AFM: A Nanotool in Membrane Biology

Intermittent contact mode AFM investigation of native plasma membrane of Xenopus laevis oocyte

Intermittent contact mode atomic force microscopy (AFM) was used to visualize the native plasma membrane of Xenopus laevis oocytes. Oocyte membranes were purified via ultracentrifugation on a sucrose gradient and adsorbed on mica leaves. AFM topographs and the corresponding phase images allowed for visualization and identification of both oocyte plasma membrane patches and pure lipid bilayer regions with a height of about 5 nm within membrane patches. The quantitative analysis showed a normal distribution for the lateral dimension and height of the protein complexes centered on 16.7 +/- 0.2 nm (mean +/- SE, n = 263) and 5.4 +/- 0.1 nm (n = 262), respectively. The phase signal, providing material-dependent information, allowed for the recognition of structural features observed in AFM topographs.

Substrate binding tunes conformational flexibility and kinetic stability of an amino acid antiporter

We used single molecule dynamic force spectroscopy to unfold individual serine/threonine antiporters SteT from Bacillus subtilis. The unfolding force patterns revealed interactions and energy barriers that stabilized structural segments of SteT. Substrate binding did not establish strong localized interactions but appeared to be facilitated by the formation of weak interactions with several structural segments. Upon substrate binding, all energy barriers of the antiporter changed thereby describing the transition from brittle mechanical properties of SteT in the unbound state to structurally flexible conformations in the substrate-bound state. The lifetime of the unbound state was much shorter than that of the substrate-bound state. This leads to the conclusion that the unbound state of SteT shows a reduced conformational flexibility to facilitate specific substrate binding and a reduced kinetic stability to enable rapid switching to the bound state. In contrast, the bound state of SteT showed an increased conformational flexibility and kinetic stability such as required to enable transport of substrate across the cell membrane. This result supports the working model of antiporters in which alternate substrate access from one to the other membrane surface occurs in the substrate-bound state.

Acute hyperoxia increases lipid peroxidation and induces plasma membrane blebbing in human U87 glioblastoma cells

Atomic force microscopy (AFM), malondialdehyde (MDA) assays, and amperometric measurements of extracellular hydrogen peroxide (H(2)O(2)) were used to test the hypothesis that graded hyperoxia induces measurable nanoscopic changes in membrane ultrastructure and membrane lipid peroxidation (MLP) in cultured U87 human glioma cells. U87 cells were exposed to 0.20 atmospheres absolute (ATA) O(2), normobaric hyperoxia (0.95 ATA O(2)) or hyperbaric hyperoxia (HBO(2), 3.25 ATA O(2)) for 60 min. H(2)O(2) (0.2 or 2 mM; 60 min) was used as a positive control for MLP. Cells were fixed with 2% glutaraldehyde immediately after treatment and scanned with AFM in air or fluid. Surface topography revealed ultrastructural changes such as membrane blebbing in cells treated with hyperoxia and H(2)O(2). Average membrane roughness (R(a)) of individual cells from each group (n=35 to 45 cells/group) was quantified to assess ultrastructural changes from oxidative stress. The R(a) of the plasma membrane was 34+/-3, 57+/-3 and 63+/-5 nm in 0.20 ATA O(2), 0.95 ATA O(2) and HBO(2), respectively. R(a) was 56+/-7 and 138+/-14 nm in 0.2 and 2 mM H(2)O(2). Similarly, levels of MDA were significantly elevated in cultures treated with hyperoxia and H(2)O(2) and correlated with O(2)-induced membrane blebbing (r(2)=0.93). Coapplication of antioxidant, Trolox-C (150 microM), significantly reduced membrane R(a) and MDA levels during hyperoxia. Hyperoxia-induced H(2)O(2) production increased 189%+/-5% (0.95 ATA O(2)) and 236%+/-5% (4 ATA O(2)) above control (0.20 ATA O(2)). We conclude that MLP and membrane blebbing increase with increasing O(2) concentration. We hypothesize that membrane blebbing is an ultrastructural correlate of MLP resulting from hyperoxia. Furthermore, AFM is a powerful technique for resolving nanoscopic changes in the plasma membrane that result from oxidative damage.

Detection of hepatitis B virus by piezoelectric biosensor

A highly sensitive piezoelectric HBV DNA biosensor has been developed based on the sensitive mass-transducing function of the quartz crystal microbalance and the speciality of nucleic acid hybridization reaction. HBV nucleic acid probe was immobilized onto the gold electrodes of a 9 MHz AT-cut piezoelectric quartz crystal with the polyethyleneimine adhesion, glutaraldehyde cross-linking (PEI-Glu) method or the physical adsorption method. The coated crystal with the PEI-Glu method to immobilized HBV nucleic acid probe showed the better results than the physical adsorption method with respect to sensitivity reproducibility and stability. The frequency shifts of hybridization have better linear relationship with the amount of HBV DNA, when the amount was in range 0.02-0.14 microg/ml. The crystal could be regenerated nearly five times without perceptible decrease of sensitivity.