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T Elliott

Publications and source records attributed to T Elliott.

At least 91 records · Page 5Linked to original sources
PubMed1996

A mathematical model of activity-dependent, anatomical segregation induced by competition for neurotrophic support.

Mathematical or computational models of activity-dependent neural competition typically impose competition in anatomically fixed networks by the use of synaptic normalisation, for which there is very little experimental support. Recent experimental evidence, however, strongly implicates neurotrophic factors in neural plasticity and competition, in addition to their well-known potent effects on neurite outgrowth and synaptogenesis. We therefore present a simple, mathematical model of anatomical segregation induces by activity-dependent competition for a limited supply of a neurotrophic factor provided by target cells to afferents. We extract the behaviour of the model in various regimes, in which the neurotrophic factor is either in critical supply or in abundant supply, by a combination of analytical and numerical methods, and study the effects of correlations in afferent inputs on competition. We apply the model to three different systems: ocular dominance column formation; elimination of polyneuronal innervation at the vertebrate neuromuscular junction; trigeminal brain stem whisker-related structure formation. Several classes of related predictions emerge, including the prediction that kittens reared with strabismus should require a higher concentration of neurotrophic factor infusion into their primary visual cortex than normally reared cats in order to induce the anatomical desegregation of ocular dominance columns. We also speculate on the mechanisms of support of inhibitory rather than excitatory neurons, and suggest the existence of a separate, (Cl-)-mediated activity-dependent pathway for their neurotrophic support.

Animals↗
PubMed1996

Tapping into tumours.

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ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1996

Efficient translation of the RpoS sigma factor in Salmonella typhimurium requires host factor I, an RNA-binding protein encoded by the hfq gene.

The RpoS transcription factor (also called sigma Sor sigma 38) is required for the expression of a number of stationary-phase and osmotically inducible genes in Escherichia coli. RpoS is also a virulence factor for several pathogenic bacteria, including Salmonella typhimurium. The activity of RpoS is regulated in response to several different signals, at the transcriptional and translational levels as well as by proteolysis. Here we report that host factor I (HF-I), the product of the hfq gene, is required for efficient expression of rpoS in S. typhimurium. HF-I is a small, heat-stable, site-specific RNA-binding protein originally characterized for its role in replication of the RNA bacteriophage Q beta of E. coli. Its role in the uninfected bacterial cell has previously been unknown. Assays of Beta-galactosidase in strains with rpoS-lac fusions, Western blot (immunoblot) analysis, and pulse-labeling and immunoprecipitation of both fusion proteins and native RpoS show that an S. typhimurium hfq mutant has a four- to sevenfold reduction in expression of rpoS that is attributable primarily to a defect in translation. These results add a new level of complexity to the regulation of RpoS activity.

Amino Acid Sequence↗
PubMed1996

Transcription of the glutamyl-tRNA reductase (hemA) gene in Salmonella typhimurium and Escherichia coli: role of the hemA P1 promoter and the arcA gene product.

In Salmonella typhimurium and Escherichia coli, the hemA gene encodes the enzyme glutamyl-tRNA reductase, which catalyzes the first committed step in the heme biosynthetic pathway. It has recently been reported that a lac operon fusion to the hemA promoter of E. coli is induced 20-fold after starvation for heme. Induction was dependent on the transcriptional regulator ArcA, with a second transcriptional regulator, FNR, playing a negative role specifically under anaerobic conditions (S. Darie and R. P. Gunsalus, J. Bacteriol. 176:5270-5276, 1994). We have investigated the generality of this effect by examining the response to heme starvation of a number of lac operon fusions to the hemA promoters of both E. coli and S. typhimurium. We confirmed that such fusions are induced during starvation of a hemA auxotroph, but the level of induction observed was maximally sixfold and for S. typhimurium fusions it was only two- to fourfold. Sequences required for high-level expression of hemA lie within 129 bp upstream of the major (P1) promoter transcriptional start site. Mutants defective in the P1 promoter had greatly reduced hemA-lac expression both in the presence and in the absence of ALA. Mutations in arcA had no effect on hemA-lac expression in E. coli during normal growth, although the increase in expression during starvation for ALA was half that seen in an arcA+ strain. Overexpression of the arcA gene had no effect on hemA-lac expression. Primer extension analysis showed that RNA 5' ends mapping to the hemA P1 and P2 promoters were not expressed at significantly higher levels in induced cultures. These results differ from those previously reported.

Aldehyde Oxidoreductases↗
PubMed1996

Stabilization of a HemA-LacZ hybrid protein against proteolysis during carbon starvation in atp mutants of Salmonella typhimurium.

Transposon insertions that stabilize the beta-galactosidase activity of a HemA-LacZ hybrid protein following carbon starvation were mapped to the atp operon of Salmonella typhimurium. This effect is similar to that seen with nuo mutants defective in the energy-conserving type I NADH dehydrogenase. Insertions in several other genes, including such highly pleiotropic mutants as rpoS, polA, and hfq, were isolated with the same phenotypic screen, but they do not affect the beta-galactosidase activity of HemA-LacZ. All of these mutants act indirectly to alter the colony color of many different fusion strains on indicator plates.

Aldehyde Oxidoreductases↗
PubMed1996

Multiple lipoprotein abnormalities in type I diabetic patients with renal disease.

The aim of this study was to characterize abnormalities of triglyceride-rich apolipoprotein (apo) B-containing lipoproteins in type I diabetic patients with elevated albumin excretion rates (AERs). Sixty-four patients (31 men, 33 women) with normoalbuminuria (AER <20 microg/min), 52 (35 men, 17 women) with microalbuminuria (AER 20-200 microg/min), and 37 (17 men, 20 women) with albuminuria (AER >200 microg/min) and 56 healthy control subjects matched for age and body weight were studied. The major finding was increased mass concentrations of the highly atherogenic intermediate-density lipoprotein fraction in patients with microalbuminuria (P < 0.05) and albuminuria (P < 0.05), compared with those with normoalbuminuria. Triglyceride, free cholesterol, cholesterol ester, and phospholipid concentrations in the VLDL, intermediate-density lipoprotein, and LDL (P < 0.05-0.01), as well as total cholesterol, total triglyceride, and apoB concentrations were higher in patients with renal disease than in those without. Notably, there were no differences between patients with microalbuminuria and albuminuria. Only minor compositional changes could be detected. Postheparin plasma lipoprotein lipase (LPL) activities were identical, but hepatic lipase activities were higher in microalbuminuric and albuminuric patients than in normoalbuminuric patients (P < 0.01). LPL activity and VLDL1, (Sf 60-400) (r = -0.528; P < 0.001) and VLDL2 (Sf 20-60) mass concentrations (r = -0.471; P < 0.001) were negatively related. In conclusion, in type I diabetic patients with early renal disease, there are multiple lipoprotein changes, which are potentially atherogenic and may contribute to the excess of macrovascular complications seen in such patients.

Adult↗
PubMed1996

NIDDM in the elderly.

OBJECTIVE: We conducted this study to assess the metabolic alterations in elderly patients with NIDDM. RESEARCH DESIGN AND METHODS: Healthy, lean (n = 15; age, 73 +/- 1 years; BMI, 23.8 +/- 0.5 kg/m2), and obese (n = 10; age, 71 +/- 1 years; BMI, 28.9 +/- 1.2 kg/m2) control subjects and lean (n = 10; age, 75 +/- 2 years; BMI, 24.0 +/- 0.5 kg/m2) and obese (n = 23; age, 73 +/- 1 years; BMI, 29.9 +/- 0.7 kg/m2) NIDDM patients underwent a 3-h glucose tolerance test, a 2-h hyperglycemic glucose clamp study, and a 3-h euglycemic glucose clamp study with tritiated glucose methodology to measure glucose production and disposal rates. RESULTS: Waist-to-hip ratio (WHR) was greater in both lean and obese NIDDM patients than in control subjects. Insulin responses during the oral glucose tolerance test were similar in obese subjects (control subjects: 417 +/- 64 pmol/l; NIDDM patients: 392 +/- 47 pmol/l) but were reduced in lean NIDDM patients (control subjects: 374 +/- 34 pmol/l; NIDDM patients: 217 +/- 20 pmol/l, P < 0.01). Lean and obese NIDDM patients had absent first-phase insulin responses during the hyperglycemic clamp. Second-phase insulin responses were reduced in lean (P < 0.01 vs. control subjects by analysis of variance) but not obese NIDDM patients. Hepatic glucose output was not increased in lean or obese NIDDM patients. Steady-state (150-180 min) glucose disposal rates were 16% less in lean NIDDM patients (control subjects: 8.93 +/- 0.37 mg.kg LBM (lean body mass)-1.min-1; NIDDM patients: 7.50 +/- 0.28 mg.kg LBM-1.min-1, P < 0.05) and 37% less in obese NIDDM patients (control subjects: 8.17 +/- 0.38 mg.kg LBM-1.min-1; NIDDM patients: 5.03 +/- 0.36 mg.kg LBM-1.min-1, P < 0.001). CONCLUSIONS: Lean elderly NIDDM patients have a profound impairment in glucose-induced insulin release but mild resistance to insulin-mediated glucose disposal. Obese elderly NIDDM patients have adequate circulating insulin, but marked resistance to insulin-mediated glucose disposal. Hepatic glucose output is not increased in elderly NIDDM patients.

Aged↗
PubMed1995

Processing of major histocompatibility class I-restricted antigens in the endoplasmic reticulum.

We have introduced long precursor peptides directly into the endoplasmic reticulum (ER) of a mutant cell line (T2-Db) that lacks the ability to transport peptides from the cytosol to the ER in a transporter associated with antigen processing (TAP) dependent way. This was done by expressing various influenza A-derived peptides containing the naturally processed epitope ASNENMDAM (366-374) preceded by the influenza hemagglutinin ER translocation sequence. Peptides derived from these minigenes that became associated with Db were isolated and identified by combined reversed phase liquid chromatography and detection by cytotoxic T lymphocytes. Our results establish that NH2-terminal extensions of at least 40 residues can be trimmed from peptides entering the ER, but that proteolysis of larger proteins may be limited.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1995

Characterization of two Epstein-Barr virus epitopes restricted by HLA-B7.

We have identified two epitopes for Epstein-Barr virus specific cytotoxic T lymphocytes (CTL) restricted by the common allele HLA-B7. They are EBNA 3C 881-9 (QPRAPIRPI) and EBNA 3A 379-387 (RPPIFIRRL). The epitopes conform well to the recently described motif for HLA-B7-binding peptides (Huckzo et al., J. Immunol. 1993. 151:2572). Titration of the peptides in CTL assays and detergent lysate binding assays revealed that extending the peptides at either the N or C terminus did not reduce their affinity for HLA-B7. This behavior contrasted with HLA-B51, which binds peptides with a similar motif to B7, and has identical amino acid residues at sites expected to form the "F" pocket of the peptide-binding groove. HLA-B51 also bound the peptide EBNA 3C 881-9, but was unable to bind peptides extended at the C terminus.

Amino Acid Sequence↗
PubMed1995

Genes encoded in the major histocompatibility complex affecting the generation of peptides for TAP transport.

The B cell line 721.174 has lost the ability to present intracellular antigens to major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes (CTL). This phenotype results from a homozygous deletion in the MHC that includes the peptide transporter genes TAP1 and TAP2, and the proteasome subunits LMP2 and LMP7. Recent work has shown that such cells transfected with TAP genes load their class I molecules with endogenous peptides, and present several viral epitopes to class I-restricted CTL. These data implied that the LMP2 and LMP7 genes were not required for the presentation of most epitopes through class I molecules. By contrast, while confirming the previous reports, we have identified several epitopes that appear to require genes in the MHC in addition to the TAP for their presentation. Further analysis localizes the defect to proteolysis in the cytosol. In one case, presentation could be partially restored by re-expression of full-length LMP7. Control experiments with LMP7, from which the putative pro-region had been removed, failed to restore presentation, and this lack of effect correlated with failure of the shortened LMP7 to incorporate into the proteasome. These results suggest a role for LMP7 in the generation of a viral epitope, but leave open the possibility that additional genes within the .174 deletion are required for full restoration of antigen presentation.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1995

Peptide anchor residue glycosylation: effect on class I major histocompatibility complex binding and cytotoxic T lymphocyte recognition.

This study extends our previous observation that glycopeptides bind to class I major histocompatibility complex (MHC) molecules and elicit carbohydrate-specific CTL responses. The Sendai virus nucleoprotein wild-type (WT) peptide (FAPGNYPAL) binds H-2Db using the P5-Asn as an anchor. The peptide K2 carrying a P5 serine substitution did not bind Db. Surprisingly, glycosylation of the serine (K2-O-GlcNAc) with N-acetylglucosamine (GlcNAc), a novel cytosolic O-linked glycosylation, partially restored peptide binding to Db. We argue that the N-acetyl group of GlcNAc may fulfil the hydrogen bonding requirements of the Db pocket which normally accomodates P5-Asn. Glycosylation of the P5-Asn residue itself abrogated binding similar to K2, probably for steric reasons. The peptide K2-O-GlcNAc readily elicited Db-restricted cytotoxic T lymphocytes (CTL), which did not cross-react with K2 or WT. However, all Db-restricted CTL raised against K2-O-GlcNAc cross-reacted strongly with another glycopeptide, K3-O-GlcNAc, where the GlcNAc substitution is on a neighboring P4-Ser. Furthermore, Db-restricted CTL clones raised against K2-O-GlcNAc or K3-O-GlcNAc displayed a striking TCR conservation. Our interpretation is that the carbohydrate of K2-O-GlcNAc not only mediates binding to Db, but also interacts with the TCR in such a way as to mimic K3-O-GlcNAc. This unusual example of molecular mimicry extends the known effects of peptide glycosylation from what we and others have previously reported: glycosylation may create a T cell neo-epitope, or, conversely, abrogate recognition. Alternatively, glycosylation may block peptide binding to MHC class I and finally, as reported here, restore binding, presumably through direct interaction of the carbohydrate with the MHC molecule.

Acetylglucosamine↗
PubMed1995

Anaerobic protoporphyrin biosynthesis does not require incorporation of methyl groups from methionine.

It was recently reported (H. Akutsu, J.-S. Park, and S. Sano, J. Am. Chem. Soc. 115:12185-12186, 1993) that in the strict anaerobe Desulfovibrio vulgaris methyl groups from exogenous L-methionine are incorporated specifically into the 1 and 3 positions (Fischer numbering system) on the heme groups of cytochrome c3. It was suggested that under anaerobic conditions, protoporphyrin IX biosynthesis proceeds via a novel pathway that does not involve coproporphyrinogen III as a precursor but instead may use precorrin-2 (1,3-dimethyluroporphyrinogen III), a siroheme and vitamin B12 precursor which is known to be derived from uroporphyrinogen III via methyl transfer from S-adenosyl-L-methionine. We have critically tested this hypothesis by examining the production of protoporphyrin IX-based tetrapyrroles in the presence of exogenous [14C]methyl-L-methionine under anaerobic conditions in a strict anaerobe (Chlorobium vibrioforme) and a facultative anaerobe (Rhodobacter capsulatus). In both organisms, 14C was incorporated into the bacteriochlorophyll precursor, Mg-protoporphyrin IX monomethyl ester. However, most of the label was lost upon base hydrolysis of this compound to yield Mg-protoporphyrin IX. These results indicate that although the administered [14C]methyl-L-methionine was taken up, converted into S-adenosyl-L-methionine, and used for methyl transfer reactions, including methylation of the 6-propionate of Mg-protoporphyrin IX, methyl groups were not transferred to the porphyrin nucleus of Mg-protoporphyrin IX. In other experiments, a cysG strain of Salmonella typhimurium, which cannot synthesize precorrin-2 because the gene encoding the enzyme that catalyzes methylation of uroporphyrinogen III at positions 1 and 3 is disrupted, was capable of heme-dependent anaerobic nitrate respiration and growth on the nonfermentable substrate glycerol, indicating that anaerobic biosynthesis of protoporphyrin IX-based hemes does not require the ability to methylate uroporphyrinogen III. Together, these results indicate that incorporation of L-methionine-deprived methyl groups into porphyrins or their precursors is not generally necessary for the anaerobic biosynthesis of protoporphyrin IX-based tetrapyrroles.

Aerobiosis↗
PubMed1995

Transcriptional control of the nuo operon which encodes the energy-conserving NADH dehydrogenase of Salmonella typhimurium.

The 14 nuo genes encode the subunits of the type I (energy-conserving) NADH dehydrogenase, a key component of the respiratory chain. Salmonella typhimurium, like Escherichia coli, has two enzymes that can oxidize NADH and transfer electrons to ubiquinone, but only the type I enzyme translocates protons across the membrane to generate a proton motive force. Cells with the type I enzyme are energetically more efficient; the role of the type II enzyme (encoded by ndh) is not established, but it may function like a relief valve to allow more rapid NADH recycling. Here, we have investigated transcription of the nuo gene cluster, primarily in S. typhimurium. Studies with polar insertion mutants demonstrate that these genes are arranged as a single, large operon that is expressed from a complex promoter region upstream of nuoA. The DNA sequence of the promoter region was determined, and primer extension analysis of nuo transcripts was used to map four major RNA 5' ends to this region. A set of lac operon fusions to various DNA segments from the nuo promoter region was also constructed. Analysis of these fusions confirmed the presence of at least two nuo promoters. Mutations in the global regulatory genes arcA, oxrA (fnr), crp, cya, and katF were tested for effects on expression of the nuo operon. However, none of the mutations tested had a large effect on expression of type I NADH dehydrogenase.

Base Sequence↗
PubMed1994

Recognition of carbohydrate by major histocompatibility complex class I-restricted, glycopeptide-specific cytotoxic T lymphocytes.

Cytotoxic T cells (CTL) recognize short peptide epitopes presented by class I glycoproteins encoded by the major histocompatibility complex (MHC). It is not yet known whether peptides containing posttranslationally modified amino acids can also be recognized by CTL. To address this issue, we have studied the immunogenicity and recognition of a glycopeptide carrying an O-linked N-acetylglucosamine (GlcNAc) monosaccharide-substituted serine residue. This posttranslational modification is catalyzed by a recently described cytosolic glycosyltransferase. We show that glycosylation does not affect peptide binding to MHC class I and that glycopeptides can elicit a strong CTL response that is glycopeptide specific. Furthermore, glycopeptide recognition by cytotoxic T cells is dependent on the chemical structure of the glycan as well as its position within the peptide.

Amino Acid Sequence↗
PubMed1994

Cloning, DNA sequence, and complementation analysis of the Salmonella typhimurium hemN gene encoding a putative oxygen-independent coproporphyrinogen III oxidase.

Coproporphyrinogen oxidation is a last step in heme biosynthesis. The biochemically characterized eukaryotic coproporphyrinogen III oxidases have an obligate requirement for molecular oxygen, and a similar enzyme is encoded by the hemF gene in Salmonella typhimurium. Anaerobic heme synthesis requires an oxygen-independent coproporphyrinogen oxidase, which is probably encoded by the hemN gene in S. typhimurium. The hemN gene has been cloned from an insertion mutant. The nucleotide sequence was obtained and used for PCR amplification of the wild-type gene. A single open reading frame was identified as the hemN gene on the basis of its interruption by the insertion mutation and plasmid complementation studies of hemF hemN double mutants. The predicted HemN protein has 38% amino acid sequence identity to a putative anaerobic Rhodobacter sphaeroides coproporphyrinogen oxidase. The hemN RNA 5' end and the inferred transcription initiation site were mapped by primer extension. The 52.8-kDa HemN protein is expressed from the second ATG codon of the hemN open reading frame. An open reading frame with an unknown function directly upstream of hemN has a striking amino acid sequence, including 11 acidic residues in a row.

Amino Acid Sequence↗
PubMed1994

Plasma cholesteryl ester transfer protein and its relationship to plasma lipoproteins and apolipoprotein A-I-containing lipoproteins in IDDM patients with microalbuminuria and clinical nephropathy.

OBJECTIVE: To study the distribution of high-density lipoprotein (HDL) subclasses in insulin-dependent diabetes mellitus (IDDM) patients with nephropathy and factors involved in the regulation of HDL, including plasma cholesteryl ester transfer protein (CETP) and postheparin plasma lipoprotein lipase (LPL) and hepatic lipase (HL) activities. RESEARCH DESIGN AND METHODS: Participants included 52 microalbuminuric IDDM patients (with a urinary albumin excretion rate [UAER] of 20-200 micrograms/min), 37 macroalbuminuric IDDM patients (UAER > 200 micrograms/min), and 64 normoalbuminuric IDDM patients (UAER < 20 micrograms/min). Groups were matched for age, body mass index, duration of diabetes, and glycemic control (HbA1). RESULTS: Median concentrations of HDL and HDL2 cholesterol were 11.6 (P = 0.01) and 22.7% (P = 0.01) less in microalbuminuric patients and 5.1 and 15.5% less in macroalbuminuric patients compared with normoalbuminuric patients. No significant differences were observed in the concentrations of apoA-I, apoA-II (apolipoprotein) or LpA-I or LpA-I:A-II (lipoprotein) particles between the groups. HDL cholesterol: apoA-I+apoA-II ratio was significantly lower in micro- (19.7 +/- 4.2 (+/- SD); P < 0.01) and macroalbuminuric patients (20.0 +/- 3.7, P < 0.05) than in normoalbuminuric patients (22.1 +/- 4.4). Postheparin plasma LPL:HL ratio was lower in microalbuminuric patients compared with normoalbuminuric patients (1.65 vs. 1.05 [median], P < 0.01). Plasma CETP activity was higher in the macroalbuminuric patients than in micro- (P < 0.05) and normoalbuminuric patients (P < 0.05) but did not correlate with HDL, HDL2, or HDL3 cholesterol. LPL:HL ratio correlated positively with HDL cholesterol (r = 0.372, P < 0.001), HDL2 cholesterol (r = 0.413, P < 0.001) and with LpA-I particles (r = 0.355, P < 0.001) but not with LpA-I:A-II particles (r = -0.065, NS). CONCLUSIONS: IDDM patients with micro- and macroalbuminuria show only trivial changes in concentrations of different HDL parameters, which cannot explain the excess risk of coronary heart disease in these patients. Data also indicate that elevation of CETP activity in IDDM patients with nephropathy is probably not responsible for the lowering of HDL cholesterol.

Adult↗
PubMed1993

Peptide selection by class I molecules of the major histocompatibility complex.

Class I molecules of the major histocompatibility complex (MHC) bind peptides derived from cytoplasmic proteins. Comparison of over 100 such peptides reveals the importance of the carboxy-terminal residue in selective binding. Recent evidence implicates the proteases and transporters of the processing pathway in providing peptides with the correct residues at the carboxyl terminus.

Journal Article↗