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

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.

Gli3 Regulates Vomeronasal Neurogenesis, Olfactory Ensheathing Cell Formation, and GnRH-1 Neuronal Migration

“…Further, these observations suggest that the near complete absence of Sox10 ϩ OECs in the nasal mucosa of Gli3 Xt/Xt mutants is not a only the result of defective vomeronasal neurogenesis (Fig. 8O) (Miller et al, 2018;Rich et al, 2018), but likely secondary to neural crest defects (Matera et al, 2008). In support of this conjecture, we observed that Gli3 Xt/WT /Ascl-1 ϩ/Ϫ double mutants were indistinguishable from Gli3 Xt/WT single mutants (data not shown).…”

“…As we detected dramatic defects in OECs formation in the developing olfactory system, we propose that the aberrant neuronal patterning and GnRH-1 migratory impairment observed in Gli3 Xt/Xt mutants is secondary to the lack of OECs in the nasal mucosa (Barraud et al, 2013;Pingault et al, 2013;Rich et al, 2018).…”

“…To determine whether the OECs in the nasal mesenchyme were missing or if they were just failing in expressing the maturation markers Sox10 (Rich et al, 2018), we performed immuno-staining against BLBP which labels mature and immature OECs even in the absence of Sox10 (Pingault et al, 2013). This immunostaining indicated almost complete absence of BLBP ϩ OECs in Gli3 Xt/Xt mice as observed with Sox10 (data not shown).…”

“…So, we tested the role of OECs in axonal misrouting and GnRH-1 migratory defects in Gli3 Xt/Xt mutants. So, we performed double staining against peripherin and Sox10 (Forni et al, 2011b;Pingault et al, 2013;Rich et al, 2018). In control animals, Sox10 ϩ OECs were localized proximal to the basal lamina of the vomeronasal epithelium and correlated with the vomeronasal bundles emerging from the vomeronasal epithelium ( Fig.…”

Erratum

“…In Rich et al (), the following error was published on page 2622, section 3.3. The heading of section 3.3 should have read: The Omp‐immunoreactive olfactory nerve layer is thinner in Frzb ‐null mice.…”