Untangling the threads: sciatic nerve and ganglion transcriptomics in prediabetic neuropathy

Aims: To investigate site-specific molecular pathways and immune responses in peripheral neuropathy (PN) progression through the analysis of gene expression in a high-fat diet-induced mouse model of obesity and pre-diabetes.

Methods: Three-week-old male C57BL/6J mice were fed either a standard diet (SD) (n=11; 12 week=6; 36 week=5) or a high-fat diet (HFD) until 12 (n=6) or 36 (n=6) weeks of age. Body weight, fasting blood glucose, and glucose tolerance and PN phenotyping were assessed at 12 and 36 weeks. PN was identified using sensory and motor nerve conduction velocities and von Frey filament testing (1-6g). The sciatic nerves (SCN) and dorsal root ganglia (DRG) were sampled, and RNA was extracted for RNA-Seq. Gene expression was analysed using self-organising maps (SOM) and differential expression, with validation by real-time polymerase chain reaction (qPCR). Mouse data were cross-references against human sural nerve RNA-Seq to identify conserved pathways.

Results: Transcriptomics of the SCN and DRG at 12 and 36 weeks in HFD mice had disease stage-specific alterations relative to SD mice. In the SCN, pathways related to muscle contraction and calcium signalling were downregulated concurrent with upregulation of cytokine signalling, reactive oxygen species processes and pain sensitisation pathways (all p<0.05). The SCN showed increased expression of fatty acid β-oxidation genes at 12 weeks and at 36 weeks, ceramide-mediated pathways were upregulated indicating cellular stress. Enrichment analysis at 12 weeks identified alterations in insulin signalling, AMP signalling, and lipid metabolism (all p<0.01). At 36 weeks, changes included glucose, glutathione, sphingolipid, fatty acid, and ketone metabolism pathways (all p<0.01), and insulin signalling (p<0.001), alongside immune and inflammatory responses (p<0.001). In the DRG at 36 weeks, altered pathways included immune response, glucose and lipid metabolism, and insulin signalling (all p<0.01). Dysregulated Hippo and Notch signalling pathways (both p<0.05) were DRG-specific and exhibited pronounced dysregulation at the proximal site. The differentially expressed gene ‘Yap1’ was downregulated indicative of impaired cell proliferation and promotion of apoptotic pathways at longer disease duration. The SOM analysis identified conserved immune/metabolism pathways across both SCN and DRG tissues providing validation. Hippo and Notch signalling pathways was site-specific to the DRG and was present during PN progression. At 36 weeks in the SCN, lymphatic and vascular epithelial cells (p<0.05) and B-cells (p=0.07) decreased, while myeloid lineage cells, macrophages (p<0.01), and fibroblasts (p<0.05) increased. Further, CD36 and Saa3 were significantly upregulated in the SCN (both p<0.01). At 36 weeks in the DRG, myelinating Schwann cells decreased, and pericytes increased (both p<0.05), with a trend towards increased B-cells (p=NS). Overlapping gene enrichment between the mouse model and human diabetic PN patients identified pathways in lysosome, inflammation, lipid metabolism, and AMPK signalling (all p<0.01). Key differentially expressed genes such as Cpt1a, Smpd, and Acaca were shared between mouse and human data (all p<0.01).

Conclusion: Derangement of distinct metabolic, neuroimmune, and inflammatory pathways occur initially at distal sites, progressing proximally, exhibiting site and disease progression specificity representing novel therapeutic targets.

Comments. This study highlights distal-to-proximal progression of PN and the role of molecular reprogramming in disease progression between the sciatic and dorsal root ganglion neurons. The relative contributions of these alterations may explain some variance in clinical signs and symptoms. Whilst further investigation will discriminate the pathways with the greatest contribution to the PN progression, these changes in the most distal projections of the DRG may be more viable targets in humans. These findings may have direct relevance to phenotypes associated with neuroanatomical alterations of the peripheral nerves (Mooshage CM et al Diabetologia 67:275-289, 2024).

Jamie Burgess

Reference. Eid SA, Elzinga SE, Guo K, Hinder LM, Hayes JM, Pacut CM, Koubek EJ, Hur J, Feldman EL. Transcriptomic profiling of sciatic nerves and dorsal root ganglia reveals site-specific effects of prediabetic neuropathy. Transl Res. 2024 Mar 29;270:24-41. doi: 10.1016/j.trsl.2024.03.009. Epub ahead of print. PMID: 38556110.

https://www.translationalres.com/article/S1931-5244(24)00060-4/abstract