Hot off the presses! Sep 02 Cell

The Sep 02 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 146(5):661, 663 (2011)
  
• Doing More with Fluorophores
  - Cell 146(5):665, 667 (2011)
  Techniques manipulating fluorophores to visualize biological processes have become increasingly sophisticated in recent years, allowing individual events to be examined within living cells. Where are these technologies going now? This issue's Select features recent methodological breakthroughs that establish new clinical tools and clever probes for exploring cellular functions.
• A New FOXO Pathway Required for Leukemogenesis
  - Cell 146(5):669-670 (2011)
  In cancer, the FOXO family of transcription factors functions as tumor suppressors and is directly inactivated by oncogenic signaling through the PI3K/AKT pathway. Now, Skykes et al. (2011) identify a paradoxical requirement for FOXOs in the maintenance of leukemia-initiating cells.
• Signaling through Chromatin: Setting the Scene at Kinetochores
  - Cell 146(5):671-672 (2011)
  Histone H3 lysine 4 trimethylation needed for transcription is mediated by the Set1 methyltransferase and requires prior monoubiquitination of histone H2B. In this issue, Latham et al. (2011) report that dimethylation of the yeast kinetochore protein Dam1 by Set1 similarly requires H2B monoubiquitination. Thus, H2B ubiquitination signals for methylation beyond chromatin.
• When T Cells Run Out of Breath: The HIF-1α Story
  - Cell 146(5):673-674 (2011)
  In this issue, Dang et al. (2011) report a role for the hypoxia-induced transcription factor HIF-1α in regulating the differentiation of T cell subsets that impact both inflammation and its resolution. Their findings illustrate how metabolic signals can alter the balance between inflammation and tolerance and present a potential therapeutic opportunity for the treatment of diseases associated with chronic inflammation.
• Basal Synaptic Transmission: Astrocytes Rule!
  - Cell 146(5):675-677 (2011)
  In this issue, Panatier et al. (2011) show that astrocytes detect synaptic activity induced by single action potentials and upregulate basal synaptic transmission through calcium-dependent mechanisms and purinergic signaling. These results demonstrate the relevance of astrocyte calcium in neurophysiology and confirm that astrocytes are actively involved in synaptic function.
• Niche Crosstalk: Intercellular Signals at the Hair Follicle
  - Cell 146(5):678-681 (2011)
  A recent series of papers, including Festa et al. (2011) in this issue, has revealed unexpected interdependent relationships among cell populations residing in and around the hair follicle. These interactions between different lineages of stem cells are crucial for hair follicle growth and cycling and point to a complex crosstalk in stem cell niches.
• Autophagy and Aging
  - Cell 146(5):682-695 (2011)
  Genetic inhibition of autophagy induces degenerative changes in mammalian tissues that resemble those associated with aging, and normal and pathological aging are often associated with a reduced autophagic potential. Pharmacological or genetic manipulations that increase life span in model organisms often stimulate autophagy, and its inhibition compromises the longevity-promoting effects of caloric restriction, Sirtuin 1 activation, inhibition of insulin/insulin growth factor signaling, or the administration of rapamycin, resveratrol, or spermidine. Here, we discuss the probable cause and effect relationship between perturbed autophagy and aging, as well as possible molecular mechanisms that may mediate the anti-aging effects of autophagy.
• AKT/FOXO Signaling Enforces Reversible Differentiation Blockade in Myeloid Leukemias
  - Cell 146(5):697-708 (2011)
  AKT activation is associated with many malignancies, where AKT acts, in part, by inhibiting FOXO tumor suppressors. We show a converse role for AKT/FOXOs in acute myeloid leukemia (AML). Rather than decreased FOXO activity, we observed that FOXOs are active in 40% of AML patient samples regardless of genetic subtype. We also observe this activity in human MLL-AF9 leukemia allele-induced AML in mice, where either activation of Akt or compound deletion of FoxO1/3/4 reduced leukemic cell growth, with the latter markedly diminishing leukemia-initiating cell (LIC) function in vivo and improving animal survival. FOXO inhibition resulted in myeloid maturation and subsequent AML cell death. FOXO activation inversely correlated with JNK/c-JUN signaling, and leukemic cells resistant to FOXO inhibition responded to JNK inhibition. These data reveal a molecular role for AKT/FOXO and JNK/c-JUN in maintaining a differentiation blockade that can be targeted to inhibit leukemias with ! a range of genetic lesions. 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 (2909 K)
• Chromatin Signaling to Kinetochores: Transregulation of Dam1 Methylation by Histone H2B Ubiquitination
  - Cell 146(5):709-719 (2011)
  Histone H3K4 trimethylation by the Set1/MLL family of proteins provides a hallmark for transcriptional activity from yeast to humans. In S. cerevisiae, H3K4 methylation is mediated by the Set1-containing COMPASS complex and is regulated in trans by prior ubiquitination of histone H2BK123. All of the events that regulate H2BK123ub and H3K4me are thought to occur at gene promoters. Here we report that this pathway is indispensable for methylation of the only other known substrate of Set1, K233 in Dam1, at kinetochores. Deletion of RAD6, BRE1, or Paf1 complex members abolishes Dam1 methylation, as does mutation of H2BK123. Our results demonstrate that Set1-mediated methylation is regulated by a general pathway regardless of substrate that is composed of transcriptional regulatory factors functioning independently of transcription. Moreover, our data identify a node of regulatory crosstalk intrans between a histone modification and modification on a nonhistone protein, dem! onstrating that changing chromatin states can signal functional changes in other essential cellular proteins and machineries.
• Control of Embryonic Stem Cell Lineage Commitment by Core Promoter Factor, TAF3
  - Cell 146(5):720-731 (2011)
  Deciphering the molecular basis of pluripotency is fundamental to our understanding of development and embryonic stem cell function. Here, we report that TAF3, a TBP-associated core promoter factor, is highly enriched in ES cells. In this context, TAF3 is required for endoderm lineage differentiation and prevents premature specification of neuroectoderm and mesoderm. In addition to its role in the core promoter recognition complex TFIID, genome-wide binding studies reveal that TAF3 localizes to a subset of chromosomal regions bound by CTCF/cohesin that are selectively associated with genes upregulated by TAF3. Notably, CTCF directly recruits TAF3 to promoter distal sites and TAF3-dependent DNA looping is observed between the promoter distal sites and core promoters occupied by TAF3/CTCF/cohesin. Together, our findings support a new role of TAF3 in mediating long-range chromatin regulatory interactions that safeguard the finely-balanced transcriptional programs underlyi! ng pluripotency.
• A Mechanism for Tunable Autoinhibition in the Structure of a Human Ca2+/Calmodulin- Dependent Kinase II Holoenzyme
  - Cell 146(5):732-745 (2011)
  Calcium/calmodulin-dependent kinase II (CaMKII) forms a highly conserved dodecameric assembly that is sensitive to the frequency of calcium pulse trains. Neither the structure of the dodecameric assembly nor how it regulates CaMKII are known. We present the crystal structure of an autoinhibited full-length human CaMKII holoenzyme, revealing an unexpected compact arrangement of kinase domains docked against a central hub, with the calmodulin-binding sites completely inaccessible. We show that this compact docking is important for the autoinhibition of the kinase domains and for setting the calcium response of the holoenzyme. Comparison of CaMKII isoforms, which differ in the length of the linker between the kinase domain and the hub, demonstrates that these interactions can be strengthened or weakened by changes in linker length. This equilibrium between autoinhibited states provides a simple mechanism for tuning the calcium response without changes in either the hub or! the kinase domains. 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 (15285 K)
• Rescue of ΔF508-CFTR Trafficking via a GRASP-Dependent Unconventional Secretion Pathway
  - Cell 146(5):746-760 (2011)
  The most prevalent disease-causing mutation of CFTR is the deletion of Phe508 (ΔF508), which leads to defects in conventional Golgi-mediated exocytosis and cell surface expression. We report that ΔF508-CFTR surface expression can be rescued in vitro and in vivo by directing it to an unconventional GRASP-dependent secretion pathway. An integrated molecular and physiological analysis indicates that mechanisms associated with ER stress induce cell surface trafficking of the ER core-glycosylated wild-type and ΔF508-CFTR via the GRASP-dependent pathway. Phosphorylation of a specific site of GRASP and the PDZ-based interaction between GRASP and CFTR are critical for this unconventional surface trafficking. Remarkably, transgenic expression of GRASP in ΔF508-CFTR mice restores CFTR function and rescues mouse survival without apparent toxicity. These findings provide insight into how unconventional protein secretion is activated, and offer a potential therapeutic strategy ! for the treatment of cystic fibrosis and perhaps diseases stemming from other misfolded proteins.
• Adipocyte Lineage Cells Contribute to the Skin Stem Cell Niche to Drive Hair Cycling
  - Cell 146(5):761-771 (2011)
  In mammalian skin, multiple types of resident cells are required to create a functional tissue and support tissue homeostasis and regeneration. The cells that compose the epithelial stem cell niche for skin homeostasis and regeneration are not well defined. Here, we identify adipose precursor cells within the skin and demonstrate that their dynamic regeneration parallels the activation of skin stem cells. Functional analysis of adipocyte lineage cells in mice with defects in adipogenesis and in transplantation experiments revealed that intradermal adipocyte lineage cells are necessary and sufficient to drive follicular stem cell activation. Furthermore, we implicate PDGF expression by immature adipocyte cells in the regulation of follicular stem cell activity. These data highlight adipogenic cells as skin niche cells that positively regulate skin stem cell activity, and suggest that adipocyte lineage cells may alter epithelial stem cell function clinically.
• Control of TH17/Treg Balance by Hypoxia-Inducible Factor 1
  - Cell 146(5):772-784 (2011)
  T cell differentiation into distinct functional effector and inhibitory subsets is regulated, in part, by the cytokine environment present at the time of antigen recognition. Here, we show that hypoxia-inducible factor 1 (HIF-1), a key metabolic sensor, regulates the balance between regulatory T cell (Treg) and TH17 differentiation. HIF-1 enhances TH17 development through direct transcriptional activation of RORγt and via tertiary complex formation with RORγt and p300 recruitment to the IL-17 promoter, thereby regulating TH17 signature genes. Concurrently, HIF-1 attenuates Treg development by binding Foxp3 and targeting it for proteasomal degradation. Importantly, this regulation occurs under both normoxic and hypoxic conditions. Mice with HIF-1α-deficient T cells are resistant to induction of TH17-dependent experimental autoimmune encephalitis associated with diminished TH17 and increased Treg cells. These findings highlight the importance of metabolic cues in T ce! ll fate determination and suggest that metabolic modulation could ameliorate certain T cell-based immune pathologies.
• Astrocytes Are Endogenous Regulators of Basal Transmission at Central Synapses
  - Cell 146(5):785-798 (2011)
  Basal synaptic transmission involves the release of neurotransmitters at individual synapses in response to a single action potential. Recent discoveries show that astrocytes modulate the activity of neuronal networks upon sustained and intense synaptic activity. However, their ability to regulate basal synaptic transmission remains ill defined and controversial. Here, we show that astrocytes in the hippocampal CA1 region detect synaptic activity induced by single-synaptic stimulation. Astrocyte activation occurs at functional compartments found along astrocytic processes and involves metabotropic glutamate subtype 5 receptors. In response, astrocytes increase basal synaptic transmission, as revealed by the blockade of their activity with a Ca2+ chelator. Astrocytic modulation of basal synaptic transmission is mediated by the release of purines and the activation of presynaptic A2A receptors by adenosine. Our work uncovers an essential role for astrocytes in the regula! tion of elementary synaptic communication and provides insight into fundamental aspects of brain function.
• Peptidoglycan Remodeling and Conversion of an Inner Membrane into an Outer Membrane during Sporulation
  - Cell 146(5):799-812 (2011)
  Two hallmarks of the Firmicute phylum, which includes the Bacilli and Clostridia classes, are their ability to form endospores and their "Gram-positive" single-membraned, thick-cell-wall envelope structure. Acetonema longum is part of a lesser-known family (the Veillonellaceae) of Clostridia that form endospores but that are surprisingly "Gram negative," possessing both an inner and outer membrane and a thin cell wall. Here, we present macromolecular resolution, 3D electron cryotomographic images of vegetative, sporulating, and germinating A. longum cells showing that during the sporulation process, the inner membrane of the mother cell is inverted and transformed to become the outer membrane of the germinating cell. Peptidoglycan persists throughout, leading to a revised, "continuous" model of its role in the process. Coupled with genomic analyses, these results point to sporulation as a mechanism by which the bacterial outer membrane may have arisen and A! . longum as a potential "missing link" between single- and double-membraned bacteria.
• Photoconversion and Nuclear Trafficking Cycles Determine Phytochrome A's Response Profile to Far-Red Light
  - Cell 146(5):813-825 (2011)
  Phytochrome A (phyA) is the only photoreceptor in plants, initiating responses in far-red light and, as such, essential for survival in canopy shade. Although the absorption and the ratio of active versus total phyA are maximal in red light, far-red light is the most efficient trigger of phyA-dependent responses. Using a joint experimental-theoretical approach, we unravel the mechanism underlying this shift of the phyA action peak from red to far-red light and show that it relies on specific molecular interactions rather than on intrinsic changes to phyA's spectral properties. According to our model, the dissociation rate of the phyA-FHY1/FHL nuclear import complex is a principle determinant of the phyA action peak. The findings suggest how higher plants acquired the ability to sense far-red light from an ancestral photoreceptor tuned to respond to red light.
• Global Proteomic Assessment of the Classical Protein-Tyrosine Phosphatome and "Redoxome"
  - Cell 146(5):826-840 (2011)
  Protein-tyrosine phosphatases (PTPs), along with protein-tyrosine kinases, play key roles in cellular signaling. All Class I PTPs contain an essential active site cysteinyl residue, which executes a nucleophilic attack on substrate phosphotyrosyl residues. The high reactivity of the catalytic cysteine also predisposes PTPs to oxidation by reactive oxygen species, such as H2O2. Reversible PTP oxidation is emerging as an important cellular regulatory mechanism and might contribute to diseases such as cancer. We exploited these unique features of PTP enzymology to develop proteomic methods, broadly applicable to cell and tissue samples, that enable the comprehensive identification and quantification of expressed classical PTPs (PTPome) and the oxidized subset of the PTPome (oxPTPome). We find that mouse and human cells and tissues, including cancer cells, display distinctive PTPomes and oxPTPomes, revealing additional levels of complexity in the regulation of protein-tyro! sine phosphorylation in normal and malignant cells.
• MAVS Forms Functional Prion-like Aggregates to Activate and Propagate Antiviral Innate Immune Response
  - Cell 146(5):841 (2011)
  
• Role of the Clathrin Terminal Domain in Regulating Coated Pit Dynamics Revealed by Small Molecule Inhibition
  - Cell 146(5):841 (2011)
  
• SnapShot: Histone Readers
  - Cell 146(5):842-842.e1 (2011)