Architecture of the mammalian mechanosensitive Piezo1 channel

Ion Channel Pharmacology for Pain Modulation

A large series of different ion channels have been identified and investigated as potential targets for new medicines for the treatment of a variety of human diseases, including pain. Among these channels, the voltage gated calcium channels (VGCC) are inhibited by drugs for the treatment of migraine, neuropathic pain or intractable pain. Transient receptor potential (TRP) channels are emerging as important pain transducers as they sense low pH media or oxidative stress and other mediators and are abundantly found at sites of inflammation or tissue injury. Low pH may also activate acid sensing ion channels (ASIC) and mechanical forces stimulate the PIEZO channels. While potent agonists of TRP channels due to their desensitizing action on pain transmission are used as topical applications, the potential of TRP antagonists as pain therapeutics remains an exciting field of investigation. The study of ASIC or PIEZO channels in pain signaling is in an early stage, whereas antagonism of the purinergic P2X₃ channels has been reported to provide beneficial effects in chronic intractable cough. The present chapter covers these intriguing channels in great detail, highlighting their diverse mechanisms and broad potential for therapeutic utility.

The mechanosensitive Piezo1 channel: a three-bladed propeller-like structure and a lever-like mechanogating mechanism

The evolutionarily conserved Piezo proteins, including Piezo1 and Piezo2, constitute a bona fide class of mechanosensitive (MS) cation channels, which play critical roles in various mammalian physiologies, including sensation of touch, proprioception and regulation of vascular development, and blood pressure. Furthermore, mutations in Piezos have been linked to various human genetic diseases, validating their potential as therapeutic targets. Thus, it is pivotal to understand how Piezo channels effectively convert mechanical force into selective cation permeation, and therefore precisely control the various mechanotransduction processes. On the basis of our recently determined cryoelectron microscopy structures of the full-length 2547-residue mouse Piezo1, structure-guided mutagenesis, and electrophysiological and pharmacological characterizations, here we focus on reviewing the key structural features and functional components that enable Piezo1 to employ a lever-like mechanogating mechanism to function as a sophisticated mechanotransduction channel.

A Model of Piezo1-Based Regulation of Red Blood Cell Volume

A red blood cell (RBC) performs its function of adequately carrying respiratory gases in blood by its volume being ∼60% of that of a sphere with the same membrane area. For this purpose, human and most other vertebrate RBCs regulate their content of potassium (K⁺) and sodium (Na⁺) ions. The focus considered here is on K⁺ efflux through calcium-ion (Ca²⁺)-activated Gárdos channels. These channels open under conditions that allow Ca²⁺ to enter RBCs through Piezo1 mechanosensitive cation-permeable channels. It is postulated that the fraction of open Piezo1 channels depends on the RBC shape as a result of the curvature-dependent Piezo1-bilayer membrane interaction. The consequences of this postulate are studied by introducing a simple model of RBC osmotic behavior supplemented by the dependence of RBC membrane K⁺ permeability on the reduced volume (i.e., the ratio of cell volume to its maximal possible volume) of RBC discoid shapes. It is assumed that because of its intrinsic curvature and strong interaction with the surrounding membrane, Piezo1 tends to concentrate in the dimple regions of these shapes, and the fraction of open Piezo1 channels depends on the membrane curvature in that region. It is shown that the properties of the described model can provide the basis for the formation of the negative feedback loop that interrelates cell volume and its content of potassium ions. The model predicts the relation, valid for each cell in an RBC population, between RBC volume and membrane area, thus explaining the large value of the measured membrane area versus the volume correlation coefficient. The mechanism proposed here for RBC volume regulation is in accord with the loss of this correlation in RBCs of Piezo1 knockout mice.

Genotype-phenotype correlation and risk stratification in a cohort of 123 hereditary stomatocytosis patients

Hereditary stomatocytoses (HSts) are a wide spectrum of hemolytic anemias in which the erythrocyte membrane cation permeability is increased. Dehydrated hereditary stomatocytosis is the most frequent among HSts. It is caused by missense mutations in PIEZO1 and KCNN4 genes. We described 123 patients enrolled in our Genetic Unit from 2013 to 2017. Overall HSt subjects exhibit macrocytic mild anemia. We found that PIEZO1 is the most frequent mutated gene within our families (47% of pedigrees). In 59.1% of cases the mutations localized in the nonpore protein domain, while in 40.9% of patients they localized in the central pore region. The genotype-phenotype correlation analysis on 29 PIEZO1-patients demonstrated that most of severely affected patients carried mutations in the pore domain, suggesting that the severity of this condition is related to the pore properties and intracellular domain that could be responsible of interactions with intracellular components. This is the first cohort study on a large set of hereditary stomatocytosis patients, stratified according to their causative gene useful for diagnosis, prognosis, and management of these patients.

Structure of the hyperosmolality-gated calcium-permeable channel OSCA1.2

In plants, hyperosmolality stimuli triggers opening of the osmosensitive channels, leading to a rapid downstream signaling cascade initiated by cytosolic calcium concentration elevation. Members of the OSCA family in Arabidopsis thaliana, identified as the hyperosmolality-gated calcium-permeable channels, have been suggested to play a key role during the initial phase of hyperosmotic stress response. Here, we report the atomic structure of Arabidopsis OSCA1.2 determined by single-particle cryo-electron microscopy. It contains 11 transmembrane helices and forms a homodimer. It is in an inactivated state, and the pore-lining residues are clearly identified. Its cytosolic domain contains a RNA recognition motif and two unique long helices. The linker between these two helices forms an anchor in the lipid bilayer and may be essential to osmosensing. The structure of AtOSCA1.2 serves as a platform for the study of the mechanism underlying osmotic stress responses and mechanosensing.

In plants, hyperosmolality stimuli triggers opening of the osmosensitive channels, leading to a rapid downstream signaling cascade. Here, the authors solve the cryo-EM structure of an osmosensitive channel from Arabidopsis OSCA1.2 in its inactivated state.

Functional analyses of heteromeric human PIEZO1 Channels

PIEZO1 and PIEZO2 are mechanosensitive channels (MSCs) important for cellular function and mutations in them lead to human disorders. We examined how functional heteromers form between subunits of PIEZO1 using the mutants E2117K, E2117D, and E2117A. Homomers of E2117K do not conduct. E2117A homomers have low conductance with rapid inactivation, and those of E2117D have high conductance with slow inactivation. Pairing E2117K with E2117D or E2117A with E2117D gave rise to new channel species representing heteromers with distinct conductances. Whole-cell currents from co-expression of E2117A and E2117D fit well with a linear-combination model of homomeric channel currents suggesting that functional channels do not form from freely-diffusing, randomly-mixed monomers in-vitro. Whole-cell current from coexpressed PIEZO1/PIEZO2 also fit as a linear combination of homomer currents. High-resolution optical images of fluorescently-tagged channels support this interpretation because coexpressed subunits segregate into discrete domains.

A Multi-model Approach to Assessing Local and Global Cryo-EM Map Quality

There does not currently exist a standardized indicator of how well cryo-EM-derived models represent the density from which they were generated. We present a straightforward methodology that utilizes freely available tools to generate a suite of independent models and to evaluate their convergence in an EM density. These analyses provide both a quantitative and qualitative assessment of the precision of the models and their representation of the density, respectively, while concurrently providing a platform for assessing both global and local EM map quality. We further use standardized datasets to provide an expected deviation within a suite of models refined against EM maps reported to be at 5 Å resolution or better. Associating multiple atomic models with a deposited EM map provides a rapid and accessible reporter of convergence, a strong indicator of highly resolved molecular detail, and is an important step toward an FSC-independent assessment of map and model quality.

Piezo1 mediates angiogenesis through activation of MT1-MMP signaling

Angiogenesis is initiated in response to a variety of external cues, including mechanical and biochemical stimuli; however, the underlying signaling mechanisms remain unclear. Here, we investigated the proangiogenic role of the endothelial mechanosensor Piezo1. Genetic deletion and pharmacological inhibition of Piezo1 reduced endothelial sprouting and lumen formation induced by wall shear stress and proangiogenic mediator sphingosine 1-phosphate, whereas Piezo1 activation by selective Piezo1 activator Yoda1 enhanced sprouting angiogenesis. Similarly to wall shear stress, sphingosine 1-phosphate functioned by activating the Ca²⁺ gating function of Piezo1, which in turn signaled the activation of the matrix metalloproteinase-2 and membrane type 1 matrix metalloproteinase during sprouting angiogenesis. Studies in mice in which Piezo1 was conditionally deleted in endothelial cells demonstrated the requisite role of sphingosine 1-phosphate-dependent activation of Piezo1 in mediating angiogenesis in vivo. These results taken together suggest that both mechanical and biochemical stimuli trigger Piezo1-mediated Ca²⁺ influx and thereby activate matrix metalloproteinase-2 and membrane type 1 matrix metalloproteinase and synergistically facilitate sprouting angiogenesis.

Infection Augments Expression of Mechanosensing Piezo1 Channels in Amyloid Plaque-Reactive Astrocytes

A defining pathophysiological hallmark of Alzheimer's disease (AD) is the amyloid plaque; an extracellular deposit of aggregated fibrillar Aβ1-42 peptides. Amyloid plaques are hard, brittle structures scattered throughout the hippocampus and cerebral cortex and are thought to cause hyperphosphorylation of tau, neurofibrillary tangles, and progressive neurodegeneration. Reactive astrocytes and microglia envelop the exterior of amyloid plaques and infiltrate their inner core. Glia are highly mechanosensitive cells and can almost certainly sense the mismatch between the normally soft mechanical environment of the brain and very stiff amyloid plaques via mechanosensing ion channels. Piezo1, a non-selective cation channel, can translate extracellular mechanical forces to intracellular molecular signaling cascades through a process known as mechanotransduction. Here, we utilized an aging transgenic rat model of AD (TgF344-AD) to study expression of mechanosensing Piezo1 ion channels in amyloid plaque-reactive astrocytes. We found that Piezo1 is upregulated with age in the hippocampus and cortex of 18-month old wild-type rats. However, more striking increases in Piezo1 were measured in the hippocampus of TgF344-AD rats compared to age-matched wild-type controls. Interestingly, repeated urinary tract infections with Escherichia coli bacteria, a common comorbidity in elderly people with dementia, caused further elevations in Piezo1 channel expression in the hippocampus and cortex of TgF344-AD rats. Taken together, we report that aging and peripheral infection augment amyloid plaque-induced upregulation of mechanoresponsive ion channels, such as Piezo1, in astrocytes. Further research is required to investigate the role of astrocytic Piezo1 in the Alzheimer's brain, whether modulating channel opening will protect or exacerbate the disease state, and most importantly, if Piezo1 could prove to be a novel drug target for age-related dementia.

Quantitative and functional posttranslational modification proteomics reveals that TREPH1 plays a role in plant touch-delayed bolting

Environmental mechanical forces, such as wind and touch, trigger gene-expression regulation and developmental changes, called "thigmomorphogenesis," in plants, demonstrating the ability of plants to perceive such stimuli. In Arabidopsis, a major thigmomorphogenetic response is delayed bolting, i.e., emergence of the flowering stem. The signaling components responsible for mechanotransduction of the touch response are largely unknown. Here, we performed a high-throughput SILIA (stable isotope labeling in Arabidopsis)-based quantitative phosphoproteomics analysis to profile changes in protein phosphorylation resulting from 40 seconds of force stimulation in Arabidopsis thaliana Of the 24 touch-responsive phosphopeptides identified, many were derived from kinases, phosphatases, cytoskeleton proteins, membrane proteins, and ion transporters. In addition, the previously uncharacterized protein TOUCH-REGULATED PHOSPHOPROTEIN1 (TREPH1) became rapidly phosphorylated in touch-stimulated plants, as confirmed by immunoblots. TREPH1 fractionates as a soluble protein and is shown to be required for the touch-induced delay of bolting and gene-expression changes. Furthermore, a nonphosphorylatable site-specific isoform of TREPH1 (S625A) failed to restore touch-induced flowering delay of treph1-1, indicating the necessity of S625 for TREPH1 function and providing evidence consistent with the possible functional relevance of the touch-regulated TREPH1 phosphorylation. Taken together, these findings identify a phosphoprotein player in Arabidopsis thigmomorphogenesis regulation and provide evidence that TREPH1 and its touch-induced phosphorylation may play a role in touch-induced bolting delay, a major component of thigmomorphogenesis.

Cell-Type-Specific Splicing of Piezo2 Regulates Mechanotransduction

Piezo2 is a mechanically activated ion channel required for touch discrimination, vibration detection, and proprioception. Here, we discovered that Piezo2 is extensively spliced, producing different Piezo2 isoforms with distinct properties. Sensory neurons from both mice and humans express a large repertoire of Piezo2 variants, whereas non-neuronal tissues express predominantly a single isoform. Notably, even within sensory ganglia, we demonstrate the splicing of Piezo2 to be cell type specific. Biophysical characterization revealed substantial differences in ion permeability, sensitivity to calcium modulation, and inactivation kinetics among Piezo2 splice variants. Together, our results describe, at the molecular level, a potential mechanism by which transduction is tuned, permitting the detection of a variety of mechanosensory stimuli.

Piezo channels

Parpaite and Coste introduce Piezo channels and their role in mechanotransduction.

The Piezo1 cation channel mediates uterine artery shear stress mechanotransduction and vasodilation during rat pregnancy

During mammalian pregnancy, the uterine circulation must undergo substantial vasodilation and growth to maintain sufficient uteroplacental perfusion. Although we and others have shown that nitric oxide (NO) is a key mediator of these processes, the mechanisms that augment uterine artery NO signaling during gestation have not been identified. We hypothesized that Piezo1, a recently discovered cation channel, may be involved in the process of shear stress mechanotransduction, as other studies have shown that it is both mechanosensitive and linked to NO production. Surprisingly, there are no studies on Piezo1 in the uterine circulation. Our aims in the present study were to determine whether this novel channel is 1) present in uterine arteries, 2) regulated by gestation, 3) functionally relevant (able to elicit rises in intracellular Ca²⁺ concentration and vasodilation), and 4) linked to NO. Immunohistochemistry confirmed that Piezo1 is present in uterine arteries, primarily but not exclusively in endothelial cells. Western blot analysis showed that its protein expression was elevated during gestation. In pressurized main uterine arteries, pharmacological activation of Piezo1 by Yoda1 produced near maximal vasodilation and was associated with significant increases in intracellular Ca²⁺ concentration in endothelial cell sheets. Shear stress induced by intraluminal flow produced reversible vasodilations that were inhibited >50% by GsMTx-4, a Piezo1 inhibitor, and by N^ω-nitro-l-arginine methyl ester/ N^ω-nitro-l-arginine, inhibitors of NO synthase. These findings are the first to implicate a functional role for Piezo1 in the uterine circulation as a mechanosensor of endothelial shear stress. Moreover, our data demonstrate that Piezo1 activation leads to vasodilation via NO and indicate that its molecular expression is upregulated during pregnancy. NEW & NOTEWORTHY This is the first study to highlight Piezo1 in the uterine circulation. As a potentially important endothelial mechanosensor of shear stress, Piezo1 may be linked to mechanisms that support increased uteroplacental perfusion during pregnancy. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/piezo1-mechanotransduction-in-the-uterine-circulation/ .

Inactivation of Mechanically Activated Piezo1 Ion Channels Is Determined by the C-Terminal Extracellular Domain and the Inner Pore Helix

Piezo proteins form mechanically activated ion channels that are responsible for our sense of light touch, proprioception, and vascular blood flow. Upon activation by mechanical stimuli, Piezo channels rapidly inactivate in a voltage-dependent manner through an unknown mechanism. Inactivation of Piezo channels is physiologically important, as it modulates overall mechanical sensitivity, gives rise to frequency filtering of repetitive mechanical stimuli, and is itself the target of numerous human disease-related channelopathies that are not well understood mechanistically. Here, we identify the globular C-terminal extracellular domain as a structure that is sufficient to confer the time course of inactivation and a single positively charged lysine residue at the adjacent inner pore helix as being required for its voltage dependence. Our results are consistent with a mechanism for inactivation that is mediated through voltage-dependent conformations of the inner pore helix and allosteric coupling with the C-terminal extracellular domain.

Sensing and transducing forces in plants with MSL10 and DEK1 mechanosensors

Mechanosensitive (MS) channels behave as microprobes that transduce mechanical tension into electric and ion signals. The plasma membrane anion-permeable channel AtMSL10 belongs to the first family of MS channels (MscS-LIKE) that has been characterized in Arabidopsis thaliana. In the same membrane, a rapidly activated calcium MS channel activity (RMA) associated with the presence of the DEFECTIVE KERNEL1 (AtDEK1) protein has been recently described. In this Review, based on the comparison of the specific properties of AtMSL10 and RMA, we put forward hypotheses on the mechanism of activation of these two channels, their respective roles in signalling and also raise the question of the molecular identity of RMA. Finally, we propose functions for these two channels within the context of plant mechanotransduction.

Squeezing for Life - Properties of Red Blood Cell Deformability

Deformability is an essential feature of blood cells (RBCs) that enables them to travel through even the smallest capillaries of the human body. Deformability is a function of (i) structural elements of cytoskeletal proteins, (ii) processes controlling intracellular ion and water handling and (iii) membrane surface-to-volume ratio. All these factors may be altered in various forms of hereditary hemolytic anemia, such as sickle cell disease, thalassemia, hereditary spherocytosis and hereditary xerocytosis. Although mutations are known as the primary causes of these congenital anemias, little is known about the resulting secondary processes that affect RBC deformability (such as secondary changes in RBC hydration, membrane protein phosphorylation, and RBC vesiculation). These secondary processes could, however, play an important role in the premature removal of the aberrant RBCs by the spleen. Altered RBC deformability could contribute to disease pathophysiology in various disorders of the RBC. Here we review the current knowledge on RBC deformability in different forms of hereditary hemolytic anemia and describe secondary mechanisms involved in RBC deformability.

Probing the gating mechanism of the mechanosensitive channel Piezo1 with the small molecule Yoda1

Piezo proteins are transmembrane ion channels which transduce many forms of mechanical stimuli into electrochemical signals. Their pore, formed by the assembly of three identical subunits, opens by an unknown mechanism. Here, to probe this mechanism, we investigate the interaction of Piezo1 with the small molecule agonist Yoda1. By engineering chimeras between mouse Piezo1 and its Yoda1-insensitive paralog Piezo2, we first identify a minimal protein region required for Yoda1 sensitivity. We next study the effect of Yoda1 on heterotrimeric Piezo1 channels harboring wild type subunits and Yoda1-insensitive mutant subunits. Using calcium imaging and patch-clamp electrophysiology, we show that hybrid channels harboring as few as one Yoda1-sensitive subunit exhibit Yoda1 sensitivity undistinguishable from homotrimeric wild type channels. Our results show that the Piezo1 pore remains fully open if only one subunit remains activated. This study sheds light on the gating and pharmacological mechanisms of a member of the Piezo channel family.

Piezo ion channels transduce mechanical stimuli into biological signals but the underlying mechanism has remained elusive. Here, the authors use the selective agonist Yoda1 to identify molecular determinants of Piezo activation, providing mechanistic insights into Piezo-mediated mechanotransduction.

Increased Piezo1 channel activity in interstitial Cajal-like cells induces bladder hyperactivity by functionally interacting with NCX1 in rats with cyclophosphamide-induced cystitis

The Piezo1 channel is a mechanotransduction mediator, and Piezo1 abnormalities have been linked to several clinical disorders. However, the role of the Piezo1 channel in cystitis-associated bladder dysfunction has not been documented. The current study aimed to discover the functional role of this channel in regulating bladder activity during cyclophosphamide (CYP)-induced cystitis. One hundred four female rats were randomly assigned to the control, CYP-4h, CYP-48h and CYP-8d groups. CYP successfully induced acute or chronic cystitis in these rats. CYP treatment for 48h or 8d significantly increased Piezo1 channel expression in bladder interstitial Cajal-like cells (ICC-LCs), and the increase in CYP-8d rats was more prominent. In addition, 2.5 μM Grammostola spatulata mechanotoxin 4 (GsMTx4) significantly attenuated bladder hyperactivity in CYP-8d rats by inhibiting the Piezo1 channel in bladder ICC-LCs. Furthermore, by using GsMTx4 and siRNA targeting the Piezo1 channel, we demonstrated that hypotonic stress-induced Piezo1 channel activation significantly triggered Ca²⁺ and Na⁺ influx into bladder ICC-LCs during CYP-induced chronic cystitis. In addition, the Piezo1 channel functionally interacted with the relatively activated reverse mode of Na⁺/Ca²⁺ exchanger 1 (NCX1) in bladder ICC-LCs from CYP-8d rats. In conclusion, we suggest that the functional role of the Piezo1 channel in CYP-induced chronic cystitis is based on its synergistic effects with NCX1, which can significantly enhance [Ca²⁺]_i and result in Ca²⁺ overload in bladder ICC-LCs, indicating that the Piezo1 channel and NCX1 are potential novel therapeutic targets for chronic cystitis-associated bladder hyperactivity.

A protein that controls the passage of ions through cell membranes is implicated in interstitial cystitis/painful bladder syndrome (IC/PBS). This condition causes chronic pelvic pain and increased urinary frequency and urgency. Current treatment options are unsatisfactory. Researchers led by Longkun Li at the Third Military Medical University in Chongqing, China, and Mingjia Tan at the University of Michigan, Ann Arbor, USA, studied the role of this membrane channel protein, called Piezo1. Increased activity of Piezo1 was linked to bladder hyperactivity in rats with drug-induced cystitis. The research also identified a synergistic interaction between Piezo1 and a second membrane channel protein. A drug that inhibits Piezo1 activity reduced bladder hyperactivity in the rats. Drugs targeting these two proteins might help to treat the chronic cystitis of patients with IC/PBS.

Multi-gene panel testing improves diagnosis and management of patients with hereditary anemias

Mutations in more than 70 genes cause hereditary anemias (HA), a highly heterogeneous group of rare/low frequency disorders in which we included: hyporegenerative anemias, as congenital dyserythropoietic anemia (CDA) and Diamond-Blackfan anemia; hemolytic anemias due to erythrocyte membrane defects, as hereditary spherocytosis and stomatocytosis; hemolytic anemias due to enzymatic defects. The study describes the diagnostic workflow for HA, based on the development of two consecutive versions of a targeted-NGS panel, including 34 and 71 genes, respectively. Seventy-four probands from 62 unrelated families were investigated. Our study includes the most comprehensive gene set for these anemias and the largest cohort of patients described so far. We obtained an overall diagnostic yield of 64.9%. Despite 54.2% of cases showed conclusive diagnosis fitting well to the clinical suspicion, the multi-gene analysis modified the original clinical diagnosis in 45.8% of patients (nonmatched phenotype-genotype). Of note, 81.8% of nonmatched patients were clinically suspected to suffer from CDA. Particularly, 45.5% of the probands originally classified as CDA exhibited a conclusive diagnosis of chronic anemia due to enzymatic defects, mainly due to mutations in PKLR gene. Interestingly, we also identified a syndromic CDA patient with mild anemia and epilepsy, showing a homozygous mutation in CAD gene, recently associated to early infantile epileptic encephalopathy-50 and CDA-like anemia. Finally, we described a patient showing marked iron overload due to the coinheritance of PIEZO1 and SEC23B mutations, demonstrating that the multi-gene approach is valuable not only for achieving a correct and definitive diagnosis, but also for guiding treatment.

Piezo1 is a mechanically activated ion channel and mediates pressure induced pancreatitis

Merely touching the pancreas can lead to premature zymogen activation and pancreatitis but the mechanism is not completely understood. Here we demonstrate that pancreatic acinar cells express the mechanoreceptor Piezo1 and application of pressure within the gland produces pancreatitis. To determine if this effect is through Piezo1 activation, we induce pancreatitis by intrapancreatic duct instillation of the Piezo1 agonist Yoda1. Pancreatitis induced by pressure within the gland is prevented by a Piezo1 antagonist. In pancreatic acinar cells, Yoda1 stimulates calcium influx and induces calcium-dependent pancreatic injury. Finally, selective acinar cell-specific genetic deletion of Piezo1 protects mice against pressure-induced pancreatitis. Thus, activation of Piezo1 in pancreatic acinar cells is a mechanism for pancreatitis and may explain why pancreatitis develops following pressure on the gland as in abdominal trauma, pancreatic duct obstruction, pancreatography, or pancreatic surgery. Piezo1 blockade may prevent pancreatitis when manipulation of the gland is anticipated.

Manipulation of the pancreas during surgery can induce acute pancreatitis due to zymogen activation. Here the authors show that the mechanoreceptor Piezo1 is activated by pressure and its activation leads to calcium dependent pancreatic injury whereas its inhibition is protective against pancreatitis.

Activation of Mechanosensitive Transient Receptor Potential/Piezo Channels in Odontoblasts Generates Action Potentials in Cocultured Isolectin B4-negative Medium-sized Trigeminal Ganglion Neurons

INTRODUCTION

Various stimuli to the dentin surface elicit dentinal pain by inducing dentinal fluid movement causing cellular deformation in odontoblasts. Although odontoblasts detect deformation by the activation of mechanosensitive ionic channels, it is still unclear whether odontoblasts are capable of establishing neurotransmission with myelinated A delta (Aδ) neurons. Additionally, it is still unclear whether these neurons evoke action potentials by neurotransmitters from odontoblasts to mediate sensory transduction in dentin. Thus, we investigated evoked inward currents and evoked action potentials form trigeminal ganglion (TG) neurons after odontoblast mechanical stimulation.

METHODS

We used patch clamp recordings to identify electrophysiological properties and record evoked responses in TG neurons.

RESULTS

We classified TG cells into small-sized and medium-sized neurons. In both types of neurons, we observed voltage-dependent inward currents. The currents from medium-sized neurons showed fast inactivation kinetics. When mechanical stimuli were applied to odontoblasts, evoked inward currents were recorded from medium-sized neurons. Antagonists for the ionotropic adenosine triphosphate receptor (P2X₃), transient receptor potential channel subfamilies, and Piezo1 channel significantly inhibited these inward currents. Mechanical stimulation to odontoblasts also generated action potentials in the isolectin B₄-negative medium-sized neurons. Action potentials in these isolectin B₄-negative medium-sized neurons showed a short duration. Overall, electrophysiological properties of neurons indicate that the TG neurons with recorded evoked responses after odontoblast mechanical stimulation were myelinated Aδ neurons.

CONCLUSIONS

Odontoblasts established neurotransmission with myelinated Aδ neurons via P2X₃ receptor activation. The results also indicated that mechanosensitive TRP/Piezo1 channels were functionally expressed in odontoblasts. The activation of P2X₃ receptors induced an action potential in the Aδ neurons, underlying a sensory generation mechanism of dentinal pain.

Yoda1 analogue (Dooku1) which antagonizes Yoda1-evoked activation of Piezo1 and aortic relaxation

Background and Purpose

The mechanosensitive Piezo1 channel has important roles in vascular physiology and disease. Yoda1 is a small‐molecule agonist, but the pharmacology of these channels is otherwise limited.

Experimental Approach

Yoda1 analogues were generated by synthetic chemistry. Intracellular Ca²⁺ and Tl⁺ measurements were made in HEK 293 or CHO cell lines overexpressing channel subunits and in HUVECs, which natively express Piezo1. Isometric tension recordings were made from rings of mouse thoracic aorta.

Key Results

Modification of the pyrazine ring of Yoda1 yielded an analogue, which lacked agonist activity but reversibly antagonized Yoda1. The analogue is referred to as Dooku1. Dooku1 inhibited 2 μM Yoda1‐induced Ca²⁺‐entry with IC₅₀s of 1.3 μM (HEK 293 cells) and 1.5 μM (HUVECs) yet failed to inhibit constitutive Piezo1 channel activity. It had no effect on endogenous ATP‐evoked Ca²⁺ elevation or store‐operated Ca²⁺ entry in HEK 293 cells or Ca²⁺ entry through TRPV4 or TRPC4 channels overexpressed in CHO and HEK 293 cells. Yoda1 caused dose‐dependent relaxation of aortic rings, which was mediated by an endothelium‐ and NO‐dependent mechanism and which was antagonized by Dooku1 and analogues of Dooku1.

Conclusion and Implications

Chemical antagonism of Yoda1‐evoked Piezo1 channel activity is possible, and the existence of a specific chemical interaction site is suggested with distinct binding and efficacy domains.

A lever-like transduction pathway for long-distance chemical- and mechano-gating of the mechanosensitive Piezo1 channel

Piezo1 represents a prototype of eukaryotic mechanotransduction channels. The full-length 2547-residue mouse Piezo1 possesses a unique 38-transmembrane-helix (TM) topology and is organized into a three-bladed, propeller-shaped architecture, comprising a central ion-conducting pore, three peripheral blade-like structures, and three 90-Å-long intracellular beam-resembling structures that bridge the blades to the pore. However, how mechanical force and chemicals activate the gigantic Piezo1 machinery remains elusive. Here we identify a novel set of Piezo1 chemical activators, termed Jedi, which activates Piezo1 through the extracellular side of the blade instead of the C-terminal extracellular domain of the pore, indicating long-range allosteric gating. Remarkably, Jedi-induced activation of Piezo1 requires the key mechanotransduction components, including the two extracellular loops in the distal blade and the two leucine residues in the proximal end of the beam. Thus, Piezo1 employs the peripheral blade-beam-constituted lever-like apparatus as a designated transduction pathway for long-distance mechano- and chemical-gating of the pore.

Never at rest: insights into the conformational dynamics of ion channels from cryo-electron microscopy

The tightly regulated opening and closure of ion channels underlies the electrical signals that are vital for a wide range of physiological processes. Two decades ago the first atomic level view of ion channel structures led to a detailed understanding of ion selectivity and conduction. In recent years, spectacular developments in the field of cryo-electron microscopy have resulted in cryo-EM superseding crystallography as the technique of choice for determining near-atomic resolution structures of ion channels. Here, we will review the recent developments in cryo-EM and its specific application to the study of ion channel gating. We will highlight the advantages and disadvantages of the current technology and where the field is likely to head in the next few years.

Structure and mechanism of mitochondrial electron transport chain

Respiration is one of the most vital and basic features of living organisms. In mammals, respiration is accomplished by respiratory chain complexes located on the mitochondrial inner membrane. In the past century, scientists put tremendous efforts in understanding these complexes, but failed to solve the high resolution structure until recently. In 2016, three research groups reported the structure of respiratory chain supercomplex from different species, and fortunately the structure solved by our group has the highest resolution. In this review, we will compare the recently solved structures of respirasome, probe into the relationship between cristae shape and respiratory chain organization, and discuss the highly disputed issues afterwards. Besides, our group reported the first high resolution structure of respirasome and medium resolution structure of megacomplex from cultured human cells this year. Definitely, these supercomplex structures will provide precious information for conquering the mitochondrial malfunction diseases.

Voltage gating of mechanosensitive PIEZO channels

Mechanosensitive PIEZO ion channels are evolutionarily conserved proteins whose presence is critical for normal physiology in multicellular organisms. Here we show that, in addition to mechanical stimuli, PIEZO channels are also powerfully modulated by voltage and can even switch to a purely voltage-gated mode. Mutations that cause human diseases, such as xerocytosis, profoundly shift voltage sensitivity of PIEZO1 channels toward the resting membrane potential and strongly promote voltage gating. Voltage modulation may be explained by the presence of an inactivation gate in the pore, the opening of which is promoted by outward permeation. Older invertebrate (fly) and vertebrate (fish) PIEZO proteins are also voltage sensitive, but voltage gating is a much more prominent feature of these older channels. We propose that the voltage sensitivity of PIEZO channels is a deep property co-opted to add a regulatory mechanism for PIEZO activation in widely different cellular contexts.

PIEZO proteins form mechanosensitive ion channels. Here the authors present electrophysiological measurements that show that PIEZO channels are also modulated by voltage and can switch to a purely voltage gated mode, which is an evolutionary conserved property of this channel family.

The mechanosensitive Piezo1 channel: structural features and molecular bases underlying its ion permeation and mechanotransduction

The evolutionarily conserved Piezo family of proteins, including Piezo1 and Piezo2, encodes the long-sought-after mammalian mechanosensitive cation channels that play critical roles in various mechanotransduction processes such as touch, pain, proprioception, vascular development and blood pressure regulation. Mammalian Piezo proteins contain over 2500 amino acids with numerous predicted transmembrane segments, and do not bear sequence homology with any known class of ion channels. Thus, it is imperative, but challenging, to understand how they serve as effective mechanotransducers for converting mechanical force into electrochemical signals. Here, we review the recent major breakthroughs in determining the three-bladed, propeller-shaped structure of mouse Piezo1 using the state-of-the-art cryo-electron microscopy (cryo-EM) and functionally dissecting out the molecular bases that define its ion permeation and mechanotransduction properties, which provide key insights into clarifying its oligomeric status and pore-forming region. We also discuss the hypothesis that the complex Piezo proteins can be deduced into discrete mechanotransduction and ion-conducting pore modules, which coordinate to fulfil their specialized function in mechanical sensing and transduction, ion permeation and selection.

Mechanosensing Piezo channels in tissue homeostasis including their role in lungs

Piezo channels are deemed to constitute one of the most important family of mechanosensing ion channels since their discovery in 2010. With recent advances in identifying their topological structure and the discovery of the agonist Yoda1 as well as the specific inhibitor GsMTx4, it is now possible to study the mechanisms by which Piezo channels are involved in physiological and pathophysiological processes. During embryonic cardiovascular development, Piezo1 senses shear stress and promotes vasculature growth. In adult mice, Piezo1 mediates the release of nitric oxide and ATP from endothelial cells to regulate blood pressure. Piezo channels also play a crucial role in cell differentiation and tissue homeostasis by exquisite mechanical force sensing.

Piezo channels are also abundantly expressed in lung tissues. As the lung is exposed to complex pulmonary hemodynamics and respiratory mechanics, cells in the lung, such as microvascular endothelial cells, bear mechanical forces from blood flow shear, pulsatile strain, static pressure, and cyclic stretch due to respiratory movement. These mechanical stimuli are involved in a serial of physiological function and pathophysiological processes of the lung, many of which Piezo channels may be the key player. Mutation of genes encoding Piezo channels are also associated with hereditary human diseases, thus highlighting the critical role of Piezo channels in both tissue homeostasis and disease.

Myotubularin related protein-2 and its phospholipid substrate PIP2 control Piezo2-mediated mechanotransduction in peripheral sensory neurons

Piezo2 ion channels are critical determinants of the sense of light touch in vertebrates. Yet, their regulation is only incompletely understood. We recently identified myotubularin related protein-2 (Mtmr2), a phosphoinositide (PI) phosphatase, in the native Piezo2 interactome of murine dorsal root ganglia (DRG). Here, we demonstrate that Mtmr2 attenuates Piezo2-mediated rapidly adapting mechanically activated (RA-MA) currents. Interestingly, heterologous Piezo1 and other known MA current subtypes in DRG appeared largely unaffected by Mtmr2. Experiments with catalytically inactive Mtmr2, pharmacological blockers of PI(3,5)P₂ synthesis, and osmotic stress suggest that Mtmr2-dependent Piezo2 inhibition involves depletion of PI(3,5)P₂. Further, we identified a PI(3,5)P₂ binding region in Piezo2, but not Piezo1, that confers sensitivity to Mtmr2 as indicated by functional analysis of a domain-swapped Piezo2 mutant. Altogether, our results propose local PI(3,5)P₂ modulation via Mtmr2 in the vicinity of Piezo2 as a novel mechanism to dynamically control Piezo2-dependent mechanotransduction in peripheral sensory neurons.

We often take our sense of touch for granted. Yet, our every-day life greatly depends on the ability to perceive our environment to alert us of danger or to further social interactions, such as mother-child bonding. Our sense of touch relies on the conversion of mechanical stimuli to electrical signals (this is known as mechanotransduction), which then travel to brain to be processed. This task is fulfilled by specific ion channels called Piezo2, which are activated when cells are exposed to pressure and other mechanical forces. These channels can be found in sensory nerves and specialized structures in the skin, where they help to detect physical contact, roughness of surfaces and the position of our body parts.

It is still not clear how Piezo2 channels are regulated but previous research by several laboratories suggests that they work in conjunction with other proteins. One of these proteins is the myotubularin related protein-2, or Mtmr2 for short. Now, Narayanan et al. – including some of the researchers involved in the previous research – set out to advance our understanding of the molecular basis of touch and looked more closely at Mtmr2.

To test if Mtmr2 played a role in mechanotransduction, Narayanan et al. both increased and reduced the levels of this protein in sensory neurons of mice grown in the laboratory. When Mtmr2 levels were low, the activity of Piezo2 channels increased. However, when the protein levels were high, Piezo2 channels were inhibited. These results suggest that Mtmr2 can control the activity of Piezo2. Further experiments, in which Mtmr2 was genetically modified or sensory neurons were treated with chemicals, revealed that Mtmr2 reduces a specific fatty acid in the membrane of nerve cells, which in turn attenuates the activity of Piezo2.

This study identified Mtmr2 and distinct fatty acids in the cell membrane as new components of the complex setup required for the sense of touch. A next step will be to test if these molecules also influence the activity of Piezo2 when the skin has become injured or upon inflammation.

Structure of the mechanically activated ion channel Piezo1

Piezo1 and Piezo2 are mechanically activated ion channels that mediate touch perception, proprioception and vascular development. Piezo proteins are distinct from other ion channels and their structure remains poorly defined, which impedes detailed study of their gating and ion permeation properties. Here we report a high-resolution cryo-electron microscopy structure of the mouse Piezo1 trimer. The detergent-solubilized complex adopts a three-bladed propeller shape with a curved transmembrane region containing at least 26 transmembrane helices per protomer. The flexible propeller blades can adopt distinct conformations, and consist of a series of four-transmembrane helical bundles that we term Piezo repeats. Carboxy-terminal domains line the central ion pore, and the channel is closed by constrictions in the cytosol. A kinked helical beam and anchor domain link the Piezo repeats to the pore, and are poised to control gating allosterically. The structure provides a foundation to dissect further how Piezo channels are regulated by mechanical force.

Mechanically activated ion channels

The ability of cells to sense and respond to their physical environment is fundamental to a broad spectrum of biological processes. Cells express an array of force sensors that can transduce mechanical inputs into biochemical signals, including the mechanically activated ion channels PIEZO1, PIEZO2 and TRPV4. The identification and characterisation of mechanically activated ion channels has proven challenging, as has identifying their mode of activation and regulation in vivo. However, the diverse channelopathies associated with the known channels highlights their physiological importance.

Portraits of a pressure sensor

Near atomic-resolution structures have provided insights into the mechanisms by which the Piezo1 ion channel senses and responds to mechanical stimuli.

Structure and mechanogating mechanism of the Piezo1 channel

The mechanosensitive Piezo channels function as key eukaryotic mechanotransducers. However, their structures and mechanogating mechanisms remain unknown. Here we determine the three-bladed, propeller-like electron cryo-microscopy structure of mouse Piezo1 and functionally reveal its mechanotransduction components. Despite the lack of sequence repetition, we identify nine repetitive units consisting of four transmembrane helices each-which we term transmembrane helical units (THUs)-which assemble into a highly curved blade-like structure. The last transmembrane helix encloses a hydrophobic pore, followed by three intracellular fenestration sites and side portals that contain pore-property-determining residues. The central region forms a 90 Å-long intracellular beam-like structure, which undergoes a lever-like motion to connect THUs to the pore via the interfaces of the C-terminal domain, the anchor-resembling domain and the outer helix. Deleting extracellular loops in the distal THUs or mutating single residues in the beam impairs the mechanical activation of Piezo1. Overall, Piezo1 possesses a unique 38-transmembrane-helix topology and designated mechanotransduction components, which enable a lever-like mechanogating mechanism.

Hereditary stomatocytosis: An underdiagnosed condition

Hereditary stomatocytoses are a wide class of hemolytic anemias characterized by alterations of ionic flux with increased cation permeability that results in inappropriate shrinkage or swelling of the erythrocytes, and water lost or gained osmotically. The last few years have been crucial for new acquisitions in this field in terms of identifying new causative genes and of studying their pathogenetic mechanisms. This review summarizes the main features of erythrocyte membrane transport diseases, dividing them into forms with either isolated erythroid phenotype (nonsyndromic) or extra-hematological manifestations (syndromic), and focusing particularly on the most recent advances regarding dehydrated forms of hereditary stomatocytosis and familial pseudohyperkalemia.

Hemodynamic Control of Endothelial Cell Fates in Development

Biomechanical forces are emerging as critical regulators of embryogenesis, particularly in the developing cardiovascular system. From the onset of blood flow, the embryonic vasculature is continuously exposed to a variety of hemodynamic forces. These biomechanical stimuli are key determinants of vascular cell specification and remodeling and the establishment of vascular homeostasis. In recent years, major advances have been made in our understanding of mechano-activated signaling networks that control both spatiotemporal and structural aspects of vascular development. It has become apparent that a major site for mechanotransduction is situated at the interface of blood and the vessel wall and that this process is controlled by the vascular endothelium. In this review, we discuss the hemodynamic control of endothelial cell fates, focusing on arterial-venous specification, lymphatic development, and the endothelial-to-hematopoietic transition, and present some recent insights into the mechano-activated pathways driving these cell fate decisions in the developing embryo.

Structure-based membrane dome mechanism for Piezo mechanosensitivity

Mechanosensitive ion channels convert external mechanical stimuli into electrochemical signals for critical processes including touch sensation, balance, and cardiovascular regulation. The best understood mechanosensitive channel, MscL, opens a wide pore, which accounts for mechanosensitive gating due to in-plane area expansion. Eukaryotic Piezo channels have a narrow pore and therefore must capture mechanical forces to control gating in another way. We present a cryo-EM structure of mouse Piezo1 in a closed conformation at 3.7Å-resolution. The channel is a triskelion with arms consisting of repeated arrays of 4-TM structural units surrounding a pore. Its shape deforms the membrane locally into a dome. We present a hypothesis in which the membrane deformation changes upon channel opening. Quantitatively, membrane tension will alter gating energetics in proportion to the change in projected area under the dome. This mechanism can account for highly sensitive mechanical gating in the setting of a narrow, cation-selective pore.

A protein interaction mechanism for suppressing the mechanosensitive Piezo channels

Piezo proteins are bona fide mammalian mechanotransduction channels for various cell types including endothelial cells. The mouse Piezo1 of 2547 residues forms a three-bladed, propeller-like homo-trimer comprising a central pore-module and three propeller-structures that might serve as mechanotransduction-modules. However, the mechanogating and regulation of Piezo channels remain unclear. Here we identify the sarcoplasmic /endoplasmic-reticulum Ca²⁺ ATPase (SERCA), including the widely expressed SERCA2, as Piezo interacting proteins. SERCA2 strategically suppresses Piezo1 via acting on a 14-residue-constituted intracellular linker connecting the pore-module and mechanotransduction-module. Mutating the linker impairs mechanogating and SERCA2-mediated modulation of Piezo1. Furthermore, the synthetic linker-peptide disrupts the modulatory effects of SERCA2, demonstrating the key role of the linker in mechanogating and regulation. Importantly, the SERCA2-mediated regulation affects Piezo1-dependent migration of endothelial cells. Collectively, we identify SERCA-mediated regulation of Piezos and the functional significance of the linker, providing important insights into the mechanogating and regulation mechanisms of Piezo channels.

Piezo1 Channels Are Inherently Mechanosensitive

The conversion of mechanical force to chemical signals is critical for many biological processes, including the senses of touch, pain, and hearing. Mechanosensitive ion channels play a key role in sensing the mechanical stimuli experienced by various cell types and are present in organisms from bacteria to mammals. Bacterial mechanosensitive channels are characterized thoroughly, but less is known about their counterparts in vertebrates. Piezos have been recently established as ion channels required for mechanotransduction in disparate cell types in vitro and in vivo. Overexpression of Piezos in heterologous cells gives rise to large mechanically activated currents; however, it is unclear whether Piezos are inherently mechanosensitive or rely on alternate cellular components to sense mechanical stimuli. Here, we show that mechanical perturbations of the lipid bilayer alone are sufficient to activate Piezo channels, illustrating their innate ability as molecular force transducers.

Highlights for the 6th International Ion Channel Conference: ion channel structure, function, disease and therapeutics

To foster communication and interactions amongst international scholars and scientists in the field of ion channel research, the 6th International Ion Channel Conference (IICC-2017) was held between June 23–27, 2017 in the eastern coastal city of Qingdao, China. The meeting consisted of 450 attendees and 130 speakers and poster presenters. The program consisted of research progress, new findings and ongoing studies that were focused on (1) Ion channel structure and function; (2) Ion channel physiology and human diseases; (3) Ion channels as targets for drug discovery; (4) Technological advances in ion channel research. An insightful overview was presented on the structure and function of the mechanotransduction channel Drosophila NOMPC (No mechanoreceptor potential C), a member of the transient receptor potential (TRP) channel family. Recent studies on Transmembrane protein 16 or Anoctamin-1 (TMEM16A, a member of the calcium-activated chloride channel [CaCC] family) were summarized as well. In addition, topics for ion channel regulation, homeostatic feedback and brain disorders were thoroughly discussed. The presentations at the IICC-2017 offer new insights into our understanding of ion channel structures and functions, and ion channels as targets for drug discovery.

The Function of the Novel Mechanical Activated Ion Channel Piezo1 in the Human Osteosarcoma Cells

Background

The Piezo1 protein ion channel is a novel mechanical activated ion channel which is related to mechanical signal transduction. However, the function of the mechanically activated ion channel Piezo1 had not been explored. In this study, we explored the function of the Piezo1 ion channel in human osteosarcoma (OS) cells related to apoptosis, invasion, and the cell proliferation.

Material/Methods

Reverse transcription polymerase chain reaction (RT-PCR) and western-blotting were used to detect the expression of the Piezo1 protein. CCK-8, Transwell experiments and AV-PI were used to detected cell proliferation, cell invasion and cell apoptosis.

Results

The Piezo1 protein ion channel was highly expressed in human OS cells. The Piezo1-shRNA inhibited the expression of the Piezo1. We explored whether LV3-PIEZO1-homo-3201 could act as Piezo1-shRNA, which could then be an inhibitor of Piezo1. The expression of Piezo1 in the 2-hour stretch group were slightly higher than the 0-hour stretch group, and the difference was not statistically significant (n=3, p>0.05, one-way ANOVA). The apoptotic gene such as the Bax, BAD, caspase-3, and caspase-9 had the same characteristics as the Piezo1 expression under the stretch force. We also explored the invasion of Piezo1 in vivo using nude mice, and found that Piezo1-shRNA could inhibit the invasion of the OS cells.

Conclusions

The Piezo1 protein may be a novel, potential therapeutic target for OS.

Disorders of erythrocyte hydration

The erythrocyte contains a network of pathways that regulate salt and water content in the face of extracellular and intracellular osmotic perturbations. This allows the erythrocyte to maintain a narrow range of cell hemoglobin concentration, a process critical for normal red blood cell function and survival. Primary disorders that perturb volume homeostasis jeopardize the erythrocyte and may lead to its premature destruction. These disorders are marked by clinical, laboratory, and physiologic heterogeneity. Recent studies have revealed that these disorders are also marked by genetic heterogeneity. They have implicated roles for several proteins, PIEZO1, a mammalian mechanosensory protein; GLUT1, the glucose transporter; SLC4A1, the anion transporter; RhAG, the Rh-associated glycoprotein; KCNN4, the Gardos channel; and ABCB6, an adenosine triphosphate-binding cassette family member, in the maintenance of erythrocyte volume homeostasis. Secondary disorders of erythrocyte hydration include sickle cell disease, thalassemia, hemoglobin CC, and hereditary spherocytosis, where cellular dehydration may be a significant contributor to disease pathology and clinical complications. Understanding the pathways regulating erythrocyte water and solute content may reveal innovative strategies to maintain normal volume in disorders associated with primary or secondary cellular dehydration. These mechanisms will serve as a paradigm for other cells and may reveal new therapeutic targets for disease prevention and treatment beyond the erythrocyte.

Clarifying the supercomplex: the higher-order organization of the mitochondrial electron transport chain

The oxidative phosphorylation electron transport chain (OXPHOS-ETC) of the inner mitochondrial membrane is composed of five large protein complexes, named CI-CV. These complexes convert energy from the food we eat into ATP, a small molecule used to power a multitude of essential reactions throughout the cell. OXPHOS-ETC complexes are organized into supercomplexes (SCs) of defined stoichiometry: CI forms a supercomplex with CIII₂ and CIV (SC I+III₂+IV, known as the respirasome), as well as with CIII₂ alone (SC I+III₂). CIII₂ forms a supercomplex with CIV (SC III₂+IV) and CV forms dimers (CV₂). Recent cryo-EM studies have revealed the structures of SC I+III₂+IV and SC I+III₂. Furthermore, recent work has shed light on the assembly and function of the SCs. Here we review and compare these recent studies and discuss how they have advanced our understanding of mitochondrial electron transport.

Piezos thrive under pressure: mechanically activated ion channels in health and disease

Cellular mechanotransduction, the process of translating mechanical forces into biological signals, is crucial for a wide range of physiological processes. A role for ion channels in sensing mechanical forces has been proposed for decades, but their identity in mammals remained largely elusive until the discovery of Piezos. Recent research on Piezos has underscored their importance in somatosensation (touch perception, proprioception and pulmonary respiration), red blood cell volume regulation, vascular physiology and various human genetic disorders.