Effect of oxygen levels on proteoglycan synthesis by intervertebral disc cells

Suspension bioprinted whole intervertebral disc analogues enable regional stiffness- and hypoxia-regulated matrix secretion by primary human nucleus pulposus and annulus fibrosus cells

Intervertebral disc (IVD) degeneration is a leading cause of back pain, and while studies have revealed the roles resident nucleus pulposus (NP) and annulus fibrosus (AF) cells play in degeneration, tissue-engineered IVD models are needed to better investigate the mechanisms underpinning these cell-driven changes. This study therefore integrated suspension baths with bioprinting to create four multi-material, whole IVD analogues and investigated the combined effect of reduced oxygen tension and increased regional matrix stiffness on disc cell phenotype since these factors correlate with IVD degeneration. Primary NP and AF cells were seeded into alginate-collagen hydrogels and bioprinted into biphasic IVD structures. The nascent area, intensity, and integrated density of pro-collagen type I, collagen type VI, aggrecan, and hyaluronic acid were quantified using immunofluorescence staining in each region. Stiffness-mediated collagen and glycosaminoglycan production was observed in the AF, and increased stiffness downregulated collagen type VI in the AF but upregulated it in NP. Oxygen tension impacted proteoglycan production, with hypoxia increasing aggrecan and hyaluronic acid in both regions. This work represents a step towards the automated biofabrication of whole IVD analogues and expands the state-of-the-art in suspension bioprinting using regionally specific matrix cues. The findings provide important insights into two key microenvironmental factors driving IVD degeneration. STATEMENT OF SIGNIFICANCE: This manuscript outlines an original application of suspended layer additive manufacturing to biofabricate novel, biphasic intervertebral disc analogues containing patient-derived primary human cells. Significantly, the bioprinted models demonstrated biological function and were used to assess the effect of stiffness and oxygen concentration on regional matrix production using a range of internationally-recognized phenotypic intervertebral disc cell markers. The study therefore furthers the state-of-the-art in suspended bioprinting using regionally specific matrix cues and paves the way for future bioprinted disc models that can serve as biosimulators capable of generating insights into key mechanisms governing tissue development, homeostasis, and degeneration.

In silico modeling the potential clinical effect of growth factor treatment on the metabolism of human nucleus pulposus cells

Background

While growth factors have the potential to halt degeneration and decrease inflammation in animal models, the literature investigating the effect of dosage on human cells is lacking. Moreover, despite the completion of clinical trials using growth differentiation factor-5 (GDF-5), no results have been publicly released.

Aims

The overall objective was to quantitatively assess the effect of three clinically relevant concentrations of GDF-5 (0.25, 1, and 2 mg) as a therapeutic for disc regeneration.

Materials and methods

Firstly, this work experimentally determined the effects of GDF-5 concentration on the metabolic and matrix synthesis rates of human nucleus pulposus (NP) cells. Secondly, in silico modeling was employed to predict the subsequent regenerative effect of different GDF-5 treatments (± cells).

Results

This study suggests a trend of increased matrix synthesis with 0.25 and 1 mg of GDF-5. However, 2 mg of GDF-5 significantly upregulates oxygen consumption. Despite this, in silico models highlight the potential of growth factors in promoting matrix synthesis compared to cell-only treatments, without significantly perturbing the nutrient microenvironment.

Discussion

This work elucidates the potential of GDF-5 on human NP cells. Although the results did not reveal statistical differences across all doses, the variability and response among donors is an interesting finding. It highlights the complexity of human response to biological treatments and reinforces the need for further human research and personalized approaches. Furthermore, this study raises a crucial question about whether these potential biologics are more regenerative in nature or better suited as prophylactic therapies for younger patient groups.

Conclusion

Biological agents exhibit unique characteristics and features, demanding tailored development strategies and individualized assessments rather than a one-size-fits-all approach. Therefore, the journey to realizing the full potential of biological therapies is long and costly. Nonetheless, it holds the promise of revolutionizing spinal healthcare and improving the quality of life for patients suffering from discogenic back pain.

Assessment of Tie2-Rejuvenated Nucleus Pulposus Cell Transplants from Young and Old Patient Sources Demonstrates That Age Still Matters

Cell transplantation is being actively explored as a regenerative therapy for discogenic back pain. This study explored the regenerative potential of Tie2+ nucleus pulposus progenitor cells (NPPCs) from intervertebral disc (IVD) tissues derived from young (<25 years of age) and old (>60 years of age) patient donors. We employed an optimized culture method to maintain Tie2 expression in NP cells from both donor categories. Our study revealed similar Tie2 positivity rates regardless of donor types following cell culture. Nevertheless, clear differences were also found, such as the emergence of significantly higher (3.6-fold) GD2 positivity and reduced (2.7-fold) proliferation potential for older donors compared to young sources. Our results suggest that, despite obtaining a high fraction of Tie2+ NP cells, cells from older donors were already committed to a more mature phenotype. These disparities translated into functional differences, influencing colony formation, extracellular matrix production, and in vivo regenerative potential. This study underscores the importance of considering age-related factors in NPPC-based therapies for disc degeneration. Further investigation into the genetic and epigenetic alterations of Tie2+ NP cells from older donors is crucial for refining regenerative strategies. These findings shed light on Tie2+ NPPCs as a promising cell source for IVD regeneration while emphasizing the need for comprehensive understanding and scalability considerations in culture methods for broader clinical applicability.

SOD2 orchestrates redox homeostasis in intervertebral discs: A novel insight into oxidative stress-mediated degeneration and therapeutic potential

Low back pain (LBP) is a pervasive global health concern, primarily associated with intervertebral disc (IVD) degeneration. Although oxidative stress has been shown to contribute to IVD degeneration, the underlying mechanisms remain undetermined. This study aimed to unravel the role of superoxide dismutase 2 (SOD2) in IVD pathogenesis and target oxidative stress to limit IVD degeneration. SOD2 demonstrated a dynamic regulation in surgically excised human IVD tissues, with initial upregulation in moderate degeneration and downregulation in severely degenerated IVDs. Through a comprehensive set of in vitro and in vivo experiments, we found a suggestive association between excessive mitochondrial superoxide, cellular senescence, and matrix degradation in human and mouse IVD cells. We confirmed that aging and mechanical stress, established triggers for IVD degeneration, escalated mitochondrial superoxide levels in mouse models. Critically, chondrocyte-specific Sod2 deficiency accelerated age-related and mechanical stress-induced disc degeneration in mice, and could be attenuated by β-nicotinamide mononucleotide treatment. These revelations underscore the central role of SOD2 in IVD redox balance and unveil potential therapeutic avenues, making SOD2 and mitochondrial superoxide promising targets for effective LBP interventions.

Preclinical to clinical translation for intervertebral disc repair: Effects of species-specific scale, metabolism, and matrix synthesis rates on cell-based regeneration

Background

A significant hurdle for potential cell-based therapies is the subsequent survival and regenerative capacity of implanted cells. While many exciting developments have demonstrated promise preclinically, cell-based therapies for intervertebral disc (IVD) degeneration fail to translate equivalent clinical efficacy.

Aims

This work aims to ascertain the clinical relevance of both a small and large animal model by experimentally investigating and comparing these animal models to human from the perspective of anatomical scale and their cellular metabolic and regenerative potential.

Materials and Methods

First, this work experimentally investigated species-specific geometrical scale, native cell density, nutrient metabolism, and matrix synthesis rates for rat, goat, and human disc cells in a 3D microspheroid configuration. Second, these parameters were employed in silico to elucidate species-specific nutrient microenvironments and predict differences in temporal regeneration between animal models.

Results

This work presents in silico models which correlate favorably to preclinical literature in terms of the capabilities of animal regeneration and predict that compromised nutrition is not a significant challenge in small animal discs. On the contrary, it highlights a very fine clinical balance between an adequate cell dose for sufficient repair, through de novo matrix deposition, without exacerbating the human microenvironmental niche.

Discussion

Overall, this work aims to provide a path towards understanding the effect of cell injection number on the nutrient microenvironment and the "time to regeneration" between preclinical animal models and the large human IVD. While these findings help to explain failed translation of promising preclinical data and the limited results emerging from clinical trials at present, they also enable the research field and clinicians to manage expectations on cell-based regeneration.

Conclusion

Ultimately, this work provides a platform to inform the design of clinical trials, and as computing power and software capabilities increase in the future, it is conceivable that generation of patient-specific models could be used for patient assessment, as well as pre- and intraoperative planning.

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.

Hypoxia Promotes Cartilage Regeneration in Cell-Seeded 3D-Printed Bioscaffolds Cultured with a Bespoke 3D Culture Device

In this study, we investigated the effect of oxygen tension on the expansion of ADMSCs and on their differentiation toward their chondrocytic phenotype, regenerating a lab-based cartilaginous tissue with superior characteristics. Controversial results with reference to MSCs that were cultured under different hypoxic levels, mainly in 2D culturing settings combined with or without other biochemical stimulus factors, prompted our team to study the role of hypoxia on MSCs chondrogenic differentiation within an absolute 3D environment. Specifically, we used 3D-printed honeycomb-like PCL matrices seeded with ADMSCs in the presence or absence of TGF and cultured with a prototype 3D cell culture device, which was previously shown to favor nutrient/oxygen supply, cell adhesion, and infiltration within scaffolds. These conditions resulted in high-quality hyaline cartilage that was distributed uniformly within scaffolds. The presence of the TGF medium was necessary to successfully produce cartilaginous tissues with superior molecular and increased biomechanical properties. Despite hypoxia's beneficial effect, it was overall not enough to fully differentiate ADMSCs or even promote cell expansion within 3D scaffolds alone.

Core-shell oxygen-releasing fibers for annulus fibrosus repair in the intervertebral disc of rats

The repair of annulus fibrosus (AF) defect after discectomy in the intervertebral disc (IVD) has presented a challenge over the past decade. Hostile microenvironments in the IVD, including, compression and hypoxia, are critical issues that require special attention. Till date, little information is available on potential strategies to cope with the hypoxia dilemma in AF defect sites. In this study, perfluorotributylamine (PFTBA) core-shell fibers were fabricated by coaxial electrospinning to construct oxygen-releasing scaffold for promoting endogenous repair in the AF after discectomy. We demonstrated that PFTBA fibers (10% chitosan, chitosan: PCL, 1:6) could release oxygen for up to 144 ​h. The oxygen released from PFTBA fibers was found to protect annulus fibrosus stem cells (AFSCs) from hypoxia-induced apoptosis. In addition, the PFTBA fibers were able to promote proliferation, migration and extracellular matrix (ECM) production in AFSCs under hypoxia, highlighting their therapeutic potential in AF defect repair. Subsequent in vivo studies demonstrated that oxygen-supplying fibers were capable of ameliorating disc degeneration after discectomy, which was evidenced by improved disc height and morphological integrity in rats with the oxygen-releasing scaffolds. Further transcriptome analysis indicated that differential expression genes (DEGs) were enriched in "oxygen transport" and "angiogenesis", which likely contributed to their beneficial effect on endogenous AF regeneration. In summary, the oxygen-releasing scaffold provides novel insights into the oxygen regulation by bioactive materials and raises the therapeutic possibility of oxygen supply strategies for defect repair in AF, as well as other aerobic tissues.

Two- and three-dimensional in vitro nucleus pulposus cultures: An in silico analysis of local nutrient microenvironments

Background

It is well established that the unique biochemical microenvironment of the intervertebral disc plays a predominant role in cell viability and biosynthesis. However, unless the effect of microenvironmental conditions is primary to a study objective, in vitro culture parameters that are critical for reproducibility are both varied and not routinely reported.

Aims

This work aims to investigate the local microenvironments of commonly used culture configurations, highlighting physiological relevance, potential discrepancies, and elucidating possible heterogeneity across the research field.

Materials and Methods

This work uses nutrient-transport in silico models to reflect on the effect of often underappreciated parameters, such as culture geometry and diffusional distance (vessel, media volume, construct size), seeding density, and external boundary conditions on the local microenvironment of two-dimensional (2D) and three-dimensional (3D) in vitro culture systems.

Results

We elucidate important discrepancies between the external boundary conditions such as the incubator level or media concentrations and the actual local cellular concentrations. Oxygen concentration and cell seeding density were found to be highly influential parameters and require utmost consideration when utilizing 3D culture systems.

Discussion

This work highlights that large variations in the local nutrient microenvironment can easily be established without consideration of several key parameters. Without careful deliberation of the microenvironment within each specific and unique system, there is the potential to confound in vitro results leading to heterogeneous results across the research field in terms of biosynthesis and matrix composition.

Conclusion

Overall, this calls for a greater appreciation of key parameters when designing in vitro experiments. Better harmony and standardization of physiologically relevant local microenvironments are needed to push toward reproducibility and successful translation of findings across the research field.

Construction of tissue-engineered nucleus pulposus by stimulation with periodic mechanical stress and BMP-2

Intervertebral disc (IVD) degeneration, which is common among elderly individuals, mainly manifests as low back pain and is caused by structural deterioration of the nucleus pulposus (NP) due to physiological mechanical stress. NP mesenchymal stem cells (NPMSCs) around the IVD endplate have multidirectional differentiation potential and can be used for tissue repair. To define favorable conditions for NPMSC proliferation and differentiation into chondroid cells for NP repair, the present study simulated periodic mechanical stress (PMS) of the NP under physiological conditions using MSC chondrogenic differentiation medium and recombinant human BMP-2 (rhBMP-2). rhBMP-2 effectively promoted NPMSC proliferation and differentiation. To clarify the mechanism of action of rhBMP-2, integrin alpha 1 (ITG A1) and BMP-2 were inhibited. PMS regulated the BMP-2/Smad1/RUNX2 pathway through ITG A1 and promoted NPMSC proliferation and differentiation. During tissue-engineered NP construction, PMS can effectively reduce osteogenic differentiation and promote extracellular matrix protein synthesis to enhance structural NP recovery.

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Extraction of NPMSCs from degenerated nucleus pulposus

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NPMSCs cultured in vitro by simulating physiological mechanical stress

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ITG A1 to promote proliferation and differentiation of NPMSCs through BMP-2/Smad1/RUNX2

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Injectable tissue-engineered nucleus pulposus

Self-Assembling Peptide Hydrogels as Functional Tools to Tackle Intervertebral Disc Degeneration

Low back pain (LBP), caused by intervertebral disc (IVD) degeneration, is a major contributor to global disability. In its healthy state, the IVD is a tough and well-hydrated tissue, able to act as a shock absorber along the spine. During degeneration, the IVD is hit by a cell-driven cascade of events, which progressively lead to extracellular matrix (ECM) degradation, chronic inflammation, and pain. Current treatments are divided into palliative care (early stage degeneration) and surgical interventions (late-stage degeneration), which are invasive and poorly efficient in the long term. To overcome these limitations, alternative tissue engineering and regenerative medicine strategies, in which soft biomaterials are used as injectable carriers of cells and/or biomolecules to be delivered to the injury site and restore tissue function, are currently being explored. Self-assembling peptide hydrogels (SAPHs) represent a promising class of de novo synthetic biomaterials able to merge the strengths of both natural and synthetic hydrogels for biomedical applications. Inherent features, such as shear-thinning behaviour, high biocompatibility, ECM biomimicry, and tuneable physiochemical properties make these hydrogels appropriate and functional tools to tackle IVD degeneration. This review will describe the pathogenesis of IVD degeneration, list biomaterials requirements to attempt IVD repair, and focus on current peptide hydrogel materials exploited for this purpose.

Consolidating and re-evaluating the human disc nutrient microenvironment

Background

Despite exciting advances in regenerative medicine, cell-based strategies for treating degenerative disc disease remain in their infancy. To maximize the potential for successful clinical translation, a more thorough understanding of the in vivo microenvironment is needed to better determine and predict how cell therapies will respond when administered in vivo.

Aims

This work aims to reflect on the in vivo nutrient microenvironment of the degenerating IVD through consolidating what has already been measured together with investigative in silico models.

Materials and Methods

This work uses in silico modeling, underpinned by more recent experimentally determined parameters of degeneration and nutrient transport from the literature, to re-evaluate the current knowledge in terms of grade-specific stages of degeneration.

Results

Through modeling only the metabolically active cell population, this work predicts slightly higher glucose concentrations compared to previous in silico models, while the predicted results show good agreement with previous intradiscal pH and oxygen measurements. Increasing calcification with degeneration limits nutrient transport into the IVD and initiates a build-up of acidity; however, its effect is compensated somewhat by a reduction in diffusional distance due to decreasing disc height.

Discussion

This work advances in silico modeling through a strong foundation of experimentally determined grade-specific input parameters. Taken together, pre-existing measurements and predicted results suggest that metabolite concentrations may not be as critically low as commonly believed, with calcification not appearing to have a detrimental effect at stages of degeneration when cell therapies are an appropriate intervention.

Conclusion

Overall, our initiative is to provoke greater deliberation and consideration of the nutrient microenvironment when performing in vitro cell culture and cell therapy development. This work highlights urgency for robust experimental glucose measurements in healthy and degenerating IVDs, not only to validate in silico models but to significantly advance the field in fully elucidating the nutrient microenvironment and refining in vitro techniques to accelerate clinical translation.

Should Degenerated Intervertebral Discs of Patients with Modic Type 1 Changes Be Treated with Mesenchymal Stem Cells?

Low back pain (LBP) has been among the leading causes of disability for the past 30 years. This highlights the need for improvement in LBP management. Many clinical trials focus on developing treatments against degenerative disc disease (DDD). The multifactorial etiology of DDD and associated risk factors lead to a heterogeneous patient population. It comes as no surprise that the outcomes of clinical trials on intradiscal mesenchymal stem cell (MSC) injections for patients with DDD are inconsistent. Intradiscal MSC injections have demonstrated substantial pain relief and significant disability-related improvements, yet they have failed to regenerate the intervertebral disc (IVD). Increasing evidence suggests that the positive outcomes in clinical trials might be attributed to the immunomodulatory potential of MSCs rather than to their regenerative properties. Therefore, patient stratification for inflammatory DDD phenotypes may (i) better serve the mechanisms of action of MSCs and (ii) increase the treatment effect. Modic type 1 changes—pathologic inflammatory, fibrotic changes in the vertebral bone marrow—are frequently observed adjacent to degenerated IVDs in chronic LBP patients and represent a clinically distinct subpopulation of patients with DDD. This review discusses whether degenerated IVDs of patients with Modic type 1 changes should be treated with an intradiscal MSC injection.

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.

Advances and Prospects in Biomaterials for Intervertebral Disk Regeneration

Low-back and neck-shoulder pains caused by intervertebral disk degeneration are highly prevalent among middle-aged and elderly people globally. The main therapy method for intervertebral disk degeneration is surgical intervention, including interbody fusion, disk replacement, and diskectomy. However, the stress changes caused by traditional fusion surgery are prone to degeneration of adjacent segments, while non-fusion surgery has problems, such as ossification of artificial intervertebral disks. To overcome these drawbacks, biomaterials that could endogenously regenerate the intervertebral disk and restore the biomechanical function of the intervertebral disk is imperative. Intervertebral disk is a fibrocartilaginous tissue, primarily comprising nucleus pulposus and annulus fibrosus. Nucleus pulposus (NP) contains high water and proteoglycan, and its main function is absorbing compressive forces and dispersing loads from physical activities to other body parts. Annulus fibrosus (AF) is a multilamellar structure that encloses the NP, comprises water and collagen, and supports compressive and shear stress during complex motion. Therefore, different biomaterials and tissue engineering strategies are required for the functional recovery of NP and AF based on their structures and function. Recently, great progress has been achieved on biomaterials for NP and AF made of functional polymers, such as chitosan, collagen, polylactic acid, and polycaprolactone. However, scaffolds regenerating intervertebral disk remain unexplored. Hence, several tissue engineering strategies based on cell transplantation and growth factors have been extensively researched. In this review, we summarized the functional polymers and tissue engineering strategies of NP and AF to endogenously regenerate degenerative intervertebral disk. The perspective and challenges of tissue engineering strategies using functional polymers, cell transplantation, and growth factor for generating degenerative intervertebral disks were also discussed.

Hypoxia Regulates the Extracellular Matrix via Mitogen-Activated Protein Kinases Pathway in Cells Retrieved from the Human Intervertebral Disc

PURPOSE

The present study aimed to identify the physiological characteristics of cells by investigating the change in gene expression and protein levels during extracellular matrix (ECM) synthesis in the intervertebral disc (IVD) under hypoxic conditions.

MATERIALS AND METHODS

To test the effect of oxygen on cell growth and ECM synthesis of chondrocyte-like cells, the cells from IVD were separated and cultured in two hypoxia-mimicking systems: chemical hypoxic conditions using deferoxamine (DFO), and physiological hypoxic conditions using a hypoxic chamber for 7 days. Chondrocyte like cells cultured without DFO and under the normal oxygen concentration (21% O₂ and 5% CO₂, 37°C) served as the controls.

RESULTS

Chondrocyte-like cells cultured in the presence of 6% oxygen demonstrated a 100% increase in cellular proliferation compared to the control. The cells treated with chemical hypoxic conditions demonstrated a dose-dependent increase in the mRNA expression of glucose transporter-1, GAPDH, aggrecan, and type II collagen on Day 1. When treated with 100 µM DFO, the cells showed a 50% increase in the levels of proteoglycan protein on Day 7. The cells treated with chemical hypoxic condition demonstrated increase in sulfated glycosaminoglycan (GAG) protein levels on Day 7. Moreover, the cells cultured in the presence of 6% oxygen showed a 120% increase in sulfated GAG levels on Day 7.

CONCLUSION

The oxygen concentration had an important role in the viability, proliferation, and maturation of chondrocyte-like cells in IVD. In addition, chondrocyte-like cells are sensitive to the concentration of oxygen.

Simulating intervertebral disc cell behaviour within 3D multifactorial environments

MOTIVATION

Low back pain is responsible for more global disability than any other condition. Its incidence is closely related to intervertebral disc (IVD) failure, which is likely caused by an accumulation of microtrauma within the IVD. Crucial factors in microtrauma development are not entirely known yet, probably because their exploration in vivo or in vitro remains tremendously challenging. In silico modelling is, therefore, definitively appealing, and shall include approaches to integrate influences of multiple cell stimuli at the microscale. Accordingly, this study introduces a hybrid Agent-based (AB) model in IVD research and exploits network modelling solutions in systems biology to mimic the cellular behaviour of Nucleus Pulposus cells exposed to a 3D multifactorial biochemical environment, based on mathematical integrations of existing experimental knowledge. Cellular activity reflected by mRNA expression of Aggrecan, Collagen type I, Collagen type II, MMP-3 and ADAMTS were calculated for inflamed and non-inflamed cells. mRNA expression over long periods of time is additionally determined including cell viability estimations. Model predictions were eventually validated with independent experimental data.

RESULTS

As it combines experimental data to simulate cell behaviour exposed to a multifactorial environment, the present methodology was able to reproduce cell death within 3 days under glucose deprivation and a 50% decrease in cell viability after 7 days in an acidic environment. Cellular mRNA expression under non-inflamed conditions simulated a quantifiable catabolic shift under an adverse cell environment, and model predictions of mRNA expression of inflamed cells provide new explanation possibilities for unexpected results achieved in experimental research.

AVAILABILITYAND IMPLEMENTATION

The AB model as well as used mathematical functions were built with open source software. Final functions implemented in the AB model and complete AB model parameters are provided as Supplementary Material. Experimental input and validation data were provided through referenced, published papers. The code corresponding to the model can be shared upon request and shall be reused after proper training.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Investigating the physiological relevance of ex vivo disc organ culture nutrient microenvironments using in silico modeling and experimental validation

BACKGROUND

Ex vivo disc organ culture systems have become a valuable tool for the development and pre-clinical testing of potential intervertebral disc (IVD) regeneration strategies. Bovine caudal discs have been widely selected due to their large availability and comparability to human IVDs in terms of size and biochemical composition. However, despite their extensive use, it remains to be elucidated whether their nutrient microenvironment is comparable to human degeneration.

AIMS

This work aims to create the first experimentally validated in silico model which can be used to predict and characterize the metabolite concentrations within ex vivo culture systems.

MATERIALS & METHODS

Finite element models of cultured discs governed by previously established coupled reaction-diffusion equations were created using COMSOL Multiphysics. Experimental validation was performed by measuring oxygen, glucose and pH levels within discs cultured for 7 days, in a static compression bioreactor.

RESULTS

The in silico model was successfully validated through good agreement between the predicted and experimentally measured concentrations. For an ex vivo organ cultured in high glucose medium (4.5 g/L or 25 mM) and normoxia, a larger bovine caudal disc (Cd1-2 to Cd3-4) had a central concentration of ~2.6 %O2, ~8 mM of glucose and a pH value of 6.7, while the smallest caudal discs investigated (Cd6-7 and Cd7-8), had a central concentration of ~6.5 %O2, ~12 mM of glucose and a pH value of 6.9.

DISCUSSION

This work advances the knowledge of ex vivo disc culture microenvironments and highlights a critical need for optimization and standardization of culturing conditions.

CONCLUSION

Ultimately, for assessment of cell-based therapies and successful clinical translation based on nutritional demands, it is imperative that the critical metabolite values within organ cultures (minimum glucose, oxygen and pH values) are physiologically relevant and comparable to the stages of human degeneration.

Investigating the role of culture conditions on hypertrophic differentiation in human cartilage endplate cells

Cartilage endplate degeneration/calcification has been linked to the onset and progression of intervertebral disc degeneration and there is a critical need to understand mechanisms, such as hypertrophic differentiation, of cartilage endplate degeneration/calcification to inform treatment strategies for discogenic back pain. In vitro cell culture conditions capable of inducing hypertrophic differentiation are used to study pathophysiological mechanisms in articular chondrocytes, but culture conditions capable of inducing a hypertrophic cartilage endplate cell phenotype have yet to be explored. The goal of this study was to investigate the role of culture conditions capable of inducing hypertrophic differentiation in articular chondrocytes on hypertrophic differentiation in human cartilage endplate cells. Isolated human cartilage endplate cells were cultured as pellets for 21 days at either 5% O₂ (physiologic for cartilage) or 20.7% O₂ (hyperoxic) and treated with 10% fetal bovine serum or Wnt agonist, two stimuli used to induce hypertrophic differentiation in articular chondrocytes. Cartilage endplate cells did not exhibit a hypertrophic cell morphology in response to fetal bovine serum or Wnt agonist but did display other hallmarks of chondrocyte hypertrophy and degeneration such as hypertrophic gene and protein expression, and a decrease in healthy proteoglycans and an increase in fibrous collagen accumulation. These findings demonstrate that cartilage endplate cells take on a degenerative phenotype in response to hypertrophic stimuli in vitro, but do not undergo classical changes in morphology associated with hypertrophic differentiation regardless of oxygen levels, highlighting potential differences in the response of cartilage endplate cells versus articular chondrocytes to the same stimuli.

A microstructure-based modeling approach to assess aging-sensitive mechanics of human intervertebral disc

BACKGROUND AND OBJECTIVE

The human body soft tissues are hierarchic structures interacting in a complex manner with the surrounding biochemical environment. The loss of soft tissues functionality with age leads to more vulnerability regarding to the external mechanical loadings and increases the risk of injuries. As a main example of the human body soft tissues, the intervertebral disc mechanical response evolution with age is explored. Although the age-dependence of the intervertebral disc microstructure is a well-known feature, no noticeable age effect on the disc stiffness is evidenced in the in-vitro experimental studies of the literature. So, if the disc intrinsic mechanics remains constant, how to explain the correlation of disc degeneration and disc functionality loss with age.

METHODS

A microstructure-based modeling approach was developed to assess in-silico the aging-sensitive mechanics of human intervertebral disc. The model considers the relationship between stress/volumetric macro-response and microstructure along with effective age effects acting at the lamellar and multi-lamellar scales. The stress-stretch and transversal responses of the different disc regions were computed for various age groups (13-18, 36, 58, 69 and 82 years old) and their evolution with age was studied.

RESULTS

While matching with in-vitro experimental data, the predicted stiffness was found to increase while passing from adolescent young discs to mature older discs and then to remain almost constant for the rest of life. Important age-related changes in the disc transversal behavior were also predicted affecting the flexibility of the disc, changing its volumetric behavior, and modifying its dimensions.

CONCLUSION

The developed approach was found able to bring new conclusions about age-dependent mechanical properties including regional dependency. The disc mechanics in terms of rigidity, radial and axial transversal responses were found to alter going from adolescent to middle age where the disc reaches a certain maturity. After reaching maturity, the mechanical properties undergo very slight changes until becoming almost constant with age.

The potential role and trend of HIF‑1α in intervertebral disc degeneration: Friend or foe? (Review)

Lower back pain (LBP) is one of the most common reasons for seeking medical advice in orthopedic clinics. Increasingly, research has shown that symptomatic intervertebral disc degeneration (IDD) is mostly related to LBP. This review first outlines the research and findings of studies into IDD, from the physiological structure of the intervertebral disc (IVD) to various pathological cascades. The vicious cycles of IDD are re-described in relation to the analysis of the relationship among the pathological mechanisms involved in IDD. Interestingly, a ‘chief molecule’ was found, hypoxia-inducible factor-1α (HIF-1α), that may regulate all other mechanisms involved in IDD. When the vicious cycle is established, the low oxygen tension activates the expression of HIF-1α, which subsequently enters into the hypoxia-induced HIF pathways. The HIF pathways are dichotomized as friend and foe pathways according to the oxygen tension of the IVD microenvironment. Combined with clinical outcomes and previous research, the trend of IDD development has been predicted in this paper. Lastly, an early precautionary diagnosis and treatment method is proposed whereby nucleus pulposus tissue for biopsy can be obtained through IVD puncture guided by B-ultrasound when the patient is showing symptoms but MRI imaging shows negative results. The assessment criteria for biopsy and the feasibility, superiority and challenges of this approach have been discussed. Overall, it is clear that HIF-1α is an indispensable reference indicator for the accurate diagnosis and treatment of IDD.

Long Noncoding RNA ANPODRT Overexpression Protects Nucleus Pulposus Cells from Oxidative Stress and Apoptosis by Activating Keap1-Nrf2 Signaling

Oxidative stress and subsequent nucleus pulposus (NP) cell apoptosis are important contributors to the development of intervertebral disc degeneration (IDD). Emerging evidences show that long noncoding RNAs (lncRNAs) play a role in the pathogenesis of IDD. In this study, we investigated the role of lncRNA ANPODRT (anti-NP cell oxidative damage-related transcript) in oxidative stress and apoptosis in human NP cells. We found that ANPODRT was downregulated in degenerative NP tissues and in NP cells treated with tert-butyl hydroperoxide (TBHP, the oxidative stress inducer). ANPODRT overexpression alleviated oxidative stress and apoptosis in NP cells exposed to TBHP, while ANPODRT knockdown exerted opposing effects. Mechanistically, ANPODRT facilitated nuclear factor E2-related factor 2 (Nrf2) accumulation and nuclear translocation and activated its target genes by disrupting the kelch-like ECH-associated protein 1- (Keap1-) Nrf2 association in NP cells. Nrf2 knockdown abolished the antioxidative stress and antiapoptotic effects of ANPODRT in NP cells treated with TBHP. Collectively, our findings suggest that ANPODRT protects NP cells from oxidative stress and apoptosis, at least partially, by activating Nrf2 signaling, implying that ANPODRT may be a potential therapeutic target for IDD.

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.

Spheroid-Like Cultures for Expanding Angiopoietin Receptor-1 (aka. Tie2) Positive Cells from the Human Intervertebral Disc

Lower back pain is a leading cause of disability worldwide. The recovery of nucleus pulposus (NP) progenitor cells (NPPCs) from the intervertebral disc (IVD) holds high promise for future cell therapy. NPPCs are positive for the angiopoietin-1 receptor (Tie2) and possess stemness capacity. However, the limited Tie2+ NPC yield has been a challenge for their use in cell-based therapy for regenerative medicine. In this study, we attempted to expand NPPCs from the whole NP cell population by spheroid-formation assay. Flow cytometry was used to quantify the percentage of NPPCs with Tie2-antibody in human primary NP cells (NPCs). Cell proliferation was assessed using the population doublings level (PDL) measurement. Synthesis and presence of extracellular matrix (ECM) from NPC spheroids were confirmed by quantitative Polymerase Chain Reaction (qPCR), immunostaining, and microscopy. Compared with monolayer, the spheroid-formation assay enriched the percentage of Tie2+ in NPCs’ population from ~10% to ~36%. Moreover, the spheroid-formation assay also inhibited the proliferation of the Tie2- NPCs with nearly no PDL. After one additional passage (P) using the spheroid-formation assay, NPC spheroids presented a Tie2+ percentage even further by ~10% in the NPC population. Our study concludes that the use of a spheroid culture system could be successfully applied to the culture and expansion of tissue-specific progenitors.

Exosomes Derived from Human Urine-Derived Stem Cells Inhibit Intervertebral Disc Degeneration by Ameliorating Endoplasmic Reticulum Stress

Objective

This study is aimed at determining the effects of human urine-derived stem cell-derived exosomes (USCs-exos) on pressure-induced nucleus pulposus cell (NPC) apoptosis and intervertebral disc degeneration (IDD) and on the ERK and AKT signaling pathways.

Methods

The NPCs were obtained from patients with herniated lumbar discs. Western blot analysis (WB) and quantitative real-time polymerase chain reaction (qRT-PCR) were used to determine endoplasmic reticulum (ER) stress levels of NPCs under stress. Human USCs were identified using an inverted microscope, three-line differentiation experiments, and flow cytometry. A transmission microscope, nanoparticle size analysis, and WB procedures were used to identify the extracted exosomes and observe NPC uptake. A control group, a 48 h group, and a USCs-exos group were established. The control group was untreated, and the 48 h group was pressure-trained for 48 h, while the USCs-exos group was pressure-trained for 48 h and treated with USCs-exos. WB, qRT-PCR, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) analysis were used to determine the ER stress levels in stress conditions and after exosomal treatment. The AKT and ERK pathways were partially detected. Magnetic Resonance Imaging (MRI) and computed tomography (CT) were used to evaluate cell degeneration while exosomal effects on the intervertebral disc (IVD) tissue were determined by hematoxylin and eosin (HE) staining, Safranin O-fast green staining, immunohistochemical staining (IHC), nuclear magnetic resonance (NMR), spectrometric detection, and total correlation spectroscopy (TOCSY). IVD metabolites were also identified and quantified.

Results

After pressure culture, ER stress markers (GRP78 and C/EBP homologous protein (CHOP)) in the NPCs were significantly elevated with time (p < 0.05). Human USCs are short and spindle-shaped. They can successfully undergo osteogenic, adipogenic, and chondrogenic differentiation. In this study, these stem cells were found to be positive for CD29, CD44, and CD73. The exosomes were centrally located with a diameter of 50-100 nm. CD63 and Tsg101 were highly expressed while the expression of Calnexin was suppressed. The exosomes can be ingested by NPCs. USCs-exos significantly improved ER stress responses and inhibited excessive activation of the unfolded protein response (UPR) as well as cell apoptosis and disc degeneration through the AKT and ERK signaling pathways (p < 0.05).

Conclusion

Through the AKT and ERK signaling pathways, USCs-exos significantly inhibit ER stress-induced cell apoptosis and IDD under pressure conditions. It is, therefore, a viable therapeutic strategy.

Intervertebral Disc Regeneration Using Stem Cell/Growth Factor-Loaded Porous Particles with a Leaf-Stacked Structure

Although biological therapies based on growth factors and transplanted cells have demonstrated some positive outcomes for intervertebral disc (IVD) regeneration, repeated injection of growth factors and cell leakage from the injection site remain considerable challenges for human therapeutic use. Herein, we prepare human bone marrow-derived mesenchymal stem cells (hBMSCs) and transforming growth factor-β3 (TGF-β3)-loaded porous particles with a unique leaf-stack structural morphology (LSS particles) as a combination bioactive delivery matrix for degenerated IVD. The LSS particles are fabricated with clinically acceptable biomaterials (polycaprolactone and tetraglycol) and procedures (simple heating and cooling). The LSS particles allow sustained release of TGF-β3 for 18 days and stable cell adhesiveness without additional modifications of the particles. On the basis of in vitro and in vivo studies, it was observed that the hBMSCs/TGF-β3-loaded LSS particles can provide a suitable milieu for chondrogenic differentiation of hBMSCs and effectively induce IVD regeneration in a beagle dog model. Thus, therapeutically loaded LSS particles offer the promise of an effective bioactive delivery system for regeneration of various tissues including IVD.

Biomaterials-Induced Stem Cells Specific Differentiation Into Intervertebral Disc Lineage Cells

Stem cell therapy, which promotes stem cells differentiation toward specialized cell types, increases the resident population and production of extracellular matrix, and can be used to achieve intervertebral disc (IVD) repair, has drawn great attention for the development of IVD-regenerating materials. Many materials that have been reported in IVD repair have the ability to promote stem cells differentiation. However, due to the limitations of mechanical properties, immunogenicity and uncontrollable deviations in the induction of stem cells differentiation, there are few materials that can currently be translated into clinical applications. In addition to the favorable mechanical properties and biocompatibility of IVD materials, maintaining stem cells activity in the local niche and increasing the ability of stem cells to differentiate into nucleus pulposus (NP) and annulus fibrosus (AF) cells are the basis for promoting the application of IVD-regenerating materials in clinical practice. The purpose of this review is to summarize IVD-regenerating materials that focus on stem cells strategies, analyze the properties of these materials that affect the differentiation of stem cells into IVD-like cells, and then present the limitations of currently used disc materials in the field of stem cell therapy and future research perspectives.

The protective effects of PI3K/Akt pathway on human nucleus pulposus mesenchymal stem cells against hypoxia and nutrition deficiency

BACKGROUND

To study the effects of hypoxia and nutrition deficiency mimicking degenerated intervertebral disc on the biological behavior of human nucleus-derived pulposus mesenchymal stem cells (hNP-MSCs) and the role of PI3K/Akt pathway in the process in vitro.

METHODS

hP-MSCs were isolated from lumbar disc and were further identified by their immunophenotypes and multilineage differentiation. Then, cells were divided into the control group, hypoxia and nutrition deficiency group, the LY294002 group, and insulin-like growth factor 1 (IGF-1) group. Then cell apoptosis, the cell viability, the caspase 3 activity, and the expression of PI3K, Akt, and functional genes (aggrecan, collagen I, and collagen II) were evaluated.

RESULT

Our work showed that isolated cells met the criteria of International Society for cellular Therapy. Therefore, cells obtained from degenerated nucleus pulposus were definitely hNP-MSCs. Our results showed that hypoxia and nutrition deficiency could significantly increase cell apoptosis, the caspase 3 activity, and inhibit cell viability. Gene expression results demonstrated that hypoxia and nutrition deficiency could increase the relative expression of PI3K and Akt gene and inhibit the expression of functional genes. However, when the PI3K/Akt pathway was inhibited by LY294002, the cell apoptosis and caspase 3 activity significantly increased while the cell viability was obviously inhibited. Quantitative real-time PCR results showed that the expression of functional genes was more significantly inhibited. Our study further verified that the above-mentioned biological activities of hNP-MSCs could be significantly improved by IGF1.

CONCLUSIONS

PI3K/Akt signal pathway may have protective effects on human nucleus pulposus-derived mesenchymal stem cells against hypoxia and nutrition deficiency.

Effect of diminished flow in rabbit lumbar arteries on intervertebral disc matrix changes using MRI T2-mapping and histology

Background

Impaired lumbar artery flow has been reported in clinical and epidemiological studies to be associated with low back pain and lumbar disc degeneration. However, it has not been experimentally demonstrated that impaired lumbar artery flow directly induces intervertebral disc (IVD) degeneration by affecting IVD matrix metabolism. The purpose of this study was to evaluate whether ligation of the lumbar artery can affect degenerative changes in the rabbit IVD.

Methods

New Zealand White rabbits (n = 20) were used in this study. Under general anesthesia, the third and fourth lumbar arteries were double-ligated using vascular clips. The blood flow to the L3/L4 disc (cranial disc) was reduced by ligation of the third lumbar artery and that of the L5/L6 disc (caudal disc) by ligation of the fourth lumbar artery. The blood flow to the L4/L5 disc (bilateral disc) was decreased by ligation of both the third and fourth lumbar arteries. The L2/L3 disc was used as the control. Disc height was radiographically monitored biweekly until 12 weeks after surgery. The rabbits were sacrificed at 4, 8, and 12 weeks after surgery and magnetic resonance imaging (MRI) T2-mapping, histology and immunohistochemistry were assessed.

Results

Lumbar artery ligation did not induce significant changes in disc height between control and ischemic discs (cranial, bilateral and caudal discs) during the 12-week experimental period. T2-values of ischemic discs had no significant trend to be lower than those of the control L2/L3 discs. Histologically, Safranin-O staining changed following ligation of corresponding IVD lumbar arteries. Histological grading scores for disc degeneration, which correlated significantly with MRI T2-values, had significant changes after the surgery. Immunohistochemical analysis showed that the ligation of lumbar arteries significantly affected a change in the percentage of HIF-1α immunoreactive cells of ischemia discs compared to that of control discs four weeks after the surgery (p < 0.05).

Conclusions

The MRI and histology results suggest that diminished flow in lumbar arteries induce mild changes in the extracellular matrix metabolism of rabbit IVDs. These matrix changes, however, were not progressive and differed from the degenerative disc changes seen in the process of human IVD degeneration.

Effects of Oxygen Concentration and Culture Time on Porcine Nucleus Pulposus Cell Metabolism: An in vitro Study

Low back pain is a common ailment that affects millions of individuals each year and is linked to degeneration of the intervertebral discs in the spine. Intervertebral disc degeneration is known to result from an imbalance in anabolic and catabolic activity by disc cells. Due to the avascular nature of the intervertebral disc, oxygen deficiency may occur in the central nucleus pulposus (NP). The resulting hypoxia affects matrix regulation and energy metabolism of disc cells, although the mechanisms are not fully understood. This study investigates in vitro glucose consumption and gene expression by NP cells over time under varying oxygen tensions. Notochordal porcine NP cells were cultured in agarose discs at 21, 5, or 1% oxygen tension for 1, 5, or 10 days. The expression of 10 key matrix genes, as well as Brachyury (T), by NP cells was analyzed using RT-PCR. Glucose consumption was measured using a two-point method. Results show that culture time and oxygen tension significantly affect glucose consumption rates by porcine NP cells. There were also significant changes in T expression based on oxygen level and culture time. The 1% oxygen tension had a significantly higher T expression on day 10 than the other two groups, which may indicate a better maintenance of the notochordal phenotype. MMP 1 and 13 expression increased over time for all groups, while only the 5% group showed an increase over time for MMP 3. TIMP expression followed the direction of MMPs but to a lesser magnitude. Five percent and twenty-one percent oxygen tensions led to decreases in anabolic gene expression while 1% led to increases. Oxygen concentration and culture time significantly impacted glucose consumption rate and the gene expression of matrix regulatory genes with hypoxic conditions most accurately maintaining the proper NP phenotype. This information is valuable not only for understanding disc pathophysiology, but also for harnessing the potential of notochordal NP cells in therapeutic applications.

The Role of Hypoxia in Angiogenesis and Extracellular Matrix Regulation of Intervertebral Disc Cells During Inflammatory Reactions

BACKGROUND

The intervertebral disc (IVD) is an avascular structure, and is therefore stable under hypoxic conditions. Previous studies have demonstrated that hypoxia might be related to symptomatic degenerative disc diseases (DDDs); however, the pathomechanism is still poorly understood.

OBJECTIVE

To identify the effect of hypoxia on the production of inflammatory mediators, angiogenic factors, and extracellular matrix-regulating enzymes of IVD cells during inflammatory reactions.

METHODS

Human nucleus pulposus (NP) and annulus fibrosus (AF) cells harvested during surgery for DDDs were cultured in macrophage conditioned media or interleukin (IL)-1β-stimulated media under hypoxic (2%) and normoxic (21%) conditions. Hypoxia-inducible factor-1α transcription factor activation was analyzed by western blotting. IL-6, IL-8, vascular endothelial growth factor (VEGF), vascular cell adhesion molecule (VCAM), matrix metalloproteinase (MMP)-1, MMP-3, tissue inhibitor of metalloprotease (TIMP)-1, and TIMP-2 in conditioned media were measured by an enzyme-linked immunosorbent assay.

RESULTS

NP cells expressed higher hypoxia-inducible factor-1α in the IL-1β-stimulated group under hypoxic condition. MMP-1 was significantly increased in the AF cells under hypoxic condition; TIMP-1 and TIMP-2 were significantly decreased in both naïve NP and AF cells during hypoxia. Both cells in macrophage conditioned media significantly diminished the production of IL-6 and VCAM, while VEGF significantly increased during hypoxia. After 1 ng/mL IL-1β stimulation, IL-8, VEGF, MMP-1, and MMP-3 were significantly increased in both cell types during hypoxia, while VCAM, TIMP-1, and TIMP-2 were decreased.

CONCLUSION

We found that hypoxia can enhance the angiogenic ability of IVD during inflammatory reactions, and cause progress in development of DDD via extracellular matrix regulation in this in vitro study.

Link protein N-terminal peptide and fullerol promote matrix production and decrease degradation enzymes in rabbit annulus cells

PURPOSE

Intervertebral disc degeneration is a major cause of back pain. Novel therapies for prevention or reversal of disc degeneration are needed. It is desirable for potential therapies to target both inflammation and matrix degeneration.

MATERIALS AND METHODS

The combined regenerative potential of link protein N-terminal peptide (LN) and fullerol on annulus fibrosus (AF) cells was evaluated in a 3D culture model.

RESULTS

Interleukin-1α (IL-1α)-induced AF cell degeneration was counteracted by fullerol, LN, and fullerol + LN, with the latter having the greatest effect on matrix production as evaluated by real-time polymerase chain reaction and glycosaminoglycan assay. IL-1α-induced increases in pro-inflammatory mediators (interleukin-6 and cyclooxygenase-2) and matrix metalloproteinases (MMP-1, -2, -9, and -13) were also counteracted by fullerol and LN.

CONCLUSION

Our data demonstrate that LN and fullerol individually, and in combination, promote matrix production and have anti-inflammatory and anti-catabolic effects on AF cells.

Influence of rabbit notochordal cells on symptomatic intervertebral disc degeneration: anti-angiogenic capacity on human endothelial cell proliferation under hypoxia

OBJECTIVES

Symptomatic degenerative disc disease (DDD) is associated with neovascularization and nerve ingrowth into intervertebral discs (IVDs). Notochordal cells (NCs) are key cells that may lead to regeneration of IVDs. However, their activities under conditions of hypoxia, the real environment of IVD, are not well known. We hypothesized that NCs may inhibit neovascularization by interacting with endothelial cells (ECs) under hypoxia.

DESIGN

Human IVDs were isolated and cultured to produce nucleus pulposus (NP) cell conditioned medium (NPCM). Immortalized human microvascular ECs were cultured in NPCM with notochordal cell-rich rabbit nucleus pulposus cells (rNC) under hypoxia. Vascular endothelial growth factor (VEGF), vascular cell adhesion molecule (VCAM), and interleukin-8 (IL-8) were analyzed by ELISA. Focal adhesion kinase (FAK), filamentous actin (F-actin), and platelet-derived growth factor (PDGF) were evaluated to investigate EC activity. Wound-healing migration assays were performed to examine EC migration.

RESULTS

The VEGF level of EC cells cultured in NPCM was significantly higher under hypoxia compared to normoxia. VEGF expression was significantly decreased, and FAK, F-actin, PDGF expression were inhibited when ECs were cocultured with rNCs under hypoxia. ECs cocultured with rNC in NPCM showed significantly decreased migratory activity compared to those without rNC under hypoxia.

CONCLUSIONS

The angiogenic capacity of ECs was significantly inhibited by NCs under hypoxia via a VEGF-related pathway. Our results suggest that NCs may play a key role in the development of IVDs by inhibiting vascular growth within the disc, and this may be a promising novel therapeutic strategy for targeting vascular ingrowth in symptomatic DDD.

Hypoxic Expansion of Human Mesenchymal Stem Cells Enhances Three-Dimensional Maturation of Tissue-Engineered Intervertebral Discs

Culture of three-dimensional (3D) constructs in hypoxic conditions (1-5% O₂) has been shown to increase production of extracellular matrix components in primary intervertebral disc (IVD) cells and drive chondrogenesis of human mesenchymal stem cells (hMSCs). Growing evidence suggests that two-dimensional (2D) expansion under hypoxic conditions may have an even greater influence on chondrogenesis in MSCs. This study aims to determine the effects of hypoxia during 2D expansion and subsequent 3D culture on the in vitro maturation of tissue-engineered IVDs (TE-IVDs) made with hMSCs, using a previously developed TE-IVD system. hMSCs were expanded in either hypoxic (5% O₂) or normoxic (21% O₂) conditions before construction of TE-IVDs. Discs were cultured in 3D in either hypoxic or normoxic conditions to create four experimental groups. Discs made from MSCs expanded in hypoxia were up to 141% stiffer than those made with normoxia-expanded MSCs. Similar patterns were seen in all mechanical properties. Increases in glycosaminoglycan content and collagen content in the nucleus pulposus (NP) were associated with 3D hypoxic culture. A boundary region between the manufactured fibrosus and NP regions developed by 2 weeks and mimicked the organization of the native disc. Hypoxic conditions in both 2D expansion and subsequent 3D culture improved the maturation of TE-IVDs made with hMSCs.

Tissue Engineering of the Intervertebral Disc's Annulus Fibrosus: A Scaffold-Based Review Study

Tissue engineering as a high technology solution for treating disc's problem has been the focus of some researches recently; however, the upcoming successful results in this area depends on understanding the complexities of biology and engineering interface. Whereas the major responsibility of the nucleus pulposus is to provide a sustainable hydrated environment within the disc, the function of the annulus fibrosus (AF) is more mechanical, facilitating joint mobility and preventing radial bulging by confining of the central part, which makes the AF reconstruction important. Although the body of knowledge regarding the AF tissue engineering has grown rapidly, the opportunities to improve current understanding of how artificial scaffolds are able to mimic the AF concentric structure-including inter-lamellar matrix and cross-bridges-addressed unresolved research questions. The aim of this literature review was to collect and discuss, from the international scientific literature, information about tissue engineering of the AF based on scaffold fabrication and material properties, useful for developing new strategies in disc tissue engineering. The key parameter of this research was understanding if role of cross-bridges and inter-lamellar matrix has been considered on tissue engineering of the AF.

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.

Inorganic polyphosphates enhances nucleus pulposus tissue formation in vitro

Disc degeneration is associated with low back pain for which currently there is no optimal therapy so there is a great need to identify new treatment approaches. Inorganic polyphosphates (polyP) are linear polymers of orthophosphate units varying in chain length and present in many cell types. As polyP has anabolic effects on chondrocytes, we hypothesized that polyP treatment would enhance matrix accumulation by nucleus pulposus (NP) cells. NP cells isolated from bovine caudal discs were grown in 3D culture under normoxic or in select experiments under hypoxic conditions, in the presence or absence of various concentrations and sizes of polyP. Gene expression was determined using RT-PCR. Matrix accumulation was quantified by measuring proteoglycan and collagen contents. DAPI fluorescence shift was used to stain for polyP in tissue. DAPI staining showed polyP present predominantly in the pericellular region of in vitro formed tissue. PolyP treatment enhanced matrix accumulation in a concentration and chain length dependant manner. NP cells exposed to polyP-22 (22 phosphate units length) showed an increase in gene expression of aggrecan, Collagen II, Sox 9, and MMP-13 which was maintained for the 14 days of culture. This suggests that polyP may enhance NP tissue formation in vitro by upregulating the expression of matrix genes. As polyP enhances proteoglycan accumulation even under hypoxic conditions, this raises the possibility that polyP may be a novel treatment to induce NP regeneration. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:41-50, 2017.

Angiopoietin-1 receptor Tie2 distinguishes multipotent differentiation capability in bovine coccygeal nucleus pulposus cells

BACKGROUND

The intervertebral disc (IVD) has limited self-healing potential and disc repair strategies require an appropriate cell source such as progenitor cells that could regenerate the damaged cells and tissues. The objective of this study was to identify nucleus pulposus-derived progenitor cells (NPPC) and examine their potential in regenerative medicine in vitro.

METHODS

Nucleus pulposus cells (NPC) were obtained from 1-year-old bovine coccygeal discs by enzymatic digestion and were sorted for the angiopoietin-1 receptor Tie2. The obtained Tie2- and Tie2+ fractions of cells were differentiated into osteogenic, adipogenic, and chondrogenic lineages in vitro. Colony-forming units were prepared from both cell populations and the colonies formed were analyzed and quantified after 8 days of culture. In order to improve the preservation of the Tie2+ phenotype of NPPC in monolayer cultures, we tested a selection of growth factors known to have stimulating effects, cocultured NPPC with IVD tissue, and exposed them to hypoxic conditions (2 % O2).

RESULTS

After 3 weeks of differentiation culture, only the NPC that were positive for Tie2 were able to differentiate into osteocytes, adipocytes, and chondrocytes as characterized by calcium deposition (p < 0.0001), fat droplet formation (p < 0.0001), and glycosaminoglycan content (p = 0.0095 vs. Tie2- NPC), respectively. Sorted Tie2- and Tie2+ subpopulations of cells both formed colonies; however, the colonies formed from Tie2+ cells were spheroid in shape, whereas those from Tie2- cells were spread and fibroblastic. In addition, Tie2+ cells formed more colonies in 3D culture (p = 0.011) than Tie2- cells. During expansion, a fast decline in the fraction of Tie2+ cells was observed (p < 0.0001), which was partially reversed by low oxygen concentration (p = 0.0068) and supplementation of the culture with fibroblast growth factor 2 (FGF2) (p < 0.0001).

CONCLUSIONS

Our results showed that the bovine nucleus pulposus contains NPPC that are Tie2+. These cells fulfilled formally progenitor criteria that were maintained in subsequent monolayer culture for up to 7 days by addition of FGF2 or hypoxic conditions. We propose that the nucleus pulposus represents a niche of precursor cells for regeneration of the IVD.

Effect of perfluorotributylamine-enriched alginate on nucleus pulposus cell: Implications for intervertebral disc regeneration

Various scaffolds have been attempted for intervertebral disc regeneration, but their effectiveness was limited by loss of nutrients within the scaffolds. It has been suggested that the disc is not severely hypoxic and limited availability of oxygen results in disc degeneration. Therefore, a certain oxygen level might be beneficial for disc regeneration, which has not been given enough attention in previous studies. Here, we used perfluorotributylamine (PFTBA) for the first time as an oxygen regulator in alginate scaffold for disc regeneration in vitro and in vivo. We found that the characteristics of alginate were not affected by PFTBA and the oxygen level of the scaffold was regulated. Then, human nucleus pulposus (NP) cells were cultured in the PFTBA-enriched alginates. It was found that PFTBA could promote NP cell survival and proliferation. In addition, 2.5% PFTBA was capable of regulating extracellular matrix (ECM) to a disc-like tissue graft with little effect on the expression of NP cell markers. Finally, 2.5% PFTBA-enriched alginate was found to restore the disc height and the ECM in a mouse disc degeneration model, indicating its beneficial effect on alleviating disc degeneration. These findings highlight the promising application of PFTBA in further intervertebral disc regeneration.

Response to tumor necrosis factor-α mediated inflammation involving activation of prostaglandin E2 and Wnt signaling in nucleus pulposus cells

The cyclooxygenase 2 (COX-2) product, prostaglandin E2 (PGE2 ), acts through a family of G protein-coupled receptors designated E-prostanoid (EP) receptors that mediate intracellular signaling by multiple pathways. However, it is not known whether crosstalk between tumor necrosis factor-α(TNF-α)-PGE2 -mediated signaling and Wnt signaling plays a role in the regulation of intervertebral disc (IVD) cells. In this study, we investigated the relationship between TNF-α-PGE2 signaling and Wnt signaling in IVD cells. TNF-α increased the expression of COX-2 in IVD cells. The EP receptors EP1, EP3, and EP4 were expressed in IVD cells, and TNF-α significantly increased PGE2 production. Stimulation with TNF-α also upregulated EP3 and EP4 mRNA and protein expression in IVD cells. The inductive effect of the EP3 and EP4 receptors on Topflash promoter activity was confirmed through gain- and loss-of-function studies using selective EP agonists and antagonists. PGE2 treatment activated Wnt-β-catenin signaling through activation of EP3. We conclude that TNF-α-induced COX-2 and PGE2 stimulate Wnt signaling and activate Wnt target genes. Suppression of the EP3 receptor via TNF-α-PGE2 signaling seems to suppress IVD degeneration by controlling the activation of Wnt signaling. These findings may help identify the underlying mechanism and role of Wnt signaling in IVD degeneration.

Electrospun scaffold containing TGF-β1 promotes human mesenchymal stem cell differentiation towards a nucleus pulposus-like phenotype under hypoxia

The study was aimed at evaluating the effect of electrospun scaffold containing TGF-β1 on promoting human mesenchymal stem cells (MSCs) differentiation towards a nucleus pulposus-like phenotype under hypoxia. Two kinds of nanofibrous scaffolds containing TGF-β1 were fabricated using uniaxial electrospinning (Group I) and coaxial electrospinning (Group II). Human MSCs were seeded on both kinds of scaffolds and cultured in a hypoxia chamber (2% O2), and then the scaffolds were characterised. Cell proliferation and differentiation were also evaluated after 3 weeks of cell culture. Results showed that both kinds of scaffolds shared similar diameter distributions and protein release. However, Group I scaffolds were more hydrophilic than that of Group II. Both kinds of scaffolds induced the MSCs to differentiate towards the nucleus pulposus-type phenotype in vitro. In addition, the expression of nucleus pulposus-associated genes (aggrecan, type II collagen, HIF-1α and Sox-9) in Group I increased more than that of Group II. These results indicate that electrospinning nanofibrous scaffolds containing TGF-β1 supports the differentiation of MSCs towards the pulposus-like phenotype in a hypoxia chamber, which would be a more appropriate choice for nucleus pulposus regeneration.

A Well-Controlled Nucleus Pulposus Tissue Culture System with Injection Port for Evaluating Regenerative Therapies

In vitro evaluation of nucleus pulposus (NP) tissue regeneration would be useful, but current systems for NP culture are not ideal for injections. The aim of this study was to develop a long-term culture system for NP tissue that allows injections of regenerative agents. Bovine caudal NPs were harvested and placed in the newly designed culture system. After equilibration of the tissue to 0.3 MPa the volume was fixed and the tissue was cultured for 28 days. The cell viability and extracellular matrix composition remained unchanged during the culture period and gene expression profiles were similar to those obtained in earlier studies. Furthermore, to test the responsiveness of bovine caudal NPs in the system, samples were cultured for 4 days and injected twice (day 1 and 3) with (1) PBS, (2) Link-N, for regeneration, and (3) TNF-α, for degeneration. It was shown that TNF-α increased COX2 gene expression, whereas no effect of Link-N was detected. In conclusion, the newly designed system allows long-term culture of NP tissue, wherein tissue reactions to injected stimulants can be observed.

Intervention of rAAV-hTERT-Transducted Nucleus Pulposus Cells in Early Stage of Intervertebral Disc Degeneration: A Study in Canine Model

OBJECTIVES

To investigate the efficacy of recombinant adeno-associated virus (rAAV)-human telomerase reverse transcriptase (hTERT)-transducted nucleus pulposus cells (NPCs) in disc degeneration process in a canine disc degeneration model.

METHODS

The intervertebral disc degeneration of lumbar (L) 1-2, L3-4, and L5-6, from 12 female mongrels was prepared with the 20-gauge biopsy gun. Four weeks after animal model preparation, intervention experiment with rAAV-hTERT-transducted NPCs was conducted: group A, L1-2, serum-free medium with rAAV-hTERT modified NPCs; group B, L3-4, serum-free medium with NPCs; group C, L5-6, serum-free medium alone. Canines underwent digital radiography and magnetic resonance imaging 1 day before intervention, and 4, 8, and 12 weeks after intervention to evaluate the change of disc height and hydration status of interventional intervertebral discs. Twelve weeks after intervention, histological, biomechanical, and biochemical studies were carried out.

RESULTS

The rAAV-hTERT-transducted NPCs were constructed successfully. The mRNA level of hTERT from rAAV-hTERT-transfected NPCs increased obviously. There was no significant change of disc height index observed between groups and within groups. The relative grayscale index (RGI) was maintained 8 weeks after the intervention in group A, whereas in group B and group C, the RGI decreased significantly (p<0.05). No significant differences of the angle of lateral bending and extension-flexion bending were observed in group A compared with other groups (p>0.05). The morphology of disc structure was preserved in group A. In group B, the structure of inner annulus was broken down and the jelly-like nucleus pulposus (NP) tissue transmitted into the fibrocartilaginous tissue. In group C, the jelly-like NP tissue was completely replaced by fibrocartilaginous tissue. In the NP, the content of proteoglycan (PG) and collagen II was higher in group A than in group C (p<0.05). The content of PG was 13, 8.9, and 15.6 times higher than the content of collagen II in group A, group B, and group C, respectively.

CONCLUSIONS

In 12 weeks of observation, rAAV-hTERT-transducted NPCs could delay the degeneration process in the canine model which was superior than the capacity of NPCs in preserving structure integrity, content of extracellular matrix, and mechanical stability.

Hypoxic regulation of functional extracellular matrix elaboration by nucleus pulposus cells in long-term agarose culture

Degeneration of the intervertebral discs is strongly implicated as a cause of low back pain. Since current treatments for discogenic low back pain show poor long-term efficacy, a number of new biological strategies are being pursued. For such therapies to succeed, it is critical that they be validated in conditions that mimic the unique biochemical microenvironment of the nucleus pulposus (NP), which include low oxygen tension. Therefore, the objective of this study was to investigate the effects of oxygen tension on NP cell functional extracellular matrix elaboration in 3D culture. Bovine NP cells were encapsulated in agarose constructs and cultured for 14 or 42 days in either 20% or 2% oxygen in defined media containing transforming growth factor beta-3. At each time point, extracellular matrix composition, biomechanics, and mRNA expression of key phenotypic markers were evaluated. Results showed that while bulk mechanics and composition were largely independent of oxygen level, low oxygen promoted improved restoration of the NP phenotype, higher mRNA expression of extracellular matrix and NP specific markers, and more uniform matrix elaboration. These findings indicate that culture under physiological oxygen levels is an important consideration for successful development of cell and growth factor-based regenerative strategies for the disc.

Extracellular matrix production by nucleus pulposus and bone marrow stem cells in response to altered oxygen and glucose microenvironments

Bone marrow (BM) stem cells may be an ideal source of cells for intervertebral disc (IVD) regeneration. However, the harsh biochemical microenvironment of the IVD may significantly influence the biological and metabolic vitality of injected stem cells and impair their repair potential. This study investigated the viability and production of key matrix proteins by nucleus pulposus (NP) and BM stem cells cultured in the typical biochemical microenvironment of the IVD consisting of altered oxygen and glucose concentrations. Culture-expanded NP cells and BM stem cells were encapsulated in 1.5% alginate and ionically crosslinked to form cylindrical hydrogel constructs. Hydrogel constructs were maintained under different glucose concentrations (1, 5 and 25 mM) and external oxygen concentrations (5 and 20%). Cell viability was measured using the Live/Dead® assay and the production of sulphated glycosaminoglycans (sGAG), and collagen was quantified biochemically and histologically. For BM stem cells, IVD-like micro-environmental conditions (5 mM glucose and 5% oxygen) increased the accumulation of sGAG and collagen. In contrast, low glucose conditions (1 mM glucose) combined with 5% external oxygen concentration promoted cell death, inhibiting proliferation and the accumulation of sGAG and collagen. NP-encapsulated alginate constructs were relatively insensitive to oxygen concentration or glucose condition in that they accumulated similar amounts of sGAG under all conditions. Under IVD-like microenvironmental conditions, NP cells were found to have a lower glucose consumption rate compared with BM cells and may in fact be more suitable to adapt and sustain the harsh microenvironmental conditions. Considering the highly specialised microenvironment of the central NP, these results indicate that IVD-like concentrations of low glucose and low oxygen are critical and influential for the survival and biological behaviour of stem cells. Such findings may promote and accelerate the translational research of stem cells for the treatment of IVD degeneration.

Delivery systems for the treatment of degenerated intervertebral discs

The intervertebral disc (IVD) is the most avascular and acellular tissue in the body and therefore prone to degeneration. During IVD degeneration, the balance between anabolic and catabolic processes in the disc is deregulated, amongst others leading to alteration of extracellular matrix production, abnormal enzyme activities and production of pro-inflammatory substances like cytokines. The established treatment strategy for IVD degeneration consists of physiotherapy, pain medication by drug therapy and if necessary surgery. This approach, however, has shown limited success. Alternative strategies to increase and prolong the effects of bioactive agents and to reverse the process of IVD degeneration include the use of delivery systems for drugs, proteins, cells and genes. In view of the specific anatomy and physiology of the IVD and depending on the strategy of the therapy, different delivery systems have been developed which are reviewed in this article.

Effect of coculturing canine notochordal, nucleus pulposus and mesenchymal stromal cells for intervertebral disc regeneration

Introduction

Early degenerative changes in the nucleus pulposus (NP) are observed after the disappearance of notochordal cells (NCs). Thus, it has been suggested that NCs play an important role in maintaining the NP and may have a regenerative potential on other cells of the NP. As the number of resident NP cells (NPCs) decreases in a degenerating disc, mesenchymal stromal (stem) cells (MSCs) may be used for cell supplementation. In this study, using cells of one species, the regenerative potential of canine NCs was assessed in long-term three-dimensional coculture with canine NPCs or MSCs.

Methods

Canine NCs and canine NPCs or MSCs were cocultured in alginate beads for 28 days under hypoxic and high-osmolarity conditions. Cell viability, cell morphology and DNA content, extracellular matrix production and expression of genes related to NC markers (Brachyury, KRT18) and NP matrix production (ACAN, COL2A1, COL1A1) were assessed after 1, 15 and 28 days of culture.

Results

NCs did not completely maintain their phenotype (morphology, matrix production, gene expression) during 28 days of culture. In cocultures of NPCs and NCs, both extracellular matrix content and anabolic gene expression remained unchanged compared with monoculture groups, whereas cocultures of MSCs and NCs showed increased glycosaminoglycan/DNA. However, the deposition of these proteoglycans was observed near the NCs and not the MSCs. Brachyury expression in the MSC and NC coculture group increased in time. The latter two findings indicate a trophic effect of MSCs on NCs rather than vice versa.

Conclusions

No regenerative potential of canine NCs on canine NPCs or MSCs was observed in this study. However, significant changes in NC phenotype in long-term culture may have resulted in a suboptimal regenerative potential of these NCs. In this respect, NC-conditioned medium may be better than coculture for future studies of the regenerative potential of NCs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0569-6) contains supplementary material, which is available to authorized users.

A unifying neuro-fasciagenic model of somatic dysfunction - underlying mechanisms and treatment - Part I

This paper offers an extensive review of the main fascia-mediated mechanisms underlying various dysfunctional and pathophysiological processes of clinical relevance for manual therapy. The concept of somatic dysfunction is revisited in light of the diverse fascial influences that may come into play in its genesis and maintenance. A change in perspective is thus proposed: from a nociceptive model that for decades has viewed somatic dysfunction as a neurologically-mediated phenomenon, to a unifying fascial model that integrates neural influences into a multifactorial and multidimensional interpretation of dysfunctional process as being partially, if not entirely, mediated by the fascia.

Activation of intervertebral disc cells by co-culture with notochordal cells, conditioned medium and hypoxia

Background

Notochordal cells (NC) remain in the focus of research for regenerative therapy for the degenerated intervertebral disc (IVD) due to their progenitor status. Recent findings suggested their regenerative action on more mature disc cells, presumably by the secretion of specific factors, which has been described as notochordal cell conditioned medium (NCCM). The aim of this study was to determine NC culture conditions (2D/3D, fetal calf serum, oxygen level) that lead to significant IVD cell activation in an indirect co-culture system under normoxia and hypoxia (2% oxygen).

Methods

Porcine NC was kept in 2D monolayer and in 3D alginate bead culture to identify a suitable culture system for these cells. To test stimulating effects of NC, co-cultures of NC and bovine derived coccygeal IVD cells were conducted in a 1:1 ratio with no direct cell contact between NC and bovine nucleus pulposus cell (NPC) or annulus fibrosus cells (AFC) in 3D alginate beads under normoxia and hypoxia (2%) for 7 and 14 days. As a positive control, NPC and AFC were stimulated with NC-derived conditioned medium (NCCM). Cell activity, glycosaminoglycan (GAG) content, DNA content and relative gene expression was measured. Mass spectrometry analysis of the NCCM was conducted.

Results

We provide evidence by flow cytometry that monolayer culture is not favorable for NC culture with respect to maintaining NC phenotype. In 3D alginate culture, NC activated NPC either in indirect co-culture or by addition of NCCM as indicated by the gene expression ratio of aggrecan/collagen type 2. This effect was strongest with 10% fetal calf serum and under hypoxia. Conversely, AFC seemed unresponsive to co-culture with pNC or to the NCCM. Further, the results showed that hypoxia led to decelerated metabolic activity, but did not lead to a significant change in the GAG/DNA ratio. Mass spectrometry identified connective tissue growth factor (CTGF, syn. CCN2) in the NCCM.

Conclusions

Our results confirm the requirement to culture NC in 3D to best maintain their phenotype, preferentially in hypoxia and with the supplementation of FCS in the culture media. Despite these advancements, the ideal culture condition remains to be identified.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2474-15-422) contains supplementary material, which is available to authorized users.