PGE exerts its functions by binding to
PGE2 exerts its functions by binding to different receptors. Four different subtypes of PGE2 receptors are found in mammals and are designated as EP1-4. These are G-protein coupled receptors that belong to a family of seven rhodopsin-like transmembrane spanning receptors. They are structurally different and share less than 35% sequence identity within one species (Sugimoto et al., 1993). EP1 and EP4 have relatively long third intracellular loops compared to EP2 and EP3. EP4 is characterized by its long intracellular domain while EP3 is distinguished by the presence of multiple isoforms (Sugimoto et al., 1993). The expression and distribution of these receptors vary between different tissues and cell types (Fennekohl et al, 1999, Sugimoto, Narumiya, 2007). Functional differences in the signaling cascade mediated by these receptors have also been reported. In general, EP2 and EP4 are considered as stimulatory receptors. They induce signaling cascade that leads to increased levels of cyclic adenosine monophosphate (cAMP) (Fujino et al., 2005). In contrast, EP3 is considered as inhibitory receptor and leads to decreased cAMP concentration (Fabre et al., 2001). Signaling through EP1 receptor, on the other hand, leads to increased intracellular calcium concentration (Katoh et al., 1995).
The heterogeneity in PGE2 induced effects can partially be explained by the presence of multiple receptors. For example, stimulation of gastric IOX4 secretion and smooth muscle relaxation in the murine gastrointestinal tract were shown to be mediated by EP4 whereas an opposite effect was mediated by EP3 (Ding et al, 1997, Okada et al, 2000). In mixed lymphocyte response, EP2 and to a lesser extent EP4 were found to be responsible for the inhibitory effect induced by PGE2 while the involvement of EP1 and EP3 was ruled out (Nataraj et al., 2001). Differences in PGE2 induced effect on the development of colorectal cancer can also be attributed to the receptor diversity (Hull et al., 2004). It is noteworthy, however, that the functions mediated by the different receptors are not always conserved and can vary between different species (Larsen et al, 2005, Takeuchi et al, 1999).
Knowledge about EP receptors in fish is still developing. EP receptors of the zebrafish are the best studied among fish species and have only been characterized recently. All the four different EP subtypes are present in zebrafish and two isoforms of EP2, three of EP1 and EP4, and five of EP3 were characterized (Iwasaki et al, 2013, Kwok et al, 2012, Tsuge et al, 2013). Analysis of the different zebrafish EP amino acid sequences revealed 40–70% sequence identity to their human counterparts. In all subtypes, the seven transmembrane regions are conserved and most of the differences are in the extra and intracellular loops (Iwasaki et al, 2013, Kwok et al, 2012, Tsuge et al, 2013). However, the distribution of these receptors or their functions in different tissues has not yet been studied in detail. For Atlantic salmon, EP4 is the only subtype that has been identified so far (Leong et al., 2010). Although the genomic sequence of Atlantic salmon EP4 (asEP4) and its corresponding amino acid are deposited in the GenBank, there is no record of any study that has been conducted to understand its functional aspects. The aim of the present study was to investigate the distribution of asEP4 receptor in different tissues by PCR and immunohistochemistry.
Materials and methods
Discussion In the present study we have investigated the expression of EP4 receptor in different Atlantic salmon tissues by RT-PCR and immunohistochemistry. To our knowledge, this is the first study to report the expression of EP4 receptor in Atlantic salmon tissues. The sequence of asEP4 has been published earlier but with no focus on receptor expression at mRNA or protein level (Leong et al., 2010). With an attempt to compare our findings to other species, we were not able to find other studies that have investigated the global EP4 expression in different tissues by immunohistochemistry although studies that focused on individual tissues are present.