Hot off the presses! Sep 18 Cell

The Sep 18 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 138(6):1035, 1037 (2009)
  
• Metabolism Select
  - Cell 138(6):1039, 1041 (2009)
  The role of adipose tissue, or fat, in the development and treatment of metabolic disease is the subject of this issue's Select. A cluster of new studies in mice suggest that dysregulation of lymphocytes and other inflammatory cells in adipose tissue contributes to obesity and metabolic diseases such as type 2 diabetes. Elucidation of how the inflammatory response affects different types of adipose tissue may shed light on new strategies for treating diseases associated with obesity.
• New Citizens for the Life Sciences
  - Cell 138(6):1043-1045 (2009)
  For years, the general public have been helping scientists to track weather, count birds, and observe the universe. Now such citizen scientists are starting to tackle research projects in the life sciences. Laura Bonetta reports.
• Reprogramming to Pluripotency: From Frogs to Stem Cells
  - Cell 138(6):1047-1050 (2009)
  This year's Albert Lasker Basic Medical Research Award goes to John Gurdon and Shinya Yamanaka for their contributions to our understanding of how to reprogram adult cells back to early embryonic states.
• Attacking Cancer at Its Root
  - Cell 138(6):1051-1054 (2009)
  This year the Lasker DeBakey Clinical Research Award will be shared by Brian Druker, Nicholas Lydon, and Charles Sawyers for their development of a targeted molecular therapy for treating chronic myeloid leukemia. Their work demonstrated the ability of drugs directed against cancer-causing oncogenes to turn a rapidly fatal malignancy into a manageable chronic disease.
• Structural Ties between Cholesterol Transport and Morphogen Signaling
  - Cell 138(6):1055-1056 (2009)
  
• Transposon Silencing by piRNAs
  - Cell 138(6):1058-1060 (2009)
  In animal cells, small RNA molecules, called piRNAs, defend the genome against selfish DNA elements such as transposons. In this issue, Klattenhoff et al. (2009) report that an HP1 family protein, Rhino, is required for piRNA generation and transposon silencing in Drosophila germline cells. The results provide a link between heterochromatin and piRNA-mediated genome defense.
• p53: A New Kingpin in the Stem Cell Arena
  - Cell 138(6):1060-1062 (2009)
  Identifying new regulators of the stem cell state offers potential for future gains in biomedicine. Evidence that the tumor suppressor p53 is a key regulator of the stem cell state (Cicalese et al., 2009) suggests a broad role for this protein and its pathways in the control of normal tissue homeostasis and tumor formation.
• Reducing the Mystery of Neuronal Differentiation
  - Cell 138(6):1062-1064 (2009)
  In the developing nervous system, neural progenitors exit the cell cycle and differentiate on a precise schedule, yet the mechanisms driving this process remain poorly defined. Yan et al. (2009) now identify a thiol-redox reaction mediated by the membrane protein GDE2 and the peroxiredoxin protein Prdx1 that promotes neurogenesis.
• A Toggle Switch in Plant Nitrate Uptake
  - Cell 138(6):1064-1066 (2009)
  In plants, the uptake of nitrate from the soil is a critical process controlled by complex regulatory networks that target nitrate transporters in the roots. In this issue, Ho et al. (2009) show that phosphorylation of the CHL1 nitrate transporter allows the plant root to sense and respond to different nitrate concentrations in the soil.
• Major Evolutionary Transitions in Centromere Complexity
  - Cell 138(6):1067-1082 (2009)
  Centromeres are chromosomal elements that are both necessary and sufficient for chromosome segregation. However, the puzzlingly broad range in centromere complexity, from simple "point" centromeres to multi-megabase arrays of DNA satellites, has defied explanation. We posit that ancestral centromeres were epigenetically defined and that point centromeres, such as those of budding yeast, have derived from the partitioning elements of selfish plasmids. We further propose that the larger centromere sizes in plants and animals and the rapid evolution of their centromeric proteins is the result of an intense battle for evolutionary dominance due to the asymmetric retention of only one product of female meiosis.
• The Tumor Suppressor p53 Regulates Polarity of Self-Renewing Divisions in Mammary Stem Cells
  - Cell 138(6):1083-1095 (2009)
  Stem-like cells may be integral to the development and maintenance of human cancers. Direct proof is still lacking, mainly because of our poor understanding of the biological differences between normal and cancer stem cells (SCs). Using the ErbB2 transgenic model of breast cancer, we found that self-renewing divisions of cancer SCs are more frequent than their normal counterparts, unlimited and symmetric, thus contributing to increasing numbers of SCs in tumoral tissues. SCs with targeted mutation of the tumor suppressor p53 possess the same self-renewal properties as cancer SCs, and their number increases progressively in the p53 null premalignant mammary gland. Pharmacological reactivation of p53 correlates with restoration of asymmetric divisions in cancer SCs and tumor growth reduction, without significant effects on additional cancer cells. These data demonstrate that p53 regulates polarity of cell division in mammary SCs and suggest that loss of p53 favors symmet! ric divisions of cancer SCs, contributing to tumor growth.
• Molecular Architecture of the Mos1 Paired-End Complex: The Structural Basis of DNA Transposition in a Eukaryote
  - Cell 138(6):1096-1108 (2009)
  A key step in cut-and-paste DNA transposition is the pairing of transposon ends before the element is excised and inserted at a new site in its host genome. Crystallographic analyses of the paired-end complex (PEC) formed from precleaved transposon ends and the transposase of the eukaryotic element Mos1 reveals two parallel ends bound to a dimeric enzyme. The complex has a trans arrangement, with each transposon end recognized by the DNA binding region of one transposase monomer and by the active site of the other monomer. Two additional DNA duplexes in the crystal indicate likely binding sites for flanking DNA. Biochemical data provide support for a model of the target capture complex and identify Arg186 to be critical for target binding. Mixing experiments indicate that a transposase dimer initiates first-strand cleavage and suggest a pathway for PEC formation.
• Recombinational Repair within Heterochromatin Requires ATP-Dependent Chromatin Remodeling
  - Cell 138(6):1109-1121 (2009)
  Heterochromatin plays a key role in protection of chromosome integrity by suppressing homologous recombination. In Saccharomyces cerevisiae, Sir2p, Sir3p, and Sir4p are structural components of heterochromatin found at telomeres and the silent mating-type loci. Here we have investigated whether incorporation of Sir proteins into minichromosomes regulates early steps of recombinational repair in vitro. We find that addition of Sir3p to a nucleosomal substrate is sufficient to eliminate yRad51p-catalyzed formation of joints, and that this repression is enhanced by Sir2p/Sir4p. Importantly, Sir-mediated repression requires histone residues that are critical for silencing in vivo. Moreover, we demonstrate that the SWI/SNF chromatin-remodeling enzyme facilitates joint formation by evicting Sir3p, thereby promoting subsequent Rad54p-dependent formation of a strand invasion product. These results suggest that recombinational repair in the context of heterochromatin presents a! dditional constraints that can be overcome by ATP-dependent chromatin-remodeling enzymes.
• Histone Crosstalk between H3S10ph and H4K16ac Generates a Histone Code that Mediates Transcription Elongation
  - Cell 138(6):1122-1136 (2009)
  The phosphorylation of the serine 10 at histone H3 has been shown to be important for transcriptional activation. Here, we report the molecular mechanism through which H3S10ph triggers transcript elongation of the FOSL1 gene. Serum stimulation induces the PIM1 kinase to phosphorylate the preacetylated histone H3 at the FOSL1 enhancer. The adaptor protein 14-3-3 binds the phosphorylated nucleosome and recruits the histone acetyltransferase MOF, which triggers the acetylation of histone H4 at lysine 16 (H4K16ac). This histone crosstalk generates the nucleosomal recognition code composed of H3K9acS10ph/H4K16ac determining a nucleosome platform for the bromodomain protein BRD4 binding. The recruitment of the positive transcription elongation factor b (P-TEFb) via BRD4 induces the release of the promoter-proximal paused RNA polymerase II and the increase of its processivity. Thus, the single phosphorylation H3S10ph at the FOSL1 enhancer triggers a cascade of events which ac! tivate transcriptional elongation.
• The Drosophila HP1 Homolog Rhino Is Required for Transposon Silencing and piRNA Production by Dual-Strand Clusters
  - Cell 138(6):1137-1149 (2009)
  Piwi-interacting RNAs (piRNAs) silence transposons and maintain genome integrity during germline development. In Drosophila, transposon-rich heterochromatic clusters encode piRNAs either on both genomic strands (dual-strand clusters) or predominantly one genomic strand (uni-strand clusters). Primary piRNAs derived from these clusters are proposed to drive a ping-pong amplification cycle catalyzed by proteins that localize to the perinuclear nuage. We show that the HP1 homolog Rhino is required for nuage organization, transposon silencing, and ping-pong amplification of piRNAs. rhi mutations virtually eliminate piRNAs from the dual-strand clusters and block production of putative precursor RNAs from both strands of the major 42AB dual-strand cluster, but not of transcripts or piRNAs from the uni-strand clusters. Furthermore, Rhino protein associates with the 42AB dual-strand cluster,but does not bind to uni-strand cluster 2 or flamenco. Rhino thus appears to promote tra! nscription of dual-strand clusters, leading to production of piRNAs that drive the ping-pong amplification cycle.
• Identification of an Aurora-A/PinsLINKER/ Dlg Spindle Orientation Pathway using Induced Cell Polarity in S2 Cells
  - Cell 138(6):1150-1163 (2009)
  Asymmetric cell division is intensely studied because it can generate cellular diversity as well as maintain stem cell populations. Asymmetric cell division requires mitotic spindle alignment with intrinsic or extrinsic polarity cues, but mechanistic detail of this process is lacking. Here, we develop a method to construct cortical polarity in a normally unpolarized cell line and use this method to characterize Partner of Inscuteable (Pins; LGN/AGS3 in mammals) -dependent spindle orientation. We identify a previously unrecognized evolutionarily conserved Pins domain (PinsLINKER) that requires Aurora-A phosphorylation to recruit Discs large (Dlg; PSD-95/hDlg in mammals) and promote partial spindle orientation. The well-characterized PinsTPR domain has no function alone, but placing the PinsTPR in cis to the PinsLINKER gives dynein-dependent precise spindle orientation. This "induced cortical polarity" assay is suitable for rapid identification of the proteins, domai! ns, and amino acids regulating spindle orientation or cell polarity.
• Detecting Folding Intermediates of a Protein as It Passes through the Bacterial Translocation Channel
  - Cell 138(6):1164-1173 (2009)
  Most bacterial exported proteins cross the cytoplasmic membrane as unfolded polypeptides. However, little is known about how they fold during or after this process due to the difficulty in detecting folding intermediates. Here we identify cotranslational and posttranslational folding intermediates of a periplasmic protein in which the protein and DsbA, a periplasmic disulfide bond-forming enzyme, are covalently linked by a disulfide bond. The cotranslational mixed-disulfide intermediate is, upon further chain elongation, resolved, releasing the oxidized polypeptide, thus allowing us to follow the folding process. This analysis reveals that two cysteines that are joined to form a structural disulfide can play different roles during the folding reaction and that the mode of translocation (cotranslational verse posttranslational) can affect the folding process of a protein in the periplasm. The latter finding leads us to propose that the activity of the ribosome (translat! ion) can modulate protein folding even in an extracytosolic compartment.
• Kinesin-8 Motors Act Cooperatively to Mediate Length-Dependent Microtubule Depolymerization
  - Cell 138(6):1174-1183 (2009)
  Motor proteins in the kinesin-8 family depolymerize microtubules in a length-dependent manner that may be crucial for controlling the length of organelles such as the mitotic spindle. We used single-molecule microscopy to understand the mechanism of length-dependent depolymerization by the budding yeast kinesin-8, Kip3p. We found that after binding at a random position on a microtubule and walking to the plus end, an individual Kip3p molecule pauses there until an incoming Kip3p molecule bumps it off. Kip3p dissociation is accompanied by removal of just one or two tubulin dimers (on average). Such a cooperative mechanism leads to a depolymerization rate that is proportional to the flux of motors to the microtubule end and accounts for the length dependence of depolymerization. This type of feedback between length and disassembly may serve as a model for understanding how an ensemble of molecules can measure and control polymer length.
• CHL1 Functions as a Nitrate Sensor in Plants
  - Cell 138(6):1184-1194 (2009)
  Ions serve as essential nutrients in higher plants and can also act as signaling molecules. Little is known about how plants sense changes in soil nutrient concentrations. Previous studies showed that T101-phosphorylated CHL1 is a high-affinity nitrate transporter, whereas T101-dephosphorylated CHL1 is a low-affinity transporter. In this study, analysis of an uptake- and sensing-decoupled mutant showed that the nitrate transporter CHL1 functions as a nitrate sensor. Primary nitrate responses in CHL1T101D and CHLT101A transgenic plants showed that phosphorylated and dephosphorylated CHL1 lead to a low- and high-level response, respectively. In vitro and in vivo studies showed that, in response to low nitrate concentrations, protein kinase CIPK23 can phosphorylate T101 of CHL1 to maintain a low-level primary response. Thus, CHL1 uses dual-affinity binding and a phosphorylation switch to sense a wide range of nitrate concentrations in the soil, thereby functioning as an i! on sensor in higher plants. For a video summary of this article, see the PaperFlick file with the Supplemental Data available online.
• Actomyosin Is the Main Driver of Interkinetic Nuclear Migration in the Retina
  - Cell 138(6):1195-1208 (2009)
  Progenitor cell nuclei in the rapidly expanding epithelium of the embryonic vertebrate central nervous system undergo a process called interkinetic nuclear migration (IKNM). Movements of IKNM are generally believed to involve smooth migration of nuclei from apical to basal and back during the G1 and G2 phases of the cell cycle, respectively. Yet, this has not been formally demonstrated, nor have the molecular mechanisms that drive IKNM been identified. Using time-lapse confocal microscopy to observe nuclear movements in zebrafish retinal neuroepithelial cells, we show that, except for brief apical nuclear translocations preceding mitosis, IKNM is stochastic rather than smooth and directed. We also show that IKNM is driven largely by actomyosin-dependent forces as it still occurs when the microtubule cytoskeleton is compromised but is blocked when MyosinII activity is inhibited.
• The Antioxidant Enzyme Prdx1 Controls Neuronal Differentiation by Thiol-Redox-Dependent Activation of GDE2
  - Cell 138(6):1209-1221 (2009)
  The six-transmembrane protein GDE2 controls the onset and progression of spinal motor neuron differentiation through extracellular glycerophosphodiester phosphodiesterase metabolism. Although this process is likely to be tightly regulated, the relevant mechanisms that modulate its activity are unknown. Here we show that the antioxidant scavenger peroxiredoxin1 (Prdx1) interacts with GDE2, and that loss of Prdx1 causes motor neuron deficits analogous to GDE2 ablation. Prdx1 cooperates with GDE2 to drive motor neuron differentiation, and this synergy requires Prdx1 thiol-dependent catalysis. Prdx1 activates GDE2 through reduction of an intramolecular disulfide bond that bridges its intracellular N- and C-terminal domains. GDE2 variants incapable of disulfide bond formation acquire independence from Prdx1 and are potent inducers of motor neuron differentiation. These findings define Prdx1 as a pivotal regulator of GDE2 activity and suggest roles for coupled thiol-redox-de! pendent cascades in controlling neuronal differentiation in the spinal cord.
• Synaptic PRG-1 Modulates Excitatory Transmission via Lipid Phosphate-Mediated Signaling
  - Cell 138(6):1222-1235 (2009)
  Plasticity related gene-1 (PRG-1) is a brain-specific membrane protein related to lipid phosphate phosphatases, which acts in the hippocampus specifically at the excitatory synapse terminating on glutamatergic neurons. Deletion of prg-1 in mice leads to epileptic seizures and augmentation of EPSCs, but not IPSCs. In utero electroporation of PRG-1 into deficient animals revealed that PRG-1 modulates excitation at the synaptic junction. Mutation of the extracellular domain of PRG-1 crucial for its interaction with lysophosphatidic acid (LPA) abolished the ability to prevent hyperexcitability. As LPA application in vitro induced hyperexcitability in wild-type but not in LPA2 receptor-deficient animals, and uptake of phospholipids is reduced in PRG-1-deficient neurons, we assessed PRG-1/LPA2 receptor-deficient animals, and found that the pathophysiology observed in the PRG-1-deficient mice was fully reverted. Thus, we propose PRG-1 as an important player in the modulatory ! control of hippocampal excitability dependent on presynaptic LPA2 receptor signaling.
• The Exosome Regulates Circadian Gene Expression in a Posttranscriptional Negative Feedback Loop
  - Cell 138(6):1236-1246 (2009)
  The eukaryotic circadian oscillators consist of autoregulatory negative feedback loops. However, little is known about the role of posttranscriptional regulation of RNA in circadian oscillators. In the Neurospora circadian negative feedback loop, FRQ and FRH form the FFC complex that represses frq transcription. Here, we show that FFC also binds frq RNA and interacts with the exosome to regulate frq RNA decay. Consequently, frq RNA is robustly rhythmic as it is more stable when FRQ levels are low. Silencing of RRP44, the catalytic subunit of the exosome, elevates frq RNA levels and impairs clock function. In addition, rrp44 is a clock-controlled gene and a direct target of the WHITE COLLAR complex, and RRP44 controls the circadian expression of some ccgs. Taken together, these results suggest that FFC and the exosome are part of a posttranscriptional negative feedback loop that regulates frq transcript levels and the circadian output pathway.
• Navigating the Deubiquitinating Proteome with a ComPASS
  - Cell 138(6):1247 (2009)
  
• SnapShot: Antibiotic Inhibition of Protein Synthesis I
  - Cell 138(6):1248-1248.e1 (2009)