[Show abstract][Hide abstract] ABSTRACT: Controversy regarding the number and function of ligand binding sites in neurotransmitter/sodium symporters arose from conflicting data in crystal structures and molecular pharmacology. Here, we have designed novel tools for atomic force microscopy that directly measure the interaction forces between the serotonin transporter (SERT) and the S- and R-enantiomers of citalopram on the single molecule level. This approach is based on force spectroscopy, which allows for the extraction of dynamic information under physiological conditions thus inaccessible via X-ray crystallography. Two distinct populations of characteristic binding strengths of citalopram to SERT were revealed in Na(+) -containing buffer. In contrast, in Li(+) -containing buffer, SERT showed only low force interactions. Conversely, the vestibular mutant SERT-G402H merely displayed the high force population. These observations provide physical evidence for the existence of two binding sites in SERT when accessed in a physiological context. Competition experiments revealed that these two sites are allosterically coupled and exert reciprocal modulation.
Full-text · Article · Dec 2015 · Angewandte Chemie International Edition
[Show abstract][Hide abstract] ABSTRACT: Neurotransmitter uptake by sodium-coupled monoamine transporters of the NSS family is required for termination of synaptic transmission. Transport is tightly regulated by protein-protein interactions involving the small cytoplasmic segments at the amino (N-) and carboxy (C-) terminal ends of the transporter. Although structures of homologs provide information about the transmembrane regions of these transporters, the structural arrangement of the terminal domains remains largely unknown. Here, we combined molecular modelling, biochemical and biophysical approaches in an iterative manner to investigate the structure of the 82-residue N-terminal and 30-residue C-terminal domains of human serotonin transporter (SERT). Several secondary structures were predicted in these domains and structural models were built using the Rosetta fragment-based methodology. 1-dimensional (1)H NMR and circular dichroism (CD) spectroscopy were consistent with the presence of helical elements in the isolated SERT N-terminal domain. Moreover, introducing helix-breaking residues within those elements altered the fluorescence resonance energy transfer (FRET) signal between terminal CFP and YFP tags in full-length SERT, consistent with the notion that the fold of the terminal domains is relatively well defined. Full-length models of SERT were generated that are consistent with these and published experimental data. The resultant models predict confined loci for the terminal domains whose separation increased during the transport-related conformational cycle, as predicted by structures of homologs and by the 'rocking bundle' hypothesis, and consistent with spectroscopic measurements. The models also suggest the nature of binding to regulatory interaction partners. This study provides a structural context for functional and regulatory mechanisms involving SERT terminal domains.
[Show abstract][Hide abstract] ABSTRACT: Addiction to psychostimulants (i.e., amphetamines and cocaine) imposes a major socioeconomic burden. Prevention and treatment represent unmet medical needs, which may be addressed, if the mechanisms underlying psychostimulant action are understood. Cocaine acts as a blocker at the transporters for dopamine (DAT), serotonin (SERT) and norepinephrine (NET), but amphetamines are substrates that do not only block the uptake of monoamines but also induce substrate efflux by promoting reverse transport. Reverse transport has been a focus of research for decades but its mechanistic basis still remains enigmatic. Recently, transporter-interacting proteins were found to regulate amphetamine-triggered reverse transport: calmodulin kinase IIα (αCaMKII) is a prominent example, because it binds the carboxyl terminus of DAT, phosphorylates its amino terminus and supports amphetamine-induced substrate efflux in vitro. Here, we investigated whether, in vivo, the action of amphetamine was contingent on the presence of αCaMKII by recording the behavioural and neurochemical effects of amphetamine. Measurement of dopamine efflux in the dorsal striatum by microdialysis revealed that amphetamine induced less dopamine efflux in mice lacking αCaMKII. Consistent with this observation, the acute locomotor responses to amphetamine were also significantly blunted in αCaMKII-deficient mice. In addition, while the rewarding properties of amphetamine were preserved in αCaMKII-deficient mice, their behavioral sensitization to amphetamine was markedly reduced. Our findings demonstrate that amphetamine requires the presence of αCaMKII to elicit a full-fledged effect on DAT in vivo: αCaMKII does not only support acute amphetamine-induced dopamine efflux but is also important in shaping the chronic response to amphetamine.Neuropsychopharmacology accepted article preview online, 29 May 2014; doi:10.1038/npp.2014.124.
Full-text · Article · May 2014 · Neuropsychopharmacology: official publication of the American College of Neuropsychopharmacology
[Show abstract][Hide abstract] ABSTRACT: The A2A-receptor is a class A/rhodopsin-like G protein-coupled receptor. Coupling to its cognate protein Gs occurs via restricted collision coupling and is contingent on the presence of cholesterol. Agonist activation slows diffusion of the A2A-adenosine receptor in the lipid bilayer. We explored the contribution of the hydrophobic core and of the extended C-terminus by examining diffusion of quantum dot labeled receptor variants in dissociated hippocampal neurons: single particle tracking of the A2A-receptor(1-311), which lacks the last 101 residues, revealed that agonist-induced confinement was abolished and that the agonist-induced decrease in diffusivity was substantially reduced. A fragment comprising the SH3- and the guanylate kinase (GUK)-domain of synapse associated protein 102 (SAP102) was identified as a candidate interactor that bound to the A2A-receptor C-terminus. Complex formation between the A2A-receptor and SAP102 was verified by coimmunoprecipitation and by tracking its impact on receptor diffusion. Analysis of all trajectories by a hidden Markov model was consistent with two diffusion states where agonist activation reduced the transition between the two states and thus promoted the accumulation of the A2A-receptor in the compartment with slow mobility. Overexpression of SAP102 precluded the access of the A2A-receptor to a compartment with restricted mobility. In contrast, a mutated A2A-receptor (with (383)DVELL(387) replaced by RVRAA) was insensitive to the action of SAP102. These observations show that the hydrophobic core per se does not fully account for the agonist-promoted change in mobility of the A2A-receptor. The extended carboxyl terminus allows for regulatory input by scaffolding molecules such as SAP102.
No preview · Article · Feb 2014 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The human serotonin transporter (hSERT) is responsible for the termination of synaptic serotonergic signaling. Although there
is solid evidence that SERT forms oligomeric complexes, the exact stoichiometry of the complexes and the fractions of different
coexisting oligomeric states still remain enigmatic. Here we used single molecule fluorescence microscopy to obtain the oligomerization
state of the SERT via brightness analysis of single diffraction-limited fluorescent spots. Heterologously expressed SERT was
labeled either with the fluorescent inhibitor JHC 1-64 or via fusion to monomeric GFP. We found a variety of oligomerization
states of membrane-associated transporters, revealing molecular associations larger than dimers and demonstrating the coexistence
of different degrees of oligomerization in a single cell; the data are in agreement with a linear aggregation model. Furthermore,
oligomerization was found to be independent of SERT surface density, and oligomers remained stable over several minutes in
the live cell plasma membrane. Together, the results indicate kinetic trapping of preformed SERT oligomers at the plasma membrane.
Full-text · Article · Jan 2014 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: Ibogaine is a psychoactive indole alkaloid. Its use as anti-addictive agent was accompanied by QT prolongation and cardiac arrhythmias, which are most likely caused by hERG potassium channel inhibition. Therefore, the interaction of ibogaine with hERG channels heterologously expressed in tsA-201 cells was studied in detail. Currents through hERG channels were blocked regardless whether ibogaine was applied via the extra- or intracellular solution. The extent of inhibition was determined by the relative pH values. Block occurred during the activation of the channels and was not observed for resting channels. With increasing depolarizations ibogaine block grew and developed faster. Steady-state activation and inactivation of the channel were shifted to more negative potentials. Deactivation was slowed, while inactivation was accelerated. Mutations in the binding site reported for other hERG channel blockers (Y652A and F656A) reduced the potency of ibogaine, whereas an inactivation-deficient double mutant (G628C/S631C) was as sensitive as wild-type channels. Molecular drug docking indicated binding within the inner cavity of the channel independently of the protonation of ibogaine. Experimental current traces were fit to a kinetic model of hERG channel gating, revealing preferential binding of ibogaine to the open and inactivated state. Taken together, ibogaine blocks hERG channels from the cytosolic side either in its charged form alone or in company with its uncharged form and alters the currents by changing the relative contribution of channel states over time.
Full-text · Article · Dec 2013 · Journal of Pharmacology and Experimental Therapeutics
[Show abstract][Hide abstract] ABSTRACT: Psychostimulants such as amphetamine and cocaine are illicitly used drugs that act on neurotransmitter transporters for dopamine, serotonin or norepinephrine. These drugs can by themselves already cause severe neurotoxicity. However, an additional health threat arises from adulterant substances which are added to the illicit compound without declaration. One of the most frequently added adulterants in street drugs sold as cocaine is the anthelmintic drug levamisole. We tested the effects of levamisole on neurotransmitter transporters heterologously expressed in HEK293 cells. Levamisole was 100 and 300-fold less potent than cocaine in blocking norepinephrine and dopamine uptake, and had only very low affinity for the serotonin transporter. In addition, levamisole did not trigger any appreciable substrate efflux. Because levamisole and cocaine are frequently co-administered, we searched for possible allosteric effects; at 30 μM, a concentration at which levamisole displayed already mild effects on norepinephrine transport it did not enhance the inhibitory action of cocaine. Levamisole is metabolized to aminorex, a formerly marketed anorectic drug, which is classified as an amphetamine-like substance. We examined the uptake-inhibitory and efflux-eliciting properties of aminorex and found it to exert strong effects on all three neurotransmitter transporters in a manner similar to amphetamine. We therefore conclude that while the adulterant levamisole itself has only moderate effects on neurotransmitter transporters, its metabolite aminorex may exert distinct psychostimulant effects by itself. Given that the half-time of levamisole and aminorex exceeds that of cocaine, it may be safe to conclude that after the cocaine effect "fades out" the levamisole/aminorex effect "kicks in".
No preview · Article · Nov 2013 · Neurochemistry International
[Show abstract][Hide abstract] ABSTRACT: The adenosine A2A receptor is a prototypical rhodopsin-like G protein-coupled receptor but has several unique structural features, in particular
a long C terminus (of >120 residues) devoid of a palmitoylation site. It is known to interact with several accessory proteins
other than those canonically involved in signaling. However, it is evident that many more proteins must interact with the
A2A receptor, if the trafficking trajectory of the receptor is taken into account from its site of synthesis in the endoplasmic
reticulum (ER) to its disposal by the lysosome. Affinity-tagged versions of the A2A receptor were expressed in HEK293 cells to identify interacting partners residing in the ER by a proteomics approach based
on tandem affinity purification. The receptor-protein complexes were purified in quantities sufficient for analysis by mass
spectrometry. We identified molecular chaperones (heat-shock proteins HSP90α and HSP70-1A) that interact with and retain partially
folded A2A receptor prior to ER exit. Complex formation between the A2A receptor and HSP90α (but not HSP90β) and HSP70-1A was confirmed by co-affinity precipitation. HSP90 inhibitors also enhanced
surface expression of the receptor in PC12 cells, which endogenously express the A2A receptor. Finally, proteins of the HSP relay machinery (e.g. HOP/HSC70-HSP90 organizing protein and P23/HSP90 co-chaperone) were recovered in complexes with the A2A receptor. These observations are consistent with the proposed chaperone/coat protein complex II exchange model. This posits
that cytosolic HSP proteins are sequentially recruited to folding intermediates of the A2A receptor. Release of HSP90 is required prior to recruitment of coat protein complex II components. This prevents premature
ER export of partially folded receptors.
Preview · Article · Aug 2013 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: Nerve functions require phosphatidylinositol-4,5-bisphosphate (PIP2) that binds to ion channels, thereby controlling their gating. Channel properties are also attributed to serotonin transporters (SERTs); however, SERT regulation by PIP2 has not been reported. SERTs control neurotransmission by removing serotonin from the extracellular space. An increase in extracellular serotonin results from transporter-mediated efflux triggered by amphetamine-like psychostimulants. Herein, we altered the abundance of PIP2 by activating phospholipase-C (PLC), using a scavenging peptide, and inhibiting PIP2-synthesis. We tested the effects of the verified scarcity of PIP2 on amphetamine-triggered SERT functions in human cells. We observed an interaction between SERT and PIP2 in pull-down assays. On decreased PIP2 availability, amphetamine-evoked currents were markedly reduced compared with controls, as was amphetamine-induced efflux. Signaling downstream of PLC was excluded as a cause for these effects. A reduction of substrate efflux due to PLC activation was also found with recombinant noradrenaline transporters and in rat hippocampal slices. Transmitter uptake was not affected by PIP2 reduction. Moreover, SERT was revealed to have a positively charged binding site for PIP2. Mutation of the latter resulted in a loss of amphetamine-induced SERT-mediated efflux and currents, as well as a lack of PIP2-dependent effects. Substrate uptake and surface expression were comparable between mutant and WT SERTs. These findings demonstrate that PIP2 binding to monoamine transporters is a prerequisite for amphetamine actions without being a requirement for neurotransmitter uptake. These results open the way to target amphetamine-induced SERT-dependent actions independently of normal SERT function and thus to treat psychostimulant addiction.
Full-text · Article · Jun 2013 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: The high-resolution crystal structure of the leucine transporter (LeuT) is frequently used as a template for homology models of the dopamine transporter (DAT). Although similar in structure, DAT differs considerably from LeuT in a number of ways: (i) when compared to LeuT, DAT has very long intracellular amino and carboxyl termini; (ii) LeuT and DAT share a rather low overall sequence identity (22%) and (iii) the extracellular loop 2 (EL2) of DAT is substantially longer than that of LeuT. Extracellular zinc binds to DAT and restricts the transporter's movement through the conformational cycle, thereby resulting in a decrease in substrate uptake. Residue H293 in EL2 praticipates in zinc binding and must be modelled correctly to allow for a full understanding of its effects. We exploited the high-affinity zinc binding site endogenously present in DAT to create a model of the complete transmemberane domain of DAT. The zinc binding site provided a DAT-specific molecular ruler for calibration of the model. Our DAT model places EL2 at the transporter lipid interface in the vicinity of the zinc binding site. Based on the model, D206 was predicted to represent a fourth co-ordinating residue, in addition to the three previously described zinc binding residues H193, H375 and E396. This prediction was confirmed by mutagenesis: substitution of D206 by lysine and cysteine affected the inhibitory potency of zinc and the maximum inhibition exerted by zinc, respectively. Conversely, the structural changes observed in the model allowed for rationalizing the zinc-dependent regulation of DAT: upon binding, zinc stabilizes the outward-facing state, because its first coordination shell can only be completed in this conformation. Thus, the model provides a validated solution to the long extracellular loop and may be useful to address other aspects of the transport cycle.
Full-text · Article · Feb 2013 · PLoS Computational Biology
[Show abstract][Hide abstract] ABSTRACT: The serotonin transporter (SERT) maintains serotonergic neurotransmission via rapid reuptake of serotonin from the synaptic cleft. SERT relies exclusively on the COPII component SEC24C for ER export. The closely related transporters for noradrenaline (NET) and dopamine (DAT) depend on SEC24D. Here, we show that discrimination between SEC24C and SEC24D is specified by the residue at position +2 downstream from the ER export motif ((607)RI(608) in SERT). Substituting K(610) by tyrosine, the corresponding residue found in NET and DAT, switched the SEC24 isoform preference: SERT-K610Y relied solely on SEC24D to reach the cell surface. This analysis was extended to other SLC6 (solute carrier 6) transporter family members: siRNA-dependent depletion of SEC24C - but not of SEC24D - reduced surface levels of the glycine transporter-1a, the betaine/GABA-transporter and the GABA-transporter-4. Experiments with dominant negative versions of SEC24C and SEC24D recapitulated these findings. We also verified that the presence of two ER export motifs (in concatemers of SERT and GABA-transporter-1) supported recruitment of both, SEC24C and SEC24D. To the best of our knowledge, this is the first report to document a change in SEC24 specificity by mutation of a single residue in the client protein. Our observations allowed for deducing a rule for SLC6 family members: a hydrophobic residue (Y or V) in the +2 position specifies interaction with SEC24D, a hydrophilic residue (K, N, Q) recruits SEC24C. Variations in SEC24C are linked to neuropsychiatric disorders. The present findings provide a mechanistic explanation. Variations in SEC24C may translate into distinct surface levels of neurotransmitter transporters.
Full-text · Article · Jan 2013 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The dopamine transporter (DAT) is a crucial regulator of dopaminergic neurotransmission, controlling the length and brevity
of dopaminergic signaling. DAT is also the primary target of psychostimulant drugs such as cocaine and amphetamines. Conversely,
methylphenidate and amphetamine are both used clinically in the treatment of attention-deficit hyperactivity disorder and
narcolepsy. The action of amphetamines, which induce transport reversal, relies primarily on the ionic composition of the
intra- and extracellular milieus. Recent findings suggest that DAT interacting proteins may also play a significant role in
the modulation of reverse dopamine transport. The pharmacological inhibition of the serine/threonine kinase αCaMKII attenuates
amphetamine-triggered DAT-mediated 1-methyl-4-phenylpyridinium (MPP+) efflux. More importantly, αCaMKII has also been shown to bind DAT in vitro and is therefore believed to be an important player within the DAT interactome. Herein, we show that αCaMKII co-immunoprecipitates
with DAT in mouse striatal synaptosomes. Mice, which lack αCaMKII or which express a permanently self-inhibited αCaMKII (αCaMKIIT305D), exhibit significantly reduced amphetamine-triggered DAT-mediated MPP+ efflux. Additionally, we investigated mice that mimic a neurogenetic disease known as Angelman syndrome. These mice possess
reduced αCaMKII activity. Angelman syndrome mice demonstrated an impaired DAT efflux function, which was comparable with that
of the αCaMKII mutant mice, indicating that DAT-mediated dopaminergic signaling is affected in Angelman syndrome.
[Show abstract][Hide abstract] ABSTRACT: Cocaine and amphetamine are psychostimulant drugs that are illicitly used; they affect sensory perception by targeting the neurotransmitter: sodium symporters (NSS) at the synapses between neurons. They both increase the concentration of the neurotransmitter in the synaptic cleft but by different means. The physiological role of NSS is the reuptake of their endogenous substrate. For this task, they exploit the pre-existing sodium-gradient across the cellular membrane that is maintained by the activity of the sodium:potassium pump. This reuptake process terminates synaptic transmission because the neurotransmitter is removed from the synaptic cleft — and its action on pre- and postsynaptic receptor molecules is stopped. Amphetamines induce the reverse operation of distinct NSS family members, whereas cocaine merely inhibits the same transporters and thereby blocks the reuptake of neurotransmitter. These effects, although completely different in molecular mechanism, lead to an increase in the synaptic concentration of non-exocytotically released neurotransmitters. While these actions have long been appreciated, the underlying mechanistic details have been surprisingly difficult to understand. The advent of a crystal structure of a prokaryotic NSS protein and the concomitant development of homology models for eukaryotic NSS family members generated novel insights into the structure-function relationships of this clinically relevant class of transporters. Ultimately, we hope to understand the effects of amphetamines and cocaine on a molecular level to elucidate their profound effects on sensory perception.