Assessment of neonatal, cord, and adult platelet granule trafficking and secretion

SARAF overexpression impairs thrombin-induced Ca2+ homeostasis in neonatal platelets

Neonatal platelets present a reduced response to the platelet agonist, thrombin (Thr), thus resulting in a deficient Thr-induced aggregation. These alterations are more pronounced in premature newborns. Here, our aim was to uncover the causes underneath the impaired Ca2+ homeostasis described in neonatal platelets. Both Ca2+ mobilization and Ca2+ influx in response to Thr are decreased in neonatal platelets compared to maternal and control woman platelets. In neonatal platelets, we observed impaired Ca2+ mobilization in response to the PAR-1 agonist (SFLLRN) or by blocking SERCA3 function with tert-butylhydroquinone. Regarding SOCE, the STIM1 regulatory protein, SARAF, was found overexpressed in neonatal platelets, promoting an increase in STIM1/SARAF interaction even under resting conditions. Additionally, higher interaction between SARAF and PDCD61/ALG2 was also observed, reducing SARAF ubiquitination and prolonging its half-life. These results were reproduced by overexpressing SARAF in MEG01 and DAMI cells. Finally, we also observed that pannexin 1 permeability is enhanced in response to Thr in control woman and maternal platelets, but not in neonatal platelets, hence, leading to the deregulation of the Ca2+ entry found in neonatal platelets. Summarizing, we show that in neonatal platelets both Ca2+ accumulation in the intracellular stores and Thr-evoked Ca2+ entry through either capacitative channels or non-selective channels are altered in neonatal platelets, contributing to deregulated Ca2+ homeostasis in neonatal platelets and leading to the altered aggregation observed in these subjects.

Correlation of bioactive components of platelet rich plasma derived from human female adult peripheral blood and umbilical cord blood with age

Platelet-rich plasma (PRP) has gained significant attention in the field of regenerative medicine due to its potential therapeutic applications. However, few studies have reported the components, especially anti-ageing-related components, of PRP derived from umbilical cord blood (UCB). It is essential to understand the influence of age on the composition and efficacy of PRP to optimize its clinical use. The present study compared the concentrations of bioactive components in PRP from healthy female adults and UCB-derived PRP. PRP was obtained from blood samples from females in four age groups (12 per group): neonates (UCB donors) and adults aged 18-25, 26-45, and 46-65 years, respectively. The concentrations of epidermal growth factor, basic fibroblast growth factor-2 (FGF-2), insulin-like growth factor-1, platelet-derived growth factor-AA (PDGF-AA), PDGF-AB/BB, vascular endothelial growth factor A, RANTES, TIMP-1, TIMP-2, GDF11, and clusterin and activity of superoxide dismutase, catalase, and glutathione peroxidase (GPx) in the PRP samples were determined and compared among groups. Pairwise comparisons between the groups showed statistically significant differences in the concentrations of some bioactive components of PRP, such as FGF-2, PDGF-AB/BB, and clusterin, and GPx activity. UCB-derived PRP contains various active ingredients such as VEGF-A, CAT activity, and TIMP-2. Contrary to expectations, UCB-derived PRP did not show higher concentrations of the anti-ageing protein GDF11. Because UCB is a rich source of bioactive components with low immunogenicity, its use in PRP preparation is an important research direction for future studies.

The Signaling Pathway of the ADP Receptor P2Y12 in the Immune System: Recent Discoveries and New Challenges

P2Y12 is a G-protein-coupled receptor that is activated upon ADP binding. Considering its well-established role in platelet activation, blocking P2Y12 has been used as a therapeutic strategy for antiplatelet aggregation in cardiovascular disease patients. However, receptor studies have shown that P2Y12 is functionally expressed not only in platelets and the microglia but also in other cells of the immune system, such as in monocytes, dendritic cells, and T lymphocytes. As a result, studies were carried out investigating whether therapies targeting P2Y12 could also ameliorate inflammatory conditions, such as sepsis, rheumatoid arthritis, neuroinflammation, cancer, COVID-19, atherosclerosis, and diabetes-associated inflammation in animal models and human subjects. This review reports what is known about the expression of P2Y12 in the cells of the immune system and the effect of P2Y12 activation and/or inhibition in inflammatory conditions. Lastly, we will discuss the major problems and challenges in studying this receptor and provide insights on how they can be overcome.

Platelets and ductus arteriosus closure in neonates

Platelet plug formation is critically involved in murine ductus arteriosus closure and thrombocytopenia in preterm infants seems to negatively affect spontaneous and pharmacologically induced ductal closure. Furthermore, platelet dysfunction may contribute to ductal patency, especially in extremely immature infants. Neonatal platelets likely have multifaceted roles during ductal closure, such as secretion of several signaling molecules and facilitation of specific cell-cell interactions. The only available randomized-controlled trial on platelet transfusions in preterm infants with patent ductus arteriosus demonstrated that a liberal transfusion regimen did not promote ductal closure, but was associated with an increased rate of intraventricular hemorrhage. Herein, we discuss the available mechanistic evidence on the role of platelets in ductus arteriosus closure and their potential clinical implications in preterm infants. We further briefly outline future research directions aimed at a better understanding of platelet-endothelial interactions in neonatal health and disease.

Platelets' morphology, metabolic profile, exocytosis, and heterotypic aggregation with leukocytes in relation to severity and mortality of COVID-19-patients

Roles of platelets during infections surpass the classical thrombus function and are now known to modulate innate immune cells. Leukocyte-platelet aggregations and activation-induced secretome are among factors recently gaining interest but little is known about their interplay with severity and mortality during the course of SARS-Cov-2 infection. The aim of the present work is to follow platelets' bioenergetics, redox balance, and calcium homeostasis as regulators of leukocyte-platelet interactions in a cohort of COVID-19 patients with variable clinical severity and mortality outcomes. We investigated COVID-19 infection-related changes in platelet counts, activation, morphology (by flow cytometry and electron microscopy), bioenergetics (by Seahorse analyzer), mitochondria function (by high resolution respirometry), intracellular calcium (by flow cytometry), reactive oxygen species (ROS, by flow cytometry), and leukocyte-platelet aggregates (by flow cytometry) in non-intensive care unit (ICU) hospitalized COVID-19 patients (Non-ICU, n=15), ICU-survivors of severe COVID-19 (ICU-S, n=35), non-survivors of severe COVID-19 (ICU-NS, n=60) relative to control subjects (n=31). Additionally, molecular studies were carried out to follow gene and protein expressions of mitochondrial electron transport chain complexes (ETC) in representative samples of isolated platelets from the studied groups. Our results revealed that COVID-19 infection leads to global metabolic depression especially in severe patients despite the lack of significant impacts on levels of mitochondrial ETC genes and proteins. We also report that severe patients' platelets exhibit hyperpolarized mitochondria and significantly lowered intracellular calcium, concomitantly with increased aggregations with neutrophil. These changes were associated with increased populations of giant platelets and morphological transformations usually correlated with platelets activation and inflammatory signatures, but with impaired exocytosis. Our data suggest that hyperactive platelets with impaired exocytosis may be integral parts in the pathophysiology dictating severity and mortality in COVID-19 patients.

Age-restricted functional and developmental differences of neonatal platelets

BACKGROUND

Developmental ontogeny of neonatal thrombopoiesis retains characteristics that are distinct from adults although molecular mechanisms remain unestablished.

METHODS

We applied multiparameter quantitative platelet responses with integrated ribosome profiling/transcriptomic studies to better define gene/pathway perturbations regulating the neonatal-to-adult transition. A bioinformatics pipeline was developed to identify stable, neonatal-restricted platelet biomarkers for clinical application.

RESULTS

Cord blood (CB) platelets retained the capacity for linear agonist-receptor coupling linked to phosphatidylserine (PS) exposure and α-granule release, although a restricted block in cross-agonist activation pathways was evident. Functional immaturity of synergistic signaling pathways was due to younger ontogenetic age and singular underdevelopment of the protein secretory gene network, with reciprocal expansion of developmental pathways (E2F, G2M checkpoint, c-Myc) important for megakaryocytopoiesis. Genetic perturbations regulating vesicle transport and fusion (TOM1L1, VAMP3, SNAP23, and DNM1L) and PS exposure and procoagulant activity (CLCN3) were the most significant, providing a molecular explanation for globally attenuated responses. Integrated transcriptomic and ribosomal footprints identified highly abundant (ribosome-protected) DEFA3 (encoding human defensin neutrophil peptide 3) and HBG1 as stable biomarkers of neonatal thrombopoiesis. Studies comparing CB- or adult-derived megakaryocytopoiesis confirmed inducible and abundant DEFA3 antigenic expression in CB megakaryocytes, ~3.5-fold greater than in leukocytes (the most abundant source in humans). An initial feasibility cohort of at-risk pregnancies manifested by maternal/fetal hemorrhage (chimerism) were applied for detection and validation of platelet HBG1 and DEFA3 as neonatal thrombopoiesis markers, most consistent for HBG1, which displayed gestational age-dependent expression.

CONCLUSIONS

These studies establish an ontogenetically divergent stage of neonatal thrombopoiesis, and provide initial feasibility studies to track disordered fetal-to-adult megakaryocytopoiesis in vivo.

Platelets in the neonate: Not just a small adult

Neonates, particularly those born preterm, have a high incidence of thrombocytopenia and bleeding, most commonly in the brain. Because of this, it has historically been accepted that neonates should be transfused at higher platelet counts than older children or adults, to decrease their bleeding risk. However, a number of observational studies and a recent large, randomized trial found a higher incidence of bleeding and mortality in neonates who received more platelet transfusions. The mechanisms underlying the deleterious effects of platelet transfusions in neonates are unknown, but it has been hypothesized that transfusing adult platelets into the very different physiological environment of a neonate may result in a “developmental mismatch” with potential negative consequences. Specifically, neonatal platelets are hyporeactive in response to multiple agonists and upon activation express less surface P‐selectin than adult platelets. However, this hyporeactivity is well balanced by factors in neonatal blood that promote clotting, such as the elevated hematocrit, elevated von Willebrand factor (VWF) levels, and a predominance of ultra‐long VWF polymers, with the net result of normal neonatal primary hemostasis. So far, most studies on the developmental differences between neonatal and adult platelets have focused on their hemostatic functions. However, it is now clear that platelets have important nonhemostatic functions, particularly in angiogenesis, immune responses, and inflammation. Whether equally important developmental differences exist with regard to those nonhemostatic platelet functions and how platelet transfusions perturb those processes in neonates remain unanswered questions.

Neonatal platelet physiology and implications for transfusion

The neonatal hemostatic system is different from that of adults. The differences in levels of procoagulant and anticoagulant factors and the evolving equilibrium in secondary hemostasis during the transition from fetal/neonatal life to infancy, childhood, and adult life are known as "developmental hemostasis." In regard to primary hemostasis, while the number (150,000-450,000/µl) and structure of platelets in healthy neonates closely resemble those of adults, there are significant functional differences between neonatal and adult platelets. Specifically, platelets derived from both cord blood and neonatal peripheral blood are less reactive than adult platelets to agonists, such as adenosine diphosphate (ADP), epinephrine, collagen, thrombin, and thromboxane (TXA2) analogs. This platelet hyporeactivity is due to differences in expression levels of key surface receptors and/or in signaling pathways, and is more pronounced in preterm neonates. Despite these differences in platelet function, bleeding times and PFA-100 closure times (an in vitro test of whole-blood primary hemostasis) are shorter in healthy full-term infants than in adults, reflecting enhanced primary hemostasis. This paradoxical finding is explained by the presence of factors in neonatal blood that increase the platelet-vessel wall interaction, such as high von Willebrand factor (vWF) levels, predominance of ultralong vWF multimers, high hematocrit, and high red cell mean corpuscular volume. Thus, the hyporeactivity of neonatal platelets should not be viewed as a developmental deficiency, but rather as an integral part of a developmentally unique, but well balanced, primary hemostatic system. In clinical practice, due to the high incidence of bleeding (especially intraventricular hemorrhage, IVH) among preterm infants, neonatologists frequently transfuse platelets to non-bleeding neonates when platelet counts fall below an arbitrary limit, typically higher than that used in older children and adults. However, recent studies have shown that prophylactic platelet transfusions not only fail to decrease bleeding in preterm neonates, but are associated with increased neonatal morbidity and mortality. In this review, we will describe the developmental differences in platelet function and primary hemostasis between neonates and adults, and will analyze the implications of these differences to platelet transfusion decisions.

Impact of Reticulated Platelets on Platelet Reactivity in Neonates

Neonatal megakaryopoiesis and platelet turnover form a developmentally unique pattern by generating a pool of newly released reticulated platelets from the bone marrow into the circulation. Reticulated platelets are more reactive and hyperaggregable compared to mature platelets, due to their high residual mRNA content, large size, increased expression of platelet surface receptors, and degranulation. The proportion of reticulated platelets in neonates is higher compared to that in adults. Due to the emergence of an uninhibited platelet subpopulation, the newly formed reticulated platelet pool is inherently hyporesponsive to antiplatelets. An elevated population of reticulated platelets is often associated with increased platelet reactivity and is inversely related to high on-treatment platelet reactivity, which can contribute to ischemia. Measurements of the reticulated platelet subpopulation could be a useful indicator of increased tendency for platelet aggregation. Future research is anticipated to define the distinct functional properties of newly formed reticulated or immature platelets in neonates, as well as determine the impact of enhanced platelet turnover and high residual platelet reactivity on the response to antiplatelet agents.

Device-Induced Hemostatic Disorders in Mechanically Assisted Circulation

Mechanically assisted circulation (MAC) sustains the blood circulation in the body of a patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) or on ventricular assistance with a ventricular assist device (VAD) or on extracorporeal membrane oxygenation (ECMO) with a pump-oxygenator system. While MAC provides short-term (days to weeks) support and long-term (months to years) for the heart and/or lungs, the blood is inevitably exposed to non-physiological shear stress (NPSS) due to mechanical pumping action and in contact with artificial surfaces. NPSS is well known to cause blood damage and functional alterations of blood cells. In this review, we discussed shear-induced platelet adhesion, platelet aggregation, platelet receptor shedding, and platelet apoptosis, shear-induced acquired von Willebrand syndrome (AVWS), shear-induced hemolysis and microparticle formation during MAC. These alterations are associated with perioperative bleeding and thrombotic events, morbidity and mortality, and quality of life in MCS patients. Understanding the mechanism of shear-induce hemostatic disorders will help us develop low-shear-stress devices and select more effective treatments for better clinical outcomes.

Shear-Induced Platelet Activation is Sensitive to Age and Calcium Availability: A Comparison of Adult and Cord Blood

INTRODUCTION

Antiplatelet therapy for neonates and infants is often extrapolated from the adult experience, based on limited observation of agonist-induced neonatal platelet hypoactivity and poor understanding of flow shear-mediated platelet activation. Therefore, thrombotic events due to device-associated disturbed flow are inadequately mitigated in critically ill neonates with indwelling umbilical catheters and infants receiving cardiovascular implants.

METHODS

Whole blood (WB), platelet-rich plasma (PRP), and gel-filtered platelets (GFP) were prepared from umbilical cord and adult blood, and exposed to biochemical agonists or pathological shear stress of 70 dyne/cm2. We evaluated α-granule release, phosphatidylserine (PS) scrambling, and procoagulant response using P-selectin expression, Annexin V binding, and thrombin generation (PAS), respectively. Activation modulation due to depletion of intracellular and extracellular calcium, requisite second messengers, was also examined.

RESULTS

Similar P-selectin expression was observed for sheared adult and cord platelets, with concordant inhibition due to intracellular and extracellular calcium depletion. Sheared cord platelet Annexin V binding and PAS activity was similar to adult values in GFP, but lower in PRP and WB. Annexin V on sheared cord platelets was calcium-independent, with PAS slightly reduced by intracellular calcium depletion.

CONCLUSIONS

Increased PS activity on purified sheared cord platelets suggest that their intrinsic function under pathological flow conditions is suppressed by cell-cell or plasmatic components. Although secretory functions of adult and cord platelets retain comparable calcium-dependence, PS exposure in sheared cord platelets is uniquely calcium-independent and distinct from adults. Identification of calcium-regulated developmental disparities in shear-mediated platelet function may provide novel targets for age-specific antiplatelet therapy.

Developmental Differences in Platelet Inhibition Response to Prostaglandin E1

BACKGROUND

The mechanisms underlying neonatal platelets hyporesponsiveness are not fully understood. While previous studies have demonstrated developmental impairment of agonist-induced platelet activation, differences in inhibitory signaling pathways have been scarcely investigated.

OBJECTIVE

To compare neonatal and adult platelets with regard to inhibition of platelet reactivity by prostaglandin E1 (PGE1).

METHODS

Platelet-rich plasma from umbilical cord (CB) or adult blood was incubated with PGE1 (0-1 μM). We assessed aggregation in response to adenosine diphosphate (ADP), collagen, and thrombin receptor activating peptide as well as cyclic adenosine 3'5'-monophosphate (cAMP) levels (ELISA). Gαs, Gαi2, and total- and phospho-protein kinase A (PKA) were evaluated in adult and CB ultrapure and washed platelets, respectively, by immunoblotting.

RESULTS

Neonatal (vs. adult) platelets display hypersensitivity to inhibition by PGE1 of platelet aggregation induced by ADP and collagen (PGE1 IC50: 14 and 117 nM for ADP and collagen, respectively, vs. 149 and 491 nM in adults). They also show increased basal and PGE1-induced cAMP levels. Mechanistically, PGE1 acts by binding to the prostanoid receptor IP (prostacyclin receptor), which couples to the Gαs protein-adenylate cyclase axis and increases intracellular levels of cAMP. cAMP activates PKA, which phosphorylates different target inhibitor proteins. Neonatal platelets showed higher basal and PGE1-induced cAMP levels, higher Gαs protein expression, and a trend to increased PKA-dependent protein phosphorylation compared to adult platelets.

CONCLUSION

Neonatal platelets have a functionally increased PGE1-cAMP-PKA axis. This finding supports a downregulation of inhibitory when going from neonate to adult contributing to neonatal platelet hyporesponsiveness.