Search PubMed⌕ Search

Biomedical subjects

D C Edwards

Publications and source records attributed to D C Edwards.

At least 19 recordsLinked to original sources
PubMed2001

Ideal observer approximation using Bayesian classification neural networks.

It is well understood that the optimal classification decision variable is the likelihood ratio or any monotonic transformation of the likelihood ratio. An automated classifier which maps from an input space to one of the likelihood ratio family of decision variables is an optimal classifier or "ideal observer." Artificial neural networks (ANNs) are frequently used as classifiers for many problems. In the limit of large training sample sizes, an ANN approximates a mapping function which is a monotonic transformation of the likelihood ratio, i.e., it estimates an ideal observer decision variable. A principal disadvantage of conventional ANNs is the potential over-parameterization of the mapping function which results in a poor approximation of an optimal mapping function for smaller training samples. Recently, Bayesian methods have been applied to ANNs in order to regularize training to improve the robustness of the classifier. The goal of training a Bayesian ANN with finite sample sizes is, as with unlimited data, to approximate the ideal observer. We have evaluated the accuracy of Bayesian ANN models of ideal observer decision variables as a function of the number of hidden units used, the signal-to-noise ratio of the data and the number of features or dimensionality of the data. We show that when enough training data are present, excess hidden units do not substantially degrade the accuracy of Bayesian ANNs. However, the minimum number of hidden units required to best model the optimal mapping function varies with the complexity of the data.

Bayes Theorem↗
PubMed1999

Structural features of LIM kinase that control effects on the actin cytoskeleton.

LIM kinase phosphorylates and inactivates the actin binding/depolymerizing factor cofilin and induces actin cytoskeletal changes. Several unique structural features within LIM kinase were investigated for their roles in regulation of LIM kinase activity. Disruption of the second LIM domain or the PDZ domain or deletion of the entire amino terminus increased activity in vivo measured as increasing aggregation of the actin cytoskeleton. A kinase-deleted alternate splice product was identified and characterized. This alternate splice product and a kinase inactive mutant inhibited LIM kinase in vivo, indicating that the amino terminus suppresses activity of the kinase domain. Mutation of threonine 508 in the activation loop to valine abolished activity whereas replacement with 2 glutamic acid residues resulted in a fully active enzyme. Dephosphorylation of LIM kinase inhibited cofilin phosphorylation. Mutation of the basic insert in the activation loop inhibited activity in vivo, but not in vitro. These results indicate phosphorylation is an essential regulatory feature of LIM kinase and indicate that threonine 508 and the adjacent basic insert sequences of the activation loop are required for this process. A combination of structural features are thus involved in receiving upstream signals that regulate LIM kinase-induced actin cytoskeletal reorganization.

Actin Depolymerizing Factors↗
PubMed1999

Activation of LIM-kinase by Pak1 couples Rac/Cdc42 GTPase signalling to actin cytoskeletal dynamics.

Extracellular signals regulate actin dynamics through small GTPases of the Rho/Rac/Cdc42 (p21) family. Here we show that p21-activated kinase (Pak1) phosphorylates LIM-kinase at threonine residue 508 within LIM-kinase's activation loop, and increases LIM-kinase-mediated phosphorylation of the actin-regulatory protein cofilin tenfold in vitro. In vivo, activated Rac or Cdc42 increases association of Pak1 with LIM-kinase; this association requires structural determinants in both the amino-terminal regulatory and the carboxy-terminal catalytic domains of Pak1. A catalytically inactive LIM-kinase interferes with Rac-, Cdc42- and Pak1-dependent cytoskeletal changes. A Pak1-specific inhibitor, corresponding to the Pak1 autoinhibitory domain, blocks LIM-kinase-induced cytoskeletal changes. Activated GTPases can thus regulate actin depolymerization through Pak1 and LIM-kinase.

Actins↗
PubMed1984

Transfer of ricin toxicity by spleen cells.

The ability of spleen cells to internalise ricin and release it in a form capable of killing an untreated cell population has been studied. Potential donor cells were incubated with a range of ricin concentrations and the amount of ricin subsequently released was found to be related to the ricin concentration of the primary incubation. Ricin release was detected either by using radiolabelled toxin or by measuring its cytotoxic effects. Cells which, after incubation with ricin, were washed with lactose or anti-ricin IgG had a slightly reduced ability to transfer the toxin to other cells, suggesting that much of the ricin had been internalised by the donor cells. The presence of 100 mM lactose in the incubation medium was unable to inhibit the uptake of ricin and did not prevent the release of competent toxin. Cells carrying the toxic material were lethal to animals and in vitro the released toxin was as susceptible to inactivation by anti-ricin IgG as was free ricin.

Animals↗
PubMed1983

Ricin B chain converts a non-cytotoxic antibody-ricin A chain conjugate into a potent and specific cytotoxic agent.

We report the conversion of a non-cytotoxic antibody-ricin A chain conjugate to one displaying specific cytotoxic effects comparable with that of native ricin, by the addition of ricin B chain as a second stage reagent. The results suggest that this conversion is achieved by the association of the added B chain with the A chain of the conjugate, and not through a primary binding of B chain at the cell surface.

Animals↗
PubMed1983

Targeting potential of antibody conjugates.

Although the use of conjugates of enzymes has been considered, their use has not been very actively pursued. Much more interest has focused on the possibilities offered by the use of toxins, their subunits or of ribosome inhibitors. Conjugates of holotoxin which were very active and specific in vitro have been prepared. High in vivo activity and some specificity together with reduced whole body toxicity has been described. When A chain subunits or ribosome inhibiting proteins are used, the results are more mixed. Some very active and specific conjugates are known but others have relatively low activity. The reasons for this may be associated with the particular antigen to which the antibody component is directed, the nature of the A chain or inhibitor and the type and physiological state of the target cell. Application to man seems likely in the first instance to involve the removal of undesirable cells from bone marrow transplants.

Animals↗
PubMed1982

A comparison of the in vitro and in vivo activities of conjugates of anti-mouse lymphocyte globulin and abrin.

Anti-mouse lymphocyte globulin and normal immunoglobulin have been conjugated to abrin using two procedures, one involving linkage through an amide bond and a piperazine ring and the other the introduction of two amide bonds flanking a disulphide bridge. The four conjugates produced were equipotent as inhibitors of protein synthesis in rabbit reticulocyte lysates. Each antibody-containing conjugate was a more effective inhibitor of protein synthesis in cultured cells than the equivalent normal immunoglobulin-containing conjugate. In addition the conjugates with disulphide linkage groups were ten times more potent than their counterparts. The disulphide conjugates were also twice as toxic to mice in an acute toxicity test but when used to suppress their immune responses to sheep red blood cells it was the non-disulphide-linked conjugates that were superior. In all instances antibody-containing conjugates were more powerful immunosuppressants than those containing normal IgG. The results are taken to indicate a relative lack of stability of the disulphide conjugates in the tissues.

Abrin↗
PubMed1981

Cytotoxicity acquired by conjugation of an anti-Thy1.1 monoclonal antibody and the ribosome-inactivating protein, gelonin.

Gelonin, a plant protein which can powerfully reduce the protein-synthetic capacity of ribosome preparations, was covalently coupled to anti-Thy1.2 antibody. The conjugate was prepared using N-succinimidyl-3-(2-pyridyldithio)propionate which generates a disulphide linkage between the component molecules. Two conjugate fractions were obtained with Mr of 180 000 and greater than 200 000. After its linkage of the antibody, gelonin suppressed those Thy1.1-bearing T lymphocytes from AKR mice which will respond to phytohaemagglutinin and concanavalin A in tissue culture. The [3H]leucine incorporation with the T-cell mitogens was inhibited by 50% with the 180 000-Mr fraction at a concentration of 0.4 nM and with the greater than 200 000-Mr fraction of pM. Unconjugated gelonin induced comparable reductions in T-cell responsiveness but at concentrations of 30 nM. The conjugates exerted little or no effect upon B lymphocytes or T lymphocytes from CBA mice (Thy1.2 + ve). Thy1.1-expressing AKR lymphoma cell lines, AKR-A and BW5147, were found to be sensitive to the conjugates, albeit much less so than the normal T lymphocytes. The conjugates injected in vivo significantly prolonged the life of CBA mice bearing in an AKR-A lymphoma allograft. It is concluded that gelonin can, by its linkage to an antibody, be rendered cytotoxic with a potency to match or exceed those of the toxins abrin and ricin.

Animals↗
PubMed1981

Abrogation of the non-specific toxicity of abrin conjugated to anti-lymphocyte globulin.

A covalent conjugate of abrin and anti-human lymphocyte globulin (AHLG) was prepared in an endeavour to create a cytotoxic agent with specificity for human lymphoid cells. The AHLG--abrin conjugate was found to be around 10-fold better able to inhibit 3H-leucine uptake by the human lymphoblastoid cell line, Daudi, in tissue culture than was the control conjugate comprising abrin and normal IgG (nIgG). Both materials were less potent than native abrin. Galactose, which is known competitively to antagonize the binding of abrin to cells, strongly inhibited the toxicities of abrin and the nIgG--abrin conjugate whereas that of ALG--abrin was unimpaired. Thus, at least for Daudi cells in tissue culture, abrin can be made selectively toxic, by linkage to AHLG, towards cells bearing antigens to which the antibody moiety of the conjugate can attach.

Abrin↗
PubMed1980

Increased toxicity of diphtheria toxin for human lymphoblastoid cells following covalent linkage to anti-(human lymphocyte) globulin or its F(ab')2 fragment.

Anti-(human lymphocyte) globulin was reacted with a mixed anhydride derivative of chlorambucil to give a product which was in turn reacted with diphtheria toxin. The resulting conjugate was partially purified and was found to possess an ability similar to that of the native antibody to bind to the human lymphoblastoid cell lines, CLA4 and Daudi. Daudi cells, as had been observed previously with CLA4 cells, lacked the high sensitivity to diphtheria toxin normally characteristic of cells of human origin. Thus treatment with free toxin at a concentration of 1 microgram/ml was without effect upon their ability to incorporate [3H]leucine. By contrast, Daudi cells were highly sensitive to toxin conjugated to anti-(human lymphocyte) globulin or to its F(ab')2 fragment. Exposure for 24 h to a solution of conjugate containing toxin at a concentration of 0.5 ng/ml caused a reduction of 50% in the leucine uptake by Daudi cells. The toxicity of the conjugate could be blocked by diphtheria antitoxin or by pretreatment of the cells with non-conjugated antibody. Toxin linked to normal horse IgG or to its F(ab')2 fragment was without cytotoxic effect upon Daudi cells. Furthermore both the conjugate with anti-(human lymphocyte) globulin and that with normal IgG were approximately 100-fold less able than non-conjugated toxin to inhibit protein synthesis by a human fibroblast cell line to which the antibody showed no appreciable binding. Thus the conjugates are relatively ineffective against cells which lack an antigen to which the antibody moiety can bind. In contrast with the greatly increased toxicity of diphtheria toxin for human lymphoblastoid cells following its linkage to anti-(human lymphocyte) globulin, a conjugate of toxin linked to anti-(mouse lymphocyte) globulin was ineffective against murine spleen cells in vitro.

Animals↗
PubMed1980

Nerve growth factor and pancreatic APUD cells.

Pancreatic endocrine cells have been considered APUD cells and been thought to be of neural crest origin. Neonatal rats were passively immunized with nerve growth factor antiserum and the development of neural crest derived superior cervical ganglia was markedly inhibited. The pancreatic content of glucagon, insulin, and somatostatin was unaffected, suggesting that pancreatic A,B, and D cells are under different developmental control than are cells of known neural crest origin.

APUD Cells↗
PubMed1977

Immunogenic contaminants in mouse nerve-growth factor.

Experiments are described that confirm the presence of gamma-globulin in standard preparations of mouse nerve growth factor (7-S complex and beta subunit) and show that antibodies to this protein are present in horse antisera to the growth factor. These antibodies may be partially removed by adsorption with soluble antigen and totally removed by affinity chromatography on columns of insolubilised antigen. These procedures do not affect the potency of the antiserum in vitro or in vivo. Contaminating globulin may be removed from samples of the isolated beta subunit by gel filtration to give an immunochemically pure preparation which will permit more meaningful studies on nerve growth factor and its antiserum.

Animals↗