A multisensory integration model of human stance control

A model is presented to study and quantify the contribution of all available sensory information to human standing based on optimal estimation theory. In the model, delayed sensory information is integrated in such a way that a best estimate of body orientation is obtained. The model approach agrees with the present theory of the goal of human balance control. The model is not based on purely inverted pendulum body dynamics, but rather on a three-link segment model of a standing human on a movable support base. In addition, the model is non-linear and explicitly addresses the problem of multisensory integration and neural time delays. A predictive element is included in the controller to compensate for time delays, necessary to maintain erect body orientation. Model results of sensory perturbations on total body sway closely resemble experimental results. Despite internal and external perturbations, the controller is able to stabilise the model of an inherently unstable standing human with neural time delays of 100 ms. It is concluded, that the model is capable of studying and quantifying multisensory integration in human stance control. We aim to apply the model in (1) the design and development of prostheses and orthoses and (2) the diagnosis of neurological balance disorders. Received: 25 August 1997 / Accepted in revised form: 8 December 1998     

Flow cytometry-assisted rapid isolation of recombinant Plasmodium berghei parasites exemplified by functional analysis of aquaglyceroporin

The most critical bottleneck in the generation of recombinant Plasmodium berghei parasites is the mandatory in vivo cloning step following successful genetic manipulation. This study describes a new technique for rapid selection of recombinant P. berghei parasites. The method is based on flow cytometry to isolate isogenic parasite lines and represents a major advance for the field, in that it will speed the generation of recombinant parasites as well as cut down on animal use significantly. High expression of GFP during blood infection, a prerequisite for robust separation of transgenic lines by flow cytometry, was achieved. Isogenic recombinant parasite populations were isolated even in the presence of a 100-fold excess of wild-type (WT) parasites. Aquaglyceroporin (AQP) loss-of-function mutants and parasites expressing a tagged AQP were generated to validate this approach. aqp^? parasites grow normally within the WT phenotypic range during blood infection of NMRI mice. Similarly, colonization of the insect vector and establishment of an infection after mosquito transmission were unaffected, indicating that AQP is dispensable for life cycle progression in vivo under physiological conditions, refuting its use as a suitable drug target. Tagged AQP localized to perinuclear structures and not the parasite plasma membrane. We suggest that flow-cytometric isolation of isogenic parasites overcomes the major roadblock towards a genome-scale repository of mutant and transgenic malaria parasite lines.     

Fructosebisphosphatase isoenzymes of the chemoautotroph Xanthobacter flavus.

Xanthobacter flavus employs two fructosebisphosphatase (FBPase)-sedoheptulosebisphosphatase (SBPase) enzymes. One of these is constitutively expressed and has a high FBPase-to-SBPase ratio. The alternative enzyme, which is encoded by cbbF, is induced during autotrophic growth. The cbbF gene was expressed in Escherichia coli, and the FBPase was purified to homogeneity. The purified enzyme has a specific FBPase activity of 114 mumol/min/mg of protein, a Michaelis constant for fructosebisphosphate of 3 microM, and a low FBPase-to-SBPase ratio. CbbF was activated by ATP and inhibited by Ca2+.     

Leucoagaricus gongylophorus uses leaf-cutting ants to vector proteolytic enzymes towards new plant substrate

The mutualism between leaf-cutting ants and their fungal symbionts revolves around processing and inoculation of fresh leaf pulp in underground fungus gardens, mediated by ant fecal fluid deposited on the newly added plant substrate. As herbivorous feeding often implies that growth is nitrogen limited, we cloned and sequenced six fungal proteases found in the fecal fluid of the leaf-cutting ant Acromyrmex echinatior and identified them as two metalloendoproteases, two serine proteases and two aspartic proteases. The metalloendoproteases and serine proteases showed significant activity in fecal fluid at pH values of 5–7, but the aspartic proteases were inactive across a pH range of 3–10. Protease activity disappeared when the ants were kept on a sugar water diet without fungus. Relative to normal mycelium, both metalloendoproteases, both serine proteases and one aspartic protease were upregulated in the gongylidia, specialized hyphal tips whose only known function is to provide food to the ants. These combined results indicate that the enzymes are derived from the ingested fungal tissues. We infer that the five proteases are likely to accelerate protein extraction from plant cells in the leaf pulp that the ants add to the fungus garden, but regulatory functions such as activation of proenzymes are also possible, particularly for the aspartic proteases that were present but without showing activity. The proteases had high sequence similarities to proteolytic enzymes of phytopathogenic fungi, consistent with previous indications of convergent evolution of decomposition enzymes in attine ant fungal symbionts and phytopathogenic fungi.     

Responses to iron deficiency in Arabidopsis thaliana: The Turbo iron reductase does not depend on the formation of root hairs and transfer cells

Arabidopsis thaliana (L.) Heynh. Columbia wild type and a root hair-less mutant RM57 were grown on iron-containing and iron-deficient nutrient solutions. In both genotypes, ferric chelate reductase (FCR) of intact roots was induced upon iron deficiency and followed a Michaelis-Menten kinetic with a K _m of 45 and 54 M Fe^III-EDTA and a V _max of 42 and 33 nmol Fe²⁺·(g FW)^–1·min^–1 for the wild type and the mutant, respectively. The pH optimum for the reaction was around pH 5.5. The approximately four fold stimulation of FCR activity was independent of formation of root hairs and/or transfer cells induced by iron deficiency. Iron-deficiency-induced chlorosis and the development of a rigid root habit disappeared when ferric chelate was applied to the leaves, while FCR activity remained unchanged. The time course of the responses to iron deficiency showed that morphological and physiological responses were controlled separately.Abbreviations FCR ferric chelate reductase - FW fresh weight Thanks are due to Klaas Sjollema (Department of Electronmicroscopy, University of Groningen, The Netherlands) for help with the electron microscopy sample preparation and especially to Dr. Uwe Santore (Heinrich-Heine-University for electron microscopy. This work was supported by the SCIENCE programm of the European community; P.R.M.) and a Personal Research Grant by the Ministerium für Wissenschaft und Forschung of Nordrhein-Westfalen (P.R.M.) and last, not least by the productive discussions in ECOTRANS B.V.     

Utilization of oligo‐ and polysaccharides at microgram‐per‐litre levels in freshwater by Flavobacterium johnsoniae

Aims: To obtain a bacterial strain that can be used to quantify biodegradable polysaccharides at concentrations of a few micrograms per litre in freshwater. Methods and Results: Flavobacterium johnsoniae strain A3 was isolated from tap water supplemented with laminarin, pectin or amylopectin at 100 μg C l^?1 and river Rhine water. The organism utilized 14 of 23 oligo‐ and polysaccharides, and 1 of 9 monosaccharides, but none of the sugar acids, sugar alcohols, carboxylic acids or aromatic acids tested at 10 μg C l^?1. Amino acids promoted growth of strain A3, but not in coculture with assimilable organic carbon (AOC) test strain Pseudomonas fluorescens P17, which utilized these compounds more rapidly than strain A3. Compounds released by strain P17 and AOC test strain Spirillum sp. NOX grown on acetate promoted the growth of strain A3 at N_max values of ≥ 2 × 10⁵ CFU ml^?1 of strain P17 and ≥ 5 × 10⁵ CFU ml^?1 of strain NOX. Significant growth of strain A3 was observed in surface water and in tap water in the presence of strain P17 (N_max P17 < 2 × 10⁵ CFU ml^?1). Conclusions: Strain A3 utilizes oligo‐ and polysaccharides at microgram‐per‐litre levels. In surface water and in tap water, the organism was able to utilize compounds that were not utilized by strain P17. These compounds may include oligo‐ and/or polysaccharides. Significance and Impact of the Study: Phytoplanktonic and bacterial polysaccharides can constitute an important biodegradable fraction of natural organic matter in water and may promote growth of heterotrophic bacteria during water treatment and drinking water distribution. Strain A3 can be used to quantify a group of compounds that includes oligo‐ and polysaccharides at microgram‐per‐litre levels in freshwater.     

Growth of Pseudomonas aeruginosa in tap water in relation to utilization of substrates at concentrations of a few micrograms per liter.

Five Pseudomonas aeruginosa strains were tested for the utilization of 47 low-molecular-weight compounds as their sole sources of carbon and energy for growth at a concentration of 2.5 g/liter. Of these compounds, 31 to 35 were consumed. Growth experiments in tap water at 15 degrees C were carried out with one particular strain (P1525) isolated from drinking water. This strain was tested for the utilization of 30 compounds supplied at a concentration of 25 microgram of C per liter. The growth rate (number of generations per hour) of strain P1525 in this tap water was approximately 0.005 h-1, and with 10 compounds it was larger than 0.03 h-1. An average yield of 6.2 x 10(9) colony-forming units per mg of C was obtained from the maximum colony counts (colony-forming units per milliliter). The average yield and maximum colony count of strain P1525 grown in tap water supplied with a mixture of 45 compounds, each at a concentration of 1 microgram of C per liter, enabled us to calculate that 28 compounds were utilized. Growth rates of two P. aeruginosa strains (including P1525) in various types of water at 15 degrees C were half of those of a fluorescent pseudomonad. The concentrations of assimilable organic carbon calculated from maximum colony counts and average yield values amounted to 0.1 to 0.7% of the total organic carbon concentrations in five types of tap water. The assimilable organic carbon percentages were about 10 times larger in river water and in water after ozonation.     

Automatic Segmentation of Eight Tissue Classes in Neonatal Brain MRI

Purpose

Volumetric measurements of neonatal brain tissues may be used as a biomarker for later neurodevelopmental outcome. We propose an automatic method for probabilistic brain segmentation in neonatal MRIs.

Materials and Methods

In an IRB-approved study axial T1- and T2-weighted MR images were acquired at term-equivalent age for a preterm cohort of 108 neonates. A method for automatic probabilistic segmentation of the images into eight cerebral tissue classes was developed: cortical and central grey matter, unmyelinated and myelinated white matter, cerebrospinal fluid in the ventricles and in the extra cerebral space, brainstem and cerebellum. Segmentation is based on supervised pixel classification using intensity values and spatial positions of the image voxels. The method was trained and evaluated using leave-one-out experiments on seven images, for which an expert had set a reference standard manually. Subsequently, the method was applied to the remaining 101 scans, and the resulting segmentations were evaluated visually by three experts. Finally, volumes of the eight segmented tissue classes were determined for each patient.

Results

The Dice similarity coefficients of the segmented tissue classes, except myelinated white matter, ranged from 0.75 to 0.92. Myelinated white matter was difficult to segment and the achieved Dice coefficient was 0.47. Visual analysis of the results demonstrated accurate segmentations of the eight tissue classes. The probabilistic segmentation method produced volumes that compared favorably with the reference standard.

Conclusion

The proposed method provides accurate segmentation of neonatal brain MR images into all given tissue classes, except myelinated white matter. This is the one of the first methods that distinguishes cerebrospinal fluid in the ventricles from cerebrospinal fluid in the extracerebral space. This method might be helpful in predicting neurodevelopmental outcome and useful for evaluating neuroprotective clinical trials in neonates.     

Early behavioral changes and quantitative analysis of neuropathological features in murine prion disease: Stereological analysis in the albino Swiss mice model

Behavioral and neuropathological changes have been widely investigated in murine prion disease but stereological based unbiased estimates of key neuropathological features have not been carried out. After injections of ME7 infected (ME7) or normal brain homogenates (NBH) into dorsal CA1 of albino Swiss mice and C57BL6, we assessed behavioral changes on hippocampal-dependent tasks. We also estimated by optical fractionator at 15 and 18 weeks post-injections (w.p.i.) the total number of neurons, reactive astrocytes, activated microglia and perineuronal nets (PN) in the polymorphic layer of dentate gyrus (PolDG), CA1 and septum in albino Swiss mice. On average, early behavioral changes in albino Swiss mice start four weeks later than in C57BL6. Cluster and discriminant analysis of behavioral data in albino Swiss mice revealed that four of nine subjects start to change their behavior at 12 w.p.i. and reach terminal stage at 22 w.p.i and the remaining subjects start at 22 w.p.i. and reach terminal stage at 26 w.p.i. Biotinylated dextran-amine BDA-tracer experiments in mossy fiber pathway confirmed axonal degeneration and stereological data showed that early astrocytosis, microgliosis and reduction in the perineuronal nets are independent of a change in the number of neuronal cell bodies. Statistical analysis revealed that the septal region had greater levels of neuroinflammation and extracellular matrix damage than CA1. This stereological and multivariate analysis at early stages of disease in an outbred model of prion disease provided new insights connecting behavioral changes and neuroinflammation and seems to be important to understand the mechanisms of prion disease progression.Key words: prion disease, optical fractionator, neuropathology, behavioral changes, albino Swiss mice     

Evolutionarily advanced ant farmers rear polyploid fungal crops

Innovative evolutionary developments are often related to gene or genome duplications. The crop fungi of attine fungus‐growing ants are suspected to have enhanced genetic variation reminiscent of polyploidy, but this has never been quantified with cytological data and genetic markers. We estimated the number of nuclei per fungal cell for 42 symbionts reared by 14 species of Panamanian fungus‐growing ants. This showed that domesticated symbionts of higher attine ants are polykaryotic with 7–17 nuclei per cell, whereas nonspecialized crops of lower attines are dikaryotic similar to most free‐living basidiomycete fungi. We then investigated how putative higher genetic diversity is distributed across polykaryotic mycelia, using microsatellite loci and evaluating models assuming that all nuclei are either heterogeneously haploid or homogeneously polyploid. Genetic variation in the polykaryotic symbionts of the basal higher attine genera Trachymyrmex and Sericomyrmex was only slightly enhanced, but the evolutionarily derived crop fungi of Atta and Acromyrmex leaf‐cutting ants had much higher genetic variation. Our opposite ploidy models indicated that the symbionts of Trachymyrmex and Sericomyrmex are likely to be lowly and facultatively polyploid (just over two haplotypes on average), whereas Atta and Acromyrmex symbionts are highly and obligatorily polyploid (ca. 5–7 haplotypes on average). This stepwise transition appears analogous to ploidy variation in plants and fungi domesticated by humans and in fungi domesticated by termites and plants, where gene or genome duplications were typically associated with selection for higher productivity, but allopolyploid chimerism was incompatible with sexual reproduction.