Production of recombinant human type I procollagen homotrimer in the mammary gland of transgenic mice.

The large scale production of recombinant collagen for use in biomaterials requires an efficient expression system capable of processing a large (> 400 Kd) multisubunit protein requiring post-translational modifications. To investigate whether the mammary gland of transgenic animals fulfills these requirements, transgenic mice were generated containing the alpha S1-casein mammary gland-specific promoter operatively linked to 37 Kb of the human alpha 1(I) procollagen structural gene and 3' flanking region. The frequency of transgenic lines established was 12%. High levels of soluble triple helical homotrimeric [(alpha 1)3] type I procollagen were detected (up to 8 mg/ml) exclusively in the milk of six out of 9 lines of lactating transgenic mice. The transgene-derived human procollagen chains underwent efficient assembly into a triple helical structure. Although proline or lysine hydroxylation has never been described for any milk protein, procollagen was detected with these post-translational modifications. The procollagen was stable in milk; minimal degradation was observed. These results show that the mammary gland is capable of expressing a large procollagen gene construct, efficiently assembling the individual polypeptide chains into a stable triple helix, and secreting the intact molecule into the milk.

Phosphatidylinositol transfer protein in murine embryonal carcinoma cells during retinoic acid-induced differentiation.

Phosphatidylinositol transfer protein (PI-TP) was studied in P19 embryonal carcinoma (EC) cells at different stages of retinoic acid (RA) induced differentiation. Western blot analysis indicated an increased expression of PI-TP (35 kDa) during differentiation. Western blots of isoelectric focusing gels showed that the 35 kDa band consisted of the PI-carrying form of PI-TP (pl 5.5) and of a novel, more acidic form of PI-TP (pl 5.4), levels of both of which increased during differentiation. These increased levels were not reflected in the in vitro PI-transfer activity of the cytosolic fraction nor in the mRNA levels as analyzed by northern blotting. By using indirect immunofluorescence it was shown that PI-TP is localized in the cytoplasm and associated with perinuclear Golgi structures and that this distribution is slightly affected during RA-induced differentiation. Immunoprecipitation of PI-TP from [32 P]Pi labeled cells demonstrated that the level of phosphorylation of PI-TP is high in undifferentiated P19 EC cells and low after 5 days of RA-induced differentiation. These results strongly suggest that changes in the levels of PI-TP are intimately connected with changes in the growth characteristics of P19 EC cells during RA-induced differentiation. It remains to be established to what extent this connection is governed by the recent finding that PI-TP is an essential cytosolic factor in stimulating phospholipase C activity.

The phosphatidylinositol transfer protein in 3T3 mouse fibroblast cells is associated with the Golgi system.

By use of indirect immunofluorescence it was shown that the phosphatidylinositol transfer protein (PI-TP) in 3T3 mouse fibroblast cells is associated with the Golgi system. This was concluded from double-labeling experiments with TRITC-labeled Ricin which binds to sugar residues that are specifically processed in the Golgi system. Independent evidence for this association was provided by the fact that dissociation of the Golgi system by brefeldin A was reflected in an extensive redistribution of PI-TP labeling. In addition, PI-TP is localized in the cytoplasm and in the nucleus. In exponentially growing cells an enhanced labeling of PI-TP was observed in the cytosol and in the cytosol and in the Golgi system in comparison with quiescent cells. By Western blot analysis and by transfer activity assays, it was confirmed that the concentration of PI-TP was increased in exponentially growing cells. These results strongly suggest that PI-TP fulfills a role in the functioning of the Golgi complex.

Efficient generation of functional transgenes by homologous recombination in murine zygotes.

To assess the feasibility of generating functional transgenes directly via homologous recombination between microinjected DNA fragments, three overlapping genomic DNA fragments, together constituting the human serum albumin (hSA) gene, were coinjected into murine zygotes. The resulting transgenic mice were analyzed for structure and expression of the transgene. All transgenic mice carried recombined hSA DNA fragments and 74% contained a reconstituted hSA gene. HSA expression could be detected in liver and serum in most (72%) of these animals. Only correctly sized hSA transcripts were observed. Transgenic hSA could not be distinguished from the human serum-derived protein by radioimmunoassay or Western blotting. The high frequency and accuracy of homologous recombination in murine zygotes reported here allows the efficient generation of relatively large transgenes.