Computation models simulating notochordal cell extinction during early ageing of an intervertebral disc

Lower back pain due to intervertebral disc (IVD) degeneration is a prevalent problem which drastically affects the quality of life of millions of sufferers. Healthy IVDs begin with high populations of notochordal cells in the nucleus pulposus, while by the second stage of degeneration, these cells will be replaced by chondrocyte-like cells. Because the IVD is avascular, these cells rely on passive diffusion of nutrients to survive. It is thought that this transition in cell phenotype causes the shift of the IVD's physical properties, which impede the flow of nutrients. Our computational model of the IVD illustrates its ability to simulate the evolving chemical and mechanical environments occurring during the early ageing process. We demonstrate that, due to the insufficient nutrient supply and accompanying changes in physical properties of the IVD, there was a resultant exponential decay in the number of notochordal cells over time.     

Regulating the fate of stem cells for regenerating the intervertebral disc degeneration

Lower back pain is a leading cause of disability and is one of the reasons for the substantial socioeconomic burden. The etiology of intervertebral disc (IVD) degeneration is complicated, and its mechanism is still not completely understood. Factors such as aging, systemic inflammation, biochemical mediators, toxic environmental factors, physical injuries, and genetic factors are involved in the progression of its pathophysiology. Currently, no therapy for restoring degenerated IVD is available except pain management, reduced physical activities, and surgical intervention. Therefore, it is imperative to establish regenerative medicine-based approaches to heal and repair the injured disc, repopulate the cell types to retain water content, synthesize extracellular matrix, and strengthen the disc to restore normal spine flexion. Cellular therapy has gained attention for IVD management as an alternative therapeutic option. In this review, we present an overview of the anatomical and molecular structure and the surrounding pathophysiology of the IVD. Modern therapeutic approaches, including proteins and growth factors, cellular and gene therapy, and cell fate regulators are reviewed. Similarly, small molecules that modulate the fate of stem cells for their differentiation into chondrocytes and notochordal cell types are highlighted.     

3D finite element analysis of nutrient distributions and cell viability in the intervertebral disc: effects of deformation and degeneration

The intervertebral disc (IVD) receives important nutrients, such as glucose, from surrounding blood vessels. Poor nutritional supply is believed to play a key role in disc degeneration. Several investigators have presented finite element models of the IVD to investigate disc nutrition; however, none has predicted nutrient levels and cell viability in the disc with a realistic 3D geometry and tissue properties coupled to mechanical deformation. Understanding how degeneration and loading affect nutrition and cell viability is necessary for elucidating the mechanisms of disc degeneration and low back pain. The objective of this study was to analyze the effects of disc degeneration and static deformation on glucose distributions and cell viability in the IVD using finite element analysis. A realistic 3D finite element model of the IVD was developed based on mechano-electrochemical mixture theory. In the model, the cellular metabolic activities and viability were related to nutrient concentrations, and transport properties of nutrients were dependent on tissue deformation. The effects of disc degeneration and mechanical compression on glucose concentrations and cell density distributions in the IVD were investigated. To examine effects of disc degeneration, tissue properties were altered to reflect those of degenerated tissue, including reduced water content, fixed charge density, height, and endplate permeability. Two mechanical loading conditions were also investigated: a reference (undeformed) case and a 10% static deformation case. In general, nutrient levels decreased moving away from the nutritional supply at the disc periphery. Minimum glucose levels were at the interface between the nucleus and annulus regions of the disc. Deformation caused a 6.2% decrease in the minimum glucose concentration in the normal IVD, while degeneration resulted in an 80% decrease. Although cell density was not affected in the undeformed normal disc, there was a decrease in cell viability in the degenerated case, in which averaged cell density fell 11% compared with the normal case. This effect was further exacerbated by deformation of the degenerated IVD. Both deformation and disc degeneration altered the glucose distribution in the IVD. For the degenerated case, glucose levels fell below levels necessary for maintaining cell viability, and cell density decreased. This study provides important insight into nutrition-related mechanisms of disc degeneration. Moreover, our model may serve as a powerful tool in the development of new treatments for low back pain.     

Gene expression profiling of early intervertebral disc degeneration reveals a down-regulation of canonical Wnt signaling and caveolin-1 expression: implications for development of regenerative strategies

Introduction

Early degeneration of the intervertebral disc (IVD) involves a change in cellular differentiation from notochordal cells (NCs) in the nucleus pulposus (NP) to chondrocyte-like cells (CLCs). The purpose of this study was to investigate the gene expression profiles involved in this process using NP tissue from non-chondrodystrophic and chondrodystrophic dogs, a species with naturally occurring IVD degeneration.

Methods

Dual channel DNA microarrays were used to compare 1) healthy NP tissue containing only NCs (NC-rich), 2) NP tissue with a mixed population of NCs and CLCs (Mixed), and 3) NP tissue containing solely CLCs (CLC-rich) in both non-chondrodystrophic and chondrodystrophic dogs. Based on previous reports and the findings of the microarray analyses, canonical Wnt signaling was further evaluated using qPCR of relevant Wnt target genes. We hypothesized that caveolin-1, a regulator of Wnt signaling that showed significant changes in gene expression in the microarray analyses, played a significant role in early IVD degeneration. Caveolin-1 expression was investigated in IVD tissue sections and in cultured NCs. To investigate the significance of Caveolin-1 in IVD health and degeneration, the NP of 3-month-old Caveolin-1 knock-out mice was histopathologically evaluated and compared with the NP of wild-type mice of the same age.

Results

Early IVD degeneration involved significant changes in numerous pathways, including Wnt/β-catenin signaling. With regard to Wnt/β-catenin signaling, axin2 gene expression was significantly higher in chondrodystrophic dogs compared with non-chondrodystrophic dogs. IVD degeneration involved significant down-regulation of axin2 gene expression. IVD degeneration involved significant down-regulation in Caveolin-1 gene and protein expression. NCs showed abundant caveolin-1 expression in vivo and in vitro, whereas CLCs did not. The NP of wild-type mice was rich in viable NCs, whereas the NP of Caveolin-1 knock-out mice contained chondroid-like matrix with mainly apoptotic, small, rounded cells.

Conclusions

Early IVD degeneration involves down-regulation of canonical Wnt signaling and Caveolin-1 expression, which appears to be essential to the physiology and preservation of NCs. Therefore, Caveolin-1 may be regarded an exciting target for developing strategies for IVD regeneration.     

Modulating Notochordal Differentiation of Human Induced Pluripotent Stem Cells Using Natural Nucleus Pulposus Tissue Matrix

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

CD24 is expressed specifically in the nucleus pulposus of intervertebral discs

Intervertebral disc (IVD) consists of a soft gelatinous material in its center, the nucleus pulposus (NP), bounded peripherally by fibrocartilage, annulus fibrosus (AF). Despite the number of patients with IVD degeneration, gene expression analysis has not been undertaken in NP and therefore little is known about the molecular markers expressed in NP. Here, we undertook a microarray screen in NP with the other nine tissues to identify the specific cell surface markers for NP. Five membrane associating molecules out of 10,490 genes were identified as highly expressing genes in NP compared with the other tissues. Among them, we identified CD24, a glycosylphosphatidylinositol (GPI) anchor protein as a cell surface marker for NP. CD24 expression was also detected in the herniated NP and chordoma, a malignant primary tumor derived from notochordal cells, while it was absent in chondrosarcoma. Therefore, CD24 is a molecular marker for NP as well as the diseases of IVD.     

Detrimental role for human high temperature requirement serine protease A1 (HTRA1) in the pathogenesis of intervertebral disc (IVD) degeneration

Human HTRA1 is a highly conserved secreted serine protease that degrades numerous extracellular matrix proteins. We have previously identified HTRA1 as being up-regulated in osteoarthritic patients and as having the potential to regulate matrix metalloproteinase (MMP) expression in synovial fibroblasts through the generation of fibronectin fragments. In the present report, we have extended these studies and investigated the role of HTRA1 in the pathogenesis of intervertebral disc (IVD) degeneration. HTRA1 mRNA expression was significantly elevated in degenerated disc tissue and was associated with increased protein levels. However, these increases did not correlate with the appearance of rs11200638 single nucleotide polymorphism in the promoter region of the HTRA1 gene, as has previously been suggested. Recombinant HTRA1 induced MMP production in IVD cell cultures through a mechanism critically dependent on MEK but independent of IL-1β signaling. The use of a catalytically inactive mutant confirmed these effects to be primarily due to HTRA1 serine protease activity. HTRA1-induced fibronectin proteolysis resulted in the generation of various sized fragments, which when added to IVD cells in culture, caused a significant increase in MMP expression. Furthermore, one of these fragments was identified as being the amino-terminal fibrin- and heparin-binding domain and was also found to be increased within HTRA1-treated IVD cell cultures as well as in disc tissue from patients with IVD degeneration. Our results therefore support a scenario in which HTRA1 promotes IVD degeneration through the proteolytic cleavage of fibronectin and subsequent activation of resident disc cells.     

Programmed cell death in intervertebral disc degeneration

Intervertebral disc (IVD) degeneration is largely a process of destruction and failure of the extracellular matrix (ECM), and symptomatic IVD degeneration is thought to be one of the leading causes of morbidity or life quality deterioration in the elderly. To date, however, the mechanism of IVD degeneration is still not fully understood. Cellular loss from cell death in the process of IVD degeneration has long been confirmed and considered to contribute to ECM degradation, but the causes and the manners of IVD cell death remain unclear. Programmed cell death (PCD) is executed by an active cellular process and is extensively involved in many physiological and pathological processes, including embryonic development and human degenerative diseases. Thus, the relationship between PCD and IVD degeneration has become a new research focus of interest in recent years. By reviewing the available literature concentrated on PCD in IVD and discussing the methodology of detecting PCD in IVD cells, its inducing factors, the relationship of cell death to ECM degradation, and the potential therapy for IVD degeneration by modulation of PCD, we conclude that IVD cells undergo PCD via different signal transduction pathways in response to different stimuli, that PCD may play a role in the process of IVD degeneration, and that modulation of PCD might be a potential therapeutic strategy for IVD degeneration.     

Modeling the role of IGF-1 on extracellular matrix biosynthesis and cellularity in intervertebral disc

The insulin-like growth factor-1 (IGF-1) is a well-known anabolic agent for intervertebral disc (IVD), promoting both proteoglycan (PG) biosynthesis and cell proliferation. Accordingly, it is believed that IGF-1 may play a central role in IVD homeostasis. Furthermore, the exogenous administration of IGF-1 has been proposed as a possible therapeutic strategy for disc degeneration. The objectives of this study were to develop a new computational framework for describing the mechanisms regulating IGF-mediated homeostasis in IVD, and to apply this numerical tool for investigating the effectiveness of exogenous administration of IGF-1 for curing disc degeneration. A diffusive–reactive model was developed for describing competitive binding of IGF-1 to its binding proteins and cell surface receptors, with the latter reaction initiating the intracellular signaling mechanism leading to PG production and cell proliferation. Because PG production increases cell metabolic rate, and cell proliferation increases nutritional demand, nutrients transport and metabolism were also included into the model, and co-regulated, together with IGF-1, IVD cellularity. The sustainability and the effectiveness of IGF-mediated anabolism were investigated for conditions of pathologically insufficient nutrient supply, and for the case of exogenous administration of IGF-1 to degenerated IVD. Results showed that pathological nutrients deprivation, by decreasing cellularity, caused a reduction of PG biosynthesis. Also, exogenous administration of IGF-1 was only beneficial in well-nourished regions of IVD, and exacerbated cell mortality in malnourished regions. These findings remark the central role of nutrition in IVD health, and suggest that adequate nutritional supply is paramount for achieving a successful IGF-based therapy for disc degeneration.     

Effects of Tobacco Smoking on the Degeneration of the Intervertebral Disc: A Finite Element Study

Tobacco smoking is associated with numerous pathological conditions. Compelling experimental evidence associates smoking to the degeneration of the intervertebral disc (IVD). In particular, it has been shown that nicotine down-regulates both the proliferation rate and glycosaminoglycan (GAG) biosynthesis of disc cells. Moreover, tobacco smoking causes the constriction of the vascular network surrounding the IVD, thus reducing the exchange of nutrients and anabolic agents from the blood vessels to the disc. It has been hypothesized that both nicotine presence in the IVD and the reduced solute exchange are responsible for the degeneration of the disc due to tobacco smoking, but their effects on tissue homeostasis have never been quantified. In this study, a previously presented computational model describing the homeostasis of the IVD was deployed to investigate the effects of impaired solute supply and nicotine-mediated down-regulation of cell proliferation and biosynthetic activity on the health of the disc. We found that the nicotine-mediated down-regulation of cell anabolism mostly affected the GAG concentration at the cartilage endplate, reducing it up to 65% of the value attained in normal physiological conditions. In contrast, the reduction of solutes exchange between blood vessels and disc tissue mostly affected the nucleus pulposus, whose cell density and GAG levels were reduced up to 50% of their normal physiological levels. The effectiveness of quitting smoking on the regeneration of a degenerated IVD was also investigated, and showed to have limited benefit on the health of the disc. A cell-based therapy in conjunction with smoke cessation provided significant improvements in disc health, suggesting that, besides quitting smoking, additional treatments should be implemented in the attempt to recover the health of an IVD degenerated by tobacco smoking.     

Cell death in intervertebral disc degeneration

Degeneration of intervertebral disc (IVD) is mainly a chronic process of excessive destruction of the extracellular matrix (ECM), and also is thought to be the primary cause of low back pain. Presently, however, the underlying mechanism of IVD degeneration is still not elucidated. Cellular loss from cell death has been believed to contribute to the degradation of ECM and plays an important role in the process of IVD degeneration, but the mechanisms of cell death in degenerated IVD remain unclear. Apoptosis, a very important type of IVD cell death, has been considered to play a crucial role in the process of degeneration. Autophagy, a non-apoptosis death type of programmed cell death, has been considered extensively involved in many pathological and physiological processes, including the degenerative diseases. Thus, the research on cell death in IVD degeneration has become a new focus recently. In this review, by analyzing the available literature pertaining to cell death in IVD and discussing the inducing factors of IVD degeneration, NP cells and ECM in IVD degeneration, apoptotic signal transduction pathways involved in IVD cell death, the relationship of cell death with IVD degeneration and potential therapeutic strategy for IVD degeneration by regulating cell death, we conclude that different stimuli induce cell death in IVD via various signal transduction pathways, and that cell death may play a key role in the degenerative process of IVD. Regulation of cell death could be a potential and attractive therapeutic strategy for IVD degeneration.     

The enigma that is the nucleus pulposus cell: the search goes on

The development of an effective treatment for degenerative disc disease has been hampered for many years by what seems a fundamental problem; what exactly defines a nucleus pulposus (NP) cell? The paper by Gilson and colleagues elegantly re-opens the debate concerning the lineage and identity of NP cells that are alike yet different from chondrocytes. As we pursue novel investigations and treatment strategies for degenerative disc disease, how do we isolate these unique cells and what is the role of the primordial notochordal cell that may well linger within the NP far longer and perhaps in a different phenotypic appearance than previously thought? The paper by Gilson and colleagues that is the subject of the present editorial presents compelling data concerning the heterogeneity of the cells of the NP, and their origin, development, maturation and function.A recent issue of Arthritis Research and Therapy contains a report by Gilson and colleagues describing their investigation of the differential cell surface marker expression found in samples of bovine intervertebral disc (IVD) and articular chondrocytes [1]. This report raises interesting questions about the identity of the residents within the nucleus pulposus (NP) and has broad implications with respect to regenerative medicine and tissue engineering of the IVD.A recent search of PubMed using the search term ''tissue engineering and intervertebral disc'' returned 263 hits, with the oldest publication dating to 1989. Although investigators have clearly been interested in developing a biological treatment for degenerative disc disease for over 30 years, we must be in the early days since we have yet to characterize the ubiquitous NP cell or to really understand the composition of the NP cellular milieu.The flexible model of cell and tissue classification whereby expression patterns reflect a functional approach rather than strict germ layer derivation suggests that, with respect to the identity of NP cells, there may be more than meets the eye [2].The notochord derives from all three germ layers as it originates in a blended fashion from primitive ectoderm, sharing mesodermal and endodermal attributes as it develops as an outgrowth from Hensen''s node between the ectoderm and endoderm [3]. This co-joined origin is particularly poignant in human and other mammals, as distinct from lower animals, because in higher mammals the developing notochord provides a pathway for migration of ectoderm to endoderm [3]. The presence of vimentin in NP cells suggests that motility may play a role in the development of the NP cellular composition; perhaps including cells that migrate inwards from the vertebral endplates [4]. As Gilson and colleagues have reported, however, the cells occupying the NP may change their appearance over time, masking their original phenotype but perhaps retaining some of their original capacity. Have these cells altered their phenotype as a consequence of maturation or pathological events, or as an adaptive response to life in the disc over time?The IVD is a hypoxic, isolated, immune-privileged compartment, the cells of which must necessarily be highly specialized in order to survive. Classically it has been thought that once the NP has been formed the notochordal cells disappear, leaving behind the fibrocartilagenous NP cell. But then along come Gilson and colleagues - who find that within adult bovine caudal discs (a tissue compartment formerly thought to be fairly homogeneous) there exists a small percentage of notochordal holdouts that continue to express their notochordal lineage markers. Is it that a small, primordial notochordal cell reservoir may linger longer than was previously thought within the mature NP?These observations raise a number of questions - notably, is the protection from degenerative disc disease seen in species that retain their notochordal cell-rich appearance, such as the nonchondrodystrophic canine and rabbit, due to the differential extracellular matrix synthesized by these cells as compared with the NP cell? [5]. Is it a dose-response issue whereby the discs that are relatively deficient in notochordal cells are therefore lacking in the necessary and sufficient molecules synthesized by these cells that may act upon the NP cell [6,7]? It is thought that the notochordal cell-rich disc NP phenotype confers superior biomechanical properties [5,8]. Do notochordal cell-deficient discs therefore fail to resist the loads imposed by daily life over time due to biomechanical or biochemical reasons - or both? Also, and importantly from the perspective of evaluating putative therapies, which cells are the best to use for in vitro assays? Should future NP cell experiments exclude cytokeratin-8⁺ cells or does this matter when evaluating the mechanisms of the IVD NP as an organ?In terms of the progression of degenerative disc disease, the NP could arguably represent the lynchpin in the degenerative cascade since many investigators consider the NP as the area demonstrating the earliest degenerative changes [9-11]. We may therefore need to look ever closer at the question of what really defines the cells within the disc. Are the current models of events leading to failure of the disc as an organ correct? What role(s) do the cells play within the NP that may mitigate or contribute to the progression of organ failure?As we look to the future and contemplate cell-based therapeutics for the treatment of degenerative disc disease, one must wonder what might be the most appropriate source of cells. Bone marrow-derived stem cells originate within an entirely different niche to cells that have adapted to survive within the NP with its tenuous nutrient supply and hypoxic environment. Along which lineage should stem cells or progenitor cells therefore be directed in order to potentially repopulate the disc and how would they best be able to restore homeostasis? For now, given that the mature disc nucleus contains holdouts of the primitive notochordal cell perhaps the best perspective from which to answer these questions is one where we take a fresh look back at the origin, development and maturation of the IVD.     

The efficacy of Link N as a mediator of repair in a rabbit model of intervertebral disc degeneration

Introduction  

Intervertebral disc (IVD) degeneration is associated with proteolytic degradation of the extracellular matrix, and its repair requires both the production of extracellular matrix and the downregulation of proteinase activity. These properties are associated with several growth factors. However, the use of growth factors in clinical practice is limited by their high cost. This cost can be circumvented using synthetic peptides, such as Link N, which can stimulate the synthesis of proteoglycan and collagen by IVD cells in vitro. The purpose of the present study was to evaluate the effect of Link N in vivo in a rabbit model of IVD degeneration.     

CD24 identifies nucleus pulposus progenitors/notochordal cells for disc regeneration

Background

Cell-based therapy by transplantation of nucleus pulposus (NP) progenitor/notochordal cells has been proposed as a promising way to halt and reverse the progression of disc degeneration. Although some studies have provided a broad panel of potential markers associated with the phenotype of notochordal cells, suitability of these markers for isolation of notochordal cells for the treatment of disc degeneration is unclear.

Results

Here, we found that the number of CD24-positive NP cells significantly decreased with increasing severity of disc degeneration. In addition, CD24-positive NP cells were shown to maintain their multipotent differentiation and self-renewal potential in vitro and to abundantly express brachyury, SHH, and GLUT-1, suggesting that CD24-positive NP cells are the progenitor/notochordal cells in the NP. Moreover, our in vivo experiments revealed that transplantation of CD24-positive NP cells enables the recovery of degenerate discs, as evidenced by increased disc height, restored magnetic resonance imaging T2-weighted signal intensity, and NP structure. In terms of the mechanism, HIF-1α–Notch1 pathway activation was essential for the maintenance of CD24-positive NP cells.

Conclusion

Our studies identify that CD24-positive NP cells are the resident progenitor/notochordal cells in disc regeneration and elucidate a crucial role of HIF-1α–Notch1 pathway in the phenotypic maintenance of CD24-positive NP cells.

     

Stiffness of photocrosslinkable gelatin hydrogel influences nucleus pulposus cell propertiesin vitro

A key early sign of degenerative disc disease (DDD) is the loss of nucleus pulposus (NP) cells (NPCs). Accordingly, NPC transplantation is a treatment strategy for intervertebral disc (IVD) degeneration. However, in advanced DDD, due to structural damage of the IVD and scaffold mechanical properties, the transplanted cells are less viable and secrete less extracellular matrix, and thus, are unable to efficiently promote NP regeneration. In this study, we evaluated the encapsulation of NPCs in a photosensitive hydrogel made of collagen hydrolysate gelatin and methacrylate (GelMA) to improve NP regeneration. By adjusting the concentration of GelMA, we prepared hydrogels with different mechanical properties. After examining the mechanical properties, cell compatibility and tissue engineering indices of the GelMA-based hydrogels, we determined the optimal hydrogel concentration of the NPC-encapsulating GelMA hydrogel for NP regeneration as 5%. NPCs effectively combined with GelMA and proliferated. As the concentration of the GelMA hydrogel increased, the survival, proliferation and matrix deposition of the encapsulated NPCs gradually decreased, which is the opposite of NPCs grown on the surface of the hydrogel. The controllability of the GelMA hydrogels suggests that these NPC-encapsulating hydrogels are promising candidates to aid in NP tissue engineering and repairing endogenous NPCs.     

Expression and regulation of neurotrophins in the nondegenerate and degenerate human intervertebral disc

Introduction

The neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) have been identified in the human intervertebral disc (IVD) and have been implicated in the mechanisms associated with nerve ingrowth and nociception in degeneration of the IVD. The aim of the current study was to investigate an association between neurotrophin expression in the IVD and the severity of disc degeneration, including the effect of disc-related proinflammatory cytokines on neurotrophin and neuropeptide expression in cells derived from the human IVD.

Methods

Immunohistochemical analysis was performed to examine the expression of NGF, BDNF and their high-affinity receptors Trk-A and Trk-B in human IVD samples, divided into three categories: non-degenerate, moderate degeneration and severe degeneration. In order to study the effect of disc-related cytokines on neurotrophin/neuropeptide gene expression, nucleus pulposus cells derived from non-degenerate and degenerate IVD samples were seeded in alginate and were stimulated with either IL-1β or TNFα for 48 hours. RNA was extracted, cDNA was synthesised and quantitative real-time PCR was performed to examine the expression of NGF, BDNF and substance P.

Results

Immunohistochemistry showed expression of NGF and BDNF in the native chondrocyte-like cells in all regions of the IVD and in all grades of degeneration. Interestingly only BDNF significantly increased with the severity of degeneration (P < 0.05). Similar expression was observed for Trk-A and Trk-B, although no association with disease severity was demonstrated. In cultured human nucleus pulposus cells, stimulation with IL-1β led to significant increases in NGF and BDNF gene expression (P < 0.05). Treatment with TNFα was associated with an upregulation of substance P expression only.

Conclusion

Our findings show that both the annulus fibrosus and nucleus pulposus cells of the IVD express the neurotrophins NGF and BDNF, factors that may influence and enhance innervation and pain in the degenerate IVD. Expression of Trk-A and Trk-B by cells of the nondegenerate and degenerate IVD suggests an autocrine role for neurotrophins in regulation of disc cell biology. Furthermore, modulation of neurotrophin expression by IL-1β and modulation of substance P expression by TNFα, coupled with their increased expression in the degenerate IVD, highlights novel roles for these cytokines in regulating nerve ingrowth in the degenerate IVD and associated back pain.     

An in vitro study investigating the survival and phenotype of mesenchymal stem cells following injection into nucleus pulposus tissue

Introduction  

The decreased disc height characteristic of intervertebral disc (IVD) degeneration has often been linked to low back pain, and thus regeneration strategies aimed at restoring the disc extracellular matrix and ultimately disc height have been proposed as potential treatments for IVD degeneration. One such therapy under investigation by a number of groups worldwide is the use of autologous mesenchymal stem cells (MSCs) to aid in the regeneration of the IVD extracellular matrix. To date, however, the optimum method of application of these cells for regeneration strategies for the IVD is unclear, and few studies have investigated the direct injection of MSCs alone into IVD tissues. In the present article, we investigated the survival and phenotype of human MSCs, sourced from aged individuals, following injection into nucleus pulposus (NP) tissue explant cultures.