Humanin

Humanin was discovered in 2001 by Nishimoto et al. while screening a human brain cDNA library (from an unaffected individual) for factors protecting against Alzheimer's disease amyloid-β (Aβ) toxicity. Its encoding sequence was traced to an open reading frame within the 16S rRNA region of the mitochondrial genome — an unexpected discovery that challenged the assumption that mitochondrial DNA encoded only respiratory chain components.

This makes humanin a member of the emerging class of mitochondria-derived peptides (MDPs), small mitochondrially-encoded proteins that function as intercellular hormones or cytokines — collectively termed "mitokines." Other MDPs discovered since include MOTS-c (already in PeptideWiki), SHLP1-6, and DALE. Humanin is the founding member and most extensively studied.

Humanin circulates in blood and cerebrospinal fluid, activates cell surface receptors on multiple tissue types, and declines with aging. The most potent natural analogue is Humanin-G (HNG), produced by substituting serine-14 with glycine (S14G-HN) — this single substitution increases receptor binding affinity by ~1000-fold. HNG is used in most preclinical research due to this superior potency.

Epidemiological data from centenarian studies (Cohen et al., aging cohort studies) consistently show higher circulating humanin levels in long-lived individuals and their offspring compared to age-matched controls — positioning humanin as a candidate longevity biomarker and therapeutic target.

// NEUROPROTECTION AND ANTI-APOPTOSIS (BAX INHIBITION)

Humanin physically interacts with Bax (a pro-apoptotic BCL-2 family protein) and prevents its translocation to the outer mitochondrial membrane (OMM). Bax oligomerization at the OMM is the key event triggering mitochondrial outer membrane permeabilization (MOMP), cytochrome c release, and caspase cascade activation. By sequestering Bax in the cytosol, humanin blocks this irreversible apoptotic commitment. In Alzheimer's disease neurons exposed to Aβ oligomers, humanin prevents the Bax-mediated apoptotic cascade that drives neuronal loss.

// SYSTEMIC INSULIN SENSITIVITY AND METABOLIC PROTECTION

Humanin signals through a heterotrimeric receptor complex on cell surfaces consisting of gp130 (IL-6 receptor common subunit), CNTFR (ciliary neurotrophic factor receptor), and WSX-1 (IL-27 receptor). This receptor complex activates JAK1/2 → STAT3 signaling and AMPK in metabolic tissues. In the liver, humanin receptor activation reduces gluconeogenesis (PEPCK, G6Pase suppression) and improves hepatic insulin sensitivity. In muscle, AMPK activation mimics exercise effects on glucose uptake. In pancreatic beta cells, humanin protects against lipotoxicity-induced apoptosis, preserving beta cell mass.

// CARDIOPROTECTION AND ISCHEMIA-REPERFUSION INJURY

Humanin reduces cardiac ischemia-reperfusion injury through multiple mechanisms: (1) Bax inhibition prevents cardiomyocyte apoptosis during reperfusion; (2) mitochondrial permeability transition pore (mPTP) opening probability is reduced, maintaining ΔΨm; (3) IGF-1 receptor transactivation (via src kinase) activates PI3K/Akt survival signaling. In rodent MI models, humanin infusion before reperfusion significantly reduces infarct size and preserves LV function.

Alzheimer's disease models: Humanin (and HNG) protect primary neurons and neuronal cell lines against Aβ(1-42), Aβ(25-35), AICD (amyloid precursor protein intracellular domain), and FAD mutant presenilin-induced neurotoxicity in vitro. The neuroprotective effect is receptor-mediated (blocked by anti-gp130 antibodies) and Bax-dependent.

Longevity and centenarian studies (Cohen et al., Aging 2023, and earlier Barzilai lab studies): Analysis of Ashkenazi Jewish centenarian cohort (mean age 97 years) and their offspring showed significantly higher circulating hCAP18/LL-37 and humanin levels vs. age-matched controls without longevity family history. Humanin levels in offspring correlated with IGF-1 sensitivity and insulin-like growth factor binding protein 3 (IGFBP-3) levels.

Mouse lifespan extension: Systemic HNG administration in aging C57BL/6 mice (starting at 12 months) delayed age-related weight loss, preserved grip strength, improved glucose tolerance, and modestly extended median lifespan vs. vehicle. Whole-body KO of humanin in mice accelerates aging phenotypes (insulin resistance, cognitive decline, mortality).

Metabolic syndrome (rodent models): HNG improved insulin sensitivity, reduced adipose tissue inflammation, and protected against diet-induced obesity in high-fat-fed mice. Liver-specific humanin overexpression reduced hepatic steatosis and improved systemic glucose metabolism.