Hypothesis

Selective autophagy of canonical JAK-STAT proteins enforces a signaling hierarchy that preserves basal ISG expression via STAT2‑IRF9 in aged tissues.

Mechanistic rationale

• In nutrient stress or oxidative stress, autophagy receptors p62/SQSTM1 and NBR1 bind phosphorylated JAK1/JAK2 and pSTAT1 through LIR motifs that become exposed after K63‑linked ubiquitination by TRIM21 (6).
• This ubiquitination is favored on canonical JAKs because their kinase domains acquire activating phosphorylations (Y1007/Y1008) that create a phosphodegron recognized by the E3 ligase, whereas STAT2 and IRF9 lack comparable motifs and remain largely nuclear, shielded from cytosolic autophagy machinery.
• Consequently, autophagic flux preferentially degrades JAK1, JAK2, and pSTAT1, dampening canonical JAK‑STAT signaling while sparing the STAT2‑IRF9 complex that can drive ISG transcription independently of JAK phosphorylation (3).
• The resulting shift maintains a low‑grade interferon tone that supports homeostatic functions but avoids the hyper‑activation that drives senescence in stem/progenitor cells (1).
• When this selective autophagy is impaired—e.g., by mutation of the LIR motif in JAK1 or by pharmacological inhibition of p62 oligomerization—canonical components accumulate, leading to erratic JAK‑STAT activation, loss of the compensatory STAT2‑IRF9 axis, and exacerbated inflammaging and cardiovascular risk (2).

Testable predictions

1. Biochemical: In lysates from old human fibroblasts or mouse tendon, immunoprecipitation of p62 will pull down JAK1, JAK2, and pSTAT1 more strongly than STAT2 or IRF9; this interaction will increase after starvation or rapamycin treatment and be abolished by mutating the LIR motif (W334A) in JAK1 (5).
2. Functional: CRISPR‑knock‑in of a non‑degradable JAK1 (LIR‑mutant) in young fibroblasts will reduce basal ISG expression despite intact STAT2‑IRF9, and will sensitize cells to IFN‑α‑induced senescence (SA‑β‑gal, p16) (1).
3. In vivo: Aged mice expressing a liver‑specific, autophagy‑resistant JAK1 will show heightened hepatic JAK2/STAT3 phosphorylation, increased SASP cytokines (IL‑6, PAI‑1), and exacerbated fibrosis compared with wild‑type littermates; concomitant overexpression of STAT2 will rescue ISG tone without restoring canonical signaling (3).
4. Pharmacological: Treatment with the autophagy inducer spermidine will enhance selective JAK1 degradation in aged tissues, lower p‑STAT3 levels, and improve tendon stem/progenitor cell viability, an effect lost in p62‑knock‑out backgrounds.

Falsifiability

If selective autophagy does not preferentially target canonical JAK‑STAT components—e.g., if p62 pulls down STAT2 and IRF9 equally with JAK1, or if LIR‑mutant JAK1 does not alter ISG tone or senescence—then the hypothesis is refuted. Likewise, if enhancing autophagy fails to modify the canonical/non‑canonical signaling balance in aged tissues, the proposed hierarchy would be unsupported.

Conclusion: Positioning autophagy as a substrate‑specific triage mechanism reframes tachyphylaxis not as signaling exhaustion but as an active, hierarchically organized adaptation that preserves interferon‑driven homeostasis during aging. Disrupting this order, rather than the autophagic process itself, should precipitate the pathogenic shift observed in age‑related inflammation and tissue dysfunction.