Analysis of lines of mice selected for fat content. 1. Correlated responses in the activities of NADPH-generating enzymes

NAD kinase sustains lipogenesis and mitochondrial metabolismthrough fatty acid synthesis

Lipid storage in fat tissue is important for energy homeostasis and cellular functions. Through RNAi screening in Drosophila fat body, we found that knockdown of a Drosophila NAD kinase (NADK), which phosphorylates NAD to synthesize NADP de novo, causes lipid storage defects. NADK sustains lipogenesis by maintaining the pool of NADPH. Promoting NADPH production rescues the lipid storage defect in the fat body of NADK RNAi animals. Furthermore, NADK and fatty acid synthase 1 (FASN1) regulate mitochondrial mass and function by altering the levels of acetyl-CoA and fatty acids. Reducing the level of acetyl-CoA or increasing the synthesis of cardiolipin (CL), a mitochondrion-specific phospholipid, partially rescues the mitochondrial defects of NADK RNAi. Therefore, NADK- and FASN1-mediated fatty acid synthesis coordinates lipid storage and mitochondrial function.

Microevolution of intermediary metabolism: evolutionary genetics meets metabolic biochemistry

During the past decade, microevolution of intermediary metabolism has become an important new research focus at the interface between metabolic biochemistry and evolutionary genetics. Increasing recognition of the importance of integrative studies in evolutionary analysis, the rising interest in ‘evolutionary systems biology’, and the development of various ‘omics’ technologies have all contributed significantly to this developing interface. The present review primarily focuses on five prominent areas of recent research on pathway microevolution: lipid metabolism and life-history evolution; the electron transport system, hybrid breakdown and speciation; glycolysis, alcohol metabolism and population adaptation in Drosophila; chemostat selection in microorganisms; and anthocyanin pigment biosynthesis and flower color evolution. Some of these studies have provided a new perspective on important evolutionary topics that have not been investigated extensively from a biochemical perspective (hybrid breakdown, parallel evolution). Other studies have provided new data that augment previous biochemical information, resulting in a deeper understanding of evolutionary mechanisms (allozymes and biochemical adaptation to climate, life-history evolution, flower pigments and the genetics of adaptation). Finally, other studies have provided new insights into how the function or position of an enzyme in a pathway influences its evolutionary dynamics, in addition to providing powerful experimental models for investigations of network evolution. Microevolutionary studies of metabolic pathways will undoubtedly become increasingly important in the future because of the central importance of intermediary metabolism in organismal fitness, the wealth of biochemical data being provided by various omics technologies, and the increasing influence of integrative and systems perspectives in biology.

Life-history evolution and the microevolution of intermediary metabolism: activities of lipid-metabolizing enzymes in life-history morphs of a wing-dimorphic cricket

Although a considerable amount of information is available on the ecology, genetics, and physiology of life-history traits, much more limited data are available on the biochemical and genetic correlates of life-history variation within species. Specific activities of five enzymes of lipid biosynthesis and two enzymes of amino acid catabolism were compared among lines selected for flight-capable (LW[f]) versus flightless (SW) morphs of the cricket Gryllus firmus. These morphs, which exist in natural populations, differ genetically in ovarian growth (100-400% higher in SW) and aspects of flight capability including the size of wings and flight muscles, and the concentration of triglyceride flight fuel (40% greater in LW[f]). Consistently higher activity of each enzyme in LW(f) versus SW-selected lines, and strong co-segregation between morph and enzyme activity, demonstrated genetically based co-variance between wing morph and enzyme activity. Developmental profiles of enzyme activities strongly paralleled profiles of triglyceride accumulation during adulthood and previous measures of in vivo lipid biosynthesis. These data strongly imply that genetically based elevation in activities of lipogenic enzymes, and enzymes controlling the conversion of amino acids into lipids, is an important cause underlying the elevated accumulation of triglyceride in the LW(f) morph, a key biochemical component of the trade-off between elevated early fecundity and flight capability. Global changes in lipid and amino-acid metabolism appear to have resulted from microevolutionary alteration of regulators of metabolism. Finally, strong genotype x environment (diet) interactions were observed for most enzyme activities. Future progress in understanding the functional causes of life-history evolution requires a more detailed synthesis of the fields of life-history evolution and metabolic biochemistry. Wing polymorphism is a powerful experimental model in such integrative studies.

Effect of the herbicide glyphosate on enzymatic activity in pregnant rats and their fetuses

To prevent health risk from environmental chemicals, particularly for progeny, we have studied the effects of the herbicide glyphosate on several enzymes of pregnant rats. Glyphosate is an organophosphorated nonselective agrochemical widely used in many countries including Argentina and acts after the sprout in a systemic way. We have studied three cytosolic enzymes: isocitrate dehydrogenase-NADP dependent, glucose-6-phosphate dehydrogenase, and malic dehydrogenase in liver, heart, and brain of pregnant Wistar rats. The treatment was administered during the 21 days of pregnancy, with 1 week as an acclimation period. The results suggest that maternal exposure to agrochemicals during pregnancy induces a variety of functional abnormalities in the specific activity of the enzymes in the studied organs of the pregnant rats and their fetuses.

Behavioural changes as a correlated response to selection

Lines of mice have been selected for up to 50 generations on the following traits: high body weight, low body weight, high fat content or low fat content. The lines selected for high or low body weight differ by a factor of 2.5 and those selected for high or low fat content differ by a factor of five, both traits measured in 10 week old males. A set of behavioural traits was measured to ascertain whether this selection had caused correlated responses in behaviour: studies included feeding behaviour, open field behaviour, ultrasound calling rates of pups, and the response to the introduction of a novel physical object. Alterations in behavioural patterns which were expected a priori were observed but there appeared to be no changes in behaviour associated with any one selection criterion. Estimates of the genetic correlations between selected and behavioural traits were, with one exception, generally less than 0.1 in magnitude and not significantly different from zero (the exception was food intake in lines selected on body weight). Assuming that mice are accurate models for commercial species, then these results have important implications for animal welfare: they demonstrate that large scale behavioural changes do not arise as an inevitable consequence of intense long-term selection on traits of economic importance in commercial species.

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

An understanding of the physiological and genetic changes which determine the response to selection is critical for both evolutionary theory and to assess the application of new molecular techniques to commercial animal breeding. We investigated an aspect of physiology, growth hormone (GH) metabolism, which might a priori have been expected to play a large part in the response of mouse lines selected for high or low body weight. Disruption of endogenous GH or addition of exogenous GH had similar proportionate effects on body weight in both lines of mice (although differences in body composition arose) suggesting that neither the production of GH nor receptor sensitivity to GH had been altered as a result of selection. This supports a 'pleiotropic model' of the response to selection: that many genes with diverse metabolic roles all contribute to the divergent phenotype. This result has significant commercial implications as it suggests that artificial selection, transgenic technology and environmental manipulation may be synergistic rather than antagonistic strategies.

Size distribution of adipocytes and variation in adipocyte number in lines of mice selected for high or low body fat

1. Adipocyte size and number were measured in epididymal fat pads from lines of mice selected for high or low body fat. 2. Epididymal fat pad weight at 10 weeks of age was 4-fold greater in the high fat than the low fat mice, accompanied by a 7-fold greater adipocyte volume but there was no difference in adipocyte number. Smaller differences in fatness and cell volume were observed at younger ages. 3. The high fat mice showed one or two populations of cells up to 5 weeks of age and at 7 weeks 3 days and 10 weeks showed two or more populations. In contrast, the low fat animals at all ages showed either one or two populations. 4. These findings imply that greater lipogenesis and cell filling, not cell proliferation, is the primary mechanism of increased fatness in these selected lines of mice.

The population genetics of alleles affecting enzyme activity

It is possible to predict the population genetics of allozymes by assuming that fitness is proportional to flux through a biochemical pathway. The model presented here extends previous work by incorporating two additional features of biological realism. Firstly, that more than one biochemical route may exist between any two metabolites. The major routes have been identified as the classical biochemical pathways but in the event of a mutation blocking a major route, minor routes become significant. These minor routes are named "bypass fluxes" and have profound effects on the population genetics of allozymes. Secondly, recent work has suggested that a metabolic cost is associated with enzyme synthesis; this will constitute an additional selective pressure on alleles which affect the amount of enzyme synthesized. The model generates a fitness curve which predicts the fitness associated with any level of enzyme activity. It can utilize data on null or near-null, structural or regulatory, mutations in the presence or absence of bypass fluxes. When data from natural populations of Drosophila are investigated, it is concluded that selection pressures acting on enzyme variants may be much higher than previously thought.

Analysis of lines of mice selected on fat content. 4. Correlated responses in growth and reproduction

Lines of mice have been selected for 32 generations for either high or low fat content, resulting in a threefold divergence in the selection criterion (estimated fat content of males at 14 weeks of age). Male mice from both lines were dissected at a series of ages between 4 and 26 weeks and the following traits measured or estimated: body weight, fat content, lean weight, and the weights of several fatpads and internal organs. The lines appeared to have a similar underlying lean weight upon which the Fat line accumulated fat at a faster rate. This accumulation continued unabated in the Fat lines for at least 26 weeks but had effectively ceased by 8 weeks of age in the Lean. The liver and kidneys were slightly larger in the Fat line but there were no differences in the weights of heart, lung or spleen. This detailed phenotypic description of the lines complemented previous reports describing correlated changes in their physiology. The threefold divergence in estimated fat content was less than that in one of its component traits, growth of gonadal fatpad, but was greater than the divergence in other physiological indicators, i.e. the activity of lipogenic enzymes in vitro and direct measurement of lipogenic flux. Testis size in the Fat line was consistently lower than in the Lean although the Fat line was slightly more fecund, apparently due to a higher prenatal survival rate.

Analysis of lines of mice selected for fat content. 3. Flux through the de novo lipid synthesis pathway

The flux through the de novo fatty acid synthesis pathway was estimated in lines of mice which differed substantially in fat content following 26 generations of selection at 10 weeks of age. Previous estimates of lipogenic enzyme activities had indicated an increase in the capacity for lipogenesis in the Fat compared to the Lean line. Therefore the in vivo flux in lipogenesis was measured in both liver and gonadal fat pad (GFP) tissues of males at 5 and 10 weeks of age, using the rate of incorporation of 3H from 3H2O and 14C from acetate and citrate into total lipids. At both ages and in both tissues the Fat line had a higher flux, about 20% increase in the liver and up to three-fold increase (range 1.2- to 3.4-fold) in the GFP. We conclude that direct selection for fatness in mice has resulted in metabolic changes in the rate of de novo fatty acid synthesis, and that the changes are largely detectable before 10 weeks, the age of selection.

Direct and correlated responses to artificial selection on lipid and glycogen contents in Drosophila melanogaster

A large outbred population of Drosophila melanogaster was subjected to artificial selection on lipid and glycogen storage. In three separate experiments, two replicates underwent sib selection for both increased and decreased storage. In the first study, flies were selected on the basis of total triacylglycerol for ten generations. This experiment resulted in no significant direct response, but there was a significant change in total body weight, underscoring the importance of concern for the allometric relationship between body weight and lipid content. In the second study, selection was performed for 15 generations on the percentage of body composition that was triacylglycerol. A significant direct response was obtained, and the two replicates revealed heritability estimates of 0.40 and 0.43. The third study selected glycogen content for 15 generations, and produced a significant response with heritabilities of 0.25 and 0.31. A series of 12 biochemical and enzyme kinetic traits was examined at five generation intervals in all experiments, and a number of correlated responses were detected. The results are interpreted with respect to the evolutionary constraints on energy storage evolution and the genetic basis of the allometric relationship between body weight and fat content.

Analysis of lines of mice selected for fat content. 2. Correlated responses in the activities of enzymes involved in lipogenesis

Estimates of the activities (Vmax) of six enzymes involved in de novo fat synthesis were made in replicated lines of mice differing in fat content. These lines had been selected high and low for 20 generations with three replicates each of Fat, Control and Lean lines and for a further eight generations high and low as an unreplicated line. The activities of ATP-citrate lyase (ACL), acetyl-CoA carboxylase (ACC), fatty acid synthetase (FAS), cytoplasmic malate dehydrogenase (MDH), malic enzyme (ME) and pyruvate kinase (PK) were determined in vitro in both liver and gonadal fatpad tissues taken at ages five and ten weeks. The activities of ACL, ACC, FAS and ME were significantly higher in the Fat than the Lean lines, and the differences were more pronounced at the earlier age and in the gonadal fatpad where activities in the Fat lines were higher by factors of 3.5, 2.4, 2.5 and 3.5 respectively. The activity of PK was unchanged in each tissue. MDH activity was significantly lower in adipose tissue in the Fat lines than the Lean lines at age ten weeks but not at age five weeks or in liver tissue. Results from replicates indicated that random genetic drift affected enzyme activities but nevertheless significant changes in activity were associated with the direction of selection. The changes in enzyme activity reported here are similar to those known to be associated with major mutations causing obesity in mice.