Glucokinase in chicken (Gallus gallus). Partial cDNA cloning, immunodetection and activity determination

Evolution of digestive enzymes and dietary diversification in birds

As the most species-rich class of tetrapod vertebrates, Aves possesses diverse feeding habits, with multiple origins of insectivory, carnivory, frugivory, nectarivory, granivory and omnivory. Since digestive enzymes mediate and limit energy and nutrient uptake, we hypothesized that genes encoding digestive enzymes have undergone adaptive evolution in birds. To test this general hypothesis, we identified 16 digestive enzyme genes (including seven carbohydrase genes (hepatic amy, pancreatic amy, salivary amy, agl, g6pc, gaa and gck), three lipase genes (cyp7a1, lipf and pnlip), two protease genes (ctrc and pgc), two lysozyme genes (lyz and lyg) and two chitinase genes (chia and chit1)) from the available genomes of 48 bird species. Among these 16 genes, three (salivary amy, lipf and chit1) were not found in all 48 avian genomes, which was further supported by our synteny analysis. Of the remaining 13 genes, eight were single-copy and five (chia, gaa, lyz, lyg and pgc) were multi-copy. Moreover, the multi-copy genes gaa, lyg and pgc were predicted to exhibit functional divergence among copies. Positively selected sites were detected in all of the analyzed digestive enzyme genes, except agl, g6pc, gaa and gck, suggesting that different diets may have favored differences in catalytic capacities of these enzymes. Furthermore, the analysis also revealed that the pancreatic amylase gene and one of the lipase genes (cyp7a1) have higher ω (the ratio of nonsynonymous to the synonymous substitution rates) values in species consuming a larger amount of seeds and meat, respectively, indicating an intense selection. In addition, the gck carbohydrase gene in species consuming a smaller amount of seeds, fruits or nectar, and a lipase gene (pnlip) in species consuming less meat were found to be under relaxed selection. Thus, gene loss, gene duplication, functional divergence, positive selection and relaxed selection have collectively shaped the evolution of digestive enzymes in birds, and the evolutionary flexibility of these enzymes may have facilitated their dietary diversification.

Insulin immuno-neutralization decreases food intake in chickens without altering hypothalamic transcripts involved in food intake and metabolism

In mammals, insulin regulates blood glucose levels and plays a key regulatory role in appetite via the hypothalamus. In contrast, chickens are characterized by atypical glucose homeostasis, with relatively high blood glucose levels, reduced glucose sensitivity of pancreatic beta cells, and large resistance to exogenous insulin. The aim of the present study was to investigate in chickens the effects of 5 h fasting and 5 h insulin immuno-neutralization on hypothalamic mRNA levels of 23 genes associated with food intake, energy balance, and glucose metabolism. We observed that insulin immune-neutralization by administration of anti-porcine insulin guinea pig serum (AI) significantly decreased food intake and increased plasma glucose levels in chickens, while 5 h fasting produced a limited and non-significant reduction in plasma glucose. In addition, 5 h fasting increased levels of NPY, TAS1R1, DIO2, LEPR, GLUT1, GLUT3, GLUT8, and GCK mRNA. In contrast, AI had no impact on the levels of any selected mRNA. Therefore, our results demonstrate that in chickens, food intake inhibition or satiety mechanisms induced by insulin immuno-neutralization do not rely on hypothalamic abundance of the 23 transcripts analyzed. The hypothalamic transcripts that were increased in the fasted group are likely components of a mechanism of adaptation to fasting in chickens.

Changes in hepatic glucose and lipid metabolism-related parameters in domestic pigeon (Columba livia) during incubation and chick rearing

This study aimed to evaluate the hepatic glucose and lipid metabolism-related parameters of adult male and female White King pigeons (Columba livia) during incubation and chick rearing. At day 4 (I4), 10 (I10) and 17 (I17) of incubation and day 1 (R1), 7 (R7), 15 (R15) and 25 (R25) of chick rearing, livers were sampled from six pigeons for each sex. Glycogen and fat contents, activities of glycolytic enzymes (hexokinase, HK; 6-phosphofructokinase, 6-PFK), and genes expressions of key enzymes involved in glycolysis (pyruvate kinase, PK; glucokinase, GK), gluconeogenesis (phosphoenolpyruvate carboxykinase cytosolic, PCK1; fructose-1,6-bisphosphatase, FBP1; glucose-6-phosphatase, G6Pase), fatty acid synthesis (fatty acid synthase, FAS; acetyl-CoA carboxylase, ACC) and fatty acid β-oxidation (carnitine palmitoyltransferase 1, CPT1; acyl-CoA 1, ACO) were measured. In male and female pigeon livers, glycogen content and HK activity dramatically increased after I17 and after R1, respectively; expressions of FBP1 and G6Pase genes were maximized at R15; activity of 6-PFK and expressions of PK and CPT1 genes were highest at R7; fat content and expressions of FAS and ACC genes steeply increased from I10 to R1. In females, hepatic expressions of GK and PCK1 genes were greatest at R7 and I17, respectively; however, in males, both of them were maximized at R15. Hepatic expression of ACO gene was significantly enhanced at R1 compared to I17 and R7 in males, whereas it was notably up-regulated at I17 and R7 in females. Furthermore, expressions of PCK1, GK, FAS and ACC genes were in significant relation to fat content in the livers of female pigeons, while fat content in male pigeons was highly correlated with expression of PCK1, ACC, CPT1 and ACO genes. In conclusion, regulations of glucose and lipid metabolic processes were enhanced in parent pigeon livers from terminal phases of incubation to mid phase of chick rearing with sexual effects.

Nutritional regulation of glucokinase: a cross-species story

The glucokinase (GK) enzyme (EC 2.7.1.1.) is essential for the use of dietary glucose because it is the first enzyme to phosphorylate glucose in excess in different key tissues such as the pancreas and liver. The objective of the present review is not to fully describe the biochemical characteristics and the genetics of this enzyme but to detail its nutritional regulation in different vertebrates from fish to human. Indeed, the present review will describe the existence of the GK enzyme in different animal species that have naturally different levels of carbohydrate in their diets. Thus, some studies have been performed to analyse the nutritional regulation of the GK enzyme in humans and rodents (having high levels of dietary carbohydrates in their diets), in the chicken (moderate level of carbohydrates in its diet) and rainbow trout (no carbohydrate intake in its diet). All these data illustrate the nutritional importance of the GK enzyme irrespective of feeding habits, even in animals known to poorly use dietary carbohydrates (carnivorous species).

Metabolic and hormonal responses of growing modern meat-type chickens to fasting

1. The present study compared the effects of fasting on circulating concentrations of glucose, insulin and glucagon in male and female modern meat-type chickens (Ross 708) at three ages (19 d, 33 d and 47 d). 2. Plasma concentrations of glucose were reduced by fasting with reductions of 24.9% (19-d-old), 22.6% (33-d-old) and 17.9% (47-d-old) in broiler chickens fasted for 12 h. 3. Plasma concentrations of insulin decreased with fasting. For instance, circulating concentrations of insulin declined after 6 h of fasting by 45.7%, 54.7% and 50.0%, respectively, in 19-d-old, 33-d-old and 47-d-old broiler chickens. 4. Plasma concentrations of glucagon were increased by fasting. Plasma concentrations of glucagon were elevated by 3.79% (19-d-old), 3.51% (33-d-old) and 3.79% (47-d-old) with 6 h of fasting and remained elevated with 12 h, 18 h and 24 h of fasting.

Evolution of glucose utilization: glucokinase and glucokinase regulator protein

Glucose is an essential nutrient that must be distributed throughout the body to provide energy to sustain physiological functions. Glucose is delivered to distant tissues via be blood stream, and complex systems have evolved to maintain the levels of glucose within a narrow physiological range. Phosphorylation of glucose, by glucokinase, is an essential component of glucose homeostasis, both from the regulatory and metabolic point-of-view. Here we review the evolution of glucose utilization from the perspective of glucokinase. We discuss the origin of glucokinase, its evolution within the hexokinase gene family, and the evolution of its interacting regulatory partner, glucokinase regulatory protein (GCKR). Evolution of the structure and sequence of both glucokinase and GCKR have been necessary to optimize glucokinase in its role in glucose metabolism.

Glucosensing and glucose homeostasis: from fish to mammals

This review is focused on two topics related to glucose in vertebrates. In a first section devoted to glucose homeostasis we describe how glucose levels fluctuate and are regulated in different classes of vertebrates. The detection of these fluctuations is essential for homeostasis and for other physiological processes such as regulation of food intake. The capacity of that detection is known as glucosensing, and the different mechanisms through which it occurs are known as glucosensors. Different glucosensor mechanisms have been demonstrated in different tissues and organs of rodents and humans whereas the information obtained for other vertebrates is scarce. In the second section of the review we describe the present knowledge regarding glucosensor mechanisms in different groups of vertebrates, with special emphasis in fish.

Glucokinase activation induces potent hypoglycemia without recruiting insulin and inhibits food intake in chicken

Glucose homeostasis exhibits several peculiarities in chickens (in short, presence of high glycemia and resistance to high doses of exogenous insulin). Though the full chicken glucokinase gene sequence is still lacking, several results suggest its existence. The functionality of chicken glucokinase (GK) has been further investigated using an activator of mammalian GK (GKA). In vitro, GKA decreased GK's S0.5(a) in a glucose-dependent manner in liver homogenates from either fasted or fed chickens; it also increased GK Vmax(a) in homogenates from fed chickens. In vivo, acute oral GKA administration (10-100 mg/kg) induced a potent and dose dependent hypoglycemic effect in fed chickens (starting between 15 and 45 min with a maximum effect at 40 mg/kg, P<0.0001). At this dose, plasma insulin levels showed erratic and minor changes in the early times (an increase at 5 min and a decrease at 10 min, P<0.05). At 90 min, when hypoglycemia had developed plasma insulin levels decreased under controls and plasma pancreatic glucagon levels increased over controls. Also at 40 mg/kg, GKA transiently inhibited food intake at about 3h (P<0.0001). In conclusion, GKA is a potent activator of chicken GK evidencing that the structure and the activity of chicken GK are similar to those of mammalian GK. At variance with results obtained in mammals, the potent GKA hypoglycemic action appears to rely mostly on an effect on liver GK in chicken. This fits with previous results and further support the hypothesis of a "deficient coupling" between Β-cell metabolism and insulin release in this species.

QTL for several metabolic traits map to loci controlling growth and body composition in an F2 intercross between high- and low-growth chicken lines

Quantitative trait loci (QTL) for metabolic and body composition traits were mapped at 7 and 9 wk, respectively, in an F2 intercross between high-growth and low-growth chicken lines. These lines also diverged for abdominal fat percentage (AFP) and plasma insulin-like growth factor-I (IGF-I), insulin, and glucose levels. Genotypings were performed with 129 microsatellite markers covering 21 chromosomes. A total of 21 QTL with genomewide level of significance were detected by single-trait analyses for body weight (BW), breast muscle weight (BMW) and percentage (BMP), AF weight (AFW) and percentage (AFP), shank length (ShL) and diameter (ShD), fasting plasma glucose level (Gluc), and body temperature (Tb). Other suggestive QTL were identified for these parameters and for plasma IGF-I and nonesterified fatty acid levels. QTL controlling adiposity and Gluc were colocalized on GGA3 and GGA5 and QTL for BW, ShL and ShD, adiposity, and Tb on GGA4. Multitrait analyses revealed two QTL controlling Gluc and AFP on GGA5 and Gluc and Tb on GGA26. Significant effects of the reciprocal cross were observed on BW, ShD, BMW, and Gluc, which may result from mtDNA and/or maternal effects. Most QTL regions for Gluc and adiposity harbor genes for which alleles have been associated with increased susceptibility to diabetes and/or obesity in humans. Identification of genes responsible for these metabolic QTL will increase our understanding of the constitutive “hyperglycemia” found in chickens. Furthermore, a comparative approach could provide new information on the genetic causes of diabetes and obesity in humans.

Induction of glucokinase in chicken liver by dietary carbohydrates

We recently provided evidence of the presence of glucokinase (GCK) in the chicken liver [Berradi, H., Taouis, M., Cassy, S., Rideau, N., 2005. Glucokinase in chicken (Gallus gallus). Partial cDNA cloning, immunodetection and activity determination. Comp. Biochem. Physiol. B Biochem. Mol. Biol. 141, 129-139]. In the present study we addressed the question of whether nutritional regulation of GCK occurs. Several nutritional conditions were compared in chickens (5 weeks old) previously trained to meal-feeding. One group was left in the fasted state (F: 24h) and one was tested at the end of the 2h meal (refed: RF). Two other 2h meal-refed groups received an acute oral saccharose load (6ml/kg BW) just before the 2h meal and were sacrificed either at the end of the meal (Saccharose refed, SRF) or 3h later (SRF+3). Liver GCK mRNA and protein levels did not differ between F, RF and SRF chickens but were significantly increased in SRF+3 chickens (2-fold, p<0.05). GCK activity did not differ between F and RF chickens but increased significantly in SRF and SRF+3 chickens (1.7-fold, p<0.05). Chicken liver GCK expression (mRNA and protein) and activity were therefore inducible in these chickens by feeding a meal with acute oral administration of carbohydrate. These and recent findings demonstrating insulin dependency of the liver GCK mRNA and protein strongly suggest that GCK may have an important role in carbohydrate metabolism, including that of the chicken. However, even in these highly stimulatory conditions, liver GCK activity remained relatively low in comparison with other species. The latter result may partly explain the high plasma glucose level in the chicken.

Pre- and post-natal hepatic glucose phosphorylation in chicks (Gallus domesticus)

1. Hepatic glycogen levels and activities of metabolic enzymes were measured 7 d and 2 d before hatching, immediately after hatching and 4 d thereafter. 2. Chicken liver has a particle-bound hexokinase with a high K(m) (8 mM) for glucose. 3. The results indicate that the high-K(m) hexokinase is involved in the mitochondrial generation of ATP for glycogen and lipid synthesis.

Molecular evolution of the vertebrate hexokinase gene family: Identification of a conserved fifth vertebrate hexokinase gene

Hexokinases (HK) phosphorylate sugar immediately upon its entry into cells allowing these sugars to be metabolized. A total of four hexokinases have been characterized in a diversity of vertebrates-HKI, HKII, HKIII, and HKIV. HKIV is often called glucokinase (GCK) and has half the molecular weight of the other hexokinases, as it only has one hexokinase domain, while other vertebrate HKs have two. Differing hypothesis has been proposed to explain the diversification of the hexokinase gene family. We used a genomic approach to characterize hexokinase genes in a diverse array of vertebrate species and close relatives. Surprisingly we identified a fifth hexokinase-like gene, HKDC1 that exists and is expressed in diverse vertebrates. Analysis of the amino acid sequence of HKDC1 suggests that it may function as a hexokinase. To understand the evolution of the vertebrate hexokinase gene family we established a phylogeny of the hexokinase domain in all of the vertebrate hexokinase genes, as well as hexokinase genes from close relatives of the vertebrates. Our phylogeny demonstrates that duplication of the hexokinase domain, yielding a HK with two hexokinase domains, occurred prior to the diversification of the hexokinase gene family. We also establish that GCK evolved from a two hexokinase domain-containing gene, but has lost its N-terminal hexokinase domain. We also show that parallel changes in enzymatic function of HKI and HKIII have occurred.

Expression of the glucokinase gene in mule duck liver and glucokinase activities in chicken and mule duck livers

The presence of glucokinase (GK), a critical enzyme controlling glucose homeostasis, particularly liver glucose utilization in mammals, has long been a matter of debate in avian species because a number of investigators have failed to detect GK activity in the livers of chickens and several other avian species. In this study, we cloned a partial GK cDNA from mule duck livers and measured GK-like activity in the livers of mule ducks and broiler chickens under 2 nutritional states. Liver samples from 5-wk-old meal-fed male broiler chickens (Ross) were obtained from overnight-fasted chickens (BC) and 5 h after an oral saccharose load (6 mL/kg of BW of a 50% saccharose solution) given just before the meal (BS). Liver samples from 15-wk-old mule ducks were collected after an overnight fast (DC) and 12 h after the last overfeeding meal (DO). A partial cDNA ( approximately 600 bp) was obtained from duck livers. It presented 99% identity with chicken partial GK cDNA (gi 44888789) and 82% identity with human GK (gi 15967158). Chicken liver weights represented 1.8 and 3.3% of BW, respectively, for BC and BS (n = 8, P < 0.05). Glucokinase and low-Michaelis constant hexokinase (HK) activity levels were similar in BC (respectively, 0.88 and 1.00 mU/mg of protein). In response to the meal load, GK activity increased significantly (+57%), whereas HK decreased (-46%) in BS. Duck liver weights represented 1.4 and 7.6% of BW, respectively, for DC and DO (n = 8, P < 0.05). In DC livers, GK activity was significantly higher than HK activity (respectively, 1.76 and 0.63 mU/mg of protein). Both activities were significantly increased in DO (2 times, n = 8, P < 0.05). In conclusion, GK is present in ducks as well as chickens, and it is nutritionally regulated in avian species as well as in mammals. Further work will determine whether the higher liver GK activity and GK:HK ratio in DC compared with BC is related to age or BW or linked to the high lipogenic capacity of the duck liver.