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A Brennan

Publications and source records attributed to A Brennan.

At least 55 records · Page 3Linked to original sources
PubMed1994

The Schwann cell precursor and its fate: a study of cell death and differentiation during gliogenesis in rat embryonic nerves.

We have characterized a cell, the Schwann cell precursor, that represents a distinct intermediate differentiation stage in the process by which Schwann cells are generated from neural crest cells. The Schwann cell precursor shows radical differences from Schwann cells which include death regulation, antigenic phenotype, pattern of cell-cell interaction, migratory behavior, and morphology. In the nerves of the rat hind limb, Schwann cells are irreversibly generated from these during a brief period, essentially embryonic days 15-17. We also provide evidence that the survival of Schwann cell precursors is regulated by neurons and identify basic fibroblast growth factor as a potential key regulator of apoptosis in Schwann cell precursors and of precursor to Schwann cell conversion. These findings have implications for our understanding of gliogenesis in the peripheral nervous system.

Animals↗
PubMed1994

Serial proton magnetic resonance spectroscopy in acute multiple sclerosis lesions.

Serial proton magnetic resonance spectroscopy (MRS), was carried out at 1-2 monthly intervals on eight patients with multiple sclerosis who presented with evidence of a large acute cerebral white matter lesion. An MRS was obtained from acute lesions (volumes of interest 4-12 ml), which at presentation showed gadolinium-diethylenetriamine penta-acetic acid enhancement, and from similar volumes of normal appearing white matter lateral to the lesion and nearer the scalp. Follow-up ranged from 4 to 9 months (mean 6 months). Short echo spectra from acute enhancing lesions invariably showed the presence of large resonances at 0.9 and 1.3 p.p.m. compared with normal appearing white matter and healthy age matched controls, indicating that these peaks were not the result of voxel contamination from scalp fat. These resonances, which probably represent lipid products of myelin breakdown, were detected in lesions which had been enhancing for < 1 month and remained elevated for a mean of 5 months (range 4-8 months). The results provide evidence that short echo proton MRS can detect myelin breakdown products and that myelin breakdown occurs during the initial inflammatory stage of lesion development. The ratio of N-acetylaspartate (NAA) (a neuronal marker) relative to creatine was reduced in acute lesions and in normal appearing white matter. In six of the lesions studied there was, however, a subsequent rise in the NAA/creatine ratio indicating that axonal loss is not the only mechanism of reduction in the NAA/creatine ratio.

Adult↗
PubMed1994

Genetic structures associated with spread of the type Ia trimethoprim-resistant dihydrofolate reductase gene amongst Escherichia coli strains isolated in the Nottingham area of the United Kingdom.

DNA probes for specific integrase genes were used to study 122 R plasmids encoding the predominant trimethoprim-insusceptible type Ia dihydrofolate reductase (DHFR) found in clinical isolates of Escherichia coli. The predominance of the type Ia DHFR was thought to result from the location of its gene on transposon Tn7, but of trimethoprim R plasmids carrying this gene that were collected between 1978 and 1983, between 1987 and 1988, and during 1992, only 49/60 (81.6%), 30/43 (69.8%) and 9/19 (47.4%) respectively hybridized with a probe for the Tn7 integrase gene. It has been suggested that novel genetic elements termed 'integrons' may play an important role in the dissemination of antibiotic resistance genes. Known integrons encode an integrase similar to that encoded by transposon Tn21, and 28 Tn7-negative plasmids (10/60 from 1978-83, 10/43 from 1987-8 and 8/19 from 1992) showed homology with a probe specific for the Tn21 integrase gene. Six plasmids were negative with both probes. It is concluded that Tn7 has played an important role in the dissemination of the gene encoding the type Ia DHFR amongst clinical isolates of E. coli in the Nottingham region of the UK, but that other genetic structures, some of which seem to have an integrase function similar to that of known integrons, may be playing an increasingly significant role.

Bacteriuria↗
PubMed1993

The effect of three dimensional collagen type I preparation on the structural organization of guinea pig enteric ganglia in culture.

In vivo, cellular relationships in the myenteric plexus are characterized by unusual compactness and by the arrangement of neurons and glia into ganglia and interconnecting strands. These features are lost when the myenteric plexus is placed in culture. In the present paper we test whether collagen type I, a major component of the matrix that surrounds the plexus in vivo, might have a role in maintaining normal neuron-glia relationships in this system. We report that a three-dimensional gel of rat tail collagen prevented the disaggregation of the guinea pig myenteric plexus in culture and induced the formation of a compact plexus-like cellular network when applied to disaggregated plexus cultures. These effects were not observed with soluble collagen. Immunohistochemical evidence was also obtained for synthesis of type I collagen by enteric glia. These observations indicate that type I collagen in a three-dimensional organization is capable of inducing and maintaining both the unusual compact organization of neurons and glial cells within myenteric ganglia and also the characteristic organization of these cells into an orderly network of ganglia and interconnecting strands.

Animals↗
PubMed1992

In the presence of dexamethasone, gamma interferon induces rat oligodendrocytes to express major histocompatibility complex class II molecules.

Cells that express major histocompatibility complex (MHC) class II molecules can interact directly with CD4 T lymphocytes and either activate immune reactions or become the targets of T-cell-mediated cytotoxic attack. Using rat optic nerve cultures combined with immunocytochemistry and in situ hybridization, we have shown that oligodendrocytes, the major myelin-forming cells of the central nervous system and the main casualty of the immune attacks associated with multiple sclerosis and experimental allergic encephalomyelitis, can be readily induced to express MHC class II mRNA and surface antigens in vitro by exposure to gamma interferon, provided the glucocorticoid dexamethasone is included in the culture medium. Oligodendrocytes exposed to gamma interferon without dexamethasone fail to express MHC class II molecules, which may account for the failure of previous attempts to induce expression in these cells. In the experiments reported here MHC class II expression can be demonstrated both on galactocerebroside-positive cells and on mature oligodendrocytes that express proteolipid protein. These findings expand possibilities for understanding immune-related oligodendrocyte killing and demyelination in human and experimental demyelinating diseases.

Animals↗
PubMed1992

The comparative role of 1,25-dihydroxycholecalciferol and phorbol esters in the differentiation of the U937 cell line.

The active metabolite of cholecalciferol, 1,25-dihydroxycholecalciferol (1,25-DHCC), is a mononuclear phagocyte product with immunoregulatory properties which can influence not only surrounding T cells but also other mononuclear phagocytes; and which acts in an autocrine fashion. In this study we have used the U937 cell line as a starting point model to investigate further the comparative role of 1,25-DHCC and phorbol myristate acetate (PMA) upon growth, differentiation and phenotype in the mononuclear phagocyte system, and have correlated our findings with changes in 1,25-DHCC metabolism and receptor expression. Both 1,25-DHCC and PMA inhibit growth and differentiate U937 cells in a dose-dependent fashion. When used together, however, at low doses of PMA, 1,25-DHCC protects against the PMA-induced growth inhibition. At high concentrations of both compounds there is a decrease (1,25-DHCC) or an increase (PMA) in 1,25-DHCC receptor expression, with either 24-OHase (1,25-DHCC) or 1-OHase (PMA) synthesis. If the compounds are used in combination the receptor levels are equivalent to controls, and both enzymes are produced. The phenotype of the 1,25-DHCC-induced cells shows light adherence, class I+, increase in CD4 and CD14 and decrease in CD71. The PMA-induced cell is tightly adherent, class I+, and strongly positive for CD13 with a concomitant decrease in both CD4 and CD71. These findings suggest another role for 1,25-DHCC in the mononuclear phagocyte system, as a potential mitogenic agent. They also suggest that 1,25-DHCC may act at both membrane and nuclear levels within this model of the mononuclear phagocyte pathway and demonstrate one possible way in which physiological peripheral macrophage heterogeneity might be induced, i.e. due to the nature of the signals which are implicated during differentiation. The presence of increased CD4 and decreased CD13 on the surface of 1,25-DHCC-differentiated cells, and vice versa on PMA-differentiated cells, illustrates how this may then be reflected in functional mononuclear phagocyte heterogeneity, which may in turn be reflected in differential peripheral function.

Antigens, Surface↗
PubMed1991

1,25(OH)2D3 regulates c-myc mRNA levels in tonsillar T lymphocytes.

The effects of 1,25(OH)2D3 on proliferation, c-myc mRNA levels and 1,25(OH)2D3 receptor expression in activated tonsillar T lymphocytes were studied. Activation of resting T cells with phytohaemagglutinin (PHA) for 72 hr led to an increase in proliferation, c-myc mRNA levels and to induction of 1,25(OH)2D3 receptor expression. However, when activation was carried out in the presence of 1,25(OH)2D3, there was inhibition of PHA-stimulated proliferation and c-myc mRNA levels. Increased cell proliferation, c-myc mRNA expression and 1,25(OH)2D3 receptor number were also observed, albeit to a lesser extent, when T cells were stimulated by phorbol myristate acetate (PMA), anti-CD3 antibody or A23187. However, in these cases 1,25(OH)2D3 was unable to prevent increased proliferation or c-myc mRNA expression. PMA and anti-CD3 used in combination produced similar or greater changes in proliferation, c-myc mRNA levels, 1,25(OH)2D3 receptor expression and responsiveness to the hormone when compared to PHA alone. Thus the inhibition of c-myc expression in activated T lymphocytes by 1,25(OH)2D3 can be related to its anti-proliferative effects. Moreover this inhibition seems to be dependent on the level of 1,25(OH)2D3 receptor expression, which in turn appears to be related to the degree of cell activation.

Calcitriol↗
PubMed1990

Mononuclear phagocytes and HSV-1 infection: increased permissivity in differentiated U937 cells is mediated by post-transcriptional regulation of viral immediate-early gene expression.

Undifferentiated U937 cells are non-permissive for herpes simplex virus (HSV) infection but can be rendered permissive by treatment with phorbol myristate acetate (PMA), which causes them to differentiate to a macrophage-like phenotype. Following infection with HSV, both PMA--treated and untreated cells correctly transcribe the viral immediate-early genes at levels comparable to those observed in fully permissive cell types, but immediate-early RNA and protein are detected only in the PMA-treated cells. Hence PMA acts by relieving an early block to HSV infection caused by the rapid turnover of immediate-early RNA. This block is not caused by the production of soluble inhibitors and can also be relieved by treatment with other agents that cause macrophage differentiation such as 1, 25 dihydroxycholecalciferol. These findings therefore indicate that the non-permissivity of undifferentiated U937 cells for HSV is mediated by post-transcriptional regulation of immediate-early gene expression.

Cell Line↗
PubMed1989

Modulation of myelomonocytic U937 cells by vitamin D metabolites.

Investigation of the effects of 1,25(OH)2D3 and 24,25(OH)2D3 on the proliferation and differentiation of the human myelomonocytic cell line U937 has been complemented with studies of the effect of the same metabolites on the number of nuclear receptors for 1,25(OH)2D3. Both 1,25(OH)2D3 and 24,25(OH)2D3 inhibit the proliferation of U937 cells in a dose-dependent manner. The concentrations of 24,25(OH)2D3 required to produce this effect were 100-times greater than those of 1,25(OH)2D3. Inhibition of proliferation was associated with increased expression of the CD14 and 200 kDa 63D3 antigens thus confirming differentiation of U937 towards a more mature cell type. Studies of the nuclear receptor for 1,25(OH)2D3 showed that pre-treatment of the cells with 1,25(OH)2D3 resulted in an apparent 40% decrease in the number of detectable 1,25(OH)2D3 receptors as compared to control U937 cells. This is due to the fact that the 1,25(OH)2D3 binds to U937 cell nuclei during culture and thus blocks the subsequent binding of radiolabelled 1,25(OH)2D3 used to measure the number of 1,25(OH)2D3 receptors. Measurement of the binding of unlabelled 1,25(OH)2D3 by radioimmunoassay indicated that pre-treatment of the cells with 1,25(OH)2D3 increased the capacity of U937 to bind the hormone, although measurement of these receptors by whole cell assay was prevented by the binding of 1,25(OH)2D3 itself. This effect was not observed with 24,25(OH)2D3 which was more easily displaced from binding sites by radiolabelled 1,25(OH)2D3 and it appears to act through low affinity binding to the 1,25(OH)2D3 receptor.

24,25-Dihydroxyvitamin D 3↗
PubMed1989

Autocrine regulation of 1,25-dihydroxycholecalciferol metabolism in myelomonocytic cells.

In this study the effects of vitamin D metabolites on the myelomonocytic precursor cell line U937 have been compared with those of phorbol myristate acetate (PMA). PMA was used as a cell modulating agent in order to avoid effects of binding of the exogenous vitamin D metabolites receptors within the cell, which would interfere with subsequent measurement of these receptors in studies of the vitamin D3 metabolic pathway. Both the active 1,25 DHCC form of vitamin D3 and the inactive 24,25 DHCC metabolite inhibit cell proliferation and induce 24-hydroxylase activity, but not 1 alpha-hydroxylase activity. These effects are dose-dependent and maximum enzyme activity is seen in the adherent cell population, which is induced by these compounds. PMA inhibits proliferation of U937 and increases receptors for 1,25 DHCC in these cells (like the vitamin D metabolites). However, unlike the vitamin D metabolites, PMA induces 1 alpha-hydroxylase activity rather than 24-hydroxylase activity. Thus, while PMA and 1,25 DHCC have some similar effects on monocyte precursor cell line differentiation, there is a difference between the effects of the two agents on the vitamin D metabolic pathway. The former promotes synthesis of the active metabolite, and the latter induces an enzyme which renders the metabolite inactive. If these results are considered together, they are consistent with the hypothesis that 1,25 DHCC has an autocrine role within the mononuclear phagocyte system.

Calcitriol↗
PubMed1987

Dendritic cells from human tissues express receptors for the immunoregulatory vitamin D3 metabolite, dihydroxycholecalciferol.

Dendritic cells have been isolated from human tonsillar tissue and shown to act as accessory cells in a mitogenic response. The dendritic cells will induced receptors for the active metabolite of vitamin D3, 1,25(OH)2D3, in the responder E+ T cells. The dendritic cells themselves constitutively express receptors for the metabolite, and this distinguishes them from other non-T cells in lymphomedullary tissue. Expression of the 1,25(OH)2D3 receptor may be a dendritic cell property that facilitates their accessory cell role within the tissue microenvironment.

Calcitriol↗
PubMed1985

Sharply timed and lateralised events at time of establishment of long term memory.

In the chick long term memory is known suddenly to become more resistant to disturbance 25 minutes after training. This was investigated using two bead pecking tasks: training involved either presentation of an ill-tasting bead (causing inhibition of spontaneous pecking) or habituation by prolonged presentation. Retention tests revealed two brief periods of improved recall, centred at 25 and at 30-32 minutes after learning. The first appeared to be associated with systems fed by the left eye and the second with right eye systems. In a final experiment memory formation in right or left hemisphere was directly disturbed by intracranial injection of the amnesic beta-antagonist sotalol. The first (and perhaps also second) period of improved recall was associated with interaction between the hemispheres. Right- and left-eyed birds differed markedly, both in the character of such interaction and in the outcome of learning.

Animals↗
PubMed1985

Generation of monoclonal antibodies to a human natural killer clone. Characterization of two natural killer-associated antigens, NKH1A and NKH2, expressed on subsets of large granular lymphocytes.

The initial characterization of two monoclonal antibodies directed at antigens selectively expressed on large granular lymphocytes (LGL) is reported in the present paper. These two reagents, anti-natural killer (NK) H1A and anti-NKH2, were obtained following immunization of mouse spleen cells with a cloned human NK cell line termed JT3. In fresh human peripheral blood, both anti-NKH1A and anti-NKH2 selectively reacted with cells that appeared morphologically as large granular lymphocytes. However, complement lysis studies and two color fluorescence analysis demonstrated that some LGL express both antigens and other cells express only NKH1A or NKH2. Functional analysis of these subsets indicated that the population of NKH1A+ cells contains the entire pool of NK active lymphocytes, whereas expression of NKH2 antigen appeared to delineate a unique subpopulation of LGL which, in a resting state, display a low degree of spontaneous cytotoxicity. Expression of NKH1A and NKH2 was also investigated using a series of nine well characterized human NK clones. All NK clones were found to be NKH1A+ and four out of nine also expressed NKH2. These results strongly supported the view that NKH1A is a "pan-NK" associated antigen, and indicated that at least a fraction of cloned NKH2 + LGL are strongly cytotoxic. Anti-NKH1A was shown to have the same specificity as the previously described N901 antibody and was found here to precipitate a 200,000-220,000-mol wt molecule in SDS-polyacrylamide gel electrophoresis (PAGE) analysis. Anti-NKH2 was specific for a structure that migrates at 60,000 mol wt in SDS-PAGE analysis under reducing conditions. Two color immunofluorescence analysis of NKH1A, NKH2, and other NK-associated antigens (Leu7 and B73.1) demonstrated variable degrees of coexpression of these antigens, which confirmed that NKH1A and NKH2 define distinct cell surface structures. Anti-NKH1A and anti-NKH2 appear to be useful reagents for characterizing LGL present in human peripheral blood and for identifying functionally relevant subsets within this heterogeneous population of cytotoxic lymphocytes.

Animals↗
PubMed1984

Identification of a 140-kDa activation antigen as a target structure for a series of human cloned natural killer cell lines.

In the present study, we have developed a monoclonal antibody termed anti-TNKTAR, able to block cytotoxicity mediated by a human natural killer (NK) clone termed JT9. Analysis of the functional effects of anti-TNKTAR indicated that alteration of the cytotoxic reactions resulted from the binding of the antibody to the membrane of target cells. In addition, it was shown that inhibition of cytotoxicity induced by anti-TNKTAR could be abolished by lectin approximation. Immunoprecipitation experiments indicated that TNKTAR antigen is a heterodimeric structure which resolves as a single band at 140 kDa under nonreducing conditions and as two bands at approximately 97 kDa and 40 kDa under reducing conditions in sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis. This heterodimer is present on lymphocytes and monocytes in human peripheral blood and perhaps more importantly, the membrane density of TNKTAR antigen increases very early and strongly following lymphocyte activation. In addition, it was shown that TNKTAR is expressed on each single cultured cell line which has been tested, although the density of the antigen varies strongly from one cell line to another. Even though the 140-kDa molecule was found to be widely distributed on activated cells, anti-TNKTAR had no blocking effects on cytotoxic reactions mediated by a series of either NK or cytotoxic T lymphocyte clones unrelated to JT9. In contrast, anti-TNKTAR blocked, in an identical fashion, cytotoxicity of JT9 and two additional clones, JT10 and JT11, against a series of 8 sensitive targets. JT9, JT10 and JT11 human cloned NK cell lines have been derived from peripheral blood of one individual donor drawn on month 0 (JT9), 12 (JT10) and 18 (JT11). Most importantly, these three clones initially selected for their capacity to kill K562 cells have been found to express the same 90-kDa clonotypic antigen receptor structure (termed NKTa) and display identical specificity when tested against a panel of randomly selected target cell lines. We have previously demonstrated that a unique subset of NK active mature T lymphocytes interact with target cells via 90-kDa clonotypic determinants in a major histocompatibility complex-independent fashion. Taken together, the present data strongly supports the view that a surface antigen of 140 kDa, linked to cell activation, serves as a specific recognition structure at the target cell level for these NK-active T lymphocytes.

Antibodies↗
PubMed1984

Natural killer-like function of activated T lymphocytes: differential blocking effects of monoclonal antibodies specific for a 90-kDa clonotypic structure.

A monoclonal antibody termed anti-NKTb has been generated following immunization of mice with cloned human cells (JT9) displaying natural killer (NK)-like activity. This antibody has the capacity to block cytotoxicity of the immunizing clone against several targets. In the present study, anti-NKTb was compared with a monoclonal antibody termed anti-NKTa that had previously been generated against JT9 cells and that had also been shown to block the NK-like function of these cells. The expression of a NKTb determinant, like that of NKTa, was found to be restricted to two NK active clones derived from the same individual, JT9 and JT10, both of which have the same mature T-cell phenotype (T3+, T8+, T11+). Comodulation, immunoprecipitation, and competitive binding experiments showed that both antibodies are directed to the same 90-kDa heterodimer associated with the T3 structure on the cell surface. However, cytotoxicity blocking studies suggested that NKTa and NKTb may represent functionally distinct epitopes of this 90-kDa molecule. Anti-NKTa uniformly blocked the cytotoxicity of both JT9 and JT10 cells when tested against 11 randomly selected target cell lines. In contrast, anti-NKTb totally blocked the cytotoxicity of these cloned cells against some targets (i.e., HPB-ALL, Nalm-1) but had very little effect when cytotoxicity was measured against other target cells (i.e., K562, U937, KG-1). This selective blocking effect, therefore, supports the notion that the heterodimer defined by the NKT antibodies is involved in the process of target cell recognition rather than in the cytolytic pathway of the cloned effector cells. Moreover, the unique functional effects of anti-NKTb suggest that additional levels of complexity exist in the specific recognition mechanisms of these clonal populations of NK active mature T lymphocytes.

Antibodies, Monoclonal↗
PubMed1984

Sex differences in lateralization in the domestic chick.

Age-related changes over the first 8 days of life in the intensity of fear response which is evoked in domestic chicks by a standard visual stimulus follow the same course in male chicks using the right eye and in female chicks using the right or left eye. Changes in left-eyed male chicks follow an entirely different course. Tests involving visual discrimination and habituation confirm the specialization of the (right-sided) neural structures fed by the left eye in males. This pattern of sex difference is formally comparable with the specialization of the right hemisphere in boys described by Witelson (Science 193, 425-427, 1976).

Aging↗