Mitochondrial health: SS-31, MOTS-c and NAD+ in the research on cellular energy

Three compounds studied in mitochondrial health research reach the same organelle at three separate points: SS-31, a tetrapeptide that binds a phospholipid on the inner mitochondrial membrane; MOTS-c, a short peptide encoded within mitochondrial DNA itself; and NAD+, the coenzyme whose reduced form, NADH, donates electrons into the respiratory chain at Complex I ^[2], ^[5], ^[8], ^[12], ^[17].

That chain sits in the inner mitochondrial membrane. NADH and FADH2, both generated by the TCA cycle, donate electrons into it at Complex I or Complex II; from Complex I, those electrons pass through a chain of cofactors to ubiquinone, and that transfer pumps protons out of the mitochondrial matrix into the intermembrane space. The resulting protonmotive force couples electron transport to Complex V, ATP synthase, which turns under a rotational mechanism to make ATP ^[17].

Although mitochondria are known as the cell's powerhouse, a 2026 review describes their role as reaching well beyond energy generation: they regulate cellular metabolism while maintaining a tightly controlled level of reactive oxygen species and redox state, and they mediate physiological and pathological processes including calcium balance, apoptosis and mitophagy ^[1]. The same review describes mitochondrial dysfunction as arising through several routes, including genetic mutations, increased production of reactive oxygen species, metabolic failure from impaired electron transport chain activity, and dysregulated mitochondrial dynamics or mitophagy, with the resulting damage implicated in ageing, metabolic syndrome, cancer, neurodegeneration and reproductive disorders ^[1].

NAD+ (nicotinamide adenine dinucleotide) is a coenzyme for redox reactions, which makes it central to energy metabolism, and it is also an essential cofactor for non-redox enzymes including sirtuins, CD38 and poly(ADP-ribose) polymerases ^[12]. A 2021 review reports that ageing is accompanied by a gradual decline in tissue and cellular NAD+ levels across multiple model organisms, including rodents and humans, a decline linked causally to numerous ageing-associated diseases such as cognitive decline, cancer, metabolic disease, sarcopenia and frailty ^[12].

Cardiolipin is an anionic phospholipid on the inner mitochondrial membrane, and it is required for cristae, the membrane's inward folds, to form; those folds are in turn required for mitochondrial ATP synthesis ^[2]. When blood flow is cut off and then restored, as in kidney injury from ischaemia, the cristae membranes are destroyed, and it is that damage, one study reports, which delays how quickly ATP recovers ^[2].

SS-31, also known by its clinical name elamipretide, is described by one trial as a mitochondrial tetrapeptide that interacts with cardiolipin ^[5]. Using a polarity-sensitive fluorescent version of the compound, researchers demonstrated that SS-31 binds with high affinity to cardiolipin on the inner mitochondrial membrane ^[2].

In rats, pretreatment with SS-31 protected cristae membranes during renal ischaemia and prevented mitochondrial swelling; the SS-31/cardiolipin complex also inhibited cytochrome c peroxidase activity, the reaction that catalyses cardiolipin peroxidation and mitochondrial damage during ischaemia, by protecting the enzyme's heme iron ^[2]. Prompt ATP recovery on reperfusion led to rapid repair of ATP-dependent processes such as the actin cytoskeleton and cell polarity, inhibited apoptosis, protected tubular barrier function and mitigated renal dysfunction ^[2].

A single treatment with SS-31 was reported to restore in vivo mitochondrial energetics to young levels in aged mice within one hour. Young (5-month-old) and old (27-month-old) mice were injected intraperitoneally with either saline or 3 mg per kg of SS-31, and skeletal muscle mitochondrial energetics were measured in vivo using a combination of optical and 31P magnetic resonance spectroscopy ^[3]. Age-related declines in resting and maximal mitochondrial ATP production, in the coupling of oxidative phosphorylation, and in cell energy state were rapidly reversed after treatment, while SS-31 had no observable effect on young muscle; aged muscle was also more fatigue resistant one hour after treatment, and eight days of treatment increased whole-animal endurance capacity ^[3].

The same compound was then tested in a randomised, double-blind, placebo-controlled trial in people. Non-invasive magnetic resonance and optical spectroscopy measured mitochondrial capacity (ATPmax) with exercise and mitochondrial coupling at rest in the first dorsal interosseous muscle of 39 healthy older adults aged 60 to 85, enrolled for having poorly functioning mitochondria ^[4]. A single dose elevated ATPmax relative to placebo immediately after a 2-hour infusion, but the difference was gone by day 7, consistent with the compound's half-life in human blood, and there was no significant effect on fatigue resistance despite the rise in ATPmax ^[4].

In Barth syndrome, a genetic disorder of mitochondrial cardiolipin metabolism, a randomised, double-blind, placebo-controlled crossover trial gave 12 subjects 40 mg per day of elamipretide or placebo for 12 weeks, with a 4-week washout before crossing over; neither of the trial's two primary endpoints, the 6-minute walk test and a Barth syndrome symptom assessment, was met in that first part ^[5]. Ten of the twelve continued into an open-label extension of the same 40 mg daily dose, with eight reaching 36 weeks, and at that point the group showed a 95.9-metre improvement on the 6-minute walk test and a 2.1-point improvement on the symptom assessment, alongside gains in secondary endpoints including knee extensor strength, patient global impression of symptoms and some cardiac parameters ^[5].

The largest trial of elamipretide to date, MMPOWER-3, randomised 218 adults with genetically confirmed primary mitochondrial myopathy, 109 to elamipretide and 109 to placebo, to 40 mg per day subcutaneously or placebo for 24 weeks ^[6]. The trial did not meet either primary endpoint: the difference in 6-minute walk distance was -3.2 metres (p=0.69) and the difference in the trial's fatigue score was -0.07 points (p=0.37), though elamipretide was reported as well tolerated, with most adverse events mild to moderate ^[6].

In September 2025, elamipretide was granted accelerated approval in the United States, to improve muscle strength in adult and paediatric patients with Barth syndrome weighing 30 kg or more, becoming the first disease-specific treatment approved for that ultra-rare, X-linked recessive genetic disorder ^[7]. The same review describes elamipretide as also under Phase III clinical development for dry age-related macular degeneration and for mitochondrial myopathies ^[7].

MOTS-c is a 16-amino-acid peptide encoded not in the cell's main genome but within a short open reading frame inside the mitochondrial 12S rRNA gene, part of mitochondrial DNA itself, first reported in 2015 ^[8]. Its discovery paper describes it as regulating insulin sensitivity and metabolic homeostasis, with skeletal muscle as its primary target organ ^[8].

The same paper describes MOTS-c's cellular action as inhibiting the folate cycle and its tethered de novo purine biosynthesis, a route that leads to activation of AMPK, the cell's central energy-sensing enzyme ^[8]. In mice, MOTS-c treatment was reported to prevent age-dependent and high-fat-diet-induced insulin resistance, as well as diet-induced obesity ^[8].

A 2021 study extended this into mice of three ages, 2 months, 12 months and 22 months, and reported that MOTS-c significantly enhanced physical performance in all three groups; treatment begun late in life, at 23.5 months, and given intermittently three times a week, still increased physical capacity ^[9]. The same paper reports that MOTS-c regulates nuclear genes related to metabolism and proteostasis, skeletal muscle metabolism, and how myoblasts, the precursor cells of muscle fibres, adapt to metabolic stress ^[9].

The same study also measured MOTS-c directly in people. In sedentary healthy young male volunteers exercising on a stationary bicycle, skeletal muscle and plasma MOTS-c were measured before, during (plasma only) and after exercise, and again after a 4-hour rest ^[9].

Two clinical programmes touching MOTS-c are registered on ClinicalTrials.gov. The first tested CB4211, an analogue of MOTS-c ^[18], in a three-part, randomised, double-blind, placebo-controlled Phase 1a/1b study of single and multiple ascending subcutaneous doses in healthy non-obese subjects and in subjects with nonalcoholic fatty liver disease; its registration lists the study as completed ^[10]. The second is testing MOTS-c itself in a Phase 2a study: adults with prediabetes and overweight or obesity are randomised 1:1 to investigational MOTS-c or placebo for 12 weeks of treatment alongside standardised lifestyle counselling, to evaluate whether it improves insulin sensitivity compared with placebo, with safety followed through week 16; its registration lists the study as recruiting ^[11].

NAD+ (nicotinamide adenine dinucleotide) is the coenzyme for the redox reactions of energy metabolism, and it is also a cofactor for non-redox enzymes including sirtuins, CD38 and poly(ADP-ribose) polymerases, influencing metabolic pathways, DNA repair, chromatin remodelling, cellular senescence and immune cell function ^[12].

A 2021 review reports that ageing brings a gradual decline in tissue and cellular NAD+ levels across multiple model organisms, including rodents and humans, and links that decline causally to numerous ageing-associated diseases ^[12]. None of the four human trials in this guide's evidence gave people NAD+ itself directly; each instead gave a precursor compound the body converts into NAD+.

Read together, the four trials point in different directions: a rise in blood-cell NAD+ ^[13]; in aged men, a metabolite rise without a change in muscle NAD+ or mitochondrial respiration, alongside an anti-inflammatory effect ^[14]; an improvement in muscle insulin sensitivity ^[15]; and, at a higher and longer niacin dose, a rise in muscle strength alongside the largest reported rise in blood NAD+ ^[16].

Set side by side, the three approaches differ in where they touch the mitochondrion, how much has been measured directly in people, and how far any of them has reached an approved use.

Elamipretide's approval, set out in the SS-31 section, is confined to Barth syndrome ^[7]. Beyond that condition the picture is less settled: MMPOWER-3 did not meet either primary endpoint in 218 adults with primary mitochondrial myopathy ^[6], and the review that records the approval still describes the compound's work in mitochondrial myopathies and dry age-related macular degeneration as Phase III development ^[7].

MOTS-c has no completed human efficacy trial in this guide's evidence. CB4211, an analogue of MOTS-c ^[18], completed a Phase 1a/1b safety study, and MOTS-c itself is being tested in a Phase 2a trial for insulin sensitivity that is still recruiting; neither carries a published human result here ^[10], ^[11].

The NAD+ precursor trials in this guide's evidence are small and short by comparison. The two nicotinamide riboside trials enrolled 30 and 12 participants, over six weeks and 21 days respectively, and neither administered NAD+ itself: both gave a precursor and measured what the body did with it ^[13], ^[14].

Where adverse events were reported, most were mild to moderate. In the Barth syndrome trial's first part, 12 of 12 participants on elamipretide and 10 of 12 on placebo had at least one treatment-emergent adverse event, most of them injection site reactions: erythema in 12 of 12 on elamipretide against 3 of 12 on placebo, pain in 9 of 12 against 4 of 12, and induration in 8 of 12 against 2 of 12; most events were mild to moderate, and the one serious event was judged unrelated to elamipretide ^[5]. MMPOWER-3 described elamipretide as well tolerated, with most adverse events mild to moderate ^[6]. The nicotinamide riboside trial in middle-aged and older adults recorded 14 treatment-emergent adverse events in 7 of 30 participants, all mild, including nausea, flushing, leg cramps and increased bruising on nicotinamide riboside, against headache, skin rash, flushing, fainting and drowsiness on placebo ^[13]. In the niacin trial, some patients showed an anaemia tendency ^[16].