Isolation and sequencing of cDNA clones coding for the catalytic unit of glucose-6-phosphatase from two haplochromine cichlid fishes

Molecular cloning and tissue distribution of glucokinase and glucose-6-phosphatase catalytic subunit paralogs in largemouth bass Micropterus salmoides: Regulation by dietary starch levels and a glucose load

The dysregulation of glucose-G6P (glucose-6-phosphate) interconversion is thought to be one of the main reasons for the low glucose disposal of carnivorous fish, but is not yet well understood in largemouth bass Micropterus salmoides (LMB). In this study, the full length cDNA sequences of genes encoding glucokinase (Gck, catalyzing glucose phosphorylation) and glucose-6-phosphatase catalytic subunit (G6pc, catalyzing glucose dephosphorylation) were cloned by the RACE method from the liver of LMB. Subsequently, the distribution of g6pc and gck as well as their transcriptional regulation by dietary starch levels and a glucose load were investigated. Only one gck gene was identified, while the tandem duplication of g6pca.1 gene was named as g6pca.2 in LMB. The full cDNA sequences of g6pca.1, g6pca.2 and gck in LMB were 1585, 1813 and 2115 bp in length, encoding 478, 352 and 359 amino acids, respectively. Gck was predicted to contain two hexokinase domains, an ATP-binding domain and multiple functional sites, while G6pca.1 and G6pca.2 contained nine transmembrane helices, a PAP2 (type-2 phosphatidic acid phosphatase) domain and multiple functional amino acid sites. Both g6pca.1 and g6pca.2 were predominantly distributed in the liver and to some extent in the intraperitoneal fat, intestine and pyloric caeca, while gck was mainly transcribed in the liver and to some extent in the heart, intestine and brain. Both feeding a high starch diet and a glucose load stimulated the mRNA expression of gck in the liver of LMB. An increase of dietary starch from 9% to 14% down-regulated the transcription of g6pca.1 in the liver of LMB. However, both the mRNA levels of hepatic g6pca.1 and g6pca.2 were sharply up-regulated in LMB during 1-3 h after a glucose load. Overall, the results of this study suggested that the functions of G6pc (G6pca.1 and G6pca.2) and Gck in LMB were highly conserved in evolution. Though hepatic glucose-G6P interconversion was well regulated at the transcript level in LMB fed high starch diets, a futile cycle between glucose and G6P was induced in the liver after a glucose load.

Genetic variation and demographic history of the Haplochromis laparogramma group of Lake Victoria-An analysis based on SINEs and mitochondrial DNA

More than 500 endemic haplochromine cichlid species inhabit Lake Victoria. This striking species diversity is a classical example of recent explosive adaptive radiation thought to have happened within the last approximately 15,000 years. In this study, we examined the population structure and historical demography of 3 pelagic haplochromine cichlid species that resemble in morphology and have similar niche, Haplochromis (Yssichromis) laparogramma, Haplochromis (Y.) pyrrhocephalus, and Haplochromis (Y.) sp. "glaucocephalus". We investigated the sequences of the mitochondrial DNA control region and the insertion patterns of short interspersed elements (SINEs) of 759 individuals. We show that sympatric forms are genetically differentiated in 4 of 6 cases, but we also found apparent weakening of the genetic differentiation in areas with turbid water. We estimated the timings of population expansion and species divergence to coincide with the refilling of the lake at the Pleistocene/Holocene boundary. We also found that estimates can be altered significantly by the choice of the shape of the molecular clock. If we employ the nonlinear clock model of evolutionary rates in which the rates are higher towards the recent, the population expansion was dated at around the event of desiccation of the lake ca. 17,000 YBP. Thus, we succeeded in clarifying the species and population structure of closely related Lake Victoria cichlids and in showing the importance of applying appropriate clock calibrations in elucidating recent evolutionary events.

Molecular cloning of hepatic glucose-6-phosphatase catalytic subunit from gilthead sea bream (Sparus aurata): response of its mRNA levels and glucokinase expression to refeeding and diet composition

To examine the relationship between structure and function of glucose-6-phosphatase (G6Pase) in fish, we undertook molecular cloning and modulation of G6Pase expression by starvation and refeeding on diets with different nutrient composition in the liver of the carnivorous fish, Sparus aurata. A cDNA encoding the full-length G6Pase catalytic subunit from the liver of S. aurata was isolated. This cDNA encodes a 350-amino acid protein, with low homology to the mammalian G6Pase, although it contains most of the key residues required for catalysis. Based on hydrophobicity and membrane structure prediction, we propose a model containing nine-transmembrane regions for S. aurata G6Pase. Northern blots showed that refeeding after a prolonged starvation rapidly reverses the glucose/glucose-6-phosphate substrate cycle flux in the fish liver through decreased G6Pase expression and strong glucokinase (GK) induction. The effect of refeeding different diets on G6Pase and GK expression, indicated that hepatic intermediary metabolism of fish fed diets with low protein/high carbohydrate diets is impelled towards utilization of dietary carbohydrates, by means of modulation of GK mRNA levels rather than G6Pase expression. These findings challenge the role attributed to dysregulation of G6Pase or GK expression in the low ability of carnivorous fish to metabolise glucose.

Extensive analysis of ORF sequences from two different cichlid species in Lake Victoria provides molecular evidence for a recent radiation event of the Victoria species flock

The Lake Victoria Cichlid fishes have diverged very rapidly. The estimated 500 species inhabiting the lake are believed to have arisen within the last 14,000 years. The fishes' jaws and teeth have diverged markedly to adapt to different feeding behaviors and environments. To examine how the genomes of these fishes differentiated during speciation, we performed comparative analysis of expressed sequenced tag (EST) sequences. We constructed cDNA libraries derived only from the jaw portions of two cichlid species endemic to Lake Victoria. We sequenced 17,280 cDNA clones from Haplochromis chilotes and 9600 cDNA clones from Haplochromis sp. "Redtailsheller" and obtained 543 different genes common to both species. Of these genes, 441 were essentially identical between species and 102 contained base replacements in their open reading frame (ORF) or untranslated (UTR) regions. Comparative analysis of 71 selected sequences has revealed that while the degree of polymorphism is 0.0054/site for H. chilotes and 0.0047/site for H. sp. "Redtailsheller", genetic distance between the two species is 0.0031/site. The genetic distance particularly indicates that the two species diverged about 890,000 years ago.

Construction of a BAC library for Haplochromis chilotes, a cichlid fish from Lake Victoria

Cichlid fishes in Lake Victoria are model organisms for studying rapid radiation and speciation. On the way to examine the molecular basis of how these cichlid fishes achieved such a remarkable morphological diversification, we constructed a bacterial artificial chromosome (BAC) library derived from a cichlid species, Haplochromis chilotes, from Lake Victoria. The library includes 157,056 clones with the average insert size of 128 kb, corresponding to a 10-fold coverage of the H. chilotes genome. Given that the cichlid fishes endemic to Lake Victoria are closely related to one another phylogenetically and their genomes are nearly identical, this BAC library can be utilized to isolate genes from the more than 200 Haplochromine cichlid species in Lake Victoria.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.