Hepatic lipid catabolism begins with the transport of lipoprotein remnants from the sinusoidal vasculature into hepatocytes by endocytosis via microvilli. To test the hypothesis that fenestrated sinusoidal endothelial cells (SECs) are crucial for this process, we selectively disrupted SECs by downregulating vascular endothelial growth factor (VEGF) signaling, using hepatocyte-specific, tetracycline-regulatable expression of a VEGF receptor that can sequester VEGF but cannot relay its signal. Newborn mutant livers appeared grossly normal, but displayed a dark-red color that was distinguishable from normal physiological lipid-rich pink livers. Mutant sinusoidal networks were reduced and their SECs lacked fenestrae. Hepatocellular lipid levels were profoundly reduced, as determined by Oil Red O staining and transmission electron microscopy, and fewer hepatocytic microvilli were evident, indicating impaired lipoprotein endocytosis. Levels of apolipoprotein (APO) E bound to mutant sinusoidal networks were significantly reduced, and fluorescently-labeled murine remnant lipoproteins injected into the blood stream failed to accumulate in the space of Disse and diffuse into hepatocytes, providing evidence that reduced hepatocellular lipid levels in mutant livers are due to impaired lipoprotein uptake. Temporal downregulation of VEGF signaling revealed that it is crucial at all developmental stages of hepatic vascular morphogenesis, and repression of the dominant-negative effect can rescue the phenotype. These findings provide the first genetic evidence that VEGF dynamically regulates SEC fenestration during liver organogenesis, a process that is required for lipoprotein uptake by the liver.

We demonstrate that the site-specific integrase encoded by phage TP901-1 of Lactococcus lactis subsp. cremoris has potential as a tool for engineering mammalian genomes. We constructed vectors that express this integrase in Escherichia coli and in mammalian cells and developed a simple plasmid assay to measure the frequency of intramolecular integration mediated by the integrase. We used the assay to document that the integrase functions efficiently in E. coli and determined that for complete reaction in E. coli, the minimal sizes of attB and attP are 31 and 50 bp, respectively. We carried out partial purification of TP901-1 integrase protein and demonstrated its functional activity in vitro in the absence of added cofactors, characterizing the time course and temperature optimum of the reaction. Finally, we showed that when expressed in human cells, the TP901-1 integrase carries out efficient intramolecular integration on a transfected plasmid substrate in the human cell environment. The TP901-1 phage integrase thus represents a new reagent for manipulating DNA in living mammalian cells.Prokaryotic enzymes have supplied us with abundant tools for engineering DNA. For example, restriction enzymes and ligases, largely derived from bacterial and phage genomes, provided the tools for recombinant DNA. This technology has allowed construction of molecules at will in vitro, causing a wholesale transformation of biomedical science over the past 25 years. More recently, in vivo engineering of the genomes of living higher eukaryotic cells is becoming possible, often through the agency of prokaryotic enzymes such as Cre, an autonomous, site-specific, tyrosine-catalyzed recombinase from phage P1 (1). Recombinases such as Cre and FLP require no host-specific cofactors and perform well in higher eukaryotic cells, carrying out efficient site-specific recombination between two identical recognition sites (18,22). These enzymes are useful for carrying out deletion and translocation-type recombination reactions in living cells (21).Another useful reaction is integration for the purpose of creating knockin and knockout alterations of the genome, such as those desirable in gene therapy, creation of transgenic organisms, and modification of cells in culture. For integration, a unidirectional recombinase such as a phage integrase is ideal, because there is no reverse reaction that could depress net integration frequency (9). Phage integrases mediate recombination between nonidentical phage attP and bacterial attB recognition sites (13). The well-studied lambda integrase is, like Cre and FLP, a member of the tyrosine-catalyzed recombinase family (17). However, the integrases from lambda phage and the closely related phage HK022 have cofactor requirements that hamper their use in eukaryotic cells (11,15).Some phage integrases are members of the unrelated serinecatalyzed family of recombinases (24) and are autonomous with no cofactor requirements, which makes them potentially ideal for use in foreign host environments, such as mammalia...

We have constructed plasmid DNA vectors that contain Epstein-Barr virus (EBV) sequences and the human gene (SERPINA1) encoding alpha1-Antitrypsin (AAT). We demonstrate that a plasmid carrying the full SERPINA1 on a 19-kb genomic fragment and the EBV gene EBNA1 and its family of repeats binding sites undergoes efficient extrachromosomal replication in dividing mammalian tissue culture cells. Therefore, use of a whole genomic therapeutic gene to provide both replication and gene expression may be an effective gene therapy vector design, if the target cells are dividing. The efficacy of this same vector for expression of AAT in vivo in the nondividing cells of mouse liver was determined by hydrodynamic injection of naked plasmid DNA by means of the tail vein. A single injection of an EBV/genomic SERPINA1 vector provided >300 microg/ml of AAT, which approached normal plasma levels and persisted for the >9-month duration of the experiment. These data exceed most previously reported values, probably due to sequences in the genomic DNA that resist silencing of gene expression, possibly in combination with favorable effects on expression provided by the EBV sequences. These results demonstrate that plasmid DNA with the correct cis-acting sequences can provide in vivo long-term expression of protein at high levels that are therapeutically relevant for gene therapy.

Phage integrases catalyze site-specific, unidirectional recombination between two short att recognition sites. Recombination results in integration when the att sites are present on two different DNA molecules and deletion or inversion when the att sites are on the same molecule. Here we demonstrate the ability of the φC31 integrase to integrate DNA into endogenous sequences in the mouse genome following microinjection of donor plasmid and integrase mRNA into mouse single-cell embryos. Transgenic early embryos and a mid-gestation mouse are reported. We also demonstrate the ability of the φC31, R4, and TP901-1 phage integrases to recombine two introduced att sites on the same chromosome in human cells, resulting in deletion of the intervening material. We compare the frequencies of mammalian chromosomal deletion catalyzed by these three integrases in different chromosomal locations. The results reviewed here introduce these bacteriophage integrases as tools for site-specific modification of the genome for the creation and manipulation of transgenic mammals.