Evaluation of renal hypoxia in diabetic mice by BOLD MRI

Abstract

Objective: Renal hypoxia has been proposed to be a pathophysiologic feature of diabetic kidney disease but it has been difficult to demonstrate in vivo, particularly in mouse models of diabetes. The objective of this work was to examine the sensitivity of blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) to assess renal oxygenation in vivo in a mouse model of diabetic kidney disease, the db/db mice.

Research design and methods: Kidney BOLD MRI studies were performed on a 3.0 T scanner using multiple gradient echo sequence with a custom-designed surface coil to acquire T2*-weighted images. Studies were performed in 10-week-old db/db mice (n = 7) and db/m controls (n = 6).

Results: R2* is a measure of the tissue deoxyhemoglobin concentration and higher values of R2* are associated with hypoxia. The db/db mice had higher medullary (43.1 ± 5.1 s⁻¹ vs. 32.3 ± 3.7⁻¹ s, P = 0.001) and cortical R2* (31.7 ± 3.1 s⁻¹ vs. 27.1 ± 4.1 s⁻¹, P = 0.04) values. Using pimonidazole staining as a marker of kidney hypoxia, in kidney sections from 10-week-old db/db mice neither cortex nor medulla had significant differences as compared with 10-week-old db/m mice (cortex: db/db 2.14 ± 0.05 vs. db/m 2.02 ± 0.28, medulla: db/db 2.81 ± 0.08 vs. db/m 2.6 ± 0.08). The db/db mice demonstrated further increased cortical and medullary hypoxia when scanned again at 15 weeks of age.

Conclusions: The report shows that renal BOLD MRI is a sensitive method for the in vivo evaluation of renal hypoxia in a mouse model of diabetic kidney disease where progressive renal hypoxia can be documented over time. BOLD MRI may be useful to monitor therapeutic interventions that may improve tissue hypoxia in the diabetic kidney.

Figure 1

Representative example of BOLD MRI data obtained in a diabetic db/db (upper panels) and a control db/m mouse (lower panels). The brightness on the R2* map reflects the relative oxygenation status of the kidney. The area of brightness in the OM of the db/db diabetic mice (right upper panel) is increased as compared with that of the db/m control mice (left lower panel), indicating a decreased tissue oxygenation.

Figure 2

Pimonidazole immunohistochemical staining of the kidney of db/m and db/db mouse. Strong pimonidazole staining of renal tubules was observed mainly in the OM (single arrows) both from db/m and db/db mouse. Weaker staining was seen in the cortex of both db/db and db/m mice (double arrows, 100×).

Figure 3

A summary of R2* values obtained by BOLD MRI in the medulla and cortex of 10-week-old db/db and db/m mice (upper panel) is shown in the upper panel. R2* is higher in medulla than cortex in both db/db and db/m reflecting lower oxygenation in the medulla. The asterisk denotes a significant difference in R2* between db/db and db/m in both cortex (P < 0.04) and medulla (P < 0.001). The lower panel shows the pimonidazole staining score for db/db and db/m mice of similar age (10 weeks old). The medulla had stronger staining than the cortex in both db/db and db/m reflecting increased hypoxia. No significant differences were found in the pimonidazole staining of the medulla and cortex between db/db and the db/m mice.