The cell volume-regulatory glycine transporter GLYT1 is activated following metallopeptidase-mediated detachment of the oocyte from the zona pellucida

Independent cell volume regulation is first acquired by the oocyte in two steps that occur during meiotic maturation: (1) activation of the glycine transporter GLYT1 (Slc6a9) that mediates the intracellular accumulation of glycine to provide osmotic support in the mature egg and early preimplantation embryo, and (2) release of the oocyte from the strong attachment to its rigid extracellular matrix shell, the zona pellucida (ZP). It was recently shown that oocyte-ZP detachment requires metallopeptidase activity that is proposed to cleave transmembrane ZP proteins connecting the oocyte to the ZP. It is unknown, however, how GLYT1 is activated. We hypothesized that oocyte-ZP detachment precedes and may be required for GLYT1 activation. In identically treated pools of oocytes, oocyte-ZP detachment occurred ~20 min before GLYT1 activation. In individual oocytes, GLYT1 activity was detected only in those that were mostly or fully detached. Blocking detachment using previously validated small molecule metallopeptidase inhibitors partly suppressed GLYT1 activation. However, removal of the ZP did not accelerate GLYT1 activation. This indicates that oocyte-ZP detachment or cleavage of transmembrane ZP proteins may be required for GLYT1 to become fully activated, or alternatively that metallopeptidase activity independently affects both detachment and GLYT1 activation.

Initial detachment of the mouse oocyte from the zona pellucida is mediated by metallopeptidase activity†

The fully grown mammalian oocyte is tightly attached to its extracellular matrix shell, the zona pellucida (ZP), but the oocyte detaches from the ZP shortly after ovulation is signaled. The mechanism by which the oocyte detaches from the ZP is unknown. Because ZP proteins are initially secreted as transmembrane proteins, we hypothesized that attachment of the oocyte to the ZP is mediated by transmembrane ZP proteins and that detachment occurs when these proteins are cleaved by peptidases. To identify potential candidates for the type of peptidase, we used mouse oocyte transcriptome data sets to identify candidate peptidases localized to the exterior of the oocyte. Screening with a set of small molecule inhibitors that broadly target the families of peptidases represented by the candidates, we found that only inhibitors of the M10 and M12 families of metallopeptidases prevented detachment. Using more selective inhibitors indicated that detachment was prevented by an inhibitor, GI254023X, developed to be selective for ADAM10 in the M12 family but not by those considered selective for the M10 family or for other M12 metallopeptidases expressed in oocytes. Using an antibody that binds to an epitope just distal to the likely cleavage site of murine ZP3 showed that this site was gradually lost from the oocyte surface during the period when detachment occurs and that inhibiting metallopeptidase activity prevented the loss of this epitope. Taken together, these results indicate that detachment of the oocyte from the ZP is mediated by a metallopeptidase.

Initiation of cell volume regulation and unique cell volume regulatory mechanisms in mammalian oocytes and embryos

The period beginning with the signal for ovulation, when a fully-grown oocyte progresses through meiosis to become a mature egg that is fertilized and develops as a preimplantation embryo, is crucial for healthy development. The early preimplantation embryo is unusually sensitive to cell volume perturbations, with even moderate decreases in volume or dysregulation of volume-regulatory mechanisms resulting in developmental arrest. To prevent this, early embryos possess mechanisms of cell volume control that are apparently unique to them. These rely on the accumulation of glycine and betaine (N, N, N-trimethylglycine) as organic osmolytes-compounds that can provide intracellular osmotic support without the deleterious effects of inorganic ions. Preimplantation embryos also have the same mechanisms as somatic cells that mediate rapid responses to deviations in cell volume, which rely on inorganic ion transport. Both the unique, embryo-specific mechanisms that use glycine and betaine and the inorganic ion-dependent mechanisms undergo major changes during meiotic maturation and preimplantation development. The most profound changes occur immediately after ovulation is triggered. Before this, oocytes cannot regulate their volume, since they are strongly attached to their rigid extracellular matrix shell, the zona pellucida. After ovulation is triggered, the oocyte detaches from the zona pellucida and first becomes capable of independent volume regulation. A complex set of developmental changes in each cell volume-regulatory mechanism continues through egg maturation and preimplantation development. The unique cell volume-regulatory mechanisms in eggs and preimplantation embryos and the developmental changes they undergo appear critical for normal healthy embryo development.