N-Cadherin-Mediated Signaling Regulates Cell Phenotype for Nucleus Pulposus Cells of the Intervertebral Disc

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.

N-cadherin Alleviates Apoptosis and Senescence of Nucleus Pulposus Cells via Suppressing ROS-dependent ERS in the Hyper-osmolarity Microenvironment

The in-situ osmolarity is an important physicochemical factor that regulates cell fate of nucleus pulposus cells (NPCs). Our previous studies demonstrated that reduced N-cadherin (NCDH) expression in nucleus pulposus cells is associated with cellular damage under hyper-osmolarity microenvironment. This study was aimed at exploring the impacts of NCDH on senescence and apoptosis of NPCs, as well as the potential molecular mechanism. By comparing NPCs from patients with lumbar fractures and lumbar disc herniation, we identified a correlation between decreased NCDH expression and increased endoplasmic reticulum stress (ERS), resulting in undesirable cell fate (senescence and apoptosis). After blocking Reactive oxygen species (ROS) or ERS, it was indicated that hyper-osmolarity microenvironment induced ERS was ROS-dependent. Further results demonstrated the correlation in rat NPCs. Upregulation of NCDH expression reduced ROS-dependent ERS, thus limiting undesirable cell fates in vitro. This was further confirmed through the rat tail acupuncture injection model. NCDH overexpression successfully mitigated ERS, preserved extracellular matrix production and alleviating intervertebral disc degeneration in vivo. Together, NCDH can alleviate senescence and apoptosis of NPCs by suppressing ROS-dependent ERS via the ATF4-CHOP signaling axis in the hyper-osmolarity microenvironment, thus highlighting the therapeutic potential of NCDH in combating degenerative disc diseases.

Biomarkers and longitudinal changes in lumbar spine degeneration and low back pain: the Johnston County Osteoarthritis Project

OBJECTIVE

To determine if baseline biomarkers are associated with longitudinal changes in the worsening of disc space narrowing (DSN), vertebral osteophytes (OST), and low back pain (LBP).

DESIGN

Paired baseline (2003-2004) and follow-up (2006-2010) lumbar spine radiographs from the Johnston County Osteoarthritis Project were graded for severity of DSN and OST. LBP severity was self-reported. Concentrations of analytes (cytokines, proteoglycans, and neuropeptides) were quantified by immunoassay. Pressure-pain threshold (PPT), a marker of sensitivity to pressure pain, was measured with a standard dolorimeter. Binary logistic regression models were used to estimate odd ratios (OR) and 95% confidence intervals (CI) of biomarker levels with DSN, OST, or LBP. Interactions were tested between biomarker levels and the number of affected lumbar spine levels or LBP.

RESULTS

We included participants (n = 723) with biospecimens, PPT, and paired lumbar spine radiographic data. Baseline Lumican, a proteoglycan reflective of extracellular matrix changes, was associated with longitudinal changes in DSN worsening (OR = 3.19 [95% CI 1.22, 8.01]). Baseline brain-derived neuropathic factor, a neuropeptide, (OR = 1.80 [95% CI 1.03, 3.16]) was associated with longitudinal changes in OST worsening, which may reflect osteoclast genesis. Baseline hyaluronic acid (OR = 1.31 [95% CI 1.01, 1.71]), indicative of systemic inflammation, and PPT (OR = 1.56 [95% CI 1.02, 2.31]) were associated with longitudinal increases in LBP severity.

CONCLUSION

These findings suggest that baseline biomarkers are associated with longitudinal changes occurring in structures of the lumbar spine (DSN vs OST). Markers of inflammation and perceived pressure pain sensitivity were associated with longitudinal worsening of LBP.

Circ0007042 alleviates intervertebral disc degeneration by adsorbing miR-369 to upregulate BMP2 and activate the PI3K/AKt pathway

Background

To identify regulatory ncRNA molecules that can cause differential expression of CDH2 in intervertebral disc degeneration (IDD) and explore whether there are other ways to affect the progression of IDD.

Methods

A primary culture of human nucleus pulposus (NP) cells was established and identified by immunofluorescence. An in vitro IDD model was constructed by compressing human NP cells, and the MTT assay was used to measure cell viability. Changes in the ncRNA group were analysed by RNA-seq. The expression levels of hsa_circ_7042, CDH2, and miR-369-3p were detected by qPCR. Cell apoptosis, senescence, and extracellular matrix (ECM) metabolism were detected by flow cytometry, β-galactosidase staining, and Western blotting. hsa_circ_7042, miR-369-3p, and bone morphogenetic protein 2 (BMP2) were verified by luciferase and RNA immunoprecipitation (RIP) analyses. The PI3K/Akt pathway was validated by transfection of BMP2 siRNA. Furthermore, a mouse model of lumbar spine instability was constructed. circ_7042 adenovirus was packaged and injected into the intervertebral discs of mice, and the influence of circ_7042 overexpression on intervertebral disc degeneration was determined.

Results

Western blotting, qPCR, and flow cytometry analyses confirmed that overexpression of circ_7042 could downregulate miR-369-3p and upregulate the expression of CDH2 and BMP2 in IDD cell and animal models. Additionally, the levels of apoptotic and senescent cells decreased, and ECM degradation decreased. The PI3K/Akt pathway was significantly activated after circ_7042 was overexpressed. The injection of circ_7042-overexpressing adenovirus effectively reduced ECM degradation and the level of apoptosis in NP tissue.

Conclusions

circ_7042 could upregulate the expression of CDH2 and BMP2 by absorbing miR-369-3p, and the increased BMP2 activated the PI3K/Akt pathway, thus improving IDD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13075-022-02895-7.

Importance of Matrix Cues on Intervertebral Disc Development, Degeneration, and Regeneration

Back pain is one of the leading causes of disability worldwide and is frequently caused by degeneration of the intervertebral discs. The discs’ development, homeostasis, and degeneration are driven by a complex series of biochemical and physical extracellular matrix cues produced by and transmitted to native cells. Thus, understanding the roles of different cues is essential for designing effective cellular and regenerative therapies. Omics technologies have helped identify many new matrix cues; however, comparatively few matrix molecules have thus far been incorporated into tissue engineered models. These include collagen type I and type II, laminins, glycosaminoglycans, and their biomimetic analogues. Modern biofabrication techniques, such as 3D bioprinting, are also enabling the spatial patterning of matrix molecules and growth factors to direct regional effects. These techniques should now be applied to biochemically, physically, and structurally relevant disc models incorporating disc and stem cells to investigate the drivers of healthy cell phenotype and differentiation. Such research will inform the development of efficacious regenerative therapies and improved clinical outcomes.

Biomarker clusters differentiate phenotypes of lumbar spine degeneration and low back pain: The Johnston County Osteoarthritis Project

Objective:

Describe the association between biomarkers and lumbar spine degeneration (vertebral osteophytes [OST], facet joint osteoarthritis [FOA], and disc space narrowing [DSN]), for persons with and without low back pain (LBP) and determine whether clusters based on biomarkers differentiate lumbar spine structure with and without LBP.

Methods:

Using data from the Johnston County Osteoarthritis Project (2006–2010), we measured serum N-cadherin, Keratin-19, Lumican, CXCL6, RANTES, HA, IL-6, BDNF, OPG, and NPY, and urinary CTX-II. Biomarkers were used to group participants using k-means cluster analysis. Logistic regression models were used to compare biomarker clusters.

Results:

The sample consisted of 731 participants with biospecimens and lumbar spine radiographic data. Three biomarker subgroups were identified: one characterized by structural degenerative changes; another characterized by structural degenerative changes and inflammation, with pain; and a referent cluster with lower levels of biomarkers, pain, and structural degenerative changes. Compared to the referent subgroup, the structural change subgroup was associated with DSN (OR = 1.94, 95% CI 1.30–2.90) and FOA (OR = 1.72, 95% CI 1.12–2.62), and the subgroup with structural degenerative change, inflammation, and pain was associated with OST with LBP (OR = 1.60, 95% CI 1.04–2.46), FOA with LBP (OR = 1.59, 95% CI 1.04–2.45), and LBP (OR = 1.63, 95% CI 1.11–2.41). The subgroup with structural degenerative changes was more likely to have OST (OR = 1.82, 95% CI 1.06–3.13) and less likely to have FOA with LBP (OR = 0.62, 95% CI 0.40–0.96) compared to the group with inflammation and pain.

Conclusion:

Clustering by biomarkers may assist in differentiating patients for specific clinical interventions aimed at decreasing LBP.

Endoplasmic reticulum stress associates with the development of intervertebral disc degeneration

Endoplasmic reticulum (ER) is an important player in various intracellular signaling pathways that regulate cellular functions in many diseases. Intervertebral disc degeneration (IDD), an age-related degenerative disease, is one of the main clinical causes of low back pain. Although the pathological development of IDD is far from being fully elucidated, many studies have been shown that ER stress (ERS) is involved in IDD development and regulates various processes, such as inflammation, cellular senescence and apoptosis, excessive mechanical loading, metabolic disturbances, oxidative stress, calcium homeostasis imbalance, and extracellular matrix (ECM) dysregulation. This review summarizes the formation of ERS and the potential link between ERS and IDD development. ERS can be a promising new therapeutic target for the clinical management of IDD.

Integrin and syndecan binding peptide-conjugated alginate hydrogel for modulation of nucleus pulposus cell phenotype

Biomaterial based strategies have been widely explored to preserve and restore the juvenile phenotype of cells of the nucleus pulposus (NP) in degenerated intervertebral discs (IVD). With aging and maturation, NP cells lose their ability to produce necessary extracellular matrix and proteoglycans, accelerating disc degeneration. Previous studies have shown that integrin or syndecan binding peptide motifs from laminin can induce NP cells from degenerative human discs to re-express juvenile NP-specific cell phenotype and biosynthetic activity. Here, we engineered alginate hydrogels to present integrin- and syndecan-binding peptides alone or in combination (cyclic RGD and AG73, respectively) to introduce bioactive features into the alginate gels. We demonstrated human NP cells cultured upon and within alginate hydrogels presented with cRGD and AG73 peptides exhibited higher cell viability, biosynthetic activity, and NP-specific protein expression over alginate alone. Moreover, the combination of the two peptide motifs elicited markers of the NP-specific cell phenotype, including N-Cadherin, despite differences in cell morphology and multicellular cluster formation between 2D and 3D cultures. These results represent a promising step toward understanding how distinct adhesive peptides can be combined to guide NP cell fate. In the future, these insights may be useful to rationally design hydrogels for NP cell-transplantation based therapies for IVD degeneration.

Bioactive in situ crosslinkable polymer-peptide hydrogel for cell delivery to the intervertebral disc in a rat model

Degeneration of the intervertebral disc (IVD) is associated with significant biochemical and morphological changes that include a loss of disc height, decreased water content and decreased cellularity. Cell delivery has been widely explored as a strategy to supplement the nucleus pulposus (NP) region of the degenerated IVD in both pre-clinical and clinical trials, using progenitor or primary cell sources. We previously demonstrated an ability for a polymer-peptide hydrogel, serving as a culture substrate, to promote adult NP cells to undergo a shift from a degenerative fibroblast-like state to a juvenile-like NP phenotype. In the current study, we evaluate the ability for this peptide-functionalized hydrogel to serve as a bioactive system for cell delivery, retention and preservation of a biosynthetic phenotype for primary IVD cells delivered to the rat caudal disc in an anular puncture degeneration model. Our data suggest that encapsulation of adult degenerative human NP cells in a stiff formulation of the hydrogel functionalized with laminin-mimetic peptides IKVAV and AG73 can promote cell viability and increased biosynthetic activity for this population in 3D culture in vitro. Delivery of the peptide-functionalized biomaterial with primary rat cells to the degenerated IVD supported NP cell retention and NP-specific protein expression in vivo, and promoted improved disc height index (DHI) values and endplate organization compared to untreated degenerated controls. The results of this study suggest the physical cues of this peptide-functionalized hydrogel can serve as a supportive carrier for cell delivery to the IVD.

Integrin-mediated interactions with a laminin-presenting substrate modulate biosynthesis and phenotypic expression for cells of the human nucleus pulposus

With aging and pathology, cells of the nucleus pulposus (NP) de-differentiate towards a fibroblast-like phenotype, a change that contributes to degeneration of the intervertebral disc (IVD). Laminin isoforms are a component of the NP extracellular matrix during development but largely disappear in the adult NP tissue. Exposing human adult NP cells to hydrogels made from PEGylated-laminin-111 (PEGLM) has been shown to regulate NP cell behaviors and promote cells to assume a biosynthetically active state with gene/protein expression and morphology consistent with those observed in juvenile NP cells. However, the mechanism regulating this effect has remained unknown. In the present study, the integrin subunits that promote adult degenerative NP cell interactions with laminin-111 are identified by performing integrin blocking studies along with assays of intracellular signaling and cell phenotype. The findings indicate that integrin α3 is a primary regulator of cell attachment to laminin and is associated with phosphorylation of signaling molecules downstream of integrin engagement (ERK 1/2 and GSK3β). Sustained effects of blocking integrin α3 were also demonstrated including decreased expression of phenotypic markers, reduced biosynthesis, and altered cytoskeletal organization. Furthermore, blocking both integrin α3 and additional integrin subunits elicited changes in cell clustering, but did not alter the phenotype of single cells. These findings reveal that integrin- mediated interactions through integrin α3 are critical in the process by which NP cells sense and alter phenotype in response to culture upon laminin and further suggest that targeting integrin α3 has potential for reversing or slowing degenerative changes to the NP cell.

The Inner Annulus Fibrosus Encroaches on the Nucleus Pulposus in the Injured Mouse Tail Intervertebral Disc

OBJECTIVE

The aim was to identify the source of cells within the center of the abnormal fibrocartilage tissue of the degenerative intervertebral disc after injury.

DESIGN

Cross-breeding of mice with an inducible type II promoter collagen construct (Col2CreER) to Rosa26-TdTomato mice has been shown to result in Cre-recombinase activity and Tomato expression in inner annulus fibrosus cells after tamoxifen injection. To investigate the role of the inner annulus fibrosus in the intervertebral disc injury response, tail intervertebral discs of Col2CreER/tdTomato mice were punctured with a needle and examined 1-4 wks after injury. N-cadherin was examined by immunostaining.

RESULTS

After the injury, the fibrocartilage in the degenerative intervertebral disc consisted of residual diseased nucleus pulposus cells and encroaching inner annulus fibrosus cells. The residual nucleus pulposus cells had lost their epithelial cell-like morphology and instead became oval shaped, with reduced adhesion to neighboring nucleus pulposus cells. This change in cellular morphology coincided with a loss of N-cadherin, which contributes to maintenance of healthy nucleus pulposus cell morphology. As expected, injured tail intervertebral discs showed reduced compressive properties as determined by biomechanical assessments.

CONCLUSIONS

The cellular composition of the degenerative intervertebral disc has been defined here, which is an important step in developing future treatments.

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.

Inflammatory, Structural, and Pain Biochemical Biomarkers May Reflect Radiographic Disc Space Narrowing: The Johnston County Osteoarthritis Project

The purpose of this work is to determine the relationship between biomarkers of inflammation, structure, and pain with radiographic disc space narrowing (DSN) in community-based participants. A total of 74 participants (37 cases and 37 controls) enrolled in the Johnston County Osteoarthritis Project during 2006-2010 were selected. The cases had at least mild radiographic DSN and low back pain (LBP). The controls had neither radiographic evidence of DSN nor LBP. The measured analytes from human serum included N-cadherin, Keratin-19, Lumican, CXCL6, RANTES, IL-17, IL-6, BDNF, OPG, and NPY. A standard dolorimeter measured pressure-pain threshold. The coefficients of variation were used to evaluate inter- and intra-assay reliability. Participants with similar biomarker profiles were grouped together using cluster analysis. The binomial regression models were used to estimate risk ratios (RR) and 95% confidence intervals (CI) in propensity score-matched models. Significant associations were found between radiographic DSN and OPG (RR = 3.90; 95% CI: 1.83, 8.31), IL-6 (RR = 2.54; 95% CI: 1.92, 3.36), and NPY (RR = 2.06 95% CI: 1.62, 2.63). Relative to a cluster with low levels of biomarkers, a cluster representing elevated levels of OPG, RANTES, Lumican, Keratin-19, and NPY (RR = 3.04; 95% CI: 1.22, 7.54) and a cluster representing elevated levels of NPY (RR = 2.91; 95% CI: 1.15, 7.39) were significantly associated with radiographic DSN. Clinical Significance: These findings suggest that individual and combinations of biochemical biomarkers may reflect radiographic DSN. This is just one step toward understanding the relationships between biochemical biomarkers and DSN that may lead to improved intervention delivery. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1027-1037, 2020.

NOTO Transcription Factor Directs Human Induced Pluripotent Stem Cell-Derived Mesendoderm Progenitors to a Notochordal Fate

The founder cells of the Nucleus pulposus, the centre of the intervertebral disc, originate in the embryonic notochord. After birth, mature notochordal cells (NC) are identified as key regulators of disc homeostasis. Better understanding of their biology has great potential in delaying the onset of disc degeneration or as a regenerative-cell source for disc repair. Using human pluripotent stem cells, we developed a two-step method to generate a stable NC-like population with a distinct molecular signature. Time-course analysis of lineage-specific markers shows that WNT pathway activation and transfection of the notochord-related transcription factor NOTO are sufficient to induce high levels of mesendoderm progenitors and favour their commitment toward the notochordal lineage instead of paraxial and lateral mesodermal or endodermal lineages. This study results in the identification of NOTO-regulated genes including some that are found expressed in human healthy disc tissue and highlights NOTO function in coordinating the gene network to human notochord differentiation.

Physical disruption of intervertebral disc promotes cell clustering and a degenerative phenotype

To test the hypothesis that physical disruption of an intervertebral disc disturbs cell-matrix binding, leading to cell clustering and increased expression of matrix degrading enzymes that contribute towards degenerative disc cell phenotype. Lumbar disc tissue was removed at surgery from 21 patients with disc herniation, 11 with disc degeneration, and 8 with adolescent scoliosis. 5 μm sections were examined with histology, and 30-µm sections by confocal microscopy. Antibodies were used against integrin α5beta1, matrix metalloproteinases (MMP) 1, MMP-3, caspase 3, and denatured collagen types I and II. Spatial associations were sought between cell clustering and various degenerative features. An additional, 11 non-herniated human discs were used to examine causality: half of each specimen was cultured in a manner that allowed free 'unconstrained' swelling (similar to a herniated disc in vivo), while the other half was cultured within a perspex ring that allowed 'constrained' swelling. Changes were monitored over 36 h using live-cell imaging. 1,9-Di-methyl methylene blue (DMMB) assay for glycosaminoglycan loss was carried out from tissue medium. Partially constrained specimens showed little swelling or cell movement in vitro. In contrast, unconstrained swelling significantly increased matrix distortion, glycosaminoglycan loss, exposure of integrin binding sites, expression of MMPs 1 and 3, and collagen denaturation. In the association studies, herniated disc specimens showed changes that resembled unconstrained swelling in vitro. In addition, they exhibited increased cell clustering, apoptosis, MMP expression, and collagen denaturation compared to 'control' discs. Results support our hypothesis. Further confirmation will require longitudinal animal experiments.

The Intervertebral Disc: Physiology and Pathology of a Brittle Joint

BACKGROUND

Intervertebral disc (ID) degeneration represents the number one cause for outpatient clinic visits worldwide. Mechanisms are discussed but not yet clearly established. Consequently, back pain management is commonly limited to symptomatic treatment therapies.

OBJECTIVES

The aim of this review is to evaluate major progress and to unravel the biology and pathology of ID discogenic pain.

METHODS

The design of this study is a systematic review. A literature search was conducted using Medline, EMBASE, and Google Scholar databases, with no time constraints to locate relevant literature. Significant articles (literature reviews, therapeutic essays, clinical-human-research studies, animal research, and laboratory research) on the intervertebral disc were identified and reviewed. The exclusion criteria were the following: case reports and clinical studies with <10 patients.

RESULTS

Through a dense review of the literature, the ID is deciphered and described as a fragile anatomic entity. For this systematic review, 132 studies were identified and 79 were retained. The main deterioration and alteration mechanisms that lead to the programmed death of the ID are summarized. In addition, the large variety of biological therapies that override surgical treatment are determined.

CONCLUSIONS

The degeneration mechanisms of the ID are well defined and decrypted. Although therapies have progressed, none has been effective. The regeneration of the ID remains highly challenging because of the complexity of its natural composition, microstructure, and mechanical properties.

Mechanotransduction and cell biomechanics of the intervertebral disc

Mechanical loading of the intervertebral disc (IVD) initiates cell-mediated remodeling events that contribute to disc degeneration. Cells of the IVD, nucleus pulposus (NP) and anulus fibrosus (AF), will exhibit various responses to different mechanical stimuli which appear to be highly dependent on loading type, magnitude, duration, and anatomic zone of cell origin. Cells of the NP, the innermost region of the disc, exhibit an anabolic response to low-moderate magnitudes of static compression, osmotic pressure, or hydrostatic pressure, while higher magnitudes promote a catabolic response marked by increased protease expression and activity. Cells of the outer AF are responsive to physical forces in a manner that depends on frequency and magnitude, as are cells of the NP, though they experience different forces, deformations, pressure, and osmotic pressure in vivo. Much remains to be understood of the mechanotransduction pathways that regulate IVD cell responses to loading, including responses to specific stimuli and also differences among cell types. There is evidence that cytoskeletal remodeling and receptor-mediated signaling are important mechanotransduction events that can regulate downstream effects like gene expression and posttranslational biosynthesis, all of which may influence phenotype and bioactivity. These and other mechanotransduction events will be regulated by known and to-be-discovered cell-matrix and cell-cell interactions, and depend on composition of extracellular matrix ligands for cell interaction, matrix stiffness, and the phenotype of the cells themselves. Here, we present a review of the current knowledge of the role of mechanical stimuli and the impact upon the cellular response to loading and changes that occur with aging and degeneration of the IVD.

Differentiation of human induced pluripotent stem cells into nucleus pulposus-like cells

Background

Intervertebral disc (IVD) degeneration is characterized by an early decrease in cellularity of the nucleus pulposus (NP) region, and associated extracellular matrix changes, reduced hydration, and progressive degeneration. Cell-based IVD therapy has emerged as an area of great interest, with studies reporting regenerative potential for many cell sources, including autologous or allogeneic chondrocytes, primary IVD cells, and stem cells. Few approaches, however, have clear strategies to promote the NP phenotype, in part due to a limited knowledge of the defined markers and differentiation protocols for this lineage. Here, we developed a new protocol for the efficient differentiation of human induced pluripotent stem cells (hiPSCs) into NP-like cells in vitro. This differentiation strategy derives from our knowledge of the embryonic notochordal lineage of NP cells as well as strategies used to support healthy NP cell phenotypes for primary cells in vitro.

Methods

An NP-genic phenotype of hiPSCs was promoted in undifferentiated hiPSCs using a stepwise, directed differentiation toward mesodermal, and subsequently notochordal, lineages via chemically defined medium and growth factor supplementation. Fluorescent cell imaging was used to test for pluripotency markers in undifferentiated cells. RT-PCR was used to test for potential cell lineages at the early stage of differentiation. Cells were checked for NP differentiation using immunohistochemistry and histological staining at the end of differentiation. To enrich notochordal progenitor cells, hiPSCs were transduced using lentivirus containing reporter constructs for transcription factor brachyury (T) promoter and green fluorescent protein (GFP) fluorescence, and then sorted on T expression based on GFP intensity by flow cytometry.

Results

Periods of pellet culture following initial induction were shown to promote the vacuolated NP cell morphology and NP surface marker expression, including CD24, LMα5, and Basp1. Enrichment of brachyury (T) positive cells using fluorescence-activated cell sorting was shown to further enhance the differentiation efficiency of NP-like cells.

Conclusions

The ability to efficiently differentiate human iPSCs toward NP-like cells may provide insights into the processes of NP cell differentiation and provide a cell source for the development of new therapies for IVD diseases.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0797-1) contains supplementary material, which is available to authorized users.

Static compression down-regulates N-cadherin expression and facilitates loss of cell phenotype of nucleus pulposus cells in a disc perfusion culture

Mechanical compression often induces degenerative changes of disc nucleus pulposus (NP) tissue. It has been indicated that N-cadherin (N-CDH)-mediated signaling helps to preserve the NP cell phenotype. However, N-CDH expression and the resulting NP-specific phenotype alteration under the static compression and dynamic compression remain unclear. To study the effects of static compression and dynamic compression on N-CDH expression and NP-specific phenotype in an in vitro disc organ culture. Porcine discs were organ cultured in a self-developed mechanically active bioreactor for 7 days and subjected to static or dynamic compression (0.4 MPa for 2 h once per day). The noncompressed discs were used as controls. Compared with the dynamic compression, static compression significantly down-regulated the expression of N-CDH and NP-specific markers (laminin, brachyury, and keratin 19); decreased the Alcian Blue staining intensity, glycosaminoglycan and hydroxyproline contents; and declined the matrix macromolecule (aggrecan and collagen II) expression. Compared with the dynamic compression, static compression causes N-CDH down-regulation, loss of NP-specific phenotype, and the resulting decrease in NP matrix synthesis.

Hyperosmolarity induces notochordal cell differentiation with aquaporin3 upregulation and reduced N-cadherin expression

The nucleus pulposus (NP) of intervertebral discs (IVD) undergoes dramatic changes with aging including loss of its gelatinous structure and large, vacuolated notochordal cells (NCs) in favor of a matrix-rich structure populated by small NP cells (sNPCs). NP maturation also involves a loading-pattern shift from pressurization to matrix deformations, and these events are thought to predispose to degeneration. Little is known of the triggering events and cellular alterations involved with NP maturation, which remains a fundamental open spinal mechanobiology question. A mouse IVD organ culture model was used to test the hypotheses that hyperosmotic overloading will induce NP maturation with transition of NCs to sNPCs while also increasing matrix accumulation and altering osmoregulatory and mechanotransductive proteins. Results indicated that static hyperosmolarity, as might occur during growth, caused maturation of NCs to sNPCs and involved a cellular differentiation process since known NC markers (cytokeratin-8, -19, and sonic hedgehog) persisted without increased cell apoptosis. Osmosensitive channels Aquaporin 3 (Aqp3) and transient receptor potential vanilloid-4 (TRPV4) expression were both modified with altered osmolarity, but increased Aqp3 with hyperosmolarity was associated with NC to sNPC differentiation. NC to sNPC differentiation was accompanied by a shift in cellular mechanotransduction proteins with decreased N-cadherin adhesions and increased Connexin 43 connexons. We conclude that hyperosmotic overloading can promote NC differentiation into sNPCs. This study identified osmolarity as a triggering mechanism for notochordal cell differentiation with associated shifts in osmoregulatory and mechanotransductive proteins that are likely to play important roles in intervertebral disc aging. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:788-798, 2018.

N‑cadherin attenuates nucleus pulposus cell senescence under high‑magnitude compression

Mechanical compression is important in disc degeneration. N-cadherin (N-CDH)-mediated signaling contributes to the maintenance of the normal nucleus pulposus (NP) cell phenotype and NP matrix biosynthesis. Our preliminary study demonstrated that a high-magnitude compression (20% deformation) promotes NP cell senescence in a three-dimensional scaffold culture system. The aim of the present study was to investigate whether N-CDH-mediated signaling alleviates NP cell senescence under the above-mentioned high-magnitude compression. NP cells were transfected with recombinant lentiviral vectors to enhance N-CDH expression. All the transfected or un-transfected NP cells were seeded into the scaffolds and subjected to 20% deformation at a frequency of 1.0 Hz for 4 h once per day for 5 days. Results indicated that N-CDH overexpressed NP cells exhibited decreased senescence-associated β-galactosidase activity and downregulated expression levels of senescence-associated markers (p16 and p53). Furthermore, the N-CDH overexpressed NP cells exhibited increased cell proliferation potency, telomerase activity and matrix biosynthesis compared with NP cells without N-CDH overexpression under high-magnitude compression. Thus, N-CDH-mediated signaling contributes to the attenuation of NP cell senescence under high-magnitude compression.

Regulation of human nucleus pulposus cells by peptide-coupled substrates

Nucleus pulposus (NP) cells are derived from the notochord and differ from neighboring cells of the intervertebral disc in phenotypic marker expression and morphology. Adult human NP cells lose this phenotype and morphology with age in a pattern that contributes to progressive disc degeneration and pathology. Select laminin-mimetic peptide ligands and substrate stiffnesses were examined for their ability to regulate human NP cell phenotype and biosynthesis through the expression of NP-specific markers aggrecan, N-cadherin, collagen types I and II, and GLUT1. Peptide-conjugated substrates demonstrated an ability to promote expression of healthy NP-specific markers, as well as increased biosynthetic activity. We show an ability to re-express markers of the juvenile NP cell and morphology through control of peptide presentation and stiffness on well-characterized polyacrylamide substrates. NP cells cultured on surfaces conjugated with α3 integrin receptor peptides P4 and P678, and on α2, α5, α6, β1 integrin-recognizing peptide AG10, show increased expression of aggrecan, N-cadherin, and types I and II collagen, suggesting a healthier, more juvenile-like phenotype. Multi-cell cluster formation was also observed to be more prominent on peptide-conjugated substrates. These findings indicate a critical role for cell-matrix interactions with specific ECM-mimetic peptides in supporting and maintaining a healthy NP cell phenotype and bioactivity.

STATEMENT OF SIGNIFICANCE

NP cells reside in a laminin-rich environment that deteriorates with age, including a loss of water content and changes in the extracellular matrix (ECM) structure that may lead to the development of a degenerated IVD. There is great interest in methods to re-express healthy, biosynthetically active NP cells using laminin-derived biomimetic peptides toward the goal of using autologous cell sources for tissue regeneration. Here, we describe a novel study utilizing several laminin mimetic peptides conjugated to polyacrylamide gels that are able to support an immature, healthy NP phenotype after culture on "soft" peptide gels. These findings can support future studies in tissue regeneration where cells may be directed to a desired regenerative phenotype using niche-specific ECM peptides.

Long-term load duration induces N-cadherin down-regulation and loss of cell phenotype of nucleus pulposus cells in a disc bioreactor culture

Long-term exposure to a mechanical load causes degenerative changes in the disc nucleus pulposus (NP) tissue. A previous study demonstrated that N-cadherin (N-CDH)-mediated signalling can preserve the NP cell phenotype. However, N-CDH expression and the resulting phenotype alteration in NP cells under mechanical compression remain unclear. The present study investigated the effects of the compressive duration on N-CDH expression and on the phenotype of NP cells in an ex vivo disc organ culture. Porcine discs were organ cultured in a self-developed mechanically active bioreactor for 7 days. The discs were subjected to different dynamic compression durations (1 and 8 h at a magnitude of 0.4 MPa and frequency of 1.0 Hz) once per day. Discs that were not compressed were used as controls. The results showed that long-term compression duration (8 h) significantly down-regulated the expression of N-CDH and NP-specific molecule markers (Brachyury, Laminin, Glypican-3 and Keratin 19), attenuated Alcian Blue staining intensity, decreased glycosaminoglycan (GAG) and hydroxyproline (HYP) contents and decreased matrix macromolecule (aggrecan and collagen II) expression compared with the short-term compression duration (1 h). Taken together, these findings demonstrate that long-term load duration can induce N-CDH down-regulation, loss of normal cell phenotype and result in attenuation of NP-related matrix synthesis in NP cells.

N-cadherin is Key to Expression of the Nucleus Pulposus Cell Phenotype under Selective Substrate Culture Conditions

Nucleus pulposus (NP) cells of the intervertebral disc are essential for synthesizing extracellular matrix that contributes to disc health and mechanical function. NP cells have a unique morphology and molecular expression pattern derived from their notochordal origin, and reside in N-cadherin (CDH2) positive cell clusters in vivo. With disc degeneration, NP cells undergo morphologic and phenotypic changes including loss of CDH2 expression and ability to form cell clusters. Here, we investigate the role of CDH2 positive cell clusters in preserving healthy, biosynthetically active NP cells. Using a laminin-functionalized hydrogel system designed to mimic features of the native NP microenvironment, we demonstrate NP cell phenotype and morphology is preserved only when NP cells form CDH2 positive cell clusters. Knockdown (CRISPRi) or blocking CDH2 expression in vitro and in vivo results in loss of a healthy NP cell. Findings also reveal that degenerate human NP cells that are CDH2 negative can be promoted to re-express CDH2 and healthy, juvenile NP matrix synthesis patterns by promoting cell clustering for controlled microenvironment conditions. This work also identifies CDH2 interactions with β-catenin-regulated signaling as one mechanism by which CDH2-mediated cell interactions can control NP cell phenotype and biosynthesis towards maintenance of healthy intervertebral disc tissues.

Decoding the intervertebral disc: Unravelling the complexities of cell phenotypes and pathways associated with degeneration and mechanotransduction

Back pain is the most common cause of pain and disability worldwide. While its etiology remains unknown, it is typically associated with intervertebral disc (IVD) degeneration. Despite the prevalence of back pain, relatively little is known about the specific cellular pathways and mechanisms that contribute to the development, function and degeneration of the IVD. Consequently, current treatments for back pain are largely limited to symptomatic interventions. However, major progress is being made in multiple research directions to unravel the biology and pathology of the IVD, raising hope that effective disease-modifying interventions will soon be developed. In this review, we will discuss our current knowledge and gaps in knowledge on the developmental origin of the IVD, the phenotype of the distinct cell types found within the IVD tissues, molecular targets in IVD degeneration identified using bioinformatics strategies, and mechanotransduction pathways that influence IVD cell fate and function.