The potential and limitations of a cell-seeded collagen/hyaluronan scaffold to engineer an intervertebral disc-like matrix

Extrusion-Based 3D Bioprinting of Bioactive and Piezoelectric Scaffolds as Potential Therapy for Treating Critical Soft Tissue Wounds

Objective: This study focuses on developing bioactive piezoelectric scaffolds that could deliver bioelectrical cues to potentially treat injuries to soft tissues such as skeletal muscles and promote active regeneration. Approach: To address the underexplored aspect of bioelectrical cues in skeletal muscle tissue engineering (SMTE), we developed piezoelectric bioink based on natural bioactive materials such as sodium alginate, gelatin, and chitosan. Extrusion-based 3D bioprinting was utilized to develop scaffolds that mimic muscle stiffness and generate electrical stimulation (E-stim) when subjected to forces. The biocompatibility of these scaffolds was tested with the C2C12 muscle cell line. Results: The bioink demonstrated suitable rheological properties for 3D bioprinting, resulting in high-resolution composite sodium alginate-gelatin-chitosan scaffolds with good structural fidelity. The scaffolds exhibited a 42-60 kPa stiffness, similar to muscle. When a controlled force of 5N was applied to the scaffolds at a constant frequency of 4 Hz, they generated electrical fields and impulses (charge), indicating their suitability as a stand-alone scaffold to generate E-stim and instill bioelectrical cues in the wound region. The cell viability and proliferation test results confirm the scaffold's biocompatibility with C2C12s and the benefit of piezoelectricity in promoting muscle cell growth kinetics. Our study indicates that our piezoelectric bioink and scaffolds offer promise as autonomous E-stim-generating regenerative therapy for SMTE. Innovation: A novel approach for treating skeletal muscle wounds was introduced by developing a bioactive electroactive scaffold capable of autonomously generating E-stim without stimulators and electrodes. This scaffold offers a unique approach to enhancing skeletal muscle regeneration through bioelectric cues, addressing a major gap in the SMTE, that is, fibrotic tissue formation due to delayed muscle regeneration. Conclusion: A piezoelectric scaffold was developed, providing a promising solution for promoting skeletal muscle regeneration. This development can potentially address skeletal muscle injuries and offers a unique approach to facilitating skeletal muscle wound healing.

Orthobiologic Management Options for Degenerative Disc Disease

Degenerative disc disease (DDD) is a pervasive condition that limits quality of life and burdens economies worldwide. Conventional pharmacological treatments primarily aimed at slowing the progression of degeneration have demonstrated limited long-term efficacy and often do not address the underlying causes of the disease. On the other hand, orthobiologics are regenerative agents derived from the patient's own tissue and represent a promising emerging therapy for degenerative disc disease. This review comprehensively outlines the pathophysiology of DDD, highlighting the inadequacies of existing pharmacological therapies and detailing the potential of orthobiologic approaches. It explores advanced tools such as platelet-rich plasma and mesenchymal stem cells, providing a historical overview of their development within regenerative medicine, from foundational in vitro studies to preclinical animal models. Moreover, the manuscript delves into clinical trials that assess the effectiveness of these therapies in managing DDD. While the current clinical evidence is promising, it remains insufficient for routine clinical adoption due to limitations in study designs. The review emphasizes the need for further research to optimize these therapies for consistent and effective clinical outcomes, potentially revolutionizing the management of DDD and offering renewed hope for patients.

Shape-memory collagen scaffold combined with hyaluronic acid for repairing intervertebral disc

BACKGROUND

Intervertebral disc degeneration (IVDD) is a common cause of chronic low back pain (LBP) and a socioeconomic burden worldwide. Conservative therapies and surgical treatments provide only symptomatic pain relief without promoting intervertebral disc (IVD) regeneration. Therefore, the clinical demand for disc regenerative therapies for disc repair is high.

METHODS

In this study, we used a rat tail nucleotomy model to develop mechanically stable collagen-cryogel and fibrillated collagen with shape-memory for use in minimally invasive surgery for effective treatment of IVDD. The collagen was loaded with hyaluronic acid (HA) into a rat tail nucleotomy model.

RESULTS

The shape-memory collagen structures exhibited outstanding chondrogenic activities, having completely similar physical properties to those of a typical shape-memory alginate construct in terms of water absorption, compressive properties, and shape-memorability behavior. The treatment of rat tail nucleotomy model with shape-memory collagen-cryogel/HA alleviated mechanical allodynia, maintained a higher concentration of water content, and preserved the disc structure by restoring the matrix proteins.

CONCLUSION

According to these results, the collagen-based structure could effectively repair and maintain the IVD matrix better than the controls, including HA only and shape-memory alginate with HA.

Intervertebral Disc Tissue Engineering Using Additive Manufacturing

Intervertebral disc (IVD) degeneration is one of the major causes of lower back pain, a common health condition that greatly affects the quality of life. With an increasing elderly population and changes in lifestyle, there exists a high demand for novel treatment strategies for damaged IVDs. Researchers have investigated IVD tissue engineering (TE) as a way to restore biological and mechanical functions by regenerating or replacing damaged discs using scaffolds with suitable cells. These scaffolds can be constructed using material extrusion additive manufacturing (AM), a technique used to build three-dimensional (3D), custom discs utilising computer-aided design (CAD). Structural geometry can be controlled via the manipulation of printing parameters, material selection, temperature, and various other processing parameters. To date, there are no clinically relevant TE-IVDs available. In this review, advances in AM-based approaches for IVD TE are briefly discussed in order to achieve a better understanding of the requirements needed to obtain more effective, and ultimately clinically relevant, IVD TE constructs.

Optimized, visible light-induced crosslinkable hybrid gelatin/hyaluronic acid scaffold promotes complete spinal cord injury repair

Severe microenvironmental changes after spinal cord injury (SCI) present serious challenges in neural regeneration and tissue repair. Gelatin (GL)- and hyaluronic acid (HA)-based hydrogels are attractive scaffolds because they are major components of the extracellular matrix and can provide a favorable adjustable microenvironment for neurogenesis and motor function recovery. In this study, three-dimensional hybrid GL/HA hydrogel scaffolds were prepared and optimized. The hybrid hydrogels could undergo in-situ gelation and fit the defects perfectly via visible light- induced crosslinking in the complete SCI rats. We found that the transplantation of the hybrid hydrogel scaffold significantly reduced the inflammatory responses and suppressed glial scar formation in an HA concentration-dependent manner. Moreover, the hybrid hydrogel with GL/HA ratios less than 8/2 effectively promoted endogenous neural stem cell migration and neurogenesis, as well as improved neuron maturation and axonal regeneration. The results showed locomotor function improved 60 days after transplantation, thus suggesting that GL/HA hydrogels can be considered as a promising scaffold for complete SCI repair.

Advanced Strategies for the Regeneration of Lumbar Disc Annulus Fibrosus

Damage to the annulus fibrosus (AF), the outer region of the intervertebral disc (IVD), results in an undesirable condition that may accelerate IVD degeneration causing low back pain. Despite intense research interest, attempts to regenerate the IVD have failed so far and no effective strategy has translated into a successful clinical outcome. Of particular significance, the failure of strategies to repair the AF has been a major drawback in the regeneration of IVD and nucleus replacement. It is unlikely to secure regenerative mediators (cells, genes, and biomolecules) and artificial nucleus materials after injection with an unsealed AF, as IVD is exposed to significant load and large deformation during daily activities. The AF defects strongly change the mechanical properties of the IVD and activate catabolic routes that are responsible for accelerating IVD degeneration. Therefore, there is a strong need to develop effective therapeutic strategies to prevent or reconstruct AF damage to support operational IVD regenerative strategies and nucleus replacement. By the way of this review, repair and regenerative strategies for AF reconstruction, their current status, challenges ahead, and future outlooks were discussed.

Intervertebral disc degeneration in mice with type II diabetes induced by leptin receptor deficiency

Background

The leptin receptor-deficient knockout (db/db) mouse is a well-established model for studying type II diabetes mellitus (T2DM). T2DM is an important risk factor of intervertebral disc degeneration (IVDD). Although the relationship between type I diabetes and IVDD has been reported by many studies, few studies have reported the effects of T2DM on IVDD in db/db mice model.

Methods

Mice were separated into 3 groups: wild-type (WT), db/db, and IGF-1 groups (leptin receptor-deficient mice were treated with insulin-like growth factor-1 (IGF-1). To observe the effects of T2DM and glucose-lowering treatment on IVDD, IGF-1 injection was used. The IVD phenotype was detected by H&E and safranin O fast green staining among db/db, WT and IGF-1 mice. The levels of blood glucose and weight in mice were also recorded. The changes in the mass of the trabecular bone in the fifth lumbar vertebra were documented by micro-computed tomography (micro-CT). Tunnel assays were used to detect cell apoptosis in each group.

Results

The weight of the mice were 27.68 ± 1.6 g in WT group, which was less than 57.56 ± 4.8 g in db/db group, and 52.17 ± 3.7 g in IGF-1 injected group (P < 0.05). The blood glucose levels were also significantly higher in the db/db mice group. T2DM caused by leptin receptor knockout showed an association with significantly decreased vertebral bone mass and increased IVDD when compared to WT mice. The db/db mice induced by leptin deletion showed a higher percentage of MMP3 expression as well as cell apoptosis in IVDD mice than WT mice (P < 0.05), while IGF-1 treatment reversed this situation (P < 0.05).

Conclusions

T2DM induced by leptin receptor knockout led to IVDD by increasing the levels of MMP3 and promoting cell apoptosis. IGF-1 treatment partially rescue the phenotype of IVDD induced by leptin receptor knockout.

Oligonucleotide-functionalized hydrogels for sustained release of small molecule (aptamer) therapeutics

Natural and synthetic hydrogels have been widely investigated as biomaterial scaffolds to promote tissue repair and regeneration. Nevertheless, the scaffold alone is often insufficient to drive new tissue growth, instead requiring continuous delivery of therapeutics, such as proteins or other biomolecules that work in concert with structural support provided by the scaffold. However, because of the high-water content, hydrogels tend to be permeable and cause rapid release of the encapsulated drug, which could lead to serious complications from local overdose and may result in the significant waste of encapsulated therapeutic(s). To this end, we designed an oligonucleotide-functionalized hydrogel that can provide sustained and controlled delivery of therapeutics for up to 4 weeks. To prove this concept, we successfully achieved sustained release (for over 28 days) of model anti-Nogo receptor (anti-NgR) RNA aptamer from oligonucleotide-functionalized hyaluronic acid-based hydrogel by changing the complementarity between the short antisense sequences and the aptamer. Furthermore, the released aptamer successfully blocked neuro-inhibitory effects of myelin-derived inhibitors and promoted neurite outgrowth from rat dorsal root ganglia in vitro. Because antisense sequences can be designed to bind to proteins, peptides, and aptamer, our oligonucleotide-functionalized hydrogel offers a promising therapeutic delivery system to obtain controlled release (both bolus and sustained) of various therapeutics for the treatment of complex diseases and injury models, such as spinal cord injury. STATEMENT OF SIGNIFICANCE: Producing a therapeutic effect often requires the administration of multiple injections with high dosages. This regimen causes discomfort to the patient and raises cost of treatment. Additionally, systemic delivery of therapeutics often results in adverse effects; therefore, local delivery at the site of injury is desirable. Therefore, in this study, we designed an oligonucleotide-functionalized biomaterial platform using ssDNA oligonucleotides (immobile species) as antisense sequences to increase residence time and fine-tune the release of anti-nogo receptor aptamer (mobile species) for spinal cord injury application. Because antisense sequences can be designed to bind proteins, peptides, and aptamer, our hydrogel offers a promising delivery system to obtain controlled release of various therapeutics for the treatment of complex diseases and injury models.

Functional cell phenotype induction with TGF-β1 and collagen-polyurethane scaffold for annulus fibrosus rupture repair

Appropriate cell sources, bioactive factors and biomaterials for generation of functional and integrated annulus fibrosus (AF) tissue analogues are still an unmet need. In the present study, the AF cell markers, collagen type I, cluster of differentiation 146 (CD146), mohawk (MKX) and smooth muscle protein 22α (SM22α) were found to be suitable indicators of functional AF cell induction. In vitro 2D culture of human AF cells showed that transforming growth factor β1 (TGF-β1) upregulated the expression of the functional AF markers and increased cell contractility, indicating that TGF-β1-pre-treated AF cells were an appropriate cell source for AF tissue regeneration. Furthermore, a tissue engineered construct, composed of polyurethane (PU) scaffold with a TGF-β1-supplemented collagen type I hydrogel and human AF cells, was evaluated with in vitro 3D culture and ex vivo preclinical bioreactor-loaded organ culture models. The collagen type I hydrogel helped maintaining the AF functional phenotype. TGF-β1 supplement within the collagen I hydrogel further promoted cell proliferation and matrix production of AF cells within in vitro 3D culture. In the ex vivo IVD organ culture model with physiologically relevant mechanical loading, TGF-β1 supplement in the transplanted constructs induced the functional AF cell phenotype and enhanced collagen matrix synthesis. In conclusion, TGF-β1-containing collagen-PU constructs can induce the functional cell phenotype of human AF cells in vitro and in situ. This combined cellular, biomaterial and bioactive agent therapy has a great potential for AF tissue regeneration and rupture repair.

Decellularised nucleus pulposus as a potential biologic scaffold for disc tissue engineering

Intervertebral disc (IVD) degeneration is associated with lower back pain, with the dysfunction of nucleus pulposus (NP) cells instigating degeneration onset. Here, we developed an optimized decellularised NP scaffold that could induce mesenchymal stem cells (MSCs) into NP-like cells in vitro and rescue the degenerated IVD in vivo. We optimized a decellularisation protocol for porcine NP and evaluated the biological properties and microstructure of the NP scaffold. Through co-culture with MSCs, we analysed scaffold bioactivity and potential signalling pathways. We tested the therapeutic efficacy of the scaffold using an IVD degeneration model in vivo. The decellularisation protocol generally removed the cellular components of the NP and preserved the majority of the biological components and regular microstructure. MSCs seeded in the NP-ECM scaffold differentiated into NP-like cells in vitro; this change was attributed to activation of the TGF-β signalling pathway. The NP-ECM exhibited good cytocompatibility ex vivo and decelerated the degeneration of the IVD in vivo. These results indicate the successful establishment of a naturally-derived ECM material that could induce MSCs into NP cells and serve as a potential treatment for degenerated IVDs.

Polyurethane as a strategy for annulus fibrosus repair and regeneration: a systematic review

Aim: Disc herniation is a spine disease that leads to suffering and disability. Discectomy is a Janus-faced approach that relieves pain symptoms but leave the intervertebral discs predisposed to herniation. This systematic review discussed the mechanical and biological requirements for a polyurethane-based biomaterial to be used in annular repair. Methods: Search strategy was performed in PubMed, Web of Science and Scopus databases to define the main mechanical properties, biological findings and follow-up aspects of these biomaterials. The range was limited to articles published from January 2000 to December 2017 in English language. Results: The search identified 82 articles. From these, a total of 18 articles underwent a full-text analysis, and 16 studies were included in the review. Conclusion: The polyurethane presents suitable properties to be used as an engineered solution to re-establish the microenvironment and biomechanical features of the intervertebral disc.

Dental pulp stem cell-derived chondrogenic cells demonstrate differential cell motility in type I and type II collagen hydrogels

BACKGROUND CONTEXT

Advances in the development of biomaterials and stem cell therapy provide a promising approach to regenerating degenerated discs. The normal nucleus pulposus (NP) cells exhibit similar phenotype to chondrocytes. Because dental pulp stem cells (DPSCs) can be differentiated into chondrogenic cells, the DPSCs and DPSCs-derived chondrogenic cells encapsulated in type I and type II collagen hydrogels can potentially be transplanted into degenerated NP to repair damaged tissue. The motility of transplanted cells is critical because the cells need to migrate away from the hydrogels containing the cells of high density and disperse through the NP tissue after implantation.

PURPOSE

The purpose of this study was to determine the motility of DPSC and DPSC-derived chondrogenic cells in type I and type II collagen hydrogels.

STUDY DESIGN/SETTING

The time lapse imaging that recorded cell migration was analyzed to quantify the cell migration velocity and distance.

METHODS

The cell viability of DPSCs in native or poly(ethylene glycol) ether tetrasuccinimidyl glutarate (4S-StarPEG)-crosslinked type I and type II collagen hydrogels was determined using LIVE/DEAD cell viability assay and AlamarBlue assay. DPSCs were differentiated into chondrogenic cells. The migration of DPSCs and DPSC-derived chondrogenic cells in these hydrogels was recorded using a time lapse imaging system. This study was funded by the Regional Institute on Aging and Wichita Medical Research and Education Foundation, and the authors declare no competing interest.

RESULT

DPSCs showed high cell viability in non-crosslinked and crosslinked collagen hydrogels. DPSCs migrated in collagen hydrogels, and the cell migration speed was not significantly different in either type I collagen or type II collagen hydrogels. The migration speed of DPSC-derived chondrogenic cells was higher in type I collagen hydrogel than in type II collagen hydrogel. Crosslinking of type I collagen with 4S-StarPEG significantly reduced the cell migration speed of DPSC-derived chondrogenic cells.

CONCLUSIONS

After implantation of collagen hydrogels encapsulating DPSCs or DPSC-derived chondrogenic cells, the cells can potentially migrate from the hydrogels and migrate into the NP tissue. This study also explored the differential cell motility of DPSCs and DPSC-derived chondrogenic cells in these collagen hydrogels.

Decellularized allogeneic intervertebral disc: natural biomaterials for regenerating disc degeneration

Intervertebral disc degeneration is associated with back pain and disc herniation. This study established a modified protocol for intervertebral disc (IVD) decellularization and prepared its extracellular matrix (ECM). By culturing mesenchymal stem cells (MSCs)(3, 7, 14 and 21 days) and human degenerative IVD cells (7 days) in the ECM, implanting it subcutaneously in rabbit and injecting ECM microparticles into degenerative disc, the biological safety and efficacy of decellularized IVD was evaluated both in vitro and in vivo. Here, we demonstrated that cellular components can be removed completely after decellularization and maximally retain the structure and biomechanics of native IVD. We revealed that allogeneic ECM did not evoke any apparent inflammatory reaction in vivo and no cytotoxicity was found in vitro. Moreover, IVD ECM can induce differentiation of MSCs into IVD-like cells in vitro. Furthermore, allogeneic ECM microparticles are effective on the treatment of rabbit disc degeneration in vivo. In conclusion, our study developed an optimized method for IVD decellularization and we proved decellularized IVD is safe and effective for the treatment of degenerated disc diseases.

Vitamin D improves the content of TGF-β and IGF-1 in intervertebral disc of diabetic rats

OBJECTIVE

The contents of transforming growth factor-β and insulin-like growth factor-1 in disc of diabetic rats were measured at three different periods after injected with 1,25-Dihydroxyvitamin D3, and compared with that in normal rats. The significance of content changes was also discussed.

METHODS

Fourty-five Sprague-Dawley (SD) rats were divided into three groups, namely the experimental group (STZ+calcitriol), control group (STZ+citrate buffer), and normal group (citrate buffer). Complete lumbar discs in these groups were obtained at the second, fourth, sixth week, respectively. After paraffin-embedded sections and HE staining, the structure and morphology changes of disc were observed. The content of transforming growth factor-β and insulin-like growth factor-1 was measured by immunohistochemical method, and the expression of transforming growth factor-β and insulin-like growth factor-1 was detected by Western Blot.

RESULTS

In hematoxylin-eosin staining, degenerative changes were observed in disc of experimental and control group at three different periods, and there were no changes in disc in normal group. Immunohistochemical method indicated the content of transforming growth factor-β and insulin-like growth factor-1 in experimental and control group was significantly lower than normal group at three different periods ( P < 0.05). And there were significant differences between experimental and control group at three different periods ( P < 0.05).

CONCLUSION

Vitamin D can protect the degeneration of intervertebral disc and improve the content of transforming growth factor-β and insulin-like growth factor-1 in the intervertebral disc, which provides a new idea for the prevention and treatment of degenerative changes of the intervertebral disc in diabetic patients. Impact statement No researchers reported Vitamin D could protect degeneration of intervertebral disc. That is to say, we found a new method to prevent and treat degenerative changes of the intervertebral disc in diabetic patients. And Vitamin D prevented the discs by improving the content of TGF-β and IGF-1.

Construction Strategy and Progress of Whole Intervertebral Disc Tissue Engineering

Degenerative disc disease (DDD) is the major cause of low back pain, which usually leads to work absenteeism, medical visits and hospitalization. Because the current conservative procedures and surgical approaches to treatment of DDD only aim to relieve the symptoms of disease but not to regenerate the diseased disc, their long-term efficiency is limited. With the rapid developments in medical science, tissue engineering techniques have progressed markedly in recent years, providing a novel regenerative strategy for managing intervertebral disc disease. However, there are as yet no ideal methods for constructing tissue-engineered intervertebral discs. This paper reviews published reports pertaining to intervertebral disc tissue engineering and summarizes data concerning the seed cells and scaffold materials for tissue-engineered intervertebral discs, construction of tissue-engineered whole intervertebral discs, relevant animal experiments and effects of mechanics on the construction of tissue-engineered intervertebral disc and outlines the existing problems and future directions. Although the perfect regenerative strategy for treating DDD has not yet been developed, great progress has been achieved in the construction of tissue-engineered intervertebral discs. It is believed that ongoing research on intervertebral disc tissue engineering will result in revolutionary progress in the treatment of DDD.

Bone morphogenetic protein-2 provokes interleukin-18-induced human intervertebral disc degeneration

OBJECTIVES

Interleukin 18 (IL-18) is a regulatory cytokine that degrades the disc matrix. Bone morphogenetic protein-2 (BMP-2) stimulates synthesis of the disc extracellular matrix. However, the combined effects of BMP-2 and IL-18 on human intervertebral disc degeneration have not previously been reported. The aim of this study was to investigate the effects of the anabolic cytokine BMP-2 and the catabolic cytokine IL-18 on human nucleus pulposus (NP) and annulus fibrosus (AF) cells and, therefore, to identify potential therapeutic and clinical benefits of recombinant human (rh)BMP-2 in intervertebral disc degeneration.

METHODS

Levels of IL-18 were measured in the blood of patients with intervertebral disc degenerative disease and in control patients. Human NP and AF cells were cultured in a NP cell medium and treated with IL-18 or IL-18 plus BMP-2. mRNA levels of target genes were measured by real-time polymerase chain reaction, and protein levels of aggrecan, type II collagen, SOX6, and matrix metalloproteinase 13 (MMP13) were assessed by western blot analysis.

RESULTS

The serum level of patients (IL-18) increased significantly with the grade of IVD degeneration. There was a dramatic alteration in IL-18 level between the advanced degeneration (Grade III to V) group and the normal group (p = 0.008) Furthermore, IL-18 induced upregulation of the catabolic regulator MMP13 and downregulation of the anabolic regulators aggrecan, type II collagen, and SOX6 at 24 hours, contributing to degradation of disc matrix enzymes. However, BMP-2 antagonised the IL-18 induced upregulation of aggrecan, type II collagen, and SOX6, resulting in reversal of IL-18 mediated disc degeneration.

CONCLUSIONS

BMP-2 is anti-catabolic in human NP and AF cells, and its effects are partially mediated through provocation of the catabolic effect of IL-18. These findings indicate that BMP-2 may be a unique therapeutic option for prevention and reversal of disc degeneration.Cite this article: S. Ye, B. Ju, H. Wang, K-B. Lee. Bone morphogenetic protein-2 provokes interleukin-18-induced human intervertebral disc degeneration. Bone Joint Res 2016;5:412-418. DOI: 10.1302/2046-3758.59.BJR-2016-0032.R1.

Hyaluronic acid scaffold has a neuroprotective effect in hemisection spinal cord injury

OBJECTIVE

Spinal cord injury occurs in 2 phases. The initial trauma is followed by inflammation that leads to fibrous scar tissue, glial scarring, and cavity formation. Scarring causes further axon death around and above the injury. A reduction in secondary injury could lead to functional improvement. In this study, hyaluronic acid (HA) hydrogels were implanted into the gap formed in the hemisected spinal cord of Sprague-Dawley rats in an attempt to attenuate damage and regenerate tissue.

METHODS

A T-10 hemisection spinal cord injury was created in adult male Sprague-Dawley rats; the rats were assigned to a sham, control (phosphate-buffered saline), or HA hydrogel–treated group. One cohort of 23 animals was followed for 12 weeks and underwent weekly behavioral assessments. At 12 weeks, retrograde tracing was performed by injecting Fluoro-Gold in the left L-2 gray matter. At 14 weeks, the animals were killed. The volume of the lesion and the number of cells labeled from retrograde tracing were calculated. Animals in a separate cohort were killed at 8 or 16 weeks and perfused for immunohistochemical analysis and transmission electron microscopy. Samples were stained using H & E, neurofilament stain (neurons and axons), silver stain (disrupted axons), glial fibrillary acidic protein stain (astrocytes), and Iba1 stain (mononuclear cells).

RESULTS

The lesions were significantly smaller in size and there were more retrograde-labeled cells in the red nuclei of the HA hydrogel–treated rats than in those of the controls; however, the behavioral assessments revealed no differences between the groups. The immunohistochemical analyses revealed decreased fibrous scarring and increased retention of organized intact axonal tissue in the HA hydrogel–treated group. There was a decreased presence of inflammatory cells in the HA hydrogel–treated group. No axonal or neuronal regeneration was observed.

CONCLUSIONS

The results of these experiments show that HA hydrogel had a neuroprotective effect on the spinal cord by decreasing the magnitude of secondary injury after a lacerating spinal cord injury. Although regeneration and behavioral improvement were not observed, the reduction in disorganized scar tissue and the retention of neurons near and above the lesion are important for future regenerative efforts. In addition, this gel would be useful as the base substrate in the development of a more complex scaffold.

Intervertebral Disc Tissue Engineering with Natural Extracellular Matrix-Derived Biphasic Composite Scaffolds

Tissue engineering has provided an alternative therapeutic possibility for degenerative disc diseases. However, we lack an ideal scaffold for IVD tissue engineering. The goal of this study is to fabricate a novel biomimetic biphasic scaffold for IVD tissue engineering and evaluate the feasibility of developing tissue-engineered IVD in vitro and in vivo. In present study we developed a novel integrated biphasic IVD scaffold using a simple freeze-drying and cross-linking technique of pig bone matrix gelatin (BMG) for the outer annulus fibrosus (AF) phase and pig acellular cartilage ECM (ACECM) for the inner nucleus pulposus (NP) phase. Histology and SEM results indicated no residual cells remaining in the scaffold that featured an interconnected porous microstructure (pore size of AF and NP phase 401.4 ± 13.1 μm and 231.6 ± 57.2 μm, respectively). PKH26-labeled AF and NP cells were seeded into the scaffold and cultured in vitro. SEM confirmed that seeded cells could anchor onto the scaffold. Live/dead staining showed that live cells (green fluorescence) were distributed in the scaffold, with no dead cells (red fluorescence) being found. The cell-scaffold constructs were implanted subcutaneously into nude mice and cultured for 6 weeks in vivo. IVD-like tissue formed in nude mice as confirmed by histology. Cells in hybrid constructs originated from PKH26-labeled cells, as confirmed by in vivo fluorescence imaging system. In conclusion, the study demonstrates the feasibility of developing a tissue-engineered IVD in vivo with a BMG- and ACECM-derived integrated AF-NP biphasic scaffold. As well, PKH26 fluorescent labeling with in vivo fluorescent imaging can be used to track cells and analyse cell--scaffold constructs in vivo.

Delivery systems for the treatment of degenerated intervertebral discs

The intervertebral disc (IVD) is the most avascular and acellular tissue in the body and therefore prone to degeneration. During IVD degeneration, the balance between anabolic and catabolic processes in the disc is deregulated, amongst others leading to alteration of extracellular matrix production, abnormal enzyme activities and production of pro-inflammatory substances like cytokines. The established treatment strategy for IVD degeneration consists of physiotherapy, pain medication by drug therapy and if necessary surgery. This approach, however, has shown limited success. Alternative strategies to increase and prolong the effects of bioactive agents and to reverse the process of IVD degeneration include the use of delivery systems for drugs, proteins, cells and genes. In view of the specific anatomy and physiology of the IVD and depending on the strategy of the therapy, different delivery systems have been developed which are reviewed in this article.

Current trends in biologics delivery to restore intervertebral disc anabolism

Low back pain is generally attributed to intervertebral disc (IVD) degeneration. This is a multifactorial disease induced by genetic and environmental factors and that progresses with aging. Disc degeneration is characterized by a limited ability of IVD cells to produce functional matrix while producing abnormal amounts of matrix-degrading enzymes. The prolonged imbalance between anabolism and catabolism in degenerative discs alters their composition and hydration. In turn, this results in increased angiogenesis and the loss of the disc's ability to maintain its aneural condition. Inflammation in the IVD, in particular the presence of pro-inflammatory cytokines, was found to favor innervation and also sensitization of the nociceptive pathways, thereby exacerbating degenerative symptoms. In this review, we discuss anti-inflammatory approaches to encounter disc catabolism, potential treatments to lower discogenic pain and pro-anabolic approaches in the form of protein delivery, gene therapy and cell delivery, to trigger regeneration in the IVD.

Screening of hyaluronic acid-poly(ethylene glycol) composite hydrogels to support intervertebral disc cell biosynthesis using artificial neural network analysis

Hyaluronic acid (HA)-poly(ethylene glycol) (PEG) composite hydrogels have been widely studied for both cell delivery and soft tissue regeneration applications. A very broad range of physical and biological properties have been engineered into HA-PEG hydrogels that may differentially affect cellular "outcomes" of survival, synthesis and metabolism. The objective of this study was to rapidly screen multiple HA-PEG composite hydrogel formulations for an effect on matrix synthesis and behaviors of nucleus pulposus (NP) and annulus fibrosus (AF) cells of the intervertebral disc (IVD). A secondary objective was to apply artificial neural network analysis to identify relationships between HA-PEG composite hydrogel formulation parameters and biological outcome measures for each cell type of the IVD. Eight different hydrogels were developed from preparations of thiolated HA (HA-SH) and PEG vinylsulfone (PEG-VS) macromers, and used as substrates for NP and AF cell culture in vitro. Hydrogel mechanical properties ranged from 70 to 489kPa depending on HA molecular weight, and measures of matrix synthesis, metabolite consumption and production and cell morphology were obtained to study relationships to hydrogel parameters. Results showed that NP and AF cell numbers were highest upon the HA-PEG hydrogels formed from the lower-molecular-weight HA, with evidence of higher sulfated glycosaminoglycan production also upon lower-HA-molecular-weight composite gels. All cells formed more multi-cell clusters upon any HA-PEG composite hydrogel as compared to gelatin substrates. Formulations were clustered into neurons based largely on their HA molecular weight, with few effects of PEG molecular weight observed on any measured parameters.

Modulation of annulus fibrosus cell alignment and function on oriented nanofibrous polyurethane scaffolds under tension

BACKGROUND CONTEXT

Annulus fibrosus (AF), a component of the intervertebral disc (IVD), is always under tension in vivo, a condition that must be taken into consideration when tissue engineering an IVD. Loss of the tensile forces has been implicated in the pathogenesis of disc degeneration characterized by mechanical and structural breakdown of the AF.

PURPOSE

In this study, we hypothesize that tensile forces modulate cellular and molecular behavior of AF cells grown on nanofibrous scaffolds in vitro.

STUDY DESIGN/SETTING

Bovine AF cells were seeded onto strained electrospun-aligned nanofibrous polycarbonate urethane (PU) scaffolds. Tension was either maintained throughout the culture duration (monotonic) or removed after 24 hours (relaxed).

METHODS

The effect of tension on AF cells cultured on PU scaffolds was evaluated over 7 days by scanning electron microscopy, biochemical assays, immunofluorescence microscopy, and quantitative polymerase chain reaction.

RESULTS

Cells grown on the relaxed scaffold were significantly more proliferative, synthesized more collagen and had increased collagen type I and TGFβ-1 gene expression; however these cells were not as aligned as were the cells and matrix on monotonic strained scaffolds. The alignment of AF cells grown on monotonic scaffolds correlated with significantly greater scaffold elastic modulus on day 7. Additionally, the cellular response to the change in strain was delayed by 3 to 5 days after tension release, which correlated with the time at which changes in scaffold length were detected.

CONCLUSIONS

This study demonstrated that AF cells respond at the molecular and cellular level to the changes in matrix/scaffold tension. This suggests that it may be necessary to determine the optimal elastic modulus and applied tensile forces to tissue engineer an AF that mimics the native tissue. Furthermore, this study provides insight into how changes in tensile forces may lead to changes in the AF cell function.

Treatment of the degenerated intervertebral disc; closure, repair and regeneration of the annulus fibrosus

Degeneration of the intervertebral disc (IVD) and disc herniation are two causes of low back pain. The aetiology of these disorders is unknown, but tissue weakening, which primarily occurs due to inherited genetic factors, ageing, nutritional compromise and loading history, is the basic factor causing disc degeneration. Symptomatic disc herniation mainly causes radicular pain. Current treatments of intervertebral disc degeneration and low back pain are based on alleviating the symptoms and comprise administration of painkillers or surgical methods such as spinal fusion. None of these methods is completely successful. Current research focuses on regeneration of the IVD and particularly on regeneration of the nucleus pulposus. Less attention has been directed to the repair or regeneration of the annulus fibrosus, although this is the key to successful nucleus pulposus, and therewith IVD, repair. This review focuses on the importance of restoring the function of the annulus fibrosus, as well as on the repair, replacement or regeneration of the annulus fibrosus in combination with restoration of the function of the nucleus pulposus, to treat low back pain.

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.

Molecular therapy for disk degeneration and pain

The nucleus pulposus of the intervertebral disk contains high amounts of the proteoglycan aggrecan, which confers the disk with a remarkable ability to resist compression. Other molecules such as collagens and noncollagenous proteins in the extracellular matrix are also essential for function. During disk degeneration, aggrecan and other molecules are lost due to proteolysis. This can result in loss of disk height, which can ultimately lead to pain. Biological therapy of intervertebral disk degeneration aims at preventing or restoring primarily aggrecan content and other molecules using therapeutic molecules. The purpose of the article is to review recent advances in biological repair of degenerate disks and pain.

Construction of a tissue-engineered annulus fibrosus

The intervertebral disc is composed of load-bearing fibrocartilage that may be subjected to compressive forces up to 10 times the body weight. The multilaminated outer layer, the annulus fibrosus (AF), is vulnerable to damage and its regenerative potential is limited, sometimes leading to nuclear herniation. Scaffold-based tissue engineering of AF using stem cell technology has enabled the development of bi-laminate constructs after 10 weeks of culture. It is difficult to know if these constructs are limited by the differentiation state of the stem cells or the culture system. In this study, we have characterized an expandable scaffold-free neoconstruct using autologous AF cells. The construct was prepared from pellet cultures derived from monolayer cultures of AF cells from mature pigs that became embedded in their own extracellular matrix. The pellet cultures were incubated for 24 h in a standardized conical tube and then carefully transferred intact to a culture flask and incubated for 21 days to allow continued matrix synthesis. Cell viability was maintained above 90% throughout the culture period. The engineered scaffold-free construct was compared with the native AF tissue by characterization of gene expression of representative markers, histological architecture, and biochemical composition. The morphological and biochemical characteristics of the cultured disc construct are very similar to that of native AF. The cell number per gram of construct was equal to that of native AF. Expression of aggrecan was elevated in the engineered construct compared with RNA extracted from the AF. The glycosaminoglycan content in the engineered construct showed no significant difference to that from native construct. These data indicate that scaffold-free tissue constructs prepared from AF cells using a pellet-culture format may be useful for in vitro expansion for transplantation into damaged discs.

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.

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.

Decellularized bovine intervertebral disc as a natural scaffold for xenogenic cell studies

Low back pain that is associated with disc degeneration contributes to a huge economic burden in the worldwide healthcare system. Traditional methods, such as spinal fusion, have been adopted to relieve mechanical back pain, but this is compromised by decreased spinal motion. Tissue engineering has attracted much attention, and aims to correct the changes fundamentally occurring in the discs by a combination of cell biology, molecular biology and engineering. Synthetic materials including poly(l-lactic acid) or poly(glycolic acid) and biomolecules like hyaluronic acid or collagen have been adopted in the development of disc scaffolds for studying therapeutic approaches. Nevertheless, the complex biological and mechanical environment of the intervertebral disc (IVD) makes the synthesis of an artificial IVD with biomaterials a difficult task. Thus the aim of this study was to develop a natural disc scaffold for culturing disc cells for future development of biological disc constructs. We adopted a combination of currently used decellularization techniques to decellularize bovine IVD to create a complete endplate-to-endplate IVD scaffold. By altering the chemical and physical decellularization parameters, we reported the removal of up to 70% of the endogenous cells, and were able to preserve the glycosaminoglycan content, collagen fibril architecture and mechanical properties of the discs. The reintroduction of nucleus pulposus cells into the scaffold indicated a high survival rate over 7days, with cell penetration. We have shown here that conventional methods used for decellularizing thin tissues can also be applied to large organs, such as IVD. Our findings suggest the potential of using decellularized IVD as a scaffold for IVD bioengineering and culturing of cells in the context of the IVD niche.

Challenges and strategies in the repair of ruptured annulus fibrosus

Lumbar discectomy is the surgical procedure most frequently performed for patients suffering from low back pain and sciatica. Disc herniation as a consequence of degenerative or traumatic processes is commonly encountered as the underlying cause for the painful condition. While discectomy provides favourable outcome in a majority of cases, there are conditions where unmet requirements exist in terms of treatment, such as large disc protrusions with minimal disc degeneration; in these cases, the high rate of recurrent disc herniation after discectomy is a prevalent problem. An effective biological annular repair could improve the surgical outcome in patients with contained disc herniations but otherwise minor degenerative changes. An attractive approach is a tissue-engineered implant that will enable/stimulate the repair of the ruptured annulus. The strategy is to develop three-dimensional scaffolds and activate them by seeding cells or by incorporating molecular signals that enable new matrix synthesis at the defect site, while the biomaterial provides immediate closure of the defect and maintains the mechanical properties of the disc. This review is structured into (1) introduction, (2) clinical problems, current treatment options and needs, (3) biomechanical demands, (4) cellular and extracellular components, (5) biomaterials for delivery, scaffolding and support, (6) pre-clinical models for evaluation of newly developed cell- and material-based therapies, and (7) conclusions. This article highlights that an interdisciplinary approach is necessary for successful development of new clinical methods for annulus fibrosus repair. This will benefit from a close collaboration between research groups with expertise in all areas addressed in this review.

Protein-hydrogel interactions in tissue engineering: mechanisms and applications

Recent advances in our understanding of the sophistication of the cellular microenvironment and the dynamics of tissue remodeling during development, disease, and regeneration have increased our appreciation of the current challenges facing tissue engineering. As this appreciation advances, we are better equipped to approach problems in the biology and therapeutics of even more complex fields, such as stem cells and cancer. To aid in these studies, as well as the established areas of tissue engineering, including cardiovascular, musculoskeletal, and neural applications, biomaterials scientists have developed an extensive array of materials with specifically designed chemical, mechanical, and biological properties. Herein, we highlight an important topic within this area of biomaterials research, protein-hydrogel interactions. Due to inherent advantages of hydrated scaffolds for soft tissue engineering as well as specialized bioactivity of proteins and peptides, this field is well-posed to tackle major needs within emerging areas of tissue engineering. We provide an overview of the major modes of interactions between hydrogels and proteins (e.g., weak forces, covalent binding, affinity binding), examples of applications within growth factor delivery and three-dimensional scaffolds, and finally future directions within the area of hydrogel-protein interactions that will advance our ability to control the cell-biomaterial interface.

Oriented lamellar silk fibrous scaffolds to drive cartilage matrix orientation: towards annulus fibrosus tissue engineering

A novel design of silk-based scaffold is developed using a custom-made winding machine, with fiber alignment resembling the anatomical criss-cross lamellar fibrous orientation features of the annulus fibrosus of the intervertebral disc. Crosslinking of silk fibroin fibers with chondroitin sulphate (CS) was introduced to impart superior biological functionality. The scaffolds, with or without CS, instructed alignment of expanded human chondrocytes and of the deposited extracellular matrix while supporting their chondrogenic redifferentiation. The presence of CS crosslinking could not induce statistically significant changes in the measured collagen or glycosaminoglycan content, but resulted in an increased construct stiffness. By offering the combined effect of cell/matrix alignment and chondrogenic support, the silk fibroin scaffolds developed with precise fiber orientation in lamellar form represent a suitable substrate for tissue engineering of the annulus fibrosus part of the intervertebral disc.

Regeneration of nucleus pulposus tissue in an ovine intervertebral disc degeneration model by cell-free resorbable polymer scaffolds

Degeneration of intervertebral discs (IVDs) occurs frequently and is often associated with lower back pain. Recent treatment options are limited and treat the symptoms rather than regenerate the degenerated disc. Cell-free, freeze-dried resorbable polyglycolic acid (PGA)-hyaluronan implants were used in an ovine IVD degeneration model. The nucleus pulposus of the IVD was partially removed, endoscopically. PGA-hyaluronan implants were immersed in autologous sheep serum and implanted into the disc defect. Animals with nucleotomy only served as controls. The T2-weighted/fat suppression sequence signal intensity index of the operated discs, as assessed by magnetic resonance imaging (MRI), showed that implantation of the PGA-hyaluronan implant improved (p = 0.0066) the MRI signal compared to controls at 6 months after surgery. Histological analysis by haematoxylin and eosin and safranin O staining showed the ingrowth of cells with typical chondrocytic morphology, even cell distribution, and extracellular matrix rich in proteoglycan. Histomorphometric analyses confirmed that the implantation of the PGA-hyaluronan scaffolds improved (p = 0.027) the formation of regenerated tissue after nucleotomy. Disc heights remained stable in discs with nucleotomy only as well as after implantation of the implant. In conclusion, implantation of cell-free polymer-based implants after nucleotomy induces nucleus pulposus tissue regeneration and improves disc water content in the ovine model.

Inflammatory response of intervertebral disc cells is reduced by fibrin sealant scaffold in vitro

Intervertebral disc (IVD) degeneration is a complex process characterized by elevated concentrations of proinflammatory cytokines and proteolytic enzymes. Because of pro-healing constituents, we hypothesized that fibrin sealant (FS) can reduce inflammation and augment soft tissue healing within the damaged or degenerative IVD. To test this, human and porcine nucleus pulposus (NP) and annulus fibrosus (AF) cells were extracted from tissues and encapsulated into alginate beads (NP cells) and type I collagen sponges (AF cells). Half of the alginate and collagen scaffolds were embedded in FS. To provoke inflammatory behaviours, the constructs were cultured with and without continuous IL-1α (10 ng/ml) for 4, 7 and 14 days. ELISA was used to quantify the cellular synthesis (ng/µg DNA) of clinically relevant cytokines, proteolytic enzymes and growth factors. In NP cell constructs with IL-1α, the syntheses of TNFα, IL-1β, IL-6, IL-8 was elevated at all culture durations. In the presence of FS, secretion of several pro-inflammatory cytokines were significantly reduced [IL-6 and IL-8 (porcine); and TNFα, IL-1β, IL-6, IL-8 (human)]. Consistent with these reductions, human NP cultures exposed to FS and FS + IL-1α synthesized significantly reduced amounts of MMP-1 and -3 compared to constructs with IL-1α. For porcine and human AF cells, there were no significant differences in the synthesis of the inflammatory or proteolytic cytokines relative to controls (without IL-1α) at any culture duration. However, the porcine AF cells exposed to FS synthesized elevated amounts of the anti-inflammatory cytokine IL-4. The results suggest that FS may have beneficial effects for patients with degenerated intervertebral discs.

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.

Metabolic effects of vitamin D active metabolites in monolayer and micromass cultures of nucleus pulposus and annulus fibrosus cells isolated from human intervertebral disc

Intragenic polymorphisms in the vitamin D receptor gene are linked to disc degeneration features, suggesting that alterations in the vitamer-mediated signalling could be involved in the pathophysiology of the disc and that interaction of disc cells with vitamin D metabolites may be critical for disc health. The vitamer-mediated modulation of disc cells proliferation, metabolic activity, extracellular matrix (ECM) genes expression and proteins production was investigated. It was stated that disc cells express vitamin D receptor and are very sensitive to metabolic stimuli. In monolayer cultures, 1,25(OH)(2)D(3), but not 24,25(OH)(2)D(3), determined an inhibition of the proliferation and regulated also the ECM genes expression in nucleus pulposus and annulus fibrosus cells. Micromass cultures induced a more physiologic expression pattern of extracellular matrix genes. Cells Treatment with vitamin D metabolites did not result in relevant modifications of glycosaminoglycans production, except for annulus cells, whose production was reduced after 1,25(OH)(2)D(3) treatment. Moreover, a reduced glycosaminoglycans staining in both cell types and a significant reduced aggrecan gene expression in annulus cells treated with 1,25(OH)(2)D(3) were observed. A reduction of collagen I and II staining in annulus cells 1,25(OH)(2)D(3) treated, in accordance with a downregulation of collagen genes expression, was also registered. Finally, the vitamin D receptor gene expression did not show significant metabolite-mediated modification in monolayer or micromass cultures. These findings could enhance new insights on the biochemical mechanisms regulated by vitamin D in disc cartilage and possibly involved in the development of physiological/pathological modifications of the disc.

Bioactive electrospun scaffold for annulus fibrosus repair and regeneration

PURPOSE

Annulus fibrosus (AF) tissue engineering is gathering increasing interest for the development of strategies to reduce recurrent disc herniation (DH) rate and to increase the effectiveness of intervertebral disc regeneration strategies. This study evaluates the use of a bioactive microfibrous poly(L-lactide) scaffold releasing Transforming Growth Factor (TGF)-β1 (PLLA/TGF) for the repair and regeneration of damaged AF.

METHODS

The scaffold was synthesized by electrospinning, with a direct incorporation of TGF-β1 into the polymeric solution, and characterized in terms of morphology and drug release profile. Biological evaluation was performed with bovine AF cells (AFCs) that were cultured on the scaffold up to 3 weeks to quantitatively assess glycosaminoglycans and total collagen production, using bare electrospun PLLA as a control. Histological evaluation was performed to determine the thickness of the deposited neo-ECM.

RESULTS

Results demonstrated that AFCs cultured on PLLA/TGF deposited a significantly greater amount of glycosaminoglycans and total collagen than the control, with higher neo-ECM thickness.

CONCLUSIONS

PLLA/TGF scaffold induced an anabolic stimulus on AFCs, mimicking the ECM three-dimensional environment of AF tissue. This bioactive scaffold showed encouraging results that allow envisaging an application for AF tissue engineering strategies and AF repair after discectomy for the prevention of recurrent DH.

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.

Intervertebral disk tissue engineering using biphasic silk composite scaffolds

Scaffolds composed of synthetic, natural, and hybrid materials have been investigated as options to restore intervertebral disk (IVD) tissue function. These systems fall short of the lamellar features of the native annulus fibrosus (AF) tissue or focus only on the nucleus pulposus (NP) tissue. However, successful regeneration of the entire IVD requires a combination approach to restore functions of both the AF and NP. To address this need, a biphasic biomaterial structure was generated by using silk protein for the AF and fibrin/hyaluronic acid (HA) gels for the NP. Two cell types, porcine AF cells and chondrocytes, were utilized. For the AF tissue, two types of scaffold morphologies, lamellar and porous, were studied with the porous system serving as a control. Toroidal scaffolds formed out of the lamellar, and porous silk materials were used to generate structures with an outer diameter of 8 mm, inner diameter of 3.5 mm, and a height of 3 mm (the interlamellar distance in the lamellar scaffold was 150-250 μm, and the average pore sizes in the porous scaffolds were 100-250 μm). The scaffolds were seeded with porcine AF cells to form AF tissue, whereas porcine chondrocytes were encapsulated in fibrin/HA hydrogels for the NP tissue and embedded in the center of the toroidal disk. Histology, biochemical assays, and gene expression indicated that the lamellar scaffolds supported AF-like tissue over 2 weeks. Porcine chondrocytes formed the NP phenotype within the hydrogel after 4 weeks of culture with the AF tissue that had been previously cultured for 2 weeks, for a total of 6 weeks of cultivation. This biphasic scaffold simulating in combination of both AF and NP tissues was effective in the formation of the total IVD in vitro.

Tissue engineering for intervertebral disk degeneration

Many challenges confront intervertebral disk engineering owing to complexity and the presence of extraordinary stresses. Rebuilding a disk of native function could be useful for removal of the symptoms and correction of altered spine kinematics. Improvement in understanding of disk properties and techniques for disk engineering brings promise to the fabrication of a functional motion segment for the treatment of disk degeneration. Increasing sophistication of techniques available in biomedical sciences will bring its application into clinics. This review provides an account of current progress and challenges of intervertebral disk bioengineering and discusses means to move forward and toward bedside translation.

Injectable thermoreversible hyaluronan-based hydrogels for nucleus pulposus cell encapsulation

INTRODUCTION

Thermoreversible hydrogels have potential in spine research as they provide easy injectability and mild gelling mechanism (by physical cross-link). The purpose of this study was to assess the potential of thermoreversible hyaluronan-based hydrogels (HA-pNIPAM) (pNIPAM Mn = 10, 20, 35 × 10(3) g mol(-1)) as nucleus pulposus cells (NPC) carrier.

MATERIALS AND METHODS

Cytocompatibility (WST-1 assay), viability (trypan blue), morphology (toluidine blue), sulphated glycosaminoglycan synthesis (DMMB assay) and gene expression profile (real-time PCR) of bovine NPC cultured in HA-pNIPAM were followed for 1 week and compared to alginate gel bead cultures. The injectability and cell survival in a whole disc organ culture model were assessed up to day 7.

RESULTS

All HA, HA-pNIPAM and their degradation products were cytocompatible to NPC. HA-pNIPAM hydrogels with no volume change upon gelling maintained NPC viability and characteristic rounded morphology. Glycosaminoglycan synthesis was similar in HA-pNIPAM and alginate gels. Following NPC expansion, both gels induced re-differentiation toward the NPC phenotype. Significant differences between the two gels were found for COLI, COLII, HAS1, HAS2 and ADAMTS4 but not for MMPs and TIMPs. Higher expression of hyaluronan synthases (HAS1, HAS2) and lower expression of COLI and COLII mRNA were noted in cells cultured in HA-pNIPAM (pNIPAM = 20 × 10(3)g mol(-1)). NPC suspension in HA-pNIPAM was injectable through a 22-G needle without loss of cell viability. Ex vivo, NPC viability was maintained in HA-pNIPAM for 1 week.

CONCLUSION

A HA-pNIPAM composition suitable for nucleus pulposus repair that provides an injectable carrier for NPC, maintains their phenotype and promotes extracellular matrix generation was identified.