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Lecture 8 genetic engineering of animal cells

Industrial Microbiology Dr. Butler 2011

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Lecture 8 genetic engineering of animal cells

  1. 1. Genetic engineering of animal cells in culture To study individual genes, it may be necessary to transfer the gene into the cells under study. In this section we will discuss some of the common methods that are used to introduce genes into mammalian cells. This is essential so that one can genetically manipulate cells so that they are able to produce large levels of the desired protein (overexpression). In the diagram, a gene is spliced into a plasmid, called shuttle vectors, and then introduced into the mammalian cell.
  2. 2. Outline <ul><li>Transfer of “Naked” DNA </li></ul><ul><li>Viral assisted DNA transfer </li></ul><ul><li>Selection and Amplification of transfected cells </li></ul><ul><li>Gene Expression </li></ul><ul><li>Expression cassettes </li></ul>
  3. 3. Lecture 7 Animal Cell Biotechnology Genetic engineering of animal cells <ul><li>Mammalian cells are the host cells of choice for the synthesis of complex glycoproteins from recombinant genes </li></ul><ul><li>have required characteristics for post-translational processing and product secretion </li></ul><ul><li>Chinese hamster ovary (CHO) most commonly used, well studied and characterized </li></ul><ul><ul><li>CHO cells are very popular for the expression of human recombinant glycoproteins because the glycosylation enzymes resemble those found in human cell lines. They can grow as anchorage dependent or as cell suspensions. </li></ul></ul><ul><li>Transfection – transfer of nucleic acid into mammalian cells -> also refers to DNA-mediated transformation </li></ul>
  4. 4. Lecture 7 Animal Cell Biotechnology Genetic engineering of animal cells <ul><li>I. Transfer of ‘naked DNA’ </li></ul><ul><li>1. Calcium phosphate method </li></ul><ul><li>2. DEAE-dextran </li></ul><ul><li>3. Encapsulation of DNA in liposome vesicles </li></ul><ul><li>4. Electroporation </li></ul><ul><li>5. Micro-injection </li></ul><ul><li>6. Protoplast fusion </li></ul>
  5. 5. <ul><li>Calcium phosphate method </li></ul><ul><li>Discovered in 1973: </li></ul><ul><ul><li>Cheap and simple method </li></ul></ul><ul><li>DNA co-precipitates with calcium phosphate and adsorbs onto the cell membrane (endocytosed) </li></ul><ul><li>Cells are treated with dimethyl sulfoxide (DMSO) or glycerol to allow DNA to pass through the membrane -> cell membrane destabilized </li></ul><ul><li>Proportion of cells taking up DNA-precipitate is high, amount incorporated into nucleus is low (1-2%) </li></ul>
  6. 6. 2. DEAE-dextran <ul><li>DNA forms complex with diethylaminoethyl dextran </li></ul><ul><li>-> cells treated with DMSO to destabilize membrane </li></ul><ul><li>-> high proportion of cells are transfected, but transient protein expression </li></ul>
  7. 7. 3. Encapsulation of DNA in liposome vesicles (lipofection) <ul><li>transfection with high levels of gene expression complexes of positively charged lipid and DNA </li></ul><ul><li>readily taken up by mammalian cells </li></ul><ul><li>high-efficiency </li></ul><ul><li>Introduction of DNA into cells with commercially available cationic lipids. Charged head groups (positive) are drawn to the phosphate backbone of DNA. </li></ul>
  8. 9. 4. Electroporation <ul><li>Exposure of cell suspension to a high-voltage electrical pulse (100-200 V, 1-2 ms) </li></ul><ul><li>Transient pores are formed in the cell membrane, allows DNA to pass through </li></ul><ul><li>Exposure of suspended cells to a pulsed electric field that causes the transient formation of pores in the cell membrane, allowing for exchange of macromolecules between the extracellular environment and the cytoplasm. Removal of the electrical field results in spontaneous sealing of the pores. </li></ul><ul><li>Microinjection </li></ul><ul><li>DNA injected with microneedles or pipettes directly into the nucleus, or into the cytoplasm. Requires highly trained and skilled personnel. Typically used when there are limited numbers of recipient cells. </li></ul>
  9. 10. 5. Micro-injection <ul><li>insertion of DNA directly into the nucleus of individual cells </li></ul><ul><li>effective for small numbers of cells, very effective </li></ul>
  10. 11. 6. Protoplast fusion <ul><li>Bacteria-grown plasmids are introduced into animal cells via fusion with bacterial protoplasts </li></ul><ul><li>Bacterial cell walls are removed using lysozyme treatment </li></ul><ul><li>Protoplasts fuse to the mammalian cells </li></ul><ul><li>DNA purification not required </li></ul><ul><li>may introduce bacterial components into recipient cells, cultures may be overgrown with bacteria </li></ul><ul><li>Bacteria are used to produce plasmids containing the human gene of interest. The cell walls are removed using lysozyme, resulting in the formation of protoplasts. The protoplasts are brought in contact with the mammalian cells, leading to protoplast fusion to mammalian cells, using Polyethylene glycol (PEG). </li></ul>
  11. 12. Lecture 7 Animal Cell Biotechnology Genetic engineering of animal cells <ul><li>II. Viral-assisted DNA transfer </li></ul><ul><li>animal viruses are very efficient for DNA transfer (transduction) </li></ul><ul><li>-> promote transfer of DNA to cells because of capsid proteins that bind to cell membrane receptors </li></ul><ul><li>-> contain promoters that allow expression of inserted genes in animal cells </li></ul><ul><li>-> replicate to high copy numbers (several copies of required genes within cell) </li></ul><ul><li>-> some viruses’ genomic material integrate into the animal cell genome </li></ul>
  12. 13. Simian virus SV40 <ul><li>most successful and widely used viral system derived from simian virus SV40 </li></ul><ul><li>contains circular DNA duplex and many well-characterized restriction enzyme cleavage sites </li></ul><ul><li>part of SV40 genome replaced with foreign DNA </li></ul><ul><li>limited host range (only monkey cells) </li></ul>
  13. 14. Retroviruses <ul><li>Retroviruses are also efficient vectors for integration of foreign genes into animal cells </li></ul><ul><li>Have RNA genome, complementary DNA copies are formed by the enzyme reverse transcriptase </li></ul><ul><li>DNA can be incorporated into host cells’ genome without causing cell lysis </li></ul><ul><li>Retroviral structural genes are replaced with foreign DNA </li></ul>
  14. 16. Selection and amplification of transfected cells <ul><li>1. Cell selection by genetic markers </li></ul><ul><li>proportion of cells that accept exogenous DNA may be as low as 1 in 10 6 </li></ul><ul><li>marker gene incorporated into a recipient cell, allows successful transfectants to be identified </li></ul><ul><li>marker can be on the same vector as target gene, or on separate vector </li></ul>
  15. 17. Genetic markers… <ul><li>non-dominant genes – selection of mutants that lack marker enzyme activity prior to transfection </li></ul><ul><ul><li>eg. DHFR converts dihydrofolate into tetrahydrofolate, a methyl group shuttle required for the de novo synthesis of purines, thymidylic acid, and certain amino acids </li></ul></ul><ul><ul><li>Methotrexate: cells produce more DHFR </li></ul></ul>eg. Glutamine synthetase: amplified by addition of methionine sulfoximine
  16. 18. <ul><li>Non-dominant genes: target cells must have endogenous genes mutated or removed, only works with mutant cells. If target genes are successfully transfected with shuttle vector containing wild-type marker gene, this will allow cells to survive in specially treated media. </li></ul><ul><li>Dominant genes: do not require mutant cells, can be used with any cell type. Dominant marker genes do not have to be mammalian, may be bacterial in origin, allows the cell to survive in special medium. </li></ul><ul><li>Antibiotic resistance genes: shuttle vector contains antibiotic, allows for successful transfectants to survive. </li></ul>
  17. 19. <ul><li>HGPRT- </li></ul><ul><li>hypoxanthine guanine phosphoribosyl transferase </li></ul><ul><ul><li>Selection of cells in hypoxanthine, aminopterin and thymidine </li></ul></ul><ul><ul><li>HAT media: contains hypoxanthine, aminopterin and thymidine </li></ul></ul><ul><ul><li>HGPRT- cells can’t survive in HAT </li></ul></ul>Simple Precursors: purine, pyrimidine and nucleotides DNA Hypoxanthine or Guanine Thymidine TK HGPRT Salvage pathway Aminopterin inhibition De novo pathway
  18. 20. Genetic Markers… <ul><li>2. dominant genes – selection of any cell type, those that acquire specific gene during transfection will survive </li></ul>eg. Xanthine-Guanine Phosphoribosyl transferase (XGPRT) -present only in bacteria naturally -only transfected mammalian cells would be able to survive in media with xanthine, hypoxanthine mycophenolic acid
  19. 21. 3. antibiotic resistance genes <ul><li>G418 is an aminoglycoside antibiotic, selects for expression of neomycin-resistant genes. G418 blocks polypeptide synthesis by inhibiting the elongation step in both prokaryotic and eukaryotic cells. </li></ul><ul><li>Blasticidin S is a peptidyl nucleoside antibiotic, selects for expression of the bsr gene. It specifically inhibits protein synthesis in both prokaryotes and eukaryotes through inhibition of peptide-bond formation in the ribosomal machinery. </li></ul>
  20. 22. <ul><li>2. Gene amplification </li></ul><ul><li>involves increasing the number of copies of a specific gene per cell </li></ul><ul><li>-> increased synthesis of gene product </li></ul><ul><li>can use inhibitors to select for cells with multiple copies of desired gene </li></ul><ul><li>inhibitors do not induce gene amplification, but selects for cells that have undergone random gene amplification </li></ul><ul><li>insert desired gene adjacent to dihydrofolate reductase or glutamine synthetase, up to 1000-fold increase in gene copy and corresponding protein expression </li></ul>
  21. 23. <ul><li>1. Transient gene expression </li></ul><ul><li>preliminary test of gene expression </li></ul><ul><li>assayed from cell culture 1-3 days after uptake of DNA </li></ul>Gene expression <ul><li>cells are not genetically stable and soon lose their expression ability </li></ul>
  22. 24. <ul><li>-> Ex. COS (monkey kidney cells) transfected with SV40 (2-5 x 105 copies of cloned genes/cell) </li></ul><ul><li>SV40 DNA not replicated due to inactivation of origin of DNA replication, transfected DNA contains SV40 origin of replication </li></ul><ul><li>leads to production of high copies of desired gene (2-5 x 10 5 copies of cloned genes/cell) </li></ul>
  23. 25. <ul><li>2. Stable gene expression </li></ul><ul><li>DNA is integrated into the nucleus of the host cell, or efficient extrachromosomal replication system is maintained during cell division </li></ul><ul><li>limited by frequency of DNA integration </li></ul><ul><li>ideal recombinant cells grow with doubling time of ~18 hours, is genetically stable, and secretes a recombinant protein at ~20% of total cellular protein </li></ul>
  24. 26. Expression cassettes <ul><li>Expression signals required to ensure gene expression in mammalian cells </li></ul><ul><li>Signals are common to wide variety of genes and used in a range of cell lines </li></ul><ul><li>Expression cassette is a DNA sequence that contains signal sequences that are incorporated into a vector used to transfect mammalian cells </li></ul>
  25. 27. Signal sequences of an expression cassette include: <ul><li>a promoter to initiate transcription of the gene </li></ul><ul><li>-> want ‘strong promoters’ that allow for a high rate of transcription (SV40, adeno- or retroviral) </li></ul><ul><li>ribosome-binding site for attachment of mRNA to ribosomes </li></ul><ul><li>a transcription terminator sequence downstream of the expressed gene </li></ul>
  26. 28. Lecture 7 Animal Cell Biotechnology Genetic engineering of animal cells <ul><li>1. retroviral promoter and termination control regions </li></ul><ul><li>2. mammalian gene (globin cDNA) </li></ul><ul><li>3. selectable gene marker (neomycin resistance) </li></ul><ul><li>4. bacterial and mammalian origin of replication sites </li></ul>Butler, M. 2004. Animal cell culture and technology 2nd ed. London and New York:Garland Science/BIOS Scientific Publishers. P106.