The Mitochondrial-Derived Peptide (MDP) Family — MOTS-c, Humanin, SS-31, and the SHLP Series
The discovery that mitochondria actively encode peptides that communicate the mitochondrial metabolic state to the rest of the cell and body represents one of the most significant paradigm shifts in cell biology of the past decade. The mitochondrial-derived peptide (MDP) family now includes multiple known members with distinct mechanisms, declining levels with age, and complementary protective functions across longevity research.
The discovery of mitochondria-encoded peptides
The human mitochondrial genome is 16,569 base pairs and was long thought to encode only 13 proteins (all electron transport chain components), 22 tRNAs, and 2 rRNAs. The discovery of Humanin in 2001 in surviving neurons from an Alzheimer's disease brain, followed by the identification of MOTS-c in 2015 and the SHLP family in 2016, revealed that the small mitochondrial genome contains additional open reading frames encoding short peptides whose existence had not been anticipated. This established a new conceptual framework: mitochondria are not simply energy factories but active signaling organelles that communicate their metabolic state through peptide hormones they encode and secrete.
All known MDPs are encoded within the 12S rRNA and 16S rRNA regions of the mitochondrial genome — the ribosomal RNA genes, not the protein-coding genes. This unusual genomic location is why these peptides were not identified by standard genomic annotation methods and required functional approaches to discover.
Humanin: the cytoprotective shield
Humanin (21 amino acids) was discovered in 2001 in neurons that remained viable in Alzheimer's disease brain tissue where surrounding neurons were dying. Its primary function is cytoprotection — it blocks the pro-apoptotic protein Bax at the mitochondrial membrane, preventing the cytochrome c release that triggers programmed cell death. Humanin also reduces beta-amyloid toxicity and improves insulin sensitivity. Like all MDPs, Humanin levels decline with age and in metabolic disease. See the Humanin research profile for dosing specifications.
MOTS-c: the metabolic adaptation signal
MOTS-c (16 amino acids) was identified in 2015 and represents the MDP most mechanistically similar to exercise — it translocates from mitochondria to the cell nucleus under metabolic stress, activating AMPK-mediated glucose uptake, fat oxidation, and mitochondrial biogenesis. A 2021 Nature Communications study showed MOTS-c levels increase 11.9-fold in skeletal muscle with exercise; a 2025 Nature study showed MOTS-c prevents pancreatic islet cell senescence. MOTS-c levels decline approximately 21% between ages 18-30 and 70-81. See the MOTS-c research profile.
The SHLP family: extending the MDP landscape
The 2016 identification of SHLP1 through SHLP6 by Yen et al. at USC extended the known MDP family significantly. All six SHLPs share structural similarities with Humanin and are encoded in overlapping reading frames in the 16S rRNA region. SHLP2 is the most studied member, demonstrating cytoprotection in retinal cells, anabolic-sparing effects in metabolic models, and additive cytoprotective effects when combined with Humanin. SHLP6 shows a unique pro-apoptotic profile in cancer cell lines while remaining protective in normal cells — a dual-edged property currently under active investigation. The remaining SHLPs (1, 3, 4, 5) have less characterization but preliminary evidence suggests each has distinct receptor binding preferences and tissue distribution patterns. See the SHLP2 and SHLP6 research profiles.
SS-31: the mitochondrial membrane repair tool
SS-31 (elamipretide, Forzinity) is not technically an MDP — it is a synthetic tetrapeptide designed to target the inner mitochondrial membrane rather than encoded by mitochondrial DNA. However, it is included in MDP research because it completes the picture of mitochondrial protection: where MDPs signal the mitochondrial state and drive adaptive responses, SS-31 directly repairs the physical structure where energy production occurs. By stabilizing cardiolipin on the inner mitochondrial membrane, SS-31 restores electron transport chain efficiency and reduces ROS production at the membrane level. Its September 2025 FDA approval for Barth syndrome validates the cardiolipin-targeting approach at a clinical level. See the SS-31 research profile and the Mito Stack Research Guide.
The complete mitochondrial research protocol
A comprehensive mitochondrial research approach combining SS-31 (membrane repair), MOTS-c (biogenesis signal), Humanin (cytoprotection), and SHLP2 (complementary cytoprotection and metabolic effects) addresses the full spectrum of age-related mitochondrial decline. The Mito Stack in the database covers SS-31, MOTS-c, and NAD+. Adding Humanin and SHLP2 on alternating days creates the most complete mitochondrial research protocol available. NAD+ underpins all of these compounds' effectiveness as the coenzyme substrate that the ETC and sirtuin systems depend on. Also see the SHLP Research Guide for the SHLP family in detail.
Research references
- Hashimoto Y, et al. (2001). A rescue factor abolishing neuronal cell death by a wide spectrum of familial Alzheimer's disease genes and Abeta — PNAS
- Lee C, et al. (2015). The mitochondrial-derived peptide MOTS-c promotes metabolic homeostasis — Cell Metabolism
- Cobb LJ, et al. (2016). Naturally occurring mitochondrial-derived peptides are age-dependent regulators of apoptosis — Aging