Correlative Imaging of Motoneuronal Cell Elasticity by Pump and Probe Spectroscopy.

Because of their role of information transmitter between the spinal cord and the muscle fibers, motor neurons are subject to physical stimulation and mechanical property modifications. We report on motoneuron elasticity investigated by time-resolved pump and probe spectroscopy. A dual picosecond geometry simultaneously probing the acoustic impedance mismatch at the cell-titanium transducer interface and acoustic wave propagation inside the motoneuron is presented. Such noncontact and nondestructive microscopy, correlated to standard atomic force microscopy or a fluorescent labels approach, has been carried out on a single cell to address some physical properties such as bulk modulus of elasticity, dynamical longitudinal viscosity, and adhesion.

Surface-induced assembly of sophorolipids.

The surface self-assembly properties of acidic sophorolipids, a bolaform microbial glycolipids with pH-responsive properties in solution, were studied based on the chemical nature of the support and pH of the solution. Sophorolipids generally form micelles in water but formation of morphologies like platelets and twisted fibers depending on pH have also been reported. The surface self-assembly was achieved using dip-coating on three different substrates namely gold, silicon(111) and TiO2 anatase. Deposition conditions (dip-coating withdrawal speed, relative humidity, temperature) were tested, and it was found that optimum self-assembly occurs at a withdrawal speed of 1 mm s-1, T of 25 °C and relative humidity of 25%. The local structure of the sophorolipid films was characterized by atomic force microscopy, while scanning electron microscopy was used to characterize the spatial homogeneity. We also attempted to correlate dispersive, electron donor and electron attractor surface energy components, using Good-van Oss's approach, and the behavior of sophorolipids. We found that when the surface energy is dominated by dispersive components, sophorolipids spontaneously assemble into entangled needles at all pH values (4, 6 and 11). However, when the surface energy is dominated by electronic components, pH has a strong influence on the surface self-assembly. We could discriminate three major organizations: homogeneous layer, isolated aggregates and a two-dimensional fibrillar network.

Substrate-induced PC12 cell differentiation without filopodial, lamellipodial activity or NGF stimulationa.

Nanoscale gradients in energy of adhesion are physical cues from the extracellular environment that can significantly affect cell functions and enhance the neuronal differentiation of PC12 cells. How such surface effects can trigger differentiation and initiate neurite outgrowth, remains to be elucidated. Here we used surface modification, atomic force microscopy and immunofluorescence to analyze PC12 cells. We studied the kinetics of neurites growth under cytochalasin-B treatment, known as an inhibitor of actin polymerization. We found that neither filopodia nor lamellipodia are involved in detecting the surface effects that induce the differentiation of PC12 cells. This finding suggests that the solution to this problem lies beyond identifying a precise cytoskeleton-associated cell-substrate intermediate. Thus, a more comprehensive model is probably required to identify the mechanism by which cell-substrate interactions are eventually translated into a differentiation signal.

Interplay between long- and short-range interactions drives neuritogenesis on stiff surfaces.

Substrate factors such as surface energy distribution can affect cell functions, such as neuronal differentiation of PC12 cells. However, the surface effects that trigger such cell responses need to be clarified and analyzed. Here we show that the total surface tension is not a critical parameter. Self-assembled monolayers of alkylsiloxanes on glass were used as culture substrates. By changing the nanoscale structure and ordering of the monolayer, we designed surfaces with a range of dispersive (γ(d) ) and nondispersive (γ(nd) ) potentials, but with a similar value for total free-energy (50 ≤ γ(d) + γ(nd) ≤ 55 mN m⁻¹). When seeded on surfaces displaying γ(d) /γ(nd) ≤ 3.7, PC12 cells underwent low level of neuritogenesis. On surfaces exhibiting γ(d) /γ(nd) ≥ 5.4, neurite outgrowth was greatly enhanced and apparent by only 24 h of culture in absence of nerve growth-factor treatment. These data indicate how the spatial distribution of surface potentials may control neuritogenesis, thus providing a new criterion to address nerve regeneration issues on rigid biocompatible surfaces.

Neuronal adhesion and differentiation driven by nanoscale surface free-energy gradients.

Recent results indicate that, in addition to chemical, spatial and mechanical cues, substrate physical cues such as gradients in surface energy may also impact cell functions, such as neuronal differentiation of PC12 cells. However, it remains to be determined what surface effect is the most critical in triggering PC12 cell differentiation. Here we show that, beyond continuously probing the surface energy landscape of their environment, PC12 cells are highly sensitive to nanoscale chemical heterogeneities. Self-assembled monolayers of alkylsiloxanes on glass were used as a culture substrate. By changing the structure, ordering and chemical nature of the monolayer, the surface energy distribution is altered. While both well-ordered CH(3) terminated substrates and bare glass (OH terminated) substrates did not favor PC12 cell adhesion, PC12 cells seeded on highly disordered CH(3)/OH substrates underwent enhanced adhesion and prompt neuritogenesis by 48 h of culture, without nerve growth factor treatment. These data illustrate that surface free-energy gradients, generated by nanoscale chemical heterogeneities, are critical to biological processes such as nerve regeneration on biomaterials.

Influence of surface energy distribution on neuritogenesis.

PC12 cells are a useful model to study neuronal differentiation, as they can undergo terminal differentiation, typically when treated with nerve growth factor (NGF). In this study we investigated the influence of surface energy distribution on PC12 cell differentiation, by atomic force microscopy (AFM) and immunofluorescence. Glass surfaces were modified by chemisorption: an aminosilane, n-[3-(trimethoxysilyl)propyl]ethylendiamine (C(8)H(22)N(2)O(3)Si; EDA), was grafted by polycondensation. AFM analysis of substrate topography showed the presence of aggregates suggesting that the adsorption is heterogeneous, and generates local gradients in energy of adhesion. PC12 cells cultured on these modified glass surfaces developed neurites in absence of NGF treatment. In contrast, PC12 cells did not grow neurites when cultured in the absence of NGF on a relatively smooth surface such as poly-L-lysine substrate, where amine distribution is rather homogeneous. These results suggest that surface energy distribution, through cell-substrate interactions, triggers mechanisms that will drive PC12 cells to differentiate and to initiate neuritogenesis. We were able to create a controlled physical nano-structuration with local variations in surface energy that allowed the study of these parameters on neuritogenesis.

Layering and prewetting in experimental systems: an assessment cross-referred to current theories.

The first aim of this review is to assess the experimental data available about surface phase transitions at the contact between fluid bulk phases and either their vapour in the case of liquids (free interfaces) or solid substrates in general and to test their "universal" behaviour out of any consideration about critical laws, which constitute a topic by themselves. As, here, the level under consideration is only the microscopic one, wetting transitions are excluded. Our second goal is to compare these experimental data to their description by models, which are far more numerous. This imposed us to privilege, here, Cahn's and DFT theories because their way of reasoning is very close to classical thermodynamics and so, the discussion of the experimental data under examination here is facilitated with no exclusion of decisive contribution by other kinds of theories. A thorough analysis of available experiments and theoretic works makes layering appear as a general and non-wetting-dependent phenomenon exhibited by simple gases or gas mixtures on solids and also by liquid mixtures. On the other hand, though prewetting is not deeply different in essence, it is fully wetting-dependent. Both phenomena can be found in systems where interactions are very different by nature and, according to very recent models and experiments, the interplay between these interactions may lead to intertwining between both surface phenomena.

Analytical approach for the Lucas-Washburn equation.

Porous media can be characterized by studying the kinetics of liquid rise within the pore spaces. Although porous media generally have a complex structure, they can be modeled as a single, vertical capillary or as an assembly of such capillaries. The main difficulties lie in separately estimating the effective mean radius of the capillaries and the contact angle between the liquid and the pore. In this paper we circumvent these obstacles by exploring another approach and suggest an analytical approach of the classical Lucas-Washburn equation (LWE). Specifically, we consider that the contact angle between the liquid meniscus and the inner surface of the capillary becomes a dynamic contact angle when the liquid front is in movement. It has previously been demonstrated that the resulting time dependence is due to frictional dissipation at the moving wetting front.