Cytoarchitectural analysis of the neuron‐to‐glia association in the dorsal root ganglia of normal and diabetic mice

Abstract

Dorsal root ganglia (DRGs) host the somata of sensory neurons which convey information from the periphery to the central nervous system. These neurons have heterogeneous size and neurochemistry, and those of small-to-medium size, which play an important role in nociception, form two distinct subpopulations based on the presence (peptidergic) or absence (non-peptidergic) of transmitter neuropeptides. Few investigations have so far addressed the spatial relationship between neurochemically different subpopulations of DRG neurons and glia. We used a whole-mount mouse lumbar DRG preparation, confocal microscopy and computer-aided 3D analysis to unveil that IB4 + non-peptidergic neurons form small clusters of 4.7 ± 0.26 cells, differently from CGRP + peptidergic neurons that are, for the most, isolated (1.89 ± 0.11 cells). Both subpopulations of neurons are ensheathed by a thin layer of satellite glial cells (SGCs) that can be observed after immunolabeling with the specific marker glutamine synthetase (GS). Notably, at the ultrastructural level we observed that this glial layer was discontinuous, as there were patches of direct contact between the membranes of two adjacent IB4 + neurons. To test whether this cytoarchitectonic organization was modified in the diabetic neuropathy, one of the most devastating sensory pathologies, mice were made diabetic by streptozotocin (STZ). In diabetic animals, cluster organization of the IB4 + non-peptidergic neurons was maintained, but the neuro-glial relationship was altered, as STZ treatment caused a statistically significant increase of GS staining around CGRP + neurons but a reduction around IB4 + neurons. Ultrastructural analysis unveiled that SGC coverage was increased at the interface between IB4 + cluster-forming neurons in diabetic mice, with a 50% reduction in the points of direct contacts between cells. These observations demonstrate the existence of a structural plasticity of the DRG cytoarchitecture in response to STZ.

Glial Cell Line-Derived Neurotrophic Factor Family Ligands, Players at the Interface of Neuroinflammation and Neuroprotection: Focus Onto the Glia

“…In rats with diabetic neuropathy induced by streptozotocin (STZ) administration (Viisanen et al, 2020) BT44 treatment resulted in the decrease in the area (Figure 2D), but did not influence in the number of IBA1positive cells (Figure 2E). This result is fully in line with recently published data of Ciglieri et al (2020). These authors observed cytoarchitectonic changes related to glia-neuronal interactions in DRGs upon the treatment with STZ, while the overall number of SGC remained largely unaffected.…”

“…These authors observed cytoarchitectonic changes related to glia-neuronal interactions in DRGs upon the treatment with STZ, while the overall number of SGC remained largely unaffected. In particular, STZ differentially affected the expression of SGC marker surrounding different subpopulations of DRG neurons and disturbed glia-neuron and neuron-neuron contacts in this tissue (Ciglieri et al, 2020).…”

2D <em>vs</em> 3D morphological analysis of dorsal root ganglia in health and painful neuropathy

“…46 The glial layer could also be discontinuous leading to a direct contact between membranes of two adjacent neurons. 47,51 SGCs around DRG neurons are connected each other by gap junctions consisting of protein connexins 46 and the most abundant so far described in SGCs are Cx43 and Cx32. 52,53 Moreover, SGCs show dye coupling as observed after injection with Lucifer yellow.…”

“…54 These features clearly indicate that SGCs carry out several important functions for DRG homeostasis, providing trophic and mechanical support for sensory neurons, regulating ion concentration, diffusion and trafficking of molecules between neurons. 48 SGCs can be identified by the expression of several glial markers such as glutamine synthetase, 51,55 GFAP, 44,56,57 S-100, 58,59 vimentin, 58 Cx42, 53 Kir4.1 potassium channels 60 transporters. 45,61 In particular, Nascimento and colleagues 58 have postulated the existence of a heterogeneous population of SGCs in DRGs after the observation that the expression pattern of S-100 and vimentin in SGCs is related to the neuron size they enwrapped.…”

“…The heterogeneity of SGCs is consistent with our recent observation that different neuron-glia arrangements can be found in DRG for specific neuronal phenotypes. 51 Due to the peculiar neuron-glia arrangement, an increasing number of studies have investigated the potential impact of transglial communication in shaping sensory transmission. The seminal work of Devor and Wall 62 demonstrated electrical coupling between DRG neurons arranged in small clusters.…”