Bertrand Coste

Piezo1 and Piezo2 Are Essential Components of Distinct Mechanically Activated Cation Channels

Mechanical Responders Identified Although many cells appear to respond to mechanical stimulation through increased conductance of ion channels in the plasma membrane, the actual channels that mediate these effects—which are important in diverse processes from hearing and touch to control of blood pressure—have remained elusive. Coste et al. (p. 55, published online 2 September) used RNA interference to decrease expression of candidate genes systematically in a mouse neuroblastoma cell line and identified two genes that encode proteins, Piezo1 and Piezo2, which are required for mechanically stimulated cation conductance in these cells and in cultured dorsal root ganglion neurons. Similar proteins are expressed in a range of species from protozoa to vertebrates. The proteins are not similar to known pore-forming proteins and thus could be unusual channels or regulatory components of a channel complex. Cation channel genes encode for a transducer molecule that converts mechanical stimuli into cell signaling. Mechanical stimuli drive many physiological processes, including touch and pain sensation, hearing, and blood pressure regulation. Mechanically activated (MA) cation channel activities have been recorded in many cells, but the responsible molecules have not been identified. We characterized a rapidly adapting MA current in a mouse neuroblastoma cell line. Expression profiling and RNA interference knockdown of candidate genes identified Piezo1 (Fam38A) to be required for MA currents in these cells. Piezo1 and related Piezo2 (Fam38B) are vertebrate multipass transmembrane proteins with homologs in invertebrates, plants, and protozoa. Overexpression of mouse Piezo1 or Piezo2 induced two kinetically distinct MA currents. Piezos are expressed in several tissues, and knockdown of Piezo2 in dorsal root ganglia neurons specifically reduced rapidly adapting MA currents. We propose that Piezos are components of MA cation channels.

Gating of the polycystin ion channel signaling complex in neurons and kidney cells

Mutations in either polycystin‐2 (PC2) or polycystin‐1 (PC1) proteins cause severe, potentially lethal, kidney disorders and multiple extrarenal (including brain) disease phenotypes. PC2, a member of the transient receptor potential channel superfamily, and PC1, an orphan membrane receptor of largely unknown function, are thought to be part of a common signaling pathway. Here, we show that in rat sympathetic neurons and kidney cells, coassembly of full‐length PC1 with PC2 forms a plasmalemmal ion channel signaling complex in which PC1 stimulation simultaneously activates PC2 ion channels and Gi/o‐proteins. PC2 activation occurs through a structural rearrangement of PC1, independent of G‐protein activation. Thus, PC1 acts as a prototypical membrane receptor that concordantly regulates PC2 channels and G‐proteins, a bimodal mechanism that may account for the multifunctional roles of polycystin proteins in fundamental cellular processes of various cell types.

Pharmacological dissection and distribution of NaN/Nav1.9, T-type Ca2+ currents, and mechanically activated cation currents in different populations of DRG neurons.

Low voltage–activated (LVA) T-type Ca2+ (ICaT) and NaN/Nav1.9 currents regulate DRG neurons by setting the threshold for the action potential. Although alterations in these channels have been implicated in a variety of pathological pain states, their roles in processing sensory information remain poorly understood. Here, we carried out a detailed characterization of LVA currents in DRG neurons by using a method for better separation of NaN/Nav1.9 and ICaT currents. NaN/Nav1.9 was inhibited by inorganic ICa blockers as follows (IC50, μM): La3+ (46) > Cd2+ (233) > Ni2+ (892) and by mibefradil, a non-dihydropyridine ICaT antagonist. Amiloride, however, a preferential Cav3.2 channel blocker, had no effects on NaN/Nav1.9 current. Using these discriminative tools, we showed that NaN/Nav1.9, Cav3.2, and amiloride- and Ni2+-resistant ICaT (AR-ICaT) contribute differentially to LVA currents in distinct sensory cell populations. NaN/Nav1.9 carried LVA currents into type-I (CI) and type-II (CII) small nociceptors and medium-Aδ–like nociceptive cells but not in low-threshold mechanoreceptors, including putative Down-hair (D-hair) and Aα/β cells. Cav3.2 predominated in CII-nociceptors and in putative D-hair cells. AR-ICaT was restricted to CII-nociceptors, putative D-hair cells, and Aα/β-like cells. These cell types distinguished by their current-signature displayed different types of mechanosensitive channels. CI- and CII-nociceptors displayed amiloride-sensitive high-threshold mechanical currents with slow or no adaptation, respectively. Putative D-hair and Aα/β-like cells had low-threshold mechanical currents, which were distinguished by their adapting kinetics and sensitivity to amiloride. Thus, subspecialized DRG cells express specific combinations of LVA and mechanosensitive channels, which are likely to play a key role in shaping responses of DRG neurons transmitting different sensory modalities.

Dehydrated hereditary stomatocytosis linked to gain-of-function mutations in mechanically activated PIEZO1 ion channels.

Dehydrated hereditary stomatocytosis (DHS) is a genetic condition with defective red blood cell (RBC) membrane properties that causes an imbalance in intracellular cation concentrations. Recently, two missense mutations inthe mechanically activated PIEZO1(FAM38A) ion channel were associated with DHS. However, it is not known how these mutations affect PIEZO1 function. Here, by combining linkage analysis and whole-exome sequencing in a large pedigree and Sanger sequencing in two additional kindreds and 11 unrelated DHS cases, we identifythree novel missense mutations and one recurrent duplication in PIEZO1, demonstrating that it is the major gene for DHS. All the DHS-associated mutations locate at C-terminal half of PIEZO1. Remarkably, we find that all PIEZO1 mutations give rise to mechanically activated currents that inactivate more slowly than wild-type currents. This gain-of-function PIEZO1 phenotype provides insight that helps to explain the increased permeability of cations in RBCs of DHS patients. Our findings also suggest a new role for mechanotransduction in RBC biology and pathophysiology.

The versatile nature of the calcium‐permeable cation channel TRPP2

TRPP2 is a member of the transient receptor potential (TRP) superfamily of cation channels, which is mutated in autosomal dominant polycystic kidney disease (ADPKD). TRPP2 is thought to function with polycystin 1—a large integral protein—as part of a multiprotein complex involved in transducing Ca2+‐dependent information. TRPP2 has been implicated in various biological functions including cell proliferation, sperm fertilization, mating behaviour, mechanosensation and asymmetric gene expression. Although its function as a Ca2+‐permeable cation channel is well established, its precise role in the plasma membrane, the endoplasmic reticulum and the cilium is controversial. Recent studies suggest that TRPP2 function is highly dependent on the subcellular compartment of expression, and is regulated by many interactions with adaptor proteins. This review summarizes the most pertinent evidence about the properties of TRPP2 channels, focusing on the compartment‐specific functions of mammalian TRPP2.

Mechano-Gated Ion Channels in Sensory Systems

Living organisms sense their physical environment through cellular mechanotransduction, which converts mechanical forces into electrical and biochemical signals. In turn, signal transduction serves a wide variety of functions, from basic cellular processes as diverse as proliferation, differentiation, migration, and apoptosis up to some of the most sophisticated senses, including touch and hearing. Accordingly, defects in mechanosensing potentially lead to diverse diseases and disorders such as hearing loss, cardiomyopathies, muscular dystrophies, chronic pain, and cancer. Here, we review the status of mechanically activated ion channel discovery and discuss current challenges to define their properties and physiological functions.

Piezo1 ion channel pore properties are dictated by C-terminal region

Piezo1 and Piezo2 encode mechanically activated cation channels that function as mechanotransducers involved in vascular system development and touch sensing, respectively. Structural features of Piezos remain unknown. Mouse Piezo1 is bioinformatically predicted to have 30–40 transmembrane (TM) domains. Here, we find that nine of the putative inter-transmembrane regions are accessible from the extracellular side. We use chimeras between mPiezo1 and dPiezo to show that ion-permeation properties are conferred by C-terminal region. We further identify a glutamate residue within a conserved region adjacent to the last two putative TM domains of the protein, that when mutated, affects unitary conductance and ion selectivity, and modulates pore block. We propose that this amino acid is either in the pore or closely associates with the pore. Our results describe important structural motifs of this channel family and lay the groundwork for a mechanistic understanding of how Piezos are mechanically gated and conduct ions.

Piezo channels: from structure to function

Mechanotransduction is the conversion of mechanical stimuli into biological signals. It is involved in the modulation of diverse cellular functions such as migration, proliferation, differentiation, and apoptosis as well as in the detection of sensory stimuli such as air vibration and mechanical contact. Therefore, mechanotransduction is crucial for organ development and homeostasis and plays a direct role in hearing, touch, proprioception, and pain. Multiple molecular players involved in mechanotransduction have been identified in the past, among them ion channels directly activated by cell membrane deformation. Most of these channels have well-established roles in lower organisms but are not conserved in mammals or fail to encode mechanically activated channels in mammals due to non-conservation of mechanotransduction property. A family of mechanically activated channels that counts only two members in human, piezo1 and 2, has emerged recently. Given the lack of valid mechanically activated channel candidates in mammals in the past decades, particular attention is given to piezo channels and their potential roles in various biological functions. This review summarizes our current knowledge on these ion channels.

Expression and localization of the Nav1.9 sodium channel in enteric neurons and in trigeminal sensory endings: implication for intestinal reflex function and orofacial pain.

The Nav1.9 sodium channel is expressed in nociceptive DRG neurons where it contributes to spontaneous pain behavior after peripheral inflammation. Here, we used a newly developed antibody to investigate the distribution of Nav1.9 in rat and mouse trigeminal ganglion (TG) nerve endings and in enteric nervous system (ENS). In TGs, Nav1.9 was expressed in the soma of small- and medium-sized, peripherin-positive neurons. Nav1.9 was present along trigeminal afferent fibers and at terminals in lip skin and dental pulp. In the ENS, Nav1.9 was detected within the soma and proximal axons of sensory, Dogiel type II, myenteric and submucosal neurons. Immunological data were correlated with the detection of persistent TTX-resistant Na(+) currents sharing similar properties in DRG, TG and myenteric neurons. Collectively, our data support a potential role of Nav1.9 in the transmission of trigeminal pain and the regulation of intestinal reflexes. Nav1.9 might therefore constitute a molecular target for therapeutic treatments of orofacial pain and gastrointestinal syndromes.

Touch sense: Functional organization and molecular determinants of mechanosensitive receptors

Cutaneous mechanoreceptors are localized in the various layers of the skin where they detect a wide range of mechanical stimuli, including light brush, stretch, vibration and noxious pressure. This variety of stimuli is matched by a diverse array of specialized mechanoreceptors that respond to cutaneous deformation in a specific way and relay these stimuli to higher brain structures. Studies across mechanoreceptors and genetically tractable sensory nerve endings are beginning to uncover touch sensation mechanisms. Work in this field has provided researchers with a more thorough understanding of the circuit organization underlying the perception of touch. Novel ion channels have emerged as candidates for transduction molecules and properties of mechanically gated currents improved our understanding of the mechanisms of adaptation to tactile stimuli. This review highlights the progress made in characterizing functional properties of mechanoreceptors in hairy and glabrous skin and ion channels that detect mechanical inputs and shape mechanoreceptor adaptation.