COVID-19 Anosmia: High Prevalence, Plural Neuropathogenic Mechanisms, and Scarce Neurotropism of SARS-CoV-2?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative pathogen of coronavirus disease 2019 (COVID-19). It is known as a respiratory virus, but SARS-CoV-2 appears equally, or even more, infectious for the olfactory epithelium (OE) than for the respiratory epithelium in the nasal cavity. In light of the small area of the OE relative to the respiratory epithelium, the high prevalence of olfactory dysfunctions (ODs) in COVID-19 has been bewildering and has attracted much attention. This review aims to first examine the cytological and molecular biological characteristics of the OE, especially the microvillous apical surfaces of sustentacular cells and the abundant SARS-CoV-2 receptor molecules thereof, that may underlie the high susceptibility of this neuroepithelium to SARS-CoV-2 infection and damages. The possibility of SARS-CoV-2 neurotropism, or the lack of it, is then analyzed with regard to the expression of the receptor (angiotensin-converting enzyme 2) or priming protease (transmembrane serine protease 2), and cellular targets of infection. Neuropathology of COVID-19 in the OE, olfactory bulb, and other related neural structures are also reviewed. Toward the end, we present our perspectives regarding possible mechanisms of SARS-CoV-2 neuropathogenesis and ODs, in the absence of substantial viral infection of neurons. Plausible causes for persistent ODs in some COVID-19 convalescents are also examined.

Lineage, Identity, and Fate of Distinct Progenitor Populations in the Embryonic Olfactory Epithelium

We defined a temporal dimension of precursor diversity and lineage in the developing mouse olfactory epithelium (OE) at mid-gestation that results in genesis of distinct cell classes. Slow, symmetrically dividing Meis1+/ Pax7+ progenitors in the early differentiating lateral OE give rise to small numbers of Ascl1+ precursors in the dorsolateral and ventromedial OE. Few of the initial progeny of the Ascl1+ precursors immediately generate olfactory receptor neurons (ORNs). Instead, most early progeny of this temporally defined precursor cohort, labeled via temporally discreet tamoxifen-dependent Ascl1Cre-driven recombination, populate a dorsomedial OE domain comprised of proliferative Ascl1+ as well as Ascl1- cells from which newly generated ORNs are mostly excluded. The most prominent early progeny of these Ascl1+ OE precursors are migratory mass cells associated with the nascent olfactory nerve (ON) in the frontonasal mesenchyme. These temporal, regional and lineage distinctions are matched by differences in proliferative capacity and modes of division in isolated, molecularly distinct lateral versus medial OE precursors. By late gestation, the progeny of the temporally and spatially defined Ascl1+ precursor cohort include few proliferating precursors. Instead, these cells generate a substantial subset of OE sustentacular cells, spatially restricted ORNs, and ensheathing cells associated with actively growing as well as mature ON axons. Accordingly, from the earliest stages of OE differentiation, distinct temporal and spatial precursor identities provide a template for acquisition of subsequent OE and ON cellular diversity.

SARS-CoV-2 infection in the mouse olfactory system

SARS-CoV-2 infection causes a wide spectrum of clinical manifestations in humans, and olfactory dysfunction is one of the most predictive and common symptoms in COVID-19 patients. However, the underlying mechanism by which SARS-CoV-2 infection leads to olfactory disorders remains elusive. Herein, we demonstrate that intranasal inoculation with SARS-CoV-2 induces robust viral replication in the olfactory epithelium (OE), not the olfactory bulb (OB), resulting in transient olfactory dysfunction in humanized ACE2 (hACE2) mice. The sustentacular cells and Bowman’s gland cells in the OE were identified as the major target cells of SARS-CoV-2 before invasion into olfactory sensory neurons (OSNs). Remarkably, SARS-CoV-2 infection triggers massive cell death and immune cell infiltration and directly impairs the uniformity of the OE structure. Combined transcriptomic and quantitative proteomic analyses revealed the induction of antiviral and inflammatory responses, as well as the downregulation of olfactory receptor (OR) genes in the OE from the infected animals. Overall, our mouse model recapitulates olfactory dysfunction in COVID-19 patients and provides critical clues for understanding the physiological basis for extrapulmonary manifestations of COVID-19.

Gross morphology, histology, and ultrastructure of the olfactory rosette of a critically endangered indicator species, the Delta Smelt, Hypomesus transpacificus

The Delta Smelt (Hypomesus transpacificus) is a small, semi-anadromous fish native to the San Francisco Bay-Delta Estuary and has been declared as critically endangered. Their olfactory biology, in particular, is poorly understood and a basic description of their sensory anatomy is needed to advance our understanding of the sensory ecology of species to inform conservation efforts to manage and protect them. We provide a description of the gross morphology, histological, immunohistochemical, and ultrastructural features of the olfactory rosette in this fish and discuss some of the functional implications in relation to olfactory ability. We show that Delta Smelt have a multilamellar olfactory rosette with allometric growth. Calretinin immunohistochemistry revealed a diffuse distribution of olfactory receptor neurons within the epithelium. Ciliated, microvillous and crypt neurons were clearly identified using morphological and immunohistochemical features. The olfactory neurons were supported by robust ciliated and secretory sustentacular cells. Although the sense of smell has been overlooked in Delta Smelt, we conclude that the olfactory epithelium has many characteristics of macrosmatic fish. With this study, we provide a foundation for future research into the sensory ecology of this imperiled fish.

Thyroid Paraganglioma- An Unusual Head and Neck Tumour

Paragangliomas are neuroendocrine tumour originating from the neural crest-derived paraganglia with majority arising from the head and neck. (1)Thyroid paraganglioma are exceedingly rare neuroendocrine tumours accounting for <0.1% of thyroid malignancy (1) We present a 57 years old gentleman who was referred to ENT surgeon following discovery of a two month history of lump on his left neckIt has not changed in size and not caused any symptoms such as anxiety, sweats, palpitations, dizziness or unexplained headaches. He has a Past medical history of epilepsy following a Road traffic accident 28 years ago leaving him seizure prone. He has no family history of neuroendocrine tumours His ultrasound scan of his thyroid gland showed a 25 x 23 x 15mm lesion lying anteriorly within the left thyroid lobe. There are two highly reflective foci which could represent microcalcification. It was classified as U5 lesion He proceeded with fine needle aspiration which confirmed carcinoma of the left thyroid gland with no clear differentiation between follicular or papillary carcinoma. He undergone a total thyroidectomy and left central level VI lymph node dissection His histology confirmed a thyroid paraganglioma staining strongly positive for neuroendocrine markers (Synaptophysin and chromogranin) while S-100 shows positivity in the sustentacular cells. He was referred for genetic testing which demonstrate no evidence of mutation in FH, SDHAF2, SDHB/C/D, RET, MAX, TMEM127 and VHL gene. He was commenced on levothyroxine replacement at a dose of 150micrograms OD. His urine metanephrines is 178.1pmol/L (0-510), urine normetanephrines 192.9pmol/L (0-1180) and 3-methoxytyramine <75pmol/ L (0-180) (all normal). His MRI neck revealed no synchronous tumour. He continues to be followed up under our endocrine clinic. Conclusion: Due to the rarity of these tumours, their natural history is mostly unknown. Nevertheless, postoperative surveillance should include plasma or urinary metanephrines and ultrasonography. References: 1 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3824793/

Urinary bladder paraganglioma metastatic to the lung in a patient with SDHB gene mutation: A case report

Introduction: Paragangliomas are tumors originating from the neural crest. Most of them are benign and arise from various locations in the body. Extra-adrenal paragangliomas arise as sporadic cases in most settings or as part of heredofamilial syndromes in about one-quarter of cases. Succinate dehydrogenase subunit B (SDHB) gene mutations are associated with an aggressive clinical disease course of pheochromocytoma/paraganglioma.Methods: We present a 41-year-old male former smoker with a history of a growing right upper lung nodule on chest imaging. He had no cough or respiratory symptoms. Twenty-seven months prior, the patient underwent a cystoprostatectomy due to paraganglioma of the bladder. Genetic testing identified a pathogenic mutation in SDHB gene, c.166_170delCCTCA (p.Pro56Tyrfs*5). He underwent a wedge resection of the lung nodule.Results: Sectioning of the lung wedge revealed a well-circumscribed, firm tan nodule. Microscopically there were nests of large neoplastic cells with round nuclei and eosinophilic granular cytoplasm. Tumor cells were positive for synaptophysin and chromogranin and negative for pan-cytokeratin. S-100 protein highlighted sustentacular cells. Morphologically, the pulmonary neoplasm was similar to the primary tumor of the bladder. These features are consistent with a bladder paraganglioma metastatic to the lung, in a background of a hereditary paraganglioma syndrome.Conclusion: Extra-adrenal paraganglioma occurring in a setting of hereditary paraganglioma syndrome has a high risk of metastasis. Lifelong surveillance even after prompt resection of the primary tumor with negative margins is required to ensure early detection of metastasis and prevent complications associated with it.

The odorant metabolizing enzyme UGT2A1: Immunolocalization and impact of the modulation of its activity on the olfactory response

Odorant metabolizing enzymes (OMEs) are expressed in the olfactory epithelium (OE) where they play a significant role in the peripheral olfactory process by catalyzing the fast biotransformation of odorants leading either to their elimination or to the synthesis of new odorant stimuli. The large family of OMEs gathers different classes which interact with a myriad of odorants alike and complementary to olfactory receptors. Thus, it is necessary to increase our knowledge on OMEs to better understand their function in the physiological process of olfaction. This study focused on a major olfactory UDP-glucuronosyltransferase (UGT): UGT2A1. Immunohistochemistry and immunogold electronic microscopy allowed to localize its expression in the apical part of the sustentacular cells and originally at the plasma membrane of the olfactory cilia of the olfactory sensory neurons, both locations in close vicinity with olfactory receptors. Moreover, using electroolfactogram, we showed that a treatment of the OE with beta-glucuronidase, an enzyme which counterbalance the UGTs activity, increased the response to eugenol which is a strong odorant UGT substrate. Altogether, the results supported the function of the olfactory UGTs in the vertebrate olfactory perireceptor process.

Evidence of SARS-CoV2 Entry Protein ACE2 in the Human Nose and Olfactory Bulb

Usually, pandemic COVID-19 disease, caused by SARS-CoV2, presents with mild respiratory symptoms such as fever, cough, but frequently also with anosmia and neurological symptoms. Virus-cell fusion is mediated by angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2) with their organ expression pattern determining viral tropism. Clinical presentation suggests rapid viral dissemination to the central nervous system leading frequently to severe symptoms including viral meningitis. Here, we provide a comprehensive expression landscape of ACE2 and TMPRSS2 proteins across human postmortem nasal and olfactory tissue. Sagittal sections through the human nose complemented with immunolabelling of respective cell types represent different anatomically defined regions including olfactory epithelium, respiratory epithelium of the nasal conchae and the paranasal sinuses along with the hardly accessible human olfactory bulb. ACE2 can be detected in the olfactory epithelium as well as in the respiratory epithelium of the nasal septum, the nasal conchae, and the paranasal sinuses. ACE2 is located in the sustentacular cells and in the glandular cells in the olfactory epithelium as well as in the basal cells, glandular cells, and epithelial cells of the respiratory epithelium. Intriguingly, ACE2 is not expressed in mature or immature olfactory receptor neurons and basal cells in the olfactory epithelium. Similarly, ACE2 is not localized in the olfactory receptor neurons albeit the olfactory bulb is positive. Vice versa, TMPRSS2 can also be detected in the sustentacular cells and the glandular cells of the olfactory epithelium. Our findings provide the basic anatomical evidence for the expression of ACE2 and TMPRSS2 in the human nose, olfactory epithelium, and olfactory bulb. Thus, they are substantial for future studies that aim to elucidate the symptom of SARS-CoV2 induced anosmia via the olfactory pathway.

SARS-CoV-2 infection causes transient olfactory dysfunction in mice

Olfactory dysfunction caused by SARS-CoV-2 infection represents as one of the most predictive and common symptoms in COVID-19 patients. However, the causal link between SARS-CoV-2 infection and olfactory disorders remains lacking. Herein we demonstrate intranasal inoculation of SARS-CoV-2 induces robust viral replication in the olfactory epithelium (OE), resulting in transient olfactory dysfunction in humanized ACE2 mice. The sustentacular cells and Bowman’s gland cells in OE were identified as the major targets of SARS-CoV-2 before the invasion into olfactory sensory neurons. Remarkably, SARS-CoV-2 infection triggers cell death and immune cell infiltration, and impairs the uniformity of OE structure. Combined transcriptomic and proteomic analyses reveal the induction of antiviral and inflammatory responses, as well as the downregulation of olfactory receptors in OE from the infected animals. Overall, our mouse model recapitulates the olfactory dysfunction in COVID-19 patients, and provides critical clues to understand the physiological basis for extrapulmonary manifestations of COVID-19.