Murine Sialidase Neu3 facilitates GM2 degradation and bypass in mouse model of Tay-Sachs disease

Lithium treatment rescues dysfunctional autophagy in the cell models of Tay-Sachs disease

Tay-Sachs disease is a rare lysosomal storage disorder (LSD) caused by a mutation in the HexA gene coding β-hexosaminidase A enzyme. The disruption of the HexA gene causes the accumulation of GM2 ganglioside resulting in progressive neurodegeneration in humans. Surprisingly, Hexa-/- mice did not show neurological phenotypes. Our group recently generated a murine model of Tay-Sachs disease exhibiting excessive GM2 accumulation and severe neuropathological abnormalities mimicking Tay-Sachs patients. Previously, we reported impaired autophagic flux in the brain of Hexa/-Neu3-/- mice. However, regulation of autophagic flux using inducers has not been clarified in Tay-Sachs disease cells. Here, we evaluated the effects of lithium treatment on dysfunctional autophagic flux using LC3 and p62 in the fibroblast and neuroglia of Hexa-/-Neu3-/- mice and Tay-Sachs patients. We discovered the clearance of accumulating autophagosomes, aggregate-prone metabolites, and GM2 ganglioside under lithium-induced conditions. Our data suggest that targeting autophagic flux with an autophagy inducer might be a rational therapeutic strategy for the treatment of Tay-Sachs disease.

Sialidase NEU3 action on GM1 ganglioside is neuroprotective in GM1 gangliosidosis

GM1 gangliosidosis is a neurodegenerative disorder caused by mutations in the GLB1 gene, which encodes lysosomal β-galactosidase. The enzyme deficiency blocks GM1 ganglioside catabolism, leading to accumulation of GM1 ganglioside and asialo-GM1 ganglioside (GA1 glycolipid) in brain. This disease can present in varying degrees of severity, with the level of residual β-galactosidase activity primarily determining the clinical course. Glb1 null mouse models, which completely lack β-galactosidase expression, exhibit a less severe form of the disease than expected from the comparable deficiency in humans, suggesting a potential species difference in the GM1 ganglioside degradation pathway. We hypothesized this difference may involve the sialidase NEU3, which acts on GM1 ganglioside to produce GA1 glycolipid. To test this hypothesis, we generated Glb1/Neu3 double KO (DKO) mice. These mice had a significantly shorter lifespan, increased neurodegeneration, and more severe ataxia than Glb1 KO mice. Glb1/Neu3 DKO mouse brains exhibited an increased GM1 ganglioside to GA1 glycolipid ratio compared with Glb1 KO mice, indicating that NEU3 mediated GM1 ganglioside to GA1 glycolipid conversion in Glb1 KO mice. The expression of genes associated with neuroinflammation and glial responses were enhanced in Glb1/Neu3 DKO mice compared with Glb1 KO mice. Mouse NEU3 more efficiently converted GM1 ganglioside to GA1 glycolipid than human NEU3 did. Our findings highlight NEU3's role in ameliorating the consequences of Glb1 deletion in mice, provide insights into NEU3's differential effects between mice and humans in GM1 gangliosidosis, and offer a potential therapeutic approach for reducing toxic GM1 ganglioside accumulation in GM1 gangliosidosis patients.

Characterization of a phenotypically severe animal model for human AB-Variant GM2 gangliosidosis

AB-Variant GM2 gangliosidosis (ABGM2) is a rare and lethal genetic disorder caused by mutations in the GM2A gene that lead to fatal accumulation of GM2 gangliosides (GM2) in neurons of the central nervous system (CNS). GM2A encodes a transport protein known as GM2 activator (GM2A) protein, which is essential for degrading GM2 into their GM3 form. ABGM2 presents in infantile-, juvenile-, and adult-onset forms; of the three, the infantile-onset is the most prominent, and by far the most severe, as evidenced by high levels of GM2 accumulation, widespread neurodegeneration, and death by the age of 4. Gm2a-/- mice are commonly used as a model of ABGM2. These mice are characterized by phenotypes most representative of predicted adult-onset form of ABGM2, which include moderate GM2 accumulation and mild neurological defects. This mild phenotype has been attributed to compensation by alternative GM2 degradation pathways mediated by sialidase, neuraminidase 3 (NEU3), a pathway that is more prominent in mice than humans. To assess the extent to which NEU3 contributes to GM2 degradation, we generated double knock-out (Gm2a-/-Neu3-/-) mice. Compellingly, these mice present with a clinical phenotype resembling that of a more severe ABGM2, including ataxia, reduced mobility and coordination, weight loss, poor body scores, and lethality by 6-7 months. Furthermore, these phenotypes correlate with a dramatic increase in GM2 accumulation in the CNS compared to levels observed in either Gm2a-/- or Neu3-/- mice. Taken together, these studies, for the first-time, confirm that the mild neurological phenotype of Gm2a-/- mice is due to compensatory activity on GM2 catabolism through an alternate breakdown pathway involving NEU3. These studies support the use of double knockout mice as a novel and highly relevant model for pre-clinical drug studies in a more severe form of ABGM2.

Sialic acids Neu5Ac and KDN in adipose tissue samples from individuals following habitual vegetarian or non-vegetarian dietary patterns

Sialic acids (Sias) are a class of sugar molecules with a parent nine-carbon neuraminic acid, generally present at the ends of carbohydrate chains, either attached to cellular surfaces or as secreted glycoconjugates. Given their position and structural diversity, Sias modulate a wide variety of biological processes. However, little is known about the role of Sias in human adipose tissue, or their implications for health and disease, particularly among individuals following different dietary patterns. The goal of this study was to measure N-Acetylneuraminic acid (Neu5Ac), N-Glycolylneuraminic acid (Neu5Gc), and 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN) concentrations in adipose tissue samples from participants in the Adventist Health Study-2 (AHS-2) and to compare the abundance of these Sias in individuals following habitual, long-term vegetarian or non-vegetarian dietary patterns. A method was successfully developed for the extraction and detection of Sias in adipose tissue. Sias levels were quantified in 52 vegans, 56 lacto-vegetarians, and 48 non-vegetarians using LC-MS/MS with Neu5Ac-D-1,2,3-13C3 as an internal standard. Dietary groups were compared using linear regression. Vegans and lacto-ovo-vegetarians had significantly higher concentrations of Neu5Ac relative to non-vegetarians. While KDN levels tended to be higher in vegans and lacto-ovo-vegetarians, these differences were not statistically significant. However, KDN levels were significantly inversely associated with body mass index. In contrast, Neu5Gc was not detected in human adipose samples. It is plausible that different Neu5Ac concentrations in adipose tissues of vegetarians, compared to those of non-vegetarians, reflect a difference in the baseline inflammatory status between the two groups. Epidemiologic studies examining levels of Sias in human adipose tissue and other biospecimens will help to further explore their roles in development and progression of inflammatory conditions and chronic diseases.

Biochemical Correction of GM2 Ganglioside Accumulation in AB-Variant GM2 Gangliosidosis

GM2 gangliosidosis is a group of genetic disorders that result in the accumulation of GM2 ganglioside (GM2) in brain cells, leading to progressive central nervous system (CNS) atrophy and premature death in patients. AB-variant GM2 gangliosidosis (ABGM2) arises from loss-of-function mutations in the GM2 activator protein (GM2AP), which is essential for the breakdown of GM2 in a key catabolic pathway required for CNS lipid homeostasis. In this study, we show that intrathecal delivery of self-complementary adeno-associated virus serotype-9 (scAAV9) harbouring a functional human GM2A transgene (scAAV9.hGM2A) can prevent GM2 accumulation in in GM2AP-deficient mice (Gm2a^-/- mice). Additionally, scAAV9.hGM2A efficiently distributes to all tested regions of the CNS within 14 weeks post-injection and remains detectable for the lifespan of these animals (up to 104 weeks). Remarkably, GM2AP expression from the transgene scales with increasing doses of scAAV9.hGM2A (0.5, 1.0 and 2.0 × 10¹¹ vector genomes (vg) per mouse), and this correlates with dose-dependent correction of GM2 accumulation in the brain. No severe adverse events were observed, and comorbidities in treated mice were comparable to those in disease-free cohorts. Lastly, all doses yielded corrective outcomes. These data indicate that scAAV9.hGM2A treatment is relatively non-toxic and tolerable, and biochemically corrects GM2 accumulation in the CNS-the main cause of morbidity and mortality in patients with ABGM2. Importantly, these results constitute proof-of-principle for treating ABGM2 with scAAV9.hGM2A by means of a single intrathecal administration and establish a foundation for future preclinical research.

Glb1 knockout mouse model shares natural history with type II GM1 gangliosidosis patients

GM1 gangliosidosis is a rare lysosomal storage disorder affecting multiple organ systems, primarily the central nervous system, and is caused by functional deficiency of β-galactosidase (GLB1). Using CRISPR/Cas9 genome editing, we generated a mouse model to evaluate characteristics of the disease in comparison to GM1 gangliosidosis patients. Our Glb1-/- mice contain small deletions in exons 2 and 6, producing a null allele. Longevity is approximately 50 weeks and studies demonstrated that female Glb1-/- mice die six weeks earlier than male Glb1-/- mice. Gait analyses showed progressive abnormalities including abnormal foot placement, decreased stride length and increased stance width, comparable with what is observed in type II GM1 gangliosidosis patients. Furthermore, Glb1-/- mice show loss of motor skills by 20 weeks assessed by adhesive dot, hanging wire, and inverted grid tests, and deterioration of motor coordination by 32 weeks of age when evaluated by rotarod testing. Brain MRI showed progressive cerebellar atrophy in Glb1-/- mice as seen in some patients. In addition, Glb1-/- mice also show significantly increased levels of a novel pentasaccharide biomarker in urine and plasma which we also observed in GM1 gangliosidosis patients. Glb1-/- mice also exhibit accumulation of glycosphingolipids in the brain with increases in GM1 and GA1 beginning by 8 weeks. Surprisingly, despite being a null variant, this Glb1-/- mouse most closely models the less severe type II disease and will guide the development of new therapies for patients with the disorder.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2017-2018

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2018. Also included are papers that describe methods appropriate to glycan and glycoprotein analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, new methods, matrices, derivatization, MALDI imaging, fragmentation and the use of arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Most of the applications are presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. The reported work shows increasing use of combined new techniques such as ion mobility and highlights the impact that MALDI imaging is having across a range of diciplines. MALDI is still an ideal technique for carbohydrate analysis and advancements in the technique and the range of applications continue steady progress.

Neuronal Ganglioside and Glycosphingolipid (GSL) Metabolism and Disease : Cascades of Secondary Metabolic Errors Can Generate Complex Pathologies (in LSDs)

Glycosphingolipids (GSLs) are a diverse group of membrane components occurring mainly on the surfaces of mammalian cells. They and their metabolites have a role in intercellular communication, serving as versatile biochemical signals (Kaltner et al, Biochem J 476(18):2623-2655, 2019) and in many cellular pathways. Anionic GSLs, the sialic acid containing gangliosides (GGs), are essential constituents of neuronal cell surfaces, whereas anionic sulfatides are key components of myelin and myelin forming oligodendrocytes. The stepwise biosynthetic pathways of GSLs occur at and lead along the membranes of organellar surfaces of the secretory pathway. After formation of the hydrophobic ceramide membrane anchor of GSLs at the ER, membrane-spanning glycosyltransferases (GTs) of the Golgi and Trans-Golgi network generate cell type-specific GSL patterns for cellular surfaces. GSLs of the cellular plasma membrane can reach intra-lysosomal, i.e. luminal, vesicles (ILVs) by endocytic pathways for degradation. Soluble glycoproteins, the glycosidases, lipid binding and transfer proteins and acid ceramidase are needed for the lysosomal catabolism of GSLs at ILV-membrane surfaces. Inherited mutations triggering a functional loss of glycosylated lysosomal hydrolases and lipid binding proteins involved in GSL degradation cause a primary lysosomal accumulation of their non-degradable GSL substrates in lysosomal storage diseases (LSDs). Lipid binding proteins, the SAPs, and the various lipids of the ILV-membranes regulate GSL catabolism, but also primary storage compounds such as sphingomyelin (SM), cholesterol (Chol.), or chondroitin sulfate can effectively inhibit catabolic lysosomal pathways of GSLs. This causes cascades of metabolic errors, accumulating secondary lysosomal GSL- and GG- storage that can trigger a complex pathology (Breiden and Sandhoff, Int J Mol Sci 21(7):2566, 2020).

Autophagic flux is impaired in the brain tissue of Tay-Sachs disease mouse model

Tay-Sachs disease is a lethal lysosomal storage disorder caused by mutations in the HexA gene encoding the α subunit of the lysosomal β-hexosaminidase enzyme (HEXA). Abnormal GM2 ganglioside accumulation causes progressive deterioration in the central nervous system in Tay-Sachs patients. Hexa-/- mouse model failed to display abnormal phenotype. Recently, our group generated Hexa-/-Neu3-/- mouse showed severe neuropathological indications similar to Tay-Sachs patients. Despite excessive GM2 ganglioside accumulation in the brain and visceral organs, the regulation of autophagy has not been clarified yet in the Tay-Sachs disease mouse model. Therefore, we investigated distinct steps of autophagic flux using markers including LC3 and p62 in four different brain regions from the Hexa-/-Neu3-/- mice model of Tay-Sachs disease. Our data revealed accumulated autophagosomes and autophagolysosomes indicating impairment in autophagic flux in the brain. We suggest that autophagy might be a new therapeutic target for the treatment of devastating Tay-Sachs disease.

NEU1 and NEU3 enzymes alter CD22 organization on B cells

The B cell membrane expresses sialic-acid-binding immunoglobulin-like lectins, also called Siglecs, that are important for modulating immune response. Siglecs have interactions with sialoglycoproteins found on the same membrane (cis-ligands) that result in homotypic and heterotypic receptor clusters. The regulation and organization of these clusters, and their effect on cell activation, is not clearly understood. We investigated the role of human neuraminidase enzymes NEU1 and NEU3 on the clustering of CD22 on B cells using confocal microscopy. We observed that native NEU1 and NEU3 activity influence the cluster size of CD22. Using single-particle tracking, we observed that NEU3 activity increased the lateral mobility of CD22, which was in contrast to the effect of exogenous bacterial NEU enzymes. Moreover, we show that native NEU1 and NEU3 activity influenced cellular Ca²⁺ levels, supporting a role for these enzymes in regulating B cell activation. Our results establish a role for native NEU activity in modulating CD22 organization and function on B cells.

Analysis of Brain Lipids in the Early-Onset Tay–Sachs Disease Mouse Model With the Combined Deficiency of β-Hexosaminidase A and Neuraminidase 3

Introduction: Tay–Sachs disease is an autosomal recessively inherited lysosomal storage disease that results from loss-of-function mutations in the HEXA gene coding β-hexosaminidase A. HEXA gene deficiency affects the central nervous system owing to GM2 ganglioside accumulation in lysosomes resulting in progressive neurodegeneration in patients. We recently generated a novel mice model with a combined deficiency of β-hexosaminidase A and neuraminidase 3 (Hexa−/−Neu3−/−) that mimics both the neuropathological and clinical abnormalities of early-onset Tay–Sachs disease. Here, we aimed to explore the secondary accumulation of lipids in the brain of Hexa−/−Neu3−/− mice.

Materials and Methods: In the cortex and hippocampus of five-month-old WT, Hexa−/−, Neu3−/−, and Hexa−/−Neu3−/− mice, lipid levels belonging to glycerolipids, glycerophospholipids, and sterol lipids were evaluated using a shotgun lipidomics approach. The levels of myelin were also assessed by luxol fast blue staining and immunohistochemistry using antibodies against myelin basic protein. We further examined glycoconjugate and cholesterol levels by periodic acid–Schiff and filipin staining, respectively. Toluidine blue staining was also performed to display axonal degeneration.

Results: Among glycerophospholipids, we demonstrated elevated levels of phosphatidylcholine-ether and lysophosphatidylcholine while decreased levels of phosphatidylcholine and phosphatidylserine in both cortex and hippocampus of Hexa−/−Neu3−/− mice. In the glycerolipid class, we showed an alleviated level of sphingomyelin in both cortex and hippocampus, but the higher levels of diacylglycerol and triacylglycerol were detected in only the hippocampus of Hexa−/−Neu3−/− mice. The lower level of sterol was also detected in the cortex of Hexa−/−Neu3−/− mice but not in the hippocampus.

Histochemical studies showed a decrease in the myelin level and axonal degeneration indicating neuronal pathology in the brain of Hexa−/−Neu3−/− mice. Although glycoconjugate accumulation was evident both in the cortex and hippocampus, we did not detect any changes in the level of cholesterol.

Conclusion: Our results indicate that alterations in lipid metabolism and neuropathology, such as demyelination and axonal degeneration, might be related to the dysfunctionality of lipid-related cellular pathways like autophagy. Understanding of brain-specific lipid alterations contributes to evaluating the effectiveness of treatments in Hexa−/−Neu3−/− mice in future studies.

Blood-brain barrier delivery for lysosomal storage disorders with IgG-lysosomal enzyme fusion proteins

The majority of lysosomal storage diseases affect the brain. Treatment of the brain with intravenous enzyme replacement therapy is not successful, because the recombinant lysosomal enzymes do not cross the blood-brain barrier (BBB). Biologic drugs, including lysosomal enzymes, can be re-engineered for BBB delivery as IgG-enzyme fusion proteins. The IgG domain of the fusion protein is a monoclonal antibody directed against an endogenous receptor-mediated transporter at the BBB, such as the insulin receptor or the transferrin receptor. This receptor transports the IgG across the BBB, in parallel with the endogenous receptor ligand, and the IgG acts as a molecular Trojan horse to ferry into brain the lysosomal enzyme genetically fused to the IgG. The IgG-enzyme fusion protein is bi-functional and retains both high affinity binding for the BBB receptor, and high lysosomal enzyme activity. IgG-lysosomal enzymes are presently in clinical trials for treatment of the brain in Mucopolysaccharidosis.

Defect in cytosolic Neu2 sialidase abrogates lipid metabolism and impairs muscle function in vivo

Sialic acid (SA) is present in glycoconjugates and important in cell–cell recognition, cell adhesion, and cell growth and as a receptor. Among the four mammalian sialidases, cytosolic NEU2 has a pivotal role in muscle and neuronal differentiation in vitro. However, its biological functions in vivo remain unclear due to its very low expression in humans. However, the presence of cytoplasmic glycoproteins, gangliosides, and lectins involved in cellular metabolism and glycan recognition has suggested the functional importance of cytosolic Neu2 sialidases. We generated a Neu2 knockout mouse model via CRISPR/Cas9-mediated genome engineering and analyzed the offspring littermates at different ages to investigate the in vivo function of cytosolic Neu2 sialidase. Surprisingly, knocking out the Neu2 gene in vivo abrogated overall lipid metabolism, impairing motor function and leading to diabetes. Consistent with these results, Neu2 knockout led to alterations in sialylated glycoproteins involved in lipid metabolism and muscle function, as shown by glycoproteomics analysis.

Reproduction in Animal Models of Lysosomal Storage Diseases: A Scoping Review

Background: Lysosomal storage diseases (LSDs) are caused by a mutation in a specific gene. Enzymatic dysfunction results in a progressive storage of substrates that gradually affects lysosomal, cellular and tissue physiology. Their pathophysiological consequences vary according to the nature of the stored substrate, making LSDs complex and multisystemic diseases. Some LSDs result in near normal life expectancies, and advances in treatments mean that more people reach the age to have children, so considering the effects of LSDs on fertility and the risks associated with having children is of growing importance.

Objectives: As there is a lack of clinical studies describing the effect of LSDs on the physiology of reproductivity, we undertook a scoping review of studies using animal models of LSDs focusing on reproductive parameters.

Methods: We searched six databases: MEDLINE, LILACS, Scopus, Web of Science, Embase and SciELO, and identified 49 articles that met our inclusion criteria.

Results: The majority of the studies used male animal models, and a number reported severe morphological and physiological damage in gametes and gonads in models of sphingolipidoses. Models of other LSDs, such as mucopolysaccharidoses, presented important morphological damage.

Conclusion: Many of the models found alterations in reproductive systems. Any signs of subfertility or morphological damage in animal models are important, particularly in rodents which are extremely fertile, and may have implications for individuals with LSDs. We suggest the use of more female animal models to better understand the physiopathology of the diseases, and the use of clinical case studies to further explore the risks of individuals with LSDs having children.

Glycosphingolipid metabolism and its role in ageing and Parkinson's disease

It is well established that lysosomal glucocerebrosidase gene (GBA) variants are a risk factor for Parkinson’s disease (PD), with increasing evidence suggesting a loss of function mechanism. One question raised by this genetic association is whether variants of genes involved in other aspects of sphingolipid metabolism are also associated with PD. Recent studies in sporadic PD have identified variants in multiple genes linked to diseases of glycosphingolipid (GSL) metabolism to be associated with PD. GSL biosynthesis is a complex pathway involving the coordinated action of multiple enzymes in the Golgi apparatus. GSL catabolism takes place in the lysosome and is dependent on the action of multiple acid hydrolases specific for certain substrates and glycan linkages. The finding that variants in multiple GSL catabolic genes are over-represented in PD in a heterozygous state highlights the importance of GSLs in the healthy brain and how lipid imbalances and lysosomal dysfunction are associated with normal ageing and neurodegenerative diseases. In this article we will explore the link between lysosomal storage disorders and PD, the GSL changes seen in both normal ageing, lysosomal storage disorders (LSDs) and PD and the mechanisms by which these changes can affect neurodegeneration.

Natural history of Tay-Sachs disease in sheep

Tay-Sachs disease (TSD) is a fatal neurodegenerative disease caused by a deficiency of the enzyme β-N-acetylhexosaminidase A (HexA). TSD naturally occurs in Jacob sheep is the only experimental model of TSD. TSD in sheep recapitulates neurologic features similar to juvenile onset and late onset TSD patients. Due to the paucity of human literature on pathology of TSD, a better natural history in the sheep TSD brain, which is on the same order of magnitude as a child's, is necessary for evaluating therapy and characterizing the pathological events that occur. To provide clinicians and researchers with a clearer understanding of longitudinal pathology in patients, we compare spectrum of clinical signs and brain pathology in mildly symptomatic (3-months), moderately symptomatic (6-months), or severely affected TSD sheep (humane endpoint at ~9-months of age). Increased GM2 ganglioside in the CSF of TSD sheep and a TSD specific biomarker on MRS (taurine) correlate with disease severity. Microglial activation and reactive astrocytes were observed globally on histopathology in TSD sheep with a widespread reduction in oligodendrocyte density. Myelination is reduced primarily in the forebrain illustrated by loss of white matter on MRI. GM2 and GM3 ganglioside were increased and distributed differently in various tissues. The study of TSD in the sheep model provides a natural history to shed light on the pathophysiology of TSD, which is of utmost importance due to novel therapeutics being assessed in human patients.

The role of neuraminidase 1 (NEU1) in cytokine release by primary mouse mesangial cells and disease outcomes in murine lupus nephritis

The importance of altered glycosphingolipid (GSL) metabolism is increasingly gaining attention as a characteristic of multiple chronic kidney diseases. Previously, we reported elevated levels of GSLs and neuraminidase (NEU) enzyme activity/expression in the urine or kidney of lupus patients and lupus-prone mice, and demonstrated NEU activity mediates the production of cytokines by lupus-prone mouse primary mesangial cells. This mediation occurs in part through TLR4 and p38/ERK MAPK signalling in response to lipopolysaccharide (LPS) and lupus serum (LS). However, the precise role of NEU1, the most abundant NEU in the kidney, is incompletely known. In this study, we investigated the effect of genetically reduced Neu1 levels in vitro and in vivo. Mesangial cells from non-autoimmune prone Neu1+/- C57BL/6 mice had significantly reduced NEU activity, cytokine expression and cytokine secretion in response to LS and LPS, thereby suggesting reducing Neu1 expression may reduce the inflammatory response in lupus nephritis. Disease was assessed in female B6.SLE1/2/3 lupus-prone mice with genetically reduced levels (Neu1+/-) or wild-type levels (Neu1+/+) of Neu1 from 28 to 44 weeks of age along with aged-matched C57BL/6 controls. Renal disease was unexpectedly mild in all B6.SLE1/2/3 mice despite evidence of systemic disease. B6.SLE1/2/3 Neu1+/- mice exhibited significantly reduced levels of renal NEU1 expression and changes in renal α-2,6 linked sialylated N-glycans compared to the Neu1+/+ or healthy C57BL/6 mice, but measures of renal and systemic disease were similar between the B6.SLE1/2/3 Neu1+/+ and Neu1+/- mice. We conclude that NEU1 is the NEU largely responsible for mediating cytokine release by mesangial cells, at least in vitro, but may not be involved in modulating renal GSL levels in vivo or impact onset of nephritis in lupus-prone mice. However, the effect of reduced NEU1 levels on disease may not be appreciated in the mild disease expression in our colony of B6.SLE1/2/3 mice. The impact of the altered renal sialylated N-glycan levels and potential role of NEU1 with respect to established nephritis (late disease) in lupus-prone mice bears further investigation.

The Function of Sialidase Revealed by Sialidase Activity Imaging Probe

Sialidase cleaves sialic acid residues from glycans such as glycoproteins and glycolipids. In the brain, desorption of the sialic acid by sialidase is essential for synaptic plasticity, learning and memory and synaptic transmission. BTP3-Neu5Ac has been developed for sensitive imaging of sialidase enzyme activity in mammalian tissues. Sialidase activity in the rat hippocampus detected with BTP3-Neu5Ac increases rapidly by neuronal depolarization. It is presumed that an increased sialidase activity in conjunction with neural excitation is involved in the formation of the neural circuit for memory. Since sialidase inhibits the exocytosis of the excitatory neurotransmitter glutamate, the increased sialidase activity by neural excitation might play a role in the negative feedback mechanism against the glutamate release. Mammalian tissues other than the brain have also been stained with BTP3-Neu5Ac. On the basis of information on the sialidase activity imaging in the pancreas, it was found that sialidase inhibitor can be used as an anti-diabetic drug that can avoid hypoglycemia, a serious side effect of insulin secretagogues. In this review, we discuss the role of sialidase in the brain as well as in the pancreas and skin, as revealed by using a sialidase activity imaging probe. We also present the detection of influenza virus with BTP3-Neu5Ac and modification of BTP3-Neu5Ac.

Sphingolipids as Regulators of Neuro-Inflammation and NADPH Oxidase 2

Neuro-inflammation accompanies numerous neurological disorders and conditions where it can be associated with a progressive neurodegenerative pathology. In a similar manner, alterations in sphingolipid metabolism often accompany or are causative features in degenerative neurological conditions. These include dementias, motor disorders, autoimmune conditions, inherited metabolic disorders, viral infection, traumatic brain and spinal cord injury, psychiatric conditions, and more. Sphingolipids are major regulators of cellular fate and function in addition to being important structural components of membranes. Their metabolism and signaling pathways can also be regulated by inflammatory mediators. Therefore, as certain sphingolipids exert distinct and opposing cellular roles, alterations in their metabolism can have major consequences. Recently, regulation of bioactive sphingolipids by neuro-inflammatory mediators has been shown to activate a neuronal NADPH oxidase 2 (NOX2) that can provoke damaging oxidation. Therefore, the sphingolipid-regulated neuronal NOX2 serves as a mechanistic link between neuro-inflammation and neurodegeneration. Moreover, therapeutics directed at sphingolipid metabolism or the sphingolipid-regulated NOX2 have the potential to alleviate neurodegeneration arising out of neuro-inflammation.

The role of neuraminidase in TLR4-MAPK signalling and the release of cytokines by lupus serum-stimulated mesangial cells

Previously, we demonstrated neuraminidase (NEU) activity or NEU1 expression, specifically, is increased in the kidneys of lupus mice and urine of human patients with nephritis. Additionally, NEU activity mediates IL-6 secretion from lupus-prone MRL/lpr primary mouse mesangial cells (MCs) in response to an IgG mimic. IL-6 mediates glomerular inflammation and promotes tissue damage in patients and mouse strains with lupus nephritis. This study further elucidates the mechanisms by which NEU activity and NEU1 specifically mediates the release of IL-6 and other cytokines from lupus-prone MCs. We demonstrate significantly increased release of multiple cytokines and NEU activity in MRL/lpr MCs in response to serum from MRL/lpr mice (lupus serum). Inhibiting NEU activity significantly reduced secretion of three of those cytokines: IL-6, GM-CSF and MIP1α. Message levels of Il-6 and Gm-csf were also increased in response to lupus serum and reduced when NEU activity was inhibited. Neutralizing antibodies to cell-surface receptors and MAPK inhibitors in lupus serum- or LPS-stimulated MCs indicate TLR4 and p38 or ERK MAP kinase signalling play key roles in the NEU-mediated secretion of IL-6. Significantly reduced IL-6 release was observed in C57BL/6 (B6) Neu1+/+ primary MCs compared with wild-type (Neu1+/+) B6 MCs in response to lupus serum. Additional results show inhibiting NEU activity significantly increases sialic acid-containing N-glycan levels. Together, our novel observations support a role for NEU activity, and specifically NEU1, in mediating release of IL-6 from lupus-prone MCs in response to lupus serum through a TLR4-p38/ERK MAPK signalling pathway that likely includes desialylation of glycoproteins.

GM2 ganglioside accumulation causes neuroinflammation and behavioral alterations in a mouse model of early onset Tay-Sachs disease

Background

Tay-Sachs disease (TSD), a type of GM2-gangliosidosis, is a progressive neurodegenerative lysosomal storage disorder caused by mutations in the α subunit of the lysosomal β-hexosaminidase enzyme. This disease is characterized by excessive accumulation of GM2 ganglioside, predominantly in the central nervous system. Although Tay-Sachs patients appear normal at birth, the progressive accumulation of undegraded GM2 gangliosides in neurons leads to death. Recently, an early onset Tay-Sachs disease mouse model, with genotype Hexa−/−Neu3−/−, was generated. Progressive accumulation of GM2 led to premature death of the double KO mice. Importantly, this double-deficient mouse model displays typical features of Tay-Sachs patients, such as cytoplasmic vacuolization of nerve cells, deterioration of Purkinje cells, neuronal death, deceleration in movement, ataxia, and tremors. GM2-gangliosidosis is characterized by acute neurodegeneration preceded by activated microglia expansion, macrophage, and astrocyte activation, along with the production of inflammatory mediators. However, the mechanism of disease progression in Hexa−/−Neu3−/− mice, relevant to neuroinflammation is poorly understood.

Method

In this study, we investigated the onset and progression of neuroinflammatory changes in the cortex, cerebellum, and retina of Hexa−/−Neu3−/− mice and control littermates by using a combination of molecular genetics and immunochemical procedures.

Results

We found elevated levels of pro-inflammatory cytokine and chemokine transcripts, such as Ccl2, Ccl3, Ccl4, and Cxcl10 and also extensive microglial and astrocyte activation and proliferation, accompanied by peripheral blood mononuclear cell infiltration in the vicinity of neurons and oligodendrocytes. Behavioral tests demonstrated a high level of anxiety, and age-dependent loss in both spatial learning and fear memory in Hexa−/−Neu3−/− mice compared with that in the controls.

Conclusion

Altogether, our data suggest that Hexa−/−Neu3−/− mice display a phenotype similar to Tay-Sachs patients suffering from chronic neuroinflammation triggered by GM2 accumulation. Furthermore, our work contributes to better understanding of the neuropathology in a mouse model of early onset Tay-Sachs disease.

GM2 Gangliosidoses: Clinical Features, Pathophysiological Aspects, and Current Therapies

GM2 gangliosidoses are a group of pathologies characterized by GM2 ganglioside accumulation into the lysosome due to mutations on the genes encoding for the β-hexosaminidases subunits or the GM2 activator protein. Three GM2 gangliosidoses have been described: Tay-Sachs disease, Sandhoff disease, and the AB variant. Central nervous system dysfunction is the main characteristic of GM2 gangliosidoses patients that include neurodevelopment alterations, neuroinflammation, and neuronal apoptosis. Currently, there is not approved therapy for GM2 gangliosidoses, but different therapeutic strategies have been studied including hematopoietic stem cell transplantation, enzyme replacement therapy, substrate reduction therapy, pharmacological chaperones, and gene therapy. The blood-brain barrier represents a challenge for the development of therapeutic agents for these disorders. In this sense, alternative routes of administration (e.g., intrathecal or intracerebroventricular) have been evaluated, as well as the design of fusion peptides that allow the protein transport from the brain capillaries to the central nervous system. In this review, we outline the current knowledge about clinical and physiopathological findings of GM2 gangliosidoses, as well as the ongoing proposals to overcome some limitations of the traditional alternatives by using novel strategies such as molecular Trojan horses or advanced tools of genome editing.

Isoenzyme-Selective Inhibitors of Human Neuraminidases Reveal Distinct Effects on Cell Migration

The human neuraminidase enzymes (NEU1, NEU2, NEU3, and NEU4) are a class of enzymes implicated in pathologies including cancer and diabetes. Several reports have linked neuraminidase activity to the regulation of cell migration in cancer cells. Using an in vitro cell migration assay on fibronectin (FN) coated surfaces, we have investigated the role of these enzymes in integrin-mediated cell migration. We observed that neuraminidase inhibition caused significant retardation of cell migration in breast cancer (MDA-MB-231) and prostate cancer (PC-3) cell lines when using inhibitors of NEU3 and NEU4. In contrast, inhibition of NEU1 caused a significant increase in cell migration for the same cell lines. We concluded that the blockade of human neuraminidase enzymes with isoenzyme-selective inhibitors can lead to disparate results and has significant potential in the development of anticancer or wound healing therapeutics.

Pre-clinical Mouse Models of Neurodegenerative Lysosomal Storage Diseases

There are over 50 lysosomal hydrolase deficiencies, many of which cause neurodegeneration, cognitive decline and death. In recent years, a number of broad innovative therapies have been proposed and investigated for lysosomal storage diseases (LSDs), such as enzyme replacement, substrate reduction, pharmacologic chaperones, stem cell transplantation, and various forms of gene therapy. Murine models that accurately reflect the phenotypes observed in human LSDs are critical for the development, assessment and implementation of novel translational therapies. The goal of this review is to summarize the neurodegenerative murine LSD models available that recapitulate human disease, and the pre-clinical studies previously conducted. We also describe some limitations and difficulties in working with mouse models of neurodegenerative LSDs.

Animal Model Contributions to Congenital Metabolic Disease

Genetic model systems allow researchers to probe and decipher aspects of human disease, and animal models of disease are frequently specifically engineered and have been identified serendipitously as well. Animal models are useful for probing the etiology and pathophysiology of disease and are critical for effective discovery and development of novel therapeutics for rare diseases. Here we review the impact of animal model organism research in three examples of congenital metabolic disorders to highlight distinct advantages of model system research. First, we discuss phenylketonuria research where a wide variety of research fields and models came together to make impressive progress and where a nearly ideal mouse model has been central to therapeutic advancements. Second, we review advancements in Lesch-Nyhan syndrome research to illustrate the role of models that do not perfectly recapitulate human disease as well as the need for multiple models of the same disease to fully investigate human disease aspects. Finally, we highlight research on the GM2 gangliosidoses Tay-Sachs and Sandhoff disease to illustrate the important role of both engineered traditional laboratory animal models and serendipitously identified atypical models in congenital metabolic disorder research. We close with perspectives for the future for animal model research in congenital metabolic disorders.

CNS myelin protein 36K regulates oligodendrocyte differentiation through Notch

In contrast to humans and other mammals, zebrafish can successfully regenerate and remyelinate central nervous system (CNS) axons following injury. In addition to common myelin proteins found in mammalian myelin, 36K protein is a major component of teleost fish CNS myelin. Although 36K is one of the most abundant proteins in zebrafish brain, its function remains unknown. Here we investigate the function of 36K using translation-blocking Morpholinos. Morphant larvae showed fewer dorsally migrated oligodendrocyte precursor cells as well as upregulation of Notch ligand. A gamma secretase inhibitor, which prevents activation of Notch, could rescue oligodendrocyte precursor cell numbers in 36K morphants, suggesting that 36K regulates initial myelination through inhibition of Notch signaling. Since 36K like other short chain dehydrogenases might act on lipids, we performed thin layer chromatography and mass spectrometry of lipids and found changes in lipid composition in 36K morphant larvae. Altogether, we suggest that during early development 36K regulates membrane lipid composition, thereby altering the amount of transmembrane Notch ligands and the efficiency of intramembrane gamma secretase processing of Notch and thereby influencing oligodendrocyte precursor cell differentiation and further myelination. Further studies on the role of 36K short chain dehydrogenase in oligodendrocyte precursor cell differentiation during remyelination might open up new strategies for remyelination therapies in human patients.

Hexb enzyme deficiency leads to lysosomal abnormalities in radial glia and microglia in zebrafish brain development

Sphingolipidoses are severe, mostly infantile lysosomal storage disorders (LSDs) caused by defective glycosphingolipid degradation. Two of these sphingolipidoses, Tay Sachs and Sandhoff diseases, are caused by β-Hexosaminidase (HEXB) enzyme deficiency, resulting in ganglioside (GM2) accumulation and neuronal loss. The precise sequence of cellular events preceding, and leading to, neuropathology remains unclear, but likely involves inflammation and lysosomal accumulation of GM2 in multiple cell types. We aimed to determine the consequences of Hexb activity loss for different brain cell types using zebrafish. Hexb deficient zebrafish (hexb-/- ) showed lysosomal abnormalities already early in development both in radial glia, which are the neuronal and glial progenitors, and in microglia. Additionally, at 5 days postfertilization, hexb-/- zebrafish showed reduced locomotor activity. Although specific oligosaccharides accumulate in the adult brain, hexb-/- ) zebrafish are viable and apparently resistant to Hexb deficiency. In all, we identified cellular consequences of loss of Hexb enzyme activity during embryonic brain development, showing early effects on glia, which possibly underlie the behavioral aberrations. Hereby, we identified clues into the contribution of non-neuronal lysosomal abnormalities in LSDs affecting the brain and provide a tool to further study what underlies the relative resistance to Hexb deficiency in vivo.

Lysosomal Glycosphingolipid Storage Diseases

Glycosphingolipids are cell-type-specific components of the outer leaflet of mammalian plasma membranes. Gangliosides, sialic acid–containing glycosphingolipids, are especially enriched on neuronal surfaces. As amphi-philic molecules, they comprise a hydrophilic oligosaccharide chain attached to a hydrophobic membrane anchor, ceramide. Whereas glycosphingolipid formation is catalyzed by membrane-bound enzymes along the secretory pathway, degradation takes place at the surface of intralysosomal vesicles of late endosomes and lysosomes catalyzed in a stepwise fashion by soluble hydrolases and assisted by small lipid-binding glycoproteins. Inherited defects of lysosomal hydrolases or lipid-binding proteins cause the accumulation of undegradable material in lysosomal storage diseases (GM1 and GM2 gangliosidosis; Fabry, Gaucher, and Krabbe diseases; and metachromatic leukodystrophy). The catabolic processes are strongly modified by the lipid composition of the substrate-carrying membranes, and the pathological accumulation of primary storage compounds can trigger an accumulation of secondary storage compounds (e.g., small glycosphingolipids and cholesterol in Niemann-Pick disease).

Substrate Reduction Therapy for Sandhoff Disease through Inhibition of Glucosylceramide Synthase Activity

Neuronopathic glycosphingolipidoses are a sub-group of lysosomal storage disorders for which there are presently no effective therapies. Here, we evaluated the potential of substrate reduction therapy (SRT) using an inhibitor of glucosylceramide synthase (GCS) to decrease the synthesis of glucosylceramide (GL1) and related glycosphingolipids. The substrates that accumulate in Sandhoff disease (e.g., ganglioside GM2 and its nonacylated derivative, lyso-GM2) are distal to the drug target, GCS. Treatment of Sandhoff mice with a GCS inhibitor that has demonstrated CNS access (Genz-682452) reduced the accumulation of GL1 and GM2, as well as a variety of disease-associated substrates in the liver and brain. Concomitant with these effects was a significant decrease in the expression of CD68 and glycoprotein non-metastatic melanoma B protein (Gpnmb) in the brain, indicating a reduction in microgliosis in the treated mice. Moreover, using in vivo imaging, we showed that the monocytic biomarker translocator protein (TSPO), which was elevated in Sandhoff mice, was normalized following Genz-682452 treatment. These positive effects translated in turn into a delay (∼28 days) in loss of motor function and coordination, as measured by rotarod latency, and a significant increase in longevity (∼17.5%). Together, these results support the development of SRT for the treatment of gangliosidoses, particularly in patients with residual enzyme activity.

New Approaches to Tay-Sachs Disease Therapy

Tay-Sachs disease belongs to the group of autosomal-recessive lysosomal storage metabolic disorders. This disease is caused by β-hexosaminidase A (HexA) enzyme deficiency due to various mutations in α-subunit gene of this enzyme, resulting in GM2 ganglioside accumulation predominantly in lysosomes of nerve cells. Tay-Sachs disease is characterized by acute neurodegeneration preceded by activated microglia expansion, macrophage and astrocyte activation along with inflammatory mediator production. In most cases, the disease manifests itself during infancy, the “infantile form,” which characterizes the most severe disorders of the nervous system. The juvenile form, the symptoms of which appear in adolescence, and the most rare form with late onset of symptoms in adulthood are also described. The typical features of Tay-Sachs disease are muscle weakness, ataxia, speech, and mental disorders. Clinical symptom severity depends on residual HexA enzymatic activity associated with some mutations. Currently, Tay-Sachs disease treatment is based on symptom relief and, in case of the late-onset form, on the delay of progression. There are also clinical reports of substrate reduction therapy using miglustat and bone marrow or hematopoietic stem cell transplantation. At the development stage there are methods of Tay-Sachs disease gene therapy using adeno- or adeno-associated viruses as vectors for the delivery of cDNA encoding α and β HexA subunit genes. Effectiveness of this approach is evaluated in α or β HexA subunit defective model mice or Jacob sheep, in which Tay-Sachs disease arises spontaneously and is characterized by the same pathological features as in humans. This review discusses the possibilities of new therapeutic strategies in Tay-Sachs disease therapy aimed at preventing neurodegeneration and neuroinflammation.

Keeping it trim: roles of neuraminidases in CNS function

The sialylated glyconjugates (SGC) are found in abundance on the surface of brain cells, where they form a dense array of glycans mediating cell/cell and cell/protein recognition in numerous physiological and pathological processes. Metabolic genetic blocks in processing and catabolism of SGC result in development of severe storage disorders, dominated by CNS involvement including marked neuroinflammation and neurodegeneration, the pathophysiological mechanisms of which are still discussed. SGC patterns in the brain are cell and organelle-specific, dynamic and maintained by highly coordinated processes of their biosynthesis, trafficking, processing and catabolism. The changes in the composition of SGC during development and aging of the brain cannot be explained based solely on the regulation of the SGC-synthesizing enzymes, sialyltransferases, suggesting that neuraminidases (sialidases) hydrolysing the removal of terminal sialic acid residues also play an essential role. In the current review we summarize the roles of three mammalian neuraminidases: neuraminidase 1, neuraminidase 3 and neuraminidase 4 in processing brain SGC. Emerging data demonstrate that these enzymes with different, yet overlapping expression patterns, intracellular localization and substrate specificity play essential roles in the physiology of the CNS.

Emerging concepts of ganglioside metabolism

Gangliosides (GGs) are sialic acid-containing glycosphingolipids (GSLs) and major membrane components enriched on cellular surfaces. Biosynthesis of mammalian GGs starts at the cytosolic leaflet of endoplasmic reticulum (ER) membranes with the formation of their hydrophobic ceramide anchors. After intracellular ceramide transfer to Golgi and trans-Golgi network (TGN) membranes, anabolism of GGs, as well as of other GSLs, is catalyzed by membrane-spanning glycosyltransferases (GTs) along the secretory pathway. Combined activity of only a few promiscuous GTs allows for the formation of cell-type-specific glycolipid patterns. Following an exocytotic vesicle flow to the cellular plasma membranes, GGs can be modified by metabolic reactions at or near the cellular surface. For degradation, GGs are endocytosed to reach late endosomes and lysosomes. Whereas membrane-spanning enzymes of the secretory pathway catalyze GSL and GG formation, a cooperation of soluble glycosidases, lipases and lipid-binding cofactors, namely the sphingolipid activator proteins (SAPs), act as the main players of GG and GSL catabolism at intralysosomal luminal vesicles (ILVs).

Genetics and Therapies for GM2 Gangliosidosis

Tay-Sachs disease, caused by impaired β-N-acetylhexosaminidase activity, was the first GM2 gangliosidosis to be studied and one of the most severe and earliest lysosomal diseases to be described. The condition, associated with the pathological build-up of GM2 ganglioside, has acquired almost iconic status and serves as a paradigm in the study of lysosomal storage diseases. Inherited as a classical autosomal recessive disorder, this global disease of the nervous system induces developmental arrest with regression of attained milestones; neurodegeneration progresses rapidly to cause premature death in young children. There is no effective treatment beyond palliative care, and while the genetic basis of GM2 gangliosidosis is well established, the molecular and cellular events, from diseasecausing mutations and glycosphingolipid storage to disease manifestations, remain to be fully delineated. Several therapeutic approaches have been attempted in patients, including enzymatic augmentation, bone marrow transplantation, enzyme enhancement, and substrate reduction therapy. Hitherto, none of these stratagems has materially altered the course of the disease. Authentic animal models of GM2 gangliodidosis have facilitated in-depth evaluation of innovative applications such as gene transfer, which in contrast to other interventions, shows great promise. This review outlines current knowledge pertaining the pathobiology as well as potential innovative treatments for the GM2 gangliosidoses.