Erroneous Silencing of the Mitotic Checkpoint by Aberrant Spindle Pole-Kinetochore Coordination

Abstract

To segregate chromosomes during cell division, microtubules that form the bipolar spindle attach to and pull on paired chromosome kinetochores. The spindle assembly checkpoint (SAC) is activated at unattached and misattached kinetochores to prevent further mitotic progression. The SAC is silenced after all the kinetochores establish proper and stable attachment to the spindle. Robust timing of SAC silencing after the last kinetochore-spindle attachment herein dictates the fidelity of chromosome segregation. Chromosome missegregation is rare in typical somatic cell mitosis, but frequent in cancer cell mitosis and in meiosis I of mammalian oocytes. In the latter cases, SAC is normally activated in response to disruptions of kinetochore-spindle attachments, suggesting that frequent chromosome missegregation ensues from faulty SAC silencing. In-depth understanding of how SAC silencing malfunctions in these cases is yet missing, but is believed to hold promise for treatment of cancer and prevention of human miscarriage and birth defects. We previously established a spatiotemporal model that, to the best of our knowledge, explained the robustness of SAC silencing in normal mitosis for the first time. In this article, we take advantage of the whole-cell perspective of the spatiotemporal model to identify possible causes of chromosome missegregation out of the distinct features of spindle assembly exhibited by cancer cells and mammalian oocytes. The model results explain why multipolar spindle could inhibit SAC silencing and spindle pole clustering could promote it-albeit accompanied by more kinetochore attachment errors. The model also eliminates geometric factors as the cause for nonrobust SAC silencing in oocyte meiosis, and instead, suggests atypical kinetochore-spindle attachment in meiosis as a potential culprit. Overall, the model shows that abnormal spindle-pole formation and its aberrant coordination with atypical kinetochore-spindle attachments could compromise the robustness of SAC silencing. Our model highlights systems-level coupling between kinetochore-spindle attachment and spindle-pole formation in SAC silencing.

Spindle Architectural Features Must Be Considered Along With Cell Size to Explain the Timing of Mitotic Checkpoint Silencing

“…Metaphase duration depends on the time required to silence the SAC after the last chromosome aligns at the metaphase plate (Rieder et al, 1994 , 1995 ; Howell et al, 2000 ), and spindle and cell sizes have been proposed to impact the dynamics of SAC silencing (Chen and Liu, 2014 , 2015 , 2016 ). Thus, it is possible that the longer metaphase duration in our small 4N DLD-1 clones may stem from a size-dependent change in the time required for SAC silencing.…”

“…The core model was formulated as a system of compartmentalized diffusion-advection-reaction partial differential equations (PDE) given in the Supplementary Materials (Chen and Liu, 2014 , 2015 , 2016 ). The core model was further expanded to an extended model, by coupling the spatial dynamics with a biochemical circuit that describes the regulatory interactions between key molecules of the SAC and a generic toggle switch circuit (Tyson et al, 2003 ) that is triggered by a threshold SAC protein concentration at the spindle pole and induces SAC silencing.…”

The frequency and consequences of multipolar mitoses in undifferentiated embryonic stem cells

“…Importantly, our data suggested that ES cells of both species underwent multipolar divisions and were able to escape nocodazole arrest faster than cells with bipolar spindles. The ability of multipolar dividing ES cells to escape SAC control more efficiently resembles that seen in cancer cells and mammalian oocytes (Chen and Liu, 2015;Holubcova et al, 2015;Nakagawa and FitzHarris, 2017). In our study we employed fluorescence microscopy to track living cells.…”

Dynamics robustness of cascading systems

“…This suggests that such robustness can be achieved by similar designs such as the two-component signaling network in bacteria, and may be a universal feature in biological signaling cascades via protein modifications. There are some biochemical processes which are known to be reliably timed, such as lysis of bacteria and chromosome segregation during mitosis [29, 30], and these reliably-timed processes might be considered as a demonstration of dynamics robustness. Such robustness should be an essential property to many biological systems, and the expansion of the present formulation will provide a future fruitful area of research.…”