Hypothesis

We hypothesize that a single systemic administration of an AAV vector combining a liver‑detargeted, myotropic capsid (e.g., AAV9‑PHP.eB variant) with an injury‑responsive promoter (TREE) driving a secreted rejuvenation factor (Klotho or FGF21) will achieve simultaneous, physiologically relevant transduction of skeletal muscle, heart, and brain while minimizing liver off‑target expression and enabling repeat dosing through transient immunosuppression.

Mechanistic Rationale

• Liver‑detargeting mutations reduce heparan sulfate binding and Kupffer cell uptake, lowering hepatocyte transduction and associated toxicity ([4]).
• Myotropic capsid variants generated by directed evolution show >10‑fold higher transduction in skeletal and cardiac muscle versus AAV9 ([2]).
• The TREE promoter is activated by chromatin changes present in senescent or damaged tissue, restricting transgene expression to areas undergoing age‑related injury ([7]).
• Coupling these elements creates a logic gate: only tissues that are both myotropic‑preferred and injury‑responsive express the therapeutic payload, providing spatial precision without needing multiple vectors.
• Secreted Klotho/FGF21 can act in an endocrine fashion, benefiting distal organs (e.g., brain, kidney) even if transduction is modest there, echoing the systemic benefits seen with liver‑derived Klotho ([5],[6]).
• Transient immunomodulation (e.g., anti‑CD20 IgG‑cleaving protease) administered before a second dose can neutralize anti‑AAV antibodies, allowing redosing despite pre‑existing immunity ([2]).

Experimental Design

1. Vector construction – Clone human Klotho cDNA downstream of a TREE enhancer‑promoter cassette into an AAV9 backbone bearing liver‑detargeting (Y731F) and myotropic (TTVR‑binding) mutations validated by AAVGen‑derived fitness scores ([1],[3]).
2. In vivo study – Use 20‑month‑old C57BL/6 mice (n=10/group). Groups: (a) saline control, (b) standard AAV9‑CMV‑Klotho, (c) liver‑detargeted myotropic AAV‑TREE‑Klotho, (d) same as (c) plus transient anti‑CD20 pretreatment before a second identical dose at month 4.
3. Readouts – At 2, 4, 8 weeks post‑dose: qPCR for vector genomes in liver, muscle, heart, brain; ELISA for circulating Klotho; histology for fibrosis, senescence (p16^INK4a); functional tests (grip strength, echocardiography, Morris water maze). Anti‑AAV IgG titers measured before and after each dose.
4. Redosing assessment – Compare transgene expression and immune markers after second dose in groups (c) vs (d).

Predictions

• Group (c) will show ≥5‑fold lower hepatic vector genomes vs group (b) while maintaining ≥2‑fold higher muscle and heart transduction ([2],[4]).
• Circulating Klotho will be elevated to levels comparable to hepatic expression in group (b) due to endocrine action, despite lower liver transduction.
• Senescence markers will be reduced in muscle, heart, and brain, correlating with improved functional outcomes.
• Group (d) will sustain or increase Klotho levels after the second dose, with no significant rise in anti‑AAV IgG relative to first dose, demonstrating successful immune reset.
• Failure to observe these outcomes would falsify the hypothesis.

Potential Pitfalls & Mitigations

• Leaky TREE activity in liver could cause off‑target expression; mitigate by incorporating liver‑specific microRNA target sites (miR‑122) in the transgene cassette.
• Capsid mutations may affect manufacturability; use AAVGen‑guided production fitness screening to select high‑yield clones ([1]).
• Immune clearance of transduced cells may limit longevity; incorporate a self‑limiting peptide degron regulated by doxycycline if needed.

By integrating capsid engineering, injury‑responsive transcriptional control, and transient immunomodulation, this approach tackles the central challenges of multi‑tissue targeting, scalable redosing, and immunogenicity that currently limit systemic AAV‑based rejuvenation therapies.