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Biomedical subjects

D B Williams

Publications and source records attributed to D B Williams.

At least 55 records · Page 3Linked to original sources
PubMed1998

Transjugular intrahepatic portosystemic shunt (TIPS) for the treatment of refractory ascites.

BACKGROUND: The Transjugular Intrahepatic Portosystemic Shunt (TIPS) corrects portal hypertension and has proven to be effective in controlling variceal bleeding in patients with cirrhosis. Several reports have now appeared suggesting a possible role in patients with refractory ascites. AIMS: To examine the outcome of TIPS for the treatment of refractory ascites in patients with cirrhosis. METHODS: Fifteen patients underwent TIPS for ascites between April 1992 and December 1996. The clinical findings, response to treatment, complications, shunt patency and survival of these patients were analysed. RESULTS: TIPS was successfully placed in all patients. The mean period of follow-up was 375 days (range: 14-1165 days). In eight patients (53%) there was a reduction in the degree of ascites after shunt insertion, with six patients (40%) having complete resolution. Age, Child-Pugh class or portal pressure gradient, before or after the procedure, were not predictive of response. Of five patients with renal insufficiency (serum creatinine > 130 umol/L), only one had improvement in ascites control. Six patients (40%) required shunt revision during follow-up, either for acute thrombotic occlusion (two) or stent stenosis (four). New or worsening encephalopathy developed in ten patients (67%). Two patients (13%) died of liver failure within 30 days. Cumulative survival was 46% at one year and 18% at two years. Treatment response was associated with increased survival (p = 0.02), with median survival of 658 days as compared with 71 days for treatment failure. CONCLUSIONS: TIPS can be effective in the treatment of refractory ascites in patients with cirrhosis. Our experience suggests the benefit may be less for patients with advanced liver disease and renal impairment. Controlled trials are needed to compare TIPS with other treatment modalities such as large volume paracentesis or peritoneovenous shunting.

Aged↗
PubMed1998

Immunoreactivity of five monoclonal antibodies against the 37-kilodalton common cell wall protein (PsaA) of Streptococcus pneumoniae.

Five monoclonal antibodies (MAbs) were produced against the Streptococcus pneumoniae pneumococcal surface adhesin A (PsaA) 37-kDa common cell wall protein. These antibodies were used in a dot immunoblot and Western blot study of clinical isolates of S. pneumoniae to detect the presence of the protein. By both assays, the MAbs reacted with clinical isolates representing the 23 type-specific serotypes present in the licensed pneumococcal polysaccharide vaccine. Western blot analysis confirmed the presence of a protein migrating in the gel with a molecular mass of 37 kDa. An extension of the study by using dot immunoblot analysis that included an analysis of the 90 serotypes of S. pneumoniae showed that all five MAbs reacted with 89 of the 90 serotypes tested. MAb 1B6, the exception, did not react with S. pneumoniae serotype 16F. Dot immunoblot analysis of the MAbs with Enterococcus faecalis and viridans streptococci showed varied reactivity patterns, depending on the species. The MAbs against the 37-kDa antigen did not react with Escherichia coli, respiratory pathogens, or nonpathogens representing 22 genera and 29 species of bacteria. All five MAbs also reacted with five multidrug-resistant strains of S. pneumoniae. In summary, these MAbs may be useful for detection of pneumococcal antigen and may lead to the development of diagnostic assays for pneumococcal disease.

Adhesins, Bacterial↗
PubMed1996

Thin foil preparation of metal particles in brittle ceramic matrices.

Thin foil preparation for metal particles in brittle ceramic matrices is particularly difficult, due to differential thinning of the component materials. This paper considers the preparation of thin foils of stony meteorites. Stony meteorites have a composite nature consisting of small metal particles (< 200 microns) embedded in an inherently brittle silicate matrix. Specimens of this type are difficult to prepare as thin foils because the area of interest is limited to specific regions within the metal particles. The problems of thin foil preparation of metal particles in the stony meteorites were overcome by developing a technique involving electrochemical polishing prior to ion beam thinning. The metal particles were first separated from the silicate matrix and then embedded in Epo-Tek H20E silver epoxy. This specific epoxy was selected for its good electron conductivity which allowed the specimen to be thinned by the electrochemical polishing technique. Specimens prepared by this technique have permitted the direct observation and characterization of the metallic phases in stony meteorites.

Ceramics↗
PubMed1996

Multiple levels of regulation for self-tolerance in beef insulin transgenic mice.

To characterize the mechanism(s) of tolerance toward soluble self-antigens (Ags), beef insulin (BI) transgenic (Tg) mice were generated in which the transgene was expressed in pancreatic beta-cells. Our previous data showed that: (i) Ag-specific tolerance can be induced and/or maintained in peripheral T cells in thymectomized BI-Tg mice and (ii) CD4+ Th2 regulatory T cells are involved in maintaining peripheral tolerance (by anti-BI antibody response). In this paper, we have further characterized the relationship of low levels of BI expression (10(-10)-10(-11) M) in Th1/Th2 activation. In addition, we have explored intrathymic events associated with tolerance to self-Ags not expressed in the thymus and/or to circulating self-Ags. Limiting dilution analysis showed that there was a significantly higher frequency of BI-specific Th2 cells in Tg mice with a corresponding higher frequency of Th1 cells in non-Tg mice. While there was no transgene expression in the thymus (by RT-PCR), independent studies showed that BI can be processed and presented in the Tg thymus, which correlated with the Ag-specific hyporesponsiveness of mature thymocyes detected in vitro. High-dose rIL-2 (150 U/ml) was able to restore in vitro peripheral T cell response of Tg mice to levels comparable to those of the non-Tg control. Collectively, our data suggest that: (i) there is a differential activation of BI-specific Th1/Th2 cells in vivo in the presence of low Ag concentration; (ii) the thymus may play a role in self-tolerance to Ags whose expression in adults is restricted to the periphery; and (iii) multiple levels of regulation such as thymic selection, peripheral anergy, and active suppression may be involved in tolerance to BI in BI-Tg mice.

Animals↗
PubMed1996

MHC class I molecules form ternary complexes with calnexin and TAP and undergo peptide-regulated interaction with TAP via their extracellular domains.

Newly assembled heavy chain-beta 2m heterodimers of class I histocompatibility molecules associate with the endoplasmic reticulum (ER) peptide transporter, TAP, and subsequently dissociate from TAP in parallel with their transport from the ER to the Golgi apparatus. It appears that TAP-associated class I molecules are waiting to bind appropriate peptides before they dissociate from TAP and leave the ER since binding of high affinity peptides to class I molecules in vitro leads to dissociation of TAP-class I complexes. In further support of this notion, we report that limiting peptide supply through inhibition of proteasome activities prolongs the association of mouse class I molecules with TAP and concomitantly slows their transport to the Golgi apparatus. By using a series of deletion mutants and hybrid class I molecules we demonstrate that the extracellular domains of class I molecules are sufficient for their peptide-regulated interaction with TAP. Furthermore, based on the inability of an alpha 3 domain-specific mAb to recognize TAP-class I complexes and the fact that a point mutant of the Dd molecule at residue 222 is unable to bind to TAP, it is likely that a major site of interaction with TAP resides in the membrane-proximal region of the heavy chain alpha 3 domain. Finally, we examined the relationship between the interaction of mouse heavy chain-beta 2m heterodimers with TAP and with the resident ER chaperone, calnexin. Most heterodimers that bound to TAP were found to associate simultaneously with calnexin. Upon delivery of peptide to class I molecules in permeabilized cells, dissociation from TAP was observed but the interaction with calnexin was largely maintained. Therefore, both TAP and calnexin may participate in the ER retention of peptide-deficient class I molecules. However, since release from calnexin occurs after dissociation from TAP, it appears that calnexin ultimately determines if a class I molecule is to be exported from the ER.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1996

The molecular chaperone calnexin facilitates folding and assembly of class I histocompatibility molecules.

Calnexin, a membrane protein of the endoplasmic reticulum, is generally thought to function as a molecular chaperone, based on indirect or correlative evidence. To examine calnexin's functions more directly, we reconstituted the assembly of class I histocompatibility molecules in the absence or presence of calnexin in Drosophila melanogaster cells. Calnexin enhanced the assembly of class I heavy chains with beta 2-microglobulin as much as 5-fold. The improved assembly appeared largely due to more efficient folding of heavy chains, as evidenced by increased reactivity with a conformation-sensitive monoclonal antibody and by a reduction in the level of aggregates. Similar findings were obtained in mouse or human cells when the interaction of calnexin with class I heavy chains was prevented by treatment with the oligosaccharide processing inhibitor castanospermine. The ability of calnexin to facilitate castanospermine. The ability of calnexin to facilitate heavy chain folding and to prevent the formation of aggregates provides compelling evidence that calnexin functions as a bona fide molecular chaperone.

Animals↗
PubMed1996

Thyroid hormonal modulation of the binding and activity of the GABAA receptor complex of brain.

Thyroid hormones, which are known to act by genomic mechanisms in peripheral tissues, were found to influence the binding and function of the GABAA receptor complex in brain membranes. Submicromolar concentrations of triiodothyronine and thyroxine stereospecifically stimulated the binding of [35S]t-butylbicyclophosphorothionate (a convulsant ligand for the GABAA receptor complex) to highly washed rat brain membranes, while higher concentrations of the hormones inhibited radioligand binding. GABA-stimulated 36Cl-flux in isolated brain membrane sacs was inhibited by L-triiodothyronine with a half-maximally inhibitory concentration (IC50) of 10(-7) M. Patch-clamp analysis of recombinant GABAA receptor subunits expressed in human embryonic kidney-293 cells showed an inhibition of chloride currents by thyroid hormones. This effect required only the alpha 1 beta 2 subunits, and was not blocked by the benzodiazepine antagonist flumazenil. Since thyroid hormones are known to be concentrated in nerve terminal preparations and subsequently released, the hormones may have non-genomic mechanisms of action as putative neurotransmitters or neuromodulators in brain and act through GABAA receptors.

Animals↗
PubMed1996

Peptide presentation by MHC class I molecules.

The presentation of peptides by class I histocompatibility molecules plays a central role in the cellular immune response to virally infected or transformed cells. The main steps in this process include the degradation of both self and 'foreign' proteins to short peptides in the cytosol, translocation of peptides into the lumen of the endoplasmic reticulum, binding of a subset of peptides to assembling class I molecules and expression of class-I-peptide complexes at the cell surface for examination by cytotoxic T cells. A molecular understanding of most of these steps is emerging, revealing a remarkable coordination between the processes of peptide translocation, delivery and binding to class I molecules.

Journal Article↗
PubMed1996

Endometrial thickness is a valid monitoring parameter in cycles of ovulation induction with menotropins alone.

OBJECTIVE: To evaluate the ability of an ultrasound (US)-measured periovulatory endometrial thickness to predict conception in hMG-stimulated cycles. DESIGN: Retrospective. SETTING: A university-based tertiary practice. PATIENTS: One hundred twelve patients undergoing 292 cycles of ovulation induction with hMG alone. MAIN OUTCOME MEASURES: A periovulatory transvaginal US measurement of endometrial thickness was obtained during cycles of ovulation induction with hMG alone. Clinical pregnancy was defined by fetal cardiac activity. Sensitivity and false-positive rates for multiple discriminatory values of endometrial thickness were calculated and a relative operating characteristic (ROC) curve was constructed to evaluate the performance of this test as a predictor of pregnancy. RESULTS: Thirty-eight of 292 cycles resulted in pregnancy. Conception and nonconception cycles showed similar demographics, diagnoses, peak E2, maximum number of follicles, midluteal P, and mean endometrial thickness. Ovulatory dysfunction was a more frequent diagnosis in the conception group. Relative operating characteristic analysis for endometrial thickness as a predictor of pregnancy yielded an area under the curve of 0.623 +/- 0.049 (mean +/- SD). CONCLUSION: Endometrial thickness is a valid screening test for conception outcome in cycles stimulated with hMG. A periovulatory endometrial thickness > or = 10 mm defined 91% of conception cycles. No pregnancy occurred when the endometrium measured < 7 mm.

Adult↗
PubMed1996

Prevention and treatment of ovarian hyperstimulation syndrome.

Ovarian hyperstimulation syndrome (OHSS) is the most serious complication of ovarian stimulation. Although the milder form is more common, particularly among patients undergoing gonadotropin stimulation for assisted reproductive technology, the severe form is rare. Classification schemes are clinically directed and useful in diagnosis and management of moderate and severe cases. The ovarian renin-angiotensin system, as well as vascular endothelial growth factor (VEGF)/vascular permeability factor (VPF) offers attractive theories as to the pathogenesis of this disorder. Unfortunately, clinical applications of these findings are not yet available. Possible prevention measures would include identification of patients at high risk, withholding human chorionic gonadotropin (hCG), using a gonadotropin-releasing hormone agonist to trigger ovulation, using a smaller dose of hCG, controlled gonadotropin drift, or avoidance of fresh embryo transfer by cryopreservation and frozen embryo transfer at a later date. Management of moderate to severe OHSS rests upon the principles of expanding intravascular volume and maintaining adequate urine output.

Female↗
PubMed1996

Dynamic measurement of the pH of the Golgi complex in living cells using retrograde transport of the verotoxin receptor.

The B subunit of verotoxin (VT1B) from enterohemorrhagic Escherichia coli is responsible for the attachment of the holotoxin to the cell surface, by binding to the glycolipid, globotriaosyl ceramide. After receptor-mediated endocytosis, the toxin is targeted to the Golgi complex by a process of retrograde transport. We took advantage of this unique property of VT1B to measure the pH of the Golgi complex in intact live cells. Purified recombinant VT1B was labeled with either rhodamine or fluorescein for subcellular localization by confocal microscopy. After 1 h at 37 degrees C, VT1B accumulated in a juxtanuclear structure that colocalized with several Golgi markers, including alpha-mannosidase II, beta-COP, and NBD-ceramide. Moreover, colchicine and brefeldin A induced dispersal of the juxtanuclear staining, consistent with accumulation of VT1B in the Golgi complex. Imaging of the emission of fluorescein-labeled VT1B was used to measure intra-Golgi pH (pHG), which was calibrated in situ with ionophores. In intact Vero cells, pHG averaged 6.45 +/- 0.03 (standard error). The acidity of the Golgi lumen dissipated rapidly upon addition of bafilomycin A1, a blocker of vacuolar-type ATPases, pHG remained constant despite acidification of the cytosol by reversal of the plasmalemmal Na+/H+ antiport. Similarly, pHG was unaffected by acute changes in cytosolic calcium. Furthermore, pHG recovered quickly toward the basal level after departures imposed with weak bases. These findings suggest that pHG is actively regulated, despite the presence of a sizable H+ "leak" pathway. The ability of VT1B to target the Golgi complex should facilitate not only studies of acid-base regulation, but also analysis of other ionic species.

Amino Acid Sequence↗
PubMed1995

Multiple proteolytic systems, including the proteasome, contribute to CFTR processing.

The molecular components of the quality control system that rapidly degrades abnormal membrane and secretory proteins have not been identified. The cystic fibrosis transmembrane conductance regulator (CFTR) is an integral membrane protein to which this quality control is stringently applied; approximately 75% of the wild-type precursor and 100% of the delta F508 CFTR variant found in most CF patients are rapidly degraded before exiting from the ER. We now show that this ER degradation is sensitive to inhibitors of the cytosolic proteasome, including lactacystin and certain peptide aldehydes. One of the latter compounds, MG-132, also completely blocks the ATP-dependent conversion of the wild-type precursor to the native folded form that enables escape from degradation. Hence, CFTR and presumably other intrinsic membrane proteins are substrates for proteasomal degradation during their maturation within the ER.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1995

The molecular chaperone calnexin binds Glc1Man9GlcNAc2 oligosaccharide as an initial step in recognizing unfolded glycoproteins.

Calnexin is a molecular chaperone that resides in the membrane of the endoplasmic reticulum. Most proteins that calnexin binds are N-glycosylated, and treatment of cells with tunicamycin or inhibitors of initial glucose trimming steps interferes with calnexin binding. To test if calnexin is a lectin that binds early oligosaccharide processing intermediates, a recombinant soluble calnexin was created. Incubation of soluble calnexin with a mixture of Glc0-3Man9GlcNAc2 oligosaccharides resulted in specific binding of the Glc1Man9GlcNAc2 species. Furthermore, Glc1Man5-7GlcNAc2 oligosaccharides bound relatively poorly, suggesting that, in addition to a requirement for the single terminal glucose residue, at least one of the terminal mannose residues was important for binding. To assess the involvement of oligosaccharide-protein interactions in complexes of calnexin and newly synthesized glycoproteins, alpha 1-antitrypsin or the heavy chain of the class I histocompatibility molecule were purified as complexes with calnexin and digested with endoglycosidase H. All oligosaccharides on either glycoprotein were accessible to this probe and could be removed without disrupting the association with calnexin. Furthermore, the addition of 1 M alpha-methyl glucoside or alpha-methyl mannoside had no effect on complex stability. These findings suggest that once complexes between calnexin and glycoproteins are formed, oligosaccharide binding does not contribute significantly to the overall interaction. However, it is likely that the binding of Glc1Man9GlcNAc2 oligosaccharides is a crucial event during the initial recognition of newly synthesized glycoproteins by calnexin.

Amino Acid Sequence↗
PubMed1995

Evidence for Th2 cell-mediated suppression of antibody responses in transgenic, beef insulin-tolerant mice.

Clonal deletion, anergy and suppression have all been considered mechanisms of immunological tolerance. Although adoptive transfer of immunosuppression has been shown to occur in the periphery, particularly for transplantation tolerance, it has proven difficult to characterize this phenomenon further, due to the lack of suppressor T cell clones. To characterize tolerance towards a physiological soluble antigen, we constructed beef insulin (BI) transgenic (Tg) BALB/c (H-2d) mice, in which the BI transgene is expressed in pancreatic beta cells. These Tg mice were tolerant to BI immunization at the level of both humoral and cell-mediated immune responses. Adoptive transfer of splenocytes from Tg mice into normal syngeneic BALB/c mice demonstrated that the reduction in antibody production is regulated by transferred T cells. The cytokine profile of T cell clones obtained after selection in vitro demonstrated dominant Th1 clones from normal non-Tg mice and dominant Th2 clones from Tg mice. Some Th2 clones (CD4+) from Tg mice produced significant suppression of antibody production after adoptive transfer into normal syngeneic BALB/c mice. These data confirm the existence of Th2 regulatory T cells in vivo in a model of peripheral tolerance to a physiological soluble antigen as a potential mechanism for self tolerance.

Animals↗
PubMed1995

Controlled ovarian hyperstimulation regimens in assisted reproductive technologies.

The goal of assisted reproductive technologies (ART) is to influence the recruitment of multiple, mature ovarian follicles. Several methods, including spontaneous cycle ART, clomiphene-based ART regimens, and gonadotropin regimens with and without adjuncts, are used. The controversies surrounding these techniques and their relative advantages and drawbacks are reviewed.

Female↗