Irisin - a myth rather than an exercise-inducible myokine

Area of Science:

• Exercise physiology
• Metabolic research
• Biochemistry

Background:

• Irisin, a myokine derived from FNDC5, is proposed to mediate exercise's metabolic benefits.
• Current evidence for circulating irisin relies heavily on commercial ELISA kits.
• These kits utilize polyclonal antibodies (pAbs) lacking prior validation for cross-reactivity.

Purpose of the Study:

• To investigate the specificity of commercial irisin ELISA kits.
• To validate the presence of circulating irisin in human and animal sera.
• To re-evaluate the physiological role of irisin.

Main Methods:

• Western blotting analysis of four commercial irisin pAbs.
• Testing pAbs against human and animal sera.
• Mass spectrometry to identify FNDC5 signatures.
• Use of recombinant glycosylated and non-glycosylated irisin as controls.

Main Results:

• Commercial pAbs exhibited significant cross-reactivity with non-specific serum proteins.
• No immune-reactive bands of the expected size for irisin were detected in biological samples.
• Mass spectrometry identified a FNDC5 signature at ~20 kDa in human serum.
• This FNDC5 signature was not detected by the tested commercial pAbs.

Conclusions:

• Commercial ELISA kits for irisin are unreliable due to antibody cross-reactivity.
• Previous studies relying on these kits may be flawed.
• Evidence does not support a significant physiological role for irisin in humans or other species.