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

Abstract

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.