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Biomedical subjects

A Engel

Publications and source records attributed to A Engel.

At least 145 records · Page 8Linked to original sources
PubMed1997

Clinical and sonographic evaluation of the risk of rupture in the Achilles tendon.

Chronic pain in the region of the Achilles tendon is a common problem and often a sign of progressive degeneration of the tendon which may lead to its rupture. We studied the clinical course and sonograms in 36 patients with achillodynia to find a prognostic parameter enabling us to estimate the risk of rupture. The patients were evaluated clinically for swelling and tenderness and by high-resolution real-time sonography. The sonograms were graded according to the tendon thickness as normal (< 6 mm), minimal (6-8 mm), moderate (8-10 mm) to high-grade (> 10 mm) in the sagittal diameter of the transverse section, and alterations of echotexture were described as diffuse, circumscribed, or inhomogenous. At the time of the primary investigation, we found thickening and alterations of the echotexture in 33 of 72 tendons. In 48 tendons we found pain and local or diffuse swelling in the Achilles tendon region (sensitivity 0.58, specificity 0.79). After a follow-up of 48 +/- 8 months, 7 tendons had ruptured spontaneously. Analysis of the sonograms of the patients taken prior to the rupture showed a high-grade thickening in 4 cases, moderate thickening in 2 cases, and a diameter between 6 and 8 nm in one patient. In no case did we find a rupture of a tendon primarily classified as normal. Patients without sonographic changes exhibited a significantly better clinical outcome following conservative treatment. Sonography was found to be a valuable tool for determination of the tendon's thickness and echotexture. In 28% of our patients with thickening, circumscribed lesions of the echotexture, and chronic pain, a spontaneous rupture occurred.

Achilles Tendon↗
PubMed1997

Imaging of avascular necrosis of bone.

The etiology of avascular necrosis (AVN) is multifactorial. Independent of its etiology and localization it shows typical pathologies and radiological images. In the early stages localized subchondral edema is characteristic. In 50 % of all cases accompanying joint effusion may be found. Due to necrosis of the cells of bone marrow and bone fibrovascular, reactions with hyperemia can be delineated. These reactions allow us to visualize necrosis indirectly. The best imaging methods are MRI and, to a lesser extent, bone scintigraphy. In later stages calcification as well as new bone formation and microfractures are typically demonstrated and visualized best with plain X-rays and CT. Why reparations in many cases, particularly in the hip, are incomplete and may stop in any stage is unknown. Over years clinically complete silent AVNs are not an uncommon finding. Prognosis depends on the localization and size of the AVN. The number of repair mechanisms is best outlined with contrast-enhanced MRI and return of fatty marrow.

Bone and Bones↗
PubMed1997

Secondary structures comparison of aquaporin-1 and bacteriorhodopsin: a Fourier transform infrared spectroscopy study of two-dimensional membrane crystals.

Aquaporins are integral membrane proteins found in diverse animal and plant tissues that mediate the permeability of plasma membranes to water molecules. Projection maps of two-dimensional crystals of aquaporin-1 (AQP1) reconstituted in lipid membranes suggested the presence of six to eight transmembrane helices in the protein. However, data from other sequence and spectroscopic analyses indicate that this protein may adopt a porin-like beta-barrel fold. In this paper, we use Fourier transform infrared spectroscopy to characterize the secondary structure of highly purified native and proteolyzed AQP1 reconstituted in membrane crystalline arrays and compare it to bacteriorhodopsin. For this analysis the fractional secondary structure contents have been determined by using several different algorithms. In addition, a neural network-based evaluation of the Fourier transform infrared spectra in terms of numbers of secondary structure segments and their interconnections [sij] has been performed. The following conclusions were reached: 1) AQP1 is a highly helical protein (42-48% alpha-helix) with little or no beta-sheet content. 2) The alpha-helices have a transmembrane orientation, but are more tilted (21 degrees or 27 degrees, depending on the considered refractive index) than the bacteriorhodopsin helices. 3) The helices in AQP1 undergo limited hydrogen/deuterium exchange and thus are not readily accessible to solvent. Our data support the AQP1 structural model derived from sequence prediction and epitope insertion experiments: AQP1 is a protein with at least six closely associated alpha-helices that span the lipid membrane.

Aquaporin 1↗
PubMed1997

The height of biomolecules measured with the atomic force microscope depends on electrostatic interactions.

In biological applications of atomic force microscopy, the different surface properties of the biological sample and its support become apparent. Observed height differences between the biomolecule and its supporting surface are thus not only of structural origin, but also depend on the different sample-tip and support-tip interactions. This can result in negative or positive contributions to the measured height, effects that are described by the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory. Experimental verification shows that the electrostatic interactions between tip and sample can strongly influence the result obtained. To overcome this problem, pH and electrolyte concentration of the buffer solution have to be adjusted to screen out electrostatic forces. Under these conditions, the tip comes into direct contact with the surface of support and biological system, even when low forces required to prevent sample deformation are applied. In this case, the measured height can be related to the thickness of the native biological structure. The observed height dependence of the macromolecules on electrolyte concentration makes it possible to estimate surface charge densities.

Aquaporin 1↗
PubMed1997

High resolution imaging of native biological sample surfaces using scanning probe microscopy.

The possibility of acquiring high resolution topographs using scanning probe microscopes under physiological conditions allows the observation of biomolecules at work. Progress has recently been made in imaging protein-DNA complexes, individual oligomers and protein arrays. Scanning probe microscopes are now tools that complement X-ray crystallography and electron microscopy.

Bacterial Outer Membrane Proteins↗
PubMed1997

Electron and atomic force microscopy of membrane proteins.

Electron crystallography is becoming a powerful tool for the resolution of membrane protein structures. The past year has seen the production of a bacteriorhodopsin model at 3.5 A and the structure of aquaporin 1 approaching atomic resolution. Determination of surface topographies of 2D crystals using the atomic force microscope is similarly advancing to a level that reveals submolecular details. As the latter is operated in solution, membrane proteins can be observed at work.

Aquaporin 1↗
PubMed1997

The ATP-dependent HslVU protease from Escherichia coli is a four-ring structure resembling the proteasome.

HslVU is a new two-component protease in Escherichia coli composed of the proteasome-related peptidase HslIV and the ATPase HsIU. We have used electron microscopy and image analysis to examine the structural organization of HslV and HslU homo-oligomers and the active HslVU enzyme. Electron micrographs of HslV reveal ring-shaped particles, and averaging of top views reveal six-fold rotational symmetry, in contrast to other beta-type proteasome subunits, which form rings with seven-fold symmetry. Side views of HslV show two rings stacked together, thus, HslV behaves as dodecamer. The ATPase HslU forms ring-shaped particles in the presence of ATP, AMP-PNP or ADP, suggesting that nucleotide binding, but not hydrolysis, is required for oligomerization. Subunit crosslinking, STEM mass estimation, and analysis of HslU top views indicate that HslU exists both as hexameric and heptameric rings. With AMP-PNP present, maximal proteolytic activity is observed with a molar ratio of HslU to HslV subunits of 1:1, and negative staining electron microscopy shows that HslV and HsIU form cylindrical four-ring structures in which the HsIV dodecamer is flanked at each end by a HslU ring.

ATP-Dependent Proteases↗
PubMed1997

Cortical tuber count: a biomarker indicating neurologic severity of tuberous sclerosis complex.

The relationship between the number of cortical tubers observed by magnetic resonance imaging (MRI) and the severity of cerebral dysfunction of tuberous sclerosis patients has been examined in a meta-analysis of the published literature. The literature review has identified five independent studies for examining the association. These studies consistently reveal that the cortical tuber count detected on MRI scans is increased among those with more severe cerebral disease. Severity of the cerebral dysfunction is measured by the seizure status and its control and by the developmental status and the level of mental retardation. Meta-analysis demonstrates that within a study population, the MRI-detected cortical tuber count is six times more likely to be above the median count for tuberous sclerosis patients with severe cerebral dysfunction (poor seizure control or moderate-severe retardation or both) than more mildly affected tuberous sclerosis patients. Similarly, across studies, moderately to severely affected patients are five times more likely to have greater than seven MRI-detected cortical tubers than those more mildly affected. These associations are both statistically significant and strong. The cortical tuber count is a biomarker that reasonably predicts the severity of cerebral dysfunction of tuberous sclerosis. Cortical tubers of tuberous sclerosis form in the early gestational period. The embryologic disruption determining the clinical severity of the cortical dysfunction of tuberous sclerosis is set in the early gestational period.

Cerebral Cortex↗
PubMed1997

Histomorphology and bone morphometry of the bone marrow edema syndrome of the hip.

From a prospective study of patients with MR imaging proven bone marrow edema syndrome of the hip, bone biopsies that were retrieved at core decompression treatment of 32 femoral heads (from 28 men and 3 women; age range, 25-63 years) were evaluated microscopically. The undecalcified microtome sections showed diffuse or spotty areas of interstitial and intrasinusoidal fluid in the marrow cavities, together with fat cell destruction or fibrovascular regeneration or both in exactly the regions exhibiting the magnetic resonance signals for bone marrow edema. The vital bone trabeculae in these edematous regions showed more or less continuous, partly osteoblast covered osteoid seams, and often, formation of irregular woven bone (microcallus), pointing to increased bone formation activity. Preceding or active osteoclastic resorption was rarely seen. Computer assisted bone morphometry revealed age related normal to elevated bone volume densities (above 20% bone volume of tissue volume); thus, no evidence for osteoporosis was present. In addition to increased osteoid volumes, a decreased maximal hydroxyapatite content and a shift to undermineralized bone was found by mineral densitometry of corresponding microradiographs, when compared with age matched femoral heads without bone pathology. These bone mineral changes, but not transient bone loss, could be the explanation for the more or less subtle and transient radiolucency in hips affected by bone marrow edema syndrome. Live trabeculae and active bone formation, however, point to increased repair capacity, which seems the key for the spontaneously reversible course of this syndrome. There is still controversy whether the bone marrow edema syndrome represents a distinct transient disease or an early reversible phase of avascular necrosis, but because of the similarities in histopathology reported for early classic avascular necrosis and bone marrow edema in the literature and in the authors' own material, a common pathophysiology is discussed for these seemingly different diseases.

Adult↗
PubMed1996

Surface topographies at subnanometer-resolution reveal asymmetry and sidedness of aquaporin-1.

Aquaporin-1 (AQP1) is an abundant protein in human erythrocyte membranes which functions as a specific and constitutively active water conducting pore. Solubilized and isolated as tetramer, it forms well-ordered two-dimensional (2D) crystals when reconstituted in the presence of lipids. Several high resolution projection maps of AQP1 have been determined, but information on its three-dimensional (3D) mass distribution is sparse. Here, we present surface reliefs at 0.9 nm resolution that were calculated from freeze-dried unidirectionally metal-shadowed AQP1 crystals as well as surface topographs recorded with the atomic force microscope of native crystals in buffer solution. Our results confirm the 3D map of negatively stained AQP1 crystals, which exhibited tetramers with four major protrusions on one side and a large central cavity on the other side of the membrane. Digestion of AQP1 crystals with carboxypeptidase Y, which cleaves off a 5 kDa intracellular C-terminal fragment, led to a reduction of the major protrusions, suggesting that the central cavity of the tetramer faces the outside of the cell. To interpret the results, sequence based structure predictions served as a guide.

Aquaporin 1↗
PubMed1996

Highly ordered two-dimensional crystals of photosystem I reaction center from Synechococcus sp.: functional and structural analyses.

The photosystem 1 reaction center complex from the thermophilic cyanobacterium Synechococcus sp. was isolated by Triton X-100 solubilization and fractional precipitation with polyethylene glycol. As shown by gel electrophoresis, the isolated complex was composed of the 83 kDa subunits A and B, and at least six other subunits with molecular mass below 20 kDa. Electron transfer from the primary electron donor P700 to the FA/FB centers was demonstrated by flash-induced absorption change of the isolated complex, while electron paramagnetic resonance (EPR) spectroscopy showed that the complex contained a full set of Fe-S clusters. Isolated complexes were reconstituted into two-dimensional crystals in the presence of phospholipids and different cations. The crystals were found to be active by flash-induced separation and EPR spectroscopy. Electron microscopy and digital image processing of negatively stained and frozen-hydrated specimens revealed orthorhombic crystals with unit cell dimensions a = 138 A, b = 145 A and p12(1) symmetry. A three-dimensional map was calculated for negatively stained crystals to 19 A resolution, whereas the projection map of frozen-hydrated crystals exhibited 8 A resolution.

Crystallization↗
PubMed1996

Tubular crystals of a photosystem II core complex.

An oxygen evolving photosystem II core complex containing all three extrinsic proteins (33, 23, 17 kDa) was isolated from spinach and reconstituted into tubular two-dimensional crystals of 72.9 nm diameter and 1-2 micrometers length. While the 17 and 23 kDa polypeptides were lost during crystallization, the extrinsic 33 kDa protein was retained. The optical spectrum of the crystallized core was characteristic of an intact PSII core complex. Immunoelectron microscopy revealed that the lumenal surface of the PSII complex was exposed at the outside of the cylindrical tubes. The projection of the complex was determined from flattened tubular crystals by negative stain electron microscopy and image analysis to 2.0 nm resolution. Rhombic unit cells (a = 16.2 nm, b = 13.7 nm; gamma = 142.4 degrees) contained one PSII complex.

Crystallization↗
PubMed1996

The micelle to vesicle transition of lipids and detergents in the presence of a membrane protein: towards a rationale for 2D crystallization.

The assembly of two-dimensional membrane protein crystals in the presence of lipids was analyzed with quasielastic light scattering and electron microscopy. Mixtures of detergent-solubilized lipids and/or proteins were submitted to slow or rapid dilution while measuring the hydrodynamic radii of the aggregates. Lipids alone exhibited lambda-shaped dilution curves with intermediate rod-shaped particles that converted into small vesicles. Depending on the protein-protein and protein-lipid interactions, detergent-solubilized protein-lipid mixtures showed a sharp transition from micelles to large densely packed proteoliposomes. Electron microscopy revealed that formation of crystals occurred shortly after this phase transition.

Crystallization↗
PubMed1996

Immuno-atomic force microscopy of purple membrane.

The atomic force microscope is a useful tool for imaging native biological structures at high resolution. In analogy to conventional immunolabeling techniques, we have used antibodies directed against the C-terminus of bacteriorhodopsin to distinguish the cytoplasmic and extracellular surface of purple membrane while imaging in buffer solution. At forces > or = 0.8 nN the antibodies were removed by the scanning stylus and the molecular topography of the cytoplasmic purple membrane surface was revealed. When the stylus was retracted, the scanned membrane area was relabeled with antibodies within 10 min. The extracellular surface of purple membrane was imaged at 0.7 nm resolution, exhibiting a major and a minor protrusion per bacteriorhodopsin monomer. As confirmed by immuno-dot blot analysis and sodium dodecyl sulfate-gel electrophoresis, labeling of the purple membrane was not observed if the C-terminus of bacteriorhodopsin was cleaved off by papain.

Amino Acid Sequence↗