Quantification of microtubule nucleation, growth and dynamics in wound-edge cells

Short-term molecular polarization of cells on symmetric and asymmetric micropatterns

The ability of cells to sense geometrical/physical constraints of local environment is important for cell movements during development, immune surveillance, and in cancer invasion. In this paper, we quantify "front-rear" polarization - the crucial step in initiating cell migration - based on cytoskeleton and substrate adhesion anisotropy in micropatterned cells of well-defined shapes. We then show that the general viewpoint that asymmetric cell shape is one of the defining characteristics of polarized cells is incomplete. Specifically, we demonstrate that cells on circular micropatterned islands can exhibit asymmetric distribution of both filamentous actin (f-actin) and focal adhesions (FAs) as well as directional, lamellipodial-like ruffling activity. This asymmetry, however, is transient and persists only for the period of several hours during which actin filaments and adhesion structures reorganize into symmetric peripheral arrangement. Cells on asymmetric tear-drop shape islands also display polarized f-actin and FAs, but polarization axes are oriented towards the wide end of the islands. Polarization of actin filaments on tear-drop islands is short-term, while focal adhesions remain asymmetrically distributed for long times. From a practical perspective, circular cells constitute a convenient experimental system, in which phenomena related to cell polarization are "decoupled" from the effects of cells' local curvature (constant along circular cell's perimeter), while asymmetric (tear-drop) micropatterned cells standardize the organization of motility machinery of polarized/ moving cells. Both systems may prove useful for the design of diagnostic tools with which to probe and quantify ex vivo the motility/invasiveness status of cells from cancer patients.

Androgen and Src signaling regulate centrosome activity

Microtubules nucleated from gamma-tubulin ring complexes located at the centrosome regulate the localization of organelles, promote vesicular transport and direct cell migration. Although several signaling mechanisms have been identified that regulate microtubule dynamics during interphase, signaling pathways that promote microtubule nucleation remain elusive. We assayed microtubule regrowth following nocodazole washout in human fibroblasts and CHO-K1 cells adhered to fibronectin in either normal serum-free medium or the serum-free, growth-promoting medium, CCM1, which contains IGF1 and androgen, as well as other nutrients. The results indicate that integrin-mediated adhesion is not sufficient to promote rapid microtubule regrowth in either cell type. The addition of androgen, but not IGF1, for 5 minutes was sufficient to promote rapid regrowth and this occurred by a mechanism requiring the androgen receptor. Since Src is a component of the cytoplasmic androgen-receptor-signaling complex, we examined its role using Src siRNA, the Src kinase inhibitor SU6656, and the expression of a constitutively active Src mutant. The data show that Src signaling is both required and sufficient to promote rapid microtubule regrowth in cells adhered to fibronectin. Measurement of the density of microtubules close to the centrosome and the rates of GFP-EB1-labeled microtubules emanating from the centrosome indicated that Src signaling promotes microtubule nucleation. Furthermore, recovery of GFP-gamma-tubulin at the centrosome following photobleaching and measurements of endogenous gamma-tubulin levels at the centrosome showed that androgen and Src signaling regulate the levels of centrosomal gamma-tubulin. Thus, we propose that androgen and Src signaling regulate microtubule nucleation during interphase by promoting the centrosomal localization of gamma-tubulin.

Effect of GFP tags on the localization of EB1 and EB1 fragments in vivo

EB1 is a microtubule plus-end tracking protein that plays a central role in the regulation of microtubule (MT) dynamics. GFP-tagged EB1 constructs are commonly used to study EB1 itself and also as markers of dynamic MT plus ends. To properly interpret these studies, it is important to understand the impact of tags on the behavior of EB1 and other proteins in vivo. To address this problem and improve understanding of EB1 function, we surveyed the localization of expressed EB1 fragments and investigated whether GFP tags alter these localizations. We found that neither N-terminal nor C-terminal tags are benign: tagged EB1 and EB1 fragments generally behave differently from their untagged counterparts. N-terminal tags significantly compromise the ability of expressed EB1 proteins to bind MTs and/or track MT plus ends, although they leave some MT-binding ability intact. C-terminally tagged EB1 constructs have localizations similar to the untagged constructs, initially suggesting that they are benign. However, most constructs tagged at either end cause CLIP-170 to disappear from MT plus ends. This effect is opposite to that of untagged full-length EB1, which recruits CLIP-170 to MTs. These observations demonstrate that although EB1-GFP can be a powerful tool for studying microtubule dynamics, it should be used carefully because it may alter the system that it is being used to study. In addition, some untagged fragments had unexpected localizations. In particular, an EB1 construct lacking the coiled-coil tracks MT plus ends, though weakly, providing evidence against the idea that EB1 +TIP behavior requires dimerization.

Microtubules growth rate alteration in human endothelial cells

To understand how microtubules contribute to the dynamic reorganization of the endothelial cell (EC) cytoskeleton, we established an EC model expressing EB3-GFP, a protein that marks microtubule plus-ends. Using this model, we were able to measure microtubule growth rate at the centrosome region and near the cell periphery of a single human EC and in the EC monolayer. We demonstrate that the majority of microtubules in EC are dynamic, the growth rate of their plus-ends is highest in the internal cytoplasm, in the region of the centrosome. Growth rate of microtubule plus-ends decreases from the cell center toward the periphery. Our data suggest the existing mechanism(s) of local regulation of microtubule plus-ends growth in EC. Microtubule growth rate in the internal cytoplasm of EC in the monolayer is lower than that of single EC suggesting the regulatory effect of cell-cell contacts. Centrosomal microtubule growth rate distribution in single EC indicated the presence of two subpopulations of microtubules with "normal" (similar to those in monolayer EC) and "fast" (three times as much) growth rates. Our results indicate functional interactions between cell-cell contacts and microtubules.

Determination of microtubule dynamic instability in living cells

The precise regulation of microtubules and their dynamics is critical for cell cycle progression, cell signaling, intracellular transport, cell polarization, and organismal development. For example, mitosis, cell migration, and axonal outgrowth all involve rapid and dramatic changes in microtubule organization and dynamics. Microtubule-associated proteins (MAPs) such as MAP2 and tau (Bunker et al., 2004; Dhamodharan and Wadsworth, 1995) and microtubule-interacting proteins such as stathmin, the kinesin MCAK, and EB1 (Cassimeris, 1999; Moore and Wordeman, 2004; Ringhoff and Cassimeris, 2009; Rusan et al., 2001) as well as numerous clinically approved or experimental anti-mitotic drugs including the taxanes, vinca alkaloids, and colchicine-like compounds modulate microtubule dynamic in cells (Jordan, 2002; Jordan and Kamath, 2007). In this chapter, we describe methods to analyze the dynamic instability of microtubules in living cells by microscopy of microinjected or expressed fluorescent tubulin, time-lapse microscopy, and analysis of time-dependent microtubule length changes.

Analysis of microtubule polymerization dynamics in live cells

The spatiotemporal regulation of intracellular microtubule polymerization dynamics, by numerous microtubule-associated proteins and other mechanisms, is central to many cell processes. Here, we give an overview and practical guide on how to acquire and analyze time-lapse sequences of dynamic microtubules in live cells by either fluorescently labeling entire microtubules or by utilizing proteins that specifically associate only with growing microtubule ends and summarize the strengths and weaknesses of different approaches. We give practical recommendations for imaging conditions, and discuss important limitations of such analysis that are dictated by the maximum achievable spatial and temporal sampling frequencies.

Microtubule network asymmetry in motile cells: role of Golgi-derived array

Cell migration requires polarization of the cell into the leading edge and the trailing edge. Microtubules (MTs) are indispensable for polarized cell migration in the majority of cell types. To support cell polarity, MT network has to be functionally and structurally asymmetric. How is this asymmetry achieved? In interphase cells, MTs form a dynamic system radiating from a centrosome-based MT-organizing center (MTOC) to the cell edges. Symmetry of this radial array can be broken according to four general principles. Asymmetry occurs due to differential modulation of MT dynamics, relocation of existing MTs within a cell, adding an asymmetric nucleation site, and/or repositioning of a symmetric nucleation site to one side of a cell. Combinations of these asymmetry regulation principles result in a variety of asymmetric MT networks typical for diverse motile cell types. Importantly, an asymmetric MT array is formed at a non-conventional MT nucleation site, the Golgi. Here, we emphasize the contribution of this array to the asymmetry of MT network.

Phosphorylation of alpha6-tubulin by protein kinase Calpha activates motility of human breast cells

Engineered overexpression of protein kinase Calpha (PKCalpha) was previously shown to endow nonmotile MCF-10A human breast cells with aggressive motility. A traceable mutant of PKCalpha (Abeyweera, T. P., and Rotenberg, S. A. (2007) Biochemistry 46, 2364-2370) revealed that alpha6-tubulin is phosphorylated in cells expressing traceable PKCalpha and in vitro by wild type PKCalpha. Gain-of-function, single site mutations (Ser-->Asp) were constructed at each PKC consensus site in alpha6-tubulin (Ser158, Ser165, Ser241, and Thr337) to simulate phosphorylation. Following expression of each construct in MCF-10A cells, motility assays identified Ser165 as the only site in alpha6-tubulin whose pseudophosphorylation reproduced the motile behavior engendered by PKCalpha. Expression of a phosphorylation-resistant mutant (S165N-alpha6-tubulin) resulted in suppression of MCF-10A cell motility stimulated either by expression of PKCalpha or by treatment with PKCalpha-selective activator diacylglycerol-lactone. MCF-10A cells treated with diacylglycerol-lactone showed strong phosphorylation of endogenous alpha-tubulin that could be blocked when S165N-alpha6-tubulin was expressed. The S165N mutant also inhibited intrinsically motile human breast tumor cells that express high endogenous PKCalpha levels (MDA-MB-231 cells) or lack PKCalpha and other conventional isoforms (MDA-MB-468 cells). Comparison of Myc-tagged wild type alpha6-tubulin and S165N-alpha6-tubulin expressed in MDA-MB-468 cells demonstrated that Ser165 is also a major site of phosphorylation for endogenously active, nonconventional PKC isoforms. PKC-stimulated motility of MCF-10A cells was nocodazole-sensitive, thereby implicating microtubule elongation in the mechanism. These findings support a model in which PKC phosphorylates alpha-tubulin at Ser165, leading to microtubule elongation and motility.

Characterization of the acute temporal changes in excisional murine cutaneous wound inflammation by screening of the wound-edge transcriptome

This work represents a maiden effort to systematically screen the transcriptome of the healing wound-edge tissue temporally using high-density GeneChips. Changes during the acute inflammatory phase of murine excisional wounds were characterized histologically. Sets of genes that significantly changed in expression during healing could be segregated into the following five sets: up-early (6-24 h; cytokine-cytokine receptor interaction pathway), up-intermediary (12-96 h; leukocyte-endothelial interaction pathway), up-late (48-96 h; cell-cycle pathway), down-early (6-12 h; purine metabolism) and down-intermediary (12-96 h; oxidative phosphorylation pathway). Results from microarray and real-time PCR analyses were consistent. Results listing all genes that were significantly changed at any specific time point were further mined for cell-type (neutrophils, macrophages, endothelial, fibroblasts, and pluripotent stem cells) specificity. Candidate genes were also clustered on the basis of their functional annotation, linking them to inflammation, angiogenesis, reactive oxygen species (ROS), or extracellular matrix (ECM) categories. Rapid induction of genes encoding NADPH oxidase subunits and downregulation of catalase in response to wounding is consistent with the fact that low levels of endogenous H2O2 is required for wound healing. Angiogenic genes, previously not connected to cutaneous wound healing, that were induced in the healing wound-edge included adiponectin, epiregulin, angiomotin, Nogo, and VEGF-B. This study provides a digested database that may serve as a valuable reference tool to develop novel hypotheses aiming to elucidate the biology of cutaneous wound healing comprehensively.

Roles of P21-activated kinases and associated proteins in epithelial wound healing

The primary function of epithelia is to provide a barrier between the extracellular environment and the interior of the body. Efficient epithelial repair mechanisms are therefore crucial for homeostasis. The epithelial wound-healing process involves highly regulated morphogenetic changes of epithelial cells that are driven by dynamic changes of the cytoskeleton. P21-activated kinases are serine/threonine kinases that have emerged as important regulators of the cytoskeleton. These kinases, which are activated downsteam of the Rho GTPases Rac and cd42, were initially mostly implicated in the regulation of cell migration. More recently, however, these kinases were shown to have many additional functions that are relevant to the regulation of epithelial wound healing. Here, we provide an overview of the morphogenetic changes of epithelial cells during wound healing and the many functions of p21-activated kinases in these processes.

The PI3K-Akt pathway promotes microtubule stabilization in migrating fibroblasts

Directed cell migration is controlled by extracellular cues such as growth factors/chemokines and extracellular matrix. In a migrating cell, a subset of microtubules becomes stabilized, and this stabilization is implicated in the establishment and maintenance of cell polarity. It is still not fully understood, however, how extracellular cues regulate the dynamics of microtubules. Here we show that the PI3K-Akt signaling pathway plays a pivotal role in growth factor regulation of microtubule stability. Treatment of NIH 3T3 fibroblasts with platelet-derived growth factor (PDGF) increases the amount of stabilized microtubules, and this increase is abrogated by the addition of a PI3K inhibitor or by expression of a dominant-negative form of Akt (DN-Akt), but not by the addition of a MEK inhibitor. Expression of an active form of Akt slightly increases the bulk amount of stabilized microtubules. Stabilization of microtubules induced in edge cells in the wounded monolayer culture is also attenuated by the PI3K inhibitor treatment or by expression of DN-Akt. Given that Akt is activated at the leading edge of a migrating cell and plays an essential role in directed cell migration, these results reveal a novel mechanism linking extracellular cues to directed cell migration, namely Akt regulation of microtubule stability.

A novel function of capsaicin-sensitive TRPV1 channels: Involvement in cell migration

Cell migration relies on a tight temporal and spatial regulation of the intracellular Ca2+ concentration ([Ca2+]i). [Ca2+]i in turn depends on Ca2+ influx via channels in the plasma membrane whose molecular nature is still largely unknown for migrating cells. A mechanosensitive component of the Ca2+ influx pathway was suggested. We show here that the capsaicin-sensitive transient receptor potential channel TRPV1, that plays an important role in pain transduction, is one of the Ca2+ influx channels involved in cell migration. Activating TRPV1 channels with capsaicin leads to an acceleration of human hepatoblastoma (HepG2) cells pretreated with hepatocyte growth factor (HGF). The speed rises by up to 50% and the displacement is doubled. Patch clamp experiments revealed the presence of capsaicin and resiniferatoxin (RTX)-sensitive currents. In contrast, HepG2 cells kept in the absence of HGF are not accelerated by capsaicin and express no capsaicin- or RTX-sensitive current. The TRPV1 antagonist capsazepine prevents the stimulation of migration and inhibits capsaicin-sensitive currents. Finally, we compared the contribution of capsaicin-sensitive TRPV1 channels to cell migration with that of mechanosensitive TRPV4 channels that are also expressed in HepG2 cells. A specific TRPV4 agonist, 4alpha-phorbol 12,13-didecanoate, does not increase the displacement. In summary, we assigned a novel role to capsaicin-sensitive TRPV1 channels. They are important Ca2+ influx channels required for cell migration.

Asymmetric CLASP-dependent nucleation of noncentrosomal microtubules at the trans-Golgi network

Proper organization of microtubule arrays is essential for intracellular trafficking and cell motility. It is generally assumed that most if not all microtubules in vertebrate somatic cells are formed by the centrosome. Here we demonstrate that a large number of microtubules in untreated human cells originate from the Golgi apparatus in a centrosome-independent manner. Both centrosomal and Golgi-emanating microtubules need gamma-tubulin for nucleation. Additionally, formation of microtubules at the Golgi requires CLASPs, microtubule-binding proteins that selectively coat noncentrosomal microtubule seeds. We show that CLASPs are recruited to the trans-Golgi network (TGN) at the Golgi periphery by the TGN protein GCC185. In sharp contrast to radial centrosomal arrays, microtubules nucleated at the peripheral Golgi compartment are preferentially oriented toward the leading edge in motile cells. We propose that Golgi-emanating microtubules contribute to the asymmetric microtubule networks in polarized cells and support diverse processes including post-Golgi transport to the cell front.

HDAC6 deacetylation of tubulin modulates dynamics of cellular adhesions

Genetic or pharmacological alteration of the activity of the histone deacetylase 6 (HDAC6) induces a parallel alteration in cell migration. Using tubacin to block deacetylation of alpha-tubulin, and not other HDAC6 substrates, yielded a motility reduction equivalent to agents that block all NAD-independent HDACs. Accordingly, we investigated how the failure to deacetylate tubulin contributes to decreased motility in HDAC6-inhibited cells. Testing the hypothesis that motility is reduced because cellular adhesion is altered, we found that inhibiting HDAC6 activity towards tubulin rapidly increased total adhesion area. Next, we investigated the mechanism of the adhesion area increase. Formation of adhesions proceeded normally and cell spreading was more rapid in the absence of active HDAC6; however, photobleaching assays and adhesion breakdown showed that adhesion turnover was slower. To test the role of hyperacetylated tubulin in altering adhesion turnover, we measured microtubule dynamics in HDAC6-inhibited cells because dynamic microtubules are required to target adhesions for turnover. HDAC6 inhibition yielded a decrease in microtubule dynamics that was sufficient to decrease focal adhesion turnover. Thus, our results suggest a scenario in which the decreased dynamics of hyperacetylated microtubules in HDAC6-inhibited cells compromises their capacity to mediate the focal adhesion dynamics required for rapid cell migration.

The von Hippel-Lindau tumor suppressor protein controls ciliogenesis by orienting microtubule growth

Cilia are specialized organelles that play an important role in several biological processes, including mechanosensation, photoperception, and osmosignaling. Mutations in proteins localized to cilia have been implicated in a growing number of human diseases. In this study, we demonstrate that the von Hippel-Lindau (VHL) protein (pVHL) is a ciliary protein that controls ciliogenesis in kidney cells. Knockdown of pVHL impeded the formation of cilia in mouse inner medullary collecting duct 3 kidney cells, whereas the expression of pVHL in VHL-negative renal cancer cells rescued the ciliogenesis defect. Using green fluorescent protein–tagged end-binding protein 1 to label microtubule plus ends, we found that pVHL does not affect the microtubule growth rate but is needed to orient the growth of microtubules toward the cell periphery, a prerequisite for the formation of cilia. Furthermore, pVHL interacts with the Par3–Par6–atypical PKC complex, suggesting a mechanism for linking polarity pathways to microtubule capture and ciliogenesis.

Microtubule acetylation promotes kinesin-1 binding and transport

Long-distance intracellular delivery is driven by kinesin and dynein motor proteins that ferry cargoes along microtubule tracks . Current models postulate that directional trafficking is governed by known biophysical properties of these motors-kinesins generally move to the plus ends of microtubules in the cell periphery, whereas cytoplasmic dynein moves to the minus ends in the cell center. However, these models are insufficient to explain how polarized protein trafficking to subcellular domains is accomplished. We show that the kinesin-1 cargo protein JNK-interacting protein 1 (JIP1) is localized to only a subset of neurites in cultured neuronal cells. The mechanism of polarized trafficking appears to involve the preferential recognition of microtubules containing specific posttranslational modifications (PTMs) by the kinesin-1 motor domain. Using a genetic approach to eliminate specific PTMs, we show that the loss of a single modification, alpha-tubulin acetylation at Lys-40, influences the binding and motility of kinesin-1 in vitro. In addition, pharmacological treatments that increase microtubule acetylation cause a redirection of kinesin-1 transport of JIP1 to nearly all neurite tips in vivo. These results suggest that microtubule PTMs are important markers of distinct microtubule populations and that they act to control motor-protein trafficking.

Cells move when ions and water flow

Cell migration is a process that plays an important role throughout the entire life span. It starts early on during embryogenesis and contributes to shaping our body. Migrating cells are involved in maintaining the integrity of our body, for instance, by defending it against invading pathogens. On the other side, migration of tumor cells may have lethal consequences when tumors spread metastatically. Thus, there is a strong interest in unraveling the cellular mechanisms underlying cell migration. The purpose of this review is to illustrate the functional importance of ion and water channels as part of the cellular migration machinery. Ion and water flow is required for optimal migration, and the inhibition or genetic ablation of channels leads to a marked impairment of migration. We briefly touch cytoskeletal mechanisms of migration as well as cell-matrix interactions. We then present some general principles by which channels can affect cell migration before we discuss each channel group separately.

Disruption of the plasma membrane stimulates rearrangement of microtubules and lipid traffic toward the wound site

Resealing of a disrupted plasma membrane requires Ca2+-regulated exocytosis. Repeated disruptions reseal more quickly than the initial wound. This facilitated response requires both Ca2+ and protein kinase C (PKC), and is sensitive to brefeldin A. There is also evidence that this response is polarized to the site where the cell membrane had previously been disrupted. Observations of GFP-tagged α-tubulin and end-binding protein 1 (EB1) revealed that membrane disruption initially induced disassembly of microtubules around the wound site, followed by elongation of microtubules toward the wound site. Recruitment of EB1 to microtubules required Ca2+ influx, but was independent of PKC. NBD C6-ceramide, a probe for the Golgi apparatus and Golgi-derived lipids, initially stained the perinuclear region, and a portion of the probe was translocated to the wound site 5 minutes after wounding. Translocation of the lipids required microtubules and PKC activity, and was suppressed by low temperature. On the other hand, constitutive traffic of the lipid was still normal in the presence of a PKC inhibitor. These findings suggest that membrane disruption stimulates regulated vesicle traffic from the region of the trans-Golgi network to the wound site along rearranged microtubules in a PKC-dependent manner.

QUANTITATIVE CELL MOTILITY FOR IN VITRO WOUND HEALING USING LEVEL SET-BASED ACTIVE CONTOUR TRACKING

Quantifying the behavior of cells individually, and in clusters as part of a population, under a range of experimental conditions, is a challenging computational task with many biological applications. We propose a versatile algorithm for segmentation and tracking of multiple motile epithelial cells during wound healing using time-lapse video. The segmentation part of the proposed method relies on a level set-based active contour algorithm that robustly handles a large number of cells. The tracking part relies on a detection-based multiple-object tracking method with delayed decision enabled by multi-hypothesis testing. The combined method is robust to complex cell behavior including division and apoptosis, and to imaging artifacts such as illumination changes.

Cell segmentation using coupled level sets and graph-vertex coloring

Current level-set based approaches for segmenting a large number of objects are computationally expensive since they require a unique level set per object (the N-level set paradigm), or [log2N] level sets when using a multiphase interface tracking formulation. Incorporating energy-based coupling constraints to control the topological interactions between level sets further increases the computational cost to O(N2). We propose a new approach, with dramatic computational savings, that requires only four, or fewer, level sets for an arbitrary number of similar objects (like cells) using the Delaunay graph to capture spatial relationships. Even more significantly, the coupling constraints (energy-based and topological) are incorporated using just constant O(1) complexity. The explicit topological coupling constraint, based on predicting contour collisions between adjacent level sets, is developed to further prevent false merging or absorption of neighboring cells, and also reduce fragmentation during level set evolution. The proposed four-color level set algorithm is used to efficiently and accurately segment hundreds of individual epithelial cells within a moving monolayer sheet from time-lapse images of in vitro wound healing without any false merging of cells.