HumaninvsSemax

Humanin is a 24-amino-acid peptide (MAPRGFSCLLLLTSEIDLPVKRRA) encoded within the 16S ribosomal RNA gene of the mitochondrial genome. Its discovery in 2001 was revolutionary — it was the first identified mitochondrial-derived peptide (MDP), challenging the long-held dogma that the mitochondrial genome only encodes 13 oxidative phosphorylation subunits, 22 tRNAs, and 2 rRNAs. Humanin, along with MOTS-C and the SHLP peptides discovered later, established mitochondria as endocrine organelles.

Humanin exerts cytoprotective effects through multiple mechanisms. Extracellularly, it binds to a trimeric receptor complex composed of CNTFR (ciliary neurotrophic factor receptor alpha), WSX-1 (IL-27 receptor alpha), and gp130 (the shared signaling subunit of the IL-6 receptor family). Activation of this complex triggers JAK/STAT3 signaling, which drives expression of anti-apoptotic genes (Bcl-2, Mcl-1) and cell survival programs. Intracellularly, humanin interacts directly with two pro-apoptotic proteins: it binds IGFBP-3, preventing IGFBP-3 from translocating to mitochondria and initiating apoptosis; and it binds BAX (Bcl-2-associated X protein), preventing BAX oligomerization and insertion into the outer mitochondrial membrane — the critical step in the intrinsic (mitochondrial) apoptosis pathway that releases cytochrome c and activates caspases.

Humanin also reduces cellular stress through multiple pathways. It decreases reactive oxygen species (ROS) production by optimizing mitochondrial electron transport chain function. It reduces endoplasmic reticulum (ER) stress by modulating the unfolded protein response (UPR). It improves insulin sensitivity through STAT3-mediated effects on hypothalamic signaling and peripheral insulin receptor substrate phosphorylation. Circulating humanin levels decline with age (approximately 40% reduction between youth and old age) and are inversely correlated with markers of age-related disease, suggesting that humanin decline contributes to the increased cellular vulnerability and apoptosis susceptibility seen in aging. Its most potent synthetic analogue, HNG (S14G-humanin), has a glycine-for-serine substitution at position 14 that increases cytoprotective potency approximately 1,000-fold.

Semax is a synthetic heptapeptide consisting of the ACTH(4-10) fragment (Met-Glu-His-Phe-Pro-Gly-Pro) — the shortest sequence of ACTH that retains neurotrophic activity — with a Pro-Gly-Pro C-terminal extension for proteolytic stability. Crucially, it contains only the neurotrophic portion of ACTH without the N-terminal amino acids (residues 1-3) required for adrenal cortex stimulation, so it has no effect on cortisol production or the HPA stress axis.

Semax's primary nootropic mechanism is upregulation of neurotrophic factors in the hippocampus and cortex. It increases expression of brain-derived neurotrophic factor (BDNF) — the most important neurotrophin for learning and memory — through activation of the TrkB receptor signaling cascade (Ras/MAPK and PI3K/Akt pathways). BDNF promotes dendritic spine formation, enhances long-term potentiation (the cellular basis of memory), and supports neuronal survival. Semax also upregulates nerve growth factor (NGF), which maintains cholinergic neurons in the basal forebrain — the same neurons that degenerate in Alzheimer's disease and are critical for attention and memory.

At the neurotransmitter level, Semax modulates three monoamine systems. It enhances dopaminergic transmission in the prefrontal cortex and striatum, improving motivation, reward processing, and executive function. It modulates serotonergic activity (5-HT) in the raphe nuclei and limbic system, affecting mood and emotional regulation. It also enhances noradrenergic signaling from the locus coeruleus, improving alertness, focused attention, and working memory. The noradrenergic effect may be particularly relevant for its clinical use in ADHD-like conditions and attention disorders. In stroke recovery (an approved indication in Russia), Semax provides neuroprotection through multiple mechanisms: BDNF-mediated anti-apoptotic signaling, reduction of glutamate excitotoxicity, decreased oxidative stress, and maintenance of blood-brain barrier integrity in the peri-infarct region.