Molecular and functional characterization of a urea transporter from the kidney of a short-finned pilot whale

Determination of water balance maintenance in Orcinus orca and Tursiops truncatus using oxygen isotopes

The secondary adaptation of Cetacea to a fully marine lifestyle raises the question of their ability to maintain their water balance in a hyperosmotic environment. Cetacea have access to four potential sources of water: surrounding salt oceanic water, dietary free water, metabolic water and inhaled water vapour to a lesser degree. Here, we measured the 18O/16O oxygen isotope ratio of blood plasma from 13 specimens belonging to two species of Cetacea raised under human care (four killer whales Orcinus orca, nine common bottlenose dolphins Tursiops truncatus) to investigate and quantify the contribution of preformed water (dietary free water, surrounding salt oceanic water) and metabolic water to Cetacea body water using a box-modelling approach. The oxygen isotope composition of Cetacea blood plasma indicates that dietary free water and metabolic water contribute to more than 90% of the total water input in weight for cetaceans, with the remaining 10% consisting of inhaled water vapour and surrounding water accidentally ingested or absorbed through the skin. Moreover, the contribution of metabolic water appears to be more important in organisms with a more lipid-rich diet. Beyond these physiological and conservation biology implications, this study opens up questions that need to be addressed, such as the applicability of the oxygen isotope composition of cetacean body fluids and skeletal elements as an environmental proxy of the oxygen isotope composition of present and past marine waters.

Physicochemical evolution and molecular adaptation of the cetacean osmoregulation-related gene UT-A2 and implications for functional studies

Cetaceans have an enigmatic evolutionary history of re-invading aquatic habitats. One of their essential adaptabilities that has enabled this process is their homeostatic strategy adjustment. Here, we investigated the physicochemical evolution and molecular adaptation of the cetacean urea transporter UT-A2, which plays an important role in urine concentration and water homeostasis. First, we cloned UT-A2 from the freshwater Yangtze finless porpoise, after which bioinformatics analyses were conducted based on available datasets (including freshwater baiji and marine toothed and baleen whales) using MEGA, PAML, DataMonkey, TreeSAAP and Consurf. Our findings suggest that the UT-A2 protein shows folding similar to that of dvUT and UT-B, whereas some variations occurred in the functional So and Si regions of the selectivity filter. Additionally, several regions of the cetacean UT-A2 protein have experienced molecular adaptations. We suggest that positive-destabilizing selection could contribute to adaptations by influencing its biochemical and conformational character. The conservation of amino acid residues within the selectivity filter of the urea conduction pore is likely to be necessary for urea conduction, whereas the non-conserved amino acid replacements around the entrance and exit of the conduction pore could potentially affect the activity, which could be interesting target sites for future mutagenesis studies.

De novo assembly and characterization of narrow-ridged finless porpoise renal transcriptome and identification of candidate genes involved in osmoregulation

During the evolutionary transition from land to water, cetaceans have undergone numerous critical challenges, with osmoregulation being the major one. Two subspecies of the narrow-ridged finless porpoise (Neophocaena asiaeorientalis), the freshwater Yangtze finless porpoise (N. a. asiaeorientalis, NAA) and the marine East Asian finless porpoise (N. a. sunameri, NAS), provide excellent subjects to understand the genetic basis of osmoregulatory divergence between freshwater and marine mammals. The kidney plays an important and well-established role in osmoregulation in marine mammals and thus, herein, we utilized RNA-seq to characterize the renal transcriptome and preliminarily analyze the divergence between the NAA and the NAS. Approximately 48.98 million clean reads from NAS and 49.40 million clean reads from NAA were obtained by RNA-Seq. And 73,449 (NAS) and 68,073 (NAA) unigenes were assembled. Among these annotations, 22,231 (NAS) and 21,849 (NAA) unigenes were annotated against the NCBI nr protein database. The ion channel complex GO term and four pathways were detected as relevant to osmoregulation by GO and KEGG pathway classification of these annotated unigenes. Although the endangered status of the study species prevented analysis of biological replicates, we identified nine differentially expressed genes (DEGs) that may be vital in the osmoregulation of the narrow-ridged finless porpoise and worthwhile for future studies. Of these DEGs, the differential expression and distribution of the aquaporin-2 (AQP2) in the collecting duct were verified using immunohistochemical experiments. Together, this work is the first report of renal transcriptome sequencing in cetaceans, and it will provide a valuable resource for future molecular genetics studies on cetacean osmoregulation.

Concentrations of osmotically related constituents in plasma and urine of finless porpoise (Neophocaena asiaeorientalis): implications for osmoregulatory strategies for marine mammals living in freshwater

Background

Most cetaceans inhabit the hyperosmotic marine environment with only a few species living in freshwater habitats. The Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) is the only freshwater subspecies of the genus. Our aim was to study whether the osmoregulation mechanism of the Yangtze finless porpoise is different from the marine subspecies, the East Asian finless porpoise (Neophocaena asiaeorientalis sunameri). We assayed and compared the concentrations of the constituents involved in osmoregulation in the blood and urine in the Yangtze finless porpoise and the East Asian finless porpoise. We also compared the corresponding urine constituents of the porpoises with existing data on fin whales (Balaenoptera physalus) and bottlenose dolphins (Tursiops truncatus).

Results

The mean plasma osmolality of Yangtze finless porpoise was significantly lower than that of the marine subspecies (P < 0.01). Similarly, the urine osmolality of Yangtze finless porpoise was also significantly lower than that of its marine counterpart (P < 0.05). However, the urine sodium concentration of freshwater finless porpoise was significantly lower than that in the marine subspecies (P < 0.01), even though their serum sodium has no significant difference. Moreover, the freshwater porpoise has significantly lower urine urea concentration but much higher serum urea than in the marine finless porpoise (P < 0.05).

Conclusions

These results suggest that the freshwater finless porpoise does have different osmoregulatory mechanism from marine cetaceans. Conserving sodium by excreting urine with low ion levels may be an essential strategy to maintain the serum electrolyte balance for the freshwater subspecies that also appears to be more susceptible to hyponatremia.

Increased gene expression of a facilitated diffusion urea transporter in the skin of the African lungfish (Protopterus annectens) during massively elevated post-terrestrialization urea excretion

The full-length cDNA sequence of a putative urea transporter (lfUT) of the facilitated diffusion UT-A type has been cloned from the African lungfish Protopterus annectens. The lfUT cDNA is 1990 bp in length and its open reading frame encodes a 409 amino acid long protein, with a calculated molecular mass of 44,723 Da. The sequence is closest to those of amphibians ( approximately 65% amino acid homology), followed by mammals and elasmobranchs ( approximately 60%), and then teleosts ( approximately 50%). lfUT was clearly expressed in gill, kidney, liver, skeletal muscle and skin. Upon re-immersion in water after 33 days of air exposure ('terrestrialization'), lungfish exhibited a massive rise in urea-N excretion which peaked at 12-30 h with rates of 2000-5000 micromol-N kg(-1) h(-1) (versus normal aquatic rates of <130 micromol-N kg(-1) h(-1)) and persisted until 70 h. This appears to occur mainly through the skin. Total 'excess' urea-N excretion amounted to approximately 81,000-91,000 micromol-N kg(-1) over 3 days. By real-time PCR, there was no difference in lfUT expression in the ventral abdominal skin between aquatic ammoniotelic controls and terrestrialized lungfish immediately after return to water (0 h), and no elevation of urea-N excretion at this time. However, skin biopsies revealed a significant 2.55-fold elevation of lfUT expression at 14 h, coincident with peak urea-N excretion. At 48 h, there was no longer any significant difference in lfUT mRNA levels from those at 0 and 14 h, or from aquatic fed controls. In accordance with earlier studies, which identified elevated urea-N excretion via the skin of P. dolloi with pharmacology typical of UT-A carriers, these results argue that transcriptional activation of a facilitated diffusion type urea transporter (lfUT) occurs in the skin during re-immersion. This serves to clear the body burden of urea-N accumulated during terrestrialization.

Molecular cloning of urea transporters from the kidneys of baleen and toothed whales

Urea transport in the kidney is important for the production of concentrated urine. This process is mediated by urea transporters (UTs) encoded by two genes, UT-A (Slc14a2) and UT-B (Slc14a1). Our previous study demonstrated that cetaceans produce highly concentrated urine than terrestrial mammals, and that baleen whales showed higher concentrations of urinary urea than sperm whales. Therefore, we hypothesized that cetaceans have unique actions of UTs to maintain fluid homeostasis in marine habitat. Kidney samples of common minke (Balaenoptera acutorostrata), sei (B. borealis), Bryde's (B. brydei) and sperm whales (Physeter macrocephalus) were obtained to determine the nucleotide sequences of mRNAs encoding UT. The sequences of 2.5-kb cDNAs encode 397-amino acid proteins, which are 90-94% identical to the mammalian UT-A2s. Two putative glycosylation sites are conserved between the whales and the terrestrial mammals, whereas consensus sites for protein kinases are not completely conserved; only a single protein kinase A consensus site was identified in the whale UT-A2s. Two protein kinase C consensus sites are present in the baleen whale UT-A2s, however, a single protein kinase C consensus site was identified in the sperm whale UT-A2. These different phosphorylation sites of whale UT-A2s may result in the high concentrations of urinary urea in whales, by reflecting their urea permeability.

Characterization of renin-angiotensin system enzyme activities in cultured mouse podocytes

Intraglomerular ANG II has been linked to glomerular injury. However, little is known about the contribution of podocytes (POD) to intraglomerular ANG II homeostasis. The aim of the present study was to examine the processing of angiotensin substrates by cultured POD. Our approach was to use matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry for peptide determination from conditioned cell media and customized AQUA peptides for quantification. Immortalized mouse POD were incubated with 1-2 microM ANG I, ANG II, or the renin substrate ANG-(1-14) for different time intervals and coincubated in parallel with various inhibitors. Human mesangial cells (MES) were used as controls. POD incubated with 1 microM ANG I primarily formed ANG-(1-9) and ANG-(1-7). In contrast, MES incubated with ANG I primarily generated ANG II. In POD, ANG-(1-7) was the predominant product, and its formation was inhibited by a neprilysin inhibitor. Modest angiotensin-converting enzyme (ACE) activity was also detected in POD, although only after cells were incubated with 2 microM ANG I. In addition, we observed that POD degraded ANG II into ANG III and ANG-(1-7). An aminopeptidase A inhibitor inhibited ANG III formation, and an ACE2 inhibitor led to ANG II accumulation. Furthermore, we found that POD converted ANG-(1-14) to ANG I and ANG-(1-7). This conversion was inhibited by a renin inhibitor. These findings demonstrate that POD express a functional intrinsic renin-angiotensin system characterized by neprilysin, aminopeptidase A, ACE2, and renin activities, which predominantly lead to ANG-(1-7) and ANG-(1-9) formation, as well as ANG II degradation. These findings may reflect a specific role of POD in maintenance of intraglomerular renin-angiotensin system balance.

Comparative renal physiology of exotic species

In this article the osmoregulatory, acid-base homeostasis, and excretory functions of the renal system of invertebrates and vertebrates are reviewed. The mammalian renal system is the most highly evolved in terms of the range of functions performed by the kidneys. Renal physiology in other animals can be very different, and a sound knowledge of these differences is important for understanding health and disease processes that involve the kidneys, as well as ion and water homeostasis. Many animals rely on multiple organs along with the kidneys to maintain osmotic, ionic, and pH balance. Some animals rely heavily on postrenal modification of urine to conserve water and salt balance; this can influence the interpretation of disease signs and treatment modalities.

The SLC14 gene family of urea transporters

Carrier-mediated urea transport allows rapid urea movement across the cell membrane, which is particularly important in the process of urinary concentration and for rapid urea equilibrium in non-renal tissues. Urea transporters mediate passive urea uptake that is inhibited by phloretin and urea analogues. Facilitated urea transporters are divided into two classes: (1) the renal tubular/testicular type of urea transporter, UT-A1 to -A5, encoded by alternative splicing of the SLC14A2 gene, and (2) the erythrocyte urea transporter UT-B1 encoded by the SLC14A1 gene. The primary structure of urea transporters is unique, consisting of two extended, hydrophobic, membrane-spanning domains and an extracellular glycosylated-connecting loop. UT-A1 is the result of a gene duplication of this two-halves-structure, and the duplicated portions are linked together by a large intracellular hydrophilic loop, carrying several putative protein kinase A (PKA) and -C (PKC) phosphorylation sites. UT-A1 is located in the apical membrane of the kidney inner medullary collecting duct cells, where it is stimulated acutely by cAMP-mediated phosphorylation in response to the antidiuretic hormone vasopressin. Vasopressin also up-regulates UT-A2 mRNA/protein expression in the descending thin limb of the loops of Henle. UT-A1 and UT-A2 are regulated independently and respond differently to changes in dietary protein content. UT-A3 and UT-A4 are located in the rat kidney medulla and UT-A5 in the mouse testis. The widely expressed UT-B participates in urea recycling in the descending vasa recta, as demonstrated by a relatively mild "urea-selective" urinary concentrating defect in transgenic UT-B null mice and individuals with the Jk(null) blood group.

Molecular and functional characterization of a urea transporter from the kidney of the Atlantic stingray

In general, marine elasmobranch fishes (sharks, skates, and rays) maintain body fluid osmolality above seawater, principally by retaining large amounts of urea. Maintenance of the high urea concentration is due in large part to efficient renal urea reabsorption. Regulation of renal urea reabsorption also appears to play a role in maintenance of fluid homeostasis of elasmobranchs that move between habitats of different salinities. We identified and cloned a novel 2.7-kb cDNA from the kidney of the euryhaline Atlantic stingray Dasyatis sabina (GenBank accession no. AF443781). This cDNA putatively encoded a 431-amino acid protein (strUT-1) that had a high degree of sequence identity (71%) to the shark kidney facilitated urea transporter (UT). However, the predicted COOH-terminal region of strUT-1 appears to contain an additional sequence that is unique among cloned renal UTs. Injection of strUT-1 cRNA into Xenopus oocytes induced a 33-fold increase in [(14)C]urea uptake that was inhibited by phloretin. Four mRNA bands were detected in kidney by Northern blot: a transcript at 2.8 kb corresponding to the expected size of strUT-1 mRNA and bands at 3.8, 4.5, and 5.5 kb. Identification of a facilitated UT in the kidney of the Atlantic stingray provides further support for the proposal that passive mechanisms contribute to urea reabsorption by elasmobranch kidney.

Gene structure of urea transporters

Urea plays various roles in the biology of diverse organisms. The past decade has produced new information on the molecular structure of several urea transporters in various species. Availability of DNA probes has revealed that the presence of urea transporters is not confined to the mammalian kidney but is also evident in testis and brain, raising new questions about the possible physiological role of urea in these organs. Cloning of the genes encoding the two closely related mammalian urea transporters UT-A and UT-B has helped in identifying molecular mechanisms affecting expression of urea transporters in the kidney, such as transcriptional control for UT-A abundance. On the basis of analysis of genomic sequences of individuals lacking the UT-B transporter, mutations have been found that explain deficits in their capacity to concentrate urine. More urea transporters are being characterized in marine organisms and lower vertebrates, and studying the role and regulation of urea transport from an evolutionary perspective can certainly enrich our understanding of renal physiology.

Molecular approaches to urea transporters

Urea plays a critical role in the urine-concentrating mechanism in the inner medulla. Physiologic data provided evidence that urea transport in red blood cells and kidney inner medulla was mediated by specific urea transporter proteins. Molecular approaches during the past decade resulted in the cloning of two gene families for facilitated urea transporters, UT-A and UT-B, encoding several urea transporter cDNA isoforms in humans, rodents, and several nonmammalian species. Polyclonal antibodies have been generated to the cloned urea transporter proteins, and the use of these antibodies in integrative animal studies has resulted in several novel findings, including: (1) the surprising finding that UT-A1 protein abundance and urea transport are increased in the inner medulla during conditions in which urine concentrating ability is reduced; (2) vasopressin increases UT-A1 phosphorylation in rat inner medullary collecting duct; (3) UT-A protein abundance is upregulated in uremia in both liver and heart; and (4) UT-B is expressed in many nonrenal tissues and endothelial cells. This review will summarize the knowledge gained from using molecular approaches to perform integrative studies into urea transporter protein regulation, both in normal animals and in animal models of human diseases, including studies of uremic rats in which urea transporter protein is upregulated in liver and heart.

Mammalian urea transporters

Urea plays a key role in the urine-concentrating mechanism. Physiologic and molecular data demonstrate that urea transport in kidney and red blood cells occurs by specific urea transporter proteins. Two gene families for facilitated urea transporters, UT-A and UT-B, and several urea transporter cDNA isoforms have been cloned from human, rat, mouse, and several non-mammalian species. Polyclonal antibodies have been generated to many of the urea transporter proteins, and several novel findings have resulted from their use in integrative animal studies. For example, (a) vasopressin increases the phosphorylation of UT-A1 in rat inner medullary collecting duct; (b) UT-A1 protein abundance is increased in the rat inner medulla during conditions in which urine-concentrating ability is reduced; and (c) urea transporters are expressed in non-renal tissues, and UT-A protein abundance is up-regulated in uremia in both liver and heart. In addition to the facilitated urea transporters, functional evidence exists for active urea transport in the kidney collecting duct. This review summarizes the physiologic evidence for the existence of facilitated and active urea transporters, the molecular biology of the facilitated urea transporter gene families and cDNAs, and integrative studies into urea transporter protein regulation, both in the kidney and in other organs.