An inflammasome-independent role for epithelial-expressed Nlrp3 in renal ischemia-reperfusion injury.

Cytoplasmic innate immune receptors are important therapeutic targets for diseases associated with overproduction of proinflammatory cytokines. One cytoplasmic receptor complex, the Nlrp3 inflammasome, responds to an extensive array of molecules associated with cellular stress. Under normal conditions, Nlrp3 is autorepressed, but in the presence of its ligands, it oligomerizes, recruits apoptosis-associated speck-like protein containing a caspase recruitment domain (Asc), and triggers caspase 1 activation and the maturation of proinflammatory cytokines such as IL-1ß and IL-18. Because ischemic tissue injury provides a potential source for Nlrp3 ligands, our study compared and contrasted the effects of renal ischemia in wild-type mice and mice deficient in components of the Nlrp3 inflammasome (Nlrp3(-/-) and Asc(-/-) mice). To examine the role of the inflammasome in renal ischemia-reperfusion injury (IRI) we also tested its downstream targets caspase 1, IL-1ß, and IL-18. Both Nlrp3 and Asc were highly expressed in renal tubular epithelium of humans and mice, and the absence of Nlrp3, but not Asc or the downstream inflammasome targets, dramatically protected from kidney IRI. We conclude that Nlrp3 contributes to renal IRI by a direct effect on renal tubular epithelium and that this effect is independent of inflammasome-induced proinflammatory cytokine production.

Nod1 and nod2 are expressed in human and murine renal tubular epithelial cells and participate in renal ischemia reperfusion injury.

Nucleotide-binding oligomerization domain (Nod) 1 and Nod2 are members of a family of intracellular innate sensors that participate in innate immune responses to pathogens and molecules released during the course of tissue injury, including injury induced by ischemia. Ischemic injury to the kidney is characterized by renal tubular epithelial apoptosis and inflammation. Among the best studied intracellular innate immune receptors known to contribute to apoptosis and inflammation are Nod1 and Nod2. Our study compared and contrasted the effects of renal ischemia in wild-type mice and mice deficient in Nod1, Nod2, Nod(1 x 2), and in their downstream signaling molecule receptor-interacting protein 2. We found that Nod1 and Nod2 were present in renal tubular epithelial cells in both mouse and human kidneys and that the absence of these receptors in mice resulted in protection from kidney ischemia reperfusion injury. Significant protection from kidney injury was seen with a deficiency of Nod2 and receptor-interacting protein 2, and the simultaneous deficiency of Nod1 and Nod2 provided even greater protection. We conclude that the intracellular sensors Nod1 and Nod2 play an important role in the pathogenesis of acute ischemic injury of the kidney, although possibly through different mechanisms.