Mitochondrial peptides — an overview of the emerging mitokine class
Mitochondrial-derived peptides (MDPs) are a recently characterised class of signalling molecules encoded in the mitochondrial genome that act as systemic stress signals. This overview covers the known MDPs — humanin, MOTS-c, SHLP2 — their receptor targets, and what the mitokine concept means for understanding mitochondria as endocrine organs.
The classical view of mitochondria as passive energy generators has been revised substantially over the past two decades. Mitochondria are now understood as dynamic signalling organelles that communicate their metabolic state to the rest of the cell and, through the release of circulating factors called mitokines, to distant tissues. The discovery that the mitochondrial genome encodes functional peptides — in addition to its 13 canonical proteins — established the mitochondrial-derived peptide (MDP) as a new molecular category with biological functions extending far beyond the mitochondrion itself.
The MDP class currently comprises three well-characterised members: humanin, MOTS-c, and the small humanin-like peptides (SHLPs 1–6). Each is encoded within the 16S ribosomal RNA gene of mitochondrial DNA (with the exception of MOTS-c, which is encoded in the 12S rRNA gene), and each appears to serve a distinct signalling role in the cellular stress response.
Humanin — the first mitochondrial peptide
Humanin was discovered in 2001 by Nishimoto and colleagues during a screen for factors that protect neurons from Alzheimer's disease-associated amyloid-beta toxicity. The 21-amino acid peptide was isolated from a cDNA library derived from surviving neurons in an Alzheimer's patient brain, and its sequence was subsequently mapped to the 16S rRNA gene of the mitochondrial genome.
The cytoprotective activity of humanin is mediated through two identified receptor systems. Extracellularly, humanin binds to a trimeric receptor complex involving gp130, IL-27Rα, and CNTFR — the same receptor used by ciliary neurotrophic factor (CNTF) and related cytokines. This binding activates JAK/STAT3 signalling, driving expression of anti-apoptotic genes and suppressing inflammatory cytokine production. Intracellularly, humanin interacts with IGFBP3 to modulate insulin-like growth factor signalling, and with Bax to directly inhibit the mitochondrial apoptosis pathway.
Humanin levels decline with age — circulating humanin is measurable in human blood and falls by approximately 50% between young adulthood and old age. This decline correlates with reduced stress tolerance and increased susceptibility to apoptosis in aged tissues, suggesting that humanin acts as a tonic cytoprotective signal whose loss contributes to age-related vulnerability.
MOTS-c — the metabolic MDP
MOTS-c (16 amino acids, encoded in the 12S rRNA gene) functions as a metabolic stress sensor, activating AMPK and translocating to the nucleus under glucose restriction and exercise. Its mechanism is covered in detail in the dedicated article on MOTS-c; within the MDP framework, it occupies the metabolic regulation niche — where humanin is primarily cytoprotective and anti-apoptotic, MOTS-c is primarily metabolic and bioenergetic.
The distinction matters for research protocol design. Humanin addresses the survival and repair dimension of mitochondrial signalling. MOTS-c addresses the metabolic adaptation dimension. Both decline with age, and both appear to contribute to the broader phenomenon of reduced stress resilience and metabolic flexibility that characterises ageing organisms.
SHLPs — the humanin relatives
Small humanin-like peptides (SHLPs 1–6) are six additional peptides encoded in the same 16S rRNA gene region as humanin. They were identified by bioinformatic analysis of the 16S rRNA gene sequence for additional open reading frames. SHLP2 is the best characterised: it shares humanin's neuroprotective activity, reduces mitochondrial ROS production, and protects against apoptosis in similar cell types.
SHLP2 is of particular research interest because it appears to have cardioprotective effects in ischaemia-reperfusion models similar to those of humanin, but through mechanisms that are partially distinct — suggesting functional redundancy in the MDP class that may provide overlapping protection against different stress modalities.
The other SHLPs (1, 3–6) have less characterised activities, and the full scope of the SHLP family's biological roles is an active research area.
Evolutionary context — why mitochondria encode signalling peptides
The evolutionary explanation for MDPs connects to the endosymbiotic origin of mitochondria. Mitochondria are derived from ancient alpha-proteobacteria, and their genome retains bacterial-style compact organisation. The discovery that regulatory peptides are encoded within what were thought to be purely structural RNA genes suggests that the mitochondrial genome is more functionally complex than its minimal protein-coding gene count implies.
From a systems biology perspective, MDPs make sense as a stress communication system: the organelle that first experiences metabolic stress (through reduced substrate availability or ETC dysfunction) releases signals that prepare the whole cell — and through circulation, the whole organism — for the adaptive response that stress requires.
Research-grade mitochondrial peptides including humanin, MOTS-c, and SHLP2 for preclinical investigation are available through ozpeps.is with verified purity documentation for Australian research applications.
The mitokine concept is one of the more consequential recent developments in mitochondrial biology — not just because MDPs have therapeutic potential, but because they require a fundamental revision of how mitochondria are understood to participate in systemic physiology. The organelle that generates the cell's energy also monitors its stress state and broadcasts that information to the rest of the body.