Intervertebral disk degeneration and homeostasis of the extracellular matrix

Intervertebral disk (IVD) degeneration is one of the major causes of low back pain and lumbar disk herniation. Biologically, the cells in the disk actively regulate the homeostasis of the IVD extracellular matrix (ECM) by maintaining a balance between anabolism and catabolism. The modulation of disk cell metabolism involves a variety of molecules (eg, cytokines, enzymes, enzyme inhibitors, and growth factors) that act in a paracrine and/or autocrine fashion. The degeneration of an IVD may result from the loss of steady state metabolism maintained in the normal disks, due to an imbalance between the anabolic and catabolic processes. The proteoglycan (PG) content of the ECM and the rate of PG synthesis decreased markedly with age and degeneration, similar to the case in articular cartilage. The anabolic regulators include polypeptide growth factors, such as insulinlike growth factor 1 (IGF-1), transforming growth factor β (TGF-β), and the bone morphogenetic proteins (BMPs). Catabolic regulators include many cytokines, notably interleukin 1 (IL-1) and tumor necrosis factor (TNF)-α), which influence the synthesis of matrix-degrading enzymes, such as the matrix metalloproteinases (MMPs). Alterations in both anabolic and catabolic processes are thought to play key roles in the onset and progression of IVD degeneration; the biochemical processes that regulate these changes are important to understand for the development of effective treatments of disk degeneration.

Catabolic mediators and enzymes in disk degeneration

Under pathologic conditions, including IVD degeneration, physical injury (eg, puncture or stab wounds), and abnormal mechanical loading, the IVD expresses cytokines and proteinases. Although macrophages that infiltrate herniated tissue or granulation tissue seem to be the major source of these cytokines, recent studies indicate that IVD cells may also synthesize these molecules in an autocrine fashion. In particular, increases in both protein and mRNA levels of IL-1α and its major regulator, TNF-α, have been observed in degenerated or herniated IVD tissues and both are spontaneously expressed by these tissues in culture.

The catabolic processes induced by cytokines are mediated by various enzymes, such as the MMPs or the aggrecanases. IL-1 has been shown to stimulate matrix degradation as well as to inhibit the synthesis of ECM macromolecules with minimum effect on cell proliferation. IL-1 induced the production of collagenase, cyclooxygenase-2 (COX-2 ), prostaglandin E2, nitric oxide, MMP-1, total and active MMP-3, MMP-13, the aggrecanase A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-4), IL-6, and monocyte chemoattractant protein-1. Recently, IL-1β was shown to increase the sensitivity of IVD cells to shear stress, suggesting that this cytokine is involved in the acceleration of degradation processes in IVDs subjected to biomechanical stress. Although IL-1 affects both synthetic and degradative processes, at lower concentrations, IL-1 was much more effective in inhibiting the synthesis of aggrecan than in stimulating its degradation in articular cartilage. An epidemiologic study has shown that IL-1 gene cluster polymorphisms significantly increased the risk of disk degeneration; this has shed more light on the possible involvement of IL-1 in IVD degeneration.

The effects of the inhibition of catabolic cytokines have been evaluated in several studies. IL-1 receptor antagonist (IL-1ra), applied in vitro to degenerated and herniated human disk tissues, reduced the expression of MMP-3. IL-1ra pretreatment of nucleus pulposus (NP) cells from moderately degenerate human disks reduced the expression of ADAMTS-4 and MMP-3 in subsequent treatment with IL-1. In another human cell culture study, the addition of IL-1ra and soluble TNF receptor significantly up-regulated PG synthesis, suggesting that IL-1 and TNF suppress PG de novo synthesis. IL-1ra and other agents may benefit from administration of slow-release agents, such as IL-1ra mixed with thermally responsive elastin-like polypeptide (ELP) or with gelatin hydrogel. In addition to IL-1, TNF-α has gained attention in association with disk herniation. TNF-α inhibition, using a monoclonal antibody in herniated human disk explants, showed suppression of MMP-3 levels ; other anti-TNF agents are beginning to be applied to reduce pain in patients with sciatica and discogenic pain. Other anticytokine therapeutics include the p38 mitogen-activated protein kinase (MAPK) inhibitor, which hindered the catabolic effects of IL-1, as well as nuclear factor κB decoy, which reduced pain in a rat lumbar disk herniation model.

Catabolic mediators and enzymes in disk degeneration

Under pathologic conditions, including IVD degeneration, physical injury (eg, puncture or stab wounds), and abnormal mechanical loading, the IVD expresses cytokines and proteinases. Although macrophages that infiltrate herniated tissue or granulation tissue seem to be the major source of these cytokines, recent studies indicate that IVD cells may also synthesize these molecules in an autocrine fashion. In particular, increases in both protein and mRNA levels of IL-1α and its major regulator, TNF-α, have been observed in degenerated or herniated IVD tissues and both are spontaneously expressed by these tissues in culture.

The catabolic processes induced by cytokines are mediated by various enzymes, such as the MMPs or the aggrecanases. IL-1 has been shown to stimulate matrix degradation as well as to inhibit the synthesis of ECM macromolecules with minimum effect on cell proliferation. IL-1 induced the production of collagenase, cyclooxygenase-2 (COX-2 ), prostaglandin E2, nitric oxide, MMP-1, total and active MMP-3, MMP-13, the aggrecanase A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-4), IL-6, and monocyte chemoattractant protein-1. Recently, IL-1β was shown to increase the sensitivity of IVD cells to shear stress, suggesting that this cytokine is involved in the acceleration of degradation processes in IVDs subjected to biomechanical stress. Although IL-1 affects both synthetic and degradative processes, at lower concentrations, IL-1 was much more effective in inhibiting the synthesis of aggrecan than in stimulating its degradation in articular cartilage. An epidemiologic study has shown that IL-1 gene cluster polymorphisms significantly increased the risk of disk degeneration; this has shed more light on the possible involvement of IL-1 in IVD degeneration.

The effects of the inhibition of catabolic cytokines have been evaluated in several studies. IL-1 receptor antagonist (IL-1ra), applied in vitro to degenerated and herniated human disk tissues, reduced the expression of MMP-3. IL-1ra pretreatment of nucleus pulposus (NP) cells from moderately degenerate human disks reduced the expression of ADAMTS-4 and MMP-3 in subsequent treatment with IL-1. In another human cell culture study, the addition of IL-1ra and soluble TNF receptor significantly up-regulated PG synthesis, suggesting that IL-1 and TNF suppress PG de novo synthesis. IL-1ra and other agents may benefit from administration of slow-release agents, such as IL-1ra mixed with thermally responsive elastin-like polypeptide (ELP) or with gelatin hydrogel. In addition to IL-1, TNF-α has gained attention in association with disk herniation. TNF-α inhibition, using a monoclonal antibody in herniated human disk explants, showed suppression of MMP-3 levels ; other anti-TNF agents are beginning to be applied to reduce pain in patients with sciatica and discogenic pain. Other anticytokine therapeutics include the p38 mitogen-activated protein kinase (MAPK) inhibitor, which hindered the catabolic effects of IL-1, as well as nuclear factor κB decoy, which reduced pain in a rat lumbar disk herniation model.

Anabolic effects of cytokines and growth factors on IVD cells

A variety of growth factors and cytokines ( Table 1 ) can alter IVD homeostasis and stimulate ECM synthesis by shifting cellular metabolism to a more anabolic state. The effects of TGF-β on PG synthesis and cell proliferation were described early in the literature. Similar effects of IGF-1 have also been reported. IGF-1 and platelet-derived growth factor (PDGF) were also shown to reduce the percentage of apoptotic anulus fibrosus (AF) cells induced by serum depletion in culture.

Osteogenic protein-1 (OP-1), which is a member of the BMP family, up-regulates the PG metabolism of IVD cells. OP-1 strongly stimulated the production and formation of the ECM by rabbit IVD cells ; similar effects were found using human IVD cells. OP-1 also replenished PGs and collagens after depletion of the ECM after exposure of IVD cells to IL-1 or chondroitinase ABC (C-ABC).

Growth and differentiation factor-5 (GDF-5), originally found to be a factor responsible for skeletal alterations in brachypodism mice, is another member of the BMP family that has anabolic effects on IVD cells. GDF-5 stimulated PG and collagen type II expression by mouse IVD cells. Recombinant human GDF-5 (rhGDF-5) enhanced cell proliferation and matrix synthesis and accumulation by cells from bovine NP and, to lesser extent, AF cells.

Because biologic molecules, prepared in an autologous fashion, avoid certain regulatory complications, they may be useful clinically. Autologous IL-1ra has been used, along with IGF-1 and PDGF proteins, to reduce apoptosis of disk cells and their production of IL-1 and IL-6. Platelet-rich plasma (PRP) can be produced by centrifugal separation of a patient’s own blood in the operating room; it contains multiple growth factors concentrated at high levels. In vitro, PRP stimulated cell proliferation and matrix synthesis, as shown using porcine disk cells. PRP induced cell proliferation and differentiation and facilitated NP-like tissue formation by human disk cells.

IntradisCal therapeutics: animal studies

To study the ability of growth factors and other therapeutic agents to stimulate repair in vivo ( Table 2 ), small animal models, in which the degree of degeneration can be controlled, are useful. Rabbit lumbar and rat caudal disks have been used extensively, although other animal models exist. Both physical injury, such as stab wound or controlled needle puncture, and chemical degradation approaches have been used.

The in vivo efficacy of OP-1 injection has been evaluated in several animal studies. In adolescent rabbits, an injection of recombinant human OP-1, but not the lactose vehicle, reversed the reduction in disk height and worsened the magnetic resonance imaging (MRI) grade caused by an anular needle puncture. In another study with the same experimental design, the injection of OP-1 restored dynamic viscoelastic biomechanical properties, such as elastic and viscous moduli, of puncture-degenerated IVDs. OP-1 treatment was also effective in restoring disks that have been chemically degraded. C-ABC has been used as an alternative to chymopapain for chemonucleolysis as well as an animal model of disk degeneration in the rat tail and goat disk. When OP-1 or vehicle was injected into rabbit disks degraded with C-ABC for 4 weeks, the disk height initially decreased (approximately 34%), then recovered and gradually approached the level of the normal control.

rhGDF-5 is another promising growth factor whose efficacy has been evaluated in animal models. The efficacy of injecting a single dose of GDF-5 has been reported in the mouse caudal disk with degeneration induced by static compression. In that study, GDF-5 induced an increase in disk height and the expansion of the inner AF fibrochondrocytic population into the NP. In a needle-puncture model of disk degeneration (4 weeks) in adolescent (5–6 months old) rabbits, a single injection of rhGDF-5 restored disk height ( Fig. 1 A) and improved MRI and histologic grading scores within approximately 6 weeks. There are concerns that aged or degenerated disks might have lower cellularity, especially in the number of notochordal cells often found in normal adolescent rabbit disks, and might respond poorly to growth factor treatment. Despite this, a study using 2-year-old rabbits showed that rhGDF-5 effectively recovered disk height (see Fig. 1 B) as well as MRI and histologic grades after a 12-week observation period. Biomechanical analyses indicated that the viscous and elastic moduli of the IVDs in the rhGDF-5-injected disks were significantly higher than those in the phosphate-buffered saline (PBS)-injected disks. Additionally, rhGDF-5 has shown efficacy in rabbit disks that have been degraded using thrombin, a serine proteinase that results in cleavage of PGs and decreased disk height. In this study, disks of adolescent rabbits were injected with thrombin (100 U/10 μL) and the rabbits were maintained for 4 weeks. A single injection of rhGDF-5 (10 μg/10 μL) or saline was then given, and endpoint measures (MRI and gene expression) were determined 12 weeks later. Quantitative MRI was used to evaluate T2 and T1rho magnetic resonance properties. T2 ( Fig. 2 A, C) and T1rho (see Fig. 2 B, D) MRI maps showed maintenance of NP morphology and MRI values in rhGDF-5–treated disks. Both T2 ( Fig. 3 A) and T1rho (see Fig. 3 B) values of the NP were higher in the rhGDF-5 group, approaching those of unoperated controls. Treatment with rhGDF-5 also reduced the level of expression for ADAMTS-5 ( Fig. 4 A), ADAMTS-4 (see Fig. 4 B), COX-2 (see Fig. 4 C), and vascular endothelial growth factor (VEGF) (see Fig. 4 D), molecules related to PG degradation, pain, and neovascularization seen in degenerated IVDs.

Changes in the IVD height index (DHI) after anular puncture and rhGDF-5 injection. In a needle-puncture model of disk degeneration (4 weeks) in adolescent (5–6 month old) rabbits, a single injection of rhGDF-5 restored disk height ( A ). In a study using 2-year-old rabbits, disk height was also effectively recovered after a rhGDF-5 injection ( B ).

( Modified from Chujo T, An HS, Akeda K, et al. Effects of growth differentiation factor-5 on the intervertebral disk—in vitro bovine study and in vivo rabbit disk degeneration model study. Spine [Phila Pa 1976] 2006;31[5]:2909; with permission; and Chujo T, An H, Takatori R, et al. In vivo effects of recombinant human growth and differentiation factor-5 on the repair of the mature rabbit intervertebral disk [abstract]. Spine J 2006;6[5]:23S–4S. [North American Spine Society, 21st Annual Meeting Proceeding, Seattle, WA]; with permission.)

Quantitative magnetic resonance parameter maps of thrombin-induced degraded rabbit lumbar spines after injection with the growth factor, rhGDF-5. Four weeks after thrombin injection (100 U/10 μL) into adolescent rabbit disks, a single injection of saline (10 μL) ( A , B ) or rhGDF-5 (10 μg/10 μL) ( C , D ) was given. T2 ( A , C ) and T1rho ( B , D ) MRI maps were obtained 12 weeks later. High magnetic resonance values in the nucleus pulposus regions are apparent in the samples treated with the growth factor ( C , D ).

( Modified from Bae WC, Yoshikawa T, Kakutani K, et al. Effect of rhGDF-5 on the thrombin model of rabbit intervertebral disk degeneration: T1ρ quantification using 3T MRI [abstract]. Radiol Soc North Am 2009;95:SSE14–02; with permission.)

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

Structure and Biology of the Intervertebral Disk in Health and Disease Management of Degenerative Disk Disease and Chronic Low Back Pain Prosthetic Total Disk Replacement—Can We Learn from Total Hip Replacement? Intervertebral Disk Nutrition: A Review of Factors Influencing Concentrations of Nutrients and Metabolites Tissue Engineering for Intervertebral Disk Degeneration Stem Cell Regeneration of the Intervertebral Disk

Stay updated, free articles. Join our Telegram channel

Join