Store Operated Calcium Entry in Cell Migration and Cancer Metastasis

Ca2+ signaling is ubiquitous in eukaryotic cells and modulates many cellular events including cell migration. Directional cell migration requires the polarization of both signaling and structural elements. This polarization is reflected in various Ca2+ signaling pathways that impinge on cell movement. In particular, store-operated Ca2+ entry (SOCE) plays important roles in regulating cell movement at both the front and rear of migrating cells. SOCE represents a predominant Ca2+ influx pathway in non-excitable cells, which are the primary migrating cells in multicellular organisms. In this review, we summarize the role of Ca2+ signaling in cell migration with a focus on SOCE and its diverse functions in migrating cells and cancer metastasis. SOCE has been implicated in regulating focal adhesion turnover in a polarized fashion and the mechanisms involved are beginning to be elucidated. However, SOCE is also involved is other aspects of cell migration with a less well-defined mechanistic understanding. Therefore, much remains to be learned regarding the role and regulation of SOCE in migrating cells.

Feedback-Driven Mechanisms Between Phosphorylated Caveolin-1 and Contractile Actin Assemblies Instruct Persistent Cell Migration

The actin cytoskeleton and membrane-associated caveolae contribute to active processes, such as cell morphogenesis and motility. How these two systems interact and control directional cell migration is an outstanding question but remains understudied. Here we identified a negative feedback between contractile actin assemblies and phosphorylated caveolin-1 (CAV-1) in migrating cells. Cytoplasmic CAV-1 vesicles display actin-associated motilities by sliding along actin filaments or/and coupling to do retrograde flow with actomyosin bundles. Inhibition of contractile stress fibers, but not Arp2/3-dependent branched actin filaments, diminished the phosphorylation of CAV-1 on site Tyr14, and resulted in substantially increased size and decreased motility of cytoplasmic CAV-1 vesicles. Reciprocally, both the CAV-1 phospho-deficient mutation on site Tyr14 and CAV-1 knockout resulted in dramatic AMPK phosphorylation, further causing reduced active level of RhoA-myosin II and increased active level of Rac1-PAK1-Cofilin, consequently led to disordered contractile stress fibers and prominent lamellipodia. As a result, cells displayed depolarized morphology and compromised directional migration. Collectively, we propose a model in which feedback-driven regulation between actin and CAV-1 instructs persistent cell migration.

Local Myo9b RhoGAP activity regulates cell motility

To migrate, cells assume a polarized morphology, extending forward with a leading edge with their trailing edge retracting back toward the cell body. Both cell extension and retraction critically depend on the organization and dynamics of the actin cytoskeleton, and the small, monomeric GTPases Rac and Rho are important regulators of actin. Activation of Rac induces actin polymerization and cell extension whereas activation of Rho enhances acto-myosin II contractility and cell retraction. To coordinate migration, these processes must be carefully regulated. The myosin Myo9b, a Rho GTPase activating protein (GAP), negatively regulates Rho activity and deletion of Myo9b in leukocytes impairs cell migration through increased Rho activity. However, it is not known whether cell motility is regulated by global or local inhibition of Rho activity by Myo9b. Here, we addressed this question by using Myo9b-deficient macrophage-like cells that expressed different recombinant Myo9b constructs. We found that Myo9b accumulates in lamellipodial extensions generated by Rac-induced actin polymerization as a function of its motor activity. Deletion of Myo9b in HL-60 derived macrophages altered cell morphology and impaired cell migration. Reintroduction of Myo9b or Myo9b motor and GAP mutants revealed that local GAP activity rescues cell morphology and migration. In summary, Rac activation leads to actin polymerization and recruitment of Myo9b, which locally inhibits Rho activity to enhance directional cell migration. In summary, Rac activation leads to actin polymerization and recruitment of Myo9b, which locally inhibits Rho activity to enhance directional cell migration.

Spatial confinement of receptor activity by tyrosine phosphatase during directional cell migration

Directional cell migration involves signaling cascades that stimulate actin assembly at the leading edge, and additional pathways must inhibit actin polymerization at the rear. During neuroblast migration inCaenorhabditis elegans, the transmembrane protein MIG-13/Lrp12 acts through the Arp2/3 nucleation-promoting factors WAVE and WASP to guide the anterior migration. Here we show that a tyrosine kinase, SRC-1, directly phosphorylates MIG-13 and promotes its activity on actin assembly at the leading edge. In GFP knockin animals, SRC-1 and MIG-13 distribute along the entire plasma membrane of migrating cells. We reveal that a receptor-like tyrosine phosphatase, PTP-3, maintains the F-actin polarity during neuroblast migration. Recombinant PTP-3 dephosphorylates SRC-1–dependent MIG-13 phosphorylation in vitro. Importantly, the endogenous PTP-3 accumulates at the rear of the migrating neuroblast, and its extracellular domain is essential for directional cell migration. We provide evidence that the asymmetrically localized tyrosine phosphatase PTP-3 spatially restricts MIG-13/Lrp12 receptor activity in migrating cells.

Morphological and Motility Features of the Stable Bleb-Driven Monopodial Form of Entamoeba and Its Importance in Encystation

ABSTRACT Entamoeba histolytica and its reptilian counterpart and encystation model Entamoeba invadens formed a polarized monopodial morphology when treated with pentoxifylline. This morphology was propelled by retrograde flow of the cell surface resulting from a cyclic sol-gel conversion of cytoplasm and a stable bleb at the leading edge. Pentoxifylline treatment switched the unpolarized, adherent trophozoites to the nonadherent, stable bleb-driven form and altered the motility pattern from slow and random to fast, directionally persistent, and highly chemotactic. Interestingly, exogenously added adenosine produced multiple protrusions and random motility, an opposite phenotype to that of pentoxifylline. Thus, pentoxifylline, an adenosine antagonist, may be inducing the monopodial morphology by preventing lateral protrusions and restricting the leading edge to one site. The polarized form of E. invadens was aggregation competent, and time-lapse microscopy of encystation revealed its appearance during early hours, mediating the cell aggregation by directional cell migration. The addition of purine nucleotides to in vitro encystation culture prevented the formation of polarized morphology and inhibited the cell aggregation and, thus, the encystation, which further showed the importance of the polarized form in the Entamoeba life cycle. Cell polarity and motility are essential in the pathogenesis of Entamoeba parasites, and the stable bleb-driven polarized morphology of Entamoeba may also be important in invasive amoebiasis.

A Relatively Small Gradient of Extracellular pH Directs Migration of MDA-MB-231 Cells In Vitro

Hematogenous tumor metastasis begins with the invasion and spread of primary tumor cells in the local tissue leading to intravasation. We hypothesized that tumor cells might actively migrate toward intratumor vessels with the extracellular metabolic gradient acting as a guiding cue. Here, we determined in vitro whether the extracellular gradient of pH can act as a cue for directional migration in MDA-MB-231 cells. Cell migration was determined by the wound-healing assay under gradients of extracellular pH (~0.2 units/mm) and oxygen concentration (~6% O2/mm) that were produced by a microfluidic device, gap cover glass (GCG). Without GCG, the migration of cells was spatially homogeneous; the same number of cells migrated to the rectangular wound space from the left and right boundaries. In contrast, when GCG generated pH/O2 gradients across the wound space, the number of cells migrating to the wound space from the boundary with higher pH/O2 values was considerably decreased, indicating a preferential movement of cells toward the region of higher pH/O2 in the gradient. The addition of hepes in the extracellular medium abolished both the extracellular pH gradient and the directional cell migration under GCG. We conclude that relatively small gradients of pH in the extracellular medium compared to those found in Na+/H+ exchanger-driven cell migration were sufficient to guide MDA-MB-231 cells. The directional cell migration as guided by the metabolic gradient could effectively elevate the probability of intravasation and, ultimately, hematogenous metastasis.