The purpose of mitosis is to take chromosomes that have been duplicated in S phase and distribute them to opposite ends of the cell prior to cell division (Figure 1). This task is accomplished by the mitotic spindle, a bipolar, football-shaped structure composed of microtubules. To ensure that each daughter cell inherits an identical copy of the genome, sister chromatids attach to microtubules emanating from opposite ends of the spindle. When the cohesion (“glue”) holding sister chromatids together is dissolved, the two copies of the genome are transported in opposite directions.
Schematic of chromosome segregation. Pairs of replicated chromosomes (blue) attach to spindle microtubules (orange) via kinetochores (red). Error-free chromosome segregation depends on each sister attaching to microtubules from opposite spindle poles, ensuring that one copy of the genome segregates to each spindle pole.
Proper cell proliferation depends on achieving both accuracy and timeliness during mitosis, and defects in either aspect may compromise cellular fitness. When accuracy is compromised, consequences include whole chromosome mis-segregation, a source of aneuploidy and genomic instability. Lengthening the time spent in mitosis can be equally detrimental, as modest delays in cell division can induce cell cycle arrest, while extensive delays can trigger apoptosis. We are interested in understanding the molecular mechanisms underlying both error-free and timely chromosome segregation.
Proper chromosome-spindle interactions are essential
Attachment of pairs of sister chromatids to opposite ends of the spindle is an important step in mitosis. The kinetochore, built on centromeric DNA and composed of approximately 100 proteins, binds directly to spindle microtubules, coupling forces generated by dynamic microtubules to power chromosome movement. The kinetochore is also a key intracellular signaling site that couples cell cycle progression to the establishment of chromosome-spindle interactions. In addition, it regulates the stability of chromosome spindle interactions such that erroneous attachments are eliminated while proper attachments persist through anaphase. Each of these processes is balanced, at least in part, through dynamic and reversible phosphorylation at the kinetochore, with essential contributions from multiple kinases and phosphatases. A focus of our work is to better understand how dynamic phosphorylation works together with microtubule-dependent forces and attachment-dependent changes in the molecular composition of the kinetochore to establish and maintain proper chromosome-spindle interactions.
Delays in mitosis are deleterious
Many essential processes in the cell, including transcription, translation, and DNA repair, are disrupted during mitosis, and it is now clear that a prolonged mitotic arrest is detrimental to the cell. Thus, juxtaposed with the need for fidelity in chromosome segregation is the need to transit through cell division rapidly to restore cellular functionality. Modest mitotic delays may arise in a variety of ways — for example, from defects associated with human cancers such as the presence of extra centrosomes, or by alterations in kinase-phosphatase signaling, either of which is expected to increase the time required to establish proper chromosome-spindle attachments. Extensive delays in mitosis are thought to be of clinical relevance, underlying the efficacy of so-called anti-mitotic drugs, which are mainstays in modern cancer chemotherapy. This class of drugs, which includes Taxol and the vinca alkaloids, perturbs chromosome-spindle attachments, triggering a prolonged mitotic arrest that can culminate in cell death. However, the molecular links between mitotic delays and subsequent cell fate remain poorly understood. Our focus is to elucidate the pathways that link delays in mitosis to decreased proliferative potential, thereby improving our understanding of the basis of cell death following anti-mitotic drug treatment.
To address these questions, we take a multidisciplinary approach, combining the use of cell biology, high-resolution live-cell microscopy, biochemistry, and proteomics.