Latest Articles Include:
- In This Issue
- Cell 147(4):707,709 (2011)
- Mitochondria: Form and Function
- Cell 147(4):711,713 (2011)
Famous as "powerhouses of the cell," mitochondria also trigger apoptosis, regulate cell growth, and integrate stress responses. This issue's Select highlights recent studies that investigate mitochondrial form and function, providing new insight into how these organelles change their numbers, remodel their shapes, and divide their membranes.
- Derepressing Nuclear Receptors for Metabolic Adaptation
- Cell 147(4):717-718 (2011)
NCoR is a corepressor of several transcription factors, including the PPAR family of nuclear receptors in fat and muscle. By specifically deleting NCoR in these tissues, Li et al. and Yamamoto et al. now uncover an important role for NCoR in regulating lipid homeostasis through the coordinated control of different nuclear receptors.
- New Driver for Lipid Synthesis
- Cell 147(4):719-721 (2011)
Cholesterol regulates activation of sterol regulatory element-binding protein (SREBP) through a classic feedback loop. Walker et al. (2011) extend the regulatory inputs governing SREBP activity to include an independent loop modulated by phosphatidylcholine (PC) and cellular methylation capacity. These findings suggest a link between lipid synthesis and cellular pathways involved in methylation.
- PINK1 and Parkin Flag Miro to Direct Mitochondrial Traffic
- Cell 147(4):721-723 (2011)
The Parkinson's disease proteins PINK1 and Parkin are proposed guardians of mitochondrial fidelity, targeting damaged mitochondria for degradation by mitophagy. In this issue of Cell, Wang et al. (2011) now show that PINK1 and Parkin also regulate mitochondrial trafficking and quarantine damaged mitochondria by severing their connection to the microtubule network.
- Feedback on Fat: p62-mTORC1-Autophagy Connections
- Cell 147(4):724-727 (2011)
Metabolic homeostasis requires integration of multiple signals and cellular activities. Without this integration, conditions of obesity and diabetes often develop. Recent in vivo studies explore the molecular basis for metabolic homestasis, showing that p62 links autophagy and mTORC1 activation to regulate adipogenesis and energy control.
- Autophagy: Renovation of Cells and Tissues
- Cell 147(4):728-741 (2011)
Autophagy is the major intracellular degradation system by which cytoplasmic materials are delivered to and degraded in the lysosome. However, the purpose of autophagy is not the simple elimination of materials, but instead, autophagy serves as a dynamic recycling system that produces new building blocks and energy for cellular renovation and homeostasis. Here we provide a multidisciplinary review of our current understanding of autophagy's role in metabolic adaptation, intracellular quality control, and renovation during development and differentiation. We also explore how recent mouse models in combination with advances in human genetics are providing key insights into how the impairment or activation of autophagy contributes to pathogenesis of diverse diseases, from neurodegenerative diseases such as Parkinson disease to inflammatory disorders such as Crohn disease.
- Programmed Cell Death in Animal Development and Disease
- Cell 147(4):742-758 (2011)
Programmed cell death (PCD) plays a fundamental role in animal development and tissue homeostasis. Abnormal regulation of this process is associated with a wide variety of human diseases, including immunological and developmental disorders, neurodegeneration, and cancer. Here, we provide a brief historical overview of the field and reflect on the regulation, roles, and modes of PCD during animal development. We also discuss the function and regulation of apoptotic proteins, including caspases, the key executioners of apoptosis, and review the nonlethal functions of these proteins in diverse developmental processes, such as cell differentiation and tissue remodeling. Finally, we explore a growing body of work about the connections between apoptosis, stem cells, and cancer, focusing on how apoptotic cells release a variety of signals to communicate with their cellular environment, including factors that promote cell division, tissue regeneration, and wound healing.
- The Hippo Transducer TAZ Confers Cancer Stem Cell-Related Traits on Breast Cancer Cells
- Cell 147(4):759-772 (2011)
Cancer stem cells (CSCs) are proposed to drive tumor initiation and progression. Yet, our understanding of the cellular and molecular mechanisms that underlie CSC properties is limited. Here we show that the activity of TAZ, a transducer of the Hippo pathway, is required to sustain self-renewal and tumor-initiation capacities in breast CSCs. TAZ protein levels and activity are elevated in prospective CSCs and in poorly differentiated human tumors and have prognostic value. Gain of TAZ endows self-renewal capacity to non-CSCs. In epithelial cells, TAZ forms a complex with the cell-polarity determinant Scribble, and loss of Scribble—or induction of the epithelial-mesenchymal transition (EMT)—disrupts the inhibitory association of TAZ with the core Hippo kinases MST and LATS. This study links the CSC concept to the Hippo pathway in breast cancer and reveals a mechanistic basis of the control of Hippo kinases by cell polarity.
- ncRNA- and Pc2 Methylation-Dependent Gene Relocation between Nuclear Structures Mediates Gene Activation Programs
- Cell 147(4):773-788 (2011)
Although eukaryotic nuclei contain distinct architectural structures associated with noncoding RNAs (ncRNAs), their potential relationship to regulated transcriptional programs remains poorly understood. Here, we report that methylation/demethylation of Polycomb 2 protein (Pc2) controls relocation of growth-control genes between Polycomb bodies (PcGs) and interchromatin granules (ICGs) in response to growth signals. This movement is the consequence of binding of methylated and unmethylated Pc2 to the ncRNAs TUG1 and MALAT1/NEAT2, located in PcGs and ICGs, respectively. These ncRNAs mediate assembly of multiple corepressors/coactivators and can serve to switch mark recognition by "readers" of the histone code. Additionally, binding of NEAT2 to unmethylated Pc2 promotes E2F1 SUMOylation, leading to activation of the growth-control gene program. These observations delineate a molecular pathway linking the actions of subnuclear structure-specific ncRNAs and nonhistone ! protein methylation to relocation of transcription units in the three-dimensional space of the nucleus, thus achieving coordinated gene expression programs. PaperClip View Within Article
- Ribosome Profiling of Mouse Embryonic Stem Cells Reveals the Complexity and Dynamics of Mammalian Proteomes
- Cell 147(4):789-802 (2011)
The ability to sequence genomes has far outstripped approaches for deciphering the information they encode. Here we present a suite of techniques, based on ribosome profiling (the deep sequencing of ribosome-protected mRNA fragments), to provide genome-wide maps of protein synthesis as well as a pulse-chase strategy for determining rates of translation elongation. We exploit the propensity of harringtonine to cause ribosomes to accumulate at sites of translation initiation together with a machine learning algorithm to define protein products systematically. Analysis of translation in mouse embryonic stem cells reveals thousands of strong pause sites and unannotated translation products. These include amino-terminal extensions and truncations and upstream open reading frames with regulatory potential, initiated at both AUG and non-AUG codons, whose translation changes after differentiation. We also define a class of short, polycistronic ribosome-associated coding RNAs (! sprcRNAs) that encode small proteins. Our studies reveal an unanticipated complexity to mammalian proteomes.
- A Quantitative Model for Ordered Cdk Substrate Dephosphorylation during Mitotic Exit
- Cell 147(4):803-814 (2011)
After sister chromatid splitting at anaphase onset, exit from mitosis comprises an ordered series of events. Dephosphorylation of numerous mitotic substrates, which were phosphorylated by cyclin-dependent kinase (Cdk), is thought to bring about mitotic exit, but how temporal ordering of mitotic exit events is achieved is poorly understood. Here, we show, using budding yeast, that dephosphorylation of Cdk substrates involved in sequential mitotic exit events occurs with ordered timing. We test different models of how ordering might be achieved by modulating Cdk and Cdk-counteracting phosphatase Cdc14 activities in vivo, as well as by kinetic analysis of Cdk substrate phosphorylation and dephosphorylation in vitro. Our results suggest that the gradual change of the phosphatase to kinase ratio over the course of mitotic exit is read out by Cdk substrates that respond by dephosphorylation at distinct thresholds. This provides an example and a mechanistic explanation for a ! quantitative model of cell-cycle progression.
- Adipocyte NCoR Knockout Decreases PPARÎ³ Phosphorylation and Enhances PPARÎ³ Activity and Insulin Sensitivity
- Cell 147(4):815-826 (2011)
Insulin resistance, tissue inflammation, and adipose tissue dysfunction are features of obesity and Type 2 diabetes. We generated adipocyte-specific Nuclear Receptor Corepressor (NCoR) knockout (AKO) mice to investigate the function of NCoR in adipocyte biology, glucose and insulin homeostasis. Despite increased obesity, glucose tolerance was improved in AKO mice, and clamp studies demonstrated enhanced insulin sensitivity in liver, muscle, and fat. Adipose tissue macrophage infiltration and inflammation were also decreased. PPARÎ³ response genes were upregulated in adipose tissue from AKO mice and CDK5-mediated PPARÎ³ ser-273 phosphorylation was reduced, creating a constitutively active PPARÎ³ state. This identifies NCoR as an adaptor protein that enhances the ability of CDK5 to associate with and phosphorylate PPARÎ³. The dominant function of adipocyte NCoR is to transrepress PPARÎ³ and promote PPARÎ³ ser-273 phosphorylation, such that NCoR deletion leads! to adipogenesis, reduced inflammation, and enhanced systemic insulin sensitivity, phenocopying the TZD-treated state.
- NCoR1 Is a Conserved Physiological Modulator of Muscle Mass and Oxidative Function
- Cell 147(4):827-839 (2011)
Transcriptional coregulators control the activity of many transcription factors and are thought to have wide-ranging effects on gene expression patterns. We show here that muscle-specific loss of nuclear receptor corepressor 1 (NCoR1) in mice leads to enhanced exercise endurance due to an increase of both muscle mass and of mitochondrial number and activity. The activation of selected transcription factors that control muscle function, such as MEF2, PPARβ/δ, and ERRs, underpins these phenotypic alterations. NCoR1 levels are decreased in conditions that require fat oxidation, resetting transcriptional programs to boost oxidative metabolism. Knockdown of gei-8, the sole C. elegans NCoR homolog, also robustly increased muscle mitochondria and respiration, suggesting conservation of NCoR1 function. Collectively, our data suggest that NCoR1 plays an adaptive role in muscle physiology and that interference with NCoR1 action could be used to improve muscle function.
- A Conserved SREBP-1/Phosphatidylcholine Feedback Circuit Regulates Lipogenesis in Metazoans
- Cell 147(4):840-852 (2011)
Sterol regulatory element-binding proteins (SREBPs) activate genes involved in the synthesis and trafficking of cholesterol and other lipids and are critical for maintaining lipid homeostasis. Aberrant SREBP activity, however, can contribute to obesity, fatty liver disease, and insulin resistance, hallmarks of metabolic syndrome. Our studies identify a conserved regulatory circuit in which SREBP-1 controls genes in the one-carbon cycle, which produces the methyl donor S-adenosylmethionine (SAMe). Methylation is critical for the synthesis of phosphatidylcholine (PC), a major membrane component, and we find that blocking SAMe or PC synthesis in C. elegans, mouse liver, and human cells causes elevated SREBP-1-dependent transcription and lipid droplet accumulation. Distinct from negative regulation of SREBP-2 by cholesterol, our data suggest a feedback mechanism whereby maturation of nuclear, transcriptionally active SREBP-1 is controlled by levels of PC. Thus, nutritional! or genetic conditions limiting SAMe or PC production may activate SREBP-1, contributing to human metabolic disorders.
- Systematic Discovery of TLR Signaling Components Delineates Viral-Sensing Circuits
- Cell 147(4):853-867 (2011)
Deciphering the signaling networks that underlie normal and disease processes remains a major challenge. Here, we report the discovery of signaling components involved in the Toll-like receptor (TLR) response of immune dendritic cells (DCs), including a previously unkown pathway shared across mammalian antiviral responses. By combining transcriptional profiling, genetic and small-molecule perturbations, and phosphoproteomics, we uncover 35 signaling regulators, including 16 known regulators, involved in TLR signaling. In particular, we find that Polo-like kinases (Plk) 2 and 4 are essential components of antiviral pathways in vitro and in vivo and activate a signaling branch involving a dozen proteins, among which is Tnfaip2, a gene associated with autoimmune diseases but whose role was unknown. Our study illustrates the power of combining systematic measurements and perturbations to elucidate complex signaling circuits and discover potential therapeutic targets.
- CD14 Controls the LPS-Induced Endocytosis of Toll-like Receptor 4
- Cell 147(4):868-880 (2011)
The transport of Toll-like Receptors (TLRs) to various organelles has emerged as an essential means by which innate immunity is regulated. While most of our knowledge is restricted to regulators that promote the transport of newly synthesized receptors, the regulators that control TLR transport after microbial detection remain unknown. Here, we report that the plasma membrane localized Pattern Recognition Receptor (PRR) CD14 is required for the microbe-induced endocytosis of TLR4. In dendritic cells, this CD14-dependent endocytosis pathway is upregulated upon exposure to inflammatory mediators. We identify the tyrosine kinase Syk and its downstream effector PLCγ2 as important regulators of TLR4 endocytosis and signaling. These data establish that upon microbial detection, an upstream PRR (CD14) controls the trafficking and signaling functions of a downstream PRR (TLR4). This innate immune trafficking cascade illustrates how pathogen detection systems operate to induce! both membrane transport and signal transduction.
- A Network of Broadly Expressed HLH Genes Regulates Tissue-Specific Cell Fates
- Cell 147(4):881-892 (2011)
Spatial and temporal expression of specific basic-helix-loop-helix (bHLH) transcription factors defines many types of cellular differentiation. We find that a distinct mechanism regulates the much broader expression of the heterodimer partners of these specific factors and impinges on differentiation. In Drosophila, a cross-interacting regulatory network links expression of the E protein Daughterless (Da), which heterodimerizes with bHLH proteins to activate them, with expression of the Id protein Extramacrochaetae (Emc), which antagonizes bHLH proteins. Coupled transcriptional feedback loops maintain the widespread Emc expression that restrains Da expression, opposing bHLH-dependent differentiation while enhancing growth and cell survival. Where extracellular signals repress emc, Da expression can increase. This defines regions of proneural ectoderm independently from the proneural bHLH genes. Similar regulation is found in multiple Drosophila tissues and in mammalian! cells and therefore is likely to be a conserved general feature of developmental regulation by HLH proteins.
- PINK1 and Parkin Target Miro for Phosphorylation and Degradation to Arrest Mitochondrial Motility
- Cell 147(4):893-906 (2011)
Cells keep their energy balance and avoid oxidative stress by regulating mitochondrial movement, distribution, and clearance. We report here that two Parkinson's disease proteins, the Ser/Thr kinase PINK1 and ubiquitin ligase Parkin, participate in this regulation by arresting mitochondrial movement. PINK1 phosphorylates Miro, a component of the primary motor/adaptor complex that anchors kinesin to the mitochondrial surface. The phosphorylation of Miro activates proteasomal degradation of Miro in a Parkin-dependent manner. Removal of Miro from the mitochondrion also detaches kinesin from its surface. By preventing mitochondrial movement, the PINK1/Parkin pathway may quarantine damaged mitochondria prior to their clearance. PINK1 has been shown to act upstream of Parkin, but the mechanism corresponding to this relationship has not been known. We propose that PINK1 phosphorylation of substrates triggers the subsequent action of Parkin and the proteasome. PaperFlick View Within Article
- Regulation of the Probability of Mouse Odorant Receptor Gene Choice
- Cell 147(4):907-921 (2011)
Each olfactory sensory neuron (OSN) in mouse chooses one of 1,200 odorant receptor (OR) genes for expression. OR genes are chosen for expression by greatly varying numbers of OSNs. The mechanisms that regulate the probability of OR gene choice remain unclear. Here, we have applied the NanoString platform of fluorescent barcodes and digital readout to measure RNA levels of 577 OR genes in a single reaction, with probes designed against coding sequences. In an inbred mouse strain with a targeted deletion in the P element, we find that this element regulates OR gene choice differentially across its cluster of 24 OR genes. Importantly, the fold changes of NanoString counts in Î"P or Î"H mice are in very close agreement with the fold changes of cell counts, determined by in situ hybridization. Thus, the P and H elements regulate the probability of OR gene choice, not OR transcript level per OSN.
- The Neural Circuits and Synaptic Mechanisms Underlying Motor Initiation in C.Â elegans
- Cell 147(4):922-933 (2011)
C. elegans is widely used to dissect how neural circuits and genes generate behavior. During locomotion, worms initiate backward movement to change locomotion direction spontaneously or in response to sensory cues; however, the underlying neural circuits are not well defined. We applied a multidisciplinary approach to map neural circuits in freely behaving worms by integrating functional imaging, optogenetic interrogation, genetic manipulation, laser ablation, and electrophysiology. We found that a disinhibitory circuit and a stimulatory circuit together promote initiation of backward movement and that circuitry dynamics is differentially regulated by sensory cues. Both circuits require glutamatergic transmission but depend on distinct glutamate receptors. This dual mode of motor initiation control is found in mammals, suggesting that distantly related organisms with anatomically distinct nervous systems may adopt similar strategies for motor control. Additionally, our! studies illustrate how a multidisciplinary approach facilitates dissection of circuit and synaptic mechanisms underlying behavior in a genetic model organism.
- A Mechanism for the Evolution of Phosphorylation Sites
- Cell 147(4):934-946 (2011)
Protein phosphorylation provides a mechanism for the rapid, reversible control of protein function. Phosphorylation adds negative charge to amino acid side chains, and negatively charged amino acids (Asp/Glu) can sometimes mimic the phosphorylated state of a protein. Using a comparative genomics approach, we show that nature also employs this trick in reverse by evolving serine, threonine, and tyrosine phosphorylation sites from Asp/Glu residues. Structures of three proteins where phosphosites evolved from acidic residues (DNA topoisomerase II, enolase, and C-Raf) show that the relevant acidic residues are present in salt bridges with conserved basic residues, and that phosphorylation has the potential to conditionally restore the salt bridges. The evolution of phosphorylation sites from glutamate and aspartate provides a rationale for why phosphorylation sometimes activates proteins, and helps explain the origins of this important and complex process.
- A Long Noncoding RNA Controls Muscle Differentiation by Functioning as a Competing Endogenous RNA
- Cell 147(4):947 (2011)
- InÂ Vivo Identification of Tumor- Suppressive PTEN ceRNAs in an Oncogenic BRAF-Induced Mouse Model of Melanoma
- Cell 147(4):948 (2011)
- Molecular Mechanisms of Fear Learning and Memory
- Cell 147(4):948 (2011)
- SnapShot: Mitochondrial Quality Control
- Cell 147(4):950-950.e1 (2011)