New Developments in Treating Glycosphingolipid Storage Diseases

Translational glycobiology: from bench to bedside

The importance of sugars to protein function is real and is of significant clinical relevance. Technology advances enable large population studies to be carried out, shedding light on individual sugar variation and variations with time. Three-dimensional mass spectroscopy on solid pathological specimens is going to open up a whole new world of pathology visualisation. The door is now open to exploit carbohydrate recognition in new therapeutics by identifying novel biomarkers in cancer to aid diagnosis, and also providing therapeutic targets for treatment. Glycan age correlates with biological age. This means we can map the reversal of biological age with exercise and diet.

Pathogenic role of ganglioside metabolism in neurodegenerative diseases

Ganglioside metabolism is altered in several neurodegenerative diseases, and this may participate in several events related to the pathogenesis of these diseases. Most changes occur in specific areas of the brain and their distinct membrane microdomains or lipid rafts. Antiganglioside antibodies may be involved in dysfunction of the blood-brain barrier and disease progression in these diseases. In lipid rafts, interactions of glycosphingolipids, including ganglioside, with proteins may be responsible for the misfolding events that cause the fibril and/or aggregate processing of disease-specific proteins, such as α-synuclein, in Parkinson's disease, huntingtin protein in Huntington's disease, and copper-zinc superoxide dismutase in amyotrophic lateral sclerosis. Targeting ganglioside metabolism may represent an underexploited opportunity to design novel therapeutic strategies for neurodegeneration in these diseases.

Therapeutic effects of stem cells and substrate reduction in juvenile Sandhoff mice

Sandhoff Disease (SD) involves the CNS accumulation of ganglioside GM2 and asialo-GM2 (GA2) due to inherited defects in the β-subunit gene of β-hexosaminidase A and B (Hexb gene). Substrate reduction therapy, utilizing imino sugar N-butyldeoxygalactonojirimycin (NB-DGJ), reduces ganglioside biosynthesis and levels of stored GM2 in SD mice. Intracranial transplantation of Neural Stem Cells (NSCs) can provide enzymatic cross correction, to help reduce ganglioside storage and extend life. Here we tested the effect of NSCs and NB-DGJ, alone and together, on brain β-hexosaminidase activity, GM2, and GA2 content in juvenile SD mice. The SD mice received either cerebral NSC transplantation at post-natal day 0 (p-0), intraperitoneal injection of NB-DGJ (500 mg/kg/day) from p-9 to p-15, or received dual treatments. The brains were analyzed at p-15. β-galactosidase staining confirmed engraftment of lacZ-expressing NSCs in the cerebral cortex. Compared to untreated and sham-treated SD controls, NSC treatment alone provided a slight increase in Hex activity and significantly decreased GA2 content. However, NSCs had no effect on GM2 content when analyzed at p-15. NB-DGJ alone had no effect on Hex activity, but significantly reduced GM2 and GA2 content. Hex activity was slightly elevated in the NSC + drug-treated mice. GM2 and GA2 content in the dual treated mice were similar to that of the NB-DGJ treated mice. These data indicate that NB-DGJ alone was more effective in targeting storage in juvenile SD mice than were NSCs alone. No additive or synergistic effect between NSC and drug was found in these juvenile SD mice.

Molecular basis of reduced glucosylceramidase activity in the most common Gaucher disease mutant, N370S

Gaucher disease is caused by the defective activity of the lysosomal hydrolase, glucosylceramidase. Although the x-ray structure of wild type glucosylceramidase has been resolved, little is known about the structural features of any of the >200 mutations. Various treatments for Gaucher disease are available, including enzyme replacement and chaperone therapies. The latter involves binding of competitive inhibitors at the active site to enable correct folding and transport of the mutant enzyme to the lysosome. We now use molecular dynamics, a set of structural analysis tools, and several statistical methods to determine the flexible behavior of the N370S Gaucher mutant at various pH values, with and without binding the chaperone, N-butyl-deoxynojirimycin. We focus on the effect of the chaperone on the whole protein, on the active site, and on three important structural loops, and we demonstrate how the chaperone modifies the behavior of N370S in such a way that it becomes more active at lysosomal pH. Our results suggest a mechanism whereby the binding of N-butyl-deoxynojirimycin helps target correctly folded glucosylceramidase to the lysosome, contributes to binding with saposin C, and explains the initiation of the substrate-enzyme complex. Such analysis provides a new framework for determination of the structure of other Gaucher disease mutants and suggests new approaches for rational drug design.

Neural precursor cell cultures from GM2 gangliosidosis animal models recapitulate the biochemical and molecular hallmarks of the brain pathology

In this work we showed that genotype-related patterns of hexosaminidase activity, isoenzyme composition, gene expression and ganglioside metabolism observed during embryonic and postnatal brain development are recapitulated during the progressive stages of neural precursor cell (NPC) differentiation to mature glia and neurons in vitro. Further, by comparing NPCs and their differentiated progeny established from Tay-Sachs (TS) and Sandhoff (SD) animal models with the wild-type counterparts, we studied the events linking the accumulation of undegraded substrates to hexosaminidase activity. We showed that similarly to what observed in brain tissues in TS NPCs and progeny, the stored GM2 was partially converted by sialidase to GA2, which can be then degraded in the lysosomes to its components. The latter can be used in a salvage pathway for the formation of GM3. Interestingly, results obtained from ganglioside feeding assays and from measurement of lysosomal sialidase activity suggest that a similar pathway might work also in the SD model.

Glycosphingolipid depletion in PC12 cells using iminosugars protects neuronal membranes from anti-ganglioside antibody mediated injury

Autoimmune neuropathies are frequently associated with pathogenic anti-ganglioside antibodies targeting ganglioside-rich neuronal and glial membranes. The extent of injury is determined by the concentration of membrane ganglioside and thus reduction might be expected to attenuate disease. In this study, we suppressed ganglioside biosynthesis in PC12 cells with the glucosylceramide synthase inhibitor, N-butyldeoxynojirimycin and observed reduced plasma membrane antibody binding and a major neuroprotective effect in complement-mediated lysis assays. These data demonstrate that iminosugar inhibitors, currently used to treat type 1 Gaucher disease, are also of potential value for depleting antigen and thereby suppressing tissue injury in anti-ganglioside antibody-associated neuropathy.

Miglustat for treatment of Niemann-Pick C disease: a randomised controlled study

BACKGROUND

Niemann-Pick type C disease (NPC) is an inherited neurodegenerative disorder characterised by an intracellular lipid-trafficking defect with secondary accumulation of glycosphingolipids. Miglustat, a small iminosugar, reversibly inhibits glucosylceramide synthase, which catalyses the first committed step of glycosphingolipid synthesis. Miglustat is able to cross the blood-brain barrier, and is thus a potential therapy for neurological diseases. We aimed to establish the effect of miglustat on several markers of NPC severity.

METHODS

Patients aged 12 years or older who had NPC (n=29) were randomly assigned to receive either miglustat 200 mg three times a day (n=20) or standard care (n=9) for 12 months. 12 children younger than 12 years of age were included in an additional cohort; all received miglustat at a dose adjusted for body surface area. All participants were then treated with miglustat for an additional year in an extension study. The primary endpoint was horizontal saccadic eye movement (HSEM) velocity, based on its correlation with disease progression. This study is registered as an International Standard Randomised Controlled Trial, number ISRCTN26761144.

FINDINGS

At 12 months, HSEM velocity had improved in patients treated with miglustat versus those receiving standard care; results were significant when patients taking benzodiazepines were excluded (p=0.028). Children showed an improvement in HSEM velocity of similar size at 12 months. Improvement in swallowing capacity, stable auditory acuity, and a slower deterioration in ambulatory index were also seen in treated patients older than 12 years. The safety and tolerability of miglustat 200 mg three times a day in study participants was consistent with previous trials in type I Gaucher disease, where half this dose was used.

INTERPRETATION

Miglustat improves or stabilises several clinically relevant markers of NPC. This is the first agent studied in NPC for which there is both animal and clinical data supporting a disease modifying benefit.

Prevention of lysosomal storage disorders in Israel

Prevention programs for the detection of heterozygotes of relatively prevalent autosomal recessive diseases in various ethnic groups are available in recent years in Israel. Several lysosomal storage disorders (LSD) are included in this program. The goal of the program is the ascertainment of high risk couples before the birth of affected offspring. This is performed by a population screening program that addresses the specific needs and requirements of various population groups in Israel. The programs are supervised and designed by medical/clinical geneticists and are accompanied by genetic counseling prior to and after testing. Three types of population screening programs are in operation. The first type is offered to the general population and is directed to premarital and married couples. High risk families mostly opt for prenatal diagnosis. The second type is performed for diseases with a frequency of about 1:1000. This occurrence is common in Israel only in various Arab communities due to the high rate of consanguinity. The third type is a premarital screening performed by the Orthodox Jewish community and is operated by a nonprofit organization--"Dor Yeshorim". Two heterozygotes for a particular disease are advised not to proceed with the marriage and thus avoid the dilemma of prenatal diagnosis. Founder mutations of the relevant genes for each ethnic group are tested and the testing is tailored for each individual according to his/her ethnic background. Genetic counseling presents family planning options to high risk couples. These programs have resulted in a significant reduction in the birth of affected patients of the tested LSD a well as other recessive diseases in recent years.