Feedback control of PLK1 by Apolo1 ensures accurate chromosome segregation

FIRRM and FIGNL1: partners in the regulation of homologous recombination

DNA repair through homologous recombination (HR) is of vital importance for maintaining genome stability and preventing tumorigenesis. RAD51 is the core component of HR, catalyzing the strand invasion and homology search. Here, we highlight recent findings on FIRRM and FIGNL1 as regulators of the dynamics of RAD51.

Sgo1 interacts with CENP-A to guide accurate chromosome segregation in mitosis

Shugoshin-1 (Sgo1) is necessary for maintaining sister centromere cohesion and ensuring accurate chromosome segregation during mitosis. It has been reported that the localization of Sgo1 at the centromere is dependent on Bub1-mediated phosphorylation of histone H2A at T120. However, it remains uncertain whether other centromeric proteins play a role in regulating the localization and function of Sgo1 during mitosis. Here, we show that CENP-A interacts with Sgo1 and determines the localization of Sgo1 to the centromere during mitosis. Further biochemical characterization revealed that lysine and arginine residues in the C-terminal domain of Sgo1 are critical for binding CENP-A. Interestingly, the replacement of these basic amino acids with acidic amino acids perturbed the localization of Sgo1 and Aurora B to the centromere, resulting in aberrant chromosome segregation and premature chromatid separation. Taken together, these findings reveal a previously unrecognized but direct link between Sgo1 and CENP-A in centromere plasticity control and illustrate how the Sgo1-CENP-A interaction guides accurate cell division.

RGCC-mediated PLK1 activity drives breast cancer lung metastasis by phosphorylating AMPKα2 to activate oxidative phosphorylation and fatty acid oxidation

BACKGROUND

More than 90% of the mortality of triple-negative breast cancer (TNBC) patients is attributed to cancer metastasis with organotropism. The lung is a frequent site of TNBC metastasis. However, the precise molecular mechanism for lung-specific metastasis of TNBC is not well understood.

METHODS

RNA sequencing was performed to identify patterns of gene expression associated with lung metastatic behavior using 4T1-LM3, MBA-MB-231-LM3, and their parental cells (4T1-P, MBA-MB-231-P). Expressions of RGCC, called regulator of cell cycle or response gene to complement 32 protein, were detected in TNBC cells and tissues by qRT-PCR, western blotting, and immunohistochemistry. Kinase activity assay was performed to evaluate PLK1 kinase activity. The amount of phosphorylated AMP-activated protein kinase α2 (AMPKα2) was detected by immunoblotting. RGCC-mediated metabolism was determined by UHPLC system. Oxidative phosphorylation was evaluated by JC-1 staining and oxygen consumption rate (OCR) assay. Fatty acid oxidation assay was conducted to measure the status of RGCC-mediated fatty acid oxidation. NADPH and ROS levels were detected by well-established assays. The chemical sensitivity of cells was evaluated by CCK8 assay.

RESULTS

RGCC is aberrantly upregulated in pulmonary metastatic cells. High level of RGCC is significantly related with lung metastasis in comparison with other organ metastases. RGCC can effectively promote kinase activity of PLK1, and the activated PLK1 phosphorylates AMPKα2 to facilitate TNBC lung metastasis. Mechanistically, the RGCC/PLK1/AMPKα2 signal axis increases oxidative phosphorylation of mitochondria to generate more energy, and promotes fatty acid oxidation to produce abundant NADPH. These metabolic changes contribute to sustaining redox homeostasis and preventing excessive accumulation of potentially detrimental ROS in metastatic tumor cells, thereby supporting TNBC cell survival and colonization during metastases. Importantly, targeting RGCC in combination with paclitaxel/carboplatin effectively suppresses pulmonary TNBC lung metastasis in a mouse model.

CONCLUSIONS

RGCC overexpression is significantly associated with lung-specific metastasis of TNBC. RGCC activates AMPKα2 and downstream signaling through RGCC-driven PLK1 activity to facilitate TNBC lung metastasis. The study provides implications for RGCC-driven OXPHOS and fatty acid oxidation as important therapeutic targets for TNBC treatment.

C1orf112 teams up with FIGNL1 to facilitate RAD51 filament disassembly and DNA interstrand cross-link repair

The recombinase RAD51 plays a core role in DNA repair by homologous recombination (HR). The assembly and disassembly of RAD51 filament need to be orderly regulated by mediators such as BRCA2 and anti-recombinases. To screen for potential regulators of RAD51, we perform RAD51 proximity proteomics and identify factor C1orf112. We further find that C1orf112 complexed with FIGNL1 facilitates RAD51 filament disassembly in the HR step of Fanconi anemia (FA) pathway. Specifically, C1orf112 physically interacts with FIGNL1 and enhances its protein stability. Meanwhile, the RAD51 filament disassembly activity of FIGNL1 is directly stimulated by C1orf112. BRCA2 directly interacts with C1orf112-FIGNL1 complex and functions upstream of this complex to protect RAD51 filament from premature disassembly. C1orf112- and FIGNL1-deficient cells are primarily sensitive to DNA interstrand cross-link (ICL) agents. Thus, these findings suggest an important function of C1orf112 in RAD51 regulation in the HR step of ICL repair by FA pathway.

FIRRM cooperates with FIGNL1 to promote RAD51 disassembly during DNA repair

Interstrand DNA cross-links (ICLs) represent complex lesions that compromise genomic stability. Several pathways have been involved in ICL repair, but the extent of factors involved in the resolution of ICL-induced DNA double-strand breaks (DSBs) remains poorly defined. Using CRISPR-based genomics, we identified FIGNL1 interacting regulator of recombination and mitosis (FIRRM) as a sensitizer of the ICL-inducing agent mafosfamide. Mechanistically, we showed that FIRRM, like its interactor Fidgetin like 1 (FIGNL1), contributes to the resolution of RAD51 foci at ICL-induced DSBs. While the stability of FIGNL1 and FIRRM is interdependent, expression of a mutant of FIRRM (∆WCF), which stabilizes the protein in the absence of FIGNL1, allows the resolution of RAD51 foci and cell survival, suggesting that FIRRM has FIGNL1-independent function during DNA repair. In line with this model, FIRRM binds preferentially single-stranded DNA in vitro, raising the possibility that it directly contributes to RAD51 disassembly by interacting with DNA. Together, our findings establish FIRRM as a promoting factor of ICL repair.

FIRRM/C1orf112 is synthetic lethal with PICH and mediates RAD51 dynamics

Joint DNA molecules are natural byproducts of DNA replication and repair. Persistent joint molecules give rise to ultrafine DNA bridges (UFBs) in mitosis, compromising sister chromatid separation. The DNA translocase PICH (ERCC6L) has a central role in UFB resolution. A genome-wide loss-of-function screen is performed to identify the genetic context of PICH dependency. In addition to genes involved in DNA condensation, centromere stability, and DNA-damage repair, we identify FIGNL1-interacting regulator of recombination and mitosis (FIRRM), formerly known as C1orf112. We find that FIRRM interacts with and stabilizes the AAA⁺ ATPase FIGNL1. Inactivation of either FIRRM or FIGNL1 results in UFB formation, prolonged accumulation of RAD51 at nuclear foci, and impaired replication fork dynamics and consequently impairs genome maintenance. Combined, our data suggest that inactivation of FIRRM and FIGNL1 dysregulates RAD51 dynamics at replication forks, resulting in persistent DNA lesions and a dependency on PICH to preserve cell viability.

FIRRM/C1orf112 mediates resolution of homologous recombination intermediates in response to DNA interstrand crosslinks

DNA interstrand crosslinks (ICLs) pose a major obstacle for DNA replication and transcription if left unrepaired. The cellular response to ICLs requires the coordination of various DNA repair mechanisms. Homologous recombination (HR) intermediates generated in response to ICLs, require efficient and timely conversion by structure-selective endonucleases. Our knowledge on the precise coordination of this process remains incomplete. Here, we designed complementary genetic screens to map the machinery involved in the response to ICLs and identified FIRRM/C1orf112 as an indispensable factor in maintaining genome stability. FIRRM deficiency leads to hypersensitivity to ICL-inducing compounds, accumulation of DNA damage during S-G2 phase of the cell cycle, and chromosomal aberrations, and elicits a unique mutational signature previously observed in HR-deficient tumors. In addition, FIRRM is recruited to ICLs, controls MUS81 chromatin loading, and thereby affects resolution of HR intermediates. FIRRM deficiency in mice causes early embryonic lethality and accelerates tumor formation. Thus, FIRRM plays a critical role in the response to ICLs encountered during DNA replication.

Multiple Roles of PLK1 in Mitosis and Meiosis

Cells are equipped with a diverse network of signaling and regulatory proteins that function as cell cycle regulators and checkpoint proteins to ensure the proper progression of cell division. A key regulator of cell division is polo-like kinase 1 (PLK1), a member of the serine/threonine kinase family that plays an important role in regulating the mitotic and meiotic cell cycle. The phosphorylation of specific substrates mediated by PLK1 controls nuclear envelope breakdown (NEBD), centrosome maturation, proper spindle assembly, chromosome segregation, and cytokinesis. In mammalian oogenesis, PLK1 is essential for resuming meiosis before ovulation and for establishing the meiotic spindle. Among other potential roles, PLK1 regulates the localized translation of spindle-enriched mRNAs by phosphorylating and thereby inhibiting the translational repressor 4E-BP1, a downstream target of the mTOR (mammalian target of rapamycin) pathway. In this review, we summarize the functions of PLK1 in mitosis, meiosis, and cytokinesis and focus on the role of PLK1 in regulating mRNA translation. However, knowledge of the role of PLK1 in the regulation of meiosis remains limited.

Acetylation of Nup62 by TIP60 ensures accurate chromosome segregation in mitosis

Stable transmission of genetic information during cell division requires faithful mitotic spindle assembly and chromosome segregation. In eukaryotic cells, nuclear envelope breakdown (NEBD) is required for proper chromosome segregation. Although a list of mitotic kinases has been implicated in NEBD, how they coordinate their activity to dissolve the nuclear envelope and protein machinery such as nuclear pore complexes was unclear. Here, we identified a regulatory mechanism in which Nup62 is acetylated by TIP60 in human cell division. Nup62 is a novel substrate of TIP60, and the acetylation of Lys432 by TIP60 dissolves nucleoporin Nup62-Nup58-Nup54 complex during entry into mitosis. Importantly, this acetylation-elicited remodeling of nucleoporin complex promotes the distribution of Nup62 to the mitotic spindle, which is indispensable for orchestrating correct spindle orientation. Moreover, suppression of Nup62 perturbs accurate chromosome segregation during mitosis. These results establish a previously uncharacterized regulatory mechanism in which TIP60-elicited nucleoporin dynamics promotes chromosome segregation in mitosis.

Phosphorylation of CENP-R by Aurora B regulates kinetochore-microtubule attachment for accurate chromosome segregation

Error-free mitosis depends on accurate chromosome attachment to spindle microtubules via a fine structure called the centromere that is epigenetically specified by the enrichment of CENP-A nucleosomes. Centromere maintenance during mitosis requires CENP-A-mediated deposition of constitutive centromere-associated network that establishes the inner kinetochore and connects centromeric chromatin to spindle microtubules during mitosis. Although previously proposed to be an adaptor of retinoic acid receptor, here, we show that CENP-R synergizes with CENP-OPQU to regulate kinetochore-microtubule attachment stability and ensure accurate chromosome segregation in mitosis. We found that a phospho-mimicking mutation of CENP-R weakened its localization to the kinetochore, suggesting that phosphorylation may regulate its localization. Perturbation of CENP-R phosphorylation is shown to prevent proper kinetochore-microtubule attachment at metaphase. Mechanistically, CENP-R phosphorylation disrupts its binding with CENP-U. Thus, we speculate that Aurora B-mediated CENP-R phosphorylation promotes the correction of improper kinetochore-microtubule attachment in mitosis. As CENP-R is absent from yeast, we reasoned that metazoan evolved an elaborate chromosome stability control machinery to ensure faithful chromosome segregation in mitosis.

The functions and effects of CUL3-E3 ligases mediated non-degradative ubiquitination

Ubiquitination of substrates usually have two fates: one is degraded by 26S proteasome, and the other is non-degradative ubiquitination modification which is associated with cell cycle regulation, chromosome inactivation, protein transportation, tumorigenesis, achondroplasia, and neurological diseases. Cullin3 (CUL3), a scaffold protein, binding with the Bric-a-Brac-Tramtrack-Broad-complex (BTB) domain of substrates recognition adaptor and RING-finger protein 1 (RBX1) form ubiquitin ligases (E3). Based on the current researches, this review has summarized the functions and effects of CUL3-E3 ligases mediated non-degradative ubiquitination.

Mad2 promotes Cyclin B2 recruitment to the kinetochore for guiding accurate mitotic checkpoint

Accurate mitotic progression relies on the dynamic phosphorylation of multiple substrates by key mitotic kinases. Cyclin-dependent kinase 1 is a master kinase that coordinates mitotic progression and requires its regulatory subunit Cyclin B to ensure full kinase activity and substrate specificity. The function of Cyclin B2, which is a closely related family member of Cyclin B1, remains largely elusive. Here, we show that Mad2 promotes the kinetochore localization of Cyclin B2 and that their interaction at the kinetochores guides accurate chromosome segregation. Our biochemical analyses have characterized the Mad2-Cyclin B2 interaction and delineated a novel Mad2-interacting motif (MIM) on Cyclin B2. The functional importance of the Cyclin B2-Mad2 interaction was demonstrated by real-time imaging in which MIM-deficient mutant Cyclin B2 failed to rescue the chromosomal segregation defects. Taken together, we have delineated a previously undefined function of Cyclin B2 at the kinetochore and have established, in human cells, a mechanism of action by which Mad2 contributes to the spindle checkpoint.

Identification of unique transcriptomic signatures and key genes through RNA sequencing and integrated WGCNA and PPI network analysis in HIV infected lung cancer

With the widespread use of highly active antiretroviral therapy (HARRT), the survival time of AIDS patients has been greatly extended. However, the incidence of lung cancer in HIV-infected patients is increasing and has become a major problem threatening the survival of AIDS patients. The aim of this study is to use Weighted Gene Co-expression Network Analysis (WGCNA) and differential gene analysis to find possible key genes involved in HIV-infected lung cancer. In this study, using lung tissue samples from five pairs of HIV-infected lung cancer patients, second-generation sequencing was performed and transcriptomic data were obtained. A total of 132 HIV-infected lung cancer-related genes were screened out by WGCNA and differential gene expression analysis methods. Based on gene annotation analysis, these genes were mainly enriched in mitosis-related functions and pathways. In addition, in protein-protein interaction (PPI) analysis, a total of 39 hub genes were identified. Among them, five genes (ASPM, CDCA8, CENPF, CEP55, and PLK1) were present in both three hub gene lists (intersection gene, DEGs, and WCGNA module) suggesting that these five genes may become key genes involved in HIV-infected lung cancer.

Recent Progress on the Localization of PLK1 to the Kinetochore and Its Role in Mitosis

The accurate distribution of the replicated genome during cell division is essential for cell survival and healthy organismal development. Errors in this process have catastrophic consequences, such as birth defects and aneuploidy, a hallmark of cancer cells. PLK1 is one of the master kinases in mitosis and has multiple functions, including mitotic entry, chromosome segregation, spindle assembly checkpoint, and cytokinesis. To dissect the role of PLK1 in mitosis, it is important to understand how PLK1 localizes in the specific region in cells. PLK1 localizes at the kinetochore and is essential in spindle assembly checkpoint and chromosome segregation. However, how PLK1 localizes at the kinetochore remains elusive. Here, we review the recent literature on the kinetochore recruitment mechanisms of PLK1 and its roles in spindle assembly checkpoint and attachment between kinetochores and spindle microtubules. Together, this review provides an overview of how the local distribution of PLK1 could regulate major pathways in mitosis.