Lack of a role of NHE4 in renal ammonia metabolism.

The cation exchanger, Na/H exchanger isoform 4 (NHE4), has been thought to have a central role in renal ammonia metabolism and handling by acting in a Na for NH exchange mode at the basolateral plasma membrane in the thick ascending limb (TAL) of the loop of Henle. These studies aimed to determine the effect of NHE4 deletion on ammonia metabolism under basal conditions and in response to metabolic acidosis. Constitutive NHE4 deletion resulted in increased urine ammonia excretion associated with significantly lower urine pH; this increase did not lead to metabolic alkalosis. In response to exogenous acid-loading, NHE4 deletion did not impair the changes in ammonia excretion. Immunoblot analysis and immunohistochemistry showed mild increases in proximal tubule ammoniagenic enzyme expression with NHE4 deletion. Both immunoblot analysis and immunohistochemistry showed no detectable NHE4 protein expression in the mouse kidney. Single-nucleus RNAseq of mouse kidneys showed no NHE4 mRNA expression in renal epithelial cells. Analysis of five publicly available mouse and human cell-specific RNAseq datasets also showed a lack of NHE4 mRNA expression in the TAL. We conclude that NHE4 is unnecessary for ammonia metabolism either under basal conditions or in response to acid-loading because it is not expressed in the TAL. NHE4 has previously been suggested to mediate a critical role in renal ammonia metabolism through its role in thick ascending limb (TAL) ammonia transport. This manuscript shows that its deletion does not alter renal ammonia handling and that neither the mouse nor human kidney expresses NHE4 in the TAL.