Effect of NBCe1 deletion on renal citrate and 2-oxoglutarate handling

Abstract

The bicarbonate transporter, NBCe1 (SLC4A4), is necessary for at least two components of the proximal tubule contribution to acid‐base homeostasis, filtered bicarbonate reabsorption, and ammonia metabolism. This study's purpose was to determine NBCe1's role in a third component of acid‐base homeostasis, organic anion metabolism, by studying mice with NBCe1 deletion. Because NBCe1 deletion causes metabolic acidosis, we also examined acid‐loaded wild‐type adult mice to determine if the effects of NBCe1 deletion were specific to NBCe1 deletion or were a non‐specific effect of the associated metabolic acidosis. Both NBCe1 KO and acid‐loading decreased citrate excretion, but in contrast to metabolic acidosis alone, NBCe1 KO decreased expression of the apical citrate transporter, NaDC‐1. Thus, NBCe1 expression is necessary for normal NaDC‐1 expression, and NBCe1 deletion induces a novel citrate reabsorptive pathway. Second, NBCe1 KO increased 2‐oxoglutarate excretion. This could not be attributed to the metabolic acidosis as experimental acidosis decreased excretion. Increased 2‐oxoglutarate excretion could not be explained by changes in plasma 2‐oxoglutarate levels, the glutaminase I or the glutaminase II generation pathways, 2‐oxoglutarate metabolism, its putative apical 2‐oxoglutarate transporter, OAT10, or its basolateral transporter, NaDC‐3. In summary: (1) NBCe1 is necessary for normal proximal tubule NaDC‐1 expression; (2) NBCe1 deletion results in stimulation of a novel citrate reabsorptive pathway; and (3) NBCe1 is necessary for normal 2‐oxoglutarate metabolism through mechanisms independent of expression of known transport and metabolic pathways.

Recent advances in understanding renal ammonia metabolism and transport

“…Excretion of another important organic anion, 2-oxoglutarate (also known as α-ketoglutarate), was also altered by NBCe1 deletion. Specifically, NBCe1 deletion dramatically increased 2-oxoglutarate excretion [29]. This change in 2-oxoglutarate excretion could not ascribed to the accompanying metabolic acidosis, as experimental metabolic acidosis in wild-type mice actually decreased 2-oxoglutarate excretion [29], as previously described by others [31;32].…”

“…Decreased ammonia excretion appeared to result from abnormal expression of multiple enzymes involved in ammonia metabolism. There was decreased expression of the major proximal tubule enzymes involved in ammonia generation, PDG, GDH and PEPCK, and increased expression of the ammonia recycling protein, glutamine synthetase [28;29], responses opposite that expected from metabolic acidosis. Thus, NBCe1 expression is necessary for normal proximal tubule ammonia metabolism.…”

“…NBCe1 deletion caused abnormal regulation of expression of the primary proximal tubule citrate transporter, NaDC-1 (Figure 5) [29]. This did not alter net citrate excretion, possibly because of increased activity of a previously described acid-stimulated, calcium-sensitive citrate transport activity [30].…”

“…Specifically, NBCe1 deletion dramatically increased 2-oxoglutarate excretion [29]. This change in 2-oxoglutarate excretion could not ascribed to the accompanying metabolic acidosis, as experimental metabolic acidosis in wild-type mice actually decreased 2-oxoglutarate excretion [29], as previously described by others [31;32]. Thus, NBCe1 is necessary for normal proximal tubule bicarbonate reabsorption, and also for two other major roles of the proximal tubule in acid-base homeostasis, namely ammonia generation and organic anion transport/metabolism.…”

“…This effect was unrelated to the associated metabolic acidosis, as experimental metabolic acidosis in the absence of NBCe1 gene deletion increased NaDC1 mRNA and immunolabel expression. Figures 2 (left panel) and 3 (right panel) from [29] were combined and are used with permission.…”