The establishment of diseased human whole organ model by normothermic machine perfusion technique: Principle of concept.

BACKGROUND: The global incidence of liver diseases is rising, yet there remains a dearth of precise research models to mimic these diseases. The use of normothermic machine perfusion (NMP) to study diseased livers recovered from liver transplantation (LT) recipients presents a promising avenue. Accordingly, we have developed a machine perfusion system tailored specifically for the human whole diseased livers and present our experience from the NMP of diseased livers. METHODS: Six diseased livers recovered from LT recipients with different diagnoses were collected. The diseased livers were connected to the machine perfusion system that circulated tailored perfusate, providing oxygen and nutrients. The pressure and flow of the system were recorded, and blood gas analysis and laboratory tests of perfusate and bile were examined to analyze the function of the diseased livers. Liver tissues before and after NMP were collected for histological analysis. RESULTS: Experiments showed that the system maintained the diseased livers in a physiological state, ensuring stable hemodynamics and a suitable partial pressure of oxygen and carbon dioxide. The results of blood gas analysis and laboratory tests demonstrated a restoration and sustenance of metabolism with minimal damage. Notably, a majority of the diseased livers exhibited bile production continuously, signifying their vivid functional integrity. The pathological characteristics remained stable before and after NMP. CONCLUSION: We successfully established the machine perfusion system tailored specifically for diseased human whole livers. Through the application of this system, we have developed a novel in vitro model that faithfully recapitulates the main features of human liver disease. This model holds immense promise as an advanced disease modeling platform, offering profound insights into liver diseases and potential implications for research and therapeutic development.

[Mechanical Design and Research of Wearable Exoskeleton Assisted Robot for Upper Limb Rehabilitation].

Based on the biomechanical mechanism of human upper limb, the disadvantages of traditional rehabilitation training and the current status of upper limb rehabilitation robot, a six degree of freedom, flexible adjustment, wearable upper limb rehabilitation exoskeleton design scheme is proposed. Firstly, the mechanics of each joint of the upper limb is analyzed, and the virtual prototype design of the whole mechanical structure of the upper limb rehabilitation wearable exoskeleton is carried out by using CATIA three-dimensional software. The tooth transmission of the forearm and the upper arm single row four point contact ball bearing with internal/external rotation and the shoulder flexible passive adjustment mechanism (viscoelastic damper) are innovatively designed. Then, the joints of the upper limb rehabilitation exoskeleton are analyzed, theoretical analysis and calculation of the driving torque, the selection of the motor and gearbox of each driving joint are carried out. Finally, the whole finite element analysis of the upper limb exoskeleton is carried out. The research and experimental results showed that the design scheme of the upper limb exoskeleton assist structure is highly feasible, which can help the patients with upper limb paralysis and motor dysfunction self-rehabilitation.

High-Temperature Requirement A1 Protease as a Rate-Limiting Factor in the Development of Osteoarthritis.

Preserving the mature articular cartilage of joints is a critical focus in the prevention and treatment of osteoarthritis. We determined whether the genetic inactivation of high-temperature requirement A1 (HtrA1) can significantly attenuate the degradation of articular or condylar cartilage. Two types of mouse models of osteoarthritis were used, a spontaneous mutant mouse model [type XI collagen-haploinsufficient (Col11a1+/-) mice] and two post-traumatic mouse models [destabilization of the medial meniscus (DMM) on the knee and a partial discectomy (PDE) on the temporomandibular joint]. Three different groups of mice were generated: i) HtrA1 was genetically deleted from Col11a1+/- mice (HtrA1-/-;Col11a1+/-), ii) HtrA1-deficient mice (HtrA1-/-) were subjected to DMM, and iii) HtrA1-/- mice were subjected to PDE. Knee and temporomandibular joints from the mice were characterized for evidence of cartilage degeneration. The degradation of articular or condylar cartilage was significantly delayed in HtrA1-/-;Col11a1+/- mice and HtrA1-/- mice after DMM or PDE. The amount of collagen type VI was significantly higher in the articular cartilage in HtrA1-/-;Col11a1+/- mice, compared with that in Col11a1+/- mice. The genetic removal of HtrA1 may delay the degradation of articular or condylar cartilage in mice.

Discoidin Domain Receptor 2 as a Potential Therapeutic Target for Development of Disease-Modifying Osteoarthritis Drugs.

Osteoarthritis (OA) is the most common form of arthritis disorders, but the identification of therapeutic targets to effectively prevent OA has been increasingly difficult. The goal of this investigation is to provide experimental evidence that discoidin domain receptor 2 (DDR2) may be an ideal target for the development of disease-modifying OA drugs. Ddr2 was conditionally deleted from articular cartilage of adult mouse knee joints. Aggrecan-CreERT2;floxed Ddr2 mice, which were generated by crossing Aggrecan-CreERT2 mice with floxed Ddr2 mice, then received tamoxifen injections at the age of 8 weeks. The mice were then subjected to destabilization of the medial meniscus (DMM) surgery. At 8 and 16 weeks after DMM, mice were euthanized for the collection of knee joints. In a separate experiment, Aggrecan-CreERT2;floxed Ddr2 mice were subjected to DMM at the age of 10 weeks. The mice then received tamoxifen injections at 8 weeks after DMM. The mice were euthanized for the collection of knee joints at 16 weeks after DMM. The progressive process of articular cartilage degeneration was significantly delayed in the knee joints of Ddr2-deficient mice in comparison to their control littermates. Articular cartilage damage in the knee joints of the mice was associated with increased expression profiles of both Ddr2 and matrix metalloproteinase 13. These findings suggest that DDR2 may be an ideal target for the development of disease-modifying OA drugs.

Attenuation of the progression of articular cartilage degeneration by inhibition of TGF-ß1 signaling in a mouse model of osteoarthritis.

Transforming growth factor beta 1 (TGF-ß1) is implicated in osteoarthritis. We therefore studied the role of TGF-ß1 signaling in the development of osteoarthritis in a developmental stage-dependent manner. Three different mouse models were investigated. First, the Tgf-ß receptor II (Tgfbr2) was specifically removed from the mature cartilage of joints. Tgfbr2-deficient mice were grown to 12 months of age and were then euthanized for collection of knee and temporomandibular joints. Second, Tgfbr2-deficient mice were subjected to destabilization of the medial meniscus (DMM) surgery. Knee joints were then collected from the mice at 8 and 16 weeks after the surgery. Third, wild-type mice were subjected to DMM at the age of 8 weeks. Immediately after the surgery, these mice were treated with the Tgfbr2 inhibitor losartan for 8 weeks and then euthanized for collection of knee joints. All joints were characterized for evidences of articular cartilage degeneration. Initiation or acceleration of articular cartilage degeneration was not observed by the genetic inactivation of Tgfbr2 in the joints at the age of 12 months. In fact, the removal of Tgfbr2 and treatment with losartan both delayed the progression of articular cartilage degeneration induced by DMM compared with control littermates. Therefore, we conclude that inhibition of Tgf-ß1 signaling protects adult knee joints in mice against the development of osteoarthritis.

Articular cartilage degeneration in the contralateral non-surgical temporomandibular joint in mice with a unilateral partial discectomy.

OBJECTIVE: The objective was to characterize the contralateral non-surgical temporomandibular joint (TMJ) in mice that had an opposing osteoarthrosis(OA)-like joint induced by unilateral partial discectomy. METHODS: TMJs on one side in mice were subjected to partial discectomy. Both surgical and contralateral non-surgical TMJs were collected at 4, 8, 12 and 16 weeks post-surgery for histological examination. The morphology of the articular cartilage of the condyle was evaluated using a scoring system. RESULTS: A progression of articular cartilage degeneration was seen in the TMJs following unilateral partial discectomy, including increased proteoglycan staining in the extracellular matrix at 4 weeks, the appearance of chondrocyte clusters at 8 weeks, reduced proteoglycan staining and fibrillation at 12 weeks and the loss of articular cartilage at 16 weeks. In the contralateral non-surgical TMJs, increased proteoglycan staining occurred in the articular cartilage of the condyle at 8 weeks and continued to age. CONCLUSION: The result indicated that OA-like changes in one TMJ by partial discectomy could initiate early-onset articular cartilage degeneration in the contralateral non-surgical TMJ in mice.

Delayed progression of condylar cartilage degeneration, by reduction of the discoidin domain receptor 2, in the temporomandibular joints of osteoarthritic mouse models.

OBJECTIVE: To determine whether reduction of the discoidin domain receptor 2 (Ddr2) delays the progression of condylar cartilage degeneration in the temporomandibular joint (TMJ) of mouse models with osteoarthritis (OA). METHODS: Double-heterozygous (Col11a1- and Ddr2-haploinsufficiency, Col11a1(+/−);Ddr2(+/−)) mice were generated. TMJs of Ddr2(+/−) mice were subjected to partial discectomy. Condylar cartilage from the TMJ of Col11a1(+/−);Ddr2(+/−) mice, surgically treated (discectomy) Ddr2(+/−) mice, and their corresponding controls was characterized by means of histology and evaluated using a scoring system specific to mouse joints. RESULTS: The progression of condylar cartilage degeneration was significantly delayed in the TMJ of Col11a1(+/−);Ddr2(+/−) mice compared with those of the Col11a1(+/−) mice. The progression of condylar cartilage degeneration in the TMJ of Ddr2(+/−) mice following discectomy was also significantly delayed when compared with their wild-type littermates. CONCLUSION: Reduced expression of Ddr2 delays the progression of condylar cartilage degeneration, induced either by type XI collagen haploinsufficiency or by a partial discectomy, in TMJ.

Substantial decline of organ preservation fluid contamination following adoption of ischemia-free liver transplantation: a post-hoc analysis.

INTRODUCTION: Preservation fluid (PF) contaminations are common in conventional liver transplantation (CLT) and presumably originate from organ or PF exposures to the external environment in a non-strict sterile manner. Such exposures and PF contamination may be avoided in ischaemia-free liver transplantation (IFLT) because of the strict sterile surgical procedures. In this study, the authors evaluated the impact of IFLT on organ PF contamination. METHODS: A post-hoc analysis using data from the first randomized controlled trial of IFLT was performed to compare the incidence, pathogenic spectrum of PF contamination, and incidence of early recipient infection between IFLT and CLT. Multivariable logistic regression was used to explore risk factors for PF contamination. RESULTS: Of the 68 cases recruited in the trial, 64 were included in this post-hoc analysis. The incidence of culture-positive PF was 9.4% (3/32) in the IFLT group versus 78.1% (25/32) in the CLT group ( P <0.001). Three microorganisms were isolated from PF in the IFLT group, while 43 were isolated in the CLT group. The recipient infection rate within postoperative day 14 was 3.1% (1/32) in the IFLT group vs 15.6% (5/32) in the CLT group, although this difference did not reach statistical significance ( P =0.196). Multivariate analysis revealed that adopting IFLT is an independent protective factor for culture-positive PF. CONCLUSION: PF contamination is substantially decreased in IFLT, and IFLT application is an independent protective factor for PF contamination. Using rigorous sterile measures and effective antibiotic therapy during IFLT may decrease PF contamination.

E74-like factor 3 (ELF3) impacts on matrix metalloproteinase 13 (MMP13) transcriptional control in articular chondrocytes under proinflammatory stress.

Matrix metalloproteinase (MMP)-13 has a pivotal, rate-limiting function in cartilage remodeling and degradation due to its specificity for cleaving type II collagen. The proximal MMP13 promoter contains evolutionarily conserved E26 transformation-specific sequence binding sites that are closely flanked by AP-1 and Runx2 binding motifs, and interplay among these and other factors has been implicated in regulation by stress and inflammatory signals. Here we report that ELF3 directly controls MMP13 promoter activity by targeting an E26 transformation-specific sequence binding site at position -78 bp and by cooperating with AP-1. In addition, ELF3 binding to the proximal MMP13 promoter is enhanced by IL-1ß stimulation in chondrocytes, and the IL-1ß-induced MMP13 expression is inhibited in primary human chondrocytes by siRNA-ELF3 knockdown and in chondrocytes from Elf3(-/-) mice. Further, we found that MEK/ERK signaling enhances ELF3-driven MMP13 transactivation and is required for IL-1ß-induced ELF3 binding to the MMP13 promoter, as assessed by chromatin immunoprecipitation. Finally, we show that enhanced levels of ELF3 co-localize with MMP13 protein and activity in human osteoarthritic cartilage. These studies define a novel role for ELF3 as a procatabolic factor that may contribute to cartilage remodeling and degradation by regulating MMP13 gene transcription.

Intact pericellular matrix of articular cartilage is required for unactivated discoidin domain receptor 2 in the mouse model.

Increased expression of the discoidin domain receptor 2 (DDR2) results from its interaction with collagen type II. This induces expression of matrix metalloproteinase (MMP)-13, leading to osteoarthritis (OA). To investigate the impact of the pericellular matrix of chondrocytes on DDR2, we generated a mouse model with inducible overexpression of DDR2 in cartilage. Conditional overexpression of DDR2 in mature mouse articular cartilage was controlled via the cartilage oligomeric matrix protein promoter using the Tet-Off-inducible system. Doxycycline was withdrawn at 1 month of age, and knee joints were examined at 2, 3, and 4 months of age. Microsurgery was performed on 3-month-old transgenic mice overexpressing DDR2 to destabilize the medial meniscus, and serial paraffin sections were examined at 2, 4, 8, and 12 weeks after surgery. DDR2 expression increased in the knee joints of transgenic mice. However, the increased DDR2 did not induce MMP-13 expression. No OA-like changes were observed in the transgenic mice at the age of 4 months. When transgenic mice were subjected to destabilizing of the medial meniscus, we observed accelerated progression to OA, which was associated with DDR2 activation. Therefore, conditionally overexpressing DDR2 in the mature articular cartilage of mouse knee joints requires activation to induce OA, and altered biomechanical stress can accelerate the onset of cartilage loss and progression to OA in transgenic mice.

Clinical Observation of Alternative Wave Electroacupuncture Combined with Lee's Naprapathy in Treating Knee Osteoarthritis (Blood Stasis due to Qi Stagnation).

Objective: We aim to explore the clinical therapeutic effect of alternative wave electroacupuncture combined with Lee's naprapathy therapy on knee osteoarthritis (KOA) (blood stasis due to qi stagnation). Method: 122 patients with KOA treated in our hospital from January 2018 to October 2021 were randomly grouped into a combined group (n = 61) and a control group (n = 61). The combined group was treated with alternative wave electroacupuncture combined with Lee's naprapathy, while the control group was treated with alternative wave electroacupuncture alone. Clinical efficacy of the two groups was observed. The Visual Analogue Scale (VAS), Lysholm Scale, Indexes of Severity for Osteoarthritis (ISOA), and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) were compared before and after treatment, followed up for 3 months and 6 months. The adverse reactions of the two groups were observed. Result: The overall response rate of the combined group (96.72%) was higher than that of the control group (81.97%), and the difference was statistically significant (P < 0.05). After treatment and follow-up for 3 months and 6 months, the Lysholm score of the combined group was higher than that of the control group, while the VAS, ISOA, and WOMAC scores were lower than those of the control group, and the difference between the two was statistically significant (P < 0.05). There were no serious adverse reactions in both groups (P > 0.05). Conclusion: The alternative wave electroacupuncture combined with Lee's naprapathy is effective and safe in treating KOA (blood stasis due to qi stagnation).

Attenuation of osteoarthritis progression by reduction of discoidin domain receptor 2 in mice.

OBJECTIVE: To investigate whether the reduction of discoidin domain receptor 2 (DDR-2), a cell membrane tyrosine kinase receptor for native type II collagen, attenuates the progression of articular cartilage degeneration in mouse models of osteoarthritis (OA). METHODS: Double-heterozygous (type XI collagen-deficient [Col11a1(+/-)] and Ddr2-deficient [Ddr2(+/-)]) mutant mice were generated. Knee joints of Ddr2(+/-) mice were subjected to microsurgical destabilization of the medial meniscus. Conditions of the articular cartilage from the knee joints of the double-heterozygous mutant and surgically treated mice were examined by histology, evaluated using a modified Mankin scoring system, and characterized by immunohistochemistry. RESULTS: The rate of progressive degeneration in knee joints was dramatically reduced in the double-heterozygous mutant mice compared with that in the type XI collagen-deficient mice. The progression in the double-heterozygous mutant mice was delayed by ∼6 months. Following surgical destabilization of the medial meniscus, the progressive degeneration toward OA was dramatically delayed in the Ddr2(+/-) mice compared with that in their wild-type littermates. The articular cartilage damage present in the knee joints of the mice was directly correlated with the expression profiles of DDR-2 and matrix metalloproteinase 13. CONCLUSION: Reduction of DDR-2 expression attenuates the articular cartilage degeneration of knee joints induced either by type XI collagen deficiency or by surgical destabilization of the medial meniscus.

Targeting Discoidin Domain Receptor 2 for the Development of Disease-Modifying Osteoarthritis Drugs.

One of the most pressing issues in osteoarthritis (OA) research is the development of disease-modifying OA drugs (DMOADs), as currently there are no such drugs available. The paucity of suitable DMOADs is mostly due to the lack of approved ideal therapeutic targets necessary for the development of these drugs. However, based on recent discoveries from our laboratory and other independent laboratories, it is indicated that a cell surface receptor tyrosine kinase for collagen type II, discoidin domain receptor 2 (DDR2), may be an ideal therapeutic target for the development of DMOADs. In this article, we review the current status of research in understanding roles of DDR2 in the development of OA.

A Molecular Cascade Underlying Articular Cartilage Degeneration.

Preserving of articular cartilage is an effective way to protect synovial joints from becoming osteoarthritic (OA) joints. Understanding of the molecular basis of articular cartilage degeneration will provide valuable information in the effort to develop cartilage preserving drugs. There are currently no disease-modifying OA drugs (DMOADs) available to prevent articular cartilage destruction during the development of OA. Current drug treatments for OA focus on the reduction of joint pain, swelling, and inflammation at advanced stages of the disease. However, based on discoveries from several independent research laboratories and our laboratory in the past 15 to 20 years, we believe that we have a functional molecular understanding of articular cartilage degeneration. In this review article, we present and discuss experimental evidence to demonstrate a sequential chain of the molecular events underlying articular cartilage degeneration, which consists of transforming growth factor beta 1, high-temperature requirement A1 (a serine protease), discoidin domain receptor 2 (a cell surface receptor tyrosine kinase for native fibrillar collagens), and matrix metalloproteinase 13 (an extracellularmatrix degrading enzyme). If, as we strongly suspect, this molecular pathway is responsible for the initiation and acceleration of articular cartilage degeneration, which eventually leads to progressive joint failure, then these molecules may be ideal therapeutic targets for the development of DMOADs.

Protective effects of the pericellular matrix of chondrocyte on articular cartilage against the development of osteoarthritis.

Understanding the pathogenesis of osteoarthritis (OA) provides invaluable information in the search of therapeutic targets for the development of disease-modifying OA drugs. Emerging results from investigations demonstrate that the pericellular matrix of chondrocytes plays important roles in protecting articular cartilages from being degraded. Thus, maintaining the structural integrity of the pericellular matrix may be an effective approach to prevent the development of osteoarthritic joints. In this review article, we discuss the consequences of lacking one or more components of the pericellular matrix, and biological effects of the destruction of the pericellular matrix in the development of OA. We believe that more attention should be directed towards the pericellular matrix for the identification of novel biomarkers and therapeutic targets for the prevention and treatment of OA.

Roles of TGF-beta 1 signaling in the development of osteoarthritis.

Osteoarthritis (OA) is a degenerative joint disorder characterized by the destruction of articular cartilage, subchondral bone and other joint tissues. Although multiple growth factors and cytokines have been shown to be involved in articular cartilage degeneration and subchondral bone destruction, which eventually leads to OA, the molecular mechanisms underlying the pathogenesis of OA are largely unknown. The canonical transforming growth factor beta 1 (TGF-ß1) signaling functions as one of the key factors in cartilage and bone formation, remodeling, and maintenance. However, the effects of TGF-ß1 signaling on the development of OA are unclear. Numerous studies provide evidence that TGF-ß1 is required for the formation of articular cartilage at early stages of joint development. In contrast, other investigations indicate that TGF-ß1 may, in fact, be a factor in joint destruction. Therefore, we, in this review article, discuss the "conflicting" roles of TGF-ß1 signaling in the development of OA.

Induced superficial chondrocyte death reduces catabolic cartilage damage in murine posttraumatic osteoarthritis.

Joints that have degenerated as a result of aging or injury contain dead chondrocytes and damaged cartilage. Some studies have suggested that chondrocyte death precedes cartilage damage, but how the loss of chondrocytes affects cartilage integrity is not clear. In this study, we examined whether chondrocyte death undermines cartilage integrity in aging and injury using a rapid 3D confocal cartilage imaging technique coupled with standard histology. We induced autonomous expression of diphtheria toxin to kill articular surface chondrocytes in mice and determined that chondrocyte death did not lead to cartilage damage. Moreover, cartilage damage after surgical destabilization of the medial meniscus of the knee was increased in mice with intact chondrocytes compared with animals whose chondrocytes had been killed, suggesting that chondrocyte death does not drive cartilage damage in response to injury. These data imply that chondrocyte catabolism, not death, contributes to articular cartilage damage following injury. Therefore, therapies targeted at reducing the catabolic phenotype may protect against degenerative joint disease.

Advances in understanding cartilage remodeling.

Cartilage remodeling is currently among the most popular topics in osteoarthritis research. Remodeling includes removal of the existing cartilage and replacement by neo-cartilage. As a loss of balance between removal and replacement of articular cartilage develops (particularly, the rate of removal surpasses the rate of replacement), joints will begin to degrade. In the last few years, significant progress in molecular understanding of the cartilage remodeling process has been made. In this brief review, we focus on the discussion of some current "controversial" observations in articular cartilage degeneration: (1) the biological effect of transforming growth factor-beta 1 on developing and mature articular cartilages, (2) the question of whether aggrecanase 1 (ADAMTS4) and aggrecanase 2 (ADAMTS5) are key enzymes in articular cartilage destruction, and (3) chondrocytes versus chondron in the development of osteoarthritis. It is hoped that continued discussion and investigation will follow to better clarify these topics. Clarification will be critical for those in search of novel therapeutic targets for the treatment of osteoarthritis.

Induction of high temperature requirement A1, a serine protease, by TGF-beta1 in articular chondrocytes of mouse models of OA.

The goal of this study is to determine whether transforming growth factor beta 1 (Tgf-ß1) induces the high temperature requirement A1 (HtrA1) in articular chondrocytes of two mouse models of osteoarthritis (OA). Early onset articular cartilage degeneration in the mouse models was characterized by histology. Expression profiles of Tgf-ß1, p-Smad1, p-Smad2/3 and HtrA1 in knee joints of the mouse models were examined by immunofluorescene. By in vitro and ex vivo experiments, human primary chondrocytes and articular cartilages from femoral condyles of mice were treated with recombinant human TGF-ß1 and an ALK5 chemical inhibitor, SB431542. The level of HTRA1 mRNA in human and mouse articular chondrocytes was examined by real-time PCR. Chondrocyte clusters were present in the articular cartilage of knee joints in the mouse models. Increased expressions of Tgf-ß1, p-Smad2/3 and HtrA1 were detected in the articular chondrocyte of knee joints in the mouse models. The increased expressions of p-Smad2/3 and HtrA1 were co-localized in the articular chondrocyte of the knee joints. The expression of p-Smad1 was hardly detectable in the mouse models and their corresponding wild-type littermates. The level of HTRA1 mRNA was increased in human and mouse articular chondrocytes treated with TGF-ß1, compared with that in chondrocytes without the treatment of TGF-ß1. The TGF-ß1-induced expression of HTRA1 in human and mouse articular chondrocytes was inhibited by SB431542. These results suggest that the Tgf-ß1 canonical signaling was activated to induce HtrA1 in the articular chondrocytes of the mouse models of OA.

Role of HTRA1, a serine protease, in the progression of articular cartilage degeneration.

This study is to investigate the possible role of high temperature requirement A 1 (HtrA1) in the articular cartilage degeneration. Paraffin sections were prepared from the knee and temporomandibular (TM) joints of four mouse OA models; two of the models had a genetic mutation (type IX collagen-deficient and type XI collagen-haploinsufficient) and two were surgically induced (destabilization of the medial meniscus of knee joint and discectomy of TM joint). The HtrA1 protein expression profiles of the prepared sections were examined by immunohistostaining. The level of HtrA1 mRNA in the articular cartilage taken from the knee joints of one of the genetically mutated OA models was determined by real-time PCR. Double immunohistostaining was used to examine the expression of co-localization of HtrA1 with type VI collagen and HtrA1 with discoidin domain receptor 2 (Ddr2) in the articular cartilage of knee joints from the genetically mutated OA model. The expression of HtrA1 was found to be increased in the knee and TM joints of these four models at early stages of the disease. An examination of the knee joint of a mutant mouse indicated an 8-fold increase in the level of HtrA1 mRNA, when compared to the levels observed in the knee joints of its wild-type littermates. Pericellular type VI collagen was not present in chondrocytes expressing HtrA1. Meanwhile, the expression of HtrA1 was associated with the expression of Ddr2 in the chondrocytes. Results indicate that HtrA1 may disrupt the pericellular matrix network, resulting in alteration of chondrocyte metabolisms. This eventually leads to OA.