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Biomedical subjects

R N Mitchell

Publications and source records attributed to R N Mitchell.

46 records · Page 3Linked to original sources
PubMed1995

Pathological considerations in cryopreserved allograft heart valves.

Since structure-function correlations have not been determined for cryopreserved allograft heart valves, we studied 20 explanted valves in place several hours to nine years, as either orthotopic aortic valves/root replacements or right ventricle to pulmonary artery conduits. They were explanted primarily due to growth-related conduit or valve stenosis, valve regurgitation, or infection. Controls included seven unused cryopreserved allograft valves and 16 aortic valves removed from transplanted allograft hearts obtained at either autopsy (n = 15) or retransplantation (n = 1), two days to four years postoperatively, following myocardial rejection (n = 4), graft coronary arteriosclerosis (n = 4), and other (n = 8). Analysis included gross inspection, radiography, light microscopy, electron microscopy, and immunohistochemical studies (to allow identification/localization of endothelial cells, mononuclear inflammatory cells, and T and B lymphocyte subsets). Cryopreserved allograft valves implanted more than one day had progressively severe loss of normal structure and were devoid of surface endothelium and stainable deep connective tissue cells. Inflammatory cellularity varied from none (most valves) to prominent (primarily T lymphocytes in one valve). Transmission electron microscopy of three long term valvular allografts revealed no viable cells, remarkable preservation of the collagenous valve matrix and focal cell-oriented calcification. In contrast, aortic valves from transplanted hearts showed remarkable structural preservation, including layered architecture, endothelium and deep connective tissue cells; inflammatory infiltrates were generally sparse, even in cases with fatal myocardial rejection. We conclude that cryopreserved allograft valves are morphologically non-viable valves, whose structural basis for function seems primarily related to the largely preserved collagen.(ABSTRACT TRUNCATED AT 250 WORDS)

Aortic Valve↗
PubMed1993

Signaling-defective mutants of the B lymphocyte antigen receptor fail to associate with Ig-alpha and Ig-beta/gamma.

The B cell receptor for antigen, the membrane immunoglobulin (mIg) molecule, is normally expressed in association with at least two other proteins, mb-1 (Ig-alpha) and B29 (Ig-beta/gamma). We have previously described a set of murine B cell clones transfected with wild-type and mutated forms of membrane-bound IgM (mIgM) that vary in their ability to transduce signals to the cell interior. Analysis of the B cell receptor complex in signaling and nonsignaling mutants of mIgM reveals complete concordance between intact signaling ability and ability of the mIgM to associate with Ig-alpha and Ig-beta/gamma. These results point to the importance of the mIgM transmembrane region in mediating binding to accessory molecules, and provide support for a role of Ig-alpha and Ig-beta/gamma that extends beyond surface expression to signal transduction in B lymphocytes.

Amino Acid Sequence↗
PubMed1991

Cytoplasmic tail deletion converts membrane immunoglobulin to a phosphatidylinositol-linked form lacking signaling and efficient antigen internalization functions.

Membrane-bound immunoglobulin (mIg) is the antigen receptor on B lymphocytes mediating early events in antigen presentation and signal transduction. Wild-type human mIgM constructs transfected into the murine B-cell lymphoma A20 are expressed as transmembrane proteins with antigen presentation and signaling functions comparable to the endogenous mIgG2A; the transfected wild-type mIgM is internalized rapidly after anti-Ig cross-linking. Transfected constructs lacking the normal three-amino acid cytoplasmic tail are expressed exclusively as phosphatidylinositol-linked proteins, lack both antigen presentation and signal transduction functions, and are internalized slowly following anti-Ig binding. The molecular mass of the cytoplasmic tail-deleted phosphatidylinositol-linked Ig molecule is consistent with cleavage of the transmembrane residues during processing. Cytoplasmic domains may therefore regulate the mode of expression of membrane proteins and thereby influence their functional capabilities.

Animals↗
PubMed1990

Mutations of immunoglobulin transmembrane and cytoplasmic domains: effects on intracellular signaling and antigen presentation.

The membrane-bound form of immunoglobulin serves as an antigen-specific receptor for B cells mediating signal transduction and antigen presentation. We have developed an assay that reconstitutes both these physiologic responses with respect to the antigen phosphorylcholine. By introducing specific mutations in the human Ig mu chain gene, we have shown that certain transmembrane residues and the short cytoplasmic domain are crucial for these two activities. Moreover, elimination of a single transmembrane hydroxyl group severely inhibits antigen presentation without affecting signal transduction, suggesting that these two functions are mediated by different protein interactions.

Amino Acid Sequence↗
PubMed1984

The use of radioactive cysteine methyl ester for labeling glycosylated molecules oxidized by periodate or neuraminidase plus galactose oxidase.

Treatment of rat lymph node cells with periodate or neuraminidase plus galactose oxidase initiates blast transformation and cell division of T lymphocytes. Either treatment introduces aldehyde functions onto glycosylated molecules of the plasma membrane. Reduction of the aldehydes with borohydride leads to a concentration-dependent inhibition of the mitogenic response. Cysteine methyl ester (Cys(Me], which can form a stable thiazolidine adduct with aldehydes, also inhibits mitogenesis in a concentration-dependent manner. Maximum inhibition is achieved at Cys(Me) concentrations about 10-fold lower than those required for borohydride (0.4 and 5 mM, respectively). [35S]Cys(Me) has been synthesized and compared with [3H]borohydride as a labeling reagent for molecules on the plasma membrane oxidized by periodate or neuraminidase plus galactose oxidase. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of labeled whole cell lysates or of crude membrane fractions prepared from labeled cells revealed that the same oxidized molecules are specifically labeled with both reagents. Homogenates of cells labeled with either radioactive reagent were fractionated by differential and isopycnic centrifugation. The fractions were analyzed for radioactivity and for a number of marker constituents localized in various subcellular organelles. Following treatment with either reagent, the radioactivity that was covalently incorporated into macromolecules was primarily associated with sedimentable components that distributed among the fractions like plasma membrane markers. When compared with [3H]borohydride, Cys(Me) offers several advantages as a surface labeling reagent for glycosylated plasma membrane molecules, chiefly the possibility of preparing reagents labeled with isotopes other than tritium, including those like 35S, which are much stronger radioactive emitters.

Animals↗
PubMed1978

Cell surface glycoproteins of rat lymphocytes. I. Correlation of mitogenic stimulation by periodate or neuraminidase and galactose oxidase with the presence of papain-sensitive glycoproteins.

Oxidation of viable rat lymph node lymphocytes with either periodate or a combination of neuraminidase and galactose oxidase (NGO), followed by reduction with tritiated sodium borohydride, labels similar sets of cell-surface molecules as assessed by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. Periodate and NGO induce blast transformation of lymph node lymphocytes (oxidative mitogenesis), and borohydride reduction inhibits the proliferative response. Thus, it is inferred that some or all of the glycoproteins that are labeled with tritiated borohydride may be involved in mediating the stimulation caused by the oxidizing agents. Treatment of lymph node lymphocytes with 5 units/ml papain abolishes the response to periodate or NGO but does not significantly affect the response to Con A. At the same time, papain treatment eliminates the labeled bands representing six high m.w. glycoproteins (175,000, 170,000, 160,000, 155,000, 100,000, and 70,000 daltons). No significant effect is seen on the labeling of the other components visualized in the slab gels. The results implicate the subset of six high m.w. papain-sensitive sialoglycoproteins in mediating oxidative mitogenesis of rat lymph node lymphocytes.

Aldehydes↗