Ethical approval of the study protocol
Animal care and all procedures involving animals were in accordance with the guidelines of the Animal Research Committee of Qingdao University (Qingdao, China). The study protocol was approved by Qingdao University.
Experimental animals and grouping
One-hundred and eight Sprague–Dawley (SD) rats (6 weeks; 60–180 g) were provided by the Animal Center of Qingdao University.
Rats were allocated randomly into four groups (n = 27): control+normal saline (NS) , experimental (EXP) +NS, EXP+Inhibitor (AMD3100), and EXP+AMD3100 groups, and each group was subdivided into three time points (n = 9).
Experimental procedures
Anesthesia was induced by 10% chloral hydrate (0.3 mL/kg bodyweight, i.p.). The removable functional appliance designed by Petrovic and colleagues18 was fitted in rats in the EXP group to create forward mandibular advancement (Fig.1). This appliance is composed of three main parts: an inclined guide plate, an upper-incision crown, and an extraoral auxiliary retention device. The inclined guide plate is made of methyl methacrylate and an aluminum alloy sheet (thickness, 0.5 mm). The labial thickness of the upper incision crown is ~1 mm. The angle between the inclined guide plate and occlusal plane is designed to be 30°–40°. The retentive part comprises the upper-incision crown and an extraoral auxiliary retention device. There are retention arms on both sides of the appliance. To prevent the removable appliance from loosening, the appliance was fixed to the nasal maxillary complex through a rubber band. A Plexiglass™ baffle with a border length of ~5 cm was obtained; the center of the baffle had a hole of diameter ~1.8 cm. The baffle was placed on the neck of rats to assist retention. When rats closed their jaw, 4 mm of incisal open bite in the vertical dimension and 3 mm mandibular advancement in the sagittal dimension was rebuilt. Appliances were checked every other day. Control groups underwent the same procedure but without the removable functional appliance. AMD3100 is an antagonist of SDF-1. Rats in EXP+AMD3100 and control+AMD3100 groups were injected bilaterally with 40 μl (1μg/μl) of AMD3100 (diluted in NS) into the TMJ area, rats in EXP + NS and control +NS groups were injected bilaterally with 40μl of NS into the TMJ area once weekly for 8 weeks.
After the experimental procedure, animals with the appliance were fed a soft diet for 3 days to allow them to get used to the appliance. Rats in all groups received a standard diet from 4 days after the experimental procedure.
Tissue preparation
Animals were sacrificed 2, 4, or 8 weeks after the beginning of the experimental procedure. Similar to a study by Wang, differences in degradation from the left and right sides were not observed19. Blocks of TMJ tissue from the left side of six mice of each group were fixed with 4.0% paraformaldehyde at 4°C for 24 h, and decalcified with 10% ethylenediaminetetraacetic acid solution for 4 weeks at room temperature. TMJs were dehydrated in a series of ethanol solutions and embedded in paraffin wax. Serial sections of thickness 5 mm were cut through the TMJ in the sagittal plane using a rotary microtome. These sections were selected randomly for hematoxylin & eosin (H&E) staining, safranin-O staining, immunohistochemical (IHC) staining of SDF-1, collagen II, MMP13 and CXCR4, and IDS of SDF-1 and OSX. The other three left-side blocks of TMJ tissue were separated from the mandible, and were fixed immediately with 4% glutaraldehyde for 24 h at 4°C; these blocks were used for micro-computed tomography (micro-CT). All condylar heads of the right-side joints in each group were isolated and preserved at –80°C for real-time polymerase chain reaction (PCR) analyses.
Histochemical and IHC staining
H&E staining and Safranin-O staining were undertaken to determine histochemical and proteoglycan changes in condyles. IHC staining involved use of four primary antibodies obtained from Abcam (Cambridge, UK): anti-SDF-1 (rabbit polyclonal; 1:50 dilution), anti-CXCR4 (rabbit polyclonal; 1:50), anti-MMP13 (rabbit polyclonal; 1:50), anti-collagen II (1:50). The avidin-biotin complex (ABC) IHC staining protocol reported by Wang and colleagues was employed20. Negative controls were stained with non-immune serum instead of the primary antibody. Images of H&E, Safranin-O and IHC staining were acquired using a DFC490 system (Leica, Wetzlar, Germany), as described previously21. Briefly, the surface of condylar cartilage was divided equally into anterior, central, and posterior thirds between the anterior and posterior attachment points of the joint disk to the condyle. three squares (300 mm × 300 mm) covering all hypertrophic layers were applied at the quarter points of the center thirds of condylar cartilage. The cartilage thickness in the central third was determined as the average length of the three short lines in each third (Fig.2). The areas of positive cells were measured with a computer-assisted image-analyses system (Qwin Plus; Leica). The average value of six squares was calculated for statistical analyses. OA severity was assessed using the Osteoarthritis Research Society International (OARSI)22 score under light microscopy by staff from the Histology or Pathology departments of Qingdao University.
IDS
Expression of SDF-1 and OSX on the cartilage and subchondral bone of condyles was measured using IDS in the EXP, EXP+inhibitor, EXP+NS, control+inhibitor and control groups 2, 4, or 8 weeks after the beginning of the experimental procedure. IDS was carried out with a three-step ABC method, as described previously3. For IDS, sections were incubated with a primary antibody against OSX monoclonal antibody (anti-rabbit; 1:100) diluted in phosphate-buffered saline at 4°C overnight. After washing, sections were incubated with the corresponding secondary antibody (goat anti-rabbit; 1:300; Google Biological Technology, Wuhan, China), Next, sections were incubated with an anti-SDF-1 primary monoclonal antibody (anti-mouse; 1:100; Abcam) and a secondary antibody (488 goat anti-mouse; 1:400; Google Biological Technology). Sections were imaged under a confocal laser scanning microscope (LSM510META; Zeiss, Jena, Germany). Positive integrated optical densities (IODs) of SDF-1 or OSX were evaluated using Image-Pro Plus 6.0 (Media Cybernetics, Rockville, MD, USA) whereby IOD = positive area in immunofluorescence image × optical density. A higher IOD corresponded to higher expression of SDF-1 or OXS.
RNA extraction and real-time PCR
Samples of condylar cartilage from each group were homogenized. Total RNA was extracted using TriPure™ Isolation Reagent (Roche, Basel, Switzerland). The primers for the target genes are listed in Table 1. Gene expression was measured with a Real-time PCR machine (7500; Applied Biosystems, Foster City, CA, USA) with glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as the internal control. Each experiment was carried out three times and the mean values were calculated. Expression of target genes, relative to that of GAPDH, was calculated using the 2−△△Ct method. Results were calculated as the relative quantification of the target gene compared with that in the control group.
Table 1 Sequences of primers of target genes used for real-time PCR
Target gene
|
Upstream primer sequence
|
Downstream primer sequence
|
Cxcl12
|
ATATTCATCCGTGCCCTCG
|
GCAATGCCACCACCTGTAAC
|
Cxcr4
|
GTCTATGTGGGTGTCTGGAT
|
ATGATGTGCTGGAACTGG
|
Col2a1
|
AAGAAGCACATCTGGTTTGGA
|
CAGTGGACAGTAGACGGAGGA
|
Mmp13
|
TGATAGACTCCGAGAAATGC
|
GTTTGGGACCATTTGAGTG
|
Gapdh
|
TTCAACGGCACAGTCAAGG
|
CTCAGCACCAGCATCACC
|
Micro-CT
Specimens were imaged and analyzed using a Micro-CT system (Latheta LCT 200, Hitachi, Tokyo, Japan). Two cubic regions of interest (ROIs; 0.25 × 0.25 × 0.25 mm) were selected at the midpoint of the central and posterior condyle of subchondral bone. These ROIs were used to measure and compare microstructural parameters (the ratio of bone volume to tissue volume (BV:TV), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), trabecular number (Tb.N), ratio of bone surface to bone volume (BS:BV), bone mineral density (BMD)) between different experimental groups at three time points.