Actin filaments and myosin I alpha cooperate with microtubules for the movement of lysosomes

RAB26 coordinates lysosome traffic and mitochondrial localization

As they mature, professional secretory cells like pancreatic acinar and gastric chief cells induce the transcription factor MIST1 (also known as BHLHA15) to substantially scale up production of large secretory granules in a process that involves expansion of apical cytoplasm and redistribution of lysosomes and mitochondria. How a scaling factor like MIST1 rearranges cellular architecture simply by regulating expression levels of its transcriptional targets is unknown. RAB26 is a MIST1 target whose role in MIST1-mediated secretory cell maturation is also unknown. Here, we confirm that RAB26 expression, unlike most Rabs which are ubiquitously expressed, is tissue specific and largely confined to MIST1-expressing secretory tissues. Surprisingly, functional studies showed that RAB26 predominantly associated with LAMP1/cathepsin D lysosomes and not directly with secretory granules. Moreover, increasing RAB26 expression - by inducing differentiation of zymogen-secreting cells or by direct transfection - caused lysosomes to coalesce in a central, perinuclear region. Lysosome clustering in turn caused redistribution of mitochondria into distinct subcellular neighborhoods. The data elucidate a novel function for RAB26 and suggest a mechanism for how cells could increase transcription of key effectors to reorganize subcellular compartments during differentiation.

Development of a Kinetic Assay for Late Endosome Movement

Automated imaging screens are performed mostly on fixed and stained samples to simplify the workflow and increase throughput. Some processes, such as the movement of cells and organelles or measuring membrane integrity and potential, can be measured only in living cells. Developing such assays to screen large compound or RNAi collections is challenging in many respects. Here, we develop a live-cell high-content assay for tracking endocytic organelles in medium throughput. We evaluate the added value of measuring kinetic parameters compared with measuring static parameters solely. We screened 2000 compounds in U-2 OS cells expressing Lamp1-GFP to label late endosomes. All hits have phenotypes in both static and kinetic parameters. However, we show that the kinetic parameters enable better discrimination of the mechanisms of action. Most of the compounds cause a decrease of motility of endosomes, but we identify several compounds that increase endosomal motility. In summary, we show that kinetic data help to better discriminate phenotypes and thereby obtain more subtle phenotypic clustering.

Myosins 1 and 6, myosin light chain kinase, actin and microtubules cooperate during antibody-mediated internalisation and trafficking of membrane-expressed viral antigens in feline infectious peritonitis virus infected monocytes

Monocytes infected with feline infectious peritonitis virus, a coronavirus, express viral proteins in their plasma membranes. Upon binding of antibodies, these proteins are quickly internalised through a new clathrin- and caveolae-independent internalisation pathway. By doing so, the infected monocytes can escape antibody-dependent cell lysis. In the present study, we investigated which kinases and cytoskeletal proteins are of importance during internalisation and subsequent intracellular transport. The experiments showed that myosin light chain kinase (MLCK) and myosin 1 are crucial for the initiation of the internalisation. With co-localisation stainings, it was found that MLCK and myosin 1 co-localise with antigens even before internalisation started. Myosin 6 co-localised with the internalising complexes during passage through the cortical actin, were it might play a role in moving or disintegrating actin filaments, to overcome the actin barrier. One minute after internalisation started, vesicles had passed the cortical actin, co-localised with microtubules and association with myosin 6 was lost. The vesicles were further transported over the microtubules and accumulated at the microtubule organising centre after 10 to 30 min. Intracellular trafficking over microtubules was mediated by MLCK, myosin 1 and a small actin tail. Since inhibiting MLCK with ML-7 was so efficient in blocking the internalisation pathway, this target can be used for the development of a new treatment for FIPV.

Lysosome transport as a function of lysosome diameter

Lysosomes are membrane-bound organelles responsible for the transport and degradation of intracellular and extracellular cargo. The intracellular motion of lysosomes is both diffusive and active, mediated by motor proteins moving lysosomes along microtubules. We sought to determine how lysosome diameter influences lysosome transport. We used osmotic swelling to double the diameter of lysosomes, creating a population of enlarged lysosomes. This allowed us to directly examine the intracellular transport of the same organelle as a function of diameter. Lysosome transport was measured using live cell fluorescence microscopy and single particle tracking. We find, as expected, the diffusive component of intracellular transport is decreased proportional to the increased lysosome diameter. Active transport of the enlarged lysosomes is not affected by the increased lysosome diameter.

Supporting a role for the GTPase Rab7 in prostate cancer progression

Invasion and subsequent metastasis is the major cause of death from most cancers including prostate cancer. Herein we report on the potential tumor suppressive properties of Rab7, a GTPase that regulates trafficking of lysosomes. The movement of lysosomes to the cell surface in response to environmental cues increases the secretion of proteinases and cell invasion. We determined that Troglitazone and other members of the Thiazolidinedione family inhibit cell-surface directed lysosome trafficking and cathepsin B secretion through a Rab7-dependent mechanism. Moreover, Rab7 shRNA expressing cells were found to be more invasive in vitro and in vivo. Increased invasiveness was accompanied by elevated expression of the c-Met receptor and prolonged downstream signaling, thereby supporting a role for Rab7 as a mediator of signaling down-regulation. Taken together, these results suggested that Rab7 acts as a negative regulator of prostate tumor growth and invasion, providing further evidence for its potential as a tumor suppressor.

Involvement of lysosomal exocytosis in the excretion of mesoporous silica nanoparticles and enhancement of the drug delivery effect by exocytosis inhibition

The exocytosis of phosphonate modified mesoporous silica nanoparticles (P-MSNs) is demonstrated and lysosomal exocytosis is identified as the mechanism responsible for this event. Regulation of P-MSN exocytosis can be achieved by inhibiting or accelerating lysosomal exocytosis. Slowing down P-MSN exocytosis enhances the drug delivery effect of CPT-loaded P-MSNs by improving cell killing.

AP-3 and Rabip4' coordinately regulate spatial distribution of lysosomes

The RUN and FYVE domain proteins rabip4 and rabip4' are encoded by RUFY1 and differ in a 108 amino acid N-terminal extension in rabip4'. Their identical C terminus binds rab5 and rab4, but the function of rabip4s is incompletely understood. We here found that silencing RUFY1 gene products promoted outgrowth of plasma membrane protrusions, and polarized distribution and clustering of lysosomes at their tips. An interactor screen for proteins that function together with rabip4' yielded the adaptor protein complex AP-3, of which the hinge region in the β3 subunit bound directly to the FYVE domain of rabip4'. Rabip4' colocalized with AP-3 on a tubular subdomain of early endosomes and the extent of colocalization was increased by a dominant negative rab4 mutant. Knock-down of AP-3 had an ever more dramatic effect and caused accumulation of lysosomes in protrusions at the plasma membrane. The most peripheral lysosomes were localized beyond microtubules, within the cortical actin network. Our results uncover a novel function for AP-3 and rabip4' in regulating lysosome positioning through an interorganellar pathway.

Identification of cytoskeleton-associated proteins essential for lysosomal stability and survival of human cancer cells

Microtubule-disturbing drugs inhibit lysosomal trafficking and induce lysosomal membrane permeabilization followed by cathepsin-dependent cell death. To identify specific trafficking-related proteins that control cell survival and lysosomal stability, we screened a molecular motor siRNA library in human MCF7 breast cancer cells. SiRNAs targeting four kinesins (KIF11/Eg5, KIF20A, KIF21A, KIF25), myosin 1G (MYO1G), myosin heavy chain 1 (MYH1) and tropomyosin 2 (TPM2) were identified as effective inducers of non-apoptotic cell death. The cell death induced by KIF11, KIF21A, KIF25, MYH1 or TPM2 siRNAs was preceded by lysosomal membrane permeabilization, and all identified siRNAs induced several changes in the endo-lysosomal compartment, i.e. increased lysosomal volume (KIF11, KIF20A, KIF25, MYO1G, MYH1), increased cysteine cathepsin activity (KIF20A, KIF25), altered lysosomal localization (KIF25, MYH1, TPM2), increased dextran accumulation (KIF20A), or reduced autophagic flux (MYO1G, MYH1). Importantly, all seven siRNAs also killed human cervix cancer (HeLa) and osteosarcoma (U-2-OS) cells and sensitized cancer cells to other lysosome-destabilizing treatments, i.e. photo-oxidation, siramesine, etoposide or cisplatin. Similarly to KIF11 siRNA, the KIF11 inhibitor monastrol induced lysosomal membrane permeabilization and sensitized several cancer cell lines to siramesine. While KIF11 inhibitors are under clinical development as mitotic blockers, our data reveal a new function for KIF11 in controlling lysosomal stability and introduce six other molecular motors as putative cancer drug targets.

The effect of nanoparticle uptake on cellular behavior: disrupting or enabling functions?

Nanoparticles (NPs) are materials with overall dimensions in the nanoscale range. They have unique physicochemical properties, and have emerged as important players in current research in modern medicine. In the last few decades, several types of NPs and microparticles have been synthesized and proposed for use as contrast agents for diagnostics and imaging and for drug delivery; for example, in cancer therapy. Yet specific targeting that will improve their delivery still represents an unsolved challenge. The mechanism by which NPs enter the cell has important implications not only for their fate but also for their impact on biological systems. Several papers in the literature discuss the potential risks related to NP exposure, and more recently the concept that even sublethal doses of NPs may elicit a cell response has been proposed. In this review, we intend to present an overall view of cell mechanisms that may be perturbed by cell-NP interaction. Published data, in fact, emphasize that NPs should no longer be viewed only as simple carriers for biomedical applications, but that they can also play an active role in mediating biological effects.

Actin cytoskeleton controls movement of intracellular organelles in pancreatic duct epithelial cells

In most eukaryotic cells, microtubules and filamentous actin (F-actin) provide tracks on which intracellular organelles move using molecular motors. Here we report that cytoplasmic movement of both mitochondria and lysosomes is slowed by F-actin meshwork formation in pancreatic duct epithelial cells (PDEC). Mitochondria and lysosomes were labeled with fluorescent Mitotracker Red CMXRos and Lysotracker Red DND-99, respectively, and their movements were monitored using epi-fluorescence and confocal microscopy. Mitochondria and lysosomes moving actively at rest stopped rapidly within several seconds after an intracellular Ca(2+) rise induced by activation of P2Y(2) purinergic receptors. The 'freezing' of the organelles was inhibited by blocking the Ca(2+) rise or by pretreatment with latrunculin B, an inhibitor of F-actin formation. Indeed, this freezing effect on the organelles was accompanied by the formation of F-actin in the whole cytoplasm as stained with Alexa 488-phalloidin in fixed PDEC. For real-time monitoring of F-actin formation in live cells, we expressed sGFP-fimbrin actin binding domain2 (fABD2) in PDEC. Rapid recruitment of the fluorescent probe near the nucleus and lysosomes suggested dense F-actin formation around intracellular structures. The development of F-actin paralleled that of organelle freezing. We conclude that rapid Ca(2+)-dependent F-actin formation physically restrains intracellular organelles and reduces their mobility non-selectively in PDEC.

Expression of axonal protein degradation machinery in sympathetic neurons is regulated by nerve growth factor

Deficiencies in protein degradation and proteolytic function within neurons are linked to a number of neurodegenerative diseases and developmental disorders. Compartmentalized cultures of peripheral neurons were used to investigate the properties and relative abundance of the proteolytic machinery in the axons and cell bodies of sympathetic and sensory neurons. Immunoblotting of axonal proteins demonstrated that LAMP2, LC3, and PSMA2 were abundant in axons, suggesting that lysosomes, autophagosomes and proteasomes were located in axons. Interestingly, the expression of proteins associated with lysosomes and proteasomes were upregulated selectively in axons by NGF stimulation of the distal axons of sympathetic neurons, suggesting that axonal growth and maintenance requires local protein turnover. The regulation of the abundance of both proteasomes and lysosomes in axons by NGF provides a link between protein degradation and the trophic status of peripheral neurons. Inhibition of proteasomes located in axons resulted in an accumulation of ubiquitinated proteins in these axons. In contrast, lysosome inhibition in axons did not result in an accumulation of ubiquitinated proteins or the transferrin receptor, a transmembrane protein degraded by lysosomes. Interestingly, lysosomes were transported both retrogradely and anterogradely, so it is likely that ubiquitinated proteins that are normally destined for degradation by lysosomes in axons can be transported to the cell bodies for degradation. In summary, proteasomal degradation occurs locally, whereas proteins degraded by lysosomes can most likely either be degraded locally in axons or be transported to cell bodies for degradation.

Cellular uptake of cyclotide MCoTI-I follows multiple endocytic pathways

Cyclotides are plant-derived proteins that naturally exhibit various biological activities and whose unique cyclic structure makes them remarkably stable and resistant to denaturation or degradation. These attributes, among others, make them ideally suited for use as drug development tools. This study investigated the cellular uptake of cyclotide, MCoTI-I in live HeLa cells. Using real time confocal fluorescence microscopy imaging, we show that MCoTI-I is readily internalized in live HeLa cells and that its endocytosis is temperature-dependent. Endocytosis of MCoTI-I in HeLa cells is achieved primarily through fluid-phase endocytosis, as evidenced by its significant colocalization with 10K-dextran, but also through other pathways as well, as evidenced by its colocalization with markers for cholesterol-dependent and clathrin-mediated endocytosis, cholera toxin B and EGF respectively. Uptake does not appear to occur only via macropinocytosis as inhibition of this pathway by Latrunculin B-induced disassembly of actin filaments did not affect MCoTI-I uptake and treatment with EIPA which also seemed to inhibit other pathways collectively inhibited approximately 80% of cellular uptake. As well, a significant amount of MCoTI-I accumulates in late endosomal and lysosomal compartments and MCoTI-I-containing vesicles continue to exhibit directed movements. These findings demonstrate internalization of MCoTI-I through multiple endocytic pathways that are dominant in the cell type investigated, suggesting that this cyclotide has ready access to general endosomal/lysosomal pathways but could readily be re-targeted to specific receptors through addition of targeting ligands.

Lysosomal membrane protein composition, acidic pH and sterol content are regulated via a light-dependent pathway in metazoan cells

In metazoans, lysosomes are characterized by a unique tubular morphology, acidic pH, and specific membrane protein (LAMP) and lipid (cholesterol) composition as well as a soluble protein (hydrolases) composition. Here we show that perturbation to the eye-color gene, light, results in impaired lysosomal acidification, sterol accumulation, altered endosomal morphology as well as compromised lysosomal degradation. We find that Drosophila homologue of Vps41, Light, regulates the fusion of a specific subset of biosynthetic carriers containing characteristic endolysosomal membrane proteins, LAMP1, V0-ATPase and the cholesterol transport protein, NPC1, with the endolysosomal system, and is then required for the morphological progression of the multivesicular endosome. Inhibition of Light results in accumulation of biosynthetic transport intermediates that contain these membrane cargoes, whereas under similar conditions, endosomal delivery of soluble hydrolases, previously shown to be mediated by Dor, the Drosophila homologue of Vps18, is not affected. Unlike Dor, Light is recruited to endosomes in a PI3P-sensitive fashion wherein it facilitates fusion of these biosynthetic cargoes with the endosomes. Depletion of the mammalian counterpart of Light, hVps41, in a human cell line also inhibits delivery of hLAMP to endosomes, suggesting an evolutionarily conserved pathway in metazoa.

Receptor sorting within endosomal trafficking pathway is facilitated by dynamic actin filaments

Early endosomes (EEs) are known to be a sorting station for internalized molecules destined for degradation, recycling, or other intracellular organelles. Segregation is an essential step in such sorting, but the molecular mechanism of this process remains to be elucidated. Here, we show that actin is required for efficient recycling and endosomal maturation by producing a motile force. Perturbation of actin dynamics by drugs induced a few enlarged EEs containing several degradative vacuoles and also interfered with their transporting ability. Actin repolymerization induced by washout of the drug caused the vacuoles to dissociate and individually translocate toward the perinuclear region. We further elucidated that cortactin, an actin-nucleating factor, was required for transporting contents from within EEs. Actin filaments regulated by cortactin may provide a motile force for efficient sorting within early endosomes. These data suggest that actin filaments coordinate with microtubules to mediate segregation in EEs.

Huntingtin coordinates the dynein-mediated dynamic positioning of endosomes and lysosomes

Huntingtin (Htt) is a membrane-associated scaffolding protein that interacts with microtubule motors as well as actin-associated adaptor molecules. We examined a role for Htt in the dynein-mediated intracellular trafficking of endosomes and lysosomes. In HeLa cells depleted of either Htt or dynein, early, recycling, and late endosomes (LE)/lysosomes all become dispersed. Despite altered organelle localization, kinetic assays indicate only minor defects in intracellular trafficking. Expression of full-length Htt is required to restore organelle localization in Htt-depleted cells, supporting a role for Htt as a scaffold that promotes functional interactions along its length. In dynein-depleted cells, LE/lysosomes accumulate in tight patches near the cortex, apparently enmeshed by cortactin-positive actin filaments; Latrunculin B-treatment disperses these patches. Peripheral LE/lysosomes in dynein-depleted cells no longer colocalize with microtubules. Htt may be required for this off-loading, as the loss of microtubule association is not seen in Htt-depleted cells or in cells depleted of both dynein and Htt. Inhibition of kinesin-1 relocalizes peripheral LE/lysosomes induced by Htt depletion but not by dynein depletion, consistent with their detachment from microtubules upon dynein knockdown. Together, these data support a model of Htt as a facilitator of dynein-mediated trafficking that may regulate the cytoskeletal association of dynamic organelles.

Intracellular uptake, transport, and processing of gold nanostructures

The emerging field of nanomedicine requires better understanding of the interface between nanotechnology and medicine. Better knowledge of the nano-bio interface will lead to better tools for diagnostic imaging and therapy. In this review, recent progress in understanding of how size, shape, and surface properties of nanoparticles (NPs) affect intracellular fate of NPs is discussed. Gold nanostructures are used as a model system in this regard since their physical and chemical properties can be easily manipulated. The NP-uptake is dependent on the physiochemical properties, and once in the cell, most of the NPs are trafficked via an endo-lysosomal path followed by a receptor-mediated endocytosis process at the cell membrane. Within the size range of 2-100 nm, Gold nanoparticles (GNPs) of diameter 50 nm demonstrate the highest uptake. Cellular uptake studies of gold nanorods (GNRs) show that there is a decrease in uptake as the aspect ratio of GNRs increases. Theoretical models support the size- and shape-dependent NP-uptake. The intracellular transport of targeted NPs is faster than untargeted NPs. The surface ligand and charge of NPs play a bigger role in their uptake, transport, and organelle distribution. Exocytosis of NPs is dependent on size and shape as well; however, the trend is different compared to endocytosis. GNPs are now being incorporated into polymer and lipid based NPs to build multifunctional devices. A multifunctional platform based on gold nanostructures, with multimodal imaging, targeting, and therapeutics; hold the possibility of promising directions in medical research.

Calmodulin dissociation regulates Myo5 recruitment and function at endocytic sites

Myosins-I are conserved proteins that bear an N-terminal motor head followed by a Tail Homology 1 (TH1) lipid-binding domain. Some myosins-I have an additional C-terminal extension (C(ext)) that promotes Arp2/3 complex-dependent actin polymerization. The head and the tail are separated by a neck that binds calmodulin or calmodulin-related light chains. Myosins-I are known to participate in actin-dependent membrane remodelling. However, the molecular mechanisms controlling their recruitment and their biochemical activities in vivo are far from being understood. In this study, we provided evidence suggesting the existence of an inhibitory interaction between the TH1 domain of the yeast myosin-I Myo5 and its C(ext). The TH1 domain prevented binding of the Myo5 C(ext) to the yeast WIP homologue Vrp1, Myo5 C(ext)-induced actin polymerization and recruitment of the Myo5 C(ext) to endocytic sites. Our data also indicated that calmodulin dissociation from Myo5 weakened the interaction between the neck and TH1 domains and the C(ext). Concomitantly, calmodulin dissociation triggered Myo5 binding to Vrp1, extended the myosin-I lifespan at endocytic sites and activated Myo5-induced actin polymerization.

Pdro, a protein associated with late endosomes and lysosomes and implicated in cellular cholesterol homeostasis

BACKGROUND

Cellular cholesterol is a vital component of the cell membrane. Its concentration is tightly controlled by mechanisms that remain only partially characterized. In this study, we describe a late endosome/lysosomes-associated protein whose expression level affects cellular free cholesterol content.

METHODOLOGY/PRINCIPAL FINDINGS

Using a restricted proteomic analysis of detergent-resistant membranes (DRMs), we have identified a protein encoded by gene C11orf59. It is mainly localized to late endosome/lysosome (LE/LY) compartment through N-terminal myristoylation and palmitoylation. We named it Pdro for protein associated with DRMs and endosomes. Very recently, three studies have reported on the same protein under two other names: the human p27RF-Rho that regulates RhoA activation and actin dynamics, and its rodent orthologue p18 that controls both LE/LY dynamics through the MERK-ERK pathway and the lysosomal activation of mammalian target of rapamycin complex 1 by amino acids. We found that, consistent with the presence of sterol-responsive element consensus sequences in the promoter region of C11orf59, Pdro mRNA and protein expression levels are regulated positively by cellular cholesterol depletion and negatively by cellular cholesterol loading. Conversely, Pdro is involved in the regulation of cholesterol homeostasis, since its depletion by siRNA increases cellular free cholesterol content that is accompanied by an increased cholesterol efflux from cells. On the other hand, cells stably overexpressing Pdro display reduced cellular free cholesterol content. Pdro depletion-mediated excess cholesterol results, at least in part, from a stimulated low-density lipoprotein (LDL) uptake and an increased cholesterol egress from LE/LY.

CONCLUSIONS/SIGNIFICANCE

LDL-derived cholesterol release involves LE/LY motility that is linked to actin dynamics. Because Pdro regulates these two processes, we propose that modulation of Pdro expression in response to sterol levels regulates LDL-derived cholesterol through both LDL uptake and LE/LY dynamics, to ultimately control free cholesterol homeostasis.

Rab11 and actin cytoskeleton participate in Giardia lamblia encystation, guiding the specific vesicles to the cyst wall

Background

Giardia passes through two stages during its life cycle, the trophozoite and the cyst. Cyst formation involves the synthesis of cyst wall proteins (CWPs) and the transport of CWPs into encystation-specific vesicles (ESVs). Active vesicular trafficking is essential for encystation, but the molecular machinery driving vesicular trafficking remains unknown. The Rab proteins are involved in the targeting of vesicles to several intracellular compartments through their association with cytoskeletal motor proteins.

Methodology and Principal Findings

In this study, we found a relationship between Rab11 and the actin cytoskeleton in CWP1 transport. Confocal microscopy showed Rab11 was distributed throughout the entire trophozoite, while in cysts it was translocated to the periphery of the cell, where it colocalized with ESVs and microfilaments. Encystation was also accompanied by changes in rab11 mRNA expression. To evaluate the role of microfilaments in encystation, the cells were treated with latrunculin A. Scanning electron microscopy showed this treatment resulted in morphological damages to encysted parasites. The intensity of fluorescence-labeled Rab11 and CWP1 in ESVs and cyst walls was reduced, and rab11 and cwp1 mRNA levels were down-regulated. Furthermore, knocking down Rab11 with a hammerhead ribozyme resulted in an up to 80% down-regulation of rab11 mRNA. Although this knockdown did not appear lethal for trophozoites and did not affect cwp1 expression during the encystation, confocal images showed CWP1 was redistributed throughout the cytosol.

Conclusions and Significance

Our results indicate that Rab11 participates in the early and late encystation stages by regulating CWP1 localization and the actin-mediated transport of ESVs towards the periphery. In addition, alterations in the dynamics of actin affected rab11 and cwp1 expression. Our results provide new information about the molecules involved in Giardia encystation and suggest that Rab11 and actin may be useful as novel pharmacological targets.

The encystation process is crucial for survival and transmission of Giardia lamblia to new hosts. During this process, vesicular trafficking and the cytoskeleton play important roles. In eukaryotic cells, intracellular transport is regulated by proteins, including Rab-GTPases and SNAREs, which regulate vesicle formation along with recognition of and binding to the target membrane. Cytoskeletal structures are also involved in these processes. In this study, we demonstrate the participation of Rab11 in the transport of encystation-specific vesicles (ESVs). Additionally, we demonstrate that disruption of actin microfilaments affects ESVs transport. The modification of actin dynamics was also correlated with a reduction in rab11 and cwp1 expression. Furthermore, down-regulation of rab11 mRNA by a specific hammerhead ribozyme caused nonspecific localization of CWP1. We thus provide new information about the molecular machinery that regulates Giardia lamblia encystation. Given our findings, Rab11 and actin may be useful targets to block Giardia encystation.

HGF-induced invasion by prostate tumor cells requires anterograde lysosome trafficking and activity of Na+-H+ exchangers

Hepatocyte growth factor (HGF) is found in tumor microenvironments, and interaction with its tyrosine kinase receptor Met triggers cell invasion and metastasis. It was previously shown that acidic extracellular pH stimulated peripheral lysosome trafficking, resulting in increased cathepsin B secretion and tumor cell invasion, which was dependent upon sodium-proton exchanger (NHE) activity. We now demonstrate that HGF induced the trafficking of lysosomes to the cell periphery, independent of HGF-induced epithelial-mesenchymal transition. HGF-induced anterograde lysosome trafficking depended upon the PI3K pathway, microtubules and RhoA, resulting in increased cathepsin B secretion and invasion by the cells. HGF-induced NHE activity via increased net acid production, and inhibition of NHE activity with 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), or a combination of the NHE1-specific drug cariporide and the NHE3-specific drug s3226 prevented HGF-induced anterograde trafficking and induced retrograde trafficking in HGF-overexpressing cells. EIPA treatment reduced cathepsin B secretion and HGF-induced invasion by the tumor cells. Lysosomes were located more peripherally in Rab7-shRNA-expressing cells and these cells were more invasive than control cells. Overexpression of the Rab7 effector protein, RILP, resulted in a juxtanuclear location of lysosomes and reduced HGF-induced invasion. Together, these results suggest that the location of lysosomes is an inherently important aspect of invasion by tumor cells.

Motion detection for subcellular structure trafficking

A detailed understanding of subcellular structure motility is critical to understanding how cells regulate the delivery of specific proteins from the site of synthesis to the site of action. Diverse modalities have been observed for subcellular dynamics, such as directional movement, random movement, tethered movement, object appearing, and disappearing. Motion modality detection is important for efficient subcelluar structure tracking and population study. In this paper, we present a new technique, called "divergence filter," for detecting subcellular structure motion modalities. The plausibility of the efficient technique was applied to caveolar membrane trafficking sequences obtained using confocal microscopy.

Na+/H+ exchangers and RhoA regulate acidic extracellular pH-induced lysosome trafficking in prostate cancer cells

Acidic extracellular pH (pHe) is a common feature of the tumor microenvironment and has been implicated in tumor invasion through the induction of protease secretion.Since lysosomes constitute the major storehouse of cellular proteases, the trafficking of lysosomes to the cell periphery may be required in order to secrete proteases. We demonstrate that a pHe of 6.4-6.8 induced the trafficking of lysosomes to membrane protrusions in the cell periphery. This trafficking event depended upon the PI3K pathway, the GTPase RhoA and sodium-proton exchange activity, resulting in lysosomal exocytosis. Acidic pHe induced a cytoplasmic acidification (although cytoplasmic acidification was not sufficient for acidic pHe-induced lysosome trafficking and exocytosis) and inhibition of NHE activity with the amiloride derivative, EIPA or the anti-diabetic agent troglitazone prevented lysosome trafficking to the cell periphery. Interestingly, using the more specific NHE1 and NHE3 inhibitors, cariporide and s3226 respectively, we show that multiple NHE isoforms are involved in acidic pHe-induced lysosome trafficking and exocytosis. Moreover, in cells expressing NHE1 shRNA, although basal NHE activity was decreased, lysosomes still underwent acidic pHe-induced trafficking,suggesting compensation by other NHE family members.Together these data implicate proton exchangers, especially NHE1 and NHE3, in acidic pHe-induced lysosome trafficking and exocytosis.

Participation of actin on Giardia lamblia growth and encystation

Background

Microfilaments play a determinant role in different cell processes such as: motility, cell division, phagocytosis and intracellular transport; however, these structures are poorly understood in the parasite Giardia lamblia.

Methodology and Principal Findings

By confocal microscopy using TRITC-phalloidin, we found structured actin distributed in the entire trophozoite, the label stand out at the ventral disc, median body, flagella and around the nuclei. During Giardia encystation, a sequence of morphological changes concurrent to modifications on the distribution of structured actin and in the expression of actin mRNA were observed. To elucidate whether actin participates actively on growth and encystation, cells were treated with Cytochalasin D, Latrunculin A and Jasplakinolide and analyzed by confocal and scanning electron microscopy. All drugs caused a growth reduction (27 to 45%) and changes on the distribution of actin. Besides, 60 to 80% of trophozoites treated with the drugs, exhibited damage at the caudal region, alterations in the flagella and wrinkles-like on the plasma membrane. The drugs also altered the cyst-yield and the morphology, scanning electron microscopy revealed diminished cytokinesis, cysts with damages in the wall and alterations in the size and on the intermembranal space. Furthermore, the drugs caused a significant reduction of the intensity of flourescence-labeled CWP1 on ESV and on cyst wall, this was coincident with a reduction of CWP1 gene expression (34%).

Conclusions and Significance

All our results, indicated an important role of actin in the morphology, growth and encystation and indirectly suggested an actin role in gene expression.

Relocation of myosin and actin, kinesin and tubulin in the acrosome reaction of bovine spermatozoa

The mammalian acrosome reaction is a specialised exocytotic event. Although molecular motors are known to be involved in exocytosis in many cell types, their potential involvement in the acrosome reaction has remained unknown. Here, it has been shown that actin is localised within the equatorial segment and in the marginal acrosomal ridge of the heads of unreacted bull spermatozoa. Myosins IIA and IIB are found within the anterior acrosomal margins of virtually all sperm cells and, less prominently, within the equatorial segment. Tubulin was detected in the equatorial segment and around the periphery of the acrosome while kinesin was prominent in the equatorial segment. After induction of the acrosome reaction by means of the calcium ionophore A23187, the number of cells exhibiting actin fluorescence intensity in the anterior acrosomal margin decreased four-fold and those displaying equatorial segment fluorescence decreased 3.5-fold; myosin IIA immunofluorescence decreased in intensity with most spermatozoa losing equatorial staining, whereas there was little change in the distribution or intensity of myosin IIB immunofluorescence, except for approximately 20% decrease in the number of cells exhibiting acrosomal staining. Tubulin became largely undetectable within the head and kinesin staining spread rostrally over the main acrosome region. A possible sequence of events that ties in these observations of molecular motor involvement with the known participation of SNARE proteins is provided.

Organelle transport: dynamic actin tracks for myosin motors

Transport of cargo by molecular motors on microtubule and actin filament tracks is a fundamental property of eukaryotic cells. A new study reports that actin dynamics are required in cells for myosin I and V motor proteins to transport their organelle cargos on actin tracks.

Alpha-E-catenin binds to dynamitin and regulates dynactin-mediated intracellular traffic

Alpha-epithelial catenin (E-catenin) is an important cell-cell adhesion protein. In this study, we show that alpha-E-catenin also regulates intracellular traffic by binding to the dynactin complex component dynamitin. Dynactin-mediated organelle trafficking is increased in alpha-E-catenin(-/-) keratinocytes, an effect that is reversed by expression of exogenous alpha-E-catenin. Disruption of adherens junctions in low-calcium media does not affect dynactin-mediated traffic, indicating that alpha-E-catenin regulates traffic independently from its function in cell-cell adhesion. Although neither the integrity of dynactin-dynein complexes nor their association with vesicles is affected by alpha-E-catenin, alpha-E-catenin is necessary for the attenuation of microtubule-dependent trafficking by the actin cytoskeleton. Because the actin-binding domain of alpha-E-catenin is necessary for this regulation, we hypothesize that alpha-E-catenin functions as a dynamic link between the dynactin complex and actin and, thus, integrates the microtubule and actin cytoskeleton during intracellular trafficking.

Actin dynamics is essential for myosin-based transport of membrane organelles

Actin filaments that serve as "rails" for the myosin-based transport of membrane organelles [1-4] continuously turn over by concurrent growth and shortening at the opposite ends [5]. Although it is known that dynamics of actin filaments is essential for many of the actin cytoskeleton functions, the role of such dynamics in myosin-mediated organelle transport was never studied before. Here, we addressed the role of turnover of actin filaments in the myosin-based transport of membrane organelles by treating cells with the drugs that suppress actin-filament dynamics and found that such a suppression significantly inhibited organelle transport along the actin filaments without inhibiting their intracellular distribution or the activity of the myosin motors. We conclude that dynamics of actin filaments is essential for myosin-based transport of membrane organelles and suggest a previously unknown role of actin-filament dynamics in providing the "rails" for continuous organelle movement resulting in the increased distances traveled by membrane organelles along the actin filaments.

Disorders of lysosome-related organelle biogenesis: clinical and molecular genetics

Lysosome-related organelles (LROs) are a heterogeneous group of vesicles that share various features with lysosomes, but are distinct in function, morphology, and composition. The biogenesis of LROs employs a common machinery, and genetic defects in this machinery can affect all LROs or only an individual LRO, resulting in a variety of clinical features. In this review, we discuss the main components of LRO biogenesis. We also summarize the function, composition, and resident cell types of the major LROs. Finally, we describe the clinical characteristics of the major human LRO disorders.

Annexin A8 regulates late endosome organization and function

Different classes of endosomes exhibit a characteristic intracellular steady-state distribution governed by interactions with the cytoskeleton. Late endosomes, organelles of the degradative lysosomal route, seem to require associated actin filaments for proper localization and function. We show here that the F-actin and phospholipid binding protein annexin A8 is associated specifically with late endosomes. Altering intracellular annexin A8 levels drastically affected the morphology and intracellular distribution of late endosomes. Trafficking through the degradative pathway was delayed in the absence of annexin A8, resulting in attenuated ligand-induced degradation of the epidermal growth factor receptor and prolonged epidermal growth factor-induced activation of mitogen-activated protein kinase. Depletion of annexin A8 reduced the association of late endosomal membranes with actin filaments. These results indicate that the defective cargo transport through the late endocytic pathway and the imbalanced signaling of activated receptors observed in the absence of annexin A8 results from the disturbed association of late endosomal membranes with the actin network, resulting in impaired actin-based late endosome motility.

Dynamic behavior of Salmonella-induced membrane tubules in epithelial cells

Salmonella Typhimurium is a facultative intracellular pathogen that causes acute gastroenteritis in man. Intracellular Salmonella survive and replicate within a modified phagosome known as the Salmonella-containing vacuole (SCV). The onset of intracellular replication is accompanied by the appearance of membrane tubules, called Salmonella-induced filaments (Sifs), extending from the SCV. Sifs are enriched in late endosomal/lysosomal membrane proteins such as lysosome-associated membrane protein 1, but their formation and ability to interact with endosomal compartments are not characterized. In this study, we use live cell imaging techniques to define the dynamics of Sif formation in infected epithelial cells. At early time-points, Sifs are simple tubules extending from the surface of SCVs. These tubules are highly dynamic and exhibit bidirectional, microtubule-dependent movement. At the distal ends of individual Sif tubules, furthest from the SCV, a distinct ‘leader’ domain was often observed. At later times, Sifs develop into highly complex tubular networks that extend throughout the cell and appear less dynamic than nascent Sifs; however, individual tubules continue to display bidirectional dynamics. Sifs can acquire endocytic content by fusion, indicating a sustained interaction with the endocytic pathway. Together, these results show that these Salmonella-induced tubules form a highly dynamic network that involves both microtubule-dependent motility and interactions with endosomal compartments.

Application of 2-dimensional difference gel electrophoresis (2D-DIGE) to the study of thrombin-activated human platelet secretome

Thrombin is an agonist inducing platelet activation. We combined two-dimensional difference gel electrophoresis (2D-DIGE) and mass spectrometry (MALDI-TOF MS) to analyse differentially expressed proteins secreted from thrombin-stimulated platelets. Human washed platelets, from healthy volunteers, were stimulated with thrombin 0.5 U/ml at 37 degrees C without stirring and the secreted proteins were resolved by 2D-DIGE. By image analysis, 1094 spots were detected in the 2D gel. The spots whose mean intensity showed at least a five-fold change intensity increase or decrease in the thrombin-activated platelet gel in comparison with unstimulated control were digested by trypsin and subjected to MALDI-TOF MS analysis. Peptides from mass spectra of in-gel digest samples were matched against available databases, using the Mascot search engine (Matrix Science) for peptide mass fingerprint. In the activated platelet secretome, transferrin, glutathione-transferase, WD repeat protein, ER-60, thrombospondin-1 precursor and thrombospondin were the most abundant. Also lamin A, a nuclear protein, not previously identified in platelets, appeared to be released. The novel strategy to combine 2D-DIGE with MALDI-TOF MS is a useful approach for a quantitative analysis of the effect of thrombin on the secretome profile of human platelets.

Actin in the endocytic pathway: from yeast to mammals

Genetic analysis of endocytosis in yeast early pointed to the essential role of actin in the uptake step. Efforts to identify the machinery involved demonstrated the important contribution of Arp2/3 and the myosins-I. Analysis of the process using live-cell fluorescence microscopy and electron microscopy have recently contributed to refine molecular models explaining clathrin and actin-dependent endocytic uptake. Increasing evidence now also indicates that actin plays important roles in post-internalization events along the endocytic pathway in yeast, including transport of vesicles, motility of endosomes and vacuole fusion. This review describes the present knowledge state on the roles of actin in endocytosis in yeast and points to similarities and differences with analogous processes in mammals.

Different microtubule motors move early and late endocytic compartments

Important progress has been made during the past decade in the identification of molecular motors required in the distribution of early and late endosomes and the proper trafficking along the endocytic pathway. There is little direct evidence, however, that these motors drive movement of the endosomes. To evaluate the contributions of kinesin-1, dynein and kinesin-2 to the movement of early and late endosomes along microtubules, we made use of a cytosol-free motility assay using magnetically isolated early and late endosomes as well as biochemical analyses and live-cell imaging. By making use of specific antibodies, we confirmed that kinesin-1 and dynein move early endosomes and we found that kinesin-2 moves both early and late endosomes in the cell-free assay. Unexpectedly, dynein did not move late endosomes in the cell-free assay. We provide evidence from disruption of dynein function and latrunculin A treatment, suggesting that dynein regulates late endosome movement indirectly, possibly through a mechanism involving the actin cytoskeleton. These data provide new insights into the complex regulation of endosomes' motility and suggest that dynein is not the major motor required to move late endosomes toward the minus end of microtubules.

Actin is required for endocytic trafficking in the malaria parasite Plasmodium falciparum

The intra-erythrocytic stages of the malaria parasite endocytose large quantities of the surrounding erythrocyte cytoplasm and deliver it to a digestive food vacuole via endocytic vesicles. Digestion provides amino acids for parasite protein synthesis and is required to maintain the osmotic integrity of the host cell. The parasite endocytic pathway has been described morphologically by electron microscopy, but the molecular mechanisms that mediate and regulate it remain elusive. Given the involvement of actin in endocytosis in other eukaryotes, we have used actin inhibitors to assess the requirement for this protein in the endocytic pathway of the human malaria parasite, Plasmodium falciparum. Treatment of cultures with cytochalasin D did not affect haemoglobin levels in the parasites when co-administered with protease inhibitors, and neither did it affect the uptake of the endocytic tracer horseradish peroxidase, suggesting the absence of actin in the mechanism of endocytosis. However, in the absence of protease inhibitors, treated parasites contained increased levels of haemoglobin due to an accumulation of enlarged endocytic vesicles, as determined by immunofluorescence and electron microscopy, suggesting a role for actin in vesicle trafficking, possibly by mediating vesicle maturation and/or fusion to the digestive vacuole. In contrast to cytochalasin D, treatment with jasplakinolide led to an inhibition of endocytosis, an accumulation of vesicles closer to the plasma membrane and a marked concentration of actin in the parasite cortex. We propose that the stabilization of cortical actin filaments by jasplakinolide interferes with normal endocytic vesicle formation and migration from the cell periphery.

Conjugated polythiophene probes target lysosome-related acidic vacuoles in cultured primary cells

Conformation-sensitive optical probes for studying biological processes and structures are of great interest. The present work shows for the first time that conjugated polyelectrolyte (CPE) probes can be used for specific targeting of chromatin, nuclear and cytoplasmatic vesicles, and cytoskeletal components in a complex system of cultured cells. One of the probes could also be used for vital staining of live cells. When bound to different entities, the polythiophene derivative probes emitted light with different colors due to the unique spectral properties of these conformation sensitive probes. The physical pre-requisites for binding could also be exploited for characterization of the target. Unexpectedly, lysosome-related acidic vacuoles were targeted in cultured primary cells by both anionic, cationic, and zwitter-ionic polythiophene derivatives. Pre-treatment with Bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase, caused redistribution of the staining. The targeting of lysosome-related acidic vesicles could not be demonstrated in transformed cells (melanoma, neuroblastoma, and prostate cancer cell lines), indicating a difference in the localization, structure, accessibility, or quantity of the target in cultured normal cells as compared with the malignant cell lines. The chemical nature of the conjugated polyelectrolyte complex in the cytoplasmatic vacuoles remains elusive.

Bioinformatic and image analyses of the cellular localization of the apoptotic proteins endonuclease G, AIF, and AMID during apoptosis in human cells

We studied the cellular localization of the apoptotic proteins endonuclease G, AIF, and AMID in silico using three prediction tools and in living cells using both single-cell colocalization image analysis and nuclear translocation analysis. We confirmed the mitochondrial localization of endonuclease G and AIF by prediction analysis and by single-cell colocalization image analysis. We found the AMID protein to be cytoplasmic, most probably incorporated into the cytoplasmic side of the membranes of various organelles. The highest concentration of AMID was observed associated with the Golgi. Colocalization of AMID with lysosomes was also indirectly confirmed by analysis of AMID-rich vesicle velocity using manual tracking analysis. Bioinformatic analysis also detected nuclear localization signals in endonuclease G and AIF, but not in AMID. A novel analysis of time-lapse fluorescence image data during staurosporine-induced apoptosis revealed nuclear translocation only for endonuclease G and AIF.

Activation of p61Hck triggers WASp- and Arp2/3-dependent actin-comet tail biogenesis and accelerates lysosomes

Secretory lysosomes exist in few cell types, but various mechanisms are involved to ensure their mobilization within the cytoplasm. In phagocytes, lysosome exocytosis is a regulated phenomenon at least in part under the control of the phagocyte-specific and lysosome-associated Src-kinase p61Hck (hematopoietic cell kinase). We show here that p61Hck activation triggered polymerization of actin at the membrane of lysosomes, which resulted in F-actin structures similar to comet tails observed on endocytic vesicles. We correlated this actin-comet biogenesis to a 35% acceleration of p61Hck-lysosomes in cells, which was dependent on actin polymerization and required an intact microtubular network. It was possible to initiate the formation of actin tails on p61Hck-positive lysosomes and on p61Hck-associated latex beads incubated in human phagocyte cytosolic extracts. The in vitro reconstitution on beads indicated that other lysosomal proteins were dispensable in this mechanism. The de novo actin polymerization process was functionally dependent on the kinase activity of Hck, WASp, the Arp2/3 complex, and Cdc42 but not Rac or Rho. Thus, we identified p61Hck as the first lysosomal protein able to recruit the molecular machinery responsible for actin tail formation. Altogether, our results suggest a new mechanism for lysosome motility involving p61Hck, actin-comet tail biogenesis, and the microtubule network.

Lysosomal exocytosis is impaired in mucolipidosis type IV

Mucolipidosis type IV (MLIV) is an autosomal recessive disease characterized by severe neurological impairment, ophthalmologic defects, and gastric dysfunction. MLIV cells have a deficiency in the late endosomal/lysosomal (LEL) pathway that results in the buildup of lysosomal inclusions. Using a Xenopus oocyte expression system, we previously showed that mucolipin-1 (MLN1), the protein encoded by the MCOLN1 gene is a Ca2+ -permeable non-selective cation channel that is transiently modulated by elevations in intracellular Ca2+. We further showed that MLN1 is translocated to the plasma membrane during lysosomal exocytosis. In this study we show that lysosomal exocytosis is impaired in fibroblasts from MLIV patients, indicating that MLN1 plays an active role in this process. Further, we show that transfection with wild type MLN1 cDNA rescues exocytosis, suggesting the possibility of treatments based on the restoration of this crucial cellular function.

Powering membrane traffic in endocytosis and recycling

Early in evolution, the diversification of membrane-bound compartments that characterize eukaryotic cells was accompanied by the elaboration of molecular machineries that mediate intercompartmental communication and deliver materials to specific destinations. Molecular motors that move on tracks of actin filaments or microtubules mediate the movement of organelles and transport between compartments. The subjects of this review are the motors that power the transport steps along the endocytic and recycling pathways, their modes of attachment to cargo and their regulation.

A role of LIM kinase 1/cofilin pathway in regulating endocytic trafficking of EGF receptor in human breast cancer cells

We have previously shown that overexpression of LIM kinase1 (LIMK1) resulted in a marked retardation of the internalization of the receptor-mediated endocytic tracer, Texas red-labeled epidermal growth factor (EGF) in low-invasive human breast cancer cell MCF-7. We thereby postulate that LIMK1 signaling plays an important role in the regulation of ligand-induced endocytosis of EGF receptor (EGFR) in tumor cells by reorganizing and influencing actin-filament dynamics. In the present study, we further assessed the effect of wild-type LIMK1, a kinase-deficient dominant negative mutant of LIMK1 (DN-LIMK1) and an active, unphosphorylatable cofilin mutant (S3A cofilin) on internalization of EGF-EGFR in MDA-MB-231, a highly invasive human breast cancer cell line. We demonstrate here that a marked delay in the receptor-mediated internalization of Texas red-labeled EGF was observed in the wild-type LIMK1 transfectants, and that most of the internalized EGF staining were accumulated within transferrin receptor-positive early endosomes even after 30 min internalization. In contrast, the expression of dominant-negative LIMK1 mutant rescued the efficient endocytosis of Texas red-EGF, and large amounts of Texas red-EGF staining already reached LIMPII-positive late endosomes/lysosomal vacuoles after 15 min internalization. We further analyzed the effect of S3A cofilin mutant on EGFR trafficking, and found an efficient delivery of Texas red-EGF into late endosomes/lysosomes at 15-30 min after internalization. Taken together, our novel findings presented in this paper implicate that LIMK1 signaling indeed plays a pivotal role in the regulation of EGFR trafficking through the endocytic pathway in invasive tumor cells.

Modulation of cell adhesion and motility in the immune system by Myo1f

Although class I myosins are known to play a wide range of roles, the physiological function of long-tailed class I myosins in vertebrates remains elusive. We demonstrated that one of these proteins, Myo1f, is expressed predominantly in the mammalian immune system. Cells from Myo1f-deficient mice exhibited abnormally increased adhesion and reduced motility, resulting from augmented exocytosis of beta2 integrin-containing granules. Also, the cortical actin that co-localizes with Myo1f was reduced in Myo1f-deficient cells. In vivo, Myo1f-deficient mice showed increased susceptibility to infection by Listeria monocytogenes and an impaired neutrophil response. Thus, Myo1f directs immune cell motility and innate host defense against infection.

PI3K-dependent lysosome exocytosis in nitric oxide-preconditioned hepatocytes

We investigated the signal mediators and the cellular events involved in the nitric oxide (NO)-induced hepatocyte resistance to oxygen deprivation in isolated hepatocytes treated with the NO donor (Z)-1-(N-methyl-N-[6-(N-methylammoniohexyl)amino])diazen-1-ium-1,2-diolate (NOC-9). NOC-9 greatly induced PI3K activation, as tested by phosphorylation of PKB/Akt. This effect was prevented by either 1H-(1,2,4)-oxadiazolo-(4,3)-quinoxalin-1-one, an inhibitor of the soluble guanylate cyclase (sGC), or KT5823, an inhibitor of cGMP-dependent kinase (cGK), as well as by farnesyl protein transferase inhibitor, which blocks the function of Ras GTPase. Bafilomycin A, an inhibitor of the lysosome-type vacuolar H+-ATPase, cytochalasin D, which disrupts the cytoskeleton-dependent organelle traffic, and wortmannin, which inhibits the PI3K-dependent traffic of lysosomes, all abolished the NOC-9-induced hepatocyte protection. The treatment with NOC-9 was associated with the PI3K-dependent peripheral translocation and fusion with the plasma membrane of lysosomes and the appearance at the cell surface of the vacuolar H+-ATPase. Inhibition of sGC, cGK, and Ras, as well as the inhibition of PI3K by wortmannin, prevented the exocytosis of lysosomes and concomitantly abolished the protective effect of NOC-9 on hypoxia-induced pHi and [Na+]i alterations and cell death. These data indicate that NO increases hepatocyte resistance to hypoxic injury by activating a pathway involving Ras, sGC, and cGK that determines PI3K-dependent exocytosis of lysosomes.

Histone deacetylase inhibitors prevent exocytosis of interleukin-1beta-containing secretory lysosomes: role of microtubules

A number of agents reducing interleukin-1beta (IL-1beta) activity are being developed as novel immunomodulatory and anti-inflammatory therapies. However, the elucidation of their molecular mechanism of action is required in the context of medical management of inflammatory diseases. Inhibitors of histone deacetylases (HDACs) are promising anticancer agents with pleiotropic activities. Of these, suberoylanilide hydroxamic acid has been reported to inhibit the production of several proinflammatory cytokines. In the present study, we investigated the effects of 2 HDAC inhibitors on IL-1beta secretion: suberoylanilide hydroxamic acid and a newly developed hydroxamic acid-derived compound ITF2357. These HDAC inhibitors do not affect the synthesis or intracellular localization of IL-1beta but both strongly reduce the levels of extracellular IL-1beta by preventing the exocytosis of IL-1beta-containing secretory lysosomes. At nanomolar concentrations, ITF2357 reduces the secretion of IL-1beta following ATP activation of the P2X7 receptor. Whereas the inhibition of HDACs results in hyperacetylation of tubulin, acetylation of HSP90 was unaffected. The reduction in IL-1beta secretion appears to be due to disruption of microtubules impairing lysosome exocytosis. Together, these observations indicate that a functional microtubule network is required for IL-1beta secretion and suggest that disruption of tubulin is the mechanism by which inhibitors of HDACs reduce the secretion of IL-1beta.

Myosin Ib modulates the morphology and the protein transport within multi-vesicular sorting endosomes

Members of at least four classes of myosin (I, II, V and VI) have been implicated in the dynamics of a large variety of organelles. Despite their common motor domain structure, some of these myosins, however, are non processive and cannot move organelles along the actin tracks. Here, we demonstrate in the human pigmented MNT-1 cell line that, (1) the overexpression of one of these myosins, myosin 1b, or the addition of cytochalasin D affects the morphology of the sorting multivesicular endosomes; (2) the overexpression of myosin 1b delays the processing of Pmel17 (the product of murine silver locus also named GP100), which occurs in these multivesicular endosomes; (3) myosin 1b associated with endosomes coimmunoprecipitates with Pmel17. All together, these observations suggest that myosin 1b controls the traffic of protein cargo in multivesicular endosomes most probably through its ability to modulate with actin the morphology of these sorting endosomes.

Distribution and dynamics of Lamp1-containing endocytic organelles in fibroblasts deficient in BLOC-3

Late endosomes and lysosomes of mammalian cells in interphase tend to concentrate in the perinuclear region that harbors the microtubule-organizing center. We have previously reported abnormal distribution of these organelles - as judged by reduced percentages of cells displaying pronounced perinuclear accumulation - in mutant fibroblasts lacking BLOC-3 (for ;biogenesis of lysosome-related organelles complex 3'). BLOC-3 is a protein complex that contains the products of the genes mutated in Hermansky-Pudlak syndrome types 1 and 4. Here, we developed a method based on image analysis to estimate the extent of organelle clustering in the perinuclear region of cultured cells. Using this method, we corroborated that the perinuclear clustering of late endocytic organelles containing Lamp1 (for ;lysosome-associated membrane protein 1') is reduced in BLOC-3-deficient murine fibroblasts, and found that it is apparently normal in fibroblasts deficient in BLOC-1 or BLOC-2, which are another two protein complexes associated with Hermansky-Pudlak syndrome. Wild-type and mutant fibroblasts were transfected to express human LAMP1 fused at its cytoplasmic tail to green fluorescence protein (GFP). At low expression levels, LAMP1-GFP was targeted correctly to late endocytic organelles in both wild-type and mutant cells. High levels of LAMP1-GFP overexpression elicited aberrant aggregation of late endocytic organelles, a phenomenon that probably involved formation of anti-parallel dimers of LAMP1-GFP as it was not observed in cells expressing comparable levels of a non-dimerizing mutant variant, LAMP1-mGFP. To test whether BLOC-3 plays a role in the movement of late endocytic organelles, time-lapse fluorescence microscopy experiments were performed using live cells expressing low levels of LAMP1-GFP or LAMP1-mGFP. Although active movement of late endocytic organelles was observed in both wild-type and mutant fibroblasts, quantitative analyses revealed a relatively lower frequency of microtubule-dependent movement events, either towards or away from the perinuclear region, within BLOC-3-deficient cells. By contrast, neither the duration nor the speed of these microtubule-dependent events seemed to be affected by the lack of BLOC-3 function. These results suggest that BLOC-3 function is required, directly or indirectly, for optimal attachment of late endocytic organelles to microtubule-dependent motors.

Activation of the Lysosome-Associated p61Hck Isoform Triggers the Biogenesis of Podosomes

Haematopoietic cell kinase (Hck) is a protein tyrosine kinase of the Src family specifically expressed in phagocytes as two isoforms, p59Hck and p61Hck, present at the plasma membrane and lysosomes, respectively. We report that ectopic expression of a constitutively active mutant of p61Hck (p61Hck(ca)) triggered the de novo formation of actin-rich rings at the ventral face of the cells that we characterized as bona fide podosome rosettes, structures involved in cell migration. Their formation required the adaptor domains and the kinase activity of p61Hck, the integrity of microfilament and microtubule networks and concerted action of Cdc42, Rac and Rho. Podosome rosette formation was either abolished when p61Hck(ca) was readdressed from lysosomes to the cytosol or triggered when p59Hck(ca) was relocalized to lysosomes. Lysosomal markers were present at podosome rosettes. By stimulating exocytosis of p61Hck(ca) lysosomes with a calcium ionophore, the formation of podosome rosettes was enhanced. Interestingly, we confirm that, in human macrophages, Hck and lysosomal markers were present at podosomes which were spatially reorganized as clusters, a foregoing step to form rosettes, upon expression of p61Hck(ca). We propose that lysosomes, under the control of p61Hck, are involved in the biogenesis of podosomes, a key phenomenon in the migration of phagocytes.

Loop 1 of transducer region in mammalian class I myosin, Myo1b, modulates actin affinity, ATPase activity, and nucleotide access

Loop 1, a flexible surface loop in the myosin motor domain, comprises in part the transducer region that lies near the nucleotide-binding site and is proposed from structural studies to be responsible for the kinetic tuning of product release following ATP hydrolysis (1). Biochemical studies have shown that loop 1 affects the affinity of actin-myosin-II for ADP, motility and the V(max) of the actin-activated Mg2+-ATPase activity, possibly through P(i) release (2-8). To test the influence of loop 1 on the mammalian class I myosin, Myo1b, chimeric molecules in which (i) loop 1 of a truncated form of Myo1b, Myo1b1IQ, was replaced with either loop 1 from other myosins; (ii) loop 1 was replaced with glycine; or (iii) some amino acids in the loop were substituted with alanine and were expressed in baculovirus, and their interactions with actin and nucleotide were evaluated. The steady-state actin-activated ATPase activity; rate of ATP-induced dissociation of actin from Myo1b1IQ; rate of ADP release from actin-Myo1b1IQ; and the affinity of actin for Myo1b1IQ and Myo1b1IQ.ADP differed in the chimeras versus wild type, indicating that loop 1 has a much wider range of effects on the coupling between actin and nucleotide binding events than previously thought. In particular, the biphasic ATP-induced dissociation of actin from actin-Myo1b1IQ was significantly altered in the chimeras. This provided evidence that loop 1 contributes to the accessibility of the nucleotide pocket and is involved in the integration of information from the actin-, nucleotide-, gamma-P(i)-, and calmodulin-binding sites and predicts that loop 1 modulates the load dependence of the motor.

MLN64 is involved in actin-mediated dynamics of late endocytic organelles

MLN64 is a late endosomal cholesterol-binding membrane protein of an unknown function. Here, we show that MLN64 depletion results in the dispersion of late endocytic organelles to the cell periphery similarly as upon pharmacological actin disruption. The dispersed organelles in MLN64 knockdown cells exhibited decreased association with actin and the Arp2/3 complex subunit p34-Arc. MLN64 depletion was accompanied by impaired fusion of late endocytic organelles and delayed cargo degradation. MLN64 overexpression increased the number of actin and p34-Arc-positive patches on late endosomes, enhanced the fusion of late endocytic organelles in an actin-dependent manner, and stimulated the deposition of sterol in late endosomes harboring the protein. Overexpression of wild-type MLN64 was capable of rescuing the endosome dispersion in MLN64-depleted cells, whereas mutants of MLN64 defective in cholesterol binding were not, suggesting a functional connection between MLN64-mediated sterol transfer and actin-dependent late endosome dynamics. We propose that local sterol enrichment by MLN64 in the late endosomal membranes facilitates their association with actin, thereby governing actin-dependent fusion and degradative activity of late endocytic organelles.