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 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 male 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 more non-A, non-B ICs; therefore their sum and the number of A-ICs were unchanged. Furthermore, 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.