New roads to a megakaryocyte inner territory

In this issue of Blood, Eckly et al describe megakaryocyte inner membranes’ origin and territories, implying an additional role they might play.¹ 

Intracellular demarcation membranes have been long recognized in mature megakaryocytes and proposed as a reservoir for platelet biogenesis. A convoluted internal system of membranes within the megakaryocyte, termed demarcation membranes, was observed decades ago, using electron microscopy.²  An early study,³  as well as subsequent ones, proposed that this system forms the plasma membrane of newly generated platelets^2,3  and that thrombopoietin promotes the development of these membranes.⁴  Live cell imaging of mature megakaryocytes showed that the intracellular demarcation membranes extend from peripheral plasma membranes.⁵  However, it has not been clear whether the demarcation membrane system is formed intracellularly and then directed to the plasma membrane or whether it is first delivered to the plasma membrane, followed by rapid invagination. Considering these possible mechanisms, the demarcation membrane system has been also recently referred to as the invaginated membrane system.⁶ 

Eckly et al use glycoprotein-Ib as a membrane tracer and an array of methods, including confocal microscopy, pulse-chase experiments, and correlative light and electron microscopy to show that the biogenesis of the demarcation membrane system starts at focal points of the cell surface. They could capture the very first moments of demarcation membrane formation, which they named the predemarcation membrane system, and determined their 3-dimensional (3D) architecture using dual axis electron tomography and large volume focused ion beam-scanning electron microscopy. Further, the authors found that a growing demarcation membrane system requires, besides invagination of the plasma membrane, insertion of Golgi-derived membrane vesicles and endoplasmic reticulum–demarcation membrane system tethering. Earlier reports showed numerous Golgi stacks targeted to the region of furrow formation during anaphase in mitotic cells, thereby contributing to active membrane delivery. This suggested that as the megakaryocyte increases in size and ploidy, it is also prepared to augment the mass of intracellular membranous territories.

Past studies proposed that the demarcation membranes define platelet territories.⁷  Do these membranes play yet unidentified additional roles? In the current study, Eckly et al mapped the location of the predemarcation membranes that start by plasma membrane invaginations, in relation to nuclear material. These membrane structures were observed between the nuclear lobes of polyploid megakaryocytes (see figure), with an intriguing correlation between the number of lobes and the plasma membrane connections. During normal mitosis, the Golgi complexes disassemble and reform during telophase. This process, however, was never studied during megakaryocyte endomitosis. Considering the origin and dynamics of demarcation membrane formation and its localization between nuclear lobes, the authors discuss the interesting hypothesis that these membranes are extended from the Golgi in consortium with endomitosis to aid in control of megakaryocyte endomitosis and polyploidy. What might argue against this contention is the uncoupling reported earlier between the development of demarcation membranes and ploidy acquisition, such as in the case of targeted expression of cyclin D3 to megakaryocytes in vivo,⁸  resulting in ploidy level similar to thrombopoietin administration, despite poorer development of demarcation membranes. Further, mutant gunmetal mice exhibit abnormal megakaryocyte demarcation membranes but also an increase in ploidy level.⁹  Naturally, each of these cases represents abnormal gene expression that might not mirror the situation in normally developing megakaryocytes. Whether or not the demarcation membranes take part in controlling megakaryocyte endomitosis warrants future examination.