Insulin/IGF-1 Signaling & the DAF-16 Longevity Switch

IIS Pathway Controls

Insulin/IGF-1 signaling (daf-2 activity) 100%

Pathway State

IIS signaling HIGH — AKT phosphorylates DAF-16, 14-3-3 holds it in the cytoplasm.

DAF-16 nuclear fraction

Stress-response gene activity

Predicted lifespan multiplier

1.00×

How it works

In C. elegans, the insulin/IGF-1 receptor DAF-2 sits on the cell membrane. When insulin-like ligands bind it, DAF-2 activates a PI3-kinase → AKT/PKB kinase cascade that phosphorylates the transcription factor DAF-16 (the worm ortholog of human FOXO3). Phosphorylated DAF-16 is bound by 14-3-3 proteins and retained in the cytoplasm, where it cannot act.

High IIS:  DAF-2 → PI3K → AKT ⊣ DAF-16-P (cytoplasm, inactive)
Low IIS:   DAF-2 quiet → DAF-16 dephosphorylated → nucleus
Nuclear DAF-16/FOXO activates: sod-3, ctl-1/2, hsp-16.2, mtl-1
Net effect: stress resistance ↑, lifespan ↑ (up to ~2× in daf-2 mutants)

Weak-signaling daf-2 mutants can roughly double median lifespan — but only if daf-16 is intact. Deleting daf-16 completely suppresses the longevity of a daf-2 mutant, proving DAF-16 is the downstream effector, not a side effect. This exact epistasis experiment (Kenyon lab, 1993 onward) launched the modern biology of ageing, and the same axis — insulin/IGF-1 → PI3K/AKT → FOXO — is evolutionarily conserved down to humans, where FOXO3 gene variants are among the most replicated genetic associations with human longevity.

Signaling slider — chronic strength of the IIS pathway. Low values mimic a hypomorphic daf-2 allele or pharmacological IGF-1R inhibition and set the steady-state lifespan multiplier.
daf-16 knockout — removes the effector entirely; DAF-16 particles vanish and the lifespan benefit collapses to 1.00× no matter how low signaling is, reproducing the classic epistasis result.
Insulin pulse — a transient ligand bolus that briefly re-phosphorylates DAF-16 and pulls it back out of the nucleus, then decays away — showing the shuttle is dynamic and reversible, not a one-way switch.

DAF-16 — cytoplasmic (phosphorylated) DAF-16 — nuclear (active) Stress-response target genes

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