Human blood platelets are essential to maintaining normal hemostasis, and platelet dysfunction often causes bleeding or thrombosis. Estimates of genome-wide platelet RNA expression using microarrays have provided insights to the platelet transcriptome but were limited by the number of known transcripts. The goal of this effort was to deep-sequence RNA from leukocyte-depleted platelets to capture the complex profile of all expressed transcripts.

Significance: MicroRNAs (miRNAs) are small ∼22-nt RNAs that are important regulators of posttranscriptional gene expression. Since their initial discovery, they have been shown to be involved in many cellular processes, and their misexpression is associated with disease etiology. Currently, nearly 2,800 human miRNAs are annotated in public repositories. A key question in miRNA research is how many miRNAs are harbored by the human genome. To answer this question, we examined 1,323 short RNA sequence samples and identified 3,707 novel miRNAs, many of which are human-specific and tissue-specific. Our findings suggest that the human genome expresses a greater number of miRNAs than has previously been appreciated and that many more miRNA molecules may play key roles in disease etiology.

Key Points: Unique dataset of human platelet mRNA, miRNA, and physiology reveals mRNAs and miRNAs that differ by age and gender. Interactive public web tool (www.plateletomics.com) provides biologic insights into platelet function and gene expression.

Significance: Immune complexes (ICs) form when antibodies encounter their antigens. ICs are present in blood in multiple pathological conditions. Given the abundance of platelets in blood and that they express a receptor for ICs, called Fcγ receptor IIA (FcγRIIA), we examined the impact of ICs in blood in a mouse model. We found that circulating ICs induced systemic shock, characterized by loss of consciousness, by activating platelet FcγRIIA. Shock was mediated by the liberation of serotonin, a molecule better known for its role in the brain, from platelet granules. During shock, platelets were sequestered in the lungs and brain and returned to the blood circulation after their degranulation. Platelets are thus crucial in response to ICs.

Heparin-induced thrombocytopenia/thrombosis (HIT/HITT) is a severe, life-threatening complication that occurs in 1% to 3% of patients exposed to heparin. Interactions between heparin, human platelet factor 4 (hPF4), antibodies to the hPF4/heparin complex, and the platelet Fc receptor (FcR) for immunoglobulin G, FcγRIIA, are the proposed primary determinants of the disease on the basis of in vitro studies. The goal of this study was to create a mouse model that recapitulates the disease process in humans in order to understand the factors that predispose some patients to develop thrombocytopenia and thrombosis and to investigate new therapeutic approaches. Mice that express both human platelet FcγRIIA and hPF4 were generated. The FcγRIIA/hPF4 mice and controls, transgenic for either FcγRIIA or hPF4, were injected with KKO, a mouse monoclonal antibody specific for hPF4/heparin complexes, and then received heparin (20 U/d). Nadir platelet counts for KKO/heparin–treated FcγRIIA/hPF4 mice were 80% below baseline values, significantly different (P < .001) from similarly treated controls. FcγRIIA/hPF4 mice injected with KKO and 50 U/d heparin developed shock and showed fibrin-rich thrombi in multiple organs, including thrombosis in the pulmonary vasculature. This is the first mouse model of HIT to recapitulate the salient features of the human disease and demonstrates that FcγRIIA and hPF4 are both necessary and sufficient to replicate HIT/HITT in an animal model. This model should facilitate the identification of factors that modulate disease expression and the testing of novel therapeutic interventions.

Objective. Thrombocytopenia is commonly observed in very low birth weight (VLBW) neonates with sepsis. Specific platelet responses to different infectious agents have not been extensively characterized. The objectives of this study were to examine platelet counts and platelet indices in preterm neonates with culture-proven sepsis to determine if there are organism-specific platelet responses. Study Design. We analyzed a cohort of all VLBW neonates (birthweight <1500 g) born over a 4-year period and admitted to a single level III neonatal intensive care unit (N = 943). Thrombocytopenia was defined as a platelet count <100 000/mm³. Platelet count, nadir, duration of thrombocytopenia, and mean platelet volume (MPV) were examined during episodes of culture-proven sepsis. Analysis of variance, Kruskal-Wallis, Mann-Whitney U, and χ² tests were used to compare groups, and data are expressed as mean ± standard deviation. Results. Sepsis was diagnosed in 154 (16%) of 943 patients in the study population. Of the sepsis episodes, 54% were associated with thrombocytopenia and 61% with an elevation in MPV. Infections were grouped by organism type: Gram-positive bacteria (117/154, 76%), Gram-negative bacteria (24/154, 16%), and fungi (13/154, 8%). When compared with patients with Gram-positive sepsis, those with Gram-negative or fungal sepsis had a significantly lower initial platelet count, a lower platelet nadir, a higher incidence of thrombocytopenia, and a greater duration of thrombocytopenia. The decrease in platelet count from baseline was also significantly less in the Gram-positive infections than in the fungal infections. Although there was an overall increase in MPV from baseline, there were no differences between groups. Conclusions. In our population of VLBW infants, sepsis is frequently associated with thrombocytopenia and an elevation in MPV. However, fungal and Gram-negative pathogens are associated with a lower platelet count and more prolonged thrombocytopenia compared with Gram-positive pathogens. We conclude that common pathogens causing sepsis have different effects on platelet kinetics.