Sturgeon Orphanin, a Molecular “Fossil” That Bridges the Gap between the Opioids and Orphanin FQ/Nociceptin

Abstract

The elucidation of the cDNA sequence for sturgeon proorphanin provides a unique window for interpreting the evolutionary history of the opioid/orphanin gene family. The molecular "fossil" status of this precursor can be seen in several ancestral sequence characteristics that point to its origin as a duplication of either a prodynorphin-or proenkephalin-like gene. The sturgeon proorphanin cDNA encodes a precursor protein of 194 residues, and the orphanin heptadecapeptide itself binds not only the opioid receptor-like 1 (ORL1) receptor but also the classical (, , and ␦) opioid receptors with near equal affinity. Allowing for this broad receptor specificity are several amino acid substitutions at key positions in the heptadecapeptide sequence, relative to its mammalian orthologs, that have been linked by amino acid scans and site-directed mutagenic studies to the exclusion of mammalian orphanin FQ/nociceptin from classic opioid ligands (i.e. F1Y and L14W). The unique receptor binding profile of sturgeon orphanin not only provides insight into the evolutionary history of the opioid and opioid-related peptides but also provides an ideal context in which to investigate the underlying mechanisms by which novel and often divergent physiological functions arise in receptor-ligand systems.The expansion and functional diversification of multigene families have been a recurring theme in the evolutionary success of complex eukaryotic organisms (1), and the opioid neuropeptides clearly illustrate this phenomenon (2). The classic opioid gene family comprises three neuropeptide precursors encoded on the following genes: proopiomelanocortin (end products: ␤-endorphin, melanocyte-stimulating hormone-associated peptides), proenkephalin (end products: Met-enkephalin, Leuenkephalin), and prodynorphin (end products: dynorphins and ␣-neoendorphin). Based on the organization of these genes it appears that they were derived from a common ancestral gene (3). This hypothesis is supported by the shared neuroinhibitory action of the opioid end products and the selective binding profiles of the opioid peptides to a family of G protein-coupled receptors (, , and ␦ opiate receptors) (4, 5).What has been difficult to resolve is the pattern of evolutionary events that has given rise to the opioid gene family. This is an issue that has been further complicated by the discovery, reported simultaneously by Meunier et al. (6) and Reinscheid et al. (7), of the 17-amino acid orphanin FQ/N, 1 the endogenous ligand for the opioid receptor-like 1 (ORL1) receptor, the "orphan" opiate receptor gene. The organization of the orphanin FQ/N precursor (proorphanin or pronociceptin) indicates that this gene is evolutionarily related to the opioid-coding precursor genes. Mammalian proorphanin has the same basic intronexon structure, conserved N-terminal cysteines, and a modified opioid-like core sequence (FGGF instead of YGGF) that are all defining features of classic opioid precursors. Similarities are also seen at the cellular level where both typical opioids ...

Cloning, heterologous expression and pharmacological characterization of a kappa opioid receptor from the brain of the rough-skinned newt, Taricha granulosa

“…Research into the physiology of mammalian opioid ligand-receptor interactions has supported the idea of a strict functional dichotomy, in which the classic opioids interact with µ, and/or receptors to produce analgesia and nociceptin-ORL1 interactions act in an opposing fashion (Reinscheid et al 1998, Danielson et al 2001. Our results suggest that, in contrast to mammals, newts have ORL1 and opioid receptors that recognize both nociceptin and dynorphin.…”

“…In mammals, nociceptin displays essentially no affinity for the canonical , and/or µ opioid receptors (Meunier et al 1995, Reinscheid et al 1995, Meng et al 1996, Pan et al 2002 and, despite a high degree of sequence homology shared among all of the opioid receptor classes, the ORL1 receptor in mammals does not recognize the classic opioid drugs that bind with high affinity to , and µ opioid receptors (Owens & Akil 2002). Such discrimination is especially interesting when it is appreciated that nociceptin (FGGFTGARK SARKLANQ) bears a significant degree of sequence identity to dynorphin A (YGGFLRRIRPKLKWDNQ) and belongs to the family of opioid peptides (Danielson et al 2001).…”

Cloning, pharmacological characterization and tissue distribution of an ORL1 opioid receptor from an amphibian, the rough-skinned newt Taricha granulosa

“…The sturgeon oFQ, with the N-terminal tyrosine, shows affinity for the mammalian ORL1 as well as the µ, , and opioid receptors. The authors suggest that the sturgeon oFQ represents an intermediate stage in the evolution of the proorphanin gene, and a ''transitional state in the eventual functional isolation of the orphanin heptadecapeptide from the classical opioids'' (Danielson et al 2001). If this is true, then receptor coevolution may have produced cognate receptor intermediates in some species capable of recognizing oFQ and the classical opioid peptides.…”

“…Recently, the first complete non-mammalian vertebrate proorphanin cDNA gene sequence was obtained from the sturgeon, Acipencer transmontanus (Danielson et al 2001). The sturgeon oFQ peptide retains the classical opioid peptide core sequence (YGGF), whereas all mammalian oFQ peptides have a modified opioid-like core (FGGF).…”

Can fish suffer?: perspectives on sentience, pain, fear and stress

“…Endogenous opioids (morphine-like substances) are also produced in tetrapods, which can suppress the intensity of pain experienced (Zieglgänsberger, 1986). A number of studies have shown that the nervous systems of teleost fish also produce chemical compounds related to adrenocorticotropic hormone and pain-mediating opiates (Ng and Chan, 1990;Vecino et al, 1992;Danielson et al, 2001). Jansen and Green (1970) and Ehrensing et al (1982) demonstrated that analgesia could be achieved in goldfish (Carassius auratus) subjected to electric shocks by adding morphine to the tank water.…”