Renal response to metabolic acidosis: role of mRNA stabilization

The renal response to metabolic acidosis is mediated, in part, by increased expression of the genes encoding key enzymes of glutamine catabolism and various ion transporters that contribute to the increased synthesis and excretion of ammonium ions and the net production and release of bicarbonate ions. The resulting adaptations facilitate the excretion of acid and partially restore systemic acid-base balance. Much of this response may be mediated by selective stabilization of the mRNAs that encode the responsive proteins. For example, the glutaminase mRNA contains a direct repeat of 8-nt AU sequences that function as a pH-response element (pHRE). This element is both necessary and sufficient to impart a pH-responsive stabilization to chimeric mRNAs. The pHRE also binds multiple RNA-binding proteins, including zeta-crystallin (zeta-cryst), AU-factor 1 (AUF1), and HuR. The onset of acidosis initiates an endoplasmic reticulum (ER)-stress response that leads to the formation of cytoplasmic stress granules. zeta-cryst is transiently recruited to the stress granules, and concurrently, HuR is translocated from the nucleus to the cytoplasm. On the basis of the cumulative data, a mechanism for the stabilization of selective mRNAs is proposed. This hypothesis suggests multiple experiments that should define better how cells in the kidney sense very slight changes in intracellular pH and mediate this essential adaptive response.