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

The role of mammalian target of rapamycin (mTOR) in the regulation of pancreatic β-cell mass: implications in the development of type-2 diabetes

Type-2 diabetes mellitus (T2DM) is a disorder that is characterized by high blood glucose concentration in the context of insulin resistance and/or relative insulin deficiency. It causes metabolic changes that lead to the damage and functional impairment of organs and tissues resulting in increased morbidity and mortality. It is this form of diabetes whose prevalence is increasing at an alarming rate due to the 'obesity epidemic', as obesity is a key risk factor in the development of insulin resistance. However, the majority of individuals who have insulin resistance do not develop diabetes due to a compensatory increase in insulin secretion in response to an increase in insulin demand. This adaptive response is sustained by an increase in both β-cell function and mass. Importantly, there is increasing evidence that the Serine/Threonine kinase mammalian target of rapamycin (mTOR) plays a key role in the regulation of β-cell mass and therefore likely plays a critical role in β-cell adaptation. Therefore, the primary focus of this review is to summarize our current understanding of the role of mTOR in stimulating pancreatic β-cell mass and thus, in the prevention of type-2 diabetes.

A Gβγ effector, ElmoE, transduces GPCR signaling to the actin network during chemotaxis

Activation of G protein-coupled receptors (GPCRs) leads to the dissociation of heterotrimeric G-proteins into Gα and Gβγ subunits, which go on to regulate various effectors involved in a panoply of cellular responses. During chemotaxis, Gβγ subunits regulate actin assembly and migration, but the protein(s) linking Gβγ to the actin cytoskeleton remains unknown. Here, we identified a Gβγ effector, ElmoE in Dictyostelium, and demonstrated that it is required for GPCR-mediated chemotaxis. Remarkably, ElmoE associates with Gβγ and Dock-like proteins to activate the small GTPase Rac, in a GPCR-dependent manner, and also associates with Arp2/3 complex and F-actin. Thus, ElmoE serves as a link between chemoattractant GPCRs, G-proteins and the actin cytoskeleton. The pathway, consisting of GPCR, Gβγ, Elmo/Dock, Rac, and Arp2/3, spatially guides the growth of dendritic actin networks in pseudopods of eukaryotic cells during chemotaxis.

Phosphoinositides in chemotaxis

Phosphatidylinositol lipids generated through the action of phosphinositide 3-kinase (PI3K) are key mediators of a wide array of biological responses. In particular, their role in the regulation of cell migration has been extensively studied and extends to amoeboid as well as mesenchymal migration. Through the emergence of fluorescent probes that target PI3K products as well as the use of specific inhibitors and knockout technologies, the spatio-temporal distribution of PI3K products in chemotaxing cells has been shown to represent a key anterior polarity signal that targets downstream effectors to actin polymerization. In addition, through intricate cross-talk networks PI3K products have been shown to regulate signals that control posterior effectors. Yet, in more complex environments or in conditions where chemoattractant gradients are steep, a variety of cell types can still chemotax in the absence of PI3K signals. Indeed, parallel signal transduction pathways have been shown to coordinately regulate cell polarity and directed movement. In this chapter, we will review the current role PI3K products play in the regulation of directed cell migration in various cell types, highlight the importance of mathematical modeling in the study of chemotaxis, and end with a brief overview of other signaling cascades known to also regulate chemotaxis.

Biased excitable networks: how cells direct motion in response to gradients

The actin cytoskeleton in motile cells has many of the hallmarks of an excitable medium, including the presence of propagating waves. This excitable behavior can account for the spontaneous migration of cells. A number of reports have suggested that the chemoattractant-mediated signaling can bias excitability, thus providing a means by which cell motility can be directed. In this review, we discuss some of these observations and theories proposed to explain them. We also suggest a mechanism for cell polarity that can be incorporated into the existing framework.

Review series: TOR kinase complexes and cell migration

Cell migration is a fundamental process in a wide array of biological and pathological responses. It is regulated by complex signal transduction pathways in response to external cues that couple to growth factor and chemokine receptors. In recent years, the target of rapamycin (TOR) kinase, as part of either TOR complex 1 (TORC1) or TOR complex 2 (TORC2), has been shown to be an important signaling component linking external signals to the cytoskeletal machinery in a variety of cell types and organisms. Thus, these complexes have emerged as key regulators of cell migration and chemotaxis.

5'-Adenosine monophosphate-activated protein kinase--mammalian target of rapamycin axis as therapeutic target for age-related macular degeneration

Age-related macular degeneration (AMD) is the most common reason for blindness in developed countries. AMD essentially involves chronic oxidative stress, increased accumulation of lipofuscin in retinal pigment epithelial (RPE) cells, and extracellular drusen formation, as well as presence of chronic inflammation in the retina. The capacity to prevent the accumulation of cellular cytotoxic protein aggregates is decreased in senescent cells, which may evoke lipofuscin accumulation into lysosomes in postmitotic RPE cells. The formation of lipofuscin, in turn, decreases the lysosomal enzyme activity and impairs the autophagic clearance of damaged proteins destined for cellular removal. 5'-Adenosine monophosphate-activated protein kinase (AMPK) is a well-known inhibitor of mammalian target of rapamycin (mTOR) that subsequently evokes induction of autophagy. This review examines the novel potential therapeutic targets on the AMPK-mTOR axis and the ways in which autophagy clearance can suppress or prevent RPE degeneration and development of AMD.

PI3K/mTOR signaling regulates prostatic branching morphogenesis

Prostatic branching morphogenesis is an intricate event requiring precise temporal and spatial integration of numerous hormonal and growth factor-regulated inputs, yet relatively little is known about the downstream signaling pathways that orchestrate this process. In this study, we use a novel mesenchyme-free embryonic prostate culture system, newly available mTOR inhibitors and a conditional PTEN loss-of-function model to investigate the role of the interconnected PI3K and mTOR signaling pathways in prostatic organogenesis. We demonstrate that PI3K levels and PI3K/mTOR activity are robustly induced by androgen during murine prostatic development and that PI3K/mTOR signaling is necessary for prostatic epithelial bud invasion of surrounding mesenchyme. To elucidate the cellular mechanism by which PI3K/mTOR signaling regulates prostatic branching, we show that PI3K/mTOR inhibition does not significantly alter epithelial proliferation or apoptosis, but rather decreases the efficiency and speed with which the developing prostatic epithelial cells migrate. Using mTOR kinase inhibitors to tease out the independent effects of mTOR signaling downstream of PI3K, we find that simultaneous inhibition of mTORC1 and mTORC2 activity attenuates prostatic branching and is sufficient to phenocopy combined PI3K/mTOR inhibition. Surprisingly, however, mTORC1 inhibition alone has the reverse effect, increasing the number and length of prostatic branches. Finally, simultaneous activation of PI3K and downstream mTORC1/C2 via epithelial PTEN loss-of-function also results in decreased budding reversible by mTORC1 inhibition, suggesting that the effect of mTORC1 on branching is not primarily mediated by negative feedback on PI3K/mTORC2 signaling. Taken together, our data point to an important role for PI3K/mTOR signaling in prostatic epithelial invasion and migration and implicates the balance of PI3K and downstream mTORC1/C2 activity as a critical regulator of prostatic epithelial morphogenesis.

A shift of the TOR adaptor from Rictor towards Raptor by semaphorin in C. elegans

The target of rapamycin (TOR), a central regulator for cell growth and metabolism, resides in the two functionally distinct complexes TORC1 and TORC2, which are defined by their adaptors Raptor and Rictor, respectively. How the formation of the two TORCs is orchestrated remains unclear. Here we show the control of TOR partnering by semaphorin-plexin signalling in Caenorhabditis elegans. In semaphorin and plexin mutants, TOR-Raptor association decreases whereas TOR-Rictor association increases, concomitantly with TORC1 down- and TORC2 up-regulation. Epidermal defects in the mutants are suppressed by inhibiting TORC2 or reinforcing TORC1 signalling. Conversely, inhibition of TORC1 signalling phenocopies the mutants. Thus, our results indicate that TORC formation is a singularly important step in semaphorin signalling that culminates in diverse outcomes including TORC1-promoted messenger RNA translation and TORC2-regulated cytoskeletal remodelling.

Quantitative phosphoproteomics of CXCL12 (SDF-1) signaling

CXCL12 (SDF-1) is a chemokine that binds to and signals through the seven transmembrane receptor CXCR4. The CXCL12/CXCR4 signaling axis has been implicated in both cancer metastases and human immunodeficiency virus type 1 (HIV-1) infection and a more complete understanding of CXCL12/CXCR4 signaling pathways may support efforts to develop therapeutics for these diseases. Mass spectrometry-based phosphoproteomics has emerged as an important tool in studying signaling networks in an unbiased fashion. We employed stable isotope labeling with amino acids in cell culture (SILAC) quantitative phosphoproteomics to examine the CXCL12/CXCR4 signaling axis in the human lymphoblastic CEM cell line. We quantified 4,074 unique SILAC pairs from 1,673 proteins and 89 phosphopeptides were deemed CXCL12-responsive in biological replicates. Several well established CXCL12-responsive phosphosites such as AKT (pS473) and ERK2 (pY204) were confirmed in our study. We also validated two novel CXCL12-responsive phosphosites, stathmin (pS16) and AKT1S1 (pT246) by Western blot. Pathway analysis and comparisons with other phosphoproteomic datasets revealed that genes from CXCL12-responsive phosphosites are enriched for cellular pathways such as T cell activation, epidermal growth factor and mammalian target of rapamycin (mTOR) signaling, pathways which have previously been linked to CXCL12/CXCR4 signaling. Several of the novel CXCL12-responsive phosphoproteins from our study have also been implicated with cellular migration and HIV-1 infection, thus providing an attractive list of potential targets for the development of cancer metastasis and HIV-1 therapeutics and for furthering our understanding of chemokine signaling regulation by reversible phosphorylation.

Signaling mechanisms for chemotaxis

Cells recognize external chemical gradients and translate these environmental cues into amplified intracellular signaling that results in elongated cell shape, actin polymerization toward the leading edge, and movement along the gradient. Mechanisms underlying chemotaxis are conserved evolutionarily from Dictyostelium amoeba to mammalian neutrophils. Recent studies have uncovered several parallel intracellular signaling pathways that crosstalk in chemotaxing cells. Here, we review these signaling mechanisms in Dictyostelium discoideum.

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.

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.

mTOR complex 2 signaling and functions

The mechanistic target of rapamycin (mTOR) plays a central role in cellular growth and metabolism. mTOR forms two distinct protein complexes, mTORC1 and mTORC2. Much is known about the regulation and functions of mTORC1 due to availability of a natural compound, rapamycin, that inhibits this complex. Studies that define mTORC2 cellular functions and signaling have lagged behind. The development of pharmacological inhibitors that block mTOR kinase activity, and thereby inhibit both mTOR complexes, along with availability of mice with genetic knockouts in mTOR complex components have now provided new insights on mTORC2 function and regulation. Since prolonged effects of rapamycin can also disrupt mTORC2, it is worth re-evaluating the contribution of this less-studied mTOR complex in cancer, metabolic disorders and aging. In this review, we focus on recent developments on mammalian mTORC2 signaling mechanisms and its cellular and tissue-specific functions.

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.

"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.

[What is your diagnosis? (Cutaneous leishmaniasis)]

Neutrophils are the most abundant, short lived, and terminally differentiated leukocytes with distinct tiers of arsenals to counter pathogens. Neutrophils were traditionally considered transcriptionally inactive cells, but recent researches in the field led to a paradigm shift in neutrophil biology and revealed subpopulation heterogeneity, and functions pivotal to immunity and inflammation. Furthermore, recent unfolding of metabolic plasticity in neutrophils has challenged the long-standing concept of their sole dependence on glycolytic pathway. Metabolic adaptations and distinct regulations have been identified which are critical for neutrophil differentiation and functions. The metabolic reprogramming of neutrophils by inflammatory mediators or during pathologies such as sepsis, diabetes, glucose-6-phosphate dehydrogenase deficiency, glycogen storage diseases (GSDs), systemic lupus erythematosus (SLE), rheumatoid arthritis, and cancer are now being explored. In this review, we discuss recent developments in understanding of the metabolic regulation, that may provide clues for better management and newer therapeutic opportunities for neutrophil centric immuno-deficiencies and inflammatory disorders.