Showing papers by "Utrecht University published in 1991"

Affine structure from motion

Abstract: A mobile observer samples sequences of narrow-field projections of configurations in ambient space. The so-called structure-from-motion problem is to infer the structure of these spatial configurations from the sequence of projections. For rigid transformations, a unique metrical reconstruction is known to be possible from three orthographic views of four points. However, human observers seem able to obtain much shape information from a mere pair of views, as is evident in the case of binocular stereo. Moreover, human observers seem to find little use for the information provided by additional views, even though some improvement certainly occurs. The rigidity requirement in its strict form is also relaxed. We indicate how solutions of the structure-from-motion problem can be stratified in such a way that one explicitly knows at which stages various a priori assumptions enter and specific geometrical expertise is required. An affine stage is identified at which only smooth deformation is assumed (thus no rigidity constraint is involved) and no metrical concepts are required. This stage allows one to find the spatial configuration (modulo an affinity) from two views. The addition of metrical methods allows one to find shape from two views, modulo a relief transformation (depth scaling and shear). The addition of a third view then merely serves to settle the calibration. Results of a numerical experiment are discussed.

Immuno-localization of the insulin regulatable glucose transporter in brown adipose tissue of the rat

Abstract: Antibodies specific for the insulin-regulatable glucose transporter (GLUT 4) were used to immunolocalize this protein in brown adipose tissue from basal- and insulin-treated rats. Cryosections of fixed tissue were incubated with antibodies, which were subsequently labeled with Protein A/gold and examined by EM. Antibodies against albumin and cathepsin D were also used with gold particles of different sizes to identify early and late endosomes, respectively. Under basal conditions 99% of the GLUT 4 labeling was located within the cell. Labeling was predominantly in the trans-Golgi reticulum and tubulo-vesicular structures elsewhere in the cytoplasm. In insulin-stimulated cells approximately 40% of the GLUT 4 labeling was at the cell surface, where it was randomly distributed, except for occasional clustering in coated pits. Moreover, after insulin treatment, GLUT 4 was also enriched in early endosomes. We conclude that translocation of GLUT 4 to the cell surface is the major mechanism by which insulin increases glucose transport. In addition, these results suggest that in the presence of insulin GLUT 4 recycles from the cell surface, probably via the coated pit-endosome pathway that has been characterized for cell surface receptors, and also that insulin causes the redistribution of GLUT 4 by stimulating exocytosis from GLUT 4-containing tubulo-vesicular structures, rather than by slowing endocytosis of GLUT 4.

Cytotoxic T lymphocyte granules are secretory lysosomes, containing both perforin and granzymes.

Abstract: Cytotoxic T lymphocytes (CTL) contain granules that are exocytosed during specific interaction with target cells (TC). In this process, the granule contents, including the lethal protein perforin, as well as granzymes, a family of serine esterases, are delivered to the TC. Information regarding the routing of these proteins towards the granule and their exact localization within the granule is of primary importance to resolve the mechanism of granule-mediated TC killing. In this study, the subcellular localization of perforin, granzymes, and known endosomal and lysosomal marker proteins was determined in human and murine CTL, by immunogold labeling of ultrathin cryosections followed by electron microscopy. Perforin and granzymes can be detected in rough endoplasmic reticulum, Golgi complex, trans-Golgi reticulum, and in all cytotoxic granules. Within the granules, they have a similar distribution and are localized not only in the so-called dense core but also over the region containing small internal vesicles. This finding implies that perforin and granzymes can be released in membrane-enveloped and/or -associated form into the intercellular cleft formed upon CTL-TC interaction. On the basis of the present evidence, additional release of these molecules in soluble form cannot be excluded. The lysosomal membrane glycoproteins lamp-1, lamp-2, and CD63, are abundantly present on the granule-delimiting outer membrane, which becomes incorporated into the CTL plasma membrane during lethal hit delivery. In contrast, the cation-dependent mannose 6-phosphate receptor, known to be present in endosomes and absent from lysosomes, is found only in a minority of the granules. Together with our previous findings that the granules are acidic and connected to the endocytic pathway, these observations define CTL granules as secretory lysosomes.

Segregation of MHC class II molecules from MHC class I molecules in the Golgi complex for transport to lysosomal compartments.

Abstract: Traffic of MHC molecules dictates the source of peptides that are presented to T cells. The intracellular distribution of MHC class I and class II molecules reflects the dichotomy in presentation of antigen from endogenous and exogenous origin, respectively. In human B lymphoblastoid cells, class I molecules are present in compartments constituting the biosynthetic pathway, whereas class II molecules enter structures related to lysosomes during their biosynthesis.

Tomographic imaging of subducted lithosphere below northwest Pacific island arcs

Abstract: The seismic tomography problem does not have a unique solution, and published tomographic images have been equivocal with regard to the deep structure of subducting slabs. An improved tomographic method, using a more realistic background Earth model and surface-reflected as well as direct seismic phases, shows that slabs beneath the Japan and Izu Bonin island arcs are deflected at the boundary between upper and lower mantle, whereas those beneath the northern Kuril and Mariana arcs sink into the lower mantle.

Speaking to many minds: on the relevance of different types of language information for the L2 learner

Abstract: This paper re-analyses the notion of consciousness raising in language learning. The process by which language input becomes salient to the learner is termed 'input enhancement'. This process can come about as a result of deliberate manipulation, or it can be the natural outcome of some internal learning strategy. It can vary quantitatively and qualitatively, not necessarily involv ing conscious analysis of rules. Externally induced salience may not neces sarily be registered by the learner and even when it is registered, it may not affect the learning mechanisms per se. Certain inferences are made about the interpretation of learner performance and ways of measuring it.

Phospholipid transfer proteins.

Abstract: INTROD UCTION AND PERSPECTI VES . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 PHOSPHOLIPID TRANSFER PROTEINS IN MAMMALS, PLANTS , A ND UNICELLULAR ORGANISMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Introductory Remarks . . . . . . . ... . . . . . . . . . . . . . . . . " . . . . . . . . .. . . . . . . . .. . " . . .... . .. . . ..... . . . . .. . . 75 Mammalian Tissues . . . . . . . . . . . . . . . . . . ... . . . . . ... . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . .. " . . . . . . " . . 75 Plams . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 77 yeast. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Bacteria. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 78 PRIMARY STRUCT URE: LACK OF SEQUEN CE HOMOLOGIES .. . . .. . "..... . . . 79 PROPERTIE S OF T HE LIPID -BINDING S ITE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 ImroduclOry Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . " . . 80 S pe cific Binding Sites for the m·J· and sn-2 Fal y Acyl Chain s . . . . . . . . . . . . . .. . . . . . . . . . 8 1 Use of P holoactivatable Analogs o f pc 82 Spe cifi c Recognition Sites for the Polar He ad Gro ups . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . 83 CARRIERS OF PHOSPHO LIPIDS BETWEEN MEMBRANES . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 84 Mode of Actio n . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . ..... . . . . . . . . . . . . . . . . . . . 85 Net Transfer versus E xchan ge : Effe ct on Memb ra ne Properties . . . . . . . . . . . . . .. . . . . . . . . . 87 TISS UE D IS TRIBUTION AND INTRACELLULAR LOCA LIZATION . . . . . . . .. . . . . . . . . . 89 Involveme nt of Pero xiso mes .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 STIMULAT ION OF LIPID METABOLISM . . .. .. . . . . . . . . . . . . . . . . . . . . " . . . . . . . . . . . . . . . . . . . . " . . 9 1 Phosphol i pids.. .. ....... " 91 Cholester ol . ...... ... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . 92 Stero idogenesis . . ... . . . . . . . . . .. . . .. . . . . . . . .... . . . . . .. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 ROLE IN MEM BRANE VESICLE FLOW. . . . . . . . . . . . . . . . . .. . . . . . . . . 93 CONCLUDING R EMARK S . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Translocation of the glucose transporter GLUT4 in cardiac myocytes of the rat.

Abstract: The insulin-regulated glucose transporter GLUT4 was immunolocalized in rat cardiac muscle under conditions of basal and stimulated glucose uptake, achieved by fasting and a combined exercise/insulin stimulus, respectively. In basal myocytes there was very little (less than 1%) GLUT4 in the different domains of the plasma membrane (sarcolemma, intercalated disk, and transverse tubular system). GLUT4 was localized in small tubulo-vesicular elements that occur predominantly near the sarcolemma and the transverse tubular system and in the trans-Golgi region. Upon stimulation approximately 42% of GLUT4 was found in the plasma membrane. Each domain of the plasma membrane contributed equally to this effect. GLUT4-positive, clathrin-coated pits were also present at each cell surface domain. The remainder of the labeling was in tubulo-vesicular elements at the same sites as in basal cells and in the intercalated disk areas. The localization of GLUT4 in cardiac myocytes is essentially the same as in brown adipocytes, skeletal muscle, and white adipocytes. We conclude that increased glucose transport in muscle and fat is accounted for by translocation of GLUT4 from the intracellular tubulo-vesicular elements to the plasma membrane. The labeling of coated pits indicates that in stimulated myocytes, as in adipocytes, GLUT4 recycles constantly between the endosomal compartment and the plasma membrane and that stimulation of the exocytotic rate constant is likely the major mechanism for GLUT4 translocation.

Real time process algebra

Abstract: We describe an axiom system ACPp that incorporates real timed actions. Many examples are provided in order to explain the intuitive contents of the notation. ACPp is a generalisation of ACP. This implies that some of the axioms have to be relaxed and that ACP can be recovered as a special case from it. The purpose of ACPp is to serve as a specification language for real time systems. The axioms of ACPp explain its operational meaning in an algebraic form.

Inactivation of Campylobacter jejuni flagellin genes by homologous recombination demonstrates that flaA but not flaB is required for invasion.

Abstract: The role of the Campylobacter jejuni flagella in adhesion to, and penetration into, eukaryotic cells was investigated. We used homologous recombination to inactivate the two flagellin genes flaA and flaB of C. jejuni, respectively. Mutants in which flaB but not flaA is inactivated remain motile. In contrast a defective flaA gene leads to immotile bacteria. Invasion studies showed that mutants without motile flagella have lost their potential to adhere to, and penetrate into, human intestinal cells in vitro. Invasive properties could be partially restored by centrifugation of the mutants onto the tissue culture cells, indicating that motility is a major, but not the only, factor involved in invasion.

The relation between above- and belowground biomass allocation patterns and competitive ability.

Abstract: In a 2-year experiment, the evergreen shrubsErica tetralix andCalluna vulgaris (dominant on nutrient-poor heathland soils) and the perennial deciduous grassMolinia caerulea (dominant on nutrient-rich heathland soils) were grown in replacement series in a factorial combination of four competition types (no competition, only aboveground competition, only belowground competition, full competition) and two levels of nutrient supply (no nutrients and 10 g N+2 g P+10 g K m−2 yr−1). Both in the unfertilized and in the fertilized treatmentsMolinia allocated about twice as much biomass to its root system than didErica andCalluna. In all three species the relative amount of biomass allocated to the roots was lower at high than at low nutrient supply. The relative decrease was larger forMolinia than forErica andCalluna. In the fertilized monocultures biomass of all three species exceeded that in the unfertilized series.Molinia showed the greatest biomass increase. In the unfertilized series no effects of interspecific competition on the biomass of each species were observed in either of the competition treatments. In the fertilized mixtures where only belowground competition was possibleMolinia increased its biomass at the expense of bothErica andCalluna. When only aboveground competition was possible no effects of interspecific competition on the biomass of the competing species were observed. However, in contrast with the evergreens,Molinia responded by positioning its leaf layers relatively higher in the canopy. The effects of full competition were similar to those of only belowground competition, so in the fertilized series belowground competition determined the outcome of competition. The high competitive ability ofMolinia at high nutrient supply can be attributed to the combination of (1) a high potential productivity, (2) a high percentage biomass allocation to the roots, (3) an extensive root system exploiting a large soil volume, and (4) plasticity in the spatial arrangement of leaf layers over its tall canopy. In the species under study the allocation patterns entailed no apparent trade-off between the abilities to compete for above- and belowground resources. This study suggests that this trade-off can be overcome by: (1) plasticity in the spatial arrangement of leaf layers and roots, and (2) compensatory phenotypic and species-specific differences in specific leaf area and specific root length.

Hypertension in the elderly is associated with white matter lesions and cognitive decline.

Abstract: Forty-two elderly patients (mean age, 66.2 +/- 5.1 yr) with hypertension, treated for an average of 17.3 years (standard deviation, 10.3), and 42 control subjects (mean age, 66.5 +/- 4.8 yr), matched for age, sex, and level of education, were studied with regard to the detection of lesions in the cerebral white matter with magnetic resonance imaging (MRI), particularly with axial T2-weighted images. The assessment of the MRI scans was blinded. Ten hypertensive patients showed confluent lesions in the white matter, versus only 1 control subject (Chi-square test, p = 0.01). The presence of diffuse lesions of the white matter was related to age but not to the known duration of hypertension, nor to the presence of any other cardiovascular risk factors. Cognitive function was measured in 34 hypertensive patients and in 18 control subjects. Results of the Mini-Mental State Examination, the Stroop color-word test, Trailmaking test, and the visual subtest of the Wechsler Memory Scale were worse in patients with confluent lesions of the white matter; there was no difference in mental functioning between hypertensive patients and control subjects with normal white matter or with only small focal lesions. Our findings suggest that long-standing hypertension in some patients may cause not only strokes but also chronic end-organ damage of the brain in the form of demyelination of the white matter, with cognitive decline.

Colony‐level sex ratio selection in the eusocial Hymenoptera

Abstract: We present an inclusive fitness model on worker-controlled sex investments in eusocial Hymenoptera which expands the existing theory for random mating populations as formulated by Trivers and Hare (1976) and Benford (1978). We assume that relatedness asymmetry is variable among colonies — owing to multiple mating, worker reproduction and polygyny — and that workers are able to assess the relatedness asymmetry in their own colony. A simple marginal value argument shows that “assessing” workers maximize their inclusive fitness by specializing on the production of the sex to which they are relatively more related than the average worker in the population is related to that sex. The model confirms our earlier verbal argument on this matter (Boomsma and Grafen, 1990) and gives further quantitative predictions of the optimal sex ratio of relatedness-asymmetry classes for both infinite and finite, random mating populations. It is shown that in large populations all but one of the relatedness-asymmetry classes should specialize on the production of one sex only. The remaining, balancing class is selected to compensate any bias induced by the other class(es) such that the population sex ratio reflects the relatedness asymmetry of that balancing class. In the absence of worker-reproduction, the sex ratio compensation by the balancing-class is generally close to 100%, unless the population is very small. In the Discussion we address explicitly the likelihood of our relatedness-assessment hypothesis and other assumptions made in the model. The relationship of our model with previous theory on sex allocation in eusocial Hymenoptera is worked out in the Appendix.

An experimental study of unstable barotropic vortices in a rotating fluid

Abstract: Laboratory experiments on barotropic vortices in a rotating fluid revealed that the instability behaviour of cyclonic and anticyclonic vortices is remarkably different. Depending on its initial vorticity distribution, the cyclonic vortex has in a number of experiments been observed to be unstable to wavenumber-2 perturbations, leading to the gradual formation of a stable tripolar vortex structure. This tripole consists of an elongated cyclonic core vortex adjoined by two anticyclonic satellite vortices.In contrast, the anticyclonic vortex shows a rather explosive instability behaviour, in the sense that it is observed to immediately split up into two dipoles. Under somewhat different circumstances the higher-order mode-3 instability is observed, in which the anticyclonic core has a triangular shape, with three smaller cyclonic satellite vortices at its sides.A modified version of Rayleigh's instability criterion offers a qualitative explanation for this apparent difference between unstable cyclonic and anticyclonic vortices.

The Analysis of Factorial Surveys

Abstract: Factorial surveys constitute a specific technique for introducing experimental designs in sample surveys. Respondents are presented with descriptions (vignettes) of a constructed world in which important factors are built in experimentally. Using balanced designs well known from the multivariate experimental tradition, it is possible to build in a relatively large number of factors and levels. Within this context, the normal hypothesis is that responses are consistent on the individual level, but not totally idiosyncratic. In the analysis, it is important to determine the influence of both the vignette and the respondent variables. Analysis models for this type of data should reflect the fact that factorial surveys produce data pertaining to two distinct levels: the individual level and the vignette level. Such models are available and are generally known as multilevel analysis models. This article discusses the properties of factorial survey designs and some analysis models that address the multilevel as...

On the complexity of some coloring games

Abstract: In this paper we consider the following game: players must alternately color the lowest numbered uncolored vertex of a given graph G= ({1, 2,…, n}, E) with a color, taken from a given set C, such that two adjacent vertices are never colored with the same color. In one variant, the first player which is unable to move, loses the game. In another variant, player 1 wins the game if and only if the game ends with all vertices colored. We show that for both variants, the problem of determining whether there is a winning strategy for player 1 is PSPACE-complete for any C with |C|≥3, but the problems are solvable in , and time, respectively, if |C|=2. We also give polynomial time algorithms for the problems with certain restrictions on the graphs and orderings of the vertices. We give some partial results for the versions where no order for coloring the vertices is specified.