Overelaborated synaptic architecture and reduced synaptomatrix glycosylation in a Drosophila classic galactosemia disease model

Brain function in classic galactosemia, a galactosemia network (GalNet) members review

Classic galactosemia (CG, OMIM #230400, ORPHA: 79,239) is a hereditary disorder of galactose metabolism that, despite treatment with galactose restriction, affects brain function in 85% of the patients. Problems with cognitive function, neuropsychological/social emotional difficulties, neurological symptoms, and abnormalities in neuroimaging and electrophysiological assessments are frequently reported in this group of patients, with an enormous individual variability. In this review, we describe the role of impaired galactose metabolism on brain dysfunction based on state of the art knowledge. Several proposed disease mechanisms are discussed, as well as the time of damage and potential treatment options. Furthermore, we combine data from longitudinal, cross-sectional and retrospective studies with the observations of specialist teams treating this disease to depict the brain disease course over time. Based on current data and insights, the majority of patients do not exhibit cognitive decline. A subset of patients, often with early onset cerebral and cerebellar volume loss, can nevertheless experience neurological worsening. While a large number of patients with CG suffer from anxiety and depression, the increased complaints about memory loss, anxiety and depression at an older age are likely multifactorial in origin.

Mucin-Type O-Glycosylation in the Drosophila Nervous System

Mucin-type O-glycosylation, a predominant type of O-glycosylation, is an evolutionarily conserved posttranslational modification in animals. Mucin-type O-glycans are often found on mucins in the mucous membranes of the digestive tract. These glycan structures are also expressed in other cell types, such as blood cells and nephrocytes, and have crucial physiological functions. Altered expression of mucin-type O-glycans is known to be associated with several human disorders, including Tn syndrome and cancer; however, the physiological roles of mucin-type O-glycans in the mammalian brain remains largely unknown. The functions of mucin-type O-glycans have been studied in the fruit fly, Drosophila melanogaster. The basic structures of mucin-type O-glycans, including Tn antigen (GalNAcα1-Ser/Thr) and T antigen (Galβ1–3GalNAcα1-Ser/Thr), as well as the glycosyltransferases that synthesize them, are conserved between Drosophila and mammals. These mucin-type O-glycans are expressed in the Drosophila nervous system, including the central nervous system (CNS) and neuromuscular junctions (NMJs). In primary cultured neurons of Drosophila, mucin-type O-glycans show a characteristic localization pattern in axons. Phenotypic analyses using mutants of glycosyltransferase genes have revealed that mucin-type O-glycans are required for CNS development, NMJ morphogenesis, and synaptic functions of NMJs in Drosophila. In this review, we describe the roles of mucin-type O-glycans in the Drosophila nervous system. These findings will provide insight into the functions of mucin-type O-glycans in the mammalian brain.

Structure-Based Optimization of Small Molecule Human Galactokinase Inhibitors

Classic galactosemia is a rare disease caused by inherited deficiency of galactose-1 phosphate uridylyltransferase (GALT). Accumulation of galactose-1 phosphate (gal-1P) is thought to be the major cause of the chronic complications associated with this disease, which currently has no treatment. Inhibiting galactokinase (GALK1), the enzyme that generates galactose-1 phosphate, has been proposed as a novel strategy for treating classic galactosemia. Our previous work identified a highly selective unique dihydropyrimidine inhibitor against GALK1. With the determination of a co-crystal structure of this inhibitor with human GALK1, we initiated a structure-based structure-activity relationship (SAR) optimization campaign that yielded novel analogs with potent biochemical inhibition (IC₅₀ < 100 nM). Lead compounds were also able to prevent gal-1P accumulation in patient-derived cells at low micromolar concentrations and have pharmacokinetic properties suitable for evaluation in rodent models of galactosemia.

Secreted C-type lectin regulation of neuromuscular junction synaptic vesicle dynamics modulates coordinated movement

The synaptic cleft manifests enriched glycosylation, with structured glycans coordinating signaling between presynaptic and postsynaptic cells. Glycosylated signaling ligands orchestrating communication are tightly regulated by secreted glycan-binding lectins. Using the Drosophila neuromuscular junction (NMJ) as a model glutamatergic synapse, we identify a new Ca2+-binding (C-type) lectin, Lectin-galC1 (LGC1), which modulates presynaptic function and neurotransmission strength. We find that LGC1 is enriched in motoneuron presynaptic boutons and secreted into the NMJ extracellular synaptomatrix. We show that LGC1 limits locomotor peristalsis and coordinated movement speed, with a specific requirement for synaptic function, but not NMJ architecture. LGC1 controls neurotransmission strength by limiting presynaptic active zone (AZ) and postsynaptic glutamate receptor (GluR) aligned synapse number, reducing both spontaneous and stimulation-evoked synaptic vesicle (SV) release, and capping SV cycling rate. During high-frequency stimulation (HFS), mutants have faster synaptic depression and impaired recovery while replenishing depleted SV pools. Although LGC1 removal increases the number of glutamatergic synapses, we find that LGC1-null mutants exhibit decreased SV density within presynaptic boutons, particularly SV pools at presynaptic active zones. Thus, LGC1 regulates NMJ neurotransmission to modulate coordinated movement.

Fragment Screening Reveals Starting Points for Rational Design of Galactokinase 1 Inhibitors to Treat Classic Galactosemia

Classic galactosemia is caused by loss-of-function mutations in galactose-1-phosphate uridylyltransferase (GALT) that lead to toxic accumulation of its substrate, galactose-1-phosphate. One proposed therapy is to inhibit the biosynthesis of galactose-1-phosphate, catalyzed by galactokinase 1 (GALK1). Existing inhibitors of human GALK1 (hGALK1) are primarily ATP-competitive with limited clinical utility to date. Here, we determined crystal structures of hGALK1 bound with reported ATP-competitive inhibitors of the spiro-benzoxazole series, to reveal their binding mode in the active site. Spurred by the need for additional chemotypes of hGALK1 inhibitors, desirably targeting a nonorthosteric site, we also performed crystallography-based screening by soaking hundreds of hGALK1 crystals, already containing active site ligands, with fragments from a custom library. Two fragments were found to bind close to the ATP binding site, and a further eight were found in a hotspot distal from the active site, highlighting the strength of this method in identifying previously uncharacterized allosteric sites. To generate inhibitors of improved potency and selectivity targeting the newly identified binding hotspot, new compounds were designed by merging overlapping fragments. This yielded two micromolar inhibitors of hGALK1 that were not competitive with respect to either substrate (ATP or galactose) and demonstrated good selectivity over hGALK1 homologues, galactokinase 2 and mevalonate kinase. Our findings are therefore the first to demonstrate inhibition of hGALK1 from an allosteric site, with potential for further development of potent and selective inhibitors to provide novel therapeutics for classic galactosemia.

Investigating Developmental and Epileptic Encephalopathy Using Drosophila melanogaster

Developmental and epileptic encephalopathies (DEEs) are the spectrum of severe epilepsies characterized by early-onset, refractory seizures occurring in the context of developmental regression or plateauing. Early infantile epileptic encephalopathy (EIEE) is one of the earliest forms of DEE, manifesting as frequent epileptic spasms and characteristic electroencephalogram findings in early infancy. In recent years, next-generation sequencing approaches have identified a number of monogenic determinants underlying DEE. In the case of EIEE, 85 genes have been registered in Online Mendelian Inheritance in Man as causative genes. Model organisms are indispensable tools for understanding the in vivo roles of the newly identified causative genes. In this review, we first present an overview of epilepsy and its genetic etiology, especially focusing on EIEE and then briefly summarize epilepsy research using animal and patient-derived induced pluripotent stem cell (iPSC) models. The Drosophila model, which is characterized by easy gene manipulation, a short generation time, low cost and fewer ethical restrictions when designing experiments, is optimal for understanding the genetics of DEE. We therefore highlight studies with Drosophila models for EIEE and discuss the future development of their practical use.

Extracellular heparan sulfate proteoglycans and glycan-binding lectins orchestrate trans-synaptic signaling

The exceedingly narrow synaptic cleft (<20 nm) and adjacent perisynaptic extracellular space contain an astonishing array of secreted and membrane-anchored glycoproteins. A number of these extracellular molecules regulate intercellular trans-synaptic signaling by binding to ligands, acting as co-receptors or modulating ligand-receptor interactions. Recent work has greatly expanded our understanding of extracellular proteoglycan and glycan-binding lectin families as key regulators of intercellular signaling at the synapse. These secreted proteins act to regulate the compartmentalization of glycoprotein ligands and receptors, crosslink dynamic extracellular and cell surface lattices, modulate both exocytosis and endocytosis vesicle cycling, and control postsynaptic receptor trafficking. Here, we focus closely on the Drosophila glutamatergic neuromuscular junction (NMJ) as a model synapse for understanding extracellular roles of the many heparan sulfate proteoglycan (HSPG) and lectin proteins that help determine synaptic architecture and neurotransmission strength. We particularly concentrate on the roles of extracellular HSPGs and lectins in controlling trans-synaptic signaling, especially that mediated by the Wnt and BMP pathways. These signaling mechanisms are causally linked to a wide spectrum of neurological disease states that impair coordinated movement and cognitive functions.

Pathophysiology and targets for treatment in hereditary galactosemia: A systematic review of animal and cellular models

Since the first description of galactosemia in 1908 and despite decades of research, the pathophysiology is complex and not yet fully elucidated. Galactosemia is an inborn error of carbohydrate metabolism caused by deficient activity of any of the galactose metabolising enzymes. The current standard of care, a galactose-restricted diet, fails to prevent long-term complications. Studies in cellular and animal models in the past decades have led to an enormous progress and advancement of knowledge. Summarising current evidence in the pathophysiology underlying hereditary galactosemia may contribute to the identification of treatment targets for alternative therapies that may successfully prevent long-term complications. A systematic review of cellular and animal studies reporting on disease complications (clinical signs and/or biochemical findings) and/or treatment targets in hereditary galactosemia was performed. PubMed/MEDLINE, EMBASE, and Web of Science were searched, 46 original articles were included. Results revealed that Gal-1-P is not the sole pathophysiological agent responsible for the phenotype observed in galactosemia. Other currently described contributing factors include accumulation of galactose metabolites, uridine diphosphate (UDP)-hexose alterations and subsequent impaired glycosylation, endoplasmic reticulum (ER) stress, altered signalling pathways, and oxidative stress. galactokinase (GALK) inhibitors, UDP-glucose pyrophosphorylase (UGP) up-regulation, uridine supplementation, ER stress reducers, antioxidants and pharmacological chaperones have been studied, showing rescue of biochemical and/or clinical symptoms in galactosemia. Promising co-adjuvant therapies include antioxidant therapy and UGP up-regulation. This systematic review provides an overview of the scattered information resulting from animal and cellular studies performed in the past decades, summarising the complex pathophysiological mechanisms underlying hereditary galactosemia and providing insights on potential treatment targets.

Human UDP-galactose 4'-epimerase (GALE) is required for cell-surface glycome structure and function

Glycan biosynthesis relies on nucleotide sugars (NSs), abundant metabolites that serve as monosaccharide donors for glycosyltransferases. In vivo, signal-dependent fluctuations in NS levels are required to maintain normal cell physiology and are dysregulated in disease. However, how mammalian cells regulate NS levels and pathway flux remains largely uncharacterized. To address this knowledge gap, here we examined UDP-galactose 4'-epimerase (GALE), which interconverts two pairs of essential NSs. Using immunoblotting, flow cytometry, and LC-MS-based glycolipid and glycan profiling, we found that CRISPR/Cas9-mediated GALE deletion in human cells triggers major imbalances in NSs and dramatic changes in glycolipids and glycoproteins, including a subset of integrins and the cell-surface death receptor FS-7-associated surface antigen. In particular, we observed substantial decreases in total sialic acid, galactose, and GalNAc levels in glycans. These changes also directly impacted cell signaling, as GALE ^-/- cells exhibited FS-7-associated surface antigen ligand-induced apoptosis. Our results reveal a role of GALE-mediated NS regulation in death receptor signaling and may have implications for the molecular etiology of illnesses characterized by NS imbalances, including galactosemia and metabolic syndrome.

Fertility in classical galactosaemia, a study of N-glycan, hormonal and inflammatory gene interactions

BACKGROUND

Classical Galactosaemia (CG) (OMIM #230400) is a rare inborn error of galactose metabolism caused by deficiency of the enzyme galactose-1-phosphate uridylyltransferase (GALT). Long-term complications persist in treated patients despite dietary galactose restriction with significant variations in outcomes suggesting epigenetic glycosylation influences. Primary Ovarian Insufficiency (POI) is a very significant complication affecting females with follicular depletion noted in early life. We studied specific glycan synthesis, leptin system and inflammatory gene expression in white blood cells as potential biomarkers of infertility in 54 adults with CG adults (27 females and 27 males) (age range 17-51 yr) on a galactose-restricted diet in a multi-site Irish and Dutch study. Gene expression profiles were tested for correlation with a serum Ultra-high Performance Liquid Chromatography (UPLC)-Immunoglobulin (IgG)-N-glycan galactose incorporation assay and endocrine measurements.

RESULTS

Twenty five CG females (93%) had clinical and biochemical evidence of POI. As expected, the CG female patients, influenced by hormone replacement therapy, and the healthy controls of both genders showed a positive correlation between log leptin and BMI but this correlation was not apparent in CG males. The strongest correlations between serum leptin levels, hormones, G-ratio (galactose incorporation assay) and gene expression data were observed between leptin, its gene and G-Ratios data (rs = - 0.68) and (rs = - 0.94) respectively with lower circulating leptin in CG patients with reduced IgG galactosylation. In CG patients (males and females analysed as one group), the key glycan synthesis modifier genes MGAT3 and FUT8, which influence glycan chain bisecting and fucosylation and subsequent cell signalling and adhesion, were found to be significantly upregulated (p < 0.01 and p < 0.05) and also the glycan synthesis gene ALG9 (p < 0.01). Both leptin signalling genes LEP and LEPR were found to be upregulated (p < 0.01) as was the inflammatory genes ANXA1 and ICAM1 and the apoptosis gene SEPT4 (p < 0.01).

CONCLUSIONS

These results validate our previous findings and provide novel experimental evidence for dysregulation of genes LEP, LEPR, ANXA1, ICAM1 and SEPT4 for CG patients and combined with our findings of abnormalities of IgG glycosylation, hormonal and leptin analyses elaborate on the systemic glycosylation and cell signalling abnormalities evident in CG which likely influence the pathophysiology of POI.

Impaired fertility and motor function in a zebrafish model for classic galactosemia

Classic galactosemia is a genetic disorder of galactose metabolism, caused by severe deficiency of galactose-1-phosphate uridylyltransferase (GALT) enzyme activity due to mutations of the GALT gene. Its pathogenesis is still not fully elucidated, and a therapy that prevents chronic impairments is lacking. In order to move research forward, there is a high need for a novel animal model, which allows organ studies throughout development and high-throughput screening of pharmacologic compounds. Here, we describe the generation of a galt knockout zebrafish model and present its phenotypical characterization. Using a TALEN approach, a galt knockout line was successfully created. Accordingly, biochemical assays confirm essentially undetectable galt enzyme activity in homozygotes. Analogous to humans, galt knockout fish accumulate galactose-1-phosphate upon exposure to exogenous galactose. Furthermore, without prior exposure to exogenous galactose, they exhibit reduced motor activity and impaired fertility (lower egg quantity per mating, higher number of unsuccessful crossings), resembling the human phenotype(s) of neurological sequelae and subfertility. In conclusion, our galt knockout zebrafish model for classic galactosemia mimics the human phenotype(s) at biochemical and clinical levels. Future studies in our model will contribute to improved understanding and management of this disorder.

COG7 deficiency in Drosophila generates multifaceted developmental, behavioral and protein glycosylation phenotypes

Congenital disorders of glycosylation (CDG) comprise a family of human multisystemic diseases caused by recessive mutations in genes required for protein N-glycosylation. More than 100 distinct forms of CDGs have been identified and most of them cause severe neurological impairment. The Conserved Oligomeric Golgi (COG) complex mediates tethering of vesicles carrying glycosylation enzymes across the Golgi cisternae. Mutations affecting human COG1, COG2 and COG4-COG8 cause monogenic forms of inherited, autosomal recessive CDGs. We have generated a Drosophila COG7-CDG model that closely parallels the pathological characteristics of COG7-CDG patients, including pronounced neuromotor defects associated with altered N-glycome profiles. Consistent with these alterations, larval neuromuscular junctions of Cog7 mutants exhibit a significant reduction in bouton numbers. We demonstrate that the COG complex cooperates with Rab1 and Golgi phosphoprotein 3 to regulate Golgi trafficking and that overexpression of Rab1 can rescue the cytokinesis and locomotor defects associated with loss of Cog7. Our results suggest that the Drosophila COG7-CDG model can be used to test novel potential therapeutic strategies by modulating trafficking pathways.

Notum coordinates synapse development via extracellular regulation of Wingless trans-synaptic signaling

Synaptogenesis requires orchestrated communication between pre- and postsynaptic cells via coordinated trans-synaptic signaling across the extracellular synaptomatrix. The first Wnt signaling ligand discovered, Drosophila Wingless (Wg; Wnt1 in mammals), plays crucial roles in synaptic development, regulating synapse architecture as well as functional differentiation. Here, we investigate synaptogenic functions of the secreted extracellular deacylase Notum, which restricts Wg signaling by cleaving an essential palmitoleate moiety. At the glutamatergic neuromuscular junction (NMJ) synapse, we find that Notum secreted from the postsynaptic muscle acts to strongly modulate synapse growth, structural architecture, ultrastructural development and functional differentiation. In Notum null flies, we find upregulated extracellular Wg ligand and nuclear trans-synaptic signal transduction, as well as downstream misregulation of both pre- and postsynaptic molecular assembly. Structural, functional and molecular synaptogenic defects are all phenocopied by Wg overexpression, suggesting that Notum acts solely by inhibiting Wg trans-synaptic signaling. Moreover, these synaptic development phenotypes are suppressed by genetically correcting Wg levels in Notum null mutants, indicating that Notum normally functions to coordinate synaptic structural and functional differentiation via negative regulation of Wg trans-synaptic signaling in the extracellular synaptomatrix.

Galactose-1-phosphate uridyltransferase deficiency: A literature review of the putative mechanisms of short and long-term complications and allelic variants

Galactosemia type 1 is an autosomal recessive disorder of galactose metabolism, determined by a deficiency in the enzyme galactose-1-phosphate uridyltransferase (GALT). GALT deficiency is classified as severe or variant depending on biochemical phenotype, genotype and potential to develop acute and long-term complications. Neonatal symptoms usually resolve after galactose-restricted diet; however, some patients, despite the diet, can develop long-term complications, in particular when the GALT enzyme activity results absent or severely decreased. The mechanisms of acute and long-term complications are still discussed and several hypotheses are presented in the literature like enzymatic inhibition, osmotic stress, endoplasmic reticulum stress, oxidative stress, defects of glycosylation or epigenetic modification. This review summarizes the current knowledge of galactosemia, in particular the putative mechanisms of neonatal and long-term complications and the molecular genetics of GALT deficiency.

Sweet and sour: an update on classic galactosemia

Classic galactosemia is a rare inherited disorder of galactose metabolism caused by deficient activity of galactose-1-phosphate uridylyltransferase (GALT), the second enzyme of the Leloir pathway. It presents in the newborn period as a life-threatening disease, whose clinical picture can be resolved by a galactose-restricted diet. The dietary treatment proves, however, insufficient in preventing severe long-term complications, such as cognitive, social and reproductive impairments. Classic galactosemia represents a heavy burden on patients' and their families' lives. After its first description in 1908 and despite intense research in the past century, the exact pathogenic mechanisms underlying galactosemia are still not fully understood. Recently, new important insights on molecular and cellular aspects of galactosemia have been gained, and should open new avenues for the development of novel therapeutic strategies. Moreover, an international galactosemia network has been established, which shall act as a platform for expertise and research in galactosemia. Herein are reviewed some of the latest developments in clinical practice and research findings on classic galactosemia, an enigmatic disorder with many unanswered questions warranting dedicated research.

The galactose-induced decrease in phosphate levels leads to toxicity in yeast models of galactosemia

Classic galactosemia is an inborn error of metabolism caused by deleterious mutations in the GALT gene. A number of evidences indicate that the galactose-1-phosphate accumulation observed in patient cells is a cause of toxicity in this disease. Nevertheless, the consequent molecular events caused by the galactose-1-phosphate accumulation remain elusive. Here we show that intracellular inorganic phosphate levels decreased when yeast models of classic galactosemia were exposed to galactose. The decrease in phosphate levels is probably due to the trapping of phosphate in the accumulated galactose-1-phosphate since the deletion of the galactokinase encoding gene GAL1 suppressed this phenotype. Galactose-induced phosphate depletion caused an increase in glycogen content, an expected result since glycogen breakdown by the enzyme glycogen phosphorylase is dependent on inorganic phosphate. Accordingly, an increase in intracellular phosphate levels suppressed the galactose effect on glycogen content and conferred galactose tolerance to yeast models of galactosemia. These results support the hypothesis that the galactose-induced decrease in phosphate levels leads to toxicity in galactosemia and opens new possibilities for the development of better treatments for this disease.

Assessment of ataxia phenotype in a new mouse model of galactose-1 phosphate uridylyltransferase (GALT) deficiency

Despite adequate dietary management, patients with classic galactosemia continue to have increased risks of cognitive deficits, speech dyspraxia, primary ovarian insufficiency, and abnormal motor development. A recent evaluation of a new galactose-1 phosphate uridylyltransferase (GALT)-deficient mouse model revealed reduced fertility and growth restriction. These phenotypes resemble those seen in human patients. In this study, we further assess the fidelity of this new mouse model by examining the animals for the manifestation of a common neurological sequela in human patients: cerebellar ataxia. The balance, grip strength, and motor coordination of GALT-deficient and wild-type mice were tested using a modified rotarod. The results were compared to composite phenotype scoring tests, typically used to evaluate neurological and motor impairment. The data demonstrated abnormalities with varying severity in the GALT-deficient mice. Mice of different ages were used to reveal the progressive nature of motor impairment. The varying severity and age-dependent impairments seen in the animal model agree with reports on human patients. Finally, measurements of the cerebellar granular and molecular layers suggested that mutant mice experience cerebellar hypoplasia, which could have resulted from the down-regulation of the PI3K/Akt signaling pathway.

Acute and long-term outcomes in a Drosophila melanogaster model of classic galactosemia occur independently of galactose-1-phosphate accumulation

Classic galactosemia (CG) is a potentially lethal inborn error of metabolism that results from the profound loss of galactose-1-phosphate uridylyltransferase (GALT), the second enzyme in the Leloir pathway of galactose metabolism. Neonatal detection and dietary restriction of galactose minimizes or resolves the acute sequelae of CG, but fails to prevent the long-term complications experienced by a majority of patients. One of the substrates of GALT, galactose-1-phosphate (Gal-1P), accumulates to high levels in affected infants, especially following milk exposure, and has been proposed as the key mediator of acute and long-term pathophysiology in CG. However, studies of treated patients demonstrate no association between red blood cell Gal-1P level and long-term outcome severity. Here, we used genetic, epigenetic and environmental manipulations of a Drosophila melanogaster model of CG to test the role of Gal-1P as a candidate mediator of outcome in GALT deficiency. Specifically, we both deleted and knocked down the gene encoding galactokinase (GALK) in control and GALT-null Drosophila, and assessed the acute and long-term outcomes of the resulting animals in the presence and absence of dietary galactose. GALK is the first enzyme in the Leloir pathway of galactose metabolism and is responsible for generating Gal-1P in humans and Drosophila. Our data confirmed that, as expected, loss of GALK lowered or eliminated Gal-1P accumulation in GALT-null animals. However, we saw no concomitant rescue of larval survival or adult climbing or fecundity phenotypes. Instead, we saw that loss of GALK itself was not benign and in some cases phenocopied or exacerbated the outcome seen in GALT-null animals. These findings strongly contradict the long-standing hypothesis that Gal-1P alone underlies pathophysiology of acute and long-term outcomes in GALT-null Drosophila and suggests that other metabolite(s) of galactose, and/or other pathogenic factors, might be involved.

Summary: In a GALT-deficient Drosophila model of classic galactosemia, Gal-1P accumulation is not required for compromised larval survival following galactose exposure or adult movement and fecundity phenotypes.

Classical Galactosaemia and CDG, the N-Glycosylation Interface. A Review

Classical galactosaemia is a rare disorder of carbohydrate metabolism caused by galactose-1-phosphate uridyltransferase (GALT) deficiency (EC 2.7.7.12). The disease is life threatening if left untreated in neonates and the only available treatment option is a long-term galactose restricted diet. While this is lifesaving in the neonate, complications persist in treated individuals, and the cause of these, despite early initiation of treatment, and shared GALT genotypes remain poorly understood. Systemic abnormal glycosylation has been proposed to contribute substantially to the ongoing pathophysiology. The gross N-glycosylation assembly defects observed in the untreated neonate correct over time with treatment. However, N-glycosylation processing defects persist in treated children and adults.Congenital disorders of glycosylation (CDG) are a large group of over 100 inherited disorders affecting largely N- and O-glycosylation.In this review, we compare the clinical features observed in galactosaemia with a number of predominant CDG conditions.We also summarize the N-glycosylation abnormalities, which we have described in galactosaemia adult and paediatric patients, using an automated high-throughput HILIC-UPLC analysis of galactose incorporation into serum IgG with analysis of the corresponding N-glycan gene expression patterns and the affected pathways.

Drosophila melanogaster Models of Galactosemia

The galactosemias are a family of autosomal recessive genetic disorders resulting from impaired function of the Leloir pathway of galactose metabolism. Type I, or classic galactosemia, results from profound deficiency of galactose-1-phosphate uridylyltransferase, the second enzyme in the Leloir pathway. Type II galactosemia results from profound deficiency of galactokinase, the first enzyme in the Leloir pathway. Type III galactosemia results from partial deficiency of UDP galactose 4'-epimerase, the third enzyme in the Leloir pathway. Although at least classic galactosemia has been recognized clinically for more than 100 years, and detectable by newborn screening for more than 50 years, all three galactosemias remain poorly understood. Early detection and dietary restriction of galactose prevent neonatal lethality, but many affected infants grow to experience a broad range of developmental and other disabilities. To date, there is no intervention known that prevents or reverses these long-term complications. Drosophila melanogaster provides a genetically and biochemically facile model for these conditions, enabling studies that address mechanism and open the door for novel approaches to intervention.

Coordinated movement, neuromuscular synaptogenesis and trans-synaptic signaling defects in Drosophila galactosemia models

The multiple galactosemia disease states manifest long-term neurological symptoms. Galactosemia I results from loss of galactose-1-phosphate uridyltransferase (GALT), which converts galactose-1-phosphate + UDP-glucose to glucose-1-phosphate + UDP-galactose. Galactosemia II results from loss of galactokinase (GALK), phosphorylating galactose to galactose-1-phosphate. Galactosemia III results from the loss of UDP-galactose 4'-epimerase (GALE), which interconverts UDP-galactose and UDP-glucose, as well as UDP-N-acetylgalactosamine and UDP-N-acetylglucosamine. UDP-glucose pyrophosphorylase (UGP) alternatively makes UDP-galactose from uridine triphosphate and galactose-1-phosphate. All four UDP-sugars are essential donors for glycoprotein biosynthesis with critical roles at the developing neuromuscular synapse. Drosophila galactosemia I (dGALT) and II (dGALK) disease models genetically interact; manifesting deficits in coordinated movement, neuromuscular junction (NMJ) development, synaptic glycosylation, and Wnt trans-synaptic signalling. Similarly, dGALE and dUGP mutants display striking locomotor and NMJ formation defects, including expanded synaptic arbours, glycosylation losses, and differential changes in Wnt trans-synaptic signalling. In combination with dGALT loss, both dGALE and dUGP mutants compromise the synaptomatrix glycan environment that regulates Wnt trans-synaptic signalling that drives 1) presynaptic Futsch/MAP1b microtubule dynamics and 2) postsynaptic Frizzled nuclear import (FNI). Taken together, these findings indicate UDP-sugar balance is a key modifier of neurological outcomes in all three interacting galactosemia disease models, suggest that Futsch homolog MAP1B and the Wnt Frizzled receptor may be disease-relevant targets in epimerase and transferase galactosemias, and identify UGP as promising new potential therapeutic target for galactosemia neuropathology.

Synaptic roles for phosphomannomutase type 2 in a new Drosophila congenital disorder of glycosylation disease model

Congenital disorders of glycosylation (CDGs) constitute a rapidly growing family of human diseases resulting from heritable mutations in genes driving the production and modification of glycoproteins. The resulting symptomatic hypoglycosylation causes multisystemic defects that include severe neurological impairments, revealing a particularly critical requirement for tightly regulated glycosylation in the nervous system. The most common CDG, CDG-Ia (PMM2-CDG), arises from phosphomannomutase type 2 (PMM2) mutations. Here, we report the generation and characterization of the first Drosophila CDG-Ia model. CRISPR-generated pmm2-null Drosophila mutants display severely disrupted glycosylation and early lethality, whereas RNAi-targeted knockdown of neuronal PMM2 results in a strong shift in the abundance of pauci-mannose glycan, progressive incoordination and later lethality, closely paralleling human CDG-Ia symptoms of shortened lifespan, movement impairments and defective neural development. Analyses of the well-characterized Drosophila neuromuscular junction (NMJ) reveal synaptic glycosylation loss accompanied by defects in both structural architecture and functional neurotransmission. NMJ synaptogenesis is driven by intercellular signals that traverse an extracellular synaptomatrix and are co-regulated by glycosylation and matrix metalloproteinases (MMPs). Specifically, trans-synaptic signaling by the Wnt protein Wingless (Wg) depends on the heparan sulfate proteoglycan (HSPG) co-receptor Dally-like protein (Dlp), which is regulated by synaptic MMP activity. Loss of synaptic MMP2, Wg ligand, Dlp co-receptor and downstream trans-synaptic signaling occurs with PMM2 knockdown. Taken together, this Drosophila CDG disease model provides a new avenue for the dissection of cellular and molecular mechanisms underlying neurological impairments and is a means by which to discover and test novel therapeutic treatment strategies.

Drosophila Collection: This work generates a new Drosophila congenital disorder of glycosylation model for the most common disease category, caused by phosphomannomutase-2 mutation, and reveals a synaptic mechanism underlying associated neurological impairments.

Mucin-type core 1 glycans regulate the localization of neuromuscular junctions and establishment of muscle cell architecture in Drosophila

T antigen (Galβ1-3GalNAcα1-Ser/Thr), a core 1 mucin-type O-glycan structure, is synthesized by Drosophila core 1 β1,3-galactosyltrasferase 1 (dC1GalT1) and is expressed in various tissues. We previously reported that dC1GalT1 synthesizes T antigen expressed in hemocytes, lymph glands, and the central nervous system (CNS) and that dC1GalT1 mutant larvae display decreased numbers of circulating hemocytes and excessive differentiation of hematopoietic stem cells in lymph glands. dC1GalT1 mutant larvae have also been shown to have morphological defects in the CNS. However, the functions of T antigen in other tissues remain largely unknown. In this study, we found that glycans contributed to the localization of neuromuscular junction (NMJ) boutons. In dC1GalT1 mutant larvae, NMJs were ectopically formed in the cleft between muscles 6 and 7 and connected with these two muscles. dC1GalT1 synthesized T antigen, which was expressed at NMJs. In addition, we determined the function of mucin-type O-glycans in muscle cells. In dC1GalT1 mutant muscles, myofibers and basement membranes were disorganized. Moreover, ultrastructural defects in NMJs and accumulation of large endosome-like structures within both NMJ boutons and muscle cells were observed in dC1GalT1 mutants. Taken together, these results demonstrated that mucin-type O-glycans synthesized by dC1GalT1 were involved in the localization of NMJ boutons, synaptogenesis of NMJs, establishment of muscle cell architecture, and endocytosis.

Classical galactosaemia: novel insights in IgG N-glycosylation and N-glycan biosynthesis

Classical galactosaemia (OMIM #230400), a rare disorder of carbohydrate metabolism, is caused by a deficient activity of galactose-1-phosphate uridyltransferase (EC 2.7.7.12). The pathophysiology of the long-term complications, mainly cognitive, neurological and female fertility problems remains poorly understood. The lack of validated biomarkers to determine prognosis, monitor disease progression and responses to new therapies, pose a huge challenge. We report the detailed analysis of an automated robotic hydrophilic interaction ultra-performance liquid chromatography N-glycan analytical method of high glycan peak resolution applied to serum IgG. This has revealed specific N-glycan processing defects observed in 40 adult galactosaemia patients (adults and adolescents), in comparison with 81 matched healthy controls. We have identified a significant increase in core fucosylated neutral glycans (P<0.0001) and a significant decrease in core fucosylated (P<0.001), non-fucosylated (P<0.0001) bisected glycans and, of specific note, decreased N-linked mannose-5 glycans (P<0.0001), in galactosaemia patients. We also report the abnormal expression of a number of related relevant N-glycan biosynthesis genes in peripheral blood mononuclear cells from 32 adult galactosaemia patients. We have noted significant dysregulation of two key N-glycan biosynthesis genes: ALG9 upregulated (P<0.001) and MGAT1 downregulated (P<0.01) in galactosaemia patients, which may contribute to its ongoing pathophysiology. Our data suggest that the use of IgG N-glycosylation analysis with matched N-glycan biosynthesis gene profiles may provide useful biomarkers for monitoring response to therapy and interventions. They also indicate potential gene modifying steps in this N-glycan biosynthesis pathway, of relevance to galactosaemia and related N-glycan biosynthesis disorders.

In Vivo NMR Studies of the Brain with Hereditary or Acquired Metabolic Disorders

Metabolic disorders, whether hereditary or acquired, affect the brain, and abnormalities of the brain are related to cellular integrity; particularly in regard to neurons and astrocytes as well as interactions between them. Metabolic disturbances lead to alterations in cellular function as well as microscopic and macroscopic structural changes in the brain with diabetes, the most typical example of metabolic disorders, and a number of hereditary metabolic disorders. Alternatively, cellular dysfunction and degeneration of the brain lead to metabolic disturbances in hereditary neurological disorders with neurodegeneration. Nuclear magnetic resonance (NMR) techniques allow us to assess a range of pathophysiological changes of the brain in vivo. For example, magnetic resonance spectroscopy detects alterations in brain metabolism and energetics. Physiological magnetic resonance imaging (MRI) detects accompanying changes in cerebral blood flow related to neurovascular coupling. Diffusion and T1/T2-weighted MRI detect microscopic and macroscopic changes of the brain structure. This review summarizes current NMR findings of functional, physiological and biochemical alterations within a number of hereditary and acquired metabolic disorders in both animal models and humans. The global view of the impact of these metabolic disorders on the brain may be useful in identifying the unique and/or general patterns of abnormalities in the living brain related to the pathophysiology of the diseases, and identifying future fields of inquiry.

IgG N-Glycosylation Galactose Incorporation Ratios for the Monitoring of Classical Galactosaemia

Classical galactosaemia (OMIM #230400) is a rare disorder of carbohydrate metabolism caused by deficiency of the galactose-1-phosphate uridyltransferase enzyme (EC 2.7.7.12). The cause of the long-term complications, including neurological, cognitive and fertility problems in females, remains poorly understood. The relatively small number of patients with galactosaemia and the lack of validated biomarkers pose a substantial challenge for determining prognosis and monitoring disease progression and responses to new therapies. We report an improved method of automated robotic hydrophilic interaction ultra-performance liquid chromatography N-glycan analysis for the measurement of IgG N-glycan galactose incorporation ratios applied to the monitoring of adult patients with classical galactosaemia. We analysed 40 affected adult patients and 81 matched healthy controls. Significant differences were noted between the G0/G1 and G0/G2 incorporation ratios between galactosaemia patients and controls (p < 0.001 and <0.01, respectively). Our data indicate that the use of IgG N-glycosylation galactose incorporation analysis may be now applicable for monitoring patient dietary compliance, determining prognosis and the evaluation of potential new therapies.