Matrix stiffness determines the fate of nucleus pulposus-derived stem cells

Viscoelastic hydrogels regulate adipose-derived mesenchymal stem cells for nucleus pulposus regeneration

Low back pain is a leading cause of disability worldwide, often attributed to intervertebral disc (IVD) degeneration with loss of the functional nucleus pulposus (NP). Regenerative strategies utilizing biomaterials and stem cells are promising for NP repair. Human NP tissue is highly viscoelastic, relaxing stress rapidly under deformation. However, the impact of tissue-specific viscoelasticity on the activities of adipose-derived stem cells (ASC) remains largely unexplored. Here, we investigated the role of matrix viscoelasticity in regulating ASC differentiation for IVD regeneration. Viscoelastic alginate hydrogels with stress relaxation time scales ranging from 100 s to 1000s were developed and used to culture human ASCs for 21 days. Our results demonstrated that the fast-relaxing hydrogel significantly enhanced ASCs long-term cell survival and NP-like extracellular matrix secretion of aggrecan and type-II collagen. Moreover, gene expression analysis revealed a substantial upregulation of the mechanosensitive ion channel marker TRPV4 and NP-specific markers such as SOX9, HIF-1α, KRT18, CDH2 and CD24 in ASCs cultured within the fast-relaxing hydrogel, compared to slower-relaxing hydrogels. These findings highlight the critical role of matrix viscoelasticity in regulating ASC behavior and suggest that viscoelasticity is a key parameter for novel biomaterials design to improve the efficacy of stem cell therapy for IVD regeneration. STATEMENT OF SIGNIFICANCE: Systematically characterized the influence of tissue-mimetic viscoelasticity on ASC. NP-mimetic hydrogels with tunable viscoelasticity and tissue-matched stiffness. Long-term survival and metabolic activity of ASCs are substantially improved in the fast-relaxing hydrogel. The fast-relaxing hydrogel allows higher rate of cell protrusions formation and matrix remodeling. ASC differentiation towards an NP-like cell phenotype is promoted in the fast-relaxing hydrogel, with more CD24 positive expression indicating NP committed cell fate. The expression of TRPV4, a molecular sensor of matrix viscoelasticity, is significantly enhanced in the fast-relaxing hydrogel, indicating ASC sensing matrix viscoelasticity during cell development. The NP-specific ECM secretion of ASC is considerably influenced by matrix viscoelasticity, where the deposition of aggrecan and type-II collagen are significantly enhanced in the fast-relaxing hydrogel.

Combining biomimetic collagen/hyaluronan hydrogels with discogenic growth factors promotes mesenchymal stroma cell differentiation into Nucleus Pulposus like cells

Based on stem cell injection into degenerated Nucleus Pulposus (NP), novel treatments for intervertebral disc (IVD) regeneration were disappointing because of cell leakage or inappropriate cell differentiation. In this study, we hypothesized that mesenchymal stromal cells encapsulated within injectable hydrogels possessing adequate physico-chemical properties would differentiate into NP like cells. Composite hydrogels consisting of type I collagen and tyramine-substituted hyaluronic acid (THA) were prepared to mimic the NP physico-chemical properties. Human bone marrow derived mesenchymal stromal cells (BM-MSCs) were encapsulated within hydrogels and cultivated in proliferation medium (supplemented with 10% fetal bovine serum) or differentiation medium (supplemented with GDF5 and TGFβ1) over 28 days. Unlike pure collagen, collagen/THA composite hydrogels were stable over 28 days in culture. In proliferation medium, the cell viability within pure collagen hydrogels was high, whereas that in composite and pure THA hydrogels was lower due to the weaker cell adhesion. Nonetheless, BM-MSCs proliferated in all hydrogels. In composite hydrogels, cells exhibited a rounded morphology similar to NP cells. The differentiation medium did not impact the hydrogel stability and cell morphology but negatively impacted the cell viability in pure collagen hydrogels. A high THA content within hydrogels promoted the gene expression of NP markers such as collagen II, aggrecan, SOX9 and cytokeratin 18 at day 28. The differentiation medium potentialized this effect with an earlier and higher expression of these NP markers. Taken together, these results show that the physico-chemical properties of collagen/THA composite hydrogels and GDF5/TGFβ1 act in synergy to promote the differentiation of BM-MSCs into NP like cells.

Mechanical Signal-Tailored Hydrogel Microspheres Recruit and Train Stem Cells for Precise Differentiation

The aberrant mechanical microenvironment in degenerated tissues induces misdirection of cell fate, making it challenging to achieve efficient endogenous regeneration. Herein, a hydrogel microsphere-based synthetic niche with integrated cell recruitment and targeted cell differentiation properties via mechanotransduction is constructed . Through the incorporation of microfluidics and photo-polymerization strategies, fibronectin (Fn) modified methacrylated gelatin (GelMA) microspheres are prepared with the independently tunable elastic modulus (1-10Kpa) and ligand density (2 and 10 µg mL-1 ), allowing a wide range of cytoskeleton modulation to trigger the corresponding mechanobiological signaling. The combination of the soft matrix (2Kpa) and low ligand density (2 µg mL-1 ) can support the nucleus pulposus (NP)-like differentiation of intervertebral disc (IVD) progenitor/stem cells by translocating Yes-associated protein (YAP), without the addition of inducible biochemical factors. Meanwhile, platelet-derived growth factor-BB (PDGF-BB) is loaded onto Fn-GelMA microspheres (PDGF@Fn-GelMA) via the heparin-binding domain of Fn to initiate endogenous cell recruitment. In in vivo experiments, hydrogel microsphere-niche maintained the IVD structure and stimulated matrix synthesis. Overall, this synthetic niche with cell recruiting and mechanical training capabilities offered a promising strategy for endogenous tissue regeneration.

The role of biomechanical factors in models of intervertebral disc degeneration across multiple length scales

Low back pain is the leading cause of disability, producing a substantial socio-economic burden on healthcare systems worldwide. Intervertebral disc (IVD) degeneration is a primary cause of lower back pain, and while regenerative therapies aimed at full functional recovery of the disc have been developed in recent years, no commercially available, approved devices or therapies for the regeneration of the IVD currently exist. In the development of these new approaches, numerous models for mechanical stimulation and preclinical assessment, including in vitro cell studies using microfluidics, ex vivo organ studies coupled with bioreactors and mechanical testing rigs, and in vivo testing in a variety of large and small animals, have emerged. These approaches have provided different capabilities, certainly improving the preclinical evaluation of these regenerative therapies, but challenges within the research environment, and compromises relating to non-representative mechanical stimulation and unrealistic test conditions, remain to be resolved. In this review, insights into the ideal characteristics of a disc model for the testing of IVD regenerative approaches are first assessed. Key learnings from in vivo, ex vivo, and in vitro IVD models under mechanical loading stimulation to date are presented alongside the merits and limitations of each model based on the physiological resemblance to the human IVD environment (biological and mechanical) as well as the possible feedback and output measurements for each approach. When moving from simplified in vitro models to ex vivo and in vivo approaches, the complexity increases resulting in less controllable models but providing a better representation of the physiological environment. Although cost, time, and ethical constraints are dependent on each approach, they escalate with the model complexity. These constraints are discussed and weighted as part of the characteristics of each model.

The role of PIEZO ion channels in the musculoskeletal system

PIEZO1 and PIEZO2 are mechanosensitive cation channels that are highly expressed in numerous tissues throughout the body and exhibit diverse, cell-specific functions in multiple organ systems. Within the musculoskeletal system, PIEZO1 functions to maintain muscle and bone mass, sense tendon stretch, and regulate senescence and apoptosis in response to mechanical stimuli within cartilage and the intervertebral disc. PIEZO2 is essential for transducing pain and touch sensations as well as proprioception in the nervous system, which can affect musculoskeletal health. PIEZO1 and PIEZO2 have been shown to act both independently as well as synergistically in different cell types. Conditions that alter PIEZO channel mechanosensitivity, such as inflammation or genetic mutations, can have drastic effects on these functions. For this reason, therapeutic approaches for PIEZO-related disease focus on altering PIEZO1 and/or PIEZO2 activity in a controlled manner, either through inhibition with small molecules, or through dietary control and supplementation to maintain a healthy cell membrane composition. Although many opportunities to better understand PIEZO1 and PIEZO2 remain, the studies summarized in this review highlight how crucial PIEZO channels are to musculoskeletal health and point to promising possible avenues for their modulation as a therapeutic target.

Development of a Nanohybrid Peptide Hydrogel for Enhanced Intervertebral Disc Repair and Regeneration

Effective therapeutic approaches to overcome the heterogeneous pro-inflammatory and inhibitory extracellular matrix (ECM) microenvironment are urgently needed to achieve robust structural and functional repair of severely wounded fibrocartilaginous tissues. Herein we developed a dynamic and multifunctional nanohybrid peptide hydrogel (NHPH) through hierarchical self-assembly of peptide amphiphile modified with biodegradable two-dimensional nanomaterials with enzyme-like functions. NHPH is not only injectable, biocompatible, and biodegradable but also therapeutic by catalyzing the scavenging of pro-inflammatory reactive oxygen species and promoting ECM remodeling. In addition, our NHPH method facilitated the structural and functional recovery of the intervertebral disc (IVD) after severe injuries by delivering pro-regenerative cytokines in a sustained manner, effectively suppressing immune responses and eventually restoring the regenerative microenvironment of the ECM. In parallel, the NHPH-enhanced nucleus pulposus cell differentiation and pain reduction in a rat nucleotomy model further validated the therapeutic potential of NHPH. Collectively, our advanced nanoscaffold technology will provide an alternative approach for the effective treatment of IVD degeneration as well as other fibrocartilaginous tissue injuries.

Rheological characterization of cell-laden alginate-gelatin hydrogels for 3D biofabrication

Biofabrication of tissue models that closely mimic the tumor microenvironment is necessary for high-throughput anticancer therapeutics. Extrusion-based bioprinting of heterogeneous cell-laden hydrogels has shown promise in advancing rapid artificial tissue development. A major bottleneck limiting the rapid production of physiologically relevant tissue models is the current limitation in effectively printing large populations of cells. However, by significantly increasing hydrogel cell-seeding densities, the time required to produce tissues could be effectively reduced. Here, we explore the effects of increasing cell seeding densities on the viscoelastic properties, printability, and cell viability of two different alginate-gelatin hydrogel compositions. Rheological analysis of hydrogels of varying cell seeding densities reveals an inverse relationship between cell concentration and zero-shear viscosity. We also observe that as cell seeding densities increases, the storage moduli decrease, thus lowering the required printing pressures for gel extrusion. We also observe that increasing cell concentration can negatively impact the structural properties of the extruded material by increasing post-print line spreading. We find that hydrogels composed of higher molecular weight alginates and the highest cell-seeding densities (107 cells/mL) yield higher cell viability (>80%) and structural uniformity after printing. The optimized printing parameters determined for the alginate-gelatin bioinks explored may aid in the future rapid fabrication of functional tissue models for therapeutic screening.

The role of microenvironment in stem cell-based regeneration of intervertebral disc

Regenerative medicine for intervertebral disc (IVD) disease, by utilizing chondrocytes, IVD cells, and stem cells, has progressed to clinical trials in the treatment of back pain, and has been studied in various animal models of disc degeneration in the past decade. Stem cells exist in their natural microenvironment, which provides vital dynamic physical and chemical signals for their survival, proliferation and function. Long-term survival, function and fate of mesenchymal stem cells (MSCs) depend on the microenvironment in which they are transplanted. However, the transplanted MSCs and the endogenous disc cells were influenced by the complicated microenvironment in the degenerating disc with the changes of biochemical and biophysical components. It is important to understand how the MSCs and endogenous disc cells survive and thrive in the harsh microenvironment of the degenerative disc. Furthermore, materials containing stem cells and their natural microenvironment have good clinical effects. However, the implantation of tissue engineering IVD (TE-IVD) cannot provide a complete and dynamic microenvironment for MSCs. IVD graft substitutes may need further improvement to provide the best engineered MSC microenvironment. Additionally, the IVD progenitor cells inside the stem cell niches have been regarded as popular graft cells for IVD regeneration. However, it is still unclear whether actual IVD progenitor cells exist in degenerative spinal conditions. Therefore, the purpose of this review is fourfold: to discuss the presence of endogenous stem cells; to review and summarize the effects of the microenvironment in biological characteristics of MSC, especially those from IVD; to explore the feasibility and prospects of IVD graft substitutes and to elaborate state of the art in the use of MSC transplantation for IVD degeneration in vivo as well as their clinical application.

Nucleus pulposus cell senescence is regulated by substrate stiffness and is alleviated by LOX possibly through the integrin β1-p38 MAPK signaling pathway

Intervertebral disc degeneration (IVDD) is a main contributor to induce low back pain, and the pathogenic mechanism of IVDD remains unclear. The nucleus pulposus (NP) is a component of the intervertebral disc (IVD) that provides protection from mechanical stimuli. The matrix stiffness of NP tissue increases during the process of disc degeneration. Although several studies have found that pathological mechanical stimuli induce NP cell senescence, which is relevant for NP degeneration, however, the effect of matrix stiffness on NP cell senescence is not clear. Therefore, in the present study, we used polyvinyl alcohol (PVA) hydrogel with controllable stiffness to mimic the matrix stiffness of normal (4 kPa) and severely degenerated (20 kPa) NP tissue. Rat NP cells were isolated and cultured on substrates with different stiffness, and the cell proliferation, SA-β-gal activity, cell cycle, telomerase activity and the phenotype markers of NP cells were analyzed. Moreover, cytoskeleton staining and NP cellular Young's modulus on different substrates were also measured. To further investigate how substrate stiffness affects NP cell senescence, lysyl oxidase (LOX) was used to restore the extracellular matrix (ECM) synthesis of NP cells. The expression levels of integrin β1 and p38 MAPK were then measured. Our results showed that the 20 kPa substrate significantly induced NP cell senescence compared to the 4 kPa substrate. NP cells cultured on the 20 kPa substrate failed to maintain the expression of their phenotype markers. Furthermore, the 20 kPa substrate induced an increase of Young's modulus of NP cells, which possibly through up regulating the expressions of integrin β1 and p38 MAPK. These results indicated that the integrin β1-p38 MAPK signaling pathway may participated in substrate stiffness induced senescence of NP cells. LOX significantly increased ECM synthesis and inhibited substrate stiffness induced NP cell senescence, which indicated that matrix mechanics may be essential for maintaining the function of NP cell. Our results may provide a new perspective on the mechanism of IVDD by pathological matrix mechanics.

Impact of Microenvironmental Changes during Degeneration on Intervertebral Disc Progenitor Cells: A Comparison with Mesenchymal Stem Cells

Intervertebral disc (IVD) degeneration occurs with natural ageing and is linked to low back pain, a common disease. As an avascular tissue, the microenvironment inside the IVD is harsh. During degeneration, the condition becomes even more compromised, presenting a significant challenge to the survival and function of the resident cells, as well as to any regeneration attempts using cell implantation. Mesenchymal stem cells (MSCs) have been proposed as a candidate stem cell tool for IVD regeneration. Recently, endogenous IVD progenitor cells have been identified inside the IVD, highlighting their potential for self-repair. IVD progenitor cells have properties similar to MSCs, with minor differences in potency and surface marker expression. Currently, it is unclear how IVD progenitor cells react to microenvironmental factors and in what ways they possibly behave differently to MSCs. Here, we first summarized the microenvironmental factors presented in the IVD and their changes during degeneration. Then, we analyzed the available studies on the responses of IVD progenitor cells and MSCs to these factors, and made comparisons between these two types of cells, when possible, in an attempt to achieve a clear understanding of the characteristics of IVD progenitor cells when compared to MSCs; as well as, to provide possible clues to cell fate after implantation, which may facilitate future manipulation and design of IVD regeneration studies.

Development, Pathogenesis, and Regeneration of the Intervertebral Disc: Current and Future Insights Spanning Traditional to Omics Methods

The intervertebral disc (IVD) is the fibrocartilaginous joint located between each vertebral body that confers flexibility and weight bearing capabilities to the spine. The IVD plays an important role in absorbing shock and stress applied to the spine, which helps to protect not only the vertebral bones, but also the brain and the rest of the central nervous system. Degeneration of the IVD is correlated with back pain, which can be debilitating and severely affects quality of life. Indeed, back pain results in substantial socioeconomic losses and healthcare costs globally each year, with about 85% of the world population experiencing back pain at some point in their lifetimes. Currently, therapeutic strategies for treating IVD degeneration are limited, and as such, there is great interest in advancing treatments for back pain. Ideally, treatments for back pain would restore native structure and thereby function to the degenerated IVD. However, the complex developmental origin and tissue composition of the IVD along with the avascular nature of the mature disc makes regeneration of the IVD a uniquely challenging task. Investigators across the field of IVD research have been working to elucidate the mechanisms behind the formation of this multifaceted structure, which may identify new therapeutic targets and inform development of novel regenerative strategies. This review summarizes current knowledge base on IVD development, degeneration, and regenerative strategies taken from traditional genetic approaches and omics studies and discusses the future landscape of investigations in IVD research and advancement of clinical therapies.

3D Bioprinting of Multifunctional Dynamic Nanocomposite Bioinks Incorporating Cu-Doped Mesoporous Bioactive Glass Nanoparticles for Bone Tissue Engineering

Bioprinting has seen significant progress in recent years for the fabrication of bionic tissues with high complexity. However, it remains challenging to develop cell-laden bioinks exhibiting superior physiochemical properties and bio-functionality. In this study, a multifunctional nanocomposite bioink is developed based on amine-functionalized copper (Cu)-doped mesoporous bioactive glass nanoparticles (ACuMBGNs) and a hydrogel formulation relying on dynamic covalent chemistry composed of alginate dialdehyde (oxidized alginate) and gelatin, with favorable rheological properties, improved shape fidelity, and structural stability for extrusion-based bioprinting. The reversible dynamic microenvironment in combination with the impact of cell-adhesive ligands introduced by aminated particles enables the rapid spreading (within 3 days) and high survival (>90%) of embedded human osteosarcoma cells and immortalized mouse bone marrow-derived stroma cells. Osteogenic differentiation of primary mouse bone marrow stromal stem cells (BMSCs) and angiogenesis are promoted in the bioprinted alginate dialdehyde-gelatin (ADA-GEL or AG)-ACuMBGN scaffolds without additional growth factors in vitro, which is likely due to ion stimulation from the incorporated nanoparticles and possibly due to cell mechanosensing in the dynamic matrix. In conclusion, it is envisioned that these nanocomposite bioinks can serve as promising platforms for bioprinting complex 3D matrix environments providing superior physiochemical and biological performance for bone tissue engineering.

Advances and Prospects in Biomaterials for Intervertebral Disk Regeneration

Low-back and neck-shoulder pains caused by intervertebral disk degeneration are highly prevalent among middle-aged and elderly people globally. The main therapy method for intervertebral disk degeneration is surgical intervention, including interbody fusion, disk replacement, and diskectomy. However, the stress changes caused by traditional fusion surgery are prone to degeneration of adjacent segments, while non-fusion surgery has problems, such as ossification of artificial intervertebral disks. To overcome these drawbacks, biomaterials that could endogenously regenerate the intervertebral disk and restore the biomechanical function of the intervertebral disk is imperative. Intervertebral disk is a fibrocartilaginous tissue, primarily comprising nucleus pulposus and annulus fibrosus. Nucleus pulposus (NP) contains high water and proteoglycan, and its main function is absorbing compressive forces and dispersing loads from physical activities to other body parts. Annulus fibrosus (AF) is a multilamellar structure that encloses the NP, comprises water and collagen, and supports compressive and shear stress during complex motion. Therefore, different biomaterials and tissue engineering strategies are required for the functional recovery of NP and AF based on their structures and function. Recently, great progress has been achieved on biomaterials for NP and AF made of functional polymers, such as chitosan, collagen, polylactic acid, and polycaprolactone. However, scaffolds regenerating intervertebral disk remain unexplored. Hence, several tissue engineering strategies based on cell transplantation and growth factors have been extensively researched. In this review, we summarized the functional polymers and tissue engineering strategies of NP and AF to endogenously regenerate degenerative intervertebral disk. The perspective and challenges of tissue engineering strategies using functional polymers, cell transplantation, and growth factor for generating degenerative intervertebral disks were also discussed.

The Cell as Matter: Connecting Molecular Biology to Cellular Functions

Viewing cell as matter to understand the intracellular biomolecular processes and multicellular tissue behavior represents an emerging research area at the interface of physics and biology. Cellular material displays various physical and mechanical properties, which can strongly affect both intracellular and multicellular biological events. This review provides a summary of how cells, as matter, connect molecular biology to cellular and multicellular scale functions. As an impact in molecular biology, we review recent progresses in utilizing cellular material properties to direct cell fate decisions in the communities of immune cells, neurons, stem cells, and cancer cells. Finally, we provide an outlook on how to integrate cellular material properties in developing biophysical methods for engineered living systems, regenerative medicine, and disease treatments.

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.

Site-Directed Differentiation of Human Adipose-Derived Mesenchymal Stem Cells to Nucleus Pulposus Cells Using an Injectable Hydroxyl-Functional Diblock Copolymer Worm Gel

Adipose-derived mesenchymal stem cells (ASCs) have been identified for their promising therapeutic potential to regenerate and repopulate the degenerate intervertebral disk (IVD), which is a major cause of lower back pain. The optimal cell delivery system remains elusive but encapsulation of cells within scaffolds is likely to offer a decisive advantage over the delivery of cells in solution by ensuring successful retention within the tissue. Herein, we evaluate the use of a fully synthetic, thermoresponsive poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) (PGMA-PHPMA) diblock copolymer worm gel that mimics the structure of hydrophilic glycosaminoglycans. The objective was to use this gel to direct differentiation of human ASCs toward a nucleus pulposus (NP) phenotype, with or without the addition of discogenic growth factors TGFβ or GDF6. Accordingly, human ASCs were incorporated into a cold, free-flowing aqueous dispersion of the diblock copolymer, gelation induced by warming to 37 °C and cell culture was conducted for 14 days with or without such growth factors to assess the expression of characteristic NP markers compared to those produced when using collagen gels. In principle, the shear-thinning nature of the biocompatible worm gel enables encapsulated human ASCs to be injected into the IVD using a 21G needle. Moreover, we find significantly higher gene expression levels of ACAN, SOX-9, KRT8, and KR18 for ASCs encapsulated within worm gels compared to collagen scaffolds, regardless of the growth factors employed. In summary, such wholly synthetic worm gels offer considerable potential as an injectable cell delivery scaffold for the treatment of degenerate disk disease by promoting the transition of ASCs toward an NP-phenotype.

6-gingerol protects nucleus pulposus-derived mesenchymal stem cells from oxidative injury by activating autophagy

BACKGROUND

To date, there has been no effective treatment for intervertebral disc degeneration (IDD). Nucleus pulposus-derived mesenchymal stem cells (NPMSCs) showed encouraging results in IDD treatment, but the overexpression of reactive oxygen species (ROS) impaired the endogenous repair abilities of NPMSCs. 6-gingerol (6-GIN) is an antioxidant and anti-inflammatory reagent that might protect NPMSCs from injury.

AIM

To investigate the effect of 6-GIN on NPMSCs under oxidative conditions and the potential mechanism.

METHODS

The cholecystokinin-8 assay was used to evaluate the cytotoxicity of hydrogen peroxide and the protective effects of 6-GIN. ROS levels were measured by 2´7´-dichlorofluorescin diacetate analysis. Matrix metalloproteinase (MMP) was detected by the tetraethylbenzimidazolylcarbocyanine iodide assay. TUNEL assay and Annexin V/PI double-staining were used to determine the apoptosis rate. Additionally, autophagy-related proteins (Beclin-1, LC-3, and p62), apoptosis-associated proteins (Bcl-2, Bax, and caspase-3), and PI3K/Akt signaling pathway-related proteins (PI3K and Akt) were evaluated by Western blot analysis. Autophagosomes were detected by transmission electron microscopy in NPMSCs. LC-3 was also detected by immunofluorescence. The mRNA expression of collagen II and aggrecan was evaluated by real-time polymerase chain reaction (RT-PCR), and the changes in collagen II and MMP-13 expression were verified through an immunofluorescence assay.

RESULTS

6-GIN exhibited protective effects against hydrogen peroxide-induced injury in NPMSCs, decreased hydrogen peroxide-induced intracellular ROS levels, and inhibited cell apoptosis. 6-GIN could increase Bcl-2 expression and decrease Bax and caspase-3 expression. The MMP, Annexin V-FITC/PI flow cytometry and TUNEL assay results further confirmed that 6-GIN treatment significantly inhibited NPMSC apoptosis induced by hydrogen peroxide. 6-GIN treatment promoted extracellular matrix (ECM) expression by reducing the oxidative stress injury-induced increase in MMP-13 expression. 6-GIN activated autophagy by increasing the expression of autophagy-related markers (Beclin-1 and LC-3) and decreasing the expression of p62. Autophagosomes were visualized by transmission electron microscopy. Pretreatment with 3-MA and BAF further confirmed that 6-GIN-mediated stimulation of autophagy did not reduce autophagosome turnover but increased autophagic flux. The PI3K/Akt pathway was also found to be activated by 6-GIN. 6-GIN inhibited NPMSC apoptosis and ECM degeneration, in which autophagy and the PI3K/Akt pathway were involved.

CONCLUSION

6-GIN efficiently decreases ROS levels, attenuates hydrogen peroxide-induced NPMSCs apoptosis, and protects the ECM from degeneration. 6-GIN is a promising candidate for treating IDD.

Nucleus Pulposus Cell Conditioned Medium Promotes Mesenchymal Stem Cell Differentiation into Nucleus Pulposus-Like Cells under Hypoxic Conditions

Low back pain (LBP) is a major physical and socioeconomic challenge worldwide. Nucleus pulposus (NP) is directly associated with LBP due to intervertebral disc (IVD) degeneration. IVD degeneration is mainly caused by structural and matrix-related changes within the IVD occurring during aging and degeneration. Mesenchymal stem cells (MSCs) can differentiate into multiple mesenchymal lineages under specific stimulatory conditions. This study is aimed at evaluating the effectiveness of the nucleus pulposus cell (NPC) conditioned medium for promoting the expression of MSCs and at confirming the expression of healthy NP phenotypic markers recently recommended by the Spine Research Interest Group. Expression was investigated using quantitative polymerase chain reaction (qPCR) and western blotting under normoxic and hypoxic conditions. qPCR and western blotting demonstrated significant upregulation of NP marker expression in MSCs cultured under hypoxic conditions and treated with the 50% or 100% NPC conditioned medium, compared with those cultured under normoxic conditions. Upregulation was highest in the presence of the 100% NPC conditioned medium compared with the control group (aggrecan, p < 0.01; brachyury, p < 0.05; collagen II, p < 0.001; KRT8, p < 0.01; KRT19, p < 0.001; and Shh, p < 0.01). The expression levels of genes in MSCs treated with the 50% NPC conditioned medium also showed upregulation compared with the control group (collagen II, p < 0.05; KRT8, p < 0.05; and KRT19, p < 0.01). These findings suggested that the NPC conditioned medium stimulated MSC differentiation into an NP-like phenotype with distinct characteristics. The results could inform strategies for IVD regeneration.

Kelly M, M. Buchowski J, P. Zebala L, Luhmann S, C. Gupta M, A. Setton L. Verteporfin treatment controls morphology, phenotype, and global gene expression for cells of the human nucleus pulposus

Cells of the nucleus pulposus (NP) are essential contributors to extracellular matrix synthesis and function of the intervertebral disc. With age and degeneration, the NP becomes stiffer and more dehydrated, which is associated with a loss of phenotype and biosynthetic function for its resident NP cells. Also, with aging, the NP cell undergoes substantial morphological changes from a rounded shape with pronounced vacuoles in the neonate and juvenile, to one that is more flattened and spread with a loss of vacuoles. Here, we make use of the clinically relevant pharmacological treatment verteporfin (VP), previously identified as a disruptor of yes-associated protein-TEA domain family member-binding domain (TEAD) signaling, to promote morphological changes in adult human NP cells in order to study variations in gene expression related to differences in cell shape. Treatment of adult, degenerative human NP cells with VP caused a shift in morphology from a spread, fibroblastic-like shape to a rounded, clustered morphology with decreased transcriptional activity of TEAD and serum-response factor. These changes were accompanied by an increased expression of vacuoles, NP-specific gene markers, and biosynthetic activity. The contemporaneous observation of VP-induced changes in cell shape and prominent, time-dependent changes within the transcriptome of NP cells occurred over all timepoints in culture. Enriched gene sets with the transition to VP-induced cell rounding suggest a major role for cell adhesion, cytoskeletal remodeling, vacuolar lumen, and MAPK activity in the NP phenotypic and functional response to changes in cell shape.

Balancing biological and biomechanical performance in intervertebral disc repair: a systematic review of injectable cell delivery biomaterials

Discogenic back pain is a common condition without approved intervertebral disc (IVD) repair therapies. Cell delivery using injectable biomaterial carriers offers promise to restore disc height and biomechanical function, while providing a functional niche for delivered cells to repair degenerated tissues. This systematic review advances the injectable IVD cell delivery biomaterials field by characterising its current state and identifying themes of promising strategies. Preferred Reporting Items for Systematic Reviews and Meta- Analyses (PRISMA) guidelines were used to screen the literature and 183 manuscripts met the inclusion criteria. Cellular and biomaterial inputs, and biological and biomechanical outcomes were extracted from each study. Most identified studies targeted nucleus pulposus (NP) repair. No consensus exists on cell type or biomaterial carrier, yet most common strategies used mesenchymal stem cell (MSC) delivery with interpenetrating network/co-polymeric (IPN/CoP) biomaterials composed of natural biomaterials. All studies reported biological outcomes with about half the studies reporting biomechanical outcomes. Since the IVD is a load-bearing tissue, studies reporting compressive and shear moduli were analysed and two major themes were found. First, a competitive balance, or 'seesaw' effect, between biomechanical and biological performance was observed. Formulations with higher moduli had inferior cellular performance, and vice versa. Second, several low-modulus biomaterials had favourable biological performance and matured throughout culture duration with enhanced extracellular matrix synthesis and biomechanical moduli. Findings identify an opportunity to develop next-generation biomaterials that provide high initial biomechanical competence to stabilise and repair damaged IVDs with a capacity to promote cell function for long-term healing.

Dual delivery of stem cells and insulin-like growth factor-1 in coacervate-embedded composite hydrogels for enhanced cartilage regeneration in osteochondral defects

Exogenous dual delivery of progenitor cell population and therapeutic growth factors (GFs) is one of alternative tissue engineering strategies for osteochondral tissue regeneration. In the present study, an implantable dual delivery platform was developed using coacervates (Coa) (i.e., a tertiary complex of poly(ethylene argininylaspartate diglyceride) (PEAD) polycation, heparin, and cargo insulin-like growth factor-1 (IGF-1), in thiolated gelatin (gelatin-SH)/ poly(ethylene glycol) diacrylate (PEGDA) interpenetrating network (IPN) hydrogels. Since Coa is able to protect cargo GF and maintain its long-term bioactivity, it is speculated that Coa-mediated delivery of chondrogenic factor IGF-1 with the aid of adipose-derived stem cells (ADSCs) would synergistically facilitate osteochondral tissue repair during physiological regeneration process. Our results indicate that gelatin-SH/PEGDA IPN hydrogels demonstrated biocompatibility and mechanical properties for a possible long-term transplantation, and PEAD-base Coa exhibited a sustained release of bioactive IGF-1 over 3 weeks. Subsequently, released IGF-1 from Coa could effectively induce chondrogenic differentiation of embedded ADSCs in the hydrogel, by showing enhanced glycosaminoglycan deposition and expression of chondrogenesis-associated genes. More importantly, at 12 weeks post-implantation in a rabbit full thickness osteochondral defect model, the quality of regenerative tissues in both chondral and subchondral layers was significantly improved in dual delivery of ADSC and IGF-1 in Coa encapsulated in gelatin-SH/PEGDA IPN hydrogels, as compared with a single delivery of ADSC only and a dual delivery without Coa. Therefore, we conclude that our Coa-embedded composite hydrogel platform could effectively augment osteochondral tissue regeneration holds promise for a feasible osteoarthritis therapeutic application.

Nanofibrous spongy microspheres to deliver rabbit mesenchymal stem cells and anti-miR-199a to regenerate nucleus pulposus and prevent calcification

Lower back pain is mainly caused by intervertebral disc degeneration, in which calcification is frequently involved. Here novel nanofibrous spongy microspheres (NF-SMS) are used to carry rabbit bone marrow mesenchymal stromal cells (MSCs) to regenerate nucleus pulposus tissues. NF-SMS are shown to significantly enhance the MSC seeding, proliferation and differentiation over control microcarriers. Furthermore, a hyperbranched polymer (HP) with negligible cytotoxicity and high microRNA (miRNAs) binding affinity is synthesized. The HP can complex with anti-miR-199a and self-assemble into "double shell" polyplexes which are able to achieve high transfection efficiency into MSCs. A double-emulsion technique is used to encapsulate these polyplexes in biodegradable nanospheres (NS) to enable sustained anti-miR-199 delivery. Our results demonstrate that MSC/HP-anti-miR-199a/NS/NF-SMS constructs can promote the nucleus pulposus (NP) phenotype and resist calcification in vitro and in a subcutaneous environment. Furthermore, injection of MSC/HP-anti-miR-199a/NS/NF-SMS can stay in place, produce functional extracellular matrix, maintain disc height and prevent intervertebral disc (IVD) calcification in a rabbit lumbar degeneration model.

Comparative investigations of in vitro and in vivo bioactivity of titanium vs. Ti-24Nb-4Zr-8Sn alloy before and after sandblasting and acid etching

To avoid the failure of clinical surgery due to "stress shielding" and the loosening of an implant, a new type of alloy, Ti-24Nb-4Zr-8Sn (TNZS), with a low Young's modulus acted as a new implant material in this work. Meanwhile, the surface characteristics, MC3T3-E1 cell behavior and in vivo osseointegration of the titanium and TNZS before and after sandblasting and acid etching were studied comparatively. TNZS and Ti had the same microstructure based on the transmission electron microscopy results. Meanwhile, the TNZS alloy had a lower Young's modulus and surface nanohardness compared with pure titanium. However, the corrosion resistance of Ti was better than that of the TNZS sample in simulated body fluid solution. In addition, the TNZS alloy after sandblasting and acid etching (SLATNZS) had excellent cell adhesion, proliferation, differentiation, ALP activity and in vivo osseointegration ability such as there being almost no soft tissue as compared with other implants. Based on the current results, the new type of Ti-24Nb-4Zr-8Sn alloy showed good potential and promising application prospects in its biochemical aspects.

Pathomechanism of intervertebral disc degeneration

Intervertebral disc degeneration (IDD) is the main contributor to low back pain, which is a leading cause of disability worldwide. Although substantial progress has been made in elucidating the molecular mechanisms of IDD, fundamental and long-lasting treatments for IDD are still lacking. With increased understanding of the complex pathomechanism of IDD, alternative strategies for treating IDD can be discovered. A brief overview of the prevalence and epidemiologic risk factors of IDD is provided in this review, followed by the descriptions of anatomic, cellular, and molecular structure of the intervertebral disc as well as the molecular pathophysiology of IDD. Finally, the recent findings of intervertebral disc progenitors are reviewed and the future perspectives are discussed.

IVD progenitor cells: a new horizon for understanding disc homeostasis and repair

Intervertebral disc (IVD) degeneration is associated with low back pain. In IVDs, a high mechanical load, high osmotic pressure and hypoxic conditions create a hostile microenvironment for resident cells. How IVD homeostasis and function are maintained under stress remains to be understood; however, several research groups have reported isolating native endogenous progenitor-like or otherwise proliferative cells from the IVD. The isolation of such cells implies that the IVD might contain a quiescent progenitor-like population that could be activated for IVD repair and regeneration. Increased understanding of endogenous disc progenitor cells will improve our knowledge of IVD homeostasis and, when combined with tissue engineering techniques, might hold promise for future therapeutic applications. In this Review, the characteristics of progenitor cells in different IVD compartments are discussed, as well as the potency of different cell populations within the IVD. The stem cell characteristics of these cells are also compared with those of mesenchymal stromal cells. On the basis of existing evidence, whether and how IVD degeneration and the hostile microenvironment might affect endogenous progenitor cell function are considered, and ways to channel the potential of these cells for IVD repair are suggested.

Biomaterials-Induced Stem Cells Specific Differentiation Into Intervertebral Disc Lineage Cells

Stem cell therapy, which promotes stem cells differentiation toward specialized cell types, increases the resident population and production of extracellular matrix, and can be used to achieve intervertebral disc (IVD) repair, has drawn great attention for the development of IVD-regenerating materials. Many materials that have been reported in IVD repair have the ability to promote stem cells differentiation. However, due to the limitations of mechanical properties, immunogenicity and uncontrollable deviations in the induction of stem cells differentiation, there are few materials that can currently be translated into clinical applications. In addition to the favorable mechanical properties and biocompatibility of IVD materials, maintaining stem cells activity in the local niche and increasing the ability of stem cells to differentiate into nucleus pulposus (NP) and annulus fibrosus (AF) cells are the basis for promoting the application of IVD-regenerating materials in clinical practice. The purpose of this review is to summarize IVD-regenerating materials that focus on stem cells strategies, analyze the properties of these materials that affect the differentiation of stem cells into IVD-like cells, and then present the limitations of currently used disc materials in the field of stem cell therapy and future research perspectives.

Biology of Tendon Stem Cells and Tendon in Aging

Both tendon injuries and tendinopathies, particularly rotator cuff tears, increase with tendon aging. Tendon stem cells play important roles in promoting tendon growth, maintenance, and repair. Aged tendons show a decline in regenerative potential coupled with a loss of stem cell function. Recent studies draw attention to aging primarily a disorder of stem cells. The micro-environment (“niche”) where stem cells resided in vivo provides signals that direct them to metabolize, self-renew, differentiate, or remain quiescent. These signals include receptors and secreted soluble factors for cell-cell communication, extracellular matrix, oxidative stress, and vascularity. Both intrinsic cellular deficits and aged niche, coupled with age-associated systemic changes of hormonal and metabolic signals can inhibit or alter the functions of tendon stem cells, resulting in reduced fitness of these primitive cells and hence more frequent injuries and poor outcomes of tendon repair. This review aims to summarize the biological changes of aged tendons. The biological changes of tendon stem cells in aging are reviewed after a systematic search of the PubMed. Relevant factors of stem cell aging including cell-intrinsic factors, changes of microenvironment, and age-associated systemic changes of hormonal and metabolic signals are examined, with findings related to tendon stem cells highlighted when literature is available. Future research directions on the aging mechanisms of tendon stem cells are discussed. Better understanding of the molecular mechanisms underlying the functional decline of aged tendon stem cells would provide insight for the rational design of rejuvenating therapies.

Matrix stiffness-regulated cellular functions under different dimensionalities

The microenvironments that cells encounter with in vitro.

Human iPSCs can be differentiated into notochordal cells that reduce intervertebral disc degeneration in a porcine model

Introduction: As many as 80% of the adult population experience back pain at some point in their lifetimes. Previous studies have indicated a link between back pain and intervertebral disc (IVD) degeneration. Despite decades of research, there is an urgent need for robust stem cell therapy targeting underlying causes rather than symptoms. It has been proposed that notochordal cells (NCs) appear to be the ideal cell type to regenerate the IVD: these cells disappear in humans as they mature, are replaced by nucleus pulposus (NP) cells, and their disappearance correlates with the initiation of degeneration of the disc. Human NCs are in short supply, thus here aimed for generation of notochordal-like cells from induced pluripotent cells (iPSCs). Methods: Human iPSCs were generated from normal dermal fibroblasts by transfecting plasmids encoding for six factors: OCT4, SOX2, KLF4, L-MYC, LIN28, and p53 shRNA. Then the iPSCs were treated with GSK3i to induce differentiation towards Primitive Streak Mesoderm (PSM). The differentiation was confirmed by qRT-PCR and immunofluorescence. PSM cells were transfected with Brachyury (Br)-encoding plasmid and the cells were encapsulated in Tetronic-tetraacrylate-fibrinogen (TF) hydrogel that mimics the NP environment (G'=1kPa), cultured in hypoxic conditions (2% O2) and with specifically defined growth media. The cells were also tested in vivo in a large animal model. IVD degeneration was induced after an annular puncture in pigs, 4 weeks later the cells were injected and IVDs were analyzed at 12 weeks after the injury using MRI, gene expression analysis and histology. Results: After short-term exposure of iPSCs to GSK3i there was a significant change in cell morphology, Primitive Streak Mesoderm (PSM) markers (Brachyury, MIXL1, FOXF1) were upregulated and markers of pluripotency (Nanog, Oct4, Sox2) were downregulated, both compared to the control group. PSM cells nucleofected with Br (PSM-Br) cultured in TF hydrogels retained the NC phenotype consistently for up to 8 weeks, as seen in the gene expression analysis. PSM-Br cells were co-cultured with bone marrow (BM)-derived mesenchymal stem cells (MSCs) which, with time, expressed the NC markers in higher levels, however the levels of expression in BM-MSCs alone did not change. Higher expression of NC and NP marker genes in human BM-MSCs was found to be induced by iNC-condition media (iNC-CM) than porcine NC-CM. The annular puncture induced IVD degeneration as early as 2 weeks after the procedure. The injected iNCs were detected in the degenerated discs after 8 weeks in vivo. The iNC-treated discs were found protected from degeneration. This was evident in histological analysis and changes in the pH levels, indicative of degeneration state of the discs, observed using qCEST MRI. Immunofluorescence stains show that their phenotype was consistent with the in vitro study, namely they still expressed the notochordal markers Keratin 18, Keratin 19, Noto and Brachyury. Conclusion: In the present study, we report a stepwise differentiation method to generate notochordal cells from human iPSCs. These cells not only demonstrate a sustainable notochordal cell phenotype in vitro and in vivo, but also show the functionality of notochordal cells and have protective effect in case of induced disc degeneration and prevent the change in the pH level of the injected IVDs. The mechanism of this effect could be suggested via the paracrine effect on resident cells, as it was shown in the in vitro studies with MSCs.

Rapid Fabrication, Microstructure, and in Vitro and in Vivo Investigations of a High-Performance Multilayer Coating with External, Flexible, and Silicon-Doped Hydroxyapatite Nanorods on Titanium

A high-performance multilayer coating with external, flexible, and silicon-doped hydroxyapatite (Si-HA) nanorods was designed using bionics. Plasma electrolytic oxidation (PEO) and the microwave hydrothermal (MH) method were used to rapidly deposit this multilayer coating on a titanium (Ti) substrate, applied for 5 and 10 min, respectively. The bioactive multilayer coating was composed of four layers, and the outermost layer was an external growth layer that consisted of many Si-HA nanorods with a single-crystal structure. The Si-HA nanorods exhibited good flexibility, likely because of their complete single-crystal structures, smooth surfaces, and suitable diameters and lengths. This multilayer coating with a high surface energy was superhydrophilic and exhibited good in vitro bioactivities, such as good apatite formation ability, good cell spreading, and high osteogenic gene expression levels. After implantation in the tibia of rabbits for 16 weeks, almost no soft tissues were formed at the MH treated PEO implant-bone interface. A direct bone contact interface was formed by a bridging effect of the flexible Si-HA nanorods, which further produced a high implant-bone interface bonding strength. The current results demonstrated that the bioactive multilayer layers with the flexible Si-HA nanorods displayed a very good osseointegration ability, showing promising applications in the biomedical field.

Fibrinogen-Based Hydrogel Modulus and Ligand Density Effects on Cell Morphogenesis in Two-Dimensional and Three-Dimensional Cell Cultures

There is a need to further explore the convergence of mechanobiology and dimensionality with systematic investigations of cellular response to matrix mechanics in 2D and 3D cultures. Here, a semisynthetic hydrogel capable of supporting both 2D and 3D cell culture is applied to investigate cell response to matrix modulus and ligand density. The culture materials are fabricated from adducts of polyethylene glycol (PEG) or PluronicF127 and fibrinogen fragments, formed into hydrogels by free-radical polymerization, and characterized by shear rheology. Control over the modulus of the materials is accomplished by changing the concentration of synthetic PEG-diacrylate crosslinker (0.5% w/v), and by altering the molecular length of the PEG (10 and 20 kDa). Control over ligand density is accomplished by changing fibrinogen concentrations from 3 to 12 mg mL^-1 . In 2D culture, cell motility parameters, including cell speed and persistence time are significantly increased with increasing modulus. In both 2D and 3D culture, cells express vinculin and there is evidence of focal adhesion formation in the high stiffness materials. The modulus- and ligand-dependent morphogenesis response from the cells in 2D culture is contradictory to the same measured response in 3D culture. In 2D culture, anchorage-dependent cells become more elongated and significantly increase their size with increasing ligand density and matrix modulus. In 3D culture, the same anchorage-dependent cells become less spindled and significantly reduce their size in response to increasing ligand density and matrix modulus. These differences arise from dimensionality constraints, most notably the encapsulation of cells in a non-porous hydrogel matrix. These insights underscore the importance of mechanical properties in regulating cell morphogenesis in a 3D culture milieu. The versatility of the hydrogel culture environment further highlights the significance of a modular approach when developing materials that aim to optimize the cell culture environment.

Autophagy mediates serum starvation-induced quiescence in nucleus pulposus stem cells by the regulation of P27

Background

Adult stem cells exist in a quiescent state (G0) within the in vivo niche; the loss of quiescence often leads to a decrease in the number and function of adult stem cells, impairing tissue regeneration and repair. Endogenous repair by nucleus pulposus-derived stem cells has recently shown promising regenerative potential for the treatment of intervertebral disc degeneration (IDD). However, the number and function of nucleus pulposus stem cells (NPSCs) declined throughout the process of IDD. This effect may have a specific relationship with quiescence. However, the biology of the quiescent NPSCs has not been reported.

Methods

First, we established an in vitro model for NPSC quiescence with serum starvation. The induction of G0 was confirmed by flow cytometry analyses of dual staining with Hoechst 33342 and Pyronin Y, immunofluorescent staining with Ki67 and Western blot analysis of P27 expression. NPSCs were cultured under serum starvation conditions for a long time period (21 days). To examine the functional phenotype of quiescent NPSCs, the cells were reactivated with 10% serum and differentiated into osteogenic and chondrogenic lineages in vitro. The number of colony-forming units was also estimated. To elucidate the role of autophagy in the quiescence of NPSCs, we activated and inhibited autophagy in starved cells with rapamycin and chloroquine, respectively. Then, the expression of P27 was evaluated by Western blot analysis, and the immunofluorescence of Ki67 was assessed. Finally, we assessed the role of P27 siRNA in NPSC quiescence by flow cytometry analyses and 5-ethynyl-20-deoxyuridine incorporation assays under normal and serum-starved conditions.

Results

NPSC quiescence was induced by 48 h of serum starvation, and they maintained quiescence for up to 21 days. Upon reactivation with serum, the quiescent NPSCs re-entered the cell cycle and exhibited enhanced clonogenic self-renewal, osteogenic differentiation and chondrogenic differentiation potentials compared to control NPSCs under normal culture conditions. We also found that autophagy underlay serum starvation-induced NPSC quiescence. Further study demonstrated that autophagy mediated the quiescence of NPSCs by regulating P27.

Conclusions

Serum starvation efficiently induces quiescence in NPSCs. Quiescent NPSCs maintain stem cell properties. Our study reveals that autophagy plays a role in maintaining NPSC quiescence and that autophagy mediates the quiescence of NPSCs by regulating P27. We conclude that the induction of quiescence in cultured NPSCs provides a useful model for the analysis of mechanisms that might be relevant to the biology of NPSCs in vivo.

Mechanisms of endogenous repair failure during intervertebral disc degeneration

Intervertebral disc (IVD) degeneration is frequently associated with Low back pain (LBP), which can severely reduce the quality of human life and cause enormous economic loss. However, there is a lack of long-lasting and effective therapies for IVD degeneration at present. Recently, stem cell based tissue engineering techniques have provided novel and promising treatment for the repair of degenerative IVDs. Numerous studies showed that stem/progenitor cells exist naturally in IVDs and could migrate from their niche to the IVD to maintain the quantity of nucleus pulposus (NP) cells. Unfortunately, these endogenous repair processes cannot prevent IVD degeneration as effectively as expected. Therefore, theoretical basis for regeneration of the NP in situ can be obtained from studying the mechanisms of endogenous repair failure during IVD degeneration. Although there have been few researches to study the mechanism of cell death and migration of stem/progenitor cells in IVD so far, studies demonstrated that the major inducing factors (compression and hypoxia) of IVD degeneration could decrease the number of NP cells by regulating apoptosis, autophagy, and necroptosis, and the particular chemokines and their receptors played a vital role in the migration of mesenchymal stem cells (MSCs). These studies provide a clue for revealing the mechanisms of endogenous repair failure during IVD degeneration. This article reviewed the current research situation and progress of the mechanisms through which IVD stem/progenitor cells failed to repair IVD tissues during IVD degeneration. Such studies provide an innovative research direction for endogenous repair and a new potential treatment strategy for IVD degeneration.

Intervertebral Disc-Derived Stem/Progenitor Cells as a Promising Cell Source for Intervertebral Disc Regeneration

Intervertebral disc (IVD) degeneration is considered to be the primary reason for low back pain. Despite remarkable improvements in both pharmacological and surgical management of IVD degeneration (IVDD), therapeutic effects are still unsatisfactory. It is because of the fact that these therapies are mainly focused on alleviating the symptoms rather than treating the underlying cause or restoring the structure and biomechanical function of the IVD. Accumulating evidence has revealed that the endogenous stem/progenitor cells exist in the IVD, and these cells might be a promising cell source in the regeneration of degenerated IVD. However, the biological characteristics and potential application of IVD-derived stem/progenitor cells (IVDSCs) have yet to be investigated in detail. In this review, the authors aim to perform a review to systematically discuss (1) the isolation, surface markers, classification, and biological characteristics of IVDSCs; (2) the aging- and degeneration-related changes of IVDSCs and the influences of IVD microenvironment on IVDSCs; and (3) the potential for IVDSCs to promote regeneration of degenerated IVD. The authors believe that this review exclusively address the current understanding of IVDSCs and provide a novel approach for the IVD regeneration.

Potential biomarkers of the mature intervertebral disc identified at the single cell level

Intervertebral disc (IVD) degeneration and trauma is a major socio-economic burden and the focus of cell-based regenerative medicine approaches. Despite numerous ongoing clinical trials attempting to replace ailing IVD cells with mesenchymal stem cells, a solid understanding of the identity and nature of cells in a healthy mature IVD is still in need of refinement. Although anatomically simple, the IVD is comprised of heterogeneous cell populations. Therefore, methods involving cell pooling for RNA profiling could be misleading. Here, by using RNA in situ hybridization and z proportion test, we have identified potential novel biomarkers through single cell assessment. We quantified the proportion of RNA transcribing cells for 50 genetic loci in the outer annulus fibrosus (AF) and nucleus pulposus (NP) in coccygeal bovine discs isolated from tails of four skeletally mature animals. Our data reconfirm existing data and suggest 10 novel markers such as Lam1 and Thy1 in the outer AF and Gli1, Gli3, Noto, Scx, Ptprc, Sox2, Zscan10 and LOC101904175 in the NP, including pluripotency markers, that indicate stemness potential of IVD cells. These markers could be added to existing biomarker panels for cell type characterization. Furthermore, our data once more demonstrate heterogeneity in cells of the AF and NP, indicating the need for single cell assessment by methods such as RNA in situ hybridization. Our work refines the molecular identity of outer AF and NP cells, which can benefit future regenerative medicine and tissue engineering strategies in humans.

Tendon-Derived Stem Cell Differentiation in the Degenerative Tendon Microenvironment

Tendinopathy is prevalent in athletic and many occupational populations; nevertheless, the pathogenesis of tendinopathy remains unclear. Tendon-derived stem cells (TDSCs) were regarded as the key culprit for the development of tendinopathy. However, it is uncertain how TDSCs differentiate into adipocytes, chondrocytes, or osteocytes in the degenerative microenvironment of tendinopathy. So in this study, the regulating effects of the degenerative tendon microenvironment on differentiation of TDSCs were investigated. TDSCs were isolated from rat Achilles tendons and were grown on normal and degenerative (prepared by stress-deprived culture) decellularized tendon slices (DTSs). Immunofluorescence staining, H&E staining, real-time PCR, and Western blot were used to delineate the morphology, proliferation, and differentiation of TDSCs in the degenerative microenvironment. It was found that TDSCs were much more spread on the degenerative DTSs than those on normal DTSs. The tenocyte-related markers, COL1 and TNMD, were highly expressed on normal DTSs than the degenerative DTSs. The expression of chondrogenic and osteogenic markers, COL2, SOX9, Runx2, and ALP, was higher on the degenerative DTSs compared with TDSCs on normal DTSs. Furthermore, phosphorylated FAK and ERK1/2 were reduced on degenerative DTSs. In conclusion, this study found that the degenerative tendon microenvironment induced TDSCs to differentiate into chondrogenic and osteogenic lineages. It could be attributed to the cell morphology changes and reduced FAK and ERK1/2 activation in the degenerative microenvironment of tendinopathy.

Latest advances in intervertebral disc development and progenitor cells

This paper is a concise review aiming to assemble the most relevant topics presented by the authors at ORS-Philadelphia Spine Research Society Fourth International Spine Research Symposium. It centers on the latest advances in disc development, its main structural entities, and the populating cells, with emphasis on the advances in pivotal molecular pathways responsible for forming the intervertebral discs (IVD). The objective of finding and emphasizing pathways and mechanisms that function to control tissue formation is to identify and to explore modifications occurring during normal aging, disease, and tissue repair. Thus, to comprehend that the cellular and molecular basis of tissue degeneration are crucial in the study of the dynamic interplay that includes cell-cell communication, gene regulation, and growth factors required to form a healthy and functional tissue during normal development.

Matrix stiffness regulates the differentiation of tendon-derived stem cells through FAK-ERK1/2 activation

Tendon derived stem cells (TDSCs) were vital in tendon homeostasis. Nevertheless, the regulation of TDSCs differentiation in tendinopathy is unclear. Matrix stiffness modulated stem cells differentiation, and matrix stiffness of tendinopathic tissues decreased significantly. In order to clarify the role of matrix stiffness in TDSCs differentiation, they were cultured on the gelatin hydrogels with the stiffness from 2.34 ± 1.48 kPa to 24.09 ± 14.03 kPa. The effect of matrix stiffness on TDSCs proliferation and differentiation were investigated with CCK8 assay, immunofluorescences, real time PCR and western blot. It was found the proliferation of TDSCs increased and more stress fibers formed with increasing matrix stiffness. The differentiation of TDSCs into tenogenic, chondrogenic, and osteogenic lineages were inhibited on stiff hydrogel evidenced by reduced expression of tenocyte markers THBS4, TNMD, SCX, chondrocyte marker COL2, and osteocyte markers Runx2, Osterix, and ALP. Furthermore, the phosphorylation of FAK and ERK1/2 were enhanced when TDSCs grew on stiff hydrogel. After FAK or ERK1/2 was inhibited, the effect of matrix stiffness on differentiation of TDSCs was inhibited as well. The above results indicated matrix stiffness modulated the proliferation and differentiation of TDSCs, and the regulation effect could correlate to the activation of FAK or ERK1/2.

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

Background

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

Methods

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

Results

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

Conclusion

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

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

AIMS

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

MATERIALS AND METHODS

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

KEY FINDINGS

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

SIGNIFICANCE

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

Biofunctionalized aligned microgels provide 3D cell guidance to mimic complex tissue matrices

Natural healing is based on highly orchestrated processes, in which the extracellular matrix plays a key role. To resemble the native cell environment, we introduce an artificial extracellular matrix (aECM) with the capability to template hierarchical and anisotropic structures in situ, allowing a minimally-invasive application via injection. Synthetic, magnetically responsive, rod-shaped microgels are locally aligned and fixed by a biocompatible surrounding hydrogel, creating a hybrid anisotropic hydrogel (Anisogel), of which the physical, mechanical, and chemical properties can be tailored. The microgels are rendered cell-adhesive with GRGDS and incorporated either inside a cell-adhesive fibrin or bioinert poly(ethylene glycol) hydrogel to strongly interact with fibroblasts. GRGDS-modified microgels inside a fibrin-based Anisogel enhance fibroblast alignment and lead to a reduction in fibronectin production, indicating successful replacement of structural proteins. In addition, YAP-translocation to the nucleus increases with the concentration of microgels, indicating cellular sensing of the overall anisotropic mechanical properties of the Anisogel. For bioinert surrounding PEG hydrogels, GRGDS-microgels are required to support cell proliferation and fibronectin production. In contrast to fibroblasts, primary nerve growth is not significantly affected by the biomodification of the microgels. In conclusion, this approach opens new opportunities towards advanced and complex aECMs for tissue regeneration.

Focal adhesion signaling affects regeneration by human nucleus pulposus cells in collagen- but not carbohydrate-based hydrogels

Hydrogel-based 3D cell cultures are an emerging strategy for the regeneration of cartilage. In an attempt to regenerate dysfunctional intervertebral discs, nucleus pulposus (NP) cells can be cultured in hydrogels of various kinds and physical properties. Stiffness sensing through focal adhesions is believed to direct chondrogenesis, but the mechanisms by which this works are largely unknown. In this study we compared focal adhesion formation and glycosaminoglycan (GAG) deposition by NP cells in a range of hydrogels. Using a focal adhesion kinase (FAK) inhibitor, we demonstrated that focal adhesion signaling is involved in the response of NP cells in hydrogels that contain integrin binding sites (i.e. methacrylated gelatin (gelMA) and type II collagen), but not in hydrogels deplete from integrin binding sites such as alginate and agarose, or CD44-binding hydrogels based on hyaluronic acid. As a result of FAK inhibition we observedenhanced proteoglycan production in gelMA, but decreased production in type II collagen hydrogels, which could be explained by alteration in cell fate as supported by the increase in the adipogenic marker peroxisome proliferator-activated receptor gamma (PPARy). Furthermore, GAG deposition was inversely proportional to polymer concentration in integrin-binding gelMA, while no direct relationship was found for the non-integrin binding gels alginate and agarose. This corroborates our finding that focal adhesion formation plays an important role in NP cell response to its surrounding matrix.

STATEMENT OF SIGNIFICANCE

Biomaterials are increasingly being investigated for regenerative medicine applications, including regeneration of the nucleus pulposus. Cells interact with their environment and are influenced by extracellular matrix or polymer properties. Insight in these interactions can improve regeneration and helps to understand degeneration processes. The role of focal adhesion formation in the regenerative response of nucleus pulposus cells is largely unknown. Therefore, the relation between materials, stiffness and focal adhesion formation is studied here.

Optimal Environmental Stiffness for Stem Cell Mediated Ischemic Myocardium Repair

Cardiovascular diseases related to myocardial infarction (MI) contribute significantly to morbidity and mortality worldwide. The loss of cardiomyocytes during MI is a key factor in the impairment of cardiac-pump functions. Employing cell transplantation has shown great potential as a therapeutic approach in regenerating ischemic myocardium. Several studies have suggested that the therapeutic effects of stem cells vary based on the timing of cell administration. It has been clearly established that the myocardium post-infarction experiences a time-dependent stiffness change, and many studies have highlighted the importance of stiffness (elasticity) of microenvironment on modulating the fate and function of stem cells. Therefore, this chapter outlines our studies and other experiments designed to establish the optimal stiffness of microenvironment that maximizes benefits for maintaining cell survival, promoting phenotypic plasticity, and improving functional specification of the engrafted stem cells.

BMP7-Based Functionalized Self-Assembling Peptides Protect Nucleus Pulposus-Derived Stem Cells From Apoptosis In Vitro

Tissue engineering has shown great success in the treatment of intervertebral disk degeneration (IVDD) in the past decade. However, the adverse and harsh microenvironment associated in the intervertebral disks remains a great obstacle for the survival of transplanted cells. Although increasing numbers of new materials have been created or modified to overcome this hurdle, a new effective strategy of biological therapy is still required. In this study, bone morphogenic protein 7 (BMP7)-based functionalized self-assembling peptides were developed by conjugating a bioactive motif from BMP-7 (RKPS) onto the C-terminal of the peptide RADARADARADARADA (RADA16-I) at a ratio of 1:1 to form a new RADARKPS peptide. Human nucleus pulposus-derived stem cells (NPDCs) were cultured in the presence of RADA-RKPS or RADA16-I in an apoptosis-promoting environment that was induced by tumor necrosis factor-alpha, and cells were cultured with RADA16-I in normal medium that served as the control group. After 48 h of apoptosis induction, the viability, proliferation, apoptosis rate, and expression of apoptosis-related genes of NPDCs in the different groups were evaluated, and the differentiation of NPDCs toward nucleus pulposus-like cells was tested. The results showed that the RADA-RKPS peptide could significantly protect the survival and proliferation of NPDCs. In addition, the application of RADA-RKPS decreased the rate of cell apoptosis, as detected by TUNEL-positive staining. Furthermore, our in vitro study confirmed the apoptosis-protecting effects of RADA-RKPS peptides, which significantly reduced the BAX/BCL-2 ratio of NPDCs and upregulated the gene expression of collagen II a1, aggrecan, and Sox-9 after 48 h of apoptosis induction. Collectively, these lines of evidence suggest that RADA-RKPS peptides confer a protective effect to NPDCs in an apoptosis environment, suggesting their potential application in the development of new biological treatment strategies for IVDD.