CTR2 is a seven-transmembrane (7TM) G protein-coupled receptor with the canonical topology of the rhodopsin-type GPCR family. Key structural features from UniProt annotation include:
Several regions and residues that are requisite for binding of vertebrate OT/VP receptors with their ligands are highly conserved in CTR2 (and CTR1), but show substitutions compared to the vertebrate consensus. Specifically, five glutamine residues in TM domains 2, 3, 4, and 6 and a lysine residue in TM3 that are essential for OT/VP interaction are conserved in both CTRs PMID:15504101. The sequences Phe-Gln-Val-Leu-Pro-Gln-Leu, Gly-Pro-Asp, Asp-Cys-Trp-Ala and Pro-Trp-Gly found in vertebrate OT/VP receptors (important for ligand binding) are substituted to Phe-Asn-Ile-Leu-Pro-Gln-Leu, Ala-Gly-Asp, Asp-Cys-Trp-Val and Ala-Trp-Val in CTR2 PMID:15504101.
CTR2 is a G protein-coupled receptor that specifically binds cephalotocin (CT), one of two oxytocin/vasopressin superfamily neuropeptides in O. vulgaris.
Functional expression of CTR2 in Xenopus oocytes demonstrated that CTR2 induces calcium-mediated inward chloride current specifically in response to cephalotocin, but not to octopressin or vertebrate OT/VP family peptides [PMID:15504101, "CTR2 and OPR expressed in Xenopus oocytes induced calcium-mediated inward chloride current in a CT- and OP-specific manner respectively"]. This selectivity contrasts with the mechanism in vertebrates, where the difference in polarity of residue 8 determines OT vs VP binding: in octopus, OP and CT share an isoleucine at position 8, and instead the different residues at positions 2-5 are responsible for receptor selectivity [PMID:15504101, "the amino acid residues of OP and CT at positions 2-5, were presumed to play crucial roles in the binding selectivity to their receptors"].
CTR2 couples to Gq/11-type G proteins, activating the inositol 1,4,5-trisphosphate-calcium signal transduction cascade, as evidenced by calcium-mediated chloride currents in Xenopus oocytes PMID:15504101. This is consistent with the signaling mode of vertebrate OxyR, V1aR, and V1bR, which also couple to Gq/11 (unlike V2R, which couples to Gs/adenylate cyclase) PMID:15504101.
Recent pharmacological studies have tested cephalotocin on mammalian receptors. Kim et al. (2022) demonstrated that cephalotocin acts as a selective agonist of human AVP type 1b (V1bR) and type 2 (V2R) receptors, and has antidiuretic activity in rats, reducing urine output similarly to desmopressin PMID:35621979. A separate study found that cephalotocin has anti-stress effects in rats via Avpr1b in hippocampal neurons, reducing immobility in the tail suspension test PMID:36050225.
CTR2 is proposed to mediate at least two major physiological roles based on tissue expression patterns and vertebrate homology:
Ammonia excretion in branchia: By analogy with vasotocin receptors in toadfish branchia that regulate urea excretion [PMID:15504101, "Arg-vasotocin caused prominent transient increases in urea excretion from branchia of toadfish, and Arg-vasotocin receptor mRNA was expressed in the branchia"], and given that ammonia is excreted from octopus branchia, CTR2 may mediate cephalotocin-regulated ammonia excretion.
Steroid hormone production/secretion in reproductive tissues: By analogy with mammalian V1aR in the epididymis, vas deferens, and Leydig cells that regulate testosterone production [PMID:15504101, "These findings, combined with the abundant expression of CTR2 in the vas deferens and testis, support a notion that CT participates in production and/or secretion of the steroid hormones via CTR2"].
Notably, unlike octopressin, cephalotocin does not induce contractile activities on smooth muscles of octopus tissues PMID:15504101, suggesting CTR2 mediates non-contractile physiological functions.
Cephalotocin receptor expression has also been detected in the gastric ganglion of octopus, the largest visceral ganglion implicated in regulation of digestive tract functions. Baldascino et al. (2017) confirmed the presence of cephalotocin and its receptor in this ganglion, and found that parasitic infection altered cephalotocin gene expression, suggesting a role in gut-brain signaling and food intake regulation PMID:29326594.
CTR2 mRNA is mainly distributed in peripheral tissues, in contrast to CTR1, which is primarily expressed in the central and peripheral nervous systems PMID:15504101:
This tissue distribution is distinct from both CTR1 (nervous system, pancreas, oviduct, ovary) PMID:14596680 and OPR (widely distributed in both nervous system and peripheral tissues) PMID:15504101.
O. vulgaris was the first invertebrate shown to possess two OT/VP superfamily peptides PMID:15504101. While most protostomes have only a single OT/VP homolog, the octopus has:
- Cephalotocin (CT): bound by CTR1 and CTR2
- Octopressin (OP): bound by OPR
The OP and CT genes are proposed to have evolved through gene duplication from a common ancestral gene, mirroring the duplication that gave rise to OT and VP in vertebrates [PMID:15504101, "the CT and OP genes, similar to the OT/VP family, evolved through duplication"].
A key finding from Kanda et al. (2005) is that the molecular basis of ligand-receptor selectivity in octopus differs fundamentally from vertebrates. In vertebrates, selectivity depends on the polarity of residue 8 (neutral in OT, basic in VP). In octopus, OP and CT share the same residue (isoleucine) at position 8, and selectivity instead depends on residues at positions 2-5 PMID:15504101. Furthermore, key ligand-binding residues in the receptor TM domains that are conserved in CTR1 and CTR2 are substituted in OPR, suggesting the OP/OPR binding mode differs from all other OT/VP systems [PMID:15504101, "the ligand-receptor selectivity were established through different evolutionary lineages from those of their vertebrate counterparts"].
| Feature | CTR1 | CTR2 | OPR |
|---|---|---|---|
| Ligand | Cephalotocin | Cephalotocin | Octopressin |
| Primary expression | CNS, reproductive tissues | Peripheral tissues (branchia, vas deferens) | Nervous system + peripheral tissues |
| Signaling | Ca2+/IP3 pathway | Ca2+/IP3 pathway | Ca2+/IP3 pathway |
| Key binding residues | Conserved with vertebrate OT/VP-R | Conserved with vertebrate OT/VP-R | Several substitutions vs vertebrate |
| Reference | PMID:14596680 | PMID:15504101 | PMID:15504101 |
The octopus OT/VP receptor system has been contextualized within broader molluscan evolution. Conopressin receptors in the land snail Theba pisana are related to the octopus system, with conopressin genes expressed in reproductive tissues and the CNS PMID:26752717. The crustacean cardioactive peptide (CCAP) signaling system in the Pacific abalone was shown to be phylogenetically related to the OT/VP receptor family, illuminating the deep evolutionary origins of these neuropeptide signaling systems PMID:37956901.
Kanda A, Satake H, Kawada T, Minakata H (2005). Novel evolutionary lineages of the invertebrate oxytocin/vasopressin superfamily peptides and their receptors in the common octopus (Octopus vulgaris). Biochem J 387:85-91. PMID:15504101 -- Primary characterization of CTR2 and OPR: cloning, functional expression, tissue distribution, evolutionary analysis.
Kanda A, Takuwa-Kuroda K, Iwakoshi-Ukena E, Furukawa Y, Matsushima O, Minakata H (2003). Cloning of Octopus cephalotocin receptor, a member of the oxytocin/vasopressin superfamily. J Endocrinol 179:281-91. PMID:14596680 -- Original characterization of CTR1, providing context for CTR2 as the second cephalotocin receptor subtype.
Baldascino E et al. (2017). The Gastric Ganglion of Octopus: Preliminary Characterization of Gene- and Putative Neurochemical-Complexity. Front Physiol 8:1001. PMID:29326594 -- Detected cephalotocin and CTR expression in gastric ganglion, implicating CT signaling in digestive regulation.
Kim YJ et al. (2022). An Octopus-Derived Peptide with Antidiuretic Activity in Rats. Mar Drugs 20:328. PMID:35621979 -- Showed cephalotocin selectively activates human V1bR and V2R, and has antidiuretic activity in rats.
Kim YJ et al. (2022). Anti-stress Effect of Octopus Cephalotocin in Rats. Exp Neurobiol 31:260-269. PMID:36050225 -- Demonstrated cephalotocin's anti-stress effects via hippocampal Avpr1b activation in rats.
Stewart MJ et al. (2016). Characterisation of two conopressin precursor isoforms in the land snail, Theba pisana. Peptides 80:32-39. PMID:26752717 -- Broader molluscan OT/VP receptor family context.
Lee SH, Kim MA, Sohn YC (2023). Crustacean cardioactive peptide signaling system in the gastropod mollusk Pacific abalone. Comp Biochem Physiol A 288:111552. PMID:37956901 -- Evolutionary relationships between CCAP, NPS, and OT/VP signaling systems.