Jean-Marc Allain

Summary: Plants respond to mechanical stimuli by a rapid increase in cytosolic calcium. The intensity and kinetics of the calcium changes define calcium signatures important for biological responses. In this study, we determine the properties of a calcium‐permeable force‐gated channel localized at the plasma membrane called Rapid Mechanically Activated (RMA). Using patch clamp and pressure clamp, we characterized the kinetics of the Arabidopsis thaliana RMA channel upon stimulation by pressure pulses applied onto the plasma membrane. Combining pressure pulse protocols at different frequencies with modeling, we investigated the channel's capacity to transduce high frequency mechanical stimuli. The RMA channel rapidly activates in response to membrane tension, then it inactivates during prolonged stimulation. Upon repeated stimulations, the RMA current amplitude decreases irreversibly indicating that it undergoes attenuation. The channel kinetics were modeled with four chemical states and the model predicts that it behaves as a pass band filter in the 10 Hz–1 kHz range. In conclusion, due to its activation/inactivation, the RMA channel is a candidate for mediating cytosolic calcium signaling in response to mechanostimulation. Its attenuation and filtering properties suggest its involvement in the transduction of high frequency mechanical stimulation, such as those produced by insects' vibrations.

Summary: Plants respond to mechanical stimuli by a rapid increase in cytosolic calcium. The intensity and kinetics of the calcium changes define calcium signatures important for biological responses . In this study, we determine the properties of a calcium permeable force-gated channel localized at the plasma membrane called Rapid Mechanically Activated (RMA). Using patch-clamp and pressure-clamp, we characterized the kinetics of activation and inactivation of RMA channel upon stimulation by pulses of pressure applied onto the plasma membrane. Combining repetitive pressure pulse protocols at different frequencies with modeling, we investigated the channel’s capacity to transduce high frequency mechanical stimuli. RMA channel rapidly activates in response to membrane tension, then it inactivates during prolonged stimulation. Upon repeated stimulations, RMA current amplitude decreases irreversibly indicating that undergoes adaptation. The channel kinetics may be modeled with four chemical states and the model predicts that it behaves as a pass band filter in the 10 Hz - 1 kHz range. In conclusion, due to its activation/inactivation characteristics, RMA channel is a candidate to mediate cytosolic calcium signaling in response to mechano-stimulation. Its adaptation and filtering properties suggest its involvement in the transduction of high frequency mechanical stimulation such as those produced by insects’ vibrations.

Everyday experience shows that cellular sheets are stiffened by the presence of a pressurized gas: from bicycle inner tubes to bubble wrap, the presence of an internal pressure increases the stiffness of otherwise floppy structures. The same is true of plants, with turgor pressure (due to the presence of water) taking the place of gas pressure; indeed, in the absence of water, many plants wilt. However, the mechanical basis of this stiffening is somewhat opaque: simple attempts to rationalize it suggest that the stiffness should be independent of the pressure, at odds with everyday experience. Here, we study the mechanics of sheets that are a single-cell thick and show how a pressure-dependent bending stiffness may arise. Our model rationalizes observations of turgor-driven shrinkage in plant cells and also suggests that turgor is unlikely to provide significant structural support in many monolayer leaves, such as those found in mosses. However, for such systems, turgor does provide a way to control leaf shape, in accordance with observations of curling upon drying of moss leaves. Guided by our results, we also present a biomimetic actuator that uncurls upon pressurization.

Biological tissues have complex mechanical properties, which are deeply related to their function. It is thus needed to quantify the volumetric displacements induced by mechanical load, rather than only surface displacements. In this article, we propose to use Optical Coherence Tomography as a simple and fast method to quantify the volumetric deformation of the cornea under pressure. Indeed, the cornea plays a key role in vision, and any mechanical defect can impact the eye as it is under the intraocular pressure. Our observations show that, in the tangential direction, the cornea deforms more in the posterior than in the anterior region. In the depth direction, we observed a strong compression, with depth-dependent heterogeneity, delineating three distinct regions. This strong compression can be explained only by important outward water fluxes. Our study shows the complexity of human corneal mechanics, highlighting the need of adequate volumetric measure to characterize its strong anisotropy and depth-dependent behavior.

The cornea, as the outer element of the human eye, plays a pivotal role in vision. Any defects in its shape can result in visual impairments. Mechanical defect manifest as shape defects, as the cornea is under to pressure. Our study presents the first comprehensive observation of human corneal deformation throughout its entire thickness during an inflation test, through Optical Coherence Tomography (OCT). Horizontal deformation reveals depth-dependent heterogeneity, suggesting that the cornea's posterior part is softer than the anterior part. Vertical deformation was observed at levels significantly higher than expected and exhibited depth-dependent heterogeneity, delineating three distinct regions. The central region of the cornea initially experienced rapid swelling, possibly due to osmotic effects, followed by increasing compressions as pressure rose. Conversely, the anterior and posterior regions showed no sign of swelling, and a difference in the compressive response, possibly due to difference in stiffness. Our study shows the complexity of human corneal mechanics, highlighting strong anisotropy and depth-dependent behavior, and implicating osmotic and poroelastic properties.

BMP-1/tolloid-like proteinases (BTPs) are major players in tissue morphogenesis, growth and repair. They act by promoting the deposition of structural extracellular matrix proteins and by controlling the activity of matricellular proteins and TGF-β superfamily growth factors. They have also been implicated in several pathological conditions such as fibrosis, cancer, metabolic disorders and bone diseases. Despite this broad range of pathophysiological functions, the putative existence of a specific endogenous inhibitor capable of controlling their activities could never be confirmed. Here, we show that procollagen C-proteinase enhancer-2 (PCPE-2), a protein previously reported to bind fibrillar collagens and to promote their BTP-dependent maturation, is primarily a potent and specific inhibitor of BTPs which can counteract their proteolytic activities through direct binding. PCPE-2 therefore differs from the cognate PCPE-1 protein and extends the possibilities to fine-tune BTP activities, both in physiological conditions and in therapeutic settings.

Hypercoagulability is a pathology that remains difficult to explain today in most cases. It is likely due to a modification of the conditions of polymerization of the fibrin, the main clot component. Using passive microrheology, we measured the mechanical properties of clots and correlated them under the same conditions with structural information obtained with confocal microscopy. We tested our approach with known alterations: an excess of fibrinogen and of coagulation Factor VIII. We observed simultaneously a rigidification and densification of the fibrin network, showing the potential of microrheology for hypercoagulability diagnosis.