HAT medium

Last updated September 01, 2021

HAT Medium ( hypoxanthine-aminopterin-thymidine medium) is a selection medium for mammalian cell culture, which relies on the combination of aminopterin, a drug that acts as a powerful folate metabolism inhibitor by inhibiting dihydrofolate reductase, with hypoxanthine (a purine derivative) and thymidine (a deoxynucleoside) which are intermediates in DNA synthesis. The trick is that aminopterin blocks DNA de novo synthesis, which is absolutely required for cell division to proceed, but hypoxanthine and thymidine provide cells with the raw material to evade the blockage (the "salvage pathway"), provided that they have the right enzymes, which means having functioning copies of the genes that encode them.

The enzyme dihydrofolate reductase, which produces tetrahydrofolate (THF) by the reduction of dihydrofolate, is specifically blocked by aminopterin. THF, acting in association with specific proteins, can receive single carbon units that are then transferred to specific targets.

One of the important targets for cellular reproduction is thymidylate synthase, which creates thymidine monophosphate (TMP) from deoxyuridine monophosphate (dUMP). By additional phosphorylation reactions, TMP can be used to make thymidine triphosphate (TTP), one of the four nucleotide precursors that are used by DNA polymerases to create DNA. Without the THF required to convert dUMP, there can be no TTP, and DNA synthesis cannot proceed, unless TMP can be produced from another source. The alternative source is the thymidine present in the HAT medium that can be absorbed by the cells and phosphorylated by thymidine kinase (TK) into TMP.

The synthesis of IMP, (precursor to GMP and GTP, and to AMP and ATP) also requires THF, and also can be bypassed. In this case hypoxanthine-guanine phosphoribosyltransferase (HGPRT) reacts hypoxanthine absorbed from the medium with PRPP, liberating pyrophosphate, to produce IMP by a salvage pathway.

Therefore, the use of HAT medium for cell culture is a form of artificial selection for cells containing working TK and HGPRT. Many useful refinements to the scheme are made possible by poisons that kill cells, but to which they are immune if they lack one of these genes. Thus, a cell lacking TK is resistant to bromodeoxyuridine (BrdU) and a cell lacking HGPRT is resistant to 6-thioguanine (6-TG) and 8-azaguanine. Thus, selection with one of the latter two drugs, followed by HAT medium, will yield revertant colonies.^[1]

Applications

HAT medium is used for preparation of monoclonal antibodies. This process is called hybridoma technology. Laboratory animals (e.g., mice) are first exposed to an antigen against which we are interested in isolating an antibody. Once splenocytes are isolated from the mammal, the B cells are fused with HGPRT negative, immortalized myeloma cells using polyethylene glycol or the Sendai virus. Fused cells are incubated in the HAT medium. Aminopterin in the medium blocks the de novo pathway. Hence, unfused myeloma cells die, as they cannot produce nucleotides by the de novo or salvage pathway. Unfused B cells die as they have a short lifespan. In this way, only the B cell-myeloma hybrids survive. These cells produce antibodies (a property of B cells) and are immortal (a property of myeloma cells). The incubated medium is then diluted into multiwell plates to such an extent that each well contains only 1 cell. Then the supernatant in each well can be checked for the desired antibody. Since the antibodies in a well are produced by the same B cell, they will be directed towards the same epitope, and are known as monoclonal antibodies.

The production of monoclonal antibodies was first invented by César Milstein and Georges J. F. Köhler, which earned them the 1984 Nobel Prize in Physiology or Medicine, shared with Niels Kaj Jerne.

Related Research Articles

Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules within all life-forms on Earth. Nucleotides are obtained in the diet and are also synthesized from common nutrients by the liver.

Thymidine is a pyrimidine deoxynucleoside. Deoxythymidine is the DNA nucleoside T, which pairs with deoxyadenosine (A) in double-stranded DNA. In cell biology it is used to synchronize the cells in G1/early S phase. The prefix deoxy- is often left out since there are no precursors of thymine nucleotides involved in RNA synthesis.

Monoclonal antibody Monospecific antibody that is made by identical immune cells that are all clones of a unique parent cell

A monoclonal antibody is an antibody made by cloning a unique white blood cell. All subsequent antibodies derived this way trace back to a unique parent cell.

In biochemistry, a ribonucleotide is a nucleotide containing ribose as its pentose component. It is considered a molecular precursor of nucleic acids. Nucleotides are the basic building blocks of DNA and RNA. The monomer itself from ribonucleotides forms the basic building blocks for RNA. However, the reduction of ribonucleotide, by enzyme ribonucleotide reductase (RNR), forms deoxyribonucleotide, which is the essential building block for DNA. There are several differences between DNA deoxyribonucleotides and RNA ribonucleotides. Successive nucleotides are linked together via phosphodiester bonds by 3'-5'.

A salvage pathway is a pathway in which a biological product is produced from intermediates in the degradative pathway of its own or a similar substance. The term often refers to nucleotide salvage in particular, in which nucleotides are synthesized from intermediates in their degradative pathway.

A nucleotide triphosphate is a molecule containing a nitrogenous base bound to a 5-carbon sugar, with three phosphate groups bound to the sugar. It is an example of a nucleotide. They are the molecular precursors of both DNA and RNA, which are chains of nucleotides made through the processes of DNA replication and transcription. Nucleoside triphosphates also serve as a source of energy for cellular reactions and are involved in signalling pathways.

Hypoxanthine-guanine phosphoribosyltransferase Enzyme that converts hypoxanthine to inosine monophosphate

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is an enzyme encoded in humans by the HPRT1 gene.

Hybridoma technology is a method for producing large numbers of identical antibodies. This process starts by injecting a mouse with an antigen that provokes an immune response. A type of white blood cell, the B cell, produces antibodies that bind to the injected antigen. These antibody producing B-cells are then harvested from the mouse and, in turn, fused with immortal B cell cancer cells, a myeloma, to produce a hybrid cell line called a hybridoma, which has both the antibody-producing ability of the B-cell and the longevity and reproductivity of the myeloma. The hybridomas can be grown in culture, each culture starting with one viable hybridoma cell, producing cultures each of which consists of genetically identical hybridomas which produce one antibody per culture (monoclonal) rather than mixtures of different antibodies (polyclonal). The myeloma cell line that is used in this process is selected for its ability to grow in tissue culture and for an absence of antibody synthesis. In contrast to polyclonal antibodies, which are mixtures of many different antibody molecules, the monoclonal antibodies produced by each hybridoma line are all chemically identical.

Thymidine kinase is an enzyme, a phosphotransferase : 2'-deoxythymidine kinase, ATP-thymidine 5'-phosphotransferase, EC 2.7.1.21. It can be found in most living cells. It is present in two forms in mammalian cells, TK1 and TK2. Certain viruses also have genetic information for expression of viral thymidine kinases. Thymidine kinase catalyzes the reaction:

Purine nucleoside phosphorylase, PNP, PNPase or inosine phosphorylase is an enzyme that in humans is encoded by the NP gene. It catalyzes the chemical reaction

Nucleic acid metabolism is the process by which nucleic acids are synthesized and degraded. Nucleic acids are the polymers of nucleotides. Nucleotide synthesis is an anabolic mechanism generally involving the chemical reaction of phosphate, pentose sugar, and a nitrogenous base. Destruction of nucleic acid is a catabolic reaction. Additionally, parts of the nucleotides or nucleobases can be salvaged to recreate new nucleotides. Both synthesis and degradation reactions require enzymes to facilitate the event. Defects or deficiencies in these enzymes can lead to a variety of diseases.

Thymidylate synthase (TS) is an enzyme that catalyzes the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP). Thymidine is one of the nucleotides in DNA. With inhibition of TS, an imbalance of deoxynucleotides and increased levels of dUMP arise. Both cause DNA damage.

Purine metabolism refers to the metabolic pathways to synthesize and break down purines that are present in many organisms.

5,10-Methylenetetrahydrofolate (N5,N10-Methylenetetrahydrofolate; 5,10-CH₂-THF) is cofactor in several biochemical reactions. It exists in nature as the diastereoisomer [6R]-5,10-methylene-THF.

Deoxyuridine monophosphate (dUMP), also known as deoxyuridylic acid or deoxyuridylate in its conjugate acid and conjugate base forms, respectively, is a deoxynucleotide.

Somatic fusion, also called protoplast fusion, is a type of genetic modification in plants by which two distinct species of plants are fused together to form a new hybrid plant with the characteristics of both, a somatic hybrid. Hybrids have been produced either between different varieties of the same species or between two different species.

Thymidylate kinase catalyzes the phosphorylation of thymidine 5'-monophosphate (dTMP) to form thymidine 5'-diphosphate (dTDP) in the presence of ATP and magnesium:

Trifluridine/tipiracil, sold under the brand name Lonsurf, is a fixed-dose combination medication that is used as a third- or fourth-line treatment of metastatic colorectal cancer or gastric cancer, after chemotherapy and targeted therapeutics have failed. It is a combination of two active pharmaceutical ingredients: trifluridine, a nucleoside analog, and tipiracil, a thymidine phosphorylase inhibitor. Tipiracil prevents rapid metabolism of trifluridine, increasing the bioavailability of trifluridine.

Thymidine kinase is an enzyme, a phosphotransferase : 2'-deoxythymidine kinase, ATP-thymidine 5'-phosphotransferase, EC 2.7.1.21 that catalyzes the reaction:

Fluorodeoxyuridylate, also known as FdUMP, 5-Fluoro-2'-deoxyuridylate, or 5-Fluoro-2'-deoxyuridine 5'-monophosphate sodium salt, is a molecule formed in vivo from 5-fluorouracil or 5-fluorodeoxyuridin.

References

↑ Holliday, R; Ho, T (1998). "evidence for gene silencing by endogenous methylation". Proc Natl Acad Sci U S A. 95 (15): 8727–32. Bibcode:1998PNAS...95.8727H. doi:10.1073/pnas.95.15.8727. PMC 21144 . PMID 9671746.

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[1] Holliday, R; Ho, T (1998). "evidence for gene silencing by endogenous methylation". Proc Natl Acad Sci U S A. 95 (15): 8727–32. Bibcode:1998PNAS...95.8727H. doi:10.1073/pnas.95.15.8727. PMC 21144 . PMID 9671746.

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