Humanin and the SHLP family: mitochondrial-derived peptides in ageing and metabolic disease

The discovery of humanin in 2001 established a new class of signalling molecules: mitochondrial-derived peptides (MDPs). Encoded not in the nuclear genome but within the mitochondrial 16S ribosomal RNA gene, humanin and its structural relatives — the small humanin-like peptides (SHLPs 1–6) — represent a direct communication channel from mitochondria to systemic physiology.

This is conceptually significant beyond the individual peptide. Mitochondria, which retain remnant bacterial DNA from their endosymbiotic origins, appear to have preserved peptide-coding capacity that functions in intercellular and hormonal signalling contexts extending far beyond the organelle's immediate metabolic role.

Discovery and basic biology

Humanin was identified through a cDNA library screen designed to find genes rescuing neurons from Alzheimer's disease-associated toxicity. Investigators expressed individual cDNA constructs in neurons exposed to familial Alzheimer's gene products; humanin was the construct providing cytoprotection (Hashimoto et al., Proceedings of the National Academy of Sciences, 2001).

The 21-amino acid peptide is encoded in the 16S rRNA region of mitochondrial DNA (mtDNA), though nuclear pseudogene copies capable of producing humanin protein also exist. This dual genomic redundancy is unusual and may reflect evolutionary pressure to preserve the peptide's function across multiple genetic backgrounds.

Humanin circulates in human plasma at measurable concentrations. Critically, circulating levels decline with age. In cohort analyses, humanin concentrations were significantly lower in older adults compared to younger controls, and levels correlated inversely with markers of insulin resistance and metabolic dysfunction (Muzumdar et al., Aging, 2009).

Cytoprotective mechanisms

Humanin operates through both intracellular and receptor-mediated pathways.

Intrinsic apoptosis inhibition: Humanin binds BAX, a pro-apoptotic BCL-2 family member, and inhibits its translocation to the mitochondrial outer membrane. Since BAX translocation initiates cytochrome c release and caspase cascade activation, humanin acts as an upstream checkpoint in the intrinsic apoptosis pathway. The S14G humanin analogue (HNG), with a single serine-to-glycine substitution at position 14, demonstrates approximately 1,000-fold greater potency than native humanin in cytoprotection assays — an important distinction for in vivo research design.

CNTFR/WSX-1/gp130 receptor complex: Humanin binds a heterotrimeric cell-surface receptor comprising ciliary neurotrophic factor receptor alpha (CNTFR-α), WSX-1, and gp130 — the same complex utilised by ciliary neurotrophic factor. This interaction activates JAK2/STAT3 and PI3K/Akt pathways, both of which have established anti-apoptotic and metabolic regulatory functions.

FPRL1 signalling: A second receptor, formyl peptide receptor-like 1 (FPRL1), mediates humanin signalling in neutrophils and macrophages, connecting the peptide to innate immune modulation.

Metabolic regulation and insulin sensitivity

The metabolic effects of humanin have become a major research focus, particularly regarding type 2 diabetes and age-related insulin resistance.

In multiple rodent models of diet-induced obesity and insulin resistance, humanin administration improved insulin sensitivity, reduced hepatic glucose output, and attenuated adiposity progression. The mechanism operates at both peripheral (skeletal muscle, adipose tissue) and central (hypothalamic) levels.

A key hepatic action is suppression of gluconeogenesis. Humanin reduces expression of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) — the rate-limiting enzymes of hepatic gluconeogenesis — through an Akt-dependent mechanism. This effect alone would be expected to reduce fasting blood glucose in insulin-resistant states.

Notably, calorie restriction — one of the most reproducible longevity interventions in model organisms — increases circulating humanin in rodent models. This parallel between humanin elevation and calorie restriction's metabolic effects suggests the peptide may mediate some of calorie restriction's metabolic benefits.

The SHLP family

Identified through systematic analysis of the mitochondrial 16S rRNA coding region, SHLPs 1–6 were characterised by Lee et al. as mitochondria-encoded peptides with distinct but overlapping biological activities (Nature Communications, 2015). Ranging from 20–38 amino acids, they share structural similarity with humanin while having divergent receptor affinities:

• SHLP2: Cytoprotective effects in pancreatic beta cells; increases oxidative phosphorylation in adipocytes; potential relevance to beta-cell preservation in type 2 diabetes.
• SHLP3: Similar cytoprotective profile to SHLP2; higher tissue expression in pancreatic tissue.
• SHLP6: Distinct from other family members — demonstrates pro-apoptotic activity in cancer cell lines, suggesting an oncostatic function not shared by the cytoprotective members.

The divergent function of SHLP6 indicates the mitochondrial-derived peptide system encompasses tumour suppression mechanisms that warrant independent investigation from the pro-survival functions of other family members.

Longevity associations

Centenarian offspring studies have provided observational human data supporting humanin's longevity relevance. Offspring of centenarians have significantly higher circulating humanin levels compared to age-matched controls without centenarian parentage (Yen et al., Aging, 2018).

Animal model data supports a causal interpretation. Transgenic mice overexpressing humanin show extended lifespan, reduced atherosclerotic lesion burden, improved insulin sensitivity, and preserved cognitive function relative to wild-type controls.

Conversely, humanin knockout mice exhibit accelerated ageing phenotypes: increased visceral adiposity, reduced muscle mass, impaired glucose homeostasis, and reduced lifespan — consistent with the peptide having a tonic regulatory role in metabolic ageing.

Alzheimer's disease research

Humanin levels in cerebrospinal fluid are lower in Alzheimer's patients than in cognitively normal age-matched controls. Post-mortem brain tissue shows reduced humanin expression in regions affected by amyloid pathology, including hippocampus and entorhinal cortex.

Mechanistically, humanin reduces amyloid beta (Aβ) oligomer-induced neurotoxicity through dual mechanisms: direct peptide-peptide interaction inhibiting Aβ fibril formation, and BAX-mediated cytoprotection downstream of Aβ toxicity. This dual mechanism positions humanin as operating at multiple nodes in the Alzheimer's pathological cascade.

Whether circulating humanin can access CNS tissue is a key translational question. Blood-brain barrier penetration by exogenous humanin has been demonstrated in rodent models using intranasal delivery, which bypasses the barrier through olfactory epithelium pathways.

Cardiovascular and renal applications

Humanin has demonstrated cardioprotective effects in ischaemia-reperfusion models, reducing infarct size and preserving ejection fraction through anti-apoptotic and anti-inflammatory mechanisms.

Renal research has identified humanin as a potentially protective signal in acute kidney injury models. Humanin reduces tubular epithelial cell apoptosis following cisplatin-induced nephrotoxicity and ischaemia-reperfusion injury — an application being explored as a nephroprotective co-treatment in chemotherapy contexts.

Research context and pharmacokinetics

Humanin and the SHLPs are research peptides without clinical approval. The primary translational barrier is pharmacokinetic: native humanin has a short plasma half-life, and achieving sustained CNS tissue concentrations requires delivery strategies beyond standard subcutaneous administration.

These mitochondrial-derived peptides complement the mitochondria-targeted synthetic compounds discussed in the SS-31 mitochondrial research article and the broader mitochondrial peptides research overview.

Summary

Humanin and the SHLPs are mitochondrial-derived peptides with well-characterised roles in cytoprotection, metabolic regulation, and ageing. Their declining circulating levels with age, association with human longevity genetics, and efficacy across multiple age-related disease models make them scientifically compelling research targets. Clinical translation faces pharmacokinetic and delivery challenges. The divergent function of SHLP6 demonstrates that individual members of this peptide family warrant independent mechanistic characterisation.