Norepinephrine transporter

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

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

Gene
The norepinephrine transporter gene, SLC6A2 is located on human is_associated_with::chromosome 16 locus 16q12.2. This gene is encoded by 14 is_associated_with::exons. 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, is_associated_with::serotonin and GABA transporters.

Single nucleotide polymorphisms
A is_associated_with::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 is_associated_with::missense mutation) could potentially be associated with various diseases that involve abnormally high or low plasma levels of is_associated_with::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 psychiatric disorders, postural tachycardia and is_associated_with::orthostatic intolerance. The SNPs rs3785143 and rs11568324 have been related to is_associated_with::attention-deficit hyperactivity disorder. Thus far, however, the only confirmed direct association between a SNP and a clinical condition is that of the SNP, Ala457Pro, and orthostatic intolerance. Thirteen NET missense mutations have been discovered so far.

Abbreviations: TMD, is_associated_with::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
An is_associated_with::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 is_associated_with::postural orthostatic tachycardia syndrome and is_associated_with::panic disorder.

Structure
The norepinephrine transporter is composed of 12 is_associated_with::transmembrane domains (TMDs). The intracellular portion contains an amino (-) group and carboxyl (-COOH) group. In addition, there is a large extracellular loop located between TMD 3 and 4. The protein is composed of 617 is_associated_with::amino acids.

Function
NET functions to transport synaptically released is_associated_with::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. Both the NET and  the is_associated_with::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.

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 is_associated_with::sympathetic nervous system. 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.

Transport mechanisms
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 is_associated_with::Na+/K+-ATPase which transports three sodium ions out and two potassium ions into the cell. NETs have conductances similar to those of is_associated_with::ligand-gated ion channels. The expression of NET results in a leak-channel activity.

Location in the nervous system
NETs are restricted to noradrenergic neurons and are not present on neurons that release dopamine or epinephrine. The transporters can be found along the cell body, is_associated_with::axons, and is_associated_with::dendrites of the neuron. 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. Norepinephrine transporters are confined to the neurons of the sympathetic system, and those innervating the adrenal medulla, lung, and placenta.

Regulation
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 is_associated_with::protein kinase C (PKC). PKC has been shown to inhibit NET function by sequestration of the transporter from the plasma membrane. The amino acid sequence of NET has shown multiple sites related to protein kinase phosphorylation. 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.

Orthostatic intolerance
Orthostatic intolerance (OI) is a disorder of the is_associated_with::autonomic nervous system (a subcategory of is_associated_with::dysautonomia) characterized by the onset of symptoms upon standing. Symptoms include fatigue, lightheadedness, headache, weakness, increased heart rate/is_associated_with::heart palpitations, anxiety, and altered vision. Often, patients have high plasma is_associated_with::norepinephrine (NE) concentrations (at least 600 pg/ml) in relation to sympathetic outflow upon standing, suggesting OI is a hyperis_associated_with::adrenergic condition. The discovery of identical twin sisters both suffering from OI suggested a genetic basis for the disorder. A is_associated_with::missense mutation on the NET gene (SLC62A) was discovered in which an is_associated_with::alanine residue was replaced with a is_associated_with::proline residue (Ala457Pro) in a highly conserved region of the transporter. The patients’ defective NET had only 2% of the activity of the wild-type version of the gene. 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. This discovery of the linkage with NET mutations that results in decreased norepinephrine is_associated_with::reuptake activity and orthostatic intolerance suggests faulty NE uptake mechanisms can contribute to cardiovascular disease.

Therapeutic uses
"The involvement of the norepinephrine transporter (NET) in the is_associated_with::pathophysiology and treatment of is_associated_with::attention deficit hyperactivity disorder (ADHD), is_associated_with::substance abuse, neurodegenerative disorders (e.g., is_associated_with::Alzheimer's disease (AD) and is_associated_with::Parkinson's disease (PD)) and is_associated_with::clinical depression has long been recognized. [2006]"

Major depressive disorder
Certain is_associated_with::antidepressant medications act to raise noradrenaline, such as serotonin-norepinephrine reuptake inhibitors (SNRIs), is_associated_with::norepinephrine-dopamine reuptake inhibitors (NDRIs), is_associated_with::norepinephrine reuptake inhibitors (NRIs or NERIs) and the is_associated_with::tricyclic antidepressants (TCAs). The mechanism by which these medications work is that the reuptake inhibitors prevent the reuptake of serotonin and norepinephrine by the is_associated_with::presynaptic neuron, paralyzing the normal function of the NET. At the same time, higher levels of is_associated_with::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, 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. For many years, the number one choice in treating mood disorders like depression was through the uptake of TCAs, such as is_associated_with::desipramine (Norpramin), is_associated_with::nortriptyline (Arentyl, Pamelor), protriptyline (Vivactil) and is_associated_with::amoxapine (Asendin). However, currently far more potent drugs have been developed in the United States, most notably the discovery of is_associated_with::selective serotonin reuptake inhibitors (SSRIs). SSRIs affect the communication between chemical messengers in the brain by regulating how much of a certain chemical messenger enters the brain. SSRIs regulate serotonin levels in the brain, which seems to aid the brain in exchanging messages more efficiently and in turn livens mood. Drugs such as is_associated_with::fluoxetine and is_associated_with::paroxetine are both very impelling in treating mood disorders because they reduce many of the side effects one receives compared to using TCAs to treat such mood disorders.

ADHD
Many drugs exist in the treatment of ADHD. is_associated_with::Dextroamphetamine (Dexedrine, Dextrostat), is_associated_with::Adderall, is_associated_with::methylphenidate (Ritalin, Metadate, Concerta, Daytrana), and is_associated_with::lisdexamfetamine (Vyvanse) block reabsorption of the is_associated_with::catecholamines dopamine and norepinephrine through is_associated_with::monoamine transporters (including NET), thereby increasing levels of these is_associated_with::neurotransmitters in the brain. The strong is_associated_with::selective norepinephrine reuptake inhibitor (NRI), is_associated_with::atomoxetine (Strattera), has been approved by the U.S. is_associated_with::Food and Drug Administration (FDA) to treat ADHD in adults. 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 who suffer from ADHD.

Cocaine
is_associated_with::Cocaine is a powerful is_associated_with::psychostimulant and known to be one of the most widely abused substances. Cocaine is a nonselective, is_associated_with::reuptake inhibitor of the is_associated_with::norepinephrine, is_associated_with::serotonin, and is_associated_with::dopamine transporters. This thwarts the absorption of these chemicals into the is_associated_with::presynaptic terminal 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 is_associated_with::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.

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

Further research
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 is_associated_with::schizophrenia, is_associated_with::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.

Schizophrenia
The role of NE in is_associated_with::schizophrenia has not been fully understood, but has stimulated research into this topic. 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. In one study, is_associated_with::clonidine, a drug used to treat medical conditions such as is_associated_with::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. This suggests that in schizophrenia, the is_associated_with::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. In addition to increased NE levels in the brain and CSF, increased levels of MHPG has also been associated with a diagnosis of schizophrenia. Impaired NE regulation in schizophrenia has been an area of interest for researchers and research on this topic is still ongoing.