FGF2 suppresses neuronogenesis of a cell line derived from rat olfactory epithelium

Chemokine signaling is required for homeostatic and injury-induced neurogenesis in the olfactory epithelium

The olfactory epithelium (OE) possesses unique lifelong neuroregenerative capacities and undergoes constitutive neurogenesis throughout mammalian lifespan. Two populations of stem cells, frequently dividing globose basal cells (GBCs) and quiescent horizontal basal cells (HBCs), readily replace olfactory neurons throughout lifetime. Although lineage commitment and neuronal differentiation of stem cells has already been described in terms of transcription factor expression, little is known about external factors balancing between differentiation and self-renewal. We show here that expression of the CXC-motif chemokine receptor 4 (CXCR4) distinguishes both types of stem cells. Extensive colocalization analysis revealed exclusive expression of CXCR4 in proliferating GBCs and their neuronal progenies. Moreover, only neuronal lineage cells were derived from CXCR4-CreER-tdTomato reporter mice in the OE. Furthermore, Cre-tdTomato mice specific for HBCs (Nestin+ and Cytokeratin14+) did not reduce CXCR4 expression when bred to mice bearing floxed CXCR4 alleles, and did not show labeling of the neuronal cells. CXCR4 and its ligand CXCL12 were markedly upregulated upon induction of GBC proliferation during injury-induced regeneration. in vivo overexpression of CXCL12 did downregulate CXCR4 levels, which results in reduced GBC maintenance and neuronal differentiation. We proved that these effects were caused by CXCR4 downregulation rather than over-activation by showing that the phenotypes of CXCL12-overexpressing mice were highly similar to the phenotypes of CXCR4 knockout mice. Our results demonstrate functional CXCR4 signaling in GBCs regulates cell cycle exit and neural differentiation. We propose that CXCR4/CXCL12 signaling is an essential regulator of olfactory neurogenesis and provide new insights into the dynamics of neurogenesis in the OE.

Translational potential of olfactory mucosa for the study of neuropsychiatric illness

The olfactory mucosa (OM) is a unique source of regenerative neural tissue that is readily obtainable from living human subjects and thus affords opportunities for the study of psychiatric illnesses. OM tissues can be used, either as ex vivo OM tissue or in vitro OM-derived neural cells, to explore parameters that have been difficult to assess in the brain of living individuals with psychiatric illness. As OM tissues are distinct from brain tissues, an understanding of the neurobiology of the OM is needed to relate findings in these tissues to those of the brain as well as to design and interpret ex vivo or in vitro OM studies. To that end, we discuss the molecular, cellular and functional characteristics of cell types within the olfactory mucosa, describe the organization of the OM and highlight its role in the olfactory neurocircuitry. In addition, we discuss various approaches to in vitro culture of OM-derived cells and their characterization, focusing on the extent to which they reflect the in vivo neurobiology of the OM. Finally, we review studies of ex vivo OM tissues and in vitro OM-derived cells from individuals with psychiatric, neurodegenerative and neurodevelopmental disorders. In particular, we discuss the concordance of this work with postmortem brain studies and highlight possible future approaches, which may offer distinct strengths in comparison to in vitro paradigms based on genomic reprogramming.

Label-retaining, quiescent globose basal cells are found in the olfactory epithelium

The vertebrate olfactory epithelium (OE) is known for its ability to renew itself throughout life as well as to reconstitute after injury. Although this remarkable capacity demonstrates the persistence of stem cells and multipotent progenitor cells, their nature in the OE remains undefined and controversial, as both horizontal basal cells (HBCs) and globose basal cells (GBCs) have features in common with each other and with stem cells in other tissues. Here, we investigate whether some among the population of GBCs satisfy a key feature of stem cells, i.e., mitotic quiescence with retention of thymidine analogue label and activation by injury. Accordingly, we demonstrate that some GBCs express p27(Kip1) , a member of the Kip/Cip family of cyclin-dependent kinase inhibitors. In addition, some GBCs retain bromodeoxyuridine or ethynyldeoxyuridine for an extended period when the pulse is administered in neonates followed by a 1-month chase. Their identity as GBCs was confirmed by electron microscopy. All spared GBCs express Ki-67 in the methyl bromide (MeBr)-lesioned OE initially after lesion, indicating that the label-retaining (LR) GBCs are activated in response to injury. LR-GBCs reappear during the acute recovery period following MeBr exposure, as demonstrated with 2- or 4-week chase periods after labeling. Taken together, our data demonstrate the existence of LR-GBCs that are seemingly activated in response to epithelial injury and then re-established after the initial phase of recovery is completed. In this regard, some among the GBCs satisfy a common criterion for functioning like stem cells.

Neuropeptide Y and extracellular signal-regulated kinase mediate injury-induced neuroregeneration in mouse olfactory epithelium

In the olfactory epithelium (OE), injury induces ATP release, and subsequent activation of P2 purinergic receptors by ATP promotes neuroregeneration by increasing basal progenitor cell proliferation. The molecular mechanisms underlying ATP-induced increases in OE neuroregeneration have not been established. In the present study, the roles of neuroproliferative factors neuropeptide Y (NPY) and fibroblast growth factor 2 (FGF2), and p44/42 extracellular signal-regulated kinase (ERK) on ATP-mediated increases of neuroregeneration in the OE were investigated. ATP increased basal progenitor cell proliferation in the OE via activation of P2 purinergic receptors in vitro and in vivo as monitored by incorporation of 5'-ethynyl-2'-deoxyuridine, a thymidine analog, into DNA, and proliferating cell nuclear antigen (PCNA) protein levels. ATP induced p44/42 ERK activation in globose basal cells (GBCs) but not horizontal basal cells (HBCs). ATP differentially regulated p44/42 ERK over time in the OE both in vitro and in vivo with transient inhibition (5-15 min) followed by activation (30 min-1 h) of p44/42 ERK. In addition, ATP indirectly activated p44/42 ERK in the OE via ATP-induced NPY release and subsequent activation of NPY Y1 receptors in the basal cells. There were no synergistic effects of ATP and NPY or FGF2 on OE neuroregeneration. These data clearly have implications for the pharmacological modulation of neuroregeneration in the olfactory epithelium.

ATP differentially upregulates fibroblast growth factor 2 and transforming growth factor α in neonatal and adult mice: effect on neuroproliferation

Multiple neurotrophic factors play a role in proliferation, differentiation and survival in the olfactory epithelium (OE); however, the signaling cascade has not been fully elucidated. We tested the hypotheses that ATP induces the synthesis and secretion of two neurotrophic factors, fibroblast growth factor 2 (FGF2) and transforming growth factor alpha (TGFα), and that these neurotrophic factors have a role in inducing proliferation. Protein levels of FGF2 and TGFα were increased 20 h post-intranasal instillation of ATP compared to vehicle control in adult Swiss Webster mice. Pre-intranasal treatment with purinergic receptor antagonist pyridoxal-phosphate-6-azophenyl-20,40-disulfonic acid (PPADS) significantly blocked this ATP-induced increase, indicating that upregulation of FGF2 and TGFα expression is mediated by purinergic receptor activation. However, in neonatal mouse, intranasal instillation of ATP significantly increased the protein levels of FGF2, but not TGFα. Likewise, ATP evoked the secretion of FGF2, but not TGFα, from neonatal mouse olfactory epithelial slices and PPADS significantly blocked ATP-evoked FGF2 release. To determine the role of FGF2 and TGFα in inducing proliferation, 5-bromo-2-deoxyuridine (BrdU) incorporation was examined in adult olfactory epithelium. Intranasal treatment with FGF receptor inhibitor PD173074 or epidermal growth factor receptor inhibitor AG1478 following ATP instillation significantly blocked ATP-induced BrdU incorporation. Collectively, these data demonstrate that ATP induces proliferation in adult mouse olfactory epithelium by promoting FGF2 and TGFα synthesis and activation of their receptors. These data suggest that different mechanisms regulate neurogenesis in neonatal and adult OE, and FGF2 and TGFα may have different roles throughout development.

Odorant receptor (OR) gene choice is biased and non-clonal in two olfactory placode cell lines, and OR RNA is nuclear prior to differentiation of these lines

We have investigated two clonal mouse olfactory placode (OP) cell lines as a model system for studying endogenous odorant receptor (OR) regulation. Both lines can be differentiated into bipolar neurons with transcriptional profiles consistent with mature sensory neurons. We show that single cells exhibit monogenic OR expression like sensory neurons in vivo. Monogenic OR expression is established in undifferentiated cells and persists through differentiation, but OR gene choice is not a clonal property of either cell line. Interestingly, OR RNA shifts from predominantly nuclear to cytoplasma during differentiation of both cell lines. Finally, our data indicate that a restricted subset of OR genes and OR clusters are over-represented in cell populations, suggesting either a pre-existing intrinsic bias in OP founder cells or extrinsic influences arising from culture conditions.

Maintaining epitheliopoietic potency when culturing olfactory progenitors

The olfactory epithelium is remarkable for the persistence of multipotent, neurocompetent progenitor and stem cells throughout life that can replace all of the various cell types of the epithelium following injury. The therapeutic exploitation of the neurocompetent stem cells of the adult olfactory epithelium would be facilitated by the development of a culture system that maintains the in vivo potency of the progenitors while they are expanded and/or manipulated. We have used an air-liquid interface culture protocol, in which a feeder cell layer of 3T3 cells is established on the underside of a culture insert and Facs-isolated or unsorted progenitor cells from the methyl bromide-lesioned adult rodent epithelium are seeded on upper side. Under these conditions, epithelial cells other than HBCs are capable of organizing themselves into complex three-dimensional, epithelium-lined spheres, which can be passaged. The spheres contain cells with the molecular phenotype of globose basal cells, horizontal basal cells, sustentacular cells and neurons. Spheres derived from mice that express the green fluorescent protein constitutively can be dissociated after 6 days in vitro and directly transplanted into the epithelium of wild-type, methyl bromide-lesioned mice via nasal infusion. The resulting clones contain the various cell types observed in aggregate when globose basal cells are transplanted acutely. In contrast, the same cells cultured as two-dimensional, submerged cultures undergo fibroblastic transition after transplantation and do not integrate into the epithelium. In conclusion, the culture system described here maintains the potency of progenitors, which can then participate in epitheliopoiesis in vivo.

Olfactory epithelium progenitors: insights from transgenic mice and in vitro biology

The rodent olfactory epithelium (OE) is capable of prolonged neurogenesis, beginning at E10 in the embryo and continuing throughout adulthood. Significant progress has been made over the last 10 years in revealing the signals that drive induction, differentiation and survival of its Olfactory Receptor Neurons (ORNs). Our understanding of the identity of specific progenitors or precursors that respond to these signals is, however, less well developed, and the search is still on for the elusive, definitive multipotent neuro-glial OE "Stem cell". Here, we review several lines of evidence that support the existence of a heterogeneous population of neural and glial progenitors in the olfactory mucosa, and highlight the differences in the identity and activity of progenitors found in the embryonic and adult OE. In particular, we show how recent advances in mouse transgenesis, and in the development of in vitro assays of progenitor activity, have helped to demonstrate the existence of multiple classes of olfactory mucosa-based progenitors.

Mouse olfactory sensory neurons express 10,000 genes

Olfactory epithelial cells from olfactory marker protein-green fluorescent protein (OMP-GFP) mice were separated by fluorescence-activated cell sorting into a GFP+ sample enriched in mature olfactory sensory neurons (OSNs) and a GFP- sample enriched in all other cells. GeneChip expression profiling of these samples provided a predictive measure of expression in OSNs. Validation tests comparing the ratio of GFP+/GFP- signal intensity against expression patterns from in situ hybridization for 189 mRNAs proved statistically significant and provided probabilities of expression in OSNs scaled according to the signal intensity ratios. These probabilities predict that, among 11,596 mRNAs detected in the GFP+ sample, more than 10,000 are expressed in OSNs. Transcripts and overrepresented categories of mRNAs detected in the GFP+ sample agreed with known properties of OSNs and predict additional properties. For example, ciliogenesis and spermatogenesis were overrepresented, consistent with similarities between OSN cilia and sperm flagella. Chromatin assembly mRNAs were expressed throughout the OSN cell lineage, consistent with the hypothesis that chromatin remodeling plays a role in OSN differentiation. We detected numerous signaling proteins and receptors, such as 30 nonchemosensory G-protein-coupled receptors, including the presynaptic glutamate receptor mGlur4 and the Wnt receptor Fzd3. The largest group of mRNAs, however, was the hundreds of transcriptional regulators that presumably determine the OSN phenotype. The absence of OMP protein in OMP-GFP mice had no detectable effect on mRNA abundance. Within limits prescribed by the nature of microarray data and the in situ hybridization validation, these data should be useful in directing further experiments on OSN function.

Nonintegrin laminin receptor precursor protein is expressed on olfactory stem and progenitor cells

The biochemical identification and immunocytochemical characterization of a cell surface antigen, expressed on globose basal cells (GBCs) of the rodent olfactory epithelium (OE), are described. The monoclonal antibody (MAb) GBC-3 recognizes a surface protein, confirmed by both live cell staining and fluorescence-activated cell sorting. Two-dimensional SDS-PAGE/Western blot followed by tandem mass spectrometry demonstrates that the cell surface GBC-3 antigen is the 40 kDa laminin receptor precursor protein. The MAb GBC-3 labels the vast majority of cells among the GBC population and does not stain either sustentacular cells or horizontal basal cells (HBCs) in the normal rat OE. After epithelial lesion by exposure to methyl bromide, the remaining cells, which are mostly GBCs, are heavily stained by GBC-3, and colabeled with GBC-3 and sustentacular cell or HBC markers. GBC-3 will be a potentially useful tool for identifying and characterizing GBCs.

Future directions in chemosensory research

The past two decades have witnessed phenomenal growth in chemosensory research in the basic sciences and clinical studies. Although chemosensory disorders today are recognized more widely for their marked impacts on QOL and public safety, the increased research focus has yet to translate into significant therapeutic advances for human olfactory or gustatory dysfunction. Nevertheless, it should be expected that active chemosensory research eventually will yield discoveries necessary to generate better treatment options for otolaryngologists and other physicians caring for smell and taste loss patients. As the understanding of chemosensory biology and disease pathophysiology grows, there is great promise that medical science will conquer the frustrating and debilitating health problems posed by olfactory and gustatory dysfunction. Moreover, it is likely that the lessons gained through chemosensory research will apply broadly to a range of human diseases affecting sensory modalities and the central nervous system.

In vivo and in vitro neurogenesis in human olfactory epithelium

The birth and differentiation of neurons have been extensively studied in the olfactory epithelium (OE) of rodents but not in humans. The goal of this study was to characterize cellular composition and molecular expression of human OE in vivo and in vitro. In rodent OE, there are horizontal basal cells and globose basal cells that are morphologically and functionally distinct. In human OE, however, there appears to be no morphological distinction among basal cells, with almost all cells having round cell bodies similar to rodent globose basal cells. Unlike the case in rodents, human basal cells, including putative neuronal precursors, express p75NGFR, suggesting a distinctive role for p75NGFR in human OE neurogenesis. Molecular expression of neuronal cells during differentiation in human OE grossly follows that in rodents. However, the topographical organization of immature and mature ORNs in human OE differs from that of rodents, in that immature and mature ORNs in humans are dispersed throughout the OE, whereas rodent counterparts have a highly laminar organization. These observations together suggest that the birth and differentiation of neuronal cells in human OE differ from those in rodents. In OE explant culture, neuronal cells derived from human OE biopsy express markers for immature and mature neurons, grossly recapitulating neuronal differentiation of olfactory neurons in vivo. Furthermore, small numbers of cells are doubly label for bromodeoxyuridine and olfactory marker protein, indicating that neuronal cells born in vitro reach maturity. These data highlight species-related differences in OE development and demonstrate the utility of explant culture for experimental studies of human neuronal development.

Multipotency of purified, transplanted globose basal cells in olfactory epithelium

By comparison with the rest of the nervous system, the olfactory epithelium has an unparalleled ability to renew and repair itself throughout life. However, the identity and capacity of the various types of progenitor cells that underlie that ability are not well established. We used selective isolation, transplantation, and engraftment of various types of marker-labeled cells into the epithelium of methyl bromide-lesioned, unmarked host mice to dissect progenitor cell capacity. Globose basal cells were purified from other potential progenitors using the monoclonal antibody GBC-2 (GBC, globose basal cell) and fluorescence activated cell sorting. Transplanted globose basal cells engraft and, in aggregate, give rise to globose basal cells, neurons, sustentacular cells, and several other kinds of non-neuronal cells. Individual clones, derived from single engrafted globose basal cells, can consist of a mixture of neurons and non-neuronal cells, only neurons, or only non-neuronal cells. Neurons that arise after transplantation mature to the point of expressing odorant receptors and olfactory marker protein and of projecting axons to the olfactory bulb. In contrast, other kinds of epithelial cells are neither neurogenic nor multipotent. For example, sustentacular and duct cells give rise only to themselves after transplantation. Furthermore, horizontal basal cells do not engraft in mice, in which the endogenous population is spared after lesion. Thus, some subtype(s) of GBC is a multipotent progenitor cell, whose multipotency is activated after destruction of both neurons and non-neuronal cells. The results suggest that progenitor cell transplantation may prove useful as a therapeutic modality as well as an analytical tool.

Expression of TGF-beta type II receptors in the olfactory epithelium and their regulation in TGF-alpha transgenic mice

Numerous in vitro studies of neurogenesis of olfactory receptor neurons (ORNs) suggest that transforming growth factor (TGF)-beta promotes the maturation/differentiation of olfactory progenitors. We demonstrate that in vivo both mature and immature ORNs, and possibly a basal neuronal progenitor cell, express the TGF-beta type II receptor (TGF-betaRII), suggesting that these cells are targets for TGF-beta signaling. In a previous study of neurogenesis in the OE of TGF-alpha overexpressing transgenic (T) mice, we observed an apparent reduction in the expression of olfactory marker protein (OMP), a marker of terminal differentiation in ORNs in T mice compared to nontransgenic (NT) littermate controls; this was confirmed by Western blotting and immunohistochemistry. In contrast, there was no apparent difference between T and NT mice in the intensity of immunoreactivity for a neuronal marker, protein gene product 9.5. Because TGF-alpha overexpression has been reported to affect TGF-beta signaling in other epithelia, we compared the expression of the TGF-beta type II receptor (TGF-betaRII) in T and NT mice. The intensity of TGF-betaRII immunoreactivity on ORNs was substantially reduced in T compared to NT mice. Similar reductions in TGF-betaRII expression in vomeronasal receptor neurons and in other epithelia in the nasal cavity of T mice were also observed. Taken together, these results indicate that TGF-beta signaling regulates terminal differentiation of ORNs in vivo and suggest ways in which interactions between TGF-alpha and TGF-beta signaling pathways may interact in the OE.

Conditionally immortalized clonal cell lines from the mouse olfactory placode differentiate into olfactory receptor neurons

To test extracellular signals that direct the development of the olfactory system, we have generated clonal temperature-sensitive cell lines that represent distinct cellular lineages derived from the E10 mouse olfactory placode. Two of these lines, OP6 and OP27, express (at the permissive temperature), a transcriptional profile representing intermediate-late developmental stages in the olfactory receptor neuron (ORN) lineage. At the nonpermissive temperature, both OP6 and OP27 cells can be induced by all-trans retinoic acid to differentiate into a population of mature bipolar ORN-like cells. In response to retinoic acid, differentiated OP6 and OP27 down-regulate neuron-specific transcription factors required for early stages of neuronal differentiation, and shift active components of the neurotrophin signaling cascade (Trk receptors) into a kinase inactive state. When morphologically mature, OP6 and OP27 express the mature ORN chemosensory signaling components, olfactory G-protein (G(olf)), Type III adenylate cyclase (ACIII), OCNC1, and the olfactory marker protein (OMP). OP27 expresses one odorant receptor, OR 27-3. OP6 expresses two very closely related receptors, OR 6-13 and OR 6-8. Voltage-gated sodium and potassium channels resembling those recorded from primary cultures of ORNs can also be recorded from a subset of differentiated OP6 cells.

Neural regeneration and the peripheral olfactory system

The peripheral olfactory system is able to recover after injury, i.e., the olfactory epithelium reconstitutes, the olfactory nerve regenerates, and the olfactory bulb is reinnervated, with a facility that is unique within the mammalian nervous system. Cell renewal in the epithelium is directed to replace neurons when they die in normal animals and does so at an accelerated pace after damage to the olfactory nerve. Neurogenesis persists because neuron-competent progenitor cells, including transit amplifying and immediate neuronal precursors, are maintained within the population of globose basal cells. Notwithstanding events in the neuron-depleted epithelium, the death of both non-neuronal cells and neurons directs multipotent globose basal cell progenitors, to give rise individually to sustentacular cells and horizontal basal cells as well as neurons. Multiple growth factors, including TGF-alpha, FGF2, BMPs, and TGF-betas, are likely to be central in regulating choice points in epitheliopoiesis. Reinnervation of the bulb is rapid and robust. When the nerve is left undisturbed, i.e., by lesioning the epithelium directly, the projection of the reconstituted epithelium onto the bulb is restored to near-normal with respect to rhinotopy and in the targeting of odorant receptor-defined neuronal classes to small clusters of glomeruli in the bulb. However, at its ultimate level, i.e., the convergence of axons expressing the same odorant receptor onto one or a few glomeruli, specificity is not restored unless a substantial number of fibers of the same type are spared. Rather, odorant receptor-defined subclasses of neurons innervate an excessive number of glomeruli in the rough vicinity of their original glomerular targets.

Regulation of olfactory neurogenesis by amidated neuropeptides

The existence of stem cells in the CNS raises issues concerning the ability of nervous tissues to regenerate in the adult mammal and provides new perspectives on the treatment of degenerative disease and traumatic injury of the nervous system. These cells have a relatively limited range of locations within the nervous system and include cells of the rostral migratory stream, hippocampus, retina, and olfactory epithelium. The olfactory epithelium has been studied as a model of adult neuronal regeneration, with neuronal precursor/basal cells serving as the olfactory "stem cells." The identification of factors that promote neuronal proliferation or regeneration within the olfactory epithelium can provide clues to the process of adult mammalian nervous system repair and treatment. Multiple factors have been examined that appear to influence the proliferation and subsequent maturation of basal cells. These factors include nerve growth factor, fibroblast growth factor-2, epidermal growth factor, and insulin/insulin-like growth factor-1. Recently, two amidated neuropeptides, neuropeptide Y (NPY) and pituitary adenylate cyclase-activating polypeptide (PACAP38), identified in the olfactory epithelium have been shown to promote dramatically neuronal proliferation. The effects of NPY and PACAP suggest that amidated neuropeptides may serve a broad developmental and regenerative role in the mammalian olfactory epithelium.

Leukemia inhibitory factor, interleukin-6, and their receptors are expressed transiently in the olfactory mucosa after target ablation

Removal of the synaptic targets of olfactory receptor neurons by olfactory bulb ablation results in apoptosis of olfactory receptor neurons and up-regulation of proliferation of their progenitors. This study focuses on the expression of the neuropoietic cytokines leukemia inhibitory factor (LIF) and its receptor (LIFR) and interleukin 6 (IL-6) and its receptor (IL-6R) in intercellular signaling pathways in the olfactory mucosa after target ablation. Olfactory bulbectomy (OBX) resulted in several transient, early-onset, temporally integrated events that were detected immunohistochemically. Macrophages infiltrated the olfactory epithelium (OE) by 16 hours post-OBX. LIF expression was up-regulated transiently at 2 days post-OBX, when up-regulated expression of LIFR also was detected on globose basal cells (GBCs), a subpopulation of which are immediate progenitors of olfactory receptor neurons. GBC proliferation peaked at 3--4 days post-OBX. In the olfactory nerve (ON), LIF-positive and IL-6-positive macrophage infiltration was followed by the transient up-regulation of expression of LIFR, IL-6, and IL-6R in ensheathing cells by 3 days post-OBX. The mRNAs for LIF/LIFR, IL-6/IL-6R, and their common signal-transduction molecule, gp130, in olfactory-nasal mucosa from control mice and from 3-day post-OBX mice were detected with reverse transcriptase-polymerase chain reaction (RT-PCR). Analysis of Northern blot and relative quantitative RT-PCR demonstrated similar temporal patterns of changes in relative mRNA levels for both LIF and IL-6, which were up-regulated by 16 hours post-OBX and peaked at 2--3 days post-OBX. These data indicate that LIF from infiltrating macrophages acts as a mitogen for GBCs and that LIF from infiltrating macrophages and IL-6 from infiltrating macrophages and ensheathing cells act as repair factors in the ON.

Basic fibroblast growth factor and fibroblast growth factor receptors in adult olfactory epithelium

Basic fibroblast growth factor (FGF2) stimulates proliferation of the globose basal cells, the neuronal precursor in the olfactory epithelium. The present study investigates the expression of basic fibroblast growth factor and fibroblast growth factor receptors in the adult olfactory epithelium. FGF2 immunoreactivity was expressed widely in the olfactory epithelium, with the highest density of immunoreactivity in the supporting cells. In contrast, most cells in the epithelium expressed FGF2 mRNA. Fibroblast growth factor receptor-1 (FGFr1) immunoreactivity was densest in the basal cell and neuronal layers of the olfactory epithelium and on the apical surface of supporting cells. In the lamina propria FGF2 immunoreactivity and mRNA were densest in cells close to the olfactory nerve bundles. FGFr1 immunoreactivity was heaviest on the olfactory ensheathing cells. Using reverse transcriptase-polymerase chain reaction analysis, the olfactory epithelium was shown to express only three receptor splice variants, including one (FGFr1c) with which basic fibroblast growth factor has high affinity. Other receptor splice variants were present in the lamina propria. Taken together, these observations indicate endogenous sources of FGF2 within the olfactory epithelium and lamina propria and suggest autocrine and paracrine pathways via which FGF2 might regulate olfactory neurogenesis. The observation of only three receptor splice variants in the olfactory epithelium limits the members of the fibroblast growth factor family which could act in the olfactory epithelium. The widespread distribution of receptors suggests that fibroblast growth factors may have roles other than proliferation of globose basal cells.

bFGF induces differentiation and death of olfactory neuroblastoma cells

Olfactory neuroblastoma (ONB) is a highly vascularized and malignant tumor arising in olfactory neuronal precursors from the paranasal sinuses. Previously, we showed that treatment of JFEN cells with transforming growth factor (TGF)-alpha caused them to differentiate and respond to chemical odorants, whereas basic fibroblast growth factor (bFGF) treated cells differentiated and died. In the present study we show that established ONB tumors treated with bFGF upregulate the bFGF receptor (FGFR1) prior to differentiation. This cellular differentiation was evidenced by bFGF-induced expression of the human runt homologue AML1 (PEBP2 alpha B, CBFA-2) that is highly expressed in developing olfactory neuroepithelium and TrkA, a preferred nerve growth factor receptor. Since TrkA is expressed in supporting cells, but not in mature olfactory neurons, we hypothesize that the expression of AML1 and TrkA in bFGF-treated JFEN cells induced supporting cell differentiation. Collectively, these results have implications for the treatment of patients afflicted with ONB.

Neurotrophic factors in the primary olfactory pathway

The number of identified growth factors continues to increase rapidly with many being implicated in the development of the nervous system, although for most of them the autocrine and paracrine pathways of cellular regulation still remain to be elucidated. The primary olfactory pathway, consisting of the olfactory epithelium and olfactory bulb, is presented here as a very useful model for the analysis of growth factor function. Review of the available literature suggests that a large proportion of neuroactive growth factors and their receptors are present in the olfactory epithelium or olfactory bulb. Furthermore, the primary olfactory pathway is one of the most plastic in the nervous system with neurogenesis continuing to contribute new sensory neurones in the olfactory epithelium and new interneurones in the olfactory bulb throughout adult life. The rich diversity of growth factors and their receptors in the olfactory system indicates that it will be useful in elucidating how these molecules regulate the formation of the nervous system. The olfactory epithelium in particular is proving useful as a model for the actions of growth factors in directing the neuronal lineage from stem cell to mature neurone.

Growth factor regulation of neurogenesis in adult olfactory epithelium

Neurogenesis continues throughout adult life in the mammalian olfactory epithelium. This process is a dynamic state of proliferation, differentiation and cell death, probably regulated by autocrine and paracrine signals such as peptide growth factors. Previous investigations have demonstrated roles for some growth factors in olfactory neurogenesis in vitro, but the assay systems used make it difficult to be certain of their effects (proliferation, differentiation, enhanced survival) or their target cells. The present study investigated the effects of growth factors in cultures of purified olfactory epithelium comprising only basal cells and supporting cells in serum-free media. The advantage of this culture system is that proliferation, differentiation and survival of the basal cells and neurons can be examined separately. Under these conditions, three growth factors exerted well-defined effects: (i) fibroblast growth factor-2 stimulated proliferation of the globose basal cells; (ii) transforming growth factor-beta2 induced these cells to differentiate into neurons; and (iii) platelet-derived growth factor promoted survival of the differentiated neurons. We conclude that fibroblast growth factor-2, transforming growth factor-beta2 and platelet-derived growth factor act sequentially on precursor cells and immature neurons during neurogenesis in the adult olfactory epithelium.

The role of the brain in metamorphosis of the olfactory epithelium in the frog, Xenopus laevis

Retrograde signaling from the brain to the olfactory sensory epithelium is important for neuronal survival, but the importance of the olfactory bulb in retrograde signaling during the naturally-induced, neuronal plasticity occurring during metamorphosis is unclear. The olfactory system of the African clawed frog (Xenopus laevis) undergoes dramatic rearrangements during metamorphosis, making this an ideal system in which to examine interactions between the brain and the olfactory sensory epithelium. The main olfactory epithelium of larvae, located in the principal cavity (PC), changes at metamorphosis in function, receptor neuron morphology, biochemistry, and axon termination sites. A new, "middle", cavity forms during metamorphosis that assumes all the characteristics of the larval PC. Using a combination of bulbectomy and olfactory transplantation, we investigated changes in expression of a marker protein (E7) and in apical ultrastructure in olfactory receptor neurons either (1) connected to the olfactory bulb, (2) connected to non-olfactory brain regions, or (3) with no apparent central nervous system (CNS) connections. We find that neurons in the middle cavity (MC) lacking connections with the CNS appear mature but neurons in the PC do not. Supporting cells in the PC undergo the changes normally observed during metamorphosis. Neurons connected to non-olfactory brain regions, either after bulbectomy or transplantation, appeared normal with regard to the changes normally expected after metamorphosis. These results suggest that influence from the brain is necessary for metamorphic changes in the X. laevis olfactory epithelium, but that these signals are not confined to the olfactory bulb; non-olfactory brain regions can also support these metamorphic changes.

Regeneration of olfactory receptor neurons following chemical lesion: time course and enhancement with growth factor administration

Although it has been known for over 50 years that olfactory receptor neuron (ORN) neurogenesis and subsequent reinnervation of the olfactory bulb (OB) occurs following ORN injury, the precise intrinsic and extrinsic factors that regulate this dynamic process have not yet been fully identified. In the first of two experiments, we characterized the time course of anatomical recovery following zinc sulfate (ZnSO(4)) lesion of ORNs in adult male Sprague-Dawley rats. ZnSO(4) produced a near complete deafferentation of OB within 3 days following intranasal administration. A time-dependent increase in ORN reinnervation of OB was observed following 10, 20, and 30 day recovery intervals. Given the evidence that bFGF, EGF, and TGF-alpha have mitogenic effects on ORNs in vitro, a second experiment examined the extent to which these growth factors (GFs) might enhance ORN regeneration and subsequent reinnervation of OB in vivo. Rats received intranasal infusions of ZnSO(4) on day 0, followed by subcutaneous injections of either bFGF (5, 10, or 50 microgram/kg), EGF (5, 10, or 50 microgram/kg), or TGF-alpha (5 or 10 microgram/kg) on days 3-6. Horseradish peroxidase (HRP) histochemistry of OB following a 10-day recovery period revealed a dose-related enhancement in reinnervation of OB for each of the three growth factors examined, with the greatest enhancement produced by TGF-alpha. These data suggest that GFs may regulate ORN mitogenesis in vivo in a way similar to that which has been characterized in vitro.

Decreased parotid saliva gustin/carbonic anhydrase VI secretion: an enzyme disorder manifested by gustatory and olfactory dysfunction

BACKGROUND

Taste and smell dysfunction has been reported to occur in patients with a variety of clinical problems. We wanted to investigate a specific group of patients in whom taste and smell dysfunction occurred putatively related to a specific biochemical abnormality in a salivary growth factor [gustin/carbonic anhydrase (CA) VI] considered responsible for maintenance of taste bud function.

METHODS

Eighteen patients developed loss and/or distortion of taste and smell after an acute influenza-type illness. They were evaluated clinically, by psychophysical tests of taste and smell function, by measurement of parotid salivary gustin/CAVI by a radioimmunoassay and by measurement of serum, urine, and salivary zinc. Biopsies of circumvallate papillae were obtained in 6 patients and examined by transmission electron microscopy. Similar studies were performed in 55 asymptomatic volunteers with biopsies of circumvallate papillae performed in 4.

RESULTS

Taste and smell acuity were impaired in patients compared with healthy volunteers and parotid gustin/CAVI, salivary, and serum zinc concentrations were lower in patients than in healthy volunteers. Taste buds in circumvallate papillae of patients exhibited severe vacuolization, cellular degeneration, and absence of dense extracellular material.

CONCLUSIONS

These results describe a clinical disorder formulated as a syndrome of hyposmia (decreased smell acuity), hypogeusia (decreased taste acuity), dysosmia (distorted smell function), dysgeusia (distorted taste function), and decreased secretion of parotid saliva gustin/CAVI with associated pathological changes in taste bud anatomy. Because gustin/CAVI is found in humans only in parotid saliva and has been associated with taste bud growth and development these results suggest that inhibition of synthesis of gustin/CAVI is associated with development of taste bud abnormalities and thereby loss of taste function.

Stress induces neurogenesis in non-neuronal cell cultures of adult olfactory epithelium

Among the basal cells of the olfactory epithelium is a stem cell which divides and whose progeny differentiate into new sensory neurons throughout adult life. Olfactory neurogenesis is highly regulated, for example it is stimulated by epithelial damage. Previous reports implicate several growth factors in progenitor cell proliferation and neuronal differentiation in vitro but these studies differ in growth conditions and age of donors making it difficult to determine precisely the roles of neurogenic stimuli and their sites of action. The aims of the present study were to develop purified basal cell cultures from adult olfactory epithelium and to stimulate neurogenesis in defined growth conditions in order to elucidate the cellular mechanisms by which neurogenesis is stimulated after epithelial damage. We show here that differentiated olfactory sensory neurons arise after biochemical or mechanical stress of rat and mouse olfactory epithelial cell cultures in the absence of growth factors, complex media (e.g., serum, conditioned media, pituitary and hypothalamic extracts), or other cells (e.g., explants, feeder layers of glia, or other non-epithelial cells). Prior to the stress, these cultures contained basal cells and supporting cells but not neurons. After the stress, some cells differentiated into bipolar neurons expressing a number of neuronal proteins including olfactory marker protein. Bromodeoxyuridine experiments show that the differentiated neurons arose from recently divided cells which did not divide again before differentiating. We conclude that stress disrupts cell surface contacts to induce the immediate neuronal precursors to undergo final differentiation into olfactory sensory neurons. This may be a mechanism for enhanced neurogenesis after epithelial damage.

Transplantation of multipotent progenitors from the adult olfactory epithelium

Mammalian olfactory epithelium produces new neurons rapidly throughout adulthood. Here, we demonstrate that precursor cells harvested from the adult olfactory epithelium, when transplanted into the nasal mucosa of host rats exposed previously to an olfactotoxic gas, engraft and participate in neuroepithelial reconstitution. In contrast to their normal neuronal fate in situ, grafted precursors harvested from bulbectomized donors produced non-neuronal cells as well as neurons. These results demonstrate that epithelial precursors activated following olfactory bulbectomy are not irreversibly committed to making neurons. Thus, olfactory progenitors are subject to a form of feedback control in vivo that regulates the types of cells that they produce within a broader-than-neuronal repertoire.