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

Abstract

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 G12/13 signaling pathway on dense granule secretion via RhoA. We comparatively analyzed secretion profiles using flow cytometry and expression levels of Gq, Gi, and G12/13 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. Gα12/13 expression was significantly decreased in neonatal platelets and correlated with lower Rho activation after thrombin stimulation. We conclude that lower expression of G12/13 in neonatal platelets results in attenuated activation of Rho and may contribute to reduced secretion of dense granules after exposure to thrombin.

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

“… 31 , 33 , 34 , 35 More recent studies have shown distinct platelet hyporeactivity upon immunoreceptor and GPCR stimulation 36 , 37 and have discussed reduced G 12 /G 13 expression as a causative factor for reduced δ-granule secretion. 38 These controversial findings could be explained by the heterogeneous study design regarding readouts (granule release, GPIIb/IIIa activation, aggregometry, and electron microscopy), the material used (peripheral vs umbilical blood), time points, or the respective control group (adults vs infants). Comprehensive studies comprising a continuum of preterm and term neonates or numerous agonists and activation markers are still very rare, owing to limited biomaterial.…”

“… 34 In addition, altered expression of G protein subunits G α12/13 and G αi2 might interfere with the responsiveness to classical agonists such as collagen or thrombin, leading to an increased basal cAMP level in neonatal platelets, which might contribute to a “hyporesponsive” platelet phenotype. 38 The diminished response to TRAP-6 might also be a consequence of the reduced expression; a future study might elaborate on thrombin responsiveness with more concentrations and including a PAR4 peptide. (4) Because of the limited blood samples available from preterm and term neonates, we decided to select concentrations according to the current (inter)national guidelines.…”