Sodium-coupled monocarboxylate transporter 2 (i.e., SMCT2, also termed SLC5A12) is a plasma membrane transport protein in the solute carrier family. It transports sodium cations (i.e., Na+) in association with the anionic forms (see conjugated base) of certain short-chain fatty acids (i.e., SC-FAs) and other agents through the plasma membrane from the outside to the inside of cells. [1] The only other member of the sodium-coupled monocarboxylate transporter group (sometimes referred to as the SLC5A family [2] ), SMCT1, similarly co-transports SC-FAs and other agents into cells. Monocarboxylate transporters (MCTs) are also transport proteins in the solute carrier family. They co-transport the anionic forms of various compounds into cells in association with hydrogen cations (i.e. H+). [3] Four of the 14 MCTs, i.e. SLC16A1 (i.e., MCT1), SLC16A7 (i.e., MCT22), SLC16A8 (i.e., MCT3), and SLC16A3 (i.e., MCT4), transport some of the same SC-FAs anions that the two SMCTs transport into cells. [2] [4] SC-FAs do diffuse into cells independently of transport proteins but at the levels normally occurring in tissues greater amounts of the SC-FAs are brought into cells that express a SC-FA transporter. [1] [3]
The human gene responsible for producing SMCT2 protein, i.e., the SLC5A12 gene, is located at position 14.2 on the "p" (i.e., short) arm of chromosome 11 (position notated as chromosome 11p14.2). [5] The full length SMCT2 protein product of this gene consist of 618 amino acids [6] and has a 57% identity with the SMCT1 protein at the amino acid level. [7] The gene for SMCT2 in mice and rats is termed the Slc5a12 gene.
Studies indicate that the SMCT2 on intestinal epithelial cells promotes their uptake of intra-intestinal SC-FAs and subsequent diffusion of the SC-FAs into the systemic circulation. These SC-FAs serve as energy sources for [8] and activators of diverse responses in [9] [10] [11] a wide range of cell types in the intestinal wall and throughout the entire body. Studies also suggest that: 1) SMCT2 in the kidney tubule cells contributes to the reabsorption of urinary SC-FAs that would otherwise be wasted in the urine; 2) the SMCT2 on the Müller cells in the eye promotes their uptake SC-FAs which they pass on to retinal neurons that use them as energy sources; [12] and 3) the SCMT2 on skeletal muscle cells may contribute to regulating their lactic acid levels. [13]
The SMCT2 protein and/or its messenger RNA is expressed by the human and/or murine: epithelial cells in the proximal (i.e., initial and middle portions) of the small intestine; epithelial cells in the renal tubules of the kidney; [7] Müeller cells in the retina, [12] and skeletal muscles throughout the body. [13]
The SC-FAs transported into cells by SMCT2 include butyric, proprionic, pyruvic, lactic, acetic, and β-hydroxybutyric acids. SMCT2 also transports into cells the anionic forms of nicotinic acid [1] [7] [13] and gamma-hydroxybutyric acid. [2] Nonsteroidal anti-inflammatory drugs such as ibuprofen, ketoprofen, and fenoprofen bind to but are not transported by SMCT2. However, their binding blocks the binding and thereby transportation of the anionic SC-FAs and, presumably, the other anionic compounds that SMTC2 normally transports into cells. SMCT1 likewise transports these SC-FAs and nicotinic acid and is blocked by the cited anti-inflammatory drugs. [8] SMCT2 does have much lower affinities than SMCT1 for binding the SC-FAs and therefore is better suited to transport larger amounts of SC-FAs when their concentrations are high. [1] [13]
SC-FAs inside the gastrointestinal tract come from the ingested food or are released by intestinal bacteria as fermentation products of the ingested food. SMCT2-bearing cells are located in the epithelial cells of the proximal portion (primarily the jejunum [14] ) of the small intestine but are not in the large intestine or cecum. Cells bearing the SMCT2 transporter, which has relatively low affinity for the SC-FAs, are suited to transport the high levels of dietary SC-FAs that are usually found in the proximal small intestine. Cells bearing the SMCT1 transporter, which has a relatively high affinity for the SC-FAs, are located in the large intestine and cecum. These cells are suited to take up the relatively low concentrations of the SC-FAs that are present in these sites. [1] [7] [14] The SC-FAs in the gastrointestinal tracts diffuse into the intestinal wall and transported by SMCT2 (and other intestinal SC-FA transporters [15] ) serve as energy sources for cells located in the intestinal wall and throughout the body. [8] They also stimulate various functions in cells of the intestine and throughout the body that express one of the SC-FA receptors, i.e., free fatty acid receptor 2, free fatty acid receptor 3, or hydroxycarboxylic acid receptor 2. (For the functions elicited by SC-FA's activation of these receptors see free fatty acid receptor 2 functions, free fatty acid receptor 3 functions, and hydroxycarboxylic acid receptor 2 functions.) [10] [11] [16] In addition, the SC-FAs that enter cells can directly activate certain signal transduction pathways and thereby elicit cellular responses independently of the three cited SC-FA receptors. [9]
Studies in mice indicate that SMCT2 is located in the brush borders of the cells lining the kidney's proximal tubules with decreasing numbers of SMTC2-exressing cells in proximal tubule segments S1, S2, and S3. [8] In contrast, SMCT1 is mostly limited to the S3, i.e., most distal, segment of the proximal tubules. [5] [13] Thus, the kidney's proximal tubules consist of SMCT1-bearing cells that transfer the higher levels of SC-FAs found in the proximal tubules while the SMCT1-bearing cells are active in transferring the lower levels of SC-FAs in more distal part of the proximal tubules. [5] Other studies have shown that 1) mice lacking SMCT1 due to the knockout of their Slc8a5 gene had massive increases in the levels of lactic acid in their urine [17] and 2)C/ebpδ gene knockout mice (these mice do not express SCMT1 or SCMT2 in their kidney tissues) likewise have a marked increase in urinary excretion of lactic and also show decreases in their lactic acid blood levels. [18] These findings indicate that, in mice, SMCT1 is involved in the reabsorption of urine SC-FAs and that SMCT2 may also do so but studies on SLC5A12 gene knockout mice are needed to prove this. [17]
Müller cells (also termed Müller glia) are critical non-neural components of the retina. They form and maintain the retina's structure, control retinal immune responses by, e.g., releasing inflammatory mediators and engulfing dead cell debris, and provide retinal neurons with essential nutrients, particularly the SC-FA, pyruvic acid. [19] SMCT2 has been detected in the Müller cells of mouse retinas and, based on indirect studies, the human rMC-1 Müeller cell line. These studies suggest that the function of SMCT2 in the retinal Müller cells of mice and humans is to take up SC-FAs and transfer of them to retinal neurons for their use as energy sources, particularly at times when other energy sources are less available. [12]
Skeletal muscles accumulate lactic acid during their contractions. This lactic acid, particularly if in excess, moves out of the skeletal muscles and either diffuses into the systemic circulation or is transferred into skeletal muscles that do not have an excessive buildup of lactic acid. The transfer of lactic acid out and into human skeletal muscle has been thought to be mediated by the H+-coupled monocarboxylate transporters, MCT1 and MCT4. [20] However, a more recent study detected SMCT2 and SMCT1 in the skeletal muscles of mice and suggested that they may contribute to the transport of SC-FAs in skeletal muscles in mice. [13] Further studies are needed to determine if SCMT2 and/or SMCT1 are expressed by human skeletal muscle and contribute to the transport of lactic acid in mouse and/or human skeletal muscle.
α-Ketoglutaric acid is a dicarboxylic acid, i.e., a short-chain fatty acid containing two carboxyl groups with C, O, and H standing for carbon, oxygen, and hydrogen, respectively. However, almost all animal tissues and extracellular fluids have a pH above 7. At these basic pH levels α-ketoglutaric acid exists almost exclusively as its conjugate base. That is, it has two negative electric charges due to its release of positively charged hydrogen from both of its now negatively charged carboxy groups, CO−2. This double negatively charge molecule is referred to as α-ketoglutarate or 2-oxoglutarate.
Butyric acid, also known under the systematic name butanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula CH3CH2CH2CO2H. It is an oily, colorless liquid with an unpleasant odor. Isobutyric acid is an isomer. Salts and esters of butyric acid are known as butyrates or butanoates. The acid does not occur widely in nature, but its esters are widespread. It is a common industrial chemical and an important component in the mammalian gut.
Ketogenesis is the biochemical process through which organisms produce ketone bodies by breaking down fatty acids and ketogenic amino acids. The process supplies energy to certain organs, particularly the brain, heart and skeletal muscle, under specific scenarios including fasting, caloric restriction, sleep, or others.
An antiporter is an integral membrane protein involved in secondary active transport. It is a type of cotransporter, which means that uses the movement of one In the case of an antiporter, two or more different molecules or ions are moved across a phospholipid membrane, such as the plasma membrane, in opposite directions, one into the cell and one out of the cell. This is in contrast to symporters, which are another type of cotransporter that moves two or more ions in the same direction.
Sodium-dependent glucose cotransporters are a family of glucose transporter found in the intestinal mucosa (enterocytes) of the small intestine (SGLT1) and the proximal tubule of the nephron. They contribute to renal glucose reabsorption. In the kidneys, 100% of the filtered glucose in the glomerulus has to be reabsorbed along the nephron. If the plasma glucose concentration is too high (hyperglycemia), glucose passes into the urine (glucosuria) because SGLT are saturated with the filtered glucose.
Sodium/glucose cotransporter 1 (SGLT1) also known as solute carrier family 5 member 1 is a protein in humans that is encoded by the SLC5A1 gene which encodes the production of the SGLT1 protein to line the absorptive cells in the small intestine and the epithelial cells of the kidney tubules of the nephron for the purpose of glucose uptake into cells. Recently, it has been seen to have functions that can be considered as promising therapeutic target to treat diabetes and obesity. Through the use of the sodium glucose cotransporter 1 protein, cells are able to obtain glucose which is further utilized to make and store energy for the cell.
Free fatty acid receptor 3 protein is a G protein coupled receptor that in humans is encoded by the FFAR3 gene. GPRs reside on cell surfaces, bind specific signaling molecules, and thereby are activated to trigger certain functional responses in their parent cells. FFAR3 is a member of the free fatty acid receptor group of GPRs that includes FFAR1, FFAR2, and FFAR4. All of these FFARs are activated by fatty acids. FFAR3 and FFAR2 are activated by certain short-chain fatty acids (SC-FAs), i.e., fatty acids consisting of 2 to 6 carbon atoms whereas FFFAR1 and FFAR4 are activated by certain fatty acids that are 6 to more than 21 carbon atoms long. Hydroxycarboxylic acid receptor 2 is also activated by a SC-FA that activate FFAR3, i.e., butyric acid.
Free fatty acid receptor 2 (FFAR2), also termed G-protein coupled receptor 43 (GPR43), is a rhodopsin-like G-protein coupled receptor. It is coded by the FFAR2 gene. In humans, the FFAR2 gene is located on the long arm of chromosome 19 at position 13.12. Like other GPCRs, FFAR2s reside on the surface membrane of cells and when bond to one of their activating ligands regulate the function of their parent cells. FFAR2 is a member of a small family of structurally and functionally related GPRs termed free fatty acid receptors (FFARs). This family includes three other receptors which, like FFAR2, are activated by certain fatty acids: FFAR1, FFAR3 (GPR41), and FFAR4 (GPR120). FFAR2 and FFAR3 are activated by short-chain fatty acids whereas FFAR1 and FFAR4 are activated by long-chain fatty acids.
Succinate receptor 1 (SUCNR1), previously named G protein-coupled receptor 91 (GPR91), is a receptor that is activated by succinate, i.e., the anionic form of the dicarboxylic acid, succinic acid. Succinate and succinic acid readily convert into each other by gaining (succinate) or losing (succinic acid) protons, i.e., H+ (see Ions). Succinate is by far the predominant form of this interconversion in living organisms. Succinate is one of the intermediate metabolites in the citric acid cycle (also termed the TCA cycle or tricarboxylic acid cycle). This cycle is a metabolic pathway that operates in the mitochondria of virtually all eucaryotic cells. It consists of a series of biochemical reactions that serve the vital function of releasing the energy stored in nutrient carbohydrates, fats, and proteins. Recent studies have found that some of the metabolites in this cycle are able to regulate various physiological and pathological functions in a wide range of cell types. The succinyl CoA in this cycle may release its bound succinate; succinate is one of these mitochondrial-formed bioactive metabolites.
Monocarboxylate transporter 5 is a protein that in humans is encoded by the SLC16A4 gene.
Monocarboxylate transporter 4 (MCT4) also known as solute carrier family 16 member 3 is a protein that in humans is encoded by the SLC16A3 gene.
Sodium-coupled monocarboxylate transporter 1 (i.e., SMCT1) and sodium-coupled monocarboxylate transporter 2 (i.e., SMCT2) are plasma membrane transport proteins in the solute carrier family. They transport sodium cations in association with the anionic forms (see conjugated base) of certain short-chain fatty acids (i.e., SC-FAs) through the plasma membrane from the outside to the inside of cells. For example, propionic acid (i.e., CH
3CH
2CO
2H) in its anionic "propionate" form (i.e., CH
3CH
2CO−
2) along with sodium cations (i.e., Na+) are co-transported from the extracellular fluid into a SMCT1-epxressing cell's cytoplasm. Monocarboxylate transporters (MCTs) are also transport proteins in the solute carrier family. They co-transport the anionic forms of various compounds into cells in association with proton cations (i.e. H+). Four of the 14 MCTs, i.e. SLC16A1 (i.e., MCT1), SLC16A7 (i.e., MCT22), SLC16A8 (i.e., MCT3), and SLC16A3 (i.e., MCT4), transport some of the same SC-FAs anions that the SMCTs transport into cells. SC-FAs do diffuse into cells independently of transport proteins but at the levels normally occurring in tissues far greater amounts of the SC-FAs are brought into cells that express a SC-FA transporter.
Monocarboxylate transporter 1 is a ubiquitous protein that in humans is encoded by the SLC16A1 gene. It is a proton coupled monocarboxylate transporter.
Iminoglycinuria is an autosomal recessive disorder of renal tubular transport affecting reabsorption of the amino acid glycine, and the imino acids proline and hydroxyproline. This results in excess urinary excretion of all three acids.
The monocarboxylate transporters, or MCTs, are a family of proton-linked plasma membrane transporters that carry molecules having one carboxylate group (monocarboxylates), such as lactate, pyruvate, and ketones across biological membranes. Acetate is actively transported to intestinal enteroendocrine cells via MCT, termed Targ. MCTs are expressed in nearly every kind of cell.
The organic anion transporter 1 (OAT1) also known as solute carrier family 22 member 6 (SLC22A6) is a protein that in humans is encoded by the SLC22A6 gene. It is a member of the organic anion transporter (OAT) family of proteins. OAT1 is a transmembrane protein that is expressed in the brain, the placenta, the eyes, smooth muscles, and the basolateral membrane of proximal tubular cells of the kidneys. It plays a central role in renal organic anion transport. Along with OAT3, OAT1 mediates the uptake of a wide range of relatively small and hydrophilic organic anions from plasma into the cytoplasm of the proximal tubular cells of the kidneys. From there, these substrates are transported into the lumen of the nephrons of the kidneys for excretion. OAT1 homologs have been identified in rats, mice, rabbits, pigs, flounders, and nematodes.
Monocarboxylate transporter 10, also known as aromatic amino acid transporter 1 and T-type amino acid transporter 1 (TAT1) and solute carrier family 16 member 10 (SLC16A10), is a protein that in humans is encoded by the SLC16A10 gene. SLC16A10 is a member of the solute carrier family.
The lactate shuttle hypothesis describes the movement of lactate intracellularly and intercellularly. The hypothesis is based on the observation that lactate is formed and utilized continuously in diverse cells under both anaerobic and aerobic conditions. Further, lactate produced at sites with high rates of glycolysis and glycogenolysis can be shuttled to adjacent or remote sites including heart or skeletal muscles where the lactate can be used as a gluconeogenic precursor or substrate for oxidation. The hypothesis was proposed 1985 by George Brooks of the University of California at Berkeley.
Dicarboxylic aminoaciduria is a rare form of aminoaciduria which is an autosomal recessive disorder of urinary glutamate and aspartate due to genetic errors related to transport of these amino acids. Mutations resulting in a lack of expression of the SLC1A1 gene, a member of the solute carrier family, are found to cause development of dicarboxylic aminoaciduria in humans. SLC1A1 encodes for EAAT3 which is found in the neurons, intestine, kidney, lung, and heart. EAAT3 is part of a family of high affinity glutamate transporters which transport both glutamate and aspartate across the plasma membrane.
The proton-coupled folate transporter is a protein that in humans is encoded by the SLC46A1 gene. The major physiological roles of PCFTs are in mediating the intestinal absorption of folate, and its delivery to the central nervous system.