Migrating fibroblasts reorient directionality by a metastable, PI3K-dependent mechanism

F-actin bundles direct the initiation and orientation of lamellipodia through adhesion-based signaling

Mesenchymal cells such as fibroblasts are weakly polarized and reorient directionality by a lamellipodial branching mechanism that is stabilized by phosphoinositide 3-kinase (PI3K) signaling. However, the mechanisms by which new lamellipodia are initiated and directed are unknown. Using total internal reflection fluorescence microscopy to monitor cytoskeletal and signaling dynamics in migrating cells, we show that peripheral F-actin bundles/filopodia containing fascin-1 serve as templates for formation and orientation of lamellipodia. Accordingly, modulation of fascin-1 expression tunes cell shape, quantified as the number of morphological extensions. Ratiometric imaging reveals that F-actin bundles/filopodia play both structural and signaling roles, as they prime the activation of PI3K signaling mediated by integrins and focal adhesion kinase. Depletion of fascin-1 ablated fibroblast haptotaxis on fibronectin but not platelet-derived growth factor chemotaxis. Based on these findings, we conceptualize haptotactic sensing as an exploration, with F-actin bundles directing and lamellipodia propagating the process and with signaling mediated by adhesions playing the role of integrator.

Mesenchymal chemotaxis requires selective inactivation of myosin II at the leading edge via a noncanonical PLCγ/PKCα pathway

Chemotaxis, migration toward soluble chemical cues, is critical for processes such as wound healing and immune surveillance and is exhibited by various cell types, from rapidly migrating leukocytes to slow-moving mesenchymal cells. To study mesenchymal chemotaxis, we observed cell migration in microfluidic chambers that generate stable gradients of platelet-derived growth factor (PDGF). Surprisingly, we found that pathways implicated in amoeboid chemotaxis, such as PI3K and mammalian target of rapamycin signaling, are dispensable for PDGF chemotaxis. Instead, we find that local inactivation of Myosin IIA, through a noncanonical Ser1/2 phosphorylation of the regulatory light chain, is essential. This site is phosphorylated by PKCα, which is activated by an intracellular gradient of diacylglycerol generated by PLCγ. Using a combination of live imaging and gradients of activators/inhibitors in the microfluidic chambers, we demonstrate that this signaling pathway and subsequent inhibition of Myosin II activity at the leading edge are required for mesenchymal chemotaxis.

Using fluctuation analysis to establish causal relations between cellular events without experimental perturbation

Experimental perturbations are commonly used to establish causal relationships between the molecular components of a pathway and their cellular functions; however, this approach suffers inherent limitations. Especially in pathways with a significant level of nonlinearity and redundancy among components, such perturbations induce compensatory responses that obscure the actual function of the targeted component in the unperturbed pathway. A complementary approach uses constitutive fluctuations in component activities to identify the hierarchy of information flow through pathways. Here, we review the motivation for using perturbation-free approaches and highlight recent advances made in using perturbation-free fluctuation analysis as a means to establish causality among cellular events.

TNFα and IL-1β influence the differentiation and migration of murine MSCs independently of the NF-κB pathway

INTRODUCTION

Mesenchymal stem cells (MSCs) have the ability to repair and regenerate tissue, home to sites of inflammation, and evade the host immune system. As such, they represent an attractive therapy for the treatment of autoimmune inflammatory diseases. However, results from in vivo murine studies in inflammatory arthritis have been conflicting, and this may be due to the genetic background of the MSCs used. It is known that the inflammatory milieu may influence properties of MSCs and that, in the case of human bone marrow-derived MSCs, this may be mediated by the nuclear factor-kappa-B (NF-κB) pathway. We sought to determine whether pro-inflammatory cytokines altered the differentiation and migration capacity of murine MSCs from different mouse strains and whether this was mediated by NF-κB.

METHODS

The differentiation and migration of FVB and BALB/c MSCs were carried out in the presence of varying concentrations of tumor necrosis factor-alpha (TNFα) and interleukin (IL)-1β, and the NF-κB pathway was inhibited in one of two ways: either by transduction of MSCs with an adenoviral vector expressing a super-repressor of NF-κB or by the addition of curcumin to culture media.

RESULTS

Both BALB/c and FVB MSCs were sensitive to the effect of pro-inflammatory cytokines in vitro. TNFα and IL-1β suppressed BALB/c osteogenesis and adipogenesis and FVB osteogenesis. The migration of both cell types toward media containing fetal bovine serum was augmented by pre-stimulation with either cytokine. In neither cell type were the cytokine effects reversed by abrogation of the NF-κB pathway.

CONCLUSIONS

These data show that murine MSCs from different genetic backgrounds may be influenced by an inflammatory milieu in a manner that is not mediated by NF-κB, as is the case for human MSCs. This is not mediated by NF-κB. These findings are important and should influence how in vivo trials of murine MSCs are interpreted and the future development of pre-clinical studies in inflammatory diseases.

Enzymatic oxidation of cholesterol: properties and functional effects of cholestenone in cell membranes

Bacterial cholesterol oxidase is commonly used as an experimental tool to reduce cellular cholesterol content. That the treatment also generates the poorly degradable metabolite 4-cholesten-3-one (cholestenone) has received less attention. Here, we investigated the membrane partitioning of cholestenone using simulations and cell biological experiments and assessed the functional effects of cholestenone in human cells. Atomistic simulations predicted that cholestenone reduces membrane order, undergoes faster flip-flop and desorbs more readily from membranes than cholesterol. In primary human fibroblasts, cholestenone was released from membranes to physiological extracellular acceptors more avidly than cholesterol, but without acceptors it remained in cells over a day. To address the functional effects of cholestenone, we studied fibroblast migration during wound healing. When cells were either cholesterol oxidase treated or part of cellular cholesterol was exchanged for cholestenone with cyclodextrin, cell migration during 22 h was markedly inhibited. Instead, when a similar fraction of cholesterol was removed using cyclodextrin, cells replenished their cholesterol content in 3 h and migrated similarly to control cells. Thus, cholesterol oxidation produces long-term functional effects in cells and these are in part due to the generated membrane active cholestenone.

Directed migration of mesenchymal cells: where signaling and the cytoskeleton meet

Cell migration directed by spatial cues, or taxis, is a primary mechanism for orchestrating concerted and collective cell movements during development, wound repair, and immune responses. Compared with the classic example of amoeboid chemotaxis, in which fast-moving cells such as neutrophils are directed by gradients of soluble factors, directed migration of slow-moving mesenchymal cells such as fibroblasts is poorly understood. Mesenchymal cells possess a distinctive organization of the actin cytoskeleton and associated adhesion complexes as its primary mechanical system, generating the asymmetric forces required for locomotion without strong polarization. The emerging hypothesis is that the molecular underpinnings of mesenchymal taxis involve distinct signaling pathways and diverse requirements for regulation.

Spatiotemporal control of fibroblast growth factor receptor signals by blue light

Fibroblast growth factor receptors (FGFRs) regulate diverse cellular behaviors that should be exquisitely controlled in space and time. We engineered an optically controlled FGFR (optoFGFR1) by exploiting cryptochrome 2, which homointeracts upon blue light irradiation. OptoFGFR1 can rapidly and reversibly control intracellular FGFR1 signaling within seconds by illumination with blue light. At the subcellular level, localized activation of optoFGFR1 induced cytoskeletal reorganization. Utilizing the high spatiotemporal precision of optoFGFR1, we efficiently controlled cell polarity and induced directed cell migration. OptoFGFR1 provides an effective means to precisely control FGFR signaling and is an important optogenetic tool that can be used to study diverse biological processes both in vitro and in vivo.

Intracellular encoding of spatiotemporal guidance cues in a self-organizing signaling system for chemotaxis in Dictyostelium cells

Even in the absence of guidance cues, chemotactic cells are often spontaneously motile, which should accompany a spontaneous symmetry breaking inside the cells. A shallow chemoattractant gradient can induce these cells to move directionally without much change in cell morphology. As the gradient becomes steeper, the accuracy of chemotaxis increases. It is not clear how the steepness is expressed or encoded internally in the signaling network, which in turn coordinately activates the motile apparatus for chemotaxis. In Dictyostelium cells, self-organizing polarization activities in the signaling network have been reported. In this paper, we conducted a theoretical study of the response of this self-organizing system to guidance cues. Our analyses indicate that self-organizing systems respond sharply to a shallow external gradient by increasing the precision of polarity direction and modulating the frequency of self-polarization. We also show how the precision increase and frequency modulation are achieved. Our results indicate that self-organizing activity, independent of external cues, is the basis for the sensitive and robust response to shallow gradients. Finally, we show that the system can sense the direction of space-time waves of a stimulus, for which Dictyostelium cells exhibit chemotaxis in the developmental process.

Rac1-dependent lamellipodial motility in prostate cancer PC-3 cells revealed by optogenetic control of Rac1 activity

The lamellipodium, an essential structure for cell migration, plays an important role in the invasion and metastasis of cancer cells. Although Rac1 recognized as a key player in the formation of lamellipodia, the molecular mechanisms underlying lamellipodial motility are not fully understood. Optogenetic technology enabled us to spatiotemporally control the activity of photoactivatable Rac1 (PA-Rac1) in living cells. Using this system, we revealed the role of phosphatidylinositol 3-kinase (PI3K) in Rac1-dependent lamellipodial motility in PC-3 prostate cancer cells. Through local blue laser irradiation of PA-Rac1-expressing cells, lamellipodial motility was reversibly induced. First, outward extension of a lamellipodium parallel to the substratum was observed. The extended lamellipodium then showed ruffling activity at the periphery. Notably, PI(3,4,5)P₃ and WAVE2 were localized in the extending lamellipodium in a PI3K-dependent manner. We confirmed that the inhibition of PI3K activity greatly suppressed lamellipodial extension, while the ruffling activity was less affected. These results suggest that Rac1-induced lamellipodial motility consists of two distinct activities, PI3K-dependent outward extension and PI3K-independent ruffling.

Phospho-NHE3 forms membrane patches and interacts with beta-actin to sense and maintain constant direction during cell migration

The Na(+)/H(+) exchanger NHE3 colocalizes with beta-actin at the leading edge of directionally migrating cells. Using human osteosarcoma cells (SaOS-2), rat osteoblasts (calvaria), and human embryonic kidney (HEK) cells, we identified a novel role for NHE3 via beta-actin in anode and cathode directed motility, during electrotaxis. NHE3 knockdown by RNAi revealed that NHE3 expression is required to achieve constant directionality and polarity in migrating cells. Phosphorylated NHE3 (pNHE3) and beta-actin complex formation was impaired by the NHE3 inhibitor S3226 (IC50 0.02µM). Fluorescence cross-correlation spectroscopy (FCCS) revealed that the molecular interactions between NHE3 and beta-actin in membrane protrusions increased 1.7-fold in the presence of a directional cue and decreased 3.3-fold in the presence of cytochalasin D. Data from flow cytometric analysis showed that membrane potential of cells (Vmem) decreases in directionally migrating, NHE3-deficient osteoblasts and osteosarcoma cells whereas only Vmem of wild type osteoblasts is affected during directional migration. These findings suggest that pNHE3 has a mechanical function via beta-actin that is dependent on its physiological activity and Vmem. Furthermore, phosphatidylinositol 3,4,5-trisphosphate (PIP3) levels increase while PIP2 remains stable when cells have persistent directionality. Both PI3 kinase (PI3K) and Akt expression levels change proportionally to NHE3 levels. Interestingly, however, the content of pNHE3 level does not change when PI3K/Akt is inhibited. Therefore, we conclude that NHE3 can act as a direction sensor for cells and that NHE3 phosphorylation in persistent directional cell migration does not involve PI3K/Akt during electrotaxis.

Nucleolar control of p53: a cellular Achilles' heel and a target for cancer therapy

Nucleoli perform a crucial cell function, ribosome biogenesis, and of critical relevance to the subject of this review, they are also extremely sensitive to cellular stresses, which can cause loss of function and/or associated structural disruption. In recent years, we have learned that cells take advantage of this stress sensitivity of nucleoli, using them as stress sensors. One major protein regulated by this role of nucleoli is the tumor suppressor p53, which is activated in response to diverse cellular injuries in order to exert its onco-protective effects. Here we discuss a model of nucleolar regulation of p53, which proposes that key steps in the promotion of p53 degradation by the ubiquitin ligase MDM2 occur in nucleoli, thus providing an explanation for the observed link between nucleolar disruption and p53 stability. We review current evidence for this compartmentalization in p53 homeostasis and highlight current limitations of the model. Interestingly, a number of current chemotherapeutic agents capable of inducing a p53 response are likely to do so by targeting nucleolar functions and these compounds may serve to inform further improved therapeutic targeting of nucleoli.

Formation of transient lamellipodia

Cell motility driven by actin polymerization is pivotal to the development and survival of organisms and individual cells. Motile cells plated on flat substrates form membrane protrusions called lamellipodia. The protrusions repeatedly appear and retract in all directions. If a lamellipodium is stabilized and lasts for some time, it can take over the lead and determine the direction of cell motion. Protrusions traveling along the cell perimeter have also been observed. Their initiation is in some situations the effect of the dynamics of the pathway linking plasma membrane receptors to actin filament nucleation, e.g. in chemotaxis. However, lamellipodia are also formed in many cells incessantly during motion with a constant state of the signaling pathways upstream from nucleation promoting factors (NPFs), or spontaneously in resting cells. These observations strongly suggest protrusion formation can also be a consequence of the dynamics downstream from NPFs, with signaling setting the dynamic regime but not initiating the formation of individual protrusions. A quantitative mechanism for this kind of lamellipodium dynamics has not been suggested yet. Here, we present a model exhibiting excitable actin network dynamics. Individual lamellipodia form due to random supercritical filament nucleation events amplified by autocatalytic branching. They last for about 30 seconds to many minutes and are terminated by filament bundling, severing and capping. We show the relevance of the model mechanism for experimentally observed protrusion dynamics by reproducing in very good approximation the repetitive protrusion formation measured by Burnette et al. with respect to the velocities of leading edge protrusion and retrograde flow, oscillation amplitudes, periods and shape, as well as the phase relation between protrusion and retrograde flow. Our modeling results agree with the mechanism of actin bundle formation during lamellipodium retraction suggested by Burnette et al. and Koestler et al.

Data-driven modeling reconciles kinetics of ERK phosphorylation, localization, and activity states

The extracellular signal‐regulated kinase (ERK) signaling pathway controls cell proliferation and differentiation in metazoans. Two hallmarks of its dynamics are adaptation of ERK phosphorylation, which has been linked to negative feedback, and nucleocytoplasmic shuttling, which allows active ERK to phosphorylate protein substrates in the nucleus and cytosol. To integrate these complex features, we acquired quantitative biochemical and live‐cell microscopy data to reconcile phosphorylation, localization, and activity states of ERK. While maximal growth factor stimulation elicits transient ERK phosphorylation and nuclear translocation responses, ERK activities available to phosphorylate substrates in the cytosol and nuclei show relatively little or no adaptation. Free ERK activity in the nucleus temporally lags the peak in nuclear translocation, indicating a slow process. Additional experiments, guided by kinetic modeling, show that this process is consistent with ERK's modification of and release from nuclear substrate anchors. Thus, adaptation of whole‐cell ERK phosphorylation is a by‐product of transient protection from phosphatases. Consistent with this interpretation, predictions concerning the dose‐dependence of the pathway response and its interruption by inhibition of MEK were experimentally confirmed.

Quantitative analysis of phosphoinositide 3-kinase (PI3K) signaling using live-cell total internal reflection fluorescence (TIRF) microscopy

This unit focuses on the use of total internal reflection fluorescence (TIRF) microscopy and image analysis methods to study the dynamics of signal transduction mediated by class I phosphoinositide 3-kinases (PI3Ks) in mammalian cells. The first four protocols cover live-cell imaging experiments, image acquisition parameters, and basic image processing and segmentation. These methods are generally applicable to live-cell TIRF experiments. The remaining protocols outline more advanced image analysis methods, which were developed in our laboratory for the purpose of characterizing the spatiotemporal dynamics of PI3K signaling. These methods may be extended to analyze other cellular processes monitored using fluorescent biosensors.

Attenuation of human lens epithelial cell spreading, migration and contraction via downregulation of the PI3K/Akt pathway

BACKGROUND

Posterior capsule opacification (PCO) represents a major challenge in the postoperative management of cataract patients. Spreading, migration and contraction of residual human lens epithelial cells play a pivotal role in the pathogenesis of PCO. Therefore, we analyzed the effect of the alkylphosphocholine (APC) erufosine on these cellular features as well as on PI3K/Akt, a crucial pathway in PCO pathogenesis.

METHODS

Human lens epithelial cells were cultured under standard cell culture conditions. Cell spreading was analyzed on fibronectin-coated wells and chemokinetic migration was assessed by time-lapse microscopy. For evaluation of cell-mediated collagen matrix contraction, the cells were seeded into collagen gels and incubated with an APC in different non-toxic concentrations before the surface area was measured on day 6. The activity of PI3K/Akt was assessed by an ELISA kit after incubation of the cells with different APC concentrations.

RESULTS

Human lens epithelial cell spreading and migration were attenuated by APCs as follows: 7 % spreading, 48 % migration (0.1 μM APC), and 32 % spreading, 68 % migration (1.0 μM APC). APC concentrations of 0.1 μM reduced collagen gel diameter by 5 %, and 1.0 μM by less than 1 %, compared to untreated, cell-populated gels that resulted in a cell diameter contraction of 36 %. PI3K was downregulated in a concentration-dependent manner.

CONCLUSIONS

The crucial cellular features of PCO pathogenesis are attenuated by the APC erufosine via downregulation of the PI3K pathway. Thus, erufosine might become a valuable tool for pharmacologic PCO prophylaxis in the future.

Chlamydia pneumoniae infection induces vascular smooth muscle cell migration via Rac1 activation

Chlamydia pneumoniae infection has been shown to be associated with the development of atherosclerosis by promoting the migration of vascular smooth muscle cells (VSMCs). However, how C. pneumoniae infection induces VSMC migration is not fully understood. A primary role of Ras-related C3 botulinum toxin substrate 1 (Rac1) is to generate a protrusive force at the leading edge that contributes to cell migration. Whether Rac1 activation plays a role in C. pneumoniae infection-induced VSMC migration is not well defined. In the present study, we therefore examined Rac1 activation in C. pneumoniae-infected rat primary VSMCs and the role of Rac1 activation in C. pneumoniae infection-induced VSMC migration. Glutathione S-transferase pull-down assay results showed that Rac1 was activated in C. pneumoniae-infected rat primary VSMCs. A Rac1 inhibitor, NSC23766 (50 µM,) suppressed Rac1 activation stimulated by C. pneumoniae infection, and thereby inhibited C. pneumoniae infection-induced VSMC migration. In addition, C. pneumoniae infection-induced Rac1 activation in the VSMCs was blocked by LY294002 (25 µM), an inhibitor of phosphatidylinositol 3-kinase (PI3K). Taken together, these data suggest that C. pneumoniae infection promotes VSMC migration, possibly through activating Rac1 via PI3K.

Bidirectional coupling between integrin-mediated signaling and actomyosin mechanics explains matrix-dependent intermittency of leading-edge motility

A physicochemical model is used to describe the coupling of adhesion, cytoskeletal, and signaling dynamics during cell migration. Analysis of stochastic simulations predicts relationships between measurable quantities that reflect partitioning of stress between F-actin–bound adhesions, which act as a molecular clutch, and retrograde F-actin flow.

Animal cell migration is a complex process characterized by the coupling of adhesion, cytoskeletal, and signaling dynamics. Here we model local protrusion of the cell edge as a function of the load-bearing properties of integrin-based adhesions, actin polymerization fostered by adhesion-mediated signaling, and mechanosensitive activation of RhoA that promotes myosin II–generated stress on the lamellipodial F-actin network. Analysis of stochastic model simulations illustrates how these pleiotropic functions of nascent adhesions may be integrated to govern temporal persistence and frequency of protrusions. The simulations give mechanistic insight into the documented effects of extracellular matrix density and myosin abundance, and they show characteristic, nonnormal distributions of protrusion duration times that are similar to those extracted from live-cell imaging experiments. Analysis of the model further predicts relationships between measurable quantities that reflect the partitioning of stress between tension on F-actin–bound adhesions, which act as a molecular clutch, and dissipation by retrograde F-actin flow.

An excitable signal integrator couples to an idling cytoskeletal oscillator to drive cell migration

It is generally believed that cytoskeletal activities drive random cell migration while signal transduction events initiated by receptors regulate the cytoskeleton to guide cells. However, we find that the cytoskeletal network, involving Scar/Wave, Arp 2/3, and actin binding proteins, is only capable of generating rapid oscillations and undulations of the cell boundary. The signal transduction network, comprising multiple pathways that include Ras GTPases, PI3K, and Rac GTPases, is required to generate the sustained protrusions of migrating cells. The signal transduction network is excitable, displaying wave propagation, refractoriness, and maximal response to suprathreshold stimuli, even in the absence of the cytoskeleton. We suggest that cell motility results from coupling of “pacemaker” signal transduction and “idling motor” cytoskeletal networks, and various guidance cues that modulate the threshold for triggering signal transduction events are integrated to control the mode and direction of migration.

Guidance of cell migration by substrate dimension

There is increasing evidence to suggest that physical parameters, including substrate rigidity, topography, and cell geometry, play an important role in cell migration. As there are significant differences in cell behavior when cultured in 1D, 2D, or 3D environments, we hypothesize that migrating cells are also able to sense the dimension of the environment as a guidance cue. NIH 3T3 fibroblasts were cultured on micropatterned substrates where the path of migration alternates between 1D lines and 2D rectangles. We found that 3T3 cells had a clear preference to stay on 2D rather than 1D substrates. Cells on 2D surfaces generated stronger traction stress than did those on 1D surfaces, but inhibition of myosin II caused cells to lose their sensitivity to substrate dimension, suggesting that myosin-II-dependent traction forces are the determining factor for dimension sensing. Furthermore, oncogene-transformed fibroblasts are defective in mechanosensing while generating similar traction forces on 1D and 2D surfaces. Dimension sensing may be involved in guiding cell migration for both physiological functions and tissue engineering, and for maintaining normal cells in their home tissue.

Migration of human dental follicle cells in vitro

BACKGROUND AND OBJECTIVES

The objective of this study was to elucidate the effects of different growth factors on the migration of dental follicle cells (DFCs). DFCs are ectomesenchymally derived easily accessible multipotent stem cells. Cell migration is a crucial step in many biological processes but also for tissue engineering. Growth factors such as epidermal growth factor (EGF), bone morphogenetic protein-2 (BMP2) or transforming growth factor β1 (TGF-β1) can be used to modify the behavior of cells.

MATERIAL AND METHODS

We used different migration assays (gel spot assay, scratch assay, transwell assay) to evaluate the influence of EGF, BMP2 and TGF-β1 on the migration of DFCs. We investigated the expression of migration-related genes after growth factor stimulation using the PCR array human cell motility.

RESULTS

DFCs treated with BMP2 or TGF-β1 migrated faster than DFCs treated with EGF. Additionally, more migration-related genes are regulated after treatment with BMP2 or TGF-β1 than with EGF. TGF-β1 additionally functions as a chemoattractant for DFCs. Osteogenic differentiation markers were regulated after BMP2 treatment only.

CONCLUSION

Whereas the strong migration induced by BMP2 was accompanied by beginning osteogenic differentiation the strong migration induced by TGF-β1 was directional. EGF exhibited not only the weakest migration stimulation but also the weakest induction of differentiation into mineralizing cells.

The small GTPase HRas shapes local PI3K signals through positive feedback and regulates persistent membrane extension in migrating fibroblasts

An inducible activation approach is used to interrogate the positive feedback circuit underlying self-amplification of PI3K signals in fibroblasts. The results show that local positive feedback between PI3K and HRas drives asymmetric membrane extension and cell migration.

Self-amplification of phosphoinositide 3-kinase (PI3K) signaling is believed to regulate asymmetric membrane extension and cell migration, but the molecular organization of the underlying feedback circuit is elusive. Here we use an inducible approach to synthetically activate PI3K and interrogate the feedback circuitry governing self-enhancement of 3′-phosphoinositide (3-PI) signals in NIH3T3 fibroblasts. Synthetic activation of PI3K initially leads to uniform production of 3-PIs at the plasma membrane, followed by the appearance of asymmetric and highly amplified 3-PI signals. A detailed spatiotemporal analysis shows that local self-amplifying 3-PI signals drive rapid membrane extension with remarkable directional persistence and initiate a robust migratory response. This positive feedback loop is critically dependent on the small GTPase HRas. Silencing of HRas abrogates local amplification of 3-PI signals upon synthetic PI3K activation and results in short-lived protrusion events that do not support cell migration. Finally, our data indicate that this feedback circuit is likely to operate during platelet-derived growth factor–induced random cell migration. We conclude that positive feedback between PI3K and HRas is essential for fibroblasts to spontaneously self-organize and generate a productive migratory response in the absence of spatial cues.

Envisioning migration: mathematics in both experimental analysis and modeling of cell behavior

The complex nature of cell migration highlights the power and challenges of applying mathematics to biological studies. Mathematics may be used to create model equations that recapitulate migration, which can predict phenomena not easily uncovered by experiments or intuition alone. Alternatively, mathematics may be applied to interpreting complex data sets with better resolution--potentially empowering scientists to discern subtle patterns amid the noise and heterogeneity typical of migrating cells. Iteration between these two methods is necessary in order to reveal connections within the cell migration signaling network, as well as to understand the behavior that arises from those connections. Here, we review recent quantitative analysis and mathematical modeling approaches to the cell migration problem.

At the leading edge of three-dimensional cell migration

Cells migrating on flat two-dimensional (2D) surfaces use actin polymerization to extend the leading edge of the plasma membrane during lamellipodia-based migration. This mode of migration is not universal; it represents only one of several mechanisms of cell motility in three-dimensional (3D) environments. The distinct modes of 3D migration are strongly dependent on the physical properties of the extracellular matrix, and they can be distinguished by the structure of the leading edge and the degree of matrix adhesion. How are these distinct modes of cell motility in 3D environments related to each other and regulated? Recent studies show that the same type of cell migrating in 3D extracellular matrix can switch between different leading edge structures. This mode-switching behavior, or plasticity, by a single cell suggests that the apparent diversity of motility mechanisms is integrated by a common intracellular signaling pathway that governs the mode of cell migration. In this Commentary, we propose that the mode of 3D cell migration is governed by a signaling axis involving cell-matrix adhesions, RhoA signaling and actomyosin contractility, and that this might represent a universal mechanism that controls 3D cell migration.