Molecular cloning and characterization of the taurine transporter of Atlantic salmon

Osmolytes are small neutral solutes that provide osmotic pressure to the cell without interacting with or denaturing enzymes and cofactors. An increase in the concentration of osmolytes in mammalian renal cells during hyperosmotic stress has been demonstrated to assist in cell volume regulation. More recently uptake of taurine, a common osmolyte, has been reported in tilapia and the carp epithelial cell line, epithelioma papulosum cyprini (EPC), exposed to hyperosmotic stress. The current report investigated whether taurine transporters were present in anadromous salmon and might assist in adaptation to the marine environment.Cloning of the cDNA coding for the taurine transporter in salmon revealed a high degree of identity with the deduced amino acid sequences of taurine transporters from tilapia (93%), carp (90%) and canine (79%). Exposure of isolated tissues to hyperosmotic conditions revealed marked upregulation (2–5 fold) of transcript for the taurine transporter in branchial lamellae, kidney and heart tissue with maximum expression at 125 mM supplemental NaCl and from 24–72 h. Juvenile salmon transferred from freshwater to seawater exhibited a similar elevation in accumulation of taurine transporter mRNA which continued at least 72 h. Supplementation of hyperosmotic tissue culture medium with 5–30 μM taurine failed to reduce cell mortality or alleviate suppression of protein synthesis in branchial lamellae. Oral administration of taurine in the diet of juvenile salmon prior to and during seawater transfer reduced the quantity of taurine transporter mRNA in smolts but not parr. The results indicate that salmon possess a transporter for the osmolyte taurine and suggest it to be one component of an array of mechanisms involved in adaptation to the dehydrating marine environment.