Genetic Evidence That Loss of Intra-Renal Angiotensinogen Is not the Cause of Lethality in Angiotensinogen Deficient Mice
Angiotensinogen (Agt) deficient mice are born in normal numbers but die before weaning presumably due to renal dysfunction caused by severe vascular lesions. Since Agt is expressed in renal proximal tubule cells, we hypothesized that its loss is responsible for the lethal phenotype. We generated two models to test this hypothesis by breeding transgenic mice expressing human renin (R+) with mice expressing human Agt either systemically (A+) or kidney-specifically (KA+). We then bred double transgenic mice (R+/A+ or R+/KA+) with Agt-/- mice to generate R+/A+ Agt+/- or R+/KA+ Agt+/-. We then intercrossed the compound heterozygotes by brother/sister mating. Each offspring of this cross was genotyped for the presence of the human transgenes and at the Agt locus. The population was then stratified into groups (double transgenic and non-double transgenic), and the data was analyzed by χ2. Based on Mendelian genetics, if survival is not an issue, 25% of the offspring should be Agt-/-. Of 63 R-/A- (non-double transgenic) offspring, only 3 survivors were genotyped as Agt-/-, whereas of 68 R+/A+ offspring, 17 were Agt -/- and all survived to adults. These data demonstrate that systemic expression of Agt and Ang-II can rescue the Agt deletion. On the contrary, of 65 R-/KA- and 63 R+/KA+ offspring, not a single mouse was genotyped as Agt-/- (χ2= 0.0001). An analysis of 159 newborns indicated that Agt-/- mice were born in normal numbers (n= 41). Collection of dead 10-day old pups (n= 27) revealed an enrichment in Agt-/- (20 of 27 mice). Importantly, using RNase protection analysis we demonstrated that the KA+ transgene was temporally expressed during renal development similar to the endogenous Agt and at comparable levels. These data strongly support the notion that the loss of intra-renal Agt is not responsible for death in the Agt-/- mouse model. Taken together with our previous results, we conclude that the intra-renal renin-angiotensin system located in the proximal tubule plays an important role in blood pressure regulation, and may cause hypertension if overexpressed, but is not required for continued development of the kidney after birth.