Neonatal expression of RNA-binding protein IGF2BP3 regulates the human fetal-adult megakaryocyte transition
Kamaleldin E. Elagib, Chih-Huan Lu, Goar Mosoyan, Shadi Khalil, Ewelina Zasadzińska, Daniel R. Foltz, Peter Balogh, Alejandro A. Gru, Deborah A. Fuchs, Lisa M. Rimsza, Els Verhoeyen, Miriam Sansó, Robert P. Fisher, Camelia Iancu-Rubin, Adam N. Goldfarb
Kamaleldin E. Elagib, Chih-Huan Lu, Goar Mosoyan, Shadi Khalil, Ewelina Zasadzińska, Daniel R. Foltz, Peter Balogh, Alejandro A. Gru, Deborah A. Fuchs, Lisa M. Rimsza, Els Verhoeyen, Miriam Sansó, Robert P. Fisher, Camelia Iancu-Rubin, Adam N. Goldfarb
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Research Article Hematology

Neonatal expression of RNA-binding protein IGF2BP3 regulates the human fetal-adult megakaryocyte transition

Abstract

Hematopoietic transitions that accompany fetal development, such as erythroid globin chain switching, play important roles in normal physiology and disease development. In the megakaryocyte lineage, human fetal progenitors do not execute the adult morphogenesis program of enlargement, polyploidization, and proplatelet formation. Although these defects decline with gestational stage, they remain sufficiently severe at birth to predispose newborns to thrombocytopenia. These defects may also contribute to inferior platelet recovery after cord blood stem cell transplantation and may underlie inefficient platelet production by megakaryocytes derived from pluripotent stem cells. In this study, comparison of neonatal versus adult human progenitors has identified a blockade in the specialized positive transcription elongation factor b (P-TEFb) activation mechanism that is known to drive adult megakaryocyte morphogenesis. This blockade resulted from neonatal-specific expression of an oncofetal RNA-binding protein, IGF2BP3, which prevented the destabilization of the nuclear RNA 7SK, a process normally associated with adult megakaryocytic P-TEFb activation. Knockdown of IGF2BP3 sufficed to confer both phenotypic and molecular features of adult-type cells on neonatal megakaryocytes. Pharmacologic inhibition of IGF2BP3 expression via bromodomain and extraterminal domain (BET) inhibition also elicited adult features in neonatal megakaryocytes. These results identify IGF2BP3 as a human ontogenic master switch that restricts megakaryocyte development by modulating a lineage-specific P-TEFb activation mechanism, revealing potential strategies toward enhancing platelet production.

Authors

Kamaleldin E. Elagib, Chih-Huan Lu, Goar Mosoyan, Shadi Khalil, Ewelina Zasadzińska, Daniel R. Foltz, Peter Balogh, Alejandro A. Gru, Deborah A. Fuchs, Lisa M. Rimsza, Els Verhoeyen, Miriam Sansó, Robert P. Fisher, Camelia Iancu-Rubin, Adam N. Goldfarb

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Figure 8

BET factor inhibition in neonatal megakaryocytes elicits adult molecular features.

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BET factor inhibition in neonatal megakaryocytes elicits adult molecular...
(A and B) BET factor inhibition in neonatal megakaryocytes downregulates IGF2BP3 and elicits adult-like molecular features. Whole cell lysates of cells treated with BET inhibitors as in Figure 7A were immunoblotted for indicated factors as in Figure 2. Arrows in A show RNAPII0 and IIA isoforms. Arrows in B show intact FLNA and the approximately 190-kDa cleavage fragment. Graphs show mean ± SEM for densitometry values from 3 independent experiments, with signal normalization as in Figure 2A. *P < 0.05; **P < 0.01; ***P < 0.005, t test. (C) Model of ontogenic regulation of megakaryocyte morphogenesis. In adult megakaryopoiesis (left arrow), downregulation of LARP7 and proteolysis of MePCE destabilize 7SK snRNA, leading to unopposed P-TEFb activation. This mode of P-TEFb activation promotes upregulation of megakaryocyte morphogenesis factors, most notably MKL1, as well as upregulation of HEXIM1 and lineage consolidation via erythroid repression. In fetal megakaryopoiesis (right arrow), expression of IGF2BP3 stabilizes 7SK snRNA despite downregulation of LARP7 and MePCE. Persistence of 7SK allows for feedback inhibition of P-TEFb, dampening both the upregulation of megakaryocyte morphogenesis factors such as MKL1 and lineage consolidation via erythroid repression. See also Supplemental Figure 11.