Quantitative Label-Free Mass Spectrometry Identifies Molecular Content and Signaling Differences between Neonatal and Adult Platelets

Observational and randomized studies have associated platelet transfusions with increased morbidity and mortality in preterm neonates. Clinical practice changes have been hindered by a limited understanding of why adult donor-derived platelets might be inappropriate or harmful to infants. Neonatal and adult platelets differ in reactivity and function, but we lack an understanding of underlying molecular differences. As proteins and kinase-driven phosphorylation signaling dictate platelet biology, we aimed to characterize altered protein and phosphoprotein content between neonatal and adult platelets to help explain differences in platelet reactivity and function.

We isolated resting platelets from full term cord blood (n=9) and adult peripheral blood (n=7) and used mass spectrometry-based platforms to ascertain protein and phosphoprotein content. We identified 4745 high-confidence proteins in adult and neonatal platelets, including 331 differentially abundant proteins (≥1.5-fold change, p<0.05). Neonatal platelets had increased metabolic and ribosomal proteins, consistent with transcriptional data. Adult platelets were enriched for inflammatory proteins, including complement components, consistent with pro-inflammatory function.

We also identified 2115 phospho-proteins (17852 unique phosphopeptides) across all samples. These impacted actin cytoskeletal biology, cell adhesion, and GTPase signaling that regulate platelet functions. Kinase enrichment analysis identified FYN, SRC, ABL1, and AKT1 as kinases most responsible for platelet phosphorylation activities.

Granule trafficking and degranulation proteins were among 1183 differentially abundant phosphoproteins in neonatal vs adult samples (p<0.05). For example, 31 different Reticulon 1 (RTN1) phosphopeptides were enriched in adult platelets (p<0.05). RTN1 regulates membrane trafficking and participates in SNARE-mediated exocytosis. Increased RTN1-directed membrane trafficking may promote increased activation and degranulation in adult vs neonatal platelets. Indeed, SNARE complex protein deficiency was previously suggested to mediate hyporeactive degranulation in neonatal platelets.

We detected 2 phosphopeptides in adult samples that were absent from any neonatal sample. We reasoned that these could represent developmental stage-specific differences that exist only in adult platelets. One of the adult-specific phosphopeptides was in Rap1GAP2 (phospho-S588). Rap1GAP2 is a highly phosphorylated GTPase-activating protein that inhibits Rap1, a small guanine-nucleotide-binding protein that facilitates integrin activation in platelets. Rap1GAP2 binds synaptotagmin-like protein 1 (Slp1) to regulate dense granule secretion. These findings may link altered developmentally regulated Rap1GAP2 activities with platelet degranulation. A Platelet Cell Adhesion Molecule (PECAM) phosphopeptide (phospho-S726) was also found exclusively in adult platelets. PECAM facilitates inflammation and leukocyte transendothelial migration. PECAM phosphorylation in adult platelets may reflect an enhanced ability to participate in these processes.

These findings reveal key differences in protein content and signaling that underlie differential neonatal vs adult platelet reactivity and function, including pro-inflammatory activities and enhanced activation and/or degranulation propensity in adult platelets. RTN1-mediated SNARE complex function and Rap1 signaling activities represent novel and potentially modifiable targets to manipulate platelet activation and/or degranulation. Our results also help provide a biological rationale for the higher morbidity and mortality observed in preterm infants transfused with adult platelets, including the inadvertent introduction of serum complement and other pro-inflammatory molecules within transfusions of donated adult platelets.