Two distinct aquaporin-4 cDNAs isolated from medullary cone of quail kidney

Water deprivation or arginine vasotocin upregulates aquaporin-2 (AQP2) expression in apical and subapical regions of medullary collecting duct (CD) cells of Coturnix coturnix quail (q) kidneys. We therefore aimed to determine whether the CD has AQPs mediating water exit from the intracellular to the extracellular (interstitial) space. Using a homologue cloning technique, we isolated two distinct qAQP4 cDNAs from quail medullary cones; long (L, open reading frames) and short (S) cDNA encoded 335 (qAQP4-L) and 301 (qAQP4-S) amino acids with, respectively, 80% and 87% identity to human long- and short-form AQP4. qAQP4-S is identical to qAQP4-L from the second initiation site. Both isoforms have two NPA motifs, but lack cysteine at the known mercury-sensitive site. qAQP4-L and qAQP4-S are expressed in membranes of Xenopus laevis oocytes, but both failed to increase the water permeability (Pf) of oocytes exposed to a hypotonic solution. Glutamate (Q242) replacement with histidine did not increase Pf. With conventional RT-PCR and real-time PCR, qAQP4-L/S mRNA signals were detected in the brain, lung, heart, intestine, adrenal gland, skeletal muscle, liver, and kidney (higher in medulla than in cortical region). qAQP4-L mRNA was detected only in the brain and adrenal gland. Orthogonal arrays of intramembranous particles were not detected in quail CDs. The results suggest that although qAQP4-L and qAQP4-S have high homology to mammalian AQP4, their physiological function may be different.