Friday, January 21, 2011

Hot off the presses! Jan 21 Cell

The Jan 21 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 144(2):159, 161 (2011)
  • Select: The Many Faces of Cancer
    - cell 144(2):163, 165 (2011)
    Although cancer is typically viewed as a consequence of unregulated cell proliferation, it has become increasingly clear that it also reflects a crisis of cellular identity. In this Select, we highlight recent papers that may help us to unravel the multiple personalities of tumor cells and, in doing so, bring us closer to conquering cancer in the clinic.
  • Funding in 2011: East Heats Up as West Cools Down
    - cell 144(2):167-169 (2011)
    The great slowdown in the global economy since 2008 is putting the brakes on government spending for health research funding worldwide. Could this be an opportunity for research centers in Asia to shift the power balance?
  • Are Polycomb Group Bodies Gene Silencing Factories?
    - cell 144(2):170-171 (2011)
    Polycomb group (PcG) proteins mediate long-range associations between Hox genes, which correlate with gene repression in vivo. Bantignies et al. (2011) identify a physiological role for the nuclear localization of Hox genes in PcG-mediated gene silencing, strengthening the evidence that nuclear positioning regulates gene expression.
  • Rallying the Exocyst as an Autophagy Scaffold
    - cell 144(2):172-174 (2011)
    Protein scaffolds coordinate the assembly of many multicomponent signaling complexes. Bodemann et al. (2011) now show that the exocyst, a protein complex involved in tethering transport vesicles to the plasma membrane, provides an assembly and activation platform for components of the autophagy machinery via a process requiring the GTPase RalB.
  • High-Resolution Genome-wide Mapping of the Primary Structure of Chromatin
    - cell 144(2):175-186 (2011)
    The genomic organization of chromatin is increasingly recognized as a key regulator of cell behavior, but deciphering its regulation mechanisms requires detailed knowledge of chromatin's primary structure—the assembly of nucleosomes throughout the genome. This Primer explains the principles for mapping and analyzing the primary organization of chromatin on a genomic scale. After introducing chromatin organization and its impact on gene regulation and human health, we then describe methods that detect nucleosome positioning and occupancy levels using chromatin immunoprecipitation in combination with deep sequencing (ChIP-Seq), a strategy that is now straightforward and cost efficient. We then explore current strategies for converting the sequence information into knowledge about chromatin, an exciting challenge for biologists and bioinformaticians.
  • Nuclear PTEN Regulates the APC-CDH1 Tumor-Suppressive Complex in a Phosphatase-Independent Manner
    - cell 144(2):187-199 (2011)
    PTEN is a frequently mutated tumor suppressor gene that opposes the PI3K/AKT pathway through dephosphorylation of phosphoinositide-3,4,5-triphosphate. Recently, nuclear compartmentalization of PTEN was found as a key component of its tumor-suppressive activity; however its nuclear function remains poorly defined. Here we show that nuclear PTEN interacts with APC/C, promotes APC/C association with CDH1, and thereby enhances the tumor-suppressive activity of the APC-CDH1 complex. We find that nuclear exclusion but not phosphatase inactivation of PTEN impairs APC-CDH1. This nuclear function of PTEN provides a straightforward mechanistic explanation for the fail-safe cellular senescence response elicited by acute PTEN loss and the tumor-suppressive activity of catalytically inactive PTEN. Importantly, we demonstrate that PTEN mutant and PTEN null states are not synonymous as they are differentially sensitive to pharmacological inhibition of APC-CDH1 targets such as PLK1 an! d Aurora kinases. This finding identifies a strategy for cancer patient stratification and, thus, optimization of targeted therapies. 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 (4924 K)
  • Global Regulation of H2A.Z Localization by the INO80 Chromatin-Remodeling Enzyme Is Essential for Genome Integrity
    - cell 144(2):200-213 (2011)
    INO80 is an evolutionarily conserved, ATP-dependent chromatin-remodeling enzyme that plays roles in transcription, DNA repair, and replication. Here, we show that yeast INO80 facilitates these diverse processes at least in part by controlling genome-wide distribution of the histone variant H2A.Z. In the absence of INO80, H2A.Z nucleosomes are mislocalized, and H2A.Z levels at promoters show reduced responsiveness to transcriptional changes, suggesting that INO80 controls H2A.Z dynamics. Additionally, we demonstrate that INO80 has a histone-exchange activity in which the enzyme can replace nucleosomal H2A.Z/H2B with free H2A/H2B dimers. Genetic interactions between ino80 and htz1 support a model in which INO80 catalyzes the removal of unacetylated H2A.Z from chromatin as a mechanism to promote genome stability.
  • Polycomb-Dependent Regulatory Contacts between Distant Hox Loci in Drosophila
    - cell 144(2):214-226 (2011)
    In Drosophila melanogaster, Hox genes are organized in an anterior and a posterior cluster, called Antennapedia complex and bithorax complex, located on the same chromosome arm and separated by 10 Mb of DNA. Both clusters are repressed by Polycomb group (PcG) proteins. Here, we show that genes of the two Hox complexes can interact within nuclear PcG bodies in tissues where they are corepressed. This colocalization increases during development and depends on PcG proteins. Hox gene contacts are conserved in the distantly related Drosophila virilis species and they are part of a large gene interaction network that includes other PcG target genes. Importantly, mutations on one of the loci weaken silencing of genes in the other locus, resulting in the exacerbation of homeotic phenotypes in sensitized genetic backgrounds. Thus, the three-dimensional organization of Polycomb target genes in the cell nucleus stabilizes the maintenance of epigenetic gene silencing.
  • Regulation of Mitochondrial Protein Import by Cytosolic Kinases
    - cell 144(2):227-239 (2011)
    Mitochondria import a large number of nuclear-encoded proteins via membrane-bound transport machineries; however, little is known about regulation of the preprotein translocases. We report that the main protein entry gate of mitochondria, the translocase of the outer membrane (TOM complex), is phosphorylated by cytosolic kinases—in particular, casein kinase 2 (CK2) and protein kinase A (PKA). CK2 promotes biogenesis of the TOM complex by phosphorylation of two key components, the receptor Tom22 and the import protein Mim1, which in turn are required for import of further Tom proteins. Inactivation of CK2 decreases the levels of the TOM complex and thus mitochondrial protein import. PKA phosphorylates Tom70 under nonrespiring conditions, thereby inhibiting its receptor activity and the import of mitochondrial metabolite carriers. We conclude that cytosolic kinases exert stimulatory and inhibitory effects on biogenesis and function of the TOM complex and thus regulate ! protein import into mitochondria.
  • Dual Action of ATP Hydrolysis Couples Lid Closure to Substrate Release into the Group II Chaperonin Chamber
    - cell 144(2):240-252 (2011)
    Group II chaperonins are ATP-dependent ring-shaped complexes that bind nonnative polypeptides and facilitate protein folding in archaea and eukaryotes. A built-in lid encapsulates substrate proteins within the central chaperonin chamber. Here, we describe the fate of the substrate during the nucleotide cycle of group II chaperonins. The chaperonin substrate-binding sites are exposed, and the lid is open in both the ATP-free and ATP-bound prehydrolysis states. ATP hydrolysis has a dual function in the folding cycle, triggering both lid closure and substrate release into the central chamber. Notably, substrate release can occur in the absence of a lid, and lid closure can occur without substrate release. However, productive folding requires both events, so that the polypeptide is released into the confined space of the closed chamber where it folds. Our results show that ATP hydrolysis coordinates the structural and functional determinants that trigger productive folding.
  • RalB and the Exocyst Mediate the Cellular Starvation Response by Direct Activation of Autophagosome Assembly
    - cell 144(2):253-267 (2011)
    The study of macroautophagy in mammalian cells has described induction, vesicle nucleation, and membrane elongation complexes as key signaling intermediates driving autophagosome biogenesis. How these components are recruited to nascent autophagosomes is poorly understood, and although much is known about signaling mechanisms that restrain autophagy, the nature of positive inductive signals that can promote autophagy remain cryptic. We find that the Ras-like small G protein, RalB, is localized to nascent autophagosomes and is activated on nutrient deprivation. RalB and its effector Exo84 are required for nutrient starvation-induced autophagocytosis, and RalB activation is sufficient to promote autophagosome formation. Through direct binding to Exo84, RalB induces the assembly of catalytically active ULK1 and Beclin1-VPS34 complexes on the exocyst, which are required for isolation membrane formation and maturation. Thus, RalB signaling is a primary adaptive response to ! nutrient limitation that directly engages autophagocytosis through mobilization of the core vesicle nucleation machinery.
  • Delay in Feedback Repression by Cryptochrome 1 Is Required for Circadian Clock Function
    - cell 144(2):268-281 (2011)
    Direct evidence for the requirement of delay in feedback repression in the mammalian circadian clock has been elusive. Cryptochrome 1 (Cry1), an essential clock component, displays evening-time expression and serves as a strong repressor at morning-time elements (E box/E′ box). In this study, we reveal that a combination of day-time elements (D box) within the Cry1-proximal promoter and night-time elements (RREs) within its intronic enhancer gives rise to evening-time expression. A synthetic composite promoter produced evening-time expression, which was further recapitulated by a simple phase-vector model. Of note, coordination of day-time with night-time elements can modulate the extent of phase delay. A genetic complementation assay in Cry1−/−:Cry2−/− cells revealed that substantial delay of Cry1 expression is required to restore circadian rhythmicity, and its prolonged delay slows circadian oscillation. Taken together, our data suggest that phase delay in ! Cry1 transcription is required for mammalian clock function.
  • RIM Proteins Tether Ca2+ Channels to Presynaptic Active Zones via a Direct PDZ-Domain Interaction
    - cell 144(2):282-295 (2011)
    At a synapse, fast synchronous neurotransmitter release requires localization of Ca2+ channels to presynaptic active zones. How Ca2+ channels are recruited to active zones, however, remains unknown. Using unbiased yeast two-hybrid screens, we here identify a direct interaction of the central PDZ domain of the active-zone protein RIM with the C termini of presynaptic N- and P/Q-type Ca2+ channels but not L-type Ca2+ channels. To test the physiological significance of this interaction, we generated conditional knockout mice lacking all multidomain RIM isoforms. Deletion of RIM proteins ablated most neurotransmitter release by simultaneously impairing the priming of synaptic vesicles and by decreasing the presynaptic localization of Ca2+ channels. Strikingly, rescue of the decreased Ca2+-channel localization required the RIM PDZ domain, whereas rescue of vesicle priming required the RIM N terminus. We propose that RIMs tether N- and P/Q-type Ca2+ channels to presynaptic a! ctive zones via a direct PDZ-domain-mediated interaction, thereby enabling fast, synchronous triggering of neurotransmitter release at a synapse.
  • Densely Interconnected Transcriptional Circuits Control Cell States in Human Hematopoiesis
    - cell 144(2):296-309 (2011)
    Though many individual transcription factors are known to regulate hematopoietic differentiation, major aspects of the global architecture of hematopoiesis remain unknown. Here, we profiled gene expression in 38 distinct purified populations of human hematopoietic cells and used probabilistic models of gene expression and analysis of cis-elements in gene promoters to decipher the general organization of their regulatory circuitry. We identified modules of highly coexpressed genes, some of which are restricted to a single lineage but most of which are expressed at variable levels across multiple lineages. We found densely interconnected cis-regulatory circuits and a large number of transcription factors that are differentially expressed across hematopoietic states. These findings suggest a more complex regulatory system for hematopoiesis than previously assumed.
  • SnapShot: Chromatin Remodeling:SWI/SNF
    - cell 144(2):310-310.e1 (2011)

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