Targeted inactivation of the mouse alpha 2-macroglobulin gene

2-Antiplasmin Gene Deficiency in Mice Is Associated With Enhanced Fibrinolytic Potential Without Overt Bleeding

2-antiplasmin (2-AP) is the main physiologic plasmin inhibitor in mammalian plasma. Inactivation of the murine 2-AP gene was achieved by replacing, through homologous recombination in embryonic stem cells, a 7-kb genomic sequence encoding the entire murine protein (exon 2 through part of exon 10, including the stop codon) with theneomycin resistance expression cassette. Germline transmission of the mutated allele was confirmed by Southern blot analysis. Mendelian inheritance of the inactivated 2-AP allele was observed, and homozygous deficient (2-AP−/−) mice displayed normal fertility, viability, and development. Reverse transcription-polymerase chain reaction confirmed the absence of 2-AP mRNA in kidney and liver from 2-AP−/− mice, in contrast to wild-type (2-AP+/+) mice. Immunologic and functional 2-AP levels were undetectable in plasma of 2-AP−/− mice, and were about half of wild-type in heterozygous littermates (2-AP+/−). Other hemostasis parameters, including plasminogen activator inhibitor-1, plasminogen, fibrinogen, hemoglobin, hematocrit, and blood cell counts were comparable for 2-AP+/+, 2-AP+/−, and 2-AP−/− mice. After amputation of tail or toe tips, bleeding stopped spontaneously in 2-AP+/+, as well as in 2-AP+/− and 2-AP−/− mice. Spontaneous lysis after 4 hours of intravenously injected 125I-fibrin–labeled plasma clots was significantly higher in 2-AP−/− than in 2-AP+/+ mice when injecting clots prepared from 2-AP+/+ plasma (78% ± 5% v 46% ± 9%; mean ± SEM, n = 6 to 7; P = .02) or from 2-AP−/−plasma (81% ± 5% v 46% ± 5%; mean ± SEM, n = 5; P = .008). Four to 8 hours after endotoxin injection, fibrin deposition in the kidneys was significantly reduced in 2-AP−/− mice, as compared with 2-AP+/+ mice (P ≤ .005). Thus, 2-AP−/− mice develop and reproduce normally; they have an enhanced endogenous fibrinolytic capacity without overt bleeding.

2-Antiplasmin Gene Deficiency in Mice Is Associated With Enhanced Fibrinolytic Potential Without Overt Bleeding

2-antiplasmin (2-AP) is the main physiologic plasmin inhibitor in mammalian plasma. Inactivation of the murine 2-AP gene was achieved by replacing, through homologous recombination in embryonic stem cells, a 7-kb genomic sequence encoding the entire murine protein (exon 2 through part of exon 10, including the stop codon) with theneomycin resistance expression cassette. Germline transmission of the mutated allele was confirmed by Southern blot analysis. Mendelian inheritance of the inactivated 2-AP allele was observed, and homozygous deficient (2-AP−/−) mice displayed normal fertility, viability, and development. Reverse transcription-polymerase chain reaction confirmed the absence of 2-AP mRNA in kidney and liver from 2-AP−/− mice, in contrast to wild-type (2-AP+/+) mice. Immunologic and functional 2-AP levels were undetectable in plasma of 2-AP−/− mice, and were about half of wild-type in heterozygous littermates (2-AP+/−). Other hemostasis parameters, including plasminogen activator inhibitor-1, plasminogen, fibrinogen, hemoglobin, hematocrit, and blood cell counts were comparable for 2-AP+/+, 2-AP+/−, and 2-AP−/− mice. After amputation of tail or toe tips, bleeding stopped spontaneously in 2-AP+/+, as well as in 2-AP+/− and 2-AP−/− mice. Spontaneous lysis after 4 hours of intravenously injected 125I-fibrin–labeled plasma clots was significantly higher in 2-AP−/− than in 2-AP+/+ mice when injecting clots prepared from 2-AP+/+ plasma (78% ± 5% v 46% ± 9%; mean ± SEM, n = 6 to 7; P = .02) or from 2-AP−/−plasma (81% ± 5% v 46% ± 5%; mean ± SEM, n = 5; P = .008). Four to 8 hours after endotoxin injection, fibrin deposition in the kidneys was significantly reduced in 2-AP−/− mice, as compared with 2-AP+/+ mice (P ≤ .005). Thus, 2-AP−/− mice develop and reproduce normally; they have an enhanced endogenous fibrinolytic capacity without overt bleeding.

Plasminogen activator inhibitor-1 is expressed in cultured rat Sertoli cells

Sertoli cells secrete plasminogen activators (PAs) on both sides of the blood-testis barrier, i.e., in the basal and apical compartments of the seminiferous tubules, whereas peritubular cells secrete plasminogen activator inhibitor-1 (PAI-1), a fast-acting and specific PA inhibitor. While it is likely that PAI-1 produced by peritubular cells counteracts the basal secretion of PA, the nature of the PA inhibitor acting in the apical compartment remains to be demonstrated. In the present study, we showed that Sertoli cells recovered from 20-day-old rats and cultured contained a transcript of 3-3.2 kilobases, which hybridized specifically to a PAI-1 cDNA probe (Northern blot). We verified that the observed PAI-1 transcript could not result solely from the peritubular cells (weakly contaminating the Sertoli cell cultures), by comparing PAI-1 mRNA levels of Sertoli and peritubular cells recovered from 20-day-old rats and cultured. We also demonstrated that cultured Sertoli cells secreted a protein that complexed with tissue-type PA (zymography), indicating that it was biologically active. This protein comigrated with purified PAI-1 as a doublet of 46 and 49 kDa (Western blot). The trophic hormone FSH decreased PAI-1 messenger RNA as well as immunoreactive PAI-1 protein (probably via the cAMP protein kinase A pathway), whereas transforming growth factor ss1 and basic fibroblast growth factor (in a nanomolar concentration) increased both of these. These observations support the hypothesis that PAI-1 is expressed by Sertoli cells and is under a complex hormonal (FSH) and paracrine and/or autocrine control exerted at least by basic fibroblast growth factor and transforming growth factor ss1.

Localization of the binding site for transforming growth factor-beta in human alpha2-macroglobulin to a 20-kDa peptide that also contains the bait region

alpha2-Macroglobulin (alpha2M) functions as a major carrier of transforming growth factor-beta (TGF-beta) in vivo. The goal of this investigation was to characterize the TGF-beta-binding site in alpha2M. Human alpha2M, which was reduced and denatured to generate 180-kDa subunits, bound TGF-beta1, TGF-beta2, and NGF-beta in ligand blotting experiments. Cytokine binding was not detected with bovine serum albumin that had been reduced and alkylated, and only minimal binding was detected with purified murinoglobulin. To localize the TGF-beta-binding site in alpha2M, five cDNA fragments, collectively encoding amino acids 122-1302, were expressed as glutathione S-transferase (GST) fusion proteins. In ligand blotting experiments, TGF-beta2 bound only to the fusion protein (FP3) that includes amino acids 614-797. FP3 bound 125I-TGF-beta1 and 125I-TGF-beta2 in solution, preventing the binding of these growth factors to immobilized alpha2M-methylamine (alpha2M-MA). The IC50 values were 33 +/- 5 and 26 +/- 6 nM for TGF-beta1 and TGF-beta2, respectively; these values were comparable with or lower than those determined with native alpha2M or alpha2M-MA. A GST fusion protein that includes amino acids 798-1082 of alpha2M (FP4) and purified GST did not inhibit the binding of TGF-beta to immobilized alpha2M-MA. FP3 (0.2 microM) neutralized the activity of TGF-beta1 and TGF-beta2 in fetal bovine heart endothelial (FBHE) cell proliferation assays; FP4 was inactive in this assay. FP3 also increased NO synthesis by RAW 264.7 cells, mimicking an alpha2M activity that has been attributed to the neutralization of endogenously synthesized TGF-beta. Thus, we have isolated a peptide corresponding to 13% of the alpha2M sequence that binds TGF-beta and neutralizes the activity of TGF-beta in two separate biological assays.

Gene disruption in mice: models of development and disease

Gene targeting technology in mice by homologous recombination has become an important method to generate loss-of-function of genes in a predetermined locus. Although the inactivation is limited to irreversible alteration of chromosomal DNA and a surprising variety of genes have given unexpected and disappointing results, modification of the basic technology now provides additional choices for a more specific and variety of manipulations of the mouse genome. This includes conditional cell-type specific gene targeting, knockin technique and the induction of the specific balanced chromosomal translocations. In the past decade this technique not only generated a wealth of knowledge concerning the roles of growth factors, oncogenes, hormone receptors and Hox genes but also helped to produce animal models for several human genetic disorders. In the future it may provide more powerful and necessary tools to dissect the psychiatric disorders, understanding the complex central nervous system and to correct the inherited disorders.

Cloning and targeted deletion of the mouse fetuin gene

We proposed that the alpha2-Heremans Schmid glycoprotein/fetuin family of serum proteins inhibits unwanted mineralization. To test this hypothesis in animals, we cloned the mouse fetuin gene and generated mice lacking fetuin. The gene consists of seven exons and six introns. The cystatin-like domains D1 and D2 of mouse fetuin are encoded by three exons each, whereas a single terminal exon encodes the carboxyl-terminal domain D3. The promoter structure is well conserved between rat and mouse fetuin genes within the regions shown to bind transcription factors in the rat system. Expression studies demonstrated that mice homozygous for the gene deletion lacked fetuin protein and that mice heterozygous for the null mutation produced roughly half the amount of fetuin protein produced by wild-type mice. Fetuin-deficient mice were fertile and showed no gross anatomical abnormalities. However, the serum inhibition of apatite formation was compromised in these mice as well as in heterozygotes. In addition, some homozygous fetuin-deficient female ex-breeders developed ectopic microcalcifications in soft tissues. These results corroborate a role for fetuin in serum calcium homeostasis. The fact that generalized ectopic calcification did not occur in fetuin-deficient mice proves that additional inhibitors of phase separation exist in serum.

The natural killer gene complex: a genetic basis for understanding natural killer cell function and innate immunity

The natural killer gene complex encodes proteins, some of which are structurally unrelated, that impact on NK-cell function. Detailed analyses have indicated that these molecules are involved in NK-cell recognition, activation, and inhibition. The importance of this genomic region is highlighted by studies indicating that NKC-associated genes significantly influence NK cell-mediated innate host defense against life-threatening pathogens and that the NKC is conserved among diverse species. Thus, further elucidation of the NKC and its gene products will provide a genetic basis for understanding innate immunity and NK-cell activity at the molecular level.

Murine alpha-macroglobulins demonstrate divergent activities as neutralizers of transforming growth factor-beta and as inducers of nitric oxide synthesis. A possible mechanism for the endotoxin insensitivity of the alpha2-macroglobulin gene knock-out mouse

alpha2-Macroglobulin null mice demonstrate increased resistance to endotoxin challenge (Umans, L., Serneels, L., Overbergh, L., Van Leuven, F., and Van den Berghe, H. (1995) J. Biol. Chem. 270, 19778-19785). We hypothesized that this phenotype might reflect the function of murine alpha2M (malpha2M) as a neutralizer of transforming growth factor-beta (TGF-beta) and inducer of nitric oxide synthesis in vivo. When incubated with wild-type mouse plasma, TGF-beta1 and TGF-beta2 bound only to malpha2M. Alternative TGF-beta-binding proteins were not detected in plasma from alpha2M(-/-) mice. Wild-type mouse plasma, but not plasma from alpha2M(-/-) mice, inhibited TGF-beta1 binding to TGF-beta receptors on fibroblasts. Purified malpha2M bound TGF-beta1 and TGF-beta2 with similar affinity; the KD values were 28 +/- 4 and 33 +/- 4 nM, respectively. Murinoglobulin, the second murine alpha-macroglobulin, bound both TGF-beta isoforms with 30-fold lower affinity. Malpha2M counteracted the activities of TGF-beta1 and TGF-beta2 in an endothelial cell growth assay. Malpha2M also induced NO synthesis when incubated with RAW 264.7 cells, an activity which probably results from the neutralization of autocrine TGF-beta activity. Human alpha2M induced NO synthesis comparably to malpha2M; however, MUG had no effect. These studies demonstrate that the ability to neutralize TGF-beta is a property of malpha2M, which is not redundant in the murine alpha-macroglobulin family or in murine plasma. Malpha2M is the only murine alpha-macroglobulin that promotes NO synthesis. The absence of malpha2M, in alpha2M(-/-) mice, may allow TGF-beta to more efficiently suppress excessive iNOS expression following endotoxin challenge.

Generation and characterization of urokinase receptor-deficient mice

Mice homozygously deficient for the urokinase-type plasminogen activator (u-PA) receptor (u-PAR-1-) were generated by homologous recombination in D3, embryonic stem cells. The genomic sequences comprising exon 2 through 5 of the u-PAR gene were replaced by the neomycin resistance gene, resulting in inactivation of both u-PAR splice variants. The inactivated u-PAR allele was transmitted via mendelian inheritance, and fertility. Inactivation of u-PAR was confirmed by the absence of binding of rabbit anti-murine u-PAR or of an aminoterminal fragment of murine u-PA (mu-PA.1-48) to u-PAR-1- embryonic fibroblasts and macrophages. u-PAR-1- mice displayed normal lysis of a murine plasma clot injected via the jugular vein. Invasion of macrophages into the peritoneal cavity after thioglycollate stimulation was similar in u-PAR-1- and u-PAR-1- mice. u-PAR-1- peritoneal macrophages had a threefold decreased initial rate of u-PA-mediated plasminogen activation in vitro but degraded extracellular matrix proteins in vitro as efficiently as u-PAR-1- macrophages.