SS-31

A Mitochondria-Targeting Peptide in Kidney Disease Research

SS-31, also known as elamipretide, MTP-131, or Bendavia, is a mitochondria-targeting tetrapeptide studied for its potential protective effects in several forms of kidney disease.[1]

The kidneys have extremely high energy requirements and therefore contain large numbers of mitochondria. These mitochondria generate adenosine triphosphate, or ATP, which supplies the energy required for filtration, electrolyte regulation, solute transport, and other essential renal functions.

When kidney mitochondria become damaged, ATP production may decline while the generation of reactive oxygen species increases. This mitochondrial dysfunction can contribute to oxidative stress, inflammation, programmed cell death, tissue injury, and fibrosis.[1,2]

The reviewed evidence suggests that SS-31 may protect kidney tissue by directly supporting mitochondrial structure and function.

What Is SS-31?

SS-31 is a small, water-soluble peptide composed of four amino acids. Its structure allows it to cross cellular membranes and concentrate within the inner mitochondrial membrane.[1,3]

Unlike mitochondria-targeting compounds that rely mainly on mitochondrial membrane potential, SS-31 appears to bind selectively to cardiolipin, a phospholipid found primarily in the inner mitochondrial membrane.[3,4]

Cardiolipin plays an important role in:

• Maintaining mitochondrial membrane structure
• Organizing the electron transport chain
• Supporting oxidative phosphorylation
• Preserving mitochondrial cristae
• Regulating cytochrome c activity
• Maintaining efficient ATP production

By interacting with cardiolipin, SS-31 may stabilize mitochondrial membranes, improve electron transport, reduce electron leakage, and limit the excessive production of reactive oxygen species.[1,3,4]

Proposed Mechanism of Action

The electron transport chain uses electrons to generate the proton gradient required for ATP production. When this system becomes damaged, electrons may leak from the respiratory chain and react with oxygen, producing excessive reactive oxygen species.

These reactive molecules can damage mitochondrial proteins, DNA, and membrane lipids. Cardiolipin is particularly vulnerable to oxidation. Its oxidation can disrupt mitochondrial respiration and facilitate the release of cytochrome c, an important step in the activation of apoptosis.[1,3]

SS-31 may help protect cardiolipin from oxidation while preserving the normal electron-transfer function of cytochrome c.[3,4]

Its proposed effects include:

• Stabilization of cardiolipin
• Preservation of mitochondrial cristae
• Improved electron transport
• Reduced mitochondrial reactive oxygen species
• Increased or preserved ATP production
• Reduced cytochrome c release
• Inhibition of mitochondrial permeability transition pore opening
• Reduction of mitochondrial calcium overload
• Reduced activation of apoptotic pathways

SS-31 is therefore considered more than a conventional antioxidant. Instead of only neutralizing reactive molecules after they have formed, it may help correct the mitochondrial dysfunction responsible for their excessive production.[1]

Acute Kidney Injury

Acute kidney injury is characterized by a rapid decline in kidney function. Mitochondrial dysfunction is believed to play an important role in several forms of acute kidney injury, including ischemia-reperfusion injury, sepsis-associated injury, and drug-induced nephrotoxicity.[1,2]

Ischemia-Reperfusion Injury

Ischemia-reperfusion injury occurs when blood flow to the kidney is temporarily interrupted and then restored.

The initial loss of blood flow deprives kidney cells of oxygen and nutrients. When circulation returns, the sudden reintroduction of oxygen may produce a burst of reactive oxygen species, mitochondrial damage, inflammation, and cell death.

In experimental models, SS-31 was associated with:

• Better preservation of mitochondrial structure
• Reduced mitochondrial swelling
• Reduced oxidative stress
• Improved ATP production
• Reduced tubular apoptosis and necrosis
• Lower levels of kidney tissue damage
• Improved serum creatinine
• Improved creatinine clearance
• Better preservation of renal function[1,5]

Some studies also suggested that early mitochondrial protection may reduce the long-term transition from acute kidney injury to chronic kidney disease, glomerulosclerosis, and renal fibrosis.[1]

Drug- and Toxin-Induced Kidney Injury

SS-31 has also been examined in experimental models of kidney injury caused by nephrotoxic drugs and compounds, including cisplatin, doxorubicin, and aristolochic acid.[1]

Cisplatin is an anticancer medication whose clinical use can be limited by kidney toxicity. Cisplatin may accumulate in renal tubular cells and contribute to mitochondrial dysfunction, oxidative stress, inflammation, and apoptosis.

In preclinical studies, SS-31 reduced several markers of mitochondrial and kidney injury. Reported effects included:

• Reduced reactive oxygen species
• Reduced tubular apoptosis
• Improved mitochondrial function
• Better preservation of kidney structure
• Improved renal-function markers[1]

However, SS-31 did not produce identical effects in every experimental model. In severe or prolonged kidney injury, it did not consistently suppress every inflammatory marker or fully prevent tissue damage.

Its protective effects may therefore depend on the cause, severity, duration, and timing of the kidney injury.

Sepsis-Associated Acute Kidney Injury

Sepsis is a severe systemic response to infection that can cause circulatory abnormalities, inflammation, metabolic dysfunction, and organ failure.

Sepsis-associated kidney injury is not caused only by reduced blood flow. It may also involve mitochondrial dysfunction, impaired cellular energy production, inflammatory signaling, and oxidative stress.

In animal models of sepsis, SS-31 was associated with:

• Partial restoration of renal ATP levels
• Reduced mitochondrial damage
• Reduced tubular-cell apoptosis
• Lower levels of tissue injury
• Improved serum creatinine
• Improved blood urea nitrogen levels[1]

These findings suggest that kidney dysfunction during sepsis may be partly related to impaired mitochondrial energy production and that protecting mitochondrial function could improve cellular recovery.

Diabetic Kidney Disease

Diabetic kidney disease is a major complication of chronic diabetes. Prolonged exposure to high glucose levels can damage glomerular cells, tubular cells, blood vessels, and mitochondrial systems within the kidneys.

Mitochondrial abnormalities associated with diabetic kidney disease may include:

• Excessive reactive oxygen species
• Reduced ATP production
• Mitochondrial membrane damage
• Abnormal mitochondrial fragmentation
• Activation of inflammatory pathways
• Increased apoptosis
• Progressive fibrosis

In experimental diabetic models, SS-31 reduced mitochondrial oxidative stress and helped preserve mitochondrial structure and function.[1,6]

Reported effects included:

• Reduced mitochondrial reactive oxygen species
• Reduced mitochondrial fragmentation
• Improved mitochondrial membrane potential
• Reduced tubular-cell apoptosis
• Reduced renal inflammation
• Lower expression of fibrotic proteins
• Reduced glomerular injury
• Reduced tubulointerstitial damage[1,6]

SS-31 may also influence proteins involved in mitochondrial fission, including dynamin-related protein 1, or Drp1. Excessive Drp1 activity can contribute to abnormal mitochondrial fragmentation and cellular injury.[1,6]

Although these results are promising, most of the evidence comes from cultured cells and animal studies. Clinical effectiveness in patients with diabetic kidney disease has not been established.

Renal Artery Stenosis

Renal artery stenosis restricts blood flow to the kidney. Chronic reductions in renal blood flow can cause tissue hypoxia, mitochondrial dysfunction, inflammation, oxidative stress, microvascular loss, and fibrosis.

Restoring blood flow through revascularization does not always fully restore kidney function. Reperfusion itself may also create additional oxidative stress.

In experimental models of atherosclerotic renal artery stenosis, SS-31 administered around the time of revascularization was associated with:

• Reduced mitochondrial damage
• Reduced oxidative stress
• Lower inflammatory activity
• Reduced apoptosis
• Reduced renal fibrosis
• Better preservation of small renal blood vessels
• Improved tubular structure
• Improved glomerular filtration[1,7]

These findings suggest that mitochondrial injury may continue even after renal blood flow is restored and that mitochondrial protection may improve recovery following revascularization.

Chronic Kidney Disease and Renal Fibrosis

Chronic kidney disease involves the progressive loss of kidney structure and function. Regardless of the original cause, persistent kidney damage frequently leads to fibrosis.

Renal fibrosis involves the accumulation of extracellular matrix components, including collagen and fibronectin. This gradually replaces healthy kidney tissue with scar tissue.

Mitochondrial dysfunction may contribute to fibrosis by promoting oxidative stress, inflammation, tubular-cell injury, metabolic dysfunction, and the activation of profibrotic pathways.

SS-31 has been associated with reduced expression of several fibrosis-related markers, including:

• Transforming growth factor beta
• Fibronectin
• Collagen
• Alpha-smooth muscle actin[1]

By maintaining mitochondrial function and reducing oxidative stress, SS-31 may interrupt some of the cellular signals that promote scar-tissue formation.

However, its antifibrotic effects remain primarily supported by preclinical evidence.

Effects on Apoptosis

Apoptosis is a regulated form of cell death. Mitochondrial damage can activate apoptosis by increasing membrane permeability and releasing cytochrome c into the cytoplasm.

Cytochrome c release can activate caspases, which are enzymes responsible for carrying out the apoptotic process.

SS-31 may regulate several apoptosis-related proteins, including:

• Bax
• Bcl-2
• Cytochrome c
• Caspase-3
• Caspase-9[1]

Bax generally promotes mitochondrial membrane permeabilization and apoptosis, while Bcl-2 helps protect cells from apoptosis.

Experimental studies suggest that SS-31 may reduce Bax activity, preserve Bcl-2 expression, limit cytochrome c release, and reduce caspase activation.[1]

These effects may help protect renal tubular cells following ischemic, toxic, diabetic, or inflammatory injury.

Effects on Inflammation

Kidney injury can activate inflammatory pathways that recruit immune cells and increase the production of inflammatory cytokines.

SS-31 has been associated with reduced activity in pathways involving:

• Nuclear factor kappa B
• Interleukin-1 beta
• Interleukin-6
• Interleukin-18
• Tumor necrosis factor alpha
• The NLRP3 inflammasome[1]

Mitochondrial reactive oxygen species can activate inflammatory signaling. By reducing mitochondrial oxidative stress, SS-31 may indirectly reduce the inflammatory response.

The anti-inflammatory effects have not been identical in every study, indicating that they may vary according to the experimental model and severity of the injury.

Mitochondrial Dynamics

Mitochondria are dynamic structures that continuously divide and fuse.

Mitochondrial fission separates mitochondria into smaller units, while fusion combines mitochondrial structures. Both processes are necessary for normal mitochondrial maintenance.

However, excessive mitochondrial fission can produce fragmented and dysfunctional mitochondria.

SS-31 may help restore the balance between mitochondrial fission and fusion by influencing proteins such as:

• Drp1
• Mitofusin 1
• Mitofusin 2
• Optic atrophy protein 1[1]

Reducing excessive fission may help preserve mitochondrial networks and protect kidney cells from apoptosis and energetic failure.

Autophagy and Mitophagy

Autophagy is the process through which cells remove damaged proteins and cellular components. Mitophagy is a specialized form of autophagy that removes damaged mitochondria.

Effective mitophagy helps prevent dysfunctional mitochondria from accumulating and producing excessive reactive oxygen species.

The reviewed evidence suggests that SS-31 may influence autophagy and mitophagy pathways, potentially improving mitochondrial quality control.[1]

However, the relationship between SS-31, autophagy, and mitophagy remains complex. Additional research is needed to determine whether SS-31 directly activates these processes or improves them indirectly by reducing mitochondrial damage.

Pharmacokinetics and Safety

SS-31 has been examined in early human studies for several mitochondrial and cardiovascular conditions.

Available studies cited in the review generally described it as well tolerated, although the amount of human safety data remained limited.[1]

A significant proportion of SS-31 appears to be eliminated through the kidneys. This may help explain its exposure within renal tissue, but it also means that impaired kidney function could influence its pharmacokinetics.

Important factors requiring further investigation include:

• Absorption
• Tissue distribution
• Renal elimination
• Effective concentration
• Treatment duration
• Long-term safety
• Effects of impaired kidney function
• Potential interactions with other treatments

The available evidence does not establish the long-term safety or clinical effectiveness of SS-31 as a treatment for kidney disease.

Limitations of the Evidence

The article is a scientific review rather than a new randomized clinical trial. It summarizes findings from multiple laboratory and experimental studies.[1]

Most of the evidence comes from:

• Cultured kidney cells
• Mouse models
• Rat models
• A limited number of larger-animal models

Animal kidney injuries are generally more controlled and less complex than human kidney diseases.

Human kidney disease may develop over many years and often occurs alongside diabetes, hypertension, cardiovascular disease, obesity, infections, medication use, and other medical conditions.

The reviewed studies also differed in their:

• SS-31 doses
• Administration routes
• Treatment schedules
• Treatment durations
• Animal species
• Kidney-injury models
• Methods used to measure outcomes

These differences make it difficult to determine an optimal protocol or predict how the findings would translate to humans.

Positive preclinical results do not guarantee clinical effectiveness. Large, controlled human studies are required before conclusions can be made regarding SS-31 as a kidney-disease therapy.

Conclusion

SS-31 is a mitochondria-targeting peptide investigated for its ability to protect mitochondrial structure and function.

Experimental evidence suggests that SS-31 may:

• Stabilize the inner mitochondrial membrane
• Bind to and protect cardiolipin
• Improve mitochondrial electron transport
• Preserve ATP production
• Reduce excessive reactive oxygen species
• Limit cytochrome c release
• Reduce apoptosis
• Reduce inflammation
• Preserve renal microvasculature
• Limit kidney fibrosis

Promising effects have been reported in experimental models of ischemia-reperfusion injury, drug-induced kidney injury, sepsis-associated kidney injury, diabetic kidney disease, chronic kidney disease, and renal artery stenosis.[1]

However, the evidence remains predominantly preclinical. SS-31 should therefore be considered a promising research compound rather than an established clinical treatment for kidney disease.

References

1. Zhu Y, Luo M, Bai X, Li J, Nie P, Li B, Luo P. SS-31, a Mitochondria-Targeting Peptide, Ameliorates Kidney Disease. Oxidative Medicine and Cellular Longevity. 2022;2022:1295509. doi:10.1155/2022/1295509.
2. Bhargava P, Schnellmann RG. Mitochondrial Energetics in the Kidney. Nature Reviews Nephrology. 2017;13(10):629–646. doi:10.1038/nrneph.2017.107.
3. Szeto HH. First-in-Class Cardiolipin-Protective Compound as a Therapeutic Agent to Restore Mitochondrial Bioenergetics. British Journal of Pharmacology. 2014;171(8):2029–2050. doi:10.1111/bph.12461.
4. Birk AV, Liu S, Soong Y, Mills W, Singh P, Warren JD, Seshan SV, Pardee JD, Szeto HH. The Mitochondrial-Targeted Compound SS-31 Re-Energizes Ischemic Mitochondria by Interacting with Cardiolipin. Journal of the American Society of Nephrology. 2013;24(8):1250–1261. doi:10.1681/ASN.2012121216.
5. Szeto HH, Liu S, Soong Y, Wu D, Darrah SF, Cheng FY, Zhao Z, Ganger M, Tow CY, Seshan SV. Mitochondria-Targeted Peptide Accelerates ATP Recovery and Reduces Ischemic Kidney Injury. Journal of the American Society of Nephrology. 2011;22(6):1041–1052. doi:10.1681/ASN.2010080808.
6. Yang SK, Li AM, Han YC, Peng CH, Song N, Yang M, Zhan M, Zeng LF, Song PA, Zhang W, et al. Mitochondria-Targeted Peptide SS-31 Attenuates Renal Injury via an Antioxidant Effect in Diabetic Nephropathy. American Journal of Physiology—Renal Physiology. 2016;310(6):F547–F559. doi:10.1152/ajprenal.00574.2014.
7. Eirin A, Ebrahimi B, Zhang X, Zhu XY, Tang H, Crane JA, Lerman A, Textor SC, Lerman LO. Mitochondrial Protection Restores Renal Function in Swine Atherosclerotic Renovascular Disease. Cardiovascular Research. 2014;103(4):461–472. doi:10.1093/cvr/cvu157.