Evidence of SARS-CoV2 entry protein ACE2 in the human nose and olfactory bulb

Abstract

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 symptom. 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 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, post-mortem 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 of via the olfactory pathway.

Histological Evidence for the Enteric Nervous System and the Choroid Plexus as Alternative Routes of Neuroinvasion by SARS-CoV2

“…This does not exclude the possibility of an ACE2-independent entry into the brain shuttled by immune cells ( Dahm et al, 2016 ). Additionally, a neuronal transmission of SARS-CoV2 via the olfactory nerve appears unlikely since ACE2 is expressed in the olfactory epithelium by sustentacular cells, but not olfactory receptor neurons ( Bilinska et al, 2020 ; Klingenstein et al, 2020 ).…”

Anosmia in COVID-19: Underlying Mechanisms and Assessment of an Olfactory Route to Brain Infection

“…This mechanism is supported by the abundant expression of the two entry proteins, ACE2 and TMPRSS2, in sustentacular cells in the olfactory epithelium ( Figs. 4 and 5 ; Bilinska and others 2020 ; Brann and others 2020 ; Chen and others 2020b ; Klingenstein and others 2020 — Table 2 ), and by the presence of the virus primarily, if not exclusively, in the sustentacular cells ( Bryche and others 2020 ; Meinhardt and others 2020 ). The initial reports of ACE2 expression in sustentacular cells based on RNAseq reported that only between 1% and 3% of these cells expressed ACE2 ( Brann and others 2020 ; Ziegler and others 2020 ), while immunocytochemistry indicated that the large majority, if not all sustentacular cells contain ACE2 protein ( Table 2 ).…”

“…They were also the first to localize the virus entry proteins to distinct cell types in the olfactory epithelium ( Bilinska et al 2020 ; Table 2 ). There is now an emerging consensus that mature olfactory neurons do not express the virus entry proteins, ACE2 and TMPRSS2, at least not at significant levels, and not in the large majority of the mature olfactory neurons in mouse and human ( Baxter and others 2020 ; Bilinska and others 2020 ; Brann and others 2020 ; Chen and others 2020b ; Fodoulian and others 2020 ; Gupta and others 2020 ; Klingenstein and others 2020 ; Ziegler and others 2020 —see Table 2 ). A recent study that localized the SARS-CoV-2 virus in the hamster olfactory epithelium confirmed this notion by showing that sustentacular cells contained the virus, but not olfactory neurons ( Bryche and others 2020 ).…”

“…It is now well established that most of the olfactory receptor neurons do not express the virus entry proteins, ACE2 and TMPRSS2 ( Baxter and others 2020 ; Bilinska and others 2020 ; Brann and others 2020 ; Chen and others 2020b ; Fodoulian and others 2020 ; Gupta and others 2020 ; Klingenstein and others 2020 ; Table 2 ), and consistent with the absence of the entry proteins, there is no convincing evidence that olfactory receptor neurons accumulate SARS-CoV-2, neither in animal models ( Bryche and others 2020 ; Sia and others 2020 ) nor in humans ( Meinhardt and others 2020 ). But just because the olfactory receptor neurons do not express the two entry proteins for the virus, or only at very low levels (TMPRSS2), is it safe to assume that SARS-CoV viruses cannot utilize the olfactory route to the brain?…”

Neurological impairments in COVID-19 pandemic

“…However, there is no strong evidence of the presence of ACE2 mRNA in the human olfactory bulb [9]. Another study found ACE2 protein in the olfactory bulb but could not localize it in neurons [17]. Furthermore, ACE2 is localized in glial cells [9,17], but the presence of SARS-CoV was reported in neurons only [18,19].…”