The response of human anulus fibrosus cells to cyclic tensile strain is frequency-dependent and altered with disc degeneration

Cellular behavior and extracellular matrix turnover in bovine annulus fibrosus cells under hydrostatic pressure and deviatoric strain

Intervertebral disc herniation is a common spinal disorder that is often treated with discectomy when conservative measures fail. To devise therapeutic strategies for tears in the annulus fibrosus (AF), the regenerative capability of AF cells under spinal loading needs to be addressed. We hypothesized that the compressive loading associated with deformation in AF cells reduces synthetic and degradative activities in extracellular matrix and cell proliferation. We evaluated expression of key matrix molecules and cell proliferation by RT-PCR and immunohistochemistry by inner and outer bovine AF cells incubated under hydrostatic pressure (HP), arc-bending strain (Strain), and combined HP and Strain (HP/Strain) mimicking spinal loading. Inner AF cells showed significantly increased levels of aggrecan core protein, chondroitin sulfate N-acetylgalactosaminyltransferase-1, and tissue inhibitor of metalloproteinases-2 by 6 days under HP (p < 0.05), with a tendency toward increased matrix metalloproteinase-13. Outer AF cells demonstrated a significant decline in collagen type-2 under Strain and HP/Strain (p < 0.05) and a tendency toward suppression of collagen type-1 and elastin expression compared to HP and unloaded control. On the other hand, proliferating cell nucleus antigen increased significantly under Strain and HP/Strain in inner AF and declined under unloaded and HP in outer AF (p < 0.05). Immunohistology findings supported reductions in gene expressions of matrix molecules. Thus, changes in HP/Strain in AF appear to diminish synthetic and degradative activities while increasing cell proliferation. To promote regeneration, continuous overloading should be avoided, as it converts the synthetic activity to a state in which tissue repair is limited.

Mechanical Factors Regulate Annulus Fibrosus (AF) Injury Repair and Remodeling: A Review

Low back pain is a common chronic disease that can severely affect the patient's work and daily life. The breakdown of spinal mechanical homeostasis caused by intervertebral disc (IVD) degeneration is a leading cause of low back pain. Annulus fibrosus (AF), as the outer layer structure of the IVD, is often the first affected part. AF injury caused by consistent stress overload will further accelerate IVD degeneration. Therefore, regulating AF injury repair and remodeling should be the primary goal of the IVD repair strategy. Mechanical stimulation has been shown to promote AF regeneration and repair, but most studies only focus on the effect of single stress on AF, and lack realistic models and methods that can mimic the actual mechanical environment of AF. In this article, we review the effects of different types of stress stimulation on AF injury repair and remodeling, suggest possible beneficial load combinations, and explore the underlying molecular mechanisms. It will provide the theoretical basis for designing better tissue engineering therapy using mechanical factors to regulate AF injury repair and remodeling in the future.

Intervertebral disc human nucleus pulposus cells associated with back pain trigger neurite outgrowth in vitro and pain behaviors in rats

Low back pain (LBP) is often associated with the degeneration of human intervertebral discs (IVDs). However, the pain-inducing mechanism in degenerating discs remains to be elucidated. Here, we identified a subtype of locally residing human nucleus pulposus cells (NPCs), generated by certain conditions in degenerating discs, that was associated with the onset of discogenic back pain. Single-cell transcriptomic analysis of human tissues showed a strong correlation between a specific cell subtype and the pain condition associated with the human degenerated disc, suggesting that they are pain-triggering. The application of IVD degeneration-associated exogenous stimuli to healthy NPCs in vitro recreated a pain-associated phenotype. These stimulated NPCs activated functional human iPSC-derived sensory neuron responses in an in vitro organ-chip model. Injection of stimulated NPCs into the healthy rat IVD induced local inflammatory responses and increased cold sensitivity and mechanical hypersensitivity. Our findings reveal a previously uncharacterized pain-inducing mechanism mediated by NPCs in degenerating IVDs. These findings could aid in the development of NPC-targeted therapeutic strategies for the clinically unmet need to attenuate discogenic LBP.

Pathogenesis and therapeutic implications of matrix metalloproteinases in intervertebral disc degeneration: A comprehensive review

Intervertebral disc (IVD) degeneration (IDD) is a common disorder that affects the spine and is a major cause of lower back pain (LBP). The extracellular matrix (ECM) is the structural foundation of the biomechanical properties of IVD, and its degradation is the main pathological characteristic of IDD. Matrix metalloproteinases (MMPs) are a group of endopeptidases that play an important role in the degradation and remodeling of the ECM. Several recent studies have shown that the expression and activity of many MMP subgroups are significantly upregulated in degenerated IVD tissue. This upregulation of MMPs results in an imbalance of ECM anabolism and catabolism, leading to the degradation of the ECM and the development of IDD. Therefore, the regulation of MMP expression is a potential therapeutic target for the treatment of IDD. Recent research has focused on identifying the mechanisms by which MMPs cause ECM degradation and promote IDD, as well as on developing therapies that target MMPs. In summary, MMP dysregulation is a crucial factor in the development of IDD, and a deeper understanding of the mechanisms involved is needed to develop effective biological therapies that target MMPs to treat IDD.

Quantifying internal intervertebral disc strains to assess nucleus replacement device designs: a digital volume correlation and ultra-high-resolution MRI study

Introduction: Nucleus replacement has been proposed as a treatment to restore biomechanics and relieve pain in degenerate intervertebral discs (IVDs). Multiple nucleus replacement devices (NRDs) have been developed, however, none are currently used routinely in clinic. A better understanding of the interactions between NRDs and surrounding tissues may provide insight into the causes of implant failure and provide target properties for future NRD designs. The aim of this study was to non-invasively quantify 3D strains within the IVD through three stages of nucleus replacement surgery: intact, post-nuclectomy, and post-treatment. Methods: Digital volume correlation (DVC) combined with 9.4T MRI was used to measure strains in seven human cadaveric specimens (42 ± 18 years) when axially compressed to 1 kN. Nucleus material was removed from each specimen creating a cavity that was filled with a hydrogel-based NRD. Results: Nucleus removal led to loss of disc height (12.6 ± 4.4%, p = 0.004) which was restored post-treatment (within 5.3 ± 3.1% of the intact state, p > 0.05). Nuclectomy led to increased circumferential strains in the lateral annulus region compared to the intact state (-4.0 ± 3.4% vs. 1.7 ± 6.0%, p = 0.013), and increased maximum shear strains in the posterior annulus region (14.6 ± 1.7% vs. 19.4 ± 2.6%, p = 0.021). In both cases, the NRD was able to restore these strain values to their intact levels (p ≥ 0.192). Discussion: The ability of the NRD to restore IVD biomechanics and some strain types to intact state levels supports nucleus replacement surgery as a viable treatment option. The DVC-MRI method used in the present study could serve as a useful tool to assess future NRD designs to help improve performance in future clinical trials.

Mechanobiology of the Human Intervertebral Disc: Systematic Review of the Literature and Future Perspectives

Low back pain is an extremely common condition with severe consequences. Among its potential specific causes, degenerative disc disease (DDD) is one of the most frequently observed. Mechanobiology is an emerging science studying the interplay between mechanical stimuli and the biological behavior of cells and tissues. The aim of the presented study is to review, with a systematic approach, the existing literature regarding the mechanobiology of the human intervertebral disc (IVD), define the main pathways involved in DDD and identify novel potential therapeutic targets. The review was carried out in accordance with the Preferential Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) guidelines. Studies were included if they described biological responses of human IVD cells under mechanical stimulation or alterations of mechanical properties of the IVD determined by different gene expression. Fifteen studies were included and showed promising results confirming the mechanobiology of the human IVD as a key element in DDD. The technical advances of the last decade have allowed us to increase our understanding of this topic, enabling us to identify possible therapeutic targets to treat and to prevent DDD. Further research and technological innovations will shed light on the interactions between the mechanics and biology of the human IVD.

The co-influence of hyaluronic acid and collagen on the development of an engineered annulus tissue model with bone marrow stromal cells

Inveterbral disc degeneration is a significant musculoskeletal disease that brings huge burden of pain, disability, psychological and social consequences to the affected population worldwide with treatments that only alleviate the pain but does not address the underlying biological problems. For the past decades, tissue engineering of the disc has been investigated with Annulus Fibrosus been one of the complicated disc component to be engineered. With the limited source of annulus cells, bone marrow stromal cells have been frequently investigated as a potental cell candidate to develop an annulus fibrosus-like tissue which often require a multi-disclipinary effort to achieve. The extracellular matrix of Annulus Fibrosus is largely make up of collagen and proteoglycan which is still unclear how these matrix proteins could influence the bone marrow stromal cells towards constructing a AF-like tissue. In this study, we adopted a coiled hydrogel microfiber that resembles the micro-architecture of the native AF tissue to encapsulate bone marrow stromal cells and incorporated collagen type 1 and hyaluronic acid which later demonstrated that the co-presence of hyaluronic acid and collagen could potentially upregulated AF-associated biomarkers and protease expression which are critical for later development of an engineered AF tissue construct.

Self-Assembling Peptide Hydrogels as Functional Tools to Tackle Intervertebral Disc Degeneration

Low back pain (LBP), caused by intervertebral disc (IVD) degeneration, is a major contributor to global disability. In its healthy state, the IVD is a tough and well-hydrated tissue, able to act as a shock absorber along the spine. During degeneration, the IVD is hit by a cell-driven cascade of events, which progressively lead to extracellular matrix (ECM) degradation, chronic inflammation, and pain. Current treatments are divided into palliative care (early stage degeneration) and surgical interventions (late-stage degeneration), which are invasive and poorly efficient in the long term. To overcome these limitations, alternative tissue engineering and regenerative medicine strategies, in which soft biomaterials are used as injectable carriers of cells and/or biomolecules to be delivered to the injury site and restore tissue function, are currently being explored. Self-assembling peptide hydrogels (SAPHs) represent a promising class of de novo synthetic biomaterials able to merge the strengths of both natural and synthetic hydrogels for biomedical applications. Inherent features, such as shear-thinning behaviour, high biocompatibility, ECM biomimicry, and tuneable physiochemical properties make these hydrogels appropriate and functional tools to tackle IVD degeneration. This review will describe the pathogenesis of IVD degeneration, list biomaterials requirements to attempt IVD repair, and focus on current peptide hydrogel materials exploited for this purpose.

A Framework for Investigating Rules of Life by Establishing Zones of Influence

The incredible complexity of biological processes across temporal and spatial scales hampers defining common underlying mechanisms driving the patterns of life. However, recent advances in sequencing, big data analysis, machine learning, and molecular dynamics simulation have renewed the hope and urgency of finding potential hidden rules of life. There currently exists no framework to develop such synoptic investigations. Some efforts aim to identify unifying rules of life across hierarchical levels of time, space, and biological organization, but not all phenomena occur across all the levels of these hierarchies. Instead of identifying the same parameters and rules across levels, we posit that each level of a temporal and spatial scale and each level of biological organization has unique parameters and rules that may or may not predict outcomes in neighboring levels. We define this neighborhood, or the set of levels, across which a rule functions as the zone of influence. Here, we introduce the zone of influence framework and explain using three examples: (a) randomness in biology, where we use a Poisson process to describe processes from protein dynamics to DNA mutations to gene expressions, (b) island biogeography, and (c) animal coloration. The zone of influence framework may enable researchers to identify which levels are worth investigating for a particular phenomenon and reframe the narrative of searching for a unifying rule of life to the investigation of how, when, and where various rules of life operate.

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.

Inhibition of RhoA/MRTF-A signaling alleviates nucleus pulposus fibrosis induced by mechanical stress overload

PURPOSE/AIM

: Intervertebral disc degeneration (IDD) is the leading cause of lower back pain, and clinically useful drugs for IDD are unavailable. Mechanical stress overload-induced fibrosis plays a critical role in IDD. RhoA/MRTF-A signaling is known to regulate tissue fibrosis; however, the effect of RhoA/MRTF-A on the development of IDD is unclear.

MATERIALS AND METHODS

: The expression of aggrecan, collagen I, collagen II, MMP-12, CTGF, and MRTF-A in nucleus pulposus (NP) samples from IDD patients and controls was detected by immunohistochemical staining. Primary nucleus pulposus cells (NPCs) were isolated and cultured to establish an overload strain model treated with or without CCG-1423. The protein levels of RhoA, ROCK2, MRTF-A, CTGF, and MMP-12 as well as fibrosis-associated proteins were detected by western blotting and immunofluorescence.

RESULTS

: Collagen I, MMP-12, and CTGF were significantly upregulated, and aggrecan and collagen II were significantly downregulated in the IDD samples. The cellular localization of MRTF-A was associated with intervertebral disc (IVD) degeneration. Overloaded strain enhanced the nuclear translocation of MRTF-A and changed the NPC morphology from spindle-shaped to long strips. Additional experiments showed that RhoA, ROCK2, MRTF-A, SRF, MMP-12, and CTGF were upregulated; however, aggrecan and collagen II were downregulated in NPCs under overload strain. CCG-1423, a RhoA/MRTF-A pathway inhibitor, reversed strain-induced fibrosis.

CONCLUSION

: Mechanical stress activates RhoA/MRTF-A signaling to promote extracellular matrix (ECM) degeneration in the NP, which is associated with the development of IDD. Our findings suggest that the RhoA/MRTF-A inhibitor CCG-1423 can alleviate NPC degeneration caused by overload stress and has potential as a therapeutic agent for IDD.

Excessive mechanical strain accelerates intervertebral disc degeneration by disrupting intrinsic circadian rhythm

Night shift workers with disordered rhythmic mechanical loading are more prone to intervertebral disc degeneration (IDD). Our results showed that circadian rhythm (CR) was dampened in degenerated and aged NP cells. Long-term environmental CR disruption promoted IDD in rats. Excessive mechanical strain disrupted the CR and inhibited the expression of core clock proteins. The inhibitory effect of mechanical loading on the expression of extracellular matrix genes could be reversed by BMAL1 overexpression in NP cells. The Rho/ROCK pathway was demonstrated to mediate the effect of mechanical stimulation on CR. Prolonged mechanical loading for 12 months affected intrinsic CR genes and induced IDD in a model of upright posture in a normal environment. Unexpectedly, mechanical loading further accelerated the IDD in an Light-Dark (LD) cycle-disrupted environment. These results indicated that intrinsic CR disruption might be a mechanism involved in overloading-induced IDD and a potential drug target for night shift workers.

Working long shifts at times when the body should be at rest can have lasting effects on the intervertebral discs in the back, leading to chronic pain. Night shift workers are susceptible to developing certain health conditions because of chronic disruption to their circadian rhythms. Now, Li-Bo Jiang at Zhongshan Hospitial, Fudan University in Shanghai and co-workers across China have uncovered a link between circadian rhythm disruption and intervertebral disc degeneration. In experiments on human tissue samples and rat models, the team found that oscillation of the expression of clock-related genes and proteins was reduced in severely degenerated disc cells. Cellular clock mechanisms were disrupted in disc cells that had been repeatedly placed under mechanical strain at night. This disruption appears to influence degradation of the extracellular matrix, which the team believe may in turn accelerate disc degeneration.

High throughput computational evaluation of how scaffold architecture, material selection, and loading modality influence the cellular micromechanical environment in tissue engineering strategies

BACKGROUND

In tissue engineering (TE) strategies, cell processes are regulated by mechanical stimuli. Although TE scaffolds have been developed to replicate tissue-level mechanical properties, it is intractable to experimentally measure and prescribe the cellular micromechanical environment (CME) generated within these constructs. Accordingly, this study aimed to fill this lack of understanding by modeling the CME in TE scaffolds using the finite element method.

METHODS

A repeating unit of composite fiber scaffold for annulus fibrosus (AF) repair with a fibrin hydrogel matrix was prescribed a series of loading, material, and architectural parameters. The distribution of CME in the scaffold was predicted and compared to proposed target mechanics based on anabolic responses of AF cells.

RESULTS

The multi-axial loading modality predicted the greatest percentage of cell volumes falling within the CME target envelope (%PTE) in the study (65 %PTE for 5.0% equibiaxial tensile strain with 50 kPa radial-direction compression; 7.6 %PTE without radial pressure). Additionally, the architectural scale had a moderate influence on the CME (maximum of 17 %PTE), with minimal change in the tissue-level properties of the scaffold. Scaffold materials and architectures had secondary influences on the predicted regeneration by modifying the tissue-level scaffold mechanics.

CONCLUSIONS

Scaffold loading modality was identified as the critical factor for TE the AF. Scaffold materials and architecture were also predicted to modulate the scaffold loading and, therefore, control the CME indirectly. This study facilitated an improved understanding of the relationship between tissue-level and cell-level mechanics to drive anabolic cell responses for tissue regeneration.

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.

Mechanical Stimulation and Diameter of Fiber Scaffolds Affect the Differentiation of Rabbit Annulus Fibrous Stem Cells

BACKGROUND

Degeneration of the annulus fibrosus (AF), an important structure of the intervertebral disc, is one of the main causes of degenerative disc disease. Fabrication of scaffolds replicating the stratified microstructure of the AF is critical for the successful regeneration of AF.

METHODS

In this study, we cultured rabbit AF-derived stem cells (AFSCs) using fabricated electrospun fibrous poly-L-lactic acid scaffolds with different diameters. We applied cyclic tensile strain (CTS) on the scaffolds to regulate the differentiation of AFSCs into specific cell types that resided at the inner, middle, and outer zones of the AF.

RESULTS

We found that the morphologies of AFSCs on the smaller-fiber-diameter scaffolds were nearly round, whereas spindle-like cells morphologies were observed on large-diameter scaffolds. CTS enhanced these phenomena and made the cells slender. The expression levels of collagen-I in cells increased as a function of the fiber diameter, whereas collagen-II and aggrecan exhibited opposite trends. Moreover, the application of CTS upregulated the gene expressions of collagen-I, collagen-II, and aggrecan.

CONCLUSION

Overlaying the scaffolds with different CTS-stimulated cells could eventually lead to engineered AF tissues with hierarchical structures that approximated the native AF tissue. Thus, the proposed methodologies could be potentially applied for AF regeneration.

Multiscale Regulation of the Intervertebral Disc: Achievements in Experimental, In Silico, and Regenerative Research

Intervertebral disc (IVD) degeneration is a major risk factor of low back pain. It is defined by a progressive loss of the IVD structure and functionality, leading to severe impairments with restricted treatment options due to the highly demanding mechanical exposure of the IVD. Degenerative changes in the IVD usually increase with age but at an accelerated rate in some individuals. To understand the initiation and progression of this disease, it is crucial to identify key top-down and bottom-up regulations' processes, across the cell, tissue, and organ levels, in health and disease. Owing to unremitting investigation of experimental research, the comprehension of detailed cell signaling pathways and their effect on matrix turnover significantly rose. Likewise, in silico research substantially contributed to a holistic understanding of spatiotemporal effects and complex, multifactorial interactions within the IVD. Together with important achievements in the research of biomaterials, manifold promising approaches for regenerative treatment options were presented over the last years. This review provides an integrative analysis of the current knowledge about (1) the multiscale function and regulation of the IVD in health and disease, (2) the possible regenerative strategies, and (3) the in silico models that shall eventually support the development of advanced therapies.

The distinct roles of myosin IIA and IIB under compression stress in nucleus pulposus cells

Objectives

Inappropriate or excessive compression applied to intervertebral disc (IVD) contributes substantially to IVD degeneration. The actomyosin system plays a leading role in responding to mechanical stimuli. In the present study, we investigated the roles of myosin II isoforms in the compression stress‐induced senescence of nucleus pulposus (NP) cells.

Material and methods

Nucleus pulposus cells were exposed to 1.0 MPa compression for 0, 12, 24 or 36 hours. Immunofluorescence and co‐immunoprecipitation analysis were used to measure the interaction of myosin IIA and IIB with actin. Western blot analysis and immunofluorescence staining were used to detect nuclear expression and nuclear localization of MRTF‐A. In addition, the expression levels of p‐RhoA/RhoA, ROCK1/2 and p‐MLC/MLC were measured in human NP cells under compression stress and in degenerative IVD tissues.

Results

Compression stress increased the interaction of myosin IIA and actin, while the interaction of myosin IIB and actin was reduced. The actomyosin cytoskeleton remodelling was involved in the compression stress‐induced fibrotic phenotype mediated by MRTF‐A nuclear translocation and inhibition of proliferation in NP cells. Furthermore, RhoA/ROCK1 pathway activation mediated compression stress‐induced human NP cells senescence by regulating the interaction of myosin IIA and IIB with actin.

Conclusions

We for the first time investigated the regulation of actomyosin cytoskeleton in human NP cells under compression stress. It provided new insights into the development of therapy for effectively inhibiting IVD degeneration.

The mechano-response of murine annulus fibrosus cells to cyclic tensile strain is frequency dependent

The intervertebral disk (IVD) is a composite structure essential for spine stabilization, load bearing, and movement. Biomechanical factors are important contributors to the IVD microenvironment regulating joint homeostasis; however, the cell type-specific effectors of mechanotransduction in the IVD are not fully understood. The current study aimed to determine the effects of cyclic tensile strain (CTS) on annulus fibrosus (AF) cells and identify mechano-sensitive pathways. Using a cell-type specific reporter mouse to differentiation NP and AF cells from the murine IVD, we characterized AF cells in dynamic culture exposed to CTS (6% strain) at specific frequencies (0.1 Hz, 1.0 Hz, or 2.0 Hz). We demonstrate that our culture model maintains the phenotype of primary AF cells and that the bioreactor system delivers uniform biaxial strain across the cell culture surface. We show that exposure of AF cells to CTS induces cytoskeleton reorganization resulting in stress fiber formation, with acute exposure to CTS at 2.0 Hz inducing a significant yet transient increase ERK1/2 pathway activation. Using SYBPR-based qPCR to assess the expression of extracellular matrix (ECM) genes, ECM-remodeling genes, candidate mechano-sensitive genes, inflammatory cytokines and cell surface receptors, we demonstrated that exposure of AF cells to CTS at 0.1 Hz increased Acan, Prg4, Col1a1 and Mmp3 expression. AF cells exposed to CTS at 1.0 Hz showed a significant increase in the expression of Acan, Myc, and Tnfα. Exposure of AF cells to CTS at 2.0 Hz induced a significant increase in Acan, Prg4, Cox2, Myc, Fos, and Tnfα expression. Among the cell surface receptors assessed, AF cells exposed to CTS at 2.0 Hz showed a significant increase in Itgβ1, Itgα5, and Trpv4 expression. Our findings demonstrate that the response of AF cells to CTS is frequency dependent and suggest that mechanical loading may directly contribute to matrix remodeling and the onset of local tissue inflammation in the murine IVD.

Inflammatory cytokine and catabolic enzyme expression in a goat model of intervertebral disc degeneration

Intervertebral disc degeneration is implicated as a leading cause of low back pain. Persistent, local inflammation within the disc nucleus pulposus (NP) and annulus fibrosus (AF) is an important mediator of disc degeneration and negatively impacts the performance of therapeutic stem cells. There is a lack of validated large animal models of disc degeneration that recapitulate clinically relevant local inflammation. We recently described a goat model of disc degeneration in which increasing doses of chondroitinase ABC (ChABC) were used to reproducibly induce a spectrum of degenerative changes. The objective of this study was to extend the clinical relevance of this model by establishing whether these degenerative changes are associated with the local expression of inflammatory cytokines and catabolic enzymes. Degeneration was induced in goat lumbar discs using ChABC at different doses. After 12 weeks, degeneration severity was determined histologically and using quantitative magnetic resonance imaging (MRI). Expression levels of inflammatory cytokines (tumor necrosis factor-α [TNF-α], interleukin-1β [IL-1β], and IL-6) and catabolic enzymes (matrix metalloproteinases-1 [MMPs-1] and 13, and a disintegrin and metalloproteinase with thrombospondin type-1 motifs-4 [ADAMTS-4]) were assessed as the percentage of immunopositive cells in the NP and AF. With the exception of MMP-1, cytokine, and enzyme expression levels were significantly elevated in ChABC-treated discs in the NP and AF. Expression levels of TNF-α, IL1-β, and ADAMTS-4 were positively correlated with histological grade, while all cytokines and ADAMTS-4 were negatively correlated with MRI T2 and T1ρ scores. These results demonstrate that degenerate goat discs exhibit elevated expression of clinically relevant inflammatory mediators, and further validate this animal model as a platform for evaluating new therapeutic approaches for disc degeneration.

Level-wise differences in in vivo lateral bending moment are associated with microstructural alterations in bovine caudal intervertebral discs

Despite its common use as a laboratory model, little is known about the in vivo forces and moments applied to the bovine caudal intervertebral disc. Such aspects are crucial, as intervertebral disc tissue is known to remodel in response to repeated loading. We hypothesized that the magnitude of loading from muscle contraction during a typical lateral bending motion varies between caudal levels and is accompanied by variations in tissue microstructure. This hypothesis was tested by estimating level-wise forces and bending moments using two independent approaches: a dynamic analytical model of the motion and analysis of muscle cross-sections obtained via computed tomography. Microstructure was assessed by measuring the collagen fiber crimp period in the annulus fibrosus, and composition was assessed via quantitative histology. Both the analytical model and muscle cross-sections indicated peak bending moments of over 3 N m and peak compressive force of over 125 N at the c1c2 level, decreasing distally. There was a significant downward trend from proximal to distal in the outer annulus fibrosus collagen crimp period in the anterior and posterior regions only, suggesting remodeling in response to the highest lateral bending moments. There were no observed trends in composition. Our results suggest that although the proximal discs in the bovine tail are subjected to forces and moments from muscle contraction that are comparable (relative to disc size) to those acting on human lumbar discs, the distal discs are not. The resulting pattern of microstructural alterations suggests that level-wise differences should be considered when using bovine discs as a research model.

Analysis of lncRNAs-miRNAs-mRNAs networks in periodontal ligament stem cells under mechanical force

OBJECTIVES

Our study aims to analyze the expression profiles of long non-coding RNAs (lncRNAs) and investigate the potential regulatory networks among lncRNAs, microRNAs (miRNAs), and mRNAs in periodontal ligament stem cells (PDLSCs) under mechanical force (MF).

MATERIALS AND METHODS

PDLSCs were isolated from human periodontal ligament tissues and identified by flow cytometry analysis. Multidirectional differentiation potential of PDLSCs was obtained by osteogenic and adipogenic induction. High-throughput RNA sequencing was used to identify the expression patterns of lncRNAs and mRNAs in PDLSCs under MF. MF-responsive miRNAs were obtained from the previous microarray data. LncRNAs-miRNAs-mRNAs networks were constructed by Cytoscape.

RESULTS

PDLSCs cultured from the periodontal ligament tissues were positive for STRO-1, CD146 and negative for CD45, CD34. Alizarin red staining and Oil Red O staining showed that PDLSCs had the ability of osteogenic and adipogenic differentiation. Then, a total of 1,339 and 1,426 differentially expressed lncRNAs and mRNAs were identified, respectively, in PDLSCs under MF. Based on the previous miRNA microarray analysis, the potential interaction networks of lncRNAs-miRNAs-mRNAs were constructed. It was found that lncRNAs and mRNAs could competitively interact with the same miRNA.

CONCLUSIONS

LncRNAs-miRNAs-mRNAs networks were involved in PDLSCs under MF, which might provide a novel mechanism in the regulation of clinical orthodontic tooth movement process.

Activation of Toll-Like Receptor 3 Induces Interleukin-1 Receptor Antagonist Expression by Activating the Interferon Regulatory Factor 3

Toll-like receptor 3 (TLR3) is a sensor of endogenous cell necrosis during the process of acute inflammation. Interleukin (IL)-1 receptor antagonist (IL-1Ra) is an anti-inflammatory cytokine and can negatively regulate the pathogenesis of inflammation. However, whether and how activation of TLR3 can regulate IL-1Ra expression has not been clarified. Here, we show that poly(I:C) induces IL-1Ra expression in primarily cultured human fibroblast-like synoviocytes and other types of cells. Induction of IL-1Ra by poly(I:C) was dependent on TLR3, but was independent of melanoma differentiation--associated protein 5 or retinoic acid-inducible gene I. Interferon regulatory factor 3 (IRF3) directly binds to the IL-1Ra promoter and promotes IL-1Ra expression in response to poly(I:C) stimulation. Induction of IL-1Ra by poly(I:C) was abolished by the inhibition of the NF-κB signaling, attenuated by the inhibition of the PI3K-Akt signaling, enhanced by inhibition of the ERK1/2 or MSK1/2 activation, but was independent of the p38 MAPK signaling. Treatment with poly(I:C) or Sendai virus elevated the levels of serum IL-1Ra in wild-type, but not in TLR3-/- or IRF3-/- mice. Our findings may provide new insights into the intrinsic anti-inflammatory function of TLR3 and double-stranded RNA-induced IL-Ra expression by TLR3 and its regulation.

Identification and characterization of circular RNAs involved in mechanical force-induced periodontal ligament stem cells

Circular RNAs (circRNAs) play critical roles in signal transduction during cell proliferation, differentiation, and apoptosis in a posttranscriptional manner. Recently, circRNAs have been proved to be a large class of animal RNAs with regulatory potency. However, whether circRNAs can respond to mechanical force (MF) and impact on human periodontal ligament stem cells (PDLSCs) and the orthodontic tooth movement (OTM) process remain unknown. Here, we investigated the circRNAs expression patterns in PDLSCs induced by MF and found that circRNAs were responsive to the MF in PDLSCs. Through the valid reads' distribution analysis, we found that the majority of reads in both the control PDLSCs and the MF-induced PDLSCs were distributed in exons. Then we analyzed Gene Ontology terms of genes that overlap with or are neighbors of the stress-responsive circRNAs and found unique enrichment patterns in biological processes, molecular function, and cellular component of PDLSCs. Next, we predicted the possible functions of circRNAs through circRNAs-miRNAs networks. We found that one circRNA may regulate one or several miRNA/miRNAs and one miRNA may interact with one or multiple circRNA/circRNAs. Importantly, a number of circRNAs were predicted to directly or indirectly regulate miRNAs-mediated osteogenic differentiation in mesenchymal stem cells. For instance, circRNA3140 was highly and widely associated with microRNA-21, which plays a critical role in MF-induced osteogenic differentiation of PDLSCs. Taken together, these findings reveal a previously unrecognized mechanism that MF can induce the expression changes of circRNAs in PDLSCs, which may modulate the OTM process and the alveolar bone remodeling.

Human notochordal cell transcriptome unveils potential regulators of cell function in the developing intervertebral disc

The adult nucleus pulposus originates from the embryonic notochord, but loss of notochordal cells with skeletal maturity in humans is thought to contribute to the onset of intervertebral disc degeneration. Thus, defining the phenotype of human embryonic/fetal notochordal cells is essential for understanding their roles and for development of novel therapies. However, a detailed transcriptomic profiling of human notochordal cells has never been achieved. In this study, the notochord-specific marker CD24 was used to specifically label and isolate (using FACS) notochordal cells from human embryonic and fetal spines (7.5–14 weeks post-conception). Microarray analysis and qPCR validation identified CD24, STMN2, RTN1, PRPH, CXCL12, IGF1, MAP1B, ISL1, CLDN1 and THBS2 as notochord-specific markers. Expression of these markers was confirmed in nucleus pulposus cells from aged and degenerate discs. Ingenuity pathway analysis revealed molecules involved in inhibition of vascularisation (WISP2, Noggin and EDN2) and inflammation (IL1-RN) to be master regulators of notochordal genes. Importantly, this study has, for the first time, defined the human notochordal cell transcriptome and suggests inhibition of inflammation and vascularisation may be key roles for notochordal cells during intervertebral disc development. The molecules and pathways identified in this study have potential for use in developing strategies to retard/prevent disc degeneration, or regenerate tissue.

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.

Degenerate intervertebral disc-like pH induces a catabolic mechanoresponse in human nucleus pulposus cells

Mechanical stimulation is known to influence intervertebral disc (IVD) cell behavior and function, but the effect on disc cells is routinely considered in isolation from other microenvironmental factors. Acidic pH has been shown to be a prominent and detrimental microenvironmental factor present in degenerate IVDs, but its influence on the human disc cell mechanoresponse has never been studied. We investigated the response of agarose-encapsulated human nucleus pulposus (NP) cells to 0.004 MPa, 1.0 Hz and 1 hour of compression (Flexcell FX4000 Compression System) under pH conditions representative of nondegenerate (pH 7.1) and degenerate (pH 6.5) IVDs. Cell viability, extracellular matrix production, and expression of anabolic/anti-catabolic and catabolic genes were assessed. We report that preculture of NP cells in agarose gels was required in order for cells to be mechanoresponsive, and this correlated with increased type VI collagen, α5β1 integrin, and fibronectin expression. Furthermore, the matrix homeostatic response observed at pH 7.1 (representative of nondegenerate IVDs; increased aggrecan [AGC], tissue inhibitor of metalloproteinases-1 [TIMP1], matrix metalloproteinase-3 [MMP3], a disintegrin and metalloproteinase with thrombospondin motif-5 [ADAMTS5] gene expression) was RGD-integrin dependent, whereas only MMP3 remained mechanoresponsive at pH 6.5, and this was independent of RGD-integrins. Our findings suggest differential mechanotransduction pathways operating for specific genes, with RGD-integrin dependent AGC expression, but not RGD-independent MMP3 expression, inhibited at pH representative of degenerate IVDs (pH 6.5), which could contribute to the catabolic phenotype observed during IVD degeneration.

CLINICAL SIGNIFICANCE

Characterizing the influence of the mechanical and chemical intervertebral disc microenvironment on disc cells, particularly in disc degeneration, could help develop future therapeutic strategies for the treatment of discogenic back pain.

Cyclic mechanical tension reinforces DNA damage and activates the p53-p21-Rb pathway to induce premature senescence of nucleus pulposus cells

Intervertebral disc (IVD) degeneration (IDD) is a widely recognized contributor to low back pain. Mechanical stress is a crucial etiological factor of IDD. During the process of IDD, a vicious circle is formed between abnormal mechanical stress and the damage of disc structure and function. Notably, the pathological process of IDD is mediated by the phenotypic shift of IVD cells from an extracellular matrix anabolic phenotype to a catabolic and pro-inflammatory phenotype. Therefore, the effects of mechanical stress on the initiation and progression of IDD depend on the mechanobiology of IVD cells. Recently, disc cell senescence was identified as a new hallmark of IDD. However, the senescent response of disc cells to mechanical stress remains unknown. In this study, we found that prolonged exposure of cyclic mechanical tension (CMT) with unphysiological magnitude generated by the Flexercell tension system markedly induced premature senescence of nucleus pulposus (NP) cells. CMT augmented the DNA damage of NP cells, but did not affect the redox homeostasis of NP cells. Moreover, the p53-p21-retinoblastoma protein (Rb) pathway was activated by CMT to mediate the CMT-induced premature senescence of NP cells. The findings are beneficial to understanding the mechanism of disc cell senescence and the mechanobiology of disc cells further. It suggests that prolonged abnormal mechanical stress accelerates the establishment and progression of disc cell senescence and consequently impairs the structural and functional homeostasis of IVDs to cause IDD. Preventing the pro-senescent effect of mechanical stress on IVD cells is a promising approach to delay the process of IDD.

Effects of Inflammation on Multiscale Biomechanical Properties of Cartilaginous Cells and Tissues

Cells within cartilaginous tissues are mechanosensitive and thus require mechanical loading for regulation of tissue homeostasis and metabolism. Mechanical loading plays critical roles in cell differentiation, proliferation, biosynthesis, and homeostasis. Inflammation is an important event occurring during multiple processes, such as aging, injury, and disease. Inflammation has significant effects on biological processes as well as mechanical function of cells and tissues. These effects are highly dependent on cell/tissue type, timing, and magnitude. In this review, we summarize key findings pertaining to effects of inflammation on multiscale mechanical properties at subcellular, cellular, and tissue level in cartilaginous tissues, including alterations in mechanotransduction and mechanosensitivity. The emphasis is on articular cartilage and the intervertebral disc, which are impacted by inflammatory insults during degenerative conditions such as osteoarthritis, joint pain, and back pain. To recapitulate the pro-inflammatory cascades that occur in vivo, different inflammatory stimuli have been used for in vitro and in situ studies, including tumor necrosis factor (TNF), various interleukins (IL), and lipopolysaccharide (LPS). Therefore, this review will focus on the effects of these stimuli because they are the best studied pro-inflammatory cytokines in cartilaginous tissues. Understanding the current state of the field of inflammation and cell/tissue biomechanics may potentially identify future directions for novel and translational therapeutics with multiscale biomechanical considerations.

Acidic pH promotes intervertebral disc degeneration: Acid-sensing ion channel -3 as a potential therapeutic target

The aetiology of intervertebral disc (IVD) degeneration remains poorly understood. Painful IVD degeneration is associated with an acidic intradiscal pH but the response of NP cells to this aberrant microenvironmental factor remains to be fully characterised. The aim here was to address the hypothesis that acidic pH, similar to that found in degenerate IVDs, leads to the altered cell/functional phenotype observed during IVD degeneration, and to investigate the involvement of acid-sensing ion channel (ASIC) -3 in the response. Human NP cells were treated with a range of pH, from that of a non-degenerate (pH 7.4 and 7.1) through to mildly degenerate (pH 6.8) and severely degenerate IVD (pH 6.5 and 6.2). Increasing acidity of pH caused a decrease in cell proliferation and viability, a shift towards matrix catabolism and increased expression of proinflammatory cytokines and pain-related factors. Acidic pH resulted in an increase in ASIC-3 expression. Importantly, inhibition of ASIC-3 prevented the acidic pH induced proinflammatory and pain-related phenotype in NP cells. Acidic pH causes a catabolic and degenerate phenotype in NP cells which is inhibited by blocking ASIC-3 activity, suggesting that this may be a useful therapeutic target for treatment of IVD degeneration.

Genetic aspects of intervertebral disc degeneration

Intervertebral disc degeneration (IVDD) is one of the common causes of low back pain. Similar to many other multifactorial diseases, it is affected by environmental and genetic factors. Although not completely understood, genetic factors include a wide spectrum of variations, such as single nucleotide polymorphisms, which could play a significant role in the etiology of this disease. Besides, the interactions with environmental factors could make the role of genetic factors more complicated. Genetic variations in disc components could participate in developing degenerative disc disease through altering the normal homeostasis of discs. Gene polymorphisms in disc proteins (collagens I, II, III, IX, and XI), proteoglycans (aggrecan), cytokines (interleukins I, VI, and X), enzymes (matrix metalloproteinases II, III, and IX), and vitamin D receptor seem to play considerable roles in the pathology of this disease. There are also many other investigated genes that could somehow take part in the process. However, it seems that more studies are needed to clarify the exact role of genetics in IVDD.

Reduced tissue osmolarity increases TRPV4 expression and pro-inflammatory cytokines in intervertebral disc cells

The mechanical behaviour and cellular metabolism of intervertebral discs (IVDs) and articular cartilage are strongly influenced by their proteoglycan content and associated osmotic properties. This osmotic environment is a biophysical signal that changes with disease and may contribute to the elevated matrix breakdown and altered biologic response to loading observed in IVD degeneration and osteoarthritis. This study tested the hypothesis that changes in osmo-sensation by the transient receptor potential vallinoid-4 (TRPV4) ion channel occur with disease and contribute to the inflammatory environment found during degeneration. Immunohistochemistry on bovine IVDs from an inflammatory organ culture model were used to investigate if TRPV4 is expressed in the IVD and how expression changes with degeneration. Western blot, live-cell calcium imaging, and qRT-PCR were used to investigate whether osmolarity changes or tumour necrosis factor α (TNFα) regulate TRPV4 expression, and how altered TRPV4 expression influences calcium signalling and pro-inflammatory cytokine expression. TRPV4 expression correlated with TNFα expression, and was increased when cultured in reduced medium osmolarity and unaltered with TNFα-stimulation. Increased TRPV4 expression increased the calcium flux following TRPV4 activation and increased interleukin-1β (IL-1β) and IL-6 gene expression in IVD cells. TRPV4 expression was qualitatively elevated in regions of aggrecan depletion in degenerated human IVDs. Collectively, results suggest that reduced tissue osmolarity, likely following proteoglycan degradation, can increase TRPV4 signalling and enhance pro-inflammatory cytokine production, suggesting changes in TRPV4 mediated osmo-sensation may contribute to the progressive matrix breakdown in disease.

Compression loading-induced stress responses in intervertebral disc cells encapsulated in 3D collagen constructs

Cells protect themselves from stresses through a cellular stress response. In the interverebral disc, such response was also demonstrated to be induced by various environmental stresses. However, whether compression loading will cause cellular stress response in the nucleus pulposus cells (NPCs) is not well studied. By using an in vitro collagen microencapsulation model, we investigated the effect of compression loading on the stress response of NPCs. Cell viability tests, and gene and protein expression experiments were conducted, with primers for the heat shock response (HSR: HSP70, HSF1, HSP27 and HSP90), and unfolded protein response (UPR: GRP78, GRP94, ATF4 and CHOP) genes and an antibody to HSP72. Different gene expression patterns occurred due to loading type throughout experiments. Increasing the loading strain for a short duration did not increase the stress response genes significantly, but over longer durations, HSP70 and HSP27 were upregulated. Longer loading durations also resulted in a continuous upregulation of HSR genes and downregulation of UPR genes, even after load removal. The rate of apoptosis did not increase significantly after loading, suggesting that stress response genes might play a role in cell survival following mechanical stress. These results demonstrate how mechanical stress might induce and control the expression of HSR and UPR genes in NPCs.

Cyclic tensile stress of human annulus fibrosus cells induces MAPK activation: involvement in proinflammatory gene expression

OBJECTIVE

To study the role of mitogen-activated protein kinases (MAPKs) in human annulus fibrosus (AF) cells subjected to cyclic tensile stress (CTS).

DESIGN

An in vitro system for CTS studies was established using AF cultures on fibronectin-coated silicone dishes. MAPK phosphorylation was studied by western analysis, while gene expression was followed by qRT-PCR. DNA synthesis was assessed by both tritiated thymidine incorporation and flow cytometry, and collagen synthesis using tritiated proline incorporation and the protease-free collagenase method.

RESULTS

All three MAPKs studied, i.e., ERK, SAPK/JNK, and p38 were found to be phosphorylated immediately after CTS application within physiological range. A second wave of phosphorylation appeared at later time points. MAPK activation was elevated at higher CTS magnitudes, but independent of the frequency. CTS did not stimulate DNA synthesis neither extracellular matrix turnover, but it stimulated the proinflammatory genes, COX-2, IL-6, and IL-8. This stimulation was more intense at the highest magnitude (8%) tested and at the median frequency (1 Hz) and time interval (12 h). Blocking of ERK, SAPK/JNK, and p38 MAPK inhibited the CTS-induced stimulation of COX-2 and IL-8, while IL-6 expression was mediated only by SAPK/JNK and p38 MAPK.

CONCLUSIONS

We have described for the first time the activation of MAPKs in human AF cells in response to CTS and showed that it drives an inflammatory reaction. These observations shed light on the mechanisms of intervertebral disc (IVD) cell responses to mechanical stress, contributing to the understanding of disc pathophysiology and possibly to the design of novel therapeutic interventions.

Fluid-induced, shear stress-regulated extracellular matrix and matrix metalloproteinase genes expression on human annulus fibrosus cells

BACKGROUND

Mechanical loading plays an important role in the regulation of extracellular matrix (ECM) homeostasis as well as pathogenesis of intervertebral disc (IVD) degeneration. The human annulus fibrosus (hAF) in the IVD is subjected to contact shear stress during body motion. However, the effects of shear stress on hAF cells remain unclear. This aim of the study was to investigate the expression of the ECM (COLI, COLIII and aggrecan) and matrix metalloproteinase (MMP-1, MMP-3 and ADAMTS-4) genes in hAF cells following fluid-induced shear stress in a custom-fabricated bio-microfluidic device.

METHODS

hAF cells were harvested from degenerated disc tissues in routine spine surgery, staged by magnetic resonance imaging, expanded in monolayers and then seeded onto the bio-microfluidic device. The experimental groups were subjected to 1 and 10 dyne/cm(2) shear stress for 4 h, and no shear stress was applied to the control group. We used real time polymerase chain reaction for gene expression.

RESULTS

Shear stress of 1 dyne/cm(2) exerted an anabolic effect on COLI and COLIII genes and catabolic effects on the aggrecan gene, while 10 dyne/cm(2) had an anabolic effect on the COLI gene and a catabolic effect on COLIII and aggrecan genes. The COLI gene was upregulated in a stress-dependent manner. Expression of MMP-1 was significantly higher in the 10 dyne/cm(2) group compared to the control group (P < 0.05), but was similar in the control and 1 dyne/cm(2) groups. Expression of MMP-3 and ADAMTS-4 were similar in all three groups.

CONCLUSION

Taken together, hAF cells responded to shear stress. The findings help us understand and clarify the effects of shear stress on IVD degeneration as well as the development of a new therapeutic strategy for IVD degeneration.

Do mechanical strain and TNF-α interact to amplify pro-inflammatory cytokine production in human annulus fibrosus cells?

During intervertebral disc (IVD) injury and degeneration, annulus fibrosus (AF) cells experience large mechanical strains in a pro-inflammatory milieu. We hypothesized that TNF-α, an initiator of IVD inflammation, modifies AF cell mechanobiology via cytoskeletal changes, and interacts with mechanical strain to enhance pro-inflammatory cytokine production. Human AF cells (N=5, Thompson grades 2-4) were stretched uniaxially on collagen-I coated chambers to 0%, 5% (physiological) or 15% (pathologic) strains at 0.5Hz for 24h under hypoxic conditions with or without TNF-α (10ng/mL). AF cells were treated with anti-TNF-α and anti-IL-6. ELISA assessed IL-1β, IL-6, and IL-8 production and immunocytochemistry measured F-actin, vinculin and α-tubulin in AF cells. TNF-α significantly increased AF cell pro-inflammatory cytokine production compared to basal conditions (IL-1β:2.0±1.4-84.0±77.3, IL-6:10.6±9.9-280.9±214.1, IL-8:23.9±26.0-5125.1±4170.8pg/ml for basal and TNF-α treatment, respectively) as expected, but mechanical strain did not. Pathologic strain in combination with TNF-α increased IL-1β, and IL-8 but not IL-6 production of AF cells. TNF-α treatment altered F-actin and α-tubulin in AF cells, suggestive of altered cytoskeletal stiffness. Anti-TNF-α (infliximab) significantly inhibited pro-inflammatory cytokine production while anti-IL-6 (atlizumab) did not. In conclusion, TNF-α altered AF cell mechanobiology with cytoskeletal remodeling that potentially sensitized AF cells to mechanical strain and increased TNF-α-induced pro-inflammatory cytokine production. Results suggest an interaction between TNF-α and mechanical strain and future mechanistic studies are required to validate these observations.

Intermittent cyclic mechanical tension promotes endplate cartilage degeneration via canonical Wnt signaling pathway and E-cadherin/β-catenin complex cross-talk

OBJECTIVE

This study aimed to investigate the role of the Wnt/β-catenin signaling pathway and E-cadherin/β-catenin complex in intermittent cyclic mechanical tension (ICMT)-induced endplate cartilage degeneration.

DESIGN

β-Catenin expression was measured in disc samples obtained from patients with disc degeneration and those with cervical vertebrae fracture or dislocation. Histological staining was performed to examine the disc tissue morphology and extracellular matrix after application of ICMT in vitro and in vivo. Multiple strategies were employed to examine activation of Wnt/β-catenin signaling after ICMT application in vivo and in vitro. Co-immunoprecipitation was performed to examine the interaction between E-cadherin and β-catenin. Pathway-specific inhibitors and an E-cadherin expression plasmid were used to regulate Wnt/β-catenin signaling and E-cadherin expression.

RESULTS

β-Catenin protein expression was elevated significantly, whereas cartilaginous genes were down-regulated in endplate cartilage samples obtained from patients with disc degeneration. ICMT loading led to Wnt/β-catenin signaling activation and the loss of the chondrogenic phenotype of endplate chondrocytes in both an in vivo rabbit model and in vitro endplate chondrocyte culture system. Inhibition of Wnt/β-catenin signaling suppressed the decrease in ICMT-induced cartilaginous gene expression. Furthermore, E-cadherin expression was inhibited by ICMT stimulation, resulting in a decrease in the interaction between E-cadherin and β-catenin proteins. Over-expression of E-cadherin rescued the cartilaginous gene expression by enhancing the interaction between E-cadherin and β-catenin proteins.

CONCLUSIONS

ICMT promotes endplate cartilage degeneration via activation of Wnt/β-catenin signaling and suppression of physical protein-protein interactions between E-cadherin and β-catenin.

Systematic study of cell isolation from bovine nucleus pulposus: Improving cell yield and experiment reliability

Differences in matrix compositions in human nucleus pulposus (NP) clinical samples demand different cell isolation protocols for optimal results but there is no clear guide about this to date. Sub-optimal protocols may result in low cell yield, limited reliability of results or even failure of experiments. Cell yield, viability and attachment of cells isolated from bovine NP tissue with different protocols were estimated by cell counting, Trypan blue staining and cell culturing respectively. RNA was extracted from isolated cells and quantified by Nanodrop spectrometry and RT-qPCR. Higher collagenase concentration, longer digestion duration and pronase pre-treatment increased the cell yield. Cell viability remained high (<5% dead cells) even after 0.2% collagenase treatment for overnight. NP cells remained to have high ACAN, COL2A1, CDH2, KRT18, and KRT19 expression compared to muscle cells for different cell isolation conditions tested. Digestion by collagenase alone without the use of pronase could isolate cells from human degenerated NP tissue but clusters of cells were observed. We suggest the use of the disappearance of tissue as an indirect measure of cells released. This study provides a guide for researchers to decide the parameters involved in NP cell isolation for optimal outcome.

Mechanotransduction pathways in bone pathobiology

The skeleton is subject to dynamic changes throughout life and bone remodeling is essential for maintenance of bone functionality. The cell populations which predominantly participate in bone and cartilage remodeling, namely osteocytes, osteoblasts, osteoclasts and chondrocytes sense and respond to external mechanical signals and via a series of molecular cascades control bone metabolism and turnover rate. The aforementioned process, known as mechanotransduction, is the underlying mechanism that controls bone homeostasis and function. A wide array of cross-talking signaling pathways has been found to play an important role in the preservation of bone and cartilage tissue health. Moreover, alterations in bone mechanotransduction pathways, due to genetic, hormonal and biomechanical factors, are considered responsible for the pathogenesis of bone and cartilage diseases. Extensive research has been conducted and demonstrated that aberrations in mechanotransduction pathways result in disease-like effects, however only few signaling pathways have actually been engaged in the development of bone disease. The aim of the present review is to present these signaling molecules and cascades that have been found to be mechano-responsive and implicated in bone disease development, as revealed by research in the last five years. In addition, the role of these molecules as prognostic or diagnostic disease markers and their potential as therapeutic targets are also discussed.

Mechanical force-induced specific MicroRNA expression in human periodontal ligament stem cells

It remains unclear how the expression of microRNAs (miRNAs) in human periodontal ligament stem cells (PDLSCs) might respond to mechanical stretch. To investigate specific miRNA expression in stretched PDLSCs, we used a Flexcell® FX-5000™ tension system to achieve external mechanical stimulation. Then, a custom-designed microarray assay was performed to investigate and describe the genome-wide differential expression of miRNAs in normal and stretched PDLSCs. Finally, we implemented integrative miRNA target prediction and network analysis approaches to construct an interaction network of the key miRNAs and their putative targets. We found that stretching induced morphological changes and increased alkaline phosphatase (ALP) activity, runt-related transcription factor 2 (RUNX2), osteocalcin (OCN), and bone sialoprotein (BSP) expression in PDLSCs. The microarray data showed that 53 miRNAs were differentially expressed with stretching. With an interaction network, we examined the connections between 10 selected key miRNAs and their putative target genes, which were related to mechanical force. The results from the interaction network provided a basis for postulating the functional roles of miRNAs in PDLSCs.

A combined biomaterial and cellular approach for annulus fibrosus rupture repair

Recurrent intervertebral disc (IVD) herniation and degenerative disc disease have been identified as the most important factors contributing to persistent pain and disability after surgical discectomy. An annulus fibrosus (AF) closure device that provides immediate closure of the AF rupture, restores disc height, reduces further disc degeneration and enhances self-repair capacities is an unmet clinical need. In this study, a poly(trimethylene carbonate) (PTMC) scaffold seeded with human bone marrow derived mesenchymal stromal cells (MSCs) and covered with a poly(ester-urethane) (PU) membrane was assessed for AF rupture repair in a bovine organ culture annulotomy model under dynamic load for 14 days. PTMC scaffolds combined with the sutured PU membrane restored disc height of annulotomized discs and prevented herniation of nucleus pulposus (NP) tissue. Implanted MSCs showed an up-regulated gene expression of type V collagen, a potential AF marker, indicating in situ differentiation capability. Furthermore, MSCs delivered within PTMC scaffolds induced an up-regulation of anabolic gene expression and down-regulation of catabolic gene expression in adjacent native disc tissue. In conclusion, the combined biomaterial and cellular approach has the potential to hinder herniation of NP tissue, stabilize disc height, and positively modulate cell phenotype of native disc tissue.

Mechanotransduction in intervertebral discs

Mechanotransduction plays a critical role in intracellular functioning—it allows cells to translate external physical forces into internal biochemical activities, thereby affecting processes ranging from proliferation and apoptosis to gene expression and protein synthesis in a complex web of interactions and reactions. Accordingly, aberrant mechanotransduction can either lead to, or be a result of, a variety of diseases or degenerative states. In this review, we provide an overview of mechanotransduction in the context of intervertebral discs, with a focus on the latest methods of investigating mechanotransduction and the most recent findings regarding the means and effects of mechanotransduction in healthy and degenerative discs. We also provide some discussion of potential directions for future research and treatments.

High mechanical strain of primary intervertebral disc cells promotes secretion of inflammatory factors associated with disc degeneration and pain

Introduction

Excessive mechanical loading of intervertebral discs (IVDs) is thought to alter matrix properties and influence disc cell metabolism, contributing to degenerative disc disease and development of discogenic pain. However, little is known about how mechanical strain induces these changes. This study investigated the cellular and molecular changes as well as which inflammatory receptors and cytokines were upregulated in human intervertebral disc cells exposed to high mechanical strain (HMS) at low frequency. The impact of these metabolic changes on neuronal differentiation was also explored to determine a role in the development of disc degeneration and discogenic pain.

Methods

Isolated human annulus fibrosus (AF) and nucleus pulposus (NP) cells were exposed to HMS (20% cyclical stretch at 0.001 Hz) on high-extension silicone rubber dishes coupled to a mechanical stretching apparatus and compared to static control cultures. Gene expression of Toll-like receptors (TLRs), neuronal growth factor (NGF) and tumour necrosis factor α (TNFα) was assessed. Collected conditioned media were analysed for cytokine content and applied to rat pheocromocytoma PC12 cells for neuronal differentiation assessment.

Results

HMS caused upregulation of TLR2, TLR4, NGF and TNFα gene expression in IVD cells. Medium from HMS cultures contained elevated levels of growth-related oncogene, interleukin 6 (IL-6), IL-8, IL-15, monocyte chemoattractant protein 1 (MCP-1), MCP-3, monokine induced by γ interferon, transforming growth factor β1, TNFα and NGF. Exposure of PC12 cells to HMS-conditioned media resulted in both increased neurite sprouting and cell death.

Conclusions

HMS culture of IVD cells in vitro drives cytokine and inflammatory responses associated with degenerative disc disease and low-back pain. This study provides evidence for a direct link between cellular strain, secretory factors, neoinnervation and potential degeneration and discogenic pain in vivo.

Integrin - dependent mechanotransduction in mechanically stimulated human annulus fibrosus cells: evidence for an alternative mechanotransduction pathway operating with degeneration

Intervertebral disc (IVD) cells derived from degenerate tissue respond aberrantly to mechanical stimuli, potentially due to altered mechanotransduction pathways. Elucidation of the altered, or alternative, mechanotransduction pathways operating with degeneration could yield novel targets for the treatment of IVD disease. Our aim here was to investigate the involvement of RGD-recognising integrins and associated signalling molecules in the response to cyclic tensile strain (CTS) of human annulus fibrosus (AF) cells derived from non-degenerate and degenerate IVDs. AF cells from non-degenerate and degenerate human IVDs were cyclically strained with and without function blocking RGD – peptides with 10% strain, 1.0 Hz for 20 minutes using a Flexercell® strain device. QRT-PCR and Western blotting were performed to analyse gene expression of type I collagen and ADAMTS -4, and phosphorylation of focal adhesion kinase (FAK), respectively. The response to 1.0 Hz CTS differed between the two groups of AF cells, with decreased ADAMTS -4 gene expression and decreased type I collagen gene expression post load in AF cells derived from non-degenerate and degenerate IVDs, respectively. Pre-treatment of non-degenerate AF cells with RGD peptides prevented the CTS-induced decrease in ADAMTS -4 gene expression, but caused an increase in expression at 24 hours, a response not observed in degenerate AF cells where RGD pre-treatment failed to inhibit the mechano-response. In addition, FAK phosphorylation increased in CTS stimulated AF cells derived from non-degenerate, but not degenerate IVDs, with RGD pre-treatment inhibiting the CTS – dependent increase in phosphorylated FAK. Our findings suggest that RGD -integrins are involved in the 1.0 Hz CTS – induced mechano-response observed in AF cells derived from non-degenerate, but not degenerate IVDs. This data supports our previous work, suggesting an alternative mechanotransduction pathway may be operating in degenerate AF cells.

Acute vibration induces transient expression of anabolic genes in the murine intervertebral disc

OBJECTIVE

Low-amplitude whole-body vibration has been adopted for the treatment of back pain and spinal disorders. However, there is limited knowledge of the impact of vibration on the intervertebral disc (IVD). This study was undertaken to examine the effects of acute vibration on anabolic and catabolic pathways in the IVD and to characterize the dependence of these changes on time and frequency.

METHODS

Custom-designed platforms were developed to apply acute vibration to ex vivo and in vivo mouse models. Spinal segments (ex vivo) or mice (in vivo) were subjected to vibration (for 30 minutes at 15-90 Hz with peak acceleration at 0.3g), and IVDs were examined at specific time points after vibration. Gene expression was quantified using real-time polymerase chain reaction, and protein levels were examined by quantitative mass spectrometry and immunofluorescence.

RESULTS

In the ex vivo model, acute vibration at 15 Hz induced expression of anabolic genes (aggrecan, biglycan, decorin, type I collagen, and Sox9) and suppressed expression of Mmp13, with the most pronounced changes detected 6 hours following vibration. These beneficial effects were frequency dependent and were no longer evident between 45 and 90 Hz. In vivo, the effects on anabolic gene expression were even more robust and were accompanied by decreased expression of Adamts4, Adamts5, and Mmp3. Moreover, significant increases in the protein levels of aggrecan, biglycan, decorin, and type I collagen were detected in vivo.

CONCLUSION

These findings demonstrate dramatic anabolic effects of acute vibration on IVD tissue, responses that are dependent on frequency. The similarity of the in vivo and ex vivo responses indicates that at least some effects of vibration are tissue autonomous.

Dynamic and static overloading induce early degenerative processes in caprine lumbar intervertebral discs

Mechanical overloading of the spine is associated with low back pain and intervertebral disc (IVD) degeneration. How excessive loading elicits degenerative changes in the IVD is poorly understood. Comprehensive knowledge of the interaction between mechanical loading, cell responses and changes in the extracellular matrix of the disc is needed in order to successfully intervene in this process. The purpose of the current study was to investigate whether dynamic and static overloading affect caprine lumbar discs differently and what mechanisms lead to mechanically induced IVD degeneration. Lumbar caprine IVDs (n = 175) were cultured 7, 14 and 21 days under simulated-physiological loading (control), high dynamic or high static loading. Axial deformation and stiffness were continuously measured. Cell viability, cell density, and gene expression were assessed in the nucleus, inner- and outer annulus. The extracellular matrix (ECM) was analyzed for water, glycosaminoglycan and collagen content. IVD height loss and changes in axial deformation were gradual with dynamic and acute with static overloading. Dynamic overloading caused cell death in all IVD regions, whereas static overloading mostly affected the outer annulus. IVDs expression of catabolic and inflammation-related genes was up-regulated directly, whereas loss of water and glycosaminoglycan were significant only after 21 days. Static and dynamic overloading both induced pathological changes to caprine lumbar IVDs within 21 days. The mechanism by which they inflict biomechanical, cellular, and extracellular changes to the nucleus and annulus differed. The described cascades provide leads for the development of new pharmacological and rehabilitative therapies to halt the progression of DDD.