Gangliosides and gangliosidoses: principles of molecular and metabolic pathogenesis

Glycans and the platelet life cycle

Platelet numbers are intricately regulated to avoid spontaneous bleeding or arterial occlusion and organ damage. The growth factor thrombopoietin (TPO) drives platelet biogenesis by inducing megakaryocyte production. A recent study in mice identified a feedback mechanism by which clearance of aged, desialylated platelets stimulates TPO synthesis by hepatocytes. This new finding generated renewed interest in platelet clearance mechanisms. Here, different established and emerging mechanisms of platelet senescence and clearance will be reviewed with specific emphasis on the role of posttranslational modifications.

Low-pH Stability of Influenza A Virus Sialidase Contributing to Virus Replication and Pandemic

The spike glycoprotein neuraminidase (NA) of influenza A virus (IAV) has sialidase activity that cleaves the terminal sialic acids (viral receptors) from oligosaccharide chains of glycoconjugates. A new antigenicity of viral surface glycoproteins for humans has pandemic potential. We found "low-pH stability of sialidase activity" in NA. The low-pH stability can maintain sialidase activity under acidic conditions of pH 4-5. For human IAVs, NAs of all pandemic viruses were low-pH-stable, whereas those of almost all human seasonal viruses were not. The low-pH stability was dependent on amino acid residues near the active site, the calcium ion-binding site, and the subunit interfaces of the NA homotetramer, suggesting effects of the active site and the homotetramer on structural stability. IAVs with the low-pH-stable NA showed much higher virus replication rates than those of IAVs with low-pH-unstable NA, which was correlated with maintenance of sialidase activity under an endocytic pathway of the viral cell entry mechanism, indicating contribution of low-pH stability to high replication rates of pandemic viruses. The low-pH-stable NA of the 1968 H3N2 pandemic virus was derived from the low-pH-stable NA of H2N2 human seasonal virus, one of two types classified by both low-pH stability in N2 NA and a phylogenetic tree of N2 NA genes. The 2009 H1N1 pandemic virus acquired low-pH-stable NA by two amino acid substitutions at the early stage of the 2009 pandemic. It is thought that low-pH stability contributes to infection spread in a pandemic through enhancement of virus replication.

Comparative Transcriptomic Exploration Reveals Unique Molecular Adaptations of Neuropathogenic Trichobilharzia to Invade and Parasitize Its Avian Definitive Host

To date, most molecular investigations of schistosomatids have focused principally on blood flukes (schistosomes) of humans. Despite the clinical importance of cercarial dermatitis in humans caused by Trichobilharzia regenti and the serious neuropathologic disease that this parasite causes in its permissive avian hosts and accidental mammalian hosts, almost nothing is known about the molecular aspects of how this fluke invades its hosts, migrates in host tissues and how it interacts with its hosts’ immune system. Here, we explored selected aspects using a transcriptomic-bioinformatic approach. To do this, we sequenced, assembled and annotated the transcriptome representing two consecutive life stages (cercariae and schistosomula) of T. regenti involved in the first phases of infection of the avian host. We identified key biological and metabolic pathways specific to each of these two developmental stages and also undertook comparative analyses using data available for taxonomically related blood flukes of the genus Schistosoma. Detailed comparative analyses revealed the unique involvement of carbohydrate metabolism, translation and amino acid metabolism, and calcium in T. regenti cercariae during their invasion and in growth and development, as well as the roles of cell adhesion molecules, microaerobic metabolism (citrate cycle and oxidative phosphorylation), peptidases (cathepsins) and other histolytic and lysozomal proteins in schistosomula during their particular migration in neural tissues of the avian host. In conclusion, the present transcriptomic exploration provides new and significant insights into the molecular biology of T. regenti, which should underpin future genomic and proteomic investigations of T. regenti and, importantly, provides a useful starting point for a range of comparative studies of schistosomatids and other trematodes.

Despite the clinical importance of Trichobilharzia regenti in bird hosts and as a cause of cercarial dermatitis in humans, almost nothing is known about the molecular aspects of this fluke and its interactions with its hosts. Here, we sequenced, assembled and annotated the transcriptome representing two life stages (cercariae and schistosomula) of T. regenti involved in the first phases of infection of the bird host. We identified key biological and metabolic pathways specific to each of these two developmental stages and also undertook comparative analyses using data available for related flukes. Detailed analyses showed the unique involvement of carbohydrate metabolism, translation and amino acid metabolism, and calcium in T. regenti cercariae during invasion and in growth and development, as well as cell adhesion molecules, microaerobic metabolism (citrate cycle and oxidative phosphorylation), peptidases (cathepsins) and other histolytic and lysozomal proteins in schistosomula during migration in neural tissues. These molecular insights into T. regenti biology should support future genomic and proteomic investigations of T. regenti, and comparative studies of flatworms.

Innate immune responses in the brain of sphingolipid lysosomal storage diseases

Lysosomal storage diseases (LSDs) are mainly caused by the defective activity of lysosomal hydrolases. A sub-class of LSDs are the sphingolipidoses, in which sphingolipids accumulate intra-cellularly. We here discuss the role of innate immunity in the sphingolipidoses, and compare the pathways of activation in two classical sphingolipidoses, namely Gaucher disease and Sandhoff disease, and in Niemann-Pick C disease, in which the main storage material is cholesterol but sphingolipids also accumulate. We discuss the mechanisms leading to neuroinflammation, and the different pathways of neuroinflammation in the different diseases, and suggest that intervention in these pathways may be a useful therapeutic approach to address these devastating human diseases.

Aβ1-25-Derived Sphingolipid-Domain Tracer Peptide SBD Interacts with Membrane Ganglioside Clusters via a Coil-Helix-Coil Motif

The Amyloid-β (Aβ)-derived, sphingolipid binding domain (SBD) peptide is a fluorescently tagged probe used to trace the diffusion behavior of sphingolipid-containing microdomains in cell membranes through binding to a constellation of glycosphingolipids, sphingomyelin, and cholesterol. However, the molecular details of the binding mechanism between SBD and plasma membrane domains remain unclear. Here, to investigate how the peptide recognizes the lipid surface at an atomically detailed level, SBD peptides in the environment of raft-like bilayers were examined in micro-seconds-long molecular dynamics simulations. We found that SBD adopted a coil-helix-coil structural motif, which binds to multiple GT1b gangliosides via salt bridges and CH–π interactions. Our simulation results demonstrate that the CH–π and electrostatic forces between SBD monomers and GT1b gangliosides clusters are the main driving forces in the binding process. The presence of the fluorescent dye and linker molecules do not change the binding mechanism of SBD probes with gangliosides, which involves the helix-turn-helix structural motif that was suggested to constitute a glycolipid binding domain common to some sphingolipid interacting proteins, including HIV gp120, prion, and Aβ.

Role of Ceramide from Glycosphingolipids and Its Metabolites in Immunological and Inflammatory Responses in Humans

Glycosphingolipids (GSLs) are composed of hydrophobic ceramide and hydrophilic sugar chains. GSLs cluster to form membrane microdomains (lipid rafts) on plasma membranes, along with several kinds of transducer molecules, including Src family kinases and small G proteins. However, GSL-mediated biological functions remain unclear. Lactosylceramide (LacCer, CDw17) is highly expressed on the plasma membranes of human phagocytes and mediates several immunological and inflammatory reactions, including phagocytosis, chemotaxis, and superoxide generation. LacCer forms membrane microdomains with the Src family tyrosine kinase Lyn and the Gαi subunit of heterotrimeric G proteins. The very long fatty acids C24:0 and C24:1 are the main ceramide components of LacCer in neutrophil plasma membranes and are directly connected with the fatty acids of Lyn and Gαi. These observations suggest that the very long fatty acid chains of ceramide are critical for GSL-mediated outside-in signaling. Sphingosine is another component of ceramide, with the hydrolysis of ceramide by ceramidase producing sphingosine and fatty acids. Sphingosine is phosphorylated by sphingosine kinase to sphingosine-1-phosphate, which is involved in a wide range of cellular functions, including growth, differentiation, survival, chemotaxis, angiogenesis, and embryogenesis, in various types of cells. This review describes the role of ceramide moiety of GSLs and its metabolites in immunological and inflammatory reactions in human.

Dissecting the Role of Anti-ganglioside Antibodies in Guillain-Barré Syndrome: an Animal Model Approach

Guillain-Barré syndrome (GBS) is an autoimmune polyneuropathy disease affecting the peripheral nervous system (PNS). Most of the GBS patients experienced neurological symptoms such as paresthesia, weakness, pain, and areflexia. There are also combinations of non-neurological symptoms which include upper respiratory tract infection and diarrhea. One of the major causes of GBS is due largely to the autoantibodies against gangliosides located on the peripheral nerves. Gangliosides are sialic acid-bearing glycosphingolipids consisting of a ceramide lipid anchor with one or more sialic acids attached to a neutral sugar backbone. Molecular mimicry between the outer components of oligosaccharide of gangliosides on nerve membrane and lipo-oligosaccharide of microbes is thought to trigger the autoimmunity. Intra-peritoneal implantation of monoclonal ganglioside antibodies secreting hybridoma into animals induced peripheral neuropathy. Recent studies demonstrated that injection of synthesized anti-ganglioside antibodies raised by hybridoma cells into mice initiates immune response against peripheral nerves, and eventually failure in peripheral nerve regeneration. Accumulating evidences indicate that the conjugation of anti-ganglioside monoclonal antibodies to activating FcγRIII present on the circulating macrophages inhibits axonal regeneration. The activation of RhoA signaling pathways is also involved in neurite outgrowth inhibition. However, the link between these two molecular events remains unresolved and requires further investigation. Development of anti-ganglioside antagonists can serve as targeted therapy for the treatment of GBS and will open a new approach of drug development with maximum efficacy and specificity.

Membrane lipids regulate ganglioside GM2 catabolism and GM2 activator protein activity

Ganglioside GM2 is the major lysosomal storage compound of Tay-Sachs disease. It also accumulates in Niemann-Pick disease types A and B with primary storage of SM and with cholesterol in type C. Reconstitution of GM2 catabolism with β-hexosaminidase A and GM2 activator protein (GM2AP) at uncharged liposomal surfaces carrying GM2 as substrate generated only a physiologically irrelevant catabolic rate, even at pH 4.2. However, incorporation of anionic phospholipids into the GM2 carrying liposomes stimulated GM2 hydrolysis more than 10-fold, while the incorporation of plasma membrane stabilizing lipids (SM and cholesterol) generated a strong inhibition of GM2 hydrolysis, even in the presence of anionic phospholipids. Mobilization of membrane lipids by GM2AP was also inhibited in the presence of cholesterol or SM, as revealed by surface plasmon resonance studies. These lipids also reduced the interliposomal transfer rate of 2-NBD-GM1 by GM2AP, as observed in assays using Förster resonance energy transfer. Our data raise major concerns about the usage of recombinant His-tagged GM2AP compared with untagged protein. The former binds more strongly to anionic GM2-carrying liposomal surfaces, increases GM2 hydrolysis, and accelerates intermembrane transfer of 2-NBD-GM1, but does not mobilize membrane lipids.

[Lysosomal storage diseases: A brief summary]

BACKGROUND

A considerable number of lysosomal storage diseases (LSD), which can occur at any age in life, should be included in the differential diagnosis of histiocytic diseases.

OBJECTIVE

To what extent can pathologists contribute to the diagnostics of LSD?

MATERIAL AND METHODS

In material collected from LSD, morphological storage phenomena in some disease forms, particularly in histiocytic cells from bone marrow smears and some tissues are highlighted, presented and described. Due to the multitude and heterogeneity of LSDs this list is by no means exhaustive.

RESULTS

In Gaucher disease, the forms of Niemann-Pick disease, cholesteryl ester storage disease (CESD), GM1 gangliosidosis and other LSDs, the histiocytic storage cells seen, for example, in bone marrow smears can be finely and ultrastructurally differentiated. Thereby, not only the presence of an LSD in general but also some individual types of LSD can be identified, even though preliminarily. To confirm the diagnosis the genetic and sometimes biochemical analysis of blood samples or fibroblast cultures from patients is usually required.

CONCLUSION

The pathologist may be the first to suspect LSD and this applies to LSDs that show storage histiocytes or one of a number of other LSDs in which only minor or absent storage is seen in histiocytes but marked storage phenomena are found in other cell systems. Some of the numerous, extremely heterogeneous LSDs may, however, be overlooked as detailed knowledge of the generally rare LSDs is the domain of LSD specialists. Clinicians, pathologists, geneticists and biochemists should cooperate in solving the diagnostic problems.

CSF N-glycan profile reveals sialylation deficiency in a patient with GM2 gangliosidosis presenting as childhood disintegrative disorder

Protein N-glycosylation consists in the synthesis and processing of the oligosaccharide moiety (N-glycan) linked to a protein and it serves several functions for the proper central nervous system (CNS) development and function. Previous experimental and clinical studies have shown the importance of proper glycoprotein sialylation for the synaptic function and the occurrence of autism spectrum disorders (ASD) in the presence of sialylation deficiency in the CNS. Late-onset Tay Sachs disease (LOTSD) is a lysosomal disorder caused by mutations in the HEXA gene resulting in GM2-ganglioside storage in the CNS. It is characterized by progressive neurological impairment and high co-occurrence of psychiatric disturbances. We studied the N-glycome profile of the cerebrospinal fluid (CSF) in a 14 year-old patient with GM2-gangliosidosis (LOTSD). At the age of 4, the patient presented regressive autism fulfilling criteria for childhood disintegrative disorder (CDD). A CSF sample was obtained in the course of diagnostic work-up for the suspicion of an underlying neurodegenerative disorder. We found definite changes of CSF N-glycans due to a dramatic decrease of sialylated biantennary and triantennary structures and an increase of asialo-core fucosylated bisected N-glycans. No changes of total plasma N-glycans were found. Herein findings highlight possible relationships between the early onset psychiatric disturbance featuring CDD in the patient and defective protein sialylation in the CNS. In conclusion, the study first shows aberrant N-glycan structures of CSF proteins in LOTSD; unveils possible pathomechanisms of GM2-gangliosidosis; supports existing relationships between neuropsychiatric disorders and unproper protein glycosylation in the CNS.

Sphingolipid transfer proteins defined by the GLTP-fold

Glycolipid transfer proteins (GLTPs) originally were identified as small (~24 kDa), soluble, amphitropic proteins that specifically accelerate the intermembrane transfer of glycolipids. GLTPs and related homologs now are known to adopt a unique, helically dominated, two-layer 'sandwich' architecture defined as the GLTP-fold that provides the structural underpinning for the eukaryotic GLTP superfamily. Recent advances now provide exquisite insights into structural features responsible for lipid headgroup selectivity as well as the adaptability of the hydrophobic compartment for accommodating hydrocarbon chains of differing length and unsaturation. A new understanding of the structural versatility and evolutionary premium placed on the GLTP motif has emerged. Human GLTP-motifs have evolved to function not only as glucosylceramide binding/transferring domains for phosphoinositol 4-phosphate adaptor protein-2 during glycosphingolipid biosynthesis but also as selective binding/transfer proteins for ceramide-1-phosphate. The latter, known as ceramide-1-phosphate transfer protein, recently has been shown to form GLTP-fold while critically regulating Group-IV cytoplasmic phospholipase A2 activity and pro-inflammatory eicosanoid production.

Cysteine cathepsins are essential in lysosomal degradation of α-synuclein

A cellular feature of Parkinson's disease is cytosolic accumulation and amyloid formation of α-synuclein (α-syn), implicating a misregulation or impairment of protein degradation pathways involving the proteasome and lysosome. Within lysosomes, cathepsin D (CtsD), an aspartyl protease, is suggested to be the main protease for α-syn clearance; however, the protease alone only generates amyloidogenic C terminal-truncated species (e.g., 1-94, 5-94), implying that other proteases and/or environmental factors are needed to facilitate degradation and to avoid α-syn aggregation in vivo. Using liquid chromatography-mass spectrometry, to our knowledge, we report the first peptide cleavage map of the lysosomal degradation process of α-syn. Studies of purified mouse brain and liver lysosomal extracts and individual human cathepsins demonstrate a direct involvement of cysteine cathepsin B (CtsB) and L (CtsL). Both CtsB and CtsL cleave α-syn within its amyloid region and circumvent fibril formation. For CtsD, only in the presence of anionic phospholipids can this protease cleave throughout the α-syn sequence, suggesting that phospholipids are crucial for its activity. Taken together, an interplay exists between α-syn conformation and cathepsin activity with CtsL as the most efficient under the conditions examined. Notably, we discovered that CtsL efficiently degrades α-syn amyloid fibrils, which by definition are resistant to broad spectrum proteases. This work implicates CtsB and CtsL as essential in α-syn lysosomal degradation, establishing groundwork to explore mechanisms to enhance their cellular activity and levels as a potential strategy for clearance of α-syn.

Bis(monoacylglycero)phosphate as a Macrophage Enriched Phospholipid

Bis(monoacylglycero)phosphate (BMP) is a structural isomer of phosphatidylglycerol (PtdGro) with an unusual sn-1:sn-1' fatty acyl configuration and is found almost exclusively in late endosomes/lysosomes. BMP comprises only about 1-2% of the total phospholipids in most mammalian cells, but accumulates in tissues of humans and animals with lysosomal storage disorders including the gangliosidoses. Total BMP content was significantly greater in cells of macrophage/microglial origin than in cells of macroglial origin. BMP composition was similar in tumorigenic/metastatic macrophages and non-tumorigenic macrophages/microglia. Finally, BMP fatty acid composition differed between cells grown in culture and obtained in vivo suggesting an influence from growth environment.

GM1 and GM2 gangliosides: recent developments

GM1 and GM2 gangliosides are important components of the cell membrane and play an integral role in cell signaling and metabolism. In this conceptual overview, we discuss recent developments in our understanding of the basic biological functions of GM1 and GM2 and their involvement in several diseases. In addition to a well-established spectrum of disorders known as gangliosidoses, such as Tay-Sachs disease, more and more evidence points at an involvement of GM1 in Alzheimer's and Parkinson's diseases. New emerging methodologies spanning from single-molecule imaging in vivo to simulations in silico have complemented standard studies based on ganglioside extraction.

Pyrimethamine Derivatives: Insight into Binding Mechanism and Improved Enhancement of Mutant β-N-acetylhexosaminidase Activity

In order to identify structural features of pyrimethamine (5-(4-chlorophenyl)-6-ethylpyrimidine-2,4-diamine) that contribute to its inhibitory activity (IC50 value) and chaperoning efficacy toward β-N-acetylhexosaminidase, derivatives of the compound were synthesized that differ at the positions bearing the amino, ethyl, and chloro groups. Whereas the amino groups proved to be critical to its inhibitory activity, a variety of substitutions at the chloro position only increased its IC50 by 2-3-fold. Replacing the ethyl group at the 6-position with butyl or methyl groups increased IC50 more than 10-fold. Surprisingly, despite its higher IC50, a derivative lacking the chlorine atom in the para-position was found to enhance enzyme activity in live patient cells a further 25% at concentrations >100 μM, while showing less toxicity. These findings demonstrate the importance of the phenyl group in modulating the chaperoning efficacy and toxicity profile of the derivatives.

A novel human model of the neurodegenerative disease GM1 gangliosidosis using induced pluripotent stem cells demonstrates inflammasome activation

GM1 gangliosidosis (GM1) is an inherited neurodegenerative disorder caused by mutations in the lysosomal β-galactosidase (β-gal) gene. Insufficient β-gal activity leads to abnormal accumulation of GM1 gangliosides in tissues, particularly in the central nervous system, resulting in progressive neurodegeneration. Here, we report an in vitro human GM1 model, based on induced pluripotent stem cell (iPSC) technology. Neural progenitor cells differentiated from GM1 patient-derived iPSCs (GM1-NPCs) recapitulated the biochemical and molecular phenotypes of GM1, including defective β-gal activity and increased lysosomes. Importantly, the characterization of GM1-NPCs established that GM1 is significantly associated with the activation of inflammasomes, which play a critical role in the pathogenesis of various neurodegenerative diseases. Specific inflammasome inhibitors potently alleviated the disease-related phenotypes of GM1-NPCs in vitro and in vivo. Our data demonstrate that GM1-NPCs are a valuable in vitro human GM1 model and suggest that inflammasome activation is a novel target pathway for GM1 drug development.

The role of gangliosides in neurodevelopment

Gangliosides are important components of neuronal cell membranes and it is widely accepted that they play a critical role in neuronal and brain development. They are functionally involved in neurotransmission and are thought to support the formation and stabilization of functional synapses and neural circuits required as the structural basis of memory and learning. Available evidence, as reviewed herein, suggests that dietary gangliosides may impact positively on cognitive functions, particularly in the early postnatal period when the brain is still growing. Further, new evidence suggests that the mechanism of action may be through an effect on the neuroplasticity of the brain, mediated through enhanced synaptic plasticity in the hippocampus and nigro-striatal dopaminergic pathway.

Role of α- and β-Synucleins in the Axonal Pathology of Parkinson's Disease and Related Synucleinopathies

Axonal swellings are histological hallmarks of axonopathies in various types of disorders in the central nervous system, including neurodegenerative diseases. Given the pivotal role of axonopathies during the early phase of neurodegenerative process, axonal swellings may be good models which may provide some clues for early pathogenesis of α-synucleinopathies, including Parkinson’s disease and dementia with Lewy bodies (DLB). In this mini-review, such a possibility is discussed based on our recent studies as well as other accumulating studies. Consistent with the current view that dysfunction in the autophagy-lysosomal system may play a major role in the formation of axonal swellings, our studies showed globule, small axonal swellings, derived from transgenic mice expressing either human wild-type α-synuclein (αS-globule) or DLB-linked P123H β-synuclein (βS-globule), contained autophagosome-like membranes. However, other pathological features, such as abnormal mitochondria, enhanced oxidative stress and LRRK2 accumulation, were observed in the αS-globules, but not in the βS-globules. Collectively, it is predicted that αS and βS may be involved in axonopathies through similar but distinct mechanisms, and thus, contribute to diverse axonal pathologies. Further studies of the axonal swellings may lead to elucidating the pathogenic mechanism of early α-synucleinopathies and illuminating a strategy for a disease-modifying therapy against these devastating disorders.

Effects of cyclodextrins on GM1-gangliosides in fibroblasts from GM1-gangliosidosis patients

OBJECTIVES

GM1-gangliosidosis is an inherited disorder characterized by the accumulation of GM1-gangliosides in many tissues and organs, particularly in the brain. Currently, there is no treatment available for patients with ganglioside storage diseases. Therefore, we investigated the effects of cyclodextrins (CyDs) on the GM1-ganglioside level in EA1 cells, fibroblasts from patients with GM1-gangliosidosis.

METHODS

The concentrations of cholesterol and phospholipids in supernatants were determined by Cholesterol E-test Wako and Phospholipid C-test Wako, respectively. The effects of CyDs on GM1-ganglioside levels in EA1 cells using fluorescence-labelled cholera toxin B-subunit, which can bind to GM1-gangliosides specifically, were investigated by flow cytometry and confocal laser scanning microscopy.

KEY FINDINGS

The treatment with methylated CyDs, hydroxypropylated CyDs and branched CyDs decreased GM1-ganglioside levels in EA1 cells at 1 mm for 24 h. Unexpectedly, there was no significant change in the efflux of cholesterol or phospholipids from the cells after treatment with CyDs under the same experimental conditions, indicating that the efflux of membrane components is not associated with down-regulation of GM1-ganglioside levels in EA1 cells upon CyDs treatment.

CONCLUSIONS

CyDs may have the potential as drugs for GM1-gangliosidosis, although the mechanism should be thereafter clarified.

Lipidomic analysis of cerebrospinal fluid by mass spectrometry-based methods

Lipids are natural substances found in all living organisms. Essential to the integrity of cell membranes, they also have many biological functions linked to energy storage and cell signaling, and are involved in a large number of heterogeneous diseases such as cancer, diabetes, neurological disorders, and inherited metabolic diseases. Lipids are challenging to analyze because of their huge structural diversity and numerous species. Up to now, lipid analysis has been achieved by targeted approaches focusing on selected families and relying on extraction protocols and chromatographic methods coupled to various detectors including mass spectrometry. Thanks to the technological improvements achieved in the fields of chromatography, high-resolution mass spectrometry and bioinformatics, it is possible to perform global lipidomic analyses enabling the concomitant detection, identification and relative quantification of many lipid species belonging to different families. The aim of this review is to focus on mass spectrometry-based methods to perform lipid and lipidomic analyses and on their application to the analysis of cerebrospinal fluid.

[Adult-onset rare diseases]

The present paper is focusing on rare diseases manifesting in late childhood or adulthood. A part of these syndromes are not of genetic origin, such as relatively or absolutely rare infections, autoimmune diseases, tumours, or diseases due to rare environmental toxic agents. In addition, even a large proportion of genetic disorders may develop in adulthood or may have adult forms as well, affecting are almost each medical specialization. Examples are storage disorders (e.g. adult form of Tay-Sachs disease, Gaucher-disease), enzyme deficiencies (e.g. ornithin-transcarbamylase deficiency of the urea cycle disorders), rare thrombophilias (e.g. homozygous factor V. Leiden mutation, antithrombin deficiency), or some rare monogenic disorders such as Huntington-chorea and many others. It is now generally accepted that at least half of the 6-8000 "rare diseases" belong either to the scope of adult-care (e.g. internal medicine, neurology), or to "age-neutral" specialities such as ophtalmology, dermatology etc.).

Sialic acids in the brain: gangliosides and polysialic acid in nervous system development, stability, disease, and regeneration

Every cell in nature carries a rich surface coat of glycans, its glycocalyx, which constitutes the cell's interface with its environment. In eukaryotes, the glycocalyx is composed of glycolipids, glycoproteins, and proteoglycans, the compositions of which vary among different tissues and cell types. Many of the linear and branched glycans on cell surface glycoproteins and glycolipids of vertebrates are terminated with sialic acids, nine-carbon sugars with a carboxylic acid, a glycerol side-chain, and an N-acyl group that, along with their display at the outmost end of cell surface glycans, provide for varied molecular interactions. Among their functions, sialic acids regulate cell-cell interactions, modulate the activities of their glycoprotein and glycolipid scaffolds as well as other cell surface molecules, and are receptors for pathogens and toxins. In the brain, two families of sialoglycans are of particular interest: gangliosides and polysialic acid. Gangliosides, sialylated glycosphingolipids, are the most abundant sialoglycans of nerve cells. Mouse genetic studies and human disorders of ganglioside metabolism implicate gangliosides in axon-myelin interactions, axon stability, axon regeneration, and the modulation of nerve cell excitability. Polysialic acid is a unique homopolymer that reaches >90 sialic acid residues attached to select glycoproteins, especially the neural cell adhesion molecule in the brain. Molecular, cellular, and genetic studies implicate polysialic acid in the control of cell-cell and cell-matrix interactions, intermolecular interactions at cell surfaces, and interactions with other molecules in the cellular environment. Polysialic acid is essential for appropriate brain development, and polymorphisms in the human genes responsible for polysialic acid biosynthesis are associated with psychiatric disorders including schizophrenia, autism, and bipolar disorder. Polysialic acid also appears to play a role in adult brain plasticity, including regeneration. Together, vertebrate brain sialoglycans are key regulatory components that contribute to proper development, maintenance, and health of the nervous system.

Multilevel structuring of ganglioside-containing aggregates: from simple micelles to complex biomimetic membranes

We revisit the structural investigation we performed over the years on gangliosides, biological amphiphiles typically found in the cell membranes of the nervous system of mammalians. Their molecular features, a large and charged saccharidic headgroup connected to a sticky and extended ceramide double tail, strongly dictate their aggregation properties and place ganglioside aggregates at the borderline between the curved world and the flatland. All along we found that unexpected interesting behaviours were induced by the hierarchical propagation of such extreme monomer properties, from the aggregate scale to the mesoscopic phases. In fact, even small changes in the monomer geometry or hindrance result in dramatic aggregate reshaping, due to collective amplification. Surface packing optimization requires preferential mutual orientation of headgroups, giving rise to trapped solid-disordered configurations. The interplay between interparticle and intraparticle interactions gives rise to unexpected behaviours and counterintuitive phase's landscape. In situ modification of monomer properties, operated by enzymatic digestion of aggregated ganglioside headgroups, either causes collective rearrangement or is overwhelmed by collective trapping, depending on their surface density. This aspect is interesting as gangliosides are not evenly distributed in cell membranes, but only in the outer leaflet, where they p]articipate in rafts, functional microdomains enriched in special lipids including cholesterol. We recently found that ganglioside GM1 forces a preferential distribution of cholesterol, constituting a collective structural pair across the membrane. In summary, ganglioside assemblies, through cooperativity, reach a structural complexity comparable or even bigger and more adaptive than that of a protein.

The development and use of small molecule inhibitors of glycosphingolipid metabolism for lysosomal storage diseases

Glycosphingolipid (GSL) storage diseases have been the focus of efforts to develop small molecule therapeutics from design, experimental proof of concept studies, and clinical trials. Two primary alternative strategies that have been pursued include pharmacological chaperones and GSL synthase inhibitors. There are theoretical advantages and disadvantages to each of these approaches. Pharmacological chaperones are specific for an individual glycoside hydrolase and for the specific mutation present, but no candidate chaperone has been demonstrated to be effective for all mutations leading to a given disorder. Synthase inhibitors target single enzymes such as glucosylceramide synthase and inhibit the formation of multiple GSLs. A glycolipid synthase inhibitor could potentially be used to treat multiple diseases, but at the risk of lowering nontargeted cellular GSLs that are important for normal health. The basis for these strategies and specific examples of compounds that have led to clinical trials is the focus of this review.

Reduced GM1 ganglioside in CFTR-deficient human airway cells results in decreased β1-integrin signaling and delayed wound repair

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) function reduces chloride secretion and increases sodium uptake, but it is not clear why CFTR mutation also results in progressive lung inflammation and infection. We previously demonstrated that CFTR-silenced airway cells migrate more slowly during wound repair than CFTR-expressing controls. In addition, CFTR-deficient cells and mouse models have been reported to have altered sphingolipid levels. Here, we investigated the hypothesis that reduced migration in CFTR-deficient airway epithelial cells results from altered sphingolipid composition. We used cell lines derived from a human airway epithelial cell line (Calu-3) stably transfected with CFTR short hairpin RNA (CFTR-silenced) or nontargeting short hairpin RNA (controls). Cell migration was measured by electric cell substrate impedance sensing (ECIS). Lipid analyses, addition of exogenous glycosphingolipids, and immunoblotting were performed. We found that levels of the glycosphingolipid, GM1 ganglioside, were ~60% lower in CFTR-silenced cells than in controls. CFTR-silenced cells exhibited reduced levels of activated β1-integrin, phosphorylated tyrosine 576 of focal adhesion kinase (pFAK), and phosphorylation of Crk-associated substrate (pCAS). Addition of GM1 (but not GM3) ganglioside to CFTR-silenced cells restored activated β1-integrin, pFAK, and pCAS to near control levels and partially restored (~40%) cell migration. Our results suggest that decreased GM1 in CFTR-silenced cells depresses β1-integrin signaling, which contributes to the delayed wound repair observed in these cells. These findings have implications for the pathology of cystic fibrosis, where altered sphingolipid levels in airway epithelial cells could result in a diminished capacity for wound repair after injury.

Neuronal expression of GalNAc transferase is sufficient to prevent the age-related neurodegenerative phenotype of complex ganglioside-deficient mice

Gangliosides are widely expressed sialylated glycosphingolipids with multifunctional properties in different cell types and organs. In the nervous system, they are highly enriched in both glial and neuronal membranes. Mice lacking complex gangliosides attributable to targeted ablation of the B4galnt1 gene that encodes β-1,4-N-acetylegalactosaminyltransferase 1 (GalNAc-transferase; GalNAcT(-/-)) develop normally before exhibiting an age-dependent neurodegenerative phenotype characterized by marked behavioral abnormalities, central and peripheral axonal degeneration, reduced myelin volume, and loss of axo-glial junction integrity. The cell biological substrates underlying this neurodegeneration and the relative contribution of either glial or neuronal gangliosides to the process are unknown. To address this, we generated neuron-specific and glial-specific GalNAcT rescue mice crossed on the global GalNAcT(-/-) background [GalNAcT(-/-)-Tg(neuronal) and GalNAcT(-/-)-Tg(glial)] and analyzed their behavioral, morphological, and electrophysiological phenotype. Complex gangliosides, as assessed by thin-layer chromatography, mass spectrometry, GalNAcT enzyme activity, and anti-ganglioside antibody (AgAb) immunohistology, were restored in both neuronal and glial GalNAcT rescue mice. Behaviorally, GalNAcT(-/-)-Tg(neuronal) retained a normal "wild-type" (WT) phenotype throughout life, whereas GalNAcT(-/-)-Tg(glial) resembled GalNAcT(-/-) mice, exhibiting progressive tremor, weakness, and ataxia with aging. Quantitative electron microscopy demonstrated that GalNAcT(-/-) and GalNAcT(-/-)-Tg(glial) nerves had significantly increased rates of axon degeneration and reduced myelin volume, whereas GalNAcT(-/-)-Tg(neuronal) and WT appeared normal. The increased invasion of the paranode with juxtaparanodal Kv1.1, characteristically seen in GalNAcT(-/-) and attributed to a breakdown of the axo-glial junction, was normalized in GalNAcT(-/-)-Tg(neuronal) but remained present in GalNAcT(-/-)-Tg(glial) mice. These results indicate that neuronal rather than glial gangliosides are critical to the age-related maintenance of nervous system integrity.

Stuck in traffic: an emerging theme in diseases of the nervous system

The past decade has seen an explosion of DNA sequencing activities and many mutations and genetic variances underlying neurological and neurodegenerative diseases have been determined. This wealth of genetic data is now placed in molecular pathways revealing the nodes that underlie the disrupted processes. Many mutations in neurological diseases affect proteins controlling endosomal/lysosomal transport. Although the age of onset of these diseases range from juvenile [i.e., Niemann-Pick type C (NPC) and Charcot-Marie-Tooth (CMT) disease] to late onset (Parkinson's and Alzheimer's disease), deregulation of endosomal transport is a common theme. This review summarizes how elucidating the genetic basis for the various neurological diseases has advanced our understanding of the endo-lysosomal system and why the various mutations all translate into similar disease phenotypes.

Ganglioside storage diseases: on the road to management

Although the biochemical and genetic basis for the GM1 and GM2 gangliosidoses has been known for decades, effective therapies for these diseases remain in early stages of development. The difficulty with many therapeutic strategies for treating the gangliosidoses comes largely from their inability to remove stored ganglioside once it accumulates in central nervous system (CNS) neurons and glia. This chapter highlights advances made using substrate reduction therapy and gene therapy in reducing CNS ganglioside storage. Information obtained from mouse and feline models provides insight on therapeutic strategies that could be effective in human clinical trials. In addition, information is presented showing how a calorie-restricted diet might facilitate therapeutic drug delivery to the CNS. The development of multiple new therapeutic approaches offers hope that longer-term management of these diseases can be achieved. It is also clear that multiple therapeutic strategies will likely be needed to provide the most complete management.

Sphingolipids and lysosomal pathologies

Endocytosed (glyco)sphingolipids are degraded, together with other membrane lipids in a stepwise fashion by endolysosomal enzymes with the help of small lipid binding proteins, the sphingolipid activator proteins (SAPs), at the surface of intraluminal lysosomal vesicles. Inherited defects in a sphingolipid-degrading enzyme or SAP cause the accumulation of the corresponding lipid substrates, including cytotoxic lysosphingolipids, such as galactosylsphingosine and glucosylsphingosine, and lead to a sphingolipidosis. Analysis of patients with prosaposin deficiency revealed the accumulation of intra-endolysosmal vesicles and membrane structures (IM). Feeding of prosaposin reverses the storage, suggesting inner membrane structures as platforms of sphingolipid degradation. Water soluble enzymes can hardly attack sphingolipids embedded in the membrane of inner endolysosomal vesicles. The degradation of sphingolipids with few sugar residues therefore requires the help of the SAPs, and is strongly stimulated by anionic membrane lipids. IMs are rich in anionic bis(monoacylglycero)phosphate (BMP). This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Niemann-pick disease type C: new aspects in a long published family - partial manifestations in heterozygotes

Decades ago, a family with three children with a neurovisceral lysosomal storage disease was described. The patient siblings died at ages 7, 9, and 11 years, respectively, and according to the current concept had the late-infantile neurologic form of Niemann-Pick type C1 (NPC) disease, given by the present molecular study that there were severe NPC1 gene variants: Blood samples preserved since that time from one patient sibling and his presently 55-year-old essentially healthy sister have now been studied, revealing the variants p.I1061T and p.G1162V in the NPC1 gene, the first long known, the second newly found but predicted to be pathogenic and similar to the known G1162A. Now, with the molecular diagnosis, that initial description warrants new interest for the following reasons. The mentioned sister carries only the I1061T variant. She had storage macrophages ("Niemann-Pick cells") in her bone marrow, but also displayed distinct splenomegaly with indurated consistency of the organ, proven in childhood and confirmed several times up to age 13, but disappeared at age 55 years. She shares the I1061T variant with her still healthy mother, and the bone marrow finding with both parents, her father having died at 66 years from a carcinoma. The present study is one of the first describing hematological and relevant clinical symptomatology, even in heterozygotes, of molecularly diagnosed human NPC. Feline NPC is known to model such a situation. For human diagnostic and clinical NPC management, the possibility of "heterozygous disease" should be kept in mind.