Thesis Format
Integrated Article
Degree
Doctor of Philosophy
Program
Physiology and Pharmacology
Supervisor
Koschinsky, Marlys L.
Affiliation
Robarts Research Institute
Abstract
Elevated levels of lipoprotein(a) (Lp(a)) are an independent and causal risk factor for the development of atherothrombotic diseases. However, it is unknown if Lp(a) directly promotes thrombus formation, inhibits thrombus clearance, or merely accelerates the underlying atherosclerotic processes that culminate in plaque rupture. Similarities between the apolipoprotein(a) (apo(a)) component of Lp(a) with the fibrinolytic proenzyme plasminogen are strongly suggestive of antifibrinolytic activity. While numerous studies indicate that apo(a) can inhibit plasminogen activation and fibrinolysis, existing evidence suggests that these effects may not be retained in Lp(a). An alternative mechanism through which Lp(a) may promote atherothrombotic events is by impacting platelet function. Corroborating this notion, recent observational clinical studies demonstrated that individuals with high Lp(a) levels derive increased benefit from dual anti-platelet therapy. However, the effects of Lp(a) on platelet function and thrombosis have never been directly assessed in blood clots formed from flowing whole blood. In our first set of experiments, we used ex vivo plasma clot lysis assays to elucidate the previously reported disparities between the antifibrinolytic effects of Lp(a) and apo(a). In these studies, we showed that the residue in apo(a) responsible for mediating its antifibrinolytic effects appears to become functionally blocked during covalent Lp(a) assembly. Next, we evaluated the prothrombotic potential of Lp(a) in human blood clots formed under arterial flow conditions using a Chandler loop apparatus. In these studies, we showed that the presence of Lp(a) during thrombogenesis promoted platelet accumulation and facilitated the development of fibrin networks that displayed features of fibrinolysis resistance. Finally, we examined the prothrombotic potential of Lp(a) in vivo using established murine models of thrombosis and hemostasis. In the absence of underlying cardiovascular disease, we showed that mice expressing human apo(a) had increased thrombus volumes, and accelerated rates of vessel occlusion and hemostasis. Importantly, we also demonstrated that the observed prothrombotic effects of Lp(a) could be mitigated by low-dose aspirin therapy. Collectively, this body of work offers novel insights into the pathophysiological mechanisms of Lp(a), and suggests that, rather than inhibiting plasminogen activation, Lp(a) contributes to atherothrombotic diseases by exerting direct prothrombotic effects on platelet function and developing fibrin networks.
Summary for Lay Audience
Cardiovascular disease (CVD) remains one of the leading causes of sickness and death worldwide. Heart attacks and strokes, which result from chronic CVD progression, can be caused by blood clots forming in blood vessels that feed the heart and brain with oxygen and nutrients. Importantly, by blocking oxygen and nutrient transport to these vital organs, these blood clots can result in disease or death if not rapidly broken down. Elevated blood levels of a circulating lipoprotein (a protein/fat complex) called lipoprotein(a) (Lp(a)) have recently been identified as a novel risk factor for heart attacks and strokes, however, it is currently unclear how Lp(a) contributes to the onset of these diseases. While some studies suggest that Lp(a) can prevent the breakdown of established blood clots, other studies suggest that Lp(a) can promote blood clot formation. In this study, our goal was to improve the understanding of the role of Lp(a) in blood clot formation and blood clot breakdown. First, in laboratory tests where we formed clots from the plasma component of blood under static conditions, we found that Lp(a) did not inhibit clot breakdown. Next, we used a rotating device that forms clots from flowing human blood, which simulates the conditions under which clots form in the body, to investigate how Lp(a) impacts blood clot formation. Our experiments showed that Lp(a) promoted the formation of clots that were harder to breakdown. Specifically, we observed that Lp(a) increased the accumulation of platelets within developing clots. Platelets are small cells that are important components of clots and help prevent bleeding. Finally, we tested the effects of Lp(a) on blood clotting in mice, and found that it increased the size and speed of blood clot formation. Interestingly, treatment of the mice with aspirin, an anti-platelet therapy, minimized the effects of Lp(a). Together, our experiments suggest that high levels of Lp(a) can lead to heart attacks and strokes by promoting the formation of stable clots capable of blocking blood flow within vessels.
Recommended Citation
Clark, Justin, "Prothrombotic Mechanisms of Lp(a)-Mediated Pathophysiology: Implications for Platelet Function, Fibrin Clot Architectures, and Atherothrombosis" (2025). Electronic Thesis and Dissertation Repository. 10679.
https://ir.lib.uwo.ca/etd/10679
