On the Aggregation of Apolipoprotein A-I

In vivo, apolipoprotein A-I (ApoA-I) is commonly found together with lipids in so-called lipoprotein particles. The protein has also been associated with several diseases—such as atherosclerosis and amyloidosis—where insoluble aggregates containing ApoA-I are deposited in various organs or arteries. The deposited ApoA-I has been found in the form of amyloid fibrils, suggesting that amyloid formation may be involved in the development of these diseases. In the present study we investigated ApoA-I aggregation into amyloid fibrils and other aggregate morphologies. We studied the aggregation of wildtype ApoA-I as well as a disease-associated mutant, ApoA-I K107Δ, under different solution conditions. The aggregation was followed using thioflavin T fluorescence intensity. For selected samples the aggregates formed were characterized in terms of size, secondary structure content, and morphology using circular dichroism spectroscopy, dynamic light scattering, atomic force microscopy and cryo transmission electron microscopy. We find that ApoA-I may form globular protein-only condensates, in which the α-helical conformation of the protein is retained. The protein in its unmodified form appears resistant to amyloid formation; however, the conversion into amyloid fibrils rich in β-sheet is facilitated by oxidation or mutation. In particular, the K107Δ mutant shows higher amyloid formation propensity, and the end state appears to be a co-existence of β-sheet rich amyloid fibrils and α-helix-rich condensates.     

A calbindin D9k mutant containing a novel structural extension: 1H nuclear magnetic resonance studies

Calbindin D9k is a small, well-studied calcium-binding protein consisting of two helix-loop-helix motifs called EF-hands. The P43MG2 mutant is one of a series of mutants designed to sequentially lengthen the largely unstructured tether region between the two EF-hands (F36-S44). A lower calcium affinity for P43MG was expected on the basis of simple entropic arguments. However, this is not the case and P43MG (-97 kJ.mol-1) has a stronger calcium affinity than P43M (-93 kJ.mol-1), P43G (-95 kJ.mol-1) and even wild-type protein (-96 kJ.mol-1). An NMR study was initiated to probe the structural basis for these calcium-binding results. The 1H NMR assignments and 3JHNH alpha values of the calcium-free and calcium-bound form of P43MG calbindin D9k mutant are compared with those of P43G. These comparisons reveal that little structure is formed in the tether regions of P43MG(apo), P43G(apo) and P43G(Ca) but a helical turn (S38-K41) appears to stabilize this part of the protein structure for P43MG(Ca). Several characteristic NOEs obtained from 2D and 3D NMR experiments support this novel helix. A similar, short helix exists in the crystal structure of calcium-bound wild-type calbindin D9k-but this is the first observation in solution for wild-type calbindin D9k or any of its mutants.     

Maps of the stress distributions in drying latex films

We report on spatially resolved measurements of the mechanical stress in drying polymer films. The technique is based on the deflection of a flexible membrane serving as the substrate. Assuming that the lateral tension of the membrane is the main source of its resistance to deformation, one can show that the local surface stress sigma f (x,y) is proportional to the vertical displacement of the membrane u z(x,y). The membrane distortion was determined by optical means. Measurements taken on drying latex dispersions revealed a maximum of film stress at the rim. The heterogeneous stress distribution often persisted after the film had become dry.     

Oscillation-induced static deflection in scanning force microscopy

Employing an atomic force microscope (AFM) in conjunction with a quartz crystal microbalance, we have investigated how a high-frequency lateral oscillation of the substrate influences the imaging process of the AFM. It was found that the time-averaged deflection of the cantilever (both vertical and lateral) changed when the oscillation of the quartz resonator was turned on. The vertical-tip-substrate distance increased, while the lateral force of sliding decreased at the same time. A mode of imaging based on this effect was demonstrated. The oscillation was periodically interrupted at a rate of 73 Hz and the corresponding periodic modulation of the deflection was filtered and amplified using lock-in amplifiers. Slowly scanning the sample and displaying the outputs of the lock-in amplifiers versus x and y produced an image of the oscillation-induced static (OIS) deflection. Various mechanisms by which a lateral oscillation can generate a time-averaged vertical force are discussed. The fact that the vertical OIS deflection scaled linearly with amplitude and, also, that the OIS deflection was stronger on the sloped portions of the sample than on the flat regions, suggests a geometric interpretation. We term the phenomenon "cobble stone effect." Experiments in liquids showed that the generation of an OIS deflection required immediate contact between the sample and the tip: a search for an OIS deflection in the presence of a liquid-filled gap between the sample and the tip yielded a negative result. Hydrodynamic forces are thereby ruled out as dominating factors.     

Beitr?ge zur Refraktometrie des Milchserums nach Ackermann

Ohne ZusammenfassungMitteilung aus der K. K. Allgem. Untersuchungsanstalt für Lebensmittel in Innsbruck. (Vorstand: Prof. Dr. Alois Lode.)     

Self-driven tunneling crack arrays—a 3D-fracture mechanics bifurcation analysis

Tunneling cracks driven by drying in a ceramic precursor confined between two glass plates represent a simple type of three-dimensional (3D) crack pattern. They arrange themselves via mutual unloading which causes some cracks to stop whereby the remaining ones get the right spacing for further propagation. By extending a 2D-model of self-driven propagation of crack arrays, a fracture mechanical bifurcation analysis for 3D-crack patterns based on calculating the post-critical contour of the alternating bifurcation mode has been developed. Shrinkage due to drying is replaced here by a simplified thermo-mechanical model based on an effective heat flow whose related temperature field and thermal stresses drive crack propagation. By means of the finite element method, the propagation velocity and the minimum spacing between the steady-state parallel tunnelling cracks are determined. Comparison of theory and experiment suggests that propagation may be non-stationary in these experiments. The observed relation between crack spacing and layer thickness, p ~ e ^2/3, follows from a scaling analysis.     

Alternate-day prednisone therapy in recipients of renal allografts. Risk and benefits

Forty-five patients receiving renal allografts were gradually converted from daily to alternate-day prednisone therapy. Indications for conversion included aseptic necrosis, growth retardation in children and adolescents, obesity, diabetes, cataract formation, and cosmetic appearance. Eight of the 45 patients developed acute or chronic rejection during or just after completion of alternate-day steroid therapy. The remaining 37 patients had a notable decrease in the degree of hypercorticism, with return of growth in children and adolescents. However, there was no improvement once aseptic necrosis or cataract formation had occurred. Although the use of alternate-day prednisone therapy is of benefit in reducing the untoward side effects of corticosteroids, the risk of precipitating allograft rejection is a significant threat and must be carefully considered whenever this form of treatment is undertaken.