Generation of a Transgenic Mouse Model Exhibiting Brain Specific Expression of Human Angiotensinogen
In order to examine the importance of the brain renin-angiotensin system (RAS) in the regulation of blood pressure and the pathogenesis of hypertension, we generated a novel transgenic model expressing human angiotensinogen selectively in the brain. In the brain, angiotensinogen is abundantly expressed in astrocytes (glia) and in some neurons. The transgene (GFAP-hAGT) used consists of the coding region of the human angiotensinogen gene fused to a 2.2 Kb fragment encoding the human glial fibrillary acidic protein (GFAP) promoter, a glial-specific promoter. Three transgenic founders were identified by PCR and confirmed by Southern blot analysis. RNAse protection assay revealed that each line exhibited human angiotensinogen mRNA expression in the brain, with variable expression in tissues outside the central nervous system. In one line, transgene expression was observed primarily in the brain (10-fold greater than any other tissue) and plasma human angiotensinogen levels were only at background levels (14.8 ± 8.5 pmol/ml in GFAP-hAGT transgenic mice vs. 13.3 ± 5.7 pmol/ml in non-transgenic mice vs. 5,236.7 ± 445.3 pmol/ml in mice containing a genomic angiotensinogen construct and expressing abundant human angiotensinogen under the control of its endogenous promoter). Its physiological activity in the brain was evident from the pressor response (12 mm Hg) caused by intracerebroventricular (ICV) injection of purified human renin in conscious, unrestrained mice. The pressor response was prevented by ICV injection of angiotensin II type 1 receptor antagonist losartan. Intravenous (IV) injection of purified human renin did not elicit a pressor response. This model when combined with similar mice expressing human renin either systemically (hRen genomic construct encoded on a P1 chromosome as recently reported) or within the brain (GFAP-hREN) may provide important tools for determining the role of brain RAS in the regulation of blood pressure and in the development and maintenance of hypertension. Similar models expressing human renin and human angiotensinogen from neuron-specific promoters have been generated and are currently being analyzed.