N-glycosylation deficiency reduces ICAM-1 induction and impairs inflammatory response.

Congenital disorders of glycosylation (CDGs) result from mutations in various N-glycosylation genes. The most common type, phosphomannomutase-2 (PMM2)-CDG (CDG-Ia), is due to deficient PMM2 (Man-6-P → Man-1-P). Many patients die from recurrent infections, but the mechanism is unknown. We found that glycosylation-deficient patient fibroblasts have less intercellular adhesion molecule-1 (ICAM-1), and because of its role in innate immune response, we hypothesized that its reduction might help explain recurrent infections in CDG patients. We, therefore, studied mice with mutations in Mpi encoding phosphomannose isomerase (Fru-6-P → Man-6-P), the cause of human MPI-CDG. We challenged MPI-deficient mice with an intraperitoneal injection of zymosan to induce an inflammatory response and found decreased neutrophil extravasation compared with control mice. Immunohistochemistry of mesenteries showed attenuated neutrophil egress, presumably due to poor ICAM-1 response to acute peritonitis. Since phosphomannose isomerase (MPI)-CDG patients and their cells improve glycosylation when given mannose, we provided MPI-deficient mice with mannose-supplemented water for 7 days. This restored ICAM-1 expression on mesenteric endothelial cells and enhanced transendothelial migration of neutrophils during acute inflammation. Attenuated inflammatory response in glycosylation-deficient mice may result from a failure to increase ICAM-1 on the vascular endothelial surface and may help explain recurrent infections in patients.

Mannose supplements induce embryonic lethality and blindness in phosphomannose isomerase hypomorphic mice.

Patients with congenital disorder of glycosylation (CDG), type Ib (MPI-CDG or CDG-Ib) have mutations in phosphomannose isomerase (MPI) that impair glycosylation and lead to stunted growth, liver dysfunction, coagulopathy, hypoglycemia, and intestinal abnormalities. Mannose supplements correct hypoglycosylation and most symptoms by providing mannose-6-P (Man-6-P) via hexokinase. We generated viable Mpi hypomorphic mice with residual enzymatic activity comparable to that of patients, but surprisingly, these mice appeared completely normal except for modest (~15%) embryonic lethality. To overcome this lethality, pregnant dams were provided 1-2% mannose in their drinking water. However, mannose further reduced litter size and survival to weaning by 40 and 66%, respectively. Moreover, ~50% of survivors developed eye defects beginning around midgestation. Mannose started at birth also led to eye defects but had no effect when started after eye development was complete. Man-6-P and related metabolites accumulated in the affected adult eye and in developing embryos and placentas. Our results demonstrate that disturbing mannose metabolic flux in mice, especially during embryonic development, induces a highly specific, unanticipated pathological state. It is unknown whether mannose is harmful to human fetuses during gestation; however, mothers who are at risk for having MPI-CDG children and who consume mannose during pregnancy hoping to benefit an affected fetus in utero should be cautious.

A zebrafish model of congenital disorders of glycosylation with phosphomannose isomerase deficiency reveals an early opportunity for corrective mannose supplementation.

Individuals with congenital disorders of glycosylation (CDG) have recessive mutations in genes required for protein N-glycosylation, resulting in multi-systemic disease. Despite the well-characterized biochemical consequences in these individuals, the underlying cellular defects that contribute to CDG are not well understood. Synthesis of the lipid-linked oligosaccharide (LLO), which serves as the sugar donor for the N-glycosylation of secretory proteins, requires conversion of fructose-6-phosphate to mannose-6-phosphate via the phosphomannose isomerase (MPI) enzyme. Individuals who are deficient in MPI present with bleeding, diarrhea, edema, gastrointestinal bleeding and liver fibrosis. MPI-CDG patients can be treated with oral mannose supplements, which is converted to mannose-6-phosphate through a minor complementary metabolic pathway, restoring protein glycosylation and ameliorating most symptoms, although liver disease continues to progress. Because Mpi deletion in mice causes early embryonic lethality and thus is difficult to study, we used zebrafish to establish a model of MPI-CDG. We used a morpholino to block mpi mRNA translation and established a concentration that consistently yielded 13% residual Mpi enzyme activity at 4 days post-fertilization (dpf), which is within the range of MPI activity detected in fibroblasts from MPI-CDG patients. Fluorophore-assisted carbohydrate electrophoresis detected decreased LLO and N-glycans in mpi morphants. These deficiencies resulted in 50% embryonic lethality by 4 dpf. Multi-systemic abnormalities, including small eyes, dysmorphic jaws, pericardial edema, a small liver and curled tails, occurred in 82% of the surviving larvae. Importantly, these phenotypes could be rescued with mannose supplementation. Thus, parallel processes in fish and humans contribute to the phenotypes caused by Mpi depletion. Interestingly, mannose was only effective if provided prior to 24 hpf. These data provide insight into treatment efficacy and the broader molecular and developmental abnormalities that contribute to disorders associated with defective protein glycosylation.

A "glyconutrient sham".

The discipline of glycobiology contributes to our understanding of human health and disease through research, most of which is published in peer-reviewed scientific journals. Recently, legitimate discoveries in glycobiology have been used as marketing tools to help sell plant extracts termed "glyconutrients." The glyconutrient industry has a worldwide sales force of over half a million people and sells nearly half a billion dollars (USD) of products annually. Here we address the relationship between glyconutrients and glycobiology, and how glyconutrient claims may impact the public and our discipline.

Ablation of mouse phosphomannose isomerase (Mpi) causes mannose 6-phosphate accumulation, toxicity, and embryonic lethality.

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 man-nose 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-[(3)H]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.

Sweet solution: sugars to the rescue.

Sugar pills are usually placebos, but Smith et al. (2002, this issue) use one to rescue designer mice unable to make GDP-Fucose. Dietary fucose enters a salvage pathway and spares the mice. Sound simple? Not so. Unknown genetic factors determine life or death.