Roles of glial cells in neural circuit formation: insights from research in insects

Functional morphology of the primary olfactory centers in the brain of the hermit crab Coenobita clypeatus (Anomala, Coenobitidae)

Terrestrial hermit crabs of the genus Coenobita display strong behavioral responses to volatile odors and are attracted by chemical cues of various potential food sources. Several aspects of their sense of aerial olfaction have been explored in recent years including behavioral aspects and structure of their peripheral and central olfactory pathway. Here, we use classical histological methods and immunohistochemistry against the neuropeptides orcokinin and allatostatin as well as synaptic proteins and serotonin to provide insights into the functional organization of their primary olfactory centers in the brain, the paired olfactory lobes. Our results show that orcokinin is present in the axons of olfactory sensory neurons, which target the olfactory lobe. Orcokinin is also present in a population of local olfactory interneurons, which may relay lateral inhibition across the array of olfactory glomeruli within the lobes. Extensive lateral connections of the glomeruli were also visualized using the histological silver impregnation method according to Holmes-Blest. This technique also revealed the structural organization of the output pathway of the olfactory system, the olfactory projection neurons, the axons of which target the lateral protocerebrum. Within the lobes, the course of their axons seems to be reorganized in an axon-sorting zone before they exit the system. Together with previous results, we combine our findings into a model on the functional organization of the olfactory system in these animals.

Analysis of Glial Distribution in Drosophila Adult Brains

Neurons and glia are the two major cell types in the nervous system and work closely with each other to program neuronal interplay. Traditionally, neurons are thought to be the major cells that actively regulate processes like synapse formation, plasticity, and behavioral output. Glia, on the other hand, serve a more supporting role. To date, accumulating evidence has suggested that glia are active participants in virtually every aspect of neuronal function. Despite this, fundamental features of how glia interact with neurons, and their spatial relationships, remain elusive. Here, we describe the glial cell population in Drosophila adult brains. Glial cells extend and tightly associate their processes with major structures such as the mushroom body (MB), ellipsoid body (EB), and antennal lobe (AL) in the brain. Glial cells are distributed in a more concentrated manner in the MB. Furthermore, subsets of glia exhibit distinctive association patterns around different neuronal structures. Whereas processes extended by astrocyte-like glia and ensheathing glia wrap around the MB and infiltrate into the EB and AL, cortex glia stay where cell bodies of neurons are and remain outside of the synaptic regions structured by EB or AL.

Morphological characterization of the antennal lobes in the Mediterranean fruit fly Ceratitis capitata

The medfly Ceratitis capitata is one of the most important pests for horticulture worldwide. The knowledge about anatomy and function of the medfly olfactory system is still limited. The first brain structure to process olfactory information in insects is the antennal lobe (AL), which is composed of its functional and morphological units, the olfactory glomeruli. Here, we present a morphological three-dimensional reconstruction of AL glomeruli in adult brains. We used unilateral antennal backfills of olfactory receptor neurons (ORNs) with neural tracers, revealing the AL structure. We recorded confocal stacks acquired from whole-mount specimens, and analyzed them with the software AMIRA. The ALs in C. capitata are organized in glomeruli which are more tightly packed in the anterior part than the posterior one. Axons of ORNs bilaterally connect the ALs through a commissure between the two ALs. This commissure is formed by several distinct fascicles. Contralateral dye transfer suggests the presence of gap junctions connecting ORNs from both antennae. There was no statistical difference between the average volumes of female ALs (204,166 ± 12,554 μm(3)) and of male ALs (190,287 ± 11,823 μm(3)). In most specimens, we counted 53 glomeruli in each AL, seven of which were sexually dimorphic in size.

GFAP expression and social deficits in transgenic mice overexpressing human sAPPα

Autistic individuals display impaired social interactions and language, and restricted, stereotyped behaviors. Elevated levels of secreted amyloid precursor protein-alpha (sAPPα), the product of α-secretase cleavage of APP, are found in the plasma of some individuals with autism. The sAPPα protein is neurotrophic and neuroprotective and recently showed a correlation to glial differentiation in human neural stem cells (NSCs) via the IL-6 pathway. Considering evidence of gliosis in postmortem autistic brains, we hypothesized that subsets of patients with autism would exhibit elevations in CNS sAPPα and mice generated to mimic this observation would display markers suggestive of gliosis and autism-like behavior. Elevations in sAPPα levels were observed in brains of autistic patients compared to controls. Transgenic mice engineered to overexpress human sAPPα (TgsAPPα mice) displayed hypoactivity, impaired sociability, increased brain glial fibrillary acidic protein (GFAP) expression, and altered Notch1 and IL-6 levels. NSCs isolated from TgsAPPα mice, and those derived from wild-type mice treated with sAPPα, displayed suppressed β-tubulin III and elevated GFAP expression. These results suggest that elevations in brain sAPPα levels are observed in subsets of individuals with autism and TgsAPPα mice display signs suggestive of gliosis and behavioral impairment.

GFAPδ expression in glia of the developmental and adolescent mouse brain

Glial fibrillary acidic protein (GFAP) is the major intermediate filament (IF) protein in astrocytes. In the human brain, GFAP isoforms have unique expression patterns, which indicate that they play distinct functional roles. One isoform, GFAPδ, is expressed by proliferative radial glia in the developing human brain. In the adult human, GFAPδ is a marker for neural stem cells. However, it is unknown whether GFAPδ marks the same population of radial glia and astrocytes in the developing mouse brain as it does in the developing human brain. This study characterizes the expression pattern of GFAPδ throughout mouse embryogenesis and into adolescence. Gfapδ transcripts are expressed from E12, but immunohistochemistry shows GFAPδ staining only from E18. This finding suggests a translational uncoupling. GFAPδ expression increases from E18 to P5 and then decreases until its expression plateaus around P25. During development, GFAPδ is expressed by radial glia, as denoted by the co-expression of markers like vimentin and nestin. GFAPδ is also expressed in other astrocytic populations during development. A similar pattern is observed in the adolescent mouse, where GFAPδ marks both neural stem cells and mature astrocytes. Interestingly, the Gfapδ/Gfapα transcript ratio remains stable throughout development as well as in primary astrocyte and neurosphere cultures. These data suggest that all astroglia cells in the developing and adolescent mouse brain express GFAPδ, regardless of their neurogenic capabilities. GFAPδ may be an integral component of all mouse astrocytes, but it is not a specific neural stem cell marker in mice as it is in humans.

Activation of glial FGFRs is essential in glial migration, proliferation, and survival and in glia-neuron signaling during olfactory system development

Development of the adult olfactory system of the moth Manduca sexta depends on reciprocal interactions between olfactory receptor neuron (ORN) axons growing in from the periphery and centrally-derived glial cells. Early-arriving ORN axons induce a subset of glial cells to proliferate and migrate to form an axon-sorting zone, in which later-arriving ORN axons will change their axonal neighbors and change their direction of outgrowth in order to travel with like axons to their target areas in the olfactory (antennal) lobe. These newly fasciculated axon bundles will terminate in protoglomeruli, the formation of which induces other glial cells to migrate to surround them. Glial cells do not migrate unless ORN axons are present, axons fail to fasciculate and target correctly without sufficient glial cells, and protoglomeruli are not maintained without a glial surround. We have shown previously that Epidermal Growth Factor receptors and the IgCAMs Neuroglian and Fasciclin II play a role in the ORN responses to glial cells. In the present work, we present evidence for the importance of glial Fibroblast Growth Factor receptors in glial migration, proliferation, and survival in this developing pathway. We also report changes in growth patterns of ORN axons and of the dendrites of olfactory (antennal lobe) neurons following blockade of glial FGFR activation that suggest that glial FGFR activation is important in reciprocal communication between neurons and glial cells.

Glial cells in the central nervous system of earthworm, Eisenia fetida

Glial elements in the central nervous system of Eisenia fetida were studied at light- and electron microscopic level. Cells were characterized with the aid of toluidine blue, Glial Fibrillary Acidic Protein (GFAP), S100 staining. We identified neurilemmal-, subneurilemmal-, supporting-nutrifying- and myelinsheath forming glial cells. Both neuronal and non-neuronal elements are S100-immunoreactive in the CNS. Among glial cells neurilemmal and subneurilemmal cells are S100-immunopositive. With the antibody against the S100 protein one band is visible at 15 kDa. GFA P-immunopositive supporting-nutrifying glial cells are localized around neurons and they often appear as cells with many vacuoles. GFA P-positive cell bodies of elongated neurilemmal glial cells are also visible. Western blot analysis shows a single 57 kDa GFA P immunoreactive band in the Eisenia sample. At ultrastructural level contacts between neuronal and glial cells are recognizable. Glial cell bodies and their filopodia contain a granular and vesicular system. Close contacts between neuronal cell membranes and glial filopodia create a special environment for material transport. Vesicles budding off glial cell granules move towards the cell membranes, probably emptying their content with kiss and run exocytosis. The secreted compounds in return may help neuronal survival, provide nutrition, and filopodia may also support neuronal terminals.

Development of a glial network in the olfactory nerve: role of calcium and neuronal activity

In adult olfactory nerves of mammals and moths, a network of glial cells ensheathes small bundles of olfactory receptor axons. In the developing antennal nerve (AN) of the moth Manduca sexta, the axons of olfactory receptor neurons (ORNs) migrate from the olfactory sensory epithelium toward the antennal lobe. Here we explore developmental interactions between ORN axons and AN glial cells. During early stages in AN glial-cell migration, glial cells are highly dye coupled, dividing glia are readily found in the nerve and AN glial cells label strongly for glutamine synthetase. By the end of this period, dye-coupling is rare, glial proliferation has ceased, glutamine synthetase labeling is absent, and glial processes have begun to extend to enwrap bundles of axons, a process that continues throughout the remainder of metamorphic development. Whole-cell and perforated-patch recordings in vivo from AN glia at different stages of network formation revealed two potassium currents and an R-like calcium current. Chronic in vivo exposure to the R-type channel blocker SNX-482 halted or greatly reduced AN glial migration. Chronically blocking spontaneous Na-dependent activity by injection of tetrodotoxin reduced the glial calcium current implicating an activity-dependent interaction between ORNs and glial cells in the development of glial calcium currents.