A novel type of urea transporter, UT-C, is highly expressed in proximal tubule of seawater eel kidney.
A new type of urea transporter was identified by a database search and shown to be highly expressed in the renal proximal tubule cells of teleosts; proximal tubule-type urea transporters have not been describe previously. We first identified urea transporter-like sequences in the fugu genome and in an EST database of rainbow trout. Based on these pieces of sequence information, we obtained a full-length cDNA for the eel ortholog, consisting of 378 amino acid residues, and named it eUT-C. Although its sequence similarity to the known urea transporters is low (approximately 35%), its heterologous expression in Xenopus laevis oocytes indicated that it is a facilitative urea transporter sensitive to phloretin. Its activity is not dependent on Na+. Northern blot analysis showed that expression of eUT-C is highly restricted to the kidney, with weak expression in the stomach. In both tissues, eUT-C mRNA was strongly induced when eels were transferred from freshwater to seawater. Immunohistochemistry and in situ hybridization histochemistry revealed proximal tubule cell localization of eUT-C. Taking into account that 1) urea is mainly secreted from the gill where another type of urea transporter (eUT) has been identified and 2) fish excrete a very small volume of urine in seawater, we propose that eUT-C cloned here is a key component working in combination with the gill transporter to achieve an efficient urea excretory system in fish, namely, eUT-C reabsorbs urea from glomerular filtrate and sends it to the gill, through the circulation, for excretion.
Mistry
Abinash Chandra
AC
Dept. of Biological Sciences, Tokyo Institute of Technology, 4259-B-19 Nagatsuta-cho, Midori-ku, Yokohama 226-8501, Japan.
Chen
Guangping
G
Kato
Akira
A
Nag
Kakon
K
Sands
Jeff M
JM
Hirose
Shigehisa
S
D000595:Amino Acid Sequence; D000811:Anguilla / Q000378:metabolism*; D000818:Animals; D015152:Blotting, Northern; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000096:biosynthesis / Q000235:genetics; D005880:Gills / Q000378:metabolism; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D009994:Osmolar Concentration; D010802:Phylogeny; D012313:RNA / Q000096:biosynthesis / Q000302:isolation & purification; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014508:Urea / Q000097:blood; D000096982:Urea Transporters
D018076:DNA, Complementary; D026901:Membrane Transport Proteins; D012313:RNA; D014508:Urea; D000096982:Urea Transporters