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(Hypertension. 1996;27:541-545.)
© 1996 American Heart Association, Inc.
Articles |
From the Departments of Physiology and Internal Medicine, the University of Michigan, Ann Arbor.
| Abstract |
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Key Words: kidney nephron rats, inbred, SHR gene expression
| Introduction |
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At present, the function of the SA gene product is entirely unknown. With the hope that localization information might provide clues to the possible function of the SA gene, we used RT-PCR methodology to study the localization of the SA gene in microdissected nephron segments and to compare the tubular expression pattern in hypertensive and normotensive animals. We found that renal expression of SA mRNA is virtually exclusively confined to the proximal tubules of both hypertensive and normotensive rats. SHRs showed high levels of expression in both the convoluted and straight portions of the proximal tubule, whereas WKY rats showed selective expression in the PSTs, while in SD rats, SA expression was restricted to the PCTs.
| Methods |
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Clipping Procedure
Four male SD rats weighing about 120 to
130 g were
anesthetized with sodium pentobarbital. The left renal artery
was clipped as previously described.13 The same surgical
procedure except for clipping of the renal artery was performed on four
rats to prepare sham-operated controls. All rats were maintained on
normal rat chow and had free access to tap water. Blood pressure was
measured in conscious animals by tail-cuff plethysmography. At 4
weeks after surgery, microdissection was performed as described above,
except that both kidneys were perfused and removed for
microdissection.
RNA Isolation
RNA from glomerular and tubular samples was
isolated
as previously described.14 15 Briefly,
glomerular and tubular samples were thawed in an ice slurry
and sonicated for 15 seconds. Twenty micrograms of ribosomal RNA from
Escherichia coli (Boehringer Mannheim) was added as
carrier, and the sample in 100 µL of GITC buffer was layered onto a
gradient of cesium chloride (100 µL of 97% and 20 µL of 40%
cesium chloride in 25 mmol/L sodium acetate buffer) in a 250-µL
polycarbonate ultracentrifuge tube. Samples were
centrifuged for 2 hours at 300 000g in a Beckman
TLA 100 ultracentrifuge (Beckman Instruments) with a
fixed-angle rotor. The RNA pellet was redissolved in 0.3 mol/L
sodium acetate and precipitated in ethanol.
Total RNA from different organs, including liver, kidney, stomach, small intestine, lung, spleen, aorta, and brain from 8-week-old SD rats, was isolated with TRI reagent (Molecular Research Center). Briefly, tissue samples were homogenized in TRI reagent solution. After addition of chloroform and centrifugation, the homogenates separated into three phases: aqueous, interphase, and organic. RNA was precipitated from the aqueous phase by addition of isopropanol. Contaminating genomic DNA was removed with RNase-free DNase I (GeneHunter). The purified RNA was redissolved in diethyl pyrocarbonatetreated water containing 20 U RNAsin.
Reverse Transcription
RT was performed in the presence of 100
IU Moloney murine
leukemia virus reverse transcriptase (Superscript; BRL), 0.5 µg oligo
(dT)12-18 (Pharmacia), 20 IU RNAsin (Promega Biotech), 10 mmol/L
dithiothreitol, 0.5 mmol/L dNTP (Pharmacia), and 1% bovine serum
albumin (Boehringer Mannheim) in the buffer provided by
the manufacturer in a total volume of 20 µL. Before the addition of
reverse transcriptase, dNTPs, and bovine serum albumin, the
reaction mixture was incubated at 65°C for 5 minutes to allow the
primers to anneal to the poly A tail of mRNA. cDNA was synthesized at
42°C for 1 hour and precipitated with 1 µL linear
acrylamide, 4 mol/L ammonium acetate, and 100% ethanol.
The pellets were redissolved in Tris-EDTA buffer at a dilution adjusted
so that each 2 µL of cDNA corresponded to 1 mm of tubule or 1
glomerulus.
Polymerase Chain Reaction
Primers for the SA gene
were chosen from the
SA-1 sequence.3 The sequence of sense primer
was 5'-GACTGTCTGTCAACGGAAGG-3' (bp 1034 to 1053) and that of the
antisense primer was 5'-TGAGAGCACTCTCTACCTCA-3' (bp 1641 to 1660).
The
primers flank a 102-bp insertion region in the SA-2
sequence.4 The two predominant products expected to be
amplified from SA-1 and SA-2 are predicted to be
630 and 732 bp, respectively.
To control for variations in RNA amount and efficiency of RT, PCR amplification for ß-actin was also performed. The primers for ß-actin were chosen from the human published sequences.16 The sequences of the ß-actin primers were as follows: sense, 5'-AACCGCGAGAAGATGACCCAGATCATGTTT-3' (bp 384 to 413); and antisense, 5'-AGCAGCCGTGGCCATCTCTTGCTCGAAGTC-3' (bp 705 to 734).
PCRs were performed in a total volume of 50 µL in the presence of 5 pmol of each oligonucleotide primer, 200 µmol/L dNTP, 10 mmol/L dithiothreitol, 50 mmol/L KCl, 1.5 mmol/L MgCl2, 10 mmol/L Tris-HCl, pH 8.3, 0.001% gelatin, 1.25 IU AmpliTaq DNA polymerase (Perkin-Elmer Cetus Corp), and 1.5 µCi [32P]dCTP (Amersham Corp). Mineral oil was layered on the top of each sample to prevent evaporation of the liquid. The samples were first denatured at 94°C for 3.5 minutes. The PCR cycle was programmed as follows: 94°C for 1 minute (melt), 55°C for 1 minute (anneal), and 72°C for 1 minute (extend). PCR was run for 25 cycles, and the last cycle was followed by additional incubation for 8 minutes at 72°C.
Analysis of PCR Products
After amplification, PCR products
were subjected to size
separation by polyacrylamide gel electrophoresis. The band
intensity was determined by phosphoimaging with Phosphor Analyst
software on a GS-250 Molecular Imager System (Bio-Rad Inc).
The identity of the PCR products was determined by restriction digestion and sequencing. The single 732-bp PCR products from PCTs of SD rats, SHRs, and WKY rats were gel-purified and digested with HaeIII and HinfI. Digestion reactions were incubated at 37°C for 1 hour in the buffer provided by the manufacturer. The gel-purified products were subcloned to pCR II (Invitrogen) and sequenced by the dideoxynucleotide chain reaction.
| Results |
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Organ Distribution of SA Gene mRNA in SD
Rats
To test the tissue-specific expression of the SA
gene, RT-PCR was performed using 1 µg of total RNA isolated from
different SD rat tissues. As shown in Fig 2
, product
could be found only in kidney and liver, and no signal could be
detected from other tissues, including heart, brain, lung, spleen,
stomach, and small intestine. RT-PCR of ß-actin was run to verify
homogeneity of mRNA amounts in each sample. A single 351-bp band with
similar intensity was detected among all the tissues.
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Localization of SA Gene mRNA in Microdissected Renal
Structures From SD Rats
RT-PCR for SA mRNA was performed on
cDNA prepared from
microdissected nephron segments from SD rats. Product abundance was
determined by serial dilutions of cDNA (x10, x100, and
x1000) and
normalized for equal levels of ß-actin expression. SA
gene mRNA was found to be abundant in PCTs. Product was
occasionally detected, at approximately 100-fold lower abundance, in
cDNA prepared from glomeruli and PSTs, and no signal was detected in
the cortical thick ascending limb, cortical collecting ducts, outer
medullary collecting ducts, inner medullary collecting ducts, and
arcuate artery (Fig 3
).
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SA mRNA Levels in Proximal Tubules From SHRs and
WKY Rats
Studies of the localization of SA gene mRNA in
microdissected nephron segments of SHRs and WKY rats showed that,
similar to SD rats, SA gene mRNA was expressed predominantly
in the proximal tubule. However, whereas SA gene mRNA in SD
rats was expressed at high levels in the PCTs, SA PCR
products in WKY rats were abundant in the PSTs. In SHRs,
SA mRNA was expressed in both PCTs and PSTs, with levels
being similar in both parts of the proximal tubule (Fig 4
, top
and
bottom; Table
). This result indicates a
higher level of SA gene expression in the proximal tubule of
hypertensive rats and a strain-dependent distribution of
SA gene mRNA along the proximal tubules of normotensive
rats.
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Nephron Localization in Kidney of Two-Kidney, One Clip Goldblatt
SD Rats
Systolic blood pressure was significantly increased in
Goldblatt rats compared with the sham-operated rats (165±15 versus
110±5 mm Hg, P<.01, n=4). SA gene mRNA was
confined to PCTs, but the expression levels in PCTs showed no
significant difference in nonclipped and clipped kidneys of SD rats and
in sham-operated controls (Fig 5
). The relative
levels of SA gene PCR products in PCTs of
nonclipped-kidney, clipped-kidney, and sham-operated
controls are 1.03±0.08, 1.02±0.28, and 1.24±0.24
(n=4,
P=NS), respectively.
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| Discussion |
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Our results demonstrate that mRNA expression of the SA gene is virtually exclusively localized to the proximal tubules in all three strains of rats tested in the present study. The demonstration that expression of the SA gene product in the kidney is confined to the proximal tubule suggests its participation in a cellular function with some specificity to this epithelium. Given the importance of proximal tubule resorption in overall fluid homeostasis; the fact that the proximal tubule is the site of action of a number of agonists potentially important in hypertension, such as angiotensin and dopamine17 ; and the fact that the proximal tubule is the site of renal synthesis of angiotensinogen and a number of other gene products with both local and systemic effects,17 18 this nephron localization was not surprising.
Unexpected, however, was the finding that localization within the proximal tubule varied markedly with strain. Whereas in WKY rats, SA gene mRNA was expressed only in the PST segment, it was found all along the proximal tubule in SHRs. This extension of SA gene mRNA expression in SHRs to the entire proximal tubule is consistent with the notion that intrarenal levels of the SA gene product correlate with the hypertensive phenotype. In a second normotensive control strain, the SD rat, however, the expression pattern was the converse of that observed in the WKY rat, restricted to the PCTs and present only at very low levels in PSTs. Furthermore, in the SD rat, the expression pattern was not altered by induction of hypertension by renal artery clamping.
This variability in PCT/PST expression is not consistent with the participation of the SA gene product in a number of transport activities of the proximal tubule. For example, a function related to organic anion transport, a transport activity localized to the PSTs,17 would be difficult to reconcile with its low levels of expression in this segment in SD rats; similarly, a key role in sodium-dependent glucose transport, which occurs along the proximal portions of the PCTs,17 would appear to be inconsistent with the low levels seen in WKY rats in this segment. Similar arguments could be made against its participation in other proximal transport activities for which well-established, species-independent PCT/PST gradients exist, such as bicarbonate transport.18
In addition to its importance for transport, the proximal tubule is also the site of synthesis of a number of bioactive metabolites with both local and systemic effects. An alternative possibility to participation in transport is that the SA gene product participates in synthetic activities of the proximal tubule. This possibility is supported, albeit somewhat indirectly, by the only known homologies of its sequence with gene products of known function. The SA gene product shows its highest overall homology with a family of prokaryotic enzymes considered to be acyl CoA synthetases (12, ie, Gen Bank P27095, P36333, and P27550). The overall homology between the SA gene and members of this family, approximately 25% to 30% nucleotide identity, is not impressive, but there are several short regions of very high homology at the amino acid level, suggesting the possible presence of conserved structural motifs. It is tempting to speculate that the SA gene product might participate in a synthetic process with some similarities to that catalyzed by the acyl CoA synthetases. Obviously, further work is needed to clarify this issue.
Published sequences for the SA gene suggest the existence of at least two different isoforms, the original form (which for convenience we call SA-1) obtained by Iwai et al3 and another isoform reported by Harris et al4 (SA-2) with a 102-bp insertion near the 3' end. In the present study, we performed RT-PCR with primers that should amplify both isoforms of the SA gene and yield distinctive products. We consistently detected only the product for the SA-2 form in the tissues from all three strains of rats. Restriction digestion and sequencing confirmed that the product was identical to SA-2. The reasons for the absence of an SA-1 product are not clear, but our results would suggest that it is in much lower abundance than the SA-2 isoform.
In summary, the present data show that mRNA expression of the SA gene is restricted to the proximal tubule; in SHRs, it is found all along the proximal tubule, whereas in normotensive control rats, it is more restricted in its expression, with variable proximal tubule expression patterns that shown strain specificity. Further study will be needed to determine whether high expression of SA gene mRNA in the proximal tubules of SHRs is causally related to the development of hypertension.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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| Footnotes |
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| References |
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