The role of growth hormone in lines of mice divergently selected on body weight

Domestication and growth hormone transgenesis cause similar changes in gene expression in coho salmon (Oncorhynchus kisutch)

Domestication has been extensively used in agricultural animals to modify phenotypes such as growth rate. More recently, transgenesis of growth factor genes [primarily growth hormone (GH)] has also been explored as a rapid approach to accelerating performance of agricultural species. Growth rates of many fishes respond dramatically to GH gene transgenesis, whereas genetic engineering of domestic mammalian livestock has resulted in relatively modest gains. The most dramatic effects of GH transgenesis in fish have been seen in relatively wild strains that have undergone little or no selection for enhanced growth, whereas genetic modification of livestock necessarily has been performed in highly domesticated strains that already possess very rapid growth. Such fast-growing domesticates may be refractory to further stimulation if the same regulatory pathways are being exploited by both genetic approaches. By directly comparing gene expression in wild-type, domestic, and GH transgenic strains of coho salmon, we have found that domestication and GH transgenesis are modifying similar genetic pathways. Genes in many different physiological pathways show modified expression in domestic and GH transgenic strains relative to wild-type, but effects are strongly correlated. Genes specifically involved in growth regulation (IGF1, GHR, IGF-II, THR) are also concordantly regulated in domestic and transgenic fish, and both strains show elevated levels of circulating IGF1. Muscle expression of GH in nontransgenic strains was found to be elevated in domesticated fish relative to wild type, providing a possible mechanism for growth enhancement. These data have implications for genetic improvement of existing domesticated species and risk assessment and regulation of emerging transgenic strains.

The contribution of animal models to the study of obesity

Obesity results from prolonged imbalance of energy intake and energy expenditure. Animal models have provided a fundamental contribution to the historical development of understanding the basic parameters that regulate the components of our energy balance. Five different types of animal model have been employed in the study of the physiological and genetic basis of obesity. The first models reflect single gene mutations that have arisen spontaneously in rodent colonies and have subsequently been characterized. The second approach is to speed up the random mutation rate artificially by treating rodents with mutagens or exposing them to radiation. The third type of models are mice and rats where a specific gene has been disrupted or over-expressed as a deliberate act. Such genetically-engineered disruptions may be generated through the entire body for the entire life (global transgenic manipulations) or restricted in both time and to certain tissue or cell types. In all these genetically-engineered scenarios, there are two types of situation that lead to insights: where a specific gene hypothesized to play a role in the regulation of energy balance is targeted, and where a gene is disrupted for a different purpose, but the consequence is an unexpected obese or lean phenotype. A fourth group of animal models concern experiments where selective breeding has been utilized to derive strains of rodents that differ in their degree of fatness. Finally, studies have been made of other species including non-human primates and dogs. In addition to studies of the physiological and genetic basis of obesity, studies of animal models have also informed us about the environmental aspects of the condition. Studies in this context include exploring the responses of animals to high fat or high fat/high sugar (Cafeteria) diets, investigations of the effects of dietary restriction on body mass and fat loss, and studies of the impact of candidate pharmaceuticals on components of energy balance. Despite all this work, there are many gaps in our understanding of how body composition and energy storage are regulated, and a continuing need for the development of pharmaceuticals to treat obesity. Accordingly, reductions in the use of animal models, while ethically desirable, will not be feasible in the short to medium term, and indeed an expansion in activity using animal models is anticipated as the epidemic continues and spreads geographically.

Genetically caused retarded growth in animals

Growth process of animals is regulated by a multitude of physiological pathways among which components of the somatotropic axis play a key role. A number of severe, simply inherited growth disturbances have been identified in humans, laboratory and farm animals. These disorders are controlled by defective alleles at major loci referring to hormones or hormone receptors, e.g. growth hormone receptor for the recessive sex-linked dwarfism (dw) in chickens and the recessive autosomal Laron-type dwarfism in man, and growth hormone releasing hormone receptor for the recessive "little" mutation (lit) in mice. Apart from these particular cases, growth rate is a quantitative polygenic trait which has a moderate heritability (close to 0.30) and is influenced by prenatal and postnatal maternal effects. Increase in the average coefficient of inbreeding in a population is also known to result in lower growth rate. Divergent selection experiments have shown that upward or downward selection on growth is effective, sometimes with asymmetrical responses, but patterns of changes in underlying physiological traits appear to differ among experiments.

Altered growth of mice divergently selected for body weight is associated with complex changes in the growth hormone/insulin-like growth factor system

Mice investigated in this study were generated by selecting a sub-population of the NMRI out-bred stock (C), for high (H) or low (L) 8-week body weight. After 58 generations of selection, 8-week body weights of the sub-populations were markedly different if compared to controls. To investigate endocrine changes causing the altered growth performance in the different lines of mice, we analysed several components of the growth hormone (GH)/insulin-like growth factor (IGF) system. Pituitary weights of male and female L mice were significantly smaller than those of C and H mice. Relative to body weight, pituitary weights of male mice from the three lines did not differ, however pituitary weight-to-body weight-ratios of female L mice were significantly greater than those of H females. Mean volume densities of somatotropic cells were significantly smaller in L mice than in C and H mice. Serum IGF-I concentrations were significantly lower in the L line than in the C and H lines. H mice displayed significantly increased serum insulin levels both after ad libitum feeding and after a 14 hour fasting period. Ligand blot analysis of serum IGF-binding proteins (IGFBPs) revealed a significant reduction of circulating IGFBP-3 in L mice as compared to C and H mice. In contrast, serum IGFBP-2 mRNA levels were significantly increased in male L mice and showed non significant increases in female L mice. Hepatic IGFBP-2 mRNA levels were significantly increased in L mice and decreased in H mice as compared to C mice. Expression of IGFBP-4 mRNA in the liver was significantly decreased in both selection lines (L, H) as compared to the random-bred controls. Our findings demonstrate that altered growth of mice resulting from selection for body weight is associated with complex changes in the endocrine network of the GH/IGF system.

Plasma cholecystokinin-8 in pigs with divergent genetic potential for feed intake and growth

Plasma cholecystokinin-8 (CCK-8) was studied in pigs with divergent genetic potential for feed intake. Differences in feed intake resulted from selection for either fast (line F) or slow (line S) postweaning gain. The hypothesis was that the relatively lesser feed intake in S versus F may be attributable, in part, to greater circulating concentrations of the putative satiety hormone CCK-8. In Experiment I, barrows from F (n = 23) and S (n = 19) were used to determine changes in CCK-8 associated with ad libitum feed consumption. Blood samples were collected after overnight feed deprivation, then periodically during a 2-hr feeding period. Averaged across sampling times, concentration of CCK-8 tended to be greater (P = 0.07) in S (6.70 pmol/l) than in F (5.06 pmol/l). Concentration of CCK-8 per unit of feed consumed (CCK-8/cumulative feed intake) was greater (P < 0.01) in S than in F during the first 30 min of the feeding period. In Experiment 2, plasma concentrations were determined for nine pairs (F, S) of the same barrows allowed an amount of feed equal to the previous ad libitum intake of the S barrow in the pair. Averaged across times, the difference between CCK-8 concentrations of S (11.65 pmol/l) and F (7.94 pmol/l) barrows was not significant (P = 0.18). A greater concentration of CCK-8 per unit of feed consumed in S than in F supports the hypothesis that satiety effects of CCK-8 may play a role in genetic differences between the lines for feed intake.

Structure of the growth hormone-encoding gene and its promoter in mice

The isolation and nucleotide sequence determination of the 5' flanking region of the mouse growth hormone (mGH)-encoding gene (mGH) is described. The mGH gene consists of five exons and four introns, as is observed in other mammalian species. The second intron in mGH is much smaller than its rat counterpart, thus being similar in size to human, bovine and porcine GH. The transcription start point was determined to be a C residue 62 bp upstream from the start codon, ATG. Analysis of 1767 bp of the 5' flanking region, with respect to putative regulatory elements, revealed a TATA box, two binding sites for growth hormone factor (GHF1), a GC box (SP1), a thyroid-response element (TRE) and a silencer (SiL) sequence motif. As expected, the mGH promoter shows a higher degree of homology with rat, as compared to the other mammalian species like pig, cattle and human, where an overall homology exists only at the proximal promoter region.

The genetic basis of response in mouse lines divergently selected for body weight or fat content. I. The relative contributions of autosomal and sex-linked genes

Lines of mice have been divergently selected for over forty generations on either body weight or fat content. Reciprocal crosses were made between the divergent lines and the offspring backcrossed to the parental lines. The resulting data allowed us to investigate the genetic basis of response, including two features of particular interest: (i) the relative contribution of autosomal and sex-linked genes and whether any significant Y chromosome or cytoplasmic effects were present (ii) the mechanism of gene action, whether predominantly additive or whether significant dominance effects were present. A large additive sex-linked effect was observed in lines selected on body weight which accounted for approximately 25% of the divergence. The remaining 75% of the divergence appeared to be autosomal. There was no apparent sex-linked effect in lines selected on fat content and the response appeared to be entirely autosomal and additive.