Simon Bulling

The Mechanistic Basis for Noncompetitive Ibogaine Inhibition of Serotonin and Dopamine Transporters

Background: Ibogaine is a noncompetitive inhibitor of SERT that stabilizes the transporter in an inward-open conformation. Results: Ibogaine binds to a site accessible from the cell exterior that does not overlap with the substrate-binding site. Conclusion: Ibogaine binds to a novel binding site on SERT and DAT. Significance: This study provides a mechanistic understanding of an unique inhibitor of SERT and DAT. Ibogaine, a hallucinogenic alkaloid proposed as a treatment for opiate withdrawal, has been shown to inhibit serotonin transporter (SERT) noncompetitively, in contrast to all other known inhibitors, which are competitive with substrate. Ibogaine binding to SERT increases accessibility in the permeation pathway connecting the substrate-binding site with the cytoplasm. Because of the structural similarity between ibogaine and serotonin, it had been suggested that ibogaine binds to the substrate site of SERT. The results presented here show that ibogaine binds to a distinct site, accessible from the cell exterior, to inhibit both serotonin transport and serotonin-induced ionic currents. Ibogaine noncompetitively inhibited transport by both SERT and the homologous dopamine transporter (DAT). Ibogaine blocked substrate-induced currents also in DAT and increased accessibility of the DAT cytoplasmic permeation pathway. When present on the cell exterior, ibogaine inhibited SERT substrate-induced currents, but not when it was introduced into the cytoplasm through the patch electrode. Similar to noncompetitive transport inhibition, the current block was not reversed by increasing substrate concentration. The kinetics of inhibitor binding and dissociation, as determined by their effect on SERT currents, indicated that ibogaine does not inhibit by forming a long-lived complex with SERT, but rather binds directly to the transporter in an inward-open conformation. A kinetic model for transport describing the noncompetitive action of ibogaine and the competitive action of cocaine accounts well for the results of the present study.

Unifying Concept of Serotonin Transporter-associated Currents

Background: hSERT is a neurotransmitter transporter driven by ion gradients with electroneutral stoichiometry but rheogenic properties. Results: hSERT displays coupled and uncoupled currents. The uncoupled current depends on internal K+. Conclusion: The conducting state of hSERT is in equilibrium with an inward facing K+-bound state. Significance: This study provides a framework for exploring transporter-associated currents. Serotonin (5-HT) uptake by the human serotonin transporter (hSERT) is driven by ion gradients. The stoichiometry of transported 5-HT and ions is predicted to result in electroneutral charge movement. However, hSERT mediates a current when challenged with 5-HT. This discrepancy can be accounted for by an uncoupled ion flux. Here, we investigated the mechanistic basis of the uncoupled currents and its relation to the conformational cycle of hSERT. Our observations support the conclusion that the conducting state underlying the uncoupled ion flux is in equilibrium with an inward facing state of the transporter with K+ bound. We identified conditions associated with accumulation of the transporter in inward facing conformations. Manipulations that increased the abundance of inward facing states resulted in enhanced steady-state currents. We present a comprehensive kinetic model of the transport cycle, which recapitulates salient features of the recorded currents. This study provides a framework for exploring transporter-associated currents.

The Two Na+ Sites in the Human Serotonin Transporter Play Distinct Roles in the Ion Coupling and Electrogenicity of Transport

Background: The Na+/Cl−-dependent serotonin transporter contains two putative Na+-binding sites (Na1 and Na2). Results: Mutations at Na1, but not Na2, allow Ca2+ to replace Na+ and alter the conducting properties of the transporter. Conclusion: Ca2+ binds at Na1, but in the Na1 mutant, it does not appear to be transported. Significance: This work uncovers distinct roles of the Na+-binding sites for serotonin transporter function. Neurotransmitter transporters of the SLC6 family of proteins, including the human serotonin transporter (hSERT), utilize Na+, Cl−, and K+ gradients to induce conformational changes necessary for substrate translocation. Dysregulation of ion movement through monoamine transporters has been shown to impact neuronal firing potentials and could play a role in pathophysiologies, such as depression and anxiety. Despite multiple crystal structures of prokaryotic and eukaryotic SLC transporters indicating the location of both (or one) conserved Na+-binding sites (termed Na1 and Na2), much remains uncertain in regard to the movements and contributions of these cation-binding sites in the transport process. In this study, we utilize the unique properties of a mutation of hSERT at a single, highly conserved asparagine on TM1 (Asn-101) to provide several lines of evidence demonstrating mechanistically distinct roles for Na1 and Na2. Mutations at Asn-101 alter the cation dependence of the transporter, allowing Ca2+ (but not other cations) to functionally replace Na+ for driving transport and promoting 5-hydroxytryptamine (5-HT)-dependent conformational changes. Furthermore, in two-electrode voltage clamp studies in Xenopus oocytes, both Ca2+ and Na+ illicit 5-HT-induced currents in the Asn-101 mutants and reveal that, although Ca2+ promotes substrate-induced current, it does not appear to be the charge carrier during 5-HT transport. These findings, in addition to functional evaluation of Na1 and Na2 site mutants, reveal separate roles for Na1 and Na2 and provide insight into initiation of the translocation process as well as a mechanism whereby the reported SERT stoichiometry can be obtained despite the presence of two putative Na+-binding sites.

Evidence for a role of transporter-mediated currents in the depletion of brain serotonin induced by serotonin transporter substrates.

Serotonin (5-HT) transporter (SERT) substrates like fenfluramine and 3,4-methylenedioxymethamphetamine cause long-term depletion of brain 5-HT, while certain other substrates do not. The 5-HT deficits produced by SERT substrates are dependent upon transporter proteins, but the exact mechanisms responsible are unclear. Here, we compared the pharmacology of several SERT substrates: fenfluramine, d-fenfluramine, 1-(m-chlorophenyl)piperazine (mCPP) and 1-(m-trifluoromethylphenyl)piperainze (TFMPP), to establish relationships between acute drug mechanisms and the propensity for long-term 5-HT depletions. In vivo microdialysis was carried out in rat nucleus accumbens to examine acute 5-HT release and long-term depletion in the same subjects. In vitro assays were performed to measure efflux of [3H]5-HT in rat brain synaptosomes and transporter-mediated ionic currents in SERT-expressing Xenopus oocytes. When administered repeatedly to rats (6 mg/kg, i.p., four doses), all drugs produce large sustained elevations in extracellular 5-HT (>5-fold) with minimal effects on dopamine. Importantly, 2 weeks after dosing, only rats exposed to fenfluramine and d-fenfluramine display depletion of brain 5-HT. All test drugs evoke fluoxetine-sensitive efflux of [3H]5-HT from synaptosomes, but d-fenfluramine and its bioactive metabolite d-norfenfluramine induce significantly greater SERT-mediated currents than phenylpiperazines. Our data confirm that drug-induced 5-HT release probably does not mediate 5-HT depletion. However, the magnitude of transporter-mediated inward current may be a critical factor in the cascade of events leading to 5-HT deficits. This hypothesis warrants further study, especially given the growing popularity of designer drugs that target SERT.

Towards an understanding of the psychostimulant action of amphetamine and cocaine

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.

Amphetamine Actions Rely on the Availability of Phosphatidylinositol-4,5-Bisphosphate

Neuronal functions, such as excitability or endo- and exocytosis, require phosphatidylinositol-4,5-bisphosphate (PIP2) since ion channels and other proteins involved in these processes are regulated by PIP2. Monoamine transporters control neurotransmission by removing monoamines from the extracellular space. They also display channel properties, but their regulation by PIP2 has not been reported. The psychostimulant amphetamine acts on monoamine transporters to stimulate transporter-mediated currents and efflux and thereby increases the levels of extracellular monoamines. Direct or receptor-mediated activation of phospholipase-C (PLC) reduced membrane PIP2 and amphetamine-evoked currents through recombinant serotonin transporters; extracellular application of a PIP2-scavenging peptide mimicked this effect. PLC activation also diminished amphetamine-induced reverse transport without altering transmitter uptake. Inhibition of reverse transport by PLC activation was also observed in brain slices and with recombinant dopamine and noradrenaline, but not GABA transporters; rises in intracellular Ca2+ or activation of protein kinase C were not involved in these effects. These data demonstrate for the first time PIP2-dependence of reverse transport and current in monoamine transporters.A toggle switch consists of two genes that mutually repress each other. This regulatory motif is active during cell differentiation and is thought to act as a memory device, being able to choose and maintain cell fate decisions. Commonly, this switch has been modeled in a deterministic framework where transcription and translation are lumped together. In this description, bistability occurs for transcription factor cooperativity, whereas autoactivation leads to a tristable system with an additional undecided state. In this contribution, we study the stability and dynamics of a two-stage gene expression switch within a probabilistic framework inspired by the properties of the Pu/Gata toggle switch in myeloid progenitor cells. We focus on low mRNA numbers, high protein abundance, and monomeric transcription-factor binding. Contrary to the expectation from a deterministic description, this switch shows complex multiattractor dynamics without autoactivation and cooperativity. Most importantly, the four attractors of the system, which only emerge in a probabilistic two-stage description, can be identified with committed and primed states in cell differentiation. To begin, we study the dynamics of the system and infer the mechanisms that move the system between attractors using both the quasipotential and the probability flux of the system. Next, we show that the residence times of the system in one of the committed attractors are geometrically distributed. We derive an analytical expression for the parameter of the geometric distribution, therefore completely describing the statistics of the switching process and elucidate the influence of the system parameters on the residence time. Moreover, we find that the mean residence time increases linearly with the mean protein level. This scaling also holds for a one-stage scenario and for autoactivation. Finally, we study the implications of this distribution for the stability of a switch and discuss the influence of the stability on a specific cell differentiation mechanism. Our model explains lineage priming and proposes the need of either high protein numbers or long-term modifications such as chromatin remodeling to achieve stable cell fate decisions. Notably, we present a system with high protein abundance that nevertheless requires a probabilistic description to exhibit multistability, complex switching dynamics, and lineage priming.

Decrypting structural and functional changes in LeuTAa at atomic level employing LRET

Background Neurotransmitter:sodium symporters (NSS) are integral membrane proteins that mediate the reuptake of monoamine neurotransmitters previously released into the synaptic cleft. They are of pharmacological significance because they are the target of many clinically important drugs. LeuTAa, a leucine/alanine transporter is a bacterial homolog to NSS. Crystal structures of LeuTAa with open to outward, occluded and inward-facing states have already been resolved at high resolution. Hence, LeuTAa serves as a good paradigm for exploring the structurefunction relationship of NSS proteins.

LRET-based distance measurements in the mammalian glutamate transporter EAAT3

Background EAAT3 (excitatory amino acid transporter 3) mediates the regulation of synaptic transmission by reuptake of glutamate in the synaptic cleft. It is distributed in neuronal membranes and is selectively enriched in the neurons of the hippocampus, cerebellum and the basal ganglia. It belongs to the family of soluble carrier family 1 member 1 (SLC1A1) and is expressed in kidney, a wide variety of epithelial tissues, brain and eyes.

Lithium Stabilizes the Inward Facing Conformation of Human Serotonin Transporter (hSERT)

According to the alternate access model of transport, a transporter such as hSERT adopts two major conformations: the outward facing conformation and the inward facing conformation. For a functional transporter the distribution of these states in equilibrium (with and without substrate) is unknown. Furthermore it is unclear how this distribution is affected by various parameters such as the temperature, the external and internal ion composition, the membrane potential etc.Here we tested the effect of various external cations such as sodium, lithium and NMDG on the conformational equilibrium.In this study we probed the conformational equilibrium using three different techniques. First we utilized intramolecular FRET measurements. This was done utilizing a construct of hSERT that had been genetically modified to contain a CFT at the N-terminal end and a YFP at the C-terminal end (CSERTY). CSERTY was heterologously expressed in HEK cells and FRET changes upon administration of different external ions were correlated with the data we obtained from binding of a radio-labeled inhibitor of SERT (second technique) as well as with the data we obtained by testing the accessibility of cysteines introduced by site directed mutagenesis to MTS reagents (third technique). Binding was conducted in membranes of HEK cells that stably expressed hSERT and for the accessibility studies we employed hSERT expressed in Xenopus laevis oocytes.In accordance with previous studies we find that external sodium supports the outward facing conformation. However for high external lithium concentrations we have evidence that the prevalent conformation is inward facing.