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Published on: November 29, 2024
Human platelets are defective in processing of cholera toxin
The Biochemical Journal
|June 15, 1983
Summary
Human platelets do not respond to cholera toxin because they cannot process it into its active form. This defect in cholera toxin processing prevents cyclic AMP elevation in platelets.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- Cholera toxin effectively elevates cyclic AMP in most mammalian cells.
- Human platelets exhibit a significantly reduced response to cholera toxin compared to other cells.
- The mechanism behind this platelet unresponsiveness to cholera toxin is not fully understood.
Purpose of the Study:
- To investigate the molecular basis for the lack of cholera toxin-induced cyclic AMP elevation in human platelets.
- To determine if human platelets can bind and process cholera toxin effectively.
- To identify potential rate-limiting steps in the cholera toxin signaling pathway within platelets.
Main Methods:
- Binding assays using 125I-labelled cholera toxin to assess receptor affinity and capacity.
- Experiments with monosialoganglioside GM1 to evaluate its effect on toxin binding and activation.
- Enzyme activity assays on platelet membranes and cytosol with activated cholera toxin subunits.
- Analysis of cholera toxin subunit processing using SDS-PAGE.
- Comparative studies with murine S49 lymphoma cells.
Main Results:
- Cholera toxin binds to human platelets with high affinity but does not significantly elevate cyclic AMP levels.
- Monosialoganglioside GM1 enhances toxin binding but not cyclic AMP response.
- Activated cholera toxin stimulates adenylate cyclase in platelet membranes, and cytosol supports subunit activity.
- Human platelets are unable to cleave cholera toxin into its active A1 subunit, unlike S49 lymphoma cells.
- The generation of the A1 subunit appears to be a rate-limiting step for cholera toxin response in platelets.
Conclusions:
- Human platelets are defective in processing surface-bound cholera toxin to its active A1 subunit.
- Binding of cholera toxin is necessary but not sufficient for eliciting a cellular response.
- The inability to generate the active A1 subunit is the primary reason for the lack of cholera toxin-mediated cyclic AMP elevation in human platelets.
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