DEVELOPMENTAL GENE-EXPRESSION AND TISSUE DISTRIBUTION OF THE CHIP28 WATER-CHANNEL PROTEIN

The CHIP28 water channel is a major component of red cell and renal tu bule membranes; however, its ontogeny and tissue distribution remain u ndefined. Three patterns of expression were identified when CHIP28 mRN A was surveyed by in situ hybridization histochemistry in rats between embryonic day 14 and maturity. (i) CHIP28 mRNA and protein were very abundant in hematopoietic tissue and kidneys of mature rats, but stron g expression did not occur until after birth, when it appeared in rena l proximal tubules and descending thin limbs, red pulp of the spleen, and membranes of circulating red cells. (ii) CHIP28 mRNA was abundant in choroid plexus epithelium throughout fetal development and maturity . (iii) CHIP28 mRNA was transiently observed in periosteum, heart, vas cular endothelium, and cornea during fetal development. The ontogeny o f kidney and red cell CHIP28 expression coincides with the ability of kidneys to concentrate urine, suggesting that CHIP28 promotes water re absorption in the proximal nephron and provides red cell osmoregulatio n needed for passage through the hypertonic medulla. Its presence in t he choroid plexus suggests that CHIP28-mediated water transport contri butes to secretion of cerebrospinal fluid. The functional role of CHIP 28 in developing bone, heart, and eye is unclear. These findings furth er establish the general physiologic role of CHIP28 as a water channel involved in reabsorption, osmoregulation, and secretion. The studies also suggest other possible functions during fetal development and pre dict that complex mechanisms will be needed for regulation of CHIP28 g ene expression in diverse tissues at distinct points in development.