The general interest is osmotic regulation of gene expression. The model system is mammalian renal medullary cells. Because of the urinary concentrating mechanism, the renal medulla is the only mammalian tissue that is routinely exposed to hyperosmotic stress under normal physiologic conditions. The cells of the renal medulla are able to survive such stress because they can accumulate non-perturbing, osmotically active organic solutes (organic osmolytes) instead of perturbing inorganic ions. The five principal organic osmolytes are: sorbitol, glycerophosphocholine (GPC), betaine, inositol, and taurine. We worked on the original identification of the organic osmolytes and have elucidated the biochemical mechanisms by which they are accumulated. Hyperosmotic stress increases sorbitol synthesis by transcriptional induction of the gene for aldose reductase (AR). GPC degradation is osmotically regulated through its degradation. Betaine, inositol and taurine transport is increased by hyperosmolality. Transcriptional regulation of the transporter genes has been also demonstrated for betaine and inositol. Recently, we identified the first eukaryotic osmotic response element (ORE) in the AR gene (PNAS 91: 10742-10746, 1994). Then, we narrowed down the length of the minimal essential ORE necessary for osmotic response (JBC 271: 18318-18321, 1996). This year we defined the functional consensus sequence for eukaryotic OREs, and demonstrated that the ORE is specific for osmotic stress (it does not mediate gene induction by other types of stress such as heat). Currently, we are characterizing cis-elements in the aldose reductase gene that act to potentiate the osmotic response mediated through the ORE. Future plans include the identification and characterization of transcription factors that interact with the OREs to elicit the osmotic response.
