Fibronectin and its fragments increase with degeneration in the human intervertebral disc

Fibronectin sensitizes activation of contractility, YAP, and NF-κB in nucleus pulposus cells

Intervertebral disc degeneration involves the breakdown of the discs of the spine due to genetics, aging, or faulty mechanical loading. As part of the progression of the disease, nucleus pulposus cells lose their phenotypic characteristics, inducing inflammation and extracellular matrix (ECM) alterations that result in a loss of disc mechanical homeostasis. Fibronectin is one ECM molecule that has been shown to be upregulated in disc degeneration and plays an important role in the progression of a wide variety of fibrotic diseases. Fragments of fibronectin have also long been associated with both osteoarthritis and disc degeneration. The goal of this work is to test the effects of fibronectin on disc cell phenotype, mechanosensing, and inflammatory signaling. We identify that fibronectin increases the activation of cellular contractility, the mechanosensitive transcription factor Yes-associated protein, and the inflammatory transcription factor nuclear factor-κB. This results in decreased production and expression of proteoglycans, which are required to maintain healthy disc function. Thus, fibronectin is a potential regulator of phenotypic changes in disc degeneration, and a potential target for treating disc degeneration at the cellular level. Understanding the role of fibronectin, and its potential as a therapeutic target, could provide new approaches for preventing or reversing disc degeneration.

MRI Markers of Degenerative Disc Disease in Young Patients With Multiple Sclerosis

Background and Purpose: Evidence has emerged for an association between degenerative disc disease (DDD) and multiple sclerosis (MS). The purpose of the current study is to determine the presence and extent of cervical DDD in young patients (age <35) with MS, an age cohort that is less well studied for these changes. Methods: Retrospective chart review of consecutive patients aged <35 referred from the local MS clinic who were MRI scanned between May 2005 and November 2014. 80 patients (51 female and 29 male) with MS of any type ranging between 16 and 32 years of age (average 26) were included. Images were reviewed by 3 raters and assessed for presence and extent of DDD, as well as cord signal abnormalities. Interrater agreement was assessed using Kendall's W and Fleiss' Kappa statistics. Results: Substantial to very good interrater agreement was observed using our novel DDD grading scale. At least some degree of DDD was found in over 91% of patients. The majority scored mild (grade 1, 30-49%) to moderate (grade 2, 39-51%) degenerative changes. Cord signal abnormality was seen in 56-63%. Cord signal abnormality, when present, occurred exclusively at degenerative disc levels in only 10-15%, significantly lower than other distributions (P < .001 for all pairwise comparisons). Conclusions: MS patients demonstrate unexpected cervical DDD even at a young age. Future study is warranted to investigate the underlying etiology, such as altered biomechanics. Furthermore, cord lesions were found to occur independently of DDD.

Machine learning algorithm predicts fibrosis-related blood diagnosis markers of intervertebral disc degeneration

BACKGROUND

Intervertebral disc cell fibrosis has been established as a contributing factor to intervertebral disc degeneration (IDD). This study aimed to identify fibrosis-related diagnostic genes for patients with IDD.

METHODS

RNA-sequencing data was downloaded from Gene Expression Omnibus (GEO) database. The diagnostic genes was identified using Random forest based on the differentially expressed fibrosis-related genes (DE-FIGs) between IDD and control samples. The immune infiltration states in IDD and the regulatory network as well as potential drugs targeted diagnostic genes were investigated. Quantitative Real-Time PCR was conducted for gene expression valifation.

RESULTS

CEP120 and SPDL1 merged as diagnostic genes. Substantial variations were observed in the proportions of natural killer cells, neutrophils, and myeloid-derived suppressor cells between IDD and control samples. Further experiments indicated that AC144548.1 could regulate the expressions of SPDL1 and CEP120 by combininghsa-miR-5195-3p and hsa-miR-455-3p, respectively. Additionally, transcription factors FOXM1, PPARG, and ATF3 were identified as regulators of SPDL1 and CEP120 transcription. Notably, 56 drugs were predicted to target these genes. The down-regulation of SPDL1 and CEP120 was also validated.

CONCLUSION

This study identified two diagnostic genes associated with fibrosis in patients with IDD. Additionally, we elucidated their potential regulatory networks and identified target drugs, which offer a theoretical basis and reference for further study into fibrosis-related genes involved in IDD.

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.

Recent advances in the repair of degenerative intervertebral disc for preclinical applications

The intervertebral disc (IVD) is a load-bearing, avascular tissue that cushions pressure and increases flexibility in the spine. Under the influence of obesity, injury, and reduced nutrient supply, it develops pathological changes such as fibular annulus (AF) injury, disc herniation, and inflammation, eventually leading to intervertebral disc degeneration (IDD). Lower back pain (LBP) caused by IDD is a severe chronic disorder that severely affects patients' quality of life and has a substantial socioeconomic impact. Patients may consider surgical treatment after conservative treatment has failed. However, the broken AF cannot be repaired after surgery, and the incidence of re-protrusion and reoccurring pain is high, possibly leading to a degeneration of the adjacent vertebrae. Therefore, effective treatment strategies must be explored to repair and prevent IDD. This paper systematically reviews recent advances in repairing IVD, describes its advantages and shortcomings, and explores the future direction of repair technology.

Ccn2a-FGFR1-SHH signaling is necessary for intervertebral disc homeostasis and regeneration in adult zebrafish

Intervertebral disc (IVD) degeneration is the primary cause of back pain in humans. However, the cellular and molecular pathogenesis of IVD degeneration is poorly understood. This study shows that zebrafish IVDs possess distinct and non-overlapping zones of cell proliferation and cell death. We find that, in zebrafish, cellular communication network factor 2a (ccn2a) is expressed in notochord and IVDs. Although IVD development appears normal in ccn2a mutants, the adult mutant IVDs exhibit decreased cell proliferation and increased cell death leading to IVD degeneration. Moreover, Ccn2a overexpression promotes regeneration through accelerating cell proliferation and suppressing cell death in wild-type aged IVDs. Mechanistically, Ccn2a maintains IVD homeostasis and promotes IVD regeneration by enhancing outer annulus fibrosus cell proliferation and suppressing nucleus pulposus cell death through augmenting FGFR1-SHH signaling. These findings reveal that Ccn2a plays a central role in IVD homeostasis and regeneration, which could be exploited for therapeutic intervention in degenerated human discs.

Comprehensive bioinformatics analyses reveal immune genes responsible for altered immune microenvironment in intervertebral disc degeneration

We sought to identify novel biomarkers and related mechanisms that might shape the immune infiltration in IDD, thereby providing novel perspective for IDD diagnosis and therapies. Gene expression data sets GSE124272 (for initial analysis) and GSE56081 (for validation analysis) involving samples from IDD patients and healthy controls were retrieved from the Gene Expression Omnibus (GEO) database. Immune genes associated with IDD were identified by GSEA; module genes that exhibited coordinated expression patterns and the strongest positive or negative correlation with IDD were identified by WGCNA. The intersection between immune genes and module genes was used for LASSO variable selection, whereby we obtained pivotal genes that were highly representative of IDD. We then correlated (Pearson correlation) the expression of pivotal genes with immune cell proportion inferred by CIBERSORT algorithm, and revealed the potential immune-regulatory roles of pivotal genes on the pathogenesis of IDD. We discovered several immune-associated pathways in which IDD-associated immune genes were highly clustered, and identified two gene modules that might promote or inhibit the pathogenesis of IDD. These candidate genes were further narrowed down to 8 pivotal genes, namely, MSH2, LY96, ADAM8, HEBP2, ANXA3, RAB24, ZBTB16 and PIK3CD, among which ANXA3, MSH2, ZBTB16, LY96, PIK3CD, ZBTB16, and ADAM8 were revealed to be correlated with the proportion of CD8 T cells and resting memory CD4 T cells. This work identified 8 pivotal genes that might be involved in the pathogenesis of IDD through triggering various immune-associated pathways and altering the composition of immune and myeloid cells in IDD patients, which provides novel perspectives on IDD diagnosis and treatment.

SNHG5/miR-299-5p/ATF2 Axis as a Biomarker in Immune Microenvironment of Intervertebral Disc Degeneration

Methods

The data sets of GSE56081 and GSE63492 in the Gene Expression Omnibus (GEO) database were used for screening and analysis, and the key gene markers were verified by GSE34095 and GSE126883. Finally, the infiltration of immune cells in the data were analyzed by MCPcounter analysis package.

Results

In this study, a ceRNA containing 15 lncRNAs, 9 miRNAs, and 103 mRNAs was constructed. After multimodel screening and verification, key gene marker was found, namely, ATF2. The lncRNA/miRNA/mRNA axis closely related to ATF2 have also been found, namely, SNHG5/miR-299-5p/ATF2. In the analysis of immune infiltration, ATF2 was negatively correlated with T cells but positively correlated with neutrophils and endothelial cells.

Conclusion

The SNHG5/miR-299-5p/ATF2 can be used as biomarker of IDD, and infiltration of immune cells plays an important role in the pathological development of IDD. In addition, as a marker of IDD, the involvement of the above-mentioned axis in the pathological development of IDD remains to be further explored.

Single-Cell RNA-Seq Analysis of Cells from Degenerating and Non-Degenerating Intervertebral Discs from the Same Individual Reveals New Biomarkers for Intervertebral Disc Degeneration

In this study, we used single-cell transcriptomic analysis to identify new specific biomarkers for nucleus pulposus (NP) and inner annulus fibrosis (iAF) cells, and to define cell populations within non-degenerating (nD) and degenerating (D) human intervertebral discs (IVD) of the same individual. Cluster analysis based on differential gene expression delineated 14 cell clusters. Gene expression profiles at single-cell resolution revealed the potential functional differences linked to degeneration, and among NP and iAF subpopulations. GO and KEGG analyses discovered molecular functions, biological processes, and transcription factors linked to cell type and degeneration state. We propose two lists of biomarkers, one as specific cell type, including C2orf40, MGP, MSMP, CD44, EIF1, LGALS1, RGCC, EPYC, HILPDA, ACAN, MT1F, CHI3L1, ID1, ID3 and TMED2. The second list proposes predictive IVD degeneration genes, including MT1G, SPP1, HMGA1, FN1, FBXO2, SPARC, VIM, CTGF, MGST1, TAF1D, CAPS, SPTSSB, S100A1, CHI3L2, PLA2G2A, TNRSF11B, FGFBP2, MGP, SLPI, DCN, MT-ND2, MTCYB, ADIRF, FRZB, CLEC3A, UPP1, S100A2, PRG4, COL2A1, SOD2 and MT2A. Protein and mRNA expression of MGST1, vimentin, SOD2 and SYF2 (p29) genes validated our scRNA-seq findings. Our data provide new insights into disc cells phenotypes and biomarkers of IVD degeneration that could improve diagnostic and therapeutic options.

Self-Assembling Peptide Hydrogels as Functional Tools to Tackle Intervertebral Disc Degeneration

Low back pain (LBP), caused by intervertebral disc (IVD) degeneration, is a major contributor to global disability. In its healthy state, the IVD is a tough and well-hydrated tissue, able to act as a shock absorber along the spine. During degeneration, the IVD is hit by a cell-driven cascade of events, which progressively lead to extracellular matrix (ECM) degradation, chronic inflammation, and pain. Current treatments are divided into palliative care (early stage degeneration) and surgical interventions (late-stage degeneration), which are invasive and poorly efficient in the long term. To overcome these limitations, alternative tissue engineering and regenerative medicine strategies, in which soft biomaterials are used as injectable carriers of cells and/or biomolecules to be delivered to the injury site and restore tissue function, are currently being explored. Self-assembling peptide hydrogels (SAPHs) represent a promising class of de novo synthetic biomaterials able to merge the strengths of both natural and synthetic hydrogels for biomedical applications. Inherent features, such as shear-thinning behaviour, high biocompatibility, ECM biomimicry, and tuneable physiochemical properties make these hydrogels appropriate and functional tools to tackle IVD degeneration. This review will describe the pathogenesis of IVD degeneration, list biomaterials requirements to attempt IVD repair, and focus on current peptide hydrogel materials exploited for this purpose.

Development, Pathogenesis, and Regeneration of the Intervertebral Disc: Current and Future Insights Spanning Traditional to Omics Methods

The intervertebral disc (IVD) is the fibrocartilaginous joint located between each vertebral body that confers flexibility and weight bearing capabilities to the spine. The IVD plays an important role in absorbing shock and stress applied to the spine, which helps to protect not only the vertebral bones, but also the brain and the rest of the central nervous system. Degeneration of the IVD is correlated with back pain, which can be debilitating and severely affects quality of life. Indeed, back pain results in substantial socioeconomic losses and healthcare costs globally each year, with about 85% of the world population experiencing back pain at some point in their lifetimes. Currently, therapeutic strategies for treating IVD degeneration are limited, and as such, there is great interest in advancing treatments for back pain. Ideally, treatments for back pain would restore native structure and thereby function to the degenerated IVD. However, the complex developmental origin and tissue composition of the IVD along with the avascular nature of the mature disc makes regeneration of the IVD a uniquely challenging task. Investigators across the field of IVD research have been working to elucidate the mechanisms behind the formation of this multifaceted structure, which may identify new therapeutic targets and inform development of novel regenerative strategies. This review summarizes current knowledge base on IVD development, degeneration, and regenerative strategies taken from traditional genetic approaches and omics studies and discusses the future landscape of investigations in IVD research and advancement of clinical therapies.

Fibrotic alterations in human annulus fibrosus correlate with progression of intervertebral disc herniation

BACKGROUND

Intervertebral disc (IVD) herniation is characterized by annulus fibrosus failure (AF) in containing the nucleus pulposus (NP). IVD herniation involves cellular and extracellular matrix (ECM) alterations that have been associated with tissue fibrosis, although still poorly investigated.

METHODS

Here, fibrotic alterations in human AF were evaluated, by characterizing the herniated ECM. Human AF samples (herniated lumbar IVD (n = 39, age 24-83) and scoliosis controls (n = 6, age 15-21)) were processed for transmission electron microscopy and histological/immunohistochemical analysis of fibrotic markers. Correlations between the fibrotic markers in AF ECM and the degree of NP containment (protused, contained and uncontained) and patients' age were conducted.

RESULTS

Our results demonstrate that with herniation progression, i.e. loss of NP containment, human AF presents less stained area of sulphated glycosaminoglycans and collagen I, being collagen I fibres thinner and disorganized. On the other hand, fibronectin stained area and percentage of α-smooth muscle actin+ cells increase in human AF, while matrix metalloproteinase-12 (MMP12) production and percentage of macrophages (CD68+ cells) remain constant. These structural and biochemical fibrotic alterations observed in human AF with herniation progression occur independently of the age.

CONCLUSIONS

The characterization of human AF here conducted evidence the presence of fibrosis in degenerated IVD, while highlighting the importance of considering the herniation progression stage, despite the patients' age, for a better understanding of the mechanisms behind AF failure and IVD herniation.

Intervertebral disc degeneration in warmblood horses: Histological and biochemical characterization

Gross morphology of healthy and degenerated intervertebral discs (IVDs) is largely similar in horses as in dogs and humans. For further comparison, the biochemical composition and the histological and biochemical changes with age and degeneration were analyzed in 41 warmblood horses. From 33 horses, 139 discs and 2 fetal vertebral columns were evaluated and scored histologically. From 13 horses, 73 IVDs were assessed for hydration, DNA, glycosaminoglycans, total collagen, hydroxyl-lysyl-pyridinoline, hydroxylysine, and advanced glycation end-product (AGE) content. From 7 horses, 20 discs were assessed for aggrecan, fibronectin, and collagen type 1 and 2 content. Histologically, tearing of the nucleus pulposus (NP) and cervical annulus fibrosus (AF), and total histological score (tearing and vascular proliferation of the AF, and chondroid metaplasia, chondrocyte-like cell proliferation, presence of notochordal cells, matrix staining, and tearing of the NP) correlated with gross degeneration. Notochordal cells were not seen in IVDs of horses. Age and gross degeneration were positively correlated with AGEs and a fibrotic phenotype, explaining gross degenerative changes. In contrast to dogs and humans, there was no consistent difference in glycosaminoglycan content and hydration between AF and NP, nor decrease of these variables with age or degeneration. Hydroxylysine decrease and collagen 1 and AGEs increase were most prominent in the NP, suggesting degeneration started in the AP. In caudal cervical NPs, AGE deposition was significantly increased in grossly normal IVDs and total collagen significantly increased with age, suggesting increased biomechanical stress and likelihood for spinal disease in this part of the vertebral column.

Elevated inflammatory gene expression in intervertebral disc tissues in mice with ADAM8 inactivated

We found ADAM8 enzymatic activity elevated in degenerative human intervertebral disc (IVD). Here, we examined the discs in ADAM8-inactivation mice that carry a mutation preventing self-activation of the enzyme. Surprisingly, elevated gene expression for inflammatory markers (Cxcl1, IL6) was observed in injured discs of ADAM8 mutant mice, along with elevated expression of type 2 collagen gene (Col2a1), compared with wild type controls. Injured annulus fibrosus of mutant and wild type mice contained a higher proportion of large collagen fibers compared with intact discs, as documented by microscopic examination under circular polarized light. In the intact IVDs, Adam8EQ mouse AF contained lower proportion of yellow (intermediate) fiber than WT mice. This suggests that ADAM8 may regulate inflammation and collagen fiber assembly. The seemingly contradictory findings of elevated inflammatory markers in mutant mice and excessive ADAM8 activity in human degenerative discs suggest that ADAM8 may interact with other enzymatic and pro-inflammatory processes needed for tissue maintenance and repair. As a future therapeutic intervention to retard intervertebral disc degeneration, partial inhibition of ADAM8 proteolysis may be more desirable than complete inactivation of this enzyme.

Fibronectin Fragments and Inflammation During Canine Intervertebral Disc Disease

Background: Canine intervertebral disc disease (IVDD) represents a significant clinical problem in veterinary medicine, with similarities to the human pathology. Host-derived damage-associated molecular patterns like fibronectin fragments (FnF) that develop during tissue dysfunction may be of specific relevance to IVD pathologies by inducing an inflammatory response in resident cells. Aim: This project aimed to determine the presence and pathobiological role of FnF during IVD herniation in dogs, with a focus on inflammation. Methods: Herniated nucleus pulposus (NP) material from five dogs as well as non-herniated adjacent NP material from three dogs was collected during spinal surgery required due to acute IVD herniation. The presence of different types of FnF were determined by Western blot analysis. NP cells isolated from six herniated canine IVDs were then exposed to 30 kDa FnF. NP cell inflammation and catabolism was examined by investigating the expression of IL-1β, IL-6, IL-8, and COX-2, as well as MMP-1 and MMP-3 by qPCR (all targets) and ELISA (IL-6, PGE2). Results: Amongst multiple sized FnF (30, 35, 45, and >170kDa), N-terminal fragments at a size of ~30 kDa were most consistently expressed in all five herniated IVDs. Importantly, these fragments were exclusively present in herniated, but not in non-herniated IVDs. Exposure of canine NP cells to 500 nM 30 kDa FnF caused a significant upregulation of IL-6 (62.5 ± 79.9, p = 0.032) and IL-8 (53.0 ± 75.7, p = 0.031) on the gene level, whereas IL-6 protein analysis was inconclusive. Donor-donor variation was observed in response to FnF treatment, whereby this phenomenon was most evident for COX-2, with three donors demonstrating a significant downregulation (0.67 ± 0.03, p = 0.003) and three donors showing upregulation (6.9 ± 5.5, p = 0.21). Co-treatment with Sparstolonin B, a TRL-2/TRL-4 antagonist, showed no statistical difference to FnF treatment alone in all tested target genes. Conclusion: Given the presence of the 30 kDa FnF in canine herniated IVDs and the proinflammatory effect of 30 kDa FnF on NP cells, we concluded that the accumulation of FnF may be involved in the pathogenesis of canine IVDD. These results correspond to the findings in humans with IVDD.

Growing a backbone - functional biomaterials and structures for intervertebral disc (IVD) repair and regeneration: challenges, innovations, and future directions

Back pain and associated maladies can account for an immense amount of healthcare cost and loss of productivity in the workplace. In particular, spine related injuries in the US affect upwards of 5.7 million people each year. The degenerative disc disease treatment almost always arises due to a clinical presentation of pain and/or discomfort. Preferred conservative treatment modalities include the use of non-steroidal anti-inflammatory medications, physical therapy, massage, acupuncture, chiropractic work, and dietary supplements like glucosamine and chondroitin. Artificial disc replacement, also known as total disc replacement, is a treatment alternative to spinal fusion. The goal of artificial disc prostheses is to replicate the normal biomechanics of the spine segment, thereby preventing further damage to neighboring sections. Artificial functional disc replacement through permanent metal and polymer-based components continues to evolve, but is far from recapitulating native disc structure and function, and suffers from the risk of unsuccessful tissue integration and device failure. Tissue engineering and regenerative medicine strategies combine novel material structures, bioactive factors and stem cells alone or in combination to repair and regenerate the IVD. These efforts are at very early stages and a more in-depth understanding of IVD metabolism and cellular environment will also lead to a clearer understanding of the native environment which the tissue engineering scaffold should mimic. The current review focusses on the strategies for a successful regenerative scaffold for IVD regeneration and the need for defining new materials, environments, and factors that are so finely tuned in the healthy human intervertebral disc in hopes of treating such a prevalent degenerative process.

Comprehensive review on intravertebral intraspinal, intrajoint, and intradiscal vacuum phenomenon: From anatomy and physiology to pathology

The term 'vacuum phenomenon' (VP), is characterized by gas-like density areas due to a rapid increase in the joint space volume ('acute VP') or represent a chronic gas collection. It can occur within a collapsed vertebral body, the spinal canal, joints but mainly the intervertebral disc. Studies support that VP is originated by a dynamic process involving the balance between tissues' liquid and gaseous components, influenced by the duration and the depth of mechanical and metabolic alterations, by the nature of neighboring tissues and the variability in both pressure and permeability of disc or vertebral or joint structures. Prevalence of VP in the general population is about 2%, reaching 20% in the elderly with disc degeneration. Although it's often a random finding in asymptomatic patients, VP is an eventually painful expression of disc degeneration, or disc or vertebral fracture, or bone lesions. In sporadic cases, intradiscal gas can be expelled (all-in-one or gradually), resulting in a gaseous cyst, causing pain and neurological symptoms. Considering that spontaneous resolution and recurrence after surgery are both possible, most of the authors recommend conservative treatment in patients with intradiscal and intravertebral VP; occasionally percutaneous CT(computed tomography) -guided aspiration or vertebral stabilization.

Alterations in ECM signature underscore multiple sub-phenotypes of intervertebral disc degeneration

The intervertebral disc is a specialized connective tissue critical for absorption of mechanical loads and providing flexibility to the spinal column. The disc ECM is complex and plays a vital role in imparting tissue its biomechanical function. The central NP is primarily composed of large aggregating proteoglycans (PGs) while surrounding AF is composed of fibrillar collagens, I and II. Aggrecan and versican in particular, due to their high concentration of sulfated GAG chains form large aggregates with hyaluronic acid (HA) and provide water binding capacity to the disc. Degradation of aggrecan core protein due to aggrecanase and MMP activity, SNPs that affect number of chondroitin sulfate (CS) substitutions and alteration in enzymes critical in synthesis of CS chains can impair the aggrecan functionality. Similarly, levels of many matrix and matrix-related molecules e.g. Col2, Col9, HAS2, ccn2 are dysregulated during disc degeneration and genetic animal models have helped establish causative link between their expression and disc health. In the degenerating and herniated discs, increased levels of inflammatory cytokines such as TNF-α, IL-1β and IL-6 are shown to promote matrix degradation through regulating expression and activity of critical proteases and stimulate immune cell activation. Recent studies of different mouse strains have better elucidated the broader impact of spontaneous degeneration on disc matrix homeostasis. SM/J mice showed an increased cell apoptosis, loss of cell phenotype, and cleavage of aggrecan during early stages followed by tissue fibrosis evident by enrichment of several collagens, SLRPs and fibronectin. In summary, while disc degeneration encompasses wide spectrum of degenerative phenotypes extensive matrix degradation and remodeling underscores all of them.

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The intervertebral disc absorbs loads and provides flexibility to the spine.

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The ECM is complex and vital for imparting tissue its biomechanical function.

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Numerous types of proteoglycans and collagens designate the quality of the disc.

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Many matrix and matrix-related molecules are dysregulated during disc degeneration.

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Matrix degradation and remodeling underscores wide spectrum of phenotype.

Profile of Matrix-Remodeling Proteinases in Osteoarthritis: Impact of Fibronectin

The extracellular matrix (ECM) is a complex and specialized three-dimensional macromolecular network, present in nearly all tissues, that also interacts with cell surface receptors on joint resident cells. Changes in the composition and physical properties of the ECM lead to the development of many diseases, including osteoarthritis (OA). OA is a chronic degenerative rheumatic disease characterized by a progressive loss of synovial joint function as a consequence of the degradation of articular cartilage, also associated with alterations in the synovial membrane and subchondral bone. During OA, ECM-degrading enzymes, including urokinase-type plasminogen activator (uPA), matrix metalloproteinases (MMPs), and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTSs), cleave ECM components, such as fibronectin (Fn), generating fibronectin fragments (Fn-fs) with catabolic properties. In turn, Fn-fs promote activation of these proteinases, establishing a degradative and inflammatory feedback loop. Thus, the aim of this review is to update the contribution of ECM-degrading proteinases to the physiopathology of OA as well as their modulation by Fn-fs.

Neonatal annulus fibrosus regeneration occurs via recruitment and proliferation of Scleraxis-lineage cells

Intervertebral disc (IVD) injuries are a cause of degenerative changes in adults which can lead to back pain, a leading cause of disability. We developed a model of neonatal IVD regeneration with full functional restoration and investigate the cellular dynamics underlying this unique healing response. We employed genetic lineage tracing in mice using Scleraxis (Scx) and Sonic hedgehog (Shh) to fate-map annulus fibrosus (AF) and nucleus pulposus (NP) cells, respectively. Results indicate functional AF regeneration after severe herniation injury occurs in neonates and not adults. AF regeneration is mediated by Scx-lineage cells that lose ScxGFP expression and adopt a stem/progenitor phenotype (Sca-1, days 3–14), proliferate, and then redifferentiate towards type I collagen producing, ScxGFP+ annulocytes at day 56. Non Scx-lineage cells were also transiently observed during neonatal repair, including Shh-lineage cells, macrophages, and myofibroblasts; however, these populations were no longer detected by day 56 when annulocytes redifferentiate. Overall, repair did not occur in adults. These results identify an exciting cellular mechanism of neonatal AF regeneration that is predominantly driven by Scx-lineage annulocytes.

Molecular basis of degenerative spinal disorders from a proteomic perspective (Review)

Intervertebral disc degeneration (IDD) and ligamentum flavum hypertrophy (LFH) are major causes of degenerative spinal disorders. Comparative and proteomic analysis was used to identify differentially expressed proteins (DEPs) in IDD and LFH discs compared with normal discs. Subsequent gene ontology term enrichment analysis and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis of the DEPs in human IDD discs or LFH samples were performed to identify the biological processes and signaling pathways involved in IDD and LFH. The PI3K-AKT signaling pathway, advanced glycation endproducts-receptor for advanced glycation endproducts signaling pathway, p53 signaling pathway, and transforming growth factor-b signaling pathway were activated in disc degeneration. This review summarizes the recently identified DEPs, including prolargin, fibronectin 1, cartilage intermediate layer protein, cartilage oligomeric matrix protein, and collagen types I, II and IV, and their pathophysiological roles in degenerative spinal disorders, and may provide a deeper understanding of the pathological processes of human generative spinal disorders. The present review aimed to summarize significantly changed proteins in degenerative spinal disorders and provide a deeper understanding to prevent these diseases.

Sex Differences in Rat Intervertebral Disc Structure and Function Following Annular Puncture Injury

STUDY DESIGN

A rat puncture injury intervertebral disc (IVD) degeneration model with structural, biomechanical, and histological analyses.

OBJECTIVE

To determine if males and females have distinct responses in the IVD after injury.

SUMMARY OF BACKGROUND DATA

Low back pain (LBP) and spinal impairments are more common in women than men. However, sex differences in IVD response to injury have been underexplored, particularly in animal models where sex differences can be measured without gender confounds.

METHODS

Forty-eight male and female Sprague Dawley rats underwent sham, single annular puncture with tumor necrosis factor α (TNFα) injection (1×), or triple annular puncture with TNFα injection (3×) surgery. Six weeks after surgery, lumbar IVDs were assessed by radiologic IVD height, spinal motion segment biomechanical testing, histological degeneration grading, second harmonic generation (SHG) imaging, and immunofluorescence for fibronectin and α-smooth muscle actin.

RESULTS

Annular puncture injuries significantly increased degenerative grade and IVD height loss for males and females, but females had increased degeneration grade particularly in the annulus fibrosus (AF). Despite IVD height loss, biomechanical properties were largely unaffected by injury at 6 weeks. However, biomechanical measures sensitive to outer AF differed by sex after 3× injury-male IVDs had greater torsional stiffness, torque range, and viscoelastic creep responses. SHG intensity of outer AF was reduced after injury only in female IVDs, suggesting sex differences in collagen remodeling. Both males and females exhibited decreased cellularity and increased fibronectin expression at injury sites.

CONCLUSION

IVD injury results in distinct degeneration and functional healing responses between males and females. The subtle sex differences identified in this animal model suggest differences in response to IVD injury that might explain some of the variance observed in human LBP, and demonstrate the need to better understand differences in male and female IVD degeneration patterns and pain pathogenesis.

LEVEL OF EVIDENCE

N/A.

Short Link N promotes disc repair in a rabbit model of disc degeneration

Background

The degeneration of the intervertebral disc (IVD) is characterized by proteolytic degradation of the extracellular matrix, and its repair requires the production of an extracellular matrix with a high proteoglycan-to-collagen ratio characteristic of a nucleus pulposus (NP)-like phenotype in vivo. At the moment, there is no medical treatment to reverse or even retard disc degeneration. The purpose of the present study was to determine if a low dose of short link N (sLN), a recently discovered fragment of the link N peptide, could behave in a manner similar to that of link N in restoring the proteoglycan content and proteoglycan-to-collagen ratio of the disc in a rabbit model of IVD degeneration, as an indication of its potential therapeutic benefit in reversing disc degeneration.

Methods

Adolescent New Zealand white rabbits received an annular puncture with an 18-gauge needle into two noncontiguous discs to induce disc degeneration. Two weeks later, either saline (10 μL) or sLN (25 μg in 10 μL saline) was injected into the center of the NP. The sLN concentration was empirically chosen at a lower molar concentration equivalent to half that of link N (100 μg in 10 μL). The effect on radiographic, biochemical and histologic changes were evaluated.

Results

Following needle puncture, disc height decreased by about 25–30% within 2 weeks and maintained this loss for the duration of the 12-week study; a single 25-μg sLN injection at 2 weeks partially restored this loss in disc height. sLN injection led to an increase in glycosaminoglycans (GAG) content 12 weeks post-injection in both the NP and annulus fibrosus (AF). There was a trend towards maintaining control disc collagen-content with sLN supplementation and the GAG-to-collagen ratio in the NP was increased when compared to the saline group.

Conclusions

When administered to the degenerative disc in vivo, sLN injection leads to an increase in proteoglycan content and a trend towards maintaining control disc collagen content in both the NP and AF. This is similar to link N when it is administered to the degenerative disc. Thus, pharmacologically, sLN supplementation could be a novel therapeutic approach for treating disc degeneration.

Modic type 1 change is an autoimmune response that requires a proinflammatory milieu provided by the 'Modic disc'

BACKGROUND CONTEXT

Modic changes (MCs) are magnetic resonance imaging (MRI) evidence of inflammatory and fibrotic vertebral bone marrow lesions that associate with adjacent disc degeneration and end plate damage. Although MC etiology is uncertain, historical data suggest a linkage to an autoimmune response of bone marrow triggered by the nucleus pulposus (NP).

PURPOSE

The aim of this study was to test whether bone marrow has an autoimmune response to NP cells that is amplified by an inflammatory milieu and ultimately leads to MC development in vivo. We hypothesized that an inflammatory co-stimulus is required for bone marrow/NP crosstalk to stimulate MC.

STUDY DESIGN

This is an in-vitro cell co-culture study plus in-vivo experiments in rat caudal vertebrae.

METHODS

In in-vitro study, bone marrow mononuclear cells (BMNCs) and NP cells (NPCs) from rats were co-cultured with and without interleukin (IL)-1α stimulation. Cell viability (n=3) of BMNCs and NPCs and gene expression (n=7) were analyzed. In in-vivo study, proinflammatory lipopolysaccharide (LPS) and control disc nucleus surrogates (NP micromass pellets) were generated in vitro from rat NPCs and implanted into rat tail vertebrae, and the response was compared with sham surgery (n=12 each). Tissue changes were investigated with T1w and T2w MRI (7T), histology, and immunohistochemistry (tumor necrosis factor, CD3) 1 (n=6) and 2 weeks (n=6) after implantation.

RESULTS

BMNC/NPC co-culture significantly increased lymphocyte viability (42%-69%, p<.05) and reduced NPC viability (96%-88%, p<.001), indicating immunogenicity of NPC. However, IL-1α was required to cause significant transcriptional upregulation of IL-1, IL-6, IL-10, and tropomyosin receptor kinase A. Therefore, an inflammatory activation is required to amplify the immune response. Immunogenicity of the NP was corroborated in vivo by CD3 cell accumulation around LPS and control disc surrogates at Day 7. However, only the LPS disc surrogate group demonstrated infiltration of CD3 cells at Day 14. Furthermore, end plate defects (p<.05, LPS: n=4/6, Ctrl: n=0/6, sham: n=0/6) and MC1-like MRI changes (T2w hyperintensity, p<.05) were only seen with LPS disc surrogates.

CONCLUSIONS

NPCs are immunogenic but cannot trigger MC without an additional proinflammatory stimulus. Our data suggest that MC requires end plate defects that allow marrow/NPC co-mingling plus an adjacent inflammatory "MC disc" that can amplify the immune response.

Link N as a therapeutic agent for discogenic pain

Neurotrophins (NTs) are the major contributors of sensory axonal sprouting, neural survival, regulation of nociceptive sensory neurons, inflammatory hyperalgesia, and neuropathic pain. Intervertebral disc (IVD) cells constitutively express NTs. Their expression is upregulated by proinflammatory cytokines present in the IVD during degeneration, which can promote peripheral nerve ingrowth and hyperinnervation, leading to discogenic pain. Currently, there are no targeted therapies that decrease hyperinnervation in degenerative disc disease. Link N is a naturally occurring peptide with a high regenerative potential in the IVD. Therefore, the suitability of Link N as a therapeutic peptide for suppressing NTs, which are known modulators and mediators of pain, was investigated. The aim of the present study is to determine the effect of Link N on NTs expression, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and their cognate receptors TrkA and TrkB as they are directly correlated with symptomatic back pain. Furthermore, the neurotransmitter (substance P) was also evaluated in human annulus fibrosus (AF) cells stimulated with cytokines. Human AF cells isolated from normal IVDs were stimulated with interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in the presence or absence of Link N. NGF release in the media was evaluated by Western blotting. Total RNA was isolated and gene expression was measured using real-time PCR. Gene expression of NGF, BDNF, TrkA, and TrkB significantly decreased in human disc cells stimulated with either IL-1β or TNF-α supplemented with Link N when compared to the cells stimulated only with IL-1β or TNF-α. NGF protein expression was also suppressed in AF cells coincubated with Link N and IL-1β when compared to the cells stimulated only with IL-1β. Link N can suppress the stimulation of NGF, BDNF, and their receptors TrkA and TrkB in AF cells in an inflammatory milieu. Thus, coupled with previous observations, this suggests that administration of Link N has the potential to not only repair the discs in early stages of the disease but also suppress pain.

Biologic therapies to enhance intervertebral disc repair

Degenerative disc disease is a progressive, chronic disorder with strong association to pain, where the dysregulated tissue environment signals disc cells, thereby leading to a low inflammatory process and slow extracellular matrix degradation and fibrosis in a perpetual vicious cycle, generating a structural and functional failure of intervertebral disc joint (IVDJ). Among current biologic therapies, there is an emerging minimally invasive strategy that consists of infiltrating plasma rich in growth factors, a safe and efficacious therapeutic approach for other musculoskeletal degenerative conditions. This review summarizes the homeostasis and degeneration of IVDJ, discusses some results on basic science and therapeutic use of platelet-rich plasma products and advances an alternative minimally invasive biologic therapy in IVDJ degeneration and chronic back pain.

Toll-like Receptor Activation Induces Degeneration of Human Intervertebral Discs

Toll-like receptors (TLR) are activated by endogenous alarmins such as fragmented extracellular matrix compounds found in the degenerating disc. TLRs regulate cytokine, neurotrophin, and protease expression in human disc cells in vitro, and thus control key factors in disc degeneration. However, whether TLR activation leads to degenerative changes in intact human discs is unclear. Nucleus pulposus (NP) cells isolated from non-degenerating discs increase IL-1β and nerve growth factor gene expression following treatment with Pam2CSK4 (TLR2/6 agonist) but not Pam3CSK4 (TLR1/2 agonist). Challenging NP cells with Pam2CSK4 or 30 kDa fibronectin fragments (FN-f, an endogenous TLR2 and TLR4 alarmin) increased secretion of proinflammatory cytokines. We then investigated the effect of TLR activation in intact, non-degenerate, ex vivo human discs. Discs were injected with PBS, Pam2CSK4 and FN-f, and cultured for 28 days. TLR activation increased proteoglycan and ECM protein release into the culture media and decreased proteoglycan content in the NP. Proteases, including MMP3, 13 and HTRA1, are secreted at higher levels following TLR activation. In addition, proinflammatory cytokine levels, including IL-6, TNFα and IFNγ, increased following TLR activation. These results indicate that TLR activation induces degeneration in human discs. Therefore, TLRs are potential disease-modifying therapeutic targets to slow disc degeneration.

Matrisome Profiling During Intervertebral Disc Development And Ageing

Intervertebral disc (IVD) degeneration is often the cause of low back pain. Degeneration occurs with age and is accompanied by extracellular matrix (ECM) depletion, culminating in nucleus pulpous (NP) extrusion and IVD destruction. The changes that occur in the disc with age have been under investigation. However, a thorough study of ECM profiling is needed, to better understand IVD development and age-associated degeneration. As so, iTRAQ LC-MS/MS analysis of foetus, young and old bovine NPs, was performed to define the NP matrisome. The enrichment of Collagen XII and XIV in foetus, Fibronectin and Prolargin in elder NPs and Collagen XI in young ones was independently validated. This study provides the first matrisome database of healthy discs during development and ageing, which is key to determine the pathways and processes that maintain disc homeostasis. The factors identified may help to explain age-associated IVD degeneration or constitute putative effectors for disc regeneration.

The role of angiopoietin-2 in nucleus pulposus cells during human intervertebral disc degeneration

Although evidence shows that intervertebral disc degeneration is generally characterized by angiogenesis, the role of angiopoietin has not been investigated. This study examined the presence of angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) within the native intervertebral disc (IVD) and elucidated their functions in the regulation of nucleus pulposus (NP) cells. Initial investigation of uncultured NP tissue revealed that Ang-1 and Ang-2 were expressed by native NP cells. Ang-2 expression was significantly increased in infiltrated and degenerate samples relative to normal samples. The ratio of Ang-2/Ang-1 in tissues from patients increased markedly with increasing age and level of degeneration of the IVD. The ratio of both Ang-2/Ang-1 mRNA and protein increased over time when cells were subjected to constant pressure at 1 Mpa in vitro. Our findings indicate that Ang-2 plays a role in suppressing cell adhesion and viability, and promotes the apoptosis of NP cells and that Ang-2 can inhibit the pathways stimulated by Ang-1 and fibronectin. Ang-2 release during IVD degeneration causes higher ratio of Ang-2 to Ang-1, further inhibits NP cell viability and adhesion, promoting apoptosis by blocking PI3K/Akt signaling. The present study therefore provides new insights into the role of the angiopoietin-Tie system in the pathogenesis of IVD degeneration.

Animal models for disc degeneration-an update

Intervertebral disc degeneration is considered a major cause of back pain that places a heavy burden on society, both because of its effect on the physiology of individuals and its consequences on the world economy. During the past few decades, research findings in the pre-clinical setting have led to a significant increase in the understanding of intervertebral disc degeneration, although many aspects of the disease remain unclear. The goal of this review is to summarize existing animal models for disc degeneration studies and the difficulties that are associated with the use of such models. A firm understanding of the cellular and molecular events that ensue as a result of injuries, as well as environmental factors, could be instrumental in the development of targeted therapies for the treatment of intervertebral disc degeneration.

Propionibacterium acnes infected intervertebral discs cause vertebral bone marrow lesions consistent with Modic changes

Modic type I change (MC1) are vertebral bone marrow lesions adjacent to degenerated discs that are specific for discogenic low back pain. The etiopathogenesis is unknown, but occult discitis, in particular with Propionibacteria acnes (P. acnes), has been suggested as a possible etiology. If true, antibiotic therapy should be considered for patients with MC1. However, this hypothesis is controversial. While some studies report up to 40% infection rate in herniated discs, others fail to detect infected discs and attribute reports of positive cultures to contamination during sampling procedure. Irrespective of the clinical controversy, whether it is biologically plausible for P. acnes to cause MC1 has never been investigated. Therefore, the objective of this study was to test if P. acnes can proliferate within discs and cause reactive changes in the adjacent bone marrow. P. acnes was aseptically isolated from a symptomatic human L4/5 disc with MC1 and injected into rat tail discs. We demonstrate proliferation of P. acnes and up-regulation of IL-1 and IL-6 within three days of inoculation. At day-7, disc degeneration was apparent along with fibrotic endplate erosion. TNF-α immunoreactivity was enhanced within the effected endplates along with cellular infiltrates. The bone marrow appeared normal. At day-14, endplates and trabecular bone close to the disc were almost completely resorbed and fibrotic tissue extended into the bone marrow. T-cells and TNF-α immunoreactivity were identified at the disc/marrow junction. On MRI, bone marrow showed MC1-like changes. In conclusion, P. acnes proliferate within the disc, induce degeneration, and cause MC1-like changes in the adjacent bone marrow. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1447-1455, 2016.

The Involvement of Protease Nexin-1 (PN1) in the Pathogenesis of Intervertebral Disc (IVD) Degeneration

Protease nexin-1 (PN-1) is a serine protease inhibitor belonging to the serpin superfamily. This study was undertaken to investigate the regulatory role of PN-1 in the pathogenesis of intervertebral disk (IVD) degeneration. Expression of PN-1 was detected in human IVD tissue of varying grades. Expression of both PN-1 mRNA and protein was significantly decreased in degenerated IVD, and the expression levels of PN-1 were correlated with the grade of disc degeneration. Moreover, a decrease in PN-1 expression in primary NP cells was confirmed. On induction by IL-1β, the expression of PN-1 in NP cells was decreased at day 7, 14, and 21, as shown by western blot analysis and immunofluorescence staining. PN-1 administration decreased IL-1β-induced MMPs and ADAMTS production and the loss of Agg and Col II in NP cell cultures through the ERK1/2/NF-kB signaling pathway. The changes in PN-1 expression are involved in the pathogenesis of IVD degeneration. Our findings indicate that PN-1 administration could antagonize IL-1β-induced MMPs and ADAMTS, potentially preventing degeneration of IVD tissue. This study also revealed new insights into the regulation of PN-1 expression via the ERK1/2/NF-kB signaling pathway and the role of PN-1 in the pathogenesis of IVD degeneration.

Development of a Large Animal Long-Term Intervertebral Disc Organ Culture Model That Includes the Bony Vertebrae for Ex Vivo Studies

Intervertebral disc (IVD) degeneration is a common cause of low back pain. Testing potential therapeutics in the regeneration of the disc requires the use of model systems. Although several animal models have been developed to investigate IVD degeneration, they are technically challenging to prepare, expensive, present with limitations when performing biomechanical studies on the disc, and are impractical in large-scale screening of novel anabolic and scaffolding agents. An IVD organ culture system offers an inexpensive alternative. In the current paradigm, the bony endplates are removed to allow for nutrient diffusion and maintenance of disc cell viability. Although this is an excellent system for testing biologics, it results in concave cartilage endplates and, as such, requires special platens for loading purposes in a bioreactor as flat ones can overload the annular disc region leading to improper loading. Furthermore, the absence of bone makes it unsuitable for applying complex cyclic loading, a topic of interest in the study of chronic progressive degeneration, as multiaxial loading is more representative of daily forces encountered by the IVD. We have developed and validated a novel long-term IVD organ culture model that retains vertebral bone and is easy to prepare. Our model is ideal for testing potential drugs and alternate-based therapies, in addition to investigating the long-term effects of loading paradigms on disc degeneration and repair.

A Review of Animal Models of Intervertebral Disc Degeneration: Pathophysiology, Regeneration, and Translation to the Clinic

Lower back pain is the leading cause of disability worldwide. Discogenic pain secondary to intervertebral disc degeneration is a significant cause of low back pain. Disc degeneration is a complex multifactorial process. Animal models are essential to furthering understanding of the degenerative process and testing potential therapies. The adult human lumbar intervertebral disc is characterized by the loss of notochordal cells, relatively large size, essentially avascular nature, and exposure to biomechanical stresses influenced by bipedalism. Animal models are compared with regard to the above characteristics. Numerous methods of inducing disc degeneration are reported. Broadly these can be considered under the categories of spontaneous degeneration, mechanical and structural models. The purpose of such animal models is to further our understanding and, ultimately, improve treatment of disc degeneration. The role of animal models of disc degeneration in translational research leading to clinical trials of novel cellular therapies is explored.

Fibrotic-like changes in degenerate human intervertebral discs revealed by quantitative proteomic analysis

OBJECTIVE

Intervertebral disc degeneration (IDD) can lead to symptomatic conditions including sciatica and back pain. The purpose of this study is to understand the extracellular matrix (ECM) changes in disc biology through comparative proteomic analysis of degenerated and non-degenerated human intervertebral disc (IVD) tissues of different ages.

DESIGN

Seven non-degenerated (11-46 years of age) and seven degenerated (16-53 years of age) annulus fibrosus (AF) and nucleus pulposus (NP) samples were used. Proteins were extracted using guanidine hydrochloride, separated from large proteoglycans (PGs) by caesium chloride (CsCl) density gradient ultracentrifugation, and identified using liquid chromatography (LC) coupled with tandem mass spectrometry (MS/MS). For quantitative comparison, proteins were labeled with iTRAQ reagents. Collagen fibrils in the NP were assessed using scanning electron microscopy (SEM).

RESULTS

In the AF, quantitative analysis revealed increased levels of HTRA1, COMP and CILP in degeneration when compared with samples from older individuals. Fibronectin showed increment with age and degeneration. In the NP, more CILP and CILP2 were present in degenerated samples of younger individuals. Reduced protein solubility was observed in degenerated and older non-degenerated samples correlated with an accumulation of type I collagen in the insoluble fibers. Characterization of collagen fibrils in the NP revealed smaller mean fibril diameters and decreased porosity in the degenerated samples.

CONCLUSIONS

Our study identified distinct matrix changes associated with aging and degeneration in the intervertebral discs (IVDs). The nature of the ECM changes, together with observed decreased in solubility and changes in fibril diameter is consistent with a fibrotic-like environment.

Nerve Growth Factor Is Regulated by Toll-Like Receptor 2 in Human Intervertebral Discs

Nerve growth factor (NGF) contributes to the development of chronic pain associated with degenerative connective tissue pathologies, such as intervertebral disc degeneration and osteoarthritis. However, surprisingly little is known about the regulation of NGF in these conditions. Toll-like receptors (TLR) are pattern recognition receptors classically associated with innate immunity but more recently were found to be activated by endogenous alarmins such as fragmented extracellular matrix proteins found in degenerating discs or cartilage. In this study we investigated if TLR activation regulates NGF and which signaling mechanisms control this response in intervertebral discs. TLR2 agonists, TLR4 agonists, or IL-1β (control) treatment increased NGF, brain-derived neurotrophic factor (BDNF), and IL-1β gene expression in human disc cells isolated from healthy, pain-free organ donors. However, only TLR2 activation or IL-1β treatment increased NGF protein secretion. TLR2 activation increased p38, ERK1/2, and p65 activity and increased p65 translocation to the cell nucleus. JNK activity was not affected by TLR2 activation. Inhibition of NF-κB, and to a lesser extent p38, but not ERK1/2 activity, blocked TLR2-driven NGF up-regulation at both the transcript and protein levels. These results provide a novel mechanism of NGF regulation in the intervertebral disc and potentially other pathogenic connective tissues. TLR2 and NF-κB signaling are known to increase cytokines and proteases, which accelerate matrix degradation. Therefore, TLR2 or NF-κB inhibition may both attenuate chronic pain and slow the degenerative progress in vivo.

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.

What is the preclinical evidence on platelet rich plasma and intervertebral disc degeneration?

PURPOSE

Intervertebral disc degeneration is a common disease that usually starts from the third decade of life and it represents a significant cause of socio-economic problems. The accepted surgical treatment for disc degeneration is disc removal and vertebral fusion or, in selected cases, intervertebral disc arthroplasty. Several studies have demonstrated the ability of platelet rich plasma (PRP) to stimulate cell proliferation and extracellular matrix regeneration. However, literature results are still limited and more studies are required to clarify the role of PRP in the prevention or in the treatment of degenerative disc disease. The aim of this review is to summarize and critically analyze the current preclinical evidence about the use of PRP in intervertebral disc degeneration.

METHODS

Literature search was performed through various combinations of the following keywords: Intervertebral Disc Degeneration, Platelet Rich Plasma, PRP, Intervertebral disc regeneration. Papers included in our review cover the period between 2006 and 2014. The PRISMA 2009 checklist was followed.

RESULTS

At the end of the review process, 12 articles were included in our final manuscript, including 6 "in vitro" and 6 "in vivo" studies. All the included studies lead to positive preclinical results. No standardization of methodological analysis was observed.

CONCLUSION

It is not possible to draw definitive evidence about the use of PRP in IVD regeneration. We advise a proper standardization of the methodological analysis in order to compare the available data and achieve definitive results. This should be the cornerstone for future clinical applications.

MiR-34a promotes Fas-mediated cartilage endplate chondrocyte apoptosis by targeting Bcl-2

Apoptosis of cartilage endplate (CEP) chondrocytes is associated with the pathogenesis of intervertebral disk degeneration (IDD). Recent studies have shown that miR-34a is crucially involved in chondrocyte apoptosis during osteoarthritic cartilage. Here, we investigated the involvement of miR-34a in CEP chondrocyte apoptosis in IDD. In human degenerated CEP chondrocytes, miRNA (miR)-34a was markedly elevated in association with increased apoptosis. Bioinformatics target prediction identified Bcl-2 as a putative target of miR-34a. Furthermore, miR-34a inhibited Bcl-2 expression by directly targeting their 3'-untranslated regions, and this inhibition was abolished by mutation of the miR-34a binding sites. In vitro, knockdown of miR-34a in human endplate chondrocytes resulted in overexpression of Bcl-2, whereas upregulation of miR-34a led to repression of Bcl-2. Fas-mediated apoptosis was decreased when antagonizing miR-34a with locked nucleotide analog-miR-34a in human endplate chondrocytes. Taken together, our results demonstrate that upregulated miR-34a potentiates Fas-mediated endplate chondrocyte apoptosis, which is associated with IDD.

Fibronectin fragments and the cleaving enzyme ADAM-8 in the degenerative human intervertebral disc

STUDY DESIGN

The presence of fibronectin fragments (FN-fs) and the cleaving enzyme, A disintegrin and metalloproteinase domain-containing protein (ADAM)-8 were examined in human intervertebral disc (IVD) tissue in vitro.

OBJECTIVE

To investigate the presence and pathophysiological concentration of FN-fs and their cleaving enzyme, ADAM-8, in the human IVD tissue.

SUMMARY OF BACKGROUND DATA

The 29-kDa FN-f has been shown to result in extracellular matrix loss in rabbit IVDs. However, the concentration of this biologically active fragment in the degenerative human IVD tissue has previously not been determined. Furthermore, it is critical to identify the enzyme(s) responsible for FN cleavage in the IVD.

METHODS

Human degenerative IVD tissues were removed during spinal surgery. A normal seeming young adult and an infant human cadaveric sample were obtained as controls. Soluble proteins were extracted, and analyzed by Western blotting using antibodies specific for the human FN neoepitope VRAA²⁷¹. A purified 29-kDa FN-f was used to allow estimation of the concentration of FN-fs in the tissues. ADAM-8, a FN-cleaving enzyme, was analyzed by Western blotting and immunostaining.

RESULTS

All adult IVD tissues contain many FN-f species, but these species were absent from the infant disc tissue. Moderately degenerative discs contained the highest amount of FN-fs; the concentration was estimated to be in the nanomolar range per gram of tissue. ADAM-8, known to cleave FN resulting in the VRAA²⁷¹ neoepitope, was present in the human disc. ADAM-8 primarily localized in the pericellular matrix of the nucleus pulposus tissue, as determined by immunostaining.

CONCLUSION

This is the first report that N-terminal FN-fs are consistently present in IVD tissues from adult subjects. The pathophysiological concentration of these fragments is estimated to be at nanomolar range per gram of IVD tissue. Furthermore, ADAM-8, known to cleave FN, is present at the pericellular matrix of disc cells.

Link N and mesenchymal stem cells can induce regeneration of the early degenerate intervertebral disc

Link N is a naturally occurring peptide that can stimulate proteoglycan synthesis in intervertebral disc (IVD) cells. IVD repair can also potentially be enhanced by mesenchymal stem cell (MSC) supplementation to maximize extracellular matrix (ECM) production. In a previous study, we have shown that Link N can inhibit osteogenesis and increase the chondrogenesis of MSCs in vitro. The aim of the present study was to determine the potential of MSCs and Link N alone or in combination with regard to tissue repair in the degenerate disc. Bovine IVDs with trypsin-induced degeneration were treated with MSCs, Link N, or a combination of MSCs and Link N. Trypsin-treated discs were also injected with phosphate-buffered saline to serve as a degeneration control. The ECM proteins and proteoglycans were extracted from the inner nucleus pulposus (NP) of the discs, and sulfated glycosaminoglycans (GAGs) were analyzed by the dimethyl methylene blue dye-binding assay. The expression of type II collagen was analyzed by western blot. To track the MSCs after injection, MSCs were labeled with PKH67 and observed under confocal microscopy after the 2 week culture period. The GAG content significantly increased compared with the degeneration control when degenerate discs were treated with MSCs, Link N, or a combination of both Link N and MSCs. Histological analysis revealed that the newly synthesized proteoglycan was able to diffuse throughout the ECM and restore tissue content even in areas remote from the cells. The quantity of extractable type II collagen was also increased when the degenerate discs were treated with MSCs and Link N, either alone or together. MSCs survived, integrated in the tissue, and were found distributed throughout the NP after the 2 week culture period. MSCs and Link N can restore GAG content in degenerate discs, when administered separately or together. Treatment with MSCs and Link N can also increase the expression of type II collagen. The results support the concept that biological repair of disc degeneration is feasible, and that the administration of either MSCs or Link N has therapeutic potential in early stages of the disease.

Fibronectin in tissue regeneration: timely disassembly of the scaffold is necessary to complete the build

Tissue injury initiates extracellular matrix molecule expression, including fibronectin production by local cells and fibronectin leakage from plasma. To benefit tissue regeneration, fibronectin promotes opsonization of tissue debris, migration, proliferation, and contraction of cells involved in the healing process, as well as angiogenesis. When regeneration proceeds, the fibronectin matrix is fully degraded. However, in a diseased environment, fibronectin clearance is often disturbed, allowing structural variants to persist and contribute to disease progression and failure of regeneration. Here, we discuss first how fibronectin helps tissue regeneration, with a focus on normal cutaneous wound healing as an example of complete tissue recovery. Then, we continue to argue that, although the fibronectin matrix generated following cartilage and central nervous system white matter (myelin) injury initially benefits regeneration, fibronectin clearance is incomplete in chronic wounds (skin), osteoarthritis (cartilage), and multiple sclerosis (myelin). Fibronectin fragments or aggregates persist, which impair tissue regeneration. The similarities in fibronectin-mediated mechanisms of frustrated regeneration indicate that complete fibronectin clearance is a prerequisite for recovery in any tissue. Also, they provide common targets for developing therapeutic strategies in regenerative medicine.

Gene expression profile analysis of human intervertebral disc degeneration

In this study, we used microarray analysis to investigate the biogenesis and progression of intervertebral disc degeneration. The gene expression profiles of 37 disc tissue samples obtained from patients with herniated discs and degenerative disc disease collected by the National Cancer Institute Cooperative Tissue Network were analyzed. Differentially expressed genes between more and less degenerated discs were identified by significant analysis of microarray. A total of 555 genes were significantly overexpressed in more degenerated discs with a false discovery rate of < 3%. Functional annotation showed that these genes were significantly associated with membrane-bound vesicles, calcium ion binding and extracellular matrix. Protein-protein interaction analysis showed that these genes, including previously reported genes such as fibronectin, COL2A1 and β-catenin, may play key roles in disc degeneration. Unsupervised clustering indicated that the widely used morphology-based Thompson grading system was only marginally associated with the molecular classification of intervertebral disc degeneration. These findings indicate that detailed, systematic gene analysis may be a useful way of studying the biology of intervertebral disc degeneration.

Genetic polymorphisms associated with intervertebral disc degeneration

BACKGROUND CONTEXT

Disc degeneration (DD) is a multifaceted chronic process that alters the structure and function of the intervertebral discs and can lead to painful conditions. The pathophysiology of degeneration is not well understood, but previous studies suggest that certain genetic polymorphisms may be important contributing factors leading to an increased risk of DD.

PURPOSE

To review the genetic factors in DD with a focus on polymorphisms and their putative role in the pathophysiology of degeneration. Elucidating the genetic components that are associated with degeneration could provide insights into the mechanism of the process. Furthermore, defining these relationships and eventually using them in a clinical setting may allow an identification and early intervention for those who are at a high risk for painful DD.

STUDY DESIGN

Literature review.

METHODS

This literature review focused on the studies concerning genetic polymorphisms and their associations with DD.

RESULTS

Genetic polymorphisms in 20 genes have been analyzed in association with DD, including vitamin D receptor, growth differentiation factor 5 (GDF5), aggrecan, collagen Types I, IX, and XI, fibronectin, hyaluronan and proteoglycan link protein 1 (HAPLN1), thrombospondin, cartilage intermediate layer protein (CILP), asporin, MMP1, 2, and 3, parkinson protein 2, E3 ubiquitin protein ligase (PARK2), proteosome subunit β type 9 (PSMB9), tissue inhibitor of metalloproteinase (TIMP), cyclooxygenase-2 (COX2), and IL1α, IL1β, and IL6. Each genetic polymorphism codes for a protein that has a functional role in the pathogenesis of DD.

CONCLUSIONS

There are known associations between several genetic polymorphisms and DD. Of the 20 genes analyzed, polymorphisms in vitamin D receptor, aggrecan, Type IX collagen, asporin, MMP3, IL1, and IL6 show the most promise as functional variants. Genetic studies are crucial for understanding the mechanism of the degeneration. This genetic information could eventually be used as a predictive model for determining a patient's risk for symptomatic DD.

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

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

Genome-wide analysis of pain-, nerve- and neurotrophin -related gene expression in the degenerating human annulus

Background

In spite of its high clinical relevance, the relationship between disc degeneration and low back pain is still not well understood. Recent studies have shown that genome-wide gene expression studies utilizing ontology searches provide an efficient and valuable methodology for identification of clinically relevant genes. Here we use this approach in analysis of pain-, nerve-, and neurotrophin-related gene expression patterns in specimens of human disc tissue. Control, non-herniated clinical, and herniated clinical specimens of human annulus tissue were studied following Institutional Review Board approval.

Results

Analyses were performed on more generated (Thompson grade IV and V) discs vs. less degenerated discs (grades I-III), on surgically operated discs vs. control discs, and on herniated vs. control discs. Analyses of more degenerated vs. less degenerated discs identified significant upregulation of well-recognized pain-related genes (bradykinin receptor B1, calcitonin gene-related peptide and catechol-0-methyltransferase). Nerve growth factor was significantly upregulated in surgical vs. control and in herniated vs. control discs. All three analyses also found significant changes in numerous proinflammatory cytokine- and chemokine-related genes. Nerve, neurotrophin and pain-ontology searches identified many matrix, signaling and functional genes which have known importance in the disc. Immunohistochemistry was utilized to confirm the presence of calcitonin gene-related peptide, catechol-0-methyltransferase and bradykinin receptor B1 at the protein level in the human annulus.

Conclusions

Findings point to the utility of microarray analyses in identification of pain-, neurotrophin and nerve-related genes in the disc, and point to the importance of future work exploring functional interactions between nerve and disc cells in vitro and in vivo. Nerve, pain and neurotrophin ontology searches identified numerous changes in proinflammatory cytokines and chemokines which also have significant relevance to disc biology. Since the degenerating human disc is primarily an avascular tissue site into which disc cells have contributed high levels of proinflammatory cytokines, these substances are not cleared from the tissue and remain there over time. We hypothesize that as nerves grow into the human annulus, they encounter a proinflammatory cytokine-rich milieu which may sensitize nociceptors and exacerbate pain production.