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 similar to 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.