Human Cartilage Oligomeric Matrix Protein Research Articles

COMP promotes pancreatic fibrosis by activating pancreatic stellate cells through CD36-ERK/AKT signaling pathways

BackgroundPancreatic fibrosis is one of the most important pathological features of chronic pancreatitis (CP) and pancreatic stellate cells (PSCs) are the key cells of fibrosis. As an extracellular matrix (ECM) glycoprotein, cartilage oligomeric matrix protein (COMP) is critical for collagen assembly and ECM stability and recent studies showed that COMP exert promoting fibrosis effect in the skin, lungs and liver. However, the role of COMP in activation of PSCs and pancreatic fibrosis remain unclear. We aimed to investigate the role and specific mechanisms of COMP in regulating the profibrotic phenotype of PSCs and pancreatic fibrosis. MethodsELISA method was used to determine serum COMP in patients with CP. Mice model of CP was established by repeated intraperitoneal injection of cerulein and pancreatic fibrosis was evaluated by Hematoxylin-Eosin staining (H&E) and Sirius red staining. Immunohistochemical staining was used to detect the expression changes of COMP and fibrosis marker such as α-SMA and Fibronectin in pancreatic tissue of mice. Cell Counting Kit-8, Wound Healing and Transwell assessed the proliferation and migration of human pancreatic stellate cells (HPSCs). Western blotting, qRT-PCR and immunofluorescence staining were performed to detect the expression of fibrosis marker, AKT and MAPK family proteins in HPSCs. RNA-seq omics analysis as well as small interfering RNA of COMP, recombinant human COMP (rCOMP), MEK inhibitors and PI3K inhibitors were used to study the effect and mechanism of COMP on activation of HPSCs. ResultsELISA showed that the expression of COMP significantly increased in the serum of CP patients. H&E and Sirius red staining analysis showed that there was a large amount of collagen deposition in the mice in the CP model group and high expression of COMP, α-SMA, Fibronectin and Vimentin were observed in fibrotic tissues. TGF-β1 stimulates the activation of HPSCs and increases the expression of COMP. Knockdown of COMP inhibited proliferation and migration of HPSCs. Further, RNA-seq omics analysis and validation experiments in vitro showed that rCOMP could significantly promote the proliferation and activation of HPSCs, which may be due to promoting the phosphorylation of ERK and AKT through membrane protein receptor CD36. rCOMP simultaneously increased the expression of α-SMA, Fibronectin and Collagen I in HPSCs. ConclusionIn conclusion, this study showed that COMP was up-regulated in CP fibrotic tissues and COMP induced the activation, proliferation and migration of PSCs through the CD36-ERK/AKT signaling pathway. COMP may be a potential therapeutic candidate for the treatment of CP. Interfering with the expression of COMP or the communication between COMP and CD36 on PSCs may be the next direction for therapeutic research.

An ultra-sensitive SPR immunosensor for quantitative determination of human cartilage oligomeric matrix protein biomarker

This paper reports the development of a novel surface plasmon resonance (SPR) immunosensor for ultra-sensitive quantitative determination of human articular cartilage oligomeric matrix protein (COMP), a major component of the extracellular matrix and an exploratory biomarker. Capture antibodies against human COMP (anti-COMP16F12) were covalently immobilized on an 11-mercaptoundecanoic acid (11-MUA) self-assembled monolayer (SAM)-coated SPR sensor disk and a dual sandwich-type signal amplification strategy using biotinylated detection antibodies against COMP (anti-COMP17C10-biot) and streptavidin-conjugated quantum dots (SAv‒QDs) were used for the development of an immunosensor. The binding of high-mass SAv‒QDs via biotin–streptavidin interaction to the surface of the immunosensor resulted in a drastic increase in the sensitivity. The developed immunosensor was able to detect concentrations of COMP in a range from 2.80 to 680.54 fM with a limit of detection (LOD) and a limit of quantification (LOQ) of 0.15 and 0.50 fM, respectively. The immunosensor exhibited good repeatability (relative standard deviation (RSD) 8.05%) and reproducibility (RSD 9.88%) as well as excellent operational stability (2.14 % decrease in SPR signal after 13 days). In addition, the analysis of secretomes of human knee articular cartilage explants from patients with osteoarthritis revealed that the immunosensor has good accuracy (analytical error less than 5 %). These results indicate that the immunosensor developed may be suitable for quantitative determination of COMP derived from articular cartilage and other synovial joint tissues in clinical studies.

Cartilage Oligomeric Matrix Protein Induced Arthritis-A New Model for Rheumatoid Arthritis in the C57BL/6 Mouse.

The most commonly used strains in experimental research, including genetically modified strains, are C57BL/6 mice. However, so far, no reliable model for rheumatoid arthritis is available, mainly due to the restriction by the MHC class II haplotype H-2b. Collagen-induced arthritis (CIA) is the most widely used animal model of rheumatoid arthritis, but C57BL/6 strain is resistant to CIA because there is no collagen II peptide associated with H-2b. To establish a rheumatoid arthritis model in C57BL/6 mice, we immunized C57BL/6NJ (B6N) mice with human cartilage oligomeric matrix protein (COMP), which induced severe arthritis with high incidence, accompanied by a strong auto-antibody response. Native COMP was required, as denatured COMP lost its ability to induce arthritis in B6N mice. An immunodominant COMP peptide was identified as the key T cell epitope, with a perfect fit into the Ab class II peptide binding pocket. A critical amino acid in this peptide was found to be phenylalanine at position 95. Recombinant COMP mutated at position 95 (COMP_F95S) lost its ability to induce arthritis or a strong immune response in the B6N mice. In conclusion, A new model for RA has been established using C57BL/6 mice through immunization with COMP, which is dependent on a COMP specific peptide binding Ab, thus in similarity with CIA in Aq expressing strains.

Cartilage oligomeric matrix protein (COMP) promotes cell proliferation in early-onset colon cancer tumorigenesis.

Colon cancer (CC) is the third most commonly diagnosed cancer in the USA. While the overall incidence is declining, it is rising alarmingly in young patients (EOCC). CC in young patients tends to be more aggressive and often diagnosed at more advanced stages and portend poorer prognosis. Our recently published data showed that EOCC is a distinct disease with unique molecular features compared to late-onset CC (LOCC). The Cartilage Oligomeric Matrix Protein (COMP) was shown to be significantly upregulated in EOCC and correlated with poor survival. However, the role of COMP in CC tumorigenesis, especially in young patients, is not well understood. Thus, the aim of this study was to elucidate the role of COMP in CC tumorigenesis by modulating COMP levels in vitro and test how it affects proliferation. Then, patient samples were evaluated by testing the levels of proliferation marker Ki67. In addition, this study investigates whether higher transcriptional mRNA levels of COMP seen in more aggressive early-onset CC correlate with protein levels compared to late-onset CC. COMP mRNA levels in fresh frozen colon tumors (young: n = 5; old: n = 5) were assessed by quantitative PCR (qPCR). Additionally, CC cell lines were profiled for COMP expression to choose an in vitro model to study the role of COMP in CC tumorigenesis. HT-29 (low COMP expression) and CaCo-2 (high COMP expression) cells were used for in vitro proliferation studies. Immunohistochemical (IHC) analysis was conducted to assess COMP and Ki67 protein levels in formalin-fixed paraffin-embedded (FFPE) colon tumors. Significantly higher COMP expression levels were observed in fresh frozen EOCC compared to LOCC tumors. This observation confirmed our previously reported results from NanoString gene expression assay using FFPE samples. Cell proliferation was significantly increased in HT-29 and CaCo-2 cells upon treatment with human recombinant COMP protein after 48 and 72h (P < 0.05). This increase was more profound in HT-29 cells. Staining for COMP and Ki67 revealed high COMP protein levels in EOCC compared to LOCC patients. COMP mRNA and protein levels are significantly higher in EOCC patients. Higher COMP levels correlate with increased proliferation suggesting a role in CC tumorigenesis.

Enhance the affinity of nanobodies to CD47 antigen by fusion expression of COMP48 self-folding peptide

Objective To modify CD47 nanobody with the self-folding peptide human cartilage oligomeric matrix protein (COMP48) so as to enhance its affinity to CD47 antigen. Methods The fusion sequences of COMP48 and CD47 nanobody (VHHB1) were designed and synthesized, and the recombinant plasmid pET22b-VHHB1-COMP48 was constructed and transformed into E. coli BL21 (DE3) to induce expression of the fusion protein. The binding specificity and affinity of the fusion protein and the antigen CD47 were detected by Western Blot, indirect enzyme-linked immunosorbent assay (ELISA) and non-competitive ELISA. Results The recombinant VHHB1-COMP48 was expressed in BL21(DE3) by inducing with 1 mmol/L IPTG and purified at 90% homogenous in IMAC. Western Blot results showed that the recombinant protein VHHB1-COMP48 specifically binds to antigen CD47 but not to unrelated protein. The indirect ELISA and non-competitive ELISA results showed that the affinity of the conjugated recombinant protein VHHB1-COMP48 was enhanced compared to that of the non-conjugated nanobody, and the difference was statistically significant (P<0.01). Through non-competitive ELISA, the constants of affinity and dissociation constants were 6.97×107 L/mol and 1.434×10-8 mol/L, respectively. Conclusions The affinity of the nanobody for the antigen can be improved by conjugating a human cartilage matrix protein (COMP48) after the nanobody. Key words: Nanobody; Human cartilage oligomeric matrix protein; Affinity maturation; Self-folding peptide; CD47

A powerful ICOS agonist that enhances anti-tumor immune responses restored by immune checkpoint inhibitors

Abstract The Inducible T cell Costimulator (ICOS, CD278) is a receptor in the CD28 family of B7-binding proteins expressed mostly by activated T cells. Upon binding to its ligand ICOS-L expressed on APCs,T cells are further activated by ICOS, resulting in their expansion and production of effector cytokines. Clinical data in patients treated with anti-CTLA-4 or anti-PD-1 mAbs have shown that ICOShiT cells correlated with an increased treatment response in these patients. Here, we report the design of a high avidity pentavalent form of the human ICOS-L extracellular domain fused to a short α-helical peptide from the human cartilage oligomeric matrix protein (COMP). This ICOS agonist, termed ICOS-L.COMP, spontaneously assembles into stable pentamers, binds tightly to both human and murine ICOS (SPR; Kd&amp;lt;10 nM) and co-stimulates (with anti CD3 mAb) the proliferation and cytokine release of both murine and human CD4+ and CD8+ T cells when added as a soluble factor (unlike ICOS-L.Fc). Soluble ICOS-L.COMP also phosphorylates Akt in activated T cells and enhances anti-tumor immune responses in tumor-bearing mice [murine MC38 or CT26 tumors] treated with either anti-murine PD-1 or anti-murine CTLA-4 mAb. Impressively, most mice treated with ICOS-L.COMP combined with an immune checkpoint inhibitor were either cured or displayed a stable tumor burden in contrast to checkpoint monotherapies (delayed tumor progression) or when ICOS-L.COMP was given as a monotherapy (comparable to no treatment). Importantly, ICOS-L.COMP lacks a Fc domain thus eliminating Fc-associated off-target adverse effects. In summary, our results suggest that ICOS-L.COMP represents a new and powerful biologic to be used in combination with existing immune checkpoint inhibitors.

Blood serum human cartilage oligomeric matrix protein (hCOMP) concentration in closed kinematic chain exercises and physiological cyclic loading using enzyme-linked immunosorbent assay (ELISA)

Patients with high levels of human cartilage oligomeric matrix protein (hCOMP) in serum might indicate increased degradation of articular cartilage. Conflicting studies recommend both open kinematic chain exercises, which imposes minimal shock load in joints of arthritic patients to prevent further cartilage degeneration, and close kinematic chain exercises among healthy individuals which can significantly increase serum hCOMP concentration. This study was conducted to establish a scientific basis in choosing between physiological cyclic loading (PCL) and closed kinematic chain exercises (CKCE) for individuals of varying body mass index having weight bearing joint conditions associated with diminishing cartilage turn over resulting from loss of hCOMP after cartilage degeneration. Blood samples were drawn from 45 healthy male adults before and after intervention. All subjects were randomly distributed to 3 groups with 15 subjects each. Group A performed CKCE for 30 minutes as many repetition as possible, Group B executed (PCL) for 30 minutes at their own pace, while the remaining 15 subjects served as control. The blood samples were centrifuged to separate the serum. The hCOMP in the serum was measured using the ELISA kit at absorbance Optical Density 450 pg/ml. There is a significant difference in pre-and post-intervention serum hCOMP concentration in CKCE, PCL, and the control group. There was also a significant difference in the post-intervention serum hCOMP concentration among the three groups. In contrast, the BMI and post-intervention serum hCOMP concentration within each group did not show any significant relationship but it was observed that BMI was inversely proportional to the hCOMP serum level 30-minutes post-exercise and resting.

NRAMPI (SLC11A1) Gene-1703 G/A Polymorphisms as Prognostic and Diagnostic Marker of Rheumatoid Arthritis in Egyptian Population

Rheumatoid arthritis (RA) is an autoimmune disease with difficulty to diagnose. The association of natural resistance-associated macrophage protein-1 (NRAMP-1) gene polymorphisms with rheumatoid arthritis (RA) is the aim of this study. Methods: Two hundred individuals who were divided into two groups: RA patients group (100 Patients with RA) and control group: (100 apparently healthy subjects). NRAMP1 polymorphisms were determined by the polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) method, including D543N. Results: Rheumatoid arthritis patients demonstrated significant increase in C-reactive protein (CRP), Rheumatoid Factor Ab, Human cartilage oligomeric matrix protein (COMP) and Anti-Cyclic Citrullinated Peptide (anti-CCP) in comparison to normal. Clinical disease activity index (CDAI) and the radiological erosion score of joints in RA patients were 29.8 ± 5.3 and 60.03 ± 38.71 respectively. The G/G, G/A and A/A genotypes were 64, 33, and 3% respectively in RA patients and were 60, 25, and 15% respectively in controls with a significant difference between RA patients and controls. The CDAI and radiological erosion score of joints had the least values in NN followed by DN while DD patients had the highest levels. The patients with phenotype DD had a grade III of disease in 40 patients and grade IV in 21 patients while grade II found in three patients. However, in patients with phenotype DN, the grading of disease II, III and IV were found to be 10, 20 and 3 respectively. All patients with NN phenotype showed a grade II of disease. There was a significant difference in percentage between RA patients and controls as regard to G or A alleles distribution. The nodule associated with G/G genotype in 58.8% while bone erosion associated with G/G genotype in 65% of RA patients. Conclusion: The present study showed that NRAMP1 1703G (543D) is a risk factor for the development of RA. The estimated NRAMP1 1703G haplotype is associated with susceptibility to RA. In RA patients with 1703A, development of rheumatoid nodule is absent.

(99m) Tc-labeled tetramer and pentamer of single-domain antibody for targeting epidermal growth factor receptor in xenografted tumors.

The single-domain antibody EG2 can be fused with right-handed coiled-coil (RHCC) and human cartilage oligomeric matrix protein (COMP), to form the multivalent antibodies EG2-RHCC and EG2-COMP. We labeled these two antibodies with (99m) Tc and assessed their targeting efficiency for epidermal growth factor receptor (EGFR). Cell binding, uptake, efflux, and blocking studies were performed with EGFR high- and/or low-expressing cells with (99m) Tc-labeled EG2-RHCC or EG2-COMP. Single photon-emission computed tomography (SPECT) imaging and biodistribution studies were further carried out. Both (99m) Tc-EG2-RHCC and (99m) Tc-EG2-COMP can specially bind to EGFR in vitro. SPECT imaging showed that A431, which expresses high levels of EGFR, was clearly visible 6 h after (99m) Tc-EG2-COMP injection; however, it was not detectable after administration of (99m) Tc-EG2-RHCC. Uptake of both antibodies by the non-EGFR-secreting OCM-1 tumors was low. EG2-COMP shows promise in identifying EGFR over-expression in tumors; however, EG2-RHCC may not be suitable for targeting EGFR in vivo.

Effects of cartilage oligomeric matrix protein on BMP-2 induced cell differentiation of mesenchymal stem cells

Objective To investigate the effect of overexpression of cartilage oligomeric matrix protein (COMP) on BMP-2 induced cell osteogenic and chondrogenic differentiation of mesenchymal stem cells (MSCs). Methods MSCs, transfected with plasmid DNA encoding recombinant human COMP, were induced to differentiate into osteocytes and chondrocytes by BMP-2. Realtime PCR of osteogenic related markers (Col1a1, RUNX2, OPN, BGP) and chondrogenic related markers (Col2a1, SOX9, Aggrecan) were performed to evaluate the process of cell differentiation. ALP staining, Alizarin red S staining for osteogenic differentiation and alcian blue staining for chondrogenic differentiation were conducted to evaluate the tendency of cell differentiation. Results Real-time PCR assay presented the significantly higher (P<0.05) COMP expression of MSCs when COMP gene was transfected into cells. The expression level of OPN was significantly (P<0.05) down-regulated at all the time points in experimental-group compared with that in control group. A final significant (P<0.05) up-regulation of expression appeared in experimental group at the late stage of induction (day 7, 14) compared with that in control group, even though a decrease (P<0.05) expression of Col1a1, RUNX2 and BGP in experimental group occurred at the early stage of induction (day 3). The expression of Aggrecan and Col2a1 in experimental group was up-regulated (P<0.05) at different time points compared with that in control group. And a significant higher (P<0.05) expression of SOX9 in experimental group only appeared at day 7 compared with that in control group. ALP staining and Alizarin red S staining were weakened while alcian blue staining was enhanced. Conclusion COMP may inhibit BMP-2 induced osteogenic differentiation and promote BMP-2 induced chondrogenic differentiation, which may provide new insight for cartilage tissue engineering. Key words: Extracellular matrix proteins; Bone morphogenetic protein 2; Mesenchymal stem cells; Cell differentiation; Gene expression

Serum Cartilage Oligometric Matrix Protein and Pettersson Scoring System As an Index for Hemophilic Arthropathy

▪The pathogenesis of hemophilic arthropathy is multifactorial, with changes occurring in the synovium, bone, cartilage, and blood vessels. Recurrent joint bleeding causes synovial proliferation and inflammation (hemophilic synovitis) that contributes to end stage degeneration (hemophilic arthropathy); with pain and limitation of motion that severely affects patients’ quality of life.The aim of the present study was to evaluate the degree of joint damage in boys with hemophilia using plain x-ray, and to measure serum level of human cartilage oligomeric matrix protein, to determine its relation to the degree of joint damage.The study was carried out on thirty boys with hemophilic arthropathy (group I) and ten healthy boys were included as control (group II).All hemophilic patients were scored for Functional independence score (FISH score) in hemophilia and radiological changes (Pettersson’s score) using conventional frontal and lateral radiographs of the most affected joint.Factor activity level was measured for all hemophilic patients while serum cartilage oligomeric matrix protein (COMP) was measured for hemophilic patients and control group.Among the thirty hemophilic patients, 26 (86.7%) patients were hemophilia A and 4 (13.3%) patients were hemophilia BAll patients were receiving on demand replacement therapy using plasma derived Factor concentrate or fresh frozen plasma (FFP) according to availability.Fifteen patients (50%) had severe hemophilia, 7 (23.3%) had moderate and 8(26.72%) had mild hemophilia.A higher serum level of COMP with a mean of 757± 211.30 in the severe hemophiliacs, and a mean of 403.57 ± 86.49 and 211.25 ± 74.26 in the moderate and mild hemophiliacs respectively the difference was statistically significant (p < 0.001).Serum level of COMP in group I was significantly higher than in group II (p=0.004) with significant negative correlation with FISH score (r=-0.435 p=0.016). COMP correlated positively with joint space narrowing of the Pettersson score (r=0.421 p<0.001) and with total Pettersson score (r=0.421 p=0.020).The number of joints affected (during life) of hemophilic patients ranged between 1-12 with a mean of 5.50 ± 2.46 Joints. A significant positive correlation between serum level of COMP and number of joints affected (r = 0.487, p = 0.006).Joint space narrowing is the most important indicator of cartilage loss. Serum COMP level is indicative of the amount of joint damage in patients with hemophilic arthropathy.The combined scoring of functional independence and Pettersson in addition with serum levels of COMP give a good overview of the degree of hemophilic arthropathy DisclosuresNo relevant conflicts of interest to declare.

Single Domain Antibody Multimers Confer Protection against Rabies Infection

Post-exposure prophylactic (PEP) neutralizing antibodies against Rabies are the most effective way to prevent infection-related fatality. The outer envelope glycoprotein of the Rabies virus (RABV) is the most significant surface antigen for generating virus-neutralizing antibodies. The small size and uncompromised functional specificity of single domain antibodies (sdAbs) can be exploited in the fields of experimental therapeutic applications for infectious diseases through formatting flexibilities to increase their avidity towards target antigens. In this study, we used phage display technique to select and identify sdAbs that were specific for the RABV glycoprotein from a naïve llama-derived antibody library. To increase their neutralizing potencies, the sdAbs were fused with a coiled-coil peptide derived from the human cartilage oligomeric matrix protein (COMP48) to form homogenous pentavalent multimers, known as combodies. Compared to monovalent sdAbs, the combodies, namely 26424 and 26434, exhibited high avidity and were able to neutralize 85-fold higher input of RABV (CVS-11 strain) pseudotypes in vitro, as a result of multimerization, while retaining their specificities for target antigen. 26424 and 26434 were capable of neutralizing CVS-11 pseudotypes in vitro by 90–95% as compared to human rabies immunoglobulin (HRIG), currently used for PEP in Rabies. The multimeric sdAbs were also demonstrated to be partially protective for mice that were infected with lethal doses of rabies virus in vivo. The results demonstrate that the combodies could be valuable tools in understanding viral mechanisms, diagnosis and possible anti-viral candidate for RABV infection.

Pentamerisation of a scFv directed against TRAIL receptor 2 increases its antitumour efficacy

Single-chain variable fragments (scFvs) have been considered as promising therapeutic drugs. However, their low affinity and rapid clearance from the blood have limited their wider application clinically. In this study, we aimed to improve scFv antibodies by multimerising a scFv against the death receptor 5 (DR5, TNFR10B) through fusion with a coiled-coil domain of human cartilage oligomeric matrix protein 48. By forming a pentamer, nick-named a combody, the avidity of the scFv increased 10⁴-fold and combody was more effective than its monomeric counterpart for antitumour activity in both in vitro and in vivo experiments.

Generation of single-domain antibody multimers with three different self-associating peptides

Conventional Y-shaped antibodies have been widely used in research, diagnostics and therapeutics. Their large size result in disadvantages in certain applications, which makes production difficult. Different parts of an antibody have been used to replace the whole antibody to make it smaller. Single-domain antibodies (sdAbs) derived from the camelid heavy chain antibodies are among the smallest antibody fragments engineered for various applications. To improve the affinity of these single- sdAbs for correspondent antigens and provide suitable size of reagents for various applications, we fused an anti-epidermal growth factor receptor sdAb EG2 to self-associating peptides, RHCC derived from a right-handed coiled-coil peptide of an archaebacterium, COMPcc from human cartilage oligomeric matrix protein and C4bpα derived from human plasma C4-binding protein α-chain, respectively, to make multimeric antibodies. The multimeric antibodies were expressed as soluble cytoplasmic proteins to spontaneously form tetramers, pentamers or heptamers and were purified by affinity chromatography. The avidity of multimeric forms of sdAbs compared with that of the monomeric form of sdAbs was increased without altering binding specificity.

Identification of a 3Kbp Mechanoresponsive Promoter Region in the Human Cartilage Oligomeric Matrix Protein Gene

Expression of chondrocyte-specific genes is regulated by mechanical force. However, despite the progress in identifying the signal transduction cascades that activate expression of mechanoresponsive genes, little is known about the transcription factors that activate transcription of mechanoresponsive genes. The DNA elements that confer mechanoresponsiveness within a cartilage gene promoter have yet to be identified. We have established an experimental system to identify the DNA elements and transcription factors that mediate the mechanoresponse of a promoter to nominal compressive stress in primary human chondrocytes and stem cells in a three-dimensional culture system. Our results demonstrate that the proximal 3 Kb of the human cartilage oligomeric matrix protein promoter is sufficient to mediate a mechanoresponse in human articular chondrocytes and stem cells, and that the magnitude of mechanoresponse correlates to the regulation of the endogenous gene at the RNA and protein level. This information is critical to understanding how mechanical force regulates the transcriptional activation of cartilage genes in three-dimensional culture.

A secreted variant of cartilage oligomeric matrix protein carrying a chondrodysplasia‐causing mutation (p.H587R) disrupts collagen fibrillogenesis

Mutations in human cartilage oligomeric matrix protein (COMP) cause multiple epiphyseal dysplasia or pseudoachondroplasia. Electron microscopic analyses of patient biopsy tissue have shown that, in most cases, mutated COMP is retained in granular or lamellar inclusions in the endoplasmic reticulum of chondrocytes. However, some mutations that do not interfere with protein trafficking, resulting in normal secretion of the mutated protein, have been identified. These mutations are likely to cause the chondrodysplasia phenotype, via events that occur after secretion. The aim of the present study was to identify such extracellular mechanisms associated with the pathogenesis of chondrodysplasias. A mutated but secreted COMP variant, p.H587R, as well as wild-type COMP were recombinantly expressed and purified from cell culture supernatants. Since recent studies have shown that COMP can facilitate collagen fibrillogenesis in vitro, the effect of the p.H587R mutation on this process was determined by analyzing the kinetics of fibrillogenesis in vitro and determining the structure of the collagen fibrils formed by immunogold electron microscopy. Mutated p.H587R COMP accelerated fibril formation by type I collagen in vitro to a slightly greater extent than that with wild-type COMP. However, p.H587R COMP induced aggregation and disorganization of fibril intermediates and end products. Mixtures of cartilage collagens or of type XI collagen alone produced similar results. The addition of p.H587R COMP to preformed fibrils induced aggregation and fusion of the fibrils, whereas wild-type COMP had little effect. The mutant COMP variant p.H587R generally interferes with normal collagen organization during fibrillogenesis. This constitutes a novel pathogenetic mechanism of COMP-associated chondrodysplasias.

COMBODY: one‐domain antibody multimer with improved avidity

Antibodies (Abs) have been engineered into small antigen-binding fragments and rebuilt into multivalent high-avidity molecules for improving in vivo pharmacokinetics and efficacy in clinical use. To increase the avidity of a T-cell receptor-like single-domain Ab (sdAb) specific for HLA-A2 complex, we fused the sdAb to a coiled-coil peptide derived from human cartilage oligomeric matrix protein (COMP48) to make an sdAb multimer, termed combody. The combody improved the binding avidity of sdAb significantly, whereas the specificity for the targeted cells was retained. The strategy was also expanded to create a bispecific combody by fusing an sdAb to the N-terminal and an anti-CD3 single-chain variable fragment to the C-terminal of COMP48. The dual-specific combody was able to efficiently mediate cytotoxicity against the target cells in vitro. Taken together, the strategy to make combody could be widely adopted to increase the avidity of Ab fragment for further application.

An Inducible Cartilage Oligomeric Matrix Protein Mouse Model Recapitulates Human Pseudoachondroplasia Phenotype

Cartilage oligomeric matrix protein (COMP) is a pentameric extracellular protein expressed in cartilage and other musculoskeletal tissues. Mutations in the COMP gene cause pseudoachondroplasia (PSACH), a severe dwarfing condition that has a growth plate chondrocyte pathology. PSACH is characterized by intracellular retention of COMP and other extracellular matrix (ECM) proteins, which form an ordered matrix within large rough endoplasmic reticulum cisternae. This accumulation is cytotoxic and causes premature chondrocyte cell death, thereby depleting chondrocytes needed for normal long bone growth. Research to define the underlying molecular mechanisms of PSACH has been hampered by the lack of a suitable model system. In this study, we achieved robust expression of human mutant (MT) or wild-type (WT) COMP in mice by using a tetracycline-inducible promoter. Normal growth plate distribution of ECM proteins was observed in 1-month-old WT-COMP and C57BL\6 control mice. In contrast, the structure of the MT-COMP growth plate recapitulated the findings of human PSACH growth plate morphology, including (1) retention of ECM proteins, (2) intracellular matrix formation in the rER cisternae, and (3) increased chondrocyte apoptosis. Therefore, we have generated the first mouse model to show extensive intracellular retention of ECM proteins recapitulating the human PSACH disease process at the cellular level.

Altered Synthesis of Cartilage-Specific Proteoglycans by Mutant Human Cartilage Oligomeric Matrix Protein

BackgroundThe mechanism by which mutant cartilage oligomeric matrix protein (COMP) induces a pseudoachondroplasia phenotype remains unknown, and the reason why a mutation of a minor protein of the growth plate cartilage causes total disruption of endochondral bone formation has not yet been determined. The current study was performed to investigate the effects of mutated COMP on the synthesis of the cartilage-specific major matrix proteins of Swarm rat chondrosarcoma chondrocytes.MethodsThe Swarm rat chondrosarcoma chondrocytes transfected with a chimeric construct, which consisted of a mutant gene of human COMP and an amino acid FLAG tag sequence, were cultured in agarose gel. Formation of extracellular proteoglycan and type-II collagen by the cells was evaluated by immunohistochemical staining and measuring the 35S-sulfate incorporation.ResultsNo difference was observed for the detection of type-II collagen among the cell lines expressing mutant COMP and the control cell lines. Histochemical staining of sulfated proteoglycans with safranin-O showed that lesser amounts of proteoglycans were incorporated into the extracellular matrix of the chondrocytes transfected with the mutant gene. 35S-sulfate incorporation into the cell/matrix fractions demonstrated markedly lower radiolabel incorporation, as compared to that of the control cells.ConclusionsMutation of COMP has an important impact on the processing of proteoglycans, rather than type-II collagen, in the three-dimensional culture of Swarm rat chondrosarcoma chondrocytes.

Cartilage Oligomeric Matrix Protein Associates with Granulin-Epithelin Precursor (GEP) and Potentiates GEP-stimulated Chondrocyte Proliferation

Mutations in human cartilage oligomeric matrix protein (COMP) have been linked to the development of pseudoachondroplasia and multiple epiphyseal dysplasia; however, the functions of both wild-type and mutant COMP in the skeletogenesis remain unknown. In an effort to define the biological functions of COMP, a functional genetic screen based on the yeast two-hybrid system was performed. This led to the identification of granulin-epithelin precursor (GEP), an autocrine growth factor, as a COMP-associated partner. COMP directly binds to GEP both in vitro and in vivo, as revealed by in vitro pull down and co-immunoprecipitation assays. GEP selectively interacts with the epidermal growth factor repeat domain of COMP but not with the other three functional domains of COMP. The granulin A repeat unit of GEP is required and sufficient for association with COMP. COMP co-localizes with GEP predominantly in the pericellular matrix of transfected rat chondrosarcoma cell and primary human chondrocytes. Staining of musculoskeletal tissues of day 19 mouse embryo with antibodies to GEP is restricted to chondrocytes in the lower proliferative and upper hypertrophic zones. Overexpression of GEP stimulates the proliferation of chondrocytes, and this stimulation is enhanced by COMP. In addition, COMP appears to be required for GEP-mediated chondrocyte proliferation, since chondrocyte proliferation induced by GEP is dramatically inhibited by an anti-COMP antibody. These findings provide the first evidence linking the association of COMP and GEP and identifying a previously unrecognized growth factor (i.e. GEP) in cartilage.