Expression of the ACE2 Virus Entry Protein in the Nervus Terminalis Reveals the Potential for an Alternative Route to Brain Infection in COVID-19

Abstract

Previous studies suggested that the SARS-CoV-2 virus may gain access to the brain by using a route along the olfactory nerve. However, there is a general consensus that the obligatory virus entry receptor, angiotensin converting enzyme 2 (ACE2), is not expressed in olfactory receptor neurons, and the timing of arrival of the virus in brain targets is inconsistent with a neuronal transfer along olfactory projections. We determined whether nervus terminalis neurons and their peripheral and central projections should be considered as a potential alternative route from the nose to the brain. Nervus terminalis neurons in postnatal mice were double-labeled with antibodies against ACE2 and two nervus terminalis markers, gonadotropin-releasing hormone (GnRH) and choline acetyltransferase (CHAT). We show that a small fraction of CHAT-labeled nervus terminalis neurons, and the large majority of GnRH-labeled nervus terminalis neurons with cell bodies in the region between the olfactory epithelium and the olfactory bulb express ACE2 and cathepsins B and L. Nervus terminalis neurons therefore may provide a direct route for the virus from the nasal epithelium, possibly via innervation of Bowman’s glands, to brain targets, including the telencephalon and diencephalon. This possibility needs to be examined in suitable animal models and in human tissues.

The route of SARS-CoV-2 to brain infection: have we been barking up the wrong tree?

“…Taken together, these three studies support the notion that SARS-CoV-2 (or its shed spike protein) does not utilize the olfactory nerve to reach the brain, but rather uses a shortcut: neurons of the nervus terminalis (the little-known cranial nerve “0” [ 12 ]) which innervates the nasal cavity, and within the olfactory epithelium specifically the ACE2-expressing (and virus-accumulating) cells in Bowman glands [ 7 ] (Fig. 1 ).…”

“…Since this cranial nerve connects the olfactory epithelium directly with the hypothalamus, the authors concluded that the nervus terminalis may provide a route for the virus to reach the brain. The variability in neuro-invasion seen in animal models, even within the same species, fits better with the known variability of the nervus terminalis system (which differs an order of magnitude between members of the same species) than with the uniform olfactory system [ 12 ]. Sauve et al, in a not yet peer-reviewed preprint [ 9 ], showed that SARS-CoV-2 can skip the olfactory bulb on its way to the brain, but the virus does appear – in an animal model and in COVID-19 patients – in significant amounts in the hypothalamus, especially in the olfactory placode-derived neurons of the hypothalamus which also express the virus entry protein ACE2.…”

“…Since this cranial nerve connects the olfactory epithelium directly with the hypothalamus, the authors concluded that the nervus terminalis may provide a route for the virus to reach the brain. The variability in neuro-invasion seen in animal models, even within the same species, fits better with the known variability of the nervus terminalis system (which differs an order of magnitude between members of the same species) than with the uniform olfactory system [ 12 ].…”

“…This little-known cranial nerve connects the olfactory epithelium directly with brain structures caudal to the olfactory bulb. It exists in humans where it contains several hundred to more than one thousand neurons [ 11 , 12 ]. Three new studies shed more light on this alternate route.…”

“…Three new studies shed more light on this alternate route. Bilinska et al [ 12 ] showed that a major fraction of the neurons in the nervus terminalis express the virus entry protein ACE2. Since this cranial nerve connects the olfactory epithelium directly with the hypothalamus, the authors concluded that the nervus terminalis may provide a route for the virus to reach the brain.…”