Regulation of intracellular glycine as an organic osmolyte in early preimplantation mouse embryos.

GLYT1, a glycine transporter belonging to the neurotransmitter transporter family, has recently been identified as a novel cell volume-regulatory mechanism in the earliest stages of the mouse preimplantation embryo. It apparently acts by regulating the steady-state intracellular concentration of glycine, which functions as an organic osmolyte in embryos, to balance external osmolarity and thus maintain cell volume. GLYT1 in embryos was the first mammalian organic osmolyte transporter identified that appears to function in cell volume control under conditions of normal osmolarity, rather than being a response to the stress of chronic hypertonicity. Its maximal rate of transport was shown to be regulated by osmolarity. However, it was not known whether this osmotic regulation of the rate of glycine transport is sufficient to account for the observed control of steady-state intracellular glycine levels as a function of osmolarity in embryos. Here, we show that the intracellular accumulation of glycine in embryos is a direct function of the rate of glycine uptake via GLYT1. In addition, we have shown that the rate of efflux, likely via the volume-regulated anion and organic osmolyte channel in embryos, is also under osmotic regulation and contributes substantially to the control of steady-state glycine concentrations. Together, control of both the rate of uptake and rate of efflux of glycine underlies the mechanism of osmotic regulation of the steady-state concentration of glycine and hence cell volume in early embryos.

The glycine neurotransmitter transporter GLYT1 is an organic osmolyte transporter regulating cell volume in cleavage-stage embryos.

Cells subjected to sustained high osmolarity almost universally respond by accumulating compatible organic osmolytes that, in contrast to inorganic ions, are not deleterious even at high intracellular concentrations. Their accumulation from the external environment by known organic osmolyte transporters, such as the four identified in mammals, occurs only slowly in response to sustained high osmolarity, by synthesis of new transporter proteins. Most cells, however, are not subject to high or varying osmolarity, and it is not clear whether organic osmolytes are generally required at normal osmolarities or how they are regulated. The fertilized egg of the mouse is protected in the oviduct from perturbations in osmolarity. However, deleterious effects of osmotic stress were evident in vitro even at normal oviductal osmolarity. Glycine was found to protect development, indicating that early mouse embryos may use glycine as an organic osmolyte at physiological osmolarity. We have now found that GLYT1, a glycine transporter of the neurotransmitter transporter gene family, functions as the organic osmolyte transporter that mediates the osmotically regulated accumulation of glycine and regulates cell volume in early embryos. Furthermore, osmotic stimulation of GLYT1 transport was immediate, without a requirement for protein synthesis, implying regulation different from known organic osmolyte transporters. Thus, GLYT1 appears to have a previously unidentified role as an organic osmolyte transporter that functions in acute organic osmolyte and volume homeostasis near normal osmolarity.