Myo-inositol is one of the major organic osmolytes in the brain and the kidney. The accumulation of intracellular organic osmolytes allows cells to regulate intracellular osmolality without altering cytoplasmic ionic strength and to adapt to hyperosmotic conditions. Two types of myo-inositol transporters, sodium/myo-inositol transporter and H(+)/myo-inositol transporter (HMIT), have been identified. Sodium/myo-inositol transporters are induced by osmotic stress and might be involved in the intracellular accumulation of myo-inositol in mammals. The role of HMIT, however, remains unknown. In the present study, we characterized three Caenorhabditis elegansHMIT genes, hmit-1.1, hmit-1.2, and hmit-1.3. hmit-1.1 was expressed in the intestine, and hmit-1.2 was expressed in the glia and the excretory canal, which is an osmotic regulatory organ that is functionally analogous to the kidney. hmit-1.3 was expressed in the intestine and the glia. The expression of hmit-1.1 and hmit-1.2 but not hmit-1.3, was markedly induced under hyperosmotic conditions. Animals with mutant hmit-1.1 and hmit-1.2 were hypersensitive to osmotic stress. The defects of hmit-1.1 and hmit-1.2 mutants were rescued by hmit-1.1 and hmit-1.2 transgenes, respectively, and by modified human HMIT. In human cell lines, HMIT expression was induced in hyperosmotic conditions. These findings indicate that the C. elegans HMIT family has a crucial role in the osmoprotective response.
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