Characterization of the p22 subunit of dynactin reveals the localization of cytoplasmic dynein and dynactin to the midbody of dividing cells

A proteome-wide association study identifies putative causal proteins for breast cancer risk

BACKGROUND

Genome-wide association studies (GWAS) have identified more than 200 breast cancer risk-associated genetic loci, yet the causal genes and biological mechanisms for most loci remain elusive. Proteins, as final gene products, are pivotal in cellular function. In this study, we conducted a proteome-wide association study (PWAS) to identify proteins in breast tissue related to breast cancer risk.

METHODS

We profiled the proteome in fresh frozen breast tissue samples from 120 cancer-free European-ancestry women from the Susan G. Komen Tissue Bank (KTB). Protein expression levels were log2-transformed then normalized via quantile and inverse-rank transformations. GWAS data were also generated for these 120 samples. These data were used to build statistical models to predict protein expression levels via cis-genetic variants using the elastic net method. The prediction models were then applied to the GWAS summary statistics data of 133,384 breast cancer cases and 113,789 controls to assess the associations of genetically predicted protein expression levels with breast cancer risk overall and its subtypes using the S-PrediXcan method.

RESULTS

A total of 6388 proteins were detected in the normal breast tissue samples from 120 women with a high detection false discovery rate (FDR) p value < 0.01. Among the 5820 proteins detected in more than 80% of participants, prediction models were successfully built for 2060 proteins with R > 0.1 and P < 0.05. Among these 2060 proteins, five proteins were significantly associated with overall breast cancer risk at an FDR p value < 0.1. Among these five proteins, the corresponding genes for proteins COPG1, DCTN3, and DDX6 were located at least 1 Megabase away from the GWAS-identified breast cancer risk variants. COPG1 was associated with an increased risk of breast cancer with a p value of 8.54 × 10-4. Both DCTN3 and DDX6 were associated with a decreased risk of breast cancer with p values of 1.01 × 10-3 and 3.25 × 10-4, respectively. The corresponding genes for the remaining two proteins, LSP1 and DNAJA3, were located in previously GWAS-identified breast cancer risk loci. After adjusting for GWAS-identified risk variants, the association for DNAJA3 was still significant (p value of 9.15 × 10-5 and adjusted p value of 1.94 × 10-4). However, the significance for LSP1 became weaker with a p value of 0.62. Stratification analyses by breast cancer subtypes identified three proteins, SMARCC1, LSP1, and NCKAP1L, associated with luminal A, luminal B, and ER-positive breast cancer. NCKAP1L was located at least 1Mb away from the GWAS-identified breast cancer risk variants. After adjusting for GWAS-identified breast cancer risk variants, the association for protein LSP1 was still significant (adjusted p value of 6.43 × 10-3 for luminal B subtype).

CONCLUSION

We conducted the first breast-tissue-based PWAS and identified seven proteins associated with breast cancer, including five proteins not previously implicated. These findings help improve our understanding of the underlying genetic mechanism of breast cancer development.

Cargo-Mediated Activation of Cytoplasmic Dynein in vivo

Cytoplasmic dynein-1 is a minus-end-directed microtubule motor that transports a variety of cargoes including early endosomes, late endosomes and other organelles. In many cell types, dynein accumulates at the microtubule plus end, where it interacts with its cargo to be moved toward the minus end. Dynein binds to its various cargoes via the dynactin complex and specific cargo adapters. Dynactin and some of the coiled-coil-domain-containing cargo adapters not only link dynein to cargo but also activate dynein motility, which implies that dynein is activated by its cellular cargo. Structural studies indicate that a dynein dimer switches between the autoinhibited phi state and an open state; and the binding of dynactin and a cargo adapter to the dynein tails causes the dynein motor domains to have a parallel configuration, allowing dynein to walk processively along a microtubule. Recently, the dynein regulator LIS1 has been shown to be required for dynein activation in vivo, and its mechanism of action involves preventing dynein from switching back to the autoinhibited state. In this review, we will discuss our current understanding of dynein activation and point out the gaps of knowledge on the spatial regulation of dynein in live cells. In addition, we will emphasize the importance of studying a complete set of dynein regulators for a better understanding of dynein regulation in vivo.

The splicing-factor Prp40 affects dynein-dynactin function in Aspergillus nidulans

The multi-component cytoplasmic dynein transports cellular cargoes with the help of another multi-component complex dynactin, but we do not know enough about factors that may affect the assembly and functions of these proteins. From a genetic screen for mutations affecting early-endosome distribution in Aspergillus nidulans, we identified the prp40A^L438* mutation in Prp40A, a homologue of Prp40, an essential RNA-splicing factor in the budding yeast. Prp40A is not essential for splicing, although it associates with the nuclear splicing machinery. The prp40A^L438* mutant is much healthier than the ∆prp40A mutant, but both mutants exhibit similar defects in dynein-mediated early-endosome transport and nuclear distribution. In the prp40A^L438* mutant, the frequency but not the speed of dynein-mediated early-endosome transport is decreased, which correlates with a decrease in the microtubule plus-end accumulations of dynein and dynactin. Within the dynactin complex, the actin-related protein Arp1 forms a mini-filament. In a pull-down assay, the amount of Arp1 pulled down with its pointed-end protein Arp11 is lowered in the prp40A^L438* mutant. In addition, we found from published interactome data that a mammalian Prp40 homologue PRPF40A interacts with Arp1. Thus, Prp40 homologues may regulate the assembly or function of dynein–dynactin and their mechanisms deserve to be further studied.

Prognostic Value of Dynactin mRNA Expression in Cutaneous Melanoma

BACKGROUND Dynactin (DCTN) is a multi-subunit protein encoded by DCTN genes for 6 subunits. In different diseases the DCTN genes may have different roles; therefore, we investigated the prognostic potential of DCTN mRNA expression in cutaneous melanoma (CM). MATERIAL AND METHODS Data for DCTN mRNA expression in CM patients were obtained from the OncoLnc database, which contains updated gene expression data for 459 CM patients based on the Cancer Genome Atlas. Kaplan-Meier analysis and a Cox regression model were used to determine overall survival (OS) with calculation of hazard ratios (HRs) and 95% confidence intervals (CIs). RESULTS The multivariate survival analysis showed that individually low expression of DCTN1, DCTN2, and DCTN5 and high expression of DCTN6 were associated with favorable OS (adjusted P=0.008, HR=0.676, 95% CI=0.506-0.903; adjusted P=0.004, HR=0.648, 95% CI=0.485-0.867; adjusted P=0.011, HR=0.686, 95% CI=0.514-0.916; and adjusted P=0.018, HR=0.706, 95% CI=0.530-0.942, respectively). In a joint-effects analysis, combinations of low expression of DCTN1, DCTN2, and DCTN5 and high expression of DCTN6 were found to be more highly correlated with favorable OS (all P<0.05). CONCLUSIONS Our findings suggest that downregulated DCTN1, DCTN2, and DCTN5 and upregulated DCTN6 mRNA expression in CM are associated with favorable prognosis and may represent potential prognostic biomarkers. Moreover, use of the 4 genes in combination can improve the sensitivity for predicting OS in CM patients.

Cloning, characterization and functional analysis of dctn5 in immune response of Chinese tongue sole (Cynoglossus semilaevis)

In mammals, microtubule-dependent trafficking could participate the immune response, where the motor proteins are suggested to play an important role in this process, while the related study in fish was rare. In this study, dctn5, a subunit of dyactin complex for docking motor protein, was obtained by previous immune QTL screening. The full-length cDNAs of two dctn5 transcript variants were cloned and identified (named dctn5_tv1 and dctn5_tv2, respectively). Tissue distribution showed that dctn5_tv1 was widely distributed and high transcription was observed in immune tissue (skin), while dctn5_tv2 was predominantly detected in gonad and very low in other tissues. Time-course expression analysis revealed that dctn5_tv1 could be up-regulated in gill, intestine, skin, spleen, and kidney after Vibrio harveyi challenge. Moreover, recombinant Dctn5_tv1 exhibited high antimicrobial activity against Escherichia coli and Streptococcus agalactiae due to binding to bacteria cells. Taken together, these data suggest Dctn5_tv1 is involved in immune response of bacterial invasion in Chinese tongue sole.

Multiple Roles, Multiple Adaptors: Dynein During Cell Cycle

Dynein is an essential protein complex present in most eukaryotes that regulate biological processes ranging from ciliary beating, intracellular transport, to cell division. Elucidating the detailed mechanism of dynein function has been a challenging task owing to its large molecular weight and high complexity of the motor. With the advent of technologies in the last two decades, studies have uncovered a wealth of information about the structural, biochemical, and cell biological roles of this motor protein. Cytoplasmic dynein associates with dynactin through adaptor proteins to mediate retrograde transport of vesicles, mRNA, proteins, and organelles on the microtubule tracts. In a mitotic cell, dynein has multiple localizations, such as at the nuclear envelope, kinetochores, mitotic spindle and spindle poles, and cell cortex. In line with this, dynein regulates multiple events during the cell cycle, such as centrosome separation, nuclear envelope breakdown, spindle assembly checkpoint inactivation, chromosome segregation, and spindle positioning. Here, we provide an overview of dynein structure and function with focus on the roles played by this motor during different stages of the cell cycle. Further, we review in detail the role of dynactin and dynein adaptors that regulate both recruitment and activity of dynein during the cell cycle.

Recurring DNA copy number gain at chromosome 9p13 plays a role in the activation of multiple candidate oncogenes in progressing oral premalignant lesions

Genomic alteration at chromosome 9p has been previously reported as a frequent and critical event in oral premalignancy. While this alteration is typically reported as a loss driven by selection for CDKN2A deactivation (at 9p21.3), we detect a recurrent DNA copy number gain of ∼2.49 Mbp at chromosome 9p13 in oral premalignant lesions (OPLs) that later progressed to invasive lesions. This recurrent alteration event has been validated using fluorescence in situ hybridization in an independent set of OPLs. Analysis of publicly available gene expression datasets aided in identifying three oncogene candidates that may have driven selection for DNA copy number increases in this region (VCP, DCTN3, and STOML2). We performed in vitro silencing and activation experiments for each of these genes in oral cancer cell lines and found that each gene is independently capable of upregulating proliferation and anchorage-independent growth. We next analyzed the activity of each of these genes in biopsies of varying histological grades that were obtained from a diseased oral tissue field in a single patient, finding further molecular evidence of parallel activation of VCP, DCTN3, and STOML2 during progression from normal healthy tissue to invasive oral carcinoma. Our results support the conclusion that DNA gain at 9p13 is important to the earliest stages of oral tumorigenesis and that this alteration event likely contributes to the activation of multiple oncogene candidates capable of governing oral cancer phenotypes.

Interaction of foot-and-mouth disease virus nonstructural protein 3A with host protein DCTN3 is important for viral virulence in cattle

Nonstructural protein 3A of foot-and-mouth disease virus (FMDV) is a partially conserved protein of 153 amino acids in most FMDVs examined to date. The role of 3A in virus growth and virulence within the natural host is not well understood. Using a yeast two-hybrid approach, we identified cellular protein DCTN3 as a specific host binding partner for 3A. DCTN3 is a subunit of the dynactin complex, a cofactor for dynein, a motor protein. The dynactin-dynein duplex has been implicated in several subcellular functions involving intracellular organelle transport. The 3A-DCTN3 interaction identified by the yeast two-hybrid approach was further confirmed in mammalian cells. Overexpression of DCTN3 or proteins known to disrupt dynein, p150/Glued and 50/dynamitin, resulted in decreased FMDV replication in infected cells. We mapped the critical amino acid residues in the 3A protein that mediate the protein interaction with DCTN3 by mutational analysis and, based on that information, we developed a mutant harboring the same mutations in O1 Campos FMDV (O1C3A-PLDGv). Although O1C3A-PLDGv FMDV and its parental virus (O1Cv) grew equally well in LFBK-αvβ6, O1C3A-PLDGv virus exhibited a decreased ability to replicate in primary bovine cell cultures. Importantly, O1C3A-PLDGv virus exhibited a delayed disease in cattle compared to the virulent parental O1Campus (O1Cv). Virus isolated from lesions of animals inoculated with O1C3A-PLDGv virus contained amino acid substitutions in the area of 3A mediating binding to DCTN3. Importantly, 3A protein harboring similar amino acid substitutions regained interaction with DCTN3, supporting the hypothesis that DCTN3 interaction likely contributes to virulence in cattle.

IMPORTANCE

The objective of this study was to understand the possible role of a FMD virus protein 3A, in causing disease in cattle. We have found that the cellular protein, DCTN3, is a specific binding partner for 3A. It was shown that manipulation of DCTN3 has a profound effect in virus replication. We developed a FMDV mutant virus that could not bind DCTN3. This mutant virus exhibited a delayed disease in cattle compared to the parental strain highlighting the role of the 3A-DCTN3 interaction in virulence in cattle. Interestingly, virus isolated from lesions of animals inoculated with mutant virus contained mutations in the area of 3A that allowed binding to DCTN3. This highlights the importance of the 3A-DCTN3 interaction in FMD virus virulence and provides possible mechanisms of virus attenuation for the development of improved FMD vaccines.

Plk1-dependent phosphorylation of optineurin provides a negative feedback mechanism for mitotic progression

Plk1 activation is required for progression through mitotic entry to cytokinesis. Here we show that at mitotic entry, Plk1 phosphorylates Optineurin (Optn) at serine 177 and that this dissociates Optn from the Golgi-localized GTPase Rab8, inducing its translocation into the nucleus. Mass spectrometry analysis revealed that Optn is associated with a myosin phosphatase complex (MP), which antagonizes the mitotic function of Plk1. Our data also indicate that Optn functionally connects this complex to Plk1 by promoting phosphorylation of the myosin phosphatase targeting subunit 1 (MYPT1). Accordingly, silencing Optn expression increases Plk1 activity and induces abscission failure and multinucleation, which were rescued upon expression of wild-type (WT) Optn, but not a phospho-deficient mutant (S177A) that cannot translocate into the nucleus during mitosis. Overall, these results highlight an important role of Optn in the spatial and temporal coordination of Plk1 activity.

Linking cytoplasmic dynein and transport of Rab8 vesicles to the midbody during cytokinesis by the doublecortin domain-containing 5 protein

Completion of mitosis requires microtubule-dependent transport of membranes to the midbody. Here, we identified a role in cytokinesis for doublecortin domain-containing protein 5 (DCDC5), a member of the doublecortin protein superfamily. DCDC5 is a microtubule-associated protein expressed in both specific and dynamic fashions during mitosis. We show that DCDC5 interacts with cytoplasmic dynein and Rab8 (also known as Ras-related protein Rab-8A), as well as with the Rab8 nucleotide exchange factor Rabin8 (also known as Rab-3A-interacting protein). Following DCDC5 knockdown, the durations of the metaphase to anaphase transition and cytokinesis, and the proportion of multinucleated cells increases, whereas cell viability decreases. Furthermore, knockdown of DCDC5 or addition of a dynein inhibitor impairs the entry of Golgi-complex-derived Rab8-positive vesicles to the midbody. These findings suggest that DCDC5 plays an important role in mediating dynein-dependent transport of Rab8-positive vesicles and in coordinating late cytokinesis.

The dynactin complex maintains the integrity of metaphasic centrosomes to ensure transition to anaphase

The dynactin complex is required for activation of the dynein motor complex, which plays a critical role in various cell functions including mitosis. During metaphase, the dynein-dynactin complex removes spindle checkpoint proteins from kinetochores to facilitate the transition to anaphase. Three components (p150(Glued), dynamitin, and p24) compose a key portion of the dynactin complex, termed the projecting arm. To investigate the roles of the dynactin complex in mitosis, we used RNA interference to down-regulate p24 and p150(Glued) in human cells. In response to p24 down-regulation, we observed cells with delayed metaphase in which chromosomes frequently align abnormally to resemble a "figure eight," resulting in cell death. We attribute the figure eight chromosome alignment to impaired metaphasic centrosomes that lack spindle tension. Like p24, RNA interference of p150(Glued) also induces prometaphase and metaphase delays; however, most of these cells eventually enter anaphase and complete mitosis. Our findings suggest that although both p24 and p150(Glued) components of the dynactin complex contribute to mitotic progression, p24 also appears to play a role in metaphase centrosome integrity, helping to ensure the transition to anaphase.

Caenorhabditis elegans ortholog of the p24/p22 subunit, DNC-3, is essential for the formation of the dynactin complex by bridging DNC-1/p150(Glued) and DNC-2/dynamitin

Dynactin is a multisubunit protein complex required for the activity of cytoplasmic dynein. In Caenorhabditis elegans, although 10 of the 11 dynactin subunits were identified based on the sequence similarities to their orthologs, the p24/p22 subunit has not been detected in the genome. Here, we demonstrate that DNC-3 (W10G11.20) is the functional counterpart of the p24/p22 subunit in C. elegans. RNAi phenotypes and subcellular localization of DNC-3 in early C. elegans embryos were nearly identical to those of the known dynactin components. All other dynactin subunits were co-immunoprecipitated with DNC-3, indicating that DNC-3 is a core component of dynactin. Furthermore, the overall secondary structure of DNC-3 resembles to those of the mammalian and yeast p24/p22. We found that DNC-3 is required for the localization of the DNC-1/p150(Glued) and DNC-2/dynamitin, the two components of the projection arm of dynactin, to the nuclear envelope of meiotic nuclei in the adult gonad. Moreover, DNC-3 physically interacted with DNC-1 and DNC-2 and significantly enhanced the binding ability between DNC-1 and DNC-2 in vitro. These results suggest that DNC-3 is essential for the formation of the projection arm subcomplex of dynactin.

Imipramine treatment and resiliency exhibit similar chromatin regulation in the mouse nucleus accumbens in depression models

Although it is a widely studied psychiatric syndrome, major depressive disorder remains a poorly understood illness, especially with regard to the disconnect between treatment initiation and the delayed onset of clinical improvement. We have recently validated chronic social defeat stress in mice as a model in which a depression-like phenotype is reversed by chronic, but not acute, antidepressant administration. Here, we use chromatin immunoprecipitation (ChIP)-chip assays--ChIP followed by genome wide promoter array analyses--to study the effects of chronic defeat stress on chromatin regulation in the mouse nucleus accumbens (NAc), a key brain reward region implicated in depression. Our results demonstrate that chronic defeat stress causes widespread and long-lasting changes in gene regulation, including alterations in repressive histone methylation and in phospho-CREB (cAMP response element-binding protein) binding, in the NAc. We then show similarities and differences in this regulation to that observed in another mouse model of depression, prolonged adult social isolation. In the social defeat model, we observed further that many of the stress-induced changes in gene expression are reversed by chronic imipramine treatment, and that resilient mice-those resistant to the deleterious effects of defeat stress-show patterns of chromatin regulation in the NAc that overlap dramatically with those seen with imipramine treatment. These findings provide new insight into the molecular basis of depression-like symptoms and the mechanisms by which antidepressants exert their delayed clinical efficacy. They also raise the novel idea that certain individuals resistant to stress may naturally mount antidepressant-like adaptations in response to chronic stress.

RACK-1 directs dynactin-dependent RAB-11 endosomal recycling during mitosis in Caenorhabditis elegans

Membrane trafficking pathways are necessary for the addition and removal of membrane during cytokinesis. In animal cells, recycling endosomes act as a major source of the additional membranes during furrow progression and abscission. However, the mechanisms and factors that regulate recycling endosomes during the cell cycle remain poorly understood. Here, we show that the Caenorhabditis elegans Receptor of Activated C Kinase 1 (RACK-1) is required for cytokinesis, germline membrane organization, and the recruitment of RAB-11-labeled recycling endosomes to the pericentrosomal region and spindle. RACK-1 is also required for proper chromosome separation and astral microtubule length. RACK-1 localizes to the centrosomes, kinetochores, the midbody, and nuclear envelopes during the cell cycle. We found that RACK-1 directly binds to DNC-2, the C. elegans p50/dynamitin subunit of the dynactin complex. Last, RACK-1 may facilitate the sequestration of recycling endosomes by targeting DNC-2 to centrosomes and the spindle. Our findings suggest a mechanism by which RACK-1 directs the dynactin-dependent redistribution of recycling endosomes during the cell cycle, thus ensuring proper membrane trafficking events during cytokinesis.

Dynactin is essential for growth cone advance

Dynactin is a multi-subunit complex that serves as a critical cofactor of the microtubule motor cytoplasmic dynein. We previously identified dynactin in the nerve growth cone. However, the function of dynactin in the growth cone is still unclear. Here we show that dynactin in the growth cone is required for constant forward movement of the growth cone. Chromophore-assisted laser inactivation (CALI) of dynamitin, a dynactin subunit, within the growth cone markedly decreases the rate of growth cone advance. CALI of dynamitin in vitro dissociates another dynactin subunit, p150(Glued), from dynamitin. These results indicate that dynactin, especially the interaction between dynamitin and p150(Glued), plays an essential role in growth cone advance.

The Saccharomyces cerevisiae homolog of p24 is essential for maintaining the association of p150Glued with the dynactin complex

Stu1 is the Saccharomyces cerevisiae member of the CLASP family of microtubule plus-end tracking proteins and is essential for spindle formation. A genomewide screen for gene deletions that are lethal in combination with the temperature-sensitive stu1-5 allele identified ldb18Delta. ldb18Delta cells exhibit defects in spindle orientation similar to those caused by a block in the dynein pathway. Consistent with this observation, ldb18Delta is synthetic lethal with mutations affecting the Kar9 spindle orientation pathway, but not with those affecting the dynein pathway. We show that Ldb18 is a component of dynactin, a complex required for dynein activity in yeast and mammalian cells. Ldb18 shares modest sequence and structural homology with the mammalian dynactin component p24. It interacts with dynactin proteins in two-hybrid and co-immunoprecipitation assays, and comigrates with them as a 20 S complex during sucrose gradient sedimentation. In ldb18Delta cells, the interaction between Nip100 (p150(Glued)) and Jnm1 (dynamitin) is disrupted, while the interaction between Jnm1 and Arp1 is not affected. These results indicate that p24 is required for attachment of the p150(Glued) arm to dynamitin and the remainder of the dynactin complex. The genetic interaction of ldb18Delta with stu1-5 also supports the notion that dynein/dynactin helps to generate a spindle pole separating force.

Kinesin and dynein-dynactin at intersecting microtubules: motor density affects dynein function

Kinesin and cytoplasmic dynein are microtubule-based motor proteins that actively transport material throughout the cell. Microtubules can intersect at a variety of angles both near the nucleus and at the cell periphery, and the behavior of molecular motors at these intersections has implications for long-range transport efficiency and accuracy. To test motor function at microtubule intersections, crossovers were arranged in vitro using flow to orient successive layers of filaments. Single kinesin and cytoplasmic dynein-dynactin molecules fused with green-fluorescent protein, and artificial bead cargos decorated with multiple motors, were observed while they encountered intersections. Single kinesins tend to cross intersecting microtubules, whereas single dynein-dynactins have a more varied response. For bead cargos, kinesin motion is independent of motor number. Dynein beads with high motor numbers pause, but their actions become more varied as the motor number decreases. These results suggest that regulating the number of active dynein molecules could change a motile cargo into one that is anchored at an intersection, consistent with dynein's proposed transport and tethering functions in the cell.

Interaction of hepatitis B viral oncoprotein with cellular target HBXIP dysregulates centrosome dynamics and mitotic spindle formation

Hepatitis B virus infection is associated with hepatocellular carcinoma, claiming 1 million lives annually worldwide. To understand the carcinogenic mechanism of hepatitis B virus-encoded oncoprotein HBx, we explored the function of HBx interaction with its cellular target HBXIP. Previously, we demonstrated that viral HBx and cellular HBXIP control mitotic spindle formation, regulating centrosome splitting. By using various fragments of HBx, we determined that residues (137)CRHK(140) within HBx are necessary for binding HBXIP. Mutation of the (137)CRHK(140) motif in HBx abolished its ability to bind HBXIP and to dysregulate centrosome dynamics in HeLa and immortal diploid RPE-1 cells. Unlike wild-type HBx, which targets to centrosomes as determined by subcellular fractionation and immunofluorescence microscopy, HBx mutants failed to localize to centrosomes. Overexpression of viral HBx wild-type protein and knockdown of endogenous HBXIP altered centrosome assembly and induced modifications of pericentrin and centrin-2, two essential proteins required for centrosome formation and function, whereas HBXIP nonbinding mutants of HBx did not. Overexpression of HBXIP or fragments of HBXIP that bind HBx neutralized the effects of viral HBx on centrosome dynamics and spindle formation. These results suggest that HBXIP is a critical target of viral HBx for promoting genetic instability through formation of defective spindles and subsequent aberrant chromosome segregation.

Breast cancer molecular signatures as determined by SAGE: correlation with lymph node status

Global gene expression measured by DNA microarray platforms have been extensively used to classify breast carcinomas correlating with clinical characteristics, including outcome. We generated a breast cancer Serial Analysis of Gene Expression (SAGE) high-resolution database of approximately 2.7 million tags to perform unsupervised statistical analyses to obtain the molecular classification of breast-invasive ductal carcinomas in correlation with clinicopathologic features. Unsupervised statistical analysis by means of a random forest approach identified two main clusters of breast carcinomas, which differed in their lymph node status (P=0.01); this suggested that lymph node status leads to globally distinct expression profiles. A total of 245 (55 up-modulated and 190 down-modulated) transcripts were differentially expressed between lymph node (+) and lymph node (-) primary breast tumors (fold change, >or=2; P<0.05). Various lymph node (+) up-modulated transcripts were validated in independent sets of human breast tumors by means of real-time reverse transcription-PCR (RT-PCR). We validated significant overexpression of transcripts for HOXC10 (P=0.001), TPD52L1 (P=0.007), ZFP36L1 (P=0.011), PLINP1 (P=0.013), DCTN3 (P=0.025), DEK (P=0.031), and CSNK1D (P=0.04) in lymph node (+) breast carcinomas. Moreover, the DCTN3 (P=0.022) and RHBDD2 (P=0.002) transcripts were confirmed to be overexpressed in tumors that recurred within 6 years of follow-up by real-time RT-PCR. In addition, meta-analysis was used to compare SAGE data associated with lymph node (+) status with publicly available breast cancer DNA microarray data sets. We have generated evidence indicating that the pattern of gene expression in primary breast cancers at the time of surgical removal could discriminate those tumors with lymph node metastatic involvement using SAGE to identify specific transcripts that behave as predictors of recurrence as well.

Mechanism of Dynamitin-mediated Disruption of Dynactin

Dynamitin is a commonly used inhibitor of cytoplasmic dynein-based motility in living cells. Dynamitin does not inhibit dynein directly but instead acts by causing disassembly of dynactin, a multiprotein complex required for dynein-based movement. In dynactin, dynamitin is closely associated with the subunits p150(Glued) and p24, which together form the shoulder and projecting arm structures of the dynactin molecule. In this study, we explore the way in which exogenous dynamitin effects dynactin disruption. We find that pure, recombinant dynamitin is an elongated protein with a strong propensity for self-assembly. Titration experiments reveal that free dynamitin binds dynactin before it causes release of subunits. When dynamitin is added to dynactin at an equimolar ratio of exogenous dynamitin subunits to endogenous dynamitin subunits (1x= 4 mol of exogenous dynamitin per mole of dynactin), exogenous dynamitin exchanges with endogenous dynamitin, and partial release of p150(Glued) is observed. When added in vast excess (> or =25x; 100 mol of exogenous dynamitin per mole of dynactin), recombinant dynamitin causes complete release of both p150(Glued) subunits, two dynamitins and one p24, but not other dynactin subunits. Our data suggest that dynamitin mediates disruption of dynactin by binding to endogenous dynamitin subunits. This binding destabilizes the shoulder structure that links the p150(Glued) arm to the Arp1 filament and leads to subunit release.

HBXIP, cellular target of hepatitis B virus oncoprotein, is a regulator of centrosome dynamics and cytokinesis

Hepatitis B virus accounts for more than 1 million cancer deaths annually, but the mechanism by which this virus promotes hepatocellular carcinoma remains unclear. The hepatitis B virus genome encodes an oncoprotein, HBx, which binds various cellular proteins including HBXIP. We show here that HBXIP is a regulator of centrosome duplication, required for bipolar spindle formation in HeLa human carcinoma cells and primary mouse embryonic fibroblast cells. We found that most cells deficient in HBXIP arrest in prometaphase with monopolar spindles whereas HBXIP overexpression causes tripolar or multipolar spindles due to excessive centrosome replication. Additionally, a defect in cytokinesis was seen in HBXIP-deficient HeLa cells, with most cells failing to complete division and succumbing eventually to apoptosis. Expression of viral HBx in HeLa cells mimicked the effects of HBXIP overexpression, causing excessive centrosome replication, resulting in tripolar and multipolar spindles and defective cytokinesis. Immunolocalization and fluorescent protein tagging experiments showed that HBXIP associates with microtubules of dividing cells and colocalizes with HBx on centrosomes. Thus, viral HBx and its cellular target HBXIP regulate centrosome dynamics and cytokinesis affecting genetic stability. In vivo experiments using antisense oligonucleotides targeting HBXIP in a mouse model of liver regeneration showed a requirement for HBXIP for growth and survival of replicating hepatocytes. Thus, HBXIP is a critical regulator of hepatocyte cell growth in vivo, making it a strong candidate for explaining the tumorigenic actions of viral HBx.

Genetic aberrations in prostate cancer by microarray analysis

The aim of this study was to screen genetic as well as expression alterations in prostate cancer. Array comparative genomic hybridization (aCGH) to a 16K cDNA microarray was performed to analyze DNA sequence copy number alterations in 5 prostate cancer cell lines and 13 xenografts. The aCGH confirmed the previously implicated common gains and losses, such as gains at 1q, 7, 8q, 16p and 17q and losses at 2q, 4p/q, 6q, 8p, 13q, 16q, 17p and 18q, which have previously been identified by chromosomal CGH (cCGH). Because of the higher resolution of aCGH, the minimal commonly altered regions were significantly narrowed-down. For example, the gain of 8q was mapped to three independent regions, 8q13.3-q21.11, 8q22.2 and 8q24.13-q24.3. In addition, a novel recurrent gain at 9p13-q21 was identified. The concomitant expression analysis indicated that genome-wide DNA sequence copy number (gene dosage) was significantly associated with the expression level (p < 0.0001). The analyses indicated several individual genes whose expression was associated with the gene copy number. For example, gains of PTK2 and FZD6, were associated with the increased expression, whereas losses of TNFRSF10B (alias DR5) and ITGA4 with decreased expression. In conclusion, the aCGH mapping data will aid in the identification of genes altered in prostate cancer. The combined expression and copy number analysis suggested that even a low-level copy number change may have significant effect on gene expression, and thus on the development of prostate cancer.

Cytoplasmic dynein's mitotic spindle pole localization requires a functional anaphase-promoting complex, gamma-tubulin, and NUDF/LIS1 in Aspergillus nidulans

In Aspergillus nidulans, cytoplasmic dynein and NUDF/LIS1 are found at the spindle poles during mitosis, but they seem to be targeted to this location via different mechanisms. The spindle pole localization of cytoplasmic dynein requires the function of the anaphase-promoting complex (APC), whereas that of NUDF does not. Moreover, although NUDF's localization to the spindle poles does not require a fully functional dynein motor, the function of NUDF is important for cytoplasmic dynein's targeting to the spindle poles. Interestingly, a gamma-tubulin mutation, mipAR63, nearly eliminates the localization of cytoplasmic dynein to the spindle poles, but it has no apparent effect on NUDF's spindle pole localization. Live cell analysis of the mipAR63 mutant revealed a defect in chromosome separation accompanied by unscheduled spindle elongation before the completion of anaphase A, suggesting that gamma-tubulin may recruit regulatory proteins to the spindle poles for mitotic progression. In A. nidulans, dynein is not apparently required for mitotic progression. In the presence of a low amount of benomyl, a microtubule-depolymerizing agent, however, a dynein mutant diploid strain exhibits a more pronounced chromosome loss phenotype than the control, indicating that cytoplasmic dynein plays a role in chromosome segregation.

Dynactin

Dynactin is a multisubunit protein complex that is required for most, if not all, types of cytoplasmic dynein activity in eukaryotes. Dynactin binds dynein directly and allows the motor to traverse the microtubule lattice over long distances. A single dynactin subunit, p150Glued, is sufficient for both activities, yet dynactin contains several other subunits that are organized into an elaborate structure. It is currently believed that the bulk of the dynactin structure participates in interactions with a wide range of cellular structures, many of which are cargoes of the dynein motor. Genetic studies verify the importance of all elements of dynactin structure to its function. Although dynein can bind some membranous cargoes independently of dynactin, establishment of a fully functional dynein-cargo link appears to depend on dynactin. In this review, I summarize what is presently known about dynactin structure, the cellular structures with which it associates, and the intermolecular interactions that underlie and regulate binding. Although the molecular details of dynactin's interactions with membranous organelles and other molecules are complex, the framework provided here is intended to distill what is presently known and to be of use to dynactin specialists and beginners alike.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

An automated in vitro protein folding screen applied to a human dynactin subunit

The preparation of proteins for structural and functional analysis using the Escherichia coli expression system is often hampered by the formation of insoluble intracellular protein aggregates (inclusion bodies). Transferring those proteins into their native states by in vitro protein folding requires screening for the best buffer conditions and suitable additives. However, it is difficult to assess the success of such a screen if no biological assay is available. We established a fully automated folding screen and a system to detect folded protein that is based on analytical hydrophobic interaction chromatography and tryptophan fluorescence spectroscopy. The system was evaluated with two model enzymes (carbonic anhydrase II and malate dehydrogenase), and was successfully applied to the folding of the p22 subunit of human dynactin, which is expressed in inclusion bodies in E. coli. The described screen allows for high-throughput folding analysis of inclusion body proteins for structural and functional analyses.

Dynamin 2 binds gamma-tubulin and participates in centrosome cohesion

Dynamin 2 (Dyn2) is a large GTPase involved in vesicle formation and actin reorganization. In this study, we report a novel role for Dyn2 as a component of the centrosome that is involved in centrosome cohesion. By light microscopy, Dyn2 localized aside centrin and colocalized with gamma-tubulin at the centrosome; by immunoelectron microscopy, however, Dyn2 was detected in the pericentriolar material as well as on centrioles. Exogenously expressed green fluorescent protein (GFP)-tagged Dyn2 also localized to the centrosome, whereas glutathione S-transferase (GST)-tagged Dyn2 pulled down a protein complex(es) containing actin, alpha-tubulin and gamma-tubulin from liver homogenate. Furthermore, gel overlay and immunoprecipitation indicated a direct interaction between gamma-tubulin and a 219-amino-acid middle domain of Dyn2. Reduction of Dyn2 protein levels with small-interfering RNA (siRNA) resulted in centrosome splitting, whereas microtubule nucleation from centrosomes was not affected, suggesting a role for Dyn2 in centrosome cohesion. Finally, fluorescence recovery after photobleaching (FRAP) analysis of a GFP-tagged Dyn2 middle domain indicated that Dyn2 is a dynamic exchangeable component of the centrosome. These findings suggest a novel function for Dyn2 as a participant in centrosome cohesion.

Inhibition of prostate tumor growth by overexpression of NudC, a microtubule motor-associated protein

Microtubules play a central role in coordinating various cellular functions that are orchestrated by their interaction with molecular motors. Anticancer drugs that target microtubule dynamics have been shown to be effective in cancer treatment. However, the effect of microtubule motor-associated molecules on cancer cell proliferation is not clear. Here, we investigated the role of NudC, a nuclear movement protein associated with the microtubule motor dynein, on prostate tumorigenesis. Recombinant adenovirus expressing NudC (Ad-NudC) was used to examine the effects of NudC on the tumorigenicity of prostate cancer cells. Expression of NudC in LNCaP cells inhibited their anchorage-independent growth in a soft agar colony assay. Expression of NudC in DU145 or PC-3 cells inhibited tumor growth in a subcutaneous xenograft model. At the cellular level, expression of NudC in DU145 and PC-3 cells inhibited cell proliferation at 48 h after Ad-NudC infection. FACS analysis of cell cycle distribution showed that 50-60% of Ad-NudC-infected PC-3 cells have a G2/M-phase DNA content compared to about 16-19% in Ad-Luciferase (Ad-Luc)-infected control cells, suggesting that NudC overexpression resulted in aberrant cell cycle progression. Immunofluorescence microscopy revealed a significant increase in cells with a single enlarged nucleus and cells exhibiting multiple nuclei, along with a concomitant increase in cell size in Ad-NudC-infected cells. These results suggest that NudC overexpression led to a block in cell division of prostate cancer cells, and that Ad-NudC may provide a new anticancer drug approach targeting the function of a microtubule motor-associated protein.

Analysis of the dynein-dynactin interaction in vitro and in vivo

Cytoplasmic dynein and dynactin are megadalton-sized multisubunit molecules that function together as a cytoskeletal motor. In the present study, we explore the mechanism of dynein-dynactin binding in vitro and then extend our findings to an in vivo context. Solution binding assays were used to define binding domains in the dynein intermediate chain (IC) and dynactin p150Glued subunit. Transient overexpression of a series of fragments of the dynein IC was used to determine the importance of this subunit for dynein function in mammalian tissue culture cells. Our results suggest that a functional dynein-dynactin interaction is required for proper microtubule organization and for the transport and localization of centrosomal components and endomembrane compartments. The dynein IC fragments have different effects on endomembrane localization, suggesting that different endomembranes may bind dynein via distinct mechanisms.

A role for Plk1 phosphorylation of NudC in cytokinesis

Polo-like kinase 1 (Plk1) plays essential roles at multiple events during cell division, yet little is known about its physiological substrates. In a cDNA phage display screen using Plk1 C-terminal affinity columns, we identified NudC (nuclear distribution gene C) as a Plk1 binding protein. Here, we characterize the interaction between Plk1 and NudC, show that Plk1 phosphorylates NudC at conserved S274 and S326 residues in vitro, and present evidence that NudC is also a substrate for Plk1 in vivo. Downregulation of NudC by RNA interference results in multiple mitotic defects, including multinucleation and cells arrested at the midbody stage, which are rescued by ectopic expression of wild-type NudC, but not by NudC with mutations in the Plk1 phosphorylation sites. These results suggest that Plk1 phosphorylation of NudC may influence cytokinesis.

Peroxisomal motility and interaction with microtubules

Recent in vivo observations have revealed that peroxisomes are more dynamic and interactive than previously assumed. The growing recognition of the tubular and reticular morphology of peroxisomes in living cells, their association with microtubules, and the dynamic movements of peroxisomes in vivo and in vitro have inspired the query into the investigation of the cellular machinery that mediates such a complex behaviour. The characterisation of the underlying molecular components of this machinery is providing insight into the mechanisms regulating peroxisomal morphology and intracellular distribution.

Role for NudC, a dynein-associated nuclear movement protein, in mitosis and cytokinesis

NudC, a nuclear movement protein that associates with dynein, was originally cloned as a mitogen-inducible early growth response gene. NudC forms a biochemical complex with components of the dynein/dynactin complex and is suggested to play a role in translocation of nuclei in proliferating neuronal progenitors as well as in migrating neurons in culture. Here, we show that NudC plays multiple roles in mitosis and cytokinesis in cultured mammalian cells. Altering NudC levels by either small interfering RNA-mediated gene silencing or adenovirus-mediated overexpression resulted in multinucleated cells and cells with persistent intercellular connections and disorganized midzone and midbody matrix. These phenotypes suggest a failure in cytokinesis in NudC altered cells. Further, a key mitotic enzyme, polo-like kinase, is mislocalized from the centrosomes and the midbody in NudC altered cells. Gene silencing of nud-1, the Caenorhabditis elegans ortholog of NudC, led to a loss of midzone microtubules and the rapid regression of the cleavage furrow, which resulted in one-celled embryos containing two nuclei. The loss of midzone microtubule organization owing to silencing of the NudC/nud-1 gene in two systems, coupled with the loss of Plk1 from mitotic structures in mammalian cells, provide clues to the cytokinesis defect and the multinucleation phenotype. Our findings suggest that NudC functions in mitosis and cytokinesis, in part by regulating microtubule organization at the midzone and midbody.

The microtubule plus-end proteins EB1 and dynactin have differential effects on microtubule polymerization

Several microtubule-binding proteins including EB1, dynactin, APC, and CLIP-170 localize to the plus-ends of growing microtubules. Although these proteins can bind to microtubules independently, evidence for interactions among them has led to the hypothesis of a plus-end complex. Here we clarify the interaction between EB1 and dynactin and show that EB1 binds directly to the N-terminus of the p150(Glued) subunit. One function of a plus-end complex may be to regulate microtubule dynamics. Overexpression of either EB1 or p150(Glued) in cultured cells bundles microtubules, suggesting that each may enhance microtubule stability. The morphology of these bundles, however, differs dramatically, indicating that EB1 and dynactin may act in different ways. Disruption of the dynactin complex augments the bundling effect of EB1, suggesting that dynactin may regulate the effect of EB1 on microtubules. In vitro assays were performed to elucidate the effects of EB1 and p150(Glued) on microtubule polymerization, and they show that p150(Glued) has a potent microtubule nucleation effect, whereas EB1 has a potent elongation effect. Overall microtubule dynamics may result from a balance between the individual effects of plus-end proteins. Differences in the expression and regulation of plus-end proteins in different cell types may underlie previously noted differences in microtubule dynamics.

The requirement of the LC8 dynein light chain for nuclear migration and septum positioning is temperature dependent in Aspergillus nidulans

In the filamentous fungus Aspergillus nidulans, the multisubunit motor complex cytoplasmic dynein plays essential roles in nuclear migration and septum positioning. The 8 kDa light chain, LC8, the smallest subunit, is conserved among eukaryotic organisms. Besides being a component in the dynein complex, LC8 also interacts with a wide spectrum of mammalian and viral proteins. To date, the function of this small polypeptide is not well understood. To address this issue, we have created a deletion mutation (DeltanudG) at the nudG locus encoding LC8 in A. nidulans. At 42 degrees C, the DeltanudG mutant forms minute colonies lacking asexual reproduction: this phenotype resembles the phenotype of the dynein heavy chain null mutant. The mutant nuclei largely clustered in the spore body after conidial germination, and the septum was often assembled distally toward the hyphal apex, whereas a control germling has its nuclei distributed along the hypha and the septum formed near the spore body. When the mutant was grown at 23 degrees C, however, its colony resembled a control one, and so did the patterns of nuclear distribution and septum positioning. Elevation of the growth temperature gradually reduced colony size and abolished asexual sporulation. After a period of growth at 23 degrees C that allowed the nuclei to move out of the spore end, a temperature shift to 42 degrees C prevented newly divided nuclei from migrating apart, suggesting that LC8/NUDG was required for both initiating and maintaining dynein motor functions at elevated temperatures. A functional GFP-NUDA fusion was used to test whether LC8/NUDG is required for DHC (dynein heavy chain)/NUDA localization. We found that at 23 degrees C GFP-NUDA localized to the hyphal apex and the septation site in DeltanudG cells as in control cells. Such localizations were absent at 42 degrees C in mutant cells, but not in control cells. We conclude that LC8 plays a role in DHC localization/function, and the requirement for such a role in A. nidulans cells is temperature dependent.

Distinct cell cycle-dependent roles for dynactin and dynein at centrosomes

Centrosomal dynactin is required for normal microtubule anchoring and/or focusing independently of dynein. Dynactin is present at centrosomes throughout interphase, but dynein accumulates only during S and G2 phases. Blocking dynein-based motility prevents recruitment of dynactin and dynein to centrosomes and destabilizes both centrosomes and the microtubule array, interfering with cell cycle progression during mitosis. Destabilization of the centrosomal pool of dynactin does not inhibit dynein-based motility or dynein recruitment to centrosomes, but instead causes abnormal G1 centriole separation and delayed entry into S phase. The correct balance of centrosome-associated dynactin subunits is apparently important for satisfaction of the cell cycle mechanism that monitors centrosome integrity before centrosome duplication and ultimately governs the G1 to S transition. Our results suggest that, in addition to functioning as a microtubule anchor, dynactin contributes to the recruitment of important cell cycle regulators to centrosomes.

Disruption of dynein/dynactin inhibits axonal transport in motor neurons causing late-onset progressive degeneration

To test the hypothesis that inhibition of axonal transport is sufficient to cause motor neuron degeneration such as that observed in amyotrophic lateral sclerosis (ALS), we engineered a targeted disruption of the dynein-dynactin complex in postnatal motor neurons of transgenic mice. Dynamitin overexpression was found to disassemble dynactin, a required activator of cytoplasmic dynein, resulting in an inhibition of retrograde axonal transport. Mice overexpressing dynamitin demonstrate a late-onset progressive motor neuron degenerative disease characterized by decreased strength and endurance, motor neuron degeneration and loss, and denervation of muscle. Previous transgenic mouse models of ALS have shown abnormalities in microtubule-based axonal transport. In this report, we describe a mouse model that confirms the critical role of disrupted axonal transport in the pathogenesis of motor neuron degenerative disease.

PLAC-24 is a cytoplasmic dynein-binding protein that is recruited to sites of cell-cell contact

We screened for polypeptides that interact specifically with dynein and identified a novel 24-kDa protein (PLAC-24) that binds directly to dynein intermediate chain (DIC). PLAC-24 is not a dynactin subunit, and the binding of PLAC-24 to the dynein intermediate chain is independent of the association between dynein and dynactin. Immunocytochemistry using PLAC-24-specific polyclonal antibodies revealed a punctate perinuclear distribution of the polypeptide in fibroblasts and isolated epithelial cells. However, as epithelial cells in culture make contact with adjacent cells, PLAC-24 is specifically recruited to the cortex at sites of contact, where the protein colocalizes with components of the adherens junction. Disruption of the cellular cytoskeleton with latrunculin or nocodazole indicates that the localization of PLAC-24 to the cortex is dependent on intact actin filaments but not on microtubules. Overexpression of beta-catenin also leads to a loss of PLAC-24 from sites of cell-cell contact. On the basis of these data and the recent observation that cytoplasmic dynein is also localized to sites of cell-cell contact in epithelial cells, we propose that PLAC-24 is part of a multiprotein complex localized to sites of intercellular contact that may function to tether microtubule plus ends to the actin-rich cellular cortex.

Differential gene expression in premalignant human trophoblast: role of IGFBP-5

Tumorigenesis results from genetic alterations that occur in a stepwise manner giving rise to cells with increasingly cancer-like characteristics. We used in vitro propagated first trimester human extravillous trophoblast (EVT) cells to identify genetic changes responsible for the transition of the EVT from a normal to premalignant stage. The model used consisted of a normal invasive EVT (HTR8) cell line and its premalignant derivative (RSVT2/C) generated by transfection with the SV40 Tag and selected using a forced crisis regimen. RSVT2/C display increased proliferative, migratory and invasive behavior, unresponsiveness to anti-proliferative and anti-invasive signals of TGFbeta and a deficiency in gap junctional intercellular communication. These cells, however, were unable to form colonies on soft agar or tumors in nude mice and are thus defined as premalignant. Differential display revealed 18 gene sequences, 7 with unknown and 11 with known identity, showing altered expression between the normal HTR8 and premalignant RSVT2/C cell lines. The known sequences include the potential tumor suppressors insulin-like growth factor binding protein (IGFBP)-5 and fibronectin (FN) and potential protooncogenes such as chromokinesin (KIF4), alternative splicing factor (SF2), dynein, DNA polymerase epsilon (DNApol epsilon) and NF-kappaB activating kinase (NAK). The role of the remaining 4 genes upregulated in the premalignant EVT is presently unknown and these are FK506 binding protein (FKBP) 25, histone protein (HP1Hs)-gamma, nucleoporin (Nup) 155 and an 82 kDa acidic human protein. The functional role of IGFBP-5 was examined in the control of proliferation, migration and invasiveness of RSVT2/C cells measured in vitro. IGFBP-5 alone had no effect on these properties of RSVT2/C cells. Furthermore, unlike normal EVT cells, RSVT2/C cells exhibited refractoriness to the migration stimulating signals of IGF-II, which was explained by the loss or downregulation of the IGF type 2 receptor (IGF-R2). RSVT2/C cells, however, expressed the IGF type 1 receptor (IGF-R1) and responded to IGF-I by increased proliferation. This response was blocked with increasing concentrations of IGFBP-5. These results suggest that the loss of IGFBP-5 and possibly IGF-R2, both of which can sequester IGF-I from IGF-R1, permits unhindered proliferation of the premalignant EVT in an IGF-I rich environment of the fetal-maternal interface. The functions of the other differentially expressed genes, some of which are essential for cell cycle progression or cell survival require further investigation.

Dynein binds to beta-catenin and may tether microtubules at adherens junctions

Interactions between microtubule and actin networks are thought to be crucial for mechanical and signalling events at the cell cortex. Cytoplasmic dynein has been proposed to mediate many of these interactions. Here, we report that dynein is localized to the cortex at adherens junctions in cultured epithelial cells and that this localization is sensitive to drugs that disrupt the actin cytoskeleton. Dynein is recruited to developing contacts between cells, where it localizes with the junctional proteins beta-catenin and E-cadherin. Microtubules project towards these early contacts and we hypothesize that dynein captures and tethers microtubules at these sites. Dynein immunoprecipitates with beta-catenin, and biochemical analysis shows that dynein binds directly to beta-catenin. Overexpression of beta-catenin disrupts the cellular localization of dynein and also dramatically perturbs the organization of the cellular microtubule array. In cells overexpressing beta-catenin, the centrosome becomes disorganized and microtubules no longer appear to be anchored at the cortex. These results identify a novel role for cytoplasmic dynein in capturing and tethering microtubules at adherens junctions, thus mediating cross-talk between actin and microtubule networks at the cell cortex.

beta III spectrin binds to the Arp1 subunit of dynactin

Cytoplasmic dynein is an intracellular motor responsible for endoplasmic reticulum-to-Golgi vesicle trafficking and retrograde axonal transport. The accessory protein dynactin has been proposed to mediate the association of dynein with vesicular cargo. Dynactin contains a 37-nm filament made up of the actin-related protein, Arp1, which may interact with a vesicle-associated spectrin network. Here, we demonstrate that Arp1 binds directly to the Golgi-associated betaIII spectrin isoform. We identify two Arp1-binding sites in betaIII spectrin, one of which overlaps with the actin-binding site conserved among spectrins. Although conventional actin binds weakly to betaIII spectrin, Arp1 binds robustly in the presence of excess F-actin. Dynein, dynactin, and betaIII spectrin co-purify on vesicles isolated from rat brain, and betaIII spectrin co-immunoprecipitates with dynactin from rat brain cytosol. In interphase cells, betaIII spectrin and dynactin both localize to cytoplasmic vesicles, co-localizing most significantly in the perinuclear region of the cell. In dividing cells, betaIII spectrin and dynactin co-localize to the developing cleavage furrow and mitotic spindle, a novel localization for betaIII spectrin. We hypothesize that the interaction between betaIII spectrin and Arp1 recruits dynein and dynactin to intracellular membranes and provides a direct link between the microtubule motor complex and its membrane-bounded cargo.

Evolutionary conservation between budding yeast and human kinetochores

Accurate chromosome segregation during mitosis requires the correct assembly of kinetochores--complexes of centromeric DNA and proteins that link chromosomes to spindle microtubules. Studies on the kinetochore of the budding yeast Saccharomyces cerevisiae have revealed functionally novel components of the kinetochore and its regulatory complexes, some of which are highly conserved in humans.

Interaction of the betaIV-tubulin isotype with actin stress fibers in cultured rat kidney mesangial cells

Microtubules and actin filaments are two of the major components of the cytoskeleton. There is accumulating evidence for interaction between the two networks. Both the alpha- and beta-subunits of tubulin exist as numerous isotypes, some of which have been highly conserved in evolution. In an effort to better understand the functional significance of tubulin isotypes, we used a double immunofluorescence labeling technique to investigate the interactions between the tubulin beta-isotypes and the actin stress fiber network in cultured rat kidney mesangial cells, smooth-muscle-like cells from the renal glomerulus. Removal of the soluble cytoplasmic and nucleoplasmic proteins by detergent extraction caused the microtubule network to disappear while the stress fiber network was still present. In these extracted cells, the betaI- and betaII-tubulin isotypes were no longer present in the cytoplasm while the betaIV-isotype co-localized with actin stress fibers. Co-localization between betaIV-tubulin and actin stress fibers was also observed when the microtubule network was disrupted by the anti-tubulin drug colchicine and also by microinjection of the betaIV-tubulin antibody. Our results suggest that the betaIV isotype of tubulin may be involved in interactions between microtubules and actin.

Immunolocalization of cytoplasmic dynein and dynactin subunits in cultured macrophages: enrichment on early endocytic organelles

Cytoplasmic dyneins and their cofactor, dynactin, work together to mediate the movement of numerous cargo organelles toward the minus-ends of microtubules. In many cases, there is compelling evidence that dynactin functions in part to attach dyneins to cargo organelles, but this may not always be the case. We have localized three dynactin subunits (Arp1, p62 and p150(Glued)) and two subunits of conventional cytoplasmic dynein (dynein intermediate chain and dynein heavy chain 1) in murine macrophages using immunogold labeling of thawed cryosections. Using stereological techniques, we have quantified the relative distributions of each of these subunits on specific membrane organelles to generate a comprehensive analysis of the distribution of these proteins in a single cell type. Our results show that each of the subunits tested exhibits the same distribution with respect to different membrane organelles, with highest levels present on early endosomes, and lower levels present on later endocytic organelles, the mitochondrial outer membrane, the plasma membrane and vesicles in the Golgi region. An additional pool of punctate dynactin labeling was detected in the cell periphery, in the absence of dynein labeling. Even when examined closely, membrane organelles could not be detected in association with these dynactin-positive sites; however, double labeling with anti-tubulin antibody revealed that at least some of these sites represent the ends of microtubules. The similarities among the labeling profiles with respect to membrane organelles suggest that dynein and dynactin bind to membrane organelles as an obligate unit. In contrast, our results show that dynactin can associate with microtubule ends in the absence of dynein, perhaps providing sites for subsequent organelle and dynein association to form a functional motility complex.

Cytoplasmic dynein is required for poleward chromosome movement during mitosis in Drosophila embryos

The movement of chromosomes during mitosis occurs on a bipolar, microtubule-based protein machine, the mitotic spindle. It has long been proposed that poleward chromosome movements that occur during prometaphase and anaphase A are driven by the microtubule motor cytoplasmic dynein, which binds to kinetochores and transports them toward the minus ends of spindle microtubules. Here we evaluate this hypothesis using time-lapse confocal microscopy to visualize, in real time, kinetochore and chromatid movements in living Drosophila embryos in the presence and absence of specific inhibitors of cytoplasmic dynein. Our results show that dynein inhibitors disrupt the alignment of kinetochores on the metaphase spindle equator and also interfere with kinetochore- and chromatid-to-pole movements during anaphase A. Thus, dynein is essential for poleward chromosome motility throughout mitosis in Drosophila embryos.

Microtubules, membranes and cytokinesis

Proper division of the cell requires coordination between chromosome segregation by the mitotic spindle and cleavage of the cell by the cytokinetic apparatus. Interactions between the mitotic spindle, the contractile ring and the plasma membrane ensure that the cleavage furrow is properly placed between the segregating chromosomes and that new membrane compartments are formed to produce two daughter cells. The microtubule midzone is able to stimulate the cortex of the cell to ensure proper ingression and completion of the cleavage furrow. Specialized microtubule structures are responsible for directing membrane vesicles to the site of cell cleavage, and vesicle fusion is required for the proper completion of cytokinesis.

HZwint-1, a novel human kinetochore component that interacts with HZW10

HZwint-1 (Human ZW10 interacting protein-1) was identified in a yeast two hybrid screen for proteins that interact with HZW10. HZwint-1 cDNA encodes a 43 kDa protein predicted to contain an extended coiled-coil domain. Immunofluorescence studies with sera raised against HZwint-1 protein revealed strong kinetochore staining in nocodazole-arrested chromosome spreads. This signal co-localizes at the kinetochore with HZW10, at a position slightly outside of the central part of the centromere as revealed by staining with a CREST serum. The kinetochore localization of HZwint-1 has been confirmed by following GFP fluorescence in HeLa cells transiently transfected with a plasmid encoding a GFP/HZwint-1 fusion protein. In cycling HeLa cells, HZwint-1 localizes to the kinetochore of prophase HeLa cells prior to HZW10 localization, and remains at the kinetochore until late in anaphase. This localization pattern, combined with the two-hybrid results, suggests that HZwint-1 may play a role in targeting HZW10 to the kinetochore at prometaphase. HZwint-1 was also found to localize to neocentromeres and to the active centromere of dicentric chromosomes. HZwint-1 thus appears to associate with all active centromeres, implying that it plays an important role in correct centromere function.

Relationships between the central spindle and the contractile ring during cytokinesis in animal cells

During late anaphase and telophase, animal cells develop a bundle of antiparallel, interdigitating microtubules between the two daughter nuclei. Recent data indicate that this structure, called the central spindle, plays an essential role during cytokinesis. Studies in Drosophila and on vertebrate cells strongly suggest that the molecular signals for cytokinesis specifically emanate from the central spindle midzone. Moreover, the analysis of Drosophila mutants defective in cytokinesis has revealed a cooperative interaction between the central spindle microtubules and the contractile ring: when either of these structures is perturbed, the proper assembly of the other is disrupted. Based on these results we propose a model for the role of the central spindle during cytokinesis. We suggest that the interaction between central spindle microtubules and cortical actin filaments leads to two early events crucial for cytokinesis: the positioning of the contractile ring, and the stabilization of the plus ends of the interdigitating microtubules that comprise the central spindle. The latter event would provide the cell with a specialized microtubule scaffold that could mediate the translocation of plus-end-directed molecular motors to the cell's equator. Among the cargoes transported by these motors could be proteins involved in the regulation and execution of cytokinesis.

A dynactin subunit with a highly conserved cysteine-rich motif interacts directly with Arp1

Dynactin is a multisubunit complex and a required cofactor for most, or all, of the cellular processes powered by the microtubule-based motor cytoplasmic dynein. Using a dynein affinity column, the previously uncharacterized p62 subunit of dynactin was isolated and microsequenced. Two peptide sequences were used to clone human cDNAs encoding p62. Sequence analysis of the predicted human polypeptide of 53 kDa revealed a highly conserved pattern of eleven cysteine residues, eight of which fit the consensus sequence for a Zn(2+)-binding RING domain. We have characterized p62 as an integral component of 20 S dynactin by biochemical and immunocytochemical methods. Affinity chromatography experiments demonstrate that p62 binds directly to the Arp1 subunit of dynactin. Immunocytochemistry with antibodies to p62 demonstrates that this polypeptide has a punctate cytoplasmic distribution as well as centrosomal distribution typical of dynactin. In transfected cells, overexpression of p62 did not disrupt microtubule organization or the integrity of the Golgi but did cause both cytosolic and nuclear distribution of the protein, suggesting that this polypeptide may be targeted to the nucleus at very high expression levels.

Cytokinesis: an emerging unified theory for eukaryotes?

In animal and fungal cells, cytokinesis involves an actomyosin ring that forms and contracts at the division plane. Important new details have emerged concerning the composition, assembly, and dynamics of these contractile rings. In addition, recent advances suggest that targeted membrane addition is a central feature of cytokinesis in animal cells - as it is in fungi and plants - and the coordination of actomyosin ring function with targeted exocytosis at the cleavage plane is being explored. Important new information has also emerged about the spatial and temporal regulation of cytokinesis, especially in relation to the function of the spindle midzone in animal cells and the control of cytokinesis by GTPase systems.

Dynactin is required for microtubule anchoring at centrosomes

The multiprotein complex, dynactin, is an integral part of the cytoplasmic dynein motor and is required for dynein-based motility in vitro and in vivo. In living cells, perturbation of the dynein-dynactin interaction profoundly blocks mitotic spindle assembly, and inhibition or depletion of dynein or dynactin from meiotic or mitotic cell extracts prevents microtubules from focusing into spindles. In interphase cells, perturbation of the dynein-dynactin complex is correlated with an inhibition of ER-to-Golgi movement and reorganization of the Golgi apparatus and the endosome-lysosome system, but the effects on microtubule organization have not previously been defined. To explore this question, we overexpressed a variety of dynactin subunits in cultured fibroblasts. Subunits implicated in dynein binding have effects on both microtubule organization and centrosome integrity. Microtubules are reorganized into unfocused arrays. The pericentriolar components, gamma tubulin and dynactin, are lost from centrosomes, but pericentrin localization persists. Microtubule nucleation from centrosomes proceeds relatively normally, but microtubules become disorganized soon thereafter. Overexpression of some, but not all, dynactin subunits also affects endomembrane localization. These data indicate that dynein and dynactin play important roles in microtubule organization at centrosomes in fibroblastic cells and provide new insights into dynactin-cargo interactions.