Occurrence of two functionally distinct proopiomelanocortin genes in all modern lampreys

Organization of the corticotropin-releasing hormone and corticotropin-releasing hormone-binding protein systems in the central nervous system of the sea lamprey Petromyzon marinus

The expression of the corticotropin-releasing hormone (PmCRH) and the CRH-binding protein (PmCRHBP) mRNAs was studied by in situ hybridization in the brain of prolarvae, larvae, and adults of the sea lamprey Petromyzon marinus. We also generated an antibody against the PmCRH mature peptide to study the distribution of PmCRH-immunoreactive cells and fibers. PmCRH immunohistochemistry was combined with antityrosine hydroxylase immunohistochemistry, PmCRHBP in situ hybridization, or neurobiotin transport from the spinal cord. The most numerous PmCRH-expressing cells were observed in the magnocellular preoptic nucleus-paraventricular nucleus and in the superior and medial rhombencephalic reticular formation. PmCRH expression was more extended in adults than in larvae, and some cell populations were mainly (olfactory bulb) or only (striatum, ventral hypothalamus, prethalamus) observed in adults. The preopto-paraventricular fibers form conspicuous tracts coursing toward the neurohypophysis, but many immunoreactive fibers were also observed coursing in many other brain regions. Brain descending fibers in the spinal cord mainly come from cells located in the isthmus and in the medial rhombencephalic reticular nucleus. The distribution of PmCRHBP-expressing neurons was different from that of PmCRH cells, with cells mainly present in the septum, striatum, preoptic region, tuberal hypothalamus, pretectum, pineal complex, isthmus, reticular formation, and spinal cord. Again, expression in adults was more extended than in larvae. PmCRH- and PmCRHBP-expressing cells are different, excluding colocalization of these substances in the same neuron. Present findings reveal a complex CRH/CRHBP system in the brain of the oldest extant vertebrate group, the agnathans, which shows similarities but important divergences with that of mammals.

Genetic and functional analysis of a Pacific hagfish opioid system

The actions of endogenous opioids and nociceptin/orphanin FQ are mediated by four homologous G protein-coupled receptors that constitute the opioid receptor family. However, little is known about opioid systems in cyclostomes (living jawless fish) and how opioid systems might have evolved from invertebrates. Here, we leveraged de novo transcriptome and low-coverage whole-genome assembly in the Pacific hagfish (Eptatretus stoutii) to identify and characterize the first full-length coding sequence for a functional opioid receptor in a cyclostome. Additionally, we define two novel endogenous opioid precursors in this species that predict several novel opioid peptides. Bioinformatic analysis shows no closely related opioid receptor genes in invertebrates with regard either to the genomic organization or to conserved opioid receptor-specific sequences that are common in all vertebrates. Furthermore, no proteins analogous to vertebrate opioid precursors could be identified by genomic searches despite previous claims of protein or RNA-derived sequences in several invertebrate species. The presence of an expressed orthologous receptor and opioid precursors in the Pacific hagfish confirms that a functional opioid system was likely present in the common ancestor of all extant vertebrates some 550 million years ago, earlier than all previous authenticated accounts. We discuss the premise that the cyclostome and vertebrate opioid systems evolved from invertebrate systems concerned with antimicrobial defense and speculate that the high concentrations of opioid precursors in tissues such as the testes, gut, and activated immune cells are key remnants of this evolutionary role.

Ancient fishes and the functional evolution of the corticosteroid stress response in vertebrates

The neuroendocrine mechanism underlying stress responses in vertebrates is hypothesized to be highly conserved and evolutionarily ancient. Indeed, elements of this mechanism, from the brain to steroidogenic tissue, are present in all vertebrate groups; yet, evidence of the function and even identity of some elements of the hypothalamus-pituitary-adrenal/interrenal (HPA/I) axis is equivocal among the most basal vertebrates. The purpose of this review is to discuss the functional evolution of the HPA/I axis in vertebrates with a focus on our understanding of this neuroendocrine mechanism in the most ancient vertebrates: the agnathan (i.e., hagfish and lamprey) and chondrichthyan fishes (i.e., sharks, rays, and chimeras). A review of the current literature presents evidence of a conserved HPA/I axis in jawed vertebrates (i.e., gnathostomes); yet, available data in jawless (i.e., agnathan) and chondrichthyan fishes are limited. Neuroendocrine regulation of corticosteroidogenesis in agnathans and chondrichthyans appears to function through similar pathways as in bony fishes and tetrapods; however, key elements have yet to be identified and the involvement of melanotropins and gonadotropin-releasing hormone in the stress axis in these ancient fishes warrants further investigation. Further, the identities of physiological glucocorticoids are uncertain in hagfishes, chondrichthyans, and even coelacanths. Resolving these and other knowledge gaps in the stress response of ancient fishes will be significant for advancing knowledge of the evolutionary origins of the vertebrate stress response.

Molecular cytogenetic characterization of chromosome site-specific repetitive sequences in the Arctic lamprey (Lethenteron camtschaticum, Petromyzontidae)

All extant lamprey karyotypes are characterized by almost all dot-shaped microchromosomes. To understand the molecular basis of chromosome structure in lampreys, we performed chromosome C-banding and silver staining and chromosome mapping of the 18S–28S and 5S ribosomal RNA (rRNA) genes and telomeric TTAGGG repeats in the Arctic lamprey (Lethenteron camtschaticum). In addition, we cloned chromosome site-specific repetitive DNA sequences and characterized them by nucleotide sequencing, chromosome in situ hybridization, and filter hybridization. Three types of repetitive sequences were detected; a 200-bp AT-rich repetitive sequence, LCA-EcoRIa that co-localized with the 18S–28S rRNA gene clusters of 3 chromosomal pairs; a 364-bp AT-rich LCA-EcoRIb sequence that showed homology to the EcoRI sequence family from the sea lamprey (Petromyzon marinus), which contains short repeats as centromeric motifs; and a GC-rich 702-bp LCA-ApaI sequence that was distributed on nearly all chromosomes and showed significant homology with the integrase-coding region of a Ty3/Gypsy family long terminal repeat (LTR) retrotransposon. All three repetitive sequences are highly conserved within the Petromyzontidae or within Petromyzontidae and Mordaciidae. Molecular cytogenetic characterization of these site-specific repeats showed that they may be correlated with programed genome rearrangement (LCA-EcoRIa), centromere structure and function (LCA-EcoRIb), and site-specific amplification of LTR retroelements through homogenization between non-homologous chromosomes (LCA-ApaI).

Posttranslational modifications of proopiomelanocortin in vertebrates and their biological significance

Proopiomelanocortin (POMC) is the precursor of several peptide hormones generated in the pituitary gland. After biosynthesis, POMC undergoes several posttranslational modifications, including proteolytic cleavage, acetylation, amidation, phosphorylation, glycosylation, and disulfide linkage formation, which generate mature POMC-derived peptides. Therefore, POMC is a useful model for the investigation of posttranslational modifications. These processes have been extensively investigated in mammals, primarily in rodents. In addition, over the last decade, much information has been obtained about the posttranslational processing of POMC in non-mammalian animals such as fish, amphibians, reptiles, and birds through sequencing and peptide identification by mass spectrometry. One POMC modification, acetylation, is known to modulate the biological activities of POMC-derived α-melanocyte-stimulating hormone (α-MSH) having an acetyl group at N-terminal through potentiation or inhibition. This bidirectional regulation depends on its intrinsic roles in the tissue or cell; for example, α-MSH, as well as desacetyl (Des-Ac)-α-MSH, stimulates pigment dispersion in the xanthophores of a flounder. In contrast, α-MSH does not stimulate pigment dispersion in the melanophores of the same species, whereas Des-Ac-α-MSH does. Regulation of pigment-dispersing activities may be associated with the subtle balance in the expression of receptor genes. In this review, we consider the posttranslational modifications of POMC in vertebrates from an evolutionary aspect, with a focus on the relationship between acetylation and the biological activities of α-MSH as an important consequence of posttranslational modification.

The pituitary-interrenal axis of fish: a review focusing on the lamprey and flounder

In fish, the pituitary-interrenal axis is associated with stress response and a variety of biological processes such as metabolism, immune response, and growth. The major hormones involved in this axis are adrenocorticotropic hormone (ACTH), released from the pars distalis of the pituitary gland, and corticosteroid, released from the interrenal gland that is embedded in the head kidney in ray-finned fish. The ACTH signal, by which corticosteroid release is stimulated, is transmitted by melanocortin (MC) receptors on interrenal cells. Thus, the interaction of ACTH and MC receptors is the pivotal event for interrenal cells. Knowledge about ACTH and MC receptors in lamprey, cartilaginous fish, and ray-finned fish is available, and it suggests the pituitary-interrenal axis was established early in vertebrate evolution. Moreover, the data, including our recent results from flounders and lampreys, provide interesting features about ligand-receptor interactions. This review focuses on the characteristics of ACTH, the proopiomelanocortin gene encoding ACTH, and the MC receptor, and it is mostly based on the results of our investigations.

Concomitant duplications of opioid peptide and receptor genes before the origin of jawed vertebrates

Background

The opioid system is involved in reward and pain mechanisms and consists in mammals of four receptors and several peptides. The peptides are derived from four prepropeptide genes, PENK, PDYN, PNOC and POMC, encoding enkephalins, dynorphins, orphanin/nociceptin and beta-endorphin, respectively. Previously we have described how two rounds of genome doubling (2R) before the origin of jawed vertebrates formed the receptor family.

Methodology/Principal Findings

Opioid peptide gene family members were investigated using a combination of sequence-based phylogeny and chromosomal locations of the peptide genes in various vertebrates. Several adjacent gene families were investigated similarly. The results show that the ancestral peptide gene gave rise to two additional copies in the genome doublings. The fourth member was generated by a local gene duplication, as the genes encoding POMC and PNOC are located on the same chromosome in the chicken genome and all three teleost genomes that we have studied. A translocation has disrupted this synteny in mammals. The PDYN gene seems to have been lost in chicken, but not in zebra finch. Duplicates of some peptide genes have arisen in the teleost fishes. Within the prepropeptide precursors, peptides have been lost or gained in different lineages.

Conclusions/Significance

The ancestral peptide and receptor genes were located on the same chromosome and were thus duplicated concomitantly. However, subsequently genetic linkage has been lost. In conclusion, the system of opioid peptides and receptors was largely formed by the genome doublings that took place early in vertebrate evolution.

Structural and functional diversity of proopiomelanocortin in fish with special reference to barfin flounder

Proopiomelanocortin (POMC) is a precursor of adrenocorticotropic hormone (ACTH), melanocyte-stimulating hormone (MSH), and endorphin (END). We have characterized POMC systems in barfin flounder. The results revealed unique aspects of POMC systems. Notable features in terms of pituitary functions are the occurrence of three functional POMC genes, the mutation of an essential sequence in the beta-END in one of the genes, occurrence of alpha-MSH in addition to ACTH in the pars distalis of the pituitary, and expression of the three genes in a single cell. While MSHs stimulate pigment dispersion, expression of the POMC gene and plasma levels of MSH do not always respond to background color changes between black and white. The functions of MSHs in skin pigmentation are very unique, because acetylation at the N-terminal of alpha-MSH inhibits its pigment dispersing activity. This is in contrast to results from other teleosts and amphibians, in which acetylation increases the activity. In the skin, the POMC gene is expressed in the non-chromatophoric dermal cells, indicating that MSH produced in the skin de novo has a paracrine function. The detection of MSH peptides in skin extracts seems to show that the control of skin pigmentation by MSHs is twofold-endocrine control by the pituitary, and paracrine control by the skin itself. Thus, fish provide an interesting model to help understand the structural and functional diversity of POMC systems. In this review, we provide an overview of our recent studies on the characterization of molecules and biological significance of POMC systems in barfin flounder.

Transcription elements and functional expression of proopiomelanocortin genes in the pituitary gland of the barfin flounder

Proopiomelanocortin (POMC) is the precursor of adrenocorticotropin, melanocyte-stimulating hormone, and endorphin. Barfin flounder Verasper moseri possesses three POMC mRNAs. In this study, we determined the amino acid sequences of POMC-A, POMC-B, and POMC-C in this fish and investigated the effects of black or white background on the expression of these genes. The three POMC genes (POMCs) were composed of three exons and two introns, wherein all the hormonal segments were encoded on the third exon, a pattern similar to that in other vertebrates. Intron B of POMC-A and POMC-B contained microsatellites of CA repeats, indicating that these two genes diverged from a common immediate ancestor by a recent duplication event. The 5'-flanking regions of the POMC-A (-1051 to -1), POMC-B (-1465 to -1), and POMC-C (-870 to -1) genes contained TATA boxes, Tpit, cyclic AMP response element-like elements, E boxes, and other elements. POMC-B and POMC-C also contained CCAAT boxes. The expression of the three POMCs seems to be regulated by synergistic interactions among a variety of transcription factors. The transfer of barfin flounder between tanks with different colors showed that in response to a black or white background, the expression of these POMCs did not always show similar profiles both in the neurointermediate lobe and pars distalis of the pituitaries. Since these POMCs are expressed in a single cell, the non-synchronous expression of these genes suggests that different sets of transcription factors are associated with the transcription of each gene.

Identity and distribution of immunoreactive adenohypophysial cells in the pituitary during the life cycle of sea lampreys, Petromyzon marinus

Adrenocorticotropin (ACTH), melanotropins (MSHs), growth hormone (GH) and gonadotropin (GTH) have been identified or cloned from the pituitary gland of sea lampreys (Petromyzon marinus). The present study was designed to gain insights into the functional significance of these hormones through a description of changes in the occurrence and distribution of cells immunoreactive to their antibodies at several different stages of the sea lamprey life cycle. ACTH-like cells and MSH-like cells were distributed in the rostral pars distalis and the pars intermedia, respectively, throughout the life cycle from ammocoetes (larvae) to pre-spawning adults. A large number of ACTH-like cells were observed during the pre-spawning period when animals may experience the highest stressful conditions. On the other hand, the number of MSH-like cells increased markedly during metamorphosis, in accordance with the completion of eye development. A small number of GH-like cells were present in the proximal pars distalis during the larval and metamorphic phases, but the number of cells increased markedly during the parasitic period, which corresponded well with the rapid somatic growth. GTH-like cells were not observed in the pituitary during the larval and metamorphic phases, but were present in the proximal pars distalis of immediately post-metamorphosed animals. Since there was a high accumulation of GTH-like cells in pre-spawning adults, these cells appeared to be involved in gonadotropic functions. The results of changing immunoreactivity during the lamprey life cycle suggest that lamprey adenohypophysial hormones, ACTH, MSH, GH and GTH, may possess biological functions similar to those of more advanced gnathostome vertebrates. Given that lampreys represent the most ancient group of vertebrates, it is most likely that these hormones have been conserved for their functions throughout vertebrate evolution.

Time scale for cyclostome evolution inferred with a phylogenetic diagnosis of hagfish and lamprey cDNA sequences

The Cyclostomata consists of the two orders Myxiniformes (hagfishes) and Petromyzoniformes (lampreys), and its monophyly has been unequivocally supported by recent molecular phylogenetic studies. Under this updated vertebrate phylogeny, we performed in silico evolutionary analyses using currently available cDNA sequences of cyclostomes. We first calculated the GC-content at four-fold degenerate sites (GC(4)), which revealed that an extremely high GC-content is shared by all the lamprey species we surveyed, whereas no striking pattern in GC-content was observed in any of the hagfish species surveyed. We then estimated the timing of diversification in cyclostome evolution using nucleotide and amino acid sequences. We obtained divergence times of 470-390 million years ago (Mya) in the Ordovician-Silurian-Devonian Periods for the interordinal split between Myxiniformes and Petromyzoniformes; 90-60 Mya in the Cretaceous-Tertiary Periods for the split between the two hagfish subfamilies, Myxininae and Eptatretinae; 280-220 Mya in the Permian-Triassic Periods for the split between the two lamprey subfamilies, Geotriinae and Petromyzoninae; and 30-10 Mya in the Tertiary Period for the split between the two lamprey genera, Petromyzon and Lethenteron. This evolutionary configuration indicates that Myxiniformes and Petromyzoniformes diverged shortly after the common ancestor of cyclostomes split from the future gnathostome lineage. Our results also suggest that intra-subfamilial diversification in hagfish and lamprey lineages (especially those distributed in the northern hemisphere) occurred in the Cretaceous or Tertiary Periods.

Evolution of melanocortin systems in fish

Proopiomelanocortin (POMC) is a common precursor of melanocortin (MC), the collective term for adrenocorticotropic hormone (ACTH) and melanophore-stimulating hormone (MSH), and of beta-endorphin (beta-END). Over the past decade, considerable progress has been made in the analysis of the POMC gene from a board taxonomic group of vertebrates and invertebrates. The results suggest that three MSHs (alpha-, beta-, and gamma-MSH) and a single END were established in ancestral invertebrates. Thereafter, unequal crossing over may have resulted in class-specific numbers of MSH segments during the radiation of fish. Moreover, duplication of the entire POMC gene may have led to the differentiation of POMC as shown in lampreys; one of the two subtypes is a precursor for ACTH and beta-END, the other is a precursor for two forms of MSH and the other form of beta-END. On the other hand, at least five subtypes of MC receptor (MCR) have been observed in fish. These are G-protein-coupled receptors with seven transmembrane domains. The ancestral MCR is suggested to have appeared before vertebrates, and then MCRs may have diverged by genome duplication and local duplication of each receptor gene during the evolution of vertebrates. They are distributed in many tissues in rather a subtype-specific manner and are responsible for a variety of biological functions. Thus, MC systems may have diverged by producing structurally different MC peptides from POMC and expressing MCR subtypes differing in ligand selectivity in a variety of tissues.

Patterns of proopiomelanotropin and proopiocortin gene expression and of immunohistochemistry for gonadotropin-releasing hormones (lGnRH-I and III) during the life cycle of a nonparasitic lamprey: relationship to this adult life history type

There are two adult life history types among lamprey species, nonparasitic and parasitic, with the former commencing the final interval of sexual maturation immediately after metamorphosis. There are no extensive studies that directly compare hormone profiles during the life cycles of nonparasitic and parasitic lamprey species, yet such data may explain differences in development, reproductive maturation, and feeding status. The present study uses immunohistochemistry to show the life cycle profiles for gonadotropin-releasing hormones (GnRH-I and -III) in the brain of the nonparasitic species, the American brook lamprey, Lampetra appendix, for comparison with the extensive, published, immunohistochemical data on these hormones in the parasitic species, the sea lamprey, Petromyzon marinus. The complete cDNAs for the two lamprey prohormones, proopiocortin (POC), and proopiomelanotropin (POM), were cloned for L. appendix and both nucleotide and deduced amino acid sequences were compared with those previously published for P. marinus. The POC and POM cDNAs for both species were used in expression studies, with Northern blotting, throughout their life cycles. Although GnRH-I and -III immunohistochemistry revealed a similar distribution of immunoreactive cells and fibers in the two species during the life cycles, a qualitative evaluation of staining intensity in L. appendix, implied early activity in the brains of metamorphosis of this species, particularly in GnRH-I. GnRH-III seems to be important in larval life and early metamorphosis in both species. A novel feature of this immunohistochemical study is the monthly observations of the distribution and relative intensity of the two GnRHs during the critical period of final sexual maturation that lead to spawning and then the spent animal. L. appendix POC and POM nucleotide sequences had 92.9 and 94.6% identity, respectively, with P. marinus POC and POM and there was an earlier increase in their expression during metamorphosis and postmetamorphic life. Since there was some correlation between the timing of metamorphic development, gonad maturation, and brain irGnRH intensity with POC and POM expression in L. appendix, it was concluded that these prohormones yield posttranslational products that likely play a substantial role in development and maturation events that lead to the nonparasitic adult life history of this species.