Na⁺/H⁺ exchange is inactivated during mouse oocyte meiosis, facilitating glycine accumulation that maintains embryo cell volume

Abstract

The coupled action of the Na(+)/H(+) exchanger NHE1 and the HCO3(-)/Cl(-) exchanger AE2 constitutes the principal mechanism for acute correction of decreased cell volume in mammalian somatic cells, while, when acting separately, they regulate intracellular pH. It was previously found that AE2 becomes inactivated during meiosis in mouse oocytes. Similarly, NHE1 activity stimulated by intracellular acidosis was present in preovulatory germinal vesicle stage (GV) mouse oocytes and then decreased during meiotic maturation. In contrast, NHE1 activity stimulated by decreased cell volume was low in GV oocytes but became active during meiotic maturation as the oocyte detached from the zona pellucida. It then decreased again in mature eggs similar to activity stimulated by acidosis. The subcellular localization of NHE1 was investigated with YFP-tagged NHE1. Exogenous NHE1 expressed in GV oocytes localized to the plasma membrane and resulted in increased Na(+)/H(+) exchanger activity, but only when co-expressed with calcineurin homologous protein 1 (CHP1). When oocytes expressing functional NHE1 were matured to eggs, however, membrane localization of NHE1 and Na(+)/H(+) exchanger activity were lost. It was unknown why NHE1 and AE2 activities are suppressed during meiotic maturation. Maintenance of cell volume in preimplantation embryos requires glycine accumulation via the GLYT1 transporter, a process unique to eggs and early embryos that is initiated during meiotic maturation. When NHE1 and AE2 activities were maintained in GV oocytes by exogenous expression, glycine accumulation was inhibited. We propose that NHE1-mediated acute cell volume regulation is inactivated during meiotic maturation to allow preferential accumulation of glycine in eggs.

Focal adhesion kinase PTK2 autophosphorylation is not required for the activation of sodium–hydrogen exchange by decreased cell volume in the preimplantation mouse embryo

“…We first determined whether maximally inhibiting FAK would affect the ability of 2-cell embryos to activate NHE1 in response to a cell volume decrease. These experiments relied on our previously validated assay that detects volume-mediated increases in NHE1 activity as a marked intracellular alkalinization when cell volume is decreased by introduction of hypertonic medium (Zhou & Baltz, 2013; Zhou et al , 2013; Xu et al , 2017). We used the selective FAK inhibitor PF-562271 at a high concentration that would be expected to completely block any FAK activity (Roberts et al , 2008) and that was similar to concentrations previously used to inhibit FAK in other types of intact cells (Wiemer et al , 2013; Yoon et al , 2014).…”

“…For pHi measurements, decreased cell volume was induced with 350 mOsM medium. We previously showed that the rate of pHi increase was robust at both 350 and 550 mOsM (Zhou et al , 2013). For western blots, we had previously shown for JAK2 that phosphorylation was more easily detected at 550 than 350 mOsM (Zhou & Baltz, 2013).…”

“…In the mouse, relatively minor dysregulation of cell volume control causes development to become irreversibly blocked at the 2-cell stage (Lawitts & Biggers, 1992; Hadi et al , 2005; Baltz & Tartia, 2010; Baltz & Zhou, 2012). It was found that this embryonic arrest was likely to be due to the accumulation of inorganic ions via NHE1 needed to balance even physiologically normal external osmolarity (Zhou & Baltz, 2013; Zhou et al , 2013; Xu et al , 2017). Because of the crucial role that cell volume regulation plays in early embryogenesis, their mechanisms have been extensively investigated (Baltz & Tartia, 2010; Baltz & Zhou, 2012).…”

Nutrition et métabolisme : quel lien avec le développement folliculaire et embryonnaire ches les mammifères ?

“…En effet, sa supplémentation dans les milieux de culture favorise la maturation de l'ovocyte chez la vache, le hamster, le chien, le lapin et le singe. Enfin, chez la souris, la glycine est impliquée dans la régulation du volume de l'ovocyte au cours de l'ovulation (Zhou et al 2013).…”

Preovulatory suppression of mouse oocyte cell volume-regulatory mechanisms is via signalling that is distinct from meiotic arrest

“…Glycine replaces inorganic ions that are instrumental in acute control of cell volume in the early embryo but are detrimental in the longer term 8 . Since early mammalian embryos are extremely sensitive to cell size perturbations, the failure of glycine-mediated cell volume control leads to developmental arrest 9–11 .…”