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Norepinephrine transporter

The norepinephrine transporter (NET), also known as noradrenaline transporter (NAT), is a protein that in humans is encoded by the solute carrier family 6 member 2 (SLC6A2) gene.[5]

SLC6A2
Identifiers
AliasesSLC6A2, NAT1, NET, NET1, SLC6A5, solute carrier family 6 member 2, Norepinephrine transporter, norepinephrine transporter gene
External IDsOMIM: 163970; MGI: 1270850; HomoloGene: 816; GeneCards: SLC6A2; OMA:SLC6A2 - orthologs
Orthologs
SpeciesHumanMouse
Entrez
Ensembl
UniProt
RefSeq (mRNA)

NM_001043
NM_001172501
NM_001172502
NM_001172504

NM_009209

RefSeq (protein)

NP_001034
NP_001165972
NP_001165973
NP_001165975

NP_033235

Location (UCSC)Chr 16: 55.66 – 55.71 MbChr 8: 93.69 – 93.73 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

NET is a monoamine transporter and is responsible for the sodium-chloride (Na+/Cl)-dependent reuptake of extracellular norepinephrine (NE), which is also known as noradrenaline. NET can also reuptake extracellular dopamine (DA). The reuptake of these two neurotransmitters is essential in regulating concentrations in the synaptic cleft. NETs, along with the other monoamine transporters, are the targets of many antidepressants and recreational drugs. In addition, an overabundance of NET is associated with ADHD.[6][7] There is evidence that single-nucleotide polymorphisms in the NET gene (SLC6A2) may be an underlying factor in some of these disorders.[7]

Gene edit

The norepinephrine transporter gene, SLC6A2 is located on human chromosome 16 locus 16q12.2. This gene is encoded by 14 exons.[7] Based on the nucleotide and amino acid sequence, the NET transporter consists of 617 amino acids with 12 membrane-spanning domains. The structural organization of NET is highly homologous to other members of a sodium/chloride-dependent family of neurotransmitter transporters, including dopamine, epinephrine, serotonin and GABA transporters.[7]

Single-nucleotide polymorphisms edit

A single-nucleotide polymorphism (SNP) is a genetic variation in which a genome sequence is altered by a single nucleotide (A, T, C or G). NET proteins with an altered amino acid sequence (more specifically, a missense mutation) could potentially be associated with various diseases that involve abnormally high or low plasma levels of norepinephrine due to altered NET function. NET SNPs and possible associations with various diseases are an area of focus for many research projects. There is evidence suggesting a relationship between NET SNPs and various disorders such as ADHD[7][8] psychiatric disorders,[7] postural tachycardia[7][9] and orthostatic intolerance.[7][9] The SNPs rs3785143 and rs11568324 have been related to attention-deficit hyperactivity disorder.[10] Thus far, however, the only confirmed direct association between a SNP and a clinical condition is that of the SNP, Ala457Pro, and orthostatic intolerance.[7] Thirteen NET missense mutations have been discovered so far.[7]

Missense Mutations in the NET Gene[7][11]
Location Amino Acid Variant TMD (if known) Related Disease
Exon 2 Val69Ile TMD 1 None
Exon 3 Thr99Ile TMD 2 None
Exon 5 Val245Ile TMD 4 None
Exon 6 Asn292Thr n/a None
Exon 8 Val356Leu n/a None
Exon 8 Ala369Pro n/a None
Exon 8 Asn375Ser n/a None
Exon 10 Val449Ile TMD 9 None
Exon 10 Ala457Pro TMD 9 Orthostatic intolerance
Exon 10 Lys463Arg n/a None
Exon 11 Gly478Ser TMD 10 None
Exon 12 Phe528Cys n/a None
Exon 13 Tyr548His n/a None
Abbreviations: TMD, transmembrane domain; n/a, non-applicable.
For the table above, refer to this table of standard amino acid abbreviations. This notation for
missense mutations, take Val69Ile for example, indicates that amino acid Val69 was changed to Ile.

Genetic variations edit

An epigenetic mechanism (hypermethylation of CpG islands in the NET gene promoter region) that results in reduced expression of the noradrenaline (norepinephrine) transporter and consequently a phenotype of impaired neuronal reuptake of norepinephrine has been implicated in both postural orthostatic tachycardia syndrome and panic disorder.[12]

rs5569 is a variant of SLC6A2.[13]

Structure edit

 
Simplified structure of the norepinephrine transporter protein.

The norepinephrine transporter is composed of 12 transmembrane domains (TMDs). The intracellular portion contains an amino (-NH
2
) group and carboxyl (-COOH) group. In addition, there is a large extracellular loop located between TMD 3 and 4.[14][6][15] The protein is composed of 617 amino acids.[14]

Function edit

NET functions to transport synaptically released norepinephrine back into the presynaptic neuron. As much as 90% of the norepinephrine released will be taken back up in the cell by NET. NET functions by coupling the influx of sodium and chloride (Na+/Cl) with the transport of norepinephrine. This occurs at a fixed ratio of 1:1:1.[16] Both the NET and the dopamine transporter (DAT) can transport norepinephrine and dopamine. The reuptake of norepinephrine and dopamine is essential in regulating the concentration of monoamine neurotransmitters in the synaptic cleft. The transporter also helps maintain homeostatic balances of the presynaptic neuron.[17]

 
Norepinephrine structure

Norepinephrine (NE) is released from noradrenergic neurons that innervate both the CNS and PNS. NE, also known as noradrenaline (NA), has an important role in controlling mood, arousal, memory, learning, and pain perception. NE is a part of the sympathetic nervous system.[6][18] Dysregulation of the removal of norepinephrine by NET is associated with many neuropsychiatric diseases, discussed below. In addition, many antidepressants and recreational drugs compete for the binding of NET with NE.[14]

Transport mechanisms edit

The transport of norepinephrine back into presynaptic cell is made possible by the cotransport with Na+ and Cl. The sequential binding of the ions results in the eventual reuptake of norepinephrine. The ion gradients of Na+ and Cl make this reuptake energetically favorable. The gradient is generated by the Na+/K+-ATPase which transports three sodium ions out and two potassium ions into the cell.[17] NETs have conductances similar to those of ligand-gated ion channels. The expression of NET results in a leak-channel activity.[16][17]

Location in the nervous system edit

NETs are restricted to noradrenergic neurons and are not present on neurons that release dopamine or epinephrine.[6][15][17] The transporters can be found along the cell body, axons, and dendrites of the neuron.[6] NETs are located away from the synapse, where norepinephrine is released. They are found closer to the plasma membrane of the cell. This requires norepinephrine to diffuse from the site it is released to the transporter for reuptake.[17] Norepinephrine transporters are confined to the neurons of the sympathetic system, and those innervating the adrenal medulla, lung, and placenta.[17]

Regulation edit

Regulation of NET function is complex and a focus of current research. NETs are regulated at both the cellular and molecular level post-translation. The most understood mechanisms include phosphorylation by the second messenger protein kinase C (PKC).[15] PKC has been shown to inhibit NET function by sequestration of the transporter from the plasma membrane.[19] The amino acid sequence of NET has shown multiple sites related to protein kinase phosphorylation.[17] Post-translational modifications can have a wide range of effects on the function of the NET, including the rate of fusion of NET-containing vesicles with the plasma membrane, and transporter turnover.[19]

Clinical significance edit

Orthostatic intolerance edit

Orthostatic intolerance (OI) is a disorder of the autonomic nervous system (a subcategory of dysautonomia) characterized by the onset of symptoms upon standing. Symptoms include fatigue, lightheadedness, headache, weakness, increased heart rate/heart palpitations, anxiety, and altered vision.[7] Often, patients have high plasma norepinephrine (NE) concentrations (at least 600 pg/ml) in relation to sympathetic outflow upon standing, suggesting OI is a hyperadrenergic condition.[7][9] The discovery of identical twin sisters who both had OI suggested a genetic basis for the disorder.[7][9] A missense mutation on the NET gene (SLC6A2) was discovered in which an alanine residue was replaced with a proline residue (Ala457Pro) in a highly conserved region of the transporter.[7] The patients’ defective NET had only 2% of the activity of the wild-type version of the gene.[7] The genetic defect in the NET protein results in decreased NET activity that could account for abnormally high NE plasma levels in OI. However, 40 other OI patients did not have the same missense mutation, indicating other factors contributed to the phenotype in the identical twins.[7] This discovery of the linkage with NET mutations that results in decreased norepinephrine reuptake activity and orthostatic intolerance suggests faulty NE uptake mechanisms can contribute to cardiovascular disease.[20]

Therapeutic uses edit

Inhibition of the norepinephrine transporter (NET) has potential therapeutic applications in the treatment of attention deficit hyperactivity disorder (ADHD), substance abuse, neurodegenerative disorders (e.g., Alzheimer's disease (AD) and Parkinson's disease (PD)) and clinical depression.[18]

Major depressive disorder edit

 
Amineptine, an NDRI tricyclic antidepressant
 
Fluoxetine, a selective serotonin reuptake inhibitor

Certain antidepressant medications act to raise noradrenaline, such as serotonin-norepinephrine reuptake inhibitors (SNRIs), norepinephrine-dopamine reuptake inhibitors (NDRIs), norepinephrine reuptake inhibitors (NRIs or NERIs) and the tricyclic antidepressants (TCAs). The mechanism by which these medications work is that the reuptake inhibitors prevent the reuptake of serotonin and norepinephrine by the presynaptic neuron, paralyzing the normal function of the NET. At the same time, higher levels of 5-HT are maintained in the synapse increasing the concentrations of the latter neurotransmitters. Since the noradrenaline transporter is responsible for most of the dopamine clearance in the prefrontal cortex,[21] SNRIs block reuptake of dopamine too, accumulating the dopamine in the synapse. However, DAT, the primary way dopamine is transported out of the cell, can work to decrease dopamine concentration in the synapse when the NET is blocked.[22] For many years, the number one choice in treating mood disorders like depression was through administration of TCAs, such as desipramine (Norpramin), nortriptyline (Arentyl, Pamelor), protriptyline (Vivactil), and amoxapine (Asendin).[18] SSRIs, which mainly regulate serotonin, subsequently replaced tricyclics as the primary treatment option for depression because of their better tolerability and lower incidence of adverse effects.[23]

ADHD edit

 
Atomoxetine, an NET inhibitor marketed as Strattera

Many drugs exist in the treatment of ADHD. Dextroamphetamine (Dexedrine, Dextrostat), Adderall, methylphenidate (Ritalin, Metadate, Concerta, Daytrana), and lisdexamfetamine (Vyvanse) block reabsorption of the catecholamines dopamine and norepinephrine through monoamine transporters (including NET), thereby increasing levels of these neurotransmitters in the brain. The strong selective norepinephrine reuptake inhibitor (NRI), atomoxetine (Strattera), has been approved by the U.S. Food and Drug Administration (FDA) to treat ADHD in adults.[24][25] The role of the NET in ADHD is similar to how it works to ease the symptoms of depression. The NET is blockaded by atomoxetine and increases NE levels in the brain. It can work to increase one's ability to focus, decrease any impulsiveness, and lessen hyperactivity in both children and adults with ADHD.[26]

Psychostimulants edit

Cocaine edit

Cocaine is a powerful psychostimulant and known to be one of the most widely used substances recreationally.[27] Cocaine is a nonselective, reuptake inhibitor of the norepinephrine, serotonin, and dopamine transporters. This thwarts the absorption of these chemicals into the presynaptic terminal[27] and allows a large concentration of dopamine, serotonin and norepinephrine to build up in the synaptic cleft. The potential for cocaine addiction is thought to be a result of its effects on dopamine transporters in the CNS, while it has been suggested that the life-threatening cardiovascular effects of cocaine may involve the inhibition of NETs at sympathetic and CNS autonomic synapses.[28]

Amphetamines edit

 
The chemical structure of MDMA or "ecstasy"

Amphetamines have an effect on norepinephrine levels similar to that of cocaine in that they both increase NE levels in the brain.[29] Amphetamine-like drugs are substrates for monoamine transporters, include NET, that cause a reversal in the direction of neurotransmitter transport.[17][30] Amphetamines cause a large accumulation of extracellular NE.[29] High levels of NE in the brain account for most of the profound effects of amphetamines, including alertness and anorectic, locomotor and sympathomimetic effects.[29] However, the effects that amphetamines have on the brain are slower but last longer than the effects cocaine has on the brain.[29] MDMA (3,4-Methylenedioxymethamphetamine or "ecstasy") is an amphetamine with wide recreational use. A study reported that the NET inhibitor reboxetine reduced the stimulant effects of MDMA in humans, demonstrating the crucial role NET has in the cardiovascular and stimulant-like effects of MDMA.[31]

Further research edit

The role of the NET in many brain disorders underlies the importance of understanding the (dys)regulation of the transporter. A complete model of the proteins that associate with the transporter will be useful in designing drug therapies for diseases such as schizophrenia, affective disorder, and autonomic disorders. Recently discovered mechanisms of the NET, including the ability to act reversibly and as an ion channel, provide other areas of research.[15][17]

Schizophrenia edit

 
Chemical structure of 3-methoxy-4-hydroxyphenylglycol (MHPG), a metabolite of NE. Abnormally high levels of MHPG are also indicative of impaired NE regulation.

The role of NE in schizophrenia has not been fully understood, but has stimulated research into this topic.[32][33][34][35] The only relationship that has been understood between researchers is that there is a positive correlation between increased NE levels in the brain and spinal fluid (CSF) and activity of schizophrenia.[32][33][34][35] In one study, clonidine, a drug used to treat medical conditions such as ADHD and high blood pressure, was shown to produce a significant decrease in plasma level MHPG (3-methoxy-4-hydroxyphenylglycol), a metabolite of NE, in the normal control group, but not in the group of schizophrenic patients.[34] This suggests that in schizophrenia, the alpha-2 adrenergic receptor, a presynaptic inhibitory receptor, may be less sensitive compared to normally functioning alpha-2 receptors and thus relate to elevated NE levels in the disorder.[34] In addition to increased NE levels in the brain and CSF, increased levels of MHPG has also been associated with a diagnosis of schizophrenia.[35] Impaired NE regulation in schizophrenia has been an area of interest for researchers and research on this topic is still ongoing.[34][35]

Imaging edit

Via positron emission tomography imaging technique, NET has been selectively investigated. 11C ME@HAPTHI and 18F-MeNER are two NET selective radio tracers for PET imaging.[36] Fluorescent substrates for the transporter can also be used to monitor the transporter rate in isolated organs or tissues,[37][38] although these are not suitable for clinical imaging.

See also edit

References edit

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  13. ^ dbSNP
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  21. ^ Morón JA, Brockington A, Wise RA, Rocha BA, Hope BT (January 2002). "Dopamine uptake through the norepinephrine transporter in brain regions with low levels of the dopamine transporter: evidence from knock-out mouse lines". The Journal of Neuroscience. 22 (2): 389–95. doi:10.1523/JNEUROSCI.22-02-00389.2002. PMC 6758674. PMID 11784783.
  22. ^ Yavich L, Forsberg MM, Karayiorgou M, Gogos JA, Männistö PT (September 2007). "Site-specific role of catechol-O-methyltransferase in dopamine overflow within prefrontal cortex and dorsal striatum". The Journal of Neuroscience. 27 (38): 10196–209. doi:10.1523/JNEUROSCI.0665-07.2007. PMC 6672678. PMID 17881525.
  23. ^ Clinical Pharmacology of SSRI's: How SSRIs as a Group Differ From TCAs, Preskorn
  24. ^ "What medications are used to treat ADHD?". National Institutes of Mental Health.
  25. ^ Simpson D, Plosker GL (2004). "Atomoxetine: a review of its use in adults with attention deficit hyperactivity disorder". Drugs. 64 (2): 205–22. doi:10.2165/00003495-200464020-00005. PMID 14717619. S2CID 195692609.
  26. ^ "Atomoxetine -". PubMed Health. Retrieved 2 November 2011.
  27. ^ a b Macey DJ, Smith HR, Nader MA, Porrino LJ (January 2003). "Chronic cocaine self-administration upregulates the norepinephrine transporter and alters functional activity in the bed nucleus of the stria terminalis of the rhesus monkey". The Journal of Neuroscience. 23 (1): 12–6. doi:10.1523/JNEUROSCI.23-01-00012.2003. PMC 6742134. PMID 12514195.
  28. ^ Barker EL, Blakely RD (1995). "Norepinephrine and Serotonin Transporters". In Kupfer DJ, Bloom FE (eds.). Psychopharmacology: the fourth generation of progress. New York: Raven Press. ISBN 978-0-7817-0166-2. Retrieved 2 November 2011.
  29. ^ a b c d Drug and Human Performance Fact Sheets. . Archived from the original on 31 October 2011. Retrieved 1 November 2011.
  30. ^ Sulzer D, Chen TK, Lau YY, Kristensen H, Rayport S, Ewing A (May 1995). "Amphetamine redistributes dopamine from synaptic vesicles to the cytosol and promotes reverse transport". The Journal of Neuroscience. 15 (5 Pt 2): 4102–8. doi:10.1523/JNEUROSCI.15-05-04102.1995. PMC 6578196. PMID 7751968.
  31. ^ Hysek CM, Simmler LD, Ineichen M, Grouzmann E, Hoener MC, Brenneisen R, Huwyler J, Liechti ME (August 2011). "The norepinephrine transporter inhibitor reboxetine reduces stimulant effects of MDMA ("ecstasy") in humans". Clinical Pharmacology and Therapeutics. 90 (2): 246–55. doi:10.1038/clpt.2011.78. PMID 21677639. S2CID 29087593.
  32. ^ a b Decaire M. . Archived from the original on 17 February 2012. Retrieved 1 November 2011.
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  34. ^ a b c d e Sternberg DE, Charney DS, Heninger GR, Leckman JF, Hafstad KM, Landis DH (March 1982). "Impaired presynaptic regulation of norepinephrine in schizophrenia. Effects of clonidine in schizophrenic patients and normal controls". Archives of General Psychiatry. 39 (3): 285–9. doi:10.1001/archpsyc.1982.04290030025004. PMID 6279049.
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  36. ^ Rami-Mark C, Berroterán-Infante N, Philippe C, Foltin S, Vraka C, Hoepping A, et al. (December 2015). "Radiosynthesis and first preclinical evaluation of the novel norepinephrine transporter pet-ligand [(11)C]ME@HAPTHI". EJNMMI Research. 5 (1): 113. doi:10.1186/s13550-015-0113-3. PMC 4467816. PMID 26061602.
  37. ^ Cao LL, Holmes AP, Marshall JM, Fabritz L, Brain KL (January 2020). "Dynamic monitoring of single-terminal norepinephrine transporter rate in the rodent cardiovascular system: A novel fluorescence imaging method". Autonomic Neuroscience. 223: 102611. doi:10.1016/j.autneu.2019.102611. PMC 6977090. PMID 31901784.
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External links edit

norepinephrine, transporter, norepinephrine, transporter, also, known, noradrenaline, transporter, protein, that, humans, encoded, solute, carrier, family, member, slc6a2, gene, slc6a2identifiersaliasesslc6a2, nat1, net1, slc6a5, solute, carrier, family, membe. The norepinephrine transporter NET also known as noradrenaline transporter NAT is a protein that in humans is encoded by the solute carrier family 6 member 2 SLC6A2 gene 5 SLC6A2IdentifiersAliasesSLC6A2 NAT1 NET NET1 SLC6A5 solute carrier family 6 member 2 Norepinephrine transporter norepinephrine transporter geneExternal IDsOMIM 163970 MGI 1270850 HomoloGene 816 GeneCards SLC6A2 OMA SLC6A2 orthologsGene location Human Chr Chromosome 16 human 1 Band16q12 2Start55 655 988 bp 1 End55 707 645 bp 1 Gene location Mouse Chr Chromosome 8 mouse 2 Band8 C5 8 44 99 cMStart93 686 707 bp 2 End93 728 295 bp 2 RNA expression patternBgeeHumanMouse ortholog Top expressed inplacentasuperior vestibular nucleuscaput epididymisglomerulusmetanephric glomerulusnippleright adrenal glandskin of abdomenganglionspinal gangliaTop expressed incarotid bodyright lung lobemaxillary nerveexternal carotid arteryupper lipsuperior cervical ganglionuterusaortic valvedorsal tegmental nucleusadrenal glandMore reference expression dataBioGPSMore reference expression dataGene ontologyMolecular functiondopamine sodium symporter activity neurotransmitter sodium symporter activity norepinephrine sodium symporter activity monoamine transmembrane transporter activity protein binding symporter activity metal ion binding beta tubulin binding alpha tubulin binding actin bindingCellular componentintegral component of membrane plasma membrane membrane membrane raft neuron projection cell surface integral component of plasma membrane neuronal cell body membrane presynaptic membraneBiological processdopamine uptake involved in synaptic transmission neurotransmitter transport response to pain monoamine transport chemical synaptic transmission norepinephrine transport transmembrane transport sodium ion transmembrane transport norepinephrine uptake neuron cellular homeostasis neurotransmitter reuptakeSources Amigo QuickGOOrthologsSpeciesHumanMouseEntrez653020538EnsemblENSG00000103546ENSMUSG00000055368UniProtP23975O55192RefSeq mRNA NM 001043NM 001172501NM 001172502NM 001172504NM 009209RefSeq protein NP 001034NP 001165972NP 001165973NP 001165975NP 033235Location UCSC Chr 16 55 66 55 71 MbChr 8 93 69 93 73 MbPubMed search 3 4 WikidataView Edit HumanView Edit Mouse NET is a monoamine transporter and is responsible for the sodium chloride Na Cl dependent reuptake of extracellular norepinephrine NE which is also known as noradrenaline NET can also reuptake extracellular dopamine DA The reuptake of these two neurotransmitters is essential in regulating concentrations in the synaptic cleft NETs along with the other monoamine transporters are the targets of many antidepressants and recreational drugs In addition an overabundance of NET is associated with ADHD 6 7 There is evidence that single nucleotide polymorphisms in the NET gene SLC6A2 may be an underlying factor in some of these disorders 7 Contents 1 Gene 1 1 Single nucleotide polymorphisms 1 2 Genetic variations 2 Structure 3 Function 4 Transport mechanisms 5 Location in the nervous system 6 Regulation 7 Clinical significance 7 1 Orthostatic intolerance 8 Therapeutic uses 8 1 Major depressive disorder 8 2 ADHD 9 Psychostimulants 9 1 Cocaine 9 2 Amphetamines 10 Further research 10 1 Schizophrenia 11 Imaging 12 See also 13 References 14 External linksGene editThe norepinephrine transporter gene SLC6A2 is located on human chromosome 16 locus 16q12 2 This gene is encoded by 14 exons 7 Based on the nucleotide and amino acid sequence the NET transporter consists of 617 amino acids with 12 membrane spanning domains The structural organization of NET is highly homologous to other members of a sodium chloride dependent family of neurotransmitter transporters including dopamine epinephrine serotonin and GABA transporters 7 Single nucleotide polymorphisms edit A single nucleotide polymorphism SNP is a genetic variation in which a genome sequence is altered by a single nucleotide A T C or G NET proteins with an altered amino acid sequence more specifically a missense mutation could potentially be associated with various diseases that involve abnormally high or low plasma levels of norepinephrine due to altered NET function NET SNPs and possible associations with various diseases are an area of focus for many research projects There is evidence suggesting a relationship between NET SNPs and various disorders such as ADHD 7 8 psychiatric disorders 7 postural tachycardia 7 9 and orthostatic intolerance 7 9 The SNPs rs3785143 and rs11568324 have been related to attention deficit hyperactivity disorder 10 Thus far however the only confirmed direct association between a SNP and a clinical condition is that of the SNP Ala457Pro and orthostatic intolerance 7 Thirteen NET missense mutations have been discovered so far 7 Missense Mutations in the NET Gene 7 11 Location Amino Acid Variant TMD if known Related Disease Exon 2 Val69Ile TMD 1 None Exon 3 Thr99Ile TMD 2 None Exon 5 Val245Ile TMD 4 None Exon 6 Asn292Thr n a None Exon 8 Val356Leu n a None Exon 8 Ala369Pro n a None Exon 8 Asn375Ser n a None Exon 10 Val449Ile TMD 9 None Exon 10 Ala457Pro TMD 9 Orthostatic intolerance Exon 10 Lys463Arg n a None Exon 11 Gly478Ser TMD 10 None Exon 12 Phe528Cys n a None Exon 13 Tyr548His n a None Abbreviations TMD transmembrane domain n a non applicable For the table above refer to this table of standard amino acid abbreviations This notation for missense mutations take Val69Ile for example indicates that amino acid Val69 was changed to Ile Genetic variations edit An epigenetic mechanism hypermethylation of CpG islands in the NET gene promoter region that results in reduced expression of the noradrenaline norepinephrine transporter and consequently a phenotype of impaired neuronal reuptake of norepinephrine has been implicated in both postural orthostatic tachycardia syndrome and panic disorder 12 rs5569 is a variant of SLC6A2 13 Structure edit nbsp Simplified structure of the norepinephrine transporter protein The norepinephrine transporter is composed of 12 transmembrane domains TMDs The intracellular portion contains an amino NH2 group and carboxyl COOH group In addition there is a large extracellular loop located between TMD 3 and 4 14 6 15 The protein is composed of 617 amino acids 14 Function editNET functions to transport synaptically released norepinephrine back into the presynaptic neuron As much as 90 of the norepinephrine released will be taken back up in the cell by NET NET functions by coupling the influx of sodium and chloride Na Cl with the transport of norepinephrine This occurs at a fixed ratio of 1 1 1 16 Both the NET and the dopamine transporter DAT can transport norepinephrine and dopamine The reuptake of norepinephrine and dopamine is essential in regulating the concentration of monoamine neurotransmitters in the synaptic cleft The transporter also helps maintain homeostatic balances of the presynaptic neuron 17 nbsp Norepinephrine structure Norepinephrine NE is released from noradrenergic neurons that innervate both the CNS and PNS NE also known as noradrenaline NA has an important role in controlling mood arousal memory learning and pain perception NE is a part of the sympathetic nervous system 6 18 Dysregulation of the removal of norepinephrine by NET is associated with many neuropsychiatric diseases discussed below In addition many antidepressants and recreational drugs compete for the binding of NET with NE 14 Transport mechanisms editThe transport of norepinephrine back into presynaptic cell is made possible by the cotransport with Na and Cl The sequential binding of the ions results in the eventual reuptake of norepinephrine The ion gradients of Na and Cl make this reuptake energetically favorable The gradient is generated by the Na K ATPase which transports three sodium ions out and two potassium ions into the cell 17 NETs have conductances similar to those of ligand gated ion channels The expression of NET results in a leak channel activity 16 17 Location in the nervous system editNETs are restricted to noradrenergic neurons and are not present on neurons that release dopamine or epinephrine 6 15 17 The transporters can be found along the cell body axons and dendrites of the neuron 6 NETs are located away from the synapse where norepinephrine is released They are found closer to the plasma membrane of the cell This requires norepinephrine to diffuse from the site it is released to the transporter for reuptake 17 Norepinephrine transporters are confined to the neurons of the sympathetic system and those innervating the adrenal medulla lung and placenta 17 Regulation editRegulation of NET function is complex and a focus of current research NETs are regulated at both the cellular and molecular level post translation The most understood mechanisms include phosphorylation by the second messenger protein kinase C PKC 15 PKC has been shown to inhibit NET function by sequestration of the transporter from the plasma membrane 19 The amino acid sequence of NET has shown multiple sites related to protein kinase phosphorylation 17 Post translational modifications can have a wide range of effects on the function of the NET including the rate of fusion of NET containing vesicles with the plasma membrane and transporter turnover 19 Clinical significance editOrthostatic intolerance edit Main article Orthostatic intolerance Orthostatic intolerance OI is a disorder of the autonomic nervous system a subcategory of dysautonomia characterized by the onset of symptoms upon standing Symptoms include fatigue lightheadedness headache weakness increased heart rate heart palpitations anxiety and altered vision 7 Often patients have high plasma norepinephrine NE concentrations at least 600 pg ml in relation to sympathetic outflow upon standing suggesting OI is a hyperadrenergic condition 7 9 The discovery of identical twin sisters who both had OI suggested a genetic basis for the disorder 7 9 A missense mutation on the NET gene SLC6A2 was discovered in which an alanine residue was replaced with a proline residue Ala457Pro in a highly conserved region of the transporter 7 The patients defective NET had only 2 of the activity of the wild type version of the gene 7 The genetic defect in the NET protein results in decreased NET activity that could account for abnormally high NE plasma levels in OI However 40 other OI patients did not have the same missense mutation indicating other factors contributed to the phenotype in the identical twins 7 This discovery of the linkage with NET mutations that results in decreased norepinephrine reuptake activity and orthostatic intolerance suggests faulty NE uptake mechanisms can contribute to cardiovascular disease 20 Therapeutic uses editInhibition of the norepinephrine transporter NET has potential therapeutic applications in the treatment of attention deficit hyperactivity disorder ADHD substance abuse neurodegenerative disorders e g Alzheimer s disease AD and Parkinson s disease PD and clinical depression 18 Major depressive disorder edit Main article Major depressive disorder nbsp Amineptine an NDRI tricyclic antidepressant nbsp Fluoxetine a selective serotonin reuptake inhibitor Certain antidepressant medications act to raise noradrenaline such as serotonin norepinephrine reuptake inhibitors SNRIs norepinephrine dopamine reuptake inhibitors NDRIs norepinephrine reuptake inhibitors NRIs or NERIs and the tricyclic antidepressants TCAs The mechanism by which these medications work is that the reuptake inhibitors prevent the reuptake of serotonin and norepinephrine by the presynaptic neuron paralyzing the normal function of the NET At the same time higher levels of 5 HT are maintained in the synapse increasing the concentrations of the latter neurotransmitters Since the noradrenaline transporter is responsible for most of the dopamine clearance in the prefrontal cortex 21 SNRIs block reuptake of dopamine too accumulating the dopamine in the synapse However DAT the primary way dopamine is transported out of the cell can work to decrease dopamine concentration in the synapse when the NET is blocked 22 For many years the number one choice in treating mood disorders like depression was through administration of TCAs such as desipramine Norpramin nortriptyline Arentyl Pamelor protriptyline Vivactil and amoxapine Asendin 18 SSRIs which mainly regulate serotonin subsequently replaced tricyclics as the primary treatment option for depression because of their better tolerability and lower incidence of adverse effects 23 ADHD edit Main article ADHD nbsp Atomoxetine an NET inhibitor marketed as Strattera Many drugs exist in the treatment of ADHD Dextroamphetamine Dexedrine Dextrostat Adderall methylphenidate Ritalin Metadate Concerta Daytrana and lisdexamfetamine Vyvanse block reabsorption of the catecholamines dopamine and norepinephrine through monoamine transporters including NET thereby increasing levels of these neurotransmitters in the brain The strong selective norepinephrine reuptake inhibitor NRI atomoxetine Strattera has been approved by the U S Food and Drug Administration FDA to treat ADHD in adults 24 25 The role of the NET in ADHD is similar to how it works to ease the symptoms of depression The NET is blockaded by atomoxetine and increases NE levels in the brain It can work to increase one s ability to focus decrease any impulsiveness and lessen hyperactivity in both children and adults with ADHD 26 Psychostimulants editCocaine edit Main article Cocaine Cocaine is a powerful psychostimulant and known to be one of the most widely used substances recreationally 27 Cocaine is a nonselective reuptake inhibitor of the norepinephrine serotonin and dopamine transporters This thwarts the absorption of these chemicals into the presynaptic terminal 27 and allows a large concentration of dopamine serotonin and norepinephrine to build up in the synaptic cleft The potential for cocaine addiction is thought to be a result of its effects on dopamine transporters in the CNS while it has been suggested that the life threatening cardiovascular effects of cocaine may involve the inhibition of NETs at sympathetic and CNS autonomic synapses 28 Amphetamines edit Main article Amphetamine nbsp The chemical structure of MDMA or ecstasy Amphetamines have an effect on norepinephrine levels similar to that of cocaine in that they both increase NE levels in the brain 29 Amphetamine like drugs are substrates for monoamine transporters include NET that cause a reversal in the direction of neurotransmitter transport 17 30 Amphetamines cause a large accumulation of extracellular NE 29 High levels of NE in the brain account for most of the profound effects of amphetamines including alertness and anorectic locomotor and sympathomimetic effects 29 However the effects that amphetamines have on the brain are slower but last longer than the effects cocaine has on the brain 29 MDMA 3 4 Methylenedioxymethamphetamine or ecstasy is an amphetamine with wide recreational use A study reported that the NET inhibitor reboxetine reduced the stimulant effects of MDMA in humans demonstrating the crucial role NET has in the cardiovascular and stimulant like effects of MDMA 31 Further research editThe role of the NET in many brain disorders underlies the importance of understanding the dys regulation of the transporter A complete model of the proteins that associate with the transporter will be useful in designing drug therapies for diseases such as schizophrenia affective disorder and autonomic disorders Recently discovered mechanisms of the NET including the ability to act reversibly and as an ion channel provide other areas of research 15 17 Schizophrenia edit Main article Schizophrenia nbsp Chemical structure of 3 methoxy 4 hydroxyphenylglycol MHPG a metabolite of NE Abnormally high levels of MHPG are also indicative of impaired NE regulation The role of NE in schizophrenia has not been fully understood but has stimulated research into this topic 32 33 34 35 The only relationship that has been understood between researchers is that there is a positive correlation between increased NE levels in the brain and spinal fluid CSF and activity of schizophrenia 32 33 34 35 In one study clonidine a drug used to treat medical conditions such as ADHD and high blood pressure was shown to produce a significant decrease in plasma level MHPG 3 methoxy 4 hydroxyphenylglycol a metabolite of NE in the normal control group but not in the group of schizophrenic patients 34 This suggests that in schizophrenia the alpha 2 adrenergic receptor a presynaptic inhibitory receptor may be less sensitive compared to normally functioning alpha 2 receptors and thus relate to elevated NE levels in the disorder 34 In addition to increased NE levels in the brain and CSF increased levels of MHPG has also been associated with a diagnosis of schizophrenia 35 Impaired NE regulation in schizophrenia has been an area of interest for researchers and research on this topic is still ongoing 34 35 Imaging editVia positron emission tomography imaging technique NET has been selectively investigated 11C ME HAPTHI and 18F MeNER are two NET selective radio tracers for PET imaging 36 Fluorescent substrates for the transporter can also be used to monitor the transporter rate in isolated organs or tissues 37 38 although these are not suitable for clinical imaging See also editNeurotransmitter transporter Sodium neurotransmitter symporter Solute carrier family Norepinephrine Monoamine transporter Orthostatic intolerance Single nucleotide polymorphismsReferences edit a b c GRCh38 Ensembl release 89 ENSG00000103546 Ensembl May 2017 a b c GRCm38 Ensembl release 89 ENSMUSG00000055368 Ensembl May 2017 Human PubMed Reference National Center for Biotechnology Information U S National Library of Medicine Mouse PubMed Reference National Center for Biotechnology Information U S National Library of Medicine Pacholczyk T Blakely RD Amara SG March 1991 Expression cloning of a cocaine and antidepressant sensitive human noradrenaline transporter Nature 350 6316 350 4 Bibcode 1991Natur 350 350P doi 10 1038 350350a0 PMID 2008212 S2CID 4333895 a b c d e Schroeter S Apparsundaram S Wiley RG Miner LH Sesack SR Blakely RD May 2000 Immunolocalization of the cocaine and antidepressant sensitive l norepinephrine transporter The Journal of Comparative Neurology 420 2 211 32 doi 10 1002 SICI 1096 9861 20000501 420 2 lt 211 AID CNE5 gt 3 0 CO 2 3 PMID 10753308 S2CID 24643588 a b c d e f g h i j k l m n o p q Tellioglu T Robertson D November 2001 Genetic or acquired deficits in the norepinephrine transporter current understanding of clinical implications Expert Reviews in Molecular Medicine 2001 29 1 10 doi 10 1017 S1462399401003878 PMID 14987367 S2CID 30965333 Kim CH Hahn MK Joung Y Anderson SL Steele AH Mazei Robinson MS Gizer I Teicher MH Cohen BM Robertson D Waldman ID Blakely RD Kim KS December 2006 A polymorphism in the norepinephrine transporter gene alters promoter activity and is associated with attention deficit hyperactivity disorder Proceedings of the National Academy of Sciences of the United States of America 103 50 19164 9 Bibcode 2006PNAS 10319164K doi 10 1073 pnas 0510836103 PMC 1748193 PMID 17146058 a b c d Shannon JR Flattem NL Jordan J Jacob G Black BK Biaggioni I Blakely RD Robertson D February 2000 Orthostatic intolerance and tachycardia associated with norepinephrine transporter deficiency The New England Journal of Medicine 342 8 541 9 doi 10 1056 NEJM200002243420803 PMID 10684912 Kim JW Biederman J McGrath CL Doyle AE Mick E Fagerness J Purcell S Smoller JW Sklar P Faraone SV June 2008 Further evidence of association between two NET single nucleotide polymorphisms with ADHD Molecular Psychiatry 13 6 624 30 doi 10 1038 sj mp 4002090 PMID 17876324 S2CID 8341997 Maarten E A Reith 2002 Neurotransmitter transporters structure function and regulation Humana Press p 120 ISBN 978 0 89603 945 2 Retrieved 27 October 2011 Esler M Alvarenga M Pier C Richards J El Osta A Barton D Haikerwal D Kaye D Schlaich M Guo L Jennings G Socratous F Lambert G July 2006 The neuronal noradrenaline transporter anxiety and cardiovascular disease Journal of Psychopharmacology 20 4 Suppl 60 6 doi 10 1177 1359786806066055 PMID 16785272 S2CID 10728780 dbSNP a b c Stober G Nothen MM Porzgen P Bruss M Bonisch H Knapp M Beckmann H Propping P November 1996 Systematic search for variation in the human norepinephrine transporter gene identification of five naturally occurring missense mutations and study of association with major psychiatric disorders American Journal of Medical Genetics 67 6 523 32 doi 10 1002 SICI 1096 8628 19961122 67 6 lt 523 AID AJMG3 gt 3 0 CO 2 I PMID 8950409 a b c d Sager JJ Torres GE August 2011 Proteins interacting with monoamine transporters current state and future challenges Biochemistry 50 34 7295 310 doi 10 1021 bi200405c PMID 21797260 a b Galli A DeFelice LJ Duke BJ Moore KR Blakely RD October 1995 Sodium dependent norepinephrine induced currents in norepinephrine transporter transfected HEK 293 cells blocked by cocaine and antidepressants The Journal of Experimental Biology 198 Pt 10 2197 212 doi 10 1242 jeb 198 10 2197 PMID 7500004 a b c d e f g h i Torres GE Gainetdinov RR Caron MG January 2003 Plasma membrane monoamine transporters structure regulation and function Nature Reviews Neuroscience 4 1 13 25 doi 10 1038 nrn1008 PMID 12511858 S2CID 21545649 a b c Zhou J December 2004 Norepinephrine transporter inhibitors and their therapeutic potential Drugs of the Future 29 12 1235 1244 doi 10 1358 dof 2004 029 12 855246 PMC 1518795 PMID 16871320 a b Gether U Andersen PH Larsson OM Schousboe A July 2006 Neurotransmitter transporters molecular function of important drug targets Trends in Pharmacological Sciences 27 7 375 83 doi 10 1016 j tips 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ADHD National Institutes of Mental Health Simpson D Plosker GL 2004 Atomoxetine a review of its use in adults with attention deficit hyperactivity disorder Drugs 64 2 205 22 doi 10 2165 00003495 200464020 00005 PMID 14717619 S2CID 195692609 Atomoxetine PubMed Health Retrieved 2 November 2011 a b Macey DJ Smith HR Nader MA Porrino LJ January 2003 Chronic cocaine self administration upregulates the norepinephrine transporter and alters functional activity in the bed nucleus of the stria terminalis of the rhesus monkey The Journal of Neuroscience 23 1 12 6 doi 10 1523 JNEUROSCI 23 01 00012 2003 PMC 6742134 PMID 12514195 Barker EL Blakely RD 1995 Norepinephrine and Serotonin Transporters In Kupfer DJ Bloom FE eds Psychopharmacology the fourth generation of progress New York Raven Press ISBN 978 0 7817 0166 2 Retrieved 2 November 2011 a b c d Drug and Human Performance Fact Sheets Methamphetamine And Amphetamine Archived from the original on 31 October 2011 Retrieved 1 November 2011 Sulzer D Chen TK Lau YY Kristensen H Rayport S Ewing A May 1995 Amphetamine redistributes dopamine from synaptic vesicles to the cytosol and promotes reverse transport The Journal of Neuroscience 15 5 Pt 2 4102 8 doi 10 1523 JNEUROSCI 15 05 04102 1995 PMC 6578196 PMID 7751968 Hysek CM Simmler LD Ineichen M Grouzmann E Hoener MC Brenneisen R Huwyler J Liechti ME August 2011 The norepinephrine transporter inhibitor reboxetine reduces stimulant effects of MDMA ecstasy in humans Clinical Pharmacology and Therapeutics 90 2 246 55 doi 10 1038 clpt 2011 78 PMID 21677639 S2CID 29087593 a b Decaire M The neurophysiology of schizophrenia Etiology and Psychopharmacological treatment Archived from the original on 17 February 2012 Retrieved 1 November 2011 a b Breier A Wolkowitz OM Roy A Potter WZ Pickar D November 1990 Plasma norepinephrine in chronic schizophrenia The American Journal of Psychiatry 147 11 1467 70 doi 10 1176 ajp 147 11 1467 PMID 2221157 a b c d e Sternberg DE Charney DS Heninger GR Leckman JF Hafstad KM Landis DH March 1982 Impaired presynaptic regulation of norepinephrine in schizophrenia Effects of clonidine in schizophrenic patients and normal controls Archives of General Psychiatry 39 3 285 9 doi 10 1001 archpsyc 1982 04290030025004 PMID 6279049 a b c d van Kammen DP Antelman S April 1984 Impaired noradrenergic transmission in schizophrenia Life Sciences 34 15 1403 13 doi 10 1016 0024 3205 84 90054 7 PMID 6323903 Rami Mark C Berroteran Infante N Philippe C Foltin S Vraka C Hoepping A et al December 2015 Radiosynthesis and first preclinical evaluation of the novel norepinephrine transporter pet ligand 11 C ME HAPTHI EJNMMI Research 5 1 113 doi 10 1186 s13550 015 0113 3 PMC 4467816 PMID 26061602 Cao LL Holmes AP Marshall JM Fabritz L Brain KL January 2020 Dynamic monitoring of single terminal norepinephrine transporter rate in the rodent cardiovascular system A novel fluorescence imaging method Autonomic Neuroscience 223 102611 doi 10 1016 j autneu 2019 102611 PMC 6977090 PMID 31901784 Parker LK Shanks JA Kennard JA Brain KL February 2010 Dynamic monitoring of NET activity in mature murine sympathetic terminals using a fluorescent substrate British Journal of Pharmacology 159 4 797 807 doi 10 1111 j 1476 5381 2009 00574 x PMC 2829205 PMID 20136837 External links editNorepinephrine transporter at the U S National Library of Medicine Medical Subject Headings MeSH Retrieved from https en wikipedia org w index php title Norepinephrine transporter amp oldid 1191012933, wikipedia, wiki, book, books, library,

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