Centractin (ARP1) associates with spectrin revealing a potential mechanism to link dynactin to intracellular organelles

Quantitatively Dissecting Triple Roles of Dynactin in Dynein-Driven Transport of Influenza Virus by Quantum Dot-Based Single-Virus Tracking

After entering host cells by endocytosis, influenza A virus (IAV) is transported along microfilaments and then transported by dynein along microtubules (MTs) to the perinuclear region for genome release. Understanding the mechanisms of dynein-driven transport is significant for a comprehensive understanding of IAV infection. In this work, the roles of dynactin in dynein-driven transport of IAV were quantitatively dissected in situ using quantum dot-based single-virus tracking. It was revealed that dynactin was essential for dynein to transport IAV toward the nucleus. After virus entry, virus-carrying vesicles bound to dynein and dynactin before being delivered to MTs. The attachment of dynein to the vesicles was dependent on dynactin and its subunits, p150Glued and Arp1. Once viruses reached MTs, dynactin-assisted dynein initiates retrograde transport of IAV. Importantly, the retrograde transport of viruses could be initiated at both plus ends (32%) and other regions on MTs (68%). Subsequently, dynactin accompanied and assisted dynein to persistently transport the virus along MTs in the retrograde direction. This study revealed the dynactin-dependent dynein-driven transport process of IAV, enhancing our understanding of IAV infection and providing important insights into the cell's endocytic transport mechanism.

Broad Influence of Mutant Ataxin-3 on the Proteome of the Adult Brain, Young Neurons, and Axons Reveals Central Molecular Processes and Biomarkers in SCA3/MJD Using Knock-In Mouse Model

Spinocerebellar ataxia type 3 (SCA3/MJD) is caused by CAG expansion mutation resulting in a long polyQ domain in mutant ataxin-3. The mutant protein is a special type of protease, deubiquitinase, which may indicate its prominent impact on the regulation of cellular proteins levels and activity. Yet, the global model picture of SCA3 disease progression on the protein level, molecular pathways in the brain, and neurons, is largely unknown. Here, we investigated the molecular SCA3 mechanism using an interdisciplinary research paradigm combining behavioral and molecular aspects of SCA3 in the knock-in ki91 model. We used the behavior, brain magnetic resonance imaging (MRI) and brain tissue examination to correlate the disease stages with brain proteomics, precise axonal proteomics, neuronal energy recordings, and labeling of vesicles. We have demonstrated that altered metabolic and mitochondrial proteins in the brain and the lack of weight gain in Ki91 SCA3/MJD mice is reflected by the failure of energy metabolism recorded in neonatal SCA3 cerebellar neurons. We have determined that further, during disease progression, proteins responsible for metabolism, cytoskeletal architecture, vesicular, and axonal transport are disturbed, revealing axons as one of the essential cell compartments in SCA3 pathogenesis. Therefore we focus on SCA3 pathogenesis in axonal and somatodendritic compartments revealing highly increased axonal localization of protein synthesis machinery, including ribosomes, translation factors, and RNA binding proteins, while the level of proteins responsible for cellular transport and mitochondria was decreased. We demonstrate the accumulation of axonal vesicles in neonatal SCA3 cerebellar neurons and increased phosphorylation of SMI-312 positive adult cerebellar axons, which indicate axonal dysfunction in SCA3. In summary, the SCA3 disease mechanism is based on the broad influence of mutant ataxin-3 on the neuronal proteome. Processes central in our SCA3 model include disturbed localization of proteins between axonal and somatodendritic compartment, early neuronal energy deficit, altered neuronal cytoskeletal structure, an overabundance of various components of protein synthesis machinery in axons.

Age-dependent ataxia and neurodegeneration caused by an αII spectrin mutation with impaired regulation of its calpain sensitivity

The neuronal membrane-associated periodic spectrin skeleton (MPS) contributes to neuronal development, remodeling, and organization. Post-translational modifications impinge on spectrin, the major component of the MPS, but their role remains poorly understood. One modification targeting spectrin is cleavage by calpains, a family of calcium-activated proteases. Spectrin cleavage is regulated by activated calpain, but also by the calcium-dependent binding of calmodulin (CaM) to spectrin. The physiologic significance of this balance between calpain activation and substrate-level regulation of spectrin cleavage is unknown. We report a strain of C57BL/6J mice harboring a single αII spectrin point mutation (Sptan1 c.3293G > A:p.R1098Q) with reduced CaM affinity and intrinsically enhanced sensitivity to calpain proteolysis. Homozygotes are embryonic lethal. Newborn heterozygotes of either gender appear normal, but soon develop a progressive ataxia characterized biochemically by accelerated calpain-mediated spectrin cleavage and morphologically by disruption of axonal and dendritic integrity and global neurodegeneration. Molecular modeling predicts unconstrained exposure of the mutant spectrin's calpain-cleavage site. These results reveal the critical importance of substrate-level regulation of spectrin cleavage for the maintenance of neuronal integrity. Given that excessive activation of calpain proteases is a common feature of neurodegenerative disease and traumatic encephalopathy, we propose that damage to the spectrin MPS may contribute to the neuropathology of many disorders.

Role of dynein-dynactin complex, kinesins, motor adaptors, and their phosphorylation in dendritogenesis

One of the characteristic features of different classes of neurons that is vital for their proper functioning within neuronal networks is the shape of their dendritic arbors. To properly develop dendritic trees, neurons need to accurately control the intracellular transport of various cellular cargo (e.g., mRNA, proteins, and organelles). Microtubules and motor proteins (e.g., dynein and kinesins) that move along microtubule tracks play an essential role in cargo sorting and transport to the most distal ends of neurons. Equally important are motor adaptors, which may affect motor activity and specify cargo that is transported by the motor. Such transport undergoes very dynamic fine-tuning in response to changes in the extracellular environment and synaptic transmission. Such regulation is achieved by the phosphorylation of motors, motor adaptors, and cargo, among other mechanisms. This review focuses on the contribution of the dynein-dynactin complex, kinesins, their adaptors, and the phosphorylation of these proteins in the formation of dendritic trees by maturing neurons. We primarily review the effects of the motor activity of these proteins in dendrites on dendritogenesis. We also discuss less anticipated mechanisms that contribute to dendrite growth, such as dynein-driven axonal transport and non-motor functions of kinesins.

βII-spectrin promotes mouse brain connectivity through stabilizing axonal plasma membranes and enabling axonal organelle transport

βII-spectrin is the generally expressed member of the β-spectrin family of elongated polypeptides that form micrometer-scale networks associated with plasma membranes. We addressed in vivo functions of βII-spectrin in neurons by knockout of βII-spectrin in mouse neural progenitors. βII-spectrin deficiency caused severe defects in long-range axonal connectivity and axonal degeneration. βII-spectrin-null neurons exhibited reduced axon growth, loss of actin-spectrin-based periodic membrane skeleton, and impaired bidirectional axonal transport of synaptic cargo. We found that βII-spectrin associates with KIF3A, KIF5B, KIF1A, and dynactin, implicating spectrin in the coupling of motors and synaptic cargo. βII-spectrin required phosphoinositide lipid binding to promote axonal transport and restore axon growth. Knockout of ankyrin-B (AnkB), a βII-spectrin partner, primarily impaired retrograde organelle transport, while double knockout of βII-spectrin and AnkB nearly eliminated transport. Thus, βII-spectrin promotes both axon growth and axon stability through establishing the actin-spectrin-based membrane-associated periodic skeleton as well as enabling axonal transport of synaptic cargo.

Spectrin regulates cell contractility through production and maintenance of actin bundles in the Caenorhabditis elegans spermatheca

Disruption to the contractility of cells, including smooth muscle cells of the cardiovascular system and myoepithelial cells of the glandular epithelium, contributes to the pathophysiology of contractile tissue diseases, including asthma, hypertension, and primary Sjögren's syndrome. Cell contractility is determined by myosin activity and actomyosin network organization and is mediated by hundreds of protein-protein interactions, many directly involving actin. Here we use a candidate RNA interference screen of more than 100 Caenorhabditis elegans genes with predicted actin-binding and regulatory domains to identify genes that contribute to the contractility of the somatic gonad. We identify the spectrin cytoskeleton composed of SPC-1/α-spectrin, UNC-70/β-spectrin, and SMA-1/β heavy-spectrin as required for contractility and actin organization in the myoepithelial cells of the C. elegans spermatheca. We use imaging of fixed and live animals as well as tissue- and developmental-stage-specific disruption of the spectrin cytoskeleton to show that spectrin regulates the production of prominent central actin bundles and is required for maintenance of central actin bundles throughout successive rounds of stretch and contraction. We conclude that the spectrin cytoskeleton contributes to spermathecal contractility by promoting maintenance of the robust actomyosin bundles that drive contraction.

The cytoplasmic dynein transport machinery and its many cargoes

Cytoplasmic dynein 1 is an important microtubule-based motor in many eukaryotic cells. Dynein has critical roles both in interphase and during cell division. Here, we focus on interphase cargoes of dynein, which include membrane-bound organelles, RNAs, protein complexes and viruses. A central challenge in the field is to understand how a single motor can transport such a diverse array of cargoes and how this process is regulated. The molecular basis by which each cargo is linked to dynein and its cofactor dynactin has started to emerge. Of particular importance for this process is a set of coiled-coil proteins - activating adaptors - that both recruit dynein-dynactin to their cargoes and activate dynein motility.

β-III-spectrin spinocerebellar ataxia type 5 mutation reveals a dominant cytoskeletal mechanism that underlies dendritic arborization

A spinocerebellar ataxia type 5 (SCA5) L253P mutation in the actin-binding domain (ABD) of β-III-spectrin causes high-affinity actin binding and decreased thermal stability in vitro. Here we show in mammalian cells, at physiological temperature, that the mutant ABD retains high-affinity actin binding. Significantly, we provide evidence that the mutation alters the mobility and recruitment of β-III-spectrin in mammalian cells, pointing to a potential disease mechanism. To explore this mechanism, we developed a Drosophila SCA5 model in which an equivalent mutant Drosophila β-spectrin is expressed in neurons that extend complex dendritic arbors, such as Purkinje cells, targeted in SCA5 pathogenesis. The mutation causes a proximal shift in arborization coincident with decreased β-spectrin localization in distal dendrites. We show that SCA5 β-spectrin dominantly mislocalizes α-spectrin and ankyrin-2, components of the endogenous spectrin cytoskeleton. Our data suggest that high-affinity actin binding by SCA5 β-spectrin interferes with spectrin-actin cytoskeleton dynamics, leading to a loss of a cytoskeletal mechanism in distal dendrites required for dendrite stabilization and arbor outgrowth.

Dynactin has two antagonistic regulatory domains and exerts opposing effects on dynein motility

Dynactin is a dynein-regulating protein that increases the processivity of dynein movement on microtubules. Recent studies have shown that a tripartite complex of dynein–dynactin–Bicaudal D2 is essential for highly processive movement. To elucidate the regulation of dynein motility by dynactin, we focused on two isoforms (A and B) of dynactin 1 (DCTN1), the largest subunit of dynactin that contains both microtubule- and dynein-binding domains. The only difference between the primary structures of the two isoforms is that DCTN1B lacks the K-rich domain, a cluster of basic residues. We measured dynein motility by single molecule observation of recombinant dynein and dynactin. Whereas the tripartite complex containing DCTN1A exhibited highly processive movement, the complex containing DCTN1B dissociated from microtubules with no apparent processive movement. This inhibitory effect of DCTN1B was caused by reductions of the microtubule-binding affinities of both dynein and dynactin, which was attributed to the coiled-coil 1 domain of DCTN1. In DCTN1A, the K-rich domain antagonized these inhibitory effects. Therefore, dynactin has two antagonistic domains and promotes or suppresses dynein motility to accomplish correct localization and functions of dynein within a cell.

Diverse mitotic functions of the cytoskeletal cross-linking protein Shortstop suggest a role in Dynein/Dynactin activity

Shortstop (Shot), an actin–microtubule cross-linking protein, interacts with the Dynactin component Arp-1 to control mitotic spindle assembly and positioning in Drosophila. Shot is important for proper chromosome congression and segregation. Loss of Shot in epithelial tissue leads to significant apoptosis, which when blocked leads to epithelial–mesenchymal transition-like changes.

Proper assembly and orientation of the bipolar mitotic spindle is critical to the fidelity of cell division. Mitotic precision fundamentally contributes to cell fate specification, tissue development and homeostasis, and chromosome distribution within daughter cells. Defects in these events are thought to contribute to several human diseases. The underlying mechanisms that function in spindle morphogenesis and positioning remain incompletely defined, however. Here we describe diverse roles for the actin-microtubule cross-linker Shortstop (Shot) in mitotic spindle function in Drosophila. Shot localizes to mitotic spindle poles, and its knockdown results in an unfocused spindle pole morphology and a disruption of proper spindle orientation. Loss of Shot also leads to chromosome congression defects, cell cycle progression delay, and defective chromosome segregation during anaphase. These mitotic errors trigger apoptosis in Drosophila epithelial tissue, and blocking this apoptotic response results in a marked induction of the epithelial–mesenchymal transition marker MMP-1. The actin-binding domain of Shot directly interacts with Actin-related protein-1 (Arp-1), a key component of the Dynein/Dynactin complex. Knockdown of Arp-1 phenocopies Shot loss universally, whereas chemical disruption of F-actin does so selectively. Our work highlights novel roles for Shot in mitosis and suggests a mechanism involving Dynein/Dynactin activation.

A PIK3C3-ankyrin-B-dynactin pathway promotes axonal growth and multiorganelle transport

Interactions between ankyrin-B and both dynactin and phosphatidylinositol 3-phosphate lipids promote fast axonal transport of organelles.

Axon growth requires long-range transport of organelles, but how these cargoes recruit their motors and how their traffic is regulated are not fully resolved. In this paper, we identify a new pathway based on the class III PI3-kinase (PIK3C3), ankyrin-B (AnkB), and dynactin, which promotes fast axonal transport of synaptic vesicles, mitochondria, endosomes, and lysosomes. We show that dynactin associates with cargo through AnkB interactions with both the dynactin subunit p62 and phosphatidylinositol 3-phosphate (PtdIns(3)P) lipids generated by PIK3C3. AnkB knockout resulted in shortened axon tracts and marked reduction in membrane association of dynactin and dynein, whereas it did not affect the organization of spectrin–actin axonal rings imaged by 3D-STORM. Loss of AnkB or of its linkages to either p62 or PtdIns(3)P or loss of PIK3C3 all impaired organelle transport and particularly retrograde transport in hippocampal neurons. Our results establish new functional relationships between PIK3C3, dynactin, and AnkB that together promote axonal transport of organelles and are required for normal axon length.

The structure of the dynactin complex and its interaction with dynein

Dynactin is an essential cofactor for the microtubule motor cytoplasmic dynein-1. We report the structure of the 23-subunit dynactin complex by cryo-electron microscopy to 4.0 angstroms. Our reconstruction reveals how dynactin is built around a filament containing eight copies of the actin-related protein Arp1 and one of β-actin. The filament is capped at each end by distinct protein complexes, and its length is defined by elongated peptides that emerge from the α-helical shoulder domain. A further 8.2 angstrom structure of the complex between dynein, dynactin, and the motility-inducing cargo adaptor Bicaudal-D2 shows how the translational symmetry of the dynein tail matches that of the dynactin filament. The Bicaudal-D2 coiled coil runs between dynein and dynactin to stabilize the mutually dependent interactions between all three components.

Autoregulatory mechanism for dynactin control of processive and diffusive dynein transport

Dynactin is the longest known cytoplasmic dynein regulator, with roles in dynein recruitment to subcellular cargo and in stimulating processive dynein movement. The latter function was thought to involve the N-terminal microtubule binding region of the major dynactin polypeptide p150^Glued, though recent results disputed this. To understand how dynactin regulates dynein we generated recombinant fragments of the N-terminal half of p150^Glued. We find that the dynein-binding coiled-coil α-helical domain CC1B is sufficient to stimulate dynein processivity, which it accomplishes by increasing average dynein step size and forward step frequency, while decreasing lateral stepping and microtubule detachment. In contrast, the immediate upstream coiled-coil domain, CC1A, activates a novel diffusive dynein state. CC1A interacts physically with CC1B and interferes with its effect on dynein processivity. We also identify a role for the N-terminal portion of p150^Glued in coordinating these activities. Our results reveal an unexpected form of long-range allosteric control of dynein motor function by internal p150^Glued sequences, and evidence for p150^Glued auto regulation.

Axonal transport: cargo-specific mechanisms of motility and regulation

Axonal transport is essential for neuronal function, and many neurodevelopmental and neurodegenerative diseases result from mutations in the axonal transport machinery. Anterograde transport supplies distal axons with newly synthesized proteins and lipids, including synaptic components required to maintain presynaptic activity. Retrograde transport is required to maintain homeostasis by removing aging proteins and organelles from the distal axon for degradation and recycling of components. Retrograde axonal transport also plays a major role in neurotrophic and injury response signaling. This review provides an overview of axonal transport pathways and discusses their role in neuronal function.

PKA-dependent dynein switching from lysosomes to adenovirus: a novel form of host-virus competition

PKA-mediated phosphorylation of a specific residue in the dynein light intermediate chain 1 releases the motor protein from lysosomes and late endosomes while activating its recruitment to adenovirus capsids.

Cytoplasmic dynein is responsible for transport of several viruses to the nucleus. Adenovirus recruits dynein directly. Transport depends on virus-induced activation of protein kinase A (PKA) and other cellular protein kinases, whose roles in infection are poorly understood. We find that PKA phosphorylates cytoplasmic dynein at a novel site in light intermediate chain 1 (LIC1) that is essential for dynein binding to the hexon capsid subunit and for virus motility. Surprisingly, the same LIC1 modification induces a slow, but specific, dispersal of lysosomes (lyso)/late endosomes (LEs) that is mediated by inhibition of a newly identified LIC1 interaction with the RILP (Rab7-interacting lysosomal protein). These results identify an organelle-specific dynein regulatory modification that adenovirus uses for its own transport. PKA-mediated LIC1 phosphorylation causes only partial lyso/LE dispersal, suggesting a role for additional, parallel mechanisms for dynein recruitment to lyso/LEs. This arrangement provides a novel means to fine tune transport of these organelles in response to infection as well as to developmental and physiological cues.

Axonal transport and neurodegenerative disease: vesicle-motor complex formation and their regulation

The process of axonal transport serves to move components over very long distances on microtubule tracks in order to maintain neuronal viability. Molecular motors - kinesin and dynein - are essential for the movement of neuronal cargoes along these tracks; defects in this pathway have been implicated in the initiation or progression of some neurodegenerative diseases, suggesting that this process may be a key contributor in neuronal dysfunction. Recent work has led to the identification of some of the motor-cargo complexes, adaptor proteins, and their regulatory elements in the context of disease proteins. In this review, we focus on the assembly of the amyloid precursor protein, huntingtin, mitochondria, and the RNA-motor complexes and discuss how these may be regulated during long-distance transport in the context of neurodegenerative disease. As knowledge of these motor-cargo complexes and their involvement in axonal transport expands, insight into how defects in this pathway contribute to the development of neurodegenerative diseases becomes evident. Therefore, a better understanding of how this pathway normally functions has important implications for early diagnosis and treatment of diseases before the onset of disease pathology or behavior.

Fodrin in centrosomes: implication of a role of fodrin in the transport of gamma-tubulin complex in brain

Gamma-tubulin is the major protein involved in the nucleation of microtubules from centrosomes in eukaryotic cells. It is present in both cytoplasm and centrosome. However, before centrosome maturation prior to mitosis, gamma-tubulin concentration increases dramatically in the centrosome, the mechanism of which is not known. Earlier it was reported that cytoplasmic gamma-tubulin complex isolated from goat brain contains non-erythroid spectrin/fodrin. The major role of erythroid spectrin is to help in the membrane organisation and integrity. However, fodrin or non-erythroid spectrin has a distinct pattern of localisation in brain cells and evidently some special functions over its erythroid counterpart. In this study, we show that fodrin and γ-tubulin are present together in both the cytoplasm and centrosomes in all brain cells except differentiated neurons and astrocytes. Immunoprecipitation studies in purified centrosomes from brain tissue and brain cell lines confirm that fodrin and γ-tubulin interact with each other in centrosomes. Fodrin dissociates from centrosome just after the onset of mitosis, when the concentration of γ-tubulin attains a maximum at centrosomes. Further it is observed that the interaction between fodrin and γ-tubulin in the centrosome is dependent on actin as depolymerisation of microfilaments stops fodrin localization. Image analysis revealed that γ-tubulin concentration also decreased drastically in the centrosome under this condition. This indicates towards a role of fodrin as a regulatory transporter of γ-tubulin to the centrosomes for normal progression of mitosis.

Ordered recruitment of dynactin to the microtubule plus-end is required for efficient initiation of retrograde axonal transport

Long-range retrograde axonal transport in neurons is driven exclusively by the microtubule motor cytoplasmic dynein. The efficient initiation of dynein-mediated transport from the distal axon is critical for normal neuronal function, and neurodegenerative disease-associated mutations have been shown to specifically disrupt this process. Here, we examine the role of dynamic microtubules and microtubule plus-end binding proteins (+TIPs) in the initiation of dynein-mediated retrograde axonal transport using live-cell imaging of cargo motility in primary mouse dorsal root ganglion neurons. We show that end-binding (EB)-positive dynamic microtubules are enriched in the distal axon. The +TIPs EB1, EB3, and cytoplasmic linker protein-170 (CLIP-170) interact with these dynamic microtubules, recruiting the dynein activator dynactin in an ordered pathway, leading to the initiation of retrograde transport by the motor dynein. Once transport has initiated, however, neither the EBs nor CLIP-170 are required to maintain transport flux along the mid-axon. In contrast, the +TIP Lis1 activates transport through a distinct mechanism and is required to maintain processive organelle transport along both the distal and mid-axon. Further, we show that the EB/CLIP-170/dynactin-dependent mechanism is required for the efficient initiation of transport from the distal axon for multiple distinct cargos, including mitochondria, Rab5-positive early endosomes, late endosomes/lysosomes, and TrkA-, TrkB-, and APP-positive organelles. Our observations indicate that there is an essential role for +TIPs in the regulation of retrograde transport initiation in the neuron.

Differential gene expression in rainbow trout (Oncorhynchus mykiss) liver and ovary after exposure to zearalenone

Zearalenone (ZEA) is a mycotoxin of worldwide occurrence, and it has been shown to produce numerous adverse effects in both laboratory and domestic animals. However, regardless of recent achievements, the molecular mechanisms underlying ZEA toxicity remain elusive, and little is known about transcriptome changes of fish cells in response to ZEA occurrence. In the present study, differential display PCR was used to generate a unique cDNA fingerprint of differentially expressed transcripts in the liver and ovary of juvenile rainbow trout after either 24, 72, or 168 h of intraperitoneal exposure to ZEA (10 mg/kg of body mass). From a total of 59 isolated cDNA bands (ESTs), 5 could be confirmed with Real-Time qPCR and their nucleotide sequences were identified as mRNAs of: acty (β-centractin), the cytoskeleton structural element; bccip, responsible for DNA repair and cell cycle control; enoa (α-enolase), encoding enzyme of the glycolysis process; proc (protein C), that takes part in the blood coagulation process; and frih, encoding the heavy chain of ferritin, the protein complex important for iron storage. Further qPCR analysis of the confirmed ESTs expression profiles revealed significant mRNA level alterations in both tissues of exposed fish during the 168 h study. The results revealed a complex network of genes associated with different biological processes that may be engaged in the cellular response to ZEA exposure, i.e. blood coagulation or iron-storage processes.

BICD2, dynactin, and LIS1 cooperate in regulating dynein recruitment to cellular structures

This study dissects the recruitment of dynein and dynactin to cargo by a conserved motor adaptor BICD2. It is shown that dynein, dynactin, and BICD2 form a triple complex in vitro and in vivo. Investigation of the properties of this complex by direct visualization of dynein in live cells shows that BICD2-induced dynein transport requires LIS1.

Cytoplasmic dynein is the major microtubule minus-end–directed cellular motor. Most dynein activities require dynactin, but the mechanisms regulating cargo-dependent dynein–dynactin interaction are poorly understood. In this study, we focus on dynein–dynactin recruitment to cargo by the conserved motor adaptor Bicaudal D2 (BICD2). We show that dynein and dynactin depend on each other for BICD2-mediated targeting to cargo and that BICD2 N-terminus (BICD2-N) strongly promotes stable interaction between dynein and dynactin both in vitro and in vivo. Direct visualization of dynein in live cells indicates that by itself the triple BICD2-N–dynein–dynactin complex is unable to interact with either cargo or microtubules. However, tethering of BICD2-N to different membranes promotes their microtubule minus-end–directed motility. We further show that LIS1 is required for dynein-mediated transport induced by membrane tethering of BICD2-N and that LIS1 contributes to dynein accumulation at microtubule plus ends and BICD2-positive cellular structures. Our results demonstrate that dynein recruitment to cargo requires concerted action of multiple dynein cofactors.

Dynactin's pointed-end complex is a cargo-targeting module

Dynactin serves as an adaptor that allows the dynein motor to bind cargoes, but how dynactin associates with its diverse complement of subcellular binding partners remains mysterious. We show that the “pointed-end complex” of dynactin is a bipartite structural domain that stabilizes dynactin and supports its binding to different subcellular structures.

Dynactin is an essential part of the cytoplasmic dynein motor that enhances motor processivity and serves as an adaptor that allows dynein to bind cargoes. Much is known about dynactin's interaction with dynein and microtubules, but how it associates with its diverse complement of subcellular binding partners remains mysterious. It has been suggested that cargo specification involves a group of subunits referred to as the “pointed-end complex.” We used chemical cross-linking, RNA interference, and protein overexpression to characterize interactions within the pointed-end complex and explore how it contributes to dynactin's interactions with endomembranes. The Arp11 subunit, which caps one end of dynactin's Arp1 filament, and p62, which binds Arp11 and Arp1, are necessary for dynactin stability. These subunits also allow dynactin to bind the nuclear envelope prior to mitosis. p27 and p25, by contrast, are peripheral components that can be removed without any obvious impact on dynactin integrity. Dynactin lacking these subunits shows reduced membrane binding. Depletion of p27 and p25 results in impaired early and recycling endosome movement, but late endosome movement is unaffected, and mitotic spindles appear normal. We conclude that the pointed-end complex is a bipartite structural domain that stabilizes dynactin and supports its binding to different subcellular structures.

Evolution of the eukaryotic dynactin complex, the activator of cytoplasmic dynein

Background

Dynactin is a large multisubunit protein complex that enhances the processivity of cytoplasmic dynein and acts as an adapter between dynein and the cargo. It is composed of eleven different polypeptides of which eight are unique to this complex, namely dynactin1 (p150^Glued), dynactin2 (p50 or dynamitin), dynactin3 (p24), dynactin4 (p62), dynactin5 (p25), dynactin6 (p27), and the actin-related proteins Arp1 and Arp10 (Arp11).

Results

To reveal the evolution of dynactin across the eukaryotic tree the presence or absence of all dynactin subunits was determined in most of the available eukaryotic genome assemblies. Altogether, 3061 dynactin sequences from 478 organisms have been annotated. Phylogenetic trees of the various subunit sequences were used to reveal sub-family relationships and to reconstruct gene duplication events. Especially in the metazoan lineage, several of the dynactin subunits were duplicated independently in different branches. The largest subunit repertoire is found in vertebrates. Dynactin diversity in vertebrates is further increased by alternative splicing of several subunits. The most prominent example is the dynactin1 gene, which may code for up to 36 different isoforms due to three different transcription start sites and four exons that are spliced as differentially included exons.

Conclusions

The dynactin complex is a very ancient complex that most likely included all subunits in the last common ancestor of extant eukaryotes. The absence of dynactin in certain species coincides with that of the cytoplasmic dynein heavy chain: Organisms that do not encode cytoplasmic dynein like plants and diplomonads also do not encode the unique dynactin subunits. The conserved core of dynactin consists of dynactin1, dynactin2, dynactin4, dynactin5, Arp1, and the heterodimeric actin capping protein. The evolution of the remaining subunits dynactin3, dynactin6, and Arp10 is characterized by many branch- and species-specific gene loss events.

Analyses of dynein heavy chain mutations reveal complex interactions between dynein motor domains and cellular dynein functions

Cytoplasmic dynein transports cargoes for a variety of crucial cellular functions. However, since dynein is essential in most eukaryotic organisms, the in-depth study of the cellular function of dynein via genetic analysis of dynein mutations has not been practical. Here, we identify and characterize 34 different dynein heavy chain mutations using a genetic screen of the ascomycete fungus Neurospora crassa, in which dynein is nonessential. Interestingly, our studies show that these mutations segregate into five different classes based on the in vivo localization of the mutated dynein motors. Furthermore, we have determined that the different classes of dynein mutations alter vesicle trafficking, microtubule organization, and nuclear distribution in distinct ways and require dynactin to different extents. In addition, biochemical analyses of dynein from one mutant strain show a strong correlation between its in vitro biochemical properties and the aberrant intracellular function of that altered dynein. When the mutations were mapped to the published dynein crystal structure, we found that the three-dimensional structural locations of the heavy chain mutations were linked to particular classes of altered dynein functions observed in cells. Together, our data indicate that the five classes of dynein mutations represent the entrapment of dynein at five separate points in the dynein mechanochemical and transport cycles. We have developed N. crassa as a model system where we can dissect the complexities of dynein structure, function, and interaction with other proteins with genetic, biochemical, and cell biological studies.

Cytoplasmic dynein

The organization and function of eukaryotic cells rely on the action of many different molecular motor proteins. Cytoplasmic dynein drives the movement of a wide range of cargoes towards the minus ends of microtubules, and these events are needed, not just at the single-cell level, but are vital for correct development. In the present paper, I review recent progress on understanding dynein's mechanochemistry, how it is regulated and how it binds to such a plethora of cargoes. The importance of a number of accessory factors in these processes is discussed.

Facilitated intracellular transport of TrkA by an interaction with nerve growth factor

Intracellular transport of neurotrophin receptors together with neurotrophins is one of the key events of neurotrophin signaling for the growth and the survival of neurons. However, the involvement of neurotrophin signaling in the regulation of intracellular transport of neurotrophin receptors has been remained unclear. We visualized the behavior of TrkA, a receptor of nerve growth factor (NGF), by labeling with GFP in PC12 cells. We found remarkable changes of the behavior of TrkA-GFP upon the application of NGF. Before the application, only ~37% of the fluorescent dots of TrkA showed translocations along neurites of PC12 cells. After the application, number of the dots showing the directional movement increased to ~65%. The averaged velocities of the directional movement of TrkA-GFP dots became higher after the application of NGF. We tested the idea whether NGF binding accelerated the translocations of TrkA by simultaneously observing TrkA-GFP and fluorescently labeled NGF, Cy3.5-NGF. The velocity of TrkA-GFP dots associated with Cy3.5-NGF was remarkably higher than that of TrkA-GFP dots without Cy3.5-NGF. On the basis of these observations, we hypothesize that there is a signaling mechanism within a single vesicle that facilitates the intracellular transport of each vesicle containing the activated TrkA.

The p25 subunit of the dynactin complex is required for dynein-early endosome interaction

The p25 subunit of the dynactin complex is required for the interaction between cytoplasmic dynein and early endosomes but is not required for dynein-mediated nuclear distribution.

Cytoplasmic dynein transports various cellular cargoes including early endosomes, but how dynein is linked to early endosomes is unclear. We find that the Aspergillus nidulans orthologue of the p25 subunit of dynactin is critical for dynein-mediated early endosome movement but not for dynein-mediated nuclear distribution. In the absence of NUDF/LIS1, p25 deletion abolished the localization of dynein–dynactin to the hyphal tip where early endosomes abnormally accumulate but did not prevent dynein–dynactin localization to microtubule plus ends. Within the dynactin complex, p25 locates at the pointed end of the Arp1 filament with Arp11 and p62, and our data suggest that Arp11 but not p62 is important for p25–dynactin association. Loss of either Arp1 or p25 significantly weakened the physical interaction between dynein and early endosomes, although loss of p25 did not apparently affect the integrity of the Arp1 filament. These results indicate that p25, in conjunction with the rest of the dynactin complex, is important for dynein–early endosome interaction.

Mitotic spindle: focus on the function of huntingtin

Mitotic spindle assembly and orientation are tightly regulated to allow the appropriate segregation of genetic material and cell fate determinants during symmetric and asymmetric divisions. Microtubules and many proteins including the dynein/dynactin complex and the large nuclear mitotic apparatus NuMA protein, are fundamental players in these mechanisms. A recent study reported that huntingtin regulates spindle orientation by ensuring the proper localization of the p150(Glued) subunit of dynactin, dynein and NuMA. This function of huntingtin is conserved in Drosophila. Among other events, spindle orientation influences the fate of daughter cells. In agreement with this, huntingtin changes the direction of division of mouse cortical progenitors and promotes neurogenesis in the neocortex. We will also discuss the involvement of mitotic spindle components in neuronal disorders.

Ankyrin-B interactions with spectrin and dynactin-4 are required for dystrophin-based protection of skeletal muscle from exercise injury

Costameres are cellular sites of mechanotransduction in heart and skeletal muscle where dystrophin and its membrane-spanning partner dystroglycan distribute intracellular contractile forces into the surrounding extracellular matrix. Resolution of a functional costamere interactome is still limited but likely to be critical for understanding forms of muscular dystrophy and cardiomyopathy. Dystrophin binds a set of membrane-associated proteins (the dystrophin-glycoprotein complex) as well as γ-actin and microtubules and also is required to align sarcolemmal microtubules with costameres. Ankyrin-B binds to dystrophin, dynactin-4, and microtubules and is required for sarcolemmal association of these proteins as well as dystroglycan. We report here that ankyrin-B interactions with β2 spectrin and dynactin-4 are required for localization of dystrophin, dystroglycan, and microtubules at costameres as well as protection of muscle from exercise-induced injury. Knockdown of dynactin-4 in adult mouse skeletal muscle phenocopied depletion of ankyrin-B and resulted in loss of sarcolemmal dystrophin, dystroglycan, and microtubules. Moreover, mutations of ankyrin-B and of dynactin-4 that selectively impaired binary interactions between these proteins resulted in loss of their costamere-localizing activity and increased muscle fiber fragility as a result of loss of costamere-associated dystrophin and dystroglycan. In addition, costamere-association of dynactin-4 did not require dystrophin but did depend on β2 spectrin and ankyrin-B, whereas costamere association of ankyrin-B required β2 spectrin. Together, these results are consistent with a functional hierarchy beginning with β2 spectrin recruitment of ankyrin-B to costameres. Ankyrin-B then interacts with dynactin-4 and dystrophin, whereas dynactin-4 collaborates with dystrophin in coordinating costamere-aligned microtubules.

Cytoplasmic gamma-tubulin complex from brain contains nonerythroid spectrin

The newer member of the tubulin superfamily, gamma-tubulin, is known to mediate microtubule nucleation from the centrosome of eukaryotic cells with the aid of some other proteins. The major amount of gamma-tubulin is believed to be located in the centrosome before the onset of mitotic division. However, a considerable amount has been found in the cytoplasm in the form of a complex whose function is not well known. Microtubules are most abundant in brain tissues and brain microtubules have been extensively used in many in vitro studies. Thus, it is relevant to use brain tissue to characterize cytoplasmic gamma-tubulin complex. Here we show that cytoplasmic gamma-tubulin in brain tissues exists as a ring complex as in other tissues. Interestingly, along with the common members of the gamma-TuRC reported from several tissues and species, the purified brain cytoplasmic complex contains some high molecular weight proteins including alpha and beta nonerythroid spectrin which are not found in other tissues. Immunohistochemical studies of brain tissue sections also show the co-localization of gamma-tubulin and spectrin. The possible implications have been discussed.

Inhibition of protein 4.1 R and NuMA interaction by mutagenization of their binding-sites abrogates nuclear localization of 4.1 R

Protein 4.1R(4.1R) is a multifunctional structural protein recently implicated in nuclear assembly and cell division. We earlier demonstrated that 4.1R forms a multiprotein complex with mitotic spindle and spindle pole organizing proteins, such as NuMA, dynein, and dynactin, by binding to residues 1788-1810 of NuMA through amino acids encoded by exons 20 and 21 in 24 kD domain. Employing random-and site-directed mutagenesis combined with glycine- and alanine-scanning, we have identified amino acids of 4.1 R and NuMA that sustain their interaction, and have analyzed the effect of mutating the binding sites on their intracellular colocalization. We found that V762, V765, and V767 of 4.1 R, and 11800, 11801,11803, Tl 804, and M1805 of NuMA are necessary for their interaction. GST-fusion peptides of the 4.1R24 kD domain bound to residues 1785-2115 of NuMA in in vitro binding assays, but the binding was inhibited by alanine substitutions of V762, V765, and V767 of 4.1 R, or residues 1800-1805 of NuMA. Additionally, expression of variants of 4.1 R or NuMA that inhibit their in vitro binding also abrogated nuclear localization of 4.1 Rand colocalization with NuMA. Our findings suggest a crucial role of 4.1 R/NuMA interaction in localization and function of 4.1 R in the nucleus.

Regulators of the cytoplasmic dynein motor

Eukaryotic cells use cytoskeletal motor proteins to transport many different intracellular cargos. Numerous kinesins and myosins have evolved to cope with the various transport needs that have arisen during eukaryotic evolution. Surprisingly, a single cytoplasmic dynein (a minus end-directed microtubule motor) carries out similarly diverse transport activities as the many different types of kinesin. How is dynein coupled to its wide range of cargos and how is it spatially and temporally regulated? The answer could lie in the several multifunctional adaptors, including dynactin, lissencephaly 1, nuclear distribution protein E (NUDE) and NUDE-like, Bicaudal D, Rod-ZW10-Zwilch and Spindly, that regulate dynein function and localization.

SUN1/2 and Syne/Nesprin-1/2 complexes connect centrosome to the nucleus during neurogenesis and neuronal migration in mice

Nuclear movement is critical during neurogenesis and neuronal migration, which are fundamental for mammalian brain development. Although dynein, Lis1, and other cytoplasmic proteins are known for their roles in connecting microtubules to the nucleus during interkinetic nuclear migration (INM) and nucleokinesis, the factors connecting dynein/Lis1 to the nuclear envelope (NE) remain to be determined. We report here that the SUN-domain proteins SUN1 and SUN2 and the KASH-domain proteins Syne-1/Nesprin-1 and Syne-2/Nesprin-2 play critical roles in neurogenesis and neuronal migration in mice. We show that SUN1 and SUN2 redundantly form complexes with Syne-2 to mediate the centrosome-nucleus coupling during both INM and radial neuronal migration in the cerebral cortex. Syne-2 is connected to the centrosome through interactions with both dynein/dynactin and kinesin complexes. Syne-2 mutants also display severe defects in learning and memory. These results fill an important gap in our understanding of the mechanism of nuclear movement during brain development.

Membrane domains based on ankyrin and spectrin associated with cell-cell interactions

Nodes of Ranvier and axon initial segments of myelinated nerves, sites of cell-cell contact in early embryos and epithelial cells, and neuromuscular junctions of skeletal muscle all perform physiological functions that depend on clustering of functionally related but structurally diverse ion transporters and cell adhesion molecules within microdomains of the plasma membrane. These specialized cell surface domains appeared at different times in metazoan evolution, involve a variety of cell types, and are populated by distinct membrane-spanning proteins. Nevertheless, recent work has shown that these domains all share on their cytoplasmic surfaces a membrane skeleton comprised of members of the ankyrin and spectrin families. This review will summarize basic features of ankyrins and spectrins, and will discuss emerging evidence that these proteins are key players in a conserved mechanism responsible for assembly and maintenance of physiologically important domains on the surfaces of diverse cells.

Myosin II motors and F-actin dynamics drive the coordinated movement of the centrosome and soma during CNS glial-guided neuronal migration

Lamination of cortical regions of the vertebrate brain depends on glial-guided neuronal migration. The conserved polarity protein Par6alpha localizes to the centrosome and coordinates forward movement of the centrosome and soma in migrating neurons. The cytoskeletal components that produce this unique form of cell polarity and their relationship to polarity signaling cascades are unknown. We show that F-actin and Myosin II motors are enriched in the neuronal leading process and that Myosin II activity is necessary for leading process actin dynamics. Inhibition of Myosin II decreased the speed of centrosome and somal movement, whereas Myosin II activation increased coordinated movement. Ectopic expression or silencing of Par6alpha inhibited Myosin II motors by decreasing Myosin light-chain phosphorylation. These findings suggest leading-process Myosin II may function to "pull" the centrosome and soma forward during glial-guided migration by a mechanism involving the conserved polarity protein Par6alpha.

Transfection-induced defects in dynein-driven transport: evidence that ICs mediate cargo-binding

Cytoplasmic dynein contributes to the localization and transport of multiple membranous organelles, including late endosomes, lysosomes, and the Golgi complex. It remains unclear which subunits of dynein are directly responsible for linking the dynein complex to these organelles, however the intermediate chain (IC), light intermediate chain (LIC) and light chain (LC) subunits are each thought to be important. Based on previous mapping of a dynein IC phosphorylation site (S84), we measured the impact of transfected ICs on dynein-driven organelle transport (Vaughan et al.,2001). Wild-type and S84A constructs disrupted organelle transport, whereas the S84D construct induced no defects. In this study we investigated the mechanisms of transfection-induced disruption of organelle transport. Transfected ICs did not: (1) disrupt the dynein holoenzyme, (2) incorporate into the native dynein complex, (3) dimerize with native dynein ICs or (4) sequester dynein LCs in a phosphorylation-sensitive manner. Consistent with saturation of dynactin as an inhibitory mechanism, truncated ICs containing only the dynactin-binding domain were as effective as full-length IC constructs in disrupting organelle transport, and this effect was influenced by phosphorylation-state. Competition analysis demonstrated that S84D ICs were less capable than dephosphorylated ICs in disrupting the dynein-dynactin interaction. Finally, two-dimensional gel analysis revealed phosphorylation of the wild-type but not S84D ICs, providing an explanation for the incomplete effects of the wild-type ICs. Together these findings suggest that transfected ICs disrupt organelle transport by competing with native dynein for dynactin binding in a phosphorylation-sensitive manner.

An ankyrin-based mechanism for functional organization of dystrophin and dystroglycan

beta-dystroglycan (DG) and the dystrophin-glycoprotein complex (DGC) are localized at costameres and neuromuscular junctions in the sarcolemma of skeletal muscle. We present evidence for an ankyrin-based mechanism for sarcolemmal localization of dystrophin and beta-DG. Dystrophin binds ankyrin-B and ankyrin-G, while beta-DG binds ankyrin-G. Dystrophin and beta-DG require ankyrin-G for retention at costameres but not delivery to the sarcolemma. Dystrophin and beta-DG remain intracellular in ankyrin-B-depleted muscle, where beta-DG accumulates in a juxta-TGN compartment. The neuromuscular junction requires ankyrin-B for localization of dystrophin/utrophin and beta-DG and for maintenance of its postnatal morphology. A Becker muscular dystrophy mutation reduces ankyrin binding and impairs sarcolemmal localization of dystrophin-Dp71. Ankyrin-B also binds to dynactin-4, a dynactin subunit. Dynactin-4 and a subset of microtubules disappear from sarcolemmal sites in ankyrin-B-depleted muscle. Ankyrin-B thus is an adaptor required for sarcolemmal localization of dystrophin, as well as dynactin-4.

Dynein and Star interact in EGFR signaling and ligand trafficking

Intracellular transport and processing of ligands is critical to the activation of signal transduction pathways that guide development. Star is an essential gene in Drosophila that has been implicated in the trafficking of ligands for epidermal growth factor (EGF) receptor signaling. The role of cytoplasmic motors in the endocytic and secretory pathways is well known, but the specific requirement of motors in EGF receptor transport has not been investigated. We identified Star in a screen designed to recover second-site modifiers of the dominant rough eye phenotype of the Glued mutation Gl(1). The Glued (Gl) locus encodes the p150 subunit of the dynactin complex, an activator of cytoplasmic dynein-driven motility. We show that alleles of Gl and dynein genetically interact with both Star and EGFR alleles. Similarly to mutations in Star, the Gl(1) mutation is capable of modifying the phenotypes of the EGFR mutation Ellipse. These genetic interactions suggest a model in which Star, dynactin and dynein cooperate in the trafficking of EGF ligands. In support of this model, overexpression of the cleaved, active Spitz ligand can partially bypass defective trafficking and suppress the genetic interactions. Our direct observations of live S2 cells show that export of Spitz-GFP from the endoplasmic reticulum, as well as the trafficking of Spitz-GFP vesicles, depends on both Star and dynein.

Novel interactions of CLN3 protein link Batten disease to dysregulation of fodrin-Na+, K+ ATPase complex

Juvenile neuronal ceroid lipofuscinosis (JNCL, Batten disease) is the most common progressive neurodegenerative disorder of childhood. CLN3, the transmembrane protein underlying JNCL, is proposed to participate in multiple cellular events including membrane trafficking and cytoskeletal functions. We demonstrate here that CLN3 interacts with the plasma membrane-associated cytoskeletal and endocytic fodrin and the associated Na(+), K(+) ATPase. The ion pumping activity of Na(+), K(+) ATPase was unchanged in Cln3(-/-) mouse primary neurons. However, the immunostaining pattern of fodrin appeared abnormal in JNCL fibroblasts and Cln3(-/-) mouse brains suggesting disturbances in the fodrin cytoskeleton. Furthermore, the basal subcellular distribution as well as ouabain-induced endocytosis of neuron-specific Na(+), K(+) ATPase were remarkably affected in Cln3(-/-) mouse primary neurons. These data suggest that CLN3 is involved in the regulation of plasma membrane fodrin cytoskeleton and consequently, the plasma membrane association of Na(+), K(+) ATPase. Most of the processes regulated by multifunctional fodrin and Na(+), K(+) ATPase are also affected in JNCL and Cln3-deficiency implicating that dysregulation of fodrin cytoskeleton and non-pumping functions of Na(+), K(+) ATPase may play a role in the neuronal degeneration in JNCL.

Control over the morphology and segregation of Zebrafish germ cell granules during embryonic development

BACKGROUND

Zebrafish germ cells contain granular-like structures, organized around the cell nucleus. These structures share common features with polar granules in Drosophila, germinal granules in Xenopus and chromatoid bodies in mice germ cells, such as the localization of the zebrafish Vasa, Piwi and Nanos proteins, among others. Little is known about the structure of these granules as well as their segregation in mitosis during early germ-cell development.

RESULTS

Using transgenic fish expressing a fluorescently labeled novel component of Zebrafish germ cell granules termed Granulito, we followed the morphology and distribution of the granules. We show that whereas these granules initially exhibit a wide size variation, by the end of the first day of development they become a homogeneous population of medium size granules. We investigated this resizing event and demonstrated the role of microtubules and the minus-end microtubule dependent motor protein Dynein in the process. Last, we show that the function of the germ cell granule resident protein the Tudor domain containing protein-7 (Tdrd7) is required for determination of granule morphology and number.

CONCLUSION

Our results suggest that Zebrafish germ cell granules undergo a transformation process, which involves germ cell specific proteins as well as the microtubular network.

Arp11 affects dynein-dynactin interaction and is essential for dynein function in Aspergillus nidulans

The dynactin complex contains proteins including p150 that interacts with cytoplasmic dynein and an actin-related protein Arp1 that forms a minifilament. Proteins including Arp11 and p62 locate at the pointed end of the Arp1 filament, but their biochemical functions are unclear (Schroer TA. Dynactin. Annu Rev Cell Dev Biol 2004;20:759-779). In Aspergillus nidulans, loss of Arp11 or p62 causes the same nuclear distribution (nud) defect displayed by dynein mutants, indicating that these pointed-end proteins are essential for dynein function. We constructed a strain with S-tagged p150 of dynactin that allows us to pull down components of the dynactin and dynein complexes. Surprisingly, while the ratio of pulled-down Arp1 to S-p150 in Arp11-depleted cells is clearly lower than that in wild-type cells, the ratio of pulled-down dynein to S-p150 is significantly higher. We further show that the enhanced dynein-dynactin interaction in Arp11-depleted cells is also present in the soluble fraction and therefore is not dependent upon the affinity of these proteins to the membrane. We suggest that loss of the pointed-end proteins alters the Arp1 filament in a way that affects the conformation of p150 required for its proper interaction with the dynein motor.

A new computational approach to analyze human protein complexes and predict novel protein interactions

A new approach to identifying interacting proteins based on gene-expression data uses hypergeometric distribution and Monte-Carlo simulations.

We propose a new approach to identify interacting proteins based on gene expression data. By using hypergeometric distribution and extensive Monte-Carlo simulations, we demonstrate that looking at synchronous expression peaks in a single time interval is a high sensitivity approach to detect co-regulation among interacting proteins. Combining gene expression and Gene Ontology similarity analyses enabled the extraction of novel interactions from microarray datasets. Applying this approach to p21-activated kinase 1, we validated α-tubulin and early endosome antigen 1 as its novel interactors.

Down-regulation of beta-centractin might be involved in dendritic cells dysfunction and subsequent hepatocellular carcinoma immune escape: a proteomic study

AIMS

Proteomic study was used to clarify the mechanism of hepatocellular carcinoma (HCC) immune escape concerning Dendritic cells (DCs') dysfunction and their association with HCC invasion.

METHODS

Human peripheral blood mononuclear cells (PBMCs) derived DCs from healthy donors were pulsed with soluble cell lysates prepared from different metastatic potential human HCC cell lines. The total protein of these DCs was analyzed by two-dimensional electrophoresis and Electro-Spray Mass Spectrometry. The allostimulatoy capacity and phenotype of these DCs were also evaluated. The clinical significance of beta-centractin, one of the largest quantitative changed spot, down-regulation in DCs was further evaluated in autologous PBMCs derived DCs pulsed with auto-tumor lysates in 26 HCC patients.

RESULTS

The expression of beta-centractin was found to be considerably lower either in DCs pulsed with HCCLM6 (high metastatic potential HCC cell line) lysates, accompanied by down-regulation of CD86 molecule and impaired allostimulatory capacity, than those of DCs pulsed with lysates from HCC cell lines with low or without metastatic potential or in DCs pulsed with lysates from HCC with invasiveness than those without invasiveness.

CONCLUSIONS

The down-regulation of beta-centractin in DCs pulsed with high metastatic potential HCC lysates might associate with DCs dysfunction and HCC invasiveness.

The ER-Golgi intermediate compartment (ERGIC): in search of its identity and function

Protein traffic moving from the endoplasmic reticulum (ER) to the Golgi complex in mammalian cells passes through the tubulovesicular membrane clusters of the ER-Golgi intermediate compartment (ERGIC), the marker of which is the lectin ERGIC-53. The dynamic nature and functional role of the ERGIC have been debated for quite some time. In the most popular current view, the ERGIC clusters are mobile transport complexes that deliver secretory cargo from ER-exit sites to the Golgi. Recent live-cell imaging data revealing the formation of anterograde carriers from stationary ERGIC-53-positive membranes, however, suggest a stable compartment model in which ER-derived cargo is first shuttled from ER-exit sites to stationary ERGIC clusters in a COPII-dependent step and subsequently to the Golgi in a second vesicular transport step. This model can better accommodate previous morphological and functional data on ER-to-Golgi traffic. Such a stationary ERGIC would be a major site of anterograde and retrograde sorting that is controlled by coat proteins, Rab and Arf GTPases, as well as tethering complexes, SNAREs and cytoskeletal networks. The ERGIC also contributes to the concentration, folding, and quality control of newly synthesized proteins.

Melanophores: A model system for neuronal transport and exocytosis?

Black pigment cells, melanophores, from lower vertebrates are specialized in bidirectional and coordinated translocation of pigment granules, melanosomes, in the cytoplasm. Melanophores develop from the neuronal crest and are most abundant in the dermal and epidermal layers of the skin, where the intracellular distribution of the pigment significantly influences the color of the animal. The transport of pigment is dependent on an intact cytoskeleton and motor proteins associated with cytoskeletal components. The easily cultured melanophores have proved to be excellent models for organelle transport because the intracellular movements of pigment can be visualized via light microscopy, and the granules move in response to defined chemical signals. The ease of achieving a combination of morphological and functional transport studies is the advantage of the melanophore system, and studies on pigment cells have revealed new components of the transport machinery, including molecular motors, their adapters, and transfer of vesicles to other cells. Many cellular components are transported with a combination of the actin- and microtubule-based transport systems, and, since all eukaryotic organisms rely on functional intracellular transport and an intact cytoskeleton, studies on melanophores are important for many aspects of cell biology, including axonal transport. In this review, we present an overview of the research on the pigment transport system and the potential use of pigment cells as a model system.