Flexoelectricity and the entropic force between fluctuating fluid membranes

Biological membranes undergo noticeable thermal fluctuations at physiological temperatures. When two membranes approach each other, they hinder the out-of-plane fluctuations of the other. This hindrance leads to an entropic repulsive force between membranes which, in an interplay with attractive and repulsive forces owing to other sources, affects a range of biological functions: cell adhesion, membrane fusion, self-assembly, binding–unbinding transition among others. In this work, we take cognizance of the fact that biological membranes are not purely mechanical entities and, owing to the phenomenon of flexoelectricity, exhibit a coupling between deformation and electric polarization. The ensuing coupled mechanics–electrostatics–statistical mechanics problem is analytically intractable. We use a variational perturbation method to analyze, in closed form, the contribution of flexoelectricity to the entropic force between two fluctuating membranes and discuss its possible physical implications. We find that flexoelectricity leads to a correction that switches from an enhanced attraction at close membrane separations and an enhanced repulsion when the membranes are further apart.

Viscosity of fluid membranes measured from vesicle deformation

Viscosity is a key mechanical property of cell membranes that controls time-dependent processes such as membrane deformation and diffusion of embedded inclusions. Despite its importance, membrane viscosity remains poorly characterized because existing methods rely on complex experimental designs and/or analyses. Here, we describe a facile method to determine the viscosity of bilayer membranes from the transient deformation of giant unilamellar vesicles induced by a uniform electric field. The method is non-invasive, easy to implement, probe-independent, high-throughput, and sensitive enough to discern membrane viscosity of different lipid types, lipid phases, and polymers in a wide range, from 10−8 to 10−4 Pa.s.m. It enables fast and consistent collection of data that will advance understanding of biomembrane dynamics.

Physical mechanisms of amyloid nucleation on fluid membranes

Biological membranes can dramatically accelerate the aggregation of normally soluble protein molecules into amyloid fibrils and alter the fibril morphologies, yet the molecular mechanisms through which this accelerated nucleation takes place are not yet understood. Here, we develop a coarse-grained model to systematically explore the effect that the structural properties of the lipid membrane and the nature of protein–membrane interactions have on the nucleation rates of amyloid fibrils. We identify two physically distinct nucleation pathways—protein-rich and lipid-rich—and quantify how the membrane fluidity and protein–membrane affinity control the relative importance of those molecular pathways. We find that the membrane’s susceptibility to reshaping and being incorporated into the fibrillar aggregates is a key determinant of its ability to promote protein aggregation. We then characterize the rates and the free-energy profile associated with this heterogeneous nucleation process, in which the surface itself participates in the aggregate structure. Finally, we compare quantitatively our data to experiments on membrane-catalyzed amyloid aggregation of α-synuclein, a protein implicated in Parkinson’s disease that predominately nucleates on membranes. More generally, our results provide a framework for understanding macromolecular aggregation on lipid membranes in a broad biological and biotechnological context.

Melatonin added to freezing diluent improves canine (Bulldog) sperm cryosurvival

Cryopreservation compromises the capacity of sperm fertilizing due to a series of alterations in the structure and physiology of the sperm. The use of antioxidants, such as melatonin, added to freezing media, may help to reduce sperm cryoinjury. To test the effect of melatonin on Bulldog (Canis lupus familiaris) sperm cryosurvival, spermatozoa were diluted in a standard freezing medium and cooled to 5°C. Then, more freezing medium was added to obtain 200 × 106 cells/mL, and 5% glycerol. Diluted spermatozoa were treated with melatonin (0.0, 0.0005, 0.002, and 0.0035 mol/L), and packaged in 0.25 mL straws, which were further cooled to −5°C before freezing in liquid nitrogen. Thawing was carried out at 70°C for 5 s, and the progressive motility, viability, plasma membrane integrity, acrosome integrity, capacitation status, and plasma membrane fluidity of the spermatozoa (at 37°C) were assessed. Data were analyzed using ANOVA to detect differences between the melatonin doses. There were statistical differences (P < 0.05) in the percentage of sperm having hyper-fluid membranes, intact acrosome, capacitated acrosome-intact, and acrosome-reacted. The values for the high melatonin doses (0.002 and 0.0035 mol/L) were better than for the low melatonin doses (0.0 and 0.0005 mol/L). In conclusion, 0.002 and 0.0035 mol/L of melatonin improved the cryosurvival of sperm from male bulldogs. Lay summary Preservation of sperm by freezing enables breeding of individuals geographically separated; protocols for the dog may be used to preserve the semen from threatened wild canids. To improve fertility of female dogs that become pregnant with frozen and then defrosted sperm, these cells must survive that process which can be damaging whilst keeping their ability to fertilize. Antioxidants are substances capable of retarding or preventing the oxidation of any oxidizing substrate such as lipids, proteins, and DNA, which are structural compounds of the sperm. The use of antioxidants, added to freezing media, may provide the sperm the capacity to neutralize oxidative compounds, such as reactive oxygen species, produced during the freezing and thawing process. In this work we tested different levels of melatonin, a natural antioxidant, on dog (English Bulldog) sperm survival and quality after freezing. We found that adding melatonin to the freezing media improved sperm quality after thawing.