A role for fasciclin II in the guidance of neuronal migration

Fasciclin 2 plays multiple roles in promoting cell migration within the developing nervous system of Manduca sexta

The coordination of neuronal and glial migration is essential to the formation of most nervous systems, requiring a complex interplay of cell-intrinsic responses and intercellular guidance cues. During the development of the enteric nervous system (ENS) in Manduca sexta (tobacco hornworm), the IgCAM Fasciclin 2 (Fas2) serves several distinct functions to regulate these processes. As the ENS forms, a population of 300 neurons (EP cells) undergoes sequential phases of migration along well-defined muscle pathways on the visceral mesoderm to form a branching Enteric Plexus, closely followed by a trailing wave of proliferating glial cells that enwrap the neurons. Initially, both the neurons and glial cells express a GPI-linked form of Fas2 (GPI-Fas2), which helps maintain cell-cell contact among the pre-migratory neurons and later promotes glial ensheathment. The neurons then switch isoforms, predominantly expressing a combination of transmembrane isoforms lacking an intracellular PEST domain (TM-Fas2 PEST-), while their muscle band pathways on the midgut transiently express transmembrane isoforms containing this domain (TM-Fas2 PEST+). Using intracellular injection protocols to manipulate Fas2 expression in cultured embryos, we found that TM-Fas2 promotes the directed migration and outgrowth of individual neurons in the developing ENS. Concurrently, TM-Fas2 expression by the underlying muscle bands is also required as a substrate cue to support normal migration, while glial expression of GPI-Fas2 helps support their ensheathment of the migratory neurons. These results demonstrate how a specific IgCAM can play multiple roles that help coordinate neuronal and glial migration in the developing nervous system.

Amyloid Precursor Proteins Are Dynamically Trafficked and Processed during Neuronal Development

Proteolytic processing of the Amyloid Precursor Protein (APP) produces beta-amyloid (Aβ) peptide fragments that accumulate in Alzheimer's Disease (AD), but APP may also regulate multiple aspects of neuronal development, albeit via mechanisms that are not well understood. APP is a member of a family of transmembrane glycoproteins expressed by all higher organisms, including two mammalian orthologs (APLP1 and APLP2) that have complicated investigations into the specific activities of APP. By comparison, insects express only a single APP-related protein (APP-Like, or APPL) that contains the same protein interaction domains identified in APP. However, unlike its mammalian orthologs, APPL is only expressed by neurons, greatly simplifying an analysis of its functions in vivo. Like APP, APPL is processed by secretases to generate a similar array of extracellular and intracellular cleavage fragments, as well as an Aβ-like fragment that can induce neurotoxic responses in the brain. Exploiting the complementary advantages of two insect models (Drosophila melanogaster and Manduca sexta), we have investigated the regulation of APPL trafficking and processing with respect to different aspects of neuronal development. By comparing the behavior of endogenously expressed APPL with fluorescently tagged versions of APPL and APP, we have shown that some full-length protein is consistently trafficked into the most motile regions of developing neurons both in vitro and in vivo. Concurrently, much of the holoprotein is rapidly processed into N- and C-terminal fragments that undergo bi-directional transport within distinct vesicle populations. Unexpectedly, we also discovered that APPL can be transiently sequestered into an amphisome-like compartment in developing neurons, while manipulations targeting APPL cleavage altered their motile behavior in cultured embryos. These data suggest that multiple mechanisms restrict the bioavailability of the holoprotein to regulate APPL-dependent responses within the nervous system. Lastly, targeted expression of our double-tagged constructs (combined with time-lapse imaging) revealed that APP family proteins are subject to complex patterns of trafficking and processing that vary dramatically between different neuronal subtypes. In combination, our results provide a new perspective on how the regulation of APP family proteins can be modulated to accommodate a variety of cell type-specific responses within the embryonic and adult nervous system.

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.

Cell adhesion molecules in Drosophila synapse development and function

Synapse is a highly specialized inter-cellular structure between neurons or between a neuron and its target cell that mediates cell-cell communications. Ample results indicate that synaptic adhesion molecules are critically important in modulating the complexity and specificity of the synapse. And disruption of adhesive properties of synapses may lead to neurodevelopmental or neurodegenerative diseases. In this review, we will use the Drosophila NMJ as a model system for glutamatergic synapses to discuss the structure and function of homophilic and heterophilic synaptic adhesion molecules with special focus on recent findings in neurexins and neuroligins in Drosophila.

A translational continuum of model systems for evaluating treatment strategies in Alzheimer's disease: isradipine as a candidate drug

A growing body of evidence supports the 'calcium hypothesis' of Alzheimer's disease (AD), which postulates that a variety of insults might disrupt the homeostatic regulation of neuronal calcium (Ca(2+)) in the brain, resulting in the progressive symptoms that typify the disease. However, despite ongoing efforts to develop new methods for testing therapeutic compounds that might be beneficial in AD, no single bioassay permits both rapid screening and in vivo validation of candidate drugs that target specific components of the Ca(2+) regulatory machinery. To address this issue, we have integrated four distinct model systems that provide complementary information about a trial compound: the human neuroblastoma MC65 line, which provides an in vitro model of amyloid toxicity; a transgenic Drosophila model, which develops age-dependent pathologies associated with AD; the 3×TgAD transgenic mouse, which recapitulates many of the neuropathological features that typify AD; and the embryonic nervous system of Manduca, which provides a novel in vivo assay for the acute effects of amyloid peptides on neuronal motility. To demonstrate the value of this 'translational suite' of bioassays, we focused on a set of clinically approved dihydropyridines (DHPs), a class of well-defined inhibitors of L-type calcium channels that have been suggested to be neuroprotective in AD. Among the DHPs tested in this study, we found that isradipine reduced the neurotoxic consequences of β-amyloid accumulation in all four model systems without inducing deleterious side effects. Our results provide new evidence in support of the Ca(2+) hypothesis of AD, and indicate that isradipine represents a promising drug for translation into clinical trials. In addition, these studies also demonstrate that this continuum of bioassays (representing different levels of complexity) provides an effective means of evaluating other candidate compounds that target specific components of the Ca(2+) regulatory machinery and that therefore might be beneficial in the treatment of AD.

A developmental study of enteric neuron migration in the grasshopper using immunological probes

Motility of enteric plexus neurons in the grasshopper Locusta migratoria depends critically on the NO/cGMP signaling cascade. This is reflected in a strong NO-dependent cGMP staining in migrating enteric midgut neurons. In contrast, first cGMP immunoreactivity (cGMP-IR) in the foregut enteric ganglia was detected clearly after the main migratory processes have taken place. Thus, expression of cGMP-IR in migrating neurons is a distinct phenomenon restricted to neurons forming midgut and foregut plexus, that does not occur during convergence of neurons forming the enteric ganglia. Analysis of time lapse video microscopy of migrating midgut neurons together with an immunofluorescence study of midgut cellular structures suggests a contribution of the midgut musculature to enteric neuron guidance. Additionally, during midgut plexus formation a fasciculating signal for enteric neuron neurites appears to be provided by the cell adhesion molecule Fasciclin I.

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.

Expression of two different isoforms of fasciclin II during postembryonic central nervous system remodeling in Manduca sexta

Insect metamorphosis serves as a useful model to investigate postembryonic development in the central nervous system, because the transformation between larval and adult life is accompanied by a remodeling of neural circuitry. Most changes are controlled by ecdysteroids, but activity-dependent mechanisms and cell surface signals also play a role. This immunocytochemical study investigates the expression patterns of two isoforms of the neural cell adhesion molecule, fasciclin II (FasII), during postembryonic ventral nerve cord remodeling in the moth, Manduca sexta. Both the expression of the glycosyl-phosphatidylinositol (GPI)-linked isoform and the transmembrane isoform of Manduca FasII (TM-MFasII) are regulated in a stereotyped spatio-temporal pattern. TM-MFasII is expressed in a stage-specific manner in a subset of neurons. Subsets of central axons express high levels during outgrowth supporting a functional role for TM-FasII during pathfinding. Dendritic localization is not found at any stage of metamorphosis, suggesting no homophilic interactions of TM-MFasII during central synapse development. GPI-MFasII is expressed in a stage-specific manner, most likely only in glial cells. The larval and adult stages show almost no GPI-MFasII expression, whereas during pupal life, positive GPI-MFasII labeling is present around synaptotagmin-negative tracts or commissures, so that either homophilic stabilization of glial boundaries or heterophilic neuron-glial interactions possibly stabilize the axons within their tracts. GPI-MFasII expression is not co-localized with synaptotagmin-positive central terminals, rendering a role for synapse development unlikely. Neither isoform is expressed in all neurons of a specific class at any developmental stage, indicating that MFasII functions are restricted to specific subsets of neurons or to individual neurons.

Hormone-dependent expression of fasciclin II during ganglionic migration and fusion in the ventral nerve cord of the moth Manduca sexta

The ventral nerve cord of holometabolous insects is reorganized during metamorphosis. A prominent feature of this reorganization is the migration of subsets of thoracic and abdominal larval ganglia to form fused compound ganglia. Studies in the hawkmoth Manduca sexta revealed that pulses of the steroid hormone 20-hydroxyecdysone (20E) regulate ganglionic fusion, but little is known about the cellular mechanisms that make migration and fusion possible. To test the hypothesis that modulation of cell adhesion molecules is an essential component of ventral nerve cord reorganization, we used antibodies selective for either the transmembrane isoform of the cell adhesion receptor fasciclin II (TM-MFas II) or the glycosyl phosphatidylinositol-linked isoform (GPI-MFas II) to study cell adhesion during ganglionic migration and fusion. Our observations show that expression of TM-MFas II is regulated temporally and spatially. GPI-MFas II was expressed on the surface of the segmental ganglia and the transverse nerve, but no evidence was obtained for regulation of GPI-MFas II expression during metamorphosis of the ventral nerve cord. Manipulation of 20E titers revealed that TM-MFas II expression on neurons in migrating ganglia is regulated by hormonal events previously shown to choreograph ganglionic migration and fusion. Injections of actinomycin D (an RNA synthesis inhibitor) or cycloheximide (a protein synthesis inhibitor) blocked ganglionic movement and the concomitant increase in TM-MFas II, suggesting that 20E regulates transcription of TM-MFas II. The few neurons that showed TM-MFas II immunoreactivity independent of endocrine milieu were immunoreactive to an antiserum specific for eclosion hormone (EH), a neuropeptide regulator of molting.

How to innervate a simple gut: familiar themes and unique aspects in the formation of the insect enteric nervous system

Like the vertebrate enteric nervous system (ENS), the insect ENS consists of interconnected ganglia and nerve plexuses that control gut motility. However, the insect ENS lies superficially on the gut musculature, and its component cells can be individually imaged and manipulated within cultured embryos. Enteric neurons and glial precursors arise via epithelial-to-mesenchymal transitions that resemble the generation of neural crest cells and sensory placodes in vertebrates; most cells then migrate extensive distances before differentiating. A balance of proneural and neurogenic genes regulates the morphogenetic programs that produce distinct structures within the insect ENS. In vivo studies have also begun to decipher the mechanisms by which enteric neurons integrate multiple guidance cues to select their pathways. Despite important differences between the ENS of vertebrates and invertebrates, common features in their programs of neurogenesis, migration, and differentiation suggest that these relatively simple preparations may provide insights into similar developmental processes in more complex systems.

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.

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.

The insect homologue of the amyloid precursor protein interacts with the heterotrimeric G protein Go alpha in an identified population of migratory neurons

The amyloid precursor protein (APP) is the source of Abeta fragments implicated in the formation of senile plaques in Alzheimer's disease (AD). APP-related proteins are also expressed at high levels in the embryonic nervous system and may serve a variety of developmental functions, including the regulation of neuronal migration. To investigate this issue, we have cloned an orthologue of APP (msAPPL) from the moth, Manduca sexta, a preparation that permits in vivo manipulations of an identified set of migratory neurons (EP cells) within the developing enteric nervous system. Previously, we found that EP cell migration is regulated by the heterotrimeric G protein Goalpha: when activated by unknown receptors, Goalpha induces the onset of Ca2+ spiking in these neurons, which in turn down-regulates neuronal motility. We have now shown that msAPPL is first expressed by the EP cells shortly before the onset of migration and that this protein undergoes a sequence of trafficking, processing, and glycosylation events that correspond to discrete phases of neuronal migration and differentiation. We also show that msAPPL interacts with Goalpha in the EP cells, suggesting that msAPPL may serve as a novel G-protein-coupled receptor capable of modulating specific aspects of migration via Goalpha-dependent signal transduction.

Plexin B3 promotes neurite outgrowth, interacts homophilically, and interacts with Rin

Background

Plexins, known to date as receptors of semaphorins, are implicated in semaphorin-mediated axon repulsion and growth cone collapse. However, subtype-specific functions of the majority of the nine members of the mammalian plexin family are largely unknown. In order to investigate functional properties of B-plexins, we analyzed the expression of human and murine plexin B3 and expressed full-length human plexins B2 (B2) and B3 (B3) in NIH-3T3 cells.

Results

Unexpectedly, B3 strongly and B2 moderately stimulate neurite outgrowth of primary murine cerebellar neurons. Both plexins mediate Ca²⁺/Mg²⁺-dependent cell aggregation due to homophilic trans-interaction, which is strong in the case of B3 and moderate for B2. Using different deletion constructs we show that the sema domain of B3 is essential for homophilic interaction. Using yeast two-hybrid analysis, we identified the neuron-specific and calmodulin-binding Ras-related GTPase Rin as an interaction partner of the intracellular part of B3, but not of B2. Rin, also known for its neurite outgrowth-inducing characteristics, co-localizes and co-immunoprecipitates with B3 in co-transfected COS-7 cells.

Conclusion

Our data suggest an involvement of homophilic interaction of B3 in semaphorin-independent signaling mechanisms positively influencing neuronal morphogenesis or function. Furthermore the neuron-specific small GTPase Rin is involved in downstream signaling of plexin B3.

The zebrafish down syndrome cell adhesion molecule is involved in cell movement during embryogenesis

The Down syndrome cell adhesion molecule (Dscam) is a protein overexpressed in the brains of Down syndrome patients and implicated in mental retardation. Dscam is involved in axon guidance and branching in Drosophila, but cellular roles in vertebrates have yet to be elucidated. To understand its role in vertebrate development, we cloned the zebrafish homolog of Dscam and showed that it shares high amino acid identity and structure with the mammalian homologs. Zebrafish dscam is highly expressed in developing neurons, similar to what has been described in Drosophila and mouse. When dscam expression is diminished by morpholino injection, embryos display few neurons and their axons do not enter stereotyped pathways. Zebrafish dscam is also present at early embryonic stages including blastulation and gastrulation. Its loss results in early morphogenetic defects. dscam knockdown results in impaired cell movement during epiboly as well as in subsequent stages. We propose that migrating cells utilize dscam to remodel the developing embryo.

Leech filamin and Tractin: markers for muscle development and nerve formation

The Lan3-14 and Laz10-1 monoclonal antibodies recognize a 400 kDa antigen that is specifically expressed by all muscle cells in leech. We show that the antigen recognized by both antibodies is a member of the filamin family of actin binding proteins. Leech filamin has two calponin homology domains and 35 filamin/ABP-repeat domains. In addition, we used the Laz10-1 antibody to characterize the development of the segmentally iterated dorsoventral flattener muscles. We demonstrate that the dorsoventral flattener muscle develops as three discrete bundles of myofibers and that CNS axons pioneering the DP nerve extend only along the middle bundle. Interestingly, the middle dorsoventral muscle anlage is associated with only non-neuronal expression of the L1-family cell adhesion molecule Tractin. This expression is transient and occurs at the precise developmental stages when DP nerve formation takes place. Based on these findings we propose that the middle dorsoventral muscle anlagen provides a substrate for early axonal outgrowth and nerve formation and that this function may be associated with differential expression of distinct cell adhesion molecules.

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.

Cell type-specific expression of fasciclin II isoforms reveals neuronal-glial interactions during peripheral nerve growth

During the formation of the insect peripheral nervous system (PNS), the cell adhesion receptor fasciclin II has been shown to play a prominent role in axonal fasciculation and synapse formation during motor neuron outgrowth. In the moth Manduca, fasciclin II (MFas II) is expressed both as a transmembrane isoform (TM-MFas II) and a glycosyl phosphatidylinositol-linked isoform (GPI-MFas II). By using RNA and antibody probes, we have shown that these two isoforms are expressed in nonoverlapping patterns: TM-MFas II is expressed exclusively by neurons and becomes localized to their most motile regions, while GPI-MFas II is expressed primarily by the glial cells that ensheath the peripheral nerves. This cell-type specificity of expression allowed us to monitor the nature of neuronal-glial interactions during PNS development. The outgrowth of TM-MFas II-positive axons in many regions preceded the arrival of GPI-MFas II-expressing glial processes that enwrapped them. In a few key locations, however, GPI-MFas II-positive glial cells differentiated before the arrival of the first axons and prefigured their subsequent trajectories. Prior inhibition of GPI-MFas II expression disrupted the subsequent outgrowth of axons at these locations but not elsewhere in the PNS. Our results suggest that the two isoforms of MFas II play distinct roles with respect to cellular motility and nerve formation.

Drosophila fasciclinII is required for the formation of odor memories and for normal sensitivity to alcohol

Drosophila fasciclinII (fasII) mutants perform poorly after olfactory conditioning due to a defect in encoding, stabilizing, or retrieving short-term memories. Performance was rescued by inducing the expression of a normal transgene just before training and immediate testing. Induction after training but before testing failed to rescue performance, showing that Fas II does not have an exclusive role in memory retrieval processes. The stability of odor memories in fasII mutants are indistinguishable from control animals when initial performance is normalized. Like several other mutants deficient in odor learning, fasII mutants exhibit a heightened sensitivity to ethanol vapors. A combination of behavioral and genetic strategies have therefore revealed a role for Fas II in the molecular operations of encoding short-term odor memories and conferring alcohol sensitivity. The preferential expression of Fas II in the axons of mushroom body neurons furthermore suggests that short-term odor memories are formed in these neurites.

Remodeling of motor terminals during metamorphosis of the moth Manduca sexta: expression patterns of two distinct isoforms of Manduca fasciclin II

During metamorphosis of the moth Manduca sexta, the neuromuscular system of the thoracic legs is reorganized dramatically. Larval leg muscles degenerate at the end of larval life, and new adult leg muscles develop during the ensuing pupal stage. Larval leg motoneurons persist, but undergo substantial remodeling of central and peripheral processes. As part of our on-going investigation of mechanisms underlying the remodeling of motor terminals, we have used antisera generated against Manduca-specific isoforms of the homophilic adhesion molecule fasciclin II (MFas II) to label motor terminals during metamorphosis. Antisera generated against the glycosyl-phosphatidylinositol (GPI) -linked isoform of MFas II (GPI-MFas II) labeled the motor nerves at all stages and seemed to be associated with glial cells ensheathing the peripheral nerves. In addition, the anti-GPI-MFas II antisera labeled regions associated with synaptic boutons at both larval and adult stages. In contrast, antisera generated against a transmembrane isoform of MFas II (TM-MFas II) only labeled specific neuronal processes at discrete intervals during remodeling. Identified leg motoneurons (such as the femoral depressor motoneuron) expressed detectable levels of TM-MFas II in their peripheral processes only during phases of motor-terminal retraction and initial stages of motor-terminal re-growth. Putative modulatory neurons (such as the unpaired median neurons), however, expressed TM-MFas II in their processes during larval stages as well as during remodeling. Use of the isoform-specific anti-MFas II antisera provided a novel method for visualizing remodeling of motor terminals during metamorphosis and helped distinguish different components of the motor nerves and neuromuscular junction.

Different isoforms of fasciclin II play distinct roles in the guidance of neuronal migration during insect embryogenesis

During the formation of the enteric nervous system (ENS) of the moth Manduca sexta, identified populations of neurons and glial cells participate in precisely timed waves of migration. The cell adhesion receptor fasciclin II is expressed in the developing ENS and is required for normal migration. Previously, we identified two isoforms of Manduca fasciclin II (MFas II), a glycosyl phosphatidylinositol-linked isoform (GPI-MFas II) and a transmembrane isoform (TM-MFas II). Using RNA and antibody probes, we found that these two isoforms were expressed in cell type-specific patterns: GPI-MFas II was expressed by glial cells and newly generated neurons, while TM-MFas II was confined to differentiating neurons. The expression of each isoform also corresponded to the motile state of the different cell types: GPI-MFas II was detected on tightly adherent or slowly spreading cells, while TM-MFas II was expressed by actively migrating neurons and was localized to their most motile regions. Manipulations of each isoform in embryo culture showed that they played distinct roles: whereas GPI-MFas II acted strictly as an adhesion molecule, TM-MFas II promoted the motility of the EP cells as well as maintaining fasciculation with their pathways. These results indicate that precisely regulated patterns of isoform expression govern the functions of fasciclin II within the developing nervous system.