MinireviewMolecular evolution of the neurohypophysial hormone precursors in mammals: Comparative genomics reveals novel mammalian oxytocin and vasopressin analogues
Highlights
► Mammalian neurohypophysial hormone precursors were surveyed using genomic data. ► Leu3 oxytocin (OT) found in armadillo and Thr4 arginine vasopressin (AVP) in tenrec. ► These two species also have additional heavily substituted AVP genes/pseudogenes. ► OT and VP gene sequences show evolutionary conservation and often gene conversion. ► Arrangement of monotreme neurohypophysial hormone genes resembles that in eutherians.
Introduction
The comparative endocrinology of the neurohypophysial hormones has been studied intensively, and a variety of related peptides has been revealed in non-mammalian vertebrates [1], [15], [29], [30]. However, in eutherians, until recently, just vasopressin (VP) and oxytocin (OT) had been reported, the only variation being that in pig and some related cetartiodactyls Lys8VP (LVP) replaces Arg8VP (AVP) [10]. Recently it has been shown that Pro8OT replaces oxytocin in many New World monkeys and in the tree shrew (Tupaia belangeri) [20]. In all cases studied genes encoding the precursors of these peptides in Eutheria are adjacent on the same chromosome (chromosome 20 in man), arranged tail-to-tail (i.e. transcribed from opposite DNA strands) [24] and separated by ∼10 kilobases (kb) in most species including rat [24], [27], but less in mouse and some other rodents (∼3.5 kb) [13].
Marsupials show a more complex picture, with at least three neurohypophysial hormones; species studied include the South American opossum (Didelphis marsupialis; OT, LVP, AVP [5]), the Eastern Grey Kangaroo (Macropus giganteus; mesotocin (MT), LVP and phenypressin [7]) and grey short-tailed opossum (Monodelphis domestica; MT, LVP and AVP [11]). In Monodelphis these are encoded by four genes closely linked on chromosome 5 (arranged: LVP-MT-AVP-MT), transcribed from the same DNA strand [11]. The monotreme platypus (Ornithorhynchus anatinus) possesses OT and AVP, like most eutherians [6]. Thus in all mammals for which information is available (and indeed for most lower vertebrates) the genes encoding OT and VP or equivalent peptides are closely linked; this indicates that the two (or more) genes arose by local tandem duplication early in vertebrate evolution rather than by whole genome duplication.
Neurohypophysial peptides are synthesized as precursors, in which a signal peptide is followed by the neurohypophysial peptide and then neurophysin (Np), and in some cases, including VP precursor, an additional peptide, copeptin. Np acts as a binding protein for the conjunct neurohypophysial peptide, though the presence in invertebrates of Np-like proteins that are not associated with VP/OT-like peptides suggest that it may have additional functions [9]. The function of copeptin is unclear. Basic residues between various components of the precursor serve as signals to allow efficient and specific cleavage, and a Gly residue following the neurohypophysial peptide allows C-terminal amidation. The genes for OT and VP each comprise three exons, the first encoding the signal peptide, neurohypophysial peptide and the start of Np, the second the bulk of Np and the third the C-terminal end of Np and (in the case of VP precursor) copeptin [17], [19], [26].
The availability of genomic sequence data for a large number of mammals provides a basis for more-extensive comparative studies than previously carried out. Here these data have been surveyed to investigate the molecular evolution of the neurohypophysial hormone precursors in mammals. Specific questions addressed include: (1) Does a survey of all the eutherian species for which genomic data are available, indicate the existence of novel neurohypophysial hormones? (2) To what extent do the various functional regions of the precursors evolve at varying rates? (3) How widely does gene conversion between the second exon of VP and OT precursor genes occur? (4) How variable is the overall organization of mammalian neurohypophysial hormone genes?
Section snippets
Mammalian neurohypophysial hormone precursor gene sequences
Complete or partial sequences for the genes encoding the OT and VP precursors were derived from genomic data available for 37 eutherian species, as summarized in Table 1. For most species just one gene was detected for the oxytocin precursor and one for the vasopressin precursor, encoding normal OT or AVP (LVP in pig) as expected. However, for marmoset (Callithrix jacchus), tree shrew, tenrec (Echinops telfairi) and armadillo (Dasypus novemcinctus), evidence for modified forms of OT or VP was
Marmoset and tree shrew
In these two species evidence for an unusual OT-like peptide was obtained – Pro8OT, confirming the recent report of Lee et al. [20]. Pro rather than Leu at position 8 would be expected to markedly change the conformation and properties of the peptide, given the difference in side-chain size and the conformational constraints associated with a prolyl residue.
Tenrec
The genomic data for the neurohypophysial hormone precursors in the Madagascan hedgehog/tenrec (Echinops telfairi) indicate a complex
Evolution of neurohypophysial hormone precursors in eutherians
Fig. 1a shows an alignment of the available eutherian OT precursors, derived by conceptual translation of the genomic sequences. Sequences are generally well conserved. Notable is the complete conservation of the sequence following OT (GKR), specifying cleavage and amidation [28], [36].
Fig. 1b shows an alignment of the available eutherian VP precursors. Again sequences are conserved, and the GKR sequence following VP is modified in only one case (armadillo), where Arg replaces Lys, a change
Gene conversion
Ruppert et al. [23] reported that the bovine OT and VP genes showed almost complete identity for most of exon 2 (encoding the bulk of Np) and the adjacent part of intron 1, and concluded that this reflected recent gene conversion. They suggested that an equivalent gene conversion had occurred in rat, but at an earlier date, since the converted sequences were less similar. Gwee et al. [11] observed that the second exons of vasotocin (VT) and MT genes in the coelacanth are almost identical, but
Organization of neurohypophysial hormone genes
Gwee et al. [11] described the organization of the neurohypophysial hormone genes in the opossum (Monodelphis domestica), showing that it differs markedly from that in eutherians and resembles that in many non-mammalian vertebrates in that all four genes are arranged tail-to-head, i.e. transcribed from the same DNA strand (Fig. 2; see [11], [12] for a summary of the organization of neurohypophysial hormone genes in lower vertebrates). This was interpreted as indicating that a genomic
Conclusions
The genomic resources currently available provide a rich source of comparative information, especially for eutherian mammals. The data are in many cases rather incomplete, but the survey carried out here has provided a number of clear conclusions, together with pointers for further work.
1. It is clear that the diversity of neurohypophysial hormones seen in Eutheria is greater than has previously been appreciated, with novel peptides having arisen on at least five independent occasions (Fig. 2).
References (36)
Neurohypophysial peptide systems: processing machinery, hydroosmotic regulation, adaptation and evolution
Regul. Pept.
(1993)- et al.
Basic local alignment search tool
J. Mol. Biol.
(1990) - et al.
The neurohypophysial hormones of the egg-laying mammals: identification of arginine vasopressin in the platypus (Ornithorhynchus anatinus)
Biochem. Biophys. Res. Commun.
(1985) - et al.
Identification of mesotocin, lysine vasopressin, and phenypressin in the eastern gray kangaroo (Macropus giganteus)
Gen. Comp. Endocrinol.
(1983) NG peptides: a novel family of neurophysin-associated neuropeptides
Gene
(2010)Genetic distribution of vasopressins in the peccary (Tayassu angulatus) and warthog (Phacochoerus aethiopicus)
Gen. Comp. Endocrinol.
(1969)- et al.
Structure of mouse vasopressin and oxytocin genes
Mol. Brain Res.
(1990) - et al.
CLUSTAL: a package for performing multiple sequence alignment on a microcomputer
Gene
(1988) Neuropeptide families: evolutionary perspectives
Regul. Pept.
(1998)- et al.
Mammalian phylogenomics comes of age
Trends Genet.
(2004)