Modifying glycosylation Add or subtract sites to your favorite protein (cis) 1a. Subtract sites: Easy, change N or S or T to A by site-directed mutagenesis 1b. Add sites: Not so easy. Consensus N-X-S does not work, e.g.: requires the insertion of a ~12 aa region encompassing
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Clone enzyme genes:Glycosyl transferases, mostlyAlso some synthetases (e.g., NAcNeu, i.e., biosynthetic path to sialic acid)
Can be complex:e.g., 7 different fucosyl transferases (FTs),with different (overlapping) substrate specificities
Hamster cells do only 2,3 sialylation.
Humans do 2,6 as well, via a 2,6 sialyl transferase (ST)
Experiment:Over-express cloned human 2,6 ST, along with a substrate protein.producing permanent transfectants of CHO cells
Get both types of structures now, substantially
(although not exactly the same ratio as in human cells).
Zhang, X., Lok, S.H., and Kon, O.L. 1998. Stable expression of human alpha-2,6-sialyltransferase in Chinese hamster ovary cells: functional consequences for human erythropoietin expression and bioactivity. Biochim Biophys Acta1425: 441-452.
Isolate mutant mammalian cell lines deficient in specific glycosylation enzymes
Stanley: Isolation of multiply mutated glycosylation mutants by selecting for lectin resistance
Lectins = carbohydrate-binding proteins
Plant lectins used mostly here (but occur widely in animals as well)
Sequential selections, push - pull on resistance, sensitivity
Resistance: enzyme deficiency failure to add the sugar need for lectin binding
Increased sensitivity: failure to add a sugar produces greater exposure of underlying sugars in a transferase-negative mutant better binding to the exposed sugar
Showed power of selection
Showed usefulness of complementation analysis via cell hybridization
All lec-R mutants were: WGA (wheat germ agglutinin) resistant (various degrees) & pro-
Tester parent was single lec-R + GAT- (req. glycine, adenine and thymidine)Select in medium lacking pro, GAT, and with +/- WGA
Complementing hybrids will have regained sensitivity to WGA
Mutants in the same gene will remain WGA resistant (non-complementation)
Could now be used as a tabla rasa (blank slate) for the introduction of a series of enzymes to build custom-tailored glycoconjugates. Complicated though (order of addition, location in the Golgi, etc. )
Potential: targeting to carbohydrate-sensitive receptors (e.g., liver asialoglycoprotein receptor); clearance rate
Review: Nature Biotechnology 19, 913 - 917 (2001) , The bittersweet promise of glycobiology. Alan Dove
Umana, P., Jean-Mairet, J., Moudry, R., Amstutz, H., and Bailey, J.E. 1999.
Engineered glycoforms of an antineuroblastoma IgG1 with optimized
antibody-dependent cellular cytotoxic activity. Nat Biotechnol17: 176-180.
Target here (bisecting NAcGlc)
(NAcG = N-acetyl-glucosamine here)
Presence of the bisecting NAcGlc enhances binding of T-cell receptor to the Fc region of antibodies.
Binding is needed for ADCC.
Mouse and hamster cell lines used for commercial production lack the glycosyltransferase needed for bisecting NAcG addition
A rat myeloma cell line does produce MAb with the bisecting NAcG.
Hypothesis: Expression of the rat enzyme in a CHO cell line will add a bisecting NacG to the anti-neuroblastoma MAb produced by these cells. The modified MAb will be a better mediator of ADCC.
Experiment: Clone the cDNA for this enzyme from the rat line and transfer it to CHO cells, driven by an inducible tet promoter.
Check sugar structure of MAb and ADCC efficiency of the MAb. Both improved.
Commercial MAb injected as a therapeutic
T-cell surface receptor binds Fc region of antibody molecule(Fc gammaR)
Protein Glycosylation Knock-out
Naoko Yamane-Ohnuki, et al.. Establishment of FUT8 knockout Chinese hamster ovary cells: an ideal host cell line for producing completely defucosylated antibodies with enhanced antibody-dependent cellular cytotoxicity. Biotechnol Bioeng. 2004 Sep 5;87(5):614-22
Kanda Y, Yamane-Ohnuki N, Sakai N, Yamano K, Nakano R, Inoue M, Misaka H, Iida S, Wakitani M, Konno Y, Yano K, Shitara K, Hosoi S, Satoh M. Comparison of cell lines for stable production of fucose-negative antibodies with enhanced ADCC. Biotechnol Bioeng. 2006 Jul 5;94(4):680-8.
Grabenhorst, E., Schlenke, P., Pohl,., Nimtz, M., and Conradt, H.S. 1999.
Genetic engineering of recombinant glycoproteins and the glycosylation
pathway in mammalian host cells. Glycoconj J16: 81-97.
Stanley, P. 1989. Chinese hamster ovary cell mutants with multiple glycosylation
defects for production of glycoproteins with minimal carbohydrate heterogeneity.
Mol Cell Biol9: 377-383.
Biotechnol Bioeng. 2004 Sep 5;87(5):614-22
Fucose interferes with binding of the T-cell Fc-gamma-3 receptor to the Fc region of an antibody molecule.
Elimination of fucose from produced MAbs will increase ADCC
Create a mutant CHO cells (starting with amplifiable dhfr- cells) in which the fucose trasnferase genes have been knocked out.
All MAbs produced in these mutant cells will be better at promoting ADCC
Double knock-out strategy for FUT8 an alpha-1,6,fucosyl transferase
Little sequence data available for Chinese hamster
Isolate CHO cDNA using mouse sequence data for primers
Use CHO cDNA to isolate CHO genomic fragments from a commercial lambda library
K.O. exon 1 translation start region
DT= diphtheria toxin gene,
Kills if integrated via
Select for G418 resistance,
Screen by PCR for homologous recomb.
108 cells 45000 colonies 40 false recombinants (extension-duplications) + 1 true recombinant
Step 2 for homozygote,
select for Pur-resistance
10 double KO homozygotes.
Lox sites for later removal of drug resistance marker
Remove drug resis. genes by
transient transfection with Cre
Recombinase. Exon 1 has undergone a 200 nt deletion.
Note: 10’s of thousands of PCRs performed to screen for homologous recomb., using 96-well plates
Double knockout evidence
Original KO’d genes have a 1.5 kb insertion
Final mRNA has 200 nt deletion
Use of a fluoresceinated lentil lectin (LCA) that binds fucose oligosaccharides to demonstrate lack of fucosylation in glycosylated proteins in the FUT8 -/- cells
Control background fluorescence(FL-anti avidin)
Surprising that fucosylation levelis down by one-half; that CHO cells do not have excess fucosylation capacity . . .
Rituxan (anti-CD20) produced in FUT -/- cells does not contain fucose
(CD20 is expressed at a high level on the surface of many lymphoma cells.)
Digestion all the way to monosaccharides
Missing d - g
Binding to CD20 membranes is the same
FUT8-/- anti CD20 = Ritxuan
In ADCC, FUT8-/- anti-CD20 >> Rituxan
Anti-CD20 from a partially FUT-deficient rat cell line
Characteristics of“FUT8-/- anti-CD20”
Fc-Receptor protein binding assay
Complement-mediated cell toxicity is the same for FUT8-/- and Rituxan(not T-cell mediated ADCC)
Rituxan = commercial product, 98% fucosylated
Very laborious, but apparently a big payoff.
Better selection?: Why not use the fluorescent LCA to select for the FUT8 KO’s along with G418 resistance (double, sequential selection) ? Avoid 100,000 PCRs. . .
Second generation protein therapeutics: improvements over nature.
Or: Isolation of recombinant protein mutants with altered binding properties.
Here: Tissue plasminogen activator (TPA, clot-buster)
Why are they doing this?
Binding of TPA to liver cells leads to clearance from the bloodstream
Want to avoid clearance in TPA therapy (anti-thrombolytic, clot buster)
Know: MAb387 blocks binding to cultured hepatoma cells (liver-like)
Know: MAb387 decreases clearance rate.
Mutate the cloned TPA gene. Mutate it in the MAb387-binding region mutant TPA that:
1) is hepatocyte binding-negative (select)
2) is still protease+ (remains catalytically active) (screen)
How could one do this? Select?
Need to characterize many mutant proteins,
and find the protein with the desired characteristic,
and then rescue the gene for that protein.
Mammalian cell transfectants: But TPA is secreted, so protein becomes divorced from the DNA that coded it.
But Coffino and Scharff had a technique for looking at secretory variants (of myeloma cells):
Immunoprecipitate secreted proteins around colonies grown in agar
(Ig secretion, precipitation by goat anti-mouse-Ig antibody):
All in soft agar
Medium in agar
Antibody in top layer = MAb387
Colonies = CHO cell permanent
co-transfectants of mutant library TPA
Colonies may not make enough.
But you don’t need a FACS ($$)
Coffino and Scharff
(Proc Natl Acad Sci U S A.
Consider an alternative:
Phage display: a way to link the variant protein to its coding DNA
Mutagenize the gene as a fusion protein to a phage coat protein
and make a library in bacteriophage. The mutants will
be displayed on the surface of the phage and can be “panned” for (or against).
Here, one would collect the members of the phage library of mutant TPAsthat don’t stick to hepatoma cells, or to immobilized MAb387.
But lots of noise in a negative selection = non-stickers could be for many uninteresting reasons (denatured, statistical, etc. )
Authors here use a mammalian cells as the carrier of the DNA and the cell surface as a display site.
Via making a fusion protein to a membrane anchor protein, DAF (peptide, really).
What did they do?
333 bp K1 (kringle-1), known to bind the MAb387, which competes for hepatocyte binding (so assuming it is the same target epitope).
How did they get it mutated?
How did they isolate just the kringle 1 region?
How did they get the mutagenized fragment back in?
Introduced restriction sites at the ends, w/o affecting the coding.
What did they put the mutagenized fragment into?
DAF – TPA fusion protein geneHow did they get it into into cells?
How many copies per cell. And why is that important?
One, by electroporation at low DNA concentration.
[In a transient transfection!]
Binding is dominant. Lack of binding is recessive.
How did they select cells making MAb387-non-binding TPA?
Sort the cells with low fluorescence
For reiteration of the process
How did they recover the plasmid carrying the mutant TPA gene from the selected cells?
Hirt extraction: Like a plasmid prep, lyse cells gently, high MW DNA entangles and forms a “clot”.
Centrifuge. Chromosomal DNA soft pellet; plasmid DNA circles stay in supernatant.
Then re-transfect, re-sort in FACS.
After 2 sorting rounds, test individual E. coli clones: 60% are binding-negative.
MAb to protease domain
Low kringle-1 reactivity
MAb to kringle-1 domain
PE = phycoerythrin (fluorescent protein)
FACS selection can also work for an internal protein
Predicted. freq. of dhfr- mutants = 10-4
Still only 1 in 10 were mutants.
Hepatoma cell binding. How?
Clone mutated regions into regular TPA gene for testing (no DAF, protein secreted)
Label WT TPA with fluorescein (FITC) (conjugated chemically)
Mix with hepatoma cells and analyze on a flow cytometer (FACS w/o the sorter part).
See specific and non-specific binding. Subtract non-specific binding: the amount not competed by excess un-labeled wt TPA.
Can’t compete (good)
But still haveprotease activity
Hepatoma cell binding assay:
measure comptetion for binding of fluorescently labeled WT TPA
Compete. So still bind.