Functional Selectivity and Partial Efficacy at the Monoamine Transporters: A Unified Model of Allosteric Modulation and Amphetamine-Induced Substrate Release.

All clinically approved drugs targeting the plasmalemmal transporters for dopamine, norepinephrine, and serotonin act either as competitive uptake inhibitors or as amphetamine-like releasers. Monoamine transporter (MAT) ligands that allosterically affect MAT-mediated substrate uptake, release, or both were recently discovered. Their modes of action have not yet been explained in a unified framework. Here, we go beyond competitive inhibitors and classic amphetamines and introduce concepts for partial efficacy at and allosteric modulation of MATs. After we elaborate on a kinetic account for amphetamine action, we provide an explanation for partial release (i.e., the observation that some amphetamines are less efficacious than others in inducing monoamine efflux). We then elucidate mechanisms of allosteric inhibition and stimulation of MATs, which can be functionally selective for either substrate uptake or amphetamine-induced release. These concepts are integrated into a parsimonious kinetic framework, which relies exclusively on physiologic transport modes (without any deviation from an alternating access mechanism). The model posits cooperative substrate and Na+ binding and functional selectivity by conformational selection (i.e., preference of the allosteric modulators for the substrate-loaded or substrate-free states of the transporter). Thus, current knowledge about the kinetics of monoamine transport is sufficiently detailed to provide a quantitative description of the releasing action of amphetamines, of substrate uptake, and of selective modulation thereof by allosteric modulators.

A kinetic account for amphetamine-induced monoamine release.

The plasmalemmal monoamine transporters for dopamine, norepinephrine, and serotonin (SERT) are targets for amphetamines. In vivo, amphetamines elicit most, if not all, of their actions by triggering monoamine efflux. This is thought to be accomplished by an amphetamine-induced switch from the forward-transport to the substrate-exchange mode. The mechanism underlying this switch has remained elusive; available kinetic models posit that substrates and cosubstrate Na+ ions bind either in a random or in a sequential order. Neither can account for all reported experimental observations. We used electrophysiological recordings to interrogate crucial conformational transitions associated with the binding of five different substrates (serotonin, para-chloroamphetamine, and the high-affinity naphthyl-propan-amines PAL-287, PAL-1045, and PAL-1046) to human SERT expressed in HEK293 cells; specifically, we determined the relaxation kinetics of SERT from a substrate-loaded to a substrate-free state at various intracellular and extracellular Na+ concentrations. These rates and their dependence on intracellular and extracellular Na+ concentrations differed considerably between substrates. We also examined the effect of K+ on substrate affinity and found that K+ enhanced substrate dissociation. A kinetic model was developed, which allowed for random, but cooperative, binding of substrate and Na+ (or K+). The synthetic data generated by this model recapitulated the experimental observations. More importantly, the cooperative binding model accounted for the releasing action of amphetamines without any digression from alternating access. To the best of our knowledge, this model is the first to provide a mechanistic framework for amphetamine-induced monoamine release and to account for the findings that some substrates are less efficacious than others in promoting the substrate-exchange mode.

Conformational state interactions provide clues to the pharmacochaperone potential of serotonin transporter partial substrates.

Point mutations in SLC6 transporters cause misfolding, which can be remedied by pharmacochaperones. The serotonin transporter (SERT/SLC6A4) has a rich pharmacology including inhibitors, releasers (amphetamines, which promote the exchange mode), and more recently, discovered partial substrates. We hypothesized that partial substrates trapped the transporter in one or several states of the transport cycle. This conformational trapping may also be conducive to folding. We selected naphthylpropane-2-amines of the phenethylamine library (PAL) including the partial substrate PAL1045 and its congeners PAL287 and PAL1046. We analyzed their impact on the transport cycle of SERT by biochemical approaches and by electrophysiological recordings; substrate-induced peak currents and steady-state currents monitored the translocation of substrate and co-substrate Na+ across the lipid bilayer and the transport cycle, respectively. These experiments showed that PAL1045 and its congeners bound with different affinities (ranging from nm to µm) to various conformational intermediates of SERT during the transport cycle. Consistent with the working hypothesis, PAL1045 was the most efficacious compound in restoring surface expression and transport activity to the folding-deficient mutant SERT-601PG602-AA. These experiments provide a proof-of-principle for a rational search for pharmacochaperones, which may be useful to restore function to clinically relevant folding-deficient transporter mutants.

Occupancy of the Zinc-binding Site by Transition Metals Decreases the Substrate Affinity of the Human Dopamine Transporter by an Allosteric Mechanism.

The human dopamine transporter (DAT) has a tetrahedral Zn2+-binding site. Zn2+-binding sites are also recognized by other first-row transition metals. Excessive accumulation of manganese or of copper can lead to parkinsonism because of dopamine deficiency. Accordingly, we examined the effect of Mn2+, Co2+, Ni2+, and Cu2+ on transport-associated currents through DAT and DAT-H193K, a mutant with a disrupted Zn2+-binding site. All transition metals except Mn2+ modulated the transport cycle of wild-type DAT with affinities in the low micromolar range. In this concentration range, they were devoid of any action on DAT-H193K. The active transition metals reduced the affinity of DAT for dopamine. The affinity shift was most pronounced for Cu2+, followed by Ni2+ and Zn2+ (= Co2+). The extent of the affinity shift and the reciprocal effect of substrate on metal affinity accounted for the different modes of action: Ni2+ and Cu2+ uniformly stimulated and inhibited, respectively, the substrate-induced steady-state currents through DAT. In contrast, Zn2+ elicited biphasic effects on transport, i.e. stimulation at 1 µm and inhibition at 10 µm A kinetic model that posited preferential binding of transition metal ions to the outward-facing apo state of DAT and a reciprocal interaction of dopamine and transition metals recapitulated all experimental findings. Allosteric activation of DAT via the Zn2+-binding site may be of interest to restore transport in loss-of-function mutants.

Electrogenic Binding of Intracellular Cations Defines a Kinetic Decision Point in the Transport Cycle of the Human Serotonin Transporter.

The plasmalemmal monoamine transporters clear the extracellular space from their cognate substrates and sustain cellular monoamine stores even during neuronal activity. In some instances, however, the transporters enter a substrate-exchange mode, which results in release of intracellular substrate. Understanding what determines the switch between these two transport modes demands time-resolved measurements of intracellular (co-)substrate binding and release. Here, we report an electrophysiological investigation of intracellular solute-binding to the human serotonin transporter (SERT) expressed in HEK-293 cells. We measured currents induced by rapid application of serotonin employing varying intracellular (co-)substrate concentrations and interpreted the data using kinetic modeling. Our measurements revealed that the induction of the substrate-exchange mode depends on both voltage and intracellular Na+ concentrations because intracellular Na+ release occurs before serotonin release and is highly electrogenic. This voltage dependence was blunted by electrogenic binding of intracellular K+ and, notably, also H+ In addition, our data suggest that Cl- is bound to SERT during the entire catalytic cycle. Our experiments, therefore, document an essential role of electrogenic binding of K+ or of H+ to the inward-facing conformation of SERT in (i) cancelling out the electrogenic nature of intracellular Na+ release and (ii) in selecting the forward-transport over the substrate-exchange mode. Finally, the kinetics of intracellular Na+ release and K+ (or H+) binding result in a voltage-independent rate-limiting step where SERT may return to the outward-facing state in a KCl- or HCl-bound form.

Binding Mode Selection Determines the Action of Ecstasy Homologs at Monoamine Transporters.

Determining the structural elements that define substrates and inhibitors at the monoamine transporters is critical to elucidating the mechanisms underlying these disparate functions. In this study, we addressed this question directly by generating a series of N-substituted 3,4-methylenedioxyamphetamine analogs that differ only in the number of methyl substituents on the terminal amine group. Starting with 3,4-methylenedioxy-N-methylamphetamine, 3,4-methylenedioxy-N,N-dimethylamphetamine (MDDMA) and 3,4-methylenedioxy-N,N,N-trimethylamphetamine (MDTMA) were prepared. We evaluated the functional activities of the compounds at all three monoamine transporters in native brain tissue and cells expressing the transporters. In addition, we used ligand docking to generate models of the respective protein-ligand complexes, which allowed us to relate the experimental findings to available structural information. Our results suggest that the 3,4-methylenedioxyamphetamine analogs bind at the monoamine transporter orthosteric binding site by adopting one of two mutually exclusive binding modes. 3,4-methylenedioxyamphetamine and 3,4-methylenedioxy-N-methylamphetamine adopt a high-affinity binding mode consistent with a transportable substrate, whereas MDDMA and MDTMA adopt a low-affinity binding mode consistent with an inhibitor, in which the ligand orientation is inverted. Importantly, MDDMA can alternate between both binding modes, whereas MDTMA exclusively binds to the low-affinity mode. Our experimental results are consistent with the idea that the initial orientation of bound ligands is critical for subsequent interactions that lead to transporter conformational changes and substrate translocation.

Nanopharmacological Force Sensing to Reveal Allosteric Coupling in Transporter Binding Sites.

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.

Dual Action of Zn2+ on the Transport Cycle of the Dopamine Transporter.

The dopamine transporter shapes dopaminergic neurotransmission by clearing extracellular dopamine and by replenishing vesicular stores. The dopamine transporter carries an endogenous binding site for Zn(2+), but the nature of the Zn(2+)-dependent modulation has remained elusive: both, inhibition and stimulation of DAT have been reported. Here, we exploited the high time resolution of patch-clamp recordings to examine the effects of Zn(2+) on the transport cycle of DAT: we recorded peak currents associated with substrate translocation and steady-state currents reflecting the forward transport mode of DAT. Zn(2+) depressed the peak current but enhanced the steady-state current through DAT. The parsimonious explanation is preferential binding of Zn(2+) to the outward facing conformation of DAT, which allows for an allosteric activation of DAT, in both, the forward transport mode and substrate exchange mode. We directly confirmed that Zn(2+) dissociated more rapidly from the inward- than from the outward-facing state of DAT. Finally, we formulated a kinetic model for the action of Zn(2+) on DAT that emulated all current experimental observations and accounted for all previous (in part contradictory) findings. Importantly, the model predicts that the intracellular Na(+) concentration determines whether substrate uptake by DAT is stimulated or inhibited by Zn(2+). This prediction was directly verified. The mechanistic framework provided by the current model is of relevance for the rational design of allosteric activators of DAT. These are of interest for treating de novo loss-of-function mutations of DAT associated with neuropsychiatric disorders such as attention deficit hyperactivity disorder (ADHD).

Ligand Selectivity among the Dopamine and Serotonin Transporters Specified by the Forward Binding Reaction.

The membrane transporters for the monoamines serotonin (SERT) and dopamine (DAT) are prominent targets of various psychoactive substances, including competitive inhibitors, such as tricyclic antidepressants, methylphenidate, and cocaine. Upon rapid application of a substrate, SERT and DAT display an inwardly directed current comprised of a peak and a steady-state component. Binding of a competitive inhibitor to the transporter leads to reduction of the peak current amplitude because occupancy of the transporter by an inhibitor prevents the induction of the peak current by the substrate. We show that the inhibitory effect on the peak current can be used to study the association rate constant (k(on)), dissociation rate constant (k(off)), and equilibrium dissociation constant (K(D)) of chemically distinct SERT and DAT inhibitors, with high temporal precision and without the need of high-affinity radioligands as surrogates. We exemplify our approach by measuring the kinetics of cocaine, methylphenidate, and desipramine binding to SERT and DAT. Our analysis revealed that the selectivity of methylphenidate and desipramine for DAT and SERT, respectively, can be accounted for by their rate of association and not by the residence time in their respective binding sites.