Immunolocalization of MMP-19 in the human intervertebral disc: implications for disc aging and degeneration

Pathogenesis and therapeutic implications of matrix metalloproteinases in intervertebral disc degeneration: A comprehensive review

Intervertebral disc (IVD) degeneration (IDD) is a common disorder that affects the spine and is a major cause of lower back pain (LBP). The extracellular matrix (ECM) is the structural foundation of the biomechanical properties of IVD, and its degradation is the main pathological characteristic of IDD. Matrix metalloproteinases (MMPs) are a group of endopeptidases that play an important role in the degradation and remodeling of the ECM. Several recent studies have shown that the expression and activity of many MMP subgroups are significantly upregulated in degenerated IVD tissue. This upregulation of MMPs results in an imbalance of ECM anabolism and catabolism, leading to the degradation of the ECM and the development of IDD. Therefore, the regulation of MMP expression is a potential therapeutic target for the treatment of IDD. Recent research has focused on identifying the mechanisms by which MMPs cause ECM degradation and promote IDD, as well as on developing therapies that target MMPs. In summary, MMP dysregulation is a crucial factor in the development of IDD, and a deeper understanding of the mechanisms involved is needed to develop effective biological therapies that target MMPs to treat IDD.

Correlational analysis of chemokine and inflammatory cytokine expression in the intervertebral disc and blood in patients with lumbar disc disease

The involvement of intervertebral disc (IVD) tissues, whole blood (WB) cytokines, and chemokines in pain in patients with lumbar degenerative disc disease (LDD) is unknown. We investigated the relationships between inflammatory cytokines and chemokines in human IVD tissues and WB samples and their association with pain. Expression levels of chemokines and cytokine gene expression were measured in samples from 20 patients with LDD and compared between IVD tissues and WB samples. The associations between WB chemokine and cytokine gene expression levels and pain intensity (numeric rating scale) were also analyzed. The mRNA of C-C chemokine ligand 20 (CCL20), C-C chemokine receptor 6 (CCR6), interleukin-6 (IL-6), IL-1β, IL-17, and tumor necrosis factor-α (TNF-α) was expressed in degenerated IVD tissues. Pearson's product-moment correlation analysis produced positive correlations between CCR6 and IL-6 expression levels in IVD tissues (r = 0.845, p < 0.001) and WB samples (r = 0.963, p < 0.001). WB IL-6 and CCR6 mRNA expression levels correlated significantly with present pain, maximum pain, and average pain. By contrast, low back pain (LBP) did not correlate with serum chemokine/cytokine expression. This is the first study to report correlations between chemokine and inflammatory cytokine gene expression levels in IVD tissues and WB samples in patients with LDD in relation to pain intensity. WB CCR6 and IL-6 gene expression levels correlated significantly with present pain, maximum pain, and average pain, but not with LBP. These data provide a new understanding of the role of chemokines and inflammatory cytokines in patients with LDD and may lead to new treatment strategies for pain.

Current Knowledge and Future Therapeutic Prospects in Symptomatic Intervertebral Disc Degeneration

Intervertebral disc (IVD) degeneration is the main source of intractable lower back pain, and symptomatic IVD degeneration could be due to different degeneration mechanisms. In this article, we describe the molecular basis of symptomatic IVD degenerative disc diseases (DDDs), emphasizing the role of degeneration, inflammation, angiogenesis, and extracellular matrix (ECM) regulation during this process. In symptomatic DDD, pro-inflammatory mediators modulate catabolic reactions, resulting in changes in ECM homeostasis and, finally, neural/vascular ingrowth-related chronic intractable discogenic pain. In ECM homeostasis, anabolic protein-regulating genes show reduced expression and changes in ECM production, while matrix metalloproteinase gene expression increases and results in aggressive ECM degradation. The resultant loss of normal IVD viscoelasticity and a concomitant change in ECM composition are key mechanisms in DDDs. During inflammation, a macrophage-related cascade is represented by the secretion of high levels of pro-inflammatory cytokines, which induce inflammation. Aberrant angiogenesis is considered a key initiative pathologic step in symptomatic DDD. In reflection of angiogenesis, vascular endothelial growth factor expression is regulated by hypoxia-inducible factor-1 in the hypoxic conditions of IVDs. Furthermore, IVD cells undergoing degeneration potentially enhance neovascularization by secreting large amounts of angiogenic cytokines, which penetrate the IVD from the outer annulus fibrosus, extending deep into the outer part of the nucleus pulposus. Based on current knowledge, a multi-disciplinary approach is needed in all aspects of spinal research, starting from basic research to clinical applications, as this will provide information regarding treatments for DDDs and discogenic pain.

Investigation of the effect of dipyrone on cells isolated from intervertebral disc tissue

The present study aimed to evaluate the effects of dipyrone, an indispensable analgesic, anti-pyretic and anti-spasmodic used in emergency departments, on nucleus pulposus and annulus fibrosus cells in vitro. After surgical biopsy, primary cell cultures were prepared from intact intervertebral disc tissues. Dipyrone was administered to the cultures in the experimental groups except for the control group. The data obtained were statistically evaluated. The proliferation was identified to be suppressed via MTT analysis. The gene expression profile of the intervertebral disc cells in the dipyrone-treated groups was significantly changed. The expression of chondroadherin, cartilage oligo matrix protein, interleukin-1β and metalloproteinase (MMP)-19 genes were decreased, but MMP-13 and MMP-7 genes expressions were increased, as determined via reverse transcription-quantitative PCR. AO/PI staining revealed that no apoptotic or other type of cell death was detectable after administration of dipyrone does not mean that the drug is innocuous. The occurrence of cellular senescence and/or the halt of cell proliferation may also be important mechanisms underlying the adverse inhibitory effects of dipyrone. Therefore, prior to administering dipyrone in clinical practice, all possible adverse effects of this drug should be considered.

Inflammation in intervertebral disc degeneration and regeneration

Intervertebral disc (IVD) degeneration is one of the major causes of low back pain, a problem with a heavy economic burden, which has been increasing in prevalence as populations age. Deeper knowledge of the complex spatial and temporal orchestration of cellular interactions and extracellular matrix remodelling is critical to improve current IVD therapies, which have so far proved unsatisfactory. Inflammation has been correlated with degenerative disc disease but its role in discogenic pain and hernia regression remains controversial. The inflammatory response may be involved in the onset of disease, but it is also crucial in maintaining tissue homeostasis. Furthermore, if properly balanced it may contribute to tissue repair/regeneration as has already been demonstrated in other tissues. In this review, we focus on how inflammation has been associated with IVD degeneration by describing observational and in vitro studies as well as in vivo animal models. Finally, we provide an overview of IVD regenerative therapies that target key inflammatory players.

MMPs and ADAMTSs in intervertebral disc degeneration

Intervertebral disc degeneration (IDD) is the most common diagnosis in patients with low back pain, a leading cause of musculoskeletal disability worldwide. The major components of extracellular matrix (ECM) within the discs are type II collagen (Col II) and aggrecan. Excessive destruction of ECM, especially loss of Col II and aggrecan, plays a critical role in promoting the occurrence and development of IDD. Matrix metalloproteinases (MMPs) and a disintegrin and metalloprotease with thrombospondin motifs (ADAMTSs) are primary enzymes that degrade collagens and aggrecan. There is a large and growing body of evidence that many members of MMPs and ADAMTSs are highly expressed in degenerative IVD tissue and cells, and are closely involved in ECM breakdown and the process of disc degeneration. In contrast, targeting these enzymes has shown promise for promoting ECM repair and mitigating disc regeneration. In the current review, after a brief description regarding the biology of MMPs and ADAMTSs, we mainly focus on their expression profiles, roles and therapeutic potential in IDD. A greater understanding of the catabolic pathways involved in IDD will help to develop potential prophylactic or regenerative biological treatment for degenerative disc disease in the future.

The molecular basis of intervertebral disc degeneration

BACKGROUND

Intervertebral disc (IVD) degeneration remains a clinically important condition for which treatment is costly and relatively ineffective. The molecular basis of degenerative disc disease has been an intense focus of research recently, which has greatly increased our understanding of the biology underlying this process.

PURPOSE

To review the current understanding of the molecular basis of disc degeneration.

STUDY DESIGN

Review article.

METHODS

A literature review was performed to identify recent investigations and current knowledge regarding the molecular basis of IVD degeneration.

RESULTS

The unique structural requirements and biochemical properties of the disc contribute to its propensity toward degeneration. Mounting evidence suggests that genetic factors account for up to 75% of individual susceptibility to IVD degeneration, far more than the environmental factors such as occupational exposure or smoking that were previously suspected to figure prominently in this process. Decreased extracellular matrix production, increased production of degradative enzymes, and increased expression of inflammatory cytokines contribute to the loss of structural integrity and accelerate IVD degeneration. Neurovascular ingrowth occurs, in part, because of the changing degenerative phenotype.

CONCLUSIONS

A detailed understanding of the biology of IVD degeneration is essential to the design of therapeutic solutions to treat degenerative discs. Although significant advances have been made in explaining the biologic mediators of disc degeneration, the inhospitable biochemical environment of the IVD remains a challenging environment for biological therapies.

Expression and regulation of metalloproteinases and their inhibitors in intervertebral disc aging and degeneration

BACKGROUND CONTEXT

Destruction of extracellular matrix (ECM) leads to intervertebral disc degeneration (IDD), which underlies many spine-related disorders. Matrix metalloproteinases (MMPs), and disintegrins and metalloproteinases with thrombospondin motifs (ADAMTSs) are believed to be the major proteolytic enzymes responsible for ECM degradation in the intervertebral disc (IVD).

PURPOSE

To summarize the current literature on gene expression and regulation of MMPs, ADAMTSs, and tissue inhibitors of metalloproteinases (TIMPs) in IVD aging and IDD.

METHODS

A comprehensive literature review of gene expression of MMP, ADAMTS, and TIMP in human IDD and reported studies on regulatory factors controlling their expressions and activities in both human and animal model systems.

RESULTS

Upregulation of specific MMPs (MMP-1, -2, -3, -7, -8, -10, and -13) and ADAMTS (ADAMTS-1, -4, and -15) were reported in human degenerated IVDs. However, it is still unclear from conflicting published studies whether the expression of ADAMTS-5, the predominant aggrecanase, is increased with IDD. Tissue inhibitors of metalloproteinase-3 is downregulated, whereas TIMP-1 is upregulated in human degenerated IVDs relative to nondegenerated IVDs. Numerous studies indicate that the expression levels of MMP and ADAMTS are modulated by a combination of many factors, including mechanical, inflammatory, and oxidative stress, some of which are mediated in part through the p38 mitogen-activated protein kinase pathway. Genetic predisposition also plays an important role in determining gene expression of MMP-1, -2, -3, and -9.

CONCLUSIONS

Upregulation of MMP and ADAMTS expression and enzymatic activity is implicated in disc ECM destruction, leading to the development of IDD. Future IDD therapeutics depends on identifying specific MMPs and ADAMTSs whose dysregulation result in pathological proteolysis of disc ECM.

Matrix metalloproteinase-26, a novel MMP, is constitutively expressed in the human intervertebral disc in vivo and in vitro

Matrix metalloproteinase (MMP) regulation and expression is important in the aging/degenerating human intervertebral disc. MMP-26 (also known as matrilysin-2 or endometase) is a newly discovered MMP which degrades type IV collagen, fibronectin, fibrinogen, vitronectin, denatured collagen types I-IV, insulin-like growth factor binding protein 1, and activated pro-MMP-9. Our objective here was to determine if it is present in human disc tissue and cultured disc cells. Immunohistochemistry and microarray gene expression analyses were used to evaluate the presence of MMP-26 in human disc tissue from healthy and degenerated discs. Immunohistochemistry was also applied to human annulus cells cultured in a collagen sponge. Cellular and matrix localization of MMP-26 was identified in the outer and inner annulus and in the nucleus pulposus. Fewer cells showed localization in the inner vs. outer annulus, and localization was sparse in the nucleus. During in vitro culture of annulus cells, MMP-26 was also expressed. Molecular analyses showed significant downregulation of expression of MMP-26 (p=0.03), and significant 9.8-fold upregulation of TGF-beta (p=0.01) in more degenerated discs vs. healthier discs. Findings document the first identification of MMP-26 in the disc at the molecular and protein levels. Results point to the potentially important role of MMP-26 in matrix modulation during disc health and degeneration.

Sequential alterations in catabolic and anabolic gene expression parallel pathological changes during progression of monoiodoacetate-induced arthritis

Chronic inflammation is one of the major causes of cartilage destruction in osteoarthritis. Here, we systematically analyzed the changes in gene expression associated with the progression of cartilage destruction in monoiodoacetate-induced arthritis (MIA) of the rat knee. Sprague Dawley female rats were given intra-articular injection of monoiodoacetate in the knee. The progression of MIA was monitored macroscopically, microscopically and by micro-computed tomography. Grade 1 damage was observed by day 5 post-monoiodoacetate injection, progressively increasing to Grade 2 by day 9, and to Grade 3-3.5 by day 21. Affymetrix GeneChip was utilized to analyze the transcriptome-wide changes in gene expression, and the expression of salient genes was confirmed by real-time-PCR. Functional networks generated by Ingenuity Pathways Analysis (IPA) from the microarray data correlated the macroscopic/histologic findings with molecular interactions of genes/gene products. Temporal changes in gene expression during the progression of MIA were categorized into five major gene clusters. IPA revealed that Grade 1 damage was associated with upregulation of acute/innate inflammatory responsive genes (Cluster I) and suppression of genes associated with musculoskeletal development and function (Cluster IV). Grade 2 damage was associated with upregulation of chronic inflammatory and immune trafficking genes (Cluster II) and downregulation of genes associated with musculoskeletal disorders (Cluster IV). The Grade 3 to 3.5 cartilage damage was associated with chronic inflammatory and immune adaptation genes (Cluster III). These findings suggest that temporal regulation of discrete gene clusters involving inflammatory mediators, receptors, and proteases may control the progression of cartilage destruction. In this process, IL-1β, TNF-α, IL-15, IL-12, chemokines, and NF-κB act as central nodes of the inflammatory networks, regulating catabolic processes. Simultaneously, upregulation of asporin, and downregulation of TGF-β complex, SOX-9, IGF and CTGF may be central to suppress matrix synthesis and chondrocytic anabolic activities, collectively contributing to the progression of cartilage destruction in MIA.

Constitutive expression of cathepsin K in the human intervertebral disc: new insight into disc extracellular matrix remodeling via cathepsin K and receptor activator of nuclear factor-κB ligand

Introduction

Cathepsin K is a recently discovered cysteine protease which cleaves the triple helical domains of type I to II collagen. It has been shown to be up-regulated in synovial tissue from osteoarthritic and rheumatoid patients, and is a component in normal and nonarthritic cartilage, where it increases with aging. Studies on heart valve development have recently shown that receptor activator of nuclear factor-κB ligand (RANKL) acts during valve remodeling to promote cathepsin K expression. Since extracellular matrix remodeling is a critical component of disc structure and biomechanical function, we hypothesized that cathepsin K and RANKL may be present in the human intervertebral disc.

Methods

Studies were performed following approval of the authors' Human Subjects Institutional Review Board. Six annulus specimens from healthier Thompson grade I to II discs, and 12 specimens from more degenerate grade III to IV discs were utilized in microarray analysis of RANKL and cathepsin K gene expression. Immunohistochemistry was also performed on 15 additional disc specimens to assess the presence of RANKL and cathepsin K.

Results

Cathepsin K gene expression was significantly greater in more degenerated grade III to IV discs compared to healthier grade I to II discs (P = 0.001). RANKL was also identified with immunohistochemistry and molecular analyses. RANKL gene expression was also significantly greater in more degenerated discs compared to healthier ones (P = 0.0001). A significant linear positive correlation was identified between expression of cathepsin K and RANKL (r² = 92.2; P < 0.0001).

Conclusions

Extracellular matrix remodeling is a key element of disc biology. Our use of an appropriate antibody and gene expression studies showed that cathepsin K is indeed present in the human intervertebral disc. Immunolocalization and molecular analyses also confirmed that RANKL is present in the human disc. Expression of RANKL was found to be significantly greater in more degenerated compared to healthier discs (P = 0.0001). Cathepsin K gene expression levels showed a positive, significant correlation with RANKL expression. Based on these data, we propose that cathepsin K plays a significant role in disc matrix remodeling and in matrix degradation in the proinflammatory cytokine-rich microenvironment of the degenerating disc.

Immunohistochemical analysis of structural changes in collagen for the assessment of osteoarthritis

Collagen fibrillation within articular cartilage (AC) plays a key role in joint osteoarthritis (OA) progression and, therefore, studying collagen synthesis changes could be an indicator for use in the assessment of OA. Various staining techniques have been developed and used to determine the collagen network transformation under microscopy. However, because collagen and proteoglycan coexist and have the same index of refraction, conventional methods for specific visualization of collagen tissue is difficult. This study aimed to develop an advanced staining technique to distinguish collagen from proteoglycan and to determine its evolution in relation to OA progression using optical and laser scanning confocal microscopy (LSCM). A number of AC samples were obtained from sheep joints, including both healthy and abnormal joints with OA grades 1 to 3. The samples were stained using two different trichrome methods and immunohistochemistry (IHC) to stain both colourimetrically and with fluorescence. Using optical microscopy and LSCM, the present authors demonstrated that the IHC technique stains collagens only, allowing the collagen network to be separated and directly investigated. Fluorescently-stained IHC samples were also subjected to LSCM to obtain three-dimensional images of the collagen fibres. Changes in the collagen fibres were then correlated with the grade of OA in tissue. This study is the first to successfully utilize the IHC staining technique in conjunction with laser scanning confocal microscopy. This is a valuable tool for assessing changes to articular cartilage in OA.

Region-dependent aggrecan degradation patterns in the rat intervertebral disc are affected by mechanical loading in vivo

STUDY DESIGN

Immunoblotting study to evaluate aggrecan degradation patterns in rat intervertebral discs (IVDs) subjected to mechanical overload.

OBJECTIVE

To evaluate the effects of in vivo dynamic compression overloading on aggrecan degradation products associated with matrix metalloproteinase (MMP) and aggrecanase activity in different regions of the IVD.

SUMMARY OF BACKGROUND DATA

Aggrecan cleavage at the MMP and aggrecanase sites is an important event in human IVD aging, with distinct cleavage patterns in the anulus and nucleus regions.No such information is available on regional variations in rat IVDs, nor on how such cleavage is affected by mechanical loading.

METHODS

Sprague-Dawley rats were instrumented with an Ilizarov-type device and subjected to dynamic compression (1 MPa and 1 Hz for 8 hours per day for 8 weeks). Control, sham, and overloaded IVDs were separated by disc region and analyzed for aggrecan degradation products using immunoblotting techniques, with antibodies specific for the aggrecanase and MMP cleavage sites in the interglobular domain of aggrecan.

RESULTS

Control IVDs exhibited strong regional variation in aggrecan degradation patterns with minimal degradation products being present in the nucleus pulposus, degradation products associated with aggrecanase cleavage predominating in the inner anulus fibrosus (AF), and degradation products associated with MMP cleavage predominating in the outer AF. Dynamic compression overloading increased the amount of aggrecan degradation products associated with MMP cleavage not only in the AF but also in the nucleus pulposus. Degradation profiles of sham IVDs were similar to control.

CONCLUSION

Aggrecan G1 regions resulting from proteolysis were found to have a strong regionally specific pattern in the rat IVD, which was altered under excessive loading. The shift from aggrecanase to MMP-induced degradation products with dynamic compression overloading suggests that protein degradation and loss can precede major structural disruption in the IVD, and that MMP-induced aggrecan degradation may be a marker of mechanically induced disc degeneration.

Differential gene expression profiling of metalloproteinases and their inhibitors: a comparison between bovine intervertebral disc nucleus pulposus cells and articular chondrocytes

STUDY DESIGN

A comparative in vitro metalloproteinases and their inhibitors gene expression profile.

OBJECTIVE

To obtain a complete expression profile of matrix metalloproteinases (MMPs), family of proteases with a disintegrin and metalloproteinase domain with thrombospondin motifs (ADAMTS), and tissue inhibitors of metalloproteinases (TIMPs) in bovine adult nucleus pulposus (NP) cells and to compare this profile with the expression profile obtained from bovine adult articular chondrocytes cultured under identical conditions.

SUMMARY OF BACKGROUND DATA

The cells of the NP resemble articular chondrocytes morphologically but produce a matrix which, though consisting of similar components, has very different biomechanical properties. No specific markers for NP cells have yet been identified; they can be distinguished from chondrocytes only by differences in gene expression. Here we compare profiles of gene expression of metalloproteinases and their inhibitors between NP cells and chondrocytes to improve understanding of the differences between these cell types.

METHODS

NP cells and articular chondrocytes were harvested respectively from bovine caudal discs and the articular cartilage of metacarpal-phalangeal joints of 18- to 24-month-old steers. These cells were cultured under identical conditions for 96 hours in alginate beads. Expression levels of MMPs, ADAMTSs, and TIMPs were detected by real-time RT-PCR.

RESULTS

Gene profiling demonstrated distinct differences between levels of MMPs, ADAMTSs, and TIMPs produced by chondrocytes and NP cells. In particular, NP cells expressed considerably more MMP-2 and MMP-14 than chondrocytes, and expression of ADAMTS-1,-2,-17 and TIMP-1 was also higher. However, expression of MMP-1,-3,-7,-8,-10,-11,-13,-16,-19,-20,-21,-23,-24,-28, ADAMTS-4,-5,-6,-14,-18,-19, and TIMP-3 was lower in NP cells than in chondrocytes. Chondrocytes but not NP cells expressed MMP12 and MMP27; this difference is a potential marker for distinguishing between NP cells and chondrocytes.

CONCLUSION

Because culture conditions and animal age were identical, differences in metalloproteinase and inhibitor expression between NP cells and chondrocytes were intrinsic to cell phenotype and not induced by differences in the in situ extracellular environment.

Matrix metalloproteinase 28, a novel matrix metalloproteinase, is constitutively expressed in human intervertebral disc tissue and is present in matrix of more degenerated discs

Introduction

The regulation and elevation in expression of the catabolic matrix metalloproteinases (MMPs) is of high importance in the human intervertebral disc since upregulation of these matrix-degrading enzymes results in matrix destruction associated with disc degeneration. MMP28 (epilysin) is a newly discovered MMP believed to play a role in matrix composition and turnover in skin. It is present in basal keratinocytes where its expression is upregulated with wound repair, and in cartilage and synovium where it is upregulated in osteoarthritis. Recent work has shown that mechanical compression can act to modulate expression of MMP28. The expression of MMP28 is unexplored in the intervertebral disc.

Methods

Following approval by our human subjects institutional review board, we employed microarray analyses to evaluate in vivo expression of MMP28 and the MMP28 precursor in human disc tissue, and utilized immunohistochemistry to determine cellular and extracellular matrix localization of MMP28 in 35 human disc tissue specimens. The percentage of cells positive for MMP28 immunocytochemical localization was also determined.

Results

The present work documents the expression and presence of MMP28 in cells and extracellular matrix (ECM) of the human intervertebral disc. Gene expression levels in human disc tissue were detectable for both MMP28 and the MMP28 precursor. MMP28 cytoplasmic localization was present in cells of the outer annulus; it was also present in some, but not all, cells of the inner annulus and nucleus. MMP28 was not found in the ECM of healthier Grade I to II discs, but was identified in the ECM of 61% of the more degenerated Grade III to V discs (P = 0.0018). There was a significant difference in cellular MMP28 distribution in the disc (P = 0.008): the outer annulus showed the largest percentage of cells positive for MMP28 immunolocalization, followed by the inner annulus and then the nucleus. Herniated discs showed a significantly greater proportion of MMP28-positive cells compared with nonherniated discs (P = 0.034).

Conclusions

Findings presented here show the first documentation of intervertebral disc expression and production of MMP28. MMP28 was found in both disc cell cytoplasm and in the ECM of more degenerated specimens, with greater cellular localization in the outer annulus and in herniated disc specimens. These findings are important because of the key role of MMPs in disc turnover and homeostasis, and previous indications of a role for this MMP in matrix repair and matrix turnover in other tissues. Our data, which show the presence of MMP28 in human disc tissue, suggest that MMP28 may have a potentially important role in ECM modulation in the healthy and degenerating disc.

Cell-based regeneration of intervertebral disc defects: review and concepts

During the last century low back pain has emerged as a widespread disease often caused by intervertebral disc degeneration (IDD). IDD is a complex problem in which a variety of causes play a role. As IDD causes high costs, corporate interest has led to a number of therapies being developed. Today, these therapies focus not only on minimizing the pain caused by this disease but also on restoring intervertebral disc function. These approaches are often biological and aim to stimulate the regeneration of the intervertebral disc by injection of activator proteins, biomaterials, different cell types or complex cell matrix composites. Genetic engineering of disc cells and in vitro tissue engineering also offer a possibility for curing IDD. This article gives an overview of these concepts.

Matrix metalloproteinase expression levels suggest distinct enzyme roles during lumbar disc herniation and degeneration

The disruption of the extracellular disc matrix is a major hallmark of disc degeneration. This has previously been shown to be associated with an up-regulation of major matrix metalloproteinase (MMP) expression and activity. However, until now hardly any data are available for MMP/TIMP regulation and thereby no concept exists as to which MMP/TIMP plays a major role in disc degeneration. The objective of this study was, therefore, to identify and quantify the putative up-regulation of MMPs/TIMPs on the mRNA and protein level and their activity in disc material in relation to clinical data and histological evidence for disc degeneration. A quantitative molecular analysis of the mRNA expression levels for the MMPs (MMPs-1, -2, -3, -7, -8, -9, -13) and the MMP inhibitors (TIMPs-1 and -2) was performed on 37 disc specimens obtained from symptomatic disc herniation or degeneration. In addition, disc specimens from patients without disc degeneration/herniation (=controls) were analyzed. Expression of MMPs-1, -2, -3, -7, -8, -9, -13 and TIMPs-1, -2 was analyzed using quantitative RT-PCR, normalized to the expression level of a house keeping gene (GAPDH). Gene expression patterns were correlated with MMP activity (in situ zymography), protein expression patterns (immunohistochemistry), degeneration score (routine histology) and clinical data. MMP-3 mRNA levels were consistently and substantially up-regulated in samples with histological evidence for disc degeneration. A similar but less pronounced up-regulation was observed for MMP-8. This up-regulation was paralleled by the expression of TIMP-1 and to a lesser extent TIMP-2. In general, these findings could be confirmed with regard to protein expression and enzyme activity. This study provides data on the gene and protein level, which highlights the key role of MMP-3 in the degenerative cascade leading to symptomatic disc degeneration and herniation. Control of the proteolytic activity of MMP-3 may, therefore, come into the focus when aiming to develop new treatment options for early disc degeneration.

Biological treatment for degenerative disc disease: implications for the field of physical medicine and rehabilitation

Spine care is a fast-growing sector of the outpatient practice for physiatrists. Current nonsurgical treatment modalities and surgical options for severe symptomatic intervertebral disc degeneration have limited and inconsistent clinical results. Thus, the development of novel approaches, such as biological treatments that offer the potential to halt or even reverse disc degeneration and restore physiologic disc function, are very attractive. In this article, we first review the structural changes that occur during intervertebral disc degeneration and their relationship with discogenic back pain. Subsequently, we review the treatment approaches currently under clinical trial and laboratory investigation. Physiatrists specializing in spine care have the skill set required for administering intradiscal injections and supervising a comprehensive rehabilitation program after the procedures. Ultimately, the clinical use of any biological treatment discussed herein would require the collective efforts of physicians (such as physiatrists and surgeons) and researchers (such as chemical and biomedical engineers, biologists, and chemists).

Anulus fibrosus tension inhibits degenerative structural changes in lamellar collagen

Mechanical stress is one of the risk factors believed to influence intervertebral disc degeneration. Animal models have shown that certain regimes of compressive loading can induce a cascade of biological effects that ultimately results in cellular and structural changes in the disc. It has been proposed that both cell-mediated breakdown of collagen and the compromised stability of collagen with loss of anular tension could result in degradation of lamellae in the anulus fibrosus (AF). To determine whether this may be important in the AF, we subjected entire rings of de-cellularized AF tissue to MMP-1 digestion with or without tension. Biomechanical testing found trends of decreasing strength and stiffness when tissues were digested without tension compared with those with tension. To determine the physiologic significance of tissue level tension in the AF, we used an established in vivo murine model to apply a disc compression insult known to cause degeneration. Afterward, that motion segment was placed in fixed-angle bending to impose tissue level tension on part of the AF and compression on the contralateral side. We found that the AF on the convex side of bending retained a healthy lamellar appearance, while the AF on the concave side resembled tissues that had undergone degeneration by loading alone. Varying the time of onset and duration of bending revealed that even a brief duration applied immediately after cessation of compression was beneficial to AF structure on the convex side of bending. Our results suggest that both cell-mediated events and cell-independent mechanisms may contribute to the protective effect of tissue level tension in the AF.

Immunolocalization and biochemical evidence of pregnancy-associated plasma protein A in the intervertebral disc

STUDY DESIGN

Studies were approved by the authors' Human Subjects Institutional Review Board and Institutional Animal Care and Use Committee. Anulus tissue was used in studies of the immunocytochemical localization of pregnancy-associated plasma protein A (PAPP-A) in disc tissue from the sand rat and from human disc surgical specimens and specimens from control donors. Cultured human disc cells were also tested for production of PAPP-A.

OBJECTIVES

(1) To determine the immunohistochemical localization of PAPP-A in human and sand rat discs; (2) To test for gene expression of PAPP-A in the human disc in vivo and in vitro production by cultured cells; and (3) To test for expression of insulin-like growth factor binding proteins (IGFBP)-2, -4, and -5 in vivo and in vitro by human disc cells.

SUMMARY OF BACKGROUND DATA

PAPP-A is a metalloproteinase expressed by several cell types, including fibroblasts, osteoblasts, and smooth muscle cells. PAPP-A has an extremely important role because it cleaves IGFBP-2, -4, and -5 in the extracellular matrix, thereby increasing the bioavailability of IGF to nearby cells. METHODS.: Specimens of human disc tissue and lumbar discs from sand rats were assessed for immunocytochemical localization of PAPP-A, and the percentage of positive cells determined. Human disc cells in three-dimensional culture were assessed for production of PAPP-A using an enzyme linked immunosorbent assay. Molecular gene expression studies were carried out using microarray analysis.

RESULTS

Positive cytoplasmic immunolocalization of PAPP-A was present in the majority of cells of the human and sand rat outer anulus (OA). In the human outer anulus, the percentage of cells positive for PAPP-A localization did not differ in Grades I-II discs vs. Grades III-V discs (OA: 77.4% +/- 10.5 vs. 75.1% +/- 7.4 [mean +/- SEM] respectively). In the inner anulus, however, the percentage of cells positive for PAPP-A localization in more degenerate discs was significantly greater than the percentage in healthier discs (60.7% +/- 10.1 vs. 15.6 +/- 5.4, P = 0.024). % positive cells in the inner anulus correlated significantly with disc grade (r = 0.579; P = 0.01). Over a 5-day three-dimensional culture period, human anulus cells produced and secreted abundant PAPP-A into the culture media. Molecular studies confirmed the expression of IGFBP-2, -4, and -5 both in vivo and in vitro.

CONCLUSIONS

Data provide important new insights into disc cell expression of PAPP-A at the translational level. The presence of a significantly greater proportion of cells positive for PAPP-A in the inner anulus of more degenerate Grade III-V discs compared with healthier Grade I-II discs supports our previous observation of increased gene expression of PAPP-A in more degenerated discs. Biochemical data shown here documented production of PAPP-A by disc cells in vitro. Production of PAPP-A by disc cells is important since PAPP-A cleaves IGF-binding proteins, and makes IGF-I, a potent mitogen and antiapoptotic agent, available to cells. Future studies are indicated to further investigate PAPP-A and IGF-BP function in the disc.

Cell therapy using articular chondrocytes overexpressing BMP-7 or BMP-10 in a rabbit disc organ culture model

STUDY DESIGN

Rabbit knee articular chondrocytes overexpressing human growth factors were injected into cultured intervertebral disc explants. Survival of the injected cells and accumulation of extracellular matrix were assessed.

OBJECTIVE

To define the utility of cell-based gene delivery approach for repair of the intervertebral disc.

SUMMARY OF BACKGROUND DATA

Back pain associated with symptomatic disc degeneration is a common clinical condition. Growth factors stimulate disc cell metabolism, but the ideal method for in vivo delivery has not been established. Cells as a vehicle for delivering growth factors to the disc offer potential advantages, including prolonged production of the growth factor within the disc and vital cells to participate in the repair process.

METHODS

New Zealand white rabbit articular chondrocytes transduced with adenovirus expressing human bone morphogenetic protein-7 and green fluorescence protein (GFP) (AdhBMP-7), human bone morphogenetic protein-10 and GFP (AdBMP-10), or GFP alone (AdGFP, as a control) were injected into whole disc explants. Discs were maintained in culture for 1 to 2 months. At the conclusion of the culture periods, cell survival was assessed by fluorescence microscopy and extracellular matrix accumulation was assessed with biochemical methods.

RESULTS

Chondrocytes achieved long-term survival in the cultured disc explants. The discs treated with chondrocytes/BMP-7 demonstrated a 50% increase in proteoglycan content within the nucleus pulposus compared to control (chondrocytes/GFP), while discs injected with chondrocytes/BMP-10 failed to show a significant increase in proteoglycan accumulation.

CONCLUSION

Our study demonstrates the ability of transduced articular chondrocytes to survive and promote proteoglycan accumulation when transplanted into the intervertebral disc. These data support the potential of a cell-based gene therapy approach for disc repair. Further studies using this approach in animal models are indicated as a step towards achieving disc repair in humans.

Interleukin-1 receptor antagonist delivered directly and by gene therapy inhibits matrix degradation in the intact degenerate human intervertebral disc: an in situ zymographic and gene therapy study

Data implicate IL-1 in the altered matrix biology that characterizes human intervertebral disc (IVD) degeneration. In the current study we investigated the enzymic mechanism by which IL-1 induces matrix degradation in degeneration of the human IVD, and whether the IL-1 inhibitor IL-1 receptor antagonist (IL-1Ra) will inhibit degradation. A combination of in situ zymography (ISZ) and immunohistochemistry was used to examine the effects of IL-1 and IL-1Ra on matrix degradation and metal-dependent protease (MDP) expression in explants of non-degenerate and degenerate human IVDs. ISZ employed three substrates (gelatin, collagen, casein) and different challenges (IL-1β, IL-1Ra and enzyme inhibitors). Immunohistochemistry was undertaken for MDPs. In addition, IL-1Ra was introduced into degenerate IVD explants using genetically engineered constructs. The novel findings from this study are: IL-1Ra delivered directly onto explants of degenerate IVDs eliminates matrix degradation as assessed by multi-substrate ISZ; there is a direct relationship between matrix degradation assessed by ISZ and MDP expression defined by immunohistochemistry; single injections of IVD cells engineered to over-express IL-1Ra significantly inhibit MDP expression for two weeks. Our findings show that IL-1 is a key cytokine driving matrix degradation in the degenerate IVD. Furthermore, IL-1Ra delivered directly or by gene therapy inhibits IVD matrix degradation. IL-1Ra could be used therapeutically to inhibit degeneration of the IVD.

Aggrecanases and aggrecanase-generated fragments in the human intervertebral disc at early and advanced stages of disc degeneration

STUDY DESIGN

A comparative study of aggrecanases and aggrecan fragmentation profile in the human intervertebral disc at early and advanced stages of disc degeneration.

OBJECTIVE

To determine differences in the content of the aggrecanases and the profile of aggrecan fragmentation in early and advanced stages of disc degeneration using cadaveric human intervertebral discs.

SUMMARY OF BACKGROUND DATA

Aggrecanases and aggrecanase-generated aggrecan fragments have been found in human degenerated discs. However, the association between the grade of disc degeneration and the content of the aggrecanases and the profile of aggrecan fragments has not been well studied.

METHODS

A total of 108 cadaveric donor spines were assessed by MRI T2 imaging and graded based on the Thompson scale. Twelve donor spines (average age, 63 years), each specifically exhibiting 2 different stages (Grade 2 and Grade 4) of disc degeneration at different disc levels, were included in this study. After harvesting the preselected discs, tissue samples were obtained from the center of the nucleus pulposus (NP) and the middle zone of the anulus fibrosus (AF). The amount of the aggrecanases, specifically ADAMTS-4 and ADAMTS-5, and the pattern of aggrecan fragmentation in the isolated tissues were assessed by western blot using specific antibodies.

RESULTS

In both NP and the AF tissues, the amount of ADAMTS-4 detected was higher in disc tissues with a higher level of degeneration (Grade 4) than in Grade 2 disc tissues with a lower level of degeneration. However, the amount of ADAMTS-5 detected did not differ between the 2 disc tissue grades. The aggrecan fragmentation analysis of these samples demonstrated the presence of aggrecanase-mediated fragmentation in both groups; however, there was no apparent difference in the aggrecan fragmentation profile between discs at early and advanced stages of disc degeneration.

CONCLUSION

Aggrecanases are involved in aggrecanolysis at both the early and advanced stages of disc degeneration. The aggrecan fragmentation profile analysis demonstrates the involvement of aggrecanases, as well as that of matrix metalloproteinases and/or cathepsins, during disc degeneration.

The cell biology of intervertebral disc aging and degeneration

Intervertebral disc degeneration, which mimics disc aging but occurs at an accelerated rate, is considered to be related to neck or low back pain and disc herniation. Degenerated discs show breakdown of the extracellular matrix and thus fail to bear the daily loadings exerted on the spine. Rather than a passive process of wear and tear, disc degeneration is an aberrant, cell-mediated response to progressive structural failure due to aging and other environmental factors such as abnormal mechanical stress. With aging and degeneration, disc cells undergo substantially biologic changes, including alternation of cell type in the nucleus pulposus, increased cell density but decreased number of viable cells as a result of increased cell death and increased cell proliferation, increased cell senescence, and altered cell phenotype which is characterized by compromised capability of synthesizing correct matrix components and by enhanced catabolic metabolism. These changes are involved in the process of disc degeneration through the complicated interactions among them. To retard or reverse disc degeneration, the abnormal conditions of the decreased viable cell population and the altered cell phenotype should be corrected. As potential therapies for disc degeneration, intradiscal protein injection, gene transfer and cell implantation are being understudied in vivo. Suppression of excessive apoptosis and accelerated senescence of disc cells may be other choices for treating disc degeneration. When performing a biologic therapy in order to repair or regenerate the degenerated disc, nutrient and biomechanical factors should also be incorporated, because they are the major causes of the biologic changes experienced by disc cells. Moreover, a very early intervention is indicated by the finding that the onset of human disc degeneration occurs as early as by adolescence.