Streptozotocin (stz)

In order to establish diabetic model, streptozotocin (STZ, Sigma, St. Louis, MO, USA) injection was applied. Male Sprague–Dawley (SD) rats (weight 200–240 g) received intraperitoneal injection of STZ (35 mg/kg) for 3 days [31 (link)]. Blood glucose levels were tested by blood glucose meter (Lifescan, Inc., Milpitas, CA, USA) one week following STZ injection. Animals with glucose levels ≥16.6 mmol/L were considered as diabetic. Diabetic rats received standard food for 4 weeks and then randomized into the following 4 groups (n = 15): (1) DM group: diabetic rats without any treatment; (2) DM + U50,488H group: diabetic rats that received daily treatment of U50,488H (Biomol, Plymouth Meeting, PA, USA) at 1 mg/kg for 4 weeks; (3) DM + vehicle group: diabetic rats that received only saline daily for 4 weeks; (4) DM + nor-BNI group: diabetic rats that received daily treatment of nor-BNI (Sigma, St. Louis, MO, USA) at 0.5 mg/kg for 4 weeks. Age-matched normal rats that received only saline daily (n = 15) were used as non-diabetic controls (CON). All experiments were conducted under the National Institutes of Health Guidelines on the Use of Laboratory Animal and were approved by the Fourth Military Medical University Committee on Animal Care.

Male FGF21KO mice with C57 BL/6J background (gift from Dr. Steve Kliewer, University of Texas Southwestern Medical Center) and wild-type (WT) C57 BL/6J mice purchased from Jackson Laboratory (Bar Harbor, ME, USA) were used in this study. Type 1 diabetic mouse model was induced in 10 week-old male FGF21KO mice and age-matched WT mice by intraperitoneal (i.p.) injection of six doses of streptozotocin (STZ, Sigma-Aldrich, St. Louis, MO, USA in 10 mM sodium citrate buffer, pH 4.5) at 60 mg/kg bw daily. Control group (Ctrl) of FGF21KO and WT mice received citrate buffer alone. Seven days after the last STZ injection, whole blood glucose obtained from the mouse tail vein was detected using a SureStep complete blood glucose monitor (LifeScan, Milpitas, CA, USA), and animals with blood glucose levels greater than 250 mg/dl were considered diabetic. At 1, 2 and 4 months after diabetes onset, heart function and blood pressure were measured and mice were then killed.

Eight‐week‐old C57BL/6 J mice (male, n = 15) and Fpr2^−/−mice (male, n = 15) were given five consecutive intraperitoneal injections of streptozotocin (STZ; 60 mg/kg body wt/day) (Sigma‐Aldrich, St. Louis, MO). Control received citrate buffer alone. At 1 week after STZ injection, hyperglycemia was confirmed in tail prick blood samples using Blood glucose monitoring system (LifeScan, CA, USA). Blood glucose values ≥16.7 mmol/L were considered as diabetic and the mice were enrolled into the study. The blood glucose and weight were monitored before and after STZ injection at 1, 4 and 12 and 24 weeks. After injection for 24 weeks, one mouse failed to reach the diabetic standard in diabetic group, two mice failed in Fpr2^−/− diabetic group. Twelve mice were survived in diabetic group and 12 mice Fpr2^−/− diabetic group. The mice were euthanized to collect retinas for retinal flat‐mount immunofluorescence staining (six retinas), frozen tissue sections immunofluorescence staining (six eyes), RT‐PCR (six retinas), and Western blot (six retinas) experiments.