Humanin

Humanin is a 24-amino-acid peptide, MAPRGFSCLLLLTSEIDLPVKRRA (PubChem CID 16131438), encoded by a short open reading frame in MT-RNR2, the mitochondrial gene for 16S ribosomal RNA (Bodzioch et al., 2009; Cobb et al., 2016). It was identified in 2001 by functional expression screening as a secreted factor that blocked neuronal cell death caused by familial Alzheimer's disease genes and amyloid-β (Hashimoto et al., 2001a), and reviews describe it as the first of the mitochondrial-derived peptides (Lee et al., 2013).

Much of the animal work used the analog S14G-humanin (HNG), so each summary names the compound tested.

PubChem CID 16131438 lists the same formula, weight and CAS number and gives the sequence.

Published work on humanin falls into six areas. Each summary below states the model used.

Discovery and the mitochondrial genome

In the founding study, cells transfected with humanin cDNA secreted the peptide, which protected neuronal cells from familial Alzheimer's disease genes and amyloid-β but not from polyglutamine or SOD1-mutant toxicity (Hashimoto et al., 2001a).

The mitochondrial genome carries an identical reading frame (Guo et al., 2003), and the human nuclear genome adds 13 MT-RNR2-like loci predicted to encode humanin-like peptides, at least ten expressed in human tissues (Bodzioch et al., 2009). In rat cells, predicted to lack such nuclear copies, humanin production fell when mitochondrial DNA was depleted, was unaffected by the cytosolic translation blocker cycloheximide, and localized to mitochondria (Paharkova et al., 2015).

Structure-activity work and the S14G analog (HNG)

In neuronal cell culture, the domain from Pro3 to Pro19 carried the rescue activity, Cys8 and Ser14 were essential, and among factors compared, including IGF-I and basic FGF, only HNG blocked all five insults tested (Hashimoto et al., 2001b). HNG, like humanin with D-serine at position 14, was two to three orders of magnitude more potent than wild-type humanin, and self-dimerization through Ser7 was essential for activity (Terashita et al., 2003).

Intracellular binding partners

Humanin bound the pro-apoptotic protein Bax, kept it from moving to mitochondria and blocked cytochrome c release from isolated mitochondria (Guo et al., 2003). It also bound Bid and its truncated form tBid, even blocking tBid-driven release of cytochrome c from Bax-deficient mitochondria (Zhai et al., 2005), and bound the extra-long isoform of Bim but not the shorter ones (Luciano et al., 2005).

Models: purified proteins, cell lines and isolated mitochondria. A yeast two-hybrid screen found IGFBP-3 as a high-affinity partner; a non-binding F6A mutant failed to block IGFBP-3-induced apoptosis in A172 glioblastoma cells, and in mouse primary neurons IGFBP-3 strengthened humanin's rescue from amyloid-β (Ikonen et al., 2003).

Cell-surface receptors

Synthetic humanin drew mononuclear phagocytes through human formyl peptide receptor-like 1 (FPRL1) and mouse FPR2, receptors amyloid-β42 also uses (Ying et al., 2004). In CHO cells expressing FPRL1, humanin had a half-maximal concentration of 3.5 nM and an N-formylated form 0.012 nM (Harada et al., 2004). In neuronal cells, gp130 was essential for protection, CNTF receptor alpha and WSX-1 took part in binding, and humanin made the three assemble in reconstituted assays (Hashimoto et al., 2009).

Rodent metabolic and cardiac models

In male Sprague-Dawley rats studied with insulin clamps, centrally infused humanin raised whole-body insulin sensitivity through hypothalamic STAT-3, and potent analogs given peripherally reproduced the effect (Muzumdar et al., 2009). In male C57BL/6J mice subjected to coronary occlusion and reperfusion, HNG reduced infarct size relative to the area at risk and raised left ventricular ejection fraction a week later (Muzumdar et al., 2010).

Aging studies in animals and human cohorts

Humanin raised cognitive test scores in aged mice, and a variant in its mitochondrial coding region (rs2854128) was tied to lower circulating humanin and, in an independent cohort of older adults, faster cognitive aging (Yen et al., 2018). Overexpression extended lifespan in C. elegans, and the same paper reported humanin falling with age in several species but higher in children of centenarians than in controls (Yen et al., 2020).

A cross-sectional study of 693 people aged 21 to 113 found the opposite, with plasma humanin highest in centenarians (Conte et al., 2019). The direction of change with age is unsettled. Authors of the first two studies declared ties to CohBar Inc.

Tumor models

In triple-negative breast cancer cells, added humanin protected the cells from apoptotic stimuli, while silencing it reduced their viability. In tumor-bearing mice, systemic humanin reduced tumor apoptosis, blunted the effect of chemotherapy and accelerated tumor growth (Moreno Ayala et al., 2020). Models: breast cancer cell lines and mice.