The addition of some growth factors improves the viability of preimplantation embryos in vitro. Sphingosine 1-phosphate (S1P) is a growth factor that, when added to culture media, improves oocyte maturation [1,2] and blastocyst formation [2], and decreases rates of apoptosis throughout development [3,4]. However, it is not known whether endogenous S1P signalling is also present in the embryo through activity of sphingosine kinase (SphK), an enzyme that catalyses the production of S1P from its precursor sphingosine.
The current study aimed to determine the expression and function of the two SphK isoenzymes, SphK1 and SphK2, in preimplantation embryos. SphK1 was expressed in the cytoplasm and nucleus at all stages of development, and staining for the phosphorylated or activated form of SphK1 was strongly cytoplasmic. Inhibition of SphK1 by PF543 reduced the percentage of embryos that cavitated and formed blastocysts. SphK2 was also expressed in the cytoplasm and nucleus at all stages of development, though staining at the 8-cell stage was weak. Phosphorylated SphK2 stained strongly in the membrane and nucleus at all stages, and inhibition of SphK2 by ABC294640 reduced embryo development at all stages.
Together, these results suggest that endogenous production of S1P by SphK is necessary for embryo development in vitro. Differences in localisation of the phosphorylated SphKs and timing of mortality upon inhibition point to SphK1 and SphK2 activating different signalling pathways to promote development, though these pathways remain to be elucidated. Identifying them would provide better insight into the molecular mechanisms underpinning preimplantation embryo development, and this knowledge could be used to improve in vitro embryo culture for clinical, research, and agricultural purposes.