Don't you forget about me(gakaryocytes)

Abstract

Platelets (small, anucleate cell fragments) derive from large precursor cells, megakaryocytes (MKs), that reside in the bone marrow. MKs emerge from hematopoietic stem cells in a complex differentiation process that involves cytoplasmic maturation, including the formation of the demarcation membrane system, and polyploidization. The main function of MKs is the generation of platelets, which predominantly occurs through the release of long, microtubule-rich proplatelets into vessel sinusoids. However, the idea of a 1-dimensional role of MKs as platelet precursors is currently being questioned because of advances in high-resolution microscopy and single-cell omics. On the one hand, recent findings suggest that proplatelet formation from bone marrow-derived MKs is not the only mechanism of platelet production, but that it may also occur through budding of the plasma membrane and in distant organs such as lung or liver. On the other hand, novel evidence suggests that MKs not only maintain physiological platelet levels but further contribute to bone marrow homeostasis through the release of extracellular vesicles or cytokines, such as transforming growth factor β1 or platelet factor 4. The notion of multitasking MKs was reinforced in recent studies by using single-cell RNA sequencing approaches on MKs derived from adult and fetal bone marrow and lungs, leading to the identification of different MK subsets that appeared to exhibit immunomodulatory or secretory roles. In the following article, novel insights into the mechanisms leading to proplatelet formation in vitro and in vivo will be reviewed and the hypothesis of MKs as immunoregulatory cells will be critically discussed.

Graphical abstract

Figure 1.

Cellular heterogeneity of MKs within the BM and of MKs present in the lung. MK subgroups within the BM. (A) Proplatelet-forming MKs close to the sinuous blood vessels are high in expression of Tubb1, Vwf, and MYH9. (B) Immunoregulatory MK subpopulation presenting MHC-II and CD18, which are responsible for the interaction with lymphocytes and neutrophils, respectively. Immunoregulatory MKs are high in expression of Chil1, Ctss, and CCL3, and secrete cytokines. (C) MKs, with suggested hematopoietic stem cell niche and osteogenic niche supporting a role evident in the high expression of Tgfb1, Egf1, COL1A3. (D) Subset of actively cycling MKs expressing Pola2, Pold1, and CCND1. (E) The lung contains MKs with an immunoregulatory phenotype expressing Ccr7, Hla-Drb1, and Icam1, in addition to intravascular MKs.

Figure 2.

Intracellular mechanisms of proplatelet formation and elongation. (A) Transmission electron micrograph of a BM MK in contact with an endothelial cell. MK is outlined in black. Arrowspoint towards other BM cells interacting with the MK. Scale bar, 3 µm. (B) Platelet production from MKs is enabled through extensive cytoskeletal rearrangements leading to breaching of the endothelium through the formation of podosomes and proplatelet generation, during which proplatelets appear like “beads on a string” and function as essential intermediate structures. Initial polarization of the DMS is dependent on actin dynamics enabled by the Rho GTPase Cdc42, which induces LIMK1 activity and activates downstream effectors such as Cof1 and Twf1 and is further indispensable to enable proplatelet formation. In addition to Cdc42 and PDK1, activity of another Rho GTPase, RhoA, which induces MLC2/NMIIA activation, is critical in enabling F-actin rearrangements including the Arp2/3-dependent formation of podosomes, which are crucial to breach the endothelium, most likely though the secretion of MMPs. Proplatelet elongation on the other hand relies on microtubule sliding within the proplatelet shaft,, which is dependent on the motor protein dynein. Cytoskeletal cross talk is enabled through proteins linking the F-actin to the microtubule cytoskeleton, such as EBs, APC, and mDia1.