Mitochondrial dynamics: molecular mechanisms and the role in the heart

Mitochondria are dynamic organelles which actively move along the cytoskeleton within the cell, change their shape and undergo fusion and fission. The heart is a metabolically active organ with high energy demands and rich in mitochondria. Mitochondria not only supply the heart with the high energy compound, adenosine triphosphate (ATP), but also actively participate in cell signaling and apoptotic events and communicate with the cytosol. Recent advantages in molecular biology and imaging techniques helped to study mitochondrial dynamics directly in the cell and under real time conditions. In this review, I will briefly summarize current knowledge about molecular machinery mediating mitochondrial fusion/ fission, its link to apoptosis and cardiovascular disease.

Mammalian SUN protein interaction networks at the inner nuclear membrane and their role in laminopathy disease processes

The nuclear envelope (NE) LINC complex, in mammals comprised of SUN domain and nesprin proteins, provides a direct connection between the nuclear lamina and the cytoskeleton, which contributes to nuclear positioning and cellular rigidity. SUN1 and SUN2 interact with lamin A, but lamin A is only required for NE localization of SUN2, and it remains unclear how SUN1 is anchored. Here, we identify emerin and short nesprin-2 isoforms as novel nucleoplasmic binding partners of SUN1/2. These have overlapping binding sites distinct from the lamin A binding site. However, we demonstrate that tight association of SUN1 with the nuclear lamina depends upon a short motif within residues 209-228, a region that does not interact significantly with known SUN1 binding partners. Moreover, SUN1 localizes correctly in cells lacking emerin. Importantly then, the major determinant of SUN1 NE localization has yet to be identified. We further find that a subset of lamin A mutations, associated with laminopathies Emery-Dreifuss muscular dystrophy (EDMD) and Hutchinson-Gilford progeria syndrome (HGPS), disrupt lamin A interaction with SUN1 and SUN2. Despite this, NE localization of SUN1 and SUN2 is not impaired in cell lines from either class of patients. Intriguingly, SUN1 expression at the NE is instead enhanced in a significant proportion of HGPS but not EDMD cells and strongly correlates with pre-lamin A accumulation due to preferential interaction of SUN1 with pre-lamin A. We propose that these different perturbations in lamin A-SUN protein interactions may underlie the opposing effects of EDMD and HGPS mutations on nuclear and cellular mechanics.

TBCD links centriologenesis, spindle microtubule dynamics, and midbody abscission in human cells

Microtubule-organizing centers recruit alpha- and beta-tubulin polypeptides for microtubule nucleation. Tubulin synthesis is complex, requiring five specific cofactors, designated tubulin cofactors (TBCs) A-E, which contribute to various aspects of microtubule dynamics in vivo. Here, we show that tubulin cofactor D (TBCD) is concentrated at the centrosome and midbody, where it participates in centriologenesis, spindle organization, and cell abscission. TBCD exhibits a cell-cycle-specific pattern, localizing on the daughter centriole at G1 and on procentrioles by S, and disappearing from older centrioles at telophase as the protein is recruited to the midbody. Our data show that TBCD overexpression results in microtubule release from the centrosome and G1 arrest, whereas its depletion produces mitotic aberrations and incomplete microtubule retraction at the midbody during cytokinesis. TBCD is recruited to the centriole replication site at the onset of the centrosome duplication cycle. A role in centriologenesis is further supported in differentiating ciliated cells, where TBCD is organized into "centriolar rosettes". These data suggest that TBCD participates in both canonical and de novo centriolar assembly pathways.

Role of GD3-CLIPR-59 association in lymphoblastoid T cell apoptosis triggered by CD95/Fas

We previously found that a directional movement of the raft component GD3 towards mitochondria, by its association with microtubules, was mandatory to late apoptogenic events triggered by CD95/Fas. Since CLIPR-59, CLIP-170-related protein, has recently been identified as a microtubule binding protein associated with lipid rafts, we analyzed the role of GD3-CLIPR-59 association in lymphoblastoid T cell apoptosis triggered by CD95/Fas. To test whether CLIPR-59 could play a role at the raft-microtubule junction, we performed a series of experiments by using immunoelectron microscopy, static or flow cytometry and biochemical analyses. We first assessed the presence of CLIPR-59 molecule in lymphoblastoid T cells (CEM). Then, we demonstrated that GD3-microtubule interaction occurs via CLIPR-59 and takes place at early time points after CD95/Fas ligation, preceding the association GD3-tubulin. GD3-CLIPR-59 association was demonstrated by fluorescence resonance energy transfer (FRET) analysis. The key role of CLIPR-59 in this dynamic process was clarified by the observation that silencing CLIPR-59 by siRNA affected the kinetics of GD3-tubulin association, spreading of GD3 towards mitochondria and apoptosis execution. We find that CLIPR-59 may act as a typical chaperone, allowing a prompt interaction between tubulin and the raft component GD3 during cell apoptosis triggered by CD95/Fas. On the basis of the suggested role of lipid rafts in conveying pro-apoptotic signals these results disclose new perspectives in the understanding of the mechanisms by which raft-mediated pro-apoptotic signals can directionally reach their target, i.e. the mitochondria, and trigger apoptosis execution.

Enhanced tumor radiosensitivity by a survivin dominant-negative mutant

Radiosensitivity of tumors is due to a complex interaction of various factors, it has been reported that survivin also acts as a constitutive and inducible radioresistance factor in a panel of tumor cells and approaches designed to inhibit survivin expression or function may lead to tumor sensitisation to chemical and physical agents. Previously, we found that the plasmid encoding the phosphorylation-defective mouse survivin threonine 34-->alanine mutant complexed to DOTAP-chol liposome (Lip-mS) can suppress murine primary breast carcinoma. However, little is known regarding the biological effect of Lip-mS combined with radiation. The present study was designed to determine whether Lip-mS could enhance the anti-tumor activity of radiation. The Lewis Lung Carcinoma (LLC) cells treated with a combination of Lip-mS and radiation displayed apparently increased apoptosis compared with those treated with Lip-mS or radiation alone. Mice bearing LLC tumors were treated with intravenous injections of Lip-mS and radiation, the combined treatment significantly reduced mean tumor volume compared with either treatment alone. Moreover, the anti-tumor effect of Lip-mS combined with radiation was greater than their additive effect when compared with the expected effect of the combined treatment. These data suggest that inhibition of survivin using a dominant-negative mutant, survivin T34A, could sensitize LLC cells to radiation efficiently and the synergistic anti-tumor activity may in part result from increasing the apoptosis of tumor cells, inhibiting tumor angiogenesis and inducing a tumor-protective immune response in the combined treatment.

Deletions and point mutations of LRRC50 cause primary ciliary dyskinesia due to dynein arm defects

Genetic defects affecting motility of cilia and flagella cause chronic destructive airway disease, randomization of left-right body asymmetry, and, frequently, male infertility in primary ciliary dyskinesia (PCD). The most frequent defects involve outer and inner dynein arms (ODAs and IDAs) that are large multiprotein complexes responsible for cilia-beat generation and regulation, respectively. Here, we demonstrate that large genomic deletions, as well as point mutations involving LRRC50, are responsible for a distinct PCD variant that is characterized by a combined defect involving assembly of the ODAs and IDAs. Functional analyses showed that LRRC50 deficiency disrupts assembly of distally and proximally DNAH5- and DNAI2-containing ODA complexes, as well as DNALI1-containing IDA complexes, resulting in immotile cilia. On the basis of these findings, we assume that LRRC50 plays a role in assembly of distinct dynein-arm complexes.

Loss-of-function mutations in the human ortholog of Chlamydomonas reinhardtii ODA7 disrupt dynein arm assembly and cause primary ciliary dyskinesia

Cilia and flagella are evolutionarily conserved structures that play various physiological roles in diverse cell types. Defects in motile cilia result in primary ciliary dyskinesia (PCD), the most prominent ciliopathy, characterized by the association of respiratory symptoms, male infertility, and, in nearly 50% of cases, situs inversus. So far, most identified disease-causing mutations involve genes encoding various ciliary components, such those belonging to the dynein arms that are essential for ciliary motion. Following a candidate-gene approach based on data from a mutant strain of the biflagellated alga Chlamydomonas reinhardtii carrying an ODA7 defect, we identified four families with a PCD phenotype characterized by the absence of both dynein arms and loss-of-function mutations in the human orthologous gene called LRRC50. Functional analyses performed in Chlamydomonas reinhardtii and in another flagellated protist, Trypanosoma brucei, support a key role for LRRC50, a member of the leucine-rich-repeat superfamily, in cytoplasmic preassembly of dynein arms.

The calmodulin-related calcium sensor CML42 plays a role in trichome branching

Calcium (Ca(2+)) is a key second messenger in eukaryotes where it regulates a diverse array of cellular processes in response to external stimuli. An important Ca(2+) sensor in both animals and plants is calmodulin (CaM). In addition to evolutionarily conserved CaM, plants possess a unique family of CaM-like (CML) proteins. The majority of these CMLs have not yet been studied, and investigation into their physical properties and cellular functions will provide insight into Ca(2+) signal transduction in plants. Here we describe the characterization of CML42, a 191-amino acid Ca(2+)-binding protein from Arabidopsis. Ca(2+) binding to recombinant CML42 was assessed by fluorescence spectroscopy, NMR spectroscopy, microcalorimetry, and CD spectroscopy. CML42 displays significant alpha-helical secondary structure, binds three molecules of Ca(2+) with affinities ranging from 30 to 430 nm, and undergoes a Ca(2+)-induced conformational change that results in the exposure of one or more hydrophobic regions. Gene expression analysis revealed CML42 transcripts at various stages of development and in many cell types, including the support cells, which surround trichomes (leaf hairs) on the leaf surface. Using yeast two-hybrid screening we identified a putative CML42 interactor; kinesin-interacting Ca(2+)-binding protein (KIC). Because KIC is a protein known to function in trichome development, we examined transgenic CML42 knockout plants and found that they possess aberrant trichomes with increased branching. Collectively, our data support a role for CML42 as a Ca(2+) sensor that functions during cell branching in trichomes.

Control of insulin granule dynamics by AMPK dependent KLC1 phosphorylation

The movement of insulin granules along microtubules, driven by kinesin-1/Kif5B, is essential for glucose-stimulated insulin secretion from pancreatic β-cells.  5΄AMP-activated protein kinase (AMPK) is a heterotrimeric serine/threonine kinase, which is activated in β-cells at low glucose concentrations, but inhibited as glucose levels increase.  AMPK activation blocks glucose-stimulated recruitment of secretory granules to the cell surface and insulin secretion, suggesting motor proteins may be targets for this kinase. Whilst both kinesin-1/Kif5B and kinesin light chain-1 (KLC1) contain consensus AMPK phosphorylation sites only a peptide corresponding to Ser520 in mouse KLC1 and purified recombinant GST-KLC1 were phosphorylated by purified AMPK in vitro.  To test the hypothesis that phosphorylation at this site may modulate kinesin-1-mediated granule movement, we developed a novel approach to study the dynamics of the granules within a cell in three-dimensions using Nokigawa spinning disc confocal microscopy.  This cell-wide approach revealed that the number of longer excursions (>10 µm) increased significantly in response to elevated glucose concentration (30 vs 3 mM) in control MIN6 cells.  However, similar changes were seen in cells over-expressing wild-type KLC1, phosphomimetic (S517/520D) or non-phosphorylatable (S517/520A) mutants of KLC1.  Moreover, no evidence for a change in the phosphorylation state of KLC1 at Ser520 after AMPK activation was obtained using an anti-phospho Ser520-specific antibody. Thus, changes in the phosphorylation state of KLC1 at Ser517/520 are unlikely to affect motor function.  In conclusion, we describe a new three-dimensional cell wide approach for the analysis of secretory granule dynamics in living β-cells.

Roles for the conserved spc105p/kre28p complex in kinetochore-microtubule binding and the spindle assembly checkpoint

BACKGROUND

Kinetochores attach sister chromatids to microtubules of the mitotic spindle and orchestrate chromosome disjunction at anaphase. Although S. cerevisiae has the simplest known kinetochores, they nonetheless contain approximately 70 subunits that assemble on centromeric DNA in a hierarchical manner. Developing an accurate picture of the DNA-binding, linker and microtubule-binding layers of kinetochores, including the functions of individual proteins in these layers, is a key challenge in the field of yeast chromosome segregation. Moreover, comparison of orthologous proteins in yeast and humans promises to extend insight obtained from the study of simple fungal kinetochores to complex animal cell kinetochores.

PRINCIPAL FINDINGS

We show that S. cerevisiae Spc105p forms a heterotrimeric complex with Kre28p, the likely orthologue of the metazoan kinetochore protein Zwint-1. Through systematic analysis of interdependencies among kinetochore complexes, focused on Spc105p/Kre28p, we develop a comprehensive picture of the assembly hierarchy of budding yeast kinetochores. We find Spc105p/Kre28p to comprise the third linker complex that, along with the Ndc80 and MIND linker complexes, is responsible for bridging between centromeric heterochromatin and kinetochore MAPs and motors. Like the Ndc80 complex, Spc105p/Kre28p is also essential for kinetochore binding by components of the spindle assembly checkpoint. Moreover, these functions are conserved in human cells.

CONCLUSIONS/SIGNIFICANCE

Spc105p/Kre28p is the last of the core linker complexes to be analyzed in yeast and we show it to be required for kinetochore binding by a discrete subset of kMAPs (Bim1p, Bik1p, Slk19p) and motors (Cin8p, Kar3p), all of which are nonessential. Strikingly, dissociation of these proteins from kinetochores prevents bipolar attachment, even though the Ndc80 and DASH complexes, the two best-studied kMAPs, are still present. The failure of Spc105 deficient kinetochores to bind correctly to spindle microtubules and to recruit checkpoint proteins in yeast and human cells explains the observed severity of missegregation phenotypes.

Isoform-selective chemical inhibition of mDia-mediated actin assembly

Formins are potent actin assembly factors. Diaphanous formins, including mDia1, mDia2, and mDia3 in mammals, are implicated in mitosis and cytokinesis, but no chemical interactors have been reported. We developed an in vitro screen for inhibitors of actin assembly by mDia1 and identified an inhibitor of mDia1 and mDia2 that does not inhibit mDia3 at the concentrations tested. These results establish the druggability of mDia formins and introduce a first-generation inhibitor.

TgICMAP1 is a novel microtubule binding protein in Toxoplasma gondii

The microtubule cytoskeleton provides essential structural support for all eukaryotic cells and can be assembled into various higher order structures that perform drastically different functions. Understanding how microtubule-containing assemblies are built in a spatially and temporally controlled manner is therefore fundamental to understanding cell physiology. Toxoplasma gondii, a protozoan parasite, contains at least five distinct tubulin-containing structures, the spindle pole, centrioles, cortical microtubules, the conoid, and the intra-conoid microtubules. How these five structurally and functionally distinct sets of tubulin containing structures are constructed and maintained in the same cell is an intriguing problem. Previously, we performed a proteomic analysis of the T. gondii apical complex, a cytoskeletal complex located at the apical end of the parasite that is composed of the conoid, three ring-like structures, and the two short intra-conoid microtubules. Here we report the characterization of one of the proteins identified in that analysis, TgICMAP1. We show that TgICMAP1 is a novel microtubule binding protein that can directly bind to microtubules in vitro and stabilizes microtubules when ectopically expressed in mammalian cells. Interestingly, in T. gondii, TgICMAP1 preferentially binds to the intra-conoid microtubules, providing us the first molecular tool to investigate the intra-conoid microtubule assembly process during daughter construction.

Perinatal survivin is essential for the establishment of pancreatic beta cell mass in mice

AIMS/HYPOTHESIS

Pancreatic beta cells undergo dynamic remodelling during the perinatal period, with enhanced neogenesis, proliferation and apoptosis observed. The molecular mechanisms responsible for these processes have yet to be elucidated. Survivin is an inhibitor of apoptosis, first described as being exclusively expressed in tumour and embryonic tissues with regulatory functions in mitosis and apoptosis. The aim of the present study was to define the essential physiological role of survivin in the pancreas.

METHODS

The expression profile of survivin was assessed in the mouse pancreas, and we generated a Pdx1 promoter-driven Survivin (also known as Birc5) knockout mouse using the Cre-loxP recombination system to determine the essential physiological function of survivin in the pancreas.

RESULTS

Survivin is transiently expressed in mouse pancreatic islets during the embryonic and neonatal periods. Targeted deletion of Survivin in the pancreas resulted in a significant decline in beta cell mass throughout the perinatal period, leading to glucose intolerance in the adult. Survivin-deficient islets showed decreased cell proliferation as a result of a delay in cell cycle progression with perturbations in cell cycle proteins. Survivin did not, however, play an essential role in beta cell apoptosis either during the physiological remodelling period or in response to streptozotocin. Islet development, islet architecture, microvasculature and apoptosis were not affected by the absence of survivin in the pancreas.

CONCLUSIONS/INTERPRETATION

Survivin expression in the pancreatic islets during the perinatal remodelling period is essential for the establishment of beta cell mass through cell cycle regulation.

Predominant regulators of tubulin monomer-polymer partitioning and their implication for cell polarization

The microtubule-system organizes the cytoplasm during interphase and segregates condensed chromosomes during mitosis. Four unrelated conserved proteins, XMAP215/Dis1/TOGp, MCAK, MAP4 and Op18/stathmin, have all been implicated as predominant regulators of tubulin monomer-polymer partitioning in animal cells. However, while studies employing the Xenopus egg extract model system indicate that the partitioning is largely governed by the counteractive activities of XMAP215 and MCAK, studies of human cell lines indicate that MAP4 and Op18 are the predominant regulators of the interphase microtubule-array. Here, we review functional interplay of these proteins during interphase and mitosis in various cell model systems. We also review the evidence that MAP4 and Op18 have interphase-specific, counteractive and phosphorylation-inactivated activities that govern tubulin subunit partitioning in many mammalian cell types. Finally, we discuss evidence indicating that partitioning regulation by MAP4 and Op18 may be of significance to establish cell polarity.

Motor-independent targeting of CLASPs to kinetochores by CENP-E promotes microtubule turnover and poleward flux

Efficient chromosome segregation during mitosis relies on the coordinated activity of molecular motors with proteins that regulate kinetochore attachments to dynamic spindle microtubules [1]. CLASPs are conserved kinetochore- and microtubule-associated proteins encoded by two paralog genes, clasp1 and clasp2, and have been previously implicated in the regulation of kinetochore microtubule dynamics [2-4]. However, it remains unknown how CLASPs work in concert with other proteins to form a functional kinetochore microtubule interface. Here we have identified mitotic interactors of human CLASP1 via a proteomic approach. Among these, the microtubule plus-end-directed motor CENP-E [5] was found to form a complex with CLASP1 that colocalizes to multiple structures of the mitotic apparatus in human cells. We found that CENP-E recruits both CLASP1 and CLASP2 to kinetochores independently of its motor activity or the presence of microtubules. Depletion of CLASPs or CENP-E by RNA interference in human cells causes a significant and comparable reduction of kinetochore microtubule poleward flux and turnover rates and rescues spindle bipolarity in Kif2a-depleted cells. We conclude that CENP-E integrates two critical functions that are important for accurate chromosome movement and spindle architecture: one relying directly on its motor activity, and the other involving the targeting of key microtubule regulators to kinetochores.

Targeted activation of innate immunity for therapeutic induction of autophagy and apoptosis in melanoma cells

Inappropriate drug delivery, secondary toxicities, and persistent chemo- and immunoresistance have traditionally compromised treatment response in melanoma. Using cellular systems and genetically engineered mouse models, we show that melanoma cells retain an innate ability to recognize cytosolic double-stranded RNA (dsRNA) and mount persistent stress response programs able to block tumor growth, even in highly immunosuppressed backgrounds. The dsRNA mimic polyinosine-polycytidylic acid, coadministered with polyethyleneimine as carrier, was identified as an unanticipated inducer of autophagy downstream of an exacerbated endosomal maturation program. A concurrent activity of the dsRNA helicase MDA-5 driving the proapoptotic protein NOXA resulted in an efficient autodigestion of melanoma cells. These results reveal tractable links for therapeutic intervention among dsRNA helicases, endo/lysosomes, and apoptotic factors.

Nucleocytoplasmic shuttling of the TACC protein Mia1p/Alp7p is required for remodeling of microtubule arrays during the cell cycle

Microtubule arrays are remodeled as cells proceed through the cell cycle. It is important to understand how remodeling is regulated in time and space. In fission yeast, the conserved microtubule associated TACC/TOG complex plays an important role in organizing microtubules throughout the cell cycle. Here we show that this complex undergoes nucleocytoplasmic shuttling through the nuclear import and export signals located in the TACC protein Mia1p/Alp7p. When the Crm1p-dependent nuclear export signal of Mia1p is disabled, Mia1p accumulates in the nucleus while its partner protein Alp14p/TOG is restricted to the cytoplasm. This leads to defects in assembly of both interphase arrays and the mitotic spindle. Artificial targeting of Alp14p to the nucleus partially rescues the mitotic spindle defects caused by lack of Mia1p nuclear export. Interestingly, the nuclear export sequence of Mia1p appears to overlap with the Alp14p binding site. We propose that intricate regulation of the subcellular distribution of TACC/TOG complexes drives microtubule array remodeling as cells progress through the cell cycle.

Mitochondrial clearance is regulated by Atg7-dependent and -independent mechanisms during reticulocyte maturation

Mitochondrial clearance is a well recognized but poorly understood biologic process, and reticulocytes, which undergo programmed mitochondrial clearance, provide a useful model to study this phenomenon. At the ultrastructural level, mitochondrial clearance resembles an autophagy-related process; however, the role of autophagy in mitochondrial clearance has not been established. Here we provide genetic evidence that autophagy pathways, initially identified in yeast, are involved in mitochondrial clearance from reticulocytes. Atg7 is an autophagy protein and an E1-like enzyme, which is required for the activity of dual ubiquitin-like conjugation pathways. Atg7 is required for the conjugation of Atg12 to Atg5, and Atg8 to phosphatidylethanolamine (PE), and is essential for autophagosome formation. In the absence of Atg7, mitochondrial clearance from reticulocytes is diminished but not completely blocked. Mammalian homologs of Atg8 are unmodified in Atg7(-/-) erythroid cells, indicating that canonical autophagy pathways are inactive. Thus, mitochondrial clearance is regulated by both autophagy-dependent and -independent mechanisms. In addition, mitochondria, which depolarize in wild-type cells before elimination, remain polarized in Atg7(-/-) reticulocytes in culture. This suggests that mitochondrial depolarization is a consequence rather than a cause of autophagosome formation in reticulocytes.

Stathmin, a microtubule regulatory protein, is associated with hypoxia-inducible factor-1alpha levels in human endometrial and endothelial cells

Local hypoxia that occurs during menstruation triggers angiogenesis that is crucial for cyclical remodeling of the endometrium during the menstrual cycle. Hypoxia is thought to be important for the expression of vascular endothelial growth factor (VEGF) via its transcriptional factor, hypoxia inducible factor (HIF)-1alpha, in the endometrium. The activation of the phosphatidylinositol-3-kinase (PI3K)/Akt signaling pathway may modulate HIF-1alpha protein levels. Stathmin, a microtubule regulatory protein, was expressed in the stroma, glandular epithelium, and vascular endothelium in human endometrium. In this study, we examined a possible role of stathmin in hypoxia-induced HIF-1alpha and VEGF expression in primary isolated and immortalized human endometrial stromal cells, glandular epithelial cells, and human umbilical venous endothelial cells (HUVEC). Knocking down stathmin expression using small interfering RNA caused microtubule stabilization and inhibited hypoxia-induced VEGF mRNA expression via the reduction of HIF-1alpha protein levels in endometrial cells and HUVEC. Treatment of the cells with a PI3K inhibitor, wortmannin, inhibited the expression of VEGF mRNA and the accumulation of HIF-1alpha protein. Silencing of stathmin expression repressed the activation (phosphorylation) of Akt in endometrial cells and HUVEC. These results suggest that endometrial stathmin is linked to HIF-1alpha protein accumulation and VEGF expression through the PI3K/Akt signaling pathway and may be involved in regeneration of the endometrium during the menstrual cycle in human uterine cells.

Extracellular, cell-permeable survivin inhibits apoptosis while promoting proliferative and metastatic potential

The tumour microenvironment is believed to be involved in development, growth, metastasis, and therapy resistance of many cancers. Here we show survivin, a member of the inhibitor of apoptosis protein (IAP) family, implicated in apoptosis inhibition and the regulation of mitosis in cancer cells, exists in a novel extracellular pool in tumour cells. Furthermore, we have constructed stable cell lines that provide the extracellular pool with either wild-type survivin (Surv-WT) or the previously described dominant-negative mutant survivin (Surv-T34A), which has proven pro-apoptotic effects in cancer cells but not in normal proliferating cells. Cancer cells grown in conditioned medium (CM) taken from Surv-WT cells absorbed survivin and experienced enhanced protection against genotoxic stresses. These cells also exhibited an increased replicative and metastatic potential, suggesting that survivin in the tumour microenvironment may be directly associated with malignant progression, further supporting survivin's function in tumourigenesis. Alternatively, cancer cells grown in CM taken from the Surv-T34A cells began to apoptose through a caspase-2- and caspase-9-dependent pathway that was further enhanced by the addition of other chemo- and radiotherapeutic modalities. Together our findings suggest a novel microenvironmental function for survivin in the control of cancer aggressiveness and spread, and should result in the genesis of additional cancer treatment modalities.

Tubulin heterodimers remain functional for one cell cycle after the inactivation of tubulin-folding cofactor D in fission yeast cells

Tubulin-folding cofactor D plays a major role in the formation of functional tubulin heterodimers, the subunits of microtubules (MTs) that are essential for cell division. Previous work has suggested that, in Schizosaccharomyces pombe, cofactor D function is required during G(1) or S phases of the cell cycle, and when it fails to function due to the temperature-sensitive mutation alp1-t1, cells are unable to segregate their chromosomes in the subsequent mitosis. Here we report that another mutation in the cofactor D gene, alp1-1315, causes failures in either the first or second mitosis in cells synchronized in G(1) or G(2) phases, respectively. Other results, however, suggest that the kinetics of viability loss in these mutants does not depend on progression through the cell cycle. When cofactor D function is perturbed in cells blocked in G(2), cytoplasmic MTs appear normal for 2-3 h but thereafter they disintegrate quickly, so that only a few short MTs remain. These residual MTs are, however, stably maintained, suggesting that they do not require active cofactor D function. The abrupt disassembly of MT cytoskeleton at restrictive temperature in non-cycling cofactor D mutant cells strongly suggests that the life-span of folded tubulin dimers might be downregulated. Indeed, this period is significantly shorter than the previously determined dissociation time of bovine tubulins in vitro. The death of mutant cells occurs inevitably after 2-3 h at restrictive temperature in the following mitosis, and is explained by the idea that MT structures formed in the absence of cofactor D cannot support normal cell division.

The WD40 repeat protein NEDD1 functions in microtubule organization during cell division in Arabidopsis thaliana

Although cells of flowering plants lack a structurally defined microtubule-organizing center like the centrosome, organization of the spindles and phragmoplasts in mitosis is known to involve the evolutionarily conserved gamma-tubulin complex. We have investigated the function of Arabidopsis thaliana NEDD1, a WD40 repeat protein related to the animal NEDD1/GCP-WD protein, which interacts with the gamma-tubulin complex. The NEDD1 protein decorates spindle microtubules (MTs) preferentially toward spindle poles and phragmoplast MTs toward their minus ends. A T-DNA insertional allele of the single NEDD1 gene was isolated and maintained in heterozygous sporophytes, and NEDD1's function in cell division was analyzed in haploid microspores produced by the heterozygote. In approximately half of the dividing microspores exhibiting aberrant MT organization, spindles were no longer restricted to the cell periphery and became abnormally elongated. After mitosis, MTs aggregated between reforming nuclei but failed to appear in a bipolar configuration. Consequently, defective microspores did not form a continuous cell plate, and two identical nuclei were produced with no differentiation into generative and vegetative cells. Our results support the notion that the plant NEDD1 homolog plays a critical role in MT organization during mitosis, and its function is likely linked to that of the gamma-tubulin complex.