Pharmacologic Approaches to Improve Mitochondrial Function in AKI and CKD

Abstract

AKI is associated with high morbidity and mortality, and it predisposes to the development and progression of CKD. Novel strategies that minimize AKI and halt the progression of CKD are urgently needed. Normal kidney function involves numerous different cell types, such as tubular epithelial cells, endothelial cells, and podocytes, working in concert. This delicate balance involves many energy-intensive processes. Fatty acids are the preferred energy substrates for the kidney, and defects in fatty acid oxidation and mitochondrial dysfunction are universally involved in diverse causes of AKI and CKD. This review provides an overview of ATP production and energy demands in the kidney and summarizes preclinical and clinical evidence of mitochondrial dysfunction in AKI and CKD. New therapeutic strategies targeting mitochondria protection and cellular bioenergetics are presented, with emphasis on those that have been evaluated in animal models of AKI and CKD. Targeting mitochondrial function and cellular bioenergetics upstream of cellular damage may offer advantages compared with targeting downstream inflammatory and fibrosis processes.

AKI is associated with high morbidity and mortality, and it predisposes to the development of CKD. The primary causes of AKI include ischemia, sepsis, and nephrotoxicity. There is, at present, no effective treatment for AKI, and therapeutic interventions targeting CKD progression are largely limited to control of hypertension and hyperglycemia, agents that interrupt the renin-angiotensin system, and the possible salutary effects of reduced salt and protein intake.¹ Novel strategies that minimize AKI and halt the progression of CKD are urgently needed.

Normal kidney function involves numerous different cell types, such as tubular epithelial cells, endothelial cells, and podocytes, working in concert. This delicate balance involves many energy-intensive processes, particularly sodium transport, necessitating close coordination between energy supply and demand.² Proximal tubular (PT) epithelial cells are rich in mitochondria and rely on oxidative phosphorylation for ATP generation. The importance of mitochondrial function as energy providers in the kidney is evident in inherited mitochondrial diseases with renal impairments.^3,4

The overwhelming evidence for mitochondrial dysfunction in AKI and CKD has led to a search for mitochondria-protective drugs. This review summarizes our current understanding of ATP production and energy demands in the kidney and available evidence of mitochondrial dysfunction in AKI and CKD. New therapeutic strategies targeting mitochondria protection and cellular bioenergetics are presented, with emphasis on those that have been evaluated in animal models of AKI and CKD.

ATP Production in the Kidney

Fatty acid (FA) oxidation is the preferred energy source for highly metabolic tissues, because it generates three times more ATP compared with glucose oxidation.^5–7 FA β-oxidation fuels the tricarboxylic acid cycle and oxidative phosphorylation (Figure 1). The uptake of long-chain FAs is facilitated by specific proteins, such as CD36.⁸ In the cytosol, FAs are activated to acyl-CoA by acyl-CoA synthetases. Because the mitochondrial membrane is impermeable to acyl-CoAs, the carnitine shuttle is needed for their import into mitochondria.⁵ Carnitine palmitoyltransferase-1 (CPT-1) on the outer mitochondrial membrane (OMM) catalyzes the transesterification of acyl-CoA to acylcarnitine. This complex then enters the mitochondrial matrix via facilitated diffusion by carnitine-acylcarnitine translocase (CACT). CPT-2 on the inner mitochondrial membrane (IMM) reconverts the acylcarnitine into an acyl-CoA. CPT-1 is the rate-limiting enzyme in FA oxidation. Deficiency of CPT-1 results in energy failure and kidney disease.⁹

Figure 1.

FA uptake and mitochondrial FA β-oxidation. FAs are the preferred energy substrates for the kidney. Uptake of FAs into kidney cells is facilitated by CD36. In the cytosol, FAs are activated to acyl-CoA and transported to mitochondria by the carnitine shuttle. CPT-1 on the OMM catalyzes the transesterification acyl-CoA to acylcarnitine. The complex then enters the mitochondrial matrix via facilitated diffusion by carnitine-acylcarnitine translocase (CACT). CPT-2 on the IMM reconverts the acylcarnitine into an acyl-CoA. In the mitochondrial matrix, acyl-CoAs undergo β-oxidation to generate acetyl-CoA to fuel the tricarboxylic acid cycle (TCA) and also NADH and FADH₂ that serve as electron donors to the ETC on the IMM for ATP production. NADH is also produced by the tricarboxylic acid.

After they are in the mitochondrial matrix, acyl-CoAs undergo β-oxidation to generate acetyl-CoA to fuel the tricarboxylic acid cycle and also NADH (reduced nicotinamide adenine dinucleotide) and FADH₂ (reduced flavin adenine dinucleotide), which serve as electron donors to the electron transport chain (ETC) on the IMM (Figure 2). The respiratory protein complexes of the ETC reside on cristae membranes in the IMM. The transfer of electrons from complex 1 to complex 4 via redox reactions is coupled with the transfer of protons into the intermembrane space. This creates an electrochemical proton gradient across the IMM that drives the production of ATP by the F₀F₁-ATPase. All of these complexes must be assembled properly for efficient electron transfer to take place. The unique conical structure of cardiolipin on the IMM promotes membrane curvature and allows the respiratory complexes to form supercomplexes that facilitates more efficient electron transfer by reducing the distance between electron donor and electron acceptor.^10,11 Cardiolipin also serves to anchor cytochrome c to the IMM via electrostatic interaction to facilitate electron transfer from complex 3 to complex 4 and as a proton trap on the outer leaflet of the IMM.

Figure 2.

Role of cardiolipin (CL) in cristae structure and supercomplex formation on the IMM. The protein complexes of the ETC reside on cristae membranes, and CL provides curvatures on the IMM to increase surface area for the respiratory complexes. CL helps to organize the respiratory complexes into supercomplexes to facilitate electron transfer among the redox partners. CL also anchors the highly cationic cytochrome c (c) via electrostatic interaction to bring it in close proximity to complex 3 and complex 4 for efficient electron transfer. In addition, CL serves as a proton trap to optimize the proton gradient for complex 5 (F₀,F₁-ATPase). CL is particularly vulnerable to oxidative damage because of its high content of unsaturated FAs. Oxidized cardiolipin (CLOOH) destabilizes CL microdomains, reduces cristae curvatures, and disrupts supercomplex formation. Peroxidation of CL also reduces its affinity for cytochrome c and sets the stage for cytochrome c release into the cytosol and apoptosis. e, electron; FAD, flavin adenine dinucleotide; FADH₂, reduced flavin adenine dinucleotide; NAD, nicotinamide adenine dinucleotide; NADH, reduced nicotinamide adenine dinucleotide; PC, phosphatidylcholine; Q, coenzyme Q.

Cellular Consequences of Mitochondrial Dysfunction

Inhibition of electron transfer in the ETC leads to electron leak at complex 1 and complex 3 and the formation of reactive oxygen species (ROS). Oxidative damage to the respiratory complexes further inhibits enzyme activity, setting up a feed-forward cycle of mitochondrial oxidative stress and progressive energy deficiency. Cardiolipin is particularly vulnerable to oxidative damage, and oxidized cardiolipin disturbs cardiolipin microdomains on the IMM, causing loss of cristae curvature and inhibition of electron transfer (Figure 2). Oxidized cardiolipin has lower affinity for cytochrome c and frees cytochrome c from the IMM.¹² Oxidized cardiolipin also breaks the Met⁸⁰-Fe ligation on its heme to turn cytochrome c into an oxygenase that can oxidize cardiolipin without ROS.^13,14

All enzymes for FA oxidation reside in the mitochondrial matrix, and tight cristae membranes serve to concentrate enzymes and substrates close to the respiratory complexes. Defective FA oxidation decreases the supply of reducing equivalents to the ETC. Furthermore, cardiolipin peroxidation can signal a number of cellular events (Figure 3). (1) Oxidized cardiolipin damages cristae curvatures, effectively reducing respiratory complexes. (2) If oxidative stress is mild, oxidized cardiolipin can translocate to the OMM, where it triggers mitophagy to selectively eliminate damaged mitochondria. Damaged mitochondria are enveloped by endoplasmic reticulum membranes to form autophagosomes, which then fuse with lysosomes to form autophagolysosomes that degrade mitochondrial content.^15,16 (3) Oxidized cardiolipin on the OMM can also recruit Bax to trigger mitochondrial permeability transition, which releases cytochrome c from mitochondria to initiate apoptosis via activation of caspase-9.¹⁷ (4) Oxidized cardiolipin on OMM can also serve as a docking station for NLRP3 (NLR family pyrin domain containing 3) inflammasome assembly, and mitochondrial ROS triggers activation of caspase-1 to cleave pro–IL-1β and pro–IL-18 to cause an inflammatory cell death called pyroptosis.^18,19 (5) Cells will undergo necrosis in the event of profound ATP depletion, and danger molecules released from necrotic cells can also trigger inflammasome activation.²⁰

Figure 3.

Cellular consequences of mitochondrial dysfunction. Mitochondrial dysfunction increases ROS and causes cardiolipin peroxidation. Oxidized cardiolipin can signal a number of cellular events. (1) Oxidized cardiolipin damages cristae curvatures, effectively decreasing substrate concentration and delivery of NADH and FADH₂ to the respiratory complexes. (2) If oxidative stress is mild, oxidized cardiolipin can translocate to the OMM, where it triggers mitophagy to selectively eliminate damaged mitochondria. Damaged mitochondria are enveloped by endoplasmic reticulum membranes to form autophagosomes, which then fuse with lysosomes to form autophagolysosomes that degrade mitochondrial content. (3) Oxidized cardiolipin on the OMM can also recruit Bax to trigger mitochondrial permeability transition, which releases cytochrome c from mitochondria to initiate apoptosome activation, activation of caspase-3, and apoptosis. (4) Oxidized cardiolipin on OMM can serve as a docking station for NLRP3 inflammasome assembly, and mitochondrial ROS triggers inflammasome activation to activate caspase-1 to cleave pro–IL-1β and pro–IL-18 to cause chronic inflammation and tissue remodeling. (5) Cells will undergo necrosis in the event of profound ATP depletion, and danger molecules released from necrotic cells can also trigger inflammasome activation. FADH₂, reduced flavin adenine dinucleotide; NADH, reduced nicotinamide adenine dinucleotide; NLRPE, NLR family pyrin domain containing 3.

Mitochondrial Dysfunction in AKI

Under ischemic conditions, the deficiency of substrates and oxygen inhibits mitochondrial respiration, and the kidney must switch to glycolytic metabolism, which greatly reduces the amount of ATP that can be produced. Mitochondrial respiration is significantly inhibited during ischemia, resulting in dramatic loss of ATP within 5–10 minutes.^21,22 In ischemia, Na⁺-K⁺-ATPase activity is inhibited within a few minutes, and intracellular Na⁺ increases three- to fourfold, causing cell swelling.²³ ATP is also required for actin polymerization, and cytoskeleton breakdown leads to detachment of endothelial cells,²³ loss of brush border and cell detachment in PT cells,^23,24 and podocyte effacement.²⁵ Ischemia can result in either apoptotic or necrotic cell death, because ATP is needed for activation of the caspase cascade.²⁶ The S3 segment of the PT is more susceptible to necrosis than S1 and S2 segments due to much lower blood flow in the outer medulla. The distal tubule has greater capacity for glycolytic metabolism and is more likely to undergo apoptosis.²⁷

Rapid recovery of ATP is essential for cell survival, and this is dependent on recovery of mitochondrial function. However, ischemic damage to mitochondria structure limits the ability of these cells to regenerate ATP rapidly on reperfusion. Cell swelling caused by ATP depletion increases the osmotic gradient that drives water into the mitochondrial matrix.²⁸ Matrix swelling is passive and can increase until the IMM becomes completely unfolded into a spherical configuration.²⁹ Matrix swelling has been described in tubular cells in ischemia,^22–24 sepsis,^30,31 and drug-induced nephrotoxicity.^32,33 The loss of cristae membranes during ischemia together with dilution of substrate concentration due to matrix swelling serve to inhibit mitochondrial respiration and delay ATP recovery on reperfusion.^22,24,29

Mitochondrial Dysfunction in CKD

There is evidence that mitochondrial damage can persist long after acute ischemia. Atrophic PTs that fail to differentiate 2 weeks after ischemia show greatly reduced mitochondria number and large autophagolysosomes containing degenerated mitochondria.³⁴ Matrix swelling and loss of cristae membranes are evident in both podocytes and PT mitochondria 1 month after ischemia,²³ and mitochondria remain small, with only remnants of cristae membranes even 9 months later.³⁵ The persistent accumulation of damaged mitochondria suggests impaired mitochondrial quality control. Autophagosomes containing mitochondria and autophagolysosomes can be seen in podocytes and PTs 9 months after original injury.³⁵ However, many mitochondria show evidence of proteolytic degradation, suggesting that mitophagy might be impaired, and damaged proteins are eliminated by mitochondrial proteases. Degenerative mitochondria were also reported in aged podocytes.³⁶

Defects in FA oxidation have been reported in experimental CKD. High-fat diet causes lipid accumulation in tubular cells and downregulation of CPT-1.^37,38 Defective FA oxidation is also seen in patients with diabetic nephropathy. Kidney biopsy samples revealed heavy lipid deposition and increased intracellular lipid droplets.³⁹ Lipid deposition was associated with downregulation of several key genes involved in FA oxidation, linking inhibition of FA oxidation to lipid accumulation. Genome-wide transcriptomic analysis of a large cohort of normal and fibrotic human kidney samples revealed lower expression of CPT-1, CPT-2, and other mitochondrial genes in the fibrotic samples.⁶ The inhibition of FA oxidation in CKD causes a switch to glucose metabolism and reduced ATP production.

Mitochondria-Targeted Approaches for Kidney Diseases

Given the abundant evidence that mitochondrial dysfunction triggers cellular injury and inflammatory and fibrotic responses, protecting mitochondria may be more effective than targeting individual downstream events⁴⁰ (Figure 3). A number of novel mitochondria-targeted approaches are currently in development, including compounds known to specifically target mitochondria and others that promote mitochondrial biogenesis or FA oxidation. These are presented below, with emphasis on those that have been evaluated in kidney disease models (a summary is in Table 1).

Table 1.

Mitochondria-targeted agents in animal models of AKI and CKD

Kidney Disease Model	Species	Experimental Compound	Major Findings	Refs.
Warm ischemia-reperfusion	Rat	MitoQ	↓Oxidative damage, ↑renal function	43
Warm ischemia-reperfusion	Rat	SkQR1	↓Oxidative damage, ↑renal function	44
Warm ischemia-reperfusion	Rat	SS-31	↓Mitochondrial swelling, protect cristae membranes, ↑mitochondrial function, ↓renal pathology, ↑renal function, ↓inflammation and fibrosis	22–24
Warm ischemia-reperfusion	Rat	MA-5	↑Renal function	61
Sepsis (cecal ligation/puncture)	Mice	SS-31	↑ATP, ↓apoptosis, ↓renal pathology	55
Sepsis (LPS)	Rat	MitoQ	↑Mitochondrial potential, ↑renal function	46
Pyelonephritis	Rat	SkQR1	↓Oxidative damage, ↓inflammation, ↓mortality	45
Cisplatin toxicity	Mice	MA-5	↓Renal pathology, ↑renal function	61
Ureteral obstruction	Rat	SS-31	↓Oxidative damage, ↓apoptosis, ↓inflammation and fibrosis	54
Aging	Mice	SS-31	Protected mitochondria in podocytes, ↓senescent markers, ↓glomerulosclerosis	36
Renal artery stenosis	Pigs	SS-31	Restored cardiolipin content, ↓oxidative damage, ↑vascular density, ↓fibrosis	57
Postischemic CKD	Rat	SS-31	Protected mitochondrial structure in all kidney cells, ↓inflammasome activation, ↓glomerulosclerosis and fibrosis	35
Obesity-related nephropathy (high-fat diet)	Mice	SS-31	Protected mitochondrial structure in all kidney cells, prevent lipid accumulation, ↓mesangial expansion, ↓glomerulosclerosis	38
Diabetic nephropathy (streptozotocin)	Mice	SS-31	↓Mesangial expansion, ↓tubular apoptosis	58
Diabetic nephropathy (Ins2±Akita)	Mice	MitoQ	↓Mesangial expansion, ↓interstitial fibrosis, ↓microalbuminuria	50
Diabetic nephropathy (db/db)	Mice	MitoQ	↓ROS, ↓apoptosis, ↓glomerular hypertrophy, ↓mesangial expansion, ↓basement membrane thickening	51

Mitochondria-Targeted Antioxidants

Mitochondrial oxidative stress is common in many kidney diseases. The disappointing clinical results with conventional antioxidants led to the development of mitochondria-targeted antioxidants.⁴¹ The initial approach involved the use of triphenylphosphonium ion conjugated to lipophilic antioxidant molecules, such as CoQ (MitoQ), TEMPO (MitoTEMPO), or plastoquinone (SkQR1). The positive charge drives the transport of these lipophilic molecules into the mitochondrial matrix in a potential-dependent manner.⁴² In warm ischemia-reperfusion, pretreatment with MitoQ 15 minutes before ischemia significantly reduced tissue carbonyl content and reduced serum creatinine in rats.⁴³ Pretreatment with SkQR1 before ischemia reduced tissue ROS and lipid peroxidation and improved renal function, but it had no effect when given after ischemia.⁴⁴ In addition, MitoQ increased mitochondrial potential and improved renal function in a sepsis model using LPS, whereas SkQR1 reduced oxidative damage and renal inflammation in a model of pyelonephritis.^45,46 However, there is little direct evidence that these compounds protect mitochondria structure or function, although MitoTEMPO did protect mitochondrial respiratory complex activity and improved renal function in a sepsis model.⁴⁷ The requirement of mitochondrial potential for uptake may limit their use in some disease states. Furthermore, lipophilic cations can depolarize mitochondria, inhibit oxidative phosphorylation, or become pro-oxidants,^48,49 which may account for loss of efficacy at higher doses.⁴⁷

Hyperglycemia is thought to be associated with increased mitochondrial ROS production. Oral administration of MitoQ in Ins2^±AkitaJ mice for 12 weeks attenuated glomerular basement membrane thickening, mesangial expansion, interstitial fibrosis, and albuminuria.⁵⁰ Because renal mitochondrial function is normal in Ins2^±AkitaJ mice, the mechanism of action of MitoQ is unclear. MitoQ also ameliorated glomerular hypertrophy and mesangial expansion in db/db mice and partially reversed mitochondrial fragmentation in tubular cells.⁵¹ Notably, MitoQ had no effect on hyperglycemia in either model. These promising results suggest that more research is warranted in mitochondria-targeted antioxidants for kidney diseases.

Cardiolipin-Targeting Peptides That Protect Cristae Structure

The Szeto–Schiller (SS) peptides are cell-permeable tetrapeptides that selectively target mitochondria but concentrate on the IMM rather than penetrate into the mitochondrial matrix.⁵² Despite their 3+ net charge, their mitochondrial uptake is potential independent, and they do not depolarize mitochondria. These peptides were originally described as mitochondria-targeted antioxidants because of their electron scavenging ability.⁵² Subsequent research revealed that these peptides interact selectively with cardiolipin to stabilize cristae curvatures.²⁴ After they are bound to cardiolipin, these peptides penetrate deep into the cytochrome c heme environment to promote electron transfer and prevent the conversion of cytochrome c to a peroxidase.^24,53 As a result, these peptides promote ATP synthesis, reduce electron leak and ROS production, and inhibit cardiolipin peroxidation.^10,13,24,53 By preventing cardiolipin peroxidation, the SS peptides prevent apoptosis, inflammation, and NLRP3 inflammasome activation.^22–24,35

SS-31 has been studied extensively in models of AKI. When given before ischemia in rats, SS-31 prevented matrix swelling and preserved cristae structure in tubular epithelial cells and endothelial cells, and it accelerated ATP recovery on reperfusion.^22–24 Cell polarity and brush border in PT cells were restored rapidly, and apoptosis and necrosis were ameliorated.²⁴ Protection of endothelial mitochondria by SS-31 mitigated capillary damage and prevented microvascular rarefaction, interstitial inflammation, and fibrosis.²³ SS-31 also prevented TGF-β upregulation and tubulointerstitial fibrosis in unilateral ureteral obstruction.⁵⁴

SS-31 may be best suited for clinical situations where AKI is anticipated, such as sepsis, partial nephrectomy, and transplantation. When administered immediately after cecal ligation and puncture in mice, SS-31 normalized kidney ATP content, decreased apoptosis, reduced histology score, and reduced serum creatinine and BUN.⁵⁵ Delayed graft function after kidney transplantation reduces long-term graft survival. A recent study of human tissue biopsies showed that grafts with delayed graft function failed to recover mitochondrial respiration.⁵⁶ Kidney biopsies subjected to warm ischemia resulted in profound reduction of oxygen consumption and complex 1 activity, but these changes were prevented when SS-31 was included in the incubation medium.⁵⁶

In addition to protecting mitochondria structure from injury, there is evidence that SS-31 can repair damaged mitochondria and restore mitochondria structure in CKD models. Two months of SS-31 treatment restored cristae structure in podocytes of aged mice, decreased senescent markers, and reduced glomerulosclerosis.³⁶ SS-31 treatment starting 1 month after acute ischemia restored normal mitochondria structure in podocytes and prevented mitophagy in PT, and it reduced NLRP3 inflammasome activation, glomerulosclerosis, and tubulointerstitial fibrosis.³⁵ Surprisingly, the kidneys continued to be protected even 6 months after terminating SS-31 treatment, suggesting that restoration of mitochondrial bioenergetics can provide long-term protection.³⁵

Atherosclerotic occlusion of the renal artery is an important cause of CKD. Renal artery stenosis causes oxidative stress, microvascular rarefaction, and fibrosis.⁵⁷ Treatment with SS-31 blunted oxidative stress, restored cardiolipin content, improved microvascular density, and diminished fibrosis.⁵⁷

Diabetes and obesity are other major causes of CKD. SS-31 has no effect on blood glucose in diabetic or obese mice.^58,59 However, SS-31 significantly reduced glomerular hypertrophy and mesangial expansion and decreased apoptosis in streptozotocin mice.⁵⁸ Treatment with SS-31 during high-fat diet protected mitochondria structure in all kidney cell types and prevented intracellular lipid accumulation.³⁸ SS-31 prevented loss of glomerular endothelial cells and podocytes, inflammation, mesangial expansion, and glomerulosclerosis.³⁸ These findings suggest that excess lipids in the kidney cause cardiolipin peroxidation and loss of cristae membranes. These damaged mitochondria limit FA β-oxidation and result in further lipid accumulation. By promoting mitochondrial function and preventing cardiolipin peroxidation, SS-31 can break this feed-forward loop that amplifies renal injury.

Mitochondria-Homing Agents

Mitochonic acid 5 (MA-5) belongs to the newest class of mitochondria-targeted agents. It is a derivative of the plant hormone indole-3-acetic acid that was discovered in a screen for compounds that can increase cellular ATP content. MA-5 increased cellular ATP and improved the survival of fibroblasts established from patients with inherited mitochondrial diseases.⁶⁰ When administered to mice 3 hours before bilateral renal ischemia, MA-5 reduced tubular necrosis, improved renal function on reperfusion. MA-5 also significantly decreased plasma BUN and tubular necrosis after cisplatin administration.⁶¹

The mechanism of action of MA-5 remains unclear. It was reported to increase ATP synthesis independently of electron transport or oxidative phosphorylation.⁶¹ MA-5 was proposed to target the mitochondrial protein mitofilin/Mic60 at the cristae junction of the IMM and facilitates oligomerization of the ATP synthase and supercomplex formation.^61,62 More research is needed to fully understand how interaction with mitofilin/Mic60 improves bioenergetics and protects the kidney.

Agents That Promote FA Oxidation

Carnitine or acetyl-l-carnitine supplements may be useful in assisting mitochondrial FA uptake. Peroxisome proliferator–activated receptor α (PPARα) is a transcriptional factor that regulates CPT-1 expression and FA oxidation, and PPARα is highly expressed in PT and mesangial cells. Pharmacologic activation of PPARα with fenofibrate reduced kidney injury and fibrosis in model of obstructive nephropathy.⁶ Fenofibrate also reduced urinary albumin excretion and interstitial fibrosis in diabetic nephropathy models by suppressing NF-κB and TGF-β signaling pathways.^63–65 Despite clear evidence that combination use of fibrates and statins can prevent cardiovascular disease in patients with diabetes, the use of fenofibrate for diabetic nephropathy is uncertain, because its use can cause a rapid increase in serum creatinine.⁶⁶

Agents That Promote Mitochondrial Biogenesis

Mitochondria are constantly being renewed through the processes of biogenesis and mitophagy. PGC-1α is a cotranscriptional regulation factor that induces mitochondrial biogenesis by activating mitochondrial transcription factor A.⁶⁷ PGC-1α is highly expressed in kidney and capable of driving all aspects of mitochondrial biogenesis, thus making it a pharmacologic target for kidney diseases.⁶⁸ The PPARγ agonist, pioglitazone, activates PGC-1α and ameliorates age-related renal injury.⁶⁹ Formoterol, a β₂-adrenergic receptor agonist, stimulates PGC-1α expression and mitochondrial respiration in PT cells and mouse renal cortex.⁷⁰ Administration of formoterol 24 hours after ischemic AKI in mice accelerated the return of mitochondrial and renal function by restoring mitochondrial protein expression and respiration via upregulation of PGC-1α.⁷¹ Interestingly, 5-HT₂ agonists also seem to induce mitochondrial biogenesis in PT cells via PGC-1α.⁷² However, the signaling pathways by which these G protein–coupled receptor ligands increase mitochondrial biogenesis remain unclear, and current efforts in developing novel compounds are limited to high-throughput physiologic-based screening.⁶⁸

Activators of AMP Kinase

AMP kinase (AMPK), the major energy-sensing enzyme, is inhibited in metabolic kidney diseases.^37,38,73,74 Reduced AMPK activity can result in decreased FA oxidation via decreased CPT-1 expression,³⁷ and AMPK activation by 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside increased FA oxidation and reduced lipid accumulation and renal pathology in mice.⁷⁴ However, 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside also improved systemic metabolic parameters⁷⁴; thus, the role of renal AMPK activation is unclear. Metformin, already widely used in diabetes treatment, can activate AMPK, but it also reduces blood glucose.⁷⁵ In contrast, SS-31 restores renal AMPK activity without affecting blood glucose or insulin sensitivity.³⁸ AMPK activation without protecting mitochondrial structure is not likely to restore FA oxidation. The relationship between mitochondrial protection, AMPK activity, and FA oxidation needs to be studied further.

Summary and Perspective

Defects in FA oxidation and mitochondria function are key players in AKI and CKD. Strategies aimed at stimulating mitochondrial FA uptake and oxidation can only be successful if mitochondria structure is intact. Targeting mitochondrial function and cellular bioenergetics upstream of cellular damage may be more effective than counteracting downstream inflammatory and fibrosis processes (Figure 4). Some of the mitochondria-targeted agents mentioned here are in early clinical development. A phase 2a trial with SS-31 (elamipretide) during renal angioplasty with stenting (NCT01755858) showed increased renal blood flow and improved cortical perfusion.⁷⁶ A phase 4 trial with MitoQ as a dietary supplement for CKD (NCT02364648) is not yet open for recruitment. In preclinical studies, these mitochondria-targeted approaches seem to protect renal structure and function without affecting systemic metabolic parameters, suggesting that it may be possible to combine these novel targeted agents with standard therapies that target BP, glucose, or lipids.

Figure 4.

Mitochondrial changes in animal models of AKI and CKD. Acute stressors, such as ischemia, sepsis, and toxins, cause mitochondrial matrix swelling and loss of cristae membrane. These structural changes are associated with impaired ATP production, oxidative stress, cellular demise, and chronic tissue remodeling. Cardiolipin peroxidation sets the stage for mitochondrial permeability transition, cytochrome c release, and apoptosis. In CKD, mitochondria are small, with reduced matrix density and loss of cristae membranes. Mitochondrial dysfunction causes a drop in ATP levels and cell injury, whereas inhibition of FA oxidation results in cellular lipid accumulation and downregulation of AMPK activity, further contributing to bioenergetics failure. In both AKI and CKD, cardiolipin peroxidation causes the translocation of cardiolipin to the OMM, where it serves as a docking station for NLRP3 inflammasome assembly. Mitochondrial ROS then triggers inflammasome activation to produce IL-1β and IL-18 that sustain tissue remodeling. MPT, mitochondrial permeabiltiy transition; NLRPE, NLR family pyrin domain containing 3.

Disclosures

H.H.S. is the inventor of SS-31 (elamipretide) and the Scientific Founder of Stealth Biotherapeutics, a clinical stage biopharmaceutical company that has licensed the SS peptides technology from the Cornell Research Foundation for commercial research and development.