In vitro analyses of interactions between olfactory receptor growth cones and glial cells that mediate axon sorting and glomerulus formation

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

Investigators over the years have noted many striking similarities in the structural organization and function of neural circuits in higher invertebrates and vertebrates. In more recent years, the discovery of similarities in the cellular and molecular mechanisms that guide development of these circuits has driven a revolution in our understanding of neural development. Cellular mechanisms discovered to underlie axon pathfinding in grasshoppers have guided productive studies in mammals. Genes discovered to play key roles in the patterning of the fruitfly's central nervous system have subsequently been found to play key roles in mice. The diversity of invertebrate species offers to investigators numerous opportunities to conduct experiments that are harder or impossible to do in vertebrate species, but that are likely to shed light on mechanisms at play in developing vertebrate nervous systems. These experiments elucidate the broad suite of cellular and molecular interactions that have the potential to influence neural circuit formation across species. Here we focus on what is known about roles for glial cells in some of the important steps in neural circuit formation in experimentally advantageous insect species. These steps include axon pathfinding and matching to targets, dendritic patterning, and the sculpting of synaptic neuropils. A consistent theme is that glial cells interact with neurons in two-way, reciprocal interactions. We emphasize the impact of studies performed in insects and explore how insect nervous systems might best be exploited next as scientists seek to understand in yet deeper detail the full repertory of functions of glia in development.

Localization of a GABA transporter to glial cells in the developing and adult olfactory pathway of the moth Manduca sexta

Glial cells have several critical roles in the developing and adult olfactory (antennal) lobe of the moth Manduca sexta. Early in development, glial cells occupy discrete regions of the developing olfactory pathway and processes of gamma-aminobutyric acid (GABA)ergic neurons extend into some of these regions. Because GABA is known to have developmental effects in a variety of systems, we explored the possibility that the glial cells express a GABA transporter that could regulate GABA levels to which olfactory neurons and glial cells are exposed. By using an antibody raised against a characterized high-affinity M. sexta GABA transporter with high sequence homology to known mammalian GABA transporters (Mbungu et al. [1995] Arch. Biochem. Biophys. 318:489-497; Umesh and Gill [2002] J. Comp. Neurol. 448:388-398), we found that the GABA transporter is localized to subsets of centrally derived glial cells during metamorphic adult development. The transporter persists into adulthood in a subset of the neuropil-associated glial cells, but its distribution pattern as determined by light-and electron-microscopic-level immunocytochemistry indicates that it could not serve to regulate GABA concentration in the synaptic cleft. Instead, its role is more likely to regulate extracellular GABA levels within the glomerular neuropil. Expression in the sorting zone glial cells disappears after the period of olfactory receptor axon ingrowth, but may be important during ingrowth if GABA regulates axon growth. Glial cells take up GABA, and that uptake can be blocked by L-2,4-diaminobutyric acid (DABA). This is the first molecular evidence that the central glial cell population in this pathway is heterogeneous.

Roles of specific membrane lipid domains in EGF receptor activation and cell adhesion molecule stabilization in a developing olfactory system

Background

Reciprocal interactions between glial cells and olfactory receptor neurons (ORNs) cause ORN axons entering the brain to sort, to fasciculate into bundles destined for specific glomeruli, and to form stable protoglomeruli in the developing olfactory system of an experimentally advantageous animal species, the moth Manduca sexta. Epidermal growth factor receptors (EGFRs) and the cell adhesion molecules (IgCAMs) neuroglian and fasciclin II are known to be important players in these processes.

Methodology/Principal Findings

We report in situ and cell-culture studies that suggest a role for glycosphingolipid-rich membrane subdomains in neuron-glia interactions. Disruption of these subdomains by the use of methyl-β-cyclodextrin results in loss of EGFR activation, depletion of fasciclin II in ORN axons, and loss of neuroglian stabilization in the membrane. At the cellular level, disruption leads to aberrant ORN axon trajectories, small antennal lobes, abnormal arrays of olfactory glomerul, and loss of normal glial cell migration.

Conclusions/Significance

We propose that glycosphingolipid-rich membrane subdomains (possible membrane rafts or platforms) are essential for IgCAM-mediated EGFR activation and for anchoring of neuroglian to the cytoskeleton, both required for normal extension and sorting of ORN axons.

Activation of the Wnt/beta-catenin signaling reporter in developing mouse olfactory nerve layer marks a specialized subgroup of olfactory ensheathing cells

Wnt reporter TOPgal mice carry a beta-galactosidase (betagal) gene under the control of the Wnt/beta-catenin signaling responsive elements. We found that the intensely immunolabeled betagal+ cells were co-immunolabeled with Nestin and formed a tangentially oriented single-cell layer in the "connecting or docking zone" where the olfactory sensory axons attached to the brain surface during mid-gestation. During early postnatal development, betagal+ cells were located in the inner olfactory nerve layer (ONLi) and co-labeled with olfactory ensheathing cell (OEC) markers S100beta and NPY but not with lineage-specific markers for neurons, oligodendrocytes, astrocytes, and microglia, demonstrating that the TOPgal marked a subpopulation of OECs. By confocal microscopy, we found that TOPgal activated processes extended along the developing glomerulus and formed multiple tunnel-like structures that ensheathe and bridge olfactory sensory axonal bundles from ONLi to the glomerulus, which may play a key role in glomerulus formation and convergent sorting of the peripheral olfactory axons.

Acetylcholine-mediated axon-glia signaling in the developing insect olfactory system

In the olfactory system of the sphinx moth Manduca sexta, migration of neuropil glial cells is triggered by olfactory receptor axons and depends on intraglial Ca(2+) signaling. It is not known, however, how receptor axons and glial cells communicate and whether Ca(2+) signaling is a consequence of this communication. We studied Ca(2+) increases in glial cells in vivo and in situ, evoked by electrical stimulation of olfactory receptor axons in pupae and by odor stimulation of receptor neurons in adult moths. Axonal activity leads to Ca(2+) increases in neuropil glial cells that are blocked by nicotinic acetylcholine receptor antagonists and can be mimicked by acetylcholine and carbachol application. In addition, Ca(2+) transients were abolished by removal of external Ca(2+) and blockage of voltage-gated Ca(2+) channels. During development, acetylcholine-mediated Ca(2+) signaling could first be elicited at stage 6, the time when neuropil glial cells start to migrate. Glial migration was reduced after injection of nicotinic antagonists into pupae. The results show that Ca(2+) signaling can be induced by acetylcholine release from olfactory receptor axons, which activates nicotinic acetylcholine receptors and leads to voltage-gated Ca(2+) influx. The results further suggest that cholinergic signaling in the olfactory system is required for glial cell migration in Manduca.

Cadherin expression in the developing mouse olfactory system

Although odor receptors have been implicated in establishing the topography of olfactory sensory neurons (OSNs) in the olfactory bulb (OB), it is likely other molecules are also involved. The cadherins (CDHs) are a large family of cell adhesion molecules that mediate cell:cell interactions elsewhere in the central nervous system. However, their distribution and role in the olfactory system have remained largely unexplored. We previously demonstrated that intracellular binding partners of cadherins, the catenins, have unique spatiotemporal patterns of expression in the developing olfactory system. To further our understanding of cadherin function within the developing olfactory system, we now report on the localization of 11 classical cadherins-CDH1, 2, 3, 4, 5, 6, 8, 10, 11, 13, and 15. We demonstrate the expression of all but CDH5 and CDH15 in neuronal and/or glial cells in primary olfactory structures. CDH1 and CDH2 are expressed by OSNs; CDH2 expression closely parallels that seen for gamma-catenin in OSN axons. CDH3 and CDH11 are expressed by olfactory ensheathing glia, which surround OSN axons in the outer OB. CDH2, CDH4, and CDH6 are expressed within neuropil. CDH2, CDH4, CDH6, CDH8, CDH10, CDH11, and CDH13 are expressed by projection neurons within the main and accessory OBs. We conclude that cadherin proteins in the developing olfactory system are positioned to underlie the formation of the odorant map and local circuits within the OB.

Insect neuronal cultures: an experimental vehicle for studies of physiology, pharmacology and cell interactions

The current status of insect neuronal cultures is discussed and their contribution to our understanding of the insect nervous system is explored. Neuronal cultures have been developed from a wide range of insect species and from all developmental stages. These have been used to study the morphological development of insect neurones and some of the extrinsic factors that affect this process. In addition, they have been used to investigate the physiology of sodium, potassium and calcium channels and the pharmacology of acetylcholine and GABA receptors. Insect neurones have also been grown in culture with muscle and glial cells to study cell interactions.

Activation of epidermal growth factor receptor mediates receptor axon sorting and extension in the developing olfactory system of the moth Manduca sexta

During development of the adult olfactory system of the moth Manduca sexta, olfactory receptor neurons extend axons from the olfactory epithelium in the antenna into the brain. As they arrive at the brain, interactions with centrally derived glial cells cause axons to sort and fasciculate with other axons destined to innervate the same glomeruli. Here we report studies indicating that activation of the epidermal growth factor receptor (EGFR) is involved in axon ingrowth and targeting. Blocking the EGFR kinase domain pharmacologically leads to stalling of many axons in the sorting zone and nerve layer as well as abnormal axonal fasciculation in the sorting zone. We also find that neuroglian, an IgCAM known to activate the EGFR through homophilic interactions in other systems, is transiently present on olfactory receptor neuron axons and on glia during the critical stages of the sorting process. The neuroglian is resistant to extraction with Triton X-100 in the sorting zone and nerve layer, possibly indicating its stabilization by homophilic binding in these regions. Our results suggest a mechanism whereby neuroglian molecules on axons and possibly sorting zone glia bind homophilically, leading to activation of EGFRs, with subsequent effects on axon sorting, pathfinding, and extension, and glomerulus development.

Embryonic origin of theDrosophila brain neuropile

Neurons of the Drosophila larval brain are formed by a stereotyped set of neuroblasts. As differentiation sets in, neuroblast lineages produce axon bundles that initially form a scaffold of unbranched fibers in the center of the brain primordium. Subsequently, axons elaborate interlaced axonal and dendritic arbors, which, together with sheath-like processes formed by glial cells, establish the neuropile compartments of the larval brain. By using markers that visualize differentiating axons and glial cells, we have analyzed the formation of neuropile compartments and their relationship to neuroblast lineages. Neurons of each lineage extend their axons as a cohesive tract ("primary axon bundle"). We generated a map of the primary axon bundles that visualizes the location of the primary lineages in the brain cortex where the axon bundles originate, the trajectory of the axon bundles into the neuropile, and the relationship of these bundles to the early-formed scaffold of neuropile pioneer tracts (Nassif et al. [1998] J. Comp. Neurol. 402:10-31). The map further shows the growth of neuropile compartments at specific locations around the pioneer tracts. Following the time course of glial development reveals that glial processes, which form prominent septa around compartments in the larval brain, appear very late in the embryonic neuropile, clearly after the compartments themselves have crystallized. This suggests that spatial information residing within neurons, rather than glial cells, specifies the location and initial shape of neuropile compartments.

Distinct types of glial cells populate the Drosophila antenna

BACKGROUND

The development of nervous systems involves reciprocal interactions between neurons and glia. In the Drosophila olfactory system, peripheral glial cells arise from sensory lineages specified by the basic helix-loop-helix transcription factor, Atonal. These glia wrap around the developing olfactory axons early during development and pattern the three distinct fascicles as they exit the antenna. In the moth Manduca sexta, an additional set of central glia migrate to the base of the antennal nerve where axons sort to their glomerular targets. In this work, we have investigated whether similar types of cells exist in the Drosophila antenna.

RESULTS

We have used different P(Gal4) lines to drive Green Fluorescent Protein (GFP) in distinct populations of cells within the Drosophila antenna. Mz317::GFP, a marker for cell body and perineural glia, labels the majority of peripheral glia. An additional approximately 30 glial cells detected by GH146::GFP do not derive from any of the sensory lineages and appear to migrate into the antenna from the brain. Their appearance in the third antennal segment is regulated by normal function of the Epidermal Growth Factor receptor and small GTPases. We denote these distinct populations of cells as Mz317-glia and GH146-glia respectively. In the adult, processes of GH146-glial cells ensheath the olfactory receptor neurons directly, while those of the Mz317-glia form a peripheral layer. Ablation of GH146-glia does not result in any significant effects on the patterning of the olfactory receptor axons.

CONCLUSION

We have demonstrated the presence of at least two distinct populations of glial cells within the Drosophila antenna. GH146-glial cells originate in the brain and migrate to the antenna along the newly formed olfactory axons. The number of cells populating the third segment of the antenna is regulated by signaling through the Epidermal Growth Factor receptor. These glia share several features of the sorting zone cells described in Manduca.

Neuron-glia interaction in the insect nervous system

In all complex organisms, glial cells are pivotal for neuronal development and function. Insects are characterized by having only a small number of these cells, which nevertheless display a remarkable molecular diversity. An intricate relationship between neurons and glia is initially required for glial migration and during axonal patterning. Recent data suggest that in organisms such as Drosophila, a prime role of glial cells lies in setting boundaries to guide and constrain axonal growth.