A unique biosynthetic pathway for gangliosides exists in Xenopus laevis oocytes

β1-subunit modulates the Nav1.4 sodium channel by changing the surface charge

Voltage-gated sodium channels, comprised of a pore-forming alpha-subunit and additional regulatory (beta) subunits, play a critical role in regulation of neuronal excitability. Mechanisms of regulation of beta-subunits remain elusive. We have tested the functional effects of beta1 sodium channel subunit on surface charges as a mechanism for channel modulation. HEK-293 cell lines permanently transfected with the sole rat skeletal muscle sodium channel alpha-subunit (Nav1.4), or co-expressing the sodium channel alpha-subunit and beta1-subunit were studied with the whole-cell mode of the patch-clamp technique. At physiological extracellular Ca2+ concentration (2 mM), expression of beta1-subunit did not produce any significant effect on the voltage-dependent properties of sodium currents. However, a shift of half-activation potentials of sodium channel by changing the extracellular Ca2+ was potentiated when beta1 was co-expressed with alpha-subunit. In contrast, the expression of beta1-subunit did not affect the Ca2+ binding to the open or to the closed sodium channel pore, difference of the effect provoked by extracellular Ca2+ could therefore be attributed to an increased in negative surface charge determined by the presence of beta1-subunit. These data are in agreement with the hypothesis of a modulation of the sodium current by the expression of the highly sialylated beta1-subunit, which would alter the channel gating by increasing the density of surface negative charges in the vicinity of the sodium channel voltage sensing machinery.

[Functional glycoconjugates involved in cellular interaction]

We first examined the involvement of the complex sphingolipids in cell-substratum adhesion using GM-95, a mutant cell line deficient in glycosphingolipids (GSLs) due to the lack of ceramide glucosyltransferase activity. We determined the adhesion of the mutant cells and stable transfectants expressing GSLs, which were established by transfection of GlcT-1 cDNA into GM-95 cells under neutral sphingomyelinase (sm) treatment. We confirmed that complex sphingolipids play critical roles in cell-substratum adhesion, and the presence of either GSLs or SM is sufficient for the adhesion. We also investigated intracellular signaling (glycosignaling) mediated by endogenous GM1a involved in the neuronal differentiation of PC12 cells using the cholera toxin B subunit (CTB) that specifically binds to ganglioside GM1a. Treatment with CTB induced neuron-like differentiation of PC12 cells. Biochemical analyses demonstrated that the tyrosine phosphorylation induced by CTB was responsible for neuron-like differentiation of PC12 cells and that the MEK-ERK cascade is a part of the biological signals mediated by endogenous ganglioside GM1a on PC12 cells. We further demonstrated that glycosignaling is mediated through a high-affinity ligand, PSGL-1, for P-selection on neutrophils. In this case, engagement of PSGL-1 on the cell surface strongly induced tyrosine phosphorylation of several cellular proteins including ERKs and activated a canonical MAP kinase pathway. Tyrosine phosphorylation induced by engagement of PSGL-1 is responsible for the secretion of interleukin-8 from neutrophils, suggesting that PSGL-1-mediated glycosignals are involved in the progression of the inflammatory response. In this review, we mainly discuss the biological and pathological significance of glycoconjugates in relation to the above issues.

Glycosylated and phosphorylated proteins--expression in yeast and oocytes of Xenopus: prospects and challenges--relevance to expression of thermostable proteins

Phosphorylation and glycosylation are important posttranslational events in the biosynthesis of proteins. The different degrees of phosphorylation and glycosylation of proteins have been an intriguing phenomenon. Advances in genetic engineering have made it possible to control the degree of glycosylation and phosphorylation of proteins. Structural biology of phosphorylated and glycosylated proteins has been advancing at a much slower pace due to difficulties in using high-resolution NMR studies in solution phase. Major difficulties have arisen from the inherent mobilities of phosphorylated and glycosylated side chains. This paper reviews molecular and structural biology of phosphorylated and glycosylated proteins expressed in eukaryotic expression systems which are especially suited for large-scale production of these proteins. In our laboratory, we have observed that eukaryotic expression systems are particularly suited for the expression of thermostable light-activated proteins, e.g., bacteriorhodopsins and plastocyanins.

Association of Src family tyrosine kinase Lyn with ganglioside GD3 in rat brain. Possible regulation of Lyn by glycosphingolipid in caveolae-like domains

Association of gangliosides with specific proteins in the central nervous system was examined by co-immunoprecipitation with anti-ganglioside antibody. Protein kinase activity was detected in precipitates with monoclonal antibody to ganglioside GD3 (R24) from membranal fraction of rat brain. Using in vitro kinase assay, several phosphorylated proteins of 40, 53, 56, and 80 kDa were isolated by gel electrophoresis. Of these proteins, the proteins of 53 and 56 kDa (p53/56) were identified as two isoforms of Src family tyrosine kinase Lyn, based on co-migration during gel electrophoresis, comparative peptide mapping, and sequential immunoprecipitation with anti-Lyn antibody. The identification was confirmed using a cDNA expression system in Chinese hamster ovary (CHO) cells, which express solely ganglioside GM3, the enzymatic substrate of GD3 synthase. In co-transfection with GD3 synthase and Lyn expression plasmids, R24 immunoprecipitated Lyn and anti-Lyn antibody immunoprecipitated GD3. R24 treatment of rat primary cerebellar cultures induced Lyn activation and rapid tyrosine phosphorylation of several substrates including mitogen-activated protein kinases. Furthermore, sucrose density gradient analysis showed that Lyn of cerebellum and CHO transfectants were detected in a low density light-scattering band, i.e. the caveolae membrane fraction. R24 immunoprecipitated caveolin from Triton X-100 extract of CHO transfectants. These observations suggest that GD3 may regulate Lyn in a caveolae-like domain on brain cell membranes.

Activities of glycolipid glycosyltransferases and sialidases during the early development of Xenopus laevis

The activities of glycosyltransferases and sialidases, together with the ganglioside content and distribution, have been extensively studied in mammals, while the informations on tissues of other animals, including amphibian, are scarce. In this paper we present data on the activities of SAT-1, SAT-2, SAT-4, SAT-5, GlcNAcT-1, GalNAcT-1, GalT-6, and sialidases studied in Xenopus laevis embryos at different stages of development. The highest activity was found at days 4 and 5 of embryogenesis for glycosyltransferases and sialidases respectively; a tentative correlation between the in vitro activity of these enzymes and the content of neutral and acidic glycolipids is discussed.

Inhibition of glycosphingolipid synthesis induces p34cdc2 activation in Xenopus oocyte

In Xenopus prophase-blocked oocytes, it is assumed that progesterone interacts with the plasma membrane to initiate a signalling cascade that ultimately leads to MPF activation. Progesterone regulates negatively the cAMP pathway through an inhibition of adenylate cyclase. However, the mechanisms linking the initial action of the hormone with adenylate cyclase activity remain to be elucidated. Here, we demonstrate that PDMP, an inhibitor of glucosphingolipid synthesis, triggers oocyte meiotic maturation in a cAMP- and cycloheximide-dependent manner, whereas exogenous ceramide is unefficient. We propose that sphingolipid metabolism and targeting represent an important regulatory process of oocyte meiosis.