Article

Unique form and osmoregulatory function of a neurohypophysial hormone in a urochordate.

Section of Behavioral Sciences, Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan.
Endocrinology (impact factor: 4.46). 07/2008; 149(10):5254-61. DOI:10.1210/en.2008-0607 pp.5254-61
Source: PubMed

ABSTRACT The cyclic nonapeptides, oxytocin and vasopressin, are neurohypophysial hormones that regulate many significant physiological processes related especially to reproduction and osmoregulation. In this study, we characterized an oxytocin-related peptide cDNA from a urochordate, Styela plicata, thought to be a sister group to vertebrates. Sequence analysis of the deduced precursor polypeptide revealed that the precursor is composed of three segments: a signal peptide, an oxytocin-like sequence flanked by a Gly C-terminal amidation signal and a Lys-Arg dibasic processing site, and a neurophysin domain, similar to other oxytocin/vasopressin family precursors. However, unlike other members of this family, the tunicate oxytocin-like peptide (CYISDCPNSRFWST-NH2) is a tetradecapeptide. We termed this peptide Styela oxytocin-related peptide (SOP). Furthermore, analyses of mass spectrometry, in situ hybridization, and immunohistochemistry demonstrated production of mature SOP in the cerebral ganglion. To elucidate the physiological action of SOP, we kept the tunicate for 2 d under the three different concentrations of seawater, 60, 100, and 130%, and measured the expression levels of SOP mRNA in the cerebral ganglion. The greatest expression of SOP mRNA was observed in the 60% seawater. In 60% seawater, but not in 100 or 130%, the tunicate mostly closed the atrial and branchial siphons. Therefore, we investigated the contractile effects of SOP on the siphons in vitro. SOP caused contractions in both siphons in a dose-dependent manner. Taken together, these results suggest that SOP acts to prevent the influx of a low concentration of seawater into the body and thus play an important role in osmoregulation.

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Keywords

cyclic nonapeptides
 
deduced precursor polypeptide
 
dose-dependent manner
 
expression levels
 
Gly C-terminal amidation signal
 
Lys-Arg dibasic processing site
 
mass spectrometry
 
mature SOP
 
neurophysin domain
 
oxytocin-like sequence
 
oxytocin-related peptide cDNA
 
oxytocin/vasopressin family precursors
 
peptide Styela oxytocin-related peptide
 
physiological action
 
Sequence analysis
 
signal peptide
 
significant physiological processes
 
sister group
 
three different concentrations
 
tunicate oxytocin-like peptide