Comment on Schulze et al, page 3868
Schulze and colleagues have used novel mice and elegant fluorescent microscopy to demonstrate that the megakaryocyte membrane demarcation system provides a reservoir for the extensive lipid membrane required for platelet biogenesis.
In 1906, James Wright1 published a morphology study in which he argued that circulating platelets (then called plates) are derived from megakaryocytes, and that megakaryocytes release platelets from elongated, cytoplasmic projections that extend into the marrow sinusoids. One hundred years later, the mysteries of the megakaryocyte and platelet biogenesis are beginning to be revealed.
Following a series of mitotic cell divisions, megakaryocyte lineage cells enter an endomitotic cycle that generates large, polyploidy megakaryocytes. Cytoplasmic maturation at the end of the endomitotic phase includes the development of the demarcation membrane system (DMS).
The DMS, first described by Yamada in 1957,2 is an extensive membrane network located throughout the cytoplasm of the mature megakaryocyte, except for a thin, uninvolved cortical zone. Staining studies have shown that the DMS is contiguous with the extracellular space, making it effectively an extensive invagination of surface membrane, but its function remains unclear. One early popular theory, which is reflected in its name, is that the DMS “marks” preformed “platelet territories” within the megakaryocyte cytoplasm, which in turn led to the cytoplasmic fragmentation hypothesis for platelet formation. However, this is incompatible with proplatelet formation as the mechanism of platelet biogenesis.
Using novel knock-in mice in which the fluorescent protein EYFP is introduced into the CD41 (integrin αIIb) locus, Schulze and colleagues provide evidence that the DMS serves as an extensive membrane reservoir to support the explosive proplatelet formation that marks the final hours of the mature megakaryocyte.3 Key to their studies is a C-terminal myristoylation site that incorporates the EYFP reporter into cellular membranes. The EYFP signal localizes to the outer cell membrane of early megakaryocyte lineage cells, but it becomes diffusely localized throughout the cytoplasm in mature megakaryocytes in a pattern that mirrors the DMS. The EYFP-expressing internal membranes were found to be contiguous with the full outline of emerging proplatelets, and platelets circulating in the EYFP mice have surface EYFP fluorescence, data compatible with the DMS providing an extensive membrane reservoir for proplatelet formation.
Using fluorescently tagged plextrin homology domain from PLCδ1 as a probe, the authors demonstrate that the phospholipid PI-4,5-P2, which typically resides in the plasma membrane of cells, localizes to the DMS in mature megakaryocytes. Small-interfering RNAs suggest that the accumulation of PI-4,5-P2 on the DMS membranes, along with DMS development and the associated increase in cell size, may all be facilitated by the lipid kinase PIP4Kα. The authors also suggest that PI-4,5-P2 signaling from the DMS may trigger actin polymerization via the WASp/WAVE pathway.
Reports over the past 5 years have significantly advanced our understanding of the structural and mechanical aspects of proplatelet formation.4,5 Schulze and colleagues provide initial insights into the formation and function of the DMS, including its role in platelet biogenesis, furthering our understanding of this most mysterious cell, the megakaryocyte. Dr Wright should be pleased, but I suspect he is wondering what took so long. ▪
