Extracellular loops of the serotonin transporter act as a selectivity filter for drug binding.

The serotonin transporter (SERT) shapes serotonergic neurotransmission by retrieving its eponymous substrate from the synaptic cleft. Ligands that discriminate between SERT and its close relative, the dopamine transporter DAT, differ in their association rate constant rather than their dissociation rate. The structural basis for this phenomenon is not known. Here we examined the hypothesis that the extracellular loops 2 (EL2) and 4 (EL4) limit access to the ligand-binding site of SERT. We employed an antibody directed against EL4 (residues 388-400) and the antibody fragments 8B6 scFv (directed against EL2 and EL4) and 15B8 Fab (directed against EL2) and analyzed their effects on the transport cycle of and inhibitor binding to SERT. Electrophysiological recordings showed that the EL4 antibody and 8B6 scFv impeded the initial substrate-induced transition from the outward to the inward-facing conformation but not the forward cycling mode of SERT. In contrast, binding of radiolabeled inhibitors to SERT was enhanced by either EL4- or EL2-directed antibodies. We confirmed this observation by determining the association and dissociation rate of the DAT-selective inhibitor methylphenidate via electrophysiological recordings; occupancy of EL2 with 15B8 Fab enhanced the affinity of SERT for methylphenidate by accelerating its binding. Based on these observations, we conclude that (i) EL4 undergoes a major movement during the transition from the outward to the inward-facing state, and (ii) EL2 and EL4 limit access of inhibitors to the binding of SERT, thus acting as a selectivity filter. This insight has repercussions for drug development.

An Electrophysiological Approach to Measure Changes in the Membrane Surface Potential in Real Time.

Biological membranes carry fixed charges at their surfaces. These arise primarily from phospholipid headgroups. In addition, membrane proteins contribute to the surface potential with their charged residues. Membrane lipids are asymmetrically distributed. Because of this asymmetry, the net-negative charge at the inner leaflet exceeds that at the outer leaflet. Changes in surface potential are predicted to give rise to apparent changes in membrane capacitance. Here, we show that it is possible to detect changes in surface potential by an electrophysiological approach; the analysis of cellular currents relies on assuming that the electrical properties of a cell are faithfully described by a three-element circuit (i.e., the minimal equivalent circuit) comprised of two resistors and one capacitor. However, to account for changes in surface potential, it is necessary to add a battery to this circuit connected in series with the capacitor. This extended circuit model predicts that the current response to a square-wave voltage pulse harbors information, which allows for separating the changes in surface potential from a true capacitance change. We interrogated our model by investigating changes in the capacitance induced by ligand binding to the serotonin transporter and to the glycine transporters (GlyT1 and GlyT2). The experimental observations were consistent with the predictions of the extended circuit. We conclude that ligand-induced changes in surface potential (reflecting the binding event) and in true membrane capacitance (reflecting the concomitant conformational change) can be detected in real time even in instances in which they occur simultaneously.

Kinetic Models of Secondary Active Transporters.

Kinetic models have been employed to understand the logic of substrate transport through transporters of the Solute Carrier (SLC) family. All SLC transporters operate according to the alternate access model, which posits that substrate transport occurs in a closed loop of partial reactions (i.e., a transport cycle). Kinetic models can help to find realistic estimates for conformational transitions between individual states of the transport cycle. When constrained by experimental results, kinetic models can faithfully describe the function of a candidate transporter at a pre-steady state. In addition, we show that kinetic models can accurately predict the intra- and extracellular substrate concentrations maintained by the transporter at a steady state, even under the premise of loose coupling between the electrochemical gradient of the driving ion and of the substrate. We define the criteria for the design of a credible kinetic model of the SLC transporter. Parsimony is the guiding principle of kinetic modeling. We argue, however, that the level of acceptable parsimony is limited by the need to account for the substrate gradient established by a secondary active transporter, and for random order binding of co-substrates and substrate. Random order binding has consistently been observed in transporters of the SLC group.

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.

Differential neuronal targeting of a new and two known calcium channel ß4 subunit splice variants correlates with their regulation of gene expression.

The ß subunits of voltage-gated calcium channels regulate surface expression and gating of CaV1 and CaV2 α1 subunits and thus contribute to neuronal excitability, neurotransmitter release, and calcium-induced gene regulation. In addition, certain ß subunits are targeted into the nucleus, where they interact directly with the epigenetic machinery. Whereas their involvement in this multitude of functions is reflected by a great molecular heterogeneity of ß isoforms derived from four genes and abundant alternative splicing, little is known about the roles of individual ß variants in specific neuronal functions. In the present study, an alternatively spliced ß4 subunit lacking the variable N terminus (ß4e) is identified. It is highly expressed in mouse cerebellum and cultured cerebellar granule cells (CGCs) and modulates P/Q-type calcium currents in tsA201 cells and CaV2.1 surface expression in neurons. Compared with the other two known full-length ß4 variants (ß4a and ß4b), ß4e is most abundantly expressed in the distal axon, but lacks nuclear-targeting properties. To determine the importance of nuclear targeting of ß4 subunits for transcriptional regulation, we performed whole-genome expression profiling of CGCs from lethargic (ß4-null) mice individually reconstituted with ß4a, ß4b, and ß4e. Notably, the number of genes regulated by each ß4 splice variant correlated with the rank order of their nuclear-targeting properties (ß4b > ß4a > ß4e). Together, these findings support isoform-specific functions of ß4 splice variants in neurons, with ß4b playing a dual role in channel modulation and gene regulation, whereas the newly detected ß4e variant serves exclusively in calcium-channel-dependent functions.

Spectrum of Cav1.4 dysfunction in congenital stationary night blindness type 2.

Defective retinal synaptic transmission in patients affected with congenital stationary night blindness type 2 (CSNB2) can result from different dysfunction phenotypes in Cav1.4 L-type calcium channels. Here we investigated two prototypical Cav1.4 variants from either end of the functional spectrum. Using whole-cell and single-channel patch-clamp techniques, we provide analysis of the biophysical characteristics of the point mutation L860P and the C-terminal truncating mutation R1827X. L860P showed a typical loss-of-function phenotype attributed to a reduced number of functional channels expressed at the plasma membrane as implied by gating current and non-stationary noise analyses. This phenotype can be rationalized, because the inserted proline is predicted to break an amphipatic helix close to the transmembrane segment IIIS1 and thus to reduce channel stability and promote misfolding. In fact, L860P was subject to an increased turnover. In contrast, R1827X displayed an apparent gain-of-function phenotype, i.e., due to a hyperpolarizing shift of the IV-curve and increased single-channel activity. However, truncation also resulted in the loss of functional C-terminal modulation and thus unmasked calcium-dependent inactivation. Thus R1827X failed to support continuous calcium influx. Current inactivation curtails the dynamic range of photoreceptors (e.g., when adapting to variation in illumination). Taken together, the analysis of two representative mutations that occur in CSNB2 patients revealed fundamental differences in the underlying defect. These may explain subtle variations in the clinical manifestation and must be taken into account, if channel function is to be restored by pharmacochaperones or related approaches.

Structural basis for hyperpolarization-dependent opening of human HCN1 channel.

Hyperpolarization and cyclic nucleotide (HCN) activated ion channels are critical for the automaticity of action potentials in pacemaking and rhythmic electrical circuits in the human body. Unlike most voltage-gated ion channels, the HCN and related plant ion channels activate upon membrane hyperpolarization. Although functional studies have identified residues in the interface between the voltage-sensing and pore domain as crucial for inverted electromechanical coupling, the structural mechanisms for this unusual voltage-dependence remain unclear. Here, we present cryo-electron microscopy structures of human HCN1 corresponding to Closed, Open, and a putative Intermediate state. Our structures reveal that the downward motion of the gating charges past the charge transfer center is accompanied by concomitant unwinding of the inner end of the S4 and S5 helices, disrupting the tight gating interface observed in the Closed state structure. This helix-coil transition at the intracellular gating interface accompanies a concerted iris-like dilation of the pore helices and underlies the reversed voltage dependence of HCN channels.

Interplay between VSD, pore, and membrane lipids in electromechanical coupling in HCN channels.

Hyperpolarized-activated and cyclic nucleotide-gated (HCN) channels are the only members of the voltage-gated ion channel superfamily in mammals that open upon hyperpolarization, conferring them pacemaker properties that are instrumental for rhythmic firing of cardiac and neuronal cells. Activation of their voltage-sensor domains (VSD) upon hyperpolarization occurs through a downward movement of the S4 helix bearing the gating charges, which triggers a break in the alpha-helical hydrogen bonding pattern at the level of a conserved Serine residue. Previous structural and molecular simulation studies had however failed to capture pore opening that should be triggered by VSD activation, presumably because of a low VSD/pore electromechanical coupling efficiency and the limited timescales accessible to such techniques. Here, we have used advanced modeling strategies, including enhanced sampling molecular dynamics simulations exploiting comparisons between non-domain swapped voltage-gated ion channel structures trapped in closed and open states to trigger pore gating and characterize electromechanical coupling in HCN1. We propose that the coupling mechanism involves the reorganization of the interfaces between the VSD helices, in particular S4, and the pore-forming helices S5 and S6, subtly shifting the balance between hydrophobic and hydrophilic interactions in a 'domino effect' during activation and gating in this region. Remarkably, our simulations reveal state-dependent occupancy of lipid molecules at this emergent coupling interface, suggesting a key role of lipids in hyperpolarization-dependent gating. Our model provides a rationale for previous observations and a possible mechanism for regulation of HCN channels by the lipidic components of the membrane.

Structural Basis for Hyperpolarization-dependent Opening of the Human HCN1 Channel.

Hyperpolarization and cyclic-nucleotide (HCN) activated ion channels play a critical role in generating self-propagating action potentials in pacemaking and rhythmic electrical circuits in the human body. Unlike most voltage-gated ion channels, the HCN channels activate upon membrane hyperpolarization, but the structural mechanisms underlying this gating behavior remain unclear. Here, we present cryo-electron microscopy structures of human HCN1 in Closed, Intermediate, and Open states. Our structures reveal that the inward motion of two gating charges past the charge transfer center (CTC) and concomitant tilting of the S5 helix drives the opening of the central pore. In the intermediate state structure, a single gating charge is positioned below the CTC and the pore appears closed, whereas in the open state structure, both charges move past CTC and the pore is fully open. Remarkably, the downward motion of the voltage sensor is accompanied by progressive unwinding of the inner end of S4 and S5 helices disrupting the tight gating interface that stabilizes the Closed state structure. This "melting" transition at the intracellular gating interface leads to a concerted iris-like displacement of S5 and S6 helices, resulting in pore opening. These findings reveal key structural features that are likely to underlie reversed voltage-dependence of HCN channels.

Descriptors of Secondary Active Transporter Function and How They Relate to Partial Reactions in the Transport Cycle.

Plasmalemmal solute carriers (SLCs) gauge and control solute abundance across cellular membranes. By virtue of this action, they play an important role in numerous physiological processes. Mutations in genes encoding the SLCs alter amino acid sequence that often leads to impaired protein function and onset of monogenic disorders. To understand how these altered proteins cause disease, it is necessary to undertake relevant functional assays. These experiments reveal descriptors of SLC function such as the maximal transport velocity (Vmax), the Michaelis constant for solute uptake (KM), potencies for inhibition of transporter function (IC50/EC50), and many more. In several instances, the mutated versions of different SLC transporters differ from their wild-type counterparts in the value of these descriptors. While determination of these experimental parameters can provide conjecture as to how the mutation gives rise to disease, they seldom provide any definitive insights on how a variant differ from the wild-type transporter in its operation. This is because the experimental determination of association between values of the descriptors and several partial reactions a transporter undergoes is casual, but not causal, at best. In the present study, we employ kinetic models that allow us to derive explicit mathematical terms and provide experimental descriptors as a function of the rate constants used to parameterize the kinetic model of the transport cycle. We show that it is possible to utilize these mathematical expressions to deduce, from experimental outcomes, how the mutation has impinged on partial reactions in the transport cycle.

Detection of Ligand-binding to Membrane Proteins by Capacitance Measurements.

In multi-cellular organisms, cells communicate with each other utilizing chemical messengers. For many of these messenger molecules, the membrane is an insurmountable barrier. Yet, they act by binding to surface proteins often triggering a cascade of reactions inside the cell. Accordingly, studying ligand-receptor interactions at the cellular surface is key to understanding important aspects of membrane biology. However, despite a multitude of approaches to study membrane features, there is a need for developing techniques that can measure ligand binding with high temporal resolution and on a single cellular level. We recently developed a label-free approach to study ligand binding in real time. This methodology capitalizes on changes of the membrane's surface potential induced by the adsorption of a charged ligand. The resulting apparent alteration of membrane capacitance is measurable by capacitance recordings. Herein, we describe the implementation of the same using recordings obtained from HEK293 cells stably expressing the human serotonin transporter (SERT), which were challenged with the inhibitor cocaine.

A label-free approach to detect ligand binding to cell surface proteins in real time.

Electrophysiological recordings allow for monitoring the operation of proteins with high temporal resolution down to the single molecule level. This technique has been exploited to track either ion flow arising from channel opening or the synchronized movement of charged residues and/or ions within the membrane electric field. Here, we describe a novel type of current by using the serotonin transporter (SERT) as a model. We examined transient currents elicited on rapid application of specific SERT inhibitors. Our analysis shows that these currents originate from ligand binding and not from a long-range conformational change. The Gouy-Chapman model predicts that adsorption of charged ligands to surface proteins must produce displacement currents and related apparent changes in membrane capacitance. Here we verified these predictions with SERT. Our observations demonstrate that ligand binding to a protein can be monitored in real time and in a label-free manner by recording the membrane capacitance.

A New Splicing Isoform of Cacna2d4 Mimicking the Effects of c.2451insC Mutation in the Retina: Novel Molecular and Electrophysiological Insights.

PURPOSE: Mutations in CACNA2D4 exon 25 cause photoreceptor dysfunction in humans (c.2406C→A mutation) and mice (c.2451insC mutation). We investigated the feasibility of an exon-skipping therapeutic approach by evaluating the splicing patterns and functional role of targeted exons. METHODS: Splicing of the targeted α2δ4 (CACNA2D4) exons in presence and absence of the mutation was assessed by RT-PCR in vivo on mouse retinae and in vitro in HEK293T cells using splicing-reporter minigenes. Whole-cell patch-clamp recordings were performed to evaluate the impact of different Cacna2d4 variants on the biophysical properties of Cav1.4 L-type calcium channels (CACNA1F). RESULTS: Splicing analysis revealed the presence of a previously unknown splicing isoform of α2δ4 in the retina that truncates the gene open reading frame (ORF) in a similar way as the c.2451insC mutation. This isoform originates from alternative splicing of exon 25 (E25) with a new exon (E25b). Moreover, the c.2451insC mutation has an effect on splicing and increases the proportion of transcripts including E25b. Our electrophysiological analyses showed that only full-length α2δ4 was able to increase Cav1.4/ß3-mediated currents while all other α2δ4 variants did not mediate such effect. CONCLUSIONS: The designed exon-skipping strategy is not applicable because the resulting skipped α2δ4 are nonfunctional. α2δ4 E25b splicing variant is normally present in mouse retina and mimics the effect of c.2451insC mutation. Since this variant does not promote significant Cav1.4-mediated calcium current, it could possibly mediate a different function, unrelated to modulation of calcium channel properties at the photoreceptor terminals.