Dynamic expression of a cell surface protein during rearrangement of epithelial cells in the Manduca wing monolayer

Butterfly pupal wing tissue with an eyespot organizer

Butterfly wing eyespots are developmentally determined at the early pupal stage, when prospective eyespot focal cells underneath the pupal cuticle focal spot function as eyespot organizers in the pupal wing tissue. Here, we performed light microscopy and transmission electron microscopy (TEM) to describe cellular structures of pupal wing tissue with an eyespot organizer immediately after pupation using the Blue Pansy butterfly Junonia orithya. The pupal forewing dorsal epidermis was a pseudostratified monolayer of vertically elongated epidermal cells. The apical portion of the cells adhered laterally to one another, but their medial and basal portions were thinner than the apical portion and were tilted to enclose cells at the center, forming a cellular cluster. The cellular cluster at the organizer was relatively large laterally and vertically. The apical portion of the cells and its corresponding cuticle at the organizer were thicker than those in the surroundings. The innermost cuticle layer was being synthesized, indicating high cuticle synthesis and secretion activity of the cells. At the medial and basal portions of the dorsal epidermis, there were many intracellular and extracellular vacuole-like globules, most likely containing extracellular matrix molecules. Some of the basal processes from epidermal cells extended to form protrusions of the basement membrane, which was often attended by hemocytes. These results suggest that the butterfly eyespot organizer is composed of a single or a few cellular clusters that secrete more cuticle than surrounding clusters, supporting the pupal cuticle hypothesis that cuticle formation is critical for eyespot color pattern determination in butterflies.

Eph receptor expression defines midline boundaries for ephrin-positive migratory neurons in the enteric nervous system of Manduca sexta

Eph receptor tyrosine kinases and their ephrin ligands participate in the control of neuronal growth and migration in a variety of contexts, but the mechanisms by which they guide neuronal motility are still incompletely understood. By using the enteric nervous system (ENS) of the tobacco hornworm Manduca sexta as a model system, we have explored whether Manduca ephrin (MsEphrin; a GPI-linked ligand) and its Eph receptor (MsEph) might regulate the migration and outgrowth of enteric neurons. During formation of the Manduca ENS, an identified set of approximately 300 neurons (EP cells) populates the enteric plexus of the midgut by migrating along a specific set of muscle bands forming on the gut, but the neurons strictly avoid adjacent interband regions. By determining the mRNA and protein expression patterns for MsEphrin and the MsEph receptor and by examining their endogenous binding patterns within the ENS, we have demonstrated that the ligand and its receptor are distributed in a complementary manner: MsEphrin is expressed exclusively by the migratory EP cells, whereas the MsEph receptor is expressed by midline interband cells that are normally inhibitory to migration. Notably, MsEphrin could be detected on the filopodial processes of the EP cells that extended up to but not across the midline cells expressing the MsEph receptor. These results suggest a model whereby MsEphrin-dependent signaling regulates the response of migrating neurons to a midline inhibitory boundary, defined by the expression of MsEph receptors in the developing ENS.

Interaxonal Eph-ephrin signaling may mediate sorting of olfactory sensory axons in Manduca sexta

We have investigated possible roles of the Eph family receptor tyrosine kinases and their ligand ephrins in the developing primary olfactory nerve pathway in the moth Manduca sexta. The Manduca homologs of the Eph receptor (MsEph) and ephrin ligand (MsEphrin) are most closely related to Drosophila Eph and ephrin, respectively. In situ labeling with Fc-fusion probes, in which IgG Fc was linked to the extracellular domain of MsEph (Eph-Fc) or MsEphrin (ephrin-Fc), reveals that both Eph receptors and ephrins are expressed on axons of olfactory receptor cells (ORCs) during their ingrowth to the primary center, the antennal lobe (AL). Interestingly, Eph receptors and ephrins are differentially distributed among identifiable glomeruli such that glomeruli with high receptor staining show little or no ligand staining, and vice versa, suggesting a complementary Eph-ephrin expression by subsets of ORC axons innervating a particular set of glomeruli. In contrast, neither Eph receptors nor ephrins are detectable in intrinsic components of the AL. In vitro, ephrin-Fc and Eph-Fc, when present homogeneously in the substratum, inhibit neurite outgrowth from olfactory epithelial explants. Moreover, in patterned substratum, neurites growing on the standard substratum turn or stop after encountering the test substratum containing ephrin-Fc. These in vitro observations indicate that MsEphrin can act as an inhibitor/repulsive cue for ORC axons. Based on results from in situ and in vitro experiments, we hypothesize that Eph receptors and ephrins mediate axon sorting and fasciculation through repulsive axon-axon interactions.

Different isoforms of fasciclin II are expressed by a subset of developing olfactory receptor neurons and by olfactory-nerve glial cells during formation of glomeruli in the moth Manduca sexta

During development of the primary olfactory projection, olfactory receptor axons must sort by odor specificity and seek particular sites in the brain in which to create odor-specific glomeruli. In the moth Manduca sexta, we showed previously that fasciclin II, a cell adhesion molecule in the immunoglobulin superfamily, is expressed by the axons of a subset of olfactory receptor neurons during development and that, in a specialized glia-rich "sorting zone," these axons segregate from nonfasciclin II-expressing axons before entering the neuropil of the glomerular layer. The segregation into fasciclin II-positive fascicles is dependent on the presence of the glial cells in the sorting zone. Here, we explore the expression patterns for different isoforms of Manduca fasciclin II in the developing olfactory system. We find that olfactory receptor axons express transmembrane fasciclin II during the period of axonal ingrowth and glomerulus development. Fascicles of TM-fasciclin II+ axons target certain glomeruli and avoid others, such as the sexually dimorphic glomeruli. These results suggest that TM-fasciclin II may play a role in the sorting and guidance of the axons. GPI-linked forms of fasciclin II are expressed weakly by glial cells associated with the receptor axons before they reach the sorting zone, but not by sorting-zone glia. GPI-fasciclin II may, therefore, be involved in axon-glia interactions related to stabilization of axons in the nerve, but probably not related to sorting.

The tetraspanin superfamily in insects

We describe four members of the tetraspanin/TM4SF superfamily of proteins that were identified in expressed sequence tag projects on the antennae of Manduca sexta moths and Apis mellifera honey bees. The three moth genes are expressed in the sensillar epithelium of male antennae, and some are expressed in female antennae, haemocytes, wing scale cell primordia and/or embryonic tissues. These proteins are probably involved in diverse cellular processes, much like their vertebrate homologues. A phylogenetic analysis of all known tetraspanins, including thirty-seven members of the superfamily revealed by the Drosophila melanogaster genome and twenty in the nematode Caenorhabditis elegans genome, reveals some phylum-specific gene amplification, in particular a contiguous array of eighteen genes in the D. melanogaster genome.

Development of a glia-rich axon-sorting zone in the olfactory pathway of the moth Manduca sexta

Olfactory receptor cells (ORCs) of a particular odor tuning are dispersed in the olfactory epithelium, but their axons converge on distinct glomeruli in primary olfactory centers. As a consequence, axon associations must change to bring axons of ORCs with the same odor specificity together. Studies in Manduca sexta have indicated that just before they enter the antennal lobe (AL), ORC axons undergo extreme reorganization, finally entering the AL in fascicles destined for subsets of glomeruli. This axon-sorting zone is heavily populated by glial cells, and ORC axon growth cones often are in close physical contact with the glia. In moths rendered glia deficient, ORC axons fail to fasciculate in this region. Using propidium iodide to label nuclei and 5-bromo-2'-deoxyuridine to monitor proliferation, we found that the glia in the sorting zone arise from the AL, appearing shortly after the first ORC axons arrive. Experimental removal of some or all of the sensory innervation revealed that proliferation of sorting-zone glia is triggered by ORC axons. A second set of glia arises in the antenna and migrates along the antennal nerve toward the brain, populating the nerve after the establishment of the sorting zone. Development of this type of glial cell is independent of contact of the ORC axons with their central targets. We conclude that the sorting zone arises from CNS glia in response to ingrowth of ORC axons, and a critical number of glia must be present in the sorting zone for axons to correctly establish new neighbor-neighbor associations.

A role for fasciclin II in the guidance of neuronal migration

The insect cell adhesion receptor fasciclin II is expressed by specific subsets of neural and non-neural cells during embryogenesis and has been shown to control growth cone motility and axonal fasciculation. Here we demonstrate a role for fasciclin II in the guidance of migratory neurons. In the developing enteric nervous system of the moth Manduca sexta, an identified set of neurons (the EP cells) undergoes a stereotyped sequence of migration along the visceral muscle bands of the midgut prior to their differentiation. Probes specific for Manduca fasciclin II show that while the EP cells express fasciclin II throughout embryogenesis, their muscle band pathways express fasciclin II only during the migratory period. Manipulations of fasciclin II in embryonic culture using blocking antibodies, recombinant fasciclin II fragments, and enzymatic removal of glycosyl phosphatidylinositol-linked fasciclin II produced concentration-dependent reductions in the extent of EP cell migration. These results support a novel role for fasciclin II, indicating that this homophilic adhesion molecule is required for the promotion or guidance of neuronal migration.

Expression of the surface antigen A2B7 in adult and developing honeybee olfactory pathway

In order to identify molecules involved in the development of the honeybee olfactory pathway, hybridoma technology has been used. Among different cell lines, A2B7 has been selected. It produces a specific antibody for a surface glycoprotein of 91 kDa. This protein is mainly expressed by both the antennal receptor cells and mushroom body neurons. Based on (i) the spatio-temporal pattern of expression during pupal development; (ii) the cell surface location of the antigen; and (iii) the partial molecular characterization of the antigen, a putative role for this protein in axonal fasciculation and guidance is discussed.

Expression of a cell surface protein during morphogenesis of the reproductive system in Manduca sexta embryos : Both moths and mammals have an indifferent stage of genital differentiation

New antibody markers have allowed more refined examinations of embryogenesis. Features are being found that were overlooked in whole and sectioned embryos stained with traditional histochemical labels. Two monoclonal antibodies that recognize two different cell surface proteins in Manduca sexta label cells of the developing reproductive system. These specific immunolabels reveal that during a brief period of Manduca embryogenesis, rudiments of both male and female genital ducts are present in a single embryo. This transient phase of genital differentiation parallels the transient indifferent stage known to occur during development of reproductive systems in many vertebrate embryos. At the end of this indifferent stage, one of the two pairs of genital ducts retracts and degenerates. The dynamic expression of the two surface proteins on cells involved in morphogenesis of both the female and male reproductive systems also suggests that these proteins are important in orchestrating the specific cellular interactions that occur between mesodermal cells of the genital ducts and the nearby ventral ectoderm.

Developmental expression of heterotrimeric G proteins in the nervous system of Manduca sexta

The heterotrimeric G proteins are a conserved family of guanyl nucleotide-binding proteins that appear in all eukaryotic cells but whose developmental functions are largely unknown. We have examined the developmental expression of representative G proteins in the developing nervous system of the moth Manduca sexta. Using affinity-purified antisera against different G alpha subunits, we found that each of the G proteins exhibited distinctive patterns of expression within the developing central nervous system (CNS), and that these patterns underwent progressive phases of spatial and temporal regulation that corresponded to specific aspects of neuronal differentiation. Several of the G proteins examined (including Gs alpha and G(o) alpha) were expressed in an apparently ubiquitous manner in all neurons, but other proteins (including Gi alpha) were ultimately confined to a more restricted subset of cells in the mature CNS. Although most of the G proteins examined could be detected within the central ganglia, only G(o) alpha-related proteins were seen in the developing peripheral nerves; manipulations of G protein activity in cultured embryos suggested that this class of G protein may contribute to the regulation of neuronal motility during axonal outgrowth. G(o) alpha-related proteins were also localized to the developing axons and terminals of the developing adult limb during metamorphosis. These intracellular signaling molecules may, therefore, play similar developmental roles in both the embryonic and postembryonic nervous system.

Developmental expression of G proteins in a migratory population of embryonic neurons

Directed neuronal migration contributes to the formation of many developing systems, but the molecular mechanisms that control the migratory process are still poorly understood. We have examined the role of heterotrimeric G proteins (guanyl nucleotide binding proteins) in regulating the migratory behavior of embryonic neurons in the enteric nervous system of the moth, Manduca sexta. During the formation of the enteric nervous system, a group of approx. 300 enteric neurons (the EP cells) participate in a precise migratory sequence, during which the undifferentiated cells populate a branching nerve plexus that lies superficially on the visceral musculature. Once migration is complete, the cells then acquire a variety of position-specific neuronal phenotypes. Using affinity-purified antisera against different G protein subtypes, we found no apparent staining for any G protein in the EP cells prior to their migration. Coincident with the onset of migration, however, the EP cells commenced the expression of one particular G protein, Go alpha. The intensity of immunostaining continued to increase as migration progressed, with Go alpha immunoreactivity being detectable in the leading processes of the neurons as well as their somata. The identity of the Go alpha-related proteins was confirmed by protein immunoblot analysis and by comparison with previously described forms of Go alpha from Drosophila. When cultured embryos were treated briefly with aluminium fluoride, a compound known to stimulate the activity of heterotrimeric G proteins, both EP cell migration and process outgrowth were inhibited. The effects of aluminium fluoride were potentiated by alpha toxin, a pore-forming compound that by itself caused no significant perturbations of migration. In preliminary experiments, intracellular injections of the non-hydrolyzable nucleotide GTP gamma-S also inhibited the migration of individual EP cells, supporting the hypothesis that G proteins play a key role in the control of neuronal motility in this system. In addition, once migration was complete, the expression of Go alpha-related proteins in the EP cells underwent a subsequent phase of regulation, so that only certain phenotypic classes among the differentiated EP cells retained detectable levels of Go alpha immunoreactivity. Thus Go may perform multiple functions within the same population of migratory neurons in the course of embryonic development.

Rearrangement of epithelial cell types in an insect wing monolayer is accompanied by differential expression of a cell surface protein

The distribution of adhesive molecules on surfaces of cells represents covert information for specifying positions of cells within a tissue. In insect wing epithelia where cells are arranged in two monolayers separated by an extracellular space, these adhesive molecules are found on basal and lateral surfaces of cells. Protein 3B11 is one surface protein whose expression changes in concert with movement and alignment of cells in wing monolayers of Manduca as well as with migration of tracheoles between the two monolayers of the wing. As epithelial cells segregate into periodic, transverse rows of alternating cell types (scale cells and generalized epithelial cells), the expression of 3B11 changes from a uniform distribution throughout the epithelial monolayer to a distribution correlated with a cell's final position and phenotype. Initially protein 3B11 is uniformly expressed on nonadherent surfaces of cells, but with the inception of cell rearrangement, differential expression of 3B11 on basolateral surfaces of cells--both adherent and nonadherent surfaces--becomes a function of epithelial cell type. At the completion of the cell movements associated with segregation of cell types, 3B11 is once again uniformly expressed throughout the wing epithelium. Also, as the upper and lower epithelial monolayers interact at their basal surfaces during adult development, 3B11 is expressed at the interface between the two epithelial monolayers and presumably functions in the nonspecific interaction between these monolayers. Examining the expression patterns of this protein as well as other adhesion molecules in wing epithelia should reveal general rules about either the simplicity or the complexity of the molecular prepatterns that orchestrate overt tissue patterns in epithelial monolayers.