Perturbation of potassium homeostasis impairs oocyte maturation†

The movement of ions, such as calcium and zinc, is a well-known regulator of oocyte maturation and fertilization. The specific contribution of potassium ions is less well understood. We tested whether acute disruption of K+ homeostasis perturbs oocyte maturation. A preliminary screen identified PA-6 (IK1/KIR2.x inhibitor) as a robust inhibitor of oocyte maturation. We compared effects of PA-6 to Valinomycin (K+ ionophore) and tetraethylammonium chloride (TEAC; broad voltage-gated K+ channel blocker). PA-6 and Valinomycin completely prevented germinal vesicle breakdown (GVBD) and polar body extrusion. The nuclear envelope remained intact but with the chromatin stretching across the nucleus (PA-6) or remained semi condensed in the nucleus (Valinomycin). TEAC permitted GVBD but 66% were arrested at MI, and both MI and MII spindles were abnormal. Thallium-based flux assays showed higher K+ transport in GV than MII oocytes with PA-6 increasing transport in GV only. Valinomycin reduced transport in MII, but not GV oocytes. TEAC enhanced transport in both stages with delayed onset at GV. PA-6 did not elevate cAMP, while Valinomycin and TEAC modestly increased cAMP. Only PA-6 increased mitochondrial membrane potential, while only Valinomycin increased plasma membrane potential. Both PA-6 and Valinomycin blunted depolarization-induced Zn2+ accumulation. Surprisingly, KCNJ2/4/12 transcripts were detected in cumulus oocyte complexes but not in denuded oocytes. However, KCNJX immunoreactivity was present in oocytes, indicating uncertainty about the PA-6 targets in oocytes. Collectively, these data identify K+ flux as a central regulator of oocyte maturation that integrates plasma membrane excitability, organelle function, chromatin remodeling, spindle integrity, and zinc dynamics.