Alternative Methods for the Detection of Superoxide Anion Generation in Platelets 

Reactive oxygen species (ROS) are highly unstable oxygen-containing molecules. Their chemical instability makes them extremely reactive and gives them the ...

University of Hull

The regulation of hematopoietic stem cell (HSC) survival and self-renewal within the bone marrow (BM) niche is not well understood. We therefore investigated global transcriptomic profiling of normal human HSC/hematopoietic progenitor cells [HPCs], revealing that several chemokine ligands (CXCL1-4, CXCL6, CXCL10, CXCL11, and CXCL13) were upregulated in human quiescent CD34(+)Hoescht(-)Pyronin Y(-) and primitive CD34(+)38(-), as compared with proliferating CD34(+)Hoechst(+)Pyronin Y(+) and CD34(+)38(+) stem/progenitor cells. This suggested that chemokines might play an important role in the homeostasis of HSCs. In human CD34(+) hematopoietic cells, knockdown of CXCL4 or pharmacologic inhibition of the chemokine receptor CXCR2, significantly decreased cell viability and colony forming cell (CFC) potential. Studies on Cxcr2(-/-) mice demonstrated enhanced BM and spleen cellularity, with significantly increased numbers of HSCs, hematopoietic progenitor cell-1 (HPC-1), HPC-2, and Lin(-)Sca-1(+)c-Kit(+) subpopulations. Cxcr2(-/-) stem/progenitor cells showed reduced self-renewal capacity as measured in serial transplantation assays. Parallel studies on Cxcl4 demonstrated reduced numbers of CFC in primary and secondary assays following knockdown in murine c-Kit(+) cells, and Cxcl4(-/-) mice showed a decrease in HSC and reduced self-renewal capacity after secondary transplantation. These data demonstrate that the CXCR2 network and CXCL4 play a role in the maintenance of normal HSC/HPC cell fates, including survival and self-renewal.

CXCR2 and CXCL4 regulate survival and self-renewal of hematopoietic stem/progenitor cells.

Prostacyclin (PGI) modulates platelet activation to regulate haemostasis. Evidence has emerged to suggest that thrombi are dynamic structures with distinct areas of differing platelet activation. It was hypothesised that PGI could reverse platelet spreading by actin cytoskeletal modulation, leading to reduced capability of platelet aggregates to withstand a high shear environment. Our data demonstrates that post-flow of PGI over activated and spread platelets on fibrinogen, identified a significant reduction in platelet surface area under high shear. Exploration of the molecular mechanisms underpinning this effect revealed that PGI reversed stress fibre formation in adherent platelets, reduced platelet spreading, whilst simultaneously promoting actin nodule formation. The effects of PGI on stress fibres were mimicked by the adenylyl cyclase activator forskolin and prevented by inhibitors of protein kinase A (PKA). Stress fibre formation is a RhoA dependent process and we found that treatment of adherent platelets with PGI caused inhibitory phosphorylation of RhoA, reduced RhoA GTP-loading and reversal of myosin light chain phosphorylation. Phospho-RhoA was localised in actin nodules with PKA type II and a number of other phosphorylated PKA substrates. This study demonstrates that PGI can reverse key platelet functions after their initial activation and identifies a novel mechanism for controlling thrombosis.

Prostacyclin reverses platelet stress fibre formation causing platelet aggregate instability.

Evidence has emerged to suggest that thrombi are dynamic structures with distinct areas of differing platelet activation and inhibition. We hypothesised that Nitric oxide (NO), a platelet inhibitor, can modulate the actin cytoskeleton reversing platelet spreading, and therefore reduce the capability of thrombi to withstand a high shear environment. Our data demonstrates that GSNO, DEANONOate, and a PKG-activating cGMP analogue reversed stress fibre formation and increased actin nodule formation in adherent platelets. This effect is sGC dependent and independent of ADP and thromboxanes. Stress fibre formation is a RhoA dependent process and NO induced RhoA inhibition, however, it did not phosphorylate RhoA at ser188 in spread platelets. Interestingly NO and PGI synergise to reverse stress fibre formation at physiologically relevant concentrations. Analysis of high shear conditions indicated that platelets activated on fibrinogen, induced stress fibre formation, which was reversed by GSNO treatment. Furthermore, preformed thrombi on collagen post perfused with GSNO had a 30% reduction in thrombus height in comparison to the control. This study demonstrates that NO can reverse key platelet functions after their initial activation and identifies a novel mechanism for controlling excessive thrombosis.

Reversal of stress fibre formation by Nitric Oxide mediated RhoA inhibition leads to reduction in the height of preformed thrombi.

Platelets are a critical cell for prevention of bleeding. Part of the response to the formation of the thrombus is the activation of the actin cytoskeleton, with an inability to effectively activate the cytoskeleton linked to thrombus formation defects and instability. The control of this process is linked to activation of the Rho GTPases, Cdc42, Rac1, and RhoA, although additional small GTPases such as Rif and Rap have been shown to play roles in platelet function.Here we describe the methodology to accurately understand how Rho GTPases are activated in platelets. Due to the technical limitations of working with platelets, such as their lack of ability to be transfected, the majority of work has been carried out either using inhibitors of Rho GTPases or within knockout mouse models. Studies can be conducted both in suspension samples and in spread platelets. In suspension the platelets will undergo a shape change response, but will not be able to spread. In spread platelets it is possible to examine the effects of the matrix environment, such as concentration, type, and stiffness on Rho GTPase function within platelet activation and platelet spreading.

Methods to Study the Roles of Rho GTPases in Platelet Function.

Fluorescence imaging has gathered interest over the recent years for its real-time response and high sensitivity. Developing probes for this modality has proven to be a challenge. Quantum dots (QDs) are colloidal nanoparticles that possess unique optical and electronic properties due to quantum confinement effects, whose excellent optical properties make them ideal for fluorescence imaging of biological systems. By selectively controlling the synthetic methodologies it is possible to obtain QDs that emit in the first (650-950 nm) and second (1000-1400 nm) near infra-red (NIR) windows, allowing for superior imaging properties. Despite the excellent optical properties and biocompatibility shown by some NIR QDs, there are still some challenges to overcome to enable there use in clinical applications. In this review, we discuss the latest advances in the application of NIR QDs in preclinical settings, together with the synthetic approaches and material developments that make NIR QDs promising for future biomedical applications.

NIR-quantum dots in biomedical imaging and their future.

Investigating human platelet function in low-oxygen environments is important in multiple settings, including hypobaric hypoxia (e.g., high altitude), sea level hypoxia-related disease, and thrombus stability. These studies often involve drawing blood from which platelets are isolated and analysed at atmospheric conditions or re-exposed to low oxygen levels in hypoxia chambers before testing. However, it remains unknown how the in vitro handling of the samples itself changes their dissolved oxygen concentration, which might affect platelet function and experimental results. Here, we prepared healthy donor platelet-rich plasma and washed platelet (WP) suspensions and exposed them to 2% oxygen. We found that the use of hypoxia pre-equilibrated tubes, higher platelet concentrations (>2 × 10/mL versus 2 × 10/mL), smaller volumes (600 µL versus 3 mL), and presence of plasma reduced the time for samples to reach 2% oxygen. Notably, oxygen levels decreased below 2% in most suspensions, but also in WP maintained at atmospheric 21% oxygen. Additionally, platelet spreading on fibrinogen was decreased when using hypoxic fibrinogen-coated culture plates regardless of the oxygen percentage (2% or 21%) in which platelet incubation took place. Thus, sample handling and experimental conditions should be carefully monitored in platelet-hypoxia studies as they might compromise results interpretation and comparison across studies.

Practical Considerations of Dissolved Oxygen Levels for Platelet Function under Hypoxia.

Approximately 17.3% of the global population exhibits an element of zinc (Zn) deficiency. One symptom of Zn deficiency is increased bleeding through impaired hemostasis. Platelets are crucial to hemostasis and are inhibited by endothelial-derived prostacyclin (prostaglandin I [PGI]), which signals via adenylyl cyclase (AC) and cyclic adenosine monophosphate signaling. In other cell types, Zn modulates cyclic adenosine monophosphate concentrations by changing AC and/or phosphodiesterase activity.

Platelet zinc status regulates prostaglandin-induced signaling, altering thrombus formation.

Platelets have a pivotal role in maintaining cardiovascular homeostasis. They are kept docile by endothelial-derived mediators. Aberration in haemostatic balance predisposes an individual to an elevated risk of a prothrombotic environment. Anti-platelet therapy has been a key component to reduce this risk. However, understanding how these medications affect the balance between the activation and inhibition of platelets is critical. There is no evidence that a key anti-platelet therapy - aspirin, may not be the most efficacious medicine of choice, as it can compromise both platelet inhibition and activation pathways. In this review, the rationale of aspirin as an anti-thrombotic drug has been critically discussed. This review looks at how recently published trials are raising key questions about the efficacy and safety of aspirin in countering cardiovascular diseases. There is an increasing portfolio of evidence that identifies that although aspirin is a very cheap and accessible drug, it may be used in a manner that is not always beneficial to a patient, and a more nuanced and targeted use of aspirin may increase its clinical benefit and maximize patient response. The questions about the use of aspirin raise the potential for changes in its clinical use for dual anti-platelet therapy. This highlights the need to ensure that treatment is targeted in the most effective manner and that other anti-platelet therapies may well be more efficacious and beneficial for CVD patients in their standard and personalized approaches.

Should aspirin be replaced with ADP blockers and anti-GPVI to manage thrombosis?

InP/ZnS quantum dots (QDs) have received a large focus in recent years as a safer alternative to heavy metal-based QDs. Given their intrinsic fluorescent imaging capabilities, these QDs can be potentially relevant for in vivo platelet imaging. The InP/ZnS QDs are synthesized and their biocompatibility investigated through the use of different phase transfer agents. Analysis of platelet function indicates that platelet-QD interaction can occur at all concentrations and for all QD permutations tested. However, as the QD concentration increases, platelet aggregation is induced by QDs alone independent of natural platelet agonists. This study helps to define a range of concentrations and coatings (thioglycolic acid and penicillamine) that are biocompatible with platelet function. With this information, the platelet-QD interaction can be identified using multiple methods. Fluorescent lifetime imaging microscopy (FLIM) and confocal studies have shown QDs localize on the surface of the platelet toward the center while showing evidence of energy transfer within the QD population. It is believed that these findings are an important stepping point for the development of fluorescent probes for platelet imaging.

Impact of Surface Ligand on the Biocompatibility of InP/ZnS Quantum Dots with Platelets.

Quantum dots (QDs) are semi-conducting nanoparticles that have been developed for a range of biological and non-biological functions. They can be tuned to multiple different emission wavelengths and can have significant benefits over other fluorescent systems. Many studies have utilised QDs with a cadmium-based core; however, these QDs have since been shown to have poor biological compatibility. Therefore, other QDs, such as indium phosphide QDs, have been developed. These QDs retain excellent fluorescent intensity and tunability but are thought to have elevated biological compatibility. Herein we discuss the applicability of a range of QDs to the cardiovascular system. Key disease states such as myocardial infarction and stroke are associated with cardiovascular disease (CVD), and there is an opportunity to improve clinical imaging to aide clinical outcomes for these disease states. QDs offer potential clinical benefits given their ability to perform multiple functions, such as carry an imaging agent, a therapy, and a targeting motif. Two key cell types associated with CVD are platelets and immune cells. Both cell types play key roles in establishing an inflammatory environment within CVD, and as such aid the formation of pathological thrombi. However, it is unclear at present how and with which cell types QDs interact, and if they potentially drive unwanted changes or activation of these cell types. Therefore, although QDs show great promise for boosting imaging capability, further work needs to be completed to fully understand their biological compatibility.

Simon Calaminus

Hull York Medical School

University of Hull

Alternative Methods for the Detection of Superoxide Anion Generation in Platelets

Simon Calaminus

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University of Hull

Alternative Methods for the Detection of Superoxide Anion Generation in Platelets

Hull York Medical School