39 research outputs found
Impact of Lipid Oxidization on Biophysical Properties of Model Cell Membranes
The
oxidization of glycerophospholipids in cell membranes due to
aging and environmental stresses may cause a variety of pathological
and physiological consequences. A variety of oxidized phospholipid
products (OxPl) are produced by the chemical oxidization of unsaturated
hydrocarbon chains, which would significantly change the physicochemical
properties of cell membranes. In this work, we constructed cell membrane
models in the absence and presence of two stable oxidized lipid products
and investigated their impact on physical properties of supported
membranes using quartz crystal microbalance with dissipation (QCM-D)
and high-energy X-ray reflectivity (XRR). Our experimental findings
suggest that the lipid oxidization up to 20 mol % leads to the rupture
of vesicles right after the adsorption. Our XRR analysis unravels
the membrane thinning and the decrease in the lateral ordering of
lipids, which can be explained by the decrease in the lateral packing
of hydrocarbon chains. Further studies on mechanics of membranes incorporating
oxidized lipids can be attributed to the decrease in the bending rigidity
and the increase in the permeability
Impact of Lipid Oxidization on Vertical Structures and Electrostatics of Phospholipid Monolayers Revealed by Combination of Specular X‑ray Reflectivity and Grazing-Incidence X‑ray Fluorescence
The influence of phospholipid oxidization
of floating monolayers
on the structure perpendicular to the global plane and on the density
profiles of ions near the lipid monolayer has been investigated by
a combination of grazing incidence X-ray fluorescence (GIXF) and specular
X-ray reflectivity (XRR). Systematic variation of the composition
of the floating monolayers unravels changes in the thickness, roughness
and electron density of the lipid monolayers as a function of molar
fraction of oxidized phospholipids. Simultaneous GIXF measurements
enable one to qualitatively determine the element-specific density
profiles of monovalent (K<sup>+</sup> or Cs<sup>+</sup>) and divalent
ions (Ca<sup>2+</sup>) in the vicinity of the interface in the presence
and absence of two types of oxidized phospholipids (PazePC and PoxnoPC)
with high spatial accuracy (±5 Å). We found the condensation
of Ca<sup>2+</sup> near carboxylated PazePC was more pronounced compared
to PoxnoPC with an aldehyde group. In contrast, the condensation of
monovalent ions could hardly be detected even for pure oxidized phospholipid
monolayers. Moreover, pure phospholipid monolayers exhibited almost
no ion specific condensation near the interface. The quantitative
studies with well-defined floating monolayers revealed how the elevation
of lipid oxidization level alters the structures and functions of
cell membranes
Morphology and Adhesion Strength of Myoblast Cells on Photocurable Gelatin under Native and Non-native Micromechanical Environments
We have quantitatively determined
how the morphology and adhesion
strength of myoblast cells can be regulated by photocurable gelatin
gels, whose mechanical properties can be fine-tuned by a factor of
10<sup>3</sup> (0.1 kPa ≤ <i>E</i> ≤ 140 kPa).
The use of such gels allows for the investigation of mechanosensing
of cells not only near the natural mechanical microenvironments (<i>E</i> ∼ 10 kPa) but also far below and beyond of the
natural condition. Optical microscopy and statistical image analysis
revealed that myoblast cells sensitively adopt their morphology in
response to the substrate elasticity at <i>E</i> ∼
1–20 kPa, which can be characterized by the significant changes
in the contact area and order parameters of actin cytoskeletons. In
contrast, the cells in contact with the gels with lower elastic moduli
remained almost round, and the increase in the elasticity beyond <i>E</i> ∼ 20 kPa caused no distinct change in morphology.
In addition to the morphological analysis, the adhesion strength was
quantitatively evaluated by measuring the critical detachment pressure
with an aid of intensive pressure waves induced by picosecond laser
pulses. This noninvasive technique utilizing extremely short pressure
waves (pulse time width ∼100 ns) enables one to determine the
critical pressure for cell detachment with reliable statistics while
minimizing the artifacts arising from the inelastic deformation of
cells. The adhesion strength also exhibited a transition from weak
adhesion to strong adhesion within the same elasticity range (<i>E</i> ∼ 1–20 kPa). A clear correlation between
the cell morphology and adhesion strength suggests the coupling of
the strain of the substrate and the mechanosensors near focal adhesion
sites
Grazing-Incidence Neutron-Induced Fluorescence Probes Density Profiles of Labeled Molecules at Solid/Liquid Interfaces
We
report on the use of characteristic prompt γ-fluorescence
after neutron capture induced by an evanescent neutron wave to probe
densities and depth profiles of labeled molecules at solid/liquid
interfaces. In contrast to classical scattering techniques and X-ray
fluorescence, this method of “grazing-incidence neutron-induced
fluorescence” combines direct chemical specificity, provided
by the label, with sensitivity to the interface, inherent to the evanescent
wave. We demonstrate that the formation of a supported lipid membrane
can be quantitatively monitored from the characteristic fluorescence
of <sup>157</sup>Gd<sup>3+</sup> ions bound to the headgroup of chelator
lipids. Moreover, we were able to localize the <sup>157</sup>Gd<sup>3+</sup> ions along the surface normal with nanometer precision.
This first proof of principle with a well-defined model system suggests
that the method has a great potential for biology and soft matter
studies where spatial resolution and chemical sensitivity are required
Generic Role of Polymer Supports in the Fine Adjustment of Interfacial Interactions between Solid Substrates and Model Cell Membranes
To
understand the generic role of soft, hydrated biopolymers in adjusting
interfacial interactions at biological interfaces, we designed a defined
model of the cell–extracellular matrix contacts based on planar lipid
membranes deposited on polymer supports (polymer-supported membranes).
Highly uniform polymer supports made out of regenerated cellulose
allow for the control of film thickness without changing the surface
roughness and without osmotic dehydration. The complementary combination
of specular neutron reflectivity and high-energy specular X-ray reflectivity
yields the equilibrium membrane–substrate distances, which
can quantitatively be modeled by computing the interplay of van der
Waals interaction, hydration repulsion, and repulsion caused by the
thermal undulation of membranes. The obtained results help to understand
the role of a biopolymer in the interfacial interactions of cell membranes
from a physical point of view and also open a large potential to generally
bridge soft, biological matter and hard inorganic materials
Combination of MD Simulations with Two-State Kinetic Rate Modeling Elucidates the Chain Melting Transition of Phospholipid Bilayers for Different Hydration Levels
The
phase behavior of membrane lipids plays an important role in
the formation of functional domains in biological membranes and crucially
affects molecular transport through lipid layers, for instance, in
the skin. We investigate the thermotropic chain melting transition
from the ordered <i>L</i><sub>β</sub> phase to the
disordered <i>L</i><sub>α</sub> phase in membranes
composed of dipalmitoylphosphatidylcholine (DPPC) by atomistic molecular
dynamics simulations in which the membranes are subject to variable
heating rates. We find that the transition is initiated by a localized
nucleus and followed by the propagation of the phase boundary. A two-state
kinetic rate model allows characterizing the transition state in terms
of thermodynamic quantities such as transition state enthalpy and
entropy. The extrapolated equilibrium melting temperature increases
with reduced membrane hydration and thus in tendency reproduces the
experimentally observed dependence on dehydrating osmotic stress
Fine Adjustment of Interfacial Potential between pH-Responsive Hydrogels and Cell-Sized Particles
We
quantitatively determined interfacial potentials between cell-sized
particles and stimulus-responsive hydrogels using a microinterferometer.
The hydrogel is based on physically interconnected ABA triblock copolymer
micelles comprising an inner biocompatible PMPC block and two outer
pH-responsive PDPA blocks. The out-of-plane temporal fluctuation in
the position of the cell-sized particles was calculated from changes
in the interference pattern measured by Reflection Interference Contrast
Microscopy (RICM), thus yielding the particle-substrate interaction
potential <i>V</i> (Δ<i>h</i>). Measurements
in pH buffers ranging from 7.0 to 7.8 resulted in a systematic reduction
in height of the potential minima ⟨Δ<i>h</i>⟩ and a concomitant increase in the potential curvature <i>V</i>″ (Δ<i>h</i>). The experimental
data were analyzed by applying the modified Ross and Pincus model
for polyelectrolytes, while accounting for gravitation, lubrication
and van der Waals interactions. Elastic moduli calculated from <i>V</i>″ (Δ<i>h</i>) were in good agreement
with those measured by Atomic Force Microscopy. The ability to fine-tune
both the gel elasticity and the interfacial potential at around physiological
pH makes such triblock copolymer hydrogels a promising biocompatible
substrate for dynamic switching of cell–material interactions
DSC thermograms of myelin (20 Mm) under different aqueous media.
<p>In the cases where homogenization takes place–bi-distilled water (black line) and near physiological medium (red line)–a maximum is reached after which a stabilization of the C<sub>p</sub> is observed near physiological temperature. In the case of high [Ca<sup>2+</sup>] (blue line), a continuous variation of C<sub>p</sub> according to a phase redistribution of components is observed.</p
Number (<i>n</i>) of correlated membranes for the different phases as a function of temperature for whole myelin (empty) and DIGs (filled).
<p>In black is marked the <i>n</i> for the native period, in red for the expanded period and in blue for the compact period. In the presence of physiological (diamonds) and high [Ca<sup>2+</sup>] (circles) media.</p
X-ray diffraction pattern of isolated myelin as a function of temperature in Ringer’s solution.
<p>At high temperature (37–46°C) two single rings Peaks 2 and 4 are observed. At 30°C the faint smallest peak (1) is observed near the beamstop. At 25°C beam splitting is more easily observed in the peak 4 and IV which is also evident in peaks 2 and II at 10°C.</p