Ion channels and ion homeostasis in the platelet and megakaryocyte

The lipid bilayer of surface and organellar cell membranes represents a barrier to simple diffusion of inorganic ions or other charged molecules between extracellular and intracellular fluid compartments. In addition, various transport proteins establish substantial gradients of many of these charged entities across the plasma and organellar membranes. Such gradients are crucial to cellular homeostasis and activation since proteins are strongly regulated by the electrostatic nature of their environment. Furthermore, nature has taken advantage of this property by using changes in the concentration of specific ions as a signal to regulate functional responses. The platelet is no exception and it is well established that an increase in cytosolic Ca²⁺ is a key signal that stimulates secretion, contraction, inside-out activation of integrins and other events crucial to hemostasis [1–3]. Two reviews within this special series consider specific advances in our understanding of the importance of platelet Ca²⁺ signaling. Millington-Burgess and Harper [4] explore how specific patterns of cytosolic and mitochondrial Ca²⁺ increases may explain why this one signal can generate two distinct populations of activated platelets: those that are mainly pro-aggregatory versus those that are pro-coagulant. The functional relevance of the complexities of Ca²⁺ signaling events that can occur downstream of different classes of surface receptors, particularly in terms of the activation status of the pro-coagulant protein anoctamin 6, are further considered by Fernandez and colleagues [5] . Interestingly, anoctamin 6 is also an ion channel [6–9], which highlights the fact that this class of transmembrane protein can modulate cellular signaling via mechanisms beyond the control of ion diffusion across membranes. This concept is further considered within the review by Wright and Mahaut-Smith [10] on K⁺-selective channels which examines recent evidence that the predominantly expressed member of this family of proteins (Kv1.3) regulates integrin function. As discussed by the authors, it is well-established that Kv1.3 sets the resting membrane potential of the platelet and its precursor cell the megakaryocyte. The negative resting membrane potential (≈-60 mV) contributes to the driving force for Ca²⁺ entry through a number of receptor-activated Ca²⁺-permeable ion channels in these electrically inexcitable cells. In addition to Ca²⁺, evidence is emerging for important roles of other ions as modifiers of protein activity in the platelet and megakaryocyte. One example is Zn²⁺, which despite its low concentration in the body, has profound effects on proteins. The emerging topic of Zn²⁺ homeostasis and signaling is extensively covered in a review by Ahmed and colleagues [11].

Ion channels represent the main mechanism whereby cells can rapidly influence the concentration of ions within the cytoplasm and organelles. Compared to other cell types, the study of these transmembrane proteins in the platelet and megakaryocyte is still in its infancy. Nevertheless, key roles are becoming apparent for a range of channels which are predominantly permeable to ions other than Ca²⁺. Furthermore, some pore-forming membrane proteins such as the gap junction family members are also permeable to quite large (<1kDa) molecular weight charged and uncharged molecules. Taylor and colleagues [12] discuss the potential roles for the pannexin and connexin gap junction proteins in regulating thrombus formation and coordinating platelet responses. Being a polar molecule, water also passes slowly across lipid bilayers and its diffusion across cell membranes is mainly controlled by pore-forming proteins known as aquaporins. Procoagulant activity has been associated with membrane ballooning that may arise from influx of water into the cytosol. In their review, Agbani and Poole [13] describe the expression and function of aquaporins in platelets, including genetic and biochemical evidence for at least 5 members of this family of water-permeable channels.

Platelets and/or megakaryocytes express a number of ion channel subtypes that are known to be essential for the core activities of excitable cells but which have less well established function in blood cells. This includes members of the ionotropic family of glutamate receptors. As discussed by Kalev-Zylinska and colleagues [14], there is now substantial evidence that specific classes of these cation-permeable channels serve to amplify platelet responses during hemostasis and thrombosis, likely following release of glutamate from dense granules. The exact mechanism whereby these channels modulate platelet responses remains unclear since some subtypes operate as Ca²⁺-impermeable cation channels, however potential hypotheses are discussed by the authors.

Platelets are known to contribute to a number of physiological processes beyond hemostasis, such as angiogenesis and immunity. Consequently they are also potential therapeutic targets for prevention of disease states beyond thrombosis, such as sepsis, cancer and stroke. This is a particular focus of the review by Oury and Wera [15] which considers the evidence for a unique role of platelet P2X1 nonselective cation channels in thromboinflammation. Substantial amounts of ATP, the physiological ligand for P2X1 channels, are released at sites of injury and inflammation and also secreted by activated platelets from their dense granules. The contribution of this nucleotide to platelet activation is often overshadowed by the central importance of co-secreted ADP which stimulates P2Y G-protein-coupled receptors. The review by Taylor and colleagues [12] is of relevance to this topic as the large pore pannexin-1 channels and connexin hemichannels proteins represent non-vesicular pathways for the release of cytosolic ATP and potentially also ADP.

The human genome encodes several hundred different types of ion channels and other proteins involved in ionic signaling or ion homeostasis. Therefore, by default the small collection of articles in this edition excludes some important subtopics, and we apologize to authors whose work has been omitted. For example, TRPM7 is a particularly interesting nonselective cation channel with intrinsic kinase activity that influences platelet and megakaryocyte function in part through Mg²⁺ movements and altered cytoskeletal activity [16]. It is hoped that future special series will be able to report on further developments of this exciting topic. An additional aspect worth including in a future collection of review articles is the regulation of platelet ion channels by accessory proteins. One very recent example is the key role of the adapter protein BIN2 in store-operated Ca²⁺ entry through Orai1 via a mechanism that involves control of IP₃-gated cation channels [17]. The central importance of both Orai1 and IP₃ receptor channels in Ca²⁺ signaling and platelet function are clearly described within the reviews by Millington-Burgess and Harper [4] and Fernandez and colleagues [5].

All reviews within this edition consider the ultimate possibility that ion channels or other proteins involved ion homeostasis represent targets for treatment of disease, in particular thrombosis. Of particular note is the article by Fernandez and colleagues [5] which discusses a novel high-throughput screening assay for inhibitors of platelet procoagulant activity based upon detection of phosphatidylserine exposure.

Declaration of interest

The authors declare no conflict of interest

Acknowledgements

The authors’ research on platelet and megakaryocyte function is funded by the British Heart Foundation.