Ultrastructural immunolocalization of alpha-elastin and keratan sulfate proteoglycan in normal and scoliotic lumbar disc

Elastic Fibers in the Intervertebral Disc: From Form to Function and toward Regeneration

Despite extensive efforts over the past 40 years, there is still a significant gap in knowledge of the characteristics of elastic fibers in the intervertebral disc (IVD). More studies are required to clarify the potential contribution of elastic fibers to the IVD (healthy and diseased) function and recommend critical areas for future investigations. On the other hand, current IVD in-vitro models are not true reflections of the complex biological IVD tissue and the role of elastic fibers has often been ignored in developing relevant tissue-engineered scaffolds and realistic computational models. This has affected the progress of IVD studies (tissue engineering solutions, biomechanics, fundamental biology) and translation into clinical practice. Motivated by the current gap, the current review paper presents a comprehensive study (from the early 1980s to 2022) that explores the current understanding of structural (multi-scale hierarchy), biological (development and aging, elastin content, and cell-fiber interaction), and biomechanical properties of the IVD elastic fibers, and provides new insights into future investigations in this domain.

Mechanisms and clinical implications of intervertebral disc calcification

Low back pain is a leading cause of disability worldwide. Intervertebral disc (IVD) degeneration is often associated with low back pain but is sometimes asymptomatic. IVD calcification is an often overlooked disc phenotype that might have considerable clinical impact. IVD calcification is not a rare finding in ageing or in degenerative and scoliotic spinal conditions, but is often ignored and under-reported. IVD calcification may lead to stiffer IVDs and altered segmental biomechanics, more severe IVD degeneration, inflammation and low back pain. Calcification is not restricted to the IVD but is also observed in the degeneration of other cartilaginous tissues, such as joint cartilage, and is involved in the tissue inflammatory process. Furthermore, IVD calcification may also affect the vertebral endplate, leading to Modic changes (non-neoplastic subchondral vertebral bone marrow lesions) and the generation of pain. Such effects in the spine might develop in similar ways to the development of subchondral marrow lesions of the knee, which are associated with osteoarthritis-related pain. We propose that IVD calcification is a phenotypic biomarker of clinically relevant disc degeneration and endplate changes. As IVD calcification has implications for the management and prognosis of degenerative spinal changes and could affect targeted therapeutics and regenerative approaches for the spine, awareness of IVD calcification should be raised in the spine community.

The role of SLRPs and large aggregating proteoglycans in collagen fibrillogenesis, extracellular matrix assembly, and mechanical function of fibrocartilage

PURPOSE

Proteoglycans, especially small leucine rich proteoglycans (SLRPs), play major roles in facilitating the development and regulation of collagen fibers and other extracellular matrix components. However, their roles in fibrocartilage have not been widely reviewed. Here, we discuss both SLRP and large aggregating proteoglycan's roles in collagen fibrillogenesis and extracellular matrix assembly in fibrocartilage tissues such as the meniscus, annulus fibrosus (AF), and TMJ disc. We also discuss their expression levels throughout development, aging and degeneration, as well as repair.

METHODS

A review of literature discussing proteoglycans and collagen fibrillogenesis in fibrocartilage was conducted and data from these manuscripts were analyzed and grouped to discuss trends throughout the tissue's architectural zones and developmental stage.

RESULTS

The spatial collagen architecture of these fibrocartilaginous tissues is reflected in the distribution of proteoglycans expressed, suggesting that each proteoglycan plays an important role in the type of architecture presented and associated mechanical function.

CONCLUSION

The unique structure-function relationship of fibrocartilage makes the varied architectures throughout the tissues imperative for their success and understanding the functions of these proteoglycans in developing and maintaining the fiber structure could inform future work in fibrocartilage replacement using tissue engineered constructs.

The Evaluation of Proteoglycan Levels and the Possible Role of ACAN Gene (c.6423T>C) Variant in Patients with Lumbar Disc Degeneration Disease

BACKGROUND/AIM

The present study aimed to investigate the role of an aggrecan (ACAN) gene variant and proteoglycan levels in the risk of lumbar degenerative disc disease (LDDD).

MATERIALS AND METHODS

A total of 108 patients with LDDD and 103 healthy controls were enrolled. Molecular assessment of the ACAN gene (c.6423T>C) variant was determined by real time-polymerase chain reaction. Proteoglycan levels in serum were measured with enzyme-linked immunosorbent assay.

RESULTS

The frequency of all alleles and genotypes in all study groups were distributed according to the Hardy-Weinberg equilibrium. In addition, no association between the ACAN gene (c.6423T>C) variant and presence of risk factors for LDDD was detected. However, proteoglycan levels were significantly lower in patients with LDDD compared to the control group (p<0.00001).

CONCLUSION

Our findings suggest that proteoglycan has emerged as a potential novel biomarker which might be used for prediction of LDDD risk.

The ultra-structural organization of the elastic network in the intra- and inter-lamellar matrix of the intervertebral disc

The inter-lamellar matrix (ILM)-located between adjacent lamellae of the annulus fibrosus-consists of a complex structure of elastic fibers, while elastic fibers of the intra-lamellar region are aligned predominantly parallel to the collagen fibers. The organization of elastic fibers under low magnification, in both inter- and intra-lamellar regions, was studied by light microscopic analysis of histologically prepared samples; however, little is known about their ultrastructure. An ultrastructural visualization of elastic fibers in the inter-lamellar matrix is crucial for describing their contribution to structural integrity, as well as mechanical properties of the annulus fibrosus. The aims of this study were twofold: first, to present an ultrastructural analysis of the elastic fiber network in the ILM and intra-lamellar region, including cross section (CS) and in-plane (IP) lamellae, of the AF using Scanning Electron Microscopy (SEM) and second, to -compare the elastic fiber orientation between the ILM and intra-lamellar region. Four samples (lumbar sheep discs) from adjacent sections (30μm thickness) of anterior annulus were partially digested by a developed NaOH-sonication method for visualization of elastic fibers by SEM. Elastic fiber orientation and distribution were quantified relative to the tangential to circumferential reference axis. Visualization of the ILM under high magnification revealed a dense network of elastic fibers that has not been previously described. Within the ILM, elastic fibers form a complex network, consisting of different size and shape fibers, which differed to those located in the intra-lamellar region. For both regions, the majority of fibers were oriented near 0° with respect to tangential to circumferential (TCD) direction and two minor symmetrical orientations of approximately±45°. Statistically, the orientation of elastic fibers between the ILM and intra-lamellar region was not different (p=0.171). The present study used extracellular matrix partial digestion to address significant gaps in understanding of disc microstructure and will contribute to multidisciplinary ultrastructure-function studies.

STATEMENT OF SIGNIFICANCE

Visualization of the intra-lamellar matrix under high magnification revealed a dense network of elastic fibers that has not been previously described. The present study used extracellular matrix partial digestion to address significant gaps in understanding of disc microstructure and will contribute to multidisciplinary ultrastructure-function studies.

Qualitative and quantitative assessment of collagen and elastin in annulus fibrosus of the physiologic and scoliotic intervertebral discs

The biophysical properties of the annulus fibrosus of the intervertebral disc are determined by collagen and elastin fibres. The progression of scoliosis is accompanied by a number of pathological changes concerning these structural proteins. This is a major cause of dysfunction of the intervertebral disc. The object of the study were annulus fibrosus samples excised from intervertebral discs of healthy subjects and patients treated surgically for scoliosis in the thoracolumbar or lumbar spine. The research material was subjected to structural analysis by light microscopy and quantitative analysis of the content of collagen types I, II, III and IV as well as elastin by immunoenzymatic test (ELISA). A statistical analysis was conducted to assess the impact of the sampling site (Mann-Whitney test, α=0.05) and scoliosis (Wilcoxon matched pairs test, α=0.05) on the obtained results. The microscopic studies conducted on scoliotic annulus fibrosus showed a significant architectural distortion of collagen and elastin fibres. Quantitative biochemical assays demonstrated region-dependent distribution of only collagen types I and II in the case of healthy intervertebral discs whereas in the case of scoliotic discs region-dependent distribution concerned all examined proteins of the extracellular matrix. Comparison of scoliotic and healthy annulus fibrosus revealed a significant decrease in the content of collagen type I and elastin as well as a slight increase in the proportion of collagen types III and IV. The content of collagen type II did not differ significantly between both groups. The observed anomalies are a manifestation of degenerative changes affecting annulus fibrosus of the intervertebral disc in patients suffering from scoliosis.

Effects of shear force on intervertebral disc: an in vivo rabbit study

PURPOSE

A new in vivo rabbit model was developed to investigate the effects of shear force on intervertebral disc (IVD).

METHODS

Japanese white rabbits (n = 38) were used for this study. The L4/5 discs in Group A (n = 10) were subjected to a constant shear force (50 N) using a custom-made external loading device for 1 month; in Group B (n = 10) for 2 months; whereas in Group C (n = 10), loading device was attached to the spine but the discs remained unloaded. Group D (n = 8) was a non-operated intact control group. After loading, the loading devices were taken out and the animals were given X-ray and MRI examination. After X-ray and MRI examination, the animals were euthanized for histological analysis.

RESULTS

After 1 and 2 months of loading, radiographic findings showed significant disc height narrowing in L4/5 discs of the animals in loading groups, and slight lumbar spondylolisthesis in some animals of Group B. MRI showed a significant decrease in nucleus pulposus (NP) area and signal intensity from T2-weighted images. Histologically, loss of normal NP cells and disorganization of the architecture of the annulus occurred, and proteoglycan stain decreased.

CONCLUSIONS

The results of this study suggest that disc degeneration can be induced by hyper-physiological shear loading in the rabbit IVD. Long-term shear loading may result in structural disc failure inducing lumbar spondylolisthesis and progressive disc degeneration, which, however, has to be proven by further studies.

Ultrastructure of Intervertebral Disc and Vertebra-Disc Junctions Zones as a Link in Etiopathogenesis of Idiopathic Scoliosis

Background Context. There is no general accepted theory on the etiology of idiopathic scoliosis (IS). An important role of the vertebrae endplate physes (VEPh) and intervertebral discs (IVD) in spinal curve progression is acknowledged, but ultrastructural mechanisms are not well understood. Purpose. To analyze the current literature on ultrastructural characteristics of VEPh and IVD in the context of IS etiology. Study Design/Setting. A literature review. Results. There is strong evidence for multifactorial etiology of IS. Early wedging of vertebra bodies is likely due to laterally directed appositional bone growth at the concave side, caused by a combination of increased cell proliferation at the vertebrae endplate and altered mechanical properties of the outer annulus fibrosus of the adjacent IVD. Genetic defects in bending proteins necessary for IVD lamellar organization underlie altered mechanical properties. Asymmetrical ligaments, muscular stretch, and spine instability may also play roles in curve formation. Conclusions. Development of a reliable, cost effective method for identifying patients at high risk for curve progression is needed and could lead to a paradigm shift in treatment options. Unnecessary anxiety, bracing, and radiation could potentially be minimized and high risk patient could receive surgery earlier, rendering better outcomes with fewer fused segments needed to mitigate curve progression.

Proteomic analysis of potential keratan sulfate, chondroitin sulfate A, and hyaluronic acid molecular interactions

PURPOSE

Corneal stroma extracellular matrix (ECM) glycosaminoglycans (GAGs) include keratan sulfate (KS), chondroitin sulfate A (CSA), and hyaluronic acid (HA). Embryonic corneal keratocytes and sensory nerve fibers grow and differentiate according to chemical cues they receive from the ECM. This study asked which of the proteins that may regulate keratocytes or corneal nerve growth cone immigration interact with corneal GAGs.

METHODS

Biotinylated KS (bKS), CSA (bCSA), and HA (bHA) were prepared and used in microarray protocols to assess their interactions with 8268 proteins and a custom microarray of 85 extracellular epitopes of nerve growth-related proteins. Surface plasmon resonance (SPR) was performed with bKS and SLIT2, and their ka, kd, and KD were determined.

RESULTS

Highly sulfated KS interacted with 217 microarray proteins, including 75 kinases, several membrane or secreted proteins, many cytoskeletal proteins, and many nerve function proteins. CSA interacted with 24 proteins, including 10 kinases and 2 cell surface proteins. HA interacted with 6 proteins, including several ECM-related structural proteins. Of 85 ECM nerve-related epitopes, KS bound 40 proteins, including SLIT, 2 ROBOs, 9 EPHs, 8 Ephrins (EFNs), 8 semaphorins (SEMAs), and 2 nerve growth factor receptors. CSA bound nine proteins, including ROBO2, 2 EPHs, 1 EFN, two SEMAs, and netrin 4. HA bound no ECM nerve-related epitopes. SPR confirmed that KS binds SLIT2 strongly. The KS core protein mimecan/osteoglycin bound 15 proteins.

CONCLUSIONS

Corneal stromal GAGs bind, and thus could alter the availability or conformation of, many proteins that may influence keratocyte and nerve growth cone behavior in the cornea.

Measurement of local strains in intervertebral disc anulus fibrosus tissue under dynamic shear: contributions of matrix fiber orientation and elastin content

Shear strain has been implicated as an initiator of intervertebral disc anulus failure, however a clear, multi-scale picture of how shear strain affects the tissue microstructure has been lacking. The purposes of this study were to measure microscale deformations in anulus tissue under dynamic shear in two orientations, and to determine the role of elastin in regulating these deformations. Bovine AF tissue was simultaneously shear loaded and imaged using confocal microscopy following either a buffer or elastase treatment. Digital image analysis was used to track through time local shear strains in specimens sheared transversely, and stretch and rotation of collagen fiber bundles in specimens sheared circumferentially. The results of this study suggest that sliding does not occur between AF plies under shear, and that interlamellar connections are governed by collagen and fibrilin rather than elastin. The transverse shear modulus was found to be approximately 1.6 times as high in plies the direction of the collagen fibers as in plies across them. Under physiological levels of in-plane shear, fiber bundles stretched and re-oriented linearly. Elastin was found to primarily stiffen plies transversely. We conclude that alterations in the elastic fiber network, as found with IVD herniation and degeneration, can therefore be expected to significantly influence the AF response to shear making it more susceptible to micro failure under bending or torsion loading.

The elastic fibre network of the human lumbar anulus fibrosus: architecture, mechanical function and potential role in the progression of intervertebral disc degeneration

Elastic fibres are critical constituents of dynamic biological structures that functionally require elasticity and resilience. The network of elastic fibres in the anulus fibrosus of the intervertebral disc is extensive, however until recently, the majority of histological, biochemical and biomechanical studies have focussed on the roles of other extracellular matrix constituents such as collagens and proteoglycans. The resulting lack of detailed descriptions of elastic fibre network architecture and mechanical function has limited understanding of the potentially important contribution made by elastic fibres to healthy disc function and their possible roles in the progression of disc degeneration. In addition, it has made it difficult to postulate what the consequences of elastic fibre related disorders would be for intervertebral disc behaviour, and to develop treatments accordingly. In this paper, we review recent and historical studies which have examined both the structure and the function of the human lumbar anulus fibrosus elastic fibre network, provide a synergistic discussion in an attempt to clarify its potentially critical contribution both to normal intervertebral disc behaviour and the processes relating to its degeneration, and recommend critical areas for future research.

Microfibrils, elastin fibres and collagen fibres in the human intervertebral disc and bovine tail disc

The distribution of microfibrils was studied immunohistochemically in intervertebral discs taken from young normal human surgical cases and from the bovine tail. Co-localization of microfibrils and elastin fibres was examined by dual immunostaining of fibrillin-1 and elastin. Collagen fibre network orientation was studied by using polarized filters. A similar microfibrillar network was seen in both bovine and human discs with network organization being completely different from region to region. In the outer annulus fibrosus (OAF), abundant microfibrils organized in bundles were mainly distributed in the interterritorial matrix. In addition, the microfibril bundles were orientated parallel to each other and co-localized highly with elastin fibres. Within each lamella, co-localized microfibrils and elastin fibres were aligned in the same direction as the collagen fibres. In the interlamellar space, a dense co-localized network, staining for both microfibrils and elastin fibres, was apparent; immunostaining for both molecules was noticeably stronger than within lamellae. In the inner annulus fibrosus, the microfibrils were predominantly visible as a filamentous mesh network, both in the interterritorial matrix and also around the cells. The microfibrils in this region co-localized with elastin fibres far less than in the OAF. In nucleus pulposus, filamentous microfibrils were organized mainly around the cells where elastin fibres were hardly detected. By contrast, sparse elastin fibres, with a few of microfibrils, were visible in the interterritorial matrix. The results of this study suggest the microfibrillar network of the annulus may play a mechanical role while that around the cells of the nucleus may be more involved in regulating cell function.

Regional variations in the density and arrangement of elastic fibres in the anulus fibrosus of the human lumbar disc

Elastic fibres are critical components of the extracellular matrix in dynamic biological structures that undergo extension and recoil. Their presence has been demonstrated in the anulus fibrosus of the human lumbar intervertebral disc; however, a detailed regional analysis of their density and arrangement has not been undertaken, limiting our understanding of their structural and functional roles. In this investigation we have quantitatively described regional variations in elastic fibre density in the anulus fibrosus of the human L3-L4 intervertebral disc using histochemistry and light microscopy. Additionally, a multiplanar comparison of patterns of elastic fibre distribution in the intralamellar and interlamellar zones was undertaken. Novel imaging techniques were developed to facilitate the visualization of elastic fibres otherwise masked by dense surrounding matrix. Elastic fibre density was found to be significantly higher in the lamellae of the posterolateral region of the anulus than the anterolateral, and significantly higher in the outer regions than the inner, suggesting that elastic fibre density in each region of the anulus is commensurate with the magnitude of the tensile deformations experienced in bending and torsion. Elastic fibre arrangments in intralamellar and interlamellar zones were shown to be architecturally distinct, suggesting that they perform multiple functional roles within the anulus matrix structural hierarchy.