A Ras signaling complex controls the RasC-TORC2 pathway and directed cell migration

mTORC1- and mTORC2-interacting proteins keep their multifunctional partners focused

The mammalian target of rapamycin, best known as mTOR, is a phylogenetically conserved serine/threonine kinase that controls life-defining cellular processes such as growth, metabolism, survival, and migration under the influence of multiple interacting proteins. Historically, the cellular activities blocked by rapamycin in mammalian cells were considered the only events controlled by mTOR. However, this paradigm changed with the discovery of two signaling complexes differentially sensitive to rapamycin, whose catalytic component is mTOR. The one sensitive to rapamycin, known as mTORC1, promotes protein synthesis in response to growth factors and nutrients via the phosphorylation of p70S6K and 4EBP1; while the other, known as mTORC2, promotes cell migration and survival via the activation of Rho GTPases and the phosphorylation of AKT, respectively. Although mTORC2 kinase activity is not inhibited by rapamycin, hours of incubation with this antibiotic can impede the assembly of this signaling complex. The direct mechanism by which mTORC2 leads to cell migration depends on its interaction with P-Rex1, a Rac-specific guanine nucleotide exchange factor, while additional indirect pathways involve the intervention of PKC or AKT, multifunctional ubiquitous serine/threonine kinases that activate effectors of cell migration upon being phosphorylated by mTORC2 in response to chemotactic signals. These mTORC2 effectors are altered in metastatic cancer. Numerous clinical trials are testing mTOR inhibitors as potential antineoplasic drugs. Here, we briefly review the actions of mTOR with emphasis on the controlling role of mTORC1 and mTORC2-interacting proteins and highlight the mechanisms linked to cell migration.

Ras trafficking, localization and compartmentalized signalling

Ras proteins are proto-oncogenes that are frequently mutated in human cancers. Three closely related isoforms, HRAS, KRAS and NRAS, are expressed in all cells and have overlapping but distinctive functions. Recent work has revealed how differences between the Ras isoforms in their trafficking, localization and protein-membrane orientation enable signalling specificity to be determined. We review the various strategies used to characterize compartmentalized Ras localization and signalling. Localization is an important contextual modifier of signalling networks and insights from the Ras system are of widespread relevance for researchers interested in signalling initiated from membranes.

Dynamic localization of the actin-bundling protein cortexillin I during cell migration

Cortexillins are actin-bundling proteins that play a critical role in regulating cell morphology and actin cytoskeleton reorganization in Dictyostelium. Here, we investigated dynamic subcellular localization of cortexillin I in chemotaxing Dictyostelium cells. Most of the cortexillin I was enriched on the lateral sides of moving cells. Upon chemoattractant stimulation, cortexillin I was rapidly released from the cortex followed by a transient translocation to the cell cortex with a peak at ~5 s and a subsequent decrease to basal levels, indicating that localization of cor-texillin I at the cortex in chemotaxing cells is controlled by two more signaling components, one for the initial delocalization from the cortex and another for the translocation to the cortex ~5 s after chemoattractant stimulation. Loss of cortexillins leads to reduced cell polarity and an increased number of lateral pseudopodia during chemotaxis, suggesting that cortexillins play an inhibitory role in producing pseudopodia along the lateral sides of the cell. Cells lacking cortexillins displayed extended chemoattrac-tant-mediated Arp2/3 complex translocation kinetics to the cortex. Our present study provides a new insight into the function of cortexillins during reorganization of the actin cytoskeleton and cell migration.

Biogenesis of the posterior pole is mediated by the exosome/microvesicle protein-sorting pathway

Biogenesis of the posterior pole is critical to directed cell migration and other polarity-dependent processes. We show here that proteins are targeted to the posterior pole on the basis of higher order oligomerization and plasma membrane binding, the same elements that target proteins to exosomes/microvesicles (EMVs), HIV, and other retrovirus particles. We also demonstrate that the polarization of the EMV protein-sorting pathway can occur in morphologically non-polarized cells, defines the site of uropod formation, is induced by increased expression of EMV cargo proteins, and is evolutionarily conserved between humans and the protozoan Dictyostelium discoideum. Based on these results, we propose a mechanism of posterior pole biogenesis in which elevated levels of EMV cargoes (i) polarize the EMV protein-sorting pathway, (ii) generate a nascent posterior pole, and (iii) prime cells for signal-induced biogenesis of a uropod. This model also offers a mechanistic explanation for the polarized budding of EMVs and retroviruses, including HIV.

Moving in the right direction: how eukaryotic cells migrate along chemical gradients

Many cells have the ability to grow or migrate towards chemical cues. Oriented growth and movement require detection of the external chemical gradient, transduction of signals, and reorganization of the cytoskeleton. Recent studies in Dictyostelium discoideum and mammalian neutrophils have revealed a complex signaling network that enables cells to migrate in chemical gradients.

Chemotaxis in cancer

Chemotaxis of tumour cells and stromal cells in the surrounding microenvironment is an essential component of tumour dissemination during progression and metastasis. This Review summarizes how chemotaxis directs the different behaviours of tumour cells and stromal cells in vivo, how molecular pathways regulate chemotaxis in tumour cells and how chemotaxis choreographs cell behaviour to shape the tumour microenvironment and to determine metastatic spread. The central importance of chemotaxis in cancer progression is highlighted by discussion of the use of chemotaxis as a prognostic marker, a treatment end point and a target of therapeutic intervention.

Fighting neurodegeneration with rapamycin: mechanistic insights

A growing number of studies point to rapamycin as a pharmacological compound that is able to provide neuroprotection in several experimental models of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease and spinocerebellar ataxia type 3. In addition, rapamycin exerts strong anti-ageing effects in several species, including mammals. By inhibiting the activity of mammalian target of rapamycin (mTOR), rapamycin influences a variety of essential cellular processes, such as cell growth and proliferation, protein synthesis and autophagy. Here, we review the molecular mechanisms underlying the neuroprotective effects of rapamycin and discuss the therapeutic potential of this compound for neurodegenerative diseases.

Putative biomarkers and targets of estrogen receptor negative human breast cancer

Breast cancer is a progressive and potentially fatal disease that affects women of all ages. Like all progressive diseases, early and reliable diagnosis is the key for successful treatment and annihilation. Biomarkers serve as indicators of pathological, physiological, or pharmacological processes. Her2/neu, CA15.3, estrogen receptor (ER), progesterone receptor (PR), and cytokeratins are biomarkers that have been approved by the Food and Drug Administration for disease diagnosis, prognosis, and therapy selection. The structural and functional complexity of protein biomarkers and the heterogeneity of the breast cancer pathology present challenges to the scientific community. Here we review estrogen receptor-related putative breast cancer biomarkers, including those of putative breast cancer stem cells, a minor population of estrogen receptor negative tumor cells that retain the stem cell property of self-renewal. We also review a few promising cytoskeleton targets for ER alpha negative breast cancer.

The LRRK2-related Roco kinase Roco2 is regulated by Rab1A and controls the actin cytoskeleton

We identify a new pathway that is required for proper pseudopod formation. We show that Roco2, a leucine-rich repeat kinase 2 (LRRK2)-related Roco kinase, is activated in response to chemoattractant stimulation and helps mediate cell polarization and chemotaxis by regulating cortical F-actin polymerization and pseudopod extension in a pathway that requires Rab1A. We found that Roco2 binds the small GTPase Rab1A as well as the F-actin cross-linking protein filamin (actin-binding protein 120, abp120) in vivo. We show that active Rab1A (Rab1A-GTP) is required for and regulates Roco2 kinase activity in vivo and that filamin lies downstream from Roco2 and controls pseudopod extension during chemotaxis and random cell motility. Therefore our study uncovered a new signaling pathway that involves Rab1A and controls the actin cytoskeleton and pseudopod extension, and thereby, cell polarity and motility. These findings also may have implications in the regulation of other Roco kinases, including possibly LRRK2, in metazoans.

PI3K and chemotaxis: a priming issue?

Although the spatiotemporal activation of phosphoinositide 3-kinases (PI3Ks) at the leading edge of chemotaxing cells represents a key marker of polarity, both Dictyostelium discoideum and neutrophils lacking measurable PI3K activity can still migrate directionally under certain conditions. Evidence from various papers suggests that the differentiation state of cells or their priming status can consolidate otherwise contradictory findings.

A brief history of TOR

The TOR (target of rapamycin) serine/threonine kinases are fascinating in that they influence many different aspects of eukaryote physiology including processes often dysregulated in disease. Beginning with the initial characterization of rapamycin as an antifungal agent, studies with yeast have contributed greatly to our understanding of the molecular pathways in which TORs operate. Recently, building on advances in quantitative MS, the rapamycin-dependent phosphoproteome in the budding yeast Saccharomyces cerevisiae was elucidated. These studies emphasize the central importance of TOR and highlight its many previously unrecognized functions. One of these, the regulation of intermediary metabolism, is discussed.

GPCR-controlled chemotaxis in Dictyostelium discoideum

Dictyostelium discoideum has been chosen as the key model organism for the study of eukaryotic chemotaxis. Studies in this lower eukaryotic organism have allowed us to discover eukaryotic chemotaxis behavior and to gradually understand the mechanism of chemotaxis. Investigations in this simple organism often guide the direction of chemotaxis studies in areas such as forming concepts, discovering molecular components, revealing pathways and networks. The cooperation between experimental approaches and computational modeling has helped us to comprehend the signaling network as a system. To further reveal the relationships among the molecular mechanisms of individual signaling steps, a continuous interplay between model development and refinement and experimental testing and verification will be useful. This article focuses on a chemoattractant G-protein-coupled receptor (GPCR)/G-protein gradient sensing machinery, which is monitored by PIP(3) responses and investigated by the interplay between live cell imaging experiments and computational modeling. We believe that such an approach will lead to a much better understanding of GPCR-controlled chemotaxis of all eukaryotic cells.

Role of a small G protein Ras in cellular immune response of the beet armyworm, Spodoptera exigua

Insect cellular immune responses accompany cytoskeletal rearrangement of hemocytes to exhibit filopodial and pseudopodial extension of their cytoplasm. Small G proteins are postulated to be implicated in the hemocyte cellular processes to perform phagocytosis, nodulation, and encapsulation behaviors. A small G protein ras gene (Se-Ras) was cloned from cDNAs prepared from hemocytes of the beet armyworm, Spodoptera exigua. The open reading frame of Se-Ras encoded 179 amino acids with a predicted molecular weight of 20.0kDa, in which 114 residues at amino terminus were predicted to be a GTP binding domain. It showed high sequence similarities (86.1-92.8%) with known ras genes in other insects. Se-Ras was constitutively expressed in all developmental stages from egg to adult without any significant change in expression levels in response to bacterial challenge. A specific double strand RNA (dsRNA) could knockdown its expression in the hemocytes after 48h post-injection. While the RNA interference (RNAi) did not show any change in total or differential hemocyte counts, it impaired hemocyte behaviors. The RNAi of Se-Ras significantly suppressed hemocyte spreading, cytoskeleton extension, and nodulation behaviors in response to bacterial challenge. Release of prophenoloxidase from oenocytoids was significantly inhibited by the RNAi, which resulted in significant suppression in PO activation in response to an inducer, PGE(2). These results suggest that Se-Ras is implicated in mediating cellular processes of S. exigua hemocytes. This is the first report of Ras role in insect cellular immune response.

Actin on disease--studying the pathobiology of cell motility using Dictyostelium discoideum

The actin cytoskeleton in eukaryotic cells provides cell structure and organisation, and allows cells to generate forces against membranes. As such it is a central component of a variety of cellular structures involved in cell motility, cytokinesis and vesicle trafficking. In multicellular organisms these processes contribute towards embryonic development and effective functioning of cells of all types, most obviously rapidly moving cells like lymphocytes. Actin also defines and maintains the architecture of complex structures such as neuronal synapses and stereocillia, and is required for basic housekeeping tasks within the cell. It is therefore not surprising that misregulation of the actin cytoskeleton can cause a variety of disease pathologies, including compromised immunity, neurodegeneration, and cancer spread. Dictyostelium discoideum has long been used as a tool for dissecting the mechanisms by which eukaryotic cells migrate and chemotax, and recently it has gained precedence as a model organism for studying the roles of conserved pathways in disease processes. Dictyostelium's unusual lifestyle, positioned between unicellular and multicellular organisms, combined with ease of handling and strong conservation of actin regulatory machinery with higher animals, make it ideally suited for studying actin-related diseases. Here we address how research in Dictyostelium has contributed to our understanding of immune deficiencies and neurological defects in humans, and briefly discuss its future prospects for furthering our understanding of neurodegenerative disorders.

"TORCing" neutrophil chemotaxis

During cell migration, chemoattractant-induced signaling pathways determine the direction of movement by controlling the spatiotemporal dynamics of cytoskeletal components. In this issue of Developmental Cell, Liu et al. report that the target of rapamycin complex 2 (TORC2) controls cell polarity and chemotaxis through regulation of both F-actin and myosin II in migrating neutrophils.

mTORC2 regulates neutrophil chemotaxis in a cAMP- and RhoA-dependent fashion

We studied the role of the target of rapamycin complex 2 (mTORC2) during neutrophil chemotaxis, a process that is mediated through the polarization of actin and myosin filament networks. We show that inhibition of mTORC2 activity, achieved via knock down (KD) of Rictor, severely inhibits neutrophil polarization and directed migration induced by chemoattractants, independently of Akt. Rictor KD also abolishes the ability of chemoattractants to induce cAMP production, a process mediated through the activation of the adenylyl cyclase 9 (AC9). Cells with either reduced or higher AC9 levels also exhibit specific and severe tail retraction defects that are mediated through RhoA. We further show that cAMP is excluded from extending pseudopods and remains restricted to the cell body of migrating neutrophils. We propose that the mTORC2-dependent regulation of MyoII occurs through a cAMP/RhoA-signaling axis, independently of actin reorganization during neutrophil chemotaxis.

mTOR: from growth signal integration to cancer, diabetes and ageing

In all eukaryotes, the target of rapamycin (TOR) signalling pathway couples energy and nutrient abundance to the execution of cell growth and division, owing to the ability of TOR protein kinase to simultaneously sense energy, nutrients and stress and, in metazoans, growth factors. Mammalian TOR complex 1 (mTORC1) and mTORC2 exert their actions by regulating other important kinases, such as S6 kinase (S6K) and Akt. In the past few years, a significant advance in our understanding of the regulation and functions of mTOR has revealed the crucial involvement of this signalling pathway in the onset and progression of diabetes, cancer and ageing.

mTOR: from growth signal integration to cancer, diabetes and ageing

In all eukaryotes, the target of rapamycin (TOR) signalling pathway couples energy and nutrient abundance to the execution of cell growth and division, owing to the ability of TOR protein kinase to simultaneously sense energy, nutrients and stress and, in metazoans, growth factors. Mammalian TOR complex 1 (mTORC1) and mTORC2 exert their actions by regulating other important kinases, such as S6 kinase (S6K) and Akt. In the past few years, a significant advance in our understanding of the regulation and functions of mTOR has revealed the crucial involvement of this signalling pathway in the onset and progression of diabetes, cancer and ageing.

Cellular responses to extracellular guidance cues

Extracellular guidance cues have a key role in orchestrating cell behaviour. They can take many forms, including soluble and cell-bound ligands (proteins, lipids, peptides or small molecules) and insoluble matrix substrates, but to act as guidance cues, they must be presented to the cell in a spatially restricted manner. Cells that recognize such cues respond by activating intracellular signal transduction pathways in a spatially restricted manner and convert the extracellular information into intracellular polarity. Although extracellular cues influence a broad range of cell polarity decisions, such as mitotic spindle orientation during asymmetric cell division, or the establishment of apical-basal polarity in epithelia, this review will focus specifically on guidance cues that promote cell migration (chemotaxis), or localized cell shape changes (chemotropism).