The role of Dlx homeogenes in early development of the olfactory pathway

Abstract

Development of the olfactory pathway requires interaction between cells and signals of different origin. Olfactory receptor neurons (ORN) in the olfactory placodes (OP) extend axons towards the forebrain (FB); with innervation taking place at a later time following degradation of the basement membrane. Cells from the OP migrate along ORN axons and differentiate into various elements, including ensheathing and Gonadotropin Releasing Hormone (GnRH)+ cells. The importance of the olfactory connection and migration is highlighted by the severe endocrine phenotype in Kallmann's patients who lack this migratory pathway. Little is known about the genetic control of intrinsic ORN properties. Inactivation of the distalless-related Dlx5 prevents connections between ORNs and FB. Using a grafting approach we show that misguidance and lack of connectivity is due to intrinsic defects in ORN neurites and migratory cells (MgC), and not to environmental factors. These data point to a cellautonomous function of Dlx5 in providing ORN axons with their connectivity properties. Dlx5 also marks a population of early MgC that partly overlaps with the GnRH+ population. In the absence of Dlx5 MgCs of the Dlx5+ lineage migrate, associated with PSA-NCAM+ axons, but fail to reach the FB as a consequence of the lack of axonal connection and not an inability to migrate. These data suggests that Dlx5 is not required to initiate migration and differentiation of MgCs.

Activation of theWnt–βCatenin Pathway in a Cell Population on the Surface of the Forebrain Is Essential for the Establishment of Olfactory Axon Connections

“…The position of Wnt-responsive cells, at the interface of the BM and FB at early stages, suggests that these cells may play a role in cell-cell and cellextracellular matrix (ECM) interactions that coordinate ECM degradation and remodeling necessary for the formation of the nerve layers and synaptic contact. In Dlx5Ϫ/Ϫ embryos, the BM remains unfenestrated (our unpublished results), and the Wnt-␤catenin response in the same region is strongly reduced, despite the fact that the mutant ORN axons do come to a close proximity with the FB (Merlo et al, 2007). Interestingly, fibronectin has been reported to be a direct target of Wnt/Wg signaling in Xenopus fibroblasts (Gradl et al, 1999), and ␤catenin is known to be associated with cell adhesion complexes and to regulate cell-substrate adhesion (Gottardi and Gumbriner, 2004).…”

“…We have characterized these cells and have established (1) the absence of typical markers for migratory neurons (DCX and PSA-NCAM), (2) the absence of the OEC marker S100, and (3) the absence of mesenchymal markers vimentin, vinculin, and talin. ␤catenin response can be restored in Dlx5Ϫ/Ϫ embryos, in which OP-derived cell migration is necessarily interrupted (Levi et al, 2003;Merlo et al, 2007), excluding a placodal migration origin of these cells. Importantly, the receptor Fzd7 is expressed in the same mesenchymal location as the ␤catenin-activated cells but is excluded from the MM.…”

“…2 A-H, available at www.jneurosci.org as supplemental material, and data not shown). Finally, grafting experiments indicate that the olfactory connectivity defect in Dlx5 null animals is linked to cell-autonomous properties of ORNs and does not depend on the OB (Levi et al, 2003;Merlo et al, 2007).…”

“…In the absence of Dlx5, ORN neurites elongate and extend toward the FB but inevitably fail to make contact and form connections, as the consequence of cellautonomous defects in ORN axons (Levi et al, 2003;Long et al, 2003;Merlo et al, 2007). If neurite-to-mesenchyme signals near the FB are critical to activate the Wnt-␤catenin pathway, the disrupted axon growth in Dlx5Ϫ/Ϫ embryos should result in an absence of ␤galϩ nuclei.…”

Competence, specification and commitment to an olfactory placode fate

“…This is similar to the situation in mouse, where ~70% of the Dlx5+ cells become GnRH+ neurons that line the olfactory nerve (Merlo et al, 2007). In Dlx5 knockout mice, GnRH neuroendocrine cells persist but are mislocalized and express GnRH at reduced levels (Merlo et al, 2007). Dlx5-null mice have greatly reduced olfactory epithelia, with few sensory neurons therein, and rudimentary vomeronasal organs in addition to grossly impaired otic vesicle derivatives (Depew et al, 1999;Long et al, 2003;Merlo et al, 2007).…”