solute carrier family 5 (sodium/glucose cotransporter), member 1 | |||||||
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Identifiers | |||||||
Symbol | SLC5A1 | ||||||
Alt. symbols | SGLT1 | ||||||
NCBI gene | 6523 | ||||||
HGNC | 11036 | ||||||
OMIM | 182380 | ||||||
RefSeq | NM_000343 | ||||||
UniProt | P13866 | ||||||
Other data | |||||||
Locus | Chr. 22 q13.1 | ||||||
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solute carrier family 5 (sodium/glucose cotransporter), member 2 | |||||||
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Identifiers | |||||||
Symbol | SLC5A2 | ||||||
Alt. symbols | SGLT2 | ||||||
NCBI gene | 6524 | ||||||
HGNC | 11037 | ||||||
OMIM | 182381 | ||||||
RefSeq | NM_003041 | ||||||
UniProt | P31639 | ||||||
Other data | |||||||
Locus | Chr. 16 p11.2 | ||||||
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solute carrier family 5 (low affinity glucose cotransporter), member four | |||||||
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Identifiers | |||||||
Symbol | SLC5A4 | ||||||
Alt. symbols | SGLT3, SAAT1, DJ90G24.4 | ||||||
NCBI gene | 6527 | ||||||
HGNC | 11039 | ||||||
RefSeq | NM_014227 | ||||||
UniProt | Q9NY91 | ||||||
Other data | |||||||
Locus | Chr. 22 q12.1-12.3 | ||||||
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Sodium-dependent glucose cotransporters (or sodium-glucose linked transporter, SGLT) are a family of glucose transporter found in the intestinal mucosa (enterocytes) of the small intestine (SGLT1) and the proximal tubule of the nephron (SGLT2 in PCT and SGLT1 in PST). They contribute to renal glucose reabsorption. In the kidneys, 100% of the filtered glucose in the glomerulus has to be reabsorbed along the nephron (98% in PCT, via SGLT2). If the plasma glucose concentration is too high (hyperglycemia), glucose passes into the urine (glucosuria) because SGLT are saturated with the filtered glucose.
The two most well known members of SGLT family are SGLT1 and SGLT2, which are members of the SLC5A gene family. In addition to SGLT1 and SGLT2, there are 10 other members in the human protein family SLC5A. [1] Of these, SLC5A4/SGLT3 (SAAT1) is a low-affinity transporter for glucose, but seems to have more of an electric function. [2]
Gene | Protein | Acronym | Tissue distribution in proximal tubule [3] | Na+:Glucose Co-transport ratio | Contribution to glucose reabsorption (%) [4] |
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SLC5A1 | Sodium/GLucose coTransporter 1 | SGLT1 | S3 segment | 2:1 | 10 |
SLC5A2 | Sodium/GLucose coTransporter 2 | SGLT2 | predominantly in the S1 and S2 segments | 1:1 | 90 |
The other SLC5 proteins transport mannose, myo-inositol, choline, iodide, vitamins, and short-chain fatty acids. [2]
SGLT2 inhibitors, also called gliflozins, [5] are used in the treatment of type 2 diabetes. SGLT2 is only found in kidney tubules and in conjunction with SGLT1 resorbs glucose into the blood from the forming urine. By inhibiting SGLT2, and not targeting SGLT1, glucose is excreted which in turn lowers blood glucose levels. Examples include dapagliflozin (Farxiga in US, Forxiga in EU), canagliflozin (Invokana) and empagliflozin (Jardiance). Certain SGLT2 inhibitors have shown to reduce mortality in type 2 diabetes. [6] The safety and efficacy of SGLT2 inhibitors have not been established in patients with type 1 diabetes, and FDA has not approved them for use in these patients. [7]
Firstly, an Na+/K+ ATPase on the basolateral membrane of the proximal tubule cell uses ATP molecules to move 3 sodium ions outward into the blood, while bringing in 2 potassium ions. This action creates a downhill sodium ion gradient from the outside to the inside of the proximal tubule cell (that is, in comparison to both the blood and the tubule itself).
The SGLT proteins use the energy from this downhill sodium ion gradient created by the ATPase pump to transport glucose across the apical membrane, against an uphill glucose gradient. These co-transporters are an example of secondary active transport. Members of the GLUT family of glucose uniporters then transport the glucose across the basolateral membrane, and into the peritubular capillaries. Because sodium and glucose are moved in the same direction across the membrane, SGLT1 and SGLT2 are known as symporters. Of course, sodium can deplete, so Sodium–hydrogen antiporter gets sodium into the cell to begin with. Therefore, glucose actually moved with net protons being pushed out of cell, sodium being the intermediate.
In August 1960, in Prague, Robert K. Crane presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption. [8]
Crane's discovery of cotransport was the first-ever proposal of flux coupling in biology. [9] [10]
In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellular energy to achieve this movement. There are two types of active transport: primary active transport that uses adenosine triphosphate (ATP), and secondary active transport that uses an electrochemical gradient. This process is in contrast to passive transport, which allows molecules or ions to move down their concentration gradient, from an area of high concentration to an area of low concentration, without energy.
The distal convoluted tubule (DCT) is a portion of kidney nephron between the loop of Henle and the collecting tubule.
Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, and other electrolytes; clearance of toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.
The proximal tubule is the segment of the nephron in kidneys which begins from the renal pole of the Bowman's capsule to the beginning of loop of Henle. At this location, the glomerular parietal epithelial cells (PECs) lining bowman’s capsule abruptly transition to proximal tubule epithelial cells (PTECs). The proximal tubule can be further classified into the proximal convoluted tubule (PCT) and the proximal straight tubule (PST).
Cotransporters are a subcategory of membrane transport proteins (transporters) that couple the favorable movement of one molecule with its concentration gradient and unfavorable movement of another molecule against its concentration gradient. They enable coupled or cotransport and include antiporters and symporters. In general, cotransporters consist of two out of the three classes of integral membrane proteins known as transporters that move molecules and ions across biomembranes. Uniporters are also transporters but move only one type of molecule down its concentration gradient and are not classified as cotransporters.
Glucose transporters are a wide group of membrane proteins that facilitate the transport of glucose across the plasma membrane, a process known as facilitated diffusion. Because glucose is a vital source of energy for all life, these transporters are present in all phyla. The GLUT or SLC2A family are a protein family that is found in most mammalian cells. 14 GLUTS are encoded by the human genome. GLUT is a type of uniporter transporter protein.
Method of glucose uptake differs throughout tissues depending on two factors; the metabolic needs of the tissue and availability of glucose. The two ways in which glucose uptake can take place are facilitated diffusion and secondary active transport. Active transport is the movement of ions or molecules going against the concentration gradient.
The solute carrier (SLC) group of membrane transport proteins include over 400 members organized into 66 families. Most members of the SLC group are located in the cell membrane. The SLC gene nomenclature system was originally proposed by the HUGO Gene Nomenclature Committee (HGNC) and is the basis for the official HGNC names of the genes that encode these transporters. A more general transmembrane transporter classification can be found in TCDB database.
The Na–K–Cl cotransporter (NKCC) is a transport protein that aids in the secondary active transport of sodium, potassium, and chloride into cells. In humans there are two isoforms of this membrane transport protein, NKCC1 and NKCC2, encoded by two different genes. Two isoforms of the NKCC1/Slc12a2 gene result from keeping or skipping exon 21 in the final gene product.
A symporter is an integral membrane protein that is involved in the transport of two different molecules across the cell membrane in the same direction. The symporter works in the plasma membrane and molecules are transported across the cell membrane at the same time, and is, therefore, a type of cotransporter. The transporter is called a symporter, because the molecules will travel in the same direction in relation to each other. This is in contrast to the antiport transporter. Typically, the ion(s) will move down the electrochemical gradient, allowing the other molecule(s) to move against the concentration gradient. The movement of the ion(s) across the membrane is facilitated diffusion, and is coupled with the active transport of the molecule(s). In symport, two molecule move in a 'similar direction' at the 'same time'. For example, the movement of glucose along with sodium ions. It exploits the uphill movement of other molecules from low to high concentration, which is against the electrochemical gradient for the transport of solute molecules downhill from higher to lower concentration.
Renal glycosuria is a rare condition in which the simple sugar glucose is excreted in the urine despite normal or low blood glucose levels. With normal kidney (renal) function, glucose is excreted in the urine only when there are abnormally elevated levels of glucose in the blood. However, in those with renal glycosuria, glucose is abnormally elevated in the urine due to improper functioning of the renal tubules, which are primary components of nephrons, the filtering units of the kidneys.
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.
The sodium/glucose cotransporter 2 (SGLT2) is a protein that in humans is encoded by the SLC5A2 gene.
Electrogenic sodium bicarbonate cotransporter 1, sodium bicarbonate cotransporter is a membrane transport protein that in humans is encoded by the SLC4A4 gene.
Renal glucose reabsorption is the part of kidney (renal) physiology that deals with the retrieval of filtered glucose, preventing it from disappearing from the body through the urine.
Robert Kellogg Crane was an American biochemist best known for his discovery of sodium–glucose cotransport.
Empagliflozin, sold under the brand name Jardiance, among others, is an antidiabetic medication used to improve glucose control in people with type 2 diabetes. It is not recommended for type 1 diabetes. It is taken by mouth.
Gliflozins are a class of drugs in the treatment of type 2 diabetes (T2D). They act by inhibiting sodium/glucose cotransporter 2 (SGLT-2), and are therefore also called SGLT-2 inhibitors. The efficacy of the drug is dependent on renal excretion and prevents glucose from going into blood circulation by promoting glucosuria. The mechanism of action is insulin independent.
SGLT2 inhibitors, also called gliflozins or flozins, are a class of medications that inhibit sodium-glucose transport proteins in the nephron, unlike SGLT1 inhibitors that perform a similar function in the intestinal mucosa. The foremost metabolic effect of this is to inhibit reabsorption of glucose in the kidney and therefore lower blood sugar. They act by inhibiting sodium-glucose transport protein 2 (SGLT2). SGLT2 inhibitors are used in the treatment of type 2 diabetes. Apart from blood sugar control, gliflozins have been shown to provide significant cardiovascular benefit in people with type 2 diabetes. As of 2014, several medications of this class had been approved or were under development. In studies on canagliflozin, a member of this class, the medication was found to enhance blood sugar control as well as reduce body weight and systolic and diastolic blood pressure.
Sodium-dependent phosphate transport protein 2A, also known as Na+-Pi cotransporter 2a (NaPi-2a), is a protein in humans that is encoded by the SLC34A1 gene. This gene encodes a member of the type II sodium-phosphate cotransporter family.
Crane in 1961 was the first to formulate the cotransport concept to explain active transport [7]. Specifically, he proposed that the accumulation of glucose in the intestinal epithelium across the brush border membrane was [is] coupled to downhill Na+ transport cross the brush border. This hypothesis was rapidly tested, refined, and extended [to] encompass the active transport of a diverse range of molecules and ions into virtually every cell type.
p. 304. "the insight from this time that remains in all current text books is the notion of Robert Crane published originally as an appendix to a symposium paper published in 1960 (Crane et al. 1960). The key point here was 'flux coupling', the cotransport of sodium and glucose in the apical membrane of the small intestinal epithelial cell. Half a century later this idea has turned into one of the most studied of all transporter proteins (SGLT1), the sodium–glucose cotransporter.