Glycosylation patterns are sexually dimorphic throughout development of the olfactory system in Manduca sexta

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.

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.

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.

Recognition, presence, and survival of locust central nervous glia in situ and in vitro

Insect glial cells serve functions for the formation, maintenance, and performance of the central nervous system in ways similar to their vertebrate counterparts. Characterization of physiological mechanisms that underlie the roles of glia in invertebrates is largely incomplete, partly due to the lack of markers that universally label all types of glia throughout all developmental stages in various species. Studies on primary cell cultures from brains of Locusta migratoria demonstrated that the absence of anti-HRP immunoreactivity, which has previously been used to identify glial cells in undissociated brains, can also serve as a reliable glial marker in vitro, but only in combination with a viability test. As cytoplasmic membranes of cultured cells are prone to degradation when they lose viability, only cells that are both anti-HRP immunonegative and viable should be regarded as glial cells, whereas the lack of anti-HRP immunoreactivity alone is not sufficient. Cell viability can be assessed by the pattern of nuclear staining with DAPI (4',6-diamidino-2-phenylindole), a convenient, sensitive labeling method that can be used in combination with other immunocytochemical cellular markers. We determined the glia-to-neuron ratio in central brains of fourth nymphal stage of Locusta migratoria to be 1:2 both in situ and in dissociated primary cell cultures. Analysis of primary cell cultures revealed a progressive reduction of glial cells and indicated that dead cells detach from the substrate and vanish from the analysis. Such changes in the composition of cell cultures should be considered in future physiological studies on cell cultures from insect nervous systems.

Olfactory axon guidance: the modified rules

The olfactory system represents a complex model for the investigation of factors that influence the guidance of sensory axon populations to specific targets in the CNS. In the mouse, the projections of approximately 1,000 neuronal subsets, each defined by expression of a distinct odorant receptor (OR), converge at unique glomerular loci in the olfactory bulb (OB). Unlike the case in other sensory systems, proper guidance is achieved without benefit of any known cues in the target itself that are capable of attracting or repelling specific axons. It has long been argued that OR proteins are the critical molecules orchestrating guidance. However, recent studies suggest that axon identity may be dependent on the graded expression of a variety of unique olfactory axon guidance cues. This review focuses attention on these non-OR factors and their roles in olfactory axon guidance.

Lactosamine differentially affects olfactory sensory neuron projections to the olfactory bulb

During embryonic development, olfactory sensory neurons extend axons that form synapses with the dendrites of projection neurons in glomeruli of the olfactory bulb (OB). The glycosyltransferase beta3GnT1 regulates the expression of 1B2-reactive lactosamine glycans that are mosaically distributed among glomeruli. In newborn beta3GnT1-/- mice, lactosamine expression is lost, and many glomeruli fail to form. To determine the role of lactosamine in OB targeting, we analyzed the trajectories of specific OR axon populations and their reactivity with 1B2 in beta3GnT1-/- mice. mI7 axons and P2 axons, both of which are weakly 1B2+ in wild-type mice, fail to grow to their normal positions in the glomerular layer during early postnatal development and never recover in adult mutant mice. In contrast, many M72 axons, which are always lactosamine negative in wild-type mice, survive but are misguided to the extreme anterior OB in neonatal mutant mice and persist as heterotypic glomeruli, even in adult null mice. These results show that the loss of lactosamine differentially affects each OR population. Those that lose their normal expression of lactosamine fail to form stable connections with mitral and tufted cells in the OB, disappear during early postnatal development, and do not recover in adults. Neurons that are normally lactosamine negative, survive early postnatal degeneration in beta3GnT1-/- mice but extend axons that converge on inappropriate targets in the mutant OB.

Patterning the developing and regenerating olfactory system

The olfactory system is a remarkable model for investigating the factors that influence the guidance of sensory axon populations to specific targets in the CNS. Since the initial discovery of the vast odorant receptor (ORs) gene family in rodents and the subsequent finding that these molecules directly influence targeting, several additional olfactory axon guidance cues have been identified. Two of these, ephrins and semaphorins, have well-established functions in patterning axon connections in other systems. In addition, lactosamine-containing glycans are also required for proper targeting and maintenance of olfactory axons, and may also function in other sensory regions. It is now apparent that these and likely other additional molecules are required along with ORs to orchestrate the complex pattern of convergence and divergence that is unique to the olfactory system.

Development-dependent and -independent ubiquitin expression in divisions of the cockroach mushroom body

It has been proposed that the alpha and beta divisions of the mushroom bodies support intermediate and long-term memory whereas the gamma lobes support short-term memory. Here we investigate developmentally dependent versus developmentally independent alterations of mushroom body structure with special emphasis on its lobes. We show that in the cockroach mushroom bodies there are two types of plastic remodeling. One is developmental, in which episodic addition of new circuitry to the alpha and beta lobes is accomplished by newly born Kenyon cells. The second is revealed as a persistent alteration of structure within the gamma lobe. In the alpha/beta lobes, newly generated Kenyon cell axons extend glutamate-immunoreactive collaterals across layers of the axons of mature Kenyon cells. At specific times in each developmental episode (instar) these collaterals express ubiquitin, undergo localized degeneration, and are scavenged by glial cells. In contrast, the mature Kenyon cells that comprise the gamma lobe express detectable ubiquitin throughout each developmental episode. This pattern of ubiquitin expression suggests that the gamma lobe circuitry undergoes continuous modification independent of development.

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.

Beta1,3-N-acetylglucosaminyltransferase 1 glycosylation is required for axon pathfinding by olfactory sensory neurons

During embryonic development, axons from sensory neurons in the olfactory epithelium (OE) extend into the olfactory bulb (OB) where they synapse with projection neurons and form glomerular structures. To determine whether glycans play a role in these processes, we analyzed mice deficient for the glycosyltransferase beta1,3-N-acetylglucosaminyltransferase 1 (beta3GnT1), a key enzyme in lactosamine glycan synthesis. Terminal lactosamine expression, as shown by immunoreactivity with the monoclonal antibody 1B2, is dramatically reduced in the neonatal null OE. Postnatal beta3GnT1-/- mice exhibit severely disorganized OB innervation and defective glomerular formation. Beginning in embryonic development, specific subsets of odorant receptor-expressing neurons are progressively lost from the OE of null mice, which exhibit a postnatal smell perception deficit. Axon guidance errors and increased neuronal cell death result in an absence of P2, I7, and M72 glomeruli, indicating a reduction in the repertoire of odorant receptor-specific glomeruli. By approximately 2 weeks of age, lactosamine is unexpectedly reexpressed in sensory neurons of null mice through a secondary pathway, which is accompanied by the regrowth of axons into the OB glomerular layer and the return of smell perception. Thus, both neonatal OE degeneration and the postnatal regeneration are lactosamine dependent. Lactosamine expression in beta3GnT1-/- mice is also reduced in pheromone-receptive vomeronasal neurons and dorsal root ganglion cells, suggesting that beta3GnT1 may perform a conserved function in multiple sensory systems. These results reveal an essential role for lactosamine in sensory axon pathfinding and in the formation of OB synaptic connections.

Standard three-dimensional glomeruli of the Manduca sexta antennal lobe: a tool to study both developmental and adult neuronal plasticity

The metamorphosing antennal lobe (AL) of the sphinx moth Manduca sexta serves as an established model system for studying neuronal development. To improve our understanding of mechanisms involved in neuronal plasticity, we have analyzed the size, shape, and localization of ten identified glomeruli at three different time points during development and in the adult, viz., (1) 13 days after pupal eclosion (P13), which reflects a time when the basic glomerular map has formed, (2) immediately after adult eclosion (A0), which represents a time when the newly formed glomeruli are uninfluenced by external odors, and (3) 4 days after adult eclosion (A4), which reflects a time when the animals have been exposed to surrounding odors. Our data from normally developing ALs of male M. sexta from P13 to A0 revealed an increase in size of all examined glomeruli of between 40% and 130%, with the strongest increases occurring in two of the three sex-specific glomeruli (cumulus, toroid). From A0 to A4, the cumulus and toroid increased significantly when correlated to AL volume, whereas the other glomeruli reached the sizes gained after A0. This study was based on antibody staining against the ubiquitous synaptic vesicle protein synaptotagmin, confocal laser scan microscopy, and the three-dimensional (3D) analysis tool AMIRA. Tissue permeability and therefore reliability of the staining quality was enhanced by using formalin/methanol fixation. The standard 3D glomeruli introduced in this study can now be used as basic tools for further examination of neuronal plasticity at the level of the identified neuropil structures, viz., the glomeruli of the AL of M. sexta.