Colloidal adhesion of phospholipid vesicles: high-resolution reflection interference contrast microscopy and theory

Unravelling soft interfaces: Visualization of gel ridges

HYPOTHESIS

Soft materials, particularly elastomers, are extensively studied, but investigations into purely soft gel contact systems are limited due to their complex dual phases consisting of polymer and free liquids. While Dual Wavelength-Reflection Interference Confocal Microscopy (DW-RICM) is effective for noninvasively visualizing interfaces from a bottom view, it faces challenges in gel studies due to close refractive indices of polymeric networks and free liquids. We hypothesize that modulating the refractive index of soft gels using nanoparticles (NPs) enhances the visualization of contact zone beneath the free surface, providing insights into the configuration of phase-separated free oil within gel-on-gel contact systems.

EXPERIMENTS

Gel-on-gel contact systems were fabricated using immiscible organogels and hydrogels. Titanium dioxide (TiO2) NPs were introduced into the organogel to modulate refractive indices. Given the lack of prior studies on the hidden contact zone between gels, various techniques, including DW-RICM, side-view imaging, and inverted optical microscopy, were employed to observe and validate our findings. Comparative analyses were conducted with elastomer-on-rigid, elastomer-on-gel, and gel-on-rigid contact systems.

FINDINGS

Our investigation demonstrated that a minimal amount of TiO2 NPs effectively delineates the direct contact radius between organogel polymeric networks and hydrogel surfaces. Comparative experiments showed that TiO2 addition did not alter the gels' mechanical and surface properties but significantly enhanced information on gel contact deformation. This enhanced visualization technique has the potential to advance our understanding of adhesive contacts in gels, providing valuable insights into interface phenomena involving biological soft tissues and cells.

Effects of neutral polymers on the mechanics of red blood cell adhesion onto coated glass surfaces

BACKGROUND

Cell-cell and cell-surface adhesion modulated by water-soluble polymers continues to be of current interest, especially since prior reports have indicated a role for depletion-mediated attractive forces.

OBJECTIVE

To determine the effects of concentration and molecular mass of the neutral polymer dextran (40 kDa to 28 MDa) on the adhesion of human red blood cells (RBC) to coated glass coverslips.

METHODS

Confocal-reflection interference contrast microscopy (C-IRM), in conjunction with phase contrast imaging, was utilized to measure the adhesion dynamics and contact mechanics of RBC during the initial stages of cell contact with several types of substrates.

RESULTS

Adhesion is markedly increased in the presence of dextran with a molecular mass ⩾ 70 kDa. This increased adhesiveness is attributed to reduced surface concentration of the large polymers and hence increased attractive forces due to depletion interaction. The equilibrium deformation of adhering RBC was modeled as a truncated sphere and the calculated adhesion energies were in close agreement with theoretical results.

CONCLUSIONS

These results clearly demonstrate that polymer depletion can promote RBC adhesion to artificial surfaces and suggest that this phenomenon may play a role in other specific and non-specific cell-cell interactions, such as rouleau formation and RBC-endothelial cell adhesion.

Effect of cytoskeleton inhibitors on deadhesion kinetics of HepG2 cells on biomimetic surface

Cytochalasin-D (Cyto-D) and latrunculin-A (Lat-A) are known inhibitors of actin microfilaments and adversely affect the physiological functions of anchorage-dependent cells. Alternatively, doxorubicin (Dox), a chemotherapeutic drug is known to induce apoptosis and cell detachment of tumor cells. However, the intricate interplay between drug administration, cytoskeletal rearrangement and biophysical responses of live cells on immobilized layer of extracellular matrix (ECM) protein remains unknown. In this study, the deadhesion kinetics and actin remodeling of live HepG2 cells following the addition of the three drugs are probed with confocal reflectance interference contrast microscopy (C-RICM) and fluorescence confocal microscopy. First, it is shown that the reduction in two-dimensional spread area of HepG2 cells is 10.5%, 15.4% and 21.9% under the influence of 5 microM of Lat-A, Cyto-D and Dox, respectively. Secondly, C-RICM demonstrates the recession of strong adhesion contact against time of cell seeding upon the addition of the three drugs. Thirdly, the initial cell detachment rate and extent of reduction in the degree of cell deformation (a/R) are dependent on both the drug types and concentration. Lastly, oscillation-like responses of a/R and adhesion energy are uniquely found in Lat-A induced cell detachment. Overall, our biophysical approaches have been proven as a highly quantitative platform for elucidating the interfacial properties of adherent cells on biomimetic surfaces under cytoskeleton disruption.

Spectral domain phase microscopy for local measurements of cytoskeletal rheology in single cells

We present spectral domain phase microscopy (SDPM) as a new tool for measurements at the cellular scale. SDPM is a functional extension of spectral domain optical coherence tomography that allows for the detection of cellular motions and dynamics with nanometer-scale sensitivity in real time. Our goal was to use SDPM to investigate the mechanical properties of the cytoskeleton of MCF-7 cells. Magnetic tweezers were designed to apply a vertical force to ligand-coated magnetic beads attached to integrin receptors on the cell surfaces. SDPM was used to resolve cell surface motions induced by the applied stresses. The cytoskeletal response to an applied force is shown for both normal cells and those with compromised actin networks due to treatment with Cytochalasin D. The cell response data were fit to several models for cytoskeletal rheology, including one- and two-exponential mechanical models, as well as a power law. Finally, we correlated displacement measurements to physical characteristics of individual cells to better compare properties across many cells, reducing the coefficient of variation of extracted model parameters by up to 50%.

Adhesion contact kinetics of HepG2 cells during Hepatitis B virus replication: Involvement of SH3-binding motif in HBX

It has been shown that Hepatitis B virus (HBV) replication directly alters the expression of key cytoskeleton-associated proteins which play key roles in mechanochemical signal transduction. Nevertheless, little is known on the correlation between HBV replication and the subsequent adhesion mechanism of HBV-replicating cells. In this study, it is demonstrated that the lag time of adhesion contact evolution of HepG2 cells with HBV replication is significantly increased by two times compared to that of normal HepG2 cell on collagen coated substrate. During the initial 20 min of cell seeding, only diffuse forms of vinculin was detected in HBV replicating cells while vinculin-associated focal complexes were found in normal and control cells. Similar delay in cell adhesion in HBV-replicating cells was observed in cells transfected with HBX, the smallest HBV protein, suggesting its involvement in this cellular process. In addition, a proline rich region found in many SH3 binding proteins was identified in HBX. HBX was found to interact with the focal adhesion protein, vinexin-beta, through the SH3 binding. Furthermore, HepG2 cells with HBV replication showed evidence of cell rounding up, possibly resulting from cytoskeletal reorganizations associated with interaction between HBX and vinexin-beta. Taken together, our results suggest that HBX is involved in the cytoskeletal reorganization in response to HBV replication.

Recognition-Induced Polymersomes: Structure and Mechanism of Formation

Random polystyrene copolymers grafted with complementary recognition elements were combined in chloroform producing vesicular aggregates, that is, recognition-induced polymersomes (RIPs). Reflection interference contrast microscopy (RICM) in solution, coupled with optical microscopy (OM) and atomic force microscopy (AFM) on solid substrates, were used to determine the wall thickness of the RIPs. Rather than a conventional mono- or bilayer structure (approximately 10 or approximately 20 nm, respectively) the RIP membrane was 43+/-7 nm thick. Structural arrangement of the polymer chains on the RIP wall were characterized by using angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The interior portion of the vesicle membrane was found to be more polar, containing more recognition units, than the exterior part. This gradient suggests that a rapid self-sorting of polymers takes place during the formation of RIPs, providing the likely mechanism for vesicle self-assembly.

Temperature dependence of vesicle adhesion

The influence of thermal fluctuations on the adhesion behavior of fluid vesicles is investigated with the help of Monte Carlo simulations. The adhesion area A(ad) of a fluid vesicle adhering to a smooth attractive substrate is studied systematically for different values of temperature, adhesion strength, and potential range. For low temperatures T , the ratio A(ad) /A between the adhesion area and the total area A of the vesicle is a linear function of T/kappa , where kappa is the bending rigidity. Linear fits of the simulation data allow an extrapolation to T=0 which corresponds well with data obtained from a simplified analytic model. A new ansatz for A(ad) (T) which is based on the eigenmodes of the adhering vesicle explains the linear behavior of A(ad) (T) for low T and helps to define a fit function which reproduces the linear behavior of the obtained simulation data. This fit function may be used in order to determine the bending rigidity and the adhesion strength from the observed adhesion geometry.

Effect of acyl chain mismatch on the contact mechanics of two-component phospholipid vesicle during main phase transition

It has been recently demonstrated that acyl chain mismatch of phospholipid bilayer composed of a binary lipid mixture induces component formation on the lateral plane of the bilayer [Biophys. J. 83 (2002) 1820-1883]. In this report, the contact mechanics of unilamellar vesicles composed of binary dimyristoyl-phosphatidylcholine (DMPC)/dipalmitoyl-phosphocholine (DPPC) mixtures on fused silica and amino-modified substrates is simultaneously probed by confocal-reflectance interference contrast microscopy (C-RICM) and cross-polarized light microscopy during gel to liquid crystalline transition of the lipid bilayer. C-RICM results indicate that the average degree of vesicle deformation for DMPC-rich and DPPC-rich vesicles adhering on fused silica substrate is increased by 30% and 14%, respectively, in comparison with that in pure DMPC and DPPC vesicles. Also, lateral heterogeneity induced by acyl chain mismatch increases the average magnitude of adhesion energy in DMPC-rich and DPPC-rich vesicles of all sizes by 6.4 times and 2.3 times, respectively. Similar modulation of adhesion mechanics induced by carbon chain difference is obtained on amino-modified substrate. Most importantly, the thermotropic transition of the mixed bilayer from gel (below T(m)) to fluid phase (above T(m)) further exemplifies the effect of acyl chain mismatch on the increases of degree of vesicle deformation and adhesion energy.

Interaction of liposome with immobilized chitosan during main phase transition

It has been recently demonstrated that chitosan in aqueous solution alters the phase behavior and structure of a phospholipid bilayer (Fang, N.; et al. Biomacromolecules 2001, 2, 1161-1168). Until now, the physical driving forces between chitosan and the phospholipid bilayer upon their initial encounter remains unknown. In this study, confocal reflectance interference contrast microscopy (C-RICM), phase contrast microscopy and bioadhesion modeling are concurrently applied to probe the interaction of phospholipid vesicle with immobilized chitosan at various temperatures, pH, and osmotic stress. First, the successful immobilization of chitosan on amino-silanized glass is indicated by the increases in both the degree of vesicle deformation and adhesion energy of vesicles adhering on chitosan modified substrate in comparison with those on amino-silanized glass. Second, the phase transition of a phospholipid bilayer does not modulate the adhesion strength at the chitosan-biomembrane interface at pH 7.4. With increase of the degree of protonation on the chitsoan backbone at pH 4, the adhesion energy is increased by 5-fold for vesicles of all sizes compared to that in pH 7.4. Furthermore, pH reduction amplifies the thermal-induced response of larger vesicles on the immobilized chitosan layer. Interestingly, a moderate increase of osmotic stress maximizes the degree of vesicle deformation and adhesion energy at 23 degrees C and dampens the effect of phase transition on vesicle adhesion. Overall, this study demonstrates the quantitation of chitosan-biomembrane interactions that will be critical for future applications of chitosan in biological systems.

Thermal induced modification of the contact mechanics of adhering liposomes on cationic substrate

The correlation between the mechanical property and the thermotropic transition of the phospholipid bilayer has been recently demonstrated (Chem. Phys. Lipids 110 (2001) 27). However, the role of thermal induced mechanical responses of phospholipid bilayer on the contact mechanics of liposome adhering on a cationic substrate has not been determined. In this study, confocal-reflectance interference contrast microscopy, phase contrast microscopy and contact mechanics modeling are applied to probe the adhesion mechanisms of liposomes in the presence of electrostatic interactions during the thermotropic transition of the lipid bilayer. When temperature increases from 23 to 49 degrees C at pH 7.4, the degree of liposome deformation (a/R) and adhesion energy of dipalmitoyl-sn-glycero-3-phosphocholine liposome increases by 10% and remains constant, respectively, on 3-amino-propyl-triethoxy-silane (APTES) modified substrate. The extents of increase in these two parameters are highly dependent on the physicochemical properties of the rigid substrate. At pH 4, the adhesion energies above and below the phase transition temperature (T(m)) are increased by one order of magnitude due to the formation of the free silanol groups on APTES substrate. In hypotonic condition, the degree of vesicle deformation remains constant and the adhesion energy reduces by 20% during sample heating. Under all conditions, the adhesion energy of the adhering liposome spans a few orders of magnitude against the increase of liposome size as the surface area to volume ratio is maximized in smallest vesicle.

Substrate-induced deformation and adhesion of phospholipid vesicles at the main phase transition

The physiochemical properties of phospholipid vesicle, e.g. permeability, elasticity, etc., are directly modulated by the chain-melting transition of the lipid bilayer. Currently, there is a lack of understanding in the relationship between thermotropic transition, mechanical deformation and adhesion strength for an adherent vesicle at temperature close to main phase transition temperature T(m). In this study, the contact mechanics of dimyristoyl-phosphatidylcholine (DMPC) vesicle at the main phase transition are probed by confocal reflectance interference contrast microscopy in combination with phase contrast microscopy. It is shown that DMPC vesicles strongly adhere on pure fused silica substrate at T(m) and the degree of deformation as well as the adhesion energy is a decreasing function against the mid-plane diameter of the vesicles. Furthermore, an increase of osmotic pressure at the gel/liquid crystalline phase co-existence imposes insignificant changes in both the degree of deformation and adhesion energy of adherent vesicles when the lipid bilayer permeability is maximized. With the reverse of substrate charge, the mechanical deformation and adhesion strength for larger vesicles (mid-plane diameter >18 microm) are significantly reduced. By monitoring the parametric response of substrate-induced vesicle adhesion during main phase transition, it is shown that the degree of deformation and adhesion energy of adhering vesicle is increased and unchanged, respectively, against the increase of temperature.