1.A.75 The Mechanical Nociceptor, Piezo (Piezo) Family
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. Coste et al. (2010) 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. Coste et al. (2010) proposed that Piezos are components of MA cation channels. Mouse piezo1 is involved in vacular system development, while piezo2 is concerned with touch sensitization (Coste et al. 2015). Ion-permeation properties are conferred by the C-terminal region, and a glutamate residue within a conserved region adjacent to the last two putative TMSs, when mutated, affects unitary conductance and ion selectivity, and modulates pore block (Coste et al. 2015).
Ion channels have a role in neuronal mechanotransduction in invertebrates, but functional conservation of these ion channels in mammalian mechanotransduction is not observed. For example, no mechanoreceptor potential C (NOMPC), a member of transient receptor potential (TRP) ion channel family, acts as a mechanotransducer in Drosophila melanogaster and Caenorhabditis elegans, and it has no orthologues in mammals. Degenerin/epithelial sodium channel (DEG/ENaC) family members are mechanotransducers in C. elegans and potentially in D. melanogaster. However, a direct role of its mammalian homologues in sensing mechanical force has not been shown. Piezo1 (also known as Fam38a) and Piezo2 (also known as Fam38b) are components of mechanically activated channels in mammals. Members of the Piezo family are evolutionarily conserved transmembrane proteins. Kim et al. (2012) studied the physiological role of the single Piezo member in D. melanogaster (Dmpiezo; also known as CG8486). Dmpiezo expression induces mechanically activated currents, similar to its mammalian counterparts. Behavioural responses to noxious mechanical stimuli were severely reduced in Dmpiezo knockout larvae, whereas responses to another noxious stimulus or touch were not affected. Knocking down Dmpiezo in sensory neurons that mediate nociception and express the DEG/ENaC ion channel pickpocket (ppk) was sufficient to impair responses to noxious mechanical stimuli. Furthermore, expression of Dmpiezo in these same neurons rescued the phenotype of the constitutive Dmpiezo knockout larvae. Accordingly, electrophysiological recordings from ppk-positive neurons revealed a Dmpiezo-dependent, mechanically activated current. Kim et al. (2012) found that Dmpiezo and ppk function in parallel pathways in ppk-positive cells, and that mechanical nociception is abolished in the absence of both channels. These data demonstrated the physiological relevance of the Piezo family in mechanotransduction in vivo, supporting a role of Piezo proteins in mechanosensory nociception.
Piezo channels are ~2500 aas long, have 24-32 TMSs, and appear to assemble into tetramers, requiring no other proteins for activity. They have a reversal potential around 0 mV and show voltage dependent inactivation. The channel is constitutively active in liposomes, indicating that no cytoskeletal elements are required. Heterologous expression of the Piezo protein can create mechanical sensitivity in otherwise insensitive cells. Piezo1 currents in outside-out patches are blocked by the extracellular MSC inhibitor peptide GsMTx4. Both enantiomeric forms of GsMTx4 inhibited channel activity in a manner similar to endogenous mechanical channels. Piezo1 can adopt a tonic (non-inactivating) form with repeated stimulation. The transition to the non-inactivating form generally occurs in large groups of channels, indicating that the channels exist in domains, and once the domain is compromised, the members simultaneously adopt new properties. Piezo proteins are associated with physiological responses in cells, such as the reaction to noxious stimulus of Drosophila larvae. Piezo1 is also essential for the removal of extra cells without apoptosis. Piezo1 mutations have been linked to the pathological response of red blood cells in a genetic disease called Xerocytosis (Gottlieb and Sachs, 2012).
Piezo homologues appear to consist of up to 9 repeat domains, each with 4 TMSs (M Saier, unpublished observations). However at the C-termini of these proteins is an addition 3 or 4 TMSs in a DUF3595 domain. These proteins can be found in a wide range of eukaryotes (animals, plants, protozoa, slime molds, ciliates etc.) but not prokaryotes. Mouse Piezo1 (TC# 1.A.75.1.14) possesses a 38-transmembrane-helix topology with mechanotransduction components that enable a lever-like mechanogating mechanism, determined by cryoEM (Zhao et al. 2018).
Ge et al. 2015 determined the cryo-EM structure of the full-length (2,547 amino acids) mouse Piezo1 (Piezo1) at a resolution of 4.8 Å. Piezo1 forms a trimeric propeller-like structure (about 900 kilodaltons), with the extracellular domains resembling three distal blades and a central cap. The transmembrane region has 14 apparently resolved segments per subunit. These segments form three peripheral wings and a central pore module that encloses a potential ion-conducting pore. The rather flexible extracellular blade domains are connected to the central intracellular domain by three long beam-like structures. This trimeric architecture suggests that Piezo1 may use its peripheral regions as force sensors to gate the central ion-conducting pore (Ge et al. 2015). In the intracellular region, three long beam-like domains ( approximately 80Å in length) support the whole transmembrane region and connect the mobile peripheral regions to the central pore module. This design suggests that the trimeric mPiezo1 may mechanistically function by similar principles as how propellers sense and transduce force to control ion conductivity (Li et al. 2017).
Piezo1 and Piezo2 mediate touch perception, proprioception and vascular development. Saotome et al. 2017 also reported a high-resolution cryo-electron microscopy structure of the mouse Piezo1 trimer. The detergent-solubilized complex adopts a three-blade propeller shape with a curved transmembrane region containing at least 26 TMSs per protomer. The flexible propeller blades can adopt distinct conformations and consist of a series of four-transmembrane helix bundles termed '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 (Saotome et al. 2017).
The transport reaction catalyzed by Piezo family members is:
cations (in) ⇌ cations (out)