Showing papers on "Signal transduction published in 2009"
TL;DR: How genetic evidence in mice has revealed complex roles for the NF-kappaB in inflammation that suggest both pro- and anti-inflammatory roles for this pathway is described.
Abstract: The nuclear factor NF-κB pathway has long been considered a prototypical proinflammatory signaling pathway, largely based on the role of NF-κB in the expression of proinflammatory genes including cytokines, chemokines, and adhesion molecules In this article, we describe how genetic evidence in mice has revealed complex roles for the NF-κB in inflammation that suggest both pro- and anti-inflammatory roles for this pathway NF-κB has long been considered the “holy grail” as a target for new anti-inflammatory drugs; however, these recent studies suggest this pathway may prove a difficult target in the treatment of chronic disease In this article, we discuss the role of NF-κB in inflammation in light of these recent studies
3,396 citations
TL;DR: Current evidence indicates that MAMPs, DAMPs, and effectors are all perceived as danger signals and induce a stereotypic defense response, and the importance of MAMP/PRR signaling for plant immunity is highlighted.
Abstract: Microbe-associated molecular patterns (MAMPs) are molecular signatures typical of whole classes of microbes, and their recognition plays a key role in innate immunity. Endogenous elicitors are similarly recognized as damage-associated molecular patterns (DAMPs). This review focuses on the diversity of MAMPs/DAMPs and on progress to identify the corresponding pattern recognition receptors (PRRs) in plants. The two best-characterized MAMP/PRR pairs, flagellin/FLS2 and EF-Tu/EFR, are discussed in detail and put into a phylogenetic perspective. Both FLS2 and EFR are leucine-rich repeat receptor kinases (LRR-RKs). Upon treatment with flagellin, FLS2 forms a heteromeric complex with BAK1, an LRR-RK that also acts as coreceptor for the brassinolide receptor BRI1. The importance of MAMP/PRR signaling for plant immunity is highlighted by the finding that plant pathogens use effectors to inhibit PRR complexes or downstream signaling events. Current evidence indicates that MAMPs, DAMPs, and effectors are all perceived as danger signals and induce a stereotypic defense response.
2,801 citations
TL;DR: Much progress has been made in the past two years revealing new insights into the regulation and functions of NF-kappaB, and this recent progress is covered in this review.
Abstract: The mammalian Rel/NF-κB family of transcription factors, including RelA, c-Rel, RelB, NF-κB1 (p50 and its precursor p105), and NF-κB2 (p52 and its precursor p100), plays a central role in the immune system by regulating several processes ranging from the development and survival of lymphocytes and lymphoid organs to the control of immune responses and malignant transformation. The five members of the NF-κB family are normally kept inactive in the cytoplasm by interaction with inhibitors called IκBs or the unprocessed forms of NF-κB1 and NF-κB2. A wide variety of signals emanating from antigen receptors, pattern-recognition receptors, receptors for the members of TNF and IL-1 cytokine families, and others induce differential activation of NF-κB heterodimers. Although work over the past two decades has shed significant light on the regulation of NF-κB transcription factors and their functions, much progress has been made in the past two years revealing new insights into the regulation and functions of NF-κB...
2,380 citations
TL;DR: It is shown that cell priming through multiple signaling receptors induces NLRP3 expression, which is identified to be a critical checkpoint for NLRP2 activation and signals provided by NF-κB activators are necessary but not sufficient forNLRP3 activation.
Abstract: The IL-1 family cytokines are regulated on transcriptional and posttranscriptional levels. Pattern recognition and cytokine receptors control pro-IL-1β transcription whereas inflammasomes regulate the proteolytic processing of pro-IL-1β. The NLRP3 inflammasome, however, assembles in response to extracellular ATP, pore-forming toxins, or crystals only in the presence of proinflammatory stimuli. How the activation of gene transcription by signaling receptors enables NLRP3 activation remains elusive and controversial. In this study, we show that cell priming through multiple signaling receptors induces NLRP3 expression, which we identified to be a critical checkpoint for NLRP3 activation. Signals provided by NF-κB activators are necessary but not sufficient for NLRP3 activation, and a second stimulus such as ATP or crystal-induced damage is required for NLRP3 activation.
2,189 citations
TL;DR: The NF-kappaB pathway is a paradigm for understanding general principles of signal transduction and gene regulation as well as other pathway-specific mediators, and the transcription factors are themselves extensively modified.
Abstract: Nuclear factor-κB (NF-κB) consists of a family of transcription factors that play critical roles in inflammation, immunity, cell proliferation, differentiation, and survival. Inducible NF-κB activation depends on phosphorylation-induced proteosomal degradation of the inhibitor of NF-κB proteins (IκBs), which retain inactive NF-κB dimers in the cytosol in unstimulated cells. The majority of the diverse signaling pathways that lead to NF-κB activation converge on the IκB kinase (IKK) complex, which is responsible for IκB phosphorylation and is essential for signal transduction to NF-κB. Additional regulation of NF-κB activity is achieved through various post-translational modifications of the core components of the NF-κB signaling pathways. In addition to cytosolic modifications of IKK and IκB proteins, as well as other pathway-specific mediators, the transcription factors are themselves extensively modified. Tremendous progress has been made over the last two decades in unraveling the elaborate regulatory networks that control the NF-κB response. This has made the NF-κB pathway a paradigm for understanding general principles of signal transduction and gene regulation.
2,093 citations
TL;DR: The mammalian target of rapamycin (mTOR) signaling pathway integrates both intracellular and extracellular signals and serves as a central regulator of cell metabolism, growth, proliferation and survival.
Abstract: The mammalian target of rapamycin (mTOR) signaling pathway integrates both intracellular and extracellular signals and serves as a central regulator of cell metabolism, growth, proliferation and survival. Discoveries that have been made over the last decade show that the mTOR pathway is activated
2,027 citations
TL;DR: G-protein-coupled receptors mediate most of the authors' physiological responses to hormones, neurotransmitters and environmental stimulants, and so have great potential as therapeutic targets for a broad spectrum of diseases.
Abstract: G-protein-coupled receptors (GPCRs) mediate most of our physiological responses to hormones, neurotransmitters and environmental stimulants, and so have great potential as therapeutic targets for a broad spectrum of diseases. They are also fascinating molecules from the perspective of membrane-protein structure and biology. Great progress has been made over the past three decades in understanding diverse GPCRs, from pharmacology to functional characterization in vivo. Recent high-resolution structural studies have provided insights into the molecular mechanisms of GPCR activation and constitutive activity.
1,965 citations
TL;DR: This study identifies interactors of ABI1 and ABI2 which are named regulatory components of ABA receptor (RCARs) in Arabidopsis and suggests that the ABA receptors may be a class of closely related complexes, which may explain previous difficulties in establishing its identity.
Abstract: The plant hormone abscisic acid (ABA) acts as a developmental signal and as an integrator of environmental cues such as drought and cold. Key players in ABA signal transduction include the type 2C protein phosphatases (PP2Cs) ABI1 and ABI2, which act by negatively regulating ABA responses. In this study, we identify interactors of ABI1 and ABI2 which we have named regulatory components of ABA receptor (RCARs). In Arabidopsis, RCARs belong to a family with 14 members that share structural similarity with class 10 pathogen-related proteins. RCAR1 was shown to bind ABA, to mediate ABA-dependent inactivation of ABI1 or ABI2 in vitro, and to antagonize PP2C action in planta. Other RCARs also mediated ABA-dependent regulation of ABI1 and ABI2, consistent with a combinatorial assembly of receptor complexes.
1,854 citations
TL;DR: The connection of AMPK with several tumour suppressors suggests that therapeutic manipulation of this pathway using established diabetes drugs warrants further investigation in patients with cancer.
Abstract: In the past decade, studies of the human tumour suppressor LKB1 have uncovered a novel signalling pathway that links cell metabolism to growth control and cell polarity. LKB1 encodes a serine-threonine kinase that directly phosphorylates and activates AMPK, a central metabolic sensor. AMPK regulates lipid, cholesterol and glucose metabolism in specialized metabolic tissues, such as liver, muscle and adipose tissue. This function has made AMPK a key therapeutic target in patients with diabetes. The connection of AMPK with several tumour suppressors suggests that therapeutic manipulation of this pathway using established diabetes drugs warrants further investigation in patients with cancer.
1,626 citations
TL;DR: This review focuses on recent advances in the understanding of the molecular and biochemical mechanisms of non-Smad pathways, and functions of these non- Smad pathways are discussed.
Abstract: Transforming growth factor-β utilizes a multitude of intracellular signaling pathways in addition to Smads to regulate a wide array of cellular functions. These non-canonical, non-Smad pathways are activated directly by ligand-occupied receptors to reinforce, attenuate, or otherwise modulate downstream cellular responses. These non-Smad pathways include various branches of MAP kinase pathways, Rho-like GTPase signaling pathways, and phosphatidylinositol-3-kinase/AKT pathways. This review focuses on recent advances in the understanding of the molecular and biochemical mechanisms of non-Smad pathways. In addition, functions of these non-Smad pathways are also discussed.
1,541 citations
TL;DR: Recent insights into pathogen sensing by PRRs are summarized and specific signaling pathways that lead to expression of genes that tailor immune responses to particular microbes are summarized.
Abstract: The mammalian innate immune system detects the presence of microbial infection through germ line-encoded pattern recognition receptors (PRRs). Toll-like receptors, retinoic acid-inducible gene-I-like receptors and nucleotide-binding oligomerization domain-like receptors serve as PRRs that recognize different but overlapping microbial components. They are expressed in different cellular compartments such as the cell surface, endosome, lysosome or cytoplasm and activate specific signaling pathways that lead to expression of genes that tailor immune responses to particular microbes. This review summarizes recent insights into pathogen sensing by these PRRs and their signaling pathways.
TL;DR: Defective autophagy is a mechanism for p62 upregulation commonly observed in human tumors that contributes directly to tumorigenesis likely by perturbing the signal transduction adaptor function of p62-controlling pathways critical for oncogenesis.
TL;DR: It is shown here that TGR5 signaling induces intestinal glucagon-like peptide-1 (GLP-1) release, leading to improved liver and pancreatic function and enhanced glucose tolerance in obese mice, and suggested that pharmacological targeting of T GR5 may constitute a promising incretin-based strategy for the treatment of diabesity and associated metabolic disorders.
TL;DR: Two novel classes of small molecules are discovered that disrupt Wnt pathway responses and contribute to Wnt-independent signal transduction pathways and thus could be broadly exploited for chemical genetics and therapeutic goals.
Abstract: The pervasive influence of secreted Wnt signaling proteins in tissue homeostasis and tumorigenesis has galvanized efforts to identify small molecules that target Wnt-mediated cellular responses. By screening a diverse synthetic chemical library, we have discovered two new classes of small molecules that disrupt Wnt pathway responses; whereas one class inhibits the activity of Porcupine, a membrane-bound acyltransferase that is essential to the production of Wnt proteins, the other abrogates destruction of Axin proteins, which are suppressors of Wnt/beta-catenin pathway activity. With these small molecules, we establish a chemical genetic approach for studying Wnt pathway responses and stem cell function in adult tissue. We achieve transient, reversible suppression of Wnt/beta-catenin pathway response in vivo, and we establish a mechanism-based approach to target cancerous cell growth. The signal transduction mechanisms shown here to be chemically tractable additionally contribute to Wnt-independent signal transduction pathways and thus could be broadly exploited for chemical genetics and therapeutic goals.
TL;DR: Nucleotides are identified as a critical find-me cue released by apoptotic cells to promote P2Y2-dependent recruitment of phagocytes, and provide evidence for a clear relationship between a find- me signal and efficient corpse clearance in vivo.
Abstract: Phagocytic removal of apoptotic cells occurs efficiently in vivo such that even in tissues with significant apoptosis, very few apoptotic cells are detectable. This is thought to be due to the release of 'find-me' signals by apoptotic cells that recruit motile phagocytes such as monocytes, macrophages and dendritic cells, leading to the prompt clearance of the dying cells. However, the identity and in vivo relevance of such find-me signals are not well understood. Here, through several lines of evidence, we identify extracellular nucleotides as a critical apoptotic cell find-me signal. We demonstrate the caspase-dependent release of ATP and UTP (in equimolar quantities) during the early stages of apoptosis by primary thymocytes and cell lines. Purified nucleotides at these concentrations were sufficient to induce monocyte recruitment comparable to that of apoptotic cell supernatants. Enzymatic removal of ATP and UTP (by apyrase or the expression of ectopic CD39) abrogated the ability of apoptotic cell supernatants to recruit monocytes in vitro and in vivo. We then identified the ATP/UTP receptor P2Y(2) as a critical sensor of nucleotides released by apoptotic cells using RNA interference-mediated depletion studies in monocytes, and macrophages from P2Y(2)-null mice. The relevance of nucleotides in apoptotic cell clearance in vivo was revealed by two approaches. First, in a murine air-pouch model, apoptotic cell supernatants induced a threefold greater recruitment of monocytes and macrophages than supernatants from healthy cells did; this recruitment was abolished by depletion of nucleotides and was significantly decreased in P2Y(2)(-/-) (also known as P2ry2(-/-)) mice. Second, clearance of apoptotic thymocytes was significantly impaired by either depletion of nucleotides or interference with P2Y receptor function (by pharmacological inhibition or in P2Y(2)(-/-) mice). These results identify nucleotides as a critical find-me cue released by apoptotic cells to promote P2Y(2)-dependent recruitment of phagocytes, and provide evidence for a clear relationship between a find-me signal and efficient corpse clearance in vivo.
TL;DR: The switching on and off of Nrf2 protects cells against free radical damage, prevents apoptosis, and promotes cell survival, and is a mechanism of critical importance for cellular protection and cell survival.
TL;DR: It is shown that tumour necrosis factor (TNF) receptor-associated factor 6 (TRAF6), an adaptor protein in the TNF-receptor and interleukin-1R/Toll-like receptor superfamily, regulates CD8 TM-cell development after infection by modulating fatty acid metabolism.
Abstract: CD8 T cells, which have a crucial role in immunity to infection and cancer, are maintained in constant numbers, but on antigen stimulation undergo a developmental program characterized by distinct phases encompassing the expansion and then contraction of antigen-specific effector (T(E)) populations, followed by the persistence of long-lived memory (T(M)) cells. Although this predictable pattern of CD8 T-cell responses is well established, the underlying cellular mechanisms regulating the transition to T(M) cells remain undefined. Here we show that tumour necrosis factor (TNF) receptor-associated factor 6 (TRAF6), an adaptor protein in the TNF-receptor and interleukin-1R/Toll-like receptor superfamily, regulates CD8 T(M)-cell development after infection by modulating fatty acid metabolism. We show that mice with a T-cell-specific deletion of TRAF6 mount robust CD8 T(E)-cell responses, but have a profound defect in their ability to generate T(M) cells that is characterized by the disappearance of antigen-specific cells in the weeks after primary immunization. Microarray analyses revealed that TRAF6-deficient CD8 T cells exhibit altered expression of genes that regulate fatty acid metabolism. Consistent with this, activated CD8 T cells lacking TRAF6 display defective AMP-activated kinase activation and mitochondrial fatty acid oxidation (FAO) in response to growth factor withdrawal. Administration of the anti-diabetic drug metformin restored FAO and CD8 T(M)-cell generation in the absence of TRAF6. This treatment also increased CD8 T(M) cells in wild-type mice, and consequently was able to considerably improve the efficacy of an experimental anti-cancer vaccine.
TL;DR: Metabolic sensors such as AMPK and SIRT1, gatekeepers of the activity of the master regulator of mitochondria, PGC-1α, are vital links in a regulatory network for metabolic homeostasis and understanding the mechanisms by which they act could guide us to identify and improve preventive and therapeutic strategies for metabolic diseases.
Abstract: PURPOSE OF REVIEW: Peroxisome proliferator-activated receptor gamma coactivator-1-alpha (PGC-1alpha) has been extensively described as a master regulator of mitochondrial biogenesis. However, PGC-1alpha activity is not constant and can be finely tuned in response to different metabolic situations. From this point of view, PGC-1alpha could be described as a mediator of the transcriptional outputs triggered by metabolic sensors, providing the idea that these sensors, together with PGC-1alpha, might be weaving a network controlling cellular energy expenditure. In this review, we will focus on how disorders such as type 2 diabetes and the metabolic syndrome might be related to an abnormal and improper function of this network. RECENT FINDINGS: Two metabolic sensors, AMP-activated protein kinase (AMPK) and SIRT1 have been described to directly affect PGC-1alpha activity through phosphorylation and deacetylation, respectively. Although the physiological relevance of these modifications and their molecular consequences are still largely unknown, recent insight from different in-vivo transgenic models clearly suggests that AMPK, SIRT1 and PGC-1alpha might act as an orchestrated network to improve metabolic fitness. SUMMARY: Metabolic sensors such as AMPK and SIRT1, gatekeepers of the activity of the master regulator of mitochondria, PGC-1alpha, are vital links in a regulatory network for metabolic homeostasis. Together, these players explain many of the beneficial effects of physical activity and dietary interventions in our battle against type 2 diabetes and related metabolic disorders. Hence, understanding the mechanisms by which they act could guide us to identify and improve preventive and therapeutic strategies for metabolic diseases.
TL;DR: The Tie receptors and their angiopoietin (Ang) ligands have been identified as the second vascular tissue-specific receptor Tyr kinase system and provide unique insights into the functions of this vascular signalling system.
Abstract: Angiogenesis, the growth of blood vessels, is a fundamental biological process that controls embryonic development and is also involved in numerous life-threatening human diseases. Much work in the field of angiogenesis research has centred on the vascular endothelial growth factor (VEGF)-VEGF receptor system. The Tie receptors and their angiopoietin (Ang) ligands have been identified as the second vascular tissue-specific receptor Tyr kinase system. Ang-Tie signalling is essential during embryonic vessel assembly and maturation, and functions as a key regulator of adult vascular homeostasis. The structural characteristics and the spatio-temporal regulation of the expression of receptors and ligands provide unique insights into the functions of this vascular signalling system.
TL;DR: Research defining the signal transduction pathways induced by IL-17R family cytokines has lagged behind that of other cytokine families, but studies in the past 2 years have begun to delineate unusual functional motifs and new proximal signalling mediators used by the IL- 17R family to mediate downstream events.
Abstract: Interleukin-17A (IL-17A), the hallmark cytokine of the newly defined T helper 17 (T(H)17) cell subset, has important roles in protecting the host against extracellular pathogens, but also promotes inflammatory pathology in autoimmune disease. IL-17A and its receptor (IL-17RA) are the founding members of a newly described family of cytokines and receptors that have unique structural features which distinguish them from other cytokine families. Research defining the signal transduction pathways induced by IL-17R family cytokines has lagged behind that of other cytokine families, but studies in the past 2 years have begun to delineate unusual functional motifs and new proximal signalling mediators used by the IL-17R family to mediate downstream events.
TL;DR: A novel mTOR-interacting protein whose deregulated overexpression in multiple myeloma cells represents a mechanism for activating PI3K/Akt signaling and promoting cell survival.
TL;DR: A model for the cellular adaptation to the hypoxic tumour microenvironment that encompasses the interplay between COX-2, hypoxia-inducible factor 1 and dynamic switches in beta-catenin function that fine-tune signalling networks to meet the ever-changing demands of a tumour is proposed.
Abstract: It is widely accepted that alterations to cyclooxygenase-2 (COX-2) expression and the abundance of its enzymatic product prostaglandin E2 (PGE2) have key roles in influencing the development of colorectal cancer. Deregulation of the COX-2/PGE2 pathway appears to affect colorectal tumorigenesis via a number of distinct mechanisms: promoting tumour maintenance and progression, encouraging metastatic spread, and perhaps even participating in tumour initiation. Here, we review the role of COX-2/ PGE2 signalling in colorectal tumorigenesis and highlight its ability to influence the hallmarks of cancer-attributes defined by Hanahan and Weinberg as being requisite for tumorigenesis. In addition, we consider components of the COX-prostaglandin pathway emerging as important regulators of tumorigenesis; namely, the prostanoid (EP) receptors, 15-hydroxyprostaglandin dehydrogenase and the prostaglandin transporter. Finally, based on recent findings, we propose a model for the cellular adaptation to the hypoxic tumour microenvironment that encompasses the interplay between COX-2, hypoxia-inducible factor 1 and dynamic switches in β-catenin function that fine-tune signalling networks to meet the ever-changing demands of a tumour. © The Author 2009. Published by Oxford University Press. All rights reserved.
TL;DR: It is shown that interleukin-1 (IL-1) signaling on T cells is critically required for the early programming of Th 17 cell lineage and Th17 cell-mediated autoimmunity and this pathway may serve as a unique target for Th17cell-mediated immunopathology.
TL;DR: It is suggested that incorporation of the CD137 signaling domain in CARs should improve the persistence of CARs in the hematologic malignancies and hence maximize their antitumor activity.
TL;DR: Genetic and pharmacological tools to decrease the production of cytokines or to diminish their effects using cytokine antagonists could provide new approaches in the management of inflammatory vascular disease.
TL;DR: Current work showing cancer-relevant complexities in the regulation of PTEN and PI3K activity, potential novel functions for PTEN, and feedback regulation within the pathway are highlighted.
Abstract: PI3-kinase and PTEN are major positive and negative regulators, respectively, of the PI3-kinase pathway, which regulates growth, survival, and proliferation. These key signaling components are two of the most frequently mutated proteins in human cancers, resulting in unregulated activation of PI3K signaling and providing irrefutable genetic evidence of the central role of this pathway in tumorigenesis. PTEN regulates PI3K signaling by dephosphorylating the lipid signaling intermediate PIP3, but PTEN may have additional phosphatase-independent activities, as well as other functions in the nucleus. In this review, we highlight current work showing cancer-relevant complexities in the regulation of PTEN and PI3K activity, potential novel functions for PTEN, and feedback regulation within the pathway. The significance and complexity of PI3K signaling make it an important but challenging therapeutic target for cancer.
TL;DR: Intriguingly, the transmembrane receptor Tyr kinases Ror2 and Ryk, as well as Frizzled receptors that act independently of LRP5 or LRP6, function as receptors for Wnt and activate β-catenin-independent pathways, which leads to changes in cell movement and polarity and to the antagonism of the β- catenin pathway.
Abstract: The Wnt family of secreted ligands act through many receptors to stimulate distinct intracellular signalling pathways in embryonic development, in adults and in disease processes. Binding of Wnt to the Frizzled family of receptors and to low density lipoprotein receptor-related protein 5 (LRP5) or LRP6 co-receptors stimulates the intracellular Wnt-beta-catenin signalling pathway, which regulates beta-cateninstability and context-dependent transcription. This signalling pathway controls many processes, such as cell fate determination, cell proliferation and self-renewal of stem and progenitor cells. Intriguingly, the transmembrane receptor Tyr kinases Ror2 and Ryk, as well as Frizzledreceptors that act independently of LRP5 or LRP6, function as receptors for Wnt and activate beta-catenin-independent pathways. This leads to changes in cell movement and polarity and to the antagonism of the beta-catenin pathway.
TL;DR: It is shown in mice that deletion of ribosomal S6 protein kinase 1 (S6K1), a component of the nutrient-responsive mTOR (mammalian target of rapamycin) signaling pathway, led to increased life span and resistance to age-related pathologies, such as bone, immune, and motor dysfunction and loss of insulin sensitivity.
Abstract: Caloric restriction (CR) protects against aging and disease, but the mechanisms by which this affects mammalian life span are unclear. We show in mice that deletion of ribosomal S6 protein kinase 1 (S6K1), a component of the nutrient-responsive mTOR (mammalian target of rapamycin) signaling pathway, led to increased life span and resistance to age-related pathologies, such as bone, immune, and motor dysfunction and loss of insulin sensitivity. Deletion of S6K1 induced gene expression patterns similar to those seen in CR or with pharmacological activation of adenosine monophosphate (AMP)–activated protein kinase (AMPK), a conserved regulator of the metabolic response to CR. Our results demonstrate that S6K1 influences healthy mammalian life-span and suggest that therapeutic manipulation of S6K1 and AMPK might mimic CR and could provide broad protection against diseases of aging.
TL;DR: It is shown that naturally occurring differences in the levels or states of proteins regulating receptor-mediated apoptosis are the primary causes of cell-to-cell variability in the timing and probability of death in human cell lines.
Abstract: Noisy gene expression or unequal partition of molecules during cell division are increasingly recognized as key sources of non-genetic cell-to-cell heterogeneity but the consequences for disease progression and drug efficiency are little understood. Through single-cell imaging, Spencer et al. now show that pre-existing cell-to-cell differences in the levels of signalling proteins determine whether the addition of an external death signal will kill a cell by apoptosis or not — and how quickly it happens. The mechanism may explain the phenomenon of 'fractional killing', in which repeated rounds of chemotherapy kill some but not all cells in a tumour. From an evolutionary perspective, such systems-level phenotypic variation — not based on genetic or epigenetic modifications — offers wider adaptive potential to populations of living organisms. Noise in gene expression gives rise to cell-to-cell variability in protein concentrations and is increasingly recognized as a key source of non-genetic differences between cells. Through single cell imaging, it has now been possible to demonstrate that pre-existing differences in the levels of signalling proteins determine whether the addition of an external death signal will kill a cell or not—and how fast. This has implications for understanding 'fractional killing' of tumour cells after chemotherapy, in which some but not all tumour cells die. In microorganisms, noise in gene expression gives rise to cell-to-cell variability in protein concentrations1,2,3,4,5,6,7. In mammalian cells, protein levels also vary8,9,10 and individual cells differ widely in their responsiveness to uniform physiological stimuli11,12,13,14,15. In the case of apoptosis mediated by TRAIL (tumour necrosis factor (TNF)-related apoptosis-inducing ligand) it is common for some cells in a clonal population to die while others survive—a striking divergence in cell fate. Among cells that die, the time between TRAIL exposure and caspase activation is highly variable. Here we image sister cells expressing reporters of caspase activation and mitochondrial outer membrane permeabilization after exposure to TRAIL. We show that naturally occurring differences in the levels or states of proteins regulating receptor-mediated apoptosis are the primary causes of cell-to-cell variability in the timing and probability of death in human cell lines. Protein state is transmitted from mother to daughter, giving rise to transient heritability in fate, but protein synthesis promotes rapid divergence so that sister cells soon become no more similar to each other than pairs of cells chosen at random. Our results have implications for understanding ‘fractional killing’ of tumour cells after exposure to chemotherapy, and for variability in mammalian signal transduction in general.
TL;DR: This work shows that by combining the recently identified ABA receptor PYR1 with the type 2C protein phosphatase (PP2C) ABI1, the serine/threonine protein kinase SnRK2, and the transcription factor ABF2/AREB1, it can reconstitute ABA-triggered phosphorylation of the transcription factors in vitro and define a minimal set of core components of a complete major ABA signalling pathway.
Abstract: The phytohormone abscisic acid (ABA) plays an important role in several physiological responses such as stomatal conductance and seed dormancy and also in protecting plants against stress conditions such as drought and cold. Recently a family of proteins known as PYR/PYL/RCAR were identified as ABA receptors, but details of the signalling pathway downstream of ABA binding remained unclear. In this paper, Zhu and colleagues reconstitute the ABA-mediated signalling in vitro and test key observations in vivo. This is the first reported reconstitution of a plant hormone signalling pathway. The plant hormone abscisic acid (ABA) is a regulator of plant growth, development and responses to environmental stresses. Although several proteins have been reported to function as ABA receptors and many more are known to be involved in ABA signalling, the identities of ABA receptors remain controversial and the mechanism of signalling unclear. ABA-mediated signalling is now reconstituted in vitro, defining a minimal set of core components of the pathway. The phytohormone abscisic acid (ABA) regulates the expression of many genes in plants; it has critical functions in stress resistance and in growth and development1,2,3,4,5,6,7. Several proteins have been reported to function as ABA receptors8,9,10,11,12,13, and many more are known to be involved in ABA signalling3,4,14. However, the identities of ABA receptors remain controversial and the mechanism of signalling from perception to downstream gene expression is unclear15,16. Here we show that by combining the recently identified ABA receptor PYR1 with the type 2C protein phosphatase (PP2C) ABI1, the serine/threonine protein kinase SnRK2.6/OST1 and the transcription factor ABF2/AREB1, we can reconstitute ABA-triggered phosphorylation of the transcription factor in vitro. Introduction of these four components into plant protoplasts results in ABA-responsive gene expression. Protoplast and test-tube reconstitution assays were used to test the function of various members of the receptor, protein phosphatase and kinase families. Our results suggest that the default state of the SnRK2 kinases is an autophosphorylated, active state and that the SnRK2 kinases are kept inactive by the PP2Cs through physical interaction and dephosphorylation. We found that in the presence of ABA, the PYR/PYL (pyrabactin resistance 1/PYR1-like) receptor proteins can disrupt the interaction between the SnRK2s and PP2Cs, thus preventing the PP2C-mediated dephosphorylation of the SnRK2s and resulting in the activation of the SnRK2 kinases. Our results reveal new insights into ABA signalling mechanisms and define a minimal set of core components of a complete major ABA signalling pathway.