Transgenic mice for studies of the renin-angiotensin system in hypertension

Liddle's syndrome (pseudoaldosteronism) is an autosomal dominant form of human hypertension characterized by a constellation of findings suggesting constitutive activation of the amiloride-sensitive distal renal epithelial sodium channel. We demonstrate complete linkage of the gene encoding the beta subunit of the epithelial sodium channel to Liddle's syndrome in Liddle's original kindred. Analysis of this gene reveals a premature stop codon that truncates the cytoplasmic carboxyl terminus of the encoded protein in affected subjects. Analysis of subjects with Liddle's syndrome from four additional kindreds demonstrates either premature termination or frameshift mutations in this same carboxy-terminal domain in all four. These findings demonstrate that Liddle's syndrome is caused by mutations in the beta subunit of the epithelial sodium channel and have implications for the regulation of this epithelial ion channel as well as blood pressure homeostasis.

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.

There are two major angiotensin II receptor isoforms, AT1 and AT2. AT1 mediates the well-known pressor and mitogenic effects of angiotensin II, but the signalling mechanism and physiological role of AT2 has not been established. Its abundant expression in fetal tissues and certain brain nuclei suggest possible roles in growth, development and neuronal functions. Here we report the unexpected finding that the targeted disruption of the mouse AT2 gene resulted in a significant increase in blood pressure and increased sensitivity to the pressor action of angiotensin II. Thus AT2 mediates a depressor effect and antagonizes the AT1-mediated pressor action of angiotensin II. In addition, disruption of the AT2 gene attenuated exploratory behaviour and lowered body temperature. Our results show that angiotensin II activates AT1 and AT2, which have mutually counteracting haemodynamic effects, and that AT2 regulates central nervous system functions, including behaviour.