Page 1878 - Williams Hematology ( PDFDrive )

1852 Part XII: Hemostasis and Thrombosis Chapter 112: Platelet Morphology, Biochemistry, and Function 1853

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a patient with an inherited platelet defect. Platelet RNA-seq was also lysates utilized two-dimensional gel electrophoresis (2D-GE). How-
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used to identify NBEAL2 as causing the gray platelet syndrome. 592 ever, technologic advances using nongel approaches with proteolytic
peptide analyses have largely replaced 2D-GE, and include surface-
Platelet mRNAs Associated with Platelet Traits enhanced laser desorption/ionization (SELDI), isotope-coded affinity
An unbiased genome-wide platelet RNA expression study identified an tags (iCAT), and isotope tags for relative and absolute quantification
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association between expression of PEAR1 with and platelet activation. (iTRAQ). 613,614 These improved technologies have provided an estimate
A similar approach identified 290 differentially expressed transcripts of approximately 20 million protein molecules per platelet and have
between hyperreactive versus hyporeactive platelets. mRNA and pro- updated estimates of the number of detectable different proteins in the
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tein levels of VAMP-8, a critical v-SNARE involved in platelet granule platelet proteome to nearly 5000. Pathway and gene ontology analyses
secretion, were significantly higher in hyperreactive platelets. Another reveal most highly expressed platelet proteins localize to the cytoplasm,
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study identified 63 genes differentially expressed according to platelet with substantial percentages in the membrane or secretome, and
activation by ADP and/or CRP. Two of these genes, COMMD7 and fall into expected functional categories of cytoskeletal rearrangement,
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LRRFIP1, were associated with early-onset MI. The Platelet RNA and membrane trafficking, and intracellular signal transduction. 615
Expression-1 (PRAX1) study phenotyped platelet function and per- Platelet protein levels are regulated by mRNA translation in
formed genome-wide platelet RNA expression profiling on 70 black and megakaryocytes and platelets, uptake of plasma proteins, and protein
84 white subjects. PAR4-mediated platelet aggregation and calcium degradation, 574,617 although the relative contribution of each mechanism
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mobilization were greater in black subjects than white subjects. A novel to the platelet proteome in health and disease is unknown. The dynamic
platelet gene encoding phosphatidylcholine transfer protein (PC-TP) nature of the platelet proteome is illustrated by alterations with disease,
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showed a strong correlation with race and with PAR-4 reactivity and aging, gender, and other environmental factors, as well as differential
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a PC-TP–specific inhibitor blocked PAR-4– but not PAR-1–mediated sorting of proteins between megakaryocytes and platelets. Infectious
platelet aggregation. This finding underscores the genetic basis for inter- agents, such as dengue virus, stimulate blood platelet mRNA translation
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individual variation in platelet function and the potential need to con- into protein. Posttranslational modifications of platelet proteins, such
sider race and genetic factors when treating patients with anti-platelet as phosphorylation, have critical effects on platelet activation. Plate-
therapies. lets from healthy individuals exhibit marked interindividual variation
in function, and unbiased genome-wide approaches have identified
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Platelet Noncoding RNAs variation in proteins regulating the corresponding function. Compo-
The best studied of the noncoding RNAs are microRNAs (miRNAs), nents of protein ubiquitination and degradation have been identified in
which regulate expression of more than 60 percent of protein cod- platelets, but their function is poorly understood.
ing genes. 597,598 Human platelets express approximately 200 annotated
miRNAs, some of which are differentially expressed according to platelet Cataloging the Platelet Proteome
reactivity and may predict platelet responsiveness to activation, and Most platelet proteomic analyses to date have studied platelets from
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some of which are differentially expressed by age, gender, and race. 582,596 small numbers of healthy donors. Analyses of resting whole platelets
Indirect evidence indicates strong correlations between megakaryo- have provided global protein profiles. 612,620 Fractionation of platelet
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cyte and platelet miRNA levels. miR-155 maintains megakaryocyte lysates has been used to assess the α granule, dense granule, and
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progenitors in an undifferentiated state, whereas miR-150 and miR- membrane proteomes. 623,624 Proteins with posttranslational modifica-
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125b-2 drive megakaryocyte differentiation. 602,603 Loss of expression of tions have been identified for phosphorylation, 625,626 palmitoylation,
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miR-145 in the 5q– syndrome leads to an increase in the megakaryocyte and glycosylation. After platelet activation, hundreds of proteins have
Fli-1 transcription factor, thus enhancing megakaryocyte production. 604 been identified in releasates (secretomes) 629,630 and microparticles. 631,632
Platelet miRNA profiles are more stable than mRNA profiles and
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are useful as biomarkers. Levels of miR-26b and miR-28 are associ- Platelet Proteome Association Studies
ated with myeloproliferative neoplasms, 605,606 whereas levels of miR-10a, Platelet proteome-wide analyses were used to identify NBEAL2 as
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miR-148a, and miR-490–5p discriminate ET from secondary thrombo- the gene responsible for the gray platelet syndrome and to unravel
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cytosis. Specific sets of platelet miRNAs have associated with MI. 608,609 the molecular basis of the Quebec platelet disorder. Differentially
Antiplatelet therapies alter platelet miRNA levels. 610,611 Relationships expressed platelet proteins involved in integrin α β signaling were
IIb 3
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between platelet miRNAs, mRNAs, and physiology in the same subjects observed in the myelodysplastic syndrome. Proteomic approaches
permit prediction of miRNA function and discovery of novel platelet have consistently identified platelet septin and actin as increasing over
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genes. This approach identified PRKAR2B as associated with plate- time in storage. 635–637 A small study suggested platelet protein post-
let reactivity, and a functional effect was confirmed in murine platelets translational modifications may be associated with acute coronary
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lacking Prkar2b. A similar approach was used to demonstrate platelet syndromes. 638
miR-376c levels were higher in white subjects compared to black sub-
jects and these levels correlated with PCTP mRNA, PC-TP protein and Relationship Between Platelet Proteome and Transcriptome
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platelet PAR-4 reactivity. miR-376c directly targets the PCTP 3′UTR Transcriptomic approaches have identified about twice as many genes
and represses its expression. 596 expressed in platelets as have proteomic approaches, primarily because
the former has greater sensitivity. Correlations between 10 platelet
RNA-seqs and the most quantitatively robust proteomic analyses to
PROTEOMICS date has been reported. Most (87.8 percent) proteins had a detect-
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Disease pathophysiology is dictated by the effects of proteins, including able corresponding mRNA and the relative abundances showed a sig-
their levels, structures and posttranslational modifications. Cataloging nificantly positive, albeit weak, correlation. Platelet proteins that lack a
platelet proteomes in health and disease and under different activation corresponding mRNA are likely to be taken up from plasma rather than
states provides information not achievable from genomics or transcrip- being synthesized in megakaryocytes, and include fibrinogen, albumin,
tomics, including protein isoforms, localization, stoichiometry, and and immunoglobulins, all of which were suspected to fall into this cate-
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posttranslational modifications. Early proteome-wide studies of platelet gory based on other studies. Platelet mRNAs that lack a corresponding
Kaushansky_chapter 112_p1829-1914.indd 1853 17/09/15 3:27 pm