Piezo1 helps bile on the pressure

JGP study shows that a mechanosensitive complex containing Piezo1 and Pannexin1 couples osmotic pressure to ATP secretion in bile duct cholangiocytes.

Cholangiocytes are epithelial cells that line the bile ducts within the liver and modify the composition of hepatocyte-derived bile. In this issue of JGP, Desplat et al. identify a mechanosensory complex that may help cholangiocytes respond to changes in osmotic pressure (1).Angélique Desplat (left), Patrick Delmas (center), and colleagues identify a mechanosensitive pathway that couples hypotonic stress to calcium influx and ATP release in cholangiocytes. Cell swelling induces calcium influx through the stretch-activated ion channel Piezo, triggering ATP release by Pannexin1 channels. This leads to the activation of P2X4 receptors and further calcium influx. Piezo1 (red) and Pannexin1 (green) colocalize in cells and may interact to form a mechanosensory complex that facilitates the hypotonic stress response.The activity of cholangiocytes can be regulated not only by chemical signals, such as hormones and bile acids, but also by mechanical cues arising from changes in bile composition and flow. “Abnormal mechanical tension is also an aggravating factor in many biliary diseases, including primary sclerosing cholangitis,” explains Patrick Delmas, a Research Director at Centre National de la Recherche Scientifique/Aix-Marseille-Université. “So, identifying the molecular players in cholangiocyte force sensing could provide a step forward for better management of biliary diseases.”Current models suggest that mechanical cues trigger an influx of calcium into cholangiocytes, leading to the release of ATP, which, by stimulating purinergic receptors at the cell surface, promotes further calcium influx and induces the secretion of anions, water, and HCO₃⁻ to modify the tonicity and pH of hepatic bile (2, 3). To identify mechanosensitive proteins that might regulate this pathway, Delmas and colleagues, including first author Angélique Desplat, purified mouse cholangiocytes from intrahepatic bile ducts and subjected them to hypotonic stress (1). The subsequent cell swelling activates calcium influx and ATP release.Desplat et al. found that depleting or inhibiting the stretch-activated ion channel Piezo1 significantly reduced this response to hypotonic stress. This mechanosensitive channel mediates the initial calcium influx into cholangiocytes when activated by cell swelling.The subsequent release of ATP is mediated by a different channel, however. Desplat et al. found that cholangiocytes express high levels of the gap junction family protein Pannexin1, and that pharmacologically inhibiting Pannexin1 channels reduced the amount of ATP released in response to hypotonic stress and Piezo1 activation.Delmas and colleagues suspect that the increase in intracellular calcium mediated by Piezo1 may activate Pannexin1 channels to release ATP, and this activation may be facilitated by a physical association between the two proteins: the researchers found that recombinant versions of the two channel proteins colocalize within the plasma membrane of cholangiocytes and can be coimmunoprecipitated.Finally, the researchers determined that the ATP released through Pannexin1 channels amplifies the signal initiated by hypotonic stress by activating purinergic P2X4 receptors, leading to further increases in intracellular calcium levels. Transfecting Piezo1-deficient HEK293 cells, which usually don’t respond to hypotonic stress, with cDNAs encoding Piezo1, Pannexin1, and P2X4R was sufficient to reconstitute the entire pathway of calcium influx and ATP release.Cholangiocytes express other mechanosensitive channels, including TRPV4, which has previously been implicated in the cells’ response to hypotonic stress (4). The functions of TRPV4 and Piezo1 may therefore be partially redundant, providing some robustness to cholangiocytes mechanical signaling pathways. However, it is also possible that, in vivo, the two channels respond to different stimuli and elicit distinct downstream effects. “Further investigation is warranted to better understand the respective roles of these two molecular players,” says Delmas. “To continue our work, we would like to challenge our model in vivo by testing whether Piezo1 agonists are able to regulate bile acid secretion.”     

Ultraviolet selection pressure on the earliest organisms

An examination of the probable photochemistry of the primitive reducing atmosphere of the Earth reveals a window, approximately bounded at 2400 Å to short wavelengths by H₂S, and possibly at 2700–2900 Å to longer wavelengths by aldehydes. The solar untraviolet flux in this 2600 Å window, delivered to unprotected organisms at the surface, corresponds to a contemporary mean lethal dose in ≤ 0·3 seconds. Extreme selection pressure for ultraviolet protection must have come into being. In addition to the early evolution of pyrimidine dimer ligases, catalase and peroxidase reduction mechanisms, and photoreactivation, it is suggested that two evolutionary paths developed. In one, heterotrophs lived below the oceanic thermocline and are suggested as perhaps the ancestors of the prokaryotes. In the other, organisms living near the oceanic surface surrounded themselves with ultraviolet absorbing layers of purines and pyrimidines. The resulting organisms have dimensions of tens to hundreds of microns (μm) and are suggested to be the ancestors of the eukaryotes. Subsequent specialization of this initially inert shielding, e.g., into ribosomal RNA, may have occurred.     

The pressure dependence of the lipid phase transitions. Effect of the pressure on the pre- and subtransition

The pressure dependence of the pre- and subtransitions is explained by a statistical physical model. Using this theoretical model the shift of the transition temperatures can be shown to be in agreement with the experimental results. Both the hysteresis effect which appears at standard pressure and the absence of the hysteresis at high pressures are explained.     

Effect of pressure on the self-association of melittin

The effect of increased hydrostatic pressure (1 bar to 1.8 kbar) on the self-association of melittin was measured by using the fluorescence anisotropy of its single tryptophan residue. The degree of self-association was found to decrease with increasing pressure. The volume change (delta V) for dissociation is surprisingly large. At low pressures, delta V for dissociation is near -150 mL/mol. The magnitude of the volume change decreased with increasing pressure, possibly as a result of pressure-induced compression of free volume trapped at the subunit interface region of the tetramer. Overall, the pressure-dependent association of melittin is comparable to that expected for hydrophobic interactions and to that found for micelle formation by detergents.     

Mechanistic studies on the high pressure neurological syndrome

The high pressure neurological syndrome (h.p.n.s.) constitutes a major barrier to deep sea exploration by man. Although the signs and symptoms of h.p.n.s. are well documented in both man and experimental animals, the underlying mechanisms remain to be elucidated. Physiological and pharmacological evidence will be presented that confirms that the principal sites of action of pressure are within the central nervous system (c.n.s.). Results from physiological studies not only indicate that there are separate sites of action of pressure within the c.n.s., which mediate the different components of h.p.n.s., but also the response to pressure may be controlled by descending inhibitory pathways. Pharmacological studies support this view and suggest that h.p.n.s. involves a failure of central inhibition.     

Influence of positive airway pressure on the pressure gradient for venous return in humans.

To study the effect of positive airway pressure (Paw) on the pressure gradient for venous return [the difference between mean systemic filling pressure (Pms) and right atrial pressure (Pra)], we investigated 10 patients during general anesthesia for implantation of defibrillator devices. Paw was varied under apnea from 0 to 15 cmH(2)O, which increased Pra from 7.3 +/- 3.1 to 10.0 +/- 2.3 mmHg and decreased left ventricular stroke volume by 23 +/- 22%. Episodes of ventricular fibrillation, induced for defibrillator testing, were performed during 0- and 15-cmH(2)O Paw to measure Pms (value of Pra 7.5 s after onset of circulatory arrest). Positive Paw increased Pms from 10.2 +/- 3.5 to 12.7 +/- 3.2 mmHg, and thus the pressure gradient for venous return (Pms - Pra) remained unchanged. Echocardiography did not reveal signs of vascular collapse of the inferior and superior vena cava due to lung expansion. In conclusion, we demonstrated that positive Paw equally increases Pra and Pms in humans and alters venous return without changes in the pressure gradient (Pms - Pra).     

Effect of peak pressure and pressure gradient on subsurface shear stresses in the neuropathic foot

The pressure distribution on the plantar surface of the foot may provide insights into the stresses within the subsurface tissues of patients with diabetes mellitus and peripheral neuropathy (PN) who are at risk for skin breakdown. The purposes of this study were to (1) estimate the stress distribution in the subsurface soft tissue from a measured surface pressure distribution and determine any differences between values in the forefoot and rearfoot, and (2) determine the relationship between maximum shear stress (MSS) (magnitude and depth) and characteristics of the pressure distribution. The measured in-shoe pressure distributions during walking characterized by the peak plantar pressure and maximum pressure gradient on the plantar surface of the feet for 20 subjects with diabetes, PN and history of a mid foot or forefoot plantar ulcer were analyzed. The effects of peak pressure and maximum pressure gradient at the peak pressure location on the stress components in the subsurface soft tissue were studied using a potential function method to estimate the subsurface tissue stress. The calculated MSSs are larger in magnitude and located closer to the surface in the forefoot, where most skin breakdown occurs, compared to the rearfoot. In addition, the MSS (magnitude and depth) is highly correlated with the pressure gradient (r=-0.77 & 0.61) and the peak pressure (r=-0.61 & 0.91). The peak pressure and the maximum pressure gradient obtained from the surface pressure distribution appear to be important variables to identify where MSSs are located in the subsurface tissues on the plantar foot that may lead to skin break down.     

Distribution of the seating pressure on unupholstered chairs

A majority of the population in industrialized countries does its work while sitting. The necessity of morphologically and physiologically adapting body-supporting systems to the human body cannot be limited to the use of traditional body measurements and certain inclination angles. An investigation of the sitting-pressure distribution is also necessary. The instruments developed for this purpose, the pressure-measuring chair and the sitting pressure simulator, are presented, and their results illustrated by specific examples. As a result of these investigations, sitting-pressure distribution and sitting behaviour have become test-criteria for the suitability of sears, office chairs, etc.