First Insights into Body Localization of an Osmoregulation-Related Cotransporter in Estuarine Annelids.

Simple Summary: The study aimed to confirm the presence of the Na+-K+-2Cl− cotransporter (NKCC) in annelids, a membrane transporter crucial for cell volume regulation, which had not been directly shown before in these worms. Initially, the presence of NKCC was first recovered in silico. Using immunofluorescence, we observed that the NKCC signal was detected in various tissues from four estuarine annelid species from southern Brazil subjected to salinity changes. Two euryhaline species and two stenohaline species were tested. While all species showed NKCC expression, its distribution varied. Euryhaline and free-living species showed widespread NKCC expression along their bodies, while the sedentary stenohaline species had it mainly localized in branchiae and internal tissues. These findings suggest NKCC plays a role in cell volume regulation, particularly in annelids experiencing habitat salinity fluctuations. The expression of the Na+-K+-2Cl− cotransporter (NKCC), widely associated with cell volume regulation, has never been directly demonstrated in annelids. Its putative presence was firstly recovered in silico, and then using immunofluorescence, its signal was retrieved for the first time in different tissues of four species of estuarine annelids from southern Brazil that are regularly subjected to salinity fluctuations. We tested two euryhaline species (wide salinity tolerance), the nereidids Alitta yarae and Laeonereis acuta (habitat salinity: ~10–28 psu), and two stenohaline species (restricted salinity tolerance), the nephtyid Nephtys fluviatilis (habitat salinity: ~6–10 psu), and the melinnid Isolda pulchella (habitat salinity: ~28–35 psu). All four species showed specific immunofluorescent labelling for NKCC-like expression. However, the expression of an NKCC-like protein was not homogeneous among them. The free-living/burrowers (both euryhaline nereidids and the stenohaline nephtyid) displayed a widespread signal for an NKCC-like protein along their bodies, in contrast to the stenohaline sedentary melinnid, in which the signal was restricted to the branchiae and the internal tissues of the body. The results are compatible with NKCC involvement in cell volume, especially in annelids that face wide variations in salinity in their habitats. [ABSTRACT FROM AUTHOR]