Complement 5's Role in Mitochondria During Kidney Ischemia-Reperfusion Injury in Rats

Background: Acute kidney injury (AKI) affects over 13 million people each year and is the primary risk factor for the development of chronic kidney disease. Disruption of mitochondrial dynamics has been confirmed in AKI, leading to cell death. The mitochondria and the complement system, an innate immune cascade, are recognized to have a complex bidirectional relationship. The complement system is aberrantly activated during AKI, although how complement contributes to mitochondrial dysfunction in this context is currently unknown. Methods: In this study, we developed a novel complement C5 knockout (C5-/-) rat model via CRISPR/Cas9 deletion of C5 exon 3. C5-/- Lewis rats underwent a model of renal ischemia-reperfusion (I/R) via 1-hour bilateral clamping of the renal pedicles and 24-hour reperfusion. Renal tissue biopsies were utilized for high-resolution respirometry, and isolated mitochondria for native and denatured protein evaluation. Further, the renal morphology was visualized via PAS and KIM-1/NGAL staining. The serum creatinine (SCr) and blood urea nitrogen (BUN) were quantified via the VetScan i-STAT system (Abbott) as a measure of kidney function. Results: I/R resulted in significant elevation of SCr and BUN compared to sham controls (p<0.001 and p<0.01, respectively); however, this effect was attenuated via C5-/-. Similarly, C5-/- rats displayed decreased KIM-1/NGAL staining and tubular necrosis after I/R compared to C5+/+ rats (p<0.05). C5-/- rats displayed decreased mitochondrial complex I and II activity from C5+/+ rats in the sham controls (p<0.01 and p<0.001), while C5+/+ rats displayed significantly decreased complex activity only after I/R. The protein level of ATPIF-1, an inhibitory factor of the ATP synthase, was significantly increased in C5-/- rats at baseline (p<0.05). Curiously, ATPIF-1 protein level significantly decreased during the 1-hour ischemia period. Conclusion: C5 gene deletion results in a protective effect for the kidney function (SCr and BUN) as well as the kidney tissue morphology (KIM-1/NGAL, tubular necrosis) during I/R. However, C5-/- displayed profound effects in the mitochondria, as ATPIF-1 was significantly increased and complex I and II activity was decreased in sham controls. These results suggest that C5 has a role in renal mitochondrial metabolism in baseline and ischemic conditions. Funding: NIDDK Support