Glycine blocks the regulatory volume response of mouse oocytes to hypoosmotic stress

Abstract

In discussing the causes of the compensatory response to hypoosmotic stress (regulatory volume decrease, RVD), the induced transport of osmolytes from the cell through the volume sensitive organic osmolyte anion channel (VSOAC) is considered first [1]. A Сl -channel with similar pharmacological prop erties, coupled to nonspecific release of amino acids, was found on zygote and oocyte membranes [2,3]. The interest to the involvement of glycine in osmotic regulation is caused by the fact that a specific trans porter, GLYT1, was found on the membrane of mouse zygote and two cell embryo [4]. This active Na + (Cl -) dependent transport of glycine was later found in mature oocytes at the stage of MII development [5]. GLYT1 provides transmembrane transport of glycine in both directions [6], which suggests the possibility of its involvement in RVD. However, the role of glycine in the compensatory response of an embryonic cell to hypotonic stress remains poorly understood [7], and only single papers are devoted to RVD regulation in the mature oocytes, the immediate precursor of early embryogenesis [3].This study shows that glycine present at a physio logical concentration (10 mM) in the extracellular space blocks the development of RVD in the oocyte. The effect is similar to the non osmotic effect of ATP, an inhibitor of VSOAC. A decrease in the concentra tion of the amino acid to the submillimolar level does not recover the compensatory reaction. This fact sug gests that oocytes have mechanisms of glycine medi ated regulation of the osmotic response.Isolated mouse MII oocytes were prepared as described in [8]. The volume of a single oocyte (Fig. 1) was measured by quantitative laser scanning micros copy (QLSM). The procedure of sample preparation for QLSM and the details of measurement of spatial characteristics were published earlier [9, 10].The obtained kinetics of the change in the cell vol ume in the oocyte in the course of hypoosmotic stress is shown in Fig. 2.In the absence of glycine, RVD is triggered in response to hypoosmotic stress, when the initial swell ing is accompanied by an adaptive response, as a result of which the cell tends to recover the intact volume (Fig. 2). Earlier, the authors of [5] demonstrated the presence of GLYT1 on the membrane of mouse oocyte MII. This glycine transporter is reversible, i.e., it per forms active transmembrane transport of amino acids in both directions, which is due to the gradient of sodium ions on the cell membrane. Hypothetically, the driving force in the case of the oocyte may be a rel atively high level of Na + , which was detected, for example, in the frog egg [11]. Unfortunately, it is not yet possible to repeat such measurements in the mouse oocyte. As a result, GLYT1, which pumps glycine from the cytoplasm to the extracellular space, may be involved in RVD. In this case, the intracellular con centration of glycine does not necessarily change, because the decrease in the cell volume compensates for the on mass decrease in the content of the amino acid.Note that th...

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

“…Furthermore, the RVD in mature eggs and 1‐cell embryos is blocked by the presence of high extracellular concentrations of a number of organic compounds to which VRAC/VSOAC is permeable. This is due to their entering the cell through the swelling‐activated channels and thus countering the overall net decrease in intracellular osmolyte concentration, which mediates RVD (Baltz, 2001; Pogorelova et al, 2012, 2015). Channel activity with the characteristics of VRAC/VSOAC was also detected in preimplantation mouse embryos by electrophysiological measurements.…”