Introduction:

In the distal nephron, where the fine-tuning of urinary acidification takes place, two types of cells are involved in acid-base transport: acid-secretory type A- intercalated cells (A-IC) and bicarbonate secreting B- intercalated cells (B-IC). While type A-IC are characterized by the presence of the anion exchanger 1 (Ae1, Slc4a1) and luminal localization of H⁺-ATPases, type B-IC express luminal pendrin (Pds, Slc26a4) with luminal, basolateral or bipolar H⁺-ATPases. At present only very little is known about the development and differentiation of these cell types during renal ontogeny or adaptation to acid-base disturbances. But lately, few studies revealed a role for the Notch signaling pathway, as well as the transcription factors Foxi1 and Cp2l1, in the terminal differentiation of the renal collecting duct (CD) system. However, detailed studies about the spatial and temporal expression and action of these candidates are still missing. Here, we describe an abnormal differentiation of IC in kidneys from AE1 deficient mice with a dysregulation of the transcription factors Foxi1 and Cp2l1. Our data suggest a new role for Ae1 in the regulated differentiation of the renal CD system that might be beyond Cl^-/HCO₃^- exchange.

Methods:

Immunofluorescent markers were used to characterize the cellular profile of renal CD of WT and Ae1 KO mice, as well as a patient carrying a mutation of AE1 and mouse models with acid-base disturbance. Furthermore, mRNA and protein levels of various genes implicated on the differentiation of mouse kidney were assessed at different time points of life.

Results:

Mice deficient for Ae1 show abnormally differentiated cells along the renal CD. Different cell populations are detected that either coexpress markers for distinct differentiated cells or lack these markers at all. In Ae1 KO mice, renal type A-IC either disappear completely or loose differentiation and express Aqp2 or no marker of differentiated CD cells, whereas Pds positive cells in the renal cortex appear to be mostly normal. Surprisingly, the differentiation of renal CD cells from other mouse models with acid-base disturbances appeared not altered. Interestingly, Foxi1 and Cp2l1 are detected in distinct cell types of the renal CD system, suggesting a modulation of terminal differentiation of adjacent cells. Furthermore, Foxi1 and Cp2l1 are abnormally expressed in renal inner medullary collecting ducts of Ae1 deficient mice. Additionally, mRNA expression levels of markers for differentiation and several other transcription factors involved in terminal differentiation of the CD are abnormally regulated in Ae1 KO mice.

Conclusion:

Our data suggest that the absence of Ae1 per se directly affects the differentiation of cells along renal collecting ducts and may indicate a function beyond Cl^-/HCO₃^-exchange.