For decades, mitochondrial DNA was thought to encode just 13 proteins, all components of the respiratory chain, along with the RNAs needed to make them. That picture changed with the discovery of short open reading frames hidden inside mitochondrial ribosomal RNA genes that produce small, bioactive peptides. These mitochondrial-derived peptides (MDPs) behave less like structural parts and more like hormones, communicating metabolic state from mitochondria to the rest of the cell and, possibly, the body. This article maps the family and the key studies, with MOTS-c as the main case. For a compound-specific summary, see our MOTS-c research overview.
How a Mitochondrial Gene Makes a Peptide
Standard gene annotation tends to ignore open reading frames shorter than about 100 codons, which is one reason these peptides were missed. The MDPs are encoded within the 16S and 12S rRNA genes of mitochondrial DNA, and the resulting peptides appear to be translated in the cytosol from mitochondrial transcripts or by mitochondrial machinery, depending on the peptide. The relevant 2013 review by Lee, Yen and Cohen (Trends Endocrinol Metab 2013;24:222-228) framed humanin as a possible "harbinger" of a broader class, and subsequent discoveries supported that.
The Family
- Humanin (24 amino acids; 16S rRNA). Identified by Hashimoto and colleagues in 2001 as a factor that protected neurons against cell death induced by Alzheimer's-disease-related genes in a cell-culture screen (Proc Natl Acad Sci USA 2001;98:6336-6341). Humanin is the best-studied MDP for cytoprotection.
- MOTS-c (16 amino acids; 12S rRNA). Described by Lee and colleagues in 2015, who reported that it promoted metabolic homeostasis and reduced diet-induced obesity and insulin resistance in mice (Cell Metab 2015;21:443-454).
- SHLP1 to SHLP6 (small humanin-like peptides; 16S rRNA). Reported by Cobb and colleagues in 2016 as age-dependent regulators of apoptosis, insulin sensitivity and inflammatory markers (Aging 2016;8:796-809).
MOTS-c: The Key Studies
Metabolic phenotype (2015). In the original report, MOTS-c treatment in mice fed a high-fat diet was associated with reduced weight gain and improved insulin sensitivity. The proposed mechanism involved the folate-purine cycle and activation of AMP-activated protein kinase (AMPK), the cell's energy sensor.
Nuclear translocation (2018). Kim and colleagues reported that under metabolic stress, such as glucose restriction, MOTS-c moves into the nucleus in an AMPK-dependent manner, where it interacts with stress-responsive transcription factors and regulates gene expression (Cell Metab 2018;28:516-524). This was notable as evidence of a mitochondrial-encoded peptide acting directly on the nuclear genome, a form of retrograde signaling.
Exercise and age (2021). Reynolds and colleagues reported that MOTS-c levels rose in human skeletal muscle and plasma after exercise, and that treating mice with MOTS-c improved physical performance across age groups, including older animals (Nat Commun 2021;12:470). The same paper discussed MOTS-c in the context of age-related decline in muscle homeostasis.
Genetic association. A variant in the MOTS-c coding region (m.1382A>C) was reported to be enriched in a Japanese cohort associated with exceptional longevity (Fuku et al., Aging Cell 2015;14:921-923). Association findings like this are suggestive only; they do not establish function.
Where the Evidence Is Thin
- Species and models. Most mechanistic and functional data come from cell lines and rodents. Human data are largely correlative: peptide levels measured in plasma or tissue, or genetic associations.
- Measurement. Detecting circulating levels of a 16-amino-acid peptide reliably is difficult, and ELISA performance varies. Mass-spectrometry-based quantification and validated antibodies are important controls.
- Replication across groups. A significant portion of the foundational work originates from a small number of laboratories. Independent replication, especially of the nuclear-translocation and exercise findings, is still developing.
- Dose and delivery. Rodent experiments typically involve injected synthetic peptide at doses that may not match physiological levels, which complicates inference about endogenous function.
- No approved applications. None of the MDPs are approved medicines. Clinical-stage data are minimal.
Why the Field Matters Even if Individual Findings Change
The broader point is conceptual: a genome long thought fully annotated still contains functional coding sequence that conventional gene finders skipped. Whatever the final verdict on individual peptides, ribosome-profiling and proteomic methods are now being applied to look for similar small ORFs in nuclear and mitochondrial genomes.
Sources
- Lee C, et al. The mitochondrial-derived peptide MOTS-c promotes metabolic homeostasis and reduces obesity and insulin resistance. Cell Metab 2015;21:443-454.
- Kim KH, Son JM, Benayoun BA, Lee C. The mitochondrial-encoded peptide MOTS-c translocates to the nucleus to regulate nuclear gene expression in response to metabolic stress. Cell Metab 2018;28:516-524.
- Reynolds JC, et al. MOTS-c is an exercise-induced mitochondrial-encoded regulator of age-dependent physical decline and muscle homeostasis. Nat Commun 2021;12:470. doi:10.1038/s41467-020-20790-0. PMC
- 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;98:6336-6341.
- Cobb LJ, et al. Naturally occurring mitochondrial-derived peptides are age-dependent regulators of apoptosis, insulin sensitivity, and inflammatory markers. Aging (Albany NY) 2016;8:796-809.
- Lee C, Yen K, Cohen P. Humanin: a harbinger of mitochondrial-derived peptides? Trends Endocrinol Metab 2013;24:222-228.
- Fuku N, et al. The mitochondrial-derived peptide MOTS-c: a player in exceptional longevity? Aging Cell 2015;14:921-923.
For research use only. Not for human or veterinary consumption. This article is educational and describes published literature; it makes no claims about the use of any product.