In vivo calcium imaging shows that satellite glial cells have increased activity in painful states.

Satellite glial cells are important for proper neuronal function of primary sensory neurons for which they provide homeostatic support. Most research on satellite glial cell function has been performed with in vitro studies, but recent advances in calcium imaging and transgenic mouse models have enabled this first in vivo study of single-cell satellite glial cell function in mouse models of inflammation and neuropathic pain. We found that in naïve conditions, satellite glial cells do not respond in a time-locked fashion to neuronal firing. In painful inflammatory and neuropathic states, we detected time-locked signals in a subset of satellite glial cells, but only with suprathreshold stimulation of the sciatic nerve. Surprisingly, therefore, we conclude that most calcium signals in satellite glial cells seem to develop at arbitrary intervals not directly linked to neuronal activity patterns. More in line with expectations, our experiments also revealed that the number of active satellite glial cells was increased under conditions of inflammation or nerve injury. This could reflect the increased requirement for homeostatic support across dorsal root ganglion neuron populations, which are more active during such painful states.

A transcriptional toolbox for exploring peripheral neuroimmune interactions.

ABSTRACT: Correct communication between immune cells and peripheral neurons is crucial for the protection of our bodies. Its breakdown is observed in many common, often painful conditions, including arthritis, neuropathies, and inflammatory bowel or bladder disease. Here, we have characterised the immune response in a mouse model of neuropathic pain using flow cytometry and cell-type-specific RNA sequencing (RNA-seq). We found few striking sex differences, but a very persistent inflammatory response, with increased numbers of monocytes and macrophages up to 3 1/2 months after the initial injury. This raises the question of whether the commonly used categorisation of pain into "inflammatory" and "neuropathic" is one that is mechanistically appropriate. Finally, we collated our data with other published RNA-seq data sets on neurons, macrophages, and Schwann cells in naive and nerve injury states. The result is a practical web-based tool for the transcriptional data mining of peripheral neuroimmune interactions. http://rna-seq-browser.herokuapp.com/.

Schwann cell p75 neurotrophin receptor modulates small fiber degeneration in diabetic neuropathy.

Diabetic neuropathy has an incidence as high as 50% of diabetic patients and is characterized by damage to neurons, Schwann cells and blood vessels within the peripheral nervous system. The low-affinity neurotrophin receptor p75 (p75NTR ), particularly expressed by the Schwann cells in the peripheral nerve, has previously been reported to play a role in developmental myelination and cell survival/death. Increased levels of p75NTR , in the endoneurium and plasma from diabetic patients and rodent models of disease, have been observed, proposing that this receptor might be involved in the pathogenesis of diabetic neuropathy. Therefore, in this study, we addressed this hypothesis by utilizing a mouse model of selective nerve growth factor receptor (Ngfr) deletion in Schwann cells (SC-p75NTR -KO). Electron microscopy of sciatic nerves from mice with high fat diet induced obesity demonstrated how loss of Schwann cell-p75NTR aggravated axonal atrophy and loss of C-fibers. RNA sequencing disclosed several pre-clinical signaling alterations in the diabetic peripheral nerves, dependent on Schwann cell p75NTR signaling, specially related with lysosome, phagosome, and immune pathways. Morphological and biochemical analyses identified abundant lysosomes and autophagosomes in the C-fiber axoplasm of the diabetic SC-p75NTR -KO nerves, which together with increased Cathepsin B protein levels corroborates gene upregulation from the phagolysosomal pathways. Altogether, this study demonstrates that Schwann cell p75NTR deficiency amplifies diabetic neuropathy disease by triggering overactivation of immune-related pathways and increased lysosomal stress.

Changes in the transcriptional fingerprint of satellite glial cells following peripheral nerve injury.

Satellite glial cells (SGCs) are homeostatic cells enveloping the somata of peripheral sensory and autonomic neurons. A wide variety of neuronal stressors trigger activation of SGCs, contributing to, for example, neuropathic pain through modulation of neuronal activity. However, compared to neurons and other glial cells of the nervous system, SGCs have received modest scientific attention and very little is known about SGC biology, possibly due to the experimental challenges associated with studying them in vivo and in vitro. Utilizing a recently developed method to obtain SGC RNA from dorsal root ganglia (DRG), we took a systematic approach to characterize the SGC transcriptional fingerprint by using next-generation sequencing and, for the first time, obtain an overview of the SGC injury response. Our RNA sequencing data are easily accessible in supporting information in Excel format. They reveal that SGCs are enriched in genes related to the immune system and cell-to-cell communication. Analysis of SGC transcriptional changes in a nerve injury-paradigm reveal a differential response at 3 days versus 14 days postinjury, suggesting dynamic modulation of SGC function over time. Significant downregulation of several genes linked to cholesterol synthesis was observed at both time points. In contrast, regulation of gene clusters linked to the immune system (MHC protein complex and leukocyte migration) was mainly observed after 14 days. Finally, we demonstrate that, after nerve injury, macrophages are in closer physical proximity to both small and large DRG neurons, and that previously reported injury-induced proliferation of SGCs may, in fact, be proliferating macrophages.