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Muscle fibers exhibit a limited capacity to alter their myosin heavy chain expression in response to various stimuli. Recent results pertinent to this observation are discussed, and a hypothetical scheme is presented whereby the contribution of myonuclei from distinct populations of myogenic precursors may account for this limited adaptive range.
A limited screen of several commercially-available and internally-produced lipases and esterases identified porcine liver esterase as a suitable biocatalyst for the enantioselective hydrolysis of a diester into its (S)-ester acid with high optical purity (99%). This (S)-ester acid is a precursor to an experimental growth hormone secretagogue. After identifying xanthan gum as the best emulsifier and optimizing the reaction conditions, hydrolysis rates of 1 g/l.h and final (S)-ester acid (ee > 99%) titers of about 8.5 g/l were routinely achieved. This process supported the production of preparative amounts of optically pure (S)-ester (ee > 99%) with a high reaction yield of 82%. Upon purification, the (S)-ester was successfully used in the subsequent synthetic steps to yield the growth hormone secretagogue.
We have begun to apply limiting dilution methods to address the cellular interactions that underlie age-associated changes in the immune response. By 18 mo of age, mice of the long-lived (BALB/c X C57BL/6)F1 strain are found to have a consistent, three to fivefold decline in precursor frequency for several different T cell-mediated responses: a) IL 2 response to T cell mitogens; b) antigen-nonspecific CTL production in mitogen-stimulated, IL 2-supplemented responses; and c) proliferation in response to concanavalin A. We also find a diminished precursor frequency for IL 2-secreting, KLH-specific helper T cells in the draining lymph nodes of KLH-immunized mice. Despite the decline in the fraction of responding precursor cells, the amount of "effect" (cytotoxicity, IL 2, or proliferation) generated per responsive cell is not altered by age. We speculate that the decline in all of these assays may reflect a common, underlying defect, one that reduces the proportion of T cells that can generate a sizeable clone of proliferating cells. This defect seems to act in several different T cell subsets, and in responses to several activating stimuli: alloantigens, soluble proteins, or mitogens.
The structural complexity of a ligand in association with the molecular chaperones SecB and SecA was investigated using three species of precursor maltose-binding protein, which differ in their stability as a result of an amino acid substitution in each that affects the rate of folding of the polypeptide. In the presence of high concentrations of both SecB and SecA, the precursors were translocated in vitro with indistinguishable kinetics. However, when SecA was limiting, the translocation was more rapid for precursor species, which had lower stability in the native state relative to the stability of the wild-type precursor. We propose that, when in complex with SecB, precursors can form an element of tertiary structure and that these tertiary contacts are blocked when SecA is bound.
Studies on the biosynthesis of the chloroplast coupling factor 1 (CF(1)) in Chlamydomonas reinhardi have been initiated. The ratio of CF(1) to chlorophyll in the cell was shown to be independent of the density of the culture. No turnover of assembled CF(1) could be detected, thus suggesting that CF(1) was synthesized at a rate equivalent to that of net chlorophyll synthesis. A lag of between 5 to 7 minutes in the incorporation of radioactive precursor sulfate into assembled CF(1) was measureable. This puts an upper limit on the pool size of any precursor to the assembled CF(1) complex. The pool size is estimated to be equivalent to 1% of the total CF(1) in the cell.
Astrocytes are intimately involved in both glutamate and gamma-aminobutyric acid (GABA) synthesis, and ischemia-induced disruption of normal neuroastrocytic interactions may have important implications for neuronal survival. The effects of middle cerebral artery occlusion (MCAO) on neuronal and astrocytic intermediary metabolism were studied in rats 30, 60, 120, and 240 minutes after MCAO using in vivo injection of [1-13C]glucose and [1,2- 13C]acetate combined with ex vivo 13C magnetic resonance spectroscopy and high-performance liquid chromatography analysis of the ischemic core (lateral caudoputamen and lower parietal cortex) and penumbra (upper frontoparietal cortex). In the ischemic core, both neuronal and astrocytic metabolism were impaired from 30 minutes MCAO. There was a continuous loss of glutamate from glutamatergic neurons that was not replaced as neuronal glucose metabolism and use of astrocytic precursors gradually declined. In GABAergic neurons astrocytic precursors were not used in GABA synthesis at any time after MCAO, and neuronal glucose metabolism and GABA-shunt activity declined with time. No flux through the tricarboxylic acid cycle was found in GABAergic neurons at 240 minutes MCAO, indicating neuronal death. In the penumbra, the neurotransmitter pool of glutamate coming from astrocytic glutamine was preserved while neuronal metabolism progressively declined, implying that glutamine contributed significantly to glutamate excitotoxicity. In GABAergic neurons, astrocytic precursors were used to a limited extent during the initial 120 minutes, and tricarboxylic acid cycle activity was continued for 240 minutes. The present study showed the paradoxical role that astrocytes play in neuronal survival in ischemia, and changes in the use of astrocytic precursors appeared to contribute significantly to neuronal death, albeit through different mechanisms in glutamatergic and GABAergic neurons.
CTL specific for the HIV belong to the CD8 subset of T lymphocytes, and their activity is restricted by class I HLA transplantation Ag. In this report, HIV-specific CTL and their precursor cells were quantified by limiting dilution analysis. CTL were recovered from the lungs, lymph nodes, and blood of asymptomatic seropositive carriers and of patients with AIDS. HIV was found to be very immunogenic. High frequencies of both HIV-specific CTL and CTL precursor cells were detected in infected individuals. These CTL killed autologous HIV-infected macrophages and T4 lymphoblasts. They also killed doubly transfected P815-A2-env-LAV mouse tumor cells, which express the human HLA-A2 gene and the HIV-1 env gene. In the longitudinal studies of two HIV-infected patients, CTL and CTL precursor cell frequencies decreased as the clinical and immunologic status of the patients deteriorated. Most surprisingly, PBL from seronegative donors also responded to HIV stimulation in vitro and generated large numbers of HLA-restricted, HIV-specific CTL.
Studies from our laboratory and others have demonstrated that alpha-interferon (IFN alpha) can regulate natural killer cells and lymphokine-activated killer (LAK) cell activation. In vitro experiments have shown that IFN alpha has differential effects on both natural killer cells and LAK activity when combined with interleukin 2 (IL2); IFN alpha synergized with IL2 to augment natural killer cells activity while it suppressed the IL2-induced LAK response. Here we demonstrated that IFN alpha A/D can also regulate IL2-induced LAK activity in vivo with enhanced or suppressed activity depending on the IFN alpha A/D dose. The enhanced response was observed with the combination when 80,000 units/day of IFN alpha A/D were used and was detectable in the spleen, lung, and peritoneum. When a high dose of IFN alpha A/D was combined with IL2, a moderate reduction in LAK activity was noted in the spleen and peritoneum. In contrast, a high dose IFN alpha A/D augmented IL2-induced LAK activity in the lung even though it reduced the level of cellular infiltration. We have also evaluated the effect that IL2, IFN alpha A/D, and IL2 plus IFN alpha A/D have on the frequency of LAK precursors in the spleen and lung using limiting dilution analysis. Treatment of normal mice with IL2 alone increased the frequency of LAK precursor (LAKp) in the lung. This increase was associated with an infiltration of Thy-1+, asialo-GM1+, Lyt-2- lymphocytes into the lungs. Moreover, treatment with IL2 plus IFN alpha A/D enhanced the frequency of LAKp over that observed with IL2 alone. Treatment with the combination did not change the phenotype of LAKp in the lung from that seen with IL2. The increase in LAKp frequency induced by the combined treatment may not be a direct effect of IFN alpha A/D on precursor cells since IFN alpha A/D alone did not increase the frequency of LAKp in vivo or in vitro when added to limiting dilution analysis cultures. In contrast to what occurred in the lung, a consistent increase in LAKp was not seen in the spleen after treatment with IL2 or with the combination, although LAK activity was observed. These results demonstrated that in addition to inducing lytic activity from LAK effectors in vivo, IL2 treatment increased the number of precursor cells within the lung. Moreover, IFN alpha A/D in combination with IL2 influenced the level of LAKp in situ.
A monoclonal antibody to a synthetic peptide consisting of residues 8-17 of amyloid beta-protein was used in immunohistochemical studies to reveal binding sites for this peptide in cytoplasmic vesicles in cells of the adrenal zona reticularis and the islets of Langerhans of the pancreas. These binding sites showed some specificity for this peptide and so may represent a membrane receptor. These results suggest that the membrane bound beta-protein precursor may be processed by limited extracellular proteolysis to release a peptide ligand containing the 8-17 sequence. It has been reported recently that the core protein of a heparin sulphate proteoglycan (HSPG) secreted by PC12 cells shows some homology with the beta-protein precursor. This suggests that the binding sites might be due to the presence of a HSPG core protein receptor. Further studies should be carried out to find out if receptors to beta-protein peptides are present in brain tissue, since these might play a role in the catabolism of the beta-protein precursor, and in the formation of cerebral amyloid in Alzheimer's disease (AD).
The import of cytochrome b2 and cytochrome c peroxidase into mitochondria was investigated by pulse-chase experiments with intact yeast cells combined with subcellular fractionation. Import and processing of the precursors of these intermembrane space proteins is blocked by uncouplers of oxidative phosphorylation, indicating that an "energized" inner membrane is required. Cytochrome b2 is processed in two steps. The first step involves energy-dependent transport across both mitochondrial membranes and cleavage by a matrix-located protease to yield an intermediate which is smaller than the precursor, but larger than the mature protein. The second step involves conversion of the intermediate to the mature form. Whereas the precursor and the mature form are soluble, the intermediate is membrane-bound and exposed to the intermembrane space. The maturation of cytochrome c peroxidase is much slower than that of cytochrome b2. Proteolytic processing rather than import is rate-limiting since cytochrome c peroxidase precursor labeled during a 3-min pulse is already found attached to the outer face of the mitochondrial inner membrane. Import of cytochrome b2 and probably also of cytochrome c peroxidase thus involves energy-dependent transport to the matrix and cleavage by a matrix-localized protease. Maturation of cytochrome b2 proceeds in the sequence: soluble precursor leads to membrane-bound intermediate form leads to soluble mature form.
Bile acid synthesis from cholesterol is tightly regulated via a feedback mechanism mediated by the farnesoid X receptor (FXR), a nuclear receptor activated by bile acids. Synthesis via the classic pathway is initiated by a series of cholesterol ring modifications and followed by the side chain cleavage. Several intermediates accumulate or are excreted as end products of the pathway in diseases involving defective bile acid biosynthesis. In this study, we investigated the ability of these intermediates to activate human FXR. In a cell-based reporter assay and coactivator recruitment assays in vitro, early intermediates possessing an intact cholesterol side chain were inactive, whereas 26- or 25-hydroxylated bile alcohols and C27 bile acids were highly efficacious ligands for FXR at a level comparable to that of the most potent physiological ligand, chenodeoxycholic acid. Treatment of HepG2 cells with these precursors repressed the rate-limiting cholesterol 7alpha-hydroxylase mRNA level and induced the small heterodimer partner and the bile salt export pump mRNA, indicating the ability to regulate bile acid synthesis and excretion. Because 26-hydroxylated bile alcohols and C27 bile acids are known to be evolutionary precursors of bile acids in mammals, our findings suggest that human FXR may have retained affinity to these precursors during evolution.
OBJECTIVE: Many patients with primary Sjögren's syndrome (SS) make high titer IgG autoantibodies to the La(SSB) antigen, suggesting antigen specific T cell-B cell interactions. T cell responses to some nuclear antigens, particularly U1RNP, have been detected in patients with systemic lupus erythematosus (SLE) and in healthy subjects. We investigated T cell reactivity to the autoantigen SSB in patients with SS and healthy controls. METHODS: Using the [3H]thymidine proliferation assay, we determined reactivity to purified recombinant SSB (rSSB) in 20 patients with SS and 19 controls. Specificity was determined using tetanus toxoid, endotoxin, and 3 other autoantigens (PBC.M2, Sc170, and GAD). Precursor frequency was calculated by limiting dilution analysis. HLA Class II dependency was investigated using anti-Class II monoclonal antibodies. HLA-DR typing was by polymerase chain reaction and sequence specific oligonucleotide typing. RESULTS: Six of 20 patients with SS and 10/19 controls proliferated to La(rSSB). Precursor frequency of anti-SSB T cells was 1:77,040 and 1:115,000 in 2 healthy subjects and 1:230,250 and 1:103,034 in two patients with SS. Anti-HLA-DR abrogated proliferation to SSB and tetanus toxoid. Thirteen of 15 patients with SS and 4/17 controls were HLA-DR3 positive, with no apparent association of HLA-DR3 with SSB reactivity in controls. CONCLUSION: Anti-La(SSB) specific T cells occur in a significant proportion of controls and in some patients with SS. The function of SSB T cells in controls remains to be defined. They may represent immunoregulatory cells, and further analysis of these cells, and a comparison to those found in patients with SS, may elucidate normal immunoregulation and the derangements that lead to Sjögren's syndrome.
Today, there are still uncertainties about the role of exogenous fat on body fat regulation. Early models of energy utilization (for example, Kleiber's, early 20th century) failed to take into account the nature of substrate oxidized in the control of food intake, whereas more recent models (e.g., Flatt's model, end of 20th century) did. Excess body fat storage is ultimately a problem of chronic positive energy balance mediated by a poor control of energy intake or/and a blunted total energy expenditure. Excess fat storage can stem from exogenous fat and to a more limited extent by nonfat substrates precursors transformed into body fat, mostly from carbohydrates, a process known as de novo lipogenesis. When considered over periods of weeks, months or years, total fat balance is closely related to energy balance. Over periods of days, the net change in fat balance is quantitatively limited as compared to the size of endogenous fat storage. The issues discussed in this article primarily include the stimulation of de novo lipogenesis after acute or prolonged CHO overfeeding and whether de novo lipogenesis is a risk factor for obesity development.
In the light of the diversity of HIV, matching the genotype of candidate HIV vaccines with the transmitted genotype may be required. Alternatively, matching the immunotype of HIV vaccines and transmitted subtypes may be the best option. Since studies of cross-subtype HIV-1 immunity are limited, subtype B specific cytolytic T lymphocyte (CTL) responses were measured in subtype C infected individuals. HIV-1 subtype B-specific CTLs, plasma viral load and absolute CD4 and CD8 lymphocyte numbers were measured in six HIV-1 subtype C infected individuals within a year of seroconversion. HIV-1 subtype B env. gag and nef-specific CTL precursor frequencies were measured by limiting dilution analysis. Three of the six subjects had demonstrable CTL directed at more than one HIV-1 subtype B antigens. One individual demonstrated CTL directed against all three HIV-1 subtype B antigens, while two individuals did not demonstrate CTL against HIV-1 subtype B antigens. The frequencies of CTL precursor were not associated with plasma viral load or absolute CD4 cell counts in peripheral blood. These findings suggest that some individuals recently infected with subtype C HIV-1 generate cross-reactive CTL that are directed against HIV-1 subtype B.
The choice of stimulator cells is crucial in determining the frequency of alloreactive cells by limiting dilution analysis (LDA). In humans, E- cells or EBV-lymphoblastoid cell lines (LCL) are available for this purpose. The E- cells have been mostly used until now, but they appear to stimulate much less than LCL in other models. Consequently, we undertook a systematic comparison of the efficiency of both types of cell for the LDA of alloreactive IL-2-secreting cells (IL-2-SC) and cytotoxic precursors (CTLp). We show that LCL are stronger stimulator cells both for IL-2 secretion and for cytotoxicity induction. However, high levels of non-allogeneic reactivity were also induced. Therefore, to measure the frequency of specific alloreactive IL-2-SC and CTLp, T cells were depleted of such irrelevant reactive cells by a suicide technique: culture with autologous LCL in the presence of BUDR and use of the remaining live cells as responding effectors in LDA. Using this methodology, no non-allogeneic reactive T cells remain in the responding cells: after restimulation by autologous LCL, no IL-2-SC could be seen and no cytotoxic activity could be observed against autologous, irrelevant or LAK sensitive targets. In contrast, the number of IL-2-SC and CTLp against allogeneic targets was preserved. Therefore with this methodology, the strong stimulator capacity of LCL could be used whereas non-specific activation could be avoided in order to assess the frequency of allo-specific IL-2-SC and CTLp.
We describe here a limiting dilution analysis to determine cytotoxic T lymphocyte precursor (CTLp) frequencies against individual HLA-A or -B antigens. This assay is reproducible and showed that the CTLp frequency of an individual remains stable with time. Significant variations in CTLp frequency against the same alloantigen were found in different individuals and even in monozygotic twins, showing that these differences were not (completely) genetically determined. Within an individual, a wide range of CTLp frequencies can be found against different allo-antigens. Serologically cross-reactivity seems not to interfere in this assay. This LDA is a practicable tool for a systematic analysis of CTLp response against selected individual HLA-A or -B antigens and can be used for the selection of HLA mismatched donors for transplantation patients.
An immediate precursor of 5S ribosomal RNA (rRNA) from Bacillus subtilis has 21 and 42 nucleotide precursor-specific segments associated with its 5' and 3' termini, respectively. On the basis of its nucleotide sequence, predicted secondary structure and location in the rRNA transcriptional unit, the 3' precursor element apparently functions during the termination of transcription. A portion of the 5' precursor element is shown to facilitate the native folding of the mature domain of the precursor. Precursor 5S rRNA molecules which lack the 5' terminal 8-9 nucleotides of the 5' precursor elements were fabricated. These abbreviated constructs assume a non-native conformation, as revealed by their behavior during polyacrylamide gel electrophoresis. The aberrant conformation is evidently forced upon the abbreviated constructs by the residual 5' precursor sequence, since its removal by the maturation endonuclease RNAase M5 precipitates the reordering of the mature domain into its native conformation. Inspection of the nucleotide sequence of the 5S precursor suggested the nature of the conformational aberration, and gel electrophoresis analyses of limited nuclease digests of end-labeled precursors in the native and aberrant conformations are consistent with the derived model. We conclude taht the 5' terminal six nucleotides in the intact 5S precursor assist in the folding of the mature domain by forming a base-paired duplex with neighboring nucleotides, thereby preventing that adjacent sequence from engendering the abnormal conformation. The involvement of precursor-specific sequences and conformational dynamics in RNA function are discussed.
It was recently shown that wines contain typically a huge diversity of structurally similar polypeptides that exhibit a high degree of homology to pathogenesis-related (PR) proteins. This observation suggested the existence of one or a few precursors in mature grapes, common to most or all the wine PR proteins. Limited proteolysis and chemical modification of the precursor(s) during fruit ripening and winemaking could then generate the large number of distinct wine polypeptides. However, the patterns of PR proteins extracted from grape berries regularly harvested from the onset of development until maturity did not confirm the previous hypothesis. Two different methodologies, involving 2-D immunoblotting and a combination of FPLC cation/anion exchange chromatographies with 1-D immunoblotting, indicate that the total concentration of PR proteins is increased but its diversity is reduced from the early stages of berry development until maturity. These results indicate that PR proteins are synthesized in a wide variety of forms from the early stages of grape development, eliminating the hypothesis previously formulated on the existence of one or few precursors common to the wine proteins.