The golgi-associated COPI-coated buds and vesicles contain beta/gamma -actin

Prolonged exposure to lactate causes TREK1 channel clustering in rat hippocampal astrocytes

Increased concentrations of lactate (15-30 mM) are associated with and found to be neuroprotective in various brain pathophysiology. In our earlier studies we showed that high levels of lactate can increase TREK1 channel activity and expression within 1 h. TREK1 channels are two pore domain leak potassium ion channels that are upregulated during cerebral ischemia, epilepsy and other brain pathologies. They play a prominent neuroprotective role against excitotoxicity. Although it has been previously shown that chronic application of lactate (6 h) causes increased gene transcription and protein expression, we observe clustering of TREK1 channels that is dependent on time of exposure (3-6 h) and concentration of lactate (15-30 mM). Using immunofluorescence techniques and image analysis, we show that the clustering of TREK1 channels is dependent on the actin cytoskeletal network of the astrocytes. Clustering of TREK1 channels can augment astrocytic functions during pathophysiological conditions and have significant implications in lactate mediated neuroprotection.

Concerted regulation of actin polymerization during constitutive secretion by cortactin and PKD2

Constitutive secretion from the trans-Golgi-network (TGN) is facilitated by a concerted regulation of vesicle biogenesis and fission processes. The protein kinase D family (PKD) has been previously described to enhance vesicle fission by modifying the lipid environment. PKD also phosphorylates the actin regulatory protein cortactin at S298 to impair synergistic actin polymerization. We here report additional functions for PKD2 (also known as PRKD2) and cortactin in the regulation of actin polymerization during the fission of transport carriers from the TGN. Phosphorylation of cortactin at S298 impairs the interaction between WIP (also known as WIPF1) and cortactin. WIP stabilizes the autoinhibited conformation of N-WASP (also known as WASL). This leads to an inhibition of synergistic Arp2/3-complex-dependent actin polymerization at the TGN. PKD2 activity at the TGN is controlled by active CDC42-GTP which directly activates N-WASP, inhibits PKD2 and shifts the balance to non-S298-phosphorylated cortactin, which can in turn sequester WIP from N-WASP. Consequently, synergistic actin polymerization at the TGN and constitutive secretion are enhanced.

ER/Golgi trafficking is facilitated by unbranched actin filaments containing Tpm4.2

We have identified novel actin filaments defined by tropomyosin Tpm4.2 at the ER. EM analysis of mouse embryo fibroblasts (MEFs) isolated from mice expressing a mutant Tpm4.2 (Tpm4^Plt53/Plt53 ), incapable of incorporating into actin filaments, revealed swollen ER structures compared with wild-type (WT) MEFs (Tpm4^+/+ ). ER-to-Golgi, but not Golgi-to-ER trafficking was altered in the Tpm4^Plt53/Plt53 MEFs following the transfection of the temperature sensitive ER-associated ts045-VSVg construct. Exogenous Tpm4.2 was able to rescue the ER-to-Golgi trafficking defect in the Tpm4^Plt53/Plt53 cells. The treatment of WT MEFs with the myosin II inhibitor, blebbistatin, blocked the Tpm4.2-dependent ER-to-Golgi trafficking. The lack of an effect on ER-to-Golgi trafficking following treatment of MEFs with CK666 indicates that branched Arp2/3-containing actin filaments are not involved in anterograde vesicle trafficking. We propose that unbranched, Tpm4.2-containing filaments have an important role in maintaining ER/Golgi structure and that these structures, in conjunction with myosin II motors, mediate ER-to-Golgi trafficking.

Actin Filaments Are Involved in the Coupling of V0-V1 Domains of Vacuolar H+-ATPase at the Golgi Complex

We previously reported that actin-depolymerizing agents promote the alkalization of the Golgi stack and thetrans-Golgi network. The main determinant of acidic pH at the Golgi is the vacuolar-type H(+)-translocating ATPase (V-ATPase), whose V1domain subunitsBandCbind actin. We have generated a GFP-tagged subunitB2construct (GFP-B2) that is incorporated into the V1domain, which in turn is coupled to the V0sector. GFP-B2 subunit is enriched at distal Golgi compartments in HeLa cells. Subcellular fractionation, immunoprecipitation, and inversal FRAP experiments show that the actin depolymerization promotes the dissociation of V1-V0domains, which entails subunitB2translocation from Golgi membranes to the cytosol. Moreover, molecular interaction between subunitsB2andC1and actin were detected. In addition, Golgi membrane lipid order disruption byd-ceramide-C6 causes Golgi pH alkalization. We conclude that actin regulates the Golgi pH homeostasis maintaining the coupling of V1-V0domains of V-ATPase through the binding of microfilaments to subunitsBandCand preserving the integrity of detergent-resistant membrane organization. These results establish the Golgi-associated V-ATPase activity as the molecular link between actin and the Golgi pH.

Regulation of calcium channels in smooth muscle: new insights into the role of myosin light chain kinase

Smooth muscle myosin light chain kinase (MLCK) plays a crucial role in artery contraction, which regulates blood pressure and blood flow distribution. In addition to this role, MLCK contributes to Ca(2+) flux regulation in vascular smooth muscle (VSM) and in non-muscle cells, where cytoskeleton has been suggested to help Ca(2+) channels trafficking. This conclusion is based on the use of pharmacological inhibitors of MLCK and molecular and cellular techniques developed to down-regulate the enzyme. Dissimilarities have been observed between cells and whole tissues, as well as between large conductance and small resistance arteries. A differential expression in MLCK and ion channels (either voltage-dependent Ca(2+) channels or non-selective cationic channels) could account for these observations, and is in line with the functional properties of the arteries. A potential involvement of MLCK in the pathways modulating Ca(2+) entry in VSM is described in the present review.

The Abl/enabled signaling pathway regulates Golgi architecture in Drosophila photoreceptor neurons

The Golgi apparatus is optimized separately in different tissues for efficient protein trafficking, but we know little of how cell signaling shapes this organelle. We now find that the Abl tyrosine kinase signaling pathway controls the architecture of the Golgi complex in Drosophila photoreceptor (PR) neurons. The Abl effector, Enabled (Ena), selectively labels the cis-Golgi in developing PRs. Overexpression or loss of function of Ena increases the number of cis- and trans-Golgi cisternae per cell, and Ena overexpression also redistributes Golgi to the most basal portion of the cell soma. Loss of Abl or its upstream regulator, the adaptor protein Disabled, lead to the same alterations of Golgi as does overexpression of Ena. The increase in Golgi number in Abl mutants arises in part from increased frequency of Golgi fission events and a decrease in fusions, as revealed by live imaging. Finally, we demonstrate that the effects of Abl signaling on Golgi are mediated via regulation of the actin cytoskeleton. Together, these data reveal a direct link between cell signaling and Golgi architecture. Moreover, they raise the possibility that some of the effects of Abl signaling may arise, in part, from alterations of protein trafficking and secretion.

Actin acting at the Golgi

The organization, assembly and remodeling of the actin cytoskeleton provide force and tracks for a variety of (endo)membrane-associated events such as membrane trafficking. This review illustrates in different cellular models how actin and many of its numerous binding and regulatory proteins (actin and co-workers) participate in the structural organization of the Golgi apparatus and in trafficking-associated processes such as sorting, biogenesis and motion of Golgi-derived transport carriers.

Intra-Golgi Transport: Roles for Vesicles, Tubules, and Cisternae

The Golgi complex is considered the central station of the secretory pathway where cargo proteins and lipids are properly modified, classified, packed into specific carriers and delivered to their final destinations. Early electron microscope studies showed the extraordinary structural complexity of this organelle. However, despite the large volume of incoming and outgoing traffic, it is able to maintain its architecture, although it is also flexible enough to adapt to the functional status of the cell. Many components of the molecular machinery involved in membrane traffic and other Golgi functions have been identified. However, some basic aspects of Golgi functioning remain unsolved. For instance, how cargo moves through the stack remains controversial and two classical models have been proposed: vesicular transport and cisternal maturation. Since neither of these models explains all the experimental data, a combination of these models as well as new models have been proposed. In this context, the specific role of the cisternae, vesicles and tubules needs to be clarified. In this review, we summarize our current knowledge of the Golgi organization and function, focusing on the mechanisms of intra-Golgi transport.

Regulation of late endosomal/lysosomal maturation and trafficking by cortactin affects Golgi morphology

Cortactin is a branched actin regulator and tumor-overexpressed protein that promotes vesicular trafficking at a variety of cellular sites, including endosomes and the trans-Golgi network. To better understand its role in secretory trafficking, we investigated its function in Golgi homeostasis. Here, we report that knockdown (KD) of cortactin leads to a dramatic change in Golgi morphology by light microscopy, dependent on binding the Arp2/3 actin-nucleating complex. Surprisingly, there was little effect of cortactin-KD on anterograde trafficking of the constitutive cargo vesicular stomatitis virus glycoprotein (VSVG), Golgi assembly from endoplasmic reticulum membranes upon Brefeldin A washout, or Golgi ultrastructure. Instead, electron microscopy studies revealed that cortactin-KD cells contained a large number of immature-appearing late endosomal/lysosomal (LE/Lys) hybrid organelles, similar to those found in lysosomal storage diseases. Consistent with a defect in LE/Lys trafficking, cortactin-KD cells also exhibited accumulation of free cholesterol and retention of the retrograde Golgi cargo mannose-6-phosphate receptor in LE. Inhibition of LE maturation by treatment of control cells with Rab7 siRNA or chloroquine led to a compact Golgi morphology similar to that observed in cortactin-KD cells. Furthermore, the Golgi morphology defects of cortactin-KD cells could be rescued by removal of cholesterol-containing lipids from the media, suggesting that buildup of cholesterol-rich membranes in immature LE/Lys induced disturbances in retrograde trafficking. Taken together, these data reveal that LE/Lys maturation and trafficking are highly sensitive to cortactin-regulated branched actin assembly and suggests that cytoskeletal-induced Golgi morphology changes can be a consequence of altered trafficking at late endosomes.

Nonredundant roles of cytoplasmic β- and γ-actin isoforms in regulation of epithelial apical junctions

The functional effects of cytoplasmic actins on epithelial junctions are examined by using isoform-specific siRNAs and cell-permeable inhibitory peptides. Unique roles of cytoplasmic actin isoforms in regulating structure and remodeling of adherens and tight junctions are revealed.

Association with the actin cytoskeleton is critical for normal architecture and dynamics of epithelial tight junctions (TJs) and adherens junctions (AJs). Epithelial cells express β-cytoplasmic (β-CYA) and γ-cytoplasmic (γ-CYA) actins, which have different cellular localization and functions. This study elucidates the roles of cytoplasmic actins in regulating structure and remodeling of AJs and TJs in model intestinal epithelia. Immunofluorescence labeling and latrunculin B treatment reveal affiliation of dynamic β-CYA filaments with newly assembled and mature AJs, whereas an apical γ-CYA pool is composed of stable perijunctional bundles and rapidly turning-over nonjunctional filaments. The functional effects of cytoplasmic actins on epithelial junctions are examined by using isoform-specific small interfering RNAs and cell-permeable inhibitory peptides. These experiments demonstrate unique roles of β-CYA and γ-CYA in regulating the steady-state integrity of AJs and TJs, respectively. Furthermore, β-CYA is selectively involved in establishment of apicobasal cell polarity. Both actin isoforms are essential for normal barrier function of epithelial monolayers, rapid AJ/TJ reassembly, and formation of three-dimensional cysts. Cytoplasmic actin isoforms play unique roles in regulating structure and permeability of epithelial junctions.

Transport of influenza virus neuraminidase (NA) to host cell surface is regulated by ARHGAP21 and Cdc42 proteins

Influenza virus neuraminidase (NA) is transported to the virus assembly site at the plasma membrane and is a major viral envelope component that plays a critical role in the release of progeny virions and in determination of host range restriction. However, little is known about the host factors that are involved in regulating the intracellular and cell surface transport of NA. Here we identified the Cdc42-specific GAP, ARHGAP21 differentially expressed in host cells infected with influenza A virus using cDNA microarray analysis. Furthermore, we have investigated the involvement of Rho family GTPases in NA transport to the cell surface. We found that expression of constitutively active or inactive mutants of RhoA or Rac1 did not significantly affect the amount of NA that reached the cell surface. However, expression of constitutively active Cdc42 or depletion of ARHGAP21 promoted the transport of NA to the plasma membranes. By contrast, cells expressing shRNA targeting Cdc42 or overexpressing ARHGAP21 exhibited a significant decrease in the amount of cell surface-localized NA. Importantly, silencing Cdc42 reduced influenza A virus replication, whereas silencing ARHGAP21 increased the virus replication. Together, our results reveal that ARHGAP21- and Cdc42-based signaling regulates the NA transport and thereby impacts virus replication.

RNAi screen of Salmonella invasion shows role of COPI in membrane targeting of cholesterol and Cdc42

A genome wide RNAi screen identifies 72 host cell genes affecting S. Typhimurium entry, including actin regulators and COPI. This study implicates COPI-dependent cholesterol and sphingolipid localization as a common mechanism of infection by bacterial and viral pathogens.

Genome-scale RNAi screen identifies 72 host genes affecting S. Typhimurium host cell invasion.

Step-specific follow-up assays assign the phenotypes to specific steps of the invasion process.

COPI effects on host cell binding, ruffling and invasion were traced to a key role of COPI in membrane targeting of cholesterol, sphingolipids, Rac1 and Cdc42.

This new role of COPI explains why COPI is required for host cell infection by numerous bacterial and viral pathogens.

Pathogens are not only a menace to public health, but they also provide excellent tools for probing host cell function. Thus, studying infection mechanisms has fueled progress in cell biology (Ridley et al, 1992; Welch et al, 1997). In the presented study, we have performed an RNAi screen to identify host cell genes required for Salmonella host cell invasion. This screen identified proteins known to contribute to Salmonella-induced actin rearrangements (e.g., Cdc42 and the Arp2/3 complex; reviewed in Schlumberger and Hardt, 2006) and vesicular traffic (e.g., Rab7) as well as unexpected hits, such as the COPI complex. COPI is a known organizer of Golgi-to-ER vesicle transport (Bethune et al, 2006; Beck et al, 2009). Here, we show that COPI is also involved in plasma membrane targeting of cholesterol, sphingolipids and the Rho GTPases Cdc42 and Rac1, essential host cell factors required for Salmonella invasion. This explains why COPI depletion inhibits infection by S. Typhimurium and illustrates how combining bacterial pathogenesis and systems approaches can promote cell biology.

Salmonella Typhimurium is a common food-borne pathogen and worldwide a major public health problem causing severe diarrhea. The pathogen uses the host's gut mucosa as a portal of entry and gut tissue invasion is a key event leading to the disease. This explains the intense interest from medicine and basic biology in the mechanism of Salmonella host cell invasion.

Tissue culture infection models have delineated a sequence of events leading host cell invasion (Figure 1;Schlumberger and Hardt, 2006): (i) pathogen binding to the host cell surface; (ii) activation of a syringe-like apparatus (‘Type III secretion system 1', T1) of the bacterium and injection of a bacterial toxin cocktail into the host cell. These toxins include SopE, a key virulence factor triggering invasion (Hardt et al, 1998), which was analyzed in our study; (iii) toxin-triggered membrane ruffling. To a significant extent, this is facilitated by SopE-triggered activation of Cdc42 and Rac1 and subsequent actin polymerization at the site of infection; (iv) engulfment of the pathogen within a vesicular compartment (SCV) and (v) maturation of the SCV, a process driven by a second Type III secretion system (T2), which is expressed by the pathogen upon bacterial entry (Figure 1). This sequence of events mediates Salmonella invasion into the gut epithelium and illustrates that this pathogen can be used for probing mechanisms of host cell actin control, membrane biogenesis, vesicle formation and vesicular trafficking.

SopE is a key virulence factor of invasion and triggers the activation of Cdc42 and Rac1 and subsequent actin polymerization at the site of infection. We have employed a SopE-expressing S. Typhimurium strain and RNAi screening technology to identify host cell factors affecting invasion. First, we developed an automated fluorescence microscopy assay to quantify S. Typhimurium entry in a high-throughput format (Figure 1C). This assay was based on a GFP reporter expressed by the pathogen after invasion and maturation of the SCV. Using this assay, we screened a ‘druggable genome' siRNA library (6978 genes, 3 oligos each, 1 oligo per well) and identified 72 invasion hits. These included established regulators of the actin cytoskeleton (Cdc42, Arp2/3, Nap1; Schlumberger and Hardt, 2006), some of which have not been implicated so far in Salmonella entry (Pfn1, Cap1), as well as proteins not previously thought to influence infection (Atp1a1, Rbx1, COPI complex). Potentially, these hits could affect any step of the invasion process (Figure 1A).

In the second stage of the study, we have assigned each ‘invasion hit' to particular steps of the invasion process. For this purpose, we developed step-specific assays for Salmonella binding, injection, ruffling and membrane engulfment and re-screened the genes found as hits in the first screen (four siRNAs per gene). As expected, a significant number of ‘hits' affected binding to the host cell, others affected binding and ruffling (e.g., Pfn1, Itgβ5, Cap1), a few were specific for the ruffling step (e.g., Cdc42) and some affected SCV maturation, namely Rab7a, the trafficking protein Vps39 and the vacuolar proton pump Atp6ap2. Thus, our experimental strategy allowed mechanistic interpretation and linked novel hits to particular phenotypes, thus providing a basis for further studies (Figure 1).

COPI depletion impaired effector injection and ruffling. This was surprising, as the COPI complex was known to regulate retrogade Golgi-to-ER transport, but was not expected to affect pathogen interactions at the plasma membrane. Therefore, we have investigated the underlying mechanism. We have observed that COPI depletion entailed dramatic changes in the plasma membrane composition (Figure 6). Cholesterol and sphingolipids, which form domains (‘lipid rafts') in the plasma membrane, were depleted from the cell surface and redirected into a large vesicular compartment. The same was true for the Rho GTPases Rac1 and Cdc42. This strong decrease in the amount of cholesterol-enriched microdomains and Rho GTPases in the plasma membrane explained the observed defects in S. Typhimurium host cell invasion and assigned a novel role for COPI in controlling mammalian plasma membrane composition. It should be noted that other viral and bacterial pathogens do show a similar dependency on host cellular COPI and plasma membrane lipids. This includes notorious pathogens such as Staphylococcus aureus (Ramet et al, 2002; Potrich et al, 2009), Listeria monocytogenes (Seveau et al, 2004; Agaisse et al, 2005; Cheng et al, 2005; Gekara et al, 2005), Mycobacterium tuberculosis (Munoz et al, 2009), Chlamydia trachomatis (Elwell et al, 2008), influenza virus (Hao et al, 2008; Konig et al, 2010), hepatitis C virus (Tai et al, 2009; Popescu and Dubuisson, 2010) and the vesicular stomatitis virus (presented study) and suggests that COPI-mediated control of host cell plasma membrane composition might be of broad importance for pathogenesis. Future work will have to address whether this might offer starting points for developing anti-infective therapeutics with a very broad spectrum of activity.

The pathogen Salmonella Typhimurium is a common cause of diarrhea and invades the gut tissue by injecting a cocktail of virulence factors into epithelial cells, triggering actin rearrangements, membrane ruffling and pathogen entry. One of these factors is SopE, a G-nucleotide exchange factor for the host cellular Rho GTPases Rac1 and Cdc42. How SopE mediates cellular invasion is incompletely understood. Using genome-scale RNAi screening we identified 72 known and novel host cell proteins affecting SopE-mediated entry. Follow-up assays assigned these ‘hits' to particular steps of the invasion process; i.e., binding, effector injection, membrane ruffling, membrane closure and maturation of the Salmonella-containing vacuole. Depletion of the COPI complex revealed a unique effect on virulence factor injection and membrane ruffling. Both effects are attributable to mislocalization of cholesterol, sphingolipids, Rac1 and Cdc42 away from the plasma membrane into a large intracellular compartment. Equivalent results were obtained with the vesicular stomatitis virus. Therefore, COPI-facilitated maintenance of lipids may represent a novel, unifying mechanism essential for a wide range of pathogens, offering opportunities for designing new drugs.

Involvement of the Rho-mDia1 pathway in the regulation of Golgi complex architecture and dynamics

A study of the role of actin cytoskeleton regulation in Golgi organization and function shows that Rho regulates Golgi fragmentation into ministacks, as well as formation of Rab6-positive Golgi-derived vesicles, via mDia1 formin activation. The Rho–mDia1 pathway affects the Golgi complex by controlling fusion and fission of Golgi membranes.

In mammalian cells, the Golgi apparatus is a ribbon-like, compact structure composed of multiple membrane stacks connected by tubular bridges. Microtubules are known to be important to Golgi integrity, but the role of the actin cytoskeleton in the maintenance of Golgi architecture remains unclear. Here we show that an increase in Rho activity, either by treatment of cells with lysophosphatidic acid or by expression of constitutively active mutants, resulted in pronounced fragmentation of the Golgi complex into ministacks. Golgi dispersion required the involvement of mDia1 formin, a downstream target of Rho and a potent activator of actin polymerization; moreover, constitutively active mDia1, in and of itself, was sufficient for Golgi dispersion. The dispersion process was accompanied by formation of dynamic F-actin patches in the Golgi area. Experiments with cytoskeletal inhibitors (e.g., latrunculin B, blebbistatin, and Taxol) revealed that actin polymerization, myosin-II–driven contractility, and microtubule-based intracellular movement were all involved in the process of Golgi dispersion induced by Rho–mDia1 activation. Live imaging of Golgi recovery revealed that fusion of the small Golgi stacks into larger compartments was repressed in cells with active mDia1. Furthermore, the formation of Rab6-positive transport vesicles derived from the Golgi complex was enhanced upon activation of the Rho–mDia1 pathway. Transient localization of mDia1 to Rab6-positive vesicles was detected in cells expressing active RhoA. Thus, the Rho–mDia1 pathway is involved in regulation of the Golgi structure, affecting remodeling of Golgi membranes.

The non-catalytic carboxyl-terminal domain of ARFGAP1 regulates actin cytoskeleton reorganization by antagonizing the activation of Rac1

Background

The regulation of the actin cytoskeleton and membrane trafficking is coordinated in mammalian cells. One of the regulators of membrane traffic, the small GTP-binding protein ARF1, also activates phosphatidylinositol kinases that in turn affect actin polymerization. ARFGAP1 is a GTPase activating protein (GAP) for ARF1 that is found on Golgi membranes. We present evidence that ARFGAP1 not only serves as a GAP for ARF1, but also can affect the actin cytoskeleton.

Principal Findings

As cells attach to a culture dish foci of actin appear prior to the cells flattening and spreading. We have observed that overexpression of a truncated ARFGAP1 that lacks catalytic activity for ARF, called GAP273, caused these foci to persist for much longer periods than non-transfected cells. This phenomenon was dependent on the level of GAP273 expression. Furthermore, cell spreading after re-plating or cell migration into a previously scraped area was inhibited in cells transfected with GAP273. Live cell imaging of such cells revealed that actin-rich membrane blebs formed that seldom made protrusions of actin spikes or membrane ruffles, suggesting that GAP273 interfered with the regulation of actin dynamics during cell spreading. The over-expression of constitutively active alleles of ARF6 and Rac1 suppressed the effect of GAP273 on actin. In addition, the activation of Rac1 by serum, but not that of RhoA or ARF6, was inhibited in cells over-expressing GAP273, suggesting that Rac1 is a likely downstream effector of ARFGAP1. The carboxyl terminal 65 residues of ARFGAP1 were sufficient to produce the effects on actin and cell spreading in transfected cells and co-localized with cortical actin foci.

Conclusions

ARFGAP1 functions as an inhibitor upstream of Rac1 in regulating actin cytoskeleton. In addition to its GAP catalytic domain and Golgi binding domain, it also has an actin regulation domain in the carboxyl-terminal portion of the protein.

Clostridium botulinum C2 toxin is internalized by clathrin- and Rho-dependent mechanisms

Clostridium botulinum C2 toxin is an ADP-ribosyltransferase, causing depolymerization of the actin cytoskeleton in eukaryotic cells. The C2 toxin is a binary toxin consisting of the enzymatic subunit C2I and the binding subunit C2II. Proteolytical activation of the binding subunit triggers the formation of heptameric structures (C2IIa), which bind to cellular receptors. C2I is able to bind to C2IIa oligomers, and it has been suggested that the whole complex is internalized by a raft-dependent mechanism. Here we analysed by which mechanism C2 toxin is endocytosed. In HeLa cells expressing a dominant-negative dynamin mutant, cytotoxicity and C2 toxin uptake were blocked. Furthermore, siRNA-mediated knockdown of flotillins or inhibition of Arf6 function, proteins suggested to be involved in dynamin-independent endocytosis, did not affect C2 toxicity. Knockdown of caveolin did not inhibit endocytosis of C2 toxin, whereas inhibition of clathrin function reduced the uptake of C2 toxin and delayed the cytotoxic effect. Finally, we found evidence for a Rho-mediated uptake of C2 toxin. In conclusion, C2 toxin is endocytosed by dynamin-dependent mechanisms and we provide evidence for involvement of clathrin and Rho.

ARF1-mediated actin polymerization produces movement of artificial vesicles

Vesicular trafficking and actin dynamics on Golgi membranes are both regulated by ADP-ribosylation factor 1 (ARF1) through the recruitment of various effectors, including vesicular coats. Actin assembly on Golgi membranes contributes to the architecture of the Golgi complex, vesicle formation, and trafficking and is mediated by ARF1 through a cascade that leads to Arp2/3 complex activation. Here we addressed the role of Golgi actin downstream of ARF1 by using a biomimetic assay consisting of liposomes of defined lipid composition, carrying an activated form of ARF1 incubated in cytosolic cell extracts. We observed actin polymerization around the liposomes resulting in thick actin shells and actin comet tails that pushed the ARF1 liposomes forward. The assay was used to characterize the ARF1-dependent pathway, leading to actin polymerization, and confirmed a dependency on CDC42 and its downstream effector N-WASP. Overall, this study demonstrates that actin polymerization driven by the complex multicomponent signaling cascade of the Golgi apparatus can be reproduced with a biomimetic system. Moreover, our results are consistent with the view that actin-based force generation at the site of vesicle formation contributes to the mechanism of fission. In addition to its well established function in coat recruitment, the ARF1 machinery also might produce movement- and fission-promoting forces through actin polymerization.

Actin filaments are involved in the maintenance of Golgi cisternae morphology and intra-Golgi pH

Here we examine the contribution of actin dynamics to the architecture and pH of the Golgi complex. To this end, we have used toxins that depolymerize (cytochalasin D, latrunculin B, mycalolide B, and Clostridium botulinum C2 toxin) or stabilize (jasplakinolide) filamentous actin. When various clonal cell lines were examined by epifluorescence microscopy, all of these actin toxins induced compaction of the Golgi complex. However, ultrastructural analysis by transmission electron microscopy and electron tomography/three-dimensional modelling of the Golgi complex showed that F-actin depolymerization first induces perforation/fragmentation and severe swelling of Golgi cisternae, which leads to a completely disorganized structure. In contrast, F-actin stabilization results only in cisternae perforation/fragmentation. Concomitantly to actin depolymerization-induced cisternae swelling and disorganization, the intra-Golgi pH significantly increased. Similar ultrastructural and Golgi pH alkalinization were observed in cells treated with the vacuolar H+ -ATPases inhibitors bafilomycin A1 and concanamycin A. Overall, these results suggest that actin filaments are implicated in the preservation of the flattened shape of Golgi cisternae. This maintenance seems to be mediated by the regulation of the state of F-actin assembly on the Golgi pH homeostasis.

Dual Regulation of Rho and Rac by p120 Catenin Controls Adipocyte Plasma Membrane Trafficking

During 3T3L1 adipogenesis there is a marked reduction in beta-catenin and N-cadherin expression with a relatively small decrease in p120 catenin protein levels. Cell fractionation demonstrated a predominant decrease in the particulate (membrane-bound) pool of p120 catenin with little effect on the soluble pool, resulting in a large redistribution from the plasma membrane to the cytosol. Reexpression of p120 catenin inhibited constitutive (transferrin receptor) and regulated mannose 6-phosphate receptor and GLUT4 trafficking to the plasma membrane. The inhibition of membrane trafficking was specific for p120 catenin function as this could be rescued by co-expression of N-cadherin. Moreover, overexpression of a p120 catenin deletion mutant (p120delta622-628) or splice variant (p120-4A), neither of which could regulate Rho or Rac activity, showed no significant effect. The inhibition of GLUT4 translocation was also observed upon the simultaneous expression of a constitutively active Rac mutant (Rac1/Val12) in combination with a dominant-interfering Rho mutant (RhoA/Asn19). This was recapitulated by expression of the Rho ADP-ribosylation factor (C3ADP) in combination with constitutively active Rac1/Val12. Moreover, siRNA-mediated knockdown of p120 catenin resulted in increased basal state accumulation of GLUT4 at the plasma membrane. Together, these data demonstrate that p120 catenin plays an important role in maintaining the basal tone of membrane protein trafficking in adipocytes through the dual regulation of Rho and Rac function and accounts for reports implicating Rho or Rac in the control of GLUT4 translocation.

Insulin receptor signals regulating GLUT4 translocation and actin dynamics

In skeletal muscle and adipose tissue, insulin-stimulated glucose uptake is dependent upon translocation of the insulin-responsive glucose transporter GLUT4 from intracellular storage compartments to the plasma membrane. This insulin-induced redistribution of GLUT4 protein is achieved through a series of highly organized membrane trafficking events, orchestrated by insulin receptor signals. Recently, several key molecules linking insulin receptor signals and membrane trafficking have been identified, and emerging evidence supports the importance of subcellular compartmentalization of signaling components at the right time and in the right place. In addition, the translocation of GLUT4 in adipocytes requires insulin stimulation of dynamic actin remodeling at the inner surface of the plasma membrane (cortical actin) and in the perinuclear region. This results from at least two independent insulin receptor signals, one leading to the activation of phosphatidylinositol (PI) 3-kinase and the other to the activation of the Rho family small GTP-binding protein TC10. Thus, both spatial and temporal regulations of actin dynamics, both beneath the plasma membrane and around endomembranes, by insulin receptor signals are also involved in the process of GLUT4 translocation.

The Actin-depolymerizing Factor Homology and Charged/Helical Domains of Drebrin and mAbp1 Direct Membrane Binding and Localization via Distinct Interactions with Actin

Cytoskeletal dynamics are important for efficient function of the secretory pathway. ADP-ribosylation factor, ARF1, triggers vesicle coat assembly and, in concert with Cdc42, regulates actin polymerization and molecular motor-based motility. Drebrin and mammalian Abp1 (mAbp1) are actin-binding proteins found previously to bind to Golgi membranes in an ARF1-dependent manner in vitro. Despite sharing homology through two shared actin binding domains, drebrin and mAbp1 have different subcellular localization and bind to distinct actin structures on the Golgi apparatus. We find that the actin-depolymerizing factor homology (ADFH) and charged/helical actin binding domains of drebrin and mAbp1 are sufficient for regulated binding to Golgi membranes and subcellular localization. We have used mutant proteins and chimeras between mAbp1 and drebrin to identify motifs that direct targeting. We find that a linker region between the ADFH and charged/helical domains confers Golgi binding properties to mAbp1. mAbp1 binds to a specific actin pool through its ADFH/linker domain that is not bound by drebrin. Drebrin localization to the cell surface was found to involve motifs within the charged/helical domain. Our results indicate that targeting of these proteins is directed through multiple distinct interactions with the actin cytoskeleton. The mechanisms for selective recruitment of mAbp1 and drebrin to Golgi membranes indicate how actin-based structures are able to select specific actin-binding proteins and, thus, carry out multiple different functions within cells.

Mechanism of Akt1 inhibition of breast cancer cell invasion reveals a protumorigenic role for TSC2

Akt1 is frequently up-regulated in human tumors and has been shown to accelerate cell proliferation and to suppress programmed cell death; consequently, inhibition of the activity of Akt1 has been seen as an attractive target for therapeutic intervention. Paradoxically, hyperactivation of the Akt1 oncogene can also prevent the invasive behavior that underlies progression to metastasis. Here we show that overexpression of activated myr-Akt1 in human breast cancer cells phosphorylates and thereby targets the tumor suppressor tuberous sclerosis complex 2 (TSC2) for degradation, leading to reduced Rho-GTPase activity, decreased actin stress fibers and focal adhesions, and reduced motility and invasion. Overexpression of TSC2 rescues the migration phenotype of myr-Akt1-expressing tumor cells, and high levels of TSC2 in breast cancer patients correlate with increased metastasis and reduced survival. These data indicate that the functional properties of genes designated as oncogenes or tumor suppressor genes depend on the context of the cell type and the tissues studied, and suggest the need for caution in designing therapies targeting the function of individual genes in epithelial tissues.

Actin dynamics at the Golgi complex in mammalian cells

Secretion and endocytosis are highly dynamic processes that are sensitive to external stimuli. Thus, in multicellular organisms, different cell types utilize specialised pathways of intracellular membrane traffic to facilitate specific physiological functions. In addition to the complex internal molecular factors that govern sorting functions and fission or fusion of transport carriers, the actin cytoskeleton plays an important role in both the endocytic and secretory pathways. The interaction between the actin cytoskeleton and membrane trafficking is not restricted to transport processes: it also appears to be directly involved in the biogenesis of Golgi-derived transport carriers (budding and fission processes) and in the maintenance of the unique flat shape of Golgi cisternae.

Membrane trafficking at the ER/Golgi interface: Functional implications of RhoA and Rac1

N-WASP and Arp2/3, the components of the actin nucleation/polymerization signaling pathway governed by Cdc42, are located in Golgi membranes and regulate ER/Golgi interface protein transport. In the present study, we examined whether RhoA and Rac1, like Cdc42, are also involved in this early secretory pathway. Unlike Cdc42, RhoA and Rac1 were not observed in the Golgi complex of different clonal cell lines nor were they present in isolated Golgi membranes. Expression of constitutively active or inactive mutants of RhoA or Rac1 proteins in HeLa cells did not alter either the disassembly or the assembly of the Golgi complex following the addition or withdrawal of BFA, respectively, the ER-to-Golgi VSV-G transport or the Sar1(dn)-induced ER accumulation of Golgi proteins. Moreover, unlike Cdc42-expressing cells, the 15 degrees C-induced subcellular redistribution of the KDEL receptor remained unaltered. Only cells that constitutively express the activated Cdc42 mutant (Cdc42Q61L), or that were microinjected with activated Cdc42Q61L protein, exhibited a significant change in Golgi complex morphology. Collectively, our results demonstrate that RhoA and Rac1 are not located in the Golgi complex, nor do they directly or indirectly regulate membrane trafficking at the ER/Golgi interface. This finding, in turn, confirms that Cdc42 is the only Rho GTPase to have a specific function on the Golgi complex.

Characterization of myosin-II binding to Golgi stacks in vitro

In addition to important roles near the actin-rich cell cortex, ample evidence indicates that multiple myosins are also involved in membrane movements in the endomembrane system. Nonmuscle myosin-II has been shown to have roles in anterograde and retrograde trafficking at the Golgi. Myosin-II is present on Golgi stacks isolated from intestinal epithelial cells and has been localized to the Golgi in several polarized and unpolarized cell lines. An understanding of roles of myosin-II in Golgi physiology will be facilitated by understanding the molecular arrangement of myosin-II at the Golgi. Salt-washing removes endogenous myosin-II from isolated Golgi and purified brush border myosin-II can bind in vitro. Brush border myosin-II binds to a tightly bound Golgi peripheral membrane protein with a K(1/2) of 75 nM and binding is saturated at 0.7 pmol myosin/microg Golgi. Binding studies using papain cleavage fragments of brush border myosin-II show that the 120-kDa rod domain, but not the head domain, of myosin heavy chain can bind directly to Golgi stacks. The 120-kDa domain does not bind to Golgi membranes when phosphorylated in vitro with casein kinase-II. These results suggest that phosphorylation in the rod domain may regulate the binding and/or release of myosin-II from the Golgi. These data support a model in which myosin-II is tethered to the Golgi membrane by its tail and actin filaments by its head. Thus, translocation along actin filaments may extend Golgi membrane tubules and/or vesicles away from the Golgi complex.

Localization and regulation of phospholipase D2 by ARF6

Phospholipase D (PLD) and ADP-ribosylation factor 6 (ARF6) have been implicated in vesicular trafficking and rearrangement of the actin cytoskeleton. We have explored the co-localization of rat PLD1b and rat PLD2 with wild type and mutant forms of ARF6 in HeLa cells and studied their activation by ARF6 and the role of the actin cytoskeleton. GFP-tagged PLD1 had a similar pattern to multivesicular and late endosomes and the trans-Golgi apparatus, but not to other organelles. When wild type or dominant negative ARF6 and PLD1 or PLD2 were co-expressed, they had a similar localization in cytosolic particles and at the cell periphery. In contrast, dominant active ARF6 caused cell shrinkage and had a similar localization with PLD1 and PLD2 in dense structures, containing the trans-Golgi apparatus and actin. Disruption of the actin cytoskeleton with cytochalasin D did not induce the formation of these structures. To determine, if ARF6 selectively activated PLD1 or PLD2, wild type and mutant forms of the ARF isoform were transfected together with PLD1 or PLD2. Wild type ARF6 did not affect either PLD isozyme, but dominant active ARF6 selectively activated PLD2 and dominant negative ARF6 selectively inhibited PLD2. In contrast, dominant active ARF1 or Rac1 stimulated both PLD isozymes but the ARF1 effect on PLD2 was very small. Cytochalasin D did not affect the activation of PLD by phorbol ester. The localizations of PLD and ARF6 were also analyzed by fractionation after methyl-beta-cyclodextrin extraction to deplete cholesterol. The results showed that all PLD isoforms and ARF6 mutants existed in the membrane fraction, but only wild type ARF6 was dependent on the presence of cholesterol. These experiments showed that wild type ARF6 had a similar location with PLD isoforms on cell staining, but it did not colocalize with PLD isoforms in fractionation experiments. It is proposed that activated ARF6 translocates to the cholesterol independent microdomain and then activates PLD2 there. It is further concluded that PLD2 is selectively activated by ARF6 in vivo and that disruption of the actin cytoskeleton does not affect this activation.

Ethanol perturbs the secretory pathway in astrocytes

Ethanol exposure induces retention of glycoproteins in growing astrocytes. We examined the intracellular sites at which this retention occurs and investigated whether this effect is accompanied by alterations in the Golgi complex and microtubular system. We studied the effects of ethanol on the Golgi complex structure, as well as on the secretory pathway functionality by monitoring both the transport of the VSV-G protein and the protein levels of several molecules involved in the regulation of this pathway. Ethanol was found to delay VSV-G transport, modify Golgi complex morphology, and reduce the number of secretory vesicles. Moreover, ethanol affected the levels of mannosidase II, p58, betaCOP, rbet1, and several Rab GTPases. It also affected microtubule organization and polymerization and the levels of the motor proteins kinesin and dynein. Most of these effects were dose-dependent. These alterations, together with those previously reported concerning biosynthesis of glycoconjugates, provide novel insights into how ethanol impairs brain development.

Binary bacterial toxins: biochemistry, biology, and applications of common Clostridium and Bacillus proteins

Certain pathogenic species of Bacillus and Clostridium have developed unique methods for intoxicating cells that employ the classic enzymatic "A-B" paradigm for protein toxins. The binary toxins produced by B. anthracis, B. cereus, C. botulinum, C. difficile, C. perfringens, and C. spiroforme consist of components not physically associated in solution that are linked to various diseases in humans, animals, or insects. The "B" components are synthesized as precursors that are subsequently activated by serine-type proteases on the targeted cell surface and/or in solution. Following release of a 20-kDa N-terminal peptide, the activated "B" components form homoheptameric rings that subsequently dock with an "A" component(s) on the cell surface. By following an acidified endosomal route and translocation into the cytosol, "A" molecules disable a cell (and host organism) via disruption of the actin cytoskeleton, increasing intracellular levels of cyclic AMP, or inactivation of signaling pathways linked to mitogen-activated protein kinase kinases. Recently, B. anthracis has gleaned much notoriety as a biowarfare/bioterrorism agent, and of primary interest has been the edema and lethal toxins, their role in anthrax, as well as the development of efficacious vaccines and therapeutics targeting these virulence factors and ultimately B. anthracis. This review comprehensively surveys the literature and discusses the similarities, as well as distinct differences, between each Clostridium and Bacillus binary toxin in terms of their biochemistry, biology, genetics, structure, and applications in science and medicine. The information may foster future studies that aid novel vaccine and drug development, as well as a better understanding of a conserved intoxication process utilized by various gram-positive, spore-forming bacteria.

Association of Cdc42/N-WASP/Arp2/3 Signaling Pathway with Golgi Membranes

Recent findings indicate that Cdc42 regulates Golgi-to-ER (endoplasmic reticulum) protein transport through N-WASP and Arp2/3 (Luna et al. 2002, Mol. Biol. Cell, 13:866-879). To analyse the components of the Cdc42-governed signaling pathway in the secretory pathway, we localized Cdc42, N-WASP and Arp2/3 in the Golgi complex by cryoimmunoelectron microscopy. Cdc42 is found throughout the Golgi stack, particularly in cis/middle cisternae, whereas N-WASP and Arp3 (a component of the Arp2/3 complex) are restricted to cis cisternae. Arp3 also colocalized in peri-Golgi tubulovesicular structures with either KDEL receptor or GM130. Even though Arp3 is not found in TGN46-positive cisternal elements, a small fraction of Arp3-labeled tubulo-vesicular elements showed TGN46 labeling. Active Cdc42 (GTP-bound form) induced relocation of N-WASP and Arp3 to the lateral rims of Golgi cisternae. These results show that the actin nucleation and polymerization signaling pathway governed by Cdc42/N-WASP/Arp operates in the Golgi complex of mammalian cells, further implicating actin dynamics in Golgi-associated membrane trafficking.

Rho-modifying C3-like ADP-ribosyltransferases

C3-like exoenzymes comprise a family of seven bacterial ADP-ribosyltransferases, which selectively modify RhoA, B, and C at asparagine-41. Crystal structures of C3 exoenzymes are available, allowing novel insights into the structure-function relationships of these exoenzymes. Because ADP-ribosylation specifically inhibits the biological functions of the low-molecular mass GTPases, C3 exoenzymes are established pharmacological tools to study the cellular functions of Rho GTPases. Recent studies, however, indicate that the functional consequences of C3-induced ADP-ribosylation are more complex than previously suggested. In the present review the basic properties of C3 exoenzymes are briefly summarized and new findings are reviewed.

The yeast dynamin-related GTPase Vps1p functions in the organization of the actin cytoskeleton via interaction with Sla1p

Recent studies have suggested that the function of the large GTPase dynamin in endocytosis in mammalian cells may comprise a modulation of actin cytoskeleton. The role of dynamin in actin cytoskeleton organization in the yeast Saccharomyces cerevisiae has remained undefined. In this report, we found that one of the yeast dynamin-related proteins, Vps1p, is required for normal actin cytoskeleton organization. At both permissive and non-permissive temperatures, the vps1 mutants exhibited various degrees of phenotypes commonly associated with actin cytoskeleton defects: depolarized and aggregated actin structures, hypersensitivity to the actin cytoskeleton toxin latrunculin-A, randomized bud site selection and chitin deposition, and impaired efficiency in the internalization of membrane receptors. Over-expression of the GTPase mutants of vps1 also led to actin abnormalities. Consistent with these actin-related defects, Vps1p was found to interact physically, and partially co-localize, with the actin-regulatory protein Sla1p. The normal cellular localization of Sla1p required Vps1p and could be altered by over-expression of a region of Vps1p that was involved in the interaction with Sla1p. The same region also promoted mis-sorting of the vacuolar protein carboxypeptidase Y upon over-expression. These findings suggest that the functions of the dynamin-related protein Vps1p in actin cytoskeleton dynamics and vacuolar protein sorting are probably related to each other.

Cytosol-derived proteins are sufficient for Arp2/3 recruitment and ARF/coatomer-dependent actin polymerization on Golgi membranes

The actin cytoskeleton has been implicated in protein trafficking at the Golgi apparatus and in Golgi orientation and morphology. Actin dynamics at the Golgi are regulated in part by recruiting Cdc42 or Rac to the membrane through a binding interaction with the coatomer-coated (COPI)-vesicle coat protein, coatomer. This leads to actin polymerization through the effector, N-WASP and the Arp2/3 complex. Here, we have used reconstitution of vesicle budding to test whether Arp2/3 is recruited to membranes during the formation of COPI vesicles. Our results revealed that ARF1 activation leads to greatly increased Arp3 levels on the membranes. Coatomer-bound Cdc42 and pre-existing F-actin are important for Arp2/3 binding. ARF1-dependent Arp2/3 recruitment and actin polymerization can be reconstituted on liposomal membranes, indicating that no membrane proteins are necessary. These results show that activated ARF1 can stimulate Arp2/3 recruitment to Golgi membranes through coatomer, Cdc42 or Rac, and N-WASP.

LIMK1 regulates Golgi dynamics, traffic of Golgi-derived vesicles, and process extension in primary cultured neurons

In this study, we examined the subcellular distribution and functions of LIMK1 in developing neurons. Confocal microscopy, subcellular fractionation, and expression of several epitope-tagged LIMK1 constructs revealed that LIMK1 is enriched in the Golgi apparatus and growth cones, with the LIM domain required for Golgi localization and the PDZ domain for its presence at neuritic tips. Overexpression of wild-type LIMK1 suppresses the formation of trans-Golgi derived tubules, and prevents cytochalasin D-induced Golgi fragmentation, whereas that of a kinase-defective mutant has the opposite effect. Transfection of wild-type LIMK1 accelerates axon formation and enhances the accumulation of Par3/Par6, insulin-like growth factor (IGF)1 receptors, and neural cell adhesion molecule (NCAM) at growth cones, while inhibiting the Golgi export of synaptophysin-containing vesicles. These effects were dependent on the Golgi localization of LIMK1, paralleled by an increase in cofilin phosphorylation and phalloidin staining in the region of the Golgi apparatus, and prevented by coexpression of constitutive active cofilin. The long-term overexpression of LIMK1 produces growth cone collapse and axon retraction, an effect that is dependent on its growth cone localization. Together, our results suggest an important role for LIMK1 in axon formation that is related with its ability to regulate Golgi dynamics, membrane traffic, and actin cytoskeletal organization.

The actin-based motor myosin Va is a component of the acroplaxome, an acrosome-nuclear envelope junctional plate, and of manchette-associated vesicles

Protein and vesicle cargos can be mobilized during spermiogenesis by intramanchette transport utilizing microtubule-based protein motors (kinesins and dyneins). However, actin-based unconventional myosin motors may also play a significant role in targeting vesicle cargos to subcellular compartments during sperm development. Here we report that myosin Va, an actin-based motor protein, is a component of the acroplaxome of rodent spermatids. The acroplaxome is an F-actin/keratin-containing scaffold plate with a marginal ring fastening the caudal recess of the developing acrosome to the nuclear envelope during spermatid nuclear shaping. In contrast to the acroplaxome, fluorescently labeled phalloidin does not produce an obvious F-actin signal in the manchette. However, immunogold electron microscopy detects moderate but specific beta-actin immunoreactivity along interconnected tube-like bundles of manchette microtubules. We also show that the membrane of vesicles co-fractionated with intact manchettes by sucrose gradient ultracentrifugation display immunogold-labeled myosin Va. Myosin Va vesicle localization is known to correlate with Rab proteins, monomeric GTPases of the Ras superfamily which recruit myosin Va/VIIa motor proteins through intermediate proteins. RT-PCR analysis demonstrates that transcripts for Rab27a and Rab27b and Slac2-c (a protein that links Rab27a/b to myosin Va/VIIa) are expressed in testis. These results indicate that two independent cytoskeletal tracks, F-actin in the acroplaxome and presumably in the manchette, and manchette microtubules, may facilitate short-range (from the Golgi to the acrosome) and long-range (from the manchette to the centrosome and axoneme) mobilization of appropriate cargos during spermiogenesis.

Maurer’s clefts—a novel secretory organelle?

During intra-erythrocytic development, the human malarial parasite Plasmodium falciparum extensively remodels its adopted cellular home by exporting proteins beyond the confines of its own plasma membrane, but is, however, faced with a major problem: the lack of an endogenous protein trafficking machinery within the host erythrocyte. Thus, in order to export proteins the parasite has to install its own protein export system within the host erythrocyte. A growing body of evidence suggests that Maurer's clefts, parasite-derived membranous structures in the cytosol of the host cell, are a crucial component of this protein sorting and trafficking machinery. In this review we summarize our current understanding of the ultra-structure of Maurer's clefts and their role in protein transport process.

Targeting of a tropomyosin isoform to short microfilaments associated with the Golgi complex

A growing body of evidence suggests that the Golgi complex contains an actin-based filament system. We have previously reported that one or more isoforms from the tropomyosin gene Tm5NM (also known as gamma-Tm), but not from either the alpha- or beta-Tm genes, are associated with Golgi-derived vesicles (Heimann et al., (1999). J. Biol. Chem. 274, 10743-10750). We now show that Tm5NM-2 is sorted specifically to the Golgi complex, whereas Tm5NM-1, which differs by a single alternatively spliced internal exon, is incorporated into stress fibers. Tm5NM-2 is localized to the Golgi complex consistently throughout the G1 phase of the cell cycle and it associates with Golgi membranes in a brefeldin A-sensitive and cytochalasin D-resistant manner. An actin antibody, which preferentially reacts with the ends of microfilaments, newly reveals a population of short actin filaments associated with the Golgi complex and particularly with Golgi-derived vesicles. Tm5NM-2 is also found on these short microfilaments. We conclude that an alternative splice choice can restrict the sorting of a tropomyosin isoform to short actin filaments associated with Golgi-derived vesicles. Our evidence points to a role for these Golgi-associated microfilaments in vesicle budding at the level of the Golgi complex.

Loss of myosin VI reduces secretion and the size of the Golgi in fibroblasts from Snell's waltzer mice

Golgi morphology and function are dependent on an intact microtubule and actin cytoskeleton. Myosin VI, an unusual actin-based motor protein moving towards the minus ends of actin filaments, has been localized to the Golgi complex at the light and electron microscopic level. Myosin VI is present in purified Golgi membranes as a peripheral membrane protein, targeted by its globular tail domain. To investigate the function of myosin VI at the Golgi complex, immortal fibroblastic cell lines of Snell's waltzer mice lacking myosin VI were established. In these cell lines, where myosin VI is absent, the Golgi complex is reduced in size by approximately 40% compared with wild-type cells. Furthermore, protein secretion of a reporter protein from Snell's waltzer cells is also reduced by 40% compared with wild-type cells. Rescue experiments showed that fully functional myosin VI was able to restore Golgi complex morphology and protein secretion in Snell's waltzer cells to the same level as that observed in wild-type cells.

Myosin motors and not actin comets are mediators of the actin-based Golgi-to-endoplasmic reticulum protein transport

We have previously reported that actin filaments are involved in protein transport from the Golgi complex to the endoplasmic reticulum. Herein, we examined whether myosin motors or actin comets mediate this transport. To address this issue we have used, on one hand, a combination of specific inhibitors such as 2,3-butanedione monoxime (BDM) and 1-[5-isoquinoline sulfonyl]-2-methyl piperazine (ML7), which inhibit myosin and the phosphorylation of myosin II by the myosin light chain kinase, respectively; and a mutant of the nonmuscle myosin II regulatory light chain, which cannot be phosphorylated (MRLC2(AA)). On the other hand, actin comet tails were induced by the overexpression of phosphatidylinositol phosphate 5-kinase. Cells treated with BDM/ML7 or those that express the MRLC2(AA) mutant revealed a significant reduction in the brefeldin A (BFA)-induced fusion of Golgi enzymes with the endoplasmic reticulum (ER). This delay was not caused by an alteration in the formation of the BFA-induced tubules from the Golgi complex. In addition, the Shiga toxin fragment B transport from the Golgi complex to the ER was also altered. This impairment in the retrograde protein transport was not due to depletion of intracellular calcium stores or to the activation of Rho kinase. Neither the reassembly of the Golgi complex after BFA removal nor VSV-G transport from ER to the Golgi was altered in cells treated with BDM/ML7 or expressing MRLC2(AA). Finally, transport carriers containing Shiga toxin did not move into the cytosol at the tips of comet tails of polymerizing actin. Collectively, the results indicate that 1) myosin motors move to transport carriers from the Golgi complex to the ER along actin filaments; 2) nonmuscle myosin II mediates in this process; and 3) actin comets are not involved in retrograde transport.

Cdc42 and Rac1 regulate late events in Salmonella typhimurium-induced interleukin-8 secretion from polarized epithelial cells

Salmonella typhimurium colonization of the intestinal epithelium initiates biochemical cross-talk between pathogen and host that results in the secretion of chemokines, such as interleukin (IL)-8, that direct neutrophil migration to the site of infection. In nonpolarized cells, Rac1 and Cdc42 have been shown to regulate both bacterial invasion and signaling events leading to nuclear responses and IL-8 secretion. However, because the underlying actin cytoskeleton and the associated signaling machinery are distributed much differently in polarized epithelial cells, we used polarized Madin-Darby canine kidney monolayers to investigate the role of Rac1 and Cdc42 in S. typhimurium-induced pro-inflammatory responses in the more physiologically relevant polarized state. In Madin-Darby canine kidney monolayers expressing dominant-negative Rac1 or Cdc42, both Salmonella- and tumor necrosis factor alpha-induced activation of NFkappaB and mitogen-activated protein kinase signaling cascades proceeded normally, but IL-8 secretion was inhibited. We found that Rac1 and Cdc42 were not involved in early pro-inflammatory signaling events, as in nonpolarized cells, but rather regulated the basolateral exocytosis and secretion of IL-8. In contrast, dominant-negative Rac1 inhibited apical actin pedestal formation, indicating that pedestal formation and nuclear signaling for pro-inflammatory activation are not linked. These findings indicate that there are significant differences in the requirements of pathogen-induced host cell signaling pathways in polarized and nonpolarized cells.

Distinct effect of actin cytoskeleton disassembly on exo- and endocytic events in a membrane patch of rat melanotrophs

We used the cell-attached mode of patch-clamp technique to measure discrete attofarad steps in membrane capacitance (C(m)), reporting area changes in the plasma membrane due to unitary exocytic and endocytic events. To investigate the role of the actin cytoskeleton in elementary exocytic and endocytic events, neuroendocrine rat melanotrophs were treated with Clostridium spiroforme toxin (CST), which specifically depolymerises F-actin. The average amplitude of exocytic events was not significantly different in control and in CST-treated cells. However, the amplitude of endocytic events was significantly smaller in CST-treated cells as compared to controls. The frequency of exocytic events increased by 2-fold in CST-treated cells relative to controls. In control cells the average frequency of exocytic events (upsilon;(exo)) was lower than the frequency of endocytic events (upsilon;(endo)) with a ratio upsilon;(exo)/upsilon;(endo) < 1. In the toxin treated cells, the predominant process was exocytosis with a ratio (upsilon;(exo)/upsilon;(endo) > 1). To study the coupling between the two processes, the slopes of regression lines relating upsilon;(exo) and upsilon;(endo) in a given patch of membrane were studied. The slopes of regression lines were similar, whereas the line intercepts with the y-axis were significantly different. The increased frequency of unitary exocytic events in CST-treated cells is consistent with the view, that the actin cytoskeleton acts as a barrier for exocytosis. While the disassembly of the actin cytoskeleton diminishes the size of unitary endocytic events, suggesting an important role of the actin cytoskeleton in determining the size of endocytic vesicles, the coupling between exocytosis and endocytosis in a given patch of membrane was independent of the state of the actin cytoskeleton.

Ethanol impairs monosaccharide uptake and glycosylation in cultured rat astrocytes

Astrocyte and glial-neuron interactions have a critical role in brain development, which is partially mediated by glycoproteins, including adhesion molecules and growth factors. Ethanol affects the synthesis, intracellular transport, subcellular distribution and secretion of these glycoproteins, suggesting alterations in glycosylation. We analyzed the effect of long-term exposure to low doses of ethanol (30 mm) on glycosylation process in growing cultured astrocytes in vitro. Cells were incubated for short (5 min) and long (90 min) periods with several radioactively labeled carbohydrate precursors. The uptake, kinetics and metabolism of these precursors, as well as the radioactivity distribution in protein gels were analyzed. The levels of GLUT1 and mannosidase II were also determined. Ethanol increased the uptake of monosaccharides and the protein levels of GLUT1 but decreased those of mannosidase II. It altered the carbohydrate moiety of proteins and increased cell surface glycoproteins containing terminal non-reduced mannose. These results indicate that ethanol impairs glycosylation in rat astrocytes, thus disrupting brain development.

Vesicular transport: the core machinery of COPI recruitment and budding

Vesicular transport is the predominant mechanism for exchange of proteins and lipids between membrane-bound organelles in eukaryotic cells. Golgi-derived COPI-coated vesicles are involved in several vesicular transport steps, including bidirectional transport within the Golgi and recycling to the ER. Recent work has shed light on the mechanism of COPI vesicle biogenesis, in particular the machinery required for vesicle formation. The new findings have allowed us to generate a model that covers the cycle of coat recruitment, coat polymerization, vesicle budding and uncoating.

Regulating the actin cytoskeleton during vesicular transport

Although the actin cytoskeleton is widely believed to play an important role in intracellular protein transport, this role is poorly understood. Recently, progress has been made toward identifying specific actin-binding proteins and signaling molecules involved in regulating actin structures that function in the secretory pathway. Studies on coat protomer I (COPI)-mediated transport at the Golgi apparatus and on clathrin-mediated endocytosis have been particularly informative in identifying such mechanisms. Important similarities between actin regulation at the Golgi and at the plasma membrane have been uncovered. The studies reveal that ADP-ribosylation factor and vesicle coat proteins are able to act through the Rho-family GTP-binding proteins, Cdc42 and Rac, and several specific actin-binding proteins to direct actin assembly through the Arp2/3 complex. Efficient function of the secretory pathway is likely to require precise temporal regulation among transport-vesicle assembly, vesicle scission, and the targeting machinery. It is proposed that numerous actin regulatory mechanisms and the connections between actin signaling and vesicle-coat formation are employed to provide such temporal regulation.

Small GTP-binding protein TC10 differentially regulates two distinct populations of filamentous actin in 3T3L1 adipocytes

TC10 is a member of the Rho family of small GTP-binding proteins that has previously been implicated in the regulation of insulin-stimulated GLUT4 translocation in adipocytes. In a manner similar to Cdc42-stimulated actin-based motility, we have observed that constitutively active TC10 (TC10/Q75L) can induce actin comet tails in Xenopus oocyte extracts in vitro and extensive actin polymerization in the perinuclear region when expressed in 3T3L1 adipocytes. In contrast, expression of TC10/Q75L completely disrupted adipocyte cortical actin, which was specific for TC10, because expression of constitutively active Cdc42 was without effect. The effect of TC10/Q75L to disrupt cortical actin was abrogated after deletion of the amino terminal extension (DeltaN-TC10/Q75L), whereas this deletion retained the ability to induce perinuclear actin polymerization. In addition, alteration of perinuclear actin by expression of TC10/Q75L, a dominant-interfering TC10/T31N mutant or a mutant N-WASP protein (N-WASP/DeltaVCA) reduced the rate of VSV G protein trafficking to the plasma membrane. Furthermore, TC10 directly bound to Golgi COPI coat proteins through a dilysine motif in the carboxyl terminal domain consistent with a role for TC10 regulating actin polymerization on membrane transport vesicles. Together, these data demonstrate that TC10 can differentially regulate two types of filamentous actin in adipocytes dependent on distinct functional domains and its subcellular compartmentalization.

Regulation of protein transport from the Golgi complex to the endoplasmic reticulum by CDC42 and N-WASP

Actin is involved in the organization of the Golgi complex and Golgi-to-ER protein transport in mammalian cells. Little, however, is known about the regulation of the Golgi-associated actin cytoskeleton. We provide evidence that Cdc42, a small GTPase that regulates actin dynamics, controls Golgi-to-ER protein transport. We located GFP-Cdc42 in the lateral portions of Golgi cisternae and in COPI-coated and non-coated Golgi-associated transport intermediates. Overexpression of Cdc42 and its activated form Cdc42V12 inhibited the retrograde transport of Shiga toxin from the Golgi complex to the ER, the redistribution of the KDEL receptor, and the ER accumulation of Golgi-resident proteins induced by the active GTP-bound mutant of Sar1 (Sar1[H79G]). Coexpression of wild-type or activated Cdc42 and N-WASP also inhibited Golgi-to-ER transport, but this was not the case in cells expressing Cdc42V12 and N-WASP(Delta WA), a mutant form of N-WASP that lacks Arp2/3 binding. Furthermore, Cdc42V12 recruited GFP-N-WASP to the Golgi complex. We therefore conclude that Cdc42 regulates Golgi-to-ER protein transport in an N-WASP-dependent manner.

Golgi vesicle proteins are linked to the assembly of an actin complex defined by mAbp1

Recent studies indicate that regulation of the actin cytoskeleton is important for protein trafficking, but its precise role is unclear. We have characterized the ARF1-dependent assembly of actin on the Golgi apparatus. Actin recruitment involves Cdc42/Rac and requires the activation of the Arp2/3 complex. Although the actin-binding proteins mAbp1 (SH3p7) and drebrin share sequence homology, they are differentially segregated into two distinct ARF-dependent actin complexes. The binding of Cdc42 and mAbp1, which localize to the Golgi apparatus, but not drebrin, is blocked by occupation of the p23 cargo-protein-binding site on coatomer. Exogenously expressed mAbp1 is mislocalized and inhibits Golgi transport in whole cells. The ability of ARF, vesicle-coat proteins, and cargo to direct the assembly of cytoskeletal structures helps explain how only a handful of vesicle types can mediate the numerous trafficking steps in the cell.

Myr 8, a novel unconventional myosin expressed during brain development associates with the protein phosphatase catalytic subunits 1alpha and 1gamma1

Directed neuronal, astroglial, and oligodendroglial cell migrations comprise a prominent feature of mammalian brain development. Because molecular motor proteins have been implicated in a wide spectrum of processes associated with cell motility, we initiated studies to define the pool of myosins in migrating cerebellar granule neurons and type-1 neocortical astrocytes. Our analyses identified two isoforms of a novel unconventional myosin, which we have cloned, sequenced, and designated myr 8a and 8b (eighth unconventional myosin from rat). Phylogenetic analysis indicates that myr 8 myosins comprise a new class of myosins, which we have designated class XVI. The head domain contains a large N-terminal extension composed of multiple ankyrin repeats, which are implicated in mediating an association with the protein phosphatase 1 (PP1) catalytic subunits 1alpha and 1gamma. The motor domain is followed by a single putative light-chain binding domain. The tail domain of myr 8a is comparatively short with a net positive charge, whereas the tail domain of myr 8b is extended, bears an overall neutral charge, and reveals several stretches of poly-proline residues. Neither the myr 8a nor the myr 8b sequence reveals alpha-helical coiled-coil motifs, suggesting that these myosins exist as monomers. Both immunoblot and Northern blot analyses indicate that myr 8b is the predominant isoform expressed in brain, principally at developmental time periods. The structural features and restricted expression patterns suggest that members of this novel class of unconventional myosins comprise a mechanism to target selectively the protein phosphatase 1 catalytic subunits 1alpha and/or 1gamma in developing brain.

Plant Golgi-associated vesicles contain a novel alpha-actinin-like protein

By using Western blotting, immunofluorescence and immunogold labeling, a novel alpha-actinin-like protein was found in pollen and pollen tubes of Lilium davidii, a model system for cytoskeleton and Golgi apparatus study of plant. As measured by Western blotting, the molecular mass of the a-actinin-like protein was about 80 kDa. Under confocal laser scanning microscopy after immunofluorescence labeling, the distribution of the alpha-actinin-like protein appeared punctated in the cytoplasm of the pollen and pollen tubes. When double labeled, the protein was co-localized with Golgi 58K protein. In addition, some fraction of the alpha-actinin-like protein was found to co-distribute with F-actin bundles in the pollen tubes. Additional studies with immuno-gold labeling and transmission electron microscopy revealed that the alpha-actinin-like protein bound mainly to the membranes of Golgi-associated vesicles. When the pollen tubes were treated with Brefeldin A (BFA), the a-actinin-like proteins were dispersed into the cytoplasm, and the growth of pollen tubes was inhibited. After BFA was removed, the protein was reversibly recovered on the Golgi apparatus. These results suggest that the novel alpha-actinin-like protein is a BFA-sensitive protein on the membranes of Golgi-associated vesicles, and may participate in Golgi-associated vesicles budding and/or sorting, together with actin microfilaments.