Pulsatile Gating of Giant Vesicles Containing Macromolecular Crowding Agents Induced by Colligative Nonideality

Abstract

The ability of large macromolecules to exhibit nontrivial deviations in colligative properties of their aqueous solutions is well-appreciated in polymer physics. Here, we show that this colligative nonideality subjects giant lipid vesicles containing inert macromolecular crowding agents to osmotic pressure differentials when bathed in small-molecule osmolytes at comparable concentrations. The ensuing influx of water across the semipermeable membrane induces characteristic swell-burst cycles: here, cyclical and damped oscillations in size, tension, and membrane phase separation occur en route to equilibration. Mediated by synchronized formation of transient pores, these cycles orchestrate pulsewise ejection of macromolecules from the vesicular interior reducing the osmotic differential in a stepwise manner. These experimental findings are fully corroborated by a theoretical model derived by explicitly incorporating the contributions of the solution viscosity, solute diffusivity, and the colligative nonideality of the osmotic pressure in a previously reported continuum description. Simulations based on this model account for the differences in the details of the noncolligatively induced swell-burst cycles, including numbers and periods of the repeating cycles, as well as pore lifetimes. Taken together, our observations recapitulate behaviors of vesicles and red blood cells experiencing sudden osmotic shocks due to large (hundreds of osmolars) differences in the concentrations of small molecule osmolytes and link intravesicular macromolecular crowding with membrane remodeling. They further suggest that any tendency for spontaneous overcrowding in single giant vesicles is opposed by osmotic stresses and requires independent specific interactions, such as associative chemical interactions or those between the crowders and the membrane boundary.

Solubilization kinetics determines the pulsatory dynamics of lipid vesicles exposed to surfactant

“…The semi-quantitative agreement between our model results and the experimental data confirms that: (i) the solubilization rate k is larger with increased concentration of surfactant [15,16,38]; (ii) the pore line tension γ is decreased from a typical value of 15 pN in the absence of surfactant [43], to 1.2 pN and 0.3 pN for low and high concentrations of surfactant respectively, in agreement with experimental measurements of line tension in DOPC lipid vesicles exposed to Tween 20 surfactant [26,44]; (iii) the value of the stretch modulus κ = 0.2×10 −4 N/m is one order of magnitude lower than the one reported for bursting vesicles in the absence of surfactant [8,53]. Although this value is significantly lower than the elastic stretch modulus of lipid membranes (∼ 0.2 N/m [12]), in this modeling approach κ should be regarded as an effective stretch modulus that accounts for the elastic membrane response as well as the unfolding of submicroscopic wrinkles produced by the sudden pore opening [8].…”

“…It was shown that the stochastic nature of membrane thermal fluctuations helps overcome the energy barrier for the formation of a pore in a load rate-dependent manner [8]: a membrane stretched faster breaks at a higher tension on average [4,5,8,11,13]. Such a consideration is important if one wishes to capture the long time dynamics of vesicle undergoing multiple swell burst cycles [8,53]. In the present study, however, we focus on the pore dynamics of the few first pores, where the rate dependence of the rupture tension only weakly influences the system's behavior.…”

“…Although this value is significantly lower than the elastic stretch modulus of lipid membranes (∼ 0.2 N/m [12]), in this modeling approach κ should be regarded as an effective stretch modulus that accounts for the elastic membrane response as well as the unfolding of submicroscopic wrinkles produced by the sudden pore opening [8]. The value of this effective modulus has been reported in the absence of surfactant to be 2×10 −3 N/m in the case of pure POPC vesicles [8], and 6×10 −3 N/m in the case of POPC/SM/Ch ternary lipid mixture [53]. Here, our results suggest that the presence of surfactant lowers κ an order of magnitude independently of the surfactant concentration.…”

Lipid unsaturation properties govern the sensitivity of membranes to photo-induced oxidative stress

“…GUVs were observed by bright field optical microscopy and photooxidation was induced by illumination with a 120 W Metal halide lamp with a 405±10 nm bandpass filter. GUVs undergoing swell-burst cycles induced by osmotic stress (45)(46)(47) or subject to surfactants (48,49). Based on these observations, we propose the following mechanism for the GUVs dynamics ( Fig.…”

Visualizing Tension and Growth in Model Membranes Using Optical Dyes

“…The lytic tension of DOPC vesicles has been reported to be 9.9 5 2.6 dyn/cm, so it is likely that vesicle lysis also contributes to the plateau in GP values we observe (40). Lipid vesicles relieve high levels of osmotically-induced membrane tension through a process of pore formation, intravesicular solute release, and membrane resealing (6,(41)(42)(43). This lytic process is expected to result in the release of some degree of membrane tension, and subsequent shifts in Laurdan fluorescence would not be expected to precisely report the total change in membrane tension from the initial unstressed state of the vesicle.…”