N-Ras induces alterations in Golgi complex architecture and in constitutive protein transport

Polyphosphoinositide metabolism and Golgi complex morphology in hippocampal neurons in primary culture is altered by chronic ethanol exposure

AIMS

Ethanol affects not only the cytoskeletal organization and activity, but also intracellular trafficking in neurons in the primary culture. Polyphosphoinositide (PPIn) are essential regulators of many important cell functions, including those mentioned, cytoskeleton integrity and intracellular vesicle trafficking. Since information about the effect of chronic ethanol exposure on PPIn metabolism in neurons is scarce, this study analysed the effect of this treatment on three of these phospholipids.

METHODS

Phosphatidylinositol (PtdIns) levels as well as the activity and/or levels of enzymes involved in their metabolism were analysed in neurons chronically exposed to ethanol. The levels of phospholipases C and D, and phosphatidylethanol formation were also assessed. The consequence of the possible alterations in the levels of PtdIns on the Golgi complex (GC) was also analysed.

RESULTS

We show that phosphatidylinositol (4,5)-bisphosphate and phosphatidylinositol (3,4,5)-trisphosphate levels, both involved in the control of intracellular trafficking and cytoskeleton organization, decrease in ethanol-exposed hippocampal neurons. In contrast, several kinases that participate in the metabolism of these phospholipids, and the level and/or activity of phospholipases C and D, increase in cells after ethanol exposure. Ethanol also promotes phosphatidylethanol formation in neurons, which can result in the suppression of phosphatidic acid synthesis and, therefore, in PPIn biosynthesis. This treatment also lowers the phosphatidylinositol 4-phosphate levels, the main PPIn in the GC, with alterations in their morphology and in the levels of some of the proteins involved in structure maintenance.

CONCLUSIONS

The deregulation of the metabolism of PtdIns may underlie the ethanol-induced alterations on different neuronal processes, including intracellular trafficking and cytoskeletal integrity.

Alcohol induces Golgi fragmentation in differentiated PC12 cells by deregulating Rab1-dependent ER-to-Golgi transport

In the present study, we analyze the effects of ethanol on the Golgi structure and membrane transport in differentiated PC12 cells, which are used as a model of neurons. Chronic exposure to moderate doses of ethanol induces Golgi fragmentation, a common characteristic of many neurodegenerative diseases. Alcohol impaired the lateral linking of stacks without causing microtubule damage. Extensive immunocytochemical and western blot analyses of representative Golgi proteins showed that few, but important, proteins are significantly affected. Thus, alcohol exposure induced a significant ER-to-Golgi transport delay, the retention of the GTPase Rab1 in the Golgi membranes and the accumulation of tethering factor p115 in the cytosol. These modifications would explain the observed fragmentation. The amount of p115 and the stacking protein GRASP65 increased in alcohol-treated cells, which might be a mechanism to reverse Golgi damage. Importantly, the overexpression of GTP-tagged Rab1 but not of a dominant-negative Rab1 mutant, restored the Golgi morphology, suggesting that this protein is the main target of alcohol. Taken together, our results support the view that alcohol and neurodegenerative diseases such as Parkinson have similar effects on intracellular trafficking and provide new clues on the neuropathology of alcoholism.

High IGFBP2 expression correlates with tumor severity in pediatric rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common childhood sarcoma and is identified as either the embryonal or alveolar (ARMS) subtype. In approximately 75% of cases, ARMSs are characterized by specific chromosomal translocations that involve PAX and FKHR genes. ARMS gene expression signatures vary, depending on the presence or absence of the translocations. Insulin-like growth factor-binding protein 2 (IGFBP2) is strongly overexpressed in translocation-negative RMS. Because IGFBP2 is associated with tumorigenesis, we investigated its functional role in RMS. An analysis of IGFBP2 distribution in RMS cell lines revealed a strong accumulation in the Golgi complex, in which morphological characteristics appeared peculiarly modified. After silencing IGFBP2 expression, our microarray analysis revealed mostly cell cycle and actin cytoskeleton gene modulations. In parallel, IGFBP2-silenced cells showed reduced cell cycle and rates of invasion and decreased seeding in the lungs after tail vein injections in immunodeficient mice. An analysis of IGFBP2 mRNA and protein localization in human tumors showed abnormal protein accumulation in the Golgi complex, mostly in PAX/FKHR-negative RMS. Moreover, an analysis of patients with RMS revealed the presence of conspicuous circulating levels of IGFBP2 proteins in children with highly aggressive RMS tumors. Taken together, our data provide evidence that IGFBP2 contributes to tumor progression and that it could be used as a marker to better classify clinical and biological risks in RMS.

Actin and microtubule regulation ofTrans-Golgi network architecture, and copper-dependent protein transport to the cell surface

The Menkes disease ATPase (MNK) is a copper transporter that localizes to the mammalian trans-Golgi network (TGN) and shows substantial co-localization wih a ubiquitous TGN resident protein and marker, TGN46. We tested our hypothesis that these two TGN residents and integral membrane proteins are localized to biochemically distinct TGN sub-compartments using constitutively active mutant proteins and drugs that disrupt membrane traffic, lumenal pH and the cellular cytoskeleton. The pH-disrupting agent, monensin, causes MNK to be more diffusely distributed with partial separation of staining patterns for these two TGN residents. Expression of a constitutively active Rho-kinase (ROCK-KIN), which causes formation of juxta-nuclear astral actin arrays, also effects separation of MNK and TGN46 staining patterns. Treatment of ROCK-KIN expressing cells with latrunculin B, an actin-depolymerizing agent, causes complete overlap of MNK and TGN46 staining patterns with concomitant disappearance of polymerized actin. When microtubules are depolymerized in ROCK-KIN expressing cells by nocodazole, both MNK and TGN46 are found in puncate structures throughout the cell. However, a substantial proportion of MNK is still found in a juxta-nuclear location in contrast to TGN46. Actin distribution in these cells reveals that juxta-nuclear MNK is distinct to the astral actin clusters in ROCK-KIN expressing cells where the microtubules were depolymerized. The TGN to cell-surface transport of MNK requires both actin and microtubules networks, whilst the constitutive trafficking of proteins is independent of actin. Taken together, our findings indicate that at least two TGN sub-domains are regulated by separate cytoskeletal dynamics involving actin and tubulin.

Docosahexaenoic acid selectively inhibits plasma membrane targeting of lipidated proteins

Membrane localization of lipidated cytosolic signaling proteins is mediated by interactions between specific lipid anchors and membranes, but little is known about the regulatory role of membrane composition in lipidated protein membrane targeting. Here, using green fluorescent protein (GFP) chimeras and quantitative fluorescence microscopy in living mouse colonocytes, we show that docosahexaenoic acid (DHA), a dietary polyunsaturated fatty acid (PUFA) with membrane lipid-modifying properties, selectively inhibits plasma membrane (PM) targeting and increases the endomembrane localization of lipidated proteins that are cytoplasmic cargo in the exocytic pathway, without affecting the exocytic pathway itself. DHA selectivity seems to be dictated by the protein trafficking route, independent of the functional state of proteins and the location and composition of membrane anchors. DHA enrichment in cell membranes was required to elicit the inhibitory effect. These data reveal that membrane lipid composition influences cell signaling by modulating intracellular trafficking and localization of membrane proteins, providing a potential molecular mechanism for the documented health benefits of DHA.

Visualizing Ras signalling in real-time

Ras GTPases are universal molecular switches that act as kinetic timers of signal transduction events. They are post-translationally modified by the addition of lipid groups to their hypervariable carboxyl termini, which plug the proteins to membranes and influence their dynamic sorting and trafficking. For the past twenty years, the plasma membrane has been considered to be the predominant platform from which Ras operates. Recent work using live-cell imaging and novel probes to visualize where and when Ras is active has supported this long-held belief. However, an equally fascinating aspect of these imaging studies has been the discovery of dynamic Ras activity, as well as distinct signal output, from intracellular organelles. Activation of Ras on the Golgi exhibits kinetics different from Ras activation on the plasma membrane, and compartmentalized Ras signalling seems particularly prominent in lymphocytes. However, data on the spatial and temporal regulation of Ras activity has frequently differed depending on the nature of the probe, the cell type and the stimulus. Nevertheless, because Ras traffics through endomembranes en route to the plasma membrane, it seems likely that Ras can signal from such compartments. The burning question in this field concerns the significance of this observation for endogenous Ras signalling output.

Spectrins and the Golgi

Several isoforms of spectrin membrane skeleton proteins have been localized to the Golgi complex. Golgi-specific membrane skeleton proteins associate with the Golgi as a detergent-resistant cytoskeletal structure that likely undergoes a dynamic assembly process that accommodates Golgi membrane dynamics. This review discusses the potential roles for this molecule in Golgi functions. In particular, it will focus on a recently identified distant cousin to conventional erythroid spectrin variously named Syne-1, Nesprin, myne, Enaptin, MSP-300, and Ank-1. Syne-1 has the novel ability to bind to both the Golgi and the nuclear envelope, a property that raises several intriguing and novel insights into Golgi structure and function. These include (1) the facilitation of interactions between Golgi and transitional ER sites on the nuclear envelope of muscle cells, and (2) an ability to impart localized specificity to the secretory pathway within large multinucleate syncytia such as skeletal muscle fibers.

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.

Cholesterol loading induces a block in the exit of VSVG from the TGN

Recent work from our laboratory demonstrated that increased cellular cholesterol content affects the structure of the Golgi apparatus. We have now investigated the functional consequences of the cholesterol-induced vesiculation of the Golgi apparatus and the role of actin for these changes. The results showed that cholesterol-induced vesiculation and dispersion of the Golgi apparatus is a reversible process and that reversal can be inhibited by cytochalasin D, an actin-disrupting reagent. Furthermore, electron microscopy revealed that jasplakinolide, which stabilizes actin filaments, prevented the dispersion, but not the vesiculation of the Golgi cisternae. Importantly, the different Golgi markers seemed to be separated even after vesiculation. To investigate whether transport through the different steps of the exocytic pathway was affected in cholesterol-treated cells, we visualized ER to plasma membrane transport by using ts045-VSVG-GFP. In COS-1 cells expressing ts045-VSVG-GFP increased cholesterol levels did not affect transport of VSVG into the vesiculated Golgi apparatus. However, increased levels of cholesterol resulted in retention of the nascent G protein in vesicles with the TGN-marker TGN46. Biotinylation of cell surface molecules to quantify arrival of VSVG at the plasma membrane confirmed that cholesterol treatment inhibited export of the VSVG protein. In conclusion, the data show that transport of VSVG into/through a vesiculated Golgi is feasible, but that cholesterol loading inhibits exit of VSVG from the vesicles containing TGN markers. Furthermore, the data illustrate the importance of actin filaments for Golgi structure.

Golgi localization of Syne-1

We have previously identified a Golgi-localized spectrin isoform by using an antibody to the beta-subunit of erythrocyte spectrin. In this study, we show that a screen of a lambdagt11 expression library resulted in the isolation of an approximately 5-kb partial cDNA from a Madin-Darby bovine kidney (MDBK) cell line, which encoded a polypeptide of 1697 amino acids with low, but detectable, sequence homology to spectrin (37%). A blast search revealed that this clone overlaps with the 5' end of a recently identified spectrin family member Syne-1B/Nesprin-1beta, an alternately transcribed gene with muscle-specific forms that bind acetylcholine receptor and associate with the nuclear envelope. By comparing the sequence of the MDBK clone with sequence data from the human genome database, we have determined that this cDNA represents a central portion of a very large gene ( approximately 500 kb), encoding an approximately 25-kb transcript that we refer to as Syne-1. Syne-1 encodes a large polypeptide (8406 amino acids) with multiple spectrin repeats and a region at its amino terminus with high homology to the actin binding domains of conventional spectrins. Golgi localization for this spectrin-like protein was demonstrated by expression of epitope-tagged fragments in MDBK and COS cells, identifying two distinct Golgi binding sites, and by immunofluorescence microscopy by using several different antibody preparations. One of the Golgi binding domains on Syne-1 acts as a dominant negative inhibitor that alters the structure of the Golgi complex, which collapses into a condensed structure near the centrosome in transfected epithelial cells. We conclude that the Syne-1 gene is expressed in a variety of forms that are multifunctional and are capable of functioning at both the Golgi and the nuclear envelope, perhaps linking the two organelles during muscle differentiation.

RhoA and lysophosphatidic acid are involved in the actin cytoskeleton reorganization of astrocytes exposed to ethanol

Astroglial cells play an important role in maintaining neuronal function in the adult and in the developing nervous system. Ethanol exposure induces profound alterations in the astrogliogenesis process, affecting important cell functions, including intracellular protein trafficking. Because the actin cytoskeleton plays a crucial role in intracellular protein transport, the aim of the present study was to analyze the effects of ethanol on actin cytoskeleton organization and the involvement of the RhoA signaling pathway in these effects. We show that RhoA and lysophosphatidic acid (LPA), an upstream activator of RhoA, stimulate the formation of stress fibers and focal adhesion in cortical astrocytes in primary culture. Exposure of cultured astrocytes to different concentrations of ethanol profoundly disorganizes the actin cytoskeleton, leading to the formation of actin rings at the cell periphery and decreasing the content of focal adhesion proteins. Furthermore, LPA treatment or RhoA transfection revert the ethanol-induced actin alterations in astrocytes, whereas transfection with an inactive mutant of RhoA is unable to revert the actin ring organization. In addition, inhibition of endogenous RhoA by C3 exoenzyme effectively blocks ethanol-induced actin ring formation. These results suggest that the effects of alcohol on actin cytoskeleton organization are mediated by the RhoA signaling pathway. Disruptions in actin organization may impair important astrocyte functions, participating in ethanol-induced astroglial and brain damage during development.

Myosin VI, an actin motor for membrane traffic and cell migration

The actin cytoskeleton and associated myosin motor proteins are essential for the transport and steady-state localization of vesicles and organelles and for the dynamic remodeling of the plasma membrane as well as for the maintenance of differentiated cell-surface structures. Myosin VI may be expected to have unique cellular functions, because it moves, unlike almost all other myosins, towards the minus end of actin filaments. Localization and functional studies indicate that myosin VI plays a role in a variety of different intracellular processes, such as endocytosis and secretion as well as cell migration. These diverse functions of myosin VI are mediated by interaction with a range of different binding partners.

Calnuc, an EF-hand Ca(2+)-binding protein, is stored and processed in the Golgi and secreted by the constitutive-like pathway in AtT20 cells

Calnuc is an ubiquitous, EF-hand Ca(2+) binding protein found in the cytoplasm where it binds to Galphai3, in the Golgi lumen where it constitutes a Ca(2+) storage pool, and secreted outside the cell. Here we investigated the pathway of secretion of calnuc in AtT20 cells. We found by pulse-chase experiments that calnuc is synthesized in the endoplasmic reticulum, transported to the Golgi where it remains greater than 12 h and undergoes posttranslational modification (O-glycosylation and sulfation) followed by secretion into the culture medium. We examined if calnuc is secreted by the constitutive or regulated secretory pathway in AtT20 cells. By immunofluorescence and immunogold labeling, endogenous calnuc is found in immature secretion granules (ISG) but not mature regulated secretory granules (RSG), whereas overexpressed calnuc-green fluorescent protein (GFP) is found in both ISG and RSG, where it colocalizes with ACTH. Neither calnuc nor calnuc-GFP are released by the regulated secretory pathway, suggesting that endogenous calnuc and calnuc-GFP are progressively removed from ISG and RSG during granule maturation. We conclude that calnuc is secreted via the constitutive-like pathway and represents a useful endogenous marker for this pathway in AtT20 cells. Together, these observations indicate that calnuc has a unique itinerary as it is retained in the Golgi and is then constitutively secreted extracellularly where it may influence cell behavior via its Ca(2+)-binding properties.

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.

Actin microfilaments facilitate the retrograde transport from the Golgi complex to the endoplasmic reticulum in mammalian cells

The morphology and subcellular positioning of the Golgi complex depend on both microtubule and actin cytoskeletons. In contrast to microtubules, the role of actin cytoskeleton in the secretory pathway in mammalian cells has not been clearly established. Using cytochalasin D, we have previously shown that microfilaments are not involved in the endoplasmic reticulum-Golgi membrane dynamics. However, it has been reported that, unlike botulinum C2 toxin and latrunculins, cytochalasin D does not produce net depolymerization of actin filaments. Therefore, we have reassessed the functional role of actin microfilaments in the early steps of the biosynthetic pathway using C2 toxin and latrunculin B. The anterograde endoplasmic reticulum-to-Golgi transport monitored with the vesicular stomatitis virus-G protein remained unaltered in cells treated with cytochalasin D, latrunculin B or C2 toxin. Conversely, the brefeldin A-induced Golgi membrane fusion into the endoplasmic reticulum, the Golgi-to-endoplasmic reticulum transport of a Shiga toxin mutant form, and the subcellular distribution of the KDEL receptor were all impaired when actin microfilaments were depolymerized by latrunculin B or C2 toxin. These findings, together with the fact that COPI-coated and uncoated vesicles contain beta/gamma-actin isoforms, indicate that actin microfilaments are involved in the endoplasmic reticulum/Golgi interface, facilitating the retrograde Golgi-to-endoplasmic reticulum membrane transport, which could be mediated by the orchestrated movement of transport intermediates along microtubule and microfilament tracks.

Arachidonyltrifluoromethy ketone, a phospholipase A(2) antagonist, induces dispersal of both Golgi stack- and trans Golgi network-resident proteins throughout the cytoplasm

Arachidonyltrifluoromethy ketone (AACOCF(3)), a phospholipase A(2) antagonist, reversibly induced dispersal of Golgi stack- and trans Golgi network (TGN)-resident proteins throughout the cytoplasm in NRK cells as followed by immunocytochemical staining of ManII and TGN38, respectively. The action of AACOCF(3) was partly blocked by other PLA(2) antagonists, suggesting it be not caused by a general inhibition of phospholipase A(2). AACOCF(3) neither dissociated beta-COP from membranes nor prevented brefeldin A-induced beta-COP release. Action of AACOCF(3) on the Golgi stack and TGN is different from that of brefeldin A and nordihydroguaiaretic acid. The most prominent difference is that the Golgi stack and TGN showed a similar sensitivity to AACOCF(3), while the TGN was dispersed more slowly than the Golgi stack in brefeldin A- or nordihydroguaiaretic acid-treated NRK cells. This novel action of AACOCF(3) may be used as pharmacological tool and give new insights into vesicle-mediated traffic and Golgi membrane dynamics.

ADPKD: a human disease altering Golgi function and basolateral exocytosis in renal epithelia

Epithelial cells explanted from autosomal dominant polycystic kidney disease (ADPKD) tissue exhibit impaired exocytosis, specifically between the Golgi and basolateral membrane (Charron A, Nakamura B, Bacallo R, Wandinger-Ness A. Compromised cytoarchitecture and polarized trafficking in autosomal dominant polycystic kidney disease cells. J Cell Biol 2000; 148: 111-124.). Here the defect is shown to result in the accumulation of the basolateral transport marker vesicular stomatitis virus (VSV) G protein in the Golgi complex. Golgi complex morphology is consequently altered in the disease cells, evident in the noticeable fenestration and dilation of the cisternae. Further detailed microscopic evaluation of normal kidney and ADPKD cells revealed that ineffective basolateral exocytosis correlated with modulations in the localization of select post-Golgi transport effectors. The cytosolic coat proteins p200/myosin II and caveolin exhibited enhanced association with the cytoskeleton or the Golgi of the disease cells, respectively. Most cytoskeletal components with known roles in vesicle translocation or formation were normally arrayed with the exception of Golgi beta-spectrin, which was less prevalent on vesicles. The rab8 GTPase, important for basolateral vesicle targeting, was redistributed from the perinuclear Golgi region to disperse vesicles in ADPKD cells. At the basolateral membrane of ADPKD cells, there was a notable loss of the exocyst components sec6/sec8 and an unidentified syntaxin. It is postulated that dysregulated basolateral transport effector function precipitates the disruption of basolateral exocytosis and dilation of the ADPKD cell Golgi as basolateral cargo accumulates within the cisternae.

Spectrin tethers and mesh in the biosynthetic pathway

The paradox of how the Golgi and other organelles can sort a continuous flux of protein and lipid but maintain temporal and morphological stability remains unresolved. Recent discoveries highlight a role for the cytoskeleton in guiding the structure and dynamics of organelles. Perhaps one of the more striking, albeit less expected, of these discoveries is the recognition that a spectrin skeleton associates with many organelles and contributes to the maintenance of Golgi structure and the efficiency of protein trafficking in the early secretory pathway. Spectrin interacts directly with phosphoinositides and with membrane proteins. The small GTPase ARF, a key player in Golgi dynamics, regulates the assembly of the Golgi spectrin skeleton through its ability to control phosphoinositide levels in Golgi membranes, whereas adapter molecules such as ankyrin link spectrin to other membrane proteins. Direct interactions of spectrin with actin and centractin (ARP1) provide a link to dynein, myosin and presumably other motors involved with intracellular transport. Building on the recognized ability of spectrin to organize macromolecular complexes of membrane and cytosolic proteins into a multifaceted scaffold linked to filamentous structural elements (termed linked mosaics), recent evidence supports a similar role for spectrin in organelle function and the secretory pathway. Two working models accommodate much of the available data: the Golgi mesh hypothesis and the spectrin ankyrin adapter protein tethering system (SAATS) hypothesis.

Ras-induced cellular events (review)

Ras is a crucial regulator of cell growth in eukaryotic cells. Activated Ras can stimulate signal transduction cascades, leading to cell proliferation, differentiation or apoptosis. It is also one of the most commonly mutated genes in both solid tumours and haematologic neoplasias. In leukaemia and tumours, aberrant Ras signalling can be induced directly by Ras mutation or indirectly by altering genes that associate with Ras or its signalling pathways. A requisite for Ras function is localization to the plasma membrane, which is induced by the post-translational modification farnesylation. Molecules that interfere with this Ras modification have been used as antitumour agents. Ras is emerging as a dual regulator of cell functions, playing either positive or negative roles in the control of proliferation or apoptosis. The diversity of Ras-mediated effects may be related in part to the differential involvement of Ras homologues in distinct cellular processes or to the expanding array of Ras effectors.

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

It has been shown previously that the morphology and subcellular positioning of the Golgi complex is controlled by actin microfilaments. To further characterize the association between actin microfilaments and the Golgi complex, we have used the Clostridium botulinum toxins C2 and C3, which specifically inhibit actin polymerization and cause depolymerization of F-actin in intact cells by the ADP ribosylation of G-actin monomers and the Rho small GTP-binding protein, respectively. Normal rat kidney cells treated with C2 showed that disruption of the actin and the collapse of the Golgi complex occurred concomitantly. However, when cells were treated with C3, the actin disassembly was observed without any change in the organization of the Golgi complex. The absence of the involvement of Rho was further confirmed by the treatment with lysophosphatidic acid or microinjection with the constitutively activated form of RhoA, both of which induced the stress fiber formation without affecting the Golgi complex. Immunogold electron microscopy in normal rat kidney cells revealed that beta- and gamma-actin isoforms were found in Golgi-associated COPI-coated buds and vesicles. Taken together, the results suggest that the Rho signaling pathway does not directly regulate Golgi-associated actin microfilaments, and that beta- and gamma-actins might be involved in the formation and/or transport of Golgi-derived vesicular or tubular intermediates.

Morphological changes in the Golgi complex correlate with actin cytoskeleton rearrangements

In this report we have studied the morphological changes of the Golgi complex (GC) that specifically accompany F-actin reorganizations. In starved rat RBL-2H3 tumor mast cells, the GC, that was visualized at immunofluorescence level with antibodies raised against the Golgi-resident proteins giantin, mannosidase II, or TGN-38, showed a compacted morphology with a supranuclear positioning. Concomitant to membrane ruffle formation induced by epidermal growth factor (EGF) or phorbol 12-myristate 13-acetate (PMA), and stress fiber formation induced by lysophosphatidic acid (LPA), specific GC morphological changes were observed. When cells were stimulated with EGF or PMA, the compacted GC morphology was transformed into a reticular network that was extended towards the cell periphery. When cells were incubated with LPA, the GC acquired a characteristic ring-shaped morphology. Brefeldin A (BFA) did not affect the PMA- or LPA-induced membrane ruffling and stress fiber formation, respectively, indicating that actin rearrangements occurred independent of the presence of the GC. Upon BFA removal, the presence of PMA or LPA during the recovery process induced the GC to acquire the morphological appearance described above for each agent. Moreover, the PMA- but not the LPA-induced GC rearrangements were sensitive to the actin perturbing agents cytochalasin D and jasplakinolide. When cells were preincubated with the phosphatidylinositide 3-kinase (PI3K) inhibitors wortmannin or LY294002, the PMA-induced GC morphological changes were inhibited but not membrane ruffles. Finally, the PMA-induced increase in the post-Golgi transport of glycosaminoglycans to the cell surface was not altered by cytochalasin D or jasplakinolide. Altogether, these data suggest that: (1) the shape of the GC is influenced by the 3D arrangement of actin microfilaments; (2) PI3K regulates the association of the GC with actin microfilaments; and (3) actin microfilaments are not essential for the post-Golgi transport to the plasma membrane.

Morphological and biochemical analysis of the secretory pathway in melanoma cells with distinct metastatic potential

In this report, we have investigated whether alterations of the morphological and functional aspects of the biosecretory membrane system are associated with the metastatic potential of tumor cells. To this end, we have analyzed the morphology of the Golgi complex, the cytoskeleton organization and membrane trafficking steps of the secretory pathway in two human melanoma A375 cell line variants with low (A375-P) and high metastatic (A375-MM) potential. Immunofluorescence analysis showed that in A375-P cells, the Golgi complex showed a collapsed morphology. Conversely, in A375-MM cells, the Golgi complex presented a reticular and extended morphology. At the ultrastructural level, the Golgi complex of A375-P cells was fragmented and cisternae were swollen. When the cytoskeleton was analyzed, the microtubular network appeared normal in both cell variants, whereas actin stress fibers were largely absent in A375-P, but not in A375-MM cells. In addition, the F-actin content in A375-P cells was significantly lower than in A375-MM cells. These morphological differences in A375-P cells were accompanied by acceleration and an increase in the endoplasmic reticulum to Golgi and the trans-Golgi network to cell surface membrane transport, respectively. Our results indicate that in human A375 melanoma cells, metastatic potential correlates with a well-structured morphofunctional organization of the Golgi complex and actin cytoskeleton.