Protamine stimulates platelet aggregation in vitro with activation of the fibrinogen receptor and alpha-granule release, but impairs secondary activation via ADP and thrombin receptors

Coronary artery bypass grafting (CABG) induces pro-inflammatory and immunomodulatory phenotype of platelets in the absence of a pro-aggregatory state

BACKGROUND

Coronary artery bypass grafting (CABG) is considered the choice treatment for patients suffering from coronary artery disease (CAD). In the inflammatory milieu of cardiopulmonary bypass (CPB), systemic inflammatory response syndrome (SIRS) can induce a platelet pro-inflammatory state which could exacerbate post-CABG inflammatory status while affecting hemostatic function in patients. Therefore, focusing on platelets, the study presented here attempted to evaluate the pro-inflammatory and immunomodulatory profile of platelets as well as pro-aggregatory status during CABG.

METHODS

Platelets from patients undergoing CABG were subjected to flowcytometry analysis to evaluate P-selectin and CD40L expressions and PAC-1 binding in five intervals of 24 h before surgery, immediately, 2 h, 24 h, and one week after surgery. Moreover, intra-platelet TGF-β1 was also examined with western blotting.

RESULTS

Data showed increases of P-selectin and CD40L expressions in patients, with the meaningful loss of platelet contents of TGF-β1 after CABG (p < 0.001), where the changes tended to recover by day 7 of surgery while remaining above baseline (p < 0.001). Meanwhile, no significant change in PAC-1 binding capacity was shown.

CONCLUSION

The study presented here suggests that although the release of pro-inflammatory substances from platelets during CABG supports the post-operative inflammatory state, platelets are not pro-aggregatory enough to enhance thrombotic events after surgery. Whilst these observations could be due to successful medical interventions to optimize hemostasis during and after surgery, post-CABG reversal of anticoagulant by protamine is considered as another factor that may also have contributed to preventing pro-aggregatory but not pro-inflammatory and immunomodulatory functions of platelets.

External Trigger Free Charge Switchable Cationic Ligands in the Design of Safe and Effective Universal Heparin Antidote

Thrombosis, the formation of blood clots within a blood vessel, can lead to severe complications including pulmonary embolism, cardiac arrest, and stroke. The most widely administered class of anticoagulants is heparin-based anticoagulants such as unfractionated heparin, low-molecular weight heparins (LMWHs), and fondaparinux. Protamine is the only FDA-approved heparin antidote. Protamine has limited efficacy neutralizing LMWHs and no reversal activity against fondaparinux. The use of protamine can lead to complications, including excessive bleeding, hypotension, and hypersensitivity, and has narrow therapeutic window. In this work, a new concept in the design of a universal heparin antidote: switchable protonation of cationic ligands, is presented. A library of macromolecular polyanion inhibitors (MPIs) is synthesized and screened to identify molecules that can neutralize all heparins with high selectivity and reduced toxicity. MPIs are developed by assembling cationic binding groups possessing switchable protonation states onto a polymer scaffold. By strategically selecting the identity and modulating the density of cationic binding groups on the polymer scaffold, a superior universal heparin reversal agent is developed with improved heparin-binding activity and increased hemocompatibility profiles leading to minimal effect on hemostasis. The activity of this heparin antidote is demonstrated using in vitro and in vivo studies.

Platelet Function is Preserved After Moderate Cardiopulmonary Bypass Times But Transiently Impaired After Protamine

OBJECTIVES

Previous studies have described impaired platelet function after cardiopulmonary bypass (CPB). Whether this is still valid in contemporary cardiac surgery is unclear. This study aimed to quantify changes in function and number of platelets during CPB in a present-day cardiac surgery cohort.

DESIGN

Prospective, controlled clinical study.

SETTING

A single-center university hospital.

PARTICIPANTS

Thirty-nine patients scheduled for coronary artery bypass graft surgery with CPB.

INTERVENTIONS

Platelet function and numbers were measured at 6 timepoints in 39 patients during and after coronary artery bypass graft surgery; at baseline before anesthesia, at the end of CPB, after protamine administration, at intensive care unit (ICU) arrival, 3 hours after ICU arrival, and on the morning after surgery.

MEASUREMENTS AND MAIN RESULTS

Platelet function was assessed with impedance aggregometry and flow cytometry. Platelet numbers are expressed as actual concentration and as numbers corrected for dilution using hemoglobin as a reference marker. There was no consistent impairment of platelet function during CPB with either impedance aggregometry or flow cytometry. After protamine administration, a decrease in platelet function was seen with impedance aggregometry and for some markers of activation with flow cytometry. Platelet function was restored 3 hours after arrival in the ICU. During CPB (85.0 ± 21 min), the number of circulating platelets corrected for dilution increased from 1.73 ± 0.42 × 10⁹/g to 1.91 ± 0.51 × 10⁹/g (p < 0.001).

CONCLUSIONS

During cardiac surgery with moderate CPB times, platelet function was not impaired, and no consumption of circulating platelets could be detected. Administration of protamine transiently affected platelet function.

Exogenous human α-Synuclein acts in vitro as a mild platelet antiaggregant inhibiting α-thrombin-induced platelet activation

α-Synuclein (αSyn) is a small disordered protein, highly conserved in vertebrates and involved in the pathogenesis of Parkinson’s disease (PD). Indeed, αSyn amyloid aggregates are present in the brain of patients with PD. Although the pathogenic role of αSyn is widely accepted, the physiological function of this protein remains elusive. Beyond the central nervous system, αSyn is expressed in hematopoietic tissue and blood, where platelets are a major cellular host of αSyn. Platelets play a key role in hemostasis and are potently activated by thrombin (αT) through the cleavage of protease-activated receptors. Furthermore, both αT and αSyn could be found in the same spatial environment, i.e. the platelet membrane, as αT binds to and activates platelets that can release αSyn from α-granules and microvesicles. Here, we investigated the possibility that exogenous αSyn could interfere with platelet activation induced by different agonists in vitro. Data obtained from distinct experimental techniques (i.e. multiple electrode aggregometry, rotational thromboelastometry, immunofluorescence microscopy, surface plasmon resonance, and steady-state fluorescence spectroscopy) on whole blood and platelet-rich plasma indicate that exogenous αSyn has mild platelet antiaggregating properties in vitro, acting as a negative regulator of αT-mediated platelet activation by preferentially inhibiting P-selectin expression on platelet surface. We have also shown that both exogenous and endogenous (i.e. cytoplasmic) αSyn preferentially bind to the outer surface of activated platelets. Starting from these findings, a coherent model of the antiplatelet function of αSyn is proposed.

Protamine-Induced Coronary Graft Thrombosis: A Review

Perioperative myocardial infarction is a serious complication affecting a significant portion of patients undergoing coronary artery bypass graft surgery. This may arise due to coronary graft thrombosis, a rare but potentially fatal phenomenon associated with both congenital and acquired risk factors. Multiple case reports implicate the role of protamine in the development of such thromboses. The role of protamine in facilitating the regulation of hemostasis by reversing the anticoagulant effects of heparin in patients undergoing cardiopulmonary bypass is well-recognized. However, discussion of its potential contribution to coronary graft thrombosis and mechanisms by which this may occur is lacking. Furthermore, its narrow therapeutic index and side effect profile are such that its appropriateness as a universal reversal agent to heparin requires reconsideration. This article reviews the current body of evidence regarding the use of protamine in cardiac surgery and the limited case reports pertaining to its potential role in the pathophysiology of coronary graft thrombosis.