Long term outcome of MPI-CDG patients on D-mannose therapy.

Mannose phosphate isomerase MPI-CDG (formerly CDG-1b) is a potentially fatal inherited metabolic disease which is readily treatable with oral D-mannose. We retrospectively reviewed long-term outcomes of patients with MPI-CDG, all but one of whom were treated with D-mannose. Clinical, biological, and histological data were reviewed at diagnosis and on D-mannose treatment. Nine patients were diagnosed with MPI-CDG at a median age of 3 months. The presenting symptoms were diarrhea (n = 9), hepatomegaly (n = 9), hypoglycemia (n = 8), and protein loosing enteropathy (n = 7). All patients survived except the untreated one who died at 2 years of age. Oral D-mannose was started in eight patients at a median age of 7 months (mean 38 months), with a median follow-up on treatment of 14 years 9 months (1.5-20 years). On treatment, two patients developed severe portal hypertension, two developed venous thrombosis, and 1 displayed altered kidney function. Poor compliance with D-mannose was correlated with recurrence of diarrhea, thrombosis, and abnormal biological parameters including coagulation factors and transferrin profiles. Liver fibrosis persisted despite treatment, but two patients showed improved liver architecture during follow-up. This study highlights (i) the efficacy and safety of D-mannose treatment with a median follow-up on treatment of almost 15 years (ii) the need for life-long treatment (iii) the risk of relapse with poor compliance, (iii) the importance of portal hypertension screening (iv) the need to be aware of venous and renal complications in adulthood.

Consensus guideline for the diagnosis and management of mannose phosphate isomerase-congenital disorder of glycosylation.

Mannose phosphate isomerase-congenital disorder of glycosylation (MPI-CDG) deficiency is a rare subtype of congenital disorders of protein N-glycosylation. It is characterised by deficiency of MPI caused by pathogenic variants in MPI gene. The manifestation of MPI-CDG is different from other CDGs as the patients suffer dominantly from gastrointestinal and hepatic involvement whereas they usually do not present intellectual disability or neurological impairment. It is also one of the few treatable subtypes of CDGs with proven effect of oral mannose. This article covers a complex review of the literature and recommendations for the management of MPI-CDG with an emphasis on the clinical aspect of the disease. A team of international experts elaborated summaries and recommendations for diagnostics, differential diagnosis, management, and treatment of each system/organ involvement based on evidence-based data and experts' opinions. Those guidelines also reveal more questions about MPI-CDG which need to be further studied.

Phosphosugar Stress in Bacillus subtilis: Intracellular Accumulation of Mannose 6-Phosphate Derepressed the glcR-phoC Operon from Repression by GlcR.

Bacillus subtilis phosphorylates sugars during or after their transport into the cell. Perturbation in the conversion of intracellular phosphosugars to the central carbon metabolites and accumulation of phosphosugars can impose stress on the cells. In this study, we investigated the effect of phosphosugar stress on B. subtilis Preliminary experiments indicated that the nonmetabolizable analogs of glucose were unable to impose stress on B. subtilis In contrast, deletion of manA encoding mannose 6-phosphate isomerase (responsible for conversion of mannose 6-phosphate to fructose 6-phosphate) resulted in growth arrest and bulged cell shape in the medium containing mannose. Besides, an operon encoding a repressor (GlcR) and a haloic acid dehalogenase (HAD)-like phosphatase (PhoC; previously YwpJ) were upregulated. Integration of the P glcR-lacZ cassette into different mutational backgrounds indicated that P glcR is induced when (i) a manA-deficient strain is cultured with mannose or (ii) when glcR is deleted. GlcR repressed the transcription of glcR-phoC by binding to the σA-type core elements of P glcR An electrophoretic mobility shift assay showed no interaction between mannose 6-phosphate (or other phosphosugars) and the GlcR-P glcR DNA complex. PhoC was an acid phosphatase mainly able to dephosphorylate glycerol 3-phosphate and ribose 5-phosphate. Mannose 6-phosphate was only weakly dephosphorylated by PhoC. Since deletion of glcR and phoC alone or in combination had no effect on the cells during phosphosugar stress, it is assumed that the derepression of glcR-phoC is a side effect of phosphosugar stress in B. subtilisIMPORTANCEBacillus subtilis has different stress response systems to cope with external and internal stressors. Here, we investigated how B. subtilis deals with the high intracellular concentration of phosphosugars as an internal stressor. The results indicated the derepression of an operon consisting of a repressor (GlcR) and a phosphatase (PhoC). Further analysis revealed that this operon is not a phosphosugar stress response system. The substrate specificity of PhoC may indicate a connection between the glcR-phoC operon and pathways in which glycerol 3-phosphate and ribose 5-phosphate are utilized, such as membrane biosynthesis and teichoic acid elongation.

Mannose impairs tumour growth and enhances chemotherapy.

It is now well established that tumours undergo changes in cellular metabolism1. As this can reveal tumour cell vulnerabilities and because many tumours exhibit enhanced glucose uptake2, we have been interested in how tumour cells respond to different forms of sugar. Here we report that the monosaccharide mannose causes growth retardation in several tumour types in vitro, and enhances cell death in response to major forms of chemotherapy. We then show that these effects also occur in vivo in mice following the oral administration of mannose, without significantly affecting the weight and health of the animals. Mechanistically, mannose is taken up by the same transporter(s) as glucose3 but accumulates as mannose-6-phosphate in cells, and this impairs the further metabolism of glucose in glycolysis, the tricarboxylic acid cycle, the pentose phosphate pathway and glycan synthesis. As a result, the administration of mannose in combination with conventional chemotherapy affects levels of anti-apoptotic proteins of the Bcl-2 family, leading to sensitization to cell death. Finally we show that susceptibility to mannose is dependent on the levels of phosphomannose isomerase (PMI). Cells with low levels of PMI are sensitive to mannose, whereas cells with high levels are resistant, but can be made sensitive by RNA-interference-mediated depletion of the enzyme. In addition, we use tissue microarrays to show that PMI levels also vary greatly between different patients and different tumour types, indicating that PMI levels could be used as a biomarker to direct the successful administration of mannose. We consider that the administration of mannose could be a simple, safe and selective therapy in the treatment of cancer, and could be applicable to multiple tumour types.

Reversible synthesis of colanic acid and O-antigen polysaccharides in Salmonella Typhimurium enhances induction of cross-immune responses and provides protection against heterologous Salmonella challenge.

Colanic Acid (CA) and lipopolysaccharide (LPS) are two major mannose-containing extracellular polysaccharides of Salmonella. Their presence on the bacterial surface can mask conserved protective outer membrane proteins (OMPs) from the host immune system. The mannose moiety in these molecules is derived from GDP-mannose, which is synthesized in several steps. The first two steps require the action of phosphomannose isomerase, encoded by pmi (manA), followed by phosphomannomutase, encoded by manB. There are two copies of manB present in the Salmonella chromosome, one located in the cps gene cluster (cpsG) responsible for CA synthesis, and the other in the rfb gene cluster (rfbK) involved in LPS O-antigen synthesis. In this study, it was demonstrated that the products of cpsG and rfbK are isozymes. To evaluate the impact of these genes on O-antigen synthesis, virulence and immunogenicity, single mutations (Δpmi, ΔrfbK or ΔcpsG) and a double mutation (ΔrfbK ΔcpsG) were introduced into both wild-type Salmonella enterica and an attenuated Δcya Δcrp vaccine strain. The Δpmi, ΔrfbK and ΔcpsG ΔrfbK mutants were defective in LPS synthesis and attenuated for virulence. In orally inoculated mice, strain S122 (Δcrp Δcya ΔcpsG ΔrfbK) and its parent S738 (Δcrp Δcya) were both avirulent and colonized internal tissues. Strain S122 elicited higher levels of anti-S. Typhimurium OMP serum IgG than its parent strain. Mice immunized with S122 were completely protected against challenge with wild-type virulent S. Typhimurium and partially protected against challenge with either wild-type virulent S. Choleraesuis or S. Enteritidis. These data indicate that deletions in rfbK and cpsG are useful mutations for inclusion in future attenuated Salmonella vaccine strains to induce cross-protective immunity.

Sex differences in the adolescent brain and body: Findings from the saguenay youth study.

This Mini-Review describes sex differences in 66 quantitative characteristics of the brain and body measured in a community-based sample of 1,024 adolescents 12-18 years of age, members of the Saguenay Youth Study. Using an extensive phenotyping protocol, we have obtained measures in a number of domains, including brain structure, cognition, mental health, substance use, body composition, metabolism, cardiovascular reactivity, and life style. For each measure, we provide estimates of effect size (Cohen's d) and sex-specific correlations with age (Pearson R). In total 59 of the 66 characteristics showed sex differences (at a nominal P < 0.05), with small (32), medium-sized (13), and large (11) effects. Some, but not all, of these sex differences increase during adolescence; this appears to be the case mostly for anatomical and physiological measures. © 2016 Wiley Periodicals, Inc.

A Novel N-Tetrasaccharide in Patients with Congenital Disorders of Glycosylation, Including Asparagine-Linked Glycosylation Protein 1, Phosphomannomutase 2, and Mannose Phosphate Isomerase Deficiencies.

BACKGROUND: Primary deficiencies in mannosylation of N-glycans are seen in a majority of patients with congenital disorders of glycosylation (CDG). We report the discovery of a series of novel N-glycans in sera, plasma, and cultured skin fibroblasts from patients with CDG having deficient mannosylation. METHOD: We used LC-MS/MS and MALDI-TOF-MS analysis to identify and quantify a novel N-linked tetrasaccharide linked to the protein core, an N-tetrasaccharide (Neu5Acα2,6Galß1,4-GlcNAcß1,4GlcNAc) in plasma, serum glycoproteins, and a fibroblast lysate from patients with CDG caused by ALG1 [ALG1 (asparagine-linked glycosylation protein 1), chitobiosyldiphosphodolichol ß-mannosyltransferase], PMM2 (phosphomannomutase 2), and MPI (mannose phosphate isomerase). RESULTS: Glycoproteins in sera, plasma, or cell lysate from ALG1-CDG, PMM2-CDG, and MPI-CDG patients had substantially more N-tetrasaccharide than unaffected controls. We observed a >80% decline in relative concentrations of the N-tetrasaccharide in MPI-CDG plasma after mannose therapy in 1 patient and in ALG1-CDG fibroblasts in vitro supplemented with mannose. CONCLUSIONS: This novel N-tetrasaccharide could serve as a diagnostic marker of ALG1-, PMM2-, or MPI-CDG for screening of these 3 common CDG subtypes that comprise >70% of CDG type I patients. Its quantification by LC-MS/MS may be useful for monitoring therapeutic efficacy of mannose. The discovery of these small N-glycans also indicates the presence of an alternative pathway in N-glycosylation not recognized previously, but its biological significance remains to be studied.

Successful liver transplantation and long-term follow-up in a patient with MPI-CDG.

Hepatopathy is the most common feature in the Congenital Disorders of Glycosylation (CDG). More than 70 subtypes have been identified in this growing group of inborn errors. Most defects present as multisystem disease, whereas phosphomannose isomerase deficiency (MPI-CDG) presents with exclusive hepato-intestinal phenotype. MPI-CDG has been considered as one of the very few treatable disorders of glycosylation; several patients showed significant improvement of their life-threatening protein-losing enteropathy and coagulation disorder on oral mannose supplementation therapy. However, patients who have MPI-CDG develop progressive liver insufficiency during a later course of disease. A patient who had MPI-CDG developed progressive liver fibrosis, despite oral mannose supplementation and repeated fractionated heparin therapy. She showed mannose therapy-associated hemolytic jaundice. She developed severe dyspnea and exercise intolerance owing to pulmonary involvement, necessitating liver transplant. After transplantation her physical exercise tolerance, pulmonary functions, and metabolic parameters became fully restored. She is still doing well 2 years after transplantation now. In conclusion, we here report on the first successful liver transplantation in CDG.

Mannose metabolism: more than meets the eye.

Mannose is a simple sugar with a complex life. It is a welcome therapy for genetic and acquired human diseases, but it kills honeybees and blinds baby mice. It could cause diabetic complications. Mannose chemistry, metabolism, and metabolomics in cells, tissues and mammals can help explain these multiple systemic effects. Mannose has good, bad or ugly outcomes depending on its steady state levels and metabolic flux. This review describes the role of mannose at cellular level and its impact on organisms.

Asymptomatic phosphomannose isomerase deficiency (MPI-CDG) initially mistaken for excessive alcohol consumption.

CASE REPORT: In a routine company health check-up, a 32-year-old woman presented a highly elevated serum level of carbohydrate-deficient transferrin (CDT), a biomarker for excessive alcohol consumption. The test result (~17% disialotransferrin, reference interval <2.0%; ~3% asialotransferrin, reference 0%) was confirmed by analysis of a second sample, while another alcohol biomarker, phosphatidylethanol (PEth) in whole-blood, was negative. This suggested that her elevated CDT may be unrelated to heavy drinking. The abnormal "type-1" transferrin glycoform pattern indicated a defect in N-glycan assembly occurring in congenital disorders of glycosylation (CDG), a family of rare inherited metabolic disorders. Probing for the underlying enzyme defect(s) using cultured skin fibroblasts demonstrated normal activity of phosphomannomutase, whereas the activity of phosphomannose isomerase (MPI) was reduced (0.64 mU/mg protein, reference 2.1-6.9), pointing to CDG of the MPI subtype (formerly called CDG-Ib). The diagnosis was confirmed by sequence analysis of the MPI gene revealing a homozygous missense mutation (c.656G>A) causing replacement of arginine by glutamine (p.R219Q). However, the woman had never experienced any clinical manifestations associated with MPI-CDG. Both parents, being distant relatives, were heterozygous mutation carriers with normal CDT values. Two of three siblings were not affected, whereas one brother was also homozygous for c.656G>A and had a highly elevated CDT and no clinical symptoms. CONCLUSION: The finding of MPI-CDG adults without clinical manifestations suggests that this type of the disorder may be underdiagnosed. If asymptomatic MPI-CDG subjects undergo CDT screening, their highly elevated test results may be wrongly interpreted as caused by excessive alcohol consumption.

Congenital disorders of glycosylation. Part I. Defects of protein N-glycosylation.

Glycosylation is the most common chemical process of protein modification and occurs in every living cell. Disturbances of this process may be either congenital or acquired. Congenital disorders of glycosylation (CDG) are a rapidly growing disease family, with about 50 disorders reported since its first clinical description in 1980. Most of the human diseases have been discovered recently. CDG result from defects in the synthesis of the N- and O-glycans moiety of glycoproteins, and in the attachment to the polypeptide chain of proteins. These defects have been found in the activation, presentation, and transport of sugar precursors, in the enzymes responsible for glycosylation, and in proteins that control the traffic of component. There are two main types of protein glycosylation: N-glycosylation and O-glycosylation. Most diseases are due to defects in the N-glycosylation pathway. For the sake of convenience, CDG were divided into 2 types, type I and II. CDG can affect nearly all organs and systems. The considerable variability of clinical features makes it difficult to recognize patients with CDG. Diagnosis can be made on the basis of abnormal glycosylation display. In this paper, an overview of CDG with a new nomenclature limited to the group of protein N-glycosylation disorders, clinical phenotype and diagnostic approach, have been presented. The location, reasons for defects, and the number of cases have been also described. This publication aims to draw attention to the possibility of occurrence of CDG in each multisystem disorder with an unknown origin.

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.

Seizures and stupor during intravenous mannose therapy in a patient with CDG syndrome type 1b (MPI-CDG).

MPI-CDG (formally called CDG 1b), caused by phosphomannose isomerase (MPI) deficiency, leads to hypoglycaemia, protein losing enteropathy, hepatopathy, and thrombotic events, whereas neurologic development remains unaffected. Dietary supplementation of mannose can reverse clinical symptoms by entering the N-glycosylation pathway downstream of MPI. When oral intake of mannose in patients with MPI-CDG is not possible, e.g. due to surgery, mannose has to be given intravenously. We report a patient with MPI-CDG on intravenous mannose therapy that showed severe depression of consciousness and seizures without apparent cause. EEG and cranial MRI findings were compatible with metabolic coma whereas extended laboratory examinations including repeated blood glucose measurements were normal. Importantly, an intravenous bolus of glucose immediately led to clinical recovery and EEG improvement. Mannose did not interfere with glucose measurement in our assay. We suggest that in patients with MPI-CDG, intravenous mannose infusion can lead to intracellular ATP deprivation due to several mechanisms: (1) in MPI deficiency, mannose 6-P cannot be isomerised to fructose 6-P and therefore is unavailable for glycolysis; (2) animal data has shown that accumulating intracellular mannose 6-P inhibits glycolysis; and (3) elevated intracellular mannose 6-P may induce an ATP wasting cycle of dephosphorylation and rephosphorylation ("honey bee effect"). The mannose-induced metabolic inhibition may be overcome by high-dose glucose treatment. We caution that, in patients with MPI-CDG, life-threatening central nervous system disturbances may occur with intravenous mannose treatment. These may be due to intracellular energy failure. Clinical symptoms of energy deficiency should be treated early and aggressively with intravenous glucose regardless of blood glucose levels.

Exogenous mannose does not raise steady state mannose-6-phosphate pools of normal or N-glycosylation-deficient human fibroblasts.

Increasing intracellular mannose-6-phosphate (Man-6-P) was previously reported to reduce the amount of the major lipid linked oligosaccharide (LLO) precursor of N-glycans; a loss that might decrease cellular N-glycosylation. If so, providing dietary mannose supplements to glycosylation-deficient patients might further impair their glycosylation. To address this question, we studied the effects of exogenous mannose on intracellular levels of Man-6-P, LLO, and N-glycosylation in human and mouse fibroblasts. Mannose (500microM) did not increase Man-6-P pools in human fibroblasts from controls or from patients with Congenital Disorders of Glycosylation (CDG), who have 90-95% deficiencies in either phosphomannomutase (CDG-Ia) or phosphomannose isomerase (MPI) (CDG-Ib), enzymes that both use Man-6-P as a substrate. In the extreme case of fibroblasts derived from Mpi null mice (<0.001% MPI activity), intracellular Man-6-P levels greatly increased in response to exogenous mannose, and this produced a dose-dependent decrease in the steady state level of the major LLO precursor. However, LLO loss did not decrease total protein N-glycosylation or that of a hypoglycosylation indicator protein, DNaseI. These results make it very unlikely that exogenous mannose could impair N-glycosylation in glycosylation-deficient CDG patients.

The clinical spectrum of phosphomannose isomerase deficiency, with an evaluation of mannose treatment for CDG-Ib.

Phosphomannose isomerase (PMI) deficiency or congenital disorders of glycosylation type Ib (CDG Ib) is the only CDG that can be treated. Despite variable severity leading to dramatically different prognoses, clinical presentation is relatively homogeneous with liver and digestive features associated with hyperinsulinism and inconstant thrombosis. A feature of CDG is that coagulation factors are decreased. In our experience, mannose given orally at least 4 times per day not only transformed lethal CDG Ib into a treatable disease, but also improved the general condition and digestive symptoms of all reported patients but one. Liver disease, however, still persisted. Heparin can be used as an alternative to mannose in certain patients, particularly in the treatment of enteropathy.

Defecto congénito de glucosilación tipo Ib. Experiencia en el tratamiento con manosa / Congenital disorder of glycosylation type 1b. Experience with mannose treatment

Los defectos congénitos de la glucosilación (CDG, por sus siglas en inglés) son enfermedades genéticas, en general multisistémicas, de herencia autosómica recesiva. Son causadas por defectos que afectan al ensamblaje, la transferencia o el procesamiento de los oligosacáridos de las proteínas u otros glucoconjugados. El CDG tipo Ib está causado por la deficiencia de la enzima citosólica fosfomanosa isomerasa (PMI), codificada por el gen MPI, que cataliza la interconversión de fructosa-6-P y manosa-6-P. Los síntomas son, fundamentalmente, gastrointestinales y hepáticos, y a diferencia de la mayoría de los pacientes con otros tipos de defectos congénitos de la glucosilación, no existe afectación neurológica. El tratamiento con manosa es muy eficaz. Describimos el primer caso de un paciente con CDG-Ib diagnosticado en España. La enfermedad se inició clínicamente a los 6 meses con hipoglucemia, fallo de medro e hipertransaminasemia; posteriormente el paciente desarrolló una enteropatía con atrofia vellositaria subtotal detectada en la biopsia. El paciente presentaba un porcentaje de transferrina deficiente en carbohidratos en el suero del 42 %, un patrón tipo 1 en el isoelectroenfoque de la transferrina sérica, una actividad PMI en fibroblastos del 16 % y las mutaciones R219Q y R56fs en el gen MPI. El tratamiento con manosa a dosis de 1 g/kg/día en 5 dosis resultó muy eficaz, y se normalizaron tanto los parámetros clínicos como los bioquímicos. El defecto congénito de la glucosilación Ib debería incluirse en el diagnóstico diferencial de hipoglucemias, hepatopatías, enteropatías y situaciones de hipercoagulabilidad, en ausencia de otras etiologías más comunes y, sobre todo, si se asocian varios de estos síntomas (AU)

Congenital disorders of glycosylation (CDG) are recessively inherited multisystemic disorders resulting from several genetic defects affecting the assembly, transfer or processing of oligosaccharides onto proteins and other glycoconjugates. CDG type Ib is due to a deficiency of phosphomannose isomerase (PMI) encoded by the MPI gene. PMI catalyzes the interconversion of fructose-6-P and mannose-6-P. The clinical phenotype is characterized by gastro-intestinal and hepatic symptoms. In contrast to most CDG patients, there is no neurological affectation. It's a mannose treatable disorder. We report the first recognised case of CDG Ib in Spain. He presented at 6 months with hypoglycaemia, failure to thrive and hypertransaminasaemia. He subsequently developed an enteropathy with subtotal villous atrophy on biopsy. The % CDT was very high and he presented with a type 1 pattern in transferrin isoelectric focusing. PMI activity in fibroblasts was very deficient. Mutations in MPI gene at R219Q and R56fs were found. Clinical and biochemical parameters normalised after treatment with mannose 1 g/kg/day in 5 doses. CDG Ib should be considered in patients with hypoglycaemia, liver disease, enteropathy and hypercoagulability, in the absence of other common causes, and particularly if some of them are combined (AU)

The relative contribution of mannose salvage pathways to glycosylation in PMI-deficient mouse embryonic fibroblast cells.

Mannose for mammalian glycan biosynthesis can be imported directly from the medium, derived from glucose or salvaged from endogenous or external glycans. All pathways must generate mannose 6-phosphate, the activated form of mannose. Imported or salvaged mannose is directly phosphorylated by hexokinase, whereas fructose 6-phosphate from glucose is converted to mannose 6-phosphate by phosphomannose isomerase (PMI). Normally, PMI provides the majority of mannose for glycan synthesis. To assess the contribution of PMI-independent pathways, we used PMI-null fibroblasts to study N-glycosylation of DNase I, a highly sensitive indicator protein. In PMI-null cells, imported mannose and salvaged mannose make a significant contribution to N-glycosylation. When these cells were grown in mannose-free medium along with the mannosidase inhibitor, swainsonine, to block the salvage pathways, N-glycosylation of DNase I was almost completely eliminated. Adding approximately 13 microm mannose to the medium completely restored normal glycosylation. Treatment with bafilomycin A(1), an inhibitor of lysosomal acidification, also markedly reduced N-glycosylation of DNase I, but in this case only 8 microm mannose was required to restore full glycosylation, indicating that a nonlysosomal source of mannose made a significant contribution. Glycosylation levels were greatly also reduced in glycoconjugate-free medium, when endosomal membrane trafficking was blocked by expression of a mutant SKD1. From these data, we conclude that PMI-null cells can salvage mannose from both endogenous and external glycoconjugates via lysosomal and nonlysosomal degradation pathways.

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.