Conditional ablation of mature olfactory sensory neurons mediated by diphtheria toxin receptor

Non-cell-autonomous disruption of nuclear architecture as a potential cause of COVID-19-induced anosmia

SARS-CoV-2 infects less than 1% of cells in the human body, yet it can cause severe damage in a variety of organs. Thus, deciphering the non-cell-autonomous effects of SARS-CoV-2 infection is imperative for understanding the cellular and molecular disruption it elicits. Neurological and cognitive defects are among the least understood symptoms of COVID-19 patients, with olfactory dysfunction being their most common sensory deficit. Here, we show that both in humans and hamsters, SARS-CoV-2 infection causes widespread downregulation of olfactory receptors (ORs) and of their signaling components. This non-cell-autonomous effect is preceded by a dramatic reorganization of the neuronal nuclear architecture, which results in dissipation of genomic compartments harboring OR genes. Our data provide a potential mechanism by which SARS-CoV-2 infection alters the cellular morphology and the transcriptome of cells it cannot infect, offering insight to its systemic effects in olfaction and beyond.

Chitinase-Like Protein Ym2 (Chil4) Regulates Regeneration of the Olfactory Epithelium via Interaction with Inflammation

The adult olfactory epithelium (OE) regenerates sensory neurons and nonsensory supporting cells from resident stem cells after injury. How supporting cells contribute to OE regeneration remains largely unknown. In this study, we elucidated a novel role of Ym2 (also known as Chil4 or Chi3l4), a chitinase-like protein expressed in supporting cells, in regulating regeneration of the injured OE in vivo in both male and female mice and cell proliferation/differentiation in OE colonies in vitro We found that Ym2 expression was enhanced in supporting cells after OE injury. Genetic knockdown of Ym2 in supporting cells attenuated recovery of the injured OE, while Ym2 overexpression by lentiviral infection accelerated OE regeneration. Similarly, Ym2 bidirectionally regulated cell proliferation and differentiation in OE colonies. Furthermore, anti-inflammatory treatment reduced Ym2 expression and delayed OE regeneration in vivo and cell proliferation/differentiation in vitro, which were counteracted by Ym2 overexpression. Collectively, this study revealed a novel role of Ym2 in OE regeneration and cell proliferation/differentiation of OE colonies via interaction with inflammatory responses, providing new clues to the function of supporting cells in these processes.SIGNIFICANCE STATEMENT The mammalian olfactory epithelium (OE) is a unique neural tissue that regenerates sensory neurons and nonsensory supporting cells throughout life and postinjury. How supporting cells contribute to this process is not entirely understood. Here we report that OE injury causes upregulation of a chitinase-like protein, Ym2, in supporting cells, which facilitates OE regeneration. Moreover, anti-inflammatory treatment reduces Ym2 expression and delays OE regeneration, which are counteracted by Ym2 overexpression. This study reveals an important role of supporting cells in OE regeneration and provides a critical link between Ym2 and inflammation in this process.

Disruption of nuclear architecture as a cause of COVID-19 induced anosmia

Olfaction relies on a coordinated partnership between odorant flow and neuronal communication. Disruption in our ability to detect odors, or anosmia, has emerged as a hallmark symptom of infection with SARS-CoV-2, yet the mechanism behind this abrupt sensory deficit remains elusive. Here, using molecular evaluation of human olfactory epithelium (OE) from subjects succumbing to COVID-19 and a hamster model of SARS-CoV-2 infection, we discovered widespread downregulation of olfactory receptors (ORs) as well as key components of their signaling pathway. OR downregulation likely represents a non-cell autonomous effect, since SARS-CoV-2 detection in OSNs is extremely rare both in human and hamster OEs. A likely explanation for the reduction of OR transcription is the striking reorganization of nuclear architecture observed in the OSN lineage, which disrupts multi-chromosomal compartments regulating OR expression in humans and hamsters. Our experiments uncover a novel molecular mechanism by which a virus with a very selective tropism can elicit persistent transcriptional changes in cells that evade it, contributing to the severity of COVID-19.

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.

Olfactory inputs modulate respiration-related rhythmic activity in the prefrontal cortex and freezing behavior

Respiration and airflow through the nasal cavity are known to be correlated with rhythmic neural activity in the central nervous system. Here we show in rodents that during conditioned fear-induced freezing behavior, mice breathe at a steady rate (~4 Hz), which is correlated with a predominant 4-Hz oscillation in the prelimbic prefrontal cortex (plPFC), a structure critical for expression of conditioned fear behaviors. We demonstrate anatomical and functional connections between the olfactory pathway and plPFC via circuit tracing and optogenetics. Disruption of olfactory inputs significantly reduces the 4-Hz oscillation in the plPFC, but leads to prolonged freezing periods. Our results indicate that olfactory inputs can modulate rhythmic activity in plPFC and freezing behavior.

Nasal airflow and olfactory bulb activity are linked to oscillations in cortical areas. This study shows olfactory input and respiration are correlated with oscillation in the prefrontal cortex during freezing behavior in mice, and attenuation of olfactory inputs can increase behavioral freezing.

The Sense of Smell Impacts Metabolic Health and Obesity

Olfactory inputs help coordinate food appreciation and selection, but their role in systemic physiology and energy balance is poorly understood. Here we demonstrate that mice upon conditional ablation of mature olfactory sensory neurons (OSNs) are resistant to diet-induced obesity accompanied by increased thermogenesis in brown and inguinal fat depots. Acute loss of smell perception after obesity onset not only abrogated further weight gain but also improved fat mass and insulin resistance. Reduced olfactory input stimulates sympathetic nerve activity, resulting in activation of β-adrenergic receptors on white and brown adipocytes to promote lipolysis. Conversely, conditional ablation of the IGF1 receptor in OSNs enhances olfactory performance in mice and leads to increased adiposity and insulin resistance. These findings unravel a new bidirectional function for the olfactory system in controlling energy homeostasis in response to sensory and hormonal signals.

Olfactory marker protein expression is an indicator of olfactory receptor-associated events in non-olfactory tissues

Olfactory receptor (OR)-associated events are mediated by well-conserved components in the olfactory epithelium, including olfactory G-protein (G_olf), adenylate cyclase III (ACIII), and olfactory marker protein (OMP). The expression of ORs has recently been observed in non-olfactory tissues where they are involved in monitoring extracellular chemical cues. The large number of OR genes and their sequence similarities illustrate the need to find an effective and simple way to detect non-olfactory OR-associated events. In addition, expression profiles and physiological functions of ORs in non-olfactory tissues are largely unknown. To overcome limitations associated with using OR as a target protein, this study used OMP with G_olf and ACIII as targets to screen for potential OR-mediated sensing systems in non-olfactory tissues. Here, we show using western blotting, real-time PCR, and single as well as double immunoassays that ORs and OR-associated proteins are co-expressed in diverse tissues. The results of immunohistochemical analyses showed OMP (+) cells in mouse heart and in the following cells using the corresponding marker proteins c-kit, keratin 14, calcitonin, and GFAP in mouse tissues: interstitial cells of Cajal of the bladder, medullary thymic epithelial cells of the thymus, parafollicular cells of the thyroid, and Leydig cells of the testis. The expression of ORs in OMP (+) tissues was analyzed using a refined microarray analysis and validated with RT-PCR and real-time PCR. Three ORs (olfr544, olfr558, and olfr1386) were expressed in the OMP (+) cells of the bladder and thyroid as shown using a co-immunostaining method. Together, these results suggest that OMP is involved in the OR-mediated signal transduction cascade with olfactory canonical signaling components between the nervous and endocrine systems. The results further demonstrate that OMP immunohistochemical analysis is a useful tool for identifying expression of ORs, suggesting OMP expression is an indicator of potential OR-mediated chemoreception in non-olfactory systems.

Transplanted human adipose tissue-derived stem cells engraft and induce regeneration in mice olfactory neuroepithelium in response to dichlobenil subministration

We used immunodeficient mice, whose dorsomedial olfactory region was permanently damaged by dichlobenil inoculation, to test the neuroregenerative properties of transplanted human adipose tissue-derived stem cells after 30 and 60 days. Analysis of polymerase chain reaction bands revealed that stem cells preferentially engrafted in the lesioned olfactory epithelium compared with undamaged mucosa of untreated transplanted mice. Although basal cell proliferation in untransplanted lesioned mice did not give rise to neuronal cells in the olfactory mucosa, we observed clusters of differentiating olfactory cells in transplanted mice. After 30 days, and even more at 60 days, epithelial thickness was partially recovered to normal values, as also the immunohistochemical properties. Functional reactivity to odorant stimulation was also confirmed through electro-olfactogram recording in the dorsomedial epithelium. Furthermore, we demonstrated that engrafted stem cells fused with mouse cells in the olfactory organ, even if heterokaryons detected were too rare to hypothesize they directly repopulated the lesioned epithelium. The data reported prove that the migrating transplanted stem cells were able to induce a neuroregenerative process in a specific lesioned sensory area, enforcing the perspective that they could become an available tool for stem cell therapy.

A lifetime of neurogenesis in the olfactory system

Neurogenesis continues well beyond embryonic and early postnatal ages in three areas of the nervous system. The subgranular zone supplies new neurons to the dentate gyrus of the hippocampus. The subventricular zone supplies new interneurons to the olfactory bulb, and the olfactory neuroepithelia generate new excitatory sensory neurons that send their axons to the olfactory bulb. The latter two areas are of particular interest as they contribute new neurons to both ends of a first-level circuit governing olfactory perception. The vomeronasal organ and the main olfactory epithelium comprise the primary peripheral olfactory epithelia. These anatomically distinct areas share common features, as each exhibits extensive neurogenesis well beyond the juvenile phase of development. Here we will discuss the effect of age on the structural and functional significance of neurogenesis in the vomeronasal and olfactory epithelia, from juvenile to advanced adult ages, in several common model systems. We will next discuss how age affects the regenerative capacity of these neural stem cells in response to injury. Finally, we will consider the integration of newborn neurons into an existing circuit as it is modified by the age of the animal.

Genetic cell ablation

Targeted cell ablation has proven to be a valuable approach to study in vivo cell functions during organogenesis, tissue homeostasis, and regeneration. Over the last two decades, various approaches have been developed to refine the control of cell ablation. In this review, we give an overview of the distinct genetic tools available for targeted cell ablation, with a particular emphasis on their respective specificity.

A sensory-labeled line for cold: TRPM8-expressing sensory neurons define the cellular basis for cold, cold pain, and cooling-mediated analgesia

Many primary sensory neurons are polymodal, responding to multiple stimulus modalities (chemical, thermal, or mechanical), yet each modality is recognized differently. Although polymodality implies that stimulus encoding occurs in higher centers, such as the spinal cord or brain, recent sensory neuron ablation studies find that behavioral responses to different modalities require distinct subpopulations, suggesting the existence of modality-specific labeled lines at the level of the sensory afferent. Here we provide evidence that neurons expressing TRPM8, a cold- and menthol-gated channel required for normal cold responses in mammals, represents a labeled line solely for cold sensation. We examined the behavioral significance of conditionally ablating TRPM8-expressing neurons in adult mice, finding that, like animals lacking TRPM8 channels (Trpm8(-/-)), animals depleted of TRPM8 neurons ("ablated") are insensitive to cool to painfully cold temperatures. Ablated animals showed little aversion to noxious cold and did not distinguish between cold and a preferred warm temperature, a phenotype more profound than that of Trpm8(-/-) mice which exhibit only partial cold-avoidance and -preference behaviors. In addition to acute responses, cold pain associated with inflammation and nerve injury was significantly attenuated in ablated and Trpm8(-/-) mice. Moreover, cooling-induced analgesia after nerve injury was abolished in both genotypes. Last, heat, mechanical, and proprioceptive behaviors were normal in ablated mice, demonstrating that TRPM8 neurons are dispensable for other somatosensory modalities. Together, these data show that, although some limited cold sensitivity remains in Trpm8(-/-) mice, TRPM8 neurons are required for the breadth of behavioral responses evoked by cold temperatures.

Glucose metabolism is altered after loss of L cells and α-cells but not influenced by loss of K cells

The enteroendocrine K and L cells are responsible for secretion of glucose-dependent insulinotropic polypeptide (GIP) and glucagon like-peptide 1 (GLP-1), whereas pancreatic α-cells are responsible for secretion of glucagon. In rodents and humans, dysregulation of the secretion of GIP, GLP-1, and glucagon is associated with impaired regulation of metabolism. This study evaluates the consequences of acute removal of Gip- or Gcg-expressing cells on glucose metabolism. Generation of the two diphtheria toxin receptor cellular knockout mice, TgN(GIP.DTR) and TgN(GCG.DTR), allowed us to study effects of acute ablation of K and L cells and α-cells. Diphtheria toxin administration reduced the expression of Gip and content of GIP in the proximal jejunum in TgN(GIP.DTR) and expression of Gcg and content of proglucagon-derived peptides in both proximal jejunum and terminal ileum as well as content of glucagon in pancreas in TgN(GCG.DTR) compared with wild-type mice. GIP response to oral glucose was attenuated following K cell loss, but oral and intraperitoneal glucose tolerances were unaffected. Intraperitoneal glucose tolerance was impaired following combined L cell and α-cell loss and normal following α-cell loss. Oral glucose tolerance was improved following L cell and α-cell loss and supernormal following α-cell loss. We present two mouse models that allow studies of the effects of K cell or L cell and α-cell loss as well as isolated α-cell loss. Our findings show that intraperitoneal glucose tolerance is dependent on an intact L cell mass and underscore the diabetogenic effects of α-cell signaling. Furthermore, the results suggest that K cells are less involved in acute regulation of mouse glucose metabolism than L cells and α-cells.

Lentivirus-mediated genetic manipulation and visualization of olfactory sensory neurons in vivo

Development of a precise olfactory circuit relies on accurate projection of olfactory sensory neuron (OSN) axons to their synaptic targets in the olfactory bulb (OB). The molecular mechanisms of OSN axon growth and targeting are not well understood. Manipulating gene expression and subsequent visualizing of single OSN axons and their terminal arbor morphology have thus far been challenging. To study gene function at the single cell level within a specified time frame, we developed a lentiviral based technique to manipulate gene expression in OSNs in vivo. Lentiviral particles are delivered to OSNs by microinjection into the olfactory epithelium (OE). Expression cassettes are then permanently integrated into the genome of transduced OSNs. Green fluorescent protein expression identifies infected OSNs and outlines their entire morphology, including the axon terminal arbor. Due to the short turnaround time between microinjection and reporter detection, gene function studies can be focused within a very narrow period of development. With this method, we have detected GFP expression within as few as three days and as long as three months following injection. We have achieved both over-expression and shRNA mediated knock-down by lentiviral microinjection. This method provides detailed morphologies of OSN cell bodies and axons at the single cell level in vivo, and thus allows characterization of candidate gene function during olfactory development.

An intrinsic vasopressin system in the olfactory bulb is involved in social recognition

Many peptides, when released as chemical messengers within the brain, have powerful influences on complex behaviours. Most strikingly, vasopressin and oxytocin, once thought of as circulating hormones whose actions were confined to peripheral organs, are now known to be released in the brain where they play fundamentally important roles in social behaviours1. In humans, disruptions of these peptide systems have been linked to several neurobehavioural disorders, including Prader-Willi syndrome, affective disorders, and obsessive-compulsive disorder, and polymorphisms of the vasopressin V1a receptor have been linked to autism2,3. Here we report that the rat olfactory bulb contains a large population of interneurones which express vasopressin, that blocking the actions of vasopressin in the olfactory bulb impairs the social recognition abilities of rats, and that vasopressin agonists and antagonists can modulate the processing of information by olfactory bulb neurones. The findings indicate that social information is processed in part by a vasopressin system intrinsic to the olfactory system.

Insulin with chondroitinase ABC treats the rat model of acute spinal cord injury

Traumatic brain injury is often associated with acute spinal cord injury (ASCI). Insulin and chondroitinase ABC (ChABC) are both therapeutically effective, but the combined therapeutic effect of insulin and ChABC is still not clear. A combination of insulin and ChABC were used to treat a rat model of ASCI. This combination therapy prevented neuronal cell death by improving motor function, increasing cell growth and inhibiting cell apoptosis in ASCI rats. Expression of growth-associated protein 43, a marker of axonal re-growth, increased after combined treatment with insulin and ChABC. These results may provide a basis for a future method of treating ASCI.

Site-specific population dynamics and variable olfactory marker protein expression in the postnatal canine olfactory epithelium

The main olfactory epithelium is a pseudostratified columnar epithelium that displays neurogenesis over the course of a lifetime. New olfactory neurons arise basally and are transferred to the middle third of the epithelium during maturation. It is generally believed that this pattern is present throughout the olfactory area. In the present study, we show that the postnatal canine olfactory epithelium is composed of two distinct types of epithelium, designated A and B, which not only differ in olfactory neuron morphology, marker expression and basal cell proliferation but also display a patchy distribution and preferential localization within the nasal cavity. Type A epithelium, abundant in the caudal part of the olfactory area, contains well-differentiated olfactory neurons positive for olfactory marker protein but low numbers of immature neurons and proliferating basal cells, as visualized by TrkB/Human Natural Killer-1 (HNK-1) glyco-epitope and Ki-67 immunostaining, respectively. In contrast, type B epithelium is mainly found in the rostral part and contains smaller and elongated neurons that display increased levels of TrkB/Human Natural Killer-1 (HNK-1) glyco-epitope immunoreactivity and a higher number of Ki-67-positive basal cells but lower and variable levels of olfactory marker protein. The vomeronasal organ displays a uniform distribution of molecular markers and proliferating basal cells. The observation that olfactory marker protein in type A and B epithelium is preferentially localized to the nucleus and cytoplasm, respectively, implies correlation between subcellular localization and olfactory neuron maturation and may indicate distinct functional roles of olfactory marker protein. Whether the site-specific population dynamics in the postnatal canine olfactory epithelium revealed in the present study are modulated by physiological parameters, such as airflow, has to be clarified in future studies.

Olfactory sensory axon growth and branching is influenced by sonic hedgehog

Olfactory sensory neuron (OSN) axons extend from the olfactory epithelium to the olfactory bulb without branching until they reach their target region, the glomerulus. In this report, we present evidence to support the involvement of sonic hedgehog in promoting rat olfactory sensory axons to branch and to enter into the glomerulus. Sonic hedgehog (Shh) protein is detected in the glomeruli of the olfactory bulb, whereas its transcript is expressed in the mitral and tufted cells, suggesting that Shh in the glomeruli is produced by mitral and tufted cells. In primary OSN cultures, Shh-N peptide promotes olfactory axon branching. When Shh function is neutralized in vivo by its antibody, growth of newly generated OSN axons into the glomeruli is vastly reduced.

Secreted TARSH regulates olfactory mitral cell dendritic complexity

Olfactory sensory neurons synapse with mitral cells to form stereotyped connections in the olfactory bulb (OB). Mitral cell apical dendrites receive input from olfactory sensory neurons expressing the same odorant receptor. During development, this restricted dendritic targeting of mitral cells is achieved through eliminating elaborated dendritic trees to a single apical dendrite. Through a genome-wide microarray screen, we identified TARSH (Target of NESH SH3) as a transiently expressed molecule in mitral cells during the dendritic refinement period. TARSH expression is restricted to pyramidal neurons along the main olfactory pathway, including the anterior olfactory nucleus and piriform cortex. The dynamic TARSH expression is not altered when odor-evoked activity is blocked by naris closure or in AC3 knockout mice. We also demonstrate that TARSH is a secreted protein. In dissociated OB cultures, secreted TARSH promotes the reduction of mitral cell dendritic complexity and restricts dendritic branching and outgrowth of interneurons. Dendritic morphological changes were also observed in mitral cells overexpressing TARSH themselves. We propose that TARSH is part of the genetic program that regulates mitral cell dendritic refinement.

Expressing exogenous functional odorant receptors in cultured olfactory sensory neurons

BACKGROUND

Olfactory discrimination depends on the large numbers of odorant receptor genes and differential ligand-receptor signaling among neurons expressing different receptors. In this study, we describe an in vitro system that enables the expression of exogenous odorant receptors in cultured olfactory sensory neurons. Olfactory sensory neurons in the culture express characteristic signaling molecules and, therefore, provide a system to study receptor function within its intrinsic cellular environment.

RESULTS

We demonstrate that cultured olfactory sensory neurons express endogenous odorant receptors. Lentiviral vector-mediated gene transfer enables successful ectopic expression of odorant receptors. We show that the ectopically expressed mouse I7 is functional in the cultured olfactory sensory neurons. When two different odorant receptors are ectopically expressed simultaneously, both receptor proteins co-localized in the same olfactory sensory neurons up to 10 days in vitro.

CONCLUSION

This culture technique provided an efficient method to culture olfactory sensory neurons whose morphology, molecular characteristics and maturation progression resembled those observed in vivo. Using this system, regulation of odorant receptor expression and its ligand specificity can be studied in its intrinsic cellular environment.

Olfactory sensory neuron-specific and sexually dimorphic expression of protocadherin 20

Olfactory sensory axons navigate from the nasal cavity to the olfactory bulb and sort from among 1,000 different odorant receptor-expressing types to converge upon the same two or three glomeruli. To achieve this task during development, it is likely that multiple classes of regulatory molecules, including cell adhesion molecules, are involved. Cell adhesion molecules have been shown to be important in controlling axon guidance, fasciculation, and synapse formation. To gain further understanding of the involvement of adhesion molecules in olfactory circuitry development, we examined the dynamic and cell type specific expression of a novel protocadherin, PCDH20, in the olfactory system. PCDH20 is specifically expressed in newly differentiated olfactory sensory neurons and their axons during development. PCDH20 expression is down-regulated in the adult olfactory system, except in a small olfactory sensory neuron population. These small, discrete numbers of PCDH20-positive glomeruli in the adult olfactory bulb are consistently clustered in the ventral-caudal region in both male and female mice. However, adult males have higher numbers of PCDH20-positive glomeruli with a broader distribution, whereas adult females have fewer PCDH20-positive glomeruli with a more restricted distribution. The gender difference in PCDH20 expression may reflect olfactory receptor expression differences for gender-specific social discrimination.