Importin β2 Mediates the Spatio-temporal Regulation of Anillin through a Noncanonical Nuclear Localization Signal

Canonical Wnt signaling promotes HSC glycolysis and liver fibrosis through an LDH-A/HIF-1α transcriptional complex

BACKGROUND AND AIMS

Aerobic glycolysis reprogramming occurs during HSC activation, but how it is initiated and sustained remains unknown. We investigated the mechanisms by which canonical Wnt signaling regulated HSC glycolysis and the therapeutic implication for liver fibrosis.

APPROACH AND RESULTS

Glycolysis was examined in HSC-LX2 cells upon manipulation of Wnt/β-catenin signaling. Nuclear translocation of lactate dehydrogenase A (LDH-A) and its interaction with hypoxia-inducible factor-1α (HIF-1α) were investigated using molecular simulation and site-directed mutation assays. The pharmacological relevance of molecular discoveries was intensified in primary cultures, rodent models, and human samples. HSC glycolysis was enhanced by Wnt3a but reduced by β-catenin inhibitor or small interfering RNA (siRNA). Wnt3a-induced rapid transactivation and high expression of LDH-A dependent on TCF4. Wnt/β-catenin signaling also stimulated LDH-A nuclear translocation through importin β2 interplay with a noncanonical nuclear location signal of LDH-A. Mechanically, LDH-A bound to HIF-1α and enhanced its stability by obstructing hydroxylation-mediated proteasome degradation, leading to increased transactivation of glycolytic genes. The Gly28 residue of LDH-A was identified to be responsible for the formation of the LDH-A/HIF-1α transcription complex and stabilization of HIF-1α. Furthermore, LDH-A-mediated glycolysis was required for HSC activation in the presence of Wnt3a. Results in vivo showed that HSC activation and liver fibrosis were alleviated by HSC-specific knockdown of LDH-A in mice. β-catenin inhibitor XAV-939 mitigated HSC activation and liver fibrosis, which were abrogated by HSC-specific LDH-A overexpression in mice with fibrosis.

CONCLUSIONS

Inhibition of HSC glycolysis by targeting Wnt/β-catenin signaling and LDH-A had therapeutic promise for liver fibrosis.

Cytoplasmic accumulation and plasma membrane association of anillin and Ect2 promote confined migration and invasion

Cells migrating in confinement experience mechanical challenges whose consequences on cell migration machinery remain only partially understood. Here, we demonstrate that a pool of the cytokinesis regulatory protein anillin is retained during interphase in the cytoplasm of different cell types. Confinement induces recruitment of cytoplasmic anillin to plasma membrane at the poles of migrating cells, which is further enhanced upon nuclear envelope (NE) rupture(s). Rupture events also enable the cytoplasmic egress of predominantly nuclear RhoGEF Ect2. Anillin and Ect2 redistributions scale with microenvironmental stiffness and confinement, and are observed in confined cells in vitro and in invading tumor cells in vivo. Anillin, which binds actomyosin at the cell poles, and Ect2, which activates RhoA, cooperate additively to promote myosin II contractility, and promote efficient invasion and extravasation. Overall, our work provides a mechanistic understanding of how cytokinesis regulators mediate RhoA/ROCK/myosin II-dependent mechanoadaptation during confined migration and invasive cancer progression.

A new Karyopherin-β2 binding PY-NLS epitope of HNRNPH2 linked to neurodevelopmental disorders

The HNRNPH2 proline-tyrosine nuclear localization signal (PY-NLS) is mutated in HNRNPH2-related X-linked neurodevelopmental disorder, causing the normally nuclear HNRNPH2 to accumulate in the cytoplasm. We solved the cryoelectron microscopy (cryo-EM) structure of Karyopherin-β2/Transportin-1 bound to the HNRNPH2 PY-NLS to understand importin-NLS recognition and disruption in disease. HNRNPH2 206RPGPY210 is a typical R-X2-4-P-Y motif comprising PY-NLS epitopes 2 and 3, followed by an additional Karyopherin-β2-binding epitope, we term epitope 4, at residues 211DRP213; no density is present for PY-NLS epitope 1. Disease variant mutations at epitopes 2-4 impair Karyopherin-β2 binding and cause aberrant cytoplasmic accumulation in cells, emphasizing the role of nuclear import defect in disease. Sequence/structure analysis suggests that strong PY-NLS epitopes 4 are rare and thus far limited to close paralogs of HNRNPH2, HNRNPH1, and HNRNPF. Epitope 4-binidng hotspot Karyopherin-β2 W373 corresponds to close paralog Karyopherin-β2b/Transportin-2 W370, a pathological variant site in neurodevelopmental abnormalities, suggesting that Karyopherin-β2b/Transportin-2-HNRNPH2/H1/F interactions may be compromised in the abnormalities.

Clinical implication and immunological landscape analyses of ANLN in pan-cancer: A new target for cancer research

BACKGROUND

Anillin is a F-actin binding protein (ANLN) mainly involved in the process of cytokinesis and known to be dysregulated in diverse cancers. However, the role of ANLN in pan-cancer prognosis and tumor immunity remains unclear.

METHODS

Gene expression profiles of 31 solid tumors were downloaded from The Cancer Genome Atlas (TCGA) database. ANLN mRNA and protein expression were quantified using quantitative real-time PCR (qRT-PCR) and immunohistochemistry (IHC). Protein expression of ANLN was further confirmed in Human Protein Atlas (HPA) database. Cox regression and Kaplan-Meier analysis were utilized to assess the prognostic value of ANLN in pan-cancer. The correlation between ANLN and different immune gene markers and infiltration cells was analyzed via ESTIMATE and CIBERSORT. A BLCA immunotherapy cohort: IMvigor (210) was used to confirm the role of ANLN in immune response.

RESULTS

ANLN upregulation was detected in 21 types of cancers and was associated with poor overall survival (OS), disease-free interval (DFI), and progression-free interval (PFI) in most cancers except in THYM (Thymoma). Additionally, correlation analysis revealed a significantly positive association between ANLN expression and tumor mutation burden (TMB), microsatellite instability (MSI), immune cells infiltration. and immune checkpoint genes in various cancers. The BLCA immunotherapy cohort confirmed that patients with higher ANLN level had better immune responses and longer OS.

CONCLUSION

ANLN may serve as a prognostic biomarker for pan-cancer. ANLN upregulation is associated with higher TMB, MSI, and immune cell infiltration in multiple types of tumors, shedding new light for cancer treatment.

An anillin-CIN85-SEPT9 complex promotes intercellular bridge maturation required for successful cytokinesis

Cleavage of one cell into two is the most dramatic event in the life of a cell. Plasma membrane fission occurs within a narrow intercellular bridge (ICB) between the daughter cells, but the mechanisms underlying ICB formation and maturation are poorly understood. Here we identify CIN85 as an ICB assembly factor and demonstrate its requirement for robust and timely cytokinesis. CIN85 interacts directly with the N-terminal region of anillin and SEPT9 and thereby facilitates SEPT9-containing filament localization to the plasma membrane of the ICB. In contrast, the C-terminal pleckstrin homology (PH) domain of anillin binds to septin units lacking SEPT9 but enriched in SEPT11. Anillin's interactions with distinct septin units are required to promote ICB elongation and maturation that, we propose, generate the physical space into which the abscission machinery is recruited to drive the final membrane scission event releasing two independent daughter cells.

Building the cytokinetic contractile ring in an early embryo: Initiation as clusters of myosin II, anillin and septin, and visualization of a septin filament network

The cytokinetic contractile ring (CR) was first described some 50 years ago, however our understanding of the assembly and structure of the animal cell CR remains incomplete. We recently reported that mature CRs in sea urchin embryos contain myosin II mini-filaments organized into aligned concatenated arrays, and that in early CRs myosin II formed discrete clusters that transformed into the linearized structure over time. The present study extends our previous work by addressing the hypothesis that these myosin II clusters also contain the crucial scaffolding proteins anillin and septin, known to help link actin, myosin II, RhoA, and the membrane during cytokinesis. Super-resolution imaging of cortices from dividing embryos indicates that within each cluster, anillin and septin2 occupy a centralized position relative to the myosin II mini-filaments. As CR formation progresses, the myosin II, septin and anillin containing clusters enlarge and coalesce into patchy and faintly linear patterns. Our super-resolution images provide the initial visualization of anillin and septin nanostructure within an animal cell CR, including evidence of a septin filament-like network. Furthermore, Latrunculin-treated embryos indicated that the localization of septin or anillin to the myosin II clusters in the early CR was not dependent on actin filaments. These results highlight the structural progression of the CR in sea urchin embryos from an array of clusters to a linearized purse string, the association of anillin and septin with this process, and provide the visualization of an apparent septin filament network with the CR structure of an animal cell.

Anillin is an emerging regulator of tumorigenesis, acting as a cortical cytoskeletal scaffold and a nuclear modulator of cancer cell differentiation

Remodeling of the intracellular cytoskeleton plays a key role in accelerating tumor growth and metastasis. Targeting different cytoskeletal elements is important for existing and future anticancer therapies. Anillin is a unique scaffolding protein that interacts with major cytoskeletal structures, e.g., actin filaments, microtubules and septin polymers. A well-studied function of this scaffolding protein is the regulation of cytokinesis at the completion of cell division. Emerging evidence suggest that anillin has other important activities in non-dividing cells, including control of intercellular adhesions and cell motility. Anillin is markedly overexpressed in different solid cancers and its high expression is commonly associated with poor prognosis of patient survival. This review article summarizes rapidly accumulating evidence that implicates anillin in the regulation of tumor growth and metastasis. We focus on molecular and cellular mechanisms of anillin-dependent tumorigenesis that include both canonical control of cytokinesis and novel poorly understood functions as a nuclear regulator of the transcriptional reprogramming and phenotypic plasticity of cancer cells.

Importin binding mediates the intramolecular regulation of anillin during cytokinesis

Cytokinesis occurs by the ingression of an actomyosin ring that cleaves a cell into two daughters. This process is tightly controlled to avoid aneuploidy, and we previously showed that active Ran coordinates ring positioning with chromatin. Active Ran is high around chromatin, and forms an inverse gradient to cargo-bound importins. We found that the ring component anillin contains a nuclear localization signal (NLS) that binds to importin and is required for its function during cytokinesis. Here we reveal the mechanism whereby importin binding favors a conformation required for anillin's recruitment to the equatorial cortex. Active RhoA binds to the RhoA-binding domain causing an increase in accessibility of the nearby C2 domain containing the NLS. Importin binding subsequently stabilizes a conformation that favors interactions for cortical recruitment. In addition to revealing a novel mechanism for the importin-mediated regulation of a cortical protein, we also show how importin binding positively regulates protein function.

Complementary functions for the Ran gradient during division

The Ran pathway has a well-described function in nucleocytoplasmic transport, where active Ran dissociates importin/karyopherin-bound cargo containing a nuclear localization signal (NLS) in the nucleus. As cells enter mitosis, the nuclear envelope breaks down and a gradient of active Ran forms where levels are highest near chromatin. This gradient plays a crucial role in regulating mitotic spindle assembly, where active Ran binds to and releases importins from NLS-containing spindle assembly factors. An emerging theme is that the Ran gradient also regulates the actomyosin cortex for processes including polar body extrusion during meiosis, and cytokinesis. For these events, active Ran could play an inhibitory role, where importin-binding may help promote or stabilize a conformation or interaction that favours the recruitment and function of cortical regulators. For either spindle assembly or cortical polarity, the gradient of active Ran determines the extent of importin-binding, the effects of which could vary for different proteins.

Cell cycle-independent furrowing triggered by phosphomimetic mutations of the INCENP STD motif requires Plk1

Timely and precise control of Aurora B kinase, the chromosomal passenger complex (CPC) catalytic subunit, is essential for accurate chromosome segregation and cytokinesis. Post-translational modifications of CPC subunits are directly involved in controlling Aurora B activity. Here, we identified a highly conserved acidic STD-rich motif of INCENP that is phosphorylated during mitosis in vivo and by Plk1 in vitro and is involved in controlling Aurora B activity. By using an INCENP conditional-knockout cell line, we show that impairing the phosphorylation status of this region disrupts chromosome congression and induces cytokinesis failure. In contrast, mimicking constitutive phosphorylation not only rescues cytokinesis but also induces ectopic furrows and contractile ring formation in a Plk1- and ROCK1-dependent manner independent of cell cycle and microtubule status. Our experiments identify the phospho-regulation of the INCENP STD motif as a novel mechanism that is key for chromosome alignment and cytokinesis.This article has an associated First Person interview with the first author of the paper.

CDK11p58-cyclin L1β regulates abscission site assembly

Rigorous spatiotemporal regulation of cell division is required to maintain genome stability. The final stage in cell division, when the cells physically separate (abscission), is tightly regulated to ensure that it occurs after cytokinetic events such as chromosome segregation. A key regulator of abscission timing is Aurora B kinase activity, which inhibits abscission and forms the major activity of the abscission checkpoint. This checkpoint prevents abscission until chromosomes have been cleared from the cytokinetic machinery. Here we demonstrate that the mitosis-specific CDK11^p58 kinase specifically forms a complex with cyclin L1β that, in late cytokinesis, localizes to the stem body, a structure in the middle of the intercellular bridge that forms between two dividing cells. Depletion of CDK11 inhibits abscission, and rescue of this phenotype requires CDK11^p58 kinase activity or inhibition of Aurora B kinase activity. Furthermore, CDK11^p58 kinase activity is required for formation of endosomal sorting complex required for transport III filaments at the site of abscission. Combined, these data suggest that CDK11^p58 kinase activity opposes Aurora B activity to enable abscission to proceed and result in successful completion of cytokinesis.

Septin and Ras regulate cytokinetic abscission in detached cells

Background

Integrin-mediated adhesion is normally required for cytokinetic abscission, and failure in the process can generate potentially oncogenic tetraploid cells. Here, detachment-induced formation of oncogenic tetraploid cells was analyzed in non-transformed human BJ fibroblasts and BJ expressing SV40LT (BJ-LT) ± overactive HRas.

Results

In contrast to BJ and BJ-LT cells, non-adherent BJ-LT-Ras cells recruited ALIX and CHMP4B to the midbody and divided. In detached BJ and BJ-LT cells regression of the cytokinetic furrow was suppressed by intercellular bridge-associated septin; after re-adhesion these cells divided by cytofission, however, some cells became bi-nucleated because of septin reorganization and furrow regression. Adherent bi-nucleated BJ cells became senescent in G1 with p21 accumulation in the nucleus, apparently due to p53 activation since adherent bi-nucleated BJ-LT cells passed through next cell cycle and divided into mono-nucleated tetraploids; the two centrosomes present in bi-nucleated BJ cells fused after furrow regression, pointing to the PIDDosome pathway as a possible mechanism for the p53 activation.

Conclusions

Several mechanisms prevent detached normal cells from generating tumor-causing tetraploid cells unless they have a suppressed p53 response by viruses, mutation or inflammation. Importantly, activating Ras mutations promote colony growth of detached transformed cells by inducing anchorage-independent cytokinetic abscission in single cells.

Common genetic variants contribute to incomplete penetrance: evidence from cancer-free BRCA1 mutation carriers

PURPOSE

The presence of pathogenic germline mutation in BRCA1 gene is considered as the most penetrant genetic predisposition for breast cancer. However, a portion of BRCA1 mutation carriers never develops breast cancer throughout their lifetime. This phenomenon is called incomplete penetrance. Genetic factor is proposed to contribute to this phenomenon, but the details regarding the genetic factor remain elusive. BRCA1 mutations were inherited from the ancestors of the mutation carrier families during human evolution, and their presence is a consistent threat to the survival of the mutation carrier population. In the present study, we hypothesize that evolution could positively select genetic components in the mutation carrier population to suppress the oncogenesis imposed by the predisposition.

EXPERIMENTAL DESIGN

To test our hypothesis, we used whole exome sequencing to compare germline variation of all genes in pairs of breast cancer-unaffected and breast cancer-affected BRCA1 mutation carriers, each pair was from the same family carrying the same BRCA1 mutation.

RESULTS

We identified a group of 'beneficial' variants enriched in the breast cancer-unaffected carrier group. These were the common variants in human population distributed in multiple genes involved in multiple functionally important pathways. We found a single-nucleotide polymorphism, rs3735400 located in ANLN gene, which plays an essential role in controlling cytokinesis and is often found to be overexpressed in cancer. The carriers of this variant had lower cumulative risk of developing breast cancer; overexpression of the variant-containing ANLN decreased ANLN nuclear localization suppressed expression of the variant-containing ANLN, and decreased the cellular proliferation respectively.

CONCLUSION

Our findings support our hypothesis that common genetic variants can be evolutionarily selected in BRCA1 mutation carrier population to counterpart the oncogenic effects imposed by mutation predisposition in BRCA1, contributing to the incomplete penetrance.

Size scaling of nucleolus in Caenorhabditis elegans embryos

Nucleolus is viewed as a plurifunctional center in the cell, tightly linked to ribosome biosynthesis. As a non-membranous structure, how the size of nucleolus is determined is a long outstanding question, and the possibility of “direct size scaling to the nucleus” was raised by genetic studies in fission yeast. Here, we used the model organism Caenorhabditis elegans to test this hypothesis in multi-cellular organisms. We depleted ani-2, ima-3, or C27D9.1 by RNAi feeding, which altered embryo sizes to different extents in ncl-1 mutant worms. DIC imaging provided evidence that in size-altering embryo nucleolar size decreases in small cells and increases in large cells. Furthermore, analyses of nucleolar size in four blastomeres (ABa, ABp, EMS, and P2) within the same embryo of ncl-1 mutants consistently demonstrated the correspondence between cell and nucleolar sizes – the small cells (EMS and P2) have smaller nucleoli in comparison to the large cells (ABa).

Cytokinesis requires localized β-actin filament production by an actin isoform specific nucleator

Cytokinesis is initiated by the localized assembly of the contractile ring, a dynamic actomyosin structure that generates a membrane furrow between the segregating chromosomal masses to divide a cell into two. Here we show that the stabilization and organization of the cytokinetic furrow is specifically dependent on localized β-actin filament assembly at the site of cytokinesis. β-actin filaments are assembled directly at the furrow by an anillin-dependent pathway that enhances RhoA-dependent activation of the formin DIAPH3, an actin nucleator. DIAPH3 specifically generates homopolymeric filaments of β-actin in vitro. By employing enhancers and activators, cells can achieve acute spatio-temporal control over isoform-specific actin arrays that are required for distinct cellular functions.

Cytokinesis is initiated by the localized assembly of the contractile ring. Here the authors show that the stabilization and organization of the cytokinetic furrow requires localized β-actin filament assembly at the site of cytokinesis by an actin isoform specific nucleator.

Characterization of the nuclear import pathway for BLM protein

Numerous studies have shown that nuclear localization of BLM protein, a member of the RecQ helicases, mediated by nuclear localization signal (NLS) is critical for DNA recombination, replication and transcription, but the mechanism by which BLM protein is imported into the nucleus remains unknown. In this study, the nuclear import pathway for BLM was investigated. We found that nuclear import of BLM was inhibited by two dominant-negative mutants of importin β1 and NTF2/E42K, which lacks the ability to bind Ran and RanGDP, respectively, but was not inhibited by the Ran/Q69L, which is deficient in GTP hydrolysis. Further studies revealed that nuclear import of BLM was reconstituted using importin β1, RanGDP and NTF2 in digitonin-permeabilized HeLa cells. Moreover, BLM had direct binding to importin β1 through its NLS domain with the 14-16 HEAT repeats of importin β1. Furthermore, importin β1, Ran or NTF2 depletion by siRNA disrupted the accumulation of BLM protein in the nucleus. These results showed that BLM enters the nucleus via the importin β1, RanGDP and NTF2 dependent pathway, demonstrating for the first time the nuclear trafficking mechanism of a DNA helicase.

Active Ran regulates anillin function during cytokinesis

We describe a novel mechanism by which active Ran regulates anillin during cytokinesis. Anillin is highly conserved and coordinates RhoA, actomyosin, microtubules, and the membrane for cytokinesis in mammalian cells. This study implicates Ran-GTP in influencing cortical contractility during anaphase by regulating anillin function.

Cytokinesis cleaves a cell into two daughters at the end of mitosis, and must be spatially coordinated with chromosome segregation to prevent aneuploidy. The dogma is that the mitotic spindle governs the assembly and constriction of an actomyosin ring. Here, we reveal a function for active Ran in spatially restricting the ring. Our model is that during anaphase, “free” importins, whose gradient inversely correlates with active Ran and chromatin position, function as a molecular ruler for the recruitment and localization of anillin, a contractile protein and a crucial regulator of cytokinesis. We found that decreasing Ran-GTP levels or tethering active Ran to the equatorial membrane affects anillin’s localization and causes cytokinesis phenotypes. Anillin contains a conserved nuclear localization signal (NLS) at its C-terminus that binds to importin-β and is required for cortical polarity and cytokinesis. Mutating the NLS decreases anillin’s cortical affinity, causing it to be more dominantly regulated by microtubules. Anillin contains a RhoA-GTP binding domain, which autoinhibits the NLS and the neighboring microtubule-binding domain, and RhoA-GTP binding may relieve this inhibition during mitosis. Retention of the C-terminal NLS in anillin homologues suggests that this is a conserved mechanism for controlling anillin function.

The Ran Pathway in Drosophila melanogaster Mitosis

Over the last two decades, the small GTPase Ran has emerged as a central regulator of both mitosis and meiosis, particularly in the generation, maintenance, and regulation of the microtubule (MT)-based bipolar spindle. Ran-regulated pathways in mitosis bear many similarities to the well-characterized functions of Ran in nuclear transport and, as with transport, the majority of these mitotic effects are mediated through affecting the physical interaction between karyopherins and Spindle Assembly Factors (SAFs)—a loose term describing proteins or protein complexes involved in spindle assembly through promoting nucleation, stabilization, and/or depolymerization of MTs, through anchoring MTs to specific structures such as centrosomes, chromatin or kinetochores, or through sliding MTs along each other to generate the force required to achieve bipolarity. As such, the Ran-mediated pathway represents a crucial functional module within the wider spindle assembly landscape. Research into mitosis using the model organism Drosophila melanogaster has contributed substantially to our understanding of centrosome and spindle function. However, in comparison to mammalian systems, very little is known about the contribution of Ran-mediated pathways in Drosophila mitosis. This article sets out to summarize our understanding of the roles of the Ran pathway components in Drosophila mitosis, focusing on the syncytial blastoderm embryo, arguing that it can provide important insights into the conserved functions on Ran during spindle formation.