Na+/K+-ATPase α-isoform switching in gills of rainbow trout (Oncorhynchus mykiss) during salinity transfer

Abstract

SUMMARYWe identified five Na+/K+-ATPase α-isoforms in rainbow trout and characterized their expression pattern in gills following seawater transfer. Three of these isoforms were closely related to other vertebrate α1 isoforms (designated α1a, α1b and α1c),one isoform was closely related to α2 isoforms (designated α2) and the fifth was closely related to α3 isoforms (designated α3). Na+/K+-ATPase α1c- and α3-isoforms were present in all tissues examined, while all others had tissue specific distributions. Four Na+/K+-ATPase α-isoforms were expressed in trout gills (α1a, α1b, α1c and α3). Na+/K+-ATPase α1c- and α3-isoforms were expressed at low levels in freshwater trout gills and their expression pattern did not change following transfer to 40% or 80% seawater. Na+/K+-ATPase α1a and α1b were differentially expressed following seawater transfer. Transfer from freshwater to 40% and 80% seawater decreased gill Na+/K+-ATPaseα1a mRNA, while transfer from freshwater to 80% seawater caused a transient increase in Na+/K+-ATPase α1b mRNA. These changes in isoform distribution were accompanied by an increase in gill Na+/K+-ATPase enzyme activity by 10 days after transfer to 80% seawater, though no significant change occurred following transfer to 40% seawater. Isoform switching in trout gills following salinity transfer suggests that the Na+/K+-ATPase α1a- andα1b-isoforms play different roles in freshwater and seawater acclimation, and that assays of Na+/K+-ATPase enzyme activity may not provide a complete picture of the role of this protein in seawater transfer.

Distinct hormonal regulation of Na+,K+-atpase genes in the gill of Atlantic salmon (Salmo salar L.)

“…As a fundamental part of the response in salmonids, a distinct switching occurs between Na C ,K C -atpase a isoforms 1a and 1b (Richards et al 2003), which may change the functional properties of the enzyme ( Jorgensen 2008) in order to maintain the appropriate ionic gradients across the gill. The primary focus of our study was the endocrine and paracrine regulation of this particular isoform switch, and the experiments were performed under both hypo-and hyperosmotic conditions in anticipation of maximal effects of the applied hormonal treatments.…”

“…Cortisol is known to stimulate gill MRC proliferation and overall gill Na C ,K C -atpase activity, and it has been puzzling for many years how cortisol in doing so may both facilitate acclimation to hyper-and hypo-osmotic conditions. The discovery by Richards et al (2003) that two isoforms of the Na C ,K C -atpase a-subunit contribute to overall enzymatic activity in the gill has led to the hypothesis, that a1a is primarily expressed in MRCs responsible for absorption of monovalent ions (and possibly Ca 2C ), while a1b is the dominant isoform in secretory-type MRCs. This has gained support from ISH analyses of transcript localisation in the salmon gill (Madsen et al 2009).…”

“…In both environments, mitochondrion-rich cells (MRCs) and their adjoining accessory and pavement cells are fundamental to compensatory ion transport for which the driving force is delivered by the Na C ,K C -atpase and in the FW mode possibly complemented by an apical V-type H C -ATPase (see Hirose et al 2003). The recent discovery of different Na C ,K C -atpase a-1 isoforms in the salmonid teleost gill (a1a, a1b and a1c; Richards et al 2003) has offered new insight into transepithelial ion transport in the gill. The mRNA expression levels of these isoforms have been investigated during salinity acclimation in several salmonids, and it is remarkable how salinity induces a reciprocal switch between a1a and a1b isoforms, a1a being augmented in FW and a1b in SW (Richards et al 2003, Mackie et al 2005, Bystrianski et al 2006.…”

“…The recent discovery of different Na C ,K C -atpase a-1 isoforms in the salmonid teleost gill (a1a, a1b and a1c; Richards et al 2003) has offered new insight into transepithelial ion transport in the gill. The mRNA expression levels of these isoforms have been investigated during salinity acclimation in several salmonids, and it is remarkable how salinity induces a reciprocal switch between a1a and a1b isoforms, a1a being augmented in FW and a1b in SW (Richards et al 2003, Mackie et al 2005, Bystrianski et al 2006. This has led to the interesting suggestion that a1a is a FW isoform driving ion uptake, whereas a1b is a SW isoform driving ion excretion.…”

Does the mineralocorticoid receptor have a functional role in the teleost gill?

“…NKA is composed of two essential subunits (a and P) and several different a isoforms (ala, alb, ale, a3) have been identified to change in rainbow trout transferred among waters of different salinity (Richards et al, 2003). The expression pattern and distribution of these a isoforms switches depending on the environment and is thought to play different roles in acclimation.…”