Animals
All the experiments involving mice conformed to the National Institutes of Health Guide for the Care and Use of Laboratory Animals and were approved by the University of Kentucky Institutional Animal Care and Use Committee (IACUC). The authors compiled with the ARRIVE guidelines. All animals were housed in a pathogen-free environment at 22°C in 14-hour light/ 10-hour dark cycle. Male six-week old C57BL6 mice (The Jackson Laboratory; Bar Harbor, ME) were fed with high fat diet (60% fat) (D12492, Research Diets, Inc, NJ) for 6 weeks to induce obesity and glucose intolerance. These diet-induced obesity (DIO) mice were then divided into three groups (n=5-7 mice/group) and injected twice a week with saline, control ASO (GGCCAATACGCCGTCA) or CD47 ASO (ATTGATTAAGTCTGAG, targeting to mouse CD47) (provided by Ionis Pharmaceuticals, Carlsbad, CA) at a 25mg/kg body weight by i.p. for 8 weeks with continuous high fat diet feeding in all groups. These ASOs are chemically stabilized with constrained ethyl modification in the wings and phosphorothioate linkages 43. At the end of study, mice were anesthetized by intraperitoneal injection of Ketamine (100 mg/kg) combined with Xylazine (10 mg/kg). After blood was drawn and several tissues were harvested, cervical dislocation of mice was performed to ensure their deaths.
Another study was conducted with C57BL6J-Lepob/Lepob (ob/ob) mice. Five-week old ob/ob mice were purchased from Jackson Laboratories. After one-week acclimation, ob/ob mice were divided into three groups and treated with saline, control ASO and CD47 ASO at 25 mg/kg body weight (i.p) twice a week for 8-9 weeks.
Metabolic analysis
Body weight was measured weekly. At the end of the study, body composition was measured by Echo MRI 13. Two weeks prior to the end of study, mice were placed in Sable Promethion system with in-cage running wheels individually for 5 days for measurement of food intake, water intake and indirect calorimetry.
Cold exposure and body temperature measurement
Implantable Programmable Temperature Transponder 300 (IPTT-300, BioMedic Data Systems, Delaware, USA) was interscapularly implanted into mice. After one day of acclimatization, mice were put in 4°C cold room individually without food for 6 hr. Body temperature of the mice was monitored at 0, 1, 2, 3, 4 and 6 hr during cold exposure.
Blood parameter analysis and glucose tolerance test
At the end of study, blood samples were obtained via cardiac puncture. Plasma alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels were measured by using ALT and AST assay kit (Connecticut, USA). Blood glucose levels were measured by using glucometer. Plasma insulin levels were measured by using ELISA Kit (Crystal Chem USA, IL). For glucose tolerance test (GTT), mice were fasted 6 hours before intraperitoneal injections of glucose (1 g/kg body weight). Blood glucose concentrations were measured using a glucometer at 0, 15, 30, 60, and 120 minutes post injection.
Lipid Analysis
Plasma total cholesterol and triglyceride, and hepatic cholesterol and triglyceride concentrations were determined enzymatically with Wako kits (Richmond, USA). For analysis of hepatic lipid, approximately 50 mg of liver was placed into 500 μl of chilled Krebs Ringer Phosphate buffer (118 mM NaCl, 5 mM KCl, 13.8 mM CaCl2, 1.2 mM MgSO4, 0.016% KH2PO4, 0.211% NaHCO2) and each sample was sonicated for ten times (3 seconds/time).
White adipose tissue cAMP and cGMP measurement
cAMP and cGMP levels in white adipose tissue were measured by using the cAMP and cGMP ELISA kits from R&D Systems (Minneapolis, USA) and Abcam (Boston, USA), respectively. In brief, 100 mg of epididymal adipose tissue was homogenized in 1 ml of 0.1N HCl and the supernatant was collected. The samples were then neutralized with 1N of NaOH or neutralizing reagents provided from the kits. cAMP or cGMP levels in the supernatant were measured and calculated based on the cAMP or cGMP standard curve following the instruction manual.
Western blotting
Liver tissues were homogenized in RIPA buffer (Sigma, St. Louis, MO, USA) plus protease and phosphatase inhibitors (Pierce, Waltham, MA, USA). After concentrations were measured using a BCA Assay (Pierce, Waltham, MA, USA), 30μg protein/well was subjected to SDS-PAGE gel under reducing conditions and transferred onto a nitrocellulose membrane. After blocking, the membrane was incubated with anti-β-actin (1:5000 dilution; Santa Cruz, Dallas, TX, USA) and anti-CD47 (1:1000 dilution; Abcam, Cambridge, MA, USA) at 4°C overnight. After washing, the membrane was incubated with horseradish peroxidase-conjugated secondary antibodies (Jackson Labs, Bar Harbor, ME, USA). The reaction was visualized by using an enhanced chemiluminescence system (Pierce, Waltham, MA, USA).
Tissue Histology
Fat tissues and liver tissues from all groups of animals were fixed in 10% Formalin for 24 hr, embedded in paraffin, sectioned at 5 μm, and stained with hematoxylin and eosin-stain (H&E) by standard method by using the service from COBRE Pathology Core at University of Kentucky. In addition, liver cryo-sections were fixed with 4% PFA and stained with 0.5% Oil red O in 60% isopropyl alcohol. All images were acquired with a Nikon Eclipse 55i.
Real-time quantitative PCR
Total RNA from tissues were extracted using TRIzol Reagent (Thermo Fisher Scientific, Massachusetts, USA). RNA was reverse transcribed to cDNA by the cDNA Synthesis Kit (Quantabio, Massachusetts, USA). Real-time quantitative PCR was performed using a MyiQ Real-time PCR Thermal Cycler (Bio-Rad) with SYBR Green PCR Master Kit (Qiagen, Valencia, CA). Relative mRNA expression was calculated using the MyiQ system software as previous reported 44 and normalized to β-actin levels. All primer sequences utilized in this study are found in Table 1.
Statistical analysis
Statistical analysis was performed using Prism 9 (GraphPad Software, San Diego, CA). All data are presented as the mean ± SEM. Statistical significance between two groups was determined using two-tailed Student’s t-test. One-way ANOVA followed by Bonferroni’s multiple comparisons test or 2-way ANOVA followed by Tukey’s multiple comparisons test was applied for multi-group comparisons. P values of less than 0.05 were considered to be significant.