KRAS is the most frequently mutated oncogene, harboring mutations in approximately one in seven cancers. Allele-specific KRASG12C inhibitors are currently changing the treatment paradigm for patients ...with KRASG12C-mutated non-small cell lung cancer and colorectal cancer. The success of addressing a previously elusive KRAS allele has fueled drug discovery efforts for all KRAS mutants. Pan-KRAS drugs have the potential to address broad patient populations, including KRASG12D-, KRASG12V-, KRASG13D-, KRASG12R-, and KRASG12A-mutant or KRAS wild-type-amplified cancers, as well as cancers with acquired resistance to KRASG12C inhibitors. Here, we review actively pursued allele-specific and pan-KRAS inhibition strategies and their potential utility.
Mutant-selective KRASG12C inhibitors target a fraction (approximately 13.6%) of all KRAS-driven cancers. A broad arsenal of KRAS drugs is needed to comprehensively conquer KRAS-driven cancers. Conceptually, we foresee two future classes of KRAS medicines: mutant-selective KRAS drugs targeting individual variant alleles and pan-KRAS therapeutics targeting a broad range of KRAS alterations.
Most metazoan cells entering mitosis undergo characteristic rounding, which is important for accurate spindle positioning and chromosome separation. Rounding is driven by contractile tension ...generated by myosin motors in the sub-membranous actin cortex. Recent studies highlight that alongside myosin activity, cortical actin organization is a key regulator of cortex tension. Yet, how mitotic actin organization is controlled remains poorly understood. To address this, we characterized the F-actin interactome in spread interphase and round mitotic cells. Using super-resolution microscopy, we then screened for regulators of cortex architecture and identified the intermediate filament vimentin and the actin-vimentin linker plectin as unexpected candidates. We found that vimentin is recruited to the mitotic cortex in a plectin-dependent manner. We then showed that cortical vimentin controls actin network organization and mechanics in mitosis and is required for successful cell division in confinement. Together, our study highlights crucial interactions between cytoskeletal networks during cell division.
•Comparison of the F-actin interactome in spread interphase and round mitotic cells•Proteomics identifies vimentin and plectin as key regulators of the mitotic cortex•Vimentin intermediate filaments localize under the actin cortex in mitosis•Sub-cortical vimentin regulates actin cortex organization and mechanics in mitosis
Mitotic rounding, which is essential for accurate spindle positioning, is controlled by the sub-membranous cortical actin network. Using proteomics, Serres et al. identify the intermediate filament vimentin as a key regulator of the mitotic cortex. Vimentin forms a sub-cortical layer in mitosis and controls actin cortex organization and mechanics.
In mammalian cell lines, the endosomal sorting complex required for transport (ESCRT)-III mediates abscission, the process that physically separates daughter cells and completes cell division. Cep55 ...protein is regarded as the master regulator of abscission, because it recruits ESCRT-III to the midbody (MB), the site of abscission. However, the importance of this mechanism in a mammalian organism has never been tested. Here we show that Cep55 is dispensable for mouse embryonic development and adult tissue homeostasis. Cep55-knockout offspring show microcephaly and primary neural progenitors require Cep55 and ESCRT for survival and abscission. However, Cep55 is dispensable for cell division in embryonic or adult tissues. In vitro, division of primary fibroblasts occurs without Cep55 and ESCRT-III at the midbody and is not affected by ESCRT depletion. Our work defines Cep55 as an abscission regulator only in specific tissue contexts and necessitates the re-evaluation of an alternative ESCRT-independent cell division mechanism.
Altered nuclear shape is a defining feature of cancer cells. The mechanisms underlying nuclear dysmorphia in cancer remain poorly understood. Here we identify PPP1R12A and PPP1CB, two subunits of the ...myosin phosphatase complex that antagonizes actomyosin contractility, as proteins safeguarding nuclear integrity. Loss of PPP1R12A or PPP1CB causes nuclear fragmentation, nuclear envelope rupture, nuclear compartment breakdown and genome instability. Pharmacological or genetic inhibition of actomyosin contractility restores nuclear architecture and genome integrity in cells lacking PPP1R12A or PPP1CB. We detect actin filaments at nuclear envelope rupture sites and define the Rho-ROCK pathway as the driver of nuclear damage. Lamin A protects nuclei from the impact of actomyosin activity. Blocking contractility increases nuclear circularity in cultured cancer cells and suppresses deformations of xenograft nuclei in vivo. We conclude that actomyosin contractility is a major determinant of nuclear shape and that unrestrained contractility causes nuclear dysmorphia, nuclear envelope rupture and genome instability.
To complete cell division with high fidelity, cytokinesis must be coordinated with chromosome segregation. Mammalian Polo-like kinase 1, Plk1, may function as a critical link because it is required ...for chromosome segregation and establishment of the cleavage plane following anaphase onset. A central spindle-localized pool of the RhoGEF Ect2 promotes activation of the small GTPase RhoA, which drives contractile ring assembly at the equatorial cortex. Here, we have investigated how Plk1 promotes the central spindle recruitment of Ect2. Plk1 phosphorylates the noncatalytic N terminus of the RhoGAP HsCyk-4 at the central spindle, creating a phospho-epitope recognized by the BRCA1 C-terminal (BRCT) repeats of Ect2. Failure to phosphorylate HsCyk-4 blocks Ect2 recruitment to the central spindle and the subsequent induction of furrowing. Microtubules, as well as the microtubule-associated protein (MAP) Prc1, facilitate Plk1 phosphorylation of HsCyk-4. Characterization of a phosphomimetic version of HsCyk-4 indicates that Plk1 promotes Ect2 recruitment through multiple targets. Collectively, our data reveal that formation of the HsCyk-4-Ect2 complex is subject to multiple layers of regulation to ensure that RhoA activation occurs between the segregated sister chromatids during anaphase.
Individuals with BRCA2 mutations are predisposed to breast cancers owing to genome instability. To determine the functions of BRCA2, the human protein was purified. It was found to bind selectively ...to single-stranded DNA (ssDNA), and to ssDNA in tailed duplexes and replication fork structures. Monomeric and dimeric forms of BRCA2 were observed by EM. BRCA2 directed the binding of RAD51 recombinase to ssDNA, reduced the binding of RAD51 to duplex DNA and stimulated RAD51-mediated DNA strand exchange. These observations provide a molecular basis for the role of BRCA2 in the maintenance of genome stability.
Fine-mapping of the cell-division cycle, notably the identification of mitotic kinase signaling pathways, provides novel opportunities for cancer-drug discovery. As a key regulator of multiple steps ...during mitotic progression across eukaryotic species, the serine/threonine-specific Polo-like kinase 1 (Plk1) is highly expressed in malignant cells and serves as a negative prognostic marker in specific human cancer types 1–4. Here, we report the discovery of a potent small-molecule inhibitor of mammalian Plk1, BI 2536, which inhibits Plk1 enzyme activity at low nanomolar concentrations. The compound potently causes a mitotic arrest and induces apoptosis in human cancer cell lines of diverse tissue origin and oncogenome signature. BI 2536 inhibits growth of human tumor xenografts in nude mice and induces regression of large tumors with well-tolerated intravenous dose regimens. In treated tumors, cells arrest in prometaphase, accumulate phosphohistone H3, and contain aberrant mitotic spindles. This mitotic arrest is followed by a surge in apoptosis, detectable by immunohistochemistry and noninvasive optical and magnetic resonance imaging. For addressing the therapeutic potential of Plk1 inhibition, BI 2536 has progressed into clinical studies in patients with locally advanced or metastatic cancers.
At the end of cell division, cytokinesis splits the cytoplasm of nascent daughter cells and partitions segregated sister genomes. To coordinate cell division with chromosome segregation, the mitotic ...spindle controls cytokinetic events at the cell envelope. The spindle midzone stimulates the actomyosin-driven contraction of the cleavage furrow, which proceeds until the formation of a microtubule-rich intercellular bridge with the midbody at its centre. The midbody directs the final membrane abscission reaction and has been proposed to attach the cleavage furrow to the intercellular bridge. How the mitotic spindle is connected to the plasma membrane during cytokinesis is not understood. Here we identify a plasma membrane tethering activity in the centralspindlin protein complex, a conserved component of the spindle midzone and midbody. We demonstrate that the C1 domain of the centralspindlin subunit MgcRacGAP associates with the plasma membrane by interacting with polyanionic phosphoinositide lipids. Using X-ray crystallography we determine the structure of this atypical C1 domain. Mutations in the hydrophobic cap and in basic residues of the C1 domain of MgcRacGAP prevent association of the protein with the plasma membrane, and abrogate cytokinesis in human and chicken cells. Artificial membrane tethering of centralspindlin restores cell division in the absence of the C1 domain of MgcRacGAP. Although C1 domain function is dispensable for the formation of the midzone and midbody, it promotes contractility and is required for the attachment of the plasma membrane to the midbody, a long-postulated function of this organelle. Our analysis suggests that centralspindlin links the mitotic spindle to the plasma membrane to secure the final cut during cytokinesis in animal cells.
The mitotic checkpoint monitors the attachment of kinetochores to microtubules and delays anaphase onset until all sister kinetochores have become attached to opposite poles 1, 2. Correct bipolar ...attachment leads to kinetochore deformation and tension and satisfies the checkpoint 3–6. What prevents mitotic checkpoint reactivation when sister centromeres are split and tension is lost at anaphase onset? Aurora B kinase, the catalytic subunit of the chromosomal passenger protein complex (CPC) 7, acts as a sensor at inner centromeres for the status of attachment 5, 8. Phosphorylation of Aurora B targets at erroneously attached kinetochores elicits the correction of these attachments and the activation of the mitotic checkpoint. At anaphase, the CPC leaves the centromeres and relocates to the spindle midzone 7. This iconic translocation might prevent the checkpoint from reengaging after anaphase onset. To test this hypothesis, we experimentally retained Aurora B and the CPC at the centromere throughout anaphase in human cells. Preventing CPC translocation caused the untimely recruitment of mitotic checkpoint proteins to kinetochores at anaphase in an Aurora B kinase activity-dependent manner. Our results suggest that the relocalization of the CPC, an evolutionarily conserved event in eukaryotes, is a key mechanism that incapacitates the mitotic checkpoint at anaphase.
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► CPC relocation prevents mitotic checkpoint engagement at anaphase onset ► Centromeric CPC retention causes recruitment of Bub1, BubR1, and Mps1 to kinetochores ► This untimely mitotic checkpoint response at anaphase requires Aurora B activity ► CPC retention at centromeres does not destabilize K fibers or delay mitotic exit
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