Biquet-Bisquert, A. | Search bioRxiv

bioRxiv2023

Mechanical characterization of regenerating Hydra tissue spheres

Hydra vulgaris, long known for its remarkable regenerative capabilities, is also a longstanding source of inspiration for models of spontaneous patterning. Recently, it became clear that early patterning during Hydra regeneration is an integrated mechano-chemical process where morphogen dynamics is influenced by tissue mechanics. One roadblock to understand Hydra self-organization is our lack of knowledge about the mechanical properties of these organisms. In this paper, we combined microfluidic developments to perform parallelized microaspiration rheological experiments and numerical simulations to characterize these mechanical properties. We found three different behaviors depending on the applied stresses: an elastic response, a visco-elastic one and tissue rupture. Using models of deformable shells, we quantify their Youngs modulus, shear viscosity as well as the critical stresses required to switch between behaviors. Based on these experimental results, we propose a description of the tissue mechanics during normal regeneration. Our results provide a first step towards the development of original mechano-chemical models of patterning grounded in quantitative, experimental data. Statement of significanceHydra vulgaris is a remarkable organism thanks to its regenerative abilities. One can cut this animal into several pieces which will reform a full Hydra in a few days. In this process, the pieces have to define a new organizing axis. Recently, researchers have shown that this axis definition is under mechanical control. One roadblock to understand the relationship between tissue mechanics and Hydra biology is our lack of knowledge about the mechanical state of this organism. Here, we perform a mechanical characterization using a combination of microaspiration setups and numerical simulations. We finally propose a description of what happens at the mechanical level during Hydra regeneration, allowing quantitative approaches questioning the role of mechanical cues in axis definition.

biophysics↗

bioRxiv2023

Spatio-temporal dynamics of the proton motive force on single bacterial cells

Electrochemical gradients established across biological membranes are fundamental in the bioenergetics of all forms of life. In bacteria, the proton motive force (PMF), the electrochemical potential associated to protons, powers an impressive array of fundamental processes, from ATP production to motility. While far from equilibrium, it has classically been considered homeostatic in time and space. Yet, recent experiments have revealed rich temporal dynamics at the single cell level and functional spatial dynamics at the scale of multicellular communities. Lateral segregation of supramolecular respiratory complexes begs the question of whether spatial heterogeneity of the PMF exists even at the single cell level. By using a light-activated proton pump as a spatially and temporally modulatable source, and the bacterial flagellar motor as a local electro-mechanical gauge, we both perturb and probe the PMF on single cells. Using global perturbations, we resolve temporal dynamics on the ms time scale and observe an asymmetrical capacitive response of the cell. Using localized perturbations, we find that the PMF is rapidly homogenized along the entire cell, faster than proton diffusion can allow. Instead, the electrical response can be explained in terms of electrotonic potential spread, as found in passive neurons and described by cable theory. This implies a global coupling between PMF sources and consumers in the bacterial membrane, excluding a sustained spatial heterogeneity while allowing for fast temporal dynamics. SignificanceStoring energy in the form of a proton gradient across a membrane is a fundamental feature of living systems. In mitochondria, spatial compartmentalization separates electrically distinct regions. In bacteria, it is unclear how this energy reservoir, the proton motive force, behaves at the single cell level: can it be heterogeneous in space as in mitochondria? How fast can it change in time? Using a light-driven proton pump and the flagellar motor as a local electro-mechanical gauge, we find that the bacterial proton motive force can change in a few tens of milliseconds, and that it is instantaneously homogenized along the membrane. This electrophysiological response is surprisingly similar to electrotonic voltage spread in passive neurons.

biophysics↗