Aneuploidy triggers a TFEB-mediated lysosomal stress response

Aneuploidy, defined as an alteration in chromosome number that is not a multiple of the haploid complement, severely affects cellular physiology. Changes in chromosome number lead to imbalances in cellular protein composition, thus disrupting cellular processes and causing proteins to misfold and aggregate. We recently reported that in mammalian cells protein aggregates are readily encapsulated within autophagosomes but are not degraded by lysosomes. This leads to a lysosomal stress response in which the transcription factor TFEB induces expression of factors needed for macroautophagy-mediated protein degradation. Our studies uncover lysosomal degradation defects as a feature of the aneuploid state, and a role for the transcription factor TFEB in the response thereto. Keywords: aneuploidy; autophagy; cancer; proteotoxicity; TFE

Short- and long-term effects of chromosome mis-segregation and aneuploidy

Dividing cells that experience chromosome mis-segregation generate aneuploid daughter cells, which contain an incorrect number of chromosomes. Although aneuploidy interferes with the proliferation of untransformed cells, it is also, paradoxically, a hallmark of cancer, a disease defined by increased proliferative potential. These contradictory effects are also observed in mouse models of chromosome instability (CIN). CIN can inhibit and promote tumorigenesis. Recent work has provided insights into the cellular consequences of CIN and aneuploidy. Chromosome mis-segregation per se can alter the genome in many more ways than just causing the gain or loss of chromosomes. The short- and long-term effects of aneuploidy are caused by gene-specific effects and a stereotypic aneuploidy stress response. Importantly, these recent findings provide insights into the role of aneuploidy in tumorigenesis.National Institutes of Health (U.S.) (Grant GM56800

A small-molecule inhibitor of Haspin alters the kinetochore functions of Aurora B

By phosphorylating Thr3 of histone H3, Haspin promotes centromeric recruitment of the chromosome passenger complex (CPC) during mitosis. Aurora B kinase, a CPC subunit, sustains chromosome bi-orientation and the spindle assembly checkpoint (SAC). Here, we characterize the small molecule 5-iodotubercidin (5-ITu) as a potent Haspin inhibitor. In vitro, 5-ITu potently inhibited Haspin but not Aurora B. Consistently, 5-ITu counteracted the centromeric localization of the CPC without affecting the bulk of Aurora B activity in HeLa cells. Mislocalization of Aurora B correlated with dephosphorylation of CENP-A and Hec1 and SAC override at high nocodazole concentrations. 5-ITu also impaired kinetochore recruitment of Bub1 and BubR1 kinases, and this effect was reversed by concomitant inhibition of phosphatase activity. Forcing localization of Aurora B to centromeres in 5-ITu also restored Bub1 and BubR1 localization but failed to rescue the SAC override. This result suggests that a target of 5-ITu, possibly Haspin itself, may further contribute to SAC signaling downstream of Aurora B

A Screen for Kinetochore-Microtubule Interaction Inhibitors Identifies Novel Antitubulin Compounds

Protein assemblies named kinetochores bind sister chromatids to the mitotic spindle and orchestrate sister chromatid segregation. Interference with kinetochore activity triggers a spindle checkpoint mediated arrest in mitosis, which frequently ends in cell death. We set out to identify small compounds that inhibit kinetochore-microtubule binding for use in kinetochore-spindle interaction studies and to develop them into novel anticancer drugs

The life and miracles of kinetochores

Kinetochores are large protein assemblies built on chromosomal loci named centromeres. The main functions of kinetochores can be grouped under four modules. The first module, in the inner kinetochore, contributes a sturdy interface with centromeric chromatin. The second module, the outer kinetochore, contributes a microtubule-binding interface. The third module, the spindle assembly checkpoint, is a feedback control mechanism that monitors the state of kinetochore–microtubule attachment to control the progression of the cell cycle. The fourth module discerns correct from improper attachments, preventing the stabilization of the latter and allowing the selective stabilization of the former. In this review, we discuss how the molecular organization of the four modules allows a dynamic integration of kinetochore–microtubule attachment with the prevention of chromosome segregation errors and cell-cycle progression

Protein aggregation mediates stoichiometry of protein complexes in aneuploid cells

Aneuploidy, a condition characterized by chromosome gains and losses, causes reduced fitness and numerous cellular stresses, including increased protein aggregation. Here, we identify protein complex stoichiometry imbalances as a major cause of protein aggregation in aneuploid cells. Subunits of protein complexes encoded on excess chromosomes aggregate in aneuploid cells, which is suppressed when expression of other subunits is coordinately altered. We further show that excess subunits are either degraded or aggregate and that protein aggregation is nearly as effective as protein degradation at lowering levels of excess proteins. Our study explains why proteotoxic stress is a universal feature of the aneuploid state and reveals protein aggregation as a form of dosage compensation to cope with disproportionate expression of protein complex subunits

Chromosome Mis-segregation Generates Cell-Cycle-Arrested Cells with Complex Karyotypes that Are Eliminated by the Immune System

Aneuploidy, a state of karyotype imbalance, is a hallmark of cancer. Changes in chromosome copy number have been proposed to drive disease by modulating the dosage of cancer driver genes and by promoting cancer genome evolution. Given the potential of cells with abnormal karyotypes to become cancerous, do pathways that limit the prevalence of such cells exist? By investigating the immediate consequences of aneuploidy on cell physiology, we identified mechanisms that eliminate aneuploid cells. We find that chromosome mis-segregation leads to further genomic instability that ultimately causes cell-cycle arrest. We further show that cells with complex karyotypes exhibit features of senescence and produce pro-inflammatory signals that promote their clearance by the immune system. We propose that cells with abnormal karyotypes generate a signal for their own elimination that may serve as a means for cancer cell immunosurveillance. By examining the immediate consequences of chromosome mis-segregation, Santaguida et al. show that aneuploidy causes genomic instability and the evolution of cells with complex karyotypes. Such cells undergo senescence and produce pro-inflammatory cytokines that promote their clearance by natural killer cells. Keywords: aneuploidy; cancer; immune system; genome instability; senescenceNational Institutes of Health (U.S.) (Grant CA206157)National Institutes of Health (U.S.) (Grant GM118066)National Institute of General Medical Sciences (U.S.) (Grant T32GM007753

Gene copy-number changes and chromosomal instability induced by aneuploidy confer resistance to chemotherapy

Mitotic errors lead to aneuploidy, a condition of karyotype imbalance, frequently found in cancer cells. Alterations in chromosome copy number induce a wide variety of cellular stresses, including genome instability. Here, we show that cancer cells might exploit aneuploidy-induced genome instability and the resulting gene copy-number changes to survive under conditions of selective pressure, such as chemotherapy. Resistance to chemotherapeutic drugs was dictated by the acquisition of recurrent karyotypes, indicating that gene dosage might play a role in driving chemoresistance. Thus, our study establishes a causal link between aneuploidy-driven changes in gene copy number and chemoresistance and might explain why some chemotherapies fail to succeed

Sustained Mps1 activity is required in mitosis to recruit O-Mad2 to the Mad1–C-Mad2 core complex

To satisfy the mitotic checkpoint and drive chromosome congression, the Mps1 kinase lets go of kinetochores by phosphorylating itself in trans (see also related papers by Maciejowski et al. and Santaguida et al. in this issue)