Sound pulses in lipid membranes and their potential function in biology

Migrasome formation is initiated preferentially in tubular junctions by membrane tension

Migrasomes, the vesicle-like membrane microstructures, arise on the retraction fibers (RFs), the branched nanotubules pulled out of cell plasma membranes during cell migration and shaped by membrane tension. Migrasomes form in two steps: a local RF bulging is followed by a protein-dependent stabilization of the emerging spherical bulge. Here, we addressed theoretically and experimentally the previously unexplored mechanism of bulging of membrane tubular systems. We assumed that the bulging could be driven by increases in membrane tension and experimentally verified this hypothesis in live-cell and biomimetic systems. We exposed RF-generating live cells to a hypotonic medium, which produced water flows into the cells and a related increase in the membrane tension. We observed the formation of migrasome-like bulges with a preferential location in the RF branching sites. Next, we developed a biomimetic system of three membrane tubules pulled out of a giant plasma membrane vesicle (GPMV), connected by a junction, and subjected to pulling forces controlled by the GPMV membrane tension. An abrupt increase in the GPMV tension resulted in the generation of migrasome-like bulges mainly in the junctions. To understand the physical forces behind these observations, we considered theoretically the mechanical energy of a membrane system consisting of a three-way tubular junction with emerging tubular arms subjected to membrane tension. Substantiating our experimental observations, the energy minimization predicted a tension increase to drive the formation of membrane bulges, preferably in the junction site, independently of the way of the tension application. We generalized the model to derive universal criteria of bulging in branched membrane tubules.

Intracellular pressure controls the propagation of tension in crumpled cell membranes

Propagation of membrane tension mediates mechanical signal transduction along surfaces of live cells and sets the time scale of mechanical equilibration of cell membranes. Recent studies in several cell types and under different conditions revealed a strikingly wide variation range of the tension propagation speeds including extremely low ones. The latter suggests a possibility of long-living inhomogeneities of membrane tension crucially affecting mechano-sensitive membrane processes. Here, we propose, analyze theoretically, and support experimentally a mechanism of tension propagation in membranes crumpled by the contractile cortical cytoskeleton. The tension spreading is mediated by the membrane flow between the crumples. We predict the pace of the tension propagation to be controlled by the intra-cellular pressure and the degree of the membrane crumpling. We provide experimental support for the suggested mechanism by monitoring the rate of tension propagation in cells exposed to external media of different osmolarities.

Cell Membrane Tension Gradients, Membrane Flows, and Cellular Processes

Cell membrane tension affects and is affected by many fundamental cellular processes, yet it is poorly understood. Recent experiments show that membrane tension can propagate at vastly different speeds in different cell types, reflecting physiological adaptations. Here we briefly review the current knowledge about membrane tension gradients, membrane flows, and their physiological context.

Mechanotransduction through membrane tension: It's all about propagation?

Over the past 25 years, membrane tension has emerged as a primary mechanical factor influencing cell behavior. Although supporting evidences are accumulating, the integration of this parameter in the lifecycle of cells, organs, and tissues is complex. The plasma membrane is envisioned as a bilayer continuum acting as a 2D fluid. However, it possesses almost infinite combinations of proteins, lipids, and glycans that establish interactions with the extracellular or intracellular environments. This results in a tridimensional composite material with non-trivial dynamics and physics, and the task of integrating membrane mechanics and cellular outcome is a daunting chore for biologists. In light of the most recent discoveries, we aim in this review to provide non-specialist readers some tips on how to solve this conundrum.

Information propagated by longitudinal pulses near a van der Waals phase transition

Longitudinal pulses that propagate in a medium near a van der Waals phase transition have a sigmoidal dependence on the strength of the stimulus arising from the phase structure. This response resembles the all-or-nothing property of action potentials, which raises the question if an acoustic system near a phase transition can be suitable for material-based neuromorphic computation. Herein, we investigate how information about the stimulus is stored within these pulses. We find that (1) the pulse propagates in parallel both digital and analog information about the stimulus amplitude; (2) the pulse encodes the type of stimulus, for instance, mechanical or thermal; and (3) a collision between two pulses stores information about both stimuli and may be used as a fading memory. Our results unravel a rich encoding of information in a phenomenon that is both common in a plethora of materials and mimics neuronal signaling. In addition, we show that these pulses carry more information than is typically considered by models of neural computation. Therefore, this phenomenon is an excellent candidate for in materio computation.

Characterizing Oscillatory and Excitability Regimes in a Protein-Free Lipid Membrane

Observations of electric potential oscillations in artificial lipid bilayers near the order-disorder transition indicate the existence of a stable limit cycle and, therefore, the possibility of producing excitable signals close to the bifurcation. We present a theoretical investigation of membrane oscillatory and excitability regimes induced by an increase in ion permeability at the order-disorder transition. The model accounts for the coupled effects of state-dependent permeability, membrane charge density, and hydrogen ion adsorption. A bifurcation diagram shows a transition between fixed-point and limit cycle solutions, enabling both oscillatory and excitability responses at different values of the acid association parameter. Oscillations are identified in terms of the membrane state, electric potential difference, and ion concentration near the membrane. The emerging voltage and time scales agree with measurements. Excitability is demonstrated by applying an external electric current stimulus, and the emerging signals display a threshold response and the appearance of repetitive signals upon using a long-lasting stimulus. The approach highlights the important role of the order-disorder transition, enabling membrane excitability in the absence of specialized proteins.

The thermodynamic theory of action potential propagation: a sound basis for unification of the physics of nerve impulses

The thermodynamic theory of action potential propagation challenges the conventional understanding of the nerve signal as an exclusively electrical phenomenon. Often misunderstood as to its basic tenets and predictions, the thermodynamic theory is virtually ignored in mainstream neuroscience. Addressing a broad audience of neuroscientists, we here attempt to stimulate interest in the theory. We do this by providing a concise overview of its background, discussion of its intimate connection to Albert Einstein's treatment of the thermodynamics of interfaces and outlining its potential contribution to the building of a physical brain theory firmly grounded in first principles and the biophysical reality of individual nerve cells. As such, the paper does not attempt to advocate the superiority of the thermodynamic theory over any other approach to model the nerve impulse, but is meant as an open invitation to the neuroscience community to experimentally test the assumptions and predictions of the theory on their validity.