Aberrant Autophagy–Senescence Axis Drives Spontaneous Abortion through HSPA6/FOXO1-Mediated Decidual Stromal Dysfunction

Technical Summary: Aberrant Autophagy–Senescence Axis Drives Spontaneous Abortion through HSPA6/FOXO1-Mediated Decidual Stromal Dysfunction

Problem Statement
Recurrent spontaneous abortion (RSA) affects 1–2% of couples, with approximately 50% of cases remaining unexplained (URSA) due to a lack of clear diagnostic causes. While cellular senescence is known to play a dual role in pregnancy—physiologically regulating implantation but pathologically disrupting homeostasis when excessive—the specific molecular mechanisms linking metabolic dysregulation to decidual stromal cell (DSC) senescence and subsequent pregnancy failure remain poorly defined. Specifically, the study addresses how aberrant metabolic signals, particularly branched-chain amino acids (BCAAs), might trigger autophagy imbalance and senescence in DSCs, leading to adverse pregnancy outcomes.

Methodology
The study employed a multi-faceted approach combining clinical sample analysis, in vitro cell models, and in vivo murine models:

• Clinical Samples: Decidual tissues were collected from women with normal early pregnancies (n=65) and RSA patients (n=40). DSCs were isolated and cultured for primary analysis.
• Cellular Models: Primary DSCs were treated with exogenous BCAAs, D-galactose (D-Gal), or hydrogen peroxide (H₂O₂) to induce senescence. Autophagy was modulated using inhibitors (3-methyladenine/3-MA) and activators (rapamycin).
• Molecular Techniques:
  • Omics: Transcriptome sequencing (RNA-seq) was performed on DSCs treated with BCAA or D-Gal to identify differentially expressed genes (DEGs) and enriched pathways.
  • Protein/Nucleic Acid Analysis: Western blot, RT-qPCR, and immunofluorescence/histochemistry were used to quantify senescence markers (CDKN2A/p16, CDKN1A/p21, TP53/p53), autophagy flux markers (LC3B, SQSTM1/p62, ATG5), and key regulators (FOXO1, HSPA6).
  • Functional Assays: SA-β-gal staining assessed senescence levels; ROS detection measured oxidative stress; lentiviral transfection (siRNA) was used to knock down HSPA6.
• In Vivo Models: C57BL/6J mice were fed a high-leucine diet (6% leucine) to mimic BCAA excess. A separate group received intraperitoneal injections of rapamycin to test therapeutic intervention. Pregnancy outcomes (embryo absorption, weight) and uterine senescence markers were evaluated at day 13.5.

Key Contributions and Results

1. Elevated DSC Senescence in RSA:
   Clinical analysis revealed that DSCs in RSA patients exhibit significantly higher levels of senescence compared to normal pregnancies. This was evidenced by increased expression of senescence markers (CDKN2A, CDKN1A, TP53) and heightened SA-β-gal activity.

2. BCAAs Induce Senescence via Oxidative Stress and Autophagy Suppression:

   • Mechanism: Exogenous BCAA treatment induced time-dependent senescence in DSCs, accompanied by increased intracellular reactive oxygen species (ROS).
   • Autophagy Blockade: RNA-seq and validation experiments showed that elevated BCAAs significantly downregulated key autophagy genes (ATG4B, ATG5, BECN1, MAP1LC3A/B).
   • Causality: Inhibiting autophagy (via 3-MA) exacerbated senescence, while activating autophagy (via rapamycin) partially rescued D-Gal-induced senescence. This establishes that BCAAs drive senescence by suppressing autophagic flux.

3. Reciprocal Regulation of Senescence and Autophagy:
   The study identified a negative feedback loop: not only does autophagy suppression induce senescence, but the senescent state itself further suppresses autophagic activity (evidenced by reduced LC3B and accumulated p62 in H₂O₂ and D-Gal treated cells).

4. The HSPA6/FOXO1 Axis:

   • FOXO1 Downregulation: High BCAA levels and senescence led to a significant reduction in FOXO1 nuclear localization and protein expression. FOXO1 downregulation was also observed in RSA patient tissues.
   • HSPA6 Mediation: Transcriptome analysis identified Heat Shock Protein A6 (HSPA6) as a key upregulated gene during senescence. HSPA6 is a member of the HSP70 family.
   • Functional Link: Knockdown of HSPA6 (siHSPA6) restored FOXO1 expression in senescent DSCs. Conversely, HSPA6 was markedly elevated in RSA tissues. This suggests that senescence-induced HSPA6 upregulation drives FOXO1 degradation, impairing decidualization.

5. In Vivo Validation and Therapeutic Potential:

   • Pathology: Mice fed a high-leucine diet exhibited increased uterine senescence, inhibited autophagic flux, reduced FOXO1, and elevated HSPA6. These mice showed significantly increased embryo absorption rates and reduced fetal/placental weights.
   • Intervention: Administration of low-dose rapamycin to high-leucine-fed mice restored autophagic flux, reduced uterine senescence, and significantly improved pregnancy outcomes (reduced embryo loss, increased fetal weight), demonstrating that enhancing autophagy can mitigate BCAA-induced pregnancy failure.

Significance and Claims
The paper claims to elucidate a novel metabolic mechanism underlying unexplained recurrent spontaneous abortion. It posits that the "BCAA-autophagy suppression-cellular senescence-HSPA6-FOXO1" signaling axis is a critical driver of decidual dysfunction.

• Mechanistic Insight: The study defines a specific pathway where metabolic disorder (excess BCAAs) triggers a cascade of autophagy inhibition, leading to excessive DSC senescence. This senescent state upregulates HSPA6, which in turn suppresses the transcription factor FOXO1, a master regulator of decidualization.
• Therapeutic Implication: The findings suggest that metabolic disorders and autophagy imbalance are actionable targets for RSA. Specifically, the study demonstrates that pharmacological activation of autophagy (using rapamycin) can reverse excessive senescence and improve pregnancy outcomes in a metabolic stress model.
• Clinical Relevance: By linking BCAA metabolism to the HSPA6/FOXO1 axis, the study offers potential new biomarkers (HSPA6, FOXO1) and therapeutic strategies for preventing spontaneous abortion in patients with unexplained causes.

The authors maintain a modest tone regarding the complexity of the maternal-fetal interface, noting that while mTORC1 is a likely upstream sensor for BCAAs, the precise molecular targets and the full extent of mTOR-independent pathways require further investigation. The study concludes that these findings provide a theoretical basis for metabolic intervention in RSA but do not yet claim immediate clinical application without further validation.