Cloning and characterization of an Eph receptor and its ligand ephrin in the developing primary olfactory pathway of the moth Manduca sexta

Abstract

This dissertation investigates possible roles of the Eph family receptor tyrosine kinases and their ligand ephrins in the developing primary olfactory 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, and biochemical assays establishes MsEphrin as a functional ligand for MsEph. In situ labeling with Fc-fusion probes in which IgG Fc was linked to the extracellular domain of MsEph and MsEphrin reveals that both Eph receptors and ephrins are expressed on olfactory receptor cell (ORC) axons during their ingrowth to the primary brain center, the antennal lobe (AL). Eph receptors and ephrins are differentially distributed among identifiable glomeruli such that glomeruli with high receptor staining show little ligand staining and vice versa, indicating a complementary Eph/ephrin expression on subsets of ORC axons innervating particular set of glomeruli. Their expression appears upregulated once ORC axons reach the region where axon sorting occurs. In contrast, neither Eph receptors nor ephrins are detectable in intrinsic components of the AL. In vitro, MsEph and MsEphrin proteins, when present homogeneously in the substratum, inhibit neurite outgrowth from olfactory epithelial explants. Moreover, in patterned substrata, neurites growing on the standard substratum avoid extending onto the substratum containing MsEphrin proteins, with behaviors characterized by turning or stopping at the border. These in vitro observations indicate that MsEphrin can act as an inhibitory/repulsive cue for ORC axons. Based on these in situ and in vitro results, I hypothesize that Eph-ephrin signaling mediates segregation of Eph-positive axons from ephrin-positive axons through repulsive inter-axonal interactions. Simultaneous labeling of ALs for Eph/ephrin and fasciclin II, a homophilic cell adhesion molecule, reveals that fasciclin-positive glomeruli are distributed in a partially overlapping pattern with Eph- or ephrin-positive glomeruli. Thus, Eph receptors and ephrins, together with fasciclin II and other adhesive/repulsive cues, might constitute a combinatorial molecular system in which sorting of ORC axons is determined by the balance of adhesive and repulsive forces.