Fertilized mouse eggs regulate their size principally by accumulating glycine as an intracellular osmolyte using the GLYT1 (SLC6A9) transporter, a mechanism of cell volume homeostasis apparently unique to early embryos before the morula stage. However, nothing was known of cell volume regulation in oocytes before fertilization. We show here that GLYT1 is quiescent in mouse germinal-vesicle-stage oocytes but becomes fully activated within hours after ovulation is triggered. This initiates accumulation of substantial amounts of intracellular glycine in oocytes during meiotic progression, reaching a maximal level in mature eggs. Measurements of endogenous free glycine showed that there were nearly undetectable levels in ovarian germinal-vesicle-stage oocytes, but high levels were present in mature ovulated eggs and in preimplantation embryos through the two-cell stage, but not in morulae. Furthermore, intracellular glycine was regulated in response to changes in external tonicity in eggs and embryos through the two-cell stage, but not in oocytes or embryos after the two-cell stage. Before activation of GLYT1, oocytes were unable to independently regulate their volume. As GLYT1 became active, however, oocyte volume decreased substantially and oocytes gained the ability to regulate their size, which required GLYT1 activity. Before ovulation, oocyte size was instead determined by a strong adhesion to the rigid extracellular matrix of the oocyte, the zona pellucida, which was released coincident with GLYT1 activation. The ability to acutely regulate cell size is thus acquired by the oocyte only after ovulation, when it first develops glycinedependent cell volume regulation.

Early preimplantation embryos are particularly sensitive to increased osmolarity, and require the importation of glycine to regulate their cell volumes using a mechanism unique to early embryos. Cell volume regulation first appears when ovulation is triggered, oocyte zona pellucida adhesion is released, and glycine transport is activated. The requirement for supporting these physiological functions in oocytes and embryos should be taken into account when developing and improving systems for in vitro oocyte maturation and embryo culture.

Oocytes grow within ovarian follicles, and only gain the ability to complete meiosis when they are nearly fully grown. We have found that both of the major types of intracellular pH regulatory mechanisms in the mammal-the Na+/H+ and HCO3-/Cl- exchangers-were essentially inactive in mouse oocytes over most of the course of their growth. However, as oocytes approached full size, Na+/H+ and HCO3-/Cl- exchangers became simultaneously active, and, at the same time, the intracellular pH of isolated oocytes increased sharply by about 0.25 pH unit. This activation of intracellular pH regulatory mechanisms and increase in pH occurred coincident with the acquisition of meiotic competence. The activation of pH regulatory mechanisms during oocyte growth represents a previously unknown milestone in the development of the capacity of the oocyte to function independently upon ovulation.

Mouse embryos employ a unique mechanism of cell volume regulation in which glycine is imported via the GLYT1 transporter to regulate intracellular osmotic pressure. Independent cell volume regulation normally becomes active in the oocyte after ovulation is triggered. This involves two steps: the first is the release of the strong adhesion between the oocyte and zona pellucida (ZP) while the second is the activation of GLYT1. In fully-grown oocytes, release of adhesion and GLYT1 activation also occur spontaneously in oocytes removed from the follicle. It is unknown, however, whether the capacity to release oocyte-ZP adhesion or activate GLYT1 first arises in the oocyte after ovulation is triggered or instead growing oocytes already possess these capabilities but they are suppressed in the follicle. Here, we assessed when during oogenesis oocyte-ZP adhesion can be released and when GLYT1 can be activated, with adhesion assessed by an osmotic assay and GLYT1 activity determined by [ H]-glycine uptake. Oocyte-ZP adhesion could not be released by growing oocytes until they were nearly fully grown. Similarly, the amount of GLYT1 activity that can be elicited in oocytes increased sharply at the end of oogenesis. The SLC6A9 protein that is responsible for GLYT1 activity and Slc6a9 transcripts are present in growing oocytes and increased over the course of oogenesis. Furthermore, SLC6A9 becomes localized to the oocyte plasma membrane as the oocyte grows. Thus, oocytes acquire the ability to regulate their cell volume by releasing adhesion to the ZP and activating GLYT1 as they approach the end of oogenesis. J. Cell. Physiol. 232: 2436-2446, 2017. © 2016 Wiley Periodicals, Inc.