Mitochondrial-Derived Peptides: MOTS-c, Humanin, and Cellular Energy Research

The Mitochondrial Genome: A Source of Signaling Peptides

For decades, mitochondria were understood primarily as cellular power plants — organelles that generate ATP through oxidative phosphorylation. The mitochondrial genome (mtDNA) was thought to encode only 13 proteins (all components of the electron transport chain), 22 transfer RNAs, and 2 ribosomal RNAs.

This view changed dramatically with the discovery of mitochondrial-derived peptides (MDPs) — small bioactive peptides encoded within previously overlooked open reading frames (ORFs) in the mitochondrial genome. These peptides are now recognized as retrograde signaling molecules that communicate mitochondrial status to the rest of the cell and to distant tissues.

The discovery of MDPs has fundamentally expanded our understanding of mitochondrial biology, revealing that mitochondria are not merely energy producers but active signaling organelles that regulate metabolism, stress responses, and aging.

The Three Families of Mitochondrial-Derived Peptides

Humanin

Humanin was the first MDP discovered, identified in 2001 by Nishimoto and colleagues during a screen for genes that protect against Alzheimer’s disease-associated neurotoxicity. It is a 24-amino-acid peptide encoded in the 16S ribosomal RNA gene of mtDNA.

Key functions:

• Neuroprotection: Protects neurons against amyloid-beta toxicity, oxidative stress, and serum deprivation-induced apoptosis
• Anti-apoptotic: Binds to BAX (a pro-apoptotic protein) and prevents its translocation to the mitochondrial membrane, blocking the intrinsic apoptosis pathway
• Insulin sensitization: Enhances insulin signaling and glucose disposal in peripheral tissues
• IGFBP-3 binding: Interacts with IGFBP-3 (insulin-like growth factor binding protein 3), modulating IGF-1 signaling
• Cytoprotection: Protects against ischemia-reperfusion injury in cardiac and cerebral models

Clinical relevance: Circulating Humanin levels decline with age and are lower in patients with Alzheimer’s disease, type 2 diabetes, and cardiovascular disease. Higher Humanin levels are associated with improved cognitive function in elderly cohorts.

MOTS-c (Mitochondrial Open Reading Frame of the Twelve S rRNA Type-c)

MOTS-c was discovered in 2015 by Pinchas Cohen’s laboratory at USC. It is a 16-amino-acid peptide encoded in the 12S rRNA gene of mtDNA. MOTS-c is unique among known peptides in that it translocates to the nucleus under stress conditions and directly regulates nuclear gene expression — making it a true retrograde (mitochondria-to-nucleus) signaling molecule.

Key functions:

• AMPK activation: MOTS-c activates AMP-activated protein kinase, the master metabolic sensor, through the folate-AICAR pathway
• Glucose regulation: Enhances glucose uptake in skeletal muscle, improves insulin sensitivity, and prevents diet-induced obesity in animal models
• Exercise mimetic: MOTS-c is released by skeletal muscle during exercise and circulates as a myokine — it partially mediates the metabolic benefits of physical activity
• Nuclear translocation: Under metabolic stress, MOTS-c enters the nucleus and interacts with transcription factors (notably NRF2, the master antioxidant regulator) to modulate gene expression
• Aging: Endogenous MOTS-c levels decline with age, and exogenous MOTS-c administration improves physical performance and metabolic parameters in aged mice

SHLP Peptides (Small Humanin-Like Peptides)

SHLPs 1-6 were discovered in 2016, encoded in the same 16S rRNA gene as Humanin but from different ORFs. These six peptides (14-38 amino acids) have distinct biological activities:

• SHLP-2: The most studied — promotes cell survival, enhances mitochondrial metabolism, and has insulin-sensitizing effects
• SHLP-3: Shows anti-apoptotic and neuroprotective properties similar to Humanin
• SHLP-6: Uniquely pro-apoptotic (promotes cell death) — the opposite of other MDPs, suggesting a role in tumor suppression

The SHLP family demonstrates that the mitochondrial genome encodes a diverse repertoire of signaling peptides with both complementary and opposing functions.

The AMPK Connection: How MOTS-c Regulates Metabolism

MOTS-c’s metabolic effects are primarily mediated through AMPK (AMP-activated protein kinase), often called the cell’s “fuel gauge.” AMPK is activated when the cellular AMP:ATP ratio rises (indicating energy deficit) and triggers adaptive responses:

• Increases glucose uptake (via GLUT4 translocation)
• Enhances fatty acid oxidation
• Inhibits lipogenesis (fat synthesis)
• Promotes mitochondrial biogenesis
• Activates autophagy (cellular recycling)

MOTS-c activates AMPK through a novel mechanism involving the folate-methionine cycle:

1. MOTS-c inhibits the folate cycle, reducing the pool of purines available for de novo nucleotide synthesis
2. This activates AICAR (5-aminoimidazole-4-carboxamide ribonucleotide), an endogenous AMPK activator
3. AICAR activates AMPK, triggering the downstream metabolic cascade

This pathway explains why MOTS-c has been described as an “exercise mimetic” — exercise also activates AMPK, and MOTS-c engages the same downstream signaling network.

MDPs and the Biology of Aging

The decline of MDP levels with age parallels other hallmarks of aging and may contribute to age-related metabolic decline:

MDP	Age-Related Change	Associated Conditions
Humanin	Declines with age	Alzheimer’s, cardiovascular disease, diabetes
MOTS-c	Declines with age	Metabolic syndrome, sarcopenia, physical decline
SHLP-2	Declines with age	Prostate cancer, metabolic dysfunction

This pattern has led to the “mitochondrial peptide hypothesis of aging,” which proposes that age-related decline in MDP production is both a consequence and a driver of metabolic aging. As mitochondria accumulate damage with age, their capacity to produce MDPs decreases, reducing the metabolic and cytoprotective signaling these peptides provide — creating a negative feedback loop.

Exogenous MDP supplementation (particularly MOTS-c) in aged animal models has shown:

• Improved insulin sensitivity and glucose tolerance
• Enhanced physical performance and endurance
• Reduced adiposity and improved body composition
• Protection against age-related metabolic decline

Mitochondrial Retrograde Signaling

MDPs are part of a broader phenomenon called mitochondrial retrograde signaling — communication from mitochondria to the nucleus about the metabolic state of the cell. This concept overturns the traditional view of mitochondria as passive organelles that merely follow nuclear instructions.

MOTS-c’s ability to physically translocate to the nucleus under stress is the most dramatic example. Once in the nucleus, MOTS-c interacts with:

• ARE/EpRE elements: Antioxidant response elements in gene promoters
• NRF2: The master transcription factor for antioxidant gene expression
• Stress-responsive transcription factors: Facilitating adaptive gene expression changes

This nuclear translocation happens specifically under metabolic stress (glucose deprivation, oxidative stress), making MOTS-c a conditional signal — it communicates that the mitochondria are under duress and that the nucleus should activate protective gene programs.

NAD+ and Mitochondrial Function

While not itself a mitochondrial-derived peptide, NAD+ (nicotinamide adenine dinucleotide) is intimately linked to mitochondrial peptide biology:

• NAD+ is essential for mitochondrial oxidative phosphorylation (as a cofactor for Complex I)
• NAD+ decline with age impairs mitochondrial function, potentially reducing MDP production
• NAD+ activates sirtuins (SIRT1, SIRT3), which regulate mitochondrial biogenesis and function
• SIRT3 (a mitochondrial sirtuin) deacetylates and activates mitochondrial enzymes, maintaining the metabolic capacity needed for MDP synthesis

This creates a potential synergy between NAD+ supplementation and MDP research — restoring NAD+ levels may support endogenous MDP production while exogenous MDPs supplement the signaling that declining mitochondria can no longer provide.

Frequently Asked Questions

What is the difference between MOTS-c and other metabolic peptides?

MOTS-c is unique in being encoded by the mitochondrial genome (not nuclear DNA) and in its ability to physically enter the nucleus to regulate gene expression. Other metabolic peptides like GLP-1 agonists work through cell-surface receptor signaling. MOTS-c’s mechanism (folate cycle → AICAR → AMPK) is also distinct from other peptide pathways.

Can MOTS-c replace exercise?

MOTS-c activates some of the same pathways as exercise (particularly AMPK), and is itself released during exercise. However, exercise produces a much broader range of physiological adaptations (cardiovascular, musculoskeletal, neurological, psychological) that no single compound can replicate. MOTS-c is more accurately described as capturing one component of the exercise response.

Why are MDPs encoded in mitochondrial DNA rather than nuclear DNA?

This is an active area of evolutionary biology research. One hypothesis is that encoding signaling peptides in mtDNA allows them to serve as direct reporters of mitochondrial status — their production is directly linked to mitochondrial health and gene expression capacity, making them honest signals of the organelle’s functional state.

How does MOTS-c relate to NAD+ supplementation?

MOTS-c and NAD+ target overlapping but distinct aspects of mitochondrial metabolism. NAD+ supports the electron transport chain and sirtuin activity. MOTS-c activates AMPK and regulates nuclear gene expression. Together, they address both the energy production (NAD+) and metabolic signaling (MOTS-c) aspects of mitochondrial function.

Are Humanin levels a biomarker for Alzheimer’s disease?

Circulating Humanin levels are lower in Alzheimer’s patients compared to age-matched controls, and lower levels correlate with greater cognitive decline. However, Humanin is not yet validated as a diagnostic biomarker — it is being studied as one component of a multi-biomarker panel for neurodegenerative disease risk assessment.

References

1. Hashimoto Y, et al. “A rescue factor abolishing neuronal cell death by a wide spectrum of familial Alzheimer’s disease genes and Abeta.” Proc Natl Acad Sci USA. 2001;98(11):6336-6341.
2. Lee C, et al. “The mitochondrial-derived peptide MOTS-c promotes metabolic homeostasis and reduces obesity and insulin resistance.” Cell Metab. 2015;21(3):443-454.
3. Kim KH, et al. “Mitochondrial peptides modulate mitochondrial function during cellular senescence.” Aging. 2018;10(6):1239-1256.
4. Reynolds JC, et al. “MOTS-c is an exercise-induced mitochondrial-encoded regulator of age-dependent physical decline and muscle homeostasis.” Nat Commun. 2021;12(1):470.
5. Cobb LJ, et al. “Naturally occurring mitochondrial-derived peptides are age-dependent regulators of apoptosis, insulin sensitivity, and inflammatory markers.” Aging. 2016;8(4):796-809.
6. Kim SJ, et al. “The mitochondrial-derived peptide MOTS-c translocates to the nucleus to regulate nuclear gene expression during metabolic stress.” Cell Metab. 2018;28(3):516-524.
7. Yen K, et al. “The mitochondrial derived peptide humanin is a regulator of lifespan and healthspan.” Aging. 2020;12(12):11185-11199.
8. Zempo H, et al. “Relation between circulating MOTS-c levels and physical fitness in healthy middle-aged adults.” Aging. 2022.