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Circulation Research. 1998;83:1279-1288

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(Circulation Research. 1998;83:1279-1288.)
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Rapid Communication

Kidney Is the Only Source of Human Plasma Renin in 45-kb Human Renin Transgenic Mice

Yan Yan, Rong Chen, Tina Pitarresi, Curt D. Sigmund, Kenneth W. Gross, Jean E. Sealey, John H. Laragh, Daniel F. Catanzaro

From the Cardiovascular Center, Weill Medical College of Cornell University (Y.Y., R.C., T.P., J.E.S., J.H.L., D.F.C.), New York, NY; Department of Physiology, University of Iowa College of Medicine (C.D.S.), Iowa City, Iowa; and Department of Molecular and Cellular Biology, Roswell Park Cancer Institute (K.W.G.), Buffalo, NY.

Correspondence to Daniel F. Catanzaro, PhD, Cardiovascular Center, Weill Medical College of Cornell University, 1300 York Ave, Room A863, New York, NY 10021. E-mail dfcatanz{at}mail.med.cornell.edu

Abstract

Abstract—Prorenin is expressed in certain extrarenal tissues, but normally only the kidneys process prorenin to renin and secrete renin into the circulation. Although transgenic animal lines containing the human renin (hREN) structural gene with either 0.9-kb or 3-kb 5'-flanking DNA express the transgene appropriately in renal juxtaglomerular cells and secrete hREN into the circulation, the source of the circulating renin is not known. In the present study, we observed that 13-kb hREN transgenic mice that contain the structural gene and 0.9-kb 5'-flanking DNA express hREN mRNA in many unusual tissues. We also observed that circulating hREN levels in 13-kb hREN mice increased after bilateral nephrectomy. These results suggested that the hREN gene is expressed at inappropriate locations where prorenin might be processed to renin. To determine if more distal sequences flanking the hREN gene might contribute to cell and tissue specificity, we used a 45-kb hREN genomic fragment that contained the structural gene and about 25-kb 5'- and 8-kb 3'-flanking DNA sequences to generate 3 separate transgenic lines that contained the intact transgene sequences. Ribonuclease protection assays revealed a much narrower tissue distribution of hREN expression than in the 13-kb hREN transgenic mice. In each 45-kb hREN line, hREN mRNA was present only in the kidney, adrenal, lung, eye, ovary, and brain. Moreover, 24 hours after nephrectomy, human plasma renin fell to very low levels, indistinguishable from those of nontransgenic littermates, indicating that their circulating hREN is of renal origin. These studies suggest that sequences flanking the structural gene, missing from previous hREN transgenic lines, suppress renin gene expression at inappropriate extrarenal sites where cellular proteases, to which prorenin is not normally exposed, could convert prorenin to renin, resulting in abnormal secretion of renin into the plasma.


Key Words: transgenic • renin • gene expression • plasma renin level • nephrectomy

Several transgenic (TG) animal lines have been developed that express the human renin (hREN) gene and secrete hREN into the circulation.1 2 3 Each of these lines expresses hREN and its mRNA in the juxtaglomerular cells of the kidney, but renin mRNA is also present at many extrarenal sites not commonly associated with renin gene expression. In human subjects, circulating renin is exclusively of renal origin, whereas plasma prorenin is derived from both renal and extrarenal sources.4 Normally, an elevation in blood pressure suppresses renin secretion by the kidney.5 However, hREN TG mice6 7 and rats8 made doubly TG for the human angiotensinogen gene develop high blood pressure associated with normal to high plasma renin levels, suggesting that these TG models cannot regulate their renin secretion appropriately. Taken together, these observations suggest that circulating renin might arise from extrarenal sites where its secretion may not be regulated correctly.

In the mouse Ren-1C gene, sequences between -4.1 and -2.3 kb upstream of the structural gene appear to be required for juxtaglomerular cell expression and may also be important for the cell specificity (for reviews, see References 9 and 109 10 ). Within this region, a transcriptional enhancer was identified between positions -2866 and -2625.11 Recently, we identified a homologous sequence in the hREN gene and showed that it can stimulate transcription from the hREN promoter, albeit less strongly than its mouse counterpart.12 Because the human enhancer is located about 12 kb upstream of the transcription start site of the hREN structural gene, it was absent from previously tested hREN transgenes that contained only 3-kb1 or 0.9-kb3 5'-flanking DNA. Although all previous hREN TG mice correctly expressed hREN in the renal juxtaglomerular cells, they also expressed the hREN gene at sites not normally associated with renin gene expression. This raised the possibility that either the human enhancer or another unidentified regulatory element is required for appropriate cell-specific expression of the hREN gene in mice.

To test this hypothesis, we developed several TG mouse lines using a 45-kb NotI-SalI hREN genomic fragment that contained approximately 25-kb 5'- and 8-kb 3'-flanking DNA sequences together with all the coding and intervening sequences. Ribonuclease protection assays showed that these new 45-kb hREN TG mice display a much stricter tissue-specific expression pattern than their 13-kb counterparts, indicating that they may have a more appropriate distribution of gene expression. To determine if circulating renin was of renal origin, we examined the effect of bilateral nephrectomy on human and mouse plasma renin and prorenin concentration in 45-kb hREN mice and 13-kb hREN3 mice and their non-TG littermates. These studies showed that plasma hREN disappeared in the 45-kb hREN mice 24 hours after bilateral nephrectomy. In contrast, in 13-kb hREN mice, plasma hREN increased after nephrectomy, suggesting inappropriate secretion from extrarenal sites. Together, these observations suggest that the extended 5'- and 3'-flanking sequences in 45-kb hREN mice that were missing from previous hREN TG lines suppress renin gene expression at inappropriate extrarenal sites. Among these sites, cellular proteases, to which prorenin is not normally exposed, could convert prorenin to renin, resulting in abnormal secretion of renin into the plasma. Thus, appropriate secretion of renin requires both the correct renal expression and processing of prorenin, and the suppression of prorenin expression at extrarenal sites where it might be inappropriately processed to renin and secreted into the plasma.

Materials and Methods

Animals
All procedures involving animals were approved by the Institutional Animal Care and Use Committee of Cornell University Medical College. Mice were kept under standard conditions and had free access to tap water and commercial mouse chow (No. 5008, Formulab), which contained 0.28% sodium.

Renin and Prorenin Assays
Blood samples were obtained by orbit puncture of mice lightly anesthetized with metaphane. Blood was collected into ice-cold microcentrifuge tubes containing EDTA and immediately centrifuged at 4°C to isolate plasma. Plasma was stored at -20°C.

Plasma renin concentration (PRC) was determined by the rate of angiotensin I (Ang I) generation from angiotensinogen at a substrate concentration close to Km.13 Mouse and hRENs were distinguished by virtue of the species specificity of the reaction between renin and angiotensinogen.14 Under the assay conditions, mouse renin failed to generate detectable levels of Ang I from human angiotensinogen, and the endogenous renin in mouse plasma was unable to generate Ang I from human angiotensinogen. Briefly, 10 µL of plasma was incubated with 10 µL of partially purified human substrate ({approx}6000 ng Ang I/mL) or 50 µL of pooled plasma from 24-hour bilaterally nephrectomized rats ({approx}3000 ng Ang I/mL) at 37°C for 1 hour or 3 hours, respectively, in a total volume of 300 µL of buffer, pH 7.5 for mouse renin15 or pH 5.6 for hREN.13 An unincubated blank reaction was set up for each of the plasma samples to control for endogenous Ang I levels. The Ang I generated was measured by radioimmunoassay.13 PRC was calculated as the difference between the incubated and unincubated samples. Total renin concentration (TRC) was determined after incubation with trypsin, as described for rat15 and human13 plasma samples, except that no treatment was applied to remove angiotensinogen fragments. Plasma prorenin concentration (ProRC) was calculated as the difference between TRC and PRC.

TG Mice Generation
The transgene used was a 45-kb NotI-SalI fragment from the P1 hREN genomic clone 3969 previously described.12 This fragment contains about 25 kb of 5'- and 8 kb of 3'-flanking DNA sequences plus all the coding and intervening sequences. P1 plasmid DNA was prepared by alkaline lysis of bacterial cells from an IPTG-induced culture followed by CsCl gradient centrifugation. The purified P1 plasmid DNA was subjected to NotI and SalI digestion and pulse-field gel electrophoresis. The 45-kb NotI-SalI fragment containing the hREN genomic sequences was excised from the gel and electroeluted. The resultant DNA was extracted with phenol/chloroform several times, dialyzed against injection buffer (5 mmol/L Tris, 100 mmol/L NaCl, and 100 µmol/L EDTA [pH 7.4]), and microinjected into the fertilized one-cell embryos derived from C57BL6xCBA mice at a concentration of 5 µg/mL. TG mice were made at the Rockefeller University Transgenic Facility (New York, NY).

Mouse Genomic DNA Analysis
Mouse genomic DNA was extracted from tail tips by digestion with proteinase K (0.5 mg/mL) in the lysis buffer (4.0 mol/L urea, 5 mmol/L CDTA, 0.5% Sarkosyl, 0.2 mol/L NaCl, and 0.1 mol/L Tris [pH 8.0]) at 55°C with rocking overnight. To 450 µL of supernatant, 1 mL 100% ethanol was added and rocked manually until the mouse chromosomal DNA aggregated into a visible clump. The DNA was then spooled with a sealed hematocrit tube and dissolved in 0.5 mL 0.1xTE (10 mmol/L Tris-Cl, 1 mmol/L EDTA; pH 8.0) by rocking for about 30 minutes. The DNA concentration was estimated by visual comparison to size markers of known concentration on a 1.0% agarose gel. Southern blots to identify founder mice were carried out with 10 µg of DNA from each mouse digested with several different restriction enzymes. Blots were probed with a full-length hREN cDNA16 or the hREN enhancer12 made by random primed synthesis.

RNA Preparation and Ribonuclease Protection Assay
Mice were killed by spinal cord dislocation and the tissue samples were removed immediately, frozen in liquid nitrogen, and stored at -70°C. Tissues were homogenized with a polytron homogenizer in Ultraspec RNA reagent (Biotecx Laboratories, Inc, Houston, Tex), and total RNA was extracted according to the manufacturer's protocol. Ten micrograms of total tissue RNA was hybridized to single-stranded labeled antisense RNA probes generated using the Maxiscript kit (Ambion, Inc). Ribonuclease protection assays were carried out using the RPAII kit (Ambion, Inc). Protected fragments were separated by electrophoresis through 5% polyacrylamide/urea sequencing gels and visualized by autoradiography. Quantification was carried out by phosphorimaging.

A set of short riboprobes was designed to detect both mouse and hREN. Regions were selected with maximal homology between human and mouse renin cDNA. The sequences used were hREN cDNA residues 550 to 730 (in the coding sequence) cloned into pCRII (Invitrogen) and mouse Ren-1C cDNA residues 184 to 418 cloned into pBSKS(-) vector. A longer hREN riboprobe spanning residues 741 to 1148 was also used.17 Probes for mouse ß-actin and 18S rRNA were made with templates supplied with the RPAII kit. The specific activity of the 18S rRNA probe was reduced by adding 0.5 mmol/L unlabeled UTP and half the usual volume of 32P-UTP to the reaction mixture. All probes were added to molar excess evidenced by the lack of any further increases in signal strength with greater amounts of probes. Size markers were produced using the Century Marker Template Set (Ambion, Inc).

Immunocytochemistry
Tissues were placed in Bouin Fixative (Poly Scientific) for 12 to 24 hours at room temperature. After fixation, the specimens were transferred to 70% ethanol for >2 hours, embedded in paraffin, cut into 5-µm sections, and mounted on slides. The sections were dewaxed, rehydrated, and washed before incubation with the primary antibody at 4°C overnight. Bound antibody was detected by light immunogold silver staining using AuroProbe LM and IntenSE M kits (Amersham). The primary antibodies used were BR1 -5,15 R15,18 and F37 2D12.19 BR1–5 and R15 are polyclonal rabbit anti-hREN antibodies, and F37 2D12 is mouse monoclonal anti-hREN antibody. The dilution factors used for each of the antibodies were 1:1000, 1:500, and 1:21, respectively.

Results

Characterization of TG Mice
To determine whether the extended flanking sequences surrounding the hREN structural gene would restrict hREN gene expression to fewer sites, TG mice were constructed using a 45-kb NotI-SalI fragment isolated from the P1 clone 396912 (Figure 1ADown). The 45-kb NotI-SalI fragment contains about 25-kb 5'- and 6-kb 3'-flanking DNA sequences plus all the coding and intervening sequences.



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Figure 1. A, Transgene sequences used to generate TG mice. A 45-kb SalI-NotI fragment was isolated from the P1 clone 3969. The location of restriction sites used to characterize transgene sequences are indicated. B, Southern blot hybridization of mouse genomic DNA run in parallel with similarly digested P1 3969 DNA. Blots were hybridized with either hREN cDNA (cDNA) or hREN enhancer sequences (ENH). The arrow indicates the hybridization signal produced by the endogenous mouse Ren-1C gene.

Forty mice were obtained from 8 surrogate mothers, of which 6 (15%) contained the transgene, and 3 transmitted the transgene to their offspring. All lines were maintained heterozygous for the transgene. Southern blot hybridization of KpnI and PstI digests of mouse genomic DNA with probes containing the hREN cDNA or enhancer sequences showed that each line contained intact copies of the structural gene and the 5'-flanking DNA containing the enhancer element (Figure 1BUp). All 3 lines gave the same hybridization pattern as the P1 3969 clone with either the cDNA or enhancer sequence probes. Notably, the contiguous KpnI fragments that contained the enhancer, 5'-flanking DNA, coding, and intervening sequences were present in each TG line. By comparison to the hybridization signal produced by the endogenous mouse renin gene in this and other experiments, it was estimated that line No. 3 contains one copy of the transgene and line Nos. 2 and 18 contained 4 and 6 copies, respectively (see Figure 1BUp; PstI digest hybridized with hREN cDNA).

Tissue Distribution of hREN mRNA
Tissue specificity of hREN transgene expression in the mice was addressed at the mRNA level using a ribonuclease protection assay. Figure 2Down shows the tissue distribution of human and mouse renin mRNAs in a mouse from the 45-kb hREN No. 2 line. Both mouse and hREN mRNAs were detectable in the kidney, ovary, adrenal, and eye of TG mice, and mouse, but not human, renin mRNA was present in the submandibular gland. hREN mRNA was also detected at high levels in the lung and at lower levels in the brain, although no mouse renin mRNA was detected at either site. Similar results were obtained with mice from No. 3 and No. 18 lines.



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Figure 2. Tissue distribution of mouse and hREN mRNA in a representative mouse from the 45-kb hREN line. The left panel shows the ribonuclease protection assay carried out with the hREN 550–730 probe and an 18S RNA probe to control for the amount of RNA. The right panel shows the ribonuclease protection assay carried out with the Ren-1C 184–418 probe. P indicates full-length probe; M, size markers in bases. The Ren-1C probe also yielded a shorter protected fragment with hREN mRNA corresponding to the homologous region of mouse and human sequences in the probe. Differences in the pattern produced by each probe and the consistency of human mRNA detected by the Ren-1C probe attest to their specificity.

The tissue distribution of hREN mRNA in 13-kb hREN mice (No. 10 line) was also examined. A representative ribonuclease protection assay is shown in Figure 3Down. In addition to the kidney, adrenal ovary, and adipose tissue reported previously,3 abundant hREN mRNA was detected in the eye, skeletal muscle, and stomach, and lower levels were detected in brain, small intestine, and spleen. However, unlike the earlier report,3 no hREN mRNA was detected in testis or submandibular gland.



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Figure 3. Tissue distribution of hREN mRNA in a representative 13-kb hREN No. 10 line mouse. The hREN probe used was hREN 741-1148.17

Renal renin mRNA levels were determined for each of the three 45-kb lines and a 13-kb hREN line (No. 10) (Figure 4Down). Among 45-kb hREN lines, hREN mRNA levels in the kidney were proportional to the copy number (Figure 5Down).



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Figure 4. Quantification of renal hREN mRNA levels in hREN TG lines. Numbers show relative levels in arbitrary units from at least 3 separate determinations with different animals.



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Figure 5. Copy number dependence of human PRC, ProRC, and hREN mRNA. Data are from Table 1Up and Figure 4Up. Units for mRNA levels are 10x the relative units in Figure 4Up.

Immunocytochemistry
The cellular localization of hREN transgene expression was examined in the kidney and lung by immunocytochemistry using antirenin antibodies (Figure 6Down, top). BR1–5, which stains both mouse and hREN, and prorenin stained the juxtaglomerular apparatus (JGA) and afferent arterioles of both TG and non-TG kidneys. However, R15, which is highly specific for hREN, stained only JGA from TG mouse kidneys. All JGA of TG animals that stained with BR1–5 also stained with R15, suggesting that there is no differential expression of mouse and hRENs between nephrons. Similar results were obtained with kidneys from the other two 45-kb hREN lines in which the intensity of staining with R15 was proportional to the plasma hREN levels (not shown).



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Figure 6. Immunocytochemical analysis of mouse and hREN/prorenin in 45-kb hREN TG mice and non-TG littermate controls. Top, Kidney tissue was stained with antiserum BR1–5, which recognizes both mouse and hREN, or R15, which recognizes only hREN. Scale bars=200 µm for upper panels and 50 µm for lower panels. Bottom, Lung tissue was stained with the antibody F37 2D12. This antibody stained only JGA of the kidney. In lung tissue from TG mice, staining was apparent in bronchiolar epithelial cells and to a lesser extent in venous endothelial cells. No staining is evident in alveoli. No staining was evident in lung tissue from non-TG mice. A indicates alveolus; B, bronchiole; and V, venule. Scale bar=50 µm.

The immunocytochemical staining on the adult TG mouse lung with antibody F37 2D12 showed hREN expression in epithelial cells of alveoli and to a lesser extent in isolated venous endothelial cells (Figure 6Up, bottom). No specific staining was detected in the lung of non-TG littermate using the same antibody.

Plasma Levels of Mouse and hREN and Prorenin
Plasma levels of mouse and hREN and prorenin in the various 45-kb hREN and 13-kb hREN TG lines and their non-TG littermates are shown in Table 1Down. Human PRC (hPRC) and ProRC differed between the TG lines and were proportional to the copy number (Figure 5Up) among the 45-kb hREN lines. The proportion of the total renin (renin+prorenin) detected as renin was about 56% in line No. 2 and No. 3 and slightly lower (47%) in line No. 18. The proportion of hREN in 13-kb hREN mice was much lower (6%), because the human plasma prorenin was about 10x higher than in 45-kb hREN mice. Mouse PRC and mouse ProRC were similar between TG lines. Mouse PRC tended to be lower in TG mice than in their non-TG littermates, but this difference was statistically significant only for the No. 3 line. Plasma ProRC accounted for 4% to 10% of the total mouse renin in 45-kb hREN mice but was undetectable in 13-kb hREN mice.


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Table 1. Human and Mouse Renin and Prorenin Levels in hREN TG Lines

Nephrectomy Studies to Determine the Source of Circulating hREN
To determine whether plasma hREN originates from the kidneys, bilateral nephrectomy was carried out on mice from each of the TG lines (Figure 7Down).



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Figure 7. Mouse and human PRC and ProRC in TG mice (solid bars) and their non-TG littermates (open bars) before and 24 hours after bilateral nephrectomy. Renin levels are plotted upward from the origin, and prorenin levels are plotted downward. Thus, the height of each bar represents the TRC. Note the different scale axis for 13-kb hREN mice. All values are nanograms of Ang I per mL/h. *P<0.05 (unpaired t test).

After nephrectomy, mouse plasma renin decreased 50- to 70-fold to very low levels in every mouse, irrespective of the TG status or line. Human plasma renin also fell in each of the 45-kb hREN lines to levels indistinguishable from their non-TG littermates. In marked contrast, in the 13-kb hREN mice, human plasma renin levels increased almost 6-fold, despite the dramatic reduction in the mouse plasma renin levels. The increased renin activity in 13-kb hREN mice after nephrectomy was inhibited by the renin inhibitor Hui Pep-27 and by the antirenin antibody BR1–5 (not shown). These data indicate that the increased renin activity is indeed due to the release of hREN.

Both mouse and human prorenin levels were unaffected by nephrectomy and in some cases actually increased, indicating that most, if not all, of the circulating prorenin in these mice originates from extrarenal sites.

Discussion

In the present study, we generated 3 mouse lines carrying a 45-kb hREN genomic fragment. These mice express the hREN transgene in a much narrower range of tissues than previous hREN TG lines and are unique in that they secrete hREN into the circulation exclusively from the kidneys, as shown previously in normal mice and humans.20 21

hREN mRNA was detected in 45-kb mice only in the kidney, eye, ovary, brain, adrenal, and lung. With the exception of lung (see below), this tissue distribution of transgene expression more closely resembles the pattern in normal human and animal tissues18 22 23 24 25 26 27 than previous mouse and rat lines (Table 2Down). Although at least some of the extrarenal sites of transgene expression in earlier models were interpreted to represent previously unidentified sites of hREN gene expression,28 failure to replicate expression at these sites with a longer transgene suggests that they are actually inappropriate sites of ectopic expression.


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Table 2. Tissue Distribution of Transgene Expression Among Various hREN TG Lines

Ectopic expression most likely explains the surprising increase in circulating human plasma renin after bilateral nephrectomy of 13-kb hREN mice, even as mouse renin disappeared. Ectopic expression of prorenin in cells that express a processing protease—to which prorenin would normally never be exposed—could lead to its conversion to renin and its secretion into the plasma. However, we do not know whether, under normal circumstances, extrarenal sites in 13-kb hREN mice actually contribute to the circulating renin. Previous observations of both appropriate and inappropriate responses of circulating hREN to physiological and pharmacological stimuli in 13-kb hREN mice17 suggest that not all of their circulating hREN originates from the kidney. In contrast, in 45-kb hREN mice, hREN mRNA and plasma hREN levels respond appropriately to changes in dietary salt, angiotensin-converting enzyme inhibition, ß1-adrenergic stimulation, and infusions of angiotensinogen that increase blood pressure and/or circulating levels of Ang II.29 Taken together, these observations indicate that for appropriate processing and secretion to occur, prorenin expression must be prevented at sites where cryptic processing of prorenin could lead to abnormal renin secretion into the plasma.

Plasma hREN levels in the three 45-kb hREN lines ranged from 9 to 66 ng of Ang I per mL/h and correlated with the transgene copy number, which ranged from 1 to 6. These levels are very low compared with the endogenous mouse renin concentration (>200 ng Ang I per mL/h). In the No. 3 line that contains a single copy of the hREN transgene, the hPRC was around 9 ng Ang I per mL/h, which is very similar to the hPRC in normal human plasma (2 to 15 ng Ang I per mL/h).13 This result suggests that the plasma renin levels are determined by sequences within the renin gene, ie, they are encoded in cis. This could be due to differences in transcription rate, mRNA turnover, or differences in the rate of renin synthesis, secretion, or degradation and clearance. Our recent studies suggest that the mechanism is posttranscriptional.29

Plasma hREN and renal hREN mRNA levels in 13-kb hREN mice were similar to the 45-kb hREN mice. It therefore seems unlikely that the extended sequences in the 45-kb hREN transgenes contribute significantly to expression levels. Despite the broad tissue distribution of transgene expression in earlier studies, hREN was correctly localized in the juxtaglomerular cells of the kidney,1 2 3 suggesting that the shorter transgenes contain all the sequences required for juxtaglomerular cell-specific expression. Therefore, the enhancer contained in the extended 5'-flanking sequence of the 45-kb hREN transgene is probably not required either to direct hREN expression to renal juxtaglomerular cells or to determine the level of expression. This is consistent with the relatively weak transcriptional activity of the human enhancer observed in transfection experiments12 and suggests that if the enhancer plays a role in directing hREN expression in vivo, then its effects are mediated outside the kidney. Thus, the enhancer or some other sequences in the 45-kb hREN transgene may function to restrict expression to appropriate physiological sites. Transcriptional suppression at inappropriate sites of expression has been described for the IgH enhancer.30

The plasma human prorenin levels were similar (47% to 56% of total renin) among the three 45-kb lines but differed from 13-kb hREN and other hREN TG lines in that the absolute level and the proportion of total renin were much lower.2 31 The high plasma human prorenin level in 13-kb hREN mice that persisted after nephrectomy is consistent with the broad tissue distribution of hREN expression in this line, suggesting that many of these extrarenal sites secrete prorenin into the plasma. Although prorenin is normally about 90% of total human plasma renin, the proportion that we measured in mice was lower (0% to 16%). The mouse prorenin levels we measured are similar to values derived from data for C57BL/6J ({approx}7%),31 although in Balb C mice,32 the prorenin levels were {approx}90%. In other studies of the 45-kb hREN No. 2 line backcrossed for at least 5 generations to C57BL/6, we measured mouse prorenin levels at {approx}50% of the total, whereas the hREN levels were similar to those reported in the present study.

Because prorenin is a derived value, we cannot be certain of the absolute levels of prorenin in mouse plasma when it comprises <10% of the total plasma renin. Nevertheless, some important conclusions can be drawn from the relative differences between TG lines and treatments. After nephrectomy, mouse and human plasma prorenin levels in 45-kb hREN were unchanged or were slightly increased (<2-fold). This indicates that plasma prorenin may arise from extrarenal sites. However, it is also possible that prorenin secreted by kidney disappears after nephrectomy and is replaced by an increase in prorenin secreted from extrarenal sites.

hREN gene expression was detected in the lung of each of the 45-kb hREN lines. However, the lung does not appear to process prorenin to renin and to secrete renin into the circulation of 45-kb hREN TG mice, although it may be a source of circulating human prorenin. hREN gene expression was also detected in the lung of all other hREN TG lines (Table 2Up) and has been reported in human fetal lung.33 However, in each case, the cellular localization differed: in the present study, we found hREN immunoreactivity primarily in bronchiolar epithelial cells; in 13-kb TG mice, hREN mRNA was localized to type II epithelial cells34 ; and in human fetal lung, renin was localized to endothelial cells.33 Renin has also been detected in human pulmonary tumors35 36 37 and in a human pulmonary carcinoma cell line (Calu-6).34 Because lung neoplasms ectopically express many other polypeptide hormones,38 lung tissue may more readily express a variety of genes, and renin gene expression may be readily activated if all the necessary regulatory mechanisms are absent or compromised. This could result from some incompatibility between the mouse transcriptional mechanisms and the hREN genomic sequences that normally prevent pulmonary expression or from the lack of essential sequences that are absent even from the 45-kb hREN transgene. This raises the possibility that in renin-secreting tumors of extrarenal origin, renin gene expression might be activated by loss of either the required regulatory sequences through mutation or genetic rearrangement or the trans-acting factors that normally interact with these sequences to prevent expression. Alternatively, some cryptic activation mechanism could result in transcription of the hREN transgene in mouse lung.

In summary, our new 45-kb hREN TG lines show copy-number dependent expression in renal juxtaglomerular cells and extrarenal expression restricted to far fewer tissues than previous models. Importantly, in 45-kb hREN mice plasma, hREN is derived exclusively from the kidneys, as shown previously in humans and other species. We conclude that the additional sequences contained in the 45-kb hREN gene restrict hREN gene expression at inappropriate sites. These new TG mice should provide a better animal model for the study of the expression and regulation of the hREN gene, the regulation of renin secretion, and the role of renin in the pathophysiology of hypertension and vascular disease.

Acknowledgments

This work was supported by NIH grants DK45982 (D.F.C.), HL48459 (K.W.G.), and HL48058 (C.D.S.) and by the generous support of the Greenburg, Wallace, and Wolk Funds (D.F.C.). Daniel F. Catanzaro and Curt D. Sigmund are Established Investigators of the American Heart Association.

Received August 14, 1998; accepted October 13, 1998.

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