PROTACs as a novel Approach to target Protein Tyrosine Kinases for degradation in human Platelets

The body has developed a finely tuned mechanism to stop blood loss at sites of vascular injury while preventing vessel blockage by excessive clot formation. It depends on a complicated series of interrelated events involving platelets and plasma coagulation factors. Platelets are small cell fragments that are normally circulating in an inactive state. However, when a blood vessel becomes injured, they rapidly stick to the site of injury and to each other to stop the bleeding. The reason that platelets can respond so efficiently, is that they have proteins on their surface called 'receptors' that can recognize molecules in the damaged blood vessel. When these molecules bind to the platelet receptors, communication signals are stimulated in the cell (=signal transduction) that tell the platelets to clump together and stop the bleeding. However, sometimes signals are stimulated that lead to too much clot formation which can lead to a heart attack or a stroke.

A lot of research has been done to investigate which molecules/proteins are involved in these signal transduction pathways. However, to study these signal transduction pathways experimentally is not straight forward as platelets do not have DNA like other cells, and therefore cannot be genetically modified. Much of the research has therefore been performed on genetically modified mouse models and/or the use of pharmacological drugs. These have the associated complication of species differences (mice are just not the same as people) and the drugs can often not discriminate between different molecules/proteins. Therefore, despite animal models showing that a set of proteins called tyrosine kinases contribute to platelet function, it is not yet clear how they contribute to platelet function in humans.

In this grant application, we therefore propose to develop and optimise a novel experimental technique called PROTAC (PROteolysis TArgeting Chimera) for use in human platelets. PROTACs are small molecules that can be added to cells and hijack their internal system to specifically break down a particular protein. This thus will help us to study the role of that particular protein in human platelet function and the stopping of a bleed. We will focus our efforts on proteins called tyrosine kinases and will use existing PROTACs and further design, develop and optimise novel PROTACs to study the role of tyrosine kinases in human platelets.

One of the complications with studying how platelets maintain normal haemostatic control is that they lack a nucleus thereby prohibiting routine molecular approaches to knockdown a protein. Most of our present knowledge therefore derives from genetic animal models and pharmacological targeting of signaling pathways, each having its own drawbacks associated with species difference and pharmacological non-specificity.

Here we propose the use of rationally designed small molecule degraders called PROTACs (PROteolysis TArgeting Chimeras) to investigate signaling pathways involved in the haemostatic function of platelets. PROTACs are small molecules that can hijack the proteasomal system by bringing the protein target of interest (POI) in the proximity of E3 ligase, leading to protein ubiquitination and proteasomal-mediated degradation. We here demonstrate the feasibility and high specificity of this chemical knockdown strategy using FAK-targeting PROTACs in human platelets.

In this proposal, we will expand on these observations and design, generate and optimize novel PROTACs in order to develop a highly efficient technology to study signaling pathways and their function in human platelets. More specifically, we will focus our attention on protein tyrosine kinases that are abundant in human platelets, including FAK, PYK2 and Src family kinases (SFK). With the PROTAC field rapidly developing and their high selectivity, potency, and efficacy, it is now very timely to develop the PROTAC approach as a novel experimental tool in platelets. This novel approach will facilitate evaluation of biological signalling pathways and their contribution to normal platelet function and haemostasis. We anticipate that the use of PROTACs will be transformative to the platelet research and wider community.