The TACC domain identifies a family of centrosomal proteins that can interact with microtubules

Inhibition of TACC3 by a small molecule inhibitor in breast cancer

Studies have shown that transforming acidic coiled-coil protein 3 (TACC3), a key component of centrosome-microtubule dynamic networks, is significantly associated with various types of human cancer. We have recently reported that high levels of TACC3 are found in breast cancer, lead to the accumulation of spontaneous DNA damage due to defective DNA damage response signaling, and confer cellular sensitivity to radiation and poly(ADP-ribose) polymerase (PARP) inhibitors. Although our study suggests a potential role of TACC3 as a biomarker in breast cancer detection and prediction of therapy outcome, its role as a therapeutic target in breast cancer is not well studied. In this study, we show that a small molecule TACC3 inhibitor, KHS101, suppresses cell growth, motility, epithelial-mesenchymal transition (EMT), and breast cancer cell stemness while it induces apoptotic cell death. Quantitative multiplexed proteomic analysis using tandem mass tags (TMTs) revealed that KHS101 alters multiple biological processes and signaling pathways, and significantly reduces the expression of mitotic kinases Aurora A and Polo-like kinase 1 (PLK1), which are closely associated with TACC3. Our findings therefore provide a new insight into the potential mechanisms of the action of KHS101 and suggest its possible use as a dual or multi-targeting mitotic inhibitor in breast cancer.

Mitotic spindle association of TACC3 requires Aurora-A-dependent stabilization of a cryptic α-helix

Aurora-A regulates the recruitment of TACC3 to the mitotic spindle through a phospho-dependent interaction with clathrin heavy chain (CHC). Here, we describe the structural basis of these interactions, mediated by three motifs in a disordered region of TACC3. A hydrophobic docking motif binds to a previously uncharacterized pocket on Aurora-A that is blocked in most kinases. Abrogation of the docking motif causes a delay in late mitosis, consistent with the cellular distribution of Aurora-A complexes. Phosphorylation of Ser558 engages a conformational switch in a second motif from a disordered state, needed to bind the kinase active site, into a helical conformation. The helix extends into a third, adjacent motif that is recognized by a helical-repeat region of CHC, not a recognized phospho-reader domain. This potentially widespread mechanism of phospho-recognition provides greater flexibility to tune the molecular details of the interaction than canonical recognition motifs that are dominated by phosphate binding.

Knockdown of TACC3 Inhibits the Proliferation and Invasion of Human Renal Cell Carcinoma Cells

Transforming acidic coiled-coil protein 3 (TACC3) is a member of the TACC family and plays an important role in regulating cell mitosis, transcription, and tumorigenesis. However, the expression pattern and roles of TACC3 in renal cell carcinoma (RCC) remain unclear. The aim of this study was to investigate the role of TACC3 in RCC. We demonstrated overexpression of TACC3 in human RCC cell lines at both RNA and protein levels. Moreover, knockdown of TACC3 repressed RCC cell proliferation, migration, and invasion in vitro. In addition, knockdown of TACC3 inactivated PI3K/Akt signaling in RCC cells. Furthermore, knockdown of TACC3 significantly reduced tumor growth in xenograft tumor-bearing mice. Taken together, our findings showed that TACC3 was increased in human RCC cell lines, and knockdown of TACC3 inhibited the ability of cell proliferation, migration, invasion, and tumorigenesis in vivo. Therefore, TACC3 may act as a therapeutic target for the treatment of human RCC.

Novel therapeutic strategy for cervical cancer harboring FGFR3-TACC3 fusions

We previously found that therapeutic targetable fusions are detected across various cancers. To identify therapeutic targetable fusion in uterine cervical cancer, for which no effective gene targeted therapy has yet been clinically applied, we analyzed RNA sequencing data from 306 cervical cancer samples. We detected 445 high confidence fusion transcripts and identified four samples that harbored FGFR3-TACC3 fusion as an attractive therapeutic target. The frequency of FGFR3-TACC3-fusion-positive cervical cancer is also 1.9% (2/103) in an independent cohort. Continuous expression of the FGFR3-TACC3 fusion transcript and protein induced anchorage-independent growth in the cervical epithelial cell line established from the ectocervix (Ect1/E6E7) but not in that from endocervix (End1/E6E7). Injection of FGFR3-TACC3 fusion-transfected-Ect1/E6E7 cells subcutaneously into NOG mice generated squamous cell carcinoma xenograft tumors, suggesting the association between FGFR3-TACC3 fusion and squamous cell carcinogenesis. Transfection of a FGFR3-TACC3 fusion transcript into four cervical cancer cell lines (SiHa, ME180, HeLa, and Ca Ski) induced activation of the MAPK pathway and enhancement of cell proliferation. Transcriptome analysis of the FGFR3-TACC3 fusion-transfected cell lines revealed that an IL8-triggered inflammatory response was increased, via activation of FGFR3–MAPK signaling. Continuous expression of FGFR3-TACC3 fusion led to activation of the PI3K–AKT pathway only in the two cell lines that harbored PIK3CA mutations. Sensitivity to the FGFR inhibitor, BGJ398, was found to depend on PIK3CA mutation status. Dual inhibition of both FGFR and AKT showed an obvious synergistic effect in cell lines that harbor mutant PIK3CA. Additionally, TACC3 inhibitor, KHS101, suppressed FGFR3-TACC3 fusion protein expression and showed antitumor effect against FGFR3-TACC3 fusion-transfected cell lines. FGFR3-TACC3 fusion-positive cancer has frequent genetic alterations of the PI3K/AKT pathway and selection of appropriate treatment based on PI3K/AKT pathway status should be required.

TACC3 transcriptionally upregulates E2F1 to promote cell growth and confer sensitivity to cisplatin in bladder cancer

Accumulating evidence has shown that transforming acidic coiled-coil 3 (TACC3) is deregulated in a broad spectrum of cancers. In the present study, we reported that TACC3 was markedly elevated in bladder cancer, especially in muscle-invasive bladder cancers (MIBCs). The upregulation of TACC3 was positively associated with tumor invasiveness, grade, T stage, and progression in patients with bladder cancer. Furthermore, a Kaplan-Meier survival analysis showed that patients with bladder cancer whose tumors had high TACC3 expression experienced a dismal prognosis compared with patients whose tumors had low TACC3 expression. Functional studies have found that TACC3 is a prerequisite for the development of malignant characteristics of bladder cancer cells, including cell proliferation and invasion. Moreover, TACC3 promoted G1/S transition, which was mediated via activation of the transcription of E2F1, eventually enhancing cell proliferation. Notably, the overexpression of TACC3 or E2F1 indicates a high sensitivity to cisplatin. Taken together, these findings define a tumor-supportive role for TACC3, which may also serve as a prognostic and therapeutic indicator in bladder cancers.

Promoter-level transcriptome in primary lesions of endometrial cancer identified biomarkers associated with lymph node metastasis

For endometrial cancer patients, lymphadenectomy is recommended to exclude rarely metastasized cancer cells. This procedure is performed even in patients with low risk of recurrence despite the risk of complications such as lymphedema. A method to accurately identify cases with no lymph node metastases (LN-) before lymphadenectomy is therefore highly required. We approached this clinical problem by examining primary lesions of endometrial cancers with CAGE (Cap Analysis Gene Expression), which quantifies promoter-level expression across the genome. Fourteen profiles delineated distinct transcriptional networks between LN + and LN- cases, within those classified as having the low or intermediate risk of recurrence. Subsequent quantitative reverse transcription polymerase chain reaction (qRT-PCR) analyses of 115 primary tumors showed SEMA3D mRNA and TACC2 isoforms expressed through a novel promoter as promising biomarkers with high accuracy (area under the receiver operating characteristic curve, 0.929) when used in combination. Our high-resolution transcriptome provided evidence of distinct molecular profiles underlying LN + /LN- status in endometrial cancers, raising the possibility of preoperative diagnosis to reduce unnecessary operations in patients with minimum recurrence risk.

Structural insight into the XTACC3/XMAP215 interaction from CD and NMR studies on model peptides

TACC3 is a centrosomal adaptor protein that plays important roles during mitotic spindle assembly. It interacts with chTOG/XMAP215, which catalyzes the addition of tubulin dimers during microtubule growth. A 3D coiled-coil model for this interaction is available but the structural details are not well described. To characterize this interaction at atomic resolution, we have designed a simplified version of the system based on small peptides. Four different peptides have been studied by circular dichroism and nuclear magnetic resonance both singly and in all possible combinations; namely, five peptide pairs and two trios. In cosolvents, all single peptides tend to adopt helical conformations resembling those of the full-length protein. However, neither the single peptides nor pairs of peptides form coiled coils. We show that the simultaneous presence of all preformed helices is a prerequisite for binding. The simplest 3D model for the interaction, based on the NMR results, is proposed. Interestingly, the peptide's structure remains unaffected by mutations at essential positions for TACC3 activity. This suggests that the lack of interaction of this TACC3 mutant with XMAP does not correlate with changes in the protein structure and that specific interactions are likely responsible for the interaction and stability of the complex.

The role of TACC3 in mitotic spindle organization

TACC3 regulates spindle organization during mitosis and also regulates centrosome-mediated microtubule nucleation by affecting γ-Tubulin ring complexes. In addition, it interacts with different proteins (such as ch-TOG, clathrin and Aurora-A) to function in mitotic spindle assembly and stability. By forming the TACC3/ch-TOG complex, TACC3 acts as a plus end-tracking protein to promote microtubule elongation. The TACC3/ch-TOG/clathrin complex is formed to stabilize kinetochore fibers by crosslinking adjacent microtubules. Furthermore, the phosphorylation of TACC3 by Aurora-A is important for the formation of TACC3/ch-TOG/clathrin and its recruitment to kinetochore fibers. Recently, the aberrant expression of TACC3 in a variety of human cancers has been linked with mitotic defects. Thus, in this review, we will discuss our current understanding of the biological roles of TACC3 in mitotic spindle organization.

FGFR3-TACC3 cancer gene fusions cause mitotic defects by removal of endogenous TACC3 from the mitotic spindle

Fibroblast growth factor receptor 3-transforming acidic coiled-coil containing protein 3 (FGFR3-TACC3; FT3) is a gene fusion resulting from rearrangement of chromosome 4 that has been identified in many cancers including those of the urinary bladder. Altered FGFR3 signalling in FT3-positive cells is thought to contribute to cancer progression. However, potential changes in TACC3 function in these cells have not been explored. TACC3 is a mitotic spindle protein required for accurate chromosome segregation. Errors in segregation lead to aneuploidy, which can contribute to cancer progression. Here we show that FT3-positive bladder cancer cells have lower levels of endogenous TACC3 on the mitotic spindle, and that this is sufficient to cause mitotic defects. FT3 is not localized to the mitotic spindle, and by virtue of its TACC domain, recruits endogenous TACC3 away from the spindle. Knockdown of the fusion gene or low-level overexpression of TACC3 partially rescues the chromosome segregation defects in FT3-positive bladder cancer cells. This function of FT3 is specific to TACC3 as inhibition of FGFR3 signalling does not rescue the TACC3 level on the spindle in these cancer cells. Models of FT3-mediated carcinogenesis should, therefore, include altered mitotic functions of TACC3 as well as altered FGFR3 signalling.

Microtubule organization within mitotic spindles revealed by serial block face scanning electron microscopy and image analysis

Serial block face scanning electron microscopy (SBF-SEM) is a powerful method to analyze cells in 3D. Here, working at the resolution limit of the method, we describe a correlative light-SBF-SEM workflow to resolve microtubules of the mitotic spindle in human cells. We present four examples of uses for this workflow that are not practical by light microscopy and/or transmission electron microscopy. First, distinguishing closely associated microtubules within K-fibers; second, resolving bridging fibers in the mitotic spindle; third, visualizing membranes in mitotic cells, relative to the spindle apparatus; and fourth, volumetric analysis of kinetochores. Our workflow also includes new computational tools for exploring the spatial arrangement of microtubules within the mitotic spindle. We use these tools to show that microtubule order in mitotic spindles is sensitive to the level of TACC3 on the spindle.

Overexpression of TACC3 is correlated with tumor aggressiveness and poor prognosis in prostate cancer

Transforming acidic coiled-coil (TACC3), a member of the TACC family, has been shown to be deregulated in various cancers and involved in tumor progression. However, its biological role and molecular mechanism in prostate cancer (PCa) have not been elucidated. Herein, we reported that TACC3 was markedly upregulated in metastatic PCa. The upregulation of TACC3 was significantly associated with the metastasis status, tumor stage, total prostate-specific antigen (PSA) level, and Gleason score in patients with PCa. Moreover, a Kaplan-Meier survival analysis showed that patients with PCa who had high TACC3 expression experienced shorter disease-free survival than patients with a low TACC3 expression. In addition, the knockdown of TACC3 dramatically reduced the migratory speed and invasiveness of PCa cells. Furthermore, silencing TACC3 markedly suppressed the Wnt/β-catenin signaling pathway. Taken together, these findings uncover a supportive role for TACC3 in PCa metastasis, which is mediated by the activation of the Wnt/β-catenin signaling pathway, suggesting that TACC3 may serve as a prognostic marker in patients with metastatic PCa.

Overexpression of Transforming Acidic Coiled Coil‑Containing Protein 3 Reflects Malignant Characteristics and Poor Prognosis of Glioma

Gliomas are malignant primary brain tumors with poor prognosis. Recently, research was indicative of a tight connection between tumor malignancy and genetic alterations. Here, we propose an oncogenic implication of transforming acidic coiled-coil-containing protein 3 (TACC3) in gliomas. By comprehensively analyzing the Chinese glioma genome atlas (CGGA) and publicly available data, we demonstrated that TACC3 were overexpressed along with glioma grade and served as an independent negative prognostic biomarker for glioma patients. Functions’ annotations and gene sets’ enrichment analysis suggested that TACC3 may participate in cell cycle, DNA repair, epithelium-mesenchymal transition and other tumor-related biological processes and molecular pathways. Patients with high TACC3 expression showed CD133⁺ stem cell properties, glioma plasticity and shorter overall survival time under chemo-/radio-therapy. Additionally, a TACC3 associated the miRNA-mRNA network was constructed based on in silico prediction and expression pattern, which provide a foundation for further detection of TACC3-miRNA-mRNA axis function. Collectively, our observations identify TACC3 as an oncogene of tumor malignancy, as well as a prognostic and motoring biomarker for glioma patients.

Systematic Identification of Oncogenic EGFR Interaction Partners

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase (TK) that—once activated upon ligand binding—leads to receptor dimerization, recruitment of protein complexes, and activation of multiple signaling cascades. The EGFR is frequently overexpressed or mutated in various cancers leading to aberrant signaling and tumor growth. Hence, identification of interaction partners that bind to mutated EGFR can help identify novel targets for drug discovery.

Here, we used a systematic approach to identify novel proteins that are involved in cancerous EGFR signaling. Using a combination of high-content imaging and a mammalian membrane two-hybrid protein–protein interaction method, we identified eight novel interaction partners of EGFR, of which half strongly interacted with oncogenic, hyperactive EGFR variants. One of these, transforming acidic coiled-coil proteins (TACC) 3, stabilizes EGFR on the cell surface, which results in an increase in downstream signaling via the mitogen-activated protein kinase and AKT pathway. Depletion of TACC3 from cells using small hairpin RNA (shRNA) knockdown or small-molecule targeting reduced mitogenic signaling in non-small cell lung cancer cell lines, suggesting that targeting TACC3 has potential as a new therapeutic approach for non-small cell lung cancer.

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A combined screening approach involving an image-based green fluorescent protein-Grb2 translocation assay and a mammalian membrane two-hybrid protein–protein interaction assay identified 11 novel interactors of EGFR.

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Eight of those were further confirmed by co-immunoprecipitation.

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TACC3 was identified as a novel EGFR interactor, which specifically binds to oncogenic EGFR variants.

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TACC3 directly modulates EGFR stability at the cell surface and hence promotes mitogen-activated protein kinase signaling.

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Targeting TACC3 in non-small cell lung cancer cells partially resensitizes TK-resistant cells to TK inhibitors.

Exploring the developmental mechanisms underlying Wolf-Hirschhorn Syndrome: Evidence for defects in neural crest cell migration

Wolf-Hirschhorn Syndrome (WHS) is a neurodevelopmental disorder characterized by mental retardation, craniofacial malformation, and defects in skeletal and heart development. The syndrome is associated with irregularities on the short arm of chromosome 4, including deletions of varying sizes and microduplications. Many of these genotypic aberrations in humans have been correlated with the classic WHS phenotype, and animal models have provided a context for mapping these genetic irregularities to specific phenotypes; however, there remains a significant knowledge gap concerning the cell biological mechanisms underlying these phenotypes. This review summarizes literature that has made recent contributions to this topic, drawing from the vast body of knowledge detailing the genetic particularities of the disorder and the more limited pool of information on its cell biology. Finally, we propose a novel characterization for WHS as a pathophysiology owing in part to defects in neural crest cell motility and migration during development.

Xenopus TACC2 is a microtubule plus end-tracking protein that can promote microtubule polymerization during embryonic development

Xenopus TACC2 is a microtubule plus end–tracking protein that localizes in front of EB1 and overlaps with TACC1 and TACC3 in cultured embryonic mesenchymal cells and neuronal growth cones. TACC2 OE can promote increased microtubule polymerization in mesenchymal cells but not growth cones, suggesting cell-type specificity to its function.

Microtubule dynamics is regulated by plus end–tracking proteins (+TIPs), which localize to the plus ends of microtubules (MTs). We previously showed that TACC1 and TACC3, members of the transforming acidic coiled-coil protein family, can act as +TIPs to regulate MT dynamics in Xenopus laevis. Here we characterize TACC2 as a +TIP that localizes to MT plus ends in front of EB1 and overlapping with TACC1 and TACC3 in multiple embryonic cell types. We also show that TACC2 can promote MT polymerization in mesenchymal cells but not neuronal growth cones, thus displaying cell-type specificity. Structure–function analysis demonstrates that the C-terminal region of TACC2 is both necessary and sufficient to localize to MT plus ends and promote increased rates of MT polymerization, whereas the N-terminal region cannot bind to MT plus ends but can act in a dominant-negative capacity to reduce polymerization rates. Finally, we analyze mRNA expression patterns in Xenopus embryos for each TACC protein and observe neural enrichment of TACC3 expression compared with TACC1 and TACC2, which are also expressed in mesodermal tissues, including somites. Overall these data provide a novel assessment of all three TACC proteins as a family of +TIPs by highlighting the unique attributes of each, as well as their collective characteristics.

Knockdown of TACC3 inhibits trophoblast cell migration and invasion through the PI3K/Akt signaling pathway

The insufficient invasion of trophoblasts is known to be correlated with the development of preeclampsia. Transforming acidic coiled‑coil protein 3 (TACC3), a member of the TACC domain family, is important in the regulation of cell differentiation, migration and invasion. However, the role of TACC3 in trophoblast function during placental development remains to be fully elucidated. The present study aimed to determine the expression and function of TACC3 in human placenta and to examine the underlying mechanisms. TACC3 expression was analyzed in preeclamptic placentas using reverse transcription‑quantitative polymerase chain reaction and western blotting. Cell proliferation was determined by the MTT assay, and cell migration and invasion were measured using Transwell assays. The expression levels of TACC3, matrix metalloproteinase (MMP)‑2, MMP‑9, tissue inhibitor of metalloproteinase (TIMP)‑1, TIMP‑2, phosphoinositide 3‑kinase (PI3K), phosphorylated (p)‑PI3K, AKT and p‑AKT were detected by western blotting. The results showed that the expression of TACC3 was downregulated in preeclamptic placentas. The knockdown of TACC3 significantly inhibited HTR8/SVneo cell proliferation, migration and invasion, and inhibited the expression of matrix metalloproteinases. In addition, the knockdown of TACC3 significantly reduced the levels of p‑PI3K and Akt in the HTR8/SVneo cells. Taken together, the results of the present study demonstrated that the knockdown of TACC3 inhibited the migration and invasion of HTR8/SVneo cells through suppression of the PI3K/Akt signaling pathway. Therefore, TACC3 may serve as a novel potential target for treating preeclampsia.

TACC3 promotes stemness and is a potential therapeutic target in hepatocellular carcinoma

Transforming acidic coiled-coil protein 3 (TACC3) is essential for cell mitosis and transcriptional functions. In the present study, we first demonstrated that both TACC3 protein and mRNA levels were elevated in HCC tissue samples compared with non-cancerous tissue biopsies according to western blot analyses, immunohistochemistry (IHC) and quantitative real-time PCR (qRT-PCR) assays. Moreover, high TACC3 expression was positively correlated with poor overall survival (OS) and disease-free survival (DFS) (p < 0.001). Using HCC cell lines, we then demonstrated that either TACC3 knockdown or treatment with the potential TACC3 inhibitor KHS101 suppressed cell growth and sphere formation as well as the expression of stem cell transcription factors, including Bmi1, c-Myc and Nanog. Silencing TACC3 may suppress the Wnt/β-catenin and PI3K/AKT signaling pathways, which regulate cancer stem cell-like characteristics. Taken together, these data suggest that TACC3 is enriched in HCC and that TACC3 down-regulation inhibits the proliferation, clonogenicity, and cancer stem cell-like phenotype of HCC cells. KHS101, a TACC3 inhibitor, may serve as a novel therapeutic agent for HCC patients with tumors characterized by high TACC3 expression.

Xenopus TACC1 is a microtubule plus-end tracking protein that can regulate microtubule dynamics during embryonic development

Microtubule plus‐end dynamics are regulated by a family of proteins called plus‐end tracking proteins (+TIPs). We recently demonstrated that the transforming acidic coiled‐coil (TACC) domain family member, TACC3, can function as a +TIP to regulate microtubule dynamics in Xenopus laevis embryonic cells. Although it has been previously reported that TACC3 is the only TACC family member that exists in Xenopus, our examination of its genome determined that Xenopus, like all other vertebrates, contains three TACC family members. Here, we investigate the localization and function of Xenopus TACC1, the founding member of the TACC family. We demonstrate that it can act as a +TIP to regulate microtubule dynamics, and that the conserved C‐terminal TACC domain is required for its localization to plus‐ends. We also show that, in Xenopus embryonic mesenchymal cells, TACC1 and TACC3 are each required for maintaining normal microtubule growth speed but exhibit some functional redundancy in the regulation of microtubule growth lifetime. Given the conservation of TACC1 in Xenopus and other vertebrates, we propose that Xenopus laevis is a useful system to investigate unexplored cell biological functions of TACC1 and other TACC family members in the regulation of microtubule dynamics. © 2015 The Authors. Cytoskeleton, Published by Wiley Periodicals, Inc.

High expression of TACC3 in esophageal squamous cell carcinoma correlates with poor prognosis

To analyze the expression of the transforming acidic coiled-coil protein 3 (TACC3) in esophageal squamous cell carcinoma (ESCC) samples, and to identify whether TACC3 can serve as a biomarker for the diagnosis and prognosis of ESCC, qPCR, western blotting and immunohistochemistry staining (IHC) were utilized to detect the expression of TACC3. Furthermore, cell growth, colony formation, migration ability and the epithelial-mesenchymal transition markers of ESCC cells in which TACC3 were knocked-down were measured. The mRNA and protein levels of TACC3 were higher in ESCC specimens compared to non-tumorous esophageal epithelial tissues. IHC results revealed TACC3 expression was significantly correlated to differentiation (p = 0.017) and lymphoid nodal status (p = 0.028). The patients with high-expression of TACC3 had a significantly poor prognosis compared to those of low-expression (p = 0.017), especially in the patients at stages I–II (p = 0.028). Multivariate analysis indicated that TACC3 expression was an independent prognostic factor for ESCC patients (p = 0.025). Knockdown of TACC3 inhibited the ability of cell proliferation, colony formation and migration. This study first identifies TACC3 not only as a useful biomarker for diagnose and prognosis of ESCC, but also as a potential therapeutic target for patients with ESCC.

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.

Polyglutamylated Tubulin Binding Protein C1orf96/CSAP Is Involved in Microtubule Stabilization in Mitotic Spindles

The centrosome-associated C1orf96/Centriole, Cilia and Spindle-Associated Protein (CSAP) targets polyglutamylated tubulin in mitotic microtubules (MTs). Loss of CSAP causes critical defects in brain development; however, it is unclear how CSAP association with MTs affects mitosis progression. In this study, we explored the molecular mechanisms of the interaction of CSAP with mitotic spindles. Loss of CSAP caused MT instability in mitotic spindles and resulted in mislocalization of Nuclear protein that associates with the Mitotic Apparatus (NuMA), with defective MT dynamics. Thus, CSAP overload in the spindles caused extensive MT stabilization and recruitment of NuMA. Moreover, MT stabilization by CSAP led to high levels of polyglutamylation on MTs. MT depolymerization by cold or nocodazole treatment was inhibited by CSAP binding. Live-cell imaging analysis suggested that CSAP-dependent MT-stabilization led to centrosome-free MT aster formation immediately upon nuclear envelope breakdown without γ-tubulin. We therefore propose that CSAP associates with MTs around centrosomes to stabilize MTs during mitosis, ensuring proper bipolar spindle formation and maintenance.

Truncated RASSF7 promotes centrosomal defects and cell death

RASSF7 protein localises to the centrosome and plays a key role in mitosis. Its expression is also increased in a range of tumour types. However, little is known about the molecular basis of RASSF7's function and it is not clear if it acts as an oncogene in the cancers where its levels are elevated. Here, we carry out the first analysis of the domains of rassf7, focusing on which of them are responsible for its localisation to the centrosome. Constructs were generated to allow the expression of a series of truncated versions of rassf7 and the level of centrosomal localisation shown by each protein quantified. This analysis was carried out in Xenopus embryos which are a tractable system where rassf7 localisation can easily be studied. Our data shows that the coiled-coil domain of rassf7 is required and sufficient to direct its centrosomal localisation. The RA domain did not appear to have a role in mediating localisation. Surprisingly, removal of the extreme C-terminus of the protein caused rassf7 to accumulate at the centrosome and drive centrosome defects, including accumulation of the centrosomal protein γ-tubulin and an amplification of the number of γ-tubulin foci. These effects required the centrosomal localisation mediated by the coiled-coil domain. Later in development cells expressing this truncated rassf7 protein underwent cell death. Finally, analysis of a database of tumour sequences identified a mutation in RASSF7 which would cause a similar C-terminal truncation of the protein. Based on our data this truncated protein might drive centrosomal defects and we propose the hypothesis that truncated RASSF7 could act as an oncogene in a small subset of tumours where it is mutated in this way.

The clinical significance of transforming acidic coiled-coil protein 3 expression in non-small cell lung cancer

The relationship between TACC3, a member of the transforming acidic coiled-coil proteins (TACCs) family, and lung carcinoma remains unclear. The present study was designed to explore the prognostic and clinical significance of TACC3 in non-small cell lung cancer (NSCLC). An immunohistochemistry (IHC) assay was performed to analyze the expression of TACC3 in 195 lung cancer cases. The mRNA and protein levels of TACC3 were examined by quantitative reverse transcription-PCR or western blotting. The correlation between TACC3 expression and clinicopathological factors was analyzed by χ2 analysis and Fisher's exact test. Kaplan-Meier analysis and the Cox proportional hazards model were used to examine the correlation of prognostic outcomes with TACC3. The results showed that the levels of TACC3 mRNA and total protein were higher in lung cancer lesions than paired non-cancerous tissues. IHC analysis revealed that TACC3 was highly expressed in 94 (48.2%) cases. The expression of TACC3 was strongly correlated with smoking status, histological classification, differentiation, cytokeratin 19 fragment levels, T stage and the clinical stage of NSCLC patients. Univariate and multivariate analyses demonstrated that TACC3 is a useful biomarker for NSCLC prognosis. The low TACC3 expression group exhibited better progression-free survival (PFS) among patients who received anti-microtubule chemotherapy. In conclusion, the results showed that a high level of TACC3 expression was correlated with advanced clinicopathological classifications, poor overall survival (OS) and poor recurrence-free survival (RFS) in NSCLC patients. Our findings indicate that TACC3 is a potential prognostic marker and therapeutic target for NSCLC.

TACC3 Is Important for Correct Progression of Meiosis in Bovine Oocytes

Transforming acidic coiled-coil (TACC) proteins are key players during mitosis via stabilization of the spindle. The roles of TACCs during meiosis are however less clear. We used bovine oocytes to study the expression and function of TACC3 during meiosis. TACC3 mRNA was detected in bovine oocytes during meiosis using qRT-PCR, and while it was also expressed in cleavage stage embryos, its expression was down-regulated at the morula and blastocyst stages. Immunofluorescence was used to demonstrate that TACC3 co-localized with tubulin in the metaphase I and II spindles. However, TACC3 was not detected at anaphase or telophase of the first meiotic division. Aurora A, which is known to phosphorylate and activate TACC3 in mitotic cells, showed a similar pattern of gene expression to that of TACC3 in meiotic oocytes and preimplantation embryos. Aurora A protein was however only very transiently associated to the meiotic spindle. Pharmaceutical inhibition of Aurora A activity inhibited TACC3 phosphorylation but did not prevent TACC3 appearance in the spindle. Inhibiting Aurora A activity did however lead to abnormal meiotic spindle formation and impaired maturation of bovine oocytes. Similar results were obtained by knock-down of TACC3 expression using siRNA injection. These results suggest that TACC3 is important for stabilizing the meiotic spindle, but phosphorylation of TACC3 by Aurora A is not required for its recruitment to the meiotic spindle although phosphorylation of TACC3 by other kinases cannot be excluded.

Aurora-A-Dependent Control of TACC3 Influences the Rate of Mitotic Spindle Assembly

The essential mammalian gene TACC3 is frequently mutated and amplified in cancers and its fusion products exhibit oncogenic activity in glioblastomas. TACC3 functions in mitotic spindle assembly and chromosome segregation. In particular, phosphorylation on S558 by the mitotic kinase, Aurora-A, promotes spindle recruitment of TACC3 and triggers the formation of a complex with ch-TOG-clathrin that crosslinks and stabilises kinetochore microtubules. Here we map the Aurora-A-binding interface in TACC3 and show that TACC3 potently activates Aurora-A through a domain centered on F525. Vertebrate cells carrying homozygous F525A mutation in the endogenous TACC3 loci exhibit defects in TACC3 function, namely perturbed localization, reduced phosphorylation and weakened interaction with clathrin. The most striking feature of the F525A cells however is a marked shortening of mitosis, at least in part due to rapid spindle assembly. F525A cells do not exhibit chromosome missegregation, indicating that they undergo fast yet apparently faithful mitosis. By contrast, mutating the phosphorylation site S558 to alanine in TACC3 causes aneuploidy without a significant change in mitotic duration. Our work has therefore defined a regulatory role for the Aurora-A-TACC3 interaction beyond the act of phosphorylation at S558. We propose that the regulatory relationship between Aurora-A and TACC3 enables the transition from the microtubule-polymerase activity of TACC3-ch-TOG to the microtubule-crosslinking activity of TACC3-ch-TOG-clathrin complexes as mitosis progresses. Aurora-A-dependent control of TACC3 could determine the balance between these activities, thereby influencing not only spindle length and stability but also the speed of spindle formation with vital consequences for chromosome alignment and segregation.

High expression of transforming acidic coiled coil-containing protein 3 strongly correlates with aggressive characteristics and poor prognosis of gastric cancer

Transforming acidic coiled coil-containing protein 3 (TACC3) is well understood to regulate mitotic spindle dynamics and centrosome integrity during mitosis. TACC3 has been suggested to be deregulated in a variety of human malignancies and may be involved in the process of cancer progression. The aim of the present study was to determine the status of TACC3 expression in gastric cancer (GC) and to clarify its clinical/prognostic significance. In the present study, we applied quantitative PCR (qPCR) and western blotting to examine TACC3 mRNA/protein expression in paired GC tissues and matched adjacent non-malignant tissues. Immunohistochemistry (IHC) was performed on a large cohort of 186 postoperative GC samples. Chi-square test, Kaplan-Meier analysis and Cox regression modelling were used to analyse the data. Upregulated mRNA and protein expression levels of TACC3 were observed in the majority of the GC tissues based on qPCR and western blotting compared to the adjacent non-cancerous gastric tissues. Specific IHC staining for TACC3 was predominantly identified in the cytoplasm of the cancer cells. A high expression of TACC3 was detected in 102 of the 186 (54.8%) tissue samples and was significantly associated with the extracapsular extension of the tumour (P<0.001), tumour relapse (P<0.001) and shortened overall survival in GC (P<0.001). Further analysis demonstrated that the TACC3 expression level stratified the patient outcome in stage II (P=0.040), stage III (P<0.001), T3/4 (P<0.001), N positive (P<0.001) and poorly differentiated/undifferentiated tumour subgroups (P<0.001). The Cox regression analysis suggested that a high expression of TACC3 was an independent prognostic factor for GC patients. The measurement of TACC3 protein expression may be beneficial for predicting clinical outcomes for GC patients.

Functions of Fibroblast Growth Factor Receptors in cancer defined by novel translocations and mutations

The four receptor tyrosine kinases (RTKs) within the family of Fibroblast Growth Factor Receptors (FGFRs) are critical for normal development but also play an enormous role in oncogenesis. Mutations and/or abnormal expression often lead to constitutive dimerization and kinase activation of FGFRs, and represent the primary mechanism for aberrant signaling. Sequencing of human tumors has revealed a plethora of somatic mutations in FGFRs that are frequently identical to germline mutations in developmental syndromes, and has also identified novel FGFR fusion proteins arising from chromosomal rearrangements that contribute to malignancy. This review details approximately 200 specific point mutations in FGFRs and 40 different fusion proteins created by translocations involving FGFRs that have been identified in human cancer. This review discusses the effects of these genetic alterations on downstream signaling cascades, and the challenge of drug resistance in cancer treatment with antagonists of FGFRs.

Pericentriolar material structure and dynamics

A centrosome consists of two barrel-shaped centrioles embedded in a matrix of proteins known as the pericentriolar material (PCM). The PCM serves as a platform for protein complexes that regulate organelle trafficking, protein degradation and spindle assembly. Perhaps most important for cell division, the PCM concentrates tubulin and serves as the primary organizing centre for microtubules in metazoan somatic cells. Thus, similar to other well-described organelles, such as the nucleus and mitochondria, the cell has compartmentalized a multitude of vital biochemical reactions in the PCM. However, unlike these other organelles, the PCM is not membrane bound, but rather a dynamic collection of protein complexes and nucleic acids that constitute the organelle's interior and determine its boundary. How is the complex biochemical machinery necessary for the myriad centrosome functions concentrated and maintained in the PCM? Recent advances in proteomics and RNAi screening have unveiled most of the key PCM components and hinted at their molecular interactions ( table 1). Now we must understand how the interactions between these molecules contribute to the mesoscale organization and the assembly of the centrosome. Among outstanding questions are the intrinsic mechanisms that determine PCM shape and size, and how it functions as a biochemical reaction hub.

TACC3-ch-TOG track the growing tips of microtubules independently of clathrin and Aurora-A phosphorylation

The interaction between TACC3 (transforming acidic coiled coil protein 3) and the microtubule polymerase ch-TOG (colonic, hepatic tumor overexpressed gene) is evolutionarily conserved. Loading of TACC3-ch-TOG onto mitotic spindle microtubules requires the phosphorylation of TACC3 by Aurora-A kinase and the subsequent interaction of TACC3 with clathrin to form a microtubule-binding surface. Recent work indicates that TACC3 can track the plus-ends of microtubules and modulate microtubule dynamics in non-dividing cells via its interaction with ch-TOG. Whether there is a pool of TACC3-ch-TOG that is independent of clathrin in human cells, and what is the function of this pool, are open questions. Here, we describe the molecular interaction between TACC3 and ch-TOG that permits TACC3 recruitment to the plus-ends of microtubules. This TACC3-ch-TOG pool is independent of EB1, EB3, Aurora-A phosphorylation and binding to clathrin. We also describe the distinct combinatorial subcellular pools of TACC3, ch-TOG and clathrin. TACC3 is often described as a centrosomal protein, but we show that there is no significant population of TACC3 at centrosomes. The delineation of distinct protein pools reveals a simplified view of how these proteins are organized and controlled by post-translational modification.

MAPping the Ndc80 loop in cancer: A possible link between Ndc80/Hec1 overproduction and cancer formation

Mis-regulation (e.g. overproduction) of the human Ndc80/Hec1 outer kinetochore protein has been associated with aneuploidy and tumourigenesis, but the genetic basis and underlying mechanisms of this phenomenon remain poorly understood. Recent studies have identified the ubiquitous Ndc80 internal loop as a protein-protein interaction platform. Binding partners include the Ska complex, the replication licensing factor Cdt1, the Dam1 complex, TACC-TOG microtubule-associated proteins (MAPs) and kinesin motors. We review the field and propose that the overproduction of Ndc80 may unfavourably absorb these interactors through the internal loop domain and lead to a change in the equilibrium of MAPs and motors in the cells. This sequestration will disrupt microtubule dynamics and the proper segregation of chromosomes in mitosis, leading to aneuploid formation. Further investigation of Ndc80 internal loop-MAPs interactions will bring new insights into their roles in kinetochore-microtubule attachment and tumourigenesis.

Genetic variation in mitotic regulatory pathway genes is associated with breast tumor grade

Mitotic index is an important component of histologic grade and has an etiologic role in breast tumorigenesis. Several small candidate gene studies have reported associations between variation in mitotic genes and breast cancer risk. We measured associations between 2156 single nucleotide polymorphisms (SNPs) from 194 mitotic genes and breast cancer risk, overall and by histologic grade, in the Breast Cancer Association Consortium (BCAC) iCOGS study (n = 39 067 cases; n = 42 106 controls). SNPs in TACC2 [rs17550038: odds ratio (OR) = 1.24, 95% confidence interval (CI) 1.16-1.33, P = 4.2 × 10(-10)) and EIF3H (rs799890: OR = 1.07, 95% CI 1.04-1.11, P = 8.7 × 10(-6)) were significantly associated with risk of low-grade breast cancer. The TACC2 signal was retained (rs17550038: OR = 1.15, 95% CI 1.07-1.23, P = 7.9 × 10(-5)) after adjustment for breast cancer risk SNPs in the nearby FGFR2 gene, suggesting that TACC2 is a novel, independent genome-wide significant genetic risk locus for low-grade breast cancer. While no SNPs were individually associated with high-grade disease, a pathway-level gene set analysis showed that variation across the 194 mitotic genes was associated with high-grade breast cancer risk (P = 2.1 × 10(-3)). These observations will provide insight into the contribution of mitotic defects to histological grade and the etiology of breast cancer.

Cancer cell death induced by novel small molecules degrading the TACC3 protein via the ubiquitin-proteasome pathway

The selective degradation of target proteins with small molecules is a novel approach to the treatment of various diseases, including cancer. We have developed a protein knockdown system with a series of hybrid small compounds that induce the selective degradation of target proteins via the ubiquitin–proteasome pathway. In this study, we designed and synthesized novel small molecules called SNIPER(TACC3)s, which target the spindle regulatory protein transforming acidic coiled-coil-3 (TACC3). SNIPER(TACC3)s induce poly-ubiquitylation and proteasomal degradation of TACC3 and reduce the TACC3 protein level in cells. Mechanistic analysis indicated that the ubiquitin ligase APC/C^CDH1 mediates the SNIPER(TACC3)-induced degradation of TACC3. Intriguingly, SNIPER(TACC3) selectively induced cell death in cancer cells expressing a larger amount of TACC3 protein than normal cells. These results suggest that protein knockdown of TACC3 by SNIPER(TACC3) is a potential strategy for treating cancers overexpressing the TACC3 protein.

Pulling it together: The mitotic function of TACC3

Transforming acidic coiled coil 3 (TACC3) is a non-motor microtubule-associated protein (MAP) that is important for mitotic spindle stability and organization. The exact mechanism by which TACC3 acts at microtubules to stabilize the spindle has been unclear. However, several recent studies identified that the TACC3 complex at microtubules contains clathrin in addition to its previously identified binding partner, colonic and hepatic tumor overexpressed gene (ch-TOG). In this complex, phosphorylated TACC3 interacts directly with both ch-TOG and clathrin heavy chain, promoting accumulation of all complex members at the mitotic spindle. This complex stabilizes kinetochore fibers within the spindle by forming cross-bridges that link adjacent microtubules in these bundles. So, TACC3 is an adaptor that recruits ch-TOG and clathrin to mitotic microtubules, in an Aurora A kinase-regulated manner. In this mini-review we will describe the recent advances in the understanding of TACC 3 function and present a model that pulls together these new data with previous observations.

Prognosis significance of HER2 status and TACC1 expression in patients with gastric carcinoma

HER2 amplification and/or expression occurs in gastric carcinoma (GC), but the role of HER2 in the prognosis of GC remains unclear. The dysregulation of transforming acidic coiled coil 1 (TACC1), a downstream gene of HER2, is thought to be involved in the development of GC. The aim of this study was to investigate the role and relationship of HER2 and TACC1 in GC. The expression of HER2 and TACC1 was analyzed using immunohistochemistry on 129 primary resected GC patients, and HER2 amplification was additionally determined by FISH. The data on clinicopathological features and relevant prognostic factors in these patients were analyzed. The expression (3+, 2+ and 1+) and the amplification of HER2 was observed in 57 cases (44.2 %) and 25 cases (19.4 %), respectively, and the correlation between HER2 expression and amplification was strong (p < 0.001). According to the FDA criteria, 24 cases (18.6 %) would have been considered as HER2 positive. A total 62 (48.1 %) GC tissues showed positive cytoplasmic staining of TACC1. There was a significant and positive association between TACC1 and HER2. HER2 positive was significantly associated with TNM stage (p = 0.019), and TACC1 expression was significantly associated with lymph node metastasis (p = 0.004) and TNM stage (p = 0.004). TNM stage, TACC1 expression and co-positive of both HER2 and TACC1 were independent prognostic factors. TACC1 expression is an independent prognostic indicator of GC. The correlation between TACC1 expression and HER2-positive status indicated a possible synergistic regulation of the two molecules and co-positive of both HER2 and TACC1 maybe a more valuable prognostic marker.

XTACC3-XMAP215 association reveals an asymmetric interaction promoting microtubule elongation

chTOG is a conserved microtubule polymerase that catalyses the addition of tubulin dimers to promote microtubule growth. chTOG interacts with TACC3, a member of the transforming acidic coiled-coil (TACC) family. Here we analyse their association using the Xenopus homologues, XTACC3 (TACC3) and XMAP215 (chTOG), dissecting the mechanism by which their interaction promotes microtubule elongation during spindle assembly. Using SAXS, we show that the TACC domain (TD) is an elongated structure that mediates the interaction with the C terminus of XMAP215. Our data suggest that one TD and two XMAP215 molecules associate to form a four-helix coiled-coil complex. A hybrid methods approach was used to define the precise regions of the TACC heptad repeat and the XMAP215 C terminus required for assembly and functioning of the complex. We show that XTACC3 can induce the recruitment of larger amounts of XMAP215 by increasing its local concentration, thereby promoting efficient microtubule elongation during mitosis.

TACC3 protein regulates microtubule nucleation by affecting γ-tubulin ring complexes

Centrosome-mediated microtubule nucleation is essential for spindle assembly during mitosis. Although γ-tubulin complexes have primarily been implicated in the nucleation process, details of the underlying mechanisms remain poorly understood. Here, we demonstrated that a member of the human transforming acidic coiled-coil (TACC) protein family, TACC3, plays a critical role in microtubule nucleation at the centrosome. In mitotic cells, TACC3 knockdown substantially affected the assembly of microtubules in the astral region and impaired microtubule nucleation at the centrosomes. The TACC3 depletion-induced mitotic phenotype was rescued by expression of the TACC3 C terminus predominantly consisting of the TACC domain, suggesting that the TACC domain plays an important role in microtubule assembly. Consistently, experiments with the recombinant TACC domain of TACC3 demonstrated that this domain possesses intrinsic microtubule nucleating activity. Co-immunoprecipitation and sedimentation experiments revealed that TACC3 mediates interactions with proteins of both the γ-tubulin ring complex (γ-TuRC) and the γ-tubulin small complex (γ-TuSC). Interestingly, TACC3 depletion resulted in reduced levels of γ-TuRC and increased levels of γ-TuSC, indicating that the assembly of γ-TuRC from γ-TuSC requires TACC3. Detailed analyses suggested that TACC3 facilitates the association of γ-TuSC-specific proteins with the proteins known to be involved in the assembly of γ-TuRC. Consistent with such a role for TACC3, the suppression of TACC3 disrupted localization of γ-TuRC proteins to the centrosome. Our findings reveal that TACC3 is involved in the regulation of microtubule nucleation at the centrosome and functions in the stabilization of the γ-tubulin ring complex assembly.

TACC3 is a microtubule plus end-tracking protein that promotes axon elongation and also regulates microtubule plus end dynamics in multiple embryonic cell types

TACC3 is a microtubule plus end–tracking protein in vertebrates. TACC3 localizes to the extreme microtubule plus end, where it interacts with XMAP215 to regulate microtubule polymerization. TACC3 is also required to promote normal axon outgrowth, likely through its regulation of microtubule dynamics within the growth cone.

Microtubule plus end dynamics are regulated by a conserved family of proteins called plus end–tracking proteins (+TIPs). It is unclear how various +TIPs interact with each other and with plus ends to control microtubule behavior. The centrosome-associated protein TACC3, a member of the transforming acidic coiled-coil (TACC) domain family, has been implicated in regulating several aspects of microtubule dynamics. However, TACC3 has not been shown to function as a +TIP in vertebrates. Here we show that TACC3 promotes axon outgrowth and regulates microtubule dynamics by increasing microtubule plus end velocities in vivo. We also demonstrate that TACC3 acts as a +TIP in multiple embryonic cell types and that this requires the conserved C-terminal TACC domain. Using high-resolution live-imaging data on tagged +TIPs, we show that TACC3 localizes to the extreme microtubule plus end, where it lies distal to the microtubule polymerization marker EB1 and directly overlaps with the microtubule polymerase XMAP215. TACC3 also plays a role in regulating XMAP215 stability and localizing XMAP215 to microtubule plus ends. Taken together, our results implicate TACC3 as a +TIP that functions with XMAP215 to regulate microtubule plus end dynamics.

Targeting Alp7/TACC to the spindle pole body is essential for mitotic spindle assembly in fission yeast

The conserved TACC protein family localises to the centrosome (the spindle pole body, SPB in fungi) and mitotic spindles, thereby playing a crucial role in bipolar spindle assembly. However, it remains elusive how TACC proteins are recruited to the centrosome/SPB. Here, using fission yeast Alp7/TACC, we have determined clustered five amino acid residues within the TACC domain required for SPB localisation. Critically, these sequences are essential for the functions of Alp7, including proper spindle formation and mitotic progression. Moreover, we have identified pericentrin-like Pcp1 as a loading factor to the mitotic SPB, although Pcp1 is not a sole platform.

Csi1p recruits alp7p/TACC to the spindle pole bodies for bipolar spindle formation

The spindle pole body (SPB) localization of the fission yeast Schizosaccharomyces pombe TACC orthologue alp7p depends on the SPB protein csi1p. Compromised interaction between csi1p and alp7p delays bipolar spindle formation and leads to abnormal chromosome segregation.

Accurate chromosome segregation requires timely bipolar spindle formation during mitosis. The transforming acidic coiled-coil (TACC) family proteins and the ch-TOG family proteins are key players in bipolar spindle formation. They form a complex to stabilize spindle microtubules, mainly dependent on their localization to the centrosome (the spindle pole body [SPB] in yeast). The molecular mechanism underlying the targeting of the TACC–ch-TOG complex to the centrosome remains unclear. Here we show that the fission yeast Schizosaccharomyces pombe TACC orthologue alp7p is recruited to the SPB by csi1p. The csi1p-interacting region lies within the conserved TACC domain of alp7p, and the carboxyl-terminal domain of csi1p is responsible for interacting with alp7p. Compromised interaction between csi1p and alp7p impairs the localization of alp7p to the SPB during mitosis, thus delaying bipolar spindle formation and leading to anaphase B lagging chromosomes. Hence our study establishes that csi1p serves as a linking molecule tethering spindle-stabilizing factors to the SPB for promoting bipolar spindle assembly.

Role of centrosomal adaptor proteins of the TACC family in the regulation of microtubule dynamics during mitotic cell division

During the mitotic division cycle, cells pass through an extensive microtubule rearrangement process where microtubules forming the mitotic spindle apparatus are dynamically instable. Several centrosomal- and microtubule-associated proteins are involved in the regulation of microtubule dynamics and stability during mitosis. Here, we focus on members of the transforming acidic coiled coil (TACC) family of centrosomal adaptor proteins, in particular TACC3, in which their subcellular localization at the mitotic spindle apparatus is controlled by Aurora-A kinase-mediated phosphorylation. At the effector level, several TACC-binding partners have been identified and characterized in greater detail, in particular, the microtubule polymerase XMAP215/ch-TOG/CKAP5 and clathrin heavy chain (CHC). We summarize the recent progress in the molecular understanding of these TACC3 protein complexes, which are crucial for proper mitotic spindle assembly and dynamics to prevent faulty cell division and aneuploidy. In this regard, the (patho)biological role of TACC3 in development and cancer will be discussed.

The centrosomal adaptor TACC3 and the microtubule polymerase chTOG interact via defined C-terminal subdomains in an Aurora-A kinase-independent manner

The cancer-associated, centrosomal adaptor protein TACC3 (transforming acidic coiled-coil 3) and its direct effector, the microtubule polymerase chTOG (colonic and hepatic tumor overexpressed gene), play a crucial function in centrosome-driven mitotic spindle assembly. It is unclear how TACC3 interacts with chTOG. Here, we show that the C-terminal TACC domain of TACC3 and a C-terminal fragment adjacent to the TOG domains of chTOG mediate the interaction between these two proteins. Interestingly, the TACC domain consists of two functionally distinct subdomains, CC1 (amino acids (aa) 414-530) and CC2 (aa 530-630). Whereas CC1 is responsible for the interaction with chTOG, CC2 performs an intradomain interaction with the central repeat region of TACC3, thereby masking the TACC domain before effector binding. Contrary to previous findings, our data clearly demonstrate that Aurora-A kinase does not regulate TACC3-chTOG complex formation, indicating that Aurora-A solely functions as a recruitment factor for the TACC3-chTOG complex to centrosomes and proximal mitotic spindles. We identified with CC1 and CC2, two functionally diverse modules within the TACC domain of TACC3 that modulate and mediate, respectively, TACC3 interaction with chTOG required for spindle assembly and microtubule dynamics during mitotic cell division.

Coordination of adjacent domains mediates TACC3-ch-TOG-clathrin assembly and mitotic spindle binding

A complex of transforming acidic coiled-coil protein 3 (TACC3), colonic and hepatic tumor overexpressed gene (ch-TOG), and clathrin has been implicated in mitotic spindle assembly and in the stabilization of kinetochore fibers by cross-linking microtubules. It is unclear how this complex binds microtubules and how the proteins in the complex interact with one another. TACC3 and clathrin have each been proposed to be the spindle recruitment factor. We have mapped the interactions within the complex and show that TACC3 and clathrin were interdependent for spindle recruitment, having to interact in order for either to be recruited to the spindle. The N-terminal domain of clathrin and the TACC domain of TACC3 in tandem made a microtubule interaction surface, coordinated by TACC3-clathrin binding. A dileucine motif and Aurora A-phosphorylated serine 558 on TACC3 bound to the "ankle" of clathrin. The other interaction within the complex involved a stutter in the TACC3 coiled-coil and a proposed novel sixth TOG domain in ch-TOG, which was required for microtubule localization of ch-TOG but not TACC3-clathrin.

TACC3 is essential for EGF-mediated EMT in cervical cancer

The third member of transforming acidic coiled-coil protein (TACC) family, TACC3, has been shown to be an important player in the regulation of centrosome/microtubule dynamics during mitosis and found to be deregulated in a variety of human malignancies. Our previous studies have suggested that TACC3 may be involved in cervical cancer progression and chemoresistance, and its overexpression can induce epithelial-mesenchymal transition (EMT) by activating the phosphatidylinositol 3-kinase (PI3K)/Akt and extracellular signal-regulated protein kinases (ERKs) signal transduction pathways. However, the upstream mechanisms of TACC3-mediated EMT and its functional/clinical importance in human cervical cancer remain elusive. Epidermal growth factor (EGF) has been shown to be a potent inducer of EMT in cervical cancer and associated with tumor invasion and metastasis. In this study, we found that TACC3 is overexpressed in cervical cancer and can be induced upon EGF stimulation. The induction of TACC3 by EGF is dependent on the tyrosine kinase activity of the EGF receptor (EGFR). Intriguingly, depletion of TACC3 abolishes EGF-mediated EMT, suggesting that TACC3 is required for EGF/EGFR-driven EMT process. Moreover, Snail, a key player in EGF-mediated EMT, is found to be correlated with the expression of TACC3 in cervical cancer. Collectively, our study highlights a novel function for TACC3 in EGF-mediated EMT process and suggests that targeting of TACC3 may be an attractive strategy to treat cervical cancers driven by EGF/EGFR signaling pathways.

Specific removal of TACC3-ch-TOG-clathrin at metaphase deregulates kinetochore fiber tension

Microtubule-associated proteins of the mitotic spindle are thought to be important for the initial assembly and the maintenance of spindle structure and function. However, distinguishing assembly and maintenance roles for a given protein is difficult. Most experimental methods for protein inactivation are slow and therefore affect both assembly and maintenance. Here, we have used 'knocksideways' to rapidly (∼5 minutes) and specifically remove TACC3-ch-TOG-clathrin non-motor complexes from kinetochore fibers (K-fibers). This method allows the complex to be inactivated at defined stages of mitosis. Removal of TACC3-ch-TOG-clathrin after nuclear envelope breakdown caused severe delays in chromosome alignment. Inactivation at metaphase, following a normal prometaphase, significantly delayed progression to anaphase. In these cells, K-fiber tension was reduced and the spindle checkpoint was not satisfied. Surprisingly, there was no significant loss of K-fiber microtubules, even after prolonged removal. TACC3-ch-TOG-clathrin removal during metaphase also resulted in a decrease in spindle length and significant alteration in kinetochore dynamics. Our results indicate that TACC3-ch-TOG-clathrin complexes are important for the maintenance of spindle structure and function as well as for initial spindle assembly.

Transforming acidic coiled-coil proteins (TACCs) in human cancer

Fine-tuned regulation of the centrosome/microtubule dynamics during mitosis is essential for faithful cell division. Thus, it is not surprising that deregulations in this dynamic network can contribute to genomic instability and tumorigenesis. Indeed, centrosome loss or amplification, spindle multipolarity and aneuploidy are often found in a majority of human malignancies, suggesting that defects in centrosome and associated microtubules may be directly or indirectly linked to cancer. Therefore, future research to identify and characterize genes required for the normal centrosome function and microtubule dynamics may help us gain insight into the complexity of cancer, and further provide new avenues for prognostic, diagnostics and therapeutic interventions. Members of the transforming acidic coiled-coil proteins (TACCs) family are emerging as important players of centrosome and microtubule-associated functions. Growing evidence indicates that TACCs are involved in the progression of certain solid tumors. Here, we will discuss our current understanding of the biological function of TACCs, their relevance to human cancer and possible implications for cancer management.