Kidney-specific chloride channel, OmClC-K, predominantly expressed in the diluting segment of freshwater-adapted tilapia kidney

Abstract

The kidney plays an important role in osmoregulation in freshwater teleosts, which are exposed to the danger of osmotic loss of Na ؉ and Cl ؊ . However, ion-transport mechanisms in the kidney are poorly understood, and ion transporters of the fish nephron have not been identified thus far. From Mozambique tilapia, Oreochromis mossambicus, we have cloned a chloride channel, which is a homologue of the mammalian kidney-specific chloride channel, ClC-K. The cDNA of the channel, named OmClC-K, encodes a protein whose amino acid sequence has high homology to Xenopus and mammalian ClC-K (Xenopus ClC-K, 41.8%; rat ClC-K2, 40.9%; rat ClC-K1, 40.1%). The mRNA of OmClC-K was expressed exclusively in the kidney, and the expression level of mRNA was increased more in freshwater-adapted fish than seawater-adapted fish. The immunohistochemical study using a specific antibody showed that OmClC-K-positive cells were specifically located in the distal nephron segments. Immunoelectron microscopy further showed that immunoreaction of OmClC-K was recognizable on the structure of basolateral membrane infoldings in the distal tubule cells. The localization of OmClC-K and its induction in hypoosmotic media suggest that OmClC-K is involved in Cl ؊ reabsorption in the distal tubule of freshwater-adapted tilapia.T he kidney plays an important role in osmoregulation in both freshwater and seawater teleosts, although its function is entirely different under the two different environmental conditions. Freshwater teleosts are exposed to the danger of osmotic water load and ion loss. Accordingly, the primary function of their kidneys is to excrete excess water, while reabsorbing most of the filtered solutes. The glomerular filtration rate of freshwater teleosts is higher than that of marine teleosts, and sometimes as much as 95% of the filtered water is excreted as copious dilute urine, which implies that the renal tubules of freshwater teleosts must generally have low water permeability. The nephron of freshwater teleosts is composed of a well-developed glomerulus, a ciliated neck segment, a proximal tubule, an intermediate segment, a distal tubule, and a collecting tubule, but it lacks the loop of Henle (1). The teleost kidney exhibits neither zonation, such as the cortex and medulla, nor a countercurrent system of tubular elements. Thus, teleosts are not able to excrete hyperosmotic urine.Nishimura et al. (2) measured water and ion transport in nephron tubules isolated from freshwater-adapted rainbow trout, Oncorhynchus mykiss. They perfused nephron tubules and measured water and ion transport in the distal tubule. These results suggest that in freshwater-adapted rainbow trout, the distal tubule acts as a diluting segment, which is equivalent to the early distal segment of the frog kidney and the mammalian thick ascending limb of Henle's loop (TALH) (3, 4).The extraction of NaCl from urine in the TALH is crucial to urinary concentration and dilution, which is accomplished by the active reabsorption of NaCl in the water-impermeable epitheli...

Morphofunctional ontogeny of the urinary system of the European sea bass Dicentrarchus labrax

“…A much higher Na + /K + -ATPase activity has been revealed in the distal segments than in the proximal segments of freshwater fish, which suggests the involvement of those tubules in ion reabsorption (Petrova 1981). In the Mozambique tilapia, Oreochromis mossambicus, a percid species closely related to the sea bass, only the distal portions of the nephrons contain Na + /K + -ATPase when the fish is kept in freshwater (Miyazaki et al 2002). In seawater-maintained tilapias, however, no immunoreaction has been detected, an observation that reinforces our hypothesis of the osmoregulatory role of the collecting tubule/duct system.…”

Ion uptake and acid secretion in zebrafish (Danio rerio)

“…Very few studies have investigated ClCs in other fish species. In tilapia, ClC3 and -5 were cloned and found to be expressed in various organs including gills, but they were suggested to function as intracellular Cl -channels based on an in vitro functional analysis (Miyazaki et al, 2002). Recently, higher protein expression analysed by an anti-rat ClC3 antibody was found in FW pufferfish gills than in SW ones (Tang and Lee, 2007).…”

Distinct Na+/K+/2Cl- cotransporter localization in kidneys and gills of two euryhaline species, rainbow trout and killifish

“…Studies on the nephrons of trout, killifish, and other species have demonstrated distinct functional differences between the tubule segments (Braun and Dantzler 1997) and specific differential localization of transporters (Miyazaki et al 2002). Nishimura et al (1983) have shown that, in the distal tubule, chloride efflux (lumen to bath) is significantly higher than chloride influx (bath to lumen), suggesting the net Cl -reabsorption from the lumen to the bath and with a lumen-positive transepithelial voltage.…”

“…The necessity of sodium in addition to chloride to generate the lumen-positive voltage in the distal tubule (Nishimura et al 1983) suggests the involvement of apical NKCC in Cl -reabsorption. The differential localization of transporters in teleosts include the cloning and characterization of a kidney-specific chloride channel, OmClC-K, cloned from tilapia (O. mossambicus; Miyazaki et al 2002); OmClC-K mRNA shows elevated transcript levels in FW-adapted tilapia compared with SW-adapted fish, whereas immunohistochemical evidence has revealed OmClC-K located in the basolateral membrane of the distal nephron suggesting that OmClC-K is involved in transepithelial Cl -reabsorption. In killifish, Cliff and Beyenbach (1992) have reported similarities in the in vitro functional properties of secretory proximal tubules by using isolated SW-and FW-adapted proximal tubules.…”