Evolution of growth hormone in primates: the GH gene clusters of the New World monkeys marmoset (Callithrix jacchus) and white-fronted capuchin (Cebus albifrons)

Gene Content and Coding Diversity of the Growth Hormone Loci of Apes

The growth hormone (GH) locus has experienced a dramatic evolution in primates, becoming multigenic and diverse in anthropoids. Despite sequence information from a vast number of primate species, it has remained unclear how the multigene family was favored. We compared the structure and composition of apes' GH loci as a prerequisite to understanding their origin and possible evolutionary role. These thorough analyses of the GH loci of the chimpanzee, gorilla, and orangutan were done by resorting to previously sequenced bacterial artificial chromosomes (BACs) harboring them, as well as to their respective genome projects data available in GenBank. The GH loci of modern man, Neanderthal, gibbon, and wild boar were retrieved from GenBank. Coding regions, regulatory elements, and repetitive sequences were identified and compared among species. The GH loci of all the analyzed species are flanked by the genes CD79B (5') and ICAM-1 (3'). In man, Neanderthal, and chimpanzee, the loci were integrated by five almost indistinguishable genes; however, in the former two, they rendered three different hormones, and in the latter, four different proteins were derived. Gorilla exhibited six genes, gibbon seven, and orangutan four. The sequences of the proximal promoters, enhancers, P-elements, and a locus control region (LCR) were highly conserved. The locus evolution might have implicated duplications of the ancestral pituitary gene (GH-N) and subsequent diversification of the copies, leading to the placental single GH-V gene and the multiple CSH genes.

Evolution of Placental Hormones: Implications for Animal Models

Human placenta secretes a variety of hormones, some of them in large amounts. Their effects on maternal physiology, including the immune system, are poorly understood. Not one of the protein hormones specific to human placenta occurs outside primates. Instead, laboratory and domesticated species have their own sets of placental hormones. There are nonetheless several examples of convergent evolution. Thus, horse and human have chorionic gonadotrophins with similar functions whilst pregnancy-specific glycoproteins have evolved in primates, rodents, horses, and some bats, perhaps to support invasive placentation. Placental lactogens occur in rodents and ruminants as well as primates though evolved through duplication of different genes and with functions that only partially overlap. There are also placental hormones, such as the pregnancy-associated glycoproteins of ruminants, that have no equivalent in human gestation. This review focusses on the evolution of placental hormones involved in recognition and maintenance of pregnancy, in maternal adaptations to pregnancy and lactation, and in facilitating immune tolerance of the fetal semiallograft. The contention is that knowledge gained from laboratory and domesticated mammals can translate to a better understanding of human placental endocrinology, but only if viewed in an evolutionary context.

Unique Aspects of Human Placentation

Human placentation differs from that of other mammals. A suite of characteristics is shared with haplorrhine primates, including early development of the embryonic membranes and placental hormones such as chorionic gonadotrophin and placental lactogen. A comparable architecture of the intervillous space is found only in Old World monkeys and apes. The routes of trophoblast invasion and the precise role of extravillous trophoblast in uterine artery transformation is similar in chimpanzee and gorilla. Extended parental care is shared with the great apes, and though human babies are rather helpless at birth, they are well developed (precocial) in other respects. Primates and rodents last shared a common ancestor in the Cretaceous period, and their placentation has evolved independently for some 80 million years. This is reflected in many aspects of their placentation. Some apparent resemblances such as interstitial implantation and placental lactogens are the result of convergent evolution. For rodent models such as the mouse, the differences are compounded by short gestations leading to the delivery of poorly developed (altricial) young.

Do some viruses use growth hormone, prolactin and their receptors to facilitate entry into cells?: Episodic evolution of hormones and receptors suggests host-virus arms races; related placental lactogens may provide protective viral decoys

The molecular evolution of pituitary growth hormone and prolactin in mammals shows two unusual features: episodes of markedly accelerated evolution and, in some species, complex families of related proteins expressed in placenta and resulting from multiple gene duplications. Explanations of these phenomena in terms of physiological adaptations seem unconvincing. Here, I propose an alternative explanation, namely that these evolutionary features reflect the use of the hormones (and their receptors) as viral receptors. Episodes of rapid evolution can then be explained as due to "arms races" in which changes in the hormone lead to reduced interaction with the virus, and subsequent changes in the virus counteract this. Placental paralogues of the hormones could provide decoys that bind viruses, and protect the foetus against infection. The hypothesis implies that the extensive changes introduced into growth hormone, prolactin and their receptors during the course of mammalian evolution reflect viral interactions, not endocrine adaptations.

Immune Regulation in Eutherian Pregnancy: Live Birth Coevolved with Novel Immune Genes and Gene Regulation

Novel regulatory elements that enabled expression of pre-existing immune genes in reproductive tissues and novel immune genes with pregnancy-specific roles in eutherians have shaped the evolution of mammalian pregnancy by facilitating the emergence of novel mechanisms for immune regulation over its course. Trade-offs arising from conflicting fitness effects on reproduction and host defenses have further influenced the patterns of genetic variation of these genes. These three mechanisms (novel regulatory elements, novel immune genes, and trade-offs) played a pivotal role in refining the regulation of maternal immune systems during pregnancy in eutherians, likely facilitating the establishment of prolonged direct maternal-fetal contact in eutherians without causing immunological rejection of the genetically distinct fetus.

Evolution of the growth hormone, prolactin, prolactin 2 and somatolactin family

Growth hormone (GH), prolactin (PRL), prolactin 2 (PRL2) and somatolactin (SL) belong to the same hormone family and have a wide repertoire of effects including development, osmoregulation, metabolism and stimulation of growth. Both the hormone and the receptor family have been proposed to have expanded by gene duplications in early vertebrate evolution. A key question is how hormone-receptor preferences have arisen among the duplicates. The first step to address this is to determine the time window for these duplications. Specifically, we aimed to see if duplications resulted from the two basal vertebrate tetraploidizations (1R and 2R). GH family genes from a broad range of vertebrate genomes were investigated using a combination of sequence-based phylogenetic analyses and comparisons of synteny. We conclude that the PRL and PRL2 genes arose from a common ancestor in 1R/2R, as shown by neighboring gene families. No other gene duplicates were preserved from these tetraploidization events. The ancestral genes that would give rise to GH and PRL/PRL2 arose from an earlier duplication; most likely a local gene duplication as they are syntenic in several species. Likewise, some evidence suggests that SL arose from a local duplication of an ancestral GH/SL gene in the same time window, explaining the lack of similarity in chromosomal neighbors to GH, PRL or PRL2. Thus, the basic triplet of ancestral GH, PRL/PRL2 and SL genes appear to be unexpectedly ancient. Following 1R/2R, only SL was duplicated in the teleost-specific tetraploidization 3R, resulting in SLa and SLb. These time windows contrast with our recent report that the corresponding receptor genes GHR and PRLR arose through a local duplication in jawed vertebrates and that both receptor genes duplicated further in 3R, which reveals a surprising asynchrony in hormone and receptor gene duplications.

Mice producing reduced levels of insulin-like growth factor type 1 display an increase in maximum, but not mean, life span

Reduced signaling through the IGF type 1 (IGF-1) receptor increases life span in multiple invertebrate organisms. Studies on mammalian longevity suggest that reducing levels of IGF-1 may also increase life span. However, the data are conflicting and complicated by the physiology of the mammalian neuroendocrine system. We have performed life-span analysis on mice homozygous for an insertion in the Igf1 gene. These mice produce reduced levels of IGF-1 and display a phenotype consistent with a significant decrease in IGF-1. Life-span analysis was carried out at three independent locations. Although the life-span data varied between sites, the maximum life span of the IGF-1-deficient mice was significantly increased and age-specific mortality rates were reduced in the IGF-1-deficient mice; however, mean life span did not differ except at one site, where mean life span was increased in female IGF-1-deficient animals. Early life mortality was noted in one cohort of IGF-1-deficient mice. The results are consistent with a significant role for IGF-1 in the modulation of life span but contrast with the published life-span data for the hypopituitary Ames and Snell dwarf mice and growth hormone receptor null mice, indicating that a reduction in IGF-1 alone is insufficient to increase both mean and maximal life span in mice.

Evolution of Placental Function in Mammals: The Molecular Basis of Gas and Nutrient Transfer, Hormone Secretion, and Immune Responses

Placenta has a wide range of functions. Some are supported by novel genes that have evolved following gene duplication events while others require acquisition of gene expression by the trophoblast. Although not expressed in the placenta, high-affinity fetal hemoglobins play a key role in placental gas exchange. They evolved following duplications within the beta-globin gene family with convergent evolution occurring in ruminants and primates. In primates there was also an interesting rearrangement of a cassette of genes in relation to an upstream locus control region. Substrate transfer from mother to fetus is maintained by expression of classic sugar and amino acid transporters at the trophoblast microvillous and basal membranes. In contrast, placental peptide hormones have arisen largely by gene duplication, yielding for example chorionic gonadotropins from the luteinizing hormone gene and placental lactogens from the growth hormone and prolactin genes. There has been a remarkable degree of convergent evolution with placental lactogens emerging separately in the ruminant, rodent, and primate lineages and chorionic gonadotropins evolving separately in equids and higher primates. Finally, coevolution in the primate lineage of killer immunoglobulin-like receptors and human leukocyte antigens can be linked to the deep invasion of the uterus by trophoblast that is a characteristic feature of human placentation.

Review: Toward an integrated evolutionary understanding of the mammalian placenta

The placenta is fundamentally important for the success of pregnancy. Disruptions outside the normal range for placental function can result in pregnancy failure and other complications. The anatomy of the placenta varies greatly across mammals, as do key parameters in pregnancy such as neonatal body mass, length of gestation and number of offspring per pregnancy. An accurate understanding of the evolution of the mammalian placenta will require at minimum the integration of anatomical, developmental, physiological, genetic, and epigenetic data. Currently available data suggest that the placenta is a dynamic organ that has evolved rapidly in a lineage specific manner. Examination of the placenta from the perspective of human evolution shows that many anatomical features of the human placenta are relatively conserved. Despite the anatomical conservation of the human placenta there are many recently evolved placenta-specific genes (e.g. CGB, LGALS13, GH2) that are important in the development and function of the human placenta. Other mammalian genomes have also evolved specific suites of placenta-expressed genes. For example, rodents have undergone expansions of the cathepsin and prolactin families, and artiodactyls have expanded their suite of pregnancy-associated glycoproteins. In addition to lineage specific birth and death of gene family members, the pattern of imprinted loci varies greatly among species. Taken together, these studies suggest that a strategy reliant upon the sampling of placentally expressed and imprinted genes from a phylogenetically diverse range of species is appropriate for unraveling the conserved and derived aspects of placental biology.

Extrapituitary growth hormone

Pituitary somatotrophs secrete growth hormone (GH) into the bloodstream, to act as a hormone at receptor sites in most, if not all, tissues. These endocrine actions of circulating GH are abolished after pituitary ablation or hypophysectomy, indicating its pituitary source. GH gene expression is, however, not confined to the pituitary gland, as it occurs in neural, immune, reproductive, alimentary, and respiratory tissues and in the integumentary, muscular, skeletal, and cardiovascular systems, in which GH may act locally rather than as an endocrine. These actions are likely to be involved in the proliferation and differentiation of cells and tissues prior to the ontogeny of the pituitary gland. They are also likely to complement the endocrine actions of GH and are likely to maintain them after pituitary senescence and the somatopause. Autocrine or paracrine actions of GH are, however, sometimes mediated through different signaling mechanisms to those mediating its endocrine actions and these may promote oncogenesis. Extrapituitary GH may thus be of physiological and pathophysiological significance.

Growth hormone-related genes from baboon (Papio hamadryas): Characterization, placental expression and evolutionary aspects

Pregnancy is a complex physiological condition, and the growth hormone (GH)-related hormones produced in the placenta, which emerged during the evolution of primates, are thought to play an important metabolic role in pregnancy that is not yet fully understood. The aim of this study was to identify the genes and transcription products of the GH family in baboon (Papio hamadryas) and to assess these in relation to the evolution of this gene family. GH-related transcripts were amplified using total RNA from placental tissue, by reverse transcription coupled to polymerase chain reaction (RT-PCR). Three different GH-related transcripts were identified in baboon placental tissue, with two encoding chorionic somatomammotropins (CSH) and one the placental variant of GH (GH-2). The CSH transcripts showed some minor allelic variation, and a splice variant of CSH-C that retains its in-frame third intron. Gene sequences for GH-1 (probably representing the GH gene expressed primarily in the pituitary gland), GH-2 and the two CSHs were identified in the baboon genomic database, together with a CSH-related pseudogene. Phylogenetic analysis of the baboon GH-related sequences, together with those of a related Old World monkey, macaque, and ape outgroup (human), showed the equivalence of the genes in baboon and macaque, and revealed evidence for several episodes of rapid adaptive evolution. Many of the substitutions seen during the evolution of these placental proteins have occurred in the receptor-binding sites, especially site 2, contrasting with the strong conservation of the hydrophobic core.

Ancient origin of placental expression in the growth hormone genes of anthropoid primates

In anthropoid primates, growth hormone (GH) genes have undergone at least 2 independent locus expansions, one in platyrrhines (New World monkeys) and another in catarrhines (Old World monkeys and apes). In catarrhines, the GH cluster has a pituitary-expressed gene called GH1; the remaining GH genes include placental GHs and placental lactogens. Here, we provide cDNA sequence evidence that the platyrrhine GH cluster also includes at least 3 placenta expressed genes and phylogenetic evidence that placenta expressed anthropoid GH genes have undergone strong adaptive evolution, whereas pituitary-expressed GH genes have faced strict functional constraint. Our phylogenetic evidence also points to lineage-specific gene gain and loss in early placental mammalian evolution, with at least three copies of the GH gene present at the time of the last common ancestor (LCA) of primates, rodents, and laurasiatherians. Anthropoid primates and laurasiatherians share gene descendants of one of these three copies, whereas rodents and strepsirrhine primates each maintain a separate copy. Eight of the amino-acid replacements that occurred on the lineage leading to the LCA of extant anthropoids have been implicated in GH signaling at the maternal-fetal interface. Thus, placental expression of GH may have preceded the separate series of GH gene duplications that occurred in catarrhines and platyrrhines (i.e., the roles played by placenta-expressed GHs in human pregnancy may have a longer evolutionary history than previously appreciated).

High divergence in primate-specific duplicated regions: human and chimpanzee chorionic gonadotropin beta genes

Background

Low nucleotide divergence between human and chimpanzee does not sufficiently explain the species-specific morphological, physiological and behavioral traits. As gene duplication is a major prerequisite for the emergence of new genes and novel biological processes, comparative studies of human and chimpanzee duplicated genes may assist in understanding the mechanisms behind primate evolution. We addressed the divergence between human and chimpanzee duplicated genomic regions by using Luteinizing Hormone Beta (LHB)/Chorionic Gonadotropin Beta (CGB) gene cluster as a model. The placental CGB genes that are essential for implantation have evolved from an ancestral pituitary LHB gene by duplications in the primate lineage.

Results

We shotgun sequenced and compared the human (45,165 bp) and chimpanzee (39,876 bp) LHB/CGB regions and hereby present evidence for structural variation resulting in discordant number of CGB genes (6 in human, 5 in chimpanzee). The scenario of species-specific parallel duplications was supported (i) as the most parsimonious solution requiring the least rearrangement events to explain the interspecies structural differences; (ii) by the phylogenetic trees constructed with fragments of intergenic regions; (iii) by the sequence similarity calculations. Across the orthologous regions of LHB/CGB cluster, substitutions and indels contributed approximately equally to the interspecies divergence and the distribution of nucleotide identity was correlated with the regional repeat content. Intraspecies gene conversion may have shaped the LHB/CGB gene cluster. The substitution divergence (1.8–2.59%) exceeded two-three fold the estimates for single-copy loci and the fraction of transversional mutations was increased compared to the unique sequences (43% versus ~30%). Despite the high sequence identity among LHB/CGB genes, there are signs of functional differentiation among the gene copies. Estimates for d_n/d_s rate ratio suggested a purifying selection on LHB and CGB8, and a positive evolution of CGB1.

Conclusion

If generalized, our data suggests that in addition to species-specific deletions and duplications, parallel duplication events may have contributed to genetic differences separating humans from their closest relatives. Compared to unique genomic segments, duplicated regions are characterized by high divergence promoted by intraspecies gene conversion and species-specific chromosomal rearrangements, including the alterations in gene copy number.

Mammalian genome projects reveal new growth hormone (GH) sequences. Characterization of the GH-encoding genes of armadillo (Dasypus novemcinctus), hedgehog (Erinaceus europaeus), bat (Myotis lucifugus), hyrax (Procavia capensis), shrew (Sorex araneus), ground squirrel (Spermophilus tridecemlineatus), elephant (Loxodonta africana), cat (Felis catus) and opossum (Monodelphis domestica)

Mammalian growth hormone (GH) sequences have been shown previously to display episodic evolution: the sequence is generally strongly conserved but on at least two occasions during mammalian evolution (on lineages leading to higher primates and ruminants) bursts of rapid evolution occurred. However, the number of mammalian orders studied previously has been relatively limited, and the availability of sequence data via mammalian genome projects provides the potential for extending the range of GH gene sequences examined. Complete or nearly complete GH gene sequences for six mammalian species for which no data were previously available have been extracted from the genome databases-Dasypus novemcinctus (nine-banded armadillo), Erinaceus europaeus (western European hedgehog), Myotis lucifugus (little brown bat), Procavia capensis (cape rock hyrax), Sorex araneus (European shrew), Spermophilus tridecemlineatus (13-lined ground squirrel). In addition incomplete data for several other species have been extended. Examination of the data in detail and comparison with previously available sequences has allowed assessment of the reliability of deduced sequences. Several of the new sequences differ substantially from the consensus sequence previously determined for eutherian GHs, indicating greater variability than previously recognised, and confirming the episodic pattern of evolution. The episodic pattern is not seen for signal sequences, 5' upstream sequence or synonymous substitutions-it is specific to the mature protein sequence, suggesting that it relates to the hormonal function. The substitutions accumulated during the course of GH evolution have occurred mainly on the side of the hormone facing away from the receptor, in a non-random fashion, and it is suggested that this may reflect interaction of the receptor-bound hormone with other proteins or small ligands.