bluuebunny/crossref_metadata_2025 huggingface dataset:
pVHL: A Multipurpose Adaptor Protein
Authors: Frew Ian J. , Krek Wilhelm
Published: June 2008

Mutations in the von Hippel-Lindau tumor suppressor gene
VHL
occur in various inherited and sporadically occurring tumors. The protein encoded by
VHL
—pVHL—bears no known enzymatic activities but interacts with numerous protein partners. With the identification of distinct pVHL-containing multiprotein complexes, a refined portrait of pVHL tumor suppressor function has arisen. In general, pVHL acts as a multipurpose adaptor protein that controls a diverse array of gene expression programs, as well as extracellular matrix assembly and microtubule-based processes, by linking various target proteins to appropriate enzymatic activities. These findings provide an evermore complex but coherent view of how pVHL functions molecularly and of the consequences of dysregulation of these diverse molecular activities on tumor formation.
An Inside Job?
Authors: Adler Elizabeth M.
Published: June 2008

Toll-like receptor (TLR) 4 interacts with the TIRAP-MyD88 pair of adaptor proteins to stimulate the production of inflammatory cytokines and with the TRAM-TRIF pair to stimulate the production of type I interferons (see Watts). Curious about how TLR4 coordinates activation of these two signaling pathways, Kagan
et al
. used tagged constructs to analyze TLR4 location in macrophages and found that it was present both at the plasma membrane and in early endosomes. Inhibition of dynamin-dependent endocytosis prevented lipopolysaccharide (LPS)-induced internalization of endogenous TLR4 and blocked TRAM-TRIF-dependent phosphorylation of the transcription factor IRF3 (and the expression of target genes), whereas TIRAP-MyD88-dependent phosphorylation of p38 mitogen-activated protein kinase and IκBα degradation were unaffected. Whereas fluorescently labeled TIRAP localized to the plasma membrane, fluorescently labeled TRAM was present at both the plasma membrane and in early endosomes. Overall TRAM localization depended on the first 20 residues, a bipartite motif consisting of a previously characterized myristoylation motif (the first 7 residues) followed by a region enriched in basic and aromatic residues, with the myristoylation motif alone sufficient for localization to endosomes. Analyses of LPS-induced cytokine production by TRAM localization mutants indicated that TLR4 signaling through TRAM-TRIF took place in endosomes. Noting that no known TLRs stimulate type I interferon production from the plasma membrane (TLR4 had been thought to be the lone exception), the authors determined that localization of TRAF3 (an adaptor critical for type I interferon production) was solely cytosolic. Moreover, targeting of TRAF3 to the plasma membrane enabled interferon-β induction by TLR2, which isn’t usually involved in this pathway. The authors thus propose that TLR4 stimulates TIRAP-MyD88 signaling from the cell surface and initiates TRAM-TRIF signaling only after internalization.

J. C. Kagan, T. Su, T. Horng, A. Chow, S. Akira, R. Medzhitov, TRAM couples endocytosis of Toll-like receptor 4 to the induction of interferon-β.
Nat Immunol.
9
, 361-368 (2008).
[PubMed]

C. Watts, Location, location, location: Identifying the neighborhoods of LPS signaling.
Nat Immunol
.
9
, 343-345 (2008).
[PubMed]
Paradoxical Signaling
Authors: Ray L. Bryan
Published: June 2008

In contrast to its role in stimulating DNA synthesis and cell division, the cyclin-dependent kinase CDK1 has been implicated in signaling that leads to cell death in nonproliferating neuronal cells. Yuan
et al.
present evidence that the CDK1 protein kinase phosphorylates the FOXO1 transcription factor, which leads to the accumulation of FOXO1 in the nucleus and activation of a FOXO1-dependent reporter gene. This conclusion contrasts with previous work, which reported that phosphorylation of FOXO1 by CDK2 caused movement of FOXO1 out of the nucleus and decreased FOXO1-dependent transcription. In proliferating cells, CDK1-induced activation of FOXO1 led to increased expression of the gene encoding another kinase that contributes to regulation of mitosis--polo-like kinase (Plk). These results may lead to better understanding of neuronal degeneration.

Z. Yuan, E. B. E. Becker, P. Merlo, T. Yamada, S. DiBacco, Y. Konishi, E. M. Schaefer, A. Bonni, Activation of FOXO1 by Cdk1 in cycling cells and postmitotic neurons.
Science
319
, 1665-1668 (2008).
[Abstract]
[Full Text]
Signaling Who Does Which Chores
Authors: Adler Elizabeth M.
Published: June 2008

Honey bees divvy up the labor of obtaining food and feeding the young by age: Young adult "nurse" bees feed the larvae, whereas older bees forage. The transition from nurse to forager, which can be accelerated or slowed in response to the needs of the hive, is accompanied by a decrease in nutrient stores. Noting that abundant nutrient stores typically promote synthesis of insulin [or insulin-like peptides (ILPs) in insects] and that insulin/insulin-like growth factor signaling (IIS) has been implicated in the central regulation of feeding-related behavior in
Drosophila
, Ament
et al
. investigated the role of IIS in honey-bee behavioral maturation. The abundance of mRNAs encoding the ILP
AmIlp1
in brain and the ILP receptors
AmInR1
and
AmInR2
in abdomen was greater in foragers than in nurses (despite their decreased adiposity). This increase depended on nutritional status; immature bees fed on sugar (which led to forager-like lipid stores) expressed more brain
ilp1
and brain and abdominal
inR1
than their counterparts fed a more nutrient-rich diet of sugar and pollen. IIS stimulates TOR (target of rapamycin), and administration of the TOR inhibitor rapamycin elicited a seasonally dependent delay in the onset of foraging. Although insulin signaling is typically associated with increased energy metabolism, the expression of genes encoding proteins involved in energy metabolism--notably, five of six in the citrate cycle--was enhanced in the brains of nurse bees compared with foragers, although foragers have higher overall metabolic rates. The authors conclude that, in the honey bee, IIS--which is involved in regulation of metabolism and of feeding behavior in a solitary insect--is associated with regulation of social behavior.

S. A. Ament, M. Corona, H. S. Pollock, G. E. Robinson, Insulin signaling is involved in the regulation of worker division of labor in honey bee colonies.
Proc. Natl. Acad. Sci. U.S.A.
105
, 4226-4231 (2008).
[Abstract]
[Full Text]
Faulty Scaffolding and Cancer
Authors: Kiberstis Paula A.
Published: June 2008

Differentiation, survival, and growth of B cells requires proper functioning of the nuclear factor κB (NF-κB) signaling pathway, including CARD11, a cytoplasmic scaffolding protein that serves as a docking site for signaling molecules. Lenz
et al
. have discovered that a certain subtype of diffuse large B cell lymphoma (DLBCL), the most common form of non-Hodgkin’s lymphoma in humans, is caused by mutations in the
CARD11
gene. In cell culture experiments, these mutant forms of CARD11 caused inappropriate activation of the NF-κB pathway.

G. Lenz, R. E. Davis, V. N. Ngo, L. Lam, T. C. George, G. W. Wright, S. S. Dave, H. Zhao, W. Xu, A. Rosenwald, G. Ott, H. K. Muller-Hermelink, R. D. Gascoyne, J. M. Connors, L. M. Rimsza, E. Campo, E. S. Jaffe, J. Delabie, E. B. Smeland, R. I. Fisher, W. C. Chan, L. M. Staudt, Oncogenic
CARD11
mutations in human diffuse large B cell lymphoma.
Science
319
, 1676-1679 (2008).
[Abstract]
[Full Text]
Matrix Metalloproteinase-7 and the 20
S
Proteasome Contribute to Cellular SenescenceA presentation from the 11th Joint Meeting of the Signal Transduction Society (STS), Signal Transduction: Receptors, Mediators and Genes, Weimar, Germany, 1 to 3 November 2007.
Authors: Bertram Catharina , Hass Ralf
Published: June 2008

A detailed understanding of aging and senescence is limited by the complex interplay of the effects of extracellular and environmental stimuli on cellular metabolic, mutational, and epigenetic phenomena. For example, STASIS (stress or aberrant signaling–induced senescence) is affected by the exposure to free radicals and conditions that cause an increased cellular production of reactive oxygen species (ROS) during normal life span. In addition, progressive telomere erosion and telomeric dysfunction contribute to a cellular feature termed replicative or cellular senescence. To focus on distinct cellular pathways that contribute to these different forms of senescence, we investigated the reversible differentiation and aging process of the human U937 leukemia cell line. This was compared to cellular senescence that occurred in normal primary human mammary epithelial cells (HMECs). These two cell systems revealed an important role of the proteolytic activity of the 20
S
proteasome and its activation by the nuclear protein poly(ADP-ribose) polymerase–1 (PARP-1) during "retrodifferentiation" and rejuvenation of the leukemic cells. Moreover, reduced extracellular proteolytic activity of certain matrix metalloproteinases—for example, MMP-7—is associated with accelerated aging and senescence in normal HMECs.
Geranylgeranylation but Not GTP-Loading of Rho GTPases Determines T Cell FunctionA presentation from the 11th Joint Meeting of the Signal Transduction Society (STS), Signal Transduction: Receptors, Mediators and Genes, Weimar, Germany, 1 to 3 November 2007.
Authors: Waiczies Sonia , Bendix Ivo , Zipp Frauke
Published: June 2008

Rho guanosine triphosphatases (GTPases) orchestrate signaling pathways leading to cell migration. They are typically responsible for the organization of actin filaments that support actomyosin contractility and cell-body translocation. The function of Rho GTPases depends on GTP-loading and isoprenylation by geranylgeranyl pyrophosphate (GGpp). The latter posttranslational modification may be manipulated by agents such as 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors (HMGCRIs) that prevent de novo synthesis of isoprenoids such as GGpp. HMGCRIs have anti-inflammatory properties and substantially reduce infiltration of inflammatory immune cells into target tissues, including the central nervous system (CNS) during neuroinflammation. The depletion of the cellular isoprenoid pool is believed to result in the regulation of antigen-specific T cells outside the target organ and also to prevent migration of these cells into target organs, such as the CNS. In vivo treatment with HMGCRI in the experimental autoimmune encephalitis (EAE) rodent model of multiple sclerosis reduces the capacity of activated T cells to traffic to and within the brain. This presentation shows that geranylgeranylation is fundamental for RhoA-mediated downstream events such as influencing cytoskeletal organization and the migration of T cells. Tethering of RhoA to the membrane by GGpp is necessary for T cell migration and provides a mechanism by which HMGCRI may prevent T cell infiltration into inflamed compartments.
The Sweet-Blind Mice
Authors: Ray L. Bryan
Published: June 2008

Understanding the brain’s reward system, mediated by the neurotransmitter dopamine, has the potential to help address the enormous health risk to humans posed by obesity. Like addictive drugs, good-tasting food increases the accumulation of dopamine in the nucleus accumbens (NAcc) of the ventral striatum. In fact, some mice with defects in the dopamine reward system fail to eat and die of starvation. But besides taste-induced or hedonic responses, animals appear also to sense the nutritional value of ingested food. De Araujo
et al.
present a series of experiments that implicate the same brain mesolimbic dopamine reward system in both processes. They distinguished the two responses by making knockout mice that lacked a functional TRPM5 (transient receptor potential channel M5) ion channel, which mediates taste sensation in taste receptor cells. Unlike wild-type animals, after being deprived of water, these mutant animals showed no preference for a sucrose solution over plain water. However, over a period of conditioning of 6 days in which mice were presented with bottles containing sucrose solution or water that could be distinguished by their sipper, the starved mutant animals had a distinct preference for the more nutritive solution. The authors implanted microdialysis probes in the NAcc of mutant or wild-type animals and showed that the mutant’s response to the sucrose solution was associated with the release of dopamine similar to that in wild-type mice drinking sucrose solution. Electrophysiological recordings from neurons in the NAcc and orbitofrontal cortex indicated that the increased dopamine release was correlated with activation of the brain reward circuitry. The authors, and Andrews and Horvath in commentary, conclude that both hedonic and nutritional or homeostatic rewards of ingested food are processed similarly, not separately as sometimes thought, in the higher brain centers that process reward responses.

I. E. de Araujo, A. J. Oliveira-Maia, T. D. Sotnikova, R. R. Gainetdinov, M. G. Caron, M. A. L. Nicolelis, S. A. Simon, Food reward in the absence of taste receptor signaling.
Neuron
57
, 930-941 (2008).
[PubMed]

Z. B. Andrews, T. L. Horvath, Tasteless food reward.
Neuron
57
, 806-808 (2008).
[PubMed]
Clocking Out
Authors: Foley John F.
Published: June 2008

Under homeostatic conditions, hematopoietic stem cells (HSCs) circulate in the blood, albeit in low numbers. Retention of HSCs in the bone marrow (BM) is mediated by the chemokine CXCL12. Pharmacological agents that mobilize HSCs from the BM cause the degradation of CXCL12; however, the mechanism responsible for physiological egress of HSCs is unknown. When mice were housed under standard light-dark cycles, Méndez-Ferrer
et
al
. noticed that the numbers of HSCs in the circulation peaked 5 hours after light was initiated and were at their lowest 5 hours after darkness began. Enzyme-linked immunosorbent assays and quantitative reverse transcription polymerase chain reaction (RT-PCR) analyses revealed that the expression of
Cxcl12
and the abundance of CXCL12 in the BM were lower when HSCs were most abundant in the circulation and higher when HSCs were retained in the BM. The regulation of circadian activity by the suprachiasmatic nucleus in the hypothalamus is mediated by the release of adrenergic neurotransmitters by the sympathetic nervous system (SNS). Pharmacological or surgical disruption of the SNS in mice abolished the circadian control of
Cxcl12
expression and HSC circulation. Experiments with pharmacological agents showed that β
3
-adrenergic receptor (β
3
-AR) stimulation resulted in reduced
Cxcl12
expression in the BM. Circadian variation of
Cxcl12
expression and the number of circulating HSCs was disrupted in β
3
-AR-deficient mice. Finally, the abundance and nuclear localization of the transcription factor Sp1, which activates
Cxcl12
expression, were lower in primary BM stromal cells treated with a β-AR agonist than in untreated cells. Together, these data provide a potential mechanism for the physiological circulation of stem cells and raise the question of why this process should be under circadian control (see commentary by Scadden).

S. Méndez-Ferrer, D. Lucas, M. Battista, P. S. Frenette, Haematopoietic stem cell release is regulated by circadian oscillations.
Nature
452
, 442-447 (2008).
[PubMed]

D. T. Scadden, Stem cells traffic in time.
Nature
452
, 416-417 (2008).
[PubMed]
Three Steps to Commitment
Authors: VanHook Annalisa
Published: June 2008

Mouse embryonic stem (ES) cells that differentiate in serum-supplemented media give rise to hematopoietic cells such as erythrocytes and macrophages. Along the way, they pass through three intermediate stages, epiblast-like, mesodermal, and hemangioblast, which are distinguishable by gene expression profile, developmental potential, and morphology. Pearson
et
al
. describe a serum-free culture method that recapitulates this differentiation pathway, and they used it to identify the stage-specific requirements for each cytokine. Cell identities were determined by quantitative reverse transcription polymerase chain reaction (RT-PCR) analysis of marker gene expression and by developmental potential as assayed by cell morphology in colonies after replating. A cocktail of bone morphogenetic protein 4 (BMP4), fibroblast growth factor 2 (FGF2), activin A, and vascular endothelial growth factor (VEGF) mimicked serum in directing ES cell fate. ES cells treated with BMP4 passed through an epiblast-like stage before expressing mesoderm-specific genes such as
Mesp2
and
Tbx6
. Without further treatment, mesodermal precursor cells gave rise to hemangioblasts and other mesodermal derivatives that were not observed in serum-directed differentiation, indicating that they retained pluripotency. However, continuing the treatment of these cells with a combination of FGF2 and activin A induced only the formation of hemangioblasts. Addition of VEGF caused the cells to differentiate into hematopoietic cells. The authors have identified a handful of factors that can direct the differentiation of ES cells to a particular subset of fates in vitro, a necessary prelude to understanding how lineage restriction may by regulated in stem cells in vivo and how it may be manipulated in vitro.

S. Pearson, P. Sroczynska, G. Lacaud, V. Kouskoff, The stepwise specification of embryonic stem cells to hematopoietic fate is driven by sequential exposure to Bmp4, activin A, bFGF and VEGF.
Development
135,
1525-1535 (2008).
[Abstract]
[Full Text]
Smell and Repel
Authors: Stern Peter
Published: June 2008

Blood-feeding insects are responsible for spreading some of the deadliest infectious diseases. Topically applied insect repellents play a crucial role in protecting humans from these insects. The most widely used of these compounds, DEET, has been around for more than 50 years, but its function is still poorly understood. Ditzen
et al.
found in both fruit flies and in the malaria mosquito that DEET acts on the insect olfactory system by inhibiting olfactory neurons that mediate responses to attractive substances. It seems that DEET functions by masking the host odor through blocking odorant receptors that require the olfactory co-receptor OR83b.

M. Ditzen, M. Pellegrino, L. B. Vosshall, Insect odorant receptors are molecular targets of the insect repellent DEET.
Science
319
, 1838-1842 (2008).
[Abstract]
[Full Text]
Clearing the Cleft
Authors: Stern Peter
Published: June 2008

Ionic currents through postsynaptic receptors can produce an electric field inside the synaptic cleft, which raises the possibility that they can affect the dwell time of electrically charged neurotransmitters. Sylantyev
et al
. analyzed excitatory postsynaptic currents in acute rat brain slice cultures and in cultured neurons. Electric fields present in the extracellular space of the synaptic cleft generated by open ion channels during synaptic transmission accelerated diffusion of charged glutamate molecules out of the cleft via electrodiffusion, which altered signal integration in hippocampal pyramidal cells.

S. Sylantyev, L. P. Savtchenko, Y.-P. Niu, A. I. Ivanov, T. P. Jensen, D. M. Kullmann, M.-Y. Xiao, D. A. Rusakov, Electric fields due to synaptic currents sharpen excitatory transmission.
Science
319
, 1845-1849 (2008).
[Abstract]
[Full Text]
Focus Issue: Mechanisms of Gene Regulation
Authors: Foley John F.
Published: June 2008

In this Focus Issue of
Science Signaling
, which complements the
Science
Special Issue on Gene Regulation (
http://www.sciencemag.org/generegulation/
), we examine the multilayered mechanisms that control how gene expression is turned on or off. Perspectives in this issue discuss how oncogenic Ras directs the silencing of tumor suppressor genes; how intrachromosomal looping brings enhancers and promoters together to stimulate gene expression; and how the abundance of a transcriptional coactivator affects nuclear receptor–mediated transcription.
Silent Assassin: Oncogenic Ras Directs Epigenetic Inactivation of Target Genes
Authors: Cheng Xiaodong
Published: June 2008

Oncogenic transformation is associated with genetic changes and epigenetic alterations. A study now shows that oncogenic Ras uses a complex and elaborate epigenetic silencing program to specifically repress the expression of multiple unrelated cancer-suppressing genes through a common pathway. These results suggest that cancer-related epigenetic modifications may arise through a specific and instructive mechanism and that genetic changes and epigenetic alterations are intimately connected and contribute to tumorigenesis cooperatively.
Shape-Shifting Chemokines
Authors: VanHook Annalisa
Published: June 2008

Lymphotactin (Ltn) is the odd-man-out amongst the chemokines. Whereas all the others have two disulfide bridges, Ltn has only one, a distinctive feature that may explain its unique structural properties. In vitro Ltn adopts two conformations: Ltn10, which predominates in high-salt conditions at low temperatures (10°C), and Ltn40, a high-temperature, low-salt (40°C) form. The two forms are in equilibrium under physiological conditions. Tuinstra
et al
. addressed the functional implications of this conformational variability. Nuclear magnetic resonance studies showed that Ltn10 and Ltn40 corresponded to distinct structures that used different combinations of amino acid residues for internal hydrogen bonding. The authors engineered recombinant Ltn10 and Ltn40 proteins by making amino acid substitutions that trapped the protein in one conformation or the other. Like other chemokines, Ltn binds to both its receptor (XCR1) and extracellular matrix glycosaminoglycans such as heparin. However, recombinant Ltn10 bound with high affinity to XCR1, but not to heparin, whereas recombinant Ltn40 bound with high affinity to heparin, but not XCR1. Neither protein mimicked the activity of Ltn in stimulating chemotaxis of human T cells in vitro. Furthermore, the authors found that Ltn could not bind to XCR1 and heparin simultaneously and suggested that Ltn must release the extracellular matrix component before it binds to and activates its receptor. This study suggests that altering the equilibrium between two conformational states of a protein by changing physiological conditions such as temperature (for example, during inflammation) may constitute a previously unrecognized means of affecting signaling pathway dynamics.

R. L. Tuinstra, F. C. Peterson, S. Kutlesa, E. S. Elgin, M. A. Kron, B. F. Volkman, Interconversion between two unrelated protein folds in the lymphotactin native state.
Proc. Natl. Acad. Sci. U.S.A.
105
, 5057-5062 (2008).
[Abstract]
[Full Text]
Sirtuin to Become Astrocytes?
Authors: Adler Elizabeth M.
Published: June 2008

Although neural progenitor cells (NPCs) can differentiate into neurons, astrocytes, or oligodendrocytes, brain injury typically stimulates production of astrocytes in preference to neurons (see Libert
et al
.). Noting that various neurological diseases are associated with inflammation, which promotes oxidative conditions, Prozorovski
et al
. investigated the effects of redox state on NPC differentiation. Pharmacological inhibition of antioxidative enzymes (oxidative conditions) decreased proliferation of mouse cortical NPCs cultured in the presence of basic fibroblast growth factor (bFGF), whereas reducing conditions enhanced NPC proliferation. The fraction of cells that differentiated into astrocytes after bFGF withdrawal was greater when NPCs had been cultured under oxidative conditions, whereas the fraction that differentiated into neurons was smaller. In contrast, cells that had been cultured under reducing conditions were more likely to differentiate into neurons and less likely to differentiate into astrocytes. Expression of the histone deacetylase Sirt1 was increased in NPCs cultured under oxidative conditions, and
Sirt1
knockdown with siRNA blocked the effects of oxidation on NPC differentiation. Oxidative conditions promoted association of Sirt1 with the transcription factor Hes1 and with the promoter of
Mash1
, which encodes a neurogenic transcription factor, and led to decreased acetylation of histone H3, residue Lys9, and a Sirt1-dependent decrease in
Mash1
expression. Mouse pups injected with a pro-oxidant showed an increase in Sirt1 immunoreactivity in a germinal region of the brain, along with a decrease in neurogenesis; in utero electroporation with shRNA directed against Sirt1 increased neurogenesis in such oxidant-treated pups. Thus, the authors conclude that nontoxic manipulation of redox conditions can influence NPC fate and that Sirt1 plays a critical role in this process.

T. Prozorovski, U. Schulze-Topphoff, R. Glumm, J. Baumgart, F. Schröter, O. Ninnemann, E. Siegert, I. Bendix, O. Brüstle, R. Nitsch, F. Zipp, O. Aktas, Sirt1 contributes critically to the redox-dependent fate of neural progenitors.
Nat. Cell Biol.
10
, 385-394 (2008).
[PubMed]

S. Libert, D. Cohen, L. Guarente, Neurogenesis directed by Sirt1.
Nat. Cell Biol.
10
, 373-374 (2008).
[PubMed]
Synthesizing a Response
Authors: Foley John F.
Published: June 2008

Arabidopsis
thaliana
is a plant that does not like the shade. If it is overshadowed, a response known as shade avoidance syndrome (SAS) is triggered. This involves adaptations, such as stem elongation, that give the plant a growth advantage over its neighbors. The growth hormone auxin is implicated in the early stages of SAS, but the mechanisms involved are poorly characterized. Tao
et
al
. performed a genetic screen of
Arabidopsis
mutants and identified plants that did not adapt to shaded conditions. These plants had mutations in
TAA1
, which encodes the enzyme tryptophan aminotransferase of
Arabidopsis
. In response to shade treatment, the abundance of auxin was lower in
TAA1
mutants than in wild-type plants; auxin target gene expression was also lower in
TAA1
mutants. In wild-type plants,
TAA1
was highly expressed in the leaves. Reporter assays performed in the presence of an auxin transport inhibitor showed the initial production of auxin in the leaves in response to shade treatment. Further biochemical studies showed that TAA1 catalyzed the conversion of tryptophan to indole-3-pyruvic acid (IPA), an initial step in the pathway to synthesize indole-3-acetic acid (auxin) that had been predicted but not previously biochemically characterized. In another study, Stepanova
et
al
. performed an alternative genetic screen and identified an
Arabidopsis
mutant with defective root growth in response to the gaseous hormone ethylene. The mutant gene responsible for the defect was
TAA1
. As well as showing the tryptophan aminotransferase activity of TAA1 in vitro, the authors demonstrated a critical role for TAA1 in tissue-specific ethylene-dependent production of auxin in plants. As Palme and Nagy discuss in commentary, these studies have identified a critical step in how both environmental and hormonal signals stimulate the localized production of auxin, which then triggers appropriate responses.

Y. Tao, J.-L. Ferrer, K. Ljung, F. Pojer, F. Hong, J. A. Long, L. Li, J. E. Moreno, M. E. Bowman, L. J. Ivans, Y. Cheng, J. Lim, Y. Zhao, C. L. Balleré, G. Sandberg, J. P. Noel, J. Chory, Rapid synthesis of auxin via a new tryptophan-dependent pathway is required for shade avoidance in plants.
Cell
133
, 164-176 (2008).
[Online Journal]

A. N. Stepanova, J. Robertson-Hoyt, J. Yun, L. M. Benavente, D.-Y. Xie, K. Doležal, A. Schlereth, G. Jürgens, J. M. Alonso,
TAA1
-mediated auxin biosynthesis is essential for hormone crosstalk and plant development.
Cell
133
, 177-191 (2008).
[Online Journal]

K. Palme, F. Nagy, A new gene for auxin synthesis.
Cell
133
, 31-32 (2008).
[Online Journal]
Cadherin Learns New Tricks
Authors: VanHook Annalisa
Published: June 2008

Calsyntenins are cadherin superfamily members that associate with scaffold proteins and amyloid precursor protein (APP) in neuronal cell membranes. Like APP, the extracellular domain of calsyntenin is released by proteolytic cleavage and the intracellular domain is endocytosed. Calsyntenins have been postulated to play a role in memory and learning in mammals, but no functional data have been reported. Ikeda
et al
. present genetic evidence that the extracellular domain of calsyntenin is required for learning in the nematode
Caenorhabditis elegans
. Animals carrying a null mutation in the single nematode calsyntenin gene
CASY-1
failed to learn in three different behavioral conditioning assays in which the animals were taught to avoid a chemical (salt or benzaldehyde) or thermal cue by learning to associate that cue with starvation.
CASY-1
mutants detected the cues in standard chemotaxis assays but could not learn to avoid them. Transient expression of
CASY-1
in a single adult neuron (ASER) was sufficient to rescue the phenotype, which suggests that the learning deficiency did not result from a developmental defect. Furthermore, expression of just the extracellular domain of CASY-1 was sufficient to rescue the learning defect, leading the authors to suggest that the cleaved protein acts locally either on ASER itself or on a nearby neighbor. Given the shared trafficking pattern and close physical association between calsyntenins and APP, it was surprising to find that the intracellular domain of CASY-1 was not necessary for learning in the nematode. It will be interesting to see which domains are essential for mammalian calsyntenin function.

D. D. Ikeda, Y. Duan, M. Matsuki, H. Kunitomo, H. Hutter, E. M. Hedgecock, Y. Iino, CASY-1, an ortholog of calsyntenins/alcadeins, is essential for learning in
Caenorhabditis elegans
.
Proc. Natl. Acad. Sci. U.S.A.
105
, 5260-5265 (2008).
[Abstract]
[Full Text]
Working Back to Pluripotency
Authors: Purnell Beverly A.
Published: June 2008

Recent studies have shown that adult mouse and human fibroblasts can be reprogrammed to a pluripotent state after the viral integration of four transcription factors. However, questions remained as to the origin of those cells, whether specific genomic integration sites are needed, and how tumorigenicity might be reduced (see the Perspective by Bang and Carpenter). Embryonic stem cells and cells of human malignancies are characterized by low levels of let-7 microRNAs (miRNAs), a phenomenon thought to be functionally linked to pluripotency and oncogenesis. In both cases, precursors of let-7 are detectable but processing to the miRNA mature form is inhibited. Viswanathan
et al
. identify Lin28 as an RNA binding protein that selectively inhibits let-7 miRNA processing. Lin28 was recently shown to function with three other factors to reprogram human fibroblasts to pluripotent stem cells. This work suggests that the regulation of miRNA processing may be critical in the dedifferentiation that occurs in both reprogramming and oncogenesis.

S. R. Viswanathan, G. Q. Daley, R. I. Gregory, Selective blockade of microRNA processing by Lin28.
Science
320
, 97-100 (2008).
[Abstract]
[Full Text]

A. G. Bang, M. K. Carpenter, Deconstructing pluripotency.
Science
320
, 58-59 (2008).
[Summary]
[Full Text]
Nitric Oxide-Mediated Oncogenic Loop
Authors: Ray L. Bryan
Published: June 2008

The gene encoding the small guanosine triphosphatase Ras was one of the first oncogenes to be discovered. Certain tumor cells require continuous presence of activated Ras oncogenes for continued proliferation. Ras has numerous targets, but protein kinase B (PKB, also known as AKT) is a critical one to sustain tumor growth. Lim
et al
. therefore explored what target or targets of AKT (it also has many) might account for sustained tumor growth. In an assay in which a mixture of cells was transplanted into mice (to provide a characteristic tumor microenvironment), the authors found that in the presence of tumor cells expressing the oncogenic form of Hras, the contribution of cells expressing an activated phosphoinositide 3-kinase (PI3K) subunit (to activate AKT) and LacZ (to allow detection of the cells by their blue staining) to tumors was inhibited if the AKT target eNOS (endothelial nitric oxide synthase) was depleted with short hairpin RNA (shRNA). If eNOS was depleted with shRNA, participation of such cells in tumor formation was rescued by wild-type eNOS but not by a mutant eNOS that was no longer phosphorylated by AKT. Activated eNOS makes nitric oxide, which promotes
S
-nitrosylation of cysteine residues in proteins, and one such target--bringing us back almost to where we started--is wild-type HRas, which is activated by such nitrosylation. Indeed, other evidence supports facilitation of the effects of oncogenic Ras by wild-type Ras proteins. Activation of AKT promoted nitrosylation of wild-type Ras, and, in assays of tumor initiation or maintenance in mice injected with transformed cells, loss of HRas inhibited tumor formation by cells with activated PI3K. This effect was overcome by expression of wild-type HRas, but an HRas in which the nitrosylated cysteine was mutated provided little recovery of tumor cell growth. In similar studies with human pancreatic tumor cells (a cancer highly dependent on Ras mutations), eNOS and the modified cysteine residue of HRas or NRas were shown to be required for initiation and maintenance of tumor growth in mice. Thus, the authors propose that oncogenic Ras proteins act through AKT and eNOS to activate wild-type Ras proteins and contribute to tumor growth. Inhibition of eNOS is thus a possible therapeutic target for human cancers such as pancreatic cancer that are promoted by Ras oncogenes.

K.-H. Lim, B. B. Ancrile, D. F. Kashatus, C. M. Counter, Tumour maintenance is mediated by eNOS.
Nature
452
, 646-649 (2008).
[PubMed]
Calpain in the CNS: From Synaptic Function to Neurotoxicity
Authors: Liu Jing , Liu Ming Cheng , Wang Kevin K. W.
Published: June 2008

The calpains are a class of cellular cysteine proteases that require calcium and are functionally active at neutral pH. Calpain activation can take place in two modes: controlled activation under physiological conditions (in which only a few molecules of calpain are activated per cell), and hyperactivation under pathological conditions that involve sustained calcium overload (in which all available calpain molecules are activated). Regulated activation of calpain in the central nervous system (CNS) may be critical to synaptic function and memory formation, with possible substrates including various structural and scaffolding proteins, enzymes, and glutamate receptors. Hyperactivation of calpain in the central nervous system is generally associated with severe cellular challenge or damage. Calpain cleavage products may thus provide useful biomarkers for the presence of neurodegenerative processes or neuronal injury.
Follow the Endothelin
Authors: Ray L. Bryan
Published: June 2008

The neurons of the sympathetic nervous system have distinct properties that allow them to regulate a particular end organ. Various models have been proposed to explain how such precise wiring of neurons to their synaptic targets might come about during development. The neurons might grow in a relatively random manner and then adopt appropriate characteristics after interacting with their target tissue, or there might be molecular markers that guide particular neurons to the right target. Makita
et al
. provide new evidence in favor of the latter scheme for a set of neurons of the superior cervical ganglia (SCG) that follow along the external carotid artery to reach their salivary gland targets. The authors examined mRNA from external carotid artery in microarray analysis to search for expression of transcripts that might encode guidance molecules. They found specific expression of mRNA encoding endothelin-converting enzyme (which converts precursors of endothelin into the active form). Endothelins were also expressed in the arteries, and endothelin receptors were present on a subset of the SCG neurons. In vitro, endothelin 3 promoted outgrowth of explanted SCG neurons. In vivo, in mice lacking expression of endothelin receptor A, the SCG axons were no longer extended along the external carotid arteries. The authors propose that smooth muscle cells of the external carotid artery may release endothelin, a neuropeptide better known for its vasoconstriction activity, which acts as a growth factor for axons of the SCG, guiding them to their salivary gland targets.

T. Makita, H. M. Sucov, C. E. Gariepy, M. Yanagisawa, D. D. Ginty, Endothelins are vascular-derived axonal guidance cues for developing sympathetic neurons.
Nature
452
, 759-763 (2008).
[PubMed]
Little Change, Large Consequence
Authors: Vinson Valda J.
Published: June 2008

Occasionally, small changes in sequence change the overall architecture of large protein assemblies. One system for understanding protein assembly is the bacterial flagellar filament, the prototype of which comes from
Salmonella
and contains 11 protofilaments. Galkin
et al.
now show that the homologous flagellar filament of
Campylobacter
contains only 7 protofilaments. The difference may be related to sequence divergence in a region of flagellin that in
Salmonella
is involved in coiled-coil formation and is recognized by the vertebrate Toll-like receptor 5 (TLR5).
Campylobacter
is not recognized by TLR5, and its evasion may have driven the change in quaternary structure.

V. E. Galkin, X. Yu, J. Bielnicki, J. Heuser, C. P. Ewing, P. Guerry, E. H. Egelman, Divergence of quaternary structures among bacterial flagellar filaments.
Science
320
, 382-385 (2008).
[Abstract]
[Full Text]
NADPH Sensor Regulates NO Production
Authors: Gough Nancy R.
Published: June 2008

The crystal structure of the protein known as HSCARG shows that it binds NADPH (nicotinamide adenine dinucleotide phosphate), and changes in the ratio of NADPH to its oxidized form NADP
+
alter the subcellular localization of HSCARG, which suggests that it may serve as a sensor for NADPH. One of the important functions of NADPH is in the production of nitric oxide (NO). The substrate for nitric oxide synthase (NOS) is arginine, which is regenerated from citrulline through a reaction involving the rate-limiting enzyme argininosuccinate synthetase (AS). Zhao
et al
. showed that AS was coimmunoprecipitated with HSCARG from HeLa cells, and this interaction was further characterized in transfected cells and in vitro assays. In transfected cells, the two proteins colocalized, and overexpression of AS resulted in the partial translocation of HSCARG from the cytoplasm to the nucleus. This translocation was also noted when the cells were treated with an NO donor and was inhibited in the presence of a cGMP inhibitor. The abundance of HSCARG was also increased in cells overexpressing AS, and the NO donor or inflammatory cytokines that raise NO concentrations, such as tumor necrosis factor-α (TNF-α) or interleukin-1β (IL-1β), also stimulated transcription of
hscarg
. In cells overexpressing HSCARG, NO production was decreased and HSCARG inhibited AS activity in vitro. Cells in which HSCARG was decreased through RNA interference exhibited elevated NO production and were more susceptible to apoptosis. Dehydroepiandrosterone (DHEA) was used to decrease the NADPH/NADP
+
ratio and, in cells cotransfected with tagged versions of HSCARG and AS, an increase in the interaction between these two proteins was observed after treatment with DHEA. Addition of NADPH to purified HSCARG and AS inhibited the interaction of the proteins in vitro. Thus, HSCARG appears to allow cells to respond to changing redox conditions and contribute to the regulation of NO production. It will be interesting to determine its function in the nucleus.

Y. Zhao, J. Zhang, H. Li, Y. Li, J. Ren, M. Luo, X. Zheng, An NADPH sensor protein (HSCARG) down-regulates nitric oxide synthesis by association with argininosuccinate synthetase and is essential for epithelial cell viability.
J. Biol. Chem.
283
, 11004-11013 (2008).
[Abstract]
[Full Text]
How Do Bugs Smell?
Authors: Ray L. Bryan
Published: June 2008

Two papers published this week report both clarifying and somewhat contradictory findings regarding signaling by insect odorant receptors. Odorant receptors (ORs) in mammals are canonical heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) and are often coupled to generation of the second messenger adenosine 3′,5′-monophosphate (cAMP), which can modulate conductance of ion channels. Insect ORs have presented a puzzle, however, as they only vaguely resemble GPCRs and assemble as heterodimers of one variable OR and a common co-receptor called Or83b. Wicher
et al
. expressed
Drosophila
receptors Or22a and Or83b in human embryonic kidney cells and used patch clamping to characterize odorant-induced ion currents in the transfected cells. They report that expression of Or83b alone conferred a nonselective cation channel whose conductance was increased by cAMP or guanosine 3′,5′-monophosphate (cGMP). When the authors transfected cells with the Or22a-Or83b pair and also with known mammalian cyclic nucleotide-gated channels, odorant-induced activation of the mammalian channel indicated that the insect OR activated cAMP signaling. Thus, they conclude that the insect ORs themselves form ligand-gated ion channels that sense odorant and amplify signals by G-protein signaling. Sato
et al
. used similar strategies to analyze various insect ORs. Human cells expressing receptor proteins from fruit fly, mosquito, or silk moth showed a response to odorant that included Ca
2+
influx and increased nonselective cation conductance. Outside-out patch clamp studies from membranes of oocytes or human cells expressing the receptor proteins confirmed that the insect ORs themselves conferred the observed currents. However, Sato
et al
.’s experiments did not detect odorant-induced generation of cAMP, and pharmacological inhibition of G-protein signaling failed to block odorant-induced currents. As noted in commentary from Chesler and Firestein, resolution of the discrepancies will require further work but may have to do with Sato
et al
.’s focus on early events (within seconds) and Wicher
et al
.’s characterization of later events—30 seconds to a minute after receptor stimulation. Insect ORs do not resemble known ion channels at all, so the work suggests there is more to learn about such proteins and raises the question of whether orphan GPCRs in other organisms might also function as ligand-gated ion channels.

K. Sato, M. Pellegrino, T. Nakagawa, T. Nakagawa, L. B. Vosshall, K. Touhara, Insect olfactory receptors are heteromeric ligand-gated ion channels.
Nature
452
, 1002-1006 (2008).
[PubMed]

D. Wicher, R. Schäfer, R. Bauernfeind, M. C. Stensmyr, R. Heller, S. H. Heinemann, B. S. Hannson,
Drosophila
odorant receptors are both ligand-gated and cyclic-nucleotide-activated cation channels.
Nature
452
, 1007-1011 (2008).
[PubMed]

A. Chesler, S. Firestein, Current views on odour receptors.
Nature
452
, 944 (2008).
[PubMed]
Notching Up Tumor Progression
Authors: Adler Elizabeth M.
Published: June 2008

Hypoxia is associated with tumor progression and may contribute to epithelial-mesenchymal transition (EMT), a process in which epithelial cells become motile and invasive. In tumors, EMT may represent the initial stages of metastasis. Sahlgren
et al
. found that hypoxia enhanced activation of a Notch reporter in tumor cell lines cocultured with cells expressing the Notch ligand Jagged1 and enhanced ligand-dependent expression of Notch target genes. Hypoxia increased Notch activation [abundance of the Notch 1 intracellular domain (N1ICD)] in SKOV-3 ovarian tumor cells and also increased expression of the mRNA and protein of the Notch ligand Delta-like 1. Hypoxic conditions stimulated morphologic changes and decreased E-cadherin mRNA and protein in SKOV-3 cells, phenotypic changes characteristic of EMT that were blocked by a γ-secretase inhibitor (GSI, to prevent release of N1ICD) or transfection with dominant-negative components of the Notch pathway. In contrast, exposure to Jagged-1 under normoxic conditions promoted morphologic changes similar to those seen in hypoxia, and introduction of N1ICD decreased E-cadherin abundance. Hypoxia stimulated recruitment of the N1ICD to the promoter of
Snail-1
, which encodes a transcription factor implicated in the EMT; moreover, introduction of N1ICD increased
Snail-1
expression, whereas GSI inhibited an increase in
Snail-1
mRNA in response to hypoxia. Further, Notch signaling enhanced the hypoxia-dependent recruitment of hypoxia-inducible factor-1α (HIF-1α) to the promoter of
lysyl oxidase
(
LOX
, which encodes a protein that increases Snail-1 stability) and the hypoxia-dependent stimulation of
LOX
expression. GSI inhibited the promotion of SKOV-3 cell migration by hypoxia in a wound-healing assay and inhibited their migration across a membrane in an assay of invasion. The authors thus propose that Notch signaling plays a critical role in initiating EMT in response to tumor cell hypoxia.

C. Sahlgren, M. V. Gustafsson, S. Jin, L. Poellinger, U. Lendahl, Notch signaling mediates hypoxia-induced tumor cell migration and invasion.
Proc. Natl. Acad. Sci. U.S.A.
105
, 6392-6397 (2008).
[Abstract]
[Full Text]
A Defensin Defense Against Diabetes?
Authors: Adler Elizabeth M.
Published: June 2008

The treatment of diabetes has focused on either replacing insulin or increasing sensitivity to it. Noting that plasma concentrations of human neutrophil α-defensins (HNPs) activate Akt, a component of the insulin signaling pathway, and that severe bacterial infections (which are associated with increased plasma HNPs) are often accompanied by hypoglycemia, Liu
et al
. investigated the role of HNPs in regulating hepatic glucose production. Intravenous administration of HNP-1 decreased blood glucose concentration in both wild-type mice and Zucker diabetic fatty rats [ZDF rats, a model of type II (insulin-resistant) diabetes]. Under conditions in which hyperinsulinemia was maintained in ZDF rats by means of the continuous infusion of insulin, HNP-1 increased the rate of glucose infusion required to maintain euglycemia. Radioisotopic analysis indicated that this depended on a decrease in glucose production, rather than an increase in uptake by skeletal muscle. HNP-1 inhibited pharmacologically stimulated gluconeogenesis and glycogenolysis in isolated mouse hepatocytes and also inhibited the pharmacologically stimulated increase in mRNAs encoding the gluconeogenic enzymes glucose-6-phosphatase and phosphoenoylpyruvate carboxyl kinase (PEPCK) and activation of a gene reporter under the control of the
PEPCK
promoter. Like insulin, HNP-1 and -2 stimulated phosphorylation of Akt and FoxO1 in hepatoma cells and primary mouse hepatocytes. Whereas insulin stimulated the phosphorylation of IRS1 but not that of c-Src, HNP-1 stimulated the phosphorylation of c-Src but not that of IRS-1. Furthermore, c-Src activation was required for the effects of HNP-1 on glucose-6-phosphatase mRNA expression and gluconeogenesis but not for those of insulin. Thus, the authors conclude that HNP-1 can inhibit hepatic glucose production through a pathway distinct from that mediated by insulin, a finding that may have implications for the treatment of diabetes.

H.-Y. Liu, Q. F. Collins, F. Moukdar, D. Zhuo, J. Han, T. Hong, S. Collins, W. Cao, Suppression of hepatic glucose production by human neutrophil α-defensins through a signaling pathway distinct from insulin.
J. Biol. Chem
.
283
, 12056-12063 (2008).
[Abstract]
[Full Text]