*4.4. Mitochondrial Dysfunction*

The remnant kidney of uraemic animals demonstrated mitochondrial dysfunction as illustrated by increased proton/electron leak. Proton/electron leaks whether due to specific uncoupling proteins or non-specific transfer of protons/electrons into the matrix results in loss of membrane potential [68,69]. The conversion of ADP to ATP by mitochondrial complex V is dependent on membrane potential, the loss of which severely compromises bioenergetics and could partly explain the exacerbation of kidney dysfunction in this study. The kidney is a metabolic organ requiring high energy (ATP) to drive its function including active reabsorption of solutes [70]. Tubular cells are rich in mitochondria, and consequently, any injury would compromise renal function and precipitate CKD [71]. The uraemic kidney in this study demonstrated several structural and biogenetic abnormalities. Transmission electron microscopic study of the remnant kidney (uraemic) revealed existence of swollen mitochondria with loss of cristae and matrix density (Figure 9). Mitochondrial matrix swelling and loss of cristae membranes were evident after renal ischaemia in rats, a ffecting ATP regeneration after reperfusion [72].

**Figure 9.** Transmission electron microscopic representation of uraemic mitochondria (**A**) with evidence of mitochondrial fragmentation (fission) as indicated by the presence of smaller mitochondrial spheres; swollen mitochondria with sparsely arranged cristae relative to sham mitochondria (**B**). Arrows indicate mitochondria.

Our observation of mitochondria with rounded morphology in the uraemic kidney is similar to the report of Lan et al. [73]. In a similar study, the remnant kidney following 5/6 nephrectomy showed fragmented and dysmorphic mitochondria with evidence of swelling and disrupted cristae architectures [20]. The authors reported diminished mitochondrial function and reduced antioxidant capacity alongside oxidative stress similar to the observation presented here. Under normal conditions, antioxidant enzymes (such as glutathione peroxidase, peroxyredoxin, glutaredoxin 2, thioredoxin and catalase) protect the mitochondria from ROS attack, and hence prevent membrane damage and peroxidation [74]. However, in conditions of diminished antioxidant activity as observed in this model of CKD, damage to mitochondrial inner membrane can occur, leading to the inexorable decline in mitochondrial function and worsen kidney function and its associated complications.

The mechanisms underlying loss of cristae in the present model are unclear but ongoing work is focused on changes in mitochondrial membrane cardiolipin (CL) content and remodelling as possible factors. Indeed, there is increased vulnerability of CL to peroxidation owing to its close proximity to site of ROS production and in part due to its relative high content of unsaturated fatty acyl chains [75,76]. This could explain the increased evidence of mitochondrial fission as shown by the presence of fragmented mitochondria (Figure 9a) in the uraemic kidney. Mitochondrial fission involved the division of the mitochondrion into two daughter organelles, and when in excess, results in mitochondrial fragmentation. Mitochondrial fragmentation is increasingly evident in many kidney diseases [77,78].

Uraemic animals showed mitochondrial complex I respiratory dysfunction in the remnant kidney, which was not improved by iron therapy. In addition, iron therapy was associated with reduced complex II and complex IV driven respiration that could be related to the increased accumulation of iron observed in the remnant kidney of uraemic animals. Indeed, dysfunctional mitochondria can potentiate excessive generation of ROS. As reported by Zhu et al. [79], mitochondrial dysfunction underlined by decreased expression of complexes I, II and IV was associated with increased generation of ROS and oxygen consumption which is not coupled to ATP production. However, despite these cellular changes iv iron appeared to increase maximal respiration and respiratory reserve capacity suggestion a degree of mitochondrial adaptation or upregulation of numbers or function at this stage of CKD. This requires further work to elicit the complex changes occurring within the kidney.
