Knockout of the V-ATPase interacting protein Tldc2 in B-type kidney intercalated cells impairs urine alkalinization

Intercalated cells (ICs) are acid-base regulatory cells in the kidney collecting duct that excrete either acid or base into the urine in response to systemic cues. A-ICs deliver protons into the tubule lumen via an apical proton pump (V-ATPase) and reabsorb base (bicarbonate) using the anion exchanger 1 (AE1) anion exchanger. B-ICs function in the opposite direction. They have basolateral V-ATPase and secrete bicarbonate into the lumen via the anion exchange protein pendrin. The function of a third IC subtype: the non-A, non-B IC, which has apical pendrin and apical V-ATPase, is less well understood. We previously reported that members of the TLDc protein family interact with the V-ATPase and may regulate its function. TLDc proteins exhibit a distinct expression pattern in the kidney with RNAseq showing high, differential expression of Tldc2 in B-ICs. Here, we show by RNAscope imaging that Tldc2 is indeed expressed in B-ICs but also in some non-A, non-B ICs. Using Tldc2 knockout (Tldc2^-/-) mice, we found that males and females had significantly lower urine pH than wild-type littermates and their ability to increase urine pH in response to a bicarbonate load was impaired. In addition, Tldc2^-/- males developed hyperbicarbonatemia. Tldc2^-/- kidneys contained fewer B-ICs than wild-type mice, but they were replaced by more non-A, non-B ICs; the number of A-ICs was unchanged. Finally, there was decreased basolateral accumulation of V-ATPase in Tldc2^-/- B-ICs. These findings suggest that Tldc2 is a novel gene involved in renal acid-base regulation and in addition, may serve as a differentiation marker for B-ICs.NEW & NOTEWORTHY Acid-base balance in the body is constantly changing but must be tightly controlled to be compatible with life. The kidney contains specialized cells that can excrete excess acid or base (bicarbonate) into the urine to maintain normal blood pH. The key protein involved in this process is called the V-ATPase. Here, we report that a novel V-ATPase interacting protein Tldc2 is critical for kidney bicarbonate secretion and is, therefore, a previously unrecognized acid-base regulatory gene.