Age-Dependent Control of Collagen-Dependent Platelet Responses by Thrombospondin-1-Comparative Analysis of Platelets from Neonates, Children, Adolescents, and Adults

Comparison of platelet proteomic profiles between children and adults reveals origins of functional differences

BACKGROUND

Platelets are blood cells responsible for the prevention of blood loss upon vessel wall disruption. It has been demonstrated that platelet functioning differs significantly between adult and pediatric donors. This study aimed to identify potential differences between the protein composition of platelets of pediatric, adolescent, and adult donors.

METHODS

Platelet functional testing was conducted with live cell flow cytometry. Using a straightforward approach to platelet washing based on the sequential platelets centrifugation-resuspension, we were able to obtain stable and robust proteomics results, which corresponded to previously published data.

RESULTS

We have identified that pediatric donors' platelets have increased amounts of proteins, responsible for mitochondrial activity, proteasome activity, and vesicle transport. Flow cytometry analysis of platelet intracellular signaling and functional responses revealed that platelets of the pediatric donors have diminished granule secretion and increased quiescent platelet calcium concentration and decreased calcium mobilization in response to ADP. We could explain the observed changes in calcium responses by the increased mitochondria protein content, and the changes in granule secretion could be explained by the differences in vesicle transport protein content.

CONCLUSIONS

Therefore, we can conclude that the age-dependence of platelet functional responses originates from the difference in platelet protein content.

IMPACT

Platelets of infants are known to functionally differ from the platelet of adult donors, although the longevity and persistivity of these differences are debatable. Pediatric donor platelets have enhanced amounts of mitochondrial, proteasomal, and vesicle transport proteins. Platelets of the pediatric donors had increased cytosolic calcium in the resting state, what is explained by the increased numbers of mitochondrial proteins. Infants had decreased platelet granule release, which resolved upon adolescence. Thus, platelets of the infants should be assessed differently from adult platelets. Differences in platelet proteomic contents persisted in adolescent groups, yet, no significant differences in platelet function were observed.

Platelet function in neonates and children

Platelets are major regulators of haemostasis and coagulation. The primary role of platelets in coagulation is to form a stable clot and stop bleeding. Studies of platelet phenotype and function in neonates and children have been restricted by the large volumes required for many common platelet function tests such as platelet aggregometry. Developmental changes in platelets have not been as well described as developmental changes in plasma coagulation proteins, and overall, platelet phenotype and function in neonates and children has been understudied when compared to adults. Recent developments in more sensitive platelet function testing methods requiring smaller blood volumes such as flow cytometry has enabled recent studies to further investigate platelet phenotype and function in neonates and children. In this review we will provide an overview of recent advances from the past five years in platelets in the context of developmental haemostasis, as well as the role of platelets in neonatal paediatric disease.

Ontogenesis of functional platelet subpopulations from preterm and term neonates to adulthood: The PLINIUS study

Erythrocytes undergo a well-defined switch from fetal to postnatal circulation, which is mainly reflected by the stage-specific expression of hemoglobin chains. Perinatal alterations in thrombopoiesis are poorly understood. We assessed the ontogenesis of platelet phenotype and function from early prematurity to adulthood. We recruited 64 subjects comprising 7 extremely preterm (27-31 weeks gestational age), 25 moderately preterm (32-36 weeks), 10 term neonates, 8 infants (<2 years), 5 children (2-13 years), and 9 adults (>13 years). Blood was withdrawn at up to 3 different time points in neonates (t1: 0-2, t2: 3-7, and t3: 8-14 days after birth). We found that the expression levels of the major surface receptors for fibrinogen, collagen, vWF, fibronectin, and laminin were reduced but correlated with decreased platelet size, indicating a normal surface density. Although CD62P and CD63 surface exposure upon stimulation with TRAP-6, ADP, or U46619 was unaltered or only slightly reduced in neonates, GPIIb/IIIa inside-out and outside-in activation was blunted but showed a continuous increase until adulthood, correlating with the expression of the GPIIb/IIIa regulating tetraspanin CD151. Platelet subpopulation analysis using automated clustering revealed that neonates presented with a CD63+/PAC-1- pattern, followed by a continuous increase in CD63+/PAC-1+ platelets until adulthood. Our findings revealed that the number of platelet-monocyte and platelet-neutrophil aggregates, but not platelet-lymphocyte aggregates, is increased in neonates and that neonatal aggregate formation depends in part on CD62P activation. Our PLatelets In Neonatal Infants Study (PLINIUS) provides several lines of evidence that the platelet phenotype and function evolve continuously from neonates to adulthood.

Circulating LPS from gut microbiota leverages stenosis-induced deep vein thrombosis in mice

OBJECTIVE AND DESIGN

An accumulating body of evidence has shown that gut microbiota is involved in regulating inflammation; however, it remains undetermined if and how gut microbiota plays an important role in modulating deep venous thrombosis (DVT), which is an inflammation-involved thrombotic event.

SUBJECTS

Mice under different treatments were used in this study.

METHODS AND TREATMENT

We induced stenosis DVT in mice by partially ligating the inferior vena cava. Mice were treated with antibiotics, prebiotics, probiotics, or inflammatory reagents to modulate inflammatory states, and their effects on the levels of circulating LPS and DVT were examined.

RESULTS

Antibiotic-treated mice or germ-free mice exhibited compromised DVT. Treatment of mice with either prebiotics or probiotics effectively suppressed DVT, which was accompanied with the downregulation of circulating LPS. Restoration of circulating LPS in these mice with a low dose of LPS was able to restore DVT. LPS-induced DVT was blocked by a TLR4 antagonist. By performing proteomic analysis, we identified TSP1 as one of the downstream effectors of circulating LPS in DVT.

CONCLUSION

These results suggest that gut microbiota may play a nonnegligible role in modulating DVT by leveraging the levels of LPS in circulation, thus shedding light on the development of gut microbiota-based strategies for preventing and treating DVT.

Neonatal Platelets: Lower G12/13 Expression Contributes to Reduced Secretion of Dense Granules

Despite fully functional primary hemostasis, platelets of healthy neonates exhibit hypoaggregability and secretion defects, which may be adaptations to specific requirements in this developmental stage. The etiologies for reduced signal transduction vary with the type of agonist. The discovered peculiarities are lower receptor densities, reduced calcium mobilization, and functional impairments of G proteins. Reduced secretion of dense granules has been attributed to lower numbers of granules. Signaling studies with adult platelets have shown a regulating effect of the G_12/13 signaling pathway on dense granule secretion via RhoA. We comparatively analyzed secretion profiles using flow cytometry and expression levels of G_q, G_i, and G_12/13 using Western blot analysis in platelets from cord blood and adults. Furthermore, we evaluated Rho activation after in vitro platelet stimulation with thrombin using a pulldown assay. We observed a markedly reduced expression of the dense granule marker CD63 on neonatal platelets after thrombin stimulation. G_α12/13 expression was significantly decreased in neonatal platelets and correlated with lower Rho activation after thrombin stimulation. We conclude that lower expression of G_12/13 in neonatal platelets results in attenuated activation of Rho and may contribute to reduced secretion of dense granules after exposure to thrombin.

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.

Healthy pediatric platelets are moderately hyporeactive in comparison with adults' platelets

Studies on platelet function in children older than neonatal period are few and their results are controversial. The pediatric platelets were alternatively reported to be more active or less active than adults' ones. We compared platelet function in the several age groups of children to adults and evaluated the age when platelet function reaches the adults' status. The study included 76 healthy children and 49 healthy adult volunteers. Types of platelet activation used included: collagen-related peptide (CRP) and PAR-1 activating peptide SFLLRN; SFLLRN, PAR-4 activating peptide AYPGKF and adenosine diphosphate (ADP); ADP. The parameters determined included forward (FSC) and side scatter (SSC), CD42b, CD61, CD62P, PAC-1, annexin V binding and mepacrine release levels. Resting pediatric platelets were similar to adults' platelets except for 1.2-fold decreased FSC and dense granules volume in youngest children, and 2.5-fold increased annexin V level in children aged 1-10 years. After CRP+SFLLRN stimulation, pediatric platelets had a 1.2-fold lower alpha- and 1.1-fold lower dense granule release than adults. For SFLLRN+AYPGKF+ADP stimulation, this was observed only for youngest children. The response to ADP stimulation was identical for pediatric platelets and adults. Pediatric platelets have lower granular release than adults' platelets, which persists until the age of 18.