Metformin-stimulated Mannose Transport in Dermal Fibroblasts

Congenital disorders of glycosylation: Still "hot" in 2020

BACKGROUND

Congenital disorders of glycosylation (CDG) are inherited metabolic diseases caused by defects in the genes important for the process of protein and lipid glycosylation. With the ever growing number of the known subtypes and discoveries regarding the disease mechanisms and therapy development, it remains a very active field of study.

SCOPE OF REVIEW

This review brings an update on the CDG-related research since 2017, describing the novel gene defects, pathobiomechanisms, biomarkers and the patients' phenotypes. We also summarize the clinical guidelines for the most prevalent disorders and the current therapeutical options for the treatable CDG.

MAJOR CONCLUSIONS

In the majority of the 23 new CDG, neurological involvement is associated with other organ disease. Increasingly, different aspects of cellular metabolism (e.g., autophagy) are found to be perturbed in multiple CDG.

GENERAL SIGNIFICANCE

This work highlights the recent trends in the CDG field and comprehensively overviews the up-to-date clinical recommendations.

New and potential strategies for the treatment of PMM2-CDG

BACKGROUND

Mutations in the PMM2 gene cause phosphomannomutase 2 deficiency (PMM2; MIM# 212065), which manifests as a congenital disorder of glycosylation (PMM2-CDG). Mutant PMM2 leads to the reduced conversion of Man-6-P to Man-1-P, which results in low concentrations of guanosine 5'-diphospho-D-mannose, a nucleotide-activated sugar essential for the construction of protein oligosaccharide chains. To date the only therapeutic options are preventive and symptomatic.

SCOPE OF REVIEW

This review covers the latest advances in the search for a treatment for PMM2-CDG.

MAJOR CONCLUSIONS

Treatments based on increasing Man-1-P levels have been proposed, along with the administration of different mannose derivates, employing enzyme inhibitors or repurposed drugs to increase the synthesis of GDP-Man. A single repurposed drug that might alleviate a severe neurological symptom associated with the disorder is now in clinical use. Proof of concept also exists regarding the use of pharmacological chaperones and/or proteostatic regulators to increase the concentration of hypomorphic PMM2 mutant proteins.

GENERAL SIGNIFICANCE

The ongoing challenges facing the discovery of drugs to treat this orphan disease are discussed.

β-Glucose-1,6-Bisphosphate Stabilizes Pathological Phophomannomutase2 Mutants In Vitro and Represents a Lead Compound to Develop Pharmacological Chaperones for the Most Common Disorder of Glycosylation, PMM2-CDG

A large number of mutations causing PMM2-CDG, which is the most frequent disorder of glycosylation, destabilize phosphomannomutase2. We looked for a pharmacological chaperone to cure PMM2-CDG, starting from the structure of a natural ligand of phosphomannomutase2, α-glucose-1,6-bisphosphate. The compound, β-glucose-1,6-bisphosphate, was synthesized and characterized via 31P-NMR. β-glucose-1,6-bisphosphate binds its target enzyme in silico. The binding induces a large conformational change that was predicted by the program PELE and validated in vitro by limited proteolysis. The ability of the compound to stabilize wild type phosphomannomutase2, as well as frequently encountered pathogenic mutants, was measured using thermal shift assay. β-glucose-1,6-bisphosphate is relatively resistant to the enzyme that specifically hydrolyses natural esose-bisphosphates.

The effects of the metformin on inhibition of UVA-induced expression of MMPs and COL-I in human skin fibroblasts

This study was to investigate the effects of metformin (MF) on ultraviolet A (UVA)-induced expression of matrix metalloproteinases (MMPs) and type I collagen (COL-I) in human skin fibroblasts (HSFs). HSFs were cultured in vitro and divided into control group, UVA group, and UVA + MF group. Cell proliferation was detected by CCK-8 method, and intracellular reactive oxygen species (ROS) level was detected by flow cytometry with fluorescent probe 2′,7′-dichlorofluorescein diacetate (DCF-DA) staining. Meanwhile, real-time polymerase chain reaction (PCR) was used to examine the relative messenger RNA (mRNA) expression of aging-related genes, including MMP1, MMP3, and COL-I. Moreover, the expression of MMP1, MMP3, and COL-I proteins was further detected by western blot. Compared with the control group, the ROS content in the UVA group was increased significantly ( P < 0.05), while the ROS content in the UVA + MF group was evidently lower than that in the UVA group ( P < 0.05). In addition, the MMP1 and MMP3 mRNA levels were significantly elevated, while the COL-I mRNA was significantly decreased in UVA-induced HSF cells compared with the control cells. However, MF could significantly inhibit the improved MMP1 and MMP3 mRNA level and increase the COL-I mRNA level. Moreover, MF could significantly reverse the increasing MMP1 and MMP3 protein level and decreasing COL-I protein level induced by UVA. In conclusion, MF can increase the antioxidant capacity of cells and increase the synthesis of COL-I by inhibiting the level of intracellular ROS and the expression of related MMPs, thereby inhibiting the UVA-induced photoaging effect of HSF.

CDG Therapies: From Bench to Bedside

Congenital disorders of glycosylation (CDG) are a group of genetic disorders that affect protein and lipid glycosylation and glycosylphosphatidylinositol synthesis. More than 100 different disorders have been reported and the number is rapidly increasing. Since glycosylation is an essential post-translational process, patients present a large range of symptoms and variable phenotypes, from very mild to extremely severe. Only for few CDG, potentially curative therapies are being used, including dietary supplementation (e.g., galactose for PGM1-CDG, fucose for SLC35C1-CDG, Mn²⁺ for TMEM165-CDG or mannose for MPI-CDG) and organ transplantation (e.g., liver for MPI-CDG and heart for DOLK-CDG). However, for the majority of patients, only symptomatic and preventive treatments are in use. This constitutes a burden for patients, care-givers and ultimately the healthcare system. Innovative diagnostic approaches, in vitro and in vivo models and novel biomarkers have been developed that can lead to novel therapeutic avenues aiming to ameliorate the patients’ symptoms and lives. This review summarizes the advances in therapeutic approaches for CDG.

A zebrafish model of PMM2-CDG reveals altered neurogenesis and a substrate-accumulation mechanism for N-linked glycosylation deficiency

PMM2-CDG patients have phosphomannomutase (Pmm2) deficiency, with developmental and N-linked glycosylation defects attributed to depletion of mannose-1-phosphate and downstream lipid-linked oligosaccharides (LLOs). This, the first PMM2-CDG zebrafish model, shows, unexpectedly, that accumulation of the Pmm2 substrate mannose-6-phosphate explains LLO deficiency.

Congenital disorder of glycosylation (PMM2-CDG) results from mutations in pmm2, which encodes the phosphomannomutase (Pmm) that converts mannose-6-phosphate (M6P) to mannose-1-phosphate (M1P). Patients have wide-spectrum clinical abnormalities associated with impaired protein N-glycosylation. Although it has been widely proposed that Pmm2 deficiency depletes M1P, a precursor of GDP-mannose, and consequently suppresses lipid-linked oligosaccharide (LLO) levels needed for N-glycosylation, these deficiencies have not been demonstrated in patients or any animal model. Here we report a morpholino-based PMM2-CDG model in zebrafish. Morphant embryos had developmental abnormalities consistent with PMM2-CDG patients, including craniofacial defects and impaired motility associated with altered motor neurogenesis within the spinal cord. Significantly, global N-linked glycosylation and LLO levels were reduced in pmm2 morphants. Although M1P and GDP-mannose were below reliable detection/quantification limits, Pmm2 depletion unexpectedly caused accumulation of M6P, shown earlier to promote LLO cleavage in vitro. In pmm2 morphants, the free glycan by-products of LLO cleavage increased nearly twofold. Suppression of the M6P-synthesizing enzyme mannose phosphate isomerase within the pmm2 background normalized M6P levels and certain aspects of the craniofacial phenotype and abrogated pmm2-dependent LLO cleavage. In summary, we report the first zebrafish model of PMM2-CDG and uncover novel cellular insights not possible with other systems, including an M6P accumulation mechanism for underglycosylation.

Neurology of inherited glycosylation disorders

Congenital disorders of glycosylation comprise most of the nearly 70 genetic disorders known to be caused by impaired synthesis of glycoconjugates. The effects are expressed in most organ systems, and most involve the nervous system. Typical manifestations include structural abnormalities (eg, rapidly progressive cerebellar atrophy), myopathies (including congenital muscular dystrophies and limb-girdle dystrophies), strokes and stroke-like episodes, epileptic seizures, developmental delay, and demyelinating neuropathy. Patients can also have neurological symptoms associated with coagulopathies, immune dysfunction with or without infections, and cardiac, renal, or hepatic failure, which are common features of glycosylation disorders. The diagnosis of congenital disorder of glycosylation should be considered for any patient with multisystem disease and in those with more specific phenotypic features. Measurement of concentrations of selected glycoconjugates can be used to screen for many of these disorders, and molecular diagnosis is becoming more widely available in clinical practice. Disease-modifying treatments are available for only a few disorders, but all affected individuals benefit from early diagnosis and aggressive management.

Improvement of dolichol-linked oligosaccharide biosynthesis by the squalene synthase inhibitor zaragozic acid

The majority of congenital disorders of glycosylation (CDG) are caused by defects of dolichol (Dol)-linked oligosaccharide assembly, which lead to under-occupancy of N-glycosylation sites. Most mutations encountered in CDG are hypomorphic, thus leaving residual activity to the affected biosynthetic enzymes. We hypothesized that increased cellular levels of Dol-linked substrates might compensate for the low biosynthetic activity and thereby improve the output of protein N-glycosylation in CDG. To this end, we investigated the potential of the squalene synthase inhibitor zaragozic acid A to redirect the flow of the polyisoprene pathway toward Dol by lowering cholesterol biosynthesis. The addition of zaragozic acid A to CDG fibroblasts with a Dol-P-Man synthase defect led to the formation of longer Dol-P species and to increased Dol-P-Man levels. This treatment was shown to decrease the pathologic accumulation of incomplete Dol pyrophosphate-GlcNAc(2)Man(5) in Dol-P-Man synthase-deficient fibroblasts. Zaragozic acid A treatment also decreased the amount of truncated protein N-linked oligosaccharides in these CDG fibroblasts. The increased cellular levels of Dol-P-Man and possibly the decreased cholesterol levels in zaragozic acid A-treated cells also led to increased availability of the glycosylphosphatidylinositol anchor as shown by the elevated cell-surface expression of the CD59 protein. This study shows that manipulation of the cellular Dol pool, as achieved by zaragozic acid A addition, may represent a valuable approach to improve N-linked glycosylation in CDG cells.

Towards a therapy for phosphomannomutase 2 deficiency, the defect in CDG-Ia patients

Phosphomannomutase (PMM2, Mannose-6-P--> Mannose-1-P) deficiency is the most frequent glycosylation disorder affecting the N-glycosylation pathway. There is no therapy for the hundreds of patients who suffer from this disorder. This review describes previous attempts at therapeutic interventions and introduces perspectives emerging from the drawing boards. Two approaches aim to increase Mannose-1-P: small membrane permeable molecules that increase the availability or/and metabolic flux of precursors into the impaired glycosylation pathway; and, phosphomannomutase enhancement and/or replacement therapy. Glycosylation-deficient cell and animal models are needed to determine which individual or combined approaches improve glycosylation and may be suitable for preclinical evaluation.

Generalized granuloma annulare in a photosensitive distribution resolving with scarring and milia formation

We describe an unusual case of generalized granuloma annulare (GA) in a 70-year-old man. This occurred in a photosensitive distribution, responded rapidly to topical and systemic treatment, and resolved leaving residual scarring and milia. To our knowledge, this is the first report of the occurrence of milia formation and scarring in association with GA.

Stimulation of N-Linked Glycosylation and Lipid-Linked Oligosaccharide Synthesis by Stress Responses in Metazoan Cells

Endoplasmic reticulum (ER) stress responses comprising the unfolded protein response (UPR) are activated by conditions that disrupt folding and assembly of proteins inside the ER lumenal compartment. Conditions known to be proximal triggers of the UPR include saturation of chaperones with misfolded protein, redox imbalance, disruption of Ca2+ levels, interference with N-linked glycosylation, and failure to dispose of terminally misfolded proteins. Potentially, ER stress responses can reprogram cells to correct all of these problems and thereby restore ER function to normal. This article will review literature on stimulation of N-linked glycosylation by ER stress responses, focusing on metazoan systems. The mechanisms involved will be contrasted with those mediating stimulation of N-linked glycosylation by cytoplasmic stress responses. This information will interest readers who study the biological roles of stress responses, the functions of N-linked glycans, and potential strategies for treatment of genetic disorders of N-linked glycosylation.

Translation attenuation by PERK balances ER glycoprotein synthesis with lipid-linked oligosaccharide flux

Endoplasmic reticulum (ER) homeostasis requires transfer and subsequent processing of the glycan Glc(3)Man(9)GlcNAc(2) (G(3)M(9)Gn(2)) from the lipid-linked oligosaccharide (LLO) glucose(3)mannose(9)N-acetylglucosamine(2)-P-P-dolichol (G(3)M(9)Gn(2)-P-P-Dol) to asparaginyl residues of nascent glycoprotein precursor polypeptides. However, it is unclear how the ER is protected against dysfunction from abnormal accumulation of LLO intermediates and aberrant N-glycosylation, as occurs in certain metabolic diseases. In metazoans phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha) on Ser(51) by PERK (PKR-like ER kinase), which is activated by ER stress, attenuates translation initiation. We use brief glucose deprivation to simulate LLO biosynthesis disorders, and show that attenuation of polypeptide synthesis by PERK promotes extension of LLO intermediates to G(3)M(9)Gn(2)-P-P-Dol under these substrate-limiting conditions, as well as counteract abnormal N-glycosylation. This simple mechanism requires eIF2alpha Ser(51) phosphorylation by PERK, and is mimicked by agents that stimulate cytoplasmic stress-responsive Ser(51) kinase activity. Thus, by sensing ER stress from defective glycosylation, PERK can restore ER homeostasis by balancing polypeptide synthesis with flux through the LLO pathway.

Quantitative measurement of events in the mammalian unfolded protein response

The accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response. IRE1, PERK, ATF6, BiP, EDEM, lipid-linked oligosaccharides (LLOs), and XBP1 directly or indirectly participate in this process. This article provides methods used in our laboratory to quantitatively measure the accumulation of mRNAs encoding BiP and EDEM; splicing of XBP-1; cleavage of ATF6; inhibition of protein synthesis by PERK; and extension of LLOs under control and stress conditions.

Analysis of glycosylation in CDG-Ia fibroblasts by fluorophore-assisted carbohydrate electrophoresis: implications for extracellular glucose and intracellular mannose 6-phosphate

Phosphomannomutase (PMM) deficiency causes congenital disorder of glycosylation (CDG)-Ia, a broad spectrum disorder with developmental and neurological abnormalities. PMM converts mannose 6-phosphate (M6P) to mannose-1-phosphate, a precursor of GDP-mannose used to make Glc(3)Man(9)GlcNAc(2)-P-P-dolichol (lipid-linked oligosaccharide; LLO). LLO, in turn, is the donor substrate of oligosaccharyltransferase for protein N-linked glycosylation. Hepatically produced N-linked glycoproteins in CDG-Ia blood are hypoglycosylated. Upon labeling with [(3)H]mannose, CDG-Ia fibroblasts have been widely reported to accumulate [(3)H]LLO intermediates. Since these are thought to be poor oligosaccharyltransferase substrates, LLO intermediate accumulation has been the prevailing explanation for hypoglycosylation in patients. However, this is discordant with sporadic reports of specific glycoproteins (detected with antibodies) from CDG-Ia fibroblasts being fully glycosylated. Here, fluorophore-assisted carbohydrate electrophoresis (FACE, a nonradioactive technique) was used to analyze steady-state LLO compositions in CDG-Ia fibroblasts. FACE revealed that low glucose conditions accounted for previous observations of accumulated [(3)H]LLO intermediates. Additional FACE experiments demonstrated abundant Glc(3)Man(9)GlcNAc(2)-P-P-dolichol, without hypoglycosylation, CDG-Ia fibroblasts grown with physiological glucose. This suggested a "missing link" to explain hypoglycosylation in CDG-Ia patients. Because of the possibility of its accumulation, the effects of M6P on glycosylation were explored in vitro. Surprisingly, M6P was a specific activator for cleavage of Glc(3)Man(9)GlcNAc(2)-P-P-dolichol. This led to futile cycling the LLO pathway, exacerbated by GDP-mannose/PMM deficiency. The possibilities that M6P may accumulate in hepatocytes and that M6P-stimulated LLO cleavage may account for both hypoglycosylation and the clinical failure of dietary mannose therapy with CDG-Ia patients are discussed.

Discordance of UPR signaling by ATF6 and Ire1p-XBP1 with levels of target transcripts

Accumulation of misfolded proteins within the lumen of the mammalian endoplasmic reticulum (ER) activates the unfolded protein response (UPR). ATF6 and Ire1p are ER-associated proteins that control UPR-specific transcription systems in mammals; UPR signaling involves cleavage of ATF6 and splicing of XBP1 mRNA initiated by Ire1p. We tested the hypothesis that activation of ATF6 and/or Ire1p determines the levels of mRNAs derived from target genes encoding GRP78/BiP and EDEM. By subjecting dermal fibroblasts to multiple stresses, strong correlations were found between ATF6 activation and XBP1 splicing, and between GRP78/BiP mRNA and EDEM mRNA accumulation. Surprisingly, there was no reasonable correlation between activation of either signal transducer with accumulation of either target transcript. Thus, ATF6 and Ire1p signaling do not define the magnitude of UPR-dependent mRNA increases, even though they may be necessary for gene activation, suggesting the existence of additional stress-sensitive factors acting as "coincidence detectors" for transcript accumulation.

Activation of glycogen phosphorylase with 5-aminoimidazole-4-carboxamide riboside (AICAR). Assessment of glycogen as a precursor of mannosyl residues in glycoconjugates

The experimental evaluation of the contribution of glycogen phosphorylase (GP) to biochemical pathways is limited to methods that raise cAMP, activating the cAMP-dependent protein kinase/phosphorylase kinase/GP cascade. Such methods convert the unphosphorylated form, "GPb," which catalyzes glycogenolysis only in the presence of appropriate allosteric activators such as AMP, to the phosphorylated, constitutively activated form, "GPa." However, activation of GP in this way is indirect, requires a functional cAMP kinase cascade, and is complicated by other actions of cAMP. Here, we demonstrate a strategy for the experimental manipulation of GP in intact dermal fibroblasts, involving activation by the membrane-permeable adenosine analog 5-aminoimidazole-4-carboxamide riboside (AICAR) and inhibition by caffeine and Pfizer compound CP-91149, which bind to GP at distinct sites. Potential complications because of activation of AMP-activated protein kinase by AICAR were assessed with metformin, which activates this kinase but does not activate GP. Using this strategy, we show that glycogen can be a significant and regulatable precursor of mannosyl units in lipid-linked oligosaccharides and glycoproteins.