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.

Remodeling of an identified motoneuron during metamorphosis: hormonal influences on the growth of dendrites and axon terminals.

During metamorphosis of the tobacco hawkmoth Manduca sexta, the femoral depressor motoneuron (FeDe MN) undergoes remodeling of its dendrites and motor terminals. Previous studies have established that remodeling of MNs during metamorphosis is mediated by the same hormones that control metamorphosis: the ecdysteroids and juvenile hormone (JH). During the pupal stage, the ecdysteroids promote adult-specific growth of MNs in the absence of JH, but JH or its synthetic mimics can interfere with ecdysteroid-mediated growth if applied during early sensitive periods. Hence, the application of a JH mimic (JHM) either systemically or locally to a target muscle has been used to distinguish those aspects of motor-terminal remodeling that are controlled by ecdysteroid action on the CNS from those that are influenced by ecdysteroid action on the peripheral targets. Here, we have extended the analysis of central and peripheral hormonal influences on MN remodeling by injecting JHM locally into the CNS thus altering the hormonal environment of the FeDe MN soma without altering the hormonal environment of its target muscle. Our results demonstrate that adult dendritic growth and motor-terminal growth can be experimentally uncoupled, suggesting that each is regulated independently. JHM application to the CNS perturbed dendritic growth, but had no measurable impact on motor-terminal growth. Peripheral actions of ecdysteroids, therefore, appear sufficient to promote the development of adult-specific motor terminals but not the development of an adult-specific dendritic arbor.

Remodeling of an identified motoneuron during metamorphosis: central and peripheral actions of ecdysteroids during regression of dendrites and motor terminals.

During metamorphosis of the moth Manduca sexta, an identified leg motoneuron, the femoral depressor motoneuron (FeDe MN), undergoes reorganization of its central and peripheral processes. This remodeling is under the control of two insect hormones: the ecdysteroids and juvenile hormone (JH). Here, we asked whether peripheral or central actions of the ecdysteroids influenced specific regressive aspects of MN remodeling. We used stable hormonal mimics to manipulate the hormonal environment of either the FeDe muscle or the FeDe MN soma. Our results demonstrate that motor-terminal retraction and dendritic regression can be experimentally uncoupled, indicating that central actions of ecdysteroids trigger dendritic regression whereas peripheral actions trigger terminal retraction. Our results further demonstrate that discrete aspects of motor-terminal retraction can also be experimentally uncoupled, suggesting that they also are regulated differently.