Coexpression of Corticotropin-Releasing Hormone and Urotensin I Precursor Genes in the Caudal Neurosecretory System of the Euryhaline Flounder (Platichthys flesus): A Possible Shared Role in Peripheral Regulation

Abstract

CRH and urotensin I (UI) are neuroendocrine peptides that belong to the superfamily of corticotropin-releasing factors. In mammals, these peptides regulate the stress response and other central nervous system functions, whereas in fish an involvement for UI in osmoregulation has also been suggested. We have identified, characterized, and localized the genes encoding these peptides in a unique fish neuroendocrine organ, the caudal neurosecretory system (CNSS). The CRH and UI precursors, isolated from a European flounder CNSS library, consist of 168 and 147 amino acid residues, respectively, with an overall homology of approximately 50%. Both precursors contain a signal peptide, a divergent cryptic region and a 41-amino acid mature peptide with cleavage and amidation sites. Genomic organization showed that whole CRH and UI coding sequences are contained in a single exon. Northern blot analysis and quantitative PCR of a range of tissues confirmed the CNSS as a major site of expression of both CRH and UI and thus serves as a likely source of circulating peptides. In situ hybridization demonstrated that CRH and UI colocalize to the same cells of the CNSS. Our findings suggest that, in euryhaline fish, the CNSS is a major site of production of CRH and probably contributes to the high circulating levels observed in response to specific environmental challenges. Furthermore, the localization of CRH and UI within the same cell population suggests an early, possibly shared role for these peptides in controlling stress-mediated adaptive plasticity.

Heads or tails? Stressor-specific expression of corticotropin-releasing factor and urotensin I in the preoptic area and caudal neurosecretory system of rainbow trout

“…Similar to the expression pattern of CRF as described above, the POA and the CNSS are among the principal sites of UI expression in the CNS. While several studies have previously identified the CNSS as a major site of UI expression in teleosts (Winter et al 2000, Lu et al 2004, the POA was only recently recognized in zebrafish as a site of CRF and UI co-expression (Alderman & Bernier 2007). We also identified the midbrain as a primary site of UI gene expression in the CNS of rainbow trout.…”

“…To what extent the CNS pattern of CRF gene expression observed in rainbow trout is characteristic of other teleosts is not known. To date, in the only other teleost species where CRF expression has been localized to the CNSS, the flounder (Lu et al 2004), a quantitative distribution pattern has not been performed.…”

“…Prior to this study, only two stressors had been investigated for their impact on the expression of CRF and UI in the CNSS, physical restraint (Lu et al 2004) and seawater transfer . Whereas physical restraint in flounder was associated with a transient increase in CRF expression and no changes in UI mRNA levels (Lu et al 2004), seawater transfer in rainbow trout elicited chronic parallel increase in CRF and UI mRNA levels ).…”

“…Teleosts differ from other vertebrates in having an additional major site of CRF-related peptide synthesis, the caudal neurosecretory system (CNSS). Located in the terminal segments of the spinal cord, the CNSS has high expression levels of CRF and UI (Lu et al 2004) and a neurohemal organ from which these peptides can be released to the circulation (Winter et al 2000). Despite its uniqueness among vertebrates and distinct advantages for studying CRF and UI neurosecretion, many fundamental questions regarding the physiological roles of the CNSS remain unresolved (McCrohan & Bernier 2007).…”

“…As seen in the PVN of mammals (for review see Bakshi & Kalin 2000), exposure to a variety of stressors in teleosts can elicit an increase in the expression of CRF in the POA (Doyon et al 2003, Huising et al 2004. In the CNSS, while an increase in osmolality is associated with increase in both CRF and UI gene expression , physical restraint transiently activates CRF but not UI neurons (Lu et al 2004). Overall, results from studies where the impact of a stressor on the expression of the CRF-related peptides was assessed in more than one region of the CNS suggest that the regulation of CRF and UI gene expression in teleosts maybe regionally specific (Lu et al 2004.…”