Humanin

Overview

Humanin (HN) is a 24-amino-acid peptide encoded by a short open reading frame within the 16S ribosomal RNA gene (MT-RNR2) of mitochondrial DNA. Discovered in 2001 by Hashimoto et al. through a functional screen for survival factors in brain tissue from an Alzheimer's disease patient, humanin was the first identified mitochondrial-derived peptide (MDP) — a class of bioactive peptides encoded within the mitochondrial genome that has since expanded to include MOTS-c and the SHLP (Small Humanin-Like Peptide) family.

The original discovery context was significant: humanin was isolated from an unaffected brain region of an Alzheimer's patient and was found to protect neurons from cell death induced by amyloid-beta peptide, the pathological hallmark of Alzheimer's disease. This neuroprotective activity catalyzed extensive research into humanin's broader cytoprotective properties, revealing that the peptide protects against apoptosis (programmed cell death) across a remarkably wide range of cell types and stress conditions.

Endogenous humanin is produced in multiple tissues and circulates in plasma, cerebrospinal fluid, and seminal fluid. Circulating levels decline with age — a finding that has positioned humanin as a potential biomarker and mediator of biological aging. Its discovery fundamentally challenged the prevailing view that the mitochondrial genome encoded only 13 proteins, 22 tRNAs, and 2 rRNAs, opening an entirely new field of mitochondrial-derived peptide biology.

Several synthetic analogs with enhanced potency have been developed, most notably HNG (S14G-Humanin or [Gly14]-Humanin), which is approximately 1,000-fold more potent than native humanin in cytoprotection assays.

Structure and Sequence

Sequence: Met-Ala-Pro-Arg-Gly-Phe-Ser-Cys-Leu-Leu-Leu-Leu-Thr-Ser-Glu-Ile-Asp-Leu-Pro-Val-Lys-Arg-Arg-Ala (MAPRGFSCLLLLTSEIDLPVKRRA)

• Molecular weight: Approximately 2,687 Da
• Encoding gene: MT-RNR2 (mitochondrial 16S rRNA gene)
• Genome of origin: Mitochondrial DNA
• Key residues: The central hydrophobic domain (Leu9-Leu12) is critical for biological activity; Ser14 is the site of the potency-enhancing Gly substitution in HNG
• Analogs:
  • HNG (S14G-Humanin): Ser14 to Gly substitution; approximately 1,000x more potent
  • HN-C8A: Cys8 to Ala substitution; eliminates dimerization; used in structural studies
  • Colivelin: A hybrid peptide incorporating the active fragment of humanin; enhanced blood-brain barrier penetration

Humanin can form dimers through Cys8-mediated disulfide bonding. Both monomeric and dimeric forms are biologically active, though they may activate different signaling pathways.

Mechanism of Action

Anti-Apoptotic Signaling via BAX Interaction

Humanin's foundational mechanism is direct interference with the mitochondrial apoptosis pathway:

• Humanin binds to BAX, a pro-apoptotic BCL-2 family member, preventing BAX oligomerization and translocation to the outer mitochondrial membrane
• By blocking BAX, humanin prevents mitochondrial outer membrane permeabilization (MOMP), cytochrome c release, and caspase cascade activation
• Humanin also interacts with BID (BH3-interacting domain death agonist), another pro-apoptotic factor, neutralizing its ability to activate BAX
• This places humanin as a direct endogenous inhibitor of the intrinsic (mitochondrial) apoptosis pathway

IGFBP-3 Binding and IGF-1 Axis Interaction

Humanin binds to insulin-like growth factor binding protein 3 (IGFBP-3):

• IGFBP-3 has IGF-independent pro-apoptotic activity; humanin neutralizes this activity
• Humanin-IGFBP-3 binding may also modulate IGF-1 bioavailability
• This interaction connects humanin biology to the growth hormone/IGF-1 axis, with implications for aging and metabolic regulation

STAT3 Signaling via CNTFR/WSX-1/gp130 Receptor Complex

Humanin signals through an extracellular receptor complex composed of:

• CNTFR (ciliary neurotrophic factor receptor alpha)
• WSX-1 (IL-27 receptor alpha)
• gp130 (glycoprotein 130, a shared cytokine receptor subunit)

Binding of humanin to this trimeric complex activates:

• JAK1/JAK2 tyrosine kinases
• STAT3 phosphorylation and nuclear translocation
• Transcription of pro-survival and anti-inflammatory genes
• This extracellular signaling pathway operates independently of the direct intracellular BAX-binding mechanism

FPRL1/FPRL2 Receptor Interactions

Humanin also interacts with formyl peptide receptor-like 1 and 2 (FPRL1/FPRL2):

• These GPCRs are involved in innate immune regulation and chemotaxis
• FPRL receptor activation may mediate some of humanin's anti-inflammatory effects
• This interaction connects humanin to the broader innate immune peptide network

Neuroprotective Mechanisms

In the specific context of neurodegeneration:

• Protection against amyloid-beta (Abeta) toxicity through multiple mechanisms including BAX inhibition and STAT3 activation
• Reduction of tau phosphorylation through modulation of GSK-3beta and PP2A
• Attenuation of oxidative stress through enhanced mitochondrial function
• Anti-neuroinflammatory effects through microglial modulation

Research Summary

Pharmacokinetics

• Endogenous plasma levels: Approximately 0.5-2 ng/mL in healthy adults; decline with age (approximately 1% per year after age 30)
• CSF levels: Detectable in cerebrospinal fluid; reduced in Alzheimer's disease patients
• Half-life: Native humanin has a relatively short plasma half-life (estimated minutes to low hours); analogs such as HNG have modified PK profiles
• Blood-brain barrier: Native humanin crosses the BBB poorly; the analog Colivelin was designed to enhance CNS penetration
• Dimerization: Humanin can form dimers via Cys8 disulfide bonding; monomeric and dimeric forms may have different pharmacokinetic and pharmacodynamic profiles
• Age-related dynamics: Levels decline approximately 1% per year; centenarians and their offspring show higher humanin levels than age-matched controls
• Tissue distribution: Endogenously produced in brain, heart, liver, kidney, skeletal muscle, and testes

The observation that centenarians maintain higher circulating humanin levels than non-centenarians has generated particular interest in humanin as both a biomarker and potential mediator of exceptional longevity.

Dosing Protocols

The following dosing information is compiled from published research and community discussion for educational purposes only. No FDA-approved human dosing guidelines exist for most research peptides. Always consult a qualified healthcare professional.

Dosing Schedule

Cycle Guidelines

• Route: Subcutaneous injection
• Note: Very limited human dosing data exists; protocols are derived almost entirely from preclinical research and early-stage investigations
• Storage: Lyophilized form at -20 °C; reconstituted solutions should be refrigerated and used promptly

Common Discussion Topics

1. Neuroprotection and Alzheimer's disease — Humanin's original discovery context and continued preclinical efficacy against amyloid-beta toxicity make it a focus of neurodegeneration research
2. Aging biomarker — The consistent age-related decline in humanin levels and the centenarian association position it as a potential biomarker of biological aging
3. Mitochondrial-derived peptide biology — Humanin's discovery opened the field of MDPs; it is discussed alongside MOTS-c as evidence that mitochondrial DNA encodes far more biological information than previously recognized
4. GH/IGF-1 axis connection — The inverse relationship between humanin and GH/IGF-1 levels connects to broader discussions of the growth-longevity trade-off
5. Cytoprotection breadth — Humanin's protective effects extend beyond neurons to cardiomyocytes, endothelial cells, pancreatic beta cells, and other cell types
6. Analog development — HNG (S14G-humanin) and Colivelin represent efforts to enhance potency and tissue targeting

Limitations of Current Research

1. No human clinical trials — All interventional data is preclinical; human data is limited to observational biomarker studies
2. Blood-brain barrier limitations — Native humanin's poor BBB penetration limits its utility for CNS applications; analogs with improved penetration are under development
3. Complex receptor pharmacology — Multiple receptor systems (trimeric complex, FPRL, intracellular BAX) complicate mechanism delineation
4. Peptide stability — Short half-life and potential dimerization complicate dosing and formulation
5. Translation uncertainty — Neuroprotective effects demonstrated in cell culture and animal models have not yet been validated in human neurodegenerative disease

Related Compounds

• MOTS-c — a metabolically focused mitochondrial-derived peptide encoded in the 12S rRNA gene
• SS-31 (Elamipretide) — a synthetic mitochondria-targeted peptide with complementary mitochondrial-protective mechanisms
• DSIP — a neuropeptide with distinct but related neuroprotective and stress-modulatory properties
• SHLP1-6 — small humanin-like peptides encoded in the 16S rRNA gene adjacent to humanin; cytoprotective and metabolic activities
• Colivelin — a hybrid peptide incorporating humanin's active domain with enhanced blood-brain barrier penetration