| Symbol |
FNDC5 |
| Full Name |
Fibronectin Type III Domain Containing 5 |
| Chromosome |
1p36.22 |
| NCBI Gene |
56852 |
| Ensembl |
ENSG00000100597 |
| UniProt |
Q8IZF6 |
| Protein Product |
[FNDC5/Irisin](/proteins/fndc5-protein) |
| Expression |
[Skeletal muscle](/brain-regions/skeletal-muscle) (primary), [hippocampus](/brain-regions/hippocampus), [cortex](/brain-regions/cortex), [heart](/brain-regions/heart) |
| Related Diseases |
[Alzheimer's Disease](/diseases/alzheimers), [Parkinson's Disease](/diseases/parkinsons-disease), [ALS](/diseases/als), [Type 2 Diabetes](/diseases/type-2-diabetes) |
¶ FNDC5 — Fibronectin Type III Domain Containing 5
FNDC5 (Fibronectin Type III Domain Containing 5) is a gene located on chromosome 1p36.22 that encodes a type I membrane protein primarily expressed in skeletal muscle. FNDC5 is the precursor protein for irisin, a circulation myokine that is cleaved and secreted in response to exercise. Irisin has garnered significant attention for its pleiotropic effects on energy metabolism and, more recently, for its neuroprotective properties in neurodegenerative disease models.
The discovery of irisin in 2012 established FNDC5 as a critical mediator of exercise-induced systemic effects, linking physical activity to benefits in distant organs including the brain. This positions FNDC5 as a compelling therapeutic target for neurodegenerative disorders where exercise has shown beneficial effects.
¶ Gene Structure and Expression
- Chromosome: 1p36.22
- Genomic coordinates: ~10,000 bp coding region
- Orientation: Plus strand
- Isoforms: Multiple splice variants identified
FNDC5 exhibits tissue-specific expression patterns:
| Tissue |
Expression Level |
Notes |
| Skeletal muscle |
High |
Primary source of circulating irisin |
| Heart |
Moderate |
Cardiac expression |
| Brain |
Low-Moderate |
Hippocampus, cortex, cerebellum |
| Brown adipose tissue |
Moderate |
Involved in thermogenesis |
| Liver |
Low |
Minor contribution |
FNDC5 expression is primarily regulated by:
- PGC-1α: Peroxisome proliferator-activated receptor gamma coactivator 1-alpha is the master regulator of exercise-induced FNDC5 expression
- PPARγ: Nuclear receptor that can induce FNDC5
- MyoD: Muscle differentiation factor
- AMP-activated protein kinase (AMPK): Energy sensor that promotes FNDC5 expression during exercise
The FNDC5 protein consists of:
- N-terminal signal peptide: Targets for secretion
- Fibronectin type III domain: The characteristic domain giving the protein its name
- C-terminal hydrophobic region: Membrane anchor
- Cleavage site: Proteolytic processing releases irisin
¶ Processing and Secretion
The cleavage of FNDC5 to produce irisin is mediated by:
- Tumor necrosis factor-alpha converting enzyme (TACE/ADAM17): Primary protease responsible for irisin release
- Proteolytic processing: Generates the soluble irisin fragment (~112 amino acids)
- Half-life: ~1 hour in circulation
- Levels: 10-100 ng/mL in human serum
- Induction: Exercise increases irisin 2-3 fold
- BBB penetration: Partial penetration of the blood-brain barrier reported
FNDC5/irisin mediates several exercise-induced beneficial effects in the brain:
- Neurogenesis: Promotes hippocampal neurogenesis
- Synaptic plasticity: Enhances long-term potentiation (LTP)
- Mitochondrial function: Increases mitochondrial biogenesis
- Neuroinflammation: Reduces inflammatory responses
- Amyloid clearance: Facilitates Aβ clearance
In Alzheimer's disease models, FNDC5/irisin has shown:
- Amyloid-beta reduction: Decreased Aβ accumulation and plaque formation
- Tau pathology: Reduced tau phosphorylation and aggregation
- Synaptic protection: Preserved synaptic markers and function
- Cognitive improvement: Enhanced memory and learning in mouse models
- Neurogenesis: Promoted adult hippocampal neurogenesis
Evidence for FNDC5/irisin in PD includes:
- Dopaminergic neuron protection: Preserved tyrosine hydroxylase (TH)-positive neurons[^7]
- Mitochondrial enhancement: Increased PGC-1α and mitochondrial DNA content
- Alpha-synuclein: Reduced aggregation and toxicity
- Motor function: Improved behavioral outcomes in models
In ALS models:
- Motor neuron protection: Reduced motor neuron degeneration
- Neuromuscular junction: Improved stability of neuromuscular junctions
- Muscle-nerve connection: Maintained muscle innervation
Given the therapeutic potential of FNDC5/irisin, several strategies are being explored:
| Approach |
Mechanism |
Status |
Challenges |
| Recombinant irisin |
Direct protein administration |
Preclinical |
BBB penetration, half-life |
| FNDC5 overexpression |
Gene therapy |
Preclinical |
Delivery, expression control |
| Small molecule agonists |
PGC-1α activators |
Research |
Specificity |
| Exercise mimetics |
Pharmacological PGC-1α induction |
Research |
Off-target effects |
Compounds being investigated:
- PGC-1α agonists: Activate FNDC5 expression
- AMPK activators: Indirect FNDC5 induction
- HDAC inhibitors: Epigenetic upregulation
- ADAM17 enhancers: Increase irisin release
Serum irisin levels have been studied as:
- Exercise compliance marker: Tracks physical activity levels
- Disease biomarker: Altered levels in AD, PD, diabetes
- Therapeutic response indicator: Monitors treatment efficacy
Current clinical investigations include:
- Exercise interventions and irisin measurement
- Recombinant irisin safety studies
- FNDC5 gene therapy trials (preclinical)