Roles of the Na,K‐ATPase α4 isoform and the Na⁺/H⁺ exchanger in sperm motility

Abstract

The Na,K-ATPase generates electrochemical gradients that are used to drive the coupled transport of many ions and nutrients across the plasma membrane. The functional enzyme is comprised of an alpha and beta subunit and families of isoforms for both subunits exist. Recent studies in this laboratory have identified a biological role for the Na,K-ATPase alpha4 isoform in sperm motility. Here we further investigate the role of the Na,K-ATPase carrying the alpha4 isoform, showing again that ouabain eliminates sperm motility, and in addition, that nigericin, a H+/K+ ionophore, and monensin, a H+/Na+ ionophore, reinitiate motility. These data, along with the observation that the K+ ionophore valinomycin has no effect on the motility of ouabain-inhibited sperm, suggest that ouabain may change intracellular H+ levels in a manner that is incompatible with sperm motility. We have also localized NHE1 and NHE5, known regulators of intracellular H+ content, to the same region of the sperm as the Na,K-ATPase alpha4 isoform. These data highlight the important role of the Na,K-ATPase alpha4 isoform in regulating intracellular H(+) levels, and provide evidence suggesting the involvement of the Na+/H+ exchanger, which is critical for maintaining normal sperm motility.

Na,K-ATPase α4 isoform is essential for sperm fertility

“…Previous studies have explored the function of α4 using selective pharmacological inhibition of this isoform with ouabain (7,11,12,20). More recently, we have investigated the function of α4 using a mouse model with increased expression of the rat α4 isoform in sperm (15).…”

Flagellar ion channels of sperm: similarities and differences between species

“…Babcock et al [106,107] suggested that the mechanism for proton efflux from bovine sperm was via a voltage-gated proton channel, based on the fact that the sperm cytosol becomes alkaline upon membrane depolarization [106,107]. Later studies focused on the role of sperm-specific Na + /H + (sNHE) and Cl − /HCO 3 − exchangers as potential mechanisms for intracellular alkalization in rodent spermatozoa [32,108,109]. Due to the increase in external Na + upon ejaculation and its flagellar localization, the NHE was proposed as potential mechanism for sperm alkalization.…”

Secretory Carrier Membrane Protein 2 Regulates Cell-surface Targeting of Brain-enriched Na+/H+ Exchanger NHE5

“…In brief, cells were treated with ammonium choline solution (50 mM NH 4 Cl, 80 mM choline chloride, 1 mM MgCl 2 , 2 mM CaCl 2 , 5 mM glucose, 20 mM HEPES-Tris, pH 7.4) for 20 min at 37°C followed by a rapid washout with isotonic choline chloride solution (130 mM choline chloride, 1 mM MgCl 2 , 2 mM CaCl 2 , 5 mM glucose, 20 mM HEPES-Tris, pH 7.4) to acutely acidify the cytosol. Assays were immediately initiated by adding radioactive 22 Na ϩ (1 Ci/ml 22 NaCl in choline chloride solution) to each well in the absence or presence of 1 mM amiloride. After 5 min, the influx of 22 Na ϩ was terminated by rapidly washing each well three times with ice-cold NaCl-saline solution (130 mM NaCl, 5 mM KCl, 1 mM MgCl 2 , 2 mM CaCl 2 , 5 mM glucose, 20 mM HEPES-NaOH, pH 7.4).…”

“…NHE5 was identified as a unique member of the NHE gene family whose mRNA is expressed almost exclusively in the brain (19,20), although more recent studies have suggested that NHE5 might be functional in other cell types such as sperm (21,22) and osteosarcoma cells (23). Curiously, mutations found in several forms of congenital neurological disorders such as spinocerebellar ataxia type 4 (24 -26) and autosomal dominant cerebellar ataxia (27)(28)(29) have been mapped to chromosome 16q22.1, a region containing NHE5.…”