Effect of oxygen levels on proteoglycan synthesis by intervertebral disc cells

Antioxidative nanofullerol prevents intervertebral disk degeneration

Compelling evidence suggests that reactive oxygen species (ROS) play a pivotal role in disk degeneration. Fullerol nanoparticles prepared in aqueous solution have been demonstrated to have outstanding ability to scavenge ROS. In this report, in vitro and in vivo models were used to study the efficacy of fullerol in preventing disk degeneration. For in vitro experiments, a pro-oxidant H₂O₂ or an inflammatory cytokine interleukin (IL)-1β was employed to induce degenerated phenotypes in human nucleus pulposus cells encapsulated in alginate beads, and fullerol was added in the culture medium. For the animal study, an annulus-puncture model with rabbit was created, and fullerol was injected into disks. It was shown that cytotoxicity and cellular ROS level induced by H₂O₂ were significantly diminished by fullerol. IL-1β-induced nitric oxide generation in culture medium was suppressed by fullerol as well. Gene-profile and biochemical assays showed that fullerol effectively reversed the matrix degradation caused by either H₂O₂ or IL-1β. The animal study delineated that intradiskal injection of fullerol prevented disk degeneration, increasing water and proteoglycan content and inhibiting ectopic bone formation. These results suggest that antioxidative fullerol may have a potential therapeutic application for disk degeneration.

Shape-memory porous alginate scaffolds for regeneration of the annulus fibrosus: effect of TGF-β3 supplementation and oxygen culture conditions

Disc herniation as a result of degenerative or traumatic injury is believed to be the primary instigator of low back pain. At present there is a lack of viable treatment options to repair damaged annulus fibrosus (AF) tissue. Developing alternative strategies to fill and repair ruptured AF tissue is a key challenge. In this work we developed a porous alginate scaffold with shape-memory properties which can be delivered using minimally invasive approaches and recover its original geometry once hydrated. Covalently cross-linked alginate hydrogels were created using carbodiimide chemistry, followed by a freeze-drying step to impart porosity and create porous scaffolds. Results showed that porous alginate scaffolds exhibited shape-memory recovery and mechanical behaviour that could be modulated depending on the cross-linker concentrations. The scaffold can be repeatedly compressed and expanded, which provides the potential to deliver the biomaterial directly to the damaged area of the AF tissue. In vitro experiments demonstrated that scaffolds were cytocompatible and supported cell seeding, penetration and proliferation under intervertebral-disc-like microenvironmental conditions (low glucose media and low oxygen concentration). Extracellular matrix (ECM) was secreted by AF cells with TGF-β3 stimulation and after 21days had filled the porous scaffold network. This biological matrix was rich in sulfated glycosaminoglycan and collagen type I, which are the main compounds of native AF tissue. Successful ECM deposition was also confirmed by the increase in the peak stress of the scaffold. However, the immaturity of the matrix network after only 21days of in vitro culture was not sufficient to attain native AF tissue mechanical properties. The ability to deliver porous scaffolds using minimal invasive approaches that can potentially promote the regeneration of AF defects provides an exciting new avenue for disc repair.

Differentiation of menstrual blood-derived stem cells toward nucleus pulposus-like cells in a coculture system with nucleus pulposus cells

STUDY DESIGN

Human stromal stem cells derived from menstrual blood (MenSCs) and nucleus pulposus (NP) cells were cocultured under normal or low oxygen (O2) condition.

OBJECTIVE

To assess the differentiation capability of MenSCs toward nucleus pulposus cells under normal or low oxygen condition.

SUMMARY OF BACKGROUND DATA

Given the proliferative capacity and pluripotentiality of mesenchymal stem cells, mesenchymal stem cells transplantation is thought to be a promising approach to managing intervertebral disc degeneration.

METHODS

Using coculture plates with 0.4-μm pore size polyethylene terephthalate track-etched inserts, MenSCs and NP cells (1:1 ratio) were cocultured with cell-to-cell contact for 2 weeks in normal (20% O2) or low oxygen tension (2% O2), respectively. Extracellular matrix accumulation was quantified by dimethylmethylene blue assay, histological staining, and quantitative reverse-transcription polymerase chain reaction. Novel characteristic human NP markers cytokeratin-19 (KRT19), carbonic anhydrase XII (CA12), and forkhead box F1 (FoxF1) were also detected by quantitative reverse-transcription polymerase chain reaction.

RESULTS

The result of quantitative reverse-transcription polymerase chain reaction showed that aggrecan and COL2A1 genes expression was significantly increased in differentiated MenSCs (P < 0.05). There was significantly more COL2A1 gene expression in normoxic group than that in low O2 group (P < 0.05). But no significant difference was observed in aggrecan gene expression between normoxic group and low O2 group. These aforementioned results were also confirmed by histological analysis. We also found that the characteristic NP markers (KRT19, CA12, FoxF1) were significantly upregulated in differentiated MenSCs. Moreover, low O2 tension (2%) further enhanced these genes expression (P < 0.05).

CONCLUSION

In our study, MenSCs were successfully differentiated into NP-like cells and may become a new source of seed cells for the treatment of intervertebral disc degeneration in the future.

LEVEL OF EVIDENCE

N/A.

Glutathione protects human nucleus pulposus cells from cell apoptosis and inhibition of matrix synthesis

Abstract Cell death (apoptosis and necrosis) and extracellular matrix destruction induced by oxidative stress have been suggested to be closely involved in the process of disc degeneration. Glutathione, a natural peptide as a powerful antioxidant in human cytoplasm, plays an important role in protecting living cells. This study is to investigate whether glutathione could retard degenerated phenotypes in cultured disc cells. Human nucleus pulposus cells were isolated and cultured in alginate beads and subsequently treated with a pro-oxidant H2O2 alone or a pro-inflammatory cytokine IL-1β alone or either of them together with glutathione. It was shown that H2O2 dose-dependently promoted nucleus pulposus cell apoptosis detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining and decreased mRNA levels of matrix proteins aggrecan and type II collagen determined by quantitative reverse transcription-polymerase chain reaction (RT-PCR). IL-1β could induce production of nitric oxide and decrease of proteoglycan, detected by the Griess reagent and the dimethyl methylene blue, respectively. The deleterious effects of either H2O2 or IL-1β could be efficiently prevented by glutathione. These results indicated that glutathione might be considered as an option for intervention of disc degeneration.

Influence of hypoxia in the intervertebral disc on the biological behaviors of rat adipose- and nucleus pulposus-derived mesenchymal stem cells

Adipose-derived mesenchymal stem cells (ADMSCs) and nucleus pulposus-derived mesenchymal stem cells (NPMSCs) are two cell candidates for cell-based therapies for intervertebral disc (IVD) regeneration. However, little work has been done to determine the influence of hypoxia in the IVD on the biological behaviors of ADMSCs and NPMSCs. This study aimed to investigate the viability, proliferation and differentiation of rat ADMSCs and NPMSCs in the hypoxic environment of IVD in vitro. ADMSCs and NPMSCs isolated from 6 SD rats were cultured under normoxia (20% O2) and hypoxia (2% O2) mimicking the standard condition and hypoxic environment of the IVD for 14 days. Cell viability was determined by the annexin-V-FITC/propidium iodide double-staining assay and cell proliferation was measured by MTT assay. The expression of hypoxia-inducible factor-1α, glucose transporter (GLUT)-1, GLUT-3 and vascular endothelial growth factor-A at the mRNA level was examined by RT-PCR. In cells cultured in three-dimensional micromass and differentiation medium, aggrecan, collagen-II and Sox-9 expression at mRNA and protein levels were examined by RT-PCR and Western blot. Hypoxia inhibited the viability and proliferation of both ADMSCs and NPMSCs, but promoted the chondrocytic differentiation of ADMSCs and NPMSCs. Compared to ADMSCs, NPMSCs showed greater viability, proliferation and chondrocytic differentiation under hypoxia. In conclusion, hypoxia in the IVD had a significant impact on the viability, proliferation and chondrocytic differentiation of ADMSCs and NPMSCs. NPMSCs exhibited more potent biological activity than ADMSCs in the hypoxic environment of the IVD and may represent another candidate for cell-based therapy for IVD regeneration.

Differential phenotypic behaviors of human degenerative nucleus pulposus cells under normoxic and hypoxic conditions: influence of oxygen concentration during isolation, expansion, and cultivation

BACKGROUND CONTEXT

Intervertebral discs (IVDs) are the largest avascular structures in the body; therefore, cells within these discs might be adapted to low-oxygen conditions. Although it has been demonstrated that a low oxygen concentration could promote synthesis of the extracellular matrix by IVD cells in the in vitro culture, isolation, expansion, and cultivation of IVD cells under classical tissue culture O2 saturation could still be detrimental.

PURPOSE

To investigate the phenotypic differences between human degenerative nucleus pulposus (NP) cells during isolation and expansion under normoxic (Nx: 21% O2) or hypoxic (Hx: 3.5% O2) conditions.

STUDY DESIGN

We investigated in vitro isolation, expansion, and cultivation of human NP cells.

METHODS

Human NP tissue samples were obtained from patients who underwent lumbar disc surgeries. Nucleus pulposus cells were then isolated, expanded, and cultivated under normoxic or hypoxic conditions. To determine whether the effects of normoxic expansion are reversible, another group of cells was isolated and expanded in normoxic conditions and then cultivated under hypoxic conditions (Nx→Hx group). Cellular proliferation, RNA expression of selected genes, and immunohistochemical staining were performed to evaluate the phenotypic behaviors of human NP cells under different conditions.

RESULTS

Expressions of Type II collagen and aggrecan in the Nx→Hx group were significantly higher than those in the normoxic group but were significantly lower than those in the hypoxic group. The normoxic group showed higher expression of matrix metalloproteinase (MMP)-2 and MMP-13 than did the other groups. Expression levels of hypoxia-inducible factors (HIFs) were significantly higher in the normoxic groups; however, a greater degree of HIF-1α staining was found in the hypoxic group, whereas a greater degree of HIF-2α staining was found in the normoxic group.

CONCLUSIONS

Human degenerative NP cells isolated, expanded, and cultivated in hypoxic conditions could better preserve the cells' regenerative potential. Compromised properties that were observed during isolation and expansion under normoxic conditions could only be partially rescued by later hypoxic cultivation. The superior phenotypic behaviors of human NP cells under hypoxia may be related to higher HIF-1α production and lower HIF-2α production. Cells that are isolated, expanded, and cultivated under hypoxic conditions may show better regenerative results when transplanted; therefore, the isolation and expansion processes of human degenerative NP cells should be managed in a hypoxic environment.

Understanding the native nucleus pulposus cell phenotype has important implications for intervertebral disc regeneration strategies

Low back pain is a leading cause of morbidity in developed societies and is strongly linked to degeneration of the intervertebral disc. The central nucleus pulposus (NP) region is most severely affected during disc degeneration and, consequently, is a focus for novel cell-based regenerative strategies. However, in order to develop such techniques, it is essential to first understand the biology and phenotype of the NP cells intended for repair. Microarray studies have highlighted novel NP markers that will allow a more accurate identification of cells for implantation, and along with other studies, have also revealed the potential importance of a developmental or immature NP cell phenotype in disseminating the optimal cell type for use. Additionally, the degenerative intervertebral disc is a harsh native environment and the effects of this on cells intended for implantation have yet to be fully elucidated; this is crucial for clinical translation of tissue engineered cell-based therapies.

Identification of novel nucleus pulposus markers: Interspecies variations and implications for cell-based therapiesfor intervertebral disc degeneration

Mesenchymal stem-cell based therapies have been proposed as novel treatments for intervertebral disc degeneration, a prevalent and disabling condition associated with back pain. The development of these treatment strategies, however, has been hindered by the incomplete understanding of the human nucleus pulposus phenotype and by an inaccurate interpretation and translation of animal to human research. This review summarises recent work characterising the nucleus pulposus phenotype in different animal models and in humans and integrates their findings with the anatomical and physiological differences between these species. Understanding this phenotype is paramount to guarantee that implanted cells restore the native functions of the intervertebral disc. Cite this article: Bone Joint Res 2013;2:169-78.

Potential of co-culture of nucleus pulposus mesenchymal stem cells and nucleus pulposus cells in hyperosmotic microenvironment for intervertebral disc regeneration

Nucleus pulposus mesenchymal stem cells (NPMSCs) are a potential cell source for intervertebral disc (IVD) regeneration, but little is known about their response to IVD-like high osmolarity (400 mOsm). This study was to investigate the viability, proliferation and protein biosynthesis of nucleus pulposus cells (NPCs), NPMSCs and co-cultured NPMSCs-NPCs under IVD-like high osmolarity conditions. NPCs and NPMSCs were isolated and cultured under standard and IVD-like high osmolarity conditions for 1 or 2 weeks. Cell viability was measured by annexin V-FITC and PI staining, and cell proliferation measured by MTT assay. The expression of SOX-9, aggrecan and collagen-II was measured by RT-PCR and Western blot analyses. IVD-like high osmolarity condition slightly inhibited cell viability and decreased the expression of SOX-9, aggrecan and collagen-II at the mRNA and protein levels in all groups compared with standard condition. NPMSCs could tolerate IVD-like high osmolarity, and NPCs-NPMSCs co-culture increased cell proliferation and the expression of SOX-9, aggrecan and collagen-II under both culture conditions, suggesting that co-culture of NPMSCs-NPCs has potential application for IVD regeneration.

Long-term culture of bovine nucleus pulposus explants in a native environment

BACKGROUND CONTEXT

Chronic low back pain is a disease with tremendous financial and social implications, and it is often caused by intervertebral disc degeneration. Regenerative therapies for disc repair are promising treatments, but they need to be tested in physiological models.

PURPOSE

To develop a physiological in vitro explant model that incorporates the native environment of the intervertebral disc, for example, hypoxia, low glucose, and high tissue osmolarity.

STUDY DESIGN

Bovine nucleus pulposus (NP) explants were cultured for 42 days in conditions mimicking the native physiological environment. Two different approaches were used to balance the swelling pressure of the NP: raised medium osmolarity or an artificial annulus.

METHODS

Bovine NP explants were either cultured in media with osmolarity balanced at isotonic and hypertonic levels compared with the native tissue or cultured inside a fiber jacket used as an artificial annulus. Oxygen and glucose levels were set at either standard (21% O2 and 4.5 g/L glucose) or physiological (5% O2 and 1 g/L glucose) levels. Samples were analyzed at Day 0, 3, and 42 for tissue composition (water, sulfated glycosaminoglycans, DNA, and hydroxyproline contents and fixed charge density), tissue histology, cell viability, and cellular behavior with messenger RNA (mRNA) expression.

RESULTS

Both the hypertonic culture and the artificial annulus approach maintained the tissue matrix composition for 42 days. At Day 3, mRNA expressions of aggrecan, collagen Type I, and collagen Type II in both hypertonic and artificial annulus cultures were not different from Day 0; however, at Day 42, the artificial annulus preserved the mRNA expression closer to Day 0. Gene expressions of matrix metalloprotease 13, tissue inhibitor of matrix metalloprotease 1, and tissue inhibitor of matrix metalloprotease 2 were downregulated under physiological O2 and glucose levels, whereas the other parameters analyzed were not affected.

CONCLUSIONS

Although the hypertonic culture and the artificial annulus approach are both promising models to test regenerative therapies, the artificial annulus was better able to maintain a cellular behavior closer to the native tissue in longer term cultures.

Hypoxia differentially regulates human nucleus pulposus and annulus fibrosus cell extracellular matrix production in 3D scaffolds

OBJECTIVE

We hypothesize that intervertebral disc (IVD) cells from distinct region respond differently to oxygen environment, and that IVD cells from patients with disc degeneration can benefit from hypoxia condition. Therefore, we aimed to determine the transcriptional response and extracellular matrix (ECM) production of nucleus pulposus (NP) and annulus fibrosus (AF) cells to different oxygen tension.

METHOD

Human NP and AF from degenerated IVD were seeded in 3D scaffolds and subjected to varying oxygen tension (2% and 20%) for 3 weeks. Changes in ECM were evaluated using quantitative real-time reverse transcriptase polymerase chain reaction, histological and immunohistological analyses.

RESULTS

Hypoxia significantly enhances NP cells phenotype, which resulted in greater production of sulfated glycosaminoglycan (GAG) and collagen type II within the constructs and the cells expressed higher levels of genes encoding NP ECM. A significantly stronger fluorescent signal for hypoxia-inducible factor (HIF-1α) as also found in the NP cells under the hypoxic than normoxic condition. However, there was little effect of hypoxia on the AF cells.

CONCLUSIONS

The NP and AF cells respond differently to hypoxia condition on the 3D scaffold, and hypoxia could enhance NP phenotype. When used in concert with appropriate scaffold material, human NP cells from degenerated disc could be regenerated for tissue engineering application.

Hypoxia-induced collagen crosslinking as a mechanism for enhancing mechanical properties of engineered articular cartilage

OBJECTIVE

The focus of tissue engineering of neocartilage has traditionally been on enhancing extracellular matrix and thus biomechanical properties. Emphasis has been placed on the enhancement of collagen type and quantity, and, concomitantly, tensile properties. The objective of this study was to improve crosslinking of the collagen network by testing the hypothesis that hypoxia could promote pyridinoline (PYR) crosslinks and, thus, improve neocartilage's tensile properties.

METHODS

Chondrocyte expression of lysyl oxidase (LOX), an enzyme responsible for the formation of collagen PYR crosslinks, was first assessed pre- and post- hypoxia application. Then, the mechanical properties of self-assembled neocartilage constructs were measured, after 4 weeks of culture, for groups exposed to 4% O2 at different initiation times and durations, i.e., during the 1st and 3rd weeks, 3rd and 4th weeks, 4th week only, continuously after cell seeding, or never.

RESULTS

Results showed that LOX gene expression was upregulated ∼20-fold in chondrocytes in response to hypoxia. Hypoxia applied during the 3rd and 4th weeks significantly increased PYR crosslinks without affecting collagen content. Excitingly, neocartilage tensile properties were increased ∼2-fold. It should be noted that these properties exhibited a distinct temporal dependence to hypoxia exposure, since upregulation of these properties was due to hypoxia applied only during the 3rd and 4th weeks.

CONCLUSION

These data elucidate the role of hypoxia-mediated upregulation of LOX and subsequent increases in PYR crosslinks in engineered cartilage. These results hold promise toward applying hypoxia at precise time points to promote tensile integrity and direct construct maturation.

Reconstruction of an in vitro niche for the transition from intervertebral disc development to nucleus pulposus regeneration

The nucleus pulposus (NP) plays a prominent role in both the onset and progression of intervertebral disc degeneration. While autologous repair strategies have demonstrated some success, their in vitro culture system is outdated and insufficient for maintaining optimally functioning cells through the required extensive passaging. Consequently, the final population of cells may be unsuitable for the overwhelming task of repairing tissue in vivo and could result in subpar clinical outcomes. Recent work has identified synovium-derived stem cells (SDSCs) as a potentially important new candidate. This population of precursors can promote matrix regeneration and additionally restore the balance of catabolic and anabolic metabolism of surrounding cells. Another promising application is their ability to produce an extracellular matrix in vitro that can be modified via decellularization to produce a tissue-specific substrate for efficient cell expansion, while retaining chondrogenic potential. When combined with hypoxia, soluble factors, and other environmental regulators, the resultant complex microenvironment will more closely resemble the in vivo niche, which further improves the cell capacity, even after extensive passaging. In this review, the adaptive mechanisms NP cells utilize in vivo are considered for insight into what factors are important for constructing a tissue-specific in vitro niche. Evidence for the use of SDSCs for NP regeneration is also discussed. Many aspects of NP behavior are still unknown, which could lead to future work yielding key information on producing sufficient numbers of a high-quality NP-specific population that is able to regenerate deteriorated NP in vivo.

Challenges and strategies in the repair of ruptured annulus fibrosus

Lumbar discectomy is the surgical procedure most frequently performed for patients suffering from low back pain and sciatica. Disc herniation as a consequence of degenerative or traumatic processes is commonly encountered as the underlying cause for the painful condition. While discectomy provides favourable outcome in a majority of cases, there are conditions where unmet requirements exist in terms of treatment, such as large disc protrusions with minimal disc degeneration; in these cases, the high rate of recurrent disc herniation after discectomy is a prevalent problem. An effective biological annular repair could improve the surgical outcome in patients with contained disc herniations but otherwise minor degenerative changes. An attractive approach is a tissue-engineered implant that will enable/stimulate the repair of the ruptured annulus. The strategy is to develop three-dimensional scaffolds and activate them by seeding cells or by incorporating molecular signals that enable new matrix synthesis at the defect site, while the biomaterial provides immediate closure of the defect and maintains the mechanical properties of the disc. This review is structured into (1) introduction, (2) clinical problems, current treatment options and needs, (3) biomechanical demands, (4) cellular and extracellular components, (5) biomaterials for delivery, scaffolding and support, (6) pre-clinical models for evaluation of newly developed cell- and material-based therapies, and (7) conclusions. This article highlights that an interdisciplinary approach is necessary for successful development of new clinical methods for annulus fibrosus repair. This will benefit from a close collaboration between research groups with expertise in all areas addressed in this review.

Modulation of in vitro microenvironment facilitates synovium-derived stem cell-based nucleus pulposus tissue regeneration

STUDY DESIGN

Two experiments were conducted. Experiment 1 evaluated the effect of 3 kinds of decellularized extracellular matrices (DECMs) deposited by synovium-derived stem cells (SDSCs) and/or nucleus pulposus cells (NPCs) on SDSC expansion and NP lineage differentiation. Experiment 2 evaluated the effect of DECM deposited by SDSCs on NPC expansion and redifferentiation capacity. In both experiments, hypoxia was evaluated in DECM preparation and pellet culture.

OBJECTIVE

Modulating the in vitro microenvironment facilitates SDSC-based NP tissue regeneration.

SUMMARY OF BACKGROUND DATA

Autologous cell therapy is a promising approach for NP regeneration. Current in vitro expansion in monolayer results in cell dedifferentiation.

METHODS

In Experiment 1, passage 3 SDSCs were expanded for 1 passage on DECM deposited by NPCs, SDSCs, or NPCs combined with SDSCs (50:50); DECM was prepared under either normoxia (21% O2) or hypoxia (5% O2). Expanded SDSCs were then cultured in a serum-free chondrogenic medium in hypoxia for 14 days. In Experiment 2, passage 2 NPCs were expanded for 1 passage on DECM deposited by SDSCs; DECM was prepared under either normoxia or hypoxia. Expanded NPCs were cultured in a serum-free chondrogenic medium under either hypoxia or normoxia for 14 days. Cell expansion on plastic flasks served as a control in both experiments. Fourteen-day pellets were evaluated for chondrogenesis using histology, immunostaining, biochemistry, and real-time polymerase chain reaction.

RESULTS

DECM deposited by NPCs combined with SDSCs effectively enhanced expanded SDSC viability and guided SDSC differentiation toward an NP lineage; this effect is comparable with DECM deposited by SDSCs but higher than that deposited by NPCs. DECM prepared under normoxia favored SDSC viability and NP lineage differentiation whereas DECM prepared under hypoxia benefited NPC viability and redifferentiation. Low oxygen in a pellet culture system enhanced NPC viability and redifferentiation.

CONCLUSION

The in vitro microenvironment can be modulated by low oxygen and tissue-specific cell-based DECM to facilitate NP tissue regeneration.

The effects of oxygen tension and antiaging factor Klotho on Wnt signaling in nucleus pulposus cells

Introduction

The goals of this study were to examine the oxemic regulation of Wnt signaling to explore whether Wnt signaling accelerates the age-related degeneration of nucleus pulposus cells, and if so, to define the mechanism underlying this effect. We investigated the expression of Klotho, a newly identified antiaging gene, and whether its regulation is attributable to the suppression of Wnt signaling.

Methods

Rat nucleus pulposus cells were cultured under normoxic (21% O₂) or hypoxic (2% O₂) conditions, and the expression and promoter activity of Wnt signaling and Klotho were evaluated. The effect of Klotho protein was examined with transfection experiments, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, senescence-associated β-galactosidase staining, and cell-cycle analysis. To determine the methylation status of the Klotho promoter region, bisulfite genomic sequencing analysis was performed. Its relation with the activation of Wnt signaling was assessed. We also examined whether the expression of Klotho could block the effects of pathological Wnt expression in nucleus pulposus cells.

Results

Nucleus pulposus cells exhibited increased β-catenin mRNA and protein under the hypoxic condition. Klotho protein was expressed in vivo, and protein and messenger RNA expression decreased under the hypoxic condition. Klotho treatment decreased cell proliferation and induced the quiescence of nucleus pulposus cells. In addition, Klotho treatment inhibited expression of β-catenin gene and protein compared with untreated control cells.

Conclusions

These data indicate that Wnt signaling and Klotho form a negative-feedback loop in nucleus pulposus cells. These results suggest that the expression of Klotho is regulated by the balance between upregulation and downregulation of Wnt signaling.

Simulated-physiological loading conditions preserve biological and mechanical properties of caprine lumbar intervertebral discs in ex vivo culture

Low-back pain (LBP) is a common medical complaint and associated with high societal costs. Degeneration of the intervertebral disc (IVD) is assumed to be an important causal factor of LBP. IVDs are continuously mechanically loaded and both positive and negative effects have been attributed to different loading conditions.

In order to study mechanical loading effects, degeneration-associated processes and/or potential regenerative therapies in IVDs, it is imperative to maintain the IVDs' structural integrity. While in vivo models provide comprehensive insight in IVD biology, an accompanying organ culture model can focus on a single factor, such as loading and may serve as a prescreening model to reduce life animal testing. In the current study we examined the feasibility of organ culture of caprine lumbar discs, with the hypothesis that a simulated-physiological load will optimally preserve IVD properties.

Lumbar caprine IVDs (n = 175) were cultured in a bioreactor up to 21 days either without load, low dynamic load (LDL), or with simulated-physiological load (SPL). IVD stiffness was calculated from measurements of IVD loading and displacement. IVD nucleus, inner- and outer annulus were assessed for cell viability, cell density and gene expression. The extracellular matrix (ECM) was analyzed for water, glycosaminoglycan and total collagen content.

IVD biomechanical properties did not change significantly with loading conditions. With SPL, cell viability, cell density and gene expression were preserved up to 21 days. Both unloaded and LDL resulted in decreased cell viability, cell density and significant changes in gene expression, yet no differences in ECM content were observed in any group.

In conclusion, simulated-physiological loading preserved the native properties of caprine IVDs during a 21-day culture period. The characterization of caprine IVD response to culture in the LDCS under SPL conditions paves the way for controlled analysis of degeneration- and regeneration-associated processes in the future.

Interactions of environmental conditions and mechanical loads have influence on matrix turnover by nucleus pulposus cells

Disc degeneration is associated with several changes in the physicochemical environment of intervertebral disc cells. Nucleus pulposus (NP) cells in the center of degenerated discs are exposed to decreased glucose supply, osmolarity, pH, and oxygen levels. To understand the complexity of these interactions on a cellular level, we designed standardized experiments in which we compared responses to these environmental factors under normal levels with those seen under two different degrees of disc degeneration. We hypothesized that these changes in environmental stimuli influence gene expression of matrix proteins and matrix degrading enzymes and alter their responses to cyclic hydrostatic pressure (HP). Our results suggest that a simulation of degenerative conditions influences the degradation of disc matrix through impairing matrix formation and accelerating matrix resorption via up- or down-regulation of the respective target genes. The greatest effects were seen for decreases in glucose concentration and pH. Low oxygen had little influence. HP had little direct effect but appeared to counteract matrix degradation by reducing or inverting some of the adverse effects of other stimuli. For ongoing in vitro studies, interactions between mechanical stimuli and factors in the physicochemical environment should not be ignored as these could markedly influence results.

Structure and biology of the intervertebral disk in health and disease

The intervertebral disks along the spine provide motion and protection against mechanical loading. The 3 structural components, nucleus pulposus, annulus fibrosus, and cartilage endplate, function as a synergistic unit, though each has its own role. The cells within each of these components have distinct origins in development and morphology, producing specific extracellular matrix proteins that are organized into unique architectures fit for intervertebral disk function. This article focuses on various aspects of intervertebral disk biology and disruptions that could lead to diseases such as intervertebral disk degeneration.