Evaluation of renal metabolic response to partial ureteral obstruction with hyperpolarized 13 C MRI

Abstract

Hyperpolarized ¹³ C magnetic resonance imaging (MRI) may be used to non-invasively image the transport and chemical conversion of ¹³ C-labeled compounds in vivo. In this study, we utilize hyperpolarized ¹³ C MRI to evaluate metabolic markers in the kidneys longitudinally in a mouse model of partial unilateral ureteral obstruction (pUUO). Partial obstruction was surgically induced in the left ureter of nine adult mice, leaving the right ureter as a control. ¹ H and hyperpolarized [1-¹³ C]pyruvate MRI of the kidneys was performed 2 days prior to surgery (baseline) and at 3, 7 and 14 days post-surgery. Images were evaluated for changes in renal pelvis volume, pyruvate, lactate and the lactate to pyruvate ratio. After 14 days, mice were sacrificed and immunohistological staining of both kidneys for collagen fibrosis (picrosirius red) and macrophage infiltration (F4/80) was performed. Statistical analysis was performed using a linear mixed effects model. Significant kidney × time interaction effects were observed for both lactate and pyruvate, indicating that these markers changed differently between time points for the obstructed and unobstructed kidneys. Both kidneys showed an increase in the lactate to pyruvate ratio after obstruction, suggesting a shift towards glycolytic metabolism. These changes were accompanied by marked hydronephrosis, fibrosis and macrophage infiltration in the obstructed kidney, but not in the unobstructed kidney. Our results show that pUUO is associated with increased pyruvate to lactate metabolism in both kidneys, with injury and inflammation specific to the obstructed kidney. The work also demonstrates the feasibility of the use of hyperpolarized ¹³ C MRI to study metabolism in renal disease.

Keywords: hyperpolarized 13C; metabolism; renal; ureteral obstruction.

FIGURE 1

Experimental time line for each mouse. Partial unilateral ureteral obstruction (pUUO) surgery was performed at day 0 (open circle). Magnetic resonance imaging and collection of blood and urine samples were performed 2 days prior to surgery (baseline) and at three separate time points afterwards (filled circles). After imaging on the final day, mice were sacrificed and both kidneys were harvested for immunohistological and biochemical evaluation

FIGURE 2

Comparison of axial ¹H and ¹³C images before and after partial ureteral obstruction. (A) Kidneys appear normal in the T₂-weighted (T₂w) image, and both pyruvate and lactate are detected in both kidneys at baseline. (B) At 14 days following obstruction, hydronephrosis is apparent in the left kidney (arrow), accompanied by a marked reduction in the pyruvate and lactate signal in this kidney. Image scaling is equivalent across (A) and (B) for pyruvate and lactate. An example region of interest is overlaid on the baseline pyruvate image (white)

FIGURE 3

Changes in renal pelvis volume, lactate and pyruvate across time points. Significant interaction effects at multiple time points were found for renal pelvis volume (A), pyruvate area under the curve (AUC) (B) and lactate AUC (C). Pyruvate AUC and lactate AUC also showed significant differences between left and right at baseline. The ratio of lactate and pyruvate AUC did not show differences between kidneys, but showed significant changes with respect to baseline (D). Data are plotted as mean ± standard error. *p ≤ 0.05 between left and right kidneys; †p ≤0.05 for main effect with respect to baseline; ‡p ≤0.05 for kidney × time interaction effect with respect to baseline

FIGURE 4

Picrosirius red (A, B) and F4/80 (C, D) immunohistological staining of kidney sections. The increased density of red pigment in (B) relative to (A) indicates greater collagen deposition, or fibrosis. Similarly, the increased density of the blue pigment in (D) relative to (C) indicates greater infiltration by macrophages

FIGURE 5

Quantitative comparison of immunohistological and biochemical assay results. Significantly greater intensity of staining was found in the left kidney relative to the right for both F4/80 and picrosirius red, although the activity of lactate dehydrogenase (LDH) in the kidneys was not different (A). LDH activity in the serum (B) and lactate concentration in the urine (C) showed no significant changes across time points. In (A), data are normalized to the value of the right kidney. Data are plotted as mean ± standard deviation. *p ≤ 0.05 between left and right kidneys