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

Notochordal cells: A potential therapeutic option for intervertebral disc degeneration

Intervertebral disc degeneration (IDD) is a prevalent musculoskeletal degenerative disorder worldwide, and ~40% of chronic low back pain cases are associated with IDD. Although the pathogenesis of IDD remains unclear, the reduction in nucleus pulposus cells (NPCs) and degradation of the extracellular matrix (ECM) are critical factors contributing to IDD. Notochordal cells (NCs), derived from the notochord, which rapidly degrades after birth and is eventually replaced by NPCs, play a crucial role in maintaining ECM homeostasis and preventing NPCs apoptosis. Current treatments for IDD only provide symptomatic relief, while lacking the ability to inhibit or reverse its progression. However, NCs and their secretions possess anti-inflammatory properties and promote NPCs proliferation, leading to ECM formation. Therefore, in recent years, NCs therapy targeting the underlying cause of IDD has emerged as a novel treatment strategy. This article provides a comprehensive review of the latest research progress on NCs for IDD, covering their biological characteristics, specific markers, possible mechanisms involved in IDD and therapeutic effects. It also highlights significant future directions in this field to facilitate further exploration of the pathogenesis of IDD and the development of new therapies based on NCs strategies.

Functional acellular matrix for tissue repair

In view of their low immunogenicity, biomimetic internal environment, tissue- and organ-like physicochemical properties, and functionalization potential, decellularized extracellular matrix (dECM) materials attract considerable attention and are widely used in tissue engineering. This review describes the composition of extracellular matrices and their role in stem-cell differentiation, discusses the advantages and disadvantages of existing decellularization techniques, and presents methods for the functionalization and characterization of decellularized scaffolds. In addition, we discuss progress in the use of dECMs for cartilage, skin, nerve, and muscle repair and the transplantation or regeneration of different whole organs (e.g., kidneys, liver, uterus, lungs, and heart), summarize the shortcomings of using dECMs for tissue and organ repair after refunctionalization, and examine the corresponding future prospects. Thus, the present review helps to further systematize the application of functionalized dECMs in tissue/organ transplantation and keep researchers up to date on recent progress in dECM usage.

Decellularized matrix for repairing intervertebral disc degeneration: Fabrication methods, applications and animal models

Intervertebral disc degeneration (IDD)-induced low back pain significantly influences the quality of life, placing a burden on public health systems worldwide. Currently available therapeutic strategies, such as conservative or operative treatment, cannot effectively restore intervertebral disc (IVD) function. Decellularized matrix (DCM) is a tissue-engineered biomaterial fabricated using physical, chemical, and enzymatic technologies to eliminate cells and antigens. By contrast, the extracellular matrix (ECM), including collagen and glycosaminoglycans, which are well retained, have been extensively studied in IVD regeneration. DCM inherits the native architecture and specific-differentiation induction ability of IVD and has demonstrated effectiveness in IVD regeneration in vitro and in vivo. Moreover, significant improvements have been achieved in the preparation process, mechanistic insights, and application of DCM for IDD repair. Herein, we comprehensively summarize and provide an overview of the roles and applications of DCM for IDD repair based on the existing evidence to shed a novel light on the clinical treatment of IDD.

Notochordal Cell-Based Treatment Strategies and Their Potential in Intervertebral Disc Regeneration

Chronic low back pain is the number one cause of years lived with disability. In about 40% of patients, chronic lower back pain is related to intervertebral disc (IVD) degeneration. The standard-of-care focuses on symptomatic relief, while surgery is the last resort. Emerging therapeutic strategies target the underlying cause of IVD degeneration and increasingly focus on the relatively overlooked notochordal cells (NCs). NCs are derived from the notochord and once the notochord regresses they remain in the core of the developing IVD, the nucleus pulposus. The large vacuolated NCs rapidly decline after birth and are replaced by the smaller nucleus pulposus cells with maturation, ageing, and degeneration. Here, we provide an update on the journey of NCs and discuss the cell markers and tools that can be used to study their fate and regenerative capacity. We review the therapeutic potential of NCs for the treatment of IVD-related lower back pain and outline important future directions in this area. Promising studies indicate that NCs and their secretome exerts regenerative effects, via increased proliferation, extracellular matrix production, and anti-inflammatory effects. Reports on NC-like cells derived from embryonic- or induced pluripotent-stem cells claim to have successfully generated NC-like cells but did not compare them with native NCs for phenotypic markers or in terms of their regenerative capacity. Altogether, this is an emerging and active field of research with exciting possibilities. NC-based studies demonstrate that cues from developmental biology can pave the path for future clinical therapies focused on regenerating the diseased IVD.

Decellularized extracellular matrix mediates tissue construction and regeneration

Contributing to organ formation and tissue regeneration, extracellular matrix (ECM) constituents provide tissue with three-dimensional (3D) structural integrity and cellular-function regulation. Containing the crucial traits of the cellular microenvironment, ECM substitutes mediate cell-matrix interactions to prompt stem-cell proliferation and differentiation for 3D organoid construction in vitro or tissue regeneration in vivo. However, these ECMs are often applied generically and have yet to be extensively developed for specific cell types in 3D cultures. Cultured cells also produce rich ECM, particularly stromal cells. Cellular ECM improves 3D culture development in vitro and tissue remodeling during wound healing after implantation into the host as well. Gaining better insight into ECM derived from either tissue or cells that regulate 3D tissue reconstruction or organ regeneration helps us to select, produce, and implant the most suitable ECM and thus promote 3D organoid culture and tissue remodeling for in vivo regeneration. Overall, the decellularization methodologies and tissue/cell-derived ECM as scaffolds or cellular-growth supplements used in cell propagation and differentiation for 3D tissue culture in vitro are discussed. Moreover, current preclinical applications by which ECM components modulate the wound-healing process are reviewed.

Hypoxia Regulates the Extracellular Matrix via Mitogen-Activated Protein Kinases Pathway in Cells Retrieved from the Human Intervertebral Disc

PURPOSE

The present study aimed to identify the physiological characteristics of cells by investigating the change in gene expression and protein levels during extracellular matrix (ECM) synthesis in the intervertebral disc (IVD) under hypoxic conditions.

MATERIALS AND METHODS

To test the effect of oxygen on cell growth and ECM synthesis of chondrocyte-like cells, the cells from IVD were separated and cultured in two hypoxia-mimicking systems: chemical hypoxic conditions using deferoxamine (DFO), and physiological hypoxic conditions using a hypoxic chamber for 7 days. Chondrocyte like cells cultured without DFO and under the normal oxygen concentration (21% O₂ and 5% CO₂, 37°C) served as the controls.

RESULTS

Chondrocyte-like cells cultured in the presence of 6% oxygen demonstrated a 100% increase in cellular proliferation compared to the control. The cells treated with chemical hypoxic conditions demonstrated a dose-dependent increase in the mRNA expression of glucose transporter-1, GAPDH, aggrecan, and type II collagen on Day 1. When treated with 100 µM DFO, the cells showed a 50% increase in the levels of proteoglycan protein on Day 7. The cells treated with chemical hypoxic condition demonstrated increase in sulfated glycosaminoglycan (GAG) protein levels on Day 7. Moreover, the cells cultured in the presence of 6% oxygen showed a 120% increase in sulfated GAG levels on Day 7.

CONCLUSION

The oxygen concentration had an important role in the viability, proliferation, and maturation of chondrocyte-like cells in IVD. In addition, chondrocyte-like cells are sensitive to the concentration of oxygen.

The potential role and trend of HIF‑1α in intervertebral disc degeneration: Friend or foe? (Review)

Lower back pain (LBP) is one of the most common reasons for seeking medical advice in orthopedic clinics. Increasingly, research has shown that symptomatic intervertebral disc degeneration (IDD) is mostly related to LBP. This review first outlines the research and findings of studies into IDD, from the physiological structure of the intervertebral disc (IVD) to various pathological cascades. The vicious cycles of IDD are re-described in relation to the analysis of the relationship among the pathological mechanisms involved in IDD. Interestingly, a ‘chief molecule’ was found, hypoxia-inducible factor-1α (HIF-1α), that may regulate all other mechanisms involved in IDD. When the vicious cycle is established, the low oxygen tension activates the expression of HIF-1α, which subsequently enters into the hypoxia-induced HIF pathways. The HIF pathways are dichotomized as friend and foe pathways according to the oxygen tension of the IVD microenvironment. Combined with clinical outcomes and previous research, the trend of IDD development has been predicted in this paper. Lastly, an early precautionary diagnosis and treatment method is proposed whereby nucleus pulposus tissue for biopsy can be obtained through IVD puncture guided by B-ultrasound when the patient is showing symptoms but MRI imaging shows negative results. The assessment criteria for biopsy and the feasibility, superiority and challenges of this approach have been discussed. Overall, it is clear that HIF-1α is an indispensable reference indicator for the accurate diagnosis and treatment of IDD.

Long Noncoding RNA ANPODRT Overexpression Protects Nucleus Pulposus Cells from Oxidative Stress and Apoptosis by Activating Keap1-Nrf2 Signaling

Oxidative stress and subsequent nucleus pulposus (NP) cell apoptosis are important contributors to the development of intervertebral disc degeneration (IDD). Emerging evidences show that long noncoding RNAs (lncRNAs) play a role in the pathogenesis of IDD. In this study, we investigated the role of lncRNA ANPODRT (anti-NP cell oxidative damage-related transcript) in oxidative stress and apoptosis in human NP cells. We found that ANPODRT was downregulated in degenerative NP tissues and in NP cells treated with tert-butyl hydroperoxide (TBHP, the oxidative stress inducer). ANPODRT overexpression alleviated oxidative stress and apoptosis in NP cells exposed to TBHP, while ANPODRT knockdown exerted opposing effects. Mechanistically, ANPODRT facilitated nuclear factor E2-related factor 2 (Nrf2) accumulation and nuclear translocation and activated its target genes by disrupting the kelch-like ECH-associated protein 1- (Keap1-) Nrf2 association in NP cells. Nrf2 knockdown abolished the antioxidative stress and antiapoptotic effects of ANPODRT in NP cells treated with TBHP. Collectively, our findings suggest that ANPODRT protects NP cells from oxidative stress and apoptosis, at least partially, by activating Nrf2 signaling, implying that ANPODRT may be a potential therapeutic target for IDD.

Current Progress in the Endogenous Repair of Intervertebral Disk Degeneration Based on Progenitor Cells

Intervertebral disk (IVD) degeneration is one of the most common musculoskeletal disease. Current clinical treatment paradigms for IVD degeneration cannot completely restore the structural and biomechanical functions of the IVD. Bio-therapeutic techniques focused on progenitor/stem cells, especially IVD progenitor cells, provide promising options for the treatment of IVD degeneration. Endogenous repair is an important self-repair mechanism in IVD that can allow the IVD to maintain a long-term homeostasis. The progenitor cells within IVD play a significant role in IVD endogenous repair. Improving the adverse microenvironment in degenerative IVD and promoting progenitor cell migration might be important strategies for implementation of the modulation of endogenous repair of IVD. Here, we not only reviewed the research status of treatment of degenerative IVD based on IVD progenitor cells, but also emphasized the concept of endogenous repair of IVD and discussed the potential new research direction of IVD endogenous repair.

Efficacy of matrilin-3-primed adipose-derived mesenchymal stem cell spheroids in a rabbit model of disc degeneration

BACKGROUND

Chronic low back pain is a prevalent disability, often caused by intervertebral disc (IVD) degeneration. Mesenchymal stem cell (MSC) therapy could be a safe and feasible option for repairing the degenerated disc. However, for successful translation to the clinic, various challenges need to be overcome including unwanted adverse effects due to acidic pH, hypoxia, and limited nutrition. Matrilin-3 is an essential extracellular matrix (ECM) component during cartilage development and ossification and exerts chondrocyte protective effects.

METHODS

This study evaluated the effects of matrilin-3-primed adipose-derived MSCs (Ad-MSCs) on the repair of the degenerated disc in vitro and in vivo. We determined the optimal priming concentration and duration and developed an optimal protocol for Ad-MSC spheroid generation.

RESULTS

Priming with 10 ng/ml matrilin-3 for 5 days resulted in the highest mRNA expression of type 2 collagen and aggrecan in vitro. Furthermore, Ad-MSC spheroids with a density of 250 cells/microwell showed the increased secretion of favorable growth factors such as transforming growth factor beta (TGF-β1), TGF-β2, interleukin-10 (IL-10), granulocyte colony-stimulating factor (G-CSF), and matrix metalloproteinase 1 (MMP1) and decreased secretion of hypertrophic ECM components. In addition, matrilin-3-primed Ad-MSC spheroid implantation was associated with optimal repair in a rabbit model.

CONCLUSION

Our results suggest that priming MSCs with matrilin-3 and spheroid formation could be an effective strategy to overcome the challenges associated with the use of MSCs for the treatment of IVD degeneration.

A prospect of cell immortalization combined with matrix microenvironmental optimization strategy for tissue engineering and regeneration

Cellular senescence is a major hurdle for primary cell-based tissue engineering and regenerative medicine. Telomere erosion, oxidative stress, the expression of oncogenes and the loss of tumor suppressor genes all may account for the cellular senescence process with the involvement of various signaling pathways. To establish immortalized cell lines for research and clinical use, strategies have been applied including internal genomic or external matrix microenvironment modification. Considering the potential risks of malignant transformation and tumorigenesis of genetic manipulation, environmental modification methods, especially the decellularized cell-deposited extracellular matrix (dECM)-based preconditioning strategy, appear to be promising for tissue engineering-aimed cell immortalization. Due to few review articles focusing on this topic, this review provides a summary of cell senescence and immortalization and discusses advantages and limitations of tissue engineering and regeneration with the use of immortalized cells as well as a potential rejuvenation strategy through combination with the dECM approach.

Functionality of decellularized matrix in cartilage regeneration: A comparison of tissue versus cell sources

Increasing evidence indicates that decellularized extracellular matrices (dECMs) derived from cartilage tissues (T-dECMs) or chondrocytes/stem cells (C-dECMs) can support proliferation and chondrogenic differentiation of cartilage-forming cells. However, few review papers compare the differences between these dECMs when they serve as substrates for cartilage regeneration. In this review, after an introduction of cartilage immunogenicity and decellularization methods to prepare T-dECMs and C-dECMs, a comprehensive comparison focuses on the effects of T-dECMs and C-dECMs on proliferation and chondrogenic differentiation of chondrocytes/stem cells in vitro and in vivo. Key factors within dECMs, consisting of microarchitecture characteristics and micromechanical properties as well as retained insoluble and soluble matrix components, are discussed in-depth for potential mechanisms underlying the functionality of these dECMs in regulating chondrogenesis. With this information, we hope to benefit dECM based cartilage engineering and tissue regeneration for future clinical application.

STATEMENT OF SIGNIFICANCE

The use of decellularized extracellular matrix (dECM) is becoming a promising approach for tissue engineering and regeneration. Compared to dECM derived from cartilage tissue, recently reported dECM from cell sources exhibits a distinct role in cell based cartilage regeneration. In this review paper, for the first time, tissue and cell based dECMs are comprehensively compared for their functionality in cartilage regeneration. This information is expected to provide an update for dECM based cartilage regeneration.

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

BACKGROUND CONTEXT

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

PURPOSE

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

STUDY DESIGN/SETTING

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

METHODS

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

RESULT

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

CONCLUSIONS

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

Meniscus, articular cartilage and nucleus pulposus: a comparative review of cartilage-like tissues in anatomy, development and function

The degradation of cartilage in the human body is impacted by aging, disease, genetic predisposition and continued insults resulting from daily activity. The burden of cartilage defects (osteoarthritis, rheumatoid arthritis, intervertebral disc damage, knee replacement surgeries, etc.) is daunting in light of substantial economic and social stresses. This review strives to broaden the scope of regenerative medicine and tissue engineering approaches used for cartilage repair by comparing and contrasting the anatomical and functional nature of the meniscus, articular cartilage (AC) and nucleus pulposus (NP). Many review papers have provided detailed evaluations of these cartilages and cartilage-like tissues individually but none have comprehensively examined the parallels and inconsistencies in signaling, genetic expression and extracellular matrix composition between tissues. For the first time, this review outlines the importance of understanding these three tissues as unique entities, providing a comparative analysis of anatomy, ultrastructure, biochemistry and function for each tissue. This novel approach highlights the similarities and differences between tissues, progressing research toward an understanding of what defines each tissue as distinctive. The goal of this paper is to provide researchers with the fundamental knowledge to correctly engineer the meniscus, AC and NP without inadvertently developing the wrong tissue function or biochemistry.

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.

A Protocol to Prepare Decellularized Stem Cell Matrix for Rejuvenation of Cell Expansion and Cartilage Regeneration

Traditional ex vivo expansion of adult stem cells yields an insufficient quantity of less potent cells. Here we describe the fabrication of decellularized matrix deposited by synovium-derived stem cells (SDSCs). This matrix could serve as a three-dimensional expansion system to rejuvenate cells for proliferation and tissue-specific differentiation potential, which could benefit cartilage regeneration. The decellularized stem cell matrix (DSCM) might be a powerful system for tissue engineering and regeneration.

Environmental preconditioning rejuvenates adult stem cells' proliferation and chondrogenic potential

Adult stem cells are a promising cell source for cartilage regeneration. Unfortunately, due to donor age and ex vivo expansion, stem cell senescence becomes a huge hurdle for these cells to be used clinically. Increasing evidence indicates that environmental preconditioning is a powerful approach in promoting stem cells' ability to resist a harsh environment post-engraftment, such as hypoxia and inflammation. However, few reports organize and evaluate the literature regarding the rejuvenation effect of environmental preconditioning on stem cell proliferation and chondrogenic differentiation capacity, which are important variables for stem cell based tissue regeneration. This report aims to identify several critical environmental factors such as oxygen concentration, growth factors, and extracellular matrix and to discuss their preconditioning influence on stem cells' rejuvenation including proliferation and chondrogenic potential as well as underlying molecular mechanisms. We believe that environmental preconditioning based rejuvenation is a simpler and safer strategy to program pre-engraftment stem cells for better survival and enhanced proliferation and differentiation capacity without the undesired effects of some treatments, such as genetic manipulation.

The influence of anisotropic nano- to micro-topography on in vitro and in vivo osteogenesis

AIM

Topographically modified substrates are increasingly used in tissue engineering to enhance biomimicry. The overarching hypothesis is that topographical cues will control cellular response at the cell-substrate interface.

MATERIALS & METHODS

The influence of anisotropically ordered poly(lactic-co-glycolic acid) substrates (constant groove width of ~1860 nm; constant line width of ~2220 nm; variable groove depth of ~35, 306 and 2046 nm) on in vitro and in vivo osteogenesis were assessed.

RESULTS & DISCUSSION

We demonstrate that substrates with groove depths of approximately 306 and 2046 nm promote osteoblast alignment parallel to underlined topography in vitro. However, none of the topographies assessed promoted directional osteogenesis in vivo.

CONCLUSION

2D imprinting technologies are useful tools for in vitro cell phenotype maintenance.

Advances in biological therapy for nucleus pulposus regeneration

OBJECTIVE

The intervertebral disc (IVD) is composed of the external annulus fibrosus (AF) and the inner gel-like center, the nucleus pulposus (NP). The elastic NP can function to relieve stress and maintain IVD function by distributing hydraulic pressure evenly to annulus and endplate. Degeneration of the NP, which leads to increased death of NP cells, the loss of proteoglycan (PG), and aberrant gene expression, may result in an overall alteration of the biomechanics of the spinal column and cause low back pain. Recent advances in biological therapy strategies that target therapy at the regeneration of degenerated and damaged NP have been investigated in in vitro and in vivo studies and demonstrated promising outcomes. In this article, we review recent studies of biological approaches for NP regeneration.

METHOD

The articles regarding NP regeneration using biomaterials, stem cells, and gene vectors were identified in PubMed databases.

RESULTS

Stem cell-mediated cell therapy demonstrates the potential to restore the function and structure of the NP. The viral or non-viral vectors encoding functional genes may generate a therapeutic effect when they are introduced into grafted cells or native cells in the NP. Biomaterial scaffolds generate an initial permissive environment for cell growth and allow the remodeling of scaffolds in the regeneration process. Biomaterial scaffolds provide structural support for NP regeneration and serve as a carrier for stem cell and gene vector delivery.

CONCLUSION

Though recent studies advance the body of knowledge needed to treat degenerated discs, many challenges need to be overcome before the application of these approaches can be successful clinically.

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.

Chondrogenic priming of human fetal synovium-derived stem cells in an adult stem cell matrix microenvironment

Cartilage defects are a challenge to treat clinically due to the avascular nature of cartilage. Low immunogenicity and extensive proliferation and multidifferentiation potential make fetal stem cells a promising source for regenerative medicine. In this study, we aimed to determine whether fetal synovium-derived stem cells (FSDSCs) exhibited replicative senescence and whether expansion on decellularized extracellular matrix (dECM) deposited by adult SDSCs (AECM) promoted FSDSCs' chondrogenic potential. FSDSCs from passage 2 and 9 were compared for chondrogenic potential, using Alcian blue staining for sulfated glycosaminoglycans (GAGs), biochemical analysis for DNA and GAG amounts, and real-time PCR for chondrogenic genes including ACAN and COL2A1. Passage 3 FSDSCs were expanded for one passage on plastic flasks (PL), AECM, or dECM deposited by fetal SDSCs (FECM). During expansion, cell proliferation was evaluated using flow cytometry for proliferation index, stem cell surface markers, and resistance to hydrogen peroxide. During chondrogenic induction, expanded FSDSCs were evaluated for tri-lineage differentiation capacity. We found that cell expansion enhanced FSDSCs' chondrogenic potential at least up to passage 9. Expansion on dECMs promoted FSDSCs' proliferative and survival capacity and adipogenic differentiation but not osteogenic capacity. AECM-primed FSDSCs exhibited an enhanced chondrogenic potential.

Delineation of in vitro chondrogenesis of human synovial stem cells following preconditioning using decellularized matrix

As a tissue-specific stem cell for chondrogenesis, synovium-derived stem cells (SDSCs) are a promising cell source for cartilage repair. However, a small biopsy can only provide a limited number of cells. Cell senescence from both in vitro expansion and donor age presents a big challenge for stem cell based cartilage regeneration. Here we found that expansion on decellularized extracellular matrix (dECM) full of three-dimensional nanostructured fibers provided SDSCs with unique surface profiles, low elasticity but large volume as well as a fibroblast-like shape. dECM expanded SDSCs yielded larger pellets with intensive staining of type II collagen and sulfated glycosaminoglycans compared to those grown on plastic flasks while SDSCs grown in ECM yielded 28-day pellets with minimal matrix as evidenced by pellet size and chondrogenic marker staining, which was confirmed by both biochemical data and real-time PCR data. Our results also found lower levels of inflammatory genes in dECM expanded SDSCs that might be responsible for enhanced chondrogenic differentiation. Despite an increase in type X collagen in chondrogenically induced cells, dECM expanded cells had significantly lower potential for endochondral bone formation. Wnt and MAPK signals were actively involved in both expansion and chondrogenic induction of dECM expanded cells. Since young and healthy people can be potential donors for this matrix expansion system and decellularization can minimize immune concerns, human SDSCs expanded on this future commercially available dECM could be a potential cell source for autologous cartilage repair.

Cell-derived matrices for tissue engineering and regenerative medicine applications

The development and application of decellularized extracellular matrices (ECM) has grown rapidly in the fields of cell biology, tissue engineering and regenerative medicine in recent years. Similar to decellularized tissues and whole organs, cell-derived matrices (CDMs) represent bioactive, biocompatible materials consisting of a complex assembly of fibrillar proteins, matrix macromolecules and associated growth factors that often recapitulate, at least to some extent, the composition and organization of native ECM microenvironments. The unique ability to engineer CDMs de novo based on cell source and culture methods makes them an attractive alternative to conventional allogeneic and xenogeneic tissue-derived matrices that are currently harvested from cadaveric sources, suffer from inherent heterogeneity, and have limited ability for customization. Although CDMs have been investigated for a number of biomedical applications, including adhesive cell culture substrates, synthetic scaffold coatings, and tissue engineered products, such as heart valves and vascular grafts, the state of the field is still at a relatively nascent stage of development. In this review, we provide an overview of the various applications of CDM and discuss successes to date, current limitations and future directions.

Hypoxia facilitates the survival of nucleus pulposus cells in serum deprivation by down-regulating excessive autophagy through restricting ROS generation

Nucleus pulposus (NP) cells reside in a hypoxic environment in vivo, while the mechanisms of how NP cells maintain survival under hypoxia are not clear. Autophagy is an important physiological response to hypoxia and implicated in the survival regulation in most types of cells. This study was designed to investigate the role of autophagy in the survival of NP cells under hypoxia. We found that appropriate autophagy activity was beneficial to the survival of NP cells in serum deprivation, while excessive autophagy led to death of the NP cells. Hypoxia facilitated the survival of NP cells in serum deprivation by down-regulating excessive autophagy. Hypoxia down-regulated the autophagy activity of NP cells through restricting the production of reactive oxygen species (ROS) and inactivating the AMPK/mTOR signaling pathway, and possibly through a pathway involving HIF-1α. We believed that understanding the autophagy response of NP cells to hypoxia and its role in cell survival had important clinical significance in the prevention and treatment of degenerative discogenic diseases.

Anti-inflammatory strategies in cartilage repair

Cartilage defects are normally concomitant with posttraumatic inflammation and pose a major challenge in cartilage repair. Due to the avascular nature of cartilage and its inability to surmount an inflammatory response, the cartilage is easily attacked by proinflammatory factors and oxidative stress; if left untreated, osteoarthritis may develop. Suppression of inflammation has always been a crux for cartilage repair. Pharmacological drugs have been successfully applied in cartilage repair; however, they cannot optimally work alone. This review article will summarize current pharmacological drugs and their application in cartilage repair. The development of extracellular matrix-based scaffolds and preconditioned tissue-specific stem cells will be emphasized because both of these tissue engineering components could contribute to an enhanced ability not only for cartilage regeneration but also for anti-inflammation. These strategies could be combined to boost cartilage repair under inflammatory conditions.

Hypoxia promotes nucleus pulposus phenotype in 3D scaffolds in vitro and in vivo: laboratory investigation

OBJECT

The role of oxygen in disc metabolism remains a matter of debate. Whether the effect of hypoxic priming on the nucleus pulposus phenotype can be maintained in vivo is not clear. The goal of the present study was to test the hypothesis that priming in a low oxygen tension in vitro could promote a nucleus pulposus phenotype in vivo.

METHODS

Bovine nucleus pulposus cells were seeded in 3D scaffolds and subjected to varying oxygen tensions (2% and 20%) for 3 weeks. The constructs were then implanted subcutaneously for 8 weeks. Changes in the extracellular matrix were evaluated using quantitative real-time reverse transcriptase polymerase chain reaction, glycosaminoglycan (GAG) assay, DNA assay, collagen quantification, and histological and immunohistological analyses.

RESULTS

Hypoxia resulted in greater production of sulfated glycosaminoglycan and higher levels of gene expression for collagen Type II, aggrecan, and SOX-9. Furthermore, after hypoxic priming, the subcutaneously implanted constructs maintained the nucleus pulposus phenotype, which was indicated by a significantly higher amount of glycosaminoglycan and collagen Type II.

CONCLUSIONS

Hypoxia enhanced the nucleus pulposus phenotype under experimental conditions both in vitro and in vivo. When used in combination with appropriate scaffold material, nucleus pulposus cells could be regenerated for tissue-engineering applications.

Significance of epigenetic landscape in cartilage regeneration from the cartilage development and pathology perspective

Regenerative therapies for cartilage defects have been greatly advanced by progress in both the stem cell biology and tissue engineering fields. Despite notable successes, significant barriers remain including shortage of autologous cell sources and generation of a stable chondrocyte phenotype using progenitor cells. Increasing demands for the treatment of degenerative diseases, such as osteoarthritis and rheumatoid arthritis, highlight the importance of epigenetic remodeling in cartilage regeneration. Epigenetic regulatory mechanisms, such as microRNAs, DNA methylation, and histone modifications, have been intensively studied due to their direct regulatory role on gene expression. However, a thorough understanding of the environmental factors that initiate these epigenetic events may provide greater insight into the prevention of degenerative diseases and improve the efficacy of treatments. In other words, if we could identify a specific factor from the environment and its downstream signaling events, then we could stop or retard degradation and enhance cartilage regeneration. A more operational definition of epigenetic remodeling has recently been proposed by categorizing the signals during the epigenetic process into epigenators, initiators, and maintainers. This review seeks to compile and reorganize the existing literature pertaining to epigenetic remodeling events placing emphasis on perceiving the landscape of epigenetic mechanisms during cartilage regeneration with the new operational definition, especially from the environmental factors' point of view. Progress in understanding epigenetic regulatory mechanisms could benefit cartilage regeneration and engineering on a larger scale and provide more promising therapeutic applications.

Rejuvenation of chondrogenic potential in a young stem cell microenvironment

Autologous cells suffer from limited cell number and senescence during ex vivo expansion for cartilage repair. Here we found that expansion on extracellular matrix (ECM) deposited by fetal synovium-derived stem cells (SDSCs) (FE) was superior to ECM deposited by adult SDSCs (AE) in promoting cell proliferation and chondrogenic potential. Unique proteins in FE might be responsible for the rejuvenation effect of FE while advantageous proteins in AE might contribute to differentiation more than to proliferation. Compared to AE, the lower elasticity of FE yielded expanded adult SDSCs with lower elasticity which could be responsible for the enhancement of chondrogenic and adipogenic differentiation. MAPK and noncanonical Wnt signals were actively involved in ECM-mediated adult SDSC rejuvenation.

Hypoxia differentially regulates human nucleus pulposus and annulus fibrosus cell extracellular matrix production in 3D scaffolds

OBJECTIVE

We hypothesize that intervertebral disc (IVD) cells from distinct region respond differently to oxygen environment, and that IVD cells from patients with disc degeneration can benefit from hypoxia condition. Therefore, we aimed to determine the transcriptional response and extracellular matrix (ECM) production of nucleus pulposus (NP) and annulus fibrosus (AF) cells to different oxygen tension.

METHOD

Human NP and AF from degenerated IVD were seeded in 3D scaffolds and subjected to varying oxygen tension (2% and 20%) for 3 weeks. Changes in ECM were evaluated using quantitative real-time reverse transcriptase polymerase chain reaction, histological and immunohistological analyses.

RESULTS

Hypoxia significantly enhances NP cells phenotype, which resulted in greater production of sulfated glycosaminoglycan (GAG) and collagen type II within the constructs and the cells expressed higher levels of genes encoding NP ECM. A significantly stronger fluorescent signal for hypoxia-inducible factor (HIF-1α) as also found in the NP cells under the hypoxic than normoxic condition. However, there was little effect of hypoxia on the AF cells.

CONCLUSIONS

The NP and AF cells respond differently to hypoxia condition on the 3D scaffold, and hypoxia could enhance NP phenotype. When used in concert with appropriate scaffold material, human NP cells from degenerated disc could be regenerated for tissue engineering application.

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.

The challenge and advancement of annulus fibrosus tissue engineering

BACKGROUND

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

MATERIAL AND METHODS

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

RESULTS

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

CONCLUSION

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