Differential expression of gill Na+,K+-ATPaseα- and β-subunits, Na+,K+,2Cl-cotransporter and CFTR anion channel in juvenile anadromous and landlocked Atlantic salmonSalmo salar

Abstract

SUMMARY This study examines changes in gill Na+,K+-ATPase(NKA) α- and β-subunit isoforms,Na+,K+,2Cl- cotransporter (NKCC) and cystic fibrosis transmembrane conductance regulator (CFTR I and II) in anadromous and landlocked strains of Atlantic salmon during parr-smolt transformation, and after seawater (SW) transfer in May/June. Gill NKA activity increased from February through April, May and June among both strains in freshwater (FW),with peak enzyme activity in the landlocked salmon being 50% below that of the anadromous fish in May and June. Gill NKA-α1b, -α3,-β1 and NKCC mRNA levels in anadromous salmon increased transiently, reaching peak levels in smolts in April/May, whereas no similar smolt-related upregulation of these transcripts occurred in juvenile landlocked salmon. Gill NKA-α1a mRNA decreased significantly in anadromous salmon from February through June, whereas α1a levels in landlocked salmon, after an initial decrease in April, remained significantly higher than those of the anadromous smolts in May and June. Following SW transfer, gill NKA-α1b and NKCC mRNA increased in both strains, whereas NKA-α1a decreased. Both strains exhibited a transient increase in gill NKA α-protein abundance, with peak levels in May. Gill α-protein abundance was lower in SW than corresponding FW values in June. Gill NKCC protein abundance increased transiently in anadromous fish, with peak levels in May, whereas a slight increase was observed in landlocked salmon in May,increasing to peak levels in June. Gill CFTR I mRNA levels increased significantly from February to April in both strains, followed by a slight,though not significant increase in May and June. CFTR I mRNA levels were significantly lower in landlocked than anadromous salmon in April/June. Gill CFTR II mRNA levels did not change significantly in either strain. Our findings demonstrates that differential expression of gill NKA-α1a,-α1b and -α3 isoforms may be important for potential functional differences in NKA, both during preparatory development and during salinity adjustments in salmon. Furthermore, landlocked salmon have lost some of the unique preparatory upregulation of gill NKA, NKCC and, to some extent, CFTR anion channel associated with the development of hypo-osmoregulatory ability in anadromous salmon.

Differential regulation of sodium–potassium pump isoforms during smolt development and seawater exposure of Atlantic salmon

“…In contrast, the abundance of gill NKAα1a decreased within 2days of seawater exposure and is only a fraction of its initial levels after 14days in seawater. This is consistent with previous research on the influence of salinity on protein abundance ) and gene transcription (Nilsen et al, 2007). This increase probably results in a greater overall capacity for salt secretion in the gill, representing the steady state needed for all activities in seawater.…”

“…While a previous study has shown that the abundance of the NKAα1a and NKAα1b isoforms in the gills of Atlantic salmon parr is tightly correlated with environmental salinity, with the NKAα1a being the predominant isoform in freshwater and NKAα1b more abundant after seawater acclimation (McCormick et al, 2009b), the present study is the first to profile the abundance of gill NKAα1a and NKAα1b isoforms during smolt development. Increased gill NKAα1b is consistent with molecular studies demonstrating that transcription of this isoform increases during smolt development of Atlantic salmon (Nilsen et al, 2007). These studies also found that gill NKAα1a mRNA decreases steadily from February to June, whereas in the present study NKAα1a protein levels are relatively stable from February to May with only a minor decrease in the late spring.…”

“…Previous research has established that salt secretion operates primarily through the action of NKA, NKCC and CFTR (Evans et al, 2005). Increased transcription of NKCC and CFTR in the gill has been observed during smolt development (Nilsen et al, 2007), and the abundance of gill NKCC and NKCC-rich ionocytes has also been found (Pelis et al, 2001). It therefore seems likely that increased abundance of NKAα1b and NKAα1b-ionocytes during smolt development is part of an ionocyte differentiation process that also includes changes in NKCC and CFTR, a possibility that is currently under investigation.…”

“…It was demonstrated that these isoforms are normally present in distinct ionocytes that are presumably involved in ion uptake and salt secretion, respectively. Recent studies on gill NKAα1a and NKAα1b mRNA levels indicate that there is differential transcription of these isoforms during smolt development (Nilsen et al, 2007). In the present study, we have examined changes in NKAα1a and NKAα1b protein abundance and the characteristics of ionocytes expressing these isoforms during smolt development.…”

“…Transcription studies indicate that there are salinity-dependent isoforms of the α-subunit expressed in the gills of salmonids and tilapia (Richards et al, 2003;Nilsen et al, 2007;Tipsmark et al, 2011). In a recent study on Atlantic salmon parr, we used specific antibodies to characterize two NKAα1 isoform proteins, NKAα1a and NKAα1b, which are the major isoforms expressed in freshwater and seawater, respectively (McCormick et al, 2009b).…”