We read with interest the recent article by Buzello et al. [1] on renal changes in apo E-/- mice after subtotal nephrectomy. They compared renal lesion development in male wild-type C57BL/6 mice with that of genetically modified male apo E-/- mice after either sham operation, unilateral nephrectomy or subtotal nephrectomy (SNX) by removal of 75% of the cortex in one kidney and removal of the contralateral kidney. They found a significantly higher baseline level of the glomerulosclerosis index, but a similar tubulo-interstitial damage score, in sham apo E-/- mice compared with sham wild-type mice. Since arterial blood pressure was higher in apo E-/- mice and since the authors demonstrated a 10-fold increase in serum cholesterol levels in apo E-/- mice compared with wild-type mice, it is reasonable to assume that both factors may have contributed to the higher glomerulosclerosis index in the former. However, after SNX no significant difference in the glomerulosclerosis index was observed between the apo E-/- and C57BL/6 groups, although the glomerulosclerosis index was higher in both uraemic groups than in non-uraemic controls.
We were surprised to see that the authors did not provide information on proteinuria in apo E-/- and wild-type mice. Although some forms of chronic kidney disease (CKD) can progress in the absence of albuminuria, in the majority of experimental CKD models proteinuria is a highly valuable marker and probably also a culprit in disease progression. Since Buzello et al. did not observe a marked acceleration of CKD progression in uraemic apo E-/- mice, it would have been very interesting to know whether this occurred in the presence or absence of a difference in proteinuria.
We recently measured urinary protein excretion in apo E-/- mice and wild-type C57BL/6 mice using a pirogallol red-colorimetric method (Hitachi 917, Roche, Meylan, France). We could not find a difference between the two strains. However, the proteinuria was significantly greater in male than in female animals of either strain (C57BL/6 mice: 2.27±0.44 vs 0.40±0.06 g/mmol creatinine, P<0.0001, respectively; apo E-/- mice: 2.33±0.54 vs 0.54±0.03 g/mmol creatinine, P<0.0001, respectively). We further examined urinary protein excretion in uraemic apo E-/- mice using an electric coagulation model to induce uraemia (Muntzel et al., unpublished data). We were surprised to see that despite a similar degree of CKD as in the uraemic male mice of Buzello et al., there was no increase in proteinuria compared with non-uraemic male controls (2.03±0.44 vs 2.27±0.44 g/mmol creatinine, NS). Neither was there a difference in proteinuria in uraemic and non-uraemic female mice (0.51±0.38 vs 0.54±0.33 g/mmol creatinine, NS). However, since one cannot easily extrapolate findings from one CKD model to another, it would be interesting to find out if the SNX model used by Buzello et al. and Bro et al. [2] leads to significant proteinuria or not.
Possible differences in the degree of proteinuria could also be due to differences in gender, as suggested by our observation above. Whereas we generally use female apo E-/- mice for studies on the influence of CKD on atherosclerosis, Buzello et al. [1] and Bro et al. [2] used exclusively male apo E-/- mice. In a previous report by Buzello et al. [3], the gender of the mice used was not mentioned. Since it was shown that in 16-week-old apo E-/- mice atherosclerotic plaque formation is two to three times greater in female than in male animals, it is important to have precise information on gender. It is only by 48 weeks of age that the lesions of male mice catch up and surpass those of age-matched females [4]. Aortic lesion progression is also more rapid in uraemic female mice than in male apo E-/- mice, as shown in our recent study on uraemia-enhanced progression of atherosclerosis (Ivanovski et al., unpublished data). In this study, female, but not male, apo E-/- mice exhibited a significant increase in atherosclerotic surface lesion area in the aortic root over a period of 8 weeks of uraemia.
In conclusion, it remains to be seen whether the failure of uraemic apo E-/- mice to have a more marked progression of CKD than the uraemic wild-type mice despite markedly increased cholesterol and blood pressure levels is related to a resistance to develop proteinuria or to other protective factors.
Conflict of interest statement. None declared.
Inserm Unit 507 Necker Hospital Paris France Email: ivanovski{at}necker.fr
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