IF a single intramuscular injection of AAV9 encoding full-length murine IL-33 (AAV9-CMV/CK8-mIL33, target dose 1×10¹¹ vg/limb) is administered to the tibialis anterior and gastrocnemius of aged (24-month-old) male C57BL/6J mice with established sarcopenia,

THEN a ≥30% expansion of Pax7⁺ satellite cells per myofiber cross-section, ≥15% increase in myofiber cross-sectional area (CSA), and ≥20% improvement in ex vivo tetanic force will be observed at 12 weeks post-injection versus AAV9-GFP controls,

BECAUSE the following causal chain operates:

1. In aged skeletal muscle, fibro-adipogenic progenitors (FAPs) — the dominant Pdgfrβ⁺ stromal cell population of the muscle interstitial niche — upregulate Pdgfrβ signaling with age, which, by direct analogy to the aged subcutaneous adipose stromal vascular compartment, suppresses endogenous IL-33 mRNA transcription and protein secretion into the niche microenvironment (Pdgfrβ deletion in aged adipose SV cells restored IL-33 mRNA and protein secretion, and rescued type 2 effector signaling in aged mice) (Reversing Pdgfrβ Signaling)[https://doi.org/10.1101/2024.06.17.599436] [SPECULATIVE — cross-tissue extrapolation; FAP-specific IL-33 suppression in muscle not yet directly demonstrated]

2. The resulting IL-33 deficiency in the aged muscle niche starves satellite cells of the ST2-activating alarmin signal required to exit quiescence and initiate symmetric expansion; AAV9-driven transgenic IL-33 restores physiologic alarmin concentration in the interstitium, bypassing FAP-mediated suppression [SPECULATIVE — mechanistic link between FAP Pdgfrβ and satellite cell ST2 deprivation is inferred]

3. IL-33 binding to ST2 on satellite cells activates MyD88→IRAK4→TRAF6→NFκB signaling, driving transcription of cyclin D1 and MyoD target genes, promoting G_alert entry and symmetric proliferative divisions that expand the Pax7⁺ pool while preserving self-renewal capacity [supported by IL-33/ST2/MyD88 pathway mechanistic data cited in the structured literature summary of the Evidence Set]

4. Concurrently, IL-33 recruits ST2⁺ regulatory T cells (Tregs) to the muscle interstitium; muscle-resident Tregs produce amphiregulin (AREG), which signals through EGFR on satellite cells to reinforce self-renewal rather than terminal differentiation, counterbalancing the pro-proliferative NFκB signal and preventing exhaustion of the expanded pool [SPECULATIVE — Treg/amphiregulin axis in aged sarcopenic muscle is extrapolated from injury-repair literature cited in the Evidence Set]

5. IL-33 simultaneously activates ST2⁺ ILC2s present in the aged muscle interstitium; ILC2s produce IL-4 and IL-13, which act directly on myoblasts to promote fusion and on FAPs to suppress fibro-adipogenic differentiation, thereby partially restoring the pro-myogenic niche (ILC2 recruitment and type 2 effector signaling restored by IL-33 induction in aged tissue) (Reversing Pdgfrβ Signaling)[https://doi.org/10.1101/2024.06.17.599436]

6. T...

SENS category: RepleniSENS

Key references: • doi.org/10.1101/2024.06.17.599436] • doi.org/10.1101/2024.06.17.599436].