Humanin

Humanin is a 21-amino acid peptide encoded by an open reading frame within the 16S ribosomal RNA (16S rRNA) gene of the mitochondrial genome — making it a member of the newly recognised class of bioactive peptides known as mitochondria-derived peptides (MDPs). Its discovery in 2001 by Nishimoto and colleagues at Osaka University arose from a cDNA library screen designed to identify factors that could protect neurons from the cytotoxic effects of familial Alzheimer's disease-associated mutant APP (amyloid precursor protein). A neuronal cDNA library from the occipital cortex of an Alzheimer's patient was screened for sequences that, when expressed, protected neurons from APP-induced death — and humanin was the protective sequence identified. Its name reflects its discovery in human tissue and its protection of human neurons. The subsequent discovery that it is encoded within the mitochondrial genome rather than the nuclear genome was a paradigm-shifting finding: the mitochondria were long thought to encode only 13 structural proteins of the respiratory chain plus the RNA machinery for their translation — finding that they also encode peptide hormones with signalling functions fundamentally revised our understanding of mitochondrial biology.

Humanin's significance extends far beyond its Alzheimer's-protective origins. It is now understood to be an endogenous mitochondrial stress signal — a peptide that is upregulated and released from cells when their mitochondria are under stress (from cytotoxic insults, metabolic challenge, oxidative stress, or mitochondrial dysfunction) and that acts in both autocrine (on the same cell) and paracrine/endocrine (on neighbouring cells and distant tissues) fashion to promote cell survival and attenuate the stressor's damage. This positions humanin as an endogenous emergency signal from the organelle where most age-related biological decline originates — the mitochondrion.

Humanin signals through multiple receptor systems, reflecting its evolutionary importance. On the cell surface, it binds to a tripartite receptor complex consisting of gp130, CNTFR (ciliary neurotrophic factor receptor alpha), and WSX-1 — a heterotrimeric receptor complex shared with interleukin-27 and CNTF. Activation of this receptor complex triggers JAK-STAT signalling (specifically STAT3 phosphorylation) and ERK1/2 (mitogen-activated protein kinase) activation, both of which promote cell survival, inhibit apoptosis, and drive expression of protective gene programmes. Humanin also binds to FPRL1 (formyl peptide receptor-like 1, now designated FPR2), a chemotactic receptor expressed on immune cells through which humanin exerts anti-inflammatory effects by modulating macrophage activity and reducing inflammatory cytokine production. Intracellularly, humanin can directly interact with the pro-apoptotic protein BAX — binding to BAX in a manner that prevents it from undergoing the conformational changes required for its translocation to the mitochondrial outer membrane and formation of the apoptotic pore, thereby directly blocking the mitochondrial apoptosis pathway at the effector level.

The neuroprotective applications of humanin extend beyond Alzheimer's disease to encompass a broad spectrum of neurotoxic and neurodegenerative challenges. In cell culture and animal model systems, humanin has demonstrated protection against: amyloid-beta 1-42 oligomer toxicity (the most neurotoxic species in Alzheimer's disease); 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity (a Parkinson's disease model); excitotoxic glutamate toxicity (relevant to ischaemic stroke); cerebral ischaemia-reperfusion injury; and oxidative stress from multiple sources. The consistency of neuroprotection across mechanistically distinct neurotoxic insults suggests that humanin is engaging fundamental, broadly cytoprotective pathways rather than narrowly counteracting one specific toxin.

The metabolic effects of humanin represent a second major functional domain with direct relevance to ageing and metabolic disease. Research by Bharat Bhatt, Dr. Pinchas Cohen, and colleagues at the University of Southern California has established that humanin has significant insulin-sensitising and glucoregulatory effects. In rodent models of diet-induced obesity and type 2 diabetes, humanin injection reduces fasting glucose, improves insulin tolerance, reduces hepatic glucose production (gluconeogenesis), and increases peripheral glucose uptake — effects mediated in part through STAT3 signalling in the hypothalamus (reducing hypothalamic insulin resistance) and in the liver (reducing FOXO1-mediated gluconeogenic gene expression). Humanin also appears to attenuate the inflammatory signalling in adipose tissue that contributes to insulin resistance in obese states.

The longevity biology of humanin was established through several converging lines of evidence. First, circulating humanin levels decline with ageing in humans — measured declines of 50% or more between young adulthood and older age have been documented in multiple independent studies. Second, Cohen and colleagues demonstrated that centenarians — individuals who have lived to age 100 or beyond — have significantly higher circulating humanin levels than age-matched older adult controls who have not achieved exceptional longevity. This correlation between high humanin and exceptional human longevity is one of the most intriguing observations in human ageing biology and suggests that humanin level maintenance may be a marker or even a mediator of successful ageing. Third, animal studies have shown that transgenic overexpression of humanin extends lifespan in C. elegans and improves multiple healthspan parameters in rodents.

The cardiovascular research on humanin complements its metabolic and neuroprotective profiles. In models of cardiac ischaemia-reperfusion injury — the cellular damage that occurs when blood flow is restored to heart tissue after a heart attack — humanin pretreatment significantly reduces cardiomyocyte death, limits infarct size, and preserves contractile function. The mechanism involves direct BAX inhibition (preventing cardiomyocyte apoptosis) and STAT3/ERK activation (promoting survival signalling), the same mechanisms operative in neuronal protection. Humanin also inhibits the formation of foam cells in atherosclerosis models (macrophages loaded with oxidised lipids that form early atherosclerotic plaques), reduces the progression of aortic atherosclerosis in ApoE-knockout mice (a standard atherosclerosis model), and has anti-inflammatory effects on vascular endothelium — collectively suggesting a protective role throughout the cardiovascular system.

The convergence of neuroprotective, cardioprotective, insulin-sensitising, anti-aging, and cytoprotective actions in a single 21-amino acid mitochondrially-encoded peptide makes humanin scientifically extraordinary. Together with MOTS-c (another mitochondria-derived peptide now in Phase 1 clinical trials), humanin represents the founding member of a new class of endocrine signals that communicate mitochondrial stress status to the broader organism — a system whose dysregulation with ageing appears to contribute fundamentally to the multi-organ decline that characterises the ageing process.