Stem cell therapy for intervertebral disc regeneration: obstacles and solutions

Leaping the hurdles in developing regenerative treatments for the intervertebral disc from preclinical to clinical

Chronic back and neck pain is a prevalent disability, often caused by degeneration of the intervertebral disc. Because current treatments for this condition are less than satisfactory, a great deal of effort is being applied to develop new solutions, including regenerative strategies. However, the path from initial promising idea to clinical use is fraught with many hurdles to overcome. Many of the keys to success are not necessarily linked to science or innovation. Successful translation to clinic will also rely on planning and awareness of the hurdles. It will be essential to plan your entire path to clinic from the outset and to do this with a multidisciplinary team. Take advice early on regulatory aspects and focus on generating the proof required to satisfy regulatory approval. Scientific demonstration and societal benefits are important, but translation cannot occur without involving commercial parties, which are instrumental to support expensive clinical trials. This will only be possible when intellectual property can be protected sufficiently to support a business model. In this manner, commercial, societal, medical, and scientific partners can work together to ultimately improve patient health. Based on literature surveys and experiences of the co-authors, this opinion paper presents this pathway, highlights the most prominent issues and hopefully will aid in your own translational endeavors.

Disc cell therapy with bone-marrow-derived autologous mesenchymal stromal cells in a large porcine disc degeneration model

PURPOSE

Disc regeneration through matrix-assisted autologous mesenchymal stromal cell therapy seems promising against disc degeneration with convincing results in small animal models. Whether these positive results can be transferred to larger animal models or humans is unclear.

METHODS

Fibrin matrix-assisted autologous bone-marrow-derived mesenchymal stromal cell therapy was compared to acellular fibrin matrix therapy in a porcine in vivo model. First, disc degeneration was induced by annular puncture and partial nucleotomy with a large 16G-needle, and 12 weeks later, disc therapy was performed in a second surgery with a thinner 26G needle. Seventy-two lumbar discs from 12 aged adult pigs were evaluated by histology, micro-CT, and gene expression analysis 13 and 24 weeks after nucleotomy and 1 and 12 weeks after treatment, respectively.

RESULTS

Radiologic disc height was not significantly different in both treatment groups. In the semi-quantitative histologic degeneration score, significant disc degeneration was still evident 1 week after treatment both in the mesenchymal stromal cell group and in the acellular fibrin matrix group. 12 weeks after treatment, degeneration was, however, not further increased and mesenchymal-stromal-cell-treated discs showed significantly less disc degeneration in the annulus fibrosus (p = 0.02), whereas reduction in the nucleus pulposus did not reach statistical significance. Cell treatment compared to matrix alone found less Col1 gene expression as a marker for fibrosis and more expression of the trophic factor BMP2 in the nucleus pulposus, whereas the inflammation marker IL1ß was reduced in the annulus fibrosus.

CONCLUSIONS

Disc treatment with fibrin matrix-assisted autologous mesenchymal stromal cells reduced degenerative findings compared to acellular fibrin matrix alone. Regenerative changes, however, were not significant for all parameters showing limitations in a large biomechanically demanding model with aged discs. These slides can be retrieved under Electronic Supplementary Material.

p38 MAPK Facilitates Crosstalk Between Endoplasmic Reticulum Stress and IL-6 Release in the Intervertebral Disc

Degenerative disc disease is associated with increased expression of pro-inflammatory cytokines in the intervertebral disc (IVD). However, it is not completely clear how inflammation arises in the IVD and which cellular compartments are involved in this process. Recently, the endoplasmic reticulum (ER) has emerged as a possible modulator of inflammation in age-related disorders. In addition, ER stress has been associated with the microenvironment of degenerated IVDs. Therefore, the aim of this study was to analyze the effects of ER stress on inflammatory responses in degenerated human IVDs and associated molecular mechanisms. Gene expression of ER stress marker GRP78 and pro-inflammatory cytokines IL-6, IL-8, IL-1β, and TNF-α was analyzed in human surgical IVD samples (n = 51, Pfirrmann grade 2-5). The expression of GRP78 positively correlated with the degeneration grade in lumbar IVDs and IL-6, but not with IL-1β and TNF-α. Another set of human surgical IVD samples (n = 25) was used to prepare primary cell cultures. ER stress inducer thapsigargin (Tg, 100 and 500 nM) activated gene and protein expression of IL-6 and induced phosphorylation of p38 MAPK. Both inhibition of p38 MAPK by SB203580 (10 µM) and knockdown of ER stress effector CCAAT-enhancer-binding protein homologous protein (CHOP) reduced gene and protein expression of IL-6 in Tg-treated cells. Furthermore, the effects of an inflammatory microenvironment on ER stress were tested. TNF-α (5 and 10 ng/mL) did not activate ER stress, while IL-1β (5 and 10 ng/mL) activated gene and protein expression of GRP78, but did not influence [Ca2+]i flux and expression of CHOP, indicating that pro-inflammatory cytokines alone may not induce ER stress in vivo. This study showed that IL-6 release in the IVD can be initiated following ER stress and that ER stress mediates IL-6 release through p38 MAPK and CHOP. Therapeutic targeting of ER stress response may reduce the consequences of the harsh microenvironment in degenerated IVD.

Stromal Cell Derived Factor-1-Mediated Migration of Mesenchymal Stem Cells Enhances Collagen Type II Expression in Intervertebral Disc

Intervertebral disc (IVD) degeneration is characterized by an unbalanced cell catabolic/anabolic activity and cell death, resulting in the degradation of extracellular matrix components and water loss. Repopulating the IVD with new cells may help in recovering tissue homeostasis and reverting the degenerative process. In this study the regenerative potential of a hyaluronan (HA)-based chemoattractant delivery system able to recruit mesenchymal stem cells (MSCs) seeded on the cartilaginous endplate (CEP) of IVD was explored. A HA delivery system containing stromal cell derived factor-1 (SDF-1) (5 ng/μL) (HAPSDF5) was injected in the cavity of nucleotomized bovine discs. Human MSCs (1 × 106) were seeded on the opposite CEP and allowed to migrate for up to 21 days. Migration of fluorescently labelled MSCs from CEP toward the IVD was enhanced by HAPSDF5. Likewise, an increase in collagen type II was detected at earlier time points, whereas no effect on proteoglycan content within the nucleotomized IVDs was found. MSCs produced an increased concentration of pro-catabolic factors, such as interleukin (IL)-6, IL-8, and monocyte chemoattractant protein-1 (MCP-1). Overall, this study demonstrates that HAPSDF5 increased MSC recruitment, while the higher number of recruited cells partially contributed to accelerate matrix remodeling in nucleotomized IVDs.

Chondrocyte death after mechanically overloading degenerated human intervertebral disk explants is associated with a structurally impaired pericellular matrix

A type VI collagen-rich pericellular matrix (PCM) encloses both intervertebral disk (IVD) and articular cartilage chondrocytes. In the latter, the PCM protects the chondrocytes from mechanical overload, whereas tissue degeneration is associated with PCM destruction. As little is known about the IVD PCM, we investigated chondrocyte survival after mechanical overload as well as PCM structural integrity as a function of clinical tissue degeneration. The hypothesis was that IVD degeneration may affect PCM integrity and overload-related chondrocyte survival. Cylindrical human IVD explants from patients undergoing surgical procedures for lumbar disk degeneration, disk prolapse, or spinal trauma were generated and scored. Mechanical overload was applied by single uniaxial 50% compression followed by immediate release, and the explants were live-dead stained (n = 20 explants). Type VI collagen, the major PCM component, was fluorescent stained and the extent was determined, in which individual cells were enclosed by a recognizable PCM; this was termed PCM fraction. More than 50% of chondrocytes in all degenerative IVD explants displayed <25% PCM fraction and a lower PCM fraction correlated with higher cell numbers (p < 0.001), suggesting a PCM structural impairment in IVD degeneration that is associated with chondrocyte clustering. Mechanical overload-induced significantly increased cell death (p = 0.005), and the PCM fraction was significantly lower in overload-induced cell death than in live cells (p = 0.042), suggesting that a fully present PCM has a protective role in mechanical overload. Collectively, human IVD degeneration is associated with a structural impairment of the PCM, which may promote cell death under mechanical overload.

Critical aspects and challenges for intervertebral disc repair and regeneration-Harnessing advances in tissue engineering

Low back pain represents the highest burden of musculoskeletal diseases worldwide and intervertebral disc degeneration is frequently associated with this painful condition. Even though it remains challenging to clearly recognize generators of discogenic pain, tissue regeneration has been accepted as an effective treatment option with significant potential. Tissue engineering and regenerative medicine offer a plethora of exploratory pathways for functional repair or prevention of tissue breakdown. However, the intervertebral disc has extraordinary biological and mechanical demands that must be met to assure sustained success. This concise perspective review highlights the role of the disc microenvironment, mechanical and clinical design considerations, function vs mimicry in biomaterial‐based and cell engineering strategies, and potential constraints for clinical translation of regenerative therapies for the intervertebral disc.

Regenerative potential of human nucleus pulposus resident stem/progenitor cells declines with ageing and intervertebral disc degeneration

Numerous studies have demonstrated the presence of resident nucleus pulposus stem/progenitor cells (NPSCs) in the tissue of the intervertebral disc (IVD). However, the cellular identity of NPSCs during IVD degeneration and ageing are poorly defined at present, despite significant progress in the understanding of NPSC biology. In the present study, NPSCs were isolated from human degenerated IVD and were characterized by flow cytometry, gene expression assays and proliferation and multipotency analysis. The results of the present study demonstrated that NPSCs isolated from human degenerated IVD may be divided into two groups according to the expression of mesenchymal stem cell (MSC) surface markers: The high expression of MSC surface markers group (H‑NPSCs) was highly positive for CD29, CD44, CD73, CD90 and CD105 at rates >95%, and the low expression of MSC markers surface markers group (L‑NPSCs), with the expression of CD29 and CD105 exhibiting individual variability, however, all at rates <95%. The donors for H‑NPSCs were aged <20 years, while the majority of donors for L‑NPSCs were aged >25 years, with one exception aged <20 years. The results highlighted that the low expression of MSC surface markers in NPSCs from aged and degenerated NP tissues were associated with a low rate of proliferation and reduced differentiation potential, as well as downregulation of the NP progenitor marker Tie2 and higher expression of NP cell‑specific markers. These findings demonstrated that the regenerative potential of human NPSCs declines with ageing and degeneration of the IVD.

Icariin Improves the Viability and Function of Cryopreserved Human Nucleus Pulposus-Derived Mesenchymal Stem Cells

Nucleus pulposus-derived mesenchymal stem cells (NPMSCs) have shown a good prospect in the regeneration of intervertebral disc (IVD) tissues. However, fresh NPMSCs are not always readily available for basic research and clinical applications. Therefore, there is a need for an effective long-term cryopreservation method for NPMSCs. The aim of this study was to determine whether adding icariin (ICA) to the conventional cryoprotectant containing dimethyl sulfoxide (DMSO) had a better cryoprotective effect for NPMSCs. The results showed that the freezing solution containing ICA along with DMSO significantly increased the postthawed cell viability, decreased the apoptosis rate, improved cell adherence, and maintained the mitochondrial functions, as compared to the freezing solution containing DMSO alone. And the inhibition of oxidative stress and upregulation of heat shock proteins (HSPs) in the presence of ICA also confirmed the beneficial effect of ICA. Furthermore, ICA had no cytotoxicity and did not alter the characteristics of postthawed NPMSCs. In conclusion, these results suggested that the addition of ICA to the conventional freezing medium could improve the viability and function of the cryopreserved human NPMSCs and provided an optimal formulated freezing solution for human NPMSC cryopreservation.

Co-culturing nucleus pulposus mesenchymal stem cells with notochordal cell-rich nucleus pulposus explants attenuates tumor necrosis factor-α-induced senescence

Background

Cell therapy for the treatment of intervertebral disc degeneration (IDD) faces serious barriers since tissue-specific adult cells such as nucleus pulposus cells (NPCs) have limited proliferative ability and poor regenerative potential; in addition, it is difficult for exogenous adult stem cells to survive the harsh environment of the degenerated intervertebral disc. Endogenous repair by nucleus pulposus mesenchymal stem cells (NPMSCs) has recently shown promising regenerative potential for the treatment of IDD. Notochordal cells (NCs) and NC-conditioned medium (NCCM) have been proven to possess regenerative ability for the treatment of IDD, but this approach is limited by the isolation and passaging of NCs. Our previous study demonstrated that modified notochordal cell-rich nucleus pulposus (NC-rich NP) has potential for the repair of IDD. However, whether this can protect NPMSCs during IDD has not been evaluated.

Methods

In the current study, tumor necrosis factor (TNF)-α was used to mimic the inflammatory environment of IDD. Human NPMSCs were cocultured with NC-rich NP explants from healthy rabbit lumbar spine with or without TNF-α. Cell proliferation and senescence were analyzed to investigate the effect of NC-rich NP explants on TNF-α-treated NPMSCs. The expression of mRNA encoding proteins related to matrix macromolecules (such as aggrecan, Sox-9, collagen Iα, and collagen IIα), markers related to the nucleus pulposus cell phenotype (including CA12, FOXF1, PAX1, and HIF-1α), and senescence markers (such as p16, p21, and p53), senescence-associated proinflammatory cytokines (IL-6), and extracellular proteases (MMP-13, ADAMTS-5) was assessed. The protein expression of CA12 and collagen II was also evaluated.

Results

After a 7-day treatment, the NC-rich NP explant was found to enhance cell proliferation, decrease cellular senescence, promote glycosaminoglycan (GAG), collagen II, and CA12 production, upregulate the expression of extracellular matrix (ECM)-related genes (collagen I, collagen II, SOX9, and ACAN), and enhance the expression of nucleus pulposus cell (NPC) markers (HIF-1α, FOXF1, PAX1, and CA12).

Conclusion

Modified NC-rich NP explants can attenuate TNF-α-induced degeneration and senescence of NPMSCs in vitro. Our findings provide new insights into the therapeutic potential of NC-rich NP for the treatment of IDD.

Immunomodulation of Human Mesenchymal Stem/Stromal Cells in Intervertebral Disc Degeneration: Insights From a Proinflammatory/Degenerative Ex Vivo Model

STUDY DESIGN

Ex vivo experimental study.

OBJECTIVE

To investigate the effect of proinflammatory/degenerative intervertebral disc (IVD) microenvironment on the regenerative and immunomodulatory behavior of mesenchymal stem/stromal cells (MSCs), using an ex vivo model from bovine origin.

SUMMARY OF BACKGROUND DATA

Low back pain is a cause of disability worldwide, most frequently associated with IVD degeneration and inflammation, and characterized by increased levels of inflammatory mediators, often disregarded. MSC-based therapies to low back pain have been advocated, but the involvement of inflammation in IVD remodeling mechanism, promoted by MSCs has not yet been explored.

METHODS

Bovine IVD organ cultures of nucleus pulposus punches were stimulated with needle puncture and culture medium supplementation with 10 ng/mL of interleukin (IL)-1β, to induce a proinflammatory/degenerative environment, as previously established. Human bone marrow-derived MSCs were cultured on top of transwells, placed above nucleus pulposus punches, for up to 16 days. MSCs were analyzed by screening cell viability/apoptosis, metabolic activity, migration, and inflammatory cytokines production in response to the proinflammatory environment. IVD extracellular matrix (ECM) remodeling, gene expression profile of IVD cells, and inflammatory cytokine profile in the presence of MSCs in basal versus proinflammatory conditions were also evaluated.

RESULTS

Proinflammatory/degenerative IVD conditions did not affect MSCs viability, but promoted cell migration, while increasing IL-6, IL-8, monocyte chemoattractant protein-1, and prostaglandin E2 and reducing transforming growth factor-β1 production by MSCs. MSCs did not stimulate ECM production (namely type II collagen or aggrecan) in neither basal nor inflammatory conditions, instead MSCs downregulated bovine proinflammatory IL-6, IL-8, and TNF-α gene expression levels in IL-1β-stimulated IVDs.

CONCLUSION

The present study provides evidence for an immunomodulatory paracrine effect of MSCs in degenerated IVD without an apparent effect in ECM remodeling, and suggest an MSCs mechanism-of-action dependent on a cytokine feedback loop.

LEVEL OF EVIDENCE

5.

Discogenic cell transplantation directly from a cryopreserved state in an induced intervertebral disc degeneration canine model

A multitude of studies has indicated the potential of cell therapy as a method for intervertebral disc (IVD) regeneration. Transplantation of a variety of cells has been assessed and shown capable of deterring the rate of degeneration in animal models and in human clinical trials. In this study, a novel approach using human discogenic nucleus pulposus cells directly from their cryopreserved state was assessed. In an established canine disc degeneration model, the degeneration process was evaluated in IVDs receiving precultured discogenic cells, thawed-only discogenic cells, and a saline sham injection after induction of degeneration. Degeneration progression was followed over time by the evaluation of the disc height index (DHI). Finally, after 12 weeks, the manipulated and control discs were explanted, histologically stained, and scored. Treated discs demonstrated retained DHI values for all treatment options. Histologic evaluations demonstrated significant improvement of matrix features compared to the sham. Moreover, thawed-only cells function at least as well as precultured discogenic cells. In short, cell transplantation of human discogenic cells directly from their cryopreserved state can arrest disc height degeneration and maintain histological matrix features in a canine disc degeneration model. The presented work demonstrates the potential of an off-the-shelf cell therapy product to treat degenerative disc disease.

CsA attenuates compression-induced nucleus pulposus mesenchymal stem cells apoptosis via alleviating mitochondrial dysfunction and oxidative stress

AIMS

This study aims to investigate the protective effects and potential mechanisms of cyclosporine A (CsA), which efficiently inhibits mitochondrial permeability transition pore (MPTP) opening, on compression-induced apoptosis of human nucleus pulposus mesenchymal stem cells (NP-MSCs).

MATERIALS AND METHODS

Human NP-MSCs were subjected to various periods of 1.0 MPa compression. Cell viability was evaluated using cell counting kit-8 (CCK-8) assay. The cellular ultrastructure and ATP level were analyzed via transmission electron microscopy (TEM) and ATP detection kit respectively. The apoptosis ratio was determined using Annexin V/PI dual staining and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assays. The levels of apoptosis-associated molecules (cleaved caspase-3, Bax and Bcl-2) were analyzed by western blot and qRT-PCR. Additionally, MPTP opening, mitochondrial membrane potential (MMP) and the levels of oxidative stress-related indicators (ROS), superoxide dismutase (SOD) and malondialdehyde (MDA) were monitored.

KEY FINDINGS

Annexin V/PI dual staining and detection of apoptosis-associated molecules demonstrated that compression significantly up-regulated apoptosis level of NP-MSCs in a time-dependent manner. CsA greatly down-regulated compression-mediated NP-MSC apoptosis and the cell death ratio. Compression also notably exacerbated mitochondrial dysfunction, ATP depletion and oxidative stress in NP-MSCs, all of which were rescued by CsA.

SIGNIFICANCE

Our results demonstrated that CsA efficiently inhibited compression-induced NP-MSCs apoptosis by alleviating mitochondrial dysfunction and oxidative stress. These findings provide new insights into intervertebral disc (IVD) degeneration (IVDD), and suggest CsA treatment as a potential strategy for delaying or even preventing IVDD.

Angiopoietin-2 promotes extracellular matrix degradation in human degenerative nucleus pulposus cells

In contrast to healthy intervertebral discs (IVDs), degenerate IVDs become vascularized. Here, we determined the role of an angiogenesis promoter, angiopoietin (Ang)-2, in the pathology of IVD degeneration (IDD). We evaluated degree of IDD using the Pfirrmann grading system. We used quantitative real-time polymerase chain reaction and western blotting to analyze ANG2 gene expression and Ang-2 protein levels, respectively. The involvement of Ang-2 in IVD degradation and regulation of nuclear factor-κB (NF-κB) signaling was examined by immunohistochemistry, western blotting and immunofluorescence. As a result, 10 samples with grades II and III IDD were categorized as the mild IDD group; for comparison, another 10 specimens with grades IV and V constituted the severe IDD group. Ang-2 expression was significantly higher in severe IDD than in mild IDD. Exogenous Ang-2 administration led to increased production of catabolic proteinases and loss of aggrecan and collagen II in degenerative NP cell cultures, which was mediated by the NF-κB signaling pathway. Elevated Ang-2 levels also increased interleukin-1β expression in degenerative NP cells. We conclude that the release of Ang-2 aggravates NP cell degradation and plays an important role in IDD. Ang-2 may thus constitute a novel therapeutic target for the treatment of IVD.

Priming and cryopreservation of microencapsulated marrow stromal cells as a strategy for intervertebral disc regeneration

A challenge in using stromal cells for intervertebral disc (IVD) regeneration is their limited differentiation capacity in vivo without exogenous growth factor (GF) supplementation. Priming of stromal cells prior to transplantation may offer a feasible strategy to overcome this limitation. Furthermore, the ability to cryopreserve cells could help alleviate logistical issues associated with storage and transport. With these critical translational challenges in mind, we aimed to develop a strategy involving priming and subsequent cryopreservation of microencapsulated bone marrow stromal cells (BMSCs). In phase one, we utilised the electrohydrodynamic atomisation process to fabricate BMSC-encapsulated microcapsules that were primed with TGF-β3 for 14 d after which they were cultured for a further 21 d under basal or GF supplemented media conditions. Results showed that priming induced differentiation of BMSC microcapsules such that they synthesised significant amounts of sGAG (61.9 ± 2.0 μg and 55.3 ± 6.1 μg for low and high cell densities) and collagen (24.4 ± 1.9 μg and 55.3 ± 4.6 μg for low and high cell densities) in continued culture without GF supplementation compared to Unprimed microcapsules. Phase two of this work assessed the extracellular matrix forming capacity of Primed BMSC microcapsules over 21 d after cryopreservation. Notably, primed and cryopreserved BMSCs successfully retained the ability to synthesise both sGAG (24.8 ± 2.7 μg and 75.1 ± 11.6 μg for low and high cell densities) and collagen (26.4 ± 7.8 μg and 93.1 ± 10.2 μg for low and high cell densities) post-cryopreservation. These findings demonstrate the significant potential of priming and cryopreservation approaches for IVD repair and could possibly open new horizons for pre-designed, 'off-the-shelf' injectable therapeutics.

Engineering Stem and Stromal Cell Therapies for Musculoskeletal Tissue Repair

Stem cells and tissue-derived stromal cells stimulate the repair of degenerated and injured tissues, motivating a growing number of cell-based interventions in the musculoskeletal field. Recent investigations have indicated that these cells are critical for their trophic and immunomodulatory role in controlling endogenous cells. This Review presents recent clinical advances where stem cells and stromal cells have been used to stimulate musculoskeletal tissue repair, including delivery strategies to improve cell viability and retention. Emerging bioengineering strategies are highlighted, particularly toward the development of biomaterials for capturing aspects of the native tissue environment, altering the healing niche, and recruiting endogenous cells.

Age-Correlated Phenotypic Alterations in Cells Isolated From Human Degenerated Intervertebral Discs With Contained Hernias

STUDY DESIGN

Human intervertebral disc (hIVD) cells were isolated from 41 surgically excised samples and assessed for their phenotypic alterations with age.

OBJECTIVE

Toward the design of novel anti-aging strategies to overcome degenerative disc disease (DDD), we investigated age-correlated phenotypic alterations that occur on primary hIVD cells.

SUMMARY OF BACKGROUND DATA

Although regenerative medicine holds great hope, much is still to be unveiled on IVD cell biology and its intrinsic signaling pathways, which can lead the way to successful therapies for IDD. A greater focus on age-related phenotypic changes at the cell level would contribute to establish more effective anti-aging/degeneration targets.

METHODS

The study was subdivided in four main steps: i) optimization of primary cells isolation technique; ii) high-throughput cell morphology analysis, by imaging flow cytometry (FC) and subsequent validation by histological analysis; iii) analysis of progenitor cell surface markers expression, by conventional FC; and iv) statistical analysis and correlation of cells morphology and phenotype with donor age.

RESULTS

Three subsets of cells were identified on the basis of their diameter: small cell (SC), large cell (LC), and super LC (SLC). The frequency of SCs decreased nearly 50% with age, whereas that of LCs increased nearly 30%. Interestingly, the increased cells size was due to an enlargement of the pericellular matrix (PCM). Moreover, the expression pattern for CD90 and CD73 was a reflexion of age, where older individuals show reduced frequencies of positive cells for those markers. Nevertheless, the elevated percentages of primary positive cells for the mesenchymal stem cells (MSCs) marker CD146 found, even in some older donors, refreshed hope for the hypothetical activation of the self-renewal potential of the IVD.

CONCLUSION

These findings highlight the remarkable morphological alterations that occur on hIVD cells with aging and degeneration, while reinforcing previous reports on the gradual disappearance of an endogenous progenitor cell population.

LEVEL OF EVIDENCE

N/A.

The potential of CRISPR/Cas9 genome editing for the study and treatment of intervertebral disc pathologies

The CRISPR/Cas9 system has emerged as a powerful tool for mammalian genome engineering. In basic and translational intervertebral disc (IVD) research, this technique has remarkable potential to answer fundamental questions on pathway interactions, to simulate IVD pathologies, and to promote drug development. Furthermore, the precisely targeted CRISPR/Cas9 gene therapy holds promise for the effective and targeted treatment of degenerative disc disease and low back pain. In this perspective, we provide an overview of recent CRISPR/Cas9 advances stemming from/with transferability to IVD research, outline possible treatment approaches for degenerative disc disease, and discuss current limitations that may hinder clinical translation.

Transcriptome and alternative splicing analysis of nucleus pulposus cells in response to high oxygen tension: Involvement of high oxygen tension in the pathogenesis of intervertebral disc degeneration

High oxygen tension caused by neovascularization in the microenvironment of intervertebral discs (IVDs) is associated with the pathogenesis of IVD degeneration (IDD). Pre-mRNAs undergo alternative splicing (AS) to produce structurally and functionally diverse mRNA and proteins. However, the precise role of high oxygen tension in IDD and the relationship between AS and high oxygen tension in disc cells remain unknown. To investigate the effect of high oxygen tension on disc cells, Affymetrix Rat Transcriptome Array 1.0 was used to determine differentially expressed genes (DEGs) and alternative splicing genes (ASGs) in rat nucleus pulposus (NP) cells treated with 20% O₂. NP cells at 1% O₂ served as the control. PCR was used for validation. GO and KEGG pathway analysis was performed. Furthermore, the reactive oxygen species (ROS) production, growth, cell cycle and matrix metabolism of NP cells were also investigated. In total, 2499 DEGs and 8451 ASGs were identified. Various GO terms and KEGG pathways were potently associated with IDD, including autophagy, mTOR signaling pathway and angiogenesis. Especially, high oxygen tension increased ROS production in NP cells. It also accelerated the matrix metabolism of NP cells and induced NP cell cycle arrest to retard cell growth. This study, for the first time, analyzes the transcriptome and AS of NP cells in response to high oxygen tension, indicating that high oxygen tension is involved in the establishment and progression of IDD through its wide effects on the viability and function of disc cells.

Cyclic mechanical tension reinforces DNA damage and activates the p53-p21-Rb pathway to induce premature senescence of nucleus pulposus cells

Intervertebral disc (IVD) degeneration (IDD) is a widely recognized contributor to low back pain. Mechanical stress is a crucial etiological factor of IDD. During the process of IDD, a vicious circle is formed between abnormal mechanical stress and the damage of disc structure and function. Notably, the pathological process of IDD is mediated by the phenotypic shift of IVD cells from an extracellular matrix anabolic phenotype to a catabolic and pro-inflammatory phenotype. Therefore, the effects of mechanical stress on the initiation and progression of IDD depend on the mechanobiology of IVD cells. Recently, disc cell senescence was identified as a new hallmark of IDD. However, the senescent response of disc cells to mechanical stress remains unknown. In this study, we found that prolonged exposure of cyclic mechanical tension (CMT) with unphysiological magnitude generated by the Flexercell tension system markedly induced premature senescence of nucleus pulposus (NP) cells. CMT augmented the DNA damage of NP cells, but did not affect the redox homeostasis of NP cells. Moreover, the p53-p21-retinoblastoma protein (Rb) pathway was activated by CMT to mediate the CMT-induced premature senescence of NP cells. The findings are beneficial to understanding the mechanism of disc cell senescence and the mechanobiology of disc cells further. It suggests that prolonged abnormal mechanical stress accelerates the establishment and progression of disc cell senescence and consequently impairs the structural and functional homeostasis of IVDs to cause IDD. Preventing the pro-senescent effect of mechanical stress on IVD cells is a promising approach to delay the process of IDD.

Accumulated Spinal Axial Biomechanical Loading Induces Degeneration in Intervertebral Disc of Mice Lumbar Spine

OBJECTIVE

To investigate the effect of accumulated spinal axial biomechanical loading on mice lumbar disc and the feasibility of applying this method to establish a mice intervertebral disc degeneration model using a custom-made hot plate cage. In previous studies, we observed that the motion pattern of mice was greatly similar to that of humans when they were standing and jumping on their lower limbs. There is little data to demonstrate whether or not accumulated spinal axial biomechanical loading could induce intervertebral disc degeneration in vivo.

METHODS

Twenty-four 0-week-old mice were randomly divided into model 1-month and 3-month groups, and control 1-month and 3-month groups (n = 6 per group). The model groups was transferred into the custom-made hot plate cage three times per day for modeling. The control group was kept in a regular cage. The intervertebral disc samples of the L₃ -L₅ were harvested for histologic, molecular, and immunohistochemical studies after modeling for 1 and 3 months.

RESULTS

Accumulated spinal axial biomechanical loading affects the histologic, molecular, and immunohistochemical changes of mice L_3- L₅ intervertebral discs. Decreased height of disc and endplate, fissures of annulus fibrosus, and ossification of cartilage endplate were found in morphological studies. Immunohistochemical studies of the protein level showed a similar expression of type II collagen at 1 month, but a slightly decreased expression at 3 months, and an increased expression level of type X collagen and matrix metalloproteinase 13 (MMP13). Molecular studies showed that ColIIa1 and aggrecan mRNA expression levels were slightly increased at 1 month (P > 0.05), but then decreased slightly (P > 0.05). ColXa1, ADAMTS-5, and MMP-13 expression levels werer increased both at 1 and 3 months (P < 0.05). In addition, increased expression of Runx2 was observed.

CONCLUSION

Accumulated spinal axial loading provided by a custom-made hot plate accelerated mice lumbar disc and especially endplate degeneration. However, this method requires further development to establish a lumbar disc degeneration model.

Dynamic imaging demonstrates that pulsed electromagnetic fields (PEMF) suppress IL-6 transcription in bovine nucleus pulposus cells

Inflammatory cytokines play a dominant role in the pathogenesis of disc degeneration. Pulsed electromagnetic fields (PEMF) are noninvasive biophysical stimulus that has been used extensively in the orthopaedic field for many years. However, the specific cellular responses and mechanisms involved are still unclear. The objective of this study was to assess the time-dependent PEMF effects on pro-inflammatory factor IL-6 expression in disc nucleus pulposus cells using a novel green fluorescence protein (GFP) reporter system. An MS2-tagged GFP reporter system driven by IL-6 promoter was constructed to visualize PEMF treatment effect on IL-6 transcription in single living cells. IL-6-MS2 reporter-labeled cells were treated with IL-1α to mimic the in situ inflammatory environment of degenerative disc while simultaneously exposed to PEMF continuously for 4 h. Time-lapse imaging was recorded using a confocal microscope to track dynamic IL-6 transcription activity that was demonstrated by GFP. Finally, real-time RT-PCR was performed to confirm the imaging data. Live cell imaging demonstrated that pro-inflammatory factor IL-1α significantly promoted IL-6 transcription over time as compared with DMEM basal medium condition. Imaging and PCR data demonstrated that the inductive effect of IL-1α on IL-6 expression could be significantly inhibited by PEMF treatment in a time-dependent manner (early as 2 h of stimulus initiation). Our data suggest that PEMF may have a role in the clinical management of patients with chronic low back pain. Furthermore, this study shows that the MS2-tagged GFP reporter system is a useful tool for visualizing the dynamic events of mechanobiology in musculoskeletal research. © 2017 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 35:778-787, 2018.

Biologic therapies to enhance intervertebral disc repair

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

Cell-Seeded Adhesive Biomaterial for Repair of Annulus Fibrosus Defects in Intervertebral Discs

Defects in the annulus fibrosus (AF) of intervertebral discs allow nucleus pulposus tissue to herniate causing painful disability. Microdiscectomy procedures remove herniated tissue fragments, but unrepaired defects remain allowing reherniation or progressive degeneration. Cell therapies show promise to enhance repair, but methods are undeveloped and carriers are required to prevent cell leakage. To address this challenge, this study developed and evaluated genipin-crosslinked fibrin (FibGen) as an adhesive cell carrier optimized for AF repair that can deliver cells, match AF material properties, and have low risk of extrusion during loading. Part 1 determined that feasibility of bovine AF cells encapsulated in high concentration FibGen (F140G6: 140 mg/mL fibrinogen; 6 mg/mL genipin) for 7 weeks could maintain high viability, but had little proliferation or matrix deposition. Part 2 screened tissue mechanics and in situ failure testing of nine FibGen formulations (fibrin: 35-140 mg/mL; genipin: 1-6 mg/mL). F140G6 formulation matched AF shear and compressive properties and significantly improved failure strength in situ. Formulations with reduced genipin also exhibited satisfactory material properties and failure behaviors warranting further biological screening. Part 3 screened AF cells encapsulated in four FibGen formulations for 1 week and found that reduced genipin concentrations increased cell viability and glycosaminoglycan production. F70G1 (70 mg/mL fibrinogen; 1 mg/mL genipin) demonstrated balanced biological and biomechanical performance warranting further testing. We conclude that FibGen has potential to serve as an adhesive cell carrier to repair AF defects with formulations that can be tuned to enhance biomechanical and biological performance; future studies are required to develop strategies to enhance matrix production.

Therapeutic effects of gefitinib-encapsulated thermosensitive injectable hydrogel in intervertebral disc degeneration

Intervertebral disc (IVD) degeneration is one of the most widespread musculoskeletal diseases worldwide, which remains an intractable clinical challenge. The aim of this study is to investigate the therapeutic potential of the small molecule gefitinib (an epidermal growth factor receptor (EGFR) inhibitor) in ameliorating IVD degeneration. Aberrant EGFR activation levels were detected in both human and rat degenerative IVDs, which prompted us to investigate the functional roles of EGFR by utilizing inducible cartilage-specific EGFR-deficient mice. We demonstrated that conditional EGFR deletion in mice increased nucleus pulposus (NP) extracellular matrix (ECM) production and autophagy marker activation while MMP13 expression decreased. These outcomes are comparable to the use of a controlled-release injectable thermosensitive hydrogel of gefitinib to block EGFR activity in a puncture-induced rat model. We also conducted a case series study involving patients with non-small cell lung cancer and IVD degeneration who received gefitinib treatment from 2010 to 2015. Gefitinib-treated patients displayed a relative slower disc degenerating progression, in contrast to control subjects. These findings thus provide evidence that suppression of EGFR by the FDA-approved drug gefitinib can protect IVD degeneration in rats, implying the potential application of gefitinib as a small molecule drug for treating IVD degeneration.

Innovative strategies for intervertebral disc regenerative medicine: From cell therapies to multiscale delivery systems

As our understanding of the physiopathology of intervertebral disc (IVD) degeneration has improved, novel therapeutic strategies have emerged, based on the local injection of cells, bioactive molecules, and nucleic acids. However, with regard to the harsh environment constituted by degenerated IVDs, protecting biologics from in situ degradation while allowing their long-term delivery is a major challenge. Yet, the design of the optimal approach for IVD regeneration is still under debate and only a few papers provide a critical assessment of IVD-specific carriers for local and sustained delivery of biologics. In this review, we highlight the IVD-relevant polymers as well as their design as macro-, micro-, and nano-sized particles to promote endogenous repair. Finally, we illustrate how multiscale systems, combining in situ-forming hydrogels with ready-to-use particles, might drive IVD regenerative medicine strategies toward innovation.

Human Mesenchymal Stem Cells Pretreated with Interleukin-1β and Stimulated with Bone Morphogenetic Growth Factor-3 Enhance Chondrogenesis

Low back pain is one of the most common ailments in western countries afflicting more than 80% of the population, and the main cause is considered to be degeneration of intervertebral discs. Interleukin-1β (IL-1β) is a vital inflammatory cytokine found in abundance in degenerated disc environment, whereas bone morphogenetic growth factor-3 (BMP-3) is believed to promote chondrogenesis through transforming growth factor-beta (TGF-β) pathway. The aim was to study the effects of BMP-3, IL-1β, and combination (pretreatment with IL-1β) on human mesenchymal stem cells (hMSCs) encapsulated in PuraMatrix™ hydrogel (Phg) especially in the absence of TGF-β in order to investigate the proliferation and differentiation ability of hMSCs over 28-day period. One hundred microliters of hMSCs' cell suspension was encapsulated between two layers of 100 μL hydrogels forming a sandwich-like structure. The encapsulated hMSCs were cultured in two sets of media, chondrogenic (C) and nonchondrogenic (nC) media, along with addition of BMP-3 (10 ng/mL) and IL-1β (10 ng/mL). To study the combined effects of BMP-3 and IL-1β, the encapsulated hMSCs were first pretreated with relevant media containing IL-1β for 24 h, and then the media was replaced by media containing BMP-3 for the remaining experimental time period. IL-1β pretreatment was carried out in both C and nC media. The samples were collected at day 7, 14, and 28. Proliferation and differentiation of hMSCs into chondrocyte-like cells were observed in all samples. Proteoglycan accumulation was observed in pretreatment samples in C media. The protein and gene expression of Sox-9 and COL2A1, respectively, showed the occurrence of chondrogenesis in all samples. High cell viability, proliferation, and differentiation were achieved in this in vitro model confirming that BMP-3 alone in the absence of TGF-β could drive hMSCs into chondrogenic lineage. Pretreatment with IL-1β followed by BMP-3 stimulation resulted in high proteoglycan accumulation compared to stimulation with growth factors or cytokine alone. This suggests that pretreatment with a pro-inflammatory cytokine before driving them into a chondrogenic lineage might be of importance also in vivo.

Spine-on-a-chip: Human annulus fibrosus degeneration model for simulating the severity of intervertebral disc degeneration

The aetiology of intervertebral disc (IVD) degeneration accompanied by low back pain (LBP) is largely unknown, and there are no effective fundamental therapies. Symptomatic IVD is known to be associated with nerve root compression. However, even in the absence of nerve compression, LBP occurs in patients with IVD degeneration. We hypothesize that this phenomenon is associated with a concentration of pro-inflammatory cytokines such as interleukin (IL)-1β and tumour necrosis factor-alpha (TNF-α), which can lead to altered histologic features and cellular phenotypes observed during IVD degeneration. This study investigated the effects of the concentration of IL-1β and macrophage derived soluble factor including IL-1β and TNF-α on the painful response of human annulus fibrosus (AF) cells using a newly developed spine-on-a-chip. Human AF cells were treated with a range of concentrations of IL-1β and macrophage soluble factors. Our results show that increasing the concentration of inflammatory initiator caused modulated expression of pain-related factors, angiogenesis molecules, and catabolic enzymes. Furthermore, accumulated macrophage derived soluble factors resulted in morphological changes in human AF cells and kinetic alterations such as velocity, dendritic length, cell area, and growth rate, similar to that reported within degenerative IVD. Thus, a better understanding of the relationships between molecular and kinetic alterations can provide fundamental information regarding the pathology of IVD degenerative progression.

Comparison of nucleus pulposus stem/progenitor cells isolated from degenerated intervertebral discs with umbilical cord derived mesenchymal stem cells

Mesenchymal stem-cell based therapies have been proposed as novel treatments for intervertebral disc (IVD) degeneration. The development of these treatment strategies, however, has been hindered by the incomplete understanding of the origin, biological properties of nucleus pulposus (NP) derived stem/progenitor cells and their effects on the IVD degeneration. The goal of this study is to explore the biological properties of NP stem/progenitor cells isolated from degenerated IVD (D-NPMSCs) regarding immunotype, proliferative capacity, multi-lineage differentiation abilities, and the expression of NP specific cell surface markers compared to human umbilical cord mesenchymal stem cells (UCMSCs). Our results indicate that although D-NPMSCs shared the mesenchymal stromal cells (MSCs) characteristics with UCMSCs, significant differences exist in phenotype signatures and biological capacities between D-NPMSCs and UCMSCs. D-NPMSCs expressed lower expression levels of CD29 and CD105, the phenotype markers of MSCs, and exhibited reduced proliferation capability and differentiation potentials, which might account for the distinct NP microenvironment and the poor capacity for disc regeneration. This study will lay a foundation for further understanding the mechanism of stem cell-based therapy for IVD degeneration.

Percutaneous posterolateral approach for the simulation of a far-lateral disc herniation in an ovine model

PURPOSE

This work describes a minimally invasive damage model for ovine lumbar discs via partial nucleotomy using a posterolateral approach.

METHODS

Two cadavers were dissected to analyze the percutaneous corridor. Subsequently, 28 ovine had their annulus fibrosus punctured via awl penetration under fluoroscopic control and nucleus pulposus tissue removed via rongeur. Efficacy was assessed by animal morbidity, ease of access to T12-S1 disc spaces, and production of a mechanical injury as verified by discography, radiography, and histology.

RESULTS

T12-S1 were accessible with minimal nerve damage morbidity. Scar tissue sealed the disc puncture site in all animals within 6 weeks, withstanding 1 MP of intradiscal pressure. Partial nucleotomy led to a significant reduction in intervertebral disk height and an increased histological degeneration score.

CONCLUSION

Inducing a reproducible injury pattern of disc degeneration required minimal time, effort, and equipment. The posterolateral approach allows operation on several discs within a single surgery and multiple animal surgeries within a single day.

Use of Limiting Dilution Method for Isolation of Nucleus Pulposus Mesenchymal Stem/Progenitor Cells and Effects of Plating Density on Biological Characteristics and Plasticity

Objectives

To evaluate the effects of the limiting dilution method and plating density in rat nucleus pulposus mesenchymal stem/progenitor cells (NPMSCs).

Materials and Methods

Nucleus pulposus tissues were isolated from 12-week-old male Sprague-Dawley rats and NPMSCs were isolated using limiting dilution method. Cells were then classified into 3 groups according to plating density. Cell morphologies were observed, and colony-forming units, migration abilities, proliferative capacities, cell cycle percentages, multilineage differentiation capacities, stem cell biomarker expression levels, and immunophenotyping were also examined in each group.

Results

Low density group (LD) had higher morphological homogeneity, stronger colony-forming ability, higher cell proliferation capacity, and enhanced cell migration ability relative to the other two groups (p < 0.05). Moreover, LD had more cells entering S phase, with fewer cells arrested in G0/G1 phase (p < 0.05). While all three density groups showed a multilineage differentiation potential, LD showed a higher degree of observed and semiquantified lineage specific staining (p < 0.05). Furthermore, LD displayed higher expression levels of stem cell biomarkers (Nanog, Oct4, and Sox2) and showed higher percentages of CD29+, CD44+, and CD90+ cells (p < 0.05) following flow cytometry analysis.

Conclusions

Limiting dilution method is suggested when isolating NPMSCs as a means of improving cell activity and plasticity.

Calcium Microcrystal Formation in Recurrent Herniation Patients After Autologous Disc Cell Transplantation

Autologous disc cell transplantation (ADCT) is a cell-based therapy aiming to initiate regeneration of intervertebral disc (IVD) tissue, but little is known about potential risks. This study aims to investigate the presence of structural phenomena accompanying the transformation process after ADCT treatment in IVD disease. Structural phenomena of ADCT-treated patients (Group 1, n = 10) with recurrent disc herniation were compared to conventionally-treated patients with recurrent herniation (Group 2, n = 10) and patients with a first-time herniation (Group 3, n = 10). For ethical reasons, a control group of ADCT patients who did not have a recurrent disc herniation was not possible. Tissue samples were obtained via micro-sequestrectomy after disc herniation and analyzed by micro-computed tomography, scanning electron microscopy, energy dispersive spectroscopy, and histology in terms of calcification zones, tissue structure, cell density, cell morphology, and elemental composition. The major differentiator between sample groups was calcium microcrystal formation in all ADCT samples, not found in any of the control group samples, which may indicate disc degradation. The incorporation of mineral particles provided clear contrast between the different materials and chemical analysis of a single particle indicated the presence of magnesium-containing calcium phosphate. As IVD calcification is a primary indicator of disc degeneration, further investigation of ADCT and detailed investigations assessing each patient's Pfirrmann degeneration grade following herniation is warranted. Structural phenomena unique to ADCT herniation prompt further investigation of the therapy's mechanisms and its effect on IVD tissue. However, the impossibility of a perfect control group limits the generalizable interpretation of the results.

Non-invasive Raman Spectroscopy and Quantitative Real-Time PCR Distinguish Among Undifferentiated Human Mesenchymal Stem Cells and Redifferentiated Nucleus Pulposus Cells and Chondrocytes In Vitro

Background:

The most common cause of lower back pain is the pathological degeneration of the nucleus pulposus (NP). Promising NP regeneration strategies involving human mesenchymal stem cells (hMSCs) would require specific markers to confirm successful differentiation into the NP lineage and to distinguish the articular cartilage (AC).

Objective:

We sought specific NP mRNA markers that are upregulated in native NP cells but not in dedifferentiated NP cells, undifferentiated hMSCs or chondrocytes. We also considered the suitability of non-invasive Raman spectroscopy to distinguish among these classes of cells.

Method:

We used quantitative real-time PCR and Raman spectroscopy to analyse undifferentiated hMSCs in monolayers and embedded in hydrogels, and compared the results with dedifferentiated and redifferentiated human NP and AC cells.

Results:

The redifferentiation of NP cells induced the expression of annexin A3 (ANXA3), collagen type II (COL2) and proteoglycan mRNAs, whereas the redifferentiation of AC cells only induced proteoglycan expression. Redifferentiated NP cells expressed higher levels of ANXA3, COL2, paired box 1 (PAX1) and OCT4 mRNA than redifferentiated AC cells. Redifferentiated NP cells and undifferentiated hMSC-TERT cells expressed similar amount of OCT4 mRNA, indicating that only ANXA3, COL2 and PAX1 are promising markers for redifferentiated NP cells. Raman spectra clearly differed among the three cell types and highlighted their differentiation status.

Conclusion:

We recommend ANXA3, COL2 and PAX1 as markers to determine the success of hMSC-based differentiation to regenerate NP cells. Raman spectroscopy can be used to determine cell type and differentiation status especially in the context of clinical trials.

Neuroinflammation, Bone Marrow Stem Cells, and Chronic Pain

Current treatments for chronic pain, such as inflammatory pain, neuropathic pain, and cancer pain are insufficient and cause severe side effects. Mounting evidence suggests that neuroinflammation in the peripheral and central nervous system (PNS and CNS) plays a pivotal role in the genesis and maintenance of chronic pain. Characteristic features of neuroinflammation in chronic pain conditions include infiltration of immune cells into the PNS [e.g., the sciatic nerve and dorsal root ganglion (DRG)], activation of glial cells such as microglia and astrocytes in the CNS (spinal cord and brain), and production and secretion of pro-inflammatory cytokines and chemokines [TNF, interleukin (IL)-1β, IL-6, CCL2, and CXCL1]. Recent studies suggest that bone marrow stem cells or bone marrow stromal cells (BMSCs) produce powerful analgesic effects in animal models of inflammatory pain, neuropathic pain, and cancer pain. We recently demonstrated that intrathecal injection of BMSCs resulted in a long-term relief of neuropathic pain for several weeks after peripheral nerve injury. Strikingly, this analgesic effect is mediated by the anti-inflammatory cytokine transforming growth factor beta secreted from BMSCs. Additionally, BMSCs exhibit potent modulation of neuroinflammation, by inhibiting monocyte infiltration, glial activation, and cytokine/chemokine production in the DRG and spinal cord. Thus, BMSCs control chronic pain by regulation of neuroinflammation in the PNS and CNS via paracrine signaling. In this review, we discuss the similar results from different laboratories of remarkable anti-nociceptive efficacy of BMSCs in animal and clinical studies. We also discuss the mechanisms by which BMSCs control neuroinflammation and chronic pain and how these cells specifically migrate to damaged tissues.

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

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

* CRISPR-Based Epigenome Editing of Cytokine Receptors for the Promotion of Cell Survival and Tissue Deposition in Inflammatory Environments

Musculoskeletal diseases have been associated with inflammatory cytokine action, particularly action by TNF-α and IL-1β. These inflammatory cytokines promote apoptosis and senescence of cells in diseased tissue and extracellular matrix breakdown. Stem cell-based therapies are being considered for the treatment of musculoskeletal diseases, but the presence of these inflammatory cytokines will have similar deleterious action on therapeutic cells delivered to these environments. Methods that prevent inflammatory-induced apoptosis and proinflammatory signaling, in cell and pathway-specific manners are needed. In this study we demonstrate the use of clustered regularly interspaced short palindromic repeats (CRISPR)-based epigenome editing to alter cell response to inflammatory environments by repressing inflammatory cytokine cell receptors, specifically TNFR1 and IL1R1. We targeted CRISPR/Cas9-based repressors to TNFR1 and IL1R1 gene regulatory elements in human adipose-derived stem cells (hADSCs) and investigated the functional outcomes of repression of these genes. Efficient signaling regulation was demonstrated in engineered hADSCs, as activity of the downstream transcription factor NF-κB was significantly reduced or maintained at baseline levels in the presence of TNF-α or IL-1β. Pellet culture of undifferentiated hADSCs demonstrated improved survival in engineered hADSCs treated with TNF-α or IL-1β, while having little effect on their immunomodulatory properties. Furthermore, engineered hADSCs demonstrated improved chondrogenic differentiation capacity in the presence of TNF-α or IL-1β, as shown by superior production of glycosaminglycans in this inflammatory environment. Overall this work demonstrates a novel method for modulating cell response to inflammatory signaling that has applications in engineering cells delivered to inflammatory environments, and as a direct gene therapy to protect endogenous cells exposed to chronic inflammation, as observed in a broad spectrum of degenerative musculoskeletal pathology.

Autologous bone marrow concentrate intradiscal injection for the treatment of degenerative disc disease with three-year follow-up

PURPOSE

The purpose of this study is to assess safety and feasibility of intradiscal bone marrow concentrate (BMC) injections to treat low back discogenic pain as an alternative to surgery with three year minimum follow-up.

METHODS

A total of 26 patients suffering from degenerative disc disease and candidates for spinal fusion or total disc replacement surgery were injected with 2 ml autologous BMC into the nucleus pulposus of treated lumbar discs. A sample aliquot of BMC was characterized by flow cytometry and CFU-F assay to determine progenitor cell content. Improvement in pain and disability scores and 12 month post-injection MRI were compared to patient demographics and BMC cellularity.

RESULTS

After 36 months, only six patients progressed to surgery. The remaining 20 patients reported average ODI and VAS improvements from 56.7 ± 3.6 and 82.1 ± 2.6 at baseline to 17.5 ± 3.2 and 21.9 ± 4.4 after 36 months, respectively. One year MRI indicated 40% of patients improved one modified Pfirrmann grade and no patient worsened radiographically. Cellular analysis showed an average of 121 million total nucleated cells per ml, average CFU-F of 2713 per ml, and average CD34+ of 1.82 million per ml in the BMC. Patients with greater concentrations of CFU-F (>2000 per ml) and CD34+ cells (>2 million per ml) in BMC tended to have significantly better clinical improvement.

CONCLUSIONS

There were no adverse events related to marrow aspiration or injection, and this study provides evidence of safety and feasibility of intradiscal BMC therapy. Patient improvement and satisfaction with this surgical alternative supports further study of the therapy.

* In Vitro Generated Intervertebral Discs: Toward Engineering Tissue Integration

The intervertebral disc (IVD) is composed of nucleus pulposus (NP) surrounded by multilamellated annulus fibrosus (AF), and is located between the vertebral bodies. Current treatments for chronic neck or low back pain do not completely restore the functionality of degenerated IVDs. Thus, developing biological disc replacements is an approach of great interest. Given the complex structure of the IVD, tissue engineering of the individual IVD components and then combining them together may be the only way to achieve this. The engineered disc must then be able to integrate into the host spine to ensure mechanical stability. The goal of this study was to generate an integrated model of an IVD in vitro. Multilamellated AF tissues were generated in vitro using aligned nanofibrous polycarbonate urethane scaffolds and AF cells. After 3 weeks in culture, it was placed around NP tissue formed on and integrated with a porous bone substitute material (calcium polyphosphate). The two tissues were cocultured to fabricate the IVD model. The AF tissue composed of six lamellae containing type I collagen-rich extracellular matrix (ECM) and the NP tissue had type II collagen- and aggrecan-rich ECM. Immunofluorescence studies showed both type I and II collagen at the AF-NP interface. There was evidence of integration of the tissues. The peel test for AF lamellae showed an interlamellar shear stress of 0.03 N/mm. The AF and NP were integrated as the pushout test demonstrated that the AF-NP interface had significantly increased mechanical stability by 2 weeks of coculture. To evaluate if these tissues remained integrated, allogeneic IVD model constructs were implanted into defects freshly made in the NP-inner AF and bone of the bovine coccygeal spine. One month postimplantation, the interfaces between the AF lamellae remained intact and there was integration with the host AF tissue. No inflammatory reaction was noted at this time period. In summary, an engineered IVD implant with mechanically stable integration between AF lamellae and AF-NP can be generated in vitro. Further study is required to scale up the size of this construct and evaluate its ability to serve as a biological disc replacement.

Injectable nanofibrous spongy microspheres for NR4A1 plasmid DNA transfection to reverse fibrotic degeneration and support disc regeneration

Safe and efficient gene therapy is highly desired for controlling pathogenic fibrosis of nucleus pulposus (NP) tissue, which would result in intervertebral disc (IVD) degeneration and disability if left untreated. In this work, a hyperbranched polymer (HP) with high plasmid DNA (pDNA) binding affinity and negligible cytotoxicity is synthesized, which can self-assemble into nano-sized polyplexes with a "double shell" structure that can transfect pDNA into NP cells with very high efficiency. These polyplexes are then encapsulated in biodegradable nanospheres (NS) to enable two-stage delivery: 1) temporally-controlled release of pDNA-carrying polyplexes and 2) highly efficient delivery of pDNA into cells by the released polyplexes. These biodegradable NS are co-injected with nanofibrous spongy microspheres (NF-SMS) to localize the cellular transfection of the pDNA encoding orphan nuclear receptor 4A1 (NR4A1), which was recently reported as a therapeutic agent to delay pathogenic fibrosis. It is shown that HP can transfect human NP cells efficiently in vitro with low cytotoxicity. The two-stage delivery system is able to present the polyplexes over a sustained time period (more than 30 days) in the tail of a rat. The NR4A1 pDNA carried by the HP polyplexes is found to therapeutically reduce the pathogenic fibrosis of NP tissue in a rat-tail degeneration model. In conclusion, the combination of the two-stage NR4A1 pDNA delivery NS and NF-SMS is able to repress fibrosis and to support IVD regeneration.

Nasal chondrocytes as a neural crest-derived cell source for regenerative medicine

Cells deriving from neural crest are generally acknowledged during embryonic development for their multipotency and plasticity, accounting for their capacity to generate various cell and tissue types even across germ layers. At least partial preservation of some of these properties in adulthood makes neural crest derived cells of large interest for regenerative purposes. Chondrocytes from fully mature nasal septum cartilage in adults are also derivatives of neural crest cells and were recently demonstrated to be able not only to maintain functionality across serial cloning, as surrogate self-renewal test, but also to respond and adapt to heterotopic transplantation sites. Based on these findings, cartilage grafts engineered by nasal chondrocytes were clinically used to reconstitute the nasal alar lobule and to repair articular cartilage defects. This article discusses further perspectives of potential clinical utility for nasal chondrocytes in musculoskeletal regeneration. It then highlights the need to derive deeper understanding of their biological properties in order to inform on possible therapeutic modes of action. This acquired knowledge will help to optimise manufacturing conditions to guarantee defined functional traits associated with safety and therapeutic potency of nasal chondrocytes in regenerative medicine.

A Histopathological Scheme for the Quantitative Scoring of Intervertebral Disc Degeneration and the Therapeutic Utility of Adult Mesenchymal Stem Cells for Intervertebral Disc Regeneration

The purpose of this study was to develop a quantitative histopathological scoring scheme to evaluate disc degeneration and regeneration using an ovine annular lesion model of experimental disc degeneration. Toluidine blue and Haematoxylin and Eosin (H&E) staining were used to evaluate cellular morphology: (i) disc structure/lesion morphology; (ii) proteoglycan depletion; (iii) cellular morphology; (iv) blood vessel in-growth; (v) cell influx into lesion; and (vi) cystic degeneration/chondroid metaplasia. Three study groups were examined: 5 × 5 mm lesion; 6 × 20 mm lesion; and 6 × 20 mm lesion plus mesenchymal stem cell (MSC) treatment. Lumbar intervertebral discs (IVDs) were scored under categories (i-vi) to provide a cumulative score, which underwent statistical analysis using STATA software. Focal proteoglycan depletion was associated with 5 × 5 mm annular rim lesions, bifurcations, annular delamellation, concentric and radial annular tears and an early influx of blood vessels and cells around remodeling lesions but the inner lesion did not heal. Similar features in 6 × 20 mm lesions occurred over a 3-6-month post operative period. MSCs induced a strong recovery in discal pathology with a reduction in cumulative histopathology degeneracy score from 15.2 to 2.7 (p = 0.001) over a three-month recovery period but no recovery in carrier injected discs.

Exosomes as potential alternatives to stem cell therapy for intervertebral disc degeneration: in-vitro study on exosomes in interaction of nucleus pulposus cells and bone marrow mesenchymal stem cells

Background

The stem cell-based therapies for intervertebral disc degeneration have been widely studied. However, the mechanisms of mesenchymal stem cells interacting with intervertebral disc cells, such as nucleus pulposus cells (NPCs), remain unknown. Exosomes as a vital paracrine mechanism in cell–cell communication have been highly focused on. The purpose of this study was to detect the role of exosomes derived from bone marrow mesenchymal stem cells (BM-MSCs) and NPCs in their interaction with corresponding cells.

Methods

The exosomes secreted by BM-MSCs and NPCs were purified by differential centrifugation and identified by transmission electron microscope and immunoblot analysis of exosomal marker proteins. Fluorescence confocal microscopy was used to examine the uptake of exosomes by recipient cells. The effects of NPC exosomes on the migration and differentiation of BM-MSCs were determined by transwell migration assays and quantitative RT-PCR analysis of NPC phenotypic genes. Western blot analysis was performed to examine proteins such as aggrecan, sox-9, collagen II and hif-1α in the induced BM-MSCs. Proliferation and the gene expression profile of NPCs induced by BM-MSC exosomes were measured by Cell Counting Kit-8 and qRT-PCR analysis, respectively.

Results

Both the NPCs and BM-MSCs secreted exosomes, and these exosomes underwent uptake by the corresponding cells. NPC-derived exosomes promoted BM-MSC migration and induced BM-MSC differentiation to a nucleus pulposus-like phenotype. BM-MSC-derived exosomes promoted NPC proliferation and healthier extracellular matrix production in the degenerate NPCs.

Conclusion

Our study indicates that the exosomes act as an important vehicle in information exchange between BM-MSCs and NPCs. Given a variety of functions and multiple advantages, exosomes alone or loaded with specific genes and drugs would be an appropriate option in a cell-free therapy strategy for intervertebral disc degeneration.

Comparison of biological characteristics of nucleus pulposus mesenchymal stem cells derived from non-degenerative and degenerative human nucleus pulposus

Cell therapy using mesenchymal stem cells provides a promising approach for the treatment of intervertebral disc degeneration (IDD). In recent years, human nucleus pulposus mesenchymal stem cells (NPMSCs) have been identified in nucleus pulposus tissue and displayed great potential for the regeneration of IDD. However, biological differences between non-degenerative and degenerative nucleus pulposus-derived NPMSCs have remained to be defined. The aim of the present study was to compare the biological characteristics of human NPMSCs derived from non-degenerative and degenerative nucleus pulposus. NPMSCs were isolated from non-degenerative and degenerative nucleus pulposus, which were assessed using the Pfirrmann grading system. The biological characteristics of the NPMSCs, including the expression of surface markers, multipotent differentiation, colony formation, chemotactic cell migration, cell activity and stemness gene expression were compared. It was found that NPMSCs could be obtained from non-degenerative and degenerative human nucleus pulposus. However, degenerative nucleus pulposus-derived NPMSCs displayed decreased ability of colony formation, chemotactic migration, cell activity and expression of stemness genes compared with non-degenerative nucleus pulposus-derived NPMSCs. Therefore, NPMSCs derived from non-degenerative and degenerative nucleus pulposus show different biological behaviors. The degenerative status of nucleus pulposus tissue should be considered when selecting NPMSCs as a source for clinical application.

Intervertebral disc response to stem cell treatment is conditioned by disc state and cell carrier: An ex vivo study

In vitro and in vivo studies evidenced that mesenchymal stem cells (MSCs) contribute to intervertebral disc (IVD) regeneration by differentiation towards the disc phenotype and matrix synthesis and/or by paracrine signalling to endogenous cells, thereby promoting a healthier disc phenotype in degenerative discs. The aim of this study was to investigate IVD response to human MSC (hMSC) treatment based on the disc degenerative state and hMSC carrier. Bovine caudal IVDs with endplates were cultured in a bioreactor under simulated physiological (0.1 Hz load and sufficient glucose) or degenerative (10 Hz load and limited glucose) conditions for 7 days. Discs were partially nucleotomised, restored with hMSCs in either fibrin gel or saline solution and cultured under physiological conditions for 7 days. Controls included fibrin and saline without hMSCs. Cell viability, histology, disc height, and gene expression analyses were performed to evaluate regeneration. hMSCs in fibrin were viable and homogenously distributed following 7 days of culture under dynamic loading in partially nucleotomised discs. IVD response to hMSCs was conditioned by both disc degenerative state and hMSC carrier. The effect of the regenerative treatment was stronger on simulated-degenerative discs than on simulated-physiological discs. hMSCs in fibrin induced a superior anabolic response in degenerative IVDs compared with fibrin alone, thus suggesting an added value of the cellular therapy compared with an acellular solution. When comparing fibrin and saline as a hMSC carrier, a significantly higher anabolic response was observed in IVDs treated with hMSCs in fibrin. Moreover, it was found that the degenerative state of the disc influenced hMSC differentiation. Indeed, a significantly higher expression of specific discogenic markers (ACAN and CA12) was observed in hMSCs implanted into physiological discs than in those implanted into degenerative discs. In conclusion, host disc cells and donor hMSC response depend on the degenerative state of the host disc and carrier used for hMSC delivery, and these two aspects need to be considered for a successful translation of hMSC therapies for the treatment of IVD degeneration.

Molecular Therapy for Degenerative Disc Disease: Clues from Secretome Analysis of the Notochordal Cell-Rich Nucleus Pulposus

Degenerative disc disease (DDD) is associated with spinal pain often leading to long-term disability. However, the non-chondrodystrophic canine intervertebral disc is protected from the development of DDD, ostensibly due to its retention of notochordal cells (NC) in the nucleus pulposus (NP). In this study, we hypothesized that secretome analysis of the NC-rich NP will lead to the identification of key proteins that delay the onset of DDD. Using mass-spectrometry, we identified 303 proteins including components of TGFβ- and Wnt-signaling, anti-angiogeneic factors and proteins that inhibit axonal ingrowth in the bioactive fractions of serum free, notochordal cell derived conditioned medium (NCCM). Ingenuity Pathway Analysis revealed TGFβ1 and CTGF as major hubs in protein interaction networks. In vitro treatment with TGFβ1 and CTGF promoted the synthesis of healthy extra-cellular matrix proteins, increased cell proliferation and reduced cell death in human degenerative disc NP cells. A single intra-discal injection of recombinant TGFβ1 and CTGF proteins in a pre-clinical rat-tail disc injury model restored the NC and stem cell rich NP. In conclusion, we demonstrate the potential of TGFβ1 and CTGF to mitigate the progression of disc degeneration and the potential use of these molecules in a molecular therapy to treat the degenerative disc.

The noncoding RNA linc-ADAMTS5 cooperates with RREB1 to protect from intervertebral disc degeneration through inhibiting ADAMTS5 expression

Previous studies have indicated the important roles of ADAMTS5 in intervertebral disc degeneration (IDD). However, the mechanisms that regulate ADAMTS5 expression in nuclear pulposus (NP) cells remain largely unknown. Evidence suggests that intergenic transcription may be associated with genes that encode transcriptional regulators. Here, we identified a long intergenic noncoding RNA, linc-ADAMTS5, which was transcribed in the opposite direction to ADAMTS5. In the present study, through mining computational algorithm programs, and publicly available data sets, we identified Ras-responsive element-binding protein 1 (RREB1) as a crucial transcription factor regulating the expression of ADAMTS5 in NP cells. RNA pull-down, RNA immunoprecipitation (RIP), in vitro binding assays, and gain- and loss-of-function studies indicated that a physical interaction between linc-ADAMTS5 and splicing factor proline/glutamine-rich (SFPQ) facilitated the recruitment of RREB1 to binding sites within the ADAMTS5 promoter to induce chromatin remodeling. This resulted in subdued ADAMTS5 levels in cultured NP cells involving histone deacetylases (HDACs). In clinical NP tissues, linc-ADAMTS5 and RREB1 were correlated negatively with ADAMTS5 expression. Taken together, these results demonstrate that RREB1 cooperates with noncoding RNA linc-ADAMTS5 to inhibit ADAMTS5 expression, thereby affecting degeneration of the extracellular matrix (ECM) of the intervertebral disc (IVD).

Biomaterials for intervertebral disc regeneration and repair

The intervertebral disc contributes to motion, weight bearing, and flexibility of the spine, but is susceptible to damage and morphological changes that contribute to pathology with age and injury. Engineering strategies that rely upon synthetic materials or composite implants that do not interface with the biological components of the disc have not met with widespread use or desirable outcomes in the treatment of intervertebral disc pathology. Here we review bioengineering advances to treat disc disorders, using cell-supplemented materials, or acellular, biologically based materials, that provide opportunity for cell-material interactions and remodeling in the treatment of intervertebral disc disorders. While a field still in early development, bioengineering-based strategies employing novel biomaterials are emerging as promising alternatives for clinical treatment of intervertebral disc disorders.

Age-Related Changes in Nucleus Pulposus Mesenchymal Stem Cells: An In Vitro Study in Rats

The functions of mesenchymal stem cells (MSCs) appear to decline with age due to cellular senescence, which could reduce the efficacy of MSCs-based therapies. Recently, MSCs have been identified in the nucleus pulposus, which offers great potential for intervertebral disc (IVD) repair. However, this potential might be affected by the senescence of nucleus pulposus MSCs (NPMSCs), but whether or not this exists remains unknown. The aim of this study was to investigate the age-related changes in NPMSCs. NPMSCs isolated from young (3-month-old) and old (14-month-old) Sprague-Dawley rats were cultured in vitro. Differences in morphology, proliferation, colony formation, multilineage differentiation, cell cycle, and expression of β-galactosidase (SA-β-gal) and senescent markers (p53, p21, and p16) were compared between groups. Both young and old NPMSCs fulfilled the criteria for definition as MSCs. Moreover, young NPMSCs presented better proliferation, colony-forming, and multilineage differentiation capacities than old NPMSCs. Old NPMSCs displayed senescent features, including significantly increased G0/G1 phase arrest, increased SA-β-gal expression, decreased S phase entry, and significant p53-p21-pRB pathway activation. Therefore, this is the first study demonstrating that senescent NPMSCs accumulate in IVD with age. The efficacy of NPMSCs is compromised by donor age, which should be taken into consideration prior to clinical application.