The Mechanism of Betaine Accumulation by Mouse Oocytes.

Abstract

Betaine (N,N,N-trimethylglycine) is transported into 1- and 2-cell embryos via SIT1, which mediates betaine transport that is dependent on Na+ and Cl-. SIT1 is activated at fertilization. In early embryos, betaine contributes to cell volume homeostasis, allowing normal development of preimplantation embryos cultured at approximately the osmolarity of the in vivo environment (~300 mOsM), and functions as a methyl donor at the blastocyst stage. We recently measured a high level of endogenous betaine in metaphase-II (MII) stage mouse eggs, before the SIT1 transporter has been activated. The mechanism by which oocytes accumulate betaine and its contribution to the betaine pool in preimplantation embryos are unknown. Since denuded oocytes from the germinal vesicle (GV) to MII stages cannot transport betaine, we hypothesise that cumulus cells of the cumulus-oocyte complex (COC) express a betaine transporter and that betaine transported through this putative transporter is transferred to the enclosed GV via gap junctions. Our data indicate that that there is indeed saturable betaine transport in COCs, since 5 mM unlabelled betaine significantly reduced the rate of transport of [3H]-betaine (1 μM). To determine the characteristics of the betaine transporter, we performed a competitive inhibition profile study. Proline is often transported by betaine transporters (e.g., SIT1). Proline (5 mM) significantly reduced the rate of [3H]-betaine transport by COCs, as did the proline derivative proline methyl ester. Alanine, methionine and methylaminoisobutyric acid all significantly reduced the rate of [3H]-betaine transport by COCs, while GABA (a substrate of the betaine/GABA transporter) and histidine did not significantly affect [3H]-betaine transport by COCs. The rate of [3H]-betaine transport by COCs was determined in the presence vs. absence of Na+ and Cl-, to determine whether co-transport of either is required. The rate of [3H]-betaine transport was significantly reduced in COCs in Na+-free media. However, omitting Cl- did not significantly affect betaine transport. These results indicate that likely candidates for the betaine transporter in COCs are either of the classical amino acid transport systems designated system A or ASC. Further inhibition profiling will determine which of these likely accounts for betaine transport in COCs. Funding: Canadian Institutes of Health Research.