NHE3 in an ancestral vertebrate: primary sequence, distribution, localization, and function in gills

Area of Science:

• Marine biology
• Comparative physiology
• Molecular biology

Background:

• The sodium-hydrogen exchanger 3 (NHE3) is crucial for pH and volume regulation in mammalian kidneys.
• In elasmobranchs, gills are primary sites for pH and osmoregulation, but the role of NHE3 is unknown.

Purpose of the Study:

• To investigate the presence and function of epithelial sodium-hydrogen exchanger (NHE) homologs in elasmobranch gills.
• To determine if NHE gene expression in stingray gills is affected by environmental salinity or hypercapnia.

Main Methods:

• Degenerate primers and RT-PCR were used to identify NHE2 and NHE3 homologs in Atlantic stingray gills.
• Real-time PCR quantified mRNA expression levels under different salinity and hypercapnia conditions.
• Rapid amplification of cDNA and antibody-based localization identified the stingray NHE3 protein and its cellular location.

Main Results:

• Partial sequences of NHE2 and NHE3 homologs were identified in the Atlantic stingray (Dasyatis sabina).
• NHE3 homolog mRNA expression significantly increased upon transfer to low salinity but not during hypercapnia.
• The complete sequence of a stingray NHE3 homolog (70% identical to human NHE3) was determined, and it localized to the apical membranes of gill epithelial cells.

Conclusions:

• This study reports the first cloned NHE3 from an elasmobranch species.
• Increased gill NHE3 expression in low salinity suggests its role in facilitating sodium absorption from ion-poor environments.
• NHE3 plays a significant role in osmoregulation in elasmobranchs, particularly in adapting to varying environmental salinities.