Post-COVID-19 Hyposmia Does Not Exhibit Main Neurodegeneration Markers in the Olfactory Pathway

The biological substrate of persistent post-COVID-19 hyposmia is still unclear. However, as many neurodegenerative diseases present with smell impairment at onset, it may theoretically reflect degeneration within the central olfactory circuits. However, no data still exist regarding the post-COVID-19 patients. As the olfactory neurons (ONs) mirror pathological changes in the brain, allowing for tracking the underlying molecular events, here, we performed a broad analysis of ONs from patients with persistent post-COVID-19 OD to identify traces of potential neurodegeneration. ONs were collected through the non-invasive brushing of the olfactory mucosa from ten patients with persistent post-COVID-19 hyposmia (lasting > 6 months after infection) and ten age/sex-matched controls. Immunofluorescence staining for protein quantification and RT-PCR for gene expression levels were combined to measure ONs markers of α-synuclein, amyloid-β, and tau pathology, axonal injury, and mitochondrial network. Patients and controls had similar ONs levels of oligomeric α-synuclein, amyloid-β peptide, tau protein, neurofilament light chain (NfL), cytochrome C oxidase subunit 3 (COX3), and the heat shock protein 60 (HSP60). Our findings thus did not provide evidence for synucleinopathy and amyloid-β mismetabolism or gross traces of neuronal injury and mitochondrial dysfunction within the olfactory system in the early phase of persistent post-COVID-19 hyposmia.

Substance P and Prokineticin-2 are overexpressed in olfactory neurons and play differential roles in persons with persistent post-COVID-19 olfactory dysfunction

Persistent olfactory dysfunction (OD) is one of the most complaining and worrying complications of long COVID-19 because of the potential long-term neurological consequences. While causes of OD in the acute phases of the SARS-CoV-2 infection have been figured out, reasons for persistent OD are still unclear. Here we investigated the activity of two inflammatory pathways tightly linked with olfaction pathophysiology, namely Substance P (SP) and Prokineticin-2 (PK2), directly within the olfactory neurons (ONs) of patients to understand mechanisms of persistent post-COVID-19 OD. ONs were collected by non-invasive brushing from ten patients with persistent post-COVID-19 OD and ten healthy controls. Gene expression levels of SP, Neurokinin receptor 1, Interleukin-1β (IL-1β), PK2, PK2 receptors type 1 and 2, and Prokineticin-2-long peptide were measured in ONs by Real Time-PCR in both the groups, and correlated with residual olfaction. Immunofluorescence staining was also performed to quantify SP and PK2 proteins. OD patients, compared to controls, exhibited increased levels of both SP and PK2 in ONs, the latter proportional to residual olfaction. This work provided unprecedented, preliminary evidence that both SP and PK2 pathways may have a role in persistent post-COVID-19 OD. Namely, if the sustained activation of SP, lasting months after infection's resolution, might foster chronic inflammation and contribute to hyposmia, the PK2 expression could instead support the smell recovery.

Insect repellents mediate species-specific olfactory behaviours in mosquitoes

Background

The species-specific mode of action for DEET and many other mosquito repellents is often unclear. Confusion may arise for many reasons. First, the response of a single mosquito species is often used to represent all mosquito species. Second, behavioural studies usually test the effect of repellents on mosquito attraction towards human odorants, rather than their direct repulsive effect on mosquitoes. Third, the mosquito sensory neuron responses towards repellents are often not directly examined.

Methods

A close proximity response assay was used to test the direct repulsive effect of six mosquito repellents on Anopheles coluzzii, Aedes aegypti and Culex quinquefasciatus mosquitoes. Additionally, the behavioural assay and calcium imaging recordings of antennae were used to test the response of An. coluzzii mosquitoes towards two human odorants (1-octen-3-ol and benzaldehyde) at different concentrations, and mixtures of the repellents lemongrass oil and p-menthane-3,8-diol (PMD) with DEET.

Results

Anopheles coluzzii mosquitoes were repelled by lemongrass oil and PMD, while Ae. aegypti and Cx. quinquefasciatus mosquitoes were repelled by lemongrass oil, PMD, eugenol, and DEET. In addition, high concentrations of 1-octen-3-ol and benzaldehyde were repellent, and activated more olfactory receptor neurons on the An. coluzzii antennae than lower concentrations. Finally, changes in olfactory responses to repellent mixtures reflected changes in repulsive behaviours.

Conclusions

The findings described here suggest that different species of mosquitoes have different behavioural responses to repellents. The data further suggest that high-odour concentrations may recruit repellent-sensing neurons, or generally excite many olfactory neurons, yielding repellent behavioural responses. Finally, DEET can decrease the neuronal and behavioural response of An. coluzzii mosquitoes towards PMD but not towards lemongrass oil. Overall, these studies can help inform mosquito repellent choice by species, guide decisions on effective repellent blends, and could ultimately identify the olfactory neurons and receptors in mosquitoes that mediate repellency.

High-Throughput Odorant Receptor Deorphanization Via Phospho-S6 Ribosomal Protein Immunoprecipitation and mRNA Profiling

We describe an approach for the high-throughput surveying of odorant receptors (ORs) expressed in olfactory sensory neurons (OSNs) that have been activated by specific odorants. When OSNs are activated, there is a molecular signature in the form of a phosphorylated-S6 (pS6) ribosomal subunit. By the immunoprecipitation of the protein-RNA complex containing pS6, we identify the OR mRNA species expressed in these activated OSNs. The one neuron - one receptor rule (mature OSN expresses a single unique OR) allows for the identification of the collection of ORs that responded toward the tested odorant. Here we detail the procedure of (1) odor stimulation, (2) tissue harvesting, (3) immunoprecipitation, and (4) mRNA profiling for the high-throughput deorphanization of ORs in vivo.

The terminal nerve plays a prominent role in GnRH-1 neuronal migration independent from proper olfactory and vomeronasal connections to the olfactory bulbs

Gonadotropin-releasing hormone-1 (GnRH-1) neurons (GnRH-1 ns) migrate from the developing olfactory pit into the hypothalamus during embryonic development. Migration of the GnRH-1 neurons is required for mammalian reproduction as these cells control release of gonadotropins from the anterior pituitary gland. Disturbances in GnRH-1 ns migration, GnRH-1 synthesis, secretion or signaling lead to varying degrees of hypogonadotropic hypogonadism (HH), which impairs pubertal onset and fertility. HH associated with congenital olfactory defects is clinically defined as Kallmann Syndrome (KS). The association of olfactory defects with HH in KS suggested a potential direct relationship between defective olfactory axonal routing, lack of olfactory bulbs (OBs) and aberrant GnRH-1 ns migration. However, it has never been experimentally proven that the formation of axonal connections of the olfactory/vomeronasal neurons to their functional targets are necessary for the migration of GnRH-1 ns to the hypothalamus. Loss-of-function of the Arx-1 homeobox gene leads to the lack of proper formation of the OBs with abnormal axonal termination of olfactory sensory neurons ( Yoshihara et al., 2005). Our data prove that correct development of the OBs and axonal connection of the olfactory/vomeronasal sensory neurons to the forebrain are not required for GnRH-1 ns migration, and suggest that the terminal nerve, which forms the GnRH-1 migratory scaffold, follows different guidance cues and differs in gene expression from olfactory/vomeronasal sensory neurons.

Summary: Our work reveals that correct olfactory bulb development is not required for GnRH-1 neuronal migration. This study challenges the idea that GnRH-1 neuronal migration to the hypothalamus relies on correct routing of the olfactory and vomeronasal neurons and supports the existence of the TN in mammals.

Vascular endothelial growth factor influences migration and focal adhesions, but not proliferation or viability, of human neural stem/progenitor cells derived from olfactory epithelium

In humans, new neurons are continuously added in the olfactory epithelium even in the adulthood. The resident neural stem/progenitor cells (hNS/PCs-OE) in the olfactory epithelium are influenced by several growth factors and neurotrophins. Among these modulators the vascular endothelial growth factor (VEGF) has attracted attention due its implicated in cell proliferation, survival and migration of other type of neural/stem progenitor cells. Interestingly, VEGFr2 receptor expression in olfactory epithelium has been described in amphibians but not in humans. Here we show that VEGFr is expressed in the hNS/PCs-OE. We also investigated the effect of VEGF on the hNS/PCs-OE proliferation, viability and migration in vitro. Additionally, pharmacological approaches showed that VEGF (0.5 ng/ml)-stimulated migration of hNS/PCs-OE was blocked with the compound DMH4, which prevents the activation of VEGFr2. Similar effects were found with the inhibitors for Rac (EHT1864) and p38MAPK (SB203850) proteins, respectively. These observations occurred with changes in focal adhesion contacts. However, no effects of VEGF on proliferation or viability were found in hNS/PCs-OE. Our results suggest that hNS/PCs-OE respond to VEGF involving VEGFr2, Rac and p38MAPK.

Mechanistic studies of the toxicity of zinc gluconate in the olfactory neuronal cell line Odora

Zinc is both an essential and potentially toxic metal. It is widely believed that oral zinc supplementation can reduce the effects of the common cold; however, there is strong clinical evidence that intranasal (IN) zinc gluconate (ZG) gel treatment for this purpose causes anosmia, or the loss of the sense of smell, in humans. Using the rat olfactory neuron cell line, Odora, we investigated the molecular mechanism by which zinc exposure exerts its toxic effects on olfactory neurons. Following treatment of Odora cells with 100 and 200μM ZG for 0-24h, RNA-seq and in silico analyses revealed up-regulation of pathways associated with zinc metal response, oxidative stress, and ATP production. We observed that Odora cells recovered from zinc-induced oxidative stress, but ATP depletion persisted with longer exposure to ZG. ZG exposure increased levels of NLRP3 and IL-1β protein levels in a time-dependent manner, suggesting that zinc exposure may cause an inflammasome-mediated cell death, pyroptosis, in olfactory neurons.

The microtubular cytoskeleton of olfactory neurons derived from patients with schizophrenia or with bipolar disorder: Implications for biomarker characterization, neuronal physiology and pharmacological screening

Schizophrenia (SZ) and Bipolar Disorder (BD) are highly inheritable chronic mental disorders with a worldwide prevalence of around 1%. Despite that many efforts had been made to characterize biomarkers in order to allow for biological testing for their diagnoses, these disorders are currently detected and classified only by clinical appraisal based on the Diagnostic and Statistical Manual of Mental Disorders. Olfactory neuroepithelium-derived neuronal precursors have been recently proposed as a model for biomarker characterization. Because of their peripheral localization, they are amenable to collection and suitable for being cultured and propagated in vitro. Olfactory neuroepithelial cells can be obtained by a non-invasive brush-exfoliation technique from neuropsychiatric patients and healthy subjects. Neuronal precursors isolated from these samples undergo in vitro the cytoskeletal reorganization inherent to the neurodevelopment process which has been described as one important feature in the etiology of both diseases. In this paper, we will review the current knowledge on microtubular organization in olfactory neurons of patients with SZ and with BD that may constitute specific cytoskeletal endophenotypes and their relation with alterations in L-type voltage-activated Ca(2+) currents. Finally, the potential usefulness of neuronal precursors for pharmacological screening will be discussed.

Brain-derived neurotrophic factor (BDNF) expression in normal and regenerating olfactory epithelium of Xenopus laevis

Olfactory epithelium has the capability to continuously regenerate olfactory receptor neurons throughout life. Adult neurogenesis results from proliferation and differentiation of neural stem cells, and consequently, olfactory neuroepithelium offers an excellent opportunity to study neural regeneration and the factors involved in the maintenance and regeneration of all their cell types. We analyzed the expression of BDNF in the olfactory system under normal physiological conditions as well as during a massive regeneration induced by chemical destruction of the olfactory epithelium in Xenopus laevis larvae. We described the expression and presence of BDNF in the olfactory epithelium and bulb. In normal physiological conditions, sustentacular (glial) cells and a few scattered basal (stem) cells express BDNF in the olfactory epithelium as well as the granular cells in the olfactory bulb. Moreover, during massive regeneration, we demonstrated a drastic increase in basal cells expressing BDNF as well as an increase in BDNF in the olfactory bulb and nerve. Together these results suggest an important role of BDNF in the maintenance and regeneration of the olfactory system.