The role of endplate poromechanical properties on the nutrient availability in the intervertebral disc

Combining the probabilistic finite element model and artificial neural network to study nutrient levels in the human intervertebral discs

BACKGROUND

Diffusion distance and diffusivity are known to affect nutrient transport rates, but the probabilistic analysis of these two factors remains vacant. There is a lack of effective tools to evaluate disc nutrient levels.

METHODS

Five-hundred-disc samples with different combinations of morphological and water content parameters were generated, which were used to evaluate nutrient levels in unloaded and loaded states. Spearman correlation coefficients between inputs and responses were calculated. Artificial neural networks were trained to predict nutrient concentrations based on the dataset generated by the probabilistic finite element model.

FINDINGS

In unloaded and loaded states, the minimum oxygen concentration of nucleus pulposus was negatively correlated with disc height (r = -0.83, p < 0.01 and r = -0.76, p < 0.01, respectively), and the minimum glucose concentration of annulus fibrosus was positively correlated with its water content (r = 0.68, p < 0.01 and r = 0.73, p < 0.01, respectively). The maximum lactate concentration of cartilage endplate was affected by endplate thickness (r = 0.94, p < 0.01 and r = 0.95, p < 0.01, respectively). For trained neural networks, nutrient concentrations could be well predicted, with coefficients of determination greater than 0.95 and mean absolute percentage errors less than 5 %.

INTERPRETATION

This study underscores the importance of disc height, annulus fibrosus water content, and endplate thickness in regulating nutrient levels, and precise control of these parameters should be prioritized in the design of tissue-engineered discs. Moreover, artificial neural networks might be a promising tool for evaluating nutrient levels.

Decellularized nucleus pulposus matrix/chitosan hybrid hydrogel combined with nucleus pulposus stem cells and GDF5-loaded microspheres for intervertebral disc degeneration prevention

BACKGROUND

Intervertebral disc degeneration (IDD) is considered an important pathological basis for spinal degenerative diseases. Tissue engineering is a powerful therapeutic strategy that can effectively restore the normal biological properties of disc units. In this study, hydrogels loaded with growth/differentiation factor 5 (GDF5) and stem cells were combined to provide an effective strategy for nucleus pulposus regeneration.

METHODS

Nucleus pulposus stem cells (NPSCs) were obtained by low-density inoculation and culture, and their stem cell characteristics were verified by flow cytometry and a tri-lineage-induced differentiation experiment. A decellularized nucleus pulposus matrix (DNPM) and chitosan hybrid hydrogel was prepared, and GDF5-loaded poly(lactic-co-glycolic acid) (PLGA) microspheres were incorporated into the hydrogels to obtain a composite hydrogels with GDF5-loaded microspheres. Taking bone marrow mesenchymal stem cells (BMSCs) as a reference, the effect of composite hydrogels with GDF5-loaded microspheres on the chondrogenic differentiation of NPSCs was evaluated. A model of intervertebral disc degeneration induced by acupuncture on the tail of rats was constructed, and the repair effect of composite hydrogels with GDF5-loaded microspheres combined with NPSCs on IDD was observed.

RESULTS

Stem cell phenotype identification, stemness gene expression and tri-lineage-induced differentiation confirmed that NPSCs had characteristics similar to those of BMSCs. The rat DNPM and chitosan hybrid hydrogels had good mechanical properties, and the GDF5-loaded microspheres sustainably released GDF5. NPSCs grew normally in the composite hydrogels and gradually expressed a chondrocyte phenotype. Animal experiments showed that the composite hydrogels with GDF5-loaded microspheres combined with NPSCs effectively promoted nucleus pulposus regeneration and that the effect of the hydrogels on the repair of IDD was significantly better than that of BMSCs.

CONCLUSION

GDF5-loaded microspheres combined with DNPM/chitosan composite hydrogels can effectively promote the differentiation of NPSCs into nucleus pulposus-like cells and effectively preventIDD.

Current status and development direction of immunomodulatory therapy for intervertebral disk degeneration

Intervertebral disk (IVD) degeneration (IVDD) is a main factor in lower back pain, and immunomodulation plays a vital role in disease progression. The IVD is an immune privileged organ, and immunosuppressive molecules in tissues reduce immune cell (mainly monocytes/macrophages and mast cells) infiltration, and these cells can release proinflammatory cytokines and chemokines, disrupting the IVD microenvironment and leading to disease progression. Improving the inflammatory microenvironment in the IVD through immunomodulation during IVDD may be a promising therapeutic strategy. This article reviews the normal physiology of the IVD and its degenerative mechanisms, focusing on IVDD-related immunomodulation, including innate immune responses involving Toll-like receptors, NOD-like receptors and the complement system and adaptive immune responses that regulate cellular and humoral immunity, as well as IVDD-associated immunomodulatory therapies, which mainly include mesenchymal stem cell therapies, small molecule therapies, growth factor therapies, scaffolds, and gene therapy, to provide new strategies for the treatment of IVDD.

Cartilaginous endplates: A comprehensive review on a neglected structure in intervertebral disc research

The cartilaginous endplates (CEP) are key components of the intervertebral disc (IVD) necessary for sustaining the nutrition of the disc while distributing mechanical loads and preventing the disc from bulging into the adjacent vertebral body. The size, shape, and composition of the CEP are essential in maintaining its function, and degeneration of the CEP is considered a contributor to early IVD degeneration. In addition, the CEP is implicated in Modic changes, which are often associated with low back pain. This review aims to tackle the current knowledge of the CEP regarding its structure, composition, permeability, and mechanical role in a healthy disc, how they change with degeneration, and how they connect to IVD degeneration and low back pain. Additionally, the authors suggest a standardized naming convention regarding the CEP and bony endplate and suggest avoiding the term vertebral endplate. Currently, there is limited data on the CEP itself as reported data is often a combination of CEP and bony endplate, or the CEP is considered as articular cartilage. However, it is clear the CEP is a unique tissue type that differs from articular cartilage, bony endplate, and other IVD tissues. Thus, future research should investigate the CEP separately to fully understand its role in healthy and degenerated IVDs. Further, most IVD regeneration therapies in development failed to address, or even considered the CEP, despite its key role in nutrition and mechanical stability within the IVD. Thus, the CEP should be considered and potentially targeted for future sustainable treatments.

Polymeric Hydrogels for Intervertebral Disc Replacement/Integration: Playing with the Chemical Composition for Tuning Shear Behavior and Hydrophilicity

Damages to the intervertebral disc (IVD) due to improper loading or degeneration result in back pain, which is a common disease affecting an increasing number of patients. Different strategies for IVD remediation have been developed, from surgical treatment to disc replacement, by using both metallic and non-metallic materials. Hydrogels are very attractive materials due to their ability to simulate the properties of many soft tissues; moreover, their chemical composition can be varied in order to assure performances similar to the natural disc. In particular, for the replacement of the IVD outer ring, namely, the anulus fibrosus, the shear properties are of paramount importance. In this work, we produced hydrogels through the photo-induced crosslinking of different mixtures composed of two hydrophilic monofunctional and difunctional polymers, namely, poly(ethyleneglycol) methyl ether methacrylate (PEGMEMA) and poly(ethyleneglycol) dimethacrylate (PEGDMA), together with a hydrophobic molecule, i.e., tert-butyl acrylate (tBA). By changing the ratio among the precursors, we demonstrated the tunability of both the shear properties and hydrophilicity. The structural properties of hydrogels were studied by solid-state nuclear magnetic resonance (NMR). These experiments provided insights on both the structure and molecular dynamics of polymeric networks and, together with information obtained by differential scanning calorimetry (DSC), allowed for correlating the physical properties of the hydrogels with their chemical composition.

Multifunctional injectable hydrogels with controlled delivery of bioactive factors for efficient repair of intervertebral disc degeneration

Millions of people worldwide suffer from intervertebral disc degeneration (IVDD), which imposes a significant socioeconomic burden on society. There is an urgent clinical demand for more effective treatments for IVDD because conventional treatments can only alleviate the symptoms rather than preventing the progression of IVDD. Hydrogels, a class of elastic biomaterials with good biocompatibility, are promising candidates for intervertebral disc repair and regeneration. In recent years, various hydrogels have been investigated in vitro and in vivo for the repair of intervertebral discs, some of which are ready for clinical testing. This review summarizes the latest findings and developments in using bioactive factors-released bioactive injectable hydrogels for the repair and regeneration of intervertebral discs. It focuses on the analysis and summary of the use of multifunctional injectable hydrogels to delivery bioactive factors (cells, exosomes, growth factors, genes, drugs) for disc regeneration, providing guidance for future study. Finally, we discussed and analyzed the optimal timing for the application of controlled-release hydrogels in the treatment of IVDD to meet the high standards required for intervertebral disc regeneration and precision medicine.

Effects of Hyperbaric Oxygen Intervention on the Degenerated Intervertebral Disc: From Molecular Mechanisms to Animal Models

MicroRNA (miRNA) 107 expression is downregulated but Wnt3a protein and β-catenin are upregulated in degenerated intervertebral disc (IVD). We investigated mir-107/Wnt3a-β-catenin signaling in vitro and in vivo following hyperbaric oxygen (HBO) intervention. Our results showed 96 miRNAs were upregulated and 66 downregulated in degenerated nucleus pulposus cells (NPCs) following HBO treatment. The 3' untranslated region (UTR) of the Wnt3a mRNA contained the "seed-matched-sequence" for miR-107. MiR-107 was upregulated and a marked suppression of Wnt3a was observed simultaneously in degenerated NPCs following HBO intervention. Knockdown of miR-107 upregulated Wnt3a expression in hyperoxic cells. HBO downregulated the protein expression of Wnt3a, phosphorylated LRP6, and cyclin D1. There was decreased TOP flash activity following HBO intervention, whereas the FOP flash activity was not affected. HBO decreased the nuclear translocation of β-catenin and decreased the secretion of MMP-3 and -9 in degenerated NPCs. Moreover, rabbit serum KS levels and the stained area for Wnt3a and β-catenin in repaired cartilage tended to be lower in the HBO group. We observed that HBO inhibits Wnt3a/β-catenin signaling-related pathways by upregulating miR-107 expression in degenerated NPCs. HBO may play a protective role against IVD degeneration and could be used as a future therapeutic treatment.

Osteochondral junction leakage and cartilage joint lubrication

BACKGROUND AND OBJECTIVES

Previous studies have shown that there is potentially interstitial fluid exchange between cartilage tissue and the subarticular spongiosa region in the case of injury or disease (e.g., osteoarthritis and osteoporosis). Interstitial flow is also required for cartilage lubrication under joint load. A key question then is how cartilage lubrication is modified by increased interstitial fluid leakage across the osteochondral junction. Thus, the purpose of this study is to develop a numerical model to investigate changes in cartilage lubrication with changes in osteochondral junction leakage.

METHODS

The multi-phase coupled model includes domains corresponding to the contact gap, cartilage tissue and subchondral bone plate region (ScBP). Each of these domains are treated as poroelastic systems, with their coupling implemented through mass and pressure continuity. The effects of osteochondral junction leakage on lubrication were investigated with a parametric study on the relative permeability between the ScBP and cartilage tissue.

RESULTS

Significant effects of ScBP permeability were predicted, especially during the early stage of the junction leakage development (early stage of the disease). There is a significant reduction in mixed-mode lubrication duration under the effect of increased junction leakage (the cartilage tissue mixed-mode lubrication duration is about 33% decrease for a relative permeability ratio of 0.1 between ScBP and cartilage tissue, and about 52% decrease under the osteoarthritis condition). In addition, the time for cartilage to reach steady-state consolidation is significantly reduced when ScBP permeability increases (the consolidation time reduces from roughly 2 h to 1.2 h when the relative permeability ratio increases from 0.001 to 0.1, and it reduces to 0.8 h for an advanced osteoarthritis condition). It is predicted that the initial friction coefficient could increase by over 60% when the ScBP permeability is consistent with an advanced osteoarthritis (OA) condition.

CONCLUSION

Increased osteochondral junction leakage induced by joint injury and disease could result in increased cartilage surface wear rates due to more rapid interstitial fluid depressurization within articular cartilage.

Silk fibroin-based biomaterials for disc tissue engineering

Low back pain is the major cause of disability worldwide, and intervertebral disc degeneration (IVDD) is one of the most important causes of low back pain. Currently, there is no method to treat IVDD that can reverse or regenerate intervertebral disc (IVD) tissue, but the recent development of disc tissue engineering (DTE) offers a new means of addressing these disadvantages. Among numerous biomaterials for tissue engineering, silk fibroin (SF) is widely used due to its easy availability and excellent physical/chemical properties. SF is usually used in combination with other materials to construct biological scaffolds or bioactive substance delivery systems, or it can be used alone. The present article first briefly outlines the anatomical and physiological features of IVD, the associated etiology and current treatment modalities of IVDD, and the current status of DTE. Then, it highlights the characteristics of SF biomaterials and their latest research advances in DTE and discusses the prospects and challenges in the application of SF in DTE, with a view to facilitating the clinical process of developing interventions related to IVD-derived low back pain caused by IVDD.

Application of stem cells combined with biomaterial in the treatment of intervertebral disc degeneration

As the world population is aging, intervertebral disc degeneration (IDD) is becoming a global health issue of increasing concern. A variety of disc degeneration diseases (DDDs) have been proven to be associated with IDD, and these illnesses have significant adverse effects on both individuals and society. The application of stem cells in regenerative medicine, such as blood and circulation, has been demonstrated by numerous studies. Similarly, stem cells have made exciting progress in the treatment of IDD. However, due to complex anatomical structures and functional requirements, traditional stem cell injection makes it difficult to meet people's expectations. With the continuous development of tissue engineering and biomaterials, stem cell combined with biomaterials has far more prospects than before. This review aims to objectively and comprehensively summarize the development of stem cells combined with contemporary biomaterials and the difficulties that need to be overcome.

Application of single and cooperative different delivery systems for the treatment of intervertebral disc degeneration

Intervertebral disc (IVD) degeneration (IDD) is the most universal pathogenesis of low back pain (LBP), a prevalent and costly medical problem across the world. Persistent low back pain can seriously affect a patient's quality of life and even lead to disability. Furthermore, the corresponding medical expenses create a serious economic burden to both individuals and society. Intervertebral disc degeneration is commonly thought to be related to age, injury, obesity, genetic susceptibility, and other risk factors. Nonetheless, its specific pathological process has not been completely elucidated; the current mainstream view considers that this condition arises from the interaction of multiple mechanisms. With the development of medical concepts and technology, clinicians and scientists tend to intervene in the early or middle stages of intervertebral disc degeneration to avoid further aggravation. However, with the aid of modern delivery systems, it is now possible to intervene in the process of intervertebral disc at the cellular and molecular levels. This review aims to provide an overview of the main mechanisms associated with intervertebral disc degeneration and the delivery systems that can help us to improve the efficacy of intervertebral disc degeneration treatment.

The Nrf2 antioxidant defense system in intervertebral disc degeneration: Molecular insights

Intervertebral disc degeneration (IDD) is a common degenerative musculoskeletal disorder and is recognized as a major contributor to discogenic lower back pain. However, the molecular mechanisms underlying IDD remain unclear, and therapeutic strategies for IDD are currently limited. Oxidative stress plays pivotal roles in the pathogenesis and progression of many age-related diseases in humans, including IDD. Nuclear factor E2-related factor 2 (Nrf2) is a master antioxidant transcription factor that protects cells against oxidative stress damage. Nrf2 is negatively modulated by Kelch-like ECH-associated protein 1 (Keap1) and exerts important effects on IDD progression. Accumulating evidence has revealed that Nrf2 can facilitate the transcription of downstream antioxidant genes in disc cells by binding to antioxidant response elements (AREs) in promoter regions, including heme oxygenase-1 (HO-1), glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), and NADPH quinone dehydrogenase 1 (NQO1). The Nrf2 antioxidant defense system regulates cell apoptosis, senescence, extracellular matrix (ECM) metabolism, the inflammatory response of the nucleus pulposus (NP), and calcification of the cartilaginous endplates (EP) in IDD. In this review, we aim to discuss the current knowledge on the roles of Nrf2 in IDD systematically.

Insights into the activity of a protein that regulates gene expression and protects cells against oxidative stress could yield novel treatments for lower back pain. Intervertebral disc degeneration (IDD) is a common cause of lower back pain, but the molecular mechanisms underlying IDD are unclear, meaning treatment options are limited. Oxidative stress is implicated in IDD, and scientists have begun exploring the role of nuclear factor E2-related factor 2 (Nrf2), a master regulator of the body’s antioxidant responses, in regulating IDD progression. In a review of recent research, Weishi Li at Peking University Third Hospital, Beijing, China, and co-workers point out that boosting the activity of Nrf2-related signaling pathways alleviates oxidative stress in intervertebral disc cells. The researchers suggest that therapies based on non-coding RNAs may prove valuable in activating Nrf2 in IDD patients.

YAP1 controls degeneration of human cartilage chondrocytes in response to mechanical tension

Disc herniation is a kind of disease caused by degenerative discs, which is common in the elderly, bringing substantial financial burden to families and society. Mechanical tension has a vital effect on the maintenance of cartilage function, however, the molecular mechanism by which mechanical tension causes degenerative discs to remain unclear. This study was the first to reveal Yes-associated protein 1(YAP1) is a key regulator in mechanical tension-mediated degenerative discs. Activation of YAP1 may be a valuable strategy to delays the degeneration of human cartilage chondrocytes. We found that YAP1 expression was significantly decreased in degenerative human endplate cartilage and tissue with the strength and time of mechanical stimulation, but the cell cycle distribution was significantly changed under the 10% cyclic mechanical tension(CMT). Besides, the degeneration of endplate cartilage can be delayed by activating the expression level of YAP1 in vitro and it has also been verified in the cartilage endplate tissue in vitro. Furthermore, We found that YAP1 and TEAD1 overexpression increased the activity of the ACAN or COL2A1 promoter to enhance the transcriptional activity of human chondrocyte collagen. The CMT activates the classic Hippo signaling pathway of YAP1, and piezo1 may regulate YAP1 expression through the Hippo signaling pathway. In conclusion, these results suggest the novel mechanism of YAP contributes to delaying the degeneration of endplate cartilage and targeting YAP in combination with Piezo1 is a potential therapeutic approach for the treatment of endplate cartilage degeneration.

Effects of Changes in Osmolarity on the Biological Activity of Human Normal Nucleus Pulposus Mesenchymal Stem Cells

The expansion and maintenance of the NPMSC (nucleus pulposus mesenchymal stem cell) phenotype are considered as potential therapeutic tools for clinical applications in intervertebral disc tissue engineering and regenerative medicine. However, the harsh microenvironment within the intervertebral disc is the main limitation of its regeneration. The osmolarity of the intervertebral disc is higher than that of other tissues, which has an important influence on the biological characteristics of NPMSCs. In this study, we observed the effect of different osmolarities on the biological characteristics of human normal NPMSCs cultured in vitro and explored the role of osmolarity in intervertebral disc degeneration. Our data demonstrated that the change in osmotic pressure has an important effect on the biological activity of NPMSCs, and this effect may occur through the P16^INK4A/Rb pathway. This study provides a theoretical basis for the future treatment of intervertebral disc degeneration.

N(6)-methyladenosine RNA methyltransferase like 3 inhibits extracellular matrix synthesis of endplate chondrocytes by downregulating sex-determining region Y-Box transcription factor 9 expression under tension

OBJECTIVE

Tension stimulation is an important inducer of endplate cartilage degeneration, but the specific regulatory mechanism remains unclear. This study was the first to reveal the mechanism by which methyltransferase-like 3 (METTL3)-mediated N(6)-methyladenosine (m6A) modification affected the extracellular matrix anabolism by tension-induced endplate chondrocytes.

METHOD

We examined the differences in METTL3 expression and m6A methylation levels in human endplate chondrocytes and human cartilage endplate tissues under in vitro tension. The effect on endplate cartilage degeneration was evaluated by manipulating m6A methylation mediated by METTL3 in vivo and in vitro. The effect of METTL3-mediated m6A methylation on the stability of sex-determining region Y-box transcription factor 9 (SOX9) gene expression was determined experimentally.

RESULTS

METTL3 expression and m6A methylation levels were significantly increased in degenerative human endplate cartilage tissue. Similarly, tension stimulation inhibited the ability of human endplate chondrocytes to synthesize extracellular matrix, which was accompanied by an increase in METTL3-mediated m6A methylation. The ability of endplate chondrocytes to resist tension was significantly enhanced by inhibiting METTL3 expression and subsequently downregulating m6A methylation in vitro and in vivo, thereby reducing intervertebral disc degeneration. Furthermore, METTL3 mediated SOX9 RNA methylation and disrupted SOX9 mRNA stability, thereby inhibiting the gene expression of the downstream collagen type II alpha 1 chain.

CONCLUSION

Tension stimulation downregulated SOX9 expression through METTL3-mediated m6A methylation, thereby inhibiting the synthesis of extracellular matrix in endplate chondrocytes.

Extracellular Vesicles Derived From Stem Cells in Intervertebral Disc Degeneration

Intervertebral disc degeneration (IVDD) is the leading cause of low back pain related to degradation of cartilaginous tissues, mainly resulting from oxidative stress, cell apoptosis, and extracellular matrix degradation. Extracellular vesicles (EVs) exist in all bodily fluids and can be produced by all types of cells. Stem cell-derived EVs (SC-EVs), which are the main paracrine components of stem cells, have gained significant attention in the field of regenerative medicine. Over the past years, accumulating evidence indicates the therapeutic and diagnostic potentials of EVs in IVDD. The main mechanisms involve the induction of regenerative phenotypes, apoptosis alleviation, and immune modulation. In addition, the efficiency of SC-EVs can be enhanced by choosing appropriate donor cells and cell phenotypes, optimizing cell culture conditions, or engineering EVs to deliver drugs and targeting molecules. Given the importance and novelty of SC-EVs, we give an overview of SC-EVs and discuss the roles of SC-EVs in IVDD.

Mechanism of Action of Mesenchymal Stem Cell-Derived Exosomes in the Intervertebral Disc Degeneration Treatment and Bone Repair and Regeneration

Exosomes are extracellular vesicles formed by various donor cells that regulate gene expression and cellular function in recipient cells. Exosomes derived from mesenchymal stem cells (MSC-Exos) perform the regulatory function of stem cells by transporting proteins, nucleic acids, and lipids. Intervertebral disc degeneration (IDD) is one of the main causes of low back pain, and it is characterized by a decreased number of nucleus pulposus cells, extracellular matrix decomposition, aging of the annulus fibrosus, and cartilage endplate calcification. Besides, nutrient transport and structural repair of intervertebral discs depend on bone and cartilage and are closely related to the state of the bone. Trauma, disease and aging can all cause bone injury. However, there is a lack of effective drugs against IDD and bone injury. Recent MSC-Exos fine tuning has led to significant progress in the IDD treatment and bone repair and regeneration. In this review, we looked at the uniqueness of MSC-Exos, and the potential treatment mechanisms of MSC-Exos with respect to IDD, bone defects and injuries.

Anti-degenerative effect of melatonin on intervertebral disc: protective contribution against inflammation, oxidative stress, apoptosis, and autophagy

Intervertebral disc (IVD) degeneration is a leading cause of lower back pain. Although the etiology of IVD degeneration (IVDD) is unclear, excessive oxidative stress, inflammation and apoptosis and disruption of autophagy play important role in the pathogenesis of IVDD. Therefore, finding a solution to mitigate these processes could stop or reduce the development of IVDD. Melatonin, a powerful antioxidant, plays an important role in regulating cartilage tissue hemostasis. Melatonin inhibits destruction of extracellular matrix (ECM) of disc. Melatonin preserves ECM contents including sox-9, aggrecan, and collagen II through inhibiting matrix degeneration enzymes such as MMP-13. These protective effects may be mediated by the inhibition of oxidative stress, inflammation and apoptosis, and regulation of autophagy in IVD cells.

Patient-Specific Variations in Local Strain Patterns on the Surface of a Trussed Titanium Interbody Cage

Introduction: 3D printed trussed titanium interbody cages may deliver bone stimulating mechanobiological strains to cells attached at their surface. The exact size and distribution of these strains may depend on patient-specific factors, but the influence of these factors remains unknown. Therefore, this study aimed to determine patient-specific variations in local strain patterns on the surface of a trussed titanium interbody fusion cage.

Materials and Methods: Four patients eligible for spinal fusion surgery with the same cage size were selected from a larger database. For these cases, patient-specific finite element models of the lumbar spine including the same trussed titanium cage were made. Functional dynamics of the non-operated lumbar spinal segments, as well as local cage strains and caudal endplate stresses at the operated segment, were evaluated under physiological extension/flexion movement of the lumbar spine.

Results: All patient-specific models revealed physiologically realistic functional dynamics of the operated spine. In all patients, approximately 30% of the total cage surface experienced strain values relevant for preserving bone homeostasis and stimulating bone formation. Mean caudal endplate contact pressures varied up to 10 MPa. Both surface strains and endplate contact pressures varied more between loading conditions than between patients.

Conclusions: This study demonstrates the applicability of patient-specific finite element models to quantify the impact of patient-specific factors such as bone density, degenerative state of the spine, and spinal curvature on interbody cage loading. In the future, the same framework might be further developed in order to establish a pipeline for interbody cage design optimizations.

Patient-specific apparent diffusion maps used to model nutrient availability in degenerated intervertebral discs

INTRODUCTION

In this study, magnetic resonance imaging data was used to (1) model IVD-specific gradients of glucose, oxygen, lactate, and pH; and (2) investigate possible effects of covariate factors (i.e., disc geometry, and mean apparent diffusion coefficient values) on the IVD's microenvironment. Mathematical modeling of the patient's specific IVD microenvironment could be important when selecting patients for stem cell therapy due to the increased nutrient demand created by that treatment.

MATERIALS AND METHODS

Disc geometry and water diffusion coefficients were extracted from MRIs of 37 patients using sagittal T1-weighted images, T2-weighted images, and ADC Maps. A 2-D steady state finite element mathematical model was developed in COMSOL Multiphysics® 5.4 to compute concentration maps of glucose, oxygen, lactate and pH.

RESULTS

Concentration of nutrients (i.e., glucose, and oxygen) dropped with increasing distance from the cartilaginous endplates (CEP), whereas acidity levels increased. Most discs experienced poor nutrient levels along with high acidity values in the inner annulus fibrosus (AF). The disc's physiological microenvironment became more deficient as degeneration progressed. For example, minimum glucose concentration in grade 4 dropped by 31.1% compared to grade 3 (p < 0.0001). The model further suggested a strong effect of the following parameters: disc size, AF and CEP diffusivities, metabolic reactions, and cell density on solute concentrations in the disc (p < 0.05).

CONCLUSION

The significance of this work implies that the individual morphology and physiological conditions of each disc, even among discs of the same Pfirrmann grade, should be evaluated when modeling IVD solute concentrations.

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.

CircSNHG5 Sponges Mir-495-3p and Modulates CITED2 to Protect Cartilage Endplate From Degradation

Background

Intervertebral disc degeneration (IDD) is a highly prevalent degenerating disease that produces tremendous amount of low back and neck pain. The cartilage endplate (CEP) is vitally important to intervertebral discs in both physiological and pathological conditions. In addition, circular RNAs (circRNAs) have been shown to be involved in the regulation of various diseases, including IDD. However, the particular role of circRNAs in cervical vertebral CEP degeneration remains unclear. Here, we examined the unique role of circRNAs in CEP of patients with cervical fracture and degenerative cervical myelopathy (DCM).

Methods

Human competitive endogenous RNA (ceRNA) microarray was performed by previous research. Western blot (WB), immunofluorescence (IF), quantitative RT-PCR (qRT-PCR), luciferase assay, and fluorescence in situ hybridization (FISH) were employed to analyze the function of circSNHG5 and its downstream effectors, miR-495-3p, and CITED2.

Results

We demonstrated that circSNHG5 expression was substantially low in degenerative CEP tissues. Knockdown of circSNHG5 in chondrocytes resulted in a loss of cell proliferation and followed by degradation of extracellular matrix (ECM). In addition, circSNHG5 was shown to sponge miR-495-3p and modulate the expression of the downstream gene CITED2. This mechanism of action was further validated via overexpression and knockdown of CITED2.

Conclusion

Our findings identified a novel circSNHG5-miR-495-3p axis responsible for IDD progression. Future investigations into IDD therapy may benefit from targeting this axis.

Multiaxial validation of a finite element model of the intervertebral disc with multigenerational fibers to establish residual strain

Finite element models of the intervertebral disc are used to address research questions that cannot be tested through typical experimentation. A disc model requires complex geometry and tissue properties to be accurately defined to mimic the physiological disc. The physiological disc possesses residual strain in the annulus fibrosus (AF) due to osmotic swelling and due to inherently pre-strained fibers. We developed a disc model with residual contributions due to swelling-only, and a multigeneration model with residual contributions due to both swelling and AF fiber pre-strain and validated it against organ-scale uniaxial, quasi-static and multiaxial, dynamic mechanical tests. In addition, we demonstrated the models' ability to mimic the opening angle observed following radial incision of bovine discs. Both models were validated against organ-scale experimental data. While the swelling only model responses were within the experimental 95% confidence interval, the multigeneration model offered outcomes closer to the experimental mean and had a bovine model opening angle within one SD of the experimental mean. The better outcomes for the multigeneration model, which allowed for the inclusion of inherently pre-strained fibers in AF, is likely due to its uniform fiber contribution throughout the AF. We conclude that the residual contribution of pre-strained fibers in the AF should be included to best simulate the physiological disc and its behaviors.

Autophagy as a potential therapeutic target in intervertebral disc degeneration

Autophagy is an evolutionarily conserved intracellular recirculation system that delivers cytoplasmic content to lysosomes for degradation, thereby maintaining metabolism and homeostasis. Recent studies have found that autophagy plays a dual role in intervertebral disc degeneration (IDD). Most studies have shown that inducing autophagy can slow down the process of IDD. A few studies have shown that extensive autophagy activation-mediated apoptosis accelerates IDD. In this review, we describe the pathophysiological characteristics of intervertebral disc (IVD), the mechanism of autophagy and the application of regulating autophagy in the treatment of IDD, hoping to provide a certain theoretical basis for the biotherapy of IDD.

Misaligned spinal rods can induce high internal forces consistent with those observed to cause screw pullout and disc degeneration

BACKGROUND CONTEXT

Manual contouring of spinal rods is often required intraoperatively for proper alignment of the rods within the pedicle screw heads. Residual misalignments are frequently reduced by using dedicated reduction devices. The forces exerted by these devices, however, are uncontrolled and may lead to excessive reaction forces. As a consequence, screw pullout might be provoked and surrounding tissue may experience unfavorable biomechanical loads. The corresponding loads and induced tissue deformations are however not well identified. Additionally, whether the forced reduction alters the biomechanical behavior of the lumbar spine during physiological movements postoperatively, remains unexplored.

PURPOSE

To predict whether the reduction of misaligned posterior instrumentation might result in clinical complications directly after reduction and during a subsequent physiological flexion movement.

STUDY DESIGN

Finite element analysis.

METHODS

A patient-specific, total lumbar (L1-S1) spine finite element model was available from previous research. The model consists of poro-elastic intervertebral discs with Pfirrmann grade-dependent material parameters, with linear elastic bone tissue with stiffness values related to the local bone density, and with the seven major ligaments per spinal motion segment described as nonlinear materials. Titanium instrumentation was implemented in this model to simulate a L4, L5, and S1 posterolateral fusion. Next, coronal and sagittal misalignments of 6 mm each were introduced between the rod and the screw head at L4. These misalignments were computationally reduced and a physiological flexion movement of 15° was prescribed. Non-instrumented and well-aligned instrumented models were used as control groups.

RESULTS

Pulling forces up to 1.0 kN were required to correct the induced misalignments of 6 mm. These forces affected the posture of the total lumbar spine, as motion segments were predicted to rotate up to 3 degrees and rotations propagated proximally to and even affect the L1-2 level. The facet contact pressures in the corrected misaligned models were asymmetrical suggesting non-physiological joint loading in the misaligned models. In addition, the discs and vertebrae experienced abnormally high forces as a result of the correction procedure. These effects were more pronounced after a 15° flexion movement following forced reduction.

CONCLUSIONS

The results of this study indicate that the correction of misaligned posterior instrumentation can result in high forces at the screws consistent with those reported to cause screw pullout, and may cause high-tissue strains in adjacent and downstream spinal segments.

CLINICAL SIGNIFICANCE

Proper alignment of spinal posterior instrumentation may reduce clinical complications secondary to unfavorable biomechanics.

Multiscale structural characterization of the vertebral endplate in animal models

Research in the field of spinal biomechanics, including analyses of the impact of implants on the stability of the spine, is conducted extensively in animal models. One of the basic problems in spinal implantation is the transfer and distribution of loads carried by the spine on the surfaces of the vertebral bodies. An important factor in proper cooperation of spinal implants with the vertebrae is the endplate (EP), which is why the EP in the animal model used for testing should be as similar as possible to the human EP. Therefore, this study involved multiscale structural and morphometric analyses of the animal models most commonly used in spinal biomechanics research, i.e. pig, ovine, and bovine tail. The tests were performed on 28 lumbar porcine, ovine, and bovine vertebrae. Both cranial and caudal EPs were analysed in three selected areas: anterior, middle, and posterior EPs. The conducted tests included a morphometric analysis of the trabecular bone (TB) layer of the EP as well as microscopic analysis at the mesoscale (total thickness) and microscale (thickness of the individual EP layers). The porcine EP had a characteristic increased circumferential thickness (~3 mm) with a significant narrowing in the central region (50%-60%). The convex cranial ovine EP had a constant thickness throughout the cross-section and the concave caudal EP showed ~35% narrowing in the central region. The thickest EPs were observed in the bovine tail model with negligibly small narrowing in the central region (~5%). The thickness of the cartilaginous layer in the porcine and bovine models reached up to 1 mm in the peripheral regions and decreased in the central part. The growth plate layer had a similar thickness in all the models. On the other hand, the narrowing of the total thickness of the EPs in the central region was mainly due to a decrease in the VEP thickness. In the ovine and bovine models, the central region of the EP was characterized by large isotropy and trabeculae of mixed or rod-like shape. By contrast, in the pig, this region had plate-like trabeculae of anisotropic nature. The porcine model was identified as best reflecting the shape and structure of the human EP and as the best surrogate model for the human EP model. This choice is particularly important in the context of biomechanical research.

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.

METTL3 promotes IL-1β-induced degeneration of endplate chondrocytes by driving m6A-dependent maturation of miR-126-5p

METTL3 is an important regulatory molecule in the process of RNA biosynthesis. It mainly regulates mRNA translation, alternative splicing and microRNA maturation by mediating m6A‐dependent methylation. Interleukin 1β (IL‐1β) is an important inducer of cartilage degeneration that can induce an inflammatory cascade reaction in chondrocytes and inhibit the normal biological function of cells. However, it is unclear whether IL‐1β is related to METTL3 expression or plays a regulatory role in endplate cartilage degeneration. In this study, we found that the expression level of METTL3 and methylation level of m6A in human endplate cartilage with different degrees of degeneration were significantly different, indicating that the methylation modification of m6A mediated by METTL3 was closely related to the degeneration of human endplate cartilage. Next, through a series of functional experiments, we found that miR‐126‐5p can play a significant role in IL‐1β–induced degeneration of endplate chondrocytes. Moreover, we found that miR‐126‐5p can inhibit the PI3K/Akt signalling pathway by targeting PIK3R2 gene, leading to the disorder of cell vitality and functional metabolism. To further determine whether METTL3 could regulate miR‐126‐5p maturation, we first confirmed that METTL3 can bind the key protein underlying pri‐miRNA processing, DGCR8. Additionally, when METTL3 expression was inhibited, the miR‐126‐5p maturation process was blocked. Therefore, we hypothesized that METTL3 can promote cleavage of pri‐miR‐126‐5p and form mature miR‐126‐5p by combining with DGCR8.

Curcumin prevents tension-induced endplate cartilage degeneration by enhancing autophagy

AIMS

Intermittent cyclic tension stimulation(ICMT) was shown to promote degeneration of endplate chondrocytes and induce autophagy. However, enhancing autophagy can alleviate degeneration partly. Studies have shown that curcumin can induce autophagy and protect chondrocytes, we speculated that regulation of autophagy by curcumin might be an effective method to improve the stress resistance of endplate cartilage. In this study, human cervical endplate cartilage specimens were collected, and expression of autophagy markers was detected and compared.

MAIN METHODS

Human cervical endplate chondrocytes were cultured to establish a tension-induced degeneration model, for which changes of functional metabolism and autophagy levels were detected under different tension loading conditions. Changes in functional metabolism of endplate chondrocytes were observed under high-intensity tension loading in the presence of inhibitors, inducers, and curcumin to regulate the autophagy level of cells. In addition, a rat model of lumbar instability was established to observe the degeneration of lumbar disc after curcumin administration.

KEY FINDINGS

Through a series of experiments, we found that low-intensity tension stimulation can maintain a stable phenotype of endplate chondrocytes, but high-intensity tension stimulation has a negative effect. Moreover, with increasing tension intensity, the degree of degeneration of endplate chondrocytes was gradually aggravated and the level of autophagy increased. Besides, curcumin upregulated autophagy, inhibited apoptosis, and reduced phenotype loss of endplate chondrocytes induced by high-intensity tension loading, thereby relieving intervertebral disc degeneration induced by mechanical imbalance.

SIGNIFICANCE

Curcumin mediated autophagy and enhanced the adaptability of endplate chondrocytes to high-intensity tension load, thereby relieving intervertebral disc degeneration.

Exosomes Derived from Bone Marrow Mesenchymal Stem Cells Prevent Acidic pH-Induced Damage in Human Nucleus Pulposus Cells

Background

The exosomes (Exo) derived from mesenchymal stem cells (MSCs) are capable of attenuating the apoptosis of nucleus pulposus cells (NPCs) elicited by proinflammatory cytokines. However, it remains unknown whether MSC-derived Exo also exert a protective effect on NPCs in the pathological acid environment.

Material/Methods

NPCs were divided into 3 groups: Group A, pH 7.1–7.3; Group B, pH 6.5–6.7 and Group C, pH 5.9–6.1. The NPCs were cultured in the above-defined acidic medium, and 3 different amounts of Exo were added into the media. Finally, the expression of the caspase-3, aggrecan, collagen II, and MMP-13 was analyzed and compared among the different groups.

Results

Compared with cells cultured at pH 7.1–7.3 (Group A), proliferation activity of NPCs cultured at pH 5.9–6.7 (Group B and C) decreased significantly. Collagen II and aggrecan expression was also obviously reduced with the decrease of cell proliferation.

Conversely, the expression of caspase-3 and MMP-13 significantly increased. Further experiments showed that proliferation activity was significantly attenuated in NPCs cultured at pH 5.9–6.1 without Exo treatment (Group E) compared with those cultured at pH 7.1–7.3 without Exo treatment (Group D).

Conclusions

In the pathological acid environment, MSC-derived Exo promotes the expression of chondrocyte extracellular matrix, collagen II, and aggrecan, and reduces matrix degradation by downregulating matrix-degrading enzymes, protecting NPCs from acidic pH-induced apoptosis. This study reveals a promising strategy for treatment of IVD degeneration.

[Research progress of hydrogel used for regeneration of nucleus pulposus in intervertebral disc degeneration]

Objective

To summarize the research progress of hydrogels for the regeneration and repair of degenerative intervertebral disc and to investigate the potential of hydrogels in clinical application.

Methods

The related literature about the role of hydrogels in intervertebral disc degeneration especially for nucleus pulposus was reviewed and analyzed.

Results

Hydrogels share similar properties with nucleus pulposus, and it plays an important role in the regeneration and repair of degenerative intervertebral disc, which can be mainly applied in nucleus pulposus prosthesis, hydrogel-based cell therapy, non-cellular therapy, and tissue engineering repair.

Conclusion

Hydrogels are widely used in the regeneration and repair of intervertebral disc, which provides a potential treatment for intervertebral disc degeneration.

The protective effects of PI3K/Akt pathway on human nucleus pulposus mesenchymal stem cells against hypoxia and nutrition deficiency

BACKGROUND

To study the effects of hypoxia and nutrition deficiency mimicking degenerated intervertebral disc on the biological behavior of human nucleus-derived pulposus mesenchymal stem cells (hNP-MSCs) and the role of PI3K/Akt pathway in the process in vitro.

METHODS

hP-MSCs were isolated from lumbar disc and were further identified by their immunophenotypes and multilineage differentiation. Then, cells were divided into the control group, hypoxia and nutrition deficiency group, the LY294002 group, and insulin-like growth factor 1 (IGF-1) group. Then cell apoptosis, the cell viability, the caspase 3 activity, and the expression of PI3K, Akt, and functional genes (aggrecan, collagen I, and collagen II) were evaluated.

RESULT

Our work showed that isolated cells met the criteria of International Society for cellular Therapy. Therefore, cells obtained from degenerated nucleus pulposus were definitely hNP-MSCs. Our results showed that hypoxia and nutrition deficiency could significantly increase cell apoptosis, the caspase 3 activity, and inhibit cell viability. Gene expression results demonstrated that hypoxia and nutrition deficiency could increase the relative expression of PI3K and Akt gene and inhibit the expression of functional genes. However, when the PI3K/Akt pathway was inhibited by LY294002, the cell apoptosis and caspase 3 activity significantly increased while the cell viability was obviously inhibited. Quantitative real-time PCR results showed that the expression of functional genes was more significantly inhibited. Our study further verified that the above-mentioned biological activities of hNP-MSCs could be significantly improved by IGF1.

CONCLUSIONS

PI3K/Akt signal pathway may have protective effects on human nucleus pulposus-derived mesenchymal stem cells against hypoxia and nutrition deficiency.

A new framework for characterization of poroelastic materials using indentation

To characterize a poroelastic material, typically an indenter is pressed onto the surface of the material with a ramp of a finite approach velocity followed by a hold where the indenter displacement is kept constant. This leads to deformation of the porous matrix, pressurization of the interstitial fluid and relaxation due to redistribution of fluid through the pores. In most studies the poroelastic properties, including elastic modulus, Poisson ratio and poroelastic diffusion coefficient, are extracted by assuming an instantaneous step indentation. However, exerting step like indentation is not experimentally possible and usually a ramp indentation with a finite approach velocity is applied. Moreover, the poroelastic relaxation time highly depends on the approach velocity in addition to the poroelastic diffusion coefficient and the contact area. Here, we extensively studied the effect of indentation velocity using finite element simulations which has enabled the formulation of a new framework based on a master curve that incorporates the finite rise time. To verify our novel framework, the poroelastic properties of two types of hydrogels were extracted experimentally using indentation tests at both macro and micro scales. Our new framework that is based on consideration of finite approach velocity is experimentally easy to implement and provides a more accurate estimation of poroelastic properties. STATEMENT OF SIGNIFICANCE: Hydrogels, tissues and living cells are constituted of a sponge-like porous elastic matrix bathed in an interstitial fluid. It has been shown that these materials behave according to the theory of 'poroelasticity' when mechanically stimulated in a way similar to that experienced in organs within the body. In this theory, the rate at which the fluid-filled sponge can be deformed is limited by how fast interstitial fluid can redistribute within the sponge in response to deformation. Here, we simulated indentation experiments at different rates and formulated a new framework that inherently captures the effects of stimulation speed on the mechanical response of poroelastic materials. We validated our framework by conducting experiments at different length-scales on agarose and polyacrylamide hydrogels.

Assessment of changes in the micro-nano environment of intervertebral disc degeneration based on Pfirrmann grade

BACKGROUND CONTEXT

Pfirrmann grading can be used to assess intervertebral disc degeneration (IVDD). There is growing evidence that IVDD is not simply a structural disorder but also involves changes to the substructural characteristics of the disc. Whether Pfirrmann grade can accurately represent these micro-nano environmental changes remains unclear.

PURPOSE

We aimed to assess the micro-nano structural characteristics of the degenerative disc to provide more specific biomechanical information than the Pfirrmann score.

STUDY DESIGN

A micro- and nano-level structural analysis of degenerative discs of rat tails.

METHODS

In this study, 12-week-old adult male Sprague-Dawley rats were divided randomly into five groups: control (no intervention to the intervertebral disc of the tail) and four intervention groups that all had caudal vertebrae immobilized using a custom-made external device to fix four caudal vertebrae (Co7-Co10) but with variable subsequent compression of Co8 and Co9 for 2, 4, 6, or 8 weeks. Magnetic resonance imaging detection of rat coccygeal vertebrae was conducted at each time node of the experiment, and the T2 signal intensity and disc space were evaluated. Animals were euthanized and the caudal vertebrae were harvested for further analysis. Histopathology, glycosaminoglycan (GAG) content, histologic score, end plate structure, and elastic modulus of the intervertebral discs were evaluated.

RESULTS

IVDD was observed at an earlier Pfirrmann grade (Pfirrmann II) under the microscope. With an increase in Pfirrmann grade to III-V, the pore structure of the bony end plate changed significantly and the number of pores decreased gradually. Furthermore, the total GAG content of the nucleus pulposus decreased from an average of 640.33 μg GAG/ng DNA in Pfirrmann grade I to 271.33 μg GAG/ng DNA in Pfirrmann grade V (p < .0001). At the early stage of clinical degeneration of intervertebral discs (Pfirrmann grades II and III), there were significant changes in mechanical properties of the outer annulus fibrosus compared with the inner layer (p < .05). Further, the fibril diameters exhibited significant changes compared with the control group (p < .05).

CONCLUSIONS

Our study found that the Pfirrmann grading system combined with intervertebral disc micro-nano structural changes more comprehensively reflected the extent of disc degeneration. These data may help improve our understanding of the pathogenesis and process of clinical disc degeneration.

Upregulation of miR-107 expression following hyperbaric oxygen treatment suppresses HMGB1/RAGE signaling in degenerated human nucleus pulposus cells

Background

The expression of both high-mobility group box 1 (HMGB1) and receptor for advanced glycation end-products (RAGE) is upregulated in degenerated discs. HMGB1 is known to function as a coupling factor between hypoxia and inflammation in arthritis, and this inflammatory response is modulated by microRNAs (miRNAs), with miR-107 expression downregulated during hypoxia. In this study, we investigated the regulation of the miR-107/HMGB1/RAGE pathway in degenerated nucleus pulposus cells (NPCs) after hyperbaric oxygen (HBO) treatment.

Methods

NPCs were separated from human degenerated intervertebral disc tissues. The control cells were maintained in 5% CO₂/95% air, and the hyperoxic cells were exposed to 100% O₂ at 2.5 atmospheres absolute. MiRNA expression profiling was performed via microarray and confirmed by real-time PCR, and miRNA target genes were identified using bioinformatics and luciferase reporter assays. The cellular protein and mRNA levels of HMGB1, RAGE, and inducible nitric oxide synthase (iNOS) were assessed, and the phosphorylation of MAPK (p38MAPK, ERK, and JNK) was evaluated. Additionally, cytosolic and nuclear fractions of the IκBα and NF-κB p65 proteins were analyzed, and secreted HMGB1 and metalloprotease (MMP) levels in the conditioned media were quantified.

Results

Using microarray analyses, 96 miRNAs were identified as upregulated and 66 downregulated following HBO treatment. Based on these results, miR-107 was selected for further investigation. Bioinformatics analyses indicated that the 3′ untranslated region of the HMGB1 mRNA contained the “seed-matched-sequence” for hsa-miR-107, which was validated via dual-luciferase reporter assays. MiR-107 was markedly induced by HBO, and simultaneous suppression of HMGB1 was observed in NPCs. Knockdown of miR-107 resulted in upregulation of HMGB1 expression in HBO-treated cells, and HBO treatment downregulated the mRNA and protein levels of HMGB1, RAGE, and iNOS and the secretion of HMGB1. In addition, HBO treatment upregulated the protein levels of cytosolic IκBα and decreased the nuclear translocation of NF-κB in NPCs. Moreover, HBO treatment downregulated the phosphorylation of p38MAPK, ERK, and JNK and significantly decreased the secretion of MMP-3, MMP-9, and MMP-13.

Conclusions

HBO inhibits pathways related to HMGB1/RAGE signaling via upregulation of miR-107 expression in degenerated human NPCs.

Theoretical Explorations Generate New Hypotheses About the Role of the Cartilage Endplate in Early Intervertebral Disk Degeneration

Altered cell nutrition in the intervertebral disk (IVD) is considered a main cause for disk degeneration (DD). The cartilage endplate (CEP) provides a major path for the diffusion of nutrients from the peripheral vasculature to the IVD nucleus pulposus (NP). In DD, sclerosis of the adjacent bony endplate is suggested to be responsible for decreased diffusion and disk cell nutrition. Yet, experimental evidence does not support this hypothesis. Hence, we evaluated how moderate CEP composition changes related to tissue degeneration can affect disk nutrition and cell viability. A novel composition-based permeability formulation was developed for the CEP, calibrated, validated, and used in a mechano-transport finite element IVD model. Fixed solute concentrations were applied at the outer surface of the annulus and the CEP, and three cycles of daily mechanical load were simulated. The CEP model indicated that CEP permeability increases with the degeneration/aging of the tissue, in accordance with recent measurements reported in the literature. Additionally, our results showed that CEP degeneration might be responsible for mechanical load-induced NP dehydration, which locally affects oxygen and lactate levels, and reduced glucose concentration by 16% in the NP-annulus transition zone. Remarkably, CEP degeneration was a condition sine-qua-non to provoke cell starvation and death, while simulating the effect of extracellular matrix depletion in DD. This theoretical study cast doubts about the paradigm that CEP calcification is needed to provoke cell starvation, and suggests an alternative path for DD whereby the early degradation of the CEP plays a key role.

TGF-β/SMAD signaling inhibits intermittent cyclic mechanical tension-induced degeneration of endplate chondrocytes by regulating the miR-455-5p/RUNX2 axis

A mechanical stimulation plays a pivotal role in maintaining normal cartilage function. Our objective was to reveal the mechanism of action of the tension-sensitive molecule miR-455-5p in the degeneration of endplate chondrocytes and to identify whether the transforming growth factor beta (TGF-β)/SMAD signaling pathway has a regulatory effect on it. The expression profiles of members of the TGF-β/SMAD pathway, miR-455-5p, and RUNX2 were determined by microRNA microarray analysis, reverse transcription quantitative polymerase chain reaction, luciferase reporter assay, and Western blot analysis. Intermittent cyclic mechanical tension (ICMT) induced the degeneration of endplate chondrocytes without affecting their viability. The tension-sensitive molecule miR-455-5p specifically bound to RUNX2, a gene involved in the degeneration of endplate chondrocytes. Activation of the TGF-β/SMAD signaling pathway upregulated miR-455-5p expression and thus inhibited RUNX2 levels. Therefore, the TGF-β/SMAD signaling pathway inhibits the ICMT-induced degeneration of endplate chondrocytes by regulating the miR-455-5p/RUNX2 axis.

Ciliary parathyroid hormone signaling activates transforming growth factor-β to maintain intervertebral disc homeostasis during aging

Degenerative disc disease (DDD) is associated with intervertebral disc degeneration of spinal instability. Here, we report that the cilia of nucleus pulposus (NP) cells mediate mechanotransduction to maintain anabolic activity in the discs. We found that mechanical stress promotes transport of parathyroid hormone 1 receptor (PTH1R) to the cilia and enhances parathyroid hormone (PTH) signaling in NP cells. PTH induces transcription of integrin α_vβ₆ to activate the transforming growth factor (TGF)-β-connective tissue growth factor (CCN2)-matrix proteins signaling cascade. Intermittent injection of PTH (iPTH) effectively attenuates disc degeneration of aged mice by direct signaling through NP cells, specifically improving intervertebral disc height and volume by increasing levels of TGF-β activity, CCN2, and aggrecan. PTH1R is expressed in both mouse and human NP cells. Importantly, knockout PTH1R or cilia in the NP cells results in significant disc degeneration and blunts the effect of PTH on attenuation of aged discs. Thus, mechanical stress-induced transport of PTH1R to the cilia enhances PTH signaling, which helps maintain intervertebral disc homeostasis, particularly during aging, indicating therapeutic potential of iPTH for DDD.

Sensory structures found in the jelly-like space between spinal discs release a hormone that helps preserve back health in aging mice. Xu Cao from Johns Hopkins University in Baltimore, Maryland, USA, and colleagues observed that levels of a critical growth factor declined in the space between adjacent vertebrae as mice aged, and that injecting a naturally occurring hormone that activates this growth factor could attenuate disc degeneration in older animals. The researchers showed, in response to mechanical stresses, receptor proteins that respond to this hormone relocate themselves to particular sensory organelles known as cilia that found within cells of the intervertebral core. That results in elevated hormone signaling—and drugs designed to have the same effect could help treat degenerative disc disease, one of the most common causes of chronic back pain.

Biological Behavior of Human Nucleus Pulposus Mesenchymal Stem Cells in Response to Changes in the Acidic Environment During Intervertebral Disc Degeneration

An acidic environment is vital for the maintenance of cellular activities but can be affected tremendously during intervertebral disc degeneration (IVDD). The effect of changes in the acidity of the environment on human nucleus pulposus mesenchymal stem cells (NP-MSCs) is, however, unknown. Thus, this study aimed to observe the biological effects of acidic conditions mimicking a degenerated intervertebral disc on NP-MSCs in vitro. NP-MSCs were isolated from patients with lumbar disc herniation and were further identified by their immunophenotypes and multilineage differentiation. Then, cells were cultured at acidic pH levels (pH 6.2, pH 6.5, pH 6.8, pH 7.1, and pH 7.4) with/without amiloride, an acid-sensing ion channel (ASIC) blocker. The proliferation and apoptosis of NP-MSCs and the expression of stem cell-related genes (Oct4, Nanog, Jagged, Notch1), ASICs, and functional genes (Aggrecan, SOX-9, Collagen-I, and Collagen-II) in NP-MSCs were evaluated. Our work showed that cells obtained from human degenerated NP met the criteria of International Society for Cellular Therapy. Therefore, cells obtained from a degenerated nucleus pulposus were definitively identified as NP-MSCs. Our results also indicated that acidic conditions could significantly inhibit cell proliferation and increase cell apoptosis. Gene expression results demonstrated that acidic conditions could decrease the expression of stem cell-related genes and inhibit extracellular matrix synthesis, whereas it could increase the expression of ASICs. Our study further verified that the above-mentioned biological activities of NP-MSCs could be significantly improved by amiloride. Therefore, the results of the study indicated that the biological behavior of NP-MSCs could be inhibited by acidic conditions during IVDD, and amiloride may meliorate IVDD by improving the activities of NP-MSCs.

Intermittent Cyclic Mechanical Tension Promotes Degeneration of Endplate Cartilage via the Nuclear Factor-κB Signaling Pathway: an in Vivo Study

OBJECTIVE

To establish a rabbit model for investigating the effects of intermittent cyclic mechanical tension (ICMT) on promoting degeneration of endplate cartilage.

METHODS

Forty New Zealand white rabbits were subjected to surgery and randomly divided into three equal groups as follows: control group (no treatment, n = 10), sham group (animals underwent a sham operation but were not subjected to mechanical tensile strain, n = 15) and loaded group (discs were subjected to 1.5 MPa external tensile loading by using an external loading device during the animals' daily activity, n = 15). Mechanical tensile strain was applied for 8 h/d. The animals were examined radiologically after 8 weeks treatment and then killed for removal of endplate cartilage tissue samples from their spines. Histological staining was performed to examine the morphology of endplate cartilage tissue. Multiple strategies were employed to examine degeneration of endplate cartilage and nuclear factor (NF)-κB signaling pathway activation.

RESULTS

After ICMT loading for 56 days, radiology revealed ossification, hyperosteogeny and stenosis in the intervertebral spaces. Examination of hematoxylin and eosin staining of sections of endplate cartilage showed significant damage as the load duration increased in the ICMT loading group. Expression of aggrecan (ACAN), type II collagen (COL-2A), SRY-related high mobility group-box gene 9 (SOX9) was down-regulated (FACAN = 21.515, P < 0.01; FCOL-2A = 6.670, P = 0.05; FSOX9 = 7.888, P < 0.05), whereas that of matrix metallopeptidase 13 (MMP13) was up-regulated (FMMP13 = 14.120, P < 0.01) after ICMT. Western blot and immunofluorescence revealed that expression of protein was consistent with gene expression results. Additionally, ICMT loading can lead to NF-κB signaling pathway activation as well as degeneration of endplate cartilage.

CONCLUSION

These experiments indicate that ICMT contributes to the activation of NF-κB signaling pathway in vivo and that the NF-κB signaling pathway further up-regulates MMP13, leading to degeneration of endplate cartilage.

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.

3D characterization of morphological changes in the intervertebral disc and endplate during aging: A propagation phase contrast synchrotron micro-tomography study

A better understanding of functional changes in the intervertebral disc (IVD) and interaction with endplate is essential to elucidate the pathogenesis of IVD degeneration disease (IDDD). To date, the simultaneous depiction of 3D micro-architectural changes of endplate with aging and interaction with IVD remains a technical challenge. We aim to characterize the 3D morphology changes of endplate and IVD during aging using PPCST. The lumbar vertebral level 4/5 IVDs harvested from 15-day-, 4- and 24-month-old mice were initially evaluated by PPCST with histological sections subsequently analyzed to confirm the imaging efficiency. Quantitative assessments of age-related trends after aging, including mean diameter, volume fraction and connectivity of the canals, and endplate porosity and thickness, reached a peak at 4 months and significantly decreased at 24 months. The IVD volume consistently exhibited same trend of variation with the endplate after aging. In this study, PPCST simultaneously provided comprehensive details of 3D morphological changes of the IVD and canal network in the endplate and the interaction after aging. The results suggest that PPCST has the potential to provide a new platform for attaining a deeper insight into the pathogenesis of IDDD, providing potential therapeutic targets.

Construction Strategy and Progress of Whole Intervertebral Disc Tissue Engineering

Degenerative disc disease (DDD) is the major cause of low back pain, which usually leads to work absenteeism, medical visits and hospitalization. Because the current conservative procedures and surgical approaches to treatment of DDD only aim to relieve the symptoms of disease but not to regenerate the diseased disc, their long-term efficiency is limited. With the rapid developments in medical science, tissue engineering techniques have progressed markedly in recent years, providing a novel regenerative strategy for managing intervertebral disc disease. However, there are as yet no ideal methods for constructing tissue-engineered intervertebral discs. This paper reviews published reports pertaining to intervertebral disc tissue engineering and summarizes data concerning the seed cells and scaffold materials for tissue-engineered intervertebral discs, construction of tissue-engineered whole intervertebral discs, relevant animal experiments and effects of mechanics on the construction of tissue-engineered intervertebral disc and outlines the existing problems and future directions. Although the perfect regenerative strategy for treating DDD has not yet been developed, great progress has been achieved in the construction of tissue-engineered intervertebral discs. It is believed that ongoing research on intervertebral disc tissue engineering will result in revolutionary progress in the treatment of DDD.

Human cartilage endplate permeability varies with degeneration and intervertebral disc site

Despite the critical functions the human cartilage endplate (CEP) plays in the intervertebral disc, little is known about its structural and mechanical properties and their changes with degeneration. Quantifying these changes with degeneration is important for understanding how the CEP contributes to the function and pathology of the disc. Therefore the objectives of this study were to quantify the effect of disc degeneration on human CEP mechanical properties, determine the influence of superior and inferior disc site on mechanics and composition, and simulate the role of collagen fibers in CEP and disc mechanics using a validated finite element model. Confined compression data and biochemical composition data were used in a biphasic-swelling model to calculate compressive extrafibrillar elastic and permeability properties. Tensile properties were obtained by applying published tensile test data to an ellipsoidal fiber distribution. Results showed that with degeneration CEP permeability decreased 50-60% suggesting that transport is inhibited in the degenerate disc. CEP fibers are organized parallel to the vertebrae and nucleus pulposus and may contribute to large shear strains (0.1-0.2) and delamination failure of the CEP commonly seen in herniated disc tissue. Fiber-reinforcement also reduces CEP axial strains thereby enhancing fluid flux by a factor of 1.8. Collectively, these results suggest that the structure and mechanics of the CEP may play critical roles in the solute transport and disc mechanics.

Involvement of acid-sensing ion channel 1a in matrix metabolism of endplate chondrocytes under extracellular acidic conditions through NF-κB transcriptional activity

Acidic conditions are present in degenerated intervertebral discs and are believed to be responsible for matrix breakdown. Acid-sensing ion channel 1a (ASIC1a) is expressed in endplate chondrocytes, and its activation is associated with endplate chondrocyte apoptosis. However, the precise role of ASIC1a in regulating the matrix metabolic activity of endplate chondrocytes in response to extracellular acid remains poorly understood. Aggrecan (ACAN), type II collagen (Col2a1), and matrix metalloproteinase (MMP) expressions were determined using reverse transcription (RT)-PCR and Western blot. ASIC1a was knocked down by transfecting endplate chondrocytes with ASIC1a siRNA. MMP activity and NF-κB transcriptional activity were measured. NF-κB transcriptional activity was assessed by examining cytosolic phosphorylated IκBα and nuclear phosphorylated p65 levels. Extracellular acidic solution (pH 6.0) resulted in a decrease in ACAN and Co12a1 expressions and an increase in MMP-1, MMP-9, and MMP-13 expressions, as well as in MMP activity; while ASIC1a siRNA blocked these effects. In addition, acid-induced increase in cytosolic levels of phosphorylated IκBα and nuclear levels of phosphorylated p65 in endplate chondrocytes were inhibited by ASIC1a siRNA. ASIC1a is involved in matrix metabolism of endplate chondrocytes under extracellular acidic conditions via NF-κB transcriptional activity.

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.

Altered swelling and ion fluxes in articular cartilage as a biomarker in osteoarthritis and joint immobilization: a computational analysis

In healthy cartilage, mechano-electrochemical phenomena act together to maintain tissue homeostasis. Osteoarthritis (OA) and degenerative diseases disrupt this biological equilibrium by causing structural deterioration and subsequent dysfunction of the tissue. Swelling and ion flux alteration as well as abnormal ion distribution are proposed as primary indicators of tissue degradation. In this paper, we present an extension of a previous three-dimensional computational model of the cartilage behaviour developed by the authors to simulate the contribution of the main tissue components in its behaviour. The model considers the mechano-electrochemical events as concurrent phenomena in a three-dimensional environment. This model has been extended here to include the effect of repulsion of negative charges attached to proteoglycans. Moreover, we have studied the fluctuation of these charges owning to proteoglycan variations in healthy and pathological articular cartilage. In this sense, standard patterns of healthy and degraded tissue behaviour can be obtained which could be a helpful diagnostic tool. By introducing measured properties of unhealthy cartilage into the computational model, the severity of tissue degeneration can be predicted avoiding complex tissue extraction and subsequent in vitro analysis. In this work, the model has been applied to monitor and analyse cartilage behaviour at different stages of OA and in both short (four, six and eight weeks) and long-term (11 weeks) fully immobilized joints. Simulation results showed marked differences in the corresponding swelling phenomena, in outgoing cation fluxes and in cation distributions. Furthermore, long-term immobilized patients display similar swelling as well as fluxes and distribution of cations to patients in the early stages of OA, thus, preventive treatments are highly recommended to avoid tissue deterioration.

On the relative relevance of subject-specific geometries and degeneration-specific mechanical properties for the study of cell death in human intervertebral disk models

Capturing patient- or condition-specific intervertebral disk (IVD) properties in finite element models is outmost important in order to explore how biomechanical and biophysical processes may interact in spine diseases. However, disk degenerative changes are often modeled through equations similar to those employed for healthy organs, which might not be valid. As for the simulated effects of degenerative changes, they likely depend on specific disk geometries. Accordingly, we explored the ability of continuum tissue models to simulate disk degenerative changes. We further used the results in order to assess the interplay between these simulated changes and particular IVD morphologies, in relation to disk cell nutrition, a potentially important factor in disk tissue regulation. A protocol to derive patient-specific computational models from clinical images was applied to different spine specimens. In vitro, IVD creep tests were used to optimize poro-hyperelastic input material parameters in these models, in function of the IVD degeneration grade. The use of condition-specific tissue model parameters in the specimen-specific geometrical models was validated against independent kinematic measurements in vitro. Then, models were coupled to a transport-cell viability model in order to assess the respective effects of tissue degeneration and disk geometry on cell viability. While classic disk poro-mechanical models failed in representing known degenerative changes, additional simulation of tissue damage allowed model validation and gave degeneration-dependent material properties related to osmotic pressure and water loss, and to increased fibrosis. Surprisingly, nutrition-induced cell death was independent of the grade-dependent material properties, but was favored by increased diffusion distances in large IVDs. Our results suggest that in situ geometrical screening of IVD morphology might help to anticipate particular mechanisms of disk degeneration.