Targeting Systemic

In order to evaluate the effect of AGT gene disruption on blood pressure regulation, conventional gene targeting in embryonic stem cells was used to delete the AGT gene (10,11). As expected, homozygous AGT knockout mice had no detectable plasma AGT whereas there was a 58% decrease in heterozygous mutant mice. Survival studies have shown that absence of a functional AGT gene is compatible with survival to birth, but postnatal survival is severely compromised, perhaps because of the altered renovascular development in these mice (17). Furthermore, compared to wild-type mice, knockout of the AGT gene resulted in a marked decrease in blood pressure.

Further evidence for the crucial role of AGT in blood pressure regulation is derived from studies of mice with different copies of the AGT gene (11). Plasma AGT levels increase with the AGT gene copy number (by 35% in 1 copy, 124% in 3 copies, and 145% in 4 copies). Interestingly, blood pressure increased in proportion to the increase in AGT

gene copy number, averaging 8 mmHg per copy. This experiment demonstrates clearly that variation in circulating AGT can affect blood pressure. It also establishes the potential for an effect on blood pressure by genetic influences arising from differences in circulating AGT.

We generated transgenic mice containing the entire hAGT gene in order to study its tissue- and cell-specific expression and its role in the pathogenesis of hypertension (14). The transgene employed contains all five exons and intervening intron sequences, and extends approx 1.2 kb upstream and 1.4 kb downstream of the gene. The expression pattern of hAGT gene was found to be very consistent with the tissue-specific expression of the mouse and rat AGT genes and with the expected pattern of AGT expression in humans. Consistent with previous reports, analysis of the cellular origin of kidney hAGT in these transgenic mice revealed that it was exclusively localized to the epithelial cells of the proximal convoluted tubules (18). All together, these data demonstrated that the hAGT transgene was appropriately regulated. It also suggests that these transgenic mice represent a valid model for examining the regulation of hAGT gene.

The hAGT released into the plasma of the transgenic mice was determined to be functional because infusion of a single bolus dose of purified human renin resulted in transient increase in blood pressure of approx 30 mmHg within 2 min. Furthermore, crossbreeding of these mice expressing hAGT with transgenic mice containing a genomic clone encoding the human renin resulted in chronic elevation of blood pressure (19). These double-transgenic mice also had an altered baroreflex response (i.e., resetting of the arterial baroreflex of heart rate to a higher pressure without significantly changing the gain or sensitivity of the reflex).

We also evaluated whether elements of the human RAS could functionally overcome the hypotension, renal pathology, and reduced survival observed in the AGT knockout mice. This was achieved by breeding the double-transgenic (hAGT/human renin) mice with the AGT knockout mice (20). We found that the presence of both human transgenes rescued the postnatal lethality in AGT knockout mice. The lower blood pressure and renal lesions observed in AGT knockout mice were also corrected by the presence of the human transgenes. These findings indicate that it is the absence of ANG II, and not AGT per se, that is responsible for the abnormalities observed in the AGT mutant mice. These data demonstrate also the ability of human renin and AGT genes to replace the mouse AGT gene.

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