Neurogenesis and cell death in olfactory epithelium

Antiviral Functions of Type I and Type III Interferons in the Olfactory Epithelium

The olfactory neuroepithelium (OE) is one of the few neuronal tissues where environmental pathogens can gain direct access. Despite this vulnerable arrangement, little is known about the protective mechanisms in the OE to prevent viral infection and its antiviral responses. We systematically investigated acute responses in the olfactory mucosa upon exposure to vesicular stomatitis virus (VSV) via RNA-seq. VSVs were nasally inoculated into C57BL/6 mice. Olfactory mucosae were dissected for gene expression analysis at different time points after viral inoculation. Interferon functions were determined by comparing the viral load in interferon receptor knockout (Ifnar1-/- and Ifnlr1-/-) with wildtype OE. Antiviral responses were observed as early as 24 h after viral exposure in the olfactory mucosa. The rapidly upregulated transcripts observed included specific type I as well as type III interferons (Ifn) and interferon-stimulated genes. Genetic analyses demonstrated that both type I and type III IFN signaling are required for the suppression of viral replication in the olfactory mucosa. Exogenous IFN application effectively blocks viral replication in the OE. These findings reveal that the OE possesses an innate ability to suppress viral infection. Type I and type III IFNs have prominent roles in OE antiviral functions.

Behavioral evidence of the functional interaction between the main and accessory olfactory system suggests a large olfactory system with a high plastic capability

Olfaction is fundamental in many species of mammals. In rodents, the integrity of this system is required for the expression of parental and sexual behavior, mate recognition, identification of predators, and finding food. Different anatomical and physiological evidence initially indicated the existence of two anatomically distinct chemosensory systems: The main olfactory system (MOS) and the accessory olfactory system (AOS). It was originally conceived that the MOS detected volatile odorants related to food, giving the animal information about the environment. The AOS, on the other hand, detected non-volatile sexually relevant olfactory cues that influence reproductive behaviors and neuroendocrine functions such as intermale aggression, sexual preference, maternal aggression, pregnancy block (Bruce effect), puberty acceleration (Vandenbergh effect), induction of estrous (Whitten effect) and sexual behavior. Over the last decade, several lines of evidence have demonstrated that although these systems could be anatomically separated, there are neuronal areas in which they are interconnected. Moreover, it is now clear that both the MOS and the AOS process both volatile and no-volatile odorants, indicating that they are also functionally interconnected. In the first part of the review, we will describe the behavioral evidence. In the second part, we will summarize data from our laboratory and other research groups demonstrating that sexual behavior in male and female rodents induces the formation of new neurons that reach the main and accessory olfactory bulbs from the subventricular zone. Three factors are essential for the neurons to reach the AOS and the MOS: The stimulation frequency, the stimulus's temporal presentation, and the release of opioids induced by sexual behavior. We propose that the AOS and the MOS are part of a large olfactory system with a high plastic capability, which favors the adaptation of species to different environmental signals.

Fluctuations in tissue growth portray homeostasis as a critical state and long-time non-Markovian cell proliferation as Markovian

Tissue growth is an emerging phenomenon that results from the cell-level interplay between proliferation and apoptosis, which is crucial during embryonic development, tissue regeneration, as well as in pathological conditions such as cancer. In this theoretical article, we address the problem of stochasticity in tissue growth by first considering a minimal Markovian model of tissue size, quantified as the number of cells in a simulated tissue, subjected to both proliferation and apoptosis. We find two dynamic phases, growth and decay, separated by a critical state representing a homeostatic tissue. Since the main limitation of the Markovian model is its neglect of the cell cycle, we incorporated a refractory period that temporarily prevents proliferation immediately following cell division, as a minimal proxy for the cell cycle, and studied the model in the growth phase. Importantly, we obtained from this last model an effective Markovian rate, which accurately describes general trends of tissue size. This study shows that the dynamics of tissue growth can be theoretically conceptualized as a Markovian process where homeostasis is a critical state flanked by decay and growth phases. Notably, in the growing non-Markovian model, a Markovian-like growth process emerges at large time scales.

Endogenous pH 6.0 β-Galactosidase Activity Is Linked to Neuronal Differentiation in the Olfactory Epithelium

The histochemical detection of β-galactosidase enzymatic activity at pH 6.0 (β-gal-pH6) is a widely used biomarker of cellular senescence in aging tissues. This histochemical assay also detects the presence of programmed cell senescence during specific time windows in degenerating structures of vertebrate embryos. However, it has recently been shown that this enzymatic activity is also enhanced in subpopulations of differentiating neurons in the developing central nervous system in vertebrates. The present study addressed the histochemical detection of β-gal-pH6 enzymatic activity in the developing postnatal olfactory epithelium in the mouse. This activity was detected in the intermediate layer of the olfactory epithelium. As development progressed, the band of β-gal-pH6 labeling in this layer increased in width. Immunohistochemistry and lectin histochemistry showed the β-gal-pH6 staining to be strongly correlated with the immunolabeling of the olfactory marker protein (OMP) that identifies mature olfactory sensory neurons. The cell somata of a subpopulation of differentiated olfactory neurons that were recognized with the Dolichos biflorus agglutinin (DBA) were always located inside this band of β-gal-pH6 staining. However, the β-gal-pH6 histochemical signal was always absent from the apical region where the cytokeratin-8 positive supporting cells were located. Furthermore, no β-gal-pH6 staining was found in the basal region of the olfactory epithelium where PCNA/pHisH3 immunoreactive proliferating progenitor cells, GAP43 positive immature neurons, and cytokeratin-5 positive horizontal basal cells were located. Therefore, β-gal-pH6 seems to be linked to neuronal differentiation and cannot be regarded as a biomarker of cellular senescence during olfactory epithelium development in mice.

The Role of the Nasal Cavity in the Pathogenesis of Prion Diseases

Prion diseases, or transmissible spongiform encephalopathies (TSEs), are a class of fatal neurodegenerative diseases caused by the entry and spread of infectious prion proteins (PrP^Sc) in the central nervous system (CNS). These diseases are endemic to certain mammalian animal species that use their sense of smell for a variety of purposes and therefore expose their nasal cavity (NC) to PrP^Sc in the environment. Prion diseases that affect humans are either inherited due to a mutation of the gene that encodes the prion protein, acquired by exposure to contaminated tissues or medical devices, or develop without a known cause (referred to as sporadic). The purpose of this review is to identify components of the NC that are involved in prion transport and to summarize the evidence that the NC serves as a route of entry (centripetal spread) and/or a source of shedding (centrifugal spread) of PrP^Sc, and thus plays a role in the pathogenesis of the TSEs.

Novel factor in olfactory ensheathing cell-astrocyte crosstalk: Anti-inflammatory protein α-crystallin B

Astrocytes are key players in CNS neuroinflammation and neuroregeneration that may help or hinder recovery, depending on the context of the injury. Although pro-inflammatory factors that promote astrocyte-mediated neurotoxicity have been shown to be secreted by reactive microglia, anti-inflammatory factors that suppress astrocyte activation are not well-characterized. Olfactory ensheathing cells (OECs), glial cells that wrap axons of olfactory sensory neurons, have been shown to moderate astrocyte reactivity, creating an environment conducive to regeneration. Similarly, astrocytes cultured in medium conditioned by cultured OECs (OEC-CM) show reduced nuclear translocation of nuclear factor kappa-B (NFκB), a pro-inflammatory protein that induces neurotoxic reactivity in astrocytes. In this study, we screened primary and immortalized OEC lines to identify these factors and discovered that Alpha B-crystallin (CryAB), an anti-inflammatory protein, is secreted by OECs via exosomes, coordinating an intercellular immune response. Our results showed that: (a) OEC exosomes block nuclear NFκB translocation in astrocytes while exosomes from CryAB-null OECs could not; (b) OEC exosomes could be taken up by astrocytes, and (c) CryAB treatment suppressed neurotoxicity-associated astrocyte transcripts. Our results indicate CryAB, as well as other factors secreted by OECs, are potential agents that can ameliorate, or even reverse, the growth-inhibitory environment created by neurotoxic reactive astrocytes following CNS injuries.

TRPM5-expressing Microvillous Cells Regulate Region-specific Cell Proliferation and Apoptosis During Chemical Exposure

The mammalian main olfactory epithelium (MOE) is exposed to a wide spectrum of external chemicals during respiration and relies on adaptive plasticity to maintain its structural and functional integrity. We previously reported that the chemo-responsive and cholinergic transient receptor potential channel M5 (TRPM5)-expressing-microvillous cells (MCs) in the MOE are required for maintaining odor-evoked electrophysiological responses and olfactory-guided behavior during two-week exposure to an inhaled chemical mixture. Here, we investigated the underlying factors by assessing the potential modulatory effects of TRPM5-MCs on MOE morphology and cell proliferation and apoptosis, which are important for MOE maintenance. In the posterior MOE of TRPM5-GFP mice, we found that two-week chemical exposure induced a significant increase in Ki67-expressing proliferating basal stem cells without a significant reduction in the thickness of the whole epithelium or mature olfactory sensory neuron (OSN) layer. This adaptive increase in stem cell proliferation was missing in chemical-exposed transcription factor Skn-1a knockout (Skn-1a^-/-) mice lacking TRPM5-MCs. In addition, a greater number of isolated OSNs from chemical-exposed Skn-1a^-/- mice displayed unhealthily high levels of resting intracellular Ca²⁺. Intriguingly, in the anterior MOE where we found a higher density of TRPM5-MCs, chemical-exposed TRPM5-GFP mice exhibited a time-dependent increase in apoptosis and a loss of mature OSNs without a significant increase in proliferation or neurogenesis to compensate for OSN loss. Together, our data suggest that TRPM5-MC-dependent region-specific upregulation of cell proliferation in the majority of the MOE during chemical exposure contributes to the adaptive maintenance of OSNs and olfactory function.

Caloric restriction reduces basal cell proliferation and results in the deterioration of neuroepithelial regeneration following olfactotoxic mucosal damage in mouse olfactory mucosa

The effects of caloric restriction (CR) on cell dynamics and gene expression in the mouse olfactory neuroepithelium are evaluated. Eight-week-old male C57BL/6 mice were fed either control pellets (104 kcal/week) or CR pellets (67 kcal/week). The cytoarchitecture of the olfactory neuroepithelium in the uninjured condition and its regeneration after injury by an olfactotoxic chemical, methimazole, were compared between mice fed with the control and CR diets. In the uninjured condition, there were significantly fewer olfactory marker protein (OMP)-positive olfactory receptor neurons and Ki67-positive proliferating basal cells at 3 months in the CR group than in the control group. The number of Ki67-positive basal cells increased after methimazole-induced mucosal injury in both the control and the CR groups, but the increase was less robust in the CR group. The recovery of the neuroepithelium at 2 months after methimazole administration was less complete in the CR group than in the control group. These histological changes were region-specific. The decrease in the OMP-positive neurons was prominent in the anterior region of the olfactory mucosa. Gene expression analysis using a DNA microarray and quantitative real-time polymerase chain reaction demonstrated that the expression levels of two inflammatory cytokines, interleukin-6 and chemokine ligand 1, were elevated in the olfactory mucosa of the CR group compared with the control group. These findings suggest that CR may be disadvantageous to the maintenance of the olfactory neuroepithelium, especially when it is injured.

Functional Consequences of Keratan Sulfate Sulfation in Electrosensory Tissues and in Neuronal Regulation

Keratan sulfate (KS) is a functional electrosensory and neuro-instructive molecule. Recent studies have identified novel low sulfation KS in auditory and sensory tissues such as the tectorial membrane of the organ of Corti and the Ampullae of Lorenzini in elasmobranch fish. These are extremely sensitive proton gradient detection systems that send signals to neural interfaces to facilitate audition and electrolocation. High and low sulfation KS have differential functional roles in song learning in the immature male zebra song-finch with high charge density KS in song nuclei promoting brain development and cognitive learning. The conductive properties of KS are relevant to the excitable neural phenotype. High sulfation KS interacts with a large number of guidance and neuroregulatory proteins. The KS proteoglycan microtubule associated protein-1B (MAP1B) stabilizes actin and tubulin cytoskeletal development during neuritogenesis. A second 12 span transmembrane synaptic vesicle associated KS proteoglycan (SV2) provides a smart gel storage matrix for the storage of neurotransmitters. MAP1B and SV2 have prominent roles to play in neuroregulation. Aggrecan and phosphacan have roles in perineuronal net formation and in neuroregulation. A greater understanding of the biology of KS may be insightful as to how neural repair might be improved.

Adaptation of olfactory receptor abundances for efficient coding

Olfactory receptor usage is highly heterogeneous, with some receptor types being orders of magnitude more abundant than others. We propose an explanation for this striking fact: the receptor distribution is tuned to maximally represent information about the olfactory environment in a regime of efficient coding that is sensitive to the global context of correlated sensor responses. This model predicts that in mammals, where olfactory sensory neurons are replaced regularly, receptor abundances should continuously adapt to odor statistics. Experimentally, increased exposure to odorants leads variously, but reproducibly, to increased, decreased, or unchanged abundances of different activated receptors. We demonstrate that this diversity of effects is required for efficient coding when sensors are broadly correlated, and provide an algorithm for predicting which olfactory receptors should increase or decrease in abundance following specific environmental changes. Finally, we give simple dynamical rules for neural birth and death processes that might underlie this adaptation.

Olfactory Ensheathing Cells for Spinal Cord Injury: Sniffing Out the Issues

Olfactory ensheathing cells (OECs) are glia reported to sustain the continuous axon extension and successful topographic targeting of the olfactory receptor neurons responsible for the sense of smell (olfaction). Due to this distinctive property, OECs have been trialed in human cell transplant therapies to assist in the repair of central nervous system injuries, particularly those of the spinal cord. Though many studies have reported neurological improvement, the therapy remains inconsistent and requires further improvement. Much of this variability stems from differing olfactory cell populations prior to transplantation into the injury site. While some studies have used purified cells, others have used unpurified transplants. Although both preparations have merits and faults, the latter increases the variability between transplants received by recipients. Without a robust purification procedure in OEC transplantation therapies, the full potential of OECs for spinal cord injury may not be realised.

Olfaction in Chronic Rhinosinusitis

Olfactory dysfunction is a frequent complaint in chronic rhinosinusitis patients and has a significant impact on quality of life. Therefore, it is essential that clinicians are aware of the importance of olfactory dysfunction in chronic rhinosinusitis (CRS) patients and know how to deal with it. Notably, the evaluation of olfactory function (i.e., using psychophysical testing) and imagery of olfactory bulb play an important role in the evaluation of patients and give essential information about the "baseline" olfactory function. Because the high impact of olfactory function on quality of life and medical and/or surgical treatment should be proposed to patients. However, it remains difficult to predict the outcome of treatment as well as long-term efficacy. The first section of this review is dedicated to the assessment of olfactory function. Secondly, we will discuss the etiopathology of olfactory dysfunction in CRS with and without nasal polyps. Finally, we will review literature findings about the efficacy of different treatments on olfactory function.

Cell migration in the developing rodent olfactory system

The components of the nervous system are assembled in development by the process of cell migration. Although the principles of cell migration are conserved throughout the brain, different subsystems may predominantly utilize specific migratory mechanisms, or may display unusual features during migration. Examining these subsystems offers not only the potential for insights into the development of the system, but may also help in understanding disorders arising from aberrant cell migration. The olfactory system is an ancient sensory circuit that is essential for the survival and reproduction of a species. The organization of this circuit displays many evolutionarily conserved features in vertebrates, including molecular mechanisms and complex migratory pathways. In this review, we describe the elaborate migrations that populate each component of the olfactory system in rodents and compare them with those described in the well-studied neocortex. Understanding how the components of the olfactory system are assembled will not only shed light on the etiology of olfactory and sexual disorders, but will also offer insights into how conserved migratory mechanisms may have shaped the evolution of the brain.

Laminar specific detection of APP induced neurodegeneration and recovery using MEMRI in an olfactory based Alzheimer's disease mouse model

Manganese enhanced MRI (MEMRI) was used to detect specific laminar changes in the olfactory bulb (OB) to follow the progression of amyloid precursor protein (APP)-induced neuronal pathology and its recovery in a reversible olfactory based Alzheimer's disease (AD) mouse model. Olfactory dysfunction is an early symptom of AD, which suggests that olfactory sensory neurons (OSNs) may be more sensitive to AD related factors than neurons in other brain areas. Previously a transgenic mouse model was established that causes degeneration of OSNs by overexpressing humanized APP (hAPP), which results in a disruption of the olfactory circuitry with changes in the glomerular structure. In the present work, OB volume and manganese enhancement of the glomerular layer in the OB were decreased in mutant mice. Turning off APP overexpression with doxycycline produced a significant increase in manganese enhancement of the glomerular layer after only 1week, and further recovery after 3weeks, while treatment with Aβ antibody produced modest improvement with MRI measurements. Thus, MEMRI enables a direct tracking of laminar specific neurodegeneration through a non-invasive in vivo measurement. The use of MRI will enable assessment of the ability of different pharmacological reagents to block olfactory neuronal loss and can serve as a unique in vivo screening tool to both identify potential therapeutics and test their efficacy.

Cell lineage branching as a strategy for proliferative control

Background

How tissue and organ sizes are specified is one of the great unsolved mysteries in biology. Experiments and mathematical modeling implicate feedback control of cell lineage progression, but a broad understanding of what lineage feedback accomplishes is lacking.

Results

By exploring the possible effects of various biologically relevant disturbances on the dynamic and steady state behaviors of stem cell lineages, we find that the simplest and most frequently studied form of lineage feedback - which we term renewal control - suffers from several serious drawbacks. These reflect fundamental performance limits dictated by universal conservation-type laws, and are independent of parameter choice. Here we show that introducing lineage branches can circumvent all such limitations, permitting effective attenuation of a wide range of perturbations. The type of feedback that achieves such performance - which we term fate control - involves promotion of lineage branching at the expense of both renewal and (primary) differentiation. We discuss the evidence that feedback of just this type occurs in vivo, and plays a role in tissue growth control.

Conclusions

Regulated lineage branching is an effective strategy for dealing with disturbances in stem cell systems. The existence of this strategy provides a dynamics-based justification for feedback control of cell fate in vivo.

See commentary article: http://dx.doi.org/10.1186/s12915-015-0123-7.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-015-0122-8) contains supplementary material, which is available to authorized users.

Olfactory functions scale with circuit restoration in a rapidly reversible Alzheimer's disease model

Neural circuits maintain a precise organization that is vital for normal brain functions and behaviors, but become disrupted during neurological disease. Understanding the connection between wiring accuracy and function to measure disease progression or recovery has been difficult because of the complexity of behavioral circuits. The olfactory system maintains well-defined neural connections that regenerate throughout life. We previously established a reversible in vivo model of Alzheimer's disease by overexpressing a humanized mutated amyloid precursor protein (hAPP) in olfactory sensory neurons (OSNs). Using this model, we currently show that hAPP is present in the OSN axons of mutant mice, which exhibit strong caspase3 signal and reduced synaptic protein expression by 3 weeks of age. In the olfactory bulb, we show that glomerular structure is distorted and OSN axonal convergence is lost. In vivo functional imaging experiments further demonstrate disruption of the glomerular circuitry, and behavioral assays reveal that olfactory function is significantly impaired. Because OSNs regenerate, we also tested if the system could recover from hAPP-induced disruption. We found that after 1 or 3 weeks of shutting-off hAPP expression, the glomerular circuit was partially restored both anatomically and functionally, with behavioral deficits similarly reversed. Interestingly, the degree of functional recovery tracked directly with circuit restoration. Together, these data demonstrate that hAPP-induced circuit disruption and subsequent recovery can occur rapidly and that behavior can provide a measure of circuit organization. Thus, olfaction may serve as a useful biomarker to both follow disease progression and gauge potential recovery.

Time course of structural and functional maturation of human olfactory epithelial cells in vitro

The unique ability of olfactory neurons to regenerate in vitro has allowed their use for the study of olfactory function, regeneration, and neurodegenerative disorders; thus, characterization of their properties is important. This present study attempts to establish the timeline of structural (protein expression) and functional (odorant sensitivity) maturation of human olfactory epithelial cells (hOE) in vitro using biopsy-derived cultured tissue. Cells were grown for 7 days; on each day, cells were tested for odorant sensitivity using calcium imaging techniques and then protein expression of each cell was tested using immunocytochemistry for proteins typically used for characterizing olfactory cells. Previous studies have shown that mature olfactory neurons in vitro attain a unique "phase-bright" morphology and express the olfactory marker protein (OMP). By day 3 in vitro, a variety of cells were odorant-sensitive, including both "phase-bright" and "phase-dark" cells that have previously been considered glial-like cells. The functional maturation of these hOEs appears to take place within 4 days. Interestingly, the emergence of an odorant sensitivity profile of both phase-bright and phase-dark cells preceded the expression of marker protein expression for OMP (which is expressed only by mature neurons in vivo). This structural maturation took 5 days, suggesting that the development of odorant sensitivity is not coincident with the expression of marker molecules that are hallmarks of structural maturation. These results have important implications for the use of hOEs as in vitro models of olfactory and neuronal function.

In vivo olfactory model of APP-induced neurodegeneration reveals a reversible cell-autonomous function

Amyloid precursor protein (APP) has long been linked to the neurodegeneration of Alzheimer's disease (AD), but the associated cell death has been difficult to capture in vivo, and the role of APP in effecting neuron loss is still unclear. Olfactory dysfunction is an early symptom of AD with amyloid pathology in the olfactory epithelium correlating well to the brain pathology of AD patients. As olfactory sensory neurons (OSNs) regenerate continuously with immature and mature OSNs coexisting in the same olfactory epithelium, we sought to use this unique system to study APP-induced neurodegeneration. Here we have developed an olfactory-based transgenic mouse model that overexpresses humanized APP containing familial AD mutations (hAPP) in either mature or immature OSNs, and found that despite the absence of extracellular plaques a striking number of apoptotic neurons were detected by 3 weeks of age. Importantly, apoptosis was restricted to the specific population overexpressing hAPP, either mature or immature OSNs, sparing those without hAPP. Interestingly, we observed that this widespread neurodegeneration could be rapidly rescued by reducing hAPP expression levels in immature neurons. Together, these data argue that overexpressing hAPP alone could induce cell-autonomous apoptosis in both mature and immature neurons, challenging the notion that amyloid plaques are necessary for neurodegeneration. Furthermore, we show that hAPP-induced neurodegeneration is reversible, suggesting that AD-related neural loss could potentially be rescued. Thus, we propose that this unique in vivo model will not only help determine the mechanisms underlying AD-related neurodegeneration but also serve as a platform to test possible treatments.

Age-related changes of the regeneration mode in the mouse peripheral olfactory system following olfactotoxic drug methimazole-induced damage

We investigated age-related changes in the mode of regeneration in the mouse peripheral olfactory system after olfactotoxic drug-induced damage. Mice at postnatal ages of 10 days, 3 months, and 16 months were given an intraperitoneal injection of methimazole to produce damage in the olfactory neuroepithelium. The olfactory neuroepithelia were harvested and analyzed immunohistochemically at various postlesion timepoints, from 1 day through to 94 days, to investigate neuroepithelial cell proliferation, the time course of neuronal differentiation, the reconstitution of neuroepithelium, and the innervation of the olfactory bulb. Functional recovery was assessed using the vanillin avoidance behavioral test. The chronological pattern in the expression of Ki67, beta III tubulin, and olfactory marker protein, molecular markers for neuronal cell proliferation and differentiation, changed similarly among the different age groups. In contrast, the extent of neuroepithelial cell proliferation after injury decreased with age, and the final histological recovery of the olfactory neuroepithelium and the innervation of the olfactory bulb were significantly smaller in the 16-month-old group compared to the younger age groups. These results suggest that the age-related decline in the capacity of olfactory neuroepithelium to reconstitute neuroepithelium is associated with its age-related decrease in proliferative activity after the neuroepithelial injury rather than changes in the process of neuronal differentiation. In spite of these incomplete anatomical recoveries, 16-month-old mice regained the ability to avoid vanillin solution by 1 month postlesion, suggesting that the extent of anatomical epithelial damage is not necessarily proportional to the threshold of olfactory perception.

Insulin but not leptin protects olfactory mucosa from apoptosis

The mammalian olfactory mucosa (OM) is continually renewed throughout life. Owing to their position in the nasal cavity, OM cells are exposed to multiple insults, including high levels of odourants that can induce their death. OM regeneration is therefore essential to maintain olfactory function, and requires the tight control of both cell death and proliferation. Apoptosis has been implicated in OM cell death. Olfaction is one of the senses involved in food intake and depends on individual nutritional status. We have previously reported the influence of hormones related to nutritional status on odour perception and have shown that the OM is a target of insulin and leptin, two hormones known for their anti-apoptotic properties. In the present study, we investigated the potential anti-apoptotic effect of these metabolic hormones on OM cells. Both Odora cells (an olfactive cell line) and OM cells treated with etoposide, a p53 activity inducer, exhibited mitochondrial-dependent apoptosis that was inhibited by the pan-caspase inhibitor zVAD-fmk. Insulin, but not leptin, impaired this apoptotic effect. Insulin addition to the culture medium reduced p53 phosphorylation, caspase-3 and caspase-9 cleavage, and caspase-3 enzymatic activity induced by etoposide. The apoptotic wave observed in the OM after interruption of the neuronal connections between the OM and the olfactory bulb by bulbectomy was impaired by intranasal insulin treatment. These findings suggest that insulin may be involved in OM cellular dynamics, through endocrine and/or paracrine-autocrine effects of circulating or local insulin, respectively.

Differential expression of RET receptor isoforms in the olfactory system

The glial cell line-derived neurotrophic factor (GDNF) family supports neurons by activating the tyrosine kinase receptor RET. The two main isoforms of RET, RET9 and RET51, differ in their carboxyl termini and have been implicated with distinct functions in the enteric and central nervous systems. Previously we reported the cellular localization of GDNF, neurturin and RET9 in the olfactory system [Maroldt H, Kaplinovsky T, Cunningham AM (2005) J Neurocytol 34:241-255]. In the current study, we examined immunohistochemical expression of RET9 and RET51 in neonatal and adult rat olfactory neuroepithelium (ON) and bulb to explore their potential functional roles. In the ON, RET9 was expressed by olfactory receptor neurons (ORNs) throughout the olfactory neuroepithelial sheet, whereas RET51 was restricted to ORNs situated in ventromedial and ventrolateral regions. Within these regions, RET51 was expressed by a subset of RET9-expressing ORNs. In olfactory bulb, RET9 expression was primarily on cell bodies, including olfactory ensheathing and periglomerular cells, and again, RET51 was expressed by a subset of RET9-expressing cells. RET51 was identified on axons in the olfactory nerve layer and glomerular neuropil, but only in the ventromedial and ventrolateral regions of the bulb. This regionalization correlated with the predicted axonal projection from expressing regions of the ON. RET51 was also expressed on dendrites in the external plexiform layer and glomerular neuropil. These results suggest RET9 may be the predominant functional isoform in the ON while RET51 plays a more selective role in a restricted region of the olfactory neuroepithelial sheet. In the bulb, RET9 is likely the main functional isoform while RET51 may be important in axonal and dendritic function/targeting.

Loss of STOP protein impairs peripheral olfactory neurogenesis

Background

STOP (Stable Tubulin-Only Polypeptide) null mice show behavioral deficits, impaired synaptic plasticity, decrease in synaptic vesicular pools and disturbances in dopaminergic transmission, and are considered a neurodevelopmental model of schizophrenia. Olfactory neurons highly express STOP protein and are continually generated throughout life. Experimentally-induced loss of olfactory neurons leads to epithelial regeneration within two months, providing a useful model to evaluate the role played by STOP protein in adult olfactory neurogenesis.

Methodology/Principal Findings

Immunocytochemistry and electron microscopy were used to study the structure of the glomerulus in the main olfactory bulb and neurogenesis in the neurosensorial epithelia. In STOP null mice, olfactory neurons showed presynaptic swellings with tubulovesicular profiles and autophagic-like structures. In olfactory and vomeronasal epithelia, there was an increase in neurons turnover, as shown by the increase in number of proliferating, apoptotic and immature cells with no changes in the number of mature neurons. Similar alterations in peripheral olfactory neurogenesis have been previously described in schizophrenia patients. In STOP null mice, regeneration of the olfactory epithelium did not modify these anomalies; moreover, regeneration resulted in abnormal organisation of olfactory terminals within the olfactory glomeruli in STOP null mice.

Conclusions/Significance

In conclusion, STOP protein seems to be involved in the establishment of synapses in the olfactory glomerulus. Our results indicate that the olfactory system of STOP null mice is a well-suited experimental model (1) for the study of the mechanism of action of STOP protein in synaptic function/plasticity and (2) for pathophysiological studies of the mechanisms of altered neuronal connections in schizophrenia.

Functional deprivation promotes amyloid plaque pathogenesis in Tg2576 mouse olfactory bulb and piriform cortex

Cerebral hypometabolism and amyloid accumulation are principal neuropathological manifestations of Alzheimer's disease (AD). Whether and how brain/neuronal activity might modulate certain pathological processes of AD are interesting topics of recent clinical and basic research in the field, and may be of potential medical relevance in regard to both the disease etiology and intervention. Using the Tg2576 transgenic mouse model of AD, this study characterized a promotive effect of neuronal hypoactivity associated with functional deprivation on amyloid plaque pathogenesis in the olfactory pathway. Unilateral naris-occlusion caused beta-secretase-1 (BACE1) elevation in neuronal terminals in the deprived relative to the non-deprived bulb and piriform cortex in young adult mice. In parallel with the overall age-related plaque development in the forebrain, locally increased BACE1 immunoreactivity co-occurred with amyloid deposition first in the piriform cortex then within the bulb, more prominent on the deprived relative to the non-deprived side. Biochemical analyses confirmed elevated BACE1 protein levels, enzymatic activity and products in the deprived relative to non-deprived bulbs. Plaque-associated BACE1 immunoreactivity in the bulb and piriform cortex was localized preferentially to swollen/sprouting glutamatergic axonal terminals, with Abeta immunoreactivity occurring inside as well as around these terminals. Together, these findings suggest that functional deprivation or neuronal hypoactivity facilitates amyloid plaque formation in the forebrain in a transgenic model of AD, which operates synergistically with age effect. The data also implicate an intrinsic association of amyloid accumulation and plaque formation with progressive axonal pathology.

Age-related changes in cell dynamics of the postnatal mouse olfactory neuroepithelium: cell proliferation, neuronal differentiation, and cell death

Age-related changes in cell proliferation, neuronal differentiation, and cell death in mouse olfactory neuroepithelium were investigated. Mice at the age of 10 days through 16 months were given a single injection of bromodeoxyuridine (BrdU). The olfactory mucosae were fixed at 9 timepoints ranging from 2 hours to 3 months after the injection and examined using double immunostaining for BrdU and olfactory marker protein (OMP), and double staining with terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end labeling (TUNEL) and immunostaining for OMP. The number of BrdU-labeled cells/mm epithelial length initially increased, peaked at 2-3 days after the BrdU injection, then declined at each age. The number of BrdU- and TUNEL-labeled neuronal cells both decreased with increasing age, suggesting that the rates of both cell proliferation and cell death in the olfactory neuroepithelium decrease with increasing age. Double-labeled cells for BrdU and OMP appeared at 7 days after injection in all age groups, suggesting that the time required for neuronal differentiation is broadly similar irrespective of age. In older age groups, smaller amounts of the newly produced cohort are integrated into the OMP-positive ORN population, and even once it is integrated it is eliminated from the population more rapidly compared to the younger age groups. Furthermore, TUNEL assay showed that the fraction of apoptotic cells distributed in the OMP-positive layer/total apoptotic cells decreased with age. This observation suggests that the turnover of mature ORNs is slower in the older neuroepithelium compared to the younger neuroepithelium.

Fate of marginal neuroblasts in the vomeronasal epithelium of adult mice

Chemical stimuli are sensed through the olfactory and vomeronasal epithelia, and the sensory cells of both systems undergo neuronal turnover during adulthood. In the vomeronasal epithelium, stem cells adjacent to the basal lamina divide and migrate to replace two classes of sensory neurons: apical neurons that express G(i2alpha)-linked V1R vomeronasal receptors and project to the anterior accessory olfactory bulb, and basal neurons that express G(oalpha)-linked V2R receptors and project to the posterior accessory olfactory bulb. Most of the dividing cells are present in the margins of the epithelium and only migrate locally. Previous studies have suggested that these marginal cells may participate in growth, sensory cell replacement or become apoptotic before maturation; however, the exact fate of these cells have remained unclear. In this work we investigated the fate of these marginal cells by analyzing markers of neurogenesis (bromodeoxyuridine incorporation), apoptosis (caspase-3), and neuronal maturation (olfactory marker protein and Neurotrace Nissl stain). Our data reveal a pool of dividing cells in the epithelial margins that predominantly give rise to mature neurons and only rarely undergo apoptosis. Newly generated cells are several times more numerous than apoptotic cells. These marginal neuroblasts could therefore constitute a net neural addition zone during adulthood.

Zicam-induced damage to mouse and human nasal tissue

Intranasal medications are used to treat various nasal disorders. However, their effects on olfaction remain unknown. Zicam (zinc gluconate; Matrixx Initiatives, Inc), a homeopathic substance marketed to alleviate cold symptoms, has been implicated in olfactory dysfunction. Here, we investigated Zicam and several common intranasal agents for their effects on olfactory function. Zicam was the only substance that showed significant cytotoxicity in both mouse and human nasal tissue. Specifically, Zicam-treated mice had disrupted sensitivity of olfactory sensory neurons to odorant stimulation and were unable to detect novel odorants in behavioral testing. These findings were long-term as no recovery of function was observed after two months. Finally, human nasal explants treated with Zicam displayed significantly elevated extracellular lactate dehydrogenase levels compared to saline-treated controls, suggesting severe necrosis that was confirmed on histology. Our results demonstrate that Zicam use could irreversibly damage mouse and human nasal tissue and may lead to significant smell dysfunction.

Bestrophin 2: an anion channel associated with neurogenesis in chemosensory systems

The chemosensory neuroepithelia of the vertebrate olfactory system share a life-long ability to regenerate. Novel neurons proliferate from basal stem cells that continuously replace old or damaged sensory neurons. The sensory neurons of the mouse and rat olfactory system specifically express bestrophin 2, a member of the bestrophin family of calcium-activated chloride channels. This channel was recently proposed to operate as a transduction channel in olfactory sensory cilia. We raised a polyclonal antibody against bestrophin 2 and characterized the expression pattern of this protein in the mouse main olfactory epithelium, septal organ of Masera, and vomeronasal organ. Comparison with the maturation markers growth-associated protein 43 and olfactory marker protein revealed that bestrophin 2 was expressed in developing sensory neurons of all chemosensory neuroepithelia, but was restricted to proximal cilia in mature sensory neurons. Our results suggest that bestrophin 2 plays a critical role during differentiation and growth of axons and cilia. In mature olfactory receptor neurons, it appears to support growth and function of sensory cilia.

Activation of purinergic receptors induces proliferation and neuronal differentiation in Swiss Webster mouse olfactory epithelium

In the central nervous system (CNS), adenosine 5'-triphosphate (ATP) induces the synthesis and release of neurotrophic factors, cell proliferation, and differentiation. The olfactory system is one site where multipotent progenitor cells continue to proliferate and differentiate into neurons throughout life. We tested the hypothesis that ATP initiates proliferation in olfactory epithelium by measuring 5-bromo-2-deoxyuridine incorporation. Adult mice were pre-treated intraperitoneally (i.p.) or intranasally with saline or purinergic receptor antagonists (pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate+suramin) 30 min prior to nasal instillation of ATP, uridine 5'-triphosphate (UTP), adenosine 5'-(3-thiotriphosphate) (ATP gamma S) or saline (0 h). Mice received three injections of 5-bromo-2-deoxyuridine between 42 and 46 h, and were sacrificed at 2, 9 or 16 days post-ATP instillation. ATP, UTP or ATP gamma S significantly increased 5-bromo-2-deoxyuridine incorporation compared to intranasal saline controls in groups pre-treated with saline. Saline, ATP, UTP or ATP gamma S instillation did not significantly increase 5-bromo-2-deoxyuridine incorporation in groups pre-treated with purinergic receptor antagonists. Similar results were observed in neonates and in a cultured slice preparation. Intranasal instillation of ATP also increased the protein levels of proliferating cell nuclear antigen in adults. Pre-treatment with purinergic receptor antagonists inhibited the ATP-induced increase in proliferating cell nuclear antigen. In adults, a subset of the cells that incorporated 5-bromo-2-deoxyuridine was immunoreactive to neuronal markers mammalian achaete-schute homolog 1, growth-associated protein 43, and olfactory marker protein at 2, 9, and 16 days, respectively. Collectively, these data indicate that purinergic receptor activation induces proliferation and neuronal differentiation in the mouse olfactory epithelium. We propose that extracellular ATP released upon injury could induce proliferation and promote the neuroregeneration of the olfactory epithelium.

Leukemia inhibitory factor promotes olfactory sensory neuronal survival via phosphoinositide 3-kinase pathway activation and Bcl-2

Leukemia inhibitory factor (LIF), a neuropoietic cytokine, has been implicated in the control of neuronal development. We previously reported that LIF plays a critical role in regulating the terminal differentiation of olfactory sensory neurons (OSNs). Here, we demonstrate that LIF plays a complementary role in supporting the survival of immature OSNs. Mature OSNs express LIF, which may be elaborated in a paracrine manner to influence adjacent neurons. LIF null mice display more apoptotic immature neurons than do their wild-type littermates. LIF treatment of dissociated OSNs in vitro significantly reduces the apoptosis of immature OSNs. Double immunocytochemical analysis indicates that the survival of immature OSNs is dependent on the presence of LIF. LIF activates the phosphoinositide 3-kinase (PI3K) pathways and induces the expression of the antiapoptotic molecule Bcl-2 in OSNs, whereas inhibition of the PI3K pathway blocks LIF-dependent OSN survival and Bcl-2 induction. Thus, LIF plays a central role in maintaining the size and integrity of the population of immature neurons within the olfactory epithelium; this population is critical to the rapid recovery of olfactory function after injury. LIF may play a similar role elsewhere in the CNS and thus be important for manipulation of stem cell populations for therapeutic interventions.

Neuroinvasion of the 263K scrapie strain after intranasal administration occurs through olfactory-unrelated pathways

The olfactory system has been implicated in the pathogenesis of transmissible spongiform encephalopathies (TSEs). To examine this issue and identify the pattern of TSE agent spread after intranasal administration, we inoculated a high-infectious dose of neurotropic scrapie strain 263K into the nasal cavity of Syrian hamsters. All animals allowed to survive became symptomatic with a mean incubation period of 162.4 days. Analysis at different time points revealed deposition of the pathological prion protein (PrP(TSE)) in nasal-associated lymphoid tissues in the absence of brain involvement from 80 days post-infection (50% of the incubation period). Olfactory-related structures and brainstem nuclei were involved from 100 days post-inoculation (62% of the incubation period) when animals were still asymptomatic. Intriguingly, vagal or trigeminal nuclei were identified as early sites of PrP(TSE) deposition in some pre-symptomatic animals. These findings indicate that the 263K scrapie agent is unable to effectively spread from the olfactory neuroepithelium to the olfactory-related structures and that, after intranasal inoculation, neuroinvasion occurs through olfactory-unrelated pathways.

Cell lineages and the logic of proliferative control

It is widely accepted that the growth and regeneration of tissues and organs is tightly controlled. Although experimental studies are beginning to reveal molecular mechanisms underlying such control, there is still very little known about the control strategies themselves. Here, we consider how secreted negative feedback factors ("chalones") may be used to control the output of multistage cell lineages, as exemplified by the actions of GDF11 and activin in a self-renewing neural tissue, the mammalian olfactory epithelium (OE). We begin by specifying performance objectives-what, precisely, is being controlled, and to what degree-and go on to calculate how well different types of feedback configurations, feedback sensitivities, and tissue architectures achieve control. Ultimately, we show that many features of the OE-the number of feedback loops, the cellular processes targeted by feedback, even the location of progenitor cells within the tissue-fit with expectations for the best possible control. In so doing, we also show that certain distinctions that are commonly drawn among cells and molecules-such as whether a cell is a stem cell or transit-amplifying cell, or whether a molecule is a growth inhibitor or stimulator-may be the consequences of control, and not a reflection of intrinsic differences in cellular or molecular character.

Distribution and severity of spontaneous lesions in the neuroepithelium and Bowman's glands in mouse olfactory mucosa: age-related progression

Age-related changes were examined in the distribution and severity of spontaneous lesions in the neuroepithelium and Bowman's glands in mouse olfactory mucosa. The olfactory mucosa of female ICR mice at postnatal ages from 10 days to 16 months were investigated histologically by hematoxylin and eosin staining, high-iron diamine-Alcian blue (HID-AB) staining, and immunohistochemistry for olfactory marker protein (OMP), betaIII tubulin (betaIIIT), and Ki67. The lesions in the neuroepithelium and Bowman's glands were quantitatively assessed by morphometric analyses of sections stained with anti-OMP antibody or HID-AB. The first appearance of neuroepithelial abnormality was observed in the dorsomedial portion of the olfactory mucosa in 5-month-old mice. The distribution and severity of lesions progressed with increasing age. In mildly affected epithelium in which OMP-positive olfactory receptor neurons (ORNs) were present but in smaller amounts, the numbers of betaIIIT-positive and Ki67-positive neuroepithelial cells tended to be increased, indicating that neurogenesis was upregulated in these areas. In contrast, severely affected epithelium in which OMP-positive ORNs were virtually absent showed high variability in the numbers of betaIIIT- and Ki67-positive cells among the areas examined, probably reflecting differences in the capacity of the basal cells remaining in the affected area to generate new neuronal cells. Histological analysis with HID-AB revealed that spontaneous lesions in Bowman's glands also occurred in aged mouse olfactory mucosa. Lesions in the neuroepithelium and underlying Bowman's glands tended to be spatially co-localized, suggesting a close association between pathogeneses in these two structures. Moreover, lesions in Bowman's glands were associated with changes in the biochemical composition of mucus on the olfactory mucosa. This information should prove useful in improving the understanding of the pathogenetic mechanisms underlying age-related changes in the peripheral olfactory system.

Subventricular zone-derived, newly generated neurons populate several olfactory and limbic forebrain regions

Neurogenesis persists in several regions of the adult mammalian brain. Although the hippocampus and olfactory bulb are most commonly studied in the context of adult neurogenesis, there is an increasing body of evidence in support of neurogenesis occurring outside of these two regions. The current study expands on previous data by showing newborn neurons with a mature phenotype are located in several olfactory and limbic structures outside of the hippocampus and olfactory bulb, where we previously described doublecortin/bromodeoxyuridine immature neurons. Notably, newborn neurons with a mature neuronal phenotype are found in the olfactory tubercles, anterior olfactory nuclei, tenia tecta, islands of Calleja, amygdala, and lateral entorhinal cortex. The appearance of newborn neurons with a mature phenotype in these regions suggests that these structures are destinations, and that newborn neurons are not simply passing through these structures. In light of the increasing body of evidence for neurogenesis in these and other olfactory, limbic, and striatal structures, we hypothesize that brain regions displaying adult neurogenesis are functionally linked.

Expression of insulin system in the olfactory epithelium: first approaches to its role and regulation

Food odours are major determinants for food choice; their detection is influenced by nutritional status. Among different metabolic signals, insulin plays a major role in food intake regulation. The aim of the present study was to investigate a potential role of insulin in the olfactory mucosa (OM), using ex vivo tissues and in vitro primary cultures. We first established the expression of insulin receptor (IR) in rat olfactory mucosa. Transcripts of IR-A and IR-B isoforms, as well as IRS-1 and IRS-2, were detected in OM extracts. Using immunocytochemistry, IR protein was located in olfactory receptor neurones, sustentacular and basal cells and in endothelium of the lamina propria vessels. Moreover, the insulin binding capacity of OM was quite high compared to that of olfactory bulb or liver. Besides the main pancreatic insulin source, we demonstrated insulin synthesis at a low level in the OM. Interestingly 48 h of fasting, leading to a decreased plasmatic insulin, increased the number of IR in the OM. Local insulin concentration was also enhanced. These data suggest a control of OM insulin system by nutritional status. Finally, an application of insulin on OM, aiming to mimic postprandial insulin increase, reversibly decreased the amplitude of electro-olfactogramme responses to odorants by approximately 30%. These data provide the first evidence that insulin modulates the most peripheral step of odour detection at the olfactory mucosa level.

Differential expression of HNK-1 and p75(NTR) in adult canine Schwann cells and olfactory ensheathing cells in situ but not in vitro

Olfactory ensheathing cells (OECs) are promising candidates for autologous cell transplantation therapies of nervous system injury and disease. Large animal models are relevant for transferring experimental data into clinical practice. In vivo studies have suggested that adult canine OECs may display similar regenerating capacities as their rodent counterpart. However, data on their molecular phenotype required for generating pure cell preparations are still scarce. In the present study, we comparatively analyzed expression of the carbohydrate HNK-1 epitope and the neurotrophin receptor p75(NTR) in adult canine Schwann cells and olfactory ensheathing cells in situ and in vitro. Myelinating and nonmyelinating Schwann cells in situ exclusively expressed HNK-1 and p75(NTR), respectively, whereas OECs were negative for both markers. In vitro, OECs and Schwann cells shared cell surface expression of p75(NTR) but not of HNK-1, which could be detected transiently in intracellular vesicles. This suggests that Schwann cells and OECs in vitro phagozytose HNK-1+ cellular debris. The cultivation-induced downregulation of HNK-1 expression in Schwann cells and upregulation of p75(NTR) in OECs argues for the possibility that axonal signals control the expression of both markers in situ. Whereas HNK-1 expression in Schwann cells is most likely controlled by signals inducing myelination, e.g., neuregulin, the mechanisms that may suppress p75(NTR) expression in OECs in situ remain to be elucidated. Interestingly, HNK-1 expression in the adult dog was found in both sensory and motor nerve myelinating Schwann cells. This is reminiscent of humans and differs from rodents; it also underscores the importance of large animal models for translational research.

Derivation of an inhalation reference concentration based upon olfactory neuronal loss in male rats following subchronic acetaldehyde inhalation

Acetaldehyde inhalation induces neoplastic and nonneoplastic responses in the rodent nasal cavity. This experiment further characterizes the dose-response relationship for nasal pathology, nasal epithelial cell proliferation, and DNA-protein cross-link formation in F-344 rats exposed subchronically to acetaldehyde. Animals underwent whole-body exposure to 0, 50, 150, 500, or 1500 ppm acetaldehyde for 6 h/day, 5 days/wk for up to 65 exposure days. Respiratory tract histopathology was evaluated after 4, 9, 14, 30, and 65 exposure days. Acetaldehyde exposure was not associated with reduced body weight gain or other evidence of systemic toxicity. Histologic evaluation of the nasal cavity showed an increased incidence of olfactory neuronal loss (ONL) following acute to subchronic exposure to > or = 150 ppm acetaldehyde and increased olfactory epithelial cell proliferation following exposure to 1500 ppm acetaldehyde. The severity of the ONL demonstrated dose- and temporal-dependent behaviors, with minimal effects noted at 150-500 ppm acetaldehyde and moderately severe lesions seen in the highest exposure group, with increased lesion severity and extent as the exposure duration increased. Acetaldehyde exposure was also associated with inflammation, hyperplasia, and squamous metaplasia of the respiratory epithelium. These responses were seen in animals exposed to > or = 500 ppm acetaldehyde. Acetaldehyde exposure was not associated with increased DNA-protein cross-link formation in the respiratory or olfactory epithelium. A model of acetaldehyde pharmacokinetics in the nose was used to derive an inhalation reference concentration (RfC) of 0.4 ppm, based on the no-observed-adverse-effect level (NOAEL) of 50 ppm for the nasal pathology seen in this study.

Macrophage-mediated neuroprotection and neurogenesis in the olfactory epithelium

Resident and recruited olfactory epithelial macrophages participate in the regulation of the survival, degeneration, and replacement of olfactory sensory neurons (OSNs). We have reported that liposome-encapsulated clodronate (Lip-C) induced selective and statistically significant depletion of macrophages in the OE of sham and 48 h OBX mice (38 and 35%, respectively) that resulted in increased OSN apoptosis and decreased numbers of mature OSNs and proliferating basal cells compared to controls (Lip-O). The aim of this study was to identify molecular mechanisms by which the selective depletion of macrophages in the OE resulted in these cellular changes by using a microarray expression pattern analysis. A 2x2 ANOVA identified 4,085 overall significantly (P < 0.01) regulated genes in the OE of Lip-O and Lip-C sham and 48 h OBX mice, and further statistical analysis using pairwise comparisons identified 4,024 genes that had either a significant (P < 0.01) treatment main effect (n = 2,680), group main effect (n = 778), or interaction effect (n = 980). The mean hybridization signals of immune response genes, e.g., Cxcr4, and genes encoding growth factors and neurogenesis regulators, e.g., Hdgf and Neurod1, respectively, were primarily lower in Lip-C mice compared with Lip-O mice. Apoptosis genes, e.g., Bak1, were also differentially regulated in Lip-C and/or OBX mice. Expression patterns of selected genes were validated with real-time RT-PCR; immunohistochemistry was used to localize selected gene products. These results identified the differential regulation of several novel genes through which alternatively activated macrophages regulate OSN progenitor cell proliferation, differentiation, and maturation, and the survival of OSNs.

Olfactory enrichment enhances the survival of newly born cortical neurons in adult mice

Neurogenesis persists in the adult rodent olfactory epithelium and olfactory bulbs. Recent studies suggest that neurogenesis might also occur in the adult rodent piriform cortex, the primary cortical projection site of the olfactory bulbs. To determine whether olfactory enrichment influences neurogenesis in the mouse piriform cortex, olfactory enrichment was used in combination with bromodeoxyuridine labeling. Quantification of the number of bromodeoxyuridine-labeled cells in the piriform cortex that double label for either the immature neuronal marker, doublecortin, or the mature neuronal marker, neuronal nuclei or NeuN, showed that olfactory enrichment increases the survival of newborn neurons in the piriform cortex. These results confirm that neurogenesis occurs in the piriform cortex of rodents and suggest that it may play a neuroplastic role there.

Macrophage depletion in the murine olfactory epithelium leads to increased neuronal death and decreased neurogenesis

Apoptosis of olfactory sensory neurons (OSNs) induced by olfactory bulbectomy (OBX) leads to the activation of resident macrophages within the olfactory epithelium (OE). These macrophages phagocytose degenerating OSNs and secrete chemokines, which recruit additional macrophages into the OE, and cytokines/growth factors, which regulate basal cell proliferation and differentiation and maturation of OSNs. In this study we apply for the first time the use of liposome-encapsulated clodronate to selectively deplete macrophages during the OSN degeneration/regeneration cycle in order to elucidate the role(s) of macrophages in regulating cellular mechanisms that lead to apoptosis and neurogenesis. Mice were injected intranasally and intravenously with either liposome-encapsulated clodronate or empty liposomes prior to and after OBX or sham OBX. At 48 hours after surgery the numbers of macrophages in the OE of both sham and OBX clodronate-treated mice were significantly reduced compared to liposome-treated controls (38% and 35%, respectively, P < 0.05). The reduction in macrophage numbers was accompanied by significant decreases in OE thickness (22% and 21%, P < 0.05), the number of mOSNs (1.2- and 1.9-fold, P < 0.05), and basal cell proliferation (7.6- and 3.8-fold, P < 0.005) in sham and OBX mice, respectively, compared to liposome-treated controls. In OBX mice there was also increased immunoreactivity for active caspase-3 in the OE and olfactory nerves of clodronate-treated OBX mice compared to liposome-treated controls. These results indicate that macrophages modulate the OSN population in the normal and target-ablated murine OE by influencing neuronal survival and basal cell proliferation, resulting in neurogenesis and replacement of mature OSNs.

Olfactory imprinting is correlated with changes in gene expression in the olfactory epithelia of the zebrafish

Odors experienced as juveniles can have significant effects on the behavior of mature organisms. A dramatic example of this occurs in salmon, where the odors experienced by developing fish determine the river to which they return as adults. Further examples of olfactory memories are found in many animals including vertebrates and invertebrates. Yet, the cellular and molecular bases underlying the formation of olfactory memory are poorly understood. We have devised a series of experiments to determine whether zebrafish can form olfactory memories much like those observed in salmonids. Here we show for the first time that zebrafish form and retain olfactory memories of an artificial odorant, phenylethyl alcohol (PEA), experienced as juveniles. Furthermore, we demonstrate that exposure to PEA results in changes in gene expression within the olfactory sensory system. These changes are evident by in situ hybridization in the olfactory epithelium of the developing zebrafish. Strikingly, our analysis by in situ hybridization demonstrates that the transcription factor, otx2, is up regulated in the olfactory sensory epithelia in response to PEA. This increase is evident at 2-3 days postfertilization and is maintained in the adult animals. We propose that the changes in otx2 gene expression are manifest as an increase in the number of neuronal precursors in the cells olfactory epithelium of the odor-exposed fish. Thus, our results reveal a role for the environment in controlling gene expression in the developing peripheral nervous system.

Dipyridamole promotes changes in calbindin-D28k and tyrosine hydroxylase expression in neonatal rats

BACKGROUND

Perinatal hypoxia alters the concentration of many neurochemicals in the brain, including adenosine, and promotes central nervous system (CNS) disorders in human infants such as periventricular leukomalacia or encephalopathy.

OBJECTIVE

Using the postnatal rat as a model of perinatal human development, we examined the effects of sustained increases in brain adenosine on CNS regions thought to be involved with both planning and execution of motor activity.

METHODS

To simulate hypoxia-induced changes in adenosine, Sprague-Dawley rats were injected twice daily from postnatal day (P) 3 to P14, with the adenosine uptake inhibitor dipyridamole (DIP) or the A(1) adenosine receptor agonist N(6)-cyclopentyladenosine (CPA). Vehicle-injected animals served as controls. Immunohistochemical and morphological analyses were then performed to examine the expression of calbindin D-28k (CB) and the thickness of the external granule cell layer (eGL) in the cerebellum. Additionally tyrosine hydroxylase (TH) expression in the caudate putamen and ventricular size were also examined.

RESULTS

In the cerebellum, both DIP and CPA reduced the number of CB-positive Purkinje cells as well as decreased the thickness of the eGL compared to vehicle. In the caudate putamen we found that DIP but not CPA decreased TH expression when compared to vehicle. Neither agent significantly altered ventricular size when compared to vehicle.

CONCLUSIONS

These observations suggest that elevations in brain adenosine, which can occur following hypoxia, leads to both neurochemical and cellular changes in regions of the brain which control the planning and execution of motor activity. Thus, therapeutic strategies that target brain regions most sensitive to adenosine may prevent or control at least some of the CNS damage observed following perinatal hypoxia.

Differential RIP antigen (CNPase) expression in peripheral ensheathing glia

The RIP monoclonal antibody is commonly used to identify oligodendrocytes. Recently, the RIP antigen was identified as 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), a known non-compact myelin protein [Watanabe, M., Sakurai, Y., Ichinose, T., Aikawa, Y., Kotani, M., Itoh, K., 2006. Monoclonal antibody Rip specifically recognizes 2',3'-cyclic nucleotide 3'-phosphodiesterase in oligodendrocytes. J. Neurosci. Res. 84, 525-533]. In the present study we characterize normal and axotomy-induced changes in RIP immunoreactivity in peripheral glia. In myelinating Schwann cells, RIP demarcated paranodal regions of myelinated axons and clearly defined Schmidt-Lantermann incisures. Surprisingly, RIP immunoreactivity was not confined to myelinating glia. Robust RIP immunoreactivity was present in Remak bundles in mixed nerves and in sympathetic ganglia and grey rami. Following peripheral nerve injury, RIP immunoreactivity was redistributed diffusely throughout de-differentiating Schwann cell cytoplasm. In uninjured rats, low levels of RIP immunoreactivity were detectable in satellite cells surrounding dorsal root ganglion (DRG) neurons and in terminal Schwann cells at neuromuscular junctions. This pattern suggested a correlation between RIP immunoreactivity and the amount of axon-glial contact. We therefore injured the L5 spinal nerve to induce sympathetic sprouting and pericellular basket formation in the DRG, and asked whether relatively RIP-negative satellite glia, which normally contact only neuronal somata, would upregulate the RIP antigen upon contact with sprouting sympathetic axons. All perineuronal sympathetic sprouts infiltrated heavily RIP-immunoreactive satellite cell sheaths. RIP immunoreactivity was absent from placode-derived olfactory ensheathing glia, indicating that the relationship between axon-glial contact and RIP-immunoreactivity is restricted to peripheral ensheathing glia of the neural crest-derived Schwann cell lineage.

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.

Immunohistochemical localization of vascular endothelial growth factor and its receptor Flk-1 in the amphibian developing principal and accessory olfactory system

In the last years several studies have shown that vascular endothelial growth factor (VEGF) is present in neural stem cells and mature neurons from different neural tissues where it may play an important role as a neuroproliferative and/or antiapoptotic factor. The olfactory neuroepithelium has the capability to replace dying neurons with new neurons formed by cell division from stem cells in the basal region of the epithelium. The present study demonstrates, for the first time, that VEGF is present in the olfactory epithelium, nerves and bulbs (both main and accessory) during the development of the toad Bufo arenarum. In this report, we detected VEGF immunoreactivity in mature olfactory neurons from early larval stages until the beginning of the metamorphic climax. VEGF expression decreases dramatically after metamorphosis. VEGF receptor Flk-1 was localized by immunohistochemistry, from premetamorphic larval stages until the climax in the neurons of the olfactory epithelium with a more intense labeling in the basal cell layer. Double-label immunofluorescence studies localized VEGF to the cytoplasm and the nucleus of mature neurons whereas Flk-1 was expressed in cell membranes. Flk-1 was present in neurons of both the main and accessory olfactory bulbs. After the end of metamorphosis, Flk-1 expression was limited to basal cells in the olfactory epithelium and Bowman's glands. The main and accessory olfactory bulbs showed the same pattern of Flk-1 immunostaining before and after the end of metamorphosis. The presence of VEGF and its receptor in the olfactory system suggests that VEGF may play an important role during neural development.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

Recovery of the olfactory receptor neurons in the African Tilapia mariae following exposure to low copper level

Low levels of Cu(2+) are known to specifically cause olfactory neuron death in fish olfactory epithelium. This study investigated the morphological changes in the olfactory mucosa of the cichlid Tilapia mariae, after a 4-day exposure to different concentrations of Cu(2+) (20, 40 and 100 microg/l), and the regeneration time-frame, when fish exposed to 20 microg/l were returned to dechlorinated tap water. Light microscopy, combined with Fluoro Jade-B staining, permitted the observation of a dose-dependent damage which became less severe and more circumscribed to receptor cells when Cu(2+) concentration decreased. The regeneration process in the olfactory tissue was examined in fish after 0, 3, and 10 days of recovery in well water. Immunostaining with PCNA showed a massive mitotic activity in the basal region of the mucosa immediately after exposure was terminated. The mitotically produced elements were immature neurons since they expressed the neural growth-associated phosphoprotein GAP-43. After 3 days of recovery the nuclei had already completed their migration to the upper portion of the epithelium and mitotic activity was much less intensive. After 10 days the olfactory tissue did not present differences when compared to the control tissue. These results suggest that after 10 days the regeneration is completed and the integrity of the tissue restored.