Mechanisms of Chromosome Congression during Mitosis

Abstract

Chromosome congression during prometaphase culminates with the establishment of a metaphase plate, a hallmark of mitosis in metazoans. Classical views resulting from more than 100 years of research on this topic have attempted to explain chromosome congression based on the balance between opposing pulling and/or pushing forces that reach an equilibrium near the spindle equator. However, in mammalian cells, chromosome bi-orientation and force balance at kinetochores are not required for chromosome congression, whereas the mechanisms of chromosome congression are not necessarily involved in the maintenance of chromosome alignment after congression. Thus, chromosome congression and maintenance of alignment are determined by different principles. Moreover, it is now clear that not all chromosomes use the same mechanism for congressing to the spindle equator. Those chromosomes that are favorably positioned between both poles when the nuclear envelope breaks down use the so-called “direct congression” pathway in which chromosomes align after bi-orientation and the establishment of end-on kinetochore-microtubule attachments. This favors the balanced action of kinetochore pulling forces and polar ejection forces along chromosome arms that drive chromosome oscillatory movements during and after congression. The other pathway, which we call “peripheral congression”, is independent of end-on kinetochore microtubule-attachments and relies on the dominant and coordinated action of the kinetochore motors Dynein and Centromere Protein E (CENP-E) that mediate the lateral transport of peripheral chromosomes along microtubules, first towards the poles and subsequently towards the equator. How the opposite polarities of kinetochore motors are regulated in space and time to drive congression of peripheral chromosomes only now starts to be understood. This appears to be regulated by position-dependent phosphorylation of both Dynein and CENP-E and by spindle microtubule diversity by means of tubulin post-translational modifications. This so-called “tubulin code” might work as a navigation system that selectively guides kinetochore motors with opposite polarities along specific spindle microtubule populations, ultimately leading to the congression of peripheral chromosomes. We propose an integrated model of chromosome congression in mammalian cells that depends essentially on the following parameters: (1) chromosome position relative to the spindle poles after nuclear envelope breakdown; (2) establishment of stable end-on kinetochore-microtubule attachments and bi-orientation; (3) coordination between kinetochore- and arm-associated motors; and (4) spatial signatures associated with post-translational modifications of specific spindle microtubule populations. The physiological consequences of abnormal chromosome congression, as well as the therapeutic potential of inhibiting chromosome congression are also discussed.

A Molecular View of Kinetochore Assembly and Function

“…Additional microtubule-binding proteins and motors identified there include the SKA and Dam1 complexes, Kif18a, MCAK, SKAP:Astrin, XMAP215/CH-TOG, CENP-E, CENP-F, and Dynein [9,254]. The functions of these proteins at kinetochores are discussed in the essays from Maiato, Lampson and Grishchuk, and Asbury and colleagues ([14,30,255]), and here we limit the discussion to a brief account of the SKA and the Dam1 complexes, two sequence and structurally unrelated microtubule binders with complementary phylogenetic distributions that have emerged as playing a fundamental role in microtubule coupling at kinetochores [256,257,258,259]. The human SKA complex and the budding yeast Dam1 complex can track dynamic microtubules, and interact with their cognate Ndc80 complex specifically when bound to microtubules [253,260,261,262,263,264,265].…”

“…As clarified in more detail in the chapter by Maiato and colleagues [255], the RZZ complex is required for kinetochore recruitment of the minus-end directed motor cytoplasmic Dynein. This function of RZZ requires an additional protein named Spindly, which additionally acts as an adaptor capable of stimulating Dynein motility [232,307,308,309,310,311,312,313].…”

Mapping the kinetochore MAP functions required for stabilizing microtubule attachments to chromosomes during metaphase

“…In summary, the motor MAPs use MTs as tracks to enable the motility of chromosomes in the “+,” anterograde and “−,” retrograde direction as required during chromosome alignment and segregation. Since several other reviews have already been published on mitotic motors (Maiato, Gomes, Sousa, & Barisic, ; Welburn, ) and the primary goal of this review is to highlight the function of the nonmotor MAPs required for stabilizing kMT attachments during metaphase and anaphase, we refrain from an extensive discussion on the function of motor MAPs during mitosis in this review (Maiato et al, ; Welburn, ).…”

In vitro reconstitution of lateral to end-on conversion of kinetochore–microtubule attachments

“…Initially, the kinetochores of most chromosomes form lateral attachments to the walls of spindle MTs, although some may be captured directly by the MT plus-end (Hayden, Bowser, & Rieder, 1990; Magidson et al, 2011; Tanaka et al, 2005). During subsequent congression of the pole-proximal chromosomes, the kinetochore-bound, MT plus end-directed kinesin CENP-E transports chromosomes by moving along the MT walls (Kapoor et al, 2006; Maiato, Gomes, Sousa, & Barisic, 2017; Wood, Sakowicz, Goldstein, & Cleveland, 1997). After chromosomes arrive at the spindle midzone, where the plus-ends of the kinetochore-bound MTs are located, the kinetochores transition from the lateral MT binding to MT end-on attachment (Gudimchuk et al, 2013; Shrestha & Draviam, 2013).…”