How the contractile perinuclear actomyosin network regulates chromosome interaction with the mitotic spindle in early mitosis

For the high-fidelity segregation of chromosomes during mitosis, it is crucial that all chromosomes correctly interact with spindle microtubules (MTs) shortly after the nuclear envelope breakdown (NEBD). Recently, our group discovered the formation of a contractile perinuclear actomyosin network on the cytoplasmic surface of the nuclear envelope (NE) during early mitosis in human cells. The contraction of this network following NEBD reduced chromosome scattering volume (CSV) (Booth et al., 2019). To address how this reduction in CSV facilitates chromosome interaction with MTs, I developed a new method to efficiently inhibit the contraction of the network. In addition, by tracking kinetochore motions in early mitosis, I identified characteristic kinetochore motions that reflect the initial kinetochore–MT interaction and subsequent chromosome congression to the metaphase plate. By combining these methods, my analyses revealed that the contraction of the network pushed chromosomes inward, facilitating the initial kinetochore–MT interaction. The contraction also promoted kinetochore interaction with other kinetochore fibres, thereby assisting in chromosome congression to the metaphase plate. Together, my analyses uncovered a novel mechanism that spatially regulates chromosome interaction with spindle MTs in early mitosis. To investigate whether NEBD triggers the contraction of the perinuclear actomyosin network, I attempted to delay NEBD occurrence by expressing a non-phosphorylatable mutant of Lamin A. However, this procedure did not affect the timing of NEBD, thus the investigation being inconclusive. Furthermore, I studied the presence or absence of the perinuclear actomyosin network in a variety of human cell lines, as well as in human colorectal cancer tissues. Some cancer cell lines with numerical chromosome instability (N-CIN) failed to form the actomyosin network in early mitosis, raising the possibility that the lack of the network may contribute to N-CIN in these cancer cell lines.