Transgenic models as tools for studying the regulation of human renin expression

The human renin kidney enhancer is required to maintain base-line renin expression but is dispensable for tissue-specific, cell-specific, and regulated expression

Renin is the rate-limiting enzyme in the renin-angiotensin system and thus dictates the level of the pressor hormone angiotensin-II. The classical site of renin expression and secretion is the renal juxtaglomerular cell, where its expression is tightly regulated by physiological cues. An evolutionarily conserved transcriptional enhancer located 11 kb upstream of the human RENIN gene has been reported to markedly enhance transcription in renin expressing cells in vitro. However, its importance in vivo remains unclear. We tested whether this enhancer is required for appropriate tissue- and cell-specific expression, or for physiological regulation of the human RENIN gene. To accomplish this, we used a retrofitting technique employing homologous recombination in bacteria to delete the enhancer from a 160-kb P1-artificial chromosome containing human RENIN, two upstream genes and one downstream gene, and then generated two lines of transgenic mice. We previously showed that human renin expression in transgenic mice containing the wild type construct is tightly regulated as is expression of the linked genes. Deletion of the enhancer had no effect on tissue-specific expression of human RENIN, but using the downstream gene as an internal control, found that human RENIN mRNA levels were 3-10-fold decreased compared with constructs containing the enhancer. Despite this decrease in expression, renin protein remained localized to renal juxtaglomerular cells and was appropriately regulated by cues that either increase or decrease expression of renin. Our results suggest that sequences other than the enhancer may be necessary for tissue-specific, cell-specific, and regulated expression of human RENIN.

Identification of a nuclear orphan receptor (Ear2) as a negative regulator of renin gene transcription

A potent transcriptional enhancer was previously identified upstream of the mouse renin gene. Within the enhancer is a TGACCT direct-repeat motif, required for enhancer activity, that is the consensus sequence recognized by members of the thyroid hormone subfamily of steroid hormone receptors. We previously reported that RAR/RXR bind to this sequence and mediate the induction of renin promoter activity by retinoids. However, gel mobility shift assays clearly show that other as yet unidentified factors also bind to this motif. In order to identify some of these TGACCT binding factors, we screened a yeast one-hybrid cDNA library derived from mouse As4.1 cells. One of these encoded the orphan nuclear receptor Ear2. Recombinant Ear2 was purified from Escherichia coli and an antipeptide antisera was generated. EMSA showed that purified recombinant Ear2 specifically binds the TGACCT direct-repeat motif. Transfection assays showed that Ear2 potently decreases both baseline and retinoid-induced mouse renin promoter activity in a dose-dependent, enhancer-dependent, and sequence-specific manner. Mutations in Ear2, which abolish its binding to the TGACCT motif, also abolish transcriptional repression. Ear2 was identified as a nuclear protein in As4.1 cells, is one of the proteins binding to the TGACCT repeat motif, and its overexpression can repress transcription of the endogenous renin gene in As4.1 cells. These data suggest that Ear2 is a negative modulator of renin gene transcription in As4.1 cells, and that the renin enhancer may actually encode a complex positive and negative regulator of transcription.

A genetically clamped renin transgene for the induction of hypertension

Experimental analysis of the effects of individual components of complex mammalian systems is frequently impeded by compensatory adjustments that animals make to achieve homeostasis. We here introduce a genetic procedure for eliminating this type of impediment, by using as an example the development and testing of a transgene for "genetically clamping" the expression of renin, the major homeostatically responding component of the renin-angiotensin system, one of the most important regulators of blood pressure. To obtain a renin transgene whose expression is genetically clamped at a constant level, we have used single-copy chosen-site gene targeting to insert into a liver-specific locus a single copy of a modified mouse renin transgene driven by a liver-specific promoter/enhancer. The resulting transgene expresses renin ectopically at a constant high level in the liver and leads to elevated plasma levels of prorenin and active renin. The transgenic mice display high blood pressure, enhanced thirst, high urine output, proteinuria, and kidney damage. Treatment with the angiotensin II type I receptor antagonist, losartan, reduces the hypertension, albuminuria, and kidney damage, but does not affect expression of the transgene. This genetically clamped renin transgene can be used in models in which hypertension and its complications need to be investigated in a high prorenin/renin environment that is not subject to homeostatic compensations by the animal when other factors are changed.

Transgenic animals as models for human disease

Transgenic animals, especially mice, have been used quite extensively as models for various human diseases. At first, the level of scientific inquiry was driven by the need to establish the model. In many cases, these models may be considered quite crude because of their limitations. More recently, transgenic models of disease have become more refined and are currently being used to study the pathological mechanisms behind the disease rather than to just provide a model of the disease. Using some examples from the recent literature, we will document the current level and complexity of inquiry using transgenic animals. New techniques and techniques that may prove promising will be discussed.

High-affinity prorenin binding to cardiac man-6-P/IGF-II receptors precedes proteolytic activation to renin

Mannose-6-phosphate (man-6-P)/insulin-like growth factor-II (man-6-P/IgF-II) receptors are involved in the activation of recombinant human prorenin by cardiomyocytes. To investigate the kinetics of this process, the nature of activation, the existence of other prorenin receptors, and binding of native prorenin, neonatal rat cardiomyocytes were incubated with recombinant, renal, or amniotic fluid prorenin with or without man-6-P. Intact and activated prorenin were measured in cell lysates with prosegment- and renin-specific antibodies, respectively. The dissociation constant (K(d)) and maximum number of binding sites (B(max)) for prorenin binding to man-6-P/IGF-II receptors were 0.6 +/- 0.1 nM and 3,840 +/- 510 receptors/myocyte, respectively. The capacity for prorenin internalization was greater than 10 times B(max). Levels of internalized intact prorenin decreased rapidly (half-life = 5 +/- 3 min) indicating proteolytic prosegment removal. Prorenin subdivision into man-6-P-free and man-6-P-containing fractions revealed that only the latter was bound. Cells also bound and activated renal but not amniotic fluid prorenin. We concluded that cardiomyocytes display high-affinity binding of renal but not extrarenal prorenin exclusively via man-6-P/IGF-II receptors. Binding precedes internalization and proteolytic activation to renin thereby supporting the concept of cardiac angiotensin formation by renal prorenin.

Identification of three human renin mRNA isoforms from alternative tissue-specific transcriptional initiation

We have reported that mice transgenic for 140- and 160-kb P1 phage artificial chromosomes (PACs) containing the human renin gene express the gene in a highly tissue-restricted and regulated manner. Herein, we demonstrate that the transgene is also expressed appropriately throughout development. In the course of this investigation, we identified the existence of three transcriptional isoforms of human renin mRNA derived from the utilization of alternative transcription start sites. The first isoform is the kidney-specific isoform, which utilizes the classic renin promoter. The second is a brain-specific isoform, which when previously identified in rats and mice was due to a transcription initiation site within intron A. However, the start site in the human gene resides approximately 1,325 bp upstream of the classic promoter and encodes a new exon 1 (termed exon 1b) that splices directly to exon 2. The third isoform is lung specific and is due to transcriptional initiation 79 bp directly upstream of exon 2, fusing additional DNA within intron A (termed exon 1c) directly to exon 2 without splicing. Importantly, the alternative first exons observed in the PAC transgenic mice were identical to those used to transcribe renin in human fetal kidney, brain, and lung, suggesting these sites are bona fide isoforms of human renin mRNA and not artifacts of transgenesis. Moreover, the subtle differences in tissue-specific transcriptional initiation observed in the renin gene of rats and humans can be faithfully and accurately emulated in a transgenic model.