Primary olfactory axons form ectopic glomeruli in mice lacking p75NTR.

The restricted expression of the low affinity nerve growth factor receptor p75NTR by olfactory ensheathing cells suggests that this molecule is involved in the development of the olfactory nerve pathway. To begin to understand the role of p75NTR, we examined the development of the primary olfactory system in p75NTR(-/-) and wild-type mice. Our results demonstrate that, although p75NTR is not essential for the initial assembly of the olfactory nerve, it plays an important role in the postnatal maturation of the olfactory bulb. In the absence of p75NTR, there is exuberant growth of some primary olfactory axons into the olfactory bulb. These axons either aberrantly bypass the glomerular layer and project into deeper lamina or grow into an abnormal bleb of tissue protruding from the medial surface of the dorsocaudal olfactory bulb. These blebs become apparent in neonatal mice and contain axons expressing olfactory marker protein that form ectopic glomerular-like tufts. Histochemical staining with the plant lectin Dolichos biflorus agglutinin revealed that axons sorted out and selectively converged on glomeruli within these blebs. Our results suggest that p75NTR indirectly influences axon growth but not glomerular targeting and plays a role in the postnatal maturation of laminar cytoarchitecture in the olfactory bulb.

Expression of EphA5 during development of the olfactory nerve pathway in rat.

The olfactory neuroepithelium is a highly plastic region of the nervous system that undergoes continual turnover of primary olfactory neurons throughout life. The mechanisms responsible for persistent growth and guidance of primary olfactory axons along the olfactory nerve are unknown. In the present study, we used antibodies against the Eph-related receptor, EphA5, to localise EphA5, and recombinant EphA5-IgG fusion protein to localise its ligands. We found that although both EphA5 and its ligands were both expressed by primary olfactory neurons within the embryonic olfactory nerve pathway, there was no graded or complementary expression pattern. In contrast, the expression patterns altered postnatally such that primary olfactory neurons expressed the ligands, whereas the second-order olfactory neurons, the mitral cells, expressed EphA5. The role of EphA5 was analysed by blocking EphA5-ligand interactions in explant cultures of olfactory neuroepithelium using anti-EphA5 antibodies and recombinant EphA5. These perturbations reduced neurite outgrowth from explant cultures and suggest that intrafascicular axon repulsion may serve to limit adhesion and optimise conditions for axon growth.

The extracellular matrix modulates olfactory neurite outgrowth on ensheathing cells.

Primary olfactory axons grow along a stereotypical pathway from the nasal cavity to the olfactory bulb through an extracellular matrix rich in laminin and heparan sulfate proteoglycans (HSPGs) and bounded by the expression of chondroitin sulfate proteoglycans (CSPGs). This pathway is pioneered by olfactory ensheathing cells, which provide a substrate conducive for axon growth during early development. In the present study, we examined the effect of several extracellular matrix constituents on the spreading and migration, as well as the neurite outgrowth-promoting properties, of olfactory ensheathing cells. Laminin and Matrigel enhanced the spreading and migration of olfactory ensheathing cells and increased their neurite outgrowth-promoting activity. In contrast, HSPG and CSPG had little effect on the spreading and migration of olfactory ensheathing cells and hence did not promote olfactory neurite outgrowth. In vitro olfactory axons grew preferentially on the surface of olfactory ensheathing cells rather than the underlying extracellular matrix. We propose that olfactory ensheathing cells secrete laminin and HSPGs, which together with other cofactors, stimulate these cells to migrate and adopt a neurite outgrowth-promoting phenotype. Expression of CSPGs in the surrounding mesenchyme confines the growth of ensheathing cells, as well as the axons, which grow on the surface of these cells, to a specific pathway. Thus, the ECM indirectly modulates the growth and guidance of olfactory axons during development.