Cells
For primary mouse bone marrow MSCs (mBMSCs) culture, bone marrow cells were isolated from femurs and tibias of 6‐week‐old C57BL/6 mice and cultured in low-glucose Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 1% penicillin and streptomycin. The cell suspension was plated in 10-cm dish and incubated at 37°C in a humidity incubator with 5% CO2. The medium was changed every 3 days to clear non-adherent cells. When the cells reached 80%–90% confluence, MSCs were detached with 0.25% trypsin/1 nM EDTA (Hyclone, USA) and passaged. After passage, MSCs received routine characterization. Except for establishing in vitro replicative senescent cell model, mBMSCs at three to eight passages were used in this study.
The detection of surface markers of mBMSCs was determined using flow cytometry. mBMSCs were incubated with anti-Sca-1, anti-CD44, anti-CD29, anti-CD45, or anti-CD34 for 30min. Then the samples were examined by flow cytometry.
C3H10T1/2 cells were maintained in DMEM containing 10% FBS, 1% penicillin and streptomycin.
Osteogenic and Adipogenic Differentiation
Osteogenic differentiation of mBMSCs and C3H10T1/2 cells were induced at approximately 80% confluence in osteogenic differentiation medium (ODM) containing 10% FBS complete hight-glucose DMEM, 0.1 µM Dexamethasone, 10 mM β-Glycerophosphate and 50 µg/ml Ascorbic acid. ODM was changed every 3 days. Matrix mineralization or calcium depositions were examined with Alizarin Red staining at day 21. Cells were observed by inverted microscopy.
Adipogenic differentiation of mBMSCs was induced at approximately 80% confluence in adipogenic differentiation medium (ADM) containing 10% FBS complete hight-glucose DMEM, 0.5 mM 3-Isobutyl-1-methylxanthine (IBMX), 1 µM Dexamethasone, 10 µM Insulin, and 200 µM Indomethacin. ADM was replaced every 3 days. Mature adipocytes or fat droplets formations were visualized by staining with Oil Red O solution. Cells were observed by inverted microscopy.
Oil Red O, Alizarin Red, Alkaline phosphatase and SA‐β‐galactosidase staining
For detection of lipid droplets, mBMSCs cultured in adipogenic medium for 2 weeks were fixed with 4% paraformaldehyde for 15 min and then stained with Oil Red O for 10 min at room temperature.
Alizarin Red and ALP staining were performed according to a previously described procedure[12].
SA‐β‐galactosidase staining was performed with Senescence β‐Galactosidase Staining Kit (Cell Signaling Technology, USA) following the manufacturer's instruction. The blue‐stained cells were regarded as positive cells.
In vitro and in vivo senescent cell model
Oxidative stress‐induced senescence. mBMSCs were incubated with 50 μM H2O2 for 2 h. Then washing with PBS, cells were maintained in low-glucose DMEM for 24 hours and then harvested for mRNA and protein expression analyses.
Replicative senescence. An in vitro replicative senescent cell model was established using long-term cultured mBMSCs.
Since we had not isolated bone marrow MSCs from ageing mice successfully, in vivo senescent bone marrow MSCs were isolated from ageing Sprague Dawley (SD) rats (25-month-old).
Transfection
For miRNAs transfection, the functional role of miR-145a was verified by transfecting C3H10T1/2 cells with miR-145a mimic, and its negative controls (RiboBio, China) using the Lipofectamine2000 transfection agent (Invitrogen, USA) according to the manufacturer’s instructions.
For plasmids transfection, either empty vector pcDNA3.1 or pcDNA3.1 expressing mouse Cbfb (pcDNA3.1-Cbfb) were transiently transfected in C3H10T1/2 cells using the Lipofectamine2000 transfection agent.
Gene expression analysis
Total RNA was extracted with the RNAiso Plus reagent (Takara, Japan). For the mRNA analysis, total RNA was reverse-transcribed into complementary cDNA using the PrimeScript RT reagent Kit (Takara, Japan). For the miRNA analysis, the forward and reverse primers for miRNAs were designed by RiboBio Corporation (Guangzhou, China). Then, the total miRNA underwent polyA tailing and reverse transcription using the miDETECT A TrackTM miRNA qRT-PCR Starter Kit (RiboBio, China). All real-time PCR was conducted with the ABI ViiA™ 7 Real-Time PCR System (Applied Biosystems) using the SYBR Green-based real-time detection method. The following thermal settings were used: 95℃ for 30 seconds followed by 40 cycles of 95℃ for 5 seconds and 60℃ for 30 seconds. The fold change was calculated with the 2-ΔΔCT method and normalized to the level of the housekeeping gene GAPDH or U6, respectively. The primer sequences used in this study are listed in Supplemental Table S1.
Western Blot Analysis
Whole-cell lysates were prepared on ice using cold RIPA lysis buffer (Beyotime, China) containing protease inhibitor (Thermo Fisher, USA). In brief, equal amounts of proteins (30 µg) were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS–PAGE) and transferred to PVDF membranes. After incubation with the primary antibody at 4℃ overnight, they were further immunoblotted with HRP-conjugated antibody at 37℃ for 1 hour, developed with enhanced chemiluminescence (ECL) substrate (Millipore, USA) and chemiluminescence detection by ChemiDocTM MP Imaging System (Bio-Rad, United Kingdom). Primary antibodies and dilutions used were as follows: p53 (1:1000; Cell Signaling Technology); p21 (1:2000; Abcam); Runx2 (1:200; Santa Cruz Biotechnology); Cbfb (1:200; Santa Cruz Biotechnology); BMP9 (1:200; Santa Cruz Biotechnology); and GAPDH (1:2000; Beyotime Biotechnology).
Luciferase assay
Mouse wild type or mutant Cbfb 3’-UTR regions were chemically synthesized by Sangon Biotech (Shanghai, China) and cloned into psiCHECK2 luciferase reporter plasmids. The recombinant plasmids were respectively named as, psiCHECK2‐Cbfb 3′ UTR‐WT, psiCHECK2‐Cbfb 3′ UTR‐mut. HEK293T cells were seeded in 6‐well plates. When the cells reached about 70% confluences, each recombinant plasmid (2μg) and either the miR-145a mimic or the mimic negative control (mimic NC) were cotransfected into the cells with Lipofectamine 2000 (Invitrogen, USA) according to the manufacturer's direction. After 48 h of transfection, firefly and Renilla luciferase activities were quantified using a dual luciferase assay system (Promega, USA).
Statistical analysis
All data are expressed as mean ± standard deviation (SD). Differences between two groups were evaluated by two-tailed Student’s t-test using GraphPad Prism 8 (GraphPad Software Inc., USA). All experiments were performed at least three times, and p<0.05 was considered statistically significant.