Association of 14-3-3 proteins with centrosomes

The Role of Non-Coding RNAs in Chromosomal Instability in Cancer

Chromosomal instability (CIN) is characterized by an increased frequency of changes in chromosome structure or number and is regarded as a hallmark of cancer. CIN plays a prevalent role in tumorigenesis and cancer progression by assisting the cancer cells' phenotypic adaptation to stress, which have been tightly linked to therapy resistance and metastasis. Both CIN-inducing and CIN-repressing agents are being clinically tested for the treatment of cancer to increase CIN levels to unsustainable levels leading to cell death or to decrease CIN levels to limit the development of drug resistance, respectively. Non-coding RNAs (ncRNAs) including microRNAs and long ncRNAs (lncRNAs) have been fundamentally implicated in CIN. The miR-22, miR-26a, miR-28, and miR-186 target important checkpoint proteins involved in mediating chromosomal stability and their expression modulation has been directly related to CIN occurrence. lncRNAs derived from telomeric, centrosomal, and enhancer regions play an important role in mediating genome stability, while specific lncRNA transcripts including genomic instability inducing RNA called Ginir, P53-responsive lncRNA termed as GUARDIN, colon cancer-associated transcript 2, PCAT2, and ncRNA activated by DNA damage called NORAD have been shown to act within CIN-associated pathways. In this review, we discuss how these ncRNAs either maintain or disrupt the stability of chromosomes and how these mechanisms could be exploited for novel therapeutic approaches targeting CIN in cancer patients. SIGNIFICANCE STATEMENT: Chromosomal instability increases tumor heterogeneity and thereby assists the phenotypic adaptation of cancer cells, causing therapy resistance and metastasis. Several microRNAs and long non-coding RNAs that have been causally linked to chromosomal instability could represent novel therapeutic targets. Understanding the role of non-coding RNAs in regulating different genes involved in driving chromosomal instability will give insights into how non-coding RNAs can be utilized toward modifying chemotherapeutic regimens in different cancers.

Structural basis for the recognition by 14-3-3 proteins of a conditional binding site within the oligomerization domain of human nucleophosmin

Nucleophosmin 1 (NPM1) is a multifunctional protein regulating ribosome biogenesis, centrosome duplication and chromatin remodeling. Being a major nucleolar protein, NPM1 can migrate to the nucleus and the cytoplasm, which is controlled by changes of NPM1 oligomerization and interaction with other cell factors. NPM1 forms a stable pentamer with its N-terminal structured domain, where two nuclear export signals and several phosphorylation sites reside. This domain undergoes dissociation and disordering upon Ser48 phosphorylation in the subunit interface. Recent studies indicated that Ser48 is important for NPM1 interaction with other proteins including 14-3-3, the well-known phosphoserine/phosphothreonine binders, but the structural basis for 14-3-3/NPM1 interaction remained unaddressed. By fusing human 14-3-3ζ with an NPM1 segment surrounding Ser48, which was phosphorylated inside Escherichia coli cells by co-expressed protein kinase A, here we obtained the desired protein/phosphopeptide complex and determined its crystal structure. While biochemical data indicated that the interaction is driven by Ser48 phosphorylation, the crystallographic 14-3-3/phosphopeptide interface reveals an NPM1 conformation distinctly different from that in the NPM1 pentamer. Given the canonical phosphopeptide-binding mode observed in our crystal structure, Ser48 emerges as a conditional binding site whose recognition by 14-3-3 proteins is enabled by NPM1 phosphorylation, disassembly and disordering under physiological circumstances.

14-3-3γ prevents centrosome duplication by inhibiting NPM1 function

14-3-3 proteins bind to ligands via phospho-serine containing consensus motifs. However, the molecular mechanisms underlying complex formation and dissociation between 14-3-3 proteins and their ligands remain unclear. We identified two conserved acidic residues in the 14-3-3 peptide-binding pocket (D129 and E136) that potentially regulate complex formation and dissociation. Altering these residues to alanine led to opposing effects on centrosome duplication. D129A inhibited centrosome duplication, whereas E136A stimulated centrosome amplification. These results were due to the differing abilities of these mutant proteins to form a complex with NPM1. Inhibiting complex formation between NPM1 and 14-3-3γ led to an increase in centrosome duplication and over-rode the ability of D129A to inhibit centrosome duplication. We identify a novel role of 14-3-3γ in regulating centrosome licensing and a novel mechanism underlying the formation and dissociation of 14-3-3 ligand complexes dictated by conserved residues in the 14-3-3 family.

Differential Subcellular Distribution and Translocation of Seven 14-3-3 Isoforms in Response to EGF and During the Cell Cycle

Multiple isoforms of 14-3-3 proteins exist in different organisms. In mammalian cells, 14-3-3 protein has seven isoforms (α/β, ε, η, γ, σ, θ/τ, and δ/ζ), with α and δ representing the phosphorylated versions of β and ζ, respectively. While the existence of multiple isoforms may represent one more level of regulation in 14-3-3 signaling, our knowledge regarding the isoform-specific functions of 14-3-3 proteins is very limited. Determination of the subcellular localization of the different 14-3-3 isoforms could give us important clues of their specific functions. In this study, by using indirect immunofluorescence, subcellular fractionation, and immunoblotting, we studied the subcellular localization of the total 14-3-3 protein and each of the seven 14-3-3 isoforms; their redistribution throughout the cell cycle; and their translocation in response to EGF in Cos-7 cells. We showed that 14-3-3 proteins are broadly distributed throughout the cell and associated with many subcellular structures/organelles, including the plasma membrane (PM), mitochondria, ER, nucleus, microtubules, and actin fibers. This broad distribution underlines the multiple functions identified for 14-3-3 proteins. The different isoforms of 14-3-3 proteins have distinctive subcellular localizations, which suggest their distinctive cellular functions. Most notably, 14-3-3ƞ is almost exclusively localized to the mitochondria, 14-3-3γ is only localized to the nucleus, and 14-3-3σ strongly and specifically associated with the centrosome during mitosis. We also examined the subcellular localization of the seven 14-3-3 isoforms in other cells, including HEK-293, MDA-MB-231, and MCF-7 cells, which largely confirmed our findings with Cos-7 cells.

14-3-3γ Prevents Centrosome Amplification and Neoplastic Progression

More than 80% of malignant tumors show centrosome amplification and clustering. Centrosome amplification results from aberrations in the centrosome duplication cycle, which is strictly coordinated with DNA-replication-cycle. However, the relationship between cell-cycle regulators and centrosome duplicating factors is not well understood. This report demonstrates that 14-3-3γ localizes to the centrosome and 14-3-3γ loss leads to centrosome amplification. Loss of 14-3-3γ results in the phosphorylation of NPM1 at Thr-199, causing early centriole disjunction and centrosome hyper-duplication. The centrosome amplification led to aneuploidy and increased tumor formation in mice. Importantly, an increase in passage of the 14-3-3γ-knockdown cells led to an increase in the number of cells containing clustered centrosomes leading to the generation of pseudo-bipolar spindles. The increase in pseudo-bipolar spindles was reversed and an increase in the number of multi-polar spindles was observed upon expression of a constitutively active 14-3-3-binding-defective-mutant of cdc25C (S216A) in the 14-3-3γ knockdown cells. The increase in multi-polar spindle formation was associated with decreased cell viability and a decrease in tumor growth. Our findings uncover the molecular basis of regulation of centrosome duplication by 14-3-3γ and inhibition of tumor growth by premature activation of the mitotic program and the disruption of centrosome clustering.

Dynamic multiprotein assemblies shape the spatial structure of cell signaling

Cell signaling underlies critical cellular decisions. Coordination, efficiency as well as fail-safe mechanisms are key elements. How the cell ensures that these hallmarks are at play are important questions. Cell signaling is often viewed as taking place through discrete and cross-talking pathways; oftentimes these are modularized to emphasize distinct functions. While simple, convenient and clear, such models largely neglect the spatial structure of cell signaling; they also convey inter-modular (or inter-protein) spatial separation that may not exist. Here our thesis is that cell signaling is shaped by a network of multiprotein assemblies. While pre-organized, the assemblies and network are loose and dynamic. They contain transiently-associated multiprotein complexes which are often mediated by scaffolding proteins. They are also typically anchored in the membrane, and their continuum may span the cell. IQGAP1 scaffolding protein which binds proteins including Raf, calmodulin, Mek, Erk, actin, and tens more, with actin shaping B-cell (and likely other) membrane-anchored nanoclusters and allosterically polymerizing in dynamic cytoskeleton formation, and Raf anchoring in the membrane along with Ras, provides a striking example. The multivalent network of dynamic proteins and lipids, with specific interactions forming and breaking, can be viewed as endowing gel-like properties. Collectively, this reasons that efficient, productive and reliable cell signaling takes place primarily through transient, preorganized and cooperative protein-protein interactions spanning the cell rather than stochastic, diffusion-controlled processes.

The 14-3-3 protein Bmh1 functions in the spindle position checkpoint by breaking Bfa1 asymmetry at yeast centrosomes

Phosphorylation of Bfa1 by Kin4 creates a docking site on Bfa1 for the 14-3-3 family protein Bmh1, which in turn weakens Bfa1–centrosome association and promotes symmetric Bfa1 localization to engage the spindle position checkpoint.

In addition to their well-known role in microtubule organization, centrosomes function as signaling platforms and regulate cell cycle events. An important example of such a function is the spindle position checkpoint (SPOC) of budding yeast. SPOC is a surveillance mechanism that ensures alignment of the mitotic spindle along the cell polarity axis. Upon spindle misalignment, phosphorylation of the SPOC component Bfa1 by Kin4 kinase engages the SPOC by changing the centrosome localization of Bfa1 from asymmetric (one centrosome) to symmetric (both centrosomes). Here we show that, unexpectedly, Kin4 alone is unable to break Bfa1 asymmetry at yeast centrosomes. Instead, phosphorylation of Bfa1 by Kin4 creates a docking site on Bfa1 for the 14-3-3 family protein Bmh1, which in turn weakens Bfa1–centrosome association and promotes symmetric Bfa1 localization. Consistently, BMH1-null cells are SPOC deficient. Our work thus identifies Bmh1 as a new SPOC component and refines the molecular mechanism that breaks Bfa1 centrosome asymmetry upon SPOC activation.

Evidence for the requirement of 14-3-3eta (YWHAH) in meiotic spindle assembly during mouse oocyte maturation

BACKGROUND

The 14-3-3 (YWHA) proteins are central mediators in various cellular signaling pathways regulating development and growth, including cell cycle regulation. We previously reported that all seven mammalian 14-3-3 isoforms are expressed in mouse oocytes and eggs and that, 14-3-3η (YWHAH) accumulates and co-localizes in the region of meiotic spindle in mouse eggs matured in vivo. Therefore, we investigated the role of 14-3-3η in spindle formation during mouse oocyte maturation.

RESULTS

Examination of oocytes matured in vitro demonstrated that 14-3-3η accumulates in both meiosis I and II spindles. To explore if 14-3-3η interacts directly with α-tubulin in meiotic spindles, we performed an in situ proximity ligation assay that can detect intracellular protein-protein interactions at the single molecule level and which allows visualization of the actual interaction sites. This assay revealed a marked interaction between 14-3-3η and α-tubulin at the metaphase II spindle. To demonstrate a functional role for 14-3-3η in oocyte maturation, mouse oocytes were microinjected with a translation-blocking morpholino oligonucleotide against 14-3-3η mRNA to reduce 14-3-3η protein synthesis during oocyte maturation. Meiotic spindles in those cells were examined by immunofluorescence staining of 14-3-3η and α-tubulin along with observation of DNA. In 76% of cells injected with the morpholino, meiotic spindles were found to be deformed or absent and there was reduced or no accumulation of 14-3-3η in the spindle region. Those cells contained clumped chromosomes, with no polar body formation. Immunofluorescence staining of 14-3-3η and α-tubulin in control eggs matured in vitro from uninjected oocytes and oocytes microinjected with the ineffective, inverted form of a morpholino against 14-3-3η, a morpholino against 14-3-3γ, or deionized water showed normal, bipolar spindles.

CONCLUSIONS

The results indicate that 14-3-3η is essential for normal meiotic spindle formation during in vitro maturation of mouse oocytes, in part by interacting with α-tubulin, to regulate the assembly of microtubules. These data add to our understanding of the roles of 14-3-3 proteins in mouse oocyte maturation and mammalian reproduction.

Expression of 14-3-3 protein isoforms in mouse oocytes, eggs and ovarian follicular development

BACKGROUND

The 14-3-3 (YWHA) proteins are a highly conserved, ubiquitously expressed family of proteins. Seven mammalian isoforms of 14-3-3 are known (β, γ, ε, ζ, η, τ and, σ). These proteins associate with many intracellular proteins involved in a variety of cellular processes including regulation of the cell cycle, metabolism and protein trafficking. We are particularly interested in the role of 14-3-3 in meiosis in mammalian eggs and the role 14-3-3 proteins may play in ovarian function. Therefore, we examined the expression of 14-3-3 proteins in mouse oocyte and egg extracts by Western blotting after polyacrylamide gel electrophoresis, viewed fixed cells by indirect immunofluorescence, and examined mouse ovarian cells by immunohistochemical staining to study the expression of the different 14-3-3 isoforms.

RESULTS

We have determined that all of the mammalian 14-3-3 isoforms are expressed in mouse eggs and ovarian follicular cells including oocytes. Immunofluorescence confocal microscopy of isolated oocytes and eggs confirmed the presence of all of the isoforms with characteristic differences in some of their intracellular localizations. For example, some isoforms (β, ε, γ, and ζ) are expressed more prominently in peripheral cytoplasm compared to the germinal vesicles in oocytes, but are uniformly dispersed within eggs. On the other hand, 14-3-3η is diffusely dispersed in the oocyte, but attains a uniform punctate distribution in the egg with marked accumulation in the region of the meiotic spindle apparatus. Immunohistochemical staining detected all isoforms within ovarian follicles, with some similarities as well as notable differences in relative amounts, localizations and patterns of expression in multiple cell types at various stages of follicular development.

CONCLUSIONS

We found that mouse oocytes, eggs and follicular cells within the ovary express all seven isoforms of the 14-3-3 protein. Examination of the differential expression of these 14-3-3 isoforms in female germ cells and ovarian follicles provides the foundation for further investigating 14-3-3 isoform-specific interactions with key proteins involved in ovarian development, meiosis and oocyte maturation. This will lead to a better understanding of the individual functional roles of the 14-3-3 protein isoforms in mammalian oogenesis and female reproductive development.

Post-translational modification of 14-3-3 isoforms and regulation of cellular function

14-3-3 is now well established as a family of dimeric proteins that can modulate interaction between proteins involved in a wide range of functions. In many cases, these proteins show a distinct preference for a particular isoform(s) of 14-3-3 and in many cases a specific repertoire of dimer formation influences the particular proteins that 14-3-3 interact. Well over 200 proteins have been shown to interact with 14-3-3. The purpose of this review is to give an overview of the recently identified post-translational modifications of 14-3-3 isoforms and how this regulates function, interaction, specificity of dimerisation between isoforms and cellular location of target proteins. The association between 14-3-3 and its targets usually involves phosphorylation of the interacting protein which has been the subject of many reviews and discussion of this is included in other reviews in this series. However, it is now realised that in some cases the phosphorylation and a number of other, novel covalent modifications of 14-3-3 isoforms may modulate interaction and dimerisation of 14-3-3. Since this aspect is now emerging to be of major importance in the mechanism of regulation by 14-3-3 isoforms and has not been the focus of previous reviews, this will be detailed here.

Mining the Giardia genome and proteome for conserved and unique basal body proteins

Giardia lamblia is a flagellated protozoan parasite and a major cause of diarrhoea in humans. Its microtubular cytoskeleton mediates trophozoite motility, attachment and cytokinesis, and is characterised by an attachment disk and eight flagella that are each nucleated in a basal body. To date, only 10 giardial basal body proteins have been identified, including universal signalling proteins that are important for regulating mitosis or differentiation. In this study, we have exploited bioinformatics and proteomic approaches to identify new Giardia basal body proteins and confocal microscopy to confirm their localisation in interphase trophozoites. This approach identified 75 homologs of conserved basal body proteins in the genome including 65 not previously known to be associated with Giardia basal bodies. Thirteen proteins were confirmed to co-localise with centrin to the Giardia basal bodies. We also demonstrate that most basal body proteins localise to additional cytoskeletal structures in interphase trophozoites. This might help to explain the roles of the four pairs of flagella and Giardia-specific organelles in motility and differentiation. A deeper understanding of the composition of the Giardia basal bodies will contribute insights into the complex signalling pathways that regulate its unique cytoskeleton and the biological divergence of these conserved organelles.

A centrosomal Cdc20-APC pathway controls dendrite morphogenesis in postmitotic neurons

The ubiquitin ligase anaphase-promoting complex (APC) recruits the coactivator Cdc20 to drive mitosis in cycling cells. However, the nonmitotic functions of Cdc20-APC have remained unexplored. We report that Cdc20-APC plays an essential role in dendrite morphogenesis in postmitotic neurons. Knockdown of Cdc20 in cerebellar slices and in postnatal rats in vivo profoundly impairs the formation of granule neuron dendrite arbors in the cerebellar cortex. Remarkably, Cdc20 is enriched at the centrosome in neurons, and the centrosomal localization is critical for Cdc20-dependent dendrite development. We also find that the centrosome-associated protein histone deacetylase 6 (HDAC6) promotes the polyubiquitination of Cdc20, stimulates the activity of centrosomal Cdc20-APC, and drives the differentiation of dendrites. These findings define a postmitotic function for Cdc20-APC in the morphogenesis of dendrites in the mammalian brain. The identification of a centrosomal Cdc20-APC ubiquitin signaling pathway holds important implications for diverse biological processes, including neuronal connectivity and plasticity.

A combined proteome and ultrastructural localization analysis of 14-3-3 proteins in transformed human amnion (AMA) cells: definition of a framework to study isoform-specific differences

The 14-3-3 proteins constitute a family of highly conserved and broadly expressed multifunctional polypeptides that are involved in a variety of important cellular processes that include cell cycle progression, growth, differentiation, and apoptosis. Although the exact cellular function(s) of 14-3-3 proteins is not fully elucidated, as a rule these proteins act by binding to protein ligands, thus regulating their activity; so far more than 300 cellular proteins have been reported to interact with 14-3-3 proteins. Binding to cognate interacting partners is isoform-specific, but redundancy also exists as several binding peptides can be recognized by all isoforms, and some functions can be carried out by any isoform indistinctly. Moreover by interacting with different ligands in a spatially and temporally regulated fashion the same isoform can play multiple possibly even opposing roles where the resultant cellular outcome will be determined by the integration of the various effects. Although there is a large body of literature on specific aspects of 14-3-3 biology, not much is known on the coordinated aspects of 14-3-3 isoform expression, post-translational modifications, and subcellular localization. To address the question of isoform-specific differences, we carried out a comparative analysis of the patterns of expression, phosphorylation, and subcellular localization of the 14-3-3 beta, epsilon, sigma, tau, and zeta protein isoforms in transformed human amnion (AMA) cells. To validate as well as broaden our observations we analyzed the occurrence of the various isoforms in a large number of established cell lines and mammary and urothelial tissue specimens. Given the systematic approach we undertook and our application of isoform-discriminating technologies to the analysis of various cellular systems, we expect the data presented in this study to serve as an enabling resource for researchers working with 14-3-3 proteins.

14-3-3 theta binding to cell cycle regulatory factors is enhanced by HIV-1 Vpr

Background

Despite continuing advances in our understanding of AIDS pathogenesis, the mechanism of CD4+ T cell depletion in HIV-1-infected individuals remains unclear. The HIV-1 Vpr accessory protein causes cell death, likely through a mechanism related to its ability to arrest cells in the G₂,M phase. Recent evidence implicated the scaffold protein, 14-3-3, in Vpr cell cycle blockade.

Results

We found that in human T cells, 14-3-3 plays an active role in mediating Vpr-induced cell cycle arrest and reveal a dramatic increase in the amount of Cdk1, Cdc25C, and CyclinB1 bound to 14-3-3 θ during Vpr_v-induced G₂,M arrest. By contrast, a cell-cycle-arrest-dead Vpr mutant failed to augment 14-3-3 θ association with Cdk1 and CyclinB1. Moreover, G₂,M arrest caused by HIV-1 infection strongly correlated with a disruption in 14-3-3 θ binding to centrosomal proteins, Plk1 and centrin. Finally, Vpr caused elevated levels of CyclinB1, Plk1, and Cdk1 in a complex with the nuclear transport and spindle assembly protein, importin β.

Conclusion

Thus, our data reveal a new facet of Vpr-induced cell cycle arrest involving previously unrecognized abnormal rearrangements of multiprotein assemblies containing key cell cycle regulatory proteins.

Reviewers

This article was reviewed by David Kaplan, Nathaniel R. Landau and Yan Zhou.

Time-course studies of 14-3-3 protein isoforms in cerebrospinal fluid and brain of primates after oral or intracerebral infection with bovine spongiform encephalopathy agent

Experimental transmission of bovine spongiform encephalopathy (BSE) to cynomolgus monkeys (Macaca fascicularis) is an animal model for variant Creutzfeldt–Jakob disease (vCJD). The presence of 14-3-3 proteins in cerebrospinal fluid (CSF) samples indicates neuronal destruction and is therefore used as a clinical biomarker. However, time-course studies using 14-3-3 proteins have not been performed until now in simian vCJD. The main goals of this study were to determine isoform patterns, to examine kinetics and to correlate the clinical course with the occurrence of this biomarker in simian vCJD. In monkeys dosed intracerebrally with BSE, the earliest clinical sign of illness was a drop in body weight that was detected months before the onset of mild neurological signs. Macaques dosed orally or intracerebrally with BSE developed neurological signs 4.3 (3.7–4.6) and 4.8 (2.9–6.0) years post-infection, respectively. 14-3-3β- and -γ-positive CSF samples were found around the time of onset of mild neurological signs, but not earlier. In contrast, 14-3-3ϵ and -ηisoforms were not detectable. 14-3-3 levels increased with time and were positively correlated with the degree of neurological symptoms. Post-mortem examination of brain samples revealed a positive correlation between PrPresand 14-3-3ϵ levels. Interestingly, florid plaques characteristic of human vCJD could not be detected in diseased monkeys. It was concluded that analysis of 14-3-3 proteins in CSF is a reliable tool to characterize the time course of brain degeneration in simian vCJD. However, there are differences in the clinical course between orally and intracerebrally infected animals that may influence the detection of other biomarkers.

Mutations in LCA5, encoding the ciliary protein lebercilin, cause Leber congenital amaurosis

Leber congenital amaurosis (LCA) causes blindness or severe visual impairment at or within a few months of birth. Here we show, using homozygosity mapping, that the LCA5 gene on chromosome 6q14, which encodes the previously unknown ciliary protein lebercilin, is associated with this disease. We detected homozygous nonsense and frameshift mutations in LCA5 in five families affected with LCA. In a sixth family, the LCA5 transcript was completely absent. LCA5 is expressed widely throughout development, although the phenotype in affected individuals is limited to the eye. Lebercilin localizes to the connecting cilia of photoreceptors and to the microtubules, centrioles and primary cilia of cultured mammalian cells. Using tandem affinity purification, we identified 24 proteins that link lebercilin to centrosomal and ciliary functions. Members of this interactome represent candidate genes for LCA and other ciliopathies. Our findings emphasize the emerging role of disrupted ciliary processes in the molecular pathogenesis of LCA.

Overexpression of LASP-1 mediates migration and proliferation of human ovarian cancer cells and influences zyxin localisation

LIM and SH3 protein 1 (LASP-1), initially identified from human breast cancer, is a specific focal adhesion protein involved in cell proliferation and migration. In the present work, we analysed the effect of LASP-1 on biology and function of human ovarian cancer cell line SKOV-3 using small interfering RNA technique (siRNA). Transfection with LASP-1-specific siRNA resulted in a reduced protein level of LASP-1 in SKOV-3 cells. The siRNA-treated cells were arrested in G(2)/M phase of the cell cycle and proliferation of the tumour cells was suppressed by 60-90% corresponding to around 70% of the cells being transfected successfully as seen by immunofluorescence. Moreover, transfected tumour cells showed a 40% reduced migration. LASP-1 silencing is accompanied by a reduced binding of the LASP-1-binding partner zyxin to focal contacts without changes in actin stress fibre and microtubule organisation or focal adhesion morphology as observed by immunofluorescence. In contrast, silencing of zyxin is not influencing cell migration and had neither influence on LASP-1 expression nor actin cytoskeleton and focal contact morphology suggesting that LASP-1 is necessary and sufficient for recruiting zyxin to focal contacts. The data provide evidence for an essential role of LASP-1 in tumour cell growth and migration, possibly through influencing zyxin localization.

CHK1 phosphorylates CDC25B during the cell cycle in the absence of DNA damage

CDC25B is one of the three human phosphatases that activate the CDK-cyclin complexes, thereby triggering cell-cycle progression and division. Commitment to early mitotic events depends on the activation of a centrosomal pool of CDK1-cyclin-B1, and CDC25B is thought to be involved in initiating this centrosomal CDK1-cyclin-B1 activity. Centrosome-associated checkpoint kinase 1 (CHK1) has been proposed to contribute to the proper timing of a normal cell division cycle by inhibiting the activation of the centrosomal pool of CDK1. Here, we show that CDC25B is phosphorylated by CHK1 in vitro on multiple residues, including S230 and S563. We demonstrate these phosphorylations occur in vivo and that they are dependent on CHK1 activity. S230 CHK1-mediated phosphorylation is detected in cell extracts during S phase and G2 phase in the absence of DNA damage. We show that the S230-phosphorylated form of CDC25B is located at the centrosome from early S phase until mitosis. Furthermore, mutation of S230 to alanine increases the mitotic-inducing activity of CDC25B. Our results support a model in which, under normal cell cycle conditions and in the absence of DNA damage, CHK1 constitutively phosphorylates CDC25B during interphase and thus prevents the premature initiation of mitosis by negatively regulating the activity of CDC25B at the centrosome.

14-3-3 proteins: a historic overview

This chapter includes a historic overview of 14-3-3 proteins with an emphasis on the differences between potentially cancer-relevant isoforms on the genomic, protein and functional level. The focus will therefore be on mammalian 14-3-3s although many important developments in the field have involved Drosophila 14-3-3 proteins for example and the cross-fertilisation from parallel studies on plant 14-3-3 should not be underestimated. In the major part of this review I will attempt to focus on some novel data and aspects of 14-3-3 structure and function, in particular regulation of 14-3-3 isoforms by oncogene-related protein kinase phosphorylation and aspects of 14-3-3 research with which newcomers to the field may be less familiar.

CPAP interacts with 14-3-3 in a cell cycle-dependent manner

We have previously reported that CPAP (Centrosomal Protein 4.1-Associated Protein) carries a novel microtubule-destabilizing motif that may regulate microtubule dynamics at the centrosome. In this study, we searched for conserved sequence motifs in CPAP and identified two classical 14-3-3 binding sites. The interaction between CPAP and 14-3-3 was demonstrated by both yeast two-hybrid and co-immunoprecipitation experiments. Further analyses revealed that the 14-3-3 binding motif is located within the C-terminal domain of CPAP. Alkaline phosphatase treatment disrupted CPAP binding to 14-3-3, suggesting that phosphorylation is required for the interaction. Mutation of serine 1109 to alanine (S1109A) in the 14-3-3 binding motif completely abolished the association of CPAP with 14-3-3. Taking together, these results imply that phosphorylation of CPAP on serine 1109 is required for 14-3-3 binding. Furthermore, we observed that the interaction between CPAP and 14-3-3 was significantly reduced in mitotic cells, suggesting that 14-3-3 binding to CPAP is regulated during cell cycle progression. In summary, our results show a direct interaction between CPAP and 14-3-3, and this interaction appears to be phosphorylation and cell cycle dependent.

Recruitment of katanin p60 by phosphorylated NDEL1, an LIS1 interacting protein, is essential for mitotic cell division and neuronal migration

LIS1 is mutated in the human neuronal migration defect lissencephaly and along with NDEL1 (formerly NUDEL) participates in the regulation of cytoplasmic dynein function during neuronal development. Targeted disruption of Ndel1 suggested that NDEL1 could have other molecular targets that regulate microtubule organization for proper neuronal migration. To further understanding the molecular mechanism of LIS1 and lissencephaly, we identified the katanin p60 microtubule-severing protein as an additional molecular target of NDEL1. We demonstrate that phosphorylation of NDEL1 by Cdk5 facilitates interaction between NDEL1 and p60, suggesting that P-NDEL1 regulates the distribution of katanin p60. Abnormal accumulation of p60 in nucleus of Ndel1 null mutants supports an essential role of NDEL1 in p60 regulation. Complete loss of NDEL1 or expression of dominant negative mutants of p60 in migrating neurons results in defective migration and elongation of nuclear-centrosomal distance. Our results suggest that NDEL1 is essential for mitotic cell division and neuronal migration not only via regulation of cytoplasmic dynein function but also by modulation of katanin p60 localization and function.

Isoform-specific subcellular localization among 14-3-3 proteins in Arabidopsis seems to be driven by client interactions

In most higher eukaryotes, the predominantly phosphoprotein-binding 14-3-3 proteins are the products of a multigene family, with many organisms having 10 or more family members. However, current models for 14-3-3/phosphopeptide interactions suggest that there is little specificity among 14-3-3s for diverse phosphopeptide clients. Therefore, the existence of sequence diversity among 14-3-3s within a single organism begs questions regarding the in vivo specificities of the interactions between the various 14-3-3s and their clients. Chief among those questions is, Do the different 14-3-3 isoforms interact with different clients within the same cell? Although the members of the Arabidopsis 14-3-3 family of proteins typically contain highly conserved regions of sequence, they also display distinctive variability with deep evolutionary roots. In the current study, a survey of several Arabidopsis 14-3-3/GFP fusions revealed that 14-3-3s demonstrate distinct and differential patterns of subcellular distribution, by using trichomes and stomate guard cells as in vivo experimental cellular contexts. The effects of client interaction on 14-3-3 localization were further analyzed by disrupting the partnering with peptide and chemical agents. Results indicate that 14-3-3 localization is both isoform specific and highly dependent upon interaction with cellular clients.

Dynamic interactions between 14-3-3 proteins and phosphoproteins regulate diverse cellular processes

14-3-3 proteins exert an extraordinarily widespread influence on cellular processes in all eukaryotes. They operate by binding to specific phosphorylated sites on diverse target proteins, thereby forcing conformational changes or influencing interactions between their targets and other molecules. In these ways, 14-3-3s 'finish the job' when phosphorylation alone lacks the power to drive changes in the activities of intracellular proteins. By interacting dynamically with phosphorylated proteins, 14-3-3s often trigger events that promote cell survival--in situations from preventing metabolic imbalances caused by sudden darkness in leaves to mammalian cell-survival responses to growth factors. Recent work linking specific 14-3-3 isoforms to genetic disorders and cancers, and the cellular effects of 14-3-3 agonists and antagonists, indicate that the cellular complement of 14-3-3 proteins may integrate the specificity and strength of signalling through to different cellular responses.

Isoform-specific differences in rapid nucleocytoplasmic shuttling cause distinct subcellular distributions of 14-3-3 sigma and 14-3-3 zeta

Nucleocytoplasmic transport of proteins plays an important role in the regulation of many cellular processes. Differences in nucleocytoplasmic shuttling can provide a basis for isoform-specific biological functions for members of multigene families, like the 14-3-3 protein family. Many organisms contain multiple 14-3-3 isoforms, which play a role in numerous processes, including signalling, cell cycle control and apoptosis. It is still unclear whether these isoforms have specialised biological functions and whether this specialisation is based on isoform-specific ligand binding, expression regulation or specific localisation. Therefore, we studied the subcellular distribution of 14-3-3 sigma and 14-3-3 zeta in vivo in various mammalian cell types using yellow fluorescent protein fusions and isoform-specific antibodies. 14-3-3 sigma was mainly localised in the cytoplasm and only low levels were present in the nucleus, whereas 14-3-3 zeta was found at relatively higher levels in the nucleus. Fluorescence recovery after photobleaching (FRAP) experiments indicated that the 14-3-3 proteins rapidly shuttle in and out of the nucleus through active transport and that the distinct subcellular distributions of 14-3-3 sigma and 14-3-3 zeta are caused by differences in nuclear export. 14-3-3 sigma had a 1.7x higher nuclear export rate constant than 14-3-3 zeta, while import rate constants were equal. The 14-3-3 proteins are exported from the nucleus at least in part by a Crm1-dependent, leptomycin B-sensitive mechanism. The differences in subcellular distribution of 14-3-3 that we found in this study are likely to reflect a molecular basis for isoform-specific biological specialisation.

The Centrosome in Higher Organisms: Structure, Composition, and Duplication

The centrosome found in higher organisms is an organelle with a complex and dynamic architecture and composition. This organelle not only functions as a microtubule-organizing center, but also is integrated with or impacts a number of cellular processes. Defects associated with this organelle have been linked to a variety of human diseases including several forms of cancer. Here we review the emerging picture of how the structure, composition, duplication, and function of the centrosome found in higher organisms are interrelated.

Proteomic characterization of the human centrosome by protein correlation profiling

The centrosome is the major microtubule-organizing centre of animal cells and through its influence on the cytoskeleton is involved in cell shape, polarity and motility. It also has a crucial function in cell division because it determines the poles of the mitotic spindle that segregates duplicated chromosomes between dividing cells. Despite the importance of this organelle to cell biology and more than 100 years of study, many aspects of its function remain enigmatic and its structure and composition are still largely unknown. We performed a mass-spectrometry-based proteomic analysis of human centrosomes in the interphase of the cell cycle by quantitatively profiling hundreds of proteins across several centrifugation fractions. True centrosomal proteins were revealed by both correlation with already known centrosomal proteins and in vivo localization. We identified and validated 23 novel components and identified 41 likely candidates as well as the vast majority of the known centrosomal proteins in a large background of nonspecific proteins. Protein correlation profiling permits the analysis of any multiprotein complex that can be enriched by fractionation but not purified to homogeneity.

14-3-3 proteins; bringing new definitions to scaffolding

The 14-3-3 proteins are a part of an emerging family of proteins and protein domains that bind to serine/threonine-phosphorylated residues in a context specific manner, analogous to the Src homology 2 (SH2) and phospho-tyrosine binding (PTB) domains. 14-3-3 proteins bind and regulate key proteins involved in various physiological processes such as intracellular signaling (e.g. Raf, MLK, MEKK, PI-3 kinase, IRS-1), cell cycling (e.g. Cdc25, Wee1, CDK2, centrosome), apoptosis (e.g. BAD, ASK-1) and transcription regulation (e.g. FKHRL1, DAF-16, p53, TAZ, TLX-2, histone deacetylase). In contrast to SH2 and PTB domains, which serve mainly to mediate protein-protein interactions, 14-3-3 proteins in many cases alter the function of the target protein, thus allowing them to serve as direct regulators of their targets. This review focuses on the various mechanisms employed by the 14-3-3 proteins in the regulation of their diverse targets, the structural basis for 14-3-3-target protein interaction with emphasis on the role of 14-3-3 dimerization in target protein binding and regulation and provides an insight on 14-3-3 regulation itself.

Accumulation of mutant huntingtin fragments in aggresome-like inclusion bodies as a result of insufficient protein degradation

The huntingtin exon 1 proteins with a polyglutamine repeat in the pathological range (51 or 83 glutamines), but not with a polyglutamine tract in the normal range (20 glutamines), form aggresome-like perinuclear inclusions in human 293 Tet-Off cells. These structures contain aggregated, ubiquitinated huntingtin exon 1 protein with a characteristic fibrillar morphology. Inclusion bodies with truncated huntingtin protein are formed at centrosomes and are surrounded by vimentin filaments. Inhibition of proteasome activity resulted in a twofold increase in the amount of ubiquitinated, SDS-resistant aggregates, indicating that inclusion bodies accumulate when the capacity of the ubiquitin-proteasome system to degrade aggregation-prone huntingtin protein is exhausted. Immunofluorescence and electron microscopy with immunogold labeling revealed that the 20S, 19S, and 11S subunits of the 26S proteasome, the molecular chaperones BiP/GRP78, Hsp70, and Hsp40, as well as the RNA-binding protein TIA-1, the potential chaperone 14-3-3, and alpha-synuclein colocalize with the perinuclear inclusions. In 293 Tet-Off cells, inclusion body formation also resulted in cell toxicity and dramatic ultrastructural changes such as indentations and disruption of the nuclear envelope. Concentration of mitochondria around the inclusions and cytoplasmic vacuolation were also observed. Together these findings support the hypothesis that the ATP-dependent ubiquitin-proteasome system is a potential target for therapeutic interventions in glutamine repeat disorders.

Centrosome abnormalities, genomic instability and carcinogenic progression

Centrosome abnormalities are a frequent finding in various malignant tumors. Since centrosomes form the poles of the mitotic spindle, these abnormalities have been implicated in chromosome missegregation and the generation of aneuploid cells which is commonly found in many human neoplasms. It is a matter of debate, however, whether centrosome alterations can drive cells into aneuploidy or simply reflect loss of genomic integrity by other mechanisms. Since these two models have fundamentally different implications for the diagnostic and prognostic value of centrosome abnormalities, we will discuss the relevance of abnormal centrosomes in the context of different oncogenic events as exemplified by high-risk human papillomavirus-associated carcinogenesis.

Active DNA topoisomerase IIalpha is a component of the salt-stable centrosome core

Recently, we reported that the monoclonal antibody specific for human DNA topoisomerase IIalpha, Ki-S1, stains not only the nuclei of human A431 cells but also extranuclear structures suggestive of centrosomes (Meyer, K. N., Kjeldsen, E., Straub, T., Knudsen, B. K., Kikuchi, A., Hickson, I. D., Kreipe, H., and Boege, F. (1997) J. Cell Biol. 136, 775-788). Here, we confirm colocalization of Ki-S1 with the centrosomal marker gamma-tubulin. In addition, we show labeling of centrosomes by peptide antibodies against the N and C termini of human topoisomerase IIalpha. Probing Western blots of isolated centrosomes with topoisomerase IIalpha antibodies, we demonstrate a protein band of 170 kDa. Moreover, isolated centrosomes exhibited DNA decatenation and relaxation activity correlated to the amount of topoisomerase IIalpha protein in the same way as seen in the pure recombinant enzyme. Topoisomerase IIalpha epitopes could not be removed from centrosomes by salt extraction, DNase treatment, or RNase treatment, procedures that completely removed the enzyme from nuclei. Taken together, these observations suggest that active topoisomerase IIalpha is bound tightly to the centrosome in a DNA-independent manner. Because such centrosomal topoisomerase IIalpha was also present in quiescent lymphocytes devoid of topoisomerase IIalpha in the nuclei, we assume that it might be a long-lived storage form.

14-3-3 proteins and growth control

The 14-3-3 proteins constitute a family that is highly conserved in a wide range of organisms, including higher eukaryotes, invertebrates and plants. Variants of 14-3-3 proteins assembled in homo- and heterodimers were found to interact with diverse cellular proteins. Until recently, the biological role of 14-3-3 members was still poorly understood. However, the results of an increasing number of studies on their structure and function are converging to define 14-3-3 proteins as a novel type of adaptor that modulates interactions between components involved in signal transduction pathway and in cell cycle control.

Integrating centrosome structure with protein composition and function in animal cells

The centrosome found in animal cells is a complex and dynamic organelle that functions as the major microtubule organizing center. Structural studies over the past several decades have defined the primary structural features of the centrosome but recent studies are now beginning to reveal structural detail previously unknown. Concurrent with these studies has been an explosion in the identification of the proteins that reside within the centrosome. Our growing understanding of how protein composition integrates with centrosome structure and hence with function is the focus of this review.

Increased levels of 14-3-3 gamma and epsilon proteins in brain of patients with Alzheimer's disease and Down syndrome

The 14-3-3 family consists of homo- and heterodimeric proteins representing a novel type of "adaptor proteins" modulating the interaction between components of signal transduction pathways. 14-3-3 isoforms interact with phosphoserine motifs on many proteins as kinases, phosphatases, apoptosis related proteins etc. Performing protein mapping by 2D electrophoresis in human brain we identified two isoforms, 14-3-3 gamma and epsilon and decided to determine these two multifunctional proteins in several brain regions of aged patients with Alzheimer's disease (AD) and Down Syndrome (DS) with AD neuropathology in comparison with control brains. 14-3-3 gamma and 14-3-3 epsilon proteins were increased in several brain regions of AD and DS patients. These changes may contribute to the complex pathomechanisms of AD and AD in DS, evolving inevitably from the fourth decade of life. Deranged 14-3-3 isoforms gamma and epsilon may reflect impaired signaling and/or apoptosis in the brain as several kinases (protein kinase C, Ras, mitogen-activated kinase MEK) involved in signaling and apoptotic factors as bcl-2-related proteins BAD and BAG-1 are binding to 14-3-3 motifs.

Modulation of the Ca(2+)-activated Cl(-) channel by 14-3-3epsilon

We have previously reported an association of 14-3-3epsilon isoform with calmodulin. Using the voltage-clamp technique, the present study investigated the potential role of 14-3-3 in modulating the Ca(2+)-activated Cl(-) channel (CaCC) endogenously expressed in Xenopus oocytes. Injection of 14-3-3epsilon antisense oligodeoxynucleotides resulted in potentiation of the ionomycin-induced Cl(-) current, while 14-3-3 peptide and calmodulin inhibitor, W13, suppressed the antisense-potentiated current. The data suggest that 14-3-3epsilon plays an inhibitory role in modulating the CaCC by interacting with the calmodulin-dependent pathway. The potential role of 14-3-3epsilon in other tissues and its therapeutic potential for cystic fibrosis are discussed.

Isoform pattern of 14-3-3 proteins in the cerebrospinal fluid of patients with Creutzfeldt-Jakob disease

Two-dimensional polyacrylamide gel electrophoresis of CSF has been used in the diagnosis of Creutzfeldt-Jakob disease (CJD). One of the two diagnostic protein spots was identified as isoform(s) of the 14-3-3 family of abundant brain proteins. This has led to the development of one-dimensional 14-3-3 sodium dodecyl sulfate polyacrylamide gel electrophoresis immunoblot, which is currently used to support the diagnosis of CJD. In the present study employing western blot analysis, we have identified the panel of 14-3-3 isoforms that appear in the CSF of 10 patients with CJD compared with 10 patients with other dementias. The results clearly show that the 14-3-3 isoforms beta, gamma, epsilon, and eta are present in the CSF of patients with CJD and can be used to differentiate other dementias. 14-3-3eta also gave a baseline signal in all patients with other dementias, including six patients with Alzheimer's disease. The presence of 14-3-3eta in the CSF of a patient with herpes simplex encephalitis was particularly noteworthy. This study has determined that isoform-specific 14-3-3 antibodies against beta, gamma, and epsilon should be considered for the neurochemical differentiation of CJD from other neurodegenerative diseases.

Recent advances in breast cancer biology

Breast cancer remains the leading cause of death in American women 30 to 70 years of age, and research in the field of breast cancer continues at an explosive pace. Our understanding of the molecular basis of familial breast cancer has advanced significantly through investigation of the tumor suppressor gene BRCA1, as has our knowledge of the role of the ATM gene and predisposition to breast cancer in ataxia-telangiectasia carriers. In addition, progress has been made in understanding the role of HER-2/neu as a prognostic factor in breast cancer. In this review, some of the recent advances in breast cancer biology that are relevant to these areas of study are highlighted.

In vivo and in vitro association of 14-3-3 epsilon isoform with calmodulin: implication for signal transduction and cell proliferation

Using a yeast two-hybrid screen, human 14-3-3 epsilon protein was found to interact with human calmodulin. In vitro binding assay between human 14-3-3 epsilon protein/peptide and calmodulin was demonstrated by native gel electrophoresis, and the interaction was shown to be calcium dependent. Our results, along with the association of the 14-3-3 epsilon protein with other signaling proteins, suggest that the 14-3-3 protein could provide a link between signal transduction and cell proliferation.

BRCA1 is associated with the centrosome during mitosis

Centrosomes and their associated microtubules direct events during mitosis and control the organization of animal cell structures and movement during interphase. The centrosome replicates during the cell cycle, directs the assembly of bipolar mitotic spindles, and plays an important role in maintaining the fidelity of cell division. Recently, tumor suppressors such as p53 and retinoblastoma protein pRB have been localized to the centrosome in a cell cycle-dependent manner. Immunofluorescence microscopy and analysis of isolated centrosomes now provide evidence that BRCA1 protein, a suppressor of tumorigenesis in breast and ovary, also is associated with centrosomes during mitosis. Our results indicate that BRCA1 localizes with the centrosome during mitosis and coimmunoprecipitates with gamma-tubulin, a centrosomal component essential for nucleation of microtubules. Furthermore, gamma-tubulin associates preferentially with a hypophosphorylated form of BRCA1.

Two unique 5' untranslated regions in mRNAs encoding human 14-3-3 zeta: differential expression in hemopoietic cells

In this report, we describe the identification and characterization of a novel 14-3-3 cDNA using the polymerase chain reaction and the screening of a human bone marrow cDNA library. This cDNA encodes the zeta isoform of 14-3-3 and contains a novel 5' untranslated region (UTR) that is G + C rich and only 50% identical to the 5' UTR in the human placental 14-3-3 zeta cDNA, suggesting that 14-3-3 zeta is encoded by at least two mRNAs. Using specific probes to the 5' UTRs of bone marrow and placental 14-3-3 zeta cDNAs, we studied the expression of each transcript in human hemopoietic cells at various stages of differentiation in the myeloid and lymphoid lineages. Differences in the expression of the bone marrow and placental 14-3-3 zeta transcripts were found, the most notable being the markedly decreased expression of both 14-3-3 zeta transcripts in HL-60 myeloid leukemic cells. Western blot analysis of 14-3-3 zeta levels in HL-60 cells revealed correspondingly decreased levels of 14-3-3 zeta protein compared to Jurkat cells. The differences among cell types of relative expression of the two 14-3-3 transcripts may reflect normal regulatory patterns, while the strikingly decreased expression of both types in HL-60 are more likely to be reflective of its multiple genetic abnormalities which contribute to its transformed phenotype.

14-3-3 is phosphorylated by casein kinase I on residue 233. Phosphorylation at this site in vivo regulates Raf/14-3-3 interaction

14-3-3 proteins mediate interactions between proteins involved in signal transduction and cell cycle regulation. Phosphorylation of target proteins as well as 14-3-3 are important for protein-protein interactions. Here, we describe the purification of a protein kinase from porcine brain that phosphorylates 14-3-3 zeta on Thr-233. This protein kinase has been identified as casein kinase Ialpha (CKIalpha) by peptide mapping analysis and sequencing. Among mammalian 14-3-3, only 14-3-3 tau possesses a phosphorylatable residue at the same position (Ser-233), and we show that this residue is also phosphorylated by CKI. In addition, we show that 14-3-3 zeta is exclusively phosphorylated on Thr-233 in human embryonic kidney 293 cells. The residue 233 is located within a region shown to be important for the association of 14-3-3 to target proteins. We showed previously that, in 293 cells, only the unphosphorylated form of 14-3-3 zeta associates with the regulatory domain of c-Raf. We have now shown that in vivo phosphorylation of 14-3-3 zeta at the CKIalpha site (Thr-233) negatively regulates its binding to c-Raf, and may be important in Raf-mediated signal transduction.