Antiporter

thumb|A comparison of transport proteins

An antiporter (also called exchanger or counter-transporter) is an integral membrane protein that uses secondary active transport to move two or more molecules in opposite directions across a phospholipid membrane. It is a type of cotransporter, which means that uses the energetically favorable movement of one molecule down its electrochemical gradient to power the energetically unfavorable movement of another molecule up its electrochemical gradient. This is in contrast to symporters, which are another type of cotransporter that moves two or more ions in the same direction, and primary active transport, which is directly powered by ATP.thumb|Illustration of an antiporter and the concentration gradients of its transport substances

Transport may involve one or more of each type of solute. For example, the Na+/Ca2+ exchanger, found in the plasma membrane of many cells, moves three sodium ions in one direction, and one calcium ion in the other. As with sodium in this example, antiporters rely on an established gradient that makes entry of one ion energetically favorable to force the unfavorable movement of a second molecule in the opposite direction. Through their diverse functions, antiporters are involved in various important physiological processes, such as regulation of the strength of cardiac muscle contraction, transport of carbon dioxide by erythrocytes, regulation of cytosolic pH, and accumulation of sucrose in plant vacuoles. In mammals, they are most commonly responsible for bringing glucose and amino acids into cells.

Symporters and antiporters are more complex because they move more than one ion and the movement of one of those ions is in an energetically unfavorable direction. As multiple molecules are involved, multiple binding processes must occur as the transporter undergoes a cycle of conformational changes to move them from one side of the membrane to the other. The mechanism used by these transporters limits their functioning to moving only a few molecules at a time. As a result, symporters and antiporters are characterized by a slower transport speed, moving between 102 and 104 molecules per second. Compare this to ion channels that provide a means for facilitated diffusion to occur and allow between 107 and 108 ions pass through the plasma membrane per second. These gating reactions limit the speed of these pumps, causing them to function even slower than transport proteins, moving between 100 and 103 ions per second. As transporters, antiporters have all of these features.

Because antiporters are highly diverse, their structure can vary widely depending upon the type of molecules being transported and their location in the cell. However, there are some common features that all antiporters share. One of these is multiple transmembrane regions that span the lipid bilayer of the plasma membrane and form a channel through which hydrophilic molecules can pass. These transmembrane regions are typically structured from alpha helices and are connected by loops in both the extracellular space and cytosol. These loops are what contain the binding sites for the molecules associated with the antiporter.

History

Antiporters were discovered as scientists were exploring ion transport mechanisms across biological membranes. The early studies took place in the mid-20th century and were focused on the mechanisms that transported ions such as sodium, potassium, and calcium across the plasma membrane. Researchers made the observation that these ions were moved in opposite directions and hypothesized the existence of membrane proteins that could facilitate this type of transport.

In the 1960's, biochemist Efraim Racker made a breakthrough in the discovery of antiporters. Through purification from bovine heart mitochondria, Racker and his colleagues found a mitochondrial protein that could exchange inorganic phosphate for hydroxide ions. The protein is located in the inner mitochondrial membrane and transports phosphate ions for use in oxidative phosphorylation. It became known as the phosphate-hydroxide antiporter, or mitochondrial phosphate carrier protein, and was the first example of an antiporter identified in living cells.

As time went on, researchers discovered other antiporters in different membranes and in various organisms. This includes the sodium-calcium exchanger (NCX), another crucial antiporter that regulates intracellular calcium levels through the exchange of sodium ions for calcium ions across the plasma membrane. It was discovered in the 1970s and is now a well-characterized antiporter known to be found in many different types of cells.

Advances in the fields of biochemistry and molecular biology have enabled the identification and characterization of a wide range of antiporters. Understanding the transport processes of various molecules and ions has provided insight into cellular transport mechanisms, as well as the role of antiporters in various physiological functions and in the maintenance of homeostasis

Role in homeostasis

Sodium-calcium exchanger

The sodium-calcium exchanger, also known as the Na+/Ca2+ exchanger or NCX, is an antiporter responsible for removing calcium from cells. This title encompasses a class of ion transporters that are commonly found in the heart, kidney, and brain. They use the energy stored in the electrochemical gradient of sodium to exchange the flow of three sodium ions into the cell for the export of one calcium ion.

Although the sodium-calcium exchanger has a low affinity for calcium ions, it can transport a high amount of the ion in a short period of time. Because of these properties, it is useful in situations where there is an urgent need to export high amounts of calcium, such as after an action potential has occurred. Its characteristics also enable NCX to work with other proteins that have a greater affinity for calcium ions without interfering with their functions. NCX works with these proteins to carry out functions such as cardiac muscle relaxation, excitation-contraction coupling, and photoreceptor activity. They also maintain the concentration of calcium ions in the sarcoplasmic reticulum of cardiac cells, endoplasmic reticulum of excitable and nonexcitable cells, and the mitochondria.

Another key characteristic of this antiporter is its reversibility. This means that if the cell is depolarized enough, the extracellular sodium level is low enough, or the intracellular level of sodium is high enough, NCX will operate in the reverse direction and begin bringing calcium into the cell. For example, when NCX functions during excitotoxicity, this characteristic allows it to have a protective effect because the accompanying increase in intracellular calcium levels enables the exchanger to work in its normal direction regardless of the sodium concentration.

The sodium-calcium exchanger's role in maintaining calcium homeostasis in cardiac muscle cells allows it to help relax the heart muscle as it exports calcium during diastole. Therefore, its dysfunction can result in abnormal calcium movement and the development of various cardiac diseases. Abnormally high intracellular calcium levels can hinder diastole and cause abnormal systole and arrhythmias. Arrhythmias can occur when calcium is not properly exported by NCX, causing delayed afterdepolarizations and triggering abnormal activity that can possibly lead to atrial fibrillation and ventricular tachycardia.

If the heart experiences ischemia, the inadequate oxygen supply can disrupt ion homeostasis. When the body tries to stabilize this by returning blood to the area, ischemia-reperfusion injury, a type of oxidative stress, occurs. If NCX is dysfunctional, it can exacerbate the increase of calcium that accompanies reperfusion, causing cell death and tissue damage. Similarly, NCX dysfunction has found to be involved in ischemic strokes. Its activity is upregulated, causing an increased cytosolic calcium level, which can lead to neuronal cell death.

The Na+/Ca2+ exchanger has also been implicated in neurological disorders such as Alzheimer's disease and Parkinson's disease. Its dysfunction can result in oxidative stress and neuronal cell death, contributing to the cognitive decline that characterizes Alzheimer's disease. The dysregulation of calcium homeostasis has been found to be a key part of neuron death and Alzheimer's pathogenesis. For example, neurons that have neurofibrillary tangles contain high levels of calcium and show hyperactivation of calcium-dependent proteins. The abnormal calcium handling of atypical NCX function can also cause the mitochondrial dysfunction, oxidative stress, and neuronal cell death that characterize Parkinson's. In this case, if dopaminergic neurons of the substantia nigra are affected, it can contribute to the onset and development of Parkinson's disease. Although the mechanism is not entirely understood, disease models have shown a link between NCX and Parkinson's and that NCX inhibitors can prevent death of dopaminergic neurons.

Sodium-hydrogen antiporter

The sodium–hydrogen antiporter, also known as the sodium-proton exchanger, Na+/H+ exchanger, or NHE, is an antiporter responsible for transporting sodium into the cell and hydrogen out of the cell. As such, it is important in the regulation of cellular pH and sodium levels. There are differences among the types of NHE antiporter families present in eukaryotes and prokaryotes. The 9 isoforms of this transporter that are found in the human genome fall under several families, including the cation-proton antiporters (CPA 1, CPA 2, and CPA 3) and sodium-transporting carboxylic acid decarboxylase (NaT-DC). Prokaryotic organisms contain the Na+/H+ antiporter families NhaA, NhaB, NhaC, NhaD, and NhaE.

Because enzymes can only function at certain pH ranges, it is critical for cells to tightly regulate cytosolic pH. When a cell's pH is outside of the optimal range, the sodium-hydrogen antiporter detects this and is activated to transport ions as a homeostatic mechanism to restore pH balance. Since ion flux can be reversed in mammalian cells, NHE can also be used to transport sodium out of the cell to prevent excess sodium from accumulating and causing toxicity.

As suggested by its functions, this antiporter is located in the kidney for sodium reabsorption regulation and in the heart for intracellular pH and contractility regulation. NHE plays an important role in the nephron of the kidney, especially in the cells of the proximal convoluted tubule and collecting duct. The sodium-hydrogen antiporter's function is upregulated by Angiotensin II in the proximal convoluted tubule when the body needs to reabsorb sodium and excrete hydrogen.

Plants are sensitive to high amounts of salt, which can halt certain necessary functions of the eukaryotic organism, including photosynthesis. One of the isoforms of the antiporter, NHE1, is essential to the function of the mammalian myocardium. NHE is involved in the case of hypertrophy and when damage to the heart muscle occurs, such as during ischemia and reperfusion. Studies have shown that NHE1 is more active in animal models experiencing myocardial infarction and left ventricular hypertrophy. For example, Christianson Syndrome (CS) is an X-linked disorder caused by a loss-of-function mutation in NHE6, which leads to the over acidification of endosomes. In studies done on postmortem brains of individuals with CS, lower NHE6 function was linked to higher levels of tau deposition. The level of tau phosphorylation was also found to be elevated, which leads to the formation of insoluble tangles that can cause neuronal damage and death. In the cardiac Purkinje fibers and smooth muscle cells of the ureters, this antiporter is the main mechanism of chloride transport into the cells. Epithelial cells such as those of the kidney use chloride-bicarbonate exchange to regulate their volume, intracellular pH, and extracellular pH. Gastric parietal cells, osteoclasts, and other acid-secreting cells have chloride-bicarbonate antiporters that function in the basolateral membrane to dispose of excess bicarbonate left behind by the function of carbonic anhydrase and apical proton pumps. However, base-secreting cells exhibit apical chloride-bicarbonate exchange and basolateral proton pumps. Protein DRA's reuptake of chloride is critical to creating an osmotic gradient that allows the intestine to reabsorb water.

Another well-studied chloride-bicarbonate antiporter is anion exchanger 1 (AE1), which is also known as band 3 anion transport protein or solute carrier family 4 member 1 (SLC4A1). This exchanger is found in red blood cells, where it helps transport bicarbonate and carbon dioxide between the lungs and tissues to maintain acid-base homeostasis.

Because of its importance to the reabsorption of water in the intestine, mutations in protein DRA cause a condition called congenital chloride diarrhea (CCD). This disorder is caused by an autosomal recessive mutation in the DRA gene on chromosome 7. CCD symptoms in newborns are chronic diarrhea with failure to thrive, and the disorder is characterized by diarrhea that causes metabolic alkalosis.

Mutations of kidney AE1 can lead to distal renal tubular acidosis, a disorder characterized by the inability to secrete acid into the urine. This causes metabolic acidosis, where the blood is too acidic. A chronic state of metabolic acidosis can the health of the bones, kidneys, muscles, and cardiovascular system. Finally, overhydrated hereditary stomatocytosis is a rare genetic disorder where red blood cells have an abnormally high volume, leading to changes in hydration status.

The proper function of AE2, an isoform of AE1, is important in gastric secretion, osteoclast differentiation and function, and the synthesis of enamel. The hydrochloric acid secretion at the apical surface of both gastric parietal cells and osteoclasts relies on chloride-bicarbonate exchange in the basolateral surface. Studies found that mice with nonfunctional AE2 did not secrete hydrochloric acid, and it was concluded that the exchanger is necessary for hydrochloric acid loading in parietal cells. The well-known chloride-hydrogen antiporters belong in the CLC family, which have isoforms from CLC-1 to CLC-7, each with a distinct tissue distribution. Their structure involves two CLC proteins coming together to form a homodimer or a heterodimer where both monomers contain an ion translocation pathway. CLC proteins can either be ion channels or anion-proton exchangers, so CLC-1 and CLC-2 are membrane chloride channels, while CLC-3 through CLC-7 are chloride-hydrogen exchangers. Dent's disease itself is one of the causes of Fanconi syndrome, which occurs when the proximal convoluted tubules of the kidney do not perform an adequate level of reabsorption. It causes molecules produced by metabolic pathways, such as amino acids, glucose, and uric acid to be excreted in the urine instead of being reabsorbed. The result is polyuria, dehydration, rickets in children, osteomalacia in adults, acidosis, and hypokalemia.

CLC-7's role in osteoclast function was revealed by studies on knockout mice that developed severe osteopetrosis. These mice were smaller, had shortened long bones, disorganized trabecular structure, a missing medullary cavity, and their teeth did not erupt. This was found to be caused by deletion mutations, missense mutations, and gain-of-function mutations that sped up the gating of CLC-7. CLC-7 is expressed in almost every neuronal cell type, and its loss led to widespread neurodegeneration in mice, especially in the hippocampus. In longer-lived models, the cortex and hippocampus had almost entirely disappeared after 1.5 years. The RFC protein is critical because folates take the form of hydrophilic anions at physiological pH, so they do not diffuse naturally across biological membranes. Folate is essential for processes such as DNA synthesis, repair, and methylation, and without entry into cells, these could not occur.

Because folates are essential for various life-sustaining processes, a deficiency in this molecule can lead to fetal abnormalities, neurological disorders, cardiovascular disease, and cancer. Folates cannot be synthesized in the body, so it must be taken in through diet and moved into cells. Without the RFC protein facilitating this movement, processes such as embryological development and DNA repair cannot occur. In mouse models, inactivating both alleles of the FRC protein gene causes death of the embryo. Even if folate is supplemented during gestation, the mice died within two weeks of birth from the failure of hematopoietic tissues. In terms of cancer, folate deficiency is related to an increased risk, especially that of colorectal cancers. In mouse models with altered RFC protein expression showed increased transcripts of genes related to colon cancer and increased proliferation of colonocytes.

Vesicle neurotransmitter antiporters

Vesicle neurotransmitter antiporters are responsible for packaging neurotransmitters into vesicles in neurons. They utilize the electrochemical gradient of hydrogen protons across the membranes of synaptic vesicles to move neurotransmitters into them. This is essential for the process of synaptic transmission, which requires neurotransmitters to be released into the synapse to bind to receptors on the next neuron.

One of the best characterized of these antiporters is the vesicular monoamine transporter (VMAT). It is responsible for the storage, sorting, and release of neurotransmitters, as well as for protecting them from autoxidation. VMAT's transport functions are dependent on the electrochemical gradient created by a vesicular hydrogen proton-ATPase.

Another important vesicle neurotransmitter antiporter is the vesicular glutamate transporter (VGLUT). This family of proteins includes three isoforms, VGLUT1, VGLUT2, and VGLUT3, that are responsible for packaging glutamate - the most abundant excitatory neurotransmitter in the brain - into synaptic vesicles. These antiporters vary by location. VGLUT1 is found in areas of the brain related to higher cognitive functions, such as the neocortex. VGLUT2 works to regulate basic physiological functions and is expressed in subcortical regions such as the brainstem and hypothalamus. Finally, VGLUT3 can be seen in neurons that also express other neurotransmitters.

VMAT2 has been found to contribute to neurological conditions such as mood disorders and Parkinson's disease. Studies done on an animal model of clinical depression showed that functional alterations of VMAT2 were associated with depression. The nucleus accumbens, pars compacta of the substantia nigra, and ventral tegmental area - all subregions of the brain involved in clinical depression - were found to have lower VMAT2 levels. The likely cause for this is VMAT's relationship with serotonin and norepinephrine, neurotransmitters that are related to depression. VMAT dysfunction may contribute to the altered levels of these neurotransmitters that occur in mood disorders.

Lower expression of VMAT2 was found to correlate with a higher susceptibility to Parkinson's disease and the antiporter's mRNA was found in all cell groups damaged by Parkinson's. This is likely because VMAT2 dysfunction can lead to a decrease in dopamine packaging into vesicles, accounting for the dopamine depletion that characterizes the disease. For this reason, the antiporter has been identified as a protective factor that could be targeted for the prevention of Parkinson's. A study was conducted where the VGLUT1 gene was inactivated in the astrocytes and neurons of an animal model. When the gene was inactivated in astrocytes, there was an 80% loss in the antiporter protein itself and, in turn, a reduction in glutamate uptake. The mice in this condition experienced seizures, lower body mass, and higher mortality rates. The researchers concluded that VGLUT1 function in astrocytes is therefore critical to epilepsy resistance and normal weight gain.

History

Role in homeostasis

Sodium-calcium exchanger

Sodium-hydrogen antiporter

Vesicle neurotransmitter antiporters

See also

References

Further reading

External links