Differential expression of proteoglycan epitopes by ovine intervertebral disc cells

Preclinical to clinical translation for intervertebral disc repair: Effects of species-specific scale, metabolism, and matrix synthesis rates on cell-based regeneration

Background

A significant hurdle for potential cell-based therapies is the subsequent survival and regenerative capacity of implanted cells. While many exciting developments have demonstrated promise preclinically, cell-based therapies for intervertebral disc (IVD) degeneration fail to translate equivalent clinical efficacy.

Aims

This work aims to ascertain the clinical relevance of both a small and large animal model by experimentally investigating and comparing these animal models to human from the perspective of anatomical scale and their cellular metabolic and regenerative potential.

Materials and Methods

First, this work experimentally investigated species-specific geometrical scale, native cell density, nutrient metabolism, and matrix synthesis rates for rat, goat, and human disc cells in a 3D microspheroid configuration. Second, these parameters were employed in silico to elucidate species-specific nutrient microenvironments and predict differences in temporal regeneration between animal models.

Results

This work presents in silico models which correlate favorably to preclinical literature in terms of the capabilities of animal regeneration and predict that compromised nutrition is not a significant challenge in small animal discs. On the contrary, it highlights a very fine clinical balance between an adequate cell dose for sufficient repair, through de novo matrix deposition, without exacerbating the human microenvironmental niche.

Discussion

Overall, this work aims to provide a path towards understanding the effect of cell injection number on the nutrient microenvironment and the "time to regeneration" between preclinical animal models and the large human IVD. While these findings help to explain failed translation of promising preclinical data and the limited results emerging from clinical trials at present, they also enable the research field and clinicians to manage expectations on cell-based regeneration.

Conclusion

Ultimately, this work provides a platform to inform the design of clinical trials, and as computing power and software capabilities increase in the future, it is conceivable that generation of patient-specific models could be used for patient assessment, as well as pre- and intraoperative planning.

Holistic Approach of Swiss Fetal Progenitor Cell Banking: Optimizing Safe and Sustainable Substrates for Regenerative Medicine and Biotechnology

Safety, quality, and regulatory-driven iterative optimization of therapeutic cell source selection has constituted the core developmental bedrock for primary fetal progenitor cell (FPC) therapy in Switzerland throughout three decades. Customized Fetal Transplantation Programs were pragmatically devised as straightforward workflows for tissue procurement, traceability maximization, safety, consistency, and robustness of cultured progeny cellular materials. Whole-cell bioprocessing standardization has provided plethoric insights into the adequate conjugation of modern biotechnological advances with current restraining legislative, ethical, and regulatory frameworks. Pioneer translational advances in cutaneous and musculoskeletal regenerative medicine continuously demonstrate the therapeutic potential of FPCs. Extensive technical and clinical hindsight was gathered by managing pediatric burns and geriatric ulcers in Switzerland. Concomitant industrial transposition of dermal FPC banking, following good manufacturing practices, demonstrated the extensive potential of their therapeutic value. Furthermore, in extenso, exponential revalorization of Swiss FPC technology may be achieved via the renewal of integrative model frameworks. Consideration of both longitudinal and transversal aspects of simultaneous fetal tissue differential processing allows for a better understanding of the quasi-infinite expansion potential within multi-tiered primary FPC banking. Multiple fetal tissues (e.g., skin, cartilage, tendon, muscle, bone, lung) may be simultaneously harvested and processed for adherent cell cultures, establishing a unique model for sustainable therapeutic cellular material supply chains. Here, we integrated fundamental, preclinical, clinical, and industrial developments embodying the scientific advances supported by Swiss FPC banking and we focused on advances made to date for FPCs that may be derived from a single organ donation. A renewed model of single organ donation bioprocessing is proposed, achieving sustained standards and potential production of billions of affordable and efficient therapeutic doses. Thereby, the aim is to validate the core therapeutic value proposition, to increase awareness and use of standardized protocols for translational regenerative medicine, potentially impacting millions of patients suffering from cutaneous and musculoskeletal diseases. Alternative applications of FPC banking include biopharmaceutical therapeutic product manufacturing, thereby indirectly and synergistically enhancing the power of modern therapeutic armamentariums. It is hypothesized that a single qualifying fetal organ donation is sufficient to sustain decades of scientific, medical, and industrial developments, as technological optimization and standardization enable high efficiency.

Keratan sulfate, a complex glycosaminoglycan with unique functional capability

From an evolutionary perspective keratan sulfate (KS) is the newest glycosaminoglycan (GAG) but the least understood. KS is a sophisticated molecule with a diverse structure, and unique functional roles continue to be uncovered for this GAG. The cornea is the richest tissue source of KS in the human body but the central and peripheral nervous systems also contain significant levels of KS and a diverse range of KS-proteoglycans with essential functional roles. KS also displays important cell regulatory properties in epithelial and mesenchymal tissues and in bone and in tumor development of diagnostic and prognostic utility. Corneal KS-I displays variable degrees of sulfation along the KS chain ranging from non-sulfated polylactosamine, mono-sulfated and disulfated disaccharide regions. Skeletal KS-II is almost completely sulfated consisting of disulfated disaccharides interrupted by occasional mono-sulfated N-acetyllactosamine residues. KS-III also contains highly sulfated KS disaccharides but differs from KS-I and KS-II through 2-O-mannose linkage to serine or threonine core protein residues on proteoglycans such as phosphacan and abakan in brain tissue. Historically, the major emphasis on the biology of KS has focused on its sulfated regions for good reason. The sulfation motifs on KS convey important molecular recognition information and direct cell behavior through a number of interactive proteins. Emerging evidence also suggest functional roles for the poly-N-acetyllactosamine regions of KS requiring further investigation. Thus further research is warranted to better understand the complexities of KS.

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.

The knee joint loose body as a source of viable autologous human chondrocytes

Loose bodies are fragments of cartilage or bone present in the synovial fluid. In the present study we assessed if loose bodies could be used as a source of autologous human chondrocytes for experimental purposes. Histochemical examination of loose bodies and differential enzymatic digestions were undertaken, the isolated cells were cultured in alginate bead microspheres and immunolocalisations were undertaken for chondrogenic markers such as aggrecan, and type II collagen. Isolated loose body cells had high viability (≥90% viable), expressed chondrogenic markers (aggrecan, type II collagen) but no type I collagen. Loose bodies may be a useful source of autologous chondrocytes of high viability.

Enrichment of committed human nucleus pulposus cells expressing chondroitin sulfate proteoglycans under alginate encapsulation

OBJECTIVE

Intervertebral disc (IVD) degeneration is associated with a malfunction of the nucleus pulposus (NP). Alginate culturing provides a favorable microenvironment for the phenotypic maintenance of chondrocyte-like NP cells. However, NP cells are recently evidenced to present heterogeneous populations, including progenitors, fibroblastic cells and primitive NP cells. The aim of this study is to profile the phenotypic changes of distinct human NP cells populations and describe the dynamic expression of chondroitin sulfate glycosaminoglycans (CS-GAGs) in extended alginate encapsulation.

METHOD

Non-degenerated (ND-NPC) and degenerated (D-NPC) NP cells were expanded in monolayers, and subject to 28-day culture in alginate after serial passaging. CS-GAG compositional expression in monolayer-/alginate-cultured NP cells was evaluated by carbohydrate electrophoresis. Cellular phenotypic changes were assessed by immunologic detection and gene expression analysis.

RESULTS

Relative to D-NPC, ND-NPC displayed remarkably higher expression levels of chondroitin-4-sulfate GAGs over the 28-day culture. Compared with monolayer culture, ND-NPC showed increased NP marker expression of KRT18, KRT19, and CDH2, as well as chondrocyte markers SOX9 and MIA in alginate culture. In contrast, expression of fibroblastic marker COL1A1, COL3A1, and FN1 were reduced. Interestingly, ND-NPC showed a loss of Tie2+ but gain in KRT19+/CD24+ population during alginate culture. In contrast, D-NPC showed more consistent expression levels of NP surface markers during culture.

CONCLUSION

We demonstrate for the first time that extended alginate culture selectively enriches the committed NP cells and favors chondroitin-4-sulfate proteoglycan production. These findings suggest its validity as a model to investigate IVD cell function.

Effect of long-term osmotic loading culture on matrix synthesis from intervertebral disc cells

The intervertebral disc is a highly hydrated tissue that acts to absorb and distribute large complex loads placed on the spine. Diurnal loading and disc degeneration causes significant changes in water volume and proteoglycan content, which alters the internal osmotic environment. Short-term osmotic loading alters disc cell gene expression; however, the long-term effect of osmotic loading on disc cell matrix synthesis is not well understood. The objective of this study was to determine the effect of long-term osmotic loading on matrix turnover and proliferation by juvenile and adult cells from the nucleus pulposus (NP) and the cartilaginous endplate (EP). Matrix synthesis was evaluated using pellets and a 3D agarose system, which has been used for developing engineered tissues. Intervertebral discs were acquired from juvenile and adult cows. Cells were acquired through enzymatic digestion and expanded in culture. Pellets were formed through centrifugation, and constructs were created by encapsulating cells within 2% w/v agarose hydrogel. Pellets and constructs were cultured up to 42 days in chemically defined medium with the osmolality adjusted to 300, 400, or 500 mOsm/kg. EP cells were evaluated as a chondrocyte comparison to chondrocyte-like NP cells. Pellet and agarose cultures of juvenile NP and EP cells demonstrated similarities with respect to cell proliferation and functional mechanical properties. Cell proliferation decreased significantly with increased osmotic loading. The final compressive Young's modulus of juvenile NP cells was 10–40× greater than initial properties (i.e., day 0) and was greater than the final Young's modulus of adult NP and juvenile EP constructs. In juvenile NP constructs, there were no significant differences in GAG content with respect to osmotic loading. However, GAG synthesis and mechanical properties were greatest for the 400 mOsm/kg group in adult NP constructs. Taken together, the results presented here suggest a tradeoff between cell proliferation and matrix production under osmotic loading conditions. In conclusion, culturing disc cells in an osmotic environment that best mimics the healthy disc environment (400 mOsm/kg) may be ideal for balancing cell proliferation, matrix production, and mechanical properties of engineered disc tissues.

Cell and molecular biology of intervertebral disc degeneration: current understanding and implications for potential therapeutic strategies

Intervertebral disc degeneration (IDD) is a chronic, complex process associated with low back pain; mechanisms of its occurrence have not yet been fully elucidated. Its process is not only accompanied by morphological changes, but also by systematic changes in its histological and biochemical properties. Many cellular and molecular mechanisms have been reported to be related with IDD and to reverse degenerative trends, abnormal conditions of the living cells and altered cell phenotypes would need to be restored. Promising biological therapeutic strategies still rely on injection of active substances, gene therapy and cell transplantation. With advanced study of tissue engineering protocols based on cell therapy, combined use of seeding cells, bio-active substances and bio-compatible materials, are promising for IDD regeneration. Recently reported progenitor cells within discs themselves also hold prospects for future IDD studies. This article describes the background of IDD, current understanding and implications of potential therapeutic strategies.

Biomechanical Conditioning Enhanced Matrix Synthesis in Nucleus Pulposus Cells Cultured in Agarose Constructs with TGFβ

Biomechanical signals play an important role in normal disc metabolism and pathology. For instance, nucleus pulposus (NP) cells will regulate metabolic activities and maintain a balance between the anabolic and catabolic cascades. The former involves factors such as transforming growth factor-β (TGFβ) and mechanical stimuli, both of which are known to regulate matrix production through autocrine and paracrine mechanisms. The present study examined the combined effect of TGFβ and mechanical loading on anabolic activities in NP cells cultured in agarose constructs. Stimulation with TGFβ and dynamic compression reduced nitrite release and increased matrix synthesis and gene expression of aggrecan and collagen type II. The findings from this work has the potential for developing regenerative treatment strategies which could either slow down or stop the degenerative process and/or promote healing mechanisms in the intervertebral disc.

Comparative immunolocalization of the elastin fiber-associated proteins fibrillin-1, LTBP-2, and MAGP-1 with components of the collagenous and proteoglycan matrix of the fetal human intervertebral disc

STUDY DESIGN

A comparative immunolocalization study of elastin-associated proteins and established intervertebral disc (IVD) extracellular matrix (ECM) components.

OBJECTIVE

To localize for the first time, elastic fiber–associated proteins with structural fibrillar components in the annulus fibrosus (AF) of the fetal IVD.

SUMMARY OF BACKGROUND DATA

Elastin has been identified histochemically in adult bovine, human, and immature rat IVDs, and in fetal human IVDs using electron microscopy; however, no immunolocalization studies have been undertaken for associated components in human fetal IVDs.

METHODS

En-bloc fixation of thoracolumbar spinal segments in formalin and Histochoice followed by standard histochemical processing, paraffin embedding, microtome sectioning, and identification of IVD ECM components using a range of specific mono- and polyclonal antibodies and bright-field and laser scanning confocal microscopy.

RESULTS

The elastic fiber-associated proteins fibrillin-1, LTBP-2, and MAGP-1 were prominently immunolocalized in the outer lamellar layers of the AF of the human fetal IVD. Dual localization of selected components by confocal microscopy demonstrated that versican and LTBP-2 were colocalized with fibrillin-1 microfibrils in the AF lamellae with a similar distribution to the elastin fibers. LTBP-2 was also associated with pericellular perlecan in the outer AF. These interconnections between elastin-associated proteins resulted in an elastic network, which connected the AF cells with the adjacent cartilaginous vertebral bodies.

CONCLUSION

Specific immunolocalization of fibrillin-1, MAGP-1, and versican with elastin in the outer AF of the fetal human IVD has been demonstrated. We deduce from the established distributions of the elastin-associated proteins and their known interactivities with matrix components that these stabilize and aid in the integration of the elastic fibers in the annular lamellae and may be responsible for the generation of tensional forces in the outer AF, which direct the assembly of this tissue.

Development of a bioreactor for axially loaded intervertebral disc organ culture

Whole disc organ culture is needed for preclinical testing of biological repair of the degenerate intervertebral disc. Such organ culture is hampered by two major limitations: first obtaining adequate nutrition though the calcified cartilage endplates adjacent to the vertebral bone, and second by loss of tissue integrity if the endplates are removed from the discs. In this work we utilize a recently described technique for whole disc isolation that overcomes these problems, by removal of vertebral bone and the adjacent calcified portion of the endplate, and the construction of a bioreactor that permits long-term viability of these discs in loaded organ culture. The bioreactor consists of a culture chamber in which the disc can be dynamically loaded in a uniform manner. The culture chamber is large enough to accommodate discs up to 60 mm in diameter, and so is amendable to study both bovine and human discs. The discs are loaded in the culture chamber by upper and lower platens, which conform to the shape of the remaining cartilaginous endplate and permit fluid flow across its surface. The bioreactor is able to load the disc under a variety of conditions ranging from static to dynamic and from physiological to pathological, and monitor induced changes in disc height. To date, bovine caudal discs have been maintained viable in the bioreactor for up to 4 weeks without any appreciable loss of disc height under physiological cyclic load and, in principle, could be maintained in such a manner for several months. Such long-term organ culture is essential for studying biological repair of the disc.

Development of an intact intervertebral disc organ culture system in which degeneration can be induced as a prelude to studying repair potential

The present work describes a novel bovine disc organ culture system with long-term maintenance of cell viability, in which degenerative changes can be induced as a prelude to studying repair. Discs were isolated with three different techniques: without endplates (NEP), with bony endplates (BEP) and with intact cartilage endplates (CEP). Swelling, deformation, and cell viability were evaluated in unloaded cultures. Degeneration was induced by a single trypsin injection into the center of the disc and the effect on cell viability and matrix degradation was followed. Trypsin-treated discs were exposed to TGFβ to evaluate the potential to study repair in this system. NEP isolated discs showed >75% maintained cell viability for up to 10 days but were severely deformed, BEP discs on the other hand maintained morphology but failed to retain cell viability having only 27% viable cells after 10 days. In CEP discs, both cell viability and morphology were maintained for at least 4 weeks where >75% of the cells were still viable. To mimic proteoglycan loss during disc degeneration, a single trypsin injection was administered to the center of the disc. This resulted in 60% loss of aggrecan, after 7 days, without affecting cell viability. When TGFβ was injected to validate that the system can be used to study a repair response following injection of a bio-active substance, proteoglycan synthesis nearly doubled compared to baseline synthesis. Trypsin-treated bovine CEP discs therefore provide a model system for studying repair of the degenerate disc, as morphology, cell viability and responsiveness to bio-active substances were maintained.

Recent advances in annular pathobiology provide insights into rim-lesion mediated intervertebral disc degeneration and potential new approaches to annular repair strategies

The objective of this study was to assess the impact of a landmark annular lesion model on our understanding of the etiopathogenesis of IVD degeneration and to appraise current IVD repairative strategies. A number of studies have utilised the Osti sheep model since its development in 1990. The experimental questions posed at that time are covered in this review, as are significant recent advances in annular repair strategies. The ovine model has provided important spatial and temporal insights into the longitudinal development of annular lesions and how they impact on other discal and paradiscal components such as the NP, cartilaginous end plates, zygapophyseal joints and vertebral bone and blood vessels. Important recent advances have been made in biomatrix design for IVD repair and in the oriented and dynamic culture of annular fibrochondrocytes into planar, spatially relevant, annular type structures. The development of hyaluronan hydrogels capable of rapid in situ gelation offer the possibility of supplementation of matrices with cells and other biomimetics and represent a significant advance in biopolymer design. New generation biological glues and self-curing acrylic formulations which may be augmented with slow delivery biomimetics in microcarriers may also find application in the non-surgical repair of annular defects. Despite major advances, significant technical challenges still have to be overcome before the biological repair of this intractable connective tissue becomes a realistic alternative to conventional surgical intervention for the treatment of chronic degenerate IVDs.

Aggrecan, versican and type VI collagen are components of annular translamellar crossbridges in the intervertebral disc

The aim of this study was to undertake a detailed analysis of the structure of the inter and intra-lamellar regions of the annulus fibrosus. A total of 30 newborn to 6 year-old lumbar ovine intervertebral discs (IVDs) were fixed and decalcified en-bloc to avoid differential swelling artifacts during processing and vertical mid-sagittal, and horizontal 4 mum sections were cut. These were stained with toluidine blue to visualise anionic proteoglycan (PG) species, H & E for cellular morphology and picro-sirius red (viewed under polarized light) to examine collagenous organization. Immunolocalisations were also undertaken using anti-PG core-protein and glycosaminoglycan (GAG) side chain antibodies to native chondroitin sulphate (CS), Delta C-4-S and C-6-S unsaturated stubs generated by chondroitinase ABC digestion of CS, keratan sulphate (KS), and with antibodies to type I, II, VI, IX and X collagens. Trans-lamellar cross bridges (TLCBs), discontinuities in annular lamellae's which provide transverse interconnections, stained prominently with toluidine blue in the adult IVDs but less so in the newborn IVDs. In adult discs TLCBs were evident in both the posterior and anterior AF where they extended from the outermost annular lamellae almost to the transitional zone extending across as many as eight lamellar layers displaying a characteristic circuitous, meandering, serpentine type course. There were significantly fewer TLCBs in 2 week-old compared with skeletally mature sheep and there was a further increase from 2 to 6 years. Immunolocalisation of perlecan delineated blood vessels in the TLBs of the newborn but not adult IVDs extending into the mid AF. In contrast adult but not 2 week-old TLCBs were immunopositive for C-4-S, C-6-S, KS, aggrecan, versican and type VI collagen. The change in number and matrix components of the trans-lamellar cross bridges with skeletal maturity and ageing suggest that they represent an adaptation to the complex biomechanical forces occurring in the annulus fibrosus.

A three-dimensional collagen matrix as a suitable culture system for the comparison of cyclic strain and hydrostatic pressure effects on intervertebral disc cells

OBJECT

To study intervertebral disc cell mechanobiology, the authors developed experimental systems that allow the application of cyclic strain and intermittent hydrostatic pressure (IHP) on isolated disc cells under equal three-dimensional (3D) culture conditions. The purpose of the study was to characterize disc cell proliferation, viability, morphology, and gene expression in 3D collagen matrices.

METHODS

The effects of cyclic strain (1, 2, 4, and 8% strain; 1 Hz) and IHP (0.25 MPa, 0.1 Hz) on gene expression (real-time polymerase chain reaction) of anabolic and catabolic matrix proteins were investigated and compared with those derived from mechanically unstimulated controls. Intervertebral disc cells proliferated in the collagen gels (mean viability 91.6%) and expressed messenger RNA for collagen I, collagen II, aggrecan, matrix metalloproteinase (MMP)-2, and MMP-3. Morphologically, both spindle-shaped cells with longer processes and rounded cells were detected in the collagen scaffolds. Cyclic strain increased collagen II and aggrecan expression and decreased MMP-3 expression of anulus fibrosus cells. No significant difference between the four strain magnitudes was found. Intermittent hydrostatic pressure tended to increase collagen I and aggrecan expression of nucleus cells and significantly decreased MMP-2 and -3 expression of nucleus cells, whereas aggrecan expression of anulus cells tended to decrease.

CONCLUSIONS

Based on these results, the collagen matrix appeared to be a suitable substrate to apply both cyclic strain and IHP to intervertebral disc cells under 3D culture conditions. Individual variations may be influenced by the extent of degeneration of the disc specimens from which the cells were isolated. This experimental setup may be suitable for studying the influence of degeneration on the disc cell response to mechanical stimuli.

Growth factors and treatment of intervertebral disc degeneration

STUDY DESIGN

Literature review.

OBJECTIVE

To review the most recent findings of the effects of growth factors on the intervertebral disc and, further, to discuss trends in the biologic repair of the degenerated intervertebral disc.

SUMMARY OF BACKGROUND DATA

Since early in 1990, advancements in molecular biology and cell culture technology have enabled researchers to accumulate knowledge about the in vitro actions of growth factors on intervertebral disc cells. More recently, the use of growth factors for the biologic regeneration of the intervertebral disc is of increasing interest to the orthopedic field, and indeed, some preliminary in vivo studies have proven their efficacy.

METHODS

Based on a literature search conducted using available databases, such as the National Library of Medicine, as well as data presented at scientific conferences held in the past 2 years, primarily in the United States, the current status of biologic therapy for disc degeneration using growth factors was summarized.

RESULTS

With increasing evidence to support the feasibility of biologically regenerating intervertebral disc tissues, the clinical application of growth factors has become more plausible. The effects of growth factors on the metabolism of intervertebral disc cells or tissues have been extensively studied using in vitro approaches. More recently, the efficacy of an injection of growth factor protein to reverse disc regeneration has been shown in vivo using a small animal disc degeneration model. The confirmation of those effects and a detailed dose-response study, as well as a long-term safety study, in a large animal model is highly anticipated. Hopefully, the expansion of the clinical use of improved imaging techniques for the early detection of disc degeneration and promising results about the effects of growth factors on intervertebral disc regeneration will benefit the human population in the near future.

CONCLUSIONS

The results from these in vitro and in vivo studies reviewed here clearly suggest the potential usefulness of growth factor injections as a new approach to restore intervertebral disc degeneration at an early stage.

Animal models of inflammatory spinal and sacroiliac joint diseases

Many cartilage matrix proteins or domains such as collagen types II, IX, and XI, GP39, AG1, VG1, and LP are potential antigens that might induce polyarthritis in susceptible animals (Table 1). Ordinarily, spondylitis is not a feature of polyarthritis induced with collagen types II, IX, and XI, GP39, cartilage matrix protein (matrilin-1) and cartilage LP. It seems that only the proteoglycans aggrecan and versican are capable of inducing sacroiliitis and spondylitis. Both molecules are structural proteins in intervertebral discs. Moreover, the arthritogenic or spondylitogenic epitopes of both molecules have been localized to the homologous N-terminal G1 globular domains. This region of versican and aggrecan is highly conserved, with 52% identity of amino acids. The homology is seen exclusively in the G1 domain and is concentrated between residues 115 and 332 (AG1 numbering) near the natural cleavage DIPEN site of aggrecan [84, 85]. Extra-articular pathology is often seen in rheumatic diseases, especially in AS. Other tissues, such as the sclera of the eye [86] and the media of the arteries [86, 87], also contain type II collagen, AG1, VG1, and LP, and versican is present in the central and peripheral nervous systems. Thus, there is the potential for an immune response against cartilage G1 and LP to be directed against related structures in extra-articular tissues. The presence of versican in the tendon and trochlea of the human superior oblique muscle might account for the occurrence of transient attacks of acquired Brown syndrome in patients with juvenile and adult forms of chronic RA [88]. Thus, it will be interesting to determine whether or not extra-articular expression of these cartilage proteins is closely related to extra-articular pathogenic expression in rheumatic diseases. Uveitis develops in VG1-immunized BALB/c mice, which is not seen in AG1-, and LP-treated animals. There is evidence that aggrecan and LP are also localized at these sites in the eye, but only immunity to versican can induce uveitis. In sacroiliitis and enthesitis of AS patients, the inflammation is associated with chondrometaplasia. In versican-induced sacroiliitis, replacement of cartilage by bone is seen with relatively little inflammation, somewhat resembling the situation in AS (Fig. 2). Versican can also stimulate chondrocyte proliferation [43]. Three conserved domains of human cartilage matrix molecules, namely VG1, AG1, and LP, show considerable homology [77, 79, 80, 89], and each is capable of inducing a unique inflammatory arthritis in BALB/c mice, with VG1 inducing only spondylitis [65], LP inducing peripheral arthritis with no spondylitis [90], and AG1 inducing axial and peripheral arthritis [66, 91]. It remains a mystery why such similar molecules cause different pathology in different target tissues. The exact immunopathogenic mechanisms deserve further study.

Cells from different regions of the intervertebral disc: effect of culture system on matrix expression and cell phenotype

STUDY DESIGN

This study examined how the culture system and region of cellular origin affect disc cell morphology and extracellular matrix production.

OBJECTIVE

To determine the role of the cell populations in the different regions of the adult intervertebral disc in maintaining gradients in composition across the disc.

SUMMARY OF BACKGROUND DATA

It is not known whether the steep profiles in composition across the intervertebral disc are maintained by distinct cell populations or whether differences in cell metabolism are determined by changes in the physical environment across the disc. Very little information exists on the matrix produced by cells from the mature, non-notochordal nucleus pulposus.

METHODS

Cells were extracted from articular cartilage, nucleus pulposus, and the inner and outer anulus fibrosus of caudal discs from 18- to 24-month-old steers cultured in alginate or collagen gels or in monolayer. The effect of culture system and cell origin on cell morphology and matrix synthesis was measured using 35S-sulphate labeling and indirect immunolocalization.

RESULTS

Distinct morphologic differences between cells from different regions cultured in monolayer were retained through two passages. The rate of sulfate incorporation varied with cell type. Immediately after isolation, it was two- to threefold greater for nucleus cells than for cells from the disc inner anulus or articular cartilage. The rate was lowest for outer anulus cells. It also varied with culture system. For all cell types, the incorporation rate was highest in alginate and lowest in monolayer. Immunolocalization showed that nucleus cells stained strongly for all proteoglycan epitopes, whereas outer anulus cells stained least and in monolayer produced little proteoglycan.

CONCLUSIONS

The disc has at least three distinct cell populations, which differ in morphology and in amount and type of matrix they produce. Cells from mature nucleus pulposus produced sulfated glycosaminoglycans at a high rate in contrast to reported results for notochordal nucleus cells. Alginate, although an appropriate culture system for inner anulus and nucleus cells, may not be a suitable medium for outer anulus cells.

Matrix protein gene expression in intervertebral disc cells subjected to altered osmolarity

Physiologic loading of the intervertebral disc may lead to changes in the osmotic pressure experienced by the resident cells. In this study, changes in gene expression levels for extracellular matrix and cytoskeletal proteins were quantified in disc cells subjected to hypo-osmotic (255 mOsm) or hyper-osmotic conditions (450 mOsm), relative to iso-osmotic conditions (293 mOsm). Important differences were observed in osmolarity and between cells of different regions, corresponding to the transition zone and nucleus pulposus. Under hypo-osmotic conditions, gene expressions for aggrecan and type II collagen were up-regulated in the transition zone, but not in the nucleus pulposus cells. Genes for the small proteoglycans, biglycan, and decorin, but not lumican, were up-regulated in transition zone cells following incubation in either hypo- or hyper-osmotic media. The same genes were down-regulated in nucleus pulposus cells under either hypo- or hyper-osmotic conditions. Differences in the response to altered osmolarity between cells of the intervertebral disc may relate to their different cytoskeletal structures or embryological origins.

Synthesis of a Kunitz-type serine proteinase inhibitory protein that shares homology with bovine pancreatic trypsin inhibitor by ovine intervertebral disc cells in serum-free alginate bead culture

The objective of this study was to determine whether disc cells could be cultured under serum-free conditions and whether they synthesized bovine pancreatic trypsin inhibitor (BPTI)-like serine proteinase inhibitory proteins (SPIs) previously demonstrated for ovine chondrocytes. Intervertebral discs from 1- to 2-year-old merino wether sheep were dissected into the annulus fibrosus and nucleus pulposus, and the cells isolated by sequential enzymatic digestion. The cells were grown encapsulated in calcium alginate microspheres under serum-free conditions for 10 days. They remained more than 92% viable as assessed using the vital fluorescent dyes chloromethyl fluorescein diacetate and ethidium homodimer-1 to delineate live/dead cells, respectively. Western and affinity blotting identified a 12-16-kDa media SPI and an additional 34-36-kDa BPTI-like species in solubilized bead samples. This study has demonstrated ovine disc cells synthesized BPTI-like SPIs in serum-free alginate bead culture similar to chondrocyte SPIs; however, the 58-kDa precursor SPI form was not detected suggesting differences in the endogenous processing of these SPIs.

The interactions of cartilage proteoglycans with collagens are determined by their structures

In the present work, the interaction of aggrecan, decorin and biglycan isolated from pig laryngeal cartilage and of the three squid cartilage proteoglycans with collagen type I and II was studied. The interaction was examined under conditions allowing the formation of collagen fibrils. It was found that biglycan interacted strongly with collagen type II and not with type I and the interaction seemed to proceed exclusively through its core proteins. Decorin interacted with collagen type I but not with type II. Aggrecan interacted very poorly with both collagen types. The two squid proteoglycans of large size, D1D1A and D1D2, interacted only with collagen type I through both glycosaminoglycans and core proteins. The third squid proteoglycan of small size, D1D1B, interacted poorly only with collagen type I. The results suggested that the interactions of cartilage proteoglycans with collagen were mainly due to the primary structure of both molecules, and would contribute to the maintenance of the integrity of the tissue. The biochemical significance of these interactions might be more critical in aged vertebrate cartilage, where loss of aggrecan and increase of the small proteoglycans was observed, a large proportion of which is found in the extracellular matrix free of glycosaminoglycan chains.

A comparative analysis of the differential spatial and temporal distributions of the large (aggrecan, versican) and small (decorin, biglycan, fibromodulin) proteoglycans of the intervertebral disc

This study provides a comparative analysis of the temporal and spatial distribution of 5 intervertebral disc (IVD) proteoglycans (PGs) in sheep. The main PGs in the 2 and 10 y old sheep groups were polydisperse chondroitin sulphate and keratan sulphate substituted species. Their proportions did not differ markedly either with spinal level or disc zone. In contrast, the fetal discs contained 2 slow migrating (by composite agarose polyacrylamide gel electrophoresis, CAPAGE), relatively monodisperse chondroitin sulphate-rich aggrecan species which were also identified by monoclonal antibody 7-D-4 to an atypical chondroitin sulphate isomer presentation previously found in chick limb bud, and shark cartilage. The main small PG detectable in the fetal discs was biglycan, whereas decorin predominated in the 2 and 10 y old IVD samples; its levels were highest in the outer annulus fibrosus (AF). Versican was most abundant in the AF of the fetal sheep group; it was significantly less abundant in the 2 and 10 y old groups. Furthermore, versican was immunolocalised between adjacent layers of annular lamellae suggesting that it may have some role in the provision of the viscoelastic properties to this tissue. Versican was also diffusely distributed throughout the nucleus pulposus of fetal IVDs, and its levels were significantly lower in adult IVD specimens. This is the first study to identify versican in ovine IVD tissue sections and confirmed an earlier study which demonstrated that ovine IVD cells synthesised versican in culture (Melrose et al. 2000). The variable distribution of the PGs identified in this study provides further evidence of differences in phenotypic expression of IVD cell populations during growth and development and further demonstrates the complexity of the PGs in this heterogeneous but intricately organised connective tissue.

Spinal biomechanics and aging are major determinants of the proteoglycan metabolism of intervertebral disc cells

STUDY DESIGN

The proteoglycan metabolism of ovine disc nucleus pulposus and anulus fibrosus cells was investigated in relation to age, spinal level, and intrinsic spinal biomechanical properties.

OBJECTIVE

To evaluate the hypothesis that with aging loss of proteoglycans from the lumbosacral disc exceeds that from upper lumbar discs because of its proximity to a rigid segment, the sacrum.

SUMMARY OF BACKGROUND DATA

The proteoglycan and associated water of the disc decreases with aging.

METHODS

Proteoglycans were extracted directly from the disc tissues using 4 M GuHCl and examined by composite agarose polyacrylamide gel electrophoresis. Disc cells were cultured in alginate beads, and their metabolic activity was assessed by 3H-thymidine incorporation into DNA and by bioreduction of a cell proliferation dye. Newly synthesized proteoglycans were radiolabeled with 35S, and their molecular weight distributions and ability to aggregate with hyaluronan were determined by Sephacryl S1000 gel chromatography. Resident proteoglycans extracted from disc tissues with 4 M GuHCl were similarly evaluated. A group of adult animals also were studied biomechanically to evaluate the range of spinal motion (L4/L5 to L7/S1).

RESULTS

In contrast to the neonatal proteoglycan samples, the biosynthesis of proteoglycans by nucleus pulposus cells of adult discs increased progressively toward the sacrum. This correlated with increased metabolic activity. Analysis of the resident proteoglycans by composite agarose polyacrylamide gel electrophoresis indicated that although the aggrecan-1 population was present almost exclusively in the neonatal group, it was the aggrecan-2 population that predominated in the adult discs, and it became progressively more heterogeneous with aging and proximity of the disc to the sacrum.

CONCLUSIONS

The proteoglycans of the lumbosacral disc of adult animals turned over faster than proteoglycans of adjacent lumbar discs. The reduced proteoglycan content and ability to aggregate, particularly in the nucleus pulposus of lumbosacral discs, indicated that proteoglycan catabolism exceeded the rate of biosynthesis. These events in the lumbosacral disc are thought to be determined mechanically by its proximity to the sacrum.