Ryan J. Petrie

Random versus directionally persistent cell migration

Directional migration is an important component of cell motility. Although the basic mechanisms of random cell movement are well characterized, no single model explains the complex regulation of directional migration. Multiple factors operate at each step of cell migration to stabilize lamellipodia and maintain directional migration. Factors such as the topography of the extracellular matrix, the cellular polarity machinery, receptor signalling, integrin trafficking, integrin co-receptors and actomyosin contraction converge on regulation of the Rho family of GTPases and the control of lamellipodial protrusions to promote directional migration.

Nonpolarized signaling reveals two distinct modes of 3D cell migration

The elastic behavior of the 3D extracellular matrix determines the relative polarization of intracellular signaling and whether cells migrate using lamellipodia or lobopodia.

Transient Translocation of the B Cell Receptor and Src Homology 2 Domain-Containing Inositol Phosphatase to Lipid Rafts: Evidence Toward a Role in Calcium Regulation

Membrane microdomains (lipid rafts) are enriched in selected signaling molecules and may compartmentalize receptor-mediated signals. Here, we report that in primary human B lymphocytes and in Ramos B cells B cell receptor (BCR) stimulation induces rapid and transient redistribution of a subset of engaged BCRs to lipid rafts and phosphorylation of raft-associated tyrosine kinase substrates. Cholesterol sequestration disrupted the lipid rafts, preventing BCR redistribution, but did not inhibit tyrosine kinase activation or phosphorylation of mitogen-activated protein kinase/extracellular regulated kinase. However, raft disruption enhanced the release of calcium from intracellular stores, suggesting that rafts may sequester early signaling events that down-regulate calcium flux. Consistent with this, BCR stimulation induced rapid and transient translocation of the Src homology 2 domain-containing inositol phosphatase, SHIP, into lipid rafts.

At the leading edge of three-dimensional cell migration

Summary 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.

Generation of compartmentalized pressure by a nuclear piston governs cell motility in a 3D matrix

Push me, pull you, thats the way to move Primary cells, derived directly from human tissue, exhibit different behaviors in shape and signaling within three-dimensional (3D) or 2D spaces. When the pressure within the cell increases, cells display limb-like bumps, which they use to move through their 3D environment. Petrie et al. now show that when the complex of actin and myosin contracts, it controls the pressure within cells and therefore the shape of those protruding structures (see the Perspective by DeSimone and Horwitz). The authors measured internal pressures in migrating mammalian cells. In the 3D matrix, those cells have higher pressure that differs between the front and back of the cell, which creates a piston effect. Science, this issue p. 1062; see also p. 1002 Human fibroblast migration in a physiological extracellular matrix relies on a polarized intracellular pressure gradient. [Also see Perspective by DeSimone and Horwitz] Cells use actomyosin contractility to move through three-dimensional (3D) extracellular matrices. Contractility affects the type of protrusions cells use to migrate in 3D, but the mechanisms are unclear. In this work, we found that contractility generated high-pressure lobopodial protrusions in human cells migrating in a 3D matrix. In these cells, the nucleus physically divided the cytoplasm into forward and rear compartments. Actomyosin contractility with the nucleoskeleton-intermediate filament linker protein nesprin-3 pulled the nucleus forward and pressurized the front of the cell. Reducing expression of nesprin-3 decreased and equalized the intracellular pressure. Thus, the nucleus can act as a piston that physically compartmentalizes the cytoplasm and increases the hydrostatic pressure between the nucleus and the leading edge of the cell to drive lamellipodia-independent 3D cell migration.

Dimensions in Cell Migration

The importance of cell migration for both normal physiological functions and disease processes has been clear for the past 50 years. Although investigations of two-dimensional (2D) migration in regular tissue culture have elucidated many important molecular mechanisms, recent evidence suggests that cell migration depends profoundly on the dimensionality of the extracellular matrix (ECM). Here we review a number of evolving concepts revealed when cell migration is examined in different dimensions.

Rab35 regulates neurite outgrowth and cell shape

MINT‐7012095: cdc42 (uniprotkb:P60953) and Rab 35 (uniprotkb:Q15286) colocalize (MI:0403) by fluorescence microscopy (MI:0416)

Colocalization of the B Cell Receptor and CD20 Followed by Activation-Dependent Dissociation in Distinct Lipid Rafts

The B cell Ag receptor (BCR) and CD20, a putative calcium channel, inducibly associate with cholesterol-dependent membrane microdomains known as lipid rafts. A functional association between the BCR and CD20 is suggested by the effects of CD20-specific mAbs, which can modulate cell cycle transitions elicited by BCR signaling. Using immunofluorescence microscopy we show here that the BCR and CD20 colocalize after receptor ligation and then rapidly dissociate at the cell surface before endocytosis of the BCR. After separation, surface BCR and CD20 were detected in distinct lipid rafts isolated as low density, detergent-resistant membrane fragments. Pretreatment with methyl-β-cyclodextrin, which we have previously shown to enhance receptor-mediated calcium mobilization, did not prevent colocalization of the BCR and CD20, but slowed their dissociation. The data demonstrate rapid dynamics of the BCR in relation to CD20 at the cell surface. Activation-dependent dissociation of the BCR from CD20 occurs before receptor endocytosis and appears to require in part the integrity of lipid rafts.

Dense fibrillar collagen is a potent inducer of invadopodia via a specific signaling network

High-density fibrillar collagen matrix induces invadopodia formation in both fibroblasts and carcinoma cell lines through a kindlin2-dependent mechanism that drives local ECM remodeling.

Spatial and temporal activation of the small GTPases RhoA and Rac1 by the netrin-1 receptor UNC5a during neurite outgrowth.

Netrin-1 attracts or repels growing axons during development. The UNC5 receptors mediate the repulsive response, either alone or in complex with DCC receptors. The signaling mechanisms activated by UNC5 are poorly understood. Here, we examined the role of Rho GTPases in UNC5a signaling. We found that UNC5a induced neurite outgrowth in N1E-115 neuroblastoma cells in a netrin-1- and Rac1-dependent manner. UNC5a lacking its cytoplasmic tail also mediated this effect. In fibroblasts, UNC5a was able to activate RhoA and to a lower extent Rac1 and Cdc42 in response to netrin-1. Using Fluorescence Resonance Energy Transfer (FRET) intermolecular probes, we visualized the spatial and temporal activation of Rac1, Cdc42 and RhoA in live N1E-115 cells expressing UNC5a during neurite outgrowth. We found that Rac1 but not Cdc42 was transiently activated at the leading edge of the cell during neurite initiation. However, at later times when well-developed neurites were formed, active RhoA was found in the cell body and at the base of the neuronal leading process in UNC5a-expressing cells. Together, these findings demonstrate that the netrin-1 receptor UNC5a is able to induce neurite outgrowth and to differentially activate RhoA and Rac1 during neurite extension in a spatial and temporal manner.