Structural mechanisms of α7 nicotinic receptor allosteric modulation and activation.

The α7 nicotinic acetylcholine receptor is a pentameric ligand-gated ion channel that plays an important role in cholinergic signaling throughout the nervous system. Its unique physiological characteristics and implications in neurological disorders and inflammation make it a promising but challenging therapeutic target. Positive allosteric modulators overcome limitations of traditional α7 agonists, but their potentiation mechanisms remain unclear. Here, we present high-resolution structures of α7-modulator complexes, revealing partially overlapping binding sites but varying conformational states. Structure-guided functional and computational tests suggest that differences in modulator activity arise from the stable rotation of a channel gating residue out of the pore. We extend the study using a time-resolved cryoelectron microscopy (cryo-EM) approach to reveal asymmetric state transitions for this homomeric channel and also find that a modulator with allosteric agonist activity exploits a distinct channel-gating mechanism. These results define mechanisms of α7 allosteric modulation and activation with implications across the pentameric receptor superfamily.

The Form and Function of PIEZO2.

Mechanosensation is the ability to detect dynamic mechanical stimuli (e.g., pressure, stretch, and shear stress) and is essential for a wide variety of processes, including our sense of touch on the skin. How touch is detected and transduced at the molecular level has proved to be one of the great mysteries of sensory biology. A major breakthrough occurred in 2010 with the discovery of a family of mechanically gated ion channels that were coined PIEZOs. The last 10 years of investigation have provided a wealth of information about the functional roles and mechanisms of these molecules. Here we focus on PIEZO2, one of the two PIEZO proteins found in humans and other mammals. We review how work at the molecular, cellular, and systems levels over the past decade has transformed our understanding of touch and led to unexpected insights into other types of mechanosensation beyond the skin.

Membrane Exporters of Fluoride Ion.

Microorganisms contend with numerous and unusual chemical threats and have evolved a catalog of resistance mechanisms in response. One particularly ancient, pernicious threat is posed by fluoride ion (F-), a common xenobiotic in natural environments that causes broad-spectrum harm to metabolic pathways. This review focuses on advances in the last ten years toward understanding the microbial response to cytoplasmic accumulation of F-, with a special emphasis on the structure and mechanisms of the proteins that microbes use to export fluoride: the CLCF family of F-/H+ antiporters and the Fluc/FEX family of F- channels.

Bioelectric signaling: Reprogrammable circuits underlying embryogenesis, regeneration, and cancer.

How are individual cell behaviors coordinated toward invariant large-scale anatomical outcomes in development and regeneration despite unpredictable perturbations? Endogenous distributions of membrane potentials, produced by ion channels and gap junctions, are present across all tissues. These bioelectrical networks process morphogenetic information that controls gene expression, enabling cell collectives to make decisions about large-scale growth and form. Recent progress in the analysis and computational modeling of developmental bioelectric circuits and channelopathies reveals how cellular collectives cooperate toward organ-level structural order. These advances suggest a roadmap for exploiting bioelectric signaling for interventions addressing developmental disorders, regenerative medicine, cancer reprogramming, and synthetic bioengineering.

Structure and gating mechanism of the α7 nicotinic acetylcholine receptor.

The α7 nicotinic acetylcholine receptor plays critical roles in the central nervous system and in the cholinergic inflammatory pathway. This ligand-gated ion channel assembles as a homopentamer, is exceptionally permeable to Ca2+, and desensitizes faster than any other Cys-loop receptor. The α7 receptor has served as a prototype for the Cys-loop superfamily yet has proven refractory to structural analysis. We present cryo-EM structures of the human α7 nicotinic receptor in a lipidic environment in resting, activated, and desensitized states, illuminating the principal steps in the gating cycle. The structures also reveal elements that contribute to its function, including a C-terminal latch that is permissive for channel opening, and an anionic ring in the extracellular vestibule that contributes to its high conductance and calcium permeability. Comparisons among the α7 structures provide a foundation for mapping the gating cycle and reveal divergence in gating mechanisms in the Cys-loop receptor superfamily.

Structural snapshots of TRPV1 reveal mechanism of polymodal functionality.

Many transient receptor potential (TRP) channels respond to diverse stimuli and conditionally conduct small and large cations. Such functional plasticity is presumably enabled by a uniquely dynamic ion selectivity filter that is regulated by physiological agents. What is currently missing is a "photo series" of intermediate structural states that directly address this hypothesis and reveal specific mechanisms behind such dynamic channel regulation. Here, we exploit cryoelectron microscopy (cryo-EM) to visualize conformational transitions of the capsaicin receptor, TRPV1, as a model to understand how dynamic transitions of the selectivity filter in response to algogenic agents, including protons, vanilloid agonists, and peptide toxins, permit permeation by small and large organic cations. These structures also reveal mechanisms governing ligand binding substates, as well as allosteric coupling between key sites that are proximal to the selectivity filter and cytoplasmic gate. These insights suggest a general framework for understanding how TRP channels function as polymodal signal integrators.

A role of PIEZO1 in iron metabolism in mice and humans.

Iron overload causes progressive organ damage and is associated with arthritis, liver damage, and heart failure. Elevated iron levels are present in 1%-5% of individuals; however, iron overload is undermonitored and underdiagnosed. Genetic factors affecting iron homeostasis are emerging. Individuals with hereditary xerocytosis, a rare disorder with gain-of-function (GOF) mutations in mechanosensitive PIEZO1 ion channel, develop age-onset iron overload. We show that constitutive or macrophage expression of a GOF Piezo1 allele in mice disrupts levels of the iron regulator hepcidin and causes iron overload. We further show that PIEZO1 is a key regulator of macrophage phagocytic activity and subsequent erythrocyte turnover. Strikingly, we find that E756del, a mild GOF PIEZO1 allele present in one-third of individuals of African descent, is strongly associated with increased plasma iron. Our study links macrophage mechanotransduction to iron metabolism and identifies a genetic risk factor for increased iron levels in African Americans.

Structural Basis of Human KCNQ1 Modulation and Gating.

KCNQ1, also known as Kv7.1, is a voltage-dependent K+ channel that regulates gastric acid secretion, salt and glucose homeostasis, and heart rhythm. Its functional properties are regulated in a tissue-specific manner through co-assembly with beta subunits KCNE1-5. In non-excitable cells, KCNQ1 forms a complex with KCNE3, which suppresses channel closure at negative membrane voltages that otherwise would close it. Pore opening is regulated by the signaling lipid PIP2. Using cryoelectron microscopy (cryo-EM), we show that KCNE3 tucks its single-membrane-spanning helix against KCNQ1, at a location that appears to lock the voltage sensor in its depolarized conformation. Without PIP2, the pore remains closed. Upon addition, PIP2 occupies a site on KCNQ1 within the inner membrane leaflet, which triggers a large conformational change that leads to dilation of the pore's gate. It is likely that this mechanism of PIP2 activation is conserved among Kv7 channels.

TACAN Is an Ion Channel Involved in Sensing Mechanical Pain.

Mechanotransduction, the conversion of mechanical stimuli into electrical signals, is a fundamental process underlying essential physiological functions such as touch and pain sensing, hearing, and proprioception. Although the mechanisms for some of these functions have been identified, the molecules essential to the sense of pain have remained elusive. Here we report identification of TACAN (Tmem120A), an ion channel involved in sensing mechanical pain. TACAN is expressed in a subset of nociceptors, and its heterologous expression increases mechanically evoked currents in cell lines. Purification and reconstitution of TACAN in synthetic lipids generates a functional ion channel. Finally, a nociceptor-specific inducible knockout of TACAN decreases the mechanosensitivity of nociceptors and reduces behavioral responses to painful mechanical stimuli but not to thermal or touch stimuli. We propose that TACAN is an ion channel that contributes to sensing mechanical pain.

Structure and Function of Stator Units of the Bacterial Flagellar Motor.

Many bacteria use the flagellum for locomotion and chemotaxis. Its bidirectional rotation is driven by a membrane-embedded motor, which uses energy from the transmembrane ion gradient to generate torque at the interface between stator units and rotor. The structural organization of the stator unit (MotAB), its conformational changes upon ion transport, and how these changes power rotation of the flagellum remain unknown. Here, we present ~3 Å-resolution cryoelectron microscopy reconstructions of the stator unit in different functional states. We show that the stator unit consists of a dimer of MotB surrounded by a pentamer of MotA. Combining structural data with mutagenesis and functional studies, we identify key residues involved in torque generation and present a detailed mechanistic model for motor function and switching of rotational direction.

Resting-State Structure and Gating Mechanism of a Voltage-Gated Sodium Channel.

Voltage-gated sodium (NaV) channels initiate action potentials in nerve, muscle, and other electrically excitable cells. The structural basis of voltage gating is uncertain because the resting state exists only at deeply negative membrane potentials. To stabilize the resting conformation, we inserted voltage-shifting mutations and introduced a disulfide crosslink in the VS of the ancestral bacterial sodium channel NaVAb. Here, we present a cryo-EM structure of the resting state and a complete voltage-dependent gating mechanism. The S4 segment of the VS is drawn intracellularly, with three gating charges passing through the transmembrane electric field. This movement forms an elbow connecting S4 to the S4-S5 linker, tightens the collar around the S6 activation gate, and prevents its opening. Our structure supports the classical "sliding helix" mechanism of voltage sensing and provides a complete gating mechanism for voltage sensor function, pore opening, and activation-gate closure based on high-resolution structures of a single sodium channel protein.

Voltage Sensor Movements during Hyperpolarization in the HCN Channel.

The hyperpolarization-activated cyclic nucleotide-gated (HCN) channel is a voltage-gated cation channel that mediates neuronal and cardiac pacemaker activity. The HCN channel exhibits reversed voltage dependence, meaning it closes with depolarization and opens with hyperpolarization. Different from Na+, Ca2+, and Kv1-Kv7 channels, the HCN channel does not have domain-swapped voltage sensors. We introduced a reversible, metal-mediated cross bridge into the voltage sensors to create the chemical equivalent of a hyperpolarized conformation and determined the structure using cryoelectron microscopy (cryo-EM). Unlike the depolarized HCN channel, the S4 helix is displaced toward the cytoplasm by two helical turns. Near the cytoplasm, the S4 helix breaks into two helices, one running parallel to the membrane surface, analogous to the S4-S5 linker of domain-swapped voltage-gated channels. These findings suggest a basis for allosteric communication between voltage sensors and the gate in this kind of channel. They also imply that voltage sensor movements are not the same in all voltage-gated channels.

A Cell-Penetrating Scorpion Toxin Enables Mode-Specific Modulation of TRPA1 and Pain.

TRPA1 is a chemosensory ion channel that functions as a sentinel for structurally diverse electrophilic irritants. Channel activation occurs through an unusual mechanism involving covalent modification of cysteine residues clustered within an amino-terminal cytoplasmic domain. Here, we describe a peptidergic scorpion toxin (WaTx) that activates TRPA1 by penetrating the plasma membrane to access the same intracellular site modified by reactive electrophiles. WaTx stabilizes TRPA1 in a biophysically distinct active state characterized by prolonged channel openings and low Ca2+ permeability. Consequently, WaTx elicits acute pain and pain hypersensitivity but fails to trigger efferent release of neuropeptides and neurogenic inflammation typically produced by noxious electrophiles. These findings provide a striking example of convergent evolution whereby chemically disparate animal- and plant-derived irritants target the same key allosteric regulatory site to differentially modulate channel activity. WaTx is a unique pharmacological probe for dissecting TRPA1 function and its contribution to acute and persistent pain.

Mechanisms for Zinc and Proton Inhibition of the GluN1/GluN2A NMDA Receptor.

N-methyl-D-aspartate receptors (NMDARs) play essential roles in memory formation, neuronal plasticity, and brain development, with their dysfunction linked to a range of disorders from ischemia to schizophrenia. Zinc and pH are physiological allosteric modulators of NMDARs, with GluN2A-containing receptors inhibited by nanomolar concentrations of divalent zinc and by excursions to low pH. Despite the widespread importance of zinc and proton modulation of NMDARs, the molecular mechanism by which these ions modulate receptor activity has proven elusive. Here, we use cryoelectron microscopy to elucidate the structure of the GluN1/GluN2A NMDAR in a large ensemble of conformations under a range of physiologically relevant zinc and proton concentrations. We show how zinc binding to the amino terminal domain elicits structural changes that are transduced though the ligand-binding domain and result in constriction of the ion channel gate.

Common PIEZO1 Allele in African Populations Causes RBC Dehydration and Attenuates Plasmodium Infection.

Hereditary xerocytosis is thought to be a rare genetic condition characterized by red blood cell (RBC) dehydration with mild hemolysis. RBC dehydration is linked to reduced Plasmodium infection in vitro; however, the role of RBC dehydration in protection against malaria in vivo is unknown. Most cases of hereditary xerocytosis are associated with gain-of-function mutations in PIEZO1, a mechanically activated ion channel. We engineered a mouse model of hereditary xerocytosis and show that Plasmodium infection fails to cause experimental cerebral malaria in these mice due to the action of Piezo1 in RBCs and in T cells. Remarkably, we identified a novel human gain-of-function PIEZO1 allele, E756del, present in a third of the African population. RBCs from individuals carrying this allele are dehydrated and display reduced Plasmodium infection in vitro. The existence of a gain-of-function PIEZO1 at such high frequencies is surprising and suggests an association with malaria resistance.

Activation and Desensitization Mechanism of AMPA Receptor-TARP Complex by Cryo-EM.

AMPA receptors mediate fast excitatory neurotransmission in the mammalian brain and transduce the binding of presynaptically released glutamate to the opening of a transmembrane cation channel. Within the postsynaptic density, however, AMPA receptors coassemble with transmembrane AMPA receptor regulatory proteins (TARPs), yielding a receptor complex with altered gating kinetics, pharmacology, and pore properties. Here, we elucidate structures of the GluA2-TARP γ2 complex in the presence of the partial agonist kainate or the full agonist quisqualate together with a positive allosteric modulator or with quisqualate alone. We show how TARPs sculpt the ligand-binding domain gating ring, enhancing kainate potency and diminishing the ensemble of desensitized states. TARPs encircle the receptor ion channel, stabilizing M2 helices and pore loops, illustrating how TARPs alter receptor pore properties. Structural and computational analysis suggests the full agonist and modulator complex harbors an ion-permeable channel gate, providing the first view of an activated AMPA receptor.

Structural Titration of Slo2.2, a Na+-Dependent K+ Channel.

The stable structural conformations that occur along the complete reaction coordinate for ion channel opening have never been observed. In this study, we describe the equilibrium ensemble of structures of Slo2.2, a neuronal Na+-activated K+ channel, as a function of the Na+ concentration. We find that Slo2.2 exists in multiple closed conformations whose relative occupancies are independent of Na+ concentration. An open conformation emerges from an ensemble of closed conformations in a highly Na+-dependent manner, without evidence of Na+-dependent intermediates. In other words, channel opening is a highly concerted, switch-like process. The midpoint of the structural titration matches that of the functional titration. A maximum open conformation probability approaching 1.0 and maximum functional open probability approaching 0.7 imply that, within the class of open channels, there is a subclass that is not permeable to ions.

Cryo-EM Structure of a KCNQ1/CaM Complex Reveals Insights into Congenital Long QT Syndrome.

KCNQ1 is the pore-forming subunit of cardiac slow-delayed rectifier potassium (IKs) channels. Mutations in the kcnq1 gene are the leading cause of congenital long QT syndrome (LQTS). Here, we present the cryoelectron microscopy (cryo-EM) structure of a KCNQ1/calmodulin (CaM) complex. The conformation corresponds to an "uncoupled," PIP2-free state of KCNQ1, with activated voltage sensors and a closed pore. Unique structural features within the S4-S5 linker permit uncoupling of the voltage sensor from the pore in the absence of PIP2. CaM contacts the KCNQ1 voltage sensor through a specific interface involving a residue on CaM that is mutated in a form of inherited LQTS. Using an electrophysiological assay, we find that this mutation on CaM shifts the KCNQ1 voltage-activation curve. This study describes one physiological form of KCNQ1, depolarized voltage sensors with a closed pore in the absence of PIP2, and reveals a regulatory interaction between CaM and KCNQ1 that may explain CaM-mediated LQTS.

Structures of the Human HCN1 Hyperpolarization-Activated Channel.

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels underlie the control of rhythmic activity in cardiac and neuronal pacemaker cells. In HCN, the polarity of voltage dependence is uniquely reversed. Intracellular cyclic adenosine monophosphate (cAMP) levels tune the voltage response, enabling sympathetic nerve stimulation to increase the heart rate. We present cryo-electron microscopy structures of the human HCN channel in the absence and presence of cAMP at 3.5 Å resolution. HCN channels contain a K+ channel selectivity filter-forming sequence from which the amino acids create a unique structure that explains Na+ and K+ permeability. The voltage sensor adopts a depolarized conformation, and the pore is closed. An S4 helix of unprecedented length extends into the cytoplasm, contacts the C-linker, and twists the inner helical gate shut. cAMP binding rotates cytoplasmic domains to favor opening of the inner helical gate. These structures advance understanding of ion selectivity, reversed polarity gating, and cAMP regulation in HCN channels.

Species-Independent Attraction to Biofilms through Electrical Signaling.

Bacteria residing within biofilm communities can coordinate their behavior through cell-to-cell signaling. However, it remains unclear if these signals can also influence the behavior of distant cells that are not part of the community. Using a microfluidic approach, we find that potassium ion channel-mediated electrical signaling generated by a Bacillus subtilis biofilm can attract distant cells. Integration of experiments and mathematical modeling indicates that extracellular potassium emitted from the biofilm alters the membrane potential of distant cells, thereby directing their motility. This electrically mediated attraction appears to be a generic mechanism that enables cross-species interactions, as Pseudomonas aeruginosa cells also become attracted to the electrical signal released by the B. subtilis biofilm. Cells within a biofilm community can thus not only coordinate their own behavior but also influence the behavior of diverse bacteria at a distance through long-range electrical signaling. PAPERCLIP.