Biomedical subjects

Eric L Barker

Publications and source records attributed to Eric L Barker.

12 recordsLinked to original sources

PubMed2006

1-Methylpyridinium-4-(4-phenylmethanethiosulfonate) iodide, MTS-MPP+, a novel scanning cysteine accessibility method (SCAM) reagent for monoamine transporter studies.

A novel substituted cysteine accessibility method (SCAM) reagent was developed for monoamine uptake transporters. The new reagent, MTS-MPP(+), was a derivative of the neurotoxin and transporter substrate MPP(+). MTS-MPP(+) labeled cysteine residues introduced into the serotonin transporter protein. Although it did not prove to be a substrate, as is MPP(+), it appears to label cysteine residues lining the permeation pore of the transporter more readily than currently available nonspecific SCAM reagents.

Cell Line↗

PubMed2006

Detergent-resistant membrane microdomains in the disposition of the lipid signaling molecule anandamide.

The endogenous cannabinoid anandamide is an agonist of the cannabinoid receptors CB1 and CB2, as well as transient receptor potential vanilloid type 1 and type 4 ion channels. In recent years, there has been a great deal of interest in the cellular processes regulating the signaling of endocannabinoids such as anandamide. This is due largely to evidence that augmentation of cannabinergic tone might be therapeutically beneficial in the treatment of multiple disease states such as chronic pain, anxiety, multiple sclerosis, and neuropsychiatric disorders. Of particular interest are the cellular processes that regulate the cellular accumulation and metabolism of anandamide. Characterization of the process by which anandamide is internalized and metabolized by the cell may identify drug targets useful in the positive modulation of cannabinergic tone. Recently, we reported that detergent-resistant membrane microdomains known as lipid rafts play a role in the cellular accumulation of anandamide by mediating an endocytic process responsible for anandamide internalization. The enzyme primarily responsible for anandamide metabolism, fatty acid amide hydrolase, is excluded from lipid rafts. However, the metabolites of anandamide accumulate in these detergent-resistant membrane microdomains. There is some preliminary evidence that makes it reasonable to propose that anandamide metabolites enriched in lipid rafts may act as precursors to anandamide synthesis. Overall, experimental evidence is mounting that detergent-resistant membrane microdomains such as lipid rafts may play a role in the cellular regulation of anandamide inactivation and production.

Animals↗

PubMed2006

Engineered zinc-binding sites confirm proximity and orientation of transmembrane helices I and III in the human serotonin transporter.

The human serotonin transporter (hSERT) regulates neurotransmission by removing released serotonin (5-HT) from the synapse. Previous studies identified residues in SERT transmembrane helices (TMHs) I and III as interaction sites for substrates and antagonists. Despite an abundance of data supporting a 12-TMH topology, the arrangement of the TMHs in SERT and other biogenic amine transporters remains undetermined. A high-resolution structure of a bacterial leucine transporter that demonstrates homology with SERT has been reported, thus providing the basis for the development of a SERT model. Zn2+-binding sites have been utilized in transporters and receptors to define experimentally TMH proximity. Focusing on residues near the extracellular ends of hSERT TMHs I and III, we engineered potential Zn2+-binding sites between V102 or W103 (TMH I) and I179-L184 (TMH III). Residues were mutated to either histidine or cysteine. TMH I/III double mutants were constructed from functional TMH I mutants, and Zn2+ sensitivity was assessed. Dose-response assays suggest an approximately twofold increase in sensitivity to Zn2+ inhibition at the hSERT V102C/M180C and approximately fourfold at the V102C/I179C mutant compared to the hSERT V102C single mutant. We propose that the increased sensitivity to Zn2+ confirms the proximity and the orientation of TMHs I and III in the membrane. Homology modeling of the proposed Zn2+-binding sites using the coordinates of the Aquifex aeolicus leucine transporter structure provided a structural basis for interpreting the results and developing conclusions.

Amino Acid Substitution↗

PubMed2005

Serotonin transporters: implications for antidepressant drug development.

Due to the complexity of the disease, several hypotheses exist to explain the etiology of depression. The monoamine theory of depression suggests that disruptions in the serotonergic and noradrenergic systems result in depressive symptoms. Therefore, the serotonin transporter (SERT) has become a pharmacological target for treating these symptoms. This review will discuss what is known about the molecular interactions of antidepressants with SERT. The effects of antidepressants on SERT regulation and expression in addition to the receptors that may be involved in mediating these effects will be addressed. Specifically, how changes to SERT expression following chronic antidepressant treatment may contribute to the therapeutic benefits of antidepressants will be discussed. Furthermore, the effects of SERT gene polymorphisms on antidepressant efficacy will be examined. Finally, a brief overview of other hypotheses of depression will be addressed as well as factors that must be considered for future antidepressant development.

Animals↗

PubMed2005

Lipid rafts: a nexus for endocannabinoid signaling?

The endocannabinoids are endogenous agonists of the cannabinoid receptors and some members of the transient receptor potential, vanilloid type (TRPV), family of cation channels. Endocannabinoids along with their target receptors comprise a signaling system that is not well characterized. There have been many advances in our collective understanding of endocannabinoid signaling in the last decade and experimental evidence is mounting that pharmacological augmentation of endocannabinoid tone might have a significant therapeutic benefit in several disease states. However, the mechanisms responsible for the biosynthesis, cellular uptake, and intracellular processing of endocannabinoids are not well understood and have been the source of much debate. Recent studies have revealed a role for detergent insoluble membrane domains called lipid rafts in various aspects of signaling associated with the endocannabinoid anandamide. Intact detergent insoluble membrane domains appear to play a role in an anandamide-induced signaling cascade that is independent of G protein-coupled cannabinoid receptors or TRPV channels. Furthermore, detergent insoluble membrane domain-related endocytosis and recycling to lipid rafts appear to regulate the organization and localization of anandamide metabolites. We will discuss the implications that these findings have on the way we view endocannabinoid signaling, trafficking, and processing.

Apoptosis↗

PubMed2004

A role for caveolae/lipid rafts in the uptake and recycling of the endogenous cannabinoid anandamide.

The mechanisms responsible for the uptake and cellular processing of the endogenous cannabinoid anandamide are not well understood. We propose that anandamide uptake may occur via a caveola/lipid raft-related endocytic process in RBL-2H3 cells. Inhibitors of caveola-related (clathrin-independent) endocytosis reduced anandamide transport by approximately 50% compared with the control. Fluorescein derived from fluorescently labeled anandamide colocalized with protein markers of caveolae at early time points following transport. In this study, we have also identified a yet unrecognized process involved in trafficking events affecting anandamide following its uptake. Following uptake of [(3)H]anandamide by RBL-2H3 cells, we found an accumulation of tritium in the caveolin-rich membranes. Inhibitors of both anandamide uptake and metabolism blocked the observed enrichment of tritium in the caveolin-rich membranes. Mass spectrometry of subcellular membrane fractions revealed that the tritium accumulation observed in the caveolin-rich membrane fraction was not representative of intact anandamide, suggesting that following metabolism by the enzyme fatty acid amide hydrolase (FAAH), anandamide metabolites are rapidly enriched in caveolae. Furthermore, HeLa cells, which do not express high levels of FAAH, showed an accumulation of tritium in the caveolin-rich membrane fraction only when transfected with FAAH cDNA. Western blot and immunocytochemistry analyses of RBL-2H3 cells revealed that FAAH was localized in intracellular compartments distinct from caveolin-1 localization. Together, these data suggest that following uptake via caveola/lipid raft-related endocytosis, anandamide is rapidly metabolized by FAAH, with the metabolites efficiently recycled to caveolin-rich membrane domains.

Amidohydrolases↗

PubMed2004

Temperature and 3,4-methylenedioxymethamphetamine alter human serotonin transporter-mediated dopamine uptake.

Although studies have suggested that dopamine can be transported by serotonin transporters (SERTs), such activity has not been characterized at the cloned SERTs. Dopamine and serotonin uptake by human SERT expressed in HEK-293 cells was compared at 37 and 40 degrees C. Elevated temperature was found to alter serotonin transport, but had no significant effect on dopamine transport. These effects led to a 10-fold increase in the serotonin:dopamine transport ratio reflecting an increased preference of SERTs for dopamine as opposed to serotonin at the higher temperature. The effects of 3,4-methylenedioxymethamphetamine (MDMA) on SERT-mediated dopamine transport were also evaluated by pre-incubating SERT-expressing cells with MDMA. The presence of intracellular MDMA caused a decrease in [3H]dopamine uptake but had no effect on [3H]serotonin transport suggesting that intracellular MDMA may be capable of inhibiting transporter function.

Biological Transport↗

PubMed2004

Anandamide transport.

N-Arachidonylethanolamine (anandamide) is an endogenous agonist of the cannabinoid CB1 and CB2 receptors and displays many of the same receptor-mediated physiological effects as delta9-tetrahydrocannabinol (delta9-THC), the active component of marijuana. As with any neurotransmitter, there must be tight control of anandamide receptor-mediated signaling and a means of rapid removal of the molecule from the system. Thus, the process by which anandamide is transported into the cell for metabolism has been a topic of much interest and has been implicated as a potential drug target in the treatment of several disease states that are reported to have an association with the endocannabinoid system. In this review, we will discuss the current models proposed for the mechanism of anandamide transport, the progress that has been made in the development of compounds that specifically inhibit anandamide transport, the observed effects of anandamide transport inhibition in vivo, and finally, potential therapeutic applications of compounds that inhibit anandamide transport.

Animals↗

PubMed2003

Interactions of antidepressants with the serotonin transporter: a contemporary molecular analysis.

One of the most prevalent disorders in present society is depression. The development of treatments for this disorder, beginning with the tricyclic antidepressants and leading to the development of selective serotonin reuptake inhibitors, has focused on compounds that block the function of the serotonin transporter (SERT). In this paper, we have performed Comparative Molecular Field Analysis (CoMFA) using data generated from rat brain synaptosomes and heterologous expression systems expressing rat SERT. Using these models, we have described the molecular requirements for the interactions of antidepressants with SERTs. In addition, molecular studies were performed using chimeric human/Drosophila SERTs and SERT point mutants. These studies focused on identifying regions or discrete amino acids on SERT that may be responsible for recognizing antidepressants.

Animals↗

PubMed2003

Distinct molecular recognition of psychostimulants by human and Drosophila serotonin transporters.

In this study, human embryonic kidney (HEK)-293 cells stably expressing human, Drosophila, or a chimeric serotonin (5-hydroxytryptamine, 5-HT) transporter (hSERT, dSERT, and H(1-281)D(282-476)H(477-638), respectively) were used to explore the ability of two libraries of structurally distinct psychostimulants to inhibit 5-HT uptake. One library consisted of 3-phenyltropane analogs, whereas the second library consisted of several substituted amphetamines. hSERT exhibited a lower K(i) value for all the compounds in both libraries compared with dSERT, whereas the chimeric SERT exhibited properties more closely resembling those of dSERT. This species selectivity was explored using computer-generated comparative molecular field analysis to model the interactions of the cocaine analogs and substituted amphetamines at hSERT, dSERT, and the cross-species chimera. Models for the 3-phenyltropane analogs indicate that a region exists around the aromatic ring where decreased electron density is favored, particularly for hSERT. This finding may indicate pi-pi stacking with an aromatic amino acid residue in SERT. Also, electronegative substituents in the 4'-position provide favorable interactions. This structural feature was demonstrated by increased potency of analogs with electronegative substituents on the aromatic ring that withdraw electron density. For the substituted amphetamines, key areas for interaction exist around the amine, an electrostatic component surrounding the 3-position on the aromatic ring, and a steric component surrounding the 4-position.

Amphetamines↗

PubMed2003

Black cohosh acts as a mixed competitive ligand and partial agonist of the serotonin receptor.

Extracts of the rhizome of black cohosh [Actaea racemosa L., formerly called Cimicifuga racemosa (L.) Nutt.] were evaluated for potential mechanisms of action in the alleviation of menopausal hot flashes. Ovariectomized Sprague-Dawley rats were administered a 40% 2-propanol extract of black cohosh [4, 40, and 400 mg/(kg.day)] by gavage for 2 weeks with or without estradiol [50 microg/(kg.day)] to determine if black cohosh could act as an estrogen or antiestrogen on the basis of an increase in uterine weight or vaginal cellular cornification. No effects were observed on uterine weight or on vaginal cellular cornification in rats treated with black cohosh alone or in combination with 17beta-estradiol, indicating this black cohosh extract had no estrogenic or antiestrogenic properties in the ovariectomized rat model. To evaluate other potential pathways by which black cohosh might reduce menopausal hot flashes, serotonin activity was first assessed by the inhibition of radioligand binding to cell membrane preparations containing recombinant human serotonin receptor (5-HT) subtypes. A 40% 2-propanol extract of black cohosh was tested against 10 subtypes of the serotonin receptor, revealing the presence of compounds with strong binding to the 5-HT(1A), 5-HT(1D), and 5-HT(7) subtypes. Subsequent binding studies were carried out using 5-HT(1A) and 5-HT(7) receptors because of their association with the hypothalamus, which has been implicated in the generation of hot flashes. The black cohosh 40% 2-propanol extract inhibited [(3)H]lysergic acid diethylamide (LSD) binding to the human 5-HT(7) receptor (IC(50) = 2.4 +/- 0.4 microg/mL) with greater potency than binding of [(3)H]-8-hydroxy-2-(di-N-propylamino)tetralin to the rat 5-HT(1A) receptor (IC(50) = 13.9 +/- 0.6 microg/mL). Analysis of ligand binding data indicated that components of a black cohosh methanol extract functioned as a mixed competitive ligand of the 5-HT(7) receptor. In addition, a black cohosh methanol extract elevated cAMP levels in 293T-5-HT(7)-transfected HEK cells, suggesting the extract acted as a partial agonist at the receptor. The elevation in cAMP mediated by the black cohosh extract could be reversed in the presence of the antagonist methiothepin, indicating a receptor-mediated process. These data suggest that reductions in hot flashes in some women taking black cohosh may not be due to estrogenic properties. This study identifies other possible biological targets of black cohosh that could account for reported biological effects.

Animals↗

PubMed2003

Serotonin 5-hydroxytryptamine 2A receptor-coupled phospholipase C and phospholipase A2 signaling pathways have different receptor reserves.

NIH3T3 cells stably expressing the rat 5-hydroxytryptamine 2A (5-HT 2A) receptor (5500 fmol/mg) were used to explore further the capacity of structurally distinct ligands to elicit differential signaling through the phospholipase C (PLC) or phospholipase A 2 (PLA 2) signal transduction pathways. Initial experiments were designed to verify that 5-HT 2A receptor-mediated PLA 2 activation in NIH3T3 cells is independent from, and not a subsequent result of, 5-HT 2A receptor-mediated PLC activation. In addition, we also explored the extent of receptor reserve for the endogenous ligand, 5-HT, for both PLC and PLA 2 activation. Finally, we employed structurally diverse ligands from the tryptamine, phenethylamine, and ergoline families of 5-HT 2A receptor agonists to test the hypothesis of agonist-directed trafficking of 5-HT 2A receptor-mediated PLC and PLA 2 activation. To measure agonist-induced pathway activation, we determined the potency and intrinsic activity of each compound to activate either the PLA 2 pathway or the PLC pathway. The results showed that a larger receptor reserve exists for 5-HT-induced PLA 2 activation than for 5-HT-induced PLC activation. Furthermore, the data support the hypothesis of agonist-directed trafficking in NIH3T3-5HT 2A cells because structurally distinct ligands were able to induce preferential activation of the PLC or PLA 2 signaling pathway. From these data we conclude that structurally distinct ligands can differentially regulate 5-HT 2A receptor signal transduction.

3T3 Cells↗