Mitochondria Damage and Kidney Disease

In human body, the kidney receives 20% of cardiac output and consumes 10% of the body’s oxygen to perform its functions. The kidney is composed of multiple cell populations which are involved in several vital functions that maintain body’s homeostasis such as acid-base and electrolytes balance, blood pressure regulation, nutrients reabsorption and hormone secretion [1]. The human adult kidney is composed with about 3 million functional units, or “nephrons”, and their primary functions and structures are subdivided into a glomerular filtration unit and several tubular segments. The glomerular filtration is a process of filtering blood circulation, retaining circulating cells and useful macromolecules. The tubular segments are involved in active transport processes to reabsorb water, electrolytes, and nutrients to maintain fluid and osmolarity homeostasis. The coordination of both processes maintains body homeostasis and results in metabolic waste excretion with only trace amount of proteins in the urine output [2]. Hence, kidney dysfunction often results in kidney disease with systemic complications.

Kidney diseases can be classified into two categories- acute kidney injury (AKI) or chronic kidney disease (CKD). Renal disease and complications are the current global health concerns due to the vital roles of kidney in body homeostasis [3–5]. AKI is characterized by rapid renal function decline along with accompanying electrolytes abnormalities, fluid overload, severe acidosis, hematologic abnormalities (e.g. anemia, uremic platelet dysfunction), and possibly multi-organs failure and poor clinical outcomes [6]. CKD is linked to gradual loss of renal function over time. CKD could derive from defects in glomerular filtration units or chronic tubular injuries which could lead to multiple complications including cardiovascular disease, high blood pressure, bone loss, and malnutrition. Both AKI and CKD are closely integrated and could serve as a risk factor for one another which often linked with increase cardiovascular risk and uprising mortality and morbidity rates [7, 8]. Additionally, kidney functions are susceptible to certain inherited genetic diseases and aging processes [9, 10]. All different forms of renal injuries could ultimately progress into severe end-stage renal disease (ESRD) with renal replacement as the only therapeutic option. Therefore, therapeutic strategies are in dire need to prevent renal diseases progression.

Mitochondria is the cell’s energy-producing organelle that maintains cellular redox and energy homeostasis, and therefore a major source of intracellular oxidative stress [11]. In addition to its role in adenosine triphosphate (ATP) generation through oxidative phosphorylation (OXPHOS), the mitochondria also play an essential role in metabolic signaling such as pyrimidine, heme biosynthesis, TCA cycle and fatty acid β-oxidation pathways, in calcium ion (Ca²⁺) homeostasis, thermogenesis, proliferation and regulating intrinsic apoptotic pathway. Mitochondria are heterogeneous and dynamic organelles. The mitochondria populations can be different in size, mass, metabolic activity, and membrane potential within a cell. Additionally, mitochondria constantly change shape, dynamics and turnover to maintain cellular homeostasis. Cellular stress can induce compromised mitochondria membrane integrity or dysfunctions, leading to cell death via different mechanisms. This could involve the release of apoptotic molecules such as cytochrome C (cyt C), and apoptosomes from mitochondrial inter-membrane spaces. Alternatively, activation of mitochondrial permeability transition (MPT) can trigger mitochondrial inner membrane potential (ΔΨm) dissipations and subsequent loss of energy production [12].

Mitochodrial damages and dysfunction are recognized as a leading factor to many chronic and acute renal diseases [11]. Therefore, it is remarkably important to understand mitochondrial biology and pathophysiology for effective therapeutics discoveries in renal diseases. In this chapter, we discuss evidence supporting mitochondrial dysfunction in the pathogenesis of kidney disease, and summarize the recent development of mitochondria-targeted therapies which hold high promise alleviating renal injury.

Mitochondria in Kidney Health

Mitochondria are especially important in metabolically active organs such as brain, heart, kidney and muscle. Kidney consumes roughly 7% of the body’s daily ATP energy expenditure [13, 14]. Due to various energy demands, different nephron segments have different mitochondria densities and distributions. It is generally accepted that renal tubule cells are rich in mitochondria, with the S1 segment containing the highest mitochondria density. These renal tubule cells require this large amount of mitochondria due to their high-energy demand for reabsorption and secretion against chemical gradients, which heavily rely on normal mitochondrial oxidative phosphorylation to supply ATP as an energy source [15]. Nevertheless, the high-energy requirement of the podocytes was only recently highlighted for possible mechanisms in structural stability, organization of cytoskeletal and extracellular matrix proteins, motility, remodeling of foot process, uptake of filtered proteins, and some other more obscure mechanisms [16]. Bioenergetic profiles studies of mitochondrial function confirmed that podocytes are very susceptible to dysfunction in energy supply during stress condtions [17]. Several factors such as mitochondrial biogenesis and turnover, bioenergetics, dynamics, and autophagy (mitophagy) regulate the conditions of mitochondria.

Mitochondrial Biogenesis

Mitochondrial turnover is an exquisitely coordinated process to maintain mitochondrial homeostasis. In this regard, dysfunctional mitochondria are selectively eliminated and replaced through biogenesis, a coordinated process between nuclear and mitochondrial genomes, to increase the mass and number of functional mitochondria in compensation for the lost or damaged mitochondria [18]. The mitochondrial DNA (mtDNA) encodes 37 genes which include 13 structural subunits of the mitochondrial respiratory chain. The molecular mechanism of mitochondrial biogenesis during tissue injury and repair are actively studied topics [19, 20] (Fig. 27.1a, b). The mitochondrial biogenesis program involves mito-nuclear communication and several key regulators have been identified such as: peroxisome proliferator-activated receptor (PPAR), PPARγ coactivator 1α (PGC-1α), sirtuin-1 and 3 (SIRT 1/3) family deacetylase, AMP-activated protein kinase (AMPK), and nuclear respiratory factors 1 and 2 (NRF1 and NRF2) [21]. AMPK and SIRT1 positively regulate PGC-1α, the master regulator of mitochondria biogenesis, through post-translational modifications with phosphorylation or deacetylation respectively. In addition, AMPK and SIRT1 are important modulators of energy metabolism. In kidney, the PGC-1α transcriptional coactivator is predominantly expressed in proximal tubules, and therefore is pivotal to tubular homeostasis. PGC-1α regulates the expression of NRF1 and NRF2 and increases the biosynthesis of nicotinamide adenine dinucleotide (NAD⁺), a central metabolic coenzyme/cosubstrate involved in cellular energy metabolism, to link oxidative metabolism to renal protection [22–24]. Both NRF1 and NRF2 are nuclear DNA (nDNA)-encoded transcription factors that activate genes coding for the OXPHOS system and genes involved in mtDNA transcription and replication [25, 26].

Fig. 27.1.

The health and disease of kidney are regulated by the pathophysiological conditions of mitochondria. Several processes regulate the health and disease states of mitochondria which include (panel a) mitochondrial biogenesis – a process requires coordinated expression of both nDNA and mtDNA and several transcriptional factors and co-activators; (panel b) OXPHOS bioenergetics – an electrochemical gradient and ATP synthesis process which depends on respiratory electron transfer chain (complexes I–V), cofactors (MPV17 and others), and cardiolipin to maintain normal oxidative phosphorylation; (panel c) mitochondrial dynamics – a process requires functional balance between fusion and fission; and (panel d) mitochondrial turnover by autophagy (or mitophagy) – an important process to clear the dysfunctional mitochondria. (Panel e) Disturbance in any steps of the processes in panels a–d will cause ROS overproduction, decrease in ATP generation, loss of IMM potential (ΔΨm), mPTP opening and cyt C release. The oxidative stress could lead to cell apoptosis or cell injury and the damaged mtDNA, nDNA could stimulate innate immune response through activation of Toll-like receptor (TLR) and inflammation which, all together, lead to AKI or CKD. See Text and Abbreviations for details

Mitochondrial Bioenergetics

The mitochondrial OXPHOS respiratory chain is composed of four protein complexes which are known as complexes I–IV. These complexes transfer electrons and protons (H⁺) across inner mitochondrial membrane (IMM) to generate an electrochemical gradient for ATP synthesis in a fifth protein complex, known as complex V (ATP synthase) (Fig. 27.1b). Some important factors were identified in this process. For example, NAD concentrations determine the rate-limiting process [27, 28] and Coenzyme Q10 (coQ10) shuttles electrons in the respiratory chain [29]. Additionally, depending on its interaction with cardiolipin, the IMM resident protein cytochrome C (cyt C) regulates an intricate balance between mitochondrial respiration and apoptosis. Cardiolipin was identified as a phospholipid that was exclusively expressed on the IMM, where it forms microdomains and plays a central structural role in cristae formation, a property of cardiolipin on membrane curvature, organization of the electron transport chain (ETC) complexes into supra-complex for OXPHOS activity, and as a platform for initiation of apoptosis [30, 31]. The interaction of cyt C and cardiolipin determines whether or not cyt C functions as an electron carrier or a peroxidase.

During normal electron transfer, some (<4%) of the consumed oxygen is converted into superoxide radicals such as reactive oxygen free radical species (ROS) and reactive nitrogen free radical species (RNS) via electron leakage, which constitutes different forms of oxidative stress [32]. However, under ischemic conditions, Ca²⁺ overflows into mitochondria during rapid loss of ATP and aggravates ROS production. These reactive radicals cause modification of biomolecules of DNAs (nDNAs and mtDNAs) and proteins, lead to lipid peroxidation, and may impair their bioactivities which eventually lead to the opening of the mitochondrial permeability transition pore (mPTP) and loss of mitochondrial membrane potential (ΔΨm) [12].

The importance of the IMM-resident MPV17 protein in mitochondrial physiology has come to light recently. MPV17 is encoded by nDNA and functions as a ΔΨm-modulating channel that contributes to mitochondrial homeostasis under different conditions. MPV17 forms weak cation-selective channesl and has shown several subconductance states in vitro [33]. Interestingly, MPV17 channel protein allows for the passage of small molecules such as deoxynucleotides triphosphates (dNTPs). It is possible that MPV17 maintains mitochondrial dNTP homeostasis, and therefore the perturbation of dNTP pools is a recognized cause of mitochondrial genomic instability [34]. Mutations of MPV17 are associated with mtDNA depletion syndrome, an inherited autosomal recessive disease in humans. Additionally, MPV-like proteins are identified as epithelial and neuronal restricted protein and implicated in ROS metabolism and apoptosis regulation through its binding and functional interaction with mitochondrial serine protease, HTRA2 [35]. Altogether, excess oxidative stress accumulation, mtDNA instability, and imbalance of bioenergetics will lead to mitochondrial dysfunction which could be important in the pathogenesis of kidney diseases [36].

Mitochondrial Dynamics

Mitochondria are highly dynamic organelles – their number, size and locations change constantly in response to energy demands. They constantly move, fuse and divide – switching between elongated interconnected networks or fragmented discrete morphologies via coordinated fusion and fission. These dynamics are essential to their size, morphology, energy biogenesis, function, and maintenance of cellular homeostasis and viability [37–39]. Perturbation of mitochondrial dynamics results in mitochondrial dysfunction and is linked to aging, end organ injury, and human diseases [40, 41]. Mitochondrial dynamics are regulated by a complex relationship between fission proteins (the large GTPase, dynamin related protein 1 (DRP1), and mitochondrial fission 1 (Fis1)) and fusion proteins (mitofusins 1 and 2 (Mfn1, Mfn2) and optical atrophy (OPA1)) [5, 42] (Fig. 27.1c). The NAD⁺-dependent SIRT family deacetylases, especially the mitochondrial matrix-resided SIRT3 protein, play an important role in regulating mitochondrial dynamics and function. In this role, the SIRT3 deacetylates and activates the mitochondrial fusion protein OPA1 to improve mitochondrial function under stress conditions [43] or acute kidney injury [44].

Mitochondrial Turnover: Autophagy and Mitophagy

Autophagy is a non-selective process where cytoplasmic contents, e.g. damaged organelles and aged protein aggregates, are sequestered into autophagosomes for lysosome delivery and bulk degradation within the cell. Autophagy occurs during nutrient stress for ATP preservation, amino acid recycling, and anabolic protein synthesis. In certain stress conditions, selective autophagy occurs to efficiently remove toxic cellular materials such as damaged organelles. Mitophagy is one of the selective autophagy processes to remove accumulated dysfunctional mitochondria. Mitophagy clearance encompasses several steps – the damaged mitochondria are first marked by a loss of mitochondrial inner membrane potential (ΔΨm) via intricate protein interactions involving kinases, E3 ubiquitin ligase, and proteins regulating mitochondrial dynamics and transportation [45] (Fig. 27.1d). The damaged mitochondria are eventually encapsulated into autophagosomes and degraded in autolysosomes. Autophagy (and mitophagy) is ROS-dependent, which is heavily regulated by cellular redox activity. Growing evidences support the notion that disturbance of autophagy/mitophagy is associated with the pathogenesis of renal diseases such as AKI [5], diabetic nephropathy [45] and glomerulosclerosis [46].

Mitochondria Damage in Kidney Diseases

Renal diseases encompassing both acute and chronic conditions of kidneys injury with declined renal functions are current global health concerns with tremendous medical burdens [3–5]. A common link between all forms of acute and chronic kidney injuries is the generation of toxic ROS and RNS when the disease manifests. This oxidative stress injury could be derived from ischemia/reperfusion, energy shortage from impaired mitochondrial biogenesis or ATP energetics, or defective clearance of damaged mitochondria (mitophagy). All of the listed conditions could link to recruitment of immune cells, inflammatory cytokines accumulation, apoptosis, and tissue injury. The renal phenotypes of mitochondrial dysfunction may manifest as proximal tubular dysfunction, tubularinterstitial disease, cystic kidney disease, podocytopathy, and nephrotic syndromes. All different forms of renal injuries might increase cardiovascular risk with uprising mortality and morbidity rates.

Acute Kidney Injury (AKI)

AKI has been a global concern with worldwide prevalence of 13 million people and death toll roughly 1.7 million annually [47]. AKI is a clinical condition that is commonly related to an acute episode of systemic injuries that occur in cases such as: kidney transplantation and septic and trauma patients. In the USA, AKI is the source of 1–2% of all hospital admissions and the mortality rates in hospitalized ICU patients with AKI could reach 50–70% [48]. Several etiologies such as ischemic, toxic, septic, and hypertensive injuries cause AKI with symptoms of abrupt reduction in kidney functions. AKI symptoms are manifested as acute decline in glomerular filtration rate, concomitant decreased urinary output, tubular necrosis, tubular interstitial inflammation and vascular permeability changes [5, 49].

Interestingly, in a polymicrobial septic AKI model, Tsuji and colleagues recently identified circulating mtDNA that stimulate systemic inflammation via Toll-like receptor 9 (TLR9) signaling as a novel pathogenic mechanism for AKI (Fig. 27.1e). The source of circulating mtDNA has been speculated to be derived from immune cells upon septic clearance or from spleen immune response [50, 51]. This study suggests that timely protection and removal of damaged mitochondria would reduce inflammation and could be an effective therapeutic strategy against AKI.

Ischemic-Reperfusion (I/R) Induced AKI

Renal ischemia-reperfusion injury (IRI) is a common cause of AKI and post-transplantation kidney allograft dysfunction resulting from an acute decline in general or localized renal oxygen and nutrients supplies to the affected tissue, and impairment of timely removal of metabolite wastes in kidney cells. Extensive oxidative stress generated during this process injures the tubular epithelial cells, causing histologically characteristic inflammation and necrotic cell death (necrosis) which was referred as acute tubule necrosis previously [52]. Emerging evidence supports the notion that multiple signaling pathways are involved in the pathobiology of AKI [52, 53]. Of these, damaged mitochondria with disrupted matrix cristae and mitochondrial permeability-mediated necrosis are crucial to ischemic AKI. Proper functional autophagy (mitophagy) is reno-protective during renal tissue regeneration if the injury is under a certain threshold, whereas defective mitophagy impedes tissue regeneration [5].

Nephrotoxic AKI

Exposure to nephrotoxins, such as some chemotherapy drugs, medications, intravascular contrast media, trace heavy metals, drug abuse, or certain chemicals/drugs combinations could induce AKI, especially in the elderly, young children, and high-risk patients [54, 55]. Drug nephrotoxicity was found to be responsible for 19% of all AKI cases on critically ill patients. The kidney is susceptible to intrinsic renal toxin injury which could result in various clinical syndromes ranging from minimal changes in tubular function to fulminant kidney failure. Injury may also occur to altered hemodynamic AKI. Some examples include: NSAIDs and renin-angiotensin system (RAS) antagonism, tubular epithelium direct cell injury from chemotherapy drugs like cisplatin, tubular obstruction of urinary flow due to precipitates of toxins or their metabolites, and angiopathogenesis from vascular injury and interstitial inflammation. Several research efforts have been dedicated to identify biomarkers of nephrotoxic AKI [56]. Cisplatin-induced renal mitochondrial structural, functional and homeostatic changes include swollen mitochondria, ultra-structurally disrupted IMM cristae, reduced mitochondrial mass, mtDNA depletion, and diminished cyt C oxidase activity in animal model [57]. Permeability of mitochondrial OMM upregulates the expression of mitochondrial dynamin related protein 1 (DRP1), an OMM resident fission protein, which induces mitochondrial fragmentation, cyt C release and apoptosis [58]. Reduction of mitochondria biogenesis regulators such as PGC-1α and SIRT3 were also documented [44]. Moreover, kidney specific overexpression of SIRT1 was protective against cisplatin-induced AKI [59]. The nephrotoxic effect and its pathophysiology of cisplatin-induced AKI had been a seminary topic of a recent review [60].

Septic AKI

Septic shock is one of the most frequent injury which could account for nearly half of the cause of AKI in critical ill patients [61]. Mitochondrial damage is an important contributing factor in the pathogenesis of septic AKI [62]. Clinical biopsies obtained from intensive care of human sepsis subjects also support this notion [63]. Apparent mitochondrial ultrastructure changes and biochemical studies revealed mitochondrial pathogenesis includes loss of mitochondrial mass, swelling, fragmentation due to aberrant mitochondrial dynamics, PGC-1α reduction, extensive mitochondrial cristae remodeling, and cyt C release during apoptosis. As aforementioned, mitochondrial damage and the release of mtDNA is linked to systemic inflammation and tissue injury. The intricate interplays between mitochondrial energy precursor NAD/NADH, mitochondrial health and kidney function were shown in several studies of septic AKI animal model [23, 24]. In summary, mitochondrial NAD concentration regulates the rate-limiting process of mitochondrial energy production and, hence, PGC-1α causes an increase of NAD biogenesis, mitochondrial numbers and subsequent renoprotection.

Chronic Kidney Disease (CKD)

CKD is characterized by persistent renal dysfunction for more than 3 months. The causes of CKD have been commonly associated with a number of comorbid conditions such as type 2 diabetes and hypertension [64]. The prevalence of CKD is estimated to be roughly 8–16% globally. According to the United States Renal Data System report, more than 13% of adults develop CKD and 30% of the population at the age of 65 and above experience some form of kidney failure in the USA [65]. CKD is staged based on estimated glomerular filtration rate (eGFR) and albuminuria with conditions ranging from asymptomatic stage to end-stage renal disease (ESRD) [66]. Different from AKI, CKD is recognized as a separate, irreversible and progressive pathologic condition which often inevitably leads to ESRD [7, 8, 49]. Individuals with diabetes or hypertension have high risk to develop CKD. Recent findings support the close relation between AKI and CKD and confirm, clinically, that one could serve as the risk factor to the other in maladaptive recovery conditions [7, 8].

Diabetic Nephropathy (DN)

Diabetic Nephropathy (DN), a progressive microvascular complication that arises from diabetes mellitus (DM) which affects individuals with both type 1 or type 2 DM, is the leading cause of CKD. DN is a chronic disease characterized by decreased glomerular filtration, proteinuria, glomerular hypertrophy, and renal fibrosis with gradual decline in renal function which contributes to 40% kidney failure and eventually requires renal replacement therapy [67–69].

The pathogenesis of DN is complex due to all four renal compartments-glomeruli, tubules, interstitium and blood vessel being involved. Moreover, etiologies of DN originated from hyperglycemia mediated microvasculate abnormalities which affect several signaling pathways. These include: increased glucose metabolite flux, advanced glycation end-products formation, endoplasmic reticulum stress, ROS overproduction, pro-inflammation and apoptotoc cell death in podocytes [68, 69]. It is well documented that aberrant mitochondrial homeostasis plays pivotal roles in DN progression [11, 68–73]. Hyperglycemia induced increased oxidative stress derived from renal ROS over-production in conditions like impaired mitoglutathione transport [74], increased mitochondrial activation of an ubiquitous NAD(P)H oxidase (Nox4) in respiratory chain [75], decreased mtDNA stability to against ROS production [76] and podocytes apoptosis [77] all contribute to cell damage and lethal effects linked to diabetic complications. Interestingly, hyperglycemia triggers mitochondrial fission and is mediated by Rho-associated coiled coil-containing protein kinase 1 (ROCK1)-dependent phosphorylation of dynamin related protein 1 (DRP1) and its recruitment to mitochondria [78]. These results suggest that mitochondrial dynamics play an important role in the pathogenesis of DN. Additionally, defects in mitochondrial biogenesis with downregulated expression or activity of PGC-1α is recognized to contribute to DN [71, 72]. It appears that the protective role of PGC-1α is associated with the inhibition of DRP1-mediated mitochondrial dynamics remodeling and ROS production [79]. Growing evidence also suggests that autophagy or mitophagy is altered in DN [45]. It is plausible that impairment in autophagic flux would lead to the DN complication associated with extracellular matrix deposition and fibrosis.

Glomerulonephritis (GN)

Chronic glomerulonephritis (GN), characterized by dysfunction in the glomerular filtration barrier, is a major feature of CKD. GN accounts for roughly 10% of CKD. GN is the inflammation of the glomeruli with clinical symptoms which may include hematuria, proteinuria, edema, and hypertension. Congenital GN can occur as genetic mutations on mitochondrial proteins (see section on “Genetic Mitochondrial Disease Affecting Kidney Functions”). Immune diseases may also cause chronic GN. Abnormal-shaped mitochondria are often found before the disease progress into secondary (or acquired) focal segmental glomerulosclerosis (FSGS), a severe form of GN [46]. Therefore, these evidence support the notion that impaired autophagic mitochondrial turnover is responsible for FSGS phenotype in mice.

Recently, Kruppel-like factors 6 (KLF6), a subfamily of DNA-binding zinc finger protein, was identified as an inducible early injury response gene that encodes a crucial transcriptional regulator of mitochondrial function in podocytes under stress response [80–83]. Podocyte-specific loss of Klf6 reduced mitochondrial function and increased the susceptibility to FSGS in mice. Likewise, podocyte-specific KLF6 expression was significantly reduced in FSGS patients when compared to healthy individuals. Mechanistic studies indicated KLF6 prevents mitochondrial injury via enhancing cyt C assembly [82].

The function roles of MPV17 were explored further as discussed earlier. Growing body of evidence indicated that the glomerulosclerosis gene mpv17 and its encoded MPV17 protein as important factors in regulating peroxisomal metabolism of ROS [84]. MPV17 was identified as an IMM- localized protein in podocyte. Loss of Mpv17 was found to be associated with glomerulosclerosis and knockout mice developed renal failure later on in their lifespan (9–12 months). The glomerular lesions were caused by toxic oxidative stress and lipid peroxidation adducts. Interestingly, expression of MPV17 is diminished in several glomerular injury models and also in human FSGS subjects [85]. Altogether, the evidence further confirmed that MPV17 is important for mitochondrial homeostasis in podocytes.

Genetic Mitochondrial Disease Affecting Kidney Functions

Genetic disorders that affect mitochondrial function can be mutations related to either the nuclear or mitochondrial genomes which encode mitochondrial proteins. They represent genetically and clinically heterogeneous disorders which affect mostly metabolic-active organs. Genetic mitochondrial mutations could be derived from primary (inherited) or secondary to environmental predisposition to drugs or oxidative stress injury. Kidney phenotypes are often manifested as tubular dysfunction, interstitial nephritis, glomerular dysfunction, and renal tumors [9, 86].

MELAS and tRNA-Leu (m.A3243G) Transition

MELAS (mitochondrial encephalopathy, lactic acidosis, and stroke like symptoms) is a mitochondrial syndrome manifested in patients that are 40-years old or younger with symptoms like seizure and stroke-like episodes, muscle myopathy, lactic acidosis, and maternally inherited diabetes with deafness. A base transition (A3243G) in mitochondrial gene tRNA-Leu accounts for 80% of MELAS cases with occasional renal phenotypes of FSGS, or less commonly with tubular interstitial nephritis, which results in progressive renal insufficiency [87, 88]. Electron microscopic images from renal biopsies of MELAS patient revealed abnormal mitochondria in podocytes and tubular cells, indicating dysfunctional mitochondria is central to disease pathogenesis.

Mitochondrial DNA Deletion

Different from the mtDNA point mutations, few mitochondrial disease syndromes which are often associated with large mtDNA deletion at different genome locations and affect tubular pathologies, are Fanconi [89], Kearns-Sayre, and Pearson syndromes [90]. The clinical features of Pearson syndrome are manifested with pancreatic fibrosis with insulin-dependent diabetes, whereas the kidney phenotype of Kearns-Sayre syndrome includes renal tubular acidosis (proximal or distal) which occasionally progress to ESRD. Although occurrences may be rare, these syndromes involve renal tubulopathy (proximal or distal) manifested as electrolyte abnormalities and fibrosis. Renal biopsies of patients with Kearns-Sayre syndrome revealed impaired cyt C oxidase activity, defect in energy metabolism, and mitochondrial ultrastructures alterations [90].

Mutations in Nuclear-Encoded Genes for Mitochondrial Proteins

The mitochondria proteome is composed with about 1,000 proteins which are mostly encoded by nDNA. nDNA mutations with prominent tubular dysfunction include mutations that affect energy metabolism, often affecting complex III and IV, and mutations that affect fatty acid metabolism. Podocyte dysfunction is often correlated with multi-mutations in coenzyme Q10 biosynthesis pathways and the pathogenic mechanism was confirmed in pdss2 gene encoding prenyl diphosphate synthase subunit 2, the first enzyme of coQ10 pathway [29, 91].

Aging in Kidney Disease

Older age is associated with an elevated risk of kidney disease and increased population of older renal transplant recipients and poor graft survival. The renal aging phenotype includes profound functional alterations leading to increased renal vascular resistance and reduced repair capacity. Aging kidney is often associated with histological changes such as tubular atrophy, interstitial fibrosis and glomerulosclerosis. Clinically, renal aging is associated with an increased susceptibility to AKI [92], and CKD may share accelerated vascular disease phenotype similar to the condition of premature aging [93]. The kidney suffers from increasing oxidative stress and disturbance in autophagy with aging [94, 95]. The molecular mechanism of renal cell senescence has been explored to certain extent. Downregulation of PGC-1α may link telomere-dysfunction to compromised renal mitochondrial function through p53 activation and alteration of transcription [96]. The renin–angiotensin–aldosterone system (RAAS) is a key regulator of blood pressure, fluid homeostasis and cardiovascular physiology. Angiotensin II and the age-dependent switch of mitochondrial expression of the two types of cell surface angiotensin receptors (AT1R vs AT2R) are important factor in determining respiratory activity during aging [97]. Further studies are needed to explore the role of the telomere–mitochondrial axis in renal physiology, aging and disease.

Mitochondria-Targeted Therapeutics

As protection of mitochondria could serve as a potential effective therapeutic strategy, it has been research endeavors focused on this development toward kidney diseases [49, 98–101]. These agents include antagonizing mitochondria oxidants, promoting mitochondrial biogenesis and ATP synthesis, regulating ROS metabolism, cardiolipin protection, inhibitors of mPTP, or inhibitors of mitochondrial fragmentation as detailed below (Table 27.1).

Table 27.1.

Mitochondria-targeted therapeutics

Therapeutics	Chemicals	Action mechanisms	Experimental model
Antioxidants
MitoQa	Mitoquinone	Anti-oxidant concentrate at matrix in a ΔΨm-dependent manner; ROS scavenger	IRI-AKI, cisplatin-AKI, sepsis-AKI
Biogenesis activator
Resveratrol (SRT501)	Small peptides	AMPK / SIRT-1 / PGC-1α axis activator	Hemorrhagic shock, sepsis-AKI
AICAR		SIRT3 activator, AMPK agonist	Cisplatin-AKI, IRI-AKI
Formoterol		β2-adrenoceptor agonist, increase PGC-1α synthesis	IRI-AKI
ROS metabolism and bioenergetics
Mitochonic acid (MA-5)	Synthetic compound	OXPHOS-independent increase of ATP synthesis, reduce ROS	Sepsis-AKI
Cardiolipin protection
Bendavia (SS-31)a	Szeto-Schiller tetrapeptide	Protect cardiolipin from peroxidation; increase ATP and reduce ROS	AKI and CKD
		Prevent cyt C transformation into peroxidase
mPTP inhibitor
cyclosporin A (CsA)a	Small molecule	Interact with cyclophilin D (a mPTP structural protein)	Adriamycin-induced podocyte injury
TDZD-8		mPTP inhibitors, GSK3β inhibitor	Podocyte injury or drug induced AKI
Fission inhibitor
Mdivil	Small molecule	Inhibitor of mitochondrial division; induce mitochondria fusion	IRI-AKI
		Block DRP1 assembly and translocation
K(ATP) channel opener
Simdax (Levosimendan)a	Small molecule	Calcium sensitizer, K(ATP) channel opener	IRI-AKI
cyt C assembly
KLF6	DNA-binding zinc finger transcriptional regulator	enhance cyt C assembly	Podocytes FSGS (animal study)

^aClinical trials. https://clinicaltrials.gov

Mitochondria-Targeted Antioxidants

Mitochondria-targeted antioxidants (e.g. MitoQ, MitoTEMPO, MitoE, Mito-CP, SkQ1 and SkQR1) have been a part of the efforts to reduce mitochondrial oxidative stress. These agents are based on the mechanism of delivery of known redox agents to mitochondrial matrix through conjugation with TPP+ (triphenylalkylphosphonium cation) moiety [100]. These ROS scavenger compounds could cross the membrane lipid bilayer and concentrate at the matrix in a membrane potential (ΔΨm) – dependent manner. MitoQ was shown to be safe in clinical trial with Parkinson’s disease, fatty acid disease, and is currently under clinical trial (NCT02364648) for CKD. In summary, several animal models have confirmed that mitochondrial-targeted antioxidants represent an effective strategy to prevent or attenuate different forms of kidney diseases.

Activator of Mitochondria Biogenesis

Activation of Mitochondria biogenesis is required for the increased metabolic and energy demands during the recovery phase of acute organ injury. The AMPK/SIRT/PGC-1α axis plays crucial roles in mitochondrial biogenesis. PGC-1α is a pivotal factor that coordinately regulates NAD biogenesiss, reduces oxidative stress, and facilitates recovery from AKI [24]. SIRT1 is a NAD-dependent deacetylase that positively regulates PGC-1α expression and activity. Agents in this category include resveratrol, AICAR, and formoterol. Treatment with resveratrol, a natural plant phytoalexin, in a rat hemorrhagic shock model increases mitochondria mass, restores mitochondrial respiratory capacity, and reduces oxidative stress [102]. Moreover, resveratrol modulates immune response by suppressing inflammation driven by macrophage in a lipopolysaccharide -induced septic AKI model [103]. Pretreatment with AICAR, an AMPK activator, attenuated injury and tubular necrosis, decreased nitrosative stress, and ameliorated renal function in a rat I/R induced AKI [104]. Formoterol, a potent agonist of β2-adrenoreceptor, induces mitochondrial biogenesis by increasing mtDNA copy numbers, oxygen consumption rate, and PGC-1α synthesis. In doing so it restores renal function, rescues renal tubules from injury, and diminishes necrosis in an I/R-induced AKI animal model [105]. In summary, agents affect mitochondrial biogenesis and NAD modulation hold great promise in treating kidney disease, but their clinical translation still awaits further investigation.

Modulation of ROS Metabolism and ATP Synthesis

Mitochonic acid 5 (MA-5), a synthetic derivative of indole acetic acid plant hormone, was identified as a compound to enhance ATP production. MA-5 was found to improve the survival of fibroblasts collected from patients with mitochondrial diseases, including Leigh syndrome and MELAS [106]. Evidence supports that MA-5 enhances ATP production by promoting assembly and oligomerization of complex V at mitochondrial crista junction, and therefore preserving the mitochondria dynamics and preventing its fragmentation. These new findings provide new mechanisms in renal and cardiac protection [106, 107].

Cardiolipin Protections

As cardiolipin regulates IMM structural and functional plasticity, peroxidation or loss of cardiolipin has been associated with aging and several forms of AKI and CKD. Development of cardiolipin-target compound to optimize efficiency of the ETC and thereby restore cellular bioenergetics has been an innovative discovery. SS-31 peptide (Szeto-Schiller peptide, also named MTP-131 or Bendavia) is a cardiolipin-targeted tetrapeptide (D-Arg-dimethylTyr-Lys-Phe-NH2) that stabilizes cardiolipin, scavenges mitochondrial ROS, regulates cyt C activity, and inhibits the mitochondrial permeability transition (MPT) pore in AKI and CKD models [108–110]. SS-31 treatment demonstrated mitochondrial protective effects on swine stenotic-kidney injury and improved renal hemodynamics and function [111]. SS-31 has entered several clinical trials for treatment with AKI renal microvascular dysfunction in hypertension, and I/R injury of myocardial infarction [108].

mPTP Inhibitors

Opening of the IMM localized mitochondrial permeability transition pore (mPTP), a Ca²⁺-dependent nonselective pore, under certain pathological conditions such as Ca²⁺ overload or oxidative stress leads to the IMM permeability to small molecules (<1,500 Da in molecular weight), loss of proton motive force, uncoupling of OXPHOS, and mitochondrial swelling which often followed by cell death. Opening of mPTP appears to be a pivotal event in AKI [112]. Cyclosporin A (CsA) is a potent inhibitor of mPTP through interaction with cyclophilin D, a structural component and mediator of mPTP which critically regulates Ca²⁺- and ROS-dependent opening. Low dose of CsA (at submicromolar concentration) suppress mPTP opening and mitochondria swelling, and is currently under clinical trial for its effect on reperfusion injury on acute myocardial infarction (NCT01595958). CsA reduced podocyte damage through mitochondrial protection in an adriamycin-induced podocyte injury animal model [113]. A clinical trial (NCT02397213) is underway to test CsA renal protection in cardiac surgery (potential AKI condition). High dose of CsA is renotoxic and thus limits its clinical use.

Evidence suggests that glycogen synthase kinase (GSK)-3β resides at the nexus of multiple signaling pathways implicated in the regulation of mitochondrial permeability transition (MPT). The selective GSK-3β inhibitor TDZD-8, a thiadiazolidinone derivative, works as a non-ATP competitive inhibitor to prevent MPT and improve cell viability during AKI [112, 114].

Fission Inhibitors

Mitochondrial fusion is required for homogenous distribution of mtDNA, matrix proteins, and lipids across all fused mitochondria. However, mitochondria fission is important in mitochondrial proliferation following mitosis and is also involved in damaged (fragmented) mitochondrial removal by mitophagy. Mdivi-1 (mitochondrial fission inhibitor-1) was identified as a small molecule that selectively and reversibly inhibits the assembly and the GTPase activity of fission protein DRP1 [115]. Mdivi-1 induces rapid and reversible formation of fused mitochondria in wide varieties of cells and several animal disease models including I/R-injury in the heart, liver, kidney, and brain [116]. Mdivi-1 ameliorates rhabdomyolysis-induced AKI in rat by reducing ROS stress and tubular epithelial cell apoposis with simultaneous increase of ATP production [117] and protection from mPTP opening in acute cardiorenal syndrome [118]. Despite its therapeutic potential, Mdivi-1 exhibits divergent functions which could be cytoprotective or cytotoxic and the reno-protective effect of Mdivi-1 in humans is still lacking [116].

K(ATP) Channel Opener

Levosimendan is a Ca²⁺ sensitizing smooth muscle vasodilator clinically used for the treatment of heart failure. Levosimendan exerts pleiotropic beneficial effects including mitochondria protection in cardiomyocytes during ischemic heart disease [119]. Possible molecular mechanisms of Levosimendan might involve its function as a mitochondrial ATP-sensitive potassium channel to favorably conserve mitochondrial energy in cardiomyocytes. Pre-clinical and clinical data also indicated positive circulatory effects in the brain, lung, and renoprotection under potentially lethal stress conditions. Currently, a clinical trial (NCT01720030) with Levosimendan in an AKI study is ongoing.

Concluding Remarks and Future Perspectives

Mitochondria are the “power house” of the high-energy demanding kidney cells. The mitochondrial networks also affect the cross-talk with other cellular organelles such as the endoplasmic reticulum, nucleus, peroxisomes, and thus impact many cellular functions. Renal mitochondrial dysfunction could derive from defects on mitochondrial biogenesis, bioenergetics, dynamics, turnover and genetic mutations which may contribute to respiratory chain-derived oxidative stress, abnormal ultra-structures, increased sensitivity to apoptosis, and accumulation of damaged mitochondria with unstable mitochondrial DNA. Disturbance of renal mitochondrial homeostasis could lead to damages to microvasculatures, inflammation, fibrosis, and kidney failure. Although kidney mitochondrial dysfunctions have been exquisitely demonstrated to be tightly linked to pathobiology in many preclinical animal studies of CKD, AKI, aging and genetic defected mitochondrial disease, many direct cause-effect mechanisms remain elusive before development of clinical translations.

The mitochondria-targeting therapeutics as effective interventions to preserve mitochondria structures and functions have been demonstrated in several animal models of kidney injuries. Among these, MitoQ and SS-31 have shown promising potential in clinical trials towards certain kidney injuries and diseases. With better understandings of mitochondrial genome and its physiological functions, advances in in vivo animal studies, and human clinical trials to confirm efficacy and safety of mitochondria-targeting therapeutics, we have high hope to translate these discoveries into therapeutic strategies to ameliorate kidney injuries and diseases in the future.

Abbreviations

AKI

Acute kidney injury

AMPK

AMP-activated protein kinase

ATP

Adenosine triphosphate

Ca²⁺

Calcium ion

CKD

Chronic kidney disease

coQ

Coenzyme Q

CsA

Cyclosporin A

cyt C

Cytochrome C

DN

Diabetic nephropathy

dNTPs

Deoxynucleotides triphosphates

DRP1

Dynamin related protein 1

ESRD

End-stage renal disease

ETC

Electron transport chain

FSGS

Focal and segmental glomerulosclerosis

GN

Glomerulonephritis

GSK

Glycogen synthase kinase

H⁺

Proton

I/R

injury or IRI Ischemic reperfusion injury

IMM

Inner mitochondrial membrane

KLF-6

Krüppel-like factor 6

MELAS

Myopathy encephalopathy lactic acidosis and stroke-like episodes

Mfn1 and 2

Mitofusins 1 and 2

MPT

Mitochondrial permeability transition

mPTP

Mitochondrial permeability transition pore

mtDNA

Mitochondrial DNA

MA-5

Mitochonic acid 5

NAD⁺

Nicotinamide adenine dinucleotide

nDNA

Nuclear DNA

NRF

Nuclear respiratory factors

OMM

Outer mitochondrial membrane

OPA1

Optical atrophy 1

OXPHOS

Oxidative phosphorylation

PGC-1α

PPARγ-coactivator −1α

PPAR

Peroxisome proliferator-activated receptor

ROS

Reactive oxygen species

SIRT

Sirtuin

TLR

Toll-like receptor

ΔΨm

Mitochondrial inner membrane potential