Uridine diphosphate-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase deletion in mice leads to lethal intracerebral hemorrhage during embryonic development

Among the enzymes of the biosynthesis of sialoglycoconjugates, uridine diphosphate-Nacetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE), catalyzing the first essential step of the sialic acid (Sia) de novo biosynthesis, and cytidine monophosphate (CMP)-Sia synthase (CMAS), activating Sia to CMP-Sia, are particularly important. The knockout of either of these enzymes in mice is embryonically lethal. While the lethality of Cmas(-/-) mice has been attributed to a maternal complement attack against asialo fetal placental cells, the cause of lethality in Gne-deficient embryos has remained elusive. Here, we advanced the significance of sialylation for embryonic development through detailed histological analyses of Gne(-/-) embryos and placentae. We found that Gne(-/-) embryonic and extraembryonic tissues are hyposialylated rather than being completely deficient of sialoglycans, which holds true for Cmas(-/-) embryos. Residual sialylation of Gne(-/-) cells can be explained by scavenging free Sia from sialylated maternal serum glycoconjugates via the lysosomal salvage pathway. The placental architecture of Gne(-/-) mice was unaffected, but severe hemorrhages in the neuroepithelium with extensive bleeding into the cephalic ventricles were present at E12.5 in the mutants. At E13.5, the vast majority of Gne(-/-) embryos were asystolic. This phenotype persisted when Gne-/- mice were backcrossed to a complement component 3-deficient background, confirming distinct pathomechanisms of Cmas(-/-) and Gne(-/-) mice. We conclude that the low level of sialylation observed in Gne(-/-) mice is sufficient both for immune homeostasis at the fetal-maternal interface and for embryonic development until E12.5. However, formation of the neural microvasculature is the first critical process, depending on a higher degree of sialylation during development of the embryo proper.