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The mouse olfactory epithelium (OE) is an ideal model system for identifying and characterizing the factors that regulate proliferation and differentiation of neurons from their stem and progenitor cells. In part, this is because the OE undergoes neurogenesis throughout life, and does so exuberantly in response to injury (Graziadei and Monti Graziadei 1978; Mackay-Sim and Kittel 1991; Calof et al. 2002). However, another advantage of great significance is the fact that numerous studies have given us a good idea of the cell types that give rise to olfactory receptor neurons (ORNs) (Cau et al. 1997; Calof et al. 2002; Kawauchi et al. 2004, 2005; Beites et al. 2005; see also Chapter 5). Thus, in the neuronal lineage of the OE, four cell stages have been identified, in vitro and in vivo: (1) Sox2-expressing stem cells, which reside in the basal compartment of the epithelium, are thought to commit to the ORN lineage via expression of the proneural gene, Mash1. (2) Mash1-expressing early progenitor cells, which divide and may act as transit-amplifying cells (Gordon et al. 1995), in turn give rise to (3) late-stage transit-amplifying cells, also known as immediate neuronal precursors (INPs), which express a second proneural gene, Ngn1 (Wu et al. 2003). INPs give rise to daughter cells that undergo terminal differentiation into (4) postmitotic Ncam-expressing ORNs. Figure 10.1A shows schematics of both the OE neuronal lineage and the spatial distribution of these cells within the OE in vivo. As is common to many epithelia, differentiation in the OE proceeds in a basal-to-apical direction: dividing stem and progenitor cells lie atop the basal lamina, and multiple layers of differentiated ORNs lie above the progenitor cells layers.

Stem cell therapies to treat central nervous system (CNS) injuries and diseases face many obstacles, one of which is the fact that the adult CNS often presents an environment hostile to the development and differentiation of neural stem and progenitor cells. Close examination of two regions of the nervous system – the olfactory epithelium (OE), which regenerates, and the neural retina, which does not – have helped identify endogenous signals, made by differentiated neurons, which act to inhibit neurogenesis by stem/progenitor cells within these tissues. In this chapter, we provide background information on these systems and their neurogenic signaling systems, with the goal of providing insight into how manipulation of endogenous signaling molecules may enhance the efficacy of stem cell neurotherapeutics.

Cornelia de Lange Syndrome (CdLS) is a multi-organ system birth defects disorder linked, in at least half of cases, to heterozygous mutations in the NIPBL gene. In animals and fungi, orthologs of NIPBL regulate cohesin, a complex of proteins that is essential for chromosome cohesion and is also implicated in DNA repair and transcriptional regulation. Mice heterozygous for a gene-trap mutation in Nipbl were produced and exhibited defects characteristic of CdLS, including small size, craniofacial anomalies, microbrachycephaly, heart defects, hearing abnormalities, delayed bone maturation, reduced body fat, behavioral disturbances, and high mortality (75-80%) during the first weeks of life. These phenotypes arose despite a decrease in Nipbl transcript levels of only ∼30%, implying extreme sensitivity of development to small changes in Nipbl activity. Gene expression profiling demonstrated that Nipbl deficiency leads to modest but significant transcriptional dysregulation of many genes. Expression changes at the protocadherin beta (Pcdhb) locus, as well as at other loci, support the view that NIPBL influences long-range chromosomal regulatory interactions. In addition, evidence is presented that reduced expression of genes involved in adipogenic differentiation may underlie the low amounts of body fat observed both in Nipbl+/- mice and in individuals with CdLS.

Foxg1, a winged-helix transcription factor, promotes the development of anterior neural structures; in mice lacking Foxg1, development of the cerebral hemispheres and olfactory epithelium (OE) is severely reduced. It has been suggested that Foxg1 acts by positively regulating the expression of growth factors, such as Fgf8, which support neurogenesis. However, Foxg1 also binds Smad transcriptional complexes, allowing it to negatively regulate the effects of TGFβ family ligands. Here, we provide evidence that this latter effect explains much of the ability of Foxg1 to drive neurogenesis in the OE. We show that Foxg1 is expressed in developing OE at the same time as the gene encoding growth differentiation factor 11 (Gdf11), a TGFβ family member that mediates negative-feedback control of OE neurogenesis. Mutations in Gdf11 rescue, to a considerable degree, the major defects in Foxg1-/-OE, including the early, severe loss of neural precursors and olfactory receptor neurons, and the subsequent collapse of both neurogenesis and nasal cavity formation. Rescue is gene-dosage dependent, with loss of even one allele of Gdf11 restoring substantial neurogenesis. Notably, we find no evidence for a disruption of Fgf8 expression in Foxg1-/-OE. However, we do observe both a failure of expression of follistatin (Fst), which encodes a secreted Gdf11 antagonist normally expressed in and around OE, and an increase in the expression of Gdf11 itself within the remaining OE in these mutants. Fst expression is rescued in Foxg1-/-;Gdf11-/-and Foxg1-/-;Gdf11+/-mice. These data suggest that the influence of Foxg1 on Gdf11-mediated negative feedback of neurogenesis may be both direct and indirect. In addition, defects in development of the cerebral hemispheres in Foxg1-/-mice are not rescued by mutations in Gdf11, nor is Gdf11 expressed at high levels within these structures. Thus, the pro-neurogenic effects of Foxg1 are likely to be mediated through different signaling pathways in different parts of the nervous system.

Highlighting common principles among various species – including humans, insects, Xenopus laevis (African frog), and Caenorhabditis elegans (nematodes) – this highly interdisciplinary book contains chapters about the most recent ...

In vertebrate olfactory epithelium (OE), neurogenesis proceeds continuously, suggesting that endogenous signals support survival and proliferation of stem and progenitor cells. We used a genetic approach to test the hypothesis that Fgf8 plays such a role in developing OE. In young embryos, Fgf8 RNA is expressed in the rim of the invaginating nasal pit (NP), in a small domain of cells that overlaps partially with that of putative OE neural stem cells later in gestation. In mutant mice in which the Fgf8 gene is inactivated in anterior neural structures, FGF-mediated signaling is strongly downregulated in both OE proper and underlying mesenchyme by day 10 of gestation. Mutants survive gestation but die at birth, lacking OE, vomeronasal organ (VNO), nasal cavity, forebrain, lower jaw, eyelids and pinnae. Analysis of mutants indicates that although initial NP formation is grossly normal, cells in the Fgf8-expressing domain undergo high levels of apoptosis, resulting in cessation of nasal cavity invagination and loss of virtually all OE neuronal cell types. These findings demonstrate that Fgf8 is crucial for proper development of the OE, nasal cavity and VNO, as well as maintenance of OE neurogenesis during prenatal development. The data suggest a model in which Fgf8 expression defines an anterior morphogenetic center, which is required not only for the sustenance and continued production of primary olfactory (OE and VNO) neural stem and progenitor cells, but also for proper morphogenesis of the entire nasal cavity.