Chronic intranasal corticosteroid treatment induces degeneration of olfactory sensory neurons in normal and allergic rhinitis mice

BACKGROUND

Nasal eosinophilic inflammation is the therapeutic target for olfactory dysfunction in allergic rhinitis (AR). Intranasal corticosteroids are commonly considered to offer targetable benefit given their immunosuppressive property. However, experimental evidence suggests that continuous corticosteroid exposure may directly cause olfactory damage by disrupting the turnover of olfactory sensory neurons (OSNs). This potentially deleterious effect of corticosteroids calls into question their long-term topical use for treating olfactory loss related to AR. The aim of this study was to assess the impacts of chronic intranasal corticosteroid treatment on olfactory function and OSN population in mice under normal and pathological conditions.

METHODS

BALB/c mice were intranasally treated with fluticasone propionate (FP, 0.3 mg/kg) for up to 8 weeks. Additional mice were used to establish an ovalbumin-induced mouse model of AR, followed by nasal challenge with ovalbumin for 8 weeks in the presence or absence of intranasal FP treatment. The authors examined olfactory function, OSN existence, neuronal turnover, and nasal inflammation using behavioral test, histological analyses, Western blotting, and enzyme-linked immunosorbent assay.

RESULTS

Intranasal treatment with FP for 8 weeks (FP-wk8) reduced odor sensitivity in normal mice. This reduction was concomitant with loss of OSNs and the axons projecting to the olfactory bulb, primarily resulting from increased neuronal apoptosis. In FP-wk8 AR mice, intranasal FP treatment attenuated olfactory impairment and eosinophilic inflammation but failed to reconstitute OSN population and axonal projections.

CONCLUSION

These results suggest that chronic intranasal corticosteroid treatment contributes to OSN degeneration that may reduce the therapeutic effectiveness for AR-related olfactory loss.

Immunohistochemical Analysis of Olfactory Sensory Neuron Populations in the Developing Olfactory Organ of the Guppy, Poecilia reticulata (Cyprinodontiformes, Poecilidae)

Olfaction is fundamental for sensing environmental chemicals and has obvious adaptive advantages. In fish, the peripheral olfactory organ is composed of lamellae in which the olfactory mucosa contains three main categories of olfactory sensory neurons (OSNs) as follows: ciliated (cOSNs), microvillous (mOSNs), and crypt cells. We studied the appearance of these different OSNs during development of Poecilia reticulata, given its growing use as animal model system. We performed immunohistochemical detection of molecular markers specific for the different OSNs, carrying out image analyses for marked-cell counting and measuring optical density. The P. reticulata olfactory organ did not show change in size during the first weeks of life. The proliferative activity increased at the onset of secondary sexual characters, remaining high until sexual maturity. Then, it decreased in both sexes, but with a recovery in females, probably in relation to their almost double body growth, compared to males. The density of both cOSNs and mOSNs remained constant throughout development, probably due to conserved functions already active in the fry, independently of the sex. The density of calretinin-positive crypt cells decreased progressively until sexual maturity, whereas the increased density of calretinin-negative crypt cell fraction, prevailing in later developmental stages, indicated their probable involvement in reproductive activities.

Axonal mRNA binding of hnRNP A/B is crucial for axon targeting and maturation of olfactory sensory neurons

Spatiotemporal control of gene expression is important for neural development and function. Here, we show that heterogeneous nuclear ribonucleoprotein (hnRNP) A/B is highly expressed in developing olfactory sensory neurons (OSNs), and its knockout results in reduction in mature OSNs and aberrant targeting of OSN axons to the olfactory bulb. RNA immunoprecipitation analysis reveals that hnRNP A/B binds to a group of mRNAs that are highly related to axon projections and synapse assembly. Approximately 11% of the identified hnRNP A/B targets, including Pcdha and Ncam2, encode cell adhesion molecules. In Hnrnpab knockout mice, PCDHA and NCAM2 levels are significantly reduced at the axon terminals of OSNs. Furthermore, deletion of the hnRNP A/B-recognition motif in the 3' UTR of Pcdha leads to impaired PCDHA expression at the OSN axon terminals. Therefore, we propose that hnRNP A/B facilitates OSN maturation and axon projection by regulating the local expression of its target genes at axon terminals.

Progressive Spread of Beta-amyloid Pathology in an Olfactory-driven Amyloid Precursor Protein Mouse Model

Years before Alzheimer's disease (AD) is diagnosed, patients experience an impaired sense of smell, and β-amyloid plaques accumulate within the olfactory mucosa and olfactory bulb (OB). The olfactory vector hypothesis proposes that external agents cause β-amyloid to aggregate and spread from the OB to connected downstream brain regions. To reproduce the slow accumulation of β-amyloid that occurs in human AD, we investigated the progressive accumulation of β-amyloid across the brain using a conditional mouse model that overexpresses a humanized mutant form of the amyloid precursor protein (hAPP) in olfactory sensory neurons. Using design-based stereology, we show the progressive accumulation of β-amyloid plaques within the OB and cortical olfactory regions with age. We also observe reduced OB volumes in these mice when hAPP expression begins prior-to but not post-weaning which we tracked using manganese-enhanced MRI. We therefore conclude that the reduced OB volume does not represent progressive degeneration but rather disrupted OB development. Overall, our data demonstrate that hAPP expression in the olfactory epithelium can lead to the accumulation and spread of β-amyloid through the olfactory system into the hippocampus, consistent with an olfactory system role in the early stages of β-amyloid-related AD progression.

Paradoxical electro-olfactogram responses in TMEM16B knock-out mice

The Ca2+-activated Cl¯ channel TMEM16B carries up to 90% of the transduction current evoked by odorant stimulation in olfactory sensory neurons and control the number of action potential firing and therefore the length of the train of action potentials. A loss of function approach revealed that TMEM16B is required for olfactory-driven behaviors such as tracking unfamiliar odors. Here, we used the electro-olfactogram (EOG) technique to investigate the contribution of TMEM16B to odorant transduction in the whole olfactory epithelium. Surprisingly, we found that EOG responses from Tmem16b knock out mice have a bigger amplitude compared to those of wild type. Moreover, the kinetics of EOG responses is faster in absence of TMEM16B, while the ability to adapt to repeated stimulation is altered in knock out mice. The larger EOG responses in Tmem16b knock out may be the results of the removal of the clamping and/or shunting action of the Ca2+-activated Cl¯ currents leading to the paradox of having smaller transduction current but larger generator potential.

Effects of zinc deficiency on the regeneration of olfactory epithelium in mice

The olfactory epithelium can regenerate after damage; however, the regeneration process is affected by various factors, such as viral infections, head trauma, and medications. Zinc is an essential trace element that has important roles in organ development, growth, and maturation. Zinc also helps regulate neurotransmission in the brain; nevertheless, its relationship with olfactory epithelium regeneration remains unclear. Therefore, we used a severe zinc deficiency mouse model to investigate the effects of zinc deficiency on olfactory epithelium regeneration. Male wild-type C57BL/6 mice were divided into zinc-deficient and control diet groups at the age of 4 weeks, and methimazole was administered at the age of 8 weeks to induce severe olfactory epithelium damage. We evaluated the olfactory epithelium before and 7, 14, and 28 days after methimazole administration by histologically analyzing paraffin sections. RNA sequencing was also performed at the age of 8 weeks before methimazole administration to examine changes in gene expression caused by zinc deficiency. In the zinc-deficient group, the regenerated olfactory epithelium thickness was decreased at all time points, and the numbers of Ki-67-positive, GAP43-positive, and olfactory marker protein-positive cells (i.e. proliferating cells, immature olfactory neurons, and mature olfactory neurons, respectively) failed to increase at some time points. Additionally, RNA sequencing revealed several changes in gene expression, such as a decrease in the expression of extracellular matrix-related genes and an increase in that of inflammatory response-related genes, in the zinc-deficient group. Therefore, zinc deficiency delays olfactory epithelium regeneration after damage in mice.

A Molecular Analysis of Neural Olfactory Placode Differentiation in Human Pluripotent Stem Cells

During embryonic development, the olfactory sensory neurons (OSNs) and the gonadotropic-releasing hormone neurons (GNRHNs) migrate from the early nasal cavity, known as the olfactory placode, to the brain. Defects in the development of OSNs and GNRHNs result in neurodevelopmental disorders such as anosmia and congenital hypogonadotropic hypogonadism, respectively. Treatments do not restore the defective neurons in these disorders, and as a result, patients have a diminished sense of smell or a gonadotropin hormone deficiency. Human pluripotent stem cells (hPSCs) can produce any cell type in the body; therefore, they are an invaluable tool for cell replacement therapies. Transplantation of olfactory placode progenitors, derived from hPSCs, is a promising therapeutic to replace OSNs and GNRHNs and restore tissue function. Protocols to generate olfactory placode progenitors are limited, and thus, we describe, in this study, a novel in vitro model for olfactory placode differentiation in hPSCs, which is capable of producing both OSNs and GNRHNs. Our study investigates the major developmental signaling factors that recapitulate the embryonic development of the olfactory tissue. We demonstrate that induction of olfactory placode in hPSCs requires bone morphogenetic protein inhibition, wingless/integrated protein inhibition, retinoic acid inhibition, transforming growth factor alpha activation, and fibroblast growth factor 8 activation. We further show that the protocol transitions hPSCs through the anterior pan-placode ectoderm and neural ectoderm regions in early development while preventing neural crest and non-neural ectoderm regions. Finally, we demonstrate production of OSNs and GNRHNs by day 30 of differentiation. Our study is the first to report on OSN differentiation in hPSCs.

Lifespan of mature olfactory sensory neurons varies with location in the mouse olfactory epithelium and age of the animal

The olfactory sensory neurons (OSNs) of the olfactory epithelium (OE) exhibit a remarkable regenerative capability, which protects the population against environmental insult and enables adjustment to new odors. The lifespan of OSNs is still open to question, with estimates ranging from 1 month to at least 1 year. However, the estimates come with some caveats, including low labeling efficiency and a focus solely on newborn neurons. We revisited the issue via the use of OMP-tTA; TetO-Cre; Rosa26-fl(stop)-Tdtomato (OMP-tTA;TdT) mice, which allowed us to selectively label ∼95% of the OMP(+) OSN population that reach maturity by a given time and, by switching to doxycycline chow, to "chase" this preexisting OSN population. Two loading protocols were used: conception to 2 months old and conception to 4.5 months old. Surviving OSNs were common up to 6 months chase time in both groups, but more neurons survived when loading for 4.5 months as compared with 2 months. A spatial difference was evident: higher percentages of OSNs survived in the dorsomedial OE as compared with ventrolateral and in posterior versus anterior OE regions. Finally, proliferation rates anticorrelated with the spatial differences in OSN survival; higher proliferation rates were observed ventrally. Together, these results demonstrate spatial and temporal differences in OSN survival, highlighting it as a dynamic system that can be studied for factors affecting neuronal survival.

Developing and maintaining a nose-to-brain map of odorant identity

Olfactory sensory neurons (OSNs) in the olfactory epithelium of the nose transduce chemical odorant stimuli into electrical signals. These signals are then sent to the OSNs' target structure in the brain, the main olfactory bulb (OB), which performs the initial stages of sensory processing in olfaction. The projection of OSNs to the OB is highly organized in a chemospatial map, whereby axon terminals from OSNs expressing the same odorant receptor (OR) coalesce into individual spherical structures known as glomeruli. This nose-to-brain map of odorant identity is built from late embryonic development to early postnatal life, through a complex combination of genetically encoded, OR-dependent and activity-dependent mechanisms. It must then be actively maintained throughout adulthood as OSNs experience turnover due to external insult and ongoing neurogenesis. Our review describes and discusses these two distinct and crucial processes in olfaction, focusing on the known mechanisms that first establish and then maintain chemospatial order in the mammalian OSN-to-OB projection.

Olfactory regulation by dopamine and DRD2 receptor in the nose

Despite the identification of neural circuits and circulating hormones in olfactory regulation, the peripheral targets for olfactory modulation remain relatively unexplored. Here we show that dopamine D2 receptor (DRD2) is expressed in the cilia and somata of mature olfactory sensory neurons (OSNs), while nasal dopamine (DA) is mainly released from the sympathetic nerve terminals, which innervate the mouse olfactory mucosa (OM). We further demonstrate that DA-DRD2 signaling in the nose plays important roles in regulating olfactory function using genetic and pharmacological approaches. Moreover, the local DA synthesis in mouse OM is reduced during hunger, which contributes to starvation-induced olfactory enhancement. Altogether, we demonstrate that nasal DA and DRD2 receptor can serve as the potential peripheral targets for olfactory modulation.

Olfactory behavior is important for animal survival, and olfactory dysfunction is a common feature of several diseases. Despite the identification of neural circuits and circulating hormones in olfactory regulation, the peripheral targets for olfactory modulation remain relatively unexplored. In analyzing the single-cell RNA sequencing data from mouse and human olfactory mucosa (OM), we found that the mature olfactory sensory neurons (OSNs) express high levels of dopamine D2 receptor (Drd2) rather than other dopamine receptor subtypes. The DRD2 receptor is expressed in the cilia and somata of mature OSNs, while nasal dopamine is mainly released from the sympathetic nerve terminals, which innervate the mouse OM. Intriguingly, genetic ablation of Drd2 in mature OSNs or intranasal application with DRD2 antagonist significantly increased the OSN response to odorants and enhanced the olfactory sensitivity in mice. Mechanistic studies indicated that dopamine, acting through DRD2 receptor, could inhibit odor-induced cAMP signaling of olfactory receptors. Interestingly, the local dopamine synthesis in mouse OM is down-regulated during starvation, which leads to hunger-induced olfactory enhancement. Moreover, pharmacological inhibition of local dopamine synthesis in mouse OM is sufficient to enhance olfactory abilities. Altogether, these results reveal nasal dopamine and DRD2 receptor as the potential peripheral targets for olfactory modulation.

Response of Olfactory Sensory Neurons to Mercury Ions in Zebrafish: An Immunohistochemical Study

Olfactory sensory neurons (OSNs) of fish belong to three main types: ciliated olfactory sensory neurons (cOSNs), microvillous olfactory sensory neurons (mOSNs), and crypt cells. Mercury is a toxic metal harmful for olfaction. We exposed the olfactory epithelium of zebrafish to three sublethal Hg2+ concentrations. Molecular markers specific for the different types of OSNs were immunohistochemically detected. Image analysis of treated sections enabled counting of marked cells and measurement of staining optical density indicative of the response of OSNs to Hg2+ exposure. The three types of OSNs reacted to mercury in a different way. Image analysis revealed that mOSNs are more susceptible to Hg2+ exposure than cOSNs and crypt cell density decreases. Moreover, while the ratio between sensory/nonsensory epithelium areas is unchanged, epithelium thickness drops, and dividing cells increase in the basal layer of the olfactory epithelium. Cell death but also reduction of apical processes and marker expression could account for changes in OSN immunostaining. Also, the differential results between dorsal and ventral halves of the olfactory rosette could derive from different water flows inside the olfactory chamber or different subpopulations in OSNs.

Heterogeneous distribution of mature olfactory sensory neurons in human olfactory epithelium

BACKGROUND

The olfactory cleft (OC) comprising the olfactory epithelium (OE) is the most important anatomical location for olfactory function. Endoscopic sinus surgery (ESS) is used to treat diseases related to the OC and improve olfactory dysfunction. However, iatrogenic OE injury occasionally occurs. Comprehensive knowledge of the olfactory region is required to avoid damage to the OE during endoscopic procedures.

METHODS

Immunohistochemistry was performed on olfactory mucosa obtained from the unaffected side of olfactory neuroblastoma surgical specimens. The OE was defined as the epithelium containing mature olfactory sensory neurons (OSNs). The distribution and cell kinetics of the OE were examined.

RESULTS

The OE was selectively localized to the anterior two-thirds of the superior turbinate (ST) and in the nasal septum (NS) just opposite to the ST; the OE was not detected within the mucosa of the superior meatus. The density of mature OSNs was high at the ethmoid tegmen but gradually decreased with distance from the ethmoid tegmen. The extent of cell death and proliferation was relatively even across the OE. Analysis of airflow profiles revealed that resection of inferior ST does not decrease airflow to the OC.

CONCLUSION

The results indicate that the distribution and degree of differentiation of mature OSNs are heterogenous throughout the OE. Epithelial resection of the anterior or superior ST has the potential to damage olfactory function. Resection of the inferior or posterior ST or widening of the superior meatus is a safer alternative that does not damage mature OSNs or alter airflow to the OC.

Olfactory Stimulation Regulates the Birth of Neurons That Express Specific Odorant Receptors

In mammals, olfactory sensory neurons (OSNs) are born throughout life, ostensibly solely to replace damaged OSNs. During differentiation, each OSN precursor “chooses,” out of hundreds of possibilities, a single odorant receptor (OR) gene, which defines the identity of the mature OSN. The relative neurogenesis rates of the hundreds of distinct OSN “subtypes” are thought to be constant, as they are determined by a stochastic process in which each OR is chosen with a fixed probability. Here, using histological, single-cell, and targeted affinity purification approaches, we show that closing one nostril in mice selectively reduces the number of newly generated OSNs of specific subtypes. Moreover, these reductions depend on an animal’s age and/or environment. Stimulation-dependent changes in the number of new OSNs are not attributable to altered rates of cell survival but rather production. Our findings indicate that the relative birth rates of distinct OSN subtypes depend on olfactory experience.

In mammals, the odorant receptor identities of newly generated olfactory sensory neurons are thought to be determined by each progenitor cell’s random choice of a single receptor. Here, van der Linden et al. show that, in mice, the birth rates of neurons expressing a subset of receptors depend on olfactory stimulation.

Sensitivity of olfactory sensory neurons to food cues is tuned to nutritional states by Neuropeptide Y signaling

Modulation of sensory perception by homeostatic feedback from physiological states is central to innate purposive behaviors. Olfaction is an important predictive modality for feeding-related behaviors and its modulation has been associated with hunger-satiety states. However, the mechanisms mapping internal states to chemosensory processing in order to modify behavior are poorly understood. In the zebrafish olfactory epithelium, a subset of olfactory sensory neurons (OSNs) and the terminal nerve projections express neuropeptide Y (NPY). Using a combination of neuronal activity and behavioral evaluation, we find that NPY signaling in the peripheral olfactory system of zebrafish is correlated with its nutritional state and is both necessary and sufficient for the olfactory perception of food-related odorants. NPY activity dynamically modulates the microvillar OSN activation thresholds and acts cooperatively with amino acid signaling resulting in a switch-like increase in OSN sensitivity in starved animals. We suggest that cooperative activation of phospholipase C by convergent signaling from NPY and amino acid receptors is central to this heightened sensitivity. This study provides ethologically relevant, physiological evidence for NPY signaling in the modulation of OSN sensitivity to food-associated amino acid cues. We demonstrate sensory gating directly at the level of OSNs and identify a novel mechanistic framework for tuning olfactory sensitivity to prevailing energy states. Cover Image for this issue: https://doi.org/10.1111/jnc.15091.

Sensitivity of olfactory sensory neurons to food cues is tuned to nutritional states by Neuropeptide Y signaling

Modulation of sensory perception by homeostatic feedback from physiological states is central to innate purposive behaviors. Olfaction is an important predictive modality for feeding-related behaviors and its modulation has been associated with hunger-satiety states. However, the mechanisms mapping internal states to chemosensory processing in order to modify behavior are poorly understood. In the zebrafish olfactory epithelium, a subset of olfactory sensory neurons (OSNs) and the terminal nerve projections express neuropeptide Y (NPY). Using a combination of neuronal activity and behavioral evaluation, we find that NPY signaling in the peripheral olfactory system of zebrafish is correlated with its nutritional state and is both necessary and sufficient for the olfactory perception of food-related odorants. NPY activity dynamically modulates the microvillar OSN activation thresholds and acts cooperatively with amino acid signaling resulting in a switch-like increase in OSN sensitivity in starved animals. We suggest that cooperative activation of phospholipase C by convergent signaling from NPY and amino acid receptors is central to this heightened sensitivity. This study provides ethologically relevant, physiological evidence for NPY signaling in the modulation of OSN sensitivity to food-associated amino acid cues. We demonstrate sensory gating directly at the level of OSNs and identify a novel mechanistic framework for tuning olfactory sensitivity to prevailing energy states.

Insulin-Dependent Maturation of Newly Generated Olfactory Sensory Neurons after Injury

Loss of olfactory sensory neurons (OSNs) after injury to the olfactory epithelium (OE) triggers the generation of OSNs that are incorporated into olfactory circuits to restore olfactory sensory perception. This study addresses how insulin receptor-mediated signaling affects the functional recovery of OSNs after OE injury. Insulin levels were reduced in mice by ablating the pancreatic β cells via streptozotocin (STZ) injections. These STZ-induced diabetic and control mice were then intraperitoneally injected with the olfactotoxic drug methimazole to selectively ablate OSNs. The OE of diabetic and control mice regenerated similarly until day 14 after injury. Thereafter, the OE of diabetic mice contained fewer mature and more apoptotic OSNs than control mice. Functionally, diabetic mice showed reduced electro-olfactogram (EOG) responses and their olfactory bulbs (OBs) had fewer c-Fos-active cells following odor stimulation, as well as performed worse in an odor-guided task compared with control mice. Insulin administered intranasally during days 8-13 after injury was sufficient to rescue recovery of OSNs in diabetic mice compared with control levels, while insulin administration between days 1 and 6 did not. During this critical time window on days 8-13 after injury, insulin receptors are highly expressed and intranasal application of an insulin receptor antagonist inhibits regeneration. Furthermore, an insulin-enriched environment could facilitate regeneration even in non-diabetic mice. These results indicate that insulin facilitates the regeneration of OSNs after injury and suggest a critical stage during recovery (8-13 d after injury) during which the maturation of newly generated OSNs is highly dependent on and promoted by insulin.

Expansion of murine and human olfactory epithelium/mucosa colonies and generation of mature olfactory sensory neurons under chemically defined conditions

Olfactory dysfunctions, including hyposmia and anosmia, affect ~100 million people around the world and the underlying causes are not fully understood. Degeneration of olfactory sensory neurons and incapacity of globose basal cells to generate olfactory sensory neurons are found in elder people and patients with smell disorders. Thus, olfactory stem cell may function as a promising tool to replace inactivated globose basal cells and to generate sensory neurons. Methods: We established clonal expansion of cells from the murine olfactory epithelium as well as colony growth from human olfactory mucosa using Matrigel-based three-dimensional system. These colonies were characterized by immunostaining against olfactory epithelium cellular markers and by calcium imaging of responses to odors. Chemical addition was optimized to promote Lgr5 expression, colony growth and sensory neuron generation, tested by quantitative PCR and immunostaining against progenitor and neuronal markers. The differential transcriptomes in multiple signaling pathways between colonies under different base media and chemical cocktails were determined by RNA-Seq. Results: In defined culture media, we found that VPA and CHIR99021 induced the highest Lgr5 expression level, while LY411575 resulted in the most abundant yield of OMP⁺ mature sensory neurons in murine colonies. Different base culture media with drug cocktails led to apparent morphological alteration from filled to cystic appearance, accompanied with massive transcriptional changes in multiple signaling pathways. Generation of sensory neurons in human colonies was affected through TGF-β signaling, while Lgr5 expression and cell proliferation was regulated by VPA. Conclusion: Our findings suggest that targeting expansion of olfactory epithelium/mucosa colonies in vitro potentially results in discovery of new source to cell replacement-based therapy against smell loss.

Anosmia: a missing link in the neuroimmunology of coronavirus disease 2019 (COVID-19)

Just before 2020 began, a novel coronavirus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), brought for humans a potentially fatal disease known as coronavirus disease 2019 (COVID-19). The world has thoroughly been affected by COVID-19, while there has been little progress towards understanding the pathogenesis of COVID-19. Patients with a severe phenotype of disease and those who died from the disease have shown hyperinflammation and were more likely to develop neurological manifestations, linking the clinical disease with neuroimmunological features. Anosmia frequently occurs early in the course of COVID-19. The prevalence of anosmia would be influenced by self-diagnosis as well as self-misdiagnosis in patients with COVID-19. Despite this, the association between anosmia and COVID-19 has been a hope for research, aiming to understand the pathogenesis of COVID-19. Studies have suggested differently probable mechanisms for the development of anosmia in COVID-19, including olfactory cleft syndrome, postviral anosmia syndrome, cytokine storm, direct damage of olfactory sensory neurons, and impairment of the olfactory perception center in the brain. Thus, the observation of anosmia would direct us to find the pathogenesis of COVID-19 in the central nervous system, and this is consistent with numerous neurological manifestations related to COVID-19. Like other neurotropic viruses, SARS-CoV-2 might be able to enter the central nervous system via the olfactory epithelium and induce innate immune responses at the site of entry. Viral replication in the nonneural olfactory cells indirectly causes damage to the olfactory receptor nerves, and as a consequence, anosmia occurs. Further studies are required to investigate the neuroimmunology of COVID-19 in relation to anosmia.

Heterogeneity of Antiviral Responses in the Upper Respiratory Tract Mediates Differential Non-lytic Clearance of Influenza Viruses

Influenza viruses initiate infection in the upper respiratory tract (URT), but early viral tropism and the importance of cell-type-specific antiviral responses in this tissue remain incompletely understood. By infecting transgenic lox-stop-lox reporter mice with a Cre-recombinase-expressing influenza B virus, we identify olfactory sensory neurons (OSNs) as a major viral cell target in the URT. These cells become infected, then eliminate the virus and survive in the host post-resolution of infection. OSN responses to infection are characterized by a strong induction of interferon-stimulated genes and more rapid clearance of viral protein relative to other cells in the epithelium. We speculate that this cell-type-specific response likely serves to protect the central nervous system from infection. More broadly, these results highlight the importance of evaluating antiviral responses across different cell types, even those within the same tissue, to more fully understand the mechanisms of viral disease.

Aversive Learning Increases Release Probability of Olfactory Sensory Neurons

Predicting danger from previously associated sensory stimuli is essential for survival. Contributions from altered peripheral sensory inputs are implicated in this process, but the underlying mechanisms remain elusive. Here, we use the mammalian olfactory system to investigate such mechanisms. Primary olfactory sensory neurons (OSNs) project their axons directly to the olfactory bulb (OB) glomeruli, where their synaptic release is subject to local and cortical influence and neuromodulation. Pairing optogenetic activation of a single glomerulus with foot shock in mice induces freezing to light stimulation alone during fear retrieval. This is accompanied by an increase in OSN release probability and a reduction in GABA_B receptor expression in the conditioned glomerulus. Furthermore, freezing time is positively correlated with the release probability of OSNs in fear-conditioned mice. These results suggest that aversive learning increases peripheral olfactory inputs at the first synapse, which may contribute to the behavioral outcome.

The Nose-To-Brain Transport of Polymeric Nanoparticles Is Mediated by Immune Sentinels and Not by Olfactory Sensory Neurons

The nose-to-brain (N-to-B) transport mechanism of nanoparticles through the olfactory epithelium (OE) is not fully understood. Most research utilized nasal epithelial cell models completely deprived of olfactory cells. Aiming to shed light into key cellular pathways, in this work, for the first time, the interaction of polymeric nanoparticles in a 17-483 nm size range and with neutral and negatively and positively charged surfaces with primary olfactory sensory neurons, cortical neurons, and microglia isolated from olfactory bulb (OB), OE, and cortex of newborn rats is investigated. After demonstrating the good cell compatibility of the different nanoparticles, the nanoparticle uptake by confocal laser scanning fluorescence microscopy is monitored. Our findings reveal that neither olfactory nor forebrain neurons internalize nanoparticles. Conversely, it is demonstrated that olfactory and cortical microglia phagocytose the nanoparticles independently of their features. Overall, our findings represent the first unambiguous evidence of the possible involvement of microglia in N-to-B nanoparticle transport and the unlikely involvement of neurons. Furthermore, this approach emerges as a completely new experimental tool to screen the biocompatibility, uptake, and transport of nanomaterials by key cellular players of the N-to-B pathway in nanosafety and nanotoxicology and nanomedicine.

Commensal Bacteria Regulate Gene Expression and Differentiation in Vertebrate Olfactory Systems Through Transcription Factor REST

Sensory systems such as the olfactory system detect chemical stimuli and thereby determine the relationships between the animal and its surroundings. Olfaction is one of the most conserved and ancient sensory systems in vertebrates. The vertebrate olfactory epithelium is colonized by complex microbial communities, but microbial contribution to host olfactory gene expression remains unknown. In this study, we show that colonization of germ-free zebrafish and mice with microbiota leads to widespread transcriptional responses in olfactory organs as measured in bulk tissue transcriptomics and RT-qPCR. Germ-free zebrafish olfactory epithelium showed defects in pseudostratification; however, the size of the olfactory pit and the length of the cilia were not different from that of colonized zebrafish. One of the mechanisms by which microbiota control host transcriptional programs is by differential expression and activity of specific transcription factors (TFs). REST (RE1 silencing transcription factor, also called NRSF) is a zinc finger TF that binds to the conserved motif repressor element 1 found in the promoter regions of many neuronal genes with functions in neuronal development and differentiation. Colonized zebrafish and mice showed increased nasal expression of REST, and genes with reduced expression in colonized animals were strongly enriched in REST-binding motifs. Nasal commensal bacteria promoted in vitro differentiation of Odora cells by regulating the kinetics of REST expression. REST knockdown resulted in decreased Odora cell differentiation in vitro. Our results identify a conserved mechanism by which microbiota regulate vertebrate olfactory transcriptional programs and reveal a new role for REST in sensory organs.

Loss of BMI1 in mature olfactory sensory neurons leads to increased olfactory basal cell proliferation

BACKGROUND

Damage to olfactory sensory neurons (OSNs), situated within the neuroepithelium of the olfactory cleft, may be associated with anosmia. Although their direct contact with the nasal airspace make OSNs vulnerable to injury and death, multiple mechanisms maintain epithelium integrity and olfactory function. We hypothesized that BMI1, a polycomb protein found to be enriched in OSNs, may function in neuroprotection. Here, we explored BMI1 function in a mouse model.

METHODS

Utilizing a mouse genetic approach to delete Bmi1 selectively in mature OSNs, we investigated changes in OE homeostasis by performing immunohistochemical, biochemical, and functional assays. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR), immunostaining, and electro-olfactograms were used to compare gene expression, cell composition, and olfactory function in OSN-specific BMI1 knockout mice (n = 3 to 5) and controls. Chromatin studies were also performed to identify protein-DNA interactions between BMI1 and its target genes (n = 3).

RESULTS

OSN-specific BMI1 knockout led to increased neuron death and basal cell activation. Chromatin studies suggested a mechanism of increased neurodegeneration due to de-repression of a pro-apoptosis gene, p19ARF. Despite the increased turnover, we found that olfactory neuroepithelium thickness and olfactory function remained intact. Our studies also revealed the presence of additional polycomb group proteins that may compensate for the loss of BMI1 in mature OSNs.

CONCLUSION

The olfactory neuroepithelium employs multiple mechanisms to maintain epithelial homeostasis. Our findings provide evidence that in a mouse model of BMI1 deletion, the overall integrity and function of the olfactory neuroepithelium are not compromised, despite increased neuronal turnover, reflecting a remarkable reparative capacity to sustain a critical sensory system.

Olfactory sensory neurons mediate ultrarapid antiviral immune responses in a TrkA-dependent manner

The nervous system regulates host immunity in complex ways. Vertebrate olfactory sensory neurons (OSNs) are located in direct contact with pathogens; however, OSNs' ability to detect danger and initiate immune responses is unclear. We report that nasal delivery of rhabdoviruses induces apoptosis in crypt OSNs via the interaction of the OSN TrkA receptor with the viral glycoprotein in teleost fish. This signal results in electrical activation of neurons and very rapid proinflammatory responses in the olfactory organ (OO), but dampened inflammation in the olfactory bulb (OB). CD8α⁺ cells infiltrate the OO within minutes of nasal viral delivery, and TrkA blocking, but not caspase-3 blocking, abrogates this response. Infiltrating CD8α⁺ cells were TCRαβ T cells with a nonconventional phenotype that originated from the microvasculature surrounding the OB and not the periphery. Nasal delivery of viral glycoprotein (G protein) recapitulated the immune responses observed with the whole virus, and antibody blocking of viral G protein abrogated these responses. Ablation of crypt neurons in zebrafish resulted in increased susceptibility to rhabdoviruses. These results indicate a function for OSNs as a first layer of pathogen detection in vertebrates and as orchestrators of nasal-CNS antiviral immune responses.

Olfactory sensory neurons mediate ultrarapid antiviral immune responses in a TrkA-dependent manner

The nervous system regulates host immunity in complex ways. Vertebrate olfactory sensory neurons (OSNs) are located in direct contact with pathogens; however, OSNs' ability to detect danger and initiate immune responses is unclear. We report that nasal delivery of rhabdoviruses induces apoptosis in crypt OSNs via the interaction of the OSN TrkA receptor with the viral glycoprotein in teleost fish. This signal results in electrical activation of neurons and very rapid proinflammatory responses in the olfactory organ (OO), but dampened inflammation in the olfactory bulb (OB). CD8α⁺ cells infiltrate the OO within minutes of nasal viral delivery, and TrkA blocking, but not caspase-3 blocking, abrogates this response. Infiltrating CD8α⁺ cells were TCRαβ T cells with a nonconventional phenotype that originated from the microvasculature surrounding the OB and not the periphery. Nasal delivery of viral glycoprotein (G protein) recapitulated the immune responses observed with the whole virus, and antibody blocking of viral G protein abrogated these responses. Ablation of crypt neurons in zebrafish resulted in increased susceptibility to rhabdoviruses. These results indicate a function for OSNs as a first layer of pathogen detection in vertebrates and as orchestrators of nasal-CNS antiviral immune responses.

Two types of sensory proliferation patterns underlie the formation of spatially tuned olfactory receptive fields in the cockroach Periplaneta americana

In the cockroach Periplaneta americana, to represent pheromone source in the receptive space, axon terminals of sex pheromone-receptive olfactory sensory neurons (pSNs) are topographically organized within the primary center, the macroglomerulus, according to the peripheral locations of sex pheromone-receptive single walled (sw)-B sensilla. In this study, we sought to determine when and where pSNs emerge in the nymphal antenna. We revealed two different pSN proliferation patterns that underlie the formation of topographic organization in the macroglomerulus. In nymphal antennae, which lack sw-B sensilla, pSNs are identified in the shorter sensilla, termed sw-A sensilla. Because new sw-A sensilla emerge on the proximal antenna at every molt, topographic organization in the macroglomerulus must be formed by adding axon terminals of newly emerged pSNs to the lateral region in the macroglomerulus at each molt. At the final molt, a huge number of new sw-B sensilla appeared throughout the whole antenna. Sw-B sensilla in the proximal part of the adult antenna were newly formed during the last instar stage, whereas those located in the distal antenna were transformed from sw-A sensilla. This transformation was accompanied by an increase in the number of pSNs. Axon terminals of newborn pSNs in new sw-B sensilla were recruited to the lateral part of the macroglomerulus, whereas those of newborn pSNs in transformed sw-B sensilla were recruited to the macroglomerulus according to the sensillar location. These mechanisms enable an increase in sensitivity to sex pheromone in adulthood while retaining the topographic map formed during the postembryonic development.

Chemical Exposure-Induced Changes in the Expression of Neurotrophins and Their Receptors in the Main Olfactory System of Mice Lacking TRPM5-Expressing Microvillous Cells

Functional maintenance of the mammalian main olfactory epithelium (MOE) is challenging because of its direct exposure to a wide spectrum of environmental chemicals. We previously reported that transient receptor potential channel M5-expressing microvillous cells (TRPM5-MCs) in the MOE play an important role in olfactory maintenance. To investigate the underpinning mechanisms, we exposed transcription factor Skn-1a knockout (Skn-1a-/-) mice lacking TRPM5-MCs, and TRPM5-GFP mice to either vehicle (water) or a mixture of odorous chemicals and chitin for two weeks and analyzed the expression of olfactory signaling proteins using immunolabeling and neurotrophin (NT) and NT receptor (NTR) gene transcripts using real-time quantitative PCR. The chemical exposure did not significantly attenuate the immunolabeling of olfactory signaling proteins. Vehicle-exposed Skn-1a-/- and TRPM5-GFP mice expressed similar levels of NT and NTR gene transcripts in the MOE and olfactory bulb. Chemical exposure significantly increased MOE expression of p75NTR in Skn-1a-/- mice, while p75NTR expression was reduced in TRPM5-GFP mice, as compared to vehicle-exposed mice. Additionally, our RNA in situ hybridization analysis and immunolabeling confirmed MOE expression of most NTs and NTRs. Together, these results indicate that TRPM5-MCs and chemical exposure influence expression of some NTs and NTRs in the MOE and olfactory bulb (OB).

A Near-Complete Spatial Map of Olfactory Receptors in the Mouse Main Olfactory Epithelium

Different regions of the mammalian nose smell different odors. In the mouse olfactory system, spatially regulated expression of >1000 olfactory receptors (ORs) along the dorsomedial-ventrolateral (DV) axis forms a topological map in the main olfactory epithelium (MOE). However, the locations of most ORs along the DV axis are currently unknown. By sequencing mRNA of 12 isolated MOE pieces, we mapped out the DV locations-as quantified by "zone indices" on a scale of 1-5-of 1033 OR genes with an estimated error of 0.3 zone indices. Our map covered 81% of all intact OR genes and 99.4% of the total OR mRNA abundance. Spatial regulation tended to vary gradually along chromosomes. We further identified putative non-OR genes that may exhibit spatial expression along the DV axis.

Combining In Vivo and In Vitro Approaches To Identify Human Odorant Receptors Responsive to Food Odorants

Olfactory perception plays an important role in food flavor. Humans have around 400 odorant receptors (ORs), which can be activated by an enormous number of odorants in a combinatorial fashion. To date, only a few odorant receptors have been linked to their respective odorants, due to the difficulties in expressing these receptor proteins in heterologous cell systems. In vivo approaches allow for the analysis of odorant-receptor interactions in their native environment and have the advantage that the complete OR repertoire is simultaneously tested. Once mouse odorant-receptor pairs are defined, one can search for the corresponding human orthologues, which can be validated against the odorants in heterologous cells. Thus, the combination of in vivo and in vitro methods should contribute to the identification of human ORs that recognize odorants of interest, such as key food odorants.

Elucidating the Neuronal Architecture of Olfactory Glomeruli in the Drosophila Antennal Lobe

Olfactory glomeruli are morphologically conserved spherical compartments of the olfactory system, distinguishable solely by their chemosensory repertoire, anatomical position, and volume. Little is known, however, about their numerical neuronal composition. We therefore characterized their neuronal architecture and correlated these anatomical features with their functional properties in Drosophila melanogaster. We quantitatively mapped all olfactory sensory neurons (OSNs) innervating each glomerulus, including sexually dimorphic distributions. Our data reveal the impact of OSN number on glomerular dimensions and demonstrate yet unknown sex-specific differences in several glomeruli. Moreover, we quantified uniglomerular projection neurons for each glomerulus, which unraveled a glomerulus-specific numerical innervation. Correlation between morphological features and functional specificity showed that glomeruli innervated by narrowly tuned OSNs seem to possess a larger number of projection neurons and are involved in less lateral processing than glomeruli targeted by broadly tuned OSNs. Our study demonstrates that the neuronal architecture of each glomerulus encoding crucial odors is unique.

Mechanosensory-Based Phase Coding of Odor Identity in the Olfactory Bulb

Mitral and tufted (M/T) cells in the olfactory bulb produce rich temporal patterns of activity in response to different odors. However, it remains unknown how these temporal patterns are generated and how they are utilized in olfaction. Here we show that temporal patterning effectively discriminates between the two sensory modalities detected by olfactory sensory neurons (OSNs): odor and airflow-driven mechanical signals. Sniff-induced mechanosensation generates glomerulus-specific oscillatory activity in M/T cells, whose phase was invariant across airflow speed. In contrast, odor stimulation caused phase shifts (phase coding). We also found that odor-evoked phase shifts are concentration invariant and stable across multiple sniff cycles, contrary to the labile nature of rate coding. The loss of oscillatory mechanosensation impaired the precision and stability of phase coding, demonstrating its role in olfaction. We propose that phase, not rate, coding is a robust encoding strategy of odor identity and is ensured by airflow-induced mechanosensation in OSNs.

Olfactory loss in chronic rhinosinusitis is associated with neuronal activation of c-Jun N-terminal kinase

BACKGROUND

Olfactory inflammation in chronic rhinosinusitis (CRS) is associated with cytokines that may result in the death of olfactory sensory neurons. The principal signaling molecules involved in the apoptotic pathway are c-Jun N-terminal kinases (JNK). Although the JNK pathway has emerged as a key player in programmed cell death in neuroinflammation, its specific role in CRS-associated olfactory loss has not been thoroughly investigated.

METHODS

JNK activation was studied in human tissue samples from 9 control and 11 CRS patients by immunohistochemical staining for phosphorylated c-Jun. A mouse model of inducible olfactory cytokine expression was used to experimentally control inflammation and assess JNK activation over time.

RESULTS

In patients with CRS, activation of c-Jun is significantly increased relative to non-CRS control subjects, and there is an associated loss of sensory neurons. In the olfactory inflammation mouse model, prolonged induction of inflammation results in elevation of c-Jun expression and neuronal apoptosis.

CONCLUSION

Activation of neuronal JNK is a feature of chronic olfactory inflammation that is associated with neuronal apoptosis. Given that inhibition of JNK activity is neuroprotective in other settings, antagonism of this pathway may have therapeutic potential in the management of inflammatory olfactory loss or other disorders linked to olfactory neuronal apoptosis.

Soft X-Ray Tomography Reveals Gradual Chromatin Compaction and Reorganization during Neurogenesis In Vivo

The realization that nuclear distribution of DNA, RNA, and proteins differs between cell types and developmental stages suggests that nuclear organization serves regulatory functions. Understanding the logic of nuclear architecture and how it contributes to differentiation and cell fate commitment remains challenging. Here, we use soft X-ray tomography (SXT) to image chromatin organization, distribution, and biophysical properties during neurogenesis in vivo. Our analyses reveal that chromatin with similar biophysical properties forms an elaborate connected network throughout the entire nucleus. Although this interconnectivity is present in every developmental stage, differentiation proceeds with concomitant increase in chromatin compaction and re-distribution of condensed chromatin toward the nuclear core. HP1β, but not nucleosome spacing or phasing, regulates chromatin rearrangements because it governs both the compaction of chromatin and its interactions with the nuclear envelope. Our experiments introduce SXT as a powerful imaging technology for nuclear architecture.

Lack of Pattern Separation in Sensory Inputs to the Olfactory Bulb during Perceptual Learning

Recent studies revealed changes in odor representations in the olfactory bulb during active olfactory learning (Chu et al., 2016; Yamada et al., 2017). Specifically, mitral cell ensemble responses to very similar odorant mixtures sparsened and became more distinguishable as mice learned to discriminate the odorants over days (Chu et al., 2016). In this study, we explored whether changes in the sensory inputs to the bulb underlie the observed changes in mitral cell responses. Using two-photon calcium imaging to monitor the odor responses of the olfactory sensory neuron (OSN) axon terminals in the glomeruli of the olfactory bulb during a discrimination task, we found that OSN inputs to the bulb are stable during discrimination learning. During one week of training to discriminate between very similar odorant mixtures in a Go/No-go task, OSN responses did not show significant sparsening, and the responses to the trained similar odorants did not diverge throughout training. These results suggest that the adaptive changes of mitral cell responses during perceptual learning are ensured by mechanisms downstream of OSN input, possibly in local circuits within olfactory bulb.

Deletion of Type 3 Adenylyl Cyclase Perturbs the Postnatal Maturation of Olfactory Sensory Neurons and Olfactory Cilium Ultrastructure in Mice

Type 3 adenylyl cyclase (Adcy3) is localized to the cilia of olfactory sensory neurons (OSNs) and is an essential component of the olfactory cyclic adenosine monophosphate (cAMP) signaling pathway. Although the role of this enzyme in odor detection and axonal projection in OSNs was previously characterized, researchers will still have to determine its function in the maturation of postnatal OSNs and olfactory cilium ultrastructure. Previous studies on newborns showed that the anatomic structure of the main olfactory epithelium (MOE) of Adcy3 knockout mice (Adcy3^-/-) is indistinguishable from that of their wild-type littermates (Adcy3^+/+), whereas the architecture and associated composition of MOE are relatively underdeveloped at this early age. The full effects of sensory deprivation on OSNs may not also be exhibited in such age. In the present study, following a comparison of postnatal OSNs in seven-, 30-, and 90-day-old Adcy3^-/- mice and wild-type controls (Adcy3^+/+), we observed that the absence of Adcy3 leads to cumulative defects in the maturation of OSNs. Upon aging, Adcy3^-/- OSNs exhibited increase in immature cells and reduction in mature cells along with elevated apoptosis levels. The density and ultrastructure of Adcy3^-/- cilia were also disrupted in mice upon aging. Collectively, our results reveal an indispensable role of Adcy3 in postnatal maturation of OSNs and maintenance of olfactory cilium ultrastructure in mice through adulthood.

The Inheritance of the Pheromone Sensory System in Two Helicoverpa Species: Dominance of H. armigera and Possible Introgression from H. assulta

Hybridization of sympatric closely related species may sometimes lead to introgression and speciation. The sister species Helicoverpa armigera and Helicoverpa assulta both use (Z)-11-hexadecenal and (Z)-9-hexadecenal as sex pheromone components but in reversed ratios. Female H. armigera and male H. assulta could hybridize and produce fertile male hybrids, which can then backcross with females of the two parent species to get backcross lines in the laboratory. In this study, we compared the olfactory responses to pheromone compounds in the periphery and in the antennal lobes (ALs) of males of the two species, as well as of their hybrids and backcrosses. Single-sensillum recordings were carried out to explore characteristics of male-specific sensilla on the antennae, and in vivo calcium imaging combined with digital 3D-reconstruction was used to describe what happens in the macroglomerular complex (MGC) of the AL. The results show that the population ratio of the two male-specific types of olfactory sensory neurons responding to two sex pheromone components are controlled by a major gene, and that the allele of H. armigera is dominant. Consistently, the study of the representative areas activated by sex pheromone components in the ALs further support the dominance of H. armigera. However, the topological structure of the MGC in the hybrid was similar but not identical to that in H. armigera. All subtypes of male-specific sensilla identified in the two species were found in the male hybrids and backcrosses. Moreover, two new subtypes with broader response spectra (the expanded A subtype and the expanded C subtype) emerged in the hybrids. Based on the inheritance pattern of the pheromone sensory system, we predict that when hybridization of female H. armigera and male H. assulta occurs in the field, male hybrids would readily backcross with female H. armigera, and introgression might occur from H. assulta into H. armigera through repeated backcrossing.

Distorted Coarse Axon Targeting and Reduced Dendrite Connectivity Underlie Dysosmia after Olfactory Axon Injury

The glomerular map in the olfactory bulb (OB) is the basis for odor recognition. Once established during development, the glomerular map is stably maintained throughout the life of an animal despite the continuous turnover of olfactory sensory neurons (OSNs). However, traumatic damage to OSN axons in the adult often leads to dysosmia, a qualitative and quantitative change in olfaction in humans. A mouse model of dysosmia has previously indicated that there is an altered glomerular map in the OB after the OSN axon injury; however, the underlying mechanisms that cause the map distortion remain unknown. In this study, we examined how the glomerular map is disturbed and how the odor information processing in the OB is affected in the dysosmia model mice. We found that the anterior–posterior coarse targeting of OSN axons is disrupted after OSN axon injury, while the local axon sorting mechanisms remained. We also found that the connectivity of mitral/tufted cell dendrites is reduced after injury, leading to attenuated odor responses in mitral/tufted cells. These results suggest that existing OSN axons are an essential scaffold for maintaining the integrity of the olfactory circuit, both OSN axons and mitral/tufted cell dendrites, in the adult.

Expression of homeobox genes in the mouse olfactory epithelium

Homeobox genes constitute a large family of genes widely studied because of their role in the establishment of the body pattern. However, they are also involved in many other events during development and adulthood. The main olfactory epithelium (MOE) is an excellent model to study neurogenesis in the adult nervous system. Analyses of homeobox genes during development show that some of these genes are involved in the formation and establishment of cell diversity in the MOE. Moreover, the mechanisms of expression of odorant receptors (ORs) constitute one of the biggest enigmas in the field. Analyses of OR promoters revealed the presence of homeodomain binding sites in their sequences. Here we characterize the expression patterns of a set of 49 homeobox genes in the MOE with in situ hybridization. We found that seven of them (Dlx3, Dlx5, Dlx6, Msx1, Meis1, Isl1, and Pitx1) are zonally expressed. The homeobox gene Emx1 is expressed in three guanylate cyclase(+) populations, two located in the MOE and the third one in an olfactory subsystem known as Grüneberg ganglion located at the entrance of the nasal cavity. The homeobox gene Tshz1 is expressed in a unique patchy pattern across the MOE. Our findings provide new insights to guide functional studies that aim to understand the complexity of transcription factor expression and gene regulation in the MOE. J. Comp. Neurol. 524:2713-2739, 2016. © 2016 Wiley Periodicals, Inc.

The Olfactory Bulb: An Immunosensory Effector Organ during Neurotropic Viral Infections

In 1935, the olfactory route was hypothesized to be a portal for virus entry into the central nervous system (CNS). This hypothesis was based on experiments in which nasophayngeal infection with poliovirus in monkeys was prevented from spreading to their CNS via transection of olfactory tracts between the olfactory neuroepithelium (ONE) of the nasal cavity and the olfactory bulb (OB). Since then, numerous neurotropic viruses have been observed to enter the CNS via retrograde transport along axons of olfactory sensory neurons whose cell bodies reside in the ONE. Importantly, this route of infection can occur even after subcutaneous inoculation of arboviruses that can cause encephalitis in humans. While the olfactory route is now accepted as an important pathway for viral entry into the CNS, it is unclear whether it provides a way for infection to spread to other brain regions. More recently, studies of antiviral innate and adaptive immune responses within the olfactory bulb suggest it provides early virologic control. Here we will review the data demonstrating that neurotropic viruses gain access to the CNS initially via the olfactory route with emphasis on findings that suggest the OB is a critical immunosensory effector organ that effectively clears virus.

The Stimulus-Dependent Gradient of Cyp26B1+ Olfactory Sensory Neurons Is Necessary for the Functional Integrity of the Olfactory Sensory Map

Stimulus-dependent expression of the retinoic acid-inactivating enzyme Cyp26B1 in olfactory sensory neurons (OSNs) forms a dorsomedial (DM)-ventrolateral (VL) gradient in the mouse olfactory epithelium. The gradient correlates spatially with different rates of OSN turnover, as well as the functional organization of the olfactory sensory map, into overlapping zones of OSNs that express different odorant receptors (ORs). Here, we analyze transgenic mice that, instead of a stimulus-dependent Cyp26B1 gradient, have constitutive Cyp26B1 levels in all OSNs. Starting postnatally, OSN differentiation is decreased and progenitor proliferation is increased. Initially, these effects are selective to the VL-most zone and correlate with reduced ATF5 expression and accumulation of OSNs that do not express ORs. Transcription factor ATF5 is known to stabilize OR gene choice via onset of the stimulus-transducing enzyme adenylyl cyclase type 3. During further postnatal development of Cyp26B1 mice, an anomalous DM(high)-VL(low) expression gradient of adenylyl cyclase type 3 appears, which coincides with altered OR frequencies and OR zones. All OR zones expand ventrolaterally except for the VL-most zone, which contracts. The expansion results in an increased zonal overlap that is also evident in the innervation pattern of OSN axon terminals in olfactory bulbs. These findings together identify a mechanism by which postnatal sensory-stimulated vitamin A metabolism modifies the generation of spatially specified neurons and their precise topographic connectivity. The distributed patterns of vitamin A-metabolizing enzymes in the nervous system suggest the possibility that the mechanism may also regulate neuroplasticity in circuits other than the olfactory sensory map.

SIGNIFICANCE STATEMENT

The mouse olfactory sensory map is functionally wired according to precise axonal projections of spatially organized classes of olfactory sensory neurons in the nose. The genetically controlled mechanisms that regulate the development of the olfactory sensory map are beginning to be elucidated. Little is known about mechanisms by which sensory stimuli shape the organization of the map after birth. We show that a stimulus-dependent gradient of a retinoic acid-inactivating enzyme Cyp26B1 modifies the composition, localization, and axonal projections of olfactory sensory neuron classes. The mechanism is novel and suggests the interesting possibility that local vitamin A metabolism could also be a mediator of stimulus-dependent modifications of precise spatial connectivity in other parts of the nervous system.

Genome-Wide Screen Reveals Rhythmic Regulation of Genes Involved in Odor Processing in the Olfactory Epithelium

Odor discrimination behavior displays circadian fluctuations in mice, indicating that mammalian olfactory function is under control of the circadian system. This is further supported by the facts that odor discrimination rhythms depend on the presence of clock genes and that olfactory tissues contain autonomous circadian clocks. However, the molecular link between circadian function and olfactory processing is still unknown. To elucidate the molecular mechanisms underlying this link, we focused on the olfactory epithelium (OE), the primary target of odors and the site of the initial events in olfactory processing. We asked whether olfactory sensory neurons (OSNs) within the OE possess an autonomous circadian clock and whether olfactory pathways are under circadian control. Employing clock gene-driven bioluminescence reporter assays and time-dependent immunohistochemistry on OE samples, we found robust circadian rhythms of core clock genes and their proteins in OSNs, suggesting that the OE indeed contains an autonomous circadian clock. Furthermore, we performed a circadian transcriptome analysis and identified several OSN-specific components that are under circadian control, including those with putative roles in circadian olfactory processing, such as KIRREL2-an established factor involved in short-term OSN activation. The spatiotemporal expression patterns of our candidate proteins suggest that they are involved in short-term anabolic processes to rhythmically prepare the cell for peak performances and to promote circadian function of OSNs.

Specific olfactory neurons and glomeruli are associated to differences in behavioral responses to pheromone components between two Helicoverpa species

Sex pheromone communication of moths helps to understand the mechanisms underlying reproductive isolation and speciation. Helicoverpa armigera and Helicoverpa assulta use (Z)-11-hexadecenal (Z11-16:Ald) and (Z)-9-hexadecenal (Z9-16:Ald) as pheromone components in reversed ratios, 97:3 and 5:95, respectively. H. armigera also produces trace amount of (Z)-9-tetradecenal (Z9-14:Ald) in the sex pheromone gland, but H. assulta does not. Wind tunnel studies revealed that the addition of small amounts (0.3%) of Z9-14:Ald to the main pheromone blend of H. armigera increased the males' attraction, but at higher doses (1%, 10%) the same compound acted as an inhibitor. In H. assulta, Z9-14:Ald reduced male attraction when presented as 1% to the pheromone blend, but was ineffective at lower concentrations (0.3%). Three types (A–C) of sensilla trichodea in antennae were identified by single sensillum recording, responding to Z11-16:Ald, Z9-14:Ald, and both Z9-16:Ald and Z9-14:Ald, respectively. Calcium imaging in the antennal lobes (ALs) revealed that the input information of the three chemicals was transmitted to three units of the macroglomerular complex (MGC) in ALs in both species: a large glomerulus for the major pheromone components, a small one for the minor pheromone components, and a third one for the behavioral antagonists. The type A and C neurons tuned to Z11-16:Ald and Z9-16:Ald had a reversed target in the MGC between the two species. In H. armigera, low doses (1, 10 μg) of Z9-14:Ald dominantly activated the glomerulus which processes the minor pheromone component, while a higher dose (100 μg) also evoked an equal activity in the antagonistic glomerulus. In H. assulta, instead, Z9-14:Ald always strongly activated the antagonistic glomerulus. These results suggest that Z9-14:Ald plays different roles in the sexual communication of two Helicoverpa species through activation of functionally different olfactory pathways.

Food odors trigger an endocrine response that affects food ingestion and metabolism

Food odors stimulate appetite and innate food-seeking behavior in hungry animals. The smell of food also induces salivation and release of gastric acid and insulin. Conversely, sustained odor exposure may induce satiation. We demonstrate novel effects of food odors on food ingestion, metabolism and endocrine signaling in Drosophila melanogaster. Acute exposure to attractive vinegar odor triggers a rapid and transient increase in circulating glucose, and a rapid upregulation of genes encoding the glucagon-like hormone adipokinetic hormone (AKH), four insulin-like peptides (DILPs) and some target genes in peripheral tissues. Sustained exposure to food odors, however, decreases food intake. Hunger-induced strengthening of synaptic signaling from olfactory sensory neurons (OSNs) to brain neurons increases food-seeking behavior, and conversely fed flies display reduced food odor sensitivity and feeding. We show that increasing the strength of OSN signaling chronically by genetic manipulation of local peptide neuromodulation reduces feeding, elevates carbohydrates and diminishes lipids. Furthermore, constitutively strengthened odor sensitivity altered gene transcripts for AKH, DILPs and some of their targets. Thus, we show that food odor can induce a transient anticipatory endocrine response, and that boosted sensitivity to this odor affects food intake, as well as metabolism and hormonal signaling.

Projection neurons in Drosophila antennal lobes signal the acceleration of odor concentrations

Temporal experience of odor gradients is important in spatial orientation of animals. The fruit fly Drosophila melanogaster exhibits robust odor-guided behaviors in an odor gradient field. In order to investigate how early olfactory circuits process temporal variation of olfactory stimuli, we subjected flies to precisely defined odor concentration waveforms and examined spike patterns of olfactory sensory neurons (OSNs) and projection neurons (PNs). We found a significant temporal transformation between OSN and PN spike patterns, manifested by the PN output strongly signaling the OSN spike rate and its rate of change. A simple two-dimensional model admitting the OSN spike rate and its rate of change as inputs closely predicted the PN output. When cascaded with the rate-of-change encoding by OSNs, PNs primarily signal the acceleration and the rate of change of dynamic odor stimuli to higher brain centers, thereby enabling animals to reliably respond to the onsets of odor concentrations.

Developmental regulation of neural map formation in the mouse olfactory system

In the mouse olfactory system, various odorants are detected by approximately 1000 different odorant receptors (ORs) expressed in the olfactory sensory neurons (OSNs). It is well established that each OSN expresses only one functional OR gene in a monoallelic manner. Furthermore, OSN axons expressing the same OR converge to a set of glomeruli in the olfactory bulb (OB). During embryonic development, a coarse map is formed by the combination of two genetically programmed processes. One is OR-independent axonal projection along the dorsal-ventral (D-V) axis, and the other is OR-dependent projection along the anterior-posterior (A-P) axis. D-V projection is regulated by the anatomical location of OSNs within the olfactory epithelium (OE), whereas A-P projection is instructed by expressed OR molecules using cyclic adenosine monophosphate (cAMP) signals. After birth, the map is further refined in an activity-dependent manner by its conversion from a continuous to a discrete map through segregation of glomerular structures. Here, we summarize recent progress from our laboratory in understanding neural map formation in the mouse olfactory system.

Dissecting the signaling mechanisms underlying recognition and preference of food odors

Food is critical for survival. Many animals, including the nematode Caenorhabditis elegans, use sensorimotor systems to detect and locate preferred food sources. However, the signaling mechanisms underlying food-choice behaviors are poorly understood. Here, we characterize the molecular signaling that regulates recognition and preference between different food odors in C. elegans. We show that the major olfactory sensory neurons, AWB and AWC, play essential roles in this behavior. A canonical Gα-protein, together with guanylate cyclases and cGMP-gated channels, is needed for the recognition of food odors. The food-odor-evoked signal is transmitted via glutamatergic neurotransmission from AWC and through AMPA and kainate-like glutamate receptor subunits. In contrast, peptidergic signaling is required to generate preference between different food odors while being dispensable for the recognition of the odors. We show that this regulation is achieved by the neuropeptide NLP-9 produced in AWB, which acts with its putative receptor NPR-18, and by the neuropeptide NLP-1 produced in AWC. In addition, another set of sensory neurons inhibits food-odor preference. These mechanistic logics, together with a previously mapped neural circuit underlying food-odor preference, provide a functional network linking sensory response, transduction, and downstream receptors to process complex olfactory information and generate the appropriate behavioral decision essential for survival.

Identification of plant semiochemicals and characterization of new olfactory sensory neuron types in a polyphagous pest moth, Spodoptera littoralis

Phytophagous insects use blends of volatiles released from plants to select hosts for feeding and oviposition. To behaviorally analyze complex blends, we need efficient and selective methods for elucidating neuron types, their ligands, and specificity. Gas chromatography-combined single sensillum recordings (GC-SSRs) from antennal olfactory sensilla of female moth, Spodoptera littoralis revealed 38 physiologically active peaks in the headspace volatile blends from both larvae-damaged cotton plants and lilac flowers. Using GC-combined mass spectrometry, 9 new physiologically active compounds were identified from damaged cotton and 11 from lilac compared with earlier electrophysiological studies using antennae of female S. littoralis. We characterized 14 novel classes of olfactory sensory neurons (OSNs). Among these, we found the first 2 ligands for a frequent type of short trichoid sensillum, for which no ligands were identified earlier. By using GC-SSR, a substantial increase in functional classes of OSNs and active compounds, 40% and 34% more, respectively, compared with recent studies using GC-electroantennogram or SSR using single compounds was detected. Compared with the estimated number of corresponding antennal olfactory receptors, the OSN classes now correspond to 83% of a likely maximum. The many specialist OSNs observed may facilitate behavioral confirmation of key plant volatiles in blends.

NaV1.5 sodium channel window currents contribute to spontaneous firing in olfactory sensory neurons

Olfactory sensory neurons (OSNs) fire spontaneously as well as in response to odor; both forms of firing are physiologically important. We studied voltage-gated Na(+) channels in OSNs to assess their role in spontaneous activity. Whole cell patch-clamp recordings from OSNs demonstrated both tetrodotoxin-sensitive and tetrodotoxin-resistant components of Na(+) current. RT-PCR showed mRNAs for five of the nine different Na(+) channel α-subunits in olfactory tissue; only one was tetrodotoxin resistant, the so-called cardiac subtype NaV1.5. Immunohistochemical analysis indicated that NaV1.5 is present in the apical knob of OSN dendrites but not in the axon. The NaV1.5 channels in OSNs exhibited two important features: 1) a half-inactivation potential near -100 mV, well below the resting potential, and 2) a window current centered near the resting potential. The negative half-inactivation potential renders most NaV1.5 channels in OSNs inactivated at the resting potential, while the window current indicates that the minor fraction of noninactivated NaV1.5 channels have a small probability of opening spontaneously at the resting potential. When the tetrodotoxin-sensitive Na(+) channels were blocked by nanomolar tetrodotoxin at the resting potential, spontaneous firing was suppressed as expected. Furthermore, selectively blocking NaV1.5 channels with Zn(2+) in the absence of tetrodotoxin also suppressed spontaneous firing, indicating that NaV1.5 channels are required for spontaneous activity despite resting inactivation. We propose that window currents produced by noninactivated NaV1.5 channels are one source of the generator potentials that trigger spontaneous firing, while the upstroke and propagation of action potentials in OSNs are borne by the tetrodotoxin-sensitive Na(+) channel subtypes.