Background
Hypertrophy of the synovial membrane accompanied by increased production of cartilage-degrading and bone-degrading enzymes is a hallmark of arthritis. Triggered by local inflammation in the synovial joint, normal fibroblast-like synoviocytes (FLS) are substituted with newly formed hypercellular granulation tissue, the pannus. Hypercellularity of the pannus could be explained by the proliferation of synovial intimal lining cells, but the pannus is largely formed by immigration into the synovium hematopoietic and mesenchymal precursors. The latter cell lineage gives rise to the phenotype of activated FLS [
1,
2].
In addition to the increased proliferation and production of a wide spectrum of cartilage-degrading proteases, activated FLS demonstrate a high level of invasiveness and motility within the affected joint, and FLS are even capable of spreading the disease to as yet unaffected cartilage using vasculature-independent routes [
3]. Invasive properties of activated FLS from patients with rheumatoid arthritis (RA) and from murine arthritis models directly correlate with the rate of joint destruction and deformities, and poor disease outcome [
4]. In that regard, FLS are similar to invasive tumor cells: FLS accumulate somatic mutations in the p53 tumor suppressor gene and acquire an activated expression status of several oncogenes that altogether contribute to the pathological FLS phenotype [
2,
5].
Activated FLS express a number of markers, but none of them alone could be called completely specific for FLS. One of the relatively unique surface markers for RA-FLS is cadherin-11 (CDH11), which plays an essential role in the acquisition of the invasive phenotype of activated FLS in arthritis [
2,
6]. However, CDH11 is expressed in many other mesenchymal-type cells and is involved in the regulation of invasiveness of, for instance, prostate and glioblastoma tumors, and also promotes bone metastasis [
7,
8].
Recently, we found that the expression of collagen triple helix repeat containing 1 (CTHRC1) encoding gene was strongly associated with the severity of murine collagen antibody-induced arthritis (CAIA) [
9]. When CAIA is genetically attenuated by the
Pgia8 locus in BALB/c.DBA/2-
Pgia8 congenic mice, expression of
Cthrc1 is decreased along with expression of mRNAs of metalloproteinase
Adamts12 and wingless (WNT)-associated pathway members r-spondyn 2 and syndecan 2.
CTHRC1 protein is expressed in a number of embryonic and neonate tissues, including developing cartilage and bone [
10]. Experiments with gene-deficient and transgenic mice indicate that CTHRC1 regulates osteoblastic bone formation [
11]. CTHRC1 can inhibit Smad2/3 phosphorylation after stimulation by transforming growth factor (TGF)-ß and can reduce production of collagen types I and III [
12]. Overexpression of
Cthrc1 in smooth muscle cells and embryonic fibroblasts correlates with increased cell migration properties [
13]. The endogenous expression of CTHRC1 has been found in more than a dozen types of metastatic solid cancer, and the inhibition of CTHRC1 expression results in decreased cell migration in vitro [
14]. Immunohistochemical analysis of various human primary cancers and metastases has revealed that CTHRC1 expression is actually limited to the stromal cells of solid tumors [
15,
16]. In this study, we analyzed CTHRC1 expression in synovium and established this protein as a novel marker of enhanced migratory potential of fibroblast-like cells, including activated FLS.
Methods
Patients
Biological samples were obtained under a protocol approved by the Institutional Research Ethics Committee (IREC) of Nazarbayev University, Astana, Kazakhstan. All subjects gave written informed consent. Patients coming to outpatient facility of the Republican Diagnostics Centre (RDC, Astana, Kazakhstan), who presented with clinically apparent synovial swelling were examined for RA symptoms. The final diagnostic outcome was based on persistent inflammatory arthritis, magnetic resonance imaging (MRI), radiographic analysis, disease duration, and number of tender and swollen joints, and resulted in the 28-joint-count disease activity score (DAS28). Clinical assessment was accompanied with peripheral blood analysis for complete blood counts with differential, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), rheumatoid factor (RF), and anti-citrullinated protein antibodies (ACPA). Venous blood was collected into heparinized tubes, and cells were removed by centrifugation at 1000 × g for 10 minutes. Plasma was stored at –80 °C.
ELISA for CTHRC1
Sandwich ELISA was used to quantify CTHRC1 in human plasma according to the manufacturer’s protocol (
www.mmcri.org/antibody, Maine Medical Center Research Institute, Scarborough, ME, USA). Briefly, 96-well plates (Maxisorp, Nunc) were coated overnight at 4 °C with capture antibody 13E09 at 1.8 μg/ml in carbonate-bicarbonate buffer pH 9.4. All subsequent procedures were performed at room temperature. The next day, wells were washed twice with PBS containing 0.1 % BSA and 0.1 % Tween 20 (buffer PBS-BT) and then blocked with PBS-BT for 1 hour. Human plasma or conditioned media were diluted at least 1:5 in PBS-BT and incubated with absorbed capture antibodies for 2 hours. Subsequently, the wells were washed and then incubated with biotinylated detection antibody Vli10G07 diluted 1:500 in PBS-BT for 1 hour. After washing, wells were treated for 1 hour with streptavidin conjugated with horseradish peroxidase (St-HRP) (Pierce High Sensitivity St-HRP, Thermo Fisher Scientific Inc., Waltham, MA, USA) diluted at 1:8,000 in PBS-BT. After the final wash, TMB (3,3′,5,5′-tetramethylbenzidine) chromogenic substrate (Amresco, Solon, OH, USA) was added, and the developed signal was measured at 450 nm using the Multiscan-FC plate reader (Thermo Fisher Scientific Inc., Waltham, MA, USA). Absorbance was converted to absolute concentration using rhCTHRC1 as a reference. ELISA was performed in triplicates.
Animals and arthritis induction
Mice were housed in a specific pathogen-free environment in the Institute for Animal Studies at the Albert Einstein College of Medicine, Bronx, NY, USA. All animal experiments were approved by the Institutional Animal Care and Use Committee (IACUC) of the Albert Einstein College of Medicine. Inbred BALB/c female mice, 2–4 months old (Charles River, Wilmington, MA, USA) were injected with ArthritoMab antibody cocktail (MD Biosciences Inc., St.Paul, MN, USA) according to the manufacturer’s protocol. Briefly, mice were injected intraperitoneally (i.p.) with 2 mg/mouse of antibodies on day 0 followed by i.p. injection of 40 μg of lipopolysaccharide (LPS) on day 3. Arthritis in the (FVB/N × BALB/c) F1 hybrid mice was induced similarly, but using CIA-MAB-2C ArthritoMab Cocktail (MD Biosciences Inc.), which was developed for murine strains that are refractory to arthritis. Mice were observed once or twice a day for paw swelling and redness by two independent observers. The arthritis scoring system was based on the number of inflamed joints in each paw producing a total score of 0–60 per mouse, as described previously [
9,
17].
Real-time PCR
Total RNA was isolated from mouse paws using the RNeasy Mini kit (Qiagen Inc., Valencia, CA, USA) according to the manufacturer’s instructions. Paw tissue was taken from digits to the ankle or to the distal radius/ulna. RNA integrity and quality was tested on an Agilent 2100 Bioanalyzer. The Superscript First-Strand Synthesis kit (Invitrogen Life Technologies, Grand Island, NY, USA) was used to generate cDNA that was assayed using an Applied Biosystems 7900HT Fast Real-Time PCR System. Expression was measured in triplicates and at least two repeats using the Fast SYBR Green system (Applied Biosystems by Life Technologies, Grand Island, NY, USA).
Cthrc1-specific primers were forward 5′-GGGATGGATTCAAAGGGGAAA-3′ and reverse 5′-AGAACTCGCAGAGCACTGTT-3′.
Gapdh-specific primers were forward 5′-ACCCAGAAGACTGTGGATGG-3′ and reverse 5′-ACACATTGGGGGTAGGAACA-3′. Relative RNA concentration was calculated using the delta-delta cycle threshold (Ct) method corrected for PCR efficiency, as described previously [
9,
18].
Cell lines
Human RA-FLS derived from the inflamed synovial tissue of patients with RA (Cell Applications Inc., San Diego, CA, USA) were used at passages three to four. RA-FLS were maintained in Dulbecco’s modified Eagle’s medium (DMEM) with GlutaMAX-1, 4.5 g/l D-glucose, and 25 mM HEPES, antibiotic and antimycotic (Gibco by Life Technologies, Grand Island, NY, USA) supplemented with 10 % heat-inactivated FBS and growth supplements (Cell Applications Inc., San Diego, CA, USA). Mouse embryonic fibroblast cell lines C3H/10T1/2 and NIH 3T3, and human skin fibroblast line Hs68 (all lines from ATCC Inc., Manassas, VA, USA) were studied.
Histopathology and immunohistochemical analysis (IHC)
Mice were killed by CO2 inhalation according to the approved IACUC protocol. For histopathological analysis, hind limbs from the digits to femur, including the knee joint, were collected. Tissues were fixed in 10 % neutral buffered formalin, decalcified using Immunocal reagent (Decal Chemical Corporation, Suffern, NY, USA) and embedded in paraffin. Tissues were sectioned at a thickness of 5 μm and stained with hematoxylin-eosin or alcian blue and nuclear fast red for histological confirmation of arthritis.
For immunohistochemical analysis, sections were deparaffinized and pretreated with 10 mM sodium citrate buffer at pH 6.0 heated to 96 °C for 20 minutes for antigen retrieval. Endogenous peroxidase activity was quenched by incubation in 0.3 % hydrogen peroxide in PBS for 5 minutes. Nonspecific binding was blocked for 1 hour at room temperature using 2 % BSA (Sigma-Aldrich, St. Louis, MO, USA) and 2 % normal goat serum (Vector laboratories Inc., Burlingame, CA, USA) in PBS and 0.05 % Tween-20 (PBS-T buffer). For detection of FLS-specific and macrophage-specific markers in arthritic pannus, we used rabbit anti-CTHRC1 antibodies (Vli-55,
www.mmcri.org/antibody, Scarborough, ME, USA), rabbit anti-IBA1 antibodies (#019-19741, Wako Chemicals USA Inc., Richmond, VA, USA), rabbit anti-CDH11 antibodies (WTID1, Invitrogen by Life Technologies, Grand Island, NY, USA), and rabbit antibodies to smooth muscle actin (α-SMA) (E184, Abcam, Cambridge, MA, USA). As a secondary antibody, donkey anti-rabbit antibodies conjugated to HRP (Abcam, Cambridge, MA, USA) were used. After three washes with PBST, slides were finally stained with ImmPact 3,3′-diaminobenzidine (DAB) and counterstained with hematoxylin (Vector laboratories Inc.) or alcian blue (Thermo Fisher Scientific Inc., Waltham, MA, USA). Sections were dehydrated and mounted with Permount (Thermo Fisher Scientific Inc.).
Recombinant protein overexpression
We used a full-length human Cthrc1 cDNA cloned in frame with turbo-GFP (clone RG204348, OriGene Technologies, Inc., Rockville, MD, USA) as a template to amplify a Cthrc1 sequence. High fidelity PCR using Herculase II fusion DNA polymerase (Agilent Technologies, Inc., Santa Clara, CA, USA) with forward 5′-ATTTGGTACC
ATGCGACCCCAGGGCCCCGCCG-3′ and reverse 5′-AATAGCGGCCG
C
TA
ATGATGATGATGATGATGTTTTGGTAGTTCTTCAAT-3′ primers/linkers to introduce sites for restriction endonucleases Kpn°I and Hind°III and 6xHis tag and stop codon (restriction sites are underlined; ATG and stop codons are boldfaced; 6xHis coding sequence is italicized and underlined) for cloning the amplicon into the pCMV6-K/N vector (OriGene Technologies, Inc.). The recombinant construct structure was confirmed with a complete sequencing (Thermo Fisher Scientific Inc., Fair Lawn, NJ, USA).
Transient transfection of the Chinese hamster ovary (CHO) cells with lipofectamine LFA2000 was performed according to the manufacturer’s protocol (Life Technologies, Grand Island, NY, USA). CHO cells were grown in Opti-CHO chemically-defined serum-free conditioned media Opti-CHO (Gibco Life Technologies, Grand Island, NY, USA) for 5 days. Conditioned media were collected and centrifuged at 1000 × g for 15 minutes to separate from floating cells.
Western immunoblotting
Cells were lysed in Laemmli sample buffer (62.5 mM Tris-HCl, 2 % sodium dodecyl sulfate (SDS), 10 % glycerol, 0.05 % bromphenol blue, and 100 mM 2-mercaptoethanol). Whole cell/tissue lysates were separated on 4–15 % gradient SDS-polyacrylamide gels (Bio-Rad Laboratories Inc., Hercules, CA, USA) and transferred onto polyvinylidene fluoride membrane (Thermo Fisher Scientific Inc., Waltham, MA, USA). Membranes were blocked for 1 hour in 1 % (w/v) casein in 25 mM Tris pH 7.4, 150 mM NaCl, 0.1 % Tween 20 (TBS-T) (casein blocking buffer, Thermo Fisher Scientific Inc.). After blocking, the membranes were probed with rabbit antibodies to CTHRC1 (Vli-55,
www.mmcri.org/antibody, Scarborough, ME, USA), rabbit antibodies to CDH11 (Invitrogen Life Technologies, Grand Island, NY, USA), rabbit antibodies to α-SMA (Abcam, Cambridge, MA, USA), mouse antibodies to α-tubulin (Sigma-Aldrich, St. Louis, MO, USA) as protein load control. As secondary antibodies, HRP-conjugated donkey anti-rabbit, anti-mouse or anti-sheep antibodies (Abcam) were used. Enhanced chemiluminescent substrate (Thermo Fisher Scientific Inc.) and HyBlot-CL film (Denville Scientific Inc., Holliston, MA, USA) were used to detect a signal. Optical density was quantified by ImageJ analysis software [
19].
Cell migration assay
Cell migration was assessed using transwell permeable supports with an 8-μm pore membrane (Corning Costar, Tewksbury, MA, USA). C3H/10 T1/2 murine embryonic fibroblasts or RA-FLS were seeded in the upper chamber in DMEM supplemented with 1 % BSA. The lower chamber was filled with DMEM containing 0 %, 1 %, 6 %, or 10 % FBS. rhCTHRC1 (Sino Biological Inc., Beijing, China) or rhCTHRC1 (OriGene Technologies Inc., Rockville, MD, USA) were added to the upper chamber at 500–2,000 ng/ml. After 18 hours at 37 °C in 5 % CO2, cells were fixed with 0.3 % glutaraldehyde in PBS for 10 minutes at room temperature and then stained with toluidine blue. Upper surface of the membrane was cleaned to remove any non-migrated cells using a cotton swab. Membranes were peeled off, and mounted onto glass slides for microscopic examination. Transmigrated cells were counted at 200 × magnification in four to eight fields of view using bright field microscopy.
Time-lapse microscopy
Murine NIH 3T3 fibroblasts were plated on collagen type I-coated μ-slides (IBIDI USA Inc., Madison, WI) and incubated for 4 hours at 37 °C in 5 % CO
2/95 % air in complete DMEM media supplemented with 10 % FBS to insure complete cell adhesion on a bridge. Subsequently, rhCTHRC1 protein (Sino Biological Inc., Beijing, China) was added at 1000 ng/ml, and imaging has been performed for 14 hours in a serum-containing medium at 37 °C using time-lapse phase contrast microscopy with a Cell Observer microscope (Carl Zeiss Microscopy GmbH, Göttingen, Germany). Recorded stacks of images were transferred to ImageJ [
19] and further analyzed with the MTrackJ plugin and Chemotaxis Tool (IBIDI USA Inc., Madison, WI, USA). The trajectories of individual cells were evaluated for accumulated distance migrated (Ad) and for cell velocity. Directness was calculated as a ratio of Euclidean distance (Ed) to the accumulated distance (Ed/Ad) (Fig.
6). Additionally, elongated morphology cell shape was studied (horizontal polarization), which was defined as maximal cell dimension using linear approximation. In preliminary experiments, we found that pro-motile and polarization effects of the CTHRC1 treatment were pronounced in the time window from 4–12 hours, and thereafter all calculations were performed for this time period.
Statistical analysis
Data were analyzed using Student’s
t test for unpaired samples, using SPSS statistical software (SPSS Inc., Chicago, IL, USA). The Mann–Whitney
U test for unpaired samples was performed using GraphPad Prism6 software (GraphPad Software Inc., La Jolla, CA, USA).
P-values less than 0.05 were considered significant. Quantitative measurement of brown-colored DAB product of IHC staining was performed using ImageJ software [
19] by color channel separation and densitometry of microscopic images at × 200–400 magnification. On each slide, at least five positively stained areas were analyzed for integrated brown-colored density; at least five slides per mouse (n = 5 per group) were analyzed.
Discussion
In this study, we discovered that CTHRC1, otherwise a recognized marker of enhanced cancer metastases, is overexpressed in inflammatory conditions of murine arthritis. Therefore, we corroborated our earlier transcriptome-based finding that
Cthrc1 mRNA is highly inducible in CAIA inflammation in synovial joints, and CTHRC1 protein content increases more than 10-fold within 3 days from a basically undetectable level [
9]. Due to clear inducibility in arthritis, specificity for the arthritic pannus, and the ability to be secreted into circulation, CTHRC1 could be a promising RA marker. In this study, the first pilot cohort of patients with RA had significantly increased circulating CTHRC1 (Fig.
2c), and high CTHRC1 correlated with increased CRP and with aggravated arthritis (Fig.
2d, e). Interestingly, plasma CTHRC1 did not increase in patients with solid tumors, where CTHRC1 is also strongly expressed [
7,
8,
16]. This discrepancy indicates that ways of regulating CTHRC1 in the circulation might depend on tissue vascularization, protein stability, and different/other sites of protein production.
In synovium, CTHRC1 expression was detected at multiple locations, while the major positive tissue was pannus. Ligaments, muscles, and fibrocartilage were sometimes positive for immunostaining, but more studies are needed to understand why these sites are affected. In general CTHRC1 is a marker of tissue remodeling, and inflammation or wounding could spark its expression. The deposition of CTHRC1 in bone and fibrocartilage at the sites of their contact with pannus could be remnants of dead cells or could represent protrusions of FLS into bone. Synovial sites occupied with CTHRC1+ cells notably overlap with the distribution of α-SMA+ cells. This overlap was detected at the tissue (Fig.
4) and at the cellular levels (Fig.
5b). Expression of α-SMA is a characteristic feature of stromal cells of different origin. In the recent study of solid tumors, it was shown that CTHRC1 is produced by a tumor stroma [
15]. Activated FLS are similar to stromal cells in their multipotency, i.e., the ability to differentiate into chondrogenic and osteogenic lineages [
1].
Because of the clear correlation between the degree of inflammation, and CTHRC1 mRNA and protein levels and because the protein is expressed in fibroblast-like cells, our first hypothesis was that CTHRC1 expression promotes bone and cartilage erosion by activated FLS. Nonetheless, we did not observe co-localization of FLS-CTHRC1+ with sites of bone erosions. Also, CTHRC1+ cells comprised an intriguingly substantial portion of the pannus (Fig.
1). An alternative hypothesis is that CTHRC1 is labeling a remodeling tissue and mesenchymal progenitor cells that influx into the arthritic synovium to fight inflammation. In the CAIA murine model, it is particularly obvious that this new pathologic tissue of the pannus could not be formed due to the proliferation of normal FLS forming normal synovial membrane. In CAIA, hypercellular pannus is formed within 2–3 days after induction. Therefore, CTHRC1+ FLS (or their progenitors) have enough time only to migrate into the synovial cavity and self-organize into new tissue within the time frame of several days. In line with this hypothesis, CTHRC1 was shown earlier to be a marker for mesenchymal lineage cells, including osteoblasts and RA-FLS [
24]. The presence of mesenchymal stem/stromal cells within the synovium has been demonstrated repeatedly [
25‐
28].
Attempts to convert dermal fibroblasts or OA-FLS into RA-FLS using stimulation with IL-1β or a mixture of pro-inflammatory cytokines under hypoxia/normoxia were not successful [
29], which suggests that the pro-inflammatory milieu is not sufficient to induce the de-differentiation of FLS in arthritic joints, and normal FLS and RA-FLS are somewhat different cell lineages. In this study, we found abundant amounts of CTHRC1 in inflamed synovium, which underlines the connection between inflammation and stromal cell activation, although more research is needed to prove that CTHRC1 is specific to synovial stromal cells and to determine cellular targets of the pro-motile activity of the protein. CTHRC1 increases cell motility, and therefore promotes the influx of cells into inflamed joints and formation of the multicellular pannus.
The effect of CTHRC1 upon cell motility is manifold and is not completely understood. Reports describe co-precipitation of CTHRC1 with components of non-canonical WNT signaling and stimulation of RhoA and Rac1 phosphorylation [
30,
31]. In hepatocellular carcinoma CTHRC1 promotes cell adhesion through activation of integrin β1 and phosphorylation of focal adhesion kinase (FAK), while the activation of RhoA, but not Rac1 or Cdc42 plays a crucial role [
32].
In this study, we demonstrated a positive effect of CTHRC1 upon motility of embryonic fibroblasts and RA-FLS. Interestingly, the protein regulated cell motility in two ways: both directness and velocity. The effect upon directness indicates that CTHRC1 induces cell polarization via recruiting potential CTHRC1 sensors/receptors at the leading edge of the lamellipodium. This effect of CTHRC1 might be connected with the redistribution of WNT signaling receptors and cell polarization (see also Fig.
6). The effect upon velocity of movement is different from the regulation of directness. The velocity effect should involve the regulation of focal adhesion formation/disassembly and stress fibers, and this is in line with CTHRC1 regulation of FAK and RhoA.
At this point, it is not clear whether CTHRC1 acts from cytoplasm or via surface receptors, or both. CTHRC1 siRNA approaches have been successfully used to deregulate cell motility, but other studies, including this one, effectively used exogenous CTHRC1 protein. The effect of the addition of rhCTHRC1 into media with murine fibroblasts lacking endogenous CTHRC1 suggests the presence of surface receptors. In contrast, in RA-FLS, which produce CTHRC1 endogenously, the mechanism of cell motility regulation could use both intracellular and extracelluar pathways.
Another possible mechanism of the regulation of cell motility might involve the Gly-X-Y motif in CTHRC1 protein. This motif is actually a ligand involved in cell adhesion by binding to α2β1 integrin [
33‐
35]. Triple helix motif-containing proteins, including CTHRC1, might compete with collagens for cell adhesion receptors and therefore attenuate cell adhesion onto the bone surface, and, therefore, modulate cell motility. Recently, a collagen-mimetic triple helical supramolecule containing the integrin-binding sequence was used to interfere with α2β1 integrin-dependent cell adhesion [
33]. Similar approaches might have potential for intra-articular interventions to control pannus activity and to treat arthritis.
Abbreviations
Ad, accumulated distance; BSA, bovine serum albumin; CAIA, collagen antibody-induced arthritis; CDH11, cadherin 11; CDNA, complementary DNA; CRP, C-reactive protein; CTHRC1, collagen triple helix repeat containing 1; DAB, 3,3′-diaminobenzidine; DAS28, 28-joint count disease activity score; DMEM, Dulbecco’s modified Eagle’s medium; Ed, Euclidean distance; ELISA, enzyme-linked immunosorbent assay; FAK, focal adhesion kinase; FBS, fetal bovine serum; FLS, fibroblast-like synoviocytes; Gapdh, glyceraldehyde 3-phosphate dehydrogenase; HRP, horseradish peroxidase; IACUC, Institutional Animal Care and Use Committee; IHC, immunohistochemistry; IL, interleukin; i.p., intraperitoneal; LPS, lipopolysaccharide; PBS, phosphate-buffered saline; PBS-BT, phosphate-buffered saline containing 0.1 % bovine serum albumin and 0.1 % Tween 20; siRNA, small interfering RNA; α-SMA, alpha smooth muscle actin
Acknowledgements
This work was supported in part by NIH/National Institute of Arthritis and Musculoskeletal and Skin Diseases grants R03-AR051101 (VAA), R01-AR053877 (VAA), NIH grant HL69182 (VL), and grant of the Program “Translational and Personalized Medicine in Biomedicine Industry Development in the Republic of Kazakhstan” (VAA). The Histopathology Core Facility was supported by NIH grants P20GM103465 and P30GM103392 (VL). We thank Dr. Rysgul M. Aitzhanova and Dr. Nurzhan K. Otarbayev (Repubican Diagnostic Center, Astana, Kazakhstan) for the help with collecting clinical samples. The authors thank Mr. Matthew V. Adarichev for assistance with manuscript editing and Ms. Sholpan Kauanova (National Laboratory Astana, Kazakstan) for technical assistance with time-lapse microscopy.