Is temporal synchrony necessary for effective Batesian mimicry?

Batesian mimicry occurs when palatable mimics gain protection from predators by evolving a phenotypic resemblance to an aposematic model species. While common in nature, the mechanisms maintaining mimicry are not fully understood. Patterns of temporal synchrony (i.

Dissociable effects of the prodrug phendimetrazine and its metabolite phenmetrazine at dopamine transporters.

Phendimetrazine (PDM) is a clinically available anorectic and a candidate pharmacotherapy for cocaine addiction. PDM has been hypothesized to function as a prodrug that requires metabolism to the amphetamine-like monoamine transporter substrate phenmetrazine (PM) to produce its pharmacological effects; however, whether PDM functions as an inactive prodrug or has pharmacological activity on its own remains unclear. The study aim was to determine PDM pharmacological mechanisms using electrophysiological, neurochemical, and behavioral procedures.

An N-terminal threonine mutation produces an efflux-favorable, sodium-primed conformation of the human dopamine transporter.

The dopamine transporter (DAT) reversibly transports dopamine (DA) through a series of conformational transitions. Alanine (T62A) or aspartate (T62D) mutagenesis of Thr62 revealed T62D-human (h)DAT partitions in a predominately efflux-preferring conformation. Compared with wild-type (WT), T62D-hDAT exhibits reduced [(3)H]DA uptake and enhanced baseline DA efflux, whereas T62A-hDAT and WT-hDAT function in an influx-preferring conformation.

A fluorescence displacement assay for antidepressant drug discovery based on ligand-conjugated quantum dots.

The serotonin (5-hydroxytryptamine, 5-HT) transporter (SERT) protein plays a central role in terminating 5-HT neurotransmission and is the most important therapeutic target for the treatment of major depression and anxiety disorders. We report an innovative, versatile, and target-selective quantum dot (QD) labeling approach for SERT in single Xenopus oocytes that can be adopted as a drug-screening platform. Our labeling approach employs a custom-made, QD-tagged indoleamine derivative ligand, IDT318, that is structurally similar to 5-HT and accesses the primary binding site with enhanced human SERT selectivity.

A conserved asparagine residue in transmembrane segment 1 (TM1) of serotonin transporter dictates chloride-coupled neurotransmitter transport.

Na(+)- and Cl(-)-dependent uptake of neurotransmitters via transporters of the SLC6 family, including the human serotonin transporter (SLC6A4), is critical for efficient synaptic transmission. Although residues in the human serotonin transporter involved in direct Cl(-) coordination of human serotonin transport have been identified, the role of Cl(-) in the transport mechanism remains unclear. Through a combination of mutagenesis, chemical modification, substrate and charge flux measurements, and molecular modeling studies, we reveal an unexpected role for the highly conserved transmembrane segment 1 residue Asn-101 in coupling Cl(-) binding to concentrative neurotransmitter uptake.

Polyspermy prevention: facts and artifacts?

The purpose of this review is to open a debate as to whether or not oocytes actively repel supernumerary sperm or in nature final sperm: oocyte ratios are so low that polyspermy preventing mechanisms are not necessary. Before encountering the oocyte, spermatozoa need to be primed, either by environmental factors as in animals exhibiting external fertilization, or by factors from the female reproductive tract, as in mammals. The spermatozoon must then recognize and interact with the outer layers of the oocyte and progression of the fertilizing spermatozoon through these layers is further controlled and modulated by a precise sequence of signals in situ.

Dopamine transporter/syntaxin 1A interactions regulate transporter channel activity and dopaminergic synaptic transmission.

The Caenorhabditis elegans (C. elegans) dopamine (DA) transporter (DAT-1) regulates DA signaling through efficient DA reuptake following synaptic release. In addition to its DA transport function, DAT-1 generates detectable DA-gated currents that may influence neuronal excitability.

All aglow about presynaptic receptor regulation of neurotransmitter transporters.

Mounting evidence supports the idea that neurotransmitter transporters are subject to many forms of post-translational regulation typically associated with receptors and ion channels, including receptor and kinase-mediated changes in transporter phosphorylation, cell surface trafficking, and/or catalytic activation. Although hints of this regulation can be achieved with traditional radiolabeled substrate flux techniques, higher resolution methods are needed that can localize transporter function in situ as well as permit real-time monitoring of transport function without confounds associated with coincident receptor activation. The elegant study by Bolan et al.

Transporters as channels.

This review investigates some key aspects of transport mechanisms and recent advances in our understanding of this ubiquitous cellular process. The prevailing model of cotransport is the alternating access model, which suggests that large conformational changes in the transporter protein accompany cotransport. This model rests on decades of research and has received substantial support because many transporter characteristics are explained using its premises.

Molecular microfluorometry: converting arbitrary fluorescence units into absolute molecular concentrations to study binding kinetics and stoichiometry in transporters.

Cotransporters use energy stored in Na+ or H+ gradients to transport neurotransmitters or other substrates against their own gradient. Cotransport is rapid and efficient, and at synapses it helps terminate signaling. Cotransport in norepinephrine (NET), epinephrine (EpiT), dopamine (DAT), and serotonin (SERT) transporters couples downhill Na+ flux to uphill transmitter flux.

Getting the message across: a recent transporter structure shows the way.

Efforts to define the mechanisms governing neurotransmitter uptake and drug action have moved into high gear with the publication of a high-resolution structure of a leucine transporter from Aquifex aeolicus, a bacterial member of the SLC6 transporter family. Solved with the substrate leucine bound, the new structure corroborates extensive biochemical and mutagenesis studies performed with related mammalian neurotransmitter transporters and provides exciting suggestions as to how coupling arises between ions and substrates to permit efficient neurotransmitter clearance.

Biogenic amine neurotransmitter transporters: just when you thought you knew them.

Plasma membrane transporters have long been known to support the reuptake of biogenic amine neurotransmitters following release in the central and peripheral nervous systems. Using high-resolution imaging, patch-clamp and amperometric approaches, as well as molecular manipulations of transporter-regulatory pathways, surprising new details have been uncovered as to how transporters work and are influenced by signaling pathways and psychostimulants.

Novel fluorescence-based approaches for the study of biogenic amine transporter localization, activity, and regulation.

Pre-synaptic norepinephrine (NE) and dopamine (DA) transporters (NET and DAT) terminate catecholamine synaptic transmission through reuptake of released neurotransmitter. Recent studies reveal that NET and DAT are tightly regulated by receptor and second messenger-linked signaling pathways. Common approaches for studying these transporters involve use of radiolabeled substrates or antagonists, methods possessing limited spatial resolution and that bear limited opportunities for repeated monitoring of living preparations.

Substrate binding stoichiometry and kinetics of the norepinephrine transporter.

The human norepinephrine (NE) transporter (hNET) attenuates neuronal signaling by rapid NE clearance from the synaptic cleft, and NET is a target for cocaine and amphetamines as well as therapeutics for depression, obsessive-compulsive disorder, and post-traumatic stress disorder. In spite of its central importance in the nervous system, little is known about how NET substrates, such as NE, 1-methyl-4-tetrahydropyridinium (MPP+), or amphetamine, interact with NET at the molecular level. Nor do we understand the mechanisms behind the transport rate.

Dopamine transporters depolarize neurons by a channel mechanism.

Neurotransmitter transporters generate larger currents than expected if one assumes fixed stoichiometry models. It remains controversial, however, whether these depolarizing currents arise from high density and rapid turnover rates of a classical transporter, or whether transporters exhibit bona fide channel behavior. Although heterologously expressed transporters show single-channel behavior and noise analysis in native cells strongly suggests channel behavior, no directly observed single-channel events associated with transporters have been reported thus far in native cells.

Transporter structure and mechanism.

During the past several decades, lac permease has assumed almost tutelary proportions as a model for cotransporters. This archetypical membrane protein now exerts its influence by the most dramatic means possible: its structure is solved. This article describes the configuration and implied transport mechanism of the bacterial lactose transporter and compares its structure and function with those of other transporters and those of ion channels.

A spontaneous, recurrent mutation in divalent metal transporter-1 exposes a calcium entry pathway.

Divalent metal transporter-1 (DMT1/DCT1/Nramp2) is the major Fe(2+) transporter mediating cellular iron uptake in mammals. Phenotypic analyses of animals with spontaneous mutations in DMT1 indicate that it functions at two distinct sites, transporting dietary iron across the apical membrane of intestinal absorptive cells, and transporting endosomal iron released from transferrin into the cytoplasm of erythroid precursors. DMT1 also acts as a proton-dependent transporter for other heavy metal ions including Mn(2+), Co(2+), and Cu(2), but not for Mg(2+) or Ca(2+).

Ionic currents in the human serotonin transporter reveal inconsistencies in the alternating access hypothesis.

We have investigated the conduction states of human serotonin transporter (hSERT) using the voltage clamp, cut-open frog oocyte method under different internal and external ionic conditions. Our data indicate discrepancies in the alternating access model of cotransport, which cannot consistently explain substrate transport and electrophysiological data. We are able simultaneously to isolate distinct external and internal binding sites for substrate, which exert different effects upon currents conducted by hSERT, in contradiction to the alternating access model.

Binding and transport in norepinephrine transporters. Real-time, spatially resolved analysis in single cells using a fluorescent substrate.

Monoamine transporters, the molecular targets for drugs of abuse and antidepressants, clear norepinephrine, dopamine, or serotonin from the synaptic cleft. Neurotransmitters, amphetamines, and neurotoxins bind before being transported, whereas cocaine and antidepressants bind to block transport. Although binding is crucial to transport, few assays separate binding from transport, nor do they provide adequate temporal or spatial resolution to describe real-time kinetics or localize sites of active uptake.

Flux coupling in the human serotonin transporter.

The serotonin (5-hydroxytryptamine; 5HT) transporter (SERT) catalyzes the movement of 5HT across cellular membranes. In the brain, SERT clears 5HT from extracellular spaces, modulating the strength and duration of serotonergic signaling. SERT is also an important pharmacological target for antidepressants and drugs of abuse.

Targeting cell surface receptors with ligand-conjugated nanocrystals.

To explore the potential for use of ligand-conjugated nanocrystals to target cell surface receptors, ion channels, and transporters, we explored the ability of serotonin-labeled CdSe nanocrystals (SNACs) to interact with antidepressant-sensitive, human and Drosophila serotonin transporters (hSERT, dSERT) expressed in HeLa and HEK-293 cells. Unlike unconjugated nanocrystals, SNACs were found to dose-dependently inhibit transport of radiolabeled serotonin by hSERT and dSERT, with an estimated half-maximal activity (EC(50)) of 33 (dSERT) and 99 microM (hSERT). When serotonin was conjugated to the nanocrystal through a linker arm (LSNACs), the EC(50) for hSERT was determined to be 115 microM.

Characterization of cocaine and antidepressant-sensitive norepinephrine transporters in rat placental trophoblasts.

1. This paper reports on a primary cell culture system that predominantly expresses native norepinephrine (NE) transporters (NETs), and is amenable to biophysical as well as biochemical analyses. 2.

Serotonin transporter function and pharmacology are sensitive to expression level: evidence for an endogenous regulatory factor.

We express mammalian serotonin transporters (SERTs) in Xenopus oocytes by cRNA injection and measure 5-hydroxytryptamine (5-HT) transport and 5-HT-induced current at varying expression levels. Transport and current both increase sigmoidally with the amount of cRNA injected, but current requires approximately 5-fold more cRNA to elicit a half-maximal response. Western blots of SERT protein demonstrate that current, but not transport, correlates linearly with the amount of SERT on the plasma membrane.

Single-file diffusion and neurotransmitter transporters: Hodgkin and Keynes model revisited.

Norepinephrine transporters (NETs) use the Na gradient to remove norepinephrine (NE) from the synaptic cleft of adrenergic neurons following NE release from the presynaptic terminal. By coupling NE to the inwardly directed Na gradient, it is possible to concentrate NE inside cells. This mechanism, which is referred to as co-transport or secondary transport (Läuger, 1991, Electrogenic Ion Pumps, Sinauer Associates) is apparently universal: Na coupled transport applies to serotonin transporters (SERTs), dopamine transporters (DATs), glutamate transporters, and many others, including transporters for osmolites, metabolites and substrates such as sugar.