To knock down HIF-1α expression in murine skeletal muscles, HIF-1α siRNA mixed with in vivo jetPEI was delivered to the skeletal muscles at two different sites (the adductor muscle and the gastrocnemius) of the bilateral hind limbs by intramuscular injection. To prepare PEI-siRNA mixtures, according to the manufacturer’s protocol, HIF-1α siRNA (10 μg) or a negative control (NC) in 5% glucose was mixed with jetPEI solution to achieve a 100-μL total injection volume. After 2 h, the mice in the different groups were subjected to brief ischemia and reperfusion.

Sepsis is a common and life-threatening systemic disorder, often leading to acute injury of multiple organs. Here, we show that remote ischemic preconditioning (rIPC), elicited by brief episodes of ischemia and reperfusion in femoral arteries, provides protective effects against sepsis-induced acute kidney injury (AKI). Methods: Limb rIPC was conducted on mice in vivo 24 h before the onset of cecal ligation and puncture (CLP), and serum exosomes derived from rIPC mice were infused into CLP-challenged recipients. In vitro, we extracted and identified exosomes from differentiated C2C12 cells (myotubes) subjected to hypoxia and reoxygenation (H/R) preconditioning, and the exosomes were administered to lipopolysaccharide (LPS)-treated mouse tubular epithelial cells (mTECs) or intravenously injected into CLP-challenged miR-21 knockout mice for rescue experiments. Results: Limb rIPC protected polymicrobial septic mice from multiple organ dysfunction, systemic accumulation of inflammatory cytokines and accelerated parenchymal cell apoptosis through upregulation of miR-21 in a hypoxia-inducible factor 1alpha (HIF-1alpha)-dependent manner in the ischemic limbs of mice. However, in miR-21 knockout mice or mice that received HIF-1alpha siRNA injection into hind limb muscles, the organ protection conferred by limb rIPC was abolished. Mechanistically, we discovered that miR-21 was transported from preischemic limbs to remote organs via serum exosomes. In kidneys, the enhanced exosomal miR-21 derived from cultured myotubes with H/R or the serum of mice treated with rIPC integrated into renal tubular epithelial cells and then targeted the downstream PDCD4/NF-kappaB and PTEN/AKT pathways, exerting anti-inflammatory and anti-apoptotic effects and consequently attenuating sepsis-induced renal injury both in vivo and in vitro. Conclusion: This study demonstrates a critical role for exosomal miR-21 in renoprotection conferred by limb rIPC against sepsis and suggests that rIPC and exosomes might serve as the possible therapeutic strategies for sepsis-induced kidney injury.