An atomic force microscopy investigation of protein crystal surface topography

Tapping mode atomic force microscopy was employed to study the surface structure of different protein crystals in a liquid environment. The (101) face of hen egg-white lysozyme crystals and the (111) face of horse spleen ferritin crystals were studied. On the (101) face of lysozyme crystals we observed islands delimitated by micro-steps and elongated in the [010] direction. The elongation direction coincides with the preferential growth direction predicted by a growth model reported in the literature. The islands observed on the ferritin (111) face are also delimitated by micro-steps but have circular symmetry. Sectioning of the images allowed us to measure the step heights. The surface free energy was estimated from the growth step morphology. Molecular resolution was achieved for ferritin crystals, showing a hexagonal surface packing, as expected for the molecular lattice of a (111) face in a fcc crystal.

Role of advanced glycation end products in aging collagen. A scanning force microscope study

The collagen structure of young and old rats was examined using a scanning force microscope (SFM). Rat tail tendons of 8- and 24-month-old Wistar rats were frayed by two blades and examined using a Nanoscope III SFM. In the same tendons, the pentosidine concentrations, a marker of the Maillard reaction, were determined by HPLC. The SFM inspection of native fibrils produces images of collagen bundles, with parallel fibrils. The diameters of old rat collagen fibrils were large in comparison to the young ones. Moreover, fibrils obtained from old rats exhibited the same band interval, while the depth of the gap between two overlap zones showed a higher mean value with respect to young collagen. The pentosidine concentration was also higher in the old than in the young tendons. In conclusion, in the presence of an increased concentration of advanced glycation end products, significant structural alterations have been observed in old fibrils.

Scanning force microscopy reveals structural alterations in diabetic rat collagen fibrils: role of protein glycation

BACKGROUND

The main functional property of collagen is to provide a supporting framework to almost all tissues: the effects of non-enzymatic glycation on this protein are deleterious and in diabetes mellitus contribute to the mechanism of late complications. The aim of this work is to provide evidence by scanning force microscopy of modifications in collagen structure caused by high glucose concentration, in vivo and in vitro, and to correlate the data with markers of non-enzymatic glycation.

METHODS

Tendon fibrils were obtained from the tails of 8-month-old rats (BB/WOR/MOL¿BB) which developed diabetes spontaneously at least 12 weeks before they were killed, and from diabetes-resistant rats of the same strain (BB/WOR/MOL¿WB). A scanning force microscope (SFM; Nanoscope III) equipped with a Contact Mode Head was used for imaging. Band interval, diameter and depth of D-band gap were measured in non-diabetic and diabetic tail tendon fibrils and in fibrils incubated with glucose (0.5 M for 2 weeks). Fructosamine was determined in the tendon fibrils by a colorimetric method and pentosidine was evaluated in acid-hydrolyzed samples by coupled reverse phase-ionic exchange column HPLC.

RESULTS

Incubated fibrils revealed modifications in radius (228+/-5 nm) and gap depth (3.65+/-0.10 nm) that closely reproduce diabetes-induced damage (236+/-3 and 3.20+/-0.04 nm respectively) and were significantly different from the pattern seen in non-diabetic fibrils (151+/-1 and 2.06+/-0.03 nm; p<0.001). Both fructosamine and pentosidine were higher in diabetic (3.82+/-1.43 nmol/mg and 2.23+/-0.24 pmol/mg collagen respectively) and in glucose-incubated fibrils (9.27+/-0.55 nmol/mg and 5.15+/-0.12 pmol/mg collagen respectively) vs non-diabetic tendons (1.29+/-0.08 nmol/mg and 0.88+/-0.11 pmol/mg collagen respectively; p<0.01); during the time course of incubation, an early increase in fructosamine was seen, whereas pentosidine increased later. The D-band parameter was similar in all three groups, indicating that axial organization is not modified by non-enzymatic glycation.

CONCLUSION

This is the first description obtained with SFM of diabetes-induced ultrastructural changes in collagen fibrils. Moreover, the data presented are consistent with the concept that chronic exposure of collagen to glucose in vivo or in vitro leads to similar structural modifications in collagen fibrils, probably through crosslinks. The correlation between morphologic parameters and both markers of glycation provides strong evidence for a crucial role of this non-enzymatic modification.

Structure of rat tail tendon collagen examined by atomic force microscope

The Atomic Force Microscope (AFM) was used to inspect collagen fibrils deposited on mica sheets at different fibrillogenesis times. Collagen was obtained from rat tail tendon fibers. Various fibril forms were observed, together with the characteristic periodic intra-fibril structure (D-bands). The fibril thickness, width, D-band periodicity and depth were measured and the statistical distribution of these parameters at 1, 2, 5, 10 and 15 days of in vitro fibril formation time was calculated. The fibrils showed an increasing size with time, but the band interval measure remained stable. The band depth, after an initial increase, exhibited a relative steadiness. The results indicate that AFM offers, at low resolution, images qualitatively similar to those obtained with electron microscopy, but with less manipulation of the sample. A quantitative evaluation of collagen structural features in the nanometer scale is made possible by AFM.

Measurements of surface roughness: use of a CCD camera to correlate doubly scattered speckle patterns

We describe an instrument, built around a commercial CCD camera and some fast image-processing boards, that evaluates roughness height by measuring the average size of doubly scattered speckle patterns. The device is a variant of a recent proposal that was based on the use of a spatial modulator to perform the Fourier transform of a speckle image. In the present setup, the Fourier transform is replaced by the direct evaluation of a second-order correlation function. Strictly speaking, the device proposed in this paper is not a real-time device but its response time (approximately 10 s) is sufficiently short to be of practical value for many applications. Updated CCD cameras that will significantly improve the performance of our prototype are already on the market.

Study of aging rat tail collagen using atomic force microscope

Collagen structure of young and old rats was examined by using atomic force microscope (AFM) images. Rat tail tendons of eight and twenty-four month-old Wistar rats were digested enzymatically (pepsin), and allowed to refibrillate for 24 hours at 37 degrees C. The samples were examined using a Nanoscope III (Digital Instruments, Santa Barbara, CA, U.S.A.) with a J scanning head and a 200 microns silicon nitride cantilever. The study was performed in air and without filters. The AFM inspection of refibrillated collagen produced images showing long fibrils with relatively homogeneous heights and widths, characterized by clear banding with a periodic interval (D band) of 67 nm. With respect to collagen extracted from young rats, collagen extracted from old rats revealed fibrils exhibiting the same band interval, but with lower widths and heights. Furthermore, the depth of gap between two overlaps showed a higher mean value in the aged rats. These data are consistent with biochemical reports of collagen modifications during aging; we suggest that post-synthetic reactions might be responsible for this as they interfere with the refibrillation process and also modify the three-dimensional structure of fibrils.

Age-related increase of collagen fluorescence in human subcutaneous tissue

Nonenzymatic glycation produces compounds with characteristic fluorescence in long-lived proteins. We recently described the influence of age in rat collagen-linked fluorescence. To examine the effect of age in humans, we studied the subcutaneous collagen-linked fluorescence in samples from 26 subjects of both sexes (age range, 42 to 78 years) who were undergoing vascular surgery. Intensity of fluorescence at 385 nm (upon excitation at 335 nm) and 440 nm (upon excitation at 370 nm) increased exponentially with age (r = .827, y = 114 + e0.038x, P less than .001; and r = .905, y = 36 + e0.039x, P less than 0.001, respectively). The two sets of data exhibited a high degree of correlation (r = .980, P less than .001, n = 26). Age-adjusted fluorescence data did not correlate with sex, body weight, or type of vascular pathology. The collagen fluorescence accumulation rate was 3.7% per year, and the characteristic time (CT) was 26 to 27 years. We conclude that the fluorescence measurement is a reliable methodology that can be used as a marker for biological age until new, more-specific tools are available.

Correlation of collagen-linked fluorescence and tendon fiber breaking time

We review in this paper some reported data on age-dependent modification of proteins pointing out the relationship between the increase of nonenzymatic glycation in abdominal skin collagen of Wistar rats, evaluated by fluorescence intensity, and tendon breaking time, used as a parameter of collagen stiffness. Fluorescence intensity data linearly correlate with the breaking times of collagen fibers from Wistar rats reported from different sources, according to the hypothesis of a common etiological mechanism. It is possible to suppose that posttranslational modifications of proteins play a role in the tissue aging and their level in collagen may be used as a parameter for quantitation.

Prevention of diabetes-increased aging effect on rat collagen-linked fluorescence by aminoguanidine and rutin

Products from the advanced Maillard reaction, which increase during aging and diabetes, may contribute to the development of the typical pathology of aging and diabetes. These compounds are detectable only by their characteristic fluorescence, and few data based on long-term studies are available. For this reason, we studied subcutaneous skin collagen fluorescence in 57 nondiabetic (10- to 110-wk-old) and 74 streptozocin-induced diabetic (10- to 22-wk-old) rats. An exponential increase (r = 0.969, P less than 0.001) of collagen-linked fluorescence (excitation at 370 nm, emission at 440 nm) was observed with aging; after a lag, diabetes induced an earlier dramatic elevation of the fluorescence, suggesting a more complicated phenomenon than simple accumulation. To prevent such increases, the effects of 1 g.kg-1.day-1 aminoguanidine, suggested to be an inhibitor of the advanced glycosylation reaction, and 1 g.kg-1.day-1 rutin, an aldose reductase inhibitor, in drinking water were tested. Both treatments had a significant lowering effect on collagen fluorescence in diabetic rats. The mechanisms by which aminoguanidine and rutin prevent the accumulation of fluorescence are unknown, but these observations raise the question of whether they could be identical. If fluorescence is a marker for age-related pathologies and diabetic sequelae, aminoguanidine and rutin could have therapeutic effects in their prevention.

Adsorption of gamma-aminobutyric acid to phosphatidylserine membranes

The interaction of the negatively-charged phosphatidylserine (PS) and gamma-Aminobutyric acid (GABA) is examined in black lipid membranes (BLM) and inverse micelles. GABA does not permeate through PS membranes and, in concentrations of 10(-5)-10(-4) M, it reduces the negative potential at the membrane-aqueous solution interface. The effect is owing to the adsorption of the GABA cationic species and the consequent decrease of the negative surface charge density of the membrane. When the intrinsic pH of the membrane-solution interface is considered, the Gouy-Chapman-Stern theory describes the GABA screening effect and makes it possible to calculate the GABA-PS binding constant. This value is compared with that obtained measuring the partition of 14C-GABA between an organic phase containing PS and the aqueous solution. The results presented strongly suggest that the electrostatic force plays a major role in GABA-PS interaction.

Leptinotoxin-h action in synaptosomes, neurosecretory cells, and artificial membranes: stimulation of ion fluxes

Leptinotoxin-h (LPTx), a neurotoxin (otherwise designated beta-leptinotarsin-h) known to stimulate the release of neurotransmitters from synapses, was purified from the hemolymph of the potato beetle, Leptinotarsa haldemani, by a simplification of the procedure originally developed by Crosland et al. [Biochemistry 23, 734-741, (1984)]. Highly and partially purified preparations of the toxin were applied to guinea pig synaptosomes and neurosecretory (PC12) cells. When applied in a Ca2+-containing Ringer medium, at concentrations in the 10(-11) - 10(-10) M range, the toxin induced: (a) rapid depolarization of the plasma membrane, which was not inhibited by organic blockers of voltage-dependent Na+ and Ca2+ channels (tetrodotoxin or verapamil); (b) large 45Ca influx; and (c) increased free cytosolic Ca2+ concentration. These latter two effects were unaffected by verapamil. In Ca2+-free media the effects of the toxin were different in the two systems investigated. In synaptosomes, depolarization was still observed, even if the toxin concentrations needed were higher (approximately 10X) than those effective in the complete medium. In contrast, in PC12 cells no effect of the toxin on membrane potential was observed. Binding of LPTx to its cellular targets could not be investigated directly because the toxin was inactivated by the procedures used for its labeling. Indirect evidence suggested however that Ca2+ is necessary for toxin binding to PC12 cells. Interaction of LPTx with air/water interfaces, as well as with cholesterol/phospholipid mono- and bilayer membranes was investigated. The results indicate that the toxin has affinity for hydrophobic surfaces, but lacks the capacity to insert across membranes unless transpositive voltage is applied. Our results are inconsistent with the previous conclusion of Crosland et al. (1984), who suggested opening of the Ca2+ channel as the mechanism of action of LPTx. The effects of the toxin resemble those of alpha-latrotoxin (alpha-LTx) of the black widow spider venom, and therefore the two toxins might act by similar mechanisms. However, the sites recognized by the two toxins might be different, because LPTx does not inhibit alpha-LTx binding.