AAV-mediated Klotho gene delivery attenuates renal fibrosis in chronic kidney disease by suppressing the TGF-β/Smad3 signaling pathway

Technical Summary: AAV-Mediated Klotho Gene Delivery Attenuates Renal Fibrosis in Chronic Kidney Disease

Problem Statement
Chronic Kidney Disease (CKD) is a global health crisis characterized by progressive renal fibrosis, a process driven by the dysregulation of the Transforming Growth Factor-beta 1 (TGF-β1)/Smad3 signaling pathway. A hallmark of CKD is the profound deficiency of Klotho, a transmembrane protein with anti-aging and nephroprotective properties. While Klotho deficiency correlates with disease progression, current standard-of-care therapies (e.g., RAAS blockade, SGLT2 inhibition) offer only partial renoprotection and fail to reverse established fibrosis. Furthermore, recombinant Klotho protein therapy is limited by a short circulating half-life, necessitating frequent administration. There is an urgent need for a disease-modifying therapy that can sustainably restore Klotho expression to directly counteract the fibrogenic program.

Methodology
The study employed a kidney-targeted gene therapy approach using an Adeno-Associated Virus serotype 9 (AAV9) vector.

• Vector Design: A self-complementary AAV9 vector was engineered to express human Klotho under the control of the kidney-specific Ksp-cadherin (Cdh16) promoter, restricting transgene expression to renal tubular epithelial cells.
• Animal Models: CKD was induced in C57BL/6J mice via two distinct models:
  1. 5/6 Nephrectomy (5/6 Nx): A two-stage surgical procedure creating a remnant kidney model.
  2. Unilateral Ureteral Obstruction (UUO): A model of obstructive nephropathy.
• Intervention: Mice received a single retrograde renal vein injection of the AAV9-KL vector at low (5×10¹⁰ vg) or high (2×10¹¹ vg) doses. Controls included sham-operated mice, vehicle-treated CKD mice, empty vector controls, and a positive control group receiving recombinant human soluble Klotho protein.
• Assessment:
  • In Vivo: Renal function (serum creatinine, BUN, proteinuria) was monitored over 12 weeks. Fibrosis markers (Col1a1, FN1, ACTA2), EMT markers (E-cadherin, Vimentin), and signaling components (TGF-β1, p-Smad3, Smad7) were quantified via qRT-PCR, Western blotting, and ELISA.
  • Transcriptomics: Bulk RNA sequencing was performed on renal cortex tissue to identify differentially expressed genes (DEGs) and pathway enrichment.
  • In Vitro: Human proximal tubular HK-2 cells were stimulated with TGF-β1 to induce epithelial-to-mesenchymal transition (EMT). Mechanistic causality was tested using Klotho restoration, Smad3 gain-of-function (overexpression), and pharmacological Smad3 inhibition (SIS3).
  • Safety: Hepatotoxicity was assessed via serum transaminases (ALT/AST) and histology; vector biodistribution was analyzed via qPCR.

Key Contributions and Results

1. Durable, Kidney-Specific Klotho Restoration:

   • AAV9 delivery via renal vein injection achieved efficient transduction of the remnant kidney (approx. 8.5 vector genomes per diploid genome) with a 5:1 kidney-to-liver ratio, minimizing off-target expression.
   • Klotho mRNA and protein levels were restored to approximately 78% and 82% of sham levels, respectively, in the high-dose group. This restoration was sustained for at least 12 weeks without significant decline.
   • Circulating soluble Klotho (sKlotho) was significantly recovered, reaching approx. 76% of sham levels.

2. Functional Improvement and Fibrosis Attenuation:

   • High-dose AAV-KL treatment significantly improved renal function in 5/6 Nx mice: serum creatinine decreased by ~47% (from ~72 to ~38 µmol/L), BUN by ~48%, and 24-hour proteinuria by ~63%.
   • Treatment suppressed fibrosis gene programs: mRNA levels of Col1a1, FN1, and ACTA2 were reduced by ~58%, ~45%, and ~55%, respectively.
   • Epithelial-to-mesenchymal transition (EMT) was reversed, evidenced by the recovery of E-cadherin and suppression of vimentin.

3. Transcriptome-Wide Validation:

   • RNA sequencing identified 924 differentially expressed genes (DEGs) in CKD kidneys versus sham. AAV-KL treatment rescued 306 of these genes, including core fibrosis effectors, EMT transcription factors, and inflammatory mediators.
   • Gene Ontology and KEGG analyses confirmed that the most significantly reversed pathways were TGF-β signaling and extracellular matrix organization.
   • GSEA revealed that the "Hallmark EMT" and "TGF-beta signaling" gene sets, highly enriched in CKD, were significantly reversed by AAV-KL, while "Oxidative Phosphorylation" was restored.

4. Mechanistic Elucidation:

   • Receptor-Level Blockade: AAV-KL treatment reduced phospho-Smad3 (p-Smad3) levels by ~47% without altering TGF-β1 ligand levels, supporting a mechanism where Klotho blocks ligand-receptor engagement rather than suppressing ligand synthesis.
   • Smad7 Restoration: Smad7, an inhibitory Smad, was upregulated ~1.9-fold, restoring the physiological negative feedback loop.
   • In Vitro Causality: In HK-2 cells, Klotho restoration inhibited TGF-β1-induced EMT and apoptosis. Crucially, forced overexpression of constitutively active Smad3 abrogated Klotho's protective effects, while the Smad3 inhibitor SIS3 phenocopied Klotho's protection. This confirms Smad3 inhibition as the obligate effector mechanism.

5. Safety Profile:

   • No hepatotoxicity was observed; serum ALT and AST levels remained within normal ranges.
   • Biodistribution analysis showed negligible vector genomes in off-target organs (heart, spleen, lung, muscle, brain).
   • Body weight remained stable throughout the study.

Significance and Claims
The paper claims to provide the first proof-of-concept that a single administration of kidney-targeted AAV9-Klotho gene therapy can durably restore Klotho expression and attenuate renal fibrosis in established CKD models. The study establishes that the therapeutic efficacy is mediated through a specific molecular cascade: Klotho acts as a receptor-level antagonist of TGF-β1/TGFβR2, leading to the inhibition of Smad3 phosphorylation and the restoration of Smad7-mediated negative feedback.

The authors emphasize that this approach offers a "disease-modifying" potential by directly counteracting the fibrogenic program, a capability lacking in current standard-of-care therapies. The findings are validated across two mechanistically distinct CKD models (5/6 Nx and UUO) and supported by integrated in vivo and in vitro mechanistic data. The study concludes that kidney-targeted AAV-Klotho gene therapy warrants further preclinical development, including evaluation in disease-specific models and non-human primates, as a potential intervention for progressive CKD.