Molecular Regulation of Ischemic Renal Failure

DESCRIPTION (provided by applicant): Kidney injury by ischemia leads to acute renal failure (ARF), a major kidney disease associated with high mortality. While the development of ARF involves multiple factors and may proceed in several phases, it is ultimately precipitated by sublethal and lethal damage to renal tubular cells. The long-term goal of our research is to delineate the apoptotic mechanism of tubular cell injury and identify novel strategies for the prevention and treatment of ARF. During the last grant period, we and others have shown the involvement of tubular cell apoptosis in ischemic and nephrotoxic renal injury. Importantly, these studies have demonstrated a pivotal role for Bax/Bak- mediated mitochondrial damage. We have further revealed a striking morphological change of mitochondria during tubular cell apoptosis, i.e. mitochondrial fragmentation. Inhibition of mitochondrial fragmentation prevents mitochondrial damage and apoptosis. Despite these findings, critical questions remain: 1) how is Bax activated? 2) how is mitochondrial fragmentation induced and regulated? Endophilin B1 (Endo-B1) was originally identified as a Bax-interacting protein by yeast two hybrid screening. Our preliminary studies have demonstrated an early Endo- B1 activation and translocation to mitochondria during ischemic renal injury. Importantly, knockdown of Endo-B1 suppresses mitochondrial fragmentation and tubular cell apoptosis. The overall objective of this project is to elucidate the regulation of Bax and mitochondrial morphological dynamics by Endo-B1, and determine the role of Endo-B1 in tubular cell apoptosis during ischemic renal injury. We hypothesize that upon apoptosis induction, Endo-B1 interacts with Bax, leading to their translocation to mitochondria. In mitochondria, Endo-B1 collaborates with fission-fusion proteins to fragment the organelles, facilitating Bax/Bak oligomerization and formation of pathological pores, resulting in the release of apoptotic factors. We propose three Specific Aims to test this hypothesis: 1) determine the role of Endo-B1 in Bax activation and analyze Endo-B1/Bax interactions during renal cell apoptosis;2) delineate the role and regulation of Endo-B1 in mitochondrial fragmentation during renal cell injury;and 3) use a newly developed knockout mouse model to determine the role of Endo-B1 in Bax activation and mitochondrial fragmentation during ischemic renal injury in vivo. Completion of the research is expected to significantly advance the mechanistic understanding of mitochondrial injury during apoptosis. In addition, it may lead to the development of novel strategies for the prevention and treatment of ischemic renal failure. PUBLIC HEALTH RELEVANCE: VA RELEVANCE: A broad range of clinical conditions including dehydration, hypotension, septic shock, trauma and operative arterial clamping leads to the reduction of blood flow to the kidneys. When blood supply to the kidneys is inadequate, renal tissues become ischemic, resulting in cell injury, tissue damage, and renal failure1-4. In the United States, over 200,000 cases of acute renal failure (ARF) are diagnosed each year, resulting in a direct medical expense in billions of dollars. Aging population, mirrored by veterans, is highly susceptible to ARF under conditions of ischemia and nephrotoxicity5-7. Data from the Austin Automation Center Patient Files show that over 3% of VA patients receiving in-patient care develop ARF (11,187 out of 359,608 patients in year 1999). ARF is also a common complication among military casualties in battle- fields3. Particularly, ischemic ARF is associated with high mortality, over 50%. This application aims at the identification of the key molecular factors that mediate ischemic ARF. While being mechanistic, it may lead to novel strategies for the prevention and treatment of this devastating disease, contributing significantly to the improvement of veterans'health. References: 1. Bonventre JV, Weinberg JM: Recent advances in the pathophysiology of ischemic acute renal failure. J Am Soc Nephrol 2003, 14:2199-2210 2. Devarajan P: Update on mechanisms of ischemic acute kidney injury. J Am Soc Nephrol 2006, 17:1503-1520 3. Schrier RW, Wang W, Poole B, Mitra A: Acute renal failure: definitions, diagnosis, pathogenesis, and therapy. J Clin Invest 2004, 114:5-14 4. Molitoris BA: Transitioning to therapy in ischemic acute renal failure. J Am Soc Nephrol 2003, 14:265-267 5. Schmitt R, Cantley LG: The impact of aging on kidney repair. Am J Physiol Renal Physiol 2008, 294:F1265-1272 6. Xue JL, Daniels F, Star RA, Kimmel PL, Eggers PW, Molitoris BA, Himmelfarb J, Collins AJ: Incidence and mortality of acute renal failure in Medicare beneficiaries, 1992 to 2001. J Am Soc Nephrol 2006, 17:1135-1142 7. Zhou XJ, Rakheja D, Yu X, Saxena R, Vaziri ND, Silva FG: The aging kidney. Kidney Int 2008, 74:710-720