Olfactory ensheathing cells abutting the embryonic olfactory bulb express Frzb, whose deletion disrupts olfactory axon targeting.

Constance A Rich,Jacqueline Andratschke,Clare V H Baker,C Claus Stolt,E Michelle Southard‐Smith,Surangi N Perera,Dennis P Buehler,Stefan Britsch,Michael Wegner

doi:10.1002/glia.23515

Constance A Rich, Jacqueline Andratschke + Show 7 more

Open Access

https://doi.org/10.1002/glia.23515

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Abstract

We and others previously showed that in mouse embryos lacking the transcription factor Sox10, olfactory ensheathing cell (OEC) differentiation is disrupted, resulting in defective olfactory axon targeting and fewer gonadotropin‐releasing hormone (GnRH) neurons entering the embryonic forebrain. The underlying mechanisms are unclear. Here, we report that OECs in the olfactory nerve layer express Frzb—encoding a secreted Wnt inhibitor with roles in axon targeting and basement membrane breakdown—from embryonic day (E)12.5, when GnRH neurons first enter the forebrain, until E16.5, the latest stage examined. The highest levels of Frzb expression are seen in OECs in the inner olfactory nerve layer, abutting the embryonic olfactory bulb. We find that Sox10 is required for Frzb expression in OECs, suggesting that loss of Frzb could explain the olfactory axon targeting and/or GnRH neuron migration defects seen in Sox10‐null mice. At E16.5, Frzb‐null embryos show significant reductions in both the volume of the olfactory nerve layer expressing the maturation marker Omp and the number of Omp‐positive olfactory receptor neurons in the olfactory epithelium. As Omp upregulation correlates with synapse formation, this suggests that Frzb deletion indeed disrupts olfactory axon targeting. In contrast, GnRH neuron entry into the forebrain is not significantly affected. Hence, loss of Frzb may contribute to the olfactory axon targeting phenotype, but not the GnRH neuron phenotype, of Sox10‐null mice. Overall, our results suggest that Frzb secreted from OECs in the olfactory nerve layer is important for olfactory axon targeting.

Highlights

During early olfactory nerve development, the “migratory mass” must forge its own route through the frontonasal mesenchyme, from the olfactory placode to the forebrain (Balmer & LaMantia, 2005)
As olfactory marker protein (Omp) expression correlates with the onset of synaptogenesis (Farbman & Margolis, 1980; Monti Graziadei et al, 1980), and can be upregulated in olfactory receptor neurons in embryonic olfactory epithelium co-cultured in direct contact with the presumptive olfactory bulb (Chuah & Farbman, 1983), these results suggest that Frzb deletion disrupts olfactory axon targeting and in consequence, the maturation of olfactory receptor neurons
We report specific expression of the secreted Wnt inhibitor gene Frzb in olfactory ensheathing cell (OEC) in the olfactory nerve layer, from E12.5 until at least E16.5, when the strongest expression of Frzb is seen in OECs in the inner ONL, next to the olfactory bulb

Summary

| INTRODUCTION

During primary mouth formation in Xenopus, the basement membrane persists after Frzb knockdown (Dickinson & Sive, 2009) Taken together, this suggested to us that Frzb secreted from OECs bordering the embryonic olfactory bulb could be involved in olfactory axon targeting and/or basement membrane breakdown around the developing olfactory bulb, which could be important for GnRH neuron entry. Loss of Frzb could explain, at least in part, the defects in olfactory axon targeting and GnRH neuron entry into the forebrain seen in Sox10-null mice, in which normal OEC differentiation is disrupted (Amaya et al, 2015; Barraud et al, 2013; Pingault et al, 2013). Our data suggest that Frzb secretion from a subpopulation of OECs abutting the embryonic olfactory bulb plays a role in olfactory axon targeting

| MATERIALS AND METHODS

| RESULTS

Findings

| DISCUSSION