ROSET Model of TonB Action in Gram-Negative Bacterial Iron Acquisition

The rotational surveillance and energy transfer (ROSET) model of TonB action suggests a mechanism by which the electrochemical proton gradient across the Gram-negative bacterial inner membrane (IM) promotes the transport of iron through ligand-gated porins (LGP) in the outer membrane (OM). TonB associates with the IM by an N-terminal hydrophobic helix that forms a complex with ExbBD. It also contains a central extended length of rigid polypeptide that spans the periplasm and a dimeric C-terminal-ββαβ-domain (CTD) with LysM motifs that binds the peptidoglycan (PG) layer beneath the OM bilayer. The TonB CTD forms a dimer with affinity for both PG- and TonB-independent OM proteins (e.g., OmpA), localizing it near the periplasmic interface of the OM bilayer. Porins and other OM proteins associate with PG, and this general affinity allows the TonB CTD dimer to survey the periplasmic surface of the OM bilayer. Energized rotational motion of the TonB N terminus in the fluid IM bilayer promotes the lateral movement of the TonB-ExbBD complex in the IM and of the TonB CTD dimer across the inner surface of the OM. When it encounters an accessible TonB box of a (ligand-bound) LGP, the monomeric form of the CTD binds and recruits it into a 4-stranded β-sheet. Because the CTD is rotating, this binding reaction transfers kinetic energy, created by the electrochemical proton gradient across the IM, through the periplasm to the OM protein. The equilibration of the TonB C terminus between the dimeric and monomeric forms that engage in different binding reactions allows the identification of iron-loaded LGP and then the internalization of iron through their trans-outer membrane β-barrels. Hence, the ROSET model postulates a mechanism for the transfer of energy from the IM to the OM, triggering iron uptake.     

Evidence that Glucose-Induced Electrical Activity in Rat Pancreatic b-Cells Does Not Require KATP Channel Inhibition

KATP-channel activity, recorded in cell-attached patches from isolated rat pancreatic beta-cells, was found to be maximally inhibited in the presence of a substimulatory concentration (5 mM) of glucose, with no further effect of higher, stimulatory glucose concentrations. KATP channel-independent effects of glucose on electrical activity were therefore investigated by incubating cells in the presence of a supramaximal concentration of tolbutamide. Addition of tolbutamide (500 mM) to cells equilibrated in the absence of glucose resulted in a rapid depolarization and electrical activity followed by a gradual repolarization and disappearence of electrical activity. Repolarization was not due to desensitization of KATP channels to the sulfonylurea, but was probably the result of activation of another K+ conductance. The subsequent application of 16 mM glucose in the continued presence of tolbutamide depolarized the cells again, leading to renewed electrical activity. Input conductance of the cells was markedly reduced by tolbutamide, reflecting KATP-channel inhibition, but was not significantly affected by the addition of glucose in the presence of the drug. In cells voltage-clamped at -70 mV, addition of glucose in the presence of tolbutamide generated a noisy inward current, probably representing activation of the volume-sensitive anion channel. KATP channel-independent activation of electrical activity by glucose was inhibited by the anion channel inhibitor 4,4'-dithiocyanatostilbene-2,2'-disulphonic acid. It is concluded that the induction of electrical activity in rat pancreatic b-cells does not require inhibition of KATP channels. The KATP channel-independent mechanism could involve, at least in part, activation by glucose of the volume-sensitive anion channel.     

The Thermophilic Yeast Hansenula polymorpha Does Not Require Trehalose Synthesis for Growth at High Temperatures but Does for Normal Acquisition of Thermotolerance

The TPS1 gene from Hansenula polymorpha, which encodes trehalose-6-phosphate (Tre6P) synthase, has been isolated and characterized. The deletion of TPS1 rendered H. polymorpha cells incapable of trehalose synthesis under conditions where wild-type cells normally accumulate high levels of trehalose. Interestingly, the loss of Tre6P synthase did not cause any obvious growth defects on a glucose-containing medium, even at high temperatures, but seriously compromised the cells' ability to acquire thermotolerance.     

Coronavirus Replication Does Not Require the Autophagy Gene ATG5

Macroautophagy (herein autophagy) is a cellular process, requiring ATG5, by which cells deliver double membrane-bound packets containing cytoplasm or cytoplasmic organelles to the lysosome. This process has been reported in some cases to be antiviral, while in other cases it has been reported to be required for efficient viral replication or release. A role for autophagy in RNA virus replication has been an attractive hypothesis because of the association of RNA virus replication with complex membrane rearrangements in the cytoplasm that can generate opposed double membranes. In this study we demonstrate that ATG5 is not required for murine hepatitis virus (MHV) replication in either bone marrow derived macrophages (BMMφ) lacking ATG5 by virtue of Cre-recombinase mediated gene deletion or primary low passage murine ATG5^-/- embryonic fibroblasts (pMEFs). We conclude that neither ATG5 nor an intact autophagic pathway are required for MHV replication or release.     

ATP Regulation of Type-1 Inositol 1,4,5-Trisphosphate Receptor Activity Does Not Require Walker A-type ATP-binding Motifs

ATP is known to increase the activity of the type-1 inositol 1,4,5-trisphosphate receptor (InsP₃R1). This effect is attributed to the binding of ATP to glycine rich Walker A-type motifs present in the regulatory domain of the receptor. Only two such motifs are present in neuronal S2+ splice variant of InsP₃R1 and are designated the ATPA and ATPB sites. The ATPA site is unique to InsP₃R1, and the ATPB site is conserved among all three InsP₃R isoforms. Despite the fact that both the ATPA and ATPB sites are known to bind ATP, the relative contribution of these two sites to the enhancing effects of ATP on InsP₃R1 function is not known. We report here a mutational analysis of the ATPA and ATPB sites and conclude neither of these sites is required for ATP modulation of InsP₃R1. ATP augmented InsP₃-induced Ca²⁺ release from permeabilized cells expressing wild type and ATP-binding site-deficient InsP₃R1. Similarly, ATP increased the single channel open probability of the mutated InsP₃R1 to the same extent as wild type. ATP likely exerts its effects on InsP₃R1 channel function via a novel and as yet unidentified mechanism.Inositol 1,4,5-trisphosphate receptors (InsP₃R)³ are a family of large, tetrameric, InsP₃-gated cation channels. The three members of this family (InsP₃R1, InsP₃R2, and InsP₃R3) are nearly ubiquitously expressed and are localized primarily to the endoplasmic reticulum (ER) membrane (1–3). Numerous hormones, neurotransmitters, and growth factors bind to receptors that stimulate phospholipase C-induced InsP₃ production (4). InsP₃ subsequently binds to the InsP₃R and induces channel opening. This pathway represents a major mechanism for Ca²⁺ liberation from ER stores (5). All three InsP₃R isoforms are dynamically regulated by cytosolic factors in addition to InsP₃ (1). Ca²⁺ is perhaps the most important determinant of InsP₃R activity besides InsP₃ itself and is known to regulate InsP₃R both positively and negatively (6). ATP, in concert with InsP₃ and Ca²⁺, also regulates InsP₃R as do numerous kinases, phosphatases, and protein-binding partners (7–10). This intricate network of regulation allows InsP₃R activity to be finely tuned by the local cytosolic environment (9). As a result, InsP₃-induced Ca²⁺ signals can exhibit a wide variety of spatial and temporal patterns, which likely allows Ca²⁺ to control many diverse cellular processes.Modulation of InsP₃-induced Ca²⁺ release (IICR) by ATP and other nucleotides provides a direct link between intracellular Ca²⁺ signaling and the metabolic state of the cell. Metabolic fluctuations could, therefore, impact Ca²⁺ signaling in many cell types given that InsP₃R are expressed in all cells (11, 12). Consistent with this, ATP has been shown to augment IICR in many diverse cell types including primary neurons (13), smooth muscle cells (14), and exocrine acinar cells (15) as well as in immortalized cell lines (16–18). The effects of ATP on InsP₃R function do not require hydrolysis because non-hydrolyzable ATP analogues are as effective as ATP (7, 14). ATP is thought to bind to distinct regions in the central, coupling domain of the receptors and to facilitate channel opening (2, 19). ATP is not required for channel gating, but instead, increases InsP₃R activity in an allosteric fashion by increasing the open probability of the channel in the presence of activating concentrations of InsP₃ and Ca²⁺ (7, 8, 20).Despite a wealth of knowledge regarding the functional effects of ATP on InsP₃R function, there is relatively little known about the molecular determinants of these actions. ATP is thought to exert effects on channel function by direct binding to glycine-rich regions containing the consensus sequence GXGXXG that are present in the receptors (2). These sequences were first proposed to be ATP-binding domains due to their similarity with Walker A motifs (21). The neuronal S2⁺ splice variant of InsP₃R1 contains two such domains termed ATPA and ATPB. A third site, ATPC, is formed upon removal of the S2 splice site (2, 22). The ATPB site is conserved in InsP₃R2 and InsP₃R3, while the ATPA and ATPC sites are unique to InsP₃R1. Our prior work examining the functional consequences of mutating these ATP-binding sites has yielded unexpected results. For example, mutating the ATPB site in InsP₃R2 completely eliminated the enhancing effects of ATP on this isoform while mutating the analogous site in InsP₃R3 failed to alter the effects of ATP (23). This indicated the presence of an additional locus for ATP modulation of InsP₃R3. In addition, mutation of the ATPC in the S2⁻ splice variant of InsP₃R1 did not alter the ability of ATP to modulate Ca²⁺ release, but instead impaired the ability of protein kinase A to phosphorylate Ser-1755 of this isoform (22).The ATPA and ATPB sites in InsP₃R1 were first identified as putative nucleotide-binding domains after the cloning of the full-length receptor (24). Early binding experiments with 8-azido-[α-³²P]ATP established that ATP cross-linked with receptor purified from rat cerebellum at one site per receptor monomer (19). Later, more detailed, binding experiments on trypsinized recombinant rat InsP₃R1 showed cross-linking of ATP to two distinct regions of the receptor that corresponded with the ATPA and ATPB sites (17). We and others (16, 22, 23) have also reported the binding of ATP analogues to purified GST fusions of small regions of InsP₃R1 surrounding the ATPA and ATPB sites. It is widely accepted, in the context of the sequence similarity to Walker A motifs and biochemical data, that the ATPA and ATPB sites are the loci where ATP exerts its positive functional effects on InsP₃R1 function (1–3, 16). Furthermore, the higher affinity of the ATPA site to ATP is thought to confer the higher sensitivity of InsP₃R1 to ATP versus InsP₃R3, which contains the ATPB site exclusively (25, 26). The purpose of this study, therefore, was to examine the contributions of the ATPA and ATPB sites to ATP modulation of the S2⁺ splice variant of InsP₃R1. We compared the effects of ATP on InsP₃R1 and on ATP-binding site mutated InsP₃R1 using detailed functional analyses in permeabilized cells and in single channel recordings. Here we report that InsP₃R1 is similar to InsP₃R3 in that ATP modulates IICR even at maximal InsP₃ concentrations and that neither the ATPA nor the ATPB site is required for this effect.     

Experience-Driven Axon Retraction in the Pharmacologically Inactivated Visual Cortex Does Not Require Synaptic Transmission

Background

Experience during early postnatal development plays an important role in the refinement of specific neural connections in the brain. In the mammalian visual system, altered visual experiences induce plastic adaptation of visual cortical responses and guide rearrangements of afferent axons from the lateral geniculate nucleus. Previous studies using visual deprivation demonstrated that the afferents serving an open eye significantly retract when cortical neurons are pharmacologically inhibited by applying a γ-aminobutyric acid type A receptor agonist, muscimol, whereas those serving a deprived eye are rescued from retraction, suggesting that presynaptic activity can lead to the retraction of geniculocortical axons in the absence of postsynaptic activity. Because muscimol application suppresses the spike activity of cortical neurons leaving transmitter release intact at geniculocortical synapses, local synaptic interaction may underlie the retraction of active axons in the inhibited cortex.

Method and Findings

New studies reported here determined whether experience-driven axon retraction can occur in the visual cortex inactivated by blocking synaptic inputs. We inactivated the primary visual cortex of kittens by suppressing synaptic transmission with cortical injections of botulinum neurotoxin type E, which cleaves a synaptic protein, SNAP-25, and blocks transmitter release, and examined the geniculocortical axon morphology in the animals with normal vision and those deprived of vision binocularly. We found that afferent axons in the animals with normal vision showed a significant retraction in the inactivated cortex, as similarly observed in the muscimol-treated cortex, whereas the axons in the binocularly deprived animals were preserved.

Conclusions

Therefore, the experience-driven axon retraction in the inactivated cortex can proceed in the absence of synaptic transmission. These results suggest that presynaptic mechanisms play an important role in the experience-driven refinement of geniculocortical axons.     

Chaperone-Like Activity of <Emphasis Type="Italic">Mycobacterium tuberculosis</Emphasis> Hsp16.3 Does Not Require Its Intact (Native) Structures

Small heat shock proteins (sHsps) were found to exhibit efficient chaperone-like activities under stress conditions although their native structures are severely disturbed. Here, using an alternative approach (site-directed mutagenesis), we obtained two structurally and functionally distinct Mycobacterium tuberculosis Hsp16.3 single-site mutant proteins. The G59W mutant protein (with Gly59 substituted by Trp) is capable of exhibiting efficient chaperone-like activity even under non-stress conditions although its secondary, tertiary, and quaternary structures are very different from that of the wild type protein. By contrast, the G59A mutant protein (with Gly59 substituted by Ala) resembles with the wild type protein in structure and function. These observations suggest that the Gly59 of the Hsp16.3 protein is critical for its folding and assembly. In particular, we propose that the exhibition of chaperone-like activity for Hsp16.3 does not require its intact (native) structures but requires the disturbance of its native structures (i.e., the native structure-disturbed Hsp16.3 retains its chaperone-like activity or even becomes more active). In addition, the behavior of such an active mutant protein (G59W) also strongly supports our previous suggestion that Hsp16.3 exhibits chaperone-like activity via oligomeric dissociation.     

Toxoplasma Co-opts Host Cells It Does Not Invade

Like many intracellular microbes, the protozoan parasite Toxoplasma gondii injects effector proteins into cells it invades. One group of these effector proteins is injected from specialized organelles called the rhoptries, which have previously been described to discharge their contents only during successful invasion of a host cell. In this report, using several reporter systems, we show that in vitro the parasite injects rhoptry proteins into cells it does not productively invade and that the rhoptry effector proteins can manipulate the uninfected cell in a similar manner to infected cells. In addition, as one of the reporter systems uses a rhoptry:Cre recombinase fusion protein, we show that in Cre-reporter mice infected with an encysting Toxoplasma-Cre strain, uninfected-injected cells, which could be derived from aborted invasion or cell-intrinsic killing after invasion, are actually more common than infected-injected cells, especially in the mouse brain, where Toxoplasma encysts and persists. This phenomenon has important implications for how Toxoplasma globally affects its host and opens a new avenue for how other intracellular microbes may similarly manipulate the host environment at large.     

Chronic Helminth Infection Does Not Exacerbate Mycobacterium tuberculosis Infection

Background

Chronic helminth infections induce a Th2 immune shift and establish an immunoregulatory milieu. As both of these responses can suppress Th1 immunity, which is necessary for control of Mycobacterium tuberculosis (MTB) infection, we hypothesized that chronic helminth infections may exacerbate the course of MTB.

Methodology/Principal Findings

Co-infection studies were conducted in cotton rats as they are the natural host for the filarial nematode Litomosoides sigmodontis and are an excellent model for human MTB. Immunogical responses, histological studies, and quantitative mycobacterial cultures were assessed two months after MTB challenge in cotton rats with and without chronic L. sigmodontis infection. Spleen cell proliferation and interferon gamma production in response to purified protein derivative were similar between co-infected and MTB-only infected animals. In contrast to our hypothesis, MTB loads and occurrence and size of lung granulomas were not increased in co-infected animals.

Conclusions/Significance

These findings suggest that chronic filaria infections do not exacerbate MTB infection in the cotton rat model. While these results suggest that filaria eradication programs may not facilitate MTB control, they indicate that it may be possible to develop worm-derived therapies for autoimmune diseases that do not substantially increase the risk for infections.     

A Model of Binge‐Like Eating Behavior in Mice That Does Not Require Food Deprivation or Stress

Binge eating disorder (BED) is characterized by excessive food intake during a short period of time and is often associated with obesity. Mouse models of binge‐like eating behavior are lacking making it difficult to employ genetic models in the identification of mechanisms regulating excessive eating. We report a rapid and simple model to induce binge‐like eating behavior in mice that does not require food deprivation or exogenous stressors. Weekly 24 h access to a nutritionally complete high energy diet (HED), along with continuous access to standard chow, resulted in a significant increase in HED intake following its presentation compared to mice that had continuous access to both diets. Mice exhibiting binge‐like eating consumed one‐third of their normal total daily caloric intake within 2.5 h of HED presentation. Moreover, total 24‐h caloric intakes were increased by 50% in mice exhibiting binge‐like eating. Following repeated cycles, binge‐like eating of the HED was maintained over several weeks with no evidence of habituation or significant alterations in body weight and adiposity. Pharmacological evaluation of binge‐like eating behavior was performed using clinically employed compounds. Interestingly, binge‐like eating was dose‐dependently decreased by fluoxetine, but not baclofen or topiramate. These data support clinical validation of this mouse model of binge‐like eating behavior, as fluoxetine has been shown to reduce binge frequency in human subjects with BED. The availability of transgenic and knockout mice will allow for the determination of genes that are involved in the initiation and maintenance of binge‐like eating behavior.     

Bacteriophage MS2 Lysis Protein Does Not Require Coat Protein to Mediate Cell Lysis

We inserted all but the extreme 5' end of a DNA copy of the bacteriophage MS2 lysis gene downstream of a lac-induced promoter on a multicopy plasmid. Upon induction, cells harboring this plasmid began to lyse, showing that phage coat protein is not required for the lytic process itself.     

Non-heme Iron as Ferrous Sulfate Does Not Interact with Heme Iron Absorption in Humans

The absorption of heme iron has been described as distinctly different from that of non-heme iron. Moreover, whether heme and non-heme iron compete for absorption has not been well established. Our objective was to investigate the potential competition between heme and non-heme iron as ferrous sulfate for absorption, when both iron forms are ingested on an empty stomach. Twenty-six healthy nonpregnant women were selected to participate in two iron absorption studies using iron radioactive tracers. We obtained the dose?Cresponse curve for absorption of 0.5, 10, 20, and 50?mg heme iron doses, as concentrated red blood cells. Then, we evaluated the absorption of the same doses, but additionally we added non-heme iron, as ferrous sulfate, at constant heme/non-heme iron molar ratio (1:1). Finally, we compare the two curves by a two-way ANOVA. Iron sources were administered on an empty stomach. One factor analysis showed that heme iron absorption was diminished just by increasing total heme iron (P?<?0.0001). The addition of non-heme iron as ferrous sulfate did not have any effect on heme iron absorption (P?=?NS). We reported evidence that heme and non-heme iron as ferrous sulfate does not compete for absorption. The mechanism behind the absorption of these iron sources is not clear.     

Acute Copper Supplementation Does Not Inhibit Non-Heme Iron Bioavailability in Humans

To measure the effect of acute copper (Cu) administration, given as an aqueous solution, on the absorption of iron (Fe), 29 healthy adult women participated in two iron absorption studies. Subjects received 0.5 mg of Fe, as ferrous sulfate, alone or with Cu, as copper sulfate, at 0.5:1, 1:1, or 2:1 Cu/Fe molar ratios (study I) or at 4:1, 6:1, or 8:1 Cu/Fe molar ratios (study II) as an aqueous solution on days 1, 2, 14, and 15 of the study. Fe absorption was assessed by erythrocyte incorporation of iron radioisotopes ⁵⁵Fe and ⁵⁹Fe. Geometric mean (range ± SD) absorption of Fe alone or at 0.5:1, 1:1, 2:1 Cu/Fe molar ratios were 34.4% (17.3–68.5%), 40.9% (24.9–67.2%), 48.3% (24.8–94.1%), and 50.2% (25.3–99.5%), respectively (ANOVA, p = 0.12). Geometric mean (range ± SD) absorption of Fe alone or at 4:1, 6:1, 8:1 Cu/Fe molar ratios were 28.7% (12.1–67.9%), 21.5% (6.5–71.5%), 29.6% (10.3–85.4%), and 36.5% (18.3–73.1%), respectively (ANOVA, p = 0.16). In conclusion, combined Cu and Fe administration in an aqueous solution does not inhibit Fe bioavailability. This information could help in the design of rational guidelines for copper and iron supplementation programs. Our results support the hypothesis that divalent metal transporter 1 is not physiologically relevant for copper absorption in humans.