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From the Division of Nephrology,*
IRCCS Policlinico San
Matteo, University of Pavia, Pavia, Italy, Renal Cell Biology
Laboratory,
Department of Medicine, University
of Miami School of Medicine, Miami, Florida, and Division of Basic
Medical Sciences,
Mercer University, School
of Medicine, Macon, Georgia
| Abstract |
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| Introduction |
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| Materials and Methods |
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Female ROP Os/+ (ROP/Le-Os Es1b/+ Es1a), ROP +/+, C57 Os/+ (C57BL/6J-Os +/+ Cchalatg-la) and C57 +/+ mice were obtained from Jackson Laboratories (Bar Harbor, Maine). ROP +/+ and C57 +/+ littermates served as controls for ROP Os/+ and C57 Os/+ mice. The left kidney was removed at age 10 weeks in each group as described.13 Control mice matched for age, sex, and strain were sham operated. Nephrectomized (NX) and sham-operated mice were fed standard diet throughout the experimental period. Each group consisted of six animals, and they were sacrificed according to NIH approved procedures either 2 weeks or 8 weeks after NX, at which time they were 12 weeks or 18 weeks old. After removing one pole for light microscopy, the remainder of the solitary kidney was perfused with a buffer containing 0.1% collagenase and RNase inhibitors for glomerular microdissection and molecular analysis as described previously.14
Light Microscopy
Kidney sections were fixed in Carnoy's fixative, embedded in glycol methacrylate, cut at a thickness of 2 µm and stained with hematoxylin/eosin (H&E) and periodic acid-Schiff (PAS). One investigator examined the sections without knowledge of the experimental groups to eliminate experimental bias. Mesangial sclerosis was graded using a scale from 0 to 4+ as described previously.10 Forty glomeruli per section were examined in six mice from each group, and the mean of scores for individual glomeruli was recorded. Arteries were examined, and the number affected was expressed as a percent of the total. The interstitial changes were mild and not quantitated.
Light Microscopic Morphometry
Glomerular volume was assessed by examining plastic-embedded sections using a digitizing tablet and video camera, as described previously.15 The mean glomerular volume was derived from the harmonic mean of the glomerular equatorial surface area.
Glomerular Cell Number and Turnover (18-Week Mice Only)
The nuclei of 50 consecutive glomerular profiles were counted by scanning H&E-stained tissue sections in a serpentine fashion. The relative glomerular cell number was calculated as described previously.15 The labeling index was determined using [3H]thymidine incorporation and autoradiography. A cell was considered labeled when the number of grains per nucleus exceeded 3 to 4. The glomerular-labeling index was calculated by counting 50 successive glomeruli per section and expressed as the percentage of labeled cells (excluding Bowman's capsular cells) divided by the total glomerular cell number.
Isolation of Glomeruli and Reverse Transcription in Situ
Glomeruli were isolated by microdissection at 4°C in a solution containing RNase inhibitors.14 After sonication, cDNA was obtained by in situ reverse transcription.
Competitive Polymerase Chain Reaction (PCR)
The primers for mouse
1 type IV collagen, laminin ß1,
tenascin, ß-actin, and 72 kd collagenase were previously
described.14,16
The expected PCR products were 484 bp for
1 type IV collagen, 443 bp for laminin ß1, 460 bp for ß-actin,
701 bp for 72 kd collagenase, and 548 bp for tenascin. Decreasing
amounts of mutant template competed for specific primers with a fixed
amount of the test glomerular cDNA during PCR amplification. The ratio
of mutant to cDNA band density was calculated by laser densitometry and
plotted as a function of the initial amount of mutant template added to
the reaction. The amount of glomerular cDNA was derived from linear
regression analysis. Duplicate or triplicate assays were performed. The
mean values obtained for all mice were expressed as attomol
x10-4/glomerulus. All reverse transcription (RT)-PCR data
were normalized to ß-actin and the results expressed as ratios.
Statistical Analysis
All values were expressed as mean ± SD. The two-tailed unpaired Student's t-test was used to evaluate differences between means for corresponding sets of data obtained from oligosyndactyly (Os) and control mice. As level of significance P < 0.05 was chosen.
| Results |
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Body weight did not differ between nephrectomized and sham-operated mice at either 2 weeks or 8 weeks post-NX. There was a significant increase (30 to 40%) in glomerular volume in ROP +/+ and C57 +/+ mice 2 weeks and 8 weeks post-NX (P < 0.01). There was no change in glomerular volume 2 weeks post-NX in either ROP Os/+ or C57 Os/+ mice. Whereas there was an increase in glomerular volume in ROP Os/+ mice (54%) 8 weeks post-NX, there were no differences between sham and NX C57 Os/+ mice 8 weeks post-NX.
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C57 +/+ Mice
There were no glomerular, tubulointerstitial, or vascular changes in sham or nephrectomized mice except for the increase in size in the NX group.
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Sham operated mice had a slight increase in the amount of
PAS-stained material in the mesangial areas of 50% of glomeruli, which
was most noticeable in the vicinity of the vascular pole 8 weeks
post-NX (score: 0.6 ± 0.4). Two weeks after nephrectomy, the
mesangial sclerosis was unchanged in ROP +/+ mice. Eight weeks post-NX,
the mesangial matrix score was 0.9 ± 0.15. The sclerotic
glomeruli were found distributed throughout the cortex. There were no
tubulointerstitial or vascular lesions (Figure 1)
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As reported previously, there was a minimal increase in the mesangial matrix in sham-operated mice (score: 1.2 ± 0.5), similar to values found in non-nephrectomized control mice.11 There was no significant change in mice post-NX (score: 1.3 ± 0.6).
ROP Os/+ Mice
Twelve- and 18-week-old sham-operated ROP Os/+ mice had glomerular
lesions (score 2.23 ± 0.26 and 2.7 ± 0.2, respectively),
similar to nonoperated mice previously described.12
At 8
weeks post-NX most glomeruli exhibited increased mesangial sclerosis.
In the more severely affected glomeruli there was a marked decrease in
the patency of the vascular loops, apparently caused by the presence of
large subendothelial masses of hyalin material (score: 3.75 ±
0.1). A small number of arterioles showed smooth muscle and intimal
cell proliferation with reduction of the lumen. There were no
tubulointerstitial lesions (Figure 2)
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Glomerular cell number and labeling index were determined 8 weeks
post-NX (Table 3)
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There was no change in the mean cell number per glomerulus between nephrectomized and sham-operated mice, despite an increase in glomerular volume in both groups of mice. Approximately 30% of the labeled cells were situated on the external surface of the glomerular tufts. It was not possible to differentiate between mesangial and endothelial cell labeling within the tufts.
ROP Os/+ and C57 Os/+ Mice
Sham-operated ROP Os/+ and C57 Os/+ mice had a higher labeling index than ROP +/+ and C57 +/+ mice. As noted above, approximately 30% of the labeled cells were situated on the external surface of the glomerular tufts. There was no increase in the labeling index post-NX in either ROP Os/+ or C57 Os/+ mice.
While there was an increased number of cells in the enlarged glomeruli
of both ROP Os/+ and C57 Os/+ mice compared with littermates without
the Os gene, no further increase was found post-NX. Glomerular cell
number did not change in ROP Os/+ mice post-NX, despite a further
increase in glomerular volume (Table 1)
.
Glomerular mRNA Levels
Glomerular
1 IV collagen, laminin ß1, tenascin, and 72 kd
collagenase mRNAs, measured by competitive PCR, were normalized to
ß-actin mRNA levels (Table 4)
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There was a significant increase in
1 IV collagen mRNA at both
2 and 8 weeks post-NX in ROP +/+ and C57 +/+ mice. The levels were
higher in ROP +/+ than in C57 +/+ mice at 8 weeks post-NX
(P < 0.05). There was no significant change
observed in laminin ß1 and tenascin mRNA levels at either 2 or 8
weeks post-NX. There was a significant increase in 72 kd collagenase
mRNA levels 8 weeks post-NX in both groups of mice.
ROP Os/+ and C57 Os/+ Mice
The levels of
1 IV collagen mRNA were higher in both groups of
mice compared with their non-Os littermates, as we had previously
found.11
In addition, the laminin ß1 and tenascin mRNA
levels were higher in ROP Os/+ compared with ROP +/+ mice, as shown
previously.12
There was a significant increase in
1 IV
collagen, laminin, and tenascin mRNAs in ROP Os/+ mice but not in C57
Os/+ mice 8 weeks post-NX. 72 kd collagenase mRNA levels were slightly
but significantly increased in both ROP Os/+ and C57 Os/+ mice at 8
weeks post-NX.
| Discussion |
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We found that the lack of a sclerotic response in the resistant strain
(C57) was not a threshold effect, because even a 75% reduction in
nephron number did not induce sclerosis. Although nephrectomy did not
induce significant sclerosis in C57 mice at either the histological or
ECM mRNA levels, glomerular volume and labeling index were both
increased. Thus, cell turnover, glomerular hypertrophy, and the
accumulation of ECM were not tightly linked. This is consistent with
our previous in vivo and in vitro data and that
of others.11,18,19
There was an increase in
1 IV
collagen mRNA in both C57 +/+ and ROP +/+ mice at both 2 and 10 weeks
NX, the increase being higher in the ROP +/+ mice at 10 weeks.
Glomerular hypertrophy has been proposed to be an important factor in the development of glomerulosclerosis in rats and mice.9,13,20,21 In mice transgenic for bovine growth hormone (bGH) we found that glomerular size was greater than expected for body size and that it continued to increase as glomerulosclerosis progressed.22,23 These mice die in renal failure.22 In the current study we found that glomerular hypertrophy occurred in both sclerosis-prone and sclerosis-resistant mice. However, there was a sharp difference between the two strains in the degree of hypertrophy ultimately attained. Whereas glomerular size increased by approximately 50% in nephrectomized adult C57 +/+ mice, nephrectomy of littermate C57 Os/+ mice resulted in no further increase in glomerular size even though there was a 75% decrease in nephron number. However, there was a substantial increase in glomerular size in ROP Os/+ mice after adult nephrectomy. This relationship between continuously increasing glomerular size and glomerulosclerosis is in agreement with our observations in mice transgenic for bGH. The data from these two animal models suggest that glomerular size does not exceed a certain size in sclerosis-resistant mice following nephron reduction. However, in sclerosis-prone mice glomerular size continues to increase as nephron number decreases, and this is associated with the development of progressive glomerulosclerosis. It is not possible, at present, to determine whether there is a causal link between the progressive glomerular lesion and the lack of a plateau in glomerular size in sclerosis-prone mice. A similar relationship between glomerular size and susceptibility to glomerulosclerosis was noted in diabetic mice following the induction of hyperglycemia.24
There was an increase in
1 IV collagen mRNA in both ROP +/+ (1.8
fold) and C57 +/+ (1.6 fold) mice (P < 0.05) 8
weeks after nephrectomy, and the levels were higher in ROP +/+ mice.
The 72-kd collagenase mRNA levels were similar in sham-operated ROP +/+
and C57 +/+ mice. Although the levels were increased 8 weeks post-NX,
they were not significantly different from one another. There was a
further increase in
1 IV collagen, laminin ß1, and tenascin mRNA
levels in ROP Os/+ mice 8 weeks after NX but not in C57 Os/+ mice.
There was an increase in 72-kd collagenase mRNA levels of similar
magnitude in both ROP Os/+ mice and C57 Os/+ mice 8 weeks after
nephrectomy. These data show that there was an increase in the
synthesis of several matrix components in the NX ROP Os/+ mice,
relative to the other groups. Furthermore, the changes in 72-kd
collagenase mRNA levels were parallel in all groups, providing evidence
that the cause of the glomerulosclerosis could have been a relative
imbalance in matrix turnover, favoring deposition.
Genetic factors have been reported to be important in determining the response to adult nephrectomy. Dahl salt sensitive rats with 5/6 renal ablation had significantly more severe hypertension and focal glomerulosclerosis than salt-resistant rats.25 Male Wistar rats and several other rat strains developed glomerulosclerosis after adult uninephrectomy,19 whereas male PVG/c rats did not.8
In summary, these data show that nephrectomy in adulthood resulted in a sclerotic response only in a susceptible mouse strain and that in this strain, a 50% reduction in nephron number during development resulted in more prominent glomerulosclerosis than did an equal reduction occurring during adulthood. The data also show that a 75% reduction in nephron number in the sclerosis-resistant strain did not induce sclerosis, suggesting that the different responses between the two strains was not simply a difference in a threshold. Finally, the data show that glomerular hypertrophy in response to nephron reduction in the sclerosis-resistant strain was limited, reaching a plateau at a 50% reduction in number. However, glomerular size continued to increase in the sclerosis-prone strain as nephron number decreased.
| Footnotes |
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C. Esposito and C-J. He contributed equally to this work.
Accepted for publication December 18, 1998.
| References |
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