Glial Sulfatides and Neuronal Complex Gangliosides Are Functionally Interdependent in Maintaining Myelinating Axon Integrity

Multiple Roles of Apolipoprotein E4 in Oxidative Lipid Metabolism and Ferroptosis During the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease worldwide and has a great socio-economic impact. Modified oxidative lipid metabolism and dysregulated iron homeostasis have been implicated in the pathogenesis of this disorder, but the detailed pathophysiological mechanisms still remain unclear. Apolipoprotein E (APOE) is a lipid-binding protein that occurs in large quantities in human blood plasma, and a polymorphism of the APOE gene locus has been identified as risk factors for AD. The human genome involves three major APOE alleles (APOE2, APOE3, APOE4), which encode for three subtly distinct apolipoprotein E isoforms (APOE2, APOE3, APOE4). The canonic function of these apolipoproteins is lipid transport in blood and brain, but APOE4 allele carriers have a much higher risk for AD. In fact, about 60% of clinically diagnosed AD patients carry at least one APOE4 allele in their genomes. Although the APOE4 protein has been implicated in pathophysiological key processes of AD, such as extracellular beta-amyloid (Aβ) aggregation, mitochondrial dysfunction, neuroinflammation, formation of neurofibrillary tangles, modified oxidative lipid metabolism, and ferroptotic cell death, the underlying molecular mechanisms are still not well understood. As for all mammalian cells, iron plays a crucial role in neuronal functions and dysregulation of iron homeostasis has also been implicated in the pathogenesis of AD. Imbalances in iron homeostasis and impairment of the hydroperoxy lipid-reducing capacity induce cellular dysfunction leading to neuronal ferroptosis. In this review, we summarize the current knowledge on APOE4-related oxidative lipid metabolism and the potential role of ferroptosis in the pathogenesis of AD. Pharmacological interference with these processes might offer innovative strategies for therapeutic interventions.

Cerebrospinal fluid sulfatide isoforms lack diagnostic utility in separating progressive from relapsing-remitting multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is an immune-mediated demyelinating disorder of the central nervous system. The glycosphingolipid sulfatide, a lipid particularly enriched in the myelin sheath, has been shown to be involved the maintenance of this specific membrane structure. Sulfatide in cerebrospinal fluid (CSF) may reflect demyelination, a dominating feature of MS. We investigated the diagnostic utility of CSF sulfatide isoform levels to separate different courses or phenotypes of MS disease.

MATERIAL AND METHODS

This was a mono-center, cross-sectional study of relapsing-remitting MS (RRMS) (n = 45) and progressive MS (PMS) (n = 42) patients (consisting of primary PMS (n = 17) and secondary PMS (n = 25)) and healthy controls (n = 19). In total, 20 sulfatide isoforms were measured in CSF by liquid chromatography-mass spectrometry.

RESULTS

CSF total sulfatide concentrations, as well as CSF sulfatide isoform distribution, did not differ across the study groups, and their levels were independent of disease course/phenotype, disease duration, time to conversion to secondary PMS, age, and disability in MS patients.

CONCLUSION

CSF sulfatide isoforms lack diagnostic and prognostic utility as a biomarker for progressive MS.

Altered plasma membrane abundance of the sulfatide-binding protein NF155 links glycosphingolipid imbalances to demyelination

Myelin is a multilayered membrane that tightly wraps neuronal axons, enabling efficient, high-speed signal propagation. The axon and myelin sheath form tight contacts, mediated by specific plasma membrane proteins and lipids, and disruption of these contacts causes devastating demyelinating diseases. Using two cell-based models of demyelinating sphingolipidoses, we demonstrate that altered lipid metabolism changes the abundance of specific plasma membrane proteins. These altered membrane proteins have known roles in cell adhesion and signaling, with several implicated in neurological diseases. The cell surface abundance of the adhesion molecule neurofascin (NFASC), a protein critical for the maintenance of myelin-axon contacts, changes following disruption to sphingolipid metabolism. This provides a direct molecular link between altered lipid abundance and myelin stability. We show that the NFASC isoform NF155, but not NF186, interacts directly and specifically with the sphingolipid sulfatide via multiple binding sites and that this interaction requires the full-length extracellular domain of NF155. We demonstrate that NF155 adopts an S-shaped conformation and preferentially binds sulfatide-containing membranes in cis, with important implications for protein arrangement in the tight axon-myelin space. Our work links glycosphingolipid imbalances to disturbance of membrane protein abundance and demonstrates how this may be driven by direct protein-lipid interactions, providing a mechanistic framework to understand the pathogenesis of galactosphingolipidoses.

Relationship of Sulfatides Physiological Function and Peroxisome Proliferator-Activated Receptor α

Sulfatides are unique sphingolipids present in the serum and the plasma membrane. Sulfatides exert important functions in a number of systems in the human body, including the nervous, immune, cardiovascular, and coagulation systems.Furthermore, it is closely related to tumor occurrence, development, and metastasis. Peroxisome proliferators-activated receptor α (PPARα) is a class of the nuclear receptor superfamily of transcription factors, which is a potential regulator of sulfatides. This review not only summarizes the current knowledge on the physiological functions of sulfatides in various systems, but also discusses the possible PPARα regulatory mechanisms in sulfatide metabolism and functions. The results of the present analysis provide deep insights and further novel ideas for expanding the research on the physiological function and clinical application of sulfatides.

Effects of the genetic knockout of the β-1,3-galactosyltransferase 2 on spatial learning and neurons in the adult mouse hippocampus and somatosensory cortex

OBJECTIVE

Glycosyltransferases contribute to the biosynthesis of glycoproteins, proteoglycans and glycolipids and play essential roles in various processes in the brain, such as learning and memory, brain development, neuronal survival and neurodegeneration. β-1,3-galactosyltransferase 2 (B3galt2) belongs to the β-1,3-galactosyltransferase gene family and is highly expressed in the brain. Recent studies have indicated that B3galt2 plays a vital role in ischemic stroke through several signaling pathways in a mouse model. However, the function of B3galt2 in the brain remains poorly understood.

METHODS

The genotypes of mice were determined by PCR. To verify B3galt2 expression in an adult mouse brain, X-gal staining was performed in 6-month-old B3galt2 heterozygous (B3galt2+/-) mice. Using adult B3galt2 homozygous (B3galt2-/-), heterozygous and wild-type (WT) littermates, spatial learning and memory were determined by the Morris Water Maze test, and neurotoxicity and synaptic plasticity were examined by immunofluorescence.

RESULTS

B3galt2 was highly expressed in the adult mouse hippocampus and cortex, especially in the hippocampal dentate gyrus. Compared to that of WT mice, the spatial learning ability of adult B3galt2-/- mice was impaired. B3galt2 mutations also caused neuronal loss and synaptic dysfunction in the hippocampus and somatosensory cortex, and these changes were more obvious in B3galt2-/- mice than in B3galt2+/- mice.

CONCLUSIONS

The findings indicate that B3galt2 plays an important role in cognitive function, neuronal maintenance and synaptic plasticity in the adult mouse brain. This study suggests that genetic and/or pharmacological manipulation of glycosyltransferases may be a novel strategy for elucidating the mechanism of and managing various brain disorders.

Functional Impairment of the Nervous System with Glycolipid Deficiencies

Patients with nervous system disorders suffer from impaired cognitive, sensory and motor functions that greatly inconvenience their daily life and usually burdens their family and society. It is difficult to achieve functional recovery for the damaged central nervous system (CNS) because of its limited ability to regenerate. Glycosphingolipids (GSLs) are abundant in the CNS and are known to play essential roles in cell-cell recognition, adhesion, signal transduction, and cellular migration, that are crucial in all phases of neurogenesis. Despite intense investigation of CNS regeneration, the roles of GSLs in neural regeneration remain unclear. Here we focus on the respective potentials of glycolipids to promote regeneration and repair of the CNS. Mice lacking glucosylceramide, lactosylceramide or gangliosides show lethal phenotypes. More importantly, patients with ganglioside deficiencies exhibit severe clinical phenotypes. Further, neurodegenerative diseases and mental health disorders are associated with altered GSL expression. Accumulating studies demonstrate that GSLs not only delimit physical regions but also play central roles in the maintenance of the biological functions of neurons and glia. We anticipate that the ability of GSLs to modulate behavior of a variety of molecules will enable them to ameliorate biochemical and neurobiological defects in patients. The use of GSLs to treat such defects in the human CNS will be a paradigm-shift in approach since GSL-replacement therapy has not yet been achieved in this manner clinically.

Sulfatide Deficiency, an Early Alzheimer's Lipidomic Signature, Causes Brain Ventricular Enlargement in the Absence of Classical Neuropathological Hallmarks

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive memory loss and a decline in activities of daily life. Ventricular enlargement has been associated with worse performance on global cognitive tests and AD. Our previous studies demonstrated that brain sulfatides, myelin-enriched lipids, are dramatically reduced in subjects at the earliest clinically recognizable AD stages via an apolipoprotein E (APOE)-dependent and isoform-specific process. Herein, we provided pre-clinical evidence that sulfatide deficiency is causally associated with brain ventricular enlargement. Specifically, taking advantage of genetic mouse models of global and adult-onset sulfatide deficiency, we demonstrated that sulfatide losses cause ventricular enlargement without significantly affecting hippocampal or whole brain volumes using histological and magnetic resonance imaging approaches. Mild decreases in sulfatide content and mild increases in ventricular areas were also observed in human APOE4 compared to APOE2 knock-in mice. Finally, we provided Western blot and immunofluorescence evidence that aquaporin-4, the most prevalent aquaporin channel in the central nervous system (CNS) that provides fast water transportation and regulates cerebrospinal fluid in the ventricles, is significantly increased under sulfatide-deficient conditions, while other major brain aquaporins (e.g., aquaporin-1) are not altered. In short, we unraveled a novel and causal association between sulfatide deficiency and ventricular enlargement. Finally, we propose putative mechanisms by which sulfatide deficiency may induce ventricular enlargement.

The role of gangliosides in the organisation of the node of Ranvier examined in glycosyltransferase transgenic mice

Gangliosides are a family of sialic acid containing glycosphingolipids highly enriched in plasma membranes of the vertebrate nervous system. They are functionally diverse in modulating nervous system integrity, notably at the node of Ranvier, and also act as receptors for many ligands including toxins and autoantibodies. They are synthesised in a stepwise manner by groups of glycosyl- and sialyltransferases in a developmentally and tissue regulated manner. In this review, we summarise and discuss data derived from transgenic mice with different transferase deficiencies that have been used to determine the role of glycolipids in the organisation of the node of Ranvier. Understanding their role at this specialised functional site is crucial to determining differential pathophysiology following directed genetic or autoimmune injury to peripheral nerve nodal or paranodal domains, and revealing the downstream consequences of axo-glial disruption.

Ganglioside lipidomics of CNS myelination using direct infusion shotgun mass spectrometry

Gangliosides are present and concentrated in axons and implicated in axon-myelin interactions, but how ganglioside composition changes during myelin formation is not known. Here, we present a direct infusion (shotgun) lipidomics method to analyze gangliosides in small amounts of tissue reproducibly and with high sensitivity. We resolve the mouse ganglioside lipidome during development and adulthood and determine the ganglioside content of mice lacking the St3gal5 and B4galnt1 genes that synthesize most ganglioside species. Our results reveal substantial changes in the ganglioside lipidome during the formation of myelinated nerve fibers. In sum, we provide insights into the CNS ganglioside lipidome with a quantitative and sensitive mass spectrometry method. Since this method is compatible with global lipidomic profiling, it will provide insights into ganglioside function in physiology and pathology.

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A sensitive direct infusion mass spectrometry method for ganglioside lipidomics

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Quantification of gangliosides in CNS myelin development

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Generation of myelin in the absence of gangliosides

White matter integrity in mice requires continuous myelin synthesis at the inner tongue

Myelin, the electrically insulating sheath on axons, undergoes dynamic changes over time. However, it is composed of proteins with long lifetimes. This raises the question how such a stable structure is renewed. Here, we study the integrity of myelinated tracts after experimentally preventing the formation of new myelin in the CNS of adult mice, using an inducible Mbp null allele. Oligodendrocytes survive recombination, continue to express myelin genes, but they fail to maintain compacted myelin sheaths. Using 3D electron microscopy and mass spectrometry imaging we visualize myelin-like membranes failing to incorporate adaxonally, most prominently at juxta-paranodes. Myelinoid body formation indicates degradation of existing myelin at the abaxonal side and the inner tongue of the sheath. Thinning of compact myelin and shortening of internodes result in the loss of about 50% of myelin and axonal pathology within 20 weeks post recombination. In summary, our data suggest that functional axon-myelin units require the continuous incorporation of new myelin membranes.

Myelin is formed of proteins of long half-lives. The mechanisms of renewal of such a stable structure are unclear. Here, the authors show that myelin integrity requires continuous myelin synthesis at the inner tongue, contributing to the maintenance of a functional axon-myelin unit.

Milk fat globule membrane attenuates high fat diet-induced neuropathological changes in obese Ldlr-/-.Leiden mice

BACKGROUND

Milk-fat globule membrane (MFGM) is a complex structure secreted by the mammary gland and present in mammalian milk. MFGM contains lipids and glycoproteins as well as gangliosides, which may be involved in myelination processes. Notably, myelination and thereby white matter integrity are often altered in obesity. Furthermore, MFGM interventions showed beneficial effects in obesity by affecting inflammatory processes and the microbiome. In this study, we investigated the impact of a dietary MFGM intervention on fat storage, neuroinflammatory processes and myelination in a rodent model of high fat diet (HFD)-induced obesity.

METHODS

12-week-old male low density lipoprotein receptor-deficient Leiden mice were exposed to a HFD, a HFD enriched with 3% whey protein lipid concentrate (WPC) high in MFGM components, or a low fat diet. The impact of MFGM supplementation during 24-weeks of HFD-feeding was examined over time by analyzing body weight and fat storage, assessing cognitive tasks and MRI scanning, analyzing myelinization with polarized light imaging and examining neuroinflammation using immunohistochemistry.

RESULTS

We found in this study that 24 weeks of HFD-feeding induced excessive fat storage, increased systolic blood pressure, altered white matter integrity, decreased functional connectivity, induced neuroinflammation and impaired spatial memory. Notably, supplementation with 3% WPC high in MFGM components restored HFD-induced neuroinflammation and attenuated the reduction in hippocampal-dependent spatial memory and hippocampal functional connectivity.

CONCLUSIONS

We showed that supplementation with WPC high in MFGM components beneficially contributed to hippocampal-dependent spatial memory, functional connectivity in the hippocampus and anti-inflammatory processes in HFD-induced obesity in rodents. Current knowledge regarding exact biological mechanisms underlying these effects should be addressed in future studies.

Sulfatide in health and disease. The evaluation of sulfatide in cerebrospinal fluid as a possible biomarker for neurodegeneration

Sulfatide (3-O-sulfogalactosylceramide, SM4) is a glycosphingolipid, highly multifunctional and particularly enriched in the myelin sheath of neurons. The role of sulfatide has been implicated in various biological fields such as the nervous system, immune system, host-pathogen recognition and infection, beta cell function and haemostasis/thrombosis. Thus, alterations in sulfatide metabolism and production are associated with several human diseases such as neurological and immunological disorders and cancers. The unique lipid-rich composition of myelin reflects the importance of lipids in this specific membrane structure. Sulfatide has been shown to be involved in the regulation of oligodendrocyte differentiation and in the maintenance of the myelin sheath by influencing membrane dynamics involving sorting and lateral assembly of myelin proteins as well as ion channels. Sulfatide is furthermore essential for proper formation of the axo-glial junctions at the paranode together with axonal glycosphingolipids. Alterations in sulfatide metabolism are suggested to contribute to myelin deterioration as well as synaptic dysfunction, neurological decline and inflammation observed in different conditions associated with myelin pathology (mouse models and human disorders). Body fluid biomarkers are of importance for clinical diagnostics as well as for patient stratification in clinical trials and treatment monitoring. Cerebrospinal fluid (CSF) is commonly used as an indirect measure of brain metabolism and analysis of CSF sulfatide might provide information regarding whether the lipid disruption observed in neurodegenerative disorders is reflected in this body fluid. In this review, we evaluate the diagnostic utility of CSF sulfatide as a biomarker for neurodegenerative disorders associated with dysmyelination/demyelination by summarising the current literature on this topic. We can conclude that neither CSF sulfatide levels nor individual sulfatide species consistently reflect the lipid disruption observed in many of the demyelinating disorders. One exception is the lysosomal storage disorder metachromatic leukodystrophy, possibly due to the genetically determined accumulation of non-metabolised sulfatide. We also discuss possible explanations as to why myelin pathology in brain tissue is poorly reflected by the CSF sulfatide concentration. The previous suggestion that CSF sulfatide is a marker of myelin damage has thereby been challenged by more recent studies using more sophisticated laboratory techniques for sulfatide analysis as well as improved sample selection criteria due to increased knowledge on disease pathology.

Cerebrospinal Fluid Sulfatide Levels Lack Diagnostic Utility in the Subcortical Small Vessel Type of Dementia

BACKGROUND

Sulfatides (STs) in cerebrospinal fluid (CSF), as well as magnetic resonance imaging (MRI)-detected white matter hyperintensities (WMHs), may reflect demyelination. Here, we investigated the diagnostic utility of CSF ST levels in the subcortical small vessel type of dementia (SSVD), which is characterized by the presence of brain WMHs.

OBJECTIVE

To study the diagnostic utility of CSF ST levels in SSVD.

METHODS

This was a mono-center, cross-sectional study of SSVD (n = 16), Alzheimer's disease (n = 40), mixed dementia (n = 27), and healthy controls (n = 33). Totally, 20 ST species were measured in CSF by liquid chromatography-mass spectrometry (LC-MS/MS).

RESULTS

CSF total ST levels, as well as CSF levels of hydroxylated and nonhydroxylated ST species, did not differ across the study groups. In contrast, CSF neurofilament light chain (NFL) levels separated the patient groups from the controls. CSF total ST level correlated with CSF/serum albumin ratio in the total study population (r = 0.64, p < 0.001) and in all individual study groups. Furthermore, CSF total ST level correlated positively with MRI-estimated WMH volume in the total study population (r = 0.30, p < 0.05), but it did not correlate with CSF NFL level.

CONCLUSION

Although there was some relation between CSF total ST level and WMH volume, CSF ST levels were unaltered in all dementia groups compared to the controls. This suggests that CSF total ST level is a poor biomarker of demyelination in SSVD. Further studies are needed to investigate the mechanisms underlying the marked correlation between CSF total ST level and CSF/serum albumin ratio.

An iPSC model of hereditary sensory neuropathy-1 reveals L-serine-responsive deficits in neuronal ganglioside composition and axoglial interactions

Hereditary sensory neuropathy type 1 (HSN1) is caused by mutations in the SPTLC1 or SPTLC2 sub-units of the enzyme serine palmitoyltransferase, resulting in the production of toxic 1-deoxysphingolipid bases (DSBs). We used induced pluripotent stem cells (iPSCs) from patients with HSN1 to determine whether endogenous DSBs are neurotoxic, patho-mechanisms of toxicity and response to therapy. HSN1 iPSC-derived sensory neurons (iPSCdSNs) endogenously produce neurotoxic DSBs. Complex gangliosides, which are essential for membrane micro-domains and signaling, are reduced, and neurotrophin signaling is impaired, resulting in reduced neurite outgrowth. In HSN1 myelinating cocultures, we find a major disruption of nodal complex proteins after 8 weeks, which leads to complete myelin breakdown after 6 months. HSN1 iPSC models have, therefore, revealed that SPTLC1 mutation alters lipid metabolism, impairs the formation of complex gangliosides, and reduces axon and myelin stability. Many of these changes are prevented by l-serine supplementation, supporting its use as a rational therapy.

Neuronally expressed a-series gangliosides are sufficient to prevent the lethal age-dependent phenotype in GM3-only expressing mice

Gangliosides are expressed on plasma membranes throughout the body and enriched in the nervous system. A critical role for complex a- and b-series gangliosides in central and peripheral nervous system ageing has been established through transgenic manipulation of enzymes in ganglioside biosynthesis. Disrupting GalNAc-transferase (GalNAc-T), thus eliminating all a- and b-series complex gangliosides (with consequent over-expression of GM3 and GD3) leads to an age-dependent neurodegeneration. Mice that express only GM3 ganglioside (double knockout produced by crossing GalNAc-T-/- and GD3 synthase-/- mice, Dbl KO) display markedly accelerated neurodegeneration with reduced survival. Degenerating axons and disrupted node of Ranvier architecture are key features of complex ganglioside-deficient mice. Previously, we have shown that reintroduction of both a- and b-series gangliosides into neurons on a global GalNAcT-/- background is sufficient to rescue this age-dependent neurodegenerative phenotype. To determine the relative roles of a- and b-series gangliosides in this rescue paradigm, we herein reintroduced GalNAc-T into neurons of Dbl KO mice, thereby reconstituting a-series but not b-series complex gangliosides. We assessed survival, axon degeneration, axo-glial integrity, inflammatory markers and lipid-raft formation in these Rescue mice compared to wild-type and Dbl KO mice. We found that this neuronal reconstitution of a-series complex gangliosides abrogated the adult lethal phenotype in Dbl KO mice, and partially attenuated the neurodegenerative features. This suggests that whilst neuronal expression of a-series gangliosides is critical for survival during ageing, it is not entirely sufficient to restore complete nervous system integrity in the absence of either b-series or glial a-series gangliosides.

Animal secretory endolysosome channel discovery

Secretory pore-forming proteins (PFPs) have been identified in organisms from all kingdoms of life. Our studies with the toad species Bombina maxima found an interaction network among aerolysin family PFPs (af-PFPs) and trefoil factors (TFFs). As a toad af-PFP, BmALP1 can be reversibly regulated between active and inactive forms, with its paralog BmALP3 acting as a negative regulator. BmALP1 interacts with BmTFF3 to form a cellular active complex called βγ-CAT. This PFP complex is characterized by acting on endocytic pathways and forming pores on endolysosomes, including stimulating cell macropinocytosis. In addition, cell exocytosis can be induced and/or modulated in the presence of βγ-CAT. Depending on cell contexts and surroundings, these effects can facilitate the toad in material uptake and vesicular transport, while maintaining mucosal barrier function as well as immune defense. Based on experimental evidence, we hereby propose a secretory endolysosome channel (SELC) pathway conducted by a secreted PFP in cell endocytic and exocytic systems, with βγ-CAT being the first example of a SELC protein. With essential roles in cell interactions and environmental adaptations, the proposed SELC protein pathway should be conserved in other living organisms.

Nanopharmaceutical-based regenerative medicine: a promising therapeutic strategy for spinal cord injury

Spinal cord injury (SCI) is a neurological disorder that can lead to loss of perceptive and athletic function due to the severe nerve damage. To date, pieces of evidence detailing the precise pathological mechanisms in SCI are still unclear. Therefore, drug therapy cannot effectively alleviate the SCI symptoms and faces the limitations of systemic administration with large side effects. Thus, the development of SCI treatment strategies is urgent and valuable. Due to the application of nanotechnology in pharmaceutical research, nanopharmaceutical-based regenerative medicine will bring colossal development space for clinical medicine. These nanopharmaceuticals (i.e. nanocrystalline drugs and nanocarrier drugs) are designed using different types of materials or bioactive molecules, so as to improve the therapeutic effects, reduce side effects, and subtly deliver drugs, etc. Currently, an increasing number of nanopharmaceutical products have been approved by drug regulatory agencies, which has also prompted more researchers to focus on the potential treatment strategies of SCI. Therefore, the purpose of this review is to summarize and elaborate the research progress as well as the challenges and future of nanopharmaceuticals in the treatment of SCI, aiming to promote further research of nanopharmaceuticals in SCI.

Synaptic Function and Dysfunction in Lysosomal Storage Diseases

Lysosomal storage diseases (LSDs) with neurological involvement are inherited genetic diseases of the metabolism characterized by lysosomal dysfunction and the accumulation of undegraded substrates altering glial and neuronal function. Often, patients with neurological manifestations present with damage to the gray and white matter and irreversible neuronal decline. The use of animal models of LSDs has greatly facilitated studying and identifying potential mechanisms of neuronal dysfunction, including alterations in availability and function of synaptic proteins, modifications of membrane structure, deficits in docking, exocytosis, recycling of synaptic vesicles, and inflammation-mediated remodeling of synapses. Although some extrapolations from findings in adult-onset conditions such as Alzheimer's disease or Parkinson's disease have been reported, the pathogenetic mechanisms underpinning cognitive deficits in LSDs are still largely unclear. Without being fully inclusive, the goal of this mini-review is to present a discussion on possible mechanisms leading to synaptic dysfunction in LSDs.

Sea urchin gangliosides exhibit neuritogenic effects in neuronal PC12 cells via TrkA- and TrkB-related pathways

Gangliosides (GLSs) are ubiquitously distributed in all tissues but highly enriched in nervous system. Currently, it is unclear how exogenous GLSs regulate neuritogenesis, although neural functions of endogenous GLSs are widely studied. Herein, we evaluated the neuritogenic activities and mechanism of sea urchin gangliosides (SU-GLSs) in vitro. These different glycosylated SU-GLSs, including GM4(1S), GD4(1S), GD4(2A), and GD4(2G), promoted differentiation of NGF-induced PC12 cells in a dose-dependent and structure-selective manner. Sulfate-type and disialo-type GLSs exhibited stronger neuritogenic effects than monosialoganglioside GM1. Furthermore, SU-GLSs might act as neurotrophic factors possessing neuritogenic effects, via targeting tyrosine-kinase receptors (TrkA and TrkB) and activating MEK1/2-ERK1/2-CREB and PI3K-Akt-CREB pathways. This activation resulted in increased expression and secretion of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF). These pathways were verified by specific inhibitors. Our results confirmed the neuritogenic functions of SU-GLS in vitro and indicated their potential roles as natural nutrition for neuritogenesis.

β-1, 3-galactosyltransferase 2 ameliorates focal ischemic cerebral injury by maintaining blood-brain barrier integrity

Blood-brain barrier (BBB) damage at the early stage of ischemic stroke is a vital cause of brain parenchymal injury. The mechanism of BBB disruption has been intensively investigated, but still not fully understood. β-1, 3-galactosyltransferase 2 (B3galt2) is expressed in the brain, but its role in the pathogenesis of cerebral ischemia remains unknown. In this study, we investigated the role of B3galt2 in cerebral ischemia in mice. Focal cerebral ischemia was induced in mice by middle cerebral artery occlusion (MCAO). B3galt2 protein levels were determined in microvessels which were isolated from ischemic brain at 12, 24 and 72 h after MCAO. Mice were administered lentiviral vectors encoding B3galt2 (LV- B3galt2) or recombinant transforming growth factor-β1 (r-TGF-β1) by intracerebroventricular injection. We assessed infarct volume and neurologic deficits on days 1, 3, and 14 after MCAO, blood-brain barrier (BBB) integrity at 12 and 24 h after MCAO, and the levels of TGF-β1, TGF-βR(Ⅱ) and p-Smad2/3 at 24 and 72 h after MCAO. Our results indicated that B3galt2 was expressed in brain microvascular endothelial cells and increased in the ischemic microvessels. Overexpression of B3galt2 by LV- B3galt2 administration reduced infarct volume and improved functional outcome after cerebral ischemia. Moreover, the neuroprotective effects were associated with preventing BBB damage. Compared with wild-type (WT) mice, heterozygous B3galt2 knockout (B3galt2^-/+) mice not only showed severe BBB damage, neurologic functional deficits, but also showed reduced expression of TGF-β1, TGF-βR(Ⅱ) and p-Smad2/3 in microvessels after cerebral ischemia. Pre-administration of r-TGF-β1 reduced BBB damage, and improved the neurological outcomes in both WT mice and B3galt2^-/+ mice after cerebral ischemia. Our results suggested B3galt2 protected against ischemic stroke in mice, and the underlying mechanism might include TGF-β signaling pathway in brain microvascular endothelial cells.

Decreased turnover of the CNS myelin protein Opalin in a mouse model of hereditary spastic paraplegia 35

Spastic paraplegia 35 (SPG35) (OMIM: 612319) or fatty acid hydroxylase-associated neurodegeneration (FAHN) is caused by deficiency of fatty acid 2-hydroxylase (FA2H). This enzyme synthesizes sphingolipids containing 2-hydroxylated fatty acids, which are particularly abundant in myelin. Fa2h-deficient (Fa2h-/-) mice develop symptoms reminiscent of the human disease and therefore serve as animal model of SPG35. In order to understand further the pathogenesis of SPG35, we compared the proteome of purified CNS myelin isolated from wild type and Fa2h-/- mice at different time points of disease progression using tandem mass tag labeling. Data analysis with a focus on myelin membrane proteins revealed a significant increase of the oligodendrocytic myelin paranodal and inner loop protein (Opalin) in Fa2h-/- mice, whereas the concentration of other major myelin proteins was not significantly changed. Western blot analysis revealed an almost 6-fold increase of Opalin in myelin of Fa2h-/- mice aged 21-23 months. A concurrent unaltered Opalin gene expression suggested a decreased turnover of the Opalin protein in Fa2h-/- mice. Supporting this hypothesis, Opalin protein half-life was reduced significantly when expressed in CHO cells synthesizing 2-hydroxylated sulfatide, compared to cells synthesizing only non-hydroxylated sulfatide. Degradation of Opalin was inhibited by inhibitors of lysosomal degradation but unaffected by proteasome inhibitors. Taken together, these results reveal a new function of 2-hydroxylated sphingolipids namely affecting the turnover of a myelin membrane protein. This may play a role in the pathogenesis of SPG35.

The role of Sphingolipids in myelination and myelin stability and their involvement in childhood and adult demyelinating disorders

Multiple sclerosis (MS) represents the most common demyelinating disease affecting the central nervous system (CNS) in adults as well as in children. Furthermore, in children, in addition to acquired diseases such as MS, genetically inherited diseases significantly contribute to the incidence of demyelinating disorders. Some genetic defects lead to sphingolipid alterations that are able to elicit neurological symptoms. Sphingolipids are essential for brain development, and their aberrant functionality may thus contribute to demyelinating diseases such as MS. In particular, sphingolipidoses caused by deficits of sphingolipid-metabolizing enzymes, are often associated with demyelination. Sphingolipids are not only structural molecules but also bioactive molecules involved in the regulation of cellular events such as development of the nervous system, myelination and maintenance of myelin stability. Changes in the sphingolipid metabolism deeply affect plasma membrane organization. Thus, changes in myelin sphingolipid composition might crucially contribute to the phenotype of diseases characterized by demyelinalization. Here, we review key features of several sphingolipids such as ceramide/dihydroceramide, sphingosine/dihydrosphingosine, glucosylceramide and, galactosylceramide which act in myelin formation during rat brain development and in human brain demyelination during the pathogenesis of MS, suggesting that this knowledge could be useful in identifying targets for possible therapies.

Molecular organization and function of vertebrate septate-like junctions

Septate-like junctions display characteristic ladder-like ultrastructure reminiscent of the invertebrate epithelial septate junctions and are present at the paranodes of myelinated axons. The paranodal junctions where the myelin loops attach to the axon at the borders of the node of Ranvier provide both a paracellular barrier to ion diffusion and a lateral fence along the axonal membrane. The septate-like junctions constrain the proper distribution of nodal Na⁺ channels and juxtaparanodal K⁺ channels, which are required for the safe propagation of the nerve influx and rapid saltatory conduction. The paranodal cell adhesion molecules have been identified as target antigens in peripheral demyelinating autoimmune diseases and the pathogenic mechanisms described. This review aims at presenting the recent knowledge on the molecular and structural organization of septate-like junctions, their formation and stabilization during development, and how they are involved in demyelinating diseases.

Myelin uncompaction and axo-glial detachment in chronic ataxic neuropathy with monospecific IgM antibody to ganglioside GD1b

To describe clinical features, disease course, treatment response, and sural nerve biopsy findings in a patient with chronic sensory ataxic neuropathy, Binet stage A chronic lymphocytic leukemia, and monoclonal IgMλ paraprotein against ganglioside GD1b. During 9 months of hospitalization at two neurologic centers, the patient underwent serial neurologic examinations, neurophysiologic studies, imaging investigations, extensive laboratory work-up, bone marrow, and sural nerve biopsies. The patient had a severe progressive sensory neuropathy accompanied by motor involvement, dysautonomia, and marked bulbar weakness with preserved ocular movements. Conduction studies were characterized by prolonged F-wave minimal latencies, prolonged distal latencies, reduction of compound motor action potentials, and absence of sensory nerve action potentials. Sural nerve biopsy showed endoneurial edema, axonal degeneration, and regeneration, in the absence of cellular inflammation, macrophagic activation, and B-lymphocyte infiltration; no IgM or complement deposition was detected. Myelinated fibers showed redundant/abnormally thickened myelin, myelin vacuolation, and frank intramyelinic edema with condensed axoplasm. Ultrastructural features included axo-glial detachment, disruption of membrane integrity, and myelin uncompaction. This study shows that monospecific anti-GD1b IgM paraprotein is associated with non-inflammatory nerve damage. We suggest that the loss of myelin and axonal integrity reflects antibody-induced disruption of membrane lipid rafts.

Coordinated internodal and paranodal adhesion controls accurate myelination by oligodendrocytes

Elazar et al. show that reduced axoglial adhesion at both the paranodal junction and the internodes results in the formation of multimyelinated axons. Their findings demonstrate that accurate ensheathment by oligodendrocytes depends on the coordinated action of these different adhesion systems.

Oligodendrocyte–axon contact is mediated by several cell adhesion molecules (CAMs) that are positioned at distinct sites along the myelin unit, yet their role during myelination remains unclear. Cadm4 and its axonal receptors, Cadm2 and Cadm3, as well as myelin-associated glycoprotein (MAG), are enriched at the internodes below the compact myelin, whereas NF155, which binds the axonal Caspr/contactin complex, is located at the paranodal junction that is formed between the axon and the terminal loops of the myelin sheath. Here we report that Cadm4-, MAG-, and Caspr-mediated adhesion cooperate during myelin membrane ensheathment. Genetic deletion of either Cadm4 and MAG or Cadm4 and Caspr resulted in the formation of multimyelinated axons due to overgrowth of the myelin away from the axon and the forming paranodal junction. Consequently, these mice displayed paranodal loops either above or underneath compact myelin. Our results demonstrate that accurate placement of the myelin sheath by oligodendrocytes requires the coordinated action of internodal and paranodal CAMs.