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*Department of Pathology, University of Erlangen-Nürnberg, Erlangen, Germany; Departments of
Pathology and
Internal Medicine, University of Heidelberg, Heidelberg, Germany; and
Department of Physiology, Universitätsklinikum Eppendorf, Hamburg, Germany.
Correspondence to Professor Dr. Kerstin Amann, Department of Pathology, University of Erlangen-Nürnberg, Krankenhausstra
e 8-10, D-91054 Erlangen, Germany. Phone: 49-9131/8522291; Fax: 49-9131/8522601;
| Abstract |
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| Introduction |
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With the exception of diabetic patients with renal failure (2), there has been little documentation that atherosclerotic complications evolve more rapidly in patients with renal failure, and it has generally been difficult to reproduce atherosclerosis in animal models of renal failure.
The apolipoprotein E knockout mouse (Apoe-/-) is a well-established model to study atherogenesis (3,4). We reasoned that it might also constitute a suitable model to address the issue whether atherosclerosis evolves more rapidly or with a different morphology in renal failure.
| Material and Methods |
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BP Measurement
Mean arterial BP and heart rate were measured in awake resting mice at the day of perfusion fixation as described previously (8). A polyethylene catheter (outer diameter, 400 µm) was inserted into the left carotid artery, and mice were placed in Plexiglas tubes. Catheters were connected to a pressure transducer (PRC-21K, amplifier MIO-0501, FMI) for continuous recording at 100 Hz (80586, DAS-0216, Keithly-Metrabyte; Lab Tech Notebook 10.2.1, Labtech) for 15 min.
Biochemistry
At the time of perfusion, fixation blood samples from the aorta were taken in EDTA tubes. Enzymatic investigations were performed using a Hitachi 917 747/400 analyser and Boehringer Mannheim reagents: serum creatinine and urea were measured. Serum cholesterol and triglycerides were measured by enzymatic methods using commercially available tests from Boehringer Co. (Mannheim, Germany).
Morphologic Evaluations
Heart.
Wall thickness of small intramyocardial arteries was determined on eight isotropic uniform random (IUR) semithin sections per animal at a magnification of 400:1 by planimetry, as described in detail previously (5).
Aorta.
Maximal plaque cross-section, plaque area per aortic circumference, and aortic wall thickness were determined by planimetry using an automatic image analyzing system (IBAS II; Kontron Co., Eching, Germany) combined with a counting grid eyepiece for defined movement over the section plane (Zeiss Co., Oberkochen; Germany) at a magnification of 400:1. Contours of the aortic plaques and profiles were marked manually with a cursor, and maximal diameter (cross-section) and plaque area per aortic circumference, as well as the thickness of aortic wall, were calculated.
Immunohistochemistry
Immunohistochemistry was performed on paraffin sections using antibodies against collagen IV (polyclonal rabbit, 1:100; Biotrend 21501470, Köln, Germany), fibronectin (polyclonal rabbit, 1:100; Sigma F3648, Deisenhofen, Germany),
-smooth muscle actin (polyclonal rabbit, 1:100; Sigma A5691, Deisenhofen, Germany), osteopontin (polyclonal rabbit, 1:100; Immundiagnostik A4226.2, Bensheim, Germany), nitrotyrosine as an index of oxidative stress (polyclonal rabbit, 1:400; Upstate Biotechnology 06 to 284, Lake Placid, NY), macrophages/monocytes (monoclonal rat, 1:5; Serotec MCA 519, Eching, Germany), proliferating cell nuclear antigen (PCNA, monoclonal mouse, 1:10; Biogenex, MU252UC, Hamburg, Germany), and the avidin-biotin technique (9). An antibody to localize RAGE-antigen (receptor for advanced glycation end products) was a kind donation of M. Shearman (Merk Sharpe and Dome, Essex, England); it was an established polyclonal goat anti-RAGE antiserum, developed by immunizing goats with Escherichia coli-expressed recombinant human RAGE.
The sections were examined by light microscopy at a magnification of x400 using a semiquantitative scoring system (scores 0 to +++). All antibodies had been tested for specificity in the mouse before. Negative controls were performed by omitting the primary antibody. To reduce run-to-run variations in the staining intensity, all stainings of the same antibody were performed in the same run for all Apoe-/- and C57/BL6 controls (6,9). All analyses were performed in a blinded manner, i.e., the observer was unaware of the experimental protocol.
Statistical Analyses
Data were normally distributed and are given as x ± SD. Differences between groups were analyzed using Kruskal-Wallis test or ANOVA followed by a post hoc Duncan multiple range analysis.
| Results |
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In the aorta, no plaque formation was found in C57/BL6 controls with sham operation or UNX; in three of eight SNX C57/BL6 control animals small plaques were found. In contrast, in Apoe-/- mice, some plaques were seen even in sham animals. In sham Apoe-/-, the average number of plaques per aortic circumference was 4.25 ± 1.06 compared with 4.5 ± 1.09 in UNX Apoe-/- and 4.6 ± 0.55 in SNX Apoe-/-. Maximal plaque thickness (from cap to bottom; cross-section) was, however, significantly higher after UNX and SNX. The total aortic plaque area, summing up all individual plaques per aortic circumference, was progressively higher from sham to UNX to SNX Apoe-/- animals (Table 2).
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-SMA staining), and virtually absent lymphocyte infiltration. Taken together, the morphology corresponded to that of an incipient plaque. By immunohistochemistry, there were no major differences in the aortic media between sham op and SNX Apoe-/- mice, with the exception of somewhat more marked staining for anti-collagen IV and fibronectin. Striking changes, in particular a markedly higher expression of nitrotyrosine in SNX Apoe-/- were noted in the non-plaque-bearing intima (Table 3). In plaques of SNX Apoe-/- as compared with C57/BL6 SNX, striking but discontinuous staining for nitrotyrosine as an index of oxidative stress was also noted. Figure 1 provides a representative example of a plaque in a SNX Apoe-/- mouse with strong staining for nitrotyrosine (A) also in the unaffected intima and strong staining for RAGE (B), osteopontin (D), and some staining of fibronectin (C). The Kossa stain consistently showed absence of calcified deposits.
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| Discussion |
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In the short period of 12 wk, no significant plaque formation was noted in the sham-operated or UNX genetic C57/BL6 controls. Minor plaque formation was noted in three of eight of the SNX wild-type controls. After SNX, similar thickening of the aortic wall was noted in wild-type and Apoe-/- mice, as described previously in SNX rats in this laboratory (11).
In contrast, plaques were found even in sham-operated Apoe-/- mice, but plaques were strikingly larger in Apoe-/- animals after uninephrectomy and particularly after subtotal nephrectomy. Similarly, maximal cap to bottom plaque thickness and total plaque area per aortic circumference were also higher. In contrast, the number of plaques per aortic circumference was not increased. This would suggest a rapid enlargement of preexistent plaques rather than their de novo generation. In view of past controversies on whether atherogenesis is truly accelerated in uremia, it is of note that the present observation strongly supports this possibility. We have to acknowledge, however, that we did not use the so-called en face approach for determination of atherosclerotic lesions, i.e., we did not open the aorta as a whole and measure the total area of atherosclerotic plaques in the various aortic segments (12). Instead, we used the method described by Patel et al. (13) and could thus collect material for different histologic techniques.
The morphology of the plaques in SNX Apoe-/- mice corresponds to that of early plaques with foam cells and virtually no lymphocyte infiltration or invasion by media cells staining positive for
-smooth muscle actin. In view of the recent interest in accelerated calcification of plaques in uremia (14,15), we emphasize the consistent absence of staining for calcium in our mice model, but we admit that the duration of the experiment was probably too short to give rise to plaque calcification. An overriding role of oxidative stress is suggested by the strongly positive staining for nitrotyrosine, which interestingly was not confined to the plaque per se, but was also found in the morphologically intact surrounding intima. Furthermore, the plaques showed massive expression of RAGE, the receptor for advanced glycation end products (AGE), but no evidence of cell proliferation (indicated by negative staining for PCNA) or infiltration by macrophages/monocytes. With respect to RAGE, it is of interest that in the model of the Apoe-/- mouse accelerated atherogenesis of diabetic mice could be abrogated by scavenging of AGE by administration of the extracellular domain of RAGE (16).
Because of the immense impact of atherosclerotic complications on survival of patients with renal failure, elucidation of the uremia-specific factors aggravating and accelerating atherogenesis is a high priority. We propose that the Apoe-/- mouse provides a suitable model to analyze the cellular and molecular mechanisms that are responsible for this complication of renal disease.
| Acknowledgments |
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| References |
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