Basal renal phenotype and response to induction of aristolochic acid nephropathy in mice lacking the neutral amino acid transporter B0AT1 (SLC6A19)

Abstract

Background B0AT1 (SLC6A19) mediates the reabsorption of neutral amino acids, including branched-chain amino acids (BCAA), in the intestine and kidney (expressed in early and mid-proximal tubule - S1/S2). Dietary restriction of BCAA has been proposed to improve metabolic health by inhibiting mTOR and inducing autophagy. Here we characterized basal kidney function and determined whether B0AT1 KO protects the kidney from aristolochic acid-induced nephropathy (AAN), a model of proximal tubular injury. Methods In male B0AT1 KO and littermate wild-type mice (WT) mice, we measured fractional renal amino acid excretion and performed renal FITC-sinistrin clearance studies under terminal anesthesia to determine renal creatinine handling. A set of female mice was injected with aristolochic acid (3 mg/kg i.p.) or vehicle every 3 days for 3 weeks, and kidneys were harvested 3 weeks later for RTqPCR analyses (n=7-16 mice per group). Results RTqPCR and Western blotting confirmed absence of renal B0AT1 mRNA in KO, associated with increased fractional renal excretion of neutral amino acids, including BCAA (to 20-40%), whereas plasma levels of BCAA were not changed. In vehicle-treated mice, plasma creatinine was slightly higher in KO vs WT (0.080±0.002 vs 0.074±0.002 mg/dl; *P<0.05). Clearance studies revealed a trend for lesser fractional excretion of creatinine in KO vs WT (P=0.057), suggesting lesser creatinine secretion contributed to higher plasma levels. Urinary albumin/creatinine ratio (UACR) and urinary glucose/creatinine ratio (UGCR) were tripled and doubled in female KO vs WT (180±18 vs 62±5 ng/μg*; 1.35±0.26 vs 0.70±0.06 mg/mg*), respectively, suggesting a modest impairment in proximal tubular albumin and glucose reabsorption. The renal expression of the alternative neutral amino acid transporter B0AT3 (SLC6A18), expressed in S1-S3, was reduced in KO (to 32±5% of WT*). This may be due to KO model generation, as B0AT1 and B0AT3 are co-expressed side-by-side on the same chromosome. Whole kidney CCR2 and KIM-1 mRNA expression were modestly higher in KO vs WT, which may reflect enhanced amino acid and glucose transport in the vulnerable S3 segment in the outer medulla. In the absence of robust effects on plasma creatinine, AAN significantly increased UACR vs vehicle in WT (+164±39 vs +52±14 ng/μg*) but not in KO (-18±30 vs +57±49 ng/μg) when measured after completion of injections. Moreover, increases in CCR2 and TIMP1 expression in response to AAN vs vehicle observed in WT (+206±28%; +203±45%) were attenuated in KO (+54±20%*; +81±39%*). Conclusions Genetic deletion of B0AT1 in male and female mice induces distinct effects on kidney transport beyond amino acids. Follow up studies with higher doses of AA are needed to more fully evaluate the nephroprotective potential of B0AT1 inhibition.