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Molecular mechanisms involved in intervertebral disc degeneration and potential new treatment strategies

Molecular mechanisms involved in intervertebral disc degeneration and potential new treatment... Volume 2 † Number 1 † March 2009 10.1093/biohorizons/hzp005 ......................................................................................................................................................................................................................................... Review Molecular mechanisms involved in intervertebral disc degeneration and potential new treatment strategies Daniel Ireland* Sheffield Hallam University, City Campus, Howard Street, Sheffield S1 1WB, UK. * Corresponding author: Tel: þ44 (0)7752867887. Email: daniel2001ireland_peru@hotmail.co.uk Supervisor: Christine Lyn Le Maitre, Molecular Cell Biology, Sheffield Hallam University, Biosciences, Faculty of Health and Well Being, City Campus, Howard Street, 730 Owen Building, Sheffield S1 1WB, UK ........................................................................................................................................................................................................................................ Lower back pain (LBP) is a major cause of pain and disability. However, current treatment strategies are focused primarily on relieving its symptoms and have varying degrees of success. For future treatments to be proactive, they must target the underlying pathogenic alterations in cellular biology. Intervertebral disc degeneration (IVDD) has been linked to a high percentage of LBP cases, therefore, inhi- bition of the processes contributing to IVDD and, regeneration of the intervertebral disc (IVD) matrix lost during IVDD are the primary focuses of current research. Therapies aimed at the inhibition of the cytokine interleukin-1 that is increased during IVDD have been investigated as potential treatments aimed at inhibiting the pathogenic processes of IVDD. In addition, the application of growth factors, such as insulin-like growth factor, transforming growth factor and bone morphogenetic protein or alternatively replacement of abnormal IVD cells, either by injection of mesenchymal stem cells or autologous disc cell transplantation, has been investigated as potential therapeutic agents aimed at regeneration of the IVD matrix. However, for research into these therapeutic techniques to pro- gress, a more detailed knowledge of the complex cellular biology of the IVD is required. Key words: intervertebral disc degeneration, cytokine, growth factor, gene therapy, mesenchymal stem cells, autologous disc cell transplantation. ........................................................................................................................................................................................................................................ asymptomatic individuals. In addition, it has been suggested Lower Back Pain that between 52 and 74% of LBP cases could arise from Lower back pain (LBP), sometimes referred to as lumbago, is genetic origin. However, although some genes have been ident- a highly prevalent cause of pain and disability in developed ified, the genetic influence is still poorly understood. countries. Epidemiological studies estimate that approxi- Existing therapies for LBP are mainly symptomatic and mately 80% of the population will suffer from LBP at achieve inconsistent results. The current therapies range some point of time in their lives. An additional factor to from simple non-operative treatments such as medication consider when assessing the morbidity of LBP is the econ- aimed at pain relief to more extreme operative procedures 4, 6 omic burden as LBP has been estimated to cost the UK aimed at alleviating the clinical symptoms of LBP. 2, 3 approximately £12 billion per annum. These therapies do not target the pathogenic changes LBP symptoms range from pain specific to areas of the lower which are seen in the degenerate disc and have been linked back, and can lead to sciatica, where the pain radiates down to approximately 40% of LBP cases. Thus, future treatment into either one or both of the legs. Although the exact pathogen- options under investigation are targeted at the underlying esis of LBP has not yet been elucidated, a variety of causes such pathogenesis of LBP rather than the symptoms which arise as mechanical stress, herniated disc, tissue injury and interver- as a result of pathogenic processes. However, for the patho- tebral disc degeneration (IVDD) have been implicated as poten- genesis of LBP to be targeted, a detailed knowledge of the tial causes, however these features can also be observed in anatomy and physiology of the spine is required. ......................................................................................................................................................................................................................................... 2009 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 83 Review Bioscience Horizons † Volume 2 † Number 1 † March 2009 ......................................................................................................................................................................................................................................... The human spine is composed of 33 individual bones, 24 of pathogenesis resulting in this condition is limited. which are movable vertebrae, divided into the cervical, thor- Degeneration of the disc occurs during the normal ageing acic and lumbar regions in addition to seven fused vertebrae process, however accelerated degeneration is seen in diseased 8 12 consisting of the sacral and coccygeal regions. The vertebrae discs. While IVD height does undergo some decrease with are separated by intervertebral discs (IVDs), which allow age, the reduction is not as significant as during IVDD, hence bending and torsion of the spine and protect it during mech- the loss of disc height is characteristic of IVDD and can be 9 15 anical loading. The individual vertebrae and IVDs are held used in the diagnosis of the condition. IVDD is a multifac- together by ‘ligaments’ that provide the tension that facilitates torial process influenced by mechanical pressures, genetic the curved posture of the spine. LBP is associated with inheritance and alterations in the cellular biology of the abnormalities in the IVDs of the lumbar region, which is IVD. under continuous pressure from bending, twisting and The chondrocyte-like cells of the NP are responsible for lifting, and supports the weight of the entire upper body. the production of growth factors such as bone morphogen- etic protein (BMP), transforming growth factor (TGF) and insulin-like growth factor (IGF) and cytokines such as inter- Intervertebral Discs leukin (IL)-1 (IL-1), IL-6 and tumour necrosis factor Collectively the IVD contribute 25% of the length of the (TNF). Growth factors are involved in the regulation of spine. Each IVD is composed of the annulus fibrosus IVD matrix by stimulating production of matrix proteins. (AF) and nucleus pulposus (NP). In contrast, the cytokines inhibit matrix synthesis and stimu- The AF consists of densely arranged, circumferential, lamel- late the production of degradative enzymes such as matrix lae rich in collagen fibres. The AF is mainly composed of col- metalloproteinases (MMPs) and a disintegrin-like and metal- lagen type I, but also contains other types of collagen loprotease with thrombospondin motifs (ADAMTS) which 11 11 fibres. The peripheral layer of the AF has a high concen- break down the extracellular matrix of the IVD. The tration of collagen type I fibres that progressively become chondrocyte-like cells of the IVD are involved in both the more proportionally collagen type II fibres towards the centre synthesis of the IVD matrix and also the production of the of the IVD. The AF is responsible for encasing the gelatinous enzymes involved in its degradation. Chondrocyte-like cell NP and enabling the IVD to sustain considerable compressive abnormalities result in alteration of the homeostatic pro- loads. The outer AF is composed of fibroblast-like cells, but is cesses within NP cells and have therefore been identified as populated by chondrocyte-like cells towards the inner AF potential pathogenic agents involved in IVDD. region. The NP core of the IVD is comprised of chondrocyte- Increased MMP and ADAMTS enzyme activity is a like cells, collagen (mainly type II) fibres and proteoglycans, characteristic of IVDD, particularly, MMP 7 and MMP 17 17 19 mainly aggrecan with smaller amounts of versican, decorin, 13 and ADAMTS 4 and ADAMTS 5, which mediate biglycan, fibromodulin and lumican. The proteoglycan-rich the degradation of the IVD matrix components collagen matrices, especially the aggrecans, are negatively charged and type II and aggrecan. attract water resulting in a hydrophilic core. This hydrophilic MMP enzymes are secreted in their latent form and so composition of the NP enables the structure to withstand com- require activation to enable their proteolytic properties. pressive force and acts as the shock absorber in the spine. In addition, MMP activity is blocked by endogenous tissue Cartilaginous end plates (CEPs) are located superior and inhibitors of metalloproteases (TIMP) of which there are inferior to the IVD and are involved in the attachment of each four types: TIMP 1, 2, 3 and 4. TIMP 1 and 2 are the individual IVD to their respective vertebrae. The CEP is a major inhibitors of MMPs. However, as production of thin structure approximately 1 mm in width, composed of TIMP 1 and 2 is also increased in the IVD during cases of 13 17 hyaline cartilage and populated by chondrocyte cells. As the IVDD, the MMPs such as MMP 7, which demonstrate IVD is mainly avascular, the CEP is heavily involved in the nutri- resistance to TIMP 1 and 2, have been implicated as key 13 18 tion of, and removal of waste products from, the IVD. enzymes involved in the pathogenesis of IVDD. Nutrients such as glucose and oxygen enter the IVD by passive ADAMTS, however, are inhibited specifically by TIMP 3, diffusion in a process which is facilitated by the blood vessels which, in contrast to TIMP 1 and 2, is not upregulated 14 17 which penetrate the CEP and the peripheral AF. Waste pro- during cases of IVDD; therefore, the aggrecanase activity ducts such as lactic acid and carbon are removed in a reciprocal of the ADAMTS is increased in cases of IVDD, because manner. As the CEP ages, calcification occurs and consequently the increased production in ADAMTS enzymes are not con- 13 11 the ability to supply the IVD with nutrients deteriorates. trolled by a concurrent increase in their inhibitor. In addition to increased matrix degradation, cells in IVDD 19, 21, 22 become abnormal and display cell senescence, which Intervertebral Disc Degeneration has been linked to the increase in matrix degradation IVDD has been identified in approximately 40% of LBP enzymes. A reduction in the number of active chondroctye- diagnoses, however current knowledge of the principal like cells leads to a decrease in the production of the growth ......................................................................................................................................................................................................................................... 84 Bioscience Horizons † Volume 2 † Number 1 † March 2009 Review ......................................................................................................................................................................................................................................... factors required to stimulate matrix synthesis. Consequently, identified as a potential solution to the administration pro- any matrix degradation cannot be repaired and will also blems as a single dose of IL-1Ra could be administered via result in a decrease in overall IVD matrix. gene therapy to the IVD which would provide long-lasting The matrix composition and cellular activity of the IVD is effects. integral to its functionality. IVDD results in loss of the IVD Genetic material can be delivered into host cells by vectors matrix, loss of chondrocyte-like cells of the NP, decreased via one of two methods in vivo or ex vivo. In vivo refers to structural integrity, hydration and ability to withstand mech- the direct injection or inhalation of the vector. Ex vivo is anical loads, angiogenesis and innervation. These biological the process where host cells are removed and the vector is changes result in IVDD and the associated decreased disc applied in vitro, and then the modified cells are returned to height, which can result in the symptoms associated with the host. There are two classifications of vectors which can LBP. be used in gene therapy: viral or non-viral vectors. Viral vectors can be further subdivided into two categories: genome incorporating, which include retroviruses and lenti- Future Treatment Strategies viruses and non-genome incorporating viruses, such as For future LBP therapies to be successful, they must be aimed herpes viruses, adenoviruses and adeno-associated viruses. at the underlying pathogenesis such as the alterations in cel- Viral vectors are genetically modified viruses which have lular biology, which have been linked to LBP. Particular con- been engineered to lack the genetic material which makes sideration has been given to the role of the cytokines IL-1 them pathogenic while retaining the genetic information and TNFa in the pathogenesis of IVDD. During IVDD, the which enables insertion of their genes into host cells. expression of both cytokines is increased, and both have been Additionally, a copy of the therapeutic gene is inserted into shown to inhibit matrix synthesis and to increase gene the viral vector so that it may be transferred into the host 23– 26 expression for matrix degradation enzymes in vitro. cell. However, although the functional receptors of IL-1 are upre- Adenoviral vectors have been utilized in various gene 6, 15, 28 –30 gulated throughout IVDD, expression of the TNFa receptors therapy investigations on the IVD. As adeno- is actually decreased. Therefore, while both cytokines are viruses do not incorporate into the host cell’s genetic expressed in instances of IVDD, IL-1 has been implicated material, the potential risk of oncogenesis associated with 25, 26 6 as the major mediator of IVDD. Indeed, using in situ genome-incorporating vectors is eliminated. One potential zymography techniques, inhibitors of IL-1 have been shown drawback of adenoviral vectors is that therapeutic genes to inhibit matrix degradation in both normal and degenerate are not transferred to daughter cells when the IVD cells repli- discs, however inhibition of TNFa failed to reduce the cate; however, due to the low cell turnover in the IVD, this matrix degradation, thus supporting the theory that IL-1 is potential disadvantage should not be a significant problem 26 6 a key cytokine involved in the pathogenesis of IVDD. in practice. The natural inhibitor of IL-1, IL-1 receptor antagonist Ex vivo administration of IL-1Ra to human IVD explants, (IL-1Ra), is also produced by IVD cells. In non-degenerate using adenovirus-mediated gene therapy, significantly inhi- IVDs, the actions of IL-1 appear to be controlled by the bits the activity of the degradative enzymes in degenerated 6, 15 levels of IL-1Ra present, however during IVDD an increase discs. In addition, other treatment strategies, such as is seen in the production of IL-1 with no parallel increase oral administration of glucosamine and chondroitin 24 31 in IL-1Ra and thus the balance is lost. As such, IL-1Ra sulfate and gene therapy targeting the anti-catabolic gene has been identified as a possible therapeutic agent to target TIMP1, which will inhibit the increased production of the inhibition of IL-1-mediated pathogenesis of IVDD. MMPs seen during disc degeneration, aimed at inhibiting IL-1Ra has been administered previously in clinical pro- the processes of IVDD have been recently reviewed, but cedures such as limiting cartilage degradation in rheumatoid will not address the increased production of ADAMTS arthritis, and consequently the related pharmacology and which also appear important in the pathogenesis of IVDD. side effects are well elucidated. However, in these circum- Preventing the loss of IVD matrix composition would stances IL-1Ra is injected subcutaneously, a technique which ideally be used in conjunction with an approach aimed at cannot be applied to the IVD due to their avascular compo- regenerating the IVD matrix in order to begin reversing the sition. Any passive diffusion into the IVD would not be matrix composition. Therefore, regeneration of the degener- sufficient to elicit a clinical effect and additionally there is ated IVD matrix must be considered as an additional and a risk that inhibition of IL-1 in the systemic circulation equally important factor in the treatment of IVDD. would occur. Direct administration of cytokines to the IVD by injection is another previously proposed method; Regeneration of Disc Matrix however, due to the short biological half-life of IL-1Ra, repeated administration would be necessary to achieve the Both the application of growth factors such as IGF, TGF and desired clinical impact. Gene therapy has therefore been BMPs and alternatively replacement of abnormal IVD cells, ......................................................................................................................................................................................................................................... 85 Review Bioscience Horizons † Volume 2 † Number 1 † March 2009 ......................................................................................................................................................................................................................................... either by injection of adult mesenchymal stem cells (MSCs) involving various transcription factors are involved in the or autologous IVD cells, have been investigated as potential production of growth factors and the matrix components therapeutic agents aimed at regeneration of the IVD matrix. of the IVD, especially collagens and proteoglycans. Sox 43 44 45 Growth factors are peptides the function of which is to 9, c-Jun and LIM mineralization protein (LMP)-1 are regulate the stimulation of cellular proliferation, differen- transcription factors involved in chondrogenesis. Sox 9 has tiation and migration and to stimulate matrix synthesis. In been shown to upregulate type II collagen synthesis. the IVD specific growth factors such as IGF, TGF and BMP Therefore, the delivery of a recombinant adenoviral vector are produced by the chondrocyte-like cells of the NP and act which expresses Sox 9 has been investigated as a novel thera- 4 43 to stimulate matrix synthesis. As loss of IVD matrix compo- peutic treatment for IVDD. Sox 9 production is signifi- sition is a characteristic feature of IVDD, growth factors have cantly increased in degenerated human IVD cells been investigated as potential therapeutic agents, aimed at transfected using adenovirus-mediated gene therapy and an promoting matrix synthesis in the degenerated IVD. increase in type II collagen synthesis is also recorded. As Preliminary work implementing exogenous application of type II collagen is an essential constituent of the NP, which 33 34 35 growth factors, such as TGF-b1, IGF-1 and BMP-7, in cases of IVDD is notably reduced, gene therapy targeting demonstrated increased IVD matrix synthesis following Sox 9 has enormous potential in the treatment for IVDD. In administration. However, as growth factors are known to addition to Sox 9, LMP-1 has been shown to mediate proteo- have short biological half-lives, this method of adminis- glycan production through its action on BMPs. In vitro tration would not be feasible in clinical scenarios as IVDD experiments using adenoviral vectors to deliver LMP-1 is a chronic condition and therefore requires the growth demonstrate increased proteoglycan aggrecan content in 36 46 factor application to be prolonged. The requirement for a NP-like cells. In addition, adenoviral vector delivery of sustainable method of growth factor application resulted in LMP-1 in vivo causes a significant increase in the levels of the development of the gene therapy delivery system for the LMP-1, BMP-2 and BMP-7. application of growth factors. Initial gene therapy studies A characteristic feature of IVDD is the reduction in meta- examined the possibility of introducing exogenous genes bolically active IVD cells and IVD cell number. A sufficient into IVD cells in vitro utilizing retroviral vectors and cell population is essential to maintain its homeostatic mech- both in vitro and in vivo studies utilizing adenoviral anisms. A suggested method of rectifying this is autologous vectors. The success of gene therapy studies on rabbit IVD cell transplantation (ADCT). ADCT involves the IVD cells was the foundation for similar experiments on explantation of IVD chondrocyte-like cells from herniated human IVD cells. Adenoviral vectors were implemented or degenerate discs, which are subsequently cultured to transduce TGFb1 genes into human IVD cells, which in vitro and replaced into the autologous IVD tissue from resulted in increased TGFb1 protein expression which in which they were derived. turn increased synthesis of proteoglycan and collagen by Promising results have been attained from clinical trials over 300%, which was significantly more than exogenous evidencing the success of ADCT. After 24 months, the application. Studies have also examined gene therapy control group of IVDD patients, who underwent surgery delivery of other growth factors such as BMP-2 and IGF-1 but did not receive ADCT, suffered a 75% reduction in which have also been implicated in the processes of IVD IVD fluid content. In comparison, the IVDs of the IVDD matrix synthesis. Future gene therapy approaches could patients that underwent ADCT only demonstrated a 59% potentially combine the various growth factors implicated reduction in fluid content. In addition, the pain suffered in the processes of IVD matrix synthesis such as TGFb1, by the ADCT-treated patients was less, according to the IGF-1 and BMP-2, in order to treat IVDD, and an additive score attained from questionnaires, in comparison to the effect on the amplification of proteoglycan synthesis has control group after 24 months. However, there was still a been observed. significant decrease in fluid content. A number of studies However, an important observation that TGF and IGF have demonstrated that the cells of a degenerated disc act 41 4, 13, 19, 21, 22 receptors are expressed in the blood vessels of the IVD abnormally and display senescence and thus suggests that the therapeutic use of these particular growth the use of degenerated cells as a therapeutic source is inher- factors to regenerate the IVD matrix could cause excessive ently flawed. angiogenesis and subsequently nerve ingrowth and therefore In addition, replacement of degenerated IVD cells with 16, 42 would not be a viable treatment strategy. As BMP recep- autologous IVD cells has a restricted potential for the treat- tor expression was not observed in the IVD blood vessels, ment of IVDD because of the limited population of IVD cells growth factors targeted at these receptors have been ident- which can be explanted for culture. Despite this, ADCT is ified as preferable therapeutic agents for therapies aimed at currently the only therapeutic technique aimed at IVDD regenerating IVD matrix in cases of IVDD. which has been clinically tested and the results demonstrate An alternative to the use of growth factors is the utiliz- that effective pain relief and changes in IVD matrix compo- ation of transcription factors. Complex signalling cascades sition are achieved long term. However, the removal of ......................................................................................................................................................................................................................................... 86 Bioscience Horizons † Volume 2 † Number 1 † March 2009 Review ......................................................................................................................................................................................................................................... autologous cells and culture in vitro is limited to a few par- At present, it is unclear if MSC differentiation can be ticular scenarios such as IVD herniation, because of the achieved long term or whether differentiated cells will repli- limited number of cells which could be harvested and the cate. In addition, the stage of degeneration may also influ- possibility that implantation of the autologous cells could ence the effectiveness of this type of therapy. These cause detrimental damage to the structure of the IVD and factors would need to be sufficiently evaluated before MSC accelerate its degeneration. More recently, attention has therapy for the treatment of IVDD could be evaluated in 48 55 turned to the potential of MSCs. clinical settings. MSC implantation therapy if used indivi- Stem cells are undifferentiated cells with two defining dually may counter the progression of IVDD, however, for properties; the ability to differentiate into specialized cells full regeneration of the IVD, an approach aimed at the cessa- and the capability of self renewal long term. After fertiliza- tion of IVDD delivered in tandem would be required. tion, a single totipotent cell is formed, which is capable of In addition to these major therapeutic approaches, other giving rise to all the cell types of the organism. During novel therapies have been proposed. Artificial IVD implan- embryonic development, this cell differentiates into pluripo- tation strategies, aimed at replacing degenerate IVDs, show tent stem cells which have the ability to differentiate into all promising clinical potential and have recently been reviewed. cell lineages. These cells then further differentiate into multi- Alternatively, whole disc transplantation has been proposed. potent stem cells which can only differentiate into cells of a However, the problems associated with identifying suitable particular germ layer. Adult MSCs are undifferentiated mul- donors make this a less viable clinical option. Tissue engin- tipotent stem cells which can be derived from several tissues eering techniques which aim to augment the function of degen- including skeletal muscle, synovial membranes, adipose, erate IVDs have also been developed. Some of these techniques dermis and bone marrow. In addition to being easily acces- utilize biomaterials that mimic the properties of the NP which sible, MSCs have a high plasticity and able to differentiate are injected into the IVD in order to provide a scaffold for the into bone, cartilage, fat and fibrous tissues. The use of existing chondrocyte-like cells and thus prevent further adult stem cells bypasses the controversial ethical issues damage such as decreased IVD matrix synthesis, which is regarding the acquisition of embryonic stem cells. Adult caused by the additional mechanical stress to the IVD as a stem cells are involved in the replacement of cells lost result of reduced IVD height. during normal matrix turnover. Various methods have It has been postulated that there may be a threshold beyond been tried in order to differentiate MSCs into the which the damage caused during IVDD becomes irrevers- 48, 50– 52 13 chondrocyte-like cells found in the NP. ible, however, recent research suggests that therapies The exogenous application of growth factors to MSCs which are delivered in later stages of IVDD are actually 51 54 in vitro can influence the differentiation of these cells more effective. Multifactorial conditions with poorly using current knowledge of the effects of growth factors on defined pathogenic mechanisms such as IVDD are a particular NP cells; studies have attempted to culture MSCs to initiate challenge for gene therapy and for the potential of gene differentiation into chondrocyte-like cells using the growth therapy to be maximized it is essential that the pathogenic factors TGFb and BMP. However, inducing MSC differen- processes of IVDD are fully elucidated to enable targeting to tiation with individual growth factors is problematic, the correct molecule or molecules. The therapeutic use of because multiple growth factors are involved in the normal growth factors has a vast potential for stimulating the syn- development of the IVD. thesis of IVD matrix components. However, for the full Co-cultivation of MSCs with IVD cells in vitro may be suf- potential to be accomplished the knowledge of the complex ficient to induce differentiation into the chondrocyte-like cell cellular biology of the IVD must be improved. For example, phenotype. However, because current knowledge of the although application of TGFb has a significant effect on pro- IVD NP cell phenotype is incomplete, a definitive conclusion teoglycan and collagen synthesis, receptors are found in the on whether differentiation has been successful is impossible. blood vessels in the periphery of the IVD. This is not the For MSC-based therapy to progress, a better knowledge of case with BMP suggesting that it has better therapeutic poten- the complex cellular biology of the IVD is required than is tial. With a greater understanding of the complex cellular currently available. The discovery that differentiation of biology of the IVD, potential agents for use in gene therapy MSC cells into IVD-like cells can be achieved by co-culture and regeneration strategies could be identified. of MSCs with IVD cells suggests that rapid production of NP cells could be possible without the removal of IVD Conclusions 48, 50 tissue. This evidence supports the theory that harvested MSCs could potentially be administered directly to the IVD, It is clear that future treatments aimed at the underlying patho- circumventing the requirement for differentiation to be genesis of IVDD are required, as current therapeutic strategies induced in vitro, however, the in vivo experiments required are purely symptomatic. The literature cited in this review high- to validate the use of this approach in clinical scenarios lights inhibition of the processes contributing to IVDD and have not been conducted to a sufficient standard to date. promotion of IVD matrix synthesis as the primary focuses of ......................................................................................................................................................................................................................................... 87 Review Bioscience Horizons † Volume 2 † Number 1 † March 2009 ......................................................................................................................................................................................................................................... 17. 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(2004) Differentiation of for intervertebral degenerative disc disease. Spine 28: 755–763. mesenchymal stem cells towards a nucleus pulposus-like phenotype in 44. Behrens A, Haigh J, Mechta-Grigoriou F et al. (2003) Impaired intervertebral vitro: implications for cell-based transplantation therapy. Spine 29: disc formation in the absence of Jun. Development 130: 103–109. 2627–2632. 45. Boden SD, Liu Y, Hair GA et al. (1998) LMP-1, an LIM-domain protein, med- 54. Ho G, Leung VY, Cheung KM et al. (2008) Effect of severity of intervertebral iates BMP-6 effects on bone formation. Endocrinology 139: 5125–5134. disc injury on mesenchymal stem cell-based regeneration. Connect Tissue Res 49: 15–21. 46. Yoon ST, Park JS, Kim KS et al. (2004) ISSLS prize winner: LMP-1 upregulates intervertebral disc cell production of proteoglycans and BMPs in vitro and in 55. Vadala G, Sobajima S, Lee JY et al. (2007) In vitro interaction between vivo. Spine 29: 2603–2611. muscle-derived stem cells and nucleus pulposus cells. Spine J 8: 804–809. ........................................................................................................................................................................................................................................ Submitted on 30 September 2008; accepted on 22 January 2009; advance access publication 24 February 2009 ......................................................................................................................................................................................................................................... http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Bioscience Horizons Oxford University Press

Molecular mechanisms involved in intervertebral disc degeneration and potential new treatment strategies

Bioscience Horizons , Volume 2 (1) – Mar 24, 2009

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Volume 2 † Number 1 † March 2009 10.1093/biohorizons/hzp005 ......................................................................................................................................................................................................................................... Review Molecular mechanisms involved in intervertebral disc degeneration and potential new treatment strategies Daniel Ireland* Sheffield Hallam University, City Campus, Howard Street, Sheffield S1 1WB, UK. * Corresponding author: Tel: þ44 (0)7752867887. Email: daniel2001ireland_peru@hotmail.co.uk Supervisor: Christine Lyn Le Maitre, Molecular Cell Biology, Sheffield Hallam University, Biosciences, Faculty of Health and Well Being, City Campus, Howard Street, 730 Owen Building, Sheffield S1 1WB, UK ........................................................................................................................................................................................................................................ Lower back pain (LBP) is a major cause of pain and disability. However, current treatment strategies are focused primarily on relieving its symptoms and have varying degrees of success. For future treatments to be proactive, they must target the underlying pathogenic alterations in cellular biology. Intervertebral disc degeneration (IVDD) has been linked to a high percentage of LBP cases, therefore, inhi- bition of the processes contributing to IVDD and, regeneration of the intervertebral disc (IVD) matrix lost during IVDD are the primary focuses of current research. Therapies aimed at the inhibition of the cytokine interleukin-1 that is increased during IVDD have been investigated as potential treatments aimed at inhibiting the pathogenic processes of IVDD. In addition, the application of growth factors, such as insulin-like growth factor, transforming growth factor and bone morphogenetic protein or alternatively replacement of abnormal IVD cells, either by injection of mesenchymal stem cells or autologous disc cell transplantation, has been investigated as potential therapeutic agents aimed at regeneration of the IVD matrix. However, for research into these therapeutic techniques to pro- gress, a more detailed knowledge of the complex cellular biology of the IVD is required. Key words: intervertebral disc degeneration, cytokine, growth factor, gene therapy, mesenchymal stem cells, autologous disc cell transplantation. ........................................................................................................................................................................................................................................ asymptomatic individuals. In addition, it has been suggested Lower Back Pain that between 52 and 74% of LBP cases could arise from Lower back pain (LBP), sometimes referred to as lumbago, is genetic origin. However, although some genes have been ident- a highly prevalent cause of pain and disability in developed ified, the genetic influence is still poorly understood. countries. Epidemiological studies estimate that approxi- Existing therapies for LBP are mainly symptomatic and mately 80% of the population will suffer from LBP at achieve inconsistent results. The current therapies range some point of time in their lives. An additional factor to from simple non-operative treatments such as medication consider when assessing the morbidity of LBP is the econ- aimed at pain relief to more extreme operative procedures 4, 6 omic burden as LBP has been estimated to cost the UK aimed at alleviating the clinical symptoms of LBP. 2, 3 approximately £12 billion per annum. These therapies do not target the pathogenic changes LBP symptoms range from pain specific to areas of the lower which are seen in the degenerate disc and have been linked back, and can lead to sciatica, where the pain radiates down to approximately 40% of LBP cases. Thus, future treatment into either one or both of the legs. Although the exact pathogen- options under investigation are targeted at the underlying esis of LBP has not yet been elucidated, a variety of causes such pathogenesis of LBP rather than the symptoms which arise as mechanical stress, herniated disc, tissue injury and interver- as a result of pathogenic processes. However, for the patho- tebral disc degeneration (IVDD) have been implicated as poten- genesis of LBP to be targeted, a detailed knowledge of the tial causes, however these features can also be observed in anatomy and physiology of the spine is required. ......................................................................................................................................................................................................................................... 2009 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 83 Review Bioscience Horizons † Volume 2 † Number 1 † March 2009 ......................................................................................................................................................................................................................................... The human spine is composed of 33 individual bones, 24 of pathogenesis resulting in this condition is limited. which are movable vertebrae, divided into the cervical, thor- Degeneration of the disc occurs during the normal ageing acic and lumbar regions in addition to seven fused vertebrae process, however accelerated degeneration is seen in diseased 8 12 consisting of the sacral and coccygeal regions. The vertebrae discs. While IVD height does undergo some decrease with are separated by intervertebral discs (IVDs), which allow age, the reduction is not as significant as during IVDD, hence bending and torsion of the spine and protect it during mech- the loss of disc height is characteristic of IVDD and can be 9 15 anical loading. The individual vertebrae and IVDs are held used in the diagnosis of the condition. IVDD is a multifac- together by ‘ligaments’ that provide the tension that facilitates torial process influenced by mechanical pressures, genetic the curved posture of the spine. LBP is associated with inheritance and alterations in the cellular biology of the abnormalities in the IVDs of the lumbar region, which is IVD. under continuous pressure from bending, twisting and The chondrocyte-like cells of the NP are responsible for lifting, and supports the weight of the entire upper body. the production of growth factors such as bone morphogen- etic protein (BMP), transforming growth factor (TGF) and insulin-like growth factor (IGF) and cytokines such as inter- Intervertebral Discs leukin (IL)-1 (IL-1), IL-6 and tumour necrosis factor Collectively the IVD contribute 25% of the length of the (TNF). Growth factors are involved in the regulation of spine. Each IVD is composed of the annulus fibrosus IVD matrix by stimulating production of matrix proteins. (AF) and nucleus pulposus (NP). In contrast, the cytokines inhibit matrix synthesis and stimu- The AF consists of densely arranged, circumferential, lamel- late the production of degradative enzymes such as matrix lae rich in collagen fibres. The AF is mainly composed of col- metalloproteinases (MMPs) and a disintegrin-like and metal- lagen type I, but also contains other types of collagen loprotease with thrombospondin motifs (ADAMTS) which 11 11 fibres. The peripheral layer of the AF has a high concen- break down the extracellular matrix of the IVD. The tration of collagen type I fibres that progressively become chondrocyte-like cells of the IVD are involved in both the more proportionally collagen type II fibres towards the centre synthesis of the IVD matrix and also the production of the of the IVD. The AF is responsible for encasing the gelatinous enzymes involved in its degradation. Chondrocyte-like cell NP and enabling the IVD to sustain considerable compressive abnormalities result in alteration of the homeostatic pro- loads. The outer AF is composed of fibroblast-like cells, but is cesses within NP cells and have therefore been identified as populated by chondrocyte-like cells towards the inner AF potential pathogenic agents involved in IVDD. region. The NP core of the IVD is comprised of chondrocyte- Increased MMP and ADAMTS enzyme activity is a like cells, collagen (mainly type II) fibres and proteoglycans, characteristic of IVDD, particularly, MMP 7 and MMP 17 17 19 mainly aggrecan with smaller amounts of versican, decorin, 13 and ADAMTS 4 and ADAMTS 5, which mediate biglycan, fibromodulin and lumican. The proteoglycan-rich the degradation of the IVD matrix components collagen matrices, especially the aggrecans, are negatively charged and type II and aggrecan. attract water resulting in a hydrophilic core. This hydrophilic MMP enzymes are secreted in their latent form and so composition of the NP enables the structure to withstand com- require activation to enable their proteolytic properties. pressive force and acts as the shock absorber in the spine. In addition, MMP activity is blocked by endogenous tissue Cartilaginous end plates (CEPs) are located superior and inhibitors of metalloproteases (TIMP) of which there are inferior to the IVD and are involved in the attachment of each four types: TIMP 1, 2, 3 and 4. TIMP 1 and 2 are the individual IVD to their respective vertebrae. The CEP is a major inhibitors of MMPs. However, as production of thin structure approximately 1 mm in width, composed of TIMP 1 and 2 is also increased in the IVD during cases of 13 17 hyaline cartilage and populated by chondrocyte cells. As the IVDD, the MMPs such as MMP 7, which demonstrate IVD is mainly avascular, the CEP is heavily involved in the nutri- resistance to TIMP 1 and 2, have been implicated as key 13 18 tion of, and removal of waste products from, the IVD. enzymes involved in the pathogenesis of IVDD. Nutrients such as glucose and oxygen enter the IVD by passive ADAMTS, however, are inhibited specifically by TIMP 3, diffusion in a process which is facilitated by the blood vessels which, in contrast to TIMP 1 and 2, is not upregulated 14 17 which penetrate the CEP and the peripheral AF. Waste pro- during cases of IVDD; therefore, the aggrecanase activity ducts such as lactic acid and carbon are removed in a reciprocal of the ADAMTS is increased in cases of IVDD, because manner. As the CEP ages, calcification occurs and consequently the increased production in ADAMTS enzymes are not con- 13 11 the ability to supply the IVD with nutrients deteriorates. trolled by a concurrent increase in their inhibitor. In addition to increased matrix degradation, cells in IVDD 19, 21, 22 become abnormal and display cell senescence, which Intervertebral Disc Degeneration has been linked to the increase in matrix degradation IVDD has been identified in approximately 40% of LBP enzymes. A reduction in the number of active chondroctye- diagnoses, however current knowledge of the principal like cells leads to a decrease in the production of the growth ......................................................................................................................................................................................................................................... 84 Bioscience Horizons † Volume 2 † Number 1 † March 2009 Review ......................................................................................................................................................................................................................................... factors required to stimulate matrix synthesis. Consequently, identified as a potential solution to the administration pro- any matrix degradation cannot be repaired and will also blems as a single dose of IL-1Ra could be administered via result in a decrease in overall IVD matrix. gene therapy to the IVD which would provide long-lasting The matrix composition and cellular activity of the IVD is effects. integral to its functionality. IVDD results in loss of the IVD Genetic material can be delivered into host cells by vectors matrix, loss of chondrocyte-like cells of the NP, decreased via one of two methods in vivo or ex vivo. In vivo refers to structural integrity, hydration and ability to withstand mech- the direct injection or inhalation of the vector. Ex vivo is anical loads, angiogenesis and innervation. These biological the process where host cells are removed and the vector is changes result in IVDD and the associated decreased disc applied in vitro, and then the modified cells are returned to height, which can result in the symptoms associated with the host. There are two classifications of vectors which can LBP. be used in gene therapy: viral or non-viral vectors. Viral vectors can be further subdivided into two categories: genome incorporating, which include retroviruses and lenti- Future Treatment Strategies viruses and non-genome incorporating viruses, such as For future LBP therapies to be successful, they must be aimed herpes viruses, adenoviruses and adeno-associated viruses. at the underlying pathogenesis such as the alterations in cel- Viral vectors are genetically modified viruses which have lular biology, which have been linked to LBP. Particular con- been engineered to lack the genetic material which makes sideration has been given to the role of the cytokines IL-1 them pathogenic while retaining the genetic information and TNFa in the pathogenesis of IVDD. During IVDD, the which enables insertion of their genes into host cells. expression of both cytokines is increased, and both have been Additionally, a copy of the therapeutic gene is inserted into shown to inhibit matrix synthesis and to increase gene the viral vector so that it may be transferred into the host 23– 26 expression for matrix degradation enzymes in vitro. cell. However, although the functional receptors of IL-1 are upre- Adenoviral vectors have been utilized in various gene 6, 15, 28 –30 gulated throughout IVDD, expression of the TNFa receptors therapy investigations on the IVD. As adeno- is actually decreased. Therefore, while both cytokines are viruses do not incorporate into the host cell’s genetic expressed in instances of IVDD, IL-1 has been implicated material, the potential risk of oncogenesis associated with 25, 26 6 as the major mediator of IVDD. Indeed, using in situ genome-incorporating vectors is eliminated. One potential zymography techniques, inhibitors of IL-1 have been shown drawback of adenoviral vectors is that therapeutic genes to inhibit matrix degradation in both normal and degenerate are not transferred to daughter cells when the IVD cells repli- discs, however inhibition of TNFa failed to reduce the cate; however, due to the low cell turnover in the IVD, this matrix degradation, thus supporting the theory that IL-1 is potential disadvantage should not be a significant problem 26 6 a key cytokine involved in the pathogenesis of IVDD. in practice. The natural inhibitor of IL-1, IL-1 receptor antagonist Ex vivo administration of IL-1Ra to human IVD explants, (IL-1Ra), is also produced by IVD cells. In non-degenerate using adenovirus-mediated gene therapy, significantly inhi- IVDs, the actions of IL-1 appear to be controlled by the bits the activity of the degradative enzymes in degenerated 6, 15 levels of IL-1Ra present, however during IVDD an increase discs. In addition, other treatment strategies, such as is seen in the production of IL-1 with no parallel increase oral administration of glucosamine and chondroitin 24 31 in IL-1Ra and thus the balance is lost. As such, IL-1Ra sulfate and gene therapy targeting the anti-catabolic gene has been identified as a possible therapeutic agent to target TIMP1, which will inhibit the increased production of the inhibition of IL-1-mediated pathogenesis of IVDD. MMPs seen during disc degeneration, aimed at inhibiting IL-1Ra has been administered previously in clinical pro- the processes of IVDD have been recently reviewed, but cedures such as limiting cartilage degradation in rheumatoid will not address the increased production of ADAMTS arthritis, and consequently the related pharmacology and which also appear important in the pathogenesis of IVDD. side effects are well elucidated. However, in these circum- Preventing the loss of IVD matrix composition would stances IL-1Ra is injected subcutaneously, a technique which ideally be used in conjunction with an approach aimed at cannot be applied to the IVD due to their avascular compo- regenerating the IVD matrix in order to begin reversing the sition. Any passive diffusion into the IVD would not be matrix composition. Therefore, regeneration of the degener- sufficient to elicit a clinical effect and additionally there is ated IVD matrix must be considered as an additional and a risk that inhibition of IL-1 in the systemic circulation equally important factor in the treatment of IVDD. would occur. Direct administration of cytokines to the IVD by injection is another previously proposed method; Regeneration of Disc Matrix however, due to the short biological half-life of IL-1Ra, repeated administration would be necessary to achieve the Both the application of growth factors such as IGF, TGF and desired clinical impact. Gene therapy has therefore been BMPs and alternatively replacement of abnormal IVD cells, ......................................................................................................................................................................................................................................... 85 Review Bioscience Horizons † Volume 2 † Number 1 † March 2009 ......................................................................................................................................................................................................................................... either by injection of adult mesenchymal stem cells (MSCs) involving various transcription factors are involved in the or autologous IVD cells, have been investigated as potential production of growth factors and the matrix components therapeutic agents aimed at regeneration of the IVD matrix. of the IVD, especially collagens and proteoglycans. Sox 43 44 45 Growth factors are peptides the function of which is to 9, c-Jun and LIM mineralization protein (LMP)-1 are regulate the stimulation of cellular proliferation, differen- transcription factors involved in chondrogenesis. Sox 9 has tiation and migration and to stimulate matrix synthesis. In been shown to upregulate type II collagen synthesis. the IVD specific growth factors such as IGF, TGF and BMP Therefore, the delivery of a recombinant adenoviral vector are produced by the chondrocyte-like cells of the NP and act which expresses Sox 9 has been investigated as a novel thera- 4 43 to stimulate matrix synthesis. As loss of IVD matrix compo- peutic treatment for IVDD. Sox 9 production is signifi- sition is a characteristic feature of IVDD, growth factors have cantly increased in degenerated human IVD cells been investigated as potential therapeutic agents, aimed at transfected using adenovirus-mediated gene therapy and an promoting matrix synthesis in the degenerated IVD. increase in type II collagen synthesis is also recorded. As Preliminary work implementing exogenous application of type II collagen is an essential constituent of the NP, which 33 34 35 growth factors, such as TGF-b1, IGF-1 and BMP-7, in cases of IVDD is notably reduced, gene therapy targeting demonstrated increased IVD matrix synthesis following Sox 9 has enormous potential in the treatment for IVDD. In administration. However, as growth factors are known to addition to Sox 9, LMP-1 has been shown to mediate proteo- have short biological half-lives, this method of adminis- glycan production through its action on BMPs. In vitro tration would not be feasible in clinical scenarios as IVDD experiments using adenoviral vectors to deliver LMP-1 is a chronic condition and therefore requires the growth demonstrate increased proteoglycan aggrecan content in 36 46 factor application to be prolonged. The requirement for a NP-like cells. In addition, adenoviral vector delivery of sustainable method of growth factor application resulted in LMP-1 in vivo causes a significant increase in the levels of the development of the gene therapy delivery system for the LMP-1, BMP-2 and BMP-7. application of growth factors. Initial gene therapy studies A characteristic feature of IVDD is the reduction in meta- examined the possibility of introducing exogenous genes bolically active IVD cells and IVD cell number. A sufficient into IVD cells in vitro utilizing retroviral vectors and cell population is essential to maintain its homeostatic mech- both in vitro and in vivo studies utilizing adenoviral anisms. A suggested method of rectifying this is autologous vectors. The success of gene therapy studies on rabbit IVD cell transplantation (ADCT). ADCT involves the IVD cells was the foundation for similar experiments on explantation of IVD chondrocyte-like cells from herniated human IVD cells. Adenoviral vectors were implemented or degenerate discs, which are subsequently cultured to transduce TGFb1 genes into human IVD cells, which in vitro and replaced into the autologous IVD tissue from resulted in increased TGFb1 protein expression which in which they were derived. turn increased synthesis of proteoglycan and collagen by Promising results have been attained from clinical trials over 300%, which was significantly more than exogenous evidencing the success of ADCT. After 24 months, the application. Studies have also examined gene therapy control group of IVDD patients, who underwent surgery delivery of other growth factors such as BMP-2 and IGF-1 but did not receive ADCT, suffered a 75% reduction in which have also been implicated in the processes of IVD IVD fluid content. In comparison, the IVDs of the IVDD matrix synthesis. Future gene therapy approaches could patients that underwent ADCT only demonstrated a 59% potentially combine the various growth factors implicated reduction in fluid content. In addition, the pain suffered in the processes of IVD matrix synthesis such as TGFb1, by the ADCT-treated patients was less, according to the IGF-1 and BMP-2, in order to treat IVDD, and an additive score attained from questionnaires, in comparison to the effect on the amplification of proteoglycan synthesis has control group after 24 months. However, there was still a been observed. significant decrease in fluid content. A number of studies However, an important observation that TGF and IGF have demonstrated that the cells of a degenerated disc act 41 4, 13, 19, 21, 22 receptors are expressed in the blood vessels of the IVD abnormally and display senescence and thus suggests that the therapeutic use of these particular growth the use of degenerated cells as a therapeutic source is inher- factors to regenerate the IVD matrix could cause excessive ently flawed. angiogenesis and subsequently nerve ingrowth and therefore In addition, replacement of degenerated IVD cells with 16, 42 would not be a viable treatment strategy. As BMP recep- autologous IVD cells has a restricted potential for the treat- tor expression was not observed in the IVD blood vessels, ment of IVDD because of the limited population of IVD cells growth factors targeted at these receptors have been ident- which can be explanted for culture. Despite this, ADCT is ified as preferable therapeutic agents for therapies aimed at currently the only therapeutic technique aimed at IVDD regenerating IVD matrix in cases of IVDD. which has been clinically tested and the results demonstrate An alternative to the use of growth factors is the utiliz- that effective pain relief and changes in IVD matrix compo- ation of transcription factors. Complex signalling cascades sition are achieved long term. However, the removal of ......................................................................................................................................................................................................................................... 86 Bioscience Horizons † Volume 2 † Number 1 † March 2009 Review ......................................................................................................................................................................................................................................... autologous cells and culture in vitro is limited to a few par- At present, it is unclear if MSC differentiation can be ticular scenarios such as IVD herniation, because of the achieved long term or whether differentiated cells will repli- limited number of cells which could be harvested and the cate. In addition, the stage of degeneration may also influ- possibility that implantation of the autologous cells could ence the effectiveness of this type of therapy. These cause detrimental damage to the structure of the IVD and factors would need to be sufficiently evaluated before MSC accelerate its degeneration. More recently, attention has therapy for the treatment of IVDD could be evaluated in 48 55 turned to the potential of MSCs. clinical settings. MSC implantation therapy if used indivi- Stem cells are undifferentiated cells with two defining dually may counter the progression of IVDD, however, for properties; the ability to differentiate into specialized cells full regeneration of the IVD, an approach aimed at the cessa- and the capability of self renewal long term. After fertiliza- tion of IVDD delivered in tandem would be required. tion, a single totipotent cell is formed, which is capable of In addition to these major therapeutic approaches, other giving rise to all the cell types of the organism. During novel therapies have been proposed. Artificial IVD implan- embryonic development, this cell differentiates into pluripo- tation strategies, aimed at replacing degenerate IVDs, show tent stem cells which have the ability to differentiate into all promising clinical potential and have recently been reviewed. cell lineages. These cells then further differentiate into multi- Alternatively, whole disc transplantation has been proposed. potent stem cells which can only differentiate into cells of a However, the problems associated with identifying suitable particular germ layer. Adult MSCs are undifferentiated mul- donors make this a less viable clinical option. Tissue engin- tipotent stem cells which can be derived from several tissues eering techniques which aim to augment the function of degen- including skeletal muscle, synovial membranes, adipose, erate IVDs have also been developed. Some of these techniques dermis and bone marrow. In addition to being easily acces- utilize biomaterials that mimic the properties of the NP which sible, MSCs have a high plasticity and able to differentiate are injected into the IVD in order to provide a scaffold for the into bone, cartilage, fat and fibrous tissues. The use of existing chondrocyte-like cells and thus prevent further adult stem cells bypasses the controversial ethical issues damage such as decreased IVD matrix synthesis, which is regarding the acquisition of embryonic stem cells. Adult caused by the additional mechanical stress to the IVD as a stem cells are involved in the replacement of cells lost result of reduced IVD height. during normal matrix turnover. Various methods have It has been postulated that there may be a threshold beyond been tried in order to differentiate MSCs into the which the damage caused during IVDD becomes irrevers- 48, 50– 52 13 chondrocyte-like cells found in the NP. ible, however, recent research suggests that therapies The exogenous application of growth factors to MSCs which are delivered in later stages of IVDD are actually 51 54 in vitro can influence the differentiation of these cells more effective. Multifactorial conditions with poorly using current knowledge of the effects of growth factors on defined pathogenic mechanisms such as IVDD are a particular NP cells; studies have attempted to culture MSCs to initiate challenge for gene therapy and for the potential of gene differentiation into chondrocyte-like cells using the growth therapy to be maximized it is essential that the pathogenic factors TGFb and BMP. However, inducing MSC differen- processes of IVDD are fully elucidated to enable targeting to tiation with individual growth factors is problematic, the correct molecule or molecules. The therapeutic use of because multiple growth factors are involved in the normal growth factors has a vast potential for stimulating the syn- development of the IVD. thesis of IVD matrix components. However, for the full Co-cultivation of MSCs with IVD cells in vitro may be suf- potential to be accomplished the knowledge of the complex ficient to induce differentiation into the chondrocyte-like cell cellular biology of the IVD must be improved. For example, phenotype. However, because current knowledge of the although application of TGFb has a significant effect on pro- IVD NP cell phenotype is incomplete, a definitive conclusion teoglycan and collagen synthesis, receptors are found in the on whether differentiation has been successful is impossible. blood vessels in the periphery of the IVD. This is not the For MSC-based therapy to progress, a better knowledge of case with BMP suggesting that it has better therapeutic poten- the complex cellular biology of the IVD is required than is tial. With a greater understanding of the complex cellular currently available. The discovery that differentiation of biology of the IVD, potential agents for use in gene therapy MSC cells into IVD-like cells can be achieved by co-culture and regeneration strategies could be identified. of MSCs with IVD cells suggests that rapid production of NP cells could be possible without the removal of IVD Conclusions 48, 50 tissue. This evidence supports the theory that harvested MSCs could potentially be administered directly to the IVD, It is clear that future treatments aimed at the underlying patho- circumventing the requirement for differentiation to be genesis of IVDD are required, as current therapeutic strategies induced in vitro, however, the in vivo experiments required are purely symptomatic. The literature cited in this review high- to validate the use of this approach in clinical scenarios lights inhibition of the processes contributing to IVDD and have not been conducted to a sufficient standard to date. promotion of IVD matrix synthesis as the primary focuses of ......................................................................................................................................................................................................................................... 87 Review Bioscience Horizons † Volume 2 † Number 1 † March 2009 ......................................................................................................................................................................................................................................... 17. 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Spine 29: 2603–2611. muscle-derived stem cells and nucleus pulposus cells. Spine J 8: 804–809. ........................................................................................................................................................................................................................................ Submitted on 30 September 2008; accepted on 22 January 2009; advance access publication 24 February 2009 .........................................................................................................................................................................................................................................

Journal

Bioscience HorizonsOxford University Press

Published: Mar 24, 2009

Keywords: Key words intervertebral disc degeneration cytokine growth factor gene therapy mesenchymal stem cells autologous disc cell transplantation

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