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Biomedical subjects

A Brennan

Publications and source records attributed to A Brennan.

At least 19 recordsLinked to original sources
PubMed1999

Developing Schwann cells acquire the ability to survive without axons by establishing an autocrine circuit involving insulin-like growth factor, neurotrophin-3, and platelet-derived growth factor-BB.

Although Schwann cell precursors from early embryonic nerves die in the absence of axonal signals, Schwann cells in older nerves can survive in the absence of axons in the distal stump of transected nerves. This is crucially important, because successful axonal regrowth in a damaged nerve depends on interactions with living Schwann cells in the denervated distal stump. Here we show that Schwann cells acquire the ability to survive without axons by establishing an autocrine survival loop. This mechanism is absent in precursors. We show that insulin-like growth factor, neurotrophin-3, and platelet-derived growth factor-BB are important components of this autocrine survival signal. The secretion of these factors by Schwann cells has significant implications for cellular communication in developing nerves, in view of their known ability to regulate survival and differentiation of other cells including neurons.

Animals↗
PubMed1997

Helix-loop-helix proteins in Schwann cells: a study of regulation and subcellular localization of Ids, REB, and E12/47 during embryonic and postnatal development.

Although basic helix-loop-helix (bHLH) proteins play an important role in transcriptional control in many cell types, the role of HLH proteins in Schwann cells has yet to be assessed. In this study, we have analyzed the expression of the dominant negative HLH genes, Id1 to Id4 and the class A gene REB, during Schwann cell development. We found that mRNA derived from these genes was present in the Schwann cell lineage throughout development including embryonic precursors and mature cells. The mRNA levels were not significantly regulated during development. Nevertheless, by using antibodies against the four different Id proteins, we found clear regulation of some of these genes at the protein level, in particular Id 2, 4, and REB, both in amount and nuclear/cytoplasmic localization. All these proteins are found in the nuclei of Schwann cell precursors but are not seen in nuclei of Schwann cells of newborn nerves. We observed extensive overlap in Id expression, especially in Schwann cell precursors that co-expressed all four Id proteins and REB. We also showed that Id 1 and 2 were up-regulated as Schwann cells progressed through the cell cycle. These data indicate that HLH transcription factors act as regulators of Schwann cell development and point to the existence of as yet unidentified cell type-specific bHLH proteins in these cells.

Aging↗
PubMed1997

Isoforms of 11beta-hydroxysteroid dehydrogenase in human granulosa-lutein cells.

To date, two isoforms of 11beta-hydroxysteroid dehydrogenase (11betaHSD) have been characterized: a low affinity, NADP+-dependent isoform (11betaHSD1) and a high affinity, NAD+-dependent isoform which metabolizes dexamethasone and is inhibited by cortisone (11betaHSD2). Having previously reported a relationship between ovarian 11betaHSD activities and conception in women undergoing in vitro fertilization (IVF-ET), the objective of the present study was to identify which isoforms of 11betaHSD metabolize glucocorticoids in cultures of human granulosa-lutein cells. In both intact cells and cell homogenates, two distinct 11betaHSD activities were identified with differing affinities for cortisol (Km = 490 nM and 2.6 microM). Even at low concentrations, cortisol oxidation was preferentially supported by NADP+ and was independent of NAD+. Although inhibited by the hemisuccinate ester of glycyrrhetinic acid, carbenoxolone, the predominant 11betaHSD activity in intact cells was resistant to end-product inhibition. Intact cells were also able to reduce [3H]cortisone (Km = 190 nM) but did not metabolize [3H]dexamethasone. 11BetaHSD1 mRNA was expressed in 23 of 28 cell cultures whereas 11betaHSD2 mRNA was not expressed in any of the 22 independent cultures studied by reverse transcriptase-polymerase chain reaction (RT-PCR). We conclude that human granulosa-lutein cells express both type 11betaHSD and a novel isoform of this enzyme. While the low affinity 11beta-dehydrogenase and 11-ketosteroid reductase activities exhibit properties consistent with 11betaHSD1, the high affinity 11beta-dehydrogenase differs from 11betaHSD2 in that it is NADP+-dependent, does not metabolize dexamethasone and is resistant to end-product inhibition.

11-beta-Hydroxysteroid Dehydrogenases↗
PubMed1997

Identification of transcriptionally regulated mRNAs from mouse Schwann cell precursors using modified RNA fingerprinting methods.

We have adopted RNA fingerprinting methods to screen for genes that are rapidly up- or down-regulated during normal mammalian development, comparing mRNA from early (embryo day 12) to late (embryo day 13) mouse Schwann cell precursors. The use of total RNA, a reduction of cDNA template for amplification, the detection of RT-PCR products with a sensitive DNA stain and polyacrylamide gel electrophoresis and rigid selection criteria involving three screening steps are significant improvements on previous methods. Of 19 differentially displayed bands, 15 represented novel genes. The four known cDNA fragments (interleukin enhancer binding factor 1, beta3 subunit of phospholipase C, brain beta-spectrin, and P21 polypeptide) consisted of coding sequences, indicating a high chance of obtaining coding regions. A semiquantitative RT-PCR analysis of three of the four known genes and a cDNA fragment randomly selected from the pool of 15 novel sequences, confirmed that they were regulated between embryo days 12 and 13, as predicted by the display gels. Our results suggest that the combination of methods described here will have wide applicability in studies of other developmental systems where precisely timed changes occur and where only small amounts of RNA can be obtained for analysis.

Animals↗
PubMed1997

P0 is constitutively expressed in the rat neural crest and embryonic nerves and is negatively and positively regulated by axons to generate non-myelin-forming and myelin-forming Schwann cells, respectively.

We show that in the rat, the major gene of PNS myelin, P0, is expressed long before myelination in the neural crest, Schwann cell precursors, and embryonic Schwann cells irrespective of whether they will myelinate or not. This myelin-independent P0 expression is constitutive and likely to serve as a specific marker for the Schwann cell lineage. The much higher P0 expression accompanying myelination is therefore not new gene expression but strong up-regulation of preexisting basal levels. We provide new evidence that the up-regulation to myelination-related levels depends on positive extrinsic signals and therefore does not represent a constitutive phenotype. P0 mRNA is not detectable in mature non-myelin-forming Schwann cells of the sympathetic trunk, but is detectable after transection, indicating that there is a P0-inhibitory signal associated with mature unmyelinated axons. Thus, the regulation of the P0 gene is complex, encompassing extrinsically signaled amplification superimposed on a highly lineage-specific and constitutive basal expression.

Animals↗
PubMed1996

A comparison of three brands of polysulfone membranes.

A prospective clinical crossover study comparing the functional performance and biocompatibility of three brands of polysulfone membranes (Fresenius Polysultone (Fresenius Ag, Bad Homburg, Germany), Polyphen (Minntech Corp., Minneapolis, MN), and Biosulfane (WR Grace Inc., Danvers, MA)) incorporated in ethylene oxide-sterilized dialyzers of comparable surface area (1.3 to 1.35 m2) was undertaken. The clearance of small molecules by each membrane was comparable. Plasma levels of beta 2 microglobulin fell to 49.9% of pretreatment values by 210 min when using the Fresenius Polysulfone membrane, 60.2% with the Polyphen membrane, and 63.1% with the Biosulfane membrane. The reduction achieved by the Fresenius Polysulfone membrane was superior (P = 0.003). The plasma reductions were associated with the recovery of 195 mg beta 2 microglobulin from the dialysate for the Fresenius Polysulfone membrane and 158 mg for the Polyphen membrane, but no beta 2 microglobulin was recovered from the dialysate with the Biosulfane membrane. The dialysate collected with the Fresenius Polysulfone membrane also contained a mean of 6853 mg of total protein, compared with 5490 mg with the Polyphen membrane and 8422 mg with the Biosulfane (P = 0.04) membrane. The neutropenia was slight and independent of membrane brand, as were the changes in C3a des arg and SC5b-9 complement components. The reduction in platelet counts was higher for the Biosulfane membrane than for the other brands (P = 0.003). This study indicates that whereas the polymer base of the membrane is the same, its production and subsequent handling during dialyzer production induce changes that attain statistical significance, most notably in the way that the membrane removes beta 2 microglobulin and interacts with proteins. The differences observed are a consequence of the different alloying polymers used during manufacture and, consequently, the membranes cannot be considered equivalent.

Adult↗
PubMed1996

Development and differentiation of Schwann cells.

Using the rat sciatic nerve as a model for the study of Schwann cell differentiation we have identified a Schwann cell precursor, a distinct cell type present in developing nerves at a time when they are projecting to their target tissues. These cells develop into Schwann cells over a relatively short time in vivo. In vitro, they can generate Schwann cells if they are cultured in neuron-conditioned medium or in the presence of neu-differentiation factors (NDF) (neuregulins, heregulins, glial growth factor), a recently discovered family of growth factors expressed at high levels in neurons. Thus neu-differentiation factors may be important neuro-glia signalling molecules in the Schwann cell lineage. Later stages in the development of Schann cells, such as differentiation towards a myelin phenotype, can be studied using cultured Schwann cells. These cells dedifferentiate both in vivo and in vitro when they are deprived of axonal contact. Elevation of intracellular cyclic AMP levels in the absence of cell division causes high levels of expression of Po, the major myelin glycoprotein. TGF beta s and FGFs suppress this induction, while IGFs promote it.

Animals↗
PubMed1995

Fibroblast growth factors and insulin growth factors combine to promote survival of rat Schwann cell precursors without induction of DNA synthesis.

In embryonic rat nerves, we recently identified an early cell in the Schwann cell lineage, the Schwann cell precursor. We found that when these cells were removed from contact with axons they underwent rapid apoptotic death, and that in a proportion of the cells this death could be prevented by basic fibroblast growth factor (bFGF, FGF-2). We now report that 100% of Schwann cell precursors isolated from peripheral nerves of 14-day-old-rat embryos can be rescued by a combination of insulin-like growth factor (IGF) 1 or 2 in combination with either acidic FGF (aFGF, FGF-1), bFGF or Kaposi's sarcoma FGF (K-FGF; FGF-4). The precursors display an absolute requirement for both an IGF and an FGF to achieve maximal survival. Elevation of intracellular levels of cAMP by forskolin does not result in a significant shift in the IGF/FGF dose-response curves. In contrast, the percentage of precursors rescued by FGF in the presence of insulin is dramatically increased by elevation of cAMP. These growth factor combinations did not stimulate DNA synthesis significantly in Schwann cell precursors. These findings show that cooperation between growth factors is required to suppress cell death in Schwann cell precursors, and suggest that survival and DNA synthesis are regulated by distinct growth factor combinations in these cells. The observations are consistent with the idea that survival regulation by FGFs and IGFs plays an important role in the development of glial cells in early embryonic nerves.

Animals↗
PubMed1995

Neu differentiation factor is a neuron-glia signal and regulates survival, proliferation, and maturation of rat Schwann cell precursors.

We show that beta forms of Neu differentiation factor (NDF), homologous to acetylcholine receptor-inducing activity, glial growth factor, and heregulin, prevent apoptotic death and stimulate DNA synthesis of the E14 Schwann cell precursor, an early cell in the rat Schwann cell lineage. When precursors are exposed to NDF in defined medium, they generate Schwann cells without the requirement for DNA synthesis and with a time course that is similar to that with which Schwann cells appear in embryonic nerves in vivo. Furthermore, a neuronal signal that also mediates precursor survival and maturation is blocked by the extracellular domain of the ErbB4 NDF receptor, a protein that specifically blocks the action of NDFs. These observations provide important evidence that NDF is one of the hitherto elusive neuron-glia signaling molecules long proposed to regulate development in the Schwann cell lineage.

Animals↗
PubMed1994

Correlation of magnetization transfer ratio with clinical disability in multiple sclerosis.

We performed spin echo magnetic resonance imaging with and without application of an off-resonance saturation pulse in 43 patients with multiple sclerosis (MS), 10 age-matched controls, and 4 elderly asymptomatic patients with the radiological diagnosis of small-vessel disease. Magnetization transfer (MT) ratio images were obtained from these. All MS subgroups (primary progressive, secondary progressive, benign, early relapsing-remitting) showed significantly lower average lesion MT ratios than small-vessel disease patients. Secondary progressive MS patients showed significantly lower lesion MT ratios than those with benign disease, and there was an inverse correlation of disability with average lesion MT ratio. The degree of reduction of MT ratios is an indicator of the extent of tissue destruction. Thus, reduced MT ratios in MS may provide an indication of the degree of demyelination and axonal loss, both of which are likely to cause functional deficits in MS. We conclude that MT measurement is (1) a robust quantitative method that may increase the pathological specificity of magnetic resonance imaging, (2) has the potential to differentiate demyelination in MS from less destructive pathological changes, and (3) may be useful in monitoring modifications in tissue structure brought about by treatment.

Adult↗
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↗