Actin and microtubules in cell motility: which one is in control?

Polarity proteins in glial cell functions

Glial cells, which include myelinating oligodendrocytes, Schwann cells and astrocytes, fulfil a large variety of functions that are critical for the development, functioning and regeneration of neurons. Some of these glial functions have been shown to require polarization of the intracellular machinery. Although the initial signals leading to glial cell polarization during development and in the adult are not completely elucidated, crucial molecules such as proteins of the extracellular matrix and their membrane receptors have been identified. A general picture of the intracellular signalling pathways controlling polarity in glial cells is also emerging and shows that highly conserved and ubiquitously expressed polarity proteins are involved.

Role of the cytoskeleton in flow (shear stress)-induced dilation and remodeling in resistance arteries

Cytoskeletal proteins determine cell shape and integrity and membrane-bound structures connected to extracellular components allow tissue integrity. These structural elements have an active role in the interaction of blood vessels with their environment. Shear stress due to blood flow is the most important force stimulating the endothelium. The role of cytoskeletal proteins in endothelial responses to flow has been studied in resistance arteries using pharmacological tools and transgenic models. Shear stress activates extracellular "flow sensing" elements associated with a thick glycocalyx communicating the signal to membrane-bound complexes (integrins and/or dystrophin-dystroglycans) and to eNOS through a pathway involving the intermediate filament vimentin, the microtubule network and actin. When blood flow increases chronically the endothelium triggers diameter enlargement and medial hypertrophy. This is facilitated by the genetic absence of the intermediate filaments, vimentin and desmin suggesting that these elements oppose the process.

Antiangiogenic vinflunine affects EB1 localization and microtubule targeting to adhesion sites

The motile behavior of endothelial cells is a crucial event for neoangiogenesis. We previously showed that noncytotoxic concentrations of vinflunine inhibit capillary-like tube formation on Matrigel and endothelial cell migration with a concomitant increase in interphase microtubule dynamic instability. In this article, we further investigated the effects of vinflunine on migration and cytoskeleton interaction dynamics in HMEC-1 endothelial cells. We confirmed that vinflunine, at low and noncytotoxic concentrations (0.01-1 nmol/L), inhibited endothelial cell random motility by 54%. This effect was associated with a decrease in the percentage of stable microtubules and in the mean duration of pauses for dynamic ones. Moreover, we found that vinflunine altered adhesion site targeting by microtubules and suppressed the microtubule (+) end pause that occurs at adhesion sites during cell migration (from 151 +/- 20 seconds in control cells to 38 +/- 7 seconds in vinflunine-treated cells, P < 0.001). This effect was associated with the inhibition of adhesion site dynamics and the formation of long-lived stress fibers. Importantly, we found that vinflunine altered EB1 localization at microtubule (+) ends. These results highlight a new mechanism of action of vinflunine, which act by disrupting the mutual control between microtubule and adhesion site dynamics and strengthen the role of +TIPs proteins such as EB1 as key regulators of endothelial cell motility.

Mediation, modulation, and consequences of membrane-cytoskeleton interactions

Elements of the cytoskeleton interact intimately and communicate bidirectionally with cellular membranes. Such interactions are critical for a host of cellular processes. Here we focus on the many types of interactions that exist between the cytoskeleton and the plasma membrane to illustrate why these cellular components can never truly be studied in isolation in vivo. We discuss how membrane-cytoskeleton interactions are mediated and modulated, and how many proteins involved in these interactions are disrupted in human disease. We then highlight key molecular and physical variables that must be considered in order to mechanistically dissect events associated with changes in plasma membrane morphology. These considerations are integrated into the context of cell migration, filopodia formation, and clathrin-mediated endocytosis to show how a holistic view of the plasma membrane-cytoskeleton interface can allow for the appropriate interpretation of experimental findings and provide novel mechanistic insight into these important cellular events.

Interference with actin dynamics is superior to disturbance of microtubule function in the inhibition of human ovarian cancer cell motility

Cellular movement is mainly orchestrated by the actin and microtubule cytoskeleton in which Rho GTPases closely collaborate. We studied whether cytoskeleton-interfering agents at subtoxic and 50% growth-inhibiting concentrations affect motility of five unselected human ovarian cancer cell lines. Cisplatin and doxorubicin as control cytotoxic agents were not effective, the microtubule-targeting agents docetaxel, epothilone B and vinblastine only marginally inhibited cell motility, while the actin-targeting agent cytochalasin D was most potent in hampering both cell migration and invasion. Disturbance of microtubule dynamics by docetaxel did not importantly affect the cellular structures of beta-tubulin and F-actin. In contrast, hindrance of actin dynamics by cytochalasin D resulted in loss of lamellipodial extensions, induced thick layers of F-actin and disorder in cellular organization. In OVCAR-3 cells the activity of Rac1 was only slightly diminished by docetaxel, but clearly reduced by cytochalasin D. In conclusion, targeting the actin cytoskeleton might provide a means to prevent metastasis formation.

The vaccinia virus F11L gene product facilitates cell detachment and promotes migration

We previously showed that infection with vaccinia virus (VV) induces cell motility, characterized by contractility and directed migration. Motility is temporally regulated because cells are motile immediately after infection, whereas late in infection motility ceases and cells resettle. Motility and its cessation are accompanied by temporal rearrangements of both the microtubule and the actin networks. Because the F11L gene has previously been implicated in VV-induced migration, we now explore the role of F11L in contractility, migration, the cessation of motility and the cytoskeletal rearrangements. By live cell imaging using a VV that lacks an intact F11L gene, we show that F11L facilitates cell detachment and is required for migration but not for contractility. By light microscopy, F11L expression induces a remodeling of the actin, but not the microtubule, network. The lack of migration correlates with smaller plaques, indicating that this process facilitates cell-to-cell spreading of VV. Late in infection, when motility ceases, cells re-establish cell-to-cell contacts in an F11L-independent manner. We finally show that VV-induced motility and its cessation correlate with a temporal regulation of the guanosine triphosphatase RhoA as well as the expression levels of F11L during the infectious cycle.

Integrins and dystroglycan regulate astrocyte wound healing: the integrin beta1 subunit is necessary for process extension and orienting the microtubular network

Monolayers of astrocytes in culture respond to a scrape wound by orienting towards the wound and extending processes that will repair it. We show here that they also upregulate the expression of extracellular matrix (ECM) proteins, laminin, and chondroitin sulfated proteoglycan, that are deposited in astrocytic scars in vivo. We have previously shown that the major functional ECM receptors on astrocytes are dystroglycan (DG) plus integrins alpha1beta1, alpha5beta1, alpha6beta1, and alphavbeta3. Consistent with this, laminin fragments that activate alpha1beta1 integrin, alpha6beta1 integrin, and DG all contribute to attachment. During astrocyte attachment, or process extension, integrins and DG are found at the leading edge of the lammelipodium, though they change in distribution with the extent of attachment and the alpha and beta subunits of DG can be spatially uncoupled. Functionally, inhibitory antibodies to DG and integrin alpha1beta1 or the RGD peptide all inhibit process extension, showing that ligand engagement of integrins and DG contribute to process extension. Astrocytes differentiated from DG or beta1 null ES cells respond very differently to wounding. The former fail to extend process and cell polarization is disrupted partially. However, beta1 null astrocytes not only fail to extend processes perpendicular to the wound, but cell polarization is completely disrupted and cells migrate randomly into the wound. We conclude that integrins are essential for astrocyte polarity.

Nuclear shape, mechanics, and mechanotransduction

In eukaryotic cells, the nucleus contains the genome and is the site of transcriptional regulation. The nucleus is the largest and stiffest organelle and is exposed to mechanical forces transmitted through the cytoskeleton from outside the cell and from force generation within the cell. Here, we discuss the effect of intra- and extracellular forces on nuclear shape and structure and how these force-induced changes could be implicated in nuclear mechanotransduction, ie, force-induced changes in cell signaling and gene transcription. We review mechanical studies of the nucleus and nuclear structural proteins, such as lamins. Dramatic changes in nuclear shape, organization, and stiffness are seen in cells where lamin proteins are mutated or absent, as in genetically engineered mice, RNA interference studies, or human disease. We examine the different mechanical pathways from the force-responsive cytoskeleton to the nucleus. We also highlight studies that link changes in nuclear shape with cell function during developmental, physiological, and pathological modifications. Together, these studies suggest that the nucleus itself may play an important role in the response of the cell to force.

Control of cell flattening and junctional remodeling during squamous epithelial morphogenesis in Drosophila

Diverse types of epithelial morphogenesis drive development. Similar cytoskeletal and cell adhesion machinery orchestrate these changes, but it is unclear how distinct tissue types are produced. Thus, it is important to define and compare different types of morphogenesis. We investigated cell flattening and elongation in the amnioserosa, a squamous epithelium formed at Drosophila gastrulation. Amnioserosa cells are initially columnar. Remarkably, they flatten and elongate autonomously by perpendicularly rotating the microtubule cytoskeleton--we call this 'rotary cell elongation'. Apical microtubule protrusion appears to initiate the rotation and microtubule inhibition perturbs the process. F-actin restrains and helps orient the microtubule protrusions. As amnioserosa cells elongate, they maintain their original cell-cell contacts and develop planar polarity. Myosin II localizes to anterior-posterior contacts, while the polarity protein Bazooka (PAR-3) localizes to dorsoventral contacts. Genetic analysis revealed that Myosin II and Bazooka cooperate to properly position adherens junctions. These results identify a specific cellular mechanism of squamous tissue morphogenesis and molecular interactions involved.

Paxillin-dependent stimulation of microtubule catastrophes at focal adhesion sites

An organized microtubule array is essential for the polarized motility of fibroblasts. Dynamic microtubules closely interact with focal adhesion sites in migrating cells. Here, we examined the effect of focal adhesions on microtubule dynamics. We observed that the probability of microtubule catastrophes (transitions from growth to shrinkage) was seven times higher at focal adhesions than elsewhere. Analysis of the dependence between the microtubule growth rate and catastrophe probability throughout the cytoplasm revealed that a nonspecific (mechanical or spatial) factor provided a minor contribution to the catastrophe induction by decreasing microtubule growth rate at adhesions. Strikingly, at the same growth rate, the probability of catastrophes was significantly higher at adhesions than elsewhere, indicative of a site-specific biochemical trigger. The observed catastrophe induction occurred at adhesion domains containing the scaffolding protein paxillin that has been shown previously to interact with tubulin. Furthermore, replacement of full-length paxillin at adhesion sites by microinjected paxillin LIM2-LIM3 domains suppressed microtubule catastrophes exclusively at adhesions. We suggest that paxillin influences microtubule dynamics at focal adhesions by serving as a scaffold for a putative catastrophe factor and/or regulating its exposure to microtubules.

Stat3 promotes metastatic progression of prostate cancer

There are currently no effective therapies for metastatic prostate cancer because the molecular mechanisms that underlie the metastatic spread of primary prostate cancer are unclear. Transcription factor Stat3 is constitutively active in malignant prostate epithelium, and its activation is associated with high histological grade and advanced cancer stage. In this work, we hypothesized that Stat3 stimulates metastatic progression of prostate cancer. We show that Stat3 is active in 77% of lymph node and 67% of bone metastases of clinical human prostate cancers. Importantly, adenoviral gene delivery of wild-type Stat3 (AdWTStat3) to DU145 human prostate cancer cells increased the number of lung metastases by 33-fold in an experimental metastasis assay compared with controls. Using various methods to inhibit Stat3, we demonstrated that Stat3 promotes human prostate cancer cell migration. Stat3 induced the formation of lamellipodia in both DU145 and PC-3 cells, further supporting the concept that Stat3 promotes a migratory phenotype of human prostate cancer cells. Moreover, Stat3 caused the rearrangement of cytoplasmic actin stress fibers and microtubules in both DU145 and PC-3 cells. Finally, inhibition of the Jak2 tyrosine kinase decreased both activation of Stat3 and prostate cancer cell motility. Collectively, these data indicate that transcription factor Stat3 is involved in metastatic behavior of human prostate cancer cells and may provide a therapeutic target to prevent metastatic spread of primary prostate cancer.

Ndel1 promotes axon regeneration via intermediate filaments

Failure of axons to regenerate following acute or chronic neuronal injury is attributed to both the inhibitory glial environment and deficient intrinsic ability to re-grow. However, the underlying mechanisms of the latter remain unclear. In this study, we have investigated the role of the mammalian homologue of aspergillus nidulans NudE, Ndel1, emergently viewed as an integrator of the cytoskeleton, in axon regeneration. Ndel1 was synthesized de novo and upregulated in crushed and transected sciatic nerve axons, and, upon injury, was strongly associated with neuronal form of the intermediate filament (IF) Vimentin while dissociating from the mature neuronal IF (Neurofilament) light chain NF-L. Consistent with a role for Ndel1 in the conditioning lesion-induced neurite outgrowth of Dorsal Root Ganglion (DRG) neurons, the long lasting in vivo formation of the neuronal Ndel1/Vimentin complex was associated with robust axon regeneration. Furthermore, local silencing of Ndel1 in transected axons by siRNA severely reduced the extent of regeneration in vivo. Thus, Ndel1 promotes axonal regeneration; activating this endogenous repair mechanism may enhance neuroregeneration during acute and chronic axonal degeneration.

Roles of Rho guanosine 5'-triphosphatase A, Rho kinases, and extracellular signal regulated kinase (1/2) in prostaglandin E2-mediated migration of first-trimester human extravillous trophoblast

Prostaglandin (PG) E(2) may regulate invasiveness of human placenta because we previously reported stimulation of migration of placental trophoblasts by PGE(2) acting through PGE receptor (EP)-1 and activating calpain. RhoA GTPase and its important effector Rho kinase (ROCK) have also been previously shown to regulate trophoblast migration. Using immortalized HTR-8/SVneo trophoblast cells and first-trimester human chorionic villus explant cultures on matrigel, we further examined the role of RhoA/ROCK and MAPK (ERK1/2) pathways on PGE(2)-mediated stimulation of trophoblast migration. Migration of cytotrophoblasts was shown to be inhibited by treatment of the trophoblast cell line and chorionic villus explants with either cell-permeable C3 transferase or selective RhoA small interfering RNA. These inhibitions were significantly mitigated by the addition of PGE(2), an EP1/EP3 agonist or an EP3/EP4 agonist, suggesting that RhoA plays an important role in trophoblast migration but may not be obligatory for PGE(2) action. Treatment of HTR-8/SVneo cells with nonselective ROCK inhibitor Y27632 or ROCK small interfering RNAs inhibited migration of these cells, which could not be rescued with PGE(2) or the other two EP agonists, suggesting the obligatory role of ROCK in PGE(2)-induced migratory response. Furthermore, U0126, an inhibitor of MAPK kinases MEK1 and MEK2, abrogated PGE(2)-induced migration of trophoblasts, and PGE(2) or the other two EP agonists stimulated ERK1/2 activation in trophoblasts, which was not abrogated by pretreatment with C3 transferase, indicating that ERK signaling pathway is an efficient alternate pathway for RhoA in PGE(2)-mediated migration of trophoblasts. These results suggest that ROCK and ERK1/2 play more important roles than RhoA in PGE(2)-mediated migration stimulation of first-trimester trophoblasts.

Stathmin activity influences sarcoma cell shape, motility, and metastatic potential

The balanced activity of microtubule-stabilizing and -destabilizing proteins determines the extent of microtubule dynamics, which is implicated in many cellular processes, including adhesion, migration, and morphology. Among the destabilizing proteins, stathmin is overexpressed in different human malignancies and has been recently linked to the regulation of cell motility. The observation that stathmin was overexpressed in human recurrent and metastatic sarcomas prompted us to investigate stathmin contribution to tumor local invasiveness and distant dissemination. We found that stathmin stimulated cell motility in and through the extracellular matrix (ECM) in vitro and increased the metastatic potential of sarcoma cells in vivo. On contact with the ECM, stathmin was negatively regulated by phosphorylation. Accordingly, a less phosphorylable stathmin point mutant impaired ECM-induced microtubule stabilization and conferred a higher invasive potential, inducing a rounded cell shape coupled with amoeboid-like motility in three-dimensional matrices. Our results indicate that stathmin plays a significant role in tumor metastasis formation, a finding that could lead to exploitation of stathmin as a target of new antimetastatic drugs.

Ndel1 controls the dynein-mediated transport of vimentin during neurite outgrowth

Ndel1, the mammalian homologue of the Aspergillus nidulans NudE, is emergently viewed as an integrator of the cytoskeleton. By regulating the dynamics of microtubules and assembly of neuronal intermediate filaments (IFs), Ndel1 promotes neurite outgrowth, neuronal migration, and cell integrity (1-6). To further understand the roles of Ndel1 in cytoskeletal dynamics, we performed a tandem affinity purification of Ndel1-interacting proteins. We isolated a novel Ndel1 molecular complex composed of the IF vimentin, the molecular motor dynein, the lissencephaly protein Lis1, and the cis-Golgi-associated protein alphaCOP. Ndel1 promotes the interaction between Lis1, alphaCOP, and the vimentin-dynein complex. The functional result of this complex is activation of dynein-mediated transport of vimentin. A loss of Ndel1 functions by RNA interference fails to incorporate Lis1/alphaCOP in the complex, reduces the transport of vimentin, and culminates in IF accumulations and altered neuritogenesis. Our findings reveal a novel regulatory mechanism of vimentin transport during neurite extension that may have implications in diseases featuring transport/trafficking defects and impaired regeneration.

Role of adenomatous polyposis coli (APC) and microtubules in directional cell migration and neuronal polarization

In response to extracellular signals during embryonic development, cells undergo directional movements to specific sites and establish proper connections to other cells to form organs and tissues. Cell extension and migration in the direction of extracellular cues is mediated by the actin and microtubule cytoskeletons, and recent results have shed new light on how these pathways are activated by neurotrophins, Wnt or extracellular matrix. These signals lead to modifications of microtubule-associated proteins (MAPs) and point to glycogen synthase kinase (GSK) 3beta as a key regulator of microtubule function during directional migration. This review will summarize these results and then focus on the role of microtubule-binding protein adenomatous polyposis coli (APC) in neuronal polarization and directed migration, and on its regulation by GSK3beta.

Prostaglandin E2-mediated migration of human trophoblast requires RAC1 and CDC42

The invasion of maternal decidua and uterine spiral arteries by a trophoblast subpopulation called extravillous trophoblast (EVT) is essential for the establishment of a normal placenta and an adequate blood flow toward the fetus. Derangements in these processes underlie pregnancy-related diseases like preeclampsia and intrauterine growth restriction. Many growth factors, growth factor binding proteins, and extracellular matrix components can positively or negatively regulate the proliferation, migration, and/or invasiveness of these EVT cells. RHO GTPases, including RHOA, RAC1, and CDC42, are ubiquitous proteins that control cytoskeletal changes by forming stress fibers and projecting lamellipodia and filopodia during cellular migration. We had previously shown that prostaglandin (PG) E(2) produced in abundance by the decidua promotes the migration of first-trimester human EVTs by increasing the intracellular concentration of calcium and activating calpain. Using our well-characterized immortalized EVT cell line, HTR-8/SVneo, as well as villus explants from first-trimester placentae, this study examined the role of RHO GTPases RAC1 and CDC42 in PGE(2)-mediated migratory responses of these cells. Though a RAC1 inhibitor, NSC23766 as well as RAC1 knockdown by siRNA decreased the migration of HTR-8/SVneo cells in a Transwell migration assay, this inhibition could not be restored by PGE(2) or 17-phenyl trinor PGE(2) (PGE receptor PTGER1 agonist) or PGE(1) Alcohol (PGE receptor PTGER4 agonist). Similar results were noted for EVT cell spreading in villus explants. Furthermore, CDC42 silencing using siRNA inhibited PGE(2)-induced migration of HTR-8/SVneo cells. Finally, the treatment of EVT cells with PGE(2), PTGER1 agonist, or PTGER4 agonist activated RAC1 and CDC42 at 10 min, suggesting that RAC1 and CDC42 play an essential role in PGE(2)-mediated migration of human EVTs.

Arf6 and microtubules in adhesion-dependent trafficking of lipid rafts

Integrin-mediated adhesion regulates membrane binding sites for Rac1 within lipid rafts. Detachment of cells from the substratum triggers the clearance of rafts from the plasma membrane through caveolin-dependent internalization. The small GTPase Arf6 and microtubules also regulate Rac-dependent cell spreading and migration, but the mechanisms are poorly understood. Here we show that endocytosis of rafts after detachment requires F-actin, followed by microtubule-dependent trafficking to recycling endosomes. When cells are replated on fibronectin, rafts exit from recycling endosomes in an Arf6-dependent manner and return to the plasma membrane along microtubules. Both of these steps are required for the plasma membrane targeting of Rac1 and for its activation. These data therefore define a new membrane raft trafficking pathway that is crucial for anchorage-dependent signalling.

Microtopography of metal surfaces influence fibroblast growth by modifying cell shape, cytoskeleton, and adhesion

Stainless Steel (SS), titanium (cpTi), and Ti-6Al-7Nb (TAN) are frequently used metals in fracture fixation, which contact not only bone, but also soft tissue. In previous soft tissue cytocompatibility studies, TAN was demonstrated to inhibit cell growth in its "standard" micro-roughened state. To elucidate a possible mechanism for this inhibition, cell area, shape, adhesion, and cytoskeletal integrity was studied. Only minor changes in spreading were observed for cells on electropolished SS, cpTi, and TAN. Cells on "standard" cpTi were similarly spread in comparison with electropolished cpTi and TAN, although the topography influenced the cell periphery and also resulted in lower numbers and shorter length of focal adhesions. On "standard" microrough TAN, cell spreading was significantly lower than all other surfaces, and cell morphology differed by being more elongated. In addition, focal adhesion numbers and mean length were significantly lower on standard TAN than on all other surfaces, with 80% of the measured adhesions below a 2-microm threshold. Focal adhesion site location and maturation and microtubule integrity were compromised by the presence of protruding beta-phase microspikes found solely on the surface of standard TAN. This led us to propose that the impairment of focal adhesion numbers, maturation (length), and cell spreading to a possibly sufficient threshold observed on standard TAN blocks cell cycle progress and eventually cell growth on the surface. We believe, as demonstrated with standard cpTi and TAN, that a difference in surface morphology is influential for controlling cell behavior on implant surfaces.

Distinct detergent-resistant membrane microdomains (lipid rafts) respectively harvest K(+) and water transport systems in brain astroglia

The detergent-resistant microdomains (DRM) of cell membranes scaffold different molecules and regulate cell functions by orchestrating various signaling pathways including G-proteins and tyrosine kinase. Here we report a novel role for DRM in astroglial cells. K(+)-buffering inwardly rectifying Kir4.1 channels and the water channel AQP4 are expressed in astrocytes and they may be functionally coupled to maintain ionic and osmotic homeostasis in the brain. Kir4.1 and AQP4 channel proteins were abundant in noncaveloar DRM in the brain and also in HEK293T cells when exogenously expressed. In HEK293T cells, depletion of membrane cholesterol by methyl-beta-cyclodextrin (MbetaCD) resulted in loss of Kir4.1 association with DRM and its channel activity but did not affect either the distribution or the function of AQP4. Immunolabeling showed that Kir4.1 and AQP4 were occasionally distributed in close proximity but in distinct compartments of the astroglial cell membrane. Astroglial noncaveolar DRM may therefore be made up of at least two distinct compartments, MbetaCD-sensitive and MbetaCD-resistant microdomains, which control localization and function of, respectively, Kir and AQP4 channels on the cell membrane.

Disorganized microtubules underlie the formation of retraction bulbs and the failure of axonal regeneration

Axons in the CNS do not regrow after injury, whereas lesioned axons in the peripheral nervous system (PNS) regenerate. Lesioned CNS axons form characteristic swellings at their tips known as retraction bulbs, which are the nongrowing counterparts of growth cones. Although much progress has been made in identifying intracellular and molecular mechanisms that regulate growth cone locomotion and axonal elongation, a comprehensive understanding of how retraction bulbs form and why they are unable to grow is still elusive. Here we report the analysis of the morphological and intracellular responses of injured axons in the CNS compared with those in the PNS. We show that retraction bulbs of injured CNS axons increase in size over time, whereas growth cones of injured PNS axons remain constant. Retraction bulbs contain a disorganized microtubule network, whereas growth cones possess the typical bundling of microtubules. Using in vivo imaging, we find that pharmacological disruption of microtubules in growth cones transforms them into retraction bulb-like structures whose growth is inhibited. Correspondingly, microtubule destabilization of sensory neurons in cell culture induces retraction bulb formation. Conversely, microtubule stabilization prevents the formation of retraction bulbs and decreases axonal degeneration in vivo. Finally, microtubule stabilization enhances the growth capacity of CNS neurons cultured on myelin. Thus, the stability and organization of microtubules define the fate of lesioned axonal stumps to become either advancing growth cones or nongrowing retraction bulbs. Our data pinpoint microtubules as a key regulatory target for axonal regeneration.

The Par-Tiam1 complex controls persistent migration by stabilizing microtubule-dependent front-rear polarity

BACKGROUND

The establishment and maintenance of cell polarity is crucial for many biological functions and is regulated by conserved protein complexes. The Par polarity complex consisting of Par3, Par6, and PKCzeta, in conjunction with Tiam1-mediated Rac signaling, controls apical-basal cell polarity in contacting epithelial cells. Here we tested the hypothesis that the Par complex, in conjunction with Tiam1, controls "front-rear" polarity during the persistent migration of freely migrating keratinocytes.

RESULTS

Wild-type (WT) epidermal keratinocytes lacking cell-cell contacts are stably front-rear polarized and migrate persistently. In contrast, Tiam1-deficient (Tiam1 KO) and (si)Par3-depleted keratinocytes are generally unpolarized and migrate randomly because front-rear polarity is short lived. Immunoprecipitation experiments show that in migrating keratinocytes, Tiam1 associates with Par3 and PKCzeta. Moreover, Par3, PKCzeta, and Tiam1 proteins are enriched at the leading edges of polarized keratinocytes. Tiam1 KO keratinocytes are impaired in chemotactic migration toward growth factors, whereaes haptotactic migration is similar to WT. Par3 depletion or the blocking of PKCzeta signaling in WT keratinocytes impairs chemotaxis but has no additional effect on Tiam1 KO cells. The migratory and morphological defects in keratinocytes with impaired Par-Tiam1 function closely resemble cells with pharmacologically destabilized microtubules (MTs). Indeed, MTs in Tiam1 KO keratinocytes and WT cells treated with a PKCzeta inhibitor are unstable, thereby negatively influencing directional but not random migration.

CONCLUSIONS

We conclude that the Par-Tiam1 complex stabilizes front-rear polarization of noncontacting migratory cells, thereby stimulating persistent and chemotactic migration, whereas in contacting keratinocytes, the same complex controls the establishment of long-lasting apical-basal polarity. These findings underscore a remarkable flexibility of the Par polarity complex that, depending on the biological context, controls distinct forms of cellular polarity.

Multiphoton microscope imaging: The behavior of neural progenitor cells in the rostral migratory stream

Neural progenitor cells (NPCs) in the subventricular zone (SVZ) travel a long distance along the rostral migratory stream (RMS) to give rise to interneurons in the olfactory bulb (OB). Using the multiphoton microscope and time-lapse recording techniques we here report the behavior of NPCs in the RMS under both intact and ischemic conditions in living brain slices. The NPCs were visualized in 3-week-old transgenic mice that carry the reporter gene, green fluorescent protein (GFP), driven by the nestin promoter. Cortical brain ischemia was induced by permanent occlusion of the right common carotid artery and the middle cerebral artery. We observed that the RMS contained two populations of NPCs: nonmigrating cells (bridge cells) and migrating cells. Bridge cells enabled migrating cells to travel and also produced new cells in the RMS. The direction of NPC migration in the RMS was bidirectional in both intact and ischemic conditions. Cortical ischemia impeded NPC travel in the RMS next to the lesion area during the early period of ischemia. Cell-cell contact was a prominent feature affecting NPC translocation and migratory direction. These data suggest that behavior and function of nestin-positive NPCs in the RMS are variable. Cell-cell contacts and microenvironmental changes influence NPC behavior in the RMS. This study may provide insights to help in understanding NPC biology.

Type I gamma phosphatidylinositol phosphate kinase is required for EGF-stimulated directional cell migration

Phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) modulates a plethora of cytoskeletal interactions that control the dynamics of actin assembly and, ultimately, cell migration. We show that the type Iγ phosphatidylinositol phosphate kinase 661 (PIPKIγ661), an enzyme that generates PI4,5P₂, is required for growth factor but not G protein–coupled receptor–stimulated directional migration. By generating PI4,5P₂ and regulating talin assembly, PIPKIγ661 modulates nascent adhesion formation at the leading edge to facilitate cell migration. The epidermal growth factor (EGF) receptor directly phosphorylates PIPKIγ661 at tyrosine 634, and this event is required for EGF-induced migration. This phosphorylation regulates the interaction between PIPKIγ661 and phospholipase Cγ1 (PLCγ1, an enzyme previously shown to be involved in the regulation of EGF-stimulated migration). Our results suggest that phosphorylation events regulating specific PIPKIγ661 interactions are required for growth factor–induced migration. These interactions in turn define the spatial and temporal generation of PI4,5P₂ and derived messengers required for directional migration.

The flare gene, which encodes the AIP1 protein of Drosophila, functions to regulate F-actin disassembly in pupal epidermal cells

Adult Drosophila are decorated with several types of polarized cuticular structures, such as hairs and bristles. The morphogenesis of these takes place in pupal cells and is mediated by the actin and microtubule cytoskeletons. Mutations in flare (flr) result in grossly abnormal epidermal hairs. We report here that flr encodes the Drosophila actin interacting protein 1 (AIP1). In other systems this protein has been found to promote cofilin-mediated F-actin disassembly. In Drosophila cofilin is encoded by twinstar (tsr). We show that flr mutations result in increased levels of F-actin accumulation and increased F-actin stability in vivo. Further, flr is essential for cell proliferation and viability and for the function of the frizzled planar cell polarity system. All of these phenotypes are similar to those seen for tsr mutations. This differs from the situation in yeast where cofilin is essential while aip1 mutations result in only subtle defects in the actin cytoskeleton. Surprisingly, we found that mutations in flr and tsr also result in greatly increased tubulin staining, suggesting a tight linkage between the actin and microtubule cytoskeleton in these cells.

Detyrosinated microtubule protrusions in suspended mammary epithelial cells promote reattachment

Breast tumor cells enter the bloodstream long before the development of clinically evident metastasis. However, the early presence of such bloodborne cells predicts poor patient outcome. Nearly 90% of human breast tumors arise as carcinomas from mammary epithelial cells, so it is important to study how these cells respond to the detached conditions that they would experience in the bloodstream. We report here that mammary epithelial cell lines produce long and dynamic protrusions of the plasma membrane when detached. Although human and mouse mammary epithelial cell lines die by apoptosis within 16 h of detachment, this protrusive response persists for days in cells overexpressing either Bcl-2 or Bcl-xL. Unlike actin-dependent invadopodia and podosomes, these protrusions are actually enhanced by actin depolymerization with Cytochalasin-D or Latrunculin-A. Immunofluorescence and Western blotting demonstrate that the protrusions are enriched in detyrosinated Glu-tubulin, a post-translationally modified form of alpha-tubulin that is found in stabilized microtubules. Video microscopy indicates that these protrusions promote cell-cell attachment, and inhibiting microtubule-based protrusions correlates with reduced extracellular matrix attachment. Since bloodborne metastasis depends on both cell-cell and cell-matrix attachment, microtubule-based protrusions in detached mammary epithelial cells provide a novel mechanism that could influence the metastatic spread of breast tumors.

Microtubule-dependent organization of subcortical microfilaments in the earlyDrosophila embryo

Dynamic alterations in the spatial organization of cytoskeletal elements constitute a prominent morphological feature of the early, syncytial stages of embryogenesis in Drosophila. Here, we describe and characterize the dynamic behavior of cytoplasmic, subcortical microfilaments, which form a series of nucleus-associated structures, at different phases of the simultaneous nuclear division cycles characteristic of early Drosophila embryos. Remodeling of the cytoplasmic microfilament arrays takes place in parallel to the established cyclic reorganization of cortical microfilament structures. We provide evidence that the cortical and subcortical microfilament populations organize independently of each other, and in response to distinct instructive cues. Specifically, formation of subcortical microfilament structures appears to rely on, and spatially mirror, the organization of polarized microtubule arrays, while cortical microfilament restructuring constitutes a centrosome-dependent process. Genetic analysis identifies a requirement for SCAR, a key mediator of Arp2/3-based microfilament dynamics, in organization of subcortical microfilament structures.

Static magnetic fields enhance skeletal muscle differentiation in vitro by improving myoblast alignment

Static magnetic field (SMF) interacts with mammal skeletal muscle; however, SMF effects on skeletal muscle cells are poorly investigated. The myogenic cell line L6, an in vitro model of muscle development, was used to investigate the effect of a 80 +/- mT SMF generated by a custom-made magnet. SMF promoted myogenic cell differentiation and hypertrophy, i.e., increased accumulation of actin and myosin and formation of large multinucleated myotubes. The elevated number of nuclei per myotube was derived from increased cell fusion efficiency, with no changes in cell proliferation upon SMF exposure. No alterations in myogenin expression, a modulator of myogenesis, occurred upon SMF exposure. SMF induced cells to align in parallel bundles, an orientation conserved throughout differentiation. SMF stimulated formation of actin stress-fiber like structures. SMF rescued muscle differentiation in the presence of TNF, a muscle differentiation inhibitor. We believe this is the first report showing that SMF promotes myogenic differentiation and cell alignment, in the absence of any invasive manipulation. SMF-enhanced parallel orientation of myotubes is relevant to tissue engineering of a highly organized tissue such as skeletal muscle. SMF rescue of muscle differentiation in the presence of TNF may have important therapeutic implications.

N-WASP regulates extension of filopodia and processes by oligodendrocyte progenitors, oligodendrocytes, and Schwann cells—implications for axon ensheathment at myelination

The molecular mechanisms used by oligodendrocyte precursor cells (OPCs), oligodendrocytes (OLs), and Schwann cells (SCs) to advance processes for motility in the developing nervous system and to ensheath axons at myelination are currently not well defined. Here we demonstrate that OPCs, OLs, and SCs express the major proteins involved in actin polymerization-driven protrusion; these key proteins including F-actin, the Arp2/3 complex, neural-Wiskott Aldrich Syndrome protein (N-WASP) and WAVE proteins, and the RhoGTPases Rac and Cdc42 are present at the leading edges of processes being extended by OPCs, OLs, and SCs. We reveal by real-time PCR that OLs and SCs have different dominant WAVE isoforms. Inhibition of the WASP/WAVE protein, N-WASP, with wiskostatin that prevents activation of the Arp2/3 complex, blocks process extension by OPCs and SCs. Inhibition of N-WASP also causes OPC and SC process retraction, which is preceded by retraction of filopodia. This implicates filopodia in OPC and SC process stability and also of N-WASP in OPC and SC process dynamics. We also demonstrate that p34 (a component of the Arp2/3 complex), WASP/WAVE proteins, actin, alpha-tubulin, Rac, Cdc42, vinculin, and focal adhesion kinase are detected in water-shocked myelin purified from brain. Inhibition of N-WASP with wiskostatin decreases the number of axons undergoing initial ensheathment in intact optic nerve samples and reduces the Po content of dorsal root ganglia:SC co-cultures. Our findings indicate that OPCs, OLs, and SCs extend processes using actin polymerization-driven protrusion dependent on N-WASP. We hypothesize that inner mesaxons of OLs and SCs use the same mechanism to ensheath axons at myelination.

Role of microtubules in tip growth of fungi

Polarized cell growth is observed ubiquitously in all living organisms. Tip growth of filamentous fungi serves as a typical model for polar growth. It is well known that the actin cytoskeleton plays a central role in cellular growth. In contrast, the role of microtubules in polar growth of fungal tip cells has not been critically addressed. Our recent study, using a green fluorescent protein (GFP)-labeled tubulin-expressing strain of the filamentous fungus Aspergillus nidulans and treatment with an anti-microtubule reagent, revealed that microtubules are essential for rapid hyphal growth. Our results indicated that microtubule organization contributes to continuous tip growth throughout the cell cycle, which in turn enables the maintenance of an appropriate mass of cytoplasm for the multinucleate system. In filamentous fungi, the microtubule is an essential component of the tip growth machinery that enables continuous and rapid growth. Recent research developments are starting to elucidate the components of the tip growth machinery and their functions in many organisms. This recent knowledge, in turn, is starting to enhance the importance of fungal systems as simple model systems to understand the polar growth of cells.

Anisotropy of cell adhesive microenvironment governs cell internal organization and orientation of polarity

Control of the establishment of cell polarity is an essential function in tissue morphogenesis and renewal that depends on spatial cues provided by the extracellular environment. The molecular role of cell-cell or cell-extracellular matrix (ECM) contacts on the establishment of cell polarity has been well characterized. It has been hypothesized that the geometry of the cell adhesive microenvironment was directing cell surface polarization and internal organization. To define how the extracellular environment affects cell polarity, we analyzed the organization of individual cells plated on defined micropatterned substrates imposing cells to spread on various combinations of adhesive and nonadhesive areas. The reproducible normalization effect on overall cell compartmentalization enabled quantification of the spatial organization of the actin network and associated proteins, the spatial distribution of microtubules, and the positioning of nucleus, centrosome, and Golgi apparatus. By using specific micropatterns and statistical analysis of cell compartment positions, we demonstrated that ECM geometry determines the orientation of cell polarity axes. The nucleus-centrosome orientations were reproducibly directed toward cell adhesive edges. The anisotropy of the cell cortex in response to the adhesive conditions did not affect the centrosome positioning at the cell centroid. Based on the quantification of microtubule plus end distribution we propose a working model that accounts for that observation. We conclude that, in addition to molecular composition and mechanical properties, ECM geometry plays a key role in developmental processes.

Orchestration of lymphocyte chemotaxis by mitochondrial dynamics

Lymphocyte traffic is required to maintain homeostasis and perform appropriate immunological reactions. To migrate into inflamed tissues, lymphocytes must acquire spatial and functional asymmetries. Mitochondria are highly dynamic organelles that distribute in the cytoplasm to meet specific cellular needs, but whether this is essential to lymphocyte functions is unknown. We show that mitochondria specifically concentrate at the uropod during lymphocyte migration by a process involving rearrangements of their shape. Mitochondrial fission facilitates relocation of the organelles and promotes lymphocyte chemotaxis, whereas mitochondrial fusion inhibits both processes. Our data substantiate a new role for mitochondrial dynamics and suggest that mitochondria redistribution is required to regulate the motor of migrating cells.

Role of RhoC in digit morphogenesis during limb development

Here we report a new role for the small GTPase RhoC in the control of limb chondrogenesis. Expression of rhoC is a precocious marker of the zeugopodial and digit blastemas and is induced by treatments with TGFbetas preceding the formation of ectopic digits. As development progresses, expression of rhoC outlines the growing distal tip of the digits, and marks the regions of interphalangeal joint formation. Functional experiments show that RhoC is a negative regulator of chondrogenesis, which controls digit outgrowth and joint segmentation. These functions appear to be mediated by reorganization of the actin cytoskeleton and modification of the adhesive properties of the mesenchymal cells.

IQGAP1 stimulates actin assembly through the N-WASP-Arp2/3 pathway

IQGAP1 is a conserved modular protein overexpressed in cancer and involved in organizing actin and microtubules in motile processes such as adhesion, migration, and cytokinesis. A variety of proteins have been shown to interact with IQGAP1, including the small G proteins Rac1 and Cdc42, actin, calmodulin, beta-catenin, the microtubule plus end-binding proteins CLIP170 (cytoplasmic linker protein) and adenomatous polyposis coli. However, the molecular mechanism by which IQGAP1 controls actin dynamics in cell motility is not understood. Quantitative co-localization analysis and down-regulation of IQGAP1 revealed that IQGAP1 controls the co-localization of N-WASP with the Arp2/3 complex in lamellipodia. Co-immunoprecipitation supports an in vivo link between IQGAP1 and N-WASP. Pull-down experiments and kinetic assays of branched actin polymerization with N-WASP and Arp2/3 complex demonstrated that the C-terminal half of IQGAP1 activates N-WASP by interacting with its BR-CRIB domain in a Cdc42-like manner, whereas the N-terminal half of IQGAP1 antagonizes this activation by association with a C-terminal region of IQGAP1. We propose that signal-induced relief of the autoinhibited fold of IQGAP1 allows activation of N-WASP to stimulate Arp2/3-dependent actin assembly.

Microtubule-targeting agents inhibit angiogenesis at subtoxic concentrations, a process associated with inhibition of Rac1 and Cdc42 activity and changes in the endothelial cytoskeleton

Conventional anticancer agents may display antiangiogenic effects, but the underlying mechanism is poorly understood. We determined the antiangiogenic properties of cisplatin, doxorubicin, and the microtubule-targeting agents docetaxel, epothilone B, and vinblastine at concentrations not affecting cell proliferation. We also assessed tubulin and actin morphology and the activity of two key molecules in cell motility, the small Rho GTPases Cdc42 and Rac1. The highest non-toxic concentration (HNTC) of each drug was defined as the concentration inhibiting a maximum of 10% human umbilical vein endothelial cell growth on a 1-hour drug exposure, being for cisplatin 10 micromol/L, doxorubicin 100 nmol/L, docetaxel 10 nmol/L, epothilone B 1 nmol/L, and vinblastine 10 nmol/L. Comparative endothelial cell functional assays using HNTCs for an exposure time of 1 hour indicated that endothelial cell migration in the wound assay, endothelial cell invasion in a transwell invasion system, and endothelial cell formation into tubelike structures on a layer of Matrigel were significantly inhibited by docetaxel, epothilone B, and vinblastine (P < 0.05), but not by cisplatin and doxorubicin. Docetaxel was slightly more efficient in the inhibition of endothelial cell motility than epothilone B and vinblastine. Fluorescence microscopy revealed that only the microtubule-targeting agents affected the integrity of the tubulin and F-actin cytoskeleton, which showed disturbed microtubule structures, less F-actin stress fiber formation, and appearance of nuclear F-actin rings. These observations were associated with early inhibition of Rac1 and Cdc42 activity. In conclusion, HNTCs of microtubule-targeting agents efficiently reduce endothelial cell motility by interference with microtubule dynamics preventing the activation of Rac1/Cdc42 and disorganizing the actin cytoskeleton.

Connexin 43-mediated modulation of polarized cell movement and the directional migration of cardiac neural crest cells

Connexin 43 knockout (Cx43α1KO) mice have conotruncal heart defects that are associated with a reduction in the abundance of cardiac neural crest cells (CNCs) targeted to the heart. In this study, we show CNCs can respond to changing fibronectin matrix density by adjusting their migratory behavior,with directionality increasing and speed decreasing with increasing fibronectin density. However, compared with wild-type CNCs, Cx43α1KO CNCs show reduced directionality and speed, while CNCs overexpressing Cx43α1 from the CMV43 transgenic mice show increased directionality and speed. Altered integrin signaling was indicated by changes in the distribution of vinculin containing focal contacts, and altered temporal response of Cx43α1KO and CMV43 CNCs to β1 integrin function blocking antibody treatment. High resolution motion analysis showed Cx43α1KO CNCs have increased cell protrusive activity accompanied by the loss of polarized cell movement. They exhibited an unusual polygonal arrangement of actin stress fibers that indicated a profound change in cytoskeletal organization. Semaphorin 3A, a chemorepellent known to inhibit integrin activation, was found to inhibit CNC motility, but in the Cx43α1KO and CMV43 CNCs, cell processes failed to retract with semaphorin 3A treatment. Immunohistochemical and biochemical analyses suggested close interactions between Cx43α1,vinculin and other actin-binding proteins. However, dye coupling analysis showed no correlation between gap junction communication level and fibronectin plating density. Overall, these findings indicate Cx43α1 may have a novel function in mediating crosstalk with cell signaling pathways that regulate polarized cell movement essential for the directional migration of CNCs.

Transiently Reorganized Microtubules Are Essential for Zippering during Dorsal Closure in Drosophila melanogaster

There is emerging evidence that microtubules in nondividing cells can be employed to remodel the intracellular space. Here, we demonstrate an essential role for microtubules in dorsal closure, which occurs toward the end of Drosophila melanogaster embryogenesis. Dorsal closure is a morphogenetic process similar to wound healing, whereby a gap in the epithelium is closed through the coordinated action of different cell types. Surprisingly, this complex process requires microtubule function exclusively in epithelial cells and only for the last step, the zippering, which seals the gap. Preceding zippering, the epithelial microtubules reorganize to attain an unusual spatial distribution, which we describe with subcellular resolution in the intact, living organism. We provide a clearly defined example where cells of a developing organism transiently reorganize their microtubules to fulfill a specialized morphogenetic task.

Myoblast attachment and spreading are regulated by different patterns by ubiquitous calpains

The calcium-dependent proteolytic system is a large family of well-conserved ubiquitous and tissue-specific proteases, known as calpains, and an endogenous inhibitor, calpastatin. Ubiquitous calpains are involved in many physiological phenomena, such as the cell cycle, muscle cell differentiation, and cell migration. This study investigates the regulation of crucial steps of cell motility, myoblast adhesion and spreading, by calpains. Inhibition of each ubiquitous calpain isoform by antisense strategy pinpointed the involvement of each of these proteases in myoblast adhesion and spreading. Moreover, the actin cytoskeleton and microtubules were observed in transfected cells, demonstrating that each ubiquitous calpain could be involved in the actin fiber organization. C2C12 cells with reduced mu- or m-calpain levels have a rounded morphology and disorganized stress fibers, but no modification in the microtubule cytoskeleton. Antisense strategy directed against MARCKS, a calpain substrate during C2C12 migration, showed that this protein could play a role in stress fiber polymerization. A complementary proteomic analysis using C2C12 cells over-expressing calpastatin indicated that two proteins were under-expressed, while six, which are involved in the studied phenomena, were overexpressed after calpain inhibition. The possible role of these proteins in adhesion, spreading, and migration was discussed.

Microtubules: forgotten players in the apoptotic execution phase

A cell entering the execution phase of apoptosis (regulated cell death) undergoes characteristic rearrangements, in which the cytoskeleton has major roles. Historically, this reorganisation has been attributed entirely to actomyosin contractility, with microtubule and intermediate filament systems both reported to be lost at an early stage. However, recent results indicate that microtubule networks re-form during the later stages of apoptosis and assist in the dispersal of nuclear and cellular fragments--steps that are thought to be important for preventing inflammation. Here, we discuss the roles of the cytoskeleton during apoptosis and challenge current thinking that actin is the sole functional component driving all major execution phase events.

Molecular regulation of cytoskeletal rearrangements during T cell signalling

Regulation of the cytoskeleton in cells of the haematopoietic system is essential for fulfilling diverse tasks such as migration towards a chemoattractant, phagocytosis or cell-cell communication. This is particularly true for the many types of T cells, which are at the foundation of the adaptive immune system in vertebrates. Deregulation of actin filament turnover is known to be involved in the development of severe immunodeficiencies or immunoproliferative diseases. Therefore, molecular dissection of signalling complexes and effector molecules, which leads to controlled cytoskeletal assembly, has been the focus of immunological research in the last decade. In the past, cytoskeletal remodelling was frequently understood as the finish line of signalling, while today it becomes increasingly evident that actin and microtubule dynamics are required for proper signal transmission in many processes such as T cell activation. Significant effort is made in many laboratories to further elucidate the contribution of cytoskeletal remodelling to immune function. The objective of this article is to summarise the current knowledge on how actin and microtubules are reorganised to support the formation of structures as diverse as the immunological synapse and peripheral protrusions during cell migration.

Antiangiogenic concentrations of vinflunine increase the interphase microtubule dynamics and decrease the motility of endothelial cells

Angiogenesis is a key event in tumor progression and metastasis. This complex process, which constitutes a potent target for cancer therapy, is inhibited by very low concentrations of microtubule-targeting drugs (MTD). However, the intimate mechanisms of the antiangiogenic activity of MTDs remain unclear. Recently, we have shown that low antiangiogenic and noncytotoxic concentrations of paclitaxel induced an unexpected increase in microtubule dynamics in endothelial cells. In this study, we showed that vinflunine, the newest Vinca alkaloid, increased microtubule dynamic instability in human endothelial cells after 4-hour incubation at low concentrations (29% and 54% at 0.1 and 2 nmol/L). The growth and shortening rates were increased, and the percentage of time spent in pause and the mean duration of pauses were decreased, as previously observed with paclitaxel. As opposed to paclitaxel, the transition frequencies were not significantly disturbed by vinflunine. Moreover, low concentrations of vinflunine did not affect mitotic index and anaphase/metaphase ratio. Interestingly, these low vinflunine concentrations that increased microtubule dynamics exhibited an antiangiogenic effect through the inhibition of both morphogenesis and random motility. Capillary tube formation on Matrigel was decreased up to 44%. The cell speed and the random motility coefficient were decreased (13% and 19% and 13% and 33% at 0.1 and 2 nmol/L, respectively) and the persistent time was statistically increased. Altogether, our results confirm that the increase in microtubule dynamics is involved in MTD antiangiogenic activity and highlight the crucial role of interphase microtubule dynamics in angiogenesis.

Mechanobiology in the third dimension

Cells are mechanically coupled to their extracellular environments, which play critical roles in both communicating the state of the mechanical environment to the cell as well as in mediating cellular response to a variety of stimuli. Along with the molecular composition and mechanical properties of the extracellular matrix (ECM), recent work has demonstrated the importance of dimensionality in cell-ECM associations for controlling the sensitive communication between cells and the ECM. Matrix forces are generally transmitted to cells differently when the cells are on two-dimensional (2D) vs. within three-dimensional (3D) matrices, and cells in 3D environments may experience mechanical signaling that is unique vis-à-vis cells in 2D environments, such as the recently described 3D-matrix adhesion assemblies. This review examines how the dimensionality of the extracellular environment can affect in vitro cell mechanobiology, focusing on collagen and fibrin systems.

Migration of bone marrow stromal cells in 3D: 4 Color methodology reveals spatially and temporally coordinated events

The cytoskeleton plays a central role in many cell processes including directed cell migration. Since most previous work has investigated cell migration in two dimensions (2D), new methods are required to study movement in three dimensions (3D) while preserving 3D structure of the cytoskeleton. Most previous studies have labeled two cytoskeletal networks simultaneously, impeding an appreciation of their complex and dynamic interconnections. Here we report the development of a 4 color method to simultaneously image vimentin, actin, tubulin and the nucleus for high-resolution confocal microscopy of bone-marrow stromal cells (BMSCs) migrating through a porous membrane. Several methods were tested for structural preservation and labeling intensity resulting in identification of an optimized simultaneous fixation and permeabilization method using glutaraldehyde, paraformaldehyde and Triton X-100 followed by a quadruple fluorescent labeling method. This procedure was then applied at a sequence of time points to migrating cells, allowing temporal progression of migration to be assessed by visualizing all three networks plus the nucleus, providing new insights into 3D directed cell migration including processes such as leading edge structure, cytoskeletal distribution and nucleokinesis. Colocalization of actin and microtubules with distinct spatial arrangements at the cellular leading edge during migration, together with microtubule axial polarization supports recent reports indicating the pivotal role of microtubules in directed cell migration. This study also provides a foundation for 3D migration studies versus 2D studies, providing precise and robust methods to attain new insights into the cellular mechanisms of motility.

Directional cell expansion--turning toward actin

Significant recent progress toward understanding directional expansion in diffusely growing plant cells concerns actin. Tools for imaging actin, including both live-cell reporters and fixation protocols, have been improved. Proteins that interact with actin have been identified and their functions probed biochemically and genetically. Specifically, members of the actin-related protein2/3 (ARP2/3) complex and the Wiskott-Aldrich syndrome Verprolin-homologous (WAVE) complex have been identified. These proteins have salient functions in shaping trichomes and leaf pavement cells. Additionally, two targets of a rho-of-plants (ROP) G-protein have been discovered that exert opposing regulatory action on actin and microtubules, a pathway that appears to be responsible for establishing the undulating shapes of pavement cells. Finally, several mutants of the fragile fiber class have revealed a link between actin organization, cell wall synthesis, and phosphoinositol signaling.

Identification, expression analysis, genomic organization and cellular location of a novel protein with a RhoGEF domain

In this study we describe the identification and characterization of a novel cytosolic protein of the guanine exchange factor (GEF) family. The human cDNA corresponds to predicted human protein FLJ00128/FLJ10357 located on chromosome 14q11.2. The deduced protein sequence contains in its C-terminus a RhoGEF domain followed by a pleckstrin domain. Its N-terminus, central region and RhoGEF/pleckstrin domain are homologous to the recently identified zebrafish Quattro protein, which is involved in morphogenetic movements mediated by the actin cytoskeleton. Based on the homology of our protein's RhoGEF domain to the RhoGEF domains of Trio, Duo and Duet and its homology with Quattro, we named it Solo. The Solo mRNA is ubiquitously expressed but enriched in brain, its expression peaks perinatally and it undergoes extensive alternative splicing. In both myoblasts and neuroblastoma cells, the Solo protein is concentrated around the nucleus.