Antonio Velázquez-Arellano

The hexokinase gene family in the zebrafish: structure, expression, functional and phylogenetic analysis.

Hexokinase-catalyzed glucose phosphorylation is the first and crucial step for glucose utilization. Although there are reported studies on glucose metabolism in commercial species, knowledge on it is almost nil in zebrafish (Danio rerio), an important model organism for biological research. We have searched these fish hexokinase genes by BLAST analysis; determined their expression in liver, muscle, brain and heart; measured their response to fasting and glucose administration; and performed homology sequences studies to glimpse their evolutionary history. We have confirmed by RT-qPCR studies that the six DNA sequences annotated as possible hexokinases in the NCBI GenBank are transcribed. The organ distribution of the HXK genes is similar in zebrafish as in mammals, to which they are distantly related. Of these, DrGLK and DrSHXK1 are expressed in the fish liver, DrHXK1 in brain and heart, and DrHXK2 in muscle. The only gene responsive to glucose was liver DrGLK. Its expression is induced approximately 1 h after glucose intraperitoneal injection, but not after saline solution injection. The comparison of the fish sequences and the corresponding mammalian ones imply that in both taxa the main muscle and brain isoforms are fusion products of the ancestral gene, their amino halves having separated before than their carboxy ones, followed by the fusion event, whereas fish and mammalian glucokinase genes remained unduplicated.

Biotin Starvation with Adequate Glucose Provision Causes Paradoxical Changes in Fuel Metabolism Gene Expression Similar in Rat (Rattus norvegicus), Nematode (Caenorhabditis elegans) and Yeast (Saccharomyces cerevisiae)

Background/Aim: Biotin affects the genetic expression of several glucose metabolism enzymes, besides being a cofactor of carboxylases. To explore how extensively biotin affects the expression of carbon metabolism genes, we studied the effects of biotin starvation and replenishment in 3 distantly related eukaryotes: yeast Saccharomyces cerevisiae, nematode Caenorhabditis elegans and rat Rattus norvegicus. Methods: Biotin starvation was produced in Wistar rats, in C. elegans N2 and S. cerevisiae W303A fed with abundant glucose. High-density oligonucleotide microarrays were used to find gene expression changes. Glucose consumption, lactate and ethanol were measured by conventional tests. Results: In spite of abundant glucose provision, the expression of fatty oxidation and gluconeogenic genes was augmented, and the transcripts for glucose utilization and lipogenesis were diminished in biotin starvation. These results were associated with diminished glucose consumption and glycolysis products (lactate and ethanol in yeast), which was consistent across 3 very different eukaryotes. Conclusion: The results point toward a strongly selected role of biotin in the control of carbon metabolism, and in adaptations to variable availability of carbon, conceivably mediated by signal transduction including soluble guanylate cyclase, cGMP and a cGMP-dependent protein kinase (PKG) and/or biotin-dependent processes.

Biotinidase knockout mice show cellular energy deficit and altered carbon metabolism gene expression similar to that of nutritional biotin deprivation: Clues for the pathogenesis in the human inherited disorder

Biotin is the prosthetic group of carboxylases that have important roles in the metabolism of glucose, fatty acids and amino acids. Biotinidase has a key role in the reutilization of the biotin, catalyzing the hydrolysis of biocytin (ε-N-biotinyl-l-lysine) and biocytin-containing peptides derived from carboxylase turnover, thus contributing substantially to the bioavailability of this vitamin. Deficient activity of biotinidase causes late-onset multiple carboxylase in humans, whose pathogenic mechanisms are poorly understood. Here we show that a knock-out biotinidase-deficient mouse from a C57BL/6 background that was fed a low biotin diet develops severe ATP deficit with activation of the energy sensor adenosine monophosphate (AMP)-activated protein kinase (AMPK), inhibition of the signaling protein mTOR, driver of protein synthesis and growth, and affecting the expression of central-carbon metabolism genes. In addition, sensitivity to insulin is augmented. These changes are similar to those observed in nutritionally biotin-starved rats. These findings further our understanding of the pathogenesis of human biotinidase deficiency.

A heuristic model for paradoxical effects of biotin starvation on carbon metabolism genes in the presence of abundant glucose

We recently showed that in biotin starvation in yeast Saccharomyces cerevisiae, nematode Caenorhabditis elegans and rat Rattus norvegicus, despite abundant glucose provision, the expression of genes for glucose utilization and lipogenesis were lowered, and for fatty acid β-oxidation and gluconeogenesis were raised, and glycolytic/fermentative flow was reduced. This work explored the mechanisms of these results. We show that they are associated with ATP deficit and activation of the energy stress sensor AMP kinase (AMPK; Snf1 in yeast). Analysis of microarray results revealed extensive changes of transcripts for signal transduction pathways and transcription factors AMPK, SREBP-1c, ChREBP, NAMPT, PGC-1α, mTORC1 in rat, and their homologs in worm. In yeast the altered factor transcripts were Adr1, Cat8, Sip4, Mig1, HXK2, and Rgt1. The insulin pathway was negatively enriched (in rat and worm), whereas the adiponectins and JAK/STAT pathways were increased (present only in the rat; they activate AMPK). Together, all these changes explain the effects of biotin starvation on glucose utilization, energy status and carbon metabolism gene expression in a coherent manner across three phylogenetically distant eukaryotes and may have clinical significance in humans, since the effects are reminiscent of insulin resistance. We propose a general model for integrating these results in regulatory circuitries, according to the biology of each species, based on impaired anaplerosis due to pyruvate carboxylase deficiency, that have a basic underlying logic. In a preliminary test in yeast, aspartate corrects all the alterations produced by biotin starvation.

Temporal development of genetic and metabolic effects of biotin deprivation. A search for the optimum time to study a vitamin deficiency

Biotin deficiency (Bt-D) is usually studied at the point at which the animal model exhibits the signs of full-blown deficiency symptoms; in rats, this typically occurs at 6-8 weeks of feeding a deficient diet. To differentiate specific deficiency effects from those of undernutrition, biotin sufficient and deficient rats were studied at 2, 3, 4, and 5 weeks on the deficiency diet, before the onset of weight loss and deficiency signs. The deficiency state was confirmed by biochemical and molecular analyses. Blood and liver metabolites were determined and western blots of signaling proteins, and qRT-PCR gene expression studies. The main effects of Bt-D were already well established by the fourth week on the diet; thus, we consider the fourth week as the optimum time to study the consequences of biotin depletion. Early effects, which were already apparent at week 2, included cellular energy deficit (as assessed by increased AMP/ATP ratio), activation of the AMPK energy sensor, and changes of carbon metabolism gene transcripts (e.g., phosphoenolpyruvate carboxykinase, carnitine palmitoyl transferase 1, liver glucokinase and fatty acid synthetase). Reduced post-prandial blood concentrations of glucose were also observed early; we speculate that these are attributable to augmented sensitivity to insulin and increased glucose utilization, a likely effect of AMPK induction of translocation of glucose transporter GLUT4 to the cell membranes and increased hexokinase expression. Other late-onset changes (week 4) included increased serum concentrations of lactate and free fatty acids and decreased liver glycogen and serum concentrations of triglycerides and total cholesterol. The identification of the early specific molecular and metabolic disturbances of biotin deficiency might be useful in identifying individuals with marginal deficiency of this vitamin, which appears to be common in normal human pregnancy. The study of time-course of other vitamin deficiencies, such as this one, might help to better understand and cope with their effects.

Functional and metabolic implications of biotin deficiency for the rat heart

The tricarboxylic acid (TCA) cycle is the main ATP provider for the heart. TCA carbons must be replenished by anaplerosis for normal cardiac function. Biotin is cofactor of the anaplerotic enzymes pyruvate and propionyl-CoA carboxylases. Here, we found that in biotin deficient rats, both carboxylases decreased 90% in adipose tissue, jejunum and spleen, but in heart they conserved about 60% residual activity. We then investigated if under biotin deficiency (BtDEF), the heart is able to maintain its function in vivo and in isolated conditions, and during ischemia and reperfusion, where metabolism drastically shifts from oxidative to mainly glycolytic. Neither glucose nor octanoate oxidation were severely affected in BtDEF hearts, as assessed by mechanical performance, oxygen uptake or high-energy metabolite content; however, myocardial hexokinase activity and lactate concentration were reduced in deficient hearts. When challenged by ischemia and reperfusion injury, BtDEF hearts did not suffer more damage than the controls, although they lowered significantly their performance, when changed to ischemic conditions, which may have clinical implications. Post-ischemic increase in ADP/ATP ratio was similar in both groups, but during reperfusion there was higher rhythm perturbation in BtDEF hearts. By being relatively insensitive to biotin deficiency, cardiac tissue seems to be able to replenish TCA cycle intermediates and to maintain ATP synthesis.

Thiamine Deprivation Produces a Liver ATP Deficit and Metabolic and Genomic Effects in Mice: Findings Are Parallel to Those of Biotin Deficiency and Have Implications for Energy Disorders

Thiamine is one of several essential cofactors for ATP generation. Its deficiency, like in beriberi and in the Wernicke-Korsakoff syndrome, has been studied for many decades. However, its mechanism of action is still not completely understood at the cellular and molecular levels. Since it acts as a coenzyme for dehydrogenases of pyruvate, branched-chain keto acids, and ketoglutarate, its nutritional privation is partly a phenocopy of inborn errors of metabolism, among them maple syrup urine disease. In the present paper, we report metabolic and genomic findings in mice deprived of thiamine. They are similar to the ones we have previously found in biotin deficiency, another ATP generation cofactor. Here we show that thiamine deficiency substantially reduced the energy state in the liver and activated the energy sensor AMP-activated kinase. With this vitamin deficiency, several metabolic parameters changed: blood glucose was diminished and serum lactate was increased, but insulin, triglycerides, and cholesterol, as well as liver glycogen, were reduced. These results indicate a severe change in the energy status of the whole organism. Our findings were associated with modified hepatic levels of the mRNAs of several carbon metabolism genes: a reduction of transcripts for liver glucokinase and fatty acid synthase and augmentation of those for carnitine palmitoyl transferase 1 and phosphoenolpyruvate carboxykinase as markers for glycolysis, fatty acid synthesis, beta-oxidation, and gluconeogenesis, respectively. Glucose tolerance was initially increased, suggesting augmented insulin sensitivity, as we had found in biotin deficiency; however, in the case of thiamine, it was diminished from the 3rd week on, when the deficient animals became undernourished, and paralleled the changes in AKT and mTOR, 2 main proteins in the insulin signaling pathway. Since many of the metabolic and gene expression effects on mice deprived of thiamine are similar to those in biotin deficiency, it may be that they result from a more general impairment of oxidative phosphorylation due to a shortage of ATP generation cofactors. These findings may be relevant to energy-related disorders, among them several inborn errors of metabolism, as well as common energy disorders like obesity, diabetes, and neurodegenerative illnesses.

Insulin sensitivity is inversely related to cellular energy status, as revealed by biotin deprivation

We have reported an early decrease in glycemia in rats fed a biotin-deficient diet with reduced cellular ATP levels, suggesting increased insulin sensitivity. Here, we show that biotin-deprived rats are more tolerant of glucose, as shown by both oral and intraperitoneal glucose tolerance tests, during which insulin plasma levels were significantly diminished in deficient rats compared with controls. Biotin-deficient rats had lower blood glucose concentrations during intraperitoneal insulin sensitivity tests than controls. Furthermore, more glucose was infused to maintain euglycemia in the biotin-deficient rats during hyperinsulinemic euglycemic clamp studies. These results demonstrate augmented sensitivity to insulin in biotin-deprived rats. They are most likely the consequence of an insulin-independent effect of AMPK activation on GLUT4 membrane translocation with increased glucose uptake. In biotin-deficient cultured L6 muscle cells, there was increased phosphorylation of the energy sensor AMPK. We have now confirmed the augmented AMPK activation in both biotin-deprived in vivo muscle and cultured muscle cells. In these cells, glucose uptake is increased by AMPK activation by AICAR and diminished by its knockdown by the specific siRNAs directed against its α1- and α2-catalytic subunits, with all of these effects being largely independent of the activity of the insulin-signaling pathway that was inhibited with wortmannin. The enhanced insulin sensitivity in biotin deficiency likely has adaptive value for organisms due to the hormone promotion of uptake and utilization of not only glucose but other nutrients such as branched-chain amino acids, whose deficiency has been reported to increase insulin tolerance.

Biotin deprivation impairs mitochondrial structure and function and has implications for inherited metabolic disorders

Certain inborn errors of metabolism result from deficiencies in biotin containing enzymes. These disorders are mimicked by dietary absence or insufficiency of biotin, ATP deficit being a major effect,whose responsible mechanisms have not been thoroughly studied. Here we show that in rats and cultured cells it is the result of reduced TCA cycle flow, partly due to deficient anaplerotic biotin-dependent pyruvate carboxylase. This is accompanied by diminished flow through the electron transport chain, augmented by deficient cytochrome c oxidase (complex IV) activity with decreased cytochromes and reduced oxidative phosphorylation. There was also severe mitochondrial damage accompanied by decrease of mitochondria, associated with toxic levels of propionyl CoA as shown by carnitine supplementation studies, which explains the apparently paradoxical mitochondrial diminution in the face of the energy sensor AMPK activation, known to induce mitochondria biogenesis. This idea was supported by experiments on AMPK knockout mouse embryonic fibroblasts (MEFs). The multifactorial ATP deficit also provides a plausible basis for the cardiomyopathy in patients with propionic acidemia, and other diseases.Additionally, systemic inflammation concomitant to the toxic state might explain our findings of enhanced IL-6, STAT3 and HIF-1α, associated with an increase of mitophagic BNIP3 and PINK proteins, which may further increase mitophagy. Together our results imply core mechanisms of energy deficit in several inherited metabolic disorders.

A high glucose diet induces autophagy in a HLH-30/TFEB-dependent manner and impairs the normal lifespan of C. elegans

A high-glucose diet (HGD) is associated with the development of metabolic diseases that decrease life expectancy, including obesity and type-2 diabetes (T2D); however, the mechanism through which a HGD does so is still unclear. Autophagy, an evolutionarily conserved mechanism, has been shown to promote both cell and organismal survival. The goal of this study was to determine whether exposure of Caenorhabditis elegans to a HGD affects autophagy and thus contributes to the observed lifespan reduction under a HGD. Unexpectedly, nematodes exposed to a HGD showed increased autophagic flux via an HLH-30/TFEB-dependent mechanism because animals with loss of HLH-30/TFEB, even those with high glucose exposure, had an extended lifespan, suggesting that HLH-30/TFEB might have detrimental effects on longevity through autophagy under this stress condition. Interestingly, pharmacological treatment with okadaic acid, an inhibitor of the PP2A and PP1 protein phosphatases, blocked HLH-30 nuclear translocation, but not TAX-6/calcineurin suppression by RNAi, during glucose exposure. Together, our data support the suggested dual role of HLH-30/TFEB and autophagy, which, depending on the cellular context, may promote either organismal survival or death.