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Transmission at the parallel fibre-Purkinje neurone synapse of the cerebellum can be depressed by a number of presynaptic receptors: endocannabinoid (CB1), metabotropic glutamate (mGluR4), adenosine (A1) and GABA (GABA(B)), which have been implicated in both short- and long-term synaptic plasticity. Stimulation of parallel fibres also activates glutamate receptors and transporters on the Bergmann glial cell that forms a sheath around the synapse. The resulting glial extrasynaptic currents (ESC) exhibit short- and long-term plasticity, which differs from the plasticity of adjacent synapses. This functional independence could arise from differential modulation of presynaptic release sites targeted to synapses or glia, but the sensitivity of glial ESC to these inhibitory pathways is unknown. Here I show that all four presynaptic receptors depress parallel fibre-Bergmann glial cell signalling with similar potency to synaptic transmission. Depression of glial ESC is accompanied by a decrease in paired pulse ratio. However, application of receptor antagonists had no effect on ESC amplitude, indicating that tonic activation of these pathways does not occur, and antagonists failed to block the activity-dependent depression of glial ESC observed during tetanic or low frequency stimulation. These data suggest that modulation of presynaptic glutamate release does not underlie glial plasticity.
Microglia are associated with neuritic plaques in Alzheimer disease (AD) and serve as a primary component of the innate immune response in the brain. Neuritic plaques are fibrous deposits composed of the amyloid beta-peptide fragments (Abeta) of the amyloid precursor protein (APP). Numerous studies have shown that the immune cells in the vicinity of amyloid deposits in AD express mRNA and proteins for pro-inflammatory cytokines, leading to the hypothesis that microglia demonstrate classical (Th-1) immune activation in AD. Nonetheless, the complex role of microglial activation has yet to be fully explored since recent studies show that peripheral macrophages enter an "alternative" activation state.
It is unclear whether TGF-beta, a critical differentiation factor for T cells producing interleukin 17 (T(H)-17 cells), is required for the initiation of experimental autoimmune encephalomyelitis (EAE) in vivo. Here we show that mice whose T cells cannot respond to TGF-beta signaling lack T(H)-17 cells and do not develop EAE despite the presence of T helper cell type 1 infiltrates in the spinal cord. Local but not systemic antibody blockade of TGF-beta prevented T(H)-17 cell differentiation and the onset of EAE. The pathogen stimulus zymosan, like mycobacterium, induced T(H)-17 cells and initiated EAE, but the disease was transient and correlated with reduced production of interleukin 23. These data show that TGF-beta is essential for the initiation of EAE and suggest that disease progression may require ongoing chronic inflammation and production of interleukin 23.
The chromosome conformation capture technique is used to monitor intra- and intermolecular chromosomal associations. By introducing an adaptation of this technique, Ling and colleagues have identified an unexpected coassociation between two loci on separate chromosomes in mouse nuclei, the imprinted Igf2-H19 locus of chromosome 7 and the Wsb1-Nf1 locus of chromosome 11. Strikingly, this interaction is CCCTC-binding factor (CTCF)-dependent and strictly allele specific. These findings extend our appreciation for genome organization and its influence on gene expression and imprinting.
The production of reactive oxygen species by the NADPH oxidase complex of phagocytes plays a critical role in our defence against bacterial and fungal infections. The PX domains of two oxidase components, p47(phox) and p40(phox), are known to bind phosphoinositide products of PI3Ks but the physiological roles of these interactions are unclear. We have created mice which carry an R58A mutation in the PX domain of their p40(phox) gene, which selectively prevents binding to PtdIns3P. p40(phoxR58A/R58A) embryos do not develop normally but p40(phoxR58A/-) mice are viable and neutrophils from these animals exhibit significantly reduced oxidase responses compared to those from their p40(phox+/-) siblings (e.g. 60% reduced in response to phagocytosis of Staphylococcus aureus). Wortmannin inhibition of the S. aureus oxidase response correlates with inhibition of phagosomal PtdIns3P accumulation and overlaps with the reduction in this response caused by the R58A mutation, suggesting PI3K regulation of this response is substantially dependent on PtdIns3P-binding to p40(phox). p40(phoxR58A/-) mice are significantly compromised in their ability to kill S. aureus in vivo, defining the physiological importance of this interaction.
Atrial cardiomyocytes make an important contribution to the refilling of ventricles with blood, which enhances the subsequent ejection of blood from the heart. The dependence of cardiac function on the contribution of atria becomes increasingly important with age and exercise. We know much less about the calcium signals that link electrical depolarisation to contraction within atrial myocytes in comparison with ventricular myocytes. Nevertheless, recent work has shed new light on calcium signalling in atrial cells. At an ultrastructural level, atrial and ventricular myocytes have many similarities. However, a few key structural differences, in particular the lack of transverse tubules (;T-tubules') in atrial myocytes, make these two cell types display vastly different calcium patterns in response to depolarisation. The lack of T-tubules in atrial myocytes means that depolarisation provokes calcium signals that largely originate around the periphery of the cells. To engage the contractile machinery, the calcium signal must propagate centripetally deeper into the cells. This inward movement of calcium is ultimately controlled by hormones that can promote or decrease calcium release within the myocytes. Enhanced centripetal movement of calcium in atrial myocytes leads to increased contraction and a more substantial contribution to blood pumping. The calcium signalling paradigm within atrial cells applies to other cardiac cell types that also do not express T-tubules, such as neonatal ventricular myocytes, and Purkinje cells that aid in the spread of electrical depolarisation. Furthermore, during heart failure ventricular myocytes progressively lose their regular T-tubule expression, and their pattern of response resembles that of atrial cells.
Abnormal clearance by the mononuclear phagocytic system of immune complexes (IC) is important in the pathogenesis of systemic lupus erythematosus (SLE). We have developed an in vitro model to investigate the cellular mechanisms involved in the transfer of soluble IC from erythrocytes to human macrophages under physiological flow conditions. In this assay, erythrocytes bearing fluorescently labelled IC are perfused over monolayers of human monocytes or monocyte-derived macrophages in a parallel-plate flow chamber, and transfer quantified using confocal microscopy and flow cytometry. Using aggregated human IgG as a model IC, we have been able to demonstrate transfer of IC from erythrocytes to macrophages. Blocking studies with specific neutralizing antibodies have shown that both complement and Fcgamma receptors are required for IC transfer. Blockade of CR4 (alpha(x)beta(2) integrin), FcgammaRIIa or FcgammaRIII reduced transfer, while anti-CR3 (alpha(m)beta(2) integrin) had no effect. Blockade of CR3, FcgammaRIIa or FcgammaRIII also reduced the number of adhesive interactions between fluorescently labelled IC-bearing erythrocytes and macrophage monolayers. Taken together with the transfer data, this suggests differing roles for these receptors in the human IC transfer reaction that includes an adhesive function which facilitates IC processing by mononuclear phagocytes. Finally, a functional effect of the FcgammaRIIa R131/H131 polymorphism, important in susceptibility to SLE, has also been demonstrated using this model. Uptake of IgG(2) but not IgG(1)-containing soluble IC was reduced by macrophages from individuals homozygous for the R131 allelic variant of the receptor.
The p53 protein has a highly evolutionarily conserved role in metazoans as 'guardian of the genome', mediating cell-cycle arrest and apoptosis in response to genotoxic injury. In large, long-lived animals with substantial somatic regenerative capacity, such as vertebrates, p53 is an important tumour suppressor--an attribute thought to stem directly from its induction of death or arrest in mutant cells with damaged or unstable genomes. Chemotherapy and radiation exposure both induce widespread p53-dependent DNA damage. This triggers potentially lethal pathologies that are generally deemed an unfortunate but unavoidable consequence of the role p53 has in tumour suppression. Here we show, using a mouse model in which p53 status can be reversibly switched in vivo between functional and inactive states, that the p53-mediated pathological response to whole-body irradiation, a prototypical genotoxic carcinogen, is irrelevant for suppression of radiation-induced lymphoma. In contrast, delaying the restoration of p53 function until the acute radiation response has subsided abrogates all of the radiation-induced pathology yet preserves much of the protection from lymphoma. Such protection is absolutely dependent on p19(ARF)--a tumour suppressor induced not by DNA damage, but by oncogenic disruption of the cell cycle.
Developmental studies support a common origin for blood and endothelial cells, while studies of adult angiogenic responses suggest that the hematopoietic system can be a source of endothelial cells later in life. Whether hematopoietic tissue is a source of endothelial cells during normal vascular development is unknown. Mouse embryos lacking the signaling proteins Syk and Slp-76 develop abnormal blood-lymphatic endothelial connections. Here we demonstrate that expression of GFPSlp-76 in a subset of hematopoietic cells rescues this phenotype, and that deficient cells confer focal vascular phenotypes in chimeric embryos consistent with a cell-autonomous mechanism. Endogenous Syk and Slp-76, as well as transgenic GFPSlp-76, are expressed in circulating cells previously proposed to be endothelial precursors, supporting a causal role for these cells. These studies provide genetic evidence for hematopoietic contribution to vascular development and suggest that hematopoietic tissue can provide a source of vascular endothelial progenitor cells throughout life.
Despite the importance of thymic stromal cells to T-cell development, relatively little is known about their biology. Here, we use single-cell analysis of stromal cells to analyze extensive changes in the number and composition of thymic stroma throughout life, revealing a surprisingly dynamic population. Phenotypic progression of thymic epithelial subsets was assessed at high resolution in young mice to provide a developmental framework. The cellular and molecular requirements of adult epithelium were studied, using various mutant mice to demonstrate new cross talk checkpoints dependent on RelB in the cortex and CD40 in the medulla. With the use of Ki67 and BrdU labeling, the turnover of thymic epithelium was found to be rapid, but then diminished on thymic involution. The various defects in stromal turnover and composition that accompanied involution were rapidly reversed following sex steroid ablation. Unexpectedly, mature cortical and medullary epithelium showed a potent capacity to stimulate naive T cells, comparable to that of thymic dendritic cells. Overall, these studies show that the thymic stroma is a surprisingly dynamic population and may have a more direct role in negative selection than previously thought.
Although ventricular cardiomyocytes express inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptors, it is unclear how these Ca2+ channels contribute to the effects of Gq-coupled agonists. Endothelin-1 augmented the amplitude of pacing-evoked Ca2+ signals (positive inotropy), and caused an increasing frequency of spontaneous diastolic Ca2+-release transients. Both effects of endothelin-1 were blocked by an antagonist of phospholipase C, suggesting that Ins(1,4,5)P3 and/or diacylglycerol production was necessary. The endothelin-1-mediated spontaneous Ca2+ transients were abolished by application of 2-aminoethoxydiphenyl borate (2-APB), an antagonist of Ins(1,4,5)P3 receptors. Incubation of electrically-paced ventricular myocytes with a membrane-permeant Ins(1,4,5)P3 ester provoked the occurrence of spontaneous diastolic Ca2+ transients with the same characteristics and sensitivity to 2-APB as the events stimulated by endothelin-1. In addition to evoking spontaneous Ca2+ transients, stimulation of ventricular myocytes with the Ins(1,4,5)P3 ester caused a positive inotropic effect. The effects of endothelin-1 were compared with two other stimuli, isoproterenol and digoxin, which are known to induce inotropy and spontaneous Ca2+ transients by overloading intracellular Ca2+ stores. The events evoked by isoproterenol and digoxin were dissimilar from those triggered by endothelin-1 in several ways. We propose that Ins(1,4,5)P3 receptors support the development of both inotropy and spontaneous pro-arrhythmic Ca2+ signals in ventricular myocytes stimulated with a Gq-coupled agonist.
Type III secretion is a widespread method whereby Gram-negative bacteria introduce toxins into eukaryotic cells. These toxins mimic or subvert a normal cellular process by interacting with a specific target, although how toxins reach their site of action is unclear. We set out to investigate the intracellular localization of a type III toxin of Pseudomonas aeruginosa called ExoU, which has phospholipase activity and requires a eukaryotic factor for activity. We found that ExoU is localized to the plasma membrane and undergoes modification within the cell by addition of two ubiquitin molecules at lysine-178. A region of five amino acids at position 679-683 near the C-terminus of the ExoU protein controls both membrane localization and ubiquitinylation. Site-directed mutagenesis identified a tryptophan at position 681 as crucial for these effects. We found that the same region at position 679-683 was also required for cell toxicity produced by ExoU as well as in vitro phospholipase activity. Localization of the phospholipase ExoU to the plasma membrane is thus required for activation and allows efficient utilization of adjacent substrate phospholipids.
The generation of reactive oxygen species (ROS) by the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex plays a critical role in the antimicrobial functions of the phagocytic cells of the immune system. The catalytic core of this oxidase consists of a complex between gp91(phox), p22(phox), p47(phox), p67(phox), p40(phox), and rac-2. Mutations in each of the phox components, except p40(phox), have been described in cases of chronic granulomatous disease (CGD), defining their essential role in oxidase function. We sought to establish the role of p40(phox) by investigating the NADPH oxidase responses of neutrophils isolated from p40(phox-/-) mice. In the absence of p40(phox), the expression of p67(phox) is reduced by approximately 55% and oxidase responses to tumor necrosis factor alpha/fibrinogen, immunoglobulin G latex beads, Staphylococcus aureus, formyl-methionyl-leucyl-phenylalanine, and zymosan were reduced by approximately 97, 85, 84, 75, and 30%, respectively. The defect in ROS production by p40(phox-/-) neutrophils in response to S. aureus translated into a severe, CGD-like defect in the killing of this organism both in vitro and in vivo, defining p40(phox) as an essential component in bacterial killing.
In multicellular eukaryotes, geminin prevents overreplication of DNA in proliferating cells. Here, we show that genetic ablation of geminin in the mouse prevents formation of inner cell mass (ICM) and causes premature endoreduplication at eight cells, rather than 32 cells. All cells in geminin-deficient embryos commit to the trophoblast cell lineage and consist of trophoblast giant cells (TGCs) only. Geminin is also down-regulated in TGCs of wild-type blastocysts during S and gap-like phases by proteasome-mediated degradation, suggesting that loss of geminin is part of the mechanism regulating endoreduplication.
The exosome complex of 3'-->5' exonucleases is an important component of the RNA-processing machinery in eukaryotes. This complex functions in the accurate processing of nuclear RNA precursors and in the degradation of RNAs in both the nucleus and the cytoplasm. However, it has been unclear how different classes of substrate are distinguished from one another. Recent studies now provide insights into the regulation and structure of the exosome, and they reveal striking similarities between the process of RNA degradation in bacteria and eukaryotes.
It is thought that the H19 imprinting control region (ICR) directs the silencing of the maternally inherited Igf2 allele through a CTCF-dependent chromatin insulator. The ICR has been shown to interact physically with a silencer region in Igf2, differentially methylated region (DMR)1, but the role of CTCF in this chromatin loop and whether it restricts the physical access of distal enhancers to Igf2 is not known. We performed systematic chromosome conformation capture analyses in the Igf2/H19 region over >160 kb, identifying sequences that interact physically with the distal enhancers and the ICR. We found that, on the paternal chromosome, enhancers interact with the Igf2 promoters but that, on the maternal allele, this is prevented by CTCF binding within the H19 ICR. CTCF binding in the maternal ICR regulates its interaction with matrix attachment region (MAR)3 and DMR1 at Igf2, thus forming a tight loop around the maternal Igf2 locus, which may contribute to its silencing. Mutation of CTCF binding sites in the H19 ICR leads to loss of CTCF binding and de novo methylation of a CTCF target site within Igf2 DMR1, showing that CTCF can coordinate regional epigenetic marks. This systematic chromosome conformation capture analysis of an imprinting cluster reveals that CTCF has a critical role in the epigenetic regulation of higher-order chromatin structure and gene silencing over considerable distances in the genome.
The S100 family member S100A9 and its heterodimeric partner, S100A8, are cytosolic Ca2+ binding proteins abundantly expressed in neutrophils. To understand the role of this EF-hand-containing complex in Ca2+ signalling, neutrophils from S100A9 null mice were investigated. There was no role for the complex in buffering acute cytosolic Ca2+ elevations. However, Ca2+ responses to inflammatory agents such as chemokines MIP-2 and KC and other agonists are altered. For S100A9 null neutrophils, signalling at the level of G proteins is normal, as is release of Ca2+ from the IP(3) receptor-gated intracellular stores. However MIP-2 and FMLP signalling in S100A9 null neutrophils was less susceptible than wildtype to PLCbeta inhibition, revealing dis-regulation of the signalling pathway at this level. Downstream of PLCbeta, there was reduced intracellular Ca2+ release induced by sub-maximal levels of chemokines. Conversely the response to FMLP was uncompromised, demonstrating different regulation compared to MIP-2 stimulation. Study of the activity of PLC product DAG revealed that chemokine-induced signalling was susceptible to inhibition by elevated DAG with S100A9 null cells showing enhanced inhibition by DAG. This study defines a lesion in S100A9 null neutrophils associated with inflammatory agonist-induced IP3-mediated Ca2+ release that is manifested at the level of PLCbeta.
Transforming growth factor (TGF)beta is most commonly considered an anti-inflammatory cytokine, a view that clearly does not correlate with the recently described role for TGFbeta1 in the differentiation of T-helper (Th)17 cells, a novel, highly inflammatory T-cell subset that produces interleukin (IL)-17. However, these recent findings endorse earlier studies, pre-dating the discovery of Th17 cells, which described a seemingly paradoxical pro-inflammatory role of TGFbeta. In this article, we propose that the administration of neutralizing anti-TGFbeta antibodies in target sites of chronic inflammation would ameliorate or abolish disease because this would limit the differentiation of Th17 cells. By contrast, similar interventions at mucosal sites, where Th17 cells seem to have a protective role, might exacerbate disease in experimental models of colitis. An excess production of Th17 cells in response to infection or trauma could result in leakage into peripheral tissues and cause autoimmune pathology.
Autotaxin (ATX), or nucleotide pyrophosphatase-phosphodiesterase 2, is a secreted lysophospholipase D that promotes cell migration, metastasis, and angiogenesis. ATX generates lysophosphatidic acid (LPA), a lipid mitogen and motility factor that acts on several G protein-coupled receptors. Here we report that ATX-deficient mice die at embryonic day 9.5 (E9.5) with profound vascular defects in yolk sac and embryo resembling the Galpha13 knockout phenotype. Furthermore, at E8.5, ATX-deficient embryos showed allantois malformation, neural tube defects, and asymmetric headfolds. The onset of these abnormalities coincided with increased expression of ATX and LPA receptors in normal embryos. ATX heterozygous mice appear healthy but show half-normal ATX activity and plasma LPA levels. Our results reveal a critical role for ATX in vascular development, indicate that ATX is the major LPA-producing enzyme in vivo, and suggest that the vascular defects in ATX-deficient embryos may be explained by loss of LPA signaling through Galpha13.
Phe-Met-Arg-Phe-NH2 (FMRFamide)-like peptides (FLPs) are the largest neuropeptide family in animals, particularly invertebrates. FLPs are characterized by a C-N-terminal gradient of decreasing amino acid conservation. Neuropeptide receptor 1 (NPR-1) is a G-protein coupled receptor (GPCR), which has been shown to be a strong regulator of foraging behavior and aggregation responses in Caenorhabditis elegans. Recently, ligands for NPR-1 were identified as neuropeptides coded by the precursor genes flp-18 and flp-21 in C. elegans. The flp-18 gene encodes eight FLPs including DFDGAMPGVLRF-NH2 and EMPGVLRF-NH2. These peptides exhibit considerably different activities on NPR-1, with the longer one showing a lower potency. We have used nuclear magnetic resonance and biological activity to investigate structural features that may explain these activity differences. Our data demonstrate that long-range electrostatic interactions exist between N-terminal aspartates and the C-terminal penultimate arginine as well as N-terminal hydrogen-bonding interactions that form transient loops within DFDGAMPGVLRF-NH2. We hypothesize that these loops, along with peptide charge, diminish the activity of this peptide on NPR-1 relative to that of EMPGVLRF-NH2. These results provide some insight into the large amino acid diversity in FLPs.
A ubiquitous pathway for cellular Ca(2+) influx involves 'store-operated channels' that respond to depletion of intracellular Ca(2+) pools via an as yet unknown mechanism. Due to its wide-spread expression, store-operated Ca(2+) entry (SOCE) has been considered a principal route for Ca(2+) influx. However, recent evidence has suggested that alternative pathways, activated for example by lipid metabolites, are responsible for physiological Ca(2+) influx. It is not clear if these messenger-activated Ca(2+) entry routes exist in all cells and what interaction they have with SOCE. In the present study we demonstrate that HEK-293 cells and Saos-2 cells express an arachidonic acid (AA)-activated Ca(2+) influx pathway that is distinct from SOCE on the basis of sensitivity to pharmacological blockers and depletion of cellular cholesterol. We examined the functional interaction between SOCE and the arachidonate-triggered Ca(2+) influx (denoted non-SOCE). Both Ca(2+) entry routes could underlie substantial long-lasting Ca(2+) elevations. However, the two pathways could not operate simultaneously. With cells that had an on-going SOCE response, addition of arachidonate gave two profound effects. Firstly, it rapidly inhibited SOCE. Secondly, the mode of Ca(2+) influx switched to the non-SOCE mechanism. Addition of arachidonate to naÃ¯ve cells resulted in rapid activation of the non-SOCE pathway. However, this Ca(2+) entry route was very slowly engaged if the SOCE pathway was already operative. These data indicate that the SOCE and arachidonate-activated non-SOCE pathways interact in an inhibitory manner. We probed the plausible mechanisms by which these two pathways may communicate.
We studied the spatial distribution, mobility, and trafficking of plasma membrane Ca2+ATPase-2 (PMCA2), a protein enriched in the hair cell apical membrane and essential for hair cell function. Using immunofluorescence, we determined that PMCA2 is enriched in the stereocilia and present at a relatively low concentration in the kinocilium and in the remaining apical membrane. Using an antibody to the extracellular domain of PMCA2 as a probe, we observed that PMCA2 diffuses laterally from the stereocilia membrane and is internalized at the apical cell border maintaining an estimated half-life of residency in the stereocilia of approximately 5-7 h. A computer simulation of our data indicates that PMCA2 has an estimated global diffusion coefficient of 0.01-0.005 microm2/s. Using a green fluorescent protein tag, we observed that PMCA2 is rapidly delivered to the apical cell border from where it diffuses to the entire stereocilia surface. Fluorescence recovery after photobleaching experiments show that approximately 60% of PMCA2 in the stereocilia exhibit high mobility with a diffusion coefficient of 0.1-0.2 microm2/s, whereas the remaining pool represents a relatively immobile fraction. These results suggest that PMCA2 molecules maintain transient interactions with other components of the stereocilia, and the mobile pool of PMCA2 mediates the exchange between the stereocilia and the removal and delivery sites at the periphery of the apical cell surface. This rapid turnover of a major stereocilia membrane protein matches the previously described rapid turnover of proteins of the stereocilia actin core, further demonstrating that these organelles undergo rapid continuous renewal.