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S Bronson

Publications and source records attributed to S Bronson.

6 recordsLinked to original sources
PubMed2000

Disrupting surfaces of nef required for downregulation of CD4 and for enhancement of virion infectivity attenuates simian immunodeficiency virus replication in vivo.

The multifunctional simian and human immunodeficiency virus (SIV and HIV) Nef proteins are important for virulence. We studied the importance of selected Nef functions using an SIV Nef with mutations in two regions that are required for CD4 downregulation. This Nef mutant is defective for downregulating CD4 and, in addition, for enhancing SIV infectivity and induction of SIV replication from infected quiescent peripheral blood mononuclear cells, but not for other known functions, including downregulation of class I major histocompatibility complex (MHC) cell surface expression. Replication of SIV containing this Nef variant in rhesus monkeys was attenuated early during infection. Subsequent increases in viral load coincided with selection of reversions and second-site compensatory changes in Nef. Our results indicate that the surfaces of Nef that mediate CD4 downregulation and the enhancement of virion infectivity are critical for SIV replication in vivo. Furthermore, these findings indicate that class I MHC downregulation by Nef is not sufficient for SIV virulence early in infection.

Amino Acid Substitution↗
PubMed1998

Detection of complement C1q receptors on human spermatozoa.

Clq, the first component of the classical complement pathway, is known to play roles in promoting phagocytic events, in addition to its role in activation of complement. Although the molecular events in fertilization leading to the entrance of the spermatozoan into the egg are not well understood, ultrastructural observations suggest that the process is quasi-phagocytic in nature. There is increasing evidence that complement components might play roles in fertilization. Previously, we have shown that C1q promoted the agglutination of capacitated human sperm as well as their adhesion to zona-free hamster eggs. In the present experiments, human spermatozoa were solubilized and, following their phase separation in Triton X-114, subject to 1-D polyacrylamide gel electrophoresis and immunoblotting for the presence of C1q receptors. Both gC1q-R and cC1q-R were detected. In addition, the ability of C1q to promote sperm agglutination was shown to be dependent upon capacitation, suggesting the increased expression of C1q receptors during this process.

Agglutination Tests↗
PubMed1997

Co-localization of HIV-1 Nef with the AP-2 adaptor protein complex correlates with Nef-induced CD4 down-regulation.

The nef gene of human and simian immunodeficiency viruses is critical for AIDS pathogenesis. Its function in vivo is unknown, but in vitro natural isolates of Nef down-regulate expression of the cell surface CD4 molecule, a component of the T cell antigen receptor and the viral receptor, by accelerating its endocytosis. We have used chimeric proteins comprised of the natural HIV-1 NA7 Nef fused to a strongly fluorescing mutant of green fluorescent protein (GFP) to correlate Nef function with intracellular localization in human CD4-positive Jurkat T cells. The NA7-GFP fusion protein co-localizes with components of the clathrin coat, including clathrin and the beta-subunit of the AP-2 adaptor protein complex, at discrete locations that are consistent with the normal cellular distribution of clathrin coats at the plasma membrane. The NA7-GFP protein is also found in the perinuclear region of the cell, which is likely to reflect the Golgi apparatus. Evidence from a CD4-negative fibroblast cell line indicates that co-localization of NA7-GFP with components of the clathrin coat does not require expression of the CD4 molecule. Analysis of a large panel of chimeric molecules containing mutant Nef moieties demonstrated that the N-terminal membrane targeting signal cooperates with additional element(s) in the disordered loops in the Nef molecule to co-localize the Nef protein with AP-2 adaptor complexes at the cell margin. This localization of NA7-GFP correlates with, but is not sufficient for, down-regulation of surface CD4 and at least one additional function of Nef is required. In T cells co-expressing CD4 and NA7-GFP, CD4 at the cell surface is redistributed into a discrete pattern that co-localizes with that of NA7-GFP. Our observations place NA7-GFP in physical proximity to AP-2-containing clathrin coat at the plasma membrane and imply that Nef interacts, either directly or indirectly, with a component of the AP-2-containing coat at this location. This evidence supports a model whereby Nef recruits CD4 to the endocytic machinery via AP-2-containing clathrin coats at the plasma membrane.

Adaptor Protein Complex alpha Subunits↗
PubMed1997

Separable functions of Nef disrupt two aspects of T cell receptor machinery: CD4 expression and CD3 signaling.

The Nef protein alters T cell receptor (TCR) signaling in T cells and is critical for the pathogenesis of AIDS. We used a transient expression assay in a human CD4+ T cell line to analyze the interaction of Nef with the TCR machinery. We show that, in addition to down-regulating CD4 expression on the cell surface, Nef blocks a receptor-proximal event in CD3 signaling. Analysis of a large number of mutant Nef proteins demonstrated that the effects of Nef on CD4 expression and on CD3 signaling are separable. The ability of Nef to block CD3 signaling was selectively abolished by mutations in the central part of the Nef protein and in particular by those known to disrupt the SH3 binding surface in the structured core of Nef. In contrast, the ability of Nef to down-regulate CD4 expression was selectively abolished by two clusters of mutations, one in the N-terminal and one in the C-terminal region of Nef. These two regions correspond to the two flexible loops in Nef as predicted by solution NMR analysis. We show that this general functional organization is conserved between the Nef proteins of the human and simian immunodeficiency viruses (HIV-1 and SIV). Our data demonstrate that Nef has at least two independent mechanisms to alter TCR function and thus may interfere with a range of T cell responses.

Amino Acid Sequence↗
PubMed1995

Echistatin, a disintegrin, inhibits sperm-oolemmal adhesion but not oocyte penetration.

OBJECTIVE: To study the effects of echistatin, a disintegrin known to block the binding of fibronectin (FN) and vitronectin to their respective integrin receptors, alpha 5 beta 1 and alpha v beta 3, on the adhesion of human spermatozoa to the oolemma of zona-free hamster eggs and their subsequent penetration. DESIGN: Motile capacitated human spermatozoa and zona-free hamster eggs were coincubated in the presence of echistatin or in its absence and observed at short serial intervals. Whole mounts of these eggs, washed out of sperm suspension and stained with acridine orange, were scored for numbers of oolemmal adherent and penetrating sperm. SETTING: University Hospital laboratories. PATIENTS: Known fertile semen donors. MAIN OUTCOME MEASURES: Numbers of spermatozoa adherent to the oolemma and those penetrating the oocyte. RESULTS: Sperm adherence to the oolemma was reduced significantly at micromolar concentrations of echistatin, in a concentration-dependent manner. In contrast, echistatin did not inhibit the penetration of oocytes by sperm that had become adherent to the oolemma despite the presence of echistatin. CONCLUSION: We propose that two processes occur in the binding of sperm to the oolemma, one that is echistatin sensitive and possibly involving the integrin receptors that recognize FN and vitronectin, and a second process, resistant to echistatin, that leads to gamete membrane fusion.

Animals↗