bluuebunny/crossref_metadata_2025 huggingface dataset:
Inhibiting Osteoclast Formation
Authors: Gough Nancy R.
Published: March 2007

Hyaluronic acid (hyaluronan, HA) is a component of the extracellular matrix and is also used clinically to treat joint and cartilage diseases such as osteoarthritis. Chang
et al
. show that high molecular mass HA (HMM-HA), but not low molecular mass HA, inhibited osteoclast differentiation of bone marrow-derived macrophages in response to macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor κB ligand (RANKL). HMM-HA did not block RANKL-stimulated osteoclastogenesis of RAW264.7 cells, which is independent of M-CSF. Although CD44 is one known receptor for HA, antibodies that block CD44 had no effect on the inhibition of osteoclast formation by HA. However, function-blocking antibodies against Toll-like receptor 4 (TLR4) blocked the inhibitory effect of HA, suggesting that HA was acting through the TLR4. TLR4-transfected cells bound labeled HA based on fluorescence-activated cell sorting analysis, and osteoclastogenesis of cells from mice with a nonfunctional TLR4 were not inhibited by HA. HA decreased signaling pathways activated by M-CSF but did not inhibit those activated by RANKL unless the cells were pretreated with HA for 2 days before RANKL exposure. HA decreased the production of reactive oxygen species stimulated by the receptor tyrosine kinase activated by M-CSF and also inhibited activation of Rac and downstream kinases of the mitogen-activated protein kinase family (p38, ERK, and JNK). HA reduced the abundance of RANK and appeared to mediate this effect by its inhibition of M-CSF signaling, because M-CSF leads to the phosphorylation and activation of two transcription factors for which binding sites exist in the
RANK
promoter. Thus, M-CSF may stimulate osteoclastogenesis by increasing the responsiveness to RANKL through increased abundance of the receptor RANK, and HA blocks this pathway. In a mouse model for bone erosion, implantation of collagen films containing RANKL in the presence or absence of HA onto mouse skulls showed that HA decreased the RANKL-induced bone loss. These results show a possible mechanism by which HA is clinically useful, expand the role of innate immune receptors to regulation of bone metabolism, and provide a potential connection between bone metabolism and the innate immune system.

E.-J. Chang, H. J. Kim. J. Ha, H. J. Kim, J. Ryu, K.-H. Park, U.-H. Kim, Z. H. Lee, H.-M. Kim, D. E. Fisher, H.-H. Kim, Hyaluronan inhibits osteoclast differentiation via Toll-like receptor 4.
J. Cell Sci.
120
, 166-176 (2007).
[Abstract]
[Full Text]
Cellular Mechanics Up Close and Personal
Authors: Hurtley Stella M.
Published: March 2007

As they move, vertebrate cells can transmit mechanical information from their insides to their outsides and vice versa. A major pathway for this is through integrin-mediated adhesive interactions between the actin cytoskeleton and the extracellular matrix. Hu
et al
. focus on defining the protein interface mediating the interaction between the cytoskeleton and the extracellular matrix that are part of integrin-based focal adhesions by using sophisticated live-cell microscopic analysis. The motion of the actin cytoskeleton is differentially transmitted through focal adhesions, with the efficiency of transmission gradually decreasing from actin-binding proteins to integrins, defining the focal adhesion as a hierarchical molecular clutch. The internal molecular dynamics of focal adhesions thus represents a key element in the mechanics of cell morphogenesis during migration.

K. Hu, L. Ji, K. T. Applegate, G. Danuser, C. M. Waterman-Storer, Differential transmission of actin motion within focal adhesions.
Science
315
, 111-115 (2007).
[Abstract]
[Full Text]
Enzyme Kinetics in Living Color
Authors: Hurtley Stella M.
Published: March 2007

Innovative methods are required to study the spatial regulation of enzymatic activity inside living cells. Yudushkin
et al
. describe such a method based on the detection of enzyme-substrate complexes using fluorescence lifetime imaging microscopy. The detection of the enzyme interacting with the substrate ensures utmost specificity and enables evaluation of the localized activity of a particular enzymatic species. The technique was used to investigate the spatial regulation of growth factor signaling by the tyrosine phosphatase PTP1B and revealed that PTP1B exists inside cells as kinetically distinct, spatially separated subpopulations.

I. A. Yudushkin, A. Schleifenbaum, A. Kinkhabwala, B. G. Neel, C. Schultz, P. I. H. Bastiaens, Live-cell imaging of enzyme-substrate interaction reveals spatial regulation of PTP1B.
Science
315
, 115-119 (2007).
[Abstract]
[Full Text]
Giving Lotus the Nodule
Authors: Hines Pamela J.
Published: March 2007

The nodulation of roots in legumes is a key factor in nitrogen fixation. Working in the leguminous plant
Lotus japonica
, Tirichine
et al
. and Murray
et al
. have identified how the hormone cytokinin fits into the signaling cascade by which leguminous plants establish nitrogen-fixation nodules filled with symbiotic bacteria. Gain-of-function mutation in a cytokinin receptor results in spontaneous formation of bacteria-free nodules, whereas loss of function results in too few nodules despite aggressive formation of bacterial infection threads.

L. Tirichine, N. Sandal, L. H. Madsen, S. Radutoiu, A. S. Albrektsen, S. Sato, E. Asamizu, S. Tabata, J. Stougaard, A gain-of-function mutation in a cytokinin receptor triggers spontaneous root nodule organogenesis.
Science
315
, 104-107 (2007).
[Abstract]
[Full Text]

J. D. Murray, B. J. Karas, S. Sato, S. Tabata, L. Amyot, K. Szczyglowski, A cytokinin perception mutant colonized by
Rhizobium
in the absence of nodule organogenesis.
Science
315
, 101-104 (2007).
[Abstract]
[Full Text]

G. E. D. Oldroyd, Nodules and hormones.
Science
315
, 52-53 (2007).
[Summary]
[Full Text]
It Takes Two to Sense CO
2
Authors: Adler Elizabeth M.
Published: March 2007

Many insects detect CO
2
, thereby obtaining critical information about their environment; bloodsucking insects like mosquitoes, for instance, follow CO
2
trails to locate potential hosts.
Drosophila
detect CO
2
through a subset of olfactory sensory neurons that do not express odorant family receptors (known to sense other volatile chemicals) but do express
Gr21a
, which is classified as belonging to the gustatory receptor family (see Wilson). Jones
et al
. showed that a second gustatory receptor gene,
Gr63a
, was coexpressed with
Gr21a
in CO
2
-sensing neurons in both adult and larval
Drosophila
. Moreover, the mosquito homologs of
Gr21a
and
Gr63a
were coexpressed in a subset of neurons in the mosquito organ through which CO
2 ­­
is sensed. When
Gr21a
and
Gr63a
were coexpressed in neurons that are normally unresponsive to CO
2
, these neurons became responsive, although neither receptor alone was sufficient to confer CO
2
sensitivity. Furthermore, mutant flies lacking
Gr63a
were insensitive to CO
2
, as determined both behaviorally and through electrophysiological recordings. Thus, the authors conclude that coexpression of
Gr21a
and
Gr63a
is required for CO
2
detection in
Drosophila
, a finding that may have implications for the development of repellants against blood-feeding insects.

W. D. Jones, P. Cayirlioglu, I. Grunwald Kadow, L. B. Vosshall, Two chemosensory receptors together mediate carbon dioxide detection in
Drosophila
.
Nature
445
, 86-90 (2007).
[PubMed]

R. I. Wilson, Scent secrets of insects.
Nature
445
, 30-31 (2007).
[PubMed]
Sensing Nuclear Glucose
Authors: Adler Elizabeth M.
Published: March 2007

The liver X receptors (LXR-α and -β) are nuclear transcription factors that have been implicated in both glucose and lipid metabolism; their activation by oxysterol ligands elicits both a decrease in atherosclerosis and antidiabetic effects. Although synthetic LXR ligands decrease hepatic gluconeogenesis and increase lipogenesis in rodent models, the normal rodent diet lacks cholesterol, which led Mitro
et al
. to search for other ligands. They found that glucose and glucose derivatives stimulated the transcriptional activation of a Gal4-responsive gene reporter in human HepG2 cells expressing constructs in which LXR ligand-binding domains (LBDs) were fused to the Gal4 DNA binding domain and transcriptionally activated LXR-RXR (retinoid X receptor) targets. Cell-free coactivator recruitment assays and scintillation proximity assays indicated that glucose and glucose-6-phosphate were direct LXR agonists that bound to the LXR LBD. Furthermore, glucose protected LXR-α from proteolytic attack and increased the LXR-β melting temperature. Glucose had effects on the transcription of LXR target genes in HEPG2 cells similar to those of known ligands, stimulating the expression of genes involved in fatty acid synthesis and cholesterol homeostasis and inhibiting expression of gluconeogenic genes; moreover, it potentiated the effects of LXR ligands. Similarly, glucose- or sucrose-feeding stimulated the expression of LXR target genes in the livers of fasted mice, even mice that were insulin deficient. Thus, glucose itself appears to act as a ligand for LXR, leading the authors to propose that LXR acts as a "transcriptional switch" to coordinate carbohydrate and lipid metabolism.

N. Mitro, P. A. Mak, L. Vargas, C. Godio, E. Hampton, V. Molteni, A. Kreusch, E. Saez, The nuclear receptor LXR is a glucose sensor.
Nature
445
, 219-223 (2007).
[PubMed]
How Flies Find Fungal Foes
Authors: Ray L. Bryan
Published: March 2007

Responses of the immune systems of plants and animals show what appears to be evidence of ancient attacks and counterattacks by pathogens and their hosts in the battle for survival.
Drosophila
have developed receptors that recognize constituents of bacterial cell walls and mount an immune response that causes proteolytic cleavage of the cytokine Spätzle. The Spätzle fragment then activates Toll receptors and leads to production of antimicrobial peptides. Gottar
et al
. explored the response of
Drosophila
to fungal infections and found a similar defense mechanism but also unveiled a second signaling pathway that detects a virulence factor produced by the fungus. The authors infected flies by pricking them with a needle dipped in fungus-containing solution and monitored survival or Toll-dependent expression of the gene encoding an antifungal peptide. They found that the receptor GNBP3 (Gram-negative binding protein 3) was required for detection of cell wall components of the fungi and consequent activation of Toll receptors. However, cells with a mutated GNB3 protein could still respond to fungi and activate Toll, but in this case cell wall-derived components were not the trigger. This response depended on the presence of live fungi and, presumably, the production of virulence factors. One such factor is the protease PR1, and the authors showed that expression of PR1 alone led to activation of the Toll pathway. Knowing that a fly protease PSH (Persephone), which is thought to participate in a cascade of proteases that lead to Spätzle cleavage and activation of the Toll pathway in response to fungi, itself requires proteolytic removal of a prodomain for activity, the authors tested whether PR1 might activate PSH. Indeed, studies in vitro and in vivo indicated that PSH appears to be a direct substrate of PR1. The fungi use the PR1 protease to break down the protective cuticle of the insect and allow infection. The authors propose that PSH acts like a sensor to monitor the status of the cuticle. If the presence of PR1 shows that the defense barrier is being broken, PSH is cleaved and the antimicrobial signaling is initiated. Whether humans have such a sensor system to recognize fungal virulence factors remains unknown.

M. Gottar, V. Gobert, A. A. Matskevich, J.-M. Reichhart, C. Wang, T. M. Butt, M. Belvin, J. A. Hoffmann, D. Ferrandon, Dual detection of fungal infections in
Drosophila
via recognition of glucans and sensing of virulence factors.
Cell
127
, 1425-1437 (2006).
[Online Journal]
Separate Ways
Authors: Simpson Stephen
Published: March 2007

Two dominant lineages of T cells (αβ and γδ T cells) are highly distinct in function and anatomical location yet share a common precursor within the thymus. Exactly how one cell fate is decided over another remains unresolved. Melichar
et al
. present evidence that selection to the γδ T cell branch in the thymus is controlled by the transcription factor SOX13, which supports and possibly even initiates γδ T cell development while opposing differentiation of their αβ T cell brethren. The authors noted that SOX13 inhibited an important effector of the central T cell developmental signaling pathway mediated by the Wnt protein.

H. J. Melichar, K. Narayan, S. D. Der, Y. Hiraoka, N. Gardiol, G. Jeannet, W. Held, C. A. Chambers, J. Kang, Regulation of γδ versus αβ T lymphocyte differentiation by the transcription factor SOX13.
Science
315
, 230-233 (2007).
[Abstract]
[Full Text]
Dead But Still Deadly
Authors: Adler Elizabeth M.
Published: March 2007

Neutrophils, which are key players in the initial response against invading pathogens, kill microbes by several mechanisms. Well known for engulfing microorganisms in phagosomes, where they are subjected to high concentrations of reactive oxygen species (ROS) and antimicrobial peptides discharged from granules, activated neutrophils also snare and kill microbes in extracellular traps. Fuchs
et al
., the researchers who initially described neutrophil extracellular traps (NETs, which consist of chromatin and granular proteins), used live-cell imaging to simultaneously monitor the morphology and viability of individual neutrophils and the appearance of NETs. Following activation with the phorbol ester phorbol 12-myristate 13-acetate (PMA), neutrophils underwent a series of morphological changes; eventually cells ruptured (indicated by the simultaneous loss of the dye calcein blue and exposure of phosphatidylserine). NETs only appeared with cell death. Morphological analysis of neutrophils fixed at various times after stimulation with PMA or
Staphyloccus aureus
indicated that the process of cell death that led to NET formation, which involved dissolution of the nuclear envelope and granular membranes so that chromatin could mix with granule contents, was distinct from apoptosis and necrosis. Moreover, cells activated to produce NET did not undergo DNA fragmentation as detected by TUNEL (terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling). NET production was enhanced by catalase inhibition and did not occur in the neutrophils of individuals with chronic granulomatous disease (associated with mutations in NADPH oxidase) and was blocked by pharmacological inhibition of NADPH. In both cases, NET production was rescued by hydrogen peroxide. Thus, the authors conclude that NET production involves a distinct form of active cell death that requires ROS generation and enables even dead neutrophils to retain antimicrobial activity.

T. A. Fuchs, U. Abed, C. Goosmann, R. Hurwitz, I. Schulze, V. Wahn, Y. Weinrauch, V. Brinkmann, A. Zychlinsky, Novel cell death program leads to neutrophil extracellular traps.
J. Cell. Biol.
176
, 231-241 (2007).
[Abstract]
[Full Text]
Chaperone Directs mRNA Stability
Authors: Ray L. Bryan
Published: March 2007

Heat-shock cognate protein 70 (HSC70) is a multifunctional protein that has known functions as an adenosine triphosphatase (ATPase) in the uncoating of clathrin-coated vesicles and in the facilitation of protein folding. Matsui
et al
. provide evidence that this versatile protein also serves to regulate stability of the mRNA encoding the proapoptotic protein Bim. During hematopoiesis, survival of the hematopoietic progenitor cells is determined, at least in part, by cytokines that decrease the abundance of Bim. Treatment of cultured Baf-3 cells with interleukin-3 (IL-3) caused destabilization of
Bim
mRNA. To explore the mechanism behind this effect, the authors analyzed proteins that interacted with the 3′ untranslated region (UTR) of
Bim
mRNA during affinity chromatography. Matrix-assisted laser desorption/ionization–time-of-flight (MALDI-TOF)/TOF mass spectrometry revealed the association of HSC70 with the
Bim
3′-UTR, and this association was decreased in cells treated with IL-3. In IL-3-treated cells, HSC70 was associated with the cochaperones Bag-4 and CHIP, whereas in IL-3-starved cells, two other cochaperones, Hsp40 and Hip, were the more prevalent partners of HSC70. Survival signals from cytokine receptors are thought to be carried by pathways activated by the small guanosine triphosphatase Ras, and cells expressing constitutively active Ras showed rapid loss of
Bim
mRNA. The authors propose a model in which cells exposed to a survival signal from IL-3 have HSC70 protein associated with its cochaperones Bag-4 and CHIP, with little potential to bind RNA. Under these conditions,
Bim
mRNA tends to interact with RNA-destabilizing factors. However, if IL-3 is removed, signals not yet resolved cause HSC70 to interact preferentially with Hsp40 and Hip. Interaction of this protein complex with
Bim
mRNA appears to protect the
Bim
mRNA from degradation. The authors also point out that these hematopoietic processes are disturbed in leukemia; thus, understanding the regulatory mechanisms involved may offer new strategies for disease intervention.

H. Matsui, H. Asou, T. Inaba, Cytokines direct the regulation of Bim mRNA stability by heat-shock cognate protein 70.
Mol. Cell
25
, 99-112 (2007).
[Online Journal]
A Protective Partnership
Authors: Adler Elizabeth M.
Published: March 2007

Kuramoto
et al
. identified the catalytic α1 subunit of adenosine monophosphate (AMP)-activated protein kinase (AMPK, which, in neurons, is activated in response to hypoxia, ischemia, or enhanced metabolic activity) in a yeast two-hybrid screen of rat brain cDNA for proteins that bind the γ-aminobutyric acid type B (GABA
B
) receptor, which is a G protein-coupled receptor. In vitro binding, coimmunoprecipitation, and immunofluorescence analysis of the subcellular distribution in cultured hippocampal neurons of AMPK and GABA
B
receptors confirmed an interaction between GABA
B
receptors and the AMPK α1 and 2 isoforms. GABA
B
receptors exist as R1 and R2 heterodimers; AMPK bound the cytoplasmic tail of the R1 subunit and phosphorylated Ser
917
of R1 and Ser
783
of R2. Using HEK-293 cells that heterologously expressed Kir3.1 and 3.2 K
+
channels and GABA
B
receptors as a system in which to assess rundown of GABA
B
receptor-dependent activation of K
+
current, the authors found that AMP decreased rundown elicited by intracellular perfusion with adenosine triphosphate (ATP) and guanosine triphosphate (GTP). Furthermore, the protective effect of AMP or of pharmacological activation of AMPK on GABA-activated K
+
current depended on phosphorylation of R2 Ser
783
. Moreover, pharmacological activation of AMPK stimulated Ser
783
phosphorylation and reduced GABA
B
receptor-activated K
+
current rundown in cultured hippocampal neurons. Ischemic brain injury (following transient occlusion of the middle cerebral artery) led to an increase in Ser
783
phosphorylation in rat hippocampus. Pharmacological inhibition of glycolysis and oxidative phosphorylation activated AMPK and stimulated Ser
783
phosphorylation in cultured hippocampal neurons; intriguingly, neurons overexpressing Ser
783
Ala R2 mutants survived this treatment less well than those overexpressing wild-type R2. Thus, the authors conclude that AMPK activation has a neuroprotective effect that depends on phosphorylation of the GABA
B
receptor. Hardie and Frenguelli discuss this research in a thoughtful Preview.

N. Kuramoto, M. E. Wilkins, B. P. Fairfax, R. Revilla-Sanchez, M. Terunuma, K. Tamaki, M. Iemata, N. Warren, A. Couve, A. Calver, Z. Horvath, K. Freeman, D. Carling, L. Huang, C. Gonzales, E. Cooper, T. G. Smart, M. N. Pangalos, S. J. Moss, Phospho-dependent functional modulation of GABA
B
receptors by the metabolic sensor AMP-dependent protein kinase.
Neuron
53
, 233-247 (2007).
[PubMed]

D. G. Hardie, B. G. Frenguelli, A neural protection racket: AMPK and the GABA
B
receptor.
Neuron
53
, 159-162 (2007).
[PubMed]
Dance, Turtle, Dance
Authors: Stern Peter
Published: March 2007

The basic spinal network mechanisms underlying limb movements are still not fully understood. Investigating spinal cord preparations from adult turtles, Berg
et al
. describe how spinal networks operate during motor pattern generation. Balanced increases in synaptic excitation and inhibition operate in spinal motor neurons to produce rhythmic bursts of action potentials that are stochastic in nature. This activity contrasts strongly with the classical notion that antiphasic inhibition and excitation produces rhythmic activity in oscillatory spinal networks (see Kristan for commentary).

R. W. Berg, A. Alaburda, J. Hounsgaard, Balanced inhibition and excitation drive spike activity in spinal half-centers.
Science
315
, 390-393 (2007).
[Abstract]
[Full Text]

W. B. Kristan, A push-me pull-you neural design.
Science
315
, 339-340 (2007).
[Summary]
[Full Text]
Cellular Morphogenesis with a Twist
Authors: Purnell Beverly
Published: March 2007

Cellular morphogenesis is important throughout much of early development in multicellular animals. During
Drosophila
gastrulation, epithelial cells on the ventral side of the embryo display apical constrictions that cause the invagination of the mesoderm and formation of a ventral furrow. Kölsch
et al
. now identify a target of the transcriptional activator Twist. This transmembrane protein, T48, coordinates with signaling factors Fog and Cta to localize the cytoskeletal modulator RhoGEF2 to the apical side of ventral cells in order to direct apical constriction and ventral furrow formation.

V. Kölsch, T. Seher, G. J. Fernandez-Ballester, L. Serrano, M. Leptin, Control of
Drosophila
gastrulation by apical localization of adherens junctions and RhoGEF2.
Science
315
, 384-386 (2007).
[Abstract]
[Full Text]
Split Personality of Transcription Factors Inside and Outside the Nuclear Border
Authors: Naranjo José R. , Mellström Britt
Published: March 2007

Growing evidence indicates that transcription factors may have functions entirely distinct from the regulation of gene transcription. Here we describe three transcription factors that, when outside the nucleus, regulate calcium homeostasis by three independent but convergent mechanisms.
Small GTPase Rac Links Receptors to cGMP Formation
Authors: Ray L. Bryan
Published: March 2007

The second messenger guanosine 3′,5′-monophosphate (cGMP) controls a broad range of cellular functions, many of which are regulated through the cGMP-dependent protein kinase. Cyclic GMP is synthesized by guanylyl cyclases (GCs) that catalyze their formation from guanosine triphosphate. A study by Guo
et al
. provides insight into the upstream regulation of the GCs. Unlike the adenyl cyclases, which are well known as targets of GPCRs [heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors], transmembrane guanylyl cyclases are thought to act as receptors themselves, although not all have identified ligands. Guo
et al
. explored how intracellular signals can also regulate the transmembrane guanylyl cyclase GC-E and observed that transient overexpression of the small GTPase Rac1 led to activation of GC-E. Activation of the platelet-derived growth factor receptor (PDGFR) also stimulated formation of cGMP, and this was inhibited when the authors used a dominant-negative form of Rac1 to inhibit activity of endogenous Rac1. The authors tested known targets of Rac1 to explore how Rac1 might be coupled to activation of GC-E. PAK (p21-activated protein kinase) is one such target, and it could substitute for Rac1 in activating formation of cGMP; expression of the autoinhibitory domain of PAK also inhibited the effect of constitutively active Rac1 to stimulate GC-E activity. The authors used in vitro and in vivo studies with purified proteins and protein fragments to show that PAK directly interacts with and stimulates GC-E. This activation required the kinase activity of PAK--but apparently only for autophosphorylation, because GC-E was not phosphorylated by PAK. This new pathway from receptor tyrosine kinase to cGMP was shown to be physiologically important in the control of cell migration. Depletion of GC-A in mouse embryo fibroblasts with small interfering RNA (siRNA) reduced the effect of PDGF on accumulation of cGMP and inhibited PDGF-dependent cell migration and formation of lamellipodia. Settleman discusses the work in a commentary and emphasizes some of the intriguing questions raised by the work, such as why Cdc42, another small GTPase that also activates PAK, fails to activate GC-E.

D. Guo, Y.-C. Tan, D. Wang, K. S. Madhusoodanan, Y. Zheng, T. Maack, J. J. Zhang, X.-Y. Huang, A Rac-cGMP signaling pathway.
Cell
128
, 341-355 (2007).
[Online Journal]

J. Settleman, PAK-in’ up cGMP for the move.
Cell
128
, 237-238 (2007).
[Online Journal]
More Than One Way to Signal
Authors: Adler Elizabeth M.
Published: March 2007

Although defined by the ability to activate heterotrimeric guanine nucleotide-binding proteins (G proteins), G protein-coupled receptors (GPCRs) can also signal through G protein-independent pathways. Sun
et al
. used mouse embryo fibroblasts lacking various proteins to investigate the mechanistic relation between G protein-dependent and -independent activation of extracellular signal-regulated kinase (ERK) in response to stimulation of the β
2
-adrenergic receptor (β
2
-AR). Analysis of ERK activation in response to various concentrations of the β
2
-AR agonist isoproterenol revealed a biphasic response. ERK activation in response to low concentrations of isoproterenol was lost in cells lacking Gα
s
, whereas ERK activation in response to high concentrations was maintained (both required expression of the β
2
-AR). Moreover, ERK activation in response to high concentrations of agonist did not depend on other heterotrimeric G proteins or on β-arrestins. In contrast, low concentrations of isoproterenol stimulated ERK activation in cells lacking Src-family tyrosine kinases, but high concentrations failed to increase the response. Isoproterenol activated Src even in cells lacking both Gα
s
and β-arrestins. In vitro analysis indicated that Src bound to and phosphorylated the C-terminal tail of the β
2
-AR; further, c-Src coimmunoprecipitated with the β
2
-AR from human embryonic kidney (HEK) 293 cells. When expressed in cells lacking the β
2
-AR, a β
2
-AR mutant that could not bind Src activated Gα
s
and ERK in response to low concentrations of isoproterenol but failed to elicit enhanced activation of ERK in response to high concentrations. Thus, the authors conclude that low concentrations of agonist activate ERK through a Gα
s
-mediated pathway, whereas high concentrations turn on a second, Src-dependent, pathway to ERK activation; they speculate that coupling to these two pathways depends on distinct conformations of the β
2
-AR .

Y. Sun, J. Huang, Y. Xiang, M. Bastepe, H. Jüppner, B. K. Kobilka, J. J. Zhang, X.-Y. Huang, Dosage-dependent switch from G protein-coupled to G protein-independent signaling by a GPCR.
EMBO J.
26
, 53–64 (2007).
[PubMed]
A New Way to Make Filopodia?
Authors: Gough Nancy R.
Published: March 2007

Filopodia are slender actin-rich protrusions that are most often associated with activation of the guanosine triphosphatase (GTPase) Cdc42 and the actin regulatory proteins, the ARP2/3 complex, Wiskott-Aldrich syndrome protein (WASP), and proteins of the Ena/VASP family. An alternate mechanism involving the GTPase Rif and the mDia2 protein, instead of Cdc42 and ARP2/3, has also been reported. Sigal
et al
. provide evidence for yet another mechanism by which filopodia may be formed. When they overexpressed lipid-phosphatase-related protein 1 (LPR1) in Cos7 or HeLa cells, they noted an increase in the formation of filopodia along the cell periphery and dorsal surface. However, LPR1-induced filopodia formed even when dominant-negative Cdc42, or peptide inhibitors of Cdc42 or ARP2/3, or dominant-negative Rif were coexpressed with LPR1 in the cells. Colocalization studies suggested that the actin structures in the LPR1-induced filopodia had a unique composition and did not include components of focal adhesion complexes characteristic of Cdc42-stimulated filopodia or VASP or two VASP homologs. The actin-bundling protein fascin was present along the shafts of the LPR1-induced filopodia, and myosin X was found in the tips of the filopodia. In MV
D7
cells, which are engineered to lack all three members of the VASP family, LPR1 stimulated filopodia formation. RNAi experiments in a human ovary surface epithelial cell line, SK-OV-3, which is an invasive and motile cell line, showed that there was a substantial decrease in the formation of filopodia when LPR1 abundance was decreased. LPR1 is predicted to have six transmembrane domains, but the putative catalytic region lacks certain conserved residues that are required for activity in other members of the lipid phosphatase and phosphotransferase (LPT) family. When expressed in Sf9 cells, which natively have low lipid phosphatase and phosphotransferase activity, detergent extracts of the LPR1-expressing cells showed no phosphatase activity against any of the four common substrates of the LPT family. However, mutation of residues in the pseudocatalytic region disrupted the filopodia-stimulating activity of LPR1; thus, lipid interactions may still be important for the function of LPR1.

Y. J. Sigal, O. A. Quintero, R. E. Cheney, A. J. Morris, Cdc42 and ARP2/3-independent regulation of filopodia by an integral membrane lipid-phosphatase-related protein.
J. Cell Sci.
120
, 340-352 (2007).
[Abstract]
[Full Text]
Mitochondrial Diversion and Aging
Authors: Ray L. Bryan
Published: March 2007

The protein p66
Shc
facilitates protein-protein interactions in growth factor signaling pathways. But mutations in Shc can enhance life span in mammals. This effect appears to depend on a different function of Shc whereby it exerts oxidoreductase activity in mitochondria and generates oxygen radicals that lead to cell death. Pinton
et al
. now show that the activity of Shc in the mitochondria depends on its phosphorylation by protein kinase C β and consequent binding of the prolyl isomerase Pin1. This leads to a conformational change in the protein and to its accumulation in mitochondria. This signaling pathway could provide a target to help delay aging.

P. Pinton, A. Rimessi, S. Marchi, F. Orsini, E. Migliaccio, M. Giorgio, C. Contursi, S. Minucci, F. Mantovani, M. R. Wieckowski, G. Del Sal, P. G. Pelicci, R. Rizzuto, Protein kinase C β and prolyl isomerase 1 regulate mitochondrial effects of the life-span determinant p66
Shc
.
Science
315
, 659-663 (2007).
[Abstract]
[Full Text]

G. Hajnóczky, J. B. Hoek, Mitochondrial longevity pathways.
Science
315
, 607-609 (2007).
[Summary]
[Full Text]
Sweet Sensor, Surprising Partners
Authors: Chen Jin-Gui
Published: March 2007

Hexokinase1 (HXK1) is an evolutionarily conserved glucose sensor in plants. However, the molecular mechanism through which HXK1 controls the expression of genes encoding proteins involved in photosynthesis is a mystery. Recent research demonstrates that a previously unknown HXK1 nuclear complex controls the expression of specific photosynthetic genes, a process that is independent of glucose metabolism but requires two unexpected partners, VHA-B1 and RPT5B. Both VHA-B1 and RPT5B have well-established and conserved functions in processes that are seemingly unrelated to glucose-dependent regulation of gene expression, and neither of them is a predominantly nuclear protein. Biochemical, genetic, and molecular evidence demonstrates that VHA-B1 and RPT5B directly interact with HXK1 in the nucleus and that the HXK1 complex binds to the cis-acting elements of
chlorophyll a/b binding protein 2
, a photosynthetic gene that is transcriptionally suppressed by glucose. The identification of the HXK1 nuclear complex reveals an unexpected glucose-signaling mechanism and reinforces the notion that metabolic enzymes can play unique roles in signal transduction by directly controlling gene expression in the nucleus.
Opening the Nuclear Gates
Authors: Adler Elizabeth M.
Published: March 2007

One little-explored mechanism whereby cell signals could influence the transcription of target genes would be by regulating the permeability of the nuclear envelope to transcription factors and other nuclear proteins. O’Brien
et al
. compared the distribution in SKHep1 liver cells of a 27-kD photoactivatable green fluorescent protein construct (PA-GFP) to that of a DsRed construct targeted to the cytoplasm (DsRed-NES) and found that PA-GFP locally excited by means of two-photon excitation did not appear to cross the nuclear envelope. PA-GFP activated in the nucleus and viewed for at least 5 minutes diffused throughout the nucleus but did not enter the cytoplasm, and PA-GFP excited within the cytoplasm did not enter the nucleus. Exposure to vasopressin, or other hormones that activate the G
q
signaling pathway (and thereby elicit an increase in cytosolic Ca
2+
concentration), increased permeability of the nuclear envelope to PA-GFP unidirectionally, so that PA-GFP activated in the cytoplasm spread into the nucleus. Increasing cytosolic calcium concentration by inhibiting the sarcoplasmic and endoplasmic reticulum Ca
2+
-ATPase also increased unidirectional nuclear membrane permeability, an effect that was blocked by a Ca
2+
chelator. Moreover, localized photorelease of caged Ca
2+
in the cytoplasm near the nuclear envelope led to a localized increase in unidirectional nuclear envelope permeability. Thus, the authors conclude that one mechanism whereby hormones that activate intracellular Ca
2+
signaling pathways may modulate transcription of target genes is through the Ca
2+
-dependent regulation of nuclear envelope permeability. These studies raise the question of how the PA-GFP activated in the nucleus got there. The authors suggest that this may have occurred during mitosis (when the nuclear envelope breaks down) or in response to previous stimulation by growth factors.

E. M. O'Brien, D. A. Gomes, S. Sehgal, M. H. Nathanson, Hormonal regulation of nuclear permeability.
J. Biol. Chem.
282
, 4210-4217 (2007).
[Abstract]
[Full Text]
The Grim Reaper on Autopilot?
Authors: Ray L. Bryan
Published: March 2007

The proteins Bax and Bak are key mediators of cell death signals that function at the mitochondria to promote apoptosis. There is evidence for multiple modes of regulation of Bax and Bak in cells. Some studies have proposed that other members of the Bcl-2 family of proteins interact with and directly activate Bax and Bak--a scenario in which cell survival seems to be the default state of the cell. However, activity of Bax and Bak is also held in check by interaction with prosurvival proteins, and it may be that relief of this inhibition determines the cell's fate. Willis
et al
. (see the Perspective by Youle) present evidence that the default state of Bax and Bak would lead to cell death. Cells lacking the direct activators of Bax and Bak still undergo apoptosis in response to overexpression of upstream components of the cell death pathway.

S. N. Willis, J. I. Fletcher, T. Kaufmann, M. F. van Delft, L. Chen, P. E. Czabotar, H. Ierino, E. F. Lee, W. D. Fairlie, P. Bouillet, A. Strasser, R. M. Kluck, J. M. Adams, D. C. S. Huang, Apoptosis initiated when BH3 ligands engage multiple Bcl-2 homologs, not Bax or Bak.
Science
315
, 856-859 (2007).
[Abstract]
[Full Text]

R. J. Youle, Cellular demolition and the rules of engagement.
Science
315
, 776-777 (2007).
[Summary]
[Full Text]
Inhibiting or Enhancing Release?
Authors: Adler Elizabeth M.
Published: March 2007

Voltage-dependent K
+
(Kv) channels can limit Ca
2+
influx through voltage-dependent Ca
2+
channels by means of their effects on membrane potential and thereby indirectly inhibit the secretion of neurotransmitters, neuropeptides, and hormones. Noting that Kv2.1 channels interact directly with syntaxin 1A and SNAP25, two components of the SNARE (soluble
N
-ethylmaleimide-sensitive factor attachment protein receptor) exocytotic apparatus, Singer-Lahat
et al
. investigated the possibility that Kv2.1-syntaxin interactions affect the release of dense-core vesicles (DCVs). Kv2.1 overexpression in undifferentiated PC12 cells (so that current density and voltage dependence of activation were similar to those characteristic of hippocampal neurons) enhanced the release of green fluorescent protein-labeled pro-ANF (atrial natriuretic factor) elicited by increasing the extracellular K
+
concentration, thereby depolarizing the cells (conditions under which the effects of K
+
flux through Kv channels on membrane potential should be minimal). Kv2.1-dependent facilitation of release did not involve effects on membrane potential or on bulk intracellular Ca
2+
concentration; moreover, a Kv2.1 mutant with a nonconducting pore enhanced release as well. Exposing permeabilized cells to calcium after priming with ATP (a treatment known to trigger DCV release) enhanced coimmunoprecipitation of syntaxin with endogenous Kv2.1. Syntaxin-binding peptides derived from cytoplasmic portions of Kv2.1 decreased syntaxin coimmunoprecipitation with Kv2.1 and inhibited Kv2.1-dependent facilitation of release. Indeed, a mutant form of Kv2.1 that lacked the syntaxin-binding C1a domain not only failed to enhance depolarization-induced release but also slightly inhibited it. The authors propose that the apparently antagonistic effects of Kv2.1 on Ca
2+
-evoked neuropeptide release (inhibiting release through effects on membrane potential and enhancing it through direct interaction with syntaxin) could contribute to the known activity dependence of DCV release.

D. Singer-Lahat, A. Sheinin, D. Chikvashvili, S. Tsuk, D. Greitzer, R. Friedrich, L. Feinshreiber, U. Ashery, M. Benveniste, E. S. Levitan, I. Lotan, K
+
channel facilitation of exocytosis by dynamic interaction with syntaxin.
J. Neurosci.
27
, 1651-1658 (2007).
[Abstract]
[Full Text]
The Queen of Dopamine
Authors: Ray L. Bryan
Published: March 2007

The queen bee controls the physiology and behavior of her fellow bees and essentially determines the workings of the entire society of insects. The queen exerts this influence by producing a cocktail of pheromones known as queen

mandibular pheromone (QMP), but it has not been clear just how the mixture produces its effects. Beggs
et al
. noted that one component of QMP, homovanillyl alcohol (4-hydroxy-3-methoxyphenylethanol, or HVA) has a chemical structure similar to that of the neurotransmitter dopamine. The authors therefore tested the effects of the pheromone on dopaminergic function in bees. Exposure of newly emerged adult bees to QMP (a commercially available strip impregnated with nine queen-equivalents of pheromone mixture) for 2 days decreased the amount of mRNA transcript encoding one of the bee’s dopamine receptors. Cultured neurons from the mushroom body normally respond to dopamine with an increase in production of cAMP (adenosine 3′,5′-monophosphate), but neurons taken from flies exposed to QMP also showed a small decrease in production of cAMP. HVA produced responses similar to those evoked by dopamine. Total amounts of dopamine in the brain were reduced in flies exposed to HVA for 2 days. The authors propose that HVA in the QMP mixture may interact directly with dopamine receptors in the bee nervous system, perhaps decreasing the expression of dopamine receptors, thus altering the response of the neurons to endogenous dopamine.

K. T. Beggs, K. A. Glendining, N. M. Marechal, V. Vergoz, I. Nakamura, K. N. Slessor, A. R. Mercer, Queen pheromone modulates brain dopamine function in worker honey bees.
Proc. Natl. Acad. Sci. U.S.A.
104
, 2460-2464 (2007).
[Abstract]
[Full Text]
And Then There Were Three
Authors: Hines Pamela J.
Published: March 2007

During development, stem cells usually generate one daughter cell that differentiates and another stem cell. Ohlstein and Spradling now describe a type of stem cell in the
Drosophila
intestine that produces three offspring fates--a stem cell, an enterocyte, and an enteroendocrine cell. The choice in fate seems to depend on the amount of the protein Delta (which can activate the signaling receptor Notch) that is expressed in the stem cell at cell division. Daughter cells with high amounts of Delta-Notch signaling become enterocytes, those with lower amounts become enteroendocrine cells, and those with the least amount retain the stem cell fate.

B. Ohlstein, A. Spradling, Multipotent
Drosophila
intestinal stem cells specify daughter cell fates by differential Notch signaling.
Science
315
, 988-992 (2007).
[Abstract]
[Full Text]
Lethal Injection
Authors: Hurtley Stella M.
Published: March 2007

Pathogenic bacteria can inject into host cells virulence factors via the so-called type III machinery. Li
et al
. describe a family of bacterial virulence factors that have a previously unknown phosphothreonine lyase activity that can remove the phosphate from signaling mitogen-activated protein kinase family members involved in innate immunity. This family of effectors is important in the virulence of a variety of animal and plant bacterial pathogens, including
Shigella
,
Salmonella
, and
Pseudomonas syringae
.

H. Li, H. Xu, Y. Zhou, J. Zhang, C. Long, S. Li, S. Chen. J.-M. Zhou, F. Shao, The phosphothreonine lyase activity of a bacterial type III effector family.
Science
315
, 1000-1003 (2007).
[Abstract]
[Full Text]
Joint Effort
Authors: Simpson Stephen
Published: March 2007

Rheumatoid arthritis (RA) is an autoimmune condition that leads to joint inflammation. Lee
et al
. (see the Perspective by Firestein) identify a new regulator of the cellular organization of the synovium that might also provide a potential therapeutic target for inflammatory arthritis. Mice lacking the cell adhesion molecule cadherin-11 showed significantly reduced growth of the synovium and were resistant to the development of an experimentally induced RA-like condition. Joint inflammation in mice could be inhibited with a monoclonal antibody to cadherin-11.

D. M. Lee, H. P. Kiener, S. K. Agarwal, E. H. Noss, G. F. M. Watts, O. Chisaka, M. Takeichi, M. B. Brenner, Cadherin-11 in synovial lining formation and pathology in arthritis.
Science
315
, 1006-1010 (2007).
[Abstract]
[Full Text]

G. S. Firestein, Every joint has a silver lining.
Science
315
, 952-953 (2007).
[Summary]
[Full Text]
Sticky Fingers
Authors: Hurtley Stella M.
Published: March 2007

As animal cells migrate across a surface, they send out processes known as filopodia that explore the substratum. Galbraith
et al
. now find that the intracellular actin network directs very local protrusions that contain clusters of cell surface-adhesion molecules at their tips that are primed to interact with molecules of the extracellular matrix. These "sticky fingers" at the leading edge of motile cells appear to search for suitable sites of adhesion that can then be used to help move the rest of the cell.

C. G. Galbraith, K. M. Yamada, J. A. Galbraith, Polymerizing actin fibers position integrins primed to probe for adhesion sites.
Science
315
, 992-995 (2007).
[Abstract]
[Full Text]
G Protein-Mediated Inhibition of Autophagy
Authors: Ray L. Bryan
Published: March 2007

Autophagy provides a mechanism for movement of cytoplasmic materials to the lysosomes, where they can be degraded or recycled. Autophagy functions to supply energy during short periods of starvation, but unrestrained autophagy causes cell death. Gohla
et al
. explored the role of the G
i
class of heterotrimeric guanine nucleotide-binding proteins in regulating autophagy. The authors induced autophagy in liver cells in culture by incubating them briefly in culture medium lacking growth factors or amino acids. Gα
i3
was normally found in the cytosol, but in cells undergoing autophagy, it was redistributed to the plasma membrane and to the endoplasmic reticulum and endosomes. Inhibition of autophagy in cells exposed to insulin or phenylalanine was associated with decreased formation of vesicles and decreased vesicular staining of Gα
i3
. Insulin or phenylalanine also reversed autophagy in mouse livers perfused with buffer lacking amino acids in vivo, and these effects were inhibited by pertussis toxin, an inhibitor of G
i
function. However, neither insulin nor amino acids are thought to directly activate G
i
signaling. Nevertheless, the antiautophagic action of insulin was almost completely absent in perfused livers from knockout mice lacking Gα
i3
. The authors note that the findings strongly implicate Gα
i3
in regulation of autophagy and raise intriguing questions about the nature of the upstream signals that may control Gα
i3
activity in response to nutrients and growth factors, given that G protein-coupled receptors are normally not found on intracellular membranes.

A. Gohla, K. Klement, R. P. Piekorz, K. Pexa, S. vom Dahl, K. Spicher, V. Dreval, D. Häussinger, L. Birnbaumer, B. Nürnberg, An obligatory requirement for the heterotrimeric G protein G
i3
in the antiautophagic action of insulin in the liver.
Proc. Natl. Acad. Sci. U.S.A.
104
, 3003-3008 (2007).
[Abstract]
[Full Text]
Death Effector Domain as a Transcriptional Regulator
Authors: Gough Nancy R.
Published: March 2007

Apoptotic cell death involves activation of the procaspases, which requires cleavage of those proteins. Thus, replenishment of the procaspase pool requires new protein synthesis. Yao
et al
. report that a cleaved fragment containing the first death effector domain (DEDa) of procaspase 8 stimulates the transcription of the gene encoding procaspase 8, thereby providing a positive feedback replenishing procaspase 8. Caspase 8 is cleaved and activated in response to ligands such as TRAIL and FasL that stimulate apoptosis. Yao
et al
. first show that the prodomain of caspase 8 (residues 1 to 233) was cleaved to produce DEDa when transfected into cultured cells or when assayed in vitro with purified caspase 8, indicating self-cleavage by caspase 8. Although in unstressed cells, DEDa is rapidly degraded (even when overexpressed), inhibition of the proteasome or exposure of the cells to TRAIL resulted in the stabilization and accumulation of DEDa in the nucleoli. DEDa interacted with extracellular signal-regulated kinase 1 and 2 (ERK1/2) by coimmunoprecipitation and knockdown of ERK1 and ERK2 with RNAi blocked nuclear translocation of DEDa, suggesting that ERK may shuttle DEDa into the nucleus. HeLa cells that overexpressed DEDa and in which the proteasome was inhibited showed increased transcription of caspase 8. A reporter gene construct containing or lacking the p53 responsive element in the caspase 8 promoter was used to show that DEDa stimulation of caspase 8 gene expression required p53. Further reporter gene experiments showed that DEDa interacted with TOPORS, a p53 binding protein, and appeared to compete with p53 for binding, thus releasing p53 from TOPORS. Cells overexpressing DEDa were more sensitive to TRAIL-induced apoptosis than were nontransfected cells, and apoptosis was inhibited if ERK1 and ERK2 were knocked down by RNAi. Thus, even the cleaved prodomain of caspase 8 appears to contribute to apoptosis by stimulating procaspase 8 biosynthesis.

Z. Yao, S. Duan, D. Hou, K. Heese, M. Wu, Death effector domain DEDa, a self-cleaved product of caspase-8/Mch5, translocates to the nucleus by binding to ERK1/2 and upregulates procaspase-8 expression via a p53-dependent mechanism.
EMBO J.
26
, 1068-1080 (2007).
[PubMed]