Abstract

Megakaryocyte (MK) differentiation is marked by the development of progressive polyploidy, facilitating platelet production by the creation of a large cytoplasmic volume. MKs become polyploid through repeated cycles of endomitosis (EnM), in which mitosis is initiated but subsequently aborted in late anaphase with failure to complete karyokinesis and cytokinesis. However, the mechanisms underlying EnM remain poorly understood. Recent hypotheses explored in the literature have focused on the possible absence or mislocalization of the chromosomal passenger protein Aurora-B kinase, as it has a pivotal role in many aspects of cytokinesis. Along with the other passenger proteins, Aurora-B kinase transits from the centromeres of metaphase chromosomes to the bundled microtubules of the spindle midzone and overlying cortex between separating chromosomes in anaphase. The midzone and its associated proteins, are thought to be critical for determining the position of the cleavage furrow. One of these proteins, the kinesin MKLP-2, is required for the translocation of Aurora-B kinase to the midzone, where it co-localizes with the GTPase MgcRacGAP and stimulates its activity towards RhoA, potentially regulating actin dynamics at the cleavage furrow. We have previously demonstrated that several chromosomal passenger proteins including Aurora-B kinase are normally expressed and localized to centromeres in EnM MKs. In this work, we use deconvolution microscopy in primary murine and human MKs to extend those findings and demonstrate that EnM MKs form midzone structures that are characteristic of late anaphase; in addition, Aurora-B kinase is clearly present on the spindle midzone, as are MKLP-2 and MgcRacGAP. Although we found images suggestive of initial cleavage furrow formation with cortical localization of Aurora-B kinase in late phase cells, we were unable to demonstrate enhanced localization of actin or anillin to the furrow in EnM cells, despite their normal localization in diploid control cells. Therefore, many of the components of the central spindle are intact during MK EnM, but the formation of the cleavage furrow appears to be incomplete. These data add to our understanding of the possible mechanisms underlying EnM and offer an alternative hypothesis to that of failed expression or localization of the chromosomal passenger proteins. Ongoing studies will focus on the assembly and function of the cleavage furrow in this enigmatic process.