The avian proghrelin system

Abstract

To understand how the proghrelin system functions in regulating growth hormone release and food intake as well as defining its pleiotropic roles in such diverse physiological processes as energy homeostasis, gastrointestinal tract function and reproduction require detailed knowledge of the structure and function of the components that comprise this system. These include the preproghrelin gene that encodes the proghrelin precursor protein from which two peptide hormones, ghrelin and obestatin, are derived and the cognate receptors that bind proghrelin-derived peptides to mediate their physiological actions in different tissues. Also key to the functioning of this system is the posttranslational processing of the proghrelin precursor protein and the individual peptides derived from it. While this system has been intensively studied in a variety of animal species and humans over the last decade, there has been considerably less investigation of the avian proghrelin system which exhibits some unique differences compared to mammals. This review summarizes what is currently known about the proghrelin system in birds and offers new insights into the nature and function of this important endocrine system. Such information facilitates cross-species comparisons and contributes to our understanding of the evolution of the proghrelin system.

Figure 1

Genomic structure and organization of a prototypical avian preproghrelin gene (based on the chicken gene). The gene exhibits five exons (four coding exons shown in black) and four introns with the positions of the proximal promoter (PR) and the start (ATG) and stop (TGA) codons indicated. The portions of the preproghrelin precursor protein encoded by each exon are indicated in boxes. Also indicated are two sequence features detected in different avian genes. These include an 8 bp insertion/deletion (INDEL) located in exon 1 and exon extensions (5′ and 3′) of exons 1 and 2 detected only in the turkey.

Figure 2

Expression of preproghrelin mRNA (a) in proventriculus and plasma total (acylated + des-acyl) ghrelin levels (b) in two groups of neonatal broiler chicks from hatch to 8 days posthatch. Feed was provided immediately after hatching (Fed) or was withheld for the first 48 hours after hatch (delayed feeding, DF). Values represent the mean ± SEM (n = 6).

Figure 3

Amino acid comparisons of preproghrelin precursor proteins for different avian species. The locations of the mature ghrelin peptide and the putative obestatin peptide are indicated by larger bold typeface. The C-terminal glutamic acid residue (E) conserved in avian obestatin peptides is shown by bold underline typeface. Also, basic amino acids delineating proteolytic cleavage sites are shown in bold italic typeface. *indicates amino acid identity across all species. Amino acid sequences shown for chicken (broiler), turkey, goose, duck, and quail preproghrelin precursor proteins were obtained from GenBank accession nos. BAC24980, AAP93133, AAQ56122, AAQ56123, and BAE54265, respectively.

Figure 4

Comparison of amino acid sequence for chicken (GenBank accession no. NP_001001131) and human (GenBank accession no. NP_001128413) perproghrelin precursor proteins and the major peptides processed from the precursor including a signal peptide, the mature ghrelin peptide, C-terminal peptide, and obestatin (shown in gray box). *indicates amino acid identity. The C-terminal glycine (G) residue used to amidate the human obestatin peptide is indicated. Basic amino acids delineating proteolytic cleavage sites are shown in bold italic typeface. The Ser³ acylation site within the “active core” of the mature ghrelin peptide is indicated. The location of a single nucleotide polymorphism (SNP) resulting in an amino acid change (glutamine/arginine) located in the C-terminal peptide of the chicken precursor is shown.

Figure 5

Genomic structure, organization, and expression of the chicken GHS-R gene consisting of two exons separated by a single intron. Different alternative splicing mechanisms are used to produce three transcript variants (GHS-R1a, GHS-R1aV, and GHS-Rtv) that encode proteins of 347, 331, and 220 amino acids, respectively. Sequence encoding a major portion of transmembrane domain 6 (TMD 6), located at the 5′-end of exon 2, is shown and is spliced out of the GHS-R1aV transcript. The GHS-Rtv variant is formed by splicing two portions of intron sequence, one of which contains an alternative stop codon (TAA_alt) which is predicted to produce a receptor protein truncated after TMD 5. Expression of each of the GHS-R mRNA transcript variants in different tissues obtained from 3-week-old broiler chickens is also shown.