Role of branched-chain amino acid catabolism in the proximal tubule

Acute kidney injury (AKI) causes significant morbidity and mortality, both of itself and as a major risk factor for development of chronic kidney disease (CKD). The proximal tubule (PT) is the primary target of AKI, triggering profound changes in PT cellular metabolism that contribute to injury. Whilst uninjured PT cells utilize fatty acid oxidation (FAO), TCA cycle and oxidative phosphorylation to generate ATP, in injury, these processes are severely downregulated, with inadequate compensation from glycolysis. Experimental upregulation of FAO can partially rescue AKI, but these strategies have so far not translated to clinical use. Furthermore, loss of the key FAO regulator PPARa does not result in PT injury at baseline, suggesting that other important PT metabolic pathways remain to be described. Branched chain amino acids (BCAA; valine, leucine, isoleucine) are catabolized by the kidney to generate TCA cycle intermediates. We recently reported that genes encoding BCAA catabolic enzymes are strongly downregulated in mouse models of AKI and CKD, and in human CKD, likely driven by transcriptional repression by Krüppel-like factor 6 (KLF6). In vitro, this led to decreased ATP production, whilst activation of BCAA catabolism increased mitochondrial respiration. However, the significance of downregulated BCAA catabolism in AKI or CKD has not been explored. Potential effects of loss of BCAA catabolism may include loss of ATP production, and toxic or detrimental accumulation of uncatabolized BCAA. In particular, leucine is a potent activator of mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) signaling, which may downregulate FAO, but this has not been demonstrated in kidney. This proposal will address these current gaps in the field by testing our central hypothesis that transcriptional suppression of PT BCAA catabolism in nephrotoxic AKI is detrimental via loss of ATP production, activation of mTORC1 signaling, and suppression of FAO. This hypothesis will be tested in two specific aims, to: 1) elucidate the mechanism by which disrupted BCAA catabolism alters FAO through mTORC1 activation; and 2) determine the contribution of BCAA catabolism to the severity of AKI and transition to fibrosis. These studies will enhance understanding of the significance of BCAA catabolism in the kidney, which is highly active yet currently unexplored. Furthermore, the link between BCAA catabolism and the critical cellular pathways of mTORC1 signaling and FAO, all of which are potentially druggable, will allow design of improved therapeutics for AKI. The long-term goal is to comprehensively define PT metabolic alterations in nephrotoxic and non-nephrotoxic AKI.