NFI in the development of the olfactory neuroepithelium and the regulation of olfactory marker protein gene expression

Opposing, spatially-determined epigenetic forces impose restrictions on stochastic olfactory receptor choice

Olfactory receptor (OR) choice represents an example of genetically hardwired stochasticity, where every olfactory neuron expresses one out of ~2000 OR alleles in the mouse genome in a probabilistic, yet stereotypic fashion. Here, we propose that topographic restrictions in OR expression are established in neuronal progenitors by two opposing forces: polygenic transcription and genomic silencing, both of which are influenced by dorsoventral gradients of transcription factors NFIA, B, and X. Polygenic transcription of OR genes may define spatially constrained OR repertoires, among which one OR allele is selected for singular expression later in development. Heterochromatin assembly and genomic compartmentalization of OR alleles also vary across the axes of the olfactory epithelium and may preferentially eliminate ectopically expressed ORs with more dorsal expression destinations from this 'privileged' repertoire. Our experiments identify early transcription as a potential 'epigenetic' contributor to future developmental patterning and reveal how two spatially responsive probabilistic processes may act in concert to establish deterministic, precise, and reproducible territories of stochastic gene expression.

Opposing, spatially-determined epigenetic forces impose restrictions on stochastic olfactory receptor choice

Olfactory receptor (OR) choice represents an example of genetically hardwired stochasticity, where every olfactory neuron expresses one out of ~2000 OR alleles in a probabilistic, yet stereotypic fashion. Here, we propose that topographic restrictions in OR expression are established in neuronal progenitors by two opposing forces: polygenic transcription and genomic silencing, both of which are influenced by dorsoventral gradients of transcription factors NFIA, B, and X. Polygenic transcription of OR genes may define spatially constrained OR repertoires, among which one OR allele is selected for singular expression later in development. Heterochromatin assembly and genomic compartmentalization of OR alleles also vary across the axes of the olfactory epithelium and may preferentially eliminate ectopically expressed ORs with more dorsal expression destinations from this "privileged" repertoire. Our experiments identify early transcription as a potential "epigenetic" contributor to future developmental patterning and reveal how two spatially responsive probabilistic processes may act in concert to establish deterministic, precise, and reproducible territories of stochastic gene expression.

Transcriptional and Epigenetic Control of Mammalian Olfactory Epithelium Development

The postnatal mammalian olfactory epithelium (OE) represents a major aspect of the peripheral olfactory system. It is a pseudostratified tissue that originates from the olfactory placode and is composed of diverse cells, some of which are specialized receptor neurons capable of transducing odorant stimuli to afford the perception of smell (olfaction). The OE is known to offer a tractable miniature model for studying the systematic generation of neurons and glia that typify neural tissue development. During OE development, stem/progenitor cells that will become olfactory sensory neurons and/or non-neuronal cell types display fine spatiotemporal expression of neuronal and non-neuronal genes that ensures their proper proliferation, differentiation, survival, and regeneration. Many factors, including transcription and epigenetic factors, have been identified as key regulators of the expression of such requisite genes to permit normal OE morphogenesis. Typically, specific interactive regulatory networks established between transcription and epigenetic factors/cofactors orchestrate histogenesis in the embryonic and adult OE. Hence, investigation of these regulatory networks critical for OE development promises to disclose strategies that may be employed in manipulating the stepwise transition of olfactory precursor cells to become fully differentiated and functional neuronal and non-neuronal cell types. Such strategies potentially offer formidable means of replacing injured or degenerated neural cells as therapeutics for nervous system perturbations. This review recapitulates the developmental cellular diversity of the olfactory neuroepithelium and discusses findings on how the precise and cooperative molecular control by transcriptional and epigenetic machinery is indispensable for OE ontogeny.

Transcriptional regulatory network during development in the olfactory epithelium

Regeneration, a process of reconstitution of the entire tissue, occurs throughout life in the olfactory epithelium (OE). Regeneration of OE consists of several stages: proliferation of progenitors, cell fate determination between neuronal and non-neuronal lineages, their differentiation and maturation. How the differentiated cell types that comprise the OE are regenerated, is one of the central questions in olfactory developmental neurobiology. The past decade has witnessed considerable progress regarding the regulation of transcription factors (TFs) involved in the remarkable regenerative potential of OE. Here, we review current state of knowledge of the transcriptional regulatory networks that are powerful modulators of the acquisition and maintenance of developmental stages during regeneration in the OE. Advance in our understanding of regeneration will not only shed light on the basic principles of adult plasticity of cell identity, but may also lead to new approaches for using stem cells and reprogramming after injury or degenerative neurological diseases.

Developmental, tract-tracing and immunohistochemical study of the peripheral olfactory system in a basal vertebrate: insights on Pax6 neurons migrating along the olfactory nerve

The olfactory system represents an excellent model for studying different aspects of the development of the nervous system ranging from neurogenesis to mechanisms of axon growth and guidance. Important findings in this field come from comparative studies. We have analyzed key events in the development of the olfactory system of the shark Scyliorhinus canicula by combining immunohistochemical and tract-tracing methods. We describe for the first time in a cartilaginous fish an early population of pioneer HuC/D-immunoreactive (ir) neurons that seemed to delaminate from the olfactory pit epithelium and migrate toward the telencephalon before the olfactory nerve was identifiable. A distinct, transient cell population, namely the migratory mass, courses later on in apposition to the developing olfactory nerve. It contains olfactory ensheathing glial (GFAP-ir) cells and HuC/D-ir neurons, some of which course toward an extrabulbar region. We also demonstrate that Pax6-ir cells coursing along the developing olfactory pathways in S. canicula are young migrating (HuC/D and DCX-ir) neurons of the migratory mass that do not form part of the terminal nerve pathway. Evidences that these Pax6 neurons originate in the olfactory epithelium are also reported. As Pax6 neurons in the olfactory epithelium show characteristics of olfactory receptor neurons, and migrating Pax6-ir neurons formed transient corridors along the course of olfactory axons at the entrance of the olfactory bulb, we propose that these neurons could play a role as guideposts for axons of olfactory receptor neurons growing toward the olfactory bulb.

A subset of mouse colonic goblet cells expresses the bitter taste receptor Tas2r131

The concept that gut nutrient sensing involves taste receptors has been fueled by recent reports associating the expression of taste receptors and taste-associated signaling molecules in the gut and in gut-derived cell lines with physiological responses induced by known taste stimuli. However, for bitter taste receptors (Tas2rs), direct evidence for their functional role in gut physiology is scarce and their cellular expression pattern remained unknown. We therefore investigated Tas2r expression in mice. RT-PCR experiments assessed the presence of mRNA for Tas2rs and taste signaling molecules in the gut. A gene-targeted mouse strain was established to visualize and identify cell types expressing the bitter receptor Tas2r131. Messenger RNA for various Tas2rs and taste signaling molecules were detected by RT-PCR in the gut. Using our knock-in mouse strain we demonstrate that a subset of colonic goblet cells express Tas2r131. Cells that express this receptor are absent in the upper gut and do not correspond to enteroendocrine and brush cells. Expression in colonic goblet cells is consistent with a role of Tas2rs in defense mechanisms against potentially harmful xenobiotics.

Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway

Pax6 is involved in the control of neuronal specification, migration, and differentiation in the olfactory epithelium and in the generation of different interneuron subtypes in the olfactory bulb. Whether these roles are conserved during evolution is not known. Cartilaginous fish are extremely useful models for assessing the ancestral condition of brain organization because of their phylogenetic position. To shed light on the evolution of development of the olfactory system in vertebrates and on the involvement of Pax6 in this process, we analyzed by in situ hybridization and immunohistochemistry the expression pattern of Pax6 in the developing olfactory system in a basal vertebrate, the lesser spotted dogfish Scyliorhinus canicula. This small shark is becoming an important fish model in studies of vertebrate development. We report Pax6 expression in cells of the olfactory epithelium and olfactory bulb, and present the first evidence in vertebrates of strings of Pax6-expressing cells extending along the developing olfactory nerve. The results indicate the olfactory epithelium as the origin of these cells. These data are compatible with a role for Pax6 in the development of the olfactory epithelium and fibers, and provide a basis for future investigations into the mechanisms that regulate development of the olfactory system throughout evolution.

NFIA controls telencephalic progenitor cell differentiation through repression of the Notch effector Hes1

The balance between self-renewal and differentiation of neural progenitor cells is an absolute requirement for the correct formation of the nervous system. Much is known about both the pathways involved in progenitor cell self-renewal, such as Notch signaling, and the expression of genes that initiate progenitor differentiation. However, whether these fundamental processes are mechanistically linked, and specifically how repression of progenitor self-renewal pathways occurs, is poorly understood. Nuclear factor I A (Nfia), a gene known to regulate spinal cord and neocortical development, has recently been implicated as acting downstream of Notch to initiate the expression of astrocyte-specific genes within the cortex. Here we demonstrate that, in addition to activating the expression of astrocyte-specific genes, Nfia also downregulates the activity of the Notch signaling pathway via repression of the key Notch effector Hes1. These data provide a significant conceptual advance in our understanding of neural progenitor differentiation, revealing that a single transcription factor can control both the activation of differentiation genes and the repression of the self-renewal genes, thereby acting as a pivotal regulator of the balance between progenitor and differentiated cell states.

Gustatory expression pattern of the human TAS2R bitter receptor gene family reveals a heterogenous population of bitter responsive taste receptor cells

Human bitter taste is mediated by approximately 25 members of the human TAS2 receptor (hTAS2R) gene family. The hTAS2R genes are expressed in taste buds of gustatory papillae on the tongue surface. Because many naturally occurring bitter compounds are toxic, bitter taste receptors are believed to serve as warning sensors against the ingestion of toxic food compounds. An important question is whether bitter taste receptor cells are a homogeneous, broadly tuned population of cells, which uniformly express all bitter taste receptor genes, or not. Gene expression analyses in rodents demonstrated an essentially overlapping expression of TAS2R genes indicating a broad tuning, whereas functional in vivo analyses suggest a narrow tuning. The present study demonstrates the expression of all 25 human TAS2R genes in taste receptor cells of human circumvallate papillae. As shown by in situ hybridization experiments, the expression of hTAS2R genes differs in both the apparent level of expression and the number of taste receptor cells expressing these genes, suggesting a heterogeneous bitter taste receptor cell population. Differences in gene expression levels were verified by quantitative reverse transcription-PCR experiments for a subset of hTAS2R genes. Direct evidence for the heterogeneity of bitter taste receptor cells is provided by dual-labeling in situ hybridizations with selected pairs of hTAS2R gene-specific probes. Functional coexpression experiments in heterologous cells show competition among hTAS2Rs, indicating a possible biological reason for the observed expression pattern. From the data, we conclude that human bitter taste receptor cells are tuned to detect a limited subset of bitter stimuli.

Mutations in Olfactory Signal Transduction Genes Are Not a Major Cause of Human Congenital General Anosmia

Anosmia affects the western world population, mostly the elderly, reaching to 5% in subjects over the age of 45 years and strongly lowering their quality of life. A smaller minority (about 0.01%) is born without a sense of smell, afflicted with congenital general anosmia (CGA). No causative genes for human CGA have been identified yet, except for some syndromic cases such as Kallman syndrome. In mice, however, deletion of any of the 3 main olfactory transduction components (guanidine triphosphate binding protein, adenylyl cyclase, and the cyclic adenosine monophosphate-gated channel) causes profound reduction of physiological responses to odorants. In an attempt to identify human CGA-related mutations, we performed whole-genome linkage analysis in affected families, but no significant linkage signals were observed, probably due to the small size of families analyzed. We further carried out direct mutation screening in the 3 main olfactory transduction genes in 64 unrelated anosmic individuals. No potentially causative mutations were identified, indicating that transduction gene variations underlie human CGA rarely and that mutations in other genes have to be identified. The screened genes were found to be under purifying selection, suggesting that they play a crucial functional role not only in olfaction but also potentially in additional pathways.

Expression of Coxsackie-Adenovirus receptor (CAR) in the developing mouse olfactory system

Interest in manipulating gene expression in olfactory sensory neurons (OSNs) has led to the use of adenoviruses (AdV) as gene delivery vectors. OSNs are the first order neurons in the olfactory system and the initial site of odor detection. They are highly susceptible to adenovirus infection although the mechanism is poorly understood. The Coxsackie-Adenovirus receptor (CAR) and members of the integrin family have been implicated in the process of AdV infection in various systems. Multiple serotypes of AdV efficiently bind to the CAR, leading to entry and infection of the host cell by a mechanism that can also involve integrins. Cell lines that do not express CAR are relatively resistant, but not completely immune to AdV infection, suggesting that other mechanisms participate in mediating AdV attachment and entry. Using in situ hybridization and western blot analyses, we show that OSNs and olfactory bulbs (OB) of mice express abundant CAR mRNA at embryonic and neonatal stages, with progressive diminution during postnatal development. By contrast to the olfactory epithelium (OE), CAR mRNA is still present in the adult mouse OB. Furthermore, despite a similar postnatal decline, CAR protein expression in the OE and OB of mice continues into adulthood. Our results suggest that the robust AdV infection observed in the postnatal olfactory system is mediated by CAR and that expression of even small amounts of CAR protein as seen in the adult rodent, permits efficient AdV infection and entry. CAR is an immunoglobulin domain-containing protein that bears homology to cell-adhesion molecules suggesting the possibility that it may participate in organization of the developing olfactory system.

Members of RTP and REEP gene families influence functional bitter taste receptor expression

Functional characterization of chemosensory receptors is usually achieved by heterologous expression in mammalian cell lines. However, many chemoreceptor genes, including bitter taste receptors (TAS2Rs), show only marginal cell surface expression. Usually, these problems are circumvented by using chimeric receptors consisting of "export tags" and the receptor sequence itself. It seems likely that chemoreceptor cells express factors for cell surface targeting of native receptor molecules in vivo. For TAS2Rs, however, such factors are still unknown. The present study investigates the influence of RTP and REEP proteins on the functional expression of human TAS2Rs in heterologous cells. We expressed hTAS2Rs in HEK 293T cells and observed dramatic differences in responsiveness to agonist stimulation. By immunocytochemistry we show accumulation of the bitter beta-glucopyranoside receptor hTAS2R16 in the Golgi compartment. Coexpression of RTP and REEP proteins changed the responses of some hTAS2Rs upon agonist stimulation, which is likely due to efficient cell surface localization as demonstrated by cell surface biotinylation experiments. The coimmunoprecipitation of hTAS2R16 and RTP3 or RTP4 suggests that the mechanism by which these cofactors influence hTAS2R16 function might involve direct protein-protein interaction. Finally, expression analyses demonstrate RTP and REEP gene expression in human circumvallate papillae and testis, both of which are sites of TAS2R gene expression.

Transcriptional Regulation of Neurogenesis in the Olfactory Epithelium

1. The olfactory epithelium (OE) is a simple structure that gives rise to olfactory sensory neurons (OSNs) throughout life. 2. Numerous transcription factors (TFs) are expressed in regions of the OE which contain progenitor cells and OSNs. The function of some of these TFs in OSN development has been elucidated with the aide of transgenic knockout mice. 3. We review here the current state of knowledge on the role of TFs in OE neurogenesis and relate the expression of these TFs, where possible, to the well-documented phenotype of the cells as they progress through the OSN lineage from progenitor cells to mature neurons.

The presence of human papillomavirus 16 in neural structures and vascular endothelial cells

Human papillomavirus (HPV) is known as a strictly epitheliotropic pathogen. Our results raised the possibility that HPV 16 is present in neural cells and in the vascular endothelium. By in situ hybridization, we have detected HPV 16 E6 ORF sequence in small blood vessels and peripheral nerves adjacent to oral and cervical cancers. The same structures have clearly shown immunohistochemical reactivity for the E6 oncoprotein. These results were verified by PCR applied to E6 and L1 ORFs following microscopic laser dissection of the immunohistochemically positive nerves and vessels. These observations suggest that HPV 16 DNA and protein are present in neurons and endothelial cells in the vicinity of HPV-associated tumors. The HPV 16 genome presumably exists in a non-replicating form in the neurons and constitutively produces high levels of E6 and E7 proteins with an unknown neuropathological outcome.

Nuclear factor-1-X regulates astrocyte-specific expression of the alpha1-antichymotrypsin and glial fibrillary acidic protein genes

Discrete tissue-specific changes in chromatin structure of the distal serpin subcluster on human chromosome 14q32.1 allow a single gene encoding alpha1-antichymotrypsin (ACT) to be expressed in astrocytes and glioma cells. This astrocyte-specific regulation involves activatory protein-1 (AP-1) because overexpression of dominant-negative c-jun(TAM67) abolishes ACT expression in glioma cells. Here we identify a new regulatory element, located within the -13-kb enhancer of the ACT gene, that binds nuclear factor-1 (NFI) and is indispensable for the full basal transcriptional activity of the ACT gene. Furthermore, down-regulation of NFI expression by siRNA abolishes basal ACT expression in glioma cells. However, NFI does not mediate astrocyte-specific expression by itself, but likely cooperates with AP-1. A detailed analysis of the 14-kb long 5'-flanking region of the ACT gene indicated the presence of adjacent NFI and AP-1 elements that colocalized with DNase I-hypersensitive sites found in astrocytes and glioma cells. Interestingly, knock-down of NFI expression also specifically abrogates the expression of glial acidic fibrillary protein (GFAP), which is an astrocyte-specific marker protein. Mutations introduced into putative NFI and AP-1 elements within the 5'-flanking region of the GFAP gene also diminished basal expression of the reporter. In addition, we found, using isoform-specific siRNAs, that NFI-X regulates the astrocyte-specific expression of ACT and GFAP. We propose that NFI-X cooperates with AP-1 by an unknown mechanism in astrocytes, which results in the expression of a subset of astrocyte-specific genes.

Bitter taste receptors for saccharin and acesulfame K

Weight-conscious subjects and diabetics use the sulfonyl amide sweeteners saccharin and acesulfame K to reduce their calorie and sugar intake. However, the intrinsic bitter aftertaste, which is caused by unknown mechanisms, limits the use of these sweeteners. Here, we show by functional expression experiments in human embryonic kidney cells that saccharin and acesulfame K activate two members of the human TAS2R family (hTAS2R43 and hTAS2R44) at concentrations known to stimulate bitter taste. These receptors are expressed in tongue taste papillae. Moreover, the sweet inhibitor lactisole did not block the responses of cells transfected with TAS2R43 and TAS2R44, whereas it did block the response of cells expressing the sweet taste receptor heteromer hTAS1R2-hTAS1R3. The two receptors were also activated by nanomolar concentrations of aristolochic acid, a purely bitter-tasting compound. Thus, hTAS2R43 and hTAS2R44 function as cognate bitter taste receptors and do not contribute to the sweet taste of saccharin and acesulfame K. Consistent with the in vitro data, cross-adaptation studies in human subjects also support the existence of common receptors for both sulfonyl amide sweeteners.

Mechanisms of axon guidance in the developing nervous system

The human brain assembles an incredible network of over a billion neurons. Understanding how these connections form during development in order for the brain to function properly is a fundamental question in biology. Much of this wiring takes place during embryonic development. Neurons are generated in the ventricular zone, migrate out, and begin to differentiate. However, neurons are often born in locations some distance from the target cells with which they will ultimately form connections. To form connections, neurons project long axons tipped with a specialized sensing device called a growth cone. The growing axons interact directly with molecules within the environment through which they grow. In order to find their targets, axonal growth cones use guidance molecules that can either attract or repel them. Understanding what these guidance cues are, where they are expressed, and how the growth cone is able to transduce their signal in a directionally specific manner is essential to understanding how the functional brain is constructed. In this chapter, we review what is known about the mechanisms involved in axonal guidance. We discuss how the growth cone is able to sense and respond to its environment and how it is guided by pioneering cells and axons. As examples, we discuss current models for the development of the spinal cord, the cerebral cortex, and the visual and olfactory systems.

A Role for Nuclear Factor I in the Intrinsic Control of Cerebellar Granule Neuron Gene Expression

Nervous system formation requires the elaboration of a complex series of differentiation events in both a spatially and maturation-regulated manner. A fundamental question is how neuronal subtype specification and developmental gene expression are controlled within maturing neurons. The alpha6 subunit of the gamma-aminobutyric acid type A (GABA(A)) receptor (GABRA6) is preferentially expressed in cerebellar granule neurons and is part of an intrinsic program directing their differentiation. We have employed a lentiviral approach to examine the transcriptional mechanisms controlling neuronal subtype-selective expression of this gene. These studies demonstrated that nuclear factor I (NFI) proteins are required for both transgenic GABRA6 promoter activity as well as endogenous expression of this gene in cerebellar granule neurons. Chromatin immunoprecipitation also showed that NFI proteins are bound to the GABRA6 promoter in these cells in vivo. Furthermore, analyses of gene knockout mice revealed that Nfia is specifically required for normal expression of the GABRA6 gene in cerebellar granule neurons. NFI expression and DNA binding activity are highly enriched in granule neurons, implicating this transcription factor family in the neuronal subtype-selective expression of the GABRA6 gene. These studies define a new role for NFI proteins as neuronal subtype-enriched transcriptional regulators that participate in an intrinsic transcriptional program directing the differentiation of cerebellar granule neurons.

The human taste receptor hTAS2R14 responds to a variety of different bitter compounds

The recent advances in the functional expression of TAS2Rs in heterologous systems resulted in the identification of bitter tastants that specifically activate receptors of this family. All bitter taste receptors reported to date exhibit a pronounced selectivity for single substances or structurally related bitter compounds. In the present study we demonstrate the expression of the hTAS2R14 gene by RT-PCR analyses and in situ hybridisation in human circumvallate papillae. By functional expression in HEK-293T cells we show that hTAS2R14 displays a, so far, unique broad tuning towards a variety of structurally diverse bitter compounds, including the potent neurotoxins, (-)-alpha-thujone, the pharmacologically active component of absinthe, and picrotoxinin, a poisonous substance of fishberries. The observed activation of heterologously expressed hTAS2R14 by low concentrations of (-)-alpha-thujone and picrotoxinin suggests that the receptor is sufficiently sensitive to caution us against the ingestion of toxic amounts of these substances.

Transcriptional regulation of rat CYP2A3 by nuclear factor 1: identification of a novel NFI-A isoform, and evidence for tissue-selective interaction of NFI with the CYP2A3 promoter in vivo

Rat CYP2A3 and its mouse and human orthologs are expressed preferentially in the olfactory mucosa. We found previously that an element in the proximal promoter region of CYP2A3 (the nasal predominant transcriptional activating (NPTA) element), which is similar to a nuclear factor 1 (NFI)-binding site, is critical for transcriptional activation of CYP2A3 in vitro. We proposed that this element might be important for tissue-selective CYP2A3 expression. The goals of the present study were to characterize NPTA-binding proteins and to obtain more definitive evidence for the role of NFI in the transcriptional activation of CYP2A3. The NPTA-binding proteins were isolated by DNA-affinity purification from rat olfactory mucosa. Mass spectral analysis indicated that isoforms corresponding to all four NFI genes were present in the purified NPTA-binding fraction. Further analysis of NPTA-binding proteins led to the identification of a novel NFI-A isoform, NFI-A-short, which was derived from alternative splicing of the NFI-A transcript. Transient transfection assay showed that NFI-A2, an NFI isoform previously identified in the olfactory mucosa, transactivated the CYP2A3 promoter, whereas NFI-A-short, which lacks the transactivation domain, counteracted the activation. Chromatin immunoprecipitation assays indicated that NFI proteins are associated with the CYP2A3 promoter in vivo, in rat olfactory mucosa, but essentially not in the liver where the CYP2A3 promoter is hypermethylated and CYP2A3 is not expressed. These data strongly support a role for NFI transcription factors in the transcriptional activation of CYP2A3.

Identification of members of the Bex gene family as olfactory marker protein (OMP) binding partners

Olfactory marker protein (OMP) expression is a hallmark of mature vertebrate olfactory receptor neurons (ORNs). Evidence for OMP function derives from altered behavioral and electrophysiological activities of OMP-KO mice. The molecular basis for the altered phenotype following the deletion of OMP is still unclear. Recent structural studies predict the involvement of OMP in protein-protein interaction. Here we report the identification of an OMP partner, Bex2, by phage-display screening of an olfactory mucosal cDNA-library. In situ hybridization demonstrates cellular co-localization of OMP mRNA with mRNAs for Bex1, Bex2, and Bex3 in ORNs of olfactory tissue of the mouse. The OMP/Bex interaction has been confirmed by demonstrating the chemical cross-linking of recombinant rat OMP with a synthetic peptide derived from the Bex amino acid sequence. The subcellular localization of Bex and OMP proteins was evaluated in transfected HEK293 cells. Bex is visualized in the nucleus and cytoplasm. Following co-transfection we observed the unexpected presence of some OMP in the nucleus along with Bex. Together, these data argue convincingly that we have identified Bex as an OMP partner whose further characterization will provide insight to the role of OMP and to the mechanism of the OMP/Bex interaction in ORN differentiation and function.

Heme oxygenase-1 and heme oxygenase-2 have distinct roles in the proliferation and survival of olfactory receptor neurons mediated by cGMP and bilirubin, respectively

Heme oxygenase (HO) is implicated in protection against oxidative stress, proliferation and apoptosis in many cell types, including neurons. We utilized olfactory receptor neurons (ORNs) as a model to define the roles of HO-1 and HO-2 in neuronal development and survival, and to determine the mediators of these effects. The olfactory system is a useful model as ORNs display neurogenesis post-natally and do not contain nitric oxide synthase (NOS) activity, which could confound results. HO isoforms were expressed in ORNs during embryogenesis and post-natally. Mice null for either HO-1 or HO-2 displayed decreased proliferation of neuronal precursors. However, apoptosis was increased only in HO-2 null mice. Cyclic GMP immunostaining was reduced in ORNs in both genotypes, providing direct evidence that HO mediates cGMP production in vivo. Bilirubin immunostaining was reduced only in HO-2 null mice. These roles for HO-1 and HO-2 were confirmed using detergent ablation of the epithelium to observe increased neurogenesis of ORNs after target disruption in HO null mice. Primary cultures of ORNs revealed that proliferative and survival effects of HO were mediated through cGMP and bilirubin, respectively. These results support a role for HO, the CO-cGMP signaling system and bilirubin in neurodevelopment and in response to injury.

Identification and characterization of a prostate-specific androgen-independent protein-binding site in the probasin promoter

In this study we investigated the combination of transcription factors and proteins binding to the proximal part of the prostate-specific probasin (PB) promoter. Using DNaseI in vitro footprinting, several protected regions were identified on the proximal PB promoter (nucleotides -286 to +28 relative to the transcription start site) when nuclear extracts from LNCaP, a human prostate cancer cell line, were used. Four of the protected areas were observed only when LNCaP nuclear extracts treated with synthetic androgen (10 nM R1881) were used. Two other regions, referred to as FPI and FPII, showed protection regardless of the presence or absence of androgen. When DNaseI footprinting was done using other prostate and non-prostate nuclear extracts, protection of the FPII region was only seen in prostate cell lines. These androgen-independent regions were further tested for tissue and binding specificity using the electrophoretic mobility-shift assay. Eight complexes formed with the FPI probe while four complexes were observed with the FPII probe on incubation with the tested nuclear extracts. Methylation protection assays reveal that prostate cancer cell lines yield slightly different protection patterns for some of the protein complexes formed with non-prostate-derived cell lines, suggesting the presence of prostate-enriched or -exclusive proteins. Site-directed mutagenesis of the protected nucleotides within FPII resulted in a significant reduction in expression from the PB promoter. Identification of proteins binding to the FPII region revealed the participation of nuclear factor I (NF-I) or a closely related protein, although other unknown proteins are also involved. Defining the DNA and protein components that dictate prostate-specific expression of the PB promoter in an androgen-independent manner would provide a strong basis for the design and development of a gene therapy for systemic treatment of androgen-independent prostate cancer.

Abnormal development of forebrain midline glia and commissural projections in Nfia knock-out mice

Nuclear factor I (NFI) genes are expressed in multiple organs throughout development (Chaudhry et al., 1997; for review, see Gronostajski, 2000). All four NFI genes are expressed in embryonic mouse brain, with Nfia, Nfib, and Nfix being expressed highly in developing cortex (Chaudhry et al., 1997). Disruption of the Nfia gene causes agenesis of the corpus callosum (ACC), hydrocephalus, and reduced GFAP expression (das Neves et al., 1999). Three midline structures, the glial wedge, glia within the indusium griseum, and the glial sling are involved in development of the corpus callosum (Silver et al., 1982; Silver and Ogawa, 1983; Shu and Richards, 2001). Because Nfia(-)/- mice show glial abnormalities and ACC, we asked whether defects in midline glial structures occur in Nfia(-)/- mice. NFI-A protein is expressed in all three midline populations. In Nfia(-)/-, mice sling cells are generated but migrate abnormally into the septum and do not form a sling. Glia within the indusium griseum and the glial wedge are greatly reduced or absent and consequently Slit2 expression is also reduced. Although callosal axons approach the midline, they fail to cross and extend aberrantly into the septum. The hippocampal commissure is absent or reduced, whereas the ipsilaterally projecting perforating axons (Hankin and Silver, 1988; Shu et al., 2001) appear relatively normal. These results support an essential role for midline glia in callosum development and a role for Nfia in the formation of midline glial structures.

BMP mRNA and protein expression in the developing mouse olfactory system

The bone morphogenetic proteins (BMPs) play fundamental roles during the organization of the central nervous system. The presence of these proteins has also been demonstrated in regions of the adult brain that are characterized by neural plasticity. In this study, we examined the expression of BMP4, 6, and 7 mRNAs and proteins in the murine olfactory system. The olfactory system is a useful model for studying cell proliferation and neural differentiation because both of these processes persist throughout life in the olfactory epithelium (OE) and olfactory bulb (OB). Our results demonstrate a differential expression of BMP4, 6, and 7 in the embryonic, postnatal, and adult olfactory system. In particular, BMP4 and BMP7 showed similar immunostaining patterns, being expressed in the olfactory region from the earliest stages studied (embryonic day 15.5) to adulthood. During development BMPs were expressed in the OE, olfactory bulb nerve layer, glomerular layer (GL), mitral cell layer (MCL), and subventricular zone. During the first postnatal week of life, BMP4 and 7 immunoreactivity (-ir) was particularly evident in the GL, MCL, and in the subependymal layer (SEL), which originates postnatally from the subventricular zone. In adults, BMP4 and 7 immunostaining was present in the GL and SEL. Within the SEL, BMP4 and 7 proteins were expressed primarily in association with the astrocytic glial compartment. BMP6-ir was always found in mature olfactory receptor neurons and their axonal projections to the OB. In summary, these data support the hypothesis that BMPs play a role in the morphogenesis of the olfactory system during development and in its plasticity during adulthood.

Leukemia inhibitory factor inhibits neuronal terminal differentiation through STAT3 activation

The discovery of stem cells in the adult central nervous system raises questions concerning the neurotrophic factors that regulate postnatal neuronal development. Olfactory receptor neurons (ORNs) are a useful model, because they are capable of robust neurogenesis throughout adulthood. We have investigated the role of leukemia inhibitory factor (LIF) in postnatal neuronal development by using ORNs as a model. LIF is a multifunctional cytokine implicated in various aspects of neuronal development, including phenotype determination, survival, and in response to nerve injury. LIF-deficient mice display significant increases, both in the absolute amount and in the number of cells expressing olfactory marker protein, a marker of mature ORNs. The maturation of ORNs was significantly inhibited by LIF in vitro. LIF activated the STAT3 pathway in ORNs, and transfection of ORNs with a dominant negative form of STAT3 abolished the effect of LIF. These findings demonstrate that LIF negatively regulates ORN maturation via the STAT3 pathway. Thus, LIF plays a critical role in controlling the transition of ORNs to maturity. Consequently, a population of ORNs is maintained in an immature state to facilitate the rapid repopulation of the olfactory epithelium with mature neurons during normal cell turnover or after injury.

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.

Nuclear factor 1-C2 contributes to the tissue-specific activation of a milk protein gene in the differentiating mammary gland

Members of the nuclear factor 1 (NF1) transcription factor family have been postulated to be involved in the regulation of milk genes. In this work we have been able to identify the splice variant NF1-C2 as an important member of a tissue-specific activating complex that regulates the milk gene encoding carboxyl ester lipase (CEL). Mutation of the NF1-binding site in the CEL gene promoter results in a drastic reduction of the gene expression to about 15% in mammary epithelial cells. Furthermore, we demonstrate that the NF1-C2 protein interacts with a higher affinity to the NF1-binding site in the CEL gene promoter than other NF1 family members do and that NF1-C2 in the mouse mammary gland is a phosphorylated protein. During development of the mouse mammary gland, binding of NF1-C2 to the CEL gene promoter is induced at midpregnancy, in correlation with the induction of CEL gene expression. The fact that the NF1-C2 involving complex remains throughout the lactation period and decreases during the weaning period, when the CEL gene is down-regulated, supports its importance in the regulation of CEL gene expression. To our knowledge, this is the first report identifying a specific, endogenously expressed NF1 isoform to be involved in the tissue-specific activation of a gene.

Olf-1/early B cell factor is a regulator of glut4 gene expression in 3T3-L1 adipocytes

A negative regulatory element in the 5'-flanking region of the murine glut4 gene mediates chronic insulin- and cAMP-induced repression in 3T3-L1 adipocytes. Previous work demonstrated that members of the nuclear factor 1 (NF1) family of transcription factors and an unidentified factor bind to and mediate repression from this regulatory element. By using a yeast one-hybrid screen, Olf-1/Early B cell factor (O/E-1) was isolated as a candidate for this unidentified factor. A protein complex from 3T3-L1 adipocyte nuclear extract that bound the negative regulatory element was recognized by O/E-specific antiserum, and binding activity was competed effectively by distinct O/E-binding sequences. O/E binding activity was also detected in nuclear extracts from insulin-responsive, GLUT4-expressing tissues including adipose, skeletal muscle, and heart. Mutations within the negative regulatory element that abolish binding of O/E proteins concomitantly blocked insulin-induced repression in reporter gene assays. These results suggest that one or more members of the O/E transcription factor family function as important regulators of glut4 gene expression and therefore may play a heretofore unanticipated role in glucose homeostasis and insulin signaling.

Activation of the NPTA element of the CYP2A3 gene by NFI-A2, a nasal mucosa-selective nuclear factor 1 isoform

The aim of this study was to determine whether the NPTA element of the olfactory mucosa-predominant CYP2A3 gene can be activated by NFI-A2, a recently identified member of the nuclear factor 1 family of transcription factors. Isoform-specific RNA-PCR confirmed that NFI-A2 is mainly expressed in rat olfactory mucosa. A full-length NFI-A2 cDNA was isolated from a cDNA library of rat olfactory mucosa and was used for preparation of a construct encoding a fusion protein of NFI-A2 with the yeast GAL4 activation domain. Expression of the fusion protein in yeast was detected with an antibody to NFI-A. The fusion protein activated the expression of a LacZ reporter gene in yeast one-hybrid assays with a reporter construct containing the NPTA element, but not with other constructs lacking the NPTA element. These findings suggest that NFI-A2 may be involved in the tissue-selective transcriptional activation of the CYP2A3 gene in the olfactory mucosa.

Identification of candidate genes for controlling development of the basilar pons by differential display PCR

The basilar pons, a major hindbrain nucleus involved in sensory-motor integration, has become a model system for studying long-distance neuronal migration, axon-target recognition by collateral branching, and the formation of patterned axonal projections. To identify genes potentially involved in these developmental events, we have performed a differential display PCR screen comparing RNA isolated from the developing basilar pons with RNA obtained from developing cerebellum and olfactory bulb, as well as the mature basilar pons. Using 400 different combinations of primers, we screened more than 11,000 labeled DNA fragments and identified 201 that exhibited higher expression in the basilar pons than in the control tissues. From these, 138 distinct gene fragments were cloned. The differential expression of a large subset of these fragments was confirmed using RNase protection assays. In situ hybridization analysis revealed that the expression of many of these genes is limited to the basilar pons and only a few other brain regions, suggesting that they may play specific roles in pontine development.

Differential and overlapping expression patterns of X-dll3 and Pax-6 genes suggest distinct roles in olfactory system development of the African clawed frog Xenopus laevis

In Xenopus laevis, the formation of the adult olfactory epithelium involves embryonic, larval and metamorphic phases. The olfactory epithelium in the principal cavity (PC) develops during embryogenesis from the olfactory placode and is thought to respond to water-borne odorants throughout larval life. During metamorphosis, the PC undergoes major transformations and is exposed to air-borne odorants. Also during metamorphosis, the middle cavity (MC) develops de novo. The olfactory epithelium in the MC has the same characteristics as that in the larval PC and is thought to respond to water-borne odorants. Using in situ hybridization, we analyzed the expression pattern of the homeobox genes X-dll3 and Pax-6 within the developing olfactory system. Early in development, X-dll3 is expressed in both the neuronal and non-neuronal ectoderm of the sense plate and in all cell layers of the olfactory placode and larval PC. Expression becomes restricted to the neurons and basal cells of the PC by mid-metamorphosis. During metamorphosis, X-dll3 is also expressed throughout the developing MC epithelium and becomes restricted to neurons and basal cells at metamorphic climax. This expression pattern suggests that X-dll3 is first involved in the patterning and genesis of all cells forming the olfactory tissue and is then involved in neurogenesis or neuronal maturation in putative water- and air-sensing epithelia. In contrast, Pax-6 expression is restricted to the olfactory placode, larval PC and metamorphic MC, suggesting that Pax-6 is specifically involved in the formation of water-sensing epithelium. The expression patterns suggest that X-dll3 and Pax-6 are both involved in establishing the olfactory placode during embryonic development, but subtle differences in cellular and temporal expression patterns suggest that these genes have distinct functions.