Rhesus glycoprotein gene expression in the mangrove killifishKryptolebias marmoratusexposed to elevated environmental ammonia levels and air

Abstract

SUMMARY The mechanism(s) of ammonia excretion in the presence of elevated external ammonia are not well understood in fish. Recent studies in other organisms have revealed a new class of ammonia transporters, Rhesus glycoprotein genes(Rh genes), which may also play a role in ammonia excretion in fish. The first objective of this study was to clone and characterize Rhgenes in a fish species with a relatively high tolerance to environmental ammonia, the mangrove killifish Kryptolebias marmoratus (formerly Rivulus marmoratus). We obtained full-length cDNAs of three Rh genes in K. marmoratus: RhBG (1736 bp), RhCG1 (1920 bp) and RhCG2 (2021 bp), which are highly homologous with other known Rh gene sequences. Hydropathy analysis revealed that all three Rh genes encode membrane proteins with 10–12 predicted transmembrane domains. RhBG, RhCG1 and RhCG2 are highly expressed in gill tissue, with RhBG also present in skin of K. marmoratus. Exposure to elevated environmental ammonia (2 mmol l–1 NH4HCO3) for 5 days resulted in a modest (+37%) increase in whole-body ammonia levels, whereas gill RhCG2 and skin RhCG1 mRNA levels were upregulated by 5.8- and 7.7-fold, respectively. RhBG mRNA levels were also increased in various tissues, with 3- to 7-fold increases in the liver and skeletal muscle. In a separate group of killifish exposed to air for 24 h, RhCG1 and RhCG2 mRNA levels were elevated by 4- to 6-fold in the skin. Thus, the multifold induction of Rh mRNA levels in excretory tissues (gills and skin) and internal tissues in response to conditions that perturb normal ammonia excretion suggests that RhBG, RhCG1 and RhCG2 may be involved in facilitating ammonia transport in this species. Furthermore, the findings support earlier studies demonstrating that the skin is an important site of ammonia excretion in K. marmoratus.

Effects of salinity on intestinal bicarbonate secretion and compensatory regulation of acid–base balance inOpsanus beta

“…Despite significant progress in the understanding of urea excretion by the gulf toadfish , the metabolic cost remains unknown. Similarly, the rapidly developing field of NH 3 /NH 4 + excretion by fish Hung et al, 2007;Nakada et al, 2007) still lacks insight into the energetic driving force and thus the metabolic cost. Unfortunately, urea excretion was not measured as part of the present study but, considering the observed NH 3 /NH 4 + excretion increase, the above discussion is relevant in the case of unaltered or reduced urea excretion and we can infer an apparent impact of intestinal osmoregulatory processes on branchial nitrogenous waste excretion.…”

Physiological and molecular responses of the goldfish kidney (Carassius auratus) to metabolic acidosis, and potential mechanisms of renal ammonia transport

“…In mammals, renal epithelial ammonia transport in the collecting duct (CD) of the nephron is facilitated by Rhesus (Rh) glycoproteins (Weiner, 2004), and in fish, recent evidence suggests that the same is true in the gills (Nakada et al, 2007b;Hung et al, 2007;Nawata et al, 2007; reviewed in Wright and Wood, 2009). These proteins appear to function as channels for the translocation of ammonia gas (NH 3 ) along favourable partial pressure gradients (Knepper and Agre, 2004;Javelle et al, 2007;Nawata et al, 2010).…”

Rh proteins and NH4+-activated Na+-ATPase in the Magadi tilapia (Alcolapia grahami), a 100% ureotelic teleost fish

“…These have been cloned (Nakada et al, 2007;Hung et al, 2007;Nawata et al, 2007) and shown by in vitro expression analysis to bind NH 4 + but facilitate the movement of NH 3 (Nawata et al, 2010a). The transepithelial movement of H + ions by other mechanisms appears to play a key role in acid-trapping of NH 3 in the external boundary layer, thereby sustaining the NH 3 movement through the Rh channels [see models in Weihrauch et al (Weihrauch et al, 2009) and Wright and Wood (Wright and Wood, 2009)].…”