The stoichiometry of P2X2/6 receptor heteromers depends on relative subunit expression levels

Enlightening activation gating in P2X receptors

P2X receptors are trimeric nonselective cation channels gated by ATP. They assemble from seven distinct subunit isoforms as either homo- or heteromeric complexes and contain three extracellularly located binding sites for ATP. P2X receptors are expressed in nearly all tissues and are there involved in physiological processes like synaptic transmission, pain, and inflammation. Thus, they are a challenging pharmacological target. The determination of crystal and cryo-EM structures of several isoforms in the last decade in closed, open, and desensitized states has provided a firm basis for interpreting the huge amount of functional and biochemical data. Electrophysiological characterization in conjugation with optical approaches has generated significant insights into structure–function relationships of P2X receptors. This review focuses on novel optical and related approaches to better understand the conformational changes underlying the activation of these receptors.

Polydisperse molecular architecture of connexin 26/30 heteromeric hemichannels revealed by atomic force microscopy imaging

Connexin (Cx) protein forms hemichannels and gap junctional channels, which play diverse and profound roles in human physiology and diseases. Gap junctions are arrays of intercellular channels formed by the docking of two hemichannels from adjacent cells. Each hexameric hemichannel contains the same or different Cx isoform. Although homomeric Cxs forms have been largely described functionally and structurally, the stoichiometry and arrangement of heteromeric Cx channels remain unknown. The latter, however, are widely expressed in human tissues and variation might have important implications on channel function. Investigating properties of heteromeric Cx channels is challenging considering the high number of potential subunit arrangements and stoichiometries, even when only combining two Cx isoforms. To tackle this problem, we engineered an HA tag onto Cx26 or Cx30 subunits and imaged hemichannels that were liganded by Fab-epitope antibody fragments via atomic force microscopy. For Cx26-HA/Cx30 or Cx30-HA/Cx26 heteromeric channels, the Fab-HA binding distribution was binomial with a maximum of three Fab-HA bound. Furthermore, imaged Cx26/Cx30-HA triple liganded by Fab-HA showed multiple arrangements that can be derived from the law of total probabilities. Atomic force microscopy imaging of ringlike structures of Cx26/Cx30-HA hemichannels confirmed these findings and also detected a polydisperse distribution of stoichiometries. Our results indicate a dominant subunit stoichiometry of 3Cx26:3Cx30 with the most abundant subunit arrangement of Cx26-Cx26-Cx30-Cx26-Cx30-Cx30. To our knowledge, this is the first time that the molecular architecture of heteromeric Cx channels has been revealed, thus providing the basis to explore the functional effect of these channels in biology.

Function of P2X4 Receptors Is Directly Modulated by a 1:1 Stoichiometric Interaction With 5-HT3A Receptors

Interacting receptors at the neuronal plasma membrane represent an additional regulatory mode for intracellular transduction pathways. P2X4 receptor triggers fast neurotransmission responses via a transient increase in intracellular Ca²⁺ levels. It has been proposed that the P2X4 receptor interacts with the 5-HT₃A receptor in hippocampal neurons, but their binding stoichiometry and the role of P2X4 receptor activation by ATP on this crosstalking system remains unknown. Via pull-down assays, total internal reflection fluorescence (TIRF) microscopy measurements of the receptors colocalization and expression at the plasma membrane, and atomic force microscopy (AFM) imaging, we have demonstrated that P2X4/5-HT₃A receptor complexes can interact with each other in a 1:1 stoichiometric manner that is preserved after ATP binding. Also, macromolecular docking followed by 100 ns molecular dynamics (MD) simulations suggested that the interaction energy of the P2X4 receptor with 5-HT₃A receptor is similar at the holo and the apo state of the P2X4 receptor, and the interacting 5-HT₃A receptor decreased the ATP binding energy of P2X4 receptor. Finally, the P2X4 receptor-dependent Ca²⁺ mobilization is inhibited by the 5-HT₃A interacting receptor. Altogether, these findings provide novel molecular insights into the allosteric regulation of P2X4/5-HT₃A receptor complex in lipid bilayers of living cells via stoichiometric association, rather than accumulation or unspecific clustering of complexes.

ATP-Gated P2X Receptor Channels: Molecular Insights into Functional Roles

In the nervous system, ATP is co-stored in vesicles with classical transmitters and released in a regulated manner. ATP from the intracellular compartment can also exit the cell through hemichannels and following shear stress or membrane damage. In the past 30 years, the action of ATP as an extracellular transmitter at cell-surface receptors has evolved from somewhat of a novelty that was treated with skepticism to purinergic transmission being accepted as having widespread important functional roles mediated by ATP-gated ionotropic P2X receptors (P2XRs). This review focuses on work published in the last five years and provides an overview of ( a) structural studies, ( b) the molecular basis of channel properties and regulation of P2XRs, and ( c) the physiological and pathophysiological roles of ATP acting at defined P2XR subtypes.

PIP Water Transport and Its pH Dependence Are Regulated by Tetramer Stoichiometry

Many plasma membrane channels form oligomeric assemblies, and heterooligomerization has been described as a distinctive feature of some protein families. In the particular case of plant plasma membrane aquaporins (PIPs), PIP1 and PIP2 monomers interact to form heterotetramers. However, the biological properties of the different heterotetrameric configurations formed by PIP1 and PIP2 subunits have not been addressed yet. Upon coexpression of tandem PIP2-PIP1 dimers in Xenopus oocytes, we can address, for the first time to our knowledge, the functional properties of single heterotetrameric species having 2:2 stoichiometry. We have also coexpressed PIP2-PIP1 dimers with PIP1 and PIP2 monomers to experimentally investigate the localization and biological activity of each tetrameric assembly. Our results show that PIP2-PIP1 heterotetramers can assemble with 3:1, 1:3, or 2:2 stoichiometry, depending on PIP1 and PIP2 relative expression in the cell. All PIP2-PIP1 heterotetrameric species localize at the plasma membrane and present the same water transport capacity. Furthermore, the contribution of any heterotetrameric assembly to the total water transport through the plasma membrane doubles the contribution of PIP2 homotetramers. Our results also indicate that plasma membrane water transport can be modulated by the coexistence of different tetrameric species and by intracellular pH. Moreover, all the tetrameric species present similar cooperativity behavior for proton sensing. These findings throw light on the functional properties of PIP tetramers, showing that they have flexible stoichiometry dependent on the quantity of PIP1 and PIP2 molecules available. This represents, to our knowledge, a novel regulatory mechanism to adjust water transport across the plasma membrane.

Heteromeric channels with different phenotypes are generated when coexpressing two P2X2 receptor isoforms

To investigate if channels with different stoichiometry are formed from P2X2 receptor isoforms during their heterologous co-expression. The two-electrode voltage-clamp technique was used to measured ATP induced currents in Xenopus laevis oocytes. We used a mutant (P2X2-2bm) because its ATP sensitivity is lower than P2X2-2b receptors, which highlights the differences with its splice variant P2X2-1a.Currents through homomeric channels had significantly different Hill coefficients. P2XR are trimeric proteins with three agonist binding sites; therefore, only two homomeric and two heteromeric stoichiometries are possible when both P2X2 isoforms are coexpressed, the heteromeric channels might be formed by: i) 2(P2X2-1a)+1(P2X2-2bm); or ii) 1(P2X2-1a)+2(P2X2-2bm). Because P2X2 channels open when two binding sites are occupied, these stoichiometries are expected to have different ATP sensitivities. Thus, co-expressing both P2X2 isoforms, two oocyte populations were distinguished based on their sensitivity to ATP and Hill coefficients. For the first population (P2X2-1a like), the ATP EC50 and the Hill coefficient were not different than those of homomeric P2X2-1a channels similarly, for the second population (P2X2-2bm like), these variables were also not different than for those of homomeric P2X2-2bm channels. Various findings indicate that homomeric channel expression is not responsible for such differences. Our observations indicate that two heteromeric channels can be assembled from two P2X2 receptor isoforms. Our data support a current model, according to which, ATP activation of two subunits can open P2X2 channel. However, PPADS appears to bind to all three subunits in order to inhibit ATP effects on P2X2 receptors.

Flexible subunit stoichiometry of functional human P2X2/3 heteromeric receptors

The aim of the present work was to clarify whether heterotrimeric P2X2/3 receptors have a fixed subunit stoichiometry consisting of one P2X2 and two P2X3 subunits as previously suggested, or a flexible stoichiometry containing also the inverse subunit composition. For this purpose we transfected HEK293 cells with P2X2 and P2X3 encoding cDNA at the ratios of 1:2 and 4:1, and analysed the biophysical and pharmacological properties of the generated receptors by means of the whole-cell patch-clamp technique. The concentration-response curves for the selective agonist α,β-meATP did not differ from each other under the two transfection ratios. However, co-expression of an inactive P2X2 mutant and the wild type P2X3 subunit and vice versa resulted in characteristic distortions of the α,β-meATP concentration-response relationships, depending on which subunit was expressed in excess, suggesting that HEK293 cells express mixtures of (P2X2)1/(P2X3)2 and (P2X2)2/(P2X3)1 receptors. Whereas the allosteric modulators H+ and Zn2+ failed to discriminate between the two possible heterotrimeric receptor variants, the α,β-meATP-induced responses were blocked more potently by the competitive antagonist A317491, when the P2X2 subunit was expressed in deficit of the P2X3 subunit. Furthermore, blue-native PAGE analysis of P2X2 and P2X3 subunits co-expressed in Xenopus laevis oocytes and HEK293 cells revealed that plasma membrane-bound P2X2/3 receptors appeared in two clearly distinct heterotrimeric complexes: a (P2X2-GFP)2/(P2X3)1 complex and a (P2X2-GFP)1/(P2X3)2 complex. These data strongly indicate that the stoichiometry of the heteromeric P2X2/3 receptor is not fixed, but determined in a permutational manner by the relative availability of P2X2 and P2X3 subunits.

Identification of P2X2/P2X4/P2X6 heterotrimeric receptors using atomic force microscopy (AFM) imaging

Seven P2X purinergic receptor subunits have been identified: P2X1-P2X7. The overlapping expression of P2X2, P2X4 and P2X6 subunits has been shown in different cell types, and functional analysis of P2X receptors in Leydig cells suggests that the three subunits might interact. Here, His6-tagged P2X2, HA-tagged P2X4 and FLAG-tagged P2X6 subunits were co-expressed in tsA 201 cells. After sequential co-immunoprecipitation using anti-HA and anti-FLAG beads, all three subunits were present, demonstrating their interaction. Atomic force microscopy (AFM) imaging revealed receptors that were specifically decorated by both an anti-His6 antibody and an anti-HA Fab fragment, indicating the presence of a P2X2/4/6 heterotrimer. To our knowledge, this is the first report of a P2X receptor containing three different subunits.

P2X7-dependent, but differentially regulated release of IL-6, CCL2, and TNF-α in cultured mouse microglia

ATP is an important regulator of microglia and its effects on microglial cytokine release are currently discussed as important contributors in a variety of brain diseases. We here analyzed the effects of ATP on the production of six inflammatory mediators (IL-6, IL-10, CCL2, IFN-γ, TNF-α, and IL-12p70) in cultured mouse primary microglia. Stimulation of P2X7 receptor by ATP (1 mM) or BzATP (500 µM) evoked the mRNA expression and release of proinflammatory cytokines IL-6, TNF-α, and the chemokine CCL2 in WT cells but not in P2X7(-/-) cells. The effects of ATP and BzATP were inhibited by the nonselective P2 receptor antagonists PPADs and suramin. Various selective P2X7 receptor antagonists blocked the P2X7-dependent release of IL-6 and CCL2, but, surprisingly, had no effect on BzATP-induced release of TNF-α in microglia. Calcium measurements confirmed that P2X7 is the main purine receptor activated by BzATP in microglia and showed that all P2X7 antagonists were functional. It is also presented that pannexin-1 hemichannel function and potential P2X4/P2X7 heterodimers are not involved in P2X7-dependent release of IL-6, CCL2, and TNF-α in microglia. How P2X7-specific antagonists only affect P2X7-dependent IL-6 and CCL2 release, but not TNF-α release is at the moment unclear, but indicates that the P2X7-dependent release of cytokines in microglia is differentially regulated.

Heteromerization of PIP aquaporins affects their intrinsic permeability

The plant aquaporin plasma membrane intrinsic proteins (PIP) subfamily represents one of the main gateways for water exchange at the plasma membrane (PM). A fraction of this subfamily, known as PIP1, does not reach the PM unless they are coexpressed with a PIP2 aquaporin. Although ubiquitous and abundantly expressed, the role and properties of PIP1 aquaporins have therefore remained masked. Here, we unravel how FaPIP1;1, a fruit-specific PIP1 aquaporin from Fragaria x ananassa, contributes to the modulation of membrane water permeability (Pf) and pH aquaporin regulation. Our approach was to combine an experimental and mathematical model design to test its activity without affecting its trafficking dynamics. We demonstrate that FaPIP1;1 has a high water channel activity when coexpressed as well as how PIP1-PIP2 affects gating sensitivity in terms of cytosolic acidification. PIP1-PIP2 random heterotetramerization not only allows FaPIP1;1 to arrive at the PM but also produces an enhancement of FaPIP2;1 activity. In this context, we propose that FaPIP1;1 is a key participant in the regulation of water movement across the membranes of cells expressing both aquaporins.

Heteromeric assembly of P2X subunits

Transcripts and/or proteins of P2X receptor (P2XR) subunits have been found in virtually all mammalian tissues. Generally more than one of the seven known P2X subunits have been identified in a given cell type. Six of the seven cloned P2X subunits can efficiently form functional homotrimeric ion channels in recombinant expression systems. This is in contrast to other ligand-gated ion channel families, such as the Cys-loop or glutamate receptors, where homomeric assemblies seem to represent the exception rather than the rule. P2XR mediated responses recorded from native tissues rarely match exactly the biophysical and pharmacological properties of heterologously expressed homomeric P2XRs. Heterotrimerization of P2X subunits is likely to account for this observed diversity. While the existence of heterotrimeric P2X2/3Rs and their role in physiological processes is well established, the composition of most other P2XR heteromers and/or the interplay between distinct trimeric receptor complexes in native tissues is not clear. After a description of P2XR assembly and the structure of the intersubunit ATP-binding site, this review summarizes the distribution of P2XR subunits in selected mammalian cell types and the biochemically and/or functionally characterized heteromeric P2XRs that have been observed upon heterologous co-expression of P2XR subunits. We further provide examples where the postulated heteromeric P2XRs have been suggested to occur in native tissues and an overview of the currently available pharmacological tools that have been used to discriminate between homo- and heteromeric P2XRs.

The trafficking and targeting of P2X receptors

The functional expression of P2X receptors at the plasma membrane is dependent on their trafficking along secretory and endocytic pathways. There are seven P2X receptor subunits, and these differ in their subcellular distributions because they have very different trafficking properties. Some are retained within the endoplasmic reticulum (ER), while others are predominantly at the cell surface or within endosomes and lysosomes. Changes in recruitment of receptors to and from the plasma membrane provides a way of rapidly up- or down-regulating the cellular response to adenosine triphosphate (ATP). An additional layer of regulation is the targeting of these receptors within the membranes of each compartment, which affects their stability, function and the nature of the effector proteins with which they form signaling complexes. The trafficking and targeting of P2X receptors is regulated by their interactions with other proteins and with lipids and we can expect this to vary in a cell-type specific manner and in response to changes in the environment giving rise to differences in receptor activity and function.

P2X2 and P2X5 subunits define a new heteromeric receptor with P2X7-like properties

Ligand-gated ion channels are prototypic oligomeric membrane proteins whose stoichiometry determines their functional properties and subcellular localization. Deciphering the quaternary structure of such protein complexes is an arduous task and usually requires the combination of multiple approaches. ATP-gated P2X receptors are formed by the association of three subunits, but the quaternary arrangement of the seven P2X subunits at the plasma membrane remains poorly characterized. By combining bioluminescence resonance energy transfer, bifunctional fluorescence complementation and protein biochemistry, we developed an experimental approach that allows precise determination of rat P2X receptor quaternary assembly. We found that P2X5 subunits associate with P2X1, P2X2, and P2X4 subunits. We demonstrate that P2X5 and P2X2 subunits interact to form as yet uncharacterized heteromeric receptors with alternate stoichiometries, both present at the plasma membrane. P2X2/5 receptors display functional properties such as pore dilatation, membrane blebbing, and phosphatidylserine exposure that were previously thought to be characteristic hallmarks of the P2X7 receptor. In mouse, P2X2 and P2X5 subunits colocalize and physically interact in specific neuronal populations suggesting that other P2X receptors might contribute to cellular responses typically attributed to P2X7 receptor.

ATP binding site mutagenesis reveals different subunit stoichiometry of functional P2X2/3 and P2X2/6 receptors

The aim of the present experiments was to clarify the subunit stoichiometry of P2X2/3 and P2X2/6 receptors, where the same subunit (P2X2) forms a receptor with two different partners (P2X3 or P2X6). For this purpose, four non-functional Ala mutants of the P2X2, P2X3, and P2X6 subunits were generated by replacing single, homologous amino acids particularly important for agonist binding. Co-expression of these mutants in HEK293 cells to yield the P2X2 WT/P2X3 mutant or P2X2 mutant/P2X3 WT receptors resulted in a selective blockade of agonist responses in the former combination only. In contrast, of the P2X2 WT/P2X6 mutant and P2X2 mutant/P2X6 WT receptors, only the latter combination failed to respond to agonists. The effects of α,β-methylene-ATP and 2-methylthio-ATP were determined by measuring transmembrane currents by the patch clamp technique and intracellular Ca(2+) transients by the Ca(2+)-imaging method. Protein labeling, purification, and PAGE confirmed the assembly and surface trafficking of the investigated WT and WT/mutant combinations in Xenopus laevis oocytes. In conclusion, both electrophysiological and biochemical investigations uniformly indicate that one subunit of P2X2 and two subunits of P2X3 form P2X2/3 heteromeric receptors, whereas two subunits of P2X2 and one subunit of P2X6 constitute P2X2/6 receptors. Further, it was shown that already two binding sites of the three possible ones are sufficient to allow these receptors to react with their agonists.

P2X4 receptors interact with both P2X2 and P2X7 receptors in the form of homotrimers

BACKGROUND AND PURPOSE

The P2X receptor family consists of seven subunit types - P2X1-P2X7. All but P2X6 are able to assemble as homotrimers. In addition, various subunit permutations have been reported to form heterotrimers. Evidence for heterotrimer formation includes co-localization, co-immunoprecipitation and the generation of receptors with novel functional properties; however, direct structural evidence for heteromer formation, such as chemical cross-linking and single-molecule imaging, is available in only a few cases. Here we examined the nature of the interaction between two pairs of subunits - P2X2 and P2X4, and P2X4 and P2X7.

EXPERIMENTAL APPROACH

We used several experimental approaches, including in situ proximity ligation, co-immunoprecipitation, co-isolation on affinity beads, chemical cross-linking and atomic force microscopy (AFM) imaging.

KEY RESULTS

Both pairs of subunits co-localize upon co-transfection, interact intimately within cells, and can be co-immunoprecipitated and co-isolated from cell extracts. Despite this, chemical cross-linking failed to show evidence for heteromer formation. AFM imaging of isolated receptors showed that all three subunits had the propensity to form receptor dimers. This self-association is likely to account for the observed close interaction between the subunit pairs, in the absence of true heteromer formation.

CONCLUSIONS AND IMPLICATIONS

We conclude that both pairs of receptors interact in the form of distinct homomers. We urge caution in the interpretation of biochemical evidence indicating heteromer formation in other cases.

Activation and regulation of purinergic P2X receptor channels

Mammalian ATP-gated nonselective cation channels (P2XRs) can be composed of seven possible subunits, denoted P2X1 to P2X7. Each subunit contains a large ectodomain, two transmembrane domains, and intracellular N and C termini. Functional P2XRs are organized as homomeric and heteromeric trimers. This review focuses on the binding sites involved in the activation (orthosteric) and regulation (allosteric) of P2XRs. The ectodomains contain three ATP binding sites, presumably located between neighboring subunits and formed by highly conserved residues. The detection and coordination of three ATP phosphate residues by positively charged amino acids are likely to play a dominant role in determining agonist potency, whereas an AsnPheArg motif may contribute to binding by coordinating the adenine ring. Nonconserved ectodomain histidines provide the binding sites for trace metals, divalent cations, and protons. The transmembrane domains account not only for the formation of the channel pore but also for the binding of ivermectin (a specific P2X4R allosteric regulator) and alcohols. The N- and C- domains provide the structures that determine the kinetics of receptor desensitization and/or pore dilation and are critical for the regulation of receptor functions by intracellular messengers, kinases, reactive oxygen species and mercury. The recent publication of the crystal structure of the zebrafish P2X4.1R in a closed state provides a major advance in the understanding of this family of receptor channels. We will discuss data obtained from numerous site-directed mutagenesis experiments accumulated during the last 15 years with reference to the crystal structure, allowing a structural interpretation of the molecular basis of orthosteric and allosteric ligand actions.

Imaging the spatial orientation of subunits within membrane receptors by atomic force microscopy

Our experimental approach is based on the atomic force microscope (AFM) imaging of epitope-tagged subunits within membrane protein complexes purified in small amounts and decorated by anti-tag antibodies. Furthermore, we can produce simultaneous decoration of protein complexes using Fab fragments and IgG antibodies, which, combined with chemical modification of the substrate, allows us to determine the protein orientation across the cell membrane. Here, we describe a detailed protocol for membrane protein purification, AFM data collection, analysis, and interpretation of results. The protocol also covers basic AFM instrument settings and best practices for both observation of membrane protein complexes by AFM and automatic detection of the structures by an in-house algorithm. Once a sufficient number of membrane protein complexes have been visualized by AFM, data acquisition and processing can be completed in approximately 10 min using a scanning surface of 1 μm(2).

Atomic force microscopy reveals the alternating subunit arrangement of the TRPP2-TRPV4 heterotetramer

There is evidence that polycystin-2 (TRPP2) interacts with two other members of the transient receptor potential (TRP) family, TRPC1 and TRPV4. We have previously shown that TRPP2 forms a heteromeric complex with TRPC1, with a 2:2 stoichiometry and an alternating subunit arrangement. Here, we used coimmunoprecipitation to show that TRPP2 also interacts with TRPV4, but not with TRPA1 or TRPM8; hence, its promiscuity is limited. We then used atomic force microscopy to study the structure of the TRPV4 homomer and the interaction between TRPP2 and TRPV4. The molecular volume of V5-tagged TRPV4 isolated from singly-transfected tsA 201 cells indicated that it assembled as a homotetramer. The distribution of angles between pairs of anti-V5 antibodies bound to TRPV4 particles had a large peak close to 90 degrees and a smaller peak close to 180 degrees , again consistent with the assembly of TRPV4 as a homotetramer. In contrast, the angle distributions for decoration of the TRPP2-TRPV4 heteromer by either anti-Myc or anti-V5 antibodies had major peaks close to 180 degrees. This result indicates that TRPP2-TRPV4 assembles identically to TRPP2-TRPC1, suggesting a common subunit arrangement among heteromeric TRP channels.

Stoichiometry of expressed alpha(4)beta(2)delta gamma-aminobutyric acid type A receptors depends on the ratio of subunit cDNA transfected

The gamma-aminobutyric acid type A receptor (GABA(A)R) is the target of many depressants, including benzodiazepines, anesthetics, and alcohol. Although the highly prevalent alphabetagamma GABA(A)R subtype mediates the majority of fast synaptic inhibition in the brain, receptors containing delta subunits also play a key role, mediating tonic inhibition and the actions of endogenous neurosteroids and alcohol. However, the fundamental properties of delta-containing GABA(A)Rs, such as subunit stoichiometry, are not well established. To determine subunit stoichiometry of expressed delta-containing GABA(A)Rs, we inserted the alpha-bungarotoxin binding site tag in the alpha(4), beta(2), and delta subunit N termini. An enhanced green fluorescent protein tag was also inserted into the beta(2) subunit to shift its molecular weight, allowing us to separate subunits using SDS-PAGE. Tagged alpha(4)beta(2)delta GABA(A)Rs were expressed in HEK293T cells using various ratios of subunit cDNA, and receptor subunit stoichiometry was determined by quantitating fluorescent alpha-bungarotoxin bound to each subunit on Western blots of surface immunopurified tagged GABA(A)Rs. The results demonstrate that the subunit stoichiometry of alpha(4)beta(2)delta GABA(A)Rs is regulated by the ratio of subunit cDNAs transfected. Increasing the ratio of delta subunit cDNA transfected increased delta subunit incorporation into surface receptors with a concomitant decrease in beta(2) subunit incorporation. Because receptor subunit stoichiometry can directly influence GABA(A)R pharmacological and functional properties, considering how the transfection protocols used affect subunit stoichiometry is essential when studying heterologously expressed alpha(4)beta(2)delta GABA(A)Rs. Successful bungarotoxin binding site tagging of GABA(A)R subunits is a novel tool with which to accurately quantitate subunit stoichiometry and will be useful for monitoring GABA(A)R trafficking in live cells.

The transient receptor potential channels TRPP2 and TRPC1 form a heterotetramer with a 2:2 stoichiometry and an alternating subunit arrangement

There is functional evidence that polycystin-2 (TRPP2) interacts with other members of the transient receptor potential family, including TRPC1 and TRPV4. Here we have used atomic force microscopy to study the structure of the TRPP2 homomer and the interaction between TRPP2 and TRPC1. The molecular volumes of both Myc-tagged TRPP2 and V5-tagged TRPC1 isolated from singly transfected tsA 201 cells indicated that they assembled as homotetramers. The molecular volume of the protein isolated from cells expressing both TRPP2 and TRPC1 was intermediate between the volumes of the two homomers, suggesting that a heteromer was being formed. The distribution of angles between pairs of anti-Myc antibodies bound to TRPP2 particles had a large peak close to 90 degrees and a smaller peak close to 180 degrees , consistent with the assembly of TRPP2 as a homotetramer. In contrast, the corresponding angle distributions for decoration of the TRPP2-TRPC1 heteromer by either anti-Myc or anti-V5 antibodies had predominant peaks close to 180 degrees . This decoration pattern indicates a TRPP2:TRPC1 subunit stoichiometry of 2:2 and an alternating subunit arrangement.

Direct observation of ATP-induced conformational changes in single P2X(4) receptors

The ATP-gated P2X₄ receptor is a cation channel, which is important in various pathophysiological events. The architecture of the P2X₄ receptor in the activated state and how to change its structure in response to ATP binding are not fully understood. Here, we analyze the architecture and ATP-induced structural changes in P2X₄ receptors using fast-scanning atomic force microscopy (AFM). AFM images of the membrane-dissociated and membrane-inserted forms of P2X₄ receptors and a functional analysis revealed that P2X₄ receptors have an upward orientation on mica but lean to one side. Time-lapse imaging of the ATP-induced structural changes in P2X₄ receptors revealed two different forms of activated structures under 0 Ca²⁺ conditions, namely a trimer structure and a pore dilation-like tripartite structure. A dye uptake measurement demonstrated that ATP-activated P2X₄ receptors display pore dilation in the absence of Ca²⁺. With Ca²⁺, the P2X₄ receptors exhibited only a disengaged trimer and no dye uptake was observed. Thus our data provide a new insight into ATP-induced structural changes in P2X₄ receptors that correlate with pore dynamics.

ATP is not only a source of intracellular energy but can act as an intercellular signal by binding membrane receptors. Purinergic receptors, which bind with nucleotides including ATP are known as P2 receptors and are divided into two types: ion channel-type P2X receptors and metabotropic-type P2Y receptors. P2X receptors are thought to undergo conformational changes in response to ATP binding, leading to the opening of transmembrane channels, through which cations enter the cells. A growing body of evidence shows that P2X receptors control various physiological and pathophysiological cellular responses. However, the receptor structure and the conformational changes it experiences upon stimulation remained to be clarified. Here, we employed an atomic force microscope (AFM) to observe P2X receptor behavior at the single channel level. We chose to analyze the P2X4 receptor, because it is known to increase the transmembrane pore size (i.e., pore dilation) in the absence of extracellular calcium. Activated P2X4 receptor exhibited a trimeric topology with a pore-like structure in the center. When calcium was present the receptor exhibited a trimer without a pore structure at its center. These structural changes corresponded well with the changes of ion permeability of P2X4 receptor.

Fast-scanning atomic force microscopy reveals the topology, ATP-induced conformational changes, and Ca²⁺ regulation of the pore-opening in P2X₄ receptors.

The subunit arrangement and assembly of ionotropic receptors

Ionotropic receptors mediate rapid communication between neurons. These receptors are oligomers and are usually assembled from multiple subunit types. Receptors built from different subunit combinations have distinct functional properties, such as single-channel conductances, rates of desensitization and sensitivities to activators and inactivators; they can also have different intracellular locations. Methods are now available for determining not only the subunit stoichiometry but also the subunit arrangement within ionotropic receptors. This information will inform experiments designed to understand the molecular basis of receptor assembly and function. It will also permit the modelling of potential ligand-binding sites at the interfaces between the subunits and should lead to a more rational approach to drug development.

Assembly and trafficking of P2X purinergic receptors (Review)

P2X receptors are cation selective ion channels gated by the binding of extracellular ATP. Seven subtypes have been identified and they have widespread and overlapping distributions throughout the body. They form homo- and heterotrimeric complexes that differ in their functional properties and subcellular localization. They form part of larger signalling complexes, interacting with unrelated ion channels and other membrane and cytosolic proteins. Up- or down-regulation of their expression is associated with several disease states. This review aims to summarize recent work on the assembly and trafficking of this family of receptors.

Direct visualization of the trimeric structure of the ASIC1a channel, using AFM imaging

There has been confusion about the subunit stoichiometry of the degenerin family of ion channels. Recently, a crystal structure of acid-sensing ion channel (ASIC) 1a revealed that it assembles as a trimer. Here, we used atomic force microscopy (AFM) to image unprocessed ASIC1a bound to mica. We detected a mixture of subunit monomers, dimers and trimers. In some cases, triple-subunit clusters were clearly visible, confirming the trimeric structure of the channel, and indicating that the trimer sometimes disaggregated after adhesion to the mica surface. This AFM-based technique will now enable us to determine the subunit arrangement within heteromeric ASICs.

Understanding ion channel biology using epitope tags: progress, pitfalls, and promise

Epitope tags have been increasingly used to understand ion channel subunit assembly and interaction, trafficking, subcellular localization, and function in living cells. In particular, epitope tags have proven extremely useful for analyses of closely related, highly homologous channel subunits in endogenous cell contexts in vitro and in vivo, where multiple channel isoforms may be expressed. However, as the variety of epitope tags that have been used has expanded, and the use of tagged channel subunits has become increasingly sophisticated and widespread, there has also been an increase in the number of examples highlighting the potential problems associated with the use of epitope tags for ion channel studies. Described here are some of the epitope tags that have been used to study ion channel subunits, including the HA, FLAG, myc, His6, and green fluorescent protein (GFP) epitopes, as well as some of the applications and avenues of research in which they have proven advantageous. Potential pitfalls and caveats associated with the use of these epitope tags are also discussed, with an emphasis on the need to include careful characterization of epitope-tagged channel subunits as part of their construction. Finally, potential avenues for future investigation and the development of this approach are considered.

Atomic force microscopy reveals the stoichiometry and subunit arrangement of the alpha4beta3delta GABA(A) receptor

The GABA(A) receptor is a chloride-selective ligand-gated ion channel of the Cys-loop superfamily. The receptor consists of five subunits arranged pseudosymmetrically around a central pore. The predominant form of the receptor in the brain contains alpha(1)-, beta(2)-, and gamma(2)-subunits in the arrangement alphabetaalphagammabeta, counter-clockwise around the pore. GABA(A) receptors containing delta-instead of gamma-subunits, although a minor component of the total receptor population, have interesting properties, such as an extrasynaptic location, high sensitivity to GABA, and potential association with conditions such as epilepsy. They are therefore attractive targets for drug development. Here we addressed the subunit arrangement within the alpha(4)beta(3)delta form of the receptor. Different epitope tags were engineered onto the three subunits, and complexes between receptors and anti-epitope antibodies were imaged by atomic force microscopy. Determination of the numbers of receptors doubly decorated by each of the three antibodies revealed a subunit stoichiometry of 2alpha:2beta:1delta. The distributions of angles between pairs of antibodies against the alpha- and beta-subunits both had peaks at around 144 degrees , indicating that these pairs of subunits were nonadjacent. Decoration of the receptor with ligands that bind to the extracellular domain (i.e., the lectin concanavalin A and an antibody that recognizes the beta-subunit N-terminal sequence) showed that the receptor preferentially binds to the mica extracellular face down. Given this orientation, the geometry of complexes of receptors with both an antibody against the delta-subunit and Fab fragments against the alpha-subunits indicates a predominant subunit arrangement of alphabetaalphadeltabeta, counter-clockwise around the pore when viewed from the extracellular space.

Determination of the architecture of ionotropic receptors using AFM imaging

Fast neurotransmission in the nervous system is mediated by ionotropic receptors, all of which contain several subunits surrounding an integral ion channel. There are three major families of ionotropic receptors: the 'Cys-loop' receptors (including the nicotinic receptor for acetylcholine, the 5-HT(3) receptor, the GABA(A) receptor and the glycine receptor), the glutamate receptors (including the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, kainate and N-methyl-D: -aspartic acid receptors) and the P2X receptors for adenosine triphosphate. These receptors are often built from multiple types of subunit, raising the question of the stoichiometry and subunit arrangement within the receptors. This question is of therapeutic significance because in some cases drug-binding sites are located at subunit-subunit interfaces. In this paper, we describe a general method, based on atomic force microscopy imaging, to solve the architecture of multi-subunit proteins, such as the ionotropic receptors. Specific epitope tags are engineered onto each receptor subunit. The subunits are then expressed exogenously in cultured cells, and the receptors are isolated from detergent extracts of membrane fractions by affinity chromatography. The receptors are imaged both alone and in complex with anti-epitope antibodies. The size of the imaged particles provides an estimate of the subunit stoichiometry, whereas the geometry of the receptor-antibody complexes produces more detailed information about the receptor architecture. We use an automated, unbiased system to identify receptors and receptor-antibody complexes and to determine the geometry of the complexes. We are also able to determine the orientation of the receptors on the mica substrate, which will allow us to solve the subunit arrangement within receptors, such as the GABA(A) receptor, which contain three types of subunits.

AFM imaging reveals the tetrameric structure of the TRPC1 channel

We have determined the subunit stoichiometry of the transient receptor potential C1 (TRPC1) channel by imaging isolated channels using atomic force microscopy (AFM). A frequency distribution of the molecular volumes of individual channel particles had two peaks, at 170 and 720 nm(3), corresponding with the expected sizes of TRPC1 monomers and tetramers, respectively. Complexes were formed between TRPC1 channels and antibodies against a V5 epitope tag present on each subunit. The frequency distribution of angles between pairs of bound antibodies had two peaks, at 88 degrees and 178 degrees. This result again indicates that the channel assembles as a tetramer.