title: Understanding the mechanisms of the regulation of platelet function by thiol isomerase enzymes
Creators: name: Aldaghmi, Abdulrahman Siran M.
Publisher: name: University of Reading
Year: 2023

description: Background: Human platelets contain a number of thiol isomerases that are released to the 
cell surface upon platelet activation, such as protein disulphide isomerase (PDI), and 
endoplasmic reticulum (ER) proteins (ERp5, ERp57, and ERp72). Previous findings indicate 
that these enzymes play a role in regulating integrin αIIbβ3, the adhesion receptor that 
mediates platelet aggregation and thrombus formation, and the similarity in their inhibitory 
effects suggests functional overlaps. However, thiol isomerases are believed to facilitate 
disulphide exchange among themselves in mammalian cells. This suggests a potential 
cooperative interaction between these enzymes to regulate platelet function.
Aims: The main aims of this research were to elucidate the mechanisms underlying the 
actions of thiol isomerases during platelet activation and explore potential interactions 
among these enzymes. The investigation aimed to identify common and distinctive 
substrates associated with PDI, ERp5, ERp57, and ERp72, and detect functional interactions
among these enzymes. To understand more completely the associations between these 
enzymes, the study aimed to identify and characterise additional thiol isomerases in platelets 
that interact with these enzymes in other human cell types. To understand how these 
interactions affect platelet function, the investigation aimed to test the effects of novel broad-spectrum inhibitors on both platelet function and thiol isomerase interactions with potential 
substrates.
Results: Experimental investigations of thiol isomerase interactions revealed inter-thiol 
complexes during platelet stimulation. The characterisation of platelet thiol isomerases 
revealed ERp18, a novel family member. Interactions among PDI, ERp5, ERp57, and ERp72 
were detected and were particularly seen during platelet activation. Proteomics experiments
revealed potential substrates and/or binding partners of these enzymes, which included key
players in integrin inside-out signalling, such as the target receptor integrin αIIbβ3, 
thioredoxin, and heat shock proteins. In experiments to explore the collective function 
effects of thiol isomerases in the control of platelet function, the effects of the newly 
identified pan-thiol isomerase inhibitor were explored. Benserazide was demonstrated to 
inhibit the enzymatic activity of PDI, ERp5, ERp57, and ERp72, along with its capacity to 
inhibit various platelet functions. In proteomics studies, benserazide was shown to block the 
interactions between thiol isomerases and their substrates.
Conclusions: These findings provide evidence that thiol isomerase enzymes participate in 
common and distinct protein complexes to regulate platelet function. The broad inhibition 
observed, resembling single inhibition of these enzymes, further supports the notion that 
platelet thiol isomerases collaborate to elicit their function roles upon their translocation to 
the cell surface. Lastly, benserazide, a well-tolerated drug with established therapeutic uses, 
holds promise as a potential fast-track option for developing a new anti-thrombotic therapy.