Blood pressures and cardiovascular homeostasis in mice having reduced or absent angiotensin-converting enzyme gene function

We studied cardiovascular phenotypes in wild-type (+/+), heterozygous (+/-), and homozygous mutant (-/-) mice for an insertional inactivation of the angiotensin-converting enzyme (ACE) gene (Ace in mice, ACE in humans). Compared with +/+ mice, baseline mean arterial pressure was not significantly altered in +/- mice but was reduced by 51+/-4 mm Hg in -/- mice. Although the pressor response to injected angiotensin II did not differ significantly in the three genotypic groups, the pressor response to angiotensin I was strongly affected by Ace genotype: Compared with the response in the +/+ group (+26% of baseline), the response to Ang I was close to half normal (+12%) in the +/- group and virtually abolished (+1%) in the -/- group. The depressor response to injected bradykinin was significantly enhanced in the +/- and -/- groups compared with the +/+ group. Ace expression and ACE activity were directly related to functional Ace copy number, and renin and angiotensinogen mRNA levels were inversely related to Ace copy number. Angiotensin type 1A receptor mRNA levels were not significantly different in the +/+, +/-, and -/- groups. We conclude that (1) ACE is essential for the maintenance of normal blood pressure; (2) subnormal levels of ACE affect the blood pressure responses to infused angiotensin I and bradykinin in vivo; and (3) compensations for inactivation of one Ace copy, which include increased expression of renin, normalize blood pressure in heterozygotes.

Angiotensin-Converting Enzyme Gene and Atherosclerosis

Common variants of the angiotensin-converting enzyme (ACE) gene ( ACE in humans, Ace in mice) associated with changes in circulating ACE activities have been suggested to confer differential risks for atherosclerosis. Using a mouse model of atherosclerosis induced by heterozygosity for apolipoprotein E gene disruption and an atherogenic diet, we have studied the impact on atherogenesis of a mutation that changes the level of function of Ace . We find that this genetically determined change does not influence the size or complexity of atherosclerotic lesions. Ace genotype was not a significant determinant of lesion size in female (+/+=12.9±1.5 and +/-=1 1.7±1.6 μm 2 ×l0 4 ) or male (+/+=0.95±0.25 and +/-=1.83±0.59 μm 2 ×l0 4 ) mice; however, lesions were significantly larger ( P <.001) in female than male mice. Ace genotype also did not affect lesion complexity; however, lesions in females showed significantly increased frequency of cholesterol clefts, acellular cores, fibrous caps, and calcifications compared with those in males. The hypothesis that genetic variation in the level of ACE gene expression affects the development of atherosclerosis is not supported by these findings. ( Arterioscler Thromb Vase Biol. 1997;17:1245-1250.)

Angiotensin-converting enzyme gene and atherosclerosis

Common variants of the angiotensin-converting enzyme (ACE) gene (ACE ie humans, Ace in mice) associated with changes in circulating ACE activities have been suggested to confer differential risks for atherosclerosis. Using a mouse model of atherosclerosis induced by heterozygosity for apolipoprotein E gene disruption and an atherogenic diet, we have studied the impact on atherogenesis of a mutation that changes the level of function of Ace. We find that this genetically determined change does not influence the size or complexity of atherosclerotic lesions. Ace genotype was not a significant determinant of lesion size in female (+/+ = 12.9 +/- 1.5 and +/- = 11.7 +/- 1.6 microns2 x 10(4)) or male (+/+ = 0.95 +/- 0.25 and +/- = 1.83 +/- 0.59 microns2 x 10(4)) mice; however, lesions were significantly larger (P < .001) in female than male mice. Ace genotype also did not affect lesion complexity; however, lesions in females showed significantly increased frequency of cholesterol clefts, acellular cores, fibrous caps, and calcifications compared with those in males. The hypothesis that genetic variation in the level of ACE gene expression affects the development of atherosclerosis is not supported by these findings.

Inactivation of the mouse HPRT locus by a 203-bp retroposon insertion and a 55-kb gene-targeted deletion: establishment of new HPRT-deficient mouse embryonic stem cell lines

To obtain useful hypoxanthine phosphoribosyl-transferase (HPRT)-deficient mouse ES cell lines, two different methods were employed: (i) selection of spontaneous 6-TG-resistant mutants and (ii) gene targeting of the HPRT locus. The first approach resulted in the establishment of E14.1TG3B1, a spontaneous HPRT-deficient cell line with an insertional mutation of 203 bp in the third exon of the HPRT gene. The insert is highly homologous to the B2 mouse repetitive element and has all the expected retroposon characteristics, thus providing an example of gene inactivation by retroposon insertion. This clone exhibited stable 6-TG resistance and high germ-line transmission frequency. Thus E14.1TG3B1 is a useful ES cell line for modifying the mouse genome using the HPRT gene as a selection marker and for transmission at a high frequency into the mouse germ line. The second approach resulted in a 55-kb deletion of the mouse HPRT locus, demonstrating the feasibility of replacement-targeting vectors to generate large genomic DNA deletions.

Angiotensin II responses in AT1A receptor-deficient mice: a role for AT1B receptors in blood pressure regulation

Most of the classic functions of the renin-angiotensin system are mediated by type 1 (AT1) angiotensin receptors, of which two subtypes, AT1A and AT1B, have been identified. However, distinct functions for these two AT1 receptors have been difficult to separate. We examined the pressor effects of angiotensin II in Agtr1A -/- mice, which lack AT1A receptors. In enalapril-pretreated Agtr1A -/- mice, angiotensin II caused significant and dose-proportional increases in mean arterial pressure. This pressor response was not blocked by pretreatment with sympatholytic agents but was completely inhibited by the AT1-receptor antagonists, losartan and candesartan, suggesting that it is directly mediated by AT1B receptors. Chronic treatment of Agtr1A -/- mice with losartan reduced systolic blood pressure from 80 +/- 5 to 72 +/- 4 mmHg (P < 0.04), suggesting a role for AT1B receptors in chronic blood pressure regulation. These studies provide the first demonstration of in vivo pressor effects mediated by AT1B receptors and demonstrate that, when AT1A receptors are absent, the AT1B receptor contributes to the regulation of resting blood pressure.

Complementation of reduced survival, hypotension, and renal abnormalities in angiotensinogen-deficient mice by the human renin and human angiotensinogen genes

The aim of this study was to determine whether elements of the human renin-angiotensin system (RAS) could functionally replace elements of the mouse RAS by complementing the reduced survival and renal abnormalities observed in mice carrying a gene-targeted deletion of the mouse angiotensinogen gene (mAgt). Double transgenic mice containing the human renin (HREN) and human angiotensinogen (HAGT) genes were bred to mice heterozygous for the mAgt deletion and the compound heterozygotes were identified and intercrossed. The resulting progeny (n = 139) were genotyped at each locus and the population was stratified into two groups: the first containing both human transgenes (RA+) and the second containing zero or one, but not both human transgenes (RA-). Despite appropriate Mendelian ratios of RA- mice that were wildtype (+/+), heterozygous (+/-), and homozygous (-/-) for the deletion of mAgt at birth, there was reduced survival of RA- mAgt-/- mice to adulthood (P < 0.001 by chi2). In contrast, we observed appropriate Mendelian ratios of RA+ mAgt+/+, RA+ mAgt+/-, and RA+ mAgt-/- mice at birth and in adults (P > 0.05 by chi2). These results demonstrate that the presence of both human transgenes rescues the postnatal lethality in mAgt-/- mice. The renal histopathology exhibited by RA- mAgt-/- mice, including thickened arterial walls, severe fibrosis, lymphocytic infiltration, and atrophied parenchyma, was also rescued in the RA+ mAgt-/- mice. Direct arterial blood pressure recordings in conscious freely moving mice revealed that BP (in mmHg) varied proportionally to mAgt gene copy number in RA+ mice (approximately 20 mmHg per mAgt gene copy, P < 0.001). BP in RA+ mAgt-/- mice (132+/-3, n = 14) was intermediate between wild-type (RA- mAgt+/+, 105+/-2, n = 9) and RA+ mAgt+/+ (174+/-3, n = 10) mice. These studies establish that the human renin and angiotensinogen genes can functionally replace the mouse angiotensinogen gene, and provides proof in principle that we can examine the regulation of elements of the human RAS and test the significance of human RAS gene variants by a combined transgenic and gene targeting approach.

Aberrant renal vascular morphology and renin expression in mutant mice lacking angiotensin-converting enzyme

To determine whether angiotensin-converting enzyme plays a role in the development and maintenance of normal renal architecture, the renal morphology of 10- to 12-month-old female mice homozygous for a disruption of the converting enzyme gene was compared with that of age-matched wild-type mice. Tubular obstruction, dilatation, and atrophy were present in all kidneys from the homozygous mutant mice but absent in wild types; two kidneys from 4 mutant mice but none from the wild types were hydronephrotic. The entire arterial vascular tree, microdissected from mice with no converting enzyme, was grossly distorted in comparison to the vasculature of wild-type mice; all intrarenal arterial vessels were widened and thickened, including the terminal (afferent) arterioles. In wild-type mice kidneys, renin-positive cells were detected exclusively in a juxtaglomerular localization. In contrast, abnormal distribution of renin immunostaining was observed in mice without converting enzyme; scattered renin-positive cells were seen along the arterial vessels, often in a perivascular localization, and interstitial renin-positive cells surrounded glomeruli. Kidney renin mRNA was increased more than 32-fold in the mutant mice compared with wild types. Northern blot analysis revealed that this increase included the accumulation of large amounts of smaller renin RNA transcripts. In summary, mice lacking the converting enzyme exhibit abnormal renal vessels and tubules. Renin synthesis is increased, accompanied by the presence of small renin mRNA species, and renin is present mainly in interstitial and perivascular cells. We conclude that angiotensin-converting enzyme is necessary to preserve normal kidney architecture and the normal pattern of renin expression.

Angiotensin-converting enzyme gene mutations, blood pressures, and cardiovascular homeostasis

A common polymorphism of the angiotensin-converting enzyme (ACE) gene (ACE in humans, Ace in mice) is associated with differences in circulating ACE levels that may confer a differential risk for cardiovascular diseases. To study the effects of genetically determined changes in Ace gene function within a defined genetic and environmental background, we have studied mice having one, two, or three functional copies of the Ace gene at its normal chromosomal location. ACE activities in the serum increased progressively from 62% of normal in the one-copy animals to 144% of normal in the three-copy animals (P < 10(-15), n = 132). The blood pressures of the mice having from one to three copies of the Ace gene did not differ significantly, but the heart rates, heart weights, and renal tubulointerstitial volumes decreased significantly with increasing Ace gene copy number. The level of kidney renin mRNA in the one-copy mice was increased to 129 +/- 9% relative to that of the normal two-copy mice (100 +/- 4%, P = .01, n = 16). We conclude that significant homeostatic adaptations successfully normalize the blood pressures of mice that have quantitative changes in Ace gene function. Our results suggest only that quantitative changes in expression of the Ace gene will observably affect blood pressures when accompanied by additional environmental or genetic factors that together with Ace exceed the capacity of the homeostatic mechanisms.

Targeted disruption of the mouse topoisomerase I gene by camptothecin selection

Topoisomerase I has ubiquitous roles in important cellular functions such as replication, transcription, and recombination. In order to further characterize this enzyme in vivo, we have used gene targeting to inactivate the mouse Top-1 gene. A selection protocol using the topoisomerase I inhibitor camptothecin facilitated isolation of embryonic stem cell clones containing an inactivated allele; isolation of correctly targeted clones was enhanced 75-fold over that achieved by normal selection procedures. The disrupted Top-1 allele is embryonic lethal when homozygous, and development of such embryos fails between the 4- and 16-cell stages. Both sperm and oocytes containing the inactive allele maintain viability through the fertilization point, and thus gene expression of topoisomerase I is not required for gamete viability. These studies demonstrate that topoisomerase I is essential for cell growth and division in vivo. The Top-1 gene was also shown to be linked to the agouti locus.

Elevated blood pressures in mice lacking endothelial nitric oxide synthase

Nitric oxide produced in endothelial cells affects vascular tone. To investigate the role of endothelial nitric oxide synthase (eNOS) in blood pressure regulation, we have generated mice heterozygous (+/-) or homozygous (-/-) for disruption of the eNOS gene. Immunohistochemical staining with anti-eNOS antibodies showed reduced amounts of eNOS protein in +/- mice and absence of eNOS protein in -/- mutant mice. Male or female mice of all three eNOS genotypes were indistinguishable in general appearance and histology, except that -/- mice had lower body weights than +/+ or +/- mice. Blood pressures tended to be increased (by approximately 4 mmHg) in +/- mice compared with +/+, while -/- mice had a significant increase in pressure compared with +/+ mice (approximately 18 mmHg) or +/- mice (approximately 14 mmHg). Plasma renin concentration in the -/- mice was nearly twice that of +/+ mice, although kidney renin mRNA was modestly decreased in the -/- mice. Heart rates in the -/- mice were significantly lower than in +/- or +/+ mice. Appropriate genetic controls show that these phenotypes in F2 mice are due to the eNOS mutation and are not due to sequences that might differ between the two parental strains (129 and C57BL/6J) and are linked either to the eNOS locus or to an unlinked chromosomal region containing the renin locus. Thus eNOS is essential for maintenance of normal blood pressures and heart rates. Comparisons between the current eNOS mutant mice and previously generated inducible nitric oxide synthase mutants showed that homozygous mutants for the latter differ in having unaltered blood pressures and heart rates; both are susceptible to lipopolysaccharide-induced death.

Two distinct apolipoprotein B alleles in mice generated by a single 'in-out' targeting

'In-out' gene targeting using a hypoxanthine phosphoribosyltransferase (HPRT) minigene was applied to generate two new alleles in the gene (Apob) coding for apolipoprotein B (apo B) in murine embryonic stem (ES) cells. Homologous integration of the targeting vector during the 'in step' disrupted the Apob gene leading to an allele encoding apo B81, having a 19% carboxyl-terminal truncation. All six targeted cells obtained had more than one insert at the locus, and the chromosomal target sequence in four of them was changed during the recombination. These results suggest that concatenation of the targeting vector prior to insertion was needed to generate sufficient gene product to yield the HPRT+ phenotype, and that recombination between the concatenated DNA and endogenous DNA was a gene replacement more frequently than a simple insertion. The 'out step' recombination event which occurs between sequences duplicated in the 'in step', was planned to replace the sequences encoding the putative LDL receptor-binding domains of apo B100 with sequences encoding human beta-globin peptides (designated apo B100-beta). 6-Thioguanine (6-TG) resistant colonies were obtained from all the 'in-step' cell lines tested at frequencies of 10(-5) to 10(-4), but the frequency of physical loss of the HPRT sequences accompanied by retention of the modified Apob sequence was variable, indicating that mechanisms other than a simple excision are responsible for the generation of 6-TG resistance. Mice from the 'in-step' produce apo B81 and display characteristics of familial hypobetalipoproteinemia; some homozygotes develop hydrocephaly or exencephaly. Mice from the 'out-step' produce apo B100-beta and secrete lipoprotein particles containing the modified protein; their phenotypic changes are subtle, suggesting the lack of the putative LDL receptor-binding domains is not sufficient to increase the steady-state level of apo B100-beta particles above that of apo B100 particles in control mice.

Leukocyte lipid body formation and eicosanoid generation: cyclooxygenase-independent inhibition by aspirin

Lipid bodies, cytoplasmic inclusions that develop in cells associated with inflammation, are inducible structures that might participate in generating inflammatory eicosanoids. Cis-unsaturated fatty acids (arachidonic and oleic acids) rapidly induced lipid body formation in leukocytes, and this lipid body induction was inhibited by aspirin and nonsteroidal antiinflammatory drugs (NSAIDs). Several findings indicates that the inhibitory effect of aspirin and NSAIDs on lipid body formation was independent of cyclooxygenase (COX) inhibition. First, the non-COX inhibitor, sodium salicylate, was as potent as aspirin in inhibiting lipid body formation elicited by cis-fatty acids. Second, cis-fatty acid-induced lipid body formation was not impaired in macrophages from COX-1 or COX-2 genetically deficient mice. Finally, NSAIDs inhibited arachidonic acid-induced lipid body formation likewise in macrophages from wild-type and COX-1- and COX-2-deficient mice. An enhanced capacity to generate eicosanoids developed after 1 hr concordantly with cis-fatty acid-induced lipid body formation. Arachidonic and oleic acid-induced lipid body numbers correlated with the enhanced levels of leukotrienes B4 and C4 and prostaglandin E2 produced after submaximal calcium ionophore stimulation. Aspirin and NSAIDs inhibited both induced lipid body formation and the enhanced capacity for forming leukotrienes as well as prostaglandins. Our studies indicate that lipid body formation is an inducible early response in leukocytes that correlates with enhanced eicosanoid synthesis. Aspirin and NSAIDs, independent of COX inhibition, inhibit cis-fatty acid-induced lipid body formation in leukocytes and in concert inhibit the enhanced synthesis of leukotrienes and prostaglandins.

Proliferation of multipotent hematopoietic cells controlled by a truncated erythropoietin receptor transgene

The long-term efficacy of gene therapy using bone marrow transplantation requires the engraftment of genetically altered totipotent hematopoietic stem cells (THSCs). Ex vivo expansion of corrected THSCs is one way to increase the efficiency of the procedure. Similarly, selective in vivo expansion of the therapeutic THSCs rather than the endogenous THSCs could favor the transplant. To test whether a conferred proliferative advantage gene can facilitate the in vitro and in vivo expansion of hematopoietic stem cells, we have generated transgenic mice expressing a truncated receptor for the growth factor erythropoietin. These mice are phenotypically normal, but when treated in vivo with exogenous erythropoietin they exhibit a marked increase in multipotent, clonogenic hematopoietic cells [colony-forming units in the spleen (CFU-S) and CFUs that give rise to granulocytes, erythroid cells, macrophages, and megakaryocytes within the same colony (CFU-GEMM)] in comparison with the wild-type mice. In addition, long-term in vitro culture of tEpoR transgenic bone marrow in the presence of erythropoietin induces exponential expansion of trilineage hematopoietic stem cells not seen with wild-type bone marrow. Thus, the truncated erythropoietin receptor gene shows promise as a means for obtaining cytokine-inducible hematopoietic stem cell proliferation to facilitate the direct targeting of THSCs and to provide a competitive repopulation advantage for transplanted therapeutic stem cells.

Single-copy transgenic mice with chosen-site integration

We describe a general way of introducing transgenes into the mouse germ line for comparing different sequences without the complications of variation in copy number and insertion site. The method uses homologous recombination in embryonic stem (ES) cells to generate mice having a single copy of a transgene integrated into a chosen location in the genome. To test the method, a single copy murine bcl-2 cDNA driven by either a chicken beta-actin promoter or a human beta-actin promoter has been inserted immediately 5' to the X-linked hypoxanthine phosphoribosyltransferase locus by a directly selectable homologous recombination event. The level of expression of the targeted bcl-2 transgene in ES cells is identical in independently isolated homologous recombinants having the same promoter yet varies between the different promoters. In contrast, the expression of bcl-2 transgenes having the same (chicken beta-actin) promoter varies drastically when they are independently integrated at random insertion sites. Both promoters direct broad expression of the single-copy transgene in mice derived from the respective targeted ES cells. In vitro and in vivo, the human beta-actin promoter consistently directed a higher level of transgene expression than the chicken beta-actin promoter.

Blood pressure and fluid-electrolyte balance in mice with reduced or absent ANP

Atrial natriuretic peptide (ANP)-gene knockout mice of three genotypes (+/+, +/-, and -/-) were maintained on a low-salt diet (0.008% NaCl). They were then fed either the same low-salt diet or a high-salt diet (8% NaCl) for 1 wk. No differences were found among genotypes in daily food and water intakes or in urinary volume and electrolyte excretions. Arterial blood pressures measured in anesthetized animals at the end of the dietary regimen were significantly and similarly increased in -/- compared with +/+ mice on each diet. Renal excretion of fluid and electrolytes was measured in anesthetized mice before and after acute blood volume expansion. No genotype differences were observed before volume expansion. After volume expansion the wild-type (+/+) mice had much greater saluretic responses than either the heterozygous (+/-) or the homozygous mutant (-/-) animals on the low-salt diet but not on the high-salt diet. We conclude that ANP lowers blood pressure in the absence of detected changes in renal function; ANP is not essential for normal salt balance, even on high-salt intake; and ANP is essential for the natriuretic response to acute blood volume expansion on a low-salt but not high-salt intake.

A mouse model for beta 0-thalassemia

We have used a "plug and socket" targeting technique to generate a mouse model of beta 0-thalassemia in which both the b1 and b2 adult globin genes have been deleted. Mice homozygous for this deletion (Hbbth-3/Hbbth-3) die perinatally, similar to the most severe form of Cooley anemia in humans. Mice heterozygous for the deletion appear normal, but their hematologic indices show characteristics typical of severe thalassemia, including dramatically decreased hematocrit, hemoglobin, red blood cell counts, mean corpuscular volume, mean corpuscular hemoglobin, and mean corpuscular hemoglobin concentration, as well as dramatically increased reticulocyte counts, serum bilirubin concentrations, and red cell distribution widths. Tissue and organ damage typical of beta-thalassemia, such as bone deformities and splenic enlargement due to increased hematopoiesis, are also seen in the heterozygous animals, as is spontaneous iron overload in the spleen, liver, and kidneys. The mice homozygous for the b1 and b2 deletions should be of great value in developing therapies for the treatment of thalassemias in utero. The heterozygous animals will be useful for studying the pathophysiology of thalassemias and have the potential of generating a model of sickle cell anemia when mated with appropriate transgenic animals.

Mice lacking inducible nitric oxide synthase are not resistant to lipopolysaccharide-induced death

Nitric oxide produced by cytokine-inducible nitric oxide synthase (iNOS) is thought to be important in the pathogenesis of septic shock. To further our understanding of the role of iNOS in normal biology and in a variety of inflammatory disorders, including septic shock, we have used gene targeting to generate a mouse strain that lacks iNOS. Mice lacking iNOS were indistinguishable from wild-type mice in appearance and histology. Upon treatment with lipopolysaccharide and interferon gamma, peritoneal macrophages from the mutant mice did not produce nitric oxide measured as nitrite in the culture medium. In addition, lysates of these cells did not contain iNOS protein by immunoblot analysis or iNOS enzyme activity. In a Northern analysis of total RNA, no iNOS transcript of the correct size was detected. No increases in serum nitrite plus nitrate levels were observed in homozygous mutant mice treated with a lethal dose of lipopolysaccharide, but the mutant mice exhibited no significant survival advantage over wild-type mice. These results show that lack of iNOS activity does not prevent mortality in this murine model for septic shock.

Prostaglandin synthase 2 gene disruption causes severe renal pathology in the mouse

The prostaglandin endoperoxide H synthase isoform 2, cyclooxygenase 2 (COX-2), is induced at high levels in migratory and other responding cells by pro-inflammatory stimuli. COX-2 is generally considered to be a mediator of inflammation. Its isoform, COX-1, is constitutively expressed in most tissues and is thought to mediate "housekeeping" functions. These two enzymes are therapeutic targets of the widely used nonsteroidal anti-inflammatory drugs (NSAIDs). To investigate further the different physiologic roles of these isoforms, we have used homologous recombination to disrupt the mouse gene encoding COX-2 (Ptgs2). Mice lacking COX-2 have normal inflammatory responses to treatments with tetradecanoyl phorbol acetate or with arachidonic acid. However, they develop severe nephropathy and are susceptible to peritonitis.

Prostaglandin synthase 1 gene disruption in mice reduces arachidonic acid-induced inflammation and indomethacin-induced gastric ulceration

Cyclooxygenases 1 and 2 (COX-1 and COX-2) are key enzymes in prostaglandin biosynthesis and the target enzymes for the widely used nonsteroidal anti-inflammatory drugs. To study the physiological roles of the individual isoforms, we have disrupted the mouse Ptgs1 gene encoding COX-1. Homozygous Ptgs1 mutant mice survive well, have no gastric pathology, and show less indomethacin-induced gastric ulceration than wild-type mice, even though their gastric prostaglandin E2 levels are about 1% of wild type. The homozygous mutant mice have reduced platelet aggregation and a decreased inflammatory response to arachidonic acid, but not to tetradecanoyl phorbol acetate. Ptgs1 homozygous mutant females mated to homozygous mutant males produce few live offspring. COX-1-deficient mice provide a useful model to distinguish the physiological roles of COX-1 and COX-2.

Analysis of transcription and estrogen insensitivity in the female mouse after targeted disruption of the estrogen receptor gene

We employed homologous recombination in mouse embryonic stem cells to disrupt the estrogen receptor (ER) gene. Subsequently generated mice that are homozygous for the gene disruption, termed ERKO, possess no demonstrable wild-type ER by Western blot analysis. However, the presence of residual high affinity binding, as detected by [3H]estradiol binding assays and sucrose gradients in uterine extracts from ERKO females prompted further investigation of transcription and translation products from the disrupted ER gene. Analysis of ERKO uterine messenger RNA (mRNA) by reverse transcriptase-polymerase chain reaction demonstrated that although no full-length wild-type ER mRNA was present, two smaller transcripts, labeled E1 and E2, were identified and partially sequenced. Both ERKO transcripts are splicing variants that result in the disrupting NEO sequence being partially or completely removed from the mRNA. In the ERKO-E2 variant, this results in a frame shift and the creation of at least two stop codons downstream. In the ERKO-E1 variant, the ER reading frame is preserved and encodes for a smaller mutant ER that could be the source of the residual estradiol binding. When this mutant form is overexpressed and characterized in vitro, it results in a smaller protein of the predicted size that possesses significantly reduced estrogen-dependent transcriptional activity compared with that of the wild-type ER. Despite residual amounts of an impaired ER variant, estrogen insensitivity in the female ERKOs was confirmed by the failure of estrogen treatment to induce known uterine markers of estrogen action, such as increased DNA synthesis, and transcription of the progesterone receptor, lactoferrin, and glucose-6-phosphate dehydrogenase genes. Furthermore, serum levels of estradiol in the ERKO female are more than 10-fold higher than those in the wild type, consistent with a syndrome of hormone insensitivity.

Requirement of MIP-1 alpha for an inflammatory response to viral infection

Macrophage inflammatory protein-1 alpha (MIP-1 alpha) is a chemokine that has pro-inflammatory and stem cell inhibitory activities in vitro. Its biologic role in vivo was examined in mice in which the gene encoding MIP-1 alpha had been disrupted. Homozygous MIP-1 alpha mutant (-/-) mice were resistant to Coxsackievirus-induced myocarditis seen in infected wild-type (+/+) mice. Influenza virus-infected -/- mice had reduced pneumonitis and delayed clearance of the virus compared with infected +/+ mice. The -/- mice had no overt hematopoietic abnormalities. These results demonstrate that MIP-1 alpha is an important mediator of virus-induced inflammation in vivo.

Gene targeting approaches to complex genetic diseases: atherosclerosis and essential hypertension

Gene targeting allows precise, predetermined changes to be made in a chosen gene in the mouse genome. To date, targeting has been used most often for generation of animals completely lacking the product of a gene of interest. The resulting "knockout" mice have confirmed some hypotheses, have upset others, but have rarely been uninformative. Models of several human genetic diseases have been produced by targeting--including Gaucher disease, cystic fibrosis, and the fragile X syndrome. These diseases are primarily determined by defects in single genes, and their modes of inheritance are well understood. When the disease under study has a complex etiology with multiple genetic and environmental components, the generation of animal models becomes more difficult but no less valuable. The problems associated with dissecting out the individual genetic factors also increases substantially and the distinction between causation and correlation is often difficult. To prove causation in a complex system requires rigorous adherence to the principle that the experiments must allow detection of the effects of changing only a single variable at one time. Gene targeting experiments, when properly designed, can test the effects of a precise genetic change completely free from the effects of differences in any other genes (linked or unlinked to the test gene). They therefore allow proofs of causation.

Male-female differences in fertility and blood pressure in ACE-deficient mice

Angiotensin-converting enzyme (ACE) is a dipeptidyl carboxy-peptidase that generates the vasoconstricting peptide angiotensin II and inactivates the vasodilating peptide bradykinin. The gene encoding ACE is composed of two homologous regions and codes for both a somatic and testis isoenzyme. Experiments with hypertensive rats and some, but not other, studies of humans suggest that sequences at or linked to the gene influence blood pressure. The testis-specific form of ACE has its own promoter within intron 12 (ref. 14), is encoded by the 3' region of the gene, and is found only in postmeiotic spermatogenic cells and sperm. Its function is unknown. Here we investigate the role of the Ace gene in blood pressure control and reproduction using mice generated to carry an insertional mutation that is designed to inactivate both forms of ACE. All homozygous female mutants were found to be fertile, but the fertility of homozygous male mutants was greatly reduced. Heterozygous males but not females had blood pressures that were 15-20 mm Hg less than normal, although both male and female heterozygotes had reduced serum ACE activity.