Roe deer uterine fluid metabolome reveals elevated glycolysis, fatty acid breakdown, and spermidine synthesis upon reactivation from diapause

The blastocyst of the European roe deer (Capreolus capreolus) undergoes a period of decelerated growth and limited metabolism. During this period known as embryonic diapause, it floats freely in the uterus encircled by the histotroph. Prior to implantation, reactivation is marked by rapid embryonic growth and conceptus elongation. We hypothesized that the uterine fluid, which is known to undergo changes in its composition to support early embryonic development, contributes to controlling embryonic growth during diapause and elongation. We therefore characterized the pre-implantation uterine fluid metabolome during diapause and at elongation by mass spectrometry and particularly assessed nonpolar lipids, polar metabolites, acylcarnitines, and polyamines. Our results show that triglycerides and diglycerides levels decreased at elongation, likely serving as a source for membrane synthesis rather than for energy production. A functional analysis identified glycolysis as a key pathway during elongation, which may compensate for the energy requirements during this phase. We also observed an increase of sphingomyelin; prostaglandin precursors; and the amino acids asparagine, glutamine, and methionine upon elongation. The sphingolipid and glycerophospholipid metabolism pathways were implicated during elongation. Particularly, spermidine, and to some extent spermine but not putrescine-levels significantly increased in the uterine fluid during elongation, indicating their significance for reactivation and/or proliferation at embryo elongation. We conclude that the roe deer uterine fluid sustained dynamic compositional changes necessary to support the energy- and resource-intensive conceptus elongation. However, it remains to be determined whether these changes are the cause or a consequence of embryo elongation. Studying the metabolic changes and molecular interactions in the roe deer during diapause and elongation not only reveals insights into aspects of its reproductive strategy, but also deepens our knowledge of embryo metabolic demands and developmental velocities across species.