Membrane ruffles in cell migration: indicators of inefficient lamellipodia adhesion and compartments of actin filament reorganization

Kinectin-dependent ER transport supports the focal complex maturation required for chemotaxis in shallow gradients

Chemotaxis in shallow gradients of chemoattractants is accomplished by preferential maintenance of protrusions oriented towards the chemoattractant; however, the mechanism of preferential maintenance is not known. Here, we test the hypothesis that kinectin-dependent endoplasmic reticulum (ER) transport supports focal complex maturation to preferentially maintain correctly oriented protrusions. We knocked down kinectin expression in MDA-MB-231 cells using small interfering RNA and observed that kinectin contributes to the directional bias, but not the speed, of cell migration. Kymograph analysis revealed that the extension of protrusions oriented towards the chemoattractant was not affected by kinectin knockdown, but that their maintenance was. Immunofluorescence staining and live-cell imaging demonstrated that kinectin transports ER preferentially to protrusions oriented towards the chemoattractant. ER then promotes the maturation of focal complexes into focal adhesions to maintain these protrusions for chemotaxis. Our results show that kinectin-dependent ER distribution can be localized by chemoattractants and provide a mechanism for biased protrusion choices during chemotaxis in shallow gradients of chemoattractants.

Prolactin Signaling Stimulates Invasion via Na(+)/H(+) Exchanger NHE1 in T47D Human Breast Cancer Cells

Prolactin (PRL) and its receptor (PRLR) are implicated in breast cancer invasiveness, although their exact roles remain controversial. The Na(+)/H(+) exchanger (NHE1) plays essential roles in cancer cell motility and invasiveness, but the PRLR and NHE1 have not previously been linked. Here we show that in T47D human breast cancer cells, which express high levels of PRLR and NHE1, exposure to PRL led to the activation of Janus kinase-2 (JAK2)/signal transducer and activator of transcription-5 (STAT5), Akt, and ERK1/2 signaling and the rapid formation of peripheral membrane ruffles, known to be associated with cell motility. NHE1 was present in small ruffles prior to PRL treatment and was further recruited to the larger, more dynamic ruffles induced by PRL exposure. In PRL-induced ruffles, NHE1 colocalized with activated Akt, ERK1/2, and the ERK effector p90Ribosomal S kinase (p90RSK), known regulators of NHE1 activity. Stimulation of T47D cells with PRL augmented p90RSK activation, Ser703-phosphorylation of NHE1, NHE1-dependent intracellular pH recovery, pericellular acidification, and NHE1-dependent invasiveness. NHE1 activity and localization to ruffles were attenuated by the inhibition of Akt and/or ERK1/2. In contrast, noncancerous MCF10A breast epithelial cells expressed NHE1 and PRLR at lower levels than T47D cells, and their stimulation with PRL induced neither NHE1 activation nor NHE1-dependent invasiveness. In conclusion, we show for the first time that PRLR activation stimulates breast cancer cell invasiveness via the activation of NHE1. We propose that PRL-induced NHE1 activation and the resulting NHE1-dependent invasiveness may contribute to the metastatic behavior of human breast cancer cells.

The cytoskeleton adaptor protein ankyrin-1 is upregulated by p53 following DNA damage and alters cell migration

The integrity of the genome is maintained by a host of surveillance and repair mechanisms that are pivotal for cellular function. The tumour suppressor protein p53 is a major component of the DNA damage response pathway and plays a vital role in the maintenance of cell-cycle checkpoints. Here we show that a microRNA, miR-486, and its host gene ankyrin-1 (ANK1) are induced by p53 following DNA damage. Strikingly, the cytoskeleton adaptor protein ankyrin-1 was induced over 80-fold following DNA damage. ANK1 is upregulated in response to a variety of DNA damage agents in a range of cell types. We demonstrate that miR-486-5p is involved in controlling G1/S transition following DNA damage, whereas the induction of the ankyrin-1 protein alters the structure of the actin cytoskeleton and sustains limited cell migration during DNA damage. Importantly, we found that higher ANK1 expression correlates with decreased survival in cancer patients. Thus, these observations highlight ANK1 as an important effector downstream of the p53 pathway.

SNX9 promotes metastasis by enhancing cancer cell invasion via differential regulation of RhoGTPases

Despite current advances in cancer research, metastasis remains the leading factor in cancer-related deaths. Here, we identify sorting nexin 9 (SNX9) as a new regulator of breast cancer metastasis. We detected an increase in SNX9 expression in human breast cancer metastases compared with primary tumors and demonstrated that SNX9 expression in MDA-MB-231 breast cancer cells is necessary to maintain their ability to metastasize in a chick embryo model. Reciprocally, SNX9 knockdown impairs the process. In vitro studies using several cancer cell lines derived from a variety of human tumors revealed a role for SNX9 in cell invasion and identified mechanisms responsible for this novel function. We showed that SNX9 controls the activation of RhoA and Cdc42 GTPases and also regulates cell motility via the modulation of well-known molecules involved in metastasis, namely RhoA-ROCK and N-WASP. In addition, we have discovered that SNX9 is required for RhoGTPase-dependent, clathrin-independent endocytosis, and in this capacity, can functionally substitute to the bona fide Rho GAP, GRAF1 (GTPase Regulator Associated with Focal Adhesion Kinase). Together, our data establish novel roles for SNX9 as a multifunctional protein scaffold that regulates, and potentially coordinates, several cellular processes that together can enhance cancer cell metastasis.

Chloride channel-3 promotes tumor metastasis by regulating membrane ruffling and is associated with poor survival

The chloride channel-3 (ClC-3) protein is known to be a component of Cl⁻ channels involved in cell volume regulation or acidification of intracellular vesicles. Here, we report that ClC-3 was highly expressed in the cytoplasm of metastatic carcinomatous cells and accelerated cell migration in vitro and tumor metastasis in vivo. High-grade expression of cytoplasmic ClC-3 predicted poor survival in cancer patients. We found that independent of its volume-activated Cl⁻ channel properties, ClC-3 was able to promote cell membrane ruffling, required for tumor metastasis. ClC-3 mediated membrane ruffling by regulating keratin 18 phosphorylation to control β1 Integrin recycling. Therefore, cytoplasmic ClC-3 plays an active and key role in tumor metastasis and may be a valuable prognostic biomarker and a therapeutic target to prevent tumor spread.

The Role of Rac1 in the Growth Cone Dynamics and Force Generation of DRG Neurons

We used optical tweezers, video imaging, immunocytochemistry and a variety of inhibitors to analyze the role of Rac1 in the motility and force generation of lamellipodia and filopodia from developing growth cones of isolated Dorsal Root Ganglia neurons. When the activity of Rac1 was inhibited by the drug EHop-016, the period of lamellipodia protrusion/retraction cycles increased and the lamellipodia retrograde flow rate decreased; moreover, the axial force exerted by lamellipodia was reduced dramatically. Inhibition of Arp2/3 by a moderate amount of the drug CK-548 caused a transient retraction of lamellipodia followed by a complete recovery of their usual motility. This recovery was abolished by the concomitant inhibition of Rac1. The filopodia length increased upon inhibition of both Rac1 and Arp2/3, but the speed of filopodia protrusion increased when Rac1 was inhibited and decreased instead when Arp2/3 was inhibited. These results suggest that Rac1 acts as a switch that activates upon inhibition of Arp2/3. Rac1 also controls the filopodia dynamics necessary to explore the environment.

Correlative Light and Electron Microscopy Reveals the HAS3-Induced Dorsal Plasma Membrane Ruffles

Hyaluronan is a linear sugar polymer synthesized by three isoforms of hyaluronan synthases (HAS1, 2, and 3) that forms a hydrated scaffold around cells and is an essential component of the extracellular matrix. The morphological changes of cells induced by active hyaluronan synthesis are well recognized but not studied in detail with high resolution before. We have previously found that overexpression of HAS3 induces growth of long plasma membrane protrusions that act as platforms for hyaluronan synthesis. The study of these thin and fragile protrusions is challenging, and they are difficult to preserve by fixation unless they are adherent to the substrate. Thus their structure and regulation are still partly unclear despite careful imaging with different microscopic methods in several cell types. In this study, correlative light and electron microscopy (CLEM) was utilized to correlate the GFP-HAS3 signal and the surface ultrastructure of cells in order to study in detail the morphological changes induced by HAS3 overexpression. Surprisingly, this method revealed that GFP-HAS3 not only localizes to ruffles but in fact induces dorsal ruffle formation. Dorsal ruffles regulate diverse cellular functions, such as motility, regulation of glucose metabolism, spreading, adhesion, and matrix degradation, the same functions driven by active hyaluronan synthesis.

Modeling the formation of cell-matrix adhesions on a single 3D matrix fiber

Cell-matrix adhesions are crucial in different biological processes like tissue morphogenesis, cell motility, and extracellular matrix remodeling. These interactions that link cell cytoskeleton and matrix fibers are built through protein clutches, generally known as adhesion complexes. The adhesion formation process has been deeply studied in two-dimensional (2D) cases; however, the knowledge is limited for three-dimensional (3D) cases. In this work, we simulate different local extracellular matrix properties in order to unravel the fundamental mechanisms that regulate the formation of cell-matrix adhesions in 3D. We aim to study the mechanical interaction of these biological structures through a three dimensional discrete approach, reproducing the transmission pattern force between the cytoskeleton and a single extracellular matrix fiber. This numerical model provides a discrete analysis of the proteins involved including spatial distribution, interaction between them, and study of the different phenomena, such as protein clutches unbinding or protein unfolding.

Multidimensional immunolabeling and 4D time-lapse imaging of vital ex vivo lung tissue

During the last decades, the study of cell behavior was largely accomplished in uncoated or extracellular matrix (ECM)-coated plastic dishes. To date, considerable cell biological efforts have tried to model in vitro the natural microenvironment found in vivo. For the lung, explants cultured ex vivo as lung tissue cultures (LTCs) provide a three-dimensional (3D) tissue model containing all cells in their natural microenvironment. Techniques for assessing the dynamic live interaction between ECM and cellular tissue components, however, are still missing. Here, we describe specific multidimensional immunolabeling of living 3D-LTCs, derived from healthy and fibrotic mouse lungs, as well as patient-derived 3D-LTCs, and concomitant real-time four-dimensional multichannel imaging thereof. This approach allowed the evaluation of dynamic interactions between mesenchymal cells and macrophages with their ECM. Furthermore, fibroblasts transiently expressing focal adhesions markers incorporated into the 3D-LTCs, paving new ways for studying the dynamic interaction between cellular adhesions and their natural-derived ECM. A novel protein transfer technology (FuseIt/Ibidi) shuttled fluorescently labeled α-smooth muscle actin antibodies into the native cells of living 3D-LTCs, enabling live monitoring of α-smooth muscle actin-positive stress fibers in native tissue myofibroblasts residing in fibrotic lesions of 3D-LTCs. Finally, this technique can be applied to healthy and diseased human lung tissue, as well as to adherent cells in conventional two-dimensional cell culture. This novel method will provide valuable new insights into the dynamics of ECM (patho)biology, studying in detail the interaction between ECM and cellular tissue components in their natural microenvironment.

Directional cell movement through tissues is controlled by exosome secretion

Directional cell movement through tissues is critical for multiple biological processes and requires maintenance of polarity in the face of complex environmental cues. Here we use intravital imaging to demonstrate that secretion of exosomes from late endosomes is required for directionally persistent and efficient in vivo movement of cancer cells. Inhibiting exosome secretion or biogenesis leads to defective tumour cell migration associated with increased formation of unstable protrusions and excessive directional switching. In vitro rescue experiments with purified exosomes and matrix coating identify adhesion assembly as a critical exosome function that promotes efficient cell motility. Live-cell imaging reveals that exosome secretion directly precedes and promotes adhesion assembly. Fibronectin is found to be a critical motility-promoting cargo whose sorting into exosomes depends on binding to integrins. We propose that autocrine secretion of exosomes powerfully promotes directionally persistent and effective cell motility by reinforcing otherwise transient polarization states and promoting adhesion assembly.

The role of myosin-II in force generation of DRG filopodia and lamellipodia

Differentiating neurons process the mechanical stimulus by exerting the protrusive forces through lamellipodia and filopodia. We used optical tweezers, video imaging and immunocytochemistry to analyze the role of non-muscle myosin-II on the protrusive force exerted by lamellipodia and filopodia from developing growth cones (GCs) of isolated Dorsal Root Ganglia (DRG) neurons. When the activity of myosin-II was inhibited by 30 μM Blebbistatin protrusion/retraction cycles of lamellipodia slowed down and during retraction lamellipodia could not lift up axially as in control condition. Inhibition of actin polymerization with 25 nM Cytochalasin-D and of microtubule polymerization with 500 nM Nocodazole slowed down the protrusion/retraction cycles, but only Cytochalasin-D decreased lamellipodia axial motion. The force exerted by lamellipodia treated with Blebbistatin decreased by 50%, but, surprisingly, the force exerted by filopodia increased by 20-50%. The concomitant disruption of microtubules caused by Nocodazole abolished the increase of the force exerted by filopodia treated with Blebbistatin. These results suggest that; i- Myosin-II controls the force exerted by lamellipodia and filopodia; ii- contractions of the actomyosin complex formed by filaments of actin and myosin have an active role in ruffle formation; iii- myosin-II is an essential component of the structural stability of GCs architecture.

Cyclin A2, a novel regulator of EMT

Our previous work showed that Cyclin A2 deficiency promotes cell invasion in fibroblasts. Given that the majority of cancers emerge from epithelia, we explored novel functions for Cyclin A2 by depleting it in normal mammary epithelial cells. This caused an epithelial to mesenchymal transition (EMT) associated with loss of cell-to-cell contacts, decreased E-Cadherin expression and increased invasive properties characterized by a reciprocal regulation of RhoA and RhoC activities, where RhoA-decreased activity drove cell invasiveness and E-Cadherin delocalization, and RhoC-increased activity only supported cell motility. Phenotypes induced by Cyclin A2 deficiency were exacerbated upon oncogenic activated-Ras expression, which led to an increased expression of EMT-related transcriptional factors. Moreover, Cyclin A2-depleted cells exhibited stem cell-like properties and increased invasion in an in vivo avian embryo model. Our work supports a model where Cyclin A2 downregulation facilitates cancer cell EMT and metastatic dissemination.

Intramolecular interactions between the Dbl homology (DH) domain and the carboxyl-terminal region of myosin II-interacting guanine nucleotide exchange factor (MyoGEF) act as an autoinhibitory mechanism for the regulation of MyoGEF functions

We have reported previously that nonmuscle myosin II-interacting guanine nucleotide exchange factor (MyoGEF) plays an important role in the regulation of cell migration and cytokinesis. Like many other guanine nucleotide exchange factors (GEFs), MyoGEF contains a Dbl homology (DH) domain and a pleckstrin homology domain. In this study, we provide evidence demonstrating that intramolecular interactions between the DH domain (residues 162-351) and the carboxyl-terminal region (501-790) of MyoGEF can inhibit MyoGEF functions. In vitro and in vivo pulldown assays showed that the carboxyl-terminal region (residues 501-790) of MyoGEF could interact with the DH domain but not with the pleckstrin homology domain. Expression of a MyoGEF carboxyl-terminal fragment (residues 501-790) decreased RhoA activation and suppressed actin filament formation in MDA-MB-231 breast cancer cells. Additionally, Matrigel invasion assays showed that exogenous expression of the MyoGEF carboxyl-terminal region decreased the invasion activity of MDA-MB-231 cells. Moreover, coimmunoprecipitation assays showed that phosphorylation of the MyoGEF carboxyl-terminal region by aurora B kinase interfered with the intramolecular interactions of MyoGEF. Furthermore, expression of the MyoGEF carboxyl-terminal region interfered with RhoA localization during cytokinesis and led to an increase in multinucleation. Together, our findings suggest that binding of the carboxyl-terminal region of MyoGEF to its DH domain acts as an autoinhibitory mechanism for the regulation of MyoGEF activation.

Regulation of lysosomal secretion by cortactin drives fibronectin deposition and cell motility

Directional cellular movement is required for various organismal processes, including immune defense and cancer metastasis. Proper navigation of migrating cells involves responding to a complex set of extracellular cues, including diffusible chemical signals and physical structural information. In tissues, conflicting gradients and signals may require cells to not only respond to the environment but also modulate it for efficient adhesion formation and directional cell motility. Recently, we found that cells endocytose fibronectin (FN) and resecrete it from a late endosomal/lysosomal (LE/Lys) compartment to provide an autocrine extracellular matrix (ECM) substrate for cell motility. Branched actin assembly regulated by cortactin was required for trafficking of FN-containing vesicles from LE/Lys to the cell surface. These findings suggest a model in which migrating cells use lysosomal secretion as a versatile mechanism to modulate the ECM environment, promote adhesion assembly and enhance directional migration.

Direct interaction of 14-3-3ζ with ezrin promotes cell migration by regulating the formation of membrane ruffle

14-3-3 proteins have been shown to regulate the actin cytoskeleton remodeling, cell adhesion and migration. In this study, we identified ezrin, a cross-linker between plasma membrane and actin cytoskeleton, as a novel 14-3-3ζ interacting partner. The direct interaction between 14-3-3ζ and ezrin was validated in the cells and by in vitro assays. We showed that the 14-3-3ζ binding region in ezrin was located within the N-terminal and central α-helical domains and that the αG-to-αI helices of 14-3-3ζ are responsible for the binding to ezrin. Functional analyses revealed that the regulation of cell migration and membrane ruffling by 14-3-3ζ is ezrin dependent, for which the integrity of ezrin protein was required. Conversely, the knockdown of 14-3-3ζ abrogates also the stimulatory effect of ezrin on cell migration and membrane ruffling. Moreover, we found that the phosphorylation of Thr567 in ezrin facilitates the 14-3-3ζ-ezrin interaction and the formation of membrane ruffles. Taken together, these results suggest strongly that the functions of these two proteins in cell migration are linked and might be mediated by their direct physical interaction, which is important for the formation of membrane ruffles.

Generation of membrane structures during phagocytosis and chemotaxis of macrophages: role and regulation of the actin cytoskeleton

Macrophages are best known for their protective search and destroy functions against invading microorganisms. These processes are commonly known as chemotaxis and phagocytosis. Both of these processes require actin cytoskeletal remodeling to produce distinct F-actin-rich membrane structures called lamellipodia and phagocytic cups. This review will focus on the mechanisms by which macrophages regulate actin polymerization through initial receptor signaling and subsequent Arp2/3 activation by nucleation-promoting factors like the WASP/WAVE family, followed by remodeling of actin networks to produce these very distinct structures.

BNIP3 supports melanoma cell migration and vasculogenic mimicry by orchestrating the actin cytoskeleton

BNIP3 is an atypical BH3-only member of the BCL-2 family of proteins with reported pro-death as well as pro-autophagic and cytoprotective functions, depending on the type of stress and cellular context. In line with this, the role of BNIP3 in cancer is highly controversial and increased BNIP3 levels in cancer patients have been linked with both good as well as poor prognosis. In this study, using small hairpin RNA (shRNA) lentiviral transduction to stably knockdown BNIP3 (BNIP3-shRNA) expression levels in melanoma cells, we show that BNIP3 supports cancer cell survival and long-term clonogenic growth. Although BNIP3-shRNA increased mitochondrial mass and baseline levels of reactive oxygen species production, which are features associated with aggressive cancer cell behavior, it also prevented cell migration and completely abolished the ability to form a tubular-like network on matrigel, a hallmark of vasculogenic mimicry (VM). We found that this attenuated aggressive behavior of these melanoma cells was underscored by severe changes in cell morphology and remodeling of the actin cytoskeleton associated with loss of BNIP3. Indeed, BNIP3-silenced melanoma cells displayed enhanced formation of actin stress fibers and membrane ruffles, while lamellopodial protrusions and filopodia, tight junctions and adherens junctions were reduced. Moreover, loss of BNIP3 resulted in re-organization of focal adhesion sites associated with increased levels of phosphorylated focal adhesion kinase. Remarkably, BNIP3 silencing led to a drop of the protein levels of the integrin-associated protein CD47 and its downstream signaling effectors Rac1 and Cdc42. These observations underscore that BNIP3 is required to maintain steady-state levels of intracellular complexes orchestrating the plasticity of the actin cytoskeleton, which is integral to cell migration and other vital processes stimulating cancer progression. All together these results unveil an unprecedented pro-tumorigenic role of BNIP3 driving melanoma cell's aggressive features, like migration and VM.

Mitochondrial reactive oxygen species accelerate gastric cancer cell invasion

Tumor invasion is the most important factor to decide patient's prognosis. The relation between reactive oxygen species and tumor invasion is mainly reported that nicotinamide adenine dinucleotide phosphate oxidase in the cell membrane is a reactive oxygen species producer for formulating an invadopodia. On the other hand, mitochondrion was known as one of the most important reactive oxygen species-producer in the cell via an energy transfer system. However, the relation between mitochondrial reactive oxygen species and the tumor invasion was not well clarified. In this study, we evaluated the relation between mitochondrial reactive oxygen species and tumor invasion using a normal gastric mucosal cell-line (RGM-1) and a cancerous mutant RGM-1 cell-line (RGK-1). Manganese superoxide dismutase-expressing RGK-1 cell-lines were used for a scavenging mitochondrial reactive oxygen species. The cells have been evaluated their movement ability as follows; cellular ruffling frequencies, wound healing assay to evaluate horizontal cellular migration, and invasion assay using matrigel to analyze vertical cellular migration. All cellular movement abilities were inhibited by scavenging mitochondrial reactive oxygen species with manganese superoxide dismutase. Therefore mitochondrial reactive oxygen species was one of factors enhancing the tumor invasion in gastric cancer.

srGAP1 regulates lamellipodial dynamics and cell migratory behavior by modulating Rac1 activity

srGAP1 limits Rac1 activity at lamellipodia in a negative feedback manner, allowing concomitant activation of Rac1 and RhoA at lamellipodia. Rho signaling causes membrane ruffling through actomyosin contractility and removes the protrusive structures. Such coordination of Rac and Rho determines migratory behavior through lamellipodial dynamics.

The distinct levels of Rac activity differentially regulate the pattern of intrinsic cell migration. However, it remains unknown how Rac activity is modulated and how the level of Rac activity controls cell migratory behavior. Here we show that Slit-Robo GAP 1 (srGAP1) is a modulator of Rac activity in locomotive cells. srGAP1 possesses a GAP activity specific to Rac1 and is recruited to lamellipodia in a Rac1-dependent manner. srGAP1 limits Rac1 activity and allows concomitant activation of Rac1 and RhoA, which are mutually inhibitory. When both GTPases are activated, the protrusive structures caused by Rac1-dependent actin reorganization are spatially restricted and periodically destabilized, causing ruffling by RhoA-induced actomyosin contractility. Depletion of srGAP1 overactivates Rac1 and inactivates RhoA, resulting in continuous spatiotemporal spreading of lamellipodia and a modal shift of intrinsic cell motility from random to directionally persistent. Thus srGAP1 is a key determinant of lamellipodial dynamics and cell migratory behavior.

The epithelial sodium channel mediates the directionality of galvanotaxis in human keratinocytes

Cellular directional migration in an electric field (galvanotaxis) is one of the mechanisms guiding cell movement in embryogenesis and in skin epidermal repair. The epithelial sodium channel (ENaC), in addition to its function of regulating sodium transport in kidney, has recently been found to modulate cell locomotory speed. Here we tested whether ENaC has an additional function of mediating the directional migration of galvanotaxis in keratinocytes. Genetic depletion of ENaC completely blocks only galvanotaxis and does not decrease migration speed. Overexpression of ENaC is sufficient to drive galvanotaxis in otherwise unresponsive cells. Pharmacologic blockade or maintenance of the open state of ENaC also decreases or increases, respectively, galvanotaxis, suggesting that the channel open state is responsible for the response. Stable lamellipodial extensions formed at the cathodal sides of wild-type cells at the start of galvanotaxis; these were absent in the ENaC knockout keratinocytes, suggesting that ENaC mediates galvanotaxis by generating stable lamellipodia that steer cell migration. We provide evidence that ENaC is required for directional migration of keratinocytes in an electric field, supporting a role for ENaC in skin wound healing.

Cytostasis and morphological changes induced by mifepristone in human metastatic cancer cells involve cytoskeletal filamentous actin reorganization and impairment of cell adhesion dynamics

Background

Changes in cell shape and plasticity in cytoskeletal dynamics are critically involved in cell adhesion, migration, invasion and the overall process of metastasis. Previous work in our laboratory demonstrated that the synthetic steroid mifepristone inhibited the growth of highly metastatic cancer cells, while simultaneously causing striking changes in cellular morphology. Here we assessed whether such morphological alterations developed in response to cytostatic concentrations of mifepristone are reversible or permanent, involve rearrangement of cytoskeletal proteins, and/or affect the adhesive capacity of the cells.

Methods

Cancer cell lines of the ovary (SKOV-3), breast (MDA-MB-231), prostate (LNCaP), and nervous system (U87MG) were exposed to cytostatic concentrations of mifepristone and studied by phase-contrast microscopy. The transient or permanent nature of the cytostasis and morphological changes caused by mifepristone was assessed, as well as the rearrangement of cytoskeletal proteins. De-adhesion and adhesion assays were utilized to determine if mifepristone-arrested and morphologically dysregulated cells had abnormal de-adhesion/adhesion dynamics when compared to vehicle-treated controls.

Results

Mifepristone-treated cells displayed a long, thin, spindle-like shape with boundaries resembling those of loosely adhered cells. Growth arrest and morphology changes caused by mifepristone were reversible in SKOV-3, MDA-MB-231 and U87MG, but not in LNCaP cells that instead became senescent. All cancer cell types exposed to mifepristone displayed greatly increased actin ruffling in association with accelerated de-adhesion from the culture plate, and delayed adhesion capacity to various extracellular matrix components.

Conclusions

Cytostatic concentrations of mifepristone induced alterations in the cellular structure of a panel of aggressive, highly metastatic cancer cells of different tissues of origin. Such changes were associated with re-distribution of actin fibers that mainly form non-adhesive membrane ruffles, leading to dysregulated cellular adhesion capacity.

Tyrosyl phosphorylated PAK1 regulates breast cancer cell motility in response to prolactin through filamin A

The p21-activated serine-threonine kinase (PAK1) is activated by small GTPase-dependent and -independent mechanisms and regulates cell motility. Both PAK1 and the hormone prolactin (PRL) have been implicated in breast cancer by numerous studies. We have previously shown that the PRL-activated tyrosine kinase JAK2 (Janus tyrosine kinase 2) phosphorylates PAK1 in vivo and identified tyrosines (Tyr) 153, 201, and 285 in the PAK1 molecule as sites of JAK2 tyrosyl phosphorylation. Here, we have used human breast cancer T47D cells stably overexpressing PAK1 wild type or PAK1 Y3F mutant in which Tyr(s) 153, 201, and 285 were mutated to phenylalanines to demonstrate that phosphorylation of these three tyrosines are required for maximal PRL-dependent ruffling. In addition, phosphorylation of these three tyrosines is required for increased migration of T47D cells in response to PRL as assessed by two independent motility assays. Finally, we show that PAK1 phosphorylates serine (Ser) 2152 of the actin-binding protein filamin A to a greater extent when PAK1 is tyrosyl phosphorylated by JAK2. Down-regulation of PAK1 or filamin A abolishes the effect of PRL on cell migration. Thus, our data presented here bring some insight into the mechanism of PRL-stimulated motility of breast cancer cells.

Withaferin A induces proteasome inhibition, endoplasmic reticulum stress, the heat shock response and acquisition of thermotolerance

In the present study, withaferin A (WA), a steroidal lactone with anti-inflammatory and anti-tumor properties, inhibited proteasome activity and induced endoplasmic reticulum (ER) and cytoplasmic HSP accumulation in Xenopus laevis A6 kidney epithelial cells. Proteasomal inhibition by WA was indicated by an accumulation of ubiquitinated protein and a decrease in chymotrypsin-like activity. Additionally, immunoblot analysis revealed that treatment of cells with WA induced the accumulation of HSPs including ER chaperones, BiP and GRP94, as well as cytoplasmic/nuclear HSPs, HSP70 and HSP30. Furthermore, WA-induced an increase in the relative levels of the protein kinase, Akt, while the levels of actin were unchanged compared to control. Northern blot experiments determined that WA induced an accumulation in bip, hsp70 and hsp30 mRNA but not eIF-1α mRNA. Interestingly, WA acted synergistically with mild heat shock to enhance HSP70 and HSP30 accumulation to a greater extent than the sum of both stressors individually. This latter phenomenon was not observed with BiP or GRP94. Immunocytochemical analysis indicated that WA-induced BiP accumulation occurred mainly in the perinuclear region in a punctate pattern, while HSP30 accumulation occurred primarily in a granular pattern in the cytoplasm with some staining in the nucleus. Prolonged exposure to WA resulted in disorganization of the F-actin cytoskeleton as well as the production of relatively large HSP30 staining structures that co-localized with F-actin. Finally, prior exposure of cells to WA treatment, which induced the accumulation of HSPs conferred a state of thermal protection since it protected the F-actin cytoskeleton against a subsequent cytotoxic thermal challenge.

FGF receptors 1 and 2 are key regulators of keratinocyte migration in vitro and in wounded skin

Efficient wound repair is essential for the maintenance of the integrity of the skin. The repair process is controlled by a variety of growth factors and cytokines, and their abnormal expression or activity can cause healing disorders. Here, we show that wound repair is severely delayed in mice lacking fibroblast growth factor receptors (FGFR) 1 and 2 in keratinocytes. As the underlying mechanism, we identified impaired wound contraction and a delay in re-epithelialization that resulted from impaired keratinocyte migration at the wound edge. Scratch wounding and transwell assays demonstrated that FGFR1/2-deficient keratinocytes had a reduced migration velocity and impaired directional persistence owing to inefficient formation and turnover of focal adhesions. Underlying this defect, we identified a significant reduction in the expression of major focal adhesion components in the absence of FGFR signaling, resulting in a general migratory deficiency. These results identify FGFs as key regulators of keratinocyte migration in wounded skin.

The physical interaction of myoblasts with the microenvironment during remodeling of the cytoarchitecture

Integrins, focal adhesions, the cytoskeleton and the extracellular matrix, form a structural continuum between the external and internal environment of the cell and mediate the pathways associated with cellular mechanosensitivity and mechanotransduction. This continuum is important for the onset of muscle tissue generation, as muscle precursor cells (myoblasts) require a mechanical stimulus to initiate myogenesis. The ability to sense a mechanical cue requires an intact cytoskeleton and strong physical contact and adhesion to the microenvironment. Importantly, myoblasts also undergo reorientation, alignment and large scale remodeling of the cytoskeleton when they experience mechanical stretch and compression in muscle tissue. It remains unclear if such dramatic changes in cell architecture also inhibit physical contact and adhesion with the tissue microenvironment that are clearly important to myoblast physiology. In this study, we employed interference reflection microscopy to examine changes in the close physical contact of myoblasts with a substrate during induced remodeling of the cytoarchitecture (de-stabilization of the actin and microtubule cytoskeleton and inhibition of acto-myosin contractility). Our results demonstrate that while each remodeling pathway caused distinct effects on myoblast morphology and sub-cellular structure, we only observed a ∼13% decrease in close physical contact with the substrate, regardless of the pathway inhibited. However, this decrease did not correlate well with changes in cell adhesion strength. On the other hand, there was a close correlation between cell adhesion and β1-integrin expression and the presence of cell-secreted fibronectin, but not with the presence of intact focal adhesions. In this study, we have shown that myoblasts are able to maintain a large degree of physical contact and adhesion to the microenvironment, even during shot periods (<60 min) of large scale remodeling and physiological stress, which is essential to their in-vivo functionality.

Examining the role of actin-plasma membrane association in Pseudomonas aeruginosa infection and type III secretion translocation in migratory T24 epithelial cells

The opportunistic pathogen Pseudomonas aeruginosa targets wounded epithelial barriers, but the cellular alteration that increases susceptibility to P. aeruginosa infection remains unclear. This study examined how cell migration contributes to the establishment of P. aeruginosa infections using (i) highly migratory T24 epithelial cells as a cell culture model, (ii) mutations in the type III secretion (T3S) effector ExoS to manipulate P. aeruginosa infection, and (iii) high-resolution immunofluorescent microscopy to monitor ExoS translocation. ExoS includes both GTPase-activating (GAP) and ADP-ribosyltransferase (ADPRT) activities, and P. aeruginosa cells expressing wild-type ExoS preferentially bound to the leading edge of T24 cells, where ExoS altered leading-edge architecture and actin anchoring in conjunction with interrupting T3S translocation. Inactivation of ExoS GAP activity allowed P. aeruginosa to be internalized and secrete ExoS within T24 cells, but as with wild-type ExoS, translocation was limited in association with disruption of actin anchoring. Inactivation of ExoS ADPRT activity resulted in significantly enhanced T3S translocation by P. aeruginosa cells that remained extracellular and in conjunction with maintenance of actin-plasma membrane association. Infection with P. aeruginosa expressing ExoS lacking both GAP and ADPRT activities resulted in the highest level of T3S translocation, and this occurred in conjunction with the entry and alignment of P. aeruginosa and ExoS along actin filaments. Collectively, in using ExoS mutants to modulate and visualize T3S translocation, we were able to (i) confirm effector secretion by internalized P. aeruginosa, (ii) differentiate the mechanisms underlying the effects of ExoS GAP and ADPRT activities on P. aeruginosa internalization and T3S translocation, (iii) confirm that ExoS ADPRT activity targeted a cellular substrate that interrupted T3S translocation, (iv) visualize the ability of P. aeruginosa and ExoS to align with actin filaments, and (v) demonstrate an association between actin anchoring at the leading edge of T24 cells and the establishment of P. aeruginosa infection. Our studies also highlight the contribution of ExoS to the opportunistic nature of P. aeruginosa infection through its ability to exert cytotoxic effects that interrupt T3S translocation and P. aeruginosa internalization, which in turn limit the P. aeruginosa infectious process.

Silencing of Eps8 blocks migration and invasion in human glioblastoma cell lines

Glioblastoma multiforme (GBM) is the most malignant human primary brain tumor, and its infiltrative nature represents the leading cause for the failure of therapies and tumor recurrences. It is therefore crucial the knowledge of the molecular mechanisms underlying GBM invasion to identify novel therapeutic targets to limit motility. In this study, we evaluated the role of Epidermal growth factor receptor Pathway Substrate 8 (Eps8), a crucial regulator of the actin cytoskeleton dynamics accompanying cell motility and invasion, in GBM migration and invasiveness. We found that silencing of the protein by small interfering RNAs (siRNAs) abrogated the migratory and invasive capacity of three different human GBM cell lines both in 2-dimensional (2-D) and 3-dimensional (3-D) in vitro assays. The inhibitory effect on invasion was maintained independently by the migration mode utilized by the cells in our 3-D model, and was accompanied by an impaired formation of actin-based cytoskeletal protrusive structures. Our data propose Eps8 as a key molecule involved in the control of the intrinsic invasive behavior of GBM cells, and suggest that this protein might represent a useful target for the design of new drugs for the treatment of these tumors.

Regulation of vascular endothelial cell polarization and migration by Hsp70/Hsp90-organizing protein

Hsp70/Hsp90-organizing protein (HOP) is a member of the co-chaperone family, which directly binds to chaperones to regulate their activities. The participation of HOP in cell motility and endothelial cell functions remains largely unknown. In this study, we demonstrate that HOP is critically involved in endothelial cell migration and angiogenesis. Tube formation and capillary sprouting experiments reveal that depletion of HOP expression significantly inhibits vessel formation from endothelial cells. Wound healing and transwell migration assays show that HOP is important for endothelial cell migration. By examination of centrosome reorientation and membrane ruffle dynamics, we find that HOP plays a crucial role in the establishment of cell polarity in response to migratory stimulus. Furthermore, our data show that HOP interacts with tubulin and colocalizes with microtubules in endothelial cells. These findings indicate HOP as a novel regulator of angiogenesis that functions through promoting vascular endothelial cell polarization and migration.

A novel function for Cyclin A2: control of cell invasion via RhoA signaling

Cyclin A2 plays a key role in cell cycle regulation. It is essential in embryonic cells and in the hematopoietic lineage yet dispensable in fibroblasts. In this paper, we demonstrate that Cyclin A2-depleted cells display a cortical distribution of actin filaments and increased migration. These defects are rescued by restoration of wild-type Cyclin A2, which directly interacts with RhoA, or by a Cyclin A2 mutant unable to associate with Cdk. In vitro, Cyclin A2 potentiates the exchange activity of a RhoA-specific guanine nucleotide exchange factor. Consistent with this, Cyclin A2 depletion enhances migration of fibroblasts and invasiveness of transformed cells via down-regulation of RhoA activity. Moreover, Cyclin A2 expression is lower in metastases relative to primary colon adenocarcinoma in matched human tumors. All together, these data show that Cyclin A2 negatively controls cell motility by promoting RhoA activation, thus demonstrating a novel Cyclin A2 function in cytoskeletal rearrangements and cell migration.

A review of models of fluctuating protrusion and retraction patterns at the leading edge of motile cells

A characteristic feature of motile cells as they undergo a change in motile behavior is the development of fluctuating exploratory motions of the leading edge, driven by actin polymerization. We review quantitative models of these protrusion and retraction phenomena. Theoretical studies have been motivated by advances in experimental and computational methods that allow controlled perturbations, single molecule imaging, and analysis of spatiotemporal correlations in microscopic images. To explain oscillations and waves of the leading edge, most theoretical models propose nonlinear interactions and feedback mechanisms among different components of the actin cytoskeleton system. These mechanisms include curvature-sensing membrane proteins, myosin contraction, and autocatalytic biochemical reaction kinetics. We discuss how the combination of experimental studies with modeling promises to quantify the relative importance of these biochemical and biophysical processes at the leading edge and to evaluate their generality across cell types and extracellular environments.

Cortactin controls cell motility and lamellipodial dynamics by regulating ECM secretion

BACKGROUND

Branched actin assembly is critical for both cell motility and membrane trafficking. The branched actin regulator cortactin is generally considered to promote cell migration by controlling leading-edge lamellipodial dynamics. However, recent reports indicate that lamellipodia are not required for cell movement, suggesting an alternate mechanism.

RESULTS

Because cortactin also regulates membrane trafficking and adhesion dynamics, we hypothesized that altered secretion of extracellular matrix (ECM) and/or integrin trafficking might underlie motility defects of cortactin-knockdown (KD) cells. Consistent with a primary defect in ECM secretion, both motility and lamellipodial defects of cortactin-KD cells were fully rescued by plating on increasing concentrations of exogenous ECM. Furthermore, cortactin-KD cell speed defects were rescued on cell-free autocrine ECM produced by control cells, but not on ECM produced by cortactin-KD cells. Investigation of the mechanism revealed that whereas endocytosed fibronectin (FN) is redeposited at the basal cell surface by control cells, cortactin-KD cells exhibit defective FN secretion and abnormal FN retention in a late endocytic/lysosomal compartment. Cortactin-KD motility and FN deposition defects were phenocopied by KD in control cells of the lysosomal fusion regulator synaptotagmin-7. Rescue of cortactin-KD cells by expression of cortactin-binding domain mutants revealed that interaction with the Arp2/3 complex and actin filaments is essential for rescue of both cell motility and autocrine ECM secretion phenotypes, whereas binding of SH3-domain partners is not required.

CONCLUSIONS

Efficient cell motility, promoted by cortactin regulation of branched actin networks, involves processing and resecretion of internalized ECM from a late endosomal/lysosomal compartment.

Differential response of arterial and venous endothelial cells to extracellular matrix is modulated by oxygen

Binding of endothelial cell (EC) integrins to extracellular-matrix (ECM) components is one of the key events to trigger intracellular signaling that will ultimately result in proper vascular development. Even within one tissue, the endothelial phenotype differs between arteries and veins. Here, we tested the hypothesis that anchorage dependent processes, such as proliferation, viability, survival and actin organization of venous (VEC) and arterial EC (AEC) differently depend on ECM proteins. Moreover,because of different oxygen tension in AEC and VEC, we tested oxygen as a co-modulator of ECM effects. Primary human placental VEC and AEC were grown in collagens I and IV, fibronectin, laminin, gelatin and uncoated plates and exposed to 12 and 21% oxygen. Our main findings revealed that VEC are more sensitive than AEC to changes in the ECM composition. Proliferation and survival of VEC, in contrast to AEC, were profoundly increased by the presence of collagen I and fibronectin when compared with gelatin or uncoated plates. These effects were reversed by inhibition of focal adhesion kinase (Fak) and modulated by oxygen. VEC were more susceptible to the oxygen dependent ECM effects than AEC. However, no differential ECM effect on actin organization was observed between the two cell types. These data provide first evidence that AEC and VEC from the same vascular loop respond differently to ECM and oxygen in a Fak-dependent manner.

Study of the actin cytoskeleton in live endothelial cells expressing GFP-actin

The microvascular endothelium plays an important role as a selectively permeable barrier to fluids and solutes. The adhesive junctions between endothelial cells regulate permeability of the endothelium, and many studies have indicated the important contribution of the actin cytoskeleton to determining junctional integrity(1-5). A cortical actin belt is thought to be important for the maintenance of stable junctions(1, 2, 4, 5). In contrast, actin stress fibers are thought to generate centripetal tension within endothelial cells that weakens junctions(2-5). Much of this theory has been based on studies in which endothelial cells are treated with inflammatory mediators known to increase endothelial permeability, and then fixing the cells and labeling F-actin for microscopic observation. However, these studies provide a very limited understanding of the role of the actin cytoskeleton because images of fixed cells provide only snapshots in time with no information about the dynamics of actin structures(5). Live-cell imaging allows incorporation of the dynamic nature of the actin cytoskeleton into the studies of the mechanisms determining endothelial barrier integrity. A major advantage of this method is that the impact of various inflammatory stimuli on actin structures in endothelial cells can be assessed in the same set of living cells before and after treatment, removing potential bias that may occur when observing fixed specimens. Human umbilical vein endothelial cells (HUVEC) are transfected with a GFP-β-actin plasmid and grown to confluence on glass coverslips. Time-lapse images of GFP-actin in confluent HUVEC are captured before and after the addition of inflammatory mediators that elicit time-dependent changes in endothelial barrier integrity. These studies enable visual observation of the fluid sequence of changes in the actin cytoskeleton that contribute to endothelial barrier disruption and restoration. Our results consistently show local, actin-rich lamellipodia formation and turnover in endothelial cells. The formation and movement of actin stress fibers can also be observed. An analysis of the frequency of formation and turnover of the local lamellipodia, before and after treatment with inflammatory stimuli can be documented by kymograph analyses. These studies provide important information on the dynamic nature of the actin cytoskeleton in endothelial cells that can used to discover previously unidentified molecular mechanisms important for the maintenance of endothelial barrier integrity.

Dynamic cell adhesion and viscoelastic signatures distinguish normal from malignant human mammary cells using quartz crystal microbalance

During transformation of a normal cell to a cell capable of forming a cancerous growth, cellular morphology, the cytoskeleton, and focal contacts undergo significant changes. These changes should be capable of being characterized via real-time monitoring of the dynamic cell adhesion process and viscoelastic properties of cells. Here, we describe use of the quartz crystal microbalance (QCM) to distinguish the dynamic cell adhesion signatures of human normal (HMEC) versus malignant (MCF-7) mammary epithelial cells. The significantly reduced QCM responses (changes in frequency [Δf] and motional resistance ΔR) of MCF-7 cells compared with those of HMECs mirror the cancer cells' morphological features as observed via optical microscope. We analyzed the initial 2-h cell adhesion kinetics, suggesting cell-cell cooperativity for HMECs and no or weak cell-cell interactions for MCF-7 cells. We propose that changes of the ΔR/Δf ratio, which we term the cell viscoelastic index (CVI), reflect the establishment of cytoskeleton structure and dynamic viscoelastic properties of living cells. The CVI decreases significantly on initiation of cell to surface interactions as cells establish their cytoskeletal structures. During the cell adhesion process, MCF-7 cells were consistently softer, exhibiting up to a 2.5-fold smaller CVI when compared with HMECs.

Filamin C plays an essential role in the maintenance of the structural integrity of cardiac and skeletal muscles, revealed by the medaka mutant zacro

Filamin C is an actin-crosslinking protein that is specifically expressed in cardiac and skeletal muscles. Although mutations in the filamin C gene cause human myopathy with cardiac involvement, the function of filamin C in vivo is not yet fully understood. Here we report a medaka mutant, zacro (zac), that displayed an enlarged heart, caused by rupture of the myocardiac wall, and progressive skeletal muscle degeneration in late embryonic stages. We identified zac to be a homozygous nonsense mutation in the filamin C (flnc) gene. The medaka filamin C protein was found to be localized at myotendinous junctions, sarcolemma, and Z-disks in skeletal muscle, and at intercalated disks in the heart. zac embryos showed prominent myofibrillar degeneration at myotendinous junctions, detachment of myofibrils from sarcolemma and intercalated disks, and focal Z-disk destruction. Importantly, the expression of γ-actin, which we observed to have a strong subcellular localization at myotendinous junctions, was specifically reduced in zac mutant myotomes. Inhibition of muscle contraction by anesthesia alleviated muscle degeneration in the zac mutant. These results suggest that filamin C plays an indispensable role in the maintenance of the structural integrity of cardiac and skeletal muscles for support against mechanical stress.

Murine missing in metastasis (MIM) mediates cell polarity and regulates the motility response to growth factors

Background

Missing in metastasis (MIM) is a member of the inverse BAR-domain protein family, and in vitro studies have implied MIM plays a role in deforming membrane curvature into filopodia-like protrusions and cell dynamics. Yet, the physiological role of the endogenous MIM in mammalian cells remains undefined.

Principal Findings

We have examined mouse embryonic fibroblasts (MEFs) derived from mice in which the MIM locus was targeted by a gene trapping vector. MIM^−/− MEFs showed a less polarized architecture characterized by smooth edges and fewer cell protrusions as compared to wild type cells, although the formation of filopodia-like microprotrusions appeared to be normal. Immunofluorescent staining further revealed that MIM^−/− cells were partially impaired in the assembly of stress fibers and focal adhesions but were enriched with transverse actin filaments at the periphery. Poor assembly of stress fibers was apparently correlated with attenuation of the activity of Rho GTPases and partially relieved upon overexpressing of Myc-RhoA^Q63L, a constitutively activated RhoA mutant. MIM^−/− cells were also spread less effectively than wild type cells during attachment to dishes and substratum. Upon treatment with PDGF MIM^−/− cells developed more prominent dorsal ruffles along with increased Rac1 activity. Compared to wild type cells, MIM^−/− cells had a slower motility in the presence of a low percentage of serum-containing medium but migrated normally upon adding growth factors such as 10% serum, PDGF or EGF. MIM^−/− cells were also partially impaired in the internalization of transferrin, fluorescent dyes, foreign DNAs and PDGF receptor alpha. On the other hand, the level of tyrosine phosphorylation of PDGF receptors was more elevated in MIM depleted cells than wild type cells upon PDGF treatment.

Conclusions

Our data suggests that endogenous MIM protein regulates globally the cell architecture and endocytosis that ultimately influence a variety of cellular behaviors, including cell polarity, motility, receptor signaling and membrane ruffling.

Adapter protein SH2B1beta binds filamin A to regulate prolactin-dependent cytoskeletal reorganization and cell motility

Prolactin (PRL) regulates cytoskeletal rearrangement and cell motility. PRL-activated Janus tyrosine kinase 2 (JAK2) phosphorylates the p21-activated serine-threonine kinase (PAK)1 and the Src homology 2 (SH2) domain-containing adapter protein SH2B1β. SH2B1β is an actin-binding protein that cross-links actin filaments, whereas PAK1 regulates the actin cytoskeleton by different mechanisms, including direct phosphorylation of the actin-binding protein filamin A (FLNa). Here, we have used a FLNa-deficient human melanoma cell line (M2) and its derivative line (A7) that stably expresses FLNa to demonstrate that SH2B1β and FLNa are required for maximal PRL-dependent cell ruffling. We have found that in addition to two actin-binding domains, SH2B1β has a FLNa-binding domain (amino acids 200-260) that binds directly to repeats 17-23 of FLNa. The SH2B1β-FLNa interaction participates in PRL-dependent actin rearrangement. We also show that phosphorylation of the three tyrosines of PAK1 by JAK2, as well as the presence of FLNa, play a role in PRL-dependent cell ruffling. Finally, we show that the actin- and FLNa-binding-deficient mutant of SH2B1β (SH2B1β 3Δ) abolished PRL-dependent ruffling and PRL-dependent cell migration when expressed along with PAK1 Y3F (JAK2 tyrosyl-phosphorylation-deficient mutant). Together, these data provide insight into a novel mechanism of PRL-stimulated regulation of the actin cytoskeleton and cell motility via JAK2 signaling through FLNa, PAK1, and SH2B1β. We propose a model for PRL-dependent regulation of the actin cytoskeleton that integrates our findings with previous studies.

Cortactin: a multifunctional regulator of cellular invasiveness

Branched actin assembly is critical for a variety of cellular processes that underlie cell motility and invasion, including cellular protrusion formation and membrane trafficking. Activation of branched actin assembly occurs at various subcellular locations via site-specific activation of distinct WASp family proteins and the Arp2/3 complex. A key branched actin regulator that promotes cell motility and links signaling, cytoskeletal and membrane trafficking proteins is the Src kinase substrate and Arp2/3 binding protein cortactin. Due to its frequent overexpression in advanced, invasive cancers and its general role in regulating branched actin assembly at multiple cellular locations, cortactin has been the subject of intense study. Recent studies suggest that cortactin has a complex role in cellular migration and invasion, promoting both on-site actin polymerization and modulation of autocrine secretion. Diverse cellular activities may derive from the interaction of cortactin with site-specific binding partners.

Extracellular ultrathin fibers sensitive to intracellular reactive oxygen species: formation of intercellular membrane bridges

Membrane bridges are key cellular structures involved in intercellular communication; however, dynamics for their formation are not well understood. We demonstrated the formation and regulation of novel extracellular ultrathin fibers in NIH3T3 cells using confocal and atomic force microscopy. At adjacent regions of neighboring cells, phorbol 12-myristate 13-acetate (PMA) and glucose oxidase induced ultrathin fiber formation, which was prevented by Trolox, a reactive oxygen species (ROS) scavenger. The height of ROS-sensitive ultrathin fibers ranged from 2 to 4 nm. PMA-induced formation of ultrathin fibers was inhibited by cytochalasin D, but not by Taxol or colchicine, indicating that ultrathin fibers mainly comprise microfilaments. PMA-induced ultrathin fibers underwent dynamic structural changes, resulting in formation of intercellular membrane bridges. Thus, these fibers are formed by a mechanism(s) involving ROS and involved in formation of intercellular membrane bridges. Furthermore, ultrastructural imaging of ultrathin fibers may contribute to understanding the diverse mechanisms of cell-to-cell communication and the intercellular transfer of biomolecules, including proteins and cell organelles.

Kinectin-mediated endoplasmic reticulum dynamics supports focal adhesion growth in the cellular lamella

Focal adhesions (FAs) control cell shape and motility, which are important processes that underlie a wide range of physiological functions. FA dynamics is regulated by cytoskeleton, motor proteins and small GTPases. Kinectin is an integral endoplasmic reticulum (ER) membrane protein that extends the ER along microtubules. Here, we investigated the influence of the ER on FA dynamics within the cellular lamella by disrupting the kinectin-kinesin interaction by overexpressing the minimal kinectin-kinesin interaction domain on kinectin in cells. This perturbation resulted in a morphological change to a rounded cell shape and reduced cell spreading and migration. Immunofluorescence and live-cell imaging demonstrated a kinectin-dependent ER extension into the cellular lamella and ER colocalisation with FAs within the cellular lamella. FRAP experiments showed that ER contact with FAs was accompanied with an increase in FA protein recruitment to FAs. Disruption of the kinectin-kinesin interaction caused a reduction in FA protein recruitment to FAs. This suggests that the ER supports FA growth within the cellular lamella. Microtubule targeting to FAs is known to promote adhesion disassembly; however, ER contact increased FA size even in the presence of microtubules. Our results suggest a scenario whereby kinectin-kinesin interaction facilitates ER transport along microtubules to support FA growth.

A role for 1-acylglycerol-3-phosphate-O-acyltransferase-1 in myoblast differentiation

AGPAT isoforms catalyze the acylation of lysophosphatidic acid (LPA) to form phosphatidic acid (PA). AGPAT2 mutations are associated with defective adipogenesis. Muscle and adipose tissue share common precursor cells. We investigated the role of AGPAT isoforms in skeletal muscle development. We demonstrate that small interference RNA-mediated knockdown of AGPAT1 expression prevents the induction of myogenin, a key transcriptional activator of the myogenic program, and inhibits the expression of myosin heavy chain. This effect is rescued by transfection with AGPAT1 but not AGPAT2. Knockdown of AGPAT2 has no effect. The regulation of myogenesis by AGPAT1 is associated with alterations on actin cytoskeleton. The role of AGPAT1 on actin cytoskeleton is further supported by colocalization of AGPAT1 to areas of active actin polymerization. AGPAT1 overexpression was not associated with an increase in PA levels. Our observations strongly implicate AGPAT1 in the development of skeletal muscle, specifically to terminal differentiation. These findings are linked to the regulation of actin cytoskeleton.

Nanofibers in a hyaluronan-based pericellular matrix

The extracellular matrix of the brain is a highly organized hyaluronan-based supramolecular assembly that is involved in neuronal pathfinding, cell migration, synaptogenesis and neuronal plasticity. Here, we analyze the structure of the hyaluronan-rich pericellular matrix of an oligodendroglial precursor cell line using helium ion beam scanning microscopy at a subnanometer resolution. We find that thin nanofibers are the ultimate building elements of this oligodendroglial pericellular matrix. These structures may participate in the regulation of oligodendroglial maturation and motility.

A critical role of conventional protein kinase C in morphological changes of rodent mast cells

In mast cells, crosslinking the high-affinity IgE receptor (FcɛRI) results in a dynamic reorganization of the actin cytoskeleton that is associated with membrane ruffling. Although the signaling involved in degranulation has been well described, it is less understood in morphological changes. In this study, we investigated the specific role of conventional protein kinase C (cPKC), a crucial signal for degranulation, in antigen-induced membrane ruffling of mast cells. In RBL-2H3 mast cells, antigen induced a long-lasting membrane ruffling, which was blocked with late-added Gö6976, a specific cPKC inhibitor, indicating that sustained activation of cPKC is required for maintaining the reaction. Immunofluorescence staining of endogenous PKCα/β and real-time imaging of transfected green fluorescent protein-tagged PKCα/β demonstrated that in response to antigen both PKCα and PKCβI quickly translocated to the plasma membrane and were colocalized with actin filaments at the ruffling sites. These reactions were blocked by expression of kinase-negative PKCβI, but not kinase-negative PKCα, and by treatment with a specific PKCβ inhibitor, LY333531. The adhesion, spreading and membrane ruffling of mouse bone marrow-derived mast cells (BMMCs), which are mostly nonadhesive, were promoted by both antigen and thymeleatoxin. Treatment with Gö6976 abolished all these reactions. Antigen-mediated migration of BMMC was also sensitive to Gö6076 and LY333531. In addition, BMMC adhesion by and migration toward stem cell factor were shown to be dependent on cPKC. Thus, cPKC, at least PKCβ subtype, may be critical for the dynamic morphological changes that lead to the migration of mast cells.