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Liang Z, Wilson CE, Teng B, Kinnamon SC, Liman ER. The proton channel OTOP1 is a sensor for the taste of ammonium chloride. Nat Commun 2023; 14:6194. [PMID: 37798269 PMCID: PMC10556057 DOI: 10.1038/s41467-023-41637-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2023] [Accepted: 09/12/2023] [Indexed: 10/07/2023] Open
Abstract
Ammonium (NH4+), a breakdown product of amino acids that can be toxic at high levels, is detected by taste systems of organisms ranging from C. elegans to humans and has been used for decades in vertebrate taste research. Here we report that OTOP1, a proton-selective ion channel expressed in sour (Type III) taste receptor cells (TRCs), functions as sensor for ammonium chloride (NH4Cl). Extracellular NH4Cl evoked large dose-dependent inward currents in HEK-293 cells expressing murine OTOP1 (mOTOP1), human OTOP1 and other species variants of OTOP1, that correlated with its ability to alkalinize the cell cytosol. Mutation of a conserved intracellular arginine residue (R292) in the mOTOP1 tm 6-tm 7 linker specifically decreased responses to NH4Cl relative to acid stimuli. Taste responses to NH4Cl measured from isolated Type III TRCs, or gustatory nerves were strongly attenuated or eliminated in an Otop1-/- mouse strain. Behavioral aversion of mice to NH4Cl, reduced in Skn-1a-/- mice lacking Type II TRCs, was entirely abolished in a double knockout with Otop1. These data together reveal an unexpected role for the proton channel OTOP1 in mediating a major component of the taste of NH4Cl and a previously undescribed channel activation mechanism.
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Affiliation(s)
- Ziyu Liang
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, 90089, USA
- Program in Neuroscience, University of Southern California, Los Angeles, CA, 90089, USA
| | - Courtney E Wilson
- Department of Otolaryngology, University of Colorado Medical School, 12700 E 19(th) Avenue, MS 8606, Aurora, CO, 80045, USA
| | - Bochuan Teng
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, 90089, USA
- Program in Neuroscience, University of Southern California, Los Angeles, CA, 90089, USA
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Sue C Kinnamon
- Department of Otolaryngology, University of Colorado Medical School, 12700 E 19(th) Avenue, MS 8606, Aurora, CO, 80045, USA
| | - Emily R Liman
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, 90089, USA.
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2
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Burendei B, Kaplan J, Forli S, Liman ER, Ward AB. Small molecule discovery for modulation of Otopetrin proton channels. Biophys J 2023; 122:522a-523a. [PMID: 36784701 DOI: 10.1016/j.bpj.2022.11.2776] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/12/2023] Open
Affiliation(s)
- Batuujin Burendei
- Department of Integrative Structural and Computational Biology, Scripps Research Institute, La Jolla, CA, USA
| | - Joshua Kaplan
- Section of Neurobiology, University of Southern California, Los Angeles, CA, USA; Program in Neuroscience, University of Southern California, Los Angeles, CA, USA
| | - Stefano Forli
- Department of Integrative Structural and Computational Biology, Scripps Research Institute, La Jolla, CA, USA
| | - Emily R Liman
- Section of Neurobiology, University of Southern California, Los Angeles, CA, USA; Program in Neuroscience, University of Southern California, Los Angeles, CA, USA
| | - Andrew B Ward
- Department of Integrative Structural and Computational Biology, Scripps Research Institute, La Jolla, CA, USA
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3
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Abstract
The evolutionary history of sour taste has been little studied. Through a combination of literature review and trait mapping on the vertebrate phylogenetic tree, we consider the origin of sour taste, potential cases of the loss of sour taste, and those factors that might have favoured changes in the valence of sour taste-from aversive to appealing. We reconstruct sour taste as having evolved in ancient fish. By contrast to other tastes, sour taste does not appear to have been lost in any major vertebrate taxa. For most species, sour taste is aversive. Animals, including humans, that enjoy the sour taste triggered by acidic foods are exceptional. We conclude by considering why sour taste evolved, why it might have persisted as vertebrates made the transition to land and what factors might have favoured the preference for sour-tasting, acidic foods, particularly in hominins, such as humans.
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Affiliation(s)
- Hannah E. R. Frank
- Department of Crop and Soil Sciences North Carolina State University, Raleigh, USA
| | - Katie Amato
- Department of Anthropology, Northwestern University, Evanston, IL, USA
| | - Michelle Trautwein
- Entomology Department, Institute for Biodiversity Science and Sustainability, California Academy of Sciences, San Francisco, USA
| | - Paula Maia
- Department of Anthropology, Northwestern University, Evanston, IL, USA
| | - Emily R. Liman
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA, USA
| | - Lauren M. Nichols
- Department of Applied Ecology, North Carolina State University, Raleigh, USA
| | - Kurt Schwenk
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT, USA
| | - Paul A. S. Breslin
- Department of Nutritional Sciences, Rutgers The State University of New Jersey, New Brunswick, NJ, USA,Monell Chemical Senses Center, Philadelphia, PA, USA
| | - Robert R. Dunn
- Department of Applied Ecology, North Carolina State University, Raleigh, USA,Center for Evolutionary Hologenomics, University of Copenhagen, Copenhagen, Denmark
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4
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Teng B, Tu YH, Kaplan J, Liang Z, Krieger Z, Liman ER. Subtype-specific gating mechanisms of OTOP channels by protons. Biophys J 2022. [DOI: 10.1016/j.bpj.2021.11.1482] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022] Open
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5
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Teng B, Kaplan JP, Liang Z, Krieger Z, Tu YH, Burendei B, Ward AB, Liman ER. Structural motifs for subtype-specific pH-sensitive gating of vertebrate otopetrin proton channels. eLife 2022; 11:77946. [PMID: 35920807 PMCID: PMC9348849 DOI: 10.7554/elife.77946] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2022] [Accepted: 07/19/2022] [Indexed: 01/05/2023] Open
Abstract
Otopetrin (OTOP) channels are proton-selective ion channels conserved among vertebrates and invertebrates, with no structural similarity to other ion channels. There are three vertebrate OTOP channels (OTOP1, OTOP2, and OTOP3), of which one (OTOP1) functions as a sour taste receptor. Whether extracellular protons gate OTOP channels, in addition to permeating them, was not known. Here, we compare the functional properties of the three murine OTOP channels using patch-clamp recording and cytosolic pH microfluorimetry. We find that OTOP1 and OTOP3 are both steeply activated by extracellular protons, with thresholds of pHo <6.0 and 5.5, respectively, and kinetics that are pH-dependent. In contrast, OTOP2 channels are broadly active over a large pH range (pH 5 pH 10) and carry outward currents in response to extracellular alkalinization (>pH 9.0). Strikingly, we could change the pH-sensitive gating of OTOP2 and OTOP3 channels by swapping extracellular linkers that connect transmembrane domains. Swaps of extracellular linkers in the N domain, comprising transmembrane domains 1-6, tended to change the relative conductance at alkaline pH of chimeric channels, while swaps within the C domain, containing transmembrane domains 7-12, tended to change the rates of OTOP3 current activation. We conclude that members of the OTOP channel family are proton-gated (acid-sensitive) proton channels and that the gating apparatus is distributed across multiple extracellular regions within both the N and C domains of the channels. In addition to the taste system, OTOP channels are expressed in the vertebrate vestibular and digestive systems. The distinct gating properties we describe may allow them to subserve varying cell-type specific functions in these and other biological systems.
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Affiliation(s)
- Bochuan Teng
- Section of Neurobiology, Department of Biological Sciences, University of Southern CaliforniaLos AngelesUnited States,Program in Neuroscience, University of Southern CaliforniaLos AngelesUnited States
| | - Joshua P Kaplan
- Section of Neurobiology, Department of Biological Sciences, University of Southern CaliforniaLos AngelesUnited States,Program in Neuroscience, University of Southern CaliforniaLos AngelesUnited States
| | - Ziyu Liang
- Section of Neurobiology, Department of Biological Sciences, University of Southern CaliforniaLos AngelesUnited States,Program in Neuroscience, University of Southern CaliforniaLos AngelesUnited States
| | - Zachary Krieger
- Section of Neurobiology, Department of Biological Sciences, University of Southern CaliforniaLos AngelesUnited States
| | - Yu-Hsiang Tu
- Diabetes, Endocrinology, and Obesity Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of HealthBethesdaUnited States
| | - Batuujin Burendei
- Department of Integrative Structural and Computational Biology, The Scripps Research InstituteLa JollaUnited States
| | - Andrew B Ward
- Department of Integrative Structural and Computational Biology, The Scripps Research InstituteLa JollaUnited States
| | - Emily R Liman
- Section of Neurobiology, Department of Biological Sciences, University of Southern CaliforniaLos AngelesUnited States,Program in Neuroscience, University of Southern CaliforniaLos AngelesUnited States
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6
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Abstract
Sour taste, the taste of acids, is one of the most enigmatic of the five basic taste qualities; its function is unclear and its receptor was until recently unknown. Sour tastes are transduced in taste buds on the tongue and palate epithelium by a subset of taste receptor cells, known as type III cells. Type III cells express a number of unique markers, including the PKD2L1 gene, which allow for their identification and manipulation. These cells respond to acid stimuli with action potentials and release neurotransmitters onto afferent nerve fibers, with cell bodies in geniculate and petrosal ganglia. Here, we review classical studies of sour taste leading up to the identification of the sour receptor as the proton channel, OTOP1. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
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Affiliation(s)
- Heather N Turner
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, California, USA; ,
| | - Emily R Liman
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, California, USA; ,
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7
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Liman ER. Insights into a receptor that lets insects sense scents. Nature 2021; 597:37-39. [PMID: 34349272 DOI: 10.1038/d41586-021-02076-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
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8
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Abstract
Sour taste, which is evoked by low pH, is one of the original four fundamental taste qualities, recognized as a distinct taste sensation for centuries, and universally aversive across diverse species. It is generally assumed to have evolved for detection of acids in unripe fruit and spoiled food. But despite decades of study, only recently have the receptor, the neurotransmitter, and the circuits for sour taste been identified. In this review, we describe studies leading up to the identification of the sour receptor as OTOP1, an ion channel that is selectively permeable to protons. We also describe advances in our understanding of how information is transmitted from the taste receptor cells to gustatory neurons, leading to behavioral aversion to acids.
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Affiliation(s)
- Emily R Liman
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, 3641 Watt Way, Los Angeles, CA 90089, USA
| | - Sue C Kinnamon
- Department of Otolaryngology and Rocky Mountain Taste and Smell Center, University of Colorado Anschutz Medical Campus, 12700 E 19(th) Avenue, Aurora, CO 80045, USA
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9
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Abstract
Detection of NaCl by the gustatory system is fundamental for salt intake and tissue homeostasis. Yet, signal transduction mechanisms for salty taste have remained obscure. In this issue of Neuron, Nomura et al. (2020) report that the epithelial sodium channel ENaC, which serves as the salty receptor, is co-expressed with the voltage-activated ATP release channel CALHM1/3 in a subset of taste cells and that these cells mediate amiloride-sensitive salty taste.
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Affiliation(s)
- Emily R Liman
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, 3641 Watt Way, Los Angeles, CA 90089, USA.
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10
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Saotome K, Teng B, Tsui CCA, Lee WH, Tu YH, Kaplan JP, Sansom MSP, Liman ER, Ward AB. Structures of the otopetrin proton channels Otop1 and Otop3. Nat Struct Mol Biol 2019; 26:518-525. [PMID: 31160780 PMCID: PMC6564688 DOI: 10.1038/s41594-019-0235-9] [Citation(s) in RCA: 29] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2019] [Accepted: 04/26/2019] [Indexed: 01/08/2023]
Abstract
Otopetrins (Otop1-Otop3) comprise one of two known eukaryotic proton-selective channel families. Otop1 is required for otoconia formation and a candidate mammalian sour taste receptor. Here we report cryo-EM structures of zebrafish Otop1 and chicken Otop3 in lipid nanodiscs. The structures reveal a dimeric architecture, with each subunit forming 12 transmembrane helices divided into structurally similar amino (N) and carboxy (C) domains. Cholesterol-like molecules occupy various sites in Otop1 and Otop3 and occlude a central tunnel. In molecular dynamics simulations, hydrophilic vestibules formed by the N and C domains and in the intrasubunit interface between N and C domains form conduits for water entry into the membrane core, suggesting three potential proton conduction pathways. By mutagenesis, we tested the roles of charged residues in each putative permeation pathway. Our results provide a structural basis for understanding selective proton permeation and gating of this conserved family of proton channels.
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Affiliation(s)
- Kei Saotome
- Department of Integrative Structural and Computational Biology, The Scripps Research Institute, La Jolla, CA, USA
- Howard Hughes Medical Institute, Department of Neuroscience, Dorris Neuroscience Center, The Scripps Research Institute, La Jolla, CA, USA
| | - Bochuan Teng
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA
| | - Che Chun Alex Tsui
- Department of Integrative Structural and Computational Biology, The Scripps Research Institute, La Jolla, CA, USA
- Department of Biochemistry, University of Oxford, Oxford, UK
| | - Wen-Hsin Lee
- Department of Integrative Structural and Computational Biology, The Scripps Research Institute, La Jolla, CA, USA
| | - Yu-Hsiang Tu
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA
| | - Joshua P Kaplan
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA
| | - Mark S P Sansom
- Department of Biochemistry, University of Oxford, Oxford, UK
| | - Emily R Liman
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA.
| | - Andrew B Ward
- Department of Integrative Structural and Computational Biology, The Scripps Research Institute, La Jolla, CA, USA.
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11
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Tu YH, Cooper AJ, Teng B, Chang RB, Artiga DJ, Turner HN, Mulhall EM, Ye W, Smith AD, Liman ER. An evolutionarily conserved gene family encodes proton-selective ion channels. Science 2018; 359:1047-1050. [PMID: 29371428 DOI: 10.1126/science.aao3264] [Citation(s) in RCA: 144] [Impact Index Per Article: 24.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/13/2017] [Accepted: 01/08/2018] [Indexed: 12/20/2022]
Abstract
Ion channels form the basis for cellular electrical signaling. Despite the scores of genetically identified ion channels selective for other monatomic ions, only one type of proton-selective ion channel has been found in eukaryotic cells. By comparative transcriptome analysis of mouse taste receptor cells, we identified Otopetrin1 (OTOP1), a protein required for development of gravity-sensing otoconia in the vestibular system, as forming a proton-selective ion channel. We found that murine OTOP1 is enriched in acid-detecting taste receptor cells and is required for their zinc-sensitive proton conductance. Two related murine genes, Otop2 and Otop3, and a Drosophila ortholog also encode proton channels. Evolutionary conservation of the gene family and its widespread tissue distribution suggest a broad role for proton channels in physiology and pathophysiology.
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Affiliation(s)
- Yu-Hsiang Tu
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA
| | - Alexander J Cooper
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA
| | - Bochuan Teng
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA
| | - Rui B Chang
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA
| | - Daniel J Artiga
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA
| | - Heather N Turner
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA
| | - Eric M Mulhall
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA
| | - Wenlei Ye
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA.
| | - Andrew D Smith
- Department of Biological Sciences, Section of Molecular and Computational Biology, University of Southern California, Los Angeles, CA 90089, USA
| | - Emily R Liman
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA. .,Bridge Institute, University of Southern California, Los Angeles, CA 90089, USA
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12
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Ye W, Chang RB, Bushman JD, Tu YH, Mulhall E, Wilson CE, Cooper AJ, Chick WS, Hill-Eubanks D, Nelson MT, Kinnamon SC, Liman ER. Inhibition of KIR2.1 by Intracellular Acidification Contributes to Sour Taste Transduction. Biophys J 2016. [DOI: 10.1016/j.bpj.2015.11.2294] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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13
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Bushman JD, Ye W, Liman ER. A proton current associated with sour taste: distribution and functional properties. FASEB J 2015; 29:3014-26. [PMID: 25857556 DOI: 10.1096/fj.14-265694] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2014] [Accepted: 03/11/2015] [Indexed: 02/06/2023]
Abstract
Sour taste is detected by taste receptor cells that respond to acids through yet poorly understood mechanisms. The cells that detect sour express the protein PKD2L1, which is not the sour receptor but nonetheless serves as a useful marker for sour cells. By use of mice in which the PKD2L1 promoter drives expression of yellow fluorescent protein, we previously reported that sour taste cells from circumvallate papillae in the posterior tongue express a proton current. To establish a correlation between this current and sour transduction, we examined its distribution by patch-clamp recording. We find that the current is present in PKD2L1-expressing taste cells from mouse circumvallate, foliate, and fungiform papillae but not in a variety of other cells, including spinal cord neurons that express PKD2L1. We describe biophysical properties of the current, including pH-dependent Zn(2+) inhibition, lack of voltage-dependent gating, and activation at modest pH values (6.5) that elicit action potentials in isolated cells. Consistent with a channel that is constitutively open, the cytosol of sour taste cells is acidified. These data define a functional signature for the taste cell proton current and indicate that its expression is mostly restricted to the subset of taste cells that detect sour.
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Affiliation(s)
- Jeremy D Bushman
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, Los Angeles, California, USA
| | - Wenlei Ye
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, Los Angeles, California, USA
| | - Emily R Liman
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, Los Angeles, California, USA
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14
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Abstract
Five canonical tastes, bitter, sweet, umami (amino acid), salty, and sour (acid), are detected by animals as diverse as fruit flies and humans, consistent with a near-universal drive to consume fundamental nutrients and to avoid toxins or other harmful compounds. Surprisingly, despite this strong conservation of basic taste qualities between vertebrates and invertebrates, the receptors and signaling mechanisms that mediate taste in each are highly divergent. The identification over the last two decades of receptors and other molecules that mediate taste has led to stunning advances in our understanding of the basic mechanisms of transduction and coding of information by the gustatory systems of vertebrates and invertebrates. In this Review, we discuss recent advances in taste research, mainly from the fly and mammalian systems, and we highlight principles that are common across species, despite stark differences in receptor types.
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Affiliation(s)
- Emily R Liman
- Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA.
| | - Yali V Zhang
- Neuroscience Research Institute, University of California Santa Barbara, Santa Barbara, CA 93106, USA
| | - Craig Montell
- Neuroscience Research Institute, University of California Santa Barbara, Santa Barbara, CA 93106, USA; Department of Molecular, Cellular and Developmental Biology, University of California Santa Barbara, Santa Barbara, CA 93106, USA.
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15
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Abstract
TRPM5 is a Ca(2+)-activated cation channel that mediates signaling in taste and other chemosensory cells. Within taste cells, TRPM5 is the final element in a signaling cascade that starts with the activation of G protein-coupled receptors by bitter, sweet, or umami taste molecules and that requires the enzyme PLCβ2. PLCβ2 breaks down PIP2 into DAG and IP3, and the ensuing release of Ca(2+) from intracellular stores activates TRPM5. Since its initial discovery in the taste system, TRPM5 has been found to be distributed in sparse chemosensory cells located throughout the digestive track, in the respiratory system, and in the olfactory system. It is also found in pancreatic islets, where it contributes to insulin secretion. This review highlights recent work on the mechanisms of the activation of the TRPM5 channel and its regulation by voltage, phosphoinositides, temperature, and pH. The distribution of the channel in the body and its functional contribution to various sensory and nonsensory processes are discussed.
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Affiliation(s)
- Emily R Liman
- Section of Neurobiology, Department of Biological Sciences, University of Southern California, 3641 Watt Way, Los Angeles, CA, 90089, USA,
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16
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Gross GG, Junge JA, Mora RJ, Kwon HB, Olson CA, Takahashi TT, Liman ER, Ellis-Davies GCR, McGee AW, Sabatini BL, Roberts RW, Arnold DB. Recombinant probes for visualizing endogenous synaptic proteins in living neurons. Neuron 2013; 78:971-85. [PMID: 23791193 DOI: 10.1016/j.neuron.2013.04.017] [Citation(s) in RCA: 194] [Impact Index Per Article: 17.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 04/10/2013] [Indexed: 11/29/2022]
Abstract
The ability to visualize endogenous proteins in living neurons provides a powerful means to interrogate neuronal structure and function. Here we generate recombinant antibody-like proteins, termed Fibronectin intrabodies generated with mRNA display (FingRs), that bind endogenous neuronal proteins PSD-95 and Gephyrin with high affinity and that, when fused to GFP, allow excitatory and inhibitory synapses to be visualized in living neurons. Design of the FingR incorporates a transcriptional regulation system that ties FingR expression to the level of the target and reduces background fluorescence. In dissociated neurons and brain slices, FingRs generated against PSD-95 and Gephyrin did not affect the expression patterns of their endogenous target proteins or the number or strength of synapses. Together, our data indicate that PSD-95 and Gephyrin FingRs can report the localization and amount of endogenous synaptic proteins in living neurons and thus may be used to study changes in synaptic strength in vivo.
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Affiliation(s)
- Garrett G Gross
- Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA
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17
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Affiliation(s)
- Emily R Liman
- Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA.
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18
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Abstract
Acetic acid produces an irritating sensation that can be attributed to activation of nociceptors within the trigeminal ganglion that innervate the nasal or oral cavities. These sensory neurons sense a diverse array of noxious agents in the environment, allowing animals to actively avoid tissue damage. Although receptor mechanisms have been identified for many noxious chemicals, the mechanisms by which animals detect weak acids, such as acetic acid, are less well understood. Weak acids are only partially dissociated at neutral pH and, as such, some can cross the cell membrane, acidifying the cell cytosol. The nociceptor ion channel TRPA1 is activated by CO(2), through gating of the channel by intracellular protons, making it a candidate to more generally mediate sensory responses to weak acids. To test this possibility, we measured responses to weak acids from heterologously expressed TRPA1 channels and trigeminal neurons with patch clamp recording and Ca(2+) microfluorometry. Our results show that heterologously expressed TRPA1 currents can be induced by a series of weak organic acids, including acetic, propionic, formic, and lactic acid, but not by strong acids. Notably, the degree of channel activation was predicted by the degree of intracellular acidification produced by each acid, suggesting that intracellular protons are the proximate stimulus that gates the channel. Responses to weak acids produced a Ca(2+)-independent inactivation that precluded further activation by weak acids or reactive chemicals, whereas preactivation by reactive electrophiles sensitized TRPA1 channels to weak acids. Importantly, responses of trigeminal neurons to weak acids were highly overrepresented in the subpopulation of TRPA1-expressing neurons and were severely reduced in neurons from TRPA1 knockout mice. We conclude that TRPA1 is a general sensor for weak acids that produce intracellular acidification and suggest that it functions within the pain pathway to mediate sensitivity to cellular acidosis.
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Affiliation(s)
- Yuanyuan Y Wang
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA
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19
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Abstract
The release of insulin by pancreatic beta cells involves a complex interplay of conductances that generate oscillations and drive secretion. A recent report identifies a new player in this process, the ion channel TRPM5. TRPM5 was originally identified in taste cells, where it forms a Ca(2+)-activated cation channel that is required for sensory responses to bitter and sweet tastes. New research now shows that TRPM5 is expressed within the pancreatic islets of Langerhans, where it regulates the frequency of Ca(2+) oscillations and contributes to insulin release by β-cells.
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Affiliation(s)
- Emily R Liman
- Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA, USA.
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20
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Abstract
BACKGROUND The main olfactory epithelium (MOE) in the nasal cavity detects a variety of air borne molecules that provide information regarding the presence of food, predators and other relevant social and environmental factors. Within the epithelium are ciliated sensory neurons, supporting cells, basal cells and microvillous cells, each of which is distinct in morphology and function. Arguably, the least understood, are the microvillous cells, a population of cells that are small in number and whose function is not known. We previously found that in a mouse strain in which the TRPM5 promoter drives expression of the green fluorescent protein (GFP), a population of ciliated olfactory sensory neurons (OSNs), as well as a population of cells displaying microvilli-like structures is labeled. Here we examined the morphology and immunocytochemical properties of these microvillous-like cells using immunocytochemical methods. RESULTS We show that the GFP-positive microvillous cells were morphologically diversified and scattered throughout the entire MOE. These cells immunoreacted to an antibody against TRPM5, confirming the expression of this ion channel in these cells. In addition, they showed a Ca2+-activated non-selective cation current in electrophysiological recordings. They did not immunoreact to antibodies that label cell markers and elements of the transduction pathways from olfactory sensory neurons and solitary chemosensory cells of the nasal cavity. Further, the TRPM5-expressing cells did not display axon-like processes and were not labeled with a neuronal marker nor did trigeminal peptidergic nerve fibers innervate these cells. CONCLUSION We provide morphological and immunocytochemical characterization of the TRPM5-expressing microvillous cells in the main olfactory epithelium. Our data demonstrate that these cells are non-neuronal and in terms of chemosensory transduction do not resemble the TRPM5-expressing olfactory sensory neurons and nasal solitary chemosensory cells.
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Affiliation(s)
- Weihong Lin
- Department of Biological Sciences, University of Maryland, Baltimore County, Baltimore, MD 21250, USA.
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Wang YY, Chang RB, Waters HN, McKemy DD, Liman ER. The nociceptor ion channel TRPA1 is potentiated and inactivated by permeating calcium ions. J Biol Chem 2008; 283:32691-703. [PMID: 18775987 DOI: 10.1074/jbc.m803568200] [Citation(s) in RCA: 203] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The transient receptor potential A1 (TRPA1) channel is the molecular target for environmental irritants and pungent chemicals, such as cinnamaldehyde and mustard oil. Extracellular Ca(2+) is a key regulator of TRPA1 activity, both potentiating and subsequently inactivating it. In this report, we provide evidence that the effect of extracellular Ca(2+) on these processes is indirect and can be entirely attributed to entry through TRPA1 and subsequent elevation of intracellular calcium. Specifically, we found that in a pore mutant of TRPA1, D918A, in which Ca(2+) permeability was greatly reduced, extracellular Ca(2+) produced neither potentiation nor inactivation. Both processes were restored by reducing intracellular Ca(2+) buffering, which allowed intracellular Ca(2+) levels to become elevated upon entry through D918A channels. Application of Ca(2+) to the cytosolic face of excised patches was sufficient to produce both potentiation and inactivation of TRPA1 channels. Moreover, in whole cell recordings, elevation of intracellular Ca(2+) by UV uncaging of 1-(4,5-dimethoxy-2-nitrophenyl)-EDTA-potentiated TRPA1 currents. In addition, our data show that potentiation and inactivation are independent processes. TRPA1 currents could be inactivated by Mg(2+), Ba(2+), and Ca(2+) but potentiated only by Ba(2+) and Ca(2+). Saturating activation by cinnamaldehyde or mustard oil occluded potentiation but did not interfere with inactivation. Last, neither process was affected by mutation of a putative intracellular Ca(2+)-binding EF-hand motif. In conclusion, we have further clarified the mechanisms of potentiation and inactivation of TRPA1 using the D918A pore mutant, an important tool for investigating the contribution of Ca(2+) influx through TRPA1 to nociceptive signaling.
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Affiliation(s)
- Yuanyuan Y Wang
- Department of Biological Sciences, University of Southern California, Los Angeles, California 90089-2520, USA
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22
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Zhang H, Xu Y, Zhang Z, Liman ER, Prestwich GD. Synthesis and biological activity of phospholipase C-resistant analogues of phosphatidylinositol 4,5-bisphosphate. J Am Chem Soc 2007; 128:5642-3. [PMID: 16637624 PMCID: PMC2531207 DOI: 10.1021/ja060621d] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) is an important regulator in cell physiology. Hydrolysis of PtdIns(4,5)P2 by phospholipase C (PLC) releases two second messengers, Ins(1,4,5)P3 and diacylglycerol. To dissect the effects of PtdIns(4,5)P2 from those resulting from PLC-generated signals, a metabolically stabilized analogue of PtdIns(4,5)P2 was required. Two analogues were designed in which the scissile O-P bond was replaced with a C-P bond that could not be hydrolyzed by PLC activity. Herein we describe the asymmetric total synthesis of the first metabolically stabilized phospholipase C-resistant analogues of PtdIns(4,5)P2. The key transformation was a Pd(0)-catalyzed coupling of a H-phosphite with a vinyl bromide to form the desired C-P linkage. The phosphonate analogues of PtdIns(4,5)P2 were found to be effective in restoring the sensitivity of the TRPM4 channel to Ca2+ activation.
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Affiliation(s)
- Honglu Zhang
- Department of Medicinal Chemistry, The University of Utah, 419 Wakara Way, Suite 205, Salt Lake City, Utah 84108-1257 USA
| | - Yong Xu
- Department of Medicinal Chemistry, The University of Utah, 419 Wakara Way, Suite 205, Salt Lake City, Utah 84108-1257 USA
| | - Zheng Zhang
- Department of Biological Sciences and Program in Neuroscience, University of Southern California, 3641 Watt Way, Los Angeles, California 90089-2520 USA
| | - Emily R. Liman
- Department of Biological Sciences and Program in Neuroscience, University of Southern California, 3641 Watt Way, Los Angeles, California 90089-2520 USA
| | - Glenn D Prestwich
- Department of Medicinal Chemistry, The University of Utah, 419 Wakara Way, Suite 205, Salt Lake City, Utah 84108-1257 USA
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23
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Young JM, Waters H, Dong C, Fülle HJ, Liman ER. Degeneration of the olfactory guanylyl cyclase D gene during primate evolution. PLoS One 2007; 2:e884. [PMID: 17849013 PMCID: PMC1964805 DOI: 10.1371/journal.pone.0000884] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2007] [Accepted: 08/20/2007] [Indexed: 02/04/2023] Open
Abstract
Background The mammalian olfactory system consists of several subsystems that detect specific sets of chemical cues and underlie a variety of behavioral responses. Within the main olfactory epithelium at least three distinct types of chemosensory neurons can be defined by their expression of unique sets of signal transduction components. In rodents, one set of neurons expresses the olfactory-specific guanylyl cyclase (GC)-D gene (Gucy2d, guanylyl cyclase 2d) and other cell-type specific molecules. GC-D-positive neurons project their axons to a small group of atypical “necklace” glomeruli in the olfactory bulb, some of which are activated in response to suckling in neonatal rodents and to atmospheric CO2 in adult mice. Because GC-D is a pseudogene in humans, signaling through this system appears to have been lost at some point in primate evolution. Principal Findings Here we used a combination of bioinformatic analysis of trace-archive and genome-assembly data and sequencing of PCR-amplified genomic DNA to determine when during primate evolution the functional gene was lost. Our analysis reveals that GC-D is a pseudogene in a large number of primate species, including apes, Old World and New World monkeys and tarsier. In contrast, the gene appears intact and has evolved under purifying selection in mouse, rat, dog, lemur and bushbaby. Conclusions These data suggest that signaling through GC-D-expressing cells was probably compromised more than 40 million years ago, prior to the divergence of New World monkeys from Old World monkeys and apes, and thus cannot be involved in chemosensation in most primates.
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Affiliation(s)
- Janet M. Young
- Division of Human Biology, Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America
- * To whom correspondence should be addressed. E-mail: (EL), (JY)
| | - Hang Waters
- Department of Biological Sciences, University of Southern California, Los Angeles, California, United States of America
| | - Cora Dong
- Department of Biological Sciences, University of Southern California, Los Angeles, California, United States of America
| | - Hans-Jürgen Fülle
- Departments of Cell and Neurobiology and Ophthalmology, Keck School of Medicine, University of Southern California, Los Angeles, California, United States of America
| | - Emily R. Liman
- Department of Biological Sciences, University of Southern California, Los Angeles, California, United States of America
- Program in Neuroscience, University of Southern California, Los Angeles, California, United States of America
- * To whom correspondence should be addressed. E-mail: (EL), (JY)
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Abstract
TRPM5 is a cation channel that it is essential for transduction of bitter, sweet and umami tastes. Signaling of these tastes involves the activation of G protein-coupled receptors that stimulate phospholipase C (PLC) beta2, leading to the breakdown of phosphatidylinositol bisphosphate (PIP2) into diacylglycerol (DAG) and inositol trisphosphate (IP3), and release of Ca2+ from intracellular stores. TRPM5 forms a nonselective cation channel that is directly activated by Ca2+ and it is likely to be the downstream target of this signaling cascade. Therefore, study of TRPM5 promises to provide insight into fundamental mechanisms of taste transduction. This review highlights recent work on the mechanisms of activation of the TRPM5 channel. The mouse TRPM5 gene encodes a protein of 1,158 amino acids that is proposed to have six transmembrane domains and to function as a tetramer. TRPM5 is structurally most closely related to the Ca(2+)-activated channel TRPM4 and it is more distantly related to the cold-activated channel TRPM8. In patch clamp recordings, TRPM5 channels are activated by micromolar concentrations of Ca2+ and are permeable to monovalent but not divalent cations. TRPM5 channel activity is strongly regulated by voltage, phosphoinositides and temperature, and is blocked by acid pH. Study of TRPM4 and TRPM8, which show similar modes of regulation, has yielded insights into possible structural domains of TRPM5. Understanding the structural basis for TRPM5 function will ultimately allow the design of pharmaceuticals to enhance or interfere with taste sensations.
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Affiliation(s)
- E R Liman
- University of Southern California, 3641 Watt Way, Los Angeles, CA 90089, USA.
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25
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Abstract
Sensory organs provide key and, in many cases, species-specific information that allows animals to effectively forage, find mates, and avoid hazards. The primary sensors for the vertebrate senses of vision, taste, and smell are G-protein-coupled receptors (GPCRs) expressed by sensory receptor cells that initiate intracellular signal transduction cascades in response to activation by appropriate stimuli. The identification of sensory GPCRs and their related downstream transduction components from a variety of species has provided an essential tool for understanding the molecular evolution of sensory systems. Expansion of the number of genes encoding sensory GPCRs has, in some cases, expanded the repertoire of signals that animals detect, allowing them to occupy new niches, while, in other cases, evolution has favored a reduction in the repertoire of receptors and their cognate signal transduction components when these signals no longer provide a selective advantage. This review will focus on recent studies that have identified molecular changes in vision, smell, taste, and pheromone detection during primate evolution.
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Affiliation(s)
- Emily R Liman
- Neuroscience Program, Molecular and Computational Biology Program, Department of Biological Sciences, University of Southern California, 3641 Watt Way, Los Angeles, CA 90089, USA.
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26
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Abstract
Members of the family of transient receptor potential (TRP) ion channels mediate a wide range of sensory modalities, including thermosensation and taste. Among the "thermo-TRPs," some, such as TRPV1, are activated by warm temperatures, whereas others, such as TRPM8, are activated by cold. How is temperature able to have such strong and opposing effects on these related channels? Although at a structural level the answer to this question is not known, an elegant biophysical model has been proposed that accounts for the different thermosensitivities of TRP channels. This model posits that temperature acts by shifting the inherent weak voltage sensitivity of TRPV1 and TRPM8 in opposite directions, thus promoting opening of TRPV1 at warm temperatures and TRPM8 at cold temperatures. TRPM5, which is distantly related to TRPM8, is a Ca2+-activated cation channel expressed in taste cells that is essential for sweet, bitter, and umami tastes. Like TRPV1 and TRPM8, TRPM5 is weakly sensitive to voltage and thus may also be temperature sensitive. A recent report shows that activity of TRPM5 is increased at warm temperatures, suggesting that heat may enhance the perception of taste through direct modulation of the putative taste transduction channel.
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Affiliation(s)
- Emily R Liman
- Department of Biological Sciences, Program in Neuroscience, University of Southern California, Los Angeles, CA 90089, USA.
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27
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Abstract
TRPM4 is a Ca(2+)-activated nonselective cation channel that regulates membrane potential in response to intracellular Ca(2+) signaling. In lymphocytes it plays an essential role in shaping the pattern of intracellular Ca(2+) oscillations that lead to cytokine secretion. To better understand its role in this and other physiological processes, we investigated mechanisms by which TRPM4 is regulated. TRPM4 was expressed in ChoK1 cells, and currents were measured in excised patches. Under these conditions, TRPM4 currents were activated by micromolar concentrations of cytoplasmic Ca(2+) and progressively desensitized. Here we show that desensitization can be explained by a loss of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) from the channels. Poly-l-lysine, a PI(4,5)P(2) scavenger, caused rapid desensitization, whereas MgATP, at concentrations that activate lipid kinases, promoted recovery of TRPM4 currents. Application of exogenous PI(4,5)P(2) to the intracellular surface of the patch restored the properties of TRPM4 currents. Our results suggest that PI(4,5)P(2) acts to uncouple channel opening from changes in the transmembrane potential, allowing current activation at physiological voltages. These data argue that hydrolysis of PI(4,5)P(2) underlies desensitization of TRPM4 and support the idea that PI(4,5)P(2) is a general regulator for the gating of TRPM ion channels.
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Affiliation(s)
- Zheng Zhang
- Division of Neurobiology, Department of Biological Sciences and Program in Neuroscience, University of Southern California, Los Angeles, California 90089, USA
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28
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Zufall F, Ukhanov K, Lucas P, Liman ER, Leinders-Zufall T. Neurobiology of TRPC2: from gene to behavior. Pflugers Arch 2005; 451:61-71. [PMID: 15971083 DOI: 10.1007/s00424-005-1432-4] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2005] [Accepted: 04/01/2005] [Indexed: 11/25/2022]
Abstract
The mammalian vomeronasal organ (VNO), a part of the accessory olfactory system, plays an essential role in the sensing of pheromonal signals. The VNO has emerged as an excellent model to investigate the functional role of transient receptor potential (TRP) channels in intact neurons and intact physiological systems. TRPC2, a member of the (canonical) TRPC subfamily, is highly localized to the dendritic tip of vomeronasal sensory neurons. Phenotypic analysis of mice exhibiting a targeted deletion in the TRPC2 gene has established that TRPC2 occupies a fundamental role in the transduction machinery underlying the detection of pheromone signals by the VNO. TRPC2-deficient mice exhibit striking behavioral defects in the regulation of sexual and social behaviors. A previously unknown Ca(2+)-permeable, diacylglycerol (DAG)-activated cation channel found at the dendritic tip of vomeronasal neurons is severely defective in TRPC2 mutants, providing the first clear example for the existence of native DAG-gated cation channels in the mammalian nervous system. The experimental strategy employed in the mouse VNO now serves as a powerful model for examining the native functions of other TRP genes.
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Affiliation(s)
- Frank Zufall
- Department of Anatomy and Neurobiology and Program in Neuroscience, University of Maryland School of Medicine, 20 Penn Street, Baltimore, MD 21201-1509, USA.
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29
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Liu D, Zhang Z, Liman ER. Extracellular acid block and acid-enhanced inactivation of the Ca2+-activated cation channel TRPM5 involve residues in the S3-S4 and S5-S6 extracellular domains. J Biol Chem 2005; 280:20691-9. [PMID: 15731110 DOI: 10.1074/jbc.m414072200] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
TRPM5, a member of the superfamily of transient receptor potential ion channels, is essential for the detection of bitter, sweet, and amino acid tastes. In heterologous cell types it forms a nonselective cation channel that is activated by intracellular Ca(2+). TRPM5 is likely to be part of the taste transduction cascade, and regulators of TRPM5 are likely to affect taste sensation. In this report we show that TRPM5, but not the related channel TRPM4b, is potently blocked by extracellular acidification. External acidification has two effects, a fast reversible block of the current (IC(50) pH = 6.2) and a slower irreversible enhancement of current inactivation. Mutation of a single Glu residue in the S3-S4 linker and a His residue in the pore region each reduced sensitivity of TRPM5 currents to fast acid block (IC(50) pH = 5.8 for both), and the double mutant was nearly insensitive to acidic pH (IC(50) pH = 5.0). Prolonged exposure to acidic pH enhanced inactivation of TRPM5 currents, and mutant channels that were less sensitive to acid block were also less sensitive to acid-enhanced inactivation, suggesting an intimate association between the two processes. These processes are, however, distinct because the pore mutant H896N, which has normal sensitivity to acid block, shows significant recovery from acid-enhanced inactivation. These data show that extracellular acidification acts through specific residues on TRPM5 to block conduction through two distinct but related mechanisms and suggest a possible interaction between extracellular pH and activation and adaptation of bitter, sweet, and amino acid taste transduction.
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Affiliation(s)
- Dan Liu
- Division of Neurobiology, Department of Biological Sciences and Program in Neuroscience, University of Southern California, Los Angeles, California 90089, USA
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30
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Abstract
The transduction of taste is a fundamental process that allows animals to discriminate nutritious from noxious substances. Three taste modalities, bitter, sweet, and amino acid, are mediated by G protein-coupled receptors that signal through a common transduction cascade: activation of phospholipase C beta2, leading to a breakdown of phosphatidylinositol-4,5-bisphosphate (PIP2) into diacylglycerol and inositol 1,4,5-trisphosphate, which causes release of Ca2+ from intracellular stores. The ion channel, TRPM5, is an essential component of this cascade; however, the mechanism by which it is activated is not known. Here we show that heterologously expressed TRPM5 forms a cation channel that is directly activated by micromolar concentrations of intracellular Ca2+ (K1/2 = 21 microM). Sustained exposure to Ca2+ desensitizes TRPM5 channels, but PIP2 reverses desensitization, partially restoring channel activity. Whole-cell TRPM5 currents can be activated by intracellular Ca2+ and show strong outward rectification because of voltage-sensitive gating of the channels. TRPM5 channels are nonselective among monovalent cations and not detectably permeable to divalent cations. We propose that the regulation of TRPM5 by Ca2+ mediates sensory activation in the taste system.
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Affiliation(s)
- Dan Liu
- Department of Biological Sciences and Program in Neuroscience, University of Southern California, Los Angeles, CA 90089, USA
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31
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Abstract
Bipolar sensory neurons within the vomeronasal organ (VNO) are thought to mediate the detection of pheromones in vertebrates. In the mouse, VNO neurons respond to pheromones with a rise in intracellular Ca2+ that accompanies a depolarization of the cell. Transduction of the pheromone appears to occur through the activation of a phosphatidylinositol signalling pathway, but the ion channels that respond to this signalling pathway have not been identified. In this report patch-clamp recording from hamster vomeronasal sensory neurons was used to identify second-messenger-gated channels that might play a role in transduction. The results demonstrate that VNO neurons show abundant expression of a Ca2+-activated non-selective (CaNS) cation channel. The CaNS channel does not discriminate between Na+ and K+ and has a slope conductance of 22 pS. Half-activation of the channel occurs at a Ca2+ concentration of 0.5 mM (at -80 mV). The probability of opening (Po) of the channel is further augmented at positive potentials, and shows an e-fold voltage dependence per 37 mV. The channel exhibits rapid rundown following patch excision with Po decreasing from near 1.0 to near 0. The adenine nucleotides ATP and cAMP block the channel with an apparent affinity of 3 and 42 microM, respectively (-80 mV). Both the activation of the channel by Ca2+ and the block of the channel by adenine nucleotides show a mild voltage dependence, which can be accounted for by the voltage dependence of Po. The properties of this channel make it a candidate to either directly mediate vomeronasal sensory transduction, or to amplify the primary sensory response.
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Affiliation(s)
- Emily R Liman
- Department of Biological Sciences, University of Southern California, Los Angeles, CA 90089, USA.
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32
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Abstract
The vomeronasal organ (VNO) detects pheromones in many vertebrate species but is likely to be vestigial in humans. TRPC2(TRP2), a gene that is essential for VNO function in the mouse, is a pseudogene in humans. Because TRPC2 is expressed only in the VNO, the loss of selective pressure on this gene can serve as a molecular marker for the time at which the VNO became vestigial. By analyzing sequence data from the TRPC2 gene of 15 extant primate species, we provide evidence that the VNO was most likely functional in the common ancestor of New World monkeys and Old World monkeys and apes, but then became vestigial in the common ancestor of Old World monkeys and apes. We propose that, at this point in evolution, other modalities, notably the development of color vision, may have largely replaced signaling by pheromones.
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Affiliation(s)
- Emily R Liman
- Department of Biological Sciences and Program in Neurosciences, University of Southern California, Los Angeles, CA 90089, USA.
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33
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Rivera JF, Ahmad S, Quick MW, Liman ER, Arnold DB. An evolutionarily conserved dileucine motif in Shal K+ channels mediates dendritic targeting. Nat Neurosci 2003; 6:243-50. [PMID: 12592409 DOI: 10.1038/nn1020] [Citation(s) in RCA: 105] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2002] [Accepted: 01/16/2003] [Indexed: 11/09/2022]
Abstract
The molecular mechanisms underlying polarized sorting of proteins in neurons are poorly understood. Here we report the identification of a 16 amino-acid, dileucine-containing motif that mediates dendritic targeting in a variety of neuronal cell types in slices of rat brain. This motif is present in the carboxy (C) termini of Shal-family K+ channels and is highly conserved from C. elegans to humans. It is necessary for dendritic targeting of potassium channel Kv4.2 and is sufficient to target the axonally localized channels Kv1.3 and Kv1.4 to the dendrites. It can also mediate dendritic targeting of a non-channel protein, CD8.
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Affiliation(s)
- Jacqueline F Rivera
- Program in Molecular Biology, University of Southern California, 835 W. 37th Street, Los Angeles, California 90089-1340, USA
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34
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Menco BP, Carr VM, Ezeh PI, Liman ER, Yankova MP. Ultrastructural localization of G-proteins and the channel protein TRP2 to microvilli of rat vomeronasal receptor cells. J Comp Neurol 2001; 438:468-89. [PMID: 11559902 DOI: 10.1002/cne.1329] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Microvilli of vomeronasal organ (VNO) sensory epithelium receptor cells project into the VNO lumen. This lumen is continuous with the outside environment. Therefore, the microvilli are believed to be the subcellular sites of VNO receptor cells that interact with incoming VNO-targeted odors, including pheromones. Candidate molecules, which are implicated in VNO signaling cascades, are shown to be present in VNO receptor cells. However, ultrastructural evidence that such molecules are localized within the microvilli is sparse. The present study provides firm evidence that immunoreactivity for several candidate VNO signaling molecules, notably the G-protein subunits G(ialpha2) and G(oalpha), and the transient receptor potential channel 2 (TRP2), is localized prominently and selectively in VNO receptor cell microvilli. Although G(ialpha2) and G(oalpha) are localized separately in the microvilli of two cell types that are otherwise indistinguishable in their apical and microvillar morphology, the microvilli of both cell types are TRP2(+). VNO topographical distinctions were also apparent. Centrally within the VNO sensory epithelium, the numbers of receptor cells with G(ialpha2)(+) and G(oalpha)(+) microvilli were equal. However, near the sensory/non-sensory border, cells with G(ialpha2)(+) microvilli predominated. Scattered ciliated cells in this transition zone resembled neither VNO nor main olfactory organ (MO) receptor cells and may represent the same ciliated cells as those found in the non-sensory part of the VNO. Thus, this study shows that, analogous to the cilia of MO receptor cells, microvilli of VNO receptor cells are enriched selectively in proteins involved putatively in signal transduction. This provides important support for the role of these molecules in VNO signaling.
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Affiliation(s)
- B P Menco
- Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208-3520, USA.
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35
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Lumsden A, Chapman S, Schubert F, Toole L, Mayford M, Hamann S, Reber PJ, Häusser M, Murthy VN, Wood JN, Liman ER, Filbin MT, Qiu J, Ashe J, Chafee M, Goodwin S, Kyriacou B, Kempermann G, Winkler J. Neurobiology. Curr Opin Neurobiol 2001; 11:1-9. [PMID: 11179863 DOI: 10.1016/s0959-4388(00)00185-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Affiliation(s)
- A Lumsden
- King's College London, Guy's Hospital, London, UK
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36
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Affiliation(s)
- E R Liman
- Dept. of Biological Sciences and Program in Neurosciences, University of Southern California, Los Angeles, 90089-2520, USA.
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37
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Lumsden A, Chapman S, Jungbluth S, Bell E, Coutinho A, Costandi M, Adams N, Dutton R, Hamann S, Reber PJ, Häusser M, Murthy VN, Wood JN, Assad JA, Liman ER, Sheng M, Filbin MT, Qiu J, Ashe J, Chafee M, El Manira A, Goodwin S, Kyriacou B, Brandon EP, Gage FH. Neurobiology. Curr Opin Neurobiol 2000. [DOI: 10.1016/s0959-4388(00)00102-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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38
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Sheng M, Qiu J, Filbin MT, Lumsden A, Zeltser L, Chapman S, Schubert F, Gilthorpe J, Wingate R, Mayford M, Knowlton B, Jackson S, Häusser M, Murthy VN, Wood JN, Assad JA, Liman ER, Ashe J, El Manira A, Brandon EP, Gage FH. Neurobiology. Curr Opin Neurobiol 1999. [DOI: 10.1016/s0959-4388(99)00001-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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Abstract
The vomeronasal organ (VNO) of terrestrial vertebrates plays a key role in the detection of pheromones, chemicals released by animals that elicit stereotyped sexual and aggressive behaviors among conspecifics. Sensory transduction in the VNO appears unrelated to that in the vertebrate olfactory and visual systems: the putative pheromone receptors of the VNO are evolutionarily independent from the odorant receptors and, in contrast to vertebrate visual and olfactory transduction, vomeronasal transduction is unlikely to be mediated by cyclic-nucleotide-gated channels. We hypothesized that sensory transduction in the VNO might instead involve an ion channel of the transient receptor potential (TRP) family, members of which mediate cyclic-nucleotide-independent sensory responses in Drosophila melanogaster and Caenorhabditis elegans and play unknown functions in mammals. We have isolated a cDNA (rTRP2) from rat VNO encoding a protein of 885 amino acids that is equally distant from vertebrate and invertebrate TRP channels (10-30% amino acid identity). rTRP2 mRNA is exclusively expressed in VNO neurons, and the protein is highly localized to VNO sensory microvilli, the proposed site of pheromone sensory transduction. The absence of Ca2+ stores in sensory microvilli suggests that, in contrast to a proposed mechanism of activation of mammalian TRP channels, but in accord with analysis of TRP function in Drosophila phototransduction, the gating of TRP2 is independent from the depletion of internal Ca2+ stores. Thus, TRP2 is likely to participate in vomeronasal sensory transduction, which may share additional similarities with light-induced signaling in the Drosophila eye.
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Affiliation(s)
- E R Liman
- Howard Hughes Medical Institute, Neurobiology Department, Massachusetts General Hospital, Boston, MA 02114, USA.
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40
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Lumsden A, Ellies D, Tucker A, Dixon M, Schubert F, Bell E, Collignon J, Jungbluth S, Adams N, Knowlton B, Häusser M, Murthy VN, Wood JN, Assad JA, Liman ER, Sheng M, Filbin MT, Ashe J, El Manira A, Brandon EP, Gage FH. Neurobiology. Curr Opin Neurobiol 1999. [DOI: 10.1016/s0959-4388(99)80001-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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41
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Liman ER, Corey DP. Electrophysiological characterization of chemosensory neurons from the mouse vomeronasal organ. J Neurosci 1996; 16:4625-37. [PMID: 8764651 PMCID: PMC6579035] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
The mechanism of sensory transduction in chemosensory neurons of the vomeronasal organ (VNO) is not known. Based on molecular data, it is likely to be different from that mediating sensory transduction in the main olfactory system. To begin to understand this system, we have characterized the electrophysiological properties of dissociated mouse VNO neurons with patch-clamp recording. Sensory neurons were distinguished from nonsensory neurons by the presence of a dendrite, by immunoreactivity for olfactory marker protein, and by the firing of action potentials. The resting potential of VNO neurons was approximately -60 mV, and the average input resistance was 3 Gomega. Current injections as small as 1-2 pA elicited steady trains of action potentials that showed no sign of elicited steady trains of action potentials that showed no sign of adaptation during a 2 sec stimulus duration. The voltage-gated conductances in VNO neurons are distinct from those in olfactory neurons. The Na+ current is composed of two components; the major component was TTX-sensitive (Ki = 3.6 nM). The outward K+ current activates at -30 mV with kinetics 10 times slower than for K+ currents in olfactory neurons. The Ca2+ current is composed of at least two components: an L-type current and a T-type current that activates at -60 mV and is not found in olfactory neurons. We find no evidence for cyclic nucleotide-gated channels in VNO neurons under a variety of experimental conditions, including those that produced large responses in mouse olfactory neurons, which is further evidence for a novel transduction pathway.
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Affiliation(s)
- E R Liman
- Department of Neurology, Harvard Medical School, Boston, Massachusetts 02114, USA
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Abstract
Single-cell physiology and cloning efforts have extended studies of the vomeronasal organ to cellular and molecular levels. Recent work has shown that transduction in the vomeronasal organ is probably mediated by signalling pathways distinct from those that mediate transduction in the main olfactory system. An advance in understanding transduction has come with the cloning from rat vomeronasal organ of a family of putative pheromone receptor genes that bear no sequence similarity to previously cloned receptors. Other work has examined the expression of putative signalling components and found a zonal organization of the epithelium. Patch-clamp studies have described the basic electrical properties of vomeronasal neurons and explored second-messenger pathways.
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Affiliation(s)
- E R Liman
- Howard Hughes Medical Institute, Wellman 414, Massachusetts General Hospital, 50 Blossom Street, Boston, 02114, USA.
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Abstract
In mammals, olfactory stimuli are detected by sensory neurons at two distinct sites: the olfactory epithelium (OE) of the nasal cavity and the neuroepithelium of the vomeronasal organ (VNO). While the OE can detect volatile chemicals released from numerous sources, the VNO appears to be specialized to detect pheromones that are emitted by other animals and that convey information of behavioral or physiological importance. The mechanisms underlying sensory transduction in the OE have been well studied and a number of components of the transduction cascade have been cloned. Here, we investigated sensory transduction in the VNO by asking whether VNO neurons express molecules that have been implicated in sensory transduction in the OE. Using in situ hybridization and Northern blot analyses, we found that most of the olfactory transduction components examined, including the guanine nucleotide binding protein alpha subunit (G-alpha-olf), adenylyl cyclase type III, and an olfactory cyclic nucleotide-gated (CNG) channel subunit (oCNC1), are not expressed by VNO sensory neurons. In contrast, VNO neurons do express a second olfactory CNG channel subunit (oCNC2). These results indicate that VNO sensory transduction is distinct from that in the OE but raise the possibility that, like OE sensory transduction, sensory transduction in the VNO might involve cyclic nucleotide-gated ion channels.
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Affiliation(s)
- A Berghard
- Department of Neurobiology, Howard Hughes Medical Institute, Harvard Medical School, Boston, MA 02115, USA
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Abstract
Sensory transduction in olfactory neurons is mediated by intracellular cAMP, which directly gates a nonselective cation channel. A cDNA encoding a cyclic nucleotide-gated (CNG) ion channel subunit (rOCNC1) has been cloned previously from rat olfactory epithelium. However, differences between the functional properties of rOCNC1 and the native olfactory CNG channel suggest that the native channel could be composed of several distinct subunit types. Here, we report the cloning and characterization of a cDNA encoding a second olfactory CNG channel subunit (rOCNC2) that is 52% identical to rOCNC1 and that is expressed specifically in olfactory sensory neurons. Expression of rOCNC2 alone in Xenopus oocytes does not lead to detectable CNG currents. However, coexpression of rOCNC2 with rOCNC1 results in a CNG conductance that differs from that detected upon expression of rOCNC1 alone and more closely resembles the native conductance in several respects, including its sensitivity to cAMP. This suggests that the native olfactory CNG channel is a hetero-oligomer composed of rOCNC1 and rOCNC2 subunits.
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Affiliation(s)
- E R Liman
- Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115
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Abstract
The subunit stoichiometry of the mammalian K+ channel KV1.1 (RCK1) was examined by linking together the coding sequences of 2-5 K+ channel subunits in a single open reading frame and tagging the expression of individual subunits with a mutation (Y379K or Y379R) that altered the sensitivity of the channel to block by external tetraethylammonium ion. Two lines of evidence argue that these constructs lead to K+ channel expression only through the formation of functional tetramers. First, currents expressed by tetrameric constructs containing a single mutant subunit have a sensitivity to tetraethylammonium that is well fitted by a single site binding isotherm. Second, a mutant subunit (Y379K) that expresses only as part of a heteromultimer contributes to the expression of functional channels when coexpressed with a trimeric construct but not a tetrameric construct.
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Affiliation(s)
- E R Liman
- Department of Cellular and Molecular Physiology, Harvard Medical School, Boston, Massachusetts 02115
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Abstract
The ability of ion-channel proteins to respond to a change of the transmembrane voltage is one of the basic mechanisms underlying electrical excitability of nerve and muscle membranes. The voltage sensor has been postulated to be the fourth putative transmembrane segment (S4) of voltage-activated Na+, Ca2+ and K+ channels. Mutations of positively charged residues within S4 alter gating of Na and Shaker-type K+ channels, but quantitative correlations between the charge or a residue in S4 and the gating valence of the channel have not yet been established. Here, with improved resolution of the voltage dependence of steady-state activation, we present estimates of the equivalent gating valence with sufficient precision to allow quantitative examination of the contribution of individual charged residues to the gating valence of a mammalian non-inactivating K+ channel. We conclude that at least part of the gating charge associated with channel activation is indeed contributed by charged residues within the S4 segment.
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Affiliation(s)
- E R Liman
- Department of Cellular and Molecular Physiology, Harvard Medical School, Boston, Massachusetts 02115
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Matsubara H, Liman ER, Hess P, Koren G. Pretranslational mechanisms determine the type of potassium channels expressed in the rat skeletal and cardiac muscles. J Biol Chem 1991; 266:13324-8. [PMID: 1712780] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
We have cloned a cDNA (RMK2) coding for a Shaker type delayed rectifier K+ channel from a rat skeletal muscle cDNA library. The clone encodes a putative protein of 602 amino acids, identical with a rat brain K+ channel Kv1 (Swanson, R., Marshall, R., Smith, J. S., Williams, J. B., Boyle, M. B., Folander, K., Luneau, C. J., Antanavage, J., Oliva, C., Burhow, S. A., Bennet, C., Stein, R. B., and Kaczmarek, L. K. (1990) Neuron 4, 929-939). Northern blot analysis showed that RMK2 is expressed in skeletal and cardiac muscle. RNase protection analysis showed that the 3'-noncoding regions of the brain, cardiac, and skeletal muscle RMK2 transcripts are identical. Cloning of the gene confirmed that the protein is encoded by a single exon (Swanson et al. (1990) Neuron 4, 929-939). We expressed RMK2 in Xenopus oocytes and showed that it encodes noninactivating delayed rectifier K+ channels, resistant to block by external tetraethylammonium, with a small unitary conductance of 8.0 picosiemens. Coinjection of RMK2 and RCK1 (RMK1) (Baumann, A., Grupe, A., Ackermann, A., and Pongs, O. (1988) EMBO J. 7, 2457-2463; Koren, G., Liman, E. R., Logothetis, D. E., Nadal-Ginard, B., and Hess, P. (1990) Neuron 4, 39-51) into Xenopus oocytes resulted in the expression of currents that have tetraethylammonium inhibition curves that differ from the linear combination of inhibition curves of the two types expressed individually. Thus, RMK2 and RCK1 (RMK1) can form heteromultimers. RNA blot hybridization analysis revealed that the RMK2 transcript is developmentally regulated in a different manner in the rat skeletal muscle, ventricle, and atrium.
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Affiliation(s)
- H Matsubara
- Division of Cardiology, Brigham and Women's Hospital, Boston, Massachusetts 02115
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Abstract
We have cloned a cDNA coding for a delayed rectifier K+ channel from rat brain (RCK1) and rat muscle (RMK1) and expressed it in Xenopus oocytes and in a myoblast cell line (Sol-8). Stably transfected Sol-8 cells exhibited large outward K+ currents, which were indistinguishable from the K+ currents induced in Xenopus oocytes by injection of mRNA transcribed in vitro. RCK1 encodes a K+ channel with a unitary conductance of approximately 14 pS. The steep voltage dependence of channel opening resides in transitions between closed states, whereas the direct transitions into and out of the open state are very rapid and not markedly voltage-dependent. Channel inactivation is very slow, voltage-independent, and occurs from the open state only. We present a simple model that incorporates our findings and is consistent with the presumed structural symmetry of a functional K+ channel.
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Affiliation(s)
- G Koren
- Department of Cellular and Molecular Physiology, Harvard Medical School, Boston, Massachusetts 02115
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Abstract
Dopamine, acting via cyclic adenosine 3':5'-monophosphate (cAMP), has been shown to enhance a kainate-gated ionic conductance in white perch retinal horizontal cells in vitro. To determine whether this effect involves stimulation of a protein kinase, kainate-gated currents were observed in cultured horizontal cells that were dialyzed with the catalytic subunit of cAMP-dependent protein kinase. Intracellular application of catalytic subunit or cAMP, but not heat-inactivated catalytic subunit, caused significant enhancement of the kainate-evoked currents. These results suggest that kainate-gated channels in horizontal cells may be modified by a phosphorylation event.
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Affiliation(s)
- E R Liman
- Department of Cellular and Developmental Biology, Harvard University, Cambridge, MA 02138
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