Satellite Glia Cells Research Articles

Satellite glia cells in dorsal root ganglia express functional NMDA receptors

Satellite glia cells (SGCs), within the dorsal root ganglia (DRG), surround the somata of most sensory neurons. SGCs have been shown to interact with sensory neurons and appear to be involved in the processing of afferent information. We found that in rat DRG various N-methyl-d-aspartate receptor (NMDAr) subunits were expressed in SGCs in intact ganglia and in vitro. In culture, when SGCs were exposed to brief pulses of NMDA they evoked transient increases in cytoplasmic calcium that were inhibited by specific NMDA blockers (MK-801, AP5) while they were Mg2+ insensitive indicating that SGCs express functional NMDAr. The percentage of NMDA responsive SGCs was similar in mixed- (SGCs plus neurons) and SGC-enriched cultures. The pattern of the magnitude changes of the NMDA-evoked response was similar in SGCs and DRG neurons when they were in close proximity, suggesting that the NMDA response of SGCs and DRG neurons is modulated by their interactions. Treating the cultures with nerve growth factor, and/or prostaglandin E2 did not alter the percentage of SGCs that responded to NMDA. Since glutamate appears to be released within the DRG, the detection of functional NMDAr in SGCs suggests that their NMDAr activity could contribute to the interactions between neurons and SGCs. In summary we demonstrated for the first time that SGCs express functional NMDAr.

Novel distribution of cluster of differentiation 200 adhesion molecule in glial cells of the peripheral nervous system of rats and its modulation after nerve injury

This study examined CD200 expression in different peripheral nerves and ganglia. Intense CD200 immunoreactivity was consistently localized in unmyelinated nerve fibers as opposed to a faint immunostaining in the myelinated nerve fibers. By light microscopy, structures resembling the node of Ranvier and Schmidt-Lanterman incisures in the myelinated nerve fibers displayed CD200 immunoreactivity. Ultrastructural study revealed CD200 expression on the neurilemma of Schwann cells whose microvilli and paranodal loops at the node of Ranvier were immunoreactive. The CD200 immunoexpression was also localized in the satellite glial cells of sensory and autonomic ganglia and in the enteric glial cells. Double labeling of CD200 with specific antigens of satellite glia or Schwann cells in the primary cultures of dorsal root ganglia had shown a differential expression of CD200 in the peripheral glial cells. The existence of CD200 in glial cells in the peripheral nervous system (PNS) was corroborated by the expression of CD200 mRNA and protein in a rat Schwann cell line RSC96. Using the model of crush or transected sciatic nerve, it was found that CD200 expression was attenuated or diminished at the site of lesion. A remarkable feature, however, was an increase in incidence of CD200-labelled Schmidt-Lanterman incisures proximal to the injured site at 7 days postlesion. Because CD200 has been reported to impart immunosuppressive signal, we suggest that its localization in PNS glial cells may play a novel inhibitory role in immune homeostasis in both normal and pathological conditions.

Calcitonin gene-related peptide differentially regulates gene and protein expression in trigeminal glia cells: Findings from array analysis

Calcitonin gene-related peptide (CGRP) is a multifunctional neuropeptide implicated in inflammatory diseases involving trigeminal ganglion nerve activation. Within trigeminal ganglia, satellite glia and Schwann cells are found in close association with neuronal cell bodies and fibers, respectively, and are known to express functional CGRP receptors. The goal of this study was to use array analysis to provide a more comprehensive understanding of CGRP regulation of inflammatory proteins and genes in trigeminal glia. Primary trigeminal ganglia cultures enriched for glia were treated with 500 nM CGRP for 8 or 24 h. CGRP caused a >3-fold increase in the level of 19 cytokines 8 h after CGRP treatment and the levels of each of these cytokines remained significantly elevated over basal unstimulated levels at 24 h. While mRNA levels of many genes involved in mitogen-activated protein (MAP) kinase signaling were increased 8 h after CGRP treatment, the number of responsive genes was greatly increased at 24 h. Specifically, CGRP was shown to temporally regulate expression of multiple MAP kinases as well as numerous MAP kinase-responsive genes including transcription factors, scaffold/anchoring proteins, and cell cycle proteins. Thus, our data provide evidence of an emerging role of CGRP as an important modulator of trigeminal ganglion glia by stimulating cytokine release as well as inducing expression of a diverse array of proteins involved in MAP kinase signaling.

How do the satellite glia cells of the dorsal root ganglia respond to stressed neurons? – nitric oxide saga from embryonic development to axonal injury in adulthood

Dorsal root ganglia (DRG) respond to peripheral nerve injury by up-regulating nitric oxide (NO) production by neurons and glia in addition to local fibroblasts, endothelium and macrophages. We hypothesise that NO produced from these cells has specific roles. We have shown that when neuronal NO synthase (nNOS) is blocked in axotomised DRG, neurons undergo degenerative changes (Thippeswamy et al., 2001, 2007a). Further, we demonstrated that increased neuronal NO production, in response to axotomy/growth factor-deprivation in vitro, signals glial cells to produce trophic factors to support neuronal survival (Thippeswamy et al., 2005a). Recently, we found that treating satellite glia-neuron co-cultures with nNOS inhibitor, 7-nitroindazole (7NI), decreases the number of nestin+ cells that show neuron-like morphology. Cultured/axotomised DRG also upregulate inducible NOS (iNOS) in non-neuronal cells. Therefore, it is plausible that degenerative changes following nNOS inhibition are also due to iNOS-mediated excessive NO production by non-neuronal cells, which indeed is cytotoxic. NG-nitro-l-arginine methylester (L-NAME), the pan NOS inhibitor did not significantly change nNOS+ neuron number in axotomised DRG compared to 7NI suggesting that iNOS-mediated NO contributes to the degenerative process. In this paper, these findings from our and others' past work on NO-mediated neuron-glia signalling in axotomised DRG are discussed.

Emerging Importance of Neuron-Satellite Glia Interactions within Trigeminal Ganglia in Craniofacial Pain

Pain in the head and face, which can be very severe and debilitating, often involves activation of trigeminal ganglion nerves. The craniofacial symptoms can manifest as acute or transient conditions such as toothaches and headaches, or can transform into more chronic conditions such as migraine, rhinosinusitis, temporomandibular joint (TMJ) disorder, or trigeminal neuralgia. Traditionally, it is known that peripheral tissue injury or inflammation leads to excitation of trigeminal nerves that release inflammatory molecules in the periphery as well as facilitate transmission of nociceptive signals to the central nervous system. However, findings from recent studies have demonstrated that peripheral tissue injury or inflammation also leads to increased interactions between neuronal cell bodies and satellite glial cells within the trigeminal ganglion. These cell-to-cell interactions, which involve the transfer of key regulatory mediators via channels or gap junctions as well as paracrine signaling, are thought to play an important role in the induction and maintenance of peripheral sensitization of trigeminal nociceptors. The focus of this review will be on understanding the importance of the increased signaling between neuronal cell bodies and satellite glia cells in trigeminal ganglia to the development of persistent pain.

Hypomorphic Sox10 alleles reveal novel protein functions and unravel developmental differences in glial lineages

The transcription factor Sox10 regulates early neural crest development, specification of neural crest-derived lineages and terminal differentiation of oligodendrocytes in the central nervous system. Here, we generated two novel hypomorphic Sox10 alleles in the mouse. Mutant mice either expressed a Sox10 protein with a triple alanine substitution in the dimerization domain, or a Sox10 protein with a deletion in the central portion that we define as a cell-specific transactivation domain. Phenotypic analysis revealed important roles for a functional dimerization domain and the newly defined novel transactivation domain in melanocyte and enteric nervous system development, whereas early neural crest development and oligodendrocyte differentiation were surprisingly little disturbed in both mutants. Unique requirements were additionally detected for the novel transactivation domain in satellite glia differentiation and during Schwann cell myelination, whereas DNA-dependent dimerization was needed for immature Schwann cells to enter the promyelinating stage. These two hypomorphic alleles thus uncover novel functions of Sox10 in satellite glia and Schwann cells during late developmental stages and reveal important developmental differences between these two types of peripheral glia and oligodendrocytes regarding their reliance on Sox10.

Evidence of Postnatal Neurogenesis in Dorsal Root Ganglion: Role of Nitric Oxide and Neuronal Restrictive Silencer Transcription Factor

The various mechanisms underlying postnatal neurogenesis from discrete CNS regions have emerged recently. However, little is known about postnatal neurogenesis in dorsal root ganglion (DRG). BrdU incorporation and subsequent immunostaining for BrdU, neural stem cell marker, nestin and neuronal marker, PGP 9.5 have provided evidence for postnatal neurogenesis in DRG. We further demonstrate, in vivo and in vitro, that nitric oxide (NO) regulates neural stem cells (nestin+) proliferation and, possibly, differentiation into neurons. Surprisingly, nerve growth factor (NGF) had no effect on nestin+ cells proliferation. Axotomy or NGF-deprivation of DRG neurons-satellite glia co-culture increases NO production by neurons and treating with a NO synthase (NOS) inhibitor, N G-nitro-L-arginine methylester (L-NAME) in vitro or 7-nitroindazole (7NI) in vivo, causes a significant increase in nestin+ cell numbers. However, a soluble guanylyl cyclase (sGC) blocker, 1H-[1, 2, 4] oxadiazolo [4, 3-a] quinoxalin-1-one (ODQ) treatment of NGF-deprived DRG neurons-satellite glia co-culture had no significant effect on nestin+ cell numbers. This implies NO regulates nestin+ cell proliferation independent of cGMP. We hypothesised that the neuronal-restrictive silencer transcription factor (NRSF, also termed REST), a master regulator of neuronal genes in non-neuronal cells, may be modulated by NO in satellite glia cultures. A NO donor, dimethyl-triamino-benzidine (DETA)-NO treatment of satellite glia cell cultures results in a significant increase in the NRSF/REST mRNA expression. The majority of cultured satellite glia cells express nestin, and also show increased levels of NOS, thus L-NAME treatment of these cultures causes a dramatic reduction in NRSF/REST mRNA. Overall these results suggest that NO inhibits neurogenesis in DRG and this is correlated with modulation of NRSF, a known modulator of differentiation.

Glial-mediated neuroprotection: evidence for the protective role of the NO-cGMP pathway via neuron-glial communication in the peripheral nervous system.

The NO-cGMP pathway has emerged as a neuroprotective signaling system involved in communication between neurons and glia. We have previously shown that axotomy or nerve growth factor (NGF)-deprivation of dorsal root ganglion (DRG) neurons leads to increased production of NO and at the same time an increase in cGMP production in their satellite glia cells. Blockade of NO or its receptor, the cGMP synthesizing enzyme soluble guanylate cyclase (sGC), results in apoptosis of neurons and glia. We now show that co-culture of neonatal DRG neurons with either Schwann cells pre-treated with an NO donor or a membrane-permeant cGMP analogue; or neurons maintained in the medium from Schwann cell cultures treated in the same way, prevents neuronal apoptosis. Both NO donor and cGMP treatment of Schwann cells results in synthesis of NGF and NT3. Furthermore, if the Schwann cells are previously infected with adenoviral vectors expressing a dominant negative sGC mutant transgene, treatment of these Schwann cells with an NO donor now fails to prevent neuronal apoptosis. Schwann cells treated in this way also fail to express neither cGMP nor neurotrophins. These findings suggest NO-sGC-cGMP-mediated NGF and NT3 synthesis by Schwann cells protect neurons.

The roles of nitric oxide in dorsal root ganglion neurons.

The functions of nitric oxide (NO) in primary somatosensory neurons are reviewed. During the early development of these neurons the neuronal isoform of nitric oxide synthase (nNOS) is expressed during neurite extension. As their axons extend peripherally and start to take up nerve growth factor (NGF), nNOS starts to disappear from the majority of these neurons. A small number (less than 5%) continue to express nNOS, and for this small population NO may have a role in synaptic transmission. Following peripheral nerve section in adult rats, nNOS is reexpressed in many small peptidergic DRG neurons that have been axotomized. At the same time cGMP synthesis is increased in satellite glia cells. From culture studies, it was established that NGF negatively regulates nNOS synthesis in DRG neurons and that block of NO production leads to neuronal death. Further recent data is reviewed that supports the view that NO has a neuroprotective action preventing loss of DRG neurons and facilitates regeneration.

Bax and caspases are inhibited by endogenous nitric oxide in dorsal root ganglion neurons in vitro.

Axotomised dorsal root ganglia (DRG) neurons show an increased expression of neuronal nitric oxide synthase (nNOS) compared with neurons from the intact ganglia. Increased nNOS expression resulted in synthesis of nitric oxide (NO) and the subsequent activation of cGMP in satellite glia cells surrounding the DRG neuron soma. In dissociated DRG we have demonstrated that the increase in nNOS expression is regulated by nerve growth factor and that the subsequent inhibition of NO production or cGMP synthesis precipitates apoptosis of neurons expressing nNOS and some non-nNOS neurons. Hence, NO or the NO-cGMP cascade appears to have a neuroprotective action in trophic factor-deprived DRG neurons. In the present study, using immunocytochemistry, we have investigated some of the factors associated with apoptosis that are activated when nNOS activity is blocked with NOS inhibitor in DRG neurons in vitro. Marked elevation of bax was observed within a few hours of NOS inhibition in nNOS containing neurons, whereas pretreatment of cultures with l-arginine completely abolished this effect in almost all nNOS neurons and 8-bromo-cGMP in some neurons. The apoptosis precipitated by NOS inhibition was also partially prevented by a number of caspase inhibitors; of those a caspase-9 blocker was the most effective. These observations further support the neuroprotective role of NO/NO-cGMP in stressed DRG neurons in an autocrine fashion that involves the suppression of bax, caspase-3 and -9 activation.

Evidence That Nitric Oxide-Induced Synthesis of cGMP Occurs in a Paracrine but Not an Autocrine Fashion and That the Site of Its Release Can Be Regulated: Studies in Dorsal Root Ganglia in Vivo and in Vitro

As nitric oxide is a gas, it cannot be stored and has to be synthesized as required. This suggests that it could be released wherever nitric oxide synthase (NOS) is activated and due to its unstable state will react with appropriate targets at this site of production. In both dissociated dorsal root ganglion (DRG) cultures and in acutely isolated, but intact, DRG, treatment with capsaicin or bradykinin caused cGMP synthesis, which could be blocked by NOS inhibitors. The cGMP was synthesized in cells different from those expressing the neuronal isoform of NOS (nNOS). In dissociated cultures many of the cells stimulated to produce cGMP were neurons, whereas in isolated ganglia they were always satellite glia cells. Surprisingly, the satellite glia cells surrounding the nNOS-containing neurons did not contain cGMP. Following nerve section in adult rats, many axotomized ganglion neurons expressed nNOS. Again in these axotomized ganglia, most cGMP was expressed in the satellite glia surrounding nNOS-negative neurons. However, an nNOS-selective inhibitor reduced the cGMP present in these axotomized ganglia, suggesting that the cGMP synthesized is stimulated by NO (nitrogen monoxide) produced by nNOS. In both dissociated cultures and axotomized ganglia, nNOS-containing processes were observed close to cGMP-positive cells. These observations lead to the suggestion that NO acts in a paracrine fashion when stimulating the synthesis of cGMP and may not be synthesized at all sites containing nNOS.

Avian transitin expression mirrors glial cell fate restrictions during neural crest development.

During development, trunk neural crest cells give rise to three primary classes of derivatives: glial cells, melanocytes, and neurons. As part of an effort to learn how neural crest diversification is regulated, we have produced monoclonal antibodies (MAbs) that recognize antigens expressed by neural crest cells early in development. One of these, MAb 7B3 (7B3), was found to recognize an avian transitin-like protein by co-immunostaining with a series of transitin-specific monoclonal antibodies and by Western blot analysis. In neural crest cell cultures, we found that 7B3 initially recognizes the majority of neural crest cells as they emerge from the neural tube. Subsequently, 7B3-immunoreactivity (IR) is progressively restricted to a smaller subpopulation of cells. In fully differentiated trunk neural crest cell cultures, 7B3-IR is expressed only by cells that do not express neuronal markers and lack melanin granules. During development in vivo, 7B3-IR is evident in neural crest cells on the medial, but not the lateral migration pathway, suggesting that it is not expressed by melanocyte precursors. Later, the antigen is detected in non-neuronal, presumptive glial cells in dorsal root ganglia (DRG) and sympathetic ganglia, as well as along ventral roots. Cultures of E5 DRG confirm that 7B3-IR is restricted to non-neuronal cells of ganglia, many of which closely associate with neuronal processes. Therefore, of the three major classes of differentiated trunk neural crest derivatives, 7B3 exclusively recognizes glial cells, including both satellite glia and Schwann cells. Since the pattern of 7B3 expression in vitro mirrors the pattern of glial cell fate-restrictions in the trunk neural crest lineage, and is expressed by neural crest-derived glia in vivo, we conclude that 7B3 is an early pan-glial marker for neural crest-derived glial cells and their precursors.

Expression of zebrafish fkd6 in neural crest-derived glia

The zebrafish fkd6 gene is a marker for premigratory neural crest. In this study, we analyze later expression in putative glia of the peripheral nervous system. Prior to neural crest migration, fkd6 expression is downregulated in crest cells. Subsequently, expression appears initially in loose clusters of cells in positions corresponding to cranial ganglia. Double labelling with a neuronal marker shows that fkd6-expressing cells are not differentiated neurones and generally lie peripheral to neurones in ganglia. Later, expression appears associated with the posterior lateral line and other cranial nerves. For the posterior lateral line nerve, we show that fkd6-labeling extends caudally along this nerve in tight correlation with lateral line primordium migration and axon elongation. Expression in colourless mutant embryos is consistent with these cells being satellite glia and Schwann cells.

Protein synthesis in neurons and satellite glial cells of rats after global cerebral ischemia caused by cardiac arrest

Protein synthesis rates are increased in light and dark neurons and satellite glia cells in various brain areas in rats with completely restituted neurological status after cardiac arrest. However, the increase is provided by different cell types. Only free light neurons are reactive in the hippocampal sector CA1, while both free light neurons as light neurons with glia are reactive in sector CA4; intense protein synthesis is observed in all types of the cerebral cortex neurons and in cortical neurons and in the satellite glial cells of the lateral cerebellar hemispheres.

Transport of glial cell acid phosphatase by endoplasmic reticulum into damaged axons

Earlier observations indicated that cutting the photoreceptor (R) cell perikarya in the retina of the flies Musca domestica and Calliphora erythrocephala resulted in an extremely rapid degeneration of the R-cell axons. The process manifested itself within minutes and secondary lysosomes appeared early on in the degenerating axons. In this study, biochemical assays of the classical lysosomal marker enzyme acid phosphatase (AcPase) showed a rapid increase in activity upon cutting the retina, reaching a peak around 5 min, followed by a drop and a subsequent slow steady rise in activity between about 4 and 24 h after the cut. Inhibitor studies indicated that at least 2 different acid phosphatases were involved, which showed identical activity changes. EM cytochemical studies with either beta-glycerophosphate or cytidine monophosphate as substrates indicated that the degeneration-dependent AcPase activity originated exclusively in one specific glial cell type in the distal lamina, the satellite glia. These cells are contiguous with R-cell axons in the pseudocartridges, directly proximal to the basement membrane of the retina. In normal uncut flies, reaction product indicative of AcPase activity was found in satellite glial cells in both rough ER (and perinuclear cisternae) and in smooth ER, the 2 types of cisternae being clearly in continuity. The electron-dense reaction product resulting from AcPase activity often showed continuity between the smooth ER of satellite glial cells and extracellular sites between the satellite glia and R-axons. This may result from the low levels of AcPase being synthesized in rough ER, and then exported to extra-cellular sites. Within minutes of cutting the retina the freqeuncy of satellite glia smooth ER staining for AcPase increases significantly. Increases in AcPase activity were subsequently found sequentially in extracellular sites, in axons in the distal lamina, and finally in the axon terminals. The time course for these increases in AcPase activity correlated well with the time course of the fine-structural pathological changes in the axons. A significant feature of the satellite glia cells is the massive packing of unidirectional microtubules (MTs), running perpendicular to the projections of the R-axons--in the same direction as AcPase transport. The MTs terminate at the glial membrane directly adjacent to the axon. The smooth ER cisternae interdigitate among the MTs and may be structurally connected to them by thin fibrils. Thick-section cytochemistry and stereology were used to reveal the 3-dimensional extent of the AcPase-reactive smooth ER system. Evidence is presented which suggests that this system represents GERL, a specialized region of smooth ER involved in lysosome formation, that in this glial cell appears to function also to export hydrolytic enzymes.