Structural analysis of the GAP-related domain from neurofibromin and its implications

Neurofibromin is the product of the NF1 gene, whose alteration is responsible for the pathogenesis of neurofibromatosis type 1 (NF1), one of the most frequent genetic disorders in man. It acts as a GTPase activating protein (GAP) on Ras; based on homology to p120GAP, a segment spanning 250-400 aa and termed GAP-related domain (NF1GRD; 25-40 kDa) has been shown to be responsible for GAP activity and represents the only functionally defined segment of neurofibromin. Missense mutations found in NF1 patients map to NF1GRD, underscoring its importance for pathogenesis. X-ray crystallographic analysis of a proteolytically treated catalytic fragment of NF1GRD comprising residues 1198-1530 (NF1-333) of human neurofibromin reveals NF1GRD as a helical protein that resembles the corresponding fragment derived from p120GAP (GAP-334). A central domain (NF1c) containing all residues conserved among RasGAPs is coupled to an extra domain (NF1ex), which despite very limited sequence homology is surprisingly similar to the corresponding part of GAP-334. Numerous point mutations found in NF1 patients or derived from genetic screening protocols can be analysed on the basis of the three-dimensional structural model, which also allows identification of the site where structural changes in a differentially spliced isoform are to be expected. Based on the structure of the complex between Ras and GAP-334 described earlier, a model of the NF1GRD-Ras complex is proposed which is used to discuss the strikingly different properties of the Ras-p120GAP and Ras-neurofibromin interactions.     

Platelet morphology in patients with mechanical circulatory support

OBJECTIVE: Mechanical circulatory support is a therapy for patients with end stage cardiac insufficiency. The thromboembolic events are feared complications during support, due to the surface thrombogenicity of the implanted device. Activated blood platelets play a major role in this context. Consequently the platelet morphology of patients was investigated. METHODS: Platelets of eight patients were observed by means of scanning electron microscopy during the period of support with the Novacor left ventricular assist system N100. Blood was collected preoperatively and daily during the first week as well as weekly during the first 3 months. Samples were fixed with cacodylic-acid buffered glutaraldehyde. Platelet alterations were classified as non-activated, activated and aggregated, based on the so-called 'shape change' morphology. In addition, blood coagulation parameters were evaluated (e.g. activated partial thromboplastin time, prothrombin time, antithrombin III). RESULTS: Preoperatively, 15.0 +/- 4.6% (overall mean values) of activated platelets were found. Within the first postoperative week, the mean level of activated platelets increased to 32.8 +/- 8.0% (P < 0.05). Comparing short- (< 30 days; n = 4) vs. long-period (> 30 days; n = 4) support, a significant difference of activated platelets was evaluated (24.3 +/- 3.3% vs. 34.8 +/- 3.4%, P = 0.004). A correlation was found between the values of activated clotting time and activated platelets. Specific platelet deformations and damages appeared during support, which could not be found preoperatively. CONCLUSIONS: The platelet morphology showed alterations in all patients probably most strongly induced by the surface activation of the implanted device. These observations should be taken into consideration in management of postoperative anticoagulation therapy.     

The V3-directed immune response in natural human immunodeficiency virus type 1 infection is predominantly directed against a variable, discontinuous epitope presented by the gp120 V3 domain

The specific binding of antibodies to the V3 loop in sera from human immunodeficiency type 1 (HIV-1)-infected individuals was investigated. Different V3 structures were analyzed as full-length loops or by pepscan. Our data show that on full-length V3 loops, both variable regions on either side of the tip of the loop (GPGRAF) contribute to a common epitope for type-specific antibodies. Type-specific antibodies bound strongly and at high titers to native V3 loops but negligibly once the loop was denatured. In contrast to the type-specific, discontinuous epitope, the linear, conserved epitopes presented by the full-length V3 loop, the tip, the amino-terminal base, and the carboxy-terminal base were not accessible to serum antibody. When the V3 sequences were analyzed with linear peptides, antibodies bound preferentially to peptides containing the conserved GPGRAF sequence. Thus, two different specificities of V3-directed antibodies were detected in patient sera. Unlike group-specific antibodies directed against GPGRAF peptides, lack of type-specific antibodies directed against the discontinuous epitope was correlated with viral escape from autologous neutralization. Our data suggest that the full-length conformation of the V3 loop is accessible predominantly to highly type-specific antibodies present in sera from HIV-1-infected individuals. These antibodies are directed against discontinuous V3 epitopes, not against conserved linear V3 targets. The implications of these findings for viral escape and blockade of infection with V3-based vaccines are discussed.     

Bench-top setup for validation of real time, digital periodic error correction

This paper provides experimental validation of the digital first-order periodic error reduction scheme described by Chu and Ray. A bench-top setup of a single-pass, heterodyne Michelson interferometer, designed to minimize common error contributors such as Abbe, dead path and environment, is described. Linear, reciprocating motion generation is achieved using a parallelogram, leaf-type flexure. Periodic error amplitude is varied through independent rotation of a half wave plate and polarizer. Experimental results demonstrate that the correction algorithm can successfully attenuate first-order error to sub-nm levels for a wide range of frequency mixing conditions.     

Cladribine (2-CDA) given as subcutaneous bolus injections is active in pretreated Waldenstr?m's macroglobulinaemia. Swiss Group for Clinical Cancer Research (SAKK)

Clinical trials with intravenous cladribine infusions in pretreated patients with Waldenstr?m's macroglobulinaemia have shown a response rate of 40%. Our pharmacokinetic studies revealed that the bioavailability of subcutaneous cladribine is complete but that the concentration-time profile is very different from intravenous administration. We designed this phase II multi-institutional trial to study the activity and toxicity of cladribine given as s.c. bolus injections in patients with symptomatic Waldenstr?m's macroglobulinaemia. Between May 1993 and October 1995, 25 patients were accrued: male/female 18/7, median age 65 years (range 44-85). All except one patient had been pretreated with more than one regimen (median 2, range 0-10). 18 patients had progressed under previous therapy and six were in relapse. All patients received cladribine for a total dose of 0.5 mg/kg per cycle as s.c. bolus injections divided over 5 d at > or = 4 week intervals, for a maximum of six cycles. All 25 patients were evaluable for toxicity and response. A total of 67 cycles were administered (median 3 cycles, range 1-6). Overall response rate including disease stabilization which had been progressive under previous therapy was 68%. 10 patients (40%, 95% CI 21-61%) achieved a partial remission. Seven responders had been progressive under previous therapy. Maximum responses were reached no later than the third cycle. Median time to treatment failure and remission duration were 4.4 (range 0.5-33) and 8 months (5-29), respectively. Four patients (16%) suffered from infections W.H.O. grade > or = 2 (pneumonia grade 2, Staphylococcus septicaemia grade 3, viral encephalitis and pneumonia, both grade 4 with complete resolution). No other severe adverse events were observed. Cladribine given as s.c. 5 d bolus injections was found to be active in pretreated Waldenstr?m's macroglobulinaemia and resulted in durable remissions.     

Potency of mixtures of polychlorinated biphenyls as inducers of dioxin receptor-regulated CYP1A activity in rat hepatocytes and H4IIE cells

Among the polychlorinated biphenyls (PCBs), a family of widespread environmental pollutants, the most toxic non-ortho-substituted coplanar (non-ortho coplanar) congeners are thought to act as strong dioxin (aryl hydrocarbon) receptor agonists leading to adverse effects, such as body weight loss, immunosuppression, thymic atrophy, hepatotoxicity, tumor promotion, and disturbances of steroid hormone action. Since PCBs are present in environmental and tissue samples as complex mixtures, we investigated the possible interaction of non-ortho coplanar congeners with other major PCBs, which are less active or inactive as dioxin receptor agonists. As a parameter for dioxin receptor activation, induction of CYP1A-catalyzed 7-ethoxyresorufin O-deethylase (EROD) was determined in rat hepatocytes in primary culture and in the rat hepatoma cell line H4IIE. In rat hepatocytes, individual EC50-values and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) equivalency factors (TEFs) for the non-ortho and mono-ortho coplanar PCBs 126, 169, 105, 118 and 156, were in good agreement with published data from in vivo experiments, while in H4IIE cells coincidence was lower. However, in both cell systems TEFs for PCB 77 were significantly higher than reported from experiments in rats. In an approximately equipotent mixture the six potent PCB congeners showed perfect additive behaviour in both cell systems. In contrast, addition of a tenfold surplus of abundant mono- and di-ortho PCBs (28, 52, 101, 138, 153 and 180) led to an almost threefold higher TEF than predicted. This finding suggests a moderate synergistic enhancement of the inducing potency of potent PCBs by less potent congeners, present in abundance in environmental and tissue samples.