Abstract
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 TUBB1 mRNA and protein levels in neonatal platelets, while TUBB2A and TUBB 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.
Keywords
platelet physiology - secretion/exocytosis - gene regulation - perinatal haemostasis
