The large GTPase dynamin is required for hepatitis B virus protein secretion from hepatocytes

BACKGROUND/AIMS: The hepatocellular transport pathways and cellular proteins utilized during the packaging and secretion of hepatitis B virus are poorly understood. In this study, we tested if the large GTPase dynamin, a protein involved in vesicle formation and secretion at the trans-Golgi network in hepatocytes, is also used by hepatitis B virus (HBV) in secreting viral proteins. METHODS: Using HepG2.2.15 cells expressing the full-length HBV genome, we tested the effects of wild-type and mutant dynamin on the localization and secretion of two hepatitis B antigens, hepatitis B surface antigen (HBsAg) and hepatitis B e antigen (HBeAg). Distribution of these two antigens was analyzed morphologically in cells transiently transfected with wild-type or mutant dynamin constructs, whereas secretion of the antigens was measured by testing for antigen levels in the media of transfected cells. RESULTS: Mutant dynamin was found to induce a striking redistribution of HBsAg and HBeAg to a perinuclear compartment, as well as a decrease in the levels of HBsAg and HBeAg present in cell culture media indicating a reduction in viral protein secretion. At the electron microscopy level, cells expressing the mutant dynamin showed a marked accumulation of viral particles in dilated cisternae of an uncharacterized cellular compartment. CONCLUSIONS: Intact dynamin function is required for secretion of HBV proteins from hepatocytes through an uncharacterized cellular compartment.     

Mobility of cytoplasmic and membrane-associated actin in living cells.

We have combined fluorescent analogue cytochemistry with fluorescence photobleaching recovery to measure the mobility of fluorescently labeled actin and other labeled test proteins microinjected into living amoebae. Bovine serum albumin, ovalbumin, and ribonuclease A have a cytoplasmic mobility, expressed as a diffusion coefficient, that is 1/2 to 1/3 of that observed in aqueous solution; 90% of the actin has a mobility 1/2 to 1/8 of that of G-actin in aqueous solution, and approximately equal to 10% of the actin has a mobility comparable to that of F-actin in aqueous solution. Therefore, no more than 10% of the actin in the cytoplasm of amoebae can exist as static filaments. Microinjection of phalloidin decreases the diffusion coefficient of the mobile component of cytoplasmic actin, and it also increases the low-mobility fraction to 50% but has no effect on the mobility of labeled ovalbumin. By comparing the mobility of actin in different parts of amoebae and by separating cytoplasm from plasmalemma-ectoplasm, we found the low-mobility fraction of actin to be enriched in the tail, along the plasmalemma-ectoplasm, and in contracted cytoplasm.     

Rho GTPase CDC42 regulates directionality and random movement via distinct MAPK pathways in neutrophils

Neutrophil transmigration into tissue is a multiple-step process that results from a coordinated rearrangement of the cytoskeleton and adhesion complexes. Assembly and disassembly of actin and adhesion structures dictate motility behavior, while polarity and gradient sensing provide directionality to the cell movement. Here, using mice deficient in the CDC42 regulator CDC42 GTPase-activating protein (CDC42GAP), we demonstrate that CDC42 activity separately regulates neutrophil motility and directionality. CDC42GAP-/- neutrophils showed increased motility, while directed migration was defective. Podosome-like structures present at the leading edge in wild-type neutrophils were significantly reduced in CDC42GAP-/- cells. CDC42GAP-/- neutrophils also showed increased lateral and tail filopodia-like formation, and excess membrane protrusions. We further suggest that CDC42GAP-mediated extracellular signal-regulated kinase (ERK) activity regulates motility associated with podosome-like structures at the cell leading edge, while CDC42GAP-induced p38(MAPK) phosphorylation regulates directed migration by antagonizing filopodia assembly. Overall, this study reveals that CDC42 activity regulates both motility and directionality in neutrophils, but via distinct mitogen-activated protein kinase (MAPK) pathways.     

Microtubules and Src homology 3 domains stimulate the dynamin GTPase via its C-terminal domain.

Dynamin is a 100-kDa GTPase that plays a critical role in the initial stages of endocytosis. Dynamin binds to microtubules, which potently stimulate its GTPase activity. Binding to Src homology 3 (SH3) domains of proteins involved in signal transduction has also recently been reported. In the present study, the protein was digested with a variety of proteases to define its functional domains. Limited digestion with papain split the protein into an approximately 7- to 9-kDa microtubule-binding fragment and a 90-kDa nonbinding fragment. Immunoblotting with an antibody to the C-terminal 20 amino acids of rat dynamin showed the small fragment to derive from the C-terminal end of the polypeptide. Microtubule-activated GTPase activity, but not basal GTPase activity, was abolished by papain digestion, identifying the basic, proline-rich C-terminal region of dynamin as an important regulatory site. Bacterially expressed growth factor receptor-bound protein 2 (GRB2) and the SH3 domain of c-Src were also found to stimulate GTPase activity, although to a lesser extent than microtubules. Stimulation of GTPase activity by the recombinant proteins was similarly abolished by papain digestion. These results identify the basic, proline-rich C-terminal region of dynamin as the binding site for both microtubules and SH3 domains and demonstrate an allosteric interaction between this region of the molecule and the N-terminal GTPase domain.     

Molecular cytochemistry: incorporation of fluorescently labeled actin into living cells.

Actin labeled with 5-iodoacetamidofluorescein has been incorporated into the functional pool of actin in Chaos carolinensis and Physarum polycephalum by direct microinjection. The functional activity of the labeled actin has been analyzed at three levels of organization as: (a) with the purified actin, (b) in motile extracts of cells, and (c) in living motile cells. The labeled actin exhibited normal polymerization and activated myosin ATPase to a similar extent as unlabeled controls. Labeled actin and endogenous actin were incorporated into contracted pellets to approximately the same extent in motile cell extracts. After labeled actin had been microinjected into single C. carolinensis cells, the fluorescent actin spread into both the endoplasm and etoplasm without forming distinct fibrils. In contrast, fluorescent bundles developed in the ectoplasm of P. polycephalum following microinjection of labeled actin. This experimental method in conjunction with fluorescence spectroscopic techniques could become a powerful tool for studying the intracellular distribution and structural changes of components in living cells.     

The actin cytoskeleton differentially regulates platelet alpha-granule and dense-granule secretion

Stimulation of platelets with strong agonists results in centralization of cytoplasmic organelles and secretion of granules. These observations have led to the supposition that cytoskeletal contraction facilitates granule release by promoting the interaction of granules with one another and with membranes of the open canalicular system. Yet, the influence of the actin cytoskeleton in controlling the membrane fusion events that mediate granule secretion remains largely unknown. To evaluate the role of the actin cytoskeleton in platelet granule secretion, we have assessed the effects of latrunculin A and cytochalasin E on granule secretion. Exposure of platelets to low concentrations of these reagents resulted in acceleration and augmentation of agonist-induced alpha-granule secretion with comparatively modest effects on dense granule secretion. In contrast, exposure of platelets to high concentrations of latrunculin A inhibited agonist-induced alpha-granule secretion but stimulated dense granule secretion. Incubation of permeabilized platelets with low concentrations of latrunculin A primed platelets for Ca(2+)- or guanosine triphosphate (GTP)-gamma-S-induced alpha-granule secretion. Latrunculin A-dependent alpha-granule secretion was inhibited by antibodies directed at vesicle-associated membrane protein (VAMP), demonstrating that latrunculin A supports soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein-dependent membrane fusion. These results indicate that the actin cytoskeleton interferes with platelet exocytosis and differentially regulates alpha-granule and dense granule secretion.     

Fluorescence staining of the actin cytoskeleton in living cells with 7-nitrobenz-2-oxa-1,3-diazole-phallacidin.

An active fluorescent derivative of the actin-binding mushroom toxin phallacidin has been synthesized. Convenient methods were developed to stain actin cytoskeletal structures in living and fixed cultured animal cells and actively streaming algal cells. Actin binding specificity was demonstrated by competitive binding experiments and comparative staining of well-known structures. Large populations of living animal cells in culture were readily stained by using a relatively mild lysolecithin permeabilization procedure facilitated by the small molecular size of the label. Actin in animal cells was stained stress fibers, ruffles, the cellular geodome, and in diffuse appearing distributions apparently associated with the plasma membrane. Staining of actin cables in algae with nitrobenzoxadiazole (NBD)-phallacidin did not inhibit cytoplasmic streaming. NBD-phallacidin provides a convenient actin-specific fluorescent label for cellular cytoskeletal structures with promise for use in studies of actin dynamics in living systems.     

Force-induced formation and propagation of adhesion nanodomains in living fungal cells

Understanding how cell adhesion proteins form adhesion domains is a key challenge in cell biology. Here, we use single-molecule atomic force microscopy (AFM) to demonstrate the force-induced formation and propagation of adhesion nanodomains in living fungal cells, focusing on the covalently anchored cell-wall protein Als5p from Candida albicans. We show that pulling on single adhesins with AFM tips terminated with specific antibodies triggers the formation of adhesion domains of 100-500 nm and that the force-induced nanodomains propagate over the entire cell surface. Control experiments (with cells lacking Als5p, single-site mutation in the protein, bare tips, and tips modified with irrelevant antibodies) demonstrate that Als5p nanodomains result from protein redistribution triggered by force-induced conformational changes in the initially probed proteins, rather than from nonspecific cell-wall perturbations. Als5p remodeling is independent of cellular metabolic activity because heat-killed cells show the same behavior as live cells. Using AFM and fluorescence microscopy, we also find that nanodomains are formed within ～30 min and migrate at a speed of ～20 nm·min(-1), indicating that domain formation and propagation are slow, time-dependent processes. These results demonstrate that mechanical stimuli can trigger adhesion nanodomains in fungal cells and suggest that the force-induced clustering of adhesins may be a mechanism for activating cell adhesion.     

Rho protein regulates tight junctions and perijunctional actin organization in polarized epithelia.

The rho family of GTP-binding proteins regulates actin filament organization. In unpolarized mammalian cells, rho proteins regulate the assembly of actin-containing stress fibers at the cell-matrix interface. Polarized epithelial cells, in contrast, are tall and cylindrical with well developed intercellular tight junctions that permit them to behave as biologic barriers. We report that rho regulates filamentous actin organization preferentially in the apical pole of polarized intestinal epithelial cells and, in so doing, influences the organization and permeability of the associated apical tight junctions. Thus, barrier function, which is an essential characteristic of columnar epithelia, is regulated by rho.     

Thymosin beta 4 (Fx peptide) is a potent regulator of actin polymerization in living cells.

Thymosin beta 4 (beta 4) is a 5-kDa polypeptide originally identified in calf thymus. Although numerous activities have been attributed to beta 4, its physiological role remains elusive. Recently, beta 4 was found to bind actin in human platelet extracts and to inhibit actin polymerization in vitro, raising the possibility that it may be a physiological regulator of actin assembly. To examine this potential function, we have increased the cellular beta 4 concentration by microinjecting synthetic beta 4 into living epithelial cells and fibroblasts. The injection induced a diminution of stress fibers and a dose-dependent depolymerization of actin filaments as indicated by quantitative image analysis of phalloidin binding. Our results show that beta 4 is a potent regulator of actin assembly in living cells.     

Morphological changes in liposomes caused by polymerization of encapsulated actin and spontaneous formation of actin bundles.

Spherical giant liposomes that had encapsulated skeletal-muscle G-actin were made by swelling a dried lipid mixture of dimyristoyl phosphatidylcholine/cardiolipin, 1:1 (wt/wt), in a solution of G-actin/CaCl2 at 0 degree C. Polymerization of the encapsulated G-actin into actin filaments was achieved by raising the temperature to 30 degrees C. We observed the subsequent shape changes of the liposomes by dark-field and differential interference-contrast light microscopy. After approximately 40 min, which was required for completion of actin polymerization, two shapes of liposome were evident: dumbbell and disk. Elongation of the dumbbell-shaped liposomes was concomitant with actin polymerization. Polarization microscopy showed that actin filaments formed thick bundles in the liposomes and that these filaments lay contiguous to the periphery of the liposome. Localization of actin filaments in the liposomes was confirmed by observation of rhodamine phalloidin-conjugated actin filaments by fluorescence microscopy. Both dumbbell- and disk-shaped liposomes were rigid and kept their shapes as far as actin filaments were stabilized. In contrast, liposomes containing bovine serum albumin were fragile, and their shapes continually fluctuated from Brownian motion, indicating that the actin bundles served as mechanical support for the liposome shapes.     

Tropomyosin synthesis accompanies formation of actin filaments in embryonal carcinoma cells induced to differentiate by hexamethylene bisacetamide.

Hexamethylene bisacetamide (HMBA) induces in vitro the cytodifferentiation of PCC3/A/1 mouse embryonal carcinoma (EC) cells. In EC cells, actin is associated with surface structures but microfilament bundles are not seen. After 2 days of HMBA treatment, rounded EC cells are converted to flat adhesive ones with a developed cytoskeleton containing actin and tropomyosin. The ratio of actin to total proteins is constant in EC cells and their HMBA derivatives; but a striking difference is observed for one of the newly synthesized proteins (Mr 34,000) identified as tropomyosin. Synthesis of tropomyosin is followed by its association with actin microfilament bundles, as revealed by indirect immunofluorescence microscopy with specific antibodies.     

CD48 on hematopoietic progenitors regulates stem cells and suppresses tumor formation

The proliferation and differentiation of adult stem cells is balanced to ensure adequate generation of differentiated cells, stem cell homeostasis, and guard against malignant transformation. CD48 is broadly expressed on hematopoietic cells but excluded from quiescent long-term murine HSCs. Through its interactions with CD244 on progenitor cells, it influences HSC function by altering the BM cytokine milieu, particularly IFNγ. In CD48-null mice, the resultant misregulation of cytokine signaling produces a more quiescent HSC, a disproportionate number of short-term progenitors, and hyperactivation of Pak1, leading to hematologic malignancies similar to those found in patients with X-linked lymphoproliferative disease. CD48 plays a vital role as an environmental sensor for regulating HSC and progenitor cell numbers and inhibiting tumor development.     

Laser-induced multiphoton processes in living cells.

It is shown that specific light-induced subcellular alterations, usually referred to as phase "paling," result from multiphoton absorption processes. For green light (532 nm), four photons are required to induce paling in chromosomes and nucleoli; a two-photon process is observed for UV light (266 nm).     

The actin binding proteins cortactin and HS1 are dispensable for platelet actin nodule and megakaryocyte podosome formation

A dynamic, properly organised actin cytoskeleton is critical for the production and haemostatic function of platelets. The Wiskott Aldrich Syndrome protein (WASp) and Actin-Related Proteins 2 & 3 Complex (Arp2/3 complex) are critical mediators of actin polymerisation and organisation in many cell types. In platelets and megakaryocytes, these proteins have been shown to be important for proper platelet production and function. The cortactin family of proteins (Cttn & HS1) are known to regulate WASp-Arp2/3-mediated actin polymerisation in other cell types and so here we address the role of these proteins in platelets using knockout mouse models.

We generated mice lacking Cttn and HS1 in the megakaryocyte/platelet lineage. These mice had normal platelet production, with platelet number, size and surface receptor profile comparable to controls. Platelet function was also unaffected by loss of Cttn/HS1 with no differences observed in a range of platelet function assays including aggregation, secretion, spreading, clot retraction or tyrosine phosphorylation. No effect on tail bleeding time or in thrombosis models was observed. In addition, platelet actin nodules, and megakaryocyte podosomes, actin-based structures known to be dependent on WASp and the Arp2/3 complex, formed normally. We conclude that despite the importance of WASp and the Arp2/3 complex in regulating F-actin dynamics in many cells types, the role of cortactin in their regulation appears to be fulfilled by other proteins in platelets.     

Changes in actin network during calcium-induced exocytosis in permeabilized GH3 cells: calcium directly regulates F-actin disassembly

Using digitonin-permeabilized GH3 cells, we investigated both the release of prolactin (PRL) and changes in the cytoskeleton. We determined that permeabilized GH3 cells released PRL in a dose-dependent manner upon addition of micromolar Ca(2+). Phalloidin, a filamentous actin (F-actin) stabilizing agent, inhibited both Ca(2+)-dependent and -independent PRL release, whereas cytochalasin B, a destabilizing agent, had almost no effect on the release. Observation with a confocal laser scanning microscope revealed that F-actin existed mainly in the cortical region in the quiescent state. Increased cytosolic Ca(2+) induced a change in F-actin distribution: F-actin in the cortical region decreased, whereas F-actin inside the cells increased. This change in F-actin distribution was not observed when phalloidin was added. Addition of cytochalasin B induced patchy F-actin spots, but the pattern of the changes of F-actin distribution did not change. The time course of change in F-actin distribution showed that the F-actin network in the cortical region was reduced within 1 min, and Ca(2+)-dependent release of PRL continued for up to 20 min. These results suggest that the F-actin network near the membrane acts as a barrier to exocytosis and that Ca(2+) directly controls the cytoskeletal changes.