Synthesis and biological activity of glycolipids, with a focus on gangliosides and sulfatide analogs

Stimulation of adult neural stem cells with a novel glycolipid biosurfactant

Glycolipids are amphipathic molecules which are highly expressed on cell membranes in skin and brain where they mediate several key cellular processes. Neural stem cells are defined as undifferentiated, proliferative, multipotential cells with extensive self-renewal and are responsive to brain injury. Di-rhamnolipid: α-L-rhamnopyranosyl-(1-2)α-L-rhamnopyranosyl-3-hydroxydecanoyl-3-hydroxydecanoic acid, also referred to as di-rhamnolipid BAC-3, is a glycolipid isolated from the bacteria Pseudomonas aeruginosa. In the previous studies, di-rhamnolipid enhanced dermal tissue healing and regeneration. The present study provides the first assessment of di-rhamnolipid, and glycolipid biosurfactants in general, on the nervous system. Treatment of neural stem cells isolated from the lateral ventricle of adult mice and cultured in defined media containing growth factors at 0.5 and 1 μg/ml of di-rhamnolipid increased the number of neurospheres (2.7- and 2.8-fold, respectively) compared to controls and this effect remained even after passaging in the absence of di-rhamnolipid. In addition, neural stem cells treated with di-rhamnolipid at 50 and 100 μg/ml in defined media supplemented with fetal calf serum and without growth factors exhibited increased cell viability, indicating an interaction between di-rhamnolipid and serum components in the regulation of neural stem cells and neuroprogenitors. Intracerebroventricular administration of di-rhamnolipid at 300 and 120 ng/day increased the number of neurospheres (1.3- and 1.63-fold, respectively) that could be derived from the anterior lateral ventricles of adult mice. These results indicate that di-rhamnolipid stimulates proliferation of neural stem cells and increases their endogenous pools which may have therapeutic potential in managing neurodegenerative or neuropsychiatric disorders and promoting nervous tissue regeneration following injury.

Chronic caloric restriction attenuates a loss of sulfatide content in PGC-1α-/- mouse cortex: a potential lipidomic role of PGC-1α in neurodegeneration

Peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α), a key regulator of energy metabolism and lipid homeostasis in multiple highly oxidative tissues, has been implicated in the metabolic derangements of diabetes and obesity. However, relatively less is known regarding its role in neurological functions. Using shotgun lipidomics, we investigated the lipidome of mouse cerebral cortex with generalized deficiency of PGC-1α (PGC-1α(-/-)) versus wild-type (WT) mice under standard diet and chronically calorically restricted conditions. Specific deficiency in sulfatide, a myelin-specific lipid class critically involved in maintaining neurological function, was uncovered in the cortex of PGC-1α(-/-) mice compared with WT mice at all ages examined. Chronic caloric restriction (CR) for 22 months essentially restored the sulfatide reduction in PGC-1α(-/-) mice compared with WT, but sulfatide reduction was not restored in PGC-1α(-/-) with CR for a short term (i.e., 3 months). Mechanistic studies uncovered and differentiated the biochemical mechanisms underpinning the two conditions of altered sulfatide homeostasis. The former is modulated through PGC-1α-MAL pathway, whereas the latter is under the control of LXR/RXR-apoE metabolism pathway. These results suggest a novel mechanistic role of PGC-1α in sulfatide homeostasis, provide new insights into the importance of PGC-1α in neurological functions, and indicate a potential therapeutic approach for treatment of deficient PGC-1α-induced alterations in sulfatide homeostasis.

The pathogenic implication of abnormal interaction between apolipoprotein E isoforms, amyloid-beta peptides, and sulfatides in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia in the aging population. Prior work has shown that the epsilon4 allele of apolipoprotein E (apoE4) is a major risk factor for "sporadic" AD, which accounts for >99% of AD cases without a defined underlying mechanism. Recently, we have demonstrated that sulfatides are substantially and specifically depleted at the very early stage of AD. To identify the mechanism(s) of sulfatide loss concurrent with AD onset, we have found that: (1) sulfatides are specifically associated with apoE-associated particles in cerebrospinal fluid (CSF); (2) apoE modulates cellular sulfatide levels; and (3) the modulation of sulfatide content is apoE isoform dependent. These findings not only lead to identification of the potential mechanisms underlying sulfatide depletion at the earliest stages of AD but also serve as mechanistic links to explain the genetic association of apoE4 with AD. Moreover, our recent studies further demonstrated that (1) apoE mediates sulfatide depletion in amyloid-beta precursor protein transgenic mice; (2) sulfatides enhance amyloid beta (Abeta) peptides binding to apoE-associated particles; (3) Abeta42 content notably correlates with sulfatide content in CSF; (4) sulfatides markedly enhance the uptake of Abeta peptides; and (5) abnormal sulfatide-facilitated Abeta uptake results in the accumulation of Abeta in lysosomes. Collectively, our studies clearly provide a link between apoE, Abeta, and sulfatides in AD and establish a foundation for the development of effective therapeutic interventions for AD.

Gangliosides are important for the preservation of the structure and organization of RBL-2H3 mast cells

Gangliosides are known to be important in many biological processes. However, details concerning the exact function of these glycosphingolipids in cell physiology are poorly understood. In this study, the role of gangliosides present on the surface of rodent mast cells in maintaining cell structure was examined using RBL-2H3 mast cells and two mutant cell lines (E5 and D1) deficient in the gangliosides, GM(1) and the alpha-galactosyl derivatives of the ganglioside GD(1b). The two deficient cell lines were morphologically different from each other as well as from the parental RBL-2H3 cells. Actin filaments in RBL-2H3 and E5 cells were under the plasma membrane following the spindle shape of the cells, whereas in D1 cells, they were concentrated in large membrane ruffles. Microtubules in RBL-2H3 and E5 cells radiated from the centrosome and were organized into long, straight bundles. The bundles in D1 cells were thicker and organized circumferentially under the plasma membrane. The endoplasmic reticulum, the Golgi complex, and the secretory granule matrix were also altered in the mutant cell lines. These results suggest that the mast cell-specific alpha-galactosyl derivatives of ganglioside GD(1b) and GM(1) are important in maintaining normal cell morphology.

Novel sulfated gangliosides, high-affinity ligands for neural siglecs, inhibit NADase activity of leukocyte cell surface antigen CD38

Three kinds of novel sulfated gangliosides structurally related to the Chol-1 (alpha-series) ganglioside GQ1balpha were synthesized. These sulfated gangliosides were potent inhibitors of NADase activity of leukocyte cell surface antigen CD38. Among the synthetic gangliosides, GSC-338 (II(3)III(6)-disulfate of iso-GM1b) was surprisingly found to be the most potent structure in both the NADase inhibition and MAG-binding activity. The present study indicates that the sulfated gangliosides are useful to study the recognition of the internal tandem sialic acid residues alpha2-3-linked to Gal(II(3)) as well as the siglec-dependent recognition including a terminal sialic acid residue.

Localization of major gangliosides in the PNS: implications for immune neuropathies

Antibodies targeting major gangliosides that are broadly distributed in the nervous system are sometimes associated with clinical symptoms that imply selective nerve damage. For example, anti-GD1a antibodies are associated with acute motor axonal neuropathy (AMAN), a form of Guillain-Barré syndrome that selectively affects motor nerves, despite reports that GD1a is present in human axons and myelin and is not expressed differentially in motor versus sensory roots. We used a series of high-affinity monoclonal antibodies (mAbs) against the major nervous system gangliosides GM1, GD1a, GD1b and GT1b to test whether any of them bind motor or sensory fibres differentially in rodent and human peripheral nerves. The following observations were made. (i) Some of the anti-GD1a antibodies preferentially stained motor fibres, supporting the association of human anti-GD1a antibodies with predominant motor neuropathies such as AMAN. (ii) A GD1b antibody preferentially stained the large dorsal root ganglion (DRG) neurones, in keeping with the proposed role of human anti-GD1b antibodies in sensory ataxic neuropathies. (iii) Two mAbs with broad structural cross-reactivity bound to both gangliosides and peripheral nerve proteins. (iv) Myelin was poorly stained; all clones stained axons nearly exclusively. Our findings suggest that anti-ganglioside antibody fine specificity as well as differences in ganglioside accessibility in axons and myelin influence the selectivity of injury to different fibre systems and cell types in human autoimmune neuropathies.

Immunoglobulin G-class mouse monoclonal antibodies to major brain gangliosides

Mice genetically engineered to lack complex gangliosides are improved hosts for raising antibodies against those gangliosides. We report the generation and characterization of nine immunoglobulin G (IgG)-class monoclonal antibodies (mAbs) raised against the four major brain gangliosides in mammals. These include (designated as ganglioside specificity-IgG subclass) two anti-GM1 mAbs (GM1-1, GM1-2b), three anti-GD1a mAbs (GD1a-1, GD1a-2a, GD1a-2b), one anti-GD1b mAb (GD1b-1), and three anti-GT1b mAbs (GT1b-1, GT1b-2a, GT1b-2b). Each mAb demonstrated high specificity, with little or no cross-reactivity with other major brain gangliosides. Enzyme-linked immunosorbent assay (ELISA) screening against 14 closely related synthetic and purified gangliosides confirmed the high specificity, with no significant cross-reactivity except that of the anti-GD1a mAbs for the closely related minor ganglioside GT1a alpha. All of the mAbs were useful for ELISA, TLC immunooverlay, and immunocytochemistry. Neural cells from wild-type rats and mice were immunostained to differing levels with the anti-ganglioside antibodies, whereas neural cells from mice engineered to lack complex gangliosides (lacking the ganglioside-specific biosynthetic enzyme UDP-GalNAc:GM3/GD3 N-acetylgalactosaminyltransferase) remained unstained, demonstrating that most of the mAbs react only with gangliosides and not with related structures on glycoproteins. These mAbs may provide useful tools for delineation of the expression and function of the major brain gangliosides and for probing the pathology of anti-ganglioside autoimmune diseases.

High-affinity anti-ganglioside IgG antibodies raised in complex ganglioside knockout mice: reexamination of GD1a immunolocalization

Gangliosides, sialic acid-bearing glycosphingolipids, are highly enriched in the vertebrate nervous system. Anti-ganglioside antibodies are associated with various human neuropathies, although the pathogenicity of these antibodies remains unproven. Testing the pathogenic role of anti-ganglioside antibodies will be facilitated by developing high-affinity IgG-class complement-fixing monoclonal anti-bodies against major brain gangliosides, a goal that has been difficult to achieve. In this study, mice lacking complex gangliosides were used as immune-naive hosts to raise anti-ganglioside antibodies. Wild-type mice and knockout mice with a disrupted gene for GM2/GD2 synthase (UDP-N-acetyl-D-galactosamine : GM3/GD3 N-acetyl-D-glactosaminyltransferase) were immunized with GD1a conjugated to keyhole limpet hemocyanin. The knockout mice produced a vigorous anti-GD1a IgG response, whereas wildtype littermates failed to do so. Fusion of spleen cells from an immunized knockout mouse with myeloma cells yielded numerous IgG anti-GD1a antibody-producing colonies. Ganglioside binding studies revealed two specificity classes; one colony representing each class was cloned and characterized. High-affinity monoclonal antibody was produced by each hybridoma : an IgG1 that bound nearly exclusively to GD1a and an IgG2b that bound GD1a, GT1b, and GT1aalpha. Both antibodies readily readily detected gangliosides via ELISA, TLC immune overlay, immunohistochemistry, and immunocytochemistry. In contrast to prior reports using anti-GD1a and anti-GT1b IgM class monoclonal antibodies, the new antibodies bound avidly to granule neurons in brain tissue sections and cell cultures. Mice lacking complex gangliosides are improved hosts for raising high-affinity, high-titer anti-ganglioside IgG antibodies for probing for the distribution and physiology of gangliosides and the pathophysiology of anti-ganglioside antibodies.