Person:

Nannas, Natalie Jo

Cells must faithfully segregate their chromosomes at division; errors in this process causes cells to inherit an incorrect number of chromosomes, a hallmark of birth defects and cancer. The machinery required to segregate chromosomes is called the spindle, a bipolar array of microtubules that attach to chromosomes through the kinetochore. Replicated chromosomes contain two sister chromatids whose kinetochores must attach to microtubules from opposite poles to ensure correct inheritance of chromosomes. The spindle checkpoint monitors the attachment to the spindle and prevents cell division until all chromatids are attached to opposite poles. Both the spindle and the checkpoint are critical for correct segregation, and we sought to understand the regulation of these two components. The spindle is assembled to a characteristic metaphase length, but it is unknown what determines this length. It has been proposed that spindle length could be regulated a balance of two forces: one generated by interaction between microtubules that elongates the spindle and a second due to interactions between kinetochores and microtubules that shortens the spindle. We tested this force-balance model which predicts that altering the number of kinetochores will alter spindle length. We manipulated the number of kinetochores and found that spindle length scales with the number of kinetochores; introducing extra kinetochores produces shorter spindles and inhibiting kinetochores produces longer spindles. Our results suggest that attachment of chromosomes to the spindle via kinetochores produces an inward force that opposes outward force. We also found that the number of microtubules in the spindle varied with the number of kinetochores. In addition to establishing a spindle, cells must also guarantee that chromosomes are correctly attached to it. Correct attachment generates tension as the chromatids are pulled toward opposite poles but held together by cohesin until anaphase. The spindle checkpoint monitors this tension which causes stretching of chromatin and kinetochores. Lack of tension on activates the checkpoint, but is unknown if the checkpoint measures stretch between kinetochores (inter-kinetochore stretch) or within kinetochores (intra-kinetochore). We tethered sister chromatids together to inhibit inter-kinetochore stretch and found that the checkpoint was not activated. Our results negate inter-kinetochore models and support intra-kinetochore models.

Organisms must faithfully segregate their chromosomes during cell division; mistakes in this process can be costly and even fatal to the organism (1, 2). During mitosis, replicated chromosomes attach to the spindle, a dynamic system of microtubules organized around two poles. Chromosomes attach to the spindle via kinetochores, structures that form on centromeres and bind the ends of microtubules. For accurate segregation, kinetochores on sister chromosomes must attach to microtubules from opposite poles; incorrect attachments lead to missegregation (3). In PNAS, Umbreit et al. (4) expand our understanding of how kinetochore–microtubule interactions can be regulated to correct improper attachments. The authors use in vitro studies to demonstrate that a component of the kinetochore, the Ndc80 complex, can directly influence the dynamics of the microtubules it is bound to and how the complex can be regulated to correct errors in chromosome attachment. Kinetochores are complicated machines. They can stay attached to microtubule ends as they grow and shrink, regulate the dynamics of microtubules, regulate their own activity, and signal to the remainder of the cell. The outer layer of the kinetochore contains the dumbbell-shaped Ndc80 complex (5): One globular domain [the N-terminal domains of Hec1 (Ndc80 in budding yeast) and Nuf2] binds microtubules (6) and is connected by a long coiled coil to the other globular domain (composed of the C-terminal domains of Spc24/Spc25), which connects to other kinetochore components (7) (Fig. 1A). Hec1 contains a conserved calponin homology domain and an unstructured N-terminal tail: Both regions can bind to microtubules independently, but they must act together to produce high-affinity binding (5⇓⇓–8). When sister kinetochores attach...(see full text).