SHLP1 (Small Humanin-Like Peptide 1)

Alzheimer's Disease Research

SHLP1 has shown neuroprotective properties in models of Alzheimer's disease pathology. In primary cortical neuron cultures exposed to amyloid-beta 1-42 oligomers, SHLP1 treatment improved cell viability in a dose-dependent manner (Kim et al., 2018). The peptide reduced amyloid-beta-induced mitochondrial fragmentation and preserved dendritic spine density, suggesting protection of both cell survival and synaptic function.

The neuroprotective effect of SHLP1 against amyloid-beta toxicity parallels that of humanin, though with lower potency. While humanin analogue HNG protects neurons at picomolar concentrations, SHLP1 requires nanomolar to low micromolar concentrations for equivalent protection. However, the additive or synergistic effects of multiple MDPs — each declining with age — may be more physiologically relevant than individual peptide potency.

Metabolic Regulation

SHLP1 contributes to metabolic homeostasis through mechanisms that partially overlap with SHLP2 and humanin. In adipocyte cell cultures, SHLP1 modulates lipid metabolism and reduces lipid accumulation under high-glucose conditions. The peptide enhances insulin receptor substrate (IRS) phosphorylation and downstream PI3K/AKT signaling, improving cellular insulin sensitivity (Cobb et al., 2016).

Animal studies have demonstrated that exogenous SHLP1 administration improves glucose tolerance in diet-induced obesity models. Mice receiving SHLP1 by intraperitoneal injection showed reduced fasting glucose and improved insulin sensitivity as measured by homeostatic model assessment (HOMA-IR), though the metabolic effects of SHLP1 were less pronounced than those observed with SHLP2.

Apoptosis Protection

The foundational discovery of SHLP1 by Cobb et al. (2016) demonstrated that the peptide protects cells from serum starvation-induced apoptosis in vitro. In mouse embryonic fibroblasts and neuronal cell lines, SHLP1 treatment reduced caspase-3/7 activation by approximately 30–40% compared to untreated controls. The anti-apoptotic effect was dose-dependent and could be partially blocked by ERK inhibitors, confirming the involvement of MAPK signaling in SHLP1-mediated protection.

Further work established that SHLP1 protects mitochondrial membrane potential under conditions of oxidative stress. When cells were exposed to hydrogen peroxide or tert-butyl hydroperoxide, SHLP1 pre-treatment preserved mitochondrial transmembrane potential (ΔΨm) and reduced cytochrome c release into the cytoplasm. This protective effect was observed across multiple cell types including neurons, cardiomyocytes, and hepatocytes, suggesting broad cytoprotective relevance.

Age-Related Decline

Circulating SHLP1 levels decline significantly with age in both mice and humans, mirroring the decline pattern observed for humanin, SHLP2, and MOTS-c. Kim et al. (2018) demonstrated that plasma SHLP1 concentrations are approximately 50% lower in individuals over 65 compared to those under 35. This decline correlates with reduced mitochondrial DNA copy number and impaired mitochondrial transcription, suggesting that age-related mitochondrial dysfunction reduces production of the entire MDP family.

Mitochondrial-derived peptide biology

SHLP1 is one of six small humanin-like peptides (SHLP1-6) encoded by short open reading frames within the mitochondrial 16S rRNA (MT-RNR2) gene, the same region that encodes humanin; SHLP1-5 are encoded on the light (L) strand. These peptides act as retrograde signaling molecules communicating mitochondrial status to the rest of the cell, and share partial functional overlap with humanin and MOTS-c.

• Encoded in the mtDNA 16S rRNA region alongside humanin; named/characterized as a family by Cobb et al. (2016).
• SHLP1 is expressed in tissues including heart, kidney, spleen and liver.
• Functions as a mitochondrial retrograde signaling peptide within the humanin/SHLP/MOTS-c family.

Metabolic and cytoprotective signaling

The SHLP family collectively modulates mitochondrial bioenergetics, insulin sensitivity, apoptosis and inflammatory markers. The best-characterized member, SHLP2, reduces apoptosis and ROS, improves mitochondrial metabolism, enhances adipocyte differentiation, and acts as an insulin sensitizer both centrally and peripherally; SHLP1's specific mechanism is comparatively under-defined and is often inferred from family-level effects.

• SHLP2/SHLP3 reduce apoptosis and ROS and improve mitochondrial metabolism in vitro.
• SHLP2 acts as a peripheral and central insulin sensitizer in clamp studies.
• SHLP1-specific bioactivity remains less characterized than SHLP2/3/6, an acknowledged knowledge gap.

Aging, evolution and disease relevance

Circulating levels of some mitochondrial-derived peptides decline with age, and evidence of natural (purifying) selection on humanin and SHLP6 supports genuine biological function for these small ORF-encoded peptides. This positions the SHLP family, including SHLP1, as candidate targets in metabolic disease, neurodegeneration and other age-related conditions.

• Related mitochondrial-derived peptide levels decrease with age.
• Signatures of natural selection support functional importance of the SHLP family.
• Family members show disease-protective effects in models such as age-related macular degeneration.