MPI encodes phosphomannose isomerase, which interconverts fructose 6-phosphate and mannose 6-phosphate (Man-6-P), used for glycoconjugate biosynthesis. MPI mutations in humans impair protein glycosylation causing congenital disorder of glycosylation Ib (CDG-Ib), but oral mannose supplements normalize glycosylation. To establish a mannose-responsive mouse model for CDG-Ib, we ablated Mpi and provided dams with mannose to rescue the anticipated defective glycosylation. Surprisingly, although glycosylation was normal, Mpi-/- embryos died around E11.5. Mannose supplementation even hastened their death, suggesting that mannose was toxic. Mpi-/- embryos showed growth retardation and placental hyperplasia. More than 90% of Mpi-/- embryos failed to form yolk sac vasculature, and 35% failed chorioallantoic fusion. We generated primary embryonic fibroblasts to investigate the mechanisms leading to embryonic lethality and found that mannose caused a concentration- and time-dependent accumulation of Man-6-P in Mpi -/- fibroblasts. In parallel, ATP decreased by more than 70% after 24 h compared with Mpi-/- controls. In cell lysates, Man-6-P inhibited hexokinase (70%), phosphoglucose isomerase (65%), and glucose-6-phosphate dehydrogenase (85%), but not phosphofructokinase. Incubating intact Mpi -/- fibroblasts with 2-[3H]deoxyglucose confirmed mannose-dependent hexokinase inhibition. Our results in vitro suggest that mannose toxicity in Mpi-/- embryos is caused by Man-6-P accumulation, which inhibits glucose metabolism and depletes intracellular ATP. This was confirmed in E10.5 Mpi-/- embryos where Man-6-P increased more than 10 times, and ATP decreased by 50% compared with Mpi-/- littermates. Because Mpi ablation is embryonic lethal, a murine CDG-Ib model will require hypomorphic Mpi alleles.
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