ζ-Crystallin: a tale of two cells

The Na+/K+/2Cl- cotransporter (BSC1/NKCC2) is the major transporter mediating sodium chloride and ammonium absorption in the medullary thick ascending limb. A loss-of-function mutation of BSC1 is responsible for Bartter's syndrome. We previously showed both in vivo and in vitro that acidosis increases the expression and activity of BSC1 and that acid pH enhances the stability of BSC1 mRNA by mechanisms involving its 3'-untranslated region (UTR). zeta-Crystallin is a pH response factor that protects the mitochondrial glutaminase mRNA by a specific interaction with AU-rich motifs. Here we identified the molecular determinant(s) within the 3'-UTR that are responsible for BSC1-mRNA expression and assessed the involvement of zeta-crystallin in this regulation. Deleting three out of six conserved AU-rich motifs drastically reduced the expression of BSC1-mRNA with maximal effect for motif 3 at position 870 of the 3'UTR. This motif was responsible for pH and zeta-crystallin-induced stability of BSC1 mRNA. The abundance of zeta-crystallin was increased by acid pH and its overexpression increased the stability of BSC1 mRNA, but its RNA silencing inhibited acid pH-induced BSC1 expression. Therefore the 3'UTR of BSC1-mRNA is a target for zeta-crystallin. The induction of zeta-crystallin by an acid pH plays an important role in preventing BSC1 mRNA decay, thus increasing its expression and activity.