Investigating the Mechanisms of Platelet Hyperactivity in Diabetes Mellitus

2017-02-07T06:07:52Z (GMT) by James David McFadyen
Background
   Diabetes is a major risk factor for cardiovascular disease, leading to arterial thrombosis and major vascular events such as ischaemic stroke and myocardial infarction. While endothelial dysfunction and the associated development of atherosclerotic lesions are thought to be the primary cause of the adverse effects of diabetes, platelet hyperactivity has also been demonstrated to contribute to the heightened risk of vascular disease. However, how diabetes influences platelet reactivity, in particular thrombus growth in vivo, to mediate the prothrombotic
phenotype remains poorly understood.
   
   Aims
   To investigate the mechanisms by which diabetes results in a prothrombotic platelet phenotype in vivo.
   
   Key findings
   Using a murine model of type 1 diabetes, in conjunction with distinct in vivo models of thrombosis and endothelial inflammation, as well as high resolution imaging techniques, the studies in this thesis provide new insights into the prothrombotic platelet phenotype in diabetes. Diabetic mice exhibit a much shorter time to thrombotic occlusion following Rose Bengal and ferric chloride induced injury of vessel wall, and a profoundly enhanced thrombotic response in a rheology-dependent needle in situ model of thrombosis. Diabetic platelets also display enhanced adhesion to inflamed endothelium induced by ischaemia or mechanical injury. Surprisingly, this prothrombotic diabetic platelet phenotype involves the heightened reactivity of platelets in a low activation state, as indicated by their discoid morphology and lack of P-selectin expression, and is mediated by the elevated adhesive function of integrin αIIbβ3 under haemodynamic shear. Total internal reflective fluorescent (TIRF) and scanning electron (SEM) microscopy studies have revealed the existence of a previously unrecognized membrane structure, termed tractopods, which are αIIbβ3- dependent and facilitate stable discoid platelet adhesion independently of soluble agonists, and prolong diabetic platelet adhesion under haemodynamic shear. Examining the underlying mechanisms using the biomembrane force probe (BFP) system demonstrates a marked elevation in the biomechanical activation of integrin αIIbβ3 in mouse and human diabetic platelets in a process linked to a force-induced intermediate conformational change in αIIbβ3 in discoid platelets. Biochemical analysis of diabetic platelets reveals the differential expression of proteins involved in PI3K signaling, coupled with the demonstration of enhanced Akt phosphorylation in diabetic platelets, suggesting that dysregulated PI3K signaling may play an important role in mediating the prothrombotic diabetic phenotype. In accordance with this, the biomechanical activation of diabetic αIIbβ3 and the shear dependent prothrombotic diabetic phenotype are exquisitely sensitive to PI3Kβ deletion and inhibition, but not to the standard antithrombotics, aspirin and clopidogrel.
   
   Conclusion
   The platelet hyperactivity and prothrombotic phenotype seen in diabetes is linked to the dysregulated biomechanical activation of integrin αIIbβ3 in discoid platelets. This is facilitated by the formation of a new membrane adhesive structure, termed tractopods. These processes lead to the marked accumulation of platelets in a low activation state at sites of thrombus formation and endothelial inflammation in vivo, and is sensitive to PI3Kβ inhibition but not standard antiplatelet agents. Together, studies in this thesis provide new insights into the mechanisms underlying the prothrombotic platelet phenotype in diabetes, and identified PI3Kβ as the potential new target to curb the heightened thrombotic risk in diabetic patients.