Hot off the presses! Dec 23 Cell

The Dec 23 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 143(7):1031, 1033 (2010)
  
• Select: Stem Cell Reprogramming
  - cell 143(7):1035, 1037 (2010)
  The remarkable progress of the stem cell field continues at a rapid pace due, in large part, to advances in understanding how somatic cells are reprogrammed to pluripotency. This issue's Select highlights recent technical developments in stem cell reprogramming and discusses newly revealed links between induced pluripotency and cancer.
• Science, Meet Poetry
  - cell 143(7):1039 (2010)
  
• Duchenne Muscular Dystrophy Models Show Their Age
  - cell 143(7):1040-1042 (2010)
  The lack of appropriate animal models has hampered efforts to develop therapies for Duchenne muscular dystrophy (DMD). A new mouse model lacking both dystrophin and telomerase (Sacco et al., 2010) closely mimics the pathological progression of human DMD and shows that muscle stem cell activity is a key determinant of disease severity.
• New Ways to Make Old Walls: Bacterial Surprises
  - cell 143(7):1042-1044 (2010)
  Two papers in this issue of Cell (Paradis-Bleau et al., 2010 and Typas et al., 2010) report that the lipoproteins LpoA and LpoB are required for the synthesis of cell walls in Escherichia coli. Attached to the bacterial outer membrane, these new cell wall components regulate penicillin-binding proteins located at the inner membrane.
• Wnt Signaling: Multivesicular Bodies Hold GSK3 Captive
  - cell 143(7):1044-1046 (2010)
  Two key events in Wnt signal transduction, receptor endocytosis and inactivation of Glycogen Synthase Kinase 3 (GSK3), remain incompletely understood. Taelman et al. (2010) discover that Wnt signaling inactivates GSK3 by sequestering the enzyme in multivesicular bodies, thus linking these two events and providing a new framework for understanding Wnt signaling.
• Breaking the Diffraction Barrier: Super-Resolution Imaging of Cells
  - cell 143(7):1047-1058 (2010)
  Anyone who has used a light microscope has wished that its resolution could be a little better. Now, after centuries of gradual improvements, fluorescence microscopy has made a quantum leap in its resolving power due, in large part, to advancements over the past several years in a new area of research called super-resolution fluorescence microscopy. In this Primer, we explain the principles of various super-resolution approaches, such as STED, (S)SIM, and STORM/(F)PALM. Then, we describe recent applications of super-resolution microscopy in cells, which demonstrate how these approaches are beginning to provide new insights into cell biology, microbiology, and neurobiology.
• Short Telomeres and Stem Cell Exhaustion Model Duchenne Muscular Dystrophy in mdx/mTR Mice
  - cell 143(7):1059-1071 (2010)
  In Duchenne muscular dystrophy (DMD), dystrophin mutation leads to progressive lethal skeletal muscle degeneration. For unknown reasons, dystrophin deficiency does not recapitulate DMD in mice (mdx), which have mild skeletal muscle defects and potent regenerative capacity. We postulated that human DMD progression is a consequence of loss of functional muscle stem cells (MuSC), and the mild mouse mdx phenotype results from greater MuSC reserve fueled by longer telomeres. We report that mdx mice lacking the RNA component of telomerase (mdx/mTR) have shortened telomeres in muscle cells and severe muscular dystrophy that progressively worsens with age. Muscle wasting severity parallels a decline in MuSC regenerative capacity and is ameliorated histologically by transplantation of wild-type MuSC. These data show that DMD progression results, in part, from a cell-autonomous failure of MuSC to maintain the damage-repair cycle initiated by dystrophin deficiency. The essential ! role of MuSC function has therapeutic implications for DMD.
• C/EBPβ Controls Exercise-Induced Cardiac Growth and Protects against Pathological Cardiac Remodeling
  - cell 143(7):1072-1083 (2010)
  The heart has the ability to grow in size in response to exercise, but little is known about the transcriptional mechanisms underlying physiological hypertrophy. Adult cardiomyocytes have also recently been proven to hold the potential for proliferation, a process that could be of great importance for regenerative medicine. Using a unique RT-PCR-based screen against all transcriptional components, we showed that C/EBPβ was downregulated with exercise, whereas the expression of CITED4 was increased. Reduction of C/EBPβ in vitro and in vivo resulted in a phenocopy of endurance exercise with cardiomyocyte hypertrophy and proliferation. This proliferation was mediated, at least in part, by the increased CITED4. Importantly, mice with reduced cardiac C/EBPβ levels displayed substantial resistance to cardiac failure upon pressure overload. These data indicate that C/EBPβ represses cardiomyocyte growth and proliferation in the adult mammalian heart and that reduction in C! /EBPβ is a central signal in physiologic hypertrophy and proliferation. 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 (3132 K)
• Paternally Induced Transgenerational Environmental Reprogramming of Metabolic Gene Expression in Mammals
  - cell 143(7):1084-1096 (2010)
  Epigenetic information can be inherited through the mammalian germline and represents a plausible transgenerational carrier of environmental information. To test whether transgenerational inheritance of environmental information occurs in mammals, we carried out an expression profiling screen for genes in mice that responded to paternal diet. Offspring of males fed a low-protein diet exhibited elevated hepatic expression of many genes involved in lipid and cholesterol biosynthesis and decreased levels of cholesterol esters, relative to the offspring of males fed a control diet. Epigenomic profiling of offspring livers revealed numerous modest (20%) changes in cytosine methylation depending on paternal diet, including reproducible changes in methylation over a likely enhancer for the key lipid regulator Ppara. These results, in conjunction with recent human epidemiological data, indicate that parental diet can affect cholesterol and lipid metabolism in offspring and def! ine a model system to study environmental reprogramming of the heritable epigenome.
• Regulation of Peptidoglycan Synthesis by Outer-Membrane Proteins
  - cell 143(7):1097-1109 (2010)
  Growth of the mesh-like peptidoglycan (PG) sacculus located between the bacterial inner and outer membranes (OM) is tightly regulated to ensure cellular integrity, maintain cell shape, and orchestrate division. Cytoskeletal elements direct placement and activity of PG synthases from inside the cell, but precise spatiotemporal control over this process is poorly understood. We demonstrate that PG synthases are also controlled from outside of the sacculus. Two OM lipoproteins, LpoA and LpoB, are essential for the function, respectively, of PBP1A and PBP1B, the major E. coli bifunctional PG synthases. Each Lpo protein binds specifically to its cognate PBP and stimulates its transpeptidase activity, thereby facilitating attachment of new PG to the sacculus. LpoB shows partial septal localization, and our data suggest that the LpoB-PBP1B complex contributes to OM constriction during cell division. LpoA/LpoB and their PBP-docking regions are restricted to γ-proteobacteria, ! providing models for niche-specific regulation of sacculus growth.
• Lipoprotein Cofactors Located in the Outer Membrane Activate Bacterial Cell Wall Polymerases
  - cell 143(7):1110-1120 (2010)
  Most bacteria surround themselves with a peptidoglycan (PG) exoskeleton synthesized by polysaccharide polymerases called penicillin-binding proteins (PBPs). Because they are the targets of penicillin and related antibiotics, the structure and biochemical functions of the PBPs have been extensively studied. Despite this, we still know surprisingly little about how these enzymes build the PG layer in vivo. Here, we identify the Escherichia coli outer-membrane lipoproteins LpoA and LpoB as essential PBP cofactors. We show that LpoA and LpoB form specific trans-envelope complexes with their cognate PBP and are critical for PBP function in vivo. We further show that LpoB promotes PG synthesis by its partner PBP in vitro and that it likely does so by stimulating glycan chain polymerization. Overall, our results indicate that PBP accessory proteins play a central role in PG biogenesis, and like the PBPs they work with, these factors are attractive targets for antibiotic devel! opment.
• Essential Role of Coiled Coils for Aggregation and Activity of Q/N-Rich Prions and PolyQ Proteins
  - cell 143(7):1121-1135 (2010)
  The functional switch of glutamine/asparagine (Q/N)-rich prions and the neurotoxicity of polyQ-expanded proteins involve complex aggregation-prone structural transitions, commonly presumed to be forming β sheets. By analyzing sequences of interaction partners of these proteins, we discovered a recurrent presence of coiled-coil domains both in the partners and in segments that flank or overlap Q/N-rich and polyQ domains. Since coiled coils can mediate protein interactions and multimerization, we studied their possible involvement in Q/N-rich and polyQ aggregations. Using circular dichroism and chemical crosslinking, we found that Q/N-rich and polyQ peptides form α-helical coiled coils in vitro and assemble into multimers. Using structure-guided mutagenesis, we found that coiled-coil domains modulate in vivo properties of two Q/N-rich prions and polyQ-expanded huntingtin. Mutations that disrupt coiled coils impair aggregation and activity, whereas mutations that enhanc! e coiled-coil propensity promote aggregation. These findings support a coiled-coil model for the functional switch of Q/N-rich prions and for the pathogenesis of polyQ-expansion diseases.
• Wnt Signaling Requires Sequestration of Glycogen Synthase Kinase 3 inside Multivesicular Endosomes
  - cell 143(7):1136-1148 (2010)
  Canonical Wnt signaling requires inhibition of Glycogen Synthase Kinase 3 (GSK3) activity, but the molecular mechanism by which this is achieved remains unclear. Here, we report that Wnt signaling triggers the sequestration of GSK3 from the cytosol into multivesicular bodies (MVBs), so that this enzyme becomes separated from its many cytosolic substrates. Endocytosed Wnt colocalized with GSK3 in acidic vesicles positive for endosomal markers. After Wnt addition, endogenous GSK3 activity decreased in the cytosol, and GSK3 became protected from protease treatment inside membrane-bounded organelles. Cryoimmunoelectron microscopy showed that these corresponded to MVBs. Two proteins essential for MVB formation, HRS/Vps27 and Vps4, were required for Wnt signaling. The sequestration of GSK3 extended the half-life of many other proteins in addition to β-Catenin, including an artificial Wnt-regulated reporter protein containing GSK3 phosphorylation sites. We conclude that mult! ivesicular endosomes are essential components of the Wnt signal-transduction pathway.
• Meningococcus Hijacks a β2-Adrenoceptor/β-Arrestin Pathway to Cross Brain Microvasculature Endothelium
  - cell 143(7):1149-1160 (2010)
  Following pilus-mediated adhesion to human brain endothelial cells, meningococcus (N. meningitidis), the bacterium causing cerebrospinal meningitis, initiates signaling cascades, which eventually result in the opening of intercellular junctions, allowing meningeal colonization. The signaling receptor activated by the pathogen remained unknown. We report that N. meningitidis specifically stimulates a biased β2-adrenoceptor/β-arrestin signaling pathway in endothelial cells, which ultimately traps β-arrestin-interacting partners, such as the Src tyrosine kinase and junctional proteins, under bacterial colonies. Cytoskeletal reorganization mediated by β-arrestin-activated Src stabilizes bacterial adhesion to endothelial cells, whereas β-arrestin-dependent delocalization of junctional proteins results in anatomical gaps used by bacteria to penetrate into tissues. Activation of β-adrenoceptor endocytosis with specific agonists prevents signaling events downstream of N.! meningitidis adhesion and inhibits bacterial crossing of the endothelial barrier. The identification of the mechanism used for hijacking host cell signaling machineries opens perspectives for treatment and prevention of meningococcal infection. 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 (14983 K)
• Nutrition-Responsive Glia Control Exit of Neural Stem Cells from Quiescence
  - cell 143(7):1161-1173 (2010)
  The systemic regulation of stem cells ensures that they meet the needs of the organism during growth and in response to injury. A key point of regulation is the decision between quiescence and proliferation. During development, Drosophila neural stem cells (neuroblasts) transit through a period of quiescence separating distinct embryonic and postembryonic phases of proliferation. It is known that neuroblasts exit quiescence via a hitherto unknown pathway in response to a nutrition-dependent signal from the fat body. We have identified a population of glial cells that produce insulin/IGF-like peptides in response to nutrition, and we show that the insulin/IGF receptor pathway is necessary for neuroblasts to exit quiescence. The forced expression of insulin/IGF-like peptides in glia, or activation of PI3K/Akt signaling in neuroblasts, can drive neuroblast growth and proliferation in the absence of dietary protein and thus uncouple neuroblasts from systemic control.
• A Tissue-Specific Atlas of Mouse Protein Phosphorylation and Expression
  - cell 143(7):1174-1189 (2010)
  Although most tissues in an organism are genetically identical, the biochemistry of each is optimized to fulfill its unique physiological roles, with important consequences for human health and disease. Each tissue's unique physiology requires tightly regulated gene and protein expression coordinated by specialized, phosphorylation-dependent intracellular signaling. To better understand the role of phosphorylation in maintenance of physiological differences among tissues, we performed proteomic and phosphoproteomic characterizations of nine mouse tissues. We identified 12,039 proteins, including 6296 phosphoproteins harboring nearly 36,000 phosphorylation sites. Comparing protein abundances and phosphorylation levels revealed specialized, interconnected phosphorylation networks within each tissue while suggesting that many proteins are regulated by phosphorylation independently of their expression. Our data suggest that the "typical" phosphoprotein is widely expres! sed yet displays variable, often tissue-specific phosphorylation that tunes protein activity to the specific needs of each tissue. We offer this dataset as an online resource for the biological research community.
• The Structure of an Arf-ArfGAP Complex Reveals a Ca2+ Regulatory Mechanism
  - cell 143(7):1190 (2010)
  
• SnapShot: Extrinsic Apoptosis Pathways
  - cell 143(7):1192-1192.e2 (2010)