Mice with disrupted GM2/GD2 synthase gene lack complex gangliosides but exhibit only subtle defects in their nervous system

Cell-specific deletion of glucosylceramide synthase in brain leads to severe neural defects after birth

Sialic acid-containing glycosphingolipids, i.e., gangliosides, constitute a major component of neuronal cells and are thought to be essential for brain function. UDP-glucose:ceramide glucosyltransferase (Ugcg) catalyzes the initial step of glycosphingolipid (GSL) biosynthesis. To gain insight into the role of GSLs in brain development and function, a cell-specific disruption of Ugcg was performed as indicated by the absence of virtually all glucosylceramide-based GSLs. Shortly after birth, mice showed dysfunction of cerebellum and peripheral nerves, associated with structural defects. Axon branching of Purkinje cells was significantly reduced. In primary cultures of neurons, dendritic complexity was clearly diminished, and pruning occurred early. Myelin sheaths of peripheral nerves were broadened and focally severely disorganized. GSL deficiency also led to a down-regulation of gene expression sets involved in brain development and homeostasis. Mice died approximately 3 weeks after birth. These results imply that GSLs are essential for brain maturation.

Characterisation of the immunoglobulin variable region gene usage encoding the murine anti-ganglioside antibody repertoire

Neuropathogenic murine antibodies reactive with terminal disialylgalactose epitopes are innate and preferentially encoded by the VH7183.3b gene. Here we have studied antibodies reactive with internal galactose-linked disialosyl epitopes and the terminal trisaccharide of GT1b. Antibodies were of moderate affinity and unmutated. Anti-GD1b antibodies were often encoded by the VH10.2b heavy and gj38c light chain genes. Anti-GT1b antibodies with broader glycan binding patterns were encoded by VHQ52 and VHJ558 family genes. These data indicate that the discrete specificities of ganglioside-binding antibodies are dictated by particular patterns of V gene usage residing within the innate B cell repertoire.

Different response of the knockout mice lacking b-series gangliosides against botulinum and tetanus toxins

We assessed the response in knockout mice lacking the b-series (G(D2), G(D1b), G(T1b) and G(Q1b)) gangliosides against Clostridium botulinum (types A, B and E) and tetani toxins. We found that botulinum toxins were fully toxic, while tetanus toxin was much less toxic in the knockout mice. Combining the present results with our previous finding that tetanus toxin and botulinum types A and B toxins showed essentially no toxic activity in the knockout mice lacking both the a-series and b-series gangliosides (complex gangliosides), we concluded that the b-series gangliosides is the major essential substance for tetanus toxin, while b-series gangliosides may be not the essential substance for botulinum toxins, at the initial step during the intoxication process in mouse.

Integration of glycosphingolipid metabolism and cell-fate decisions in cancer and stem cells: review and hypothesis

The metabolism of glycosphingolipids is strictly regulated during the mitotic cell cycle. Before the G1-to-S transition, the ceramide and glucosylceramide concentration is elevated. Ceramide induces apoptosis synergistically with the pro-apoptotic protein prostate apoptosis response 4 (PAR-4) that may be asymmetrically inherited during cell division. Only one daughter cell dies shortly after mitosis, a mechanism we suggested to regulate the number of neural stem cells during embryonic development. The progeny cells, however, may protect themselves by converting ceramide to glucosylceramide and other glycosphingolipids. In particular, complex gangliosides have been found to sustain cell survival and differentiation. The cell cycle may thus be a turning point for (glyco)sphingolipid metabolism and explain rapid changes of the sphingolipid composition in cells that undergo mitotic cell-fate decisions. In the proposed model termed "Shiva cycle", progression through the cell cycle, differentiation, or apoptosis may rely on a delicate balance of (glyco)sphingolipid second messengers that modulate the retinoblastoma-dependent G1-to-S transition or caspase-dependent G1-to-apoptosis program. Ceramide-induced cell cycle delay at G0/G1 is either followed by ceramide-induced apoptosis or by conversion of ceramide to glucosylceramide, a proposed key regulatory rheostat that rescues cells from re-entry into a life/death decision at G1-to-S. We propose a mechanistic model for sphingolpid-induced protein scaffolds ("slip") that regulate cell-fate decisions and will discuss the biological consequences and pharmacological potential of manipulating the (glyco)sphingolipid-dependent cell fate program in cancer and stem cells.

Interruption of ganglioside synthesis produces central nervous system degeneration and altered axon-glial interactions

Gangliosides, which are sialylated glycosphingolipids, are the major class of glycoconjugates on neurons and carry the majority of the sialic acid within the central nervous system (CNS). To determine the role of ganglioside synthesis within the CNS, mice carrying null mutations in two critical ganglioside-specific glycosyltransferase genes, Siat9 (encoding GM3 synthase) and Galgt1 (encoding GM2 synthase), were generated. These double-null mice were unable to synthesize gangliosides of the ganglio-series of glycosphingolipids, which are the major ganglioside class in the CNS. Soon after weaning, viable mice developed a severe neurodegenerative disease that resulted in death. Histopathological examination revealed striking vacuolar pathology in the white matter regions of the CNS with axonal degeneration and perturbed axon-glia interactions. These results indicate that ganglioside synthesis is essential for the development of a stable CNS, possibly by means of the promotion of interactions between axon and glia.

Overexpression of GD1a ganglioside sensitizes motor nerve terminals to anti-GD1a antibody-mediated injury in a model of acute motor axonal neuropathy

Anti-GD1a ganglioside antibodies (Abs) are the serological hallmark of the acute motor axonal form of the post-infectious paralysis, Guillain-Barre syndrome. Development of a disease model in mice has been impeded by the weak immunogenicity of gangliosides and the apparent resistance of GD1a-containing neural membranes to anti-GD1a antibody-mediated injury. Here we used mice with altered ganglioside biosynthesis to generate such a model at motor nerve terminals. First, we bypassed immunological tolerance by immunizing GD1a-deficient, beta-1,4-N-acetylgalactosaminyl transferase knock-out mice with GD1a ganglioside-mimicking antigens from Campylobacter jejuni and generated high-titer anti-GD1a antisera and complement fixing monoclonal Abs (mAbs). Next, we exposed ex vivo nerve-muscle preparations from GD1a-overexpressing, GD3 synthase knock-out mice to the anti-GD1a mAbs in the presence of a source of complement and investigated morphological and electrophysiological damage. Dense antibody and complement deposits were observed only over presynaptic motor axons, accompanied by severe ultrastructural damage and electrophysiological blockade of motor nerve terminal function. Perisynaptic Schwann cells and postsynaptic membranes were unaffected. In contrast, normal mice were not only unresponsive to immunization with GD1a but also resistant to neural injury during anti-GD1a Ab exposure, demonstrating the central role of membrane antigen density in modulating both immune tolerance to GD1a and axonal susceptibility to anti-GD1a Abmediated injury. Identical paralyzing effects were observed when testing mouse and human anti-GD1a-positive sera. These data indicate that anti-GD1a Abs arise via molecular mimicry and are likely to be clinically relevant in injuring peripheral nerve axonal membranes containing sufficiently high levels of GD1a.

The crucial role of Campylobacter jejuni genes in anti-ganglioside antibody induction in Guillain-Barre syndrome

Molecular mimicry of Campylobacter jejuni lipo-oligosaccharides (LOS) with gangliosides in nervous tissue is considered to induce cross-reactive antibodies that lead to Guillain-Barre syndrome (GBS), an acute polyneuropathy. To determine whether specific bacterial genes are crucial for the biosynthesis of ganglioside-like structures and the induction of anti-ganglioside antibodies, we characterized the C. jejuni LOS biosynthesis gene locus in GBS-associated and control strains. We demonstrated that specific types of the LOS biosynthesis gene locus are associated with GBS and with the expression of ganglioside-mimicking structures. Campylobacter knockout mutants of 2 potential GBS marker genes, both involved in LOS sialylation, expressed truncated LOS structures without sialic acid, showed reduced reactivity with GBS patient serum, and failed to induce an anti-ganglioside antibody response in mice. We demonstrate, for the first time, to our knowledge, that specific bacterial genes are crucial for the induction of anti-ganglioside antibodies.

The crucial role of Campylobacter jejuni genes in anti-ganglioside antibody induction in Guillain-Barre syndrome

Molecular mimicry of Campylobacter jejuni lipo-oligosaccharides (LOS) with gangliosides in nervous tissue is considered to induce cross-reactive antibodies that lead to Guillain-Barre syndrome (GBS), an acute polyneuropathy. To determine whether specific bacterial genes are crucial for the biosynthesis of ganglioside-like structures and the induction of anti-ganglioside antibodies, we characterized the C. jejuni LOS biosynthesis gene locus in GBS-associated and control strains. We demonstrated that specific types of the LOS biosynthesis gene locus are associated with GBS and with the expression of ganglioside-mimicking structures. Campylobacter knockout mutants of 2 potential GBS marker genes, both involved in LOS sialylation, expressed truncated LOS structures without sialic acid, showed reduced reactivity with GBS patient serum, and failed to induce an anti-ganglioside antibody response in mice. We demonstrate, for the first time, to our knowledge, that specific bacterial genes are crucial for the induction of anti-ganglioside antibodies.

Ganglioside GD1a restores infectibility to mouse cells lacking functional receptors for polyomavirus

Recent investigations on the pathway of cell entry by polyomavirus (Py) and simian virus 40 (SV40) have defined specific gangliosides as functional receptors mediating virus binding and transport from the plasma membrane to the endoplasmic reticulum (B. Tsai et al., EMBO J. 22:4346-4355, 2003; Gilbert and Benjamin, in press). These studies were carried out with C6 rat glioma cells, a heterologous host chosen for its known deficiency in ganglioside biosynthesis. Here, a cell genetic approach was undertaken to identify components required for the early steps of infection using mouse cells as the natural host for Py. Receptor-negative (R-) mouse cells, screened based on resistance to Py infection, were shown to bind Py but failed to allow entry of the virus. R- cells were also found to be resistant to SV40. Infectibility was restored or enhanced by the addition of the same specific gangliosides found in earlier studies with C6 cells. In one R- line, overexpression of caveolin-1 also increased infectibility. These results support and extend findings on gangliosides in lipid rafts as functional receptors and mediators of internalization for Py and SV40.

Sphingolipid metabolites in neural signalling and function

Sphingolipid metabolites, such as ceramide, sphingosine, sphingosine-1-phosphate (S1P) and complex sphingolipids (gangliosides), are recognized as molecules capable of regulating a variety of cellular processes. The role of sphingolipid metabolites has been studied mainly in non-neuronal tissues. These studies have underscored their importance as signals transducers, involved in control of proliferation, survival, differentiation and apoptosis. In this review, we will focus on studies performed over the last years in the nervous system, discussing the recent developments and the current perspectives in sphingolipid metabolism and functions.

Role of glycosylation in development

Researchers have long predicted that complex carbohydrates on cell surfaces would play important roles in developmental processes because of the observation that specific carbohydrate structures appear in specific spatial and temporal patterns throughout development. The astounding number and complexity of carbohydrate structures on cell surfaces added support to the concept that glycoconjugates would function in cellular communication during development. Although the structural complexity inherent in glycoconjugates has slowed advances in our understanding of their functions, the complete sequencing of the genomes of organisms classically used in developmental studies (e.g., mice, Drosophila melanogaster, and Caenorhabditis elegans) has led to demonstration of essential functions for a number of glycoconjugates in developmental processes. Here we present a review of recent studies analyzing function of a variety of glycoconjugates (O-fucose, O-mannose, N-glycans, mucin-type O-glycans, proteoglycans, glycosphingolipids), focusing on lessons learned from human disease and genetic studies in mice, D. melanogaster, and C. elegans.

Innate murine B cells produce anti-disialosyl antibodies reactive with Campylobacter jejuni LPS and gangliosides that are polyreactive and encoded by a restricted set of unmutated V genes

In Guillain-Barré syndrome following Campylobacter enteritis, anti-lipopolysaccharide antibodies cross-react with neural gangliosides, thereby precipitating autoimmune neuropathy. We examined the properties of 15 murine anti-LPS/ganglioside mAbs specific for NeuAc(alpha2-8)NeuAc-Gal disialosyl epitopes. Many mAbs displayed features of an innate B cell origin including polyreactivity (13/15), hybridoma CD5 mRNA expression (5/15), predominance of IgM (9/15) or IgG3 (3/6) isotype, low affinity, and utilisation of unmutated VH and VL VDJ rearrangements. Antibody specificity resided in highly selective V gene usage, with 6/15 mAbs being encoded by the VH7183.3b gene. These data indicate that neuropathogenic antiganglioside autoantibodies can arise from the natural autoantibody repertoire.

Alterations in neuroblastoma ganglioside synthesis by induction of GD1b synthase by retinoic acid

Recent findings link increased expression of the structurally complex 'b' pathway gangliosides GD1b, GT1b, GQ1b (CbG) to a favourable clinical and biological behaviour in human neuroblastoma (NB). Seeking a model to probe these observations, we evaluated four human NB cell lines. Very low CbG content (4-10%) in three of the four cell lines (LAN-5, LAN-1, SMS-KCNR) reflected the ganglioside pattern observed in the most aggressive NB tumours. Pharmacological alterations of complex ganglioside synthesis in vitro by a 5-7 day exposure to 5-10 microM retinoic acid, which is employed in maintenance therapy of disseminated NB, included markedly increased (i) relative expression of CbG (6.6+/-2.0-fold increase, P=0.037), (ii) relative expression of the analogous 'a' pathway gangliosides, termed CaG (6.4+/-1.4-fold increase in GM1a and GD1a; P=0.010), and (iii) total cellular ganglioside content (2.0-6.3-fold), which in turn amplified the accumulation of structurally complex gangliosides. Substantial increases (2.7-2.9-fold) in the activity of GD1b/GM1a synthase (beta-1,3-galactosyltransferase), which initiates the synthesis of CbG and CaG, accompanied the all-trans retinoic acid (ATRA)-induced ganglioside changes. Thus, increased CbG synthesis in NB cell lines is attributable to a specific effect of ATRA, namely induction of GD1b/GM1a synthase activity. Since the shift towards higher expression of CbG and CaG during retinoic acid-induced cellular differentiation reflects a ganglioside pattern found in clinically less-aggressive tumours, our studies suggest that complex gangliosides may play a role in the biological and clinical behaviour of NB.

Carbohydrate mimicry between human ganglioside GM1 and Campylobacter jejuni lipooligosaccharide causes Guillain-Barre syndrome

Molecular mimicry between microbial and self-components is postulated as the mechanism that accounts for the antigen and tissue specificity of immune responses in postinfectious autoimmune diseases. Little direct evidence exists, and research in this area has focused principally on T cell-mediated, antipeptide responses, rather than on humoral responses to carbohydrate structures. Guillain-Barré syndrome, the most frequent cause of acute neuromuscular paralysis, occurs 1-2 wk after various infections, in particular, Campylobacter jejuni enteritis. Carbohydrate mimicry [Galbeta1-3GalNAcbeta1-4(NeuAcalpha2-3)Galbeta1-] between the bacterial lipooligosaccharide and human GM1 ganglioside is seen as having relevance to the pathogenesis of Guillain-Barré syndrome, and conclusive evidence is reported here. On sensitization with C. jejuni lipooligosaccharide, rabbits developed anti-GM1 IgG antibody and flaccid limb weakness. Paralyzed rabbits had pathological changes in their peripheral nerves identical with those present in Guillain-Barré syndrome. Immunization of mice with the lipooligosaccharide generated a mAb that reacted with GM1 and bound to human peripheral nerves. The mAb and anti-GM1 IgG from patients with Guillain-Barré syndrome did not induce paralysis but blocked muscle action potentials in a muscle-spinal cord coculture, indicating that anti-GM1 antibody can cause muscle weakness. These findings show that carbohydrate mimicry is an important cause of autoimmune neuropathy.

Glycolipid-mediated cell-cell recognition in inflammation and nerve regeneration

Cell surface complex carbohydrates have emerged as key recognition molecules, mediating physiological interactions between cells. Typically, glycans on one cell surface are engaged by complementary carbohydrate binding proteins (lectins) on an apposing cell, initiating appropriate cellular responses. Although many cell surface lectins have been identified in vertebrates, only a few of their endogenous carbohydrate ligands have been established. Each major class of cell surface glycans-glycoproteins, glycolipids, and proteoglycans-has been implicated as physiologically relevant lectin ligands. The current minireview focuses on findings that implicate glycosphingolipids as especially important molecules in cell-cell recognition in two different systems: the recognition of human leukocytes by E-selectin on the vascular endothelium during inflammation and the recognition of nerve cell axons by myelin-associated glycoprotein in myelin-axon stabilization and the regulation of axon regeneration.

Novel biological function of sialic acid (N -acetylneuraminic acid) as a hydrogen peroxide scavenger

We have found that N-acetylneuraminic acid (NANA) consumes toxic hydrogen peroxide (H(2)O(2)) under physiological conditions. Close investigation of this finding revealed that NANA was oxidized by an equimolar amount of H(2)O(2) to provide its decarboxylated product, 4-(acetylamino)-2,4-dideoxy-D-glycero-D-galacto-octonic acid (ADOA). To date, there have been little data on this reaction, and its physiological significance has not been discussed. Examining the detoxification of H(2)O(2) in cultured cells with NANA, we were able to confirm that the cell death caused by H(2)O(2) was suppressed by NANA in a dose-dependent manner. These results revealed a novel role for NANA as a reactive oxygen scavenger. It is known that terminal NANA residues are removed by neuraminidase and that free NANA molecules are recycled or degraded by enzymes. We propose that released monomeric NANA is the potent defense molecule against oxidative damage.

Expression of gangliosides in an immortalized neural progenitor/stem cell line

Glycosphingolipids (GSLs) are known to play important roles in cellular growth and differentiation in the nervous system. The change in expression of gangliosides is correlated with crucial developmental events and is evolutionarily conserved among many vertebrate species. The emergence of neural progenitors represents a crucial step in neural development, but little is known about the exact composition and subcellular localization of gangliosides in neural progenitor cells. The C17.2 cell line was derived after v-myc transformation of neural progenitor cells isolated from neonatal mouse cerebellar cortex. The developmental potential of C17.2 cells is similar to that of endogenous neural progenitor/stem cells in that they are multipotential and capable of differentiating into all neural cell types. We characterized the GSL composition of C17.2 cells and found the presence of only a-series gangliosides. Subcellular localization studies revealed that GM1 and GD1a are localized mainly on the plasma membrane and partly in the cytoplasm, both as punctate clusters. Reverse transcription-polymerase chain reaction revealed the absence of ST-II transcripts in C17 cells, which most likely accounts for the lack of expression of b- and c-series complex gangliosides in this cell line. These data suggest that the divergence in ganglioside expression in C17.2 cells is regulated at the transcriptional level.

A genetic approach to Mammalian glycan function

The four essential building blocks of cells are proteins, nucleic acids, lipids, and glycans. Also referred to as carbohydrates, glycans are composed of saccharides that are typically linked to lipids and proteins in the secretory pathway. Glycans are highly abundant and diverse biopolymers, yet their functions have remained relatively obscure. This is changing with the advent of genetic reagents and techniques that in the past decade have uncovered many essential roles of specific glycan linkages in living organisms. Glycans appear to modulate biological processes in the development and function of multiple physiologic systems, in part by regulating protein-protein and cell-cell interactions. Moreover, dysregulation of glycan synthesis represents the etiology for a growing number of human genetic diseases. The study of glycans, known as glycobiology, has entered an era of renaissance that coincides with the acquisition of complete genome sequences for multiple organisms and an increased focus upon how posttranslational modifications to protein contribute to the complexity of events mediating normal and disease physiology. Glycan production and modification comprise an estimated 1% of genes in the mammalian genome. Many of these genes encode enzymes termed glycosyltransferases and glycosidases that reside in the Golgi apparatus where they play the major role in constructing the glycan repertoire that is found at the cell surface and among extracellular compartments. We present a review of the recently established functions of glycan structures in the context of mammalian genetic studies focused upon the mouse and human species. Nothing tends so much to the advancement of knowledge as the application of a new instrument. The native intellectual powers of men in different times are not so much the causes of the different success of their labours, as the peculiar nature of the means and artificial resources in their possession. T. Hager: Force of Nature (1)

Ganglioside/calmodulin kinase II signal inducing cdc42-mediated neuronal actin reorganization

Cell surface glycoconjugates are thought to mediate cell-cell recognition and play roles in neuronal development and functions. We demonstrated here that exposure of neuronal cells to nanomolar levels of gangliosides Neu5Acalpha 8Neu5Acalpha 3Galbeta 4GlcCer, Galbeta 3GalNAcbeta 4(Neu5Acalpha 8Neu5Acalpha 3)Galbeta 4GlcCer (GD1b), Neu5Acalpha 3Galbeta 3GalNAcbeta 4(Neu5Acalpha 8Neu5Acalpha 3)Galbeta 4GlcCer (GT1b) or its oligosaccharide portion induced a rapid and transient activation of Ca2+/calmodulin-dependent protein kinase II (CaM-KII) in the subplasmalemma. Galbeta 3GalNAcbeta 4(Neu5Acalpha 3)Galbeta 4GlcCer (GM1), GalNAcbeta 4(Neu5Acalpha 3)Galbeta 4GlcCer, Neu5Acalpha 3Galbeta 4GlcCer, Neu5Acalpha 3Galbeta 3GalNAcbeta 4(Neu5Acalpha 3)Galbeta 4GlcCer (GD1a), and Neu5Acalpha 8Neu5Acalpha 3Galbeta 3GalNAcbeta 4(Neu5Acalpha 8Neu5Acalpha 3)-Galbeta 4GlcCer were ineffective. GT1b and GD1b stimulated transient elevation of bulk cytosolic Ca2+ levels while GM1 slightly elevated the levels and GD1a did not. Thus, the cytosolic Ca2+ elevation by the gangliosides may trigger the CaM-KII activation. The treatment was accompanied by peripheral actin polymerization and filopodia formation in NG108-15 cells and primary hippocampal neurons, but not in glial cells. CaM-KII inhibitors blocked both CaM-KII activation and the subsequent filopodia formation. A small G-protein cdc42 was a potential downstream target of CaM-KII activated by the gangliosides. These results suggest that oligosaccharides of the gangliosides serve as potential regulators of the filopodia formation in neuronal cells by triggering the activation of CaM-KII followed by cdc42 up-regulation via a cell surface receptor-like component. The filopodia formation induced by the gangliosides may have a physiological relevance because long-term exposure of hippocampal neurons to GT1b oligosaccharide induced advanced dendritogenesis. Furthermore, exposure of cerebellar neurons to GT1b oligosaccharide facilitated CaM-KII-dependent dendritic outgrowth and branch formation of cerebellar Purkinje neurons, in which actin isoforms were localized to motile structures in dendrites. Thus, the ganglioside/CaM-KII signal plays a role in modulating dendritic morphogenesis by inducing cdc42-mediated actin reorganization.

Functional role of a specific ganglioside in neuronal migration and neurite outgrowth

Cell migration occurs extensively during mammalian brain development and persists in a few regions in the adult brain. Defective migratory behavior of neurons is thought to be the underlying cause of several congenital disorders. Knowledge of the dynamics and molecular mechanisms of neuronal movement could expand our understanding of the normal development of the nervous system as well as help decipher the pathogenesis of neurological developmental disorders. In our studies we have identified and characterized a specific ganglioside (9-O-acetyl GD3) localized to the membrane of neurons and glial cells that is expressed in regions of cell migration and neurite outgrowth in the developing and adult rat nervous system. In the present article we review our findings that demonstrate the functional role of this molecule in neuronal motility.

Biosynthesis and degradation of mammalian glycosphingolipids

Glycolipids are a large and heterogeneous family of sphingolipids that form complex patterns on eukaryotic cell surfaces. This molecular diversity is generated by only a few enzymes and is a paradigm of naturally occurring combinatorial synthesis. We report on the biosynthetic principles leading to this large molecular diversity and focus on sialic acid-containing glycolipids of the ganglio-series. These glycolipids are particularly concentrated in the plasma membrane of neuronal cells. Their de novo synthesis starts with the formation of the membrane anchor, ceramide, at the endoplasmic reticulum (ER) and is continued by glycosyltransferases of the Golgi complex. Recent findings from genetically engineered mice are discussed. The constitutive degradation of glycosphingolipids (GSLs) occurs in the acidic compartments, the endosomes and the lysosomes. Here, water-soluble glycosidases sequentially cleave off the terminal carbohydrate residues from glycolipids. For glycolipid substrates with short oligosaccharide chains, the additional presence of membrane-active sphingolipid activator proteins (SAPs) is required. A considerable part of our current knowledge about glycolipid degradation is derived from a class of human diseases, the sphingolipidoses, which are caused by inherited defects within this pathway. A new post-translational modification is the attachment of glycolipids to proteins of the human skin.

Glycosphingolipid functions: insights from engineered mouse models

Glycosphingolipids are remarkable for their structural complexity and their distinctive patterns that mark different tissues and stages of development. The physiological functions of glycosphingolipids are likely to be profound and are only beginning to emerge. Much of this new information is due to the study of mutant mice lacking specific sets of glycosphingolipids, the topic of this review.

Morphological study of disordered myelination and the degeneration of nerve fibers in the spinal cord of mice lacking complex gangliosides

Gangliosides, a family of glycosphingolipids that contain sialic acid, are abundant on the neuronal cell membranes, but their precise functions in the central nervous system remain largely undefined. In a previous study of GalNAc-T(-/-) mice engineered to lack beta1,4-N-acetylgalactos-aminyltransferase (GM2/GD2 synthase) to abolish any, complex gangliosides, we observed the reduction of nerve conduction velocity but did not find any obvious morphological change in the brain. In the present study, we observed morphological changes in the nerve fiber tracts of the spinal cord in these mice. In GalNAc-T(-/-) mice, the number of degenerated axons was markedly increased in the dorsal funiculus, tract of Lissauer, and dorsolateral funiculus of the cervical segment of the spinal cord as well as the dorsal funiculus and tract of Lissauer of the lumbar segment of the spinal cord. There were also increased numbers of unmyelinated fibers in GalNAc-T(-/-) mice. Loosened myelin sheaths and myelin sheaths separated from axons by wide spaces were also observed in GalNAc-T(-/-) mice. These results provide a morphological basis for the previously observed reduction in the nerve conduction velocity and suggest that complex gangliosides are essential for the maintenance of myelin and the integrity of nerve fibers of the spinal cord.

A synthetic ceramide analog ameliorates spatial cognition deficit and stimulates biosynthesis of brain gangliosides in rats with cerebral ischemia

A synthetic ceramide analog, L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (L-PDMP) upregulates ganglioside biosynthesis in several cell lines. In cultured cortical neurons, neurotrophic effects of L-PDMP on neurite outgrowth and synaptic activity were demonstrated. In addition, it was found that L-PDMP could ameliorate the spatial cognition deficit in rats with ischemia. To elucidate this effect, we evaluated the effect of L-PDMP on brain ganglioside biosynthesis and its therapeutic efficacy against spatial cognition deficit in rats made ischemic. Rats were trained for 2 weeks, using an 8-arm radial maze task, and then forebrain ischemia was induced. L-PDMP was injected i.p. at 40 mg/kg twice a day starting from day 1 or 3 after ischemia induction for 6 or 4 days, respectively. The first study showed significantly reduced spatial cognition deficit at 12 h after the final drug administration, and L-PDMP tended to attenuate apoptosis in hippocampal CA1. To examine the effect of L-PDMP on brain ganglioside biosynthesis, N-[3H]acetyl-D-mannosamine was infused into the lateral ventricle via an injection cannula at 12 h after the final drug administration. After 4 h, the brain gangliosides were purified and analyzed. Upregulation of ganglioside biosynthesis by L-PDMP was observed on days 3 and 5 after ischemia. These results are an indication that L-PDMP may ameliorate spatial cognition deficit by upregulating ganglioside biosynthesis in ischemic brain.

Drosophila egghead encodes a beta 1,4-mannosyltransferase predicted to form the immediate precursor glycosphingolipid substrate for brainiac

The neurogenic Drosophila genes brainiac and egghead are essential for epithelial development in the embryo and in oogenesis. Analysis of egghead and brainiac mutants has led to the suggestion that the two genes function in a common signaling pathway. Recently, brainiac was shown to encode a UDP-N-acetylglucosamine:beta Man beta 1,3-N-acetylglucosaminyltransferase (beta 3GlcNAc-transferase) tentatively assigned a key role in biosynthesis of arthroseries glycosphingolipids and forming the trihexosylceramide, GlcNAc beta 1-3Man beta 1-4Glc beta 1-1Cer. In the present study we demonstrate that egghead encodes a Golgi-located GDP-mannose:beta Glc beta 1,4-mannosyltransferase tentatively assigned a biosynthetic role to form the precursor arthroseries glycosphingolipid substrate for Brainiac, Man beta 1-4Glc beta 1-1Cer. Egghead is unique among eukaryotic glycosyltransferase genes in that homologous genes are limited to invertebrates, which correlates with the exclusive existence of arthroseries glycolipids in invertebrates. We propose that brainiac and egghead function in a common biosynthetic pathway and that inactivating mutations in either lead to sufficiently early termination of glycolipid biosynthesis to inactivate essential functions mediated by glycosphingolipids.

Beta1,4-N-acetylgalactosaminyltransferase--GM2/GD2 synthase: a key enzyme to control the synthesis of brain-enriched complex gangliosides

Beta1,4-N-acetylgalactosaminyltransferase (GM2/GD2 synthase) is a key enzyme which catalyzes the conversion of GM3, GD3 and lactosylceramide (LacCer) to GM2, GD2 and asialo-GM2 (GA2), respectively. This step is critical for the synthesis of all complex gangliosides enriched in the nervous system of vertebrates. Following the cloning of cDNAs encoding GM2/GD2 synthase by an expression cloning approach, substantial evidence for the roles of complex gangliosides have been obtained. Above all, knock-out mice lacking all complex gangliosides revealed important roles of complex gangliosides in vivo, i.e., in the maintenance and repair of nervous tissues, in the intact differentiation of spermatocytes via the transport of testosterone, and in the regulation of interleukin-2 receptor complex. Molecular mechanisms for these functions of complex gangliosides in vivo remain to be clarified.

Differential effects of three inhibitors of glycosphingolipid biosynthesis on neuronal differentiation of embryonal carcinoma stem cells

Gangliosides have been implicated in having important roles in neural development. It has been shown that disruption of ganglioside biosynthesis inhibits neurite outgrowth. However, many contradictory results have been reported. The inconsistency of these reports may result from the differential use of neuronal cell lines and inhibitors for ganglioside biosynthesis. In order to clarify the inconsistency in these studies, we utilized an in vitro neuronal differentiation model using an embryonic caricinoma (EC) stem cell line to elucidate the relationship between ganglioside expression and neural development. These cells were exposed to three different inhibitors of glucosylceramide synthase, the first enzyme committed for the biosynthesis of most of the brain gangliosides. All three inhibitors, D-threo-1-phenyl-2-decanoylamino-3-morphlino-1-propanol (D-PDMP), D-threo-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (D-PPPP), and N-butydeoxynojirimycin (NB-DNJ) can inhibit greater than 90% of ganglioside biosynthesis at certain concentrations, respectively. D-PDMP significantly slowed down cellular proliferation in undifferentiated P19 EC cells, inhibited neurite outgrowth, and eventually caused cell death in differentiated cells. However, no retardation in cell growth, neuronal differentiation, and neurite outgrowth was observed in cultures treated with D-PPPP or NB-DNJ despite the depletion of gangliosides. These results indicate that the effect of D-PDMP on cellular proliferation, neurite outgrowth, and survival of differentiated cells is independent of the inhibition of ganglioside biosynthesis.

Complex gangliosides at the neuromuscular junction are membrane receptors for autoantibodies and botulinum neurotoxin but redundant for normal synaptic function

One specialization of vertebrate presynaptic neuronal membranes is their multifold enrichment in complex gangliosides, suggesting that these sialoglycolipids may play a major functional role in synaptic transmission. We tested this hypothesis directly by studying neuromuscular synapses of mice lacking complex gangliosides attributable to deletion of the gene coding for beta1,4 GalNAc-transferase (GM2/GD2 synthase), which catalyzes an early step in ganglioside synthesis. Our studies show that complex gangliosides are surprisingly redundant for regulated neurotransmitter release under normal physiological conditions. In contrast, we show that they are membrane receptors for both the paralytic botulinum neurotoxin type-A and human neuropathy-associated anti-ganglioside autoantibodies that arise through molecular mimicry with microbial structures. These data prove the critical importance of complex gangliosides in mediating pathophysiological events at the neuromuscular synapse.

Tolerance to self gangliosides is the major factor restricting the antibody response to lipopolysaccharide core oligosaccharides in Campylobacter jejuni strains associated with Guillain-Barré syndrome

Guillain-Barré syndrome following Campylobacter jejuni infection is frequently associated with anti-ganglioside autoantibodies mediated by molecular mimicry with ganglioside-like oligosaccharides on bacterial lipopolysaccharide (LPS). The regulation of antibody responses to these T-cell-independent antigens is poorly understood, and only a minority of Campylobacter-infected individuals develop anti-ganglioside antibodies. This study investigates the response to gangliosides and LPS in strains of mice by using a range of immunization strategies. In normal mice following intraperitoneal immunization, antibody responses to gangliosides and LPS are low level but can be enhanced by the antigen format or coadministration of protein to recruit T-cell help. Class switching from the predominant immunoglobulin M (IgM) response to IgG3 occurs at low levels, suggesting B1-cell involvement. Systemic immunization results in poor responses. In GalNAc transferase knockout mice that lack all complex gangliosides and instead express high levels of GM3 and GD3, generation of anti-ganglioside antibodies upon immunization with either complex gangliosides or ganglioside-mimicking LPS is greatly enhanced and exhibits class switching to T-cell-dependent IgG isotypes and immunological memory, indicating that tolerance to self gangliosides is a major regulatory factor. Responses to GD3 are suppressed in knockout mice compared with wild-type mice, in which responses to GD3 are induced specifically by GD3 and as a result of polyclonal B-cell activation by LPS. The anti-ganglioside response generated in response to LPS is also dependent on the epitope density of the ganglioside mimicked and can be further manipulated by providing secondary signals via lipid A and CD40 ligation.

Refractory skin injury in complex knock-out mice expressing only the GM3 ganglioside

We generated double knock-out mice lacking the GM2/GD2 and the GD3 synthase gene by mating single gene mutants, and we analyzed the abnormal phenotypes of the mutant mice expressing only the GM3 ganglioside. We observed a refractory skin lesion that appeared primarily on the face of the mutant mice at 25 weeks after birth or later. Frequent scratching of the wound sites was observed in mutant mice with the skin injury, suggesting that it is a triggering factor that exacerbates the injury. This was confirmed by isolating mice in special cages for metabolic study in which the skin injury developed more rapidly. Characteristic proliferation of nerve fibers was found in the epidermis and subepidermis at the injured sites of the mutants, probably a result of continuous skin injury. Peripheral nerve degeneration was observed in young mutant mice, suggesting that reduced sensory function induced over-scratching and the resulting skin lesion. The fact that sensory response to mechanical stimuli decreased while that to hot stimuli increased in the mutant mice supports this interpretation. Thus, only GM3-expressing mice displayed the important role of gangliosides in maintaining skin integrity via regulation of the peripheral nerves.

The role of sphingolipids in neuronal development: lessons from models of sphingolipid storage diseases

The study of sphingolipids has undergone a renaissance over the past decade due to the realization that these lipids are involved in a variety a biological processes, such as differentiation, apoptosis, cell growth, and cell migration. In the nervous system, sphingolipids, particularly gangliosides, have attracted particular attention as they occur at high levels and their levels change in a developmentally regulated program. Despite the fact that a large body of data has accumulated on the expression and metabolism of individual gangliosides within specific brain regions, the role of individual gangliosides in neuronal development is still poorly understood, and their specific functions are only now beginning to be elucidated. In the present article, we discuss various aspects of our current knowledge concerning the involvement of sphingolipids and gangliosides in neuronal development, and then discuss some recent findings that shed light on the role of sphingolipids and gangliosides obtained with animal models of sphingolipid and other lysosomal storage diseases.

Ganglioside function in calcium homeostasis and signaling

Ganglioside function in eukaryotic cells encompasses a variety of modulatory interactions related to both development and mature cellular behavior. In relation to the nervous system this includes induction of neurite outgrowth and trophic/neuroprotective phenomena; more generally this applies to ganglioside effects on receptor function, adhesion reactions, and signal transduction mechanisms in neural and extraneural systems. Underlying many of these trophic effects are ganglioside-induced changes in cellular calcium, accomplished through modulation of Ca2+ influx channels, Ca2+ exchange proteins, and various Ca2+-dependent enzymes that are altered through association with gangliosides. A clear distinction needs to be drawn between intrinsic functions of gangliosides as naturally expressed by the cell and activities created by application of exogenous ganglioside(s) that may or may not reflect natural function. This review attempts to summarize findings in this area and point to possible future directions of research.

Glycosyltransferases in different brain regions during chick embryo development

Glycosphingolipids, in particular gangliosides, play a crucial role in neuronal development and are known to change dramatically in total content and distribution in different brain areas during embryogenesis. In the present work we analyzed the activity of enzymes involved in the metabolism of gangliosides, at different periods of functional maturation in different regions of chick embryo brain. Our data demonstrate differences in the enzymatic activities in the examined areas; these differences might be correlated with the functional lateralization occurring in the brain during development. Significative differences were found in glycosphingolipid composition between controlateral cerebral hemispheres and optic lobes; these results together with previous data we found, contribute to reinforce our hypothesis on the occurrence of biochemical lateralization during early brain development.

Kidney sulfatides in mouse models of inherited glycosphingolipid disorders: determination by nano-electrospray ionization tandem mass spectrometry

Sulfatides show structural, and possibly physiological similarities to gangliosides. Kidney dysfunction might be correlated with changes in sulfatides, the major acidic glycosphingolipids in this organ. To elucidate their in vivo metabolic pathway these compounds were analyzed in mice afflicted with inherited glycosphingolipid disorders. The mice under study lacked the genes encoding either beta-hexosaminidase alpha-subunit (Hexa-/-), the beta-hexosaminidase beta-subunit (Hexb-/-), both beta-hexosaminidase alpha and beta-subunits (Hexa-/- and Hexb-/-), GD3 synthase (GD3S-/-), GD3 synthase and GalNAc transferase (GD3S-/- and GalNAcT-/-), GM2 activator protein (Gm2a-/-), or arylsulfatase A (ASA-/-). Quantification of the sulfatides, I(3)SO(3)(-)-GalCer (SM4s), II(3)SO(3)(-)-LacCer (SM3), II(3)SO(3)(-)-Gg(3)Cer (SM2a), and IV(3,) II(3)-(SO(3)(-))(2)-Gg(4)Cer (SB1a), was performed by nano-electrospray tandem mass spectrometry. We conclude for the in vivo situation in mouse kidneys that: 1) a single enzyme (GalNAc transferase) is responsible for the synthesis of SM2a and GM2 from SM3 and GM3, respectively. 2) In analogy to GD1a, SB1a is degraded via SM2a. 3) SM2a is hydrolyzed to SM3 by beta-hexosaminidase S (Hex S) and Hex A, but not Hex B. Both enzymes are supported by GM2-activator protein. 4) Arylsulfatase A is required to degrade SB1a. It is probably the sole sphingolipid-sulfatase cleaving the galactosyl-3-sulfate bond. In addition, a human Tay-Sachs patient's liver was investigated, which showed accumulation of SM2a along with GM2 storage. The different ceramide compositions of both compounds indicated they were probably derived from different cell types. These data demonstrate that in vivo the sulfatides of the ganglio-series follow the same metabolic pathways as the gangliosides with the replacement of sulfotransferases and sulfatases by sialyltransferases and sialidases. Furthermore, a novel neutral GSL, IV(6)GlcNAcbeta-Gb(4)Cer, was found to accumulate only in Hexa-/- and Hexb-/- mouse kidneys. From this we conclude that Hex S also efficiently cleaves terminal beta1-6-linked HexNAc residues from neutral GSLs in vivo.

Ganglioside alterations in the central nervous system of rats chronically injected with methylmalonic and propionic acids

Neurological dysfunction and structural cerebral abnormalities are commonly found in patients with methylmalonic and propionic acidemia. However, the mechanisms underlying the neuropathology of these disorders are poorly understood. We have previously demonstrated that methylmalonic and propionic acids induce a significant reduction of ganglioside N-acetylneuraminic acid in the brain of rats subjected to chronic administration of these metabolites. In the present study, we investigated the in vivo effects of chronic administration of methylmalonic (MMA) and propionic (PA) acids (from the 6th to the 28th day of life) on the distribution and composition of gangliosides in the cerebellum and cerebral cortex of rats. Control rats were treated with the same volumes of saline. It was first verified that MMA and PA treatment did not modify body, cerebellum, or cortical weight, nor the ganglioside concentration in the cerebral cortex of the animals. In contrast, a significant reduction in total ganglioside content in the cerebellum of approximately 20-30% and 50% of control levels occurred in rats injected with MMA and PA, respectively. Moreover, chronic MMA and PA administration did not interfere with the ganglioside pattern in the cerebral cortex, whereas the distribution of individual gangliosides was altered in the cerebellum of MMA- and PA-treated animals. Rats injected with MMA demonstrated a marked decrease in GM1 and GD3, whereas chronic PA treatment provoked a significant reduction of all ganglioside species, with the exception of an increase in GM2. Since gangliosides are closely related to the dendritic surface and other neural membranes, indirectly reflecting synaptogenesis, these ganglioside abnormalities may be associated with the brain damage found in methylmalonic and propionic acidemias.

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.

b-series Ganglioside deficiency exhibits no definite changes in the neurogenesis and the sensitivity to Fas-mediated apoptosis but impairs regeneration of the lesioned hypoglossal nerve

The polymorphic carbohydrate structures of gangliosides play regulatory roles. In particular, b-series gangliosides, all of which contain alpha-2,8 sialic acids, have been considered to be critical in various biological events such as adhesion, toxin binding, neurite extension, cell growth, and apoptosis. To clarify the physiological functions of b-series gangliosides in vivo, we have established a gene knockout mouse of GD3 synthase. Although all b-series structures were deleted in the mutant mice, they showed an almost complete nervous tissue morphology with no apparent abnormal behavior. Moreover, no differences in Fas-mediated apoptotic reaction of lymphocytes between wild type and the mutant mice were detected. However, the mutant mice exhibited clearly reduced regeneration of axotomized hypoglossal nerves compared with the wild type, suggesting that b-series gangliosides are more important in the repair rather than in the differentiation of the nervous system and apoptotic process induced via Fas.

Lactosylceramide is essential for the osteoclastogenesis mediated by macrophage-colony-stimulating factor and receptor activator of nuclear factor-kappa B ligand

Glycosphingolipids and their metabolites play important roles in a variety of biological processes. Several signal molecules are localized in a glycolipid-enriched microdomain on the cell surface, and their signals are regulated by the glycolipid composition. However, the function of glycolipids in osteoclastogenesis has not been clearly understood. We found that D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), a glucosylceramide synthase inhibitor, completely inhibits the osteoclast formation induced by macrophage-colony-stimulating factor and receptor activator of nuclear factor-kappa B ligand (RANKL) in a dose-dependent manner. Expression of RANK, the receptor of RANKL, induced by macrophage colony-stimulating factor, was reduced markedly in D-PDMP-treated cells. d-PDMP also inhibited the phosphorylation of the inhibitor of nuclear factor-kappa B and extracellular signal-regulated kinase 1/2 induced by RANKL. In several experiments with the addition of glycolipids to D-PDMP-treated purified bone marrow cells, lactosylceramide (LacCer) strongly affected the differentiation into tartrate-resistant acid phosphatase mononucleated cells, but not positive multinucleated cells. GM3 and GM1 also recovered, but less effectively compared with LacCer. Moreover, exogenous LacCer recovered the reduced expression of RANK and the phosphorylation of inhibitor of NF-kappa B and extracellular signal-regulated kinase 1/2 after stimulation by RANKL at the same level of cells without D-PDMP treatment. Our data suggest that glycosphingolipids, especially LacCer, are necessary for the initiation step of RANKL-induced osteoclastogenesis.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

Brain gangliosides: functional ligands for myelin stability and the control of nerve regeneration

Gangliosides, sialylated glycosphingolipids which are the predominant glycans on vertebrate nerve cell surfaces, are emerging as components of membrane rafts, where they can mediate important physiological functions. Myelin associated glycoprotein (MAG), a minor constituent of myelin, is a sialic acid binding lectin with two established physiological functions: it is involved in myelin-axon stability and cytoarchitecture, and controls nerve regeneration. MAG is found selectively on the myelin membranes directly apposed to the axon surface, where it has been proposed to mediate myelin-axon interactions. Although the nerve cell surface ligands for MAG remain to be established, evidence supports a functional role for sialylated glycoconjugates. Here we review recent studies that reflect on the role of gangliosides, sialylated glycosphingolipids, as functional MAG ligands. MAG binds to gangliosides with the terminal sequence 'NeuAc alpha 3Gal beta 3GalNAc' which is found on the major nerve gangliosides GD1a and GT1b. Gangliosides lacking that terminus (e.g., GM1 or GD1b), or having any biochemical modification of the terminal NeuAc residue fail to support MAG binding. Genetically engineered mice lacking the GalNAc transferase required for biosynthesis of the 'NeuAc alpha 3Gal beta 3GalNAc' terminus have grossly impaired myelination and progressive neurodegeneration. Notably the MAG level in these animals is dysregulated. Furthermore, removal of NeuAc residues from nerve cells reverses MAG-mediated inhibition of neuritogenesis, and neurons from mice lacking the 'NeuAc alpha 3 Gal beta 3GalNAc' terminus have an attenuated response to MAG. Cross-linking nerve cell surface gangliosides can mimic MAG-mediated inhibition of nerve regeneration. Taken together these observations implicate gangliosides as functional MAG ligands.

Engagement of endogenous ganglioside GM1a induces tyrosine phosphorylation involved in neuron-like differentiation of PC12 cells

Using the cholera toxin B subunit (CTB) that specifically binds to ganglioside GM1a on the plasma membrane, we investigated intracellular signaling mediated by endogenous GM1a involved in neuronal differentiation of PC12 cells. The treatment with CTB induced morphological alternations of PC12 cells, such as augmentation of the cell body, neurite extension, and branched spikes of tips of neurites. The neurite extension induced with CTB was strongly suppressed by the pretreatment of tyrosine kinase inhibitors in a dose-dependent manner. Western blotting analysis showed that CTB induced tyrosine phosphorylation of several cellular proteins with molecular masses around 120, 70, and 45-40 kDa in PC12 cells. Some of the proteins identified were extracellular-signal regulated kinase (ERKs) (ERK1 and ERK2). The peak activation of ERKs lasted for 60-90 min and gradually decreased thereafter. Immunoprecipitation analysis demonstrated that the intracellular events induced with CTB are not related with the activation of Trk proteins, suggesting that signals evoked by ligation of endogenous GM1a are unique and distinct from those induced with exogenous GM1a. Although the presence of a tyrosine kinase inhibitor, genistein, at a concentration of 10 microM diminished the neurite extension of PC12 cells induced with CTB, ERK activation was still observed. However, pretreatment with a MEK inhibitor, PD98059, abolished the activation of ERKs induced with CTB in a dose-dependent manner and only attenuated the morphological alternations of PC12 cells. Considered together, we concluded that tyrosine phosphorylation induced with CTB was responsible for neuron-like differentiation of PC12 cells and that the MEK-ERK cascade is part of the biological signals mediated by endogenous ganglioside GM1a on PC12 cells.

Mice expressing only monosialoganglioside GM3 exhibit lethal audiogenic seizures

Gangliosides are a family of glycosphingolipids that contain sialic acid. Although they are abundant on neuronal cell membranes, their precise functions and importance in the central nervous system (CNS) remain largely undefined. We have disrupted the gene encoding GD3 synthase (GD3S), a sialyltransferase expressed in the CNS that is responsible for the synthesis of b-series gangliosides. GD3S-/- mice, even with an absence of b-series gangliosides, appear to undergo normal development and have a normal life span. To further restrict the expression of gangliosides, the GD3S mutant mice were crossbred with mice carrying a disrupted GalNAcT gene encoding beta1,4-N-acetylgalactosaminyltransferase. These double mutant mice expressed GM3 as their major ganglioside. In contrast to the single mutant mice, the double mutants displayed a sudden death phenotype and were extremely susceptible to induction of lethal seizures by sound stimulus. These results demonstrate unequivocally that gangliosides play an essential role in the proper functioning of the CNS.

Substrate reduction therapy for glycosphingolipid storage disorders

Substrate reduction therapy is a novel approach to treating glycosphingolipid (GSL) lysosomal storage disorders. These diseases are caused by mutations in the genes coding for enzymes involved in GSL catabolism and are characterised by the accumulation of GSL substrates within the lysosomes of cells. The aim of substrate reduction therapy is to inhibit the rate of synthesis of GSLs to levels where the residual activity of the mutant catabolic enzyme is sufficient to prevent pathological storage. In this review we discuss the development of N-butyldeoxynojirimycin (NB-DNJ), an imino sugar that inhibits the ceramide-specific glucosyltransferase which catalyses the first committed step of GSL synthesis. This agent has been shown to slow accumulation of stored glycolipid in an in vitro model of Gaucher's disease and in knockout mouse models of Tay-Sachs and Sandhoff diseases. Furthermore, administration of NB-DNJ to Sandhoff mice delays the onset of neurological disease and also slows its progression. We discuss safety and efficacy data from the clinical trial of substrate reduction with NB-DNJ which has been undertaken in patients with Type 1 Gaucher's disease. This trial provides a proof-of-principle for the use of this approach in a wide range of GSL lysosomal storage diseases.

Novel functions of complex carbohydrates elucidated by the mutant mice of glycosyltransferase genes

Complex carbohydrates consist of carbohydrate moieties and protein or lipid portions, resulting in the formation of glycoproteins, proteoglycans or glycosphingolipids. The polymorphic carbohydrate structures are believed to contain profound biological implications which are important in cell-cell or cell-extracellular matrix interactions. A number of studies to delineate the roles of carbohydrates have been performed, and demonstrated definite changes in their profiles, cellular phenotypic changes or, sometimes, morphological and functional changes in tissues after modification of their structures. Recent successes in the isolation of glycosyltransferase genes and their modification enzyme genes has enabled clearer demonstrations of the roles of complex carbohydrates. In particular, genetic modification of glycosyltransferase genes in mice can elucidate the biological significances of their products in vivo. Here, we summarize recent advances in the understanding of the roles of complex carbohydrates provided from studies of gene knock-out mice of glycosyltransferase and modification enzyme genes focusing on novel functions which had not been expected.

Specificity of carbohydrate structures of gangliosides in the activity to regenerate the rat axotomized hypoglossal nerve

We previously reported that a ganglioside mixture from bovine brain could prevent neuronal death and promote regeneration in rats with hypoglossal nerve resection. In the present study, we have compared the neurotrophic effects of various glycosphingolipids including lactosyl-ceramide. The findings revealed that GT1b had the activity of neuronal death prevention equivalent to a ganglioside mixture or autograft, while other glycolipids exhibited about 60% activity. However, the capability to promote the regeneration varied among glycolipids, that is, GT1b (86%), GD1b (55%), GD1a (35%), GQ1b (34%), GM1 (20%), lactosyl-ceramide (17%) in the number of horseradish peroxidase-positive neurons as an indicator of regeneration. The experiments with oligosaccharides of GT1b or GD1b and ceramide showed that the carbohydrate moiety mainly exerts neurotrophic effects. These findings suggested that fine structures of carbohydrate moiety in gangliosides are critical in the regenerative activity in this hypoglossal nerve regeneration system.