Production and secretion of recombinant human fibrinogen by the transgenic murine mammary gland

Abstract

The mammary gland of lactating transgenic animals has several advantages for production of heterologous proteins including a high cell density that results in high concentrations of secreted protein and the ability to perform several types of post-translational modifications. Transgenes were constructed from the 4.1 kbp murine Whey Acidic Protein promoter (mWAP) and the three cDNAs coding for the A, B$ and ( fibrinogen chains to evaluate the requirements of the transgenic murine mammary gland for high level secretion of fully assembled human fibrinogen. After introducing the constructs into the murine zygotes by microinjection, secretion of fully assembled fibrinogen into milk was measured at concentrations between 10 :g/ml to 200 :g/ml. In one line of mice the total secretion of fibrinogen and unassembled subunits approached 700 :g/ml in milk. The level of assembled fibrinogen was proportional to the lowest amount of subunit produced where both the B$ and ( chains were rate limiting. Also, the subunit complexes (2 , A(2 and the individual subunits A, B$ and ( were found as secretion products. This is the first time that secretion of individual B$-subunits by any cell type has been reported and suggests the organization of the secretion pathway in mammary epithelia is different from that in liver. Glycosylated forms of individual B$-chain contained a complex saccharide with low mannose. Glycosylation of the (-chain was also observed. These results suggest the 4.1 mWAP promoter can drive expression of fibrinogen cDNAs to high levels and that the amount of fully assembled fibrinogen secreted is equal to the level of the lowest expressing chain.