Hot off the presses! Apr 15 Cell

The Apr 15 issue of the Cell is now up on Pubget (About Cell): if you're at a subscribing institution, just click the link in the latest link at the home page. (Note you'll only be able to get all the PDFs in the issue if your institution subscribes to Pubget.)

Latest Articles Include:

• In This Issue
  - cell 145(2):163, 165 (2011)
  
• Royal Wedding
  - cell 145(2):167, 169 (2011)
  On Friday the 29th of April, 2011, England's Prince William will marry the graceful and poised Miss Catherine Middleton in London's hallowed Westminster Abbey. As the world tunes in to watch the "wedding of the century," Cell Culture explores the more biological aspects of this historic union, including the neurocircuits that strengthen a marriage, the epigenetic changes that transform a "commoner" into a queen, and the search process for finding a high-affinity partner in a sea of weak interactions.
• FEN Nucleases: Bind, Bend, Fray, Cut
  - cell 145(2):171-172 (2011)
  In this issue, Orans et al. (2011) and Tsutakawa et al. (2011) report exciting insights into the molecular principles governing diverse endo- and exonucleolytic cleavage specificities of members of the RAD2/FEN superfamily of nucleases, which have critical roles in DNA replication and maintenance.
• Argonaute10 as a miRNA Locker
  - cell 145(2):173-174 (2011)
  The stability and translation efficiency of many messenger RNAs is regulated by microRNAs (miRNAs), which exert their effects through associated Argonaute proteins. In this issue, Zhu, Zhang, and colleagues reveal that plants also exploit miRNA binding by Argonautes as a sequestering mechanism that prevents miRNAs from fulfilling their normal roles.
• An Exciting Calcium Sensor for Smell
  - cell 145(2):175-177 (2011)
  Formation of sensory maps within the olfactory bulb depends on insulin-like growth factor (IGF) signaling. Cao et al. (2011) now show that neuronal IGF secretion is regulated by neural activity through the calcium sensor synaptotagmin-10 and is required in the olfactory bulb for the sensation of smell.
• No-Nonsense Functions for Long Noncoding RNAs
  - cell 145(2):178-181 (2011)
  The mysterious secrets of long noncoding RNAs, often referred to as the Dark Matter of the genome, are gradually coming to light. Several recent papers dig deep to reveal surprisingly complex and diverse functions of these enigmatic molecules.
• Wdr5 Mediates Self-Renewal and Reprogramming via the Embryonic Stem Cell Core Transcriptional Network
  - cell 145(2):183-197 (2011)
  The embryonic stem (ES) cell transcriptional and chromatin-modifying networks are critical for self-renewal maintenance. However, it remains unclear whether these networks functionally interact and, if so, what factors mediate such interactions. Here, we show that WD repeat domain 5 (Wdr5), a core member of the mammalian Trithorax (trxG) complex, positively correlates with the undifferentiated state and is a regulator of ES cell self-renewal. We demonstrate that Wdr5, an "effector" of H3K4 methylation, interacts with the pluripotency transcription factor Oct4. Genome-wide protein localization and transcriptome analyses demonstrate overlapping gene regulatory functions between Oct4 and Wdr5. The Oct4-Sox2-Nanog circuitry and trxG cooperate in activating transcription of key self-renewal regulators, and furthermore, Wdr5 expression is required for the efficient formation of induced pluripotent stem (iPS) cells. We propose an integrated model of transcriptional and ep! igenetic control, mediated by select trxG members, for the maintenance of ES cell self-renewal and somatic cell reprogramming.
• Human Flap Endonuclease Structures, DNA Double-Base Flipping, and a Unified Understanding of the FEN1 Superfamily
  - cell 145(2):198-211 (2011)
  Flap endonuclease (FEN1), essential for DNA replication and repair, removes RNA and DNA 5′ flaps. FEN1 5′ nuclease superfamily members acting in nucleotide excision repair (XPG), mismatch repair (EXO1), and homologous recombination (GEN1) paradoxically incise structurally distinct bubbles, ends, or Holliday junctions, respectively. Here, structural and functional analyses of human FEN1:DNA complexes show structure-specific, sequence-independent recognition for nicked dsDNA bent 100° with unpaired 3′ and 5′ flaps. Above the active site, a helical cap over a gateway formed by two helices enforces ssDNA threading and specificity for free 5′ ends. Crystallographic analyses of product and substrate complexes reveal that dsDNA binding and bending, the ssDNA gateway, and double-base unpairing flanking the scissile phosphate control precise flap incision by the two-metal-ion active site. Superfamily conserved motifs bind and open dsDNA; direct the target region into! the helical gateway, permitting only nonbase-paired oligonucleotides active site access; and support a unified understanding of superfamily substrate specificity.
• Structures of Human Exonuclease 1 DNA Complexes Suggest a Unified Mechanism for Nuclease Family
  - cell 145(2):212-223 (2011)
  Human exonuclease 1 (hExo1) plays important roles in DNA repair and recombination processes that maintain genomic integrity. It is a member of the 5′ structure-specific nuclease family of exonucleases and endonucleases that includes FEN-1, XPG, and GEN1. We present structures of hExo1 in complex with a DNA substrate, followed by mutagenesis studies, and propose a common mechanism by which this nuclease family recognizes and processes diverse DNA structures. hExo1 induces a sharp bend in the DNA at nicks or gaps. Frayed 5′ ends of nicked duplexes resemble flap junctions, unifying the mechanisms of endo- and exonucleolytic processing. Conformational control of a mobile region in the catalytic site suggests a mechanism for allosteric regulation by binding to protein partners. The relative arrangement of substrate binding sites in these enzymes provides an elegant solution to a complex geometrical puzzle of substrate recognition and processing.
• Widespread Negative Response Elements Mediate Direct Repression by Agonist- Liganded Glucocorticoid Receptor
  - cell 145(2):224-241 (2011)
  The glucocorticoid (GC) receptor (GR), when liganded to GC, activates transcription through direct binding to simple (+)GRE DNA binding sequences (DBS). GC-induced direct repression via GR binding to complex "negative" GREs (nGREs) has been reported. However, GR-mediated transrepression was generally ascribed to indirect "tethered" interaction with other DNA-bound factors. We report that GC-induces direct transrepression via the binding of GR to simple DBS (IR nGREs) unrelated to (+)GRE. These DBS act on agonist-liganded GR, promoting the assembly of cis-acting GR-SMRT/NCoR repressing complexes. IR nGREs are present in over 1000 mouse/human ortholog genes, which are repressed by GC in vivo. Thus variations in the levels of a single ligand can coordinately turn genes on or off depending in their response element DBS, allowing an additional level of regulation in GR signaling. This mechanism suits GR signaling remarkably well, given that adrenal secretion of GC f! luctuates in a circadian and stress-related fashion.
• Arabidopsis Argonaute10 Specifically Sequesters miR166/165 to Regulate Shoot Apical Meristem Development
  - cell 145(2):242-256 (2011)
  The shoot apical meristem (SAM) comprises a group of undifferentiated cells that divide to maintain the plant meristem and also give rise to all shoot organs. SAM fate is specified by class III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) transcription factors, which are targets of miR166/165. In Arabidopsis, AGO10 is a critical regulator of SAM maintenance, and here we demonstrate that AGO10 specifically interacts with miR166/165. The association is determined by a distinct structure of the miR166/165 duplex. Deficient loading of miR166 into AGO10 results in a defective SAM. Notably, the miRNA-binding ability of AGO10, but not its catalytic activity, is required for SAM development, and AGO10 has a higher binding affinity for miR166 than does AGO1, a principal contributor to miRNA-mediated silencing. We propose that AGO10 functions as a decoy for miR166/165 to maintain the SAM, preventing their incorporation into AGO1 complexes and the subsequent repression of HD-ZIP III g! ene expression. PaperFlick To view the video inline, enable JavaScript on your browser. However, you can download and view the video by clicking on the icon below Download this Video (9011 K)
• Single-Molecule Protein Unfolding and Translocation by an ATP-Fueled Proteolytic Machine
  - cell 145(2):257-267 (2011)
  All cells employ ATP-powered proteases for protein-quality control and regulation. In the ClpXP protease, ClpX is a AAA+ machine that recognizes specific protein substrates, unfolds these molecules, and then translocates the denatured polypeptide through a central pore and into ClpP for degradation. Here, we use optical-trapping nanometry to probe the mechanics of enzymatic unfolding and translocation of single molecules of a multidomain substrate. Our experiments demonstrate the capacity of ClpXP and ClpX to perform mechanical work under load, reveal very fast and highly cooperative unfolding of individual substrate domains, suggest a translocation step size of 5–8 amino acids, and support a power-stroke model of denaturation in which successful enzyme-mediated unfolding of stable domains requires coincidence between mechanical pulling by the enzyme and a transient stochastic reduction in protein stability. We anticipate that single-molecule studies of the mechanica! l properties of other AAA+ proteolytic machines will reveal many shared features with ClpXP.
• A Genome-wide Multidimensional RNAi Screen Reveals Pathways Controlling MHC Class II Antigen Presentation
  - cell 145(2):268-283 (2011)
  MHC class II molecules (MHC-II) present peptides to T helper cells to facilitate immune responses and are strongly linked to autoimmune diseases. To unravel processes controlling MHC-II antigen presentation, we performed a genome-wide flow cytometry-based RNAi screen detecting MHC-II expression and peptide loading followed by additional high-throughput assays. All data sets were integrated to answer two fundamental questions: what regulates tissue-specific MHC-II transcription, and what controls MHC-II transport in dendritic cells? MHC-II transcription was controlled by nine regulators acting in feedback networks with higher-order control by signaling pathways, including TGFβ. MHC-II transport was controlled by the GTPase ARL14/ARF7, which recruits the motor myosin 1E via an effector protein ARF7EP. This complex controls movement of MHC-II vesicles along the actin cytoskeleton in human dendritic cells (DCs). These genome-wide systems analyses have thus identified fact! ors and pathways controlling MHC-II transcription and transport, defining targets for manipulation of MHC-II antigen presentation in infection and autoimmunity.
• The AAA+ ATPase Thorase Regulates AMPA Receptor-Dependent Synaptic Plasticity and Behavior
  - cell 145(2):284-299 (2011)
  The synaptic insertion or removal of AMPA receptors (AMPAR) plays critical roles in the regulation of synaptic activity reflected in the expression of long-term potentiation (LTP) and long-term depression (LTD). The cellular events underlying this important process in learning and memory are still being revealed. Here we describe and characterize the AAA+ ATPase Thorase, which regulates the expression of surface AMPAR. In an ATPase-dependent manner Thorase mediates the internalization of AMPAR by disassembling the AMPAR-GRIP1 complex. Following genetic deletion of Thorase, the internalization of AMPAR is substantially reduced, leading to increased amplitudes of miniature excitatory postsynaptic currents, enhancement of LTP, and elimination of LTD. These molecular events are expressed as deficits in learning and memory in Thorase null mice. This study identifies an AAA+ ATPase that plays a critical role in regulating the surface expression of AMPAR and thereby regulates! synaptic plasticity and learning and memory.
• Activity-Dependent IGF-1 Exocytosis Is Controlled by the Ca2+-Sensor Synaptotagmin-10
  - cell 145(2):300-311 (2011)
  Synaptotagmins Syt1, Syt2, Syt7, and Syt9 act as Ca2+-sensors for synaptic and neuroendocrine exocytosis, but the function of other synaptotagmins remains unknown. Here, we show that olfactory bulb neurons secrete IGF-1 by an activity-dependent pathway of exocytosis, and that Syt10 functions as the Ca2+-sensor that triggers IGF-1 exocytosis in these neurons. Deletion of Syt10 impaired activity-dependent IGF-1 secretion in olfactory bulb neurons, resulting in smaller neurons and an overall decrease in synapse numbers. Exogenous IGF-1 completely reversed the Syt10 knockout phenotype. Syt10 colocalized with IGF-1 in somatodendritic vesicles of olfactory bulb neurons, and Ca2+-binding to Syt10 caused these vesicles to undergo exocytosis, thereby secreting IGF-1. Thus, Syt10 controls a previously unrecognized pathway of Ca2+-dependent exocytosis that is spatially and temporally distinct from Ca2+-dependent synaptic vesicle exocytosis controlled by Syt1. Our findings thereby! reveal that two different synaptotagmins can regulate functionally distinct Ca2+-dependent membrane fusion reactions in the same neuron.
• Thermal Robustness of Signaling in Bacterial Chemotaxis
  - cell 145(2):312-321 (2011)
  Temperature is a global factor that affects the performance of all intracellular networks. Robustness against temperature variations is thus expected to be an essential network property, particularly in organisms without inherent temperature control. Here, we combine experimental analyses with computational modeling to investigate thermal robustness of signaling in chemotaxis of Escherichia coli, a relatively simple and well-established model for systems biology. We show that steady-state and kinetic pathway parameters that are essential for chemotactic performance are indeed temperature-compensated in the entire physiological range. Thermal robustness of steady-state pathway output is ensured at several levels by mutual compensation of temperature effects on activities of individual pathway components. Moreover, the effect of temperature on adaptation kinetics is counterbalanced by preprogrammed temperature dependence of enzyme synthesis and stability to achieve nearl! y optimal performance at the growth temperature. Similar compensatory mechanisms are expected to ensure thermal robustness in other systems. PaperClip To listen to this audio, enable JavaScript on your browser. However, you can download and play the audio by clicking on the icon below Download this Audio (4733 K)
• Autophagy Suppresses Tumorigenesis through Elimination of p62
  - cell 145(2):322 (2011)
  
• SnapShot: Nonsense-Mediated mRNA Decay
  - cell 145(2):324-324.e2 (2011)