Expression of glucose transporters in the endometrium and early conceptus membranes of the horse

INTRODUCTION: Glucose is the primary energy substrate for early conceptus development and, for the first 40 days of gestation, the equine conceptus depends solely on glucose available in the histotroph; thereafter, histotrophic glucose provision continues to support transport across the definitive placenta.

METHODS: To investigate glucose provision routes during early equine pregnancy we examined expression of glucose transporters in conceptus membranes and endometrium recovered on days 7, 14, 21 and 28 after ovulation. To further differentiate the contributions of maternal progesterone priming and conceptus-endometrium crosstalk in regulating glucose transporter expression, day 8 embryos were transferred to recipient mares on day 8 (synchronous) or day 3 (asynchronous) after ovulation; conceptuses and endometrium were recovered 6 or 11 days later.

RESULTS: The glucose transporters SLC2A1, 2A3, 2A4, 2A8, 2A10 and 5A1 were expressed in equine endometrium. In conceptus membranes, expression of SLC2A1-3, 2A5, 2A8, 2A10, 5A1 and 5A11 increased from day 14, and SLC2A1 protein was highly abundant on the apical trophectodermal membrane and in the endoderm. Asynchronous embryo transfer (ET) resulted in reduced SLC2A1 expression in both the endometrium and conceptus membranes.

DISCUSSION: A wide range of glucose transporters are expressed in the pre-implantation equine conceptus and endometrium, presumably to ensure adequate glucose provision to the developing embryo. Endometrial expression of SLC2A1 appears to be regulated by a combination of progesterone-priming and conceptus signalling, and its delayed upregulation after asynchronous ET may contribute to the observed delay in conceptus development.