The open canalicular system (OCS) is an internal membrane structure found in platelets. First identified 50 years ago, the OCS comprises a tunneling network of surface-connected channels that appear to play an important role in platelet function. Yet, our understanding of how the OCS forms, how it functions, and what might regulate its structure and behavior remains fairly rudimentary. Structural abnormalities of the OCS are observed in some human platelet disorders. Yet, because platelets from these patients display multiple defects, the specific contribution of any OCS dysregulation to the impaired platelet function is unclear. However, recent studies have begun to shed light on mechanisms that regulate the OCS structure and to understand what influence the OCS has on overall platelet function. Advances in cellular imaging techniques have allowed whole-cell visualization of the OCS, providing the opportunity for a more detailed structural examination. Furthermore, recent work indicates that the modulation of the OCS structure may be sufficient to impact in vivo platelet function, opening up the intriguing possibility of manipulating the OCS structure as an anti-thrombotic approach. On the 50 anniversary of its discovery, we review here what is known about OCS structure and function, and outline some of the key microscopy tools for studying this intriguing internal membrane system.

The phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases central to regulating a wide range of important intracellular processes. Despite the vast knowledge around class I PI3Ks, the class II PI3Ks have been neglected, seemingly only due to the chronology of their discovery. Here we focus on the cellular functions of the three class II PI3K isoforms, PI3KC2α, PI3KC2β, and PI3KC2γ, in different cell systems and underline the emerging importance of these enzymes in different physiological and pathological contexts. We provide an overview on the current development of class II PI3 kinase inhibitors and outline the potential use for such inhibitors. The field is in its infancy as compared to their class I counterparts. Nevertheless, recent advances in understanding the roles of class II PI3 kinases in different pathological contexts is leading to an increased interest in the development of specific inhibitors that can provide potential novel pharmacological tools.

Arterial thrombosis causes heart attacks and most strokes and is the most common cause of death in the world. Platelets are the cells that form arterial thrombi, and antiplatelet drugs are the mainstay of heart attack and stroke prevention. Yet, current drugs have limited efficacy, preventing fewer than 25% of lethal cardiovascular events without clinically relevant effects on bleeding. The key limitation on the ability of all current drugs to impair thrombosis without causing bleeding is that they block global platelet activation, thereby indiscriminately preventing platelet function in hemostasis and thrombosis. Here, we identify an approach with the potential to overcome this limitation by preventing platelet function independently of canonical platelet activation and in a manner that appears specifically relevant in the setting of thrombosis. Genetic or pharmacological targeting of the class II phosphoinositide 3-kinase (PI3KC2α) dilates the internal membrane reserve of platelets but does not affect activation-dependent platelet function in standard tests. Despite this, inhibition of PI3KC2α is potently antithrombotic in human blood ex vivo and mice in vivo and does not affect hemostasis. Mechanistic studies reveal this antithrombotic effect to be the result of impaired platelet adhesion driven by pronounced hemodynamic shear stress gradients. These findings demonstrate an important role for PI3KC2α in regulating platelet structure and function via a membrane-dependent mechanism and suggest that drugs targeting the platelet internal membrane may be a suitable approach for antithrombotic therapies with an improved therapeutic window.