Humanin

Humanin was identified in 2001 by Hashimoto et al. (2001, PNAS) through a functional expression screen of a cDNA library constructed from surviving neurons in the occipital lobe of an autopsy-confirmed Alzheimer's disease brain — a region relatively spared from disease pathology. The screen sought transcripts with cytoprotective activity against familial-AD gene insults (V642I-APP, NL-APP, M146L-PS1, N141I-PS2) and against amyloid-beta peptide in neuronal cell-death assays. The lead clone mapped to a short open reading frame within the mitochondrial 16S rRNA region (MT-RNR2) rather than a nuclear gene — an unexpected genomic locus for a cytoprotective signaling peptide. A companion paper (Hashimoto et al., 2001, J. Neurosci.) characterized the specificity of this cytoprotective activity, examining which AD-relevant and non-AD insults (polyglutamine Q79, SOD1-mutant) fell within versus outside the assay's scope. Humanin became the founding member of the mitochondria-derived peptide (MDP) family; related MDPs — MOTS-c and the small humanin-like peptides SHLPs 1–6 — were subsequently identified from the same genomic neighborhood by the Cohen laboratory at USC. Because the MT-RNR2 ORF is read by both cytosolic and mitochondrial ribosomes (with the mitochondrial genetic code truncating the reading frame three codons earlier), two forms are described: a 24-residue cytosolic form (canonical UniProt Q8IVG9) and a 21-residue mitochondrial form, both examined in the literature.

A major mechanistic line of research positioned Humanin as a binding partner for proteins within the intrinsic (mitochondrial) apoptosis pathway. Guo et al. (2003, Nature) examined the interaction between Humanin and BAX, characterizing the relationship between Humanin engagement and BAX translocation from the cytosol to mitochondrial membranes — a step involved in mitochondrial outer-membrane permeabilization (MOMP). The study also used RNAi-mediated knockdown of endogenous Humanin to assess pathway sensitivity. Zhai et al. (2005, JBC) extended this biochemical framework to BID/tBID, examining the impact on cytochrome c and SMAC release from isolated mitochondria in the context of BID activation. Luciano et al. (2005, JBC) used co-immunoprecipitation and cell-based assays to identify the BimEL isoform as an additional Humanin-binding partner. In parallel, Ikonen et al. (2003, PNAS) used a yeast two-hybrid screen to identify IGFBP-3 as a Humanin-interacting protein, and alanine scanning mapped residues important for this binding interface. More recent biophysical work (Morris et al., 2019, JBC) characterized the conformational changes Humanin induces in BAX and demonstrated that the complex can be recruited into β-sheet fiber assemblies — work that provides structural context for the MOMP-suppression model, later extended to BID.

Extracellular Humanin has been examined as a ligand for two receptor systems with distinct pharmacologies. Ying et al. (2004, J. Immunol.) examined Humanin's interaction with the G-protein-coupled formyl peptide receptor FPRL1 (FPR2), measuring calcium mobilization and ERK1/2 activation and studying competitive binding with amyloid-beta-42 at the receptor. Harada et al. (2004, BBRC) characterized N-formylated Humanin (fHN) at FPRL1 and FPRL2, with the formylated form showing markedly greater potency in receptor activation assays. The structural basis for receptor engagement was subsequently addressed by Zhu et al. (2022, Nat. Commun.), who used cryo-EM to resolve FPR2 in complex with Gi and with either N-formyl Humanin or Aβ42, mapping the binding sites and characterizing the structural basis for the competitive model at atomic resolution. Separately, Hashimoto et al. (2009, Mol. Biol. Cell) demonstrated that Humanin signals through a trimeric cytokine receptor complex comprising CNTFR, WSX-1, and gp130, examining the contribution of individual subunits in neuronal model systems and identifying gp130 as essential for the downstream signaling response. STAT3 was identified as a downstream mediator in this receptor axis, a finding used in subsequent metabolic studies.

Muzumdar et al. (2009, PLoS ONE) examined Humanin in the context of insulin action, using hyperinsulinemic-euglycemic clamp methodology combined with intracerebroventricular infusion to study how central Humanin signals relate to peripheral glucose handling. The study examined hypothalamic STAT3 pathway involvement by testing the effect of STAT3 inhibition on the response. A follow-up study (Kuliawat et al., 2013, FASEB J.) examined the F6A analog HNGF6A in pancreatic islet and β-cell line preparations, measuring glucose-stimulated insulin secretion as the primary endpoint. Both studies also characterized circulating Humanin levels in rodent and human samples as a function of age, framing Humanin as a candidate signal at the intersection of metabolic aging and neurodegeneration research.

Muzumdar et al. (2010, Arterioscler. Thromb. Vasc. Biol.) examined the S14G Humanin analog (HNG) in a murine myocardial ischemia/reperfusion preparation, measuring infarct size, left-ventricular function by echocardiography, and apoptotic signaling markers. The study also assessed endogenous Humanin protein levels in left-ventricular tissue at time points after the ischemic insult. Subsequent cardiomyocyte and in vivo studies in related research groups examined mitochondrial membrane potential, reactive oxygen species generation, and caspase-3 activity in additional cardioprotection model systems, generally using the HNG analog owing to its extended in vivo stability relative to the native sequence.

Yen et al. (2018, Sci. Rep.) examined circulating Humanin levels as a function of age in a human cohort and studied the association between an mtDNA single-nucleotide variant (rs2854128) and circulating Humanin levels, together with cognitive measures, in cross-sectional analysis. Yen et al. (2020, Aging) examined the relationship between Humanin expression and lifespan in C. elegans, probing daf-16/FOXO pathway dependence by testing transgenic overexpression against daf-16 mutant backgrounds; the study also surveyed Humanin levels across species with differing longevity characteristics including the naked mole-rat and offspring of centenarians. Miller et al. (2024, Aging Cell) characterized the P3S variant, identified as enriched in APOE4 centenarian cohorts, examining its association with brain pathology in related model systems. On the structural side, NMR studies have characterized Humanin's solution conformation — predominantly unstructured in aqueous conditions with nascent helical character, adopting a defined α-helix in membrane-mimetic environments — with structural entries deposited in RCSB PDB (1Y32: native humanin in 30% TFE; 2GD3: S14G form; 5GIW: D-Ser14 form). Dimerization studies (Terashita et al., 2003) examined how specific residues, including an L12A substitution, influence receptor interaction and can function as dominant-negative modulators in model systems, linking the dimerization interface to receptor pharmacology.