Stephen P. Butler

Transgenesis in mice by cytoplasmic injection of polylysine/DNA mixtures

Pronuclear injection is currently the most often used method to make transgenic animals, but in some animal species it is temporally restrictive due to difficulty in visualizing pronuclei. However, the injection of construct DNA into the cytoplasm does not result in transgenesis. The production of transgenic mice by a cytoplasmic microinjection technique of polylysine complexed DNA into pronuclear stage zygotes is described. Transgenic mice were produced from cytoplasmic microinjection of mixtures of a 5.3 kb linearized DNA and poly-l-lysine (degree of polymerization=51). Effects on transgenic frequency of both the lysine to phosphate ratio of polylysine to DNA and DNA concentration were studied. About 12.8% of the pups born from zygotes cytoplasmically microinjected with a polylysine/DNA mixture having a lysine to phosphate ratio (L:P) of 1∶1 microinjection positive control of DNA alone was 21.7%. No transgenic pups were born from microinjection of DNA alone into the cytoplasm. Complexes of polylysine/DNA were detected using agarose gel electrophoresis at the conditions which produced transgenic mice. The presence of polylysine with construct DNA altered thein vitro activities of restriction endonuclease and DNA ligase on the construct DNA. The production of transgenic animals using DNA and polylysine in the absence of any other signal protein suggests that a DNA/polylysine complex but not DNA alone can act as a substrate for transgenesis from the cytoplasm.

Transgenic pigs as bioreactors: a comparison of gamma-carboxylation of glutamic acid in recombinant human protein C and factor IX by the mammary gland

The mammary gland of transgenic livestock can be used as a bioreactor for producing complex therapeutic proteins. However, the capacity for making a given post-translational modification upon any given polypeptide is uncertain. For example, the efficiency of gamma-carboxylation of glutamic acid in the amino terminal regions of recombinant human protein C (rhPC) and recombinant human Factor IX (rhFIX) is different at similar expression levels. At an expression level of about 200 microg/ml in the milk of transgenic pigs, rhFIX is highly gamma-carboxylated as indicated by pro-coagulant activity and amino acid sequencing. However, only about 20-35% of rhPC has a native, gamma-carboxyglutamic acid-dependent conformation and anti-coagulant activity. Thus, this work provides an example of apparent differences in substrate specificity between two homologous proteins to the endogenous carboxylase of porcine mammary epithelium which leads to varying degrees of post-translational modification.

Phenotypic and genotypic stability of multiple lines of transgenic pigs expressing recombinant human protein C

The genotypic and phenotypic stability of four lines of transgenic pigs expressing recombinant human protein C in milk was examined. Two lines were established with a construct consisting of a 2.6 kb mouse WAP promoter and a 9.4 kb human protein C genomic DNA. Two lines were established with another construct consisting of a 4.1 kb mouse WAP promoter and a 9.4 kb human protein C genomic DNA. Genotypic stability was measured by transgene copy number transmission. Outbred offspring having a single transgene integration locus were established from a founder having three independent, multicopy loci. Phenotypic stability over multiple lactations was defined by the combination of recombinant human protein C expression levels and the isoform signature of recombinant human protein C in western blots. Both cDNA and genomic human protein C transgenes gave similar ranges of expression levels of about 100--1800 μg ml−1. Within a given outbred lineage having a single loci for the cDNA transgene, the expression levels ranged between 100--400 μg ml−1. Western blots of reduced recombinant protein C revealed that single chain content was not dependent on expression level and was consistent within each transgenic line, but varied between transgenic lines. This suggests that native swine genetics may play a role in selection of production herds with optimal post-translational proteolytic processing capability. Although swine are not conventional dairy livestock, it is agreed that the short generation times, multiple offspring per litter, stable paternal transmission of the transgene, and milk production capabilities of swine offer distinct advantages over conventional dairy livestock for the establishment of a herd producing a therapeutic recombinant protein

Functional factor VIII made with von Willebrand factor at high levels in transgenic milk

Summary.  Background: Current manufacturing methods for recombinant human factor VIII (rFVIII) within mammalian cell cultures are inefficient, hampering the production of sufficient amounts for affordable, worldwide treatment of hemophilia A. However, rFVIII has been expressed at very high levels by the transgenic mammary glands of mice, rabbits, sheep, and pigs. Unfortunately, it is secreted into milk with low specific activity, owing in part to the labile, heterodimeric structure that results from furin processing of its B domain. Objectives: To express biologically active rFVIII in the milk of transgenic mice through targeted bioengineering. Methods: Transgenic mice were made with a mammary‐specific FVIII gene (226/N6) bioengineered for efficient expression and stability, encoding a protein containing a B domain with no furin cleavage sites. 226/N6 was expressed with and without von Willebrand factor (VWF). 226/N6 was evaluated by ELISA, SDS‐PAGE, western blot, and one‐stage and two‐stage clotting assays. The hemostatic activity of immunoaffinity‐enriched 226/N6 was studied in vivo by infusion into hemophilia A knockout mice. Results and conclusions: With or without coexpression of VWF, 226/N6 was secreted into milk as a biologically active single‐chain molecule that retained high specific activity, similar to therapeutic‐grade FVIII. 226/N6 had > 450‐fold higher IU mL−1 than previously reported in cell culture for rFVIII. 226/N6 exhibited similar binding to plasma‐derived VWF as therapeutic‐grade rFVIII, and intravenous infusion of transgenic 226/N6 corrected the bleeding phenotype of hemophilia A mice. This provides proof‐of‐principle for the study of expression of 226/N6 and perhaps other single‐chain bioengineered rFVIIIs in the milk of transgenic livestock.

Engineering protein processing of the mammary gland to produce abundant hemophilia B therapy in milk

Both the low animal cell density of bioreactors and their ability to post-translationally process recombinant factor IX (rFIX) limit hemophilia B therapy to <20% of the world’s population. We used transgenic pigs to make rFIX in milk at about 3,000-fold higher output than provided by industrial bioreactors. However, this resulted in incomplete γ-carboxylation and propeptide cleavage where both processes are transmembrane mediated. We then bioengineered the co-expression of truncated, soluble human furin (rFurin) with pro-rFIX at a favorable enzyme to substrate ratio. This resulted in the complete conversion of pro-rFIX to rFIX while yielding a normal lactation. Importantly, these high levels of propeptide processing by soluble rFurin did not preempt γ-carboxylation in the ER and therefore was compartmentalized to the Trans-Golgi Network (TGN) and also to milk. The Golgi specific engineering demonstrated here segues the ER targeted enhancement of γ-carboxylation needed to biomanufacture coagulation proteins like rFIX using transgenic livestock.

Expression of human fibrinogen in transgenic mice

Human fibrinogen (hfib) is a 340 kDa plasma protein which is a complex precursor substrate to fibrin during clot formation. The present work evaluates the potential of the mammary gland of transgenic mice to coordinately express three separate cDNAs for hfib and then assemble hexameric, functional recombinant human fibrinogen (rhfib). Trigenic mice were made by pronuclear co-microinjection of separate constructs for each cDNA of the A/spl alpha/, B/spl beta/ and /spl gamma/ hfib polypeptides. Each transgene contained a 2.6 kb promoter sequence from the murine whey acidic protein (mWAP). Trigenic mice expressing rhfib at about 5-35 /spl mu/g/ml milk were generated. The rhfib showed an apparent molecular weight by SDS-PAGE similar to that of hfib under non-reducing conditions. Under reducing conditions, the /spl alpha/-chain of rhfib had slightly greater mobility than /spl alpha/-chain from hfib. The rhfib was converted to fibrin by thrombin in a manner similar to that of hfib where a normal fibrin clot and a lower apparent mobility of the /spl alpha/-chain from the clot relative to the starting precursor chains were observed for both rhfib and hfib. In summary, these results show that functional fibrinogen can be synthesized and secreted by the mammary gland of transgenic mice.

Development of murine embryos following electroporation

AbstractPurpose: The objective was to establish the parameters for reversible electroporation of murine embryos. Methods: In Trial 1, murine presumptive zygotes received an electrical pulse of 5, 10, or 20-μs duration, and one of five voltages (100, 200, 250, 300, or 400 V). In Trial 2, embryo orientation within the electroporation chamber was evaluated with 250 or 400 V at a pulse period of 10 μs. Results: Presumptive zygotes that received 400 V at each pulse length and zygotes exposed to 20 μs at each voltage had the lowest embryonic development (P < 0.05). Presumptive zygotes that received 250 V had higher development compared to 400 V, irrespective of orientation (P < 0.01), but development was lower than the controls (P < 0.01). Conclusions: Electrical stimulation of presumptive zygotes can have a detrimental impact on early embryo development, but low amounts of stimulation may allow for potential gene transfer in transgenic experimentation.