Abstract
Enteric glial cells (EGCs) influence nitric oxide (NO)− and adenosine diphosphate (ADP)− mediated signaling in the enteric nervous system (ENS). Since Toll-like receptor 4 (TLR4) participates to EGC homoeostasis, this study aimed to evaluate the possible involvement of EGCs in the alterations of the inhibitory neurotransmission in TLR4−/− mice. Ileal segments from male TLR4−/− and wild-type (WT) C57BL/6J mice were incubated with the gliotoxin fluoroacetate (FA). Alterations in ENS morphology and neurochemical coding were investigated by immunohistochemistry whereas neuromuscular responses were determined by recording non-adrenergic non-cholinergic (NANC) relaxations in isometrically suspended isolated ileal preparations. TLR4−/− ileal segments showed increased iNOS immunoreactivity associated with enhanced NANC relaxation, mediated by iNOS-derived NO and sensitive to P2Y1 inhibition. Treatment with FA diminished iNOS immunoreactivity and partially abolished NO− and ADP− mediated relaxation in the TLR4−/− mouse ileum, with no changes of P2Y1 and connexin-43 immunofluorescence distribution in the ENS. After FA treatment, S100β and GFAP immunoreactivity in TLR4−/− myenteric plexus was reduced to levels comparable to those observed in WT. Our findings show the involvement of EGCs in the alterations of ENS architecture and in the increased purinergic and nitrergic-mediated relaxation, determining gut dysmotility in TLR4−/− mice.
Highlights
The enteric nervous system (ENS), like the central nervous system (CNS), is composed of neurons and enteric glial cells (EGCs)
In the myenteric plexus of Toll-like receptor 4 (TLR4)−/− mouse ileum, the immunoreactivities of glial markers S100β, SOX10 and glial fibrillary acidic protein (GFAP) were significantly higher compared to those found in WT mice
A higher total number of HuC/D+ neurons associated with the absence of changes in SOX10 immunoreactivity was observed in the myenteric plexus of FA-treated ileal segments of TLR4−/− animals, suggesting the onset of adaptive neuroplastic changes following glia disruption [16,40]
Summary
The enteric nervous system (ENS), like the central nervous system (CNS), is composed of neurons and enteric glial cells (EGCs). EGCs are considered the central regulators of ENS homeostasis and the disruption of glial functions has been implicated in several gastrointestinal disorders [1], such as functional gastrointestinal disorders, inflammatory bowel diseases (IBDs) [6,7], chronic idiopathic intestinal pseudo-obstruction, Cells 2020, 9, 838; doi:10.3390/cells9040838 www.mdpi.com/journal/cells. The expression of a common set of biomarkers, including S100β [13], glial fibrillary acidic protein (GFAP) [14], and the transcription factor, SRY-related HMG-box (Sox) 10 [2], reflects the origin of EGCs from neural crest-derived progenitors [15]. The EGCs, like all glial cells, are highly active and may undergo plastic changes in response to harmful stimuli, for example during an inflammatory injury, characterized by “reactive gliosis”
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