Acid increases NHE8 surface expression and activity in NRK cells.

Area of Science:

• Nephrology
• Cell Physiology
• Molecular Biology

Background:

• Neonates have low levels of the primary adult sodium-hydrogen exchanger (NHE3) but higher levels of NHE8.
• Metabolic acidosis in neonatal rodents increases brush-border membrane NHE8 expression and activity.
• NHE8 plays a crucial role in renal sodium and acid-base homeostasis.

Purpose of the Study:

• To investigate the regulation of NHE8 by acid conditions in kidney cells.
• To determine the mechanisms underlying acid-induced changes in NHE8 activity and surface expression.

Main Methods:

• Incubation of NRK cells (expressing NHE8) with acidic (pH 6.6) or control (pH 7.4) media.
• Measurement of Na(+)/H(+) exchanger activity via pH recovery assay (dpH(i)/dt).
• Assessment of surface NHE8 protein expression using surface biotinylation and streptavidin precipitation.
• Evaluation of the role of protein synthesis using actinomycin D and cycloheximide.
• Investigation of cytoskeletal involvement and exocytosis using colchicine and cytochalasin D.

Main Results:

• Acidic media significantly increased Na(+)/H(+) exchanger activity within 6 hours, persisting for at least 24 hours.
• This increase in activity was paralleled by enhanced surface expression of NHE8.
• Acid-induced increases in NHE8 surface expression and activity were independent of new mRNA or protein synthesis.
• Total cellular NHE8 protein and mRNA abundance did not change with acidic media.
• Acid-induced exocytosis, rather than synthesis, mediated the increase in surface NHE8 expression, involving cytoskeletal elements.

Conclusions:

• Acidic conditions regulate NHE8 activity and surface expression in kidney cells.
• The primary mechanism involves increasing the rate of NHE8 trafficking and exocytosis to the apical membrane.
• These findings highlight a novel regulatory pathway for NHE8 in response to acid-base disturbances.