The c-Myc target glycoprotein1balpha links cytokinesis failure to oncogenic signal transduction pathways in cultured human cells

Isoform-specific functions of Mud/NuMA mediate binucleation of Drosophila male accessory gland cells

Background

In standard cell division, the cells undergo karyokinesis and then cytokinesis. Some cells, however, such as cardiomyocytes and hepatocytes, can produce binucleate cells by going through mitosis without cytokinesis. This cytokinesis skipping is thought to be due to the inhibition of cytokinesis machinery such as the central spindle or the contractile ring, but the mechanisms regulating it are unclear. We investigated them by characterizing the binucleation event during development of the Drosophila male accessory gland, in which all cells are binucleate.

Results

The accessory gland cells arrested the cell cycle at 50 hours after puparium formation (APF) and in the middle of the pupal stage stopped proliferating for 5 hours. They then restarted the cell cycle and at 55 hours APF entered the M-phase synchronously. At this stage, accessory gland cells binucleated by mitosis without cytokinesis. Binucleating cells displayed the standard karyokinesis progression but also showed unusual features such as a non-round shape, spindle orientation along the apico-basal axis, and poor assembly of the central spindle. Mud, a Drosophila homolog of NuMA, regulated the processes responsible for these three features, the classical isoform Mud^PBD and the two newly characterized isoforms Mud^L and Mud^S regulated them differently: Mud^L repressed cell rounding, Mud^PBD and Mud^S oriented the spindle along the apico-basal axis, and Mud^S and Mud^L repressed central spindle assembly. Importantly, overexpression of Mud^S induced binucleation even in standard proliferating cells such as those in imaginal discs.

Conclusions

We characterized the binucleation in the Drosophila male accessory gland and examined mechanisms that regulated unusual morphologies of binucleating cells. We demonstrated that Mud, a microtubule binding protein regulating spindle orientation, was involved in this binucleation. We suggest that atypical functions exerted by three structurally different isoforms of Mud regulate cell rounding, spindle orientation and central spindle assembly in binucleation. We also propose that Mud^S is a key regulator triggering cytokinesis skipping in binucleation processes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-014-0046-5) contains supplementary material, which is available to authorized users.

Role of tumor suppressor p53 in megakaryopoiesis and platelet function

The pathobiological role of p53 has been widely studied, however, its role in normophysiology is relatively unexplored. We previously showed that p53 knock-down increased ploidy in megakaryocytic cultures. This study aims to examine the effect of p53 loss on in vivo megakaryopoiesis, platelet production, and function, and to investigate the basis for greater ploidy in p53(-/-) megakaryocytic cultures. Here, we used flow cytometry to analyze ploidy, DNA synthesis, and apoptosis in murine cultured and bone marrow megakaryocytes following thrombopoietin administration and to analyze fibrinogen binding to platelets in vitro. Culture of p53(-/-) marrow cells for 6 days with thrombopoietin gave rise to 1.7-fold more megakaryocytes, 26.1% ± 3.6% of which reached ploidy classes ≥64 N compared to 8.2% ± 0.9% of p53(+/+) megakaryocytes. This was due to 30% greater DNA synthesis in p53(-/-) megakaryocytes and 31% greater apoptosis in p53(+/+) megakaryocytes by day 4 of culture. Although the bone marrow and spleen steady-state megakaryocytic content and ploidy were similar in p53(+/+) and p53(-/-) mice, thrombopoietin administration resulted in increased megakaryocytic polyploidization in p53(-/-) mice. Although their platelet counts were normal, p53(-/-) mice exhibited significantly longer bleeding times and p53(-/-) platelets were less sensitive than p53(+/+) platelets to agonist-induced fibrinogen binding and P-selectin secretion. In summary, our in vivo and ex vivo studies indicate that p53 loss leads to increased polyploidization during megakaryopoiesis. Our findings also suggest for the first time a direct link between p53 loss and the development of fully functional platelets resulting in hemostatic deficiencies.