Coculture of synovium-derived stem cells and nucleus pulposus cells in serum-free defined medium with supplementation of transforming growth factor-beta1: a potential application of tissue-specific stem cells in disc regeneration

Co-culture engineering: a promising strategy for production of engineered extracellular vesicle for osteoarthritis treatment

The therapeutic effects of extracellular vesicles (EVs) have been identified as a significant factor in intercellular communication in different disease treatments, including osteoarthritis (OA). Compared to the conventional approaches in treating OA, EV therapy is a non-invasive and cell-free method. However, improving the yield of EVs and their therapeutic effects are the main challenges for clinical applications. In this regard, researchers are using the EV engineering potential to overcome these challenges. New findings suggest that the co-culture strategy as an indirect EV engineering method efficiently increases EV production and quality. The co-culture of mesenchymal stem cells (MSCs) and chondrocytes has improved their chondrogenesis, anti-inflammatory effects, and regenerative properties which are mediated by EVs. Hence, co-culture engineering by considerable systems could be useful in producing engineered EVs for different therapeutic applications. Here, we review various co-culture approaches, including diverse direct and indirect, 2D and 3D cell cultures, as well as static and dynamic systems. Meanwhile, we suggest and discuss the advantages of combined strategies to achieve engineered EVs for OA treatment.

A Review: Methodologies to Promote the Differentiation of Mesenchymal Stem Cells for the Regeneration of Intervertebral Disc Cells Following Intervertebral Disc Degeneration

Intervertebral disc (IVD) degeneration (IDD), a highly prevalent pathological condition worldwide, is widely associated with back pain. Treatments available compensate for the impaired function of the degenerated IVD but typically have incomplete resolutions because of their adverse complications. Therefore, fundamental regenerative treatments need exploration. Mesenchymal stem cell (MSC) therapy has been recognized as a mainstream research objective by the World Health Organization and was consequently studied by various research groups. Implanted MSCs exert anti-inflammatory, anti-apoptotic, and anti-pyroptotic effects and promote extracellular component production, as well as differentiation into IVD cells themselves. Hence, the ultimate goal of MSC therapy is to recover IVD cells and consequently regenerate the extracellular matrix of degenerated IVDs. Notably, in addition to MSC implantation, healthy nucleus pulposus (NP) cells (NPCs) have been implanted to regenerate NP, which is currently undergoing clinical trials. NPC-derived exosomes have been investigated for their ability to differentiate MSCs from NPC-like phenotypes. A stable and economical source of IVD cells may include allogeneic MSCs from the cell bank for differentiation into IVD cells. Therefore, multiple alternative therapeutic options should be considered if a refined protocol for the differentiation of MSCs into IVD cells is established. In this study, we comprehensively reviewed the molecules, scaffolds, and environmental factors that facilitate the differentiation of MSCs into IVD cells for regenerative therapies for IDD.

Human 3D nucleus pulposus microtissue model to evaluate the potential of pre-conditioned nasal chondrocytes for the repair of degenerated intervertebral disc

Introduction: An in vitro model that appropriately recapitulates the degenerative disc disease (DDD) microenvironment is needed to explore clinically relevant cell-based therapeutic strategies for early-stage degenerative disc disease. We developed an advanced 3D nucleus pulposus (NP) microtissues (µT) model generated with cells isolated from human degenerating NP tissue (Pfirrmann grade: 2-3), which were exposed to hypoxia, low glucose, acidity and low-grade inflammation. This model was then used to test the performance of nasal chondrocytes (NC) suspension or spheroids (NCS) after pre-conditioning with drugs known to exert anti-inflammatory or anabolic activities. Methods: NPµTs were formed by i) spheroids generated with NP cells (NPS) alone or in combination with ii) NCS or iii) NC suspension and cultured in healthy or degenerative disc disease condition. Anti-inflammatory and anabolic drugs (amiloride, celecoxib, metformin, IL-1Ra, GDF-5) were used for pre-conditioning of NC/NCS. The effects of pre-conditioning were tested in 2D, 3D, and degenerative NPµT model. Histological, biochemical, and gene expression analysis were performed to assess matrix content (glycosaminoglycans, type I and II collagen), production and release of inflammatory/catabolic factors (IL-6, IL-8, MMP-3, MMP-13) and cell viability (cleaved caspase 3). Results: The degenerative NPµT contained less glycosaminoglycans, collagens, and released higher levels of IL-8 compared to the healthy NPµT. In the degenerative NPµT, NCS performed superior compared to NC cell suspension but still showed lower viability. Among the different compounds tested, only IL-1Ra pre-conditioning inhibited the expression of inflammatory/catabolic mediators and promoted glycosaminoglycan accumulation in NC/NCS in DDD microenvironment. In degenerative NPµT model, preconditioning of NCS with IL-1Ra also provided superior anti-inflammatory/catabolic activity compared to non-preconditioned NCS. Conclusion: The degenerative NPµT model is suitable to study the responses of therapeutic cells to microenvironment mimicking early-stage degenerative disc disease. In particular, we showed that NC in spheroidal organization as compared to NC cell suspension exhibited superior regenerative performance and that IL-1Ra pre-conditioning of NCS could further improve their ability to counteract inflammation/catabolism and support new matrix production within harsh degenerative disc disease microenvironment. Studies in an orthotopic in vivo model are necessary to assess the clinical relevance of our findings in the context of IVD repair.

Co-aggregation of MSC/chondrocyte in a dynamic 3D culture elevates the therapeutic effect of secreted extracellular vesicles on osteoarthritis in a rat model

Extracellular vesicles (EVs) have therapeutic effects on osteoarthritis (OA). Some recent strategies could elevate EV's therapeutic properties including cell aggregation, co-culture, and 3D culture. It seems that a combination of these strategies could augment EV production and therapeutic potential. The current study aims to evaluate the quantity of EV yield and the therapeutic effect of EVs harvested from rabbit mesenchymal stem cells (MSCs) aggregates, chondrocyte aggregates, and their co-aggregates in a dynamic 3D culture in a rat osteoarthritis model. MSC and chondrocytes were aggregated and co-aggregated by spinner flasks, and their conditioned medium was collected. EVs were isolated by size exclusion chromatography and characterized in terms of size, morphology and surface markers. The chondrogenic potential of the MSC-ag, Cho-ag and Co-ag EVs on MSC micromass differentiation in chondrogenic media were assessed by qRT-PCR, histological and immunohistochemical analysis. 50 μg of MSC-ag-EVs, Cho-ag-EVs and Co-ag-EVs was injected intra-articularly per knee of OA models established by monoiodoacetate in rats. After 8 weeks follow up, the knee joints were harvested and analyzed by radiographic, histological and immunohistochemical features. MSC/chondrocyte co-aggregation in comparison to MSC or chondrocyte aggregation could increase EV yield during dynamic 3D culture by spinner flasks. Although MSC-ag-, Cho-ag- and Co-ag-derived EVs could induce chondrogenesis similar to transforming growth factor-beta during in vitro study, Co-ag-EV could more effectively prevent OA progression than MSC-ag- and Cho-ag-EVs. Our study demonstrated that EVs harvested from the co-aggregation of MSCs and chondrocytes could be considered as a new therapeutic potential for OA treatment.

Hypoxia Modulates Regenerative Potential of Fetal Stem Cells

Adult mesenchymal stem cells (MSCs) are prone to senescence, which limits the scope of their use in tissue engineering and regeneration and increases the likelihood of post-implantation failure. As a robust alternative cell source, fetal stem cells can prevent an immune reaction and senescence. However, few studies use this cell type. In this study, we sought to characterize fetal cells' regenerative potential in hypoxic conditions. Specifically, we examined whether hypoxic exposure during the expansion and differentiation phases would affect human fetal nucleus pulposus cell (NPC) and fetal synovium-derived stem cell (SDSC) plasticity and three-lineage differentiation potential. We concluded that fetal NPCs represent the most promising cell source for chondrogenic differentiation, as they are more responsive and display stronger phenotypic stability, particularly when expanded and differentiated in hypoxic conditions. Fetal SDSCs have less potential for chondrogenic differentiation compared to their adult counterpart. This study also indicated that fetal SDSCs exhibit a discrepancy in adipogenic and osteogenic differentiation in response to hypoxia.

Anti-hypertrophic effect of synovium-derived stromal cells on costal chondrocytes promotes cartilage repairs

Background

Costal chondrocytes (CCs), as a promising donor cell source for cell-based therapy for cartilage repair, have strong tendency of hypertrophy and calcification, which limited CCs from further application in cartilage regenerative medicine. Synovium-derived stromal cells (SDSCs), have shown their beneficial effect for chondrocytes to maintain phenotype. This study aims to investigate whether SDSCs could help CCs to maintain chondrogenic phenotype and suppress hypertrophic differentiation in cartilage repairs.

Methods

CCs were directly cocultured with SDSCs in pellet or indirectly cocultured using a conditioned medium in vitro for 3 weeks. Cartilage matrix formation and hypertrophic differentiation of CCs were analyzed by RT-PCR, biochemical assays, and histological staining. Cocultured pellets were implanted into the osteochondral defects made on the femoral groove of the rats. Then, macroscopic and histological evaluations were performed.

Results

Pellets formed by CCs alone and CCs cocultured with SDSCs reveal equal cartilage matrix deposition. However, the gene expression of type X collagen was significantly downregulated in cocultured pellets. Immunohistochemistry analysis revealed suppressed expression of type X collagen in cocultured pellets, indicating SDSCs may suppress hypertrophic differentiation of chondrocytes. Further in indirect coculture experiment, SDSCs suppressed type X collagen expression as well and promoted the proliferation of CCs, indicating SDSCs may influence CCs by paracrine mechanism. The pellets implanted in the osteochondral defects showed good restoration effects, whereas the grafts constructed with CCs and SDSCs showed lower type X expression levels.

Conclusion

These results suggest that SDSCs may maintain the phenotype of CCs and prevent the hypertrophic differentiation of CCs in cartilage repair.

The Translational Potential of this Article: CCs is a promising donor cell source for cell-based therapy for cartilage repair. Based on our study, cocultured with SDSCs weakened the tendency of hypertrophy and calcification of CCs, which provide a potential usage of SDSCs in CCs-based cartilage repair therapy to suppress newly formed cartilage calcification and improve clinical outcomes.

Cell sources proposed for nucleus pulposus regeneration

Lower back pain (LBP) occurs in 80% of adults in their lifetime; resulting in LBP being one of the biggest causes of disability worldwide. Chronic LBP has been linked to the degeneration of the intervertebral disc (IVD). The current treatments for chronic back pain only provide alleviation of symptoms through pain relief, tissue removal, or spinal fusion; none of which target regenerating the degenerate IVD. As nucleus pulposus (NP) degeneration is thought to represent a key initiation site of IVD degeneration, cell therapy that specifically targets the restoration of the NP has been reviewed here. A literature search to quantitatively assess all cell types used in NP regeneration was undertaken. With key cell sources: NP cells; annulus fibrosus cells; notochordal cells; chondrocytes; bone marrow mesenchymal stromal cells; adipose-derived stromal cells; and induced pluripotent stem cells extensively analyzed for their regenerative potential of the NP. This review highlights: accessibility; expansion capability in vitro; cell survival in an IVD environment; regenerative potential; and safety for these key potential cell sources. In conclusion, while several potential cell sources have been proposed, iPSC may provide the most promising regenerative potential.

Advances in Tissue Engineering for Disc Repair

Intervertebral disc (IVD) degeneration is a leading cause of chronic low back pain (LBP) that results in serious disability and significant economic burden. IVD degeneration alters the disc structure and spine biomechanics, resulting in subsequent structural changes throughout the spine. Currently, treatments of chronic LBP due to IVD degeneration include conservative treatments, such as pain medication and physiotherapy, and surgical treatments, such as removal of herniated disc without or with spinal fusion. However, none of these treatments can completely restore a degenerated disc and its function. Thus, although the exact pathogenesis of disc degeneration remains unclear, there are studies examining the effectiveness of biological approaches, such as growth factor injection, gene therapy, and cell transplantation, in promoting IVD regeneration. Furthermore, tissue engineering using a combination of cell transplantation and biomaterials has emerged as a promising new approach for repair or restoration of degenerated discs. The main purpose of this review was to provide an overview of the current status of tissue engineering applications for IVD regenerative therapy by performing literature searches using PubMed. Significant advances in tissue engineering have opened the door to a new generation of regenerative therapies for the treatment of chronic discogenic LBP.

Transforming growth factor-β1 (TGF-β1) induces mouse precartilaginous stem cell differentiation through TGFRII-CK1ε-β-catenin signalling

Precartilaginous stem cells (PSCs) are adult stem cells which could self-renew or differentiate into chondrocytes to promote bone growth. In this study, we aimed to understand the role of transforming growth factor-β1 (TGF-β1) in precartilaginous stem cell (PSC) differentiation and to study the mechanisms that underlie this role. We purified PSCs from the neonatal murine perichondrial mesenchyme using immunomagnetic beads, and primary cultured them. Their phenotype was confirmed by the PSC marker fibroblast growth factor receptor-3 (FGFR-3) overexpression. TGF-β1 was added to induce PSCs differentiation. TGF-β1 increased mRNA expression of chondrogenesis-related genes (collagen type II, Sox 9 and aggrecan) in the cultured PSCs. This was abolished by TGF-β receptor II (TGFRII) and Casein kinase 1 epsilon (CK1ε) lentiviral shRNA depletion. Meanwhile, we found that TGF-β1 induced CK1ε activation, glycogen synthase kinase-3β (GSK3β) phosphorylation and β-catenin nuclear translocation in the mouse PSCs, which was almost completely blocked by TGFRII and CK1ε shRNA knockdown. Based on these results, we suggest that TGF-β1 induces CK1ε activation to promote β-catenin nuclear accumulation, which then regulates chondrogenesis-related gene transcription to eventually promote mouse PSC differentiation.

Direct Coculture of Human Chondrocytes and Synovium-Derived Stem Cells Enhances In Vitro Chondrogenesis

Objective

Coculture of chondrocytes and mesenchymal stem cells (MSCs) has been developed as a strategy to overcome the dedifferentiation of chondrocytes during in vitro expansion in autologous chondrocyte transplantation. Synovium-derived stem cells (SDSCs) can be a promising cell source for coculture due to their superior chondrogenic potential compared to other MSCs and easy accessibility without donor site morbidity. However, studies on coculture of chondrocytes and SDSCs are very limited. The aim of this study was to investigate whether direct coculture of human chondrocytes and SDSCs could enhance chondrogenesis compared to monoculture of each cell.

Materials and Methods

In this experimental study, passage 2 chondrocytes and SDSCs were directly cocultured using different ratios of chondrocytes to SDSCs (3:1, 1:1, or 1:3). glycosaminoglycan (GAG) synthetic activity was assessed using GAG assays and Safranin-O staining. Expression of chondrogenesis-related genes (collagen types I, II, X, Aggrecan, and Sox-9) were analyzed by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and immunohistochemistry staining.

Results

GAG/DNA ratios in 1:1 and 1:3 coculture groups were significantly increased compared to those in the chondrocyte and SDSC monoculture groups. Type II collagen and SOX-9 were significantly upregulated in the 1:1 coculture group compared to those in the chondrocyte and SDSC monoculture groups. On the other hand, osteogenic marker (type I collagen) and hypertrophic marker (type X collagen) were significantly downregulated in the coculture groups compared to those in the SDSC monoculture group.

Conclusion

Direct coculture of human chondrocytes and SDSCs significantly enhanced chondrogenic potential, especially at a ratio of 1:1, compared to chondrocyte or SDSC monocultures.

Effect of adenovirus-mediated TGF-β1 gene transfer on the function of rabbit articular chondrocytes

BACKGROUND

Articular chondrocytes are important in maintaining normal cartilage tissue and preventing articular degeneration. Exogenous genes have previously been transduced into articular cells using adenoviral vectors to contribute to the maintenance of cell function. This study aimed to transfer the transforming growth factor-β1 gene (TGF-β1) into rabbit articular chondrocytes by adenovirus infection to elucidate its effects on cell function.

METHODS

Rabbit chondrocytes were isolated and cultured both as monolayers and three-dimensional culture systems. To achieve overexpression, TGF-β1 was transfected by adenovirus infection, using the LacZ gene as a control. TGF-β1 protein expression was analyzed by western blotting. Quantitative DNA fluorometric analysis evaluated cell proliferation, and quantitative reverse transcriptase PCR determined the mRNA expression of related chondrocyte marker genes. Western blotting and glycosaminoglycan quantitative testing were used to examine changes in extracellular matrix components.

RESULTS

TGF-β1 protein expression was found to increase in Adv-TGF-β1-transduced cells, reaching a maximum after chondrocytes had been cultured for 4 weeks. Adv-hTGF-β1 transduction altered chondrocyte morphology from fibrocyte-like long spindle-shaped to round or oval. TGF-β1-transduced cells showed an increase in DNA synthesis, glycosaminoglycan content, and increased aggrecan and collagen II protein expression, while collagen I was significantly decreased. Moreover, TGF-β1 overexpression significantly promoted the mRNA expression of the chondrogenic gene SOX9, and inhibited that of the hypertrophic marker COL10A1 and the mineralization marker MMP-13.

CONCLUSIONS

TGF-β1 overexpression positively improved the phenotype, function, and proliferation of chondrocytes, even after several generations.

Strategies in regenerative medicine for intervertebral disc repair using mesenchymal stem cells and bioscaffolds

The intervertebral disc (IVD) is a major weight bearing structure that undergoes degenerative changes with aging limiting its ability to dissipate axial spinal loading in an efficient manner resulting in the generation of low back pain. Low back pain is a number one global musculoskeletal disorder with massive socioeconomic impact. The WHO has nominated development of mesenchymal stem cells and bioscaffolds to promote IVD repair as primary research objectives. There is a clear imperative for the development of strategies to effectively treat IVD defects. Early preclinical studies with mesenchymal stem cells in canine and ovine models have yielded impressive results in IVD repair. Combinatorial therapeutic approaches encompassing biomaterial and cell-based therapies promise significant breakthroughs in IVD repair in the near future.

Use of FGF-2 and FGF-18 to direct bone marrow stromal stem cells to chondrogenic and osteogenic lineages

Aim:

Intervertebral disc degeneration/low back pain is the number one global musculoskeletal condition in terms of disability and socioeconomic impact.

Materials & methods

Multipotent mesenchymal stem cells (MSCs) were cultured in micromass pellets ± FGF-2 or -18 up to 41 days, matrix components were immunolocalized and gene expression monitored by quantitative-reverse transcription PCR.

Results:

Chondrogenesis occurred earlier in FGF-18 than FGF-2 cultures. Lower COL2A1, COL10A1 and ACAN expression by day 41 indicated a downregulation in chondrocyte hypertrophy. MEF2c, ALPL, were upregulated; calcium, decorin and biglycan, and 4C3 and 7D4 chondroitin sulphate sulfation motifs were evident in FGF-18 but not FGF-2 pellets.

Conclusion:

FGF-2 and -18 preconditioned MSCs produced cell lineages which promoted chondrogenesis and osteogenesis and may be useful in the production of MSC lineages suitable for repair of cartilaginous tissue defects.

Intervertebral disc degeneration and low back pain is the number one global musculoskeletal disorder effecting 80% of the general population. A remedy for this condition is being eagerly sought as part of a WHO research priority. Stem cells are one potential therapy that shows promise in animal models, laboratory studies, and preclinical and early clinical trials. Conditioning of stem cells in the laboratory before injection may improve their efficacy for the alleviation of low back pain. In the present study we have developed a means of improving how stem cells form cartilage and bone, which should be of application in the repair of spinal defects.

The trans-well coculture of human synovial mesenchymal stem cells with chondrocytes leads to self-organization, chondrogenic differentiation, and secretion of TGFβ

Background

Synovial mesenchymal stem cells (SMSC) possess a high chondrogenic differentiation potential, which possibly supports natural and surgically induced healing of cartilage lesions. We hypothesized enhanced chondrogenesis of SMSC caused by the vicinity of chondrocytes (CHDR).

Methods

Human SMSC and CHDR interactions were investigated in an in-vitro trans-well monolayer coculture over a time period of up to 21 days. Protein expression was analyzed using histology, immunostaining, or enzyme-linked immunosorbent assay. Additionally, mRNA expression was assessed by quantitative PCR.

Results

After 7 days, phase-contrast microscopy revealed cell aggregation of SMSC in coculture with CHDR. Afterwards, cells formed spheres and lost adherence. However, this phenomenon was not observed when culturing SMSC alone. Fluorescence labeling showed concurrent collagen type II expression. Addition of transforming growth factor beta (TGFβ) to the cocultures induced SMSC aggregation in less time and with higher intensity. Additionally, alcian blue staining demonstrated enhanced glycosaminoglycan expression around SMSC aggregates after 1 and 2 weeks. Although TGFβ mRNA was expressed in all SMSC, the protein was measured with constantly increasing levels over 21 days only in supernatants of the cocultures. Considering the enhanced mRNA levels following supplementation with TGFβ, a positive feedback mechanism can be supposed. In line with the development of a chondrogenic phenotype, aggrecan mRNA expression increased after 7 and 14 days in the cocultures with and without TGFβ. Coculture conditions also amplified collagen type II mRNA expression after 2 weeks without and already after 1 week with TGFβ. There was no difference in collagen type I and type X expression between SMSC alone and the coculture with CHDR. Expression of both collagens increased following addition of TGFβ. mRNA data correlated with the intensity of immunofluorescence staining.

Conclusions

Paracrine effects of CHDR induce a chondrogenic phenotype in SMSC possibly mimicking joint homeostasis. Coculture approaches may lead to a better understanding of cellular interactions with potential implications for cartilage repair procedures.

Tissue Engineering a Biological Repair Strategy for Lumbar Disc Herniation

The intervertebral disc is a critical part of the intersegmental soft tissue of the spinal column, providing flexibility and mobility, while absorbing large complex loads. Spinal disease, including disc herniation and degeneration, may be a significant contributor to low back pain. Clinically, disc herniations are treated with both nonoperative and operative methods. Operative treatment for disc herniation includes removal of the herniated material when neural compression occurs. While this strategy may have short-term advantages over nonoperative methods, the remaining disc material is not addressed and surgery for mild degeneration may have limited long-term advantage over nonoperative methods. Furthermore, disc herniation and surgery significantly alter the mechanical function of the disc joint, which may contribute to progression of degeneration in surrounding tissues. We reviewed recent advances in tissue engineering and regenerative medicine strategies that may have a significant impact on disc herniation repair. Our review on tissue engineering strategies focuses on cell-based and inductive methods, each commonly combined with material-based approaches. An ideal clinically relevant biological repair strategy will significantly reduce pain and repair and restore flexibility and motion of the spine.

Cell-based cartilage repair strategies in the horse

Damage to the articular cartilage surface is common in the equine athlete and, due to the poor intrinsic healing capabilities of cartilage, can lead to osteoarthritis (OA). Joint disease and OA are the leading cause of retirement in equine athletes and currently there are no effective treatments to stop the progression of OA. Several different cell-based strategies have been investigated to bolster the weak regenerative response of chondrocytes. Such techniques aim to restore the articular surface and prevent further joint degradation. Cell-based cartilage repair strategies include enhancement of endogenous repair mechanisms by recruitment of stem cells from the bone marrow following perforation of the subchondral bone plate; osteochondral implantation; implantation of chondrocytes that are maintained in defects by either a membrane cover or scaffold, and transplantation of mesenchymal stem cells into cartilage lesions. More recently, bioengineered cartilage and scaffoldless cartilage have been investigated for enhancing repair. This review article focuses on the multitude of cell-based repair techniques for cartilage repair across several species, with special attention paid to the horse.

Stem Cell Approaches to Intervertebral Disc Regeneration: Obstacles from the Disc Microenvironment

Intervertebral disc (IVD) degeneration results in segmental instability and irritates neural compressive symptoms, such as low back pain and motor deficiency. The transplanting of stem cell into degenerative discs has attracted increasing clinical attention, as a new and proven approach to alleviating disc degeneration and to relieving discogenic pains. Aside from supplementation with stem cells, the IVD itself already contains a pool of stem and progenitor cells. Since the resident disc stem cells are incapable of reversing the pathologic changes that occur during aging and disc degeneration, it has been debated as to whether transplanted stem cells are capable of providing an efficient and durable therapeutic effect, even though there have been positive outcomes in both animal models and in clinical trials. This review aims to decipher the interactions between the stem cell and the disc microenvironment. Within their new niches in the IVD, the exogenous stem cell shows metabolic adaptation to the low-glucose supply, hypoxia, and compressive loadings, but demonstrates little tolerance to the disc-like acidity and hypertonicity. Similarly, the survival of endogenous stem cells is threatened as well by the harsh disc microenvironment, which may exhaust the stem cell resources and restrict the self-repair capacity of a degenerating IVD. To eliminate the intrinsic obstacles within the stressful disc niches, stem cells should be delivered with an injectable scaffold that provides both survival and mechanical support. Quick healing or concretion of the injection injuries, which minimizes stem cell leakage and disturbance to disc homeostasis, is of equal importance toward achieving efficient stem cell-based disc regeneration.

Differentiation of Human Ligamentum Flavum Stem Cells Toward Nucleus Pulposus-Like Cells Induced by Coculture System and Hypoxia

STUDY DESIGN

Human ligamentum flavum (LF)-derived stem cells (LFSCs) and nucleus pulposus cells (NPCs) were cocultured under normoxia or hypoxia.

OBJECTIVE

To isolate and identify human LFSCs and determine whether they can differentiate into NPCs when cocultured with NPCs under hypoxia.

SUMMARY OF BACKGROUND DATA

Mesenchymal stem cell (MSC)-based therapies have been proposed as a biological treatment for intervertebral disc degeneration. MSCs derived from various tissues are leading candidates for cell-based therapies, but such cells have not been reported in LF.

METHODS

LF cells were isolated from patient samples and cultured using culture flasks coated with fibronectin, and their identity was confirmed using flow cytometry. The cells were induced to differentiate into osteoblasts, chondrocytes, and adipocytes, and their morphology, immunophenotype, cell proliferation capacity, cell cycle, and expression of stem cell-specific genes were compared with those of bone marrow-MSCs (BM-MSCs) derived from the same patients. NPCs and LFSCs were cocultured in 1-μm-pore-size insert transwell-culture systems under hypoxia (2% O2) or normoxia. CD24 expression was measured by flow cytometry and confocal microscopy assay. On day 14, reverse transcription-polymerase chain reaction was used for comparing the expression of chondrogenic genes (Sox-9, collagen-II, aggrecan) and novel marker genes (KRT19, CA12, FOXF1, HIF-1α) between the 2 groups.

RESULTS

LFSCs were obtained using the fibronectin differential-adhesion assay. The morphology of LFSCs was altered, and their immunophenotype, multilineage induction, cell proliferation capacity, cell cycle, and stem cell-specific gene expression were closely related-but not identical-to BM-MSCs, CD24 expression was highly significant in the differentiated LFSCs. RT/Real-time polymerase chain reaction revealed that compared with LFSCs grown under normoxia, hypoxia-treated LFSCs expressed higher levels of Sox-9, collagen-II, aggrecan, KRT19, CA12, and HIF-1α genes except FOXF1.

CONCLUSION

Stem cells were identified in human LF, and LFSCs cocultured with NPCs were successfully differentiated into NP-like cells under hypoxia. This potentially provides new cell candidates for cell-based regenerative medicine and tissue engineering.

LEVEL OF EVIDENCE

N/A.

Differentiation of menstrual blood-derived stem cells toward nucleus pulposus-like cells in a coculture system with nucleus pulposus cells

STUDY DESIGN

Human stromal stem cells derived from menstrual blood (MenSCs) and nucleus pulposus (NP) cells were cocultured under normal or low oxygen (O2) condition.

OBJECTIVE

To assess the differentiation capability of MenSCs toward nucleus pulposus cells under normal or low oxygen condition.

SUMMARY OF BACKGROUND DATA

Given the proliferative capacity and pluripotentiality of mesenchymal stem cells, mesenchymal stem cells transplantation is thought to be a promising approach to managing intervertebral disc degeneration.

METHODS

Using coculture plates with 0.4-μm pore size polyethylene terephthalate track-etched inserts, MenSCs and NP cells (1:1 ratio) were cocultured with cell-to-cell contact for 2 weeks in normal (20% O2) or low oxygen tension (2% O2), respectively. Extracellular matrix accumulation was quantified by dimethylmethylene blue assay, histological staining, and quantitative reverse-transcription polymerase chain reaction. Novel characteristic human NP markers cytokeratin-19 (KRT19), carbonic anhydrase XII (CA12), and forkhead box F1 (FoxF1) were also detected by quantitative reverse-transcription polymerase chain reaction.

RESULTS

The result of quantitative reverse-transcription polymerase chain reaction showed that aggrecan and COL2A1 genes expression was significantly increased in differentiated MenSCs (P < 0.05). There was significantly more COL2A1 gene expression in normoxic group than that in low O2 group (P < 0.05). But no significant difference was observed in aggrecan gene expression between normoxic group and low O2 group. These aforementioned results were also confirmed by histological analysis. We also found that the characteristic NP markers (KRT19, CA12, FoxF1) were significantly upregulated in differentiated MenSCs. Moreover, low O2 tension (2%) further enhanced these genes expression (P < 0.05).

CONCLUSION

In our study, MenSCs were successfully differentiated into NP-like cells and may become a new source of seed cells for the treatment of intervertebral disc degeneration in the future.

LEVEL OF EVIDENCE

N/A.

Stem cells in preclinical spine studies

BACKGROUND CONTEXT

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

PURPOSE

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

STUDY DESIGN

Review of the literature.

METHODS

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

RESULTS

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

CONCLUSIONS

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

Potential regenerative treatment strategies for intervertebral disc degeneration in dogs

Pain due to spontaneous intervertebral disc (IVD) disease is common in dogs. In chondrodystrophic (CD) dogs, IVD disease typically develops in the cervical or thoracolumbar spine at about 3–7 years of age, whereas in non-chondrodystrophic (NCD) dogs, it usually develops in the caudal cervical or lumbosacral spine at about 6–8 years of age. IVD degeneration is characterized by changes in the biochemical composition and mechanical integrity of the IVD. In the degenerated IVD, the content of glycosaminoglycan (GAG, a proteoglycan side chain) decreases and that of denatured collagen increases. Dehydration leads to tearing of the annulus fibrosus (AF) and/or disc herniation, which is clinically characterized by pain and/or neurological signs. Current treatments (physiotherapy, anti-inflammatory/analgesic medication, surgery) for IVD disease may resolve neurological deficits and reduce pain (although in many cases insufficient), but do not lead to repair of the degenerated disc. For this reason, there is interest in new regenerative therapies that can repair the degenerated disc matrix, resulting in restoration of the biomechanical function of the IVD. CD dogs are considered a suitable animal model for human IVD degeneration because of their spontaneous IVD degeneration, and therefore studies investigating cell-, growth factor-, and/or gene therapy-based regenerative therapies with this model provide information relevant to both human and canine patients. The aim of this article is to review potential regenerative treatment strategies for canine IVD degeneration, with specific emphasis on cell-based strategies.

Molecular and genetic advances in the regeneration of the intervertebral disc

Background:

Owing to the debilitating nature of degenerative disc disease (DDD) and other spine pathologies, significant research has been performed with the goal of healing or regenerating the intervertebral disc (IVD). Structural complexity, coupled with low vascularity and cellularity, make IVD regeneration an extremely challenging task.

Methods:

Tissue engineering-based strategies utilize three components to enhance tissue regeneration; scaffold materials to guide cell growth, biomolecules to enhance cell migration and differentiation, and cells (autologous, or allogeneic) to initiate the process of tissue formation. Significant advances in IVD regeneration have been made utilizing these tissue engineering strategies.

Results:

The current literature demonstrates that members of the transforming growth factor beta (TGF-β) superfamily are efficacious in the regeneration of an anabolic response in the IVD and to facilitate chondrogenic differentiation. Gene therapy, though thwarted by safety concerns and the risk of ectopic transfection, has significant potential for a targeted and sustained regenerative response. Stem cells in combination with injectable, biocompatible, and biodegradable scaffolds in the form of hydrogels can differentiate into de novo IVD tissue and facilitate regeneration of the existing matrix. Therapies that address both anabolism and the inherent catabolic state of the IVD using either direct inhibitors or broad-spectrum inhibitors show extensive promise.

Conclusion:

This review article summarizes the genetic and molecular advances that promise to play an integral role in the development of new strategies to combat DDD and promote healing of injured discs.

Reconstruction of an in vitro niche for the transition from intervertebral disc development to nucleus pulposus regeneration

The nucleus pulposus (NP) plays a prominent role in both the onset and progression of intervertebral disc degeneration. While autologous repair strategies have demonstrated some success, their in vitro culture system is outdated and insufficient for maintaining optimally functioning cells through the required extensive passaging. Consequently, the final population of cells may be unsuitable for the overwhelming task of repairing tissue in vivo and could result in subpar clinical outcomes. Recent work has identified synovium-derived stem cells (SDSCs) as a potentially important new candidate. This population of precursors can promote matrix regeneration and additionally restore the balance of catabolic and anabolic metabolism of surrounding cells. Another promising application is their ability to produce an extracellular matrix in vitro that can be modified via decellularization to produce a tissue-specific substrate for efficient cell expansion, while retaining chondrogenic potential. When combined with hypoxia, soluble factors, and other environmental regulators, the resultant complex microenvironment will more closely resemble the in vivo niche, which further improves the cell capacity, even after extensive passaging. In this review, the adaptive mechanisms NP cells utilize in vivo are considered for insight into what factors are important for constructing a tissue-specific in vitro niche. Evidence for the use of SDSCs for NP regeneration is also discussed. Many aspects of NP behavior are still unknown, which could lead to future work yielding key information on producing sufficient numbers of a high-quality NP-specific population that is able to regenerate deteriorated NP in vivo.

Differentiation of Human Adipose-derived Stem Cells along the Keratocyte Lineage In vitro

PURPOSE

To evaluate differentiation of human adipose-derived stem cells (hASCs) to the keratocyte lineage by co-culture with primary keratocytes in vitro.

MATERIALS AND METHODS

A co-culture system using transwell inserts to grow hASCs on bottom and keratocytes on top in keratocyte differentiating medium (KDM) was developed. hASCs that were cultured in complete culture medium (CCM) and KDM were used as control. After 16 days, hASCs were examined for morphologic changes and proliferation by cell count. qRT-PCR and flow cytometry were used to detect the expression of aldehyde dehydrogenase 3 family, member A1 (ALDH3A1) and keratocan.

RESULTS

hASCs became more dendritic and elongated in co-culture system relative to CCM and KDM. The doubling time of the cells was longer as differentiation progressed. qRT-PCR showed a definite trend towards increased expression of both ALDH3A1 and keratocan in co-culture system despite statistically non-significant p-values. Flow cytometry showed significantly increased protein levels of ALDH3A1 and keratocan in co-culture system relative to CCM group (p < 0.001) and even relative to KDM group (p < 0.001 for ALDH3A1 and p < 0.01 for keratocan).

CONCLUSION

The co-culture method is a promising approach to induce differentiation of stem cell populations prior to in vivo applications. This study reveals an important potential for bioengineering of corneal tissue using autologous multi-potential stem cells.

Modulation of in vitro microenvironment facilitates synovium-derived stem cell-based nucleus pulposus tissue regeneration

STUDY DESIGN

Two experiments were conducted. Experiment 1 evaluated the effect of 3 kinds of decellularized extracellular matrices (DECMs) deposited by synovium-derived stem cells (SDSCs) and/or nucleus pulposus cells (NPCs) on SDSC expansion and NP lineage differentiation. Experiment 2 evaluated the effect of DECM deposited by SDSCs on NPC expansion and redifferentiation capacity. In both experiments, hypoxia was evaluated in DECM preparation and pellet culture.

OBJECTIVE

Modulating the in vitro microenvironment facilitates SDSC-based NP tissue regeneration.

SUMMARY OF BACKGROUND DATA

Autologous cell therapy is a promising approach for NP regeneration. Current in vitro expansion in monolayer results in cell dedifferentiation.

METHODS

In Experiment 1, passage 3 SDSCs were expanded for 1 passage on DECM deposited by NPCs, SDSCs, or NPCs combined with SDSCs (50:50); DECM was prepared under either normoxia (21% O2) or hypoxia (5% O2). Expanded SDSCs were then cultured in a serum-free chondrogenic medium in hypoxia for 14 days. In Experiment 2, passage 2 NPCs were expanded for 1 passage on DECM deposited by SDSCs; DECM was prepared under either normoxia or hypoxia. Expanded NPCs were cultured in a serum-free chondrogenic medium under either hypoxia or normoxia for 14 days. Cell expansion on plastic flasks served as a control in both experiments. Fourteen-day pellets were evaluated for chondrogenesis using histology, immunostaining, biochemistry, and real-time polymerase chain reaction.

RESULTS

DECM deposited by NPCs combined with SDSCs effectively enhanced expanded SDSC viability and guided SDSC differentiation toward an NP lineage; this effect is comparable with DECM deposited by SDSCs but higher than that deposited by NPCs. DECM prepared under normoxia favored SDSC viability and NP lineage differentiation whereas DECM prepared under hypoxia benefited NPC viability and redifferentiation. Low oxygen in a pellet culture system enhanced NPC viability and redifferentiation.

CONCLUSION

The in vitro microenvironment can be modulated by low oxygen and tissue-specific cell-based DECM to facilitate NP tissue regeneration.

Structured bilaminar coculture outperforms stem cells and disc cells in a simulated degenerate disc environment

STUDY DESIGN

This study explores the use of bilaminar coculture pellets of mesenchymal stem cells (MSCs) and nucleus pulposus cells (NPCs) as a cell-based therapy for intervertebral disc regeneration. The pellets were tested under conditions that mimic the degenerative disc.

OBJECTIVE

Our goal was to optimize our cell-based therapy in vitro under conditions representative of the eventual diseased tissue.

SUMMARY OF BACKGROUND DATA

Harnessing the potential of stem cells is an important strategy for regenerative medicine. Our approach directed the behavior of stem cells by mimicking embryonic processes underlying cartilage and intervertebral disc development. Prior experiments have shown that bilaminar coculture can help differentiate MSC and substantially improve new matrix deposition.

METHODS

We have designed a novel spherical bilaminar cell pellet (BCP) where MSCs are enclosed in a shell of NPC. There were 3 groups: MSC, NPC, and BCP. The pellets were tested under 3 different culture conditions: 1) in a bioreactor that provides pressure and hypoxia (mimicking normal disc conditions): 2) with inflammatory cytokines (IL-1b and TNF-a); and 3) a bioreactor with inflammation (mimicking painful disc conditions).

RESULTS

When cultured in the bioreactor, the NPC pellets produced significantly more glycosaminoglycans (GAGs) per cell than the other groups: 70% to 80% more than the BCP and the MSC alone. When cultured in an inflammatory environment, the MSC and BCP groups produced 30% to 34% more GAGs per cell than NPC (P < 0.05). When the pellets were cultured in a bioreactor with inflammation, the BCP made 25% more GAGs per cell than the MSC and 57% more than the NPC (P < 0.05).

CONCLUSION

This study shows that BCPs outperform controls in a simulated degenerated disc environment. Adapting inductive mechanisms from development to trigger differentiation and restore diseased tissue has many advantages. As opposed to strategies that require growth factor supplements or genetic manipulations, our method is self-sustaining, targeted, and minimally invasive injection.

Structured coculture of mesenchymal stem cells and disc cells enhances differentiation and proliferation

PURPOSE

During in vivo stem cell differentiation, mature cells often induce the differentiation of nearby stem cells. Accordingly, prior studies indicate that a randomly mixed coculture can help transform mesenchymal stem cells (MSC) into nucleus pulposus cells (NPC). However, because in vivo signaling typically occurs heterotopically between adjacent cell layers, we hypothesized that a structurally organized coculture between MSC and NPC will result in greater cell differentiation and proliferation over single cell-type controls and cocultures with random organization.

METHODS

We developed a novel bilaminar cell pellet (BCP) system where a sphere of MSC is enclosed in a shell of NPC by successive centrifugation. Controls were made using single cell-type pellets and coculture pellets with random organization. The pellets were evaluated for DNA content, gene expression, and histology.

RESULTS

A bilaminar 3D organization enhanced cell proliferation and differentiation. BCP showed significantly more cell proliferation than pellets with one cell type and those with random organization. Enhanced differentiation of MSC within the BCP pellet relative to single cell-type pellets was demonstrated by quantitative RT-PCR, histology, and in situ hybridization.

CONCLUSIONS

The BCP culture system increases MSC proliferation and differentiation as compared to single cell type or randomly mixed coculture controls.

Engineered microenvironments for self-renewal and musculoskeletal differentiation of stem cells

Stem cells hold great promise for therapies aimed at regenerating damaged tissue, drug screening and studying in vitro models of human disease. However, many challenges remain before these applications can become a reality. One such challenge is developing chemically defined and scalable culture conditions for derivation and expansion of clinically viable human pluripotent stem cells, as well as controlling their differentiation with high specificity. Interaction of stem cells with their extracellular microenvironment plays an important role in determining their differentiation commitment and functions. Regenerative medicine approaches integrating cell-matrix and cell-cell interactions, and soluble factors could lead to development of robust microenvironments to control various cellular responses. Indeed, several of these recent developments have provided significant insight into the design of microenvironments that can elicit the targeted cellular response. In this article, we will focus on some of these developments with an emphasis on matrix-mediated expansion of human pluripotent stem cells while maintaining their pluripotency. We will also discuss the role of matrix-based cues and cell-cell interactions in the form of soluble signals in directing stem cell differentiation into musculoskeletal lineages.

Chondrogenesis and cartilage tissue engineering: the longer road to technology development

Joint injury and disease are painful and debilitating conditions affecting a substantial proportion of the population. The idea that damaged cartilage in articulating joints might be replaced seamlessly with tissue-engineered cartilage is of obvious commercial interest because the market for such treatments is large. Recently, a wealth of new information about the complex biology of chondrogenesis and cartilage has emerged from stem cell research, including increasing evidence of the role of physical stimuli in directing differentiation. The challenge for the next generation of tissue engineers is to identify the key elements in this new body of knowledge that can be applied to overcome current limitations affecting cartilage synthesis in vitro. Here we review the status of cartilage tissue engineering and examine the contribution of stem cell research to technology development for cartilage production.

Stem cell regeneration of the intervertebral disk

The use of stem cell applications has been explored and aimed at regenerating the intervertebral disk. The microenvironment in which cells of the intervertebral disk reside is harsh; however, researchers have reported on many applications for stem cells, including research aimed at defining and stimulating endogenous stem cell populations, methods to induce stem cell differentiation toward intervertebral disk cell phenotype in vivo, and direct transplantation of stem cells into damaged intervertebral disk to promote transplanted site-dependant differentiation. Successful results have been reported, although limitations remain. This article reviews the current status of stem cell research as applied to the intervertebral disk.

Effects of hypoxias and scaffold architecture on rabbit mesenchymal stem cell differentiation towards a nucleus pulposus-like phenotype

Nucleus pulposus grafts are needed for patients requiring replacement of their degenerated intervertebral discs. Bone marrow-derived mesenchymal stem cells (MSCs) are a potential autologous stem cell source for the nucleus pulposus regeneration. One of the key issues of constructing functional nucleus pulposus using MSCs, however, is to differentiate MSCs into nucleus pulposus phenotype in vitro and to maintain their phenotypic stability in vivo. In this study, three-dimensional (3D) nanofibrous poly(l-lactide) (PLLA) scaffolds were seeded with multi-potent rabbit MSCs and the constructs were induced along nucleus pulposus development routes in a hypoxia chamber (2% O(2)) in the presence of TGF-β1. It was found that nanofibrous scaffold could support the differentiation of rabbit MSCs towards a nucleus pulposus-like phenotype in vitro, as evidenced by upregulated expression of a few important nucleus pulposus-associated genes (aggrecan, type II collagen and Sox-9), abundant deposition of extracellular matrix (glycosaminoglycan (GAG) and type II collagen), and the continuous expression of the nucleus pulposus-specific marker, hypoxia-inducible factor (HIF)-1α. The subcutaneous implantation results confirmed that hypoxic induction before implantation could help the constructs to retain their phenotype and resist calcification in vivo. Therefore, the above data showed the promise of using 3D nanofibrous scaffolds in combination with TGF-β1 and hypoxic induction to regenerate functional nucleus pulposus grafts for intervertebral disc replacement.

Differential response of human bone marrow stromal cells to either TGF-β(1) or rhGDF-5

Cell therapy along with growth factor injection is currently widely investigated to restore the intervertebral disc. However, there is increasing evidence that transplanted unconditioned bone marrow-derived stromal cells (BMSCs) cannot thrive in the intervertebral disc "niche". Moreover, uncertainty exists with respect to the cell phenotype that would be suitable to inject. The intervertebral disc cell phenotype only recently has been started to be characterised using transcriptomics profiling. Recent findings suggest that cytokeratin 19 (KRT-19) could be used as a potential candidate marker for the intervertebral disc, or more specifically the nucleus pulposus cell (NPC) phenotype. We present in vitro cell culture data using alginate bead culture of primary human BMSCs exposed to the standard chondrogenic stimulus, transforming growth factor beta-1 (TGF-β), the growth and differentiation factor 5 and/or bovine NPCs to induce a potential "discogenic" pathway. Chondrogenic induction via TGF-β pathway provoked down-regulation of KRT-19 gene expression in four out of five donors after 18 days of culture, whereas KRT-19 expression remained unchanged in the "discogenic" groups. In addition, the ratio of aggrecan/collagen II gene expression showed a remarkable difference (of at least 3 magnitudes) between the chondrogenic stimulus (low ratio) and the discogenic stimulus (high ratio). Therefore, KRT-19 and aggrecan/collagen II ratio may be potential markers to distinguish chondrogenic from "discogenic" differentiation.

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

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

Exploiting notochord cells for stem cell-based regeneration of the intervertebral disc

The nucleus pulposus is an avascular and aneural tissue that has significant influence on the homeostasis and overall function of the intervertebral disc. The nucleus pulposus is comprised of a heterogeneous population of cells including large notochord cells and smaller chondrocyte-like cells. Loss of notochord cells has been correlated with the pathogenesis of disc degeneration and consequently, it has been hypothesized that regeneration of the disc could be mediated by notochord cells. Attempts to grow and expand notochord cells in vitro have thus far been limited by cell availability and ineffective culturing methodologies. As a result, co-culturing techniques have been developed in order to exploit notochord-derived signals for the differentiation of proliferative mesenchymal stem cells. A recent study by Korecki et al. has demonstrated that notochord cell conditioned medium has the ability to differentiate mesenchymal stem cells toward a nucleus pulposus-like fate, producing high levels of glycosaminoglycans and type III collagen. These findings suggest that growth factors and other soluble proteins may be able to stimulate endogenous IVD tissue maintenance in vivo. While this study advances our understanding of intervertebral disc cell-cell interactions, limitations remain in our ability to determine the phenotype of terminally differentiated cells within the nucleus pulposus (ie mature notochord cells) and therefore assess the relevance of differentiated mesenchymal stem cells for disc regeneration. In order for the field to progress, elucidation of the notochord phenotype remains of utmost importance.

Intradiscal transplantation of synovial mesenchymal stem cells prevents intervertebral disc degeneration through suppression of matrix metalloproteinase-related genes in nucleus pulposus cells in rabbits

INTRODUCTION

Synovial mesenchymal stem cells (MSCs) have high proliferative and chondrogenic potentials, and MSCs transplanted into the articular cartilage defect produce abundant extracellular matrix. Because of similarities between the articular cartilage and the intervertebral disc cartilage, synovial MSCs are a potential cell source for disc regeneration. Here, we examined the effect of intradiscal transplantation of synovial MSCs after aspiration of nucleus pulposus in rabbits.

METHODS

The nucleus pulposus tissues of rabbit's intervertebral discs were aspirated to induce disc degeneration, and allogenic synovial MSCs were transplanted. At 2, 4, 6, 8, 16, 24 weeks postoperatively, we evaluated with imaging analyses such as X-ray and magnetic resonance imaging (MRI), and histological analysis. To investigate interaction between synovial MSCs and nucleus pulposus cells, human synovial MSCs and rat nucleus pulposus cells were co-cultured, and species specific microarray were performed.

RESULTS

The existence of transplanted cells labeled with DiI or derived from green fluorescent protein (GFP)-expressing transgenic rabbits was confirmed up until 24 weeks. X-ray analyses demonstrated that intervertebral disc height in the MSC group remained higher than that in the degeneration group. T2 weighted MR imaging showed higher signal intensity of nucleus pulposus in the MSC group. Immunohistological analyses revealed higher expression of type II collagen around nucleus pulposus cells in the MSC group compared with even that of the normal group. In co-culture of rat nucleus pulposus cells and human synovial MSCs, species specific microarray revealed that gene profiles of nucleus pulposus were altered markedly with suppression of genes relating matrix degradative enzymes and inflammatory cytokines.

CONCLUSIONS

Synovial MSCs injected into the nucleus pulposus space promoted synthesis of the remaining nucleus pulposus cells to type II collagen and inhibition of expressions of degradative enzymes and inflammatory cytokines, resulting in maintaining the structure of the intervertebral disc being maintained.