TRIM36 Catalyzes K63-Linked Ubiquitination of TAK1 to Drive Septic Inflammation

Technical Summary: TRIM36 Catalyzes K63-Linked Ubiquitination of TAK1 to Drive Septic Inflammation

Problem Statement
Sepsis remains a leading cause of global mortality, driven by a dysregulated host immune response characterized by a hyperinflammatory phase. While the Toll-like receptor 4 (TLR4) pathway is central to initiating innate immunity, its pathological overactivation leads to excessive cytokine production and organ failure. The specific molecular mechanisms governing the amplification of TLR4 signaling, particularly the post-translational regulation of key kinases like TGF-β-activated kinase 1 (TAK1) during sepsis, remain incompletely understood. Specifically, the E3 ubiquitin ligases responsible for the K63-linked ubiquitination required for TAK1 activation in this context have not been fully defined.

Methodology
The study employed a multi-faceted approach combining in vivo animal models, in vitro cellular assays, and bioinformatic analysis:

• Animal Models: An LPS-induced endotoxemia model was established in Trim36 knockout (Trim36⁻/⁻) and wild-type (Trim36⁺/⁺) C57BL/6 mice. Outcomes included survival rates, hypothermia monitoring, histological analysis of lung tissue, and quantification of inflammatory mediators in serum and bronchoalveolar lavage fluid (BALF).
• Cellular Models: Bone marrow-derived macrophages (BMDMs) from mice and human THP-1-derived macrophages were utilized. Cells were subjected to LPS stimulation, adenoviral overexpression, and siRNA-mediated knockdown of TRIM36 or TAK1.
• Molecular Mechanisms:
  • Interaction Mapping: Co-immunoprecipitation (Co-IP) assays using truncation mutants in HEK293T cells and endogenous proteins in BMDMs were used to identify binding domains between TRIM36 and TAK1.
  • Ubiquitination Analysis: Ubiquitination assays were performed using linkage-specific antibodies (K63 vs. K48) and lysine-to-arginine point mutants of TAK1 (K34R, K158R, K209R, K562R) to pinpoint the specific ubiquitination site.
  • Signaling Profiling: Western blotting assessed the phosphorylation status of key signaling molecules (TAK1, IκBα, p65, TBK1, IRF3, AKT, JNK, p38) following LPS stimulation.
  • Bioinformatics: Public transcriptomic datasets (GSE13904, GSE28750, GSE54514) were analyzed to correlate TRIM36 expression levels with sepsis patient outcomes. Molecular docking simulations (HDOCK) were used to model the TRIM36-TAK1 interface.

Key Contributions and Results

1. Identification of TRIM36 as an LPS-Induced Amplifier: Screening of TRIM family members revealed that TRIM36 is the most significantly upregulated gene in macrophages upon LPS challenge. This upregulation was confirmed in both mouse BMDMs and human THP-1 cells. Public data analysis showed elevated TRIM36 transcript levels in sepsis patients compared to healthy controls.
2. Protective Role of TRIM36 Deficiency: Trim36⁻/⁻ mice challenged with LPS exhibited significantly improved survival rates, attenuated hypothermia, and reduced lung injury compared to wild-type controls. These mice also displayed lower levels of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) and nitric oxide (NO) in serum and BALF. Flow cytometry indicated a reduced proportion of M1-like macrophages in the peritoneum of knockout mice.
3. Specificity for the TAK1-NF-κB Axis: Mechanistic studies demonstrated that TRIM36 deficiency selectively impaired the phosphorylation of TAK1, IκBα, and p65, while leaving other pathways (TBK1/IRF3, JAK/STAT, MAPKs) largely unaffected. RNA-seq analysis confirmed a negative enrichment of inflammatory signaling pathways in Trim36⁻/⁻ cells.
4. Direct Interaction and K63-Ubiquitination: TRIM36 was found to directly bind TAK1 via an extensive multi-domain interface involving the RING and SPRY domains of TRIM36. TRIM36 acts as an E3 ubiquitin ligase, catalyzing K63-linked polyubiquitination of TAK1. This activity is dependent on the RING domain of TRIM36.
5. Identification of the Ubiquitination Site: Through mutagenesis, Lysine 158 (K158) on TAK1 was identified as the primary site for TRIM36-mediated K63-linked ubiquitination.
6. Functional Dependency: Overexpression of TRIM36 enhanced inflammatory responses, an effect that was completely abrogated by the knockdown of TAK1 for IL-1β and IL-6 production, and partially attenuated for TNF-α and NO. This confirms TAK1 as the primary downstream effector.
7. Clinical Correlation: Analysis of human sepsis data (GSE54514) revealed that higher TRIM36 expression levels were significantly associated with non-survival in patients.

Significance
The paper establishes TRIM36 as a critical positive regulator of TLR4-induced hyperinflammation in sepsis. By elucidating the mechanism wherein TRIM36 directly ubiquitinates TAK1 at K158 via K63-linkage, the study defines a specific molecular bottleneck in the inflammatory cascade. The findings suggest that the TRIM36-TAK1 axis is not only a mechanistic driver in murine models but also clinically relevant in human disease, as evidenced by the correlation between high TRIM36 levels and mortality. Consequently, the authors propose that targeting TRIM36 or its interaction with TAK1 represents a potential therapeutic strategy for precision anti-inflammatory therapy in sepsis. The study highlights the TRIM family's role as positive amplifiers in innate immunity, expanding the understanding of ubiquitination dynamics in septic pathophysiology.