Regulation of chemotaxis by the orchestrated activation of Ras, PI3K, and TOR

Excitable Ras dynamics-based screens reveal RasGEFX is required for macropinocytosis and random cell migration

Excitable systems of eukaryotic chemotaxis can generate asymmetric signals of Ras-GTP-enriched domains spontaneously to drive random cell migration without guidance cues. However, the molecules responsible for the spontaneous signal generation remain elusive. Here, we characterized RasGEFs encoded in Dictyostelium discoideum by live-cell imaging of the spatiotemporal dynamics of Ras-GTP and hierarchical clustering, finding that RasGEFX is primarily required for the spontaneous generation of Ras-GTP-enriched domains and is essential for random migration in combination with RasGEFB/M/U in starved cells, and they are dispensable for chemotaxis to chemoattractant cAMP. RasGEFX and RasGEFB that co-localize with Ras-GTP regulate the temporal periods and spatial sizes of the oscillatory Ras-GTP waves propagating along the membrane, respectively, and thus control the protrusions of motile cells differently, while RasGEFU and RasGEFM regulate adhesion and migration speed, respectively. Remarkably, RasGEFX is also important for Ras/PIP3-driven macropinocytosis in proliferating cells, but RasGEFB/M/U are not. These findings illustrate a specific and coordinated control of the cytoskeletal dynamics by multiple RasGEFs for spontaneous motility and macropinocytosis.

EhRacM differentially regulates macropinocytosis and motility in the enteric protozoan parasite Entamoeba histolytica

Macropinocytosis is an evolutionarily conserved endocytic process that plays a vital role in internalizing extracellular fluids and particles in cells. This non-selective endocytic pathway is crucial for various physiological functions such as nutrient uptake, sensing, signaling, antigen presentation, and cell migration. While macropinocytosis has been extensively studied in macrophages and cancer cells, the molecular mechanisms of macropinocytosis in pathogens are less understood. It has been known that Entamoeba histolytica, the causative agent of amebiasis, exploits macropinocytosis for survival and pathogenesis. Since macropinocytosis is initiated by actin polymerization, leading to the formation of membrane ruffles and the subsequent trapping of solutes in macropinosomes, actin cytoskeleton regulation is crucial. Thus, this study focuses on unraveling the role of well-conserved actin cytoskeleton regulators, Rho small GTPase family proteins, in macropinocytosis in E. histolytica. Through gene silencing of highly transcribed Ehrho/Ehrac genes and following flow cytometry analysis, we identified that silencing EhracM enhances dextran macropinocytosis and affects cellular migration persistence. Live imaging and interactome analysis unveiled the cytosolic and vesicular localization of EhRacM, along with its interaction with signaling and membrane traffic-related proteins, shedding light on EhRacM's multiple roles. Our findings provide insights into the specific regulatory mechanisms of macropinocytosis among endocytic pathways in E. histolytica, highlighting the significance of EhRacM in both macropinocytosis and cellular migration.

Mathematical Modeling of PI3K/Akt Pathway in Microglia

The motility of microglia involves intracellular signaling pathways that are predominantly controlled by changes in cytosolic Ca2+ and activation of PI3K/Akt (phosphoinositide-3-kinase/protein kinase B). In this letter, we develop a novel biophysical model for cytosolic Ca2+ activation of the PI3K/Akt pathway in microglia where Ca2+ influx is mediated by both P2Y purinergic receptors (P2YR) and P2X purinergic receptors (P2XR). The model parameters are estimated by employing optimization techniques to fit the model to phosphorylated Akt (pAkt) experimental modeling/in vitro data. The integrated model supports the hypothesis that Ca2+ influx via P2YR and P2XR can explain the experimentally reported biphasic transient responses in measuring pAkt levels. Our predictions reveal new quantitative insights into P2Rs on how they regulate Ca2+ and Akt in terms of physiological interactions and transient responses. It is shown that the upregulation of P2X receptors through a repetitive application of agonist results in a continual increase in the baseline [Ca2+], which causes the biphasic response to become a monophasic response which prolongs elevated levels of pAkt.

CRISPR-mediated reversion of oncogenic KRAS mutation results in increased proliferation and reveals independent roles of Ras and mTORC2 in the migration of A549 lung cancer cells

Although the RAS oncogene has been extensively studied, new aspects concerning its role and regulation in normal biology and cancer continue to be discovered. Recently, others and we have shown that the mechanistic Target of Rapamycin Complex 2 (mTORC2) is a Ras effector in Dictyostelium and mammalian cells. mTORC2 plays evolutionarily conserved roles in cell survival and migration and has been linked to tumorigenesis. Because RAS is often mutated in lung cancer, we investigated whether a Ras-mTORC2 pathway contributes to enhancing the migration of lung cancer cells expressing oncogenic Ras. We used A549 cells and CRISPR/Cas9 to revert the cells' KRAS G12S mutation to wild-type and establish A549 revertant (REV) cell lines, which we then used to evaluate the Ras-mediated regulation of mTORC2 and cell migration. Interestingly, our results suggest that K-Ras and mTORC2 promote A549 cell migration but as part of different pathways and independently of Ras's mutational status. Moreover, further characterization of the A549REV cells revealed that loss of mutant K-Ras expression for the wild-type protein leads to an increase in cell growth and proliferation, suggesting that the A549 cells have low KRAS-mutant dependency and that recovering expression of wild-type K-Ras protein increases these cells tumorigenic potential.

Unraveling the molecular landscape: a comparative analysis of PI3K and MAPK signaling pathways in plasmablastic lymphoma and diffuse large B-cell lymphoma with therapeutic implications

Plasmablastic lymphoma (PBL) is a rare and aggressive subtype of non-Hodgkin lymphoma that shares features with diffuse large B-cell lymphoma (DLBCL). While significant progress has been made in treating DLBCL, the prognosis for PBL remains poor, highlighting the need to identify new therapeutic targets. Using RNA expression analysis, we compared the expression of genes involved in the phosphatidylinositol-3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling pathways between PBL and DLBCL. We used critical PI3K (n = 201) and MAPK (n = 57) signaling probe sets to achieve this objective. Our results demonstrate unique molecular mechanisms underlying PBL pathogenesis compared to DLBCL, particularly within the PI3K and MAPK signaling pathways. We found that elevated STAT3 expression in PBL correlates with hyperactive MAPK and PI3K pathways, unlike DLBCL. Additionally, the hyperactivation of the PI3K signaling axis in PBL is unrelated to B-cell receptor or phosphatase and tensin homolog activity, indicating a distinct mechanism compared to DLBCL. Furthermore, we observed unique activation patterns in MAPK pathways between PBL and DLBCL, with PBL exhibiting high expression of the neurotrophic tyrosine kinase receptor (NTKR) family, specifically NTRK1 and NTRK2 genes, which have therapeutic potential. We also found that neither human immunodeficiency virus nor Epstein-Barr virus infection influences gene expression profiles linked to PI3K and MAPK signaling in PBL. These findings could lead to adapting targeted therapies developed for DLBCL to address the specific needs of PBL patients better and contribute to developing novel, targeted therapeutic strategies to improve patient outcomes.

Dynamic subcellular localization of DydA in Dictyostelium cells

DydA plays an important role in chemotaxis, development, and cell growth as an adaptor protein that connects Ras signaling and cytoskeletal rearrangement. DydA is a downstream effector of RasG and is involved in controlling cell polarity and pseudopodia formation during chemoattractant-directed cell migration. To understand the mechanism by which DydA functions on the cell migration, we investigated the dynamic subcellular localization of DydA in response to chemoattractant stimulation and found that DydA rapidly and transiently translocated to the cell cortex through the RA domain and the PRM region in DydA in response to chemoattractant stimulation. The PRM region appears to play a primary role in the translocation of DydA to the cell cortex and in its localization to the actin foci at the bottom of cells. Colocalization experiments of GFP-PRM with RFP-coronin indicated that GFP-PRM preceded GFP-coronin by 2-3 s in response to chemoattractant stimulation. These results suggest that the PRM region plays an indispensable role in relaying upstream regulators, such as RasG, to downstream effectors by mediating the localization of DydA to the cell cortex upon chemoattractant stimulation.

Ras-mediated activation of mTORC2 promotes breast epithelial cell migration and invasion

We previously identified the mechanistic target of rapamycin complex 2 (mTORC2) as an effector of Ras for the control of directed cell migration in Dictyostelium. Recently, the Ras-mediated regulation of mTORC2 was found to be conserved in mammalian cells, and mTORC2 was shown to be an effector of oncogenic Ras. Interestingly, mTORC2 has been linked to cancer cell migration, and particularly in breast cancer. Here, we investigated the role of Ras in promoting the migration and invasion of breast cancer cells through mTORC2. We observed that both Ras and mTORC2 promote the migration of different breast cancer cells and breast cancer cell models. Using HER2 and oncogenic Ras-transformed breast epithelial MCF10A cells, we found that both wild-type Ras and oncogenic Ras promote mTORC2 activation and an mTORC2-dependent migration and invasion in these breast cancer models. We further observed that, whereas oncogenic Ras-transformed MCF10A cells display uncontrolled cell proliferation and invasion, disruption of mTORC2 leads to loss of invasiveness only. Together, our findings suggest that, whereas the Ras-mediated activation of mTORC2 is expected to play a minor role in breast tumor formation, the Ras-mTORC2 pathway plays an important role in promoting the migration and invasion of breast cancer cells.

The Crossroads between RAS and RHO Signaling Pathways in Cellular Transformation, Motility and Contraction

Ras and Rho proteins are GTP-regulated molecular switches that control multiple signaling pathways in eukaryotic cells. Ras was among the first identified oncogenes, and it appears mutated in many forms of human cancer. It mainly promotes proliferation and survival through the MAPK pathway and the PI3K/AKT pathways, respectively. However, the myriad proteins close to the plasma membrane that activate or inhibit Ras make it a major regulator of many apparently unrelated pathways. On the other hand, Rho is weakly oncogenic by itself, but it critically regulates microfilament dynamics; that is, actin polymerization, disassembly and contraction. Polymerization is driven mainly by the Arp2/3 complex and formins, whereas contraction depends on myosin mini-filament assembly and activity. These two pathways intersect at numerous points: from Ras-dependent triggering of Rho activators, some of which act through PI3K, to mechanical feedback driven by actomyosin action. Here, we describe the main points of connection between the Ras and Rho pathways as they coordinately drive oncogenic transformation. We emphasize the biochemical crosstalk that drives actomyosin contraction driven by Ras in a Rho-dependent manner. We also describe possible routes of mechanical feedback through which myosin II activation may control Ras/Rho activation.

PIP4K2A as a negative regulator of PI3K in PTEN-deficient glioblastoma

Glioblastoma (GBM) is the most malignant brain tumor with profound genomic alterations. Tumor suppressor genes regulate multiple signaling networks that restrict cellular proliferation and present barriers to malignant transformation. While bona fide tumor suppressors such as PTEN and TP53 often undergo inactivation due to mutations, there are several genes for which genomic deletion is the primary route for tumor progression. To functionally identify putative tumor suppressors in GBM, we employed in vivo RNAi screening using patient-derived xenograft models. Here, we identified PIP4K2A, whose functional role and clinical relevance remain unexplored in GBM. We discovered that PIP4K2A negatively regulates phosphoinositide 3-kinase (PI3K) signaling via p85/p110 component degradation in PTEN-deficient GBMs and specifically targets p85 for proteasome-mediated degradation. Overexpression of PIP4K2A suppressed cellular and clonogenic growth in vitro and impeded tumor growth in vivo. Our results unravel a novel tumor-suppressive role of PIP4K2A for the first time and support the feasibility of combining oncogenomics with in vivo RNAi screen.

PI3K: A Crucial Piece in the RAS Signaling Puzzle

RAS proteins are key signaling switches essential for control of proliferation, differentiation, and survival of eukaryotic cells. RAS proteins are mutated in 30% of human cancers. In addition, mutations in upstream or downstream signaling components also contribute to oncogenic activation of the pathway. RAS proteins exert their functions through activation of several signaling pathways and dissecting the contributions of these effectors in normal cells and in cancer is an ongoing challenge. In this review, we summarize our current knowledge about how RAS regulates type I phosphatidylinositol 3-kinase (PI3K), one of the main RAS effectors. RAS signaling through PI3K is necessary for normal lymphatic vasculature development and for RAS-induced transformation in vitro and in vivo, especially in lung cancer, where it is essential for tumor initiation and necessary for tumor maintenance.

Using biomaterials to modulate chemotactic signaling for central nervous system repair

Chemotaxis enables cellular communication and movement within the body. This review focuses on exploiting chemotaxis as a tool for repair of the central nervous system (CNS) damaged from injury and/or degenerative diseases. Chemokines and factors alone may initiate repair following CNS injury/disease, but exogenous administration may enhance repair and promote regeneration. This review will discuss critical chemotactic molecules and factors that may promote neural regeneration. Additionally, this review highlights how biomaterials can impact the presentation and delivery of chemokines and growth factors to alter the regenerative response.

AKT as a key target for growth promoting functions of neutral ceramidase in colon cancer cells

Despite advances in the field, colorectal cancer (CRC) remains a leading cause of cancer-related mortality worldwide. Research into bioactive sphingolipids over the past two decades has played an important role in increasing our understanding of the pathogenesis and therapeutics of CRC. In the complex metabolic network of sphingolipids, ceramidases (CDases) have a key function. These enzymes hydrolyze ceramides into sphingosine (SPH) which in turn is phosphorylated by sphingosine kinases (SK) 1 and 2 to generate sphingosine-1 phosphate (S1P). Importantly, we have recently shown that inhibition of neutral CDase (nCDase) induces an increase of ceramide in colon cancer cells which decreases cellular growth, increases apoptosis and modulates the WNT/β-catenin pathway. We have also shown that the deletion of nCDase protected mice from the onset and progression of colorectal cancer in the AOM carcinogen model. Here we demonstrate that AKT is a key target for the growth suppressing functions of ceramide. The results show that inhibition of nCDase activates GSK3β through dephosphorylation, and thus is required for the subsequent phosphorylation and degradation of β-catenin. Our findings show that inhibition of nCDase also inhibits the basal activation status of AKT, and we further establish that a constitutively active AKT (AKT T308D, S473D; AKT^DD) reverses the effect of nCDase on β-catenin degradation. Functionally, the AKT^DD mutant is able to overcome the growth suppressive effects of nCDase inhibition in CRC cells. Moreover, nCDase inhibition induces a growth delay of xenograft tumors from control cells, whereas xenograft tumors from constitutively active AKT cells become resistant to nCDase inhibition. Taken together, these results provide important mechanistic insight into how nCDase regulates cell proliferation. These findings demonstrate a heretofore unappreciated, but critical, role for nCDase in enabling/maintaining basal activation of AKT and also suggest that nCDase is a suitable novel target for colon cancer therapy.

The Scribble Cell Polarity Module in the Regulation of Cell Signaling in Tissue Development and Tumorigenesis

The Scribble cell polarity module, comprising Scribbled (Scrib), Discs-large (Dlg) and Lethal-2-giant larvae (Lgl), has a tumor suppressive role in mammalian epithelial cancers. The Scribble module proteins play key functions in the establishment and maintenance of different modes of cell polarity, as well as in the control of tissue growth, differentiation and directed cell migration, and therefore are major regulators of tissue development and homeostasis. Whilst molecular details are known regarding the roles of Scribble module proteins in cell polarity regulation, their precise mode of action in the regulation of other key cellular processes remains enigmatic. An accumulating body of evidence indicates that Scribble module proteins play scaffolding roles in the control of various signaling pathways, which are linked to the control of tissue growth, differentiation and cell migration. Multiple Scrib, Dlg and Lgl interacting proteins have been discovered, which are involved in diverse processes, however many function in the regulation of cellular signaling. Herein, we review the components of the Scrib, Dlg and Lgl protein interactomes, and focus on the mechanism by which they regulate cellular signaling pathways in metazoans, and how their disruption leads to cancer.

Role of the small GTPase Rap1 in signal transduction, cell dynamics and bacterial infection

Rap1 belongs to the Ras family of small GTPases, which are involved in a multitude of cellular signal transduction pathways and have extensively been linked to cancer biogenesis and metastasis. The small GTPase is activated in response to various extracellular and intracellular cues. Rap1 has conserved functions in Dictyostelium discoideum amoeba and mammalian cells, which are important for cell polarity, substrate and cell-cell adhesion and other processes that involve the regulation of cytoskeletal dynamics. Moreover, our recent study has shown that Rap1 is required for the formation of the replication-permissive vacuole of an intracellular bacterial pathogen. Here we review the function and regulation of Rap1 in these distinct processes, and we discuss the underlying signal transduction pathways.

Extracellular polyphosphate signals through Ras and Akt to prime Dictyostelium discoideum cells for development

Linear chains of five to hundreds of phosphates called polyphosphate are found in organisms ranging from bacteria to humans, but their function is poorly understood. In Dictyostelium discoideum, polyphosphate is used as a secreted signal that inhibits cytokinesis in an autocrine negative feedback loop. To elucidate how cells respond to this unusual signal, we undertook a proteomic analysis of cells treated with physiological levels of polyphosphate and observed that polyphosphate causes cells to decrease levels of actin cytoskeleton proteins, possibly explaining how polyphosphate inhibits cytokinesis. Polyphosphate also causes proteasome protein levels to decrease, and in both Dictyostelium and human leukemia cells, decreases proteasome activity and cell proliferation. Polyphosphate also induces Dictyostelium cells to begin development by increasing expression of the cell-cell adhesion molecule CsA (also known as CsaA) and causing aggregation, and this effect, as well as the inhibition of proteasome activity, is mediated by Ras and Akt proteins. Surprisingly, Ras and Akt do not affect the ability of polyphosphate to inhibit proliferation, suggesting that a branching pathway mediates the effects of polyphosphate, with one branch affecting proliferation, and the other branch affecting development.

Crosstalk of cell polarity signaling pathways

Cell polarity, the asymmetric organization of cellular components along one or multiple axes, is present in most cells. From budding yeast cell polarization induced by pheromone signaling, oocyte polarization at fertilization to polarized epithelia and neuronal cells in multicellular organisms, similar mechanisms are used to determine cell polarity. Crucial role in this process is played by signaling lipid molecules, small Rho family GTPases and Par proteins. All these signaling circuits finally govern the cytoskeleton, which is responsible for oriented cell migration, cell shape changes, and polarized membrane and organelle trafficking. Thus, typically in the process of cell polarization, most cellular constituents become polarized, including plasma membrane lipid composition, ion concentrations, membrane receptors, and proteins in general, mRNA, vesicle trafficking, or intracellular organelles. This review gives a brief overview how these systems talk to each other both during initial symmetry breaking and within the signaling feedback loop mechanisms used to preserve the polarized state.

Signaling Pathways Controlling Microglia Chemotaxis

Microglia are the primary resident immune cells of the central nervous system (CNS). They are the first line of defense of the brain's innate immune response against infection, injury, and diseases. Microglia respond to extracellular signals and engulf unwanted neuronal debris by phagocytosis, thereby maintaining normal cellular homeostasis in the CNS. Pathological stimuli such as neuronal injury induce transformation and activation of resting microglia with ramified morphology into a motile amoeboid form and activated microglia chemotax toward lesion site. This review outlines the current research on microglial activation and chemotaxis.

Mechanotransduction as an Adaptation to Gravity

Gravity has played a critical role in the development of terrestrial life. A key event in evolution has been the development of mechanisms to sense and transduce gravitational force into biological signals. The objective of this manuscript is to review how living organisms on Earth use mechanotransduction as an adaptation to gravity. Certain cells have evolved specialized structures, such as otoliths in hair cells of the inner ear and statoliths in plants, to respond directly to the force of gravity. By conducting studies in the reduced gravity of spaceflight (microgravity) or simulating microgravity in the laboratory, we have gained insights into how gravity might have changed life on Earth. We review how microgravity affects prokaryotic and eukaryotic cells at the cellular and molecular levels. Genomic studies in yeast have identified changes in genes involved in budding, cell polarity, and cell separation regulated by Ras, PI3K, and TOR signaling pathways. Moreover, transcriptomic analysis of late pregnant rats have revealed that microgravity affects genes that regulate circadian clocks, activate mechanotransduction pathways, and induce changes in immune response, metabolism, and cells proliferation. Importantly, these studies identified genes that modify chromatin structure and methylation, suggesting that long-term adaptation to gravity may be mediated by epigenetic modifications. Given that gravity represents a modification in mechanical stresses encounter by the cells, the tensegrity model of cytoskeletal architecture provides an excellent paradigm to explain how changes in the balance of forces, which are transmitted across transmembrane receptors and cytoskeleton, can influence intracellular signaling pathways and gene expression.

Connecting G protein signaling to chemoattractant-mediated cell polarity and cytoskeletal reorganization

The directional movement toward extracellular chemical gradients, a process called chemotaxis, is an important property of cells. Central to eukaryotic chemotaxis is the molecular mechanism by which chemoattractant-mediated activation of G-protein coupled receptors (GPCRs) induces symmetry breaking in the activated downstream signaling pathways. Studies with mainly Dictyostelium and mammalian neutrophils as experimental systems have shown that chemotaxis is mediated by a complex network of signaling pathways. Recently, several labs have used extensive and efficient proteomic approaches to further unravel this dynamic signaling network. Together these studies showed the critical role of the interplay between heterotrimeric G-protein subunits and monomeric G proteins in regulating cytoskeletal rearrangements during chemotaxis. Here we highlight how these proteomic studies have provided greater insight into the mechanisms by which the heterotrimeric G protein cycle is regulated, how heterotrimeric G proteins-induced symmetry breaking is mediated through small G protein signaling, and how symmetry breaking in G protein signaling subsequently induces cytoskeleton rearrangements and cell migration.

A Gα-Stimulated RapGEF Is a Receptor-Proximal Regulator of Dictyostelium Chemotaxis

Chemotaxis, or directional movement toward extracellular chemical gradients, is an important property of cells that is mediated through G-protein-coupled receptors (GPCRs). Although many chemotaxis pathways downstream of Gβγ have been identified, few Gα effectors are known. Gα effectors are of particular importance because they allow the cell to distinguish signals downstream of distinct chemoattractant GPCRs. Here we identify GflB, a Gα2 binding partner that directly couples the Dictyostelium cyclic AMP GPCR to Rap1. GflB localizes to the leading edge and functions as a Gα-stimulated, Rap1-specific guanine nucleotide exchange factor required to balance Ras and Rap signaling. The kinetics of GflB translocation are fine-tuned by GSK-3 phosphorylation. Cells lacking GflB display impaired Rap1/Ras signaling and actin and myosin dynamics, resulting in defective chemotaxis. Our observations demonstrate that GflB is an essential upstream regulator of chemoattractant-mediated cell polarity and cytoskeletal reorganization functioning to directly link Gα activation to monomeric G-protein signaling.

Life Is Simple-Biologic Complexity Is an Epiphenomenon

Life originated from unicellular organisms by circumventing the Second Law of Thermodynamics using the First Principles of Physiology, namely negentropy, chemiosmosis and homeostatic regulation of calcium and lipids. It is hypothesized that multicellular organisms are merely contrivances or tools, used by unicellular organisms as agents for the acquisition of epigenetic inheritance. The First Principles of Physiology, which initially evolved in unicellular organisms are the exapted constraints that maintain, sustain and perpetuate that process. To ensure fidelity to this mechanism, we must return to the first principles of the unicellular state as the determinants of the primary level of selection pressure during the life cycle. The power of this approach is reflected by examples of its predictive value. This perspective on life is a "game changer", mechanistically rendering transparent many dogmas, teleologies and tautologies that constrain the current descriptive view of Biology.

The impact of RGS and other G-protein regulatory proteins on Gαi-mediated signaling in immunity

Leukocyte chemoattractant receptors are members of the G-protein coupled receptor (GPCR) family. Signaling downstream of these receptors directs the localization, positioning and homeostatic trafficking of leukocytes; as well as their recruitment to, and their retention at, inflammatory sites. Ligand induced changes in the molecular conformation of chemoattractant receptors results in the engagement of heterotrimeric G-proteins, which promotes α subunits to undergo GTP/GDP exchange. This results in the functional release of βγ subunits from the heterotrimers, thereby activating downstream effector molecules, which initiate leukocyte polarization, gradient sensing, and directional migration. Pertussis toxin ADP ribosylates Gαi subunits and prevents chemoattractant receptors from triggering Gαi nucleotide exchange. The use of pertussis toxin revealed the essential importance of Gαi subunit nucleotide exchange for chemoattractant receptor signaling. More recent studies have identified a range of regulatory mechanisms that target these receptors and their associated heterotrimeric G-proteins, thereby helping to control the magnitude, kinetics, and duration of signaling. A failure in these regulatory pathways can lead to impaired receptor signaling and immunopathology. The analysis of mice with targeted deletions of Gαi isoforms as well as some of these G-protein regulatory proteins is providing insights into their roles in chemoattractant receptor signaling.

The Emergence of Physiology and Form: Natural Selection Revisited

Natural Selection describes how species have evolved differentially, but it is descriptive, non-mechanistic. What mechanisms does Nature use to accomplish this feat? One known way in which ancient natural forces affect development, phylogeny and physiology is through gravitational effects that have evolved as mechanotransduction, seen in the lung, kidney and bone, linking as molecular homologies to skin and brain. Tracing the ontogenetic and phylogenetic changes that have facilitated mechanotransduction identifies specific homologous cell-types and functional molecular markers for lung homeostasis that reveal how and why complex physiologic traits have evolved from the unicellular to the multicellular state. Such data are reinforced by their reverse-evolutionary patterns in chronic degenerative diseases. The physiologic responses of model organisms like Dictyostelium and yeast to gravity provide deep comparative molecular phenotypic homologies, revealing mammalian Target of Rapamycin (mTOR) as the final common pathway for vertical integration of vertebrate physiologic evolution; mTOR integrates calcium/lipid epistatic balance as both the proximate and ultimate positive selection pressure for vertebrate physiologic evolution. The commonality of all vertebrate structure-function relationships can be reduced to calcium/lipid homeostatic regulation as the fractal unit of vertebrate physiology, demonstrating the primacy of the unicellular state as the fundament of physiologic evolution.

Biologic relativity: Who is the observer and what is observed?

When quantum physics and biological phenomena are analogously explored, it emerges that biologic causation must also be understood independently of its overt appearance. This is similar to the manner in which Bohm characterized the explicate versus the implicate order as overlapping frames of ambiguity. Placed in this context, the variables affecting epigenetic inheritance can be properly assessed as a key mechanistic principle of evolution that significantly alters our understanding of homeostasis, pleiotropy, and heterochrony, and the purposes of sexual reproduction. Each of these become differing manifestations of a new biological relativity in which biologic space-time becomes its own frame. In such relativistic cellular contexts, it is proper to question exactly who has observer status, and who and what are being observed. Consideration within this frame reduces biology to cellular information sharing through cell-cell communication to resolve ambiguities at every scope and scale. In consequence, it becomes implicit that eukaryotic evolution derives from the unicellular state, remaining consistently adherent to it in a continuous evolutionary arc based upon elemental, non-stochastic physiologic first principles. Furthermore, the entire cell including its cytoskeletal apparatus and membranes that participate in the resolution of biological uncertainties must be considered as having equivalent primacy with genomes in evolutionary terms.

A novel role for flotillin-1 in H-Ras-regulated breast cancer aggressiveness

Elevated expression and aberrant activation of Ras have been implicated in breast cancer aggressiveness. H-Ras, but not N-Ras, induces breast cell invasion. A crucial link between lipid rafts and H-Ras function has been suggested. This study sought to identify the lipid raft protein(s) responsible for H-Ras-induced tumorigenicity and invasiveness of breast cancer. We conducted a comparative proteomic analysis of lipid raft proteins from invasive MCF10A human breast epithelial cells engineered to express active H-Ras and non-invasive cells expressing active N-Ras. Here, we identified a lipid raft protein flotillin-1 as an important regulator of H-Ras activation and breast cell invasion. Flotillin-1 was required for epidermal growth factor-induced activation of H-Ras, but not that of N-Ras, in MDA-MB-231 triple-negative breast cancer (TNBC) cells. Flotillin-1 knockdown inhibited the invasiveness of MDA-MB-231 and Hs578T TNBC cells in vitro and in vivo. In xenograft mouse tumor models of these TNBC cell lines, we showed that flotillin-1 played a critical role in tumor growth. Using human breast cancer samples, we provided clinical evidence for the metastatic potential of flotillin-1. Membrane staining of flotillin-1 was positively correlated with metastatic spread (p = 0.013) and inversely correlated with patient disease-free survival rates (p = 0.005). Expression of flotillin-1 was associated with H-Ras in breast cancer, especially in TNBC (p < 0.001). Our findings provide insight into the molecular basis of Ras isoform-specific interplay with flotillin-1, leading to tumorigenicity and aggressiveness of breast cancer.

Homo-FRET Based Biosensors and Their Application to Multiplexed Imaging of Signalling Events in Live Cells

Multiplexed imaging of Förster Resonance Energy Transfer (FRET)-based biosensors potentially presents a powerful approach to monitoring the spatio-temporal correlation of signalling pathways within a single live cell. Here, we discuss the potential of homo-FRET based biosensors to facilitate multiplexed imaging. We demonstrate that the homo-FRET between pleckstrin homology domains of Akt (Akt-PH) labelled with mCherry may be used to monitor 3'-phosphoinositide accumulation in live cells and show how global analysis of time resolved fluorescence anisotropy measurements can be used to quantify this accumulation. We further present multiplexed imaging readouts of calcium concentration, using fluorescence lifetime measurements of TN-L15-a CFP/YFP based hetero-FRET calcium biosensor-with 3'-phosphoinositide accumulation.

Girdin, an actin-binding protein, is critical for migration, adhesion, and invasion of human glioblastoma cells

Girdin, an actin-binding protein, possesses versatile functions in a multitude of cellular processes. Although several studies have shown that Girdin is involved in the cell DNA synthesis, actin cytoskeleton rearrangement, and cell motility, the molecular mechanisms of Girdin in tumor development and progression remain elusive. In this study, through over-expression and siRNA experiments, we found that Girdin increased migration of LN229 human glioblastoma cells. On the other hand, reducing Girdin impaired F-actin polymerization, which is essential for cell morphogenesis and motility. Matrix metalloproteinase 2, critical in human glioma migration and invasion, was down-regulated upon Girdin reduction and led to decreased invasion in vitro and in vivo. In addition, silencing Girdin expression impaired the phosphorylation of two important adhesion molecules, integrin β1 and focal adhesion kinase, resulting in cell adhesion defects. Our immunohistochemical study on human gliomas tissue sections indicated that Girdin expression was positively related with glioma malignancy, supporting the in vitro and in vivo results from cell lines. Collectively, our findings suggest a critical role for Girdin in glioma infiltration. We show that reduction of Girdin, an actin-binding protein, leads to impaired F-actin polymerization and down-regulated expression of matrix metallopeptidase protein 2 (MMP-2), phosphorylated integrin β1, and phosphorylated focal adhesion kinase (FAK), which resulted in decreased migration, adhesion, and invasion of glioblastoma cells. Girdin was positively correlated with glioma malignancy and negatively associated with clinical prognosis, suggesting Girdin as a critical regulator in glioma infiltration.

Adipose tissue immune response: novel triggers and consequences for chronic inflammatory conditions

Adipose tissue inflammation mediates the association between excessive body fat accumulation and several chronic inflammatory diseases. A high prevalence of obesity-associated adipose tissue inflammation was observed not only in patients with cardiovascular conditions but also in patients with inflammatory bowel diseases, abdominal aortic aneurysm, or cardiorenal syndrome. In addition to excessive caloric intake, other triggers promote visceral adipose tissue inflammation followed by chronic, low-grade systemic inflammation. The infiltration and accumulation of immune cells in the inflamed and hypertrophied adipose tissue promote the production of inflammatory cytokines, contributing to target organ damages. This comorbidity seems to delimit subgroups of individuals with systemic adipose tissue inflammation and more severe chronic inflammatory diseases that are refractory to conventional treatment. This review highlights the association between adipose tissue immune response and the pathophysiology of visceral adiposity-related chronic inflammatory diseases, while suggesting several new therapeutic strategies.

Insulin exerts direct, IGF-1 independent actions in growth plate chondrocytes

Insufficient insulin production or action in diabetic states is associated with growth retardation and impaired bone healing, while the underling mechanisms are unknown. In this study, we sought to define the role of insulin signaling in the growth plate. Insulin treatment of embryonic metatarsal bones from wild-type mice increased chondrocyte proliferation. Mice lacking insulin receptor (IR) selectively in chondrocytes (CartIR^−/−) had no discernable differences in total femoral length compared to control littermates. However, CartIR^−/− mice exhibited an increase in chondrocyte numbers in the growth plate than that of the controls. Chondrocytes lacking IR had elevated insulin-like growth factor (IGF)-1R mRNA and protein levels. Subsequently, IGF-1 induced phosphorylation of Akt and ERK was enhanced, while this action was eliminated when the cells were treated with IGF-1R inhibitor Picropodophyllin. Deletion of the IR impaired chondrogenic differentiation, and the effect could not be restored by treatment of insulin, but partially rescued by IGF-1 treatment. Intriguingly, the size of hypertrophic chondrocytes was smaller in CartIR^−/− mice when compared with that of the control littermates, which was associated with upregulation of tuberous sclerosis complex 2 (TSC2). These results suggest that deletion of the IR in chondrocytes sensitizes IGF-1R signaling and action, IR and IGF-1R coordinate to regulate the proliferation, differentiation and hypertrophy of growth plate chondrocytes.

Cytoskeletal Mechanics Regulating Amoeboid Cell Locomotion

Migrating cells exert traction forces when moving. Amoeboid cell migration is a common type of cell migration that appears in many physiological and pathological processes and is performed by a wide variety of cell types. Understanding the coupling of the biochemistry and mechanics underlying the process of migration has the potential to guide the development of pharmacological treatment or genetic manipulations to treat a wide range of diseases. The measurement of the spatiotemporal evolution of the traction forces that produce the movement is an important aspect for the characterization of the locomotion mechanics. There are several methods to calculate the traction forces exerted by the cells. Currently the most commonly used ones are traction force microscopy methods based on the measurement of the deformation induced by the cells on elastic substrate on which they are moving. Amoeboid cells migrate by implementing a motility cycle based on the sequential repetition of four phases. In this paper we review the role that specific cytoskeletal components play in the regulation of the cell migration mechanics. We investigate the role of specific cytoskeletal components regarding the ability of the cells to perform the motility cycle effectively and the generation of traction forces. The actin nucleation in the leading edge of the cell, carried by the ARP2/3 complex activated through the SCAR/WAVE complex, has shown to be fundamental to the execution of the cyclic movement and to the generation of the traction forces. The protein PIR121, a member of the SCAR/WAVE complex, is essential to the proper regulation of the periodic movement and the protein SCAR, also included in the SCAR/WAVE complex, is necessary for the generation of the traction forces during migration. The protein Myosin II, an important F-actin cross-linker and motor protein, is essential to cytoskeletal contractility and to the generation and proper organization of the traction forces during migration.

Activation of PI3K and R-Ras signaling promotes the extension of sensory axons on inhibitory chondroitin sulfate proteoglycans

Chondroitin sulfate proteoglycans (CSPGs) are extracellular inhibitors of axon extension and plasticity, and cause growth cones to exhibit dystrophic behaviors. Phosphoinositide 3-kinase (PI3K) is a lipid kinase activated by axon growth promoting signals. In this study, we used embryonic chicken dorsal root ganglion neurons to determine if CSPGs impair signaling through PI3K. We report that CSPGs inhibit PI3K signaling in axons and growth cones, as evidenced by decreased levels of phosphorylated downstream kinases (Akt and S6). Direct activation of PI3K signaling, using a cell permeable phosphopeptide (PI3Kpep), countered the effects of CSPGs on growth cones and axon extension. Both overnight and acute treatment with PI3Kpep promoted axon extension on CSPG-coated substrates. The R-Ras GTPase is an upstream positive regulator of PI3K signaling. Expression of constitutively active R-Ras promoted axon extension and growth cone elaboration on CSPGs and permissive substrata. In contrast, an N-terminus-deleted constitutively active R-Ras, deficient in PI3K activation, promoted axon extension but not growth cone elaboration on CSPGs and permissive substrata. These data indicate that activation of R-Ras-PI3K signaling may be a viable approach for manipulating axon extension on CSPGs.

SILAC-based proteomic quantification of chemoattractant-induced cytoskeleton dynamics on a second to minute timescale

Cytoskeletal dynamics during cell behaviours ranging from endocytosis and exocytosis to cell division and movement is controlled by a complex network of signalling pathways, the full details of which are as yet unresolved. Here we show that SILAC-based proteomic methods can be used to characterize the rapid chemoattractant-induced dynamic changes in the actin–myosin cytoskeleton and regulatory elements on a proteome-wide scale with a second to minute timescale resolution. This approach provides novel insights in the ensemble kinetics of key cytoskeletal constituents and association of known and novel identified binding proteins. We validate the proteomic data by detailed microscopy-based analysis of in vivo translocation dynamics for key signalling factors. This rapid large-scale proteomic approach may be applied to other situations where highly dynamic changes in complex cellular compartments are expected to play a key role.

Actin-dependent motility is driven by the rapid changes in the recruitment of many different structural and regulatory proteins at the cell’s cortex. Sobczyk et al. characterize these changes in the cytoskeletal proteome on a second to minute timescale during chemotactic response in Dictyostelium using SILAC-based proteomics.

Increased mammalian target of rapamycin complex 2 signaling promotes age-related decline in CD4 T cell signaling and function

CD4 T cell function declines significantly during aging. Although the mammalian target of rapamycin (TOR) has been implicated in aging, the roles of the TOR complexes (TORC1, TORC2) in the functional declines of CD4 T cells remain unknown. In this study, we demonstrate that aging increases TORC2 signaling in murine CD4 T cells, a change blocked by long-term exposure to rapamycin, suggesting that functional defects may be the result of enhanced TORC2 function. Using overexpression of Rheb to activate TORC1 and Rictor plus Sin1 to augment TORC2 in naive CD4 T cells from young mice, we demonstrated that increased TORC2, but not TORC1, signaling results in aging-associated biochemical changes. Furthermore, elevated TORC2 signaling in naive CD4 T cells from young mice leads to in vivo functional declines. The data presented in this article suggest a novel model in which aging increases TORC2 signaling and leads to CD4 T cell defects in old mice.

Ras activation and symmetry breaking during Dictyostelium chemotaxis

Central to chemotaxis is the molecular mechanism by which a shallow spatial gradient of chemoattractant induces symmetry breaking of activated signaling molecules. Previously, we have used Dictyostelium mutants to investigate the minimal requirements for chemotaxis, and identified a basal signaling module providing activation of Ras and F-actin at the leading edge. Here, we show that Ras activation after application of a pipette releasing the chemoattractant cAMP has three phases, each depending on specific guanine-nucleotide-exchange factors (GEFs). Initially a transient activation of Ras occurs at the entire cell boundary, which is proportional to the local cAMP concentrations and therefore slightly stronger at the front than in the rear of the cell. This transient Ras activation is present in gα2 (gpbB)-null cells but not in gβ (gpbA)-null cells, suggesting that Gβγ mediates the initial activation of Ras. The second phase is symmetry breaking: Ras is activated only at the side of the cell closest to the pipette. Symmetry breaking absolutely requires Gα2 and Gβγ, but not the cytoskeleton or four cAMP-induced signaling pathways, those dependent on phosphatidylinositol (3,4,5)-triphosphate [PtdIns(3,4,5)P3], cGMP, TorC2 and PLA2. As cells move in the gradient, the crescent of activated Ras in the front half of the cell becomes confined to a small area at the utmost front of the cell. Confinement of Ras activation leads to cell polarization, and depends on cGMP formation, myosin and F-actin. The experiments show that activation, symmetry breaking and confinement of Ras during Dictyostelium chemotaxis uses different G-protein subunits and a multitude of Ras GEFs and GTPase-activating proteins (GAPs).

Ras and rheb signaling in survival and cell death

One of the most obvious hallmarks of cancer is uncontrolled proliferation of cells partly due to independence of growth factor supply. A major component of mitogenic signaling is Ras, a small GTPase. It was the first identified human protooncogene and is known since more than three decades to promote cellular proliferation and growth. Ras was shown to support growth factor-independent survival during development and to protect from chemical or mechanical lesion-induced neuronal degeneration in postmitotic neurons. In contrast, for specific patho-physiological cases and cellular systems it has been shown that Ras may also promote cell death. Proteins from the Ras association family (Rassf, especially Rassf1 and Rassf5) are tumor suppressors that are activated by Ras-GTP, triggering apoptosis via e.g., activation of mammalian sterile 20-like (MST1) kinase. In contrast to Ras, their expression is suppressed in many types of tumours, which makes Rassf proteins an exciting model for understanding the divergent effects of Ras activity. It seems likely that the outcome of Ras signaling depends on the balance between the activation of its various downstream effectors, thus determining cellular fate towards either proliferation or apoptosis. Ras homologue enriched in brain (Rheb) is a protein from the Ras superfamily that is also known to promote proliferation, growth, and regeneration through the mammalian target of rapamycin (mTor) pathway. However, recent evidences indicate that the Rheb-mTor pathway may switch its function from a pro-growth into a cell death pathway, depending on the cellular situation. In contrast to Ras signaling, for Rheb, the cellular context is likely to modulate the whole Rheb-mTor pathway towards cellular death or survival, respectively.

Dictyostelium Ric8 is a nonreceptor guanine exchange factor for heterotrimeric G proteins and is important for development and chemotaxis

Heterotrimeric G proteins couple external signals to the activation of intracellular signal transduction pathways. Agonist-stimulated guanine nucleotide exchange activity of G-protein-coupled receptors results in the exchange of G-protein-bound GDP to GTP and the dissociation and activation of the complex into Gα-GTP and a Gβγ dimer. In Dictyostelium, a basal chemotaxis pathway consisting of heterotrimeric and monomeric G proteins is sufficient for chemotaxis. Symmetry breaking and amplification of chemoattractant sensing occurs between heterotrimeric G protein signaling and Ras activation. In a pull-down screen coupled to mass spectrometry, with Gα proteins as bait, we have identified resistant to inhibitors of cholinesterase 8 (Ric8) as a nonreceptor guanine nucleotide exchange factor for Gα-protein. Ric8 is not essential for the initial activation of heterotrimeric G proteins or Ras by uniform chemoattractant; however, it amplifies Gα signaling, which is essential for Ras-mediated symmetry breaking during chemotaxis and development.

A homogeneous, high-throughput assay for phosphatidylinositol 5-phosphate 4-kinase with a novel, rapid substrate preparation

Phosphoinositide kinases regulate diverse cellular functions and are important targets for therapeutic development for diseases, such as diabetes and cancer. Preparation of the lipid substrate is crucial for the development of a robust and miniaturizable lipid kinase assay. Enzymatic assays for phosphoinositide kinases often use lipid substrates prepared from lyophilized lipid preparations by sonication, which result in variability in the liposome size from preparation to preparation. Herein, we report a homogeneous 1536-well luciferase-coupled bioluminescence assay for PI5P4Kα. The substrate preparation is novel and allows the rapid production of a DMSO-containing substrate solution without the need for lengthy liposome preparation protocols, thus enabling the scale-up of this traditionally difficult type of assay. The Z’-factor value was greater than 0.7 for the PI5P4Kα assay, indicating its suitability for high-throughput screening applications. Tyrphostin AG-82 had been identified as an inhibitor of PI5P4Kα by assessing the degree of phospho transfer of γ-³²P-ATP to PI5P; its inhibitory activity against PI5P4Kα was confirmed in the present miniaturized assay. From a pilot screen of a library of bioactive compounds, another tyrphostin, I-OMe tyrphostin AG-538 (I-OMe-AG-538), was identified as an ATP-competitive inhibitor of PI5P4Kα with an IC₅₀ of 1 µM, affirming the suitability of the assay for inhibitor discovery campaigns. This homogeneous assay may apply to other lipid kinases and should help in the identification of leads for this class of enzymes by enabling high-throughput screening efforts.

A novel chemotaxis assay in 3-D collagen gels by time-lapse microscopy

The directional cell response to chemical gradients, referred to as chemotaxis, plays an important role in physiological and pathological processes including development, immune response and tumor cell invasion. Despite such implications, chemotaxis remains a challenging process to study under physiologically-relevant conditions in-vitro, mainly due to difficulties in generating a well characterized and sustained gradient in substrata mimicking the in-vivo environment while allowing dynamic cell imaging. Here, we describe a novel chemotaxis assay in 3D collagen gels, based on a reusable direct-viewing chamber in which a chemoattractant gradient is generated by diffusion through a porous membrane. The diffusion process has been analysed by monitoring the concentration of FITC-labelled dextran through epifluorescence microscopy and by comparing experimental data with theoretical and numerical predictions based on Fick's law. Cell migration towards chemoattractant gradients has been followed by time-lapse microscopy and quantified by cell tracking based on image analysis techniques. The results are expressed in terms of chemotactic index (I) and average cell velocity. The assay has been tested by comparing the migration of human neutrophils in isotropic conditions and in the presence of an Interleukin-8 (IL-8) gradient. In the absence of IL-8 stimulation, 80% of the cells showed a velocity ranging from 0 to 1 µm/min. However, in the presence of an IL-8 gradient, 60% of the cells showed an increase in velocity reaching values between 2 and 7 µm/min. Furthermore, after IL-8 addition, I increased from 0 to 0.25 and 0.25 to 0.5, respectively, for the two donors examined. These data indicate a pronounced directional migration of neutrophils towards the IL-8 gradient in 3D collagen matrix. The chemotaxis assay described here can be adapted to other cell types and may serve as a physiologically relevant method to study the directed locomotion of cells in a 3D environment in response to different chemoattractants.

Pleckstrin homology domain of Akt kinase: a proof of principle for highly specific and effective non-enzymatic anti-cancer target

While pharmacological inhibition of Akt kinase has been regarded as a promising anti-cancer strategy, most of the Akt inhibitors that have been developed are enzymatic inhibitors that target the kinase active site of Akt. Another key cellular regulatory event for Akt activation is the translocation of Akt kinase to the cell membrane from the cytoplasm, which is accomplished through the pleckstrin homology (PH) domain of Akt. However, compounds specifically interacting with the PH domain of Akt to inhibit Akt activation are currently limited. Here we identified a compound, lancemaside A (LAN-A), which specifically binds to the PH domain of Akt kinase. First, our mass spectra analysis of cellular Akt kinase isolated from cells treated with LAN-A revealed that LAN-A specifically binds to the PH domain of cellular Akt kinase. Second, we observed that LAN-A inhibits the translocation of Akt kinase to the membrane and thus Akt activation, as examined by the phosphorylation of various downstream targets of Akt such as GSK3β, mTOR and BAD. Third, in a co-cultured cell model containing human lung epithelial cancer cells (A549) and normal human primary lung fibroblasts, LAN-A specifically restricts the growth of the A549 cells. LAN-A also displayed anti-proliferative effects on various human cancer cell lines. Finally, in the A549-luciferase mouse transplant model, LAN-A effectively inhibited A549 cell growth with little evident cytotoxicity. Indeed, the therapeutic index of LAN-A in this mouse model was >250, supporting that LAN-A is a potential lead compound for PH domain targeting as a safe anti-cancer Akt inhibitor.

Molecular signatures of amyotrophic lateral sclerosis disease progression in hind and forelimb muscles of an SOD1(G93A) mouse model

AIMS

This study utilized proteomics, biochemical and enzymatic assays, and bioinformatics tools that characterize protein alterations in hindlimb (gastrocnemius) and forelimb (triceps) muscles in an amyotrophic lateral sclerosis (ALS) (SOD1(G93A)) mouse model. The aim of this study was to identify the key molecular signatures involved in disease progression.

RESULTS

Both muscle types have in common an early down-regulation of complex I. In the hindlimb, early increases in oxidative metabolism are associated with uncoupling of the respiratory chain, an imbalance of NADH/NAD(+), and an increase in reactive oxygen species (ROS) production. The NADH overflow due to complex I inactivation induces TCA flux perturbations, leading to citrate production, triggering fatty acid synthase (FAS), and lipid peroxidation. These early metabolic changes in the hindlimb followed by sustained and comparatively higher metabolic and cytoskeletal derangements over time precede and may catalyze the progressive muscle wasting in this muscle at the late stage. By contrast, in the forelimb, there is an early down-regulation of complexes I and II that is associated with the reduction of oxidative metabolism, which promotes metabolic homeostasis that is accompanied by a greater cytoskeletal stabilization response. However, these early compensatory systems diminish by a later time point.

INNOVATION

The identification of potential early- and late-stage disease molecular signatures in an ALS model: muscle albumin, complex I, complex II, citrate synthase, FAS, and phosphoinositide 3-kinase functions as diagnostic markers and peroxisome proliferator-activated receptor γ co-activator 1α (PGC1α), Sema-3A, and Rho-associated protein kinase 1 (ROCK1) play the role of disease progression markers.

CONCLUSION

The differing pattern of cellular metabolism and cytoskeletal derangements in the hind and forelimb identifies the potential dysmetabolism/hypermetabolism molecular signatures associated with disease progression, which may serve as diagnostic/disease progression markers in ALS patients.

Daydreamer, a Ras effector and GSK-3 substrate, is important for directional sensing and cell motility

Daydreamer (DydA), a new Mig10/RIAM/lamellipodin family adaptor protein, is a Ras effector required for cell polarization and directional movement during chemotaxis. DydA is phosphorylated by glycogen synthase kinase-3, which is required for some, but not all, of DydA's functions. gskA⁻ cells exhibit very strong chemotactic phenotypes, a subset of which are exhibited by dydA⁻ cells.

How independent signaling pathways are integrated to holistically control a biological process is not well understood. We have identified Daydreamer (DydA), a new member of the Mig10/RIAM/lamellipodin (MRL) family of adaptor proteins that localizes to the leading edge of the cell. DydA is a putative Ras effector that is required for cell polarization and directional movement during chemotaxis. dydA⁻ cells exhibit elevated F-actin and assembled myosin II (MyoII), increased and extended phosphoinositide-3-kinase (PI3K) activity, and extended phosphorylation of the activation loop of PKB and PKBR1, suggesting that DydA is involved in the negative regulation of these pathways. DydA is phosphorylated by glycogen synthase kinase-3 (GSK-3), which is required for some, but not all, of DydA's functions, including the proper regulation of PKB and PKBR1 and MyoII assembly. gskA⁻ cells exhibit very strong chemotactic phenotypes, as previously described, but exhibit an increased rate of random motility. gskA⁻ cells have a reduced MyoII response and a reduced level of phosphatidylinositol (3,4,5)-triphosphate production, but a highly extended recruitment of PI3K to the plasma membrane and highly extended kinetics of PKB and PKBR1 activation. Our results demonstrate that GSK-3 function is essential for chemotaxis, regulating multiple substrates, and that one of these effectors, DydA, plays a key function in the dynamic regulation of chemotaxis.

Afamin secreted from nonresorbing osteoclasts acts as a chemokine for preosteoblasts via the Akt-signaling pathway

Although it is well known that osteoclastic bone resorption is followed by osteoblastic bone formation, questions remain as to when coupling factors are produced during bone resorption and which stages of bone formation are affected by these factors. To clarify these mechanisms, we established an in vitro system to investigate the coupling phenomenon. We obtained conditioned media (CM) from osteoclasts in the early and late stages of differentiation and from bone resorption stages. The collected CM was used to treat primary mouse calvarial osteoblasts and preosteoblastic MC3T3-E1 cells and to evaluate its influence on the migration, viability, proliferation, and differentiation of osteoblasts. We found that CM from osteoclasts in the early stage of differentiation predominantly stimulated the migration of osteoblastic lineages. By further performing fractional analyses of the CM with liquid chromatography-tandem mass spectrometry, we identified afamin, which has binding activity with vitamin E, as a possible coupling factor. The CM collected from afamin siRNA-transfected osteoclasts significantly suppressed preosteoblast migration. Afamin activated Akt in preosteoblasts, and pretreatment with Akt inhibitor significantly blocked afamin-stimulated preosteoblast migration. In conclusion, these results indicate that osteoclasts themselves play a central role in the coupling of bone resorption and formation by stimulating preosteoblast migration. In addition, we identified afamin as one of osteoclast-derived chemokines that affect preosteoblasts through the activation of the Akt-signaling pathway.

Metastasis-associated protein 1/histone deacetylase 4-nucleosome remodeling and deacetylase complex regulates phosphatase and tensin homolog gene expression and function

Metastasis-associated protein 1 (MTA1) is widely overexpressed in human cancers and is associated with malignant phenotypic changes contributing to morbidity in the associated diseases. Here we discovered for the first time that MTA1, a master chromatin modifier, transcriptionally represses the expression of phosphatase and tensin homolog (PTEN), a tumor suppressor gene, by recruiting class II histone deacetylase 4 (HDAC4) along with the transcription factor Yin-Yang 1 (YY1) onto the PTEN promoter. We also found evidence of an inverse correlation between the expression levels of MTA1 and PTEN in physiologically relevant breast cancer microarray datasets. We found that MTA1 up-regulation leads to a decreased expression of PTEN protein and stimulation of PI3K as well as phosphorylation of its signaling targets. Accordingly, selective down-regulation of MTA1 in breast cancer cells increases PTEN expression and inhibits stimulation of the PI3K/AKT signaling. Collectively, these findings provide a mechanistic role for MTA1 in transcriptional repression of PTEN, leading to modulation of the resulting signaling pathways.

AKT-mediated regulation of polarization in differentiated human neutrophil-like HL-60 cells

OBJECTIVES

Neutrophil polarization is critical for the inflammatory response. AKT is a serine/threonine protein kinase and has been implicated in cell migration. However, it is not completely clear whether AKT affects neutrophil polarization. In this study, we tested the hypothesis that AKT regulates the polarization of neutrophil-like differentiated HL-60 cells (dHL-60) in response to fMLP.

METHODS

HL-60 cells were differentiated into dHL-60 by incubation in medium containing 1.3 % DMSO for up to 6 days. Polarization of dHL-60 cells and primary human neutrophils were measured by Zigmond chamber. Phospho-Akt was analyzed by immunofluorescence and Western blot analysis. F-actin polymerization was detected by Rhodamine-Phalloidine staining. Rac2 activation was evaluated using GST Pull-down assay.

RESULTS

We found that changes in the rate of cell polarization were consistent with the changes in AKT phosphorylation levels during HL-60 cell differentiation in response to fMLP. Moreover, cell polarization and AKT phosphorylation were reduced in fMLP-stimulated dHL-60 cells pretreated with the PI3 kinase inhibitors or the AKT inhibitors, which was confirmed in the primary human neutrophils. The AKT inhibitors altered fMLP-induced F-actin polymerization. Rac2 GTPases was also decreased by the AKT inhibitors in fMLP-stimulated dHL-60 cells.

CONCLUSION

This study demonstrates that AKT activation plays a crucial role in dHL-60 cell polarization.

Regulation of actin cytoskeleton by Rap1 binding to RacGEF1

Rap1 is rapidly and transiently activated in response to chemoattractant stimulation and helps establish cell polarity by locally modulating cytoskeletons. Here, we investigated the mechanisms by which Rap1 controls actin cytoskeletal reorganization in Dictyostelium and found that Rap1 interacts with RacGEF1 in vitro and stimulates F-actin polymerization at the sites where Rap1 is activated upon chemoattractant stimulation. Live cell imaging using GFP-coronin, a reporter for F-actin, demonstrates that cells expressing constitutively active Rap1 (Rap1CA) exhibit a high level of F-actin uniformly distributed at the cortex including the posterior and lateral sides of the chemotaxing cell. Examination of the localization of a PH-domain containing PIP3 reporter, PhdA-GFP, and the activation of Akt/Pkb and other Ras proteins in Rap1CA cells reveals that activated Rap1 has no effect on the production of PIP3 or the activation of Akt/Pkb and Ras proteins in response to chemoattractant stimulation. Rac family proteins are crucial regulators in actin cytoskeletal reorganization. In vitro binding assay using truncated RacGEF1 proteins shows that Rap1 interacts with the DH domain of RacGEF1. Taken together, these results suggest that Rap1-mediated F-actin polymerization probably occurs through the Rac signaling pathway by directly binding to RacGEF1.

Mast cell chemotaxis - chemoattractants and signaling pathways

Migration of mast cells is essential for their recruitment within target tissues where they play an important role in innate and adaptive immune responses. These processes rely on the ability of mast cells to recognize appropriate chemotactic stimuli and react to them by a chemotactic response. Another level of intercellular communication is attained by production of chemoattractants by activated mast cells, which results in accumulation of mast cells and other hematopoietic cells at the sites of inflammation. Mast cells express numerous surface receptors for various ligands with properties of potent chemoattractants. They include the stem cell factor (SCF) recognized by c-Kit, antigen, which binds to immunoglobulin E (IgE) anchored to the high affinity IgE receptor (FcεRI), highly cytokinergic (HC) IgE recognized by FcεRI, lipid mediator sphingosine-1-phosphate (S1P), which binds to G protein-coupled receptors (GPCRs). Other large groups of chemoattractants are eicosanoids [prostaglandin E₂ and D₂, leukotriene (LT) B₄, LTD₄, and LTC₄, and others] and chemokines (CC, CXC, C, and CX3C), which also bind to various GPCRs. Further noteworthy chemoattractants are isoforms of transforming growth factor (TGF) β1–3, which are sensitively recognized by TGF-β serine/threonine type I and II β receptors, adenosine, C1q, C3a, and C5a components of the complement, 5-hydroxytryptamine, neuroendocrine peptide catestatin, tumor necrosis factor-α, and others. Here we discuss the major types of chemoattractants recognized by mast cells, their target receptors, as well as signaling pathways they utilize. We also briefly deal with methods used for studies of mast cell chemotaxis and with ways of how these studies profited from the results obtained in other cellular systems.