Expression Analysis of a Pro-Inflammatory Pathway in Neuropathic Pain
Patricia Garcia Fernandez University Hospital of Würzburg, Germany
Patricia Garcia Fernandez, University Hospital of Würzburg, Germany
Colette Reinhold, University Hospital of Würzburg, Germany
Nurcan Üçeyler, University Hospital of Würzburg, Germany
Claudia Sommer, University Hospital of Würzburg, Germany
Background and Aims:
Polyneuropathy describes a group of diseases with multiple causes where degeneration affects the peripheral nervous system. While neuropathies are characterized by a myriad of symptoms that can affect the motor, sensory and/or autonomic system, only 50% of the patients report pain, independent of the etiology. Moreover, in some types of neuropathies such as small fiber neuropathy (SFN), pain is reported to be located in the skin distally in the leg, while the proximal regions are mostly pain free. This suggests a local regulatory process leading to pain. In this study, we hypothesize that the immune system may be involved in the development of pain in patients with neuropathies. Based on previous studies, we have proposed a pro-inflammatory pathway that might be activated in patients with painful polyneuropathies. The activation of the toll-like receptor 4 (TLR4) and the transient receptor potential vanilloid 1 (TRPV1) would induce a pro-inflammatory cascade mediated by nuclear factor 'kappa-light-chain-enhancer' of activated B-cells (NF?B). Furthermore, the modulation of regulatory factors, such as microRNA (miR) 132, 146 or 155, would promote the activation of this cascade via sirtuin 1 (SIRT1) and the interleukin-1 receptor-associated kinase 1 (IRAK1) / tumor necrosis factor receptor associated factor 6 (TRAF6) complex. This would release inflammatory factors such as tumor necrosis factor ? (TNF?) that can activate the same receptors, creating a pro-inflammatory loop.
To test this hypothesis, we have recruited a cohort of 68 informed patients with polyneuropathies of different etiologies who had biopsies of skin from the lower leg and upper thigh and sural nerve for diagnostic reasons. Patients were stratified in subgroups according to the inflammatory component of their neuropathy based on clinical, electrophysiological, and histological criteria, and the levels of pain: No pain, mild pain and severe pain. With their consent, we performed RT-qPCR in a small part of these biopsies to analyze the gene expression of the proposed inflammatory pathway components in sural nerve and between distal and proximal skin regions. The results were analyzed with SPSS and considered significant when the p-value was higher than 0.05.
Our results showed that, in sural nerve, patients with severe pain, in comparison to those with mild or no pain, presented a higher expression of TNF? and TLR4, suggesting a relationship between inflammation and the development of pain. Furthermore, we found a positive correlation between TNF? and the severity of the disease (correlation coefficient: 0.4543; p< 0.0001). In addition, patients with an inflammatory neuropathy presented lower expression of SIRT1 compared to those with a noninflammatory neuropathy, which suggests an upregulation of the NFKB-mediated pathway. In distal skin samples, our results showed a higher expression of miR-132 and miR-155 in patients with reduced nerve fibers in the skin, and lower expression of TRPV1 and SIRT1 in patients with mild and severe pain in comparison to patients with no pain. Moreover, a correlation was found in the expression of SIRT1 between distal and proximal skin regions, suggesting a systemic modulation of this component (correlation coefficient: 0.5759; p< 0.01). Finally, we found lower expression of miR-146 and TLR4 in distal skin regions in comparison to proximal, in patients with mild and severe pain, not present in patients with no pain or healthy controls.
Although some of these results open further questions that should be studied in the future, the gene expression of TNF?, miR-132, miR-146 and miR-155 found in patients with painful polyneuropathies clearly suggests an involvement of the immune system in the development of neuropathies and pain. Nonetheless, further research is ongoing to elucidate the biological relevance of our findings.
Conflicts of Interest:
The authors declare that they have no conflicts of interest.
Source of Financial Support for the Project:
This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sk?odowska-Curie Grant Agreement No 764860.