Nucleus pulposus tissue engineering: a brief review

Characterization of mechanics and cytocompatibility of fibrin-genipin annulus fibrosus sealant with the addition of cell adhesion molecules

There is an unmet clinical need for a biomaterial sealant capable of repairing small annulus fibrosus (AF) defects. Causes of these defects include painful intervertebral disc herniations, microdiscectomy procedures, morbidity associated with needle puncture injury from discography, and future nucleus replacement procedures. This study describes the enhancements of a fibrin gel through genipin crosslinking (FibGen) and the addition of the cell adhesion molecules (CAMs), fibronectin and collagen. The gel's performance as a potential AF sealant is assessed using a series of in vitro tests. FibGen gels with CAMs had equivalent adhesive strength, gene expression, cytomorphology, and cell proliferation as fibrin alone. However, FibGen gels had enhanced material behaviors that were tunable to higher shear stiffness values and approximated human annulus tissue as compared with fibrin alone, were more dimensionally stable, and had a slower in vitro degradation rate. Cytomorphology of human AF cells cultured on FibGen gels exhibited increased elongation compared with fibrin alone, and the addition of CAMs to FibGen did not significantly affect elongation. This FibGen gel offers the promise of being used as a sealant material to repair small AF defects or to be used in combination with other biomaterials as an adhesive for larger defects.

Stem cells in preclinical spine studies

BACKGROUND CONTEXT

The recent identification and characterization of mesenchymal stem cells have introduced a shift in the research focus for future technologies in spinal surgery to achieve spinal fusion and treat degenerative disc disease. Current and past techniques use allograft to replace diseased tissue or rely on host responses to recruit necessary cellular progenitors. Adult stem cells display long-term proliferation, efficient self-renewal, and multipotent differentiation.

PURPOSE

This review will focus on two important applications of stem cells in spinal surgery: spine fusion and the management of degenerative disc disease.

STUDY DESIGN

Review of the literature.

METHODS

Relevant preclinical literature regarding stem cell sources, growth factors, scaffolds, and animal models for both osteogenesis and chondrogenesis will be reviewed, with an emphasis on those studies that focus on spine applications of these technologies.

RESULTS

In both osteogenesis and chondrogenesis, adult stem cells derived from bone marrow or adipose show promise in preclinical studies. Various growth factors and scaffolds have also been shown to enhance the properties and eventual clinical potential of these cells. Although its utility in clinical applications has yet to be proven, gene therapy has also been shown to hold promise in preclinical studies.

CONCLUSIONS

The future of spine surgery is constantly evolving, and the recent advancements in stem cell-based technologies for both spine fusion and the treatment of degenerative disc disease is promising and indicative that stem cells will undoubtedly play a major role clinically. It is likely that these stem cells, growth factors, and scaffolds will play a critical role in the future for replacing diseased tissue in disease processes such as degenerative disc disease and in enhancing host tissue to achieve more reliable spine fusion.

Silk fibroin porous scaffolds for nucleus pulposus tissue engineering

Intervertebral discs (IVDs) are structurally complex tissue that hold the vertebrae together and provide mobility to spine. The nucleus pulposus (NP) degeneration often results in degenerative IVD disease that is one of the most common causes of back and neck pain. Tissue engineered nucleus pulposus offers an alternative approach to regain the function of the degenerative IVD. The aim of this study is to determine the feasibility of porous silk fibroin (SF) scaffolds fabricated by paraffin-sphere-leaching methods with freeze-drying in the application of nucleus pulposus regeneration. The prepared scaffold possessed high porosity of 92.38±5.12% and pore size of 165.00±8.25μm as well as high pore interconnectivity and appropriate mechanical properties. Rabbit NP cells were seeded and cultured on the SF scaffolds. Scanning electron microscopy, histology, biochemical assays and mechanical tests revealed that the porous scaffolds could provide an appropriate microstructure and environment to support adhesion, proliferation and infiltration of NP cells in vitro as well as the generation of extracellular matrix. The NP cell-scaffold construction could be preliminarily formed after subcutaneously implanted in a nude mice model. In conclusion, The SF porous scaffold offers a potential candidate for tissue engineered NP tissue.

Nonviral Gene Delivery of Growth and Differentiation Factor 5 to Human Mesenchymal Stem Cells Injected into a 3D Bovine Intervertebral Disc Organ Culture System

Intervertebral disc (IVD) cell therapy with unconditioned 2D expanded mesenchymal stem cells (MSC) is a promising concept yet challenging to realize. Differentiation of MSCs by nonviral gene delivery of growth and differentiation factor 5 (GDF5) by electroporation mediated gene transfer could be an excellent source for cell transplantation. Human MSCs were harvested from bone marrow aspirate and GDF5 gene transfer was achieved by in vitro electroporation. Transfected cells were cultured as monolayers and as 3D cultures in 1.2% alginate bead culture. MSC expressed GDF5 efficiently for up to 21 days. The combination of GDF5 gene transfer and 3D culture in alginate showed an upregulation of aggrecan and SOX9, two markers for chondrogenesis, and KRT19 as a marker for discogenesis compared to untransfected cells. The cells encapsulated in alginate produced more proteoglycans expressed in GAG/DNA ratio. Furthermore, GDF5 transfected MCS injected into an IVD papain degeneration organ culture model showed a partial recovery of the GAG/DNA ratio after 7 days. In this study we demonstrate the potential of GDF5 transfected MSC as a promising approach for clinical translation for disc regeneration.

Effects of releasing recombinant human growth and differentiation factor-5 from poly(lactic-co-glycolic acid) microspheres for repair of the rat degenerated intervertebral disc

Purpose:

The objective of this study was to investigate the therapeutic potential of poly(lactic- co-glycolic acid) (PLGA) microspheres loaded with recombinant human growth and differentiation factor-5 (rhGDF-5) on the disc degeneration induced by needle puncture in a rat caudal disc model.

Methods:

The rhGDF-5-loaded PLGA microspheres were prepared by the water-oil-water double-emulsion solvent evaporation method, and release kinetics was determined over 42 days. Rats that underwent 21-G needle puncture at rat tail discs were injected with rhGDF-5/PLGA microspheres at four weeks after needle injury. At eight weeks after the injection, disc height, glycosaminoglycans content, and DNA content of the discs were evaluated. In addition, gene expression analysis of aggrecan, collagen type I, and collagen type II in the rat nucleus pulposus was measured by real-time polymerase chain reaction. Rat discs were also assessed by histology using hematoxylin and eosin stain.

Results:

Encapsulation of rhGDF-5 in PLGA microspheres guaranteed a sustained release of active rhGDF-5 for more than 42 days. The injection of GDF-5/PLGA microspheres resulted in a statistically significant restoration of disc height ( p < 0.01), improvement of sulfated glycosaminoglycan ( p < 0.05), DNA content ( p < 0.05), and significantly increased mRNA levels of collagen type II ( p < 0.01), and the differentiation index (the ratio of collagen type II to collagen type I, p < 0.01). In addition, rhGDF-5/PLGA microspheres treatment also improved histological changes induced by needle puncture.

Conclusions:

The results of this study suggest that injection of rhGDF-5 loaded in PLGA microspheres into rat tail discs may be as a promising therapy strategy to regenerate or repair the degenerative disc.

Novel cartilage-derived biomimetic scaffold for human nucleus pulposus regeneration: a promising therapeutic strategy for symptomatic degenerative disc diseases

Because current therapies have not always been successful and effective, the possibility of regenerating the nucleus pulposus (NP) through a tissue-engineered construct offers a novel therapeutic possibility for symptomatic degenerative disc diseases (DDDs). However, more research is necessary to identify the optimal scaffold, cell type and mixture of signal factors needed for NP regeneration. Numerous possible scaffolds for NP regeneration have been investigated; they have many shortcomings in common. Various biological scaffolds derived from decellularized tissue and organs have been successfully used in tissue engineering and received approval for use in humans. Regretfully, harvesting of human NP is difficult and only small amounts can be obtained. The macromolecules of cartilage, which include collagen and proteoglycan aggrecan, are similar to those of the extracellular matrix of immature NP. Recent studies have shown that adipose-derived stem cells (ADSC) can be induced to develop NP-like phenotypes when stimulated by appropriate signals. We thus reasonably postulated that an ideal NP scaffold for tissue engineering could be fabricated from decellularized cartilage matrix (DCM). Furthermore, a combination of ADSCs and DCM-derived biomimetic scaffolds would be advantageous in NP tissue engineering and, in the long run, could become an effective treatment option for symptomatic DDD.

Regulatory role of hypoxia inducible factor in the biological behavior of nucleus pulposus cells

Intervertebral disc (IVD) degeneration is implicated as a major cause of low back pain. The alternated phenotypes, reduced cell survival, decreased metabolic activity, loss of matrix production and dystrophic mineralization of nucleus pulposus (NP) cells may be key contributors to progressive IVD degeneration. IVD is the largest avascular structure in the body, characterized by low oxygen tension in vivo. Hypoxia-inducible factor (HIF) is a master transcription factor that is induced upon hypoxia and directs coordinated cellular responses to hypoxic environments. This review summarizes relevant studies concerning the involvement of HIF in the regulation of biological behaviors of NP cells. We describe current data on the expression of HIF in NP cells and further discuss the various roles that HIF plays in the regulation of the phenotype, survival, metabolism, matrix production and dystrophic mineralization of NP cells. Here, we conclude that HIF may be a promising target for the prevention and treatment of IVD degeneration.

Intervertebral disc regeneration: from the degenerative cascade to molecular therapy and tissue engineering

Low back pain is one of the major health problems in industrialized countries, as a leading source of disability in the working population. Intervertebral disc degeneration has been identified as its main cause, being a progressive process mainly characterized by alteration of extracellular matrix composition and water content. Many factors are involved in the degenerative cascade, such as anabolism/catabolism imbalance, reduction of nutrition supply and progressive cell loss. Currently available treatments are symptomatic, and surgical procedures consisting of disc removal are often necessary. Recent advances in our understanding of intervertebral disc biology led to an increased interest in the development of novel biological treatments aimed at disc regeneration. Growth factors, gene therapy, stem cell transplantation and biomaterials-based tissue engineering might support intervertebral disc regeneration by overcoming the limitation of the self-renewal mechanism. The aim of this paper is to overview the literature discussing the current status of our knowledge from the degenerative cascade of the intervertebral disc to the latest molecular, cell-based therapies and tissue-engineering strategies for disc regeneration.

Papain-induced in vitro disc degeneration model for the study of injectable nucleus pulposus therapy

BACKGROUND CONTEXT

Proteolytic enzyme digestion of the intervertebral disc (IVD) offers a method to simulate a condition of disc degeneration for the study of cell-scaffold constructs in the degenerated disc.

PURPOSE

To characterize an in vitro disc degeneration model (DDM) of different severities of glycosaminoglycans (GAG) and water loss by using papain, and to determine the initial response of the human mesenchymal stem cells (MSCs) introduced into this DDM.

STUDY DESIGN

Disc degeneration model of a bovine disc explant with an end plate was induced by the injection of papain at various concentrations. Labeled MSCs were later introduced in this model.

METHODS

Phosphate-buffered saline (PBS control) or papain in various concentrations (3, 15, 30, 60, and 150 U/mL) were injected into the bovine caudal IVD explants. Ten days after the injection, GAG content of the discs was evaluated by dimethylmethylene blue assay and cell viability was determined by live/dead staining together with confocal microscopy. Overall matrix composition was evaluated by histology, and water content was visualized by magnetic resonance imaging. Compressive and torsional stiffness of the DDM were also recorded. In the second part, MSCs were labeled with a fluorescence cell membrane tracker and injected into the nucleus of the DDM or a PBS control. Mesenchymal stem cell viability and distribution were evaluated by confocal microscopy.

RESULTS

A large drop of GAG and water content of the bovine disc were obtained by injecting >30 U/mL papain. Magnetic resonance imaging showed Grade II, III, and IV disc degeneration by injecting 30, 60, and 150 U/mL papain. A cavity in the center of the disc could facilitate later injection of the nucleus pulposus tissue engineering construct while retaining an intact annulus fibrosus. The remaining disc cell viability was not affected. Mesenchymal stem cells injected into the protease-treated DDM disc showed significantly higher cell viability than when injected into the PBS-injected control disc.

CONCLUSIONS

By varying the concentration of papain for injection, an increasing amount of GAG and water loss could be induced to simulate the different severities of disc degeneration. MSC suspension introduced into the disc has a very low short-term survival. However, it should be clear that this bovine IVD DDM does not reflect a clinical situation but offers exciting possibilities to test novel tissue engineering protocols.

Role of biomechanics in intervertebral disc degeneration and regenerative therapies: what needs repairing in the disc and what are promising biomaterials for its repair?

BACKGROUND CONTEXT

Degeneration and injuries of the intervertebral disc (IVD) result in large alterations in biomechanical behaviors. Repair strategies using biomaterials can be optimized based on the biomechanical and biological requirements of the IVD.

PURPOSE

To review the present literature on the effects of degeneration, simulated degeneration, and injury on biomechanics of the IVD, with special attention paid to needle puncture injuries, which are a pathway for diagnostics and regenerative therapies and the promising biomaterials for disc repair with a focus on how those biomaterials may promote biomechanical repair.

STUDY DESIGN

A narrative review to evaluate the role of biomechanics on disc degeneration and regenerative therapies with a focus on what biomechanical properties need to be repaired and how to evaluate and accomplish such repairs using biomaterials. Model systems for the screening of such repair strategies are also briefly described.

METHODS

Articles were selected from two main PubMed searches using keywords: intervertebral AND biomechanics (1,823 articles) and intervertebral AND biomaterials (361 articles). Additional keywords (injury, needle puncture, nucleus pressurization, biomaterials, hydrogel, sealant, tissue engineering) were used to narrow the articles down to the topics most relevant to this review.

RESULTS

Degeneration and acute disc injuries have the capacity to influence nucleus pulposus (NP) pressurization and annulus fibrosus (AF) integrity, which are necessary for an effective disc function and, therefore, require repair. Needle injection injuries are of particular clinical relevance with the potential to influence disc biomechanics, cellularity, and metabolism, yet these effects are localized or small and more research is required to evaluate and reduce the potential clinical morbidity using such techniques. NP replacement strategies, such as hydrogels, are required to restore the NP pressurization or the lost volume. AF repair strategies including cross-linked hydrogels, fibrous composites, and sealants offer promise for regenerative therapies to restore AF integrity. Tissue engineered IVD structures, as a single implantable construct, may promote greater tissue integration due to the improved repair capacity of the vertebral bone.

CONCLUSIONS

IVD height, neutral zone characteristics, and torsional biomechanics are sensitive to specific alterations in the NP pressurization and AF integrity and must be addressed for an effective functional repair. Synthetic and natural biomaterials offer promise for NP replacement, AF repair, as an AF sealant, or whole disc replacement. Meeting mechanical and biological compatibilities are necessary for the efficacy and longevity of the repair.

The challenge and advancement of annulus fibrosus tissue engineering

BACKGROUND

Intervertebral disc degeneration, a main cause of back pain, is an endemic problem and a big economic burden for the health care system. Current treatments are symptom relieving but do not address underlying problems-biological and structural deterioration of the disc. Tissue engineering is an emerging approach for the treatment of intervertebral disc degeneration since it restores the functionality of native tissues. Although numerous studies have focused on the nucleus pulposus tissue engineering and achieved successes in laboratory settings, disc tissue engineering without annulus fibrosus for the end stage of disc degeneration is deemed to fail. The purpose of this article is to review the advancement of annulus fibrosus tissue engineering.

MATERIAL AND METHODS

Relevant articles regarding annulus fibrosus tissue engineering were identified in PubMed and Medline databases.

RESULTS

The ideal strategy for disc regeneration is to restore the function and integrity of the disc by using biomaterials, native matrices, growth factors, and cells that producing matrices. In the past decades there are tremendous advancement in annulus fibrosus tissue engineering including cell biology, biomaterials, and whole disc replacement. The recent promising results on whole disc tissue engineering-a composite of annulus fibrosus and nucleus pulposus-make the tissue engineering approach more appealing.

CONCLUSION

Despite the promising results in disc tissue engineering, there is still much work to be done regarding the clinical application.

Evaluation of thiol-modified hyaluronan and elastin-like polypeptide composite augmentation in early-stage disc degeneration: comparing 2 minimally invasive techniques

STUDY DESIGN

An in vitro biomechanical and imaging study generated from an in vivo porcine model of early stage degenerative disc disease was used to evaluate mechanical property restoration, comparing 2 minimally invasive injection techniques.

OBJECTIVE

To evaluate the ability of an injectable hydrogel to restore the mechanical properties of spinal motion segments with early stage disc degeneration, comparing 2 minimally invasive injection techniques.

SUMMARY OF BACKGROUND DATA

Treatment of early-stage disc degeneration may benefit from a combination of tissue engineering and minimally invasive therapeutic approaches. A recently developed hydrogel, thiol-modified hyaluronan elastin-like polypeptide (TMHA/EP) composite, has demonstrated potential as an injectable nucleus replacement.

METHODS

From a total of thirteen 35-kg Yorkshire boars, early-stage lumbar disc degeneration was introduced into 10 pigs via injection of chondroitinase ABC. After degeneration, 8 pigs received TMHA/EP augmentation; 1 disc via direct needle injection and a second using a modified kyphoplasty approach. High-resolution magnetic resonance images were acquired of the excised spinal motion segments, followed by biomechanical testing in axial compression, flexion-extension, lateral bending, and torsion.

RESULTS

The degenerate control motion segments were generally less stiff and more flexible than healthy controls. The injection of TMHA/EP into the degenerated nucleus produced similar mechanical stiffness to healthy controls. The direct-injected discs showed a dispersive pattern of TMHA/EP within the nucleus, whereas the modified kyphoplasty method yielded a bolus of hydrogel. Yet, mechanical behavior was comparable considering the 2 minimally invasive augmentation techniques.

CONCLUSION

The TMHA/EP composite can restore initial mechanical behavior in early-stage disc degeneration. Although both augmentation methods yielded mechanical properties comparable with healthy controls, direct injection represents a simpler technique, uses a smaller-gauge needle, does not introduce air into the disc, and yields a dispersive pattern that may be beneficial for future delivery of cells or growth factors.

Characterization of the annulus fibrosus-vertebral body interface: identification of new structural features

Current surgical treatments for degenerative intervertebral disc disease do not restore full normal spinal movement. Tissue engineering a functional disc replacement may be one way to circumvent this limitation, but will require an integration of the different tissues making up the disc for this approach to be successful. Hence, an in-depth characterization of the native tissue interfaces, including annulus insertion into bone is necessary, as knowledge of this interface is limited. The objective of this study was to characterize the annulus fibrosus-vertebral bone (AF-VB) interface in immature (6-9 months old) and mature (18-24 months old) bovine discs, as well as to define these structures for normal adult human (22 and 45 years old) discs. Histological assessment showed that collagen fibers in the inner annulus, which are predominantly type II collagen, all appear to insert into the mineralized endplate zone. In contrast, some of the collagen fibers of the outer annulus, predominantly type I collagen, insert into this endplate, while other fibers curve laterally, at an ∼ 90° angle, to the outer aspect of the bone, and merge with the periosteum. This is seen in both human and bovine discs. Where the AF inserts into the calcified zone of the AF-VB interface, it passes through a chondroid region, rich in type II collagen and proteoglycans. Annulus cells (elongated cells that are not surrounded by proteoglycans) are present at this interface. This cartilage zone is evident in both human and bovine discs. Type X collagen and alkaline phosphatase are localized to the interface region. Age-associated differences in bovine spines are observed when examining the interface thickness and the matrix composition of the cartilaginous endplate, as well as the thickness of the mineralized endplate. These findings will assist with the design of the AF-VB interface in the tissue engineered disc.

Development of poly(lactide-co-glycolide) scaffold-impregnated small intestinal submucosa with pores that stimulate extracellular matrix production in disc regeneration

The pore size and microstructure of scaffolds influences cell attachment, migration, proliferation and ingrowth, but the optimal pore size of scaffolds for disc tissue formation is not clearly understood. We developed porous poly(lactide-co-glycolide) (PLGA) scaffolds with various pore sizes for nucleus pulposus (NP) cell cultures and examined the effects of pore size on cell ingrowth and extracellular matrix (ECM) synthesis. High cell density in the small pores of scaffolds promotes collagen synthesis and cell migration through interconnected pores. Scaffolds with large pores exhibited slower cell proliferation and collagen synthesis. Guided by these results, we investigated a novel, biodegradable, synthetic/natural hybrid scaffold composed of PLGA and small intestinal submucosa (SIS) (PLGA-SIS) with the proper pore size for NP regeneration. We tested the morphological and physical properties of PLGA-SIS scaffolds and initial cell attachment and ECM production of NP in scaffolds. The mechanical and degradable properties of the PLGA-SIS scaffold were superior to those of SIS sponge and were similar to the properties of PLGA scaffold. NP cells grown on PLGA-SIS scaffold exhibited higher initial cell adhesion and ECM production than those grown on pure PLGA scaffold in a biological assay. In conclusion, this study suggests that a proper pore size of scaffolds is critical in NP regeneration, and that PLGA-SIS scaffolds with suitable pores might be useful as substrates for tissue-engineered biodiscs.

Fabrication of a Layered Microstructured Polymeric Microspheres as a Cell Carrier for Nucleus Pulposus Regeneration

This study aimed to investigate the feasibility of nanostructured 3D poly(lactide-co-glycolide) (PLGA) constructs, which are loaded with dexamethasone (DEX) and growth factor embedded hepaiin/poly(L-lysine) nanoparticles by a layer-by-layer system, to serve as an effective scaffold for nucleus pulposus (NP) tissue engineering. Our results demonstrated that the microsphere constructs were capable of simultaneously releasing basic fibroblast growth factor and DEX with approximately zero-order kinetics. The dual bead microspheres showed no cytotoxicity, and promoted the proliferation of the rat mesenchymal stem cells (rMSCs) by lactate dehydrogenase assay and CCK-8 assay. After 4 weeks of culture in vitro, the rMSCs- scaffold hybrids contained significantly higher levels of sulfated GAG/DNA and type-II collagen than the control samples. Moreover, quantity real-time PCR analysis revealed that the expression of disc-matrix proteins, including type-II collagen, aggrecan and versican, in the rMSCs-scaffold hybrids was significantly higher than the control group, whereas the expression of osteogenic differentiation marker type-I collagen was decreased. Taken together, these data indicate that the heparin bound bFGF-coated and DEX-loaded PLGA microsphere constructs is an effective bioactive scaffold for the regeneration of NP tissue.

Demineralized bone particle impregnated poly(l-lactide-co-glycolide) scaffold for application in tissue-engineered intervertebral discs

Demineralized bone particle (DBP) contains powerful bioactive molecules that facilitate new bone or cartilage growth. We developed hybrid scaffolds of poly(l-lactide-co-glycolide) (PLGA) with various concentrations of DBP (DBP/PLGA), of which phenotypes on intervertebral disc (IVD) cells were investigated. The hybrid scaffold has a cylindrical donut shape with two distinct parts; the inner is for the nucleus pulposus (NP) and the outer is for annulus fibrosus (AF). Rabbit NP and AF cells were seeded into the inner and outer regions of the DBP/PLGA scaffolds separately. Disc cell viability in DBP/PLGA scaffolds was superior to pure PLGA scaffold and increased with increasing DBP concentration. In vitro- and in vivo-formed tissues were characterized by RT-PCR, Safranin-O, Masson's trichrome staining and immunohistochemi- cal staining for type-I and type-II collagen. DBP/PLGA hybrid scaffolds revealed more active expression of disc phenotypes, as characterized by protein and mRNA expression, than the PLGA control. This study provides valuable information for potential disc replacement using DBP and PLGA.

Nucleus pulposus replacement and regeneration/repair technologies: present status and future prospects

Degenerative disc disease is implicated in the pathogenesis of many painful conditions of the back, chief among which is low back pain. Acute and/or chronic low back pain (A/CLBP) afflicts a large number of people, thus making it a major healthcare issue with concomitant cost ramifications. When conservative treatments for A/CLBP, such as bed rest, anti-inflammatory medications, and physical therapy, prove to be ineffectual, surgical options are recommended. The most popular of these is discectomy followed by fusion. Although there are many reports of good to excellent outcomes with this method, there are concerns, such as long-term adverse biomechanical consequences to adjacent functional spinal unit(s). A surgical option that has been attracting much attention recently is replacement or regeneration/repair of the nucleus pulposus, an approach that holds the prospect of not compromising either mobility or function and causing no adjacent-level injury. There is a sizeable body of literature highlighting this option, comprising in vitro biomechanical studies, finite element analyses, animal-model studies, and limited clinical evaluations. This work is a review of this body of literature and is organized into four parts, with the focus being on replacement technologies, regeneration/repair technologies, and detailed expositions on 14 areas for future study. This review ends with a summary of the salient points made.

Stem cell therapy for degenerative disc disease

Low back pain is widely recognized as one of the most prevalent pathologies in the developed world. In the United States, low back pain is the most common health problem for adults under the age of 50, resulting in significant societal and personal costs. While the causes of low back pain are myriad, it has been significantly associated with intervertebral disc (IVD) degeneration. Current first-line therapies for IVD degeneration such as physical therapy and spinal fusion address symptoms, but do not treat the underlying degeneration. The use of tissue engineering to treat IVD degeneration provides an opportunity to correct the pathological process. Novel techniques are currently being investigated and have shown mixed results. One major avenue of investigation has been stem cell injections. Mesenchymal stem cells (MSCs) have shown promise in small animal models, but results in larger vertebrates have been mixed.

Mesenchymal stem cell for prevention and management of intervertebral disc degeneration

Intervertebral disc degeneration (IVD) is a frequent pathological condition. Conservative management often fails, and patients with IVD degeneration may require surgical intervention. Several treatment strategies have been proposed, although only surgical discectomy and arthrodesis have been proved to be predictably effective. The aim of biological strategies is to prevent and manage IVD degeneration, improve the function, the anabolic and reparative capabilities of the nucleus pulposus and annulus fibrosus cells, and inhibit matrix degradation. At present, clinical applications are still in their infancy. Further studies are required to clarify the role of mesenchymal stem cells and gene therapy for the prevention and treatment of IVD degeneration.

Nucleus pulposus cells synthesize a functional extracellular matrix and respond to inflammatory cytokine challenge following long-term agarose culture

Intervertebral disc degeneration is characterized by a cascade of cellular, biochemical and structural changes that may lead to functional impairment and low back pain. Interleukin-1 beta (IL-1β) is strongly implicated in the etiology of disc degeneration, however there is currently no direct evidence linking IL-1β upregulation to downstream biomechanical changes. The objective of this study was to evaluate long-term agarose culture of nucleus pulposus (NP) cells as a potential in vitro model system to investigate this. Bovine NP cells were cultured in agarose for 49 days in a defined medium containing transforming growth factor-beta 3, after which both mechanical properties and composition were evaluated and compared to native NP. The mRNA levels of NP cell markers were compared to those of freshly isolated NP cells. Glycosaminoglycan (GAG) content, aggregate modulus and hydraulic permeability of mature constructs were similar to native NP, and aggrecan and SOX9 mRNA levels were not significantly different from freshly isolated cells. To investigate direct links between IL-1β and biomechanical changes, mature agarose constructs were treated with IL-1β, and effects on biomechanical properties, extracellular matrix composition and mRNA levels were quantified. IL-1β treatment resulted in upregulation of a disintegrin and metalloproteinase with thrombospondin motifs 4, matrix metalloproteinase-13 and inducible nitric oxide sythase, decreased GAG and modulus, and increased permeability. To evaluate the model as a test platform for therapeutic intervention, co-treatment with IL-1β and IL-1 receptor antagonist (IL-1ra) was evaluated. IL-1ra significantly attenuated degradative changes induced by IL-1β. These results suggest that this in vitro model represents a reliable and cost-effective platform for evaluating new therapies for disc degeneration.

Hydrogels in acellular and cellular strategies for intervertebral disc regeneration

Low back pain is an extremely common illness syndrome that causes patient suffering and disability and requires urgent solutions to improve the quality of life of these patients. Treatment options aimed to regenerate the intervertebral disc (IVD) are still under development. The cellular complexity of IVD, and consequently its fine regulatory system, makes it a challenge to the scientific community. Biomaterials-based therapies are the most interesting solutions to date, whereby tissue engineering and regenerative medicine (TE&RM) strategies are included. By using such strategies, i.e., combining biomaterials, cells, and biomolecules, the ultimate goal of reaching a complete integration between native and neo-tissue can be achieved. Hydrogels are promising materials for restoring IVD, mainly nucleus pulposus (NP). This study presents an overview of the use of hydrogels in acellular and cellular strategies for intervertebral disc regeneration. To better understand IVD and its functioning, this study will focus on several aspects: anatomy, pathophysiology, cellular and biomolecular performance, intrinsic healing processes, and current therapies. In addition, the application of hydrogels as NP substitutes will be addressed due to their similarities to NP mechanical properties and extracellular matrix. These hydrogels can be used in cellular strategies when combined with cells from different sources, or in acellular strategies by performing the functionalization of the hydrogels with biomolecules. In addition, a brief summary of therapies based on simple injection for primary biological repair will be examined. Finally, special emphasis will focus on reviewing original studies reporting on the use of autologous cells and biomolecules such as platelet-rich plasma and their potential clinical applications.

Difference in Energy Metabolism of Annulus Fibrosus and Nucleus Pulposus Cells of the Intervertebral Disc

Low back pain is associated with intervertebral disc degeneration. One of the main signs of degeneration is the inability to maintain extracellular matrix integrity. Extracellular matrix synthesis is closely related to production of adenosine triphosphate (i.e. energy) of the cells. The intervertebral disc is composed of two major anatomical regions: annulus fibrosus and nucleus pulposus, which are structurally and compositionally different, indicating that their cellular metabolisms may also be distinct. The objective of this study was to investigate energy metabolism of annulus fibrosus and nucleus pulposus cells with and without dynamic compression, and examine differences between the two cell types. Porcine annulus and nucleus tissues were harvested and enzymatically digested. Cells were isolated and embedded into agarose constructs. Dynamically loaded samples were subjected to a sinusoidal displacement at 2 Hz and 15% strain for 4 h. Energy metabolism of cells was analyzed by measuring adenosine triphosphate content and release, glucose consumption, and lactate/nitric oxide production. A comparison of those measurements between annulus and nucleus cells was conducted. Annulus and nucleus cells exhibited different metabolic pathways. Nucleus cells had higher adenosine triphosphate content with and without dynamic loading, while annulus cells had higher lactate production and glucose consumption. Compression increased adenosine triphosphate release from both cell types and increased energy production of annulus cells. Dynamic loading affected energy metabolism of intervertebral disc cells, with the effect being greater in annulus cells.

Enhancement of matrix production and cell proliferation in human annulus cells under bioreactor culture

Tissue engineering is a promising approach for treatment of disc degeneration. Herein, we evaluated effects of rotating bioreactor culture on the extracellular matrix production and proliferation of human annulus fibrosus (AF) cells. AF cells were embedded into alginate beads, and then cultured up to 3 weeks in a rotating wall vessel bioreactor or a static vessel. By real-time reverse transcription-polymerase chain reaction, expression of aggrecan, collagen type I and type II, and collagen prolyl 4-hydroxylase II was remarkably elevated, whereas expression of matrix metalloproteinase 3 and a disintegrin and metalloproteinase with thrombospondin motifs 5 was significantly decreased under bioreactor. Biochemical analysis revealed that the levels of the whole cell-associated proteoglycan and collagen were approximately five- and twofolds in rotating bioreactor, respectively, compared to those in static culture. Moreover, AF cell proliferation was augmented in rotating bioreactor. DNA contents were threefolds higher in rotating bioreactor than that in static culture. Expression of the proliferating cell nuclear antigen was robustly enhanced in rotating bioreactor as early as 1 week. Our findings suggested that rotating bioreactor culture would be an effective technique for expansion of human annulus cells for tissue engineering driven treatment of disc degeneration.

Structured coculture of stem cells and disc cells prevent disc degeneration in a rat model

BACKGROUND CONTEXT

Harnessing the potential of stem cells is an important strategy for regenerative medicine. This study explores the use of bilaminar coculture pellets (BCPs) of mesenchymal stem cells (MSCs) and nucleus pulposus cells (NPCs) as a cell-based therapy for intervertebral disc regeneration. Prior in vitro experiments have shown that BCP can help differentiate MSCs and substantially improve new matrix deposition.

PURPOSE

To evaluate the clinical relevance of BCPs by testing the system in vivo.

STUDY DESIGN/SETTING

We have designed a novel spherical BCP where MSCs are enclosed in a shell of NPCs. The pellets were tested in vivo in a rat tail model of disc degeneration.

METHODS

Rat caudal intervertebral discs were denucleated and treated with BCP in a fibrin sealant (FS) carrier (controls were MSCs suspended in FS; NPCs suspended in FS; MSCs and NPCs suspended in FS; FS only; and surgery only). At 14 and 35 days after implantation, the animals were euthanized and discs were evaluated for proteoglycan content, enzyme-linked immunosorbent assay for inflammatory cytokines, cell retention using polymerase chain reaction, disc height, histology, and disc grade based on a blinded scoring system.

RESULTS

The proteoglycan and cytokine levels were not significantly different among groups. The BCP group had higher cell retention than controls. Disc height and disc grade increased over time only in the BCP group. Bilaminar coculture pellets were the only treatment to show proteoglycan staining in the nucleus space at 35 days.

CONCLUSIONS

This study shows that BCPs may prevent postnucleotomy disc degeneration in vivo. Larger animals and longer time points will be necessary to further judge potential clinical impact. As opposed to strategies that require growth factor supplements, predifferentiation, or genetic manipulations, BCPs are a self-sustaining and targeted method for tissue regeneration in situ.

Degeneration and regeneration of the intervertebral disc: lessons from development

Degeneration of the intervertebral discs, a process characterized by a cascade of cellular, biochemical, structural and functional changes, is strongly implicated as a cause of low back pain. Current treatment strategies for disc degeneration typically address the symptoms of low back pain without treating the underlying cause or restoring mechanical function. A more in-depth understanding of disc degeneration, as well as opportunities for therapeutic intervention, can be obtained by considering aspects of intervertebral disc development. Development of the intervertebral disc involves the coalescence of several different cell types through highly orchestrated and complex molecular interactions. The resulting structures must function synergistically in an environment that is subjected to continuous mechanical perturbation throughout the life of an individual. Early postnatal changes, including altered cellularity, vascular regression and altered extracellular matrix composition, might set the disc on a slow course towards symptomatic degeneration. In this Perspective, we review the pathogenesis and treatment of intervertebral disc degeneration in the context of disc development. Within this scope, we examine how model systems have advanced our understanding of embryonic morphogenesis and associated molecular signaling pathways, in addition to the postnatal changes to the cellular, nutritional and mechanical microenvironment. We also discuss the current status of biological therapeutic strategies that promote disc regeneration and repair, and how lessons from development might provide clues for their refinement.

Pulsed electromagnetic field stimulates cellular proliferation in human intervertebral disc cells

PURPOSE

The purpose of this study is to investigate the mechanism of cellular proliferation of electromagnetic field (EMF) on human intervertebral disc (IVD) cells.

MATERIALS AND METHODS

Human IVD cells were cultured three-dimensionally in alginate beads. EMF was exposed to IVD cells with 650 Ω, 1.8 millitesla magnetic flux density, 60 Hz sinusoidal wave. Cultures were divided into a control and EMF group. Cytotoxicity, DNA synthesis and proteoglycan synthesis were measured by MTT assay, [(3)H]-thymidine, and [(35)S]-sulfate incorporation. To detect phenotypical expression, reverse transcription-polymerase chain reactions (RT-PCR) were performed for aggrecan, collagen type I, and type II mRNA expression. To assess action mechanism of EMF, IVD cells were exposed to EMF with N(G)-Monomethyl-L-arginine (NMMA) and acetylsalicylic acid (ASA).

RESULTS

There was no cytotoxicity in IVD cells with the EMF group in MTT assay. Cellular proliferation was observed in the EMF group (p < 0.05). There was no difference in newly synthesized proteoglycan normalized by DNA synthesis between the EMF group and the control. Cultures with EMF showed no significant change in the expression of aggrecan, type I, and type II collagen mRNA compared to the control group. Cultures with NMMA (blocker of nitric oxide) or ASA (blocker of prostaglandin E2) exposed to EMF demonstrated decreased DNA synthesis compared to control cultures without NMMA or ASA (p < 0.05).

CONCLUSION

EMF stimulated DNA synthesis in human IVD cells while no significant effect on proteoglycan synthesis and chondrogenic phenotype expressions. DNA synthesis was partially mediated by nitric oxide and prostaglandin E2. EMF can be utilized to stimulate proliferation of IVD cells, which may provide efficient cell amplification in cell therapy to degenerative disc disease.

The response of annulus fibrosus cell to fibronectin-coated nanofibrous polyurethane-anionic dihydroxyoligomer scaffolds

Tissue engineering of the annulus fibrosus(AF), a component of the intervertebral disc, has proven to be challenging due to its complex oriented lamellar structure. Previously it was demonstrated that polyurethane (PU) scaffolds containing an anionic dihydroxy oligomers (ADO) may be suitable to use in this application. The current study examines whether matrix protein(s) coatings (collagen type I, collagen type I and fibronectin, fibronectin, or vitronectin) would promote cell and collagen orientation that more closely mimics native AF. The greatest cell attachment occurred when scaffolds were pre-coated with Fn. Cells on Fn-coated scaffolds were aligned parallel to scaffold fibers, a process that involved α5β1 integrin, as determined by integrin-specific blocking antibodies, which in turn reduced AF cell spreading and alignment. Cell shape was regulated by the actin cytoskeleton as cells grown in the presence of cytochalasin D did not spread. Cells on Fn-coated PU scaffolds formed fibrillar Fn, synthesized significantly more collagen, and showed linear alignment of the secreted type I collagen when compared to cells grown on the other protein-coated scaffolds and the non-coated control. Thus Fn-coating of PU-ADO scaffolds appears to promote properly oriented AF cells and collagen, which should facilitate developing AF tissue that more closely mimics the native tissue.

Mechanical design criteria for intervertebral disc tissue engineering

Due to the inability of current clinical practices to restore function to degenerated intervertebral discs, the arena of disc tissue engineering has received substantial attention in recent years. Despite tremendous growth and progress in this field, translation to clinical implementation has been hindered by a lack of well-defined functional benchmarks. Because successful replacement of the disc is contingent upon replication of some or all of its complex mechanical behaviors, it is critically important that disc mechanics be well characterized in order to establish discrete functional goals for tissue engineering. In this review, the key functional signatures of the intervertebral disc are discussed and used to propose a series of native tissue benchmarks to guide the development of engineered replacement tissues. These benchmarks include measures of mechanical function under tensile, compressive, and shear deformations for the disc and its substructures. In some cases, important functional measures are identified that have yet to be measured in the native tissue. Ultimately, native tissue benchmark values are compared to measurements that have been made on engineered disc tissues, identifying where functional equivalence was achieved, and where there remain opportunities for advancement. Several excellent reviews exist regarding disc composition and structure, as well as recent tissue engineering strategies; therefore this review will remain focused on the functional aspects of disc tissue engineering.

Construction of collagen II/hyaluronate/chondroitin-6-sulfate tri-copolymer scaffold for nucleus pulposus tissue engineering and preliminary analysis of its physico-chemical properties and biocompatibility

To construct a novel scaffold for nucleus pulposus (NP) tissue engineering, The porous type II collagen (CII)/hyaluronate (HyA)-chondroitin-6-sulfate (6-CS) scaffold was prepared using 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and N-hydroxysuccinimide (NHS) cross-linking system. The physico-chemical properties and biocompatibility of CII/HyA-CS scaffolds were evaluated. The results suggested CII/HyA-CS scaffolds have a highly porous structure (porosity: 94.8 +/- 1.5%), high water-binding capacity (79.2 +/- 2.8%) and significantly improved mechanical stability by EDC/NHS crosslinking (denaturation temperature: 74.6 +/- 1.8 and 58.1 +/- 2.6 degrees C, respectively, for the crosslinked scaffolds and the non-crosslinked; collagenase degradation rate: 39.5 +/- 3.4 and 63.5 +/- 2.0%, respectively, for the crosslinked scaffolds and the non-crosslinked). The CII/HyA-CS scaffolds also showed satisfactory cytocompatibility and histocompatibility as well as low immunogenicity. These results indicate CII/HyA-CS scaffolds may be an alternative material for NP tissue engineering due to the similarity of its composition and physico-chemical properties to those of the extracellular matrices (ECM) of native NP.