Understanding the underlying mechanisms of the well-substantiated platelet hyporeactivity in neonates is of interest given their implications for the clinical management of newborns, a population at higher bleeding risk than adults (especially sick and preterm infants), as well as for gaining insight into the regulatory mechanisms of platelet biology. Transcriptome analysis is useful in identifying mRNA signatures affecting platelet function. However, human fetal/neonatal platelet transcriptome analysis has never before been reported. We have used mRNA expression array for the first time to compare platelet transcriptome changes during development. Microarray analysis was performed in pure platelet RNA obtained from adult and cord blood, using the same platform in two independent laboratories. A high correlation was obtained between array results for both adult and neonate platelet samples. There was also good agreement between results in our adult samples and outcomes previously reported in three different studies. Gene enrichment analysis showed that immunity- and platelet function-related genes are highly expressed at both developmental stages. Remarkably, 201 genes were found to be differentially expressed throughout development. In particular, neonatal platelets contain higher levels of mRNA that are associated with protein synthesis and processing, while carrying significantly lower levels of genes involved in calcium transport/metabolism and cell signaling (including GNAZ). Overall, our results point to variations in platelet transcriptome as possibly underlining the hypo-functional phenotype of neonatal platelets and provide further support for the role of platelets in cellular immune response. Better characterization of the platelet transcriptome throughout development can contribute to elucidate how transcriptome changes impact different pathological conditions.

Neonatal platelets are hyporeactive and show impaired agonist-induced secretion despite no obvious abnormalities in their granules. Here, we examined, for the first time, the ultrastructure of neonatal and adult platelets following agonist activation. Under resting conditions, neonatal and adult platelets appeared ultrastructurally identical. Following agonist stimulation, however, noticeable degranulation occurred in adult platelets, while granules in neonatal platelets remained clearly visible and apparently unable to centralize or fuse. To investigate the underlying mechanisms, we first examined the expression levels of the main SNARE proteins, which mediate the membrane fusion events required for exocytosis. Neonatal platelets showed significantly reduced levels of syntaxin-11 and its regulator, Munc18b. Since granule centralization depends on contraction of the microtubule ring, we also examined the expression of its main component, β1-tubulin. Noteworthy, we found decreased mRNA and protein levels in neonatal platelets, while and isoforms were overexpressed, partially compensating for that deficiency. Finally, supporting the functional consequences of defective exocytosis, adhesion kinetic assays, performed in plasma-free medium, demonstrated delayed adhesion and spreading of neonatal platelets. This is the first report showing marked reductions of syntaxin-11-Munc18b complex and β1-tubulin in neonatal platelets, indicating that these proteins, required for platelet degranulation, are developmentally regulated.

Exosomes are extracellular vesicles that contain nucleic acids, lipids and metabolites, and play a critical role in health and disease as mediators of intercellular communication. The majority of extracellular vesicles in the blood are platelet-derived. Compared to adults, neonatal platelets are hyporeactive and show impaired granule release, associated with defects in Soluble N-ethylmaleimide-sensitive fusion Attachment protein REceptor (SNARE) proteins. Since these proteins participate in biogenesis of exosomes, we investigated the potential differences between newborn and adult plasma-derived exosomes. Plasma-derived exosomes were isolated by ultracentrifugation of umbilical cord blood from full-term neonates or peripheral blood from adults. Exosome characterization included size determination by transmission electron microscopy and quantitative proteomic analysis. Plasma-derived exosomes from neonates were significantly smaller and contained 65% less protein than those from adults. Remarkably, 131 proteins were found to be differentially expressed, 83 overexpressed and 48 underexpressed in neonatal (vs. adult) exosomes. Whereas the upregulated proteins in plasma exosomes from neonates are associated with platelet activation, coagulation and granule secretion, most of the underexpressed proteins are immunoglobulins. This is the first study showing that exosome size and content change with age. Our findings may contribute to elucidating the potential “developmental hemostatic mismatch risk” associated with transfusions containing plasma exosomes from adults.

<b><i>Background:</i></b> 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. <b><i>Objective:</i></b> To compare neonatal and adult platelets with regard to inhibition of platelet reactivity by prostaglandin E1 (PGE₁). <b><i>Methods:</i></b> 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). G_αs, G_αi2, and total- and phospho-protein kinase A (PKA) were evaluated in adult and CB ultrapure and washed platelets, respectively, by immunoblotting. <b><i>Results:</i></b> 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 G_αs 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 G_αs protein expression, and a trend to increased PKA-dependent protein phosphorylation compared to adult platelets. <b><i>Conclusion:</i></b> 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.