TRIM37 controls cancer-specific vulnerability to PLK4 inhibition.

Centrosomes catalyse the formation of microtubules needed to assemble the mitotic spindle apparatus1. Centrosomes themselves duplicate once per cell cycle, in a process that is controlled by the serine/threonine protein kinase PLK4 (refs. 2,3). When PLK4 is chemically inhibited, cell division proceeds without centrosome duplication, generating centrosome-less cells that exhibit delayed, acentrosomal spindle assembly4. Whether PLK4 inhibitors can be leveraged as a treatment for cancer is not yet clear. Here we show that acentrosomal spindle assembly following PLK4 inhibition depends on levels of the centrosomal ubiquitin ligase TRIM37. Low TRIM37 levels accelerate acentrosomal spindle assembly and improve proliferation following PLK4 inhibition, whereas high TRIM37 levels inhibit acentrosomal spindle assembly, leading to mitotic failure and cessation of proliferation. The Chr17q region containing the TRIM37 gene is frequently amplified in neuroblastoma and in breast cancer5-8, rendering these cancer types highly sensitive to PLK4 inhibition. We find that inactivating TRIM37 improves acentrosomal mitosis because TRIM37 prevents PLK4 from self-assembling into centrosome-independent condensates that serve as ectopic microtubule-organizing centres. By contrast, elevated TRIM37 expression inhibits acentrosomal spindle assembly through a distinct mechanism that involves degradation of the centrosomal component CEP192. Thus, TRIM37 is an essential determinant of mitotic vulnerability to PLK4 inhibition. Linkage of TRIM37 to prevalent cancer-associated genomic changes-including 17q gain in neuroblastoma and 17q23 amplification in breast cancer-may offer an opportunity to use PLK4 inhibition to trigger selective mitotic failure and provide new avenues to treatments for these cancers.

Inhibition of dopamine receptor D1 signaling promotes human bile duct cancer progression via WNT signaling.

Bile duct cancer (BDC) frequently invades the nerve fibers, making complete surgical resection difficult. A single tumor mass contains cells of variable malignancy and cell-differentiation states, with cancer stem cells (CSCs) considered responsible for poor clinical outcomes. This study aimed to investigate the contribution of autosynthesized dopamine to CSC-related properties in BDC. Sphere formation assays using 13 commercially available BDC cell lines demonstrated that blocking dopamine receptor D1 (DRD1) signaling promoted CSC-related anchorage-independent growth. Additionally, we newly established four new BDC patient-derived organoids (PDOs) and found that blocking DRD1 increased resistance to chemotherapy and enabled xenotransplantation in vivo. Single-cell analysis revealed that the BDC PDO cells varied in their cell-differentiation states and responses to dopamine signaling. Further, DRD1 inhibition increased WNT7B expression in cells with bile duct-like phenotype, and it induced proliferation of other cell types expressing Wnt receptors and stem cell-like signatures. Reagents that inhibited Wnt function canceled the effect of DRD1 inhibition and reduced cell proliferation in BDC PDOs. In summary, in BDCs, DRD1 is a crucial protein involved in autonomous CSC proliferation through the regulation of endogenous WNT7B. As such, inhibition of the DRD1 feedback signaling may be a potential treatment strategy for BDC.

Establishment of patient-derived organoids and a characterization-based drug discovery platform for treatment of pancreatic cancer.

BACKGROUND: Pancreatic cancer is one of the most lethal tumors. The aim of this study is to provide an effective therapeutic discovery platform for pancreatic cancer by establishing and characterizing patient-derived organoids (PDOs). METHODS: PDOs were established from pancreatic tumor surgical specimens, and the mutations were examined using a panel sequence. Expression of markers was assessed by PCR, immunoblotting, and immunohistochemistry; tumorigenicity was examined using immunodeficient mice, and drug responses were examined in vitro and in vivo. RESULTS: PDOs were established from eight primary and metastatic tumors, and the characteristic mutations and expression of cancer stem cell markers and CA19-9 were confirmed. Tumorigenicity of the PDOs was confirmed in subcutaneous transplantation and in the peritoneal cavity in the case of PDOs derived from disseminated nodules. Gemcitabine-sensitive/resistant PDOs showed consistent responses in vivo. High throughput screening in PDOs identified a compound effective for inhibiting tumor growth of a gemcitabine-resistant PDO xenograft model. CONCLUSIONS: This PDO-based platform captures important aspects of treatment-resistant pancreatic cancer and its metastatic features, suggesting that this study may serve as a tool for the discovery of personalized therapies.

mDia1/3 generate cortical F-actin meshwork in Sertoli cells that is continuous with contractile F-actin bundles and indispensable for spermatogenesis and male fertility.

Formin is one of the two major classes of actin binding proteins (ABPs) with nucleation and polymerization activity. However, despite advances in our understanding of its biochemical activity, whether and how formins generate specific architecture of the actin cytoskeleton and function in a physiological context in vivo remain largely obscure. It is also unknown how actin filaments generated by formins interact with other ABPs in the cell. Here, we combine genetic manipulation of formins mammalian diaphanous homolog1 (mDia1) and 3 (mDia3) with superresolution microscopy and single-molecule imaging, and show that the formins mDia1 and mDia3 are dominantly expressed in Sertoli cells of mouse seminiferous tubule and together generate a highly dynamic cortical filamentous actin (F-actin) meshwork that is continuous with the contractile actomyosin bundles. Loss of mDia1/3 impaired these F-actin architectures, induced ectopic noncontractile espin1-containing F-actin bundles, and disrupted Sertoli cell-germ cell interaction, resulting in impaired spermatogenesis. These results together demonstrate the previously unsuspected mDia-dependent regulatory mechanism of cortical F-actin that is indispensable for mammalian sperm development and male fertility.

Citron kinase mediates transition from constriction to abscission through its coiled-coil domain.

Cytokinesis is initiated by constriction of the cleavage furrow, and completed with separation of the two daughter cells by abscission. Control of transition from constriction to abscission is therefore crucial for cytokinesis. However, the underlying mechanism is largely unknown. Here, we analyze the role of Citron kinase (Citron-K) that localizes at the cleavage furrow and the midbody, and dissect its action mechanisms during this transition. Citron-K forms a stable ring-like structure at the midbody and its depletion affects the maintenance of the intercellular bridge, resulting in fusion of two daughter cells after the cleavage furrow ingression. RNA interference (RNAi) targeting Citron-K reduced accumulation of RhoA, Anillin, and septins at the intercellular bridge in mid telophase, and impaired concentration and maintenance of KIF14 and PRC1 at the midbody in late telophase. RNAi rescue experiments revealed that these functions of Citron-K are mediated by its coiled-coil (CC) domain, and not by its kinase domain. The C-terminal part of CC contains a Rho-binding domain and a cluster-forming region and is important for concentrating Citron-K from the cleavage furrow to the midbody. The N-terminal part of CC directly binds to KIF14, and this interaction is required for timely transfer of Citron-K to the midbody after furrow ingression. We propose that the CC-domain-mediated translocation and actions of Citron-K ensure proper stabilization of the midbody structure during the transition from constriction to abscission.

Liprin-α controls stress fiber formation by binding to mDia and regulating its membrane localization.

Regulation of the actin cytoskeleton is crucial for cell morphology and migration. mDia is an actin nucleator that produces unbranched actin filaments downstream of Rho. However, the mechanisms by which mDia activity is regulated in the cell remain unknown. We pulled down Liprin-α as an mDia-binding protein. The binding is mediated through the central region of Liprin-α and through the N-terminal Dia-inhibitory domain (DID) and dimerization domain (DD) of mDia. Liprin-α competes with Dia autoregulatory domain (DAD) for binding to DID, and binds preferably to the open form of mDia. Overexpression of a Liprin-α fragment containing the mDia-binding region decreases localization of mDia to the plasma membrane and attenuates the Rho-mDia-mediated formation of stress fibers in cultured cells. Conversely, depletion of Liprin-α by RNA interference (RNAi) increases the amount of mDia in the membrane fraction and enhances formation of actin stress fibers. Thus, Liprin-α negatively regulates the activity of mDia in the cell by displacing it from the plasma membrane through binding to the DID-DD region.

Augmin deficiency in neural stem cells causes p53-dependent apoptosis and aborts brain development.

Microtubules that assemble the mitotic spindle are generated by centrosomal nucleation, chromatin-mediated nucleation, and nucleation from the surface of other microtubules mediated by the augmin complex. Impairment of centrosomal nucleation in apical progenitors of the developing mouse brain induces p53-dependent apoptosis and causes non-lethal microcephaly. Whether disruption of non-centrosomal nucleation has similar effects is unclear. Here, we show, using mouse embryos, that conditional knockout of the augmin subunit Haus6 in apical progenitors led to spindle defects and mitotic delay. This triggered massive apoptosis and abortion of brain development. Co-deletion of Trp53 rescued cell death, but surviving progenitors failed to organize a pseudostratified epithelium, and brain development still failed. This could be explained by exacerbated mitotic errors and resulting chromosomal defects including increased DNA damage. Thus, in contrast to centrosomes, augmin is crucial for apical progenitor mitosis, and, even in the absence of p53, for progression of brain development.

Centriole-independent mitotic spindle assembly relies on the PCNT-CDK5RAP2 pericentriolar matrix.

Centrosomes, composed of centrioles that recruit a pericentriolar material (PCM) matrix assembled from PCNT and CDK5RAP2, catalyze mitotic spindle assembly. Here, we inhibit centriole formation and/or remove PCNT-CDK5RAP2 in RPE1 cells to address their relative contributions to spindle formation. While CDK5RAP2 and PCNT are normally dispensable for spindle formation, they become essential when centrioles are absent. Acentriolar spindle assembly is accompanied by the formation of foci containing PCNT and CDK5RAP2 via a microtubule and Polo-like kinase 1-dependent process. Foci formation and spindle assembly require PCNT-CDK5RAP2-dependent matrix assembly and the ability of CDK5RAP2 to recruit γ-tubulin complexes. Thus, the PCM matrix can self-organize independently of centrioles to generate microtubules for spindle assembly; conversely, an alternative centriole-anchored mechanism supports spindle assembly when the PCM matrix is absent. Extension to three cancer cell lines revealed similar results in HeLa cells, whereas DLD1 and U2OS cells could assemble spindles in the absence of centrioles and PCNT-CDK5RAP2, suggesting cell type variation in spindle assembly mechanisms.

An essential role of Cdc42-like GTPases in mitosis of HeLa cells.

Here we used RNA interference and examined possible redundancy amongst Rho GTPases in their mitotic role. Chromosome misalignment is induced significantly in HeLa cells by Cdc42 depletion and not by depletion of either one or all of the other four Cdc42-like GTPases (TC10, TCL, Wrch1 or Wrch2), four Rac-like GTPases or three Rho-like GTPases. Notably, combined depletion of Cdc42 and all of the other four Cdc42-like GTPases significantly enhances chromosomal misalignment. These observations suggest that Cdc42 is the primary GTPase functioning during mitosis but that the other four Cdc42-like GTPases can also assume the mitotic role in its absence.

Intra-spindle Microtubule Assembly Regulates Clustering of Microtubule-Organizing Centers during Early Mouse Development.

Errors during cell division in oocytes and early embryos are linked to birth defects in mammals. Bipolar spindle assembly in early mouse embryos is unique in that three or more acentriolar microtubule-organizing centers (MTOCs) are initially formed and are then clustered into two spindle poles. Using a knockout mouse and live imaging of spindles in embryos, we demonstrate that MTOC clustering during the blastocyst stage requires augmin, a critical complex for MT-dependent MT nucleation within the spindle. Functional analyses in cultured cells with artificially increased numbers of centrosomes indicate that the lack of intra-spindle MT nucleation, but not loss of augmin per se or overall reduction of spindle MTs, is the cause of clustering failure. These data suggest that onset of mitosis with three or more MTOCs is turned into a typical bipolar division through augmin-dependent intra-spindle MT assembly.

Friction on MAP determines its traveling direction on microtubules.

Microtubule networks generate various forces, and the forces are applied to microtubule-associated proteins (MAPs). Forth et al. (2014) show in a recent issue of Cell that asymmetric frictional force between MAPs and microtubules leads to directional movement of MAPs along microtubules, providing insight into the mechanism of microtubule network self-organization.

Physiological roles of Rho and Rho effectors in mammals.

Rho GTPase is a master regulator controlling cytoskeleton in multiple contexts such as cell migration, adhesion and cytokinesis. Of several Rho GTPases in mammals, the best characterized is the Rho subfamily including ubiquitously expressed RhoA and its homologs RhoB and RhoC. Upon binding GTP, Rho exerts its functions through downstream Rho effectors, such as ROCK, mDia, Citron, PKN, Rhophilin and Rhotekin. Until recently, our knowledge about functions of Rho and Rho effectors came mostly from in vitro studies utilizing cultured cells, and their physiological roles in vivo were largely unknown. However, gene-targeting studies of Rho and its effectors have now unraveled their tissue- and cell-specific roles and provide deeper insight into the physiological function of Rho signaling in vivo. In this article, we briefly describe previous studies of the function of Rho and its effectors in vitro and then review and discuss recent studies on knockout mice of Rho and its effectors.

Loss of a Rho-regulated actin nucleator, mDia2, impairs cytokinesis during mouse fetal erythropoiesis.

The small GTPase Rho and mDia2, a Rho-regulated actin nucleator, function as critical regulators of cytokinesis in cultured cells. However, their involvement in cytokinesis during mammalian development remains unknown. Here, we generated mice deficient in mDia2 and examined the role of Rho signaling in cytokinesis during development. mDia2-deficient mice survive until embryonic day 11.5 (E11.5), exhibit severe anemia with multinucleate erythroblasts, and die in utero by E12.5. mDia2-deficient erythroid cells differentiate normally, though in a delayed manner, but exhibit cytokinesis failure with decreased accumulation of F-actin in the cleavage furrow during late differentiation from proerythroblasts. On the other hand, inactivation of Rho induces cytokinesis failure from the earlier progenitor stage. mDia2-deficient erythroblasts, however, are able to enucleate their nuclei. Our findings have thus revealed that mDia2 functions critically in cytokinesis in vivo during erythropoiesis and further suggest that the cytokinesis mechanism in development diverges downstream of Rho. They also demonstrate that cytokinesis and enucleation utilize different mechanisms.

Rho and anillin-dependent control of mDia2 localization and function in cytokinesis.

Diaphanous-related formin, mDia, is an actin nucleation/polymerization factor functioning downstream of the small GTPase Rho. Although Rho is critically involved in cytokinesis, it remains elusive how Rho effectors and other regulators of cytoskeletons work together to accomplish this process. Here we focused on mDia2, an mDia isoform involved in cytokinesis of NIH 3T3 cells, and analyzed mechanisms of its localization in cytokinesis. We found that targeting of mDia2 to the cleavage furrow requires not only its binding to RhoA but also its diaphanous-inhibitory domain (DID). We then performed pulldown assays using a fragment containing the latter domain as a bait and identified anillin as a novel mDia2 interaction partner. The anillin-binding is competitive with the diaphanous autoregulatory domain (DAD) of mDia2 in its autoinhibitory interaction. A series of RNA interference and functional rescue experiments has revealed that, in addition to the Rho GTPase-mediated activation, the interaction between mDia2 and anillin is required for the localization and function of mDia2 in cytokinesis.

mDia2 shuttles between the nucleus and the cytoplasm through the importin-{alpha}/{beta}- and CRM1-mediated nuclear transport mechanism.

Mammalian homolog of Drosophila diaphanous (mDia) consisting of three isoforms, mDia1, mDia2, and mDia3, is an effector of Rho GTPases that catalyzes actin nucleation and polymerization. Although the mDia actions on actin dynamics in the cytoplasm have been well studied, whether mDia accumulates and functions in the nucleus remains largely unknown. Given the presence of actin and actin-associated proteins in the nucleus, we have examined nuclear localization of mDia isoforms. We expressed each of mDia isoforms as a green fluorescent protein fusion protein and examined their localization. Although all the mDia isoforms were localized predominantly in the cytoplasm under the steady-state conditions, mDia2 and not mDia1 or mDia3 accumulated extensively in the nucleus upon treatment with leptomycin B (LMB), an inhibitor of CRM1-dependent nuclear export. The LMB-induced nuclear accumulation was confirmed for endogenous mDia2 by using an antibody specific to mDia2. Studies using green fluorescent protein fusions of various truncation mDia2 mutants and point mutants of some of these proteins identified a functional nuclear localization signal in the N terminus of mDia2 and at least one functional nuclear export signal in the C terminus. The nuclear localization signal of mDia2 bound to importin-alpha and was imported into the nucleus by importin-alpha/beta complex in an in vitro transport assay. Consistently, depletion of importin-beta with RNA interference suppressed the LMB-induced nuclear localization of endogenous mDia2. These results suggest that mDia2 continuously shuttles between the nucleus and the cytoplasm using specific nuclear transport machinery composing of importin-alpha/beta and CRM1.

mDia2 induces the actin scaffold for the contractile ring and stabilizes its position during cytokinesis in NIH 3T3 cells.

mDia proteins are mammalian homologues of Drosophila diaphanous and belong to the formin family proteins that catalyze actin nucleation and polymerization. Although formin family proteins of nonmammalian species such as Drosophila diaphanous are essential in cytokinesis, whether and how mDia proteins function in cytokinesis remain unknown. Here we depleted each of the three mDia isoforms in NIH 3T3 cells by RNA interference and examined this issue. Depletion of mDia2 selectively increased the number of binucleate cells, which was corrected by coexpression of RNAi-resistant full-length mDia2. mDia2 accumulates in the cleavage furrow during anaphase to telophase, and concentrates in the midbody at the end of cytokinesis. Depletion of mDia2 induced contraction at aberrant sites of dividing cells, where contractile ring components such as RhoA, myosin, anillin, and phosphorylated ERM accumulated. Treatment with blebbistatin suppressed abnormal contraction, corrected localization of the above components, and revealed that the amount of F-actin at the equatorial region during anaphase/telophase was significantly decreased with mDia2 RNAi. These results demonstrate that mDia2 is essential in mammalian cell cytokinesis and that mDia2-induced F-actin forms a scaffold for the contractile ring and maintains its position in the middle of a dividing cell.