During the human lifetime it is estimated that 10 000 trillion cell divisions take place by a cellular process known as ‘mitosis’ to ensure tissue homeostasis, the renewal of epithelia and an efficient immune response against pathogens. Due to the stochastic nature of chromosome/kinetochore interactions with mitotic spindle microtubules, mitosis is prone to errors that can lead to aneuploidy, a condition that results from whole chromosome mis-segregation and is the main cause of prenatal human death. Aneuploidy is also the most common abnormality in human cancers. Thus, understanding the cellular mechanisms that normally ensure mitotic fidelity is not only important for our comprehension of life, but also represents major social and economic challenges with strong implications to human health and well-being in modern societies. In this project we tested two original concepts with strong implications for chromosome segregation fidelity. The first concept was based on the “tubulin code” hypothesis, which predicts that molecular motors “read” specific modifications on spindle microtubules. In this project we found that peripheral chromosome alignment in human cells is driven by the coordinated activities of kinetochore motors that are regulated by a navigation system based on tubulin tyrosination/detyrosination of specific spindle microtubule populations. In addition, we found that tubulin tyrosination/detyrosination works as a mitotic error code to ensure proper chromosome segregation by regulating the microtubule depolymerizing enzyme MCAK. This regulation turned out to be instrumental for how cancer cells respond to the microtubule stabilizing drug taxol. The second concept was centered on the recently uncovered chromosome separation checkpoint that delays the completion of mitosis in response to incompletely separated chromosomes. In this regard, we validated the role of a chromosome separation checkpoint in human cells to spatially and temporally regulate nuclear envelope formation at the exit from mitosis. In particular, we uncovered an Aurora B-mediated surveillance mechanism that ensures proper error correction during anaphase, thereby preventing micronuclei formation. Surprisingly, we uncovered a new route for micronuclei formation in cancer cells based on the missegregation of misaligned chromosomes that satisfy the spindle-assembly checkpoint. Lastly, this project established Indian muntjac cells as a model system for mitosis, uncovering a key role for the Augmin complex in kinetochore fiber maturation and demonstrating that chromosome (mis)segregation in mammals is biased by kinetochore size. Overall, this work established a paradigm shift in our understanding of how spatial information is conveyed to faithfully segregate chromosomes during mitosis.
