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Involvement of lipid rafts in G protein-coupled monoamine receptor trafficking and signaling : A pharmacological approach
| Content Provider | Semantic Scholar |
|---|---|
| Author | Sjögren, Benita |
| Copyright Year | 2008 |
| Abstract | The present work focused on lipid raft-mediated modulation of signaling and trafficking of serotonin (5-HT) and dopamine receptors. The 5-HT system is one of the most complex neurotransmitter systems, with a wide distribution both in the CNS and in the periphery. It is involved in the regulation of many biological functions as diverse as mood, metabolism and cardiovascular tone. 5-HT acts via at least 14 receptors, divided into seven subgroups according to sequence homology and mode of signal transduction. All of them are G protein-coupled receptors (GPCRs), except for the 5-HT3 receptor, which is a ligand-gated ion channel. The dopamine system is involved in the motor, cognitive and reward systems of the brain. Dopamine acts via five receptors, all GPCRs, divided into D1-like and D2-like receptors according to their mode of signal transduction. Dysregulation of the dopamine system is the underlying cause of addiction, schizophrenia and Parkinson’s disease (PD). The Singer-Nicholson fluidic mosaic model has for many years been the general model for the structure of biological membranes. In recent years this view has been revised with the discovery of liquid ordered microdomains, lipid rafts, enriched in cholesterol and sphingolipids. These microdomains play important roles in regulating signal transduction and trafficking events. A subgroup of lipid rafts, caveolae, is flask-shaped invaginations of the plasma membrane. They contain caveolin (Cav) proteins that can interact with other proteins, including GPCRs. Recently it was discovered that lipid rafts and caveolae participate in endocytosis and receptor trafficking. Although the exact mechanism for these processes is not yet fully understood, it seems clear that there can be alternative routes of receptor internalization than the classical endosomatic pathway via clathrin coated vesicles. In the present work we found that decreases of cholesterol, gangliosides, sphingomyelin or Cav-1 (by RNA interference) levels, attenuated maximum agonist and antagonist binding to 5-HT7 receptors. This could not be explained by a mere reduction in total receptor protein levels. Furthermore, signaling via 5HT7 receptors was attenuated by cholesterol depletion with HMG-CoA reductase inhibitors, statins, which are widely used in the treatment of atherosclerosis. Reduction of cholesterol levels also attenuated signaling via 5-HT1A receptors in primary neuronal cultures. Furthermore, we could detect dopamine D1 receptors in low density regions in a sucrose gradient, suggesting lipid raft localization. This was further confirmed by the finding that reduction of cholesterol levels inhibited signaling via dopamine D1 receptors in mouse striatal slices. These results give further indications that lipid raft play important roles in regulating monoamine GPCRs in vivo. We have also found that Cav-1, a key protein in caveolae, regulates cell surface expression of 5-HT7 receptors. Biochemical and cellular assays showed that 5-HT7 receptors are located in low density regions in a sucrose gradient together with Cav-1, indicating lipid raft localization. Furthermore, it was found that 5-HT7 receptors undergo agonist induced internalization and that this could be inhibited by reducing Cav1 expression with RNA interference. The internalization could also be inhibited with genistein, known to block lipid raft-mediated internalization. In contrast, hypertonic sucrose solution could not inhibit internalization. Altogether this suggests that 5-HT7 receptors are internalized through a clathrinindependent mechanism that is dependent on Cav-1. SVENSK SAMMANFATTNING I detta arbete studerades hur receptorer for de monoaminerga signalsubstanserna serotonin (5-HT) och dopamin regleras via s.k. lipid rafts. Serotoninsystemet ar ett av de mest komplexa neurotransmittorsystem som finns beskrivet och 5-HT finns distribuerat i manga omraden av det centrala och perifera nervsystemet. 5-HT ar involverat i regleringen av manga biologiska funktioner, sasom sinnesstamningar, metabolism och kardiovaskulart tonus. Det finns minst 14 receptorer for 5-HT, som ar indelade i sju grupper, med avseende pa sekvenshomologi och signaleringsegenskaper. Dessa ar alla G-proteinkopplade receptorer (GPCR), med undantag for 5-HT3 receptorn som ar en jonkanal. Dopaminsystemet ar involverat i motoriska och kognitiva funktioner i hjarnan, samt en essentiell del av hjarnans beloningssystem. Det finns fem receptorer for dopamin, alla GPCRer, uppdelade i tva grupper, D1och D2-lika. Indelningen grundar sig liksom for 5-HT-receptorer pa likhet i sekvens och signaleringsegenskaper. Felaktig dopaminreglering ar den underliggande orsaken till bl.a. substansberoende, schizofreni och Parkinsons sjukdom. Den generella bilden av biologiska membraner har lange varit den s.k. Singer-Nicholson fluidic mosaic model (ung. halvflytande mosaik). Detta synsatt har pa senare ar blivit reviderat med upptackten av mikrodomaner i cellmembraner, s.k. lipid rafts, rika pa mattade fettsyror och kolesterol. Dessa domaner spelar en viktig roll i cellsignalering och kontroll av transport av proteiner till och fran cellytan. En undergrupp av lipid rafts, caveoler, innehaller caveolinproteiner, som genom sin speciella struktur skapar karakteristiska flaskformade inbuktningar i cellmembranet. Caveolinproteiner kan interagera med andra proteiner, inklusive GPCRer. Nyligen upptacktes det att lipid rafts och caveoler kan mediera internalisering och endocytos. De exakta mekanismerna for detta ar fortfarande inte helt klarlagda, men det verkar alltmer troligt att det kan finnas andra mekanismer for internalisering av receptorer an den klassiska endosomala vagen, som medieras av clatrin. I detta arbete fann vi att minskning av nivaerna av olika lipid raft komponenter, som kolesterol, sphingomyelin, gangliosider eller Cav-1, ledde till minskad maximal agonistoch antagonistbindning till 5-HT7 receptorer. Vidare fann vi ocksa att signalering via 5-HT7 receptorer paverkades av behandling med HMG-CoA reduktashammare, statiner, som anvands kliniskt som behandling av aderforkalkning. Minskning av kolesterolnivaer ledde ocksa till reducering av signalering via 5-HT1A receptorer i primara humana nervceller. Vidare kunde vi detektera dopamin D1 receptorer i fraktioner med lag densitet i en sukrosgradient, vilket antyder att dessa receptorer ar lokaliserade i lipid rafts. Denna uppfattning styrktes av att sankning av kolesterolnivaer reducerade signalering via dopamin D1 receptorer i striatala hjarnskivor fran mus. Dessa resultat styrker uppfattningen att lipid rafts spelar en viktig roll for monoaminreceptorer in vivo. Vi har aven funnit att Cav-1, ett nyckelprotein i formationen av caveoler, reglerar uttryck av 5-HT7 receptorer pa cellytan. 5-HT7 receptorer ar lokaliserade i fraktioner med lag densitet i en sukrosgradient tillsammans med Cav-1, vilket ar en indikation pa att receptorerna ar lokaliserade i lipid rafts. Vidare fann vi att 5-HT7 receptorer internaliseras vid aktivering och att denna internalisering blockeras av minskade Cav-1 nivaer. Internaliseringen kunde ocksa blockeras med genistein, som blockerar all internalisering via lipid rafts. Daremot hade behandling med hypertonisk sukros, som blockerar clatrinmedierad internalisering, ingen effekt pa internalisering av 5-HT7 receptorer. Tillsammans talar dessa resultat for att 5-HT7 receptorer internaliseras via en mekanism som ar beroende av Cav-1, men oberoende av clatrin. LIST OF PUBLICATIONS I. Sjogren, B., Hamblin, M.W., Svenningsson, P. (2006) Cholesterol depletion reduces serotonin binding and signaling via human 5-HT7(a) receptors. Eur. J. Pharm. 552:1-10. II. Sjogren, B., Svenningsson P. (2007) Depletion of the lipid raft constituents, sphingomyelin and ganglioside, decreases serotonin binding at human 5-HT7(a) receptors in HeLa cells. Acta Phys. 190:47-53. III. Sjogren, B., Svenningsson P. (2007) Caveolin-1 affects binding and cell surface levels of human 5-HT7(a) receptors. FEBS lett. 581:5115-1521. IV. Sjogren, B., Csoregh, L., Svenningsson, P. Cholesterol reduction attenuates 8OH-DPAT-mediated signaling in human primary neuronal cultures. Submitted manuscript. V. Sjogren, B., Zhang, X., Svenningsson, P. Dopamine D1 receptor-mediated signaling in brain slices from an animal model of Parkinsonism is modulated by sequestration of cholesterol. Submitted manuscript. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://openarchive.ki.se/xmlui/bitstream/handle/10616/39451/thesis.pdf?sequence=1 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
