A serotonergic axon-cilium synapse drives nuclear signaling to alter chromatin accessibility.

Chemical synapses between axons and dendrites mediate neuronal intercellular communication. Here, we describe a synapse between axons and primary cilia: the axo-ciliary synapse. Using enhanced focused ion beam-scanning electron microscopy on samples with optimally preserved ultrastructure, we discovered synapses between brainstem serotonergic axons and the primary cilia of hippocampal CA1 pyramidal neurons. Functionally, these cilia are enriched in a ciliary-restricted serotonin receptor, the 5-hydroxytryptamine receptor 6 (5-HTR6). Using a cilia-targeted serotonin sensor, we show that opto- and chemogenetic stimulation of serotonergic axons releases serotonin onto cilia. Ciliary 5-HTR6 stimulation activates a non-canonical Gαq/11-RhoA pathway, which modulates nuclear actin and increases histone acetylation and chromatin accessibility. Ablation of this pathway reduces chromatin accessibility in CA1 pyramidal neurons. As a signaling apparatus with proximity to the nucleus, axo-ciliary synapses short circuit neurotransmission to alter the postsynaptic neuron's epigenetic state.

The Structure of the Polycystic Kidney Disease Channel PKD2 in Lipid Nanodiscs.

The Polycystic Kidney Disease 2 (Pkd2) gene is mutated in autosomal dominant polycystic kidney disease (ADPKD), one of the most common human monogenic disorders. Here, we present the cryo-EM structure of PKD2 in lipid bilayers at 3.0 Å resolution, which establishes PKD2 as a homotetrameric ion channel and provides insight into potential mechanisms for its activation. The PKD2 voltage-sensor domain retains two of four gating charges commonly found in those of voltage-gated ion channels. The PKD2 ion permeation pathway is constricted at the selectivity filter and near the cytoplasmic end of S6, suggesting that two gates regulate ion conduction. The extracellular domain of PKD2, a hotspot for ADPKD pathogenic mutations, contributes to channel assembly and strategically interacts with the transmembrane core, likely serving as a physical substrate for extracellular stimuli to allosterically gate the channel. Finally, our structure establishes the molecular basis for the majority of pathogenic mutations in Pkd2-related ADPKD.

The TRPM7 chanzyme is cleaved to release a chromatin-modifying kinase.

TRPM7 is a ubiquitous ion channel and kinase, a unique "chanzyme," required for proper early embryonic development. It conducts Zn(2+), Mg(2+), and Ca(2+) as well as monovalent cations and contains a functional serine/threonine kinase at its carboxyl terminus. Here, we show that in normal tissues and cell lines, the kinase is proteolytically cleaved from the channel domain in a cell-type-specific manner. These TRPM7 cleaved kinase fragments (M7CKs) translocate to the nucleus and bind multiple components of chromatin-remodeling complexes, including Polycomb group proteins. In the nucleus, the kinase phosphorylates specific serines/threonines of histones. M7CK-dependent phosphorylation of H3Ser10 at promoters of TRPM7-dependent genes correlates with their activity. We also demonstrate that cytosolic free [Zn(2+)] is TRPM7 dependent and regulates M7CK binding to transcription factors containing zinc-finger domains. These findings suggest that TRPM7-mediated modulation of intracellular Zn(2+) concentration couples ion-channel signaling to epigenetic chromatin covalent modifications that affect gene expression patterns. PAPERCLIP:

Structurally distinct Ca(2+) signaling domains of sperm flagella orchestrate tyrosine phosphorylation and motility.

Spermatozoa must leave one organism, navigate long distances, and deliver their paternal DNA into a mature egg. For successful navigation and delivery, a sperm-specific calcium channel is activated in the mammalian flagellum. The genes encoding this channel (CatSpers) appear first in ancient uniflagellates, suggesting that sperm use adaptive strategies developed long ago for single-cell navigation. Here, using genetics, super-resolution fluorescence microscopy, and phosphoproteomics, we investigate the CatSper-dependent mechanisms underlying this flagellar switch. We find that the CatSper channel is required for four linear calcium domains that organize signaling proteins along the flagella. This unique structure focuses tyrosine phosphorylation in time and space as sperm acquire the capacity to fertilize. In heterogeneous sperm populations, we find unique molecular phenotypes, but only sperm with intact CatSper domains that organize time-dependent and spatially specific protein tyrosine phosphorylation successfully migrate. These findings illuminate flagellar adaptation, signal transduction cascade organization, and fertility.

mTOR regulates lysosomal ATP-sensitive two-pore Na(+) channels to adapt to metabolic state.

Survival in the wild requires organismal adaptations to the availability of nutrients. Endosomes and lysosomes are key intracellular organelles that couple nutrition and metabolic status to cellular responses, but how they detect cytosolic ATP levels is not well understood. Here, we identify an endolysosomal ATP-sensitive Na(+) channel (lysoNa(ATP)). The channel is a complex formed by two-pore channels (TPC1 and TPC2), ion channels previously thought to be gated by nicotinic acid adenine dinucleotide phosphate (NAADP), and the mammalian target of rapamycin (mTOR). The channel complex detects nutrient status, becomes constitutively open upon nutrient removal and mTOR translocation off the lysosomal membrane, and controls the lysosome's membrane potential, pH stability, and amino acid homeostasis. Mutant mice lacking lysoNa(ATP) have much reduced exercise endurance after fasting. Thus, TPCs make up an ion channel family that couples the cell's metabolic state to endolysosomal function and are crucial for physical endurance during food restriction.

TPC proteins are phosphoinositide- activated sodium-selective ion channels in endosomes and lysosomes.

Mammalian two-pore channel proteins (TPC1, TPC2; TPCN1, TPCN2) encode ion channels in intracellular endosomes and lysosomes and were proposed to mediate endolysosomal calcium release triggered by the second messenger, nicotinic acid adenine dinucleotide phosphate (NAADP). By directly recording TPCs in endolysosomes from wild-type and TPC double-knockout mice, here we show that, in contrast to previous conclusions, TPCs are in fact sodium-selective channels activated by PI(3,5)P(2) and are not activated by NAADP. Moreover, the primary endolysosomal ion is Na(+), not K(+), as had been previously assumed. These findings suggest that the organellar membrane potential may undergo large regulatory changes and may explain the specificity of PI(3,5)P(2) in regulating the fusogenic potential of intracellular organelles.

TRPV4 is a regulator of adipose oxidative metabolism, inflammation, and energy homeostasis.

PGC1α is a key transcriptional coregulator of oxidative metabolism and thermogenesis. Through a high-throughput chemical screen, we found that molecules antagonizing the TRPVs (transient receptor potential vanilloid), a family of ion channels, induced PGC1α expression in adipocytes. In particular, TRPV4 negatively regulated the expression of PGC1α, UCP1, and cellular respiration. Additionally, it potently controlled the expression of multiple proinflammatory genes involved in the development of insulin resistance. Mice with a null mutation for TRPV4 or wild-type mice treated with a TRPV4 antagonist showed elevated thermogenesis in adipose tissues and were protected from diet-induced obesity, adipose inflammation, and insulin resistance. This role of TRPV4 as a cell-autonomous mediator for both the thermogenic and proinflammatory programs in adipocytes could offer a target for treating obesity and related metabolic diseases.

TRP channel regulates EGFR signaling in hair morphogenesis and skin barrier formation.

A plethora of growth factors regulate keratinocyte proliferation and differentiation that control hair morphogenesis and skin barrier formation. Wavy hair phenotypes in mice result from naturally occurring loss-of-function mutations in the genes for TGF-alpha and EGFR. Conversely, excessive activities of TGF-alpha/EGFR result in hairless phenotypes and skin cancers. Unexpectedly, we found that mice lacking the Trpv3 gene also exhibit wavy hair coat and curly whiskers. Here we show that keratinocyte TRPV3, a member of the transient receptor potential (TRP) family of Ca(2+)-permeant channels, forms a signaling complex with TGF-alpha/EGFR. Activation of EGFR leads to increased TRPV3 channel activity, which in turn stimulates TGF-alpha release. TRPV3 is also required for the formation of the skin barrier by regulating the activities of transglutaminases, a family of Ca(2+)-dependent crosslinking enzymes essential for keratinocyte cornification. Our results show that a TRP channel plays a role in regulating growth factor signaling by direct complex formation.

A STIMulus Package puts orai calcium channels to work.

Activation of plasma membrane calcium channels by depletion of endoplasmic reticulum (ER) calcium stores is important for calcium entry in many cell types. Park et al. (2009) now show that direct binding of the ER protein STIM to tetramers of the Orai1 calcium channel in the plasma membrane triggers opening of this channel.

Essential role for TRPC5 in amygdala function and fear-related behavior.

The transient receptor potential channel 5 (TRPC5) is predominantly expressed in the brain where it can form heterotetrameric complexes with TRPC1 and TRPC4 channel subunits. These excitatory, nonselective cationic channels are regulated by G protein, phospholipase C-coupled receptors. Here, we show that TRPC5(-/-) mice exhibit diminished innate fear levels in response to innately aversive stimuli. Moreover, mutant mice exhibited significant reductions in responses mediated by synaptic activation of Group I metabotropic glutamate and cholecystokinin 2 receptors in neurons of the amygdala. Synaptic strength at afferent inputs to the amygdala was diminished in P10-P13 null mice. In contrast, baseline synaptic transmission, membrane excitability, and spike timing-dependent long-term potentiation at cortical and thalamic inputs to the amygdala were largely normal in older null mice. These experiments provide genetic evidence that TRPC5, activated via G protein-coupled neuronal receptors, has an essential function in innate fear.

Employing NaChBac for cryo-EM analysis of toxin action on voltage-gated Na+ channels in nanodisc.

NaChBac, the first bacterial voltage-gated Na+ (Nav) channel to be characterized, has been the prokaryotic prototype for studying the structure-function relationship of Nav channels. Discovered nearly two decades ago, the structure of NaChBac has not been determined. Here we present the single particle electron cryomicroscopy (cryo-EM) analysis of NaChBac in both detergent micelles and nanodiscs. Under both conditions, the conformation of NaChBac is nearly identical to that of the potentially inactivated NavAb. Determining the structure of NaChBac in nanodiscs enabled us to examine gating modifier toxins (GMTs) of Nav channels in lipid bilayers. To study GMTs in mammalian Nav channels, we generated a chimera in which the extracellular fragment of the S3 and S4 segments in the second voltage-sensing domain from Nav1.7 replaced the corresponding sequence in NaChBac. Cryo-EM structures of the nanodisc-embedded chimera alone and in complex with HuwenToxin IV (HWTX-IV) were determined to 3.5 and 3.2 Å resolutions, respectively. Compared to the structure of HWTX-IV-bound human Nav1.7, which was obtained at an overall resolution of 3.2 Å, the local resolution of the toxin has been improved from ∼6 to ∼4 Å. This resolution enabled visualization of toxin docking. NaChBac can thus serve as a convenient surrogate for structural studies of the interactions between GMTs and Nav channels in a membrane environment.

Structure of full-length human TRPM4.

Transient receptor potential melastatin subfamily member 4 (TRPM4) is a widely distributed, calcium-activated, monovalent-selective cation channel. Mutations in human TRPM4 (hTRPM4) result in progressive familial heart block. Here, we report the electron cryomicroscopy structure of hTRPM4 in a closed, Na+-bound, apo state at pH 7.5 to an overall resolution of 3.7 Å. Five partially hydrated sodium ions are proposed to occupy the center of the conduction pore and the entrance to the coiled-coil domain. We identify an upper gate in the selectivity filter and a lower gate at the entrance to the cytoplasmic coiled-coil domain. Intramolecular interactions exist between the TRP domain and the S4-S5 linker, N-terminal domain, and N and C termini. Finally, we identify aromatic interactions via π-π bonds and cation-π bonds, glycosylation at an N-linked extracellular site, a pore-loop disulfide bond, and 24 lipid binding sites. We compare and contrast this structure with other TRP channels and discuss potential mechanisms of regulation and gating of human full-length TRPM4.

Structure of the mammalian TRPM7, a magnesium channel required during embryonic development.

The transient receptor potential ion channel subfamily M, member 7 (TRPM7), is a ubiquitously expressed protein that is required for mouse embryonic development. TRPM7 contains both an ion channel and an α-kinase. The channel domain comprises a nonselective cation channel with notable permeability to Mg2+ and Zn2+ Here, we report the closed state structures of the mouse TRPM7 channel domain in three different ionic conditions to overall resolutions of 3.3, 3.7, and 4.1 Å. The structures reveal key residues for an ion binding site in the selectivity filter, with proposed partially hydrated Mg2+ ions occupying the center of the conduction pore. In high [Mg2+], a prominent external disulfide bond is found in the pore helix, which is essential for ion channel function. Our results provide a structural framework for understanding the TRPM1/3/6/7 subfamily and extend the knowledge base upon which to study the diversity and evolution of TRP channels.

Molecular basis of ion permeability in a voltage-gated sodium channel.

Voltage-gated sodium channels are essential for electrical signalling across cell membranes. They exhibit strong selectivities for sodium ions over other cations, enabling the finely tuned cascade of events associated with action potentials. This paper describes the ion permeability characteristics and the crystal structure of a prokaryotic sodium channel, showing for the first time the detailed locations of sodium ions in the selectivity filter of a sodium channel. Electrostatic calculations based on the structure are consistent with the relative cation permeability ratios (Na(+) ≈ Li(+) â‰« K(+), Ca(2+), Mg(2+)) measured for these channels. In an E178D selectivity filter mutant constructed to have altered ion selectivities, the sodium ion binding site nearest the extracellular side is missing. Unlike potassium ions in potassium channels, the sodium ions in these channels appear to be hydrated and are associated with side chains of the selectivity filter residues, rather than polypeptide backbones.

Histone phosphorylation by TRPM6's cleaved kinase attenuates adjacent arginine methylation to regulate gene expression.

TRPM6 and TRPM7 are members of the melastatin-related transient receptor potential (TRPM) subfamily of ion channels. Deletion of either gene in mice is embryonically lethal. TRPM6/7 are the only known examples of single polypeptides containing both an ion channel pore and a serine/threonine kinase (chanzyme). Here we show that the C-terminal kinase domain of TRPM6 is cleaved from the channel domain in a cell type-specific fashion and is active. Cleavage requires that the channel conductance is functional. The cleaved kinase translocates to the nucleus, where it is strictly localized and phosphorylates specific histone serine and threonine (S/T) residues. TRPM6-cleaved kinases (M6CKs) bind subunits of the protein arginine methyltransferase 5 (PRMT5) molecular complex that make important epigenetic modifications by methylating histone arginine residues. Histone phosphorylation by M6CK results in a dramatic decrease in methylation of arginines adjacent to M6CK-phosphorylated amino acids. Knockout of TRPM6 or inactivation of its kinase results in global changes in histone S/T phosphorylation and changes the transcription of hundreds of genes. We hypothesize that M6CK associates with the PRMT5 molecular complex in the nucleus, directing M6CK to a specific genomic location and providing site-specific histone phosphorylation. M6CK histone phosphorylation, in turn, regulates transcription by attenuating the effect of local arginine methylation.

TRPM7 senses oxidative stress to release Zn2+ from unique intracellular vesicles.

TRPM7 (transient receptor potential cation channel subfamily M member 7) regulates gene expression and stress-induced cytotoxicity and is required in early embryogenesis through organ development. Here, we show that the majority of TRPM7 is localized in abundant intracellular vesicles. These vesicles (M7Vs) are distinct from endosomes, lysosomes, and other familiar vesicles or organelles. M7Vs accumulate Zn2+ in a glutathione-enriched, reduced lumen when cytosolic Zn2+ concentrations are elevated. Treatments that increase reactive oxygen species (ROS) trigger TRPM7-dependent Zn2+ release from the vesicles, whereas reduced glutathione prevents TRPM7-dependent cytosolic Zn2+ influx. These observations strongly support the notion that ROS-mediated TRPM7 activation releases Zn2+ from intracellular vesicles after Zn2+ overload. Like the endoplasmic reticulum, these vesicles are a distributed system for divalent cation uptake and release, but in this case the primary divalent ion is Zn2+ rather than Ca2.

Isomeric Tuning Yields Bright and Targetable Red Ca2+ Indicators.

Targeting small-molecule fluorescent indicators using genetically encoded protein tags yields new hybrid sensors for biological imaging. Optimization of such systems requires redesign of the synthetic indicator to allow cell-specific targeting without compromising the photophysical properties or cellular performance of the small-molecule probe. We developed a bright and sensitive Ca2+ indicator by systematically exploring the relative configuration of dye and chelator, which can be targeted using the HaloTag self-labeling tag system. Our "isomeric tuning" approach is generalizable, yielding a far-red targetable indicator to visualize Ca2+ fluxes in the primary cilium.

Direct recording and molecular identification of the calcium channel of primary cilia.

A primary cilium is a solitary, slender, non-motile protuberance of structured microtubules (9+0) enclosed by plasma membrane. Housing components of the cell division apparatus between cell divisions, primary cilia also serve as specialized compartments for calcium signalling and hedgehog signalling pathways. Specialized sensory cilia such as retinal photoreceptors and olfactory cilia use diverse ion channels. An ion current has been measured from primary cilia of kidney cells, but the responsible genes have not been identified. The polycystin proteins (PC and PKD), identified in linkage studies of polycystic kidney disease, are candidate channels divided into two structural classes: 11-transmembrane proteins (PKD1, PKD1L1 and PKD1L2) remarkable for a large extracellular amino terminus of putative cell adhesion domains and a G-protein-coupled receptor proteolytic site, and the 6-transmembrane channel proteins (PKD2, PKD2L1 and PKD2L2; TRPPs). Evidence indicates that the PKD1 proteins associate with the PKD2 proteins via coiled-coil domains. Here we use a transgenic mouse in which only cilia express a fluorophore and use it to record directly from primary cilia, and demonstrate that PKD1L1 and PKD2L1 form ion channels at high densities in several cell types. In conjunction with an accompanying manuscript, we show that the PKD1L1-PKD2L1 heteromeric channel establishes the cilia as a unique calcium compartment within cells that modulates established hedgehog pathways.

Primary cilia are specialized calcium signalling organelles.

Primary cilia are solitary, non-motile extensions of the centriole found on nearly all nucleated eukaryotic cells between cell divisions. Only ∼200-300 nm in diameter and a few micrometres long, they are separated from the cytoplasm by the ciliary neck and basal body. Often called sensory cilia, they are thought to receive chemical and mechanical stimuli and initiate specific cellular signal transduction pathways. When activated by a ligand, hedgehog pathway proteins, such as GLI2 and smoothened (SMO), translocate from the cell into the cilium. Mutations in primary ciliary proteins are associated with severe developmental defects. The ionic conditions, permeability of the primary cilia membrane, and effectiveness of the diffusion barriers between the cilia and cell body are unknown. Here we show that cilia are a unique calcium compartment regulated by a heteromeric TRP channel, PKD1L1-PKD2L1, in mice and humans. In contrast to the hypothesis that polycystin (PKD) channels initiate changes in ciliary calcium that are conducted into the cytoplasm, we show that changes in ciliary calcium concentration occur without substantially altering global cytoplasmic calcium. PKD1L1-PKD2L1 acts as a ciliary calcium channel controlling ciliary calcium concentration and thereby modifying SMO-activated GLI2 translocation and GLI1 expression.

Mitochondrial calcium uniporter regulator 1 (MCUR1) regulates the calcium threshold for the mitochondrial permeability transition.

During the mitochondrial permeability transition, a large channel in the inner mitochondrial membrane opens, leading to the loss of multiple mitochondrial solutes and cell death. Key triggers include excessive reactive oxygen species and mitochondrial calcium overload, factors implicated in neuronal and cardiac pathophysiology. Examining the differential behavior of mitochondrial Ca(2+) overload in Drosophila versus human cells allowed us to identify a gene, MCUR1, which, when expressed in Drosophila cells, conferred permeability transition sensitive to electrophoretic Ca(2+) uptake. Conversely, inhibiting MCUR1 in mammalian cells increased the Ca(2+) threshold for inducing permeability transition. The effect was specific to the permeability transition induced by Ca(2+), and such resistance to overload translated into improved cell survival. Thus, MCUR1 expression regulates the Ca(2+) threshold required for permeability transition.