Fetal vs adult megakaryopoiesis

Abstract

Fetal and neonatal megakaryocyte progenitors are hyperproliferative compared with adult progenitors and generate a large number of small, low-ploidy megakaryocytes. Historically, these developmental differences have been interpreted as "immaturity." However, more recent studies have demonstrated that the small, low-ploidy fetal and neonatal megakaryocytes have all the characteristics of adult polyploid megakaryocytes, including the presence of granules, a well-developed demarcation membrane system, and proplatelet formation. Thus, rather than immaturity, the features of fetal and neonatal megakaryopoiesis reflect a developmentally unique uncoupling of proliferation, polyploidization, and cytoplasmic maturation, which allows fetuses and neonates to populate their rapidly expanding bone marrow and blood volume. At the molecular level, the features of fetal and neonatal megakaryopoiesis are the result of a complex interplay of developmentally regulated pathways and environmental signals from the different hematopoietic niches. Over the past few years, studies have challenged traditional paradigms about the origin of the megakaryocyte lineage in both fetal and adult life, and the application of single-cell RNA sequencing has led to a better characterization of embryonic, fetal, and adult megakaryocytes. In particular, a growing body of data suggests that at all stages of development, the various functions of megakaryocytes are not fulfilled by the megakaryocyte population as a whole, but rather by distinct megakaryocyte subpopulations with dedicated roles. Finally, recent studies have provided novel insights into the mechanisms underlying developmental disorders of megakaryopoiesis, which either uniquely affect fetuses and neonates or have different clinical presentations in neonatal compared with adult life.

Graphical abstract

Figure 1.

Schematic representation of the traditional and current models of hematopoiesis and developmental changes in progenitor composition. (A) In the traditional model of hematopoiesis, the initial separation segregated myelopoiesis and lymphopoiesis, and MKs emerged exclusively from MEPs. (B) Current literature supports a model in which the initial step separates MK-E/myeloid and lymphomyeloid progenitors, and MKs arise directly from the HSC compartment, as well as from classically defined MEPs (adapted from Psaila and Mead²⁸). (C) There is a gradual decrease in the ratio of multipotent to unipotent progenitors in the course of human development. FL CD34⁺ cells give rise to multipotent, oligopotent, and unipotent progenitors, whereas adult BM CD34⁺ cells primarily generate multipotent and unipotent progenitors, with a striking paucity of oligopotent progenitors. CLP, common lymphoid progenitor; CMP, common myeloid progenitor; EoMP, eosinophil-basophil-mast cell progenitor; Ery, erythroid cells; GMP, granulocyte-monocyte progenitor; Gran, granulocytes; LMPP, lymphomyeloid progenitors; Ly, lymphocytes; Mono, monocytes; MPP, multipotent progenitor; MyeP, myeloid progenitor. Professional illustration by Somersault 18:24.

Figure 2.

Key features of neonatal megakaryopoiesis. (A) Hematopoietic progenitors from full-term CB or adult PB were cultured in a collagen-based semisolid culture medium (Megacult; StemCell Technologies, Burnaby BC, Canada) in the presence of TPO only. MK colonies generated from CB progenitors were significantly larger than those generated from adult PB. Photomicrographs were taken at a magnification of 200x. (B) CD34⁺ cells from CB and PB were cultured in a serum-free liquid culture medium with 50 ng/mL of recombinant human TPO as the only growth factor. MKs were evaluated after a 14-day culture period. Representative photomicrographs and ploidy levels by flow cytometry of CB- and PB-MKs demonstrated the smaller size and lower ploidy levels of CB-MKs. Both pictures were taken at a magnification of ×600. (C) Despite their small size, cultured CB-MKs exhibit abundant α-granules containing von Willebrand factor and P-selectin. (D) Flow sorted 2N/4N CB-MKs (left) contain abundant granules (Gr) and a well-developed demarcation membrane system (DMS), consistent with mature MKs and similar to flow-sorted PB MKs with ploidy ≥8N (right). Adapted from Liu et al and Davenport et al.

Figure 3.

Model of ontogenic regulation of MK morphogenesis. In most cells, most P-TEFb is sequestered in an inactive state within the 7SK small nuclear ribonucleoprotein (snRNP) complex. 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 MK morphogenesis factors and repression of erythroid markers. In fetal megakaryopoiesis (right arrow), IGF2BP3 stabilizes 7SK snRNA despite the downregulation of LARP7 and proteolysis of MePCE. Persistence of 7SK allows for inhibition of P-TEFb, dampening both the upregulation of MK morphogenesis factors and lineage consolidation via erythroid repression. Professional illustration by Somersault 18:24.

Figure 4.

Schematic of the combined effects of trisomy 21 and GATA-1s mutations on the pathogenesis of DS-TMD (preleukemia) and the progression to leukemia. Left panel, in normal FL HSPCs, the transcription factor ARID3A functionally cooperates with full length GATA 1 to induce normal MK proliferation and differentiation. Right panel, in FL HSCs isolated from fetuses with trisomy 21 (T21), 3 chromosome 21 miRNAs are upregulated: miR-99a, miR-155, and miR-125b. The elevated miR125b levels posttranscriptionally repress ARID3A. The combination of low ARID3A levels and GATA-1s in T21 fetuses carrying GATA1s mutations leads to MK hyperproliferation and maturational arrest, which characterize DS-TMD (preleukemia). The progression from preleukemia to leukemia requires additional mutations, most frequently in the cohesin genes STAG2, RAD21, and NIPBL. CD117 (KIT) is a marker of the cells that mediate the propagation of GATA1s-induced preleukemia and GATA1s/STAG2ko-induced leukemia. ARID3A and CD117 (KIT) are potential therapeutic targets for DS-TMD and to avoid the progression to leukemia. Professional illustration by Somersault 18:24.