Cdc42, dynein, and dynactin regulate MTOC reorientation independent of Rho-regulated microtubule stabilization

In migrating adherent cells such as fibroblasts and endothelial cells, the microtubule-organizing center (MTOC) reorients toward the leading edge [1-3]. MTOC reorientation repositions the Golgi toward the front of the cell [1] and contributes to directional migration [4]. The mechanism of MTOC reorientation and its relation to the formation of stabilized microtubules (MTs) in the leading edge, which occurs concomitantly with MTOC reorientation [3], is unknown. We show that serum and the serum lipid, lysophosphatidic acid (LPA), increased Cdc42 GTP levels and triggered MTOC reorientation in serum-starved wounded monolayers of 3T3 fibroblasts. Cdc42, but not Rho or Rac, was both sufficient and necessary for LPA-stimulated MTOC reorientation. MTOC reorientation was independent of Cdc42-induced changes in actin and was not blocked by cytochalasin D. Inhibition of dynein or dynactin blocked LPA- and Cdc42-stimulated MTOC reorientation. LPA also stimulates a Rho/mDia pathway that selectively stabilizes MTs in the leading edge [5, 6]; however, activators and inhibitors of MTOC reorientation and MT stabilization showed that each response was regulated independently. These results establish an LPA/Cdc42 signaling pathway that regulates MTOC reorientation in a dynein-dependent manner. MTOC reorientation and MT stabilization both act to polarize the MT array in migrating cells, yet these processes act independently and are regulated by separate Rho family GTPase-signaling pathways.     

Localized cdc42 activation,detected using a novel assay,mediates microtubule organizing center positioning in endothelial cells in response to fluid shear stress

Fluid flow regulates morphology, physiology, and pathophysiology of vascular endothelial cells (reviewed in Ref. 1). The small GTPase Cdc42 mediates polarity in several systems including migrating cells and early embryos, which involve reorientation of the microtubule organizing center (MTOC) and Golgi apparatus toward the direction of movement. Here, we show that Cdc42 is activated by fluid shear stress and that activation is a consequence of integrins binding to extracellular matrix. A novel fluorescence energy transfer assay to visualize Cdc42 activation in single cells shows that Cdc42 activity is polarized in the direction of flow. Localized activation of Cdc42 as well as the activity of Par6 and protein kinase Czeta direct the reorientation of the MTOC to a position on the downstream side of the nucleus relative to the direction of flow. Thus, shear-stimulated integrin dynamics induce polarized Cdc42 activity, which induces MTOC localization through the Par6-protein kinase Czeta complex.     

Microtubule-organizing centers and cell migration: effect of inhibition of migration and microtubule disruption in endothelial cells

We have previously shown that microtubule-organizing centers (MTOC's) become preferentially oriented towards the leading edge of migrating endothelial cells (EC's) at the margin of an experimentally induced wound made in a confluent EC monolayer. To learn more about the mechanism responsible for the reorientation of MTOC's and to determine whether a similar reorientation takes place when cell migration is inhibited, we incubated the wounded cultures with colcemid (C) and cytochalasin B (CB), which disrupt microtubules (MT's) and microfilaments (MF's), respectively. The results obtained showed that the MTOC reorientation can occur independent of cell migration since MTOC's reoriented preferentially toward the wound edge in the CB- treated cultures, even though forward migration of the EC was inhibited. In addition, the MTOC reorientation is inhibited by C, indicating that it requires an intact system of MT's and/or other intracellular structures whose distribution is dependent on that of MT's.     

Distribution of microtubule organizing centers in migrating sheets of endothelial cells

This study was designed to investigate the relationship between the position of the microtubule organizing center (MTOC) and the direction of migration of a sheet of endothelial cells (EC). Using immunofluorescence and phase microscopy the MTOC's of migrating EC were visualized as the cells moved into an in vitro experimental wound produced by mechanical denudation of part of a confluent monolayer culture. Although the MTOC's in nonmigrating EC were randomly positioned in relation to the nucleus, in migrating cells the position of the MTOC's changed so that 80% of the cells had the MTOC positioned in front of the nucleus toward the direction of movement of the endothelial sheet. This repositioning of the MTOC occurred within the first 4 h after wounding and was associated with the beginning of migration of EC's into the wounded area as seen by time-lapse cinemicrophotography. These studies focus attention on the MTOC as a cytoskeletal structure that may play a role in determining the direction of cell movement.     

Emerin organizes actin flow for nuclear movement and centrosome orientation in migrating fibroblasts

In migrating fibroblasts, rearward movement of the nucleus orients the centrosome toward the leading edge. Nuclear movement results from coupling rearward-moving, dorsal actin cables to the nucleus by linear arrays of nesprin-2G and SUN2, termed transmembrane actin-associated nuclear (TAN) lines. A-type lamins anchor TAN lines, prompting us to test whether emerin, a nuclear membrane protein that interacts with lamins and TAN line proteins, contributes to nuclear movement. In fibroblasts depleted of emerin, nuclei moved nondirectionally or completely failed to move. Consistent with these nuclear movement defects, dorsal actin cable flow was nondirectional in cells lacking emerin. TAN lines formed normally in cells lacking emerin and were coordinated with the erratic nuclear movements, although in 20% of the cases, TAN lines slipped over immobile nuclei. Myosin II drives actin flow, and depletion of myosin IIB, but not myosin IIA, showed similar nondirectional nuclear movement and actin flow as in emerin-depleted cells. Myosin IIB specifically coimmunoprecipitated with emerin, and emerin depletion prevented myosin IIB localization near nuclei. These results show that emerin functions with myosin IIB to polarize actin flow and nuclear movement in fibroblasts, suggesting a novel function for the nuclear envelope in organizing directional actin flow and cytoplasmic polarity.     

The position of the microtubule-organizing center relative to the nucleus is independent of the direction of cell migration in Dictyostelium discoideum

Dictyostelium amoebae can migrate in several different modes. We tested for correlations of the direction of cell locomotion with the relative positions of the nucleus and microtubule-organizing center (MTOC). Five cases were analyzed on electron micrographs with a microcomputer. Each mode of movement showed characteristic locations of the MTOC relative to the nucleus; however, they differed in the various cases. In randomly migrating interphase amoebae, the number of cells with the MTOC located behind the nucleus was twice as great as those with the MTOC located ahead of the nucleus. During chemotactic migration toward folic acid, cells with the MTOC behind the nucleus were more numerous, with a concomitant reduction of anterior MTOCs. When amoebae aggregated on agar plates, a posterior location of the MTOC was most strikingly favored, whereas in cells aggregating under submerged conditions, the MTOC was indifferently anterior or posterior to the nucleus. (It may be significant that EDTA-resistant cell-cell adhesion was fully expressed in the former cells, but weaker in the latter.) Finally, in the case of chemotactically migrating cells from dissociated pseudoplasmodia, which adhere by means of other molecules, the MTOC was consistently ahead of the nucleus. Thus the MTOC shows no necessary preferential position anterior or posterior to the nucleus; its position, rather, correlates with the type of migration and perhaps with the nature of cell-cell adhesion.     

Auto-reverse nuclear migration in bipolar mammalian cells on micropatterned surfaces

A novel assay based on micropatterning and time-lapse microscopy has been developed for the study of nuclear migration dynamics in cultured mammalian cells. When cultured on 10-20-microm wide adhesive stripes, the motility of C6 glioma and primary mouse fibroblast cells is diminished. Nevertheless, nuclei perform an unexpected auto-reverse motion: when a migrating nucleus approaches the leading edge, it decelerates, changes the direction of motion, and accelerates to move toward the other end of the elongated cell. During this process, cells show signs of polarization closely following the direction of nuclear movement. The observed nuclear movement requires a functioning microtubular system, as revealed by experiments disrupting the main cytoskeletal components with specific drugs. On the basis of our results, we argue that auto-reverse nuclear migration is due to forces determined by the interplay of microtubule dynamics and the changing position of the microtubule organizing center as the nucleus reaches the leading edge. Our assay recapitulates specific features of nuclear migration (cell polarization, oscillatory nuclear movement), while it allows the systematic study of a large number of individual cells. In particular, our experiments yielded the first direct evidence of reversive nuclear motion in mammalian cells, induced by attachment constraints.     

Events in the movement of newt epidermal cells across implanted substrates

Pieces of coverslip glass, polycarbonate filters, or coverslip plastic, coated with fibrinogen or type I collagen, were implanted under one edge of a fresh skin wound on adult newt hind limbs so that the implant served as wound bed for migrating epidermal cells as they attempted to form a wound epithelium. Migratory events were then analyzed by phase contrast and electron microscopy. Phase-contrast microscopy revealed two types of lamellipodia on leading edge cells: one which was attached broadly to the cell body and one attached by a long, thin stalk. Stalkless forms were by far the most common type and we believe they provide the motive force for cell movement. Stalked-forms often moved at distinct angles to the direction of sheet movement, suggesting that they may be sensory appendages. Phase photographs of the leading edge of migrating sheet 4 hours and 8 hours after implantation showed that all cells that were on the leading edge at 4 hours continued to advance for the next 4 hours, demonstrating clearly that under these circumstances the distalmost cells do not become immobile upon contact with the substrate as others have suggested. TEM revealed that migrating sheets were modified monolayers and that regardless of proximodistal location in the sheet, and even in the intact skin adjoining a wound, each epidermal cell adjacent to the substrate puts forth a lamellipodium which underlaps the cell in front. This and the behavior of sheets as they were teased or pulled from the implant suggest strongly that all basal cells contribute to movement of the sheet by interacting with the substrate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Exo70-Mediated Recruitment of Nucleoporin Nup62 at the Leading Edge of Migrating Cells is Required for Cell Migration

Nucleoporin Nup62 localizes at the central channel of the nuclear pore complex and is essential for nucleocytoplasmic transport. Through its FG-repeat domain, Nup62 regulates nuclear pore permeability and binds nuclear transport receptors. Here, we report that Nup62 interacts directly with Exo70 and colocalizes with Exo70 at the leading edge of migrating cells. Nup62 binds the N-terminal domain of Exo70 through its coiled-coil domain but not through its FG-repeat domain. Selective inhibition of leading edge Nup62 using RNA interference significantly reduces cell migration. Furthermore, Exo70 recruits Nup62 at the plasma membrane and at filopodia. Removal of the Exo70-binding domain of Nup62 prevents leading edge localization of Nup62. Analogous to Exo70, Nup62 cycles between the plasma membrane and the perinuclear recycling compartment. Altogether, we propose that Nup62 not solely regulates access to the cell nucleus, but additionally functions in conjunction with Exo70, a key regulator of exocytosis and actin dynamics, at the leading edge of migrating cells.     

Reorientation and fusion of cytotoxic T lymphocyte granules after interaction with target cells as determined by high resolution cinemicrography

The interaction of murine cytotoxic T lymphocyte (CTL) clones with human lymphoblastoid target cells was studied in thin preparations by using high resolution cinemicrography. CTL not bound to target cells were morphologically polar, possessing a broad leading edge containing the nucleus, and a tapered tail containing a large number of granules. The CTL were observed to move by the extension of pseudopods from the leading edge. Initial contact with a target cell was made via the leading edge of the CTL. If the human target cell expressed the appropriate HLA antigen, distinct morphologic changes occurred in the CTL as early as 2 min after initial contact. The CTL rounded up, and the nucleus moved from a position adjacent to the zone of contact to be replaced by the cytoplasmic granules. Redistribution of the granules was completed as early as 10 min after initial contact was made. These morphologic changes did not occur when the CTL made contact with other CTL, or with target cells that did not express the appropriate HLA antigens. In studies that make use of Nomarski optics, an apparent fusion of CTL cytoplasmic granules with the membrane in the vicinity of the target cell contact area was observed 4 min after binding, and before granule reorientation was complete. These data provide direct evidence for the occurrence of both reorientation of the cytoplasmic contents and granule fusion in CTL with a time course similar to that of administration of the lethal hit.     

MTOC reorientation occurs during FcgammaR-mediated phagocytosis in macrophages

Cell polarization is essential for targeting signaling elements and organelles to active plasma membrane regions. In a few specialized cell types, cell polarity is enhanced by reorientation of the MTOC and associated organelles toward dynamic membrane sites. Phagocytosis is a highly polarized process whereby particles >0.5 microm are internalized at stimulated regions on the cell surface of macrophages. Here we provide detailed evidence that the MTOC reorients toward the site of particle internalization during phagocytosis. We visualized MTOC proximity to IgG-sRBCs in fixed RAW264.7 cells, during live cell imaging using fluorescent chimeras to label the MTOC and using frustrated phagocytosis assays. MTOC reorientation in macrophages is initiated by FcgammaR ligation and is complete within 1 h. Polarization of the MTOC toward the phagosome requires the MT cytoskeleton and dynein motor activity. cdc42, PI3K, and mPAR-6 are all important signaling molecules for MTOC reorientation during phagocytosis. MTOC reorientation was not essential for particle internalization or phagolysosome formation. However Golgi reorientation in concert with MTOC reorientation during phagocytosis implicates MTOC reorientation in antigen processing events in macrophages.     

Emerging role for nuclear rotation and orientation in cell migration

Nucleus movement, positioning, and orientation is precisely specified and actively regulated within cells, and it plays a critical role in many cellular and developmental processes. Mutation of proteins that regulate the nucleus anchoring and movement lead to diverse pathologies, laminopathies in particular, suggesting that the nucleus correct positioning and movement is essential for proper cellular function. In motile cells that polarize toward the direction of migration, the nucleus undergoes controlled rotation promoting the alignment of the nucleus with the axis of migration. Such spatial organization of the cell appears to be optimal for the cell migration. Nuclear reorientation requires the cytoskeleton to be anchored to the nuclear envelope, which exerts pulling or pushing torque on the nucleus. Here we discuss the possible molecular mechanisms regulating the nuclear rotation and reorientation and the significance of this type of nuclear movement for cell migration.     

Emerging role for nuclear rotation and orientation in cell migration

Nucleus movement, positioning, and orientation is precisely specified and actively regulated within cells, and it plays a critical role in many cellular and developmental processes. Mutation of proteins that regulate the nucleus anchoring and movement lead to diverse pathologies, laminopathies in particular, suggesting that the nucleus correct positioning and movement is essential for proper cellular function. In motile cells that polarize toward the direction of migration, the nucleus undergoes controlled rotation promoting the alignment of the nucleus with the axis of migration. Such spatial organization of the cell appears to be optimal for the cell migration. Nuclear reorientation requires the cytoskeleton to be anchored to the nuclear envelope, which exerts pulling or pushing torque on the nucleus. Here we discuss the possible molecular mechanisms regulating the nuclear rotation and reorientation and the significance of this type of nuclear movement for cell migration.     

Nuclear lamin A/C deficiency induces defects in cell mechanics, polarization, and migration

Lamin A/C is a major constituent of the nuclear lamina, a thin filamentous protein layer that lies beneath the nuclear envelope. Here we show that lamin A/C deficiency in mouse embryonic fibroblasts (Lmna(-/-) MEFs) diminishes the ability of these cells to polarize at the edge of a wound and significantly reduces cell migration speed into the wound. Moreover, lamin A/C deficiency induces significant separation of the microtubule organizing center (MTOC) from the nuclear envelope. Investigations using ballistic intracellular nanorheology reveal that lamin A/C deficiency also dramatically affects the micromechanical properties of the cytoplasm. Both the elasticity (stretchiness) and the viscosity (propensity of a material to flow) of the cytoplasm in Lmna(-/-) MEFs are significantly reduced. Disassembly of either the actin filament or microtubule networks in Lmna(+/+) MEFs results in decrease of cytoplasmic elasticity and viscosity down to levels found in Lmna(-/-) MEFs. Together these results show that both the mechanical properties of the cytoskeleton and cytoskeleton-based processes, including cell motility, coupled MTOC and nucleus dynamics, and cell polarization, depend critically on the integrity of the nuclear lamina, which suggest the existence of a functional mechanical connection between the nucleus and the cytoskeleton. These results also suggest that cell polarization during cell migration requires tight mechanical coupling between MTOC and nucleus, which is mediated by lamin A/C.     

Mechanism of centrosome positioning during the wound response in BSC-1 cells

Locomoting cells are characterized by a pronounced external and internal anterior-posterior polarity. One of the events associated with cell polarization at the onset of locomotion is a shift of the centrosome, or MTOC, ahead of the nucleus. This position is believed to be of strategic importance for directional cell movement and cell polarity. We have used BSC-1 cells at the edge of an in vitro wound to clarify the causal relationship between MTOC position and the initiation of cell polarization. We find that pronounced cell polarization (the extension of a lamellipod) can take place in the absence of MTOC repositioning or microtubules. Conversely, MTOCs will reposition even after lamellar extension and cell polarization have occurred. Repositioning requires microtubules that extend to the cell periphery and is independent of selective detyrosination of microtubules extending towards the cell front. Significantly, MTOCs maintain, or at least attempt to maintain, a position at the cell's centroid. This is most clearly demonstrated in wounded monolayers of enucleated cells where the MTOC closely follows the centroid position. We suggest that the primary response to the would is the biased extension of a lamellipod, which can occur in the absence of microtubules and MTOC repositioning. Lamellipod extension leads to a shift of the cell's centroid towards the wound. The MTOC, in an attempt to maintain a position near the cell center, will follow. This will automatically put the MTOC ahead of the nucleus in the vast majority of cells. The nucleus as a reference for MTOC position may not be as meaningful as previously thought.     

Actomyosin Pulls to Advance the Nucleus in a Migrating Tissue Cell

The cytoskeletal forces involved in translocating the nucleus in a migrating tissue cell remain unresolved. Previous studies have variously implicated actomyosin-generated pushing or pulling forces on the nucleus, as well as pulling by nucleus-bound microtubule motors. We found that the nucleus in an isolated migrating cell can move forward without any trailing-edge detachment. When a new lamellipodium was triggered with photoactivation of Rac1, the nucleus moved toward the new lamellipodium. This forward motion required both nuclear-cytoskeletal linkages and myosin activity. Apical or basal actomyosin bundles were found not to translate with the nucleus. Although microtubules dampen fluctuations in nuclear position, they are not required for forward translocation of the nucleus during cell migration. Trailing-edge detachment and pulling with a microneedle produced motion and deformation of the nucleus suggestive of a mechanical coupling between the nucleus and the trailing edge. Significantly, decoupling the nucleus from the cytoskeleton with KASH overexpression greatly decreased the frequency of trailing-edge detachment. Collectively, these results explain how the nucleus is moved in a crawling fibroblast and raise the possibility that forces could be transmitted from the front to the back of the cell through the nucleus.     

Receptor tyrosine kinase Ror2 mediates Wnt5a-induced polarized cell migration by activating c-Jun N-terminal kinase via actin-binding protein filamin A

The receptor tyrosine kinase Ror2 has recently been shown to act as an alternative receptor or coreceptor for Wnt5a and to mediate Wnt5a-induced migration of cultured cells. However, little is known about the molecular mechanism underlying this migratory process. Here we show by wound-healing assays that Ror2 plays critical roles in Wnt5a-induced cell migration by regulating formation of lamellipodia and reorientation of microtubule-organizing center (MTOC). Wnt5a stimulation induces activation of the c-Jun N-terminal kinase JNK at the wound edge in a Ror2-dependent manner, and inhibiting JNK activity abrogates Wnt5a-induced lamellipodia formation and MTOC reorientation. Additionally, the association of Ror2 with the actin-binding protein filamin A is required for Wnt5a-induced JNK activation and polarized cell migration. We further show that Wnt5a-induced JNK activation and MTOC reorientation can be suppressed by inhibiting PKCzeta. Taken together, our findings indicate that Wnt5a/Ror2 activates JNK, through a process involving filamin A and PKCzeta, to regulate polarized cell migration.     

Actomyosin Pulls to Advance the Nucleus in a Migrating Tissue Cell

The cytoskeletal forces involved in translocating the nucleus in a migrating tissue cell remain unresolved. Previous studies have variously implicated actomyosin-generated pushing or pulling forces on the nucleus, as well as pulling by nucleus-bound microtubule motors. We found that the nucleus in an isolated migrating cell can move forward without any trailing-edge detachment. When a new lamellipodium was triggered with photoactivation of Rac1, the nucleus moved toward the new lamellipodium. This forward motion required both nuclear-cytoskeletal linkages and myosin activity. Apical or basal actomyosin bundles were found not to translate with the nucleus. Although microtubules dampen fluctuations in nuclear position, they are not required for forward translocation of the nucleus during cell migration. Trailing-edge detachment and pulling with a microneedle produced motion and deformation of the nucleus suggestive of a mechanical coupling between the nucleus and the trailing edge. Significantly, decoupling the nucleus from the cytoskeleton with KASH overexpression greatly decreased the frequency of trailing-edge detachment. Collectively, these results explain how the nucleus is moved in a crawling fibroblast and raise the possibility that forces could be transmitted from the front to the back of the cell through the nucleus.     

Arterial shear stress regulates endothelial cell-directed migration, polarity, and morphology in confluent monolayers

Hemodynamic regulation of directional endothelial cell (EC) migration implies an essential role of shear stress in governing EC polarity. Shear stress induces reorientation of the microtubule organizing center toward the leading edge of migrating cells in a Cdc42-dependent manner. We have characterized the global patterns of EC migration in confluent monolayers as a function of shear stress direction and exogenous pleiotropic factors. Results demonstrate the presence of mitogenic factors significantly affects the flow-induced dynamics of movement by prolonging the onset of monolayer quiescence up to 4 days, but not shear stress-induced morphology. In conjunction with increased motility, exogenous growth factors contributed to the directed migration of ECs in the flow direction. ECs exposed to arterial flow in serum/growth factor-free media and then supplemented with growth factors rapidly increased directional migration to 85% of cells migrating in the direction of flow and induced an increase in the distance traveled with the flow direction. This response was modulated by the directionality of flow and inhibited by the expression of dominant-negative Par6, a major downstream effector of Cdc42-induced polarity. Shear stress-induced directed migratory polarity is modulated by exogenous growth factors and dependent on Par6 activity and shear stress direction.     

Association of adenovirus with the microtubule organizing center

Adenoviruses (Ad) must deliver their genomes to the nucleus of the target cell to initiate an infection. Following entry into the cell and escape from the endosome, Ad traffics along the microtubule cytoskeleton toward the nucleus. In the final step in Ad trafficking, Ad must leave the microtubule and establish an association with the nuclear envelope. We hypothesized that in cells lacking a nucleus, the capsid moves to and associates with the microtubule organizing center (MTOC). To test this hypothesis, we established an experimental system to examine Ad trafficking in enucleated cells compared to Ad trafficking in intact, mock-enucleated cells. Enucleation of a monolayer of A549 human lung epithelial cells was accomplished by depolymerization of the actin cytoskeleton followed by centrifugation. Upon infection of enucleated cells with Cy3-labeled Ad, the majority of Ad capsid trafficked to a discrete, centrally located site which colocalized with pericentrin, a component of the MTOC. MTOC-associated Ad had escaped from endosomes and thus had direct access to MTOC components. Ad localization at this site was sensitive to the microtubule-depolymerizing agent nocodazole, but not to the microfilament-depolymerizing agent cytochalasin B, indicating that intact microtubules were required to maintain the localization with the MTOC. Ad localization to the MTOC in the enucleated cells was stable, as demonstrated by continuing Ad localization with pericentrin for more than 5 h after infection, a strong preference for Ad arrival at rather than Ad departure from the MTOC, and minimal redistribution of Ad between MTOCs within a single cell. In summary, the data demonstrate that the Ad capsid establishes a stable interaction with the MTOC when a nucleus is not present, suggesting that dissociation of Ad from microtubules likely requires nuclear factors.