Proteomics Characteristics of Sciatic Nerves in Diabetic Peripheral Neuropathy, Key Gene DDI2 Identification and Comprehensive Therapeutic Strategy Exploration

Technical Summary: Proteomics Characteristics of Sciatic Nerves in Diabetic Peripheral Neuropathy, Key Gene DDI2 Identification and Comprehensive Therapeutic Strategy Exploration

Problem Statement
Diabetic Peripheral Neuropathy (DPN) affects over 50% of diabetic patients and is characterized by progressive peripheral nerve damage, leading to sensory loss, pain, and a high risk of non-traumatic lower-limb amputations. Despite extensive investigation, the precise molecular mechanisms driving DPN remain elusive. While previous proteomic studies have focused on dorsal root ganglia (DRGs), which contain a complex mixture of neurons, satellite cells, and immune cells, there is a lack of comprehensive proteomic analysis specifically targeting peripheral nerve fibers. Given that DPN symptoms typically originate at the distal ends of peripheral nerves, the authors propose that analyzing the sciatic nerves of diabetic mice offers a more direct approach to identifying initial molecular alterations and potential disease drivers.

Methodology
The study employed a multi-omics and functional validation approach:

1. Proteomic Profiling: Label-free quantitative proteomics (DIA-MS) was performed on sciatic nerves from db/db (Type 2 diabetic) and db/m (control) mice. Differentially expressed proteins (DEPs) were identified and subjected to KEGG and GO functional enrichment analyses.
2. Target Identification & Localization: The study focused on DDI2 (DNA damage-inducible 1 homolog 2). Its expression was validated via immunohistochemistry and double-labeling immunofluorescence (co-staining with S100 for Schwann cells and NF200 for neurons) to determine tissue specificity.
3. In Vitro Modeling: Rat (RSC96) and human (sNF96.2) Schwann cells were exposed to high glucose and palmitic acid (PA) to mimic hyperglycemic and lipotoxic stress. DDI2 expression was assessed via qPCR and immunofluorescence.
4. Functional Mechanism: RSC96 cells were transfected with DDI2 overexpression or knockdown (shRNA) plasmids. Autophagic flux was evaluated using an LC3-RFP-GFP reporter system and Western blotting for LC3-II/LC3-I and P62 levels.
5. Transcriptomic & Therapeutic Screening: RNA-seq was conducted on DDI2-overexpressed RSC96 cells. To identify therapeutic interventions, the authors utilized two bioinformatics strategies:
   • Connectivity Map (CMap): To screen for small-molecule compounds that counteract the DDI2-induced gene expression signature.
   • KnockTF 2.0: To identify transcription factors (TFs) capable of reversing the DDI2-regulated transcriptional profile.
6. Functional Validation of Candidates: The effects of identified compounds (fraxetin, cytisine) and transcription factors (ATF3) on autophagy in DDI2-overexpressed cells were validated via Western blot and reporter assays.

Key Results

• Proteomic Landscape: The study identified 80 DEPs in the sciatic nerves of db/db mice. Functional enrichment revealed three core metabolic abnormalities: lipid metabolism imbalance, protein/amino acid homeostasis disruption, and nucleotide metabolism perturbation.
• DDI2 Upregulation: DDI2 was identified as a significantly upregulated protein in the sciatic nerves of diabetic mice. Double immunofluorescence confirmed that this upregulation occurs predominantly in Schwann cells (S100+), not neurons. In vitro, both high glucose and palmitic acid treatments significantly upregulated DDI2 mRNA and protein levels in Schwann cells.
• Mechanism of Action: DDI2 overexpression was found to inhibit autophagic flux in Schwann cells. This was evidenced by a significant decrease in the LC3-II/LC3-I ratio and reduced autolysosome formation (red puncta in the reporter assay), alongside P62 accumulation. Conversely, DDI2 knockdown enhanced autophagy.
• Therapeutic Screening:
  • Small Molecules: CMap screening identified fraxetin and cytisine as natural compounds with strong negative connectivity scores, indicating their potential to reverse DDI2-induced transcriptional dysregulation. Functional assays confirmed that both compounds restored autophagy (increased LC3-II/LC3-I and P62) in DDI2-overexpressed cells.
  • Transcription Factors: KnockTF 2.0 analysis identified ATF3 as a top candidate TF. Overexpression of ATF3 in DDI2-overexpressed cells significantly rescued autophagic capacity, whereas ATF3 knockdown exacerbated the inhibition.

Significance and Claims
The paper claims to provide a comprehensive therapeutic paradigm for DPN by linking metabolic dysregulation to Schwann cell dysfunction via a specific molecular axis.

• Novel Mechanism: The study establishes DDI2 as a novel regulator in DPN, demonstrating that hyperglycemic and lipotoxic stress upregulates DDI2 in Schwann cells, which subsequently impairs autophagic flux via suppressed LC3-II/LC3-I conversion.
• Integrated Strategy: The authors propose a dual-targeting strategy that moves beyond single-gene inhibition. By combining small-molecule screening (fraxetin, cytisine) and transcription factor modulation (ATF3), the study offers a method to comprehensively reverse the aberrant transcription patterns and biological processes (specifically autophagy inhibition) driven by DDI2.
• Clinical Relevance: The identification of natural compounds and endogenous transcription factors that can restore autophagy in diabetic Schwann cells suggests potential therapeutic avenues to mitigate the progression of peripheral nerve damage in diabetic patients.

The authors conclude that their work elucidates the convergence of multiple metabolic abnormalities on Schwann cell autophagy inhibition through a DDI2-centric mechanism and validates a strategy to screen for agents that can reverse this pathological state.