Analysis of self-assembly of S-layer protein slp-B53 from Lysinibacillus sphaericus.

The formation of stable and functional surface layers (S-layers) via self-assembly of surface-layer proteins on the cell surface is a dynamic and complex process. S-layers facilitate a number of important biological functions, e.g., providing protection and mediating selective exchange of molecules and thereby functioning as molecular sieves. Furthermore, S-layers selectively bind several metal ions including uranium, palladium, gold, and europium, some of them with high affinity. Most current research on surface layers focuses on investigating crystalline arrays of protein subunits in Archaea and bacteria. In this work, several complementary analytical techniques and methods have been applied to examine structure-function relationships and dynamics for assembly of S-layer protein slp-B53 from Lysinibacillus sphaericus: (1) The secondary structure of the S-layer protein was analyzed by circular dichroism spectroscopy; (2) Small-angle X-ray scattering was applied to gain insights into the three-dimensional structure in solution; (3) The interaction with bivalent cations was followed by differential scanning calorimetry; (4) The dynamics and time-dependent assembly of S-layers were followed by applying dynamic light scattering; (5) The two-dimensional structure of the paracrystalline S-layer lattice was examined by atomic force microscopy. The data obtained provide essential structural insights into the mechanism of S-layer self-assembly, particularly with respect to binding of bivalent cations, i.e., Mg2+ and Ca2+. Furthermore, the results obtained highlight potential applications of S-layers in the fields of micromaterials and nanobiotechnology by providing engineered or individual symmetric thin protein layers, e.g., for protective, antimicrobial, or otherwise functionalized surfaces.

Local topographic influences on vision restoration hot spots after brain damage.

PURPOSE: Vision restoration training (VRT) in hemianopia patients leads to visual field enlargements, but the mechanisms of this vision restoration are not known. To investigate the role of residual vision in recovery, we studied topographic features of visual field charts and determined residual functions in local regions and their immediate surround. METHODS: We analyzed High Resolution Perimetry visual field charts of hemianopic stroke patients (n = 23) before and after 6 months of VRT and identified all local visual field regions with ("hot spots", n = 688) or without restoration ("cold spots", n = 3426). Topographic features of these spots at baseline where then related to (i) their respective local residual function, (ii) residual activity in their spatial neighbourhood, and (iii) their distance to the scotoma border estimated in cortical coordinates following magnification factor transformation. RESULTS: Visual field areas had a greater probability of becoming vision restoration hot spots if they had more residual activity in both local areas and in a spatially limited surround of 5° of visual angle. Hot spots were typically also located closer than 4 mm from the scotoma border in cortical coordinates. Thus, restoration depended on residual activity in both the local region and its immediate surround. CONCLUSIONS: Our findings confirm the special role of residual structures in visual field restoration which is likely mediated by partially surviving neuronal elements. Because the immediate but not distant surround influenced outcome of individual spots, we propose that lateral interactions, known to play a role in perceptual learning and receptive field plasticity, also play a major role in vision restoration.

A treatment outcome prediction model of visual field recovery using self-organizing maps.

Brain injuries caused by stroke, trauma, or tumor often affect the visual system that leads to perceptual deficits. After intense visual stimulation of the damaged visual field or its border region, recovery may be achieved in some sectors of the visual field, but the extent of restoration is highly variable between patients and is not homogeneously distributed in the visual field. We now assess the visual field loss and its dynamics by perimetry, a standard diagnostic procedure in medicine, to measure the detectability of visual stimuli in the visual field. Subsequently, a treatment outcome prediction model (TOPM) has been developed, using features that were extracted from the baseline perimetric charts. The features in the TOPM were either empirically associated with treatment outcomes or were based on findings in the vision-restoration literature. Among other classifiers, the self-organizing map (SOM) was selected because it implicitly supports data exploration. Using a data pool of 52 patients with visual field defects, the TOPM was constructed to predict areas of improvement in the visual field topography. To evaluate the predictive validity of the TOPM, we propose a method to calculate the receiver operating characteristic graph, whereby the SOM is used in combination with a nearest neighbor classifier. We discuss issues relevant for medical TOPMs, such as appropriateness to the patient sample, clinical relevance, and incorporation of a priori knowledge.

Inverse stimuli in perimetric performance reveal larger visual field defects: implications for vision restoration.

PURPOSE: When studying the efficacy of vision restoration training (VRT), near-threshold and super-threshold perimetry revealed visual field enlargements whereas the Scanning Laser Ophthalmoscope (SLO) did not. Because the SLO procedure differs in many parameters from the other perimetric tests (task difficulty, inability to reveal relative defects, inverse stimulus presentation, bright red background) the question arises which of these parameters might be responsible for such discrepancies in outcome. We have therefore simulated with a computer-based campimetry test some of the SLO parameters and compared performance with that in standard perimetry. METHODS: A 46-year old female patient was evaluated with computer-based high resolution perimetry (HRP) using detection tasks of "positive" (bright) stimuli on grey background. Performance was compared with an SLO-like task using "inverse" black target stimuli on red background. RESULTS: Detection rate was 89% when the stimuli were positive (HRP) but dropped to 79.6% and 80.4% in the SLO-like "inverse" stimulation mode with red background, and striped red background, respectively. The number of false positives increased from 8.5 when a grey background was used, to 9.8 and 9.5 for plain red and striped red background, respectively. Reaction times were prolonged from 384 ms using a grey background to 412 ms and 391 ms using a plain red and striped red background, respectively. Thus, visual fields tested with SLO-like "inverse" stimuli showed larger scotomata and prolonged reaction time. CONCLUSIONS: Inverse stimulus detection on red background is apparently a more difficult task for hemianopic patients than standard perimetric protocols (such as those used in Tuebinger Automatic Perimetry or HRP). The difference in stimulus features might explain why VRT-induced visual field enlargements could not be observed with the SLO. Our findings also suggest that vision restoration training does not improve all aspects of vision, such as inverse, chromatic stimulus detection.

Perimetry while moving the eyes: implications for the variability of visual field defects.

BACKGROUND: In standard perimetry, subjects fixate so that saccades are reduced and testing precision is increased. However, because vision in daily life requires eye movements, it is appropriate to assess visual fields during eye movement. METHODS: Perimetry was carried out in 8 healthy subjects and in 16 patients with visual field defects under conditions of a stable and moving fixation spot. Eye movements were simultaneously recorded with an eye tracker. Outcome measures included stimulus detection, variability of visual field border, and saccade amplitudes. RESULTS: Perimetric performance during stable fixation was comparable to that during eye movement. All subjects showed 92%-96% correct detections of the fixation controls and a stable and comparable blind spot position in the stable and moving fixation spot conditions. The eye tracker revealed that 97% of the time the eyes were positioned within +/-1 from fixation. CONCLUSIONS: Visual fields obtained by perimetry while moving the eyes is comparable to standard perimetry in which a stable fixation spot minimizes eye movements.

Coherent nonlinear optical response of single quantum dots studied by ultrafast near-field spectroscopy.

The nonlinear response of single GaAs quantum dots is studied in femtosecond near-field pump-probe experiments. At negative time delays, transient reflectivity spectra show pronounced oscillatory structure around the quantum dot exciton line, providing the first evidence for a perturbed free induction decay of the excitonic polarization. Phase-disturbing Coulomb interactions between the excitonic polarization and continuum excitations dominate the optical nonlinearity on ultrafast time scales. A theoretical analysis based on the semiconductor Bloch equations accounts for this behavior.