Claudin-6, -10d, and -10e contribute to seawater acclimation in the euryhaline puffer fishTetraodon nigroviridis

Abstract

Expression profiles of claudin-6, -10d and -10e in the euryhaline teleost fish Tetraodon nigroviridis revealed claudin-6 in brain, eye, gill and skin tissue, while claudin-10d and -10e were found in brain, gill and skin only. In fishes, the gill and skin are important tissue barriers that interface directly with surrounding water, but these organs generally function differently in osmoregulation. Therefore, roles for gill and skin claudin-6, -10d and -10e in the osmoregulatory strategies of T. nigroviridis were investigated. In the gill epithelium, claudin-6, -10d and -10e co-localized with Na + -K + -ATPase immunoreactive (NKA-ir) ionocytes, and differences in sub-cellular localization could be observed in hypoosmotic (freshwater, FW) versus hyperosmotic (seawater, SW) environments. Claudin-10d and -10e abundance increased in the gills of fish acclimated to SW versus FW, while claudin-6 abundance decreased in the gills of fish acclimated to SW. Taken together with our knowledge of claudin-6 and -10 function in other vertebrates, data support the idea that in SW-acclimated T. nigroviridis, these claudins are abundant in gill ionocytes, where they contribute to the formation of a Na + shunt and 'leaky' epithelium, both of which are characteristic of salt-secreting SW fish gills. Skin claudin10d and -10e abundance also increased in fish acclimated to SW versus those in FW, but so did claudin-6. In skin, claudin-6 was found to co-localize with NKA-ir cells, but claudin-10d and -10e did not. This study provides direct evidence that the gill epithelium contains salinity-responsive tight junction proteins that are abundant primarily in ionocytes. These same proteins also appear to play a role in the osmoregulatory physiology of the epidermis.

Tight junction protein gene expression patterns and changes in transcript abundance during development of model fish gill epithelia

“…Therefore, the differences observed between these genes in SSI versus DSI epithelia seem more likely to relate to differences in cell composition. In the case of cldn-10e, MRCs are likely to possess a rich complement of this TJ isoform, as has been shown in other fish species (Bui and Kelly, 2014). Furthermore, an ~60-fold increase in cldn-31 strongly supports the idea that this cldn is highly expressed in MRCs, although it is not absent from PVCs.…”

“…As previously discussed, it seems very probable that cldn-10c and -10d may be associated exclusively with gill ionocytes as there is evidence to support the exclusive presence of Cldn-10 isoforms (i.e. Cldn-10d and -10e) in the MRCs of the euryhaline puffer fish Tetraodon nigroviridis (Bui and Kelly, 2014). It has been proposed that in gill tissue Cldn-10d and -10e are involved in SW-induced mechanisms of salt secretion because their abundance is greater in SW than in freshwater (FW), and CLDN-10 in mammalian epithelia has been reported to be poreforming and selective for Na + movement (Breiderhoff et al, 2012;Günzel et al, 2009).…”

“…Previous studies have associated changes in cldn-10e mRNA abundance with acclimation to seawater (SW), smoltification and elevated levels of cortisol (Tipsmark et al, 2008;Bui et al, 2010;Bui and Kelly, 2011). In an SSI preparation derived from the gills of T. nigroviridis, cldn-10e was found to be absent (Bui et al, 2010), and it was subsequently shown that in gill tissue of this species Cldn-10e localizes exclusively to mitochondria-rich cells (Bui and Kelly, 2014). The presence of cldn-10e in rainbow trout PVCs therefore suggests that species-specific differences in the presence of 'ionocyte-specific' Cldn proteins may be expected, but that does not necessarily mean that a major role of proteins such as Cldn-10e may not be associated with ionocytes.…”

“…1) it seems probable that these proteins are associated with other gill cell types. Transcript encoding Cldn-10d is also absent from cultured puffer fish (Tetraodon nigroviridis) gill epithelia composed entirely of PVCs (Bui et al, 2010;Bui and Kelly, 2011), and recently it has been shown that this Cldn is associated exclusively with gill ionocytes (Bui and Kelly, 2014). In puffer fish, Cldn-10e is also absent from primary cultured gill PVC epithelia and associated exclusively with ionocytes in gill tissue (Bui et al, 2010;Bui and Kelly, 2011).…”

“…It has been proposed that in gill tissue Cldn-10d and -10e are involved in SW-induced mechanisms of salt secretion because their abundance is greater in SW than in freshwater (FW), and CLDN-10 in mammalian epithelia has been reported to be poreforming and selective for Na + movement (Breiderhoff et al, 2012;Günzel et al, 2009). Therefore, the association of Cldn-10 isoforms with gill MRCs (and in vivo possibly also accessory cells), together with the elevated levels of transcript and protein abundance that follow SW acclimation (see Bui et al, 2010;Bui and Kelly, 2014) may indicate their contribution to the organization of a gill Na + shunt, which is necessary for salt secretion in SW fishes.…”