Molecular characterization of the ghrelin and ghrelin receptor genes and effects on fat deposition in chicken and duck

Decoding the role of ghrelin and its interactions with central signaling pathways in avian appetite regulation

Ghrelin, a peptide hormone primarily produced in the enteroendocrine cells of the gastrointestinal tract, plays a vital role in regulating food intake, and energy balance in avian species. This review examines the complex interactions between ghrelin and the central signaling pathways associated with hunger regulation in birds. In contrast to mammals, where ghrelin typically promotes feeding behavior, its effects in birds appear more nuanced, exhibiting anorexigenic properties under certain conditions. The interactions of ghrelin with central signaling pathways, particularly within the hypothalamus, are explored, highlighting its influence on various neuropeptide systems, including GABAergic, corticotropinergic, opioidergic, dopaminergic, serotonergic, cannabinoidergic, and adrenergic pathways. This article synthesizes current knowledge regarding ghrelin's structure and physiological functions, as well as its interactions with other neuropeptides and hormones that collectively govern avian feeding behaviors. Furthermore, this review proposes future research directions aimed at elucidating the intricate mechanisms underlying appetite control in birds. Insights gained from this analysis may not only enhance our understanding of avian biology and the optimal regulation of their food intake but also inform wildlife management and conservation strategies in response to environmental changes.

Investigating the Impact of Fasting and Refeeding on Blood Biochemical Indicators and Transcriptional Profiles in the Hypothalamus and Subcutaneous Adipose Tissue in Geese

Fasting and refeeding systems can cause significant short-term fluctuations in nutrient and energy levels, triggering adaptive physiological responses in animals. This study examines the effects of fasting and refeeding on blood biochemical indicators and transcriptional profiles in the hypothalamus and subcutaneous adipose tissue of geese. Biochemical assays reveal that fasting significantly increases levels of free fatty acids and glucagon, while reducing concentrations of triglycerides, leptin, and insulin. Transcriptomic analyses identify a complex transcriptional response in both the hypothalamus and subcutaneous adipose tissue, affecting several metabolic pathways and key genes associated with feed intake and energy metabolism. In subcutaneous adipose tissue, fasting downregulates genes involved in fatty acid synthesis (LPL, SCD, and ACSL1) and upregulates PLIN2, a gene promoting lipid droplet degradation. Fasting affects a variety of metabolic pathways and critical genes in the hypothalamus, including Apelin, insulin, and mTOR signaling pathways. After fasting, the mRNA expression of NOG, GABRD, and IGFBP-1 genes in the hypothalamus are significantly upregulated, while proopiomelanocortin (POMC) gene expression is markedly downregulated. This study highlights the intricate biological responses to nutritional changes in geese, which adds to our understanding of energy balance and metabolic regulation in avian species.

Comparative transcriptome analysis reveals mechanisms of restriction feeding on lipid metabolism in ducks

Presently, excessive fat deposition is the main reason to limit the development of duck industry. In the production, the methods of restricted feeding (RF) were widely used to reduce the lipid deposition of ducks. The liver (L), abdominal adipose (AA), and subcutaneous adipose (SA) were the main tissues of lipid metabolism and deposition of ducks. However, the mechanisms of lipid metabolism and deposition of ducks under RF have not been fully clarified. In this study, in order to better understand the mechanisms of lipid metabolism and deposition in ducks under RF, a total of 120 male Nonghua ducks were randomly divided into a free feeding group (FF, n = 60) and RF group (RF, n = 60), then comparative transcriptomic analysis of L, AA, and SA between FF (n = 3) and RF (n = 3) ducks was performed at 56 d of age. Phenotypically, L, AA, and SA index of FF group was higher than that in RF group. There were 279, 390, and 557 differentially expressed genes (DEGs) in L, AA, and SA. Functional enrichment analysis revealed that ECM-receptor interaction and metabolic pathways were significantly enriched in L, AA, and SA. Lipid metabolism-related pathways including fatty acid metabolism, unsaturated fatty acid synthesis, and steroidogenesis were significantly enriched in AA and SA. Moreover, through integrated analysis weighted gene coexpression network (WGCNA) and protein-protein interaction network, 10 potential candidate genes involved in the ECM-receptor interaction and lipid metabolism pathways were identified, including 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2), aldolase B (ALDOB), formimidoyltransferase cyclodeaminase(FTCD), phosphoenolpyruvate carboxykinase 1 (PCK1), tyrosine aminotransferase (TAT), stearoyl-CoA desaturase (SCD), squalene epoxidase (SQLE), phosphodiesterase 4B (PDE4B), choline kinase A (CHKA), and elongation of very-long-chain fatty acids-like 2 (ELOVL2), which could play a key role in lipid metabolism and deposition of ducks under RF. Our study reveals that the liver might regulate the lipid metabolism of abdominal adipose and subcutaneous adipose through ECM-receptor interaction and metabolic pathways (fatty acid metabolism, unsaturated fatty acid synthesis, and steroid synthesis), thus to reduce the lipid deposition of ducks under RF. These results provide novel insights into the avian lipid metabolism and will help better understand the underlying molecular mechanisms.

Association of ghrelin gene polymorphisms with slaughter traits in pig

It has been hypothesized that mutations in the ghrelin gene in pigs may play a similar role as in humans and may be associated with obesity. The aim of study was to analyse the polymorphisms of ghrelin gene and to evaluate its effect on pigs’ carcass traits. The effect of c.-93A>G, 4428T>C and g.4486C>T polymorphisms at the ghrelin gene on slaughter performance were analysed in 346 gilts represented by three breeds (Polish Landrace, Duroc, Pietrain). Animals were fattened from 30 to 100 (±2.5) kg body weight. After slaughter, the carcasses were chilled for 24 hours (4ºC), weighted and the right half-carcasses were dissected and evaluated. A number of data were obtained including: meat weight in primary cuts, weight of ham, backfat thickness and carcass yield. From breeding and production point of view, the favourable results were obtained for pigs with the GG genotype at the c.-93A>G locus, characterized by better carcass results than those with the AA genotype, e.g. higher ham weight and lower average backfat thickness. In pigs with the TT genotype at the g.4428T>C locus, we found lower mean backfat thickness than pigs with the CC genotype.

Transcriptional Insights into Key Genes and Pathways Underlying Muscovy Duck Subcutaneous Fat Deposition at Different Developmental Stages

Simple Summary

Subcutaneous fat is an important factor affecting the meat quality and feed conversion rate of waterfowl. The current study compared the transcriptome data of Muscovy duck subcutaneous fat among three developmental stages, aiming at exploring the key regulatory genes for subcutaneous fat deposition. The results generated abundant candidate genes and pathways involving in subcutaneous fat deposition in Muscovy duck. This study provides an important reference for revealing the developmental mechanisms of subcutaneous fat in duck.

Abstract

Subcutaneous fat is a crucial trait for waterfowl, largely associated with meat quality and feed conversion rate. In this study, RNA-seq was used to identify differentially expressed genes of subcutaneous adipose tissue among three developmental stages (12, 35, and 66 weeks) in Muscovy duck. A total of 138 and 129 differentially expressed genes (DEGs) were identified between 35 and 12 weeks (wk), and 66 and 35 wk, respectively. Compared with 12 wk, subcutaneous fat tissue at 35 wk upregulated several genes related to cholesterol biosynthesis and fatty acid biosynthesis, including HSD17B7 and MSMO1, while it downregulated fatty acid beta-oxidation related genes, including ACOX1 and ACSL1. Notably, most of the DEGs (92.2%) were downregulated in 66 wk compared with 35 wk, consistent with the slower metabolism of aging duck. Protein network interaction and function analyses revealed GC, AHSG, FGG, and FGA were the key genes for duck subcutaneous fat from adult to old age. Additionally, the PPAR signaling pathway, commonly enriched between the two comparisons, might be the key pathway contributing to subcutaneous fat metabolism among differential developmental stages in Muscovy duck. These results provide several candidate genes and pathways potentially involved in duck subcutaneous fat deposition, expanding our understanding of the molecular mechanisms underlying subcutaneous fat deposition during development.

Genomic Insights Into the Multiple Factors Controlling Abdominal Fat Deposition in a Chicken Model

Genetic selection for an increased growth rate in meat-type chickens has been accompanied by excessive fat accumulation particularly in abdominal cavity. These progressed to indirect and often unhealthy effects on meat quality properties and increased feed cost. Advances in genomics technology over recent years have led to the surprising discoveries that the genome is more complex than previously thought. Studies have identified multiple-genetic factors associated with abdominal fat deposition. Meanwhile, the obesity epidemic has focused attention on adipose tissue and the development of adipocytes. The aim of this review is to summarize the current understanding of genetic/epigenetic factors associated with abdominal fat deposition, or as it relates to the proliferation and differentiation of preadipocytes in chicken. The results discussed here have been identified by different genomic approaches, such as QTL-based studies, the candidate gene approach, epistatic interaction, copy number variation, single-nucleotide polymorphism screening, selection signature analysis, genome-wide association studies, RNA sequencing, and bisulfite sequencing. The studies mentioned in this review have described multiple-genetic factors involved in an abdominal fat deposition. Therefore, it is inevitable to further study the multiple-genetic factors in-depth to develop novel molecular markers or potential targets, which will provide promising applications for reducing abdominal fat deposition in meat-type chicken.

Gut Hormones and Regulation of Food Intake in Birds

Gut hormones act as appetite regulatory hormones in mammals. For example, the hunger hormone ghrelin, which is released from the stomach before food intake, stimulates appetite. In contrast, satiety hormones such as cholecystokinin, glucagon-like peptide-1, and peptide YY, which are released from the intestines after food intake, suppress appetite. The effects of these peptides on food intake have been shown to be similar in both mammals and fishes. However, evidence suggests that the physiological roles of these gut hormones may be different between birds and other vertebrates. This review summarizes the current information on the roles of gut hormones in the regulation of food intake in birds, especially in chickens.

Regulation of Agouti-Related Protein and Pro-Opiomelanocortin Gene Expression in the Avian Arcuate Nucleus

The arcuate nucleus is generally conserved across vertebrate taxa in its neuroanatomy and neuropeptide expression. Gene expression of agouti-related protein (AGRP), neuropeptide Y (NPY), pro-opiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcript (CART) has been established in the arcuate nucleus of several bird species and co-localization demonstrated for AGRP and NPY. The proteins encoded by these genes exert comparable effects on food intake in birds after central administration to those seen in other vertebrates, with AGRP and NPY being orexigenic and CART and α-melanocyte-stimulating hormone anorexigenic. We have focused on the measurement of arcuate nucleus AGRP and POMC expression in several avian models in relation to the regulation of energy balance, incubation, stress, and growth. AGRP mRNA and POMC mRNA are, respectively, up- and downregulated after energy deprivation and restriction. This suggests that coordinated changes in the activity of AGRP and POMC neurons help to drive the homeostatic response to replace depleted energy stores in birds as in other vertebrates. While AGRP and POMC expression are generally positively and negatively correlated with food intake, respectively, we review here situations in some avian models in which AGRP gene expression is dissociated from the level of food intake and may have an influence on growth independent of changes in appetite. This suggests the possibility that the central melanocortin system exerts more pleiotropic functions in birds. While the neuroanatomical arrangement of AGRP and POMC neurons and the sensitivity of their activity to nutritional state appear generally conserved with other vertebrates, detailed knowledge is lacking of the key nutritional feedback signals acting on the avian arcuate nucleus and there appear to be significant differences between birds and mammals. In particular, recently identified avian leptin genes show differences between bird species in their tissue expression patterns and appear less closely linked in their expression to nutritional state. It is presently uncertain how the regulation of the central melanocortin system in birds is brought about in the situation of the apparently reduced importance of leptin and ghrelin compared to mammals.

Effect of ghrelin and leptin receptors genes polymorphisms on production results and physicochemical characteristics of M. pectoralis superficialis in broiler chickens

Ghrelin and leptin and their receptors GHSR and LEPR regulate food intake, the processes in adipose tissue, and the body’s energy homeostasis in mammals. The aim of the present study was to determine the effect of GHSR/Csp6I and LEPR/Bsh1236I polymorphisms on the meat production parameters of broiler chickens reared to 42 days of age. In 318 fast-growing Hubbard Flex and Ross 308 chickens, g.3051C > T substitution at the GHSR locus and a GGTCAA deletion at positions g.3407_3409del and g.3411_3413del were identified. The use of restriction enzyme Bsh1236I showed the presence of two transitions g.352C > T and g.427G > A in LEPR locus. The chickens were classified into four GHSR/Csp6I and into five LEPR/Bsh1236I diplotypes. GHSR and LEPR polymorphisms were found to influence final bodyweight, daily gain, dressing percentage without giblets, proportion of giblets and the quality characteristics of M. pectoralis superficialis. GHSR/Csp6I and LEPR/Bsh1236I had an effect on pH24 h (P < 0.05) and lightness (L*) of M. pectoralis superficialis (P < 0.05), whereas GHSR/Csp6I influenced shear force (P < 0.05) and thawing loss (P < 0.05). GHSR/Csp6I and LEPR/Bsh1236I were found to have no effect on the abdominal fat content in chicken carcasses. Single nucleotide polymorphisms reported in the present study could be used in breeding programs as selection markers for growth traits and poultry meat quality.

Is really endogenous ghrelin a hunger signal in chickens? Association of GHSR SNPs with increase appetite, growth traits, expression and serum level of GHRL, and GH

Chicken growth hormone secretagogue receptor (GHSR) is a receptor for ghrelin (GHRL), a peptide hormone produced by chicken proventriculus, which stimulates growth hormone (GH) release and food intake. The purpose of this study was to search for single nucleotide polymorphisms (SNPs) in exon 2 of GHSR gene and to analyze their effect on the appetite, growth traits and expression levels of GHSR, GHRL, and GH genes as well as serum levels of GH and GHRL in Mandara chicken. Two adjacent SNPs, A239G and G244A, were detected in exon 2 of GHSR gene. G244A SNP was non-synonymous mutation and led to replacement of lysine amino acid (aa) by arginine aa, while A239G SNP was synonymous mutation. The combined genotypes of A239G and G244A SNPs produced three haplotypes; GG/GG, GG/AG, AG/AG, which associated significantly (P<0.05) with growth traits (body weight, average daily gain, shank length, keel length, chest circumference) at age from >4 to 16w. Chickens with the homozygous GG/GG haplotype showed higher growth performance than other chickens. The two SNPs were also correlated with mRNA levels of GHSR and GH (in pituitary gland), and GHRL (in proventriculus and hypothalamus) as well as with serum level of GH and GHRL. Also, chickens with GG/GG haplotype showed higher mRNA and serum levels. This is the first study to demonstrate that SNPs in GHSR can increase appetite, growth traits, expression and level of GHRL, suggesting a hunger signal role for endogenous GHRL.

The effect of peripheral administration of ghrelin on the performance of growing geese

Abstract. The purpose of the present study is to investigate the effect of intraperitoneal (IP) injection of ghrelin on goose performance. Hence, forty-eight 28-day-old geese were assigned into three treatments which lasted 40 days. The first intact group included no injection; that is, treatment 1 was characterized by G0; treatment 2, given to the second intact group, was characterized by G50, 50 ng kg−1 ghrelin body weight (BW); and treatment 3, given to the third intact group, was characterized by G100, 100 ng kg−1 ghrelin BW. Ghrelin was injected at the outset of the experimental rearing period (28-day-old birds). Blood samples were taken at two different times: (1) 12 h after the injection and (2) at the end of the rearing period. The effects of the injections were examined and evaluated during two rearing periods (28–48 days old and 48–68 days old). In the second treatment (G50), ghrelin injection caused an increase in the feed intake during the growing period but not the finishing period. Body weight gain and feed conversion ratio (FCR) of the growing period and finishing period did not change following any ghrelin treatments (P > 0.05). Injection of ghrelin at G100 increased breast muscle (pectoral) weight. The results of the present study indicate that ghrelin has a significant impact on feeding regulation and muscle growth at a certain period in geese. Nevertheless, it should be noted that ghrelin may have different effects on feeding of avian species.

Comparative analysis reveals loss of the appetite-regulating peptide hormone ghrelin in falcons

Ghrelin and leptin are key peripherally secreted appetite-regulating hormones in vertebrates. Here we consider the ghrelin gene (GHRL) of birds (class Aves), where it has been reported that ghrelin inhibits rather than augments feeding. Thirty-one bird species were compared, revealing that most species harbour a functional copy of GHRL and the coding region for its derived peptides ghrelin and obestatin. We provide evidence for loss of GHRL in saker and peregrine falcons, and this is likely to result from the insertion of an ERVK retrotransposon in intron 0. We hypothesise that the loss of anorexigenic ghrelin is a predatory adaptation that results in increased food-seeking behaviour and feeding in falcons.

Association of single nucleotide polymorphism of GHSR and TGFB2 genes with growth and body composition traits in sire and dam lines of a broiler chicken

Growth hormone secretagogue receptor (GHSR) modulates many physiological processes by binding to its ligand, as well as transforming growth factor-beta 2 (TGFB2) regulates cell growth and development in animals and, therefore, are candidate genes for performance in chickens. In the current study, single nucleotide polymorphisms of GHSR C3286 > T and TGFB2 T(-640) > C were genotyped in sire and dam lines of a broiler chicken to evaluate the association with the growth and body composition traits. Least squares means analysis showed that the GHSR C3286 > T SNP was significantly (P < 0.01) associated with growth (DFI and ADG) and body composition traits (AFW and %AFW). In addition, the TGFB2 T(-640) > C SNP was associated with ADG (P < 0.05) and DFI and body composition traits (DW, LBW, BAKWT, %BMW, %HNDWT and %CW) (P < 0.01). Significant associations of the single nucleotide polymorphisms (SNPs) on the traits reported in the present study might be the distinct usage of codons in avian, or relating to an enhancer element and modulating the expression of the gene in chicken. The data indicated that these SNPs could be valuable genetic elements for selection of chickens for better performance in the population.

Ghrelin receptors in non-Mammalian vertebrates

The growth hormone secretagogue-receptor (GHS-R) was discovered in humans and pigs in 1996. The endogenous ligand, ghrelin, was discovered 3 years later, in 1999, and our understanding of the physiological significance of the ghrelin system in vertebrates has grown steadily since then. Although the ghrelin system in non-mammalian vertebrates is a subject of great interest, protein sequence data for the receptor in non-mammalian vertebrates has been limited until recently, and related biological information has not been well organized. In this review, we summarize current information related to the ghrelin receptor in non-mammalian vertebrates.

Ghrelin effect on nutritional indices, midgut and fat body of Lymantria dispar L. (Lymantriidae)

Ghrelin is a 28-amino acid peptide that has significant effects on appetite and growth in humans and animals. The aim of this study was to examine 4th instar larvae of the pest insect Lymantria dispar L. after ghrelin treatment. Parameters included changes in nutritional indices (efficiency of conversion of ingested food, efficiency of conversion of digested food, approximate digestibility); midgut and fat body mass; total proteases, trypsin and leucine aminopeptidase activities in the midgut; number, height and width of columnar and goblet cells and their nuclei in the midgut epithelium and detection of ghrelin-like immunoreactivity in the midgut tissue. Four subpicomolar injections of ghrelin (0.3pmol) or physiological saline (control) were applied every 24h. The nutritional indices were higher in the ghrelin treated than in the control group. Ghrelin treatment was also associated with elevation of midgut mass, induced digestive enzyme activities, increased fat body mass and morphometric changes in columnar and goblet cells. This is the first report of the presence of ghrelin-like hormone in endocrine cells of an insect midgut. Such information provides additional evidence for application of this relatively simple model system in the future studies of the mechanisms underlying of digestion and energy balance in more complex organisms.

Recent advances in the phylogenetic study of ghrelin

To understand fully the biology of ghrelin, it is important to know the evolutionary history of ghrelin and its receptor. Phylogenetic and comparative genomic studies of mammalian and non-mammalian vertebrates are a useful approach to that end. Ghrelin is a hormone that has apparently evaded natural selection during a long evolutionary history. Surely ghrelin plays crucial physiological roles in living animals. Phylogenetic studies reveal the nature and evolutionary history of this important signaling system.