The protective role of the pericellular matrix in chondrocyte apoptosis

Matrix metalloproteinases accelerate pericellular matrix breakdown and disrupt mechanotransduction in osteoarthritis

The pericellular matrix (PCM) is a specialized, narrow matrix surrounding each chondrocyte in articular cartilage, together constituting the chondron - the fundamental metabolic and functional unit of cartilage. The PCM plays a vital role in mediating biomechanical and biochemical signals essential for chondrocyte function. In osteoarthritis (OA), a chronic joint disorder characterized by progressive cartilage degradation, the PCM is one of the earliest sites of catabolic degradation, primarily driven by matrix metalloproteinases (MMPs). This study aims to investigate the functional relationship between PCM degradation and chondrocyte mechanosignaling, with an emphasis on MMP-driven changes in mechanotransduction in osteoarthritic cartilage. Human chondrons (N = 64) were incubated with MMP-2, MMP-3, and MMP-7, and structural changes were assessed histologically by evaluating perlecan and collagen type VI. Cellular elasticity was measured using atomic force microscopy (AFM), and mechanically evoked intracellular Ca2+ transients were assessed via AFM single-cell indentations (500 nN). All three MMPs induced pronounced catabolic effects on the PCM structure, showing distinct impacts on collagen type VI and perlecan, as well as on the biomechanical properties (p < 0.001). MMP-driven alterations in PCM integrity significantly reduced the Ca2+ transients of chondrons in response to mechanical stimuli (p < 0.001). While TRPV4 activation was elevated in intact chondrons, PIEZO channels were involved in mechanotransduction in both healthy and MMP-treated chondrons. In osteoarthritic stages, the mechanotransduction dynamics shifted significantly towards PIEZO channels. This study elucidates the interplay between MMP-mediated PCM degradation, structural-functional dynamics, and chondrocyte mechanotransduction, underscoring the critical role of the PCM in maintaining normal chondrocyte functionality and mechanosensing. STATEMENT OF SIGNIFICANCE: Osteoarthritis (OA) is a prevalent degenerative joint disease affecting millions worldwide. Central to its pathology is the degradation of the pericellular matrix (PCM) by matrix metalloproteinases (MMPs), which disrupts chondrocyte mechanotransduction, altering cellular responses to mechanical stimuli. This study explores the impact of MMP-2, MMP-3, and MMP-7 on PCM structure and chondrocyte mechanosensing. Our results reveal that MMP-induced degradation significantly compromises PCM structural integrity, leading to altered mechanotransduction dynamics in chondrocytes. Degradation specifically redirects the primary function of ion channels from TRPV4 to PIEZO channels in cells impacted by MMPs. This highlights the interplay between MMP-mediated PCM degradation, chondrocyte mechanotransduction and as thus structural-functional dynamics, underscoring the critical role of the PCM in maintaining normal chondrocyte functionality and mechanosensing.

Identification and culture of meniscons, meniscus cells with their pericellular matrix

BACKGROUND AIMS

Meniscus injury is highly debilitating and often results in osteoarthritis. Treatment is generally symptomatic; no regenerative treatments are available. "Chondrons," articular chondrocytes with preserved pericellular matrix, produce more hyaline cartilage extracellular matrix and improve cartilage repair. If meniscons exist in the meniscus and have similar therapeutic potential as chondrons, employing these cells has potential for meniscus cell therapy and tissue engineering. In this study, we isolated and cultured "meniscons," meniscus cells surrounded by their native pericellular matrix, and investigated cell behavior in culture compared with chondrons.

METHODS

Human meniscons were enzymatically isolated from osteoarthritic menisci and cultured up to 28 days in fibrin glue. Freshly isolated meniscons and chondrons were analyzed by histology and transmission electron microscopy. We used 5-([4,6-dichlorotriazin-2-yl]amino)fluorescein hydrochloride labeling and type VI collagen immunohistochemistry to image pericellular matrix after 0 and 28 days of culture. Gene expression was quantified using real-time polymerase chain reaction and DNA content and proteoglycan production were analyzed using biochemical assays.

RESULTS

Meniscons were successfully isolated from human meniscus tissue. The pericellular matrix of meniscons and chondrons was preserved during 28 days of culture. Meniscons and chondrons had similar cell proliferation and proteoglycan production. Meniscons and chondrons expressed similar levels of collagen type I alpha 1 chain, whereas collagen type II alpha 1 chain and aggrecan expression was lower in the meniscon population.

CONCLUSIONS

Freshly isolated meniscons and meniscons cultured for 28 days share similarities with chondrons with regard to cell proliferation, morphology and biochemical activity. Rapid isolation of meniscons (45 min) demonstrates potential for one-stage meniscus regeneration and repair, which should be confirmed in vivo.

Cartilage-derived cells display heterogeneous pericellular matrix synthesis in agarose microgels

The pericellular matrix (PCM) surrounding chondrocytes is essential for articular cartilage tissue engineering. As the current isolation methods to obtain chondrocytes with their PCM (chondrons) result in a heterogeneous mixture of chondrocytes and chondrons, regenerating the PCM using a tissue engineering approach could prove beneficial. In this study, we aimed to discern the behavior of articular chondrocytes (ACs) in regenerating the PCM in such an approach and whether this would also be true for articular cartilage-derived progenitor cells (ACPCs), as an alternative cell source. Bovine ACs and ACPCs were encapsulated in agarose microgels using droplet-based microfluidics. ACs were stimulated with TGF-β1 and dexamethasone and ACPCs were sequentially stimulated with BMP-9 followed by TGF-β1 and dexamethasone. After 0, 3, 5, and 10 days of culture, PCM components, type-VI collagen and perlecan, and ECM component, type-II collagen, were assessed using flow cytometry and fluorescence microscopy. Both ACs and ACPCs synthesized the PCM before the ECM. It was seen for the first time that synthesis of type-VI collagen always preceded perlecan. While the PCM synthesized by ACs resembled native chondrons after only 5 days of culture, ACPCs often made less well-structured PCMs. Both cell types showed variations between individual cells and donors. On one hand, this was more prominent in ACPCs, but also a subset of ACPCs showed superior PCM and ECM regeneration, suggesting that isolating these cells may potentially improve cartilage repair strategies.

Proteolysis of the pericellular matrix: Pinpointing the role and involvement of matrix metalloproteinases in early osteoarthritic remodeling

The pericellular matrix (PCM) serves a critical role in signal transduction and mechanoprotection in chondrocytes. Osteoarthritis (OA) leads to a gradual deterioration of the cartilage, marked by a shift in the spatial arrangement of chondrocytes from initially isolated strands to large cell clusters in end-stage degeneration. These changes coincide with progressive enzymatic breakdown of the PCM. This study aims to assess the role and involvement of specific matrix metalloproteinases (MMPs) in PCM degradation during OA. We selected cartilage samples from 148 OA patients based on the predominant spatial chondrocyte patterns. The presence of various MMPs (-1,-2,-3,-7,-8,-9,-10,-12,-13) was identified by multiplexed immunoassays. For each pattern and identified MMP, the levels and activation states (pro-form vs. active form) were measured by zymograms and western blots. The localization of these MMPs was determined using immunohistochemical labeling. To verify these results, healthy cartilage was exposed to purified MMPs, and the consecutive structural integrity of the PCM was analyzed through immunolabeling and proximity ligation assay. Screening showed elevated levels of MMP-1,-2,-3,-7, and -13, with their expression profile showing a clear dependency of the degeneration stage. MMP-2 and -7 were localized in the PCM, whereas MMP-1,-7, and -13 were predominantly intracellular. We found that MMP-2 and -3 directly disrupt collagen type VI, and MMP-3 and -7 destroy perlecan. MMP-2, -3, and -7 emerge as central players in early PCM degradation in OA. With the disease's initial stages already displaying elevated peaks in MMP expression, this insight may guide early targeted therapies to halt abnormal PCM remodeling. STATEMENT OF SIGNIFICANCE: Osteoarthritis (OA) causes a gradual deterioration of the articular cartilage, accompanied by a progressive breakdown of the pericellular matrix (PCM). The PCM's crucial function in protecting and transmitting signals within chondrocytes is impaired in OA. By studying 148 OA-patient cartilage samples, the involvement of matrix metalloproteinases (MMPs) in PCM breakdown was explored. Findings highlighted elevated levels of certain MMPs linked to different stages of degeneration. Notably, MMP-2, -3, and -7 were identified as potent contributors to early PCM degradation, disrupting key components like collagen type VI and perlecan. Understanding these MMPs' roles in initiating OA progression, especially in its early stages, provides insights into potential targets for interventions to preserve PCM integrity and potentially impeding OA advancement.

Biomechanics of Chondrocytes and Chondrons in Healthy Conditions and Osteoarthritis: A Review of the Mechanical Characterisations at the Microscale

Biomechanical studies are expanding across a variety of fields, from biomedicine to biomedical engineering. From the molecular to the system level, mechanical stimuli are crucial regulators of the development of organs and tissues, their growth and related processes such as remodelling, regeneration or disease. When dealing with cell mechanics, various experimental techniques have been developed to analyse the passive response of cells; however, cell variability and the extraction process, complex experimental procedures and different models and assumptions may affect the resulting mechanical properties. For these purposes, this review was aimed at collecting the available literature focused on experimental chondrocyte and chondron biomechanics with direct connection to their biochemical functions and activities, in order to point out important information regarding the planning of an experimental test or a comparison with the available results. In particular, this review highlighted (i) the most common experimental techniques used, (ii) the results and models adopted by different authors, (iii) a critical perspective on features that could affect the results and finally (iv) the quantification of structural and mechanical changes due to a degenerative pathology such as osteoarthritis.

Changes in stiffness of the extracellular and pericellular matrix in the anulus fibrosus of lumbar intervertebral discs over the course of degeneration

Analogous to articular cartilage, changes in spatial chondrocyte organisation have been proposed to be a strong indicator for local tissue degeneration in the intervertebral disc (IVD). While a progressive structural and functional degradation of the extracellular (ECM) and pericellular (PCM) matrix occurs in osteoarthritic cartilage, these processes have not yet been biomechanically elucidated in the IVD. We aimed to evaluate the local stiffness of the ECM and PCM in the anulus fibrosus of the IVD on the basis of local chondrocyte spatial organisation. Using atomic force microscopy, we measured the Young's modulus of the local ECM and PCM in human and bovine disc samples using the spatial chondrocyte patterns as an image-based biomarker. By measuring tissue from 31 patients and six bovine samples, we found a significant difference in the elastic moduli (E) of the PCM in clusters when compared to the healthy patterns single cells (p = 0.029), pairs (p = 0.016), and string-formations (p = 0.010). The ECM/PCM ratio ranged from 0.62-0.89. Interestingly, in the bovine IVD, the ECM/PCM ratio of the E significantly varied (p = 0.002) depending on the tissue origin. Overall the reduced E in clusters demonstrates that cluster formation is not only a morphological phenomenon describing disc degeneration, but it marks a compromised biomechanical functioning. Immunohistochemical analyses indicate that collagen type III degradation might be involved. This study is the first to describe and quantify the differences in the E of the ECM in relation to the PCM in the anulus fibrosus of the IVD by means of atomic force microscopy on the basis of spatial chondrocyte organisation.

Encapsulation of rat bone marrow-derived mesenchymal stem cells (rBMMSCs) in collagen type I containing platelet-rich plasma for osteoarthritis treatment in rat model

Osteoarthritis (OA) is the most common form of degenerative joint disease, affecting more than 25% of the adults despite its prevalence in the elderly population. Most of the current therapeutic modalities aim at symptomatic treatment which lingers the disease progression. In recent years, regenerative medicine such as stem cell transplantation and tissue engineering has been suggested as a potential curative intervention for OA. The objective of this current study was to assess the safety and efficacy of an injectable tissue-engineered construct composed of rat bone marrow mesenchymal stem cells (rBMMSCs), platelet-rich plasma (PRP), and collagen type I in rat model of OA. To produce collagen type I, PRP and rBMMSCs, male Wistar rats were ethically euthanized. After isolation, culture, expansion and characterization of rBMMSCs, tissue-engineered construct was formed by a combination of appropriate amount of collagen type I, PRP and rBMMSCs. In vitro studies were conducted to evaluate the effect of PRP on chondrogenic differentiation capacity of encapsulated cells. In the following, the tissue-engineered construct was injected in knee joints of rat models of OA (24 rats in 4 groups: OA, OA + MSC, OA + collagen + MSC + PRP, OA + MSC + collagen). After 6 weeks, the animals were euthanized and knee joint histopathology examinations of knee joint samples were performed to evaluate the effect of each treatment on OA. Tissue-engineered construct was successfully manufactured and in vitro assays demonstrated the relevant chondrogenic genes and proteins expression were higher in PRP group than that of others. Histopathological findings of the knee joint samples showed favorable regenerative effect of rBMMSCs + PRP + collagen group compared to others. We introduced an injectable tissue-engineered product composed of rBMMSCs + PRP + collagen with potential regenerative effect on cartilage that has been damaged by OA.

The Current Status of Clinical Trials on Biologics for Cartilage Repair and Osteoarthritis Treatment: An Analysis of ClinicalTrials.gov Data

OBJECTIVE

Biologics are increasingly used for cartilage repair and osteoarthritis (OA) treatment. This study aimed to provide an overview of the clinical trials conducted on this subject.

DESIGN

Two-word combinations of two sets of key words "cartilage"; "joint"; "osteoarthritis" and "biologics"; "stem cells"; "cell implantation" were used to search the database of ClinicalTrials.gov and supplemented with searches of PubMed and EMbase. The registered trials were analyzed for clinical conditions, completion status, phases, and investigated biologics. Recently completed trials with posted/published results were summarized.

RESULTS

From 2000 to 2022, a total of 365 clinical trials were registered at ClinicalTrials.gov to use biologics for cartilage repair and OA treatment. Since 2006, the number of registered trials accelerated at an annual rate of 16.4%. Of the 265 trials designated with a phase, 72% were early Phase 1, Phase 1, and Phase 2. Chondrocytes and platelet-rich plasma (PRP) were studied in nearly equal number of early- and late-stage trials. Mesenchymal stem/stromal cells (MSCs) were the most commonly investigated biologics (38%) and mostly derived from bone marrow and adipose tissue (70%). In last 5 years, 32 of the 72 completed trials posted/published results, among which seven Phase 3 trials investigated chondrocytes, PRP, bone marrow aspirate concentrate, hyaluronic acid, collagen membrane, and albumin.

CONCLUSIONS

There was a rapid increase in the number of registered clinical trials in recent years, using a variety of biologics for cartilage repair and OA treatment. Majority of the biologics still require late-stage trials to validate their clinical effectiveness.

Targeted mesenchymal stem cell therapy equipped with a cell-tissue nanomatchmaker attenuates osteoarthritis progression

Mesenchymal stem cells (MSCs) are at the forefront of research for a wide range of diseases, including osteoarthritis (OA). Despite having attracted the attention of orthopedists, current MSC therapy techniques are limited by poor MSC implantation in tissue defects and lack of lateral tissue integration, which has restricted the efficacy of cell therapy to alleviate OA symptoms only. Here, we developed targeted MSC therapy for OA cartilage using a cell-tissue matchmaking nanoconstruct (C-TMN). C-TMN, as an MSC vehicle, consists of a central iron oxide nanoparticle armed with two types of antibodies, one directed at the MSC surface and the other against articular cartilage. We treated rat OA articular cartilage with intra-articular injections of C-TMN with and without exogenous MSCs. We observed substantial improvements in both symptomatic and radiographic OA caused by C-TMN, which was independent of exogenous MSCs. This new approach could predict a promising future for OA management.

Painful temporomandibular joint overloading induces structural remodeling in the pericellular matrix of that joint's chondrocytes

Mechanical stress to the temporomandibular joint (TMJ) is an important factor in cartilage degeneration, with both clinical and preclinical studies suggesting that repeated TMJ overloading could contribute to pain, inflammation, and/or structural damage in the joint. However, the relationship between pain severity and early signs of cartilage matrix microstructural dysregulation is not understood, limiting the advancement of diagnoses and treatments for temporomandibular joint-osteoarthritis (TMJ-OA). Changes in the pericellular matrix (PCM) surrounding chondrocytes may be early indicators of OA. A rat model of TMJ pain induced by repeated jaw loading (1 h/day for 7 days) was used to compare the extent of PCM modulation for different loading magnitudes with distinct pain profiles (3.5N-persistent pain, 2N-resolving pain, or unloaded controls-no pain) and macrostructural changes previously indicated by Mankin scoring. Expression of PCM structural molecules, collagen VI and aggrecan NITEGE neo-epitope, were evaluated at Day 15 by immunohistochemistry within TMJ fibrocartilage and compared between pain conditions. Pericellular collagen VI levels increased at Day 15 in both the 2N (p = 0.003) and 3.5N (p = 0.042) conditions compared to unloaded controls. PCM width expanded to a similar extent for both loading conditions at Day 15 (2N, p < 0.001; 3.5N, p = 0.002). Neo-epitope expression increased in the 3.5N group over levels in the 2N group (p = 0.041), indicating pericellular changes that were not identified in the same groups by Mankin scoring of the pericellular region. Although remodeling occurs in both pain conditions, the presence of pericellular catabolic neo-epitopes may be involved in the macrostructural changes and behavioral sensitivity observed in persistent TMJ pain.

Scientific Developments and Clinical Applications Utilizing Chondrons and Chondrocytes with Matrix for Cartilage Repair

Injuries to articular cartilage of the knee are increasingly common. The operative management of these focal chondral lesions continues to be problematic for the treating orthopedic surgeon secondary to the limited regenerative capacity of articular cartilage. The pericellular matrix (PCM) is a specialized, thin layer of the extracellular matrix that immediately surrounds chondrocytes forming a unit together called the chondron. The advancements in our knowledge base with regard to the PCM/chondrons as well as interterritorial matrix has permeated and led to advancements in product development in conjunction with minced cartilage, marrow stimulation, osteochondral allograft, and autologous chondrocyte implantation (ACI). This review intends to summarize recent progress in chondrocytes with matrix research, with an emphasis on the role the PCM/extracellular matrix (ECM) plays for favorable chondrogenic gene expression, as a barrier/filtration unit, and in osteoarthritis. The bulk of the review describes cutting-edge and evolving clinical developments and discuss these developments in light of underlying basic science applications. Clinical applications of chondrocytes with matrix science include Reveille Cartilage Processor, Cartiform, and ACI with Spherox (which was recently recommended for the treatment of grade III or IV articular cartilage defects over 2 cm² by the National Institute of Health and Care Excellence [NICE] in the United Kingdom). The current article presents a comprehensive overview of both the basic science and clinical results of these next-generation cartilage repair techniques by focusing specifically on the scientific evolution in each category as it pertains with underlying chondrocytes with matrix theory.

Protective and proliferative effect of Aesculus indica extract on stressed human adipose stem cells via downregulation of NF-κB pathway

Inflammatory microenvironment after transplantation affects the proliferation and causes senescence of adipose-derived mesenchymal stem cells (hADMSCs) thus compromising their clinical efficacy. Priming stem cells with herbal extracts is considered very promising to improve their viability in the inflammatory milieu. Aesculus indica (A. indica) is used to treat many inflammatory diseases in Asia for decades. Herein, we explored the protective role of A. indica extract on human adipose-derived Mesenchymal Stem Cells (hADMSCs) against Monosodium Iodoacetate (MIA) induced stress in vitro. A. indica ameliorated the injury as depicted by significantly enhanced proliferation, viability, improved cell migration and superoxide dismutase activity. Furthermore, reduced lactate dehydrogenase activity, reactive oxygen species release, senescent and apoptotic cells were detected in A. indica primed hADMSCs. Downregulation of NF-κB pathway and associated inflammatory genes, NF-κB p65/RelA and p50/NF-κB 1, Interleukin 6 (IL-6), Interleukin 1 (IL-1β), Tumor necrosis factor alpha (TNF-α) and matrix metalloproteinase 13 (MMP-13) were observed in A. indica primed hADMSCs as compared to stressed hADMSCs. Complementary to gene expression, A. indica priming reduced the release of transcription factor p65, inhibitory-κB kinase (IKK) α and β, IL-1β and TNF-α proteins expression. Our data elucidates that A. indica extract preconditioning rescued hADMSCs against oxidative stress and improved their therapeutic potential by relieving inflammation through regulation of NF-κB pathway.

Combination of chondrocytes and chondrons improves extracellular matrix production to promote the repairs of defective knee cartilage in rabbits

Background

Chondrons are composed of chondrocytes and the surrounding pericellular matrix (PCM) and function to enhance chondrocyte-mediated cartilage tissue engineering. This study aimed at investigating the potential effect of combined chondrocytes with chondrons on the production of proteoglycan and collagen-II (Col-2) and the repair of defective knee cartilage in rabbits.

Methods

Chondrocytes and chondrons were isolated from the knee cartilage of rabbits, and cultured alone or co-cultured for varying periods in vitro. Their morphology was characterized by histology. The levels of aggrecan (AGG), Col-2 and glycosaminoglycan (GAG) expression were quantified by qRT-PCR, Alcian blue-based precipitation and ELISA. The effect of combined chondrocytes with chondrons in alginate spheres on the repair of defective knee cartilage was examined in rabbits.

Results

The isolated chondrocytes and chondrons displayed unique morphology and began to proliferate on day 3 and 6 post culture, respectively, accompanied by completely degenerated PCM on day 6 post culture. Evidently, chondrocytes had stronger proliferation capacity than chondrons. Longitudinal analyses indicated that culture of chondrons, but not chondrocytes, increased AGG mRNA transcripts and GAG levels with time and Col-2 mRNA transcripts only on day 3 post culture. Compared with chondrocytes or chondrons alone, co-culture of chondrocytes and chondrons significantly up-regulated AGG and Col-2 expression and GAG production, particularly at a ratio of 1:1. Implantation with chondrocytes and chondrons at 1:1 significantly promoted the repair of defective knee cartilage in rabbits, accompanied by reduced the Wakiteni scores with time.

Conclusion

Combined chondrons with chondrocytes promoted the production of extracellular matrix and the repair of defective knee cartilage in rabbits.

The translational potential of this article

This study explores that the combination of chondrons and chondrocytes may be new therapeutic strategy for cartilage tissue engineering and repair of defective cartilage.

Biochemical changes of the pericellular matrix and spatial chondrocyte organization-Two highly interconnected hallmarks of osteoarthritis

During osteoarthritis, chondrocytes change their spatial arrangement from single to double strings, then to small and big clusters. This change in pattern has recently been established as an image-based biomarker for osteoarthritis. The pericellular matrix (PCM) appears to degrade together alongside cellular reorganization. The aim of this study was to characterize this PCM-degradation based on different cellular patterns. We additionally wanted to identify the earliest time point of PCM-breakdown in this physiopathological model. To this end, cartilage samples were selected according to their predominant cellular pattern. Qualitative analysis of PCM degradation was performed immunohistochemically by analysing five main PCM components: collagen type VI, perlecan, collagen type III, biglycan, and fibrillin-1 (n = 6 patients). Their protein content was quantified by enzyme-linked immunosorbent assay (127 patients). Accompanying spatial cellular rearrangement, the PCM is progressively destroyed, with a pericellular signal loss in fluorescence microscopy for collagen type VI, perlecan, and biglycan. This loss in protein signal is accompanied by a reduction in total protein content from single strings to big clusters (P < .001 for collagen type VI, P = .003 for perlecan, and P < .001 for biglycan). As a result of an increase in the number of cells from single strings to big clusters, the amount of protein available per cell also decreases for collagen type III and fibrillin-1, where total protein levels remain constant. Biochemical changes of the PCM and cellular rearrangement are thus highly interconnected hallmarks of osteoarthritis. Interestingly, the earliest point in time for a relevant PCM impairment appears to be at the transition to small clusters.

Proteome Alterations in Equine Osteochondrotic Chondrocytes

Osteochondrosis is a failure of the endochondral ossification that affects developing joints in humans and several animal species. It is a localized idiopathic joint disorder characterized by focal chondronecrosis and growing cartilage retention, which can lead to the formation of fissures, subchondral bone cysts, or intra-articular fragments. Osteochondrosis is a complex multifactorial disease associated with extracellular matrix alterations and failure in chondrocyte differentiation, mainly due to genetic, biochemical, and nutritional factors, as well as traumas. This study describes the main proteomic alterations occurring in chondrocytes isolated from osteochondrotic cartilage fragments. A comparative analysis performed on equine osteochondrotic and healthy chondrocytes showed 26 protein species as differentially represented. In particular, quantitative changes in the extracellular matrix, cytoskeletal and chaperone proteins, and in cell adhesion and signaling molecules were observed in osteochondrotic cells, compared to healthy controls. Functional group analysis annotated most of these proteins in “growth plate and cartilage development”, while others were included in “glycolysis and gluconeogenesis”, “positive regulation of protein import”, “cell–cell adhesion mediator activity”, and “mitochondrion nucleoid”. These results may help to clarify some chondrocyte functional alterations that may play a significant role in determining the onset and progression of equine osteochondrosis and, being related, of human juvenile osteochondrosis.

Changes in stiffness and biochemical composition of the pericellular matrix as a function of spatial chondrocyte organisation in osteoarthritic cartilage

OBJECTIVE

During osteoarthritis (OA), chondrocytes seem to change their spatial arrangement from single to double strings, small and big clusters. Since the pericellular matrix (PCM) appears to degrade alongside this reorganisation, it has been suggested that spatial patterns act as an image-based biomarker for OA. The aim of this study was to establish the functional relevance of spatial organisation in articular cartilage.

METHOD

Cartilage samples were selected according to their predominant spatial cellular pattern. Young's modulus of their PCM was measured by atomic force microscopy (AFM) (∼500 measurements/pattern). The distribution of two major PCM components (collagen type VI and perlecan) was analysed by immunohistochemistry (8 patients) and protein content quantified by enzyme-linked immunosorbent assay (ELISA) (58 patients).

RESULTS

PCM stiffness significantly decreased with the development from single to double strings (p = 0.030), from double strings to small clusters (p = 0.015), and from small clusters to big clusters (p < 0.001). At the same time, the initially compact collagen type VI and perlecan staining progressively weakened and was less focalised. The earliest point with a significant reduction in protein content as shown by ELISA was the transition from single strings to small clusters for collagen type VI (p = 0.016) and from double strings to small clusters for perlecan (p = 0.008), with the lowest amounts for both proteins seen in big clusters.

CONCLUSIONS

This study demonstrates the functional relevance of spatial chondrocyte organisation as an image-based biomarker. At the transition from single to double strings PCM stiffness decreases, followed by protein degradation from double strings to small clusters.

Promoting effective tendon healing and remodeling

Daily activities subject our tendons to accumulation of sub-rupture fatigue injury which can lead to tendon rupture. Consequently, tendinopathies account for over 30% of musculoskeletal consultations. We adopted a multidisciplinary approach to determine the role of the extracellular matrix (ECM) in the pathogenesis of tendinopathy and impaired healing of ruptured tendons. We have been investigating three main areas: (i) the pathogenesis of tendon degeneration; (ii) approaches to promoting remodeling of sub-rupture fatigue injuries; and the (iii) role of the ECM in promoting scarless tendon healing. In this Kappa Delta Young Investigator award paper, we describe the key discoveries made in each of our three research areas of focus. Briefly, we discovered that sub-rupture fatigue damage can accumulate from just one bout of fatigue loading. Furthermore, any attempt to repair the fatigue damage diminishes as the severity of induced damage increases. We have utilized exercise to develop animal models of exercise-led degeneration and exercise-led repair of sub-rupture fatigue damage injuries, wherein underlying mechanisms can be uncovered, thereby overcoming a major hurdle to development of therapeutics. Since damage accumulation ultimately leads to rupture that is characterized by formation of a mechanically inferior scar, we have used the MRL/MpJ mouse to evaluate the role of the systemic environment and the local tendon environment in driving regeneration to identify new therapeutic pathways to promote scarless healing. Our data suggests that the therapeutic potential of the MRL/MpJ provisional ECM should be further explored as it may harness biological and structural mechanisms to promote scarless healing. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3115-3124, 2018.

Chondrocyte death after mechanically overloading degenerated human intervertebral disk explants is associated with a structurally impaired pericellular matrix

A type VI collagen-rich pericellular matrix (PCM) encloses both intervertebral disk (IVD) and articular cartilage chondrocytes. In the latter, the PCM protects the chondrocytes from mechanical overload, whereas tissue degeneration is associated with PCM destruction. As little is known about the IVD PCM, we investigated chondrocyte survival after mechanical overload as well as PCM structural integrity as a function of clinical tissue degeneration. The hypothesis was that IVD degeneration may affect PCM integrity and overload-related chondrocyte survival. Cylindrical human IVD explants from patients undergoing surgical procedures for lumbar disk degeneration, disk prolapse, or spinal trauma were generated and scored. Mechanical overload was applied by single uniaxial 50% compression followed by immediate release, and the explants were live-dead stained (n = 20 explants). Type VI collagen, the major PCM component, was fluorescent stained and the extent was determined, in which individual cells were enclosed by a recognizable PCM; this was termed PCM fraction. More than 50% of chondrocytes in all degenerative IVD explants displayed <25% PCM fraction and a lower PCM fraction correlated with higher cell numbers (p < 0.001), suggesting a PCM structural impairment in IVD degeneration that is associated with chondrocyte clustering. Mechanical overload-induced significantly increased cell death (p = 0.005), and the PCM fraction was significantly lower in overload-induced cell death than in live cells (p = 0.042), suggesting that a fully present PCM has a protective role in mechanical overload. Collectively, human IVD degeneration is associated with a structural impairment of the PCM, which may promote cell death under mechanical overload.

Inhibition of apoptosis exacerbates fatigue-damage tendon injuries in an in vivo rat model

Tendinopathy is a common and progressive musculoskeletal disease. Increased apoptosis is an end-stage tendinopathy manifestation, but its contribution to the pathology of the disease is unknown. A previously established in vivo model of fatigue damage accumulation shows that increased apoptosis is correlated with the severity of induced tendon damage, even in early onset of the disease, supporting its implication in the pathogenesis of the disease. Consequently, this study aimed to determine: (1) whether apoptosis could be inhibited after fatigue damage and (2) whether its inhibition could lead to remodeling of the extracellular matrix (ECM) and pericellular matrix (PCM), to ultimately improve the mechanical properties of fatigue-damaged tendons. The working hypothesis was that, despite the low vascular nature of the tendon, apoptosis would be inhibited, prompting increased production of matrix proteins and restoring tendon mechanical properties. Rats received 2 or 5 d of systemic pan-caspase inhibitor (Q-VD-OPh) or dimethyl sulfoxide (DMSO) carrier control injections starting immediately prior to fatigue loading and were sacrificed at days 7 and 14 post-fatigue-loading. Systemic pan-caspase inhibition for 2 d led to a surprising increase in apoptosis, but inhibition for 5 d increased the population of live cells that could repair the fatigue damage. Further analysis of the 5 d group showed that effective inhibition led to an increased population of cells producing ECM and PCM proteins, although typically in conjunction with oxidative stress markers. Ultimately, inhibition of apoptosis led to further deterioration in mechanical properties of fatigue-damaged tendons.

Pericellular collagen I coating for enhanced homing and chondrogenic differentiation of mesenchymal stem cells in direct intra-articular injection

Background

Direct intra-articular injection (DIAI) of mesenchymal stem cells (MSCs) is a promising technique for cartilage repair. However, the repair process was hindered by the absence of scaffold and poor cell–matrix interactions.

Methods

In this study, we developed a pericellular collagen I coating (PCC) on MSCs. The overall performances of MSC-PCC homing, chondrogenic differentiation, and cartilage regeneration have been comprehensively evaluated in a New Zealand rabbit model. Firstly, we examined the morphology and physical characteristics of PCC. Secondly, MSC ex-vivo cartilage slice adhesion and in-vivo cartilage defect homing were observed using multiscale methods. Thirdly, the precartilage condensation of cell pellets formed by aggregation of MSCs was examined to evaluate the cartilage-inducing potential of PCC. Finally, the cartilage regeneration by DIAI of PCC-coated MSCs was observed and scored macroscopically and histologically.

Results

In general, the cell adhesion and homing assay revealed that PCC facilitated MSC adhesion on cartilage slices, enhancing MSC homing and retention to cartilage defect. This increased homing ratio was accompanied by an increasing cell–cell contact. Compared with naked MSCs, the cell pellets formed by PCC-coated MSCs exhibited more evident appearance of condensation. In pellets, cell–cell interaction has been significantly stimulated, inducing the expression of condensation marker N-cadherin, and subsequent chondrogenic marker collagen II and aggrecan. By 12 weeks after DIAI, cartilage defects have been repaired by MSCs to varying degrees. Overall, PCC significantly enhances the quality of cartilage regeneration judging from macroscopic observation, ICRS score, histological examination, and collagen type I, II, and X immunohistochemical staining.

Conclusions

The capacity and viability of MSCs can be enhanced by collagen I coating, which provides cues for enhancing cell homing and differentiation. Our method provides a novel strategy for stem cell therapy.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0916-z) contains supplementary material, which is available to authorized users.

A potential new role for myofibroblasts in remodeling of sub-rupture fatigue tendon injuries by exercise

Tendons are ineffective at repairing sub-rupture fatigue injuries. Accordingly, we evaluated whether an exercise protocol that we have previously found to decrease structural damage kinks in fatigue damaged tendons, leads to improvement in mechanical properties. We hypothesized that exercise that promotes repair of fatigue damage will decrease apoptosis and increase the population of myofibroblasts. Rat patellar tendons underwent in vivo fatigue loading for 500 or 7200 cycles. Animals resumed cage activity for 2-weeks, then either remained cage active or began treadmill running until sacrifice at 4- or 10-weeks post-fatigue loading. Exercise following fatigue damage increased the stiffness back towards naïve levels, decreased apoptosis and increased the population of myofibroblasts. Next, proteins associated with inhibition of apoptosis (Collagen VI) or activation of myofibroblast (pSmad 2/3, fibrillin, integrin subunits αV and α5) were evaluated. Data suggests that collagen VI may not be integral to inhibition of apoptosis in this context. Exercise increased pSmad 2/3 and fibrillin in the insertion region for the 7200-cycles group. In addition, exercise decreased integrin αV and increased integrin α5 in fatigue damaged tendons. Data suggests that a decrease in apoptosis and an increase in population of myofibroblasts may be integral to remodeling of fatigue damaged tendons.

Osteoarthritis as a disease of the cartilage pericellular matrix

Osteoarthritis is a painful joint disease characterized by progressive degeneration of the articular cartilage as well as associated changes to the subchondral bone, synovium, and surrounding joint tissues. While the effects of osteoarthritis on the cartilage extracellular matrix (ECM) have been well recognized, it is now becoming apparent that in many cases, the onset of the disease may be initially reflected in the matrix region immediately surrounding the chondrocytes, termed the pericellular matrix (PCM). Growing evidence suggests that the PCM - which along with the enclosed chondrocytes are termed the "chondron" - acts as a critical transducer or "filter" of biochemical and biomechanical signals for the chondrocyte, serving to help regulate the homeostatic balance of chondrocyte metabolic activity in response to environmental signals. Indeed, it appears that alterations in PCM properties and cell-matrix interactions, secondary to genetic, epigenetic, metabolic, or biomechanical stimuli, could in fact serve as initiating or progressive factors for osteoarthritis. Here, we discuss recent advances in the understanding of the role of the PCM, with an emphasis on the reciprocity of changes that occur in this matrix region with disease, as well as how alterations in PCM properties could serve as a driver of ECM-based diseases such as osteoarthritis. Further study of the structure, function, and composition of the PCM in normal and diseased conditions may provide new insights into the understanding of the pathogenesis of osteoarthritis, and presumably new therapeutic approaches for this disease.

Lutein prevents osteoarthritis through Nrf2 activation and downregulation of inflammation

INTRODUCTION

Osteoarthritis is an inflammatory disorder associated with oxidative stress and apoptosis leading to cartilage destruction and impairment of cartilage formation. In the present study, we studied the protective effect of lutein against monosodium iodoacetate (MIA)-induced osteoarthritis in primary chondrocyte cells.

MATERIAL AND METHODS

Oxidative stress was determined through testing antioxidant status, reactive oxygen species levels and lipid peroxide content. Also, Nrf2 expression and its downstream target genes HO-1 and NQO-1 were determined. Inflammation was analyzed through NF-κB, COX-2 and pro-inflammatory cytokines (IL-6, TNF-α, IL-1β). In addition, the effects of MIA and lutein on mitochondrial membrane potential and caspase-3 levels were analyzed.

RESULTS

The results showed that lutein treatment significantly increased the cell viability of chondrocytes and offered significant cytoprotection by enhancing the antioxidant defense mechanisms and reducing oxidative stress (reactive oxygen species and lipid peroxidation). Lutein treatment showed anti-inflammatory effects by downregulating inflammatory proteins (NF-κB, COX-2) and pro-inflammatory cytokines (IL-6, TNF-α, IL-1β). Lutein reduced MIA-induced apoptosis through maintaining mitochondrial membrane potential and downregulating caspase-3 activity.

CONCLUSIONS

The present study shows significant cytoprotection offered by lutein against MIA-induced oxidative stress, inflammation and apoptosis by the modulatory effect of NF-κB and Nrf2 activation.

Human osteoarthritic chondrons outnumber patient- and joint-matched chondrocytes in hydrogel culture-Future application in autologous cell-based OA cartilage repair?

Autologous chondrocyte implantation (ACI) is used in 34-60% for osteoarthritic (OA) cartilage defects, although ACI is neither recommended nor designed for OA. Envisioning a hydrogel-based ACI for OA that uses chondrons instead of classically used chondrocytes, we hypothesized that human OA chondrons may outperform OA chondrocytes. We compared patient- and joint surface-matched human OA chondrons with OA chondrocytes cultured for the first time in a hydrogel, using a self-assembling peptide system. We determined yield, viability, cell numbers, mRNA expression, GAPDH mRNA enzyme activity, Collagen II synthesis (CPII) and degradation (C2C), and sulfated glycosaminoglycan. Ex vivo, mRNA expression was comparable. Over time, significant differences in survival led to 3.4-fold higher OA chondron numbers in hydrogels after 2 weeks (p = .002). Significantly, more enzymatically active GAPDH protein indicated higher metabolic activity. The number of cultures that expressed mRNA for Collagen Types I and VI, COMP, aggrecan, VEGF, TGF-β1, and FGF-2 (but not Collagen Types II and X) was different, resulting in a 3.5-fold higher number of expression-positive OA chondron cultures (p < .05). Measuring CPII and C2C per hydrogel, OA chondron hydrogels synthesized more than they degraded Collagen Type II, the opposite was true for OA chondrocytes. Per cell, OA chondrons but not OA chondrocytes displayed more synthesis than degradation. Thus, OA chondrons displayed superior biosynthesis and mRNA expression of tissue engineering and phenotype-relevant genes. Moreover, human OA chondrons displayed a significant survival advantage in hydrogel culture, whose presence, drastic extent, and timescale was novel and is clinically significant. Collectively, these data highlight the high potential of human OA chondrons for OA ACI, as they would outnumber and, thus, surpass OA chondrocytes.

Distribution of pericellular matrix molecules in the temporomandibular joint and their chondroprotective effects against inflammation

The objectives of this study were to (1) determine the distribution and synthesis of pericellular matrix (PCM) molecules (collagen VI, collagen IV and laminin) in rat temporomandibular joint (TMJ) and (2) investigate the effects of PCM molecules on chondrocytes against inflammation in osteoarthritis. Four zones (fibrous, proliferating, mature and hypertrophic) of condylar cartilage and three bands (anterior, intermediate and posterior) of disc were analysed by immunohistochemistry for the presence of PCM molecules in rat TMJs. Isolated chondrocytes were pre-treated with PCM molecules before being subjected to interleukin (IL)-1β treatment to stimulate inflammation. The responses of the chondrocytes were analysed using gene expression, nitric oxide release and matrix metalloproteinase (MMP)-13 production measures. Histomorphometric analyses revealed that the highest areal deposition of collagen VI (67.4%), collagen IV (45.7%) and laminin (52.4%) was in the proliferating zone of TMJ condylar cartilage. No significant difference in the distribution of PCM molecules was noted among the three bands of the TMJ disc. All three PCM molecules were expressed intracellularly by chondrocytes cultured in the monolayer. Among the PCM molecules, pre-treatment with collagen VI enhanced cellular proliferation, ameliorated IL-1β-induced MMP-3, MMP-9, MMP-13 and inducible nitric oxide synthase gene expression, and attenuated the downregulation of cartilage matrix genes, including collagen I, aggrecan and cartilage oligomeric matrix protein (COMP). Concurrently, collagen VI pretreatment inhibited nitric oxide and MMP-13 production. Our study demonstrates for the first time the distribution and role of PCM molecules, particularly collagen VI, in the protection of chondrocytes against inflammation.

Cellular senescence in aging and osteoarthritis

- It is well accepted that age is an important contributing factor to poor cartilage repair following injury, and to the development of osteoarthritis. Cellular senescence, the loss of the ability of cells to divide, has been noted as the major factor contributing to age-related changes in cartilage homeostasis, function, and response to injury. The underlying mechanisms of cellular senescence, while not fully understood, have been associated with telomere erosion, DNA damage, oxidative stress, and inflammation. In this review, we discuss the causes and consequences of cellular senescence, and the associated biological challenges in cartilage repair. In addition, we present novel strategies for modulation of cellular senescence that may help to improve cartilage regeneration in an aging population.

Loss of spatial organization and destruction of the pericellular matrix in early osteoarthritis in vivo and in a novel in vitro methodology

OBJECTIVES

Current repair procedures for articular cartilage (AC) cannot restore the tissue's original form and function because neither changes in its architectural blueprint throughout life nor the respective biological understanding is fully available. We asked whether two unique elements of human cartilage architecture, the chondrocyte-surrounding pericellular matrix (PCM) and the superficial chondrocyte spatial organization (SCSO) beneath the articular surface (AS) are congenital, stable or dynamic throughout life. We hypothesized that inducing chondrocyte proliferation in vitro impairs organization and PCM and induces an advanced osteoarthritis (OA)-like structural phenotype of human cartilage.

METHODS

We recorded propidium-iodine-stained fetal and adult cartilage explants, arranged stages of organization into a sequence, and created a lifetime-summarizing SCSO model. To replicate the OA-associated dynamics revealed by our model, and to test our hypothesis, we transduced specifically early OA-explants with hFGF-2 for inducing proliferation. The PCM was examined using immuno- and auto-fluorescence, multiphoton second-harmonic-generation (SHG), and scanning electron microscopy (SEM).

RESULTS

Spatial organization evolved from fetal homogeneity, peaked with adult string-like arrangements, but was completely lost in OA. Loss of organization included PCM perforation (local micro-fibrillar collagen intensity decrease) and destruction [regional collagen type VI (CollVI) signal weakness or absence]. Importantly, both loss of organization and PCM destruction were successfully recapitulated in FGF-2-transduced explants.

CONCLUSION

Induced proliferation of spatially characterized early OA-chondrocytes within standardized explants recapitulated the full range of loss of SCSO and PCM destruction, introducing a novel in vitro methodology. This methodology induces a structural phenotype of human cartilage that is similar to advanced OA and potentially of significance and utility.

Chondrons and the pericellular matrix of chondrocytes

In cartilage, chondrocytes are embedded within an abundant extracellular matrix (ECM). A typical chondron consists of a chondrocyte and the immediate surrounding pericellular matrix (PCM). The PCM has a patent structure, defined molecular composition, and unique physical properties that support the chondrocyte. Given this spatial position, the PCM is pivotal in mediating communication between chondrocytes and the ECM and, thus, plays a critical role in cartilage homeostasis. The biological function and mechanical properties of the PCM have been extensively studied, mostly in the form of chondrons. This review intends to summarize recent progress in chondron and chondrocyte PCM research, with emphasis on the re-establishment of the PCM by isolated chondrocytes or mesenchymal stem cells during chondrogenic differentiation, and the effects of the PCM on cartilage tissue formation.

The structure and function of the pericellular matrix of articular cartilage

Chondrocytes in articular cartilage are surrounded by a narrow pericellular matrix (PCM) that is both biochemically and biomechanically distinct from the extracellular matrix (ECM) of the tissue. While the PCM was first observed nearly a century ago, its role is still under investigation. In support of early hypotheses regarding its function, increasing evidence indicates that the PCM serves as a transducer of biochemical and biomechanical signals to the chondrocyte. Work over the past two decades has established that the PCM in adult tissue is defined biochemically by several molecular components, including type VI collagen and perlecan. On the other hand, the biomechanical properties of this structure have only recently been measured. Techniques such as micropipette aspiration, in situ imaging, computational modeling, and atomic force microscopy have determined that the PCM exhibits distinct mechanical properties as compared to the ECM, and that these properties are influenced by specific PCM components as well as disease state. Importantly, the unique relationships among the mechanical properties of the chondrocyte, PCM, and ECM in different zones of cartilage suggest that this region significantly influences the stress-strain environment of the chondrocyte. In this review, we discuss recent advances in the measurement of PCM mechanical properties and structure that further increase our understanding of PCM function. Taken together, these studies suggest that the PCM plays a critical role in controlling the mechanical environment and mechanobiology of cells in cartilage and other cartilaginous tissues, such as the meniscus or intervertebral disc.

Genome wide analysis indicates genes for basement membrane and cartilage matrix proteins as candidates for hip dysplasia in Labrador Retrievers

Hip dysplasia, an abnormal laxity of the hip joint, is seen in humans as well as dogs and is one of the most common skeletal disorders in dogs. Canine hip dysplasia is considered multifactorial and polygenic, and a variety of chromosomal regions have been associated with the disorder. We performed a genome-wide association study in Dutch Labrador Retrievers, comparing data of nearly 18,000 single nucleotide polymorphisms (SNPs) in 48 cases and 30 controls using two different statistical methods. An individual SNP analysis based on comparison of allele frequencies with a χ(2) statistic was used, as well as a simultaneous SNP analysis based on Bayesian variable selection. Significant association with canine hip dysplasia was observed on chromosome 8, as well as suggestive association on chromosomes 1, 5, 15, 20, 25 and 32. Next-generation DNA sequencing of the exons of genes of seven regions identified multiple associated alleles on chromosome 1, 5, 8, 20, 25 and 32 (p<0.001). Candidate genes located in the associated regions on chromosomes 1, 8 and 25 included LAMA2, LRR1 and COL6A3, respectively. The associated region on CFA20 contained candidate genes GDF15, COMP and CILP2. In conclusion, our study identified candidate genes that might affect susceptibility to canine hip dysplasia. These genes are involved in hypertrophic differentiation of chondrocytes and extracellular matrix integrity of basement membrane and cartilage. The functions of the genes are in agreement with the notion that disruptions in endochondral bone formation in combination with soft tissue defects are involved in the etiology of hip dysplasia.

Annexin A2 mediates secretion of collagen VI, pulmonary elasticity and apoptosis of bronchial epithelial cells

The annexins are an evolutionarily conserved family of phospholipid-binding proteins of largely unknown function. We observed that the AnxA2(-/-) lung basement membrane specifically lacks collagen VI (COL6), and postulated that ANXA2 directs bronchial epithelial cell secretion of COL6, an unusually large multimeric protein. COL6 serves to anchor cells to basement membranes and, unlike other collagens, undergoes multimerization prior to secretion. Here, we show that AnxA2(-/-) mice have reduced exercise tolerance with impaired lung tissue elasticity, which was phenocopied in Col6a1(-/-) mice. In vitro, AnxA2(-/-) fibroblasts retained COL6 within intracellular vesicles and adhered poorly to their matrix unless ANXA2 expression was restored. In vivo, AnxA2(-/-) bronchial epithelial cells underwent apoptosis and disadhesion. Immunoprecipitation and immunoelectron microscopy revealed that ANXA2 associates with COL6 and the SNARE proteins SNAP-23 and VAMP2 at secretory vesicle membranes of bronchial epithelial cells, and that absence of ANXA2 leads to retention of COL6 in a late-Golgi, VAMP2-positive compartment. These results define a new role for ANXA2 in the COL6 secretion pathway, and further show that this pathway establishes cell-matrix interactions that underlie normal pulmonary function and epithelial cell survival.

Sodium fluoride induces apoptosis through the downregulation of hypoxia-inducible factor-1α in primary cultured rat chondrocytes

It has been reported that sodium fluoride (NaF) suppresses the proliferation and induces apoptosis of chondrocytes. However, the cellular and molecular mechanisms of the effect have not been elucidated. Therefore, the aim of this study was to evaluate the mechanisms of the effects of NaF on primary cultured rat chondrocytes in vitro. Chondrocytes were treated with NaF at concentrations of 0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 mM. Cell viability decreased and the rate of apoptotic cells increased significantly with the gradient concentration of NaF in a time- and dose-dependent manner. Electron microscopy revealed cytoplasmic, organelle and nuclear alterations in the ultrastructure of chondrocytes exposed to various NaF concentrations. The cell cycle distribution was analyzed by flow cytometry, and the results indicated that NaF induced G2 cell cycle arrest. Western blotting was used to detect the apoptotic pathways. Downregulation of the Bcl-2 protein and upregulation of Bax, cleaved caspase-9, -12 and -3 proteins suggested that NaF was capable of inducing apoptosis through the mitochondrial and endoplasmic reticulum pathways. The results also showed that the levels of hypoxia-inducible factor 1α (HIF-1α), sex determining region Y box gene 9 (Sox9) and the collagen II (Col II) protein of the NaF groups were lower compared to those of the control groups. Thus, NaF may induce apoptosis through the downregulation of HIF-1α and disrupt the synthesis of extracellular matrix (ECM) through the downregulation of HIF-1α via the Sox9 pathway in primary cultured rat chondrocytes.

Matrix-embedded cytokines to simulate osteoarthritis-like cartilage microenvironments

In vivo, cytokines noncovalently bind to the extracellular matrix (ECM), to facilitate intimate interactions with cellular receptors and potentiate biological activity. Development of a biomaterial that simulates this type of physiological binding and function is an exciting proposition for designing controlled advanced delivery systems for simulating in vivo conditions in vitro. We have decorated silk protein with sulfonated moieties through diazonium coupling reactions to noncovalently immobilize pro-inflammatory cytokines interleukin-1 beta (IL-1β) and tumor necrosis factor alpha (TNF-α) in such a biomimetic manner. After adsorption of the cytokines to the diazonium-modified silk matrix, constant release of cytokines up to at least 3 days was demonstrated, as an initial step to simulate an osteoarthritic (OA) microenvironment in vitro. Matrix-embedded cytokines induced the formation of multiple elongated processes in chondrocytes in vitro, akin to what is seen in OA cartilage in vivo. Gene expression profiles with this in vitro tissue model of OA showed significant similarities to profiles from explanted OA cartilage tissues collected from patients who underwent total knee replacement surgery. The common markers of OA, including COL, MMP, TIMP, ADAMTS, and metallothioneins, were upregulated at least 35-fold in the in vitro model when compared to the control-non-OA in vitro generated tissue-engineered cartilage. The microarray data were validated by reverse transcriptase-polymerase chain reaction. Mechanistically, protein interaction studies indicated that TNF-α and IL-1β synergistically controlled the equilibrium between MMPs and their inhibitors, TIMPs, resulting in ECM degradation through the MAPK pathway. This study offers a promising initial step toward establishing a relevant in vitro OA disease model, which can be further modified to assess signaling mechanisms, responses to cell or drug treatments and patient-specific features.

Computational investigation of in situ chondrocyte deformation and actin cytoskeleton remodelling under physiological loading

Previous experimental studies have determined local strain fields for both healthy and degenerate cartilage tissue during mechanical loading. However, the biomechanical response of chondrocytes in situ, in particular the response of the actin cytoskeleton to physiological loading conditions, is poorly understood. In the current study a three-dimensional (3-D) representative volume element (RVE) for cartilage tissue is created, comprising a chondrocyte surrounded by a pericellular matrix and embedded in an extracellular matrix. A 3-D active modelling framework incorporating actin cytoskeleton remodelling and contractility is implemented to predict the biomechanical behaviour of chondrocytes. Physiological and abnormal strain fields, based on the experimental study of Wong and Sah (J. Orthop. Res. 2010; 28: 1554-1561), are applied to the RVE. Simulations demonstrate that the presence of a focal defect significantly affects cellular deformation, increases the stress experienced by the nucleus, and alters the distribution of the actin cytoskeleton. It is demonstrated that during dynamic loading cyclic tension reduction in the cytoplasm causes continuous dissociation of the actin cytoskeleton. In contrast, during static loading significant changes in cytoplasm tension are not predicted and hence the rate of dissociation of the actin cytoskeleton is reduced. It is demonstrated that chondrocyte behaviour is affected by the stiffness of the pericellular matrix, and also by the anisotropy of the extracellular matrix. The findings of the current study are of particular importance in understanding the biomechanics underlying experimental observations such as actin cytoskeleton dissociation during the dynamic loading of chondrocytes.

Monosodium iodoacetate induces apoptosis via the mitochondrial pathway involving ROS production and caspase activation in rat chondrocytes in vitro

Monosodium iodoacetate (MIA) is an inhibitor of glyceraldehyde-3-phosphate dehydrogenase activity, and causes dose-dependent cartilage degradation resembling the pathological changes of human osteoarthritis (OA). In this study, we assessed the apoptosis induced by MIA and clarified the underlying mechanisms using the primary rat chondrocytes. The apoptosis of primary rat chondrocytes was analyzed by flow cytometry. The levels of mitochondrial membrane potential (ΔΨm) were evaluated using fluorescence spectrophotometer. The production of reactive oxygen species (ROS) was determined by fluorescence spectrophotometer. Apoptosis-related protein cytochrome c and procaspase-3 expressions were examined by Western blotting. We found that MIA treatment induces apoptosis in chondrocytes, as confirmed by increases in the percent of apoptotic cells, up-regulation of cytochrome c and caspase-3 protein levels. Treatment with MIA increases ROS production and decreases the levels of ΔΨm. The antioxidant, N-acetylcysteine (NAC), significantly prevented the production of ROS, the reduction of ΔΨm, the release of cytochrome c and the activation of caspase-3. Further, NAC completely protected the cells from MIA-induced apoptosis. Together these observations suggest that the mechanisms of MIA-induced apoptosis are primarily via ROS production and mitochondria-mediated caspase-3 activation in primary rat chondrocytes.

A proteomic approach for identification and localization of the pericellular components of chondrocytes

Although the pericellular matrix (PCM) plays a central role in the communication between chondrocytes and extracellular matrix, its composition is largely unknown. In this study, the PCM was investigated with a proteomic approach using chondrons, which are enzymatically isolated constructs including the chondrocyte and its surrounding PCM. Chondrons and chondrocytes alone were isolated from human articular cartilage. Proteins extracted from chondrons and chondrocytes were used for two-dimensional electrophoresis. Protein spots were quantitatively compared between chondron and chondrocyte gels. Cellular proteins, which had similar density between chondron and chondrocyte gels, did not proceed for analysis. Since chondrons only differ from chondrocytes in association of the PCM, protein spots in the chondron gels that had higher quantity than that in the chondrocyte gels were selected as candidates of the PCM components and processed for mass spectrometry. Among 15 identified peptides, several were fragments of the three type VI collagen chains (α-1, α-2, and α-3). Other identified PCM proteins included triosephosphate isomerase, transforming growth factor-β induced protein, peroxiredoxin-4, ADAM (A disintegrin and metalloproteinases) 28, and latent-transforming growth factor beta-binding protein-2. These PCM components were verified with immunohisto(cyto)chemistry for localization in the PCM region of articular cartilage. The abundance of type VI collagen in the PCM emphasizes its importance to the microenvironment of chondrocytes. Several proteins were localized in the PCM of chondrocytes for the first time and that warrants further investigation for their functions in cartilage biology.