Association of the crosslinked IgE receptor with the membrane skeleton is independent of the known signaling mechanisms in rat basophilic leukemia cells

The involvement of the 67 kDa laminin receptor-mediated modulation of cytoskeleton in the degranulation inhibition induced by epigallocatechin-3-O-gallate

Recently, we have reported that (-)-epigallocatechin-3-O-gallate (EGCG) acts as an inhibitor of degranulation. However, the inhibitory mechanism for degranulation is still poorly understood. Here we show that suppression of exocytosis-related myosin II regulatory light chain phosphorylation and alteration of actin remodeling are involved in the inhibitory effect of EGCG on the calcium ionophore-induced degranulation from human basophilic KU812 cells. Surface plasmon resonance assay also revealed that EGCG binds to the cell surface, and the disruption of lipid rafts resulted in reduction of EGCG's ability. We have previously identified the raft-associated 67kDa laminin receptor (67LR) as an EGCG receptor on the cell surface. Treatment of the cells with anti-67LR antibody or RNA interference-mediated downregulation of 67LR expression abolished the effects of EGCG. These findings suggest that EGCG-induced inhibition of the degranulation includes the primary binding of EGCG to the cell surface 67LR and subsequent modulation of cytoskeleton.

Characterization of the detergent insolubility of the T cell receptor for antigen

Aggregation of cell surface receptors plays an important role in signal transduction in many receptor systems. In the T cell receptor (TCR), as in many other cell surface receptors, this aggregation results in insolubility in certain nonionic detergents. We have characterized this insolubility for TCR, and we show it is not preexisting in HPB-ALL cells but increases with increasing TCR aggregation. It is not likely to be due to a direct interaction with cellular cytoskeletal elements, as it is not affected by inhibitors of actin or tubulin polymerization. It may be due to interaction with detergent-resistant membrane domains that have been found in various cell types and contain tyrosine kinases, the earliest known participants in TCR signal transduction. This aggregation-dependent insolubility occurs as rapidly as the anti-TCR antibody binds, so the kinetics are consistent with an involvement in signal transduction. It is not, however, dependent on signal transduction, as inhibitors of tyrosine kinases do not inhibit the insolubility. Insolubility is also enhanced by preaggregation of CD4, an important T cell surface molecule which also associates with the tyrosine kinase p56lck.

Demonstration of membrane-associated phospholipase A2 in cultivated heart muscle cells by immunogold-technique in surface replicas

Recently we produced mAb's to phospholipase A2 (PIA2) from bee venom for different purposes (allergen-identification and -standardization). Obviously one of these monoclonal antibodies "5D5" binds to a conserved (cross reactive) epitope of membrane PIA2 of intact cells. In this study we demonstrate binding of 5D5 to membrane associated phospholipase A2 of rat heart muscle cells by means of two-step immunogold technique in combination with replica technique.

A transient rise in intracellular Ca2+ is a precursor reaction to the zona pellucida-induced acrosome reaction in mouse sperm and is blocked by the induced acrosome reaction inhibitor 3-quinuclidinyl benzilate

The acrosome reaction induced by the zona pellucida in mouse sperm has been shown to proceed in two stages experimentally distinguishable by the fluorescent probe chlortetracycline. Entry into the first stage of sperm bound to isolated, structurally intact zonae pellucidae is blocked by the compound 3-quinuclidinyl benzilate. In this study, we show, utilizing the fluorescent Ca2+ indicator fluo-3, that the first stage of the zona-induced acrosome reaction is characterized by an increase in intracellular Ca2+, followed by a decrease as the acrosome reaction proceeds. This calcium transient is completely suppressed by 3-quinuclidinyl benzilate. We conclude that the Ca2+ transient is induced by the zona pellucida and is required for the zona-induced acrosome reaction. Blockage of this sperm intracellular Ca2+ transient provides a mechanism for the inhibitory action of 3-quinuclidinyl benzilate on the zona-induced acrosome reaction in mouse sperm.

Interaction of aggregated native and mutant IgE receptors with the cellular skeleton

When aggregated, cell surface proteins become resistant to solubilization by detergents, presumably because of aggregation-induced or -stabilized interactions between the membrane protein and the cytoskeleton or plasma membrane skeleton. We genetically engineered variants of the tetrameric high-affinity receptor for IgE (Fc epsilon RI) to identify a site on its alpha, beta, or gamma chains that mediates such putative interactions. Using flow cytofluorometry, we studied rat basophilic leukemia cells, transiently transfected COS cells, and stably transfected P815 cells bearing wild-type and mutated receptors. We observed that (i) solubilization was markedly dependent on the degree of aggregation, the extent varying somewhat with the cell type and, particularly at lower levels of aggregation, with the time after addition of detergent; (ii) truncation of no single cytoplasmic domain of the alpha, beta, or gamma chains ablated the insolubilization effect; and (iii) incomplete receptors were also efficiently insolubilized by aggregation. Thus receptors consisting only of alpha and gamma chains, a "receptor" consisting of only the ectodomain of the alpha chain attached to the plasma membrane by a glycosyl-phosphatidyl inositol anchor, and "receptors" consisting only of minimally modified gamma chains were resistant to solubilization after aggregation. We conclude that no unique subunit or domain of Fc epsilon RI mediates the insolubilization phenomenon. Our results support a model in which the bridging of membrane proteins leads to their becoming nonspecifically enmeshed in a network of membrane skeletal proteins on either the outside and/or the inside of the membrane so that dissolution of the lipid bilayer becomes irrelevant.