Triphenylmethylphosphonium is an ion channel ligand of the nicotinic acetylcholine receptor

Mitochondria-Targeted Triphenylphosphonium-Based Compounds: Syntheses, Mechanisms of Action, and Therapeutic and Diagnostic Applications

Mitochondria are recognized as one of the most important targets for new drug design in cancer, cardiovascular, and neurological diseases. Currently, the most effective way to deliver drugs specifically to mitochondria is by covalent linking a lipophilic cation such as an alkyltriphenylphosphonium moiety to a pharmacophore of interest. Other delocalized lipophilic cations, such as rhodamine, natural and synthetic mitochondria-targeting peptides, and nanoparticle vehicles, have also been used for mitochondrial delivery of small molecules. Depending on the approach used, and the cell and mitochondrial membrane potentials, more than 1000-fold higher mitochondrial concentration can be achieved. Mitochondrial targeting has been developed to study mitochondrial physiology and dysfunction and the interaction between mitochondria and other subcellular organelles and for treatment of a variety of diseases such as neurodegeneration and cancer. In this Review, we discuss efforts to target small-molecule compounds to mitochondria for probing mitochondria function, as diagnostic tools and potential therapeutics. We describe the physicochemical basis for mitochondrial accumulation of lipophilic cations, synthetic chemistry strategies to target compounds to mitochondria, mitochondrial probes, and sensors, and examples of mitochondrial targeting of bioactive compounds. Finally, we review published attempts to apply mitochondria-targeted agents for the treatment of cancer and neurodegenerative diseases.

Dynamic Structural Investigations on the Torpedo Nicotinic Acetylcholine Receptor by Time-Resolved Photoaffinity Labeling

An increasing number of high-resolution structures of membrane-embedded ion channels (or soluble homologues) have emerged during the last couple of years. The most pressing need now is to understand the complex mechanism underlying ion-channel function. Time-resolved photoaffinity labeling is a suitable tool for investigating the molecular function of membrane proteins, especially when high-resolution structures of related proteins are available. However until now this methodology has only been used on the Torpedo nicotinic acetylcholine receptor (nAChR). nAChRs are allosteric cation-selective receptor channels that are activated by the neurotransmitter acetylcholine (ACh) and implicated in numerous physiological and pathological processes. Time-resolved photoaffinity labeling has already enabled local motions of nAChR subdomains (i.e. agonist binding sites, ion channel, subunit interface) to be understood at the molecular level, and has helped to explain how small molecules can exert their physiological effect, an important step toward the development of drug design. Recent analytical and technical improvements should allow the application of this powerful methodology to other membrane proteins in the near future.

Location of the polyamine binding site in the vestibule of the nicotinic acetylcholine receptor ion channel

To map the structure of a ligand-gated ion channel, we used the photolabile polyamine-containing toxin MR44 as photoaffinity label. MR44 binds with high affinity to the nicotinic acetylcholine receptor in its closed channel conformation. The binding stoichiometry was two molecules of MR44 per receptor monomer. Upon UV irradiation of the receptor-ligand complex, (125)I-MR44 was incorporated into the receptor alpha-subunit. From proteolytic mapping studies, we conclude that the site of (125)I-MR44 cross-linking is contained in the sequence alpha His-186 to alpha Leu-199, which is part of the extracellular domain of the receptor. This sequence partially overlaps in its C-terminal region with one of the three loops that form the agonist-binding site. The agonist carbachol and the competitive antagonist alpha-bungarotoxin had only minor influence on the photocross-linking of (125)I-MR44. The site where the hydrophobic head group of (125)I-MR44 binds must therefore be located outside the zone that is sterically influenced by agonist bound at the nicotinic acetylcholine receptor. In binding and photocross-linking experiments, the luminal noncompetitive inhibitors ethidium and triphenylmethylphosphonium were found to compete with (125)I-MR44. We conclude that the polyamine moiety of (125)I-MR44 interacts with the high affinity noncompetitive inhibitor site deep in the channel of the nicotinic acetylcholine receptor, while the aromatic ring of this compound binds in the upper part of the ion channel (i.e. in the vestibule) to a hydrophobic region on the alpha-subunit that is located in close proximity to the agonist binding site. The region of the alpha-subunit labeled by (125)I-MR44 should therefore be accessible from the luminal side of the vestibule.

Nicotinic receptors modulating somatodendritic and terminal dopamine release differ pharmacologically

Ascending dopaminergic and noradrenergic neurons possess somatodendritic and terminal nicotinic cholinoceptors in the rat. Each neuronal population expresses mRNA for several types of nicotinic cholinoceptor subunit, including alpha6 and beta3. In superfused rat striatal synaptosomes, epibatidine evoked release of [3H]dopamine with similar efficacy to ACh, whereas nicotine and cytisine were weaker (70+/-6% and 58+/-6%, respectively). The four agonists were equi-efficacious in evoking [3H]noradrenaline release from hippocampal synaptosomes. Nicotine-evoked synaptosomal release was tetrodotoxin-insensitive. Somatodendritic nicotinic cholinoceptors on dopaminergic neurons were studied using a dendrosomal [3H]dopamine release assay and also in locomotor activity tests. In both assays, nicotine appeared more efficacious than epibatidine. Furthermore, with repeated nicotine exposure, the acute locomotor stimulant response to nicotine increased, whereas the epibatidine response became undetectable. In conclusion, somatodendritic nicotinic cholinoceptors located on dopaminergic neurons appear to differ pharmacologically from those on striatal dopaminergic terminals and hippocampal noradrenergic terminals.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Agonist self-inhibitory binding site of the nicotinic acetylcholine receptor

A major focus of current research on the nicotinic acetylcholine receptor (AChR) has been to understand the molecular mechanism of ion channel inhibition. In particular, we put special emphasis on the description of the localization of the agonist self-inhibitory binding site. Binding of agonist in the millimolar concentration range to this particular site produces inhibition of the ion flux activity previously elicited by the same agonist at micromolar concentrations. Due to the similitude in the pharmacological and electrophysiological behavior in inhibiting the ion channel of both high agonist concentrations and noncompetitive antagonists, we first describe the localization of noncompetitive inhibitor binding sites on the AChR. There is a great body of experimental evidence for the existence and location of luminal high-affinity noncompetitive inhibitor binding sites. In this regard, the most simple mechanism to describe the action of noncompetitive inhibitors which bind to luminal sites and, by its semblance, the agonist self-inhibition itself, is based on the assumption that these compounds enter the open channel, bind to different rings within the M2 transmembrane domain of the receptor, and block cation flux by occluding the receptor pore. However, the existence of high-affinity nonluminal noncompetitive inhibitor binding sites is not consistent with the open-channel-blocking mechanism. Instead, the presence of the quinacrine locus at the lipid-protein (alpha M1) interface approximately 7 A from the lipid-water interface and the ethidium domain located approximately 46 A from the membrane surface in the wall of the vestibule open the possibility for the regulation of cation permeation by an allosteric process. Additionally, the observed (at least partially) overlapping between the quinacrine and the agonist self-inhibitory binding site also suggests an allosteric process for agonist self-inhibition. For this alternative mechanism, cholinergic agonist molecules first need to be partitioned into (or to be adsorbed onto) the lipid membrane to further interact with its binding site located at the lipid-protein interface.

Luminal and non-luminal non-competitive inhibitor binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of the alpha subunit exist the binding sites for agonists such as the neurotransmitter acetylcholine, which upon binding trigger the channel opening, and for competitive antagonists such as d-tubocurarine, which compete for the former inhibiting its pharmacological action. For non-competitive inhibitors, a population of low-affinity binding sites have been found at the lipid-protein interface of the nicotinic acetylcholine receptor. In addition, at the M2 transmembrane domain, several high-affinity binding sites have been found for non-competitive inhibitors such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222 and the hydrophobic probe trifluoromethyl-iodophenyldiazirine. They are known as luminal binding sites. Although the local anaesthetic meproadifen seems to be located between the hydrophobic domains M2-M3, this locus is considered to form part of the channel mouth, thus this site can also be called a luminal binding site. In contraposition, experimental evidences support the hypothesis of the existence of other high-affinity binding sites for non-competitive inhibitors located not at the channel lumen, but at non-luminal binding domains. Among them, we can quote the binding site for quinacrine, which is located at the lipid-protein interface of the alpha M1 domain, and the binding site for ethidium, which is believed to interact with the wall of the vestibule very far away from both the lumen channel and the lipid membrane surface. The aim of this review is to discuss these recent findings relative to both structurally and functionally relevant aspects of non-competitive inhibitors of the nicotinic acetylcholine receptor. We will put special emphasis on the description of the localization of molecules with non-competitive antagonist properties that bind with high-affinity to luminal and non-luminal domains. The information described herein was principally obtained by means of methods such as photolabelling and site-directed mutagenesis in combination with patch-clamp. Our laboratory has contributed with data obtained by using biophysical approaches such as paramagnetic electron spin resonance and quantitative fluorescence spectroscopy.

Cyanotriphenylborate: subtype-specific blocker of glycine receptor chloride channels

The inhibitory glycine receptor is a ligand-gated ion-channel protein existing in different homo- and heterooligomeric isoforms. Here we show that the chloride channel of the recombinant alpha 1-subunit homooligomeric glycine receptor is efficiently blocked by cyanotriphenylborate (CTB) with a concentration effecting 50% inhibition (IC50) of 1.3 microM in the presence of 50 microM glycine. The antagonistic effect of CTB is noncompetitive, use dependent, and more pronounced at positive membrane potentials, suggesting open-channel block. In contrast to alpha 1-subunit receptors, alpha 2-subunit homooligomers are resistant to CTB (IC50 >> 20 microM). By exchanging the channel-lining transmembrane segment M2 of the alpha 1 polypeptide by that of the alpha 2 polypeptide, we could transfer this resistance to alpha 1 channels, indicating that a single glycine residue at position 254 of the alpha 1 subunit is critical for CTB sensitivity. The blocker did not affect the cation-selective channel of the nicotinic acetylcholine receptor. Thus, CTB may prove useful as a tool to probe the subunit structure of native glycine receptors in mammalian neurons.

Extracytoplasmic disposition of lysine beta 165 of acetylcholine receptor

The location, with respect to the membrane, of Lys 165 in the folded beta polypeptide of native nicotinic acetylcholine receptor has been determined by site-directed immunochemistry. Sealed, right-side-out vesicles rich in acetylcholine receptor were modified with pyridoxal phosphate and sodium [3H]-borohydride. Saponin was added to one portion of the vesicles to make them permeable to the pyridoxal phosphate and sodium borohydride; the other portion was modified in the absence of saponin. Both samples were then exhaustively succinylated and digested with trypsin and thermolysin to produce the peptide LDAKGER, which contains Lys beta 165. The digests were passed over an immunoadsorbent specific for peptides with the sequence LDAXGER, where X represents any modified or unmodified amino acid, and specifically bound peptides were eluted with 0.1 M sodium phosphate, pH 2.5. The eluates were submitted to high-pressure liquid chromatography, and two peptides, N epsilon-phospho[3H]pyridoxalLDAKGER and N epsilon-succinylLDAKGER, modified at the epsilon amino group of lysine with pyridoxal phosphate and sodium [3H]-borohydride or succinic anhydride, respectively, were identified by comparison to standards. The relative specific radioactivity of N epsilon-phospho[3H]pyridoxalLDAKGER modified in the presence or absence of saponin, respectively, was 0.9 +/- 0.4. The incorporation of phospho[3H]pyridoxyl groups into Lys alpha 380, a residue located on the cytoplasmic surface of acetylcholine receptor, was also monitored. The relative specific radioactivity of the peptide that contains the modified Lys alpha 380, N epsilon-phospho[3H]pyridoxalGVKYIAE, increased 3.6-fold when the modification was performed in the presence of saponin. This result verifies that the vesicles used in these experiments were sealed and right-side-out. Because the incorporation of [3H]pyridoxyl groups into Lys beta 165 is the same in the presence or absence of saponin, Lys beta 165 must have been located on the outside surface of the sealed, right-side-out vesicles, and therefore on the extracytoplasmic surface of native acetylcholine receptor.

Neuronal-type nicotinic receptors in human neuroblastoma and small-cell lung carcinoma cell lines

A beta subunit of the neuronal nicotinic receptor, sharing 88% homology with the rat beta 4 subunit, has been cloned from a human neuroblastoma cell line. The gene encoding the human beta 4 subunit is expressed in association with the alpha 3 gene in neuroblastoma and small-cell lung carcinoma cell lines. Patch-clamp experiments and radioligand binding assays confirm that these neuroendocrine tumor cell lines express functional neuronal nicotinic receptors. We suggest that these receptors might play a crucial role in the control of neurotransmitter and hormone secretion from neurosecretory human tumors.

The mechanism by which cyclopiazonic acid potentiates accumulation of tetraphenylphosphonium in cultured renal epithelial cells

Cyclopiazonic acid (CPA), a fungal metabolite produced by Aspergillus and Penicillium, potentiated the accumulation of the quaternary cation tetraphenylphosphonium (TPP+) in cultured pig renal epithelial cells. This is the first report of a natural product mediating the tight and apparently nonsaturable binding of a membrane potential probe to subcellular compartments. The potentiated TPP+ accumulation was dose dependent, nonsaturable, and not a result of hyperpolarization across the plasma membrane. Cyclopiazonic acid-potentiated accumulation was completely inhibited by the protonophore carbonylcyanide-m-chlorophenylhydrazone (CCCP). Dinitrophenol (DNP), tetrahexylammonium (THA), and n-ethylmaleimide (NEM) were also effective inhibitors of CPA-potentiated TPP+ accumulation. Although CPA-potentiated TPP+ uptake appeared to be energy dependent, TPP+ efflux (in the presence of CCCP) from CPA-treated cells was incomplete and most of the TPP+ accumulated in the presence of CPA was tightly bound. Dicyclohexylcarbodiimide (DCC), verapamil, and monensin also stimulated TPP+ accumulation, but the TPP+ which accumulated in the presence of these compounds was not tightly bound. As with controls, fractionation of cells which had accumulated TPP+ in the presence of DCC, verapamil, or monensin always resulted in near complete recovery (greater than 93%) of the TPP+ in the cytosolic fraction, whereas with CPA, greater than 88% of the TPP+ was recovered noncovalently bound in the plasma membrane and mitochondrial fractions. These results are consistent with the hypothesis that CPA-potentiated TPP+ accumulation is a result of potentiated partitioning of TPP+ into the plasma membranes and mitochondria of LLC-PK1 cells.

The ion channel of the nicotinic acetylcholine receptor is formed by the homologous helices M II of the receptor subunits

A binding site for the channel-blocking noncompetitive antagonist [3H]triphenylmethylphosphonium ([3H]TPMP+) was localized in the alpha-, beta- and delta-chains of the nicotinic acetylcholine receptor (AChR) from Torpedo marmorata electric tissue. The photolabel was found in homologous positions of the highly conserved sequence helix II, alpha 248, beta 254, and delta 262. The site of the photoreaction appears to not be affected by the functional state of the receptor. [3H]TPMP+ was found in position delta 262 independent of whether photolabeling was performed with the receptor in its resting, desensitized or antagonist state. A model of the AChR ion channel is proposed, according to which the channel is formed by the five helices II contributed by the five receptor subunits.

Covalent labeling of functional states of the acetylcholine receptor. Effects of antagonists on the receptor conformation

Photoaffinity labeling of membrane-bound nicotinic acetylcholine receptor from Torpedo marmorata electric tissue with the ion-channel blocker [3H]TPMP+ reveals various functional states of the receptor protein if labeling is performed with ms time resolution. In the resting and in the activated state most of the label is incorporated into the alpha-polypeptide chains of the receptor complex. When equilibrated with agonists and antagonists, predominantly the delta-polypeptide chain (and to a lesser extent the beta-chain) reacts with the photolabel. Reactivity of the delta-chain increases after exposure to cholinergic effectors with a half-life slower than the kinetics of receptor activation or rapid desensitization. Agonists and antagonists stimulate photolabelling of the delta-chain with different kinetics. For acetylcholine, carbamoylcholine and suberyldicholine the half-life of the reactivity increases is 400 - 500 ms; for the antagonists hexamethonium, d-tubocurarine and flaxedil it is about 10 s. The latter slow kinetics are also observed when the receptor is preequilibrated with agonists or antagonists prior to mixing with [3H]TPMP+ and starting the photoreaction. We conclude that time-resolved photoaffinity labeling can convalently mark protein structures involved in receptor functions. Of special interest is the observation that antagonists also induce a conformational change in the receptor protein.

Rapid laser flash photoaffinity labeling of binding sites for a noncompetitive inhibitor of the acetylcholine receptor

Photoaffinity labeling of the nicotinic acetylcholine receptor from Torpedo marmorata electric tissue was performed in the presence of cholinergic effectors in the millisecond to second time range by a combination of a stopped-flow apparatus and a high-energy pulse laser. The label applied was [3H]triphenylmethylphosphonium, a lipophilic cation previously shown to be a specific blocker of the acetylcholine receptor ion channel. With the receptor in the resting state most of the label was incorporated into the alpha polypeptide chains. In the presence of agonists and antagonists increasing incorporation into the delta- and (less pronounced) the beta-chain was observed. The time course of this increase had a half-life of about 0.4 s, being slower than receptor activation and channel opening. in the resting, active, and even rapidly desensitized state, the alpha polypeptide chains appear to be the primary targets of the photoaffinity reaction. The action spectrum of the photolabeling has a sharp maximum at lambda = 270 nm and a small-side maximum at lambda = 290 nm. It does not resemble the absorption spectrum of the label and may hint at amino acid side chains as the moieties activated by UV light causing the photolabeling. The effector specificity of the observed slow increase of label incorporation into the delta polypeptide chain was investigated. It does not prove that slow desensitization is the underlying event. The agonists acetylcholine and carbamoylcholine as well as treatment of receptor-rich membranes with phospholipase A2 (but not phospholipase D) triggered labeling of delta, but antagonists such as D-tubocurarine and most conspicuously flaxedil had a similar effect.

Photoaffinity labeling of acetylcholine receptor in millisecond time scale

Photoaffinity labeling of acetylcholine receptors can be performed with a time resolution allowing to discriminate reaction sites within the receptor protein in its different functional states. This is achieved by a combination of a stopped-flow apparatus with a high energy pulse laser. The photoaffinity label used is the lipophilic cation [3H]TPMP+ which has been shown to be a non-competitive antagonist and a specific ion channel blocker. AChR in its resting (channel closed) and active (channel open) state incorporates the label mainly into the alpha-polypeptide chain of the receptor. Only several hundred milliseconds after mixing AChR with agonist labeling of delta-chains becomes significant.