accelerated rate in some individuals. To understand the initiation and progression of this disease, it is crucial to identify key top-down and bottom-up regulations' processes, across the cell, tissue, and organ levels, in health and disease. Owing to unremitting investigation of experimental research, the comprehension of detailed cell signaling pathways and their effect on matrix turnover significantly rose. Likewise, in silico research substantially contributed to a holistic understanding of spatiotemporal effects and complex, multifactorial interactions within the IVD. Together with important achievements in the research of biomaterials, manifold promising approaches for regenerative treatment options were presented over the last years. This review provides an integrative analysis of the current knowledge about (1) the multiscale function and regulation of the IVD in health and disease, (2) the possible regenerative strategies, and (3) the in silico models that shall eventually support the development of advanced therapies.

Hypoxia Protects Rat Bone Marrow Mesenchymal Stem Cells Against Compression-Induced Apoptosis in the Degenerative Disc Microenvironment Through Activation of the HIF-1α/YAP Signaling Pathway

Stem cell therapy provides an attractive solution for intervertebral disc (IVD) degeneration. However, the degenerative microenvironment, characterized by excessive mechanical loading and hypoxia, remains an obstacle for the long-lasting survival of exogenous transplanted stem cells. Whether and how bone marrow mesenchymal stem cells (BMSCs) adapt to the hostile microenvironment remain unclear. In this study, CoCl₂ and mechanical compression were simultaneously used to simulate the hypoxic and overloaded microenvironment of IVDs in vitro. Compression had a proapoptotic effect through activation of the mitochondrial apoptotic pathway, while hypoxia exerted a prosurvival effect counteracting compression-induced apoptosis. Inhibiting the transcriptional activity of hypoxia inducible factor 1 subunit alpha (HIF-1α) by chetomin reversed the antiapoptotic effect of hypoxia. Furthermore, HIF-1α promoted dephosphorylation and activation of yes-associated protein (YAP) in hypoxic conditions. Conversely, both YAP inhibition and increased cell apoptosis were observed after inhibition through chetomin or YAP inhibitor verteporfin. Immunofluorescence staining and coimmunoprecipitation assays revealed that YAP could interact directly with HIF-1α and colocalize in the nucleus. Taken together, our results demonstrated that hypoxia protected BMSCs against compression-induced apoptosis in the degenerative disc microenvironment through activation of the HIF-1α/YAP signaling pathway. Thus, regulation of HIF-1α/YAP signaling might provide novel insights for promoting long-lasting BMSC survival and optimizing stem cell therapy for IVD degeneration.

Histomorphology and immunohistochemical patterns in degenerative disc disease and clinical-radiological correlations: a prospective study

PURPOSE

Degenerative disc disease (DDD) is a common condition causing low-back pain, disability and, eventually, neurological symptoms. This investigation aimed to investigate intervertebral disc DDD-related changes, evaluating histomorphology and cytokines secretion, and their clinical-radiological correlations.

METHODS

This is a monocentric prospective observational study. A cohort of patients who underwent microdiscectomy for DDD, from June 2018 to January 2019, were enrolled. Discs samples were examined for histomorphology, chondrons count, immunohistochemistry for Hif-1α, Nf200 and Egr-1. Demographical and clinical data were also collected.

RESULTS

Twenty patients were finally included. MRI evaluation showed a Modic I alteration in nine patients and a Modic II in 11. The disability grade was low-moderate (ODI score was ≤ 40%) in eight patients and high (ODI score > 40%) in 12. The Modic I was associated with a low-moderate disability in two (22%) patients and to a high disability in seven (88%) (p < 0.01). In Modic I group and in ODI > 40% groups, there were a significative higher mean disability grade 48.4 (± 8.3)%, number of chondrons per section, cells per chondron, Nf200+ nerve fibers and Hif-1α expression, compared with Modic II and ODI ≤ 40% groups, respectively. There were no differences in terms of Egr-1 expression.

CONCLUSIONS

The discs with Modic I MRI signal could represent potential targets for medical treatments, whereas Modic II seems to be a more likely point of no return in a degenerative process. Therefore, further investigations are to better investigate inflammatory pathways and degenerative mechanisms in DDD.

Time-Course Investigation of Intervertebral Disc Degeneration Induced by Different Sizes of Needle Punctures in Rat Tail Disc

Background

This study aimed to determine the best size needle to use in inducing IVDD and to find the proper time point of disc degeneration suitable for further biologic treatment study.

Material/Methods

First, rat tail level 5/6, 7/8, and 9/10 discs were punctured by 18G, 21G, or 25G needles. Then, degeneration was assessed by radiography, MRI, and histological evaluation at 2, 4, and 6 weeks after puncture. Later, real-time reverse transcriptase (RT-PCR) was used to examine mRNA expressions of aggrecan, collagen type II, hypoxia-inducible factor-1α (HIF-1α), glucose transporter1 (GLUT-1), and vascular endothelial growth factor (VEGF).

Results

Significant differences were identified in almost all parameters compared with the control group in the 18G and 21G group at almost all time points. To assess the effect of different needle sizes on DHI, we used magnetic resonance imaging (MRI), grade, and mRNA expression. We found significant differences between different groups, except for DHI between the 21G group and 25G group and MRI grade between the 18G and 21G group at the 2-week time point. In assessing the effect of different needle sizes on HE staining score and toluidine blue staining grade, statistical differences were observed at some time points. The effects of time on all parameters were significant at almost all time points in all groups.

Conclusions

The middle-size needle (21G) performed better in inducing disc degeneration. The 2-week time point may be better for use in further experimental studies.

ENERGY SUPPLY AND DEMAND IN THE INTERVERTEBRAL DISC

ABSTRACT The intervertebral disc (IVD) is one of the parts of the body most commonly affected by disease, and it is only recently that we have come closer to understanding the reasons for its degeneration, in which nutrient supply plays a crucial role. In this literature review, we discuss the basic principles and characteristics of energy supply and demand to the IVD. Specifically, we review how different metabolites influence IVD cell activity, the effects of mechanical loading on IVD cell metabolism, and differences in energy metabolism of the annulus fibrous and nucleus pulposus cell phenotypes. Determining the factors that influence nutrient supply and demand in the IVD will enhance our understanding of the IVD pathology, and help to elucidate new therapeutic targets for IVD degeneration treatment.

Identification of a new mineralized tissue in the notochord of reared Siberian sturgeon (Acipenser baerii)

In a study aiming to improve knowledge on the mineralization of the axial skeleton in reared Siberian sturgeon (Acipenser baerii Brandt, 1869), we discovered a new mineralized tissue within the notochord. To our knowledge, such a structure has never been reported in any vertebrate species with the exception of the pathological mineralization of the notochord remains in degenerative intervertebral disks of mammals. Here, we describe this enigmatic tissue using X-ray microtomography, histological analyses and solid state NMR-spectroscopy. We also performed a 1-year monitoring of the mineral content (MC) of the notochord in relation with seasonal variations of temperature. In all specimens studied from 2-year-old juveniles onwards, this mineralized structure was found within a particular region of the notochord called funiculus. This feature first appears in the abdominal region then extends posteriorly with ageing, while the notochord MC also increases. The mineral phase is mainly composed of amorphous calcium phosphate, a small amount of which changes into hydroxyapatite with ageing. The putative role of this structure is discussed as either a store of minerals available for the phosphocalcic metabolism, or a mechanical support in a species with a poorly mineralized axial skeleton. A pathological feature putatively related to rearing conditions is also discussed.

Time course investigation of intervertebral disc degeneration in a rat-tail puncture model

AIMS

Intervertebral disc (IVD) degeneration was believed to contribute to lower back pain. The aim of the study was to investigate the pathogenesis and regulatory mechanism of puncture-induced IVD degeneration.

MAIN METHODS

We established a rat-tail puncture model using Kirschner wire and a homemade stopper. The progress of disc degeneration was evaluated by histological examination and the quantitative measurement of type I, type II collagen and other factors expression at 0.5, 1, 2, 6, and 12weeks after puncture and was compared with control rats of the same age.

KEY FINDINGS

Histological examination and Safranin-O staining revealed progressive degeneration of the punctured disc. Matrix metalloproteinase 13 (MMP13) was increased at 1week after puncture but did not change in the control group. The interleukin-1 beta (IL-1β) mRNA expression level was elevated at the acute stage after puncture compared with the control group. The hypoxia inducible factor 2 (HIF-2) increased expression in punctured groups. Additionally, compare to adjacent non-punctured segments, HIF-2α expression level transiently increased and then decreased in the nucleus pulposus immediately following puncture, and it then increased 12weeks after puncture.

SIGNIFICANCE

The degenerative changes observed in this rat-tail puncture model are similar to human disc degeneration and that this model may be valuable for elucidating the molecular mechanisms and pathways underlying disc degeneration.

Hypoxia Induces Epithelial-Mesenchymal Transition in Follicular Thyroid Cancer: Involvement of Regulation of Twist by Hypoxia Inducible Factor-1α

PURPOSE

Although follicular thyroid cancer (FTC) has a relatively fair prognosis, distant metastasis sometimes results in poor prognosis and survival. There is little understanding of the mechanisms contributing to the aggressiveness potential of thyroid cancer. We showed that hypoxia inducible factor-1α (HIF-1α) induced aggressiveness in FTC cells and identified the underlying mechanism of the HIF-1α-induced invasive characteristics.

MATERIALS AND METHODS

Cells were cultured under controlled hypoxic environments (1% O₂) or normoxic conditions. The effect of hypoxia on HIF-1α, and epithelial-to-mesenchymal transition (EMT) related markers were evaluated by quantitative real-time PCR, Western blot analysis and immunocytochemistry. Invasion and wound healing assay were conducted to identify functional character of EMT. The involvement of HIF-1α and Twist in EMT were studied using gene overexpression or silencing. After orthotopic nude mouse model was established using the cells transfected with lentiviral shHIF-1α, tissue analysis was done.

RESULTS

Hypoxia induces HIF-1α expression and EMT, including typical morphologic changes, cadherin shift, and increased vimentin expression. We showed that overexpression of HIF-1α via transfection resulted in the aforementioned changes without hypoxia, and repression of HIF-1α with RNA interference suppressed hypoxia-induced HIF-1α and EMT. Furthermore, we also observed that Twist expression was regulated by HIF-1α. These were confirmed in the orthotopic FTC model.

CONCLUSION

Hypoxia induced HIF-1α, which in turn induced EMT, resulting in the increased capacity for invasion and migration of cells via regulation of the Twist signal pathway in FTC cells. These findings provide insight into a possible therapeutic strategy to prevent invasive and metastatic FTC.

Decreased Zn(2+) Influx Underlies the Protective Role of Hypoxia in Rat Nucleus Pulposus Cells

Zn(2+) is an essential component of metalloproteinases, and is required for their activity in cartilage; however, the effect of Zn(2+) on nucleus pulposus (NP) cells has not been widely investigated. The aim of this paper was to investigate the effect of intracellular Zn(2+) concentration ([Zn(2+)]i) in hypoxia-induced regulation of metalloproteinases (MMPs) and extracellular matrix (ECM) production in NP cells. NP cells from Sprague-Dawley (SD) rats were cultured as monolayers or in alginate beads. [Zn(2+)]i was assayed by FluoZin-3 AM staining. Alcian Blue staining, immunochemistry, 1,9-dimethylmethylene blue (DMMB) assay, and real-time PCR were used to assay collagen II, proteoglycan, and COL2A1, MMP-13, and ADAMTS-5 mRNA expression. ZIP8, a main Zn(2+) transporter in chondrocytes, was assayed by immunochemistry and in Western blotting. Interleukin (IL)-1β- and ZnCl2-induced increases of [Zn(2+)]i were significantly inhibited by hypoxia. Hypoxia did not reverse a decline of ECM expression caused by IL-1β and ZnCl2 in monolayer cultures, but did significantly attenuate the decreases of proteoglycan, glycosaminoglycan (GAG), and COL2A1 mRNA expression following IL-1β and ZnCl2 treatment in alginate bead cultures. However, ZnCl2 inhibited the protective effect of hypoxia. Both an intracellular Zn(2+) chelator and hypoxia prevented the increase in MMP-13 mRNA expression. IL-1β and ZnCl2 treatment increased ZIP8 expression in NP cells, and hypoxia inhibited ZIP8 expression. In conclusion, decrease of Zn(2+) influx mediates the protective role of hypoxia on ECM and MMP-13 expression. Consequently, changes in intracellular Zn(2+) concentration maybe involved in intervertebral disc degeneration.

Hypoxia facilitates the survival of nucleus pulposus cells in serum deprivation by down-regulating excessive autophagy through restricting ROS generation

Nucleus pulposus (NP) cells reside in a hypoxic environment in vivo, while the mechanisms of how NP cells maintain survival under hypoxia are not clear. Autophagy is an important physiological response to hypoxia and implicated in the survival regulation in most types of cells. This study was designed to investigate the role of autophagy in the survival of NP cells under hypoxia. We found that appropriate autophagy activity was beneficial to the survival of NP cells in serum deprivation, while excessive autophagy led to death of the NP cells. Hypoxia facilitated the survival of NP cells in serum deprivation by down-regulating excessive autophagy. Hypoxia down-regulated the autophagy activity of NP cells through restricting the production of reactive oxygen species (ROS) and inactivating the AMPK/mTOR signaling pathway, and possibly through a pathway involving HIF-1α. We believed that understanding the autophagy response of NP cells to hypoxia and its role in cell survival had important clinical significance in the prevention and treatment of degenerative discogenic diseases.

HIF-1α polymorphism in the susceptibility of cervical spondylotic myelopathy and its outcome after anterior cervical corpectomy and fusion treatment

Background

To investigate the association between the single nucleotide polymorphism (SNP) of hypoxia-inducible factor1 α (HIF-1α) and the susceptibility to cervical spondylotic myelopathy (CSM) and its outcome after surgical treatment.

Method

A total of 230 CSM patients and 284 healthy controls were recruited. All patients received anterior cervical corpectomy and fusion (ACF) and were followed for 12 months. The genotypes for two HIF-1α variants (1772C>T and 1790G>A) were determined.

Results

In the present study, we found that the HIF-1α polymorphism at 1790G>A significantly affects the susceptibility to CSM and its clinical features, including severity and onset age. In addition, the 1790A>G polymorphism also determines the prognosis of CSM patients after ACF treatment. The GG genotype of 1790G>A polymorphism is associated with a higher risk to develop CSM, higher severity and earlier onset age. More importantly, we found that the 1790G>A polymorphism determines the clinical outcome in CSM patients who underwent ACF treatment.

Conclusion

Our findings suggest that the HIF-1α 1790G>A polymorphism is associated with the susceptibility to CSM and can be used as predictor for the clinical outcome in CSM patients receiving ACF treatment.

Responses and adaptations of intervertebral disc cells to microenvironmental stress: a possible central role of autophagy in the adaptive mechanism

Intervertebral discs comprise the largest avascular cartilaginous organ in the body, and its nutrient condition can be impaired by degeneration, aging and even metabolic disease. The unique microenvironment brings special stresses to various disc cell types, including nucleus pulposus cells, notochordal cells, annulus fibrosus cells and endplate chondrocytes. These cells experience nutrient starvation, acidic stress, hypoxic stress, hyperglycemic stress, osmotic stress and mechanical stress. Understanding the detailed responses and complex adaptive mechanisms of disc cells to various stresses might provide some clues to guide therapy for disc degeneration. By reviewing the published literatures describing disc cells under different hostile microenvironments, we conclude that these cells exhibit different responses to microenvironmental stresses with different mechanisms. Moreover, the interaction and combination of these stresses create a complex environment that synergistically increase or decrease influences on disc cells, compared with the effects of a single stress. Interestingly, most of these stresses activate autophagy, a self-protective mechanism by which dysfunctional protein and organelles are degraded. It is becoming clear that autophagy facilitates the cellular adaptation to stresses and might play a central role in regulating the adaptation of disc cells under stress. Therefore, autophagy modulation might be a potential therapeutic method to treat disc degeneration.

Modulating notochordal differentiation of human induced pluripotent stem cells using natural nucleus pulposus tissue matrix

Human induced pluripotent stem cells (hiPSCs) can differentiate into notochordal cell (NC)-like cells when cultured in the presence of natural porcine nucleus pulposus (NP) tissue matrix. The method promises massive production of high-quality, functional cells to treat degenerative intervertebral discs (IVDs). Based on our previous work, we further examined the effect of cell-NP matrix contact and culture medium on the differentiation, and further assessed the functional differentiation ability of the generated NC-like. The study showed that direct contact between hiPSCs and NP matrix can promote the differentiation yield, whilst both the contact and non-contact cultures can generate functional NC-like cells. The generated NC-like cells are highly homogenous regarding the expression of notochordal marker genes. A culture medium containing a cocktail of growth factors (FGF, EGF, VEGF and IGF-1) also supported the notochordal differentiation in the presence of NP matrix. The NC-like cells showed excellent functional differentiation ability to generate NP-like tissue which was rich in aggrecan and collagen type II; and particularly, the proteoglycan to collagen content ratio was as high as 12.5–17.5 which represents a phenotype close to NP rather than hyaline cartilage. Collectively, the present study confirmed the effectiveness and flexibility of using natural NP tissue matrix to direct notochordal differentiation of hiPSCs, and the potential of using the generated NC-like cells for treating IVD degeneration.