The physiology and pathophysiology of branched-chain amino acids in the kidney

Abstract

Background:

Kidneys require large amounts of energy to maintain function, and are highly metabolically active. Acute kidney injury (AKI) and diseases including chronic kidney disease (CKD), diabetic kidney disease (DKD), and polycystic kidney disease (PKD) are strongly associated with metabolic disturbances.

Summary:

Whilst most research to date has focused on glucose and fatty acid metabolism, the catabolism of the branched-chain amino acids (BCAAs) leucine, isoleucine, and valine, is an emerging area of importance across different kidney pathologies. BCAAs can be used in protein synthesis, or catabolized to provide tricarboxylic acid (TCA) cycle intermediates. BCAAs and their metabolites can also act as signaling molecules. Disturbances of BCAA catabolism have recently been described in AKI, CKD, DKD, and PKD, driven by both transcriptional and post-translational mechanisms. This results in accumulation of BCAAs in the kidney and loss of a source of TCA cycle intermediates. In addition, accumulated BCAAs, especially leucine, can activate mechanistic target of rapamycin complex 1 (mTORC1) signaling. In addition to the described disturbances in BCAA catabolism, recent preclinical studies have shown that reactivation of BCAA catabolism could be a potential therapeutic strategy.

Key messages:

This review will describe the process of BCAA catabolism, and its disturbances in AKI, CKD, DKD, and PKD.

Introduction

Kidneys are highly metabolically active, consuming as much oxygen as the heart in individuals at rest [1]. Metabolic disturbances are therefore detrimental to kidney function, and have been shown to drive injury and dysfunction in a number of settings, including acute kidney injury (AKI), chronic kidney disease (CKD), diabetic kidney disease (DKD), glomerular diseases, and polycystic kidney disease (PKD) [2–5]. Most studies to date have focused on glucose and fatty acid metabolism, as these are generally considered to account for the bulk of ATP generated across the different cell types within the kidney. Relatively little attention has been paid to other potential sources of ATP production such as amino acids. However, an emerging area that is rapidly gaining attention is the catabolism of the branched-chain amino acids (BCAAs) valine, leucine, and isoleucine. For several years, disturbances of circulating BCAAs have been linked both epidemiologically and experimentally with obesity, insulin resistance, and type 2 diabetes, as well as in inborn errors of metabolism such as maple syrup urine disease (MSUD), caused by mutations in BCAA catabolic enzymes [6–12]. Isotope tracing studies in vivo showed that the kidney can utilize BCAAs to generate tricarboxylic acid (TCA) cycle intermediates and indeed is the organ with the fourth highest tissue oxidation flux for BCAA after heart, brown adipose tissue, and pancreas in mice [13]. It is therefore not surprising that recent studies have shown dysregulated kidney BCAA catabolism across several kidney pathologies. Furthermore, dysregulated BCAA catabolism can result in BCAA accumulation, which may have secondary effects both systemically and locally within kidney cells, due to the signaling roles of both BCAA themselves, and their catabolic intermediates. This review will give an overview of BCAA catabolism and its regulation, explore the kidney-intrinsic disturbances in BCAA catabolism in various types of injury and disease, and briefly describe roles of systemic BCAA disturbances in relation to kidney outcomes.

BCAA Distribution and Catabolism

BCAAs are essential amino acids in mammals because they cannot be synthesized due to loss of the synthetic enzymes during evolution, and must be obtained through dietary intake [14]. After intake, BCAAs are rapidly absorbed in the bloodstream, with skeletal muscle rather than liver accounting for most of the whole body catabolism [14]. Notably, almost no BCAA are excreted in the urine, with fractional excretions of <1% for all three BCAAs [15], confirming that the main route of BCAA disposal is through their catabolism. Within the kidney, filtered BCAAs are absorbed via SLC6A18 (mice) or SLC36A2 (humans) and SLC6A19 (B⁰AT1) neutral amino acid transporters, which are expressed at highest levels in the proximal tubule (PT) and descending thin limb (DTL) in mice (https://esbl.nhlbi.nih.gov/Databases/SLC-kidney/). PT cells can also take up BCAAs from the basolateral side via SLC7A8 (L-type amino acid transporter 2; LAT2). Once in the cytoplasm, they can act as signaling molecules, and/or be used in protein synthesis or catabolized. Quantitative proteomics analysis of isolated mouse tubular segments showed high levels of BCAA catabolic enzymes in the PT, indicating that a large proportion of kidney BCAA catabolism likely occurs in the PT [16].

The first step in BCAA catabolism is a freely reversible transamination reaction catalyzed by branched chain amino acid transaminase (BCAT) enzymes, which transfer the BCAA amino group to α-ketoglutarate to generate glutamate plus an α-branched chain ketoacid (BCKA): α-ketoisocaproate (KIC) from leucine; α-keto-β-methylvalerate (KMV) from isoleucine; and α-ketoisovalerate (KIV) from valine (Fig. 1A) [17]. There are two distinct mammalian BCAT isoforms resulting from two genes: BCAT1 which is cytosolic, and BCAT2 which is mitochondrial and appears to be the more highly expressed isoform in the kidney (Fig. 1B). Cells expressing only BCAT2 also require a mitochondrial BCAA transporter, e.g. SLC25A44. The remaining steps of BCAA catabolism take place entirely within the mitochondrial matrix. The first irreversible and rate-limiting step involves oxidative decarboxylation of the BCKAs by the inner mitochondrial membrane-associated complex branched chain ketoacid dehydrogenase (BCKDH), generating branched chain acyl-coAs and CO₂, and requiring reduction of NAD⁺ to NADH. BCKDH is a large complex formed from three separate enzymes: branched chain α-keto acid decarboxylase (E1), an α₂β₂ heterotetramer encoded by BCKDHA and BCKDHB genes; dihydrolipoyltransacylase (E2) encoded by the DBT gene; and FAD-dependent dihydrolipoyl dehydrogenase (E3) encoded by the DLD gene (Fig. 1A). The overall complex is composed of E1, E2, and E3 enzymes in a stoichiometry of 12:24:12, with the 12 E3 proteins arranged as 6 homodimers. BCKDH activity can be measured directly in isolated tissues by measuring ¹⁴CO₂ release from ¹⁴C-labeled KIC. In humans, the kidney was found to have the highest BCKDH activity per mg of tissue across multiple organs [18]. Interestingly, the same study found that in rat tissues, the overall levels of BCKDH activity were much higher than in humans, and the kidney had the second highest activity after liver [18], highlighting the importance of caution when interpreting pre-clinical studies. After the production of branched chain acyl-CoAs, the rest of BCAA catabolism continues in a series of steps that are similar to mitochondrial fatty acid oxidation (FAO). Indeed, some enzymes such as acyl-CoA dehydrogenase medium chain (ACADM) are shared between BCAA catabolism and FAO. Complete oxidation results in generation of succinyl-CoA from valine, acetyl-CoA from leucine, and both succinyl-CoA and acetyl-CoA from isoleucine, thus feeding the TCA cycle and contributing to ATP production (Fig. 1A). Given the high energy demand of the kidney, it is most likely that the acetyl-CoA produced preferentially enters the TCA cycle rather than other metabolic pathways such as ketogenesis. This is supported experimentally by the observation that ¹³C-labeled leucine (the only purely ketogenic BCAA) generates more labeled malate (TCA cycle intermediate) than ¹³C-labeled valine or isoleucine [13].

Fig. 1. BCAA catabolic pathway and expression of enzymes.

(A) Simple schematic of BCAA catabolism. BCAT: branched chain amino acid transaminase; BCKDH: branched chain ketoacid dehydrogenase; KIC: ketoisocaproate, KIV: ketoisovalerate; KMV: keto-β-methylvalerate. (B) Expression of BCAA catabolic genes and proteins in mouse kidney cells, data mined from GSE268344 [33] and Lyu et al [16]. CD.PC: collecting duct principal cells; CNT: connecting tubule; DCT: distal convoluted tubule; EC: endothelial cells; IC: intercalated cells; LH.AL: loop of Henlé ascending limb; LH.DL: loop of Henlé descending limb; MC: mesangial cells; PT: proximal tubule. (C) Post-translational modification of BCKDH activity via BCKDK (branched chain ketoacid dehydrogenase kinase) and PP2Cm (protein phosphatase 2C of mitochondrial matrix).

Regulation of BCAA Catabolism

BCAA catabolism has been shown to be regulated by multiple mechanisms including transcriptional, post-translational, and allosterically by metabolic intermediates. However, the relative contributions of these different mechanisms in determining the flux through the pathway are not well defined, either in the kidney or in other organs. mRNA expression of BCAA catabolic genes may be regulated by several transcription factors, although most have been described in other organs and have not been confirmed in the kidney. Studies in knockout mice demonstrated that the zinc finger transcription factor Krüppel-like factor 15 (KLF15) was partially responsible for driving mRNA expression of multiple BCAA catabolic enzymes including Bckdha and Bckdhb in mouse cardiac muscle [19]. Interestingly, high BCAA levels have been reported to suppress KLF15 mRNA expression in skeletal muscle and adipose tissue [20]; however, the physiological significance of this mechanism is unclear, as this would be hypothesized to result in a feed-forward mechanism causing progressively higher intracellular BCAA concentrations. Studies utilizing knockout and activation of peroxisome proliferator-activated receptor gamma (PPARγ) have similarly shown a role for PPARγ in transcriptional regulation of BCAA catabolic enzyme expression in adipose tissue in rats and humans [21–23]. Indeed several targets of PPARγ were the same as those reported for KLF15; however, further studies will be needed to determine whether KLF15 and PPARγ directly or functionally interact to control BCAA catabolism, and if so, whether this mechanism is relevant in kidney cells such as podocytes and PT where KLF15 and PPARγ are coexpressed. In the setting of AKI, KLF6 is likely to act as a transcriptional suppressor of BCAA catabolism in PT cells. Genes encoding multiple catabolic enzymes, including Bckdhb, were less downregulated in Klf6 PT-specific knockdown mice after AKI, and were found to have KLF6 binding sites in their promoters, suggesting direct regulation by KLF6 [24]. These studies demonstrate that BCAA enzyme expression is likely regulated by a complex transcriptional network that requires further elucidation, and indeed mRNA expression of BCAA catabolic enzymes can vary significantly in different cell types in the kidney (Fig. 1B). Furthermore, although mRNA expression is often reduced in AKI (Supplementary Fig. 1A) and CKD (Supplementary Fig. 1B), some enzymes are transcriptionally upregulated and others show segment-specific changes.

The major mechanism by which BCAA catabolism is post-translationally regulated is by phosphorylation of the BCKDH complex. Phosphorylation of the E1α (BCKDHA) subunit on Ser293 is inhibitory, and is performed by BCKDH kinase (BCKDK) (Fig. 1C). Dephosphorylation is undertaken by protein phosphatase 2C of mitochondrial matrix (PP2Cm; also known as protein phosphatase, Mg²⁺/Mn²⁺ dependent 1K, PPM1K). BCKDH is also allosterically inhibited by the products of its activity, i.e. the branched chain acyl-CoAs and NADH, thus forming a feedback loop [14]. Additionally, BCKAs can allosterically inhibit BCKDK, leading to reduced phosphorylation of BCKDH and increased flux through the pathway [14].

Signaling Roles of BCAAs and their Catabolic Intermediates

BCAAs have long been known to have systemic signaling roles, including in insulin resistance, cardiometabolic disease, and stimulation of muscle protein synthesis (for recent reviews, see [14, 25, 26]). Similarly, circulating BCKAs are associated with insulin resistance and thought to play a major role in causing the neurological symptoms in patients with MSUD [12]. The valine catabolic intermediate 3-hydroxyisobutyrate (3-HIB) is the only BCAA catabolic intermediate except for acetoacetate that can exit the mitochondria and be secreted, resulting in systemic effects [27]. Intracellularly, BCAA, particularly leucine, are activators of mechanistic target of rapamycin complex 1 (mTORC1) signaling. Indeed, three different mechanisms of leucine sensing have been described to activate mTORC1 signaling, via Sestrin-2, leucyl-tRNA synthase, and secretion associated Ras-related GTPase 1B (SAR1B) (for recent review see [28]). Recently, valine was shown to be a key regulator of the intracellular localization of histone deacetylase 6 (HDAC6). Valine restriction led to inappropriate trapping of HDAC6 in the nucleus, with activation of DNA methylation and DNA damage [29]. This mechanism has not yet been shown to be active in the kidney.

Disturbances of BCAA Catabolism in AKI

AKI, a rapid loss of kidney function, includes injury to PT cells, which is accompanied by rapid mitochondrial dysfunction and metabolic reprogramming. Although PT predominantly utilize FAO to generate ATP, they also express high levels of BCAA catabolic enzymes [16]. Bulk and single cell RNA-sequencing studies have shown that mRNA levels of BCAA catabolic enzymes and regulators are downregulated in kidney cortex, and specifically in the PT, in AKI, including in nephrotoxic and ischemia-reperfusion injury (IRI) models [24, 30]. For example, administration of a single dose of the PT-specific toxin aristolochic acid I (AAI) induced an ~30–50% decrease in mRNA expression of several enzymes after 48 hours [31], whilst in IRI, key enzymes such as Bckdha and Bckdhb were reduced by >50% by 6 hours [30]. Interestingly, mRNA expression of Ppm1k was also downregulated, potentially exacerbating the loss of catabolic enzymes through additional loss of ability to dephosphorylate and activate BCKDH [30]. These gene expression disturbances resulted in accumulation of BCAA in kidney cortex, measured both by direct BCAA assay, and by NMR-based metabolomics in septic mice [32, 30, 31]. Mice with PT-specific knockdown of Klf6 had attenuated loss of expression of catabolic enzymes, whilst mice overexpressing KLF6 either globally or in the nephron, had exacerbated downregulation, suggesting that the increased expression of KLF6 in AKI is partially responsible for the transcriptional repression of BCAA catabolic enzymes [24, 33] (Fig. 2A). The small molecule 3,6-dichlorobenzo[b]thiophene-2-carboxylic acid (BT2) is an inhibitor of BCKDK, and therefore enhances BCAA catabolic flux by limiting phosphorylation of BCKDH [34]. Simultaneous administration of BT2 with AAI in mice showed that activation of BCAA catabolism could protect against AKI [31]. Furthermore, BT2 attenuated the accumulation of BCAA in kidney cortex and consequently reduced mTORC1 activation back to normal levels, as well as restoring mitochondrial bioenergetics in isolated primary PT cells injured with AAI. These studies therefore demonstrated that dysregulation of BCAA catabolism contributes to AKI both through loss of ATP production and through intracellular activation of mTORC1 signaling [31] (Fig. 2A). Furthermore, activation of BCAA catabolism appears to have dual benefits in attenuating both of these consequences, in contrast to possible alternative approaches such as inhibition of BCAA uptake, which would reduce accumulation but not restore mitochondrial energy production. Future studies will show whether activation of BCAA catabolism after induction of AKI is a potential therapeutic strategy.

Fig. 2. Intracellular disturbances of BCAA catabolism in kidney injury and disease.

(A) In AKI, KLF6 suppresses mRNA expression of catabolic enzymes, leading to loss of ATP production and BCAA accumulation which activates mTORC1 signaling. (B) In CKD, upregulation of miR-429–3p leads to suppression of mRNA expression of catabolic enzymes and upregulation of genes in the ferroptotic pathway. (C) In DKD, downregulation of BCAA catabolic and regulatory enzymes leads to accumulation of BCAA which shifts PKM2 to its dimeric state and transcriptionally increases serine and folate metabolism and apoptosis, whilst suppressing oxidative phosphorylation (OxPhos). (D) In PKD, cyst-lining cells have downregulation of BCAA catabolic enzymes and activated mTORC1 signaling, leading to increased proliferation.

BCAA Catabolism in CKD

CKD, which affects over 10% of adults globally, can arise from diverse causes, including as a result of maladaptive repair after AKI, or as a result of systemic disturbances such as hypertension, obesity, and cardiovascular disease [35]. CKD is accompanied by profound alterations in cellular metabolism, including mitochondrial fragmentation, altered FAO, and lipid droplet accumulation [4]. Several recent studies have shown roles for altered BCAA handling and catabolism in CKD. A novel mechanism for suppression of BCAA catabolism by microRNA (miR)-429–3p was identified in repeated low dose cisplatin-induced CKD by utilization of chimeric enhanced cross-linking immunoprecipitation (eCLIP)-seq [36]. Several BCAA catabolic enzymes were found to be direct targets for miR-429–3p, and their expression was upregulated by a miR-429–3p inhibitor (Fig. 2B). Furthermore, treatment with the BCKDK inhibitor BT2 during cisplatin administration attenuated ferroptosis pathways, showing for the first time a potential link between disrupted BCAA catabolism and ferroptosis via upregulation of Acsl4, which incorporates polyunsaturated fatty acids into phospholipids, and Tfrc, which facilitates iron uptake by cells. Both of these were suppressed after BT2 treatment (Fig. 2B), although the mechanistic link was not further explored [36]. In a chronic model of AAI toxicity, mice lacking B⁰AT1 had markedly attenuated fibrosis, cellular senescence and inflammation specifically in the fibrotic phase after injury. In this phase, BCAA catabolism would already likely be disrupted, and additional knockout of B⁰AT1 may have had beneficial effects as a result of less accumulation of BCAA within the PT cells, thus further suggesting that accumulation of BCAA in PT cells may also be a key driver of fibrosis and AKI–CKD transition [37] (Fig. 2B). Studies of dietary supplementation of amino acids in a rat 5/6 nephrectomy model of CKD showed that BCAA supplementation at 4.5x normal levels decreased renal plasma flow and GFR by ~25%, and increased inflammatory and fibrotic markers after 5 weeks [38]. However, the extent to which these effects were intrinsic to the kidney versus the result of systemic effects is unclear. BCAA catabolism was also found to be dysregulated in kidneys from rats with cardiovascular-kidney-metabolic syndrome (CKM). In the ZSF1 obese rat model, numerous BCAA catabolic enzymes were downregulated at both the mRNA and protein levels in the kidney [39]. In both the ZSF1 rats, and in a second model of CKM, uninephrectomized spontaneously diabetic Torii (SDT) rats, BT2 treatment improved kidney function, and reduced injury, proteinuria, and inflammation. Intriguingly, treatment of the ZSF1 rats with a combination of the SGLT2 inhibitor empagliflozin and BT2 resulted in a further decrease in proteinuria and kidney weight, and a further improvement in kidney histology, compared to empagliflozin alone, suggesting that activation of BCAA catabolism may be a promising therapeutic strategy in conjunction with other established treatments [39].

BCAA Catabolism in DKD

DKD is a leading cause of end stage kidney disease (ESKD) and is marked by metabolic inflexibility, oxidative stress, and mitochondrial dysfunction, which affects podocytes, endothelial cells and PT cells [3]. Disturbances of circulating BCAA have been previously described in systemic metabolic diseases, with increased levels reported in insulin resistance and type 1 and type 2 diabetic patients compared to healthy subjects [6–11]. Furthermore, plasma BCAAs were reported to be lower in DKD patients than in type 2 diabetic patients without DKD, although this was also true of several other amino acids [40]. However, a subsequent study identified that elevated BCAA levels were associated with increased risk of incident CKD but lower odds of prevalent CKD in type 2 diabetes patients. These findings were confirmed in a larger number of patients from the UK Biobank but Mendelian randomization analysis did not suggest that altered BCAAs may cause CKD [41]. Despite these conflicting clinical associations, emerging evidence from preclinical studies now implicates altered BCAA catabolism as a direct modulator of podocyte injury and DKD progression, and suggests activation as a potential therapeutic strategy [42–44]. mRNA expression of key BCAA catabolic enzymes was decreased in kidneys of diabetic db/db mice versus controls, and interestingly podocyte expression of phospho (p-)BCKDHA was increased in db/db mice and in DKD versus non-DKD diabetic patients, in conjunction with reduced expression of PPM1K, suggesting that post-translational control of BCAA catabolism may also be disrupted in DKD [42, 44]. Upregulation of BCKDK, increased phospho-BCKDHA, and downregulation of catabolic enzymes were also shown in the tubules of DKD patients and db/db mice [43]. Furthermore, inhibition of BCAA catabolism by global Ppm1k knockout exacerbated DKD in db/db mice, whilst podocyte-specific Ppm1k knockdown induced glomerular injury in mice on high fat diet [42, 44]. Conversely, activation of BCAA catabolism, either by using BT2 in db/db or streptozotocin-treated mice, or by PT-specific knockdown of BCKDK, attenuated tubular injury and fibrosis, and restored kidney function [42–44]. Mechanistically, in podocytes high BCAA concentrations caused a shift in pyruvate kinase M2 isoform (PKM2) away from its tetrameric form towards increased abundance of the dimeric and monomeric forms, possibly through direct allosteric regulation of PKM2 by the BCAA themselves (Fig. 2C). This resulted in a metabolic switch with loss of oxidative phosphorylation but a gain in serine and folate biosynthesis, a finding that was confirmed in DKD patients [44]. Increased nuclear shuttling of the dimeric/monomeric PKM2 also resulted in increased transcription of proapoptotic genes. In tubular cells, treatment with high leucine shifted cells towards a more glycolytic metabolic phenotype via activation of mTORC1 signaling, whilst treatment with the LAT2 inhibitor JPH203 attenuated mTORC1 signaling and the switch to glycolysis [43].

BCAA Catabolism in PKD

PKD is an inherited disorder characterized by the progressive formation of kidney cysts and a gradual decline in kidney function leading to CKD, and is predominantly cause by mutations in PKD1 and PKD2 [5]. Metabolic reprogramming, such as a switch away from oxidative phosphorylation and towards anerobic glycolysis, have been well described [5]; however, more recent studies have also elucidated a role for altered BCAA catabolism in cyst growth and disease progression. Dietary BCAA supplementation increased the cystic index in both the kidney and liver of Pkd1-deficient mice, in association with increased proliferation [45]. Mechanistically, cyst-lining cells expressed high levels of LAT1, and after BCAA supplementation, showed significant upregulation of mTORC1 signaling compared to non-cyst lining cells [45] (Fig. 2D). Defects in BCAA catabolism have also recently been confirmed in cystic tissue from PKD patients, in which multiple enzymes were found to be downregulated at the mRNA level compared to adjacent non-cystic tissue [46] (Fig. 2D). Notably, other key metabolic pathways, including FAO, TCA cycle, and oxidative phosphorylation components were also strongly suppressed, so further studies are needed to determine whether the defects in BCAA catabolism are due to a global metabolic derangement, or have separate specific mechanisms that may be amenable to therapeutic intervention.

Future directions and conclusions

Understanding of the role of BCAA catabolism in various kidney pathologies has rapidly evolved in the past 5 years. However, many aspects have not yet been explored. Other nephron segments such as the thick ascending limb (TAL) also express high levels of BCAA catabolic enzymes, but the role of BCAA catabolism in TAL function has not been studied, either in normal physiology or in pathological situations such as AKI. Similarly, the signaling roles of BCAAs, BCKAs and other BCAA catabolic intermediates such as 3-HIB directly within the kidney have not been studied. Intracellularly, downregulation of BCAA catabolism does appear to exacerbate mTORC1 activation, as shown by the corresponding decrease in mTORC1 upon administration of BT2. However, the mechanism of BCAA sensing to modulate mTORC1 activity has not been definitively shown in the kidney. Furthermore, alterations of mTORC1 signaling may also affect other metabolic pathways such as fatty acid synthesis and degradation, and it remains to be seen how many or to what extent the concomitant metabolic alterations such as downregulation of FAO in AKI, CKD, and PKD may be secondary to BCAA accumulation and mTORC1 activation, or possibly more directly linked via changes in BCAA catabolic components. As an example, it was recently shown that BCKDK can promote hepatic gluconeogenesis independently of its role in phosphorylation of BCKDH in the liver [47], and that both BCKDK and PPM1K can act on ATP citrate lyase to modulate de novo lipogenesis [48]. These studies also highlight the major limitation of using BT2 to restore BCAA catabolism, as it has systemic effects that may also contribute to the changes in phenotypes observed. More studies utilizing genetic methods of activating BCAA catabolism, such as cell-specific Bckdk knockdown will help to further elucidate the cell-specific effects of restoring BCAA catabolism. Finally, there are potential sex differences which have not been explored in the kidney. Kidneys in males and females are estimated to have similar energy requirements per Kg of tissue, but kidneys in males are larger and therefore require more energy [49], which may result in them being more vulnerable to changes in energy metabolism. There is evidence of important sex differences in liver BCAA catabolism [50]. However, in common with many preclinical studies, BCAA catabolism in kidney pathology has only been studied in male mice to date, partly because females are relatively resistant to developing injury and disease. It is therefore unclear whether the same mechanisms are important in kidney pathology in females, or indeed whether sex differences in BCAA catabolism may contribute to the resistance to kidney injury and disease in females compared to males.

In conclusion, suppression of BCAA catabolism is becoming an increasingly recognized factor in various forms of kidney injury and disease, and preclinical studies suggest that restoration of BCAA catabolism may represent a viable therapeutic strategy. However, many aspects of defective BCAA catabolism and its downstream consequences for other intracellular signaling and metabolic pathways remain to be elucidated.