Background- Chronic kidney disease (CKD) affects 15% of the general population. Cardiorenal syndrome (CRS) type 4 is characterized by underlying CKD leading to impairment of cardiac function and to major cardiovascular events. Using an animal model for CKD, we have recently demonstrated significantly aberrant ultrastructure of cardiac mitochondria compared to Sham; merely enlarged mitochondria that occupy larger volume of the cardiomyocyte. We wished to characterize the mechanisms leading to this pathological presentation of cardiac mitochondria.
Methods- Male Lewis rats underwent 5/6 nephrectomy allowing CKD development. At time of necropsy, eight months later, cardiac samples (left septum) of Sham and CKD rats were obtained. Total mitochondrial DNA content was determined by measuring the expression of the mitochondrial gene, cytochrome B. Active mitochondrial content was assessed by citrate synthase enzymatic activity (a mitochondrial marker enzyme) in tissue homogenates. Respiratory chain function in isolated mitochondria was determined by enzymatic assays of complexes I-IV. Expression levels of specific microRNAs (miRs) known to target functional mitochondrial genes were tested using real-time PCR.
Results- Mitochondrial DNA content was decreased by 78% in cardiac tissue of CKD rats compared to Sham. Concomitantly, active mitochondrial content was significantly reduced. Conversely, no differences were observed in respiratory chain enzymes` functions (complexes I-IV) in isolated active mitochondria. Interestingly, we found that miR15b was exclusively upregulated in the cardiac tissue of CKD rats.
Conclusions- The data strongly suggest that while part of the cardiac mitochondria are degraded and hence inactive in the CKD setting, the remaining mitochondria present similar activity as the control group. These significant changes in cardiac mitochondrial structure and function may be explained, at least in part, by the upregulation of miR15b, whose known targets include genes affecting mitochondrial function. AntagomiRs to miR15b may thus hold therapeutic potential in attenuating mitochondrial deterioration and disease progression to CRS.
