The isolation and characterization of DFCs and dNC-PCs were described in previous studies [4,7,12]. DFCs were routinely cultivated in DMEM (Sigma-Aldrich, St. Louis, MO, USA) supplemented with 10% fetal bovine serum (Sigma-Aldrich, St. Louis, MO, USA) and 100 μg/ml penicillin/streptomycin (standard cell culture medium). dNC-PCs were cultivated in DMEM (Sigma-Aldrich) supplemented with 15% fetal bovine serum (Sigma-Aldrich) and 100 μg/ml penicillin/streptomycin (standard cell culture medium). For experiments, both cell types were used after passage 6. DFCs and dNC-PCs expressed typical markers for dental stem cells such as CD105, Nestin, and STRO-1 (Additional file 1: Figure S1).
Preparation of polyacrylamide materials
Five milliliter of PA gel solution with the concentration of 8% acrylamide and 0.06% bis-acrylamide (Bio-Rad, Hercules, CA, USA) were mixed and degas under vacuum for at least 20 min to remove oxygen. Then, 30 μl of 0.1 mg/mL ammonium persulfate (Sigma-Aldrich, St. Louis, MO, USA) and 20 μl TEMED (Applichem, Omaha, NE, USA) were added and placed into the mini protean casting strand and frame (Bio-Rad) to form 1-mm thickness of substrate. After letting the gel to polymerize for 30 to 45 min, gently remove and rinse gel with 50-mM HEPES, pH 8.5 (Applichem, Omaha, NE, USA). PA gel was then cut into circular shape with 14 mm diameters and placed in 24 well plates for the experiment. Sulfo-SANPAH (Pierce Biotechnologies, Rockford, IL USA) 0.5 mg/mL in 50-mM HEPES, pH 8.5 was pipetted onto the surface and exposed to the UV light for photoactivation procedure. After photoactivation, the substrate was washed several times in 50-mM HEPES. A 0.2 mg/mL of type I collagen (Sigma-Aldrich, St. Louis, MO, USA) was then layered onto the surface of gel and incubated 4 h at room temperature or overnight at 4°C on a shaker. After washing with PBS, the gels were stored in PBS at 4°C. Before platting the cells, the gel was exposed to UV for 15 min for the sterilization and replace PBS with complete culture medium for 1 h at 37°C.
The bone substitutes Maxgraft® (AP) and Maxresorb® (SB) were obtained from the company Botiss (botiss dental GmbH, Berlin, Germany). Maxgraft® is a sterile, high-safety allograft product (AP), derived from human donor bone. It is processed by an audited and certified bone bank (Cells+ Tissue Bank Austria, Berlin, Germany). In contrast, Maxresorb® is a fully synthetic bone graft substitute (SB) with controlled resorption properties. It is a homogenous composition of 60% hydroxyapatite and 40% beta-tri-calcium phosphate. SB maintains the volume and mechanical stability over a long time period. The osteoconductivity of SB is achieved by a matrix of interconnecting pores and a very high porosity of approximately 80%, as well as pore sizes from 200 to 800 μm (www.botiss.com). Experiments with AP and SB were done with solid blocks (10 × 10 × 20 mm cancellous block). PA was produced in our lab (see above), and silicone-based implant materials were obtained from Vivomed (Downpatrick, UK) as tubes. Silicone tubes were cut in pieces with a size which is similar to that of AP and SB.
Implant materials were washed with PBS or cell culture medium before use. DFCs and dNC-PCs were seeded onto materials for indicated periods of time. For the isolation of total RNA and the estimation of vital cell numbers, implant materials with cells were transferred to a fresh well with cell culture medium.
For the evaluation of apoptosis induction, cell culture eluates were produced by incubating 0.1 mL of bone substitutes or soft materials in 1-mL standard medium at 37°C for 24 h. This incubation step with the implant material was repeated twice with fresh cell culture media. Three eluates were pooled for cell culture experiments. DFCs were seeded onto cell culture plates and cultivated in standard cell culture media. After cell seeding (12 to 24 h), cell culture media were changed, and cells were cultivated in cell culture media with material eluates. After 24 h of cultivation, cells were harvested for flow cytometry analyses or protein isolation for Western blots (see below).
Cell counting kit 8 assay
Numbers of vital cells were evaluated after days 1, 2, 3, and 6. For cell counting, cell cultures were incubated with the cell counting kit 8 (CCK8) ready to use solution according to manufactures instructions (Dojindo, Rockville, MD, USA). The optical density (O.D.) was measured at a wavelength of 450 nm. For the evaluation of the cell adherence (normalized to standard cell culture dishes), cell proliferation (normalized to cell number at day 1 of cell culture) relative cell numbers were calculated.
Flow cytometry analysis
The induction of apoptosis in DFCs and dNC-PCs was evaluated by measuring the Cell Event® Caspase3/7 Green Flow cytometry assay (Life Technologies, Carlsbad, CA, USA). For the Caspase3/7assay, cells were cultivated in eluates as described above. After 24 h, cells were harvested by trypsin-EDTA treatment, washed with PBS, and stained first with Caspase3/7 Green Detection Reagent (25 min, 37°C). After this step 1-mM SYTOX® AADvanced dead cell stain solution was added to the sample (5 min, 37°C). Cell fluorescence was analyzed at 488-nm excitation and applied to standard fluorescence compensation. Emission of fluorescence was measured with 530/30 BP (Caspase3/7 Green Detection Reagent) and with 690/50 BP (SYTOX® AADvanced dead cell stain) filters. Cells positive for Caspase3/7 Green Detection Reagent were identified as apoptotic cells, while dead cells were positive for SYTOX® AADvanced dead cell stain. However, vital cells were negatively stained for both staining solutions.
For protein isolation, cells were treated with lysis buffer (250 μl phosphatase, 100 mM Na3VO4, 137 mM NaCl, 200 mM Tris, 480 mM NaF, 1% NP-40, 10% Glycerol) on ice for 2 min. A protease-inhibitor (1 Protease Inhibitor Cocktail tablet from Roche) was included to minimize protein degradation. Cell lysates were placed on ice for 10 min. Protein samples were separated by SDS-polyacrylamide gel electrophoresis in pre-casted 12% Tris-glycine gels (Invitrogen, Waltham, MA, USA) and blotted to nitrocellulose membranes. Membranes were blocked with skimmed milk for 1 h and incubated with primary antibodies that were specific for proteins BAX (pro apoptotic protein), BCL2 (anti apoptotic protein), and β-Actin (housekeeper protein). Washed membranes were then incubated with a horseradish peroxidase-labeled secondary antibody. The detection of the secondary antibody was performed via chemiluminescence and X-ray films (GE Healthcare, Pewaukee, WI, USA).
DFCs were cultivated until sub-confluence (>80%) in standard cell culture medium before the differentiation starts with the osteogenic differentiation medium (ODM) comprised DMEM (PAA) supplemented with 10% fetal bovine serum (Sigma-Aldrich), 100 μmol/L ascorbic acid 2-phosphate, 10 mmol/L KH2PO4, 1 × 10−8 mol/L dexamethasone sodium phosphate (Sigma-Aldrich, St. Louis, MO, USA), HEPES (20 mmol/L) and 100 μg/ml penicillin/streptomycin. The differentiation was evaluated by qRT-PCR and ALP activity detection.
ALP activity detection
Cells were washed with PBS buffer and lysed by shock freezing (−80°C). Diluted 1:1 in 1 × PBS, 100 mM p-nitrophenyl phosphate (Sigma) was added to each sample. After incubation at 37°C for 60 min, the reaction was stopped by adding 300 μL of 0.3 M NaOH and the liberated p-nitrophenol was measured at 405 nm. ALP activity values were normalized to total DNA concentration, which were determined by the Quant-iT PicoGreen dsDNA Assay (Invitrogen).
Prime PCR arrays
For the evaluation of osteogenic marker expression, the Biorad PrimePCR array (Development - Hedgehog and PTH signaling pathways in bone and cartilage development) was used, which consists of the most important markers for the osteogenic differentiation. Total RNAs, which were isolated from differentiated dental cells at day 7, were reverse-transcripted with the iScript™ Advanced cDNA Synthesis Kit for RT-qPCR (Biorad) according to the manufacturers protocol. PCRs were made with SsoAdvanced™ Universal SYBR® Green Supermix (Biorad) on the StepOne real-time PCR machine (Life Technologies, Carlsbad, CA, USA). Results were analyzed with the PrimePCR™ Analysis Software (Biorad), and the output is presented as Clustergrams. While red tiles signify a high gene expression, black/gray and green tiles show a middle gene expression and a low gene expression, respectively. Black tiles with a cross designate no gene expression.
Combinations of SB with dNC-PCs and AP with dental cells yielded from cell cultures after 7 days of osteogenic differentiation were fixed in 4% formaldehyde/0.1 M PBS at 4°C for at least 24 h. Tissues were decalcified with EDTA and subsequently dehydrated in an ascending series of ethanol and embedded in paraffin. Serial sections of 5 μm were cut in different planes for orientation and stained with hematoxylin-eosin (HE).