Exogenous GM3 ganglioside stimulates process formation and glycoprotein release by cultured bovine oligodendrocytes

Oligodendroglial glycolipids in (Re)myelination: implications for multiple sclerosis research

Covering: up to 2020 This short review surveys aspects of glycolipid-based natural products and their biological relevance in multiple sclerosis (MS). The role of isolated gangliosides in disease models is discussed together with an overview of ganglioside-inspired small molecule drugs and imaging probes. The discussion is extended to neurodegeneration in a more general context and addresses the need for more efficient synthetic methods to generate (glyco)structures that are of therapeutic relevance.

Selective cell-cycle arrest and induction of apoptosis in proliferating neural cells by ganglioside GM3

Control of cell proliferation and cell survival is critical during development of the vertebrate central nervous system (CNS). Much of the cell death seen during early stages of CNS development occurs through apoptosis; however, the factors that induce this early apoptosis are not clearly understood. Gangliosides, sialylated glycosphingolipids, are expressed in the CNS and have been proposed to regulate cell growth and differentiation. Here we show that the simple ganglioside GM3 selectively inhibits the proliferation of and induces apoptosis of actively dividing astrocyte precursors and other neural progenitors. The inhibition of astrocyte precursor proliferation by GM3 appears to be mediated in part by the cyclin-dependent kinase (Cdk) inhibitor p27(Kip1). During neonatal development there is extensive cell proliferation and little apoptosis in the ventricular and subventricular zones of the CNS. This proliferation was dramatically inhibited and the degree of apoptosis dramatically increased following intraventricular administration of GM3. These data suggest that GM3, a simple ganglioside, may regulate cell proliferation and death in the CNS and as such may have potential for brain tumor therapy.

The role of globo-series glycolipids in neuronal cell differentiation--a review

Alterations in glycolipid composition as well as glycosyltransferase activities during cellular differentiation and growth have been well documented. However, the underlying mechanisms for the regulation of glycolipid expression remain obscure. One of the major obstacles has been the lack of a well defined model system for studying these phenomena. We have chosen PC12 pheochromocytoma cells as a model because (a) the properties of these cells have been well characterized, and (b) they respond to nerve growth factor (NGF) by differentiating into sympathetic-like neurons and are amenable to well-controlled experimentation. Thus, PC12 cells represent a suitable model for studying changes in glycolipid metabolism in relation to cellular differentiation. We have previously shown that subcloned PC12 cells accumulate a unique series of globo-series neutral glycolipids which are not expressed in parental PC12 cells. This unusual change in glycolipid distribution is accompanied by changes in the activities of specific glycosyltransferases involved in their synthesis and is correlated with neuritogenesis and/or cellular differentiation in this cell line. We have further demonstrated that changes in the glycosyltransferase activities may be modulated by the phosphorylation states of the cells via protein kinase systems. We conclude that these unique globo-series glycolipids may play a functional role in the initiation and/or maintenance of neurite outgrowth in PC12 cells.

Regulation of glycolipid synthesis in HL-60 cells by antisense oligodeoxynucleotides to glycosyltransferase sequences: effect on cellular differentiation

Treatment of the human promyelocytic leukemia cell line HL-60 with antisense oligodeoxynucleotides to UDP-N-acetylgalactosamine:beta-1,4-N-acetylgalactosaminyl-transferase (GM2-synthase; EC 2.4.1.92) and CMP-sialic acid:alpha-2,8-sialyltransferase (GD3-synthase; EC 2.4.99.8) sequences effectively down-regulated the synthesis of more complex gangliosides in the ganglioside synthetic pathways after GM3, resulting in a remarkable increase in endogenous GM3 with concomitant decreases in more complex gangliosides. The treated cells underwent monocytic differentiation as judged by morphological changes, adherent ability, and nitroblue tetrazolium staining. These data provide evidence that the increased endogenous ganglioside GM3 may play an important role in regulating cellular differentiation and that the antisense DNA technique proves to be a powerful tool in manipulating glycolipid synthesis in the cell.

Differentiation of oligodendrocytes cultured from developing rat brain is enhanced by exogenous GM3 ganglioside

Cultures consisting primarily of O-2A progenitor cells and immature oligodendrocytes with a few microglia and astrocytes were obtained by shaking primary cultures from neonatal rat brain after 12-14 days in vitro. Addition of 50 micrograms/ml exogenous Neu-NAc alpha 2-3Gal beta 1-4Glc beta 1-1'ceramide (GM3 ganglioside) to the cultures resulted in an increase in the number and thickness of cell processes that stained intensely for sulfatide and galactocerebroside (galC) in comparison to control cultures without added GM3. The treated cultures also contained fewer astrocytes than control cultures as revealed by immunostaining for glial fibrillary acidic protein (GFAP). Cells that immunostained for both GFAP and sulfatide/galC were very rare in control cultures but were frequently seen in the GM3-treated cultures, suggesting that these may represent cells changing their direction of differentiation away from type II astrocytes toward oligodendrocytes under the influence of GM3. These effects on the developing rat oligodendrocytes were specific for GM3 ganglioside and were not produced by adding GM1, GM2, GD3, or GD1a to the cultures. Lactosyl ceramide and neuraminyl lactose were also ineffective. When control cultures were initially plated on polylysine and incubated with [14C]galactose, GD3 was the principal labeled ganglioside. However, as the control cells differentiated over time in culture without the addition of exogenous GM3 and produced increasing amounts of myelin-related components, the incorporation of [14C]galactose into endogenous GM3 increased to become the predominant labeled ganglioside by 6 days after plating. Metabolic labeling of the GM3-treated oligodendrocytes with [14C]galactose revealed increased incorporation into galC and sulfatide in comparison to control cultures, but a decreased labeling of endogenous GM3. Similarly, incorporation of an amino acid precursor into the myelin-associated glycoprotein (MAG) was increased by GM3 treatment, but incorporation into myelin basic protein (MBP) was not affected. Although the overall effect of added GM3 was to decrease the phosphorylation of most proteins in the oligodendrocytes, including MBP, GM3 enhanced the phosphorylation of MAG. These findings indicate that GM3 ganglioside has an important role in the differentiation of cells of the O-2A lineage toward myelin production, since differentiation is associated with increased metabolic labeling of endogenous GM3 in control cultures and is enhanced by the addition of exogenous GM3.

Cyclic AMP causes differentiation and decreases the expression of neutral glycosphingolipids in cell cultures derived from a malignant glioma

Cultures derived from a malignant glioma (U-87 MG) were treated with 3 mM dibutyryl cAMP. The treatment resulted in morphological differentiation of the cultures and a decrease in cell proliferation. Biochemically, dibutyryl cAMP treatment caused a general reduction in the concentration of neutral glycosphingolipids in the U-87 MG cells. The concentration of individual neutral glycosphingolipids in the untreated cells was 1.8- to 3.0-fold higher than in cells treated for 72 h with 3 mM dibutyryl cAMP. Cells were labeled with [3H]galactose to monitor synthesis of the neutral glycosphingolipids. Decreased synthesis was noted in cells treated with dibutyryl cAMP as compared with untreated cells as indicated by decreased uptake of [3H]galactose label. The ganglioside composition of the cells was essentially unchanged after dibutyryl cAMP treatment.

GM3 regulates protein kinase systems in cultured brain microvascular endothelial cells

The barrier function of endothelial cells is known to be positively regulated by protein kinase A (PKA) and negatively regulated by protein kinase C (PKC). We found that exogenously administered GM3(NeuAc) promoted PKA activity in cultured brain microvascular endothelial cells (BMECs). Other glycolipids, including GM1, sulfoglucuronyl paragloboside, and GM3(NeuGc), did not have any effect on the PKA activity of BMECs. PC12 cells did not respond to exogenously applied GM3(NeuAc). GM3(NeuAc) also suppressed the PKC activity of BMECs. Thus, GM3(NeuAc) may function as a modulator of blood-brain barrier function via the two different kinase systems.

Gangliosides stimulate synthesis of prostaglandin E2 and prostacyclin in fetal rat brain hemispheres after episodes of global intrauterine ischemia

The ability of brain preparations from 20-day-old rat fetuses to synthesize prostanoids in vitro before and after interruption of the maternal-fetal blood flow was examined using a radioimmunoassay technique. Synthesis of thromboxane B2 (TxB; the stable thromboxane A2 metabolite) decreased with increasing restriction time; conversely, it was elevated with reperfusion. Synthesis of 6-keto prostaglandin F1 alpha (PGF; the stable prostacyclin metabolite) and prostaglandin E2 (PGE) prostanoids remained unchanged after 20 min restriction and through a 2 hr reperfusion period. Intraperitoneal administration of GM1 (45 mg/kg) into the pregnant rat, 3 hr before restriction, stimulated synthesis of PGE and reduced synthesis of TxB. A prostanoid vasoactive index (PVI), which reflects the relative proportion of the three prostanoids synthesized and asserts the vasoactive potential of the brain tissue, was established. A rise in this value was attained after intrafetal administration into the peritoneal cavity of either GM1, GM3, or isopropyl-GM1 (AGF44) gangliosides, each given at 40 micrograms dose in 5 microliters volume, and N-dichloroacetyl-sphingosine (LIGA20; 15 micrograms/5 microliters) ganglioside analog, 1 hr before restriction. The effect was primarily due to an increase in the capacity of fetal brain tissue to synthesize PGE and, to a lesser extent PGF, vasodilating prostanoids. The N-methyl-D-aspartate (NMDA) receptor-blocker MK801 (6.6 micrograms/2 microliters) and the platelet activating factor (PAF) receptor antagonist BN52021 (0.1 mumol/2 microliters), given by the same route, effectively raised by 60-80% the vasodilating potential of the brain tissue following ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Biosynthesis and expression of the myelin-associated glycoprotein in cultured oligodendrocytes from adult bovine brain

The biosynthesis and expression of myelin-associated glycoprotein (MAG) were investigated in cultured oligodendrocytes isolated from adult bovine brain. Western blotting revealed two prominent MAG bands that were present in comparable amounts; the larger component electrophoresed above the 97 kD standard but was slightly smaller than the MAG band in purified bovine myelin, and the smaller component electrophoresed below the 97 kD standard. In comparison to other precursors of oligosaccharides, inorganic [35S]sulfate was a relatively specific isotope for labeling MAG relative to other glycoproteins in the cells. Sulfate labeled only the larger of the two MAG components, which contains complex N-linked oligosaccharides, but which appears to be glycosylated to a lesser extent than MAG in vivo. The smaller MAG band in the cells is a form with high-mannose oligosaccharides and was not detected in purified bovine myelin. Both the large and small MAG components were expressed on the oligodendrocyte surface as indicated by their sensitivity to neuraminidase and/or trypsin treatment of live cells. MAG was also released by the oligodendrocytes into the culture medium. The MAG in the medium was slightly smaller than that in the cells, suggesting that it may be released from the cell surface by limited proteolysis. The release of MAG by myelin-forming cells could be relevant to physiological roles that have been postulated for soluble forms of MAG and other adhesion proteins.