Bimodal regulation of protein phosphorylation by a ganglioside in rat brain membrane

The down-modulation of CD4 induced by the GM1 ganglioside is regulated by phosphatases and kinases: evidence from enzyme inhibitors and anti-CD45 antibodies

The involvement of protein kinases and phosphatases in the down-modulation of expression of CD4 molecules on peripheral blood lymphocytes (PBL) by gangliosides was studied. Exposure of PBL either to genistein or to H7 practically abolished the down-modulation of CD4 induced by GM1 and diminished their susceptibility to CD4+ down-modulation by exposure to GD1a. Staurosporine had no effect on the down-modulation of CD4 by either GM1 or GD1a. Orthovanadate treatment drastically inhibited the down-regulation of CD4 induced by GM1 but had no effect on down-modulation of CD4 induced by GD1a. Exposure to monoclonal antibodies (mAbs) against CD45 and CD45RA but not against CD45RO abrogated the down-modulation of CD4 by GM1. The down-modulation of CD4 elicited by GD1a, GD1b, or GT1b was not inhibited by anti-CD45RA and anti-CD45RO mAbs. MAbs against CD3, CD2, or HLA-DR had no effect on the GM1-induced down-modulation of CD4. In view of the differences obtained between GM1 and GD1a it was of interest to check whether these gangliosides competed for cellular binding sites. When PBL were first treated with anti-CD45RA and GM1 or with orthovanadate and GM1, which had a negligible effect on CD4 expression, and subsequently treated with GD1a the expression of CD4 was down-modulated. This demonstrated that GD1a binds to sites on the cell membrane to which GM1 does not bind. The present study indicates that the capacity of GM1 to down-modulate CD4 depended on the CD45 and particularly CD45RA molecules, while other gangliosides may utilize different cell surface structures to down-modulate the expression of CD4.

Functional roles of gangliosides in bio-signaling

Brain gangliosides, a sialic acid-containing glycosphingolipid family enriched in brain, are discriminated from those of extra neural tissues by their characteristic structures of carbohydrate chain with large molecular diversity. Numerous minor components and monoclonal antibodies to them are useful to identify type, distribution and lineage of the cells, as shown in the recent finding of the ganglioside epitope of cholinergic neuron-specific Chol-1 antigens. Various cell biological effects of exogenous gangliosides (bioactive gangliosides) particularly with regard to cell growth and differentiation strongly suggest involvement of gangliosides and possibly their metabolic intermediates as second messenger in signaling pathways. The neuritogenic as well as synaptogenic effects of gangliosides may be interpreted by their action on protein kinases. The analysis of the neuritogenic activity of GQ1b ganglioside on human neuroblastoma cell lines strongly indicates the possibility that the action is carried out by coupling of GQ1b sugar-specific glycoreceptor of cell surface membrane and a unique, cell surface localized protein kinase (ecto-protein kinase) to phosphorylate cell surface protein(s) with extracellular ATP. This cell surface (ecto) type of protein phosphorylation system which is in contrast to intracellular (endo) type of protein phosphorylation seems to highly develop in neuron. Possible involvement of gangliosides in synaptic function including ion-transport and long-term potentiation is also suggested.

A novel glycosignaling system: GQ1b-dependent neuritogenesis of human neuroblastoma cell line, GOTO, is closely associated with GQ1b-dependent ecto-type protein phosphorylation

Previously, we reported that ganglioside GQ1b specifically promoted neuritogenesis of human neuroblastoma cells (GOTO), and also that is specifically stimulated the phosphorylation of several cell surface proteins on the same cells. To disclose the relationship between the two events, we examined them using a novel protein kinase inhibitor, K-252b, which is a derivative of K-252a and cannot pass through cell membrane. K-252b inhibited the GQ1b-dependent neuritogenesis as well as the GQ1b-stimulated phosphorylation. This suggests the direct coupling between the two cell events and the occurrence of a new biosignal transduction system.