Expression of Laminin Isoforms, Receptors, and Binding Proteins Unique to Nucleus Pulposus Cells of Immature Intervertebral Disc

Extracellular matrix: Dystroglycan interactions-Roles for the dystrophin-associated glycoprotein complex in skeletal tissue dynamics

Contributions made by the dystrophin-associated glycoprotein complex (DGC) to cell-cell and cell-extracellular matrix (ECM) interactions are vital in development, homeostasis and pathobiology. This review explores how DGC functions may extend to skeletal pathophysiology by appraising the known roles of its major ECM ligands, and likely associated DGC signalling pathways, in regulating cartilage and bone cell behaviour and emergent skeletal phenotypes. These considerations will be contextualised by highlighting the potential of studies into the role of the DGC in isolated chondrocytes, osteoblasts and osteoclasts, and by fuller deliberation of skeletal phenotypes that may emerge in very young mice lacking vital, yet diverse core elements of the DGC. Our review points to roles for individual DGC components-including the glycosylation of dystroglycan itself-beyond the establishment of membrane stability which clearly accounts for severe muscle phenotypes in muscular dystrophy. It implies that the short stature, low bone mineral density, poor bone health and greater fracture risk in these patients, which has been attributed due to primary deficiencies in muscle-evoked skeletal loading, may instead arise due to primary roles for the DGC in controlling skeletal tissue (re)modelling.

The proteomic landscape of extracellular vesicles derived from human intervertebral disc cells

Background

Extracellular vesicles (EVs) function as biomarkers and are crucial in cell communication and regulation, with therapeutic potential for intervertebral disc (IVD)-related low back pain (LBP). EV cargo is often affected by tissue health, which may affect the therapeutic potential. There is currently limited knowledge of how the cargo of IVD cell-derived EVs varies with tissue health and how differences in proteomic profile affect the predicted biological functions.

Methods

Our study purified EVs from human IVD cell conditioned media by size-exclusion chromatography. Nanoparticle tracking analysis was conducted to measure EV size and concentration. Transmission electron microscopy and Western blot were performed to examine EV structure and markers. Tandem mass tag-mass spectrometry was conducted to determine protein cargo.

Results

Most EVs were exosomes and intermediate microvesicles with an increasing amount linked to disease progression. Of the proteins detected, 88.6% were shared across the non-degenerate, mildly-degenerate, and degenerate samples. GO and KEGG analyses revealed that cargo from the mildly-degenerate samples was the most distinct, with the proteins in high abundance strongly associated with extracellular matrix (ECM) organization and structure. Shared proteins, highly expressed in the non-degenerate and degenerate samples, showed strong associations with cell adhesion, ECM-receptor interaction, and vesicle-mediated transport, respectively.

Conclusions

Our findings indicate that EVs from IVD cells from tissue with different degrees of degeneration share a majority of the cargo proteins. However, the level of expression differs with degeneration grade. Cargo from the mildly-degenerate samples exhibits the most differences. A better understanding of changes in EV cargo in the degenerative process may provide novel information related to molecular mechanisms underlying IVD degeneration and suggest new potential treatment modalities for IVD-related LBP.

HIF-1α protects nucleus pulposus cells from oxidative stress-induced mitochondrial impairment through PDK-1

The pathogenesis of intervertebral disc degeneration (IVDD) involves complex signaling networks and various effector molecules, and our understanding of the pathogenesis of IVDD is limited. Hypoxia inducible factor-1α (HIF-1α) is closely related to IVDD, and there is excessive oxidative stress concurrent with IVDD. In this study, we found that HIF-1α could protect nucleus pulposus cells from excessive oxidative stress by reversing the imbalance between oxidants and antioxidants and thus mitigating the oxidative stress-induced mitochondrial impairment. With further exploration, we found that pyruvate dehydrogenase kinase 1 (PDK-1) was involved in the protective effect of HIF-1α on nucleus pulposus cells under oxidative stress. We suggested that HIF-1α could preserve the mitochondrial integrity and activate glycolysis in nucleus pulposus cells via PDK-1, and the addition of DCA, a PDK-1 inhibitor, could blunt the protective effect of HIF-1α. In addition, the HIF-1α/PDK-1 regulatory axis was also confirmed in vivo through HIF-1α knockout mice model. Therefore, we propose that HIF-1α protects nucleus pulposus cells from excessive oxidative stress by maintaining the mitochondrial integrity and glycolysis via PDK-1, thus enriching the insight into the protective mechanism of HIF-1α against IVDD, and providing a novel therapeutic target for the treatment of IVDD.

In vitro and in vivo models define a molecular signature reference for human embryonic notochordal cells

Understanding the emergence of human notochordal cells (NC) is essential for the development of regenerative approaches. We present a comprehensive investigation into the specification and generation of bona fide NC using a straightforward pluripotent stem cell (PSC)-based system benchmarked with human fetal notochord. By integrating in vitro and in vivo transcriptomic data at single-cell resolution, we establish an extended molecular signature and overcome the limitations associated with studying human notochordal lineage at early developmental stages. We show that TGF-β inhibition enhances the yield and homogeneity of notochordal lineage commitment in vitro. Furthermore, this study characterizes regulators of cell-fate decision and matrisome enriched in the notochordal niche. Importantly, we identify specific cell-surface markers opening avenues for differentiation refinement, NC purification, and functional studies. Altogether, this study provides a human notochord transcriptomic reference that will serve as a resource for notochord identification in human systems, diseased-tissues modeling, and facilitating future biomedical research.

Recombinant Laminin-511 Fragment (iMatrix-511) Coating Supports Maintenance of Human Nucleus Pulposus Progenitor Cells In Vitro

The angiopoietin-1 receptor (Tie2) marks specific nucleus pulposus (NP) progenitor cells, shows a rapid decline during aging and intervertebral disc degeneration, and has thus sparked interest in its utilization as a regenerative agent against disc degeneration. However, the challenge of maintaining and expanding these progenitor cells in vitro has been a significant hurdle. In this study, we investigated the potential of laminin-511 to sustain Tie2+ NP progenitor cells in vitro. We isolated cells from human NP tissue (n = 5) and cultured them for 6 days on either standard (Non-coat) or iMatrix-511 (laminin-511 product)-coated (Lami-coat) dishes. We assessed these cells for their proliferative capacity, activation of Erk1/2 and Akt pathways, as well as the expression of cell surface markers such as Tie2, GD2, and CD24. To gauge their regenerative potential, we examined their extracellular matrix (ECM) production capacity (intracellular type II collagen (Col2) and proteoglycans (PG)) and their ability to form spherical colonies within methylcellulose hydrogels. Lami-coat significantly enhanced cell proliferation rates and increased Tie2 expression, resulting in a 7.9-fold increase in Tie2-expressing cell yields. Moreover, the overall proportion of cells positive for Tie2 also increased 2.7-fold. Notably, the Col2 positivity rate was significantly higher on laminin-coated plates (Non-coat: 10.24% (±1.7%) versus Lami-coat: 26.2% (±7.5%), p = 0.010), and the ability to form spherical colonies also showed a significant improvement (Non-coat: 40.7 (±8.8)/1000 cells versus Lami-coat: 70.53 (±18.0)/1000 cells, p = 0.016). These findings demonstrate that Lami-coat enhances the potential of NP cells, as indicated by improved colony formation and proliferative characteristics. This highlights the potential of laminin-coating in maintaining the NP progenitor cell phenotype in culture, thereby supporting their translation into prospective clinical cell-transplantation products.

Rat tail models for the assessment of injectable nucleus pulposus regeneration strategies

Back pain is a global epidemiological and socioeconomic problem often associated with intervertebral disc degeneration; a condition believed to initiate in the nucleus pulposus (NP). There is considerable interest in developing early therapeutic interventions to target the NP and halt degeneration. Rat caudal models of disc degeneration have demonstrated significant utility in the study of disease progression and its impact on tissue structure, composition, and mechanical performance. One significant advantage of the caudal model is the ease of access and high throughput nature. However, considerable variability exists across the literature in terms of experimental setup and parameters. The objective of this article is to aid researchers in the design and development of caudal puncture models by providing details and insight into the most reported experimental parameters. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were employed to screen the existing literature and 80 manuscripts met the inclusion criteria. Disc geometry, surgical approaches, effect of needle gauge size to induce degeneration, therapeutic volume, outcome measures, and associated limitations are considered and discussed, and a range of recommendations based on different research questions are presented.

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.

The Proteolysis of ECM in Intervertebral Disc Degeneration

Intervertebral disc (IVD) degeneration (IDD) is a pathological process that commonly occurs throughout the human life span and is a major cause of lower back pain. Better elucidation of the molecular mechanisms involved in disc degeneration could provide a theoretical basis for the development of lumbar disc intervention strategies. In recent years, extracellular matrix (ECM) homeostasis has received much attention due to its relevance to the mechanical properties of IVDs. ECM proteolysis mediated by a variety of proteases is involved in the pathological process of disc degeneration. Here, we discuss in detail the relationship between the IVD as well as the ECM and the role of ECM proteolysis in the degenerative process of the IVD. Targeting ECM proteolysis-associated proteases may be an effective means of intervention in IDD.

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.

Increased Expression of Integrin Alpha 6 in Nucleus Pulposus Cells in Response to High Oxygen Tension Protects against Intervertebral Disc Degeneration

The destruction of the low oxygen microenvironment in nucleus pulposus (NP) cells played a critical role in the pathogenesis of intervertebral disc degeneration (IVDD). The purpose of this study was to determine the potential role of integrin alpha 6 (ITG α6) in NP cells in response to high oxygen tension (HOT) in IVDD. Immunofluorescence staining and western blot analysis showed that the levels of ITG α6 expression were increased in the NP tissue from IVDD patients and the IVDD rat model with mild degeneration, which were reduced as the degree of degeneration increases in severity. In NP cells, the treatment of HOT resulted in upregulation of ITG α6 expression, which could be alleviated by blocking the PI3K/AKT signaling pathway. Further studies found that ITG α6 could protect NP cells against HOT-induced apoptosis and oxidative stress and protect NP cells from HOT-inhibited ECM protein synthesis. Upregulation of ITG α6 expression by HOT contributed to maintaining NP tissue homeostasis through the interaction with hypoxia-inducible factor-1α (HIF-1α). Furthermore, silencing of ITG α6 in vivo could obviously accelerate puncture-induced IVDD. Taken together, these results revealed that the increase of ITG α6 expression by HOT in NP cells might be a protective factor in IVD degeneration as well as restore NP cell function.

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.

Simulating intervertebral disc cell behaviour within 3D multifactorial environments

MOTIVATION

Low back pain is responsible for more global disability than any other condition. Its incidence is closely related to intervertebral disc (IVD) failure, which is likely caused by an accumulation of microtrauma within the IVD. Crucial factors in microtrauma development are not entirely known yet, probably because their exploration in vivo or in vitro remains tremendously challenging. In silico modelling is, therefore, definitively appealing, and shall include approaches to integrate influences of multiple cell stimuli at the microscale. Accordingly, this study introduces a hybrid Agent-based (AB) model in IVD research and exploits network modelling solutions in systems biology to mimic the cellular behaviour of Nucleus Pulposus cells exposed to a 3D multifactorial biochemical environment, based on mathematical integrations of existing experimental knowledge. Cellular activity reflected by mRNA expression of Aggrecan, Collagen type I, Collagen type II, MMP-3 and ADAMTS were calculated for inflamed and non-inflamed cells. mRNA expression over long periods of time is additionally determined including cell viability estimations. Model predictions were eventually validated with independent experimental data.

RESULTS

As it combines experimental data to simulate cell behaviour exposed to a multifactorial environment, the present methodology was able to reproduce cell death within 3 days under glucose deprivation and a 50% decrease in cell viability after 7 days in an acidic environment. Cellular mRNA expression under non-inflamed conditions simulated a quantifiable catabolic shift under an adverse cell environment, and model predictions of mRNA expression of inflamed cells provide new explanation possibilities for unexpected results achieved in experimental research.

AVAILABILITYAND IMPLEMENTATION

The AB model as well as used mathematical functions were built with open source software. Final functions implemented in the AB model and complete AB model parameters are provided as Supplementary Material. Experimental input and validation data were provided through referenced, published papers. The code corresponding to the model can be shared upon request and shall be reused after proper training.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Development of a library of laminin-mimetic peptide hydrogels for control of nucleus pulposus cell behaviors

The nucleus pulposus (NP) of the intervertebral disc plays a critical role in distributing mechanical loads to the axial skeleton. Alterations in NP cells and, consequently, NP matrix are some of the earliest changes in the development of disc degeneration. Previous studies demonstrated a role for laminin-presenting biomaterials in promoting a healthy phenotype for human NP cells from degenerated tissue. Here we investigate the use of laminin-mimetic peptides presented individually or in combination on a poly(ethylene) glycol hydrogel as a platform to modulate the behaviors of degenerative human NP cells. Data confirm that NP cells attach to select laminin-mimetic peptides that results in cell signaling downstream of integrin and syndecan binding. Furthermore, the peptide-functionalized hydrogels demonstrate an ability to promote cell behaviors that mimic that of full-length laminins. These results identify a set of peptides that can be used to regulate NP cell behaviors toward a regenerative engineering strategy.

Simultaneous detection of multiple mRNAs and proteins in bovine IVD cells and tissue with single cell resolution

OBJECTIVES

Interactions of cells with their neighbors and influences by the surrounding extracellular matrix (ECM) is reflected in a cells transcriptome and proteome. In tissues comprised of heterogeneous cell populations or cells depending on ECM signalling cues such as those of the intervertebral disc (IVD), this information is obscured or lost when cells are pooled for the commonly used transcript analysis by quantitative PCR or RNA sequencing. Instead, these cells require means to analyse RNA transcript and protein distribution at a single cell or subcellular level to identify different cell types and functions, without removing them from their surrounding signalling cues.

RESULTS

We developed a simple, sequential protocol combining RNA is situ hybridisation (RISH) and immunohistochemistry (IHC) for the simultaneous analysis of multiple transcripts alongside proteins. This allows one to characterize heterogeneous cell populations at the single cell level in the natural cell environment and signalling context, both in vivo and in vitro. This protocol is demonstrated on cells of the bovine IVD, for transcripts and proteins involved in mechanotransduction, stemness and cell proliferation.

CONCLUSIONS

A simple, sequential protocol combining RISH and IHC is presented that allows for simultaneous information on RNA transcripts and proteins to characterize cells within a heterogeneous cell population and complex signalling environments such as those of the IVD.

Protocol for parallel proteomic and metabolomic analysis of mouse intervertebral disc tissues

The comprehensiveness of data collected by "omics" modalities has demonstrated the ability to drastically transform our understanding of the molecular mechanisms of chronic, complex diseases such as musculoskeletal pathologies, how biomarkers are identified, and how therapeutic targets are developed. Standardization of protocols will enable comparisons between findings reported by multiple research groups and move the application of these technologies forward. Herein, we describe a protocol for parallel proteomic and metabolomic analysis of mouse intervertebral disc (IVD) tissues, building from the combined expertise of our collaborative team. This protocol covers dissection of murine IVD tissues, sample isolation, and data analysis for both proteomics and metabolomics applications. The protocol presented below was optimized to maximize the utility of a mouse model for "omics" applications, accounting for the challenges associated with the small starting quantity of sample due to small tissue size as well as the extracellular matrix-rich nature of the tissue.

Control of adhesive ligand density for modulation of nucleus pulposus cell phenotype

Cells of the nucleus pulposus have been observed to undergo a shift from their notochordal-like juvenile phenotype to a more fibroblast-like state with age and maturation. It has been demonstrated that culture of degenerative adult human nucleus pulposus cells upon soft (<1 kPa) full length laminin-containing hydrogel substrates promotes increased levels of a panel of markers associated with the juvenile nucleus pulposus cell phenotype. In the current work, we observed an ability to use soft polymeric substrates functionalized with short laminin-mimetic peptide sequences to recapitulate the behaviors elicited by soft, full-length laminin containing materials. Furthermore, our work suggests an ability to mimic features of soft systems through control of peptide density upon stiffer substrates. Specifically, results suggest that stiffer polymer-peptide hydrogel substrates can be used to promote the expression of a more juvenile-like phenotype for cells of the nucleus pulposus by reducing adhesive ligand presentation. Here we show how polymer stiffness combined with adhesive ligand presentation can be controlled to be supportive of nucleus pulposus cell phenotype and biosynthesis.

Alterations in ECM signature underscore multiple sub-phenotypes of intervertebral disc degeneration

The intervertebral disc is a specialized connective tissue critical for absorption of mechanical loads and providing flexibility to the spinal column. The disc ECM is complex and plays a vital role in imparting tissue its biomechanical function. The central NP is primarily composed of large aggregating proteoglycans (PGs) while surrounding AF is composed of fibrillar collagens, I and II. Aggrecan and versican in particular, due to their high concentration of sulfated GAG chains form large aggregates with hyaluronic acid (HA) and provide water binding capacity to the disc. Degradation of aggrecan core protein due to aggrecanase and MMP activity, SNPs that affect number of chondroitin sulfate (CS) substitutions and alteration in enzymes critical in synthesis of CS chains can impair the aggrecan functionality. Similarly, levels of many matrix and matrix-related molecules e.g. Col2, Col9, HAS2, ccn2 are dysregulated during disc degeneration and genetic animal models have helped establish causative link between their expression and disc health. In the degenerating and herniated discs, increased levels of inflammatory cytokines such as TNF-α, IL-1β and IL-6 are shown to promote matrix degradation through regulating expression and activity of critical proteases and stimulate immune cell activation. Recent studies of different mouse strains have better elucidated the broader impact of spontaneous degeneration on disc matrix homeostasis. SM/J mice showed an increased cell apoptosis, loss of cell phenotype, and cleavage of aggrecan during early stages followed by tissue fibrosis evident by enrichment of several collagens, SLRPs and fibronectin. In summary, while disc degeneration encompasses wide spectrum of degenerative phenotypes extensive matrix degradation and remodeling underscores all of them.

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The intervertebral disc absorbs loads and provides flexibility to the spine.

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The ECM is complex and vital for imparting tissue its biomechanical function.

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Numerous types of proteoglycans and collagens designate the quality of the disc.

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Many matrix and matrix-related molecules are dysregulated during disc degeneration.

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Matrix degradation and remodeling underscores wide spectrum of phenotype.

Transcriptional profiling of the murine intervertebral disc and age-associated changes in the nucleus pulposus

Purpose/Aim: The intervertebral disc (IVD) is composed of cell types whose subtle phenotypic differences allow for the formation of distinct tissues. The role of the nucleus pulposus (NP) in the initiation and progression of IVD degeneration is well established; however, the genes and pathways associated with NP degeneration are poorly characterized.Materials and Methods: Using a genetic strategy for IVD lineage-specific fluorescent reporter expression to isolate cells, gene expression and bioinformatic analysis was conducted on the murine NP at 2.5, 6, and 21 months-of-age and the annulus fibrosus (AF) at 2.5 and 6 months-of-age. A subset of differentially regulated genes was validated by qRT-PCR.Results: Transcriptome analysis identified distinct profiles of NP and AF gene expression that were remarkably consistent at 2.5 and 6 months-of-age. Prg4, Cilp, Ibsp and Comp were increased >50-fold in the AF relative to NP. The most highly enriched NP genes included Dsc3 and Cdh6, members of the cadherin superfamily, and microRNAs mir218-1 and mir490. Changes in the NP between 2.5 and 6 months-of-age were associated with up-regulation of molecular functions linked to laminin and Bmp receptor binding (including up-regulation of Bmp5 & 7), with the most up-regulated genes being Mir703, Shh, and Sfrp5. NP degeneration was associated with molecular functions linked to alpha-actinin binding (including up-regulation of Ttn & Myot) and cytoskeletal protein binding, with the overall most up-regulated genes being Rnu3a, Snora2b and Mir669h.Conclusions: This study provided insight into the phenotypes of NP and AF cells, and identified candidate pathways that may regulate degeneration.

Mechanosensitive transcriptional coactivators MRTF-A and YAP/TAZ regulate nucleus pulposus cell phenotype through cell shape

Cells of the adult nucleus pulposus (NP) are critically important in maintaining overall disc health and function. NP cells reside in a soft, gelatinous matrix that dehydrates and becomes increasingly fibrotic with age. Such changes result in physical cues of matrix stiffness that may be potent regulators of NP cell phenotype and may contribute to a transition toward a senescent and fibroblastic NP cell with a limited capacity for repair. Here, we investigate the mechanosignaling cues generated from changes in matrix stiffness in directing NP cell phenotype and identify mechanisms that can potentially preserve a biosynthetically active, juvenile NP cell phenotype. Using a laminin-functionalized polyethylene glycol hydrogel, we show that when NP cells form rounded, multicell clusters, they are able to maintain cytosolic localization of myocardin-related transcription factor (MRTF)-A, a coactivator of serum-response factor (SRF), known to promote fibroblast-like behaviors in many cells. Upon preservation of a rounded shape, human NP cells similarly showed cytosolic retention of transcriptional coactivator Yes-associated protein (YAP) and its paralogue PDZ-binding motif (TAZ) with associated decline in activation of its transcription factor TEA domain family member-binding domain (TEAD). When changes in cell shape occur, leading to a more spread, fibrotic morphology associated with stronger F-actin alignment, SRF and TEAD are up-regulated. However, targeted deletion of either cofactor was not sufficient to overcome shape-mediated changes observed in transcriptional activation of SRF or TEAD. Findings show that substrate stiffness-induced promotion of F-actin alignment occurs concomitantly with a flattened, spread morphology, decreased NP marker expression, and reduced biosynthetic activity. This work indicates cell shape is a stronger indicator of SRF and TEAD mechanosignaling pathways than coactivators MRTF-A and YAP/TAZ, respectively, and may play a role in the degeneration-associated loss of NP cellularity and phenotype.-Fearing, B. V., Jing, L., Barcellona, M. N., Witte, S. E., Buchowski, J. M., Zebala, L. P., Kelly, M. P., Luhmann, S., Gupta, M. C., Pathak, A., Setton, L. A. Mechanosensitive transcriptional coactivators MRTF-A and YAP/TAZ regulate nucleus pulposus cell phenotype through cell shape.

Galectins-1 and -3 in Human Intervertebral Disc Degeneration: Non-Uniform Distribution Profiles and Activation of Disease Markers Involving NF-κB by Galectin-1

Degeneration of the human intervertebral disc (IVD) is assumed to underlie severe clinical symptoms, in particular chronic back pain. Since adhesion/growth-regulatory galectins are linked to arthritis/osteoarthritis pathogenesis by activating a pro-degradative/-inflammatory gene expression signature, we hypothesized a similar functional involvement of galectins in IVD degeneration. Immunohistochemical evidence for the presence of galectins-1 and -3 in IVD is provided comparatively for specimens of spondylochondrosis, spondylolisthesis, and spinal deformity. Immunopositivity was detected in sections of fixed IVD specimens in each cellular compartment with age-, disease-, and galectin-type-related differences. Of note, presence of both galectins correlated with IVD degeneration, whereas correlation with age was seen only for galectin-3. In addition, staining profiles for these two galectins showed different distribution patterns in serial sections, an indication for non-redundant functionalities. In vitro, both galectins bound to IVD cells in a glycan-dependent manner. However, exclusively galectin-1 binding triggered a significant induction of functional disease markers (i.e., IL6, CXCL8, and MMP1/3/13) with involvement of the nuclear factor-kB pathway. This study thus gives direction to further network analyses and functional studies on galectins in IVD degeneration. © 2019 The Authors. Journal of Orthopaedic Research® published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 37:2204-2216, 2019.

Disordered Mechanical Stress and Tissue Engineering Therapies in Intervertebral Disc Degeneration

Low back pain (LBP), commonly induced by intervertebral disc degeneration, is a lumbar disease with worldwide prevalence. However, the mechanism of degeneration remains unclear. The intervertebral disc is a nonvascular organ consisting of three components: Nucleus pulposus, annulus fibrosus, and endplate cartilages. The disc is structured to support our body motion and endure persistent external mechanical pressure. Thus, there is a close connection between force and intervertebral discs in LBP. It is well established that with aging, disordered mechanical stress profoundly influences the fate of nucleus pulposus and the alignment of collagen fibers in the annulus fibrosus. These support a new understanding that disordered mechanical stress plays an important role in the degeneration of the intervertebral discs. Tissue-engineered regenerative and reparative therapies are being developed for relieving disc degeneration and symptoms of lower back pain. In this paper, we will review the current literature available on the role of disordered mechanical stress in intervertebral disc degeneration, and evaluate the existing tissue engineering treatment strategies of the current therapies.

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.

EGFR is a pivotal regulator of thrombin-mediated inflammation in primary human nucleus pulposus culture

We found that the coagulation and cytokine pathways were important mechanisms involve in the degeneration of intervertebral discs (IVD) using a microarray approach to analyze gene expression in different grades of specimens. Furthermore, using a cytokine/chemokine array, a significant increase in CXCL8 expression was observed in human nucleus pulposus (NP) cells after thrombin treatment. The enhancement of CXCL8 expression by thrombin was activated by the PAR1 receptor. Importantly, analysis of degenerated human NP tissue samples showed that EGFR expression positively correlated with the grade of tissue degeneration. In NP cells, thrombin caused an increase in phosphorylation of the EGFR at the Tyr1068, and treatment with the pharmacological EGFR inhibitor, AG1473 effectively blocked thrombin-enhanced CXCL8 production. Surprisingly, inhibition of STAT3 for 24 h decreased expression of EGFR. Treatment with thrombin also increased Akt and GSK3α/β activation; this activation was also blocked by EGFR inhibitor. Although c-Src, ERK, and FAK were activated by thrombin, only c-Src and ERK were involved in the STAT3/CXCL8 induction. Our findings indicate that stimulation of an inflammatory response in NP cells by thrombin is part of a specific pathophysiology that modulates the EGFR activation through activation of Src/ERK/STAT3 signaling.

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.

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

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

Biomaterials for intervertebral disc regeneration and repair

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

Identifying molecular phenotype of nucleus pulposus cells in human intervertebral disc with aging and degeneration

Previous study claimed that disc degeneration may be preceded by structure and matrix changes in the intervertebral disc (IVD) which coincide with the loss of distinct notochordally derived nucleus pulposus (NP) cells. However, the fate of notochordal cells and their molecular phenotype change during aging and degeneration in human are still unknown. In this study, a set of novel molecular phenotype markers of notochordal NP cells during aging and degeneration in human IVD tissue were revealed with immunostaining and flow cytometry. Furthermore, the potential of phenotype juvenilization and matrix regeneration of IVD cells in a laminin-rich pseudo-3D culture system were evaluated at day 28 by immunostaining, Safranin O, and type II collagen staining. Immunostaining and flow cytometry demonstrated that transcriptional factor Brachyury T, neuronal-related proteins (brain abundant membrane attached signal protein 1, Basp1; Neurochondrin, Ncdn; Neuropilin, Nrp-1), CD24, and CD221 were expressed only in juvenile human NP tissue, which suggested that these proteins may be served as the notochordal NP cell markers. However, the increased expression of CD54 and CD166 with aging indicated that they might be referenced as the potential biomarker for disc degeneration. In addition, 3D culture maintained most of markers in juvenile NP, and rescued the expression of Basp1, Ncdn, and Nrp 1 that disappeared in adult NP native tissue. These findings provided new insight into molecular profile that may be used to characterize the existence of a unique notochordal NP cells during aging and degeneration in human IVD cells, which will facilitate cell-based therapy for IVD regeneration. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1316-1326, 2016.

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.

High glucose regulates LN expression in human liver sinusoidal endothelial cells through ROS/integrin αvβ3 pathway

Diabetes mellitus can cause a wide variety of vascular complications and is one of the major risk factors for Non Alcoholic Fatty Liver Disease (NAFLD). The present study was designed investigate the expression of laminin (LN) in human liver sinusoidal endothelial cells (HLSECs) induced by high glucose and the role of reactive oxygen species (ROS) and integrin αvβ3 in the regulation of LN expression. HLSECs were cultured and treated with media containing 25 mM glucose in the presence or absence of N-acetylcysteine (NAC) or clone LM609. The level of intracellular ROS of HLSECs was measured with 2',7' dichloro-fluorescein diacetate (DCFH-DA) probe. Expression of integrin αvβ3 was measured using RT-PCR and Western blot. Expression of LN was testified by immunofluorescence assay. Compared with that in control group, ROS level and the expression of integrin αvβ3 and LN increased in high glucose group. Compared with that in high glucose group, antioxidant NAC inhibited the expression of integrin αvβ3, NAC and the anti-body for blocking integrin αvβ3 (clone LM609) down-regulated the expression of LN. However, the above parameters did not differ between control and mannitol groups. High glucose up-regulates expression of LN in HLSECs through ROS/integrin αvβ3 pathway.

Advancing biomaterials of human origin for tissue engineering

Biomaterials have played an increasingly prominent role in the success of biomedical devices and in the development of tissue engineering, which seeks to unlock the regenerative potential innate to human tissues/organs in a state of deterioration and to restore or reestablish normal bodily function. Advances in our understanding of regenerative biomaterials and their roles in new tissue formation can potentially open a new frontier in the fast-growing field of regenerative medicine. Taking inspiration from the role and multi-component construction of native extracellular matrices (ECMs) for cell accommodation, the synthetic biomaterials produced today routinely incorporate biologically active components to define an artificial in vivo milieu with complex and dynamic interactions that foster and regulate stem cells, similar to the events occurring in a natural cellular microenvironment. The range and degree of biomaterial sophistication have also dramatically increased as more knowledge has accumulated through materials science, matrix biology and tissue engineering. However, achieving clinical translation and commercial success requires regenerative biomaterials to be not only efficacious and safe but also cost-effective and convenient for use and production. Utilizing biomaterials of human origin as building blocks for therapeutic purposes has provided a facilitated approach that closely mimics the critical aspects of natural tissue with regard to its physical and chemical properties for the orchestration of wound healing and tissue regeneration. In addition to directly using tissue transfers and transplants for repair, new applications of human-derived biomaterials are now focusing on the use of naturally occurring biomacromolecules, decellularized ECM scaffolds and autologous preparations rich in growth factors/non-expanded stem cells to either target acceleration/magnification of the body's own repair capacity or use nature's paradigms to create new tissues for restoration. In particular, there is increasing interest in separating ECMs into simplified functional domains and/or biopolymeric assemblies so that these components/constituents can be discretely exploited and manipulated for the production of bioscaffolds and new biomimetic biomaterials. Here, following an overview of tissue auto-/allo-transplantation, we discuss the recent trends and advances as well as the challenges and future directions in the evolution and application of human-derived biomaterials for reconstructive surgery and tissue engineering. In particular, we focus on an exploration of the structural, mechanical, biochemical and biological information present in native human tissue for bioengineering applications and to provide inspiration for the design of future biomaterials.

The role of extracellular matrix elasticity and composition in regulating the nucleus pulposus cell phenotype in the intervertebral disc: a narrative review

Intervertebral disc (IVD) disorders are a major contributor to disability and societal health care costs. Nucleus pulposus (NP) cells of the IVD exhibit changes in both phenotype and morphology with aging-related IVD degeneration that may impact the onset and progression of IVD pathology. Studies have demonstrated that immature NP cell interactions with their extracellular matrix (ECM) may be key regulators of cellular phenotype, metabolism and morphology. The objective of this article is to review our recent experience with studies of NP cell-ECM interactions that reveal how ECM cues can be manipulated to promote an immature NP cell phenotype and morphology. Findings demonstrate the importance of a soft (&lt;700 Pa), laminin-containing ECM in regulating healthy, immature NP cells. Knowledge of NP cell-ECM interactions can be used for development of tissue engineering or cell delivery strategies to treat IVD-related disorders.

Molecular characterization of chordoma xenografts generated from a novel primary chordoma cell source and two chordoma cell lines

OBJECT

Chordoma cells can generate solid-like tumors in xenograft models that express some molecular characteristics of the parent tumor, including positivity for brachyury and cytokeratins. However, there is a dearth of molecular markers that relate to chordoma tumor growth, as well as the cell lines needed to advance treatment. The objective in this study was to isolate a novel primary chordoma cell source and analyze the characteristics of tumor growth in a mouse xenograft model for comparison with the established U-CH1 and U-CH2b cell lines.

METHODS

Primary cells from a sacral chordoma, called "DVC-4," were cultured alongside U-CH1 and U-CH2b cells for more than 20 passages and characterized for expression of CD24 and brachyury. While brachyury is believed essential for driving tumor formation, CD24 is associated with healthy nucleus pulposus cells. Each cell type was subcutaneously implanted in NOD/SCID/IL2Rγ(null) mice. The percentage of solid tumors formed, time to maximum tumor size, and immunostaining scores for CD24 and brachyury (intensity scores of 0-3, heterogeneity scores of 0-1) were reported and evaluated to test differences across groups.

RESULTS

The DVC-4 cells retained chordoma-like morphology in culture and exhibited CD24 and brachyury expression profiles in vitro that were similar to those for U-CH1 and U-CH2b. Both U-CH1 and DVC-4 cells grew tumors at rates that were faster than those for U-CH2b cells. Gross tumor developed at nearly every site (95%) injected with U-CH1 and at most sites (75%) injected with DVC-4. In contrast, U-CH2b cells produced grossly visible tumors in less than 50% of injected sites. Brachyury staining was similar among tumors derived from all 3 cell types and was intensely positive (scores of 2-3) in a majority of tissue sections. In contrast, differences in the pattern and intensity of staining for CD24 were noted among the 3 types of cell-derived tumors (p < 0.05, chi-square test), with evidence of intense and uniform staining in a majority of U-CH1 tumor sections (score of 3) and more than half of the DVC-4 tumor sections (scores of 2-3). In contrast, a majority of sections from U-CH2b cells stained modestly for CD24 (scores of 1-2) with a predominantly heterogeneous staining pattern.

CONCLUSIONS

This is the first report on xenografts generated from U-CH2b cells in which a low tumorigenicity was discovered despite evidence of chordoma-like characteristics in vitro. For tumors derived from a primary chordoma cell and U-CH1 cell line, similarly intense staining for CD24 was observed, which may correspond to their similar potential to grow tumors. In contrast, U-CH2b tumors stained less intensely for CD24. These results emphasize that many markers, including CD24, may be useful in distinguishing among chordoma cell types and their tumorigenicity in vivo.

Distribution of Basement Membrane Molecules, Laminin and Collagen Type IV, in Normal and Degenerated Cartilage Tissues

OBJECTIVE

The objective of the present study was to investigate the presence and distribution of 2 basement membrane (BM) molecules, laminin and collagen type IV, in healthy and degenerative cartilage tissues.

DESIGN

Normal and degenerated tissues were obtained from goats and humans, including articular knee cartilage, the intervertebral disc, and meniscus. Normal tissue was also obtained from patella-tibial enthesis in goats. Immunohistochemical analysis was performed using anti-laminin and anti-collagen type IV antibodies. Human and goat skin were used as positive controls. The percentage of cells displaying the pericellular presence of the protein was graded semiquantitatively.

RESULTS

When present, laminin and collagen type IV were exclusively found in the pericellular matrix, and in a discrete layer on the articulating surface of normal articular cartilage. In normal articular (hyaline) cartilage in the human and goat, the proteins were found co-localized pericellularly. In contrast, in human osteoarthritic articular cartilage, collagen type IV but not laminin was found in the pericellular region. Nonpathological fibrocartilaginous tissues from the goat, including the menisci and the enthesis, were also positive for both laminin and collagen type IV pericellularly. In degenerated fibrocartilage, including intervertebral disc, as in degenerated hyaline cartilage only collagen type IV was found pericellularly around chondrocytes but with less intense staining than in non-degenerated tissue. In calcified cartilage, some cells were positive for laminin but not type IV collagen.

CONCLUSIONS

We report differences in expression of the BM molecules, laminin and collagen type IV, in normal and degenerative cartilaginous tissues from adult humans and goats. In degenerative tissues laminin is depleted from the pericellular matrix before collagen type IV. The findings may inform future studies of the processes underlying cartilage degeneration and the functional roles of these 2 extracellular matrix proteins, normally associated with BM.

Photocrosslinkable laminin-functionalized polyethylene glycol hydrogel for intervertebral disc regeneration

Intervertebral disc (IVD) disorders and age-related degeneration are believed to contribute to lower back pain. There is significant interest in cell-based strategies for regenerating the nucleus pulposus (NP) region of the disc; however, few scaffolds have been evaluated for their ability to promote or maintain an immature NP cell phenotype. Previous studies have shown that NP cell-laminin interactions promote cell adhesion and biosynthesis, which suggests a laminin-functionalized biomaterial may be useful for promoting or maintaining the NP cell phenotype. Here, a photocrosslinkable poly(ethylene glycol)-laminin 111 (PEG-LM111) hydrogel was developed. The mechanical properties of PEG-LM111 hydrogel could be tuned within the range of dynamic shear moduli values previously reported for human NP. When primary immature porcine NP cells were seeded onto PEG-LM111 hydrogels of varying stiffnesses, LM111-presenting hydrogels were found to promote cell clustering and increased levels of sGAG production as compared to stiffer LM111-presenting and PEG-only gels. When cells were encapsulated in 3-D gels, hydrogel formulation was found to influence NP cell metabolism and expression of proposed NP phenotypic markers, with higher expression of N-cadherin and cytokeratin 8 observed for cells cultured in softer (<1kPa) PEG-LM111 hydrogels. Overall, these findings suggest that soft, LM111-functionalized hydrogels may promote or maintain the expression of specific markers characteristic of an immature NP cell phenotype.

Human umbilical cord mesenchymal stromal cells exhibit immature nucleus pulposus cell phenotype in a laminin-rich pseudo-three-dimensional culture system

Introduction

Cell supplementation to the herniated or degenerated intervertebral disc (IVD) is a potential strategy to promote tissue regeneration and slow disc pathology. Human umbilical cord mesenchymal stromal cells (HUCMSCs) – originating from the Wharton’s jelly – remain an attractive candidate for such endeavors with their ability to differentiate into multiple lineages. Previously, mesenchymal stem cells (MSCs) have been studied as a potential source for disc tissue regeneration. However, no studies have demonstrated that MSCs can regenerate matrix with unique characteristics matching that of immature nucleus pulposus (NP) tissues of the IVD. In our prior work, immature NP cells were found to express specific laminin isoforms and laminin-binding receptors that may serve as phenotypic markers for evaluating MSC differentiation to NP-like cells. The goal of this study is to evaluate these markers and matrix synthesis for HUCMSCs cultured in a laminin-rich pseudo-three-dimensional culture system.

Methods

HUCMSCs were seeded on top of Transwell inserts pre-coated with Matrigel™, which contained mainly laminin-111. Cells were cultured under hypoxia environment with three differentiation conditions: NP differentiation media (containing 2.5% Matrigel™ solution to provide for a pseudo-three-dimensional laminin culture system) with no serum, or the same media supplemented with either insulin-like growth factor-1 (IGF-1) or transforming growth factor-β1 (TGF-β1). Cell clustering behavior, matrix production and the expression of NP-specific laminin and laminin-receptors were evaluated at days 1, 7, 13 and 21 of culture.

Results

Data show that a pseudo-three-dimensional culture condition (laminin-1 rich) promoted HUCMSC differentiation under no serum conditions. Starting at day 1, HUCMSCs demonstrated a cell clustering morphology similar to that of immature NP cells in situ and that observed for primary immature NP cells within the similar laminin-rich culture system (prior study). Differentiated HUCMSCs under all conditions were found to contain glycosaminoglycan, expressed extracellular matrix proteins of collagen II and laminin α5, and laminin receptors (integrin α3 and β4 subunits). However, neither growth factor treatment generated distinct differences in NP-like phenotype for HUCMSC as compared with no-serum conditions.

Conclusions

HUCMSCs have the potential to differentiate into cells sharing features with immature NP cells in a laminin-rich culture environment and may be useful for IVD cellular therapy.

Integrin-mediated interactions with extracellular matrix proteins for nucleus pulposus cells of the human intervertebral disc

The extracellular matrix (ECM) of the human intervertebral disc is rich in molecules that interact with cells through integrin-mediated attachments. Porcine nucleus pulposus (NP) cells have been shown to interact with laminin (LM) isoforms LM-111 and LM-511 through select integrins that regulate biosynthesis and cell attachment. Since human NP cells lose many phenotypic characteristics with age, attachment and interaction with the ECM may be altered. Expression of LM-binding integrins was quantified for human NP cells using flow cytometry. The cell-ECM attachment mechanism was determined by quantifying cell attachment to LM-111, LM-511, or type II collagen after functionally blocking specific integrin subunits. Human NP cells express integrins β1, α3, and α5, with over 70% of cells positive for each subunit. Blocking subunit β1 inhibited NP cell attachment to all substrates. Blocking subunits α1, α2, α3, and α5 simultaneously, but not individually, inhibits NP cell attachment to laminins. While integrin α6β1 mediated porcine NP cell attachment to LM-111, we found integrins α3, α5, and β1 instead contributed to human NP cell attachment. These findings identify integrin subunits that may mediate interactions with the ECM for human NP cells and could be used to promote cell attachment, survival, and biosynthesis in cell-based therapeutics.

Differentiation of mouse induced pluripotent stem cells (iPSCs) into nucleus pulposus-like cells in vitro

A large percentage of the population may be expected to experience painful symptoms or disability associated with intervertebral disc (IVD) degeneration - a condition characterized by diminished integrity of tissue components. Great interest exists in the use of autologous or allogeneic cells delivered to the degenerated IVD to promote matrix regeneration. Induced pluripotent stem cells (iPSCs), derived from a patient's own somatic cells, have demonstrated their capacity to differentiate into various cell types although their potential to differentiate into an IVD cell has not yet been demonstrated. The overall objective of this study was to assess the possibility of generating iPSC-derived nucleus pulposus (NP) cells in a mouse model, a cell population that is entirely derived from notochord. This study employed magnetic activated cell sorting (MACS) to isolate a CD24(+) iPSC subpopulation. Notochordal cell-related gene expression was analyzed in this CD24(+) cell fraction via real time RT-PCR. CD24(+) iPSCs were then cultured in a laminin-rich culture system for up to 28 days, and the mouse NP phenotype was assessed by immunostaining. This study also focused on producing a more conducive environment for NP differentiation of mouse iPSCs with addition of low oxygen tension and notochordal cell conditioned medium (NCCM) to the culture platform. iPSCs were evaluated for an ability to adopt an NP-like phenotype through a combination of immunostaining and biochemical assays. Results demonstrated that a CD24(+) fraction of mouse iPSCs could be retrieved and differentiated into a population that could synthesize matrix components similar to that in native NP. Likewise, the addition of a hypoxic environment and NCCM induced a similar phenotypic result. In conclusion, this study suggests that mouse iPSCs have the potential to differentiate into NP-like cells and suggests the possibility that they may be used as a novel cell source for cellular therapy in the IVD.

Basement membrane molecule expression attendant to chondrogenesis by nucleus pulposus cells and mesenchymal stem cells

Bone marrow-derived mesenchymal stem cells (MSCs) represent an autologous cell source for nucleus pulposus (NP) tissue engineering and regeneration. Although studies have demonstrated the ability of MSCs to differentiate to NP-like chondrocytic cells, few have comparatively studied the matrix synthesis and composition of the cartilaginous tissue formed in vitro from both cell types, particularly with respect to the expression of basement membrane (BM) molecules. The objective of this study was to evaluate chondrogenesis and expression of BM molecules, laminin and type IV collagen, in monolayer and in pellet cultures of caprine NP cells and MSCs. Both cell types demonstrated comparable levels of chondrogenesis, indicated by the percentage of chondrocytic cells, and the amounts of glycosaminoglycan and type II collagen. Laminin and type IV collagen were expressed intracellularly by NP cells and MSCs cultured in monolayer. During chondrogenesis in pellet cultures, the deposition of BM molecules in NP and MSC pellets followed an orderly spatiotemporal shift in pattern from a diffuse territorial and interterritorial distribution to a defined pericellular localization, as seen in normal adult NP. These results inform the use of MSCs for NP regeneration and suggest the possible involvement of certain BM molecules in chondrogenesis and cartilage regeneration.

Effects of nucleus pulposus cell-derived acellular matrix on the differentiation of mesenchymal stem cells

Recent attempts to treat disc degeneration with mesenchymal stem cells (MSCs) showed encouraging results. Differentiating MSCs towards nucleus pulposus cell (NPC)-like lineages represents a speculative mechanism. Niche factors including hypoxia, growth factors and cell-cell interactions have been suggested but the matrix niche factor has not been studied. Our collagen microencapsulation provides a 3D model to study matrix niche as it enables the encapsulated cells to remodel the template matrix. We previously demonstrated the chondro-inductive role of of chondrocytes-derived matrix in MSCs and showed that NPCs maintained their phenotype and remodeled the template matrix of collagen microspheres into a glycosaminoglycan (GAG)-rich one. Here we aim to study the effects of NPC-derived matrix on MSC differentiation towards NPC-like lineages by firstly producing an NPC-derived matrix in collagen microspheres, secondly optimizing a decellularization protocol to discard NPCs yet retaining the matrix, thirdly repopulating the acellular NPC-derived matrix with MSCs and fourthly evaluating their phenotype. Finally, we injected these microspheres in a pilot rabbit disc degeneration model. Results showed that NPCs survived, maintained their phenotypic markers and produced GAGs. A decellularization protocol with maximal removal of the NPCs, minimal loss in major matrix components and partial retention of NPC-specific markers was identified. The resulting acellular matrix supported MSC survival and matrix production, and up-regulated the gene expression of NPC markers including type II collagen and glypican 3. Finally, injection of MSC in these microspheres in rabbit degenerative disc better maintained hydration level with more pronounced staining of GAGs and type II collagen than controls.

Changes in the molecular phenotype of nucleus pulposus cells with intervertebral disc aging

Intervertebral disc (IVD) disorder and age-related degeneration are believed to contribute to low back pain. Cell-based therapies represent a promising strategy to treat disc degeneration; however, the cellular and molecular characteristics of disc cells during IVD maturation and aging still remain poorly defined. This study investigated novel molecular markers and their age-related changes in the rat IVD. Affymetrix cDNA microarray analysis was conducted to identify a new set of genes characterizing immature nucleus pulposus (NP) cells. Among these markers, select neuronal-related proteins (Basp1, Ncdn and Nrp-1), transcriptional factor (Brachyury T), and cell surface receptors (CD24, CD90, CD155 and CD221) were confirmed by real-time PCR and immunohistochemical (IHC) staining for differential expression between IVD tissue regions and among various ages (1, 12 and 21 months). NP cells generally possessed higher levels of mRNA or protein expression for all aforementioned markers, with the exception of CD90 in anulus fibrosus (AF) cells. In addition, CD protein (CD24 and CD90) and Brachyury (T) expression in immature disc cells were also confirmed via flow cytometry. Similar to IHC staining, results revealed a higher percentage of immature NP cells expressing CD24 and Brachyury, while higher percentage of immature AF cells was stained positively for CD90. Altogether, this study identifies that tissue-specific gene expression and age-related differential expression of the above markers do exist in immature and aged disc cells. These age-related phenotype changes provide a new insight for a molecular profile that may be used to characterize NP cells for developing cell-based regenerative therapy for IVD regeneration.

Modulation of mesenchymal stem cell chondrogenesis in a tunable hyaluronic acid hydrogel microenvironment

An injectable and biodegradable hydrogel system comprising hyaluronic acid-tyramine (HA-Tyr) conjugates can safely undergo covalent cross-linking in vivo by the addition of small amounts of peroxidase and hydrogen peroxide (H(2)O(2)), with the independent tuning of the gelation rate and degree of cross-linking. Such hydrogel networks with tunable mechanical and degradation properties may provide the additional level of control needed to enhance chondrogenesis and overall cartilage tissue formation in vitro and in vivo. In this study, HA-Tyr hydrogels were explored as biomimetic matrices for caprine mesenchymal stem cells (MSCs) in cartilage tissue engineering. The compressive modulus, equilibrium swelling and degradation rate could be controlled by varying the concentration of H(2)O(2) as the oxidant in the oxidative coupling reaction. Cellular condensation reflected by the increase in effective number density of rounded cells in lacunae was greater in softer hydrogel matrices with lower cross-linking that displayed enhanced scaffold contracture. Conversely, within higher cross-linked matrices, cells adopted a more elongated morphology, with a reduced degree of cellular condensation. Furthermore, the degree of hydrogel cross-linking also modulated matrix biosynthesis and cartilage tissue histogenesis. Lower cross-linked matrix enhanced chondrogenesis with increases in the percentage of cells with chondrocytic morphology; biosynthetic rates of glycosaminoglycan and type II collagen; and hyaline cartilage tissue formation. With increasing cross-linking degree and matrix stiffness, a shift in MSC differentiation toward fibrous phenotypes with the formation of fibrocartilage and fibrous tissues was observed. These findings suggest that the tunable three-dimensional microenvironment of the HA-Tyr hydrogels modulates cellular condensation during chondrogenesis and has a dramatic impact on spatial organization of cells, matrix biosynthesis, and overall cartilage tissue histogenesis.