Indexing Changes in Soma-Gila Microcontact Associated with Pain Severity

Background & Aims

Satellite glial cells (SGCs) surround primary sensory neurons in the dorsal root ganglion (DRG) and are implicated in peripheral nerve injury and pain modulation (Hanani, 2005; Pannese, 2010). Recent studies highlight their role in pain thresholds (Jasmin et al., 2010; Kim et al., 2016). Under conditions like neuropathy or inflammation, SGCs may undergo structural and functional changes impacting DRG-SGC communication (Cherkas et al., 2004; Hanani, 2005; Warwick and Hanani, 2013).

This research focuses on specific structural modifications in SGCs during pain, utilizing Transmission Electron Microscope (TEM) images. Genetic changes specific to each cell type were explored through mining GEO Data (GSE154659), confirming significant gene expression alterations in the pain model. The study suggests that observing morphological changes in SGCs could serve as a novel pain indicator marker.

Methods

Transmission Electron Microscope
Imaging was carried out using a JEOL (JEM-F200) electron microscope and a Hitachi (H-7650) electron microscope. To assess the condition of neurons in relation to SGCs, three measurements were recorded: the distance between the neuron and SGCs (the largest visible gap), the neuron’s diameter (the largest measurement across the neuron), and the neuron’s area (Avraham et al., 2020; Avraham et al., 2021)

Nociceptive Behavior Tests
The chronic constriction injury (CCI) model was established as previously described (Bennett and Xie, 1988). To induced inflammation pain using complete Freund’s adjuvant (CFA). To create the chemotherapy-induced peripheral neuropathy (CIPN) model, mice were administered 4mg/kg of paclitaxel. After creating each model, behavioral experiments were conducted to assess mechanical (von_Frey test) and thermal (Hargreaves test and Acetone evaporation test) pain responses in the mice.

Results

Our study revealed distinctive structural changes in SGCs under pain conditions, evident in TEM images. Notably, there is a noticeable increase in the gap between neurons and SGCs, accompanied by a deformation in SGC shape. These changes were consistently observed in all pain models, indicating the specificity of these morphological alterations to neuropathic pain. Subsequently, our focus shifted to examining gene expression changes specific to each neuropathic model. We aimed to identify alterations in paired genes involved in interactions, particularly between neurons and SGCs. Reanalyzing existing GEO data (GSE154659), we found an overall decrease in adhesion molecules of neurons in the nerve injury model. Furthermore, specific clusters within neuronal populations exhibited changes, suggesting active involvement in the pain experience. The observed structural changes between neurons and SGCs within these clusters are significantly associated with pain.

Conclusions

In conclusion, our study highlights the diverse structural changes in SGCs under conditions of neuronal injury that induce pain. The notably increased distance suggests significant alterations in the interaction between DRG neurons and SGCs, and we posit that this phenomenon may be indicative of the pain state. Additionally, we discovered that a specific cluster of DRG neuron is involved in this process. In the cluster, the gene expression of molecules maintaining the structure significantly decreases, and this change is expected to pose challenges to maintaining a stable structure between neurons and SGCs. The gene exhibiting changes have sufficient potential to become new pain modulating targets. Though this research, we proposed a new morphological indicator for pain and simultaneously identify key genes and neuronal clusters involved in these changes.

References

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