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

Canonical and noncanonical Wnt signaling: Multilayered mediators, signaling mechanisms and major signaling crosstalk

Wnt signaling plays a major role in regulating cell proliferation and differentiation. The Wnt ligands are a family of 19 secreted glycoproteins that mediate their signaling effects via binding to Frizzled receptors and LRP5/6 coreceptors and transducing the signal either through β-catenin in the canonical pathway or through a series of other proteins in the noncanonical pathway. Many of the individual components of both canonical and noncanonical Wnt signaling have additional functions throughout the body, establishing the complex interplay between Wnt signaling and other signaling pathways. This crosstalk between Wnt signaling and other pathways gives Wnt signaling a vital role in many cellular and organ processes. Dysregulation of this system has been implicated in many diseases affecting a wide array of organ systems, including cancer and embryological defects, and can even cause embryonic lethality. The complexity of this system and its interacting proteins have made Wnt signaling a target for many therapeutic treatments. However, both stimulatory and inhibitory treatments come with potential risks that need to be addressed. This review synthesized much of the current knowledge on the Wnt signaling pathway, beginning with the history of Wnt signaling. It thoroughly described the different variants of Wnt signaling, including canonical, noncanonical Wnt/PCP, and the noncanonical Wnt/Ca2+ pathway. Further description involved each of its components and their involvement in other cellular processes. Finally, this review explained the various other pathways and processes that crosstalk with Wnt signaling.

Branching out in different directions: Emerging cellular functions for the Arp2/3 complex and WASP-family actin nucleation factors

The actin cytoskeleton impacts practically every function of a eukaryotic cell. Historically, the best-characterized cytoskeletal activities are in cell morphogenesis, motility, and division. The structural and dynamic properties of the actin cytoskeleton are also crucial for establishing, maintaining, and changing the organization of membrane-bound organelles and other intracellular structures. Such activities are important in nearly all animal cells and tissues, although distinct anatomical regions and physiological systems rely on different regulatory factors. Recent work indicates that the Arp2/3 complex, a broadly expressed actin nucleator, drives actin assembly during several intracellular stress response pathways. These newly described Arp2/3-mediated cytoskeletal rearrangements are coordinated by members of the Wiskott-Aldrich Syndrome Protein (WASP) family of actin nucleation-promoting factors. Thus, the Arp2/3 complex and WASP-family proteins are emerging as crucial players in cytoplasmic and nuclear activities including autophagy, apoptosis, chromatin dynamics, and DNA repair. Characterizations of the functions of the actin assembly machinery in such stress response mechanisms are advancing our understanding of both normal and pathogenic processes, and hold great promise for providing insights into organismal development and interventions for disease.

Hepatitis B Virus Utilizes a Retrograde Trafficking Route via the Trans-Golgi Network to Avoid Lysosomal Degradation

BACKGROUND & AIMS

Hepatitis B virus (HBV) infection is difficult to cure owing to the persistence of covalently closed circular viral DNA (cccDNA). We performed single-cell transcriptome analysis of newly established HBV-positive and HBV-negative hepatocellular carcinoma cell lines and found that dedicator of cytokinesis 11 (DOCK11) was crucially involved in HBV persistence. However, the roles of DOCK11 in the HBV lifecycle have not been clarified.

METHODS

The cccDNA levels were measured by Southern blotting and real-time detection polymerase chain reaction in various hepatocytes including PXB cells by using an HBV-infected model. The retrograde trafficking route of HBV capsid was investigated by super-resolution microscopy, proximity ligation assay, and time-lapse analysis. The downstream molecules of DOCK11 and underlying mechanism were examined by liquid chromatography-tandem mass spectrometry, immunoblotting, and enzyme-linked immunosorbent assay.

RESULTS

The cccDNA levels were strongly increased by DOCK11 overexpression and repressed by DOCK11 suppression. Interestingly, DOCK11 functionally associated with retrograde trafficking proteins in the trans-Golgi network (TGN), Arf-GAP with GTPase domain, ankyrin repeat, and pleckstrin homology domain-containing protein 2 (AGAP2), and ADP-ribosylation factor 1 (ARF1), together with HBV capsid, to open an alternative retrograde trafficking route for HBV from early endosomes (EEs) to the TGN and then to the endoplasmic reticulum (ER), thereby avoiding lysosomal degradation. Clinically, DOCK11 levels in liver biopsies from patients with chronic hepatitis B were significantly reduced by entecavir treatment, and this reduction correlated with HBV surface antigen levels.

CONCLUSIONS

HBV uses a retrograde trafficking route via EEs-TGN-ER for infection that is facilitated by DOCK11 and serves to maintain cccDNA. Therefore, DOCK11 is a potential therapeutic target to prevent persistent HBV infection.

WASP family proteins: Molecular mechanisms and implications in human disease

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family play a central role in regulating actin cytoskeletal dynamics in a wide range of cellular processes. Genetic mutations or misregulation of these proteins are tightly associated with many diseases. The WASP-family proteins act by transmitting various upstream signals to their conserved WH2-Central-Acidic (WCA) peptide sequence at the C-terminus, which in turn binds to the Arp2/3 complex to stimulate the formation of branched actin networks at membranes. Despite this common feature, the regulatory mechanisms and cellular functions of distinct WASP-family proteins are very different. Here, we summarize and clarify our current understanding of WASP-family proteins and how disruption of their functions is related to human disease.

Ubiquitination-Dependent Regulation of Small GTPases in Membrane Trafficking: From Cell Biology to Human Diseases

Membrane trafficking is critical for cellular homeostasis, which is mainly carried out by small GTPases, a class of proteins functioning in vesicle budding, transport, tethering and fusion processes. The accurate and organized membrane trafficking relies on the proper regulation of small GTPases, which involves the conversion between GTP- and GDP-bound small GTPases mediated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Emerging evidence indicates that post-translational modifications (PTMs) of small GTPases, especially ubiquitination, play an important role in the spatio-temporal regulation of small GTPases, and the dysregulation of small GTPase ubiquitination can result in multiple human diseases. In this review, we introduce small GTPases-mediated membrane trafficking pathways and the biological processes of ubiquitination-dependent regulation of small GTPases, including the regulation of small GTPase stability, activity and localization. We then discuss the dysregulation of small GTPase ubiquitination and the associated human membrane trafficking-related diseases, focusing on the neurological diseases and infections. An in-depth understanding of the molecular mechanisms by which ubiquitination regulates small GTPases can provide novel insights into the membrane trafficking process, which knowledge is valuable for the development of more effective and specific therapeutics for membrane trafficking-related human diseases.

Regulation of the Actin Cytoskeleton via Rho GTPase Signalling in Dictyostelium and Mammalian Cells: A Parallel Slalom

Both Dictyostelium amoebae and mammalian cells are endowed with an elaborate actin cytoskeleton that enables them to perform a multitude of tasks essential for survival. Although these organisms diverged more than a billion years ago, their cells share the capability of chemotactic migration, large-scale endocytosis, binary division effected by actomyosin contraction, and various types of adhesions to other cells and to the extracellular environment. The composition and dynamics of the transient actin-based structures that are engaged in these processes are also astonishingly similar in these evolutionary distant organisms. The question arises whether this remarkable resemblance in the cellular motility hardware is accompanied by a similar correspondence in matching software, the signalling networks that govern the assembly of the actin cytoskeleton. Small GTPases from the Rho family play pivotal roles in the control of the actin cytoskeleton dynamics. Indicatively, Dictyostelium matches mammals in the number of these proteins. We give an overview of the Rho signalling pathways that regulate the actin dynamics in Dictyostelium and compare them with similar signalling networks in mammals. We also provide a phylogeny of Rho GTPases in Amoebozoa, which shows a variability of the Rho inventories across different clades found also in Metazoa.

Cross-Kingdom Activation of Vibrio Toxins by ADP-Ribosylation Factor Family GTPases

Pathogenic Vibrio species use many different approaches to subvert, attack, and undermine the host response. The toxins they produce are often responsible for the devastating effects associated with their diseases. These toxins target a variety of host proteins, which leads to deleterious effects, including dissolution of cell organelle integrity and inhibition of protein secretion. Becoming increasingly prevalent as cofactors for Vibrio toxins are proteins of the small GTPase families. ADP-ribosylation factor small GTPases (ARFs) in particular are emerging as a common host cofactor necessary for full activation of Vibrio toxins. While ARFs are not the direct target of Vibrio cholerae cholera toxin (CT), ARF binding is required for its optimal activity as an ADP-ribosyltransferase. The makes caterpillars floppy (MCF)-like and the domain X (DmX) effectors of the Vibrio vulnificus multifunctional autoprocessing repeats-in-toxin (MARTX) toxin also both require ARFs to initiate autoprocessing and activation as independent effectors. ARFs are ubiquitously expressed in eukaryotes and are key regulators of many cellular processes, and as such they are ideal cofactors for Vibrio pathogens that infect many host species. In this review, we cover in detail the known Vibrio toxins that use ARFs as cross-kingdom activators to both stimulate and optimize their activity. We further discuss how these contrast to toxins and effectors from other bacterial species that coactivate, stimulate, or directly modify host ARFs as their mechanisms of action.

Targeting and signaling of Rho of plants guanosine triphosphatases require synergistic interaction between guanine nucleotide inhibitor and vesicular trafficking

Most eukaryotic cells are polarized. Common toolbox regulating cell polarization includes Rho guanosine triphosphatases (GTPases), in which spatiotemporal activation is regulated by a plethora of regulators. Rho of plants (ROPs) are the only Rho GTPases in plants. Although vesicular trafficking was hinted in the regulation of ROPs, it was unclear where vesicle-carried ROP starts, whether it is dynamically regulated, and which components participate in vesicle-mediated ROP targeting. In addition, although vesicle trafficking and guanine nucleotide inhibitor (GDI) pathways in Rho signaling have been extensively studied in yeast, it is unknown whether the two pathways interplay. Unclear are also cellular and developmental consequences of their interaction in multicellular organisms. Here, we show that the dynamic targeting of ROP through vesicles requires coat protein complex II and ADP-ribosylation factor 1-mediated post-Golgi trafficking. Trafficking of vesicle-carried ROPs between the plasma membrane and the trans-Golgi network is mediated through adaptor protein 1 and sterol-mediated endocytosis. Finally, we show that GDI and vesicle trafficking synergistically regulate cell polarization and ROP targeting, suggesting that the establishment and maintenance of cell polarity is regulated by an evolutionarily conserved mechanism.

Emerging roles for Rho GTPases operating at the Golgi complex

Rho GTPases are known to play an essential role in fundamental processes such as defining cell shape, polarity and migration. As such, the majority of Rho GTPases localize and function at, or close to, the plasma membrane. However, it is becoming increasingly clear that a number of Rho family proteins are also associated with the Golgi complex, where they not only regulate events at this organelle but also more widely across the cell. Given the central location of this organelle, and the numerous membrane trafficking pathways that connect it to both the endocytic and secretory systems of cells, it is clear that the Golgi is fundamental for maintaining cellular homoeostasis. In this review, we describe these GTPases in the context of how they regulate Golgi architecture, membrane trafficking into and away from this organelle, and cell polarity and migration. We summarize the key findings that show the growing importance of the pool of Rho GTPases associated with Golgi function, namely Cdc42, RhoA, RhoD, RhoBTB1 and RhoBTB3, and we discuss how they act in concert with other key families of molecules associated with the Golgi, including Rab GTPases and matrix proteins.

RNA Interference Screening Identifies Novel Roles for RhoBTB1 and RhoBTB3 in Membrane Trafficking Events in Mammalian Cells

In the endomembrane system of mammalian cells, membrane traffic processes require a high degree of regulation in order to ensure their specificity. The range of molecules that participate in trafficking events is truly vast, and much attention to date has been given to the Rab family of small GTPases. However, in recent years, a role in membrane traffic for members of the Rho GTPase family, in particular Cdc42, has emerged. This prompted us to develop and apply an image-based high-content screen, initially focussing on the Golgi complex, using RNA interference to systematically perturb each of the 21 Rho family members and assess their importance to the overall organisation of this organelle. Analysis of our data revealed previously unreported roles for two atypical Rho family members, RhoBTB1 and RhoBTB3, in membrane traffic events. We find that depletion of RhoBTB3 affects the morphology of the Golgi complex and causes changes in the trafficking speeds of carriers operating at the interface of the Golgi and endoplasmic reticulum. In addition, RhoBTB3 was found to be present on these carriers. Depletion of RhoBTB1 was also found to cause a disturbance to the Golgi architecture, however, this phenotype seems to be linked to endocytosis and retrograde traffic pathways. RhoBTB1 was found to be associated with early endosomal intermediates, and changes in the levels of RhoBTB1 not only caused profound changes to the organisation and distribution of endosomes and lysosomes, but also resulted in defects in the delivery of two different classes of cargo molecules to downstream compartments. Together, our data reveal new roles for these atypical Rho family members in the endomembrane system.

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.

Spatiotemporal Control of Intracellular Membrane Trafficking by Rho GTPases

As membrane-associated master regulators of cytoskeletal remodeling, Rho GTPases coordinate a wide range of biological processes such as cell adhesion, motility, and polarity. In the last years, Rho GTPases have also been recognized to control intracellular membrane sorting and trafficking steps directly; however, how Rho GTPase signaling is regulated at endomembranes is still poorly understood. In this review, we will specifically address the local Rho GTPase pools coordinating intracellular membrane trafficking with a focus on the endo- and exocytic pathways. We will further highlight the spatiotemporal molecular regulation of Rho signaling at endomembrane sites through Rho regulatory proteins, the GEFs and GAPs. Finally, we will discuss the contribution of dysregulated Rho signaling emanating from endomembranes to the development and progression of cancer.

A New Look at the Functional Organization of the Golgi Ribbon

A characteristic feature of vertebrate cells is a Golgi ribbon consisting of multiple cisternal stacks connected into a single-copy organelle next to the centrosome. Despite numerous studies, the mechanisms that link the stacks together and the functional significance of ribbon formation remain poorly understood. Nevertheless, these questions are of considerable interest, since there is increasing evidence that Golgi fragmentation – the unlinking of the stacks in the ribbon – is intimately connected not only to normal physiological processes, such as cell division and migration, but also to pathological states, including neurodegeneration and cancer. Challenging a commonly held view that ribbon architecture involves the formation of homotypic tubular bridges between the Golgi stacks, we present an alternative model, based on direct interaction between the biosynthetic (pre-Golgi) and endocytic (post-Golgi) membrane networks and their connection with the centrosome. We propose that the central domains of these permanent pre- and post-Golgi networks function together in the biogenesis and maintenance of the more transient Golgi stacks, and thereby establish “linker compartments” that dynamically join the stacks together. This model provides insight into the reversible fragmentation of the Golgi ribbon that takes place in dividing and migrating cells and its regulation along a cell surface – Golgi – centrosome axis. Moreover, it helps to understand transport pathways that either traverse or bypass the Golgi stacks and the positioning of the Golgi apparatus in differentiated neuronal, epithelial, and muscle cells.

Abnormal Golgi morphology and decreased COPI function in cells with low levels of SMN

We report here the finding of abnormal Golgi apparatus morphology in motor neuron like cells depleted of SMN as well as Golgi apparatus morphology in SMA patient fibroblasts. Rescue experiments demonstrate that this abnormality is dependent on SMN, but can also be rescued by expression of the COPI coatomer subunit alpha-COP. A motor neuron-like cell line containing an inducible alpha-COP shRNA was created to generate a parallel system to study knockdown of SMN or alpha-COP. Multiple assays of COPI-dependent intracellular trafficking in cells depleted of SMN demonstrate that alpha-COP function is suboptimal, including failed sequestration of plasma membrane proteins, altered binding of mRNA, and defective targeting and transport of Golgi-resident proteins.

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.

MiR-199a Inhibits Secondary Envelopment of Herpes Simplex Virus-1 Through the Downregulation of Cdc42-specific GTPase Activating Protein Localized in Golgi Apparatus

Because several studies have shown that exogenous miR-199a has antiviral effects against various viruses, including herpesviruses, we examined how miR-199a exerts its antiviral effects using epithelial tumour cell lines infected with herpes simplex virus-1 (HSV-1). We found that both miR-199a-5p and -3p impair the secondary envelopment of HSV-1 by suppressing their common target, ARHGAP21, a Golgi-localized GTPase-activating protein for Cdc42. We further found that the trans-cisternae of the Golgi apparatus are a potential membrane compartment for secondary envelopment. Exogenous expression of either pre-miR-199a or sh-ARHGAP21 exhibited shared phenotypes i.e. alteration of Golgi function in uninfected cells, inhibition of HSV-1 secondary envelopment, and reduction of trans-Golgi proteins upon HSV-1 infection. A constitutively active form of Cdc42 also inhibited HSV-1 secondary envelopment. Endogenous levels of miR-199a in epithelial tumour cell lines were negatively correlated with the efficiency of HSV-1 secondary envelopment within these cells. These results suggest that miR-199a is a crucial regulator of Cdc42 activity on Golgi membranes, which is important for the maintenance of Golgi function and for the secondary envelopment of HSV-1 upon its infection.

Spatial analysis of Cdc42 activity reveals a role for plasma membrane-associated Cdc42 in centrosome regulation

The ability of the small GTPase Cdc42 to regulate diverse cellular processes depends on tight spatial control of its activity. Cdc42 function is best understood at the plasma membrane (PM), where it regulates cytoskeletal organization and cell polarization. Active Cdc42 has also been detected at the Golgi, but its role and regulation at this organelle are only partially understood. Here we analyze the spatial distribution of Cdc42 activity by moni-toring the dynamics of the Cdc42 FLARE biosensor using the phasor approach to FLIM-FRET. Phasor analysis revealed that Cdc42 is active at all Golgi cisternae and that this activity is controlled by Tuba and ARHGAP10, two Golgi-associated Cdc42 regulators. To our surprise, FGD1, another Cdc42 GEF at the Golgi, was not required for Cdc42 regulation at the Golgi, although its depletion decreased Cdc42 activity at the PM. Similarly, changes in Golgi morphology did not affect Cdc42 activity at the Golgi but were associated with a substantial reduction in PM-associated Cdc42 activity. Of interest, cells with reduced Cdc42 activity at the PM displayed altered centrosome morphology, suggesting that centrosome regulation may be mediated by active Cdc42 at the PM. Our study describes a novel quantitative approach to determine Cdc42 activity at specific subcellular locations and reveals new regulatory principles and functions of this small GTPase.

Mechanism of cell-intrinsic adaptation to Adams-Oliver Syndrome gene DOCK6 disruption highlights ubiquitin-like modifier ISG15 as a regulator of RHO GTPases

DOCK6 is a RAC1/CDC42 guanine nucleotide exchange factor, however, little is known about its function and sub-cellular localization. DOCK6 regulates the balance between RAC1 and RHOA activity during cell adhesion and is important for CDC42-dependent mitotic chromosome alignment. Surprisingly, a cell intrinsic adaptation mechanism compensates for errors in these DOCK6 functions that arise as a consequence of prolonged DOCK6 depletion or complete removal in DOCK6 knockout cells. Down-regulation of the ubiquitin-like modifier ISG15 accounts for this adaptation. Strikingly, although most other DOCK family proteins are deployed on the plasma membrane, here we show that DOCK6 localizes to the endoplasmic reticulum (ER) in dependence of its DHR-1 domain. ER localization of DOCK6 opens up new insights into its functions.

Globozoospermia and lack of acrosome formation in GM130-deficient mice

Globozoospermia is a common reproductive disorder that causes male infertility in humans, and the malformation or loss of acrosomes is the prominent feature of this disease. Although the acrosome is thought to be derived from the Golgi apparatus, the detailed molecular mechanisms remain unclear. GM130 is a cis-side localized Golgi matrix protein,whereas the physiological functions of this protein remain elusive. Here we showed that inactivation of GM130-caused male infertility in mouse model. The primary defects were the absence of acrosomes, round sperm heads, and aberrant assembly of the mitochondrial sheath, which comprise the characteristic features of human globozoospermia. Further investigation indicated that loss of GM130 did not affect the secretion of pro-acrosomic vesicles, whereas the vesicles failed to fuse into a single large acrosome vesicle. Co-localization of the adaptor protein complex AP1 and trans-Golgi network (TGN) protein TGN46 was disrupted, suggesting that the malformation of acrosomes is most likely due to the defect in the sorting and coating of Golgi-derived pro-acrosomic vesicles. Thus, the GM130-deficient mouse provides a valuable model for investigating the etiology of human globozoospermia.

Rho GTPases operating at the Golgi complex: Implications for membrane traffic and cancer biology

The Golgi complex is the central unit of the secretory pathway, modifying, processing and sorting proteins and lipids to their correct cellular localisation. Changes to proteins at the Golgi complex can have deleterious effects on the function of this organelle, impeding trafficking routes through it, potentially resulting in disease. It is emerging that several Rho GTPase proteins, namely Cdc42, RhoBTB3, RhoA and RhoD are at least in part localised to the Golgi complex, and a number of studies have shown that dysregulation of their levels or activity can be associated with cellular changes which ultimately drive cancer progression. In this mini-review we highlight some of the recent work that explores links between form and function of the Golgi complex, Rho GTPases and cancer.

The Diverse Family of Arp2/3 Complexes

The Arp2/3 complex has so far been considered to be a single seven-subunit protein complex required for actin nucleation and actin filament polymerization in diverse critical cellular functions including phagocytosis, vesicular trafficking and lamellipodia extension. The Arp2/3 complex is also exploited by bacterial pathogens and viruses during cellular infectious processes. Recent studies suggest that some subunits of the complex are dispensable in specific cellular contexts, pointing to the existence of alternative 'hybrid Arp2/3 complexes' containing other components such as vinculin or α-actinin, as well as different isoforms or phosphorylation variants of canonical Arp2/3 subunits. Therefore, this diversity should be now considered when assigning specific Arp2/3 assemblies to different actin-dependent cellular processes.

Novel Coronin7 interactions with Cdc42 and N-WASP regulate actin organization and Golgi morphology

The contribution of the actin cytoskeleton to the unique architecture of the Golgi complex is manifold. An important player in this process is Coronin7 (CRN7), a Golgi-resident protein that stabilizes F-actin assembly at the trans-Golgi network (TGN) thereby facilitating anterograde trafficking. Here, we establish that CRN7-mediated association of F-actin with the Golgi apparatus is distinctly modulated via the small Rho GTPase Cdc42 and N-WASP. We identify N-WASP as a novel interaction partner of CRN7 and demonstrate that CRN7 restricts spurious F-actin reorganizations by repressing N-WASP 'hyperactivity' upon constitutive Cdc42 activation. Loss of CRN7 leads to increased cellular F-actin content and causes a concomitant disruption of the Golgi structure. CRN7 harbours a Cdc42- and Rac-interactive binding (CRIB) motif in its tandem β-propellers and binds selectively to GDP-bound Cdc42N17 mutant. We speculate that CRN7 can act as a cofactor for active Cdc42 generation. Mutation of CRIB motif residues that abrogate Cdc42 binding to CRN7 also fail to rescue the cellular defects in fibroblasts derived from CRN7 KO mice. Cdc42N17 overexpression partially rescued the KO phenotypes whereas N-WASP overexpression failed to do so. We conclude that CRN7 spatiotemporally influences F-actin organization and Golgi integrity in a Cdc42- and N-WASP-dependent manner.

Rab1 recruits WHAMM during membrane remodeling but limits actin nucleation

Small G-proteins regulate the recruitment and activation of WASP-family actin nucleation factors at the plasma membrane. The G-protein Rab1 interacts with the nucleation factor WHAMM to remodel internal membranes into tubules. Unlike other G-proteins that recruit nucleation factors, Rab1 inhibits actin assembly.

Small G-proteins are key regulatory molecules that activate the actin nucleation machinery to drive cytoskeletal rearrangements during plasma membrane remodeling. However, the ability of small G-proteins to interact with nucleation factors on internal membranes to control trafficking processes has not been well characterized. Here we investigated roles for members of the Rho, Arf, and Rab G-protein families in regulating WASP homologue associated with actin, membranes, and microtubules (WHAMM), an activator of Arp2/3 complex–mediated actin nucleation. We found that Rab1 stimulated the formation and elongation of WHAMM-associated membrane tubules in cells. Active Rab1 recruited WHAMM to dynamic tubulovesicular structures in fibroblasts, and an active prenylated version of Rab1 bound directly to an N-terminal domain of WHAMM in vitro. In contrast to other G-protein–nucleation factor interactions, Rab1 binding inhibited WHAMM-mediated actin assembly. This ability of Rab1 to regulate WHAMM and the Arp2/3 complex represents a distinct strategy for membrane remodeling in which a Rab G-protein recruits the actin nucleation machinery but dampens its activity.

Shiga toxins: from structure and mechanism to applications

Shiga toxins are a group of type 2 ribosome-inactivating proteins (RIPs) produced in several types of bacteria. The toxins possess an AB5 structure, which comprises a catalytic A chain with N-glycosidase activity, and five identical B chains and recognize and bind to the target cells with specific carbohydrate moieties. In humans, the major molecular target which recognizes the Shiga toxins is the Gb3 receptor, which is mainly expressed on the cell surface of endothelial cells of the intestine, kidney, and the brain. This causes these organs to be susceptible to the toxicity of Shiga toxins. When a person is infected by Shiga toxin-producing bacteria, the toxin is produced in the gut, translocated to the circulatory system, and carried to the target cells. Toxicity of the toxin causes inflammatory responses and severe cell damages in the intestine, kidneys, and brain, bringing about the hemolytic uremic syndrome (HUS), which can be fatal. The Shiga toxin requires a couple of steps to exert its toxicity to the target cells. After binding with the target cell surface receptor, the toxin requires a complicated process to be transported into the cytosol of the cell before it can approach the ribosomes. The mechanisms for the interactions of the toxin with the cells are described in this review. The consequences of the toxin on the cells are also discussed. It gives an overview of the steps for the toxin to be produced and transported, expression of catalytic activity, and the effects of the toxin on the target cells, as well as effects on the human body.

Cdc42 and Cellular Polarity: Emerging Roles at the Golgi

Cdc42 belongs to the Rho family of small GTPases and plays key roles in cellular events of polarity. This role of Cdc42 has typically been attributed to its function at the plasma membrane. However, Cdc42 also exists at the Golgi complex. Here we summarize major insights that have been gathered in studying the Golgi pool of Cdc42 and propose that Golgi-localized Cdc42 enables the cell to diversify the function of Cdc42, which in some cases represents new roles and in other cases acts to complement the established roles of Cdc42 at the plasma membrane. Studies on how Cdc42 acts at the Golgi also suggest key questions to address in the future.

Mena-GRASP65 interaction couples actin polymerization to Golgi ribbon linking

GRASP65 plays a role in Golgi ribbon formation. Because the gaps between Golgi stacks are heterogeneous and large, it is possible that other proteins may help GRASP65 in ribbon linking. Mena is a novel GRASP65-binding protein that promotes actin elongation and enhances GRASP65 oligomerization to link Golgi stacks into a ribbon.

In mammalian cells, the Golgi reassembly stacking protein 65 (GRASP65) has been implicated in both Golgi stacking and ribbon linking by forming trans-oligomers through the N-terminal GRASP domain. Because the GRASP domain is globular and relatively small, but the gaps between stacks are large and heterogeneous, it remains puzzling how GRASP65 physically links Golgi stacks into a ribbon. To explore the possibility that other proteins may help GRASP65 in ribbon linking, we used biochemical methods and identified the actin elongation factor Mena as a novel GRASP65-binding protein. Mena is recruited onto the Golgi membranes through interaction with GRASP65. Depleting Mena or disrupting actin polymerization resulted in Golgi fragmentation. In cells, Mena and actin were required for Golgi ribbon formation after nocodazole washout; in vitro, Mena and microfilaments enhanced GRASP65 oligomerization and Golgi membrane fusion. Thus Mena interacts with GRASP65 to promote local actin polymerization, which facilitates Golgi ribbon linking.

Regulation of vesicle transport and cell motility by Golgi-localized Dbs

DBS/MCF2L has been recently identified as a risk locus for osteoarthritis. It encodes a guanine nucleotide exchange factor (Dbs) that has been shown to regulate both normal and tumor cell motility. In the current study, we have determined that endogenous Dbs is predominantly expressed as 2 isoforms, a 130 kDa form (Dbs-130) that is localized to the Golgi complex, and an 80 kDa form (Dbs-80) that is localized to the endoplasmic reticulum (ER). We have previously described an inhibitor that binds to the RhoGEF domain of Dbs and blocks its transforming activity. Here we show that the inhibitor localizes to the Golgi, where it specifically interacts with Dbs-130. Inhibition of endogenous Dbs-130 activity is associated with reduced levels of activated Cdc42, enlarged Golgi, and resistance to Brefeldin A-mediated Golgi dispersal, suggesting a role for Dbs in vesicle transport. Cells treated with the inhibitor exhibit normal protein transport from the ER to the Golgi, but are defective in transport from the Golgi to the plasma membrane. Inhibition of Dbs-130 in MDA-MB-231 human breast tumor cells limits motility in both transwell and wound healing assays, but appears to have no effect on the organization of the microtubule cytoskeleton. The reduced motility is associated with a failure to reorient the Golgi toward the leading edge. This is consistent with the Golgi localization, and suggests that the Dbs-130 regulates aspects of the secretory pathway that are required to support cell polarization during directed migration.

The Rho-specific GAP protein DLC3 coordinates endocytic membrane trafficking

Membrane trafficking is known to be coordinated by small GTPases, but the identity of their regulators, the guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that ensure balanced GTPase activation at different subcellular sites is largely elusive. Here, we show in living cells that deleted in liver cancer 3 (DLC3, also known as STARD8) is a functional Rho-specific GAP protein, the loss of which enhances perinuclear RhoA activity. DLC3 is recruited to Rab8-positive membrane tubules and is required for the integrity of the Rab8 and Golgi compartments. Depletion of DLC3 impairs the transport of internalized transferrin to the endocytic recycling compartment (ERC), which is restored by the simultaneous downregulation of RhoA and RhoB. We further demonstrate that DLC3 loss interferes with epidermal growth factor receptor (EGFR) degradation associated with prolonged receptor signaling. Taken together, these findings identify DLC3 as a novel component of the endocytic trafficking machinery, wherein it maintains organelle integrity and regulates membrane transport through the control of Rho activity.

PAR3 and aPKC regulate Golgi organization through CLASP2 phosphorylation to generate cell polarity

A PAR complex (PAR3, PAR6, and aPKC) plays a central role in the establishment of cell polarity. Another polarity protein, CLASP2, binds directly with PAR3 and is phosphorylated by aPKC. Through CLASP2 phosphorylation, aPKC and PAR3 regulate the localization of CLASP2 to the trans-Golgi network, thereby controlling the Golgi organization.

The organization of the Golgi apparatus is essential for cell polarization and its maintenance. The polarity regulator PAR complex (PAR3, PAR6, and aPKC) plays critical roles in several processes of cell polarization. However, how the PAR complex participates in regulating the organization of the Golgi remains largely unknown. Here we demonstrate the functional cross-talk of the PAR complex with CLASP2, which is a microtubule plus-end–tracking protein and is involved in organizing the Golgi ribbon. CLASP2 directly interacted with PAR3 and was phosphorylated by aPKC. In epithelial cells, knockdown of either PAR3 or aPKC induced the aberrant accumulation of CLASP2 at the trans-Golgi network (TGN) concomitantly with disruption of the Golgi ribbon organization. The expression of a CLASP2 mutant that inhibited the PAR3-CLASP2 interaction disrupted the organization of the Golgi ribbon. CLASP2 is known to localize to the TGN through its interaction with the TGN protein GCC185. This interaction was inhibited by the aPKC-mediated phosphorylation of CLASP2. Furthermore, the nonphosphorylatable mutant enhanced the colocalization of CLASP2 with GCC185, thereby perturbing the Golgi organization. On the basis of these observations, we propose that PAR3 and aPKC control the organization of the Golgi through CLASP2 phosphorylation.

Cdc42 induces EGF receptor protein accumulation and promotes EGF receptor nuclear transport and cellular transformation

Cdc42 is a Ras-related small GTP-binding protein. A previous study has shown that Cdc42 binding to the γ subunit of the coatomer protein complex (γCOP) is essential for Cdc42-regulated cellular transformation, but the molecular mechanism involved is not well understood. Here, we demonstrate that constitutively-active Cdc42 binding to γCOP induced the accumulation of epithelial growth factor receptor (EGFR) in the cells, sustained EGF-stimulated extracellular signal-regulated kinase (ERK), JUN amino-terminal kinase (JNK) and phosphoinositide 3-kinase (PI3K) signaling and promoted cell division. Moreover, constitutive Cdc42 activity facilitated the nuclear translocation of EGFR, and this indicates a novel mechanism through which Cdc42 might promote cellular transformation.

Rho GTPases, phosphoinositides, and actin: a tripartite framework for efficient vesicular trafficking

Rho GTPases are well known regulators of the actin cytoskeleton that act by binding and activating actin nucleators. They are therefore involved in many actin-based processes, including cell migration, cell polarity, and membrane trafficking. With the identification of phosphoinositide kinases and phosphatases as potential binding partners or effectors, Rho GTPases also appear to participate in the regulation of phosphoinositide metabolism. Since both actin dynamics and phosphoinositide turnover affect the efficiency and the fidelity of vesicle transport between cell compartments, Rho GTPases have emerged as critical players in membrane trafficking. Rho GTPase activity, actin remodeling, and phosphoinositide metabolism need to be coordinated in both space and time to ensure the progression of vesicles along membrane trafficking pathways. Although most molecular pathways are still unclear, in this review, we will highlight recent advances made in our understanding of how Rho-dependent signaling pathways organize actin dynamics and phosphoinositides and how phosphoinositides potentially provide negative feedback to Rho GTPases during endocytosis, exocytosis and membrane exchange between intracellular compartments.

The pathogenic mechanism of the Mycobacterium ulcerans virulence factor, mycolactone, depends on blockade of protein translocation into the ER

Infection with Mycobacterium ulcerans is characterised by tissue necrosis and immunosuppression due to mycolactone, the necessary and sufficient virulence factor for Buruli ulcer disease pathology. Many of its effects are known to involve down-regulation of specific proteins implicated in important cellular processes, such as immune responses and cell adhesion. We have previously shown mycolactone completely blocks the production of LPS-dependent proinflammatory mediators post-transcriptionally. Using polysome profiling we now demonstrate conclusively that mycolactone does not prevent translation of TNF, IL-6 and Cox-2 mRNAs in macrophages. Instead, it inhibits the production of these, along with nearly all other (induced and constitutive) proteins that transit through the ER. This is due to a blockade of protein translocation and subsequent degradation of aberrantly located protein. Several lines of evidence support this transformative explanation of mycolactone function. First, cellular TNF and Cox-2 can be once more detected if the action of the 26S proteasome is inhibited concurrently. Second, restored protein is found in the cytosol, indicating an inability to translocate. Third, in vitro translation assays show mycolactone prevents the translocation of TNF and other proteins into the ER. This is specific as the insertion of tail-anchored proteins into the ER is unaffected showing that the ER remains structurally intact. Fourth, metabolic labelling reveals a near-complete loss of glycosylated and secreted proteins from treated cells, whereas cytosolic proteins are unaffected. Notably, the profound lack of glycosylated and secreted protein production is apparent in a range of different disease-relevant cell types. These studies provide a new mechanism underlying mycolactone's observed pathological activities both in vitro and in vivo. Mycolactone-dependent inhibition of protein translocation into the ER not only explains the deficit of innate cytokines, but also the loss of membrane receptors, adhesion molecules and T-cell cytokines that drive the aetiology of Buruli ulcer.

Buruli ulcer is a progressive necrotic skin lesion caused by infection with the human pathogen Mycobacterium ulcerans. Mycolactone, a small compound produced by the mycobacterium, is the root cause of the disease pathology, but until now there has been no unifying mechanism explaining why. We have been using a model system to investigate the reason for the selective loss of protein that is a common feature of mycolactone exposure. Specifically, this involves identifying the point at which it stops immune cells making inflammatory mediators. In this work, we demonstrate that mycolactone inhibits production of such proteins by blocking the first step of protein export: translocation into a cellular compartment called the endoplasmic reticulum (ER). Proteins due for export are instead made in the cell cytosol where they are recognised as being in the wrong place and are rapidly degraded, causing a general cessation of the production of proteins that have to travel through the ER, including almost all secreted and surface proteins. This has a profound effect on basic cell functions such as growth, adhesion and survival. Therefore, we have identified the molecular basis underlying the key features of Buruli ulcer, and this will transform our understanding of disease progression.

Molecular mechanisms of cell-cell spread of intracellular bacterial pathogens

Several bacterial pathogens, including Listeria monocytogenes, Shigella flexneri and Rickettsia spp., have evolved mechanisms to actively spread within human tissues. Spreading is initiated by the pathogen-induced recruitment of host filamentous (F)-actin. F-actin forms a tail behind the microbe, propelling it through the cytoplasm. The motile pathogen then encounters the host plasma membrane, forming a bacterium-containing protrusion that is engulfed by an adjacent cell. Over the past two decades, much progress has been made in elucidating mechanisms of F-actin tail formation. Listeria and Shigella produce tails of branched actin filaments by subverting the host Arp2/3 complex. By contrast, Rickettsia forms tails with linear actin filaments through a bacterial mimic of eukaryotic formins. Compared with F-actin tail formation, mechanisms controlling bacterial protrusions are less well understood. However, recent findings have highlighted the importance of pathogen manipulation of host cell–cell junctions in spread. Listeria produces a soluble protein that enhances bacterial protrusions by perturbing tight junctions. Shigella protrusions are engulfed through a clathrin-mediated pathway at ‘tricellular junctions’—specialized membrane regions at the intersection of three epithelial cells. This review summarizes key past findings in pathogen spread, and focuses on recent developments in actin-based motility and the formation and internalization of bacterial protrusions.

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.

Crosstalk of small GTPases at the Golgi apparatus

Small GTPases regulate a wide range of homeostatic processes such as cytoskeletal dynamics, organelle homeostasis, cell migration and vesicle trafficking, as well as in pathologic conditions such as carcinogenesis and metastatic spreading. Therefore, it is important to understand the regulation of small GTPase signaling, but this is complicated by the fact that crosstalk exists between different GTPase families and that we have to understand how they signal in time and space. The Golgi apparatus represents a hub for several signaling molecules and its importance in this field is constantly increasing. In this review we will discuss small GTPases signaling at the Golgi apparatus. Then, we will highlight recent work that contributed to a better understanding of crosstalk between different small GTPase families, with a special emphasis on their crosstalk at the Golgi apparatus. Finally, we will give a brief overview of available methods and tools to investigate spatio-temporal small GTPase crosstalk.

Roles of rho GTPases in intracellular transport and cellular transformation

Rho family GTPases belong to the Ras GTPase superfamily and transduce intracellular signals known to regulate a variety of cellular processes, including cell polarity, morphogenesis, migration, apoptosis, vesicle trafficking, viral transport and cellular transformation. The three best-characterized Rho family members are Cdc42, RhoA and Rac1. Cdc42 regulates endocytosis, the transport between the endoplasmic reticulum and Golgi apparatus, post-Golgi transport and exocytosis. Cdc42 influences trafficking through interaction with Wiskott-Aldrich syndrome protein (N-WASP) and the Arp2/3 complex, leading to changes in actin dynamics. Rac1 mediates endocytic and exocytic vesicle trafficking by interaction with its effectors, PI3kinase, synaptojanin 2, IQGAP1 and phospholipase D1. RhoA participates in the regulation of endocytosis through controlling its downstream target, Rho kinase. Interestingly, these GTPases play important roles at different stages of viral protein and genome transport in infected host cells. Importantly, dysregulation of Cdc42, Rac1 and RhoA leads to numerous disorders, including malignant transformation. In some cases, hyperactivation of Rho GTPases is required for cellular transformation. In this article, we review a number of findings related to Rho GTPase function in intracellular transport and cellular transformation.

Shiga toxins

Shiga toxins are virulence factors produced by the bacteria Shigella dysenteriae and certain strains of Escherichia coli. There is currently no available treatment for disease caused by these toxin-producing bacteria, and understanding the biology of the Shiga toxins might be instrumental in addressing this issue. In target cells, the toxins efficiently inhibit protein synthesis by inactivating ribosomes, and they may induce signaling leading to apoptosis. To reach their cytoplasmic target, Shiga toxins are endocytosed and transported by a retrograde pathway to the endoplasmic reticulum, before the enzymatically active moiety is translocated to the cytosol. The toxins thereby serve as powerful tools to investigate mechanisms of intracellular transport. Although Shiga toxins are a serious threat to human health, the toxins may be exploited for medical purposes such as cancer therapy or imaging.

Epstein-Barr virus-encoded LMP1 interacts with FGD4 to activate Cdc42 and thereby promote migration of nasopharyngeal carcinoma cells

Epstein-Barr virus (EBV) is closely associated with nasopharyngeal carcinoma (NPC), a human malignancy notorious for its highly metastatic nature. Among EBV-encoded genes, latent membrane protein 1 (LMP1) is expressed in most NPC tissues and exerts oncogenicity by engaging multiple signaling pathways in a ligand-independent manner. LMP1 expression also results in actin cytoskeleton reorganization, which modulates cell morphology and cell motility— cellular process regulated by RhoGTPases, such as Cdc42. Despite the prominent association of Cdc42 activation with tumorigenesis, the molecular basis of Cdc42 activation by LMP1 in NPC cells remains to be elucidated. Here using GST-CBD (active Cdc42-binding domain) as bait in GST pull-down assays to precipitate active Cdc42 from cell lysates, we demonstrated that LMP1 acts through its transmembrane domains to preferentially induce Cdc42 activation in various types of epithelial cells, including NPC cells. Using RNA interference combined with re-introduction experiments, we identified FGD4 (FYVE, RhoGEF and PH domain containing 4) as the GEF (guanine nucleotide exchange factor) responsible for the activation of Cdc42 by LMP1. Serial deletion experiments and co-immunoprecipitation assays further revealed that ectopically expressed FGD4 modulated LMP1-mediated Cdc42 activation by interacting with LMP1. Moreover, LMP1, through its transmembrane domains, directly bound FGD4 and enhanced FGD4 activity toward Cdc42, leading to actin cytoskeleton rearrangement and increased motility of NPC cells. Depletion of FGD4 or Cdc42 significantly reduced (∼50%) the LMP1-stimulated cell motility, an effect that was partially reversed by expression of a constitutively active mutant of Cdc42. Finally, quantitative RT-PCR and immunohistochemistry analyses showed that FGD4 and LMP1 were expressed in NPC tissues, supporting the potential physiologically relevance of this mechanism in NPC. Collectively, our results not only uncover a novel mechanism underlying LMP1-mediated Cdc42 activation, namely LMP1 interaction with FGD4, but also functionally link FGD4 to NPC tumorigenesis.

Epstein-Barr virus (EBV) is closely associated with human malignancies, including nasopharyngeal carcinoma (NPC). Among EBV-expressed genes, latent membrane protein 1 (LMP1) has been detected in most NPC tissues and has the ability to transform cell growth and drive cell migration, both of which are highly associated with tumorigenesis and tumor progression. Previous reports have demonstrated that cell migration primarily involves cytoskeleton rearrangement, and the RhoGTPase Cdc42 is known to actively mediate such rearrangement processes. Using LMP1-expressing NPC cells, we discovered that LMP1 induces Cdc42 activation by directly binding to FGD4, a positive regulator of Cdc42, thereby promoting motility of NPC cells. The observed correlation between FGD4 and LMP1 expression in NPC tissues provides support of physiological relevance. Notably, FGD4 has recently been shown to be responsible for a type of inherited neural disease. Our findings not only provide a novel insight into EBV pathogenesis, but also suggest a role for FGD4 in tumorigenesis.

Charcot-Marie-Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot-Marie-Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.

The small GTPase Cdc42 modulates the number of exocytosis-competent dense-core vesicles in PC12 cells

Although the small GTPase Rho family Cdc42 has been shown to facilitate exocytosis through increasing the amount of hormones released, the precise mechanisms regulating the quantity of hormones released on exocytosis are not well understood. Here we show by live cell imaging analysis under TIRF microscope and immunocytochemical analysis under confocal microscope that Cdc42 modulated the number of fusion events and the number of dense-core vesicles produced in the cells. Overexpression of a wild-type or constitutively-active form of Cdc42 strongly facilitated high-KCl-induced exocytosis from the newly recruited plasma membrane vesicles in PC12 cells. By contrast, a dominant-negative form of Cdc42 inhibited exocytosis from both the newly recruited and previously docked plasma membrane vesicles. The number of intracellular dense-core vesicles was increased by the overexpression of both a wild-type and constitutively-active form of Cdc42. Consistently, activation of Cdc42 by overexpression of Tuba, a Golgi-associated guanine nucleotide exchange factor for Cdc42 increased the number of intracellular dense-core vesicles, whereas inhibition of Cdc42 by overexpression of the Cdc42/Rac interactive binding domain of neuronal Wiskott-Aldrich syndrome protein decreased the number of them. These findings suggest that Cdc42 facilitates exocytosis by modulating both the number of exocytosis-competent dense-core vesicles and the production of dense-core vesicles in PC12 cells.

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.

Role of β-catenin in post-meiotic male germ cell differentiation

Though roles of β-catenin signaling during testis development have been well established, relatively little is known about its role in postnatal testicular physiology. Even less is known about its role in post-meiotic germ cell development and differentiation. Here, we report that β-catenin is highly expressed in post-meiotic germ cells and plays an important role during spermiogenesis in mice. Spermatid-specific deletion of β-catenin resulted in significantly reduced sperm count, increased germ cell apoptosis and impaired fertility. In addition, ultrastructural studies show that the loss of β-catenin in post-meiotic germ cells led to acrosomal defects, anomalous release of immature spermatids and disruption of adherens junctions between Sertoli cells and elongating spermatids (apical ectoplasmic specialization; ES). These defects are likely due to altered expression of several genes reportedly involved in Sertoli cell-germ cell adhesion and germ cell differentiation, as revealed by gene expression analysis. Taken together, our results suggest that β-catenin is an important molecular link that integrates Sertoli cell-germ cell adhesion with the signaling events essential for post-meiotic germ cell development and maturation. Since β-catenin is also highly expressed in the Sertoli cells, we propose that binding of germ cell β-catenin complex to β-catenin complex on Sertoli cell at the apical ES surface triggers a signaling cascade that regulates post-meiotic germ cell differentiation.

Lipid-mediated endocytosis

Receptor-mediated endocytosis is used by a number of viruses and toxins to gain entry into cells. Some have evolved to use specific lipids in the plasma membrane as their receptors. They include bacterial toxins such as Shiga and Cholera toxin and viruses such as mouse polyoma virus and simian virus 40. Through multivalent binding to glycosphingolipids, they induce lipid clustering and changes in membrane properties. Internalization occurs by unusual endocytic mechanisms involving lipid rafts, induction of membrane curvature, trans-bilayer coupling, and activation of signaling pathways. Once delivered to early endosomes, they follow diverse intracellular routes to the lumen of the ER, from which they penetrate into the cytosol. The role of the lipid receptors is central in these well-studied processes.

Identification of a novel virulence factor in Clostridium difficile that modulates toxin sensitivity of cultured epithelial cells

Two glucosylating toxins named toxins A and B play a role in the pathogenesis of Clostridium Difficile infection. The interaction of the toxins with host cell factors proceeds to downstream stages of cytotoxic effects in cells, in which involvement of other C. difficile factors remains unknown. We utilized culture filtrate of C. difficile with a low dilution to characterize the influence of putative minor proteins on the organization of the actin cytoskeleton in cultured epithelial cells and found a previously uncharacterized F-actin aggregated structure, termed "actin aggregate," at the juxtanuclear region. We reasoned that formation of actin aggregate was due to an additional factor(s) in the culture filtrate rather than the glucosylating toxins, because treatment of purified toxins rarely caused actin aggregate in cells. We focused on a previously uncharacterized hypothetical protein harboring a KDEL-like sequence as a candidate. The product of the candidate gene was detected in culture filtrate of C. difficile ATCC 9689 and was renamed Srl. Purified glutathione S-transferase-tagged Srl triggered formation of actin aggregate in the cells in the presence of either toxin A or B and enhanced cytotoxicity of each of the two toxins, including decreases in both cell viability and transepithelial resistance of cultured epithelial monolayer, although the recombinant Srl alone did not show detectable cytotoxicity. Srl-neutralized culture filtrate partially inhibited morphological changes of the cells in parallel with decreased actin aggregate formation in the cells. Thus, Srl might contribute to the modulation of toxin sensitivity of intestinal epithelial cells by enhancing cytotoxicity of C. difficile toxins.

Reelin signals through apolipoprotein E receptor 2 and Cdc42 to increase growth cone motility and filopodia formation

Lipoprotein receptor signaling regulates the positioning and differentiation of postmitotic neurons during development and modulates neuronal plasticity in the mature brain. Depending on the contextual situation, the lipoprotein receptor ligand Reelin can have opposing effects on cortical neurons. We show that Reelin increases growth cone motility and filopodia formation, and identify the underlying signaling cascade. Reelin activates the Rho GTPase Cdc42, known for its role in neuronal morphogenesis and directed migration, in an apolipoprotein E receptor 2-, Disabled-1-, and phosphatidylinositol 3-kinase-dependent manner. We demonstrate that neuronal vesicle trafficking, a Cdc42-controlled process, is increased after Reelin treatment and further provide evidence that the peptidergic VIP/PACAP38 system and Reelin can functionally interact to promote axonal branching. In conclusion, Reelin-induced activation of Cdc42 contributes to the regulation of the cytoskeleton of individual responsive neurons and converges with other signaling cascades to orchestrate Rho GTPase activity and promote neuronal development. Our data link the observation that defects in Rho GTPases and Reelin signaling are responsible for developmental defects leading to neurological and psychiatric disorders.