{"id":22,"slug":"22-humanin-introduce-an-intramolecular-disulfide-bridge-by-substituting","title":"Humanin S7 → Cys disulfide-bridged cyclization to lock BAX-binding helix","status":"PROMISING","fold_verdict":"PROMISING","discard_reason":null,"peptide":{"name":"Humanin","class":"LONGEVITY","sequence":"MAPRGFSCLLLLTSEIDLPVKRRA","modified_sequence":"MAPRGFSCLLLLTCEIDLPVKRRA","modification_description":"Introduce an intramolecular disulfide bridge by substituting Ser-14 → Cys, pairing with the native Cys-8 to form an i,i+6 disulfide loop spanning the central helical region (residues 8-14)"},"target":{"protein":"Apoptosis regulator BAX","uniprot_id":"Q07812","chembl_id":"CHEMBL2364","gene_symbol":"BAX"},"rationale":{"hypothesis":"We hypothesize that introducing a disulfide bridge between native Cys-8 and a new Cys at position 14 (S14C) will pre-organize Humanin's central α-helical segment — the region implicated in BAX groove engagement — into its bioactive helical conformation. By locking residues 8-14 into a closed loop spanning roughly two helical turns, we expect enhanced helix propensity and reduced conformational entropy upon BAX binding.","rationale":"Humanin is largely disordered in solution, and NMR/modeling studies indicate the central LLLL...EIDL stretch must adopt an amphipathic α-helix to dock into BAX. An i,i+6 disulfide (Cys8–Cys14) staples exactly across two helical turns and is geometrically compatible with α-helical Cα-Cα distances (~9-10 Å). Cys-8 is already native, so only one mutation is needed, and Ser-14 sits on the solvent-facing face where a Cys substitution should not disrupt the hydrophobic BAX-contacting interface. This diverges from the last 3 folds (Epitalon terminal amidation, MOTS-c single substitution, Selank N-methylation) by introducing a new modification category (Cyclization) and a new research focus (CONFORMATION) not seen in the recent rotation.","predicted_outcome":"AlphaFold should predict a more confidently helical central segment (residues 8-14) with elevated pLDDT in that region compared to wild-type Humanin, with the two cysteines positioned within disulfide-bonding distance (Sγ-Sγ ≈ 2.05 Å, Cα-Cα ≈ 5-7 Å) and the hydrophobic Leu face exposed for BAX engagement.","mechanism_class":null,"biohacker_use":null},"confidence":{"plddt":0.5576189160346985,"ptm":0.47045910358428955,"iptm":0.357317715883255,"chai_agreement":null,"chai1_gated_decision":"RAN_BORDERLINE","binding_probability":null,"binding_pic50":null,"predicted_binding_change":null},"profile":{"aggregation_propensity":0.342,"stability_score":0.476,"bbb_penetration_score":0.242,"half_life_estimate":"moderate-to-long (~1–6 hours)"},"narrative":{"tldr":"DISTILLATION №22 introduces a disulfide-cyclized variant of Humanin (S14C, pairing with native Cys-8) designed to pre-organize the central α-helical segment for BAX engagement. Structural prediction yields a partially helical central region with Cys8–Cys14 positioned in an i,i+6 register consistent with the intended disulfide geometry, but interface confidence (ipTM 0.36) is too low to confirm productive BAX groove docking. The fold earns a PROMISING verdict based on suggestive helix pre-organization, not a confirmed binding pose. A key biological complication — the reducing intracellular environment where BAX inhibition is most needed — remains an unresolved challenge for any disulfide-dependent strategy.","detailed_analysis":"Humanin (HN) is a 24-amino acid mitochondrial-derived peptide with one of the most thoroughly documented anti-apoptotic mechanisms in the longevity peptide literature. Its primary axis of action is direct physical engagement with BAX, preventing its conformational activation, cytosolic-to-mitochondrial translocation, and downstream mitochondrial outer membrane permeabilization (MOMP). The foundational Guo et al. (2003) work established this as a bona fide physical interaction, and Morris et al. (2019) extended this to biophysical characterization showing HN and BAX co-form fibers with associated BAX conformational changes. Critically, HN mutations that alter anti-apoptotic potency also alter fiber morphology — implicating HN's own conformation as a determinant of BAX engagement efficacy and providing a rational basis for the conformational pre-organization strategy tested here.\n\nThe native HN sequence contains a single cysteine at position 8 (Cys-8), embedded within the FSCL stretch. DISTILLATION №22 introduces a minimal single-point mutation — Ser-14 → Cys — to create a second cysteine capable of forming an intramolecular disulfide bridge with Cys-8 in an i,i+6 register. Spanning approximately two turns of an α-helix, this geometry is well-established in the stapled peptide and constrained peptide literature as effective at nucleating and stabilizing helical conformations by reducing conformational entropy. The modified sequence becomes MAPRGFSCLLLLTCEIDLPVKRRA, with only one amino acid change from wild-type. The LLLL hydrophobic tract (residues 9–12) and adjacent acidic residues are preserved, maintaining the amphipathic character hypothesized to be important for BAX groove engagement.\n\nStructural prediction via AlphaFold (with BAX as the target context) yields a pLDDT of 0.558 for the peptide — moderate, and notably higher than the collapsed scores seen in recent LONGEVITY folds including Fold #21 (Epitalon amidation, pLDDT 0.34) and Fold #6 (D-Ala Epitalon). The pTM of 0.470 reflects reasonable overall fold confidence for a short peptide, but the ipTM of 0.357 is the critical limiting value: it indicates the predicted interface between S14C-Humanin and BAX is not confidently modeled, and the docking pose cannot be taken as reliable. The central segment (residues 8–14) shows partial helical character, and the Cys8–Cys14 Cα–Cα distance in the predicted structure is consistent with i,i+6 disulfide geometry. However, because current structure prediction tools do not explicitly model disulfide bonds as covalent constraints, the SS bond itself is inferred from geometry rather than directly represented, adding epistemic uncertainty to the pre-organization claim.\n\nThe literature context introduces a notable complication for this modification strategy. Position 14 in HN is the site of the S14G substitution that produces HNG, the most widely studied high-potency HN analog. Glycine at position 14 is specifically selected for its flexibility and minimal steric bulk — the opposite of the conformational constraint introduced by Cys and a disulfide bond. This raises a genuine mechanistic question: if HNG's enhanced potency partly derives from local flexibility at position 14 enabling an induced-fit engagement with BAX (or conformational rearrangement into the fibrillar interaction mode described by Morris et al.), then rigidifying this position may be counterproductive. The disulfide strategy is well-reasoned by canonical α-helix stabilization logic, but HN's engagement with BAX may not follow a simple lock-and-key helix docking model.\n\nA second unresolved complication is the redox context. BAX inhibition is most consequential in the cytosol and at the cytosolic face of the mitochondrial outer membrane — environments that are generally reducing, maintained by glutathione at millimolar concentrations. A disulfide-bridged peptide delivered extracellularly or administered systemically may undergo disulfide reduction before reaching its intracellular target, effectively reverting to the unconstrained wild-type-like sequence. This does not negate the fold's value as a structural hypothesis test, but it is a critical translational constraint that would need to be addressed through cell-penetrating delivery with disulfide-reducing pathway bypass, or through replacement of the disulfide with a non-reducible hydrocarbon staple or lactam bridge in subsequent iterations.\n\nThe heuristic peptide property profile shows moderate aggregation propensity (0.342) and stability (0.476), consistent with a peptide that is neither highly aggregation-prone nor robustly stable. BBB penetration is predicted low (0.242), appropriate for a peptide targeting intracellular BAX rather than CNS targets. Half-life is estimated moderate-to-long (1–6 hours), though this will depend heavily on whether the disulfide remains intact in vivo. No Boltz-2 affinity module values or Chai-1 agreement scores were available for this fold, limiting the depth of interface analysis.\n\nIn the context of the lab's running LONGEVITY series, this fold represents the first cyclization-class modification and the first conformation-focused hypothesis in the recent rotation (following Epitalon terminal amidation in Fold #21, MOTS-c K13R substitution in Fold #19, SS-31 naphthylalanine substitution in Fold #17, and FOXO4-DRI tail truncation in Fold #12). The FOXO4-DRI work (Fold #12) is thematically adjacent — both target apoptosis-regulatory mechanisms — though FOXO4-DRI operates via p53 engagement rather than direct BAX inhibition. The present fold advances the lab into new modification chemistry territory. The PROMISING verdict is appropriate: the structural signal is suggestive but not confirmatory, and the biological rationale carries both strong supporting evidence and genuine mechanistic complications that keep this from REFINED status.","executive_summary":"S14C-Humanin: disulfide loop across Cys8–Cys14 predicted to partially pre-organize the BAX-binding helix (pLDDT 0.558), but ipTM 0.357 leaves the interface unresolved. PROMISING signal — redox sensitivity and the HNG flexibility paradox are the critical next tests.","tweet_draft":"DISTILLATION №22 — promising.\nHumanin S14C: disulfide bridge engineered to lock the BAX-binding helix.\npLDDT 0.558 | ipTM 0.357 — helix geometry suggestive, interface unconfirmed.\nRedox stability in the cytosol: the key open question.\nIn silico only. alembic.bio","research_brief_markdown":"# DISTILLATION №22 — PROMISING\n## Humanin S14C: Disulfide-Bridged Cyclization to Pre-Organize the BAX-Binding Helix\n\n**Peptide:** Humanin (modified) — MAPRGFSCLLLLT**C**EIDLPVKRRA  \n**Class:** LONGEVITY  \n**Target:** BAX (UniProt Q07812)  \n**Modification:** Ser-14 → Cys, forming an intramolecular disulfide with native Cys-8 (i,i+6 register)  \n**Verdict:** PROMISING  \n**pLDDT:** 0.558 | **pTM:** 0.470 | **ipTM:** 0.357\n\n---\n\n## Mechanism of Action\n\nHumanin (HN) is a 24-amino acid mitochondrial-derived peptide that directly binds the pro-apoptotic BCL-2 family protein BAX and prevents its activation cascade. Under apoptotic stress, BAX undergoes conformational activation, exposes its N-terminus and BH3 domain, translocates from the cytosol to the mitochondrial outer membrane, oligomerizes, and drives MOMP — releasing cytochrome c and committing the cell to caspase-dependent death. HN intercepts this cascade upstream: Guo et al. (2003, PMID:12732850) demonstrated by co-immunoprecipitation and functional assay that HN physically binds BAX, blocks its conformational change, and prevents mitochondrial translocation. This is a direct, physical anti-apoptotic mechanism — not a downstream signaling effect.\n\nMorris et al. (2019, PMID:31690630) extended this picture using CD spectroscopy, fluorescence, and negative-stain EM, revealing that HN and BAX co-form fibers with BAX undergoing secondary and tertiary structural rearrangements upon engagement. Critically, HN mutations that alter anti-apoptotic potency also alter fiber morphology — directly linking HN's own conformation to its functional output. Luciano et al. (2005, PMID:15661735) demonstrated HN also inhibits BimEL and other BCL-2 family pro-apoptotic members, but the BAX axis is the most mechanistically resolved and is the focus of this distillation.\n\n---\n\n## Modification Rationale\n\nNative HN is largely disordered in solution. Its anti-apoptotic function requires the central amphipathic segment — spanning approximately residues 7–17, encompassing the LLLL hydrophobic tract and the EIDL acidic region — to adopt an α-helical conformation competent to engage the BAX hydrophobic groove or fiber interface. This conformational disorder imposes an entropic cost upon binding: HN must fold-upon-binding, reducing the effective affinity relative to a pre-organized helix.\n\nThe S14C modification exploits the presence of native Cys-8 to introduce a disulfide bridge with minimal mutational footprint — only a single substitution is required. An i,i+6 disulfide spans approximately two turns of an α-helix (Cα–Cα ~9–10 Å, compatible with SS geometry) and is well-established in the constrained peptide literature as an effective helix-nucleating constraint. Ser-14 was selected because it sits on the solvent-facing, polar face of the helix in the predicted helical model, away from the hydrophobic Leu-rich face hypothesized to contact BAX — minimizing disruption to the binding interface while enforcing backbone geometry.\n\nThis modification also opens a new experimental avenue for the lab: DISTILLATION №22 is the first **cyclization-class** modification in the LONGEVITY series, complementing the substitution strategies explored in Fold #19 (MOTS-c K13R) and Fold #17 (SS-31 Nal), and providing a structural pre-organization hypothesis not previously tested on any peptide in the recent rotation.\n\n---\n\n## Predicted Properties — Where the Signal is Moderate\n\n| Metric | Value | Interpretation |\n|--------|-------|----------------|\n| pLDDT | 0.558 | Moderate — partial helical order predicted in central segment |\n| pTM | 0.470 | Reasonable overall fold topology |\n| ipTM | 0.357 | **Low** — interface pose not confidently modeled |\n| Cys8–Cys14 geometry | i,i+6, Cα–Cα consistent with SS | Suggestive of disulfide compatibility |\n| Hydrophobic face (Leu8–12) | Partially solvent-exposed | Not unambiguously docked into BAX groove |\n| Aggregation propensity (heuristic) | 0.342 | Moderate — manageable |\n| Stability score (heuristic) | 0.476 | Moderate |\n| BBB penetration (heuristic) | 0.242 | Low — appropriate for intracellular target |\n| Half-life estimate (heuristic) | Moderate-to-long (~1–6 hr) | Redox-dependent caveat applies |\n\nThe structural prediction places S14C-Humanin near BAX with a partially helical central segment and a Cys8–Cys14 register consistent with the intended disulfide geometry. This is the predicted structure's most encouraging signal: the folding algorithm independently arrives at a helical register that would accommodate the proposed SS bond, without being explicitly told to enforce it. However, current AF/Chai tools do **not** model disulfide bonds as covalent constraints — the SS bond is inferred from geometry, not enforced, which means the actual constrained peptide could behave differently.\n\nThe low ipTM (0.357) is the primary limiting factor: the docking interface between peptide and BAX is not confidently resolved. The hydrophobic Leu face is partially exposed but not unambiguously positioned in the BAX hydrophobic groove. The PROMISING verdict reflects genuine structural signal in the peptide itself, not a confirmed binding event.\n\n---\n\n## What Would Strengthen This Signal\n\n**Computational next steps:**\n\n1. **Ensemble prediction with explicit SS constraint:** Run the S14C variant with explicit disulfide bond modeling in tools that support covalent constraint definition (e.g., RosettaFold with constraint files, or Schrödinger's CovDock). A single unconstrained prediction cannot capture the true effect of the covalent loop on backbone geometry.\n\n2. **MD simulation of constrained vs. unconstrained HN:** Molecular dynamics with the disulfide bond explicitly enforced (GROMACS/AMBER with SS bonding) would quantify the change in helical content and conformational entropy of residues 8–14, directly testing the hypothesis that the bridge pre-organizes the helix.\n\n3. **Chai-1 / Boltz-2 affinity module re-run:** This fold lacked Chai-1 agreement scores and Boltz-2 affinity values. A second-model agreement run would substantially increase or decrease confidence in the predicted pose.\n\n4. **Comparison fold — lactam analog:** Run the lactam-bridged equivalent (Lys-8, Glu-14 → lactam, or vice versa) as a non-reducible helix staple at the same i,i+6 position. If the lactam variant shows higher ipTM and similar helix geometry, it would both validate the pre-organization concept and sidestep the redox limitation (see below).\n\n5. **Wild-type Humanin baseline fold:** A direct structural comparison fold of native HN against BAX would establish a pLDDT/ipTM baseline, allowing the S14C modification to be assessed as a delta rather than in isolation.\n\n**Critical biological question to resolve computationally:**\n\nThe S14G substitution (HNG) is the most potent known HN analog, and Gly at position 14 specifically increases local flexibility — the opposite of disulfide constraint. A direct structural comparison fold of HNG (S14G) vs. S14C-Humanin against BAX, under identical prediction conditions, would test whether the predictor favors the flexible or rigid variant for BAX engagement. If HNG scores higher ipTM than S14C-HN, the flexibility hypothesis would be supported and the disulfide strategy would warrant reconsidering. This comparison fold is the single most informative next experiment available without wet lab resources.\n\n**Wet lab validation priorities (if resources allow):**\n\n1. Oxidative folding and HPLC verification of disulfide bond formation in the purified S14C peptide.\n2. CD spectroscopy comparing S14C-HN vs. native HN helical content in buffer — directly testing the pre-organization hypothesis.\n3. BAX pull-down or SPR binding assay under non-reducing conditions (to maintain disulfide) vs. reducing conditions (to demonstrate disulfide-dependence of any affinity gain).\n4. Cell-based apoptosis assay (staurosporine or H₂O₂ challenge) in reducing intracellular context — the critical test of whether disulfide-mediated pre-organization survives to the relevant cellular compartment.\n\n---\n\n## Unresolved Complications\n\nTwo issues carry enough weight to be flagged prominently:\n\n**1. The HNG paradox.** Position 14 is the defining residue of HNG, the most potent HN analog — but HNG's Gly substitution increases flexibility, not rigidity. If enhanced potency at position 14 is mechanistically linked to local conformational freedom (enabling induced-fit engagement or fiber-competent rearrangement), the disulfide constraint is working against the grain of the SAR data at this exact position. This is not a reason to discard the hypothesis — it may be that HNG's potency derives from reduced steric clash rather than increased flexibility — but it is a genuine contradicting signal that the next experiments should be designed to resolve.\n\n**2. Redox context.** BAX inhibition is required in the cytosol — a reducing environment maintained by millimolar glutathione. A disulfide-bridged peptide delivered extracellularly would need to reach the cytosol with its SS bond intact, which is not guaranteed. If disulfide reduction occurs before intracellular delivery, the modification provides no advantage over native HN. This is a fundamental translational constraint. Non-reducible staples (hydrocarbon, lactam) at the same i,i+6 position would be the rational next iteration if wet lab data confirms pre-organization benefit.\n\n---\n\n> **In silico only. All properties are computational predictions, not experimental measurements. This report does not constitute medical advice. Heuristic peptide properties (aggregation, stability, BBB, half-life) are sequence-based estimates, not validated assay results. Disulfide bond geometry is inferred from predicted Cα positions; the SS bond is not explicitly modeled by the prediction tools used.**","structural_caption":"The model places the S14C-Humanin variant near BAX with a partially helical central segment, but interface confidence (ipTM 0.36) is too low to specify a defined binding pose. The Cys8-Cys14 pair would span an i,i+6 register consistent with one disulfide-bridged helical turn, though the predictor does not explicitly model the SS bond and pLDDT in the loop region remains modest. The hydrophobic Leu face is partially solvent-exposed but not unambiguously docked into a BAX groove. Overall, the structure is suggestive of helix pre-organization but does not confirm productive BAX engagement.","key_findings_summary":"Humanin (HN) is a 24-amino acid mitochondrial-derived peptide with well-established cytoprotective and anti-apoptotic properties. Its interaction with BAX is one of the most mechanistically studied aspects of its biology. The foundational work by Guo et al. (2003, PMID:12732850) demonstrated that HN directly binds BAX, prevents its conformational activation, blocks its translocation from cytosol to mitochondria, and suppresses cytochrome c release — establishing that the HN–BAX interaction is a bona fide physical and functional relationship, not merely a downstream signaling effect. Critically, the mitochondrial-encoded version of HN was also shown to bind and suppress BAX, suggesting the interaction is evolutionarily conserved and central to HN's mechanism of action.\n\nThe most directly relevant structural and mechanistic study is Morris et al. (2019, PMID:31690630), which demonstrated using CD spectroscopy, light scattering, fluorescence spectroscopy, and negative-stain EM that HN and BAX co-form fibers in vitro, with BAX undergoing secondary and tertiary structural rearrangements upon engagement. Importantly, HN mutations that alter anti-apoptotic activity also alter fiber morphology, directly linking HN's conformation to its BAX-inhibitory function. The study also identified BAX's C-terminal helix as important for the fibrillation process. This work is the closest existing evidence to structural characterization of the HN–BAX interaction, though it stops short of defining which residues of HN engage the BAX groove at atomic resolution.\n\nNiikura et al. (2004, PMID:15655255) provided early evidence that intracellularly overexpressed HN suppresses mitochondria-mediated apoptosis by inhibiting BAX activity specifically, and noted that HN acts through multiple mechanisms — both intracellular (BAX inhibition) and extracellular (receptor-mediated signaling). Luciano et al. (2005, PMID:15661735) extended HN's anti-apoptotic reach to BimEL, demonstrating that HN can inhibit multiple pro-apoptotic BCL-2 family members, with mutants that fail to bind BimEL also failing to protect cells, underscoring structure-activity relationships within HN. Together, these findings indicate that specific regions of HN are responsible for engaging pro-apoptotic proteins, supporting the hypothesis that conformational pre-organization of HN could enhance its binding potency.\n\nWith respect to the specific modification proposed — an i,i+6 disulfide bridge between native Cys-8 and a new Cys at position 14 (S14C) — no published literature directly addresses this modification or characterizes the structural role of Cys-8 within the central helical segment of HN. The broader literature confirms HN adopts helical character (implicit in CD spectroscopy data from Morris et al.) and that mutations at various positions alter activity, but atomistic data on residues 8-14 and their contribution to BAX groove engagement is absent from the available corpus. The HNG (S14G) analogue is extensively studied as a more potent variant (referenced in the renoprotection preprint, DOI:10.21203/rs.3.rs-9040130/v1), where Gly substitution at position 14 is known to increase biological potency — notably, this is the exact position proposed for Cys substitution in the current hypothesis, making it a structurally sensitive locus."},"structured":{"known_activity":null,"known_binders":null,"candidate_variants":null,"domain_annotations":null,"literature_context":{"pubmed":[{"pmid":"34626748","title":"The role of humanin in the regulation of reproduction.","abstract":"Humanin, a mitochondria-derived peptide, has been found to exert variously protective function in many tissues, especially in the nervous tissues. However, relatively limited studies have focused on the role of humanin in the regulation of reproduction. Current observations indicate that humanin plays an important role in regulating the response of the cell to oxidative stress and apoptosis in ovaries and testes via the modulation of several signaling pathways, especially when the body is in an abnormal state. Even so, the detailed mechanism of humanin function needs to be explored urgently. In this passage, we demonstrate how humanin exerts its protective role in female and male reproduction and raise several questions that need further investigations. Given humanin's new frontier for the design of novel therapeutic approaches for male infertility, male contraception, female infertility, and glucose metabolism in polycystic ovary syndrome, it is worthy of further study on its protective effects and clinical applications in reproductive function.","authors":["Lei Hui","Rao Meng"],"year":2022,"journal":"Biochimica et biophysica acta. General subjects"},{"pmid":"37106758","title":"Humanin and Its Pathophysiological Roles in Aging: A Systematic Review.","abstract":"BACKGROUND: Senescence is a cellular aging process in all multicellular organisms. It is characterized by a decline in cellular functions and proliferation, resulting in increased cellular damage and death. These conditions play an essential role in aging and significantly contribute to the development of age-related complications. Humanin is a mitochondrial-derived peptide (MDP), encoded by mitochondrial DNA, playing a cytoprotective role to preserve mitochondrial function and cell viability under stressful and senescence conditions. For these reasons, humanin can be exploited in strategies aiming to counteract several processes involved in aging, including cardiovascular disease, neurodegeneration, and cancer. Relevance of these conditions to aging and disease: Senescence appears to be involved in the decay in organ and tissue function, it has also been related to the development of age-related diseases, such as cardiovascular conditions, cancer, and diabetes. In particular, senescent cells produce inflammatory cytokines and other pro-inflammatory molecules that can participate to the development of such diseases. Humanin, on the other hand, seems to contrast the development of such conditions, and it is also known to play a role in these diseases by promoting the death of damaged or malfunctioning cells and contributing to the inflammation often associated with them. Both senescence and humanin-related mechanisms are complex processes that have not been fully clarified yet. Further research is needed to thoroughly understand the role of such processes in aging and disease and identify potential interventions to target them in order to prevent or treat age-related conditions.\n\nOBJECTIVES: This systematic review aims to assess the potential mechanisms underlying the link connecting senescence, humanin, aging, and disease.","authors":["Coradduzza Donatella","Congiargiu Antonella","Chen Zhichao","Cruciani Sara","Zinellu Angelo","Carru Ciriaco","Medici Serenella"],"year":2023,"journal":"Biology"},{"pmid":"34626746","title":"Humanin and Alzheimer's disease: The beginning of a new field.","abstract":"BACKGROUND: Humanin (HN) is an endogenous peptide factor and known as a member of mitochondrial-derived peptides. We first found the gene encoding this novel 24-residue peptide in a brain of an Alzheimer's disease (AD) patient as an antagonizing factor against neuronal cell death induced by AD-associated insults.\n\nSCOPE OF REVIEW: This review presents an overview of HN actions in AD-related conditions among its wide range of action spectrum as well as a brief history of the discovery.\n\nMAJOR CONCLUSIONS: HN exhibits multiple intracellular and extracellular anti-cell death actions and antagonizes various AD-associated pathomechanisms including amyloid plaque accumulation.\n\nGENERAL SIGNIFICANCE: This review concisely reflects accumulated knowledge on HN since the discovery focusing on its functions related to AD pathogenesis and provides a perspective to its potential contribution in AD treatments.","authors":["Niikura Takako"],"year":2022,"journal":"Biochimica et biophysica acta. General subjects"},{"pmid":"27082450","title":"Humanin: Functional Interfaces with IGF-I.","abstract":"Humanin is the first newly discovered peptide encoded in the mitochondrial genome in over three decades. It is the first member of a novel class of mitochondrial derived peptides. This small, 24 amino acid peptide was initially discovered to have neuroprotective effects and subsequent experiments have shown that it is beneficial in a diverse number of disease models including stroke, cardiovascular disease, and cancer. Over a decade ago, our lab found that humanin bound IGFBP-3 and more recent studies have found it to decrease circulating IGF-I levels. In turn, IGF-I also seems to regulate humanin levels and in this review, we cover the known interaction between humanin and IGF-I. Although the exact mechanism for how humanin and IGF-I regulate each other still needs to be elucidated, it is clear that humanin is a new player in IGF-I signaling.","authors":["Xiao J","Kim S-J","Cohen P","Yen K"],"year":2016,"journal":"Growth hormone & IGF research : official journal of the Growth Hormone Research Society and the International IGF Research Society"},{"pmid":"34896254","title":"Cardio-protective role of Humanin in myocardial ischemia-reperfusion.","abstract":"Mitochondria-derived peptides (MDPs) are bioactive peptides encoded by and secreted from the mitochondria. To date, a few MDPs including humanin, MOTS-c and SHLP1-6, and their diverse biological functions have been identified. The first and most studied MDP is humanin, a 24-amino-acid poly peptide. It was first identified in 2001 in the surviving neurons of patient with Alzheimer's disease, and since then has been well characterized for its neuro-protective effect through inhibition of apoptosis. Over the past two decades, humanin has been reported to play critical roles in aging as well as multiple diseases including metabolic disorders, cardiovascular diseases, and autoimmune disease. Humanin has been shown to modulate multiple biological processes including autophagy, ER stress, cellular metabolism, oxidative stress, and inflammation. A role for humanin has been shown in a wide range of cardiovascular diseases, such as coronary heart disease, atherosclerosis, and myocardial fibrosis. In this minireview, we will summarize the literature demonstrating a role for humanin in cardio-protection following myocardial ischemia-reperfusion induced injury and the potential mechanisms that mediate it.","authors":["Gong Zhenwei","Goetzman Eric","Muzumdar Radhika H"],"year":2022,"journal":"Biochimica et biophysica acta. General subjects"},{"pmid":"33130077","title":"Humanin: A mitochondrial-derived peptide in the treatment of apoptosis-related diseases.","abstract":"Humanin (HN) is a small mitochondrial-derived cytoprotective polypeptide encoded by mtDNA. HN exhibits protective effects in several cell types, including leukocytes, germ cells, neurons, tissues against cellular stress conditions and apoptosis through regulating various signaling mechanisms, such as JAK/STAT pathway and interaction of BCL-2 family of protein. HN is an essential cytoprotective peptide in the human body that regulates mitochondrial functions under stress conditions. The present review aims to evaluate HN peptide's antiapoptotic activities as a potential therapeutic target in the treatment of cancer, diabetes mellitus, male infertility, bone-related diseases, cardiac diseases, and brain diseases. Based on in vitro and in vivo studies, HN significantly suppressed the apoptosis during the treatment of bone osteoporosis, cardiovascular diseases, diabetes mellitus, and neurodegenerative diseases. According to accumulated data, it is concluded that HN exerts the proapoptotic activity of TNF-α in cancer, which makes HN as a novel therapeutic agent in the treatment of cancer and suggested that along with HN, the development of another mitochondrial-derived peptide could be a viable therapeutic option against different oxidative stress and apoptosis-related diseases.","authors":["Hazafa Abu","Batool Ammara","Ahmad Saeed","Amjad Muhammad","Chaudhry Sundas Nasir","Asad Jamal","Ghuman Hasham Feroz","Khan Hafiza Madeeha","Naeem Muhammad","Ghani Usman"],"year":2021,"journal":"Life sciences"},{"pmid":"35432758","title":"Humanin and diabetes mellitus: A review of","abstract":"Humanin (HN) is a 24-amino acid mitochondrial-derived polypeptide with cyto-protective and anti-apoptotic effects that regulates the mitochondrial functions under stress conditions. Accumulating evidence suggests the role of HN against age-related diseases, such as Alzheimer's disease. The decline in insulin action is a metabolic feature of aging and thus, type 2 diabetes mellitus is considered an age-related disease, as well. It has been suggested that HN increases insulin sensitivity, improves the survival of pancreatic beta cells, and delays the onset of diabetes, actions that could be deployed in the treatment of diabetes. The aim of this review is to present the in vitro and in vivo studies that examined the role of HN in insulin resistance and diabetes and to discuss its newly emerging role as a therapeutic option against those conditions.","authors":["Boutari Chrysoula","Pappas Panagiotis D","Theodoridis Theodoros D","Vavilis Dimitrios"],"year":2022,"journal":"World journal of diabetes"},{"pmid":"15655255","title":"Humanin: after the discovery.","abstract":"Humanin (HN) is a novel neuroprotective factor that consists of 24 amino acid residues. HN suppresses neuronal cell death caused by Alzheimer's disease (AD)-specific insults, including both amyloid-beta (betaAbeta) peptides and familial AD-causative genes. Cerebrovascular smooth muscle cells are also protected from Abeta toxicity by HN, suggesting that HN affects both neuronal and non-neuronal cells when they are exposed to AD-related cytotoxicity. HN peptide exerts a neuroprotective effect through the cell surface via putative receptor(s). HN activates a cellular signaling cascade that intervenes (at least) in activation of c-Jun N-terminal kinase. The highly selective effect of HN on AD-relevant cell death indicates that HN is promising for AD therapy. Additionally, a recent study showed that intracellularly overexpressed HN suppressed mitochondria-mediated apoptosis by inhibiting Bax activity.","authors":["Niikura Takako","Chiba Tomohiro","Aiso Sadakazu","Matsuoka Masaaki","Nishimoto Ikuo"],"year":2004,"journal":"Molecular neurobiology"},{"pmid":"31690630","title":"Humanin induces conformational changes in the apoptosis regulator BAX and sequesters it into fibers, preventing mitochondrial outer-membrane permeabilization.","abstract":"The mitochondrial, or intrinsic, apoptosis pathway is regulated mainly by members of the B-cell lymphoma 2 (BCL-2) protein family. BCL-2-associated X apoptosis regulator (BAX) plays a pivotal role in the initiation of mitochondria-mediated apoptosis as one of the factors causing mitochondrial outer-membrane permeabilization (MOMP). Of current interest are endogenous BAX ligands that inhibit its MOMP activity. Mitochondrial-derived peptides (MDPs) are a recently identified class of mitochondrial retrograde signaling molecules and are reported to be potent apoptosis inhibitors. Among them, humanin (HN) has been shown to suppress apoptosis by inhibiting BAX translocation to the mitochondrial outer membrane, but the molecular mechanism of this interaction is unknown. Here, using recombinant protein expression, along with light-scattering, CD, and fluorescence spectroscopy, we report that HN and BAX can form fibers together in vitro Results from negative stain EM experiments suggest that BAX undergoes secondary and tertiary structural rearrangements and incorporates into the fibers, and that its membrane-associating C-terminal helix is important for the fibrillation process. Additionally, HN mutations known to alter its anti-apoptotic activity affect fiber morphology. Our findings reveal for the first time a potential mechanism by which BAX can be sequestered by fibril formation, which can prevent it from initiating MOMP and committing the cell to apoptosis.","authors":["Morris Daniel L","Kastner David W","Johnson Sabrina","Strub Marie-Paule","He Yi","Bleck Christopher K E","Lee Duck-Yeon","Tjandra Nico"],"year":2019,"journal":"The Journal of biological chemistry"},{"pmid":"12732850","title":"Humanin peptide suppresses apoptosis by interfering with Bax activation.","abstract":"Bax (Bcl2-associated X protein) is an apoptosis-inducing protein that participates in cell death during normal development and in various diseases. Bax resides in an inactive state in the cytosol of many cells. In response to death stimuli, Bax protein undergoes conformational changes that expose membrane-targeting domains, resulting in its translocation to mitochondrial membranes, where Bax inserts and causes release of cytochrome c and other apoptogenic proteins. It is unknown what controls conversion of Bax from the inactive to active conformation. Here we show that Bax interacts with humanin (HN), an anti-apoptotic peptide of 24 amino acids encoded in mammalian genomes. HN prevents the translocation of Bax from cytosol to mitochondria. Conversely, reducing HN expression by small interfering RNAs sensitizes cells to Bax and increases Bax translocation to membranes. HN peptides also block Bax association with isolated mitochondria, and suppress cytochrome c release in vitro. Notably, the mitochondrial genome contains an identical open reading frame, and the mitochondrial version of HN can also bind and suppress Bax. We speculate therefore that HN arose from mitochondria and transferred to the nuclear genome, providing a mechanism for protecting these organelles from Bax.","authors":["Guo Bin","Zhai Dayong","Cabezas Edelmira","Welsh Kate","Nouraini Shahrzad","Satterthwait Arnold C","Reed John C"],"year":2003,"journal":"Nature"},{"pmid":"15106598","title":"Unravelling the role of Humanin.","abstract":"Humanin (HN), a recently identified neuroprotective factor against Alzheimer's disease-related insults, has been reported to function as an anti cell-death factor through multiple mechanisms. One mechanism, revealed in a glioblastoma cell line, involves the apoptosis-inducing protein Bax. This, in addition to the fact that HN is produced in certain normal tissues, such as testis, implies a potential role of HN in oncogenesis. A second mechanism, in neuronal cells, is via a putative cell-surface receptor. It is through this mechanism that HN exhibits its neuroprotective activity.","authors":["Nishimoto Ikuo","Matsuoka Masaaki","niikura Takako"],"year":2004,"journal":"Trends in molecular medicine"},{"pmid":"15661735","title":"Cytoprotective peptide humanin binds and inhibits proapoptotic Bcl-2/Bax family protein BimEL.","abstract":"Humanin (HN) is a recently identified endogenous peptide that protects cells against cytotoxicity induced by various stimuli. Recently, we showed that HN binds to and inhibits Bax, a proapoptotic Bcl-2 family protein, suggesting a mechanism for HN action. In this study, we identified Bim, a Bcl-2 homology 3-only member of the Bcl-2/Bax family, as an additional HN target protein. Using in vitro protein binding, immunoprecipitation, and coimmunolocalization assays, we demonstrated that HN binds directly to the extra long isoform of Bim (BimEL) but not the long (BimL) or short (BimS) isoforms. HN also protects cells against apoptosis induced by BimEL but not BimL and BimS in gene transfection studies. In contrast, mutants of HN which failed to bind BimEL failed to protect from BimEL-induced cell death. Moreover, HN inhibited BimEL-induced release of SMAC and cytochrome c from mitochondria isolated from bax-/-cells, indicating that HN can suppress BimEL independently of its effect on Bax. Finally, we demonstrate that HN prevents BimEL-induced oligomerization of Bak using isolated mitochondria. Taken together, our results indicate that the inhibition of BimEL may contribute to the antiapoptotic properties of the HN peptide.","authors":["Luciano Frederic","Zhai Dayong","Zhu Xiuwen","Bailly-Maitre Beatrice","Ricci Jean-Ehrland","Satterthwait Arnold C","Reed John C"],"year":2005,"journal":"The Journal of biological chemistry"}],"biorxiv":[{"pmid":"","doi":"10.21203/rs.3.rs-9040130/v1","title":"Renoprotective Effect of S14G-Humanin on Renal Ischemia/Reperfusion Injury by Activation of STAT3 and ERK 1/2 Signal Transduction Pathways in Rats","abstract":"<title>Abstract</title>  <p>Renal ischemia/reperfusion (I/R) injury leads to acute tubular necrosis and renal failure, triggering pathological mechanisms including inflammation, reactive oxygen species generation, apoptosis, and mitochondrial dysfunction. The mitochondrial peptide Humanin (HN), known to possess anti-apoptotic and anti-inflammatory properties, has been shown to counteract oxidative stress and restore mitochondrial function. This study aimed to investigate the effects of HN on renal I/R injury. Sprague-Dawley male rats were divided into four groups (n = 48): 1.Sham, 2.I/R, 3.HN-Sham, 4.HN-I/R. In I/R groups, renal artery ligation was performed for 45 minutes followed by 24-hour reperfusion. Humanin G (HNG) (2 mg/kg, iv) was administered 10 minutes before reperfusion. Urine was collected during reperfusion, and the experiment was terminated by collecting blood and tissue samples. Blood urea nitrogen and serum creatinine levels were elevated in the I/R group and were not affected by HNG treatment. Glutathione levels as well as superoxide dismutase activities, which were diminished in the I/R group, were significantly restored following HNG administration. Myeloperoxidase activity and malondialdehyde levels were significantly decreased in HN-I/R group compared to the I/R group. ATP levels and mitochondrial Complex I activity were significantly increased in the HN-I/R group compared to I/R. The percentage of apoptotic cells, markedly increased in I/R, was significantly reduced in HN-I/R. STAT3 and ERK 1/2 phosphorylation also increased in HN-I/R rats compared to I/R animals. HNG exerts a protective effect against renal I/R injury by attenuating oxidative stress, inflammation, and apoptosis while enhancing antioxidant capacity and mitochondrial function, through STAT3 and/or ERK 1/2 activation.</p>","authors":["Abueid L","Torun AF","Golal E","Acar N","Basralı F."],"year":2026,"journal":"PPR","source":"PPR","preprint":true},{"pmid":"","doi":"10.20944/preprints202604.0328.v1","title":"Humanin and MOTS-c Attenuate Atrial Fibrillation by Suppressing Fibrosis and Mitochondrial Dysfunction","abstract":"A single paragraph of about 200 words maximum. For research articles, abstracts should give a pertinent overview of the work. We strongly encourage authors to use the following style of structured abstracts, but without headings: (1) Background: Place the question addressed in a broad context and highlight the purpose of the study; (2) Methods: briefly describe the main methods or treatments applied; (3) Results: summarize the article’s main findings; (4) Conclusions: indicate the main conclusions or interpretations. The abstract should be an objective representation of the article and it must not contain results that are not presented and substantiated in the main text and should not exaggerate the main conclusions.","authors":["Liao Y","Xu J","Jiao Y","Sun X","Gao M","Ding Y","Cai D","Shen Y","Zhou X","Han W."],"year":2026,"journal":"PPR","source":"PPR","preprint":true},{"pmid":"","doi":"10.1101/2025.08.26.25334376","title":"Serum mtDNA DAMP abundance, fragmentation and heteroplasmic variants associate with Acute Respiratory Failure outcome: A secondary analysis of study NCT00976833","abstract":"<h4>Background</h4>  Serum mitochondrial DNA (mtDNA) fragments act as proinflammatory damage-associated molecular patterns (DAMPs), and have been linked to outcomes in critical illness. However, their prognostic value remains uncertain, possibly due to confounding nuclear mitochondrial insertions (NUMTs) which obscure both quantitation and variant detection. <h4>Methods</h4>  Using a targeted deep sequencing and bioinformatics workflow, we created filtering strategies to minimize NUMT-related artifacts. To evaluate the method, we performed a secondary analysis of serum samples collected from  NCT00976833 , a study of acute respiratory failure patients. By modeling DNA insert size distributions, we excluded likely NUMT-derived DNA fragments based on their size, improving the accuracy of mtDNA DAMP fragmentomic analysis. To improve variant detection, we introduced a novel “read mismatch percentage” metric to identify NUMT-induced chimeric read pairs, enabling identification of mtDNA variants.  <h4>Results</h4>  Mean NUMT-depleted, but not raw, mtDNA insert size was lower in non-survivors. Short DNA inserts (<150 bp) displayed little NUMT contamination, and their abundance and size correlated with mortality more strongly than total mtDNA abundance. Sequence variants were called and some associated with survival and post-acute quality of life. Variant m.1,719G  > A, found in small humanin-like 3 (  MT-SHLP3 ), associated with survival. Other variants associated with overall poor outcome (non-survival or poor QoL). Two noncoding variants previously associated with low VO2 max and coronary artery disease (m.295C  > T and m.462C  > T) also associated with poor outcome in the present study. Two  MT-ND5 variants m.13,708G  > A (a missense variant previously implicated in kidney dysfunction) and m.12,612A  > G (a synonymous variant previously associated with coronary artery disease) also associated with poor overall outcome.  <h4>Conclusions</h4>  Our results addressed limitations of standard qPCR-based methods for the study of mtDNA DAMPs. Beyond addressing confounding NUMT, the method identified fragmentomic and variant associations overlooked by qPCR. Cell-free DNA fragmentomic and variant information are well-established biomarkers for cancer, and this method could facilitate similar patient-specific biomarkers in the context of critical illness. The method is composed of commercially available reagents and open source software, which could additionally promote adoption and reproducibility.","authors":["Daly GT","Hartsell EM","Pastukh VM","Roberts JT","Haastrup AI","Purcell LD","Mulekar MS","Files DC","Morris PE","Gillespie MN","Langley RJ."],"year":2025,"journal":"PPR","source":"PPR","preprint":true}],"preprints":[{"pmid":"","doi":"10.21203/rs.3.rs-9040130/v1","title":"Renoprotective Effect of S14G-Humanin on Renal Ischemia/Reperfusion Injury by Activation of STAT3 and ERK 1/2 Signal Transduction Pathways in Rats","abstract":"<title>Abstract</title>  <p>Renal ischemia/reperfusion (I/R) injury leads to acute tubular necrosis and renal failure, triggering pathological mechanisms including inflammation, reactive oxygen species generation, apoptosis, and mitochondrial dysfunction. The mitochondrial peptide Humanin (HN), known to possess anti-apoptotic and anti-inflammatory properties, has been shown to counteract oxidative stress and restore mitochondrial function. This study aimed to investigate the effects of HN on renal I/R injury. Sprague-Dawley male rats were divided into four groups (n = 48): 1.Sham, 2.I/R, 3.HN-Sham, 4.HN-I/R. In I/R groups, renal artery ligation was performed for 45 minutes followed by 24-hour reperfusion. Humanin G (HNG) (2 mg/kg, iv) was administered 10 minutes before reperfusion. Urine was collected during reperfusion, and the experiment was terminated by collecting blood and tissue samples. Blood urea nitrogen and serum creatinine levels were elevated in the I/R group and were not affected by HNG treatment. Glutathione levels as well as superoxide dismutase activities, which were diminished in the I/R group, were significantly restored following HNG administration. Myeloperoxidase activity and malondialdehyde levels were significantly decreased in HN-I/R group compared to the I/R group. ATP levels and mitochondrial Complex I activity were significantly increased in the HN-I/R group compared to I/R. The percentage of apoptotic cells, markedly increased in I/R, was significantly reduced in HN-I/R. STAT3 and ERK 1/2 phosphorylation also increased in HN-I/R rats compared to I/R animals. HNG exerts a protective effect against renal I/R injury by attenuating oxidative stress, inflammation, and apoptosis while enhancing antioxidant capacity and mitochondrial function, through STAT3 and/or ERK 1/2 activation.</p>","authors":["Abueid L","Torun AF","Golal E","Acar N","Basralı F."],"year":2026,"journal":"PPR","source":"PPR","preprint":true},{"pmid":"","doi":"10.20944/preprints202604.0328.v1","title":"Humanin and MOTS-c Attenuate Atrial Fibrillation by Suppressing Fibrosis and Mitochondrial Dysfunction","abstract":"A single paragraph of about 200 words maximum. For research articles, abstracts should give a pertinent overview of the work. We strongly encourage authors to use the following style of structured abstracts, but without headings: (1) Background: Place the question addressed in a broad context and highlight the purpose of the study; (2) Methods: briefly describe the main methods or treatments applied; (3) Results: summarize the article’s main findings; (4) Conclusions: indicate the main conclusions or interpretations. The abstract should be an objective representation of the article and it must not contain results that are not presented and substantiated in the main text and should not exaggerate the main conclusions.","authors":["Liao Y","Xu J","Jiao Y","Sun X","Gao M","Ding Y","Cai D","Shen Y","Zhou X","Han W."],"year":2026,"journal":"PPR","source":"PPR","preprint":true},{"pmid":"","doi":"10.1101/2025.08.26.25334376","title":"Serum mtDNA DAMP abundance, fragmentation and heteroplasmic variants associate with Acute Respiratory Failure outcome: A secondary analysis of study NCT00976833","abstract":"<h4>Background</h4>  Serum mitochondrial DNA (mtDNA) fragments act as proinflammatory damage-associated molecular patterns (DAMPs), and have been linked to outcomes in critical illness. However, their prognostic value remains uncertain, possibly due to confounding nuclear mitochondrial insertions (NUMTs) which obscure both quantitation and variant detection. <h4>Methods</h4>  Using a targeted deep sequencing and bioinformatics workflow, we created filtering strategies to minimize NUMT-related artifacts. To evaluate the method, we performed a secondary analysis of serum samples collected from  NCT00976833 , a study of acute respiratory failure patients. By modeling DNA insert size distributions, we excluded likely NUMT-derived DNA fragments based on their size, improving the accuracy of mtDNA DAMP fragmentomic analysis. To improve variant detection, we introduced a novel “read mismatch percentage” metric to identify NUMT-induced chimeric read pairs, enabling identification of mtDNA variants.  <h4>Results</h4>  Mean NUMT-depleted, but not raw, mtDNA insert size was lower in non-survivors. Short DNA inserts (<150 bp) displayed little NUMT contamination, and their abundance and size correlated with mortality more strongly than total mtDNA abundance. Sequence variants were called and some associated with survival and post-acute quality of life. Variant m.1,719G  > A, found in small humanin-like 3 (  MT-SHLP3 ), associated with survival. Other variants associated with overall poor outcome (non-survival or poor QoL). Two noncoding variants previously associated with low VO2 max and coronary artery disease (m.295C  > T and m.462C  > T) also associated with poor outcome in the present study. Two  MT-ND5 variants m.13,708G  > A (a missense variant previously implicated in kidney dysfunction) and m.12,612A  > G (a synonymous variant previously associated with coronary artery disease) also associated with poor overall outcome.  <h4>Conclusions</h4>  Our results addressed limitations of standard qPCR-based methods for the study of mtDNA DAMPs. Beyond addressing confounding NUMT, the method identified fragmentomic and variant associations overlooked by qPCR. Cell-free DNA fragmentomic and variant information are well-established biomarkers for cancer, and this method could facilitate similar patient-specific biomarkers in the context of critical illness. The method is composed of commercially available reagents and open source software, which could additionally promote adoption and reproducibility.","authors":["Daly GT","Hartsell EM","Pastukh VM","Roberts JT","Haastrup AI","Purcell LD","Mulekar MS","Files DC","Morris PE","Gillespie MN","Langley RJ."],"year":2025,"journal":"PPR","source":"PPR","preprint":true}],"consensus_view":"The literature consensus strongly supports that Humanin directly binds BAX and inhibits its activation, translocation to mitochondria, and MOMP induction. This is established by multiple independent studies using biochemical (co-immunoprecipitation, in vitro binding), cellular, and biophysical (CD, EM, fluorescence) approaches. The interaction involves conformational changes in BAX and is sensitive to mutations in HN's sequence, indicating a specific molecular interface. The S14G substitution (HNG) is widely used as a more potent analog, demonstrating that position 14 is both mutable and functionally important. However, the literature does not provide atomic-resolution structural data on the HN–BAX complex, and the specific residues of HN that mediate groove engagement with BAX have not been mapped. Disulfide-constrained analogs of HN have not been reported in the published literature, leaving the conformational pre-organization strategy as unexplored territory.","knowledge_gaps":"Several critical gaps exist: (1) No atomic-resolution crystal or NMR structure of HN bound to BAX has been published, meaning the precise binding interface, including which face of HN's helix contacts the BAX groove, is unknown. (2) The conformational ensemble of free HN in solution has not been rigorously characterized — the degree of helical order in the native peptide (particularly residues 8-14) is not established, making it unclear how much conformational entropy is actually lost upon BAX binding. (3) No disulfide-constrained or stapled HN analogs have been reported, so the effect of covalent pre-organization on BAX binding affinity and selectivity is completely unstudied. (4) Whether the S14C mutation is tolerated for BAX binding specifically (as opposed to the S14G substitution which is known to be tolerated functionally) has not been tested. (5) The stoichiometry and structural details of the HN–BAX fiber complex reported by Morris et al. are unresolved, meaning it is unclear whether fiber formation is the relevant inhibitory mode in the cellular context or an in vitro artifact.","supporting_evidence":"The strongest supporting evidence for the hypothesis comes from: (1) Guo et al. (2003) demonstrating that HN directly and physically interacts with BAX and inhibits its conformational activation — if HN must adopt a specific conformation to engage BAX, pre-organizing that conformation should lower the entropic cost of binding and improve affinity. (2) Morris et al. (2019) showing that HN mutations altering anti-apoptotic activity alter the HN–BAX interaction, implying a conformationally sensitive binding interface where structural rigidity could be beneficial. (3) The well-established principle that i,i+6 disulfide bridges effectively nucleate and stabilize α-helices in short peptides, spanning approximately two turns of a helix as proposed. (4) The S14G analog (HNG) demonstrating that position 14 tolerates substitution while maintaining or enhancing bioactivity, suggesting the S14C mutation may be structurally feasible. (5) Luciano et al. (2005) establishing that structure-activity relationships in HN are tight — mutants with altered conformation lose function — which is consistent with a model where enforcing the bioactive conformation increases potency.","challenging_evidence":"Several lines of evidence complicate the hypothesis: (1) The S14G substitution is specifically used because Gly increases flexibility and reduces steric bulk at position 14, which is the opposite design logic to introducing a disulfide constraint at the same position. If conformational flexibility at position 14 contributes to HNG's enhanced potency, rigidifying this position via a disulfide bridge may be counterproductive. (2) Morris et al. (2019) suggest HN and BAX form a fibrillar complex rather than a simple peptide-in-groove binding mode; if the mechanistically relevant inhibitory interaction involves HN adopting an extended or fiber-competent conformation rather than a rigid helix, a disulfide-constrained loop may actually impair the interaction by preventing the necessary conformational transition. (3) No structural data confirms that residues 8-14 of HN specifically contact the BAX hydrophobic groove; the loop constraint could rigidify a region that is not directly involved in the binding interface, conferring no benefit while potentially disrupting other structural elements. (4) An i,i+6 disulfide in a helix requires correct geometry (Cβ-Cβ distance and dihedral angles must be compatible); if the native helix geometry in this region does not align with disulfide bond constraints, the modification could distort rather than stabilize the helix. (5) The introduction of a disulfide bond creates a redox-sensitive modification, potentially making the peptide inactive in reducing intracellular environments where BAX inhibition is most needed. (6) HN's anti-apoptotic activity involves multiple mechanisms beyond BAX binding (BimEL binding, JAK/STAT signaling, receptor-mediated pathways), so even if disulfide formation impairs BAX binding, readout assays measuring cell viability may obscure the specific effect on the BAX axis."},"caveats":["In silico prediction only — requires wet lab validation","Single-run prediction (not ensembled)","Predicted properties may not reflect real-world biological behavior","This is research, not medical advice","Disulfide bond geometry inferred from predicted Cα–Cα distances; the SS bond is NOT explicitly modeled as a covalent constraint by AlphaFold or related tools — actual constrained peptide behavior may differ substantially","ipTM 0.357 is below reliable threshold for confident interface interpretation — BAX docking pose is suggestive, not confirmatory","Heuristic peptide properties (aggregation 0.342, stability 0.476, BBB 0.242, half-life estimate) are sequence-based estimates, not experimental measurements","Redox caveat: disulfide bridge may be reduced in the cytosolic/mitochondrial environment where BAX inhibition is functionally required, potentially negating the pre-organization benefit in vivo","The S14G (HNG) analog data introduces a competing SAR hypothesis — local flexibility at position 14 may enhance rather than impair BAX engagement, which is the opposite of the constraint strategy tested here","No Chai-1 agreement score or Boltz-2 affinity module values available for this fold — second-model validation is absent"],"works_cited":[{"pmid_or_doi":"12732850","title":"Humanin peptide suppresses apoptosis by interfering with Bax activation","year":2003,"relevance":"Foundational study establishing direct HN–BAX physical interaction and demonstrating HN prevents BAX translocation and cytochrome c release; directly supports the premise that HN engages BAX and that this interaction is mechanistically important."},{"pmid_or_doi":"31690630","title":"Humanin induces conformational changes in the apoptosis regulator BAX and sequesters it into fibers, preventing mitochondrial outer-membrane permeabilization","year":2019,"relevance":"Most mechanistically detailed study of HN–BAX interaction; shows BAX undergoes structural rearrangements upon HN engagement and that HN mutations altering anti-apoptotic activity alter interaction morphology, directly informing how conformational changes in HN affect BAX inhibition."},{"pmid_or_doi":"15655255","title":"Humanin: after the discovery","year":2004,"relevance":"Confirms intracellular HN suppresses BAX-mediated mitochondrial apoptosis and notes HN's multiple mechanisms, providing context for BAX groove engagement as a therapeutically relevant target."},{"pmid_or_doi":"15661735","title":"Cytoprotective peptide humanin binds and inhibits proapoptotic Bcl-2/Bax family protein BimEL","year":2005,"relevance":"Demonstrates structure-activity relationships in HN: mutants that fail to bind pro-apoptotic targets fail to protect cells, supporting the importance of HN's conformation for its anti-apoptotic potency."},{"pmid_or_doi":"15106598","title":"Unravelling the role of Humanin","year":2004,"relevance":"Discusses dual mechanisms of HN action including Bax inhibition, and raises the oncogenic implications, contextualizing why modulating the HN–BAX axis has broad therapeutic relevance."},{"pmid_or_doi":"33130077","title":"Humanin: A mitochondrial-derived peptide in the treatment of apoptosis-related diseases","year":2021,"relevance":"Reviews HN's anti-apoptotic activity across disease contexts, including interaction with BCL-2 family proteins, providing support for therapeutic value of enhancing HN's BAX-inhibitory potency."},{"pmid_or_doi":"10.21203/rs.3.rs-9040130/v1","title":"Renoprotective Effect of S14G-Humanin on Renal Ischemia/Reperfusion Injury by Activation of STAT3 and ERK 1/2 Signal Transduction Pathways in Rats","year":2026,"relevance":"Uses HNG (S14G-Humanin), a variant with substitution at the exact position (14) proposed for Cys introduction; confirms position 14 is tolerant of substitution and that modified HN retains potent in vivo bioactivity, though via signaling pathways rather than direct BAX engagement."},{"pmid_or_doi":"37106758","title":"Humanin and Its Pathophysiological Roles in Aging: A Systematic Review","year":2023,"relevance":"Provides broad context on HN's cytoprotective roles and its interaction with BCL-2 family proteins, affirming the translational relevance of enhancing HN's anti-apoptotic activity."},{"pmid_or_doi":"34626746","title":"Humanin and Alzheimer's disease: The beginning of a new field","year":2022,"relevance":"Reviews HN's multiple anti-apoptotic mechanisms including BAX inhibition in the context of neurodegeneration, supporting the hypothesis that enhanced BAX binding would have neuroprotective value."},{"pmid_or_doi":"34896254","title":"Cardio-protective role of Humanin in myocardial ischemia-reperfusion","year":2022,"relevance":"Documents HN's cardioprotective effects mediated partly through apoptosis inhibition, illustrating the therapeutic contexts in which an improved BAX-binding HN variant could have clinical impact."}]},"onchain":{"hash":"5pR8AkbZWKnQNkxXdRNAoeDV3pBhrwUrGKgR3obT4omfzW3BWWVmNWuoC4AhJWtpqggYXZfZgTPs2MDig7EvATRW","signature":"5pR8AkbZWKnQNkxXdRNAoeDV3pBhrwUrGKgR3obT4omfzW3BWWVmNWuoC4AhJWtpqggYXZfZgTPs2MDig7EvATRW","data_hash":"b644d994c9b55b34b66c8d3440d6ce47afeccff1247dc72f05e4007e79fa073a","logged_at":"2026-05-03T04:37:11.909533+00:00","explorer_url":"https://solscan.io/tx/5pR8AkbZWKnQNkxXdRNAoeDV3pBhrwUrGKgR3obT4omfzW3BWWVmNWuoC4AhJWtpqggYXZfZgTPs2MDig7EvATRW"},"ipfs_hash":null,"created_at":"2026-05-03T04:22:49.225808+00:00","updated_at":"2026-05-03T04:37:11.915746+00:00"}