The abdominal fat contribution to adiposity in chickens divergently selected for fatness or growth: Cross-model elucidation and validation of gene expression
Date
2013
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Publisher
University of Delaware
Abstract
The domestic chicken (Gallus domesticus) is an important global source of high-quality dietary protein and a widely used biological model. Decades of intensive genetic selection have established the remarkable growth rate of the commercial broiler today; however, increased growth rate has been accompanied by the magnification of several unfavorable traits. This thesis addresses abdominal fatness, one of the traits that are incidentally amplified by selection for increased growth. Excessive fatness, coupled with several unique avian features of metabolism (i.e., fasting hyperglycemia and insulin insensitivity), parallels conditions observed in humans with metabolic diseases. Thus, understanding the genetic influence on excessive fatness in chickens will not only serve to improve the quality of production from an agricultural standpoint, but will also advance the knowledge of metabolic disorders in humans.
Four experimental lines of meat-type chickens that were divergently selected for either a large difference in abdominal (visceral) fatness or in growth rate were used to characterize the role of adipose tissue (classically thought to have a minimal lipogenic contribution) in regulating adiposity. At the age of selection (9 weeks), the fat line (FL) and lean line (LL) chickens exhibit a 2.5-fold difference in abdominal adipose weight, while their body weight and feed intake are similar. The high growth (HG) and low growth (LG) chickens were divergently selected for either high (HG) or low (LG) body weight at 8 and 32 weeks of age resulting in a 2.7-fold increase in bodyweight and an 8-fold increase in abdominal fatness (as a percentage of BW) in HG chickens on average from 1 through 11 weeks. The adipose transcriptomes of these four genotypes (FL compared to LL and HG compared to LG) were analyzed at 1 through 11 weeks of age using the Del-Mar 14K Chicken Integrated Systems microarray, and at a single age (7 weeks) by RNA sequencing.
Microarray analysis of abdominal fat in FL and LL chickens revealed 131 differentially expressed (DE) genes (FDR≤0.05) as the main effect of genotype, 254 DE genes as an interaction of age and genotype and 3,194 DE genes (FDR≤0.01) as the main effect of age. The most notable discoveries in the abdominal fat transcriptome during juvenile development were higher expression of many genes involved in hemostasis in the LL and up-regulation of numerous adipogenic and lipogenic genes in FL chickens. Many of these DE genes belong to pathways controlling the synthesis, metabolism and transport of lipids or endocrine signaling pathways activated by adipokines, retinoids and thyroid hormones. The importance of these processes in regulating adiposity in abdominal fat of FL and LL chickens was reinforced by the deep RNA sequencing analysis at 7 weeks. Remarkably, the highest expressed genes at this age included those involved in the metabolism of lipid and carbohydrates which are functionally associated with endocrine system and metabolic disorders. There were 1,687 DE genes between fat and lean chickens at 7 weeks including transcription factors and metabolic enzymes which have direct influences on lipogenesis and adipogenesis. The findings of the microarray analysis were further verified by the abundance of DE hemostatic factors uncovered by RNA sequencing analysis. This deep sequencing analysis also revealed a number of ectopically expressed genes suggesting that visceral fat functions autonomously as well as an endocrine organ in the regulation of lipid metabolism and perhaps feed intake.
Microarray analysis of HG and LG chickens at 1 through 11 weeks of age revealed DE genes (FDR≤0.05) as the main effect of genotype (321 genes), the interaction of age and genotype (718 genes), and the main effect of age (2,918 genes). RNA sequencing at 7 weeks uncovered 280 DE genes (FDR≤0.1). Similar to the FL, HG chickens over-express many genes involved in adipogenesis and lipogenesis (including biosynthesis of fatty acids, cholesterol and triglycerides) which could at least partially account for their increase in abdominal fatness. Conversely, LG chickens up-regulate several energy producing processes (i.e., peroxisomal -oxidation, mitochondrial -oxidation, ketogenesis and oxidative phosphorylation) early on in juvenile development which are likely responsible for their extreme leanness. Hemostasis also appears to have a critical role in the maintenance of the lean phenotype at the age of maximal difference in adiposity in these chickens (7 weeks). These findings validate abdominal fat as a major contributor to adiposity in response to either divergent selection on abdominal fatness in the FL and LL or body weight in the HG and LG chickens.