Sequence, expression and evolutionary relationships of carbamoyl phosphate synthetase I in the toadXenopus laevis

Metabolic cost of osmoregulation in a hypertonic environment in the invasive African clawed frog Xenopus laevis

Studies of aquatic invertebrates reveal that salinity affects feeding and growth rates, reproduction, survival, and diversity. Little is known, however, about how salinity impacts the energy budget of vertebrates and amphibians in particular. The few studies focused on this topic in vertebrates suggest that the ingestion of salts and the resulting osmoregulatory activity is energetically expensive. We analyzed the effect of saline acclimation on standard metabolic rates (SMR) and the activities of metabolic enzymes of internal organs and osmoregulatory variables (plasma osmolality and urea plasma level) in females of Xenopus laevis by means of acclimating individuals to an isosmotic (235 mOsm NaCl; ISO group) and hyper-osmotic (340 mOsm NaCl; HYP group) environment for 40 days. After acclimation, we found that total and mass-specific SMR was approximately 80% higher in the HYP group than those found in the ISO group. These changes were accompanied by higher citrate synthase activities in liver and heart in the HYP group than in the ISO group. Furthermore, we found a significant and positive correlation between metabolic rates and plasma urea, and citrate synthase activity in liver and heart. These results support the notion that the cost of osmoregulation is probably common in most animal species and suggest the existence of a functional association between metabolic rates and the adjustments in osmoregulatory physiology, such as blood distribution and urea synthesis.

Phylogenetic aspects of carbamoyl phosphate synthetase in lungfish: a transitional enzyme in transitional fishes

Carbamoyl phosphate synthetase (CPS) catalyses the formation of carbamoyl phosphate from glutamine or ammonia, bicarbonate and ATP. There are three different isoforms of CPS that play vital roles in two metabolic pathways, pyrimidine biosynthesis (CPS II) and arginine/urea biosynthesis (CPS I and CPS III). Gene duplication has been proposed as the evolutionary mechanism creating this gene family with CPS II likely giving rise to the CPS I/III clade. In the evolutionary history of this gene family it is still undetermined when CPS I diverged from CPS III on the path to terrestriality in the vertebrates. Transitional organisms such as lungfishes are of particular interest because they are capable of respiring via gills and with lungs and therefore can be found in both aquatic and terrestrial environments. Notably, enzymatic characterization of the mitochondrial CPS isoforms in this transitional group has not led to clear conclusions. In order to determine which CPS isoform is present in transitional animals, we examined partial sequences for liver CPS amplified from five species of lungfish, and a larger fragment of CPS from one lungfish species (Protopterus annectens) and compared them to CPS isoforms from other fish and mammals. Enzyme activities for P. annectens liver were also examined. While enzyme activities did not yield a clear distinction between isoforms (virtually equal activities were obtained for either CPS I or III), CPS sequences from the lungfishes formed a monophyletic clade within the CPS I clade and separate from the CPS III clade of other vertebrates. This finding implies that the mitochondrial isoform of CPS in lungfish is derived from CPS I and is likely to have a physiological function similar to CPS I. This finding is important because it supports the hypothesis that lungfish employ a urea cycle similar to terrestrial air-breathing vertebrates.

Analysis of ven3 and ven6 reticulate mutants reveals the importance of arginine biosynthesis in Arabidopsis leaf development

Arabidopsis thaliana reticulate mutants exhibit differential pigmentation of the veinal and interveinal leaf regions, a visible phenotype that often indicates impaired mesophyll development. We performed a metabolomic analysis of one ven6 (venosa6) and three ven3 reticulate mutants that revealed altered levels of arginine precursors, namely increased ornithine and reduced citrulline levels. In addition, the mutants were more sensitive than the wild-type to exogenous ornithine, and leaf reticulation and mesophyll defects of these mutants were completely rescued by exogenous citrulline. Taken together, these results indicate that ven3 and ven6 mutants experience a blockage of the conversion of ornithine into citrulline in the arginine pathway. Consistent with the participation of VEN3 and VEN6 in the same pathway, the morphological phenotype of ven3 ven6 double mutants was synergistic. Map-based cloning showed that the VEN3 and VEN6 genes encode subunits of Arabidopsis carbamoyl phosphate synthetase (CPS), which is assumed to be required for the conversion of ornithine into citrulline in arginine biosynthesis. Heterologous expression of the Arabidopsis VEN3 and VEN6 genes in a CPS-deficient Escherichia coli strain fully restored bacterial growth in minimal medium, demonstrating the enzymatic activity of the VEN3 and VEN6 proteins, and indicating a conserved role for CPS in these distinct and distant species. Detailed study of the reticulate leaf phenotype in the ven3 and ven6 mutants revealed that mesophyll development is highly sensitive to impaired arginine biosynthesis.

The serotonin subtype 1A receptor regulates cortisol secretion in the Gulf toadfish, Opsanus beta

It is well established that serotonin (5-HT; 5-hydroxytryptamine) plays a role in mammalian regulation of the hypothalamic-pituitary-adrenal (HPA) axis via the 5-HT receptor subtype 1A (5-HT(1A)). To date, there has not been a comprehensive investigation of the molecular, pharmacological and physiological aspects of the 5-HT(1A) receptor and its role in the activation of the hypothalamic-pituitary-interrenal (HPI) axis in teleost fish. The 5-HT(1A) receptor of the Gulf toadfish (Opsanus beta) was cloned and sequenced, showing 67.5% amino acid similarity to the human homologue. The 5-HT(1A) receptor was distributed throughout the brain, with the whole brain containing significantly higher levels of 5-HT(1A) mRNA compared to all other tissues and the midbrain/diencephalon region containing significantly higher levels of transcript than any other brain region. Substantial levels of transcript were also found in the pituitary, while very low levels were in the kidney that contains the interrenal cells. Xenopus oocytes injected with toadfish 5-HT(1A) receptor cRNA displayed significantly higher binding of [(3)H]5-HT that was abolished by the mammalian 5-HT(1A) receptor agonist, 8-OH-DPAT, indicating a conserved binding site of the toadfish 5-HT(1A) receptor and a high specificity for the agonist. Supporting this, binding of [(3)H]5-HT was not affected by the mammalian 5-HT(1B) receptor agonist, 5-nonyloxytryptamine, the 5-HT(7) receptor antagonist, SB269970, or the 5-HT(2) receptor agonist, alpha-methylserotonin. Confirming these molecular and pharmacological findings, intravenous injection of 8-OH-DPAT stimulated the HPI axis to cause a 2-fold increase in circulating levels of cortisol. The present study of the 5-HT(1A) receptor in a single teleost species illustrates the high conservation of this 5-HT receptor amongst vertebrates.

Occludin and hydromineral balance in Xenopus laevis

To investigate the response of the tight junction (TJ) protein occludin to environmental change in an anuran amphibian, we examined occludin tissue distribution, immunolocalization and alterations in mRNA expression in African clawed frogs (Xenopus laevis) acclimated to brackish water (BW) conditions (from freshwater to 2 per thousand, 5 per thousand or 10 per thousand salt water). Occludin mRNA is widely expressed in Xenopus and is abundant in tissues involved in regulating salt and water balance, such as the gastrointestinal (GI) tract, kidney and urinary bladder. Immunohistochemical analyses revealed strong occludin immunolabelling in the apicolateral region of epithelia lining the GI tract and mRNA expression increased along the longitudinal axis of the gut. In kidney tissue, occludin was differentially expressed on the luminal side of the nephron tubule, appearing in the distal tubules and collecting ducts only. In response to BW acclimation, Xenopus exhibited a significant loss of tissue water as well as salinity-dependent elevations in serum osmolality as a result of increased urea levels followed by elevated serum Na(+) and Cl(-) levels. Tissue-specific alterations in the ionomotive enzyme Na(+),K(+)-ATPase were also observed in Xenopus in response to BW acclimation. Most notably, Na(+),K(+)-ATPase activity in the rectum increased in response to elevated environmental salt concentrations while renal activity decreased. Furthermore, acclimation to BW caused tissue-specific and salinity-dependent alterations in occludin mRNA expression within select Xenopus osmoregulatory organs. Taken together, these studies suggest that alterations in occludin, in conjunction with active transport processes, may contribute to amphibian hydromineral homeostasis during environmental change.

Inversion of allosteric effect of arginine on N-acetylglutamate synthase, a molecular marker for evolution of tetrapods

Background

The efficient conversion of ammonia, a potent neurotoxin, into non-toxic metabolites was an essential adaptation that allowed animals to move from the aquatic to terrestrial biosphere. The urea cycle converts ammonia into urea in mammals, amphibians, turtles, snails, worms and many aquatic animals and requires N-acetylglutamate (NAG), an essential allosteric activator of carbamylphosphate synthetase I (CPSI) in mammals and amphibians, and carbamylphosphate synthetase III (CPSIII) in fish and invertebrates. NAG-dependent CPSI and CPSIII catalyze the formation of carbamylphosphate in the first and rate limiting step of ureagenesis. NAG is produced enzymatically by N-acetylglutamate synthase (NAGS), which is also found in bacteria and plants as the first enzyme of arginine biosynthesis. Arginine is an allosteric inhibitor of microbial and plant NAGS, and allosteric activator of mammalian NAGS.

Results

Information from mutagenesis studies of E. coli and P. aeruginosa NAGS was combined with structural information from the related bacterial N-acetylglutamate kinases to identify four residues in mammalian NAGS that interact with arginine. Substitutions of these four residues were engineered in mouse NAGS and into the vertebrate-like N-acetylglutamate synthase-kinase (NAGS-K) of Xanthomonas campestris, which is inhibited by arginine. All mutations resulted in arginine losing the ability to activate mouse NAGS, and inhibit X. campestris NAGS-K. To examine at what point in evolution inversion of arginine effect on NAGS occur, we cloned NAGS from fish and frogs and examined the arginine response of their corresponding proteins. Fish NAGS were partially inhibited by arginine and frog NAGS were activated by arginine.

Conclusion

Difference in arginine effect on bacterial and mammalian NAGS most likely stems from the difference in the type of conformational change triggered by arginine binding to these proteins. The change from arginine inhibition of NAGS to activation was gradual, from complete inhibition of bacterial NAGS, to partial inhibition of fish NAGS, to activation of frog and mammalian NAGS. This change also coincided with the conquest of land by amphibians and mammals.

Transcriptional profiling of the model Archaeon Halobacterium sp. NRC-1: responses to changes in salinity and temperature

Background

The model halophile Halobacterium sp. NRC-1 was among the first Archaea to be completely sequenced and many post-genomic tools, including whole genome DNA microarrays are now being applied to its analysis. This extremophile displays tolerance to multiple stresses, including high salinity, extreme (non-mesophilic) temperatures, lack of oxygen, and ultraviolet and ionizing radiation.

Results

In order to study the response of Halobacterium sp. NRC-1 to two common stressors, salinity and temperature, we used whole genome DNA microarrays to assay for changes in gene expression under differential growth conditions. Cultures grown aerobically in rich medium at 42°C were compared to cultures grown at elevated or reduced temperature and high or low salinity. The results obtained were analyzed using a custom database and microarray analysis tools. Growth under salt stress conditions resulted in the modulation of genes coding for many ion transporters, including potassium, phosphate, and iron transporters, as well as some peptide transporters and stress proteins. Growth at cold temperature altered the expression of genes involved in lipid metabolism, buoyant gas vesicles, and cold shock proteins. Heat shock showed induction of several known chaperone genes. The results showed that Halobacterium sp. NRC-1 cells are highly responsive to environmental changes at the level of gene expression.

Conclusion

Transcriptional profiling showed that Halobacterium sp. NRC-1 is highly responsive to its environment and provided insights into some of the specific responses at the level of gene expression. Responses to changes in salt conditions appear to be designed to minimize the loss of essential ionic species and abate possible toxic effects of others, while exposure to temperature extremes elicit responses to promote protein folding and limit factors responsible for growth inhibition. This work lays the foundation for further bioinformatic and genetic studies which will lead to a more comprehensive understanding of the biology of a model halophilic Archaeon.