MOTS-c and SS-31: two peptides at the centre of mitochondrial research

Mitochondria as signalling hubs

Mitochondria are best known as the power plants of the cell, but they also carry their own small genome and communicate with the rest of the cell. The discovery that mitochondrial DNA contains short open reading frames, small stretches that can code for peptides, opened a new field of mitochondrial-derived peptides. MOTS-c is one of them. A second line of research has produced synthetic peptides that target mitochondria from the outside, of which SS-31, also known as elamipretide, is the most advanced.

MOTS-c: a peptide from the mitochondrial genome

Lee and colleagues reported in 2015 that a short open reading frame within the mitochondrial 12S ribosomal RNA gene encodes a 16-amino-acid peptide, which they named MOTS-c (mitochondrial open reading frame of the 12S rRNA type-c). Its main target appears to be skeletal muscle. Inside cells, MOTS-c inhibits the folate pathway and the de novo purine synthesis that depends on it, which leads to activation of AMPK, a central energy sensor. In mice, MOTS-c prevented insulin resistance caused by ageing and by a high-fat diet, as well as diet-induced obesity.

MOTS-c, exercise and physical capacity

Reynolds and colleagues extended this work in 2021. MOTS-c improved physical performance in young (2 months), middle-aged (12 months) and old (22 months) mice. It regulated nuclear genes, including genes for metabolism and protein maintenance, influenced skeletal muscle metabolism and helped muscle precursor cells adapt to metabolic stress. Intermittent administration started late in life, at 23.5 months, three times a week, increased physical capacity and healthspan in the mice.

They also found that exercise raises the body's own MOTS-c in human skeletal muscle and in the blood, which places MOTS-c among the signals that link physical activity to metabolism. The authors conclude that ageing is regulated by genes in both the mitochondrial and nuclear genomes.

SS-31: protecting cardiolipin

SS-31 belongs to the Szeto-Schiller peptides, short synthetic peptides with alternating aromatic and basic amino acids (D-Arg-Dmt-Lys-Phe-NH2) that enter cells and concentrate in the inner mitochondrial membrane. Szeto's 2014 review explains its target: cardiolipin, a phospholipid found only in the inner mitochondrial membrane, which shapes the cristae (the folds where energy is produced) and organises the respiratory complexes. When cardiolipin is oxidised or depleted, cytochrome c can switch from carrying electrons to acting as a peroxidase that damages the membrane further.

SS-31 binds cardiolipin through electrostatic and hydrophobic interactions. By doing so it stops cardiolipin from converting cytochrome c into a peroxidase while preserving its electron-carrying role, which protects the cristae and supports oxidative phosphorylation.

SS-31 in a model of ischaemia

Birk and colleagues studied SS-31 in rat kidneys deprived of blood flow, a model of acute kidney injury in which ischaemia destroys the cristae and ATP returns slowly once blood flow resumes. Using a fluorescent analogue, they showed that SS-31 binds cardiolipin with high affinity and that the complex inhibits cytochrome c peroxidase activity. Rats given SS-31 beforehand kept their cristae intact and avoided mitochondrial swelling, ATP returned quickly after blood flow was restored, and cell structure, barrier function and kidney function were better preserved.

Elamipretide in Barth syndrome

Elamipretide's most advanced clinical research is in Barth syndrome, a rare X-linked genetic disorder caused by defects in tafazzin, the enzyme that remodels cardiolipin. As Sabbah reviews, people with Barth syndrome can have up to 95% less mature cardiolipin, leading to heart muscle disease, muscle weakness and fatigue.

The TAZPOWER trial combined a 28-week randomised, placebo-controlled phase with a 168-week open-label extension. Thompson and colleagues reported that ten participants entered the extension and eight reached week 168. Over that time, the distance walked in six minutes improved by a cumulative 96.1 metres, fatigue scores stayed below baseline, several three-dimensional heart volume measures improved, and the ratio of immature to mature cardiolipin improved. Reactions at the administration site were the most common adverse events. These are small numbers in a rare disease, but they are among the few human data for a cardiolipin-targeted peptide.

How the two compare

• Origin: MOTS-c is encoded in mitochondrial DNA; SS-31 is fully synthetic.
• Where they act: MOTS-c signals to the nucleus and to skeletal muscle metabolism through AMPK; SS-31 binds cardiolipin in the inner mitochondrial membrane.
• Main research themes: MOTS-c in metabolism, exercise and ageing in mice; SS-31 in ischaemia, mitochondrial disease and Barth syndrome.

Key terms

• Open reading frame: a stretch of DNA that can be read into a protein or peptide.
• 12S rRNA: a ribosomal RNA encoded by mitochondrial DNA, within which the MOTS-c sequence lies.
• AMPK: AMP-activated protein kinase, a central sensor of cellular energy.
• Cardiolipin: a phospholipid found only in the inner mitochondrial membrane.
• Cristae: the folds of the inner mitochondrial membrane where most ATP is made.
• Healthspan: the part of life spent in good health, as opposed to total lifespan.

Open questions

For MOTS-c, the key questions are how a peptide made in mitochondria acts on the nucleus and on distant tissues, and how far the mouse findings apply to people. For SS-31, research is extending from rare mitochondrial diseases to conditions where cardiolipin damage may play a part, such as ischaemia and the age-related decline of muscle and heart function.

Notes for laboratory work

Both peptides are supplied lyophilized and should be stored frozen, dry and dark. MOTS-c contains methionine and is more prone to oxidation, so prepare its solutions shortly before use. In cell experiments with SS-31, mitochondrial readouts such as membrane potential, oxygen consumption or ATP levels are the usual endpoints; for MOTS-c, AMPK activation and glucose handling in muscle cells are common starting points. We offer MOTS-c and SS-31 with current batch reports. For the cofactor at the heart of mitochondrial energy production, see our NAD+ research overview and NAD+.

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