Expression pattern of aquaporin water channels in the inner ear of the rat- The molecular basis for a water regulation system in the endolymphatic sac

Mammalian aquaporins constitute a family of so far 10 related water channel proteins which mediate osmotically driven water fluxes across the plasma m embrane. Because regulation of the ionic composition and osmolality of inne r ear fluids is of great functional significance, we investigated the expre ssion patterns of aquaporins in five defined areas of the rat inner ear by RT-PCR. The tissues used were stria vascularis, endolymphatic sac, Reissner 's membrane, vestibulum and organ of Corti. Aquaporin 1 transcripts were de tected in all tissues and are probably constitutive. Aquaporin 5 was only e xpressed in the organ of Corti and in Reissner's membrane. We show that aqu aporin 2, so far considered to be specific to the principal cells of the re nal collecting duct, is expressed in the endolymphatic sac. Aquaporin 2 exp ression was not detected in any other inner ear region. The postnatal appea rance of aquaporin 2 transcripts in the endolymphatic sac resembled that in the kidney, i.e, it increased postnatally until day 4. The full-length DNA for aquaporin 2 was cloned from cDNA of the endolymphatic sac. It had an i rrelevant Ile54Thr mutation because it could be functionally expressed in X enopus oocytes. Also exclusively in the endolymphatic sac of the inner ear, we detected transcripts for aquaporin isoforms 3 and 4 which are known to be expressed in the renal principal cells. In the kidney, aquaporin 2 regul ation involves vasopressin-stimulated, cAMP-dependent phosphorylation of Se r256 of aquaporin 2 which is stored in cytosolic vesicles. These storage ve sicles also contain a serpentine calcium/polycation-sensing receptor. Vesic le shuffling to the plasma membrane involves proteins such as vesicle-assoc iated membrane protein VAMP2, syntaxin-4 and the small GTPase Rab3a. Using RT-PCR we were able to demonstrate the expression of all of these component s. By analogy the data suggest that in the endolymphatic sac of the inner e ar a system for cellular water permeability is in place which may share man y similarities with that characterized in the principal cells of the renal collecting duct. These findings may have a number of interesting pharmacolo gical implications which need to be addressed in future studies. (C) 1999 E lsevier Science B.V. All rights reserved.