Role of APC and its binding partners in regulating microtubules in mitosis

Spatio-temporal transcriptome dynamics coordinate rapid transition of core crop functions in 'lactating' pigeon

Pigeons (Columba livia) are among a select few avian species that have developed a specialized reproductive mode wherein the parents produce a 'milk' in their crop to feed newborn squabs. Nonetheless, the transcriptomic dynamics and role in the rapid transition of core crop functions during 'lactation' remain largely unexplored. Here, we generated a de novo pigeon genome assembly to construct a high resolution spatio-temporal transcriptomic landscape of the crop epithelium across the entire breeding stage. This multi-omics analysis identified a set of 'lactation'-related genes involved in lipid and protein metabolism, which contribute to the rapid functional transitions in the crop. Analysis of in situ high-throughput chromatin conformation capture (Hi-C) sequencing revealed extensive reorganization of promoter-enhancer interactions linked to the dynamic expression of these 'lactation'-related genes between stages. Moreover, their expression is spatially localized in specific epithelial layers, and can be correlated with phenotypic changes in the crop. These results illustrate the preferential de novo synthesis of 'milk' lipids and proteins in the crop, and provides candidate enhancer loci for further investigation of the regulatory elements controlling pigeon 'lactation'.

β-Tubulin Isotype, TUBB4B, Regulates The Maintenance of Cancer Stem Cells

Recent advancements in cancer research have shown that cancer stem cell (CSC) niche is a crucial factor modulating tumor progression and treatment outcomes. It sustains CSCs by orchestrated regulation of several cytokines, growth factors, and signaling pathways. Although the features defining adult stem cell niches are well-explored, the CSC niche is poorly characterized. Since membrane trafficking proteins have been shown to be essential for the localization of critical proteins supporting CSCs, we investigated the role of TUBB4B, a probable membrane trafficking protein that was found to be overexpressed in the membranes of stem cell enriched cultures, in sustaining CSCs in oral cancer. Here, we show that the knockdown of TUBB4B downregulates the expression of pluripotency markers, depletes ALDH1A1+ population, decreases in vitro sphere formation, and diminishes the tumor initiation potential in vivo. As TUBB4B is not known to have any role in transcriptional regulation nor cell signaling, we suspected that its membrane trafficking function plays a role in constituting a CSC niche. The pattern of its expression in tissue sections, forming a gradient in and around the CSCs, reinforced the notion. Later, we explored its possible cooperation with a signaling protein, Ephrin-B1, the abrogation of which reduces the self-renewal of oral cancer stem cells. Expression and survival analyses based on the TCGA dataset of head and neck squamous cell carcinoma (HNSCC) samples indicated that the functional cooperation of TUBB4 and EFNB1 results in a poor prognosis. We also show that TUBB4B and Ephrin-B1 cohabit in the CSC niche. Moreover, depletion of TUBB4B downregulates the membrane expression of Ephrin-B1 and reduces the CSC population. Our results imply that the dynamics of TUBB4B is decisive for the surface localization of proteins, like Ephrin-B1, that sustain CSCs by their concerted signaling.

The kinesin KIF1C transports APC-dependent mRNAs to cell protrusions

RNA localization and local translation are important for numerous cellular functions. In mammals, a class of mRNAs localize to cytoplasmic protrusions in an APC-dependent manner, with roles during cell migration. Here, we investigated this localization mechanism. We found that the KIF1C motor interacts with APC-dependent mRNAs and is required for their localization. Live cell imaging revealed rapid, active transport of single mRNAs over long distances that requires both microtubules and KIF1C. Two-color imaging directly revealed single mRNAs transported by single KIF1C motors, with the 3'UTR being sufficient to trigger KIF1C-dependent RNA transport and localization. Moreover, KIF1C remained associated with peripheral, multimeric RNA clusters and was required for their formation. These results reveal a widespread RNA transport pathway in mammalian cells, in which the KIF1C motor has a dual role in transporting RNAs and clustering them within cytoplasmic protrusions. Interestingly, KIF1C also transports its own mRNA, suggesting a possible feedback loop acting at the level of mRNA transport.

Adenomatous Polyposis Coli loss controls cell cycle regulators and response to paclitaxel in MDA-MB-157 metaplastic breast cancer cells

Adenomatous Polyposis Coli (APC) is lost in approximately 70% of sporadic breast cancers, with an inclination towards triple negative breast cancer (TNBC). TNBC is treated with traditional chemotherapy, such as paclitaxel (PTX); however, tumors often develop drug resistance. We previously created APC knockdown cells (APC shRNA1) using the human TNBC cells, MDA-MB-157, and showed that APC loss induces PTX resistance. To understand the mechanisms behind APC-mediated PTX response, we performed cell cycle analysis and analyzed cell cycle related proteins. Cell cycle analysis indicated increased G2/M population in both PTX-treated APC shRNA1 and parental cells, suggesting that APC expression does not alter PTX-induced G2/M arrest. We further studied the subcellular localization of the G2/M transition proteins, cyclin B1 and CDK1. The APC shRNA1 cells had increased CDK1, which was preferentially localized to the cytoplasm, and increased baseline CDK6. RNA-sequencing was performed to gain a global understanding of changes downstream of APC loss and identified a broad mis-regulation of cell cycle-related genes in APC shRNA1 cells. Our studies are the first to show an interaction between APC and taxane response in breast cancer. The implications include designing combination therapy to re-sensitize APC-mutant breast cancers to taxanes using the specific cell cycle alterations.

Microtubules: Evolving roles and critical cellular interactions

Microtubules are cytoskeletal elements known as drivers of directed cell migration, vesicle and organelle trafficking, and mitosis. In this review, we discuss new research in the lens that has shed light into further roles for stable microtubules in the process of development and morphogenesis. In the lens, as well as other systems, distinct roles for characteristically dynamic microtubules and stabilized populations are coming to light. Understanding the mechanisms of microtubule stabilization and the associated microtubule post-translational modifications is an evolving field of study. Appropriate cellular homeostasis relies on not only one cytoskeletal element, but also rather an interaction between cytoskeletal proteins as well as other cellular regulators. Microtubules are key integrators with actin and intermediate filaments, as well as cell–cell junctional proteins and other cellular regulators including myosin and RhoGTPases to maintain this balance.

Impact statement

The role of microtubules in cellular functioning is constantly expanding. In this review, we examine new and exciting fields of discovery for microtubule’s involvement in morphogenesis, highlight our evolving understanding of differential roles for stabilized versus dynamic subpopulations, and further understanding of microtubules as a cellular integrator.

Dlg1 activates beta-catenin signaling to regulate retinal angiogenesis and the blood-retina and blood-brain barriers

Beta-catenin (i.e., canonical Wnt) signaling controls CNS angiogenesis and the blood-brain and blood-retina barriers. To explore the role of the Discs large/membrane-associated guanylate kinase (Dlg/MAGUK) family of scaffolding proteins in beta-catenin signaling, we studied vascular endothelial cell (EC)-specific knockout of Dlg1/SAP97. EC-specific loss of Dlg1 produces a retinal vascular phenotype that closely matches the phenotype associated with reduced beta-catenin signaling, synergizes with genetically-directed reductions in beta-catenin signaling components, and can be rescued by stabilizing beta-catenin in ECs. In reporter cells with CRISPR/Cas9-mediated inactivation of Dlg1, transfection of Dlg1 enhances beta-catenin signaling ~4 fold. Surprisingly, Frizzled4, which contains a C-terminal PDZ-binding motif that can bind to Dlg1 PDZ domains, appears to function independently of Dlg1 in vivo. These data expand the repertoire of Dlg/MAGUK family functions to include a role in beta-catenin signaling, and they suggest that proteins other than Frizzled receptors interact with Dlg1 to enhance beta-catenin signaling.

Adenomatous Polyposis Coli as a Scaffold for Microtubule End-Binding Proteins

End-binding proteins (EBs), referred to as the core components of the microtubule plus-end tracking protein network, interact with the C-terminus of the adenomatous polyposis coli (APC) tumor suppressor. This interaction is disrupted in colon cancers expressing truncated APC. APC and EBs act in synergy to regulate microtubule dynamics during spindle formation, chromosome segregation and cell migration. Since EBs autonomously end-track microtubules and partially co-localize with APC at microtubule tips in cells, EBs have been proposed to direct APC to microtubule ends. However, the interdependency of EB and APC localization on microtubules remains elusive. Here, using in vitro reconstitution and single-molecule imaging, we have investigated the interplay between EBs and the C-terminal domain of APC (APC-C) on dynamic microtubules. Our results show that APC-C binds along the microtubule wall but does not accumulate at microtubule tips, even when EB proteins are present. APC-C was also found to enhance EB binding at the extremity of growing microtubules and on the microtubule lattice: APC-C promotes EB end-tracking properties by increasing the time EBs spend at microtubule growing ends, whereas a pool of EBs with a fast turnover accumulates along the microtubule surface. Overall, our results suggest that APC is a promoter of EB interaction with microtubules, providing molecular determinants to reassess the relationship between APC and EBs.

Striatin is a novel modulator of cell adhesion

The adherens junctions (AJs) and tight junctions (TJs) provide critical adhesive contacts between neighboring epithelial cells and are crucial for epithelial adhesion, integrity, and barrier functions in a wide variety of tissues and organisms. The striatin protein family, which are part of the striatin interaction phosphatases and kinases complex, are multidomain scaffolding proteins that play important biologic roles. We have previously shown that striatin colocalizes with the tumor suppressor protein adenomatous polyposis coli in the TJs of epithelial cells. Here we show that striatin affects junction integrity and cell migration, probably through a mechanism that involves the adhesion molecule E-cadherin. Cells engaged in cell-cell adhesion expressed a high MW-modified form of striatin that forms stable associations with detergent-insoluble, membrane-bound cellular fractions. In addition, striatin has recently been suggested to be a target of the poly (ADP-ribose) polymerases Tankyrase 1, and we have found that striatin interacts with Tankyrase 1 and is subsequently poly-ADP-ribosylated. Taken together, our results suggest that striatin is a novel cell-cell junctional protein that functions to maintain correct cell adhesion and may have a role in establishing the relationship between AJs and TJs that is fundamental for epithelial cell-cell adhesion.-Lahav-Ariel, L., Caspi, M., Nadar-Ponniah, P. T., Zelikson, N., Hofmann, I., Hanson, K. K., Franke, W. W., Sklan, E. H., Avraham, K. B., Rosin-Arbesfeld, R. Striatin is a novel modulator of cell adhesion.

Tumor suppressor APC is an attenuator of spindle-pulling forces during C. elegans asymmetric cell division

The adenomatous polyposis coli (APC) tumor suppressor has dual functions in Wnt/β-catenin signaling and accurate chromosome segregation and is frequently mutated in colorectal cancers. Although APC contributes to proper cell division, the underlying mechanisms remain poorly understood. Here we show that Caenorhabditis elegans APR-1/APC is an attenuator of the pulling forces acting on the mitotic spindle. During asymmetric cell division of the C. elegans zygote, a LIN-5/NuMA protein complex localizes dynein to the cell cortex to generate pulling forces on astral microtubules that position the mitotic spindle. We found that APR-1 localizes to the anterior cell cortex in a Par-aPKC polarity-dependent manner and suppresses anterior centrosome movements. Our combined cell biological and mathematical analyses support the conclusion that cortical APR-1 reduces force generation by stabilizing microtubule plus-ends at the cell cortex. Furthermore, APR-1 functions in coordination with LIN-5 phosphorylation to attenuate spindle-pulling forces. Our results document a physical basis for the attenuation of spindle-pulling force, which may be generally used in asymmetric cell division and, when disrupted, potentially contributes to division defects in cancer.

Germline mutation contribution to chromosomal instability

Genomic instability is a feature of cancer that fuels oncogenesis through increased frequency of genetic disruption, leading to loss of genomic integrity and promoting clonal evolution as well as tumor transformation. A form of genomic instability prevalent across cancer types is chromosomal instability, which involves karyotypic changes including chromosome copy number alterations as well as gross structural abnormalities such as transversions and translocations. Defects in cellular mechanisms that are in place to govern fidelity of chromosomal segregation, DNA repair and ultimately genomic integrity are known to contribute to chromosomal instability. In this review, we discuss the association of germline mutations in these pathways with chromosomal instability in the background of related cancer predisposition syndromes. We will also reflect on the impact of genetic predisposition to clinical management of patients and how we can exploit this vulnerability to promote catastrophic genomic instability as a therapeutic strategy.

The connections of Wnt pathway components with cell cycle and centrosome: side effects or a hidden logic?

Wnt signaling cascade has developed together with multicellularity to orchestrate the development and homeostasis of complex structures. Wnt pathway components - such as β-catenin, Dishevelled (DVL), Lrp6, and Axin-- are often dedicated proteins that emerged in evolution together with the Wnt signaling cascade and are believed to function primarily in the Wnt cascade. It is interesting to see that in recent literature many of these proteins are connected with cellular functions that are more ancient and not limited to multicellular organisms - such as cell cycle regulation, centrosome biology, or cell division. In this review, we summarize the recent literature describing this crosstalk. Specifically, we attempt to find the answers to the following questions: Is the response to Wnt ligands regulated by the cell cycle? Is the centrosome and/or cilium required to activate the Wnt pathway? How do Wnt pathway components regulate the centrosomal cycle and cilia formation and function? We critically review the evidence that describes how these connections are regulated and how they help to integrate cell-to-cell communication with the cell and the centrosomal cycle in order to achieve a fine-tuned, physiological response.

Insulin signaling regulates a functional interaction between adenomatous polyposis coli and cytoplasmic dynein

Diabetes is linked to an increased risk for colorectal cancer, but the mechanistic underpinnings of this clinically important effect are unclear. Here we describe an interaction between the microtubule motor cytoplasmic dynein, the adenomatous polyposis coli tumor suppressor protein (APC), and glycogen synthase kinase-3β (GSK-3β), which could shed light on this issue. GSK-3β is perhaps best known for glycogen regulation, being inhibited downstream in an insulin-signaling pathway. However, the kinase is also important in many other processes. Mutations in APC that disrupt the regulation of β-catenin by GSK-3β cause colorectal cancer in humans. Of interest, both APC and GSK-3β interact with microtubules and cellular membranes. We recently demonstrated that dynein is a GSK-3β substrate and that inhibition of GSK-3β promotes dynein-dependent transport. We now report that dynein stimulation in intestinal cells in response to acute insulin exposure (or GSK-3β inhibition) is blocked by tumor-promoting isoforms of APC that reduce an interaction between wild-type APC and dynein. We propose that under normal conditions, insulin decreases dynein binding to APC to stimulate minus end-directed transport, which could modulate endocytic and secretory systems in intestinal cells. Mutations in APC likely impair the ability to respond appropriately to insulin signaling. This is exciting because it has the potential to be a contributing factor in the development of colorectal cancer in patients with diabetes.

Regulation of microtubule (MT)-based cytoskeleton in the seminiferous epithelium during spermatogenesis

In rodents and humans, testicular cells, similar to other mammalian cells, are supported by actin-, microtubule (MT)- and intermediate filament-based cytoskeletons. Although the cytoskeletal network of the testis serves an important role in regulating spermatogenesis during the epithelial cycle, most of the published findings in the literature are limited to studies that only visualize these cytoskeletons in the seminiferous epithelium. Few focus on the underlying molecular mechanism that regulates their organization in the epithelium in response to changes in the stages of the epithelial cycle. Functional studies in the last decade have begun to focus on the role of binding proteins that regulate these cytoskeletons, with some interesting findings rapidly emerging in the field. Since the actin- and intermediate filament-based cytoskeletons have been recently reviewed, herein we focus on the MT-based cytoskeleton for two reasons. First, besides serving as a structural support cytoskeleton, MTs are known to serve as the track to support and facilitate the transport of germ cells, such as preleptotene spermatocytes connected in clones and elongating/elongated spermatids during spermiogenesis, across the blood-testis barrier (BTB) and the adluminal compartment, respectively, during spermatogenesis. While these cellular events are crucial to the completion of spermatogenesis, they have been largely ignored in the past. Second, MT-based cytoskeleton is working in concert with the actin-based cytoskeleton to provide structural support for the transport of intracellular organelles across the cell cytosol, such as endosome-based vesicles, and phagosomes, which contain residual bodies detached from spermatids, to maintain the cellular homeostasis in the seminiferous epithelium. We critically evaluate some recent published findings herein to support a hypothesis regarding the role of MT in conferring germ cell transport in the seminiferous epithelium.

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.

Two Chinese pedigrees for adenomatous polyposis coli: new mutations at codon 1309 and predisposition to phenotypic variations

Familial adenomatous polyposis (FAP) is an autosomal dominant inherited disease caused by a mutation in the adenomatous polyposis coli (APC) gene. Some studies have attempted to correlate mutations at codon 1309 with classic FAP (≥100 colorectal polyps). We report two Chinese FAP pedigrees with new frameshift mutations at codon 1309, in which affected individuals manifest phenotypic variations. Comprehensive physical examinations were performed for all living individuals and the medical data of deceased patients were collected. Screening of the APC and human mutY homolog (MUTYH) genes for germline mutations was conducted by direct polymerase chain reaction (PCR) sequencing. In two pedigrees, a heterozygous deletion in exon 16 of the APC gene was present in all FAP patients but absent in the unaffected individuals. There were no changes to the MUTYH gene. The first pedigree, with a new frameshift mutation at c.3926_3930 del AAAAG (p. Glu1309Aspfs X4), exhibited obvious differences in the polyp number such that the proband manifested only three colorectal polyps, whereas another patients showed the symptoms of classic FAP. The second pedigree, also traced a new mutation at c.3922_3925 del AAAG (p. Glu1309Argfs X11). Although all of the patients presented with classic polyposis, one of them exhibited a delayed onset of colorectal cancer in his 50s. Two novel mutations at codon 1309 in two Chinese families suffering from FAP could enrich the germline mutation spectrum of the APC gene. Families of individuals might manifest different phenotypes, even with an identical codon 1309 mutation, unlike in previous studies.

Wnt/β-catenin signaling pathway and colorectal cancer

The Wnt/β-catenin signaling pathway is an extensively studied Wnt signaling pathway. It plays a critical role in embryogenesis, organogenesis and tumorigenesis. Colorectal cancer has high rates of incidence and mortality, and it is critical to elucidate the mechanisms responsible for the pathogenesis of colorectal cancer. Molecular studies indicate that aberrant activation of the Wnt/β-catenin signaling pathway occurs in colorectal cancer frequently, and this signaling pathway may provide new potential targets for the treatment of colorectal cancer. In this article, we give an overview of the Wnt/β-catenin signaling pathway and discuss its relationship with colorectal cancer and its potential use as therapeutic targets for this malignancy.

The special case of hepatocytes: unique tissue architecture calls for a distinct mode of cell division

Columnar epithelia (e.g., kidney, intestine) and hepatocytes embody the two major organizational phenotypes of non-stratified epithelial cells. Columnar epithelia establish their apical and basal domains at opposing poles and organize in monolayered cysts and tubules, in which their apical surfaces form a single continuous lumen whereas hepatocytes establish their apical domains in the midst of their basolateral domains and organize a highly branched capillary luminal network, the bile canaliculi, in which a single hepatocyte can engage in lumen formation with multiple neighbors. To maintain their distinct tissue architectures, columnar epithelial cells bisect their luminal domains during symmetric cell divisions, while the cleavage furrow in dividing hepatocytes avoids bisecting the bile canalicular domains. We discuss recently discovered molecular mechanisms that underlie the different cell division phenotypes in columnar and hepatocytic model cell lines. The serine/threonine kinase Par1b determines both the epithelial lumen polarity and cell division phenotype via cell adhesion signaling that converges on the small GTPase RhoA.

MicroRNA-452 contributes to the docetaxel resistance of breast cancer cells

MicroRNA-452 (miRNA-452) was overexpressed in docetaxel-resistant human breast cancer MCF-7 cells (MCF-7/DOC). However, its role in modulating the sensitivity of breast cancer cells to docetaxel (DOC) remains unclear. The aim of this study is to investigate the role of miRNA-452 in the sensitivity of breast cancer cells to DOC.Real-time quantitative PCR (RT-qPCR) were used to identify the differential expression of miRNA-452 between MCF-7/DOC and MCF-7 cells. MiRNA-452 mimic was transfected into MCF-7 cells and miRNA-452 inhibitor was transfected into MCF-7/DOC cells. The role of miRNA-452 in these transfected cells was evaluated using RT-qPCR, MTT assay, and flow cytometry assay. The relationship of miRNA-452 and its predictive target gene "anaphase-promoting complex 4" (APC4) was analyzed by RT-qPCR and Western blot.MiRNA-452 showed significantly higher expression (78.9-folds) in MCF-7/DOC cells compared to parental MCF-7 cells. The expression of miRNA-452 in the mimic transfected MCF-7 cells was upregulated 212.2-folds (P < 0.05) compared to its negative control (NC), and the half maximal inhibitory concentration (IC50) value of DOC (1.98 ± 0.15 μM) was significantly higher than that in its NC (0.85 ± 0.08 μM, P < 0.05) or blank control (1.01 ± 0.19 μM, P < 0.05). Furthermore, its apoptotic rate (6.3 ± 1.3 %) was distinctly decreased compared with that in its NC (23.8 ± 6.6 %, P < 0.05) or blank control (18.6 ± 4.7 %, P < 0.05). In contrast, the expression of miRNA-452 in the inhibitor-transfected MCF-7/DOC cells was downregulated 0.58-fold (P < 0.05) compared to its NC, the IC50 value of DOC (44.5 ± 3.2 μM) was significantly lower than that in its NC (107.3 ± 6.63 μM, P < 0.05) or blank control (102.22 ± 11.34 μM, P < 0.05), and the apoptotic rate (45.5 ± 10.8 %) was distinctly increased compared with its NC (9.9 ± 2.2 %, P < 0.05) and blank control (9.4 ± 2.5 %, P < 0.05). Further, there was an inverse association between miRNA-452 and APC4 expression in breast cancer cells in vitro.Dysregulation of miRNA-452 involved in the DOC resistance formation of breast cancer cells may be, in part, via targeting APC4.

Wnt signaling proteins associate with the nuclear pore complex: implications for cancer

Several components of the Wnt signaling pathway have in recent years been linked to the nuclear pore complex. β-catenin, the primary transducer of Wnt signals from the plasma membrane to the nucleus, has been shown to transiently associate with different FG-repeat containing nucleoporins (Nups) and to translocate bidirectionally through pores of the nuclear envelope in a manner independent of classical transport receptors and the Ran GTPase. Two key regulators of β-catenin, IQGAP1 and APC, have also been reported to bind specific Nups or to locate at the nuclear pore complex. The interaction between these Wnt signaling proteins and different Nups may have functional implications beyond nuclear transport in cellular processes that include mitotic regulation, centrosome positioning and cell migration, nuclear envelope assembly/disassembly, and the DNA replication checkpoint. The broad implications of interactions between Wnt signaling proteins and Nups will be discussed in the context of cancer.

APC2 and Axin promote mitotic fidelity by facilitating centrosome separation and cytoskeletal regulation

To ensure the accurate transmission of genetic material, chromosome segregation must occur with extremely high fidelity. Segregation errors lead to chromosomal instability (CIN), with deleterious consequences. Mutations in the tumor suppressor adenomatous polyposis coli (APC) initiate most colon cancers and have also been suggested to promote disease progression through increased CIN, but the mechanistic role of APC in preventing CIN remains controversial. Using fly embryos as a model, we investigated the role of APC proteins in CIN. Our findings suggest that APC2 loss leads to increased rates of chromosome segregation error. This occurs through a cascade of events beginning with incomplete centrosome separation leading to failure to inhibit formation of ectopic cleavage furrows, which result in mitotic defects and DNA damage. We test several hypotheses related to the mechanism of action of APC2, revealing that APC2 functions at the embryonic cortex with several protein partners, including Axin, to promote mitotic fidelity. Our in vivo data demonstrate that APC2 protects genome stability by modulating mitotic fidelity through regulation of the cytoskeleton.

A relay mechanism between EB1 and APC facilitate STIM1 puncta assembly at endoplasmic reticulum-plasma membrane junctions

The assembly of STIM1 protein puncta near endoplasmic reticulum-plasma membrane (ER-PM) junctions is required for optimal activation of store-operated channels (SOC). The mechanisms controlling the translocation of STIM1 puncta to ER-PM junctions remain largely unknown. In the present study, we have explored the role of the microtubule binding protein adenomatous polyposis coli (APC), on STIM1 puncta and store-operated calcium entry (SOCE). APC-depleted cells showed reduced STIM1 puncta near ER-PM junctions, instead puncta is found at the ER surrounding the cell nucleus. Reduced STIM1 puncta near ER-PM junctions in APC-depleted cells correlates with a strong inhibition of SOCE and diminished Orai whole-cell currents. Immunoprecipitation and confocal microscopy co-localization studies indicate that, upon depletion of the ER, STIM1 dissociates from EB1 and associates to APC. Deletion analysis identified an APC-binding domain in the carboxyl terminus of STIM1 (STIM1 650-685). These results together position APC as an important element in facilitating the translocation of STIM1 puncta near ER-PM junctions, which in turn is required for efficient SOCE and Orai activation upon depletion of the ER.

The evolution and diversification of plant microtubule-associated proteins

Plant evolution is marked by major advances in structural characteristics that facilitated the highly successful colonization of dry land. Underlying these advances is the evolution of genes encoding specialized proteins that form novel microtubular arrays of the cytoskeleton. This review investigates the evolution of plant families of microtubule-associated proteins (MAPs) through the recently sequenced genomes of Arabidopsis thaliana, Oryza sativa, Selaginella moellendorffii, Physcomitrella patens, Volvox carteri and Chlamydomonas reinhardtii. The families of MAPs examined are AIR9, CLASP, CRIPT, MAP18, MOR1, TON, EB1, AtMAP70, SPR2, SPR1, WVD2 and MAP65 families (abbreviations are defined in the footnote to Table 1). Conjectures are made regarding the evolution of MAPs in plants in relation to the evolution of multicellularity, oriented cell division and vasculature. Angiosperms in particular have high numbers of proteins that are involved in promotion of helical growth or its suppression, and novel plant microtubular structures may have acted as a catalyst for the development of novel plant MAPs. Comparisons of plant MAP gene families with those of animals show that animals may have more flexibility in the structure of their microtubule cytoskeletons than plants, but with both plants and animals possessing many MAP splice variants.

Mitotic spindle (DIS)orientation and DISease: cause or consequence?

Correct alignment of the mitotic spindle during cell division is crucial for cell fate determination, tissue organization, and development. Mutations causing brain diseases and cancer in humans and mice have been associated with spindle orientation defects. These defects are thought to lead to an imbalance between symmetric and asymmetric divisions, causing reduced or excessive cell proliferation. However, most of these disease-linked genes encode proteins that carry out multiple cellular functions. Here, we discuss whether spindle orientation defects are the direct cause for these diseases, or just a correlative side effect.

+TIPs: SxIPping along microtubule ends

+TIPs are a heterogeneous class of proteins that specifically bind to growing microtubule ends. Because dynamic microtubules are essential for many intracellular processes, +TIPs play important roles in regulating microtubule dynamics and microtubule interactions with other intracellular structures. End-binding proteins (EBs) recognize a structural cap at growing microtubule ends, and have emerged as central adaptors that mediate microtubule plus-end tracking of potentially all other +TIPs. The majority of these +TIPs bind to EBs through a short hydrophobic (S/T)x(I/L)P sequence motif (SxIP) and surrounding electrostatic interactions. These recent discoveries have resulted in a rapid expansion of the number of possible +TIPs. In this review, we outline our current understanding of the molecular mechanism of plus-end tracking and provide an overview of SxIP-recruited +TIPs.

Regulation of Wnt signaling by the tumor suppressor adenomatous polyposis coli does not require the ability to enter the nucleus or a particular cytoplasmic localization

Wnt signaling plays key roles in development and disease. The tumor suppressor adenomatous polyposis coli (APC) is an essential negative regulator of Wnt signaling. Its best-characterized role is as part of the destruction complex, targeting the Wnt effector β-catenin (βcat) for phosphorylation and ultimate destruction, but several studies suggested APC also may act in the nucleus at promoters of Wnt-responsive genes or to shuttle βcat out for destruction. Even in its role in the destruction complex, APC's mechanism of action remains mysterious. We have suggested APC positions the destruction complex at the appropriate subcellular location, facilitating βcat destruction. In this study, we directly tested APC's proposed roles in the nucleus or in precisely localizing the destruction complex by generating a series of APC2 variants to which we added tags relocalizing otherwise wild-type APC to different cytoplasmic locations. We tested these for function in human colon cancer cells and Drosophila embryos. Strikingly, all rescue Wnt regulation and down-regulate Wnt target genes in colon cancer cells, and most restore Wnt regulation in Drosophila embryos null for both fly APCs. These data suggest that APC2 does not have to shuttle into the nucleus or localize to a particular subcellular location to regulate Wnt signaling.

Regulation of Wnt signaling by the tumor suppressor adenomatous polyposis coli does not require the ability to enter the nucleus or a particular cytoplasmic localization

Wnt signaling plays key roles in development and disease. The tumor suppressor adenomatous polyposis coli (APC) is an essential negative regulator of Wnt signaling. Its best-characterized role is as part of the destruction complex, targeting the Wnt effector β-catenin (βcat) for phosphorylation and ultimate destruction, but several studies suggested APC also may act in the nucleus at promoters of Wnt-responsive genes or to shuttle βcat out for destruction. Even in its role in the destruction complex, APC's mechanism of action remains mysterious. We have suggested APC positions the destruction complex at the appropriate subcellular location, facilitating βcat destruction. In this study, we directly tested APC's proposed roles in the nucleus or in precisely localizing the destruction complex by generating a series of APC2 variants to which we added tags relocalizing otherwise wild-type APC to different cytoplasmic locations. We tested these for function in human colon cancer cells and Drosophila embryos. Strikingly, all rescue Wnt regulation and down-regulate Wnt target genes in colon cancer cells, and most restore Wnt regulation in Drosophila embryos null for both fly APCs. These data suggest that APC2 does not have to shuttle into the nucleus or localize to a particular subcellular location to regulate Wnt signaling.

APC as a mobile scaffold: regulation and function at the nucleus, centrosomes, and mitochondria

Genetic mutations of adenomatous polyposis coli (APC) predispose to high risk of human colon cancer. APC is a large tumor suppressor protein and truncating mutations disrupt its normal roles in regulating cell migration, DNA replication/repair, mitosis, apoptosis, and turnover of oncogenic β-catenin. APC is targeted to multiple subcellular sites, and here we discuss recent evidence implicating novel protein interactions and functions of APC in the nucleus and at centrosomes and mitochondria. The ability of APC to shuttle between these and other cell locations is hypothesized to be integral to its cellular function.

Modulation of the ligand-independent traffic of Notch by Axin and Apc contributes to the activation of Armadillo in Drosophila

There is increasing evidence for close functional interactions between Wnt and Notch signalling. In many instances, these are mediated by convergence of the signalling events on common transcriptional targets, but there are other instances that cannot be accounted for in this manner. Studies in Drosophila have revealed that an activated form of Armadillo, the effector of Wnt signalling, interacts with, and is modulated by, the Notch receptor. Specifically, the ligand-independent traffic of Notch serves to set up a threshold for the amount of this form of Armadillo and therefore for Wnt signalling. In the current model of Wnt signalling, a complex assembled around Axin and Apc allows GSK3 (Shaggy) to phosphorylate Armadillo and target it for degradation. However, genetic experiments suggest that the loss of function of any of these three elements does not have the same effect as elevating the activity of β-catenin. Here, we show that Axin and Apc, but not GSK3, modulate the ligand-independent traffic of Notch. This finding helps to explain unexpected differences in the phenotypes obtained by different ways of activating Armadillo function and provides further support for the notion that Wnt and Notch signalling form a single functional module.